(349a) High Temperature Decomposition and Reactivity of Bronsted Acid Sites in Zeolites

Zeolites
are crystalline materials that have wide application in industry as solid acid
catalyst. Their importance stem from the presence of microporosity, the high
surface area and the tunable Bronsted acid sites (BAS) Si?OH?Al¹.
The OH group in the BAS sites absorbs IR in the range of 3600-3660 cm^-1
in which its absorption intensity decreases until it disappears upon heating
above 973 K². The dehydroxilation mechanism is believed to proceed
via a dehydration path of the acid sites³. However, M. J. Nash⁴
et al recently reported that hydrogen is also formed during the dehydroxilation
process. Moreover, the amount of hydrogenis related to the silica
to alumina ratio. Using ZSM5 with silica to alumina ratios of 20 and 40, M. J.
Nash⁴ et al showed that more hydrogen is produced in the first
sample with some water but essentially hydrogen is produced from the second
sample.

A new mechanism was proposed based on the new findings in
which dehydrogenation by homolytic decomposition of BAS and dehydration by
heterolytic decomposition of BAS are parallel paths to describe the
dehydroxilation process (see scheme 1). Specifically hydrogen and [AlO₄]⁰
are formed by the dehydrogenation channel⁴. Which path is
predominant depends on the silica to alumina ratio. At high silica to alumina
ratio as in the case of ZSM5 homolytic decomposition pathway is the dominant
one while at low ratio as in the case of zeolite Y dehydration mechanism is the
dominant one.

We are studying the dehydroxilation mechanism using FTIR, XAFS
and TPD-MS. We found out that for Beta substituted with Al or Ga, a fraction of
the BAS can be recovered upon exposing the sample to H₂ gas and to
humid atmosphere for short periods. That is, both of these mechanisms are
contributing to the dehydroxilation.  For other types of zeolites (ZSM5 in Al
and Ga form and MOR in Al form), only a small fraction of the BAS can be
recovered by rehydration. The reactivity of the dehydroxilated and
un-dehydroxilated samples is studied using propane cracking as a probe molecule
to investigate the nature of the active sites under different activation
methods.

Significance

This
work will shed the light on the role of redox chemistry on the catalytic
cracking mechanism which is important for fluid catalytic cracking (FCC)
process.

References

1.      S.
M. Auerbach, K. A. Carrado, P. K. Dutta (Eds), Handbook of Zeolite Science and
Technology, Marcel Dekker, New York, 2003.

2.      Szanyi,
J.; Paffett, M. T., Micropor. Mater. 1996, 7, 201-218.

3.      Stamires,
D. N.; Turkevich, J., J. Am. Chem. Soc. 1964, 86, 749

4.      Nash,
M. J.; Shough, A. M.; Fickel, D. W.; Doren, D. J.; Lobo, R. F., J. Am.
Chem.   Soc. 2008,130, 2460.