Selectivity-Switchable Conversion of Cellulose to Glycols over Ni–Sn

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Selectivity-Switchable Conversion of Cellulose to Glycols over Ni−Sn Catalysts Ruiyan Sun,†,‡ Mingyuan Zheng,*,† Jifeng Pang,† Xin Liu,† Junhu Wang,† Xiaoli Pan,† Aiqin Wang,† Xiaodong Wang,† and Tao Zhang*,† †

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, State Key Laboratory of Catalysis, Zhongshan Road 457, Dalian 116023, P.R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: The direct hydrogenolysis of cellulose represents an attractive and promising route for green polyol production. Designing a catalyst system that could control the selectivity of polyols of this process is highly desirable. In this work, we realized the selectivity-switchable production of ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG) by using Sn species with different valences in combination with Ni catalysts. The combination of Ni/AC and metallic Sn powders exhibited a superior activity toward EG (57.6%) with up to 86.6% total polyol yield, while the combination of Ni/AC and SnO favored the formation of 1,2-PG (32.2%) with a 22.9% yield of EG. The Sn species in NiSn alloy in situ formed from metallic Ni and Sn powders was found to be the active sites for the high selectivity of EG as evidenced by control experiments and characterizations including X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, energy dispersive X-ray mapping, and 119Sn Mössbauer spectroscopy. The effects of Sn loading, reaction temperature, reaction time, and the concentration of cellulose were investigated for Ni/AC + Sn powders. Because of the formation of NiSn alloy, the Ni−Sn catalyst showed good stability during repeated use. Experimental results disclosed that the Sn species with different valence possessed distinct catalytic functions. Both SnO and the alloyed Sn species could catalyze the retro-aldol condensation of glucose to glycolaldehyde, and meanwhile, SnO was also active for the isomerization of glucose to fructose. Therefore, controlling the glycol products distribution could be realized using SnO or the alloyed Sn species as catalysts. KEYWORDS: cellulose, ethylene glycol, propylene glycol, nickel, tin, valence

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INTRODUCTION

catalyst which fulfills catalytic functions for both retro-aldol condensation and hydrogenation.9 Mechanism studies showed that the active sites of above catalysts for the retro-aldol condensation of glucose to glycolaldehyde proved to be the tungsten bronze (HxWO3) species and La(III) cations that dissolved in water, which behaved as homogeneous catalysts.9,17 For the hydrogenation of glycolaldehyde to EG, heterogeneous Ni and Ru catalysts were usually adopted in this process. Additionally, other catalysts, such as Ru/C + WO3/Al2O3 + Cact,11 CuCr + Ca(OH)2,18,19 and Ni/ZnO20 catalysts, were explored for the conversion of cellulose and glucose, and the main product was 1,2-PG rather than EG. Obviously, these reports demonstrated that the required catalyst systems for conversion of cellulose to 1,2-PG are different from that for the conversion of cellulose to EG and a base (Cact, Ca(OH)2, and

The limited availability of fossil resources demands developing efficient catalysts and processes for conversion of renewable lignocellulosic feedstock into chemicals and fuels.1,2 In the past decade, great progress has been made regarding the valorization of cellulose, which is the primary component in lignocellulosic biomass, and not competing with food.3,4 For the production of value-added chemicals from cellulose, many catalytic routes have been developed,5−7 among which the direct hydrolytic hydrogenation of cellulose to glycols, e.g. ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG), has attracted extensive attention due to the ever-increasing market demand for the synthesis of polyester.8−13 In previous studies, a series of highly efficient catalysts including tungstenic (tungsten carbides,8 bimetallic catalysts,14 ammonium metatungstate,15 and tungstic acid16) and nickel− lanthanum17 catalysts were developed for the selective production of EG from cellulose. The conversion process involves cascade reactions and therefore requires a bifunctional © XXXX American Chemical Society

Received: August 17, 2015 Revised: November 10, 2015

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Table 1. Product Distribution for Cellulose Conversion over Different Ni Catalysts in Combination with Sn Speciesa yield (%)b entry

catalyst

conv (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

20% Ni/AC 20% Ni/AC + SnO2 20% Ni/AC + SnOd 20% Ni/AC + Sn powders 20% Ni/Al2O3+ Sn powders 20% Ni/MgO + Sn powders 20% Ni/ZnO + Sn powders 20% Ni/TiO2 + Sn powders 20% Ni/SiO2 + Sn powders 20% Ni/AC + Sn granules Ni−Sn(90)/AC 20% Ni/AC + Sn powderse 20% Ni/AC + Sn powdersf 20% Ni/AC + SnO2f 20% Ni/AC + SnOf

