Reductive Amination of Furanic Aldehydes in Aqueous Solution over Versatile Ni y AlO x Catalysts.

We disclose in this study a Ni6AlO x catalyst prepared by coprecipitation for the reductive amination of biomass-derived aldehydes and ketones in aqueous ammonia under mild reaction conditions. The catalyst exhibited 99% yield toward 5-aminomethyl-2-furylmethanol in the reaction of 5-hydroxymethyl furfural with ammonia at 100 °C for 6 h under 1 bar H2. The catalyst was further extended to the reductive amination of a library of aromatic and aliphatic aldehydes and ketones with a yield in the range 81-90% at optimized reaction conditions. Besides, 5-hydroxymethylfurfural could react with a library of primary and secondary amines with yields in the range 76-88%. The catalyst could be easily recycled and reused without apparent loss of activity in four consecutive runs.

Introduction

Furanic aldehydes (X-furfural with X = H, CH2OH) are important chemicals readily accessible from carbohydrates (hexoses or pentoses) contained in lignocellulosic biomass.[1] In particular, furfural (FF) is a versatile platform chemical for the production of fuel additives, solvents, polymers, surfactants, perfumes, and agrochemical ingredients.[2] The current world production of FF is estimated at 280 kton/year with the largest plant being based in the Dominican Republic with a capacity of 35 kton/year.[3] Likewise, 5-hydroxymethylfurfural (HMF) has attracted great interest in recent years with extensive research on its production and further transformation as a platform chemical. Unlike FF, no industrial production of HMF is available today. However, as key intermediate for FDCA production, HMF is on a fast track for commercialization.[4]

Although the transformation of biomass into N-containing compounds is highly desired for the synthesis of surfactants, pharmaceuticals, and intermediates, only few examples have been reported on furanic aldehydes.[5] At industrial scale, FF can be selectively converted into furfurylamine (FAM) by reductive amination (RA) with NH3 over Ni and Co catalysts using dioxane or alcohols as solvents.[6] In the RA mechanism, NH3 or an amine condense with a carbonyl compound to generate an imine intermediate that is reduced in situ to form the amine product and water.[7,8] FAM can be further used for the synthesis of pharmaceuticals such as antiseptic reagents, antihypertensives, and diuretics (e.g., furosemide).[9] Ionic liquids were also synthesized by the RA of furanic aldehydes.[10] At laboratory scale, Au, Pt, and Ir colloids supported over sulfonic acid-functionalized silica were developed for the RA of FF using aniline.[11] Besides, the electrochemical RA of HMF with ethanolamine using water as a hydrogen source was also reported over a Ag electrode, achieving 92% yield.[12]

The amination of furanic aldehydes in aqueous solution is drawing more and more attention for applications in biorefineries. A key challenge in these transformations is how to develop stable and robust non-noble metal catalysts with high activity and selectivity toward the desired amines. Kawanami and co-workers used commercial 5%Rh/Al2O3 to catalyze the RA of furanic aldehydes in 28% aqueous solution of NH3 to achieve high selectivity at mild reaction conditions without addition of solvents or additives.[13] Ebitani and co-workers reported an active poly(N-vinyl-2-pyrrolidone)-capped Ru-supported hydroxyapatite (Ru-PVP/HAP) catalyst for the RA of FF with aqueous NH3, achieving a yield up to 60% at optimized reaction conditions.[14] Starting from diformylfuran (DFF), Girka et al. prepared a new family of tetrahydrofuran-derived amines as biobased surfactants.[15] The authors developed a two-step process comprising first the RA of DFF with aliphatic amines in water using NaBH4 as reducing agent followed by the reduction of the furan ring with Raney Ni. Hara and co-workers reported a highly active and durable Ru/Nb2O5 catalyst for the RA of carbonyl compounds containing reduction-sensitive functional groups to primary amines.[16,17] Ru/ZrO2 was also developed for the RA of 5-methylfurfural with aqueous NH3, leading to 5-methylfurfurylamine with 61% yield. The coexistence of Ru and RuO2 on the surface of Ru/ZrO2 provided both strong Lewis acid and metal hydrogenation sites. The cooperation between both sites led to an excellent performance for the production of primary amines.[18] Finally, CO-assisted RA of HMF using a variety of amines in methanol/water (1:1 v/v) could proceed fast over Au/TiO2.[19]

Ni is known as an active non-noble metal for the direct amination of alcohols and the RA of aldehydes and ketones.[20,21] Herein, a series of NiAlO catalysts were easily prepared by coprecipitation of starting precursors and were further employed in the RA of furanic aldehydes with aqueous NH3. More importantly, the Ni/Al molar ratio could be easily tuned by adjusting the relative amount of the starting precursors in the synthesis solution.

