(3q) Building Hierarchical Catalysts with Ultra-Small Units

Zeolites are aluminosilicates with ordered micropores, where the size of the micropores (0.5-2 nm) are similar to industrial molecules. It has long been recognized that in the processing of certain petrochemical products, fine chemicals, and biomass, the ability to combine mesoporosity with the zeolitic microporosity of conventional zeolites could  provide new opportunities to fine tune reaction rates, selectivity and resistance to deactivation.  Hierarchical zeolties are a new class of zeolites with precisely defined and often ordered mesoporosity. In addition to hierarchical pore architecture, ultra-small (down to the size of a unit cell) building units can lead to catalysts with higher specific surface area, vastly extending the degree of  manipulation of their surface catalytic properties.

My research involves the synthesis of hierarchical zeolite catalysts with diffusion lengths at various scales. Hard and soft templates were involved in the preparation of hierarchical zeolites, such as 3DOm-i zeolites [1] and supported lamellar zeolites [2], where the diffusion length can be tuned from 20 nm to 1.6 nm. In addition to the synthesis of these materials, characterization techniques (such as electron microscopy, X-ray diffraction and scattering) suitable for the small length scales are also involved in my research. These techniques, along with state-of-the-art modeling methods, enable comprehensive understanding of the structure and properties of these materials [3]. Repetitive branching strategy originally used in the preparation of semiconductor tetrapods was also applied to zeolites for the first time, enabling the direct-synthesis of self-pillared pentasil (MFI/MEL) zeolite nanosheets with 2 nm-thick MFI lamellae, where the diffusion length of zeolite catalysts was further reduced to 1 nm [4]. Preliminary results show that this self-pillared material enables higher reaction rates for bulky molecules compared to those of other hierarchical and conventional MFI zeolites [4].

In my future research, the repetitive branching strategy is expected to be applied to other catalysts. For example, other intergrown zeolite structures can be prepared in a self-pillared pattern to allow catalytic reactions of bulky molecules over Brønsted and Lewis acid sites. Moreover, metal and metal oxides that show twinning growth pattern can be prepared for hydrogenation reactions, photocatalysis and base catalysis.

[1] P.-S. Lee, X. Zhang, J.A. Stoeger, A. Malek, W. Fan, S. Kumar, W.C. Yoo, S. Al Hashimi, R.L. Penn, A. Stein, M. Tsapatsis, Sub-40 nm zeolite suspensions via disassembly of three-dimensionally ordered mesoporous-imprinted silicalite-1, J. Am. Chem. Soc., 133 (2011) 493-502.

[2] X. Zhang, M. Tsapatsis, Mesoporous silica nanoparticles from a clear sol and their transformation to lamellar silicalite-1 particles and films, Microporous and Mesoporous Materials, 138 (2011) 239-242.

[3] K. Varoon, X. Zhang, B. Elyassi, D.D. Brewer, M. Gettel, S. Kumar, J.A. Lee, S. Maheshwari, A. Mittal, C.-Y. Sung, M. Cococcioni, L.F. Francis, A.V. McCormick, K.A. Mkhoyan, M. Tsapatsis, Dispersible exfoliated zeolite nanosheets and their application as a selective membrane, Science, 333 (2011) 72-75.

[4] X. Zhang, D. Liu, D. Xu, S. Asahina, K. A. Cychosz, K. V. Agrawal, Y. Al Wahedi, A. Bhan, S. Al Hashimi, O. Terasaki, M. Thommes, M. Tsapatsis. Synthesis of self-pillared zeolite nanosheets by repetitive branching. To be presented at AIChE Annual Meeting, Pittsburgh, PA (2012).