Cellulose Hydrolysis Using Oxidized Carbon ... - ACS Publications

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Cite This: Ind. Eng. Chem. Res. 2017, 56, 14471−14478

Cellulose Hydrolysis Using Oxidized Carbon Catalyst in a Plug-Flow Slurry Process Abhijit Shrotri, Hirokazu Kobayashi, Hiroyuki Kaiki, Mizuho Yabushita, and Atsushi Fukuoka* Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-Ku, Sapporo, Hokkaido 001-0021, Japan S Supporting Information *

ABSTRACT: The catalytic conversion of cellulose to glucose at the industrial scale is a sustainable approach to the production of fuels and chemicals. Herein, we report the hydrolysis of cellulose to glucose using an inexpensive carbon catalyst in a continuous slurry process. A carbon catalyst prepared by air oxidation showed the highest activity for cellulose hydrolysis owing to the large number of weakly acidic functional groups. The air-oxidized carbon catalyst hydrolyzed cellulose in a plug-flow slurry reactor after mix-milling to produce soluble β-1,4-glucans. Further hydrolysis of the β-1,4glucans to glucose was achieved using a fixed-bed reactor containing Amberlyst-70 catalyst in series with the slurry reactor to obtain glucose in 59% yield. Another approach was to use dilute H3PO4 for the hydrolysis of the β-1,4-glucans to glucose with a 70% yield, resulting in a space time yield of glucose of 456 kg m−3 h−1. The simple design, short residence time, and high space time yield will enable the scaleup of this process using existing chemical technology.

1. INTRODUCTION Lignocellulosic biomass, abundantly available as an agricultural waste and energy crop, is a carbon-based renewable feedstock for the synthesis of liquid fuels and chemicals.1−4 Large-scale processing of lignocellulose is necessary to replace the enormous global demand for fossil-fuel-derived chemicals.5 The design of a feasible and cost-effective catalytic process for lignocellulose conversion is challenging because of the recalcitrant structure and disparate composition of lignocellulose.6,7 Hydrolysis of cellulose, the largest and the most recalcitrant component of lignocellulose, is the first step in the synthesis of many value-added chemicals (Figure 1).8 The commercial application of catalytic cellulose hydrolysis is currently prevented by many factors including the efficient synthesis of an active catalyst and the lack of a scalable continuous hydrolysis process. Carbon catalysts containing acidic functional groups have the potential for cellulose hydrolysis at a large scale. The polyaromatic surface of carbon materials strongly adsorbs cellulosic molecules through CH−π and hydrophobic interactions.9 The adsorbed molecules are then hydrolyzed by acidic functional groups present on the carbon surface.10−12 Sulfonic acid groups introduced by sulfuric acid treatment can hydrolyze β-1,4-glycosidic bonds.10,11 However, the stability of sulfonic acid groups under hydrothermal conditions is not very high, and the active sites are lost at the temperatures required to achieve high rates of reaction.13 Moreover, the synthesis of catalysts containing sulfonic acid groups generates large amounts of acid waste that must be treated before disposal. Alternatively, carbon catalysts decorated with weakly acidic © 2017 American Chemical Society

carboxyl and hydroxyl groups are hydrothermally stable and show high activity for cellulose hydrolysis.12,14 Weakly acidic carboxyl and hydroxyl groups can be introduced through the oxidation of carbon materials. Oxidizing agents such as HNO3 and H2O2 are commonly used for the oxidation of carbon materials.15 However, disposal of the spent oxidizing agents causes an economic and environmental burden on the process. Air oxidation is a promising method for the synthesis of weakly acidic carbon catalysts because it utilizes atmospheric oxygen as the oxidant.16 Nevertheless, processes for carbon-catalyzed cellulose hydrolysis are limited to laboratory-scale demonstrations that cannot be scaled up for commercialization. The development of a continuous hydrolysis process using a carbon catalyst is necessary to achieve the processing capacity required to replace the demand for fossil-derived fuels and chemicals. The nature of cellulose, an insoluble polymer, makes the development of a continuous process rather difficult. Pilotand industrial-scale continuous hydrolyses of cellulose were previously attempted using homogeneous acid catalysts.17 A plug-flow process with H2SO4 catalyst (0.5−2.0%) could process concentrated lignocellulose to yield glucose with 52% yield.18 However, the economic viability of these processes was difficult because of the costs of acid separation and neutralization. It has been claimed that heterogeneous carbon catalysts for cellulose hydrolysis have overcome these issues Received: Revised: Accepted: Published: 14471

