C6 Sugar Alcohols by Hydrolytic Hydrogenation of

May 26, 2017 - Producing chemicals from lignocellulosic biomass is important in view of the huge availability of biomass and positive environmental si...
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Research Article pubs.acs.org/journal/ascecg

Production of C5/C6 Sugar Alcohols by Hydrolytic Hydrogenation of Raw Lignocellulosic Biomass over Zr Based Solid Acids Combined with Ru/C Qiying Liu,*,† Tao Zhang,†,‡ Yuhe Liao,† Chiliu Cai,† Jin Tan,† Tiejun Wang,† Songbai Qiu,† Minghong He,† and Longlong Ma*,† †

CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, 510640, P.R. China ‡ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, P.R. China S Supporting Information *

ABSTRACT: Producing chemicals from lignocellulosic biomass is important in view of the huge availability of biomass and positive environmental significance by reducing carbon emission due to fast carbon cycle during biomass growth and applications. Here, we prepared zirconium based solid acids for hydrolytic hydrogenation of raw lignocelluloses to coproduce C5/ C6 sugar alcohols (the important platform for downstream chemicals and fuel production) as combined with commercial Ru/C. Among these solid acids, the amorphous zirconium phosphate (ZrP) presented the largest acidic sites, with medium and strong acidity as the majority, showing the highest goal sugar alcohols yield of 70% at optimal reaction conditions. During pennisetum transformation, this combined catalyst was reusable despite the activity of the second run being lower than the initial one, and the activity could be recovered by recalcination of spent ZrP. The primary structure of surviving lignin remained after cellulose and hemicellulose were converted, showing the significance for fractional biomass applications if considering the further transformation of lignin. KEYWORDS: Biomass, Sugar alcohol, Hydrolytic hydrogenation, Solid combined catalyst

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INTRODUCTION Lignocellulosic biomass produced by photosynthesis is the most abundant, renewable resource in nature.1 Using lignocellulosic biomass as the feedstock to produce C5/C6 sugar alcohols, which are widely used for downstream chemicals and fuels production, is of importance and attracted considerable concerns regarding the positive environment and resource significance.2 Among kinds of biomass resources, cellulose and hemicellulose possess about 60−80% of the total weight.3 Thus, efficient conversion of cellulose and hemicellulose into C5/C6 sugar alcohols is the key for biomass application. Traditionally, C5/C6 sugar alcohols (mainly xylitol and sorbitol) were mainly dependent on hydrogenation of glucose/ xylose at mild reaction conditions.4,5 Although high target products yield can be obtained by using supported metal © 2017 American Chemical Society

catalysts, the limited sugars from fermentation of edible starches are necessary for this process. Comparatively, using cellulosic biomass to produce C5/C6 sugar alcohols shows significant source preference. Microcrystalline and ball-milled cellulose are the most used feedstocks for producing sorbitol by hydrolytic hydrogenation, due to the relatively simple molecular structure and less side products.6 The catalysts used in this process mainly contained acidic catalysts for cellulose hydrolysis and metal catalysts for glucose hydrogenation to sugar alcohols.7,8 Mineral acids such as H2SO4,9 HCl,9 H3PO4,9,10 and heteropoly acids11,12 combined with Ru, Pd, Ir, and Pt7,13−18 catalysts showed high sugar alcohol yields Received: March 7, 2017 Revised: April 17, 2017 Published: May 26, 2017 5940