100 100 100 100 82.6 93.7 100 97.6 100 100 100 18.4 69.4 60.4 71.2

c

EG

1,2-PG

Gly

1,2-BD

Ery

hexitol

acetol

GC

LC

7.3 5.6 22.9 57.6 5.4 11.9 13.6 6.0 5.0 16.4 48.9 2.6 28.9 4.6 12.0

6.6 6.3 32.2 9.2 9.6 17.4 21.2 10.2 6.9 9.4 11.5 2.3 4.4 4.3 9.1

2.4 1.9 1.9 2.4 5.1 2.7 1.7 5.0 3.7 3.2 1.4 1.6 1.0 1.5 1.1

3.4 3.1 9.3 7.2 2.8 2.9 6.4 3.6 2.8 3.9 6.6 0.5 2.7 2.5 2.9

2.5 2.2 2.4 3.5 2.6 0.9 1.2 2.4 4.2 2.9 2.1 1.0 1.9 1.3 1.0

24.4 27.3 3.3 6.7 8.9 0.6 4.3 9.4 21.0 20.7 6.2 81.6 7.6 12.4 3.6

0.5 ND 2.6 ND 2.1 7.7 ND ND ND ND ND ND 1.3 2.9 24.7

2.1 1.5 2.1 5.4 4.1 4.9 1.4 2.4 1.2 3.0 1.0 1.0 0.8 0.7 1.2

87.7 83.6 80.0 92.0 77.6 85.6 88.4 94.0 98.5 84.2 87.1 98.0 57.3 49.5 58.5

a

Reaction conditions: Ni catalysts 0.1 g, Sn compounds 0.09 g, cellulose 0.25 g, H2O 25 mL, 518 K, 5 MPa H2, 95 min, 800 rpm. EG, 1,2-PG, Gly, 1,2-BD, and Ery are abbreviations for ethylene glycol, 1,2-propylene glycol, glycerol, 1,2-butanediol, and erythritol, respectively. Hexitol mentioned in this paper included sorbitol and mannitol. ND = not detected. bThe yields of polyols were calculated by using the equation: yield (C%) = (masses of carbon in the products)/(masses of carbon in cellulose charged into the reactor) × 100%; GC and LC indicate the total carbon in the gas and liquid products, respectively, and were calculated on the basis of carbon. cCellulose conversion (wt %) was calculated by the change of cellulose weight before and after the reaction. d0.18 g of SnO was added. e0.25 g of sorbitol was employed as substrate. fReaction time was 0 min.

resulting material was then reduced in a stream of H2 at 723 K for 2 h. Prior to exposure to air, the catalyst was passivated under a stream of 1% O2 in N2 for at least 5 h. Other nickel catalysts supported on metal oxides (Al2O3, ZnO, MgO, SiO2, and TiO2) were prepared by a similar process but the supports were changed to metal oxides and the reduction temperature was changed to 773 K. Ni−Sn/AC catalyst was prepared by a hydrothermal method. 20% Ni/AC (0.1 g) and a designed amount of metallic Sn powders were added to 25 mL of H2O, and the mixture was heated in a sealed autoclave (Parr Instrument Company) at 518 K for 20 min under stirring and in a H2 atmosphere. After the autoclave treatment, the resulting solid catalyst and the Sn residues were separated from the aqueous solution by magnetism and filtration and then dried at 393 K for 8 h. The Ni−Sn/AC catalysts were denoted as Ni− Sn(n)/AC, and n in the parentheses represents the amount (mg) of Sn powders put into the autoclave. Catalyst Characterization. X-ray diffraction (XRD) patterns were recorded on a PANalytical X’ Pert PRO diffractometer equipped with a Cu Kα radiation source (λ = 0.15432 nm), operating at 40 kV and 40 mA. XPS (X-ray photoelectron spectroscopy) signals were obtained on an ESCALAB-250 spectrometer equipped with a monochromated 150 W Al Kα source (Thermo VG Scientific Co., Ltd. USA). Binding energy (BE) values were referenced to the binding energy of the C 1 s core level (284.6 eV). A JEOL JEM-2100F microscope equipped with HAADF (high angle annular dark field) and Oxford detectors was employed to acquire the HRTEM (high-resolution transmission electron microscopy) and STEM (scanning transmission electron microscopy) images and energy dispersive X-ray (EDX) spectra under 200 keV. The Mössbauer spectra were recorded using 119Sn as the radiation source (BaSnO3 matrix), and the isomer shifts were shown with respect to BaSnO3 at room temperature. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was conducted on an IRIS intrepid II XSP instrument (Thermo Electron Corporation). The carbon

ZnO) is usually added to the catalyst system, due to its promotion effect in the isomerization of glucose to fructose. Other than the commonly used base catalyst, noble metal supported on solid Lewis acid catalysts such as Pt/AlW and PdWOx/Al2O3 were also found effective for the production of 1,2PG, which afforded a 20% yield and a 60.8% selectivity to 1,2PG, respectively.21,22 Deng et al. reported that, when using PtSnOx/Al2O3 as a catalyst, the conversion of cellulose with the product selectivity tunable from hexitol to C2 and C3 products could be achieved.23 Although various catalysts have been developed for the conversion of cellulose to EG and 1,2-PG and some efforts have been made to control product selectivity from hexitol to C2−C3 polyols, effectively controlling the selectivity of EG and 1,2-PG over one single catalyst system is still a great challenge. In view of the practical application, it is of great significance to realize the flexible switching of products according to market demand. Herein, we designed Ni−Sn catalysts which could not only convert cellulose to EG with high selectivity (57.6%) but also switch the selectivity between EG and 1,2-PG by changing the Sn valence in the Ni−Sn catalysts. The active site of Ni−Sn catalysts as well as the functions of Sn species in different valence in the conversion of cellulose to EG and 1,2-PG are elucidated in detail.

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EXPERIMENTAL SECTION

Preparation of Catalysts. Activated carbon (AC, SBET = 1203 m2/g) was supplied by Beijing Guanghua-Jingke Activated Carbon Co. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was purchased from Sinopharm Chemical Reagent Co. Sn powders (100 meshes), Sn granules (10 meshes), stannous oxide (SnO), and tin oxide (SnO2) were purchased from Aladdin Chemical Co. Raney Ni was purchased from Anshan Zhongli catalyst factory. Nickel catalyst (20% Ni/AC) was prepared according to the incipient wetness impregnation method using an aqueous solution of Ni(NO3)2 as a precursor and active carbon as a support, followed with drying at room temperature overnight and a period of heating at 393 K for 8 h. The 192