September 22, 2017 November 20, 2017 November 21, 2017 November 21, 2017 DOI: 10.1021/acs.iecr.7b03918 Ind. Eng. Chem. Res. 2017, 56, 14471−14478

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Industrial & Engineering Chemistry Research

Figure 1. Glucose produced by the hydrolysis of cellulose is a gateway for the synthesis of valuable chemicals and precursors in biomass-based industries.

Figure 2. Schematic showing the steps of cellulose hydrolysis using a carbon catalyst. Initial hydrolysis occurs at the solid−solid interface, followed by reaction at the solid−liquid interface when the β-1,4-glucans dissolve in water. After the reaction, the dissolved products and the catalyst are easily separated.

through the easy separation of the catalyst.10,11,19,20 The choice of the reactor for the continuous hydrolysis of cellulose using a carbon catalyst is crucial. Initial hydrolysis occurs at the solid− solid interface between cellulose and the catalyst particles until the β-1,4-glucans formed are small enough to dissolve in water (Figure 2).21 Subsequent hydrolysis occurs on the solid−liquid interface through the adsorption of dissolved β-1,4-glucans on the catalyst surface. Vigorous mechanical mixing is important to promote the interactions between the solid catalyst and the cellulose particles in the first step. Therefore, laboratory-scale batch reactors equipped with mechanical stirrers are used for catalyst design and screening. Scaling up a batch reactor to a continuous stirred tank reactor (CSTR) has limitations because of the energy required for stirring and the difficulty in designing large pressurized CSTRs. A plug-flow slurry reactor is ideally suited for the large-scale conversion of cellulose. However, the lack of a stirring mechanism in a plug-flow reactor limits the catalyst and cellulose interactions. The preferential adsorption of cellulose onto carbon catalysts offers a solution to facilitate the use of a plug-flow continuous process for cellulose hydrolysis. Longer-chain β-1,4-glucans present in cellulose are favorably adsorbed on carbon, as the adsorption equilibrium constant increases with the degree of polymerization.22 The strong interaction of β-1,4-glucans with the catalyst surface can eliminate the requirement for vigorous mechanical stirring to promote interparticle contact. Therefore, a simple plug-flow slurry reactor can be used after the adsorption of cellulose onto the carbon surface. However, the insolubility of cellulose in conventional solvents prevents the adsorption of cellulose on the carbon catalyst. Alternatively, cellulose can also be adsorbed on carbon by ball-milling them

together in a pretreatment step called mix-milling.12,23 Ballmilling alters the crystalline structure of cellulose and makes a solid mixture of cellulose and carbon. In this study, we report the synthesis of carbon catalysts using different oxidizing methods and compare their activities for cellulose hydrolysis. We also demonstrate a process for the hydrolysis of cellulose and lignocellulose to soluble β-1,4glucans and the monomeric sugars glucose and xylose in a continuous plug-flow reactor using an air-oxidized carbon catalyst.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Oxidized Carbon Catalyst. Dried activated carbon denoted as BA (Hokuetsu BA, Ajinomoto Fine-Techno) was used for the preparation of all oxidized carbon catalysts. Air oxidation was performed by spreading 4 g of BA to a thickness of 3−5 mm on a Pyrex dish (ø130 mm) and calcining under air in an electric furnace (DenkenHighdental, KDF S90) with the following program: increase from 25 to 425 °C at 5 °C min−1 and hold at 425 °C for 10 h. This sample is denoted as BA-Air. Nitric acid oxidation was performed by slowly adding 80 mL of ice-cold 68% HNO3 (aqueous) to 4 g of BA and stirring the mixture at room temperature in the absence of light for 24 h. The recovered solid was washed with hot and cold water until the pH of the filtrate was greater than 5. This sample was dried under a vacuum and is denoted as BA-HNO3. Hydrogen peroxide oxidation was performed by adding 30 mL of cold 30% H2O2 (aqueous) solution to 3 g of BA and stirring the mixture for 48 h at room temperature. The solid was recovered by filtration and then washed with 0.1 M oxalic acid and then repeatedly 14472