DOI: 10.1021/acssuschemeng.7b00702 ACS Sustainable Chem. Eng. 2017, 5, 5940−5950

Research Article

ACS Sustainable Chemistry & Engineering

prepared by the same procedure but with NH4NO3 saturated solution as the precipitator. The NH4NO3 saturated solution was directly added to the Na2SiO3 solution. Zr(OH)4 and Si(OH)4 were then blended with vigorous stirring at a molar ratio of Si/Zr = 3. The mixed precipitate was aged at 348 K. After filtration and complete washing with deionized water, the solid was dried at 393 K and calcined at 773 K for 5 h before use. SO42−/ZrO2 was prepared by a typical procedure. 5 g of ZrO2 (bought from Aladdin Industrial Inc., Shanghai, China) precalcined at 723 K was dispersion in 50 mL of H2O under agitation for several minutes. 50 mL of 0.5 M H2SO4 solution was then introduced. The mixed solution was further aged for 1 h at ambient temperature. The solid was recovered by filtering and drying at 353 K, and was calcined at 873 K for 3 h before use. WO3/ZrO2 catalyst was prepared by initial wetness impregnation. 0.27 g of ammonium metatungstate hydrate ((NH4)6H2W12O40· nH2O) was dissolved in 20 mL of H2O, and then 5 g of ZrO2 (obtained from Aladdin Industrial Inc., Shanghai, China) precalcined at 723 K was introduced into the above solution under continuous stirring. After 3 h, the excessive water was removed by evaporating at 333 K. The obtained solid was dried at 373 K overnight and calcined in air at 923 K for 3 h prior to reaction. The WO3 loading in the final catalyst was calculated as 5% by weight. Catalyst Characterization. X-ray powder diffraction (XRD) of the sample was measured with an X-ray diffractometer (X’Pert Pro MPD, Philip) with Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 100 mA. The BET specific surface area was measured by N2 isothermal adsorption−desorption profiles at 77 K using a QUADRASORB SIMP-10/PoreMaster 33 analyzer equipped with QuadraWin software. The mesoporous volume and pore size distribution were calculated by the desorption branch using the BJH method. Ammonia-temperature-programmed desorption (NH3-TPD) was conducted in a U-tube quartz reactor using an ASIQACIV200-2 automatic physical/chemical adsorption analyzer (Quantachrome, US). MS was used to determine the effluent NH3 from catalyst. The quantitative analysis of acidic sites was performed by using a calibration loop of 250 μL. H2-chemisorption of Ru/C catalyst was tested using a Quantachrome-ASIQACIV200-2 automated gas sorption analyzer. The catalyst was treated at 573 K for 2 h under hydrogen atmosphere. To clean the catalyst surface, the samples were further heated to 623 K for 0.5 h under nitrogen, and then cooled to 323 K for the H2-chemisorption test. TEM images were 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. SEM images were recorded using an S-4800 instrument operated at 2 kV. The samples were placed on a conductive carbon tape adhered to an aluminum sample holder. EDS was used to determine the P/Zr molar ratio of ZrP. Elemental analyses for Zr and Ru were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Plasma-Spec-II spectrometer. The filtrate was collected after hydrothermal pretreatment and diluted by water to the appropriate concentration to meet the detecting range of the instrument. FT-IR spectra were recorded on a Bruker TENSOR27 spectrometer with a resolution of 4 cm−1. The samples were pelleted with KBr before measurement. The pyridine absorbed FT-IR spectra were recorded on the same instrument. Prior to measurement, the selfsupported pellet was pretreated at 673 K for 1 h in He and then cooled to 423 K for recording the pristine FT-IR spectrum. Afterward, the pyridine vapor was introduced into the cell at 423 K and kept for 2 h until the saturated state was achieved. The excess pyridine was purged by He at the same temperature for 12 h. The spectra were obtained by subtracting the pristine ones. TG analyses were implemented on a NETZSCH-STA 409PC DSCSP thermal analyzer by increasing the temperature from 300 to 1000 K at 10 K/min under an air flow rate of 30 mL/min.

because of the efficient hydrolysis for cellulose by the abovementioned acids. However, these homogeneous acids are hardly recovered and result in wastewater emission. The supported metals Ru, Ir, Pt, Ni, and Rh on acidic supports were also used as bifunctional catalysts for this process, but the sugar alcohol yield and/or cellulose conversion are relatively low.19−22 Very recently, Fukuoka et al. prepared biomass derived carbon based catalyst for eucalyptus hydrolysis to sugars.23 This catalyst was highly efficient to obtain a target sugars yield of more than 90%. In addition, hemicellulose was also used as the feedstock to produce the corresponding sugar alcohols. Due to the amorphous nature, hemicellulose is easily hydrolyzed and hydrogenated to the goal products with the yield of more than 80% under relatively mild conditions.24,25 Compared to cellulose, raw lignocellulosic biomass has a more complex structure, which mainly consists of cellulose, hemicellulose, and lignin. Converting the former two components into C5/C6 sugar alcohols (mainly xylitol and sorbitol) by hydrolytic hydrogenation is more feasible based on resource priority. Different kinds of raw biomass, such as beet fiber and inulin (starch biomass), hardwood powder, wood chips, bagasse, rice husk, wheat straw, birch wood, etc., were used as the feedstock to directly produce sugar alcohols by using supported Ru, Pt, and Pt contained bimetal catalysts.26−31 The sugar alcohols yield is generally dependent on the class and component content of original biomass, and a high sorbitol/ xylitol yield of 50% could be obtained with assistance of watersoluble heteropoly acid for efficiently decomposing cellulose and hemicellulose.28,30 Using solid acids for this process shows significance, owing to their easy recovery, but relative research is rarely reported hitherto. Metal phosphates showed wide applications in hydrolysis of lignocellulosic biomass and dehydration of monosaccharides to furan derivatives as particular solid acid catalysts.32,33 Followed by our previous work for cellulose transformation to C6 sugar alcohols,34,35 we used kinds of raw lignocellulosic biomass as the feedstock to coproduce C5/C6 sugar alcohols by different zirconium based solid acids combined with commercial Ru/C in aqueous phase. The solid acids were screened based on their activity and target products yield. The process parameters of typical biomass were studied and the possible reaction pathway was discussed based on the synergistic effect of acid and metal catalyst.

