One-pot, in situ reductive catalytic approach for biomass furfural-based secondary amines under ambient conditions

Abstract

Biomass-based secondary amines are the versatile structural scaffolds of various drug molecules. This study reports a highly practical catalytic process for the room-temperature synthesis of renewable secondary amines from furfural (an inexpensive and readily available hemicellulose biomass-derived platform chemical) via one-pot, in situ reductive amination using an efficiently recyclable nanoalloy PdNi/Nb₂O₅ catalyst. The in situ hydrogenation of the imine intermediate obtained from the C–N coupling of furfural with benzylamine was carried out using a hydrogen donor, i.e., a mixture of readily available triethylsilane and methanol, which provides a valuable silyl ether product during the reaction as well. The synergy of PdNi nanoalloys and their strong interaction with shape-controlled Nb₂O₅ nanorods facilitated optimum amounts of active metal and redox sites, as elucidated by CO-chemisorption and H₂-TPR studies, respectively. Controlled reactions confirmed that metallic Pd facilitated in situ hydrogen generation from the hydrogen donor and the subsequent hydrogenation of the imine intermediate. The PdNi/Nb₂O₅ nanocatalyst (2 wt% Pd and 3 wt% Ni) gave a 97% yield of N-benzyl-1-(furan-2-yl)methanamine at room temperature. In contrast, only a 39% product yield was obtained over the Pd/Nb₂O₅ catalyst, whereas the Ni/Nb₂O₅ catalyst did not show any activity, confirming the significant role of the PdNi nanoalloy. The widespread substrate scope, gram-scale synthesis of secondary amines, robust structural stability and excellent reusability even after 10 cycles of the PdNi/Nb₂O₅ nanocatalyst confirmed the practical applicability of the developed catalytic methodology for producing furfural-based drug scaffolds under ambient conditions.

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Green foundation

1. This study developed a synergistic PdNi nanoalloy catalyst supported on rod-shaped Nb₂O₅ for the reductive amination of hemicellulose-derived furfural with benzylamine towards pharmaceutically benign secondary amines. This reductive amination proceeds through one-pot C–N coupling/in situ hydrogenation using readily available hydrogen donors under ambient conditions, providing promising solutions for sustainable bioprocessing.

2. The PdNi/Nb₂O₅ nanomaterial is significantly cost-effective and catalytically versatile, as the alloying of noble metal (Pd) with a base metal (Ni) reduces the cost of the catalyst, as well as providing substantial advancement and control over the hydrogenation sites and reactivity by tuning the geometrical/electronic structure of the nanoalloy for various applications.

3. The gram-scale synthesis, excellent catalyst stability, and remarkable catalyst reusability (10 cycles) emphasize the practical viability of the catalytic process for biomass valorization. Using a nanoalloy catalyst, consisting of non-noble metals encapsulated in a porous support (SiO₂ or Al₂O₃), with desirable catalytic properties, can provide a more sustainable and greener catalytic process for biorefinery.

1. Introduction

Valorization of renewable biomass into fuels and chemicals provides a sustainable pathway to achieve carbon neutrality while reducing our dependence on depleting fossil resources.^1–7 Although several biomass substrates are readily available, enormous attention has been given to lignocellulosic biomass (cellulose, hemicellulose, and lignin) because of its non-edible nature, huge availability as waste from agriculture and forest residues, and diverse functionality for producing numerous platform molecules, biofuels, and value-added chemicals.^8–10 Furan-2-carbaldehyde (furfural) is one of the essential platform chemicals, commercially produced (>200 kT per year) from hemicellulose, with a market value of only 1–1.2 euros per kilogram.^11–15 Thus, the practical conversion of furfural into chemicals and drug motifs can boost the lignocellulosic biorefinery industry.

Heterogeneous catalysis can provide sustainable methods for converting biomass-derived platform molecules, including furfural, because of the easy synthesis, robust chemical/thermal stability, and efficient reusability of heterogeneous solid catalysts compared to homogeneous liquid catalysts.^3,16–19 Several heterogeneous catalytic processes, such as hydrogenation, hydrogenolysis, hydrodeoxygenation, decarbonylation, and reductive amination, have been reported for furfural conversion.^20–25 Nitrogenated furfural derivatives can find significant applications in the pharma industry, as more than 80% of the top 200 drugs contain nitrogen-containing motifs.^26–30 The increased demand for producing bioactive compounds with amine moieties emphasizes the importance of furfural-based amines. Reductive amination using ammonia is widely studied for converting furfural into furfurylamine.^25,28,31,32 In contrast, furfural-based secondary amines are scarcely reported, but they are more valuable building blocks for several drugs (Fig. 1).^23,33–41

Fig. 1 The industrial importance of furfural-based secondary amines as pharmaceuticals.

The synthesis of furfural-based secondary amines involves one-pot reductive amination, i.e., C–N coupling/hydrogenation strategies using various primary amines.^{33–35,42–44} The C–N coupling between furfural and a primary aromatic amine gives an imine intermediate, which needs to be hydrogenated to obtain a secondary amine. The catalyst plays a pivotal role in achieving elevated reaction rates and optimum yields of the products in C–N coupling/hydrogenation of furfural with primary amines. Usually, molecular hydrogen gas is used for hydrogenation reactions. However, high-pressure/temperature conditions are required to facilitate dissociative hydrogen adsorption on the catalyst surface. For example, the C–N coupling/hydrogenation of furfural with aniline was reported by Brijaldo et al.³⁴ and Martínez et al.³⁵ over Pt/Nb₂O₅–Al₂O₃ (Scheme 1, Fig. 2) and Pt/SiO₂–SO₃H (Scheme 2, Fig. 2) catalysts, respectively. The Pt/Nb₂O₅–Al₂O₃ catalyst gave only 47% conversion with 99% selectivity of N-(furan-2-ylmethyl)aniline at 50 bar of H₂ pressure and 90 °C, whereas the Pt/SiO₂–SO₃H catalyst showed 71% conversion with only 21% selectivity of N-(furan-2-ylmethyl)aniline although the reaction was carried out at 50 bar of H₂ pressure and room temperature for 8 h. García-Ortiz and coworkers studied the C–N coupling/hydrogenation of furfural with aniline over the Pd/C (50 mg) catalyst (Scheme 3, Fig. 2).⁴⁴ The catalyst gave 99% conversion of furfural and 99% product selectivity at 100 °C for 4 h using 3 bar H₂ pressure and trifluorotoluene (TFT) solvent. Similarly, a Pd/CNT (50 mg) catalyst was explored for C–N coupling/hydrogenation using formic acid as a hydrogen donor (Scheme 4, Fig. 2).⁴³ The catalyst gave 99% furfural conversion with 99% product selectivity, but the reaction was performed at elevated temperature (120 °C). In these studies, not only the use of higher H₂ pressures and reaction temperatures but also the use of noble metal catalysts are key concerns, adversely affecting the sustainability of the catalytic processes for producing furfural-based secondary amines.

