Strategy for Conversion of Cellulose, Hemicellulose, and Lignin into

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Cascade Utilization of Biomass: Strategy for Conversion of Cellulose, Hemicellulose, and Lignin into Useful Chemicals Aritomo Yamaguchi,*,† Naoki Mimura,† Masayuki Shirai,†,‡ and Osamu Sato† †

Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai, Miyagi 983-8551, Japan ‡ Department of Chemistry and Biological Sciences, Faculty of Science and Engineering, Iwate University, Ueda 4-3-5, Morioka, Iwate 020-8551, Japan

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ABSTRACT: Efficient utilization of all three components of lignocellulosecellulose, hemicellulose, and ligninimpels efficient utilization of carbon dioxide fixed by photosynthesis. Conversion of all three of these polymeric components of lignocellulosic biomass into the corresponding monomeric chemicals was investigated using reusable solid catalysts in water without organic solvents, acids, or bases. Cellulose and hemicellulose were first converted into sugar alcohols such as sorbitol and xylitol using a carbon-supported platinum catalyst (Pt/C), H2, and bagasse as a reactant at 463 K for 16 h. The sugar alcohol yield was 64%. The solid residue from this process, which was mainly lignin and Pt/C, was recovered by simple filtration. Lignin in the solid residue was then converted into aromatic products at 673 K for 1 h. The yield of monomeric aromatics was 40%. After this process, only Pt/C catalyst was recovered as a solid residue, and it retained its activity for the conversion of all three components of lignocellulosic biomass into the corresponding monomeric chemicals throughout four runs. The two-step conversion of bagasse using Pt/C was thereby shown to be a powerful technique for converting the cellulose, hemicellulose, and lignin in lignocellulosic biomass into useful chemicals. KEYWORDS: Lignocellulosic biomass, Catalytic conversion, Depolymerization, Sugar alcohol, Aromatic monomer

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INTRODUCTION Conversion of lignocellulosic biomass into chemicals has attracted the attention of persons and organizations interested in establishing a sustainable society because it is abundant, renewable, and inedible.1,2 Lignocellulose consists of three main components: cellulose (40−50%), hemicellulose (20− 30%), and lignin (10−35%). The efficient separation of these three components is a difficult challenge.3,4 Efficient use of all three lignocellulose components impels efficient utilization of carbon dioxide fixed by photosynthesis. In the papermaking industry, lignin and hemicellulose in wood chips are removed by dissolution in an aqueous solution with chemicals to produce pulp. The lignin dissolved in the solution, the “black liquor”, is reconstructed and degraded; it is difficult to use as a chemical feedstock and is usually only burned for heat recovery. The conversion of waste lignin into valuable aromatics is also a possibility during the production of bioethanol from lignocellulosic biomass.5 Cellulose and hemicellulose are polysaccharides made from C5 and C6 sugars. They can be converted into valuable products via bioprocesses or chemical processes.6−10 Conversely, lignin is a complex, three-dimensional polymer of aromatic compounds connected by cross-linking via C−O−C © XXXX American Chemical Society

ether bonds and C−C bonds. Depolymerization of lignin to obtain valuable aromatic compounds has become common in recent years.11,12 However, use of lignin is generally difficult in combination with cellulose and hemicellulose use.13 Shikinaka et al. have reported enzymatic saccharification of the polysaccharide in lignocellulosic biomass such as cellulose and hemicellulose to soluble sugars (yield 69%) by wet-type bead milling. Almost all of the lignin fraction was recovered as solid residue.14 However, heterogeneous catalysts play a major role in the conversion of lignocellulosic biomass because the reaction rates are high and the catalysts are relatively stable and reusable compared with enzymes and homogeneous catalysts. Heterogeneous catalysts can convert cellulose and hemicellulose into useful chemicals. However, contamination of the catalysts by lignin after the reaction causes catalyst deactivation. Kobayashi et al. reported that a carbon-based catalyst converted lignocellulosic biomass in the form of Eucalyptus to glucose and xylose. The remaining lignin and the used catalyst could be converted to a fresh carbon-based Received: February 8, 2019 Revised: April 19, 2019 Published: April 23, 2019 A

DOI: 10.1021/acssuschemeng.9b00786 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Cascade utilization of biomass using solid catalysts. First, cellulose and hemicellulose are converted into sugar alcohols. Then, lignin is converted into aromatic compounds. The catalyst is reusable.