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EXPERIMENTAL SECTION

Catalyst Preparation. Ru/C and ZrO2 (5 wt %) were purchased from Aladdin Industrial Inc. (Shanghai, China). ZrO2 was calcined at 723 K for 4 h before use. Zirconium phosphate (ZrP) was prepared by precipitation of ZrOCl2.8H2O with NH4H2PO4 at the P/Zr molar ratio of 2. The flocculent mixture was stirred for 1 h and then filtered. The white precipitate was washed with deionized water until the pH of the filtrate of 7, and was then dried at 373 K overnight. Before reaction, the ZrP was calcined at 673 K for 4 h in air. Comparatively, the 5 wt % Ru/ZrP was prepared by initial wetness impregnation. 0.135 g of RuCl3·xH2O (Ru > 37%) was dissolved in 30 mL of H2O to form a homogeneous solution under agitation. 1 g of freshly calcined ZrP was then introduced into the solution. The mixture was stirred for 5 h at room temperature followed by removing water at 333 K. The solid remaining was dried at 373 K overnight and calcined at 673 K for 4 h. Prior to reaction, the catalyst was reduced at 623 K for 2 h by flowing H2. SiO2−ZrO2 composite oxide was prepared by our previous report.36 Briefly, Zr(OH)4 precipitate was obtained by adding NH3·H2O into ZrOCl2·8H2O solution with continuous agitation. Si(OH)4 was 5941

DOI: 10.1021/acssuschemeng.7b00702 ACS Sustainable Chem. Eng. 2017, 5, 5940−5950

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ACS Sustainable Chemistry & Engineering Table 1. Conversion of Pennisetum to Sugar Alcohols by Different Solid Acids Combined with Ru/Ca Product yieldb (C-mol %) Solid acid

Conv (%)

C6 SG

C5 SG

c ZrO2 SiO2−ZrO2 WO3/ZrO2 SO42−/ZrO2 ZrP Ru/ZrPd Ru/C Single ZrPe

7.9 16.8 35.6 36.8 48.8 54.3 42.7 20.3 39.9

2.0 1.9 1.7 1.1 3.0 5.1 6.4 2.2 7.3

6.1

7.8

C6 SA

C5 SA

ERY

GLY

PD

EG

3.3 15.8 11.6 22.5 34.6 28.1 5.8 0.6

9.6 16.7 8.3 25.4 26.2 24.4 7.0 1.0

0.2 1.4 6.0 3.2 5.8 7.7 0.5

0.5 0.9 6.7 1.2 1.8 3.6 0.3

0.3 0.1 3.8 2.3 1.0 2.5

0.1 1.2 0.3

a

Reaction conditions: 473 K, 6 MPa of initial H2 pressure (room temperature), 2.5 h, pennisetum 1 g, H2O 50 mL, solid acid 0.5 g, Ru/C 0.1 g. bC6 SG: C6 sugar, C5 SG: C5 sugar, C6 SA: C6 sugar alcohol, C5 SA: C5 sugar alcohol, ERY: erythritol, GLY: glycerol, PD: 1,2-propanediol, EG: ethylene glycol. cNo catalyst was introduced. dCatalyst loading 0.5 g. eThe sole ZrP was introduced without Ru/C. Index Detector, RID 2414). A SUGAR SH1011 column (8 mm × 300 mm) was used to analyze the polyols and sugars using 0.5% H2SO4 aqueous solution as the mobile phase. An external standard method was used for quantification. The biomass conversion was determined by the weight difference of biomass before and after reaction, and was calculated as given by mbiomass,0 − mbiomass Conversion (%) = × 100% mbiomass,0 (1)

