Promoting Hydrolytic Hydrogenation of Cellulose to Sugar Alcohols by

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Promoting Hydrolytic Hydrogenation of Cellulose to Sugar Alcohols by Mixed Ball Milling of Cellulose and Solid Acid Catalyst Yuhe Liao,†,‡ Qiying Liu,† Tiejun Wang,† Jinxing Long,† Qi Zhang,† Longlong Ma,*,† Yong Liu,†,‡ and Yuping Li† †

CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Enhancing the contact or interaction between cellulose and solid catalyst is a significant aspect in its efficient catalytic conversion. Herein, mixed ball milling of cellulose and solid catalyst was presented to achieve this goal, and the promotion effect was measured by hydrolytic hydrogenation of cellulose to sugar alcohols (the platform compounds for biogasoline) with solid acid and commercial 5 wt % Ru/C in water. The effects of ball-milling modes, time, and reaction parameters were studied. The properties of cellulose and solid acid catalyst before and after treatment were also analyzed. The yield of sugar alcohols reached 90.3% at 463 K with amorphous zirconium phosphate and Ru/C (mixed ball-milling time of 2 h). This high yield of sugar alcohols achieved in the mixed ball-milling time of 2 h was 12 times faster than that by the single ball milling of 24 h under the same reaction conditions. It is ascribed to the enhanced contact between cellulose and catalyst, resulting in promoting cellulose depolymerization. The high concentration of sugar alcohols up to 67 mg/mL was obtained by augmenting the mass ratio of cellulose/catalyst.

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INTRODUCTION Lignocellulosic biomass as a renewable resource has attracted increasing attention in recent years. In comparison to hemicellulose and lignin, cellulose possesses the greatest proportion of 30−50% in lignocellulosic biomass.1 Cellulose is formed by the linkage of β-1,4-glucosidic bonds between glucose units and contains numerous inter- and intramolecular hydrogen bonds.2 Cellulose can be converted into platform compounds, such as glucose,3−5 levulinic acid,6 5-hydroxymethylfurfural,7,8 lactic acid,9,10 and polyols,11 through kinds of technologies. Among these compounds, sugar alcohols (sorbitol and mannitol) can be converted to liquid fuel (biogasoline) through aqueous-phase hydrodeoxygenation.12 Besides, they also show wide application in the fields of chemistry, medicine, food industry, etc.13,14 For example, isosorbide, the dehydration product of sorbitol, is an ideal, versatile chemical, which can be used for synthesizing biopolymers.15 One-pot conversion of cellulose to sugar alcohols is a continuous two-step reaction, involving depolymerization of cellulose to sugars and in situ hydrogenation of soluble sugars to sugar alcohols. In comparison to hydrogenation of sugars, depolymerization of cellulose is the rate-controlling step.16 However, cellulose depolymerization is hampered by the inertness and inaccessibility of cellulose to catalyst because of its crystallinity and insolubility in traditional solvents. The recent advances in cellulose depolymerization can be divided into three main research lines. In the first, enzymatic hydrolysis of cellulose has been proven to be viable,17,18 but the hydrolysis efficiency and cellulase cost need to be considered. In the second line of research, liquid acids were rationally used for cellulose hydrolysis.19,20 Although liquid acids are efficient and the contact between cellulose and catalyst is improved, recover © 2014 American Chemical Society

