Catalytic Transformation of Biomass-Derived Furfurals to Cyclopentanones and Their Derivatives: A Review.

Furfural (FF) and 5-(hydroxymethyl)furfural (HMF) are well-recognized biomass-derived chemical building blocks with established applications and markets for several of their derivatives. Attaining a wide spectrum of petrochemicals is the primary target of a biorefinery that employs FF and HMF as the chemical feedstock. In this regard, cyclopentanone (CPN) is a crucial petrochemical intermediate used for synthesizing a diverse range of compounds with immense commercial prospects. The hydrogenative ring rearrangement of FF to CPN in an aqueous medium under catalytic hydrogenation conditions was first reported in 2012, whereas the first report on the catalytic conversion of HMF to 3-(hydroxymethyl)cyclopentanone (HCPN) was published in 2014. Over the past decade, several investigations have been undertaken in converting FF and HMF to CPN and HCPN, respectively. The research studies aimed to improve the scalability, selectivity, environmental footprint, and cost competitiveness of the process. A blend of theoretical and experimental studies has helped to develop efficient, inexpensive, and recyclable heterogeneous catalysts that work under mild reaction conditions while providing excellent yields of CPN and HCPN. The time is ripe to consolidate the data in this area of research and analyze them rigorously in a review article. This work will assist both beginners and experts of this field in acknowledging the accomplishments to date, recognize the challenges, and strategize the way forward.

Introduction

Chemical industries are undergoing an exhilarating transformation where the manufacturing processes, both revamped and newly developed, emphasize not only the process economics but also environmental perspectives.[1] Since the advent of green chemistry in the early 1990s, ecological aspects of the industrial processes are thoroughly evaluated from time to time.[2,3] The primary obstacle for the chemical industries to become truly sustainable is their unyielding dependence on exhaustible fossilized resources like petroleum. With the demand for petrofuels and petrochemicals reaching newer heights, concerns over the fast depleting petroleum reserves and the seemingly irreversible environmental degradations have intensified.[4] The full potential of green chemistry can be comprehended by utilizing a renewable carbon-based feedstock in the chemical industries. In this aspect, biomass is the only organic carbon in nature with the commercial prospect to replace petroleum, at least partially, in many futuristic scenarios.[5,6] Cellulosic biomasses (e.g., terrestrial lignocellulosic, freshwater and marine algae) are of particular interest since they are available in large excess, not considered food for humans, often regarded as wastes, geographically diverse, and inexpensive.[7] Lignocellulosic biomass primarily consists of biopolymers, such as cellulose (35–50%), hemicellulose (15–25%), and lignin (20–30%).[8] Hemicellulose is a nonlinear polymer consisting of hexose (e.g., glucose) and pentose (e.g., xylose) sugars, whereas cellulose is a linear, crystalline polymer of glucose connected by the β-1,4-glycosidic bond. Lignin is a complex polymer of substituted phenolic compounds connected by ether and ester linkages. The chemical-catalytic pathways for biomass value addition are fast, selective, and biomass-independent and work under reagent-economic and energy-efficient conditions.[9,10] A frequently used strategy initially hydrolyzes the hemicellulose and cellulose fractions into the constituent sugars, which are subsequently dehydrated into furanic compounds under acid catalysis for downstream synthetic upgrading.[11,12] The lignin fraction is typically separated and combusted to generate process heat and electricity. The catalytic deconstruction of lignin to phenolic fuels and chemicals has gained traction in recent years.[13,14]

The acid-catalyzed dehydration of pentose sugars in hemicellulose forms furfural (FF), whereas dehydration of hexoses in cellulose and hemicellulose produces 5-(hydroxymethyl)furfural (HMF).[15] The acid-catalyzed dehydration process sequentially eliminates water molecules from the sugar moieties without forming any significant waste streams. Both FF and HMF are known for over a century and have significant literature presence as renewable chemical platforms.[16] The production and derivative chemistry of FFs have been explored and perfected over the past several years. Many of the derivatives of FF, such as furfuryl alcohol (FAL), have well-known applications and established markets.[17] Many of the synthetic transformations of FF and HMF to bulk and fine chemicals employ chemocatalytic pathways.[18] Catalysis that grew as a separate field in its initial years of development has found widespread applications in organic synthesis and the chemistry of renewables.[19] A new generation of inexpensive, eco-friendly, and recyclable catalysts (homogeneous and heterogeneous) are being developed that can produce the targeted compound(s) in desirable selectivity and yield.[20,21] A wide range of transportation fuels, chemicals, and polymers have been synthesized by the structural modifications of FF and HMF in the presence of a suitable catalyst.[22−25] Interestingly, FF can be converted into HMF by formylation at the C5-position, whereas HMF can be converted to FAL by the decarbonylation reaction.[26,27] Therefore, the derivatives of HMF can potentially be accessed from FF and vice versa.

Two major categories of products are sourced from biomass. The first category of bio-based products is structurally relatable to the existing products of the petrochemical origin, has analogous properties, and functions as their renewable replacement of the latter.[28,29] In contrast, the second category of bio-based products is the drop-in equivalent of the petrochemicals that can be seamlessly integrated into the existing infrastructure and markets.[30] Novel synthetic strategies must be developed that allow transforming FF and HMF into drop-in petrochemicals, widen their derivative chemistry, and strengthen their value chains in a biorefinery setup.

In this regard, cyclopentanone (CPN) is a chemical intermediate for synthesizing several compounds of commercial significance, such as solvents, fragrances, cosmetic products, and agrochemicals.[31,32] CPN is commercially produced by the intramolecular decarboxylative ketonization of adipic acid.[33] The process has been optimized over the years, and Ba(OH)2 was found to be an efficient and recyclable base catalyst.[33] The diesters of adipic acid, such as dimethyl adipate, have also been used to synthesize CPN in the vapor phase.[34] However, the feedstock (i.e., adipic acid) is relatively expensive, requires a multistep synthesis, and produces significant waste streams during its preparation.[35] CPN can also be produced by the hydration of cyclopentene (obtained by the steam cracking of naphtha) and dehydrogenation of the cyclopentanol (CPL) intermediate. Cyclopentene can be reacted with acetic acid by addition–esterification and then transesterified with methanol to form CPL. However, the processes require an expensive catalyst and harsh reaction conditions (280–300 °C, 25–40 MPa) and produce significant waste streams, adversely affecting the scalability and process economy. Besides, both the processes described above typically use starting materials from exhaustible fossilized resources and have questionable sustainability.

In this regard, FF has attracted attention as a plausible renewable chemical feedstock for CPN since the former is a C5 compound of renewable origin (Figure ).

Figure 1

Synthetic route of CPN from petroleum and its prospective preparation from biomass.

FF has an oxidation level of 6 (three π-bonds, one ring, and two oxygen atoms), whereas CPN has 3 (one π-bond, one ring, and one oxygen atom).[36] The chemical equation for transforming FF into CPN reveals that 3 moles of hydrogen are required per mole of FF, forming 1 mole of water as the byproduct (Scheme A). Analogously, HMF has an oxidation level of 7 (three π-bonds, one ring, and three oxygen atoms) compared to 4 in 3-(hydroxymethyl)cyclopentanone (HCPN) (one π-bond, one ring, and two oxygen atoms) (Scheme B).

Scheme 1

Catalytic Conversion of (A) Xylose to CPN via FF and (B) Glucose to HCPN via HMF

CPL is generally formed as a minor side product by the over-reduction of CPN during its preparation from FF. CPL has potential applications as dyes, pharmaceutical products, fragrance agents, and solvents. Besides, it is a promising feedstock for various biofuels, including jet and aviation fuels. Targeted production of CPL from FF has also been attempted and is discussed in detail in Section .

Mechanistic Details of Forming CPN and HCPN

Several research groups have investigated the mechanism of forming CPN and HCPN starting from FF and HMF, respectively. Detailed understanding of the mechanistic pathway is crucial for process development by assisting in designing more efficient catalysts and fine-tuning the reaction parameters. The reaction starts with hydrogenating the aldehyde group of FF and HMF to FAL and 2,5-bis(hydroxymethyl)furan (BHMF), respectively. Selectivity of the first step primarily depends on the type of metal used, morphology and dispersity of metal atom clusters, and hydrogen gas pressure. The intermediates then undergo an acid-promoted ring-rearrangement reaction (Scheme ) named Piancatelli rearrangement reaction. The Piancatelli rearrangement of FAL and BHMF in the presence of acidic catalysts results in the formation of 4-hydroxy-2-cyclopentanone (1) and 4-hydroxy-(4-hydroxymethyl)-2-cyclopentanone (2) (Scheme A). The second step mainly consists of breaking the C–O bond and forming a C–C bond. Hydrogenation and dehydration of 1 form 2-cyclopentenone (CPEN), whereas 2 leads to 4-hydroxymethyl-2-cyclopentenone (HCPEN). Finally, hydrogenation of the olefinic group in CPEN and HCPEN leads to CPN and HCPN, respectively (Scheme B).[37−40]

Scheme 2

(A) Acid-Catalyzed Piancatelli Rearrangement of FF and HMF to 1 and 2, Respectively; (B) Catalytic Reduction of 1 and 2 to CPN and HCPN, Respectively

The production of HCPN from HMF via the 1-hydroxyhexane-2,5-dione (HHD) intermediate using a combination of hydrogenation, molecular rearrangement via ring opening, and aldol condensation reactions is an alternative mechanism (Scheme ). HHD is formed by the acid-catalyzed rehydration of BHMF, followed by ring-opening and hydrogenation steps. Subsequent intramolecular aldol condensation of HHD, followed by hydrogenation, forms HCPN.[41,42]

Scheme 3

Alternative Mechanistic Pathway for the Synthesis of HCPN from HMF

Catalytic Conversion of FF into CPN

The selective production of CPN from FF without involving any C–C bond breaking could be visualized under conditions that permit catalytic hydrogenation and molecular rearrangement reactions. Table tabulates the catalyst details, optimized reaction parameters, and the best yield of CPN for all published work on the targeted conversion of FF into CPN since its discovery in 2012.

Table 1

Catalytic Preparation of CPN from FF (in Reverse Chronological Order of the Date of Publication)a

entry	catalyst	reaction conditions	yield (%)b	refs
1	5% Pt/C	FF (1.0 g), catalyst (0.05 g), water (20 mL), 160 °C, 8 MPa H2, 0.5 h	76.5	(43)
2	5% Pt/C0.05	FF (1.0 g), catalyst (0.05 g), water (20 mL), 160 °C, 8 MPa H2, 0.5 h	76.5	(44)
3	NiCu-50/SBA-15	FF (0.5 g), catalyst (0.20 g), water (9.5 mL), 160 °C, 4 MPa H2, 4 h	62	(45)
4	5% Pd/C	FF (1.0 g), catalyst (0.10 g), water (20 mL), 175 °C, 8 MPa H2, 0.5 h	67	(46)
5	CuZnAl-500-0.5	FF (0.480 g), catalyst (0.2 g), water (15 mL), 150 °C, 4 MPa H2, 6 h	62	(47)
6	5% Cu-Co-CP-500	FA (0.192 g), catalyst (0.05 g), water (10 mL), 170 °C, 2 MPa H2, 1 h	67	(48)
7	Cu–Ni–Al–HT	FF (5.8 g), catalyst (1.5 g), water (95 mL), 140 °C, 4 MPa H2, 8 h	95.8	(49)
8	20 wt % Ni/HY-0.018	FF (5 wt %), catalyst (1.5 wt %), water, 150 °C, 4 MPa H2, 9 h	86.5	(50)
9	3% Ru/MIL-101	FF (0.5 g), catalyst (Ru 0.28 mol %), water (5 mL), 160 °C, 4 MPa H2, 2.5 h	96	(51)
10	5% Pd–10% Cu/C	FF (1.0 g), catalyst (0.010 g), water (20 mL), 160 °C, 3 MPa H2, 1 h	92.1	(37)
11	Au/TiO2-A	FF (4.804 g), catalyst (0.0048 g), water (100 mL), 160 °C, 4 MPa H2, 15 h	99	(52)
12	CuNi0.5@C	FF (5.0 g), catalyst (0.1 g), water (95 mL), 130 °C, 5 MPa H2, 5 h	96.9	(53)
13	6 wt % Ru/CNTs	FF (0.46 g), catalyst (0.02 g), water (40 mL), 160 °C, 1 MPa H2, 5 h	90	(54)
14	Cu/ZnO	FF (5.8 g), catalyst (0.8 g), water (95 mL), 140 °C, 4 MPa H2, 6 h	85	(55)
15	5 wt % Pt/NC-BS-800	FF (0.24 g), catalyst (0.025 g), water (10 mL), 150 °C, 3 MPa H2, 4 h	76	(56)
16	(17 + 3)% CuZn/CNT	FF (2.5 g), catalyst (0.05 g), water (50 mL), 140 °C, 4 MPa H2, 10 h	85.3	(57)
17	10% Co–10% Ni/TiO2	FF (0.5 g), catalyst (0.3 g), water (10 mL), 150 °C, 4 MPa H2, 4 h	53.3	(58)
18	Cu/ZrO2-500	FF (0.480 g), catalyst (0.05 g), water (15 mL), 150 °C, 1.5 MPa H2, 4 h	91.3	(40)
19	4% Pd/f-SiO2	FF (5 g), catalyst (0.5 g), water (95 mL), 165 °C, 3.44 MPa H2, 5 h	89	(59)
20	Pt(3)Co(3)/C	FF (0.35 g), catalyst (0.078 g), toluene/water (15/20 mL), 180 °C, 1 MPa H2, 5 h	75	(60)
21	Co@NCNTs-600-800	FF (0.096 g), catalyst (0.03 g), water (10 mL), 140 °C, 4 MPa H2, 5 h.	75.3	(61)
22	5% Ru/AC	FF (0.096 g), catalyst (0.03 g), iron powder (0.39 g), water (20 mL), 165 °C, 0.5 MPa N2, 5 h	25	(62)
23	Ru/C (0.5 wt %) + Al11.6PO23.7	FF (1.921 g), Ru/C (0.25 g), Al11.6PO23.7 (0.25 g), water (50 mL), 160 °C, 2 MPa H2, 4 h.	78	(63)
24	Zn/Co-ZIF-700	FF (0.25 g), catalyst (0.05 g), water (30 mL), 130 °C, 0.7 MPa H2, 2 h	50.5	(64)
25	Pd/Fe-MIL-100	FF (1.0 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 6 h	92.2	(65)
26	Cu0.4Mg5.6Al2	FF (0.096 g), catalyst (0.064 g), water (2 mL), 180 °C, 0.2 MPa H2, 4 h	98.1	(66)
27	Pd/Cu–BTC	FF (1.0 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 6 h	93	(67)
28	Pd@N–C	FF (0.776 g), catalyst (0.1 g), water/2-propanol (19/1 mL), 120 °C, 1 MPa H2, 6 h	85	(68)
29	10% Cu/Fe3O4	FF (0.1 g), catalyst (0.05 g), water (10 mL), 170 °C, 3 MPa H2, 4 h	91	(69)
30	NiFe/SBA-15	FF (6.0 g), catalyst (1.2 g), water (100 mL), 160 °C, 3.4 MPa H2, 6 h	90	(70)
31	Pd/FeZn–DMC	FF (1.0 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 6 h	96.6	(71)
32	Pd–Bi/SiO2	FF (100 mM), catalyst (0.001–0.05 g), water (90 mL), 150 °C, 5 MPa H2, 2.3 h	54.6	(72)
33	Pd–Co@UiO-66	FF (0.172 g), catalyst (0.03 g), water (20 mL), 120 °C, 3 MPa H2, 12 h	95	(73)
34	Ni–Fe(3.0)/TiO2	FF (0.1 g), catalyst (0.05 g), ethanol/water (1.5/2 mL), 170 °C, 3 MPa H2, 6 h	27.2	(74)
35	Pd/Y2(Sn0.7Ce0.3)2O7−δ	FF (1.0 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 6 h	95	(75)
36	1 wt % Pd/CMK-5	FF (1.0 g), catalyst (0.02 g), water (20 mL), 160 °C, 3 MPa H2, 5 h	40.2	(76)
37	10% Ni/CNTox	FF (0.232 mol/L), catalyst (0.05 g), dodecane/water (8/8 mL), 200 °C, 2 MPa H2, 1 h	25	(77)
38	1% Pd/CNT	FF (2.32 g), catalyst (0.2 g), water (50 mL), 150 °C, 3 MPa H2, 1 h	79	(38)
39	Pd/TiO2	FF (2.5 g), catalyst (0.1 g), water (100 mL), 170 °C, 2 MPa H2, 4 h	55.5	(78)
40	Ni–NiO/TiO2-Re450	FF (0.096 g), catalyst (0.05 g), water (10 mL), 140 °C, 1 MPa H2, 6 h	87.4	(79)
41	Ni/SiC + CrCl3	FF (0.093 g), catalyst (0.00093 g), CrCl3 (0.25 wt %), water (4 mL), 160 °C, 3 MPa H2, 2 h.	88.1	(80)
42	Ni2Cu1/Al2O3	FF (0.5 g), catalyst (0.02 g), water (15 mL), 140 °C, 1 MPa H2, 1 h	89.5	(81)
43	CuZnAl-LDH	FF (0.116 g), catalyst (40 cm2), water (20 mL), 140 °C, 2 MPa H2, 2 h	86.5	(82)
44	15% Ni–10% P/γ-Al2O3	FF (0.040 g), catalyst (0.04 g), water (3.96 mL), 150 °C, 3 MPa H2, 2 h	85.8	(83)
45	2Co-1Ni@NC-800	FF (0.3 g), catalyst (0.05 g), water (12 mL), 150 °C, 1.5 MPa H2, 6 h	92.5	(84)
46	CoNP@N–CNTs	FF (0.096 g), catalyst (0.015 g), water (10 mL), 160 °C, 0.5 MPa H2, 8 h	95	(85)
47	Pd/7.74% Y2(Sn0.65Al0.35)2O7−δ/Al2O3	FF (0.480 g), catalyst (0.05 g), water (20 mL), 150 °C, 4 MPa H2, 6 h	98.1	(86)
48	Ni(10%)/CuFe2O4	FF (0.0960 g), catalyst (0.05 g), water (10 mL), 150 °C, 1 MPa H2, 9 h	80	(87)
49	2% Ni/SiO2	FF (4.0 g), catalyst (0.04 g), water (3.96 mL), 160 °C, 3 MPa H2, 3 h	83.5	(88)
50	CuNi/Al-MCM-41	FF (0.5 g), catalyst (0.0625 g), water (10 mL), 160 °C, 2 MPa H2, 5 h	96.7	(89)
51	Pd/La2Ti2O7	FF (1.0 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 6 h	98	(42)
52	Pd/UiO-66-NO2	FF (0.1 g), catalyst (0.005 g), water (9.9 mL), 150 °C, 1 MPa H2, 5 h	95.5	(90)

