Degradation And Dissolution Of Carbohydrate During Bamboo Prehydrolysis

Pretreatment is a primary step for dissolving pulp and bio-ethanol production, it can degrade and dissolve hemicellulose/or open fiber structure facilitating the subsequent process. The targets of the present study were to investigate the cellulose and hemicellulose dissolution and degradation. Based on the combination analysis of the pretreated substrate and hydrolysis liquor, cellulose and hemicellulose degradation behavior and mechanism were elucidated. The mass transfer effects were evaluated upon the hemicellulose degradation respect to the interior and exterior of the bamboo chips. The factors affecting hemicellulose removal was proposed according to the analysis of the chemical and morphology analysis. Pseudo lignin formation was investigated by using holocellulose as model for hydrothermal pretreatment; whereas, pseudo lignin was isolated from pretreated holocellulose substrate and the characteristics was studied. The present study could provide promise informaiton for the subsequent process selection such as cooking, bleaching for dissolving pulp or enzyme hydrolysis for ethanol production. The study of the hydrolysis liquor was important for the utilization of hemicellulose and lignin.The pretreatment conditions effects on sugar component and the characteristics of the dissolved polysaccharides and lignin were studied. The results showed that elevating temperature could accelerate the hemicellulose degradation and dissolution, but also promote the decomposition of the sugar and formation of the aldehydes. Pretreatment at170℃and90min could provide maximum xylan extraction (21g/L). The soluble hemicellulose and insoluble hemicellulose fractions both involved in the prehydrolysis liquor. The turbidity of the prehydrolysis liquor was controlled by dissolved lignin and suspended long chain hemicelluloses; hence, turbidity increased first and then decreased with the decreasing of lignin and hemicellulose content. The dissolved lignin with low molecular weight had higher reactivity that could be condensed readily.The rule of hemicelluloses degradation and dissolution and the mass transfer effects on hemicellulose removal were investigated. The results showed that hemicellulose degradation occurred continually but with a greater rate in the initial stage of the pretreatment. Elevating temperature could improve the hemicellulose removal rate substantially but without an obvious removal amount. Lingering dissolution at the later stage might attribute to the facts that some soluble fractions were still entrapped in the substrate.5～15%hemicellulose fractions remained in the pretreated substrate at the later stage were composed of soluble species. With the enhancing mass transfer resistance, the degradation solubles were intercepted inside of the chips and indicated as low MW of hemicellulose for interior. Hemicellulose removal was a combination result of the degradation and diffusion. Hemicellulose removal process mainly involved the hydrolysis of the hemicellulose to soluble species and diffusion and/or transportation of degradation hemicellulose oligomers.A systematic pretreatment of bamboo chips had been conducted with an aim to trace the cellulose degradation; while the changes of crystalline structure of cellulose were also investigated. The results showed that cellulose chain cleavage basically occurred when the temperature exceeded150℃. A slightly higher DP (degree of polymerization) than starting material had been observed at low temperature pretreatment. Treatment at higher temperature (≥170℃) caused severe cleavage of cellulose and therefore gave rise to low DP with more soluble species. In oder to preserve cellulose with high yield and high DP, the pretreatment conditions might be controlled at temperature lower than170℃and duration less than90min. DP of cellulose declined drastically without additional hemicelluloses dissolution when hemicelluloses removal reached to the limit level. Cellulose degradation under pretreatment generally followed the zero reaction kinetics with the activity energy of121.0kJ/mol. Besides, the increase of cellulose crystalline index and the conversion of Iα to Iβ had also observed at the hydrothermal pretreatment.The surface morphology and chemical changes of the bamboo substrates during pretreatment were characterized and thus to explore the factors which could control the hemicellulose removal. The electron beam irradiation effects on hemicellulose removal during bamboo pretreatment were also studied. No visible changes could be found at the initial stage of the pretreatment, the droplet appeared when pretreatment time exceeded60min. The hydrophobic lignin coatings could impede the water attack and diffusion of dissolved hemicellulose fraction. The low dose electron beam irradiation preferentially degraded cellulose in amorphous region; whereas the cellulose in crystalline region remained stable. Moreover, the cleavage of cellulose could open the biomass structure and thus promote the hemicellulose removal.The method was established for lignin quantitative determination of the pretreated bamboo substrate; pseudo lignin formation was investigated by using holocellulose pretreatment and the structure characters of the pseudo lignin were determined. The results showed that benzene alcohol extraction (BAE) could remove some of the lignin fragments with low molecular weight from the pretreated substrate and therefore caused a decrease in the lignin content of the substrate. In order to exclude the BAE effects, the none BAE procedure for lignin determination was proposed. During the holocellulose pretreatment, droplets were generally appeared onto the surface of holocellulose; besides, the lignin content of the pretreated substrate increased with pretreatment time. Although pseudo lignin has equivalent total oxygen element and O/C with reference, it is rich in C1and less in C2, suggesting that more cyclic structure and hydroxyl groups presented in the pseudo lignin. More alicyclic structures were involved in the pseudo lignin which were formed initially; whereas, the subsequent pretreatment convert the alicyclic structure to aromatic-leading structure. The pretreatment also resulted in higher Mw and Mn of the pseudo lignin, especially for Mn.