and corrosion of liquid acids are the puzzles. Finally, tailormade solid catalysts were used to degrade cellulose.3 The main advantages of solid catalysts are that the catalyst can be easily recycled and neutralization is not needed when treating waste liquid after reaction. Despite these advantages, solid catalysts require severe reaction conditions or pretreatment of cellulose to achieve high conversion. Meanwhile, the yield of sugar alcohols is reduced at high temperatures (e.g., higher than 463 K) because of significant side reactions, such as hydrogenolysis and dehydration.21 Moreover, traditional pretreatment, such as ball milling, can only reduce cellulose crystallinity and depolymerize cellulose to a certain degree, and normally, a large amount of energy is consumed to achieve this process. Additionally, depolymerization of cellulose by solid catalysts is a solid−solid phase reaction, and the degradation rate is limited by the inaccessibility of the cellulose surface to interact with the catalysts. Therefore, enhancing the contact between cellulose and solid catalyst, which could promote the interaction between them contributed to the reduction of the reaction temperature and time,22 is a prerequisite for cellulose valorization by heterogeneous catalysis. Traditional ball milling has been extensively used in chemical and enzymatic processes to improve the reactivity of cellulose for (bio)catalysis. Actually, the primary aim of mechanical pretreatment is to amorphitize cellulose, thus improving the depolymerization rate of cellulose. However, achieving this aim is often accompanied by a long time (e.g., 24 h). Blair et al. have developed a concept of a mechanocatalytic process for cellulose depolymerization in the presence of solid acid catalyst, Received: March 31, 2014 Revised: August 5, 2014 Published: August 5, 2014 5778

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precipitate was filtered, washed with water to remove chloride ions completely, and dried at 393 K overnight. Before reaction, SiO2−ZrO2 was calcined at 773 K in air for 5 h. Ball-Milling Treatment of Cellulose. Ball milling of cellulose was conducted using a planetary ball mill (QM-3SP04, Nanjing NanDa Instrument Plant) at room temperature. Single Ball Milling. Microcrystalline cellulose was charged in a 100 mL ZrO2 container, and the ball milling was carried out at a speed of 500 rpm with ZrO2 balls (15 balls; Φ = 7 mm) for a certain period. Mixed Ball Milling. Microcrystalline cellulose and solid acid catalyst were charged in a 100 mL ZrO2 container, and the ball milling was carried out at a speed of 500 rpm with ZrO2 balls (15 balls; Φ = 7 mm) for a certain time. To avoid thermal degradation of the microcrystalline cellulose, the temperature of cellulose was not higher than 333 K during the ball milling. Hydrolytic Hydrogenation of Cellulose to Sugar Alcohols. The conversion of ball-milled cellulose was conducted in a 100 mL stainless autoclave (316L stainless) equipped with magnetic agitation. In a typical reaction, solid acid catalyst, commercial 5 wt % Ru/C, ballmilled cellulose, and H2O were charged in the autoclave and pressured with H2 to 6 MPa at room temperature. The residual air was removed by hydrogen purging several times before heating it. The reactor was heated to a given temperature with stirring and held at this temperature for a certain period. After the reaction, the reaction system was quickly cooled with water. The aqueous products and solid residue were separated by filtration and decantation. The aqueous products were analyzed with high-performance liquid chromatography (HPLC). Hydrothermal treatment of ball-milled ZPA was performed by treating the ball-milled ZPA under the reaction conditions of cellulose transformation. The hydrothermal-treated ZPA was washed 3 times with deionized water and then calcined at 673 K for 4 h in air before reuse. Characterization. Scanning electron microscopy (SEM) images were monitored using a Hitachi S-4800 instrument operated at 10 kV. The sample was placed on a conductive carbon tape adhered to an aluminum sample holder. The particle size distribution was measured by two instruments, Zetasizer Nano ZS and Mastersizer 2000E, made by Malvern Instruments, Ltd., allowing for measurements in the range of 0.1− 1000 μm. 13 C cross-polarization/magic angle spinning (CP/MAS) NMR analysis was conducted by a Bruker AVANCE III 300 WB spectrometer (7.05 T). All of the spectra were acquired with 2048 scans, a recycle delay of 5 s, and a contact time of 4.5 ms. The sample was charged in 4 mm rotors, and the spinning frequency of the rotor was 5000 Hz. X-ray powder diffraction (XRD) patterns were measured by an Xray diffractometer (X’Pert 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°. The Brunauer−Emmett−Teller (BET) specific surface area and average pore diameter were measured by N2 isothermal adsorption− desorption profiles at 77 K using a QUADRASORB SI-MP-10/ PoreMaster 33 analyzer equipped with a QuadraWin software system. The mesoporous volume and pore size distribution were calculated by the desorption branch. Ammonia-temperature programmed desorption (NH3-TPD) was conducted in a U-tube quartz reactor using an ASIQACIV200-2 automatic physical/chemical adsorption analyzer (Quantachrome, Boynton Beach, FL). The quantitative analysis of acidic sites of solid catalyst was performed using a calibration loop of 250 μL. Gravimetric Analysis. The solubility of cellulose after ball milling was monitored gravimetrically. The content of water-soluble intermediates was determined by stirring the mixed ball-milled sample in 50 mL of water. The solution was filtered by a Millipore filter with a pore size of 0.22 μm. The residue was dried at 343 K overnight and then weighed. Product Analysis. The aqueous products were detected by HPLC (Waters e2695, refractive index detector). An InertSustain C18

