Correlation of the Product Yield with the Total Organic Carbon Yield in

Aug 26, 2016 - based on the total organic carbon (TOC) yield were determined as first- ... plots of the product yield versus TOC yield were correlated...
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Correlation of the Product Yield with the Total Organic Carbon Yield in the Hydrothermal Conversion of Pure Celluloses in the Absence of Additives Toshitaka Funazukuri,* Yuki Asaoka, Kengo Hirajima, and Minori Taguchi Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, Tokyo 112-8551, Japan S Supporting Information *

ABSTRACT: Three cellulosic samples (CP, cellulose powder; CC, cotton cellulose; FP, filter paper) were subjected to liquid water at 493 to 553 K and 10 MPa in a semibatch reactor. The solubilization rates based on the total organic carbon (TOC) yield were determined as firstorder reaction kinetics, and the rates for CP were 2−4 times higher than those for CC and FP. The yields of the products, such as glucose, cellooligosaccharides with degrees of polymerization (DP) from 2 to 9, fructose, 5-(hydroxymethyl)furfural, and levoglucosan, were proportional to the TOC of the products over nearly the entire range of conversion, irrespective of the reaction temperature. The slopes in the plots of the product yield versus TOC yield were correlated with DP of up to 9, and the correlations for CP, CC, and FP were consistent and independent of the temperature and cellulose species.

1. INTRODUCTION Cellulose is one of the renewable biomass resources abundantly available on Earth, and the conversion of biomass to chemical feedstocks or fuels,1−4 as an alternative to fossil fuels, has been increasingly in demand. Glucose and cellooligosaccharides can be produced from hydrolysis of cellulosic samples. Cellooligosaccharides are important as dietary supplementation5−7 and have possible versatile applications in the pharmaceutical, nutraceutical, and cosmetic industries. Hydrothermal conversion, by hydrolysis in compressed liquid water, is attractive for producing glucose and cellooligosaccharides from cellulose without additives or with a small amount of acids.8,9 While cellulose has a structure similar to that of starch, their reactivities are quite different. Cellulose is a polysaccharide made of glucan bonded via β-1,4-glycosidic linkages, and starch is made of glucan bonded via α-1,4-glycosidic linkages.10 The former has inter- and intramolecular hydrogen bonds. This brings about high crystallinity, which leads to resistance to chemical or enzymatic hydrolysis. Thus, a higher reaction temperature is required to hydrolyze cellulose than starch.11,12 Using an in situ polarized microscope, Deguchi et al. directly observed that crystalline cellulose fibers heated at a rate of 11− 14 °C/min10,13 undergo a crystalline-to-amorphous transformation in water at about 320 °C and disappear at 340 °C. Thus, supercritical water (critical temperature Tc = 647.1 K and critical pressure Pc = 22.1 MPa14) is preferable for hydrolysis of cellulose to make it noncrystalline. In fact, cellulose hydrolysis with water in the absence of additives has been carried out at supercritical or subcritical conditions, i.e., slightly lower than the critical temperature.15−20 © XXXX American Chemical Society

Because products formed from the degradation of cellulose have less crystallinity, the reactivity of the products is higher than that of the initial cellulosic sample when it is treated with water alone. The products formed are more readily decomposed than the initial cellulosic sample. To avoid or further reduce reactions of the products, a flow reactor is essential, such that products formed from celluloses are instantly swept out of the reaction zone. In fact, Kumar and Gupta pointed out that kinetics obtained from the batch reactor often can give misleading information on the design of industrial-level reactors.20 The authors carried out cellulose hydrolysis in a semibatch reactor, using dilute (less than 1 wt % acid concentration) aqueous solutions of formic acid8 and various organic acids.9 As a result, 86% of total sugars, which included glucose, 61% of the initial cellulose, were obtained in a 1 wt % formic acid solution.9 Although kinetic studies on actual biomass materials such as agricultural wastes,21−25 wood,21,26 grasses,21,27−29 medicinal plants,30 and waste newspaper31 have been reported under hydrothermal conditions, mostly in the presence of acids, those on pure fibrous cellulose samples32 rather than cellulose powder16,33−36 are limited. From a practical point of view, these studies on the actual cellulosic biomass are of importance, but because of other ingredients such as hemicellulose and lignin in the systems, it is difficult to study the effects of reaction conditions on both the rates and product distributions. While Received: May 10, 2016 Revised: August 3, 2016 Accepted: August 17, 2016

