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. 2021 May 27;6(22):14252–14259. doi: 10.1021/acsomega.1c01026

Hydrolysis of Lignocellulosic Biomass in Hot-Compressed Water with Supercritical Carbon Dioxide

Masahide Sasaki †,*, Kurumi Ohsawa ‡
PMCID: PMC8190814  PMID: 34124448

Abstract

graphic file with name ao1c01026_0011.jpg

This study investigated the decomposition behavior of bamboo under hydrothermal and hydrolysis conditions with H2O/CO2 in a semicontinuous-flow reactor at 9.8 MPa. At 255 °C, with and without CO2, xylan in bamboo completely decomposed into xylo-oligosaccharide (XOD). The yield of glucan degradation products with CO2 was significantly higher compared with that under the hydrothermal reaction (25.7 vs 14.9 wt %, respectively). The reaction rate of glucan decomposition with CO2 was slightly higher than the rate of hydrothermal reaction (kH2O/CO2/kH2O = 1.3). Increasing the fluid velocity of the hydrothermal reaction (3–10 mL/min) significantly accelerated the solubilization rate, but the ultimate yield of the soluble fraction was unchanged. The ultimate yield of the soluble fraction was slightly affected by physical effects. Hydrolysis with CO2 under severe conditions exhibited effective degradation of glucan. The catalytic activity of the H2O/CO2 system under hydrolysis can be explained by the system’s chemical effect.

1. Introduction

The conversion of lignocellulosic biomass into chemicals has attracted mounting interest during the last 20 years. Considering that <10% consumption of crude oil is attributed to the production chemicals,1,2 synthesizing chemicals using lignocellulosic biomass would not contribute to decreasing greenhouse gas emissions. However, because of its substitutable molecules, biomass has high potential as renewable feedstock for chemical production that requires fewer steps compared with chemical production of fossil fuels.1 Sustainable chemical production from biomass should reinforce the public’s confidence in the chemical industry.2 Moreover, the development of flexible and integrated biorefineries to produce biofuels and bioproducts from renewable biomass sources represents a key tool to perform the transition from a petroleum-based economy to a novel bioeconomy that looks for a more efficient and sustainable global development.3,4 Therefore, converting biomass into useful chemicals through economically viable processes is crucial for sustainable development.5

Depolymerization typically involves a pretreatment process (one of the most important steps in the use of lignocellulosic materials), since it improves the efficiency and economy of the overall process.6,7 Available biomass pretreatment techniques include dilute acid,8 alkaline hydrolysis,9 ammonia fiber explosion (AFEX),10 ionic liquids,11,12 hot-compressed water (HCW) hydrolysis,13−16 and CO2.17−31 HCW has unique physical and chemical properties.32,33 The ionic product (Kw) of water changes as the temperature changes. HCW can be used simultaneously as a solvent and a reactant in applications of biomass, such as lignocellulosic pretreatment.13,14 Previous studies revealed that HCW can completely solubilize hemicellulose from biomass.15 However, the robust crystalline structure of cellulose creates an obstacle for hydrolysis. The cellulose hydrolysis rate and glucose yield are much lower in cellulose hydrolysis in HCW compared with acid hydrolysis.9 The hydrothermal reaction requires a relatively high temperature (>250 °C) or an acidic catalyst, which accelerates the degradation of sugar products.

Carbon dioxide is nontoxic, nonflammable, and inexpensive reagents.34 Adding CO2 to lignocellulosic treatments has two effects, which are physical and chemical.35 As a physical effect, a high CO2 pressure can easily penetrate small pores of the lignocellulosic structure, which results in structural changes in feedstock.29 In addition, the supercritical CO2 phase offers high diffusivity and has a swelling effect on plant materials.31

