THE CHARACTERIZATION AND ELIMINATION OF THE EXTERNAL ACIDITY OF ZSM-5

The external acidity of ZSM-5 was modified either by coating the cryst als with a silicalite shell or by chemical vapour deposition (CVD) of tetraethoxysilane (TEOS). Temperature programmed desorption (TPD) tech niques were used to characterize the acidity changes arising from thes e modifications. Ammonia and pyridine were used as probes for the tota l acidity, and 4-methylquinoline was used to probe the external acidit y of the modified and unmodified ZSM-5 samples. Si/Al ratios of the mo dified samples were also used to characterize the samples. Both method s of modification were able to make the external acid sites inert. Che mical vapour deposition was found to be a better method, because the e xternal surface acidity was eliminated without significantly changing the catalyst in terms of total acidity. Optimum elimination of the ext ernal surface acidity by CVD was obtained for the sample with the high er deposition time (16 h) and temperature (400 degrees C). Optimum eli mination of the external surface acidity using the silicalite shell me thod 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.