Hydrogenation of Furfural with Nickel Nanoparticles Stabilized on

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Hydrogenation of furfural with nickel nanoparticles stabilized on nitrogen-rich carbon core-shell and its transformations for the synthesis of #-valerolactone in aqueous conditions Sekhar Nandi, Arka Saha, Parth Patel, Noorul H. Khan, Rukhsana I. Kureshy, and Asit B Panda ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

Hydrogenation of furfural with nickel nanoparticles stabilized on nitrogenrich carbon core-shell and its transformations for the synthesis of γvalerolactone in aqueous conditions Sekhar Nandi,a,b Arka Saha,a,b Parth Patel,a,c Noor-ul H. Khan,a,b,c* Rukhsana I. Kureshya,b and Asit B. Pandaa,b a

Inorganic Materials and Catalysis Division, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar- 364002, Gujarat, India. * [email protected] b

Academy of Scientific and Innovative Research (AcSIR), Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar 364002, Gujarat, India. c

Charotar University of Science and Technology, Changa, Anand-388 421, Gujarat, India.

Abstract In this article, we report the synthesis of nitrogen-rich carbon layer encapsulated Ni(0) nanoparticles as a core-shell structure (Ni@N/C-g-800) for the catalytic hydrogenation of furfural to furfuryl alcohol. The nickel nanoparticles were stabilized by the nitrogen-rich graphitic framework, which formed during the agitation of nickel acetate impregnated cucurbit[6]uril surface in reducing atmosphere. Further, the catalyst was characterized using various physicochemical methods such as P-XRD, Raman, FE-SEM, HR-TEM, XPS, BETsurface area, CO2-TPD, ICP and CHN analysis. The nitrogen-rich environment of the solid support with metallic Ni nanoparticles was found to be active and selective for the catalytic hydrogenation of furfural with molecular H2 in an aqueous medium at 100 oC. To understand the reaction mechanism, DRIFT study was performed which revealed that the C=O bond is activated in the presence of a catalyst. In addition, we have extended our methodology towards the synthesis of “Levulinic acid” and “γ-valerolactone”, by successive hydrolysis and hydrogenation of furfuryl alcohol and levulinic acid respectively in an aqueous medium. Moreover, the

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heterogeneous catalysts used in all the three consecutive steps help in recovery, recycling of catalyst and easy separation of products. Keywords nickel nanoparticles. furfural. levulinic acid. γ-valerolactone. catalytic hydrogenation. cucurbit[6]uril. heterogeneous catalysis Introduction With the upsurging demand of fuel and bulk chemicals arising due to the growing population, the entire world needs sustainable, economical, energy-efficient fuel and chemicals.16

Though the underground crude oil satisfies the demands of human need, which is the

nonrenewable source and may be exhausted in near future. Therefore, biomass serves as a substitute, sustainable source of organic chemicals, exist in enormous amount, inexpensive, safe and renewable carbon source in nature.1 In the present scenario, furfural (FFA) is imperative biomass, forthright derived from the sugars and can be transformed into many useful products from chain length of C4-C5.7-8 The FFA transformation was first reported in 19219 and later became an essential commodity chemical with annual production of an of >300 kTon which utilize for the synthesis of series of chemicals, additives and biofuels (Scheme 1). Among the numerous C4-C5 chemicals, furfuryl alcohol (FA), levulinic acid (LA) and γvalerolactone (GVL) treated as the most promising feedstock used for numerous applications.1013

FA is the most consuming chemical i.e. >65% of its overall production from FFA, used for the

preparation of LA, GVL, resins, building blocks, pharmaceuticals etc.7


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Furfural derived C4 chemicals OH

HO

O

O

O

O

O

O OH

HO O

O

OH

O

Biofuels

O FA derived resins O

Biomass

OH

OH

OHC

O Furfural

O

O O GVL

LA O

FA O

OH LA derived biofuels

HO O

O

O

OH OH

Furfural derived C5 chemicals

Scheme 1. Furfural as a feedstock for production of wide range of chemicals and biofuels. In addition, LA and GVL also considered as valuable platform molecules for the production of biofuels and chemical intermediates.7 Moreover, GVL considered as a potential solvent for the transformation of biomass.14-16 Currently, these chemicals are synthesized from furfural by consecutive hydrogenation and acid hydrolysis process.7 Chemoselective hydrogenation of FFA to FA is highly valuable due because of its extensive applications.7 Thus, the choice of catalyst and catalytic conditions are very crucial.17 On an industrial scale, FFA to FA is synthesized by gas or liquid phase catalytic hydrogenation using copper chromite as a catalyst.7,18 However, the chromite based catalyst is well known for its environmental toxicity, which indorses extensive research for appropriate alternatives. Recently, metal nanoparticles based catalytic systems have gained much attention due to its high activity compared to the conventional ligated metal catalysts and successfully employed for various hydrogenation reactions.19-20 Further, depending on the parameters, these nanoparticles need some tunings such as selection of appropriate metal source, solid support and