On the other hand, water and carbon dioxide form carbonic acid

1. 1

The pH of the H2O/CO2 mixture decreases to ∼2.9, which creates a strongly acidic environment that facilitates biomass hydrolysis.36 Studies have reported differences in the effects of CO2 on aspen wood21 and corn stover.22 Van Walsum found that CO2 in the pretreatment of corn stover enhances xylose and furan concentrations.22 There are numerous studies for hemicellulose or xylan decomposition with CO2.37−42 Bogel-Lukasik and co-workers demonstrated that pretreatment of lignocellulosic biomass (wheat straw) with H2O/CO2 in a batch reactor28−30 produces a liquid fraction that is rich in hemicellulose (mainly in oligomer form) and a solid mainly containing glucan and lignin. A decrease in the solid product yield was also observed using the H2O/CO2 system. On the other hand, a continuous-flow reactor used in pretreatment with H2O/CO2 at 100 bar did not enhance the degree of biomass (rye straw) dissolution.27

In a previous paper,43 hydrolysis with CO2 under severe conditions exhibited a remarkable decrease in the solid yield. A linear relationship existed between the catalytic activity of the H2O/CO2 system and the severity factor. This study investigated H2O/CO2 hydrolysis of bamboo in a semicontinuous (percolating) flow reactor at 9.8 MPa pressure. The research focus was on the catalytic function of supercritical CO2 for hydrolysis of lignocellulosic material as a function of glucan and xylan degradation. In addition, we considered the effect of fluid velocity on the yield of solid products for hydrothermal reactions.

2. Results and Discussion

2.1. Effect of the Heating Period at 255 °C on Solid Yield

A series of hydrolysis experiments were conducted under variable holding periods with and without CO2. Figure 1 illustrates the solid yield of hydrolysis at 255 °C as a function of the holding period. We did not observe a difference in the solid yield of hydrolysis with or without CO2 at 0 min of the holding period. However, this difference gradually increased as the holding period increased. At 30 min, the solid yield of hydrolysis with CO2 was substantially lower compared with that of the hydrothermal reaction. The solid yields at 30 min were consistent with the results of our preceding paper.43

Figure 1.

Figure 1

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Solid yields of bamboo in a semicontinuous-flow reactor at 255 °C, 9.8 MPa, depending on a holding period.

Results of hydrolysis with and without CO2 are summarized in Table 1. For the hydrothermal reaction (without CO2), the liquid yield was calculated using the carbon yield, which was also specified in this table. The liquid product yield was calculated using the weight of the sample after freeze-drying. The sum of the solid and liquid yields was ∼93%. However, the liquid product yield estimated by the carbon yield of eluents was much higher compared with freeze-drying. Approximately 100 wt % of feed material was recovered as solid and liquid (carbon yield) products up to 10 min of the holding period. Losses observed in yield for freeze-drying were attributed to the formation of lower-Mw compounds, such as the hydroxymethylfurfural (HMF) and furfural,14 which easily evaporate under freeze-drying conditions. However, the loss obtained at 30 min for the hydrothermal reaction was ascribed to gaseous products.

Table 1. Results of Solid and Liquid Yields by Hydrolysis with and without CO2.

holding period, min log(R0) solvent solid yield, wt % liquid yield, wt % total yield, wt % differencea liquid yield (TOC), wt % solid + liquid (TOC), wt %
0.0 4.87 H2O 41.2 54.3 95.5 4.5 60.2 101.4
5.0 4.98 H2O 36.3 56.8 93.1 6.9 64.4 100.7
9.8 5.01 H2O 30.5 63.5 94.0 6.0 70.0 100.5
29.3 5.29 H2O 24.5 70.3 94.8 5.2 70.7 95.2
0.0 4.87 H2O/CO2 41.6 52.8 94.4 5.6    
4.9 4.98 H2O/CO2 31.6 64.3 95.9 4.1    
10.0 5.03 H2O/CO2 26.1 69.1 95.2 4.8    
20.2 5.15 H2O/CO2 16.5 76.4 92.9 7.1    
29.9 5.24 H2O/CO2 9.4 83.3 92.7 7.3    
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a

Difference = 100-(solid yield + liquid yield).

