Cellulose Hydrolysis Using Oxidized Carbon Catalyst in a Plug-Flow

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Cellulose hydrolysis using oxidized carbon catalyst in a plug flow slurry process Abhijit Shrotri, Hirokazu Kobayashi, Hiroyuki Kaiki, Mizuho Yabushita, and Atsushi Fukuoka Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03918 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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Industrial & Engineering Chemistry Research

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Cellulose hydrolysis using oxidized carbon catalyst

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in a plug flow slurry process

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Abhijit Shrotri, Hirokazu Kobayashi, Hiroyuki Kaiki, Mizuho Yabushita and Atsushi Fukuoka*

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Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-Ku, Sapporo, Hokkaido 001-

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0021, Japan. E-mail: [email protected].

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KEYWORDS: Cellulose, Carbon Catalyst, Air-oxidation, Hydrolysis, Slurry process,

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ABSTRACT: Catalytic conversion of cellulose to glucose at industrial scale is a sustainable way

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to produce fuels and chemicals. Here, we report hydrolysis of cellulose to glucose using an

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inexpensive carbon catalyst in a continuous slurry process. Carbon catalyst prepared by air-

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oxidation showed the highest activity for cellulose hydrolysis owing to the large number of

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weakly acidic functional groups. Air-oxidized carbon catalyst hydrolyzed cellulose in a plug

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flow slurry reactor after mix-milling to produce soluble β-1,4-glucans. Further hydrolysis of β-

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1,4-glucans to glucose was achieved using a fixed bed reactor containing Amberlyst-70 catalyst

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in series with the slurry reactor to obtain glucose in 59% yield. Another approach was to use

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dilute H3PO4 for hydrolysis of β-1,4-glucans to glucose with 70% yield resulting in space time

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yield of 456 kg m-3 h-1 of glucose. Simple design, short residence time and high space time yield

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will enable scale up of this process using existing chemical technology.

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1. Introduction Lignocellulosic biomass, abundantly available as agricultural waste and energy crop, is a

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carbon-based renewable feedstock for the synthesis of liquid fuels and chemicals.1–4 Large-scale

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processing of lignocellulose is necessary to replace the enormous global demand of fossil fuel

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derived chemicals.5 Design of a feasible and cost effective catalytic process for lignocellulose

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conversion is challenging due to its recalcitrant structure and disparate composition.6,7

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Hydrolysis of cellulose, the largest and the most recalcitrant component of lignocellulose, is the

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first step for synthesis of many value added chemicals (Figure 1).8 Commercial application of

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catalytic cellulose hydrolysis is currently prohibited by many factors including efficient synthesis

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of an active catalyst and lack of a scalable continuous hydrolysis process.

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Figure 1. Glucose produced by hydrolysis of cellulose is the gateway for synthesis of valuable

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chemicals and precursors in biomass based industries.

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Carbon catalysts containing acidic functional groups have the potential for cellulose hydrolysis

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at large scale. The polyaromatic surface of carbon materials strongly adsorbs cellulosic

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molecules through CH−π and hydrophobic interactions.9 The adsorbed molecules are then

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hydrolyzed by acidic functional groups present on the carbon surface.10–12 Sulfonic acid groups

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introduced by sulfuric acid treatment can hydrolyze β-1,4-glycosidic bonds.10,11 However,


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stability of sulfonic acid groups under hydrothermal conditions is not very high and the active

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sites are lost at temperature required to achieve high rate of reaction.13 Moreover, synthesis of

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catalyst containing sulfonic acid groups generates large amount of acid waste that must be

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treated before disposal. Alternatively, carbon catalysts decorated with weakly acidic carboxyl

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and hydroxyl groups are hydrothermally stable and show high activity for cellulose

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hydrolysis.12,14 Weakly acidic carboxyl and hydroxyl groups can be introduced by oxidation of

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carbon materials. Oxidizing agents such as HNO3 and H2O2 are commonly used for the oxidation

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of carbon materials.15 However, disposal of spent oxidizing agents causes an economic and

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environmental burden on the process. Air-oxidation is a promising method for synthesis of

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weakly acidic carbon catalyst because it utilizes atmospheric oxygen as oxidant.16 Nevertheless,

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processes for carbon catalyzed cellulose hydrolysis are limited to lab scale demonstrations that

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cannot be scaled up for commercialization.

