Hierarchical ZSM-5 zeolite with uniform mesopores and improved catalytic properties

Abstract

Hierarchical ZSM-5 with uniform mesoporosity was synthesized by the sequential coupling of two strategies: generation of secondary porosity by the crystallization of silanized protozeolitic units and a subsequent treatment with a basic surfactant-containing solution. The ZSM-5 zeolite obtained exhibited high crystallinity and contained two levels of uniform porosities within the micro- and mesopore ranges, respectively. The uniform mesoporosity is the result of the reorganization of irregular mesopores, created initially from the silanized protozeolitic units, via the local rearrangement of zeolitic fragments, and promoted by contact with a cationic surfactant under mild basic conditions. Interestingly, this second treatment was less effective when it is applied to a non-hierarchical ZSM-5 sample, showing that the presence of initial secondary porosity is essential for allowing the surfactant/ammonia solution to modify the zeolite and form uniform mesopores. Characterization of the zeolite samples using different techniques showed that the crystallinity, Al coordination and acidic features of the zeolite do not change significantly after the mesopore narrowing treatment, even though significant variations in the textural properties are observed as expected. The effects of the occurrence of regular mesoporosity on the catalytic properties of the hierarchical ZSM-5 zeolite were proven using the acylation of 2-methoxynaphthalene as a test reaction. The material with uniform mesopores showed both the highest activity and selectivity towards 6-acetyl-2-methoxynaphthalene, which was interpreted as the result of the presence of a more regular and less rough mesopore surface, which in turn facilitated the interaction between the reactant molecules and the active sites located and distributed over the mesopores.