THERMODYNAMIC AND MECHANISTIC STUDIES OF INITIAL-STAGES IN PROPANE AROMATIZATION OVER GA-MODIFIED H-ZSM-5 CATALYSTS

C-13 MAS NMR has been performed in situ to investigate the early stage s in the conversion of propane to aromatics on Ga-containing ZSM-5 cat alysts. Propane 2-C-13 was used as labelled reactant. The scrambling o f the C-13 label in the very early stages of the propane conversion, e ven at 573 K, indicates that the first reaction intermediate is a prot onated pseudocyclopropane (PPCP) species formed by activation of propa ne on a (Ga3+,O2-) ion pair and its protonation by a nearby Bronsted a cidic site. This PPCP species can decompose in several ways leading to H-2, CH4, C2H4, C2H6, and C3H6 as primary products. The very same mol ecules can also be produced as secondary products by cracking and hydr ogen transfer at high conversion. CH3+, C2H5+ and C3H7+ carbenium ions which are formed by decomposition of PPCP can react further with alka ne or olefinic species. Reaction of CH3+ (stabilised by a basic anioni c framework oxygen) with propane (activated on a Ga site) may yield n- butane as indicated by the increase in the n-butane/i-butane ratio whe n the catalyst contains gallium.