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1,2-PG by hydrogenation over Ni/AC.24 Therefore, these comparison results could further identify that the conversion of cellulose over Ni/AC + SnO and Ni/AC + Sn powders followed the different pathway, which realized the control of EG and 1,2-PG selectivity. In addition, the nickel catalysts supported on different carries also imposed notable effects on the product selectivity as they were used in combination with metallic Sn powders. Compared to the Ni/AC, all the catalysts of Ni/Al2O3, Ni/MgO, Ni/ZnO, Ni/TiO2, and Ni/SiO2 (Table 1, entries 5−9) showed much lower activity toward EG and 1,2-PG formation. Among them, Ni/ZnO and Ni/MgO showed notable promotion effects on EG and 1,2-PG formation, which should be ascribed to the catalytic function of ZnO and MgO supports20 rather than the addition of Sn powders. Active Site of Ni−Sn Catalyst. On the basis of reaction mechanism of cellulose conversion to 1,2-PG,24−26 we proposed that SnO was active for the isomerization of glucose to fructose and the subsequent retro-aldol condensation of fructose to C3 products which resulted in 1,2-PG dominant over the EG production (more detailed discussion on the reaction mechanism was presented in a latter section). This is consistent with the conjecture on the catalytic function of SnOx in Liu’s work.23 Surprisingly, it was noted that much high yield of EG but low yield of 1,2-PG were obtained over Ni/AC + Sn powders. Metallic Sn seemed to provide the active site for the production of EG. However, when metallic Sn granules instead of Sn powders were used in the reaction (Table 1, entry 10), the yields of EG and 1,2-PG only increased slightly as compared with that of the sole nickel catalyst used. Thus, this excluded the possibility that metallic Sn and the soluble Sn cations possibly released from metallic Sn powders or granules acted as the active site. These results pose a question: what is the genuine active site in the composite catalyst of Ni/AC and Sn powders for the production of EG with a high yield? In order to find factors responsible for the promoting effect of metallic Sn powders introduced, the spent catalysts of Ni/ AC and Ni/metal oxide (Ni supported on metal oxides) were characterized by XRD after they were separated from Sn powders by filtration and magnetism (denoted as Ni/AC (spent, powder) and Ni/metal oxide (spent, powder)). As shown in Figure 1a, the intensity of Ni crystalline diffraction peaks (2θ = 44.5° and 51.8°) of all used Ni/AC catalysts increased, indicating the agglomeration of Ni particles after reaction probably due to the Ostwald ripening of the Ni particles under hydrothermal conditions. In addition, a set of new diffraction peaks at a 2θ of 30.6°, 43.3°, 54.9°, 57.5°, 59.6°, and 63.6° appeared in the XRD patterns of Ni/AC (spent, powder), which can be assigned to the alloy phase of NiSn (atomic ratio = 1:1) (JCPDS: 03-1004). In contrast, no obvious diffraction peaks of NiSn alloy were found on the Ni/AC (spent, granule) which was derived from Ni/AC + Sn granules, and nor on the Ni/metal oxide (spent, powder) (Figures 1b and c). Such little difference in the NiSn alloy formation might be the clue to identifying the active site. For the purpose of verifying our conjecture that NiSn alloy was the active site for the conversion of cellulose to EG, we prepared the Ni−Sn(90)/AC catalyst by hydrothermally treating the Ni/AC in the presence of Sn powders (90 mg) under the same reaction conditions but without adding cellulose substrate. The Ni−Sn(90)/AC catalyst was characterized by different techniques (XRD, HRTEM, STEM-EDX, XPS, and Sn Mössbauer) for achieving the information on the

content of the liquid products was determined by total organic carbon (TOC) analysis on a Vario EL III element analyzer (Elementar). Chemisorption measurements of CO were taken on a Quantachrome Autosorb-1C automated catalyst characterization system equipped with a thermal conductivity detector. Catalytic Reaction. The catalytic conversion of microcrystalline cellulose (ACROS, extra pure with an average particle size of 90 μm) was performed in a stainless-steel autoclave (Parr Instrument Company). Typically, 0.25 g of cellulose, a proper amount of catalyst, and 25 mL deionized water were placed in a 75 mL autoclave, and the resulting mixture was stirred at 800 rpm and 518 K under 5 MPa of H2 pressure (measured at room temperature). For the recycling test, the catalysts were recovered after each run by magnetic separation and washed several times with deionized water; then, they were put into the reactor for the next reaction together with the same amounts of cellulose and water used in the first run. The liquid-phase products were analyzed by HPLC on an Agilent 1200 system equipped with a Phenomenex Rezex RQAOrganic Acid H+ (8%) column and a differential refractive index detector (RID). HPLC analysis was conducted at a column temperature of 318 K using water as the mobile phase with a flow rate of 0.5 mL min−1. The samples were injected with an injection volume of 5.0 μL. Gaseous products were analyzed by GC on an Agilent 6890N system equipped with a TDX-1 column and a TCD. Cellulose conversions were determined by measuring the difference in the cellulose weight before and after the reaction. The polyols yields were calculated based on the amount of carbon according to the equation: yield (C%) = (mass of carbon in the products)/(mass of carbon in the cellulose charged into the reactor) × 100%. The carbon content of the cellulose fed into the reactor was 4230 ppm.