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Development of a continuous hydrolysis process using carbon catalyst is necessary to achieve

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the processing capacity required to replace the demand of fossil derived fuels and chemicals. The

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nature of cellulose, an insoluble polymer, makes the development of a continuous process rather

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difficult. Pilot and industrial scale continuous hydrolysis of cellulose was previously tried using

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homogenous acid catalysts.17 A plug flow process with H2SO4 catalyst (0.5–2.0 %) could process

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concentrated lignocellulose to yield glucose with 52% yield.18 However, economic viability of

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these processes was difficult due to the cost of acid separation and neutralization. Heterogeneous

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carbon catalysts for cellulose hydrolysis claim to have overcome these issues by easy separation

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of catalyst.10,11,19,20 Choice of the reactor is crucial for continuous hydrolysis of cellulose using

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carbon catalyst. Initial hydrolysis occurs at the solid-solid interface between cellulose and

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catalyst particles until the β-1,4-glucans formed are small enough to dissolve in water (Figure


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2).21 Subsequent hydrolysis occurs on the solid-liquid interface by adsorption of dissolved β-1,4-

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glucans on the catalyst surface. Vigorous mechanical mixing is important to promote the

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interaction between solid catalyst and cellulose particles in the first step. Therefore, lab-scale

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batch reactors equipped with mechanical stirrers are used for catalyst design and screening.

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Scaling up a batch reactor to continuous stirred tank reactor (CSTR) has its limitations due to the

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energy required for stirring and difficulty in designing large pressurized CSTRs. A plug flow

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slurry reactor is ideally suited for large-scale conversion of cellulose. However, lack of stirring

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mechanism in a plug flow reactor limits the catalyst and cellulose interaction.

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Figure. 2. Schematic showing steps of cellulose hydrolysis using carbon catalyst. Initial

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hydrolysis occurs at the solid-solid interface followed with reaction at solid-liquid interface when

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the β-1,4-glucans dissolve in water. After reaction, the dissolved products and the catalyst is

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easily separated.

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The preferential adsorption of cellulose on carbon catalyst offers a solution to facilitate the use

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of a plug flow continuous process for cellulose hydrolysis. Longer chains of β-1,4-glucans

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present in cellulose are favorably adsorbed on carbon as the adsorption equilibrium constant

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increases with the degree of polymerization.22 The strong interaction of β-1,4-glucans with

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catalyst surface can eliminate the requirement for vigorous mechanical stirring to promote inter-


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particle contact. Therefore, a simple plug flow slurry reactor can be used after the adsorption of

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cellulose on carbon surface. However, the insolubility of cellulose in conventional solvents

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prevents the adsorption of cellulose on the carbon catalyst. Alternatively, cellulose can also be

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adsorbed on carbon by ball-milling them together in a pre-treatment step called mix-milling.12,23

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Ball-milling alters the crystalline structure of cellulose and makes a solid mixture of cellulose

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and carbon.

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In this study, we report the synthesis of carbon catalysts using different oxidizing methods and

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compare their activity for cellulose hydrolysis. We also demonstrate a process for hydrolysis of

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cellulose and lignocellulose to soluble β-1,4-glucans and monomeric sugars, glucose and xylose,

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in a continuous plug flow reactor using air-oxidized carbon catalyst.