Results and Discussion

Figure plots the X-ray diffraction (XRD) patterns of the different NiAlO catalysts prepared in this study. The catalysts were prereduced ex situ before analysis. The patterns suggest that finely crystallized metal Ni is formed on NiAlO, especially at high Ni loading, as can be inferred from the characteristic Ni(111), Ni(200), Ni(220), Ni(311), and Ni(222) reflections appearing at 44°, 53°, 77°, 93°, and 98°, respectively (Ni, JCPDS card no: 04-0850, Fm3m cubic unit cell). Unexpectedly, metal Ni is stable after air exposure, suggesting potential encapsulation by alumina or passivation, especially at higher Ni content. Additional reflections are also visible at 37°, 43°, 63°, 76°, and 79°, which can be assigned to NiO(111), NiO(200), NiO(220), NiO(311), and NiO(222), respectively (NiO, JCPDS card no: 47-1049, Fm3m cubic unit cell). By decreasing the Ni/Al molar ratio, the reflections ascribed to metal Ni become significantly weakened at the expense of the reflections belonging to NiO. No reflections attributed to either alumina or the spinel NiAl2O4 can be discerned, suggesting the presence of amorphous aluminum oxides or hydroxides.

Figure 1

XRD patterns of NiAlO catalysts. From top to bottom: Ni10AlO, Ni8AlO, Ni6AlO, Ni4AlO, Ni2AlO, and Ni1AlO. The catalysts were prereduced ex situ before analysis.

Figure shows representative transmission electron microscopy (TEM) micrographs of Ni6AlO after reduction at 450 °C for 2 h under H2 flow. Because the catalyst was prepared by the coprecipitation method, it is difficult to distinguish Ni/NiO nanoparticles on the catalyst surface. The average particle size of the catalyst ranges from 10 to 30 nm. The magnified TEM micrographs of Ni6AlO reveal the presence of highly crystalline Ni/NiO nanoparticles.

Figure 2

TEM micrographs of Ni6AlO at different magnification levels.

The surface composition of the NiAlO catalysts was analyzed by X-ray photoelectron spectroscopy (XPS). Figure plots representative XPS spectra of the Ni 2p, Ni 3p, and Al 2p core levels for Ni6AlO. Additional spectra for the remaining catalysts can be found in the Supporting Information (Figures S1–S5). The deconvoluted Ni 2p core level (Figure a) can be assigned to two spin–orbit coupling levels for the Ni 2p3/2 and Ni 2p1/2 states. Both states show a main band in the range 855.7–856.9 eV (Ni 2p3/2) and 873.4–874.5 eV (Ni 2p1/2) corresponding to Ni2+, whereas the small band (<5% surface) in the range 852.0–853.4 eV (Ni 2p3/2) and 871.2–872.5 eV (Ni 2p1/2) can be attributed to Ni0.[23−26] A broad band is also present at 861.7–862.7 eV (Ni 2p3/2) and 879.7–881.5 eV (Ni 2p1/2) which can be assigned to the shake-up satellite band of NiO. Distinctive bands ascribed to Ni2+ and shake-up satellites can also be observed in the deconvoluted spectra of the Ni 3p core level (Figure b). This level is also constituted by two states with BEs for Ni2+ in the range 67.3–68.5 eV (Ni 3p3/2) and 68.8–69.6 eV (Ni 3p1/2).[25,26] Finally, the Al 2p core level also exhibits two spin–orbit coupling levels with BEs in the range 73.6–75.0 eV (Al 2p3/2) and 74.3–76.3 eV (Al 2p1/2), reflecting the presence of Al3+ either in the oxide, oxyhydroxide, or hydroxide form (Figure b).[25,26]

Figure 3

XPS spectra of (a) Ni 2p core level and (b) Ni 3p and Al 2p core levels for Ni6AlO.

