ISSN 1070-4272, Russian Journal of Applied Chemistry, 2016, Vol. 89, No. 8, pp. 1360−1364. © Pleiades Publishing, Ltd., 2016.

VARIOUS TECHNOLOGICAL PROCESSES

Preparation and Characterization of Biopolyol via Liquefaction of Rice Straw1 Guizhen Gong and Xiucheng Zou* School of Chemical Engineering, Xuzhou Institute of Technology, Xuzhou 221111, China *e-mail: [email protected] Received July 13, 2016

Abstract—Biopolyols were prepared by the liquefaction of rice straw under the mild condition. The optimum liquefaction effect was obtained at 5 : 1 volume ratio of PEG400 to DEG, 4 : 1 liquid–solid ratio, H2SO4 3%, time 2.5 h, and reaction temperature120°C. Products were characterized by FTIR and gel permeation chromatograms (GPC) measurements. The hydroxyl value and weight-average molecular weight of the biopolyol produced based on the above optimal conditions were 260 mg KOH/g polyol and 420 g mol–1, respectively. Biopolyol obtained is suitable for the preparation of rigid polyurethane foam. This study has certain significance for the high added value use of rice straw and reducing the production cost and improvement biodegradability of polyurethane foams. DOI: 10.1134/S1070427216080231

INTRODUCTION Polyol is an important polymer that has wide application such as in detergent, emulsifier, dispersant, and antistatic agent. One of its most important applications is the preparation of polyurethane (PU), which has wide application in coatings, packing, automobile, elastomers, aviation, sealants, and aircraft manufacturing [1–3]. However, most of polyol is produced from petroleum, and the price heavily depends upon the oil markets in the world. Also, in view of petroleum reserves and environmental issues, it should search alternative resources for producing polyol. Cellulose, semi-cellulose, and lignin, as three major components of biomass, have lots of hydroxyl groups. So, biopolyols with sufficient amount of hydroxyl groups may be produced from various biomass feedstocks. The subject has been paid more and more attentions. Untill now, a variety of biomass considering as raw material has been studied for the production of biopolyols [4–10]. Generally polyols produced from biomass possess promising properties for the production of PU and the resulting PU shows properties comparable to that based 1

The text was submitted by the authors in English.

on petroleum [11–13]. Rice straw, as agricultural wastes, is an abundant, renewable, and cheap resource. Most of them are not fully utilized such as by direct combustion without any disposal, that pollutes the environment and wastes the biomass resource. Thus, converting rice straw into polyols by liquefaction may produce great social, economic, and environmental benefits, therefore, it is important in theory and valuable in practice to research in this respect [14]. In the study, biopolyols prepared by the liquefaction of rice straw were investigated under the mild condition. The effects of different liquefaction conditions were discussed. The characteristic of biopolyols was also tested. EXPERIMENTAL Materials. Rice straw used in the study was collected from the farm at Xuzhou city, Jiangsu, China. It was washed with de-ionized water, air-dryed, and then ground to pass through 40 mesh sieve. All the raw materials were dried in an oven at 105°C for 24 h before liquefaction, and then stored in a desiccator at room temperature before using. All chemicals used in the experiment are of analytical grade, obtained from the commercial source, and used without further purification.

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Pretreatment of rice straw. Rice straw (5.0 g) and 150 mL 5% sodium hydroxide solution were added into a three-necked flask, equipped with a magnetic stirrer, a thermometer, and a reflux condenser. The mixture was heated to boiling temperature in an electric heating sleeve for 1.5 h, then cooled to room temperature, and filtrated under vacuum. Filter cake was washed with de-ionized water until the filtrate became neutral. The filter cake scraped from the filter paper, dried in an oven, and weighed is called rice straw treated with alkali solution. Rice straw treated with alkali solution and 3% hydrochloric acid of a 10 : 1 mass ratio were weighed into 3-necked flask, and heated to boiling temperature for 0.5 h. The reaction mixture cooled to room temperature and filtrated through filter paper under vacuum, washed with deionized water until the filtrate became neutral, dried in an oven and weighed is called rice straw treated with acid solution. The rice straw treated with acid solution was decolorized with 6% hydrogen peroxide and 6.5% aqueous sodium hypochlorite solution, sequentially. The solid material obtained was called pretreated rice straw. Preparation of biopolyol. The preparation of biopolyol was carried out in a three-neck flask with a reflux condenser, thermometer and stirrer under atmospheric pressure. Polyethylene glycol (PEG400) and diethylene glycol (DEG) were added into reactor according to preset ratio. Then, when the reaction mixture reached the desired temperature a certain amount of concentrated 98% sulfuric acid and 2.0 g pretreated rice straw were placed and kept for desired time. After ending the reaction, the liquefaction mixture was cooled to room temperature, and filtrated under vacuum. Filter cake fried into oven and then weighed is called residue. To the filtrate adjusted to about pH 7 with magnesium oxide a certain amount of anhydrous CaCl2 was added and filtrated. Biopolyols were obtained upon recovery from the flask. FTIR and gel permeation chromatograph analysis. Biopolyol, residue, and pretreated raw were analyzed by FTIR spectroscopy (Alpha, Bruker company, German) using potassium bromide tableting method over a scan range of 500 to 4000 cm−1. Molecular weight analysis was performed on an Agilent PL-GPC 50 gel permeation chromatograph. Chloroform was used as the mobile phase at a flow rate of 1.0 mL min–1, column temperature of 35°C under 10.0 MPa pressure. The concentration of the samples was 1.5 mg mL–1 in the chloroform solution, and the amount injected was 100 μL. The molecular weights of the samples were calibrated by use of monodisperse polystyrene standards.