Biomass Transformation. 0.5 g of solid acid, 0.1 g of 5 wt % Ru/ C, 1 g of biomass (40−60 mesh and dried at 378 K to remove the moisture), and 50 mL of H2O were added into a 100 mL stainless steel autoclave. Before reaction, the reactor was flushed with hydrogen for several times to remove the residue air and pressurized to 6 MPa of hydrogen pressure at room temperature. Then, the reactor was heated to a given temperature under rigor agitation and kept at the given temperature for a certain period. After reaction, the reactor was quickly cooled to room temperature with iced water. The product mixture was separated by filtration, and the water-soluble products were detected by HPLC instrument. For reusability, the mixed catalyst (ZrP and Ru/C) was recovered by centrifuging the reaction mixture and thoroughly rinsing with deionic water, and it was directly used for the next cycle without any treatment. 93 wt % of the collected catalyst mixture could be recovered by drying at 373 K followed by calcination at 673 K, showing the efficiency of this separation procedure. The liquid was obtained with a 0.22 μm syringe filter for HPLC analysis, and the solid residue was rinsed by deionic water, and dried in vacuum for further analysis. After reaction, the dissolved total organic carbon (TOC) was analyzed by using an elemental analyzer (vario EL cube, Germany). The liquid was obtained by centrifuging catalyst followed by filtering solid residue, and then was diluted to a proper concentration for the TOC measurement. Materials and Product Analysis. Five raw biomass materials (switchgrass, corncob, cornstalk, pennisetum, and sugar cane bagasse) were collected from Guangdong province, China. These materials were air-dried and ground to 40−60 mesh before measurement. Analytic grade glucose and xylose were purchased from Aladdin Industrial Inc. (Shanghai, China). Microcrystalline cellulose (MCC, AvicelPH101, Fluka) was obtained from Sigma-Aldrich and dried overnight at 343 K prior to use. Biomass Composition. The moisture in raw materials was determined by drying at 378 K until constant weight obtained. The content of ash was analyzed by completely combusting the materials at 823 K for 4 h in air. The extractives in biomass were determined by Soxhlet extraction with 95% ethanol aqueous solution. The contents of glucan, xylan, and lignin of raw materials were determined according to the laboratory analytical procedure (LAP) for biomass analysis offered by the US National Renewable Energy Laboratory (NREL).37 The total sugars in the pretreatment liquid were calculated after a secondary hydrolysis of oligosaccharide into monosaccharide with 4 wt % sulfuric acid at 394 K for 45 min.38 Sugars and other degradation products were quantitatively determined by HPLC. The content of lignin was calculated by subtracting moisture, total sugars, ash, and extractives from raw biomass. The contents of inorganic elements in biomass materials were determined by wavelength dispersive X-ray fluorescence spectrometery (AXIOS mAX petro, Netherlands). Product Analysis. The aqueous products were detected by high performance liquid chromatography (HPLC; Waters e2695, Refractive

where mbiomass,0 is the weight of biomass charged in the reactor, mbiomass is the weight of residual biomass after reaction, and mbiomass was calculated as follows: mbiomass = mresidue − mcatalyst (2) where mresidue is the weight of solid residue after reaction and mcatalyst is the weight of charged solid catalysts (solid acid and Ru/C). The yield of product originated from cellulose and hemicellulose was determined by the carbon moles in the product and the carbon moles in the cellulose and hemicellulose charged in the reactor (the content of cellulose and hemicellulose was determined by biomass composition). The yield was calculated as given by Yproduct, i(%) =

C product, i Ccellulose + C hemi − cellulose

× 100%

(3)

where Cproduct,i is the carbon moles of product i, and Ccellulose and Chemi‑cellulose are the carbon moles of cellulose and hemicellulose charged in the reactor, respectively, which were calculated as follows: mcellulose Ccellulose = ×6 M(C6H10O5) (4)

C hemi ‐ cellulose =

mhemi ‐ cellulose ×5 M(C5H8O4 )

(5)

wh1ere M(C6H10O5) and M(C5H8O4) are the molecule weights of anhydroglucose and the anhydroxylose unit, respectively.