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2. Experimental Section

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2.1 Oxidized Carbon Catalyst Synthesis

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Dried activated carbon named BA (Hokuetsu BA, Ajinomoto Fine-Techno) was used for

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preparation of all oxidized carbon catalysts. Air-oxidation was done by spreading 4 g of BA with

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a thickness of 3-5 mm on a Pyrex dish (ø130) and calcining under air in an electric furnace

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(Denken-Highdental, KDF S90) with the following program: 25 to 425 °C at 5 °C min-1 and 425

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°C for 10 h. This sample was named BA-Air. Nitric acid oxidation was done by slowly adding

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80 mL of ice cold 68% HNO3 aq. to 4 g of BA and stirring it at room temperature in the absence

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of light for 24 h. The recovered solid was washed with hot and cold water until the pH of filtrate

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was above 5. After drying under vacuum this sample was named BA-HNO3. Hydrogen peroxide

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oxidation was done by adding 30 mL of cold 30% H2O2 aq. solution to 3 g of BA and stirring it

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for 48 h at room temperature. The solid was recovered by filtration and then washed with 0.1 M


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oxalic acid followed by repeated washing with water. After drying under vacuum this sample

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was named BA-H2O2. Alkali oxidation was done by mixing 0.6 g of BA with 1.8 g of KOH in a

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mortar. The sample was spread on a stainless steel film and then heated under N2 flow (20 mL

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min-1) at 700 °C for 1 h in a tube furnace. The recovered solid was first washed with water then

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with 50 mL of 1 M HCl solution and then again repeatedly with water. The washed solid was

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dried under vacuum and named BA-KOH.

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2.2 Catalyst characterization

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Surface oxygenated functional groups were characterized by titration method reported by

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Boehm.24 For the titration, oxidized carbon (200 mg) was stirred in 20 mL of 0.05 M solutions of

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NaOH and NaHCO3 for 24 h. Subsequently, the solutions were filtered and 10 mL of 0.05 M

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HCl solution was added to 5 mL of filtrates. The resulting solutions were titrated with 0.05 M

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NaOH using methyl orange as an indicator to determine the presence of oxygenated functional

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groups. Consumption of NaHCO3 and NaOH was attributed to carboxyl groups and total acidic

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functional groups, respectively. Temperature programmed desorption (TPD) of oxygenated

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functional groups was measured in a MicrotracBel instrument by heating the catalyst under He

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flow (30 mL min-1) up to 1000 °C at 10 °C min-1 and detecting the evolution of gases using a

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mass spectrometer. Surface area was calculated by applying BET approximation to the N2

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adsorption isotherm of catalysts measured at -196 °C using a Belsorp mini analyzer. Raman

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spectra were collected using a Renishaw inVia Reflex spectroscope with a 532 nm laser.

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2.3 Cellulose hydrolysis in batch reactor

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Batch reactions were performed in a MMJ-100 batch reactor procured from OM Lab-tech.

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Microcrystalline cellulose (10 g, Merck column chromatography grade) was ball milled for 96 h


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at 60 rpm prior to the reaction with 1 kg of ø10 zirconia balls in a ceramic pot using an ANZ-51S

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rotator. For each hydrolysis reaction 324 mg of ball milled cellulose and 50 mg of catalyst were

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added to the reactor with 40 mL of water. The reactor was heated under stirring to 230 °C and

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then the temperature was immediately reduced back to room temperature using an air blower.

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The solid was separated by centrifugation, dried and weighed to determine cellulose conversion

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and the liquid was analyzed to determine the product yield.

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2.4 Mix-milling

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Microcrystalline cellulose (Avicel PH-101) or Eucalyptus (5 g) and BA-Air (0.77 g) were

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milled together in an Al2O3 pot (250 mL) with alumina balls (ø5, 200 g) using a Fritsch P-6

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planetary ball mill. Milling condition was 500 rpm for 2 h with a 10-min interval after every 10

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min of milling.

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2.5 Hydrolysis in flow reactor

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The hydrolysis reaction was performed by mixing 1.87 g of mix-milled substrate in 200 mL

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solvent (water or 4.5 mM H3PO4) and feeding it to the pneumatic pump reservoir (Figure 3). The

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flow reaction system was pressurized to 3.0–3.5 MPa using N2 gas flow of 35 mL min-1 through

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a bypass gas line. After the system pressure and reactor temperature were stable, the N2 gas flow

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was diverted inside the pneumatic pump to start the slurry flow. The oil bath temperature for the

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slurry reactor was maintained at 220 °C for mix-milled cellulose hydrolysis and 240 °C for mix-

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milled Eucalyptus hydrolysis. The fixed bed reactor was filled with 4 g of as received

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Amberlyst-70. The catalyst bed temperature was maintained at 140 °C. The fixed bed reactor

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was bypassed using three-way valves when not in operation. Samples were collected from the

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outlet of back pressure regulator at regular interval and analyzed.


