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 (COx) 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/COx ratio, 1-3; CO2/COx ratio, 0-1; space time, 0-10 gIn2O3-ZrO2 h molC-1, 0-20 gSAPO-34 h molC-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/COx ratio). The model predicts an olefin yield higher than 7% (selectivity above 60%), a COx conversion of 12% and a CO2 conversion of 16% at 415 °C and 50 bar, for a CO2/COx = 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.

Role of Zr loading into In2O3 catalysts for the direct conversion of CO2/CO mixtures into light olefins

The effect of the ZrO₂ content on the performance (activity, selectivity, stability) of In₂O₃-ZrO₂ catalyst has been studied on the hydrogenation of CO₂/CO mixtures. This effect is a key feature for the viability of using In₂O₃-ZrO₂/SAPO-34 tandem catalysts for the direct conversion of CO₂ and syngas into olefins via oxygenates as intermediates. The interest of co-feeding syngas together with CO₂ resides in jointly valorizing syngas derived from biomass or wastes (via gasification) and supplying the required H₂. The experiments of methanol synthesis and direct synthesis of olefins, with In₂O₃-ZrO₂ and In₂O₃-ZrO₂/SAPO-34 catalysts, respectively, have been carried out under the appropriate conditions for the direct olefins synthesis (400 °C, 30 bar, H₂/CO_X ratio = 3) in an isothermal fixed bed reactor at low space time values (kinetic conditions) to evaluate the behavior and deactivation of the catalysts. The Zr/In ratio of 1/2 favors the conversion of CO₂ and CO_X, attaining good oxygenates selectivity, and prevents the sintering attributable to the over-reduction of the In₂O₃ (more significant for syngas feeds). The improvement is more remarkable in the direct olefins synthesis, where the thermodynamic equilibrium of methanol formation is displaced, and methanation suppressed (in a greater extent for feeds with high CO content). With the In₂O₃-ZrO₂/SAPO-34 tandem catalysts, the conversion of CO_x almost 5 folds respect oxygenates synthesis with In₂O₃-ZrO₂ catalyst, meaning the yield of the target products boosts from ∼0.5% of oxygenates to >3% of olefins (selectivity >70%) for mixtures of CO₂/CO_X of 0.5, where an optimum performance has been obtained.

Conditions for the Joint Conversion of CO2 and Syngas in the Direct Synthesis of Light Olefins Using In2O3–ZrO2/SAPO-34 Catalyst

The conditions for promoting the joint conversion of CO₂ and syngas in the direct synthesis of light olefins have been studied. In addition, given the relevance for the viability of the process, the stability of the In₂O₃–ZrO₂/SAPO-34 (InZr/S34) catalyst has also been pursued. The CO+CO₂ (CO_x) hydrogenation experimental runs were conducted in a packed bed isothermal reactor under the following conditions: 375–425 °C; 20–40 bar; space time, 1.25–20 g_catalyst h mol_C^–1; H₂/(CO_x) ratio in the feed, 1–3; CO₂/(CO_x) ratio in the feed, 0.5; time on stream (TOS), up to 24 h. Analyzing the reaction indices (CO₂ and CO_x conversions, yield and selectivity of olefins and paraffins, and stability), the following have been established as suitable conditions: 400 °C, 30 bar, 5–10 g_cat h mol_C^–1, CO₂/CO_x = 0.5, and H₂/CO_x = 3. Under these conditions, the catalyst is stable (after an initial period of deactivation by coke), and olefin yield and selectivity surpass 4 and 70%, respectively, with light paraffins as byproducts. Produced olefin yields follow propylene > ethylene > butenes. The conditions of the process (low pressure and low H₂/CO_x ratio) may facilitate the integration of sustainable H₂ production with PEM electrolyzers and the covalorization of CO₂ and syngas obtained from biomass.