Abbreviations: DMC, double-metal cyanide; NPs, nanoparticles; CuZnAl-500-0.5 (Cu/Zn = 0.5, calcined at 500 °C); CP, coprecipitation method; CNTs, carbon nanotubes; Au/TiO2-A, Au deposited on a single-phase anatase; ZIF, zeolite imidazolate framework; ML-101, chromium terephthalate metal–organic framework; NC-BS, heteroatom-doped carbon materials from biomass; SBA, Santa Barbara amorphous; CNTox, functionalized carbon nanotube; Re, rutile; f-SiO2, fumed silica; HT, hydrotalcite; LDH, layered double hydroxide; AC, activated carbon; N–C, N-doped graphitic carbon; CMK-3, ordered mesoporous carbon; BTC, 1,3,5-benzene-tricarboxylate.

In cases where yield data was not mentioned, it was calculated by multiplying conversion of FF with the selectivity toward CPN.

Hronec et al. reported the first preparation of CPN in an attempt to catalytically hydrogenate FF in the aqueous medium using noble and non-noble metal catalysts supported on carbon.[43] When the reaction was performed in an organic solvent like n-butanol or tetrahydrofuran (THF), FAL and 2-methylfuran (2MF) were obtained as the major products. However, CPN was obtained in decent yields in the aqueous medium. When 5% Pt/C was used as the catalyst in a 10 wt % loading, quantitative conversion of FF was achieved at 175 °C for 30 min under 8 MPa H2, where CPN and CPL were obtained in 40 and 36% yields, respectively. Palladium, ruthenium, and nickel catalysts showed similar trends. CPN was obtained in a maximum of 76.5% yield when the reaction was conducted for 30 min at 160 °C under 8 MPa of H2 pressure and 5 wt % Pt/C catalyst.

In a later report, the authors studied the effect of solvents and various carbon-supported noble metal catalysts on CPN selectivity starting from FF and FAL.[44] When the temperature was increased beyond 160 °C, the selectivity toward CPL increased. Other metals like Pd and Ru also afforded lesser selectivity and yield of CPN. When the solvent was changed to 1-butanol, almost no CPN was observed. The use of mineral acids or inorganic bases as additives had a detrimental effect on the yield of CPN.

Yang et al. studied the efficiency of Ni–Cu bimetallic catalysts supported on SBA-15 (NiCu/SBA-15) for the hydrogenation of FF to CPN in an aqueous medium.[45] The Ni–Cu molar ratio was varied, and the best activity was achieved at the equimolar ratio (NiCu-50/SBA-15). The nickel catalyst formed CPN in poor yield, whereas the monometallic copper catalyst did not produce CPN at all. When 40 wt % NiCu-50/SBA-15 catalyst was used (compared to the amount of FF used), CPN was obtained in 62% yield under optimized conditions (160 °C, 4 MPa H2, 4 h). No CPN or CPL was observed when CH2Cl2, CH3OH, or 1,4-dioxane was used as the solvent. However, a mixture of 1,4-dioxane and water formed CPN, validating the requirement of water for the transformation. When basic salts like Na2HPO4 were used, tetrahydrofurfuryl alcohol (THFAL) was formed preferably. A slightly acidic medium using NaH2PO4 or acetic acid had decent selectivity toward CPN. However, stronger acids led to extensive resinification of FF.

Hronec et al. also studied the effects of the FAL polymer (FALP), formed by the acidified water formed at elevated reaction temperatures and deposited on the surface of commercial catalysts (e.g., 5% Pt/C, 5% Pd/C) during the synthesis of CPN from FF.[46] Adsorption and desorption of FALP depend on the type of metal used in the catalyst for the hydrogenation/hydrogenolysis reactions.

A series of CuZnAl catalysts with different Cu/Zn molar ratios and calcination temperatures were prepared, characterized, and investigated for their performance in converting FF to CPN in the aqueous medium.[47] A 62% yield of CPN was achieved under optimized conditions (150 °C, 4 MPa H2, 6 h) using the CuZnAl-500-0.5 catalyst (Cu/Zn = 0.5, calcined at 500 °C). The conversion of FF and the selectivity toward CPN decreased with increasing Cu/Zn molar ratio. The results were explained by the high Cu dispersion, the high Cu specific surface area, and the small Cu particle size in the case of the CuZnAl-500-0.5 catalyst. The alumina support was also found to improve the catalytic activity by affording higher dispersion and a smaller particle size of the active metal for hydrogenation, that is, copper. The catalyst calcined at 350 °C had noticeably lower stability, whereas the catalyst calcined at 700 °C led to a small pore size and pore volume. The CuZnAl-500 catalyst, having good stability and a large pore volume, had the best performance. The inexpensive catalyst showed good activity, stability, and recyclability for five consecutive catalytic cycles.

A copper–cobalt bimetallic catalyst was prepared by the coprecipitation method, calcined at 500 °C (5% Cu@Co-CP-500), characterized, and used to prepare CPN from FF.[48] CPN was obtained in 67% yield under optimized reaction conditions. The process was optimized on several process parameters, including temperature, hydrogen pressure, loading of Cu, and calcination temperature. The calcination temperature and the process of catalyst preparation were crucial for the selectivity toward CPN. For example, the catalyst prepared by the sol–gel method provided mostly CPL (ca. 68%) under similar reaction conditions. The catalyst calcined at 500 °C showed marginally better performance than that calcined at 400 °C. However, a calcination temperature of 600 °C or more markedly lowered the catalyst activity.

A series of Cu–Ni–Al hydrotalcite (Cu–Ni–Al HT) catalysts with a (Cu + Ni)/Al mole ratio of 3 and varying molar ratios of Cu/Ni were synthesized via the coprecipitation method and used for the preparation of CPN from FF in the aqueous medium.[49] The reaction was optimized on various parameters like temperature, duration, the molar ratio of Cu/Ni, and hydrogen pressure. The yield of CPN reached 95.8% when the reaction was conducted at 140 °C for 8 h under a 4 MPa of H2 pressure. The conversion of FF and the yield of CPN decreased roughly by 10% after each reuse due to the agglomeration of Cu and Ni particles on the catalyst surface and lowering their hydrogenating capability.

Nickel-bearing hierarchical Y zeolite (HY) catalysts were synthesized using sodium alginate as the template.[50] The FF to CPN transformation was parameterized on the reaction temperature, pressure of H2 gas, and catalyst loading. Under optimized conditions (20 wt %Ni/HY-0.018 catalyst, 150 °C, 4 MPa H2, 9 h), an 86.5% yield of CPN was obtained at a 96.5% conversion of FF. The excellent activity of the HY catalyst was explained by its high crystallinity, high Brunauer–Emmett–Teller (BET) surface area, abundant mesopores, and suitable acidity that are controlled by the template (i.e., sodium alginate) used.

Ruthenium nanoparticles (NPs) supported on acidic MOF (MIL-101) afforded a quantitative conversion of FF with a 96% selectivity toward CPN under optimized parameters (3% Ru/MIL-101, 160 °C, 4 MPa H2, 2.5 h).[51] The Ru/MIL-101 catalyst had a large BET surface area (ca. 3000 m2/g), a pore size of 3 nm, and thermal stability up to 310 °C. The catalyst showed excellent recyclability, and no noticeable loss of catalytic activity was observed even after the sixth cycle.

Pd–Cu bimetallic catalysts supported on activated carbon (Pd–Cu/C) with varying Pd–Cu molar ratios were prepared using a prereduced 5% Pd/C catalyst.[37] Pd(0) and Cu(I) sites on the catalyst surface were responsible for excellent catalytic activity in transforming FF to CPN. A 92.1% yield of CPN was achieved under optimized conditions using a 5% Pd–10% Cu/C (1 wt % loading) catalyst. The Cu/C catalyst favored ring hydrogenation of FF, resulting in a poor yield (ca. 7%) of CPN. The 5% Pd/C catalyst, on the other hand, gave around a 57% yield of CPN. The addition of copper sites on the 5% Pd/C catalyst significantly improved the activity and selectivity of the bimetallic catalyst. The preparation method also had a significant impact on catalyst activity. The catalyst prepared by the coprecipitation method had lower activity than that prepared by the electroless plating method starting from a copper(II) tartrate complex.

Gold NPs supported on anatase TiO2 (Au/TiO2-A) were used as a versatile catalyst for converting FF to CPN in the aqueous medium.[52] The Lewis acidic sites on TiO2 minimized the side products and improved the selectivity toward CPN. A 99% yield of CPN was achieved by conducting the reaction under optimized parameters (160 °C, 4 MPa H2, 1.2 h) using 0.10 wt % Au/TiO2-A catalyst. The high selectivity toward CPN was explained by the low concentration of FAL in the reaction mixture, minimizing the formation of side products like THFAL. The catalyst was successfully reused for five consecutive cycles without any noticeable loss of activity. Other substrates like 5-methylfurfural (MFF) and 2-acetylfuran have also been converted to the corresponding CPN derivatives in excellent (>95%) yields.

Wang et al. reported using CuNi bimetallic NPs embedded in the carbon matrix (CuNi@C), prepared by the thermolysis of Cu-based MOFs impregnated with Ni(NO3)2, as the catalyst for transforming FF to CPN.[53] Cu catalysts showed a strong ability to polarize the C=O bond while leaving the furan ring unaffected, and the addition of Ni particles improved the catalytic performance due to the synergistic effect between the two metals. The catalyst had a high surface area (ca. 91.6 m2/g) containing nanosized (15 nm) copper and nickel particles when the molar ratio of nickel to copper was adjusted to 0.5. The CuNi0.5@C catalyst showed the best catalytic performance with a 96.9% yield of CPN under optimum conditions (130 °C, 5 MPa H2, 5 h), and the catalyst was recycled for four consecutive cycles.

Ruthenium NPs supported on carbon nanotubes (CNTs) (Ru/CNT) were efficient catalysts for converting FF to CPN under relatively mild reaction conditions.[54] CPN was obtained in a 90% yield under optimized conditions (160 °C, 1 MPa H2, 0.26 wt % Ru, 5 h) at a 99% conversion of FF. Water was found as the best medium for synthesizing CPN. The catalyst was recycled for three cycles with only a marginal loss of CPN yield due to the leaching of Ru particles from the support or aggregation of Ru NPs.

Various metal-oxide and zeolite-supported Cu catalysts were prepared and applied for the hydrogenative ring rearrangement of FF to CPN.[55] Cu/ZnO, having a good balance of surface acidity, gave an 85% yield of CPN at the quantitative conversion of FF and an excellent mass balance at 97.2% when the reaction was conducted at 140 °C under a 4 MPa H2 pressure. When an acidic support like ZSM-5 was used, the reaction led to oligomers, whereas basic supports like MgO led to FAL. Supports containing both acid and basic sites, such as SiO2, Al2O3, and ZnO, formed CPN as the major product.

Liu et al. reported the preparation, characterization, and application of Pt NPs supported on bamboo shoot-derived porous heteroatom-doped carbon materials (Pt/NC-BS) as the catalyst to synthesize CPN from FF.[56] The Pt/NC-BS-500 catalyst, prepared at the calcination temperature of 500 °C, gave a nearly quantitative yield of FAL at 100 °C and a H2 pressure of 1 MPa. However, the selectivity changed to CPN at 150 °C and a 3 MPa H2 pressure when Pt/NC-BS-800 was used as the catalyst, affording >76% yield of CPN. The large surface area explained the high activity of the catalyst with a hierarchical porous structure, a high content of nitrogen and oxygen functionalities, high dispersion of the Pt NPs, good water dispensability, and stability under the reaction conditions.

Copper-based catalysts supported on CNTs and promoted by metals like Co and Zn were prepared. The CuZn/CNT catalyst provided better selectivity and yield of CPN under relatively mild conditions.[57] The effects of process parameters, including metal compositions, reaction temperature, duration, and hydrogen pressure, on the product distribution were investigated in detail. Under optimized conditions (140 °C, 4 MPa H2, 10 h), the (17 + 3)% CuZn/CNT catalyst provided an 85.3% yield of CPN. The catalyst was successfully recycled for three catalytic cycles with a <5% lowering in the yield of CPN. The slight loss of catalytic activity was attributed to the coke formation on the active sites, corroborated by the thermogravimetric data.

