Coking, aging, and regeneration of zeolites: IX. Location of coke formed on hydrogen offretite during n-heptane cracking: Mode of deactivation

Abstract

Adsorption experiments were carried out on a series of hydrogen offretite (H-OFF) samples coked during n-heptane cracking at 450 †C. Three adsorbates of different molecular size were chosen: nitrogen, n-hexane, and 3-methylpentane. The decrease, caused by coke, in the diffusion rate and in the pore volume accessible to these adsorbates allows confirmation of the location of coke deduced from its composition. At a low content (<1 wt%), coke has practically no effect on the volume of the pores (large channels) accessible to 3-methylpentane but causes a decrease in the pore volume accessible to n-hexane (large channels + gmelinite cages); therefore, coke is located in the gmelinite cages. With coke contents between 1 and 3.5 wt%, part of the coke is also located in the large channels since it limits or even blocks the access of n-hexane and, above all, of 3-methylpentane to part of the large channels. With a high content, coke blocks the access of all the adsorbates to the entire pore volume because it is deposited on the outer surface. Whatever the coke content the deactivation of H-OFF occurs through pore blockage. With a low content the small molecules located in the gmelinite cages occupy about 40% of the volume and block the access of n-heptane to their acid sites whereas with a high content the highly polyaromatic molecules located on the outer surface block the access of n-heptane to all the inner acid sites.