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.