2.2. Decomposition Behavior of Xylan

CO2 addition in hydrolysis at 255 °C enhanced the biomass dissolution degree as a function of the solid yield. To reveal the effect of CO2 addition on lignocellulosic materials dissolution, we focused on xylan decomposition. Figure 2 illustrates the xylo-oligosaccharide (XOD) yield of hydrolysis as a function of the holding period. All values of the XOD yield were constant and located at approximately 24 wt %. The broken line in Figure 2 indicated the inherent content of xylan in feed material. In addition, we did not detect xylose in the liquid products of any of the cases. These data provide evidence that xylan degradation completely proceeded to XOD even under hydrothermal reaction (without CO2) at 255 °C. This result was consistent with the fact that CO2-enhanced biomass dissolution was attributed to the degradation of another component, such as glucan.

Figure 2.

Figure 2

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Yields of xylo-oligosaccharides (XOS) at 255 °C as a function of the holding period.

To determine the effect of CO2 on xylan degradation, feed material was hydrolyzed at a relatively low reaction temperature (<200 °C) for 30 min. Figure 3 illustrates the solid yield as a function of the reaction temperature.

Figure 3.

Figure 3

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Solid yields of bamboo at 9.8 MPa of pressure, depending on the reaction temperature.

The solid yield of hydrolysis with and without CO2 gradually decreased as the reaction temperature increased. These data are summarized in Table 2. We did not observe this difference at 200 °C. Hydrolysis reactions with CO2 at a lower reaction temperature had a small effect on biomass dissolution. Typical posthydrolysis results for the solid products are summarized in Table 3. Hydrolysis with CO2 at 136 °C did not show degradation of xylan, nor a hydrothermal reaction (without CO2). At 200 °C, the degradation of xylan completely proceeded irrespective of whether CO2 was present or not. Therefore, xylan in the biomass was easily decomposed by a hydrothermal reaction at 200 °C. Our result is consistent with reports from Mok15 and Allen et al.44 On the other hand, solubilization of glucan did not occur at 200 °C in either case. Fockink et al.45 found that the CO2/H2O system used in pretreatment contributed positively to the selective removal of hemicelluloses from sugarcane bagasse. Our results specified in Table 3, over 96 wt % of xylan were removed at 200 °C even for hydrothermal reaction. The solid products after hydrothermal reaction with a high content of glucan would easily convert to glucose subsequent enzymatic reaction.

Table 2. Results of Solid and Liquid Yields at Lower Temperatures.

  solid yield, wt %
reaction temperature, °C H2O H2O/CO2 differencea
136 88.3 83.6 4.7
150 79.1 74.9 4.2
170 62.2 58.9 3.3
200 49.0 48.2 0.8
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a

Difference = solid yield of H2O – solid yield of H2O/CO2.

Table 3. Results of Xylan and Glucan Yields after Hydrolysis.

  xylan yield, wt %
 glucan yield, wt %
reaction temperature, °C H2O H2O/CO2 H2O H2O/CO2
136 87.2 79.4 98.5 98.8
200 2.5 3.3 98.3 95.8
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2.3. Decomposition Behavior of Glucan

Figure 4 illustrates typical changes in glucan decomposition products over time. The glucan yield of the hydrothermal reaction (without CO2) decreased with the increasing holding period, as shown in Figure 4a. The GlcOS yield increased corresponding to the decreasing yield of glucan. The glucose yields after the hydrothermal reaction were less than 3 wt % based on feed. The degradation behaviors for CO2 addition (Figure 4b) were analogous to those for the hydrothermal reaction. However, the absolute yield value of GlcOS remarkably increased with the addition of CO2. These data are summarized in Table 4. At 30 min, the liquid yield drastically increased from 70 to 83 wt % with the addition of CO2. The increments of liquid yield (13 wt %) roughly corresponded to increasing yields of GlcOS and glucose (11 wt %). To exhibit the effect of CO2 addition on biomass degradation, we carried out kinetic analysis of the glucan decomposition reaction.

Figure 4.

Figure 4

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Typical change in the holding period for testing glucan degradation products at 255 °C, 9.8 MPa.

