One-Pot Transformation of Cellulose to Sugar Alcohols over Acidic

Article pubs.acs.org/IECR

One-Pot Transformation of Cellulose to Sugar Alcohols over Acidic Metal Phosphates Combined with Ru/C Qi-Ying Liu,† Yu-He Liao,†,‡ Tie-Jun Wang,*,† Chi-Liu Cai,† Qi Zhang,† Noritatsu Tsubaki,§ and Long-Long Ma*,† †

CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, P. R. China ‡ University of Chinese Academy of Sciences, Beijing, 100049, P. R. China § Department of Applied Chemistry, School of Engineering, Toyama University, Toyama, 9308555, Japan S Supporting Information *

ABSTRACT: ZrP with large surface, mesoporous volume, and size was prepared. Its catalytic performance was evaluated in hydrolytic hydrogenation of cellulose to sorbitol/mannitol combined with commercial Ru/C. A significantly enhanced sorbitol/ mannitol yield was obtained as compared with that of the water-soluble acidic phosphates and buffer solution of potassium acid phthalate combined with the same Ru/C. The enhanced yield was attributed to the Lewis and Brönsted acid on ZrP surface improving the surface’s absorption and activation for cellulose and accelerating the rate-determined hydrolysis step. The maximal sorbitol/mannitol yield of 63% and 81% could be obtained by using microcrystalline and ball milling cellulose as the feedstock, respectively. The ZrP combined with Ru/C was adaptable to the high concentrated cellulose and obtained 5.8 wt % of sorbitol/ mannitol concentration. The remarkably enhanced cellulose hydrolysis by ZrP sharply decreased the use of Ru/C, which is significant for the essential application.

1. INTRODUCTION Fossil resources are nonrenewable and the utilization of fossil energy poses increasing environmental problems due to huge greenhouse gas and sulfur/nitrogen contained pollutants emissions. These problems drive people to pursue alternatives to these fuels to overcome the mentioned drawbacks.1,2 Biomass is a kind of renewable and sustainable resource produced by natural photosynthesis. Owing to domestic availability, resource abundance, and CO2 neutrality (CO2 released during biomass combustion can be used for biomass growth in the next cycle), using biomass as the feedstock for chemical and fuel production has attracted worldwide attention.3 Cellulose is the main component in lignocellulosic biomass, but its crystalline nature and huge inter/intrahydrogen bonds make it hardly dissolved in water and other conventional solvents, providing a significant challenge for fuels and chemicals production from direct cellulose transformations.4 C6 sugar alcohols (sorbitol/mannitol) originating from cellulosic biomass are key platform molecules for producing various downstream chemicals that show wide applications in the organic and fine chemical industries.5 A general production from cellulose depends on cellulose hydrolysis to mono/oligosaccharides under mineral acids and harsh reaction conditions, followed by further hydrogenation to the final polyols. However, the widely used percolation process for cellulose hydrolysis results in a low concentration of saccharides in the hydrolysate and the erosion of equipment from the use of mineral acids, both of which renders the two-step transformation process an open drawback.6,7 Comparatively, sorbitol/mannitol from a one-pot conversion of cellulose could circumvent the mentioned drawback and is thus regarded © 2014 American Chemical Society

as a promising route because the saccharides in situ produced could be hydrogenated to the stable goal products in time and the side reactions of the saccharide intermediates can be significantly suppressed, if the proper hybrid catalyst is chosen.8,9 One-pot sorbitol/mannitol production from cellulose relies on the tandem acid catalyzed hydrolysis of cellulose to mono/ oligo-saccharides followed by hydrogenation over metals. Mineral acids such as HCl,10 H2SO4 and H3PO4,10,11 and heteropoly acids8,12 combined with supported metals such as Ru, Pt, and Pd13,14 are used as the effective catalysts for this process, and the acceptable yields of sorbitol/mannitol are obtained. However, the catalyst recovery and wastewater emission problems from using water-soluble acids present an obvious obstacle. By comparison, bifunctional solid catalysts containing metals supported on acidic supports can be easily recovered and significantly reduce negative impacts on the environment.15,16 Pt/γ-Al2O3,9,17 Ru/HZSM-5,18 Ni/CNF,19 and bimetallic Ir−Ni/MC and Rh−Ni/MC20 are employed as the candidates, but the sorbitol/mannitol yield and/or conversion of cellulose are rather low when the microcrystalline cellulose is used, and the reaction usually takes a long time. The accessibility of cellulose can be significantly enhanced through ball milling to significantly decrease its crystallinity and polymerization degree, which promotes its hydrolysis and obtains the obviously higher sorbitol/mannitol yield.21,22 When acid Received: Revised: Accepted: Published: 12655

April 21, 2014 June 19, 2014 July 15, 2014 July 15, 2014 dx.doi.org/10.1021/ie5016238 | Ind. Eng. Chem. Res. 2014, 53, 12655−12664

Industrial & Engineering Chemistry Research

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mesoporous volume and pore size distribution were calculated by the desorption branch. The SEM image of ZrP was recorded using an S-4800 instrument operated at 2 kV. The sample was placed on a conductive carbon tape adhered to an aluminum sample holder. The TEM image of Ru/C was gained on a Gatan Ultra scan camera and a JEOL JEM-2100F instrument operated at 200 kV. The sample was ultrasonically dispersed in ethanol, and drops of the suspension were placed on a carbon-coated copper grid and then dried in air. The Ru particle size distribution was determined by using approximately 150 particles, and the average particle size was calculated with eq 1,28 where n refers to the number of particles with diameter d.