obtaining 84% of cellulose conversion by layered mineral delaminated kaolinite during the ball milling.23 This approach greatly reduces the energy required for cleavage of glycosidic bonds. However, the selectivity and yield of specific products (glucose, fructose, or levoglucosan) is very low.23 Recently, mechanocatalytic depolymerization of impregnating cellulose has attracted attention and deemed to be an efficient method.24 The impregnation of cellulose with liquid acid (e.g., HCl and H2SO4) improves the efficiency of ball milling, and the product mainly includes low-molecular-weight carbohydrates.25,26 The oligomerization of in-situ-produced glucose was found to be responsible for the remarkably enhanced selectivity. By means of high-resolution nuclear magnetic resonance (NMR) spectroscopy, Beltramini et al. have proven that branched cellooligomers were attributed to form α(1 → 6) linkage.27 However, the acidic solution would promote the side reactions of sugar alcohols.27 In addition, Fukuoka et al. reported that mixed ball milling of activated carbons and cellulose could promote the cellulose depolymerization, and this process combined with HCl could efficiently convert cellulose to glucose.28 In this work, a series of solid acid catalysts were blended with cellulose during the ball milling. After mixed ball milling, hydrolytic hydrogenation of cellulose to sugar alcohols was conducted in the presence of 5 wt % Ru/C and ball-milled solid acid catalyst. The effects of the ball-milling modes, time, and reaction parameters were investigated, and the characteristics of cellulose and catalyst after mixed ball milling were monitored. Interestingly, this mixed ball-milling technology could obtain more than 90% of yield of sugar alcohols under mild reaction conditions.

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EXPERIMENTAL SECTION

Materials. Microcrystalline cellulose (AvicelPH101) was purchased from Sigma-Aldrich (Shanghai, China) and dried at 343 K for 12 h prior to use. ZrOCl2·8H2O was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). NH4H2PO4 was obtained from Tianjin Fu Chen Chemical Reagents Factory (Tianjin, China). Na2SiO3·9H2O, NH3·H2O, and NH4NO3 were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Unless otherwise indicated, all chemicals were used without further treatment. 5 wt % Ru/C was obtained from Aladdin Industrial, Inc. (Shanghai, China) and used as received. HZSM-5 (Si/Al = 38), HMOR (Si/Al = 10), and Hβ (Si/Al = 25) were purchased from the Catalyst Plant of Nankai University (Tianjin, China) and calcined at 773 K in air for 4 h prior to use. Kaolinite was obtained from Tianjin Da Mao Chemical Reagents Factory (Tianjin, China) and calcined at 673 K in air for 4 h prior to use. ZrO2 was purchased from Aladdin Industrial, Inc. (Shanghai, China) and calcined at 773 K in air for 4 h prior to use. Catalyst Preparation. The amorphous zirconium phosphate catalyst (ZPA) was prepared according to the reported method.29,30 Namely, ZPA was obtained by precipitation of NH4H2PO4 (1.0 mol/ L, 200 mL) with ZrOCl2·8H2O (1.0 mol/L, 100 mL) at the mole ratio of P/Zr = 2. The solution was vigorous stirred for 1 h and then filtered. The precipitate was washed with deionized water until the pH of the filtrate was 4. The filter cake was dried at 373 K for 12 h and calcined at 673 K for 4 h in air. The SiO2−ZrO2 composite oxide was obtained according to our previous report.31 Zr(OH)4 was obtained by adding NH3·H2O into the ZrOCl2·8H2O solution with continuous agitation until the pH of solution was 8. Si(OH)4 was prepared by the same method but with NH4NO3-saturated solution as the precipitator. Namely, the NH4NO3saturated solution was added to the Na2SiO3 solution with continuous agitation until the pH of solution was 8. Zr(OH)4 and Si(OH)4 were blended with vigorous stirring at a mole ratio of Si/Zr = 3. The mixed precipitate was aged overnight at the temperature of 348 K. The 5779