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RESULTS AND DISCUSSION Catalytic Performance of Ni−Sn Catalysts. As shown in Table 1, in combination with nickel catalysts, different Sn species showed different catalytic performance. In detail, compared to the blank experiment in which 20% Ni/AC was solely used (Table 1, entry 1), the yields of polyols remain almost unchanged over the Ni/AC + SnO2, and hexitol was the main product (Table 1, entry 2). However, the addition of metallic Sn powders or SnO to Ni/AC significantly changed the products distribution. In the presence of metallic Sn powders, 57.6% EG yield and 9.2% 1,2-PG yield were obtained after 95 min reaction at 518 K (Table 1, entry 4). The total yield of EG and 1,2-PG reached up to 66.8%, which was comparable to the best result of Ni−W/SBA-15 (69.5%, carbon yield) in the conversion of cellulose to EG.14 In contrast, the addition of SnO afforded 22.9% EG yield and 32.2% 1,2-PG yield (Table 1, entry 3), which was nearly 5-fold increase in 1,2-PG selectivity (32.2% vs 6.6%). Evidently, Sn species in different valence showed different synergistic effect with Ni/AC catalyst and gave different polyols distribution. To more convictively show the difference in the reaction selectivity of the three catalysts, we decreased the conversion of cellulose to 60−70% through reducing the reaction time to 0 min (the point of heating to 518 K). It was found that the product distributions over Ni/AC + Sn powders and Ni/AC + SnO2 (Table 1, entries 13 and 14) were still similar to that when the conversions of cellulose were 100% (Table 1, entries 2 and 4). Differently, over Ni/AC + SnO, the main product was acetol at the low conversion of cellulose (Table 1, entry 15). As reported in our previous work, acetol is the precursor of 1,2-PG which could be transformed to 193

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peak of Ni and (101) peak of NiSn alloy. In the fresh Ni/AC catalyst, the calculated Ni crystallite size was 7.2 nm. Evidently, the crystallite size of unalloyed Ni sharply increased to 6 times larger while that of the alloyed NiSn was merely doubled. It could be concluded that the addition of Sn retarded the sintering of Ni particles, which was also found in the Sn modified Raney Ni catalysts as reported in the literature.27 TEM image of Ni/AC and STEM images of Ni−Sn(90)/AC catalyst are shown in Figure 2. For the Ni/AC catalyst (Figure 2a), metallic Ni particles had a high dispersion throughout AC with the mean particle size of 9.7 ± 1.8 nm. However, for the Ni−Sn(90)/AC catalyst, there were two kinds of different particle size distributions. In one case as shown in Figure 2b, the metal particles were homogeneously distributed with the average particle size of 20.6 ± 6.1 nm, which was slightly larger than the data (17.2 nm) obtained from XRD. In the other case (Figure 2c), the metal particles were nonuniformly distributed with the particle size in a range of 10 to 150 nm. To get a clear vision of the elemental distribution of Ni and Sn on the Ni− Sn(90)/AC catalyst, these two representative regions (Figures 2b and c) were subjected to EDX mapping (Figure 3). In the Ni and Sn EDX mapping images (Figures 3a and b) of the whole zone in Figure 2b, it can be observed that Sn species was mixed and coexisted closely with Ni species and the Ni/Sn mole ratio was 1.0 (in line with the nominal ratio of NiSn alloy) calculated form the EDX results (Figure S1). The HRTEM image of a single particle selected from Figure 2b and the corresponding fast Fourier transform (FFT) image (inset in Figure 2d) are displayed in Figure 2d. The calculated lattice distances were 2.0, 2.1, and 2.1 Å, respectively, which well matched the d-spacing of (110), (01̅2), and (102) planes of hexagonal crystalline structure of NiSn alloy phase. The above results provided direct evidence to demonstrate the formation of NiSn alloy particle in the Ni−Sn(90)/AC catalyst. As for the red box zone in Figure 2c, the Ni and Sn EDX mapping results (Figures 3c and d) clearly showed that Ni was accompanied by Sn except for the green box region. The EDX analysis results (Figure S2) showed that the content of Sn in the green box zone was zero, which indicates not all Ni particles alloy with Sn and there are some metallic Ni particles retained (around 15 nm) in the Ni−Sn(90)/AC catalyst. Besides, the EDX line scan image of the individual large particle showed that the Ni element was rich in the particle center (Figure 3e). The Ni/Sn mole ratio of this particle was higher than the nominal NiSn alloy (2.6 vs 1.0), indicating that this NiSn particle was not homogeneously alloyed (Figure S3). Taking into account the XRD results (average crystalline size of Ni was 45.2 nm), we proposed that on large particles of Ni−Sn, NiSn alloy most likely locates on the surface while metallic Ni constitutes the core of particle to form a core−shell like structure. The XPS spectra of Ni−Sn(90)/AC with different etching time are displayed in Figure 4, and the composition of Ni− Sn(90)/AC analyzed by XPS and ICP are summarized in Table 2. Two groups of peaks were detected both on the surface and in the bulk. The peaks with binding energy at 486.9 ev (Sn 3d5/2) and 496.3 ev (Sn 3d3/2) are assigned to the oxidized Sn species (Sn(II, IV)), and the other group of peaks with binding energy at 484.9 ev (Sn 3d5/2) and 493.1 ev (Sn 3d3/2) are attributable to the reduced Sn species (Sn(0)).28,29 The contents of Sn and Ni in the Ni−Sn(90)/AC were quantified by the spectra area of Sn 3d peaks and Ni 2p peaks. With the etching time increasing from 0 to 420 s, the mole ratio of Ni/Sn increased from 2.3 to 3.2, which reveals that the amount of Sn

Figure 1. XRD patterns of different fresh and spent catalysts: (a) Ni/ AC and Ni−Sn(90)/AC, (b) Ni/ZnO, Ni/TiO2, and Ni/SiO2, (c) Ni/ MgO and Ni/Al2O3.

catalyst composition, structure, and chemical state of Sn species. The XRD pattern of Ni−Sn(90)/AC was presented in Figure 1a. Similar to the XRD peaks of Ni/AC (spent, powder) (Figure 1a), Ni−Sn(90)/AC showed significantly narrowed diffraction peaks of Ni accompanied by the weak peaks of NiSn alloy, which indicates coexistence of large Ni particles and small NiSn particles on the AC support. The crystallite sizes of Ni and NiSn were ca. 45.2 and 17.2 nm, respectively, estimated by using the Debye−Scherrer equation according to the (111) 194