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DOI: 10.1021/acs.iecr.6b01782 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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up to 180 min. The temperature of the molten salt bath was maintained at the prescribed temperature to within ±2 K. The system pressure was maintained at 10 ± 0.1 MPa by adjusting the back-pressure regulator. Distilled water at room temperature was also fed in by another HPLC pump (3) to quench the product solution instantly. The residence times of fluid between A and B designated in Figure 1 were measured at 20−30 s by a tracer response technique.10,37 An aqueous galacturonic acid solution was pulse-injected via an HPLC injector installed at position A, and the response was measured by an electronic conductance detector placed at position B. 2.3. Analysis. The three celluloses employed were characterized by their CI values, which were estimated by X-ray diffraction measurements,9 and their DP values, which were estimated with an Ubbelohde viscometer via viscosity measurements of a 0.5 M copper ethylenediamine solution,9 in which the cellulose sample was dissolved at ambient temperature. Organic carbon in the product solution was measured by a TC analyzer (5000A, Shimadz, Tokyo, Japan), and the yields of glucose, fructose, cellooligosaccharides with DP = 2−9, levoglucosan, and 5-HMF were quantified by high-performance anionexchange chromatography.8,9 Cellooligosaccharides were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, as described elsewhere.8,9

microcrystalline cellulose powder has been hydrolyzed in supercritical water,15−19,33−35 hydrolysis under hydrothermal conditions (subcritical conditions) is rare. Because of higher reaction temperatures, hydrolysis in supercritical water may be preferable for producing glucose and further decomposition products such as levoglucosan, 5-(hydroxymethyl)furfural, and glycolaldehyde rather than cellooligosaccharides. Moreover, the effects of cellulose species on the product distribution and rates have not been well investigated because microcrystalline cellulose powder has mostly been employed in continuousflow reactors because of technical difficulties in feeding fibrous celluloses into the reactor with rapid heating. In particular, studies8,9,35 on the yields of cellooligosaccharides are limited, and the effects of the reaction conditions on the yields and rates are not well understood. In the present study, a small semibatch reactor was employed, so that the products formed could be instantly removed from the reaction zone and further decomposition of the products could be suppressed. Using the reactor, the objectives are to measure the solubilization rates based on the total organic carbon (TOC) and the yields of major products in the resulting solutions from hydrolysis of pure cellulosic samples (cellulose powder, CP; cotton cellulose, CC; filter paper, FP) under hydrothermal conditions and to study the effects of the reaction temperature and cellulose species on the product distribution.

3. RESULTS AND DISCUSSION 3.1. Product Yield. The yields of product and TOC are defined by eqs 1 and 2, respectively, as in previous studies.8,9 yield of the product component (%)

2. EXPERIMENTAL SECTION 2.1. Chemicals. Glucose (99.5%), cellobiose (99.0%), cellotriose (99.0%), levoglucosan (99.0%), 5-(hydroxymethyl)furfural (99.0%), and ashless cellulose powder (Avicel, 100−200 mesh) were obtained from Sigma-Aldrich (Japan) and used as received. Ashless filter paper (no. 7) was purchased from Advantech (Tokyo, Japan), and standard dewaxed sanitary cotton cellulose, compliant with the pharmacopeia of Japan, was used. The characteristics of the three cellulosic samples are listed in Table 1. The degree of polymerization (DP) and crystallinity index (CI) for CP are lower than those for CC and FP. The measurements of DP and CI are described later.

= 100 ×

(1)

Herein, each product component whose yield was measured is glucose, cellooligosaccharides with DP = 2−9, fructose, levoglucosan, and 5-HMF. TOC yield (%) = 100 ×

Table 1. Characteristics of Cellulose Samplesa

a

carbon in the product component (g) carbon in the initial cellulose sample (g)

carbon in the aqueous solution (g) carbon in the initial cellulose sample (g) (2)

cellulose

DP

CI

filter paper (FP) cotton (CC) microcrystalline cellulose (CP)

960 2040 210

89 81 66

Figure 2 shows variations over the flow time of (a) TOC, (b) glucose yields, and (c) unrecovered component yield from CP at temperatures from 513 to 553 K. The TOC values at temperatures higher than 533 and 543 K reach nearly 100%, and those below 523 K may reach 100% but only after longer flow times. The maximum yield at the highest temperature, 553 K, was lower than those at 533 and 543 K. This may be caused by the production of gaseous products at the higher temperature. The glucose yields show the same tendency as that for the TOC yield, but the yields were lower. The maximum glucose yields were 18 wt % of the initial sample at 543 and 553 K. The solubilization rates increased with increasing temperature. Similar to the TOC yield over time, the glucose yield also increased with increasing temperature. At 543 K, the TOC yield leveled off for 20 min, as did the glucose yield. The unrecovered component yield was defined as the difference between the TOC yield and total mass of the products quantified, namely, glucose, fructose, cellooligosaccharides with DP = 2−9, levoglucosan, and 5-HMF. The tendency of the yields was similar to those of the TOC and glucose yield. At 543 and 553 K, the yields leveled off at the same values of flow times as the TOC and glucose yields did. The unrecovered components were expected to be cellooligo-

DP = degree of polymerization; CI = crystallinity index.