Fig. 2 Literature comparison for the synthesis of furfural-based amines over various heterogeneous catalysts. ^a Conversion (%) and ^b selectivity (%).

Liquid hydrogen carriers, such as methanol, ethanol, isopropanol, formic acid, and organosilanes, are alternative sources of hydrogen that facilitate in situ hydrogenation under mild reaction conditions.^28,45 The key to the efficient synthesis of furfural-based secondary amines lies in multifunctional heterogeneous catalysts with an optimum number of active sites to accelerate the C–N coupling/in situ hydrogenation (one-pot, in situ reductive amination) of furfural with primary amines. Supported nanoalloy catalysts having two different metals with optimum metal ratios and uniform dispersion have received significant interest in heterogeneous catalysis because of their synergetic metal–metal and nanoalloy–support properties.^3,46–51 The supported nanoalloy catalysts can provide several advantages, such as enhanced lattice strain, tunable composition, larger surface area, and selective redox-active metal sites to achieve the optimum yield of furfural-based secondary amines. In addition, alloying an Earth-abundant base metal (viz., Ni) with a noble metal (viz., Pd) by optimizing the metal amount and ensuring uniform distribution not only reduces the cost of the catalysts but also provides significant control over the selectivity of furfural-based secondary amines.

The homogeneous anchoring of nanoalloy particles throughout the surface of shape-controlled metal oxides leads to the development of multifunctional catalysts with tunable active sites.^52,53 Shape-controlled metal oxides with nanosized particles offer abundant surface-exposed crystal facets and more truncated surfaces (corners, edges, kinks, and steps).^54–56 Specifically, metal oxides having an acidic nature are suitable supports for developing bifunctional nanoalloy/metal oxide catalysts. In this line, nanorod-shaped Nb₂O₅ material could be an outstanding support for nanoalloy catalysts as it possesses a distinctive morphology, a high surface area, and excellent acid properties.^57–60 Besides, unsaturated surface Nb^x+ sites and oxygen vacancies in Nb₂O₅ are additional advantages, which not only stabilize the nanoalloy species but also act as active sites in heterogeneous catalysis. Our recent work elucidated the role of triethylsilane and methanol as hydrogen donors for furfural hydrogenation in the presence of a Pd catalyst at room temperature.⁶¹ This strategy can be extended to the one-pot, in situ reductive amination of furfural with amines using efficient heterogeneous catalysts.

Herein, we report the development of an efficient and multifunctional nanoalloy PdNi/Nb₂O₅ catalyst, which showed excellent activity for the synthesis of a furfural-based secondary amine, i.e., N-benzyl-1-(furan-2-yl)methanamine, with 97% yield via one-pot, in situ reductive amination of furfural with benzylamine under ambient conditions (Scheme 5, Fig. 2). The detailed characterization of the catalysts in conjunction with controlled experiments revealed that the synergy of PdNi nanoalloy particles on Nb₂O₅ nanorods plays a key role in achieving optimum yields of furfural-based secondary amines at ambient temperature and without the requirement of elevated-pressure conditions. The nanoalloy PdNi/Nb₂O₅ catalyst is highly stable and can be reused ten times effectively without any variation in the yield of N-benzyl-1-(furan-2-yl)methanamine.

2. Experimental section

2.1 Materials

All the reagents and chemicals were used as received. Ammonium niobate oxalate hydrate (Sigma Aldrich, 99% purity), H₂O₂ (SRL Chemicals, extra pure), sodium hydroxide pellets (SRL Chemicals, 97%), palladium acetate (Sigma Aldrich, 99.98%, trace metal basis), nickel acetate dihydrate (Sigma Aldrich, 98%), ethylene glycol (SRL Chemicals, extra pure AR, 99%), hydrazine hydrate (Sigma Aldrich, reagent grade, 50–60%), polyvinylpyrrolidone (average mol. wt 40 000, Sigma Aldrich), furfural (SRL Chemicals, extra pure AR, 99%), benzylamine (SRL Chemicals, extra pure AR, 99%), aniline (SRL Chemicals, 98%), triethylsilane (BLDpharma, 99.98%), methanol (SRL Chemicals, HPLC grade), and dodecane (TCI Chemicals, >99%, GC grade) were used. All the furfural and benzylamine derivatives were purchased from BLDpharma and used as received.

2.2 Catalyst synthesis

2.2.1 Preparation of Nb₂O₅ nanorods. A hydrothermal method was used to synthesize Nb₂O₅ nanorods by following the procedure described in previous reports.^62,63 The appropriate amount of Nb precursor (ammonium niobate oxalate hydrate, Sigma Aldrich, 99% purity) was dissolved in 60 mL of DI water. Consequently, 30 w/v% H₂O₂ (15 mL) solution was added dropwise at room temperature for 10 min, and the resultant mixture was stirred further for up to 30 min. Then, the mixture was scrupulously transferred to a 100 mL PTFE-lined hydrothermal reactor and treated at 175 °C for 15 h. The hydrothermal autoclave was removed from the oven and cooled to room temperature. The solid precipitate was carefully separated by centrifugation and washed with DI water multiple times until the pH became neutral. The solid sample was dried overnight in static air at 80 °C, followed by calcination at 400 °C for 4 h.