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Figure 3. Cellulose hydrolysis process with plug flow slurry reactor and fixed bed reactor for

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continuous hydrolysis to β-1,4-glucans and monomers.

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2.6 Product analysis

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Liquid products obtained from batch and flow reactor were analyzed by high-performance

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liquid chromatography (Shimadzu; refractive index and ultraviolet detectors) with a SUGAR

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SH1011 (Shodex; ø8×300 mm; eluent: water 0.5 mL min-1; 50 °C) column. Composition of

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Eucalyptus was determined by NREL/TP-510-42618 method on dry basis.25 Yields of glucose

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and xylose from Eucalyptus were based on the glucan and xylan content determined by the

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NREL method.25

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3. Results and Discussion

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3.1 Carbon oxidation and characterization

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Activated carbon is the cheapest source of high surface area carbon material suitable for

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catalysis. Therefore, we used a biomass derived activated carbon named BA for the synthesis of

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oxygenated carbon catalysts. This carbon was oxidized using air, HNO3, H2O2 and KOH (Table

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1). After air oxidation 52% wt. of catalyst was lost in the form of CO and CO2 due to the partial


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combustion of BA. The surface area of BA-Air was 870 m2 g-1, a slight reduction from 1230 m2

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g-1 of BA. After nitric acid oxidation, 11% carbon was lost and the surface area decreased to

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1080 m2 g-1. Weight loss after H2O2 oxidation was only 1% as it is the weakest of all oxidizing

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agents we used. The surface area of BA-H2O2 was 1020 m2 g-1. Treatment of BA with KOH

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resulted in weight loss of 42 % and the surface area increased to 2040 m2 g-1. Alkali treatment

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creates micropores in the carbon materials that increases the surface area of catalyst.26

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Table 1. Characterization and cellulose hydrolysis activity of oxidized carbon catalysts.a

Entry

Catalyst

BET Weight surface lossb Area (%) (m2 g-1)

Total acidity

Carboxyl group density

Cellulose conversion

Glucose yield

(mmol g1 ) (mmol g-1)

(%)

(%)

1

BA

-

1230

0.3

0.1

48

15

2

BA-Air

52

870

2.6

1.0

68

35

3

BA-HNO3

11

1080

1.1

0.5

59

27

4

BA-H2O2

1

1020

0.6

0.2

55

22

5

BA-KOH

42

2040

1.9

0.8

60

23

6

Blank

-

-

-

-

33

2.1

a

Hydrolysis reaction condition: ball-milled cellulose (324 mg), carbon catalyst (50 mg) and water (40 mL) were added to high-pressure reactor and heated to 230 °C and then rapidly cooled. b

Weight loss of carbon during oxidation.

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Oxidation of BA introduced varying amount of acidic functional groups on carbon surface.

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The number of acidic functional groups were determined by titration of solid with NaOH and

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NaHCO3. NaOH reacts with all acidic functional groups and it represents the total acidity of the

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catalyst. NaHCO3 is a weak base and it reacts with carboxylic acids and sulfonic acids. As the

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presence of sulfonic groups is unlikely in our catalysts, the consumption of NaHCO3 was


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attributed to the presence of carboxylic acids. Among weak acid sites, carboxylic acid is the most

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active functional group for hydrolysis of β-1,4 glycosidic bond.27 Unoxidized BA contained 0.30

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mmol g-1 of acidic functional groups with 0.10 mmol g-1 of carboxyl groups. Among all the

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oxidation methods, air oxidation was the most effective for introduction of acidic functional

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groups (Table 1). BA-Air had 2.6 mmol g-1 of acidic functional groups with 1.0 mmol g-1 of

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carboxyl groups. Alkali treatment introduced 1.9 mmol g-1 of acidic functional groups with 0.8

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mmol g-1 of carboxyl groups. Both HNO3 and H2O2 were less effective for introducing acidic

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functional groups. Consequently, the degree of oxidation was related to weight loss as the

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severity of oxidation increased.