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Figure 2. TEM image of 20% Ni/AC (a), STEM images of Ni−Sn(90)/AC (b, c) and HRTEM image of a single NiSn alloy particle (d).

between Ni and Sn in unsupported NiSn alloy was also observed in the XPS spectra of Ni 3d near the Fermi level by Onda et al.34 The electron state and chemical properties of alloyed Sn in the NiSn alloy are slightly modified by nearby Ni atoms, that is to say the alloyed Sn species actually show a little positive rather than zero valence, which might endow it a different catalytic property from the monometallic Sn. As for the catalytic performance of presynthesized Ni− Sn(90)/AC in cellulose conversion (Table 1, entry 11), it was very similar to that of the physically mixed Ni/AC and Sn powders. The above characterization results have shown that NiSn alloy was finely formed in Ni−Sn(90)/AC after hydrothermal treatment. Albeit SnO2 was also found in the Ni−Sn(90)/AC, the catalytic activity of SnO2 toward EG was rather poor as shown in Table 1, entry 2. Therefore, all of these results provide strong evidence to confirm that the alloyed Sn species in NiSn alloy played a crucial catalytic role in the EG production. Effect of Reaction Conditions. To gain more insight into the catalytic performance of the mechanical combination of Ni/ AC and Sn powders, the effects of reaction conditions including reaction temperature, time, the amount of Sn powders, and cellulose concentration were investigated. The effect of reaction temperature on the cellulose conversion and products selectivity is presented in Figure 6. The total selectivity of EG and 1,2-PG increased as the temperature increasing from 488 to 518 K at the expense of hexitol selectivity. When the temperature increased to 503 K, the selectivity of hexitol decreased drastically while the selectivity of EG increased drastically. Cellulose was completely converted at 518 K, which was the optimum temperature for the production of EG and 1,2-PG. The hydrogenolysis of polyols to gases accounted for

species locating on the surface was more than that in bulk. This is in agreement with the STEM-EDX results that Sn was enriched on the surface of some large particles (Figure 3e). The ICP measurement showed that the whole Ni/Sn mole ratio of Ni−Sn(90)/AC was 4:1, which is much higher than the nominal ratio 1:1 of NiSn alloy. Therefore, it could be speculated that not all Ni particles formed NiSn alloy with Sn. That is to say that some NiSn alloy particles might be formed nonuniformly or some metallic Ni particles might exist on the Ni−Sn(90)/AC. This is very consistent with the STEM results. Besides, the surface mole ratio of Sn(0)/Sn(II, IV) was 0.3 and the bulk mole ratio of Sn(0)/Sn(II, IV) increased to 1.8, suggesting that the surface of NiSn alloy was partially oxidized to the Sn(II, IV) species due to the XPS measurement was conducted under ex-situ reaction conditions. Since Sn(II) and Sn(IV) cannot be distinguished by XPS,30 119 Sn Mössbauer spectroscopy was used to determine the chemical environment of the Sn species in the Ni−Sn(90)/AC due to its high sensitivity to the different states of Sn (Figure 5). The value of isomer shift (IS) is closely related to the valence state of Sn and can act as an indicator of the density of s electron cloud. As shown in Figure 5, the doublet with an IS of approximately 1.8 was attributed to the NiSn alloy and the singlet with an IS of about −0.02 was attributed to the Sn (IV).31,32 The relative spectral areas of NiSn alloy and Sn(IV) were 85.6 and 14.4%, respectively. These results indicate that the oxidized Sn species observed in XPS should be assigned to Sn (IV) (i.e., SnO2) and the Ni−Sn(90)/AC catalyst largely consisted of NiSn alloy with small amount of SnO2. In addition, the IS value of Sn in NiSn alloy is lower than that of monometallic Sn (1.8 vs 2.5),31 suggesting that some Sn 2s orbital electron transferred to Ni.33 The electron transfer 195

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Table 2. Composition of Ni−Sn(90)/AC Catalyst Measured by Different Methods XPS analysis Sn(0)/Sn(II, IV) (mole ratio) Ni/Sn (mole ratio) Sn (wt %) Ni (wt %)

Figure 5.

119

0s

180 s

420 s

0.3 2.3

1.5 2.5

1.8 3.2

ICP analysis 4.0 8.6 17.0

Sn Mössbauer spectra of Ni−Sn(90)/AC catalyst.

Figure 3. EDX images of Ni−Sn(90)/AC: (a) Ni EDX mapping of the Figure 2b, (b) Sn EDX mapping of the Figure 2b, (c) Ni EDX mapping of the selected red box area in Figure 2c, (d) Sn EDX mapping of the selected red box area in Figure 2c, (e) EDX line scans across a large NiSn alloy particle as dedicated by the white line. Figure 6. Effect of reaction temperature on the product selectivity for the hydrolytic hydrogenation of cellulose over Ni/AC + Sn powders. Reaction conditions: Ni/AC 0.1 g, Sn powders 0.09 g, cellulose 0.25 g, H2O 25 mL, 95 min, 5 MPa H2, 800 rpm.

