Kinetic Model for the Direct Conversion of CO₂/CO into Light Olefins over an In₂O₃-ZrO₂/SAPO-34 Tandem Catalyst

An original kinetic model is proposed for the direct production of light olefins by hydrogenation of CO₂/CO (CO) mixtures over an In₂O₃-ZrO₂/SAPO-34 tandem catalyst, quantifying deactivation by coke. The reaction network comprises 12 individual reactions, and deactivation is quantified with expressions dependent on the concentration of methanol (as coke precursor) and H₂O and H₂ (as agents attenuating coke formation). The experimental results were obtained in a fixed-bed reactor under the following conditions: In₂O₃-ZrO₂/SAPO-34 mass ratio, 0/1-1/0; 350-425 °C; 20-50 bar; H₂/CO ratio, 1-3; CO₂/CO ratio, 0-1; space time, 0-10 g_In2O3-ZrO2 h mol_C^-1, 0-20 g_SAPO-34 h mol_C^-1; time, up to 500 h; H₂O and CH₃OH in the feed, up to 5% vol. The utility of the model for further scale-up studies is demonstrated by its application in optimizing the process variables (temperature, pressure, and CO₂/CO ratio). The model predicts an olefin yield higher than 7% (selectivity above 60%), a CO conversion of 12% and a CO₂ conversion of 16% at 415 °C and 50 bar, for a CO₂/CO = 0.5 in the feed. Additionally, an analysis of the effect of In₂O₃-ZrO₂ and SAPO-34 loading in the configuration of the tandem catalyst is conducted, yielding 17% olefins and complete conversion of CO₂ under full water removal conditions.