Heterogeneous Nickel Catalysts Derived from 2D Metal-Organic Frameworks for Regulating the Selectivity of Furfural Hydrogenation.

Hydrolysis of the biomass platform compound furfural can produce a bulk of fine chemicals because of its multiple functional groups. Developing an efficient catalytic system to regulate the process toward some desirable products has always been a hot research area. Herein, the novel Ni-based catalysts (Ni-MFC-X, X = 300, 400...800) synthesized by pyrolysis of the 2D Ni-based metal-organic framework (MOF) in the temperature range 300-800 °C show good performance for selective hydrogenation of furfural (FUR). Interestingly, the calcination temperature of the MOF precursor plays an important role in hydrogenation of furfural with controllable selectivity toward furfuryl alcohol (FOL) and tetrahydro FOL (THFOL). Ni-MFC-500 affords us 92.5% conversion of furfural and 59.5% selectivity of FOL. Ni-MFC-700 can promote hydrogenation of furfural with 91.8% conversion and 51.0% selectivity of THFOL. Furthermore, the stability of as-obtained Ni-MFC-500 and Ni-MFC-700 was also very impressive in this reaction system.

Introduction

Furfural is widely identified as a promising biomass platform compound with huge potential to produce a large number of valuable chemicals because of its abundant and inexpensive source, high reactivity based on the carbonyl (C=O), π-conjugated (C=C–C=C) groups, and a five-membered ring structure.[1−5] Hydrogenation of furfural can afford us with a variety of fine chemicals in the following pathways (see Scheme ). (i) 2-Methylfuran and 2-methyl tetrahydrofuran (THF) via hydrodeoxygenation. (ii) Furfuryl alcohol (FOL) and tetrahydro FOL (THFOL) via hydrogenation. (iii) 1,4-Pentanediol and 1,2-pentanediol via ring-opening hydrogenolysis.[6−13] (iv) Furan and THF via hydrodeocaboxylation. Selective hydrogenation of furfural to FOL and THFOL via path (ii) is highly desired because of their significant applications in the production of pentanediols, environmental benign solvents, industrial resins, and fuel additives. Developing the efficient catalytic system to control the hydrogenation of furfural has been the research hotspot for many years. A large number of noble metallic catalysts (e.g., Pt, Pd, Rh, Ir, Pt, Au, and Ru) have been reported for effective hydrogenation of furfural to FOL.[14−22] Appropriate Pd-, Pt-, or Ru-based catalysts can effectively promote the transformation from furfural (FUR) to THFOL.[23−25] However, it is more preferable to utilize some nonprecious metal catalysts, especially some supported heterogeneous catalysts because of their abundant resources and sustainable development. Cu-, Co-, and Fe-based catalysts show an advantage for selectively catalyzing the hydrogenation of the C=O bond to afford FOL as the main product, avoiding the hydrogenation of the furan ring, which may be because of their lower hydrogenation ability compared with noble metals.[26−28] Among non-noble metals, Ni catalysts seldom favor the selective hydrogenation of furfural toward FOL or THFOL because of their excellent hydrogenation activity. Modification of nickel is thus necessary to improve the selective hydrogenation along the aforementioned second path.[29−31] Three modification methods of Ni-based catalysts were summarized by Royer et al.[11] (i) Adding oxophilic metals, such as Fe, Mg, and Co, to precisely control the amount and localize the additive. (ii) Using oxide supports to interact with nickel strongly, such as TiO2. (iii) Controlling the reaction conditions to suppress the furan ring hydrogenation. However, the components and synthesis of these catalysts are very complicated.