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Table 1. Conversion of Cellulose to Sugar Alcohols by Kinds of Solid Catalysts Combined with Ru/Ca cellulose conversion (%) entry

selectivity of sugar alcohols (%)

single ball milling

mixed ball milling

single ball milling

mixed ball milling

single ball milling

mixed ball milling

kaolinite ZrO2 SiO2−ZrO2 H-MOR Hβ HZSM-5 ZPA

16.8 23.4 21.8 39.8 75.6 77.1 78.2 77.6

30.0 31.1 50.1 81.5 85.1 86.9 100.0

1.5 0.03 1.0 4.1 35.8 41.5 41.7 65.1

3.7 4.7 8.3 40.0 51.5 48.9 90.3

8.9 0.13 4.6 10.3 47.4 53.8 53.3 83.9

12.3 14.8 16.6 49.1 60.5 56.3 90.3

b

1 2 3 4 5 6 7 8

yield of sugar alcohols (%)

solid acid catalyst

a

Reaction conditions: cellulose, 1 g; solid acid catalyst, 0.9 g; 5 wt % Ru/C, 0.075 g; H2O, 50 mL; reaction temperature, 463 K; H2, 6 MPa; reaction time, 2.5 h; and ball-milling time, 2 h. bNo solid acid catalyst.

column was used to analyze the polyols, and water was used as the mobile phase. The external standard method was used for quantification. The cellulose conversion was determined by the weight difference of cellulose before and after reaction and was calculated as given by mcellulose,0 − mcellulose conversion (%) = × 100% mcellulose,0 (1)

16.8% over the sole Ru/C (entry 1). It can be clearly seen that the conversions of the mixed ball-milled cellulose were higher than those obtained with the single ball-milling conversions, which illustrated that blending cellulose and solid acid catalyst during ball milling could promote cellulose transformation (entries 2−8). Because hydrogenation of glucose and hydrogenolysis of sugar alcohols take place at the surface of Ru/C simultaneously, hydrogenation of glucose will be preferable if the glucose is offered quickly.32 More water-soluble sugars occupy the active sites of Ru/C as the depolymerization rate of cellulose is accelerated in mixed ball-milled cellulose, which could inhibit hydrogenolysis of sugar alcohols over Ru/C. In addition, improving the contact between cellulose and solid acid catalyst could make the catalyst be covered by cellulose, thus suppressing the dehydration of sugar alcohols.34 Both factors play an essential role in enhancing the selectivity of sugar alcohols (entries 2−8). Results of Table 1 show that the cellulose conversion was influenced significantly by different solid acid catalysts. For example, for single ball-milled cellulose, less than 40% of conversion was achieved with kaolinite, ZrO2, and SiO2−ZrO2. It also shows that 75−78% of conversions were obtained by HMOR, Hβ, HZSM-5, and ZPA. This difference might be caused by the intrinsic properties of the catalyst. Table 2 shows the textual properties of solid acid catalysts. The surface area and average pore diameter of the catalyst have no direct relationship to cellulose conversion. However, the acidic properity of solid acid catalyst is the important parameter. Increasing the acidity of the catalyst, the cellulose conversion increased correspondingly. Among the solid acid catalysts explored, ZPA presented