DOI: 10.1021/acs.iecr.7b03918 Ind. Eng. Chem. Res. 2017, 56, 14471−14478

Article


Figure 3. Cellulose hydrolysis process in a plug-flow slurry reactor and fixed-bed reactor for continuous hydrolysis to β-1,4-glucans and monomers.

Table 1. Characterization and Cellulose Hydrolysis Activity of Oxidized Carbon Catalystsa entry

catalyst

weight lossb (%)

BET surface area (m2 g−1)

total acidity (mmol g−1)

carboxyl group density (mmol g−1)

cellulose conversion (%)

glucose yield (%)

1 2 3 4 5 6

BA BA-Air BA-HNO3 BA-H2O2 BA-KOH blank

− 52 11 1 42 −

1230 870 1080 1020 2040 −

0.3 2.6 1.1 0.6 1.9 −

0.1 1.0 0.5 0.2 0.8 −

48 68 59 55 60 33

15 35 27 22 23 2.1

a

Hydrolysis reaction conditions: ball-milled cellulose (324 mg), carbon catalyst (50 mg), and water (40 mL) were added to the high-pressure reactor and heated to 230 °C and then rapidly cooled. bWeight loss of carbon during oxidation.

mg of catalyst were added to the reactor with 40 mL of water. The reaction mixture was heated under stirring to 230 °C, and then the temperature was immediately reduced back to room temperature using an air blower. The solid was separated by centrifugation, dried, and weighed to determine the cellulose conversion, and the liquid was analyzed to determine the product yield. 2.4. Mix-Milling. Microcrystalline cellulose (Avicel PH101) or Eucalyptus (5 g) and BA-Air (0.77 g) were milled together in an Al2O3 pot (250 mL) with alumina balls (ø5 mm, 200 g) using a Fritsch P-6 planetary ball mill. The milling conditions were 500 rpm for 2 h with a 10-min interval after every 10 min of milling. 2.5. Hydrolysis in a Flow Reactor. The hydrolysis reaction was performed by mixing 1.87 g of mix-milled substrate in 200 mL of solvent (water or 4.5 mM H3PO4) and feeding the solution into the pneumatic pump reservoir (Figure 3). The flow reaction system was pressurized to 3.0− 3.5 MPa using N2 gas at a flow rate of 35 mL min−1 through a bypass gas line. After the system pressure and reactor temperature had become stable, the N2 gas flow was diverted inside the pneumatic pump to start the slurry flow. The oil bath temperature for the slurry reactor was maintained at 220 °C for the hydrolysis of mix-milled cellulose and 240 °C for the hydrolysis of mix-milled Eucalyptus. The fixed-bed reactor was filled with 4 g of as-received Amberlyst-70. The catalyst bed temperature was maintained at 140 °C. The fixed-bed reactor was bypassed using three-way valves when not in operation. Samples were collected from the outlet of the back-pressure regulator at regular intervals and analyzed. 2.6. Product Analysis. Liquid products obtained from the batch and flow reactors were analyzed by high-performance liquid chromatography (Shimadzu; refractive index and ultraviolet detectors) with a SUGAR SH1011 column (Shodex; ø8 × 300 mm; eluent, water at 0.5 mL min−1; 50 °C). The composition of Eucalyptus was determined by the NREL/TP510-42618 method on a dry basis.25 The yields of glucose and