2.2.2 Preparation of a PdNi/Nb₂O₅ nanocatalyst. A PdNi/Nb₂O₅ catalyst (2 wt% Pd and 3 wt% Ni w.r.t. Nb₂O₅) was prepared by a solvothermal method. Appropriate amounts of Pd precursor (palladium acetate, Sigma Aldrich, 99.98% purity) and Ni precursor (nickel acetate dihydrate, Sigma Aldrich, 99% purity) were added to 50 mL of ethylene glycol (SRL Chemicals, 99% purity) and stirred for 10 min to completely dissolve the metal precursors. The required amount of hydrazine hydrate solution (N₂H₄·H₂O, Sigma Aldrich, 80% in H₂O) was added dropwise for 10 min, followed by the addition of 1.5 mol% of polyvinylpyrrolidone (PVP-40, Sigma Aldrich, 99% purity) and continuous stirring for another 10 min. Then, the Nb₂O₅ sample was added and stirred for 1 h at room temperature. It was transferred to a PPL-lined autoclave reactor for hydrothermal treatment at 160 °C for 12 h in an oven. After cooling the autoclave to room temperature, the solid sample was collected by centrifugation and washed with a mixture of DI water and methanol until the pH reached neutral. The PdNi/Nb₂O₅ material was oven-dried at 80 °C for 12 h and then calcined at 400 °C for 4 h. The schematic illustration of the preparation of both Nb₂O₅ nanorods and PdNi/Nb₂O₅ catalysts is given in Fig. 3.

Fig. 3 Synthesis of Nb₂O₅ nanorods and the nanoalloy PdNi/Nb₂O₅ catalyst (NR = nanorod).

Details of the characterization and catalytic activity of Nb₂O₅ nanorods and nanoalloy PdNi/Nb₂O₅ catalysts are provided in the ESI.†

3. Results and discussion

3.1 Catalyst characterization studies

3.1.1 TEM and powder XRD studies. To elucidate the morphology of Nb₂O₅, particle size, and dispersion of Pd and Ni in the catalysts, TEM and STEM-EDAX analyses were carried out (Fig. 4).^62,63 The Nb₂O₅ material has a rod morphology with a mean width of 5.7 ± 1 nm and a length range of 24–65 nm (Fig. 4a). The lattice fringes of the Nb₂O₅ nanorods are clearly visible in the TEM image, with a d-spacing of 0.39 nm, corresponding to the (001) plane of pseudohexagonal Nb₂O₅. The TEM image of commercial Nb₂O₅ shows larger particles (115–650 nm) with irregular shapes (Fig. S1, ESI†). The TEM images of the PdNi/Nb₂O₅ catalyst show both Nb₂O₅ nanorods and PdNi particles with a mean size of 4.5 ± 1 nm (Fig. 4b and Fig. S2, ESI†). The STEM-EDAX images reveal that both Pd and Ni species are homogeneously dispersed throughout the surfaces of the Nb₂O₅ nanorods without any segregation, indicating the formation of the PdNi alloy particles (Fig. 4d–h).⁶⁴ The powder XRD diffraction patterns of Nb₂O₅, Ni/Nb₂O₅, Pd/Nb₂O₅, and PdNi/Nb₂O₅ are presented in Fig. S3, ESI.† All the materials show similar XRD peaks with the diffraction angles (2θ) of 22.8, 28.6, 36.4, 46.4, and 55.7°, corresponding to the (001), (100), (101), (002), and (111) crystal planes of the pseudohexagonal phase of Nb₂O₅ (JCPDS 28-0317).^62,65,66 Besides, the Ni/Nb₂O₅ nanomaterial shows diffraction peaks at 2θ = 44.4, 51.8, and 76.2°, which correspond to the (111), (200), and (220) crystal planes of metallic Ni crystallized in the face-centered cubic (FCC) structure (PDF# 04-0850).⁶⁷ The Pd/Nb₂O₅ nanomaterial shows two XRD peaks at 33.9° and 40.1°, attributed to the (101) and (111) crystal planes having the FCC crystal structure of the metallic Pd.^68–70 There are no diffraction patterns with respect to the PdO. The PdNi/Nb₂O₅ nanomaterial exhibits all the aforementioned diffraction patterns with slight shifting due to the formation of nanoalloyed PdNi particles.^71–74

Fig. 4 (a) TEM images of Nb₂O₅ nanorods and (b) the PdNi/Nb₂O₅ nanoalloy catalyst. (c–h) STEM-EDAX maps of the PdNi/Nb₂O₅ nanoalloy catalyst.