Table 4. Results of Degradation Products Yields by Posthydrolysis of Products.

holding period, min log(R0) solvent solid yield, wt % liquid yield, wt % total yield, wt % differencea glucan, wt % GlcOS, wt % glucose, wt % XOS, wt %
0.0 4.87 H2O 41.2 54.3 95.5 4.5 35.8 4.4 0.7b 23.6
5.0 4.98 H2O 36.3 56.8 93.1 6.9 31.7 6.6 1.2b 22.9
9.8 5.01 H2O 30.5 63.5 94.0 6.0 25.8 9.6 2.1b 23.7
29.3 5.29 H2O 24.5 70.3 94.8 5.2 19.8 12.4 2.5b 25.2
0.0 4.87 H2O/CO2 41.6 52.8 94.4 5.6 34.9 3.5 0.7b 23.8
4.9 4.98 H2O/CO2 31.6 64.3 95.9 4.1 26.6 8.4 2.1b 23.4
10.0 5.03 H2O/CO2 26.1 69.1 95.2 4.8 21.0 11.7 3.0 23.9
20.2 5.15 H2O/CO2 16.5 76.4 92.9 7.1 12.3 17.3 4.8 23.5
29.9 5.24 H2O/CO2 9.4 83.3 92.7 7.3 5.8 20.9 4.8 23.9
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a

Difference = 100 – total yield.

b

Below the detection limit of high-performance liquid chromatography (HPLC).

The glucan concentration in the feed material was 40.4 wt %. The glucan yield at 0 min as a holding period was 35.8 and 34.9 wt %, respectively. Therefore, more than 12 wt % of reactants had already decomposed during the heat-up period. To provide kinetic analysis, the decomposition reaction during the heating period was suppressed as much as possible. One of the methods of suppressing glucan decomposition during the heating period, we have focused on heating rate for the hydrolysis reaction. We examined the hydrolysis reaction with a higher heating rate using the bypass method.

The effects of the bypass method on heating rate are summarized in Table 5. The heating rate using the bypass method was approximately 3 times that of the normal procedure. As a result, the suppression of glucan decomposition during the heating period can be achieved through the reaction with higher heating rate with the bypass method. A series of hydrolysis experiments with higher heating rates were conducted under variable holding periods with and without CO2. Figure 5 illustrates the solid yield of hydrolysis at 255 °C as a function of the holding period.

Table 5. Comparison of Glucan Yield and Concentration by Heating Methoda.

solvent method heating rate, °C/min glucan yield, wt % CA/CA0
H2O normal 7.1 35.8 0.884
H2O bypass 21.2 39.4 0.973
H2O/CO2 normal 7.5 34.9 0.862
H2O/CO2 bypass 21.2 37.8 0.933
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a

Feed: glucan contents = 40.5 wt %.

Figure 5.

Figure 5

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Solid yields of bamboo in a semicontinuous-flow reactor using the bypass method at 255 °C, 9.8 MPa, depending on the holding period.

As an analogy with the former result (Figure 1), the effect of CO2 addition on biomass decomposition can be observed for hydrolysis with a higher heating rate. Data in Table 6 show the kinetics analysis of a glucan decomposition reaction was examined by assuming a first-order mechanism.

Table 6. Results of Degradation Products Yields by Posthydrolysis of Products Using the Bypass Method.

holding period, min log(R0) solvent solid yield, wt % liquid yield, wt % total, wt % differencea glucan, wt % GlcOS, wt % glucose, wt %
0.0 4.34 H2O 50.7 43.1 93.8 6.2 39.4 1.1b 0.1b
5.4 4.71 H2O 37.4 56.2 93.6 6.4 32.1 6.0 1.3b
10.0 4.81 H2O 31.5 61.1 92.6 7.4 26.1 9.1 2.2b
19.9 5.00 H2O 22.3 69.7 92.0 8.0 18.2 13.5 3.5
25.0 5.08 H2O 18.1 69.8 87.9 12.1      
30.0 5.15 H2O 18.1 71.8 89.9 10.1      
0.0 4.44 H2O/CO2 46.4 48.2 94.6 5.4 37.8 1.9 0.3b
5.0 4.63 H2O/CO2 35.7 59.1 94.8 5.2 30.3 6.6 1.7b
9.7 4.75 H2O/CO2 29.3 66.1 95.4 4.6 25.4 9.5 2.2b
19.7 5.01 H2O/CO2 19.6 75.7 95.3 4.7 15.8 15.1 3.9
29.9 5.12 H2O/CO2 12.2 83.2 95.4 4.6 8.9 20.1 5.9
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a

Difference = 100 – total yield.

b

Below the detection limit of HPLC.

