Resolving the Intricate Mechanism and Selectivity of Syngas Conversion on Reduced ZnCr₂O_x: A Quantitative Study from DFT and Microkinetic Simulations

Abstract

By combining reduced oxide and zeolite (e.g., ZnCrO x /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 ZnCrO x 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.

Steering the reaction pathway of syngas-to-light olefins with coordination unsaturated sites of ZnGaOx spinel

“…8a ). It indicates that the reaction equilibrium is successfully shifted and the reaction channel from intermediates to light olefins is opened up in the presence of SAPO-34, consistent with a recent theoretical study 21 . Thus, it forms a tandem catalytic process.…”

“…The above results demonstrate that the presence of coordination unsaturated Ga 3+ together with oxygen vacancies and zinc vacancies lead to a much more active ZnGaO x spinel in generating ketene-acetate (acetyl) intermediates. Interestingly, Lai et al recently also revealed the essential role of coordination unsaturated Cr 3+ together with the oxygen vacancies in the cleavage of the C–O bond over the highly reduced ZnCr 2 O 4 (110) surface 21 . Thus, incorporation of SAPO-34 would direct the reaction pathway towards light olefins (Fig.…”

H2 Activation on Pristine and Substitutional ZnO(101̅0) and Cr2O3(001) Surfaces by Density Functional Theory Calculations

“…Different from transition-metal catalysts, the bifunctional OX-ZEO catalysts separate the activation of syngas on the oxide part from the subsequent carbon chain growth and hydrogenation on the zeolite part. For the oxide part, the metal–oxide mixtures and/or complex metal oxides were found to be more active than the separated metal oxides. − Identification of the active sites and their role in syngas conversion remain challenging. − Mechanistic investigation and gaining an understanding of the intrinsic activity of prototype oxides such as ZnO and Cr 2 O 3 for syngas activation and the influence of the defects such as heterometal atoms are valuable for understanding the mechanism behind this phenomenon but not well explored yet.…”

Direct Conversion Syngas to Isobutanol over Ce/ZC Catalysts: Effect of Ce Promoter on the Catalytic Performance

“…With the raising amount of Ce 3+ dopant on catalyst surface, the electron transferring from Zn, Cr, and O to Ce was stimulated, which led to decreased electron density of surrounding metal ions, and obvious higher binding energy of Zn over 2.5‐Ce/ZC catalyst. This preferable electronic structure may facilitate the adsorption of electron‐rich intermediates (formate or bicarbonate species) [20] …”

“…This preferable electronic structure may facilitate the adsorption of electron-rich intermediates (formate or bicarbonate species). [20]…”