and metal were simultaneously used, the high sorbitol/mannitol yield usually depended on the low molar ratio of acid/metal.19 For gaining a high yield of sorbitol/mannitol from direct cellulose conversion, preparation of the solid catalyst with the matched hydrolysis and hydrogenation functionalities is the key. Acidic zirconium phosphate (ZrP) is widely used in the hydrolysis of softwood sawdust,23 dehydration of glucose/ fructose,24,25 and transformation of sorbitol/xylitol to highquality diesel.26 However, using ZrP cooperated with metal as combined catalyst for hydrolytic hydrogenation of cellulose to sorbitol/mannitol is rarely reported hitherto. In this work, we prepared ZrP by simply mixing the aqueous solution of ZrOCl2·8H2O and NH4H2PO4 to obtain the precipitate followed by drying and calcination, and the catalytic performance of the resultant ZrP combined with commercial 5 wt % Ru/C was tested in one-pot transformation of microcrystalline and ball milling cellulose to sorbitol/mannitol. As a reference, the commercial NaH2PO4·2H2O, KH2PO4·3H2O, Ca(H2PO4)2·H2O, and the buffer solution contained potassium acid phthalate combined with 5 wt % Ru/C were also used in this process. The physicochemical properties of the ZrP and the 5 wt % Ru/C were characterized by using XRD, BET, SEM/TEM, NH3-TPD, ICP-AES, FT-IR, and 31P MAS NMR techniques and the proposed reaction pathway was discussed.

d̅ =

∑ nd3/(∑ nd2)

(1)

The ammonia temperature-programmed desorption (NH3-TPD) of ZrP was conducted in a U-tube quartz reactor using an ASIQACIV200-2 automated physical/chemical adsorption analyzer (Quantachrome, US). A sample of 300 mg was loaded and degassed at 673 K in pure helium for 1 h. After the sample was cooled to 373 K, 10%NH3/He was introduced until the absorption saturation was reached. The helium purging was used to remove physically adsorbed NH3. NH3-TPD was performed at the heating rate of 10 K/min from 373 to 973 K in helium using a mass spectroscopy analyzer (Ametek, LC-D300) to monitor the desorbed NH3. The quantitative analysis of acidic sites of fresh and treated ZrP was performed by using a calibration loop of 250 μL and a flowing 10%NH3/He. The deconvolution of the NH3-TPD profile was conducted by an Origin 7.0 software and Gaussian Fitting procedure. The elemental analyses of ZrP and Ru/C were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Plasma-Spec-II spectrometer (Leeman, USA). The samples were dissolved into aqua regia, and the solution was then diluted with 2% HNO3 to meet the detection range of the instrument. The FT-IR and pyridine absorbed FT-IR spectra were recorded on a Bruker TENSOR27 spectrometer with a resolution of 4 cm−1. Prior to measurement, the pellet was pretreated at 673 K for 1 h in He and then cooled to 373 K for recording the pristine FT-IR spectrum. Alternatively, the pyridine vapor was introduced into the cell stepwise at 373 K and kept for 2 h until the saturated state was achieved. The excess pyridine was purged by He for 12 h. The PO43− in the final products was analyzed by an ion chromatograph (883 Basic IC plus 1) equipped with a Metrosep A Supp4-250/4.0 column. A mixed aqueous solution of 1.8 mM Na2CO3 and 1.7 mM NaHCO3 was used as the mobile phase (the flow rate, 1.0 mL/min). External standard method was used for quantification. The 31P solid-state NMR experiments of ZrP were performed on a Bruker Avance-400 spectrometer (400 MHz for 1H nuclei) using a 4 mm magic angle spinning (MAS) probe head. The samples were placed in a ZrO2 rotor, and the spinning frequency was fixed with 10 kHz. The reference sample was adenosine diphosphate. 2.5. Hydrolytic Hydrogenation of Cellulose. In a typical reaction, ZrP, 5 wt % Ru/C, cellulose, and H2O were mixed into a 100 mL stainless steel autoclave. The reactor was flushed several times with hydrogen to remove the residue air and was pressurized to 6 MPa of hydrogen pressure at room temperature. The reaction system was then heated to a given