A series of Co–Ni bimetallic catalysts on various heterogeneous supporting materials were prepared by the wetness impregnation method.[58] The catalytic efficiency and selectivity depended on Co–Ni leadings and the supporting material used. When 10% Co–10% Ni/TiO2 was employed as the catalyst, CPN was isolated in around 53% yield under the optimized conditions (150 °C, 4 MPa H2, 4 h). The catalyst showed excellent reusability for three consecutive runs with only marginal loss of yield for CPN. The lowering of catalytic activity in the subsequent runs was attributed to the leaching of metals from the support. Interestingly, switching the catalyst to 20% Ni/TiO2 changed the selectivity, and CPL was obtained as the main product in a 45.4% yield.

Favorable interactions between surface Cu species (Cu0 and Cu+) and the ZrO2 support greatly facilitated the formation of the Cu+–O–Zr-like structure at the metal–support interface and attributed to the high catalytic performance of the Cu/ZrO2 catalyst in synthesizing CPN from FF.[40] A 91.3% yield of CPN was ensured under relatively mild reaction conditions (150 °C, 1.5 MPa H2). The calcination temperature of 500 °C was found appropriate for satisfactory surface properties and catalytic activity. The catalyst prepared at the calcination temperature of 400 °C had comparable selectivity toward CPN. However, when the calcination temperature was increased to 600 °C, the selectivity toward CPN decreased markedly due to the lower surface area of copper and fewer Cu+ species on the catalyst surface. Other supporting materials like Al2O3 and ZnO provided inferior yields of CPN.

Pd NPs (5–13 nm) supported on different types of silica were examined as catalysts for converting FF to CPN in the aqueous medium.[59] Palladium supported on fumed silica (4% Pd/f-SiO2) showed the best activity that afforded an 89% yield of CPN at 165 °C and 3.44 MPa H2, whereas other silica-based supports led to hydrogenation of the furan ring. Lower reaction temperatures gave both low conversion of FF and low selectivity toward CPN. An increase in the pressure of H2 led to form THFAL and 2MF. The authors proposed the formation of CPN from FF via the Piancatelli mechanistic pathway. The catalyst was recycled for three cycles without significant loss of activity or selectivity, and the regenerated catalyst regained the original activity.

The PtCo bimetallic catalyst supported on carbon (PtCo/C) showed excellent activity for the hydrogenative ring opening of FF and selectivity toward CPN.[60] More specifically, the Pt(3)Co(3)/C catalyst (∼3 wt % loading for both Pt and Co metals) provided a 75% yield of CPN in a toluene/water biphasic system when the reaction was performed for 5 h at 180 °C under a 1 MPa H2 pressure. The bimetallic catalyst in other weight ratios and the monometallic catalysts gave inferior yields of CPN. With the increase in Pt loading in the bimetallic catalyst, both conversion and yield of CPN increased rapidly. The idea of using toluene was to extract CPN as it forms in the reaction mixture and safeguard it from possible side reactions. The hypothesis was proved by studying the effect of stirring on the yield of CPN. At a relatively low stirring speed of 300 rpm, the yield of CPN was only 20% due to insufficient extraction of CPN in the toluene layer, which increased rapidly to 75% at 900 rpm. Unfortunately, the catalyst suffered a significant loss of activity after the first run but partly recovered after prereduction at 400 °C.

Bamboo-like N-doped CNT-encapsulated Co and Ni NPs were efficient catalysts for the aqueous-phase transformation of FF to CPN.[61] The CNTs were created using melamine as the carbon source. The Co@NCNTs-600-800 catalyst gave 75.3% CPN at 140 °C, whereas FAL was obtained in a quantitative yield at 80 °C. The high catalytic activity was explained by the synergistic effect between metallic cobalt and N-doped CNTs. The catalyst showed excellent stability under the reaction conditions and good recyclability. On the other hand, the Ni@NCNTs-600-800 catalyst provided a nearly quantitative yield of THFAL at 100 °C and a 4 MPa H2 pressure in the aqueous medium.

The synthesis of CPN from FF was carried in a batch reactor using a combination of 5% Ru/AC (AC: activated carbon) and iron powder. The reaction provided a 25% yield of CPN when conducted for 5 h at 165 °C under a 0.5 MPa N2 pressure.[62]

A combination of Ru/C (0.5 wt %) and Al11.6PO23.7 was used for FF to CPN transformation in the aqueous medium.[63] A good balance of the Brønsted and Lewis acid sites on Al11.6PO23.7 was credited for the high activity and selectivity of the catalyst. A 78% yield of CPN was obtained at the quantitative conversion of FF under optimized conditions. When the H2 pressure was increased to 4 MPa, CPL was isolated in 84% yield. Interestingly, when a combination of Pt/C and Al11.6PO23.7 was used, CPN was isolated in 81% yield after 8 h at 160 °C and a 4 MPa H2 pressure. Therefore, the hydrogenating metal and the hydrogen pressure greatly influenced the selectivity of CPN and CPL. The catalyst was simply filtered and washed with methanol before resubmitting for the consecutive catalytic cycles. The catalyst retained its catalytic activity for six cycles.

Direct calcination of the bimetallic Zn/Co zeolitic imidazolate framework (Zn/Co-ZIF) produced ZnO/Co@N–CNTs, which was used as a magnetically separable catalyst for the hydrogenation of various biomass-derived compounds.[64] The catalyst calcined at 700 °C gave a 50.5% yield of CPN, along with 45.7% FAL and 3.8% CPL. The catalyst morphology and activity could be maintained even after five consecutive runs, which was attributed to the strong interaction between ZnO and Co NPs.

Li et al. uniformly dispersed the NPs of noble metals like Ru, Pt, Pd, and Au on the internal surface of the MOF supports (e.g., Fe-MIL-100, Fe-MIL-101, Cr-MIL-101).[65] Among the MOFs studied, Fe-MIL-101 showed the fastest hydrogenation rate, explained by the highest dispersion of metal NPs on its surface. Better selectivity toward CPN using the Fe-MIL-101 catalyst, compared to Cr-MIL-101, was due to the oxophilic nature of the Fe centers within the catalyst that promoted the hydrolytic ring rearrangement reaction forming CPN. A 92.2% yield of CPN was obtained under optimized reaction conditions (150 °C, 4.0 MPa H2, 6 h).

An inexpensive mixed-metal catalyst Cu0.4Mg5.6Al2 (6 mol % Cu) was prepared and used to convert FF to CPN.[66] At 110 °C and a 2 MPa H2 pressure, FAL was obtained in nearly quantitative yield. However, by increasing the temperature to 180 °C and lowering the pressure to 0.2 MPa, CPN formed in a 98.1% yield. The selectivity changed to CPL when the reaction was conducted at 190 °C under a 2 MPa H2 pressure. In all processes, no coke formation was observed. The catalyst was successfully reused for five consecutive cycles, and only marginal loss of activity was observed.

Palladium NPs supported on Cu-MOFs [e.g., Cu3(BTC)2] with Lewis acidity were synthesized, characterized, and applied as catalysts for the transformation of FF and HMF into CPN and HCPN, respectively.[67] Acidic sites on the catalyst were crucial for the hydrolysis reaction, whereas dispersion of Pd NPs onto the catalyst surface was important for the hydrogenation reaction. Palladium NPs had a high affinity toward Cu, and the clusters of Pd were mostly found in the pores of Cu–BTC MOF. The supports with Brønsted acidity were found to have lesser selectivity toward CPNs than the Lewis acidic catalysts and promoted humin formation instead. CPN and HCPN were obtained in >90% yields under optimized reaction parameters.

Palladium NPs were uniformly dispersed on chitosan-derived N-doped graphitic carbon (N–C) by carbonizing the Pd–chitosan complex at 800 °C under a nitrogen atmosphere.[68] The catalyst was used for the selective hydrogenation of various functional groups like olefin, alkyne, epoxide, nitro, and aldehyde. When the catalyst was applied for hydrogenating FF in the aqueous medium, CPN formed as the major product. The reaction was conducted for 6 h at 120 °C in a mixture of water and 2-propanol (19:1, v/v) under a 1 MPa H2 pressure. The reaction gave an 85% yield of CPN at the quantitative conversion of FF. The catalyst was recycled for five consecutive cycles without much loss of activity. The well-dispersed metallic Pd NPs in the carbonaceous network provided abundant reactive sites for adsorption of reactant molecules by diffusion and the activation of H2 by bond dissociation.

Magnetic Cu/Fe3O4 catalysts with varying Cu loading were prepared by the coprecipitation method and used to synthesize CPN starting from FF.[69] X-ray photoelectron spectroscopy depicted a robust electronic interaction between Fe and Cu. The 10% Cu/Fe3O4 catalyst afforded a 91% yield of CPN under optimized conditions (170 °C, 3 MPa H2, 4 h). When the copper loading was increased to 50%, CPL was obtained in 82% yield due to further hydrogenation of CPN. Reaction temperature had a crucial role in the yield of CPN and mass balance of the reaction. Analogous to other studies, a higher temperature promoted the rearrangement reaction of FF forming CPN. In contrast, a temperature around 200 °C led to a sharp decrease in the carbon yield and coking on the catalyst surface. The catalyst was conveniently separated by exploiting its magnetism, and no noticeable loss of catalytic activity was observed even after five cycles.

A bimetallic NiFe/SBA-15 was used as the catalyst for converting FF to CPN.[70] The NiFe catalyst over other supporting materials like SiO2 and Al2O3 provided inferior yields of CPN, which can be correlated with their lesser BET surface area and pore volume. The yield of CPN increased from 56.2% at 120 °C to as high as 90.1% at 160 °C. When the reaction temperature reached 170 °C, a dip in CPN yield was observed due to side reactions like polymerization and over-reduction of CPN to CPL. The solvent effect of the transformation was studied by using a mixture of methanol and water in different volume ratios. The use of methanol as a solvent favored forming THFAL over CPN, and pure water was found to provide the best selectivity and yield of CPN.

A highly efficient, bifunctional Pd NPs (ca. 8 nm) supported on FeZn–double-metal cyanide (DMC) catalyst was developed for the hydrogenative ring rearrangement of FF to CPN.[71] The moderate Lewis acidity of Pd/FeZn–DMC was responsible for catalyzing the ring rearrangement reaction. The Pd/FeNi–DMC and Pd/FeCo–DMC catalysts with low Lewis acidity formed FAL as the major product and failed to provide CPN in satisfactory yield under comparable conditions. The catalyst showed good stability under the reaction conditions employed, and no leaching of Pd particles or structure degradation of DMC was observed. Even after six consecutive runs, and the yield of CPN remained high (>90%).

A 5 wt % Pd/SiO2 catalyst, poisoned by bismuth(III), was used to prepare CPN from FF in the aqueous medium.[72] Bismuth helped to isolate the active sites on the palladium surface and minimized the oligomerization of FF. As the reaction temperature was increased, the major product changed from THFAL (at 50 °C) to FAL (100–150 °C) to CPN (>150 °C). Under optimized reaction conditions (150 °C, 5 MPa H2, 2.3 h), CPN was obtained in a 54.6% yield. Bi(III) blocked some of the active sites on the Pd surface so that simultaneous hydrogenation of multiple olefinic groups (e.g., THFAL from FF) was disfavored.

Bimetallic Pd–Co NPs were encapsulated within UiO-66, an archetypal MOF, to form a core–shell Pd–Co@UiO-66 catalyst.[73] The catalyst showed better performance than the monometallic Pd@UiO-66 catalyst, and a 95% yield of CPN was ensured under moderate reaction conditions (120 °C, 3 MPa H2, 12 h). The catalyst retained its activity even after the fifth run.

Nickel–cobalt and nickel–iron bimetallic catalysts supported on titanium dioxide were prepared, characterized, and applied for the hydrogenation of FF.[74] The catalysts were prepared by the hydrothermal method at 150 °C, followed by reduction with molecular hydrogen. The molar ratio of Ni/M (M = Co, Fe) was fixed to 3.0, and the catalysts were denoted as Ni–M(3.0)/TiO2. Both catalysts gave good yields of FAL and THFAL when the reaction was carried out at 110 °C and a 3 MPa of H2 pressure in ethanol or 2-propanol. The Ni–Fe(3.0)/TiO2 catalyst gave 27.2% CPN and 41% CPL by performing the reaction at 170 °C in an ethanol/water medium. The result is relatable to other reports that showed that the presence of water in the reaction medium is crucial for the selectivity toward CPN. The catalyst slightly lost activity on recycling but was used for three consecutive cycles.

Deng et al. reported three pure Lewis acidic pyrochlore supports of the form A2B2O7 [La2Sn2O7, Y2Sn2O7, Y2(Sn0.7Ce0.3)2O7−δ] with the same crystal structures, and different metals were synthesized, characterized, and used as catalytic supports for synthesizing CPN and HCPN from FF and HMF, respectively.[75] A reduced metal for the selective hydrogenation of the carbonyl functionality and a Lewis acidic support for ring rearrangement under hydrolytic conditions are both indispensable for the synthesis of CPN and HCPN. Pd NPs with appropriate particle sizes were uniformly loaded on the surface of pyrochlore by the impregnation method. Among these pyrochlore-based catalysts, Pd/Y2Sn2O7 exhibited the best activity and selectivity. Moreover, the Y2Sn2O7-based catalyst partially substituted with Ce3+ ions at the B site is more efficient. The yield of CPN and HCPN reached as high as 95.0 and 92.5%, respectively, under optimized conditions. The catalyst remained stable and effective even after four consecutive runs.

Various types of carbon, such as CMK-3, CMK-5, and CNTs, were used as supporting materials for palladium catalysts.[76] The surface area, pore volume, and average pore diameter of the Pd/C catalysts were characterized by N2 physisorption, and the physical values were converted by the BET method. Among the carbon materials studied, CMK-5 was found to be the most efficient due to its largest surface area and hexagonal hollow tubular framework. Aluminum-substituted mesoporous SBA-15 (Al-SBA-15) was used as the template and FAL as the carbon source for the preparation of CMK-15. The mixture of FAL and Al-SBA-15 was carbonized, and then, the latter was etched with HF to leave behind CMK-15. Highly dispersed Pd particles were generated by using trisodium citrate as the anchoring agent and sodium borohydride as the reducing agent. The product distribution was strongly dependent on the solvent employed. In the aqueous medium, under optimized conditions, the 1 wt % Pd/CMK-5 catalyst afforded a 40.2% yield of CPN.

Nickel NPs supported on functionalized CNT (Ni/CNTox) was used as a novel and interface-active catalyst to synthesize CPN.[77] The reaction was optimized on various parameters like temperature, hydrogen pressure, and duration. The conversion of FF and selectivity toward CPN were favored as the temperature was increased from 150 to 200 °C. However, temperatures above 200 °C lowered the yield of CPN. The formation of CPN was favored at low hydrogen pressures (ca. 2 MPa), whereas the products formed via the hydrogenation of the furan ring (e.g., THFAL) were favored at higher H2 pressures (ca. 5 MPa). Although the conversion of FF was rather low (ca. 35%), CPN was obtained in a 25% yield (i.e., 71% selectivity) within 1 h reaction at 200 °C and a 2 MPa H2 pressure.

Mironenko et al. performed detailed mechanistic studies on the FF to CPN conversion catalyzed by 1% Pd/CNT under a hydrogen atmosphere using hydrothermal conditions.[38] Isotope labeling experiments were performed during the hydrogenation process using D2O as a tracer, which proved the formation of CPN from the FAL intermediate following the Piancatelli rearrangement. The mechanistic pathways for forming side products like 2-methyltetrahydrofuran (MTHF) and THFAL were also proposed.

Byun et al. synthesized a series of Pd catalysts supported on various oxides (e.g., SiO2, TiO2, Al2O3) by chemical reduction and applied for the FF to CPN transformation in the aqueous medium.[78] The dispersion and size distribution of Pd particles were dependent on the properties of the oxide support and the rate of nucleation. The product distribution was influenced by the physicochemical properties of the catalyst and the reaction parameters employed. Catalysts with high Pd dispersion and strong acidity promoted the C=C hydrogenation that led to higher THFAL selectivity. 5 wt % Pd/TiO2 was found as the most suitable catalyst candidate for the high-yielding preparation of CPN from FF. Under optimized conditions (170 °C, 2 MPa H2, 4 h), CPN was obtained in a 55.5% yield at the near quantitative conversion of FF.

The Ni–NiO heterojunction supported by TiO2 with the optimized composition of anatase and rutile prepared by pyrolysis at 450 °C (Ni–NiO/TiO2-Re450) was used as the catalyst to synthesize CPN from FF.[79] Under relatively mild conditions (140 °C, 1 MPa H2, 6 h), a 87.4% yield of CPN was achieved at the quantitative conversion of FF. The catalyst showed superior catalytic performance to Ni/TiO2-450 or NiO/TiO2-450. The combination of anatase and rutile as a supporting material performed better than the pure phases. The high catalytic performance of Ni–NiO/TiO2-Re450 mainly attributes to the synergistic effect of the Ni–NiO heterojunction and the TiO2 support. The catalyst showed excellent stability and was successfully recycled for five consecutive runs without noticeable loss of activity.

Yu et al. studied the effect of Lewis and Brønsted acids on the transformation of FF to CPN using nickel on silicon carbide (Ni/SiC) as the acid-free catalyst.[80] The absence of acidic sites on the catalyst led to a negligible yield of CPN, and FAL was obtained as the major product. The effect of acidic additives like Lewis acidic metal salt and a Brønsted acidic ion-exchange resin was then investigated. Both Lewis and Brønsted acid additives significantly improved the yield of CPN compared to the Ni/SiC catalyst under otherwise identical conditions. Among the various metal salts examined, CrCl3 was found to be the most effective Lewis acid. The CrCl3 additive not only suppressed the formation of THFAL but also catalyzed CPN formation. The reaction temperature was found to have a crucial role in altering the product selectivity, and the formation of CPN was favored at a higher temperature. The yield of CPN rapidly increased from 58.2 to 88.1% by increasing the reaction temperature from 120 to 160 °C. Under optimized conditions (160 °C, 3 MPa H2, 2 h, 1 wt % catalyst, 0.25 wt % additive), CPN was obtained in an 88.1% yield at the near quantitative conversion of FF.

Surface oxygen-decorated bimetallic NiCu NPs were derived from ternary Ni–Cu–Al layered double hydroxide precursors, which showed excellent catalytic properties for the hydrogenative ring rearrangement of FF to CPN in water.[81] The Ni2Cu1 composition behaved better than other compositions and also monometallic Ni or Cu catalysts. A maximum of 89.5% yield of CPN was achieved under optimized reaction conditions. The catalytic activity was explained by the mode of adsorption of the carbonyl group on the catalyst due to abundant surface oxygen species promoting their hydrogenation.

Structured Cu-based film catalysts with a novel surface superstructure composed of large quantities of interlaced and nearly vertically grown nanosheets were fabricated from structured Cu–Zn–Al layered double hydroxide (CuZnAl-LDH) film precursors.[82] The catalyst containing a Cu/(Cu + Zn) ratio of 4:5 showed the best activity for CPN synthesis from FF compared to other Cu-rich or Zn-rich catalysts examined. Different morphologies like pristine CuZnAl-LDH-derived power showed lower catalytic activity. The acidic sites in the mixed Zn–Al oxides and synergistic interactions between the highly dispersed Cu metal sites with the metal oxide support were responsible for the high activity and selectivity of the catalyst toward CPN. With increasing copper amount and reaction temperature, the selectivity of the reaction changed from FAL to CPN. When the reaction was performed at 140 °C for 2 h under 2 MPa of H2 pressure, an 86.5% yield of CPN was obtained. The catalyst also successfully reduced other functional groups like olefins, nitro, and aldehydes.

Phosphorus and Ni were introduced into γ-Al2O3 by the wetness impregnation method using (NH4)H2PO4 and Ni(NO3)2 as the precursors.[83] The catalyst was then reduced at elevated temperature under flowing H2 gas to produce metallic Ni sites on the catalyst. The introduced phosphorous modified the distribution of acidic sites over the catalyst surface and tailored the activity of metal sites for the hydrogenation process. The formation of AlPO4 increased the acidic nature of the catalyst, whereas nickel phosphide species (Ni2P, Ni3P, and Ni12P5) tailored the selectivity of the hydrogenation sites. Unfortunately, the catalyst activity dropped noticeably after four cycles due to coke deposition on the active sites.

Highly dispersed Co–Ni alloy NPs embedded in porous nitrogen-containing carbon (xCo–yNi@NC) were prepared by pyrolyzing the MOF template and used as an active catalyst for the preparation of CPN from FF.[84] The synergistic effect between metal components and N-species influenced the physicochemical properties of the bimetallic catalyst. The Lewis acidic sites in the metal oxide promoted hydrogenation of the carbonyl group and ring-opening rearrangement of FAL. The solvent was found to influence the selectivity of the product significantly. 2-Propanol gave FAL as the primary product at a lower reaction temperature. When conducted at a higher temperature in an aqueous medium, the reaction provided CPN as the major product. Under the optimized reaction conditions, a 92.5% CPN yield was obtained in the water solvent over 2Co–1Ni@NC-800 (pyrolysis temperature = 800 °C). The catalyst showed excellent stability and successfully recycled for five runs without significant loss of activity.

Cobalt NPs confined in N-doped CNTs (N–CNTs) were used as a magnetically recoverable catalyst to synthesize CPN from FF under a considerably low pressure of hydrogen gas (ca. 0.5 MPa).[85] The selectivity toward FAL decreased, and that of CPN increased as the temperature was increased from 100 to 160 °C. Within 2 h of reaction at 160 °C, a quantitative conversion of FF was reached, and the FAL intermediate started converting to CPN. Increasing the H2 pressure to >0.5 MPa led to over-reduction of CPN to CPL. Under optimized conditions, a 95% yield of CPN was achieved.

In a later report, pyrochlore/Al2O3 composites [Y2(Sn0.65Al0.35)2O7−δ/Al2O3] with Lewis acidity and rich oxygen defects were prepared via a dissolution–precipitation method, and Pd NPs were impregnated on it.[86] The catalyst showed excellent catalytic activity for transforming FF to CPN by catalyzing both hydrogenation and ring-rearrangement steps. Compared to their corresponding single-metal-oxide-based catalysts (Pd/Al2O3, Pd/Y2Sn2O7), the superior catalytic activity was attributed to the coordinatively unsaturated metal ions present on the catalytic surface, promoting selective adsorption and hydrogenation of the carbonyl functionality. Palladium NPs supported on 7.74 wt % Y2(Sn0.65Al0.35)2O7−δ/Al2O3 showed a CPN yield of 98.1% starting from FF, whereas HCPN formed in a 90.6% yield from HMF.

Ni(10%)/CuFe2O4 has been used as an inexpensive and magnetically separable catalyst for the selective preparation of CPN from FF in an aqueous medium.[87] The catalyst afforded >90% selectivity toward CPN when the reaction was performed at 150 °C under a 1 MPa H2 pressure. Lewis acidity of the CuFe2O4 support was attributed to the high catalytic activity. The effect of the solvent was studied, and water was found to be the best solvent for CPN, compared to alcoholic and hydrocarbon solvents. The catalyst was successfully recycled for five consecutive cycles without much loss of activity.

Nickel supported on silica (Ni/SiO2) with control over the number of exposed metallic nickel sites on the catalyst surface was used as a selective catalyst for FF to CPN transformation.[88] The authors showed that the selectivity toward THFAL or CPN depends on the orientation of adsorption of the FAL intermediate on the catalyst surface and the number of metallic sites involved in the hydrogenation process. Too many metallic sites lead to the hydrogenation of the furan ring forming THFAL. Therefore, 2 wt % Ni/SiO2 gave the best catalyst for CPN synthesis, providing a yield as high as 83.5%. Higher loading of Ni led to THFAL formation, whereas lower loading of Ni led to incomplete conversion of FF. The catalyst was superior to other Ni-based catalysts reported in the literature for CPN synthesis, providing a better yield of CPN at lower loading of Ni. The catalyst was reused for five cycles without catastrophic loss of activity.

The CuNi/Al-MCM-41 catalyst afforded a 96.7% yield of CPN when the reaction was performed at 160 °C for 5 h under a 2 MPa H2 pressure. The catalyst worked better than other molecular sieve supports, including MCM-41, SBA-15, HY, and ZSM-5. A small amount of Al in the MCM-41 framework promoted forming highly dispersed CuNi bimetallic NPs. The synergistic effects in the catalyst and the near-neutral conditions minimized polymerization and achieved high selectivity of CPN.[89]

Recently, Tong et al. prepared a series of mixed metal oxide La2B2O7 (B = Ti, Zr, and Ce)-supported Pd catalysts for the ring rearrangement of FF into CPN. Among the various catalysts examined, Pd/La2Ti2O7 gave a 99.5% conversion of HMF with 98.7% of selectivity toward CPN. The Pd/La2Zr2O7 catalyst also showed very good conversion with moderate selectivity. However, the use of Pd/La2Ce2O7 showed good conversion but very low selectivity toward CPN.[42]

Pd NPs were encapsulated into the cavities of UiO-66-NO2 (a Zr-based MOF) via the impregnation method using a double-solvent method, and the material was used as a catalyst for preparing CPN from FF in the aqueous medium. Only 5 wt % catalyst loading afforded a 95.5% yield of CPN at 150 °C under a 1 MPa H2 pressure. High Lewis acidity of the MOF support was responsible for catalyzing the molecular rearrangement reaction under moderate conditions. The catalyst was stable and successfully recycled for four catalytic cycles.[90]

Effect of Reaction Parameters on the Yield of CPN from FF

Detailed analysis of the reaction conditions reported on the targeted synthesis of CPN from FF can extract crucial information about the effect of various reaction parameters on the selectivity and yield of CPN (Figure ). The reaction was predominantly carried out in the aqueous medium. In several studies, it was found that the catalytic hydrogenation of FF in organic solvents led to form hydrogenation products like 2MF, FAL, and THFAL. The presence of water is a necessary requirement for the hydrogenative ring rearrangement of FF to CPN. When the reaction was attempted in a miscible solvent mixture like ethanol/water, the higher proportions of water favored CPN formation. Reaction temperature plays a guiding role in the selectivity and yield of CPN, and temperatures ranging from 110 to 180 °C have been examined (Figure a). In the majority of the report, 140–150 °C was chosen, which points at a relatively high activation barrier for the hydrolysis and ring-rearrangement steps. Hydration of the FAL intermediate is catalyzed by the protons formed during the splitting of water molecules, especially at elevated temperatures. The overpressure of hydrogen gas is another crucial parameter that governs the selectivity and yield of CPN over other hydrogenation products. Only a few reports reported CPN formation at a hydrogen pressure less than 1 MPa, but most reports used 3–4 MPa (Figure d). The mechanism of CPN formation from FF suggests that hydrogenation of both olefin and carbonyl groups is required, although a higher pressure of H2 led to the over-reduction of CPN into CPL. A metal-based heterogeneous catalyst is routinely employed for the FF to CPN transformation. Both noble metals (e.g., Pd, Ru, Pt, Au) and earth-abundant transition metals (e.g.e.g., Cu, Ni, Co) have been successfully employed for the process (Figure b). Although only one report used the Au catalyst for the FF to CPN transformation to date, it provided a nearly quantitative yield of the latter. The result demonstrates superior physicochemical and recycling properties of gold NPs and their relevance in the chemocatalytic value addition of biomass, in general. However, the relatively high cost of Au-based catalysts remains a concern for the process economics, which may be resolved if the catalytic loading can be brought down to an acceptable level and the catalyst can be recycled satisfactorily. Palladium is the most commonly employed noble metal for catalytic transformation of FF into CPN, followed by ruthenium and platinum. Among the transition metals, copper- and nickel-based catalysts are most frequently used due to their capability in hydrogenating both olefinic and carbonyl functionalities. In most cases, the catalysts were successfully recovered and recycled for multiple cycles without drastic loss of activity. However, some common deactivation routes were found that include coke formation and deposition of humin blocking the active sites on the catalyst surface, agglomeration of metal NPs leading to lowering of the surface area and active sites, and leaching of metal particles from the catalyst support. The problem is more severe in the noble metal-based catalyst due to their high cost. However, the problem is no less severe for inexpensive transition-metal-based catalysts since their deactivation leads to the waste formation, and their regeneration requires elaborate synthetic procedures. Interestingly, there is no clear indication that the noble metal catalysts require less stringent reaction conditions than the transition-metal catalysts. Supporting materials used for the metal catalysts play vital roles in modifying various characteristics that lead to desired changes in product distribution and selectivity (Figure c). The supporting materials offer to separate the metal particles, which are often in the nanodimensions. The acidic sites (e.g., Lewis, Brønsted) on the supporting materials promote catalyzing the hydrolysis and dehydration steps. In some cases, the support has electronic interactions with the metal sites that modulate their activity. The support materials typically possess a high surface area, porosity, and channels that assist in reversibly binding the molecules of hydrogen gas, the starting material, and intermediates. In addition, the support helps in catalyst recovery by introducing additional properties (e.g., magnetism) and increasing the bulk of the catalyst. In the FF to CPN transformation, carbon was the most-used catalyst support. Carbon was used in many forms, such as mesoporous ACs prepared from various biomass feedstocks, CNTs, graphitic carbon, and some functionalized CNTs. Various MOFs were used as robust, well-structured, mesoporous, and recyclable supports for the catalysts and provided satisfactory yields of CPN. Inorganic oxides like SiO2, ZrO2, zeolites, and other mixed metal oxides were also employed. In many cases, Lewis acidic sites on the supporting material were found responsible for their selectivity toward CPN in the aqueous medium. For example, the addition of a Lewis acidic salt like CrCl3 in SiC (a supporting material with acid sites on the surface) led to a dramatic improvement in the yield of CPN. The use of a Brønsted acid additive or supporting material containing Brønsted acidic sites is discouraged since they led to the polymerization (or resinification) of FF and FAL. The duration of the reaction also altered the product distribution. Therefore, detailed kinetic analyses were carried out in many studies. Although CPN was stable in the aqueous medium under the reaction parameters studied, longer duration often promoted its over-reduction to CPL and some Aldol products. Although quantitative conversion of FF was obtained in many cases, the duration of reaction was often manipulated in arriving at an optimum value for the best selectivity of CPN. Stirring speed was mentioned in most reports, where the reaction was performed in a batch reactor. A high stirring speed was typically used to minimize mass-transfer limitations.

