Article pubs.acs.org/IECR
Semibatch Hydrothermal Hydrolysis of Cellulose in a Filter Paper by Dilute Organic Acids Kengo Hirajima, Minori Taguchi,* and Toshitaka Funazukuri* Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *
ABSTRACT: Cellulose in a filter paper is hydrolyzed by six dilute organic acids (0.01−1 wt %) under hydrothermal conditions (260 °C and 5 MPa) using a semibatch reactor. The rate of degradation increases with an increase in the concentration of the acid. The cellulose sample is completely degraded by the acids (1 wt %) in 60 min. While the yields of the different products are dependent on the nature of the organic acid used, cellulose is essentially converted to mono- and oligosaccharides, with glucose as the major product. The observed increase in the rate of degradation in the presence of formic or citric acid at 1 wt % allows for the degradation of the cellulose sample in a short time (20 min) and results in the highest yield of glucose (approximately 60%) observed in this study. The rate of conversion of the cellulose sample to glucose correlates directly with [H+].
1. INTRODUCTION Biomass is a renewable organic resource, and biomass conversion is a promising alternative for the sustainable supply of medical, fuel, and valuable chemicals.1−4 This process can lead to low environmental impact because it involves the recycling of resources.1−4 Polysaccharide cellulosic materials in foods, plants, woods, and seaweeds, and their materials, residues, and wastes, constitute the most abundant biomass resource.5−7 Cellulose is converted into chemicals, such as glucose and oligosaccharides, via enzymatic8 or concentrated acid hydrolyses.9,10 The degradation or depolymerization of cellulose does not progress rapidly due to the robust crystal structure of cellulose, which results from the network of intra- and intermolecular hydrogen bonds.5−7 Due to its high crystallinity, cellulose takes longer to be hydrolyzed by enzymes. Furthermore, acidic hydrolysis of cellulose affords low yields of glucose, since the obtained products are decomposed further under harsh reaction conditions. In addition to these conventional methods, cellulose is broken down using supercritical water in the absence of any additives and with extremely short contact times.11−16 In this process, the use of fine particles of cellulosic materials can be beneficial because it allows for the accurate regulation of the contact times and the constant feeding rate of cellulose materials (samples) to the reactor. However, examples of samples containing lignocellulose include textiles, woods, and grass plants; these samples, with varying compositions and structures of their lignocellulosic components, require pulverizing and crushing, which are neither easy nor economical. In practice, with such cellulosic materials, processes with relatively longer contact times (several minutes) and mild temperatures (e.g., hydrothermal conditions using subcritical water) are suitable, since the conversion can be performed in conventional reactors. In fact, under hydrothermal conditions, cellulose degrades slowly in water (absence of any additives). The reaction temperature can be reduced to © XXXX American Chemical Society
suppress the decomposition of the desired products, while the presence of an acid can further accelerate the reaction. Cellulosic biomass, such as woods and grass plants, is pretreated with aqueous, dilute, or concentrated acids, predominantly sulfuric acid, to moderately accelerate the hydrolysis of the materials.9,10,17−28 Recently, organic acids, such as formic, acetic, and succinic acids, have also been utilized for the hydrolysis of lignocellulosic samples under hydrothermal conditions.29−34 There are several carboxylic acids, such as formic, acetic, and citric acids, that are nature derived and barely cause any corrosion to stainless steel reactors (when compared to that caused by inorganic acids at identical concentration). Considering the separation and refinement of the products, most organic acids with low boiling points can be easily vaporized, resulting in products that can be separated from organic acids.34 In contrast, the use of inorganic acids mandates the neutralization of product solutions. Given these advantages, organic acids are suitable for the pretreatment of cellulosic biomass. While a few conducted organic acids have been used previously,29−34 other organic acids have not yet been evaluated. Previously, we have developed techniques for the conversion of polysaccharide samples to glucose via hydrolysis under hydrothermal conditions.35−42 Recently, we have also evaluated the impact of organic acids (i.e., formic and acetic acids) on the hydrolysis of cellulose in a pure cotton sample using a semibatch reactor.41,42 These studies reveal that the proton (H+) provided by the acid is positively correlated to the degradation of the cellulose sample via hydrolysis. In this study, the effect of various carboxylic acids on the degradation of cellulose is examined under hydrothermal conditions using a semibatch reactor, and the influence of [H+] on the kinetics of Received: March 9, 2015 Revised: May 20, 2015 Accepted: May 22, 2015