2. EXPERIMENTAL SECTION 2.1. Chemicals. Microcrystalline cellulose (MCC, Avicel PH101, Fluka) was purchased from Sigma-Aldrich and dried overnight at 343 K prior to use. ZrOCl2·8H2O was purchased from Sinopharm Chemical Reagent Co., Ltd. Ru/C (5 wt %), NH4H2PO4, NaH2PO4, KH2PO4, Ca(H2PO4)2 and buffer solution containing potassium acid phthalate of pH = 4 (1 wt % of aqueous solution) were obtained from Aladdin Chemicals Company. All the chemicals and catalyst were used as received. 2.2. Ball Milling Cellulose. Ball milling of cellulose was carried out by using a ball miller (QM-3SP04, Nanjing Nanda Instrument Company) at room temperature. Microcrystalline cellulose was loaded in a 100 mL ZrO2 container and the milling was carried out at a speed of 500 rpm with ZrO2 balls. To avoid thermal degradation, the temperature of cellulose was not higher than 333 K. The ball milling cellulose was obtained as a white powder and dried overnight at 343 K prior to use. 2.3. Catalyst Preparation. ZrP was prepared by using the previously reported procedure with minor modification.27 Typically, 100 mL of aqueous solution of 1 M ZrOCl2·8H2O was directly mixed with 200 mL of aqueous solution of 1 M NH4H2PO4 (the mole ration of P/Zr = 2) to obtain a white precipitation. The precipitate was then aged for 1 h at room temperature under stirring. The suspension solution was filtered, and the precipitate was washed with deionized water until the pH of the filtrate was 7. Before use, the solid was dried at 373 K overnight and was calcined at 673 K for 4 h in air. 2.4. Catalyst Characterization. X-ray powder diffraction (XRD) patterns were measured by a X-ray diffractometer (XPert Pro MPD, Philip) with Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 100 mA. The 2θ angles were scanned from 5° to 80°. Specific surface area, pore volume and pore diameter distribution of ZrP and Ru/C were measured by the isothermal adsorption−desorption curves of N2 at 77 K, using a QUADRASORB SI-MP-10/PoreMaster 33 analyzer equipped with QuadraWin software system. The samples were degassed under vacuum at 523 K for 20 h prior to measurement. The 12656

dx.doi.org/10.1021/ie5016238 | Ind. Eng. Chem. Res. 2014, 53, 12655−12664


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temperature under strong agitation and kept at this temperature for a certain period. After the reaction, the reactor was quickly cooled with iced water. The liquid products were separated by filtration and detected by using a HPLC instrument. To evaluate the catalytic stability of ZrP and 5 wt % Ru/C, the reaction time was lengthened to 3 h to ensure complete cellulose conversion. The catalyst was obtained through filtering and washing the mixture, and was directly used for the next run. For estimating the hydrothermal stability of ZrP and Ru/C, the fresh ZrP and Ru/C were treated at the reaction condition, followed by drying and recalcination at 673 K for 4 h in air (for Ru/C, the recalcination is avoided). The treated ZrP and Ru/C was used as the catalyst for cellulose conversion. 2.6. Liquid Product Analysis. The aqueous products were analyzed by a high performance liquid chromatograph (HPLC; Waters e2695) with an autosampler and refractive index detector (RID 2414). An Inertsustain C18 column was used to detect the products, and the mobile phase was water. The external standard method was used for quantification. The cellulose conversion was determined by the weight difference of celloluse before and after reaction: mcellulose,0 − mcellulose conversion (%) = × 100 mcellulose,0 (2)

Figure 1. Product yields in direct transformation of microcrystalline cellulose to sorbitol/mannitol over different metal phosphates combined with 5 wt % Ru/C catalysts. The buffer solution is aqueous potassium acid phthalate of pH = 4. Reaction conditions: cellulose, 0.5 g; metal phosphates, 0.5 g; 5 wt % Ru/C, 0.075 g; reaction temperature, 488 K; initial H2 pressure, 6 MPa; reaction time, 2.5 h.

Ru/C, the yield of sorbitol/mannitol and the side products by hydrogenolysis significantly increased. As solid ZrP and 5 wt % Ru/C was used, however, the highest yield of 58% for sorbitol/ mannitol was observed, and the total side products were the lowest at 5%. In comparison, we also used buffer solution including potassium acid phthalate of pH = 4 for this process, and 44% of sorbitol/mannitol yield together with 23% of side products were produced. Obviously, ZrP combined with 5 wt % Ru/C shows the best catalytic performance based on the yield of sorbitol/mannitol and the side products. We further used ZrP and 5 wt % Ru/C for one-pot transformation of microcrystalline cellulose, and the influence of reaction temperature was evaluated. As shown in Figure 2, the conversion of cellulose increased with an increase in the reaction temperature and the highest conversion, 99%, was observed at 518 K. On the other hand, the selectivity of sorbitol/mannitol presented the opposite trend. At the relatively low temperatures of no more than 488 K, the selectivity of sorbitol/mannitol was more than 83%, and no hydrogenolysis products were detected. As temperature increased, the selectivity of sorbitol/mannitol dropped accompanied with incremental side products. At the highest temperature, 518 K, the catalyst showed the lowest selectivity 14% for sorbitol/mannitol and the highest 10% for the side products. Apparently, reaction temperature shows significant influence in this process, and the higher temperature leads to remarkably reduced sorbitol/mannitol selectivity because of the side products produced by sorbitol/mannitol hydrogenolysis and other transformations. We have provided the results from typical HPLC with different reaction temperatures (Supporting Information, Figure S1). At a low temperature of 478 K, the products mainly contained the target sorbitol/mannitol as well as trace amounts of 1,4-/ 3,6-sorbitans by dehydration of sorbitaol/mannitol.29 Comparatively, the elevated temperature accelerated the kinetics of both dehydration and hydrogenolysis of the sorbitol/mannitol and produced the remarkably increased sorbitans and hydrogenolysis products, which is the main reason for the decreased sorbitol/mannitol selectivity. No water-soluble oligosaccharides

where mcellulose,0 is the weight of cellulose before reaction, mcellulose is the weight of cellulose after the reaction, and mcellulose was calculated as follows: mcellulose = mremain − mcatalyst (3) where mremain is the total weight of catalysts and unconverted cellulose after reaction and mcatalyst is the weight of catalysts. The yield of product was determined by moles of carbon in the products and the moles of carbon in the cellulose before the reaction, as indicated by the following equation: Yproduct% =

