Overcoming phase separation in dual templating: A homogeneous hierarchical ZSM-5 zeolite with flower-like morphology, synthesis and in-depth acidity study

Abstract

Dual templating approach, using hexadecyltrimethylammonium bromide (CTAB), was employed in an attempt to synthesize hierarchical ZSM-5 zeolite. Amount of mesoporogen and the duration of aging of the precursor were varied. Majority of the synthesis routes resulted in phase separation, yielding separate ZSM-5 and amorphous mesoporous material. The relative amounts of the two phases were dependent on the CTAB amount ratio and also significantly on the duration of precursor aging before CTAB addition. One particular combination of the two factors led to the formation of a homogeneous hierarchical form of ZSM-5 with leafy morphology, consisting of intergrown thin crystalline sheets which formed flower-like structures. The hierarchical ZSM-5 possessed significant microporous (≈95 m2/g) and highly developed mesoporous surface (≈470 m2/g), with a relatively broad distribution of mesopore sizes (<20 nm). The acidity of all samples was studied in detail. Isothermal microcalorimetry/volumetry of ammonia adsorption provided quantitative data on the number and distribution of strength of acidic sites. In situ FTIR of pyridine and collidine adsorption was used to quantify Brønsted and Lewis acid sites, and to provide information on their location - in the micropores or mesopores/external surface. The hierarchical ZSM-5 possessed both Lewis and Brønsted acidity, with Brønsted sites located mainly in the micropores. All samples were fully characterized using XRD, low temperature nitrogen adsorption, FESEM and EDS. The synthetic route used for obtaining the ZSM-5 zeolite with flower-like morphology is a simple strategy for preparing hierarchical ZSM-5 forms targeting enhanced diffusivity and accessibility of catalytically active sites.