2.2. Apparatus and Procedure. The experimental setup, which is similar to that employed in the previous studies,8,9 is shown in Figure 1 and described briefly below. A reactor with an inner volume of 3.6 mL was made of stainless steel tubing with an inside diameter of 6.7 mm and a length of 8.5 mm and plugged at the exit with a frit disk (pore size of 2 μm). Both sides were connected via reducing unions to a 3.18 mm (1/8-in.-o.d.) preheating column (6 m) and a water jacket in which water was flowing via 3.18-mm-o.d. stainless steel tubing. Prior to a hydrolysis run, the cellulosic sample (CP, CC, or FP) was loosely packed in the reactor at room temperature, and the flow lines and reactor were filled with distilled water. Once the temperature of the molten salt bath, heated by an electric furnace, reached the intended value and was stable for longer than 30 min, liquid distilled water was supplied by switching on two HPLC pumps (2), and the reactor was quickly immersed in the molten salt bath. The resulting solution eluted from the exit of the back-pressure regulator (JASCO 880-81, Japan) was collected every 3−15 min while distilled water was continuously supplied to the reactor for flow times of mainly 60 min B

DOI: 10.1021/acs.iecr.6b01782 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic drawing of the experimental apparatus: (1) reservoir of distilled water; (2) HPLC pump; (3) HPLC pump; (4) preheating column; (5) reactor; (6) stirrer; (7) thermocouple; (8) molten salt bath; (9) cooler jacket; (10) back-pressure regulator; (11) sampling. k1

cellulose → water‐soluble components k2

cellulose → glucose −

dC = kC dt

(3) (3a)

for k = k1 or k 2

(4)

where C is the cellulose remaining and is defined as W/W0. W and W0 are cellulose weights at flow time t and the initial time zero, respectively, and C was assumed to be equal to (1 − [TOC (%)]/100) for k1 and (1 − [glucose (%)]/100) for k2, respectively. Herein, k1 and k2 are first-order rate constants (min−1) based on the TOC and glucose yield, respectively. Figure 3 shows (1 − [TOC]/100) versus time for CP at

Figure 3. 1 − [TOC (%)]/100 versus flow time for CP at various temperatures: 513 K (open circles); 523 K (solid circles); 533 K (open triangles); 543 K (solid triangles); 553 K (open squares).

temperatures from 513 to 553 K. As depicted, eq 4 is valid over a wide range of solubilizations up to (1 − [TOC]/100) = 0.08, corresponding to conversions higher than 90%. Figure 4 shows Arrhenius plots for the first-order rate constants for solubilization rates, based on the TOC yield, for the three celluloses, CP, CC, and FP, compared to those for CC by Schwald and Bobleter.32 The first-order rate constants reported in the literature were obtained by measuring the glucose yields in eq 3a. Equation 3 is more suitable for the cellulose decomposition rates than that used for glucose because products other than glucose are produced, and the solid weight changes are less accurate when a small amount of sample is loaded in the reactor. The values for CC and FP were consistent and substantially agreed with those in eq 3a by

Figure 2. Variations of (a) TOC, (b) glucose, and (c) unrecovered mass over flow time at temperatures from 513 to 553 K for CP.

saccharides having DP higher than 10 and further degradation products. 3.2. Reaction Rate. The reaction rate for the reaction pathway shown in eq 3 is expressed as first-order reaction kinetics, and the rate is described in eq 4. C

DOI: 10.1021/acs.iecr.6b01782 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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were observed at different flow times, although the conversions were similar. This could be significantly affected by mixing or dispersion in the reactor, even though the flow rates were the same. It can be considered that the small difference in the residence time in the reactor results in a difference in the heating/reaction time and affects each product yield. Thus, it is difficult to accurately predict the production rates for each product without information on the dispersion in the reactor, even a simple tubular reactor. 3.4. Correlation. Figure 6a shows yield cross-plots of total sugar versus TOC and glucose versus TOC at temperatures from 513 to 553 K, including three data sets at the same reaction temperature of 523 K, as shown in Figure 5. Although variations of the glucose yields over flow time for the three runs at 523 K were less reproducible, as shown in Figure 5b, yield cross-plots of glucose versus TOC were consistent irrespective of the reaction temperature. Yields of total sugar also exhibited the same behavior. This implies that the product distribution, i.e., the selectivity for each product, was almost the same irrespective of conversion. In Figure 6b, the cellobiose yields were also proportional to the TOC yield, but the slope was lower than that for glucose. Figure 6c plots the fructose yield. Because fructose is produced by isomerization of glucose,38 the yields were also proportional to the TOC yield. However, the slopes were affected by the reaction conditions. The slopes for CP were not directly related to the reaction temperature. Parts d and e of Figure 6 show the levoglucosan and 5-HMF yields. Yields of both products were also proportional to the TOC yield, and the slope for levoglucosan increased with the temperature; however, that for 5-HMF increased with the temperature only up to 523 K and decreased at higher temperatures. Levoglucosan and 5-HMF are produced by the dehydration reaction from glucose and intermediate products. Thus, intercepts of the TOC yield were observed corresponding to the time delay required to form the major products such as glucose. In Figure S1a−g, plots are shown for DP = 3−9. Yields of these products were also proportional to the TOC yield. The product yields decreased with increasing DP, and yields were more scattered. As depicted in Figure 6, the yield of each product component y is proportional to the TOC yield, as described by eq 5.