Figure 2

Graphs showing the various reaction parameters used in synthesizing CPN from FF: (a) temperature dependence, (b) active hydrogenation metal, (c) hydrogen pressure, and (d) supporting material used in the catalyst.

Catalytic Conversion of HMF to HCPN

HCPN can be produced by the ring rearrangement of HMF in water using molecular hydrogen and a suitable metal-based catalyst for the hydrogenation steps. Ohyama et al. reported the synthesis of HCPN from HMF for the first time using Au NPs supported on various metal oxides (e.g., Nb2O5, Al2O3, ZrO2, TiO2) as catalysts that contained both acidic and basic sites (Table , entry 1). The Au/Nb2O5 catalyst worked best and afforded a maximum of 86% yield of HCPN. The control reactions with only Nb2O5 or without H2 did not produce any HCPN. The use of basic supports like La2O3, CeO2, and HT afforded only a trace yield (ca. 1%) of HCPN, signifying the promoting effect of acidic sites on the HCPN yield. The catalyst was recycled and reused successfully without loss of Au metal loading from the support and afforded 84% of HCPN in the second cycle.[91] In a later study, the same research group reported the effect of various metal oxides as additives with the Pt/SiO2 catalyst for the aqueous-phase conversion of HMF to HCPN. In the absence of any additive, the catalyst produced only a trace amount of HCPN and formed mostly BHMF, among other products. Weakly acidic metal oxides like CeO2 and La2O3 led to only low yields (<25%) of HCPN. The addition of metal oxides with Lewis acidity (e.g., ZrO2, Nb2O5) led to good yields (>60%) of HCPN. Under the optimized reaction conditions (140 °C, 3 MPa H2, 12 h), the combination of Pt/SiO2 and Ta2O5 provided an 82% yield of HCPN.[41]

Table 2

Catalytic Preparation of HCPN from HMFa

entry	catalyst	reaction conditions	yield (%)	refs
1	Au/Nb2O5	HMF (0.025 g), catalyst (0.01 g), water (3 mL), 140 °C, 8 MPa H2, 12 h	86	(91)
2	Pt/SiO2 + Ta2O5	HMF (0.025 g), catalyst (0.01 g), Ta2O5 (0.01 g), water (3 mL), 140 °C, 3 MPa H2, 12 h	82	(41)
3	NiAl-4R	HMF (0.113–0.226 g), catalyst (0.035 g), water (45 mL), 140 °C, 2 MPa H2, 6 h	81	(92)
4	Cu–Al2O3	HMF (0.23 g), catalyst (0.06 g), water (45 mL), 180 °C, 2 MPa H2, 6 h	86	(39)
5	Pt/SiO2 + Ta2O5	HMF (0.025 g), catalyst (0.01 g), Ta2O5 (0.01 g), water (3 mL), 140 °C, 3 MPa H2, 30 h	61	(93)
6	[Cp*Ir(4,4′-(OH)2-bpy)(OH2)]SO4 + Al2O3	HMF (0.025 g), catalyst (0.1 mol %), Al2O3 (0.01 g), water (3 mL), 130 °C, 3 MPa H2, 4 h	82	(94)
7	Pd/Fe-MIL-100	HMF (1.3 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 24 h	85.4	(65)
8	Ni–Cu/MOF-74	HMF (0.252 g), catalyst (0.05 g), water (5 mL), 140 °C, 2 MPa H2, 5 h	70.3	(95)
9	Pd/Cu–BTC	HMF (1.3 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 24 h	90.4	(67)
10	Pd/FeZn–DMC	HMF (1.3 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 24 h	87.5	(71)
11	Pd/Y2(Sn0.7Ce0.3)2O7−δ	HMF (1.3 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 12 h	92.5	(75)
12	Pd/7.74% Y2(Sn0.65Al0.35)2O7−δ/Al2O3	HMF (0.630 g), catalyst (0.05 g), water (20 mL), 150 °C, 4 MPa H2, 12 h	90.6	(86)
13	Ni–Fe/Al2O3	HMF (0.63 g), catalyst (0.12 g), water (30 mL), 160 °C, 2 MPa H2, 4 h	86	(96)
14	Pd/La2Ti2O7	HMF (1.3 g), catalyst (0.1 g), water (40 mL), 150 °C, 4 MPa H2, 6 h	82	(42)

Abbreviations: NiAl-4R, Ni0.53Al0.47O1.10H0.39; Cp*, pentamethylcyclopentane; MIL-101, chromium terephthalate metal–organic framework; MOF, metal organic framework; DMC, double-metal cyanide.

The Ni on Al2O3 catalysts were prepared from layered double hydroxides via the precipitation method for hydrogenating HMF reaction. Further, the catalytic activity on ring rearrangement of HMF has been studied over the NiAl-4R (Ni0.53Al0.47O1.10H0.39) catalyst, and 81% of HCPN was obtained.[92]

Al-rich amorphous alumina supported on the five mixed metal oxides consisting of Ni, Cu, Co, Zn, and Mg catalysts was synthesized and tested for one-pot conversion of HMF to HCPN in water. Under optimized conditions (180 °C, 2 MPa H2, 6 h), the use of the Cu–Al2O3 catalyst afforded a 100% conversion of HMF with an 86% selectivity toward HCPN. However, the catalyst lost activity in the recycling experiments due to the lowering of acidic sites, the decreased surface area of the catalyst, and aggregation of Cu metal species on the supporting material.[39]

In 2017, the combination of Pt/SiO2 and lanthanoid oxides (Pt/SiO2 + LO) was used as the catalyst for the hydrogenative ring rearrangement of HMF to HCPN in the aqueous medium. The use of Nd2O3, La2O3, CeO2, Dy2O3, Yb2O3, and Al2O3 gave a maximum yield of 5-(hydroxymethyl)cyclopentanol (HCPL) (88–79%) and less yield of HCPN (0–24%). However, the use of Ta2O5 gave 61% of HCPN and only a 1% yield of HCPL.[93]

The combination of a half-sandwich cyclopentyl–iridium complex and an acidic co-catalyst was applied as a catalyst for the synthesis of HCPN from HMF. The effect of Brønsted and Lewis acidity on the ring rearrangement reaction was studied by using Ta2O5, ZrO2, Nb2O5, TiO2, and Al2O3. Among all acidic oxides, Al2O3 afforded 82% of HCPN, whereas TiO2 and Ta2O5 resulted in 60% yield, and Nb2O5 afforded a relatively low yield (35%). The result may be correlated with the number of acidic sites available on the catalyst surface. The authors proposed a different mechanism since HHD, believed to be an important intermediate for HCPN synthesis, did not form under the reaction conditions employed.[94]

As discussed earlier in Table (entry 25) in synthesizing CPN from FF, the Pd NPs supported on Fe-MIL-100 showed good catalytic activity in producing HCPN from HMF due to the oxophilic properties of the MOF support. Among the noble metals studied, Pd and Au showed good catalytic activity compared to Pt and Ru. The Pd/Fe-MIL-100 catalyst resulted in a 99.9% conversion of HMF with 85.5% selectivity toward HCPN when the reaction was performed at 150 °C for 24 h.[65]

Zhang et al. synthesized MOF-derived nickel and copper-based monometallic and nickel–copper bimetallic catalysts to prepare HCPN from HMF without any Lewis acidic support or additives. The use of monometallic catalysts like Ni, Cu, and Fe on MOF did not result in any HCPN formation. In comparison, Co on the MOF support afforded only a 19% yield of HCPN at the near quantitative conversion of HMF. Furthermore, the use of a bimetallic catalyst (Ni–Cu/MOF-64) afforded a maximum 70.3% yield of HCPN under optimized conditions (140 °C, 5 h, and 2.0 MPa).[95]

The Pd NPs supported on Cu-MOFs catalyst (Pd/Cu–BTC) afforded a 94% yield of HCPN (150 °C, 24 h), much higher than that of other MOFs like Cr-MIL-101, due to higher dispersion of the Pd NPs in the former.[67]

As discussed in Table (entry 31), the Pd/FeZn–DMC catalyst was used for the hydrogenative ring arrangement of HMF to form HCPN. Under optimized conditions, an 87.5% yield of HCPN was obtained at the near quantitative conversion of HMF.[71]

The Pd-supported Lewis acidic pyrochlore of the form A2B2O7 [ca. La2Sn2O7, Y2Sn2O7, and Y2(Sn0.7Ce0.3)2O7−δ] was synthesized, characterized, and used for hydrogenative ring rearrangement reaction of HMF. The Pd/Y2(Sn0.7Ce0.3)2O7−δ/Al2O3 catalyst was found to be best and showed 99.9% of HMF conversion with a 90.7% HCPN selectivity after 12 h of reaction at 150 °C.[75]

As discussed in Table (entry 47), the Pd/7.74% Y2(Sn0.65Al0.35)2O7−δ/Al2O3 catalyst was used for the hydrogenative ring rearrangement of FFs, and a 90.6% yield of HCPN was obtained from HMF under optimized conditions.[86]

A non-noble bimetallic Ni–Fe/Al2O3 catalyst was fabricated and characterized for HCPN synthesis from HMF. The alloy formation between Ni and Fe resulted in high catalytic activity with an 86% yield of HCPN. The catalyst was conveniently separated by using magnets after the reaction. Moreover, the catalyst was recycled and reused for three cycles without significant loss of activity.[96]

As discussed in the previous section (Table , entry 51), a series of Pd-based catalysts were prepared for hydrogenating FFs. Detailed mechanistic and kinetic studies were carried out on the ring-rearrangement reaction of HMF. The use of the Pd/La2TiO7 catalyst provided an 82% yield of HCPN. The catalyst was stable up to five cycles without significance loss of the catalytic activity.[42]

Analogous to the preparation of CPN, both noble metal catalysts (e.g., Pd, Pt) and non-noble metal catalysts (e.g., Cu, Ni) were employed for synthesizing HCPN by the hydrogenative ring rearrangement of HMF in the aqueous medium. Even though both types of metal catalysts afforded an excellent yield of HCPN yield, a prolonged reaction time is typically required for synthesizing HCPN compared to CPN.

The attempt to hydrogenate FF and HMF in an aqueous medium leads to many partially hydrogenated compounds, whose relative amounts depend on the catalyst and reaction conditions employed (Scheme ). Hydrogenation of the carbonyl carbon in FF and HMF leads to FAL and BHMF, respectively. These compounds act as reactive intermediates for further hydrogenation and rearrangement reactions. Hydrogenolysis reaction of the C–O bond in FAL forms 2MF. Analogously, hydrogenolysis of both the hydroxymethyl groups in BHMF forms 2,5-dimethylfuran. Hydrogenation of the furan ring in FAL and BHMF forms THFAL and 2,5-bis(hydroxymethyl)tetrahydrofuran, respectively. The hydrogenative ring rearrangement of FF and HMF forms CPN and HCPN following the Piancatelli reaction or some other alternative mechanistic pathway. During the formation of CPN and HCPN, some isolable intermediates are formed, such as CPEN from FF and HCPEN from HMF. On the other hand, over-reduction of CPN and HCPN forms CPL and HCPL. Hydrogenolysis of the hydroxymethyl group in HCPN forms 3-methylcyclopentanone (MCPN).

Scheme 4

Side Products that are Frequently Formed during the Synthesis of CPN and HCPN from FF and HMF, Respectively

Synthesis of CPN Derivatives from Furaldehydes and Furanic Ketones

Various 5-substituted FFs and furanic ketones have also been transformed into CPN derivatives by hydrogenative ring opening in the aqueous medium. For example, MFF has been converted into MCPN in a yield as high as 96% using Au- or Co-based nanocatalysts (Table , entry 1). Both catalytic processes were performed in the aqueous medium using molecular hydrogen as the reducing agent. The Au/TiO2-A catalyst required a 4 MPa H2 pressure, whereas the CoNP@N–CNTs catalyst required only a 0.5 MPa H2 pressure, demonstrating the superior hydrogenating ability of the latter catalyst. The Au/TiO2-A catalyst gave 3-ethylcyclopentanone and 2-ethylcyclopentanone starting from 5-ethylfurfural and ethyl 2-furyl ketone (entries 2, 4). Both the catalysts mentioned above converted 2-acetylfuran to 2-methylcyclopentanone in a 98% yield (entry 3). The CoNP@N–CNTs catalyst provided good yields of bromo-substituted CPNs starting from the corresponding substituted FFs (entries 5, 6). Therefore, the synthetic versatility of the process and its wide substrate scope are demonstrated.

Table 3

Preparation of Some Substituted CPNs from Furanic Aldehydes/Ketonesa

Reaction conditions: [a] Au/TiO2-A catalyst (0.10 wt %), substrate/catalyst (2000), H2O (10 mL), 160 °C, 4 MPa H2; [b] substrate (100 mmol/L), CoNP@N–CNT catalyst (0.015 g), H2O (10 mL), 160 °C, 0.5 MPa H2.

Catalytic Conversion of FF to CPL

CPL is frequently formed as a minor side product during the preparation of CPN. In some studies, a one-pot preparation of CPL was reported from FF by reduction of CPN in situ. The catalysts used for the process are similar to those used for synthesizing CPN. Both noble and non-noble metal-based monometallic and bimetallic catalysts have shown excellent activity in producing CPL. The reaction temperature used is typically in the range of 140–170 °C and the H2 pressure is in the range of 2–5 MPa, although higher temperatures and higher pressures have also been experimented. Table lists the literature examples where CPL was prepared as the targeted, isolable compound.

Table 4

Targeted Preparation of CPL from FF under Catalytic Conditionsa

entry	catalyst	reaction conditions	yield (%)	refs
1	30 wt % Ni/CNT	FF (5.8 g), catalyst (1.5 g), water (95 mL), 140 °C, 5 MPa H2, 10 h	83.6	(97)
2	Cu–Mg–Al (2.5:12.5:5) HT	FF (5.8 g), catalyst (1.5 g), water (95 mL), 140 °C, 4 MPa H2, 10 h	93.4	(98)
3	Cu/Zn/Al (6:9:5)-600	FF (5.8 g), catalyst (0.116 g), water (95 mL), 150 °C, 4 MPa H2, 10 h	84	(99)
4	5% Cu–Co-OG-500	FF (0.192 g), catalyst (0.05 g), water (10 mL), 170 °C, 2 MPa H2, 1 h	68	(48)
5	Raney Ni	FF (3.843 g), catalyst (0.5 g), additive (0.02 mol phenol), water (30 mL), methanol (0.2 mol), 180 °C, 1 MPa N2, 4 h	75.2	(100)
6	Co/ZrO2–La2O3	FF (0.096 g), catalyst (0.05 g), water (10 mL), 160 °C, 2 MPa H2, 4 h	82	(101)
7	PdRu/CNT	FF (3.5 g), catalyst (0.3 g), water (100 mL), 200 °C, 8 MPa H2, 3 h	77	(102)
8	20% Co/TiO2	FF (0.5 g), catalyst (0.3 g), water (10 mL), 150 °C, 4 MPa H2, 4 h	45.4	(58)
9	Pd–Ru/C	FF (5.8 g), catalyst (0.5 g), water (95 mL), 200 °C, 8 MPa H2, 3 h	77	(103)
10	50% Cu/Fe3O4	FF (0.1 g), catalyst (0.05 g), water (10 mL), 170 °C, 3 MPa H2, 3 h	82	(69)
11	Cu0.4Mg5.6Al2	FF (0.096 g), catalyst (0.064 g), water (2 mL), 190 °C, 2 MPa H2, 12 h	98.6	(66)
12	RuMo/CNT	FF (0.5 g), catalyst (0.05 g), water (10 mL), 160 °C, 4 MPa H2, 4 h	74.3	(104)
13	4LH-Co@NC	FF (0.019 g), catalyst (0.04 mmol Co), water (3 mL), 160 °C, 2 MPa H2, 8 h	97	(105)
14	Ni–Co(3.0)/TiO2	FF (0.105 g), catalyst (0.05 g), ethanol/water (1.5/2 mL), 170 °C, 3 MPa H2, 6 h	41	(74)
15	trans-[(dppt)(CH3CN)2Ru](OTf)2	furfuryl acetate (1.752 g), catalyst (0.1 mol %), water/dioxane (1/1, v/v), 200 °C, 5.12 MPa H2, 5 h	35	(106)
16	1% Ru–2.5% Mo/CNT	FF (0.25 g), catalyst (0.05 g), water (5 mL), 180 °C, 4 MPa H2, 4 h	89.1	(107)
17	Ni2P	FF (0.192 g), catalyst (0.05 g), water (10 mL), 150 °C, 4 MPa H2, 12 h	62.8	(108)
18	Pt/SiO2 + Nd2O3	HMF (0.025 g), catalyst (0.01 g), Nd2O3 (0.01 g), water (3 mL), 140 °C, 3 MPa H2, 30 h	88b	(93)

Abbreviations: HT, hydrotalcite; CNT, carbon nanotube; OG, oxalate sol–gel method; NC, nitrogen-doped carbon; 4LH, 4-layered solid ZIF(67); dppt, 2,9-dipyridyl-1,10-phenanthroline.

