Criação de mesoporos em zeólitas ZSM-5, mordenita e ferrierita e análise de seu efeito na reação de hidroisomerização do n-heptano

Abstract

Zeolites have received great academic and industrial attention due to their acidic and structural characteristics. The structure of the zeolite can be used to conduct a catalytic reaction selectively to the desired product, thus avoiding undesirable side reactions. However, this structure could create diffusive restrictions regarding the access to the active sites inside the micropores. In this work two methods of mesopores generation (thermal and basic) were studied in order to modify the accessibility of the catalytic sites of the zeolites. The reaction of hydroisomerization of n-heptane was selected to evaluate the modified zeolites. The thermal treatment was employed for ZSM-5, mordenite and ferrierite zeolites and a negligible increase in mesoporosity has been observed. However, this treatment produced a significant dealumination of the samples along with the reduction of the acid sites density and the formation of significant quantities of extraframework Al species (EFAL), which resulted in a partial blockage of the pre-existent micropores and of the mesopores generated. Furthermore, the scale-up of the thermal treatment (increasing the amount of treated sample from 2 g to 30 g) was not reproducible, producing less mesopores than that was observed in the small-scale preparation. The basic treatment with NaOH solution was also tested for ZSM-5 zeolite. It increased significantly the mesoporosity of the zeolite and formed less amount of extra-framework aluminum. Moreover, it was more reproducible when larger amounts of sample were treated. The performance of Pt/Al2O3 + zeolite catalysts in hydroisomerization/hydrocracking of n-heptane was influenced by both the strong acid sites density and the pore structure of the zeolite. As to the effect of the thermal and basic treatments on the performance of the ZSM-5-based catalysts, the results showed that the behavior of that the basically treated samples was similar to those of heat-treated was similar regarding the product distribution particularly with respect to light hydrocarbons and monobranched C7 isomers. However, the larger mesoporosity of sample basically treated slightly favored the formation polybranched isomers