Figure 7 shows the time dependence of cellulose conversion to EG and 1,2-PG over Ni/AC + Sn powders at 518 K. The yield of EG and 1,2-PG first increased and then leveled off at 57.6 and 9.2% with the reaction time increasing from 0 to 95 min, respectively. Interestingly, it can be noticed that both the yield of EG and the conversion of cellulose increased drastically during the heating ramp of the reactor from the temperature of 503 to 518 K in 16 min. The temperature of 503 K was close to the melting point of Sn powder (505 K). It was reported that the diffusion of molten Sn into the Ni particles to form NiSn alloy occurred more quickly than the solid Sn.27,35 Therefore, the formation of NiSn alloy would be greatly promoted when the temperature increased to 503 K, at which the conversion of cellulose to EG would be initiated. Thus, the NiSn alloy catalyst showed the highest activity from 503 to 518 K (0 min). The formation of NiSn alloy was accelerated at higher than 503 K

Figure 4. XPS spectra of Sn 3d level for Ni−Sn(90)/AC with different etching time.

the decrease in the total selectivity of EG and 1,2-PG at higher temperatures (above 518 K). 196

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Table 3. CO Uptake and the Loading of Ni and Sn on Ni− Sn/AC with Different Amounts of Sn Powders

a

catalyst

amount of CO adsorption (μmol g−1)

Sn (wt %)

Ni (wt %)

Ni/Sn (mole ratio)a

Ni−Sn(20)/AC Ni−Sn(90)/AC Ni−Sn(170)/AC Ni−Sn(290)/AC

513.1 270.0 89.6 24.4

1.4 8.6 11.3 16.0

17.3 17.0 18.0 18.3

25.0 4.0 3.1 2.3

a

a

Determined by ICP.

The XRD patterns of these synthesized catalysts showed that the diffraction peaks of NiSn alloy were observed (Figure S4), revealing that NiSn alloy phase was formed. It was reported that the formation of NiSn alloy significantly suppressed the adsorption of CO on the metallic Ni sites.27,36,37 Therefore, for Ni−Sn/AC catalysts, the CO uptakes largely represent the amount of unalloyed metallic Ni sites on the catalyst. As shown in Table 3, the CO uptakes decreased from 513.1 to 24.4 μmol g−1 as the amount of Sn powders increasing from 20 to 290 mg, which indicated that more NiSn alloy was formed at expense of metallic Ni. The hydrogenation activity of NiSn alloy was much lower than that of metallic Ni.34,38 For the conversion of cellulose to EG, adequate hydrogenation activity is required. Otherwise, the unsaturated reaction intermediate would undergo side reactions to form humins, etc.9,14 The point of 90 mg Sn powders (Ni/Sn mole ratio was 4.0) seemed to be the best amount to meet the balance between sugars cracking and intermediate hydrogenation, which gave the highest yield of total polyols up to 86.6%. Further increasing the amount of Sn powders to 290 mg produced overmuch NiSn alloy on the catalyst, which greatly depressed the activity for hydrogenation and led to the decrease in yields of EG and total polyols. The effect of the cellulose concentration on product yield was shown in Table S1. The yield of EG slightly decreased from 55.7 to 49.1% with the increase in cellulose concentration from 1 to 3 wt %. Further increasing the cellulose concentration to 6 wt % remarkably deceased the yield of EG to 13.9%, accompanied by the formation of large amount of humins in the solution. This might be related to the insufficiency of catalyst amount for the reaction.39 When we increased the amounts of Ni/AC (0.25 g) and Sn powders (0.24 g) for conversion of 6 wt % cellulose, the yield of EG and 1,2-PG was largely recovered to 40.8 and 7.5%, respectively. Therefore, when the ratio of Ni−Sn catalyst amount and cellulose concentration was kept in a suitable range, a high yield of EG could be obtained in the conversion of high-concentration cellulose. Reusability of Raney Ni + Sn Powders. The stability of catalyst is a crucial character for practical application. Generally, Ni based catalysts undergo fast deactivation under hydrothermal conditions40 except for skeletal Ni catalysts (Raney Ni), which is much more stable against hydrothermal attack than Ni/AC.41 Hence, Raney Ni instead of Ni/AC in combination with Sn powder was employed to conduct the recycling experiments. As shown in Figure 9, over four consecutive runs the total yield of EG and 1,2-PG merely decreased slightly from 64.5 to 50.1%, suggesting that the catalytic performance is relatively stable. The ICP results showed that only a little amount of Ni species (2−6 ppm) and less than 1 ppm of Sn species leached into the solution during each recycle (Table S2). This is quite different from the

Figure 7. Time course of the cellulose conversion to EG and 1,2-PG over Ni/AC + Sn powders. Reaction conditions: Ni/AC 0.1 g, Sn powders 0.09 g, cellulose 0.25 g, H2O 25 mL, 518 K, 5 MPa H2, 800 rpm.

and led to the drastic increase in the selectivity of EG and drastic decrease in the selectivity of hexitol. This could account for the effect of reaction temperature on the cellulose conversion and products selectivity as mentioned above. Figure 8 shows the effect of the amount of Sn powders for the hydrolytic hydrogenation of cellulose. The amount of Sn

Figure 8. Effect of the amount of Sn powders on the product distribution for the conversion of cellulose to polyols. Reaction conditions: Ni/AC 0.1 g, cellulose 0.25 g, H2O 25 mL, 518 K, 95 min, 5 MPa H2, 800 rpm.

powders notably influenced the polyols yields and afforded a volcano shape of EG yield. With increasing the amount of Sn powders from 0 mg to 290 mg, the total yield of EG and 1,2PG monotonically increased to the highest value of 66.8% at a point of 90 mg Sn powders and then decreased gradually. In contrast, the yield of hexitol decreased continuously from 24.4 to 3.1% with the Sn amount increase. The trend for the yield of total polyols was the same as that of EG. Four Ni−Sn/AC catalysts derived from different amounts of Sn powders added (20, 90, 170, 290 mg) were characterized. From results of ICP analysis for these catalysts (Table 3), it could be seen that the increase in the amount of Sn powders from 20 to 290 mg led to the Sn loading increasing from 1.4 to 16.0% and the Ni/Sn mole ratio decreasing from 25.0 to 2.3. 197

