(475f) Synthesis of Hierarchically Structured Zeolites with Superior Catalytic Activity Guided by Numerical Optimization of the Broad Pore Network

Rational design of catalysts has mostly focused on the nanoscale. However, performance of nanoporous catalysts, such as zeolites, is often diffusion limited in industrial operations. A validated rational design methodology that reduces transport limitations and maximizes particle-based, overall catalytic performance is lacking. There is a wide gap between theoretical [1] and experimental efforts [2-4], which this paper aims to narrow by presenting, for the first time, theoretical optimization side by side with experimental catalytic results guided by our numerical optimization. Combining theory with experiments provides a sound basis for the rational design of catalysts at all scales, reducing cost and development time compared to empirical approaches.

We illustrate this approach by addressing the optimal design and synthesis of hierarchically structured zeolite composites using a combination of multiscale modeling, optimization and experiments. The composites contain zeolite crystals dispersed in an amorphous mesoporous matrix. The industrially important alkylation of benzene by ethylene over H-ZSM-5 to form ethylbenzene is used to illustrate our approach. The slow diffusion of benzene and ethylbenzene within a zeolite crystal induces strong diffusion limitations, which can be overcome by introducing an optimized network of mesopores surrounding the zeolite crystals. However, the introduction of mesopores reduces the volume of zeolite available for reaction. Therefore, the mesoporosity, the mesopore diameter and the zeolite crystal size are theoretically optimized to maximize the catalytic activity of the zeolite composite.

To describe diffusion and reaction within a catalyst pellet, a hierarchical approach is employed. Intrinsic kinetics from density functional theory (DFT) calculations are used, and adsorption and diffusion inside the zeolite crystals are accounted for by using a combination of the dusty gas model adapted for zeolites and the Ideal Adsorbed Solution Theory [5]. Single pellet optimization studies using finite element simulations are used to determine the optimal pore network properties of the zeolite composite.  The optimized hierarchically structured zeolite composites are then synthesized using a combination of hydrothermal treatment and sol-gel methods.  Properties of the zeolite crystals, such as size and Si/Al ratio, are tuned independently of the meso/macropore properties, providing greater flexibility. Fixed bed reactor experiments are used to compare the catalytic activity of the synthesized zeolite composites with the theoretical predictions from fixed bed reactor simulations.

This methodology could speed up catalyst discovery by complementing experiments for screening catalyst candidates, and can be adapted to other industrially important reactions such as isomerizations and cracking.

References:

[1] Rao, S. M.; Coppens, M.-O. Increasing Robustness against Deactivation of Nanoporous Catalysts by Introducing an Optimized Hierarchical Pore Network—Application to Hydrodemetalation, Chem. Eng. Sci., (2012) DOI: http://dx.doi.org/10.1016/j.ces.2011.11.044

[2] Christensen, C. H.; Johannsen, K.; Schmidt, I.; Christensen, C. H. Catalytic Benzene Alkylation over Mesoporous Zeolite Single Crystals: Improving Activity and Selectivity with a New Family of Porous Materials. J. Am. Chem. Soc., 2003, 125, 13370-13371.

[3] Wang, J.; Groen, J. C.; Yue, W. B.; Zhou, W. Z.; Coppens M.-O. Single Template Synthesis of Zeolite ZSM-5 Composites with Tunable Mesoporosity. Chem. Comm., 2007, 44, 4653-4655.

[4] Fan, W.; Snyder, M. A.; Kumar, S.; Lee, P.-S.; Yoo, W. C.; McCormick, A. V.; Penn, R. L.; Stein, A.; Tsapatsis, M. Hierarchical Nanofabrication of Microporous Crystals with Ordered Mesoporosity. Nat. Mater., 2008, 7, 984-991.

[5] Hansen, N.; van Baten, J. M.; Krishna, R.; Bell, A. T.; Keil, F. J. Analysis of Diffusion Limitation in the Alkylation of Benzene over H-ZSM-5 by Combining Quantum Chemical Calculations, Molecular Simulations and a Continuum Approach. J. Phys. Chem. C, 2009, 113, 235-246.