3.1.2 XPS analysis. XPS analysis of the Nb₂O₅, Ni/Nb₂O₅, Pd/Nb₂O₅, and PdNi/Nb₂O₅ nanocatalysts elucidated the oxidation states of all the elements as well as the metal–support and metal–metal interactions, which are emphasized in terms of changes in the binding energies (BEs) of Nb, O, Ni, and Pd species. The Nb 3d XP spectra show two peaks at 211.0–208.3 eV (Nb 3d_3/2) and 208.4–205.4 eV (Nb 3d_5/2), which belong to Nb⁵⁺ (Fig. 5a).^62,65,75 Two distinct species of oxygen are evidenced in all the catalysts (Fig. 5b).^76,77 The peaks at 533.9–528.9 and 531.5–528.4 eV correspond to the hydroxyl species (which interacted with the oxygen vacancies, O_v) and the lattice oxygen (O_L), respectively.^63,78–80 The addition of Ni and Pd led to decreased BEs of O and Nb species, especially in the PdNi/Nb₂O₅ catalyst, indicating the strong nanoalloy–support interaction. The Ni 2p XP spectra of Ni/Nb₂O₅ and PdNi/Nb₂O₅ reveal the presence of various Ni species (Fig. 5c). Literature reports reveal that the Ni 2p XPS peaks observed at 855.8–857.5 and 854.2–855.6 eV belong to Ni³⁺ and Ni²⁺ species, respectively.^81–83 The presence of Ni³⁺ species in the Ni/Nb₂O₅ catalyst is attributed to the formation of Ni–Nb–O species on the catalyst surface.⁸¹ The PdNi/Nb₂O₅ catalyst exhibited Ni⁰ and Ni²⁺ species.^84–86 The absence of Ni³⁺ in the PdNi/Nb₂O₅ catalyst indicates the strong Pd–Ni interaction, which inhibited the formation of surface Ni–Nb–O species. There are also two satellite peaks at 878.5 eV and 860.9 eV, confirming the presence of Ni²⁺ species.⁸⁷ The Pd 3d XP spectra showed two peaks at 336.2 eV and 341.5 eV for Pd/Nb₂O₅ and 335.9 eV and 341.2 eV for PdNi/Nb₂O₅ (Fig. 5d). These values, along with a BE difference of around 5.3 eV, indicate the presence of Pd⁰ species in both the catalysts.^88–91 The lower binding energy of Pd in the PdNi/Nb₂O₅ catalyst compared to the Pd/Nb₂O₅ catalyst is due to the charge transfer from Ni to Pd through strong metal–metal interaction (Fig. 5d).⁷⁴

Fig. 5 XPS profiles of (a) Nb 3d, (b) O 1s, (c) Ni 2p, and (d) Pd 3d of the catalysts.

3.1.3 N₂ adsorption–desorption and Raman spectroscopy analyses. The N₂ adsorption–desorption analysis determined the surface area, pore size, and pore volume of Nb₂O₅, Ni/Nb₂O₅, Pd/Nb₂O₅, and PdNi/Nb₂O₅ catalysts (Table S1, ESI†). All the nanomaterials exhibited a type IV isotherm and an H3-hysteresis loop, which belong to the narrow slit-shaped pores in the Nb₂O₅ aggregates of the nanorod assembly (Fig. S4a, ESI†). The pores in these nanomaterials represent the gaps between the particles.^57,76 The BET surface area of the Nb₂O₅ nanorods is found to be 175 m² g⁻¹ with a 1.28 nm pore diameter and a 0.073 cc g⁻¹ pore volume (entry 1, Table S1, ESI†). The BET surface area decreased slightly after the individual dispersion of Ni (159 m² g⁻¹, entry 2, Table S1, ESI†) and Pd (148 m² g⁻¹, entry 3, Table S1, ESI†) as they blocked the gaps between the particles. The decrease in the BET surface area was also insignificant when both Pd and Ni were dispersed on the Nb₂O₅ nanorods (159 m² g⁻¹) with a 1.18 nm pore diameter and a 0.05 cc g⁻¹ pore volume (entry 4, Table S1, ESI†). This might be due to the alloy formation and the strong interaction between the PdNi nanoalloys and Nb₂O₅ nanorods. Pure Nb₂O₅ nanorods show three Raman peaks at 231, 704, and 960 cm⁻¹ (Fig. S4b, ESI†). The band at 231 cm⁻¹ belongs to the bending vibrational mode of Nb–O–Nb, and the narrow peak at 704 cm⁻¹ is attributed to the subtly distorted NbO₆ octahedral structure of Nb₂O₅.⁹² The feeble Raman band at 960 cm⁻¹ indicates the presence of edge-shared NbO₆ octahedral structures.⁷⁵ The monometallic Pd/Nb₂O₅ and Ni/Nb₂O₅ catalysts do not exhibit any characteristic Raman bands. This is because these catalysts contain metallic Ni and Pd species with a single-atom primitive unit cell and thus do not show any change in polarizability.^93,94 However, the peak at 863 cm⁻¹ in the case of PdNi/Nb₂O₅ is attributed to the combination of two-phonon transverse optical and two-phonon longitudinal optical (2P_TO+LO) NiO vibrational modes. This indicates the charge transfer from Ni to Pd in the PdNi/Nb₂O₅ catalyst, which aligns with the XPS analysis (Fig. 5c and d). The amounts of Pd and Ni in all the catalysts were quantitatively determined via ICP-OES analysis (Table S1, ESI†).