Table (columns 5 and 6) lists the bulk and surface Ni/Al molar ratios [that is, (Ni/Al)b and (Ni/Al)s] of the NiAlO catalysts measured by inductively coupled plasma optical emission spectrometry (ICP–OES) and XPS, respectively. In all cases, the (Ni/Al)b bulk ratios are slightly lower than the nominal ratios. Interestingly, the (Ni/Al)s surface ratios are higher than the (Ni/Al)b bulk ratios for Ni1AlO (3.8 vs 0.9), Ni2AlO (4.0 vs 1.6), and Ni4AlO (4.0 vs 3.0), whereas the opposite trend is observed for the highly Ni-loaded catalysts, that is, Ni8AlO (6.2 vs 7.2) and Ni10AlO (7.3 vs 8.9). In the case of Ni6AlO, both ratios show similar values (5.5 vs 5.1). In line with the XRD patterns, these results can be interpreted by a partial encapsulation of Ni and NiO nanoparticles by alumina for Ni6AlO, Ni8AlO, and Ni10AlO, whereas Ni and NiO nanoparticles are expected to be mostly exposed on the alumina surface for Ni1AlO, Ni2AlO, and Ni4AlO.

Table 1

Composition and Textural Properties of NiAlO Catalysts

	textural propertiesc			composition
catalyst	BET area (m2/g)	pore size (nm)	pore volume (cm3/g)	(Ni/Al)b (mol/mol)a	(Ni/Al)s (mol/mol)b
Ni1AlOx	273	3.8	0.51	0.9	3.8
Ni2AlOx	277	4.0	0.55	1.6	4.0
Ni4AlOx	203	4.0	0.41	3.0	4.0
Ni6AlOx	133	5.5	0.36	5.1	5.5
Ni8AlOx	115	6.2	0.36	7.2	6.2
Ni10AlOx	81	7.3	0.30	8.9	7.3

Measured by ICP–OES.

Measured by XPS.

Measured by N2 adsorption/desorption at −196 °C.

Table (columns 2–4) also lists the Brunauer–Emmett–Teller (BET) surface area, average pore size, and pore volume of the different catalysts measured by N2 adsorption/desorption at −196 °C. The BET surface area decreases gradually with the Ni content from 273 m2/g for Ni1AlO to 81 m2/g for Ni10AlO. Likewise, the pore volume declines with the Ni content from 0.51 cm3/g for Ni1AlO to 0.30 cm3/g for Ni10AlO. In parallel, the average pore size increases from 3.8 nm for Ni1AlO to 7.3 nm for Ni10AlO.

We conducted a first series of catalytic tests for the hydrogenation and RA of HMF over Ni6AlO (Table ). Ni6AlO can efficiently catalyze the hydrogenation of HMF into 2,5-furandimethanol (FMA) at 70 °C and 10 bar H2 with 92% yield (entry 1). At 100 °C, 2,5-tetrahydrofuran-dimethanol (THFMA) is obtained with 95% yield (entry 2). When NH3 is added to the reaction system at 1 bar H2, 5-aminomethyl-2-furylmethanol (FAA) is generated as main product with 98% yield at the expense of FMA and THFMA (entry 3). At higher H2 pressure (10 bar) and temperature (150 °C), 5-aminomethyl-2-tetrahydro-furylmethanol (THFAA) is obtained as main product with 95% yield (entry 4) at the expense of FAA. FAA is fast generated during the initial 3 h reaction and becomes gradually hydrogenated to THFAA during the first 10 h at full HMF conversion (Figure ).

Table 2

Hydrogenation and RA of HMF over Ni6Al1Oa

entry	NH3/HMF ratio (—)	H2 pressure (bar)	total pressure (bar)	T (°C)	time (h)	product	product yield (%)
1		10	10.5	70	6	FMA	92
2		10	12.5	100	6	THFMA	95
3	49:1	1	5.0	100	6	FAA	98
4	49:1	10	25.0	150	16	THFAA	95

Other reaction conditions: 1 mmol of HMF, 50 mg of cat, 1 bar H2, and 3 mL of H2O.

Figure 4

Product distribution during the RA of HMF over Ni6AlO. Reaction conditions: 1 mmol of HMF, 49:1 NH3/HMF, 3 mL of H2O, 40 mg of cat, 150 °C, and 10 bar H2.