Cproduct Ccellulose,0

× 100 (4)

where Cproduct is the carbon mole of products; Ccellulose,0 is the carbon mole of cellulose and was calculated as follows: mcellulose,0 Ccellulose,0 = ×6 M(C6H10O5) (5) where M(C 6 H 10 O 5 ) is the molecular weight of the anhydroglucose unit of cellulose. The selectivity of the product was calculated by the following equation: Sproduct% =

Yproduct conversion

× 100

(6)

3. RESULTS AND DISCUSSION 3.1. From Microcrystalline Cellulose to Sorbitol/ Mannitol. Figure 1 shows the catalytic performance of ZrP, acidic metal phosphates with water solubility and buffer solution of potassium acid phthalate of pH = 4 combined with 5 wt % Ru/C in one-pot transformation of microcrystalline cellulose. For NaH2PO4 and 5 wt % Ru/C, the yield of sorbitol/mannitol was only 8.4%, and 7.6% of the side products mainly involved glycerol, 1,2-/1,3-propylene glycol, and ethylene glycol, which resulted from the C−C bond splitting of sorbitol/mannitol. With the use of KH2PO4 and Ca(H2PO4)2 along with 5 wt % 12657



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Figure 2. Influence of reaction temperature on the conversion of microcrystalline cellulose and product selectivity over ZrP combined with 5 wt % Ru/C. Reaction condition: cellulose, 0.5 g; ZrP, 0.5 g; 5 wt % Ru/C, 0.0375 g; initial H2 pressure, 6 MPa; reaction time, 1.5 h.

Figure 3. Morphology and textural properties of ZrP and 5 wt % Ru/C: (A) SEM image of the fresh ZrP, (B) N2 isothermal adsorption−desorption curves of fresh and hydrothermally treated ZrP at 77 K (the inset presents the pore size distributions of the fresh and treated ZrP), (C) TEM image of 5 wt % Ru/C, and (D) Ru particle size distribution.

and cellubiose were detected in our cases, indicating that the prepared ZrP can effectively hydrolyze microcrystalline cellulose to glucose as a solid acid catalyst. The absence of glucose

intermediate is attributed to its fast hydrogenation to the target products over 5 wt % Ru/C, which is consistent with a previous report. 30 12658



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respectively. In terms of the catalytic performance of the metal phosphates and the buffer solution (Figure 1), the lower pH value presented the higher sorbitol/mannitol yield, indicating that more dissociated H+ promotes cellulose hydrolysis, which is regarded as the rate-determining step, and accelerates the total transformation rate, no matter what kinds of acid are used. 3.2. ZrP Combined with Ru/C Catalyst. The SEM image showed that the particle size of ZrP was below 50 nm (Figure 3A) and the average Ru particle size was about 3 nm as measured by TEM and statistical analysis (Figure 3C,D). The element analysis of ZrP by ICP-AES showed that the actual molar ratio of P/Zr is 1.37, which is much lower than the stoichiometric value of the initial introduction. The Ru content in the commercial Ru/C was measured as 4.92 wt % by the same instrument, and its respective surface area and mesopore size were 1171 m2/g and 3.8 nm. Because ZrP is hardly separated from the unconverted cellulose and 5 wt % Ru/C after reaction, the fresh ZrP was simulantly obtained by treating the mixture of fresh 0.5 g of ZrP and 50 mL of H2O at 488 K, 6 MPa of initial H2 pressure, and 3 h, followed by drying and recalcination. The P/Zr of the treated ZrP was 1.31, indicating 4.38% of P loss. The textural properties of the fresh and treated ZrP were presented in Figure 3B. Both ZrP samples show the type IV profiles with adsorption hysteresis (induced by capillary condensation) in the P/ Po of 0.45−0.99. The surface and mesopore volume of the fresh and treated ZrP were 164 m2/g, 0.4 mL/g, and 116 m2/g, 0.38 mL/g, respectively, with the pore size of both being of 12 nm. The textural properties of ZrP are possibly responsible for the superior performance in the transformation of cellulose to sorbitol/mannitol (Figures 1 and 2) owing to the large surface and mesopores. Figure 4 A shows the XRD pattern of the fresh ZrP, and the amorphous character is observed, which is consistent with a previous report.32 The NH3-TPD profiles of the fresh and treated ZrP are presented in Figure 4B,C. Four deconvoluted NH3 desorption peaks were observed at 474, 532, 644, and 787 K, respectively, indicating that weak, medium, and strong acidities simultaneously exist on it. Quantitative calculations revealed that the total acidic comprised 1.0 mmol/g with the weak (474 and 532 K), medium (644 K), and strong (787 K) acid percentages of 46%, 34%, and 20%, respectively. Comparatively, the total acidic sites of the treated ZrP slight increased to 1.06 mmol/g with the remarkably increased strong acid (781 K). Meanwhile, the weak acidic sites (464 and 569 K) were almost unchangeable but the NH3 desorption peaks shifted to higher temperature, demonstrating this acidity was strengthened. The measured molar ratio of P/Zr of 1.37 in fresh ZrP means that H+ loss takes place from the original NH4H2PO4 during ZrP preparation. The remaining P−OH groups act as a Brönsted acid with strong acidity that has widely adaptable ion exchange properties with K+ and Ni2+.32,33 This decreased acid amount is possibly attributed to dehydration of adjacent P−OH groups to P−O−P bonds and P−OH and Zr−OH groups to Zr−O−P bonds at the calcination temperature of 673 K,34 besides the inaccessible acidic sites that cannot be detected by NH3-TPD measurement. Pyridine absorbed FT-IR measurement shows two distinct 1542 and 1444 cm−1 absorbencies over the ZrP surface, indicating simultaneous existence of both Brönsted and Lewis acidic sites, respectively (Supporting Information, Figure S2). These two typical acidic sites possibly originate from the P−OH groups and the vacant orbits of framework Zr4+ species with