with water. This sample was dried under a vacuum and is denoted as BA-H2O2. Alkali oxidation was performed by mixing 0.6 g of BA with 1.8 g of KOH in a mortar. The mixture was spread on a stainless steel film and then heated under a N2 flow (20 mL min−1) at 700 °C for 1 h in a tube furnace. The recovered solid was washed first with water, then with 50 mL of 1 M HCl solution, and then again repeatedly with water. The washed solid was dried under a vacuum and is denoted as BAKOH. 2.2. Catalyst Characterization. Surface oxygenated functional groups were characterized by the titration method reported by Boehm.24 For the titration, oxidized carbon (200 mg) was stirred in 20 mL of 0.05 M solutions of NaOH and NaHCO3 for 24 h. Subsequently, the solutions were filtered, and 10 mL of 0.05 M HCl solution was added to 5 mL of each of the filtrates. The resulting solutions were titrated with 0.05 M NaOH using methyl orange as an indicator to determine the presence of oxygenated functional groups. The amounts of NaHCO3 and NaOH consumed were attributed to carboxyl groups and total acidic functional groups, respectively. Temperature-programmed desorption (TPD) of oxygenated functional groups was performed in a MicrotracBel instrument by heating the catalyst under a He flow (30 mL min−1) to 1000 °C at 10 °C min−1 and detecting the evolution of gases using a mass spectrometer. Surface area was calculated by applying the Brunauer−Emmett−Teller (BET) approximation to the N2 adsorption isotherms of the catalysts measured at −196 °C using a Belsorp mini analyzer. Raman spectra were collected using a Renishaw inVia Reflex spectroscope with a 532-nm laser. 2.3. Cellulose Hydrolysis in a Batch Reactor. Batch reactions were performed in an MMJ-100 batch reactor obtained from OM Lab-tech. Microcrystalline cellulose (10 g, Merck, column chromatography grade) was ball-milled for 96 h at 60 rpm prior to the reaction with 1 kg of 10-mm-diameter zirconia balls in a ceramic pot using an ANZ-51S rotator. For each hydrolysis reaction, 324 mg of ball-milled cellulose and 50 14473


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Figure 4. Characterization of BA and BA-Air catalysts. (A) CO2 and (B) CO evolution during TPD under He flow. (C) N2 adsorption isotherm. (D) Raman spectra measured using a 532-nm laser.

xylose from Eucalyptus were based on the glucan and xylan contents determined by the NREL method.25

treatment introduced 1.9 mmol of acidic functional groups and 0.8 mmol of carboxyl groups per gram. Both HNO3 and H2O2 were less effective for introducing acidic functional groups. Consequently, the degree of oxidation was related to the weight loss as the severity of oxidation increased. 3.2. Cellulose Hydrolysis in a Batch Reactor. The activities of the carbon catalysts for cellulose hydrolysis were tested in a batch reactor (Table 1). In the absence of carbon catalyst, the cellulose conversion was 33% with a glucose yield of only 2.1% . Unoxidized BA catalyst increased the conversion to 48% and the glucose yield to 15%. The activity of BA was attributed to the presence of a small number of acidic functional groups and the favorable interactions between cellulose and the carbon surface. The cellulose conversion in the presence of BA-Air was 68% with a glucose yield of 35% (Table 1, entry 2). The activities of other catalysts for cellulose conversion were lower than that of BA-Air. The cellulose conversions were proportional to the total acidities of the carbon catalysts. The glucose yield in the presence of BA-KOH was lower than that in the presence of BA-HNO3, even though the former had a higher conversion and a greater number of oxygenated functional groups. The lower glucose yield on BAKOH was attributed to the strong adsorption of products in the micropores of BA-KOH. Therefore, simple air oxidation was determined to be the most effective way for introducing a large number of oxygenated functional groups that show a high activity for cellulose hydrolysis. 3.3. Characterization of BA-Air. BA-Air was characterized in more detail to identify the structural and chemical changes occurring during air oxidation. Temperature-programmed desorption (TPD) of a carbon catalyst under an inert atmosphere releases CO and CO2. The evolution of CO and CO2 can be used to determine the concentrations of functional groups. The TPD of BA released 0.17 mmol of CO2 g−1, which increased to 1.2 mmol g−1 after air oxidation (Figure 4A). The amount of CO2 evolved from BA-Air agreed with the number of carboxyl groups determined by titration. The amounts of CO evolved from BA and BA-Air during TPD were 0.41 and 5.3