Yield of HCPL.

Nickel supported on HNO3-pretreated CNTs with different nickel loadings was prepared by the impregnation method and used for the one-pot conversion of FF to CPL. The effects of various reaction parameters, such as the Ni loading on CNTs, reaction temperature, hydrogen pressure, and reaction duration, were studied on product distribution. Under optimized reaction conditions (140 °C, 10 h, 5.0 MPa H2), the 30 wt % Ni/CNT catalyst gave up to an 83.6% yield of CPL at a 96.5% conversion of FF.[97]

Various Cu–Mg–Al HT-derived oxides were prepared via the coprecipitation method for the hydrogenation of FF in the aqueous phase. Reaction temperatures below 120 °C produced a mixture of CPN, CPL, and FAL. The conversion of FF reached a maximum of 98.5% with a 94.8% selectivity toward CPL when the reaction was performed for 10 h at 140 °C under 4.0 MPa of initial H2 pressure using Cu–Mg–Al HT (Cu/Mg molar ratio of 0.2) as the catalyst. The loss of catalytic activity over three consecutive cycles was explained by coke formation over the active metal surface.[98]

The targeted synthesis of CPL from FF was carried out by using Cu/Zn/Al catalysts. The Cu/Zn/Al catalysts were prepared from HT-like compounds via the calcination and reduction methods. The calcination temperature had a significant influence on the catalytic activity of the Cu/Zn/Al catalysts. The process was optimized on other parameters like temperature, duration, and H2 pressure. Quantitative conversion of FF was achieved with 84% CPL selectivity using the Cu/Zn/Al (6:9:5) catalyst calcined at 600 °C. The catalyst was reused up to four catalytic cycles without further treatment.[99]

As discussed in Table (entry 6), the Cu–Co catalysts synthesized by the oxalate sol–gel method (OG) displayed a high catalytic activity for FF to CPL conversion due to the presence of Co0 and Cu2O sites on the catalyst surface. A 64% yield of CPL was obtained using 5% Cu–Co-OG-600 catalysts calcined at 600 °C.[48]

The transfer hydrogenation of FF was performed using Raney Ni as the catalyst and methanol as the hydrogen donor.[100] The influence of various organic additives on product distribution was studied. When phenol was used as an additive, CPN formation and CPL formation were favored over hydrogenated furanic products like THFAL. At 160 °C, the conversion of FF was low (ca. 53%), which increased to >90% at 180 °C. The selectivity toward rearrangement products (i.e., CPO and CPL) was also high. With a further increase in reaction temperatures (ca. 200 °C), the decarbonylation reaction of FF was favored, forming THF as the major product. The reaction parameters like temperature and initial pressure of N2 were also studied. The conversion of FF increased 51 to >97% by increasing the initial N2 pressure from 0.1 to 1 MPa. However, at higher pressures, THF formation was more. Under optimized reaction conditions (180 °C, 1 MPa N2, 4 h) and using phenol as an additive, the combined selectivity of CPN and CPL reached 88.97 at a 94.4% conversion of FF.

Metallic cobalt supported on ZrO2–La2O3 was used as a heterogeneous catalyst for the one-pot transformation of FF to CPL.[101] Under optimized conditions (50 wt % catalyst, 160 °C, 2 MPa H2, 4 h), CPL was obtained in a 82% yield. A lower temperature (ca. 150 °C) and a lower hydrogen pressure (ca. 1 MPa) favored the formation of CPN. Interestingly, when the recycled catalyst was used, CPN was obtained in a 56% yield at a 92% conversion of FF. This can be explained by the diminished activity of the catalyst upon recycling.

Mironenko et al. demonstrated that the Pd–Ru bimetallic catalyst supported on CNTs (PdRu/CNT) had excellent performance for the one-pot conversion of FF to CPL in an aqueous medium. A 77% yield of CPL was ensured by using the bimetallic Pd–Ru/CNT catalyst, whereas the yield did not exceed 18% when monometallic Pd/CNT and Ru/CNT were used as catalysts under identical conditions (200 °C, 3 h, 8 MPa H2). The result was justified by smaller bimetallic particles (average particle size 1.6 nm) and more extensive availability of active sites.[102]

As listed in Table , entry 17, Co–Ni bimetallic catalysts were synthesized, characterized, and screened for the hydrogenative ring-rearrangement reactions of FF to CPN and CPL. By changing the Co–Ni catalyst to monometallic Co-supported TiO2 (20% Co/TiO2), the product selectivity switched from CPN to CPL. When the reaction was carried out at 150 °C for 4 h under a 4 MPa H2 pressure, CPL was obtained in a 45.4% yield.[58]

The aqueous-phase hydrogenation of FF to CPN using the Pd–Ru/CNT bimetallic catalyst was reported and compared with Pd/C and Ru/C catalysts.[103] The catalyst was prepared by the wet impregnation method using the corresponding metal chlorides as the precursors and contained 0.85 wt % Pd and 0.80 wt % Ru (a Pd/Ru molar ratio of 1.0). When the reaction was performed for 3 h at 200 °C under an 8 MPa H2 pressure, a quantitative conversion of FF was achieved, and CPL was obtained in a 77% yield with CPN in a trace amount (0.8%). Under identical conditions, Pd/CNT and Ru/CNT catalysts provided CPN as the major product with 36.2 and 51.3% yields, respectively. The superior activity of the bimetallic catalyst was explained by the synergistic interaction between Pd and Ru particles, including the possibility of electronic interactions and alloy formation.

As discussed in Table (entry 29), a series of magnetically separable Cu/Fe3O4 catalysts containing Cu NPs were synthesized for transforming FF to CPN and CPL. The effect of reaction time, Cu loading, and hydrogen pressure on the product distribution were studied. CPL was obtained in 82% yield when the reaction was performed for 3 h at 170 °C under a 3 MPa H2 pressure using 50% Cu/Fe3O4.[69]

The Cu/Mg/Al-based HT catalyst was prepared and examined for the hydrogenation of FF. The reaction was initially optimized for the preparation of CPL from FF, and the same protocol was extended for the synthesis of CPN and FAL. Cu0.4Mg5.6Al2 gave a 98.6% yield of CPL, whereas Cu0.8Mg5.2Al2 and Cu1.2Mg4.8Al2 resulted in 97.9 and 85.1% yields of CPL, respectively, under identical conditions. The use of Cu/HT gave only 72.9% CPL, and when Cu is supported on activated carbon, CPN was obtained in 92% yield with only trace CPL (ca. 1.2%).[66]

Wang et al. reported a Ru–Mo bimetallic catalyst supported on CNTs (RuMo/CNT) for the direct synthesis of CPL from FF.[104] Under optimized conditions, a quantitative conversion of FF was achieved, and CPL was obtained in a 74% yield along with 9% CPN. The catalyst was prereduced before applying on FF to CPL reaction, and the temperature used for the prereduction significantly affected the product distribution. A temperature of 600 °C gave the optimum yield of CPL. A lower temperature (ca. 400–500 °C) gave lower yields of both CPN and CPL, whereas higher temperatures (ca. 700–800 °C) improved the selectivity of CPN over CPL, although their combined yield remained low. The catalyst could be reused without requiring any regeneration process, and no noticeable loss of catalytic activity was observed.

In 2019, Chen et al. synthesized and characterized the fabricated hallow multishell metal@NC dodecahedrons with a controlled nanoarchitecture and tunable composition. The catalysts were screened for the hydrogenative ring-rearrangement reaction of FF. The synthesized catalysts showed good catalytic activity due to highly dispersed Co NPs and superior properties of N-doped carbon. The highest yield (97%) of CPL was obtained after 8 h at 160 °C. The catalyst could be reused up to six times without significant loss of activity.[105]

As discussed in previous Section (Table , entry 34), Astuti et al. synthesized the nickel-based heterogeneous bimetallic catalysts for the hydrogenation of FF in the aqueous medium. Up to a 41% yield of CPL was obtained by using the Ni–Fe(3.0)/TiO2 catalyst for 6 h at 170 °C under 3.0 MPa of H2 pressure.[74]

Stones et al. synthesized the trans-[(2,9-dipyridyl-1,10-phenanthroline)(CH3CN)2Ru](OTf)2 complex and applied as a homogeneous catalyst for the hydrogenative ring-rearrangement reaction of furfuryl acetate to 1,4-pentanediol and CPL. The reaction was carried out for 3 h at 200 °C using trans-[(dppt)(CH3CN)2Ru](OTf)2 as a catalyst under a 5.12 MPa H2 pressure, which resulted in a 35% yield of CPL and a 48% yield of 1,4-pentanediol.[106]

A ruthenium–molybdenum bimetallic catalyst supported on CNTs (1% Ru–2.5% Mo/CNT) was used for the one-pot preparation of CPL from FF in the aqueous medium. When the reaction was performed for 4 h at 160 °C under a 4 MPa H2 pressure, an 81% yield of CPL was obtained. Increasing the temperature to 180 °C improved the yield of CPL to 89% at the quantitative conversion of FF. The product distribution was found to depend on reaction parameters like temperature and hydrogen pressure. The reduction temperature of the catalyst influenced its Lewis acidity and hydrogenation activity, affecting the product distribution.[107]

Metal phosphate NPs such as CoP, Co2P, and Ni2P with different compositions and morphologies were used as a catalyst to prepare bio-derived polyols from FF. The CoP catalyst showed the highest selectivity toward 1,2,5-pentanetriol, whereas the Ni2P catalyst showed good selectivity toward CPL. The hydrogenative ring rearrangement of FF in the aqueous phase using the Ni2P catalyst produced 62.8% CPL under optimized conditions (150 °C, 4 MPa H2, 12 h).[108]

Ohyama et al. reported the synthesis of HCPL from HMF using the combination of Pt/SiO2 and lanthanoid oxides (Pt/SiO2 + LO). The maximum yield (88%) of HCPL was obtained using a combination of Pt/SiO2 and Nd2O3 as a catalyst and conducting the reaction for 30 h at 140 °C under a 3.0 MPa of initial H2 pressure.[93]

Catalytic Preparation of CPN from FAL

The synthesis of CPN starting from FAL has also been attempted. Hronec et al. synthesized CPN from FAL in an aqueous medium using Ru, Pd, Pt, and Ni-based metal catalysts. Acetic acid (AcOH) enhanced the catalytic activity of metal catalysts and the selectivity of CPN. Under optimized conditions (160 °C, 0.8 MPa H2, 1 h), >95% yield of CPN was reported using the nickel catalyst (G-134) using AcOH as an additive.[109] Although an excellent yield of CPN was obtained under mild reaction conditions, the reaction requires one additional step of preparing and purifying FAL from FF.

Preparation and Recycling of Catalysts

Catalysts containing both metal sites (for hydrogenation) and acid sites (for hydrolytic ring rearrangement) are required for the transformation of FFs to CPNs and CPLs. The type of metal(s) and supporting material used, loading of the metal, the surface distribution of active sites, the particle size distribution of the metal sites, and the surface area of the catalyst are crucial parameters governing the activity and selectivity of the catalyst. The synthetic procedure of the catalysts plays a pivotal role in determining/modifying the above characteristics. Virtually, all the catalysts reported for the synthesis of CPN, HCPN, or their derivatives are heterogeneous catalysts due to their relatively straightforward separation and recycling compared to homogeneous catalysts. In many catalysts, the metal particles are in the nanoscale and have shown high catalytic activity. Commonly used bottom-up techniques like sol–gel, hydrothermal, and wet-impregnation processes have routinely been employed. The type of metal precursors and the molecular template used during the preparation of the supporting material often modify its physicochemical, thermal, and catalytic properties. The 3D network of oxide-based supports (e.g., Al2O3, TiO2, SiO2), reducible metal oxides (e.g., PdO), and carbonization (for carbon-based supports) are formed by calcination. The calcination temperature and the temperature ramp used for reaching the same regulate the properties (e.g., acidity, surface area, porosity, particle size distribution) of the supporting material produced. It has been reported that although the metal type and supporting material remain identical, catalysts synthesized by different preparatory methods provide significantly different results.

Recovery and recycling of catalysts are of utmost importance for the process development of any catalytic synthesis. The efficient recovery of catalysts assists in working up the reaction mixture, minimizing waste generation, and avoiding contamination of the product. The recovery of noble metal catalysts is particularly important since their high cost substantially affects the process economy. Attempts are made to reuse catalysts in the consecutive catalytic cycles without undergoing any extensive regeneration process. For example, magnetic supporting materials (e.g., Fe3O4) enable the catalyst to be separated from the reaction mixture conveniently by using an external magnet. Common deactivation pathways of the heterogeneous catalysts used in the transformation of FFs to CPN and its derivatives involved the blockage of active sites by coke formation and poisoning the catalyst by side products. Leaching of the active metal or loss of acidic sides from the supporting material was found to be a major deactivation pathway, especially because the transformation is carried out in the water at elevated temperatures. Agglomeration of the metal NPs under the reaction conditions deactivated the nanocatalysts by decreasing the surface area, hydrogenation sites, and lesser electronic interactions with the support. Various strategies are adopted to reactivate the catalyst. Calcination helps burn off the coke deposit on the surface and pores and free the catalyst’s active sites. However, repeated calcination often changes the morphology, crystallinity, and particle size distribution of the catalyst. Therefore, the calcination temperature and other conditions must be chosen cautiously. Washing the solvent with a suitable solvent helps remove adsorbed intermediates and side products and free the active sites. The design of the catalyst with stronger interactions between the metal particles and supporting material minimizes the leaching issue.