Scheme 1

Main Pathways of Furfural Hydrogenation

Metal–organic frameworks (MOFs) have emerged as a novel attractive class of catalytic materials in view of their outstanding designability.[32−34] Noticeably, the active metal centers can be predesigned as the metallic nodes of MOFs, in which the active metal centers are confined to the matrix materials in the single-atom state. Therefore, when MOFs were used as precursors of carbon matrix nanocomposites, the active centers were incorporated and dispersed uniformly in the carbon matrix.[35,36] In our previous study, we used one 2D Co-based MOF ([Co(tia)(H2O)2], tia2– = 5-(1H-1,2,3-triazol-1-yl)isophthalate) as the precursor to fabricate Co-MOF-700 catalysts, in which Co-MOF-700 displayed good performance in catalytic oxidation condensation of FUR with aliphatic alcohols-O2.[37] Herein, we fabricated Ni-MFC catalysts using 2D Ni-based MOFs ([Ni(tia)(H2O)2]) as the precursor and investigated their catalytic effect on selective hydrogenation of furfural. It is found that the hydrogenation pathway is dominant when using Ni-MFC catalysts. Interestingly, the calcination temperature of the MOF precursor plays an important role in hydrogenation of furfural with controllable selectivity toward FOL and THFOL.

Results and Discussion

Catalytic Evaluation

A 2D Ni-based MOF ([Ni(tia)(H2O)2]) was calcined at different temperatures under a N2 atmosphere to afford us with Ni-MFC catalysts (Ni-MFC-300, 350...800, corresponding to the calcining temperature at 300, 350, and 800 °C, respectively). Ni-MFC catalysts were investigated for catalyzing hydrogenation of furfural, and the results are displayed in Table . In our reaction system, FOL, THFOL, and acetal were detected (see Scheme ). Obviously, Ni-MFC catalysts have significant catalytic effect on selective hydrogenation conversion of furfural (see entries 1 and 2–7 in Table ). Very interestingly, the calcination temperature of the MOF precursor plays an important role in hydrogenation of furfural with controllable selectivity toward FOL and THFOL. Below 500 °C, the selectivity of FOL and THFOL increased with the increasing calcination temperature (entries 2–5 in Table ). For further increasing calcination temperature, the selectivity of FOL began to decrease but the selectivity of THFOL continuously increased. The selectivity of FOL was up to the maximum (59.5%), and then began to decrease with the increasing calcination temperature (entries 6–8 in Table ). Herein, Ni-MFC-500 affords us 92.5% conversion of furfural, 59.5% selectivity of FOL, 30.9% selectivity of THFOL, and 9.6% selectivity of acetal. Ni-MFC-700 can promote hydrogenation of furfural with 91.8% conversion, 51.0% selectivity of THFOL, 39.1% selectivity of FOL, and 9.9% selectivity of acetal.

Table 1

Effect of the Calcination Temperature of the 2D Ni-Based MOFs on the Selective Hydrogenation of Furfuralc

				selectivity (%)b
entry	catalyst	T (°C)	conc (%)b	FOL	THFOL	others
1	none	160	84.2	22.1	0	77.9
2	Ni-MFC-300a	160	91.2	24.2	17.0	58.8
3	Ni-MFC-350a	160	90.8	54.2	26.3	19.5
4	Ni-MFC-400a	160	92.3	59.3	35.6	5.1
5	Ni-MFC-500a	160	92.5	59.5	30.9	9.6
6	Ni-MFC-600a	160	83.6	52.2	42.9	4.9
7	Ni-MFC-700a	160	91.8	39.1	51.0	9.9
8	Ni-MFC-800a	160	76.0	43.5	42.6	13.9

The catalysts were achieved by calcining the 2D Ni-based MOF in a N2 flow at different temperatures.

The results were acquired by GC analysis with the internal standard technique.

Reaction conditions: 0.1 g of furfural, 0.025 g of the catalyst, in 15 mL of methanol, under 2 MPa H2, at 160 °C for 4 h.

Scheme 2

Pathways of Hydrogenation of Furfural

Optimization of Reaction Conditions

With the best catalysts for producing FOL and THFOL in hand, respectively, we investigated the effect of the reaction temperature, reaction time, and reaction pressure in the Ni-MFC-500 and Ni-MFC-700 catalytic reaction system, respectively. In the Ni-MFC-500 catalytic system, the conversion of furfural was up to maximum (about 92%) in 4 h and the selectivity of FOL increased at first and then decreased. When the reaction time was prolonged to 3 h, the selectivity of FOL was up to 63.3%. However, in the Ni-MFC-700 catalytic system, the selectivity of FOL was higher than that of THFOL at the beginning. After 2 h, the selectivity of FOL decreased, but the selectivity of THFOL increased gradually. When the reaction time was prolonged to 6 h, the selectivity of THFOL was up to 57.3%. Obviously, the Ni-MFC catalytic system can reach the transformation equilibrium more quickly (Figure a, b).

