Role of intermediates in reaction pathways from ethene to hydrocarbons over H-ZSM-5

Insight in ethene to hydrocarbon transformation over a H-ZSM-5 catalyst was obtained by means of temporal analysis of products (TAP) in the temperature range 598–698 K with pulses of higher olefins, dienes, cyclodienes and aromatics. Pulses of propene, 1-butene and 1-hexene allowed to identify the cracking routes from ethene oligomerization products. When pulsing benzene or ethylbenzene, only accumulation of aromatics occurred. In-situ temperature programmed desorption (TPD) experiments after pulsing identified aromatics as long-lived surface species. The role of intermediates was assessed by means of pre-adsorption of the different feeds before pulsing ethene, in so-called pump-probe experiments. Butene enhanced propene formation, while all other olefins favored butene production via aliphatic surface intermediates. The latter were also intermediates in the conversion of hexadiene to butene and aromatics, while cyclohexadiene was converted to propene and aromatics via aromatic surface intermediates. In contrast to ethylbenzene pulses alone, aromatics alkylation participated towards light olefin production via sidechain/paring mechanisms. Isotope experiments of ¹³C₂H₄ over a catalyst coked during continuous flow experiments with ¹²C only showed scrambling in both propene and butene products, stressing the role of long-lived aromatic surface intermediates.