solvent selection. Among the well-known solid supports (silica, alumina and carbon), modified nitrogen doped graphitic carbon are promising solid supports and used extensively in recent years.21 These nitrogen doped graphitic supports shows good dispersion and stabilization of metal nanoparticles with improved chemical and electrical properties.22-26 In addition, due to the nitrogen-rich environment, these supports provides a basic environment which facilitates the catalytic hydrogenation reactions with the heterolytic cleavage of molecular hydrogen in polar media.27 The first liquid-phase hydrogenation of FFA was reported by Ni/MgO catalyst by Quaker Oats Company in 1928.28 The process was susceptible and required accurate control over the byproducts formation. Since then, several precious/ noble metals (Pd, Pt, Rh, Ru, Ir) and nonprecious metals (Ni, Cu) were extensively used for the FFA hydrogenation reactions.7,10,16,29 However, the reported procedures required harsh conditions, organic solvents and expensive catalytic systems. In recent time, the scientific community gives more attention towards the non-precious metal-based catalytic systems, in which nickel-based catalysts are excellent for the FFA hydrogenation, as reported by various research groups. Li et al. have reported the synthesis of bimetallic NiFeB amorphous catalyst by chemical reduction method for the hydrogenation of FFA to FA.30 Similarly, Liaw et al. have synthesized the CoNiB catalyst for catalytic hydrogenation of FFA at 20 bar of H2 pressure.31 Recently, Resasco and his group have reported the Ni-Fe bimetallic catalyst and silica supported Ni catalyst for similar hydrogenation reactions but it required high temperature.32 Moreover, Zhang and his group reported the Raney® Nicatalyzed furfural hydrogenation under N2 atmosphere with methanol as a hydrogen donor in aqueous media.33 Despite the notable results for FFA hydrogenation, the reported catalytic


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systems and reaction protocol require tedious methods and harsh conditions limit its efficiency and sustainability. Due to these demerits, these catalytic systems require much improvement to make it sustainable with increased efficiency. Again, for the synthesis of GVL from FFA, only few reports are available. Zhu et al. reported the direct synthesis of GVL from hemicellulose catalyzed by Au nanoparticles via transfer hydrogenation.34 Further, Jae and co-workers have used Sn-Al-β zeolite as a bi-functional catalyst for GVL synthesis from FFA in 2-butanol at 180 o

C.35 Though the catalytic systems demonstrate direct GVL synthesis, but existing protocols are

neither efficient nor economical. On the basis of this background and our previous involvements towards the hydrogenation reactions,36-37 here we report the complete catalytic protocol for the synthesis of GVL from FFA by successive hydrogenation and hydrolysis reactions with heterogeneous catalysts. The three-step catalysis follows hydrogenation of FFA to FA over Ni(0) nanoparticles followed by hydrolysis of intermediate FA to LA by heterogeneous acidic resin (IER-130) and finally hydrogenation of LA to GVL with the same Ni(0) nanoparticles. The entire catalytic procedure is conceded in an aqueous medium with the heterogeneous catalytic system, makes the process more economical and viable. The nickel nanoparticles on a nitrogen-rich graphitic support is prepared by in-situ pyrolysis of nickel impregnated cucurbit[6]uril CB[6] material, which contents an enormous number of nitrogen atoms along with carbon and oxygen. The above protocol is very efficient, robust, and selective for the transformations of furfural to GVL in three successive steps. Further, different reaction parameters such as the effect of support, solvent, pressure, temperature and furfural amount on the catalytic hydrogenation for FA synthesis were explored in detail. Moreover, the mechanistic pathway for the FFA hydrogenation has been proposed by using catalytic reactions and DRIFT study. Notably, the catalysts and



catalytic protocols are very stable, efficient and exhibit the significant advantages over the previously reported methods. Experimental Material and reagent All the chemicals were purchased from commercial suppliers and used as received. Furfural was purchased from TCI chemicals and used without purification. Glycoluril and CB[6] were prepared from the following reported method.37 Hydrogen gas was taken from Hydro Gas India Pvt. Ltd., India. Ion-exchanged resin (IER-130/ Indion-130) was purchased from the local vendors. Impregnation of Ni(OAc)2 on different solid supports 1.

On CB[6]: To impregnate the metal precursor on the CB[6], initially, 2 g of CB[6] was dispersed in methanol (100 mL) and sonicated for 30 min, then the Ni(OAc)2.4H2O (250 mg in 5 mL methanol) solution was added dropwise to the dispersed solution. The resulting mixture was stirred for 30 min at 27 oC (r.t) and allowed to dry at 70 oC on a hot plate. After complete evaporation of methanol, the solid impregnated catalyst was taken out and ground on the mortar-pestle to a fine powder.

2.

On carbon: Nickel precursor was impregnated on carbon support by caramelization process. At first, sucrose (2g, reagent grade) was dissolved in water (50 mL), then Ni(OAc)2.4H2O (296 mg in 5 mL water) was added dropwise onto the dispersed solution. The resulting mixture was stirred for 30 min at 27 oC (r.t) and allowed to dry/ caramelize at 120 oC on a hot plate. After caramelizing the mixture (black color solid material after


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complete evaporation of water) the solid material was taken out and ground on the mortar-pestle to a fine powder. N.B- For better dispersion, the solution (metal precursor with sucrose solution) should be clear from start to end (no agglomeration) on caramelizing process. 3.

On urea: Initially, urea (2g, reagent grade) was dissolved in water (50 mL), then Ni(OAc)2.4H2O (43 mg in 5 mL water) was added dropwise onto the dispersed solution. The resulting mixture was stirred for 30 min at 27 oC (r.t) and allowed to dry at 120 oC on a hot plate. After drying the mixture the solid material was taken out and ground on the mortar-pestle to a fine powder.

4.

