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 hydrogen is 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.

	Scheme 1: proposed dehydrogenation path

                                                                                                                         

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.