where mcellulose,0 is the weight of cellulose charged in the reactor, mcellulose is the weight of residual cellulose after reaction, and mcellulose was calculated as follows: mcellulose = mresidue − mcatalyst (2) where mresidue is the weight of residual solid after reaction and mcatalyst is the weight of solid catalysts (solid acid catalyst and Ru/C). The yield of sugar alcohols was determined by the carbon moles in sugar alcohols and carbon moles in the cellulose charged in the reactor. The yield was calculated as given by Ysugar alcohols =

Csugar alcohols Ccellulose,0

× 100% (3)

where Csugar alcohols is the carbon moles of sugar alcohols and Ccellulose,0 is the carbon moles of cellulose charged in the reactor and was calculated as follows: mcellulose,0 Ccellulose,0 = ×6 M(C6H10O5) (4) where M(C6H10O5) is the molecule weight of anhydroglucose units. The selectivity of the product was calculated by the following equation: Ssugar alcohols =

Ysugar alcohols conversion

× 100%

(5)

The concentration of sugar alcohols was determined by the following equation:

concentration sugar alcohols =

Ysugar alcoholsCcellulose,0 × 182 6Vliquid

Table 2. Textural Property and Acid Density of the Solid Acid Catalysts entry

catalyst

surface areaa (m2/g)

1 2 3 4 5 6 7 8

kaolinite ZrO2 SiO2−ZrO2 H-MOR Hβ HZSM-5 ZPA Ru/C

9.1 5.1 253.1 574.1 541.3 319.5 126.7 1175.7

× 100%

(6) All of the experiments were conducted 3 times, and the relative errors are less than ±2%. All data of figures and tables are the average value of 3 times.

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RESULTS AND DISCUSSION Comparison of Ball-Milling Modes. Several solid acid catalysts combined with 5 wt % Ru/C were tested in hydrolytic hydrogenation of single and mixed ball-milled cellulose under the same reaction conditions, and the corresponding results are shown in Table 1. Solid acid catalyst was important for cellulose transformation because the conversion of cellulose is only

volumeb (cm3/g)

average pore diameterb (nm)

acidity density (mmol/g)

0.07 0.05 0.91 0.21 0.19 0.22 0.31 0.28

26.12 27.49 7.88 0.61 0.49 2.75 4.83 3.82

0.11 0.42 0.82 1.15 1.24 1.19 1.80

a Surface area, calculated by the BET method. bVolume and average pore diameter, calculated from the desorption branch by the Barrett− Joyner−Halenda (BJH) method.

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this time (2 h) is chosen to study the effects of other reaction parameters. Effects of the Reaction Time and Temperature on Mixed Ball-Milled Cellulose Transformation. The effect of the reaction temperature on mixed ball-milled cellulose transformation was studied by performing the reaction in temperature range of 433−473 K. According to Figure 2a, the

the superior performance with the significantly enhanced cellulose conversion and yield of sugar alcohols based on the two ball-milling modes; therefore, ZPA was selected as the solid acid for the subsequent experiments. Figure 1 shows the influence of ball-milling time on cellulose transformation. The ball milling can significantly improve the

Figure 1. Influence of ball-milling time on hydrolytic hydrogenation of cellulose to sugar alcohols. Reaction conditions: cellulose, 1 g; ZPA, 0.9 g; 5 wt % Ru/C, 0.075 g; H2O, 50 mL; reaction temperature, 463 K; H2, 6 MPa; and reaction time, 2.5 h. For 0 h, the reaction time was 4.5 h.