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RESULTS AND DISCUSSION Compositional Analysis of Raw Lignocellulosic Biomass. Table S1 gives the glucan, xylan, and lignin contents in different lignocellulosic biomass. Although these contents varied by different species, the cellulose (mainly glucan) and hemicellulose (mainly xylan) were in the range of 34−39%, with sugar cane bagasse having the maximum, and 17−31%, with corncob showing the maximum, respectively. The lignin content of biomass varied in 14−26%, and switchgrass possessed the largest one. In addition, all the biomass contained water residue of 6−11% and extractives, mainly including protein and lipid species, with the total content of 3−10%. The 5942

DOI: 10.1021/acssuschemeng.7b00702 ACS Sustainable Chem. Eng. 2017, 5, 5940−5950

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ACS Sustainable Chemistry & Engineering Table 2. Textural Property and Acid Density of Solid Acid Catalysts Entry

Catalyst

Surface areaa (m2/g)

Pore volumeb (cm3/g)

Average pore diameterb (nm)

Acidic densityc (mmol/g)

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

ZrO2 Spent ZrO2e SiO2−ZrO2 Spent SiO2−ZrO2e WO3/ZrO2 Spent WO3/ZrO2e SO42−/ZrO2 Spent SO42−/ZrO2e ZrP Spent ZrPe Ru/C Spent Ru/Ce Ru/ZrP Spent Ru/ZrPe

20.8 23.2 233.1 211.1 17.3 19.9 21.5 18.0 126.7 107.0 1125.4 982.1 104.1 87.2

0.04 0.05 0.96 0.80 0.07 0.06 0.12 0.10 0.31 0.27 0.28 0.25 0.27 0.22

27.5 30.0 7.9 8.6 21.1 23.8 24.7 26.1 4.8 5.6 3.8 4.3 5.1 6.1

0.12 0.10 0.62 0.55 0.44 0.29 1.19 0.67 1.75 1.71

1.10 0.90

Dd

0.51 0.39 0.19 0.13

a The surface area was estimated by the BET method using N2 at 77 K. bThe volume and average pore diameter indicate the mesoporous ones determined by using the desorption branch and BJH method. cThe acidic amount was calculated by NH3-TPD measurement and a calibration loop of 250 μL. dThe dispersion of Ru by H2-chemisorption. Due to the Ru/C was hardly isolated from ZrP after reaction; the spent Ru/C indicated that the fresh Ru/C was hydrothermally treated to simulate the reaction at the condition in Table 1. eThe spent solid acids after hydrothermal treatment were conducted at 473 K for 2.5 h, followed by drying and recalcination at the same temperature as to the preparation for the respective fresh ones. The spent Ru/C after hydrothermal treatment was dried and calcined at 623 K under N2 flow.

erythritol, glycerol, 1,2-propanediol, and ethylene glycol. Among sugar alcohols, the C5 counterparts presented as the majority, indicating that amorphous hemicellulose is more easily hydrolyzed to the relative sugars serving as precursor for C 5 sugar alcohols when compared to the crystalline cellulose.28,39 Over SiO2−ZrO2 and WO3/ZrO2 catalysts, both increased activities and C5/C6 sugar alcohols yield were observed. Meanwhile, enhanced side products by sugar/sugar alcohol hydrogenolysis were obtained over WO3/ZrO2. This is possibly responsible for the W species promoted hydrocracking of sugar intermediates and/or target sugar alcohols to the small molecular C2−C4 polyols.41 When SO42−/ZrO2 was used, both pennisetum conversion and total target sugar alcohols yield were significantly increased to 49% and 48%, respectively. Meanwhile, the increased C6 sugars intermediate and the reduced C2−C4 hydrogenolysis products were observed as compared to WO3/ZrO2. Interestingly, the pennisetum conversion of 54% and the highest C5/C6 sugar alcohols yield of 61% were obtained by using ZrP as the solid acid. It is noted that no C5 sugars intermediate was detected over all solid acids, again demonstrating the easier hydrolysis of hemicellulose and hydrogenation of C5 sugars. The sole Ru/C was used for this process, and much lower activity was observed as compared to the combined catalyst, implying that acid catalyst plays the important role in obtaining the target products with high yield by enhancing polysaccharides hydrolysis. Comparatively, over the only ZrP without Ru/C, despite the pennisetum conversion being about 40%, the target C5/C6 sugar alcohols were trace amounts due to the absence of metal for hydrogenation. In addition to the C5 and C6 sugars with the yield of about 7%, the dehydration products 5-hydroxymethyl furfural (the yield of 5%) and furfural (the yield of 2%) and rehydration product levulinic acid (the yield of 12%) and a trace amount formic acid/acetic acid (the total yield of 3%) were detected. This low yield is ascribed to the fact that the sugar intermediates and/or furan derivatives can be easily condensed into humins in acidic environment and single aqueous phase.