catalyst.15 In this case, lignin was not converted into useful chemicals. The separation of solid lignin from the catalysts is challenging. Lignin should therefore be removed from lignocellulose before the reaction by pretreatment so that the heterogeneous catalysts can be reused. In the Kraft pulping process, for example, pretreatment is achieved with the use of sodium hydroxide and sodium sulfide. In that case, however, the dissolved lignin is difficult to convert into valuable aromatics. One-pot conversion of lignocellulosic biomass into alkylcyclohexane and polyols using Ru/C and H2 in water was reported by Wang et al.16 In the paper, high yield (97%) of alkylcyclohexane was obtained from lignin. In this paper, we aim to obtain aromatic products from lignin. The aromatic products can be obtained by upgrading bio-oils derived from fast pyrolysis of lignocellulosic biomass.17,18 In that case, the products are a mixture of a wide variety of aromatic compounds having several functional groups. We have reported that the cellulose and hemicellulose in lignocellulosic biomass can be converted into sugar alcohols such as sorbitol and xylitol without delignification by using supported metal catalysts with hydrogen gas. The lignin remains as a solid after the reaction.19,20 In that case, separation of the solid lignin from the supported metal catalysts in the solid residue is required. We have also reported cleavage of the C−O−C ether bonds in lignin model compounds using supported metal catalysts at 673 K in supercritical water without hydrogenation of the aromatic rings.21,22 We therefore applied this technique in supercritical water to separation of the solid lignin from the supported metal catalysts by converting the lignin into soluble useful chemicals. Here, we describe a new strategy for conversion of cellulose, hemicellulose, and lignin into chemicals by using heterogeneous catalysts: cascade utilization of lignocellulose, which consists of (1) conversion of cellulose and hemicellulose, followed by (2) conversion of lignin into soluble aromatics at higher temperatures (Figure 1). We succeeded in converting lignin into aromatic compounds after the conversion of cellulose and hemicellulose. We also found that the supported metal catalyst was reusable. To the best of our knowledge, this is the first report of efficient conversion of all three components of lignocellulosic biomass into the corresponding chemicals using reusable solid catalysts in water without organic solvents, acids, or bases.

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solution (20 cm3) of cis-[Pt(NH3)2(NO2)2] with nitric acid (Furuya Metal Co., Ltd.) and carbon black (2 g) (BP2000, Cabot Corporation) was stirred at 293 K for 12 h. Water in the mixture was evaporated under reduced pressure at 323 K with a rotary evaporator. The samples were dried at 373 K for 10 h in an oven and then treated at 673 K for 2 h under flowing hydrogen gas to reduce platinum species (denoted as Pt/C). The amount of platinum in the catalyst was 4 wt %. The XRD patterns of the catalysts were recorded with a Rigaku SmartLab using Cu Kα radiation (λ = 0.15406 nm) with a current and voltage of 30 mA and 40 kV, respectively, in the 2θ range 30−80° with a 2θ step size of 0.02°. The TEM images were measured with an electron microscope (FEI TECNAI-G20) operated at an accelerating voltage of 200 kV. For TEM measurements, the catalyst powders were dispersed in methanol and then loaded onto a grid coated with carbon films. Reactant Preparation and Characterization. Bagasse chips (10 g) with a size < 2 mm were pulverized with a ball mill (ball diameter 15 mm, weight 2 kg) at 60 rpm for 48 h and then used as a reactant. Amounts of cellulose, hemicellulose, and lignin in bagasse were estimated in accord with published procedures.23 The total amount of cellulose and hemicellulose was defined as the amount of insoluble material when the bagasse was treated at 348 K in a NaClO2 aqueous solution containing dilute acetic acid. Cellulose was defined as the insoluble material when the cellulose and hemicellulose from the bagasse were treated at 293 K in an aqueous NaOH solution. Lignin was defined as the insoluble material when the bagasse was treated at 293 K in an aqueous H2SO4 solution. The sugar content of bagasse was determined in accord with published methods.24 The sugars were obtained from the cellulose and hemicellulose in bagasse by hydrolysis in an aqueous H2SO4 solution. The solution containing the sugars was analyzed with a highperformance liquid chromatograph (HPLC) equipped with a refractive index (RI) detector and a chromatography column (HPX87P, Aminex). Reaction Procedure. Figure 2 shows a flow diagram of the cascade utilization of bagasse. The conversion of the cellulose and hemicellulose in the bagasse to sugar alcohols was carried out in a batch reactor (OM Lab-Tech, MMJ-100, heater 600 W), the inner volume of which was 100 cm3.19,20 Bagasse (0.324 g), the Pt/C catalyst (0.3 g), and water (40 g) were loaded into the reactor. Hydrogen gas was then loaded into the reactor (5 MPa). The reactor was heated to 463 K and kept at that temperature for 16 h. Throughout that time, the contents were stirred with a screw at 600 rpm. After the reaction was complete, the contents of the reactor were filtered to separate the solid residue from the liquid fraction. The solid residue was used for the subsequent reaction (lignin conversion), as described in the following paragraph. The lignin content of this same solid residue was equated to the amount of solid residue insoluble in an aqueous solution of H2SO4 at 293 K. Quantitative analysis of the

