Hydrolysis of Wood Hemicelluloses at Ultra-Low Sulfuric Acid Concentrations

Abstract

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.