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ACS Catalysis

was rather stable and the total yield of EG and 1,2-PG was as low as 4.9% (Table 1, entry 12). This result ruled out the possibility of sorbitol degradation to EG and 1,2-PG over the Ni−Sn catalyst. Accordingly, on the basis of the above conclusion that the alloyed Sn species in NiSn alloy was the active site in this process, we proposed that the NiSn alloy particles were active for the retro-aldol condensation of glucose to glycolaldehyde and the unalloyed Ni particles were responsible for the hydrogenation of glycolaldehyde to EG. To validate our viewpoint, we monitored the intermediates of cellulose conversion to EG over Ni/AC + Sn powders as a function of reaction time (Figure 7). Notably, glycolaldehyde was found in this reaction. Its yield got to the highest (5.9%) at 10 min and then decreased to 0 rapidly due to the hydrogenation to EG proceeding much more quickly than the retro-aldol condensation.43,44 The variation of glycolaldehyde yield with reaction time confirms it as an intermediate in the conversion of cellulose to EG over the Ni/AC + Sn powders. Therefore, the above findings manifest our hypothesis that glycolaldehyde is produced over NiSn alloy. Clearly, the conversion of cellulose to EG over Ni/AC + Sn powders follows the first pathway (i), which is very similar to that of tungsten-based catalyst. The conversion of cellulose to 1,2-PG was believed to be a multistep cascade reaction, involving the hydrolysis of cellulose to glucose, the isomerization of glucose to fructose, the retroaldol condensation of fructose to 1,3-dihydeoxyacetone and glyceraldehyde, hydrodeoxygenation to form acetol, and the final hydrogenation to 1,2-PG (Scheme 1iii).24−26 To confirm the presence of above intermediates in the reaction, the time course of cellulose conversion to 1,2-PG over Ni/AC + SnO was studied. As shown in Figure 10, acetol was present in products and reached the yield summit as high as 24.7% at the starting time of 518 K, and then gradually decreased to 2.4% with the process of the reaction. In contrast, the yield of 1,2-PG monotonically increased to 32.2%, at the expense of acetol

Figure 9. Recycling test of Raney Ni + Sn powders in the conversion of cellulose to EG and 1,2-PG. Reaction conditions: Raney Ni 0.1 g, Sn powders 0.06 g, cellulose 0.25 g, H2O 25 mL, 518 K, 5 MPa H2, 120 min, 800 rpm.

previous catalysts including Ni−W and Ni−La, in which tungsten and lanthanum ions readily leach into the solution in remarkably larger amounts (hundreds of ppm) and play homogeneous catalytic roles in the cellulose conversion.17,41 The Ni−Sn catalyst most likely acts as a stable heterogeneous catalyst even under a harsh hydrothermal reaction environment, which is a very attractive property for the industrial application. Reaction Pathway. Generally, there are two pathways for the conversion of cellulose to EG. One pathway includes the retro-aldol condensation of glucose (derived from cellulose hydrolysis) to glycolaldehyde followed by the hydrogenation to EG (Scheme 1i);16 the other pathway is the hydrogenolysis of hexitol (production of glucose hydrogenation) to EG and 1,2PG (Scheme 1ii).17,42 We conducted the conversion of sorbitol in the presence of Ni/AC + Sn powders and found that sorbitol

Scheme 1. Proposed Reaction Pathway for the Conversion of Cellulose to EG and 1,2-PG over Ni−Sn Catalysts

198

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Figure 10. Time course of the cellulose conversion to 1,2-PG and EG over Ni/AC + SnO. Reaction conditions: 20% Ni/AC 0.1 g, SnO 0.18 g, cellulose 0.25 g, H2O 25 mL, 518 K, 5 MPa H2, 800 rpm.

yield. Therefore, the conversion of cellulose to 1,2-PG over Ni/ AC + SnO should follow route iii in Scheme 1.

■

DISCUSSION We confirmed that the active site for the selective production of EG from cellulose was not the metallic Sn powders but the in situ formed alloyed Sn species in NiSn alloy. The readily formed NiSn alloy from metallic Ni particles and Sn powders was responsible for the high selectivity to EG over Ni/AC + Sn powder. However, the reason for the low activity of Ni/AC + Sn granule and Ni/metal oxides + Sn powders toward EG was still unclear. Thus, XPS measurements for these two spent catalysts were carried out and the results were compared with the Ni/AC (spent, powder) (Figure 11). The Ni/AC (spent, powder), Ni/AC (spent, granule), and Ni/Al2O3 (spent, powder) were separated from Sn powders before subjecting to XPS measurements. The relative amounts of Sn(0) and Sn(II, IV) in each catalyst were obtained according to the spectra area of Sn(II, IV) and Sn(0) and shown in Table 4. Two different chemical states of Sn existed in all catalysts, i.e., Sn(0) and Sn(II, IV). For the Ni/Al2O3 (spent, powder), Sn(II, IV) accounted for the majority of Sn both in bulk (54.5%) and on the surface (64.4%), and the Sn atomic percent was much lower than that of Ni/AC (spent, powder).These results indicated that NiSn alloy was difficult to form and Sn(II, IV) was dominant in Ni/Al2O3 (spent, powder). Similarly, the Sn atomic percent in Ni/AC (spent, granule) was far lower than that in Ni/AC (spent, powder) regardless of in bulk or on the surface of catalyst. The Ni/Sn mole ratio was as high as 32.0 on the surface and 19.0 in bulk of Ni/AC (spent, granule), which was much more than that (1.5 on the surface and 2.4 in bulk) of Ni/AC (spent, powder). The high Ni/Sn mole ratio and low Sn atomic percent for the Ni/AC (spent, granule) suggested the main composition in the used Ni/AC was metallic Ni rather than NiSn alloy, which was in accord with the XRD results (Figure 1a). From these results, it could be supposed that the low activity toward EG in the presence of Ni/metal oxides + Sn powders or Ni/AC + Sn granules mainly resulted from too low content of NiSn alloy formed in these catalysts. The strong interaction between metal oxides (e.g., Al2O3) and Ni particles would inhibit the formation of NiSn alloy.45 Differently, compared to

Figure 11. XPS spectra of Sn 3d level for the spent Ni/AC and Ni/ Al2O3 with etching time at (a) 0 and (b) 420 s.

