Thermal behaviour of the tpa IONS in ZSM-5 zeolite: II. Effect of crystal size and grinding of silicalite-1

Abstract

RENDE, 87030 RENDE (CS), ITALYThermal decomposition of Silicalite-1 crystals of various size was studied. The crystals were in as-made form and upon grinding. The DTA and DTG curves show differences in the temperature of thermal decomposition of TPA species occluded in the as-made crystals. These differences disappear upon grinding. The explanation of this phenomena is given.Silicalite-1, the Al-free analogue of ZSM-5 zeolite was first prepared by E. M. Flanigen et al. [1]. It has the same crystal structure as the ZSM-5 zeolite which has been applied as a catalyst in MTG and other petrochemical processes [2].Synthesis of the ZSM-5 zeolite and its aluminium deficient end-member silicalite has been the topic of a large number of investigations [3]. The parameters affecting the growth of large silicalite crystals and the effect of reactant concentration on crystal morphology have been reported by Hayhurst et aL [4,5].The organic compounds used as either structure-directing and/or space filling species in a high-silica zeolite synthesis are generally occluded in the zeolite channels and cavities either in an intact or in a modified state [3,6]. The state of tetrapropylammonium (TPA) ions, occluded in ZSM-5 channels has been most thoroughly investigated by thermal analysis combined with mass spectrometry [7] or 13C-NMR spectroscopy [8]. TPA ions decompose by the Hoffman reaction to give an olefin and trialkylamine [7].The three DTA peaks (B, C1 and C2) have been assigned [8] to decomposition of different TPA species occluded into the zeolitic channels. In the case of Al-free silicalite, the low temperature (ca 380 ~ peak B is attributed [9] to less strongly held or "outer shell" strained TPA ions. A peak C1 (ca. 425 ~ is assigned to inner ions neutralizing SiO-negative charges of John Wiley & Sons, Limited, Chichester Akadgrm'ai Kiad6, Budapest