Figure 6 plots the logarithmic function of hydrolysis degradation of glucan versus the holding period, which exhibited a linear relationship in a correlation coefficient of 0.999 and 0.998, respectively. Therefore, glucan was equal by disintegrated in the pseudo-first-order reaction in both cases.

Figure 6.

Figure 6

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Determination of the first-order rate constant for the glucan decomposition reaction.

By calculating the rate constant for these experiments, values of kH2O = 6.4 × 10–4 and kH2O/CO2 = 8.0 × 10–4 s–1 were obtained. Hydrolysis with CO2 led to a 1.3-fold increase in the constant rate of the glucan decomposition reaction. The effect of CO2 addition on glucan decomposition caused a modest increase in the rate constant. There are no kinetic data available from the literature on lignocellulosic biomass hydrolysis, especially glucan degradation. These data are significant for future practical reaction designs.

2.4. Effect of the Linear Velocity on Biomass Degradation (Physical Effect)

In the H2O/CO2 experiments, CO2 was added to the water flow at 1.0 mL/min. The water flow was fixed at 3.0 mL/min. For this reason, the volume of fluid per unit time of the CO2 additive experiments was much higher than that of the hydrothermal reaction. Each fluid volume per minute under the experimental condition (255 °C, 9.8 MPa) was 3.8 mL of H2O and 9.7 mL of CO2, respectively. The CO2 volume drastically increased under supercritical conditions due to a remarkable decrease in its density.46 Therefore, the total volume of fluids was 13.5 mL, which was 3.5-fold of the hydrothermal reaction. This volume, 13.5 mL, did not consider the solubility of CO2 in water. The solubility of CO2 in water under experimental conditions was calculated by the estimated formula of Henry’s constant proposed by Crovetto.47

2.4. 2

where k0 is Henry’s constant and T# = T/Tc, where Tc is a critical temperature of water. On the basis of Henry’s constant from eq 2, we can calculate the molar fraction of CO2 under experimental conditions. These results are illustrated in Figure 7 and are summarized in Table 7.

Figure 7.

Figure 7

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Molar fraction of CO2 under high pressure as a function of temperature.

Table 7. Summary of Molar Fraction of CO2 under High Pressure.

pressure, MPa molar fraction, xB solubility, g/g-H2O
8.0 0.014 0.034
9.8 0.016 0.039
12.0 0.018 0.044
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The solubility of CO2 in water was 0.039 g/g-H2O under 9.8 MPa at 255 °C. The corrected total volume was 12.3 mL, which corresponded to 12.4 mm/s as a linear velocity. To reproduce a similar linear velocity with H2O by itself, the flow rate requires 10.0 mL/min at room temperature. To exhibit the effect of linear velocity on lignocellulosic material dissolution, we focused on a hydrothermal reaction with 10.0 mL/min as the flow rate. The results of reactions with 10.0 mL/min are depicted in Figure 8 with a red circle. The solid yields with 10.0 mL/min were remarkably similar to those observed with 3.0 mL/min for 25 min, which was an equilibrium value of the solid yield at 3.0 mL/min as a flow rate. We did not observe any improvement in biomass degradation with 10.0 mL/min as a function of the solid yield. In addition, water volume during the holding period was approximately 75 mL in both cases. The observation (Figure 8) was consistent with the fact that the degradation of lignocellulosic materials corresponded to the water volume during the holding period. These results suggest that lignocellulosic material dissolution with HCW progresses in a reaction-controlled mechanism. A change in the linear velocity of a reactor was insensitive to the ultimate yield of soluble fractions.

