Abstract:
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In the context of vinyl chloride monomer (VCM) production, an oxychlorination catalyst that allows
direct VCM formation from gas-derived ethane instead of expensive oil-derived ethene is intensively
sought after. A wide range of stable ethane oxychlorination catalysts for this purpose have recently been
reported, yet they mainly yield ethene, while VCM remains a minor by-product. Strikingly, the same catalysts
are active in ethene oxychlorination, resulting in selective VCM formation under equivalent reaction
conditions. This work reveals the origin of these diverging selectivity patterns by combining
quantitative catalytic tests, temporal analysis of products (TAP), and density functional theory (DFT)
on iron phosphate. Ethane oxychlorination is found to proceed sequentially through ethyl chloride
(EtCl) dehydrochlorination to ethene, while ethene oxychlorination directly yields VCM without formation
of the intermediate dichloroethane (EDC) on iron phosphate. Furthermore, by co-feeding ethane in
ethene oxychlorination, we demonstrate that the alkane suppresses the formation of VCM in ethene
oxychlorination. The reason for this VCM inhibition is found in the hydrocarbon competition for a combination
of the active, free and chlorinated iron centers. As ethane activation exhibits half of the barrier of
ethene activation, the presence of ethane leads to active site depletion, hindering VCM formation. These
observations are extended by ethane co-feeding tests in ethene oxychlorination over a wide range of
known oxychlorination catalysts (EuOCl, LaOCl, CeO2, and CuCl2-KCl-LaCl3/c-Al2O3), and corresponding
DFT calculations, indicating that the described phenomenon is material independent. The gathered
molecular-level understanding explains the major hurdle of using ethane as feedstock for vinyl chloride
production. |