Thermoelectrically polarized amorphous silica promotes sustainable carbon dioxide conversion into valuable chemical products†

Electrically polarized amorphous silica (aSiO₂) is demonstrated to be an efficient and viable metal-free heterogeneous catalyst for the conversion of CO₂ into valuable chemical products. The catalyst was prepared applying a thermoelectric polarization process in air to commercially available aSiO₂ nanoparticles. Four polarization temperatures were assayed (150, 500, 800 and 1000 °C), the larger structural and chemical changes induced by the polarization treatment being observed at 150 and 500 °C. The polarization at such temperatures reduced considerably the electrical resistance of calcined aSiO₂, while no significant change was detected at 800 and 1000 °C. Polarized aSiO₂ was tested as heterogeneous catalysts for the reaction of CO₂ with water at mild reaction conditions (120 °C, 6 bar of CO₂, 40 mL of water, 72 h). The highest catalytic activity was observed with aSiO₂ polarized at 150 °C, which was attributed to the structural defects induced during the thermoelectric polarization treatment. Thus, CO₂ was converted into a mixture of formic acid (39.9%), acetic acid (44.4%) and dioxane (15.7%). Although the catalytic process was not selective, the yields were not only very high but also allowed obtaining a significant amount of dioxane, a product with four carbon atoms, which is very unusual in processes catalyzed by polarized ceramics. In summary, polarized aSiO₂ can be used as a sustainable and low-cost raw material to prepare metal-free catalysts by means of a thermoelectric polarization process at 150 °C. This catalyst is capable of capturing CO₂ to produce valuable chemical products by applying mild reaction conditions.