Kinetics of Direct Olefin Synthesis from Syngas over Mixed Beds of Zn–Zr Oxides and SAPO-34

A packed bed containing a physical mixture of both Zn–Zr mixed oxide catalyst and SAPO-34 converts syngas directly into a mixture of C2–C5 olefins and paraffins. Specifically, the mixed oxide catalyst is responsible for intermediate oxygenate synthesis from syngas while the molecular sieve catalyzes olefin synthesis from the oxygenate intermediates. Kinetic measurements with cofed propylene over each catalyst independently confirm olefin hydrogenation activity over both components of the composite bed. The addition of either water or CO to the feed drops the activity of propylene hydrogenation over the Zn–Zr oxide. In sum, under reaction conditions of syngas feed and produced water, olefin hydrogenation predominantly occurs over the SAPO-34 catalyst, rather than over the catalyst responsible for hydrogenating CO into oxygenate intermediates. A developed kinetic model consistent with this conclusion describes measurements at differing feed compositions, temperatures, space velocities, and bed catalyst mixing ratios. Technoeconomic analysis of the process indicates that the olefin-to-paraffin ratio is a key performance metric for commercial scale syngas conversion and highlights the importance of considering olefin hydrogenation rates over the molecular sieve component

Support Effect and Surface Reconstruction in In₂O₃/<i>m-</i>ZrO₂ Catalyzed CO₂ Hydrogenation

We investigate the chemical and structural dynamics at the interface of In2O3/m-ZrO2 and their consequences on the CO2 hydrogenation reaction (CO2HR) under reaction conditions. While acting to enrich CO2, monoclinic zirconia (m-ZrO2) was also found to serve as a chemical and structural modifier of In2O3 that directly governs the outcome of the CO2HR. These modifying effects include the following: (1) Under reaction conditions (above 623 K), partially reduced In2O3, i.e., InOx (0 x < 1.5), was found to migrate in and out of the subsurface of m-ZrO2 in a semireversible manner, where m-ZrO2 accommodates and stabilizes InOx by serving as a reservoir. The decreased concentration of surface InOx under elevated temperatures coincides with significantly decreased selectivity toward methanol and a sharp increase of the reverse water–gas shift reaction. The reconstruction-induced variation of InOx concentration appears to be one of the most important factors contributing to the altered catalytic performance of CO2HR at different reaction conditions. (2) The strong interactions and reactions between m-ZrO2 and In2O3 result in the activation of a pool of In–O bonds at the In2O3/m-ZrO2 interface to form oxygen vacancies. On the other hand, the high dispersity of In2O3 nanostructures onto m-ZrO2 prevents their over-reduction under catalytically relevant conditions (up to 673 K), when bare In2O3 is unavoidably reduced into the metallic phase (In0). The relationship between the extent of reduction of In2O3 and catalytic performance (CO2 conversion, CH3OH selectivity, or yield of CH3OH) suggests the presence of an optimum coverage of surface InOx and oxygen vacancies under reaction conditions. The conventional model that links catalytic performance solely to the coverage of oxygen vacancies appears invalid in the present case. In situ analysis also allows the observation of surface reaction intermediates and their interconversions, including the reduction of CO3* into formate, a precursor for the formation of methanol and CO. The combinative ex situ and in situ study sheds light on the reaction mechanism of the CO2HR on In2O3/m-ZrO2-based catalysts. Our findings on the large-scale surface reconstructions, support effect, and the reaction mechanism of In2O3/m-ZrO2 for CO2HR may apply to other related metal oxide catalyzed CO2 reduction reactions