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3.2 Cellulose hydrolysis in batch reactor

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Activity of carbon catalysts were tested for cellulose hydrolysis in a batch reactor (Table 1). In

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the absence of carbon catalyst, cellulose conversion was 33% with a glucose yield of only 2.1% .

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Unoxidized BA catalyst increased the conversion to 48% and the glucose yield increased to 15%.

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The activity of BA was attributed to the presence of small number of acidic functional groups

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and the favorable interactions between cellulose and carbon surface. Cellulose conversion in the

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presence of BA-Air was 68% with glucose yield of 35% (Table 1 Entry 2). The activity of other

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catalysts for cellulose conversion was lower than BA-Air. Cellulose conversion was proportional

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to the total acidity of carbon catalyst. Glucose yield in the presence of BA-KOH was lower than

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BA-HNO3 despite having a higher conversion and more number of oxygenated functional

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groups. Strong adsorption of products in the micropores of BA-KOH was attributed to the lower

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glucose yield. Therefore, simple air-oxidation was the most effective way for introducing large

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number of oxygenated functional groups that show high activity for cellulose hydrolysis.


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3.3 Characterization of BA-Air

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BA-Air was characterized in more detail to identify the structural and chemical changes during

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air-oxidation. Temperature programmed desorption (TPD) of carbon catalyst under inert

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atmosphere releases CO and CO2. The evolution of CO and CO2 is used to determine the

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concentration of functional groups. TPD of BA released 0.17 mmol g-1 of CO2 that increased to

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1.2 mmol g-1 after air oxidation (Figure 4A). The amount of CO2 evolved from BA-Air agreed

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with the number of carboxyl groups determined by titration. CO evolved from BA and BA-Air

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during TPD was 0.41 mmol g-1 and 5.3 mmol g-1 (Figure 4B). The evolution of CO from BA-Air

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was twice that of the total acidic functional groups determined titration. This result indicates that

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the non-acidic functional groups such as carbonyl and ether groups were also present in BA-Air.

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However, these functional groups are not expected to be active for acid catalyzed hydrolysis of

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cellulose. The porous structure was not remarkably changed after air oxidation apart from the

13

reduction in surface area (Figure 4C). Raman spectra of BA and BA-Air did not show an

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increase in the degree of graphitization due to air-oxidation (Figure 4D). Therefore, the effect of

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air oxidation on BA was primarily introduction of oxygenated functional groups that catalyzed

16

the hydrolysis of cellulose to glucose.


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Figure 4. Characterization of BA and BA-Air catalysts. (A) and (B): CO2 and CO evolution

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during TPD under He flow. (C) N2 adsorption isotherm (D) Raman spectra measure using 532

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nm laser.

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3.4 Hydrolysis in flow reactor

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BA-Air was chosen as the preferred catalyst for cellulose hydrolysis in a lab-scale continuous

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plug flow slurry reactor. The plug flow slurry reactor is crucial for high throughput reactions. At

8

lab scale, the process design is difficult due to the lack of slurry pumping systems at high

9

pressure. The design of flow reactor system is shown in Figure 3, which consisted of the

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following elements:

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1. Pneumatic feeder: The slurry pump was built in-house by modifying a high-pressure

12

batch reactor. Slurry was continuously stirred before being fed into the reactor by

13

overhead N2 gas pressure. The slurry flow rate was controlled by N2 gas flow rate.

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2. Slurry reactor: Stainless steel (SS) 316 tube of OD 1/16 inch (1.58 mm) and ID 1.0 mm was coiled and dipped in a stirred oil bath.


