Pore Size Modification, Characterization And Acid Catalytic Performance Of Zeolite Beta

Terpenes, as a sustainable and versatile feedstock, are found abundantly in essential oils. And a-pinene is one of the most widespread bicyclic monoterpenes. A large number of fine chemicals and raw materials could be obtained by deep processing of a-pinene. In this work, the commercial zeolite beta and immobilized solid acid were chosen as the catalysts for the isomerization of a-pinene, the relation between the acidity and pore structures of catalysts and the reaction performance of a-pinene were investigated. And the optimal molar Si/Al ratio of parent materials and the conditions of alkaline treatment for achieving the hierarchically structured zeolite beta were studied systematically.The commercial zeolite beta in small crystal and of low molar Si/Al ratio (Si/Al=12) was subjected to alkaline treatment with NaOH solution. Firstly, the effect of alkaline concentration on the pore structures, compositions, acidity and catalytic performace was investigated. The results showed that the hierarchically structured zeolite beta can be effectively obtained under the conditions even much harshly compared with that reported in the literature, while with microporosity and crystallinity better preserved. The amount of the acid sites somewhat decreased, but most of the medium strong acid sites was preserved. Secondly, different post-synthesis treatment methods (citric acid treatment, alkaline treatment and the sequential citric acid-alkaline treatment) for the creation of hierarchically structured zeolite beta were studied. The results manifested that the mesoporosity was effectively developed with the mesopore surface area increase to298cm2/g while91%of the micropore volume was preserved for the sample obtained by the sequential acid-alkaline treatment, indicating that the citric acid treatment prior to the alkaline treatment is an efficient method to tune the hierarchical structure of alkaline treated zeolite beta. The catalytic performances of all the samples after modification were highly improved. And the sample possess the optimal hierarchical structure gave the best catalytic performance:100%convension of a-pinene and about95%selectivity towards the main products such as camphene, limonene, terpinolene, a-and y-terpinene, were obtained at70℃and30min reaction time.The commercial zeolite beta in small crystal and of low molar Si/Al ratio (Si/Al=14.5) was selected as the raw materials. Zeolite beta with different molar Si/Al ratio was obtained by oxalic acid leaching of the commercial zeolite beta. And then the effect of the molar Si/Al ratio on the formation of mesopore, the acidity and catalytic performance of the alkaline treated samples was investigated systematically. The results indicated that when the molar Si/Al ratio of zeolite beta is lower than30, the hierarchical structure of zeolite beta will be achieved without significantly change in the crystallinity and acidity; when the molar Si/Al ratio of zeolite beta is30, the most porous zeolite beta will be obtained with the mesopore surface area up to488m2/g, but the crystallinity of zeolite beta decreased obviously; when the molar Si/Al ratio of zeolite beta exceeds30, though substantial mesopore surface area of373m2/g can still be obtained, the crystalline of BEA totally disappeared. Therefore, the molar Si/Al ratio of zeolite beta in small crystal should be less than30in order to obtain the hierarchical structure by NaOH treatment, that is, enhancing the mesopore surface area without severely penalization of the crystallinity and micropore volume of zeolite beta. The catalytic performace of alkaline treated samples were improved evidently, about90%conversion of a-pinene and about97%selectivity towards the main products can be achieved at70℃and30min reaction time for the sample with the optimal hierarchical system.The traditional Lewis acid such as AICI3, FeCl3, SnC4, SbCl5and BC33etc were transformed into environmentally friendly catalysts by immobilizing them on γ-Al2O3or SiO2. And the immobilized Lewis acids, for the first time, were applied in the isomerization of a-pinene. The results indicated that the immobilized Lewis acid exhibit high catalytic performance and selectivity towards main products (such as camphene and limonene) under much milder conditions.100%conversion of a-pinene, and over90%selectivity towards camphene and limonene could be obtained at30-50℃.