Figure 1

Effect of reaction time on the selective hydrogenation of furfural: (a) using Ni-MFC-500 as a catalyst, (b) using Ni-MFC-700 as a catalyst [reaction conditions: furfural (0.1 g), catalyst (0.025 g), H2 pressure (2 MPa), and temperature (160 °C)].

We have carried out the reaction in the pressure range 0.5–3 MPa, and the results are shown in Figure . The conversion of furfural increased in both catalytic reaction systems, when the reaction pressure was increased. The selectivity of THFOL increased, but the selectivity of FOL and acetal declined with the increasing reaction pressure. Therefore, high pressure favors the production of THFOL. Under 3 MPa H2 pressure, the selectivity of THFOL was up to 55.2 and 53.8% in the Ni-MFC-500 and Ni-MFC-700 catalytic reaction system, respectively (Figure a,b).

Figure 2

Effect of reaction pressure on the selective hydrogenation of furfural: (a) using Ni-MFC 500 as a catalyst, (b) using Ni-MFC-700 as a catalyst [reaction conditions: furfural (0.1 g), catalyst (0.025 g), temperature (160 °C), and time (4 h)].

With the increasing reaction temperature, the conversion of furfural and the selectivity of THFOL increased, while the selectivity of FOL decreased (see Figure ). Below 140 °C, the lower conversion was achieved and FOL dominated in both the Ni-MFC-500 and Ni-MFC-700 catalytic systems. For example, the selectivity of FOL was up to 80.7 and 55.0% at 120 °C, respectively. The high yield of FOL was obtained using Ni-MFC-500 as a catalyst at 160 °C. Upon further increasing the temperature, the yield of the deep hydrogenation product (THFOL) continuously increased in Ni-MFC-700 catalytic reaction systems. However, in the Ni-MFC-500 catalytic reaction system, the selectivity of THFOL remained unchanged with the increasing temperature after 160 °C.

Figure 3

Effect of reaction temperature on the selective hydrogenation of furfural: (a) using Ni-MFC-500 as a catalyst; (b) using Ni-MFC-700 as a catalyst [reaction conditions: furfural (0.1 g), catalyst (0.025 g), pressure (2 MPa), and time (4 h)].

Recyclability of Ni-MFC-500 and Ni-MFC-700

After being centrifuged and washed with a solvent, the used Ni-MFC-500 and Ni-MFC-700 catalysts were obtained for the next catalytic cycles. As shown in Tables and 3, the conversion of furfural and the amount of the desirable products were kept almost constant in the consecutive six runs.

Table 2

Results of Durability Experiments of the Ni-MFC-500 Catalysta

		selectivity (%)b
run	conc (%)b	FOL	THFOL	others
1	92.5	59.5	30.9	9.6
2	90.4	57.6	35.8	6.6
3	87.6	58.8	30.1	11.1
4	91.2	55.4	41.5	3.1
5	89.7	56.2	37.3	6.5
6	90.3	56.4	34.9	8.7

Reaction conditions: furfural (0.1 g), catalyst (0.025 g), in 15 mL of methanol, under 2 MPa H2, at 160 °C for 4 h.

The results were acquired by GC analysis with the internal standard technique.

Table 3

Results of Durability Experiments of the Ni-MFC-700 Catalysta

		selectivity (%)b
run	conc (%)b	FOL	THFOL	others
1	91.8	39.1	51.0	9.9
2	90.5	35.6	53.5	10.9
3	87.8	41.3	49.8	8.9
4	91.2	40.7	52.9	6.4
5	89.3	43.2	48.5	8.3
6	87.9	42.7	47.3	10.0

Reaction conditions: furfural (0.1 g), catalyst (0.025 g), in 15 mL methanol, under 2 MPa H2, at 160 °C for 4 h.

The results were acquired by GC analysis with the internal standard technique.