3.1.4 H₂-TPR and CO-chemisorption analyses. The H₂-TPR and CO-chemisorption analyses were conducted to estimate the nature and strength of the redox and active metal sites, respectively (Fig. 6a–d). As shown in Fig. 6a, Nb₂O₅ nanorods exhibited two broad reduction peaks in the ranges of 327–637 and 640–780 °C, attributed to the reduction of surface and bulk Nb⁵⁺ to Nb⁴⁺, respectively.⁵⁷ The addition of Pd (2 wt%) to Nb₂O₅ lowered the reduction temperature range of Nb₂O₅ compared to the pure Nb₂O₅ nanorods, indicating strong Pd–Nb₂O₅ interaction.^95,96 The absence of a reduction peak attributed to PdO in the Pd/Nb₂O₅ catalyst confirms the presence of metallic Pd, which is in line with the XPS studies (Fig. 5d).⁹⁷ The addition of Ni (5 wt%) to Nb₂O₅ nanorods resulted in three reduction peaks upon deconvolution (Fig. 6a).^98–100 The peak at 330 °C corresponds to the reduction of Ni²⁺ to Ni⁺, whereas the remaining two peaks at 360 °C and 391 °C correspond to the reduction of Ni⁺ to Ni⁰ and the strong interaction of NiO_x species with the support, respectively.⁹⁹ The alloying of Pd and Ni on Nb₂O₅ nanorods resulted in three distinct reduction peaks. The reduction profiles at 310 °C and 384 °C correspond to the reduction of Ni²⁺ to Ni⁺ and Ni⁺ to Ni⁰, respectively. The absence of the NiO_x reduction peak in the nanoalloy PdNi/Nb₂O₅ catalyst indicates the dominant Pd–Ni interaction compared to the Ni–Nb₂O₅ interaction. As shown in Fig. 6c, the larger amount of H₂ uptake of the Ni/Nb₂O₅ catalyst (695 μmol g⁻¹) than the PdNi/Nb₂O₅ catalyst (623 μmol g⁻¹) is due to the higher Ni (5 wt%) loading in the Ni/Nb₂O₅ catalyst than in the bimetallic catalyst (3 wt% Ni). It should be noted that the amount of hydrogen uptake by the nanoalloyed PdNi particles is solely determined by the Ni^2+/1+ species, as Pd is entirely present in the metallic phase, as evidenced by the XRD (Fig. S3, ESI†) and XPS (Fig. 5d) studies. In addition, the peak at 539.3 °C in PdNi/Nb₂O₅ corresponds to the reduction of Nb⁵⁺ to Nb⁴⁺. The higher H₂ uptake of Nb₂O₅ in PdNi/Nb₂O₅ than in Pd/Nb₂O₅ clearly indicates the enhanced reducibility at lower temperatures as a result of alloyed PdNi having a strong interaction with the Nb₂O₅ support through the hydrogen spillover effect.¹⁰¹ The CO-chemisorption profiles of the catalyst and their active site concentrations are shown in Fig. 6b and d. About 1155 μmol g⁻¹ of CO uptake was achieved over the 5 wt% Ni/Nb₂O₅ catalyst (Fig. 6d). The 2 wt% Pd/Nb₂O₅ catalyst exhibited 1179 μmol g⁻¹ of CO uptake, which is higher than that of 5 wt% Ni/Nb₂O₅, indicating the significance of Pd over Ni (Fig. 6d). The nanoalloying of Pd and Ni on the Nb₂O₅ nanorods’ surfaces increased the number of active sites significantly compared with that of the monometallic catalysts, as evidenced by the increased CO uptake (1372 μmol g⁻¹) of the PdNi/Nb₂O₅ catalyst (Fig. 6d).¹⁰²

Fig. 6 (a) H₂-TPR profiles, (b) CO-chemisorption profiles, (c) number of redox sites (H₂-TPR), and (d) number of active sites (CO-chemisorption) of the catalysts.

4. Catalytic activity studies for one-pot, in situ reductive amination of furfural

4.1 Screening of the catalysts’ activity and reaction conditions optimization

A model reaction using furfural and benzylamine was conducted at room temperature to develop an efficient and practical catalytic process for synthesizing furfural-based secondary amines. The in situ hydrogenation of the intermediate imine, N-benzyl-1-(furan-2-yl)methanimine, was achieved using hydrogen donors (a mixture of methanol and triethylsilane). The product was not formed in the absence of a catalyst (blank; entry 1, Table 1) and in the presence of commercial Nb₂O₅ (entry 2, Table 1) and Nb₂O₅ nanorods (entry 3, Table 1). Although Ni is well known for the hydrogenation reactions, no furfural conversion and product formation were observed over a 5%Ni/Nb₂O₅ catalyst (entry 4, Table 1), as it is not capable of hydrogen dissociation at room temperature. The 2%Pd/Nb₂O₅ catalyst gave a 38% yield of N-benzyl-1-(furan-2-yl)methanamine with 99% selectivity (entry 5, Table 1). The 1%Pd4%Ni/Nb₂O₅ catalyst gave a 58% yield of N-benzyl-1-(furan-2-yl)methanamine with 99% selectivity (entry 6, Table 1). On the other hand, the 2%Pd3%Ni/Nb₂O₅ catalyst gave a 97% yield of N-benzyl-1-(furan-2-yl)methanamine with 99% selectivity (entry 7, Table 1), indicating the cooperative effect of PdNi nanoalloys for the one-pot, in situ reductive amination of furfural under ambient conditions. The 2%Pd3%Ni/Nb₂O₅ nanocatalyst didn't show any activity in the presence of molecular hydrogen (entry 8, Table 1). This is due to the lack of sufficient pressure to dissolve the hydrogen gas in the reaction mixture at ambient temperature for the hydrogenation of the imine intermediate. The PdNi/commercial Nb₂O₅ catalyst, prepared by the solvothermal method, gave a 78% product yield (entry 9, Table 1). The TEM image of the commercial Nb₂O₅ shows irregular-shaped particles (Fig. S1, ESI†). Thus, this work emphasizes the significance of the Nb₂O₅ morphology (i.e., nanorods) in achieving higher activity (entry 7, Table 1). When EtOH (5 mL) was used in combination with TES, the PdNi/Nb₂O₅ catalyst gave a 71% product yield (entry 10, Table 1), whereas the use of i-PrOH (5 mL) resulted in a 53% product yield (entry 11, Table 1). The lower activity of the PdNi/Nb₂O₅ catalyst in the presence of EtOH and i-PrOH is due to their inefficiency in hydrogen release during the reaction. Hence, the combination of MeOH with TES is most efficient in releasing hydrogen in the presence of the PdNi/Nb₂O₅ catalyst for the reductive amination of furfural with benzylamine. The main product, N-benzyl-1-(furan-2-yl)methanamine, was confirmed by ¹H, ¹³C, DEPT-135, and HR-MS analyses (Fig. S5a–d, ESI†). The byproduct, triethyl(methoxy)silane, obtained from methanol and triethylsilane, was confirmed by the HR-MS analysis (Fig. S6, ESI†).

