Unraveling the Mechanism of the Initiation Reaction of the Methanol to Olefins Process Using ab lnitio and DFT Calculations

We report a theoretical investigation of the initiation of the methanol to olefin process, where we study the full reaction mechanism from methanol to propylene. The zeolite H-SSZ-13 is investigated with periodic density functional theory (DFT) calculations. These calculations are corrected with MP2-calculations on large (46T) cluster models, which is found to be crucial for sufficient accuracy. Our calculations clearly demonstrate that initiation via the formation of carbon monoxide is a realistic mechanism and is more likely than the methane-formaldehyde mechanism or variants thereof. A kinetic model of the autocatalytic carbon pool mechanism is employed to investigate the initiation kinetics in more detail, demonstrating that an assessment of the feasibility of an initiation reaction needs to be based on kinetic modeling of both the initiation reaction and autocatalysis. This model gives further evidence that initiation proceeds via oxidation of methanol to carbon monoxide, which subsequently forms the first carbon-carbon bond via carbonylation of methanol. The kinetic model also shows that only extremely small amounts of an olefin need to be formed for autocatalysis to start, implying that small impurities will dominate over initiation mechanisms.