3. RESULTS AND DISCUSSION 3.1. Carbon Oxidation and Characterization. Activated carbon is the cheapest source of high-surface-area carbon material suitable for catalysis. Therefore, we used a biomassderived activated carbon denoted as BA for the synthesis of oxygenated carbon catalysts. This carbon was oxidized using air, HNO3, H2O2, and KOH (Table 1). After air oxidation, 52 wt % of the catalyst was lost in the form of CO and CO2 as a result of the partial combustion of BA. The surface area of BA-Air was 870 m2 g−1, a slight reduction from the 1230 m2 g−1 of BA. After nitric acid oxidation, 11 wt % of the carbon was lost, and the surface area decreased to 1080 m2 g−1. The weight loss after H2O2 oxidation was only 1% as H2O2 is the weakest of all of the oxidizing agents used. The surface area of BA-H2O2 was 1020 m2 g−1. Treatment of BA with KOH resulted in a weight loss of 42% and an increase in the surface area to 2040 m2 g−1. Alkali treatment creates micropores in carbon materials that increase the surface areas of such catalysts.26 The different oxidations of BA introduced varying amounts of acidic functional groups on carbon surface. The numbers of acidic functional groups were determined by titration of the solids with NaOH and NaHCO3. NaOH reacts with all acidic functional groups, and it represents the total acidity of the catalyst. NaHCO3 is a weak base that reacts with carboxylic acids and sulfonic acids. As the presence of sulfonic groups is unlikely in our catalysts, the consumption of NaHCO3 was attributed to the presence of carboxylic acids. Among weak acid sites, carboxylic acid sites are the most active functional group for the hydrolysis of β-1,4 glycosidic bonds.27 Unoxidized BA contained 0.30 mmol of acidic functional groups per gram, with 0.10 mmol of carboxyl groups per gram. Among all of the oxidation methods, air oxidation was the most effective for the introduction of acidic functional groups (Table 1). The concentrations of acidic functional groups and carboxyl groups in BA-Air were 2.6 and 1.0 mmol g−1, respectively. Alkali 14474


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Industrial & Engineering Chemistry Research mmol g−1, respectively (Figure 4B). The evolution of CO from BA-Air was twice the amount of total acidic functional groups determined by titration. This result indicates that nonacidic functional groups such as carbonyl and ether groups were also present in BA-Air. However, these functional groups are not expected to be active for the acid-catalyzed hydrolysis of cellulose. The porous structure was not markedly changed after air oxidation, apart from the reduction in surface area (Figure 4C). Raman spectra of BA and BA-Air did not show an increase in the degree of graphitization due to air oxidation (Figure 4D). Therefore, the effect of air oxidation on BA was primarily the introduction of oxygenated functional groups that catalyzed the hydrolysis of cellulose to glucose. 3.4. Hydrolysis in a Flow Reactor. BA-Air was chosen as the preferred catalyst for cellulose hydrolysis in a laboratoryscale continuous plug-flow slurry reactor. The plug-flow slurry reactor is crucial for high-throughput reactions. At the laboratory scale, the process design is difficult because of the lack of slurry pumping systems at high pressure. The design of the flow reactor system is shown in Figure 3; it consisted of the following elements: (1) Pneumatic feeder: The slurry pump was built in-house by modifying a high-pressure batch reactor. Slurry was continuously stirred before being fed into the reactor by overhead N2 gas pressure. The slurry flow rate was controlled by the N2 gas flow rate. (2) Slurry reactor: Stainless steel (SS) 316 tube with an o.d. of 1/16 in. (1.58 mm) and and i.d. of 1.0 mm was coiled and dipped in a stirred oil bath. (3) Solid separator: The solid catalyst and unreacted residue were separated from the product solution by gravity settling and removed through the drain valve. (4) Fixed-bed reactor: SS 316 tube with an o.d. of 3/8 in. (9.52 mm) was filled with Amberlyst-70 catalyst. (5) Product removal: The clear product solution was passed through an inline filter before exiting the reactor through a back-pressure regulator. Slurry pump design was challenging because of the frequent clogging that occurs in piston-type pumps. The slurry particles settle within tubes and compact under compression to block the lines. Therefore, we devised a pneumatic pump to continuously stir the slurry and feed it into the reactor. A continuous tube without any fittings was used from the feed inlet to the solid separator to avoid clogging due to the buildup of particles on obstructions. The inner diameter of tube was small (1 mm) to achieve a high linear velocity. Under the reaction conditions, the flow was laminar (Reynolds number = 1020), and turbulent mixing did not occur. Unlike in the batch reactor, contact between the catalyst and the cellulose cannot be initiated by mechanical stirring in a plug-flow reactor. Therefore, prior adsorption of the cellulose on the carbon surface is necessary. Cellulose was adsorbed on the carbon surface under solvent-free conditions by ball-milling them together in a planetary ball mill. Adsorption by mixmilling does not degrade the structure of cellulose. Only a small amount of soluble products was detected (3.7%) when the mixmilled substrate was dispersed in water, suggesting that the polymeric cellulose structure was preserved. The large cellulose molecules are insoluble in water and remain adsorbed on the carbon surface under aqueous conditions. In contrast, when strong acid catalysts are used during ball-milling, significant depolymerization occurs, and soluble products are ob-

