The Study Of Pore Structure And Acidity Of Zeolite On The Catalytic Hydrolysis Of Cellulose

With the depletion of fossil resources including coal, petroleum and natural gas,and the environmental problems related to using fossil resources, the development ofnew and clean resources to replacement of fossil resources has became challenge forthe world. Biomass is a promising alternative source for fossil resources due to itslarge output per year. Cellulose is the main component of lignocelluloses. Theproduct of hydrolysis of cellulose is glucose, which is an important chemicalmaterial. Glucose can be further converted to biofuel and high value-added products.So, hydrolysis of cellulose to glucose is the critical step for the replacement of fossilresources by lignocelluloses biomass.The hydrolysis of cellulose is a process catalyzed by acid, so development ofproper acid as catalyst is the key. Zeolite is a kind of microporous materials which iswidely applied in petrochemical engineering. Zeolites have abstracted more andmore attention in hydrolysis cellulose due to its strong acidity, good hydrothermalstability, nontoxicity, and easy separation from solution.The molecule of cellulose is difficult to diffuse into the micropore of zeolite dueto the large molecule size. Therefore, in this thesis, we first selected HUSY as modelcatalyst. HUSY zeolite was treated by oxalic acid to introduce a large number ofmesopore to improve mass transfer. Following, the obtained hierarchical HUSYzeolite was grafted with sulfonate acid groups to improve its acidity. The porestructure, acidity and elemental composition were characterized. From the reactionresults for hydrolysis of cellulose, it can be seen that sulfonated hierarchical HUSYzeolite showed higher activity than HUSY and hierarchical HUSY, implying thatpore structure and acidity of zeolite played main roles in hydrolysis cellulose.When hierarchical zeolites were used as catalyst in the hydrolysis of cellulose,the reaction process can be divided into two steps:(1) cellulose was hydrolyzed intooligosaccharide at acid sites located on the outer surface, mesopores and macroporesof zeolite;(2) the generated oligosaccharides were diffused into micropores and were further hydrolyzed to glucose. Mesopore and macropore are beneficial for thehydrolyzed cellulose into oligosaccharide. However, the role of micropore structureand acidity of zeolite in the following hydrolysis of oligosaccharides to glucose isstill not clear. To understand the role of micropore structure and acidity of zeolite inhydrolysis of oligosaccharides is helpful to design an effective catalyst for thehydrolysis of biomass.The influence of micropore structure and acidity of zeolite for hydrolysis ofcellobiose was carried out. The results showed that strong Br nsted acid sites is theactive site while weak acid sites of Br nsted acidity do not showed any activity inhydrolysis of cellobiose. Lewis acid sites catalyzed the isomerization of glucose, soit decreased the selectivity of glucose. When we investigated on the other possiblefactors in hydrolysis of cellobiose catalyzed by zeolite, it was found that silicon oraluminate species dissolved by acidic products could inhibit the catalytic activity ofzeolite. Acid sites on the outer surface of zeolite had little influence on thehydrolysis reaction, and the hydrolysis reaction mainly occurs in the active sites inmicropore of zeolite. Micropores with middle size were favorable to hydrolysis ofcellobiose. The activation energy was only71.10kJ·mol-1when H-BEA, the bestcatalyst, was used. It was much lower than homogeneous catalysts such as mineralacids, organic acids etc.