EXPERIMENTAL SECTION

Catalyst Preparation and Characterization. Preparation of the catalyst has been described in previous papers.19,20 An aqueous B

DOI: 10.1021/acssuschemeng.9b00786 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Yield for sugar alcohol (%) = (sugar alcohol in product (mol))

/(sum of sugars in introduced bagasse (mol)) × 100

(1)

Yield for aromatic product (%) = (aromatic compound in product (mol))

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/(sum of aromatic rings in introduced bagasse (mol)) × 100 (2)

RESULTS AND DISCUSSION Conversion of the Cellulose and Hemicellulose in Lignocellulose. The weight percentages of cellulose, hemicellulose, and lignin in the bagasse were 37.0%, 31.4%, and 22.2%, respectively. The percentages of sugars in the bagasse (percentage of bagasse by weight) were 45.9%, 22.7%, and 1.4% for glucose, xylose, and arabinose, respectively.20 Table 1

Figure 2. Flow diagram of cascade utilization of biomass.

Table 1. Chemical and Elemental Composition of Bagasse water-soluble products in the liquid fraction was carried out with an HPLC equipped with a RI detector, a UV−vis detector, and a Phenomenex SC1211 column. The yield of total sugar alcohols was calculated based on the amount of total sugars in the bagasse reactant. The amount of total organic carbon in the liquid fraction was measured with a total organic carbon analyzer (Shimadzu, TOCVCSN). The amount of the other water-soluble products was defined as the difference between the amount of total organic carbon in the liquid fraction and the amount of total carbon in sugar alcohols. Amounts of metal species in the liquid fraction were measured using an inductively coupled plasma emission spectrometry (ICP) (Seiko, ICP-SPS 1500R). Conversion of lignin in the solid residue from the first reaction was carried out in a stainless steel 316 tube batch reactor with an inner volume of 6.0 cm3. The wet, solid residue and water were loaded into the reactor (total amount of water: 3.0 g), and the reactor was purged with inert gas (argon gas). The reactor was put into a molten-salt bath at 673 K for 1 h and then moved to a water bath to cool the reactor quickly. Gaseous products were analyzed by gas chromatography (Shimadzu, GC-8A) with a thermal conductivity detector and a chromatography column (Shincarbon ST). Liquid products in the reactor were collected with additional tetrahydrofuran (THF) and filtered to separate the solid materials (solid catalysts) from the liquid. The solid catalysts recovered in this way were reused, as described in the next paragraph. Liquid products were quantitatively analyzed by gas chromatography (HP-6890, Agilent) with a flame ionization detector and a capillary column (DB-WAX, Agilent). The yields of aromatic products based on carbon were calculated based on the moles of aromatic rings in the lignin of the reactant bagasse. The moles of aromatic rings in the reactant bagasse were calculated on the assumption that the lignin was composed of coniferyl alcohols connected with O-4 linkages. The molecular weight distributions of products in the THF were determined by a gel permeation chromatograph (GPC, Shimadzu) with an RI detector, a UV−vis detector, and GPC columns (KF801 and KF803L, Shimadzu). The recovered solid residue, mainly Pt/C, was dried at 373 K in an oven overnight and used again for the conversion of bagasse to sugar alcohols by hydrogenolysis at 463 K. The reaction procedures were the same in all runs, except that in runs 2−4 dried solid residue was used instead of fresh Pt/C catalyst. The yield of sugar alcohol was calculated based on the sugar content of the fresh bagasse reactant. The solid residue from this step, mainly Pt/C and lignin, was again treated at 673 K to convert lignin into aromatic products. The Pt/C catalyst was used four times for the hydrogenolysis of cellulose and hemicellulose at 463 K and for conversion of lignin at 673 K. The yields of sugar alcohols in the first step and aromatic products in the second step were defined as the following equations.