reactivity of cellulose. The cellulose conversion increased from 34% for microcrystalline cellulose (without ball milling) to 87 and 63% for mixed and single ball-milled (ball-milling time of 1 h) cellulose, respectively. The overall time (the sum for reaction and ball-milling time) of ball-milled cellulose (3.5 h) is shorter than that of microcrystalline cellulose (4.5 h). For the single ball milling, the cellulose conversion and yield of sugar alcohols experienced a gradually increase with increasing the ball-milling time. A total of 90.3% of cellulose conversion and 70.7% of yield of sugar alcohols were obtained at ball milling of 4 h. Comparatively, the mixed ball-milled cellulose was completely converted at the ball milling of 2 h, and the highest yield of sugar alcohols reached 90.3%. This yield obtained is the highest yield in one-pot transformation of cellulose to sugar alcohols using a solid catalyst.33 Moreover, this technology also shows the obvious advantage with the remarkably shortened ball-milling and reaction time (total ballmilling and reaction time of 4.5 h), because the reported performance in hydrolytic hydrogenation of cellulose to sugar alcohols was accompanied by a longer reaction and/or ballmilling time (the overall time is more than 24 h).32,33 We have demonstrated that 85.5% of yield of sugar alcohols can be obtained from single ball-milled cellulose with using ZPA and Ru/C at the same reaction conditions, but the ball-milling time was 24 h.34 The above-mentioned results demonstrate that mixed ball milling of cellulose and solid acid catalyst is highly effective to promote cellulose conversion. In comparison to the previous reported results of transformation of acidulated cellulose,26,27 although our experiments require a higher reaction temperature, there is no neutralizing products or acid solution, which is eco-friendly and beneficial to suppress the side reaction of sugar alcohols. Considering the highest yield of sugar alcohols obtained at the ball-milling time of 2 h,

Figure 2. Effects of the (a) reaction temperature and (b) time on hydrolytic hydrogenation of mixed ball-milled cellulose to sugar alcohols. Reaction conditions: cellulose, 1 g; ZPA, 0.9 g; 5 wt % Ru/C, 0.075 g; H2O, 50 mL; H2, 6 MPa; ball-milling time, 2 h, (a) reaction time, 2.5 h; and (b) reaction temperature, 463 K.

cellulose conversion increased with increasing the reaction temperature and the cellulose can be completely converted at 463 K. As the reaction temperature increased, the yield of sugar alcohols also gradually grew and became the highest at 90.3% at the same temperature (463 K). However, as the temperature further increased to 473 K, the yield of sugar alcohols decreased to 82%, which is due to side products resulting from the produced sugar alcohols by dehydration and hydrogenolysis, as indicated by our previous report.34 The effect of the reaction time on the mixed ball-milled cellulose conversion was also investigated, and the corresponding results are shown in Figure 2b. In comparison to reaction temperature, the reaction time shows the moderate effect on cellulose conversion and yield of sugar alcohols. When cellulose conversion was lower than 100%, the yield of sugar alcohols experienced a similar upward trend. However, when cellulose 5781

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results indicate that both ball-milling modes reduce the crystallinity of cellulose. Apart from reducing the crystallinity, the particle size distribution of cellulose was also investigated (Figure 4). The

conversion was 100%, the target product yield decreased with prolonging the reaction time because the produced sugar alcohols were converted to byproducts. To ascertain whether the solid cellulose was degraded to water-soluble intermediates during the mixed ball milling, as indicated by the investigation by Blair and co-workers,23 the solubility of cellulose was tested after mixed ball milling. The result showed that the detectable soluble intermediates are no more than 1%, which is different from the report conducting the mechanocatalytic depolymerization of cellulose.23 Water is essential to disrupt the β−O−4 bond of cellulose during this process. The trace water-soluble intermediate after mixed ball milling is possibly attributed to trace water remaining with cellulose dried at 343 K and catalyst calcined at a higher temperature (such as 673 K). Characterization of Cellulose and ZPA. To further understand the mixed ball-milling process, the crystallinity of cellulose (microcrystalline cellulose and single and mixed ballmilled cellulose) was determined by the solid-state 13C NMR.35 As shown in Figure 3, the peaks between 60 and 110 ppm are