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3. Solid separator: The solid catalyst and unreacted residue was separated from the product solution by gravity settling and removed through the drain valve. 4. Fixed bed reactor: SS 316 tube of OD 3/8 inch (9.52 mm) filled with Amberlyst-70 catalyst. 5. Product removal: The clear product solution was passed through an inline filter before exiting through a backpressure regulator. Slurry pump design was challenging due to frequent clogging in piston type pumps. The slurry

8

particles settle within tubes and compact under compression to block the lines. Therefore, we

9

devised a pneumatic pump to continuously stir the slurry and feed it into the reactor. A

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continuous tube without any fittings was used from the feed inlet until the solid separator to

11

avoid clogging due to the build-up of particles on obstructions. The inner diameter of tube was

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small (1 mm) to achieve high linear velocity. Under the reaction condition, the flow was laminar

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(Reynolds number = 1020) and turbulent mixing did not occur.

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Unlike batch reactor the catalyst and cellulose contact cannot be initiated by mechanical

15

stirring in a plug flow reactor. Therefore, prior adsorption of cellulose on the carbon surface is

16

necessary. Cellulose was adsorbed on carbon surface under solvent free condition by ball milling

17

them together in a planetary ball mill. Adsorption by mix-milling does not degrade the structure

18

of cellulose. Only small amount of soluble products were detected (3.7%) when the mix-milled

19

substrate was dispersed in water, suggesting the polymeric cellulose structure was preserved. The

20

large cellulose molecules are insoluble in water and remain adsorbed on carbon surface under

21

aqueous condition. In contrast, when strong acid catalysts are used during ball milling significant

22

depolymerization occurs and soluble products are obtained.28–30 When the mix-milled substrate is


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subjected to hydrothermal conditions the β-1,4-glycosidic bonds linking cellulose monomers are

2

rapidly cleaved by neighboring weakly acidic functional groups.

3

Cellulose hydrolysis in our process occurs in two steps. Initially, cellulose undergoes

4

hydrolysis to soluble β-1,4-glucans over carbon catalyst. At first, we tested the performance of

5

slurry reactor for hydrolysis of cellulose to β-1,4-glucans. Time on stream of mix-milled

6

cellulose hydrolysis in the slurry reactor with the fixed bed reactor bypassed is shown in Figure

7

5A. The slurry concentration during this experiment was 9.3 g L-1 and the slurry reactor

8

temperature was 220 °C. The residence time of reactants within the slurry reactor was 90 s. The

9

process approached steady state within short time and was stable for the course of the reaction.

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β-1,4-Glucans were the primary products (57%, DP = 2−6, See supporting information (SI) for

11

characterization of β-1,4-glucans) followed by glucose (25%). By-product yield was 9%, which

12

included fructose, mannose, levoglucosan and 5-hydroxymethylfurfural. β-1,4-Glucan is a high

13

value product with potential application in livestock farming to enhance the growth rate of

14

animals and as an alternative to antibiotics for controlling pathogenic gut bacteria.31,32 β-1,4-

15

Glucans are also used in food industry to enhance the taste, texture and physiological effect of

16

human food products.33


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Figure 5. Yield of products for cellulose hydrolysis over the course of reaction in the slurry

3

reactor with fixed bed reactor bypassed. Residence time within the reactor was 90 s (A) and 155

4

s (B). Reaction condition: substrate mix milled cellulose, slurry concentration 9.3 g L-1, slurry

5

reactor temperature 220 °C, pressure 3.2 MPa.

6

Conversion of β-1,4-glucans to glucose is required to produce chemical platforms. Further

7

hydrolysis of β-1,4-glucan did not occur by simply increasing the residence time of reactants in

8

the slurry reactor to 155 s. Under this condition the yield of by-products increased to 14% with

9

only a small improvement in the glucose yield (30%) (Figure 5B). Therefore, at high

10

temperatures the weakly acidic carbon catalyst alone is not suitable for synthesis of glucose due

11

to the slow hydrolysis rate and increase in by-product formation.