Characterization of Ni-MFC-500 and Ni-MFC-700 Catalysts

As shown in Figure a,g, the block shape of the MOF remains unchanged before and after being calcined. The lamellar structures are clearly visible in both Ni-MFC-500 and Ni-MFC-700 catalysts (see Figure b,h). Comparing Figure c with 4i, it is found that a large quantity of Ni nanoparticles with uniform particle size was dispersed uniformly in the lamellar of Ni-MFC-700. Although in Ni-MFC-500, Ni nanoparticles were not fully grown. This explains that the hydrogenation capacity of Ni-MFC-700 is stronger than Ni-MFC-500, which may be responsible for the higher selectivity for THFOL. Unlike the traditional supported Ni-based catalysts, Ni nanoparticles were not only dispersed on the surface but also inside the interior of Ni-MFC-700 because the uniform Ni nanoparticles can be observed on the fracture surface (see Figure i,4l). The elementary composition of the spent Ni-MFC-500 and Ni-MFC-700 was determined by inductively coupled plasma–atomic emission spectroscopy (see Table S1). It was found that no Ni leaching occurred in the course of the reaction. In addition, the X-ray diffraction (XRD) pattern of the spent catalyst was similar to that of the fresh sample (Figure ). All these explain that Ni-MFC-500 and Ni-MFC-700 have a good catalytic cycling performance.

Figure 4

SEM images of Ni-MFC-500 and Ni-MFC-700 catalysts: (a–c) for the fresh Ni-MFC-500; (d–f) for the used Ni-MFC-500; (g–i) for the fresh Ni-MFC-700; and (j–l) for the used Ni-MFC-700.

Figure 5

XRD patterns of Ni-MFC-500 and Ni-MFC-700 catalysts.

Transmission electron microscopy, energy-dispersive spectroscopy (EDS) analysis, and EDS elemental mapping of the Ni-MFC-500 catalyst have been performed and incorporated into Figure S5 in the Supporting Information. It was found that Ni nanoparticles were uniformly dispersed in the Ni-MFC-500 (Figure S5a,b in the Supporting Information). At the same time, their lattice fringes are clearly seen in Figure S5c in the Supporting Information. The EDS spectrum of Ni-MFC-500 showed that the main components of the Ni-MFC-500 catalyst are Ni, C, N, and O (Figure S5d in the Supporting Information). Element mapping also verified that the Ni nanoparticles dispersed uniformly on the Ni-MFC-500 catalysts (Figure S5e in the Supporting Information). These results were in agreement with the results from high-resolution scanning electron microscopy (SEM) mapping of the Ni-MFC-500 catalyst (Figure S6 in the Supporting Information).

The X-ray photoelectron spectroscopy (XPS) analysis of Ni-MFC-500 and Ni-MFC-700 catalysts is supplemented in Figures S7 and S8 in the Supporting Information. The best fitted peaks of Ni 2p3/2 in Ni-MFC-500 at 853.1 and 856 eV may be ascribed to the adsorption of Ni(0) and Ni(II) coordinated with pyridine nitrogen, respectively. The peaks at 857.1 and 861.9 eV should be assigned to Ni(II) coordinated with oxygen, respectively (Figure S8b in the Supporting Information). While the best fitted peaks of Ni 2p3/2 in Ni-MFC-700 at 852.1, 853.3 eV were attributed to metallic nickel, the peaks at 855.1 and 859.8 eV were ascribed to NiO and Ni2O3 (see Figure S8b in the Supporting Information).[43−45] Obviously, the content of Ni(0) in Ni-MFC-700 is much larger than that in Ni-MFC-500, which may be responsible for the higher yield of the deep hydrogenolysis product THFOL.

The specific surface area (SBET) and pore volume (Vpore) of Ni-MFC-700 and Ni-MFC-500 were determined by the N2 physisorption technique, and the results are supplemented in Figure S9 and Table S2 in the Supporting Information. The adsorption and desorption hysteresis loop of Ni-MFC-500 and Ni-MFC-700 catalysts were assigned to H3 and H2, respectively, which showed the relatively uniform pores in the latter. The pore size and pore volume of Ni-MFC-700 were also larger than those of Ni-MFC-500.