Figure 4. Particle diameter distributions of cellulose. BMC, single ballmilled cellulose; MCC, microcrystalline cellulose; ZPA + BMC, mixed ball-milled cellulose; and ball-milling time, 2 h.

average particle size of microcrystalline cellulose is around 20 μm. Surprisingly, regardless of ball-milling modes, the average particle size of cellulose is not significantly changed after ball milling. Besides, the single and mixed ball-milled cellulose have a similar size of about 22 μm, which was close to the reported results.28 To verify the diameter distribution of cellulose, the cellulose morphology was analyzed by SEM (Figure 5). The shape of

Figure 5. SEM micrograph of cellulose: (a) microcrystalline cellulose, (b) single ball-milled cellulose, and (c) mixed ball-milled cellulose. Ball-milling time = 2 h.

microcrystalline cellulose is fibrous, and the diameter is around 20 μm. After ball milling, the cellulose becomes a spherical particle and the diameter is also approximate to 22 μm. These results are well-consistent with those in Figure 4. Apparently, the length of cellulose decreased along the direction of the fiber after ball milling, illustrating ball milling breaking the cellulose fiber and resulted in microcrystalline cellulose to amorphous cellulose.36 The above-mentioned results illustrated that the mixed ballmilled cellulose was almost the same as the single ball-milled cellulose. Therefore, the excellent performance of the mixed ball milling might be ascribed to the solid acid catalyst. The properties of ZPA were displayed in Figure 6. ZPA is an amorphous structure before and after ball milling, and the surface area decreased from the fresh at 126.7 m2/g to the ball milled at 12.3 m2/g (Figure 6b). The single and mixed ball-milled cellulose have the similar crystallinity, average diameter distribution, and morphology, and the lower surface area of ZPA is supposed to worsen the catalytic performance of cellulose depolymerization. However, the catalytic performance of the mixed ball milling of cellulose and ZPA is better than the single ball milling in our cases, which infers that the mixed ball milling not only reduces the

Figure 3. 13C CP/MAS NMR spectra of (a) MCC and (b) mixed and single ball-milled cellulose. MCC, microcrystalline cellulose; BMC, single ball-milled cellulose; ZPA + BMC, mixed ball-milled cellulose; and ball-milling time, 2 h.

due to the C1−C6 carbons of the pyranose ring in cellulose. The signal from 86 to 92 ppm is assigned to the C4 carbon from the crystalline part of cellulose, whereas the field peak at 82−86 ppm corresponds to the amorphous part of cellulose. It can be seen that the peaks representing the crystalline part of cellulose clearly decreased by ball milling and the single and mixed ball-milled cellulose have a similar structure. These 5782

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Figure 6. Properties of ZPA before and after ball milling (2 h): (a) XRD and (b) N2 isothermal adsorption−desorption curves at 77 K.

Figure 7. (a) Conversion of concentrated cellulose (mixed ball milling) to sugar alcohols and (b) corresponding concentration of sugar alcohols. The cellulose concentration is the mass ratio of cellulose/water. Reaction conditions: H2O, 50 mL; reaction temperature, 463 K; H2, 6 MPa; reaction time, 2.5 h; ZPA, 0.9 g; 5 wt % Ru/ C, 0.075 g (5 wt %), 0.1 g (10 wt %), 0.125 g (15 wt %), and 0.15 g (20 wt %); and ball-milling time, 2 h.