Catalyst Deactivation and Recyclability

The catalyst is an indispensable part of the catalytic preparation of CPN and its derivatives from FFs and contributes significantly to the operational expenditure. Heterogeneous catalysts are routinely employed for the above chemical transformations, and significant research has been undertaken in designing tailor-made catalysts for expedited kinetics under moderate reaction conditions, high selectivity, superior stability, and high recyclability. Although in theory the catalyst can be recovered and recycled indefinitely, various deactivation processes diminish the catalytic efficiency over repeated use. The deactivated catalyst also leads to possible contamination of products and waste disposal problems. Therefore, a comprehensive understanding of the deactivation mechanisms of the catalyst candidate is vital for the commercial feasibility of a catalytic transformation. In terms of the deactivation mechanism, the deactivation pathways of heterogeneous catalysts can be classified into five general areas: (1) chemical degradation including volatilization and leaching, (2) fouling, (3) mechanical degradation, (4) poisoning, and (5) thermal degradation.[110] Fouling is the physical blockage of the active sites in the catalyst by nonselective substances. For example, humin and coke are often formed during the formation of CPN and its derivatives from FFs and deposit on the catalyst surface. Poisoning of the catalyst is caused by the strong chemisorption of species (which are often side products of the reaction) on the active sites blocking them for catalytic reaction. Poisoning can be both selective and nonselective, whereas its reversibility or irreversibility depends on the type of poison involved. Chemical degradation (e.g., oxidation of the active metal during hydrogenation) can be disastrous for the stability and recyclability of the catalysts. Leaching of active species (e.g., metal NPs) from the catalyst support is commonly encountered while producing CPN from FUR in the aqueous medium. Leaching refers to the loss of active species from the solid catalyst into the liquid reaction medium, causing partial or complete deactivation of the catalyst. Sintering is frequently encountered in supported metal catalysts used in producing CPN from FUR. The active surface area is reduced via agglomeration and coalescence of small metal crystallites (e.g., metal NPs) into larger ones with lower surface-to-volume ratios.[111] The change of size of metal crystallites lowers the catalyst activity with lower conversion and inferior selectivity toward CPN.

In general, the deactivation of the catalyst can be minimized by modifying the catalyst (i.e., catalytically active species, support, promoter), the process, or both. Modifying the catalyst candidate includes changing the support, the synthetic procedure, and the composition. The catalytic process can be modified by altering the operational conditions, reactor design, and purification of feed/reaction mixture. When the catalyst gets significantly deactivated even after adopting the preventive measures, there are various curative treatments to recuperate the catalytic activity. The fouling process is generally reversible and could be minimized by process optimization. The coke deposition can be minimized by using supports with large pores, lower reaction temperatures, additives that hinder coke formation/deposition, and design catalysts with suitable pore structure and acidity. The coke deposited on the catalyst can be removed by calcination of the catalyst at a suitable temperature, where the carbon-rich particles burn off, forming gaseous products. The poisoning of the catalyst can be controlled by modifying reaction conditions, optimizing pore structures, applying a diffusion barrier (e.g., coating) on the catalyst, and using additives that selectively absorb poison. The poisoned catalyst may be regenerated by rinsing repeatedly in a suitable organic solvent or deionized water under agitation (e.g., sonication). Chemical degradation of catalysts can be minimized by lowering the reaction temperature, using promoters to stabilize the catalyst, and incorporating additives to minimize deactivation. Leaching may be minimized by altering the reaction parameters, using coating on the catalyst surface, and using supporting materials that have stronger interactions with the active metal. Agglomeration of metal particles or the supporting material can be minimized by changing the process of catalyst synthesis, using lower reaction temperature, and adding binders such as carbon to the support material. Interestingly, the early publications (2012–2013) on producing CPN from FF concentrated largely on the process development and mechanistic studies. However, the later publications put significant emphasis on the development, stability studies, and recyclability of efficient catalysts. Some publications reported excellent stability of the catalyst up to five consecutive catalytic cycles without noticeable loss of activity.[51,63,82,87] However, leaching of the active metal from the support,[58,75] coke formation,[47,53] poisoning of active sites by side products, and agglomeration of active metal particles[54,67] were the major reasons for catalyst deactivation.

Applications of CPN and HCPN

CPN and HCPN have a wide array of applications and markets for their derivatives (Figure ). Traditionally, CPN has applications as a starting material for various fragrances and perfumes in personal care products. It is also a common laboratory reagent and a key reagent for multiple classes of products, including agrochemicals and pharmaceuticals. CPN can also be converted into important monomers for making high-volume polymers. Some emerging applications for the derivatives of renewable CPN include green solvents and biofuels. Although high-value applications of CPN and HCPN are already existing, the commercial prospect for their preparation from FFs will get benefited upon expanding the horizon of their applications.

Figure 3

Some commercially relevant derivatives of CPN and HCPN.

Applications as a Precursor to Hydrocarbon Fuels

The carbonyl group in CPN can be hydrogenated to a methylene group to form cyclopentane, whereas the reduction of HCPN forms methylcyclopentane. The naphthenes like cyclopentane and methylcyclopentane are desirable fluidized catalytic cracking feedstocks to produce high-octane gasoline and to make aromatic chemicals for downstream value addition. A major strategy for producing diesel- and jet-fuel-like hydrocarbons from biomass-derived carbohydrates starts with FF and HMF. In the initial step, the carbon backbone in the FFs is extended by employing various C–C bond-forming reactions like aldol condensation. The aldol condensation often involves FFs and a renewable ketone like acetone. In the second step, the purified aldol products are subjected to the HDO conditions in the presence of a suitable catalyst (preferably heterogeneous) to form straight-chain or cyclic diesel- or jet-fuel-like hydrocarbons. CPN and HCPN could potentially be used as the renewable ketone partner for extending the C–C backbone of FF and HMF (Scheme a). When FF is the coupling partner, 1,3-dipentylcyclopentane (1,3-DPCP) is produced as the end product after the HDO process.

Scheme 5

Synthesis of Jet Fuels and Diesel-Range Hydrocarbons from CPN and HCPN through (a) Condensation with Furanics, (b) Condensation with Straight-Chain Aldehydes, and (c) Self-Condensation of CPN

CPN can also be coupled with 2MF by Friedel–Crafts (F–C) reaction (Scheme a). HDO of the coupled product forms C15 hydrocarbons with the cyclopentane ring residing at the middle of the hydrocarbon chain. The solvent-free hydroxyalkylation/alkylation of 2MF and CPO by F–C reaction to form 2,2′-cyclopentylidenebis(5-methylfuran) [CPDMF] was performed using various solid acid catalysts, and Nafion-212 was found to be the most effective due to its high acid strength. The HDO chemistry in the second step used the Ni catalyst supported on acidic materials. The acidic sites on the supporting material were crucial to promote dehydration and ring-opening steps. Ni/SiO2–Al2O3 showed the best catalyst for this purpose, providing a high carbon yield of jet-fuel-range cycloalkanes, including 1,1-dipentylcyclopentane (1,1-DPCP).[112] Sulfonic acid-functionalized biochar with superacidity and hydrophobicity has been employed as an efficient catalyst for preparing CPDMF from 2MF and CPN.

Jet-fuel-range cycloalkanes have been synthesized in high overall yields (ca. 80%) by condensing CPN with butanal and then subjecting the aldol product to HDO chemistry.[113] Magnesium–aluminum hydrotalcite (MgAl-HT) was an effective catalyst with both acidic and basic sites for the solvent-free aldol condensation between CPN and butanal and provided an 87% carbon yield (Scheme b). In the next step, the condensed products were hydrogenated in the presence of a bimetallic 4% Ni–1% Pd/SiO2 catalyst to provide butylcyclopentane (BCP) and 1,3-dibutylcyclopentane (1,3-DBCP) in 88% carbon yield under optimized reaction conditions (230 °C, 6 MPa H2). The catalyst was found to be superior to Ni/SiO2 and Pd/SiO2 catalysts and was explained by the formation of the Ni–Pd alloy under the reaction conditions. Butanal required for the process can be sourced renewably by the partial oxidation of biobutanol.[114] CPN can be self-condensed in the presence of acid or base catalysts to form dimers (C10) or trimers (C15), which on hydrogenation result in jet-fuel-range hydrocarbons (Scheme c). CPN can be dimerized by aldol condensation in the presence of 10% NaOH, which forms bi(cyclopentane) (a C10 oxygenate) when subjected to the HDO chemistry. Recently, bimetallic FeNi3 NPs enriched with Ni (FeNi3@Ni) have been used as catalysts as well as magnetic heating agents for the oligomerization of CPN to yield C10 and C15 oligocyclopentyl products.[115] Cycloalkanes were produced in two steps in ∼84.8% overall carbon yield starting from CPN and FF.[116] In the initial step, 2,5-di(furylmethyl)cyclopentanone (DFMCPN) was prepared in a 98.5% yield by the aldol condensation/hydrogenation reaction of CPN and FF using Pd/C–CaO as a solid bifunctional base as well as a hydrogenation catalyst. In the final step, DFMCPN was subjected to HDO chemistry over the Pd/H-ZSM-5 catalyst under solvent-free conditions to provide cycloalkanes in 86.1% yield. The mixture of cycloalkanes showed interesting properties like a high density and a low freezing point. Mg–Zr mixed oxide prepared by the sol–gel method was used as a solid base catalyst for the aldol condensation between FF and CPN to produce 2,5-di(furylmethylidene)cyclopentanone (DFCPN).[117] Selectivity toward the C15 compound was studied at different temperatures (20–50 °C) and different FF/CPN molar ratios (1:1, 3:1, 5:1, and 10:1). More than 60% yield of the C15 condensed product was obtained under the optimum conditions (30 °C, 4 h). The efficiencies of various solid acid catalysts were examined and compared for the Aldol condensation reaction between CPN and FF.[118] Nafion was the most efficient, robust, and recyclable solid acid catalyst that provided a 37.48% yield of the C15 oxygenate and 23.77% of the C10 oxygenate. Several other studies have reported using solid acid and base catalysts for the self-aldol condensation of CPN, crossed-aldol condensation between CPN with appropriate furanic compounds, and even F–C reaction to make oxygenates.[119,120] Complete deoxygenation of the oxygenated compounds produces high-density jet fuel–fuel range cycloalkanes.[121] Muldoon and Harvey have recently reviewed the renewable synthesis of cycloalkanes for jet fuel applications starting from various biomass-derived chemical intermediates like FF, HMF, and CPN.[122]

The acid- or base-catalyzed crossed-aldol condensation between FF and CPN leads to FC (furfural-cyclopentanone) and FCF (furfural-cyclopentanone-furfural)-type oxygenates (Table ), which on extensive HDO forms high-density cyclic hydrocarbons in the jet fuel range. The reaction can be carried out in the presence of a suitable solvent or under solvent-free conditions. The most frequently used catalysts are alkali because of their strong basicity and faster kinetics under relatively mild reaction conditions. However, there are concerns over their large-scale use due to reactor corrosion, poor recyclability of the catalyst, and waste generation. Heterogeneous base catalysts like mixed-metal oxides have shown promising results and have been used as greener alternatives to alkali catalysts (Table , entries 7, 8, & 9). Solid acids like Nafion, Amberlyst 15, and zeolites have also been used as catalysts for the aldol condensation reaction. Strong alkali catalysts produce the trimer almost exclusively, whereas solid acid or base catalysts often lead to a mixture of dimer and trimer products.

Table 5

Crossed-Aldol Condensation between FF and CPN as Precursors for Hydrocarbon Fuels

entry	catalyst	reaction conditions	yield (%)	refs
1	NaOH	FF and CPN (2:1), catalyst (20–75 mol % of CPN), water (15 & 100 mL), 25–50 °C, 0.5–2 h	96–97b	(123−125)
2	KOH	FF (0.192 g), CPN (0.084 g), catalyst (10 mol % of CPN), water (15 mL), 30 °C, 40 min	93b	(125)
3	Ba(OH)2	FF (0.192 g), CPN (0.084 g), catalyst (10 mol % relative to CPN), water (15 mL), 30 °C, 40 min	92b	(125)
4	CaO	FF (1.92 g), CPN (0.84 g), catalyst (0.089 mol % relative to CPN), 150 °C, 10 h	95.4b	(116)
5	Nafion	FF (1.92 g), CPN (5.047 g), catalyst (0.4 g), 60 °C, 6 h	23.77a, 37.48b	(118)
6	Amberlyst 15	FF (1.92 g), CPN (5.047 g), catalyst (0.4 g), 60 °C, 6 h	∼26a, ∼38b	(118)
7	Mg–Zr mixed oxide	FF and CPN (1:1 to 5:1), catalyst (0.1 g), water or water/ethanol (1:2), 30 °C, 4–24 h	36–65c	(117, 126)
8	MgZr	FF and CPN (1:10), catalyst (0.5 g), KHCO3, water/ethanol (1:2), 50 °C, pH 8, 8 h	76a, 14b	(127)
9	5.7% Na–MgAlO	FF (1.92 g), CPN (1.68 g), catalyst (0.4 g), 80 °C, 2 h	96.9c	(128)
10	33 wt % KF/Al2O3	FF (1.92 g), CPN (0.84 g), catalyst (0.2 g), ethanol (40 mL), 60 °C, 2 h	95.4b	(129)

2-Furylmethylidenecyclopentanone (FCPN, FC).

DFCPN (F2C).

Total yield (FC and F2C).

The self-aldol condensation of CPN forms C10 and C15 oxygenates (Table ), which can then be transformed to C10 and C15 cycloalkanes by HDO chemistry. The synthetic strategy does not require multiple precursors, and only CPN is the feedstock. Besides, the crossed aldol of CPN with furanic compounds makes oxygenates bearing the furan ring, requiring several moles of hydrogen to get reduced and multiple hydrogenolysis steps.

Table 6

Synthesis of Fuel Precursors by the Acid- or Base-Catalyzed Self-Condensation of CPNa

entry	catalyst	reaction conditions	yield (%)	refs
1	10% NaOH	CPN (1045 g), catalyst (104.5 g), reflux, 10 h	65b	(45)
2	MgAl-HT	CPN (4.0 g), catalyst (0.4 g), 150 °C, 8 h	86b	(130)
3	PTA/MIL-101	CPN (28.5 g), catalyst (1.5 g), 130 °C, 48 h	45.5b	(121)
4	MgZr41	CPN (4.0 g), catalyst (0.1 g), 130 °C, 4.5 h	84.6b	(131)
5	S-shell-750	CPN (4.0 g), catalyst (1.0 g), 180 °C, 2 h	92.1b	(132)
6	Nb2O5	CPN (2.1 g), catalyst (0.1 g), 130 °C, 12 h	29.2b, 57.8c	(133)
7	EAOAc	CPN (0.841 g), catalyst (10 mol %), 100 °C, 6 h	83.5b	(134)
8	KOH/diatomite	CPN (4.0 g), catalyst (1.0 g), 180 °C, 2 h	∼58b	(135)
9	TZ-ST	CPN (1.0 g), catalyst (0.2 g), 140 °C, 6 h	75.6b	(136)
10	NaH	CPN (0.096 g), catalyst (0.029 g), toluene (5 mL), 120 °C, 3 h	26.2b, 73.6c	(137)
11	TiO2–ZrO2 (uncalcined)	CPN (1.0 g), catalyst (0.2 g), 120 °C, 6 h	86b	(138)
12	CaO	CPN (4.0 g), catalyst (5.0 g), 2 h, milling ball (∼45 °C, 195 g, 5 mm ZrO2)	98.9b	(139)

Abbreviations: HT: hydrotalcite; MgAl41: MgO and Al2O3 mixed oxide; PTA: phosphotungstic acid; S-shell-750: waste sea-shell-derived CaO; EAOAc: ethanolamine acetate; TZ-ST: TiO2–ZrO2 mixed oxide prepared from the solvothermal method.