tained.28−30 When the mix-milled substrate was subjected to hydrothermal conditions, the β-1,4-glycosidic bonds linking cellulose monomers were rapidly cleaved by neighboring weakly acidic functional groups. Cellulose hydrolysis in our process occurs in two steps: Initially, cellulose undergoes hydrolysis to soluble β-1,4-glucans over the carbon catalyst. First, we tested the performance of the slurry reactor for the hydrolysis of cellulose to β-1,4-glucans. The product yields with time on stream of the mix-milled cellulose hydrolysis in the slurry reactor with the fixed-bed reactor bypassed are shown in Figure 5A. The slurry

Figure 5. Yields of products for cellulose hydrolysis over the course of the reaction in the slurry reactor with the fixed-bed reactor bypassed. The residence time within the reactor was (A) 90 and (B) 155 s. Reaction condition: substrate, mix-milled cellulose; slurry concentration, 9.3 g L−1; slurry reactor temperature, 220 °C; pressure, 3.2 MPa.

concentration during this experiment was 9.3 g L−1, and the slurry reactor temperature was 220 °C. The residence time of the reactants within the slurry reactor was 90 s. The process approached steady state within a short time and was stable for the course of the reaction. β-1,4-Glucans were the primary products [57%, degree of polymerization (DP) = 2−6; see Supporting Information (SI) for characterization of the β-1,4glucans], followed by glucose (25%). The byproduct yield was 9%, which included fructose, mannose, levoglucosan, and 5hydroxymethylfurfural. β-1,4-Glucans represent a high-value product with potential applications in livestock farming to enhance the growth rate of animals and as an alternative to antibiotics for controlling pathogenic gut bacteria.31,32 β-1,4Glucans are also used in the food industry to enhance the taste, texture, and physiological effects of human food products.33 Conversion of the β-1,4-glucans to glucose is required to produce chemical platforms. Further hydrolysis of the β-1,4glucan did not occur upon simply increasing the residence time of reactants in the slurry reactor to 155 s. Under these conditions, the yield of byproducts increased to 14% with only a small improvement in the glucose yield (30%) (Figure 5B). Therefore, at high temperatures, the weakly acidic carbon 14475


