Process optimization for preparation of carboxymethyl cellulose by homogeneous etherification of wheat straw

Abstract: In this study, carboxymethyl cellulose (CMC) was synthesized from wheat straw cellulose by a homogeneous reaction. The main process was as follows: wheat straw was cut into 1–2 cm-long pieces, rinsed with water 3–5 times to remove dust and other impurities, and washed first with deionized water for a further 3–5 times and then with boiling distilled water. The wheat straw was then preliminarily treated for 1 h, and dried in an oven. Crushing was then performed using a high-speed grinder with a 40-mesh screen to obtain the wheat straw powder. Sodium hydroxide and hydrogen peroxide were used to treat the wheat straw powder to obtain cellulose. An alkali/urea/thiourea system was used to dissolve ZnO. The extracted wheat straw cellulose was dissolved by freezing and melt circulation, following which the cellulose was stored for 4 h at −20°C, then thawed, and a clear cellulose solution was obtained by rapid stirring in an ice water bath. The optimal composition of the alkali/urea/thiourea aqueous solution was obtained through orthogonal experiments. The orthogonal design were four factors, five levels. The four factors with five levels were sodium hydroxide (5%, 6%, 7%, 8%, and 9%), thiourea (5%, 7%, 9%, 11%, and 13%), urea (5%, 7%, 9%, 11%, and 13%), and zinc oxide (0, 0.05%, 0.1%, 0.15%, and 0.2%). After dissolving wheat straw cellulose with the optimal dissolving system, CMC samples were prepared with sodium chloroacetate. The CMC samples were characterized by Fourier transform-infrared (FTIR) spectroscopy and the degree of substitution (DS). The results can be summarized as follows: first, the wheat straw powder was treated at a solid-liquid (10% NaOH solution) ratio of 1:20 g/mL, at a reaction temperature of 85 ℃, reflux time of 3.5 h, and with a 10% sodium hydroxide solution, followed by treatment with a 10% hydrogen peroxide solution. For the reaction conditions of a solid-liquid (the wheat straw powder treated with 10% NaOH and 3% H2O2) ratio of 1:30 g/mL, at a reaction temperature of 85 ℃, and a reflux time of 3 h, the highest proportion of cellulose that can be extracted from the wheat straw was 84.61 %. The removal rate of hemicellulose was 84.44%, while that of lignin was 91.14%. In the orthogonal experiment, we assessed the influence of the R value of each factor on the experimental indicators, with a greater R value indicating a greater impact. The R values for the dissolution system (NaOH, thiourea, urea, ZnO) were 1.190 7, 0.217 4, 0.170 8, and 0.219 9, respectively; thus, the most influential factor was NaOH, followed by ZnO, thiourea, and urea with minimal impact. Variance analysis also showed that NaOH was the most influential factor, followed by ZnO. After comparing NaOH, thiourea, urea, and ZnO at the level of solubility, a comprehensive consideration of the cost and environmental factors can be developed. The optimum values of the mass fraction for the solution components were 7%, 11%, 5%, and 0.05% for NaOH, thiourea, urea and ZnO, respectively, for a total solubility of 2.8801 g. The FTIR spectra of the cellulose straw and cellulose showed characteristic absorption peaks of pure cellulose. The etherification reaction of wheat straw cellulose results in the formation of CMC. The characteristic absorption peaks of wheat straw CMC and pure CMC were very similar. The DS of CMC was dependent on the cellulose dosage, temperature, and the molar ratio of sodium chloroacetate to cellulose AGU. Increased cellulose dosage, molar ratios of sodium chloroacetate to cellulose AGU, and temperature initially increased the DS and then caused it to decrease. For a temperature of 55 ℃, the amount of cellulose is 2.8 g, the molar ratio of sodium chloroacetate and cellulose AGU is 10.5:1, and the DS is the highest at 0.45.