Reactions on ZSM-5-type zeolite catalysts

Catalytic reactions and sorption measurements have been carried out with ZSM-5 and silicalite catalysts which are distinguished by variation in skeletal heteroatom concentration. The catalysts were used in both the hydrogen-exchanged and sodium-exchanged forms. Sorption measurements were made with the hydrocarbons n-hexane, 3-methylpentane, and 2,2-dimethylbutane, and with the bases ammonia, n-butylamine, t-butylamine, and 4-methylquinoline. Catalytic reactions were carried out on both unpoisoned and base-poisoned catalysts using methanol, propylene, and 3-methylpentane reactants. In addition, the behavior of ethylene and ethanol reactants was also explored. The ease of base sorption and hydrocarbon sorption has been assessed in terms of effective molecular size in relation to the channel size of the catalyst, and this factor is also used as a basis for explaining the effectiveness of bases for poisoning the catalytic reactions. Temperature-programmed desorption (TPD) measurements with ammonia have been used to assess the energetic distribution of sorption sites for bases, and very strong binding sites with a TPD maximum for ammonia at about 780 °K have been identified as the probable sites used in the conversion processes. The main features of the catalytic conversion process are discussed. It is concluded that sorbed C3,C4 olefinic residues are general intermediates leading to aromatic formation. Under most circumstances, ethylene was relatively unreactive, and it is inferred that a sorbed C2 residue, which is related to ethylene by sorption and desorption, is an unlikely general intermediate. A mechanism is suggested for the formation of sorbed C3,C4 olefinic residues, and for initial carbon-carbon bond formation from methanol. Catalyst self-poisoning was observed with all catalysts except hydrogen-exchanged ZSM-5.