Impact of Forming and Modification with Phosphoric Acid on the Acid Sites of HZSM-5

The individual and combined effect of phosphoric acid modification, the addition of an alumina-based binder, and steaming on the acid-base and catalytic properties of HZSM-5 have been investigated. Whereas all three processes reduce the concentration of Bronsted acid sites, the underlying mechanisms and the extent of the effects vary. Upon addition of an alumina-based binder or during steaming of the parent zeolite dealumination is the sole process occurring. Impregnation of HZSM-5 with phosphoric acid leads to hydrolysis of framework aluminum and to the condensation of Bronsted acid sites with POH groups. This latter process can be reversed in the presence of an aqueous phase. In the forming process, this leads to the reversible formation of phosphate anions and phosphoric acid and the redistribution of phosphate species between the zeolite and the alumina binder. This mobility is mitigated by introducing phosphate species to the (alumina) binder indicating that in aqueous phase equilibrium between phosphate ions and phosphoric acid the zeolite and the (alumina) binder exists. Whatever the first step of modification of the parent material is (doping with H3PO4 or addition of either alumina or phosphated alumina binder), steaming at elevated temperatures essentially reduces and equalizes the concentration of Bronsted acid sites. For all samples derived from one common parent zeolite, the rate of protolytic cracking of propane depends linearly on the concentration of strong Bronsted acid sites; that is, all modified materials have the same turnover frequency. This turnover frequency is notably higher than that of the parent material.