12

We studied homogeneous and heterogeneous catalysts for hydrolysis of β-1,4-glucans to

13

glucose to evaluate their process feasibility. Heterogeneous approach used a fixed bed reactor

14

with Amberlyst-70 sulfonic acid resin catalyst. When the oligomer feed obtained from slurry


15


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Page 16 of 29

1

reactor was directly fed to the fixed bed reactor (4 g Amberlyst-70, catalyst bed temperature 140

2

°C) the glucose yield increased to 59 % and β-1,4-glucan yield was 7.7 % (Table 2 entry 2). In

3

the homogenous approach, we used diluted H3PO4 solution (4.5 mM, pH 2.5) as a solvent instead

4

of pure water. Under this condition the β-1,4-glucans rapidly underwent hydrolysis in the slurry

5

reactor itself to form glucose in high yield (67%) (Table 2 entry 3). Use of H3PO4, a weak acid,

6

is in contrast with acid hydrolysis processes that utilize HCl and H2SO4, which are corrosive and

7

not compatible with downstream processes. Furthermore, neutralization of H3PO4 produces

8

phosphate salts that are useful as fertilizer for biomass production.

9

Table 2: Steady state yield of products in the continuous flow process using cellulose and Ecalyptus as substrate.a

10

Yield of products (%) Entry Substrate

Solvent

β-1,4Glucans

Glucose Xylose

C6 isomers Levoglucosan 5-HMF Furfural

STYb (kg m3 h-1)

1

Cellulose

H2O

57

25

-

3.8

2.7

2.3

-

82

2

Cellulosec

H2O

7.7

59

-

3.2

4.2

2.5

-

-

3

Cellulose

dil. H3PO4

1.8

67

-

3.8

5.0

2.6

-

219

4

Cellulosed

dil. H3PO4

9.6

70

-

4.5

4.5

1.0

-

456

5

Cellulosee

dil. H3PO4

6.5

36

-

2.6

2.4

0.5

5

Eucalyptusf

H2O

53

20

46

-

1.2

2.6

9.5

32

6

Eucalyptusc,f

H2O

3.0

46

40

-

5.0

1.0

20

-

7

Eucalyptusf dil. H3PO4

5.1

71

73

-

9.9

4.1

17

116

a

-1

472

-1

Slurry concentration 9.3 g L , slurry flow rate 1 mL min , 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 slurry reactor. cFixed bed reactor was used for conversion of β-1,4-glucans to glucose using Amberlyst-70 catalyst with catalyst bed temperature of 140 °C. dSlurry concentration 18.7 g L-1. eSlurry concentration 37.4 g L-1. fYield of C6 products was based on glucan content in Eucalyptus and yield of xylose and furfural was based on xylan content.

11


16

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

We tested the performance of slurry reactor to handle higher slurry concentration to achieve

2

practical applicability (Table 2 entry 4 and 5). Using the dilute H3PO4 as solvent, we obtained

3

similar glucose yield of 70% when slurry concentration was doubled to 18.7 g L-1 (SI Figure S4

4

A). Further increasing the slurry concentration to 37.4 g L-1 did not cause any clogging in the

5

reactor. However, the glucose yield under the same reaction condition (220 °C, 90 s residence

6

time) reduced to 36% (SI Figure S4 B). The decrease in yield is expected as it is known that the

7

rate of cellulose hydrolysis is slower at higher cellulose concentration.34 Furthermore, the β-1,4-

8

glucans are poorly soluble in water and their limited dissolution may also reduce the product

9

yield.

10

Space time yield (STY) for slurry reactor, defined here as yield of glucose (kg) obtained from

11

a reactor of volume 1 m3 in 1 h, was 219 kg m-3 h-1 using H3PO4 solvent and slurry concentration

12

of 9.3 g L-1. In comparison, the STY for the same reaction utilizing carbon catalyst and dilute

13

HCl solvent in a batch reactor would be 8 kg m-3 h-1.16 Therefore, the flow reactor shows 27

14

times higher rate of glucose synthesis for the same reactor volume and slurry concentration per

15

unit time. STY increased to 456 kg m-3 h-1 when the slurry concentration was 18.7 g L-1. The

16

high STY obtained in our process will reduce the operation energy requirement and cost of the

17

process at an industrial scale.