chemical composition (%)

elemental composition (wt %)

cellulose hemicellulose lignin oil/fat ash C H N S Cl

37.0 31.4 22.2 4.6 2.4 45.7 6.0 0.3 0.04 0.03

shows the chemical and elemental composition of the bagasse. The conversion of cellulose and hemicellulose in the bagasse was carried out in a batch reactor at 463 K for 16 h using Pt/C with H2, as reported in previous papers.19,20 The amounts of sorbitol, mannitol, xylitol, and arabitol produced in the reactor were 0.25, 0.044, 0.14, and 0.065 g, respectively, per gram of dry bagasse. The yield of total sugar alcohols was 64.0% based on the total moles of sugar in the reactant bagasse. This yield indicated that the cellulose and hemicellulose in the bagasse could be converted into sugar alcohols by Pt/C with H2 in water. We confirmed that platinum species were not detected in the aqueous solution by ICP measurement, indicating that platinum species remained on the carbon support. The solid residue remaining after the reaction and the aqueous solution were separated by filtration. We treated the wet, solid residue at 673 K without drying, as described in the next section. When the solid residue was dried, the weight of solid residue from the bagasse, which was calculated by subtracting the weight of Pt/C from the recovered solid residue, was 0.268 g per gram of dry bagasse.20 The 22.2 wt % of lignin in the bagasse indicated that the recovered solid residue was composed of mainly lignin, Pt/C, and a small amount of unreacted cellulose. This conclusion is consistent with (1) the 90.1% conversion of cellulose under the same reaction conditions when pure cellulose was the reactant instead of bagasse and (2) the ease with which hemicellulose can be converted into sugar alcohols, even under milder conditions (428 K).25 Actually, the weight percent of Klason lignin in the solid residue recovered from the bagasse conversion was 77.0%. The Klason lignin was insoluble in an aqueous H2SO4 solution at 293 K. After the conversion of cellulose and C

DOI: 10.1021/acssuschemeng.9b00786 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Table 2. Amount of Product, Total Yield of Sugar Alcohol, and Solid Remaining after Conversion of Bagasse over 4%Pt/C with 5 MPa H2 at 463 K for 16 ha amount of product (g/biomass-g) run number

sorbitol

mannitol

xylitol

arabitol

total

sugar alcohol yield (%)

solid remaining (g/biomass-g)

1 2 3 4

0.252 0.294 0.278 0.260

0.044 0.037 0.040 0.044

0.140 0.185 0.168 0.166

0.065 0.038 0.053 0.059

0.502 0.554 0.540 0.529

64.0 70.5 68.8 67.6

0.268 -

a

First run, 0.324 g of bagasse and 0.3 g of 4%Pt/C; second through fourth runs, 0.324 g of bagasse and solid residue (mainly Pt/C) after treatment at 673 K (Table 3).