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DOI: 10.1021/acs.iecr.5b00920 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
were compared with those of a pure cotton sample used in our previous study.41,42 The XRD patterns were recorded using a SmartLab diffractometer (Rigaku) with Cu Kα (λ = 1.5416 Å) radiation at a 2θ scan speed of 2°/min. The crystallinities of cellulose in filter paper and cotton samples were approximately 86 and 75%, respectively (Supporting Information, Figure S1), as determined using the intensity ratio of (002) peak to amorphous part in the pattern.44 This indicates that the cellulose in filter paper has higher crystallinity when compared to that in cotton; i.e., the amount of amorphous component in cotton is greater than that in filter paper. Magnified images of the samples were obtained by SEM using a TM3030 instrument (Hitachi). SEM images indicated that cellulose was denser in the filter paper sample than in the cotton sample (Supporting Information, Figure S2). The DPs of these cellulose samples were estimated by a viscosity measurement.45 Each sample was dissolved in aqueous copper ethylenediamine solution (0.5 M), and its viscosity was measured using an Ubbelohde viscometer; the DPs of the cellulose in filter paper and cotton samples were approximately 1020 and 2040, respectively. To determine the amount of carbon of the cellulose in filter paper sample, elemental analysis of the cellulose sample was performed on a PerkinElmer 2400 II CHN analyzer. 2.2. Hydrothermal Treatment Using a Semibatch Reactor. The cellulose sample was depolymerized in a stainless steel semibatch reactor that was similar to the one used in our previous studies (Figure 1).37,39−42 The reactor with an inner volume of 3.6 mL was connected to a preheating stainless steel coiled column (6 m, 1/8 in. o.d.). The strip of filter paper (0.5 g; 2 × 50 mm2) was placed in the reactor, and frit disks (pore size, 2 μm) were placed at the inlet and outlet of the reactor to contain the unreacted cellulose sample and/or water-insoluble substances. Each acid (i.e., formic, acetic, oxalic, malonic, succinic, and citric (Scheme 2)) was independently dissolved in distilled water at different concentrations (0.01, 0.1, and 1 wt %; corresponding to the molar concentrations of 0.5−220 mmol/kg) at room temperature, and the solution was pumped by two high-performance liquid chromatography (HPLC) pumps (L-6000, Hitachi), each with a volumetric flow rate of 7.5 mL/min. Distilled water was also fed by another HPLC pump (PU-2080, JASCO) at a volumetric flow rate of 5 mL/ min to rapidly quench the reaction solution. The reaction was heated to 260 °C using a molten salt bath under a constant pressure of 5 MPa for 120 min. The pressure was controlled by a backpressure regulator (880-81, JASCO). The reaction was initiated by immersing both the preheating column and semibatch reactor in the molten salt bath. The semibatch reactor was equipped with a thermocouple, which indicated that the temperature reached the set value (260 °C) within 1 min. Fractions of product eluting from the backpressure regulator were collected at 3−15 min (mainly 5 min) intervals. The residence time for the water fluid between the reactor inlet and the exit at the backpressure regulator was approximately 15
the degradation is examined. In addition, complex cellulosic materials, such as textiles, woods, or grass plants, are used rather than pure cellulose materials, such as microcrystalline and cotton cellulose samples that are predominantly used for studying saccharification in a flow-type reactor.40−43 While we already hydrolyzed cellulose in a pure cotton sample without lignin and hemicellulose under hydrothermal conditions, there is little fundamental information and knowledge for the degradation of cellulose in order to apply the present technique to industrial production. To understand the fundamental properties and kinetics for the degradation of cellulose under the present conditions is necessary. Subsequently, a filter paper, a pure cellulose sample without lignin and hemicellulose, representing wastes such as printed papers, cardboards, and woods in terms of the degree of polymerization (DP) and crystallinity, is selected as a sample for saccharification.
2. EXPERIMENTAL SECTION 2.1. Materials. Formic acid (98%), malonic acid (98%), and citric acid (98%) were purchased from Wako Chemical. Acetic acid (99.7%), oxalic acid (97%), and succinic acid (99.5%) were purchased from Kanto Chemical (Scheme 1). Chemicals used Scheme 1. Structure of Organic Acids
as reference materials include glucose (Glu; 99.5%) (Scheme 2), fructose (Fru; 99%), 5-hydroxymethylfurfural (5-HMF; 99%), and levoglucosan (Lev; 99%). These chemicals were purchased from Aldrich, and cellobiose (glucose dimer) was purchased from Wako Chemical. Monogalacturonic acid (97%) was purchased from Wako Chemical. For use in instrumental analyses, aqueous copper ethylenediamine solution (1 M), NaOH (97%), and 2,5-dihydroxybenzoic acid (DHB; 98%) were purchased from Wako Chemical, and sodium acetate (AcONa; 98.5%) was purchased from Kanto Chemical. The cellulose sample (Scheme 2), quantitative filter paper (No. 7; 0.18 mm thickness and 87 g/m2, ash content