On nitrogen-rich polymer: Nitrogen-rich polymer was synthesized from the reported procedure.38 To impregnate the metal precursor on the polymer, 2 g of polymer was dispersed in methanol (100 mL) and sonicated for 30 min, then the Ni(OAc)2.4H2O (86 mg in 5 mL methanol) solution was added dropwise onto the dispersed solution. The resulting mixture was stirred for 30 min at 27 oC (r.t) and allowed to dry at 70 oC on a hot plate. After complete evaporation of methanol, the solid impregnated catalyst was taken out and ground on the mortar-pestle to a fine powder.

Pyrolysis of nickel impregnated material The solid materials obtained from the previous impregnation process were transferred to the crucible and placed in a tubular furnace for pyrolysis at 800 oC for 2 h with a temperature gradient 5 oC/min under reducing atmosphere (10% H2 with argon). The entire pyrolysis process was carried out in presence of inert gas. Once the furnace was cooled, it was opened and calcined material was taken out for further characterization and catalytic reactions. The catalysts with



different solid supports labeled as: CB[6] support- “Ni@N/C-g-800”, yield- 40%; Carbon support- “Ni@C-g-800”, yield- 35%; Urea support- “Ni@U-800”, yield- 5%; Nitrogen-rich polymer- “Ni@NCp-800”, yield- 12% N.B- The temperature of the calcination furnace should be cooled down to r.t under inert atmosphere before opening it. Else, the activated carbon powder burns rapidly during contact with air. Characterization 1

H and

13

C NMR spectra of products were measured in CDCl3 by using TMS as an

internal standard, on a Bruker Avance 500 MHz FT-NMR. In situ Drift FTIR experiments were performed using Perkin-Elmer, G-FTIR spectrometer and data was collected in the range of 4000-700 cm-1 at 4 cm-1 resolution over 30 scans. Scanning Electron Microscopy images were recorded on a microscope (JEOL JSM 7100F) with an accelerating voltage of 18 kV with a probe current of 102 AMP. The samples were coated with gold using sputter coating to avoid charging. RAMAN spectra’s were carried out with LabRam HR Evolution Raman Spectrometer, Horiba Scientific, Japan. Laser frequency was kept at 532 nm with 10x resolution. Transmission Electron Microscopy images were carried out on a JEOL JEM-2100 microscope with an acceleration voltage of 200 kV using carbon coated 200 mesh copper grids. The samples were ultrasonically dispersed in acetone for 30 min and deposited on the copper grid using capillary and drying overnight in air. Thermogravimetric analysis (TGA) was carried out by Mettler TGA/SDTA 851e equipment in flowing N2 (flow rate = 50 ml/min), at a heating rate of 10 o

C/min and data were processed by Stare software. XRD analysis of samples was carried out on

a Philips X’pert X-ray powder diffractometer by using Cu Kα (λ= 1.54178Å) radiation. XPS analysis was done on Omicron Nanotechnology. Inductive Coupled Plasma (ICP) of the catalyst


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was carried out on Perkin Elmer, Optima 2000. Gas Chromatography of the samples was carried out on a Shimadzu GC-2010 with a capillary column, column oven temperature of 110 oC, FID of 200 oC, Pressure was 110 kPa. General procedure for the hydrogenation reaction In a 100 mL stainless steel high-pressure reactor, Ni@N/C-g-800 (100 mg), solvent (35 mL), FA/ LA (12 mmol) and dodecane (50 µL, internal standard for GC) were charged. The reactor was flushed three times with nitrogen gas (1 bar) and then pressurized with H2 gas (2 MPa). After that, the reactor was heated to 100 oC with continue stirring the reaction mass at 600 rpm. The reaction was initiated instantly as evident by the pressure drop. Once the pressure drop ceases or no noticeable change in pressure drop, then the reactor was brought to r.t and depressurized. The solid catalyst was then separated by the centrifugation and washed it carefully with deionized water and acetone. The supernatant was extracted with dichloromethane (4 X 10 mL), and organic part was separated. Further, the organic portion was dried by mixing with sodium sulfate which then evaporated to get the pure product. The final product was analyzed by the gas chromatography and NMR (1H and 13C). General procedure for the hydrolysis reaction In a 25 mL single neck round bottom flask water-THF (1:1, 5 mL), IER-130 (100 mg) and FA (6 mmol) were taken. The temperature of the reaction was increased to 100 oC on oil bath and stirred for 2 h at 600 rpm. The progress of the reaction was monitored by GC with dodecane as an internal standard. After the completion of the reaction, the catalyst was separated by simple filtration and the liquid phase was separated by liquid-liquid phase separation process by adding 1 mL of diethyl ether. The organic layer was extracted with diethyl ether (3 X 10 mL),



and total organic part was combined. Further, the organic part was dried by adding the sodium sulfate. Then, the solvent was evaporated to get the pure product, which is further characterized by 1H and 13C. Result and discussions CB[6] is an active molecule, synthesized by commonly available reagents i.e. urea, glyoxal, formaldehyde and mineral acids.36-37 Initially, glycoluril was synthesized from urea and glyoxal by a condensation process, which on reaction with formaldehyde in strong acidic medium gives CB[6] product.36-37 Its purification process contains several important steps which reported in our previous report.37 The beauty of this molecule is the enormous number of carbon, nitrogen and oxygen atoms39-41 available for the stabilization of the metal nanoparticles.36,42-43

Scheme 2. Synthesis of support and Ni@N/C-g-800 catalyst. In contrast, we have utilized CB[6] for the synthesis of nitrogen-rich graphitic-carbon by pyrolysis process, reported for the first time by our group.37 In this report, we have infused Ni(OAc)2.4H2O on CB[6] support in the presence of methanol solvent with continuous stirring for 30 min at r.t and then evaporated to dryness at 70 oC. After complete drying, the nickel impregnated material was pyrolyzed/calcined at 800 oC for 2 h (temperature gradient 5 oC/min)