Table 1 One-pot, in situ reductive amination of hemicellulose-derived furfural with benzylamine using triethylsilane as a green hydrogen donor^a

S. no.	Catalyst	Furfural conversion (%)	Yield (%)	Selectivity (%)
				N-Benzyl-1-(furan-2-yl)methanamine	N-Benzyl-1-(tetrahydrofuran-2-yl)methanamine
a Reaction conditions: furfural = 1 mmol, TES = 2 mmol, MeOH = 5 mL, catalyst = 20 mg, room temperature, and 6 h. b Commercial Nb2O5. c H2 gas. d TES and EtOH. e TES and i-PrOH. GC error = ±2%.
1	Blank	—	—	—	—
2^b	Nb2O5	—	—	—	—
3	Nb2O5 NR	—	—	—	—
4	5%Ni/Nb2O5	—	—	—	—
5	2%Pd/Nb2O5	39	38	99	1
6	1%Pd4%Ni/Nb2O5	59	58	99	1
7	2%Pd3%Ni/Nb2O5	98	97	99	1
8^c	2%Pd3%Ni/Nb2O5	—	—	—	—
9^b	2%Pd3%Ni/Nb2O5	79	78	99	1
10^d	2%Pd3%Ni/Nb2O5	72	71	99	1
11^e	2%Pd3%Ni/Nb2O5	54	53	99	1

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Using the best catalyst, 2%Pd3%Ni/Nb₂O₅, the effects of the reaction temperature, catalyst amount, and amounts of TES and MeOH were studied for the one-pot, in situ reductive amination of furfural with benzylamine at room temperature (Fig. 7). Indeed, the selectivity to N-benzyl-1-(furan-2-yl)methanamine was found to be 99% in all these cases, indicating the excellent selective performance of the catalyst. The catalyst showed 15% and 35% furfural conversion in 1 h and 2 h, respectively (Fig. 7a). Moreover, conversions of 58%, 74%, 84%, and 98% were obtained in the cases of 3, 4, 5, and 6 h, respectively. This signifies that the furfural conversion increases relatively slowly until 2 h, whereas the conversion rate is faster after 3 h. The hydrogen generation requires a specific time period, after which it is utilized to hydrogenate the N-benzyl-1-(furan-2-yl)methanimine intermediate to N-benzyl-1-(furan-2-yl)methanamine. On the other hand, to determine the significance of the catalyst amount, catalytic activity studies were performed using the 2%Pd3%Ni/Nb₂O₅ catalyst under the optimized reaction conditions (Fig. 7b). When the reductive amination of furfural with benzylamine was carried out using 10 mg of catalyst, 57% conversion was obtained, whereas the 15 mg catalyst showed 72% conversion. With the 20 mg catalyst, an optimum conversion of 98% with 99% selectivity was achieved. When the source of hydrogen for the reductive amination of furfural was TES, the influence of varying its amount from 1 mmol to 2 mmol was studied by taking 5 mL of methanol (Fig. 7c). When the reaction was carried out using 1 mmol and 1.25 mmol of TES by keeping the remaining conditions constant, the PdNi/Nb₂O₅ catalyst gave 35% and 60% conversion of furfural with 99% product selectivity, respectively. The catalyst gave 81% conversion with 99% product selectivity in the presence of 1.75 mmol of TES, whereas the optimum catalytic activity of 98% conversion and 99% selectivity was obtained when 2 mmol of TES was used. The effect of the MeOH amount (1–5 mL) was studied for the reductive amination of furfural with benzylamine, by keeping the remaining conditions unaltered (Fig. S7, ESI†). The PdNi/Nb₂O₅ nanocatalyst gave 32%, 49%, 65%, and 86% conversions of furfural with 99% selectivity of the product when 1, 2, 3, and 4 mL of MeOH was used, respectively. Using 5 mL of MeOH resulted in the maximum furfural conversion (98%) with 99% product selectivity, confirming the necessity of 5 mL of MeOH along with 2 mmol of TES for obtaining the optimum product yield.

Fig. 7 (a) Effect of the reaction duration (reaction conditions: furfural = 1 mmol, benzylamine = 1 mmol, PdNi/Nb₂O₅ = 20 mg, TES = 2 mmol, methanol = 5 mL, and room temperature), (b) effect of the catalyst amount (reaction conditions: furfural = 1 mmol, benzylamine = 1 mmol, TES = 2 mmol, methanol = 5 mL, room temperature, and 6 h), and (c) effect of the TES amount on the reductive amination of furfural (reaction conditions: furfural = 1 mmol, benzylamine = 1 mmol, PdNi/Nb₂O₅ = 20 mg, methanol = 5 mL, room temperature, and 6 h). Product = N-benzyl-1-(furan-2-yl)methanamine. GC error = ±2%.

4.2 Reaction scalability and catalyst stability studies

Process scalability, hot filtration, and catalyst reusability studies were performed to determine the realistic applicability of the process for the synthesis of furfural-based secondary amines (Fig. 8). The hot filtration test was conducted to determine the active site leaching and heterogeneity of the reaction (Fig. 8a). After separating the PdNi/Nb₂O₅ catalyst from the reaction mixture by centrifugation at 2 h, the reaction was continued for another 4 h. The furfural conversion was 35% after 2 h, whereas the change in the conversion was negligible (36%) after 6 h, confirming the catalyst stability. The gram-scale synthesis was performed by increasing all the reactants, reagents, and 2%Pd3%Ni/Nb₂O₅ catalyst 10 times (Fig. 8b). When the reaction was carried out for 6 h, the catalyst gave only 78% conversion with 99% selectivity. However, when the reaction duration was extended from 6 h to 8 h, the catalyst gave 98% conversion with a selectivity of 99%. Increasing the reaction duration (2 h) is necessary and viable to overcome the mass diffusion and adsorption limitations. The reusability of the PdNi/Nb₂O₅ nanocatalyst was investigated for up to 10 cycles under the optimized conditions (Fig. 8c). The catalyst was carefully separated by centrifugation after each cycle, and then washed three times with methanol, followed by oven drying at 80 °C overnight. The PdNi/Nb₂O₅ nanocatalyst showed outstanding reusability efficiency for 10 cycles at room temperature without the necessity of the active sites regeneration step. The catalyst showed a maximum conversion of 97 ± 2% with 99% product selectivity. This confirms that the catalyst is highly stable and reusable for many cycles without the need for activation or pre-reduction.