Figure 8.

Figure 8

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Effects of the flow rate of water on the solid yield of a hydrothermal reaction.

3. Conclusions

This report detailed the degradation of lignocellulosic material, especially glucan decomposition by H2O/CO2, at 255 °C, under high-pressure conditions. For hydrothermal reactions, an increasing flow rate of water resulted in an accelerated degradation reaction, but the ultimate yield of the soluble fraction was unchanged. These results suggested that lignocellulosic material dissolution by HCW progresses as a reaction control mechanism.

In the case of glucan decomposition, the yield of the products degraded with CO2 was significantly higher compared with those of the hydrothermal reaction (25.7 vs 14.9 wt %, respectively). The reaction rate of glucan decomposition by CO2 was slightly higher than that of the hydrothermal reaction (kH2O/CO2/kH2O = 1.3). Observation of the effect of linear velocity was consistent with the fact that a change under physical condition, such as flow rate, was insensitive to the ultimate yield of soluble fraction under our experimental condition. The catalytic activity of the H2O/CO2 system in hydrolysis can be explained by the system’s chemical effect.

We proposed a mechanism for the decomposition of lignocellulosic material in the presence of supercritical CO2. From the viewpoint of practical pretreatment of lignocellulosic materials, this mechanism should be carried out using a larger particle size sample to consider pulverizing cost. However, these data would be of great importance to future reactor designs.

4. Experimental Section

4.1. Materials

The lignocellulosic biomass sample used in this study was dried bamboo (phyllostachys) pulverized to 149–297 μm. The bamboo used in this study consisted of approximately 57% (weight) cellulose, 12% hemicellulose, 25% Klason lignin, 4% diethyl ether extracts, and 12% ash. The bamboo moisture was determined to be 9.2% (weight). To evaluate biomass degradation under hydrothermal conditions, the water-soluble fraction of the sample was removed using a Soxhlet extractor. The water-insoluble yield of the sample was ∼90 wt %. The result of the elemental analysis of the water-insoluble fraction was: C, 48.2 wt %; H, 6.2 wt %; and O (diff.), 4.6 wt %.

4.2. Experimental Apparatus and Procedure

Hydrothermal and hydrolysis reactions with CO2 were conducted in a semicontinuous-flow reactor, as described by Mok et al.15 and Ando et al.13 Further details of the reactor system design can be found in Sasaki et al.43 A high-performance liquid chromatography pump delivered the solvent into a preheater, which consisted of a 7.53 mm i.d. × 185 mm, 2.7 mL stainless steel tube (SUS316). Approximately 250 mg of the sample was charged in a 4.6 mm i.d. × 39.4 mm, 0.7 mL reactor (SUS316) capped with gasket filters (average pore size: 5 μm) at the inlet and outlet. The reactor was covered with heat-insulating material to prevent heat release. The effluent from the reactor was quenched using a duplex-tube-type cooler. Temperature data were measured using thermocouples at three different locations: preheater outside, preheater exit (inside), and reactor exit (inside). The thermocouple of the preheater outside was fixed at the preheater’s center and controlled the electric heater output. Pressure in the reactor system was kept constant using a back-pressure regulator (BPR).

To define the reaction temperature, we conducted another experiment using a stainless steel tube instead of a reactor. A thermocouple was inserted at the vertical center of the tube and fixed with silver solder welding. The setting temperature and the temperature inside the reactor were measured using the tube to determine the correlation between the two parameters.43

First, we loaded ∼250 mg of the sample into the reactor. After the reactor was assembled in the apparatus, we purged it thrice with N2 gas at 3.0 MPa to remove air. The pump delivered deionized water through the preheater to the reactor at a flow rate of 3.0 mL/min. Next, the BPR was set at 9.8 MPa pressure. Heat was applied at a heating rate of ∼5 °C/min up to the desired temperature, at which it was maintained for 30 min (holding phase). Next, the electric heater was turned off and the reactor cooled to room temperature for 10 min. A siphon-type CO2 cylinder and a liquid CO2 pump (PU-2086 plus, JASCO, Tokyo, Japan) were used for hydrolysis with CO2. A spiral-type gas–liquid separator was used for efficient eluent recovery, and the CO2 flow rate was fixed at 1.0 mL/min.