Scheme 1. Proposed Reaction Pathway for One-Pot Transformation of Cellulose to Sorbitol/Mannitol over ZrP Combined with Ru/C

On the basis of the experiments, the pathway for cellulose conversion to sorbitol/mannitol is proposed in Scheme 1. Briefly, cellulose is first hydrolyzed to glucose by the acidic metal phosphates, followed by transformation of partial glucose to mannose by the acid-catalyzed C2 epimerization.31 Glucose and mannose were then rapidly hydrogenated over 5 wt % Ru/C to obtain the targeted sorbitol and mannitol (these two products cannot be identified by the present HPLC analysis), thus making the two intermediates hardly observed at the end of the reaction. If the transformation proceeds with more 5 wt % Ru/C and/or higher reaction temperature, sorbitol and mannitol may be further converted into sorbitans by dehydration over acid and hydrogenolysis products by C−C cracking over metal in parallel, resulting in the significantly decreased sorbitol/mannitol yields (Figures 1 and 2). To measure the pH values of water-soluble NaH 2PO4, KH2PO4, and Ca(H2PO4)2 by a pH meter, 0.5 g of metal phosphates were added into 50 mL of H2O to obtain the transparent solution. The pH values of NaH2PO4, K H2PO4, and Ca(H2PO4)2 solutions were measured as 5.1, 4.2, and 3.8, 12659



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Figure 4. XRD pattern of the fresh ZrP (A) and NH3-TPD profiles of the treated (B) and the fresh (C) ZrP.

four-coordination in ZrP.35 Regarding the superior performance of ZrP to those of the water-soluble NaH2PO4, KH2PO4, Ca(H2PO4)2, and buffer solution with the sole Brönsted acid, the Lewis acid may play a role in this process (Figure 1). The investigation on the promotion effect of the Lewis acid is in progress. For hydrolytic hydrogenation of cellulose to sorbitol/ mannitol, the matched acid and metal functionality of the ZrP and 5 wt % Ru/C catalysts is prerequisite. On the basis of the surface acids (determined by NH3-TPD measurement) and the surface Ru atoms (the dispersion of Ru was dependent on the 3 nm of particle size and the calculation from a previous report36), we studied the acid−metal balance by changing the relative amount of ZrP while fixing the 5 wt % Ru/C. As shown in Figure 5, without using ZrP, the microcrystalline cellulose conversion and sorbitol/mannitol yield are only 10% and 8%, respectively, indicating that the catalytic cooperation effect containing acid and metal is inevitable. However, once the acid was introduced, the cellulose conversion and sorbitol/mannitol yield sharply increased, and both values increased with an increase in the molar ratio nA/nRu. At the low molar ratios of no more than 89, the metal is dominant but the acid is insufficient, which is not enough to hydrolyze cellulose efficiently and thus obtain the low cellulose conversion and sorbitol/mannitol yield. On the other hand, at the high ratios of more than 196, despite the cellulose conversion further increasing, the yield of sorbitol/mannitol was slightly reduced, which is ascribed possibly to the accelerated dehydration of sorbitol/mannitol to obtain sorbitans over the dominant acid centers. Compared to that of the metal, the influence of acid is predominant, which suggests that the kinetics for sorbitol/mannitol production is determined by the acid. At the proper 196 ratio, the matched

Figure 5. Acid/metal balance in hydrolytic hydrogenation of microcrystalline cellulose to sorbitol/mannitol over ZrP and 5 wt % Ru/C. Reaction conditions: cellulose, 0.5 g; 5 wt % Ru/C, 0.0375 g; reaction temperature, 488 K; initial H2 pressure, 6 MPa; reaction time, 1.5 h.

acid/metal is reached and the highest sorbitol/mannitol yield of 56% is obtained, which is comparable to the best reported result in literature which was obtained by using Rh−Ni/MC as the catalyst.20 It is worth noting that the proper nA/nRu used here is more than 20 times higher than the reported value obtained by using the metal−organic framework encapsulated Ru and H3PW12O40 as the catalyst,37 indicating that ZrP combined with 5 wt % Ru/C could act as the highly active solid catalyst for this process, but the Ru used is sharply decreased. 12660



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3.3. ZrP Combined with Ru/C for Ball Milling and High Concentrated Cellulose. Ball milling is an effective way to decrease the crystallinity and polymerization degree of microcrystalline cellulose and thus significantly improve the degradability. For the transformation of ball milling cellulose, an obvious advantage is to decrease the reaction temperature to suppress the side reactions and thus enhance the yield of the goal products at a high conversion of cellulose.38,39 Here, we used ball milling cellulose with different ball milling time to produce sorbitol/mannitol. The XRD patterns are shown in Figure 6. The untreated cellulose presented a broadened diffraction at 15.4°, two

