Tailoring the properties of hierarchical TS-1 zeolite synthesized from silanized protozeolitic units

Abstract

Hierarchical TS-1 zeolites, characterized by having a secondary porosity within the supermicro/mesopore region (1.5-6 nm), have been synthesized following a procedure based on the silanization of protozeolitic units, which are previously generated by means of a precrystallization step. The silanization agent, phenylaminopropyltrimethoxysilane (PHAPTMS) acts as crystal growth inhibitor, hindering partially the protozeolitic units growth and aggregation during the crystallization treatment. Both the duration of the precrystallization step and the proportion of the organosilane compound added to the synthesis gel have a significant influence on the physicochemical and textural properties of the resultant materials. Thus, the best precrystallization time, leading to the most enhanced textural properties, is comprised between 22-24 hours. Using this time an appropriate balance between the number of protozeolitic units formed and their size is reached. On the other hand, by controlling the organosilane compound proportion added to synthesis gel, the contribution of the secondary porosity can be tailored. Likewise, amounts of organosilane larger than 5 mol% provide to a secondary porous system more uniform in size. The catalytic activity of these materials was evaluated in 1-octene and cyclohexene epoxidation reactions, using tert-butylhydroperoxide (TBHP) as oxidant. The olefin conversion and TOF values reached by hierarchical TS-1 zeolites are remarkably superior to that obtained with the conventional microporous TS-1 zeolite, being higher when the modification degree of the textural properties is more pronounced. These results can be ascribed to the higher accessibility of both TBHP and olefin to the titanium active sites located in the secondary porous system. Likewise, these zeolites exhibit a high oxidant efficiency and total selectivity to epoxide, parameters which are not affected by the presence of the secondary porosity.