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Fig. 1. Effect of acid loading on rice straw liquefaction. (VPEG400 : VDEG = 5 : 1, liquid–solid ratio 4 : 1, reaction temperature 120°C, reaction time 2 h).

Determination of yield of residues. The yields of residues y (%) were calculated using the following equation: m y = ––– × 100％, m0 where m is weight of residues obtained from liquefaction process; m0, weight of rice straw added in liquefaction process. RESULTS AND DISCUSSION Effects of catalyst. Effects of hydrochloric, phosphoric, and sulfuric acids as a catalyst on liquefaction reaction were investigated. The results showed that the effects of hydrochloric acid and phosphoric acid on liquefaction were small, while catalytic effect of sulfuric acid was the best. Figure 1 illustrates the change in the yield of liquefaction residue as a function of sulfuric acid loadings from 0 to 4%. It is obvious that addition of catalyst significantly reduces the residual yield. The yield of liquefaction residue was 96.5% without catalyst indicating that straw rice was almost not liquefied under this condition. Then, yields of residue dramatically decreased from 96.5% to 43.3% upon an increase in the sulfuric acid loading from 0 to 3% and became flat when the acid loading increased from 3 to 4%. Meanwhile, 5% sulfuric acid loading was also examined. It was found that

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Fig. 2. Effect of the liquid–solid ratio on liquefaction. VDEG/ VPEG400 = 5 : 1, H2SO4 3%, reaction temperature120°C, reaction time 2 h.

Fig. 3. Effect of ancillary reagent DEG on liquefaction. The liquid–solid ratio 4 : 1, H2SO4 3%, reaction temperature120°C, reaction time 2 h.

during the liquefaction a liquefaction rate was great at the beginning, then temperature gradually increased and can not be controlled; residue was black solid material, and its amount also increased. This is might due to the fact that more acid resulted in more condensation reactions occurring of the liquefied components, leading to higher residue yield. Thus, 3% value was determined to be a suitable acid loading concentration for rice straw liquefying. Effect of liquid–solid ratio. Liquefaction of rice straw at different liquid-solid ratios was studied and is shown in Fig. 2. The yield of liquefaction residue decreased with increasing liquid–solid ratio. When the liquid–solid ratio was at lower ratio (2 : 1) the yield of liquefaction residue was very high (76.0%). After the great decrease in the yield of liquefaction residue with increasing the liquid–solid ratio from 2 : 1 to 4 : 1 it became slow. That is because liquid can disperse rice straw into liquefaction solvent. The low liquid–solid ratio means high raw material concentration, which could lead to re-condensation reactions between the liquefied components. An increase in the liquid–solid ratio could promote diffusion of rice straw into the liquefaction solvent, which means more consumption of the solvent that could rise the cost of the liquefaction process and at the same time will affect PU synthesis. So the liquid-solid ratio 4 : 1 was considered as the most suitable for this liquefaction process.

Effects of DEG. To evaluate the effect of DEG as ancillary reagent to PEG400, a series of experiments was carried out. Figure 3 shows the results. The amount of the solid residue sharply decreased from 50.2 to 41.3% as DEG and PEG400 volume ratio (V DEG/V PEG400) changing from 0.1 to 0.5. A further increase in the VDEG/ VPEG400 ratio, from 1 to 5, did not affect the amount significantly. Hence, the addition of DEG as ancillary reagent to PEG400 plays a key role on liquefaction processes, which is because the fact that DEG has good permeability into the rice straw, thereby accelerating the liquefaction rate. In addition, DEG played a role in the suppress of re-condensation reaction resulting in a decrease in the amount of residue. But a large number of small molecule DEG can reduce PU molecular weight. Based on the above results, the optimal volume ratio of DEG to PEG400 was determined to be 1 : 5. This ratio was used for further optimization studies. Effects of reaction temperature. The effect of temperature on the yield of the solid residue is shown in Fig. 4. It is obvious that the influence of temperature is more complicated. When the temperature was 110°C, the yield of the solid residue was 86.7%, indicating poor liquefaction. When the liquefaction temperature increased from 100 to 120°C, the yield of the solid residue sharply decreased to 43.3%, and remained constant at 130°C. At further increase in temperature to over 150°C, the amount of residue reaches to the minimum. Then, the amount of

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Fig. 4. Effect of temperature on liquefaction. VPEG400 : VDEG = 5 : 1, the liquid–solid ratio 4 : 1, H2SO4 3%, reaction time 2 h.