Table 4. Atomic Percent of Sn and Ni Measured by XPS at Different Etching Times relative percentage of Sn(0) and Sn(II, IV) (%)

sample Ni/AC (spent, powder) Ni/AC (spent, granule) Ni/Al2O3 (spent, powder)

Ni atomic percent (%)

Ni/Sn (mole ratio)

etching time (s)

Sn(0)

Sn(II, IV)

Sn atomic percent (%)

0 420

24.1 66.0

75.9 34.0

1.1 2.1

1.7 5.1

1.5 2.4

0 420

25.7 67.3

74.3 32.7

0.1 0.3

3.2 5.7

32.0 19.0

0 420

35.6 45.5

64.4 54.5

0.4 0.6

1.7 3.4

4.3 5.7

the Sn powders (100 meshes), the extremely low surface area of Sn granules (10 meshes) might account for the much less formation of NiSn alloy under the reaction conditions. Catalytic Function of Sn Species with Different Valence. As discussed above, the Sn species with different valence in combination with Ni catalysts showed distinctly different catalytic performance in the transformation of cellulose. The main products were EG, 1,2-PG and hexitol over Ni−Sn(90)/AC, Ni/AC + SnO and Ni/AC + SnO2, respectively, which were produced through the intermediates of 199

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ACS Catalysis glycolaldehyde, acetol and glucose correspondingly. To illuminate the different catalytic functions of Sn species, the conversion of cellulose over Ni−Sn(90)/AC, SnO2, and SnO without hydrogenation catalyst were carried out under the same reaction conditions (Table 5). In the presence of SnO2,

retro-aldol condensation and isomerization reaction. This promising Ni−Sn catalyst may provide interesting guidance for designing a catalyst capable of rationally controlling the product distributions in the transformation of biomass to polyols.

Table 5. Yields of Main Intermediates in the Cellulose Conversion over Different Sn Species without Hydrogenation Catalysta

S Supporting Information *

■

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01807. XRD patterns and EDX analysis of Ni−Sn/AC (Figures S1−S4). Results of higher concentration cellulose conversion (Table S1). ICP results (Table S2) (PDF)

yield (%) catalyst

conv (%)

glucose

glycolaldehyde

acetol

SnO2 SnO Ni−Sn(90)/ACb

72 69 75

13.4 4.1 4.4

2.6 5.6 7.3

2.7 20.0 3.9

ASSOCIATED CONTENT

■

a

Reaction conditions: Catalyst 0.05 g, cellulose 0.25 g, H2O 25 mL, 518 K, 2 MPa H2, 10 min, 800 rpm. bThe reaction was conducted under 2 MPa N2.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.Z.). *E-mail: [email protected] (T.Z.).

considerable amount of glucose (13.4%) was produced but the yield of acetol (2.7%) and glycolaldehyde (2.6%) was very low. We speculated that the little amount of acetol and glycolaldehyde should arise from the catalysis of subcritical water26 on the basis of the fact that the catalytic performance of SnO2 was the same as the blank test (Table 1, entries 1 and 2). This further verified the result that SnO2 was inactive for the production of EG and 1,2-PG from cellulose. For the Ni− Sn(90)/AC, the yield of acetol was not nearly promoted but the yield of glycolaldehyde was 3 times higher than that over SnO2. On the contrary, the yield of acetol and glycolaldehyde over SnO was 7 and 2 times higher than those over SnO2, respectively. These results implied that SnO were not only active for the retro-aldol condensation but also could promote the isomerization of glucose to fructose probably due to the Lewis acidity of SnO·nH2O (SnO easily hydrated to SnO·nH2O under hydrothermal conditions),46,47 like the case of Nb2O5· nH2O.48 Unlike SnO, the alloyed Sn species in NiSn alloy only possesses the catalytic function of retro-aldol condensation, thus enabling the Ni−Sn(90)/AC to afford a high yield of EG.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (21376239, 21306191, and 21176235). REFERENCES

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CONCLUSIONS In summary, we have developed a highly effective Ni−Sn catalyst for controlling the product selectivity in the transformation of cellulose to EG and 1,2-PG. The main products could be switched from EG (57.6%) to 1,2-PG (32.2%) using metallic Sn powders or SnO in combination with Ni/AC, respectively. The active site for the high selectivity of EG over Ni/AC + Sn powders proved to be the alloyed Sn species in NiSn alloy by experimental characterizations and conditional experiments. Two kinds of metallic sites were found on the Ni−Sn/AC, one of which was the alloyed Sn species responsible for the retro-aldol condensation of glucose to glycolaldehyde and the other was unalloyed metallic Ni responsible for the hydrogenation of glycolaldehyde to EG. The loading of Sn controlled the balance between the retroaldol condensation and the following hydrogenation reaction, and the best performance (the total polyols yield up to 86.6%) was obtained at a Ni/Sn mole ratio of 4.0. The stability of Ni− Sn alloy catalyst was reasonably good owing to the rather low leaching amounts of Ni and Sn species in the solution. In addition, through the controlled experiments we concluded that the alloyed Sn species was effective for the retro-aldol condensation of sugars, while SnO could catalyze both the 200

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