Effects of dry-air calcination on the physico-chemical and catalytic properties of HZSM-5 zeolite

Abstract

A series of HZSM-5 zeolites has been prepared by dry-air calcination, in the range 833 to 1163 K, of samples from a single preparation of NH₄ZSM-5 containing 5.5 framework Al_T atoms per unit cell. A similar series of HZSM-5 zeolites has been prepared from NH₄ZSM-5 that had been bound with colloidal silica (Ludox) prior to dry-air calcination. The zeolites were characterised using mid-IR spectroscopy, ²⁷Al magic-angle-spinning nuclear magnetic resonance (²⁷Al MAS NMR) and temperature-programmed desorption (TPD) of NH₃. It was shown that the zeolites are highly crystalline and that extensive structural dealumination does not occur at calcination temperatures below ca. 1000 K. Framework aluminium contents determined by ²⁷Al MAS NMR are considered to be a better measure of acid-site concentration that those given by TPD of NH₃. The activity of HZSM-5 zeolites as catalysts for ethylbenzene conversion at 573 K, to ethene, benzene and diethylbenzene, was only slightly suppressed by binding with silica. Reaction is believed to proceed by a dealkylation–alkylation mechanism; a maximum in activity for unbound HZSM-5 at Al_T= 2.8 suggests synergic interaction between framework and non-framework aluminium atoms. The conversion of propane at 808 K, to mainly methane, ethene and propene, showed an activity pattern closely similar to that of ethylbenzene.