Fig. 8 (a) Hot filtration study (reaction conditions: furfural = 1 mmol, benzylamine = 1 mmol, PdNi/Nb₂O₅ = 20 mg, TES = 2 mmol, methanol = 5 mL, room temperature, and 6 h). (b) Process scalability (reaction conditions: furfural = 10 mmol, benzylamine = 10 mmol, PdNi/Nb₂O₅ = 200 mg, TES = 20 mmol, methanol = 50 mL, room temperature, and 8 h). (c) Catalyst reusability study (reaction conditions: furfural = 1 mmol, benzylamine = 1 mmol, PdNi/Nb₂O₅ = 20 mg, TES = 2 mmol, methanol = 5 mL, room temperature, and 6 h). Product = N-benzyl-1-(furan-2-yl)methanamine. GC error = ±2%.

4.3 Substrate scope studies

The one-pot, in situ reductive amination of furfural with benzylamine was extended to various derivatives of furfural and benzylamine under the optimized conditions. The model reaction gave 98% furfural conversion and 99% product selectivity (entry 1, Table 2). The reactivity is influenced by electronic effects, such as the inductive and mesomeric effects of the substituents and their position on the rings of furfural and benzylamine. In general, the electron-releasing group on the furfural ring decreases the electropositive nature of the carbonyl group, and thus, the rate of nucleophilic attack (benzylamine) decreases. In contrast, the C–N coupling rate between furfural and benzylamine increases in the presence of the electron-withdrawing substituent on furfural. Similarly, this effect is opposite in the case of substituents on benzylamine. However, this is wholly based on the position of the substituent on the rings of furfural and benzylamine. For instance, 85% conversion and 90% selectivity were achieved when a methyl group was present on the 5th position of furfural (entry 2, Table 2). The presence of –Cl on the 5th position of furfural gave 78% conversion with 85% selectivity (entry 3, Table 2). The mesomeric effect of –Cl lowered the carbonyl carbon's electropositive strength of the furfural and thus reduced the activity to a moderate extent. Similarly, the presence of –Br on the 4th position of furfural resulted in lower conversion (65%) and selectivity (75%), which is due to the steric effect of the bulky –Br group (entry 4, Table 2). The reactivity decreased further (60% conversion and 72% selectivity) in the case of the –I group (entry 5, Table 2), as it is a more efficient electron-releasing group (less electronegative) than –Br (entry 4, Table 2). The hydroxymethyl group present on the 5th position of furfural gave 92% conversion with 75% selectivity (entry 6, Table 2). In this case, the decrease in selectivity is due to the hydroxysilylation of the –OH group of the hydroxymethyl group. 2-Chlorobenzylamine showed a lower conversion of 50% with 92% selectivity (entry 7, Table 2) due to steric hindrance, regardless of the mesomeric effect of –Cl. On the other hand, 3-chlorobenzylamine gave 80% conversion with 94% selectivity (entry 8, Table 2). This is due to the decrease in the nucleophilicity of benzylamine through the electron-withdrawing inductive effect of the –Cl group. 3,4-Dichlorobenzylamine gave 75% conversion with 90% selectivity (entry 9, Table 2) due to the steric hindrance of the two Cl groups, as the electronic effects are nullified by each other. Similarly, the –OMe group in the 2nd position of the benzene ring lowered the reactivity to 46% conversion with 97% selectivity due to the significant extent of the steric effect of the OMe group (entry 10, Table 2). However, the –OMe group in the 4th position increased the reactivity and gave 70% conversion and 92% selectivity due to the electron-releasing mesomeric effect (entry 11, Table 2). 3,4-Dimethoxybenzylamine gave 51% conversion and 94% selectivity (entry 12, Table 2). The decreased conversion is mainly due to the steric effect of both methoxy groups, as they nullified each other's electronic effects. 3-Bromobenzylamine showed a relatively lower activity of 50% conversion and 91% selectivity (entry 13, Table 2) compared with 3-chlorobenzylamine due to the larger size of –Br compared to –Cl. The presence of the trifluoromethyl (–CF₃) group on the 3rd position showed 93% conversion with 95% selectivity (entry 14, Table 2), whereas that on the 4th position showed 81% conversion with 93% selectivity (entry 15, Table 2). The –CF₃ group in the 4th position reduced the nucleophilicity of the amine group via the electron-withdrawing inductive effect significantly from the para position, whereas the effect is moderate from the 3rd position. Similarly, the dominant electron-withdrawing nature of the –NO₂ group on the para position of benzylamine reduced the nucleophilicity to a greater extent, and thus, a lower activity of 60% conversion and 85% selectivity was obtained (entry 16, Table 2). The presence of a methyl group at the 5th position of furfural and the 4th position of benzylamine does not impose a significant electronic effect. However, they showed decreased reactivity (65% conversion and 92% selectivity) due to the moderate steric hindrance, which slowed the hydrogenation of the intermediate imine to a moderate extent (entry 17, Table 2). The PdNi/Nb₂O₅ catalyst showed 95% conversion with 97% selectivity when aniline was used instead of benzylamine (entry 18, Table 2). The derivatives were confirmed by the HR-MS analysis (Fig. S8–S24, ESI†).

Table 2 Substrate scope studies for the reductive amination of various furfurals with benzylamine derivatives and aniline

Reaction conditions: furfural = 1 mmol, aromatic amine = 1 mmol, PdNi/Nb₂O₅ = 20 mg, TES = 2 mmol, methanol = 5 mL, room temperature, and 6 h. ^aConversion (%). ^bSelectivity (%).