Figure 7. Influence of ball milling time on the conversion of cellulose and selectivity of sorbitol/mannitol over ZrP combined with 5 wt % Ru/C. Reaction condition: cellulose, 0.5 g; ZrP, 0.5 g; 5 wt % Ru/C, 0.0375 g; reaction temperature, 463 K; initial H2 pressure, 6 MPa; reaction time, 2.5 h.

the previously reported Ru/C combined with Cs3.5H0.5SiW at the similar temperature and H2 pressure but much shortened reaction time. During cellulose transformation, improving the cellulose concentration in the aqueous phase can potentially increase the concentration of the goal product, which is preferable to decreasing the energy consumption for further conversion. Over ZrP combined with 5 wt % Ru/C, we used a high concentration of microcrystalline and ball milling cellulose to produce sorbitol/mannitol, and the results are listed in Table 1. 42 12O40

Figure 6. XRD patterns of cellulose with different ball milling time.

resolved at 22.4° and 34.4°, corresponding to the (101)/(10), (002), and (004) crystal planes, respectively. This indicates the typical I crystalline form.40 Cellulose has crystalline and amorphous parts in its structure. By using the previously reported calculation,41 the crystallinity index (CrI) of the untreated cellulose was 61%. As the ball milling time lengthened, the diffractions of cellulose significantly reduced, demonstrating that crystallinity decreased and more amorphous regions formed. As the ball milling time was more than 2 h, however, the diffractions of the ball milling cellulose changed a little. The CrI was estimated as 25% for the 1 h sample and about 17% for the 2, 6, and 12 h samples. Figure 7 shows the influence of ball milling time on cellulose conversion and sorbitol/mannitol selectivity at the reaction temperature of 463 K. With the use of untreated cellulose, the conversion and sorbitol/mannitol selectivity was 26% and 83%, respectively, gaining a product yield of 22%. For 1 h of ball milling, the conversion of cellulose significantly increased to 68% with the increased sorbitol/mannitol selectivity of 90%. At the ball milling time of more than 1 h, the cellulose conversions gradually increased from 72% at 2 h of ball milling to 90% at 12 h of ball milling, while keeping the constant 90% of sorbitol/mannitol selectivity. The highest yield of 81% was obtained at the 12 h ball milling time. The high sorbitol/ mannitol selectivity observed at the higher cellulose conversion indicates that the side products from dehydration and hydrogenolysis of sorbitol/mannitol are significantly suppressed at this low reaction temperature. Here, ZrP combined with 5 wt % Ru/C is highly active based on the goal products yield, which shows the much higher productivity as comparing with

Table 1. Hydrolytic Hydrogenolysis of Highly Concentration of Cellulose to Sorbitol/Mannitol over ZrP Combined with Ru/Ca product selectivity (C-mol %) entry

5 wt % Ru/ C (g)

conversion (%)

sorbtol/ mannitol

glycerol

1,2-/1,3PDO

1b 2b 3c 4c

0.075 0.125 0.075 0.125

62.0 61.6 67.5 72.0

56.0 67.5 70.7 86.6

3.9 4.4 6.7 2.1

2.7 0.8 0.8 1.4

EG 2.5 1.7

a Reaction conditions: cellulose, 5 g; ZrP, 0.5 g; initial H2 pressure, 6.0 MPa; H2O, 50 mL; 2.5 h. PDO, propanediol; EG, ethylene glycol. b Microcrystalline cellulose; reaction temperature, 488 K. c12 h of ball milling cellulose; reaction temperature, 463 K.

For the microcrystalline cellulose, even its concentration was increased to 10 times higher, and more than 60% of cellulose conversion and 56% of sorbitol/mannitol selectivity were observed with 9% of side products by sorbitol/mannitol hydrogenolysis. Raising the amount of 5 wt % Ru/C can not promote cellulose conversion but significantly increase the selectivity of sorbitol/mannitol. When the same concentration of 12 h ball milling cellulose was used, both the cellulose conversion and sorbitol/mannitol selectivity were increased. With increasing 5 wt % Ru/C, limited promotion for cellulose conversion proposes again that the acid-catalyzed hydrolysis of cellulose is the ratedetermining step no matter what kinds of cellulose are used. The highest yield and weight concentration of sorbitol/mannitol 12661