With these results in hand, we explored the performance of the NiAlO catalysts toward FAA formation in the RA of HMF with NH3 at 100 °C for 6 h under 1 bar H2 (Table ). Ni1AlO exhibits low HMF conversion (65%) and FAA yield (32%) together with a poor mass balance (70%). FMA issued from HMF hydrogenation is generated as minor byproduct with only 2.3% yield. An increase of the Ni/Al molar ratio to 2 and 4 enhances moderately the HMF conversion, the FAA yield, and the mass balance. The best performance is achieved when the nominal Ni/Al molar ratio is set at either 6 or 8, resulting in full HMF conversion and 99% FAA yield together with 99% mass balance. FMA only appears as trace byproduct. A further increase of the Ni/Al molar ratio to 10 leads to a slight decline of the FMA yield (96%) at full HMF conversion with 96% mass balance. For comparison, the RA of HMF with NH3 was also studied over a NiO catalyst prereduced at 450 °C for 2 h under H2 flow and over Raney-Ni. In both cases, low FAA yield is obtained (33 and 48%, respectively) at high HMF conversion with a modest mass balance (45 and 53%, respectively). This body of results points out a positive effect of Al in NiAlO on the FAA selectivity, which is especially pronounced for Ni6AlO and Ni8AlO, encompassing partial encapsulation of metal Ni by alumina. In light of these results and to its lower Ni content, Ni6AlO was further used for optimizing the catalyst loading targeting FAA (Table ). In these tests, a catalyst loading in the range 40–50 mg afforded 99% FAA yield at full HMF conversion while keeping the other reaction conditions unchanged. This catalyst loading was further used for expanding the reaction scope.

Table 3

RA of HMF over NiAlO Catalystsa

		yield (%)
catalyst	HMF conversion (%)	FAA	FMA	mass balance
Ni1AlOx	65	32	2.3	70
Ni2AlOx	74	38	1.5	66
Ni4AlOx	93	85	0.6	92
Ni6AlOx	100	99	0.1	99
Ni8AlOx	100	99	0.1	99
Ni10AlOx	100	96	0.1	96
NiOx	91	33	3.2	45
Raney-Ni	100	48	5.6	53

Reaction conditions: 1 mmol of HMF, 49:1 NH3/HMF, 3 mL of H2O, 50 mg of cat, 100 °C, 1 bar H2, and 6 h.

Table 4

Influence of the Ni6AlO Loading on the RA of HMFa

catalyst loading (mg)	HMF conversion (%)	FAA yield (%)	mass balance (%)
10	100	2	2
20	79	26	47
30	88	82	94
40	100	99	99
50	100	99	99

Reaction conditions: 1 mmol of HMF, 49:1 NH3/HMF, 3 mL of H2O, 100 °C, 1 bar H2, and 6 h.

In a next step of our study, we assessed the catalytic performance of Ni6AlO in the RA of a series of aldehydes and ketones with NH3 (Table ). FF can be aminated to give FAM with excellent yield (90%) at 100 °C for 5 h under 4 bar H2 (entry 2). Likewise, aromatic aldehydes can be successfully aminated to the corresponding primary amines. Indeed, benzaldehydes substituted with electron-donating and electron-withdrawing groups (e.g., 4-anisaldehyde and 4-chloro-benzaldehyde) can be transformed into the target amines with >80% yield (entries 3 and 4). Ni6AlO also enables the RA of vanillin, which is an important flavor in sweet foods, into the corresponding primary amine by reaction at 100 °C for 10 h under 2 bar H2 with 82% yield (entry 5). Aliphatic aldehydes such as butyraldehyde can be converted into 1-butylamine at low temperature (80 °C) with 83% yield (entry 6). Unlike aldehydes, the RA of ketones is often challenging because of their lower reactivity. However, Ni6AlO can provide high amine yields for several important biomass-derived ketones such as cyclohexanone and isobutyl ketone (89 and 84%, respectively) (entries 7 and 8). Finally, glycolaldehyde (dimer), which can be produced by hydrolysis and retro-aldol condensation of cellulose, can be transformed into ethanolamine with 85% yield by reaction at 80 °C for 3 h under 5 bar H2 (entry 9).

Table 5

RA of Different Carbonyl Compounds over Ni6AlO

Reaction conditions: 1 mmol of the substrate, 49: 1 NH3/aldehyde, 50 mg of cat, and 3 mL of H2O.

Isolated yields in entries 1–5 and GC yields in entries 6–9.

0.5 mmol of the substrate.

We also explored the credentials of Ni6AlO for preparing HMF-derived amines (Table ). The reaction of HMF with primary and secondary amines can afford the corresponding secondary and tertiary amines in good yield by reaction at 100 °C for 6 h under 3 bar H2. Aromatic amines such as benzylamine and aniline can react with HMF to produce the corresponding secondary amines with 76 and 85% yield (entries 1 and 2), respectively. A secondary amine such as morpholine can also react with HMF to generate the tertiary amine with high yield (88%, entry 3). Finally, a yield >80% of secondary amines can be achieved with aliphatic amines such as 1-butylamine and ethanolamine (entries 4 and 5).