The H2-pulse chemisorption of the catalyst is shown in Figure S11 in the Supporting Information. The H2-pulse chemisorption parameters in Table S3 in the Supporting Information showed that the metal dispersion, metallic surface area (sample), and metallic surface area (metal) of Ni-MFC-700 were larger than those of Ni-MFC-500, which ascribed to the increasing exposure Ni nanoparticles along with the increasing calcination temperature. Moreover, the average particle size of Ni-MFC-700 was smaller than that of Ni-MFC-500.

The CO2-temperature-programmed desorption (TPD) profiles of Ni-MFC-500 and Ni-MFC-700 catalysts are displayed in Figure S10 in the Supporting Information. No peaks were observed below 600 °C, which were likely associated to decomposition of carbonate-like species. Hence, the surface Lewis basic sites of Ni-MFC-500 and Ni-MFC-700 catalysts were very weak. The broad peak with the maximum at 630 °C of Ni-MFC-500 can be assigned to the decomposition of the remnant organic framework. One strong peak at about 750 °C was observed on the desorption curves of Ni-MFC-500 and Ni-MFC-700 catalysts, which may be ascribed to the reaction of C component with CO2.

The result of the NH3-TPD experiment is shown in Figure S13 in the Supporting Information. The peak at 375 °C was ascribed to the acidity of the catalyst, in which a broad peak of Ni-MOF-500 may be attributed to the corporation between weak acid sites and the decomposition of the remnant MOFs.

The result of the NH3-TPD experiment is shown in Figure S12 in the Supporting Information. The fitting reduction peak of the catalyst at 525 °C would be assigned to the presence of NiO and Ni2O3, which was in agreement with the result of XPS.

Based on the aforementioned results of CO2-TPD, NH3-TPD, H2-pulse chemisorption, and BET, the effect of the calcination temperature on the structure of catalysts can be summarized as follows: (i) the lower calcination temperature of 500 °C leads to the incomplete decomposition of the organic framework, which was verified by the results of CO2-TPD and NH3-TPD of Ni-MFC-500 and Ni-MFC-700; (ii) the content of Ni(0) element in Ni-MFC-700 was higher than that in Ni-MFC-500; (iii) the pore size and pore volume in Ni-MFC-700 were larger than those in Ni-MFC-500; (iv) the acidic sites of Ni-MFC-500 were more than those of Ni-MFC-700.

Advantage of Our Reaction System Compared with the Previous Report

Monometallic Ni-based and noble metallic catalysts seldom favor the selective hydrogenation of furfural toward FOL or THFOL because of their too high hydrogenation activity. Although some modification strategies were used to functionalize these catalysts for controlling the hydrogenation direction to produce FOL or THFOL and the results were very good, developing the none-noble metal catalysts are still desirable. The main work on hydrogenation of furfural to FOL and THFOL using none-noble metal catalysts is displayed in Table . The none-noble metal of the Co-based catalyst can achieve high selectivity of FOL and full conversion of furfural in the report of Liu co-workers and Zhao co-workers.[38−42] Herein, the developed Ni-MFC catalysts showed good performance in catalyzing the transformation of furfural to FOL and THFOL..

Table 4

Reported Non-noble Catalysts for Catalyzing the Selective Hydrogenation of Furfural toward FOL or THFOL

							sel. (%)
entry	catalyst	feed conditions	p (MPa)	T (°C)	T (h)	conv (%)	FOL	THFOL	others	refs
1	Ni-MFC-500	0.1 g furfural, 15 mL CH3OH, 0.025 g catalyst	2	160	4	92.5	59.5	30.9	9.6
2	Ni-MFC-700	0.1 g furfural, 15 mL CH3OH, 0.025 g catalyst	2	160	4	91.8	39.1	51.0	9.9
3	Co–B	10 mL furfural, 30 mL CH3CH2OH; 1 g catalyst	1	80		100	100	0	0	(38)
4	Co/CN	0.0480 g furfural, 10 mL water; 0.06 g ammonia–borane, 0.02 g catalyst (30 wt % Co)	0.1 (air)	25	7	>99	>99	0	0	(39)
5	Ni/SiO2-773	FFR/H2/N2 ratio = 1:36:72, W/F = 0.884 g catalyst/(mol/h)	0.1	140	0.5	100	0	94.0	6.0	(40)
6	Ni/NAC-1-1073	mass ratio of catalyst and furfural mass ratio = 1:1, 5 mL 2-propanol	4	80	3	100	0	100	0	(41)
7	Ni–Fe–B	10 mL furfural, 30 mL CH3CH2OH, 1 g catalyst	1	100	4	100	100	0	0	(42)