Figure 4. Arrhenius plots for the first-order rate constants k1 for solubilization of CP, CC, and FP, together with k2 for CC by Schwald and Bobleter.32

Schwald and Bobleter, while the rates at lower temperatures were slightly higher. However, the rates for CP were 2−4 times higher than those for CC and FP. The preexponential factors and activation energies were 8.28 × 1011 min−1 and 159 kJ/mol and 8.61 × 1017 min−1 and 195 kJ/mol for CC and FP and for CP, respectively. 3.3. Reproducibility in the Product Yields with Time. Figure 5 shows variations of the TOC and glucose yields for

y = α(TOC)

(5)

where α is the slope and TOC is the TOC yield. Equation 5 is valid for glucose and cellooligosaccharides with DP = 2−9. Note that eq 5 was also valid for further decomposition products such as levoglucosan and 5-HMF, but the slopes varied at the temperatures, and threshold TOC values (about 10% TOC yield) were found. Figure 7 shows slope α versus DP for CP at various temperatures, together with ±20% α values shown with dotted lines. The α values decrease with increasing DP and can be expressed with a function of DP as

Figure 5. Variations of Δy/Δt over flow time for (a) TOC and (b) glucose at 523 K and 10 MPa for the three identical reaction conditions.

each fraction over flow time at three identical conditions, at 513 K and 10 MPa, where Δy is the yield of TOC or glucose in each fraction during the interval Δt. The results are quite interesting. The variations in Δy/Δt for TOC were similar for the three runs at the same reaction conditions, and the maximum values of Δy/Δt were ca. 10 min and thereafter decreased with the flow time for the three runs. Thus, the conditions of the fluid contacting solid cellulose are reproducible. Note that cellulosic samples were solid and did not melt in the course of the reaction, and the color of the residual solids was not changed. On the other hand, the maximum values of Δy/Δt for glucose

α = exp[−(2.578 × 10−2x 2 + 2.559 × 10−1x + 1.276)] (6)

where x is the DP. Note that α values for cellobiose from the three cellulosic samples below deviated from eq 6, and the reason is not clarified. While the data deviated somewhat, most data were correlated with a single line and ranged within ±20% deviation. Moreover, the three cellulosic samples were consistent (see Figure S2a,b), although those for FP and CC deviate more than those for CP. D

DOI: 10.1021/acs.iecr.6b01782 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. Slope α versus DP for CP at temperatures from 513 to 553 K and 10 MPa. The solid line represents eq 6, and the broken lines are ±20% values.

were not affected by cellulose species and DP, as shown in Figure S2a,b.

4. CONCLUSIONS Three pure cellulosic samples were hydrolytically decomposed without any additives under hydrothermal conditions in a semibatch reactor. The solubilization rates based on the TOC yield were expressed by first-order reaction kinetics. The rates for CP were 2−4 times higher than those for CC and CP. The yields of major products such as glucose, cellooligosaccharides with DP = 2−9, fructose, levoglucosan, and 5-HMF were measured. Each product yield was proportional to the TOC yield and found to be independent of the reaction temperature and cellulose species. The correlation of the slopes with DP was obtained and was identical for the three celluloses CP, FP, and CC.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01782. Plots of the product yield versus TOC yield for DP = 3− 9 from cellulose powder (Figure S1a−g) and slope α versus DP for CC and FP (parts a and b of Figure S2, respectively) (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-3-3817-1914. Fax: +81-3-3817-1895. Notes

The authors declare no competing financial interest.

■

Figure 6. Yield cross-plots of (a) total sugar and glucose, (b) cellobiose, (c) fructose, (d) levoglucosan, and (e) 5-HMF versus TOC yield at temperatures from 513 to 553 K, including the data obtained for three runs at 523 K and 10 MPa.

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DOI: 10.1021/acs.iecr.6b01782 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX