Methylation of olefins with ketene in zeotypes and its implications for the direct conversion of syngas to light olefins: a periodic DFT study

Abstract

The direct conversion of syngas to light olefins with high selectivity is of great significance as it offers an option to produce ethene, propene, or butenes from nonpetroleum resources. Recent studies (Science, 2016, 351, 1065–1068) reported a process named OX-ZEO, activating CO and H₂ to light olefins with selectivity as high as 80% using bifunctional catalysts. It was verified that ketene, produced from partially reduced oxide (ZnCrO_x), is an important intermediate to be transformed into the desired olefins in acidic zeolite (H-SAPO-34). In this work, we theoretically illustrated the evolution pathway of ketene with olefins, a key step in the hydrocarbon pool mechanism for chain propagation, to understand the conversion from ketene to olefins in H-SAPO-34. We revealed that the framework-bounded CH₃CO species (CH₃COZ), an intermediate produced via the protonation of ketene, is an important methylating agent towards hydrocarbon pool in zeotypes. It is the direct associative pathway other than the sequential dissociative pathway that contributes to the methylation between CH₃COZ and tetramethylethene (TME) as a representative olefin-based hydrocarbon pool. The effect of acid strength is also studied in a series of metal isomorphically substituted CHA-structured zeolites or zeotypes. The scaling relations of the transition state enthalpies with the acid strength using the adsorption enthalpy of ammonia as a descriptor can be established in both key elementary steps, i.e. the decarbonylation of CH₃COZ and the methylation of CH₃COZ with TME; the enthalpy barrier of the latter step is more sensitive to acid strength than the former one while both decreases with the increase of acid strength. These theoretical results may provide some implications to understand the key role of ketene and tailor catalyst structures in the OX-ZEO process.