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and Ru/C continued to the second run, the cellulose conversion and sorbitol/mannitol selectivity remained almost unchanged (Figure 8, recycle 4), indicating the good hydrothermal stability of ZrP and Ru/C. As compared with the NH3-TPD profiles of the fresh and the hydrothermally treated ZrP for the first run (Figure 4B,C), both total acidic sites are nearly identical. The hydrothermally treated 5 wt % Ru/C was conducted by using the same procedure for ZrP without the recalcination step. The treated Ru/C gave an Ru residue of 4.87 wt % (very close to the fresh 4.92 wt %), and the surface and mesopore size of 1023 m2/g and 3.7 nm, respectively. The XRD pattern showed that no distinct diffractions relative to Ru are observed after hydrothermal treatment (Supporting Information, Figure S4), indicating the high dispersion of Ru. Therefore, it is highly speculated that the decreased cellulose conversion may not be attributed to P loss and/or Ru leaching/sintering, but possibly due to the reduced surface of ZrP after the reaction. The structure change of ZrP was characterized by the solidstate 31P MAS NMR experiments. As shown in Figure 9A, the fresh ZrP had a main peak at −25.7 ppm with two shoulders centered at −20.4 ppm and −12.0 ppm, respectively. The chemical shifts at −20.4 ppm and −12.0 ppm indicate the tetrahedral (Zr−O)2PO(OH) and (Zr−O)PO(OH)2, respectively.35 The peak at −25.7 ppm implies the phosphates with tetrahedral (Zr−O)4-P bonds and polyphosphate with P−O−P bonds.43,44 After the fresh ZrP was hydrothermally treated and drying at 373 K, the peaks relative to (Zr−O)4-P (−23.6 ppm) and (Zr−O)2PO(OH) (−18.2 ppm) decreased and shifted to low magnetic field, meaning the P−O−P bonds dissolved into P−OH groups but the Zr−O−P bonds were preserved. The FT-IR spectra of fresh ZrP showed the presence of the stretching vibrations of P−OH groups (2355 cm−1) and Zr−O−P (1063 cm−1) and the bending vibration of P−O−P (754 cm−1) (Figure 9B).45,46 After hydrothermal treatment, the peak intensity relative to P−O−P reduced while that corresponding to P−OH groups increased, which is well consistent with NMR results. The formation of P−OH groups indicates P leaching, which possibly influences the chemicophysical properties of the treated ZrP and obtains the inferior performance. As the treated ZrP was recalcined at 673 K, however, the structures corresponding to (Zr−O)4-P, (Zr−O)2PO(OH), and (Zr−O)PO(OH)2 were partially recovered, and about 90% of initial activity can be regenerated accordingly.

were calculated as 62% and 5.8 wt %, respectively (entry 4, Table 1), demonstrating that ZrP combined with 5 wt % Ru/C can be adaptable to highly concentrated cellulose. 3.4. Reuse of ZrP Combined with Ru/C. The reusability of the ZrP and 5 wt % Ru/C used with the microcrystalline cellulose is shown in Figure 8. For the fresh ZrP and 5 wt %

Figure 8. Reusability of ZrP combined with 5 wt % Ru/C catalyst in transformation of microcrystalline cellulose to sorbitol/mannitol. The third recycle used the recalcination of hydrothermal pretreatment ZrP combined with the commercial 5 wt % Ru/C (reaction conditions: cellulose, 0.5 g; ZrP, 0.5 g; 5 wt % Ru/C, 0.0375 g; reaction temperature, 488 K; initial H2 pressure, 6 MPa; reaction time, 3 h).

Ru/C, more than 99% of cellulose and 63% of sorbitol/ mannitol selectivities were observed, respectively. After the second run, the cellulose conversion significantly decreased to 48% while a comparable sorbitol/mannitol selectivity remained. At the end of the second run, the residue PO43− in the final products was also measured, and 3.3% of P leaching was observed (Supporting Information, Figure S3). This P loss is close to that obtained by ICP-AES measurement from hydrothermally treated ZrP (4.38% of P loss). However, as the fresh ZrP and Ru/C was hydrothermally treated, and the treated ZrP was recalcined at 673 K for regeneration, nearly 90% of activity and target products yield can be recovered (Figure 8, recycle 3). When the hydrothermal treatment of ZrP

Figure 9. 31P NMR (A) and FT-IR (B) spectra of ZrP samples. The asterisk (∗) denotes spinning side bands. 12662