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under reducing atmosphere (10% H2 on Ar). The material thus obtained from the pyrolysis was labeled as Ni@N/C-g-800 and analyzed by various physicochemical techniques. The complete schematic representation of catalyst synthesis was shown in Scheme 2. The catalysts with other solid supports such as sucrose (Ni@C-g-800), urea (Ni@U-800) and nitrogen-rich polymer (Ni@NCp-800) synthesized to see the role of the different type of solid support. However, while using urea and nitrogen-rich polymer as a solid support, very low yield (5 and 12%) of final catalyst was observed. Material characterization Initially, the synthesized Ni@N/C-g-800 catalyst was characterized by PXRD and Raman to get the structural and phase information. The XRD pattern of Ni@N/C-g-800 before reaction shown in Figure 1a. The three peaks at 2θ = 44.32o, 51.54o and 76.31o can be assigned to the diffractions of (111), (200), and (220) crystal planes of pure face-centered cubic (FCC) Ni(0) (JCDPS No.7440-02-0).44-45 The intensity of the diffraction peak evidences the highly crystalline nature of Ni(0) nanoparticles. The characteristic broad peak around 2θ = 25.71o (JCPDS no. 010646), representing the crystal structure of amorphous and/or stacked graphitic carbon formed during calcination (Figure 1a). The synthesis of amorphous carbon/or stacked graphitic carbon was done by annealing of CB[6] in reducing atmosphere at 800 oC by our previously reported method (Figure S1).37



Figure 1. (a) PXRD of Ni@N/C-g-800 catalyst; (b) Raman spectra in the graphitic range of Ni@N/C-g-800 catalyst. In the respective Raman spectra (Figure 1b), two peaks located at 1324 cm-1 (D band) and 1556 cm-1 (G band), responsible for graphitic carbon of calcined CB[6] was recognized. The D and G band is accompanying with defective graphitic carbon and sp2 carbon atoms respectively.46 The intensity ratio found between the D band and the G band is nearly 1. This value is comparable to that of reduced graphene oxide (1.024) which suggested the graphitic nature of carbon material. The corresponding D and G band of Ni@N/C-g-800 catalyst was shifted to 1352 cm-1 and 1575 cm-1. This shifting was due to the interaction of encapsulated Ni(0) nanoparticles with the graphitic carbon. For detailed morphological and microstructural identification, field emission-scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) analysis were performed. Through FE-SEM analysis, it was very difficult to reach any conclusions regarding the orientation of carbon around the Ni nanoparticle. Figure S2 (a, b) shows only the thick platelike morphology for both Calcined CB[6] and Ni@N/C-g-800 materials with the mesoporous surface (inset figure S2a). Homogeneous distribution of C, N and O in calcined CB[6] (Figure S2 c-g) and the distribution of C, N, and Ni in the prepared catalyst was observed (Figure S2 h-


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l). Further, TEM images of Ni@N/C-g-800 catalyst were shown in Figure 2. Homogeneous distribution of nearly spherical 5-10 nm Ni(0) nanoparticles was observed on the carbon matrix (Figure 2a).

Figure 2. TEM image of (a) fresh Ni@N/C-g-800 catalyst; (b) HR-TEM image of Ni@N/C-g800 shows the arrangement of carbon over the Ni(0) nanoparticles; (c) distribution of carbon without Ni (d) existence of 002 plane of graphitic carbon material and (111) plane Ni nanoparticles. The orientation of carbon over the Nickel nanoparticle was observed from HR-TEM images (Figure 2b). It shows that Ni(0) nanoparticles were covered by onion slice layered carbon material. The porous nature of the carbon was proved by HR-TEM image (Figure 2b). In the absence of Ni(0) nanoparticles, random orientation of carbon material was observed (Figure



2c). It concluded that the presence of Ni(0) nanoparticle facilitates the carbon layer formation during carbothermal reduction aiding extra stability to Ni(0) nanoparticles. The interplanar distance between the two carbon layers was found to be 0.359 nm, representative of the (002) plane of graphitic carbon and the highly crystalline (Figure 2d, S3b),47 well exposed (111) plane of Ni(0) nanoparticle was also observed form HR-TEM and corroborates the PXRD result (Figure 3d, S3a).48 Further, the synthesized catalyst was analyzed by ICP-OES, to determine the exact metal (nickel) composition in the material. It was found that nickel present in 9.93% by weight. C, H, N analysis results revealed the elemental composition of the catalyst, which shows that carbon, nitrogen, and hydrogen are in 61.05, 7.86, and 1.03% respectively (see ESI, TableS1).

Figure 3. (a) XPS survey spectrum of Ni@N/C-g-800; (b-d) High-resolution XPS spectrum of Ni2p, N1s and C1s region of the proposed catalyst.