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5. Structure–activity relationship and characterization of the spent PdNi/Nb₂O₅ catalyst

The PdNi/Nb₂O₅ catalyst showed remarkable activity for the one-pot, in situ reductive amination of furfural with benzylamine at room temperature (entry 7, Table 1). It is very important to understand the role of catalysts and their active sites in both hydrogen release from the hydrogen donor and in situ hydrogenation of the imine intermediate to the desired secondary amine. This in-depth elucidation will provide valuable insights for the rational design of advanced functional catalysts toward sustainable biorefinery. The plausible reaction mechanism and role of the catalyst in the reductive amination of furfural with benzylamine are shown in Fig. 9. The formation of triethyl(methoxy)silane is the basis for the hydrogen release from TES and methanol (step 2, Fig. 9). Thus, the silyl ether formation is not possible without hydrogen release from methanol and TES, which, in turn, has no possibility of hydrogenation of the imine intermediate. The controlled reactions confirm that hydrogen release from both TES and methanol is possible in the presence of Pd-based catalysts (entries 3 and 5, Table S2, ESI†). The Ni/Nb₂O₅ catalyst doesn't show any activity in terms of furfural conversion (entry 4, Table 1), even though hydrogen is released from TES and methanol to a moderate extent (entry 4, Table S2, ESI†). This indicates that this catalyst is incapable of chemisorbing the released hydrogen during the reaction. On the other hand, the Pd/Nb₂O₅ catalyst gives 39% conversion along with the equivalent amount of triethyl(methoxy)silane and 38% N-benzyl-1-(furan-2-yl)methanamine (entry 5, Table 1). This indicates that metallic Pd (XPS studies, Fig. 5d) is essential for both the hydrogen release and chemisorption of the released hydrogen to facilitate the subsequent hydrogenation of the imine intermediate. Thus, it can be concluded that the formation of triethyl(methoxy)silane and dissociative adsorption of hydrogen takes place on the Pd of the PdNi nanoalloy predominantly (steps 1 and 2, Fig. 9). The adsorption of furfural and benzylamine on the PdNi/Nb₂O₅ catalyst furnishes N-benzyl-1-(furan-2-yl)methanimine (steps 3 and 4, Fig. 9). The intermediate imine further undergoes hydrogenation, giving the desired N-benzyl-1-(furan-2-yl)methanamine product (steps 5 and 6, Fig. 9), which regenerates the PdNi/Nb₂O₅ catalyst for the next catalytic cycle. The PdNi/Nb₂O₅ catalyst shows an optimum catalytic activity of 99% conversion with 98% yield of N-benzyl-1-(furan-2-yl)methanamine (entry 7, Table 1). This is ascribed to the synergistic and alloying effect of Pd and Ni, which are finely dispersed on the surfaces of the Nb₂O₅ nanorods, as confirmed by the STEM-EDAX mapping analysis (Fig. 4d–h). The smaller crystallite size of PdNi (PdNi/Nb₂O₅) compared with Pd (Pd/Nb₂O₅), as confirmed by powder XRD studies (Fig. S3, ESI†), a higher number of redox sites (Fig. 6c), and a higher BET surface area (Table S1, ESI†) contributed to the higher activity of the PdNi/Nb₂O₅ catalyst. The significant charge transfer from Ni to Pd in PdNi/Nb₂O₅ due to the alloying effect is evidenced by the XPS analysis (Fig. 5c and d). The enhanced reducibility of the Nb₂O₅ nanorods in the case of PdNi/Nb₂O₅ confirms the hydrogen spillover effect (H₂-TPR analysis, Fig. 6a). In addition, the PdNi/Nb₂O₅ catalyst contains a maximum concentration of active sites (1372 μmol g⁻¹, Fig. 6d), which is responsible for the observed catalytic activity (entry 7, Table 1). The phenomenal activity of the PdNi/Nb₂O₅ nanoalloy catalyst in the one-pot, in situ reductive amination of furfural with benzylamine under ambient conditions, together with its high stability (hot filtration test, Fig. 8a), scalability (Fig. 8b), and remarkable reusability (Fig. 8c), shows the practical viability of this catalytic approach for synthesizing bio-based secondary amines.

Fig. 9 Schematic representation of the structure–activity relationship and role of the catalyst in the reductive amination of furfural with benzylamine. Note: FFAL – furfural, BAM – benzylamine, TES – triethylsilane, TEMS – triethyl(methoxy)silane, NBFMI – N-benzyl-1-(furan-2-yl)methanimine, NBFMA – N-benzyl-1-(furan-2-yl)methanamine.

6. Conclusions

As secondary amines are considered highly valuable pharmaceutical precursors, their synthesis from biomass using heterogeneous catalysis is a sustainable approach. Hence, we developed an efficient one-pot strategy to synthesize secondary amines from biomass-derived furfural with benzylamine over a PdNi/Nb₂O₅ nanocatalyst at room temperature using a hydrogen donor. The catalyst gave 98% conversion of furfural with 99% product selectivity without the need for higher pressures and temperatures. The XPS, H₂-TPR, and CO chemisorption studies confirmed the significant charge transfer from Ni to Pd, and their synergistic effect, along with optimum redox and active metal sites, which are the reasons for the excellent activity of the PdNi/Nb₂O₅ catalyst. Moreover, the PdNi/Nb₂O₅ catalyst showed very good tolerance towards various derivatives of furfural and benzylamine, giving optimum yields of the desired products at room temperature. The hot filtration study confirmed the stability and heterogeneity of the PdNi/Nb₂O₅ catalyst. Thus, the catalyst showed excellent catalytic activity even after 10 cycles of reuse. The gram-scale synthesis of furfural-based secondary amines was demonstrated using the PdNi/Nb₂O₅ catalyst. Overall, this work provided valuable information for synthesizing biomass-based drug scaffolds at room temperature using an efficiently reusable supported nanoalloy catalyst.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

P. S. is thankful to the UGC for providing a fellowship. P. S. acknowledges the funding support from the SERB-CRG (CRG/2022/005932).

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Footnote

† Electronic supplementary information (ESI) available: Quantitative measurements of XPS analysis of the catalysts, N₂-adsorption–desorption isotherms, Raman spectra, BET surface areas, pore volumes, pore sizes, ICP-OES analysis of the catalysts, and HR-MS spectra of triethyl(methoxy)silane, N-benzyl-1-(furan-2-yl)methanamine, and the derivatives, including the controlled reactions. See DOI: https://doi.org/10.1039/d5gc02500a

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