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(4) Stö cker, M. Biofuels and biomass-to-liquid fuels in the biorefinery: Catalytic conversion of lignocellulosic biomass using porous materials. Angew. Chem., Int. Ed. 2008, 47, 9200−9211. (5) Zhang, J.; Li, J. B.; Wu, S. B.; Liu, Y. Advances in the catalytic production and utilization of sorbitol. Ind. Eng. Chem. Res. 2013, 52, 11799−11815. (6) Agbor, V. B.; Cicek, N.; Sparling, R.; Berlin, A.; Levin, D. B. Biomass pretreatment: Fundamentals toward application. Biotechnol. Adv. 2011, 29, 675−685. (7) Zhu, Y. M.; Lee, Y. Y.; Elander, R. T. Optimization of dilute-acid pretreatment of corn stover using a high-solids percolation reactor. Appl. Biochem. Biotechnol. 2005, 121, 1045−1054. (8) Geboers, J.; Van de Vyver, S.; Carpentier, K.; de Blochouse, K.; Jacobs, P.; Sels, B. F. Efficient catalytic conversion of concentrated cellulose feeds to hexitols with heteropoly acids and Ru on carbon. Chem. Commun. 2010, 46, 3577−3579. (9) Fukuoka, A.; Dhepe, P. L. Catalytic conversion of cellulose into sugar alcohols. Angew. Chem., Int. Ed. 2006, 45, 5161−5163. (10) Geboers, J.; Van de Vyver, S.; Carpentier, K.; Jacobs, P.; Sels, B. F. Efficient hydrolytic hydrogenation of cellulose in the presence of Ru-loaded zeolites and trace amounts of mineral acid. Chem. Commun. 2011, 47, 5590−5592. (11) Palkovits, R.; Tajvidi, K.; Procelewska, J.; Rinaldi, R.; Ruppert, A. Hydrogenolysis of cellulose combining mineral acids and hydrogenation catalysts. Green Chem. 2010, 12, 972−978. (12) Palkovits, R.; Tajvidi, K.; Ruppert, A. M.; Procelewska, J. Heteropoly acids as efficient acid catalysts in the one-step conversion of cellulose to sugar alcohols. Chem. Commun. 2011, 47, 576−578. (13) Liu, M.; Deng, W.; Zhang, Q.; Wang, Y. L.; Wang, Y. Polyoxometalate-supported ruthenium nanoparticles as bifunctional heterogeneous catalysts for the conversions of cellobiose and cellulose into sorbitol under mild conditions. Chem. Commun. 2011, 47, 9717− 9719. (14) Liang, G. F.; Wu, C.; He, L. M.; Ming, J.; Cheng, H. Y.; Zhuo, L.; Zhao, F. Y. Selective conversion of concentrated microcrystalline cellulose to isosorbide over Ru/C catalyst. Green Chem. 2011, 13, 839−842. (15) Lai, D. M.; Deng, L.; Guo, Q. X.; Fu, Y. Hydrolysis of biomass by magnetic solid acid. Energy Environ. Sci. 2011, 4, 3552−3557. (16) Huang, Y. B.; Fu, Y. Hydrolysis of cellulose to glucose by solid acid catalysts. Green Chem. 2013, 15, 1095−1111. (17) Jolle, V.; Chambon, F.; Rataboul, F.; Cabiac, A.; Pinel, C.; Guillon, E.; Essayem, N. Non-catalyzed and Pt/gamma-Al2O3catalyzed hydrothermal cellulose dissolution-conversion: Influence of the reaction parameters and analysis of the unreacted cellulose. Green Chem. 2009, 11, 2052−2060. (18) Liang, G.; Cheng, H. Y.; Li, W.; He, L. M.; Yu, Y. C.; Zhao, F. Y. Selective conversion of microcrystalline cellulose into hexitols on nickel particles encapsulated within ZSM-5 zeolite. Green Chem. 2012, 14, 2146−2149. (19) Van de Vyver, S.; Geboers, J.; Schutyser, W.; Dusselier, M.; Eloy, P.; Dornez, E.; Seo, J. W.; Courtin, C. M.; Gaigneaux, E. M.; Jacobs, P. A.; Sels, B. F. Tuning the acid/metal balance of carbon nanofiber-supported nickel catalysts for hydrolytic hydrogenation of cellulose. ChemSusChem 2012, 5, 1549−1558. (20) Pang, J. F.; Wang, A. Q.; Zheng, M. Y.; Zhang, Y. H.; Huang, Y. Q.; Chen, X. W.; Zhang, T. Catalytic conversion of cellulose to hexitols with mesoporous carbon supported Ni-based bimetallic catalysts. Green Chem. 2012, 14, 614−617. (21) Kobayashi, H.; Matsuhashi, H.; Komanoya, T.; Hara, K.; Fukuoka, A. Transfer hydrogenation of cellulose to sugar alcohols over supported ruthenium catalysts. Chem. Commun. 2011, 47, 2366−2368. (22) Hilgert, J.; Meine, N.; Rinaldi, R.; Schüth, F. Mechanocatalytic depolymerization of cellulose combined with hydrogenolysis as a highly efficient pathway to sugar alcohols. Energy Environ. Sci. 2013, 6, 92−96. (23) Gliozzi, G.; Innorta, A.; Mancini, A.; Bortolo, R.; Perego, C.; Ricci, M.; Cavani, F. Zr/P/O catalyst for the direct acid chemo-

4. CONCLUSIONS We prepared acidic ZrP by simply mixing the aqueous solution of ZrOCl2·8H2O and NH4H2PO4 and tested its catalytic performance in a one-pot transformation of cellulose to sorbitol/ mannitol by hydrolytic hydrogenation, as combined with the commercial Ru/C. The ZrP combined with Ru/C was highly active in this process and showed significantly improved sorbitol/ mannitol yield as compared with that of the water-soluble acidic metal phosphates and buffer solution containing potassium acid phthalate combined with the same Ru/C. The Lewis and Brönsted acids on the surface of ZrP possibly play the essential role in accelerating the rate-determined hydrolysis of cellulose, and the maximal yield of 63% for sorbitol/mannitol under the optimal conditions is obtained. When ball-milling cellulose was used, the highest sorbitol/mannitol yield could further increase to 81% due to the promoted accessibility of cellulose and the suppressed side products at 463 K. The ZrP combined with Ru/C is also adaptable to highly concentrated cellulose and gained 5.8 wt % of sorbitol/mannitol concentration. The remarkably improved hydrolysis of cellulose sharply reduced the use of Ru/C while maintaining the high sorbitol/mannitol yield, which affords an important inspiration for preparing low cost catalysts in this process.

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ASSOCIATED CONTENT

S Supporting Information *

Typical HPLC analyses of products obtained by using ZrP and 5 wt % Ru/C combined catalyst at different reaction temperatures; pyridine absorbed FT-IR spectrum of ZrP; ion chromatography profile of final products obtained at the end of the first run; XRD pattern of hydrothermally treated 5 wt % Ru/C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-20-87057751. Fax: +86-20-87057737. *E-mail: [email protected]. Tel: +86-20-87057673. Fax: +8620-87057673. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51376185 and 51161140331), the National Basic Research Program of China (2012CB215304), the Natural Science Foundation of Guangdong Province (S2013010011612), and the Creative Foundation of President of Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (y307p21001).

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REFERENCES

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One-Pot Transformation of Cellulose to Sugar Alcohols over Acidic

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