Studies in the field of hydrolysis of plant polysaccharides are ordinary classified according to protic reactions with diluted or concentrated acids. Such classification is based on the significant difference in the mechanisms of the reactions. The hydrolysis of polysaccharides of plant materials with the diluted acids is indicated by the concentrations of the mineral acids 0.5–10.0 % or happens by acid-free autohydrolysis, without any use of acids. Each of these reactions has considerably different kinetic and temperature-time parameters. They have both advantages and disadvantages. In particular, the hydrolysis using dilute acids is specified by a significant consumption of reagents and the presence of a large amount of carbohydrate degradation products in the hydrolysate. Autohydrolysis is characterized by a relatively low monosaccharide yield, high energy consumption for the process and the formation of many by-products. To date, studies regarding hydrolysis of polysaccharides of plant materials with acids in a concentration range of less than 0.5 % are absent. The reason for the lack of interest in research in this area, in our opinion, was the statement that acid in the process of hydrolysis is spent on the neutralization of ash components of plant materials at a flow rate of 5 to 20 g/kg of dry raw materials. Accordingly, when hydrolysis is carried out with ultra-low concentrations of acid, it is possible to completely neutralize it and switch the hydrolysis process from acid to acid-free autohydrolysis. The purpose of the work was to establish the efficiency of the hydrolysis process at ultra-low acid consumption. A study of the process of hydrolysis of hemicelluloses of birch wood at ultra-low concentrations of sulfuric acid was carried out. The possibility of almost complete hydrolysis of hemicelluloses with sulfuric acid with concentration of 0.10–0.25 % is shown. The process of hydrolysis of hemicelluloses with ultra-low acid concentrations is well described by the first order reaction. The general kinetic constants are calculated according to the experimental data. They show that the process occupies an intermediate position between acid-free autohydrolysis and traditional hydrolysis of hemicelluloses with sulfuric acid with a concentration of more than 0.5 %. The developed technique is advantageously different from the known methods of hydrolysis of hemicelluloses by low consumption of sulfuric acid (more than 5 times) and energy resources. Hemicellulose hydrolysates obtained by ultra-low acid concentration regimes have high benign properties and can be used in xylitol production.

Abstract. Microcrystalline cellulose (MCC) is a common product used in pharmaceutical, food and other industries. MCC is obtained by liquid-phase hydrolysis of cotton or wood bleached pulp with diluted 0.5–10.0 % mineral acids at a temperature of 100–140 °C. This process requires significant consumption of acid, water, and heat energy. Production of MCC in general is very expensive, which determines its high cost and the need to find alternative methods of cellulose hydrolysis. It is proposed to hydrolyze cellulose with concentrated hydrochloric acid produced by absorption of hydrogen chloride. We studied the processes of hydrogen chloride adsorption by bleached wood pulp with 8–18 % humidity. It is shown that adsorption of hydrogen chloride is determined by pulp humidity and is 3–5 % of the dry pulp mass. The sorption of hydrogen chloride leads to the formation of hydrochloric acid with a concentration of 25–40 % in the raw material moisture, significant heating of the mass and rapid hydrolysis of the amorphous cellulose fraction. It has been found that the use of pure hydrogen chloride for saturation causes strong darkening and humification of pulp. We recommend the use of hydrogen chloride gas-air mixtures to saturate the pulp, which will significantly reduce the sorption temperature and eliminate the strong darkening of the pulp during hydrolysis. The adsorption of hydrogen chloride by cellulose proceeds at an extremely high rate and is accompanied by the formation of a clearly visible sorption front at a temperature of 45–60 °C. Hydrolysis occurs for 15–30 min at 40–60 °C until the amorphous cellulose fraction is completely decomposed. A very small amount of monosaccharides is formed (4 % of dry pulp). The yield of MCC is high, more than 95 %. These circumstances are probably related to the recrystallization of a part of the amorphous fragments of cellulose macromolecules, which is a characteristic of hydrolysis with concentrated acids. The product obtained by cellulose hydrolysis is identical to MCC according to the data of IR spectroscopy, X-ray diffraction and viscometry. The article shows the high efficiency of cellulose hydrolysis with hydrogen chloride gas-air mixtures compared to traditional methods of MCC production.

One of the main consumers of microcrystalline cellulose (MCC) is the pharmaceutical industry, where MCC is used as a binder and filler in direct compression of tablets. MCC is produced by acidic hydrolysis of cellulose, which usually results in a decrease in whiteness. This is due to the destruction of sugars formed during hydrolysis and the subsequent formation of colored products. The composition and properties of these products depend on the method of hydrolysis, acid concentration, temperature, and process duration. One of the most promising methods for producing MCC is gas-phase hydrolysis of cellulose with hydrogen chloride gas-air mixtures. The method has a high rate of hydrolysis, low reagent and energy consumption. The requirements of the pharmaceutical industry determine the need to produce MCC with high whiteness. The research purpose is to select bleaching modes for MCC using sodium hypochlorite and hydrogen peroxide as bleaching agents. MCC produced by gas-phase hydrolysis of bleached wood pulp was used during the study. The whiteness and intensity of the yellow tint of MCC in the bleaching process were determined by digital colorimetry on a flatbed scanner. The paper shows that sodium hypochlorite and hydrogen peroxide allow achieving the whiteness not less than 90 % and the intensity of the yellow tint not more than 3 standard units. High-quality bleaching can be carried out even for MCC samples with an initial whiteness of about 40 %. The most effective bleaching agent is sodium hypochlorite when the pH of the bleaching solution is 2–3. Hydrogen peroxide also provides high whiteness of MCC at pH of 10–11. However, the consumption of active oxygen (AO) for bleaching is more than three times higher in comparison with the consumption of active chlorine (ACh). It was found that the dyes of MCC produced by gas-phase hydrolysis consist of two chromophore groups that decolorize at different rates. The easily oxidized group of components makes up about 90 % of the total amount of dyes, and the resistant to oxidation components make up about 10 % and determine the intensity of the yellow tint of MCC. The modes of bleaching MCC with sodium hypochlorite and hydrogen peroxide to product samples with whiteness comparable to that of imported samples were determined. For citation: Sizov A.I., Pimenov S.D., Stroiteleva A.D., Stroiteleva K.D. Bleaching of Microcrystalline Cellulose Produced by Gas-Phase Hydrolysis. Lesnoy Zhurnal [Russian Forestry Journal], 2021, no. 6, pp. 173–183. DOI: 10.37482/0536-1036-2021-6-173-183