Lignocellulosic materials in hydrolysis and spent liquors

Abstract

Biomass pretreatment is widely used for softening biomass prior to its disintegration for value added product production. There are numerous methods for biomass pre-treatment, among which alkaline pulping and acid hydrolysis are the most widely applied. Hydrothermal treatment or autohydrolysis has a similar concept of action to acid hydrolysis, while autohydrolysis is the chemical-free and environmentally friendly technology. Hydrothermal and alkaline pulping pretreatments lead to lignin and polysaccharide dissolutions in hydrolysate and spent liquor (SL), respectively. Lignin and carbohydrates presented in hydrolysates and SL can be used for manufacturing value-added products. Lignin can be employed in carbon fiber, phenol formaldehyde, and hydrogen productions, for example, and hemicelluloses could be used for ethanol or xylitol production. However, the direct conversion of lignocellulosic materials present in hydrolysates and SL to value-added products is expensive due to their low concentrations. Lignocelluloses can be isolated from these liquors via acidification, solvent precipitation and membrane filtration. It is well known that lignocelluloses have different properties. Pulping and pretreatment processes have also great impacts on the properties of the extracted lignocellulose in liquors. Despite their effectiveness in isolating lignocelluloses, the impact of lignocelluloses properties on the efficiency of extraction processes is unknown. In this dissertation, the effect of autohydrolysis parameters on the properties, structure and composition of extracted lignocellulosic material was investigated. Also, the effect of lignocellulose's properties presented in hydrolysates and SL on the efficiency of acidification and solvent extraction was examined. The efficiency of lignocelluloses extraction from softwood chips via flow through autohydrolysis pretreatment was investigated in this PhD study. The highest temperatures applied in the autohydrolysis process were found to yield maximum removal of lignin from wood; whereas, prolonged hydrothermal treatment increased the removal of hemicelluloses from wood. In addition, it was discovered that a low flow velocity led to higher lignocelluloses removal. However, at high liquid flow rates hemicelluloses with larger molecular weight (Mw) were extracted. Gel permeation chromatography (GPC) analysis revealed the presence of lignin-carbohydrate complexes (LCC) in the hydrolysates. The GPC analysis showed that the hydrolysate generated in autohydrolysis treatment with a high liquid to solid (L/S) ratio contained a significantly lower amount of lignin presented in the LCC form. Moreover, 2D HSQC NMR spectroscopy revealed the existence of LCC linkages only in lignocellulosic materials obtained via lyophilisation of hydrolysate produced at the lowest autohydrolysis severity. The material generated as the result of mixing ethanol with hydrolysates produced at low autohydrolysis intensity showed the highest sugar content and negligible lignin content. Acidification of hydrolysates generated at high severity conditions led to extraction of almost pure lignin. The isolated material produced from hydrolysates due to ethanol or acid addition showed approximately twice as much heat capacity values as that of dried hydrolysates. The 1H-NMR analysis revealed that the extracted materials via acidification contained more methoxyl groups and a lesser degree of cross-linking than those present in hydrolysate. The GPC analysis also suggested the presence of LCC in the hydrolysates before and after treatment processes. In addition, the current research studied the properties, structures, and composition of lignocelluloses in spent liquors before and after isolation. Furthermore, the effect of organic solvents (i.e., ethanol, acetone, isopropyl alcohol) on lignocelluloses removal and the properties of the isolated lignocelluloses were determined. In examining the impact of the solvent type, it was discovered that hemicelluloses isolation had the highest isolation at the lowest solvent concentrations. While acetone and isopropyl were found to be efficient for lignosulfonate extraction, the highest hemicelluloses removal was achieved with ethanol treatment. The molecular weight and charge density of isolated lignocelluloses was higher when ethanol was used in comparison with acetone and isopropyl. The examination of the heating values, ignition temperatures, and heat capacity values of the precipitates revealed the possibility of using these materials as fuel or additives in briquette production. Additionally, a direct correlation between the inorganic content of the extracted lignocelluloses and their Tg values was discovered. The results achieved in this dissertation can be used as guidelines for developing biorefining processes for generating lignocelluloses with different properties from hydrolysate or spent liquors of different pulping processes. In addition, information available in literature on the structure and properties of lignin-carbohydrate complexes (LCC) as well as their production, analysis and application were discussed and critically evaluated.