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Further, the surface elements and the chemical states of Ni and carbon species of Ni@N/C-g-800 was investigated by XPS (Figure 3). Figure 3a showed the coexistence of C, Ni, N, O elements in Ni@N/C-g-800 catalyst, which was consistent with the XRD and SEM elemental mapping (Figure S2). Figure 3b exhibited the high-resolution XPS spectra of Ni2p region. Two main peaks with a binding energy of 853.75 eV and 871.34 eV for Ni2p3/2 and Ni2p1/2 were attributed to the metallic Ni.49-50 In the N1s spectrum (Figure 3c), the broad peak at ~399 eV could be deconvoluted, generating three new peaks at 398.47 eV, 400.46 eV and 403.88 eV assigned to the coexistence of pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen respectively.51 Figure 3d exhibited deconvoluted high-resolution XPS spectra of C1s region. Three key peaks with the binding energies of 284.6, 285.24 and 286.29 eV were assigned to graphitic carbon (CC/C=C), C-N coordination and C-N=C coordination in Ni@N/C-g-800, respectively.52 BET surface area and pore diameter of the Ni@N/C-g-800 catalyst were measured by the Brunauer–Emmett–Teller (BET) method using N2 adsorption-desorption at 77 K. The BET surface area of the Ni supported catalyst was 83.8 m2g−1 with an average pore diameter of 39.7 nm. The low-temperature N2 adsorption isotherms of the catalyst have a profile of type IV (followed by IUPAC) which typically behave as mesoporous materials (pore diameters 2–50 nm) (Figure 4a). Additionally, the CO2-TPD (temperature programmed desorption) analysis was carried out to understand the nature of nitrogen atoms (basic nitrogen) on the catalyst surface. In TPD spectra, we observed that two intense peaks at a temperature range of 100-500 oC, suggests that the catalyst/ support has a strong ability to adsorb/ interact with CO2 (0.275 mmol/g) (Figure 4b). Two distinct adsorption peaks suggest two major type of basic nitrogen on the support which corroborates with the XPS analysis.



Figure 4. (a) BET surface area of Ni@N/C-g-800; (b) CO2-TPD of Ni@N/C-g-800. Further, the thermal stability of the catalyst was analyzed by thermogravimetric analysis, and it was found that the catalyst is stable up to ∼400 °C. A slight weight loss after 100 °C is due to physically adsorbed water molecules, and a subsequent weight loss after 250 °C is due to organic impurities on the catalyst. The weight loss after 450 °C is due to decomposition of the nickel complex of the catalyst (see ESI, Figure S4). Pulse chemisorption analysis was carried out to understand the nickel dispersion on the catalyst. In which, the catalyst (0.063 g) was pretreated in H2/Ar flow gas at temperature 45 oC. The amount of H2 adsorbed on the catalyst at 45 oC was 0.266 x 10-3 mmol/g, which gives approximately 31.45% nickel dispersion on the catalyst with 21.75 m2/g of the metallic surface area. Catalytic tests After successful characterization of the catalyst, it was then illustrated for hydrogenation of furfural as a model substrate with molecular H2 in an aqueous medium. Due to the inherent reactivity of FFA molecule towards both C=O group and a furan ring, chemoselective hydrogenation of FFA to yield desired products is of great interest. To achieve the selective FA


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from hydrogenation of FFA, water was chosen as a reaction media to study its effect and catalytic activity of the Ni@N/C-g-800 catalyst (Scheme 3).10

Scheme 3. Hydrogenation of furfural. The catalytic hydrogenation reactions were performed in 100 mL high-pressure reactors. To find the ideal conditions for selective hydrogenation of FFA to achieve FA, the catalytic reactions were initially accomplished with 500 µL of furfural at 140 οC using 500 psi H2 and 150 mg of a metallic nickel catalyst (Ni@N/C-g-800) in an aqueous solution (35 mL). After 12 h, the crude reaction mass was taken out and analyzed by gas chromatography. From GC analysis, we found 80% conversion of FFA with >99% of FA selectivity (Table 1, entry 1). No further byproducts were detected, inferring that reaction might have proceeded via selective hydrogenation of C=O bonds (in FFA) to form FA. It was further confirmed by 1H and

13

C NMR analysis (see ESI,

Figure S5 and S6). In absence of a catalyst, FFA has not converted to FA suggests that Ni(0) catalyst was responsible and necessary for the catalytic hydrogenation reaction of FFA (Table 1, entry 2). With this promising results in hand, we have further optimized the reaction conditions to achieve better yield and selectivity of FA. The catalytic hydrogenation reactions are mostly dependent on solvent, pressure and temperature hence, we started our optimization studies with varying different solvents such as ethanol, water, isopropanol (IPA), tetrahydrofuran (THF) and toluene. The results were >98, 34, 88, 35 and 22% of conversion with 12, >99, 40, 95 and >99% of FA selectivity respectively (Table 1, entries 3-10).



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Table 1. Optimization studies.a Entry

FFA vol. Solvent Time H2 press. Temp. Conv. Sel. TON (µL) (mL) (h) (psi) (oC) (%) (%) 1 500 H2O (35) 12 500 140 80 98 86 b 2 500 H2O (35) 12 500 140 0 0 0 3 500 EtOH (35) 6 500 140 98 12 13 4 500 H2O (35) 6 500 140 34 99 37 5 500 IPA (35) 6 500 140 88 40 39 6 500 THF (35) 6 500 140 35 95 36 7 500 Toluene (35) 6 500 140 22 99 24 8 500 H2O+IPA (20+15) 6 500 140 99 85 92 9 500 H2O+IPA (30+05) 6 500 140 99 94 101 10 500 H2O+IPA (34+01) 6 500 140 99 98 106 11 500 H2O+IPA (34+01) 6 300 140 99 98 106 12 500 H2O+IPA (34+01) 6 200 140 95 97 107 13 500 H2O+IPA (34+01) 6 100 140 43 98 46 14 500 H2O+IPA (34+01) 7 200 100 99 98 106 15 500 H2O+IPA (34+01) 12 200 50 37 98 40 16 1000 H2O+IPA (34+01) 12 200 100 97 97 206 17 1500 H2O+IPA (34+01) 24 200 100 67 98 215 18 2000 H2O+IPA (34+01) 24 200 100 23 98 98 c 19 500 H2O+IPA (34+01) 7 200 100 4 98 d 20 500 H2O+IPA (34+01) 7 200 100 5 98 e 21 500 H2O+IPA (34+01) 7 200 100 12 99 a Reaction conditions: Catalyst- 150 mg, stirring speed- 600 rpm. Conversion and selectivity are determined by GC using dodecane as an internal standard, turnover numbers were calculated by mmol of product formed per mmol of nickel (available active sites determined by chemisorption analysis).