and methoxy groups. The products of lignin depolymerization are thus distributed mainly among phenolic aromatic compounds in most cases.26−36 Most of the products of lignin depolymerization by the method we used were aromatic compounds lacking phenolic groups. We believe that the process we used to convert the cellulose and hemicellulose explains why there were fewer phenolic aromatic compounds than aromatic compounds lacking phenolic groups. In the first step, the cellulose and hemicellulose in the bagasse were converted into sugar alcohols via hydrogenolysis at 463 K with hydrogen and Pt/C. During this process, most of the lignin was recovered as a solid; however, functional groups such as phenolic and methoxy moieties may be reduced with hydrogen and Pt/C. When bagasse was treated with Pt/C at 673 K for 1 h without prior hydrogenolysis of cellulose and hemicellulose at 463 K, the yields of phenol, methylphenol, ethylphenol, and propylphenol were 10.9%, 5.3%, 8.2%, and 0.24%, respectively. Conversely, the yields of benzene, toluene, ethylbenzene, and 1-propylbenzene were 0.73, 3.6%, 1.4%, and 0.33%, respectively. Phenolic aromatic compounds were therefore clearly the main products of depolymerizing the lignin in bagasse without prior hydrogenolysis of the cellulose and hemicellulose at 463 K. The indication is that functional groups such as phenolic and methoxy moieties were reduced with hydrogen and Pt/C in the first step. During treatment of the solid residue at 673 K for 1 h, small amounts of gaseous products were also observed. The number of carbon atoms in the gases corresponded to 15.0% of the lignin carbon in the reactant bagasse. The gas composition was 46.1%, 12.7%, 38.7%, and 2.6% CH4, CO2, C2H6, and H2, respectively. The H2 caused the lignin to depolymerize via hydrogenolysis of C−O−C ether bonds between lignin monomer units.21 Treatment of the solid residue from the first step at 673 K allowed us to use gas chromatography flame ionization detection (GC-FID) and gas chromatography thermal conductivity detection (GC-TCD) to identify the products as aromatic monomers and gases. However, the materials balance of the products determined by GC was less than 100%. After treatment of the solid residue at 673 K, we recovered the contents in the reactor with THF. Organic materials with a few aromatic rings, which could not be identified with GC-FID, may have been dissolved in the THF recovery solution. The observation of a peak at a molecular weight of ca. 370 in the gel permeation chromatograph (GPC) of the recovery solution indicated that organic molecules having two or three aromatic rings were dissolved in the recovery solution, which could not be determined by GC. We therefore succeeded in depolymerizing the lignin and recovering the supported metal catalyst in the solid residue.

hemicellulose into sugar alcohols, comparison of the amounts of lignin in the recovered solid residue and reactant bagasse indicated that 94% of the lignin remained in the solid residue. Conversion of the bagasse was investigated without Pt/C with 5 MPa H2 for 16 h at 463 K to understand the effects of the Pt/C catalyst. Any sugar alcohols were not detected in the recovered solution; however, the amount of recovered solid residue was almost the same as that with Pt/C. In the first step, cellulose and hemicellulose were hydrolyzed to the corresponding sugars (e.g., glucose, mannose, xylose) at 463 K. The produced sugars were immediately hydrogenated to sugar alcohols with H2 on platinum surfaces. The yield of the other water-soluble products such as ethylene glycol, propylene glycol, glycerin, and erythritol was ca. 29% based on the total organic carbon content of the recovered aqueous liquid and the amount of carbon in the cellulose and hemicellulose. The cellulose and hemicellulose in the bagasse were converted into sugar alcohols (64%) and other water-soluble products (ca. 29%). We confirmed that the degradation of the sugar alcohols into other water-soluble products occurred slowly.19 Conversion of Lignin in the Solid Residue. Lignin in bagasse is a complex polymer of three aromatic monomers such as p-coumaryl, coniferyl, and sinapyl alcohols with several types of linkages. A rigorous calculation of moles of aromatic rings from the lignin weight is therefore difficult. In this paper, the moles of aromatic rings and yield of aromatic products were calculated on the assumption that coniferyl alcohol, which has a molecular weight equal to the average of three aromatic monomers, is connected with a β-O-4 linkage, which is the dominant linkage (ca. 50%) in lignin. In this case, the moles of aromatic rings are calculated to be 1.2 × 10−3 mol in 1 g of the reactant bagasse. The wet, solid residue from the first reaction, which was composed of mainly lignin, Pt/C, and a small amount of unreacted cellulose, was treated at 673 K for 1 h with water (0.5 g cm−3) to depolymerize the lignin into aromatic compounds and to recover the Pt/C catalyst. The partial pressure of water at 673 K and water density of 0.5 g cm−3 is 37.1 MPa in the supercritical phase. The lignin was successfully depolymerized. The fact that the weight of the solid residue after treatment at 673 K was only 1.9% of the weight of the reactant bagasse indicated that only the Pt/C catalyst was recovered as solid. The yields of benzene, toluene, ethylbenzene, and propylbenzene were 2.5%, 4.8%, 16.7%, and 11.9%, respectively. Conversely, phenol, methylphenol, ethylphenol, and propylphenol were observed at yields of