Effects of Zeolite Growth Modifiers on Different Stages of ZSM-5 Crystallization

Effects of Zeolite Growth Modifiers on Different Stages of ZSM-5 Crystallization

Wei Qin and Jeffrey D. Rimer

Chemical and Biomolecular Engineering, University of Houston, 4726 Calhoun Rd., Houston, Texas 77204 USA

Zeolite ZSM-5 is a common heterogeneous catalyst in refinery and petrochemical processes, owing to its excellent shape-selectivity, strong Brønsted acidity, and high thermal/hydrothermal stability. It has an MFI structure with 3-dimensional pores consisting of interconnecting straight and sinusoidal channels. Molecular diffusion is much faster in the straight channels oriented along the [010] direction. To this end, it is desirable to selectively control the [010] thickness of ZSM-5 crystals.

Here, we will present a novel approach to tailor ZSM-5 morphology that incorporates zeolite growth modifiers (ZGMs). The use of ZGMs has proven to be an efficient method to tailor the morphology of silicalite-1, which is the siliceous isostructural analogue of ZSM-5.¹ For our study of ZSM-5 synthesis in the presence of ZGMs, we assessed a library of modifiers previously screened for slicalite-1 to determine the influence of aluminum incorporation within the MFI framework. It was observed that subtle changes in the synthesis conditions can alter ZGM binding specificity to different crystallographic faces of ZSM-5. Using atomic force microscopy (AFM), we show that chemical force microscopy can be used to efficiently probe modifier-crystal interactions in order to elucidate ZGM molecular recognition for different zeolite surfaces.²

We also investigated the effects of ZGMs on the initial stages of amorphous precursor self-assembly and evolution (pre-nucleation) as well as crystal growth using a combination of two techniques: small angle X-ray scattering (SAXS) ³ and dynamic light scattering (DLS). We observed that polycations have a unique effect of promoting precursor aggregation during the induction period.

Through the judicious selection of ZGMs under different synthesis conditions, we were able to synthesize ZSM-5 crystals with various sizes and morphologies. We will present details of these syntheses as well as catalytic tests of ZSM-5 catalysts with a model reaction to gain a better understanding of the relationship between ZSM-5 morphology and catalyst performance. Collectively, this work provides quantitative guidelines for designing synthesis protocols to tailor ZSM-5 crystallization and improve catalyst properties for a wide range of applications.

(1) Lupulescu, A. I.; Rimer, J. D. Angew. Chemie - Int. Ed. 2012, 51 (14), 3345–3349.

(2) Lupulescu, A. I.; Rimer, J. D. Science. 2014, 729.

(3)  Fedekyo, J.; Rimer, J. D.; Lobo, R. et al. J. Physical Chemistry C. 2004, 108.