(629e) 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