Resolving the Intricate Mechanism and Selectivity of Syngas Conversion on Reduced ZnCr2Ox: A Quantitative Study from DFT and Microkinetic Simulations

Abstract

By combining reduced oxide and zeolite (e.g., ZnCrOx/MSAPO), bifunctional catalysts achieve outstanding olefin selectivity for syngas conversion via a relay process (CO + H2 → IM → olefin) and are one of the hottest catalytic systems. However, the reaction mechanism on the reduced ZnCrOx as well as the intermediate (IM = ketene versus methanol) entering the zeolite are under heated debate owing to the complexity of this system. Herein, we perform systematic density functional theory (DFT) calculations and microkinetic simulations to inspect the possible elementary steps on the highly reduced ZnCr2O4(110) surface, which quantitatively unveils the favored reaction pathways for CO activation and conversion. We find that CO tends to adsorb at the two-coordinate O vacancy sites (VO 2C) and reacts with H at VO 2C to form CHO, while the disproportionation reaction and the direct C–O cleavage are less favored. Importantly, the conversion of CHO plays a vital role in product selectivity; the dissociation of CHO is identified to proceed easily and constitutes a major route responsible for the formation of CH4 and CH2CO, through the following pathway: CHO → CH + O; CH + H → CH2; CH2 + CO → CH2CO; or CH2 + 2H → CH4. Alternatively, CHO could undergo hydrogenation to give CH2O and CH3O intermediates, eventually leading to the formation of CH3OH and CH4. The kinetic analyses on such a complex reaction network disclose that CH4 is the dominant product, while both CH2CO and CH3OH (i.e., two experimentally controversial intermediates) exist in minority with CH2CO being relatively more readily formed. More interestingly, the kinetic model illustrates that the selectivity for CH2CO and the formation of triggered light olefins can be significantly improved over CH4 if a reaction channel converts CH2CO to light olefins when zeolite is added, providing insight into the bifunctionality of the oxide/zeolite system. Also, we demonstrate that high VO 2C coverage is a prerequisite for high activity of CO activation, and the reason for high selectivity of CO2 and low selectivity of H2O is identified to the easier removal of the lattice oxygen by CO to generate CO2 than by H2 to generate H2O. The understanding derived from this work could lay a solid theoretical foundation for comprehending the oxides for the syngas conversion mechanism.