Table 6

RA of HMF with Primary and Secondary Amines over Ni6AlOa

Reaction conditions: 1 mmol of HMF, 1.2 mmol of the amine, 40 mg of cat, 100 °C, 3 bar H2, and 6 h.

100 °C.

Co-solvent (1 mL of H2O + 2 mL of ethanol) was used due to insolubility of fatty amines in H2O.

More importantly, we found that Ni6AlO could catalyze the RA of HMF with NH3 toward FAA in a gram-scale synthesis with high yield. As shown in Scheme , by stirring a mixture of 3 g of HMF and 30 mL of aqueous NH3 (28 et %) at 90 °C for 12 h under 10 bar H2 over Ni6AlO, FAA can be obtained with 81% isolated yield.

Scheme 1

Scale-Up Synthesis of FAA from the RA of HMF

Finally, we explored the catalyst recycling and reuse using the same operation conditions as in Table (entry 3). The catalyst can be separated and reused in four consecutive runs without apparent loss of activity (Figure ). Nonetheless, after the fifth and sixth runs, a drop in the HMF conversion is observed, which can be ascribed to a progressive catalyst loss during operation as the turnover number (TON) of the catalyst measured with respect to the bulk Ni keeps almost unchanged at a value of 2.3 mmol FAA/mmol Ni. Noticeably, the catalytic performance can be recovered if the lost catalyst, that is, 20 mg, is compensated by the addition of fresh Ni6AlO. The catalyst keeps its integrity during operation, as can be deduced by the comparison of the XRD patterns and XPS spectra of the Ni 2p, Ni 3p, and Al 2p core levels before and after the first and fifth runs (Figures S6–S8). However, after the fifth run, characteristic reflections ascribed to γ-alumina can be distinguished at 25°, 34°, 37°, 54°, 65°, and 67°, which can be assigned to AL(111), AL(220), AL(311), AL(400), AL(511), and AL (440) (γ-Al2O3, JCPDS no: 10-0425, Fd3m cubic unit cell), respectively. This observation suggests a partial recrystallization of the initially amorphous alumina phase during the reaction. The (Ni/Al)b bulk molar ratio of the fresh and spent catalyst after the fifth run as analyzed by ICP–OES is 5.2 and 4.9, respectively, suggesting a slight leaching of nickel during the reaction.

Figure 5

Recycling test of Ni6AlO in the RA of HMF. Reaction conditions as in Table (entry 3). The experiment at run 7 was conducted by adding 20 mg of the catalyst to compensate the catalyst loss along runs 1–6.

Conclusions

Along this study, we have demonstrated that NiAlO formulations can behave as efficient and versatile catalysts for the RA of a broad series of biomass-derived aldehydes and ketones in aqueous ammonia at mild reaction conditions. By tuning the Ni/Al molar ratio, Ni6AlO was found to be an optimal formulation. This catalyst offered high selectivity and yield to primary amines by reacting carbonyl compounds with aqueous ammonia. In particular, FF and HMF could react with a variety of primary and secondary amines to afford highly substituted amines. The catalyst could be easily recycled and reused without apparent loss of activity in four consecutive runs.

Experimental Section

Chemicals

Ni(NO3)2·6H2O, Al(NO3)3·9H2O, and Na2CO3, all supplied by Sinopharm Chemical Reagent Co., Ltd. were used for the preparation of the NiAlO formulations. A Ni–Al alloy (50:50 w/w) supplied by J&K Chemical Co., Ltd. was used for preparing RaneyNi. FF, 4-anisaldehyde, 4-chlorobenzaldehyde, vanillin, butyraldehyde, 1-butylamine, 2-butylamine, cyclohexanone, cyclohexylamine, butanone, ethanolamine, aniline, benzylamine, and morpholine were also supplied by Sinopharm Chemical Reagent Co., Ltd. HMF, FMA, THFMA, FAA, and THFAA were procured from Shanghai Shaoyuan Co., Ltd. Glycolaldehyde was purchased from Ark Pharm, Inc. Dioxane, ethanol and aqueous ammonia solution (28 wt %) were purchased from Damao Chemical Reagent Factory. Hydrogen was obtained commercially from Lanzhou Yulong Gas Co., LTD. All the chemicals were used as received without further purification.