Residue in the reactor (solid products) was dried at 80 °C in a vacuum for 8 h, after which the residue yield was calculated using the reactor weight before and after the reaction. Effluents that were collected during heating, holding, and cooling (liquid products) were then freeze-dried and weighed. The experimental error of our reaction procedure was within 3% of each severity.

For solid and liquid products, posthydrolysis using oligomeric reaction products took place in the second half of quantitative saccharification. First, 4% sulfuric acid solution was prepared from the reactor effluent,48 as shown in Figure 9. The reactant water-insoluble fraction of bamboo was treated with 72 and 4% sulfuric acid49 for a comparison with the yield of monomeric sugars in liquid products. Solid products after posthydrolysis defined glucan and xylan, respectively. GlcOS and XOS yields were calculated by glucose and xylose yields after posthydrolysis of liquid products.

Figure 9.

Figure 9

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Schematic diagram of the apparatus and posthydrolysis procedure. TC: thermocouple; cooler: duplex-tube-type cooler; BPR: back-pressure regulator.

To conduct a kinetic analysis of the glucan decomposition, a reaction with a higher heating rate was carried out. For higher heating rate experiments, we added a bypass channel around the reactor, as shown in Figure 9. The bypass channel was equipped with two needle valves (Swegelok, SS-3NBS4-G) and a three-way ball valve (Swegelok, SS-H83XPS4), which were resistant to high temperatures and pressures. The pump delivered deionized water to the bypass channel at a flow rate of 3.0 mL/min. Next, the BPR was set at 9.8 MPa pressure. Heat was applied at a heating rate of ∼30 °C/min up to the desired temperature. After the internal temperature (TC-2) was constant (∼60 min), the channel changed from the bypass to the reactor. As a result, the heating rate remarkably increased from 7 °C/min up to 21 °C/min based on the internal temperature (see the temperature profile for the bypass method in the Supporting Information).

4.3. Analytical Method

Liquid products were analyzed for oligosaccharides using a JASCO liquid chromatography system (JASCO) with Shodex sugar KS-804 (Shodex, Tokyo, Japan). Molecular weights of the samples were calculated by Pullulan standards. The posthydrolysis samples were analyzed for glucose (98.0%), xylose (98.0%), arabinose (99.0%), furfural (98%), and 5-hydroxymethylfurfural (5-HMF) (99%) using the same liquid chromatography system as used for Shodex sugar SP-0810 (Shodex). This system was also equipped with an ionic-form H+/CO3– de-ashing guard column (Bio-Rad, CA). Standards for the analysis and reagents were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). The carbon concentration in the liquid products was measured using a TOC-5000A total organic carbon (TOC) analyzer (Shimadzu, Tokyo, Japan).

Overend and Chornet first defined the log severity factor.50 This factor can only be used if the temperature remains constant during the reaction. Therefore, this work aimed to evaluate the temperature effect on the compositions of liquid and solid products as a function of the modified severity factor proposed by Rogalinski et al.27

4.3. 3

where t is the residence time(min), T(t) is the temperature (°C) in dependency on the residence time considering the heat-up period, and 14.75 is an empirical parameter related to temperature and activation energy. In this study, we used the modified severity factor to evaluate the temperature and holding time effects on the decomposition behavior of the H2O/CO2 system.

Acknowledgments

This work was supported by JSPS KAKENHI [grant number JP26620151]. The authors thank Prof. Kazuhiro Mae of Kyoto University for helpful discussions. They also thank Katsunori Matsushita (HBM, Japan) and Hiroshi Kimiwada (JASCO, Japan) for technical support and the fabrication of the semicontinuous-flow reactor used for this work.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c01026.

  • Temperature profiles of bypass method (Figure S1) and reproducibility of preheater inside temperature of bypass method (Figure S2) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c01026_si_001.pdf (220.9KB, pdf)

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