2-Cyclopentylidenecyclopentanone (CPCPN, dimer).

2,5-Dicyclopentylidene-cyclopentanone (DCPCPN, trimer).

The reaction was typically carried out at elevated temperatures (>120 °C) in the presence of strong bases like NaOH and a solvent (Table , entries 1, 8, 10). Solid base catalysts like waste sea-shell-derived CaO showed good catalytic properties for the self-aldol reaction (Table , entry 5). Mixed metal oxides, heteropolyacid-supported MOF, and mixed oxide-supported HT were also reported (Table , entries 2–4, 9, 11). Solvent-free methods like ball milling also afforded a satisfactory yield of the C10 oxygenate (Table , entry 12).

Scheme depicts various pathways of making C10 and C15 oxygenates for converting them into jet-fuel-range cycloalkanes.

Scheme 6

Synthesis of Jet-Fuel-Range Cycloalkanes from CPL via (a) Guerbet Reaction and (b) Coupling and Rearrangement

CPL has also been used as a starting material for the preparation of potential aviation fuels like bi(cyclopentane) and tri(cyclopentane).[140] The two-step process started with the concomitant dehydrogenation of CPL and Aldol condensation of the resulting CPN into C10 and C15 α,β-unsaturated ketones (Guerbet reaction) using a combination of Raney Ni and MgAl-HT as catalysts (Scheme a). Raney Ni catalyzed the dehydrogenation reaction, whereas the solid base MgAl-HT promoted the aldol condensation reaction. The efficacy of various solid bases was examined and compared. The C10 and C15 oxygenated compounds were then submitted to hydrogenation and HDO chemistry using molecular hydrogen as the reducing agent and Raney Ni as the catalyst under flowing conditions. The Guerbet reaction was performed in a batch reactor for 8 h at 170 °C under an argon atmosphere. The subsequent hydrogenation step was conducted in a flow reactor at 230 °C under a 6 MPa hydrogen pressure, resulting in a 95.6% combined yield of bi(cyclopentane) and tri(cyclopentane). CPL can be dehydrated to cyclopentene under acid catalysis. Cyclopentene can then dimerize and undergo acid-catalyzed rearrangement reactions through the carbocation intermediates. Xu et al. reported Guerbet reaction of CPL using waste sea-shell-derived CaO and Raney Ni catalysts for the synthesis of renewable jet fuels. Conch shells (C-shells), Scapharca broughtonii (S-shells), and oyster shells (O-shells) were selected as precursors for the synthesis of CaO. The combination of C-shell-950 (calcination at 950 °C) and Raney Ni exhibited excellent catalytic properties and provided the highest 94.6% yield of CPL.[141]

The synthesis of decalin, a promising high-density jet fuel, was accomplished starting from CPL (Scheme b). In one report, Amberlyst-36 resin was used as a stable and efficient acid catalyst for cyclopentene oligomerization/rearrangement.[142] A 74.2% carbon yield of C10 and C15 polycycloalkenes was achieved. The products were then catalytically hydrogenated to form C10 and C15 polycycloalkanes with a 77% selectivity toward decalin. Dicyclopentyl ether, formed by the dehydrative etherification from two molecules of CPL, is formed as a secondary product during the production of decalin. The acid-catalyzed molecular rearrangement of cyclopentene, followed by hydrogenation, also forms bi(cyclopentane) as a secondary product.

Applications as Renewable Solvents

CPN is a widely used solvent in the electronic industry due to its high solvent power for various resins (e.g., epoxy).[143] Solvents like cyclopentyl methyl ether (CPME) and cyclopentyl ethyl ether (CPEE) are renewable ether-based solvents with superior properties like increased hydrophobicity (compared to diethyl ether); higher stability to acids, bases, and peroxides; a higher boiling point; and low water solubility.[144] The solvents are increasingly being used for both bench-scale and plant-scale processes in various organic transformations.[144,145] CPME can be synthesized by the etherification of CPL using a methylating agent like dimethyl sulfate.[146] Alternatively, the acid-catalyzed methanolysis of cyclopentene forms CPME.[147] A mixed solvent system consisting of alcohols (e.g., methanol, ethanol) and cyclic ketones (e.g., CPN) makes a hydrogen bond donor–hydrogen bond acceptor solvent pair for synthesizing pharmaceuticals and biorefinery products and for chemical processing.[147,148] CPME was found to have relatively lower toxicity than many traditional solvents.[149]Table compares some of the physicochemical properties of diethyl ether, THF, methyl t-butyl ether (MTBE), and CPME. CPME possesses many desirable properties as a solvent, such as a high flash point, a high boiling point, low water solubility, and low heat of vaporization.

Table 7

Comparison of the Physicochemical Properties of Some Frequently Used Ethereal Solvents

properties	diethyl ether	THF	MTBE	CPME
BP (°C)	34.6	65	55	106
heat of vaporization (kcal/kg)	86.08	98.1	81.7	69.2
flash point (°C)	–45	–14.5	–28	–1
solubility in water (g/g)	0.065	miscible	0.048	0.011
azeotrope point with water (°C)	34	64		83
density (g/cm3)	0.71	0.89	0.74	0.86
specific heat (kcal kg–1 K–1)	0.5385	0.469	0.51	0.4346
instability (NFPA)	1	1	0	0

Applications in Resins and Renewable Polymers

Cyclic ketones like CPN have established applications as solvents for various resins. Synthetic resins have been prepared by the base-catalyzed polymerization of CPN in a high-pressure batch reactor.[150] The brown, highly viscous resin is thermally stable up to 180 °C and proposed to be a viable substitute for phenol–formaldehyde resin for paint and coating applications. A photoreactive epoxy resin with the CPN moiety in the main chain has been synthesized, and its thermal characteristics have been explored. The polymer was found to be stable even at 300 °C and soluble in polar aprotic solvents.[151] Due to the structural similarity of CPN with cyclohexanone, the former can be converted into δ-valerolactone for synthesizing polyamides.[152] A novel polymer was synthesized by the polycondensation reaction between CPN and formaldehyde, followed by sulfation with sodium sulfite, and finally replacing the sodium ion with a proton using an ion-exchange resin. The acidic polymer was used as the catalyst for the efficient preparation of HMF from fructose and inulin.[153] CPN can be oxidized into glutaric acid, a dicarboxylic acid used as a monomer for polyesters and polyamides.[154] Dimethyl glutarate is a green solvent used as an ingredient in cleaning products, paints, and coating formulations.[155] Glutaric acid can be reduced to 1,5-pentanediol, a promising diol-based monomer for polyesters and polyurethanes.[156,157] Dimethyl adipate has been synthesized in an 85% yield by reacting CPN with dimethyl carbonate in the aqueous medium using surface-exposed ceria (CeO2) nanorods as the base catalyst. The transformation goes through condensation between CPN and dimethyl carbonate, followed by nucleophilic attack of methoxide at the ketonic carbon, leading to ring opening.[52]

Applications in Personal Care Products

CPN has widespread applications and established markets as a key starting material for preparing various volatile, aroma-producing compounds for fragrances and perfumes used as components in numerous personal care products.[31] Jasmonic acid is a jasmonate class of plant hormones and is commonly found in plants of the jasmine family (Figure ). The plants use them for attracting certain insects and also as an internal defense mechanism to repel others. The esters of jasmonic acids, such as methyl jasmonate, are found in the essential oil extracted from jasmine flowers. Decarboxylation of jasmonic acid leads to jasmone, a related fragrance compound of natural origin. The naturally occurring compounds and various synthetic analogues of jasmone and jasmonate have been synthesized.[158,159] The desired properties in the jasmine class of artificial fragrances, also recognized as jasmonoids, include higher chemical stability, synthetic versatility, process scalability, and a low odor recognition threshold. For example, hydrogenation of the olefinic group in methyl jasmonate produces methyl dihydrojasmonate or Hedione (Figure ). As a racemic mixture, Hedione has a floral aroma with an odor threshold of 15 ppb. Magnolione (Magnolia ketone) is a structurally related compound with higher stability than Hedione and higher odor strength.[160] All the four stereoisomers of Magnolione have been synthesized, and their aroma properties studied.[161]

Figure 4

Structure of some jasmine classes of naturally occurring and synthetic fragrances coming from CPN.

Applications in Making Pesticides

CPN is a key starting material for several pesticides, including cyclopentamine, pencycuron, and pentethylcyclanone.[162] Pentethylcyclanone is an antitussive medication to ease common cold symptoms. It is prepared by the nucleophilic substitution reaction between the anion of the self-condensation product of CPN and N-(2-chloroethyl)-morpholine.

Applications as a Chemical Intermediate

Apart from the above applications, CPN is a common laboratory reagent and is used in many organic transformations as a reactant or reagent. Besides, CPN is a crucial chemical intermediate for various natural products, molecules with biological activities, and pharmaceuticals.[163] Methylenomycin B, a cyclopentenoid antibiotic isolated from the culture broth of Streptomyces species, is effective against both Gram-negative and Gram-positive bacteria.[164,165] Various water-soluble benzylidene CPN-based photosensitizers have been synthesized and studied for in vitro and in vivo antimicrobial photodynamic therapy. The compounds showed exciting antibacterial activities, which may help in controlling antibiotic-resistant bacteria.[166] Various heterocycle systems containing the CPN moiety have been synthesized that show antimicrobial (e.g., antibacterial, antifungal) activity.[167]

Conclusions

Biomass-derived FF and HMF can be used as renewable chemical feedstocks for synthesizing CPN and HCPN, respectively, which are otherwise sourced from petroleum. A major motivation behind this research is the commercial prospect of CPN, HCPN, and many of their derivatives. The conversion of FFs to CPNs involves a combination of hydrogenation and rearrangement reactions. The process is routinely performed in an aqueous medium at elevated temperatures in the presence of a metal-based heterogeneous catalyst and a source of hydrogen. Molecular hydrogen is routinely employed for the hydrogenation reaction, even though hydrogen donors like methanol and 2-propanol have also been examined. Both noble metal and non-noble metal-based heterogeneous catalysts have been studied, and few of them afforded near-quantitative (>95%) yields of CPN and HCPN under optimized reaction parameters. Nanocatalysts have shown remarkable activity compared to traditional heterogeneous catalysts. Both monometallic and bimetallic NPs have been employed as efficient catalysts for transforming FF and HMF into CPN and HCPN, respectively. The supporting materials for the active metal catalyst play multifaceted roles. Apart from playing conventional roles like increasing the surface area, uniformly dispersing the metal particles, and giving favorable mechanical properties, the supporting materials often establish electronic communications between the metal sites modifying their hydrogenating activity, provide acidic sites for the rearrangement reaction, and also provide adsorption sites for substrate and hydrogen molecules. Water was found to be the best solvent for producing CPN by molecular rearrangement of the furan ring, whereas the use of organic solvents (e.g., alcohol, ether) promoted hydrogenation. In some cases, the mixture of two solvents like 2-propanol/water and toluene/water provided promising results. The reaction required elevated temperatures in the range of 140–170 °C for the best selectivity and yield of CPN and HCPN. High activation energy is needed for the ring-opening molecular rearrangement reaction involving hydration and dehydration steps. An overpressure of hydrogen gas in the range of 1–5 MPa is typically used during the hydrogenation step. The acidic sites on the catalyst promote hydration and rearrangement reactions. Lewis acidic sites are very efficient for the reaction, whereas the Brønsted acidic sites lead to side products and decomposition reactions. Many catalysts have been reported that contained both metal sites and acid sites for hydrogenation and rearrangement reactions. Interestingly, a physical mixture of the metal catalyst and Lewis acid catalyst also showed comparable results. Many studies have reported excellent yields of CPN and HCPN using recyclable catalysts. The reaction is believed to proceed via the Piancatelli mechanism, although alternative mechanisms have also been proposed and substantiated with experimental data. As a petrochemical, CPN has several applications and established markets for its derivatives. Novel applications of biorenewable CPN, such as jet-fuel-range cycloalkanes and green solvents, have been intended. For example, the high-density, jet-fuel-range, cyclic hydrocarbon fuels have been synthesized from lignocellulosic biomass via CPN in three catalytic steps. It may be concluded that remarkable advances have been made toward the renewable synthesis of CPNs from FFs since their discovery in the past decade, and exciting discoveries are awaited in the coming years.

Future Perspectives

Further research in this area should focus on improving the process scalability, design inexpensive but efficient catalysts, and aim for excellent yields of CPN and HCPN under energy-efficient conditions. A detailed theoretical understanding of the interactions between the reaction intermediates in the mechanistic pathway of forming CPN and HCPN with the catalytic sites and their experimental validation will help make the catalytic processes more efficient and cost-competitive. A one-pot production of CPN and HCPN from hemicellulose and cellulose would be ideal since the processes would avoid the complications and expenses associated with the isolation of FF and HMF. The process would require a multifunctional catalyst containing both acidic and metal sites for the tandem sequence of dehydration, hydrogenation, and hydrolysis reactions. The reaction parameters and catalysts may be tweaked to produce related derivatives like CPEN and CPL, which will increase the synthetic versatility of the process and make it commercially more appealing. Novel applications of CPN and HCPN, such as the synthesis of hydrocarbon-based transportation fuels and renewable solvents, must be explored to expand their scope of applications toward a sustainable bio-based economy. The carbon chain has been extended by the self-aldol condensation of CPN or by crossed aldol with FF using various solid acid and base catalysts. In principle, HMF can also be condensed with CPN, and the condensed products can be reduced to cyclic hydrocarbons. However, no publication has reported this work to date. The scalability and cost of producing FFs from biomass contribute significantly to the downstream processing of CPNs. Therefore, attempts to produce FF and HMF with satisfactory yields and scalability must reach the commercialization stage. Although FF is produced commercially at present, the commercial production of HMF from inexpensive biomass is yet to begin. Most literature studies used pressurized hydrogen gas for CPN and HCPN synthesis, which requires a more elaborate and expensive reactor setup, increasing both the capital expenditure and operational expenditure. Therefore, an efficient catalyst that allows the reaction to occur at an acceptable rate and selectivity under moderate hydrogen gas pressure would be ideal. A combination of the hydrogen donor (e.g., 2-propanol) and H2 gas could be a plausible way to lower the requirement of H2 overpressure. In the case of noble metal catalysts, emphasis should be given to lowering the active metal loading. Although non-noble metal catalysts are less expensive and generally perceived as more eco-friendly than noble metal catalysts, their catalytic efficiency and stability should be thoroughly examined. Irrespective of the type of catalyst used, special emphasis must be given to the efficient separation from the reaction mixture and recyclability. Nanocatalysts have shown outstanding performance with high activity, selectivity, and recyclability. Comprehensive research should be performed to design and synthesize novel and efficient nanocatalysts with tunable properties and comprehend their catalytic functions. Single-atom catalysts with a remarkable efficiency are increasingly used in organic transformations, and they can be utilized in the renewable preparation and value addition of CPN and its derivatives. Detailed technoeconomic analysis for the synthesis of CPN and its derivatives from biomass via FFs must be conducted. It is anticipated that the economic prospects of the existing as well as novel derivative chemistry of renewable CPN and HCPN will encourage their commercial production in the near future.