Catalyst Preparation

The NiAlO formulations were prepared as follows. In a typical preparation, Ni(NO3)2·6H2O (4.66 g, 16 mmol) and Al(NO3)3·9H2O (1 g, 2.67 mmol) were added to deionized water (30 mL) at room temperature and stirred until complete dissolution. Then, aqueous Na2CO3 (20 mL, 1.25 M) was added dropwise and the mixture was stirred for 5 h, followed by centrifugation and washing with deionized water until neutral pH was reached. Finally, the solid was dried at 100 °C in air for 5 h, calcined at 450 °C for 4 h, and reduced at 450 °C for 2 h under 10 cm3(STP)/min H2 flow using a 10 °C/min heating ramp. The Ni/Al ratio in the formulations was tuned by adjusting the Ni/Al molar ratio in the system. A NiO catalyst was also prepared using the same protocol, but without addition of the Al(NO3)3·9H2O precursor.

RaneyNi was prepared according to a previous study.[22] Briefly, 10.0 g of Ni–Al alloy (50:50 w/w) was slowly added to an aqueous solution of NaOH (6.0 M, 50 mL) under stirring over ice water and a controlled temperature in the range 10–20 °C for 1 h, followed by 30 min at room temperature and heating to 90 °C under stirring for 2 h until no H2 bubbles were observed. Then, the mixture was cooled down to room temperature and the final black precipitate was washed with distilled water until pH 7 and kept in water or ethanol.

Catalyst Characterization

The bulk metal composition of the catalysts was quantified by ICP analysis using an Activa (HORIBA Jobin-Yvon) Optical Emission Spectrometer. Before the measurements, the samples were dissolved using a mixture of inorganic acids (H2SO4, HNO3, and HF).

TEM was carried out using a Tecnai G2 F30 S-Twin Field microscope operating at 300 kV equipped with energy-dispersive X-ray spectroscopy analysis in the scanning TEM mode. For TEM inspection, the prereduced catalysts were suspended in ethanol by ultrasonication and deposited on carbon-coated copper grids.

The XRD patterns were measured using a STADIP-automated transmission diffractometer (STOE) equipped with an incident-beam curved Ge monochromator with Cu Kα1 radiation and operated at 40 kV and 150 mA.

XPS was used for measuring the surface composition of the catalysts on a Kratos Axis Ultra DLD apparatus equipped with a hemispherical analyzer and a delay line detector. The spectra were recorded using an Al monochromated X-ray source (10 kV, 15 mA) with a pass energy of 40 eV (0.1 eV/step) and a pass energy of 160 eV in the hybrid mode. The adventitious C 1s binding energy (285.0 eV) was used as an internal reference.

The specific surface area of the catalysts was measured by N2 adsorption/desorption at −196 °C on a Quantachrome IQ2 instrument. The pore size distribution was calculated from the desorption isotherms using the Barrett–Joyner–Halenda method. Prior to the measurements, the samples were degassed at 300 °C for 10 h to remove adsorbed moisture and vapors.

Catalytic Tests

The catalysts were tested in the liquid-phase amination reaction of HMF with NH3. The catalytic tests were conducted in an 80 mL stainless steel autoclave equipped with a pressure gauge and a safety valve. In a typical experiment, the reactor was charged with 1 mmol of HMF, 3 mL of NH3·H2O (28 wt %) solution and 50 mg of the prereduced catalyst. The reactor was sealed and evacuated by applying vacuum followed by H2 charging. The reactor was then placed on a hot plate equipped with a magnetic stirrer at the desired temperature, the pressure was equilibrated, and the reaction was conducted at variable times.

The reactant (HMF) and the N-products were analyzed and quantified using an Agilent 7890A GC equipped with a HP-5 capillary column with 5 wt % phenyl groups and an FID detector, using dioxane as the internal standard. Besides, the isolated yields were measured by flash column chromatography. The 1H NMR spectra were measured using a Bruker ARX 400 or ARX 100 spectrometer at 400 MHz (1H) and 100 MHz (13C). All the spectra were recorded in CD3OD.

The HMF conversion, the selectivity and yield to N-products, the mass balance, and the TON (FAA) were defined as followswhere nHMF0 and nHMF refer to the initial and final HMF mole number, respectively, n indicates the mole number of the N-products, and nNi (bulk) refers to the mole number of bulk Ni in the reactor.