crystallinity of cellulose but also possibly promotes the contact and interaction between cellulose and ZPA, thus enhancing the rate-determined cellulose hydrolysis to obtain the higher sugar alcohol yield. Hydrolytic Hydrogenation of Concentrated Cellulose. Transformation of concentrated cellulose is a challenge for heterogeneous catalysis,3 because the weak/medium acidity of solid acid and limited contact between solid catalyst and cellulose lead to its insufficient depolymerization. Here, we used this mixed ball-milling technology to convert concentrated cellulose while maintaining the amount of ZPA. Although the amount of ZPA is kept constant, sugars production will increase with increasing the cellulose concentration when the acidic sites of ZPA are not fully covered by cellulose. To completely hydrogenate the produced sugars, the dosage of Ru/C increased slightly with increasing the cellulose concentration. As shown in Figure 7a, the cellulose conversion was continuously decreased, while with the sugar alcohols, selectivity slightly increased when the cellulose concentration increased from 5 to 20 wt %. This led to the yield of sugar alcohols to decrease from 60.8% at 5 wt % of the cellulose concentration to 29.8% at 20 wt % of the cellulose concentration. The decreased cellulose conversion with its increasing concentration is potentially responsible for the limited acid sites of ZPA because its amount is fixed. Improving the concentration of sugar alcohols is an advantage for further using them. For further conversion, the concentrated sugar alcohols consume less energy during the condensation process compared to the dilute counterpart.

According to Figure 7b, the concentration of sugar alcohols increased with increasing the cellulose concentration and the highest concentration reached 67 mg/mL at the cellulose concentration of 20 wt %, which is similar to the result from hydrolytic hydrogenation of ball-milled cellulose using H4SiW12O40 and Ru/C catalysts with a longer overall time (24.33 h).32 The recovery of used ball-milled solid acid catalyst was difficult, because Ru/C was blended with the solid acid after reaction. To investigate the hydrothermal stability of the ballmilled ZPA, the sole ball-milled ZPA was subject to hydrothermal treatment under reaction conditions and then calcined at 673 K for 4 h before the mixed ball-milling process with cellulose. Figure 8 shows the stability of ZPA after ball milling. The cellulose conversion and yield of sugar alcohols slightly decreased after one hydrothermal treatment. These declines are perhaps caused by P leaching in the aqueous medium and/or the surface area decreasing with further ball milling. The former hypothesis is more likely, because the surface area of ball-milled ZPA with 4 h was the same as that with 2 h. Our previous report demonstrated that leaching P from ZPA was observed from the first hydrothermal treatment, and ZPA showed excellent hydrothermal stability after the first 5783

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Figure 8. Hydrothermal stability of ball-milled ZPA. Reaction conditions: cellulose, 1 g; ZPA, 0.9 g; 5 wt % Ru/C, 0.075 g; H2O, 50 mL; reaction time, 2.5 h; reaction temperature, 463 K; H2, 6 MPa; and mixed ball-milling time, 2 h. Hydrothermal treatment conditions: ball-milled ZPA, 0.9 g; H2O, 50 mL; reaction time, 2.5 h; reaction temperature, 463 K; and H2, 6 MPa.

hydrothermal treatment of ZPA.34 Therefore, ball-milled ZPA would also show favorable hydrothermal stability.

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CONCLUSION Mixed ball milling of cellulose and solid acid catalyst was proven as a highly effective pretreatment technology in hydrolytic hydrogenation of cellulose to sugar alcohols. In comparison to traditional single ball milling, the mixed ballmilling process showed the higher yield of sugar alcohols at a shorter ball-milling time because of the significantly enhanced contact between them. Among the solid acid catalysts, ZPA was demonstrated as the best catalyst, obtaining the highest 90.3% yield of sugar alcohols, when combined with 5 wt % Ru/C. This mixed ball-milling technology also adapted to the conversion of concentrated cellulose, and the concentration of sugar alcohols reached 67 mg/mL at 463 K. Besides, ZPA showed good hydrothermal stability.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-20-87057673. E-mail: [email protected]. cn. 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), and the Natural Science Foundation of Guangdong Province (S2013010011612 and S2012040006992).

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REFERENCES

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dx.doi.org/10.1021/ef500717p | Energy Fuels 2014, 28, 5778−5784