(41a) Tailoring Zeolite ZSM-5 Crystal Morphology and Spatial Distribution of Acid Sites

Authors: 
Qin, W., University of Houston
Patton, M., University of Houston
Rimer, J., University of Houston
The micropores of large zeolite crystals often impose severe mass transport limitations that adversely affect catalyst activity.1 One of the challenges in zeolite design is to predictively control crystal size and habit to improve molecular diffusion. Here, we present a novel approach to tailor ZSM-5 crystallization using zeolite growth modifiers (ZGMs), which are molecules that bind to specific crystal surfaces and alter the anisotropic growth rates as a means of tailoring crystal habit.2,3 We have observed that subtle changes in synthesis conditions (e.g., pH) can alter ZGM binding specificity and produce crystals with different morphology. This is due to the fact that modifier speciation can be controlled by adjusting alkalinity, which could prove to be a facile and highly effective method of tailoring zeolite growth.4

The unique properties of ZSM-5 catalysts, such as hydrothermal stability and shape-selectivity, are ideal for many reactions. Catalyst performance is dependent upon its chemical composition. A phenomenon known as aluminum zoning in ZSM-5 crystals has been reported in many studies; however, its origin remains elusive.5 To address this topic, we have performed parametric investigations to assess the role of synthesis conditions on Al siting. We show that ZSM-5 crystals can be prepared with either Si- or Al-rich exteriors, which impacts their hydrothermal stability and product selectivity in catalytic reactions.6

The ability to design ZSM-5 catalysts with tunable morphology and Al distribution opens new avenues for tailoring catalyst performance, and understanding property-performance relationships. ZSM-5 properties can be selectively optimized for a wide range of applications. To this end, this study provides a general platform for zeolite design and optimization that could potentially be applied to other microporous framework types.

(1) Pérez-Ramirez, J. et al.; Chem. Soc. Rev. 37 (2008) 2530-2542

(2) Lupulescu, A. I.; Rimer, J. D.; Angew. Chem. Int. Ed. 51 (2012) 3345-3349

(3) Lupulescu, A. I.; Qin, W.; Rimer, J. D.; Langmuir 32 (2016) 11888-11898

(4) Qin, W.; Clark, R.J.; Palmer, J.C.; Rimer, J.D. (in preparation)

(5) Danillina, N. et al.; J. Phys. Chem. C 114 (2010) 6640-6645

(6) Qin, W.; Patton, M.D.; Rimer, J.D. (in preparation)