Catalyst sites and active species in the early stages of MTO conversion over cobalt AlPO-18 followed by IR spectroscopy

The isomorphic insertion of cobalt(II) confers strong acidity to the AlPO-18 zeotype. This makes it a suitable catalyst for the acid catalysed conversion of methanol to olefins (MTO). However, due to the tendency of Co(II) to be oxidised into Co(III) or to acquire distorted tetrahedral coordination, the concentration of strong Brønsted acidic sites (BAS) in CoAPO-18 is highly susceptible to catalyst activation conditions. The use of vibrational and electron spectroscopy coupled with the use of probe molecules sheds light on the conditions under which the concentration of active sites can be maximized, limiting the formation of species of different nature. The MTO reaction was followed by in situ FT-IR spectroscopy, revealing that the population of strong BAS profoundly affects the methanol conversion in the very early stages of the process. Indeed, spectroscopic evidence of the consumption of CO (produced in situ from the decomposition of methanol itself) was obtained, when the concentration and availability of BAS are optimized. This gives support to the occurrence of Koch carbonylation of surface methoxides, at the early stages of MTO.