The characterization and elimination of the external acidity of ZSM-5.

Weber, R; Fletcher, J; Möller, K; O'Connor, C

The characterization and elimination of the external acidity of ZSM-5.

Journal Article

1996

Journal Title

Microporous Materials (now incoporated in Microporous and Mesoporous Materials)

Link to Journal

http://www.journals.elsevier.com/microporous-and-mesoporous-materials/

Publisher

Elsevier

Publisher

University of Cape Town

License

http://creativecommons.org/licenses/by-nc-nd/4.0/

Abstract

The external acidity of ZSM-5 was modified either by coating the crystals with a silicalite shell or by chemical vapour deposition (CVD) of tetraethoxysilane (TEOS). Temperature programmed desorption (TPD) techniques were used to characterize the acidity changes arising from these modifications. Ammonia and pyridine were used as probes for the total acidity, and 4-methylquinoline was used to probe the external acidity of the modified and unmodified ZSM-5 samples. Si/Al ratios of the modified samples were also used to characterize the samples. Both methods of modification were able to make the external acid sites inert. Chemical vapour deposition was found to be a better method, because the external surface acidity was eliminated without significantly changing the catalyst in terms of total acidity. Optimum elimination of the external surface acidity by CVD was obtained for the sample with the higher deposition time (16 h) and temperature (400°C). Optimum elimination of the external surface acidity using the silicalite shell method was obtained using the detemplated NH4-form of the parent crystal as seed material. The channel structures of crystals modified by both processes remained accessible to ammonia and pyridine.