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Table 2. Steady-State Yields of Products in the Continuous-Flow Process Using Cellulose and Eucalyptus as Substratesa yield of products (%) entry

substrate

1 2 3 4 5 5 6 7

cellulose cellulosec cellulose cellulosed cellulosee Eucalyptusf Eucalyptusc,f Eucalyptusf

solvent H2O H2O dilute dilute dilute H2O H2O dilute

H3PO4 H3PO4 H3PO4

H3PO4

β-1,4-glucans

glucose

xylose

C6 isomers

levoglucosan

5-HMF

furfural

STYb (kg m3 h−1)

57 7.7 1.8 9.6 6.5 53 3.0 5.1

25 59 67 70 36 20 46 71

− − − − − 46 40 73

3.8 3.2 3.8 4.5 2.6 − − −

2.7 4.2 5.0 4.5 2.4 1.2 5.0 9.9

2.3 2.5 2.6 1.0 0.5 2.6 1.0 4.1

− − − −

82 − 219 456 472 32 − 116

9.5 20 17

Slurry concentration, 9.3 g L−1; slurry flow rate, 1 mL min−1; reactor temperature, 220 °C for cellulose feed and 240 °C for Eucalyptus feed; pressure, 3.2−3.4 MPa. bSpace time yield of glucose production in the slurry reactor. cFixed-bed reactor was used for the conversion of β-1,4-glucans to glucose using Amberlyst-70 catalyst with a catalyst bed temperature of 140 °C. dSlurry concentration of 18.7 g L−1. eSlurry concentration of 37.4 g L−1. fYield of C6 products was based on the glucan content in Eucalyptus, and yields of xylose and furfural were based on the xylan content. a

The slurry flow reactor can also process lignocellulosic biomass such as Eucalyptus, containing 42.2% glucan and 13.9% xylan by weight as analyzed by the NREL TP-510-42618 method. (See Table S1 in the SI for a detailed composition.) The hydrolysis of mix-milled Eucalyptus in water without the fixed-bed reactor provided a 53% yield of β-1,4-glucans and 20% glucose (9.3 g L−1 slurry, 240 °C) (Table 2, entry 5). Xylan present as hemicellulose was also hydrolyzed to xylose with a 46% yield. With the fixed-bed reactor in line, the glucose yield increased to 46%, and the β-1,4-glucans yield was 3.0%. Using dilute H3PO4 as the solvent, without the fixed-bed reactor, the β-1,4-glucans were hydrolyzed to yield 71% glucose and 73% xylose. Lignin present in Eucalyptus adsorbs on the surface of carbon and partially inhibits the catalytic activity. The activity can be recovered through the air oxidation of the lignincontaining carbon residue after the reaction.14 Many factors influence the scaleup capability of this process for large-scale application. Clogging of reactor is a major issue for continuous operation. Turbulent flow is a straightforward approach to prevent clogging due to stratification of the biomass slurry.35 Our process worked under laminar flow because of the absence of any obstruction in the slurry flow path, thereby reducing the energy requirements to achieve turbulent flow. The accumulation of humin compound, complex polymers formed by the degradation of glucose, is another reason for clogging after prolonged use. The humin formation in our process was limited by the short residence time of the products. Use of the fixed-bed reactor also reduces humin formation by using lower temperatures for the hydrolysis of the β-1,4-glucans. The carbon catalyst recovered during the reaction can be recycled as is or after reoxidation to remove residual lignin from the biomass.14 The carbon catalyst did not show any decline in activity after reuse experiments in the batch reactor at 230 °C (Figure S5). The stability of the Amberlyst-70 catalyst was studied separately using cellobiose as a model oligomer feed. The cellobiose conversion was 98% in the fixed-bed reactor, and the glucose yield was 90% (8.1 g L−1 cellobiose feed, 1 mL min−1, 140 °C bed temperature). The Amberlyst-70 catalyst did not show any decline in activity after 30 h of continuous operation (Figure S6).

