Thermoanalytical studies of high silica ZSM-5 zeolites containing organic templateS

Abstract

Novel shape-selective ZSM-5 zeolites acting as versatile catalysts for a variety of hydrocarbon conversion reactions are synthesised using quaternary ammonium compounds such as tetrapropylammonium bromide (TPA-Br), tetrapropylammonium hydroxide (TPA-OH), tetrabutylammonium hydroxide (TBA-OH), triethyl- n-propylammonium bromide (TEPA-Br) and triethyl- n-butylammonium bromide (TEBA-Br). These templating agents, occluded during synthesis, are found to play a structure-directing role and are found to influence structural, sorption and catalytic properties of these zeolite catalysts in their active form. A course of the decomposition pattern of these quaternary ammonium compounds in order to achieve the zeolite in the active form is followed systematically by TG, DTA and DTG. TG and DTA curves indicate that pure quaternary ammonium compounds decompose in air around 180–200°C, while these, when occluded in the channels of the crystalline zeolites, decompose at much higher temperature around 400–500°C. This indicates that these templating organic species are localised and/or stabilised when occluded in zeolitic channels by some sort of physico-chemical binding forces. DTA curves further characterise the decomposition of these species by two pronounced, distinct exotherm maximas (at 410 and 470°C) due to their oxidative decomposition in air. This clearly indicates that occluded organic species possess two different environments. The weakly bound species indicated by exotherm maxima around 410°C, are probably situated near the channel intersections and not associated with acid sites. Comparatively more stabilised species decomposing by exotherm maxima at 470°C are believed to be associated with aluminium acid sites and acting as charge compensating cations. These observations are further supported by TG and DTG curves. The nature of the decomposition products of these organic species are discussed with the help of IR spectra and sorption properties are found to complement these results.