Thermodynamic and mechanistic studies of initial stages in propane aromatisation over Ga-modified H-ZSM-5 catalysts

Add time:08/31/2019    Source:sciencedirect.com

13C MAS NMR has been performed in situ to investigate the early stages in the conversion of propane to aromatics on Ga-containing ZSM-5 catalysts. Propane 2-13C was used as labelled reactant. The scrambling of the 13C label in the very early stages of the propane conversion, even at 573 K, indicates that the first reaction intermediate is a protonated pseudocyclopropane (PPCP) species formed by activation of propane on a (Ga3+,O2−) ion pair and its protonation by a nearby Brønsted acidic site. This PPCP species can decompose in several ways leading to H2, CH4, C2H4, C2H6, and C3H6 as primary products. The very same molecules can also be produced as secondary products by cracking and hydrogen transfer at high conversion. CH+3, C2H+5 and C3H+7 carbenium ions which are formed by decomposition of PPCP can react further with alkane or olefinic species. Reaction of CH+3 (stabilised by a basic anionic 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 when the catalyst contains gallium.

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