ash residue was obtained by combusting biomass at 823 K to remove organic species. As shown in Table S2, the elements were mainly composed of K, Si, Ca, Mg, Al, and Fe with the former three as the majority. Besides, a trace amount of P was also detected, but the contents were lower than 10 ppm no matter what kind of biomass was measured. Pennisetum Conversion to Sugar Alcohols. For lignocellulosic biomass transformation to C5/C6, sugar alcohols in aqueous solution, and H2 atmosphere, acid is necessary to hydrolyze cellulose and hemicellulose macromolecules to the relative C5/C6 sugars followed by hydrogenation to the target products over metal catalyst. Owing to the complex of sugar composition in a raw lignocellulosic biomass (glucose in cellulose and xylose, glucose, arabinose, and galactose in hemicellulose) and sugar isomerization to another one during acid catalyzed depolymerization (for example, from glucose to fructose),39,40 we proposed the possible glucose, fructose, and galactose as C6 sugars and the relative hydrogenated sorbitol, mannitol, and galactitol as C6 sugar alcohols; similarly, the xylose and arabinose were defined as C5 sugars, and the corresponding xylitol and arabitol were indicated as C5 sugar alcohols. The blank experiment was first implemented without using catalyst (Table 1). The pennisetum conversion was lower than 10% with minor C5/C6 sugars observed, indicating the essential role of catalyst in this process. We prepared different zirconium contained solid acids for coproducing C5 and C6 sugar alcohols from pennitsetum (a typical breeding grass for renewable chemical and fuel production) as combined with the commercial Ru/C. Due to the fact that acid catalyzed hydrolysis of cellulose and hemicellulose to relative sugars is proposed as the rate-determined step as compared to sugars hydrogenation,9,13 it is reasonable to focus on Ru/C as the hydrogenation catalyst and carry out the influence of solid acids in this process. As shown in Table 1, only 17% of pennisetum conversion was obtained by using ZrO2; meanwhile, the total C5/C6 sugar alcohols yield was about 12% with minor C6 sugars intermediate (mainly glucose and trace fructose and galactose) and hydrogenolysis products such as 5943

DOI: 10.1021/acssuschemeng.7b00702 ACS Sustainable Chem. Eng. 2017, 5, 5940−5950

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ACS Sustainable Chemistry & Engineering

Figure 1. SEM image (A) and FT-IR spectrum (B) of lignin residue originated from pennisetum transformation. The inset in (A) presented the pristine pennisetum.

of phenolic OH groups and side hydrocarbon chains of the phenolic ring, respectively, which are well consistent with the previous report.42 These findings demonstrated that the primary lignin structure was basically kept after the cellulose and hemicellulose were converted, showing significance for fractional and comprehensive utilization of biomass if considering further applications of the lignin component. The balance of acid and metal is the key for obtaining the target C5/C6 sugar alcohols in this process. According to the previous reports,21,25 the influence of molar ratios of surface acid density in ZrP (determined by NH3-TPD) to surface Ru atom in fresh Ru/C (determined by dispersion by H2chemisorption) on C5/C6 sugar alcohols yield was conducted to monitor this balance (Figure 2 and Table S3). When no ZrP