18

The slurry flow reactor can also process lignocellulosic biomass such as Eucalyptus,

19

containing 42.2% glucan and 13.9% xylan by weight as analyzed by NREL TP-510-42618

20

method. (See SI Table S1 for detailed composition). Hydrolysis of mix milled Eucalyptus in

21

water without the fixed bed reactor provided 53% yield of β-1,4-glucans and 20% glucose (9.3 g

22

L-1 slurry, 240 °C) (Table 2 entry 5). Xylan present as hemicellulose was also hydrolyzed to

23

xylose with 46% yield. With the fixed bed reactor in line, the glucose yield increased to 46% and


17


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Page 18 of 29

1

β-1,4-glucans yield was 3.0%. Using dilute H3PO4 as solvent, and without the fixed bed reactor,

2

the β-1,4-glucans were hydrolyzed to yield 71% glucose and 73% xylose. Lignin present in

3

Eucalyptus adsorbs on the surface of carbon and partially inhibits the catalytic activity. The

4

activity can be recovered by air-oxidation of lignin containing carbon residue after the reaction.14

5

Many factors influence the scale up capability of this process for large-scale. Clogging of

6

reactor is a major issue for continuous operation. Turbulent flow is a straightforward approach to

7

prevent clogging due to stratification of biomass slurry.35 Our process worked under laminar

8

flow rate due to the absence of any obstruction in the slurry flow path, thereby reducing the

9

energy requirement to achieve turbulent flow. Build-up of humin compound, complex polymers

10

formed by degradation of glucose, is another reason for clogging after prolonged use. The humin

11

formation in our process was limited by the short residence time of products. Use of the fixed

12

bed reactor also reduces humin formation by using lower temperature for hydrolysis of β-1,4-

13

glucans. The carbon catalyst recovered during the reaction can be recycled as it is or after re-

14

oxidation to remove residual lignin from biomass.14 The carbon catalyst did not show any

15

decline in activity after reuse experiments in the batch reactor at 230 °C (Figure S5). The

16

stability of Amberlyst-70 catalyst was studied separately using cellobiose as a model oligomer

17

feed. Cellobiose conversion was 98% in the fixed bed reactor and glucose yield was 90% (8.1 g

18

L-1 cellobiose feed, 1 mL min-1, bed temperature 140 °C). The Amberlyst-70 catalyst did not

19

show any decline in activity after 30 h of continuous operation (Figure S6).

20

4. Conclusions

21

We report the synthesis of an inexpensive carbon catalyst by air-oxidation of activated carbon

22

and its application for cellulose and lignocellulose hydrolysis in a plug flow slurry reactor. BA-


18

Page 19 of 29

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1

Air showed higher total acidity and abundance of carboxyl groups in comparison to catalysts

2

prepared by other oxidation methods. Consequently, BA-Air was the most active catalyst for

3

cellulose hydrolysis in a batch reactor. Application of BA-Air in a plug flow slurry reactor

4

required adsorption of cellulose on carbon by mix-milling. β-1,4-Glucans, high value products

5

used as food additives, were primary product after hydrolysis of mix-milled cellulose and

6

lignocellulose in the plug flow reactor. Monomeric sugars, glucose and xylose, were easily

7

obtained in high yield by using a fixed bed reactor in series or by using a dilute acid solution as

8

solvent to catalyze the sequential hydrolysis within the slurry reactor itself. Space time yield of

9

glucose was as high as 456 kg m-3 h-1 in our process. Mix-milling used in our process is an

10

energy intensive method and development of an alternative pre-treatment for adsorption of

11

cellulose on carbon catalyst is the next challenge.

12

Supporting Information. Characterization of β-1,4 glucans using NMR spectroscopy and LC-

13

MS, time on stream data for reactions in flow reactor, catalyst reusability test and composition of

14

Eucalyptus.

15

Corresponding Author

16

*[email protected]

17

ACKNOWLEDGMENT

18

This work was supported by funding received from Japan Science and Technology Agency (JST)

19

through the Advanced Low Carbon Technology Research and Development Program (ALCA).

20

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Figure 1 42x14mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3 161x56mm (300 x 300 DPI)


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Figure 5 82x114mm (300 x 300 DPI)


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TOC 39x19mm (300 x 300 DPI)


Cellulose Hydrolysis Using Oxidized Carbon Catalyst in a Plug-Flow

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