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

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 AlT 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 AlT= 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.