Preparation And Acid Catalytic Performance Of Hierarchical Zeolite Beta By Urea Solution Treatment

Zeolites, as highly ordered crystalline aluminosilicates with unique compositions, have been widely used in petrochemical and fine chemical industries owing to the properties such as ion exchanging, adsorption, and the strong acidity. Zeolite beta is a three-dimensional intersecting pore system with highly defects and high Si/Al ratio. The relatively small size of the micropores in zeolite imposes diffusion limitations on the reaction and therefore impedes its catalytic performance. Desilication, as one of the post synthesis treatment methods, is widely investigated in the preparation of hierarchical zeolites owing to its simplicity and easy operation.In this dissertation, zeolites beta (Si/Al ratio=12.5) in two morphologies (agglomeration and small crystal) were employed as the parent samples. The Si/Al ratios were adjusted by citric acid solution treatment, and the predealuminated zeolites were then subjected to urea solution treatment to achieve hierarchical zeolites beta. It was found that, compared with small crystal zeolite beta, the aggregated zeolite beta was prone to dealumination under the same citric acid treatment condition. The commercial zeolite beta in aggregation state was employed to investigate the effect of treatment conditions (including the concentration and amount of urea solution, temperature, and time) and the Si/Al ratios of the starting material on the textural structure, compositions and acid property of the hierarchical zeolite beta. The results displayed that urea solution treatment preferred to remove framework silicon and the crystalline structure was well maintained. The mesopore distribution was in the range of 3-30 nm for the resultant hierarchical zeolite beta, and the acid amount was slightly declined. Intracrystal mesopores can be successfully introduced by desilication with urea solution for both the small crystal and the agglomeration zeolites beta. So urea solution treatment is a very gentle and efficient post synthesis method to develop mosoporosity in zeolite beta. The advantage of urea solution treatment is that hierarchical zeolite Beta can be directly used in protonic form in acid-catalysed reactions, without subsequently tedious ammonium ion exchange.The acid catalytic performances of both the microporous and the hierarchical zeolites beta were evaluated in terms of nopol synthesis. It was found that the hierarchical zeolite beta displayed significantly improved conversion of β-pinene but slightly decreased selectivity to nopol. The introduction of some zinc ions into hierarchical zeolite beta by ion exchanging led to slight change in the crystalline and textural structure but evident increase in the Lewis acid amount, which significantly improved the selectivity to nopol. The reaction conditions were optimized using zinc ion exchanged hierarchical zeolite beta, and the catalytic performance with conversion of β-pinene=43% and selectivity to nopol=71% was achieved under the optical conditions, paraformaldehyde:β-pinene=3,1.40 mL toluene,10 wt% catalyst,70℃,5 h.The acid catalytic performances of both the microporous and the hierarchical zeolites beta were also evaluated by the acylation of anisole with acetic anhydride, which is a typical strong Br(?)nsted acid catalyzed reaction. The results showed that hierarchical zeolite beta displayed increased initial anisole conversion but little improvement in the catalytic stability. This indicated that the deactivation of the catalyst didn’t arise from the textural structure and it may be related to the strong adsorption of larger product molecules on the surface of the catalyst.