Microporous–mesoporous ZSM-12 zeolites: Synthesis by using a soft template and textural, acid and catalytic properties

Abstract

The influence of the amount of amphiphilic organosilane TPOAC {[3-(trimethoxy-silyl)propyl] octadecyldimethylammonium chloride} added in the synthesis and of the crystallization time in the textural, acid and catalytic properties of microporous–mesoporous ZSM-12 (MMZ12) zeolites was investigated. XRD/Rietveld Refinement, N2 sorption measurements, SEM, EDX, and NH3-TPD were used to sample characterization and the performance as acid catalysts verified using the transformation of cyclohexane as a model reaction. WAXRD data showed that the usage of short synthesis times and increasing amounts of TPOAC in the synthesis led to ZSM-12 zeolites with poor crystallinity due to the organosilane steric constraints to MTW nuclei formation and crystal growth. On the other hand, SAXRD and N2 sorption data evidenced that TPOAC firstly generates a pseudo-ordered mesoporous phase, which is transformed into well-crystalline hierarchical MMZ12 zeolites tailoring the synthesis conditions. Were obtained samples with a relatively broad mesopore size distribution having mean diameters close to 4.0nm and slightly higher external surface areas and lower amounts of strong acid sites than the only microporous ZSM-12 one. The MMZ12 zeolites showed similar cyclohexane conversion and lower deactivation rate due to coke formation than a conventional ZSM-12 that were attributed to a balance of two opposite effects, a lower strong acidity and the presence of HRTEM evidenced interconnected micropores and mesopores that reduce the 3D internal diffusion limitations. The yield and selectivity on the conventional ZSM-12 were coherent with the transformation of cyclohexane on 12-ring H-zeolites, nevertheless on the microporous–mesoporous ones the yield to C6 saturated hydrocarbons increased with the increase of the mesoporous volume, then disfavoring the yield to saturated and unsaturated light hydrocarbons.