b

Without catalyst.c With “Ni@C-g-800”.

d

With “Ni@U-800”.

e

With “Ni@NCp-

800”. Among the solvents scrutinized, water shows the better activity for furfural hydrogenation with 34% conversion and >99% selectivity toward FA in 6 h (Table 1, entry 4). Other solvents like EtOH, IPA, THF and toluene ended with a poor result for desired FA (Table 1, entries 3, 5-7). Even though the effect of solvents on the catalyst was not explicit, yet these


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affect the catalytic activity pertaining to solvent polarity.10 Since the conversion of FFA was relatively low in the water, we decided to conduct the hydrogenation reaction by mixing two different solvents and for that, we have chosen water and IPA as a solvent of choice. The reason behind this it could be attributed to the fact that with IPA being polar resulted in high yield which is due to the solubility factor and water thus minimizing the byproduct formation. So, we carried out the reaction using 20 and 15 mL of water/ IPA mixture (Table 1, entry 8). Interestingly, the reaction rate improved in 6 h of reaction time with conversion (>99%) and selectivity (85%). Further, we reduced the amount of IPA for the hydrogenation of FFA. The experiments conducted with 5 and 1 mL of IPA, revealed that the reaction time does not suffer (6 h) but selectivity increases up to 98% (Table 1, entries 9 and 10). The above results showed that the little amount of IPA is enough to accelerate the FFA hydrogenation with excellent selectivity. The hydrogen pressure is more crucial for any hydrogenation reaction in lab scale as well as for industrial scale. Thus, we have examined the hydrogenation reaction at higher (500 psi) to lower H2 pressure (100 psi) in 6 h of reaction time (Table 1, entries 11-13). On declining the H2 pressure from 500-200 psi, FFA conversion and selectivity remained same (Table 1, entries 11 and 12). However, on continue decreasing the H2 pressure below 200 psi reaction time significantly increases (Table 1, entry 13). Further, we varied the reaction temperature by maintaining 200 psi of H2 pressure (Table 1, entries 12, 14 and 15). On reducing the reaction temperature to 100 oC selectivity of FA was not altered and observed the complete conversion of FFA in just 7 h (Table 1, entry 14). However, below this temperature reaction time increased significantly (Table 1, entry 15). Also, we have carried out the furfural amount variation (5002000 µL) under the above-optimized conditions i.e. with 200 psi of pressure, 150 mg of catalyst, 35 mL of W:IPA (34+1) mixture at 100 oC (Table 1, entries 14, 16-18). Good conversion of



FFA to FA was obtained while taking 500 and 1000 µL of furfural (Table 1, entries 14 and 16), but longer reaction time (12 h) was observed for 1000 µl of FFA. On further increasing the furfural amount beyond 1000 µL, complete conversion did not achieve even after longer reaction time (>24 h) (Table 1, entries 17-18). Thus, for 1 mL of furfural to be converted to FA, 150 mg of nickel catalyst, 35 mL of mixed solvent at 100 oC and 200 psi of H2 pressure was optimized for excellent selectivity. We have calculated the turnover numbers for all the respective reactions and found moderate to good under the various reaction conditions (Table 1). Additionally, the optimized reaction condition obtained from “Ni@N/C-g-800” was tested for the catalyst prepared from other supports (“Ni@C-g-800”, “Ni@U-800” and “Ni@NCp-800”) to know the role of support, carbon and nitrogen (Table 1, entries 19-21). The results obtained by the catalysts “Ni@C-g-800”, “Ni@U-800” and “Ni@NCp-800” are 4, 5 and 12% respectively. The above results with different types of supports give inferior results might be due to the absence of nitrogen (sucrose) or very low carbon to nitrogen ratio (urea and polymer), which decomposes at the time of pyrolysis (results low yield) and gives inferior results for FFA hydrogenation. Thus, it suggests that the sufficiency of graphitic carbon, nitrogen atoms with crystalline nickel nanoparticles on “Ni@N/C-g-800” altogether responsible/ facilitate the hydrogenation of FFA, which is absent in the catalysts prepared from other solid support (sucrose, urea and nitrogenrich polymer). Consequently, the nitrogen-rich carbon core-shell prepared from CB[6] matrix provides extensive stability to the crystalline nickel nanoparticles of the active catalyst53-55 (Ni@N/C-g-800) and sufficient basicity for the catalytic hydrogenation reaction facilitates the heterolytic cleavage of molecular hydrogen.56