Mechanism for Selective Hydrogenation of Furfural toward FOL and THFOL

The kinetic curves of the transformation are shown in Figure . The low conversion of furfural and 100% selectivity of acetal were obtained at the beginning. The conversion of furfural and the selectivity of FOL and THFOL increased with prolonging the reaction time but the selectivity of acetal decreased. Based on these results, we assumed that furfural condensed with alcohol to produce acetal with a weak acidic catalyst at the beginning. Then, the active Ni(0) sites promoted hydrolysis of acetals to FOL or THFOL. Of course, THFOL can also be obtained by hydrogenolysis of the intermediate time-of-flight.

Figure 6

Reaction mechanism: (a) pathway of the reaction; (b) kinetic curves of the transformation of furfural in methanol (reaction conditions: 0.1 g of furfural, 0.025 g of the Ni-MFC-700 catalyst, in 15 mL of methanol, under 2 MPa of H2, at 160 °C).

Conclusions

The novel Ni-MFC catalysts were prepared by simply pyrolysis of one Ni-based MOF in our report, which can orient the reaction in the direction of hydrogenation and avoid the processes of ring opening, hydrodecarboxylation, and hydrodeoxygenation. Notably, we can control the degree of hydrogenation of furfural by altering the calcination temperature. Ni-MFC-500 affords us with 92.5% conversion of furfural and 59.5% selectivity of FOL. Ni-MFC-700 can promote hydrogenation of furfural with 91.8% conversion and 51.0% selectivity of THFOL.

Materials and Methods

Materials

Furfural (99.0%), FOL (98.0%), THFOL (99.0%), NaN3 (98.0%), and methanol (99.5%) were purchased from Shanghai Aladdin Bio-Chem. Technology Co. LTD. Ni(NO3)2·6H2O (98.0%) was purchased from Tianjin Guangfu Technology Development Co. Ltd. Propiolic acid (95%) was purchased from Beijing Hwrk Chem. Co. Ltd. 5-Aminoisophthalic acid (98.0%) was purchased from Chem. Great Wall.

Catalyst Preparation

The 2D Ni-based MOF ([Ni(tia)(H2O)2]) was synthesized according to the literature.[37] The as-synthesized Ni-based MOFs were put into a quartz boat and placed in the tubular furnace. With the calcination temperature of 300, 350, 400, 500, 600, 700, and 800 °C under a nitrogen flow (50 mL/min) for 4 h, the Ni-MFC-300, 350, 400, 500, 600, 700, and 800 catalysts were obtained, respectively.

Catalytic Tests

The hydrogenation of furfural was carried out in 100 mL Parr stainless autoclave equipped with a heater and a mechanical stirrer. Furfural (0.1 g), catalyst (0.025 g), and methanol (15 mL) were placed in the reactor. After being purged with nitrogen several times to eliminate the air, H2 was purged into the reactor four times at room temperature to replace N2. Then, the autoclave was pressurized with hydrogen up to 2 MPa and heated to 160 °C under vigorous stirring at 550 rpm and retained for 4 h. After the reaction was over, the reaction mixture was weighed and diluted with acetonitrile. The products were qualitatively analyzed by using an Agilent 7890A/5975C gas chromatography–mass spectrometer equipped with a flame ionization detector (FID) and quantitatively determined using GC (Agilent 7820A) employing a FID and a HP-5 capillary column (30 m × 0.32 mm × 0.25 mm). The carrier gas used was N2 at a flow of 2 mL/min.

Recycling Tests

Recycling tests were performed in the following manner. After each reaction, the catalyst was recovered by centrifugation, washed with methanol six times, dried, and then activated at the corresponding temperature. The recycled catalysts were recharged in the reactor following the same procedure as described above.