We measured the textural and acidic properties of solid catalysts in Table 2. The commercial ZrO2 showed low surface area of 21 m2/g, and its pore volume and size were 0.04 mL/g and 28 nm, respectively. Meanwhile, its acid density was the lowest, 0.12 mmol/g, with the weak acidity of below 523 K based on the NH3 desorption peak. After supporting WO3 and H2SO4, the textural properties changed a little, but the acid density and strength remarkably increased. For SiO2−ZrO2 prepared by sol−gel technology, the surface area and volume were the largest among those five solid acids and the pore size was about 8 nm. The acid density was 0.62 mmol/g with the weak acidity (the NH3 desorption peak was below 623 K). ZrP showed large surface area of more than 120 m2/g with moderate pore volume and size. Interestingly, the acid density presented the maximal 1.75 mmol/g with the medium and strong acidity as majority (Figure 4C). Dependent on the results in Table 2, it seems that pennisetum conversion and sugar alcohol products yield are not straightforward to the surface area and porosity, but determined by the acidic properties of solid acids, with those possessing more acidic sites and strong acidity showing higher activity and target products yield. The Ru/ZrP was tested under the same reaction condition (Table 1), but the obviously reduced activity and target sugar alcohols yield were observed as compared to ZrP combined with Ru/C. This is due to the fact that supporting Ru on ZrP decreased the acidic sites as well as the acidity of Ru/ ZrP besides the lower dispersion of Ru to Ru/C, despite its similar textural properties to pure ZrP (Table 2 and Figure S1). After reaction by ZrP combined with Ru/C, we collected the solid residue by centrifuging catalyst followed by filtration, rinsing with deionic water to remove organic compounds (this process was repeated for three times), and drying in vacuum at 323 K for 8 h. Compositional analysis showed that cellulose and hemicellulose in pennisetum were largely converted and lignin remained lignin as the majority (Table S1), indicating this combined catalyst is highly efficient for converting polysaccharides into sugar alcohols. As indicated in Figure 1A, the pristine pennisetum surface was dense and rather smooth (the inset in Figure 1A). After reaction, the surface became loose, and porous and irregular blocks were observed, which is due to the tracks remaining by inner hemicellulose and cellulose hydrolysis. Furthermore, the groups surviving in the lignin residue were detected by FT-IR (Figure 1B). The substituted phenolic ring (1609 cm−1, 1512 cm−1, 1211 cm−1, and 1110 cm−1) and carbonyl group (1705 cm−1) in lignin linked by esters were clearly observed. Meanwhile, a broad peak at 3410 cm−1 and a peak at 2930 cm−1 proposed the existence

Figure 2. Influence of molar acid/metal ratios on sugar alcohols yield in hydrolytic hydrogenation of pennisetum. The molar ratio was determined by fixing Ru/C while changing ZrP. Reaction conditions: 473 K, 6 MPa of initial H2 pressure (room temperature), 2.5 h, pennisetum 1 g, H2O 50 mL.

was introduced, both yields were lower than 7%, demonstrating the essential role of acidic ZrP for effectively hydrolyzing cellulose and hemicellulose. As ZrP was added in increasing amounts, both yields were raised significantly. The highest C5 and C6 sugar alcohols yields were observed at the ratios of 28.6 and 34.7, respectively. However, as the ratio was further increased, both yields reduced but the C5 sugar alcohols dropped more remarkably. Compared to C6 sugar alcohols, the maximal C5 sugar alcohols obtained at relatively low acid to metal ratio indicated that the easy decomposition of hemicellulose needed less acid and more metal. In this process, the monosaccharide intermediates encounter competitive acid5944

DOI: 10.1021/acssuschemeng.7b00702 ACS Sustainable Chem. Eng. 2017, 5, 5940−5950

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ACS Sustainable Chemistry & Engineering

Figure 3. Processing parameters including reaction temperature (A), reaction time (B), reaction pressure (C), and biomass concentration (D) in conversion of pennisetum to sugar alcohols. The reaction condition is the same as that in Table 1 apart from the individually varied parameter tested. For (B) the reaction temperature was 458 K, and for (D) the concentrations of 1%, 2%, 5%, and 10% mean 0.5 g, 1 g, 2.5 g,and 5 g of pennisetum were introduced in 50 mL of H2O, respectively.