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Reaction mechanism: Drift study In order to understand the reaction pathway of the FFA hydrogenation, diffuse reflectance infrared fourier transform spectroscopy (DRIFT) experiment was performed under the H2 pressure. Initially, only Ni(0) catalyst’s spectra was recorded and kept constant to study the other interactions. Figure 5a shows the interactions of the catalyst with H2 at room temperature (27 oC) and higher temperature (80 oC). The catalyst under the contact with H2 mainly shows three strong absorption peaks at 3499, 3393 and 3444 cm-1 at 27 oC, it might be due to the interaction of hydrogen with nitrogen (νN-H), oxygen (νO-H), and carbon (νC-H) present on the catalyst. These observations were supported by the XPS analysis i.e. presence of different nitrogen and oxygen atoms and CO2-TPD analysis where the presence of basic nitrogen confirmed. Further, when we increased the temperature (at 80 oC), a number of absorption peaks increases in the range of 3626-3204 cm-1 due to the temperature and anharmonic effects on the IR spectrum.57-58 Alternatively, these effects/ interactions have enormous physical explanations. The energy for the vibrational levels consists of two major effects. First, the transition energies depend only on the mode of frequency in harmonic systems and second, the equilibrium spectrum at finite temperature, which mainly depends on the population of vibrational states.57 Further, the peaks at lower wavenumbers < 800 cm-1 might be due to the interactions of metallic Ni(0) with the dihydrogen molecule. Moreover, we examined the type of adsorption took place when furfural was dispersed on the catalyst surface to understand the activation of substrates at a different temperature over the catalyst surface. Spectra’s were recorded initially at 50 oC, then the temperature was increased to 100 oC (Figure 5b).



Figure 5. DRIFT- FTIR spectra of (a) interaction of H2 with a catalyst; (b) furfural adsorption on the catalyst. Before exposure of H2 gas, all the absorption peaks belong to adsorbed FFA on Ni(0) catalyst surface. The methylene stretching was detected at 2857 cm-1 and can relate with the fcc(111) surface of the catalyst.59 The absorbance peaks at 3130 and 2811 cm-1 present on all the Ni(0) catalyst after contact of FFA, could be ascribed to the symmetric and asymmetric νC-H stretching of unsaturated FFA ring and aldehyde integral respectively.59 These stretching frequencies corroborate with the liquid like FFA, representing for physisorbed or very weakly chemisorbed FFA on the catalyst. Further, the spectra obtained also indicates that only C=O (carbonyl) group was interacting with Ni(0) catalyst (νC-O = 1898 and 1780 cm-1) and not the furan ring.59-60 Moreover, we have observed these stretching frequencies even at a higher temperature in presence of continuous H2 flow up to 1 h. Interestingly, there was no peak shifting observed which confirmed that in the catalytic conditions only C=O (carbonyl group) interacts with the catalyst and not the furan ring. Thus, the selective hydrogenation of FFA to FA (νO-H = 3349 cm1

) takes place.


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Figure 6. Probable mechanistic pathway for FFA hydrogenation. In the light of the DRIFT studies and above experimental results a plausible reaction mechanism was depicted for chemoselective hydrogenation of FFA catalyzed by Ni@N/C-g-800 in water (Figure 6). The hydrogenation of FFA to FA would proceed through successive steps. Initially, FFA was adsorbed on the catalyst surface and interacted with the metallic Ni(0) surface via C=O bond. Meanwhile, hydrogen molecule was adsorbed and heterolytically decomposed into hydrogen atoms on nitrogen-rich support and metallic Ni(0) surface. Finally, the C=O bond gets hydrogenated selectively to form FA by the attack of activated hydrogen atoms. Thus, once the furfuryl alcohol (FA) was prepared on the catalyst surface, it would leave the catalytic site rapidly for the new incoming reactant molecules. Additionally, the nitrogen-rich environment and metallic phase of nickel in combination facilitated the catalytic FFA hydrogenation in an aqueous medium. Transformation of FA to LA After understanding the reaction mechanism of FFA hydrogenation. Next, we have utilized the as-synthesized FA for the synthesis of LA. It was reported extensively in the literature, whereby simple hydrolysis of FA yields LA.11 In this report, we have chosen cation exchanged resin (IER-130) as a heterogeneous catalyst for FA hydrolysis in an aqueous medium (scheme 4). The IER-130 or INDION-130 was commercially available ion exchange resins and utilized



previously for the synthesis of industrially important molecule 3-methyl-5-phenylpentanol commonly known as “Mefrosol” by our group.61

Scheme 4. Hydrolysis of furfuryl alcohol. We have utilized its acidic property for the transformation of FA to LA by catalytic hydrolysis process. Typically, 5.7 mmol of FA, 5 mL of water/ THF mixture and 100 mg of catalyst (IER130) was added in a 50 mL RBF. The reaction temperature was increased to 100 oC while stirring at the rate of 600 rpm. The reaction was completed within 4 h with >99% conversion and selectivity, confirmed by 1H and 13C NMR (see ESI, Figure S7 and S8). Transformation of LA to GVL The LA synthesized by the hydrolysis process was further used for the synthesis of industrial important γ-valerolactone (GVL). Basically, the transformation of GVL from LA was carried out by hydrogenation in the presence of various precious and non-precious transition metals. In this report, we have performed this hydrogenation reaction by as-synthesized metallic nickel nanoparticles (scheme 5).

Scheme 5. Hydrogenation of levulinic acid.