Fig. 5. Effect of liquefaction time on liquefaction.

residue increased with further increasing the temperature from 160 to170°C. The results indicate that degradation and re-condensation reaction occurred simultaneously in liquefaction process, and temperature plays an important role in the two kinds of reactions. Degradation reaction takes place at low temperature, resulting in low the amount of residue, while recondensation reaction occurs at high temperature, leading to high the amount of residue. At low temperature, liquefaction effect is poor, and higher temperature leads to carbonation as well as waste energy. So the temperature of 150°C is suitable. Effects of reaction time. In order to study the effect of the liquefaction time on the yield of the solid residue, a series of experiments was performed at different liquefaction time under the following liquefaction conditions: PEG400 : DEG volume ratio 5 : 1, liquid– solid ratio 4 : 1, H2SO4 3%, and reaction temperature 120°C, as shown in Fig. 5. The yield of the solid residue decreased from 73.1% to 20.6% with the increase in the reaction time from 0.5 to 2.5 h. Along with the extension of liquefaction time, the yield of the solid residue has a tendency to grow. This might be due to the fact that degradation and re-condensation reaction occurred simultaneously during liquefaction process, with prolonging time the recondensation reaction was the more dominant reaction, producing a high amount of residue. Hence, it can be said that 2.5 h is considered as the most suitable liquefaction time to use for this system. Hydroxyl number and weight-average molecular weight. To characterize the products, biopolyol was obtained by the following optimized conditions: PEG400 :

DEG volume ratio 5 : 1, liquid–solid ratio of 4 : 1, H2SO4 3%, time 2.5 h, and reaction temperature 120°C. The hydroxyl value of biopolyols was determined in accordance with the method of Yao [15]. The hydroxyl value of the biopolyol produced based on liquefaction process was 260 mg KOH/g polyol. Weight-average molecular weight of which was 420 g mol–1. So, the biopolyol obtained from liquefaction rice straw is suitable for the production of rigid PU foams [16]. FTIR analysis of biopolyol, residue and pretreated raw. Figure 6 illustrates the FTIR spectra of biopolyols, residue, and pretreated raw material. The broad band at about 3350 cm–1 attributed to stretching vibration of hydroxyl group, which indicates the presence of OH groups in the three samples, while, the two solid samples had relative small peak form at this location, indicating a large amount of hydrogen groups in biopolyols. Another strong peak at 2900 cm–1 was the characteristic C–H stretching modes of the aliphatic –CH3, –CH2, and CH groups. The wave at 1640 cm–1 was corresponded to carbonyl bond which was characteristic of ketone, aldehyde, or carbonyl acid and, ester, which are the characteristic of lignin structure, showing that lignin had not been completely degraded, or hydroxyl group was oxidized by concentrated sulfuric acid. The band at 1460 cm–1 was attributed to bending vibration of carbon hydrogen bonds in CH3 or CH2 moieties. Absorption at about 1100 cm–1 was assigned to C–O–C asymmetry stretching. Chemical groups, such as –OH, C–H, and C–O, are characteristic of biopolyols, implying that biopolyols were successfully prepared by liquefying rice straw under the above conditions.

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Fig. 6. FTIR spectra of biopolyol, residue and pretreated raw.

CONCLUSIONS Biopolyols were successfully prepared by the liquefaction of rice straw in the presence of PEG400/ DEG blended solvents and sulfuric acid as catalyst. The influence factors on liquefaction reaction, including catalyst, liquid-solid ratios, the PEG400/DEG volume ratio, acid loading, liquefaction temperature, and liquefaction time were discussed. The optimal conditions are as follows: PEG400 : DEG volum ratio of 5 : 1, liquid–solid ratio 4 : 1, H2SO4 3%, time 2.5 h, and reaction temperature120°C. FTIR confirmed biopolyols were successfully obtained. The hydroxyl value and weightaverage molecular weight of the biopolyol produced based on the above optimal conditions were 260 mg KOH/g polyol and 420 g mol–1, respectively. Which is suitable for the production of rigid PU foams. This study has certain significance for the high added value use of rice straw and reducing the production cost of PU foams. ACKNOWLEDGMENTS This work was supported by the China Building Material Federation (2014-M3-4) and Xuzhou Information Institute (XKQ016). REFERENCES 1. Sarkar, D, Chen, Y.J., and Lopina, S., J. Appl. Polym. Sci., 2008, vol. 108, pp. 2345-2355.

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