Density Functional Theory Study of External Zeolite Surfaces

Density Functional Theory Study of External Zeolite Surfaces

Junbo Chen, N. Aaron Deskins; Department of Chemical Engineering;

Worcester Polytechnic Institute; Worcester, MA 01543

Zeolites have been used extensively in the petroleum industry to process crude hydrocarbons into desirable fuel materials. Catalytic cracking, hydro-cracking, and dewaxing(1, 2)⁠ are common catalytic reactions that take place over zeolites. The reactivity of zeolite materials is largely attributed to their internal pore structure which leads to a large internal surface area, or high density of reactive sites. Large molecules, however, cannot enter the zeolite's pores and therefore do not react within the zeolite, but at external surface sites. This phenomena has been well-documented in the literature. For example one phenomena discussed in the literature is pore mouth catalysis(3-6) where reactions do not occur in the pore (due to steric hindrance) but outside the pore or at the pore mouth. Another example is the alkylation of biphenyl over several zeolites, which was attributed mainly to external surface reaction sites(7)⁠. Work on ZSM-5 has also shown the importance of surfaces for larger molecules that cannot enter the pore structure(8, 9)⁠. Several reaction models involving large hydrocarbon were shown to agree much better with experimental data when surface sites were included in the models(10, 11)⁠.

Many details however of the surface reactivity of zeolites are largely unknown, particularly the atomic-level structures and mechanisms of these surface reactions. The structural complexity of zeolites has precluded many theoretical surface studies in the past due to computational limitations. The focus of this work is to simulate, using density functional theory, the adsorption and reactivity of hydrocarbons over zeolite external surfaces. Various (001) surface terminations of zeolite LTA are considered, at various Si/Al ratios and with several extra-framework cations. The most stable location of Brønsted acid sites (H⁺) was determined, and results show a energetic preference for the surface, as opposed to bulk internal area, indicating that the surface may be the most active region of the catalyst. The role of surface defects in surface stability is also presented. Both small and large prototypical molecules (e. g. H₂O, NH₃, benzene, naphthalene) are adsorbed at the external surfaces in order to determine precursor states to reactivity. Bulk calculations allow a comparison of internal versus external catalytic activity. Ultimately, understanding the importance and role of surfaces of zeolites in hydrocarbon reactivity is the aim of this work.

⁠References

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