catalyst alone is not suitable for the synthesis of glucose because of the low hydrolysis rate and increase in byproduct formation. We studied homogeneous and heterogeneous catalysts for the hydrolysis of β-1,4-glucans to glucose to evaluate their process feasibility. The heterogeneous approach used a fixedbed reactor with Amberlyst-70 sulfonic acid resin catalyst. When the oligomer feed obtained from the slurry reactor was directly fed to the fixed-bed reactor (4 g of Amberlyst-70, 140 °C catalyst bed temperature), the glucose yield increased to 59%, and the β-1,4-glucan yield was 7.7% (Table 2, entry 2). In the homogeneous approach, we used dilute H3PO4 solution (4.5 mM, pH 2.5) as the solvent instead of pure water. Under these conditions, the β-1,4-glucans rapidly underwent hydrolysis in the slurry reactor itself to form glucose in high yield (67%) (Table 2, entry 3). Use of H3PO4, a weak acid, is in contrast to acid hydrolysis processes that utilize HCl and H2SO4, which are corrosive and not compatible with downstream processes. Furthermore, neutralization of H3PO4 produces phosphate salts that are useful as fertilizer for biomass production. We tested the performance of the slurry reactor in handling higher slurry concentrations to achieve practical applicability (Table 2, entries 4 and 5). Using dilute H3PO4 as the solvent, we obtained a similar glucose yield of 70% when the slurry concentration was doubled to 18.7 g L−1 (Figure S4A, SI). Further increasing the slurry concentration to 37.4 g L−1 did not cause any clogging in the reactor. However, the glucose yield under the same reaction conditions (220 °C, 90-s residence time) decreased to 36% (Figure S4B, SI). This decrease in yield was expected, as it is known that the rate of cellulose hydrolysis is lower at higher cellulose concentrations.34 Furthermore, the β-1,4-glucans are poorly soluble in water, and their limited dissolution might also reduce the product yield. The space time yield (STY) of the slurry reactor, defined here as the yield of glucose (kg) obtained from a reactor volume of 1 m3 in 1 h, was 219 kg m−3 h−1 using H3PO4 solvent and a slurry concentration of 9.3 g L−1. In comparison, the STY for the same reaction utilizing carbon catalyst and dilute HCl solvent in a batch reactor would be 8 kg m−3 h−1.16 Therefore, the flow reactor provided a 27 times higher rate of glucose synthesis for the same reactor volume and slurry concentration per unit time. The STY increased to 456 kg m−3 h−1 when the slurry concentration was 18.7 g L−1. The high STY obtained in our process will reduce the operation energy requirements and costs of the process at an industrial scale.

4. CONCLUSIONS We report the synthesis of an inexpensive carbon catalyst by air oxidation of activated carbon and its application for cellulose and lignocellulose hydrolysis in a plug-flow slurry reactor. The catalyst prepared by oxidation in air, denoted as BA-Air, 14476


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showed a higher total acidity and abundance of carboxyl groups than catalysts prepared by other oxidation methods. BA-Air was the most active catalyst for cellulose hydrolysis in a batch reactor. Application of BA-Air in a plug-flow slurry reactor required the adsorption of cellulose on the carbon by mixmilling. β-1,4-Glucans, high-value products used as food additives, were the primary product after the hydrolysis of mix-milled cellulose and lignocellulose in the plug-flow reactor. Monomeric sugars, glucose and xylose, were easily obtained in high yield by using a fixed-bed reactor in series or by using a dilute acid solution as the solvent to catalyze the sequential hydrolysis within the slurry reactor itself. The space time yield of glucose was as high as 456 kg m−3 h−1 in our process. The mix-milling used in our process is an energy-intensive method, and the development of an alternative pretreatment for the adsorption of cellulose onto the carbon catalyst is the next challenge.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03918. Characterization of β-1,4 glucans using NMR spectroscopy and LC-MS, time-on-stream data for reactions in the flow reactor, catalyst reusability test, and composition of Eucalyptus (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Abhijit Shrotri: 0000-0001-9850-7325 Hirokazu Kobayashi: 0000-0001-8559-6509 Mizuho Yabushita: 0000-0001-9739-0954 Atsushi Fukuoka: 0000-0002-8468-7721 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding received from the Japan Science and Technology Agency (JST) through the Advanced Low Carbon Technology Research and Development Program (ALCA).

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REFERENCES

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