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The hydrogenation of LA was carried out in 100 mL high-pressure reactor with molecular hydrogen as a green hydrogen source. The hydrogenation of LA was carried out by using 1000 µl (10 mmol) of LA, 35 mL of water, 500 psi H2 pressure and 150 mg of a metallic nickel catalyst at 100 oC. The conversion of LA was excellent with high selectivity of GVL, only in 6 h, confirmed by 1H and 13C NMR (see ESI, Figure S9 and S10). Furthermore, it is essential to investigate the reusability of the heterogeneous catalyst (Ni@N/C-g-800), which is an important part of any catalytic reaction. The recycling experiments of the catalyst was studied for FFA hydrogenation under above-optimized conditions. After completion of the reaction, the catalyst was recovered by the filtration method, followed by washing with acetone and drying at 70 oC. With the same treatment, we have tested Ni@N/C-g800 catalyst for five catalytic cycles and found no significant loss of its catalytic activity and selectivity (Figure 7).

Figure 7. Recyclability of the Ni@N/C-g-800 catalyst.



The recycled catalyst was further characterized by PXRD, HR-TEM, ICP and CHN analysis to understand it’s structural and surface properties. The PXRD analysis suggests, with continuous utilizing the catalyst crystallinity of the carbon material decrease. The decreasing peak intensity of Ni(0) nanoparticles after first and second catalytic cycle was due to shrinkage of metal nanoparticles. Moreover, the P-XRD pattern of the recycled catalyst was similar to the fresh catalyst hence, proves the stability of Ni(0) nanoparticle (Figure S3 c-d) where no oxide peak was detected. On TEM analysis, the particle distribution remained unchanged after consecutive cycles, which also proves that carbon coated Ni(0) nanoparticles were hindered towards agglomeration (Figure S3 c-d). Additionally, we have done ICP and elemental analysis for both the catalytic hydrogenation processes (FFA to FA and LA to GVL), which confirms that there is no metal leaching and structural damage on the catalyst (Table S1 and S2). These physicochemical analysis results illustrated that the Ni(0) supported catalyst was efficient, water tolerant and stable for the hydrogenation in the catalytic conditions. Conclusion In conclusion, metallic Ni nanoparticles on the nitrogen-rich carbon core-shell have been successfully synthesized by successive pyrolysis of Ni(II) impregnated CB[6]. The assynthesized catalyst was characterized by various analytical tools and utilized for the hydrogenation reaction of furfural in water. The effective Ni(0) catalyst exhibited very high activity with desired product selectivity in the water-mediated hydrogenation of furfural, an exciting biomass-derived chemical. The high catalytic activity of the system could be attributed to the metallic nickel nanoparticles with basic nitrogen-rich carbon support. Further, the possible mechanistic pathway was determined from the optimization and in-situ IR experiments of furfural. Moreover, the catalytic protocol was extended for the synthesis of γ-valerolactone,


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followed by hydrolysis of furfuryl alcohol and hydrogenation of levulinic acid intermediate in separate catalytic steps. Excitingly, the heterogeneous catalyst in all the catalytic steps helps for product separation with catalyst recovery and reusability. In short, the highly efficient catalytic protocols for the successive synthesis of γ-valerolactone from furfural using stable and reusable catalytic system may open up the new viewpoint towards the biomass transformation and utilization. Conflict of interest The authors declare no conflict of interest. Supporting Information Supporting information consist of P-XRD, SEM, TEM, ICP, CHNS, TGA and NMR characterization of products. Author Information Corresponding author *Email- [email protected] ORCID Sekhar Nandi- 0000-0001-5380-4537 Arka Saha- 0000-0002-0571-1835 Parth Patel- 0000-0001-9742-2784 Noor-ul H. Khan- 0000-0002-5204-5943 Rukhsana I. Kureshy- 0000-0001-6611-7605 Notes The authors declare no competing financial interest. Acknowledgments



CSIR-CSMCRI registration No is 068/2018. Authors are thankful to DST and CSIR for financial support under project GAP-2025 and HCP-0009. SN and AS are thankful to AcSIR for Ph.D. enrolment. PP is thankful to Charutar University for Ph.D registration. CSMCRI-ADCIF Division is highly acknowledged for all the characterization facilities. References 1. Jiang, Y.; Wang, X.; Cao, Q., Dong, L.; Guan, J.; Mu, X. Sustainable Production of Bulk Chemicals, 2015, Springer, pp 19-49. 2. Jakob, M.; Hilaire, J. Climate Science: Unburnable Fossil-Fuel Reserves. Nature, 2015, 517, 150–152. 3. Xu, G.; Han, J.; Ding, B.; Nie, P.; Pan, J.; Dou, H.; Li, H.; Zhang, X. Biomass-Derived Porous Carbon Materials with Sulfur and Nitrogen Dual-Doping for Energy Storage, Green Chem., 2015, 17, 1668–1674. 4. Meiri, N.; Dinburg, Y.; Amoyal, M.; Koukouliev, V.; Nehemya, R. V.; Landau, M. V.; Herskowitz, M. Novel Process and Catalytic Materials for Converting CO2 and H2 Containing Mixtures to Liquid Fuels and Chemicals, Faraday Discuss., 2015, 183, 197– 215. 5. Kamm, B. Production of Platform Chemicals and Synthesis Gas from Biomass, Angew. Chem. Int. Ed., 2007, 46, 5056–5058. 6. Huber, GW.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering, Chem. Rev., 2006, 106, 4044–4098. 7. Mariscal, R.; Maireles-Torres, P.; Ojeda, M.; Sádaba, I.; Granados, M. L. Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels, Energy Environ. Sci., 2016, 9, 1144-1189.


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TOC

Multi-catalysis

Aqueous medium


Milder conditions

Hydrogenation of Furfural with Nickel Nanoparticles Stabilized on

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