5%, indicating the hydrogenolysis is suppressed at the temperature of 458 K. The influence of H2 pressure was presented in Figure 3C. Because H2 was consumed by hydrogenation, the higher H2 pressure promoted cellulose and hemicellulose transformation with obtaining the increased conversion and target sugar alcohols yield. On the other hand, the hydrogenolysis products were significantly reduced from initial 17% to final 4% as the H2 pressure increased from 4 to 8 MPa, which is responsible for the restraint of such kind of volume expanded hydrogenolysis. In Figure 3D, we also implemented a pennisetum concentration test over the same combined catalyst. As the concentration increased from 1% to 5% by weight, the pennisetum conversion reduced from nearly 60% to 32% and the C5/C6 sugar alcohols yield decreased from 68% to 38%, demonstrating this combined catalyst can be adaptable to high biomass concentration for producing target sugar alcohols. As the concentration was further increased to 10%, the pennisetum conversion and the total sugar alcohols yield reduced to 16% and 11%, respectively, and the C5/C6 sugars presented as the major intermediates, indicating incomplete conversion as well as partial hydrogenation occurred for the large amount of feedstock used in this case. Despite the fact that the individual C5/C6 sugar alcohol yield is lower than the previous reports with using cellulose/hemicellulose as the feedstock,16,17,25 the total yield of 70% produced from the raw biomass over the current solid combined catalyst is comparable to the previous report which was obtained in methanol and at higher reaction temperature.30 Considering lignin could be possibly degraded into oligomeric species and dissolved in water under the present reaction conditions (these species cannot be detected by HPLC), the TOC analyses of the aqueous product mixture were carried out to calculate carbon balance. The carbon

catalyzed degradation and metal-catalyzed hydrogenation simultaneously. It is possible that the excessive acid resulted in the sugar intermediates subjected to the other acid-catalyzed reactions rather than being hydrogenated to sugar alcohols, showing the reduced C5/C6 sugar alcohols. The results are consistent with the previous report.25 Taking that the ZrP combined with Ru/C showed the best performance in pennisetum conversion to C5/C6 sugar alcohols, we further implemented controlled experiments including reaction temperature, time, H2 pressure, and pennisetum concentration, focusing on this catalyst. As shown in Figure 3A, the pennisetum conversion increased with rising reaction temperature from 443 to 488 K, and the maximum C5/C6 sugar alcohols yield was obtained at 473 K. At low temperatures, C5 and C6 sugars presented as intermediates while levels were significantly reduced for C6 sugars and vanished for C5 sugars at temperatures higher than 473 K. The high temperature also resulted in increased side products, such as C2−C4 polyols from sugars and sugar alcohols, and isosorbide from sorbitol dehydration over acid catalyst.43 Considering more side products were detected at 473 K, we implemented the experiments at 458 K to more clearly monitor the intermediates tendency and possibly higher sugar alcohols yield at different times. As shown in Figure 3B, the pennisetum conversion and C5/C6 sugar alcohols yield increased with increasing reaction time and obtained the highest ones of 58% and 70%, respectively, at the time of 6 h. More than 20% of C5 sugars were observed, but no obvious C6 sugars were detected at initial reaction. This is due to the fact that the amorphous hemicellulose encompassed around crystalline cellulose first contacted the catalyst and thus lead to the preferable conversion to the relative sugars. As reaction proceeded, the C2−C4 polyols increased but the total yield was no more than 5945

DOI: 10.1021/acssuschemeng.7b00702 ACS Sustainable Chem. Eng. 2017, 5, 5940−5950

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. Physicochemical properties of fresh ZrP: (A) XRD pattern, (B) TEM image, (C) NH3-TPD profile, and (D) pyridine absorbed FT-IR spectra (a, pristine ZrP, and b, hydrothermally pretreated ZrP followed by recalcination).

Figure 5. Properties of fresh commercial Ru/C: XRD pattern (A) and TEM image (B).

of furans,44 are significantly suppressed. This is further evidenced by the comparison experiment using cellulose as the feedstock over the same combined catalyst (this is because the black humins could be easily resolved from the white cellulose if humins were produced). The photograph after reaction showed that no black solid was observed in solution and on the wall (the black solid at the bottom is originated from Ru/C, Figure S4), demonstrating no humins formed over the present catalyst. The physicochemical properties of ZrP and Ru/C catalyst are shown in Figures 4 and 5, respectively. The ZrP synthesized by our technology was amorphous with regard to the XRD pattern. The TEM images showed that its particle size was hardly resolved, and the HRTEM showed no well-defined lattice fringes, indicating the amorphous nature of ZrP. This is well consistent with the XRD measurement. NH3-TPD revealed that ZrP possesses one main desorption peak at 580 K with a wide shoulder extended to 850 K, which is responsible for significant medium and strong acidic sites on the surface.

balance was higher than 90% for all experiments investigated, indicating