Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates

Abstract

The synthesis of many industrial bulk and fine chemicals frequently involves electrophilic aromatic substitution (SEAr) reactions. The most widely practiced example of the SEAr mechanism is the zeolite-catalysed ethylation of benzene, using ethylene as an alkylating agent. However, the current production route towards ethylbenzene is completely dependent on fossil resources, making the recent commercial successes in the zeolite-catalysed benzene ethylation process using bioethanol (instead of ethylene) very encouraging and noteworthy. Unfortunately, there is no information available on the reaction mechanism of this alternative synthesis route. Here, by employing a combination of advanced solid-state NMR spectroscopy and operando UV-Vis diffuse reflectance spectroscopy with on-line mass spectrometry, we have obtained detailed mechanistic insights into the bioethanol-mediated benzene ethylation process through the identification of active surface ethoxy species, surface-adsorbed zeolite-aromatic pi-complexes, as well as the more controversial Wheland-type sigma-complex. Moreover, we distinguish between rigid and mobile zeolite-trapped organic species, providing further evidence for distinctive host-guest chemistry during catalysis.