Thermocatalytic transcarbonation with diethyl carbonate for valorization of glycerol : recycling siliceous bauxite residue waste components and exploring the potential of sodalite hydroxide reservoir

Bauxite residues contain katoites and desilication products (sodalite and cancrinite), which are excellent candidates for Earth-abundant heterogeneous base catalysts from industrial waste sources. For the first time, they were tested in synthetizing the renewable and promising fuel or fuel additive glycerol carbonate for the purpose of recycling glycerol by-products from biodiesel production. Incorporation of silicate was found to enhance the previously established excellent catalytic activity of tricalcium aluminate (Si-free katoite). Performance of sodalites containing four different cage anion-designed (OH, CO3, SO4, Cl) and carbonated cancrinite was determined to a much greater extent by their Brönsted OH content than by the basicity of the caged anions. Direct catalytic use of bauxite residue resulted in a glycerol carbonate yield of over 80%, but it was quickly deactivated due to the limited reusability of many of its most active components (i.e., Si-free and Si-containing katoites, chloride sodalites). However, hydroxysodalites showed excellent reusability, with a glycerol carbonate yield of around 80% even after 5 reuses. The activity of katoites and sodalities overtook that of many complex systems containing relatively rare metals, such as Ni, Cu, Zr, and Ce. Their excellent activity has made it possible to use diethyl carbonate solvent/reagent, which is more favourable in terms of sustainability, instead of the more widely applied dimethyl carbonate, even with as short reaction times as 30–60 min.