Fading memory of MgAl hydrotalcites at mild rehydration conditions deteriorates their performance in aldol condensation

Abstract

MgAl hydrotalcites (HTCs) are widely used as basic heterogenous catalysts. Their calcination followed by rehydration enhances their basicity and catalytic activity. The effect of repetitive “calcination-rehydration” treatments on their properties and catalytic performance needs to be established to understand regeneration and reusability of HTCs. Therefore, we investigated the properties of MgAl HTCs in dependence on the rehydration conditions and the number of repeated calcination-rehydration cycles. We found that regardless of the rehydration conditions, all catalysts after the first rehydration recovered their properties and furfural conversion was > 80%. After successive treatments at 25 °C, the HTCs properties deteriorated significantly and furfural conversion dropped < 5%. When rehydrating at 75 °C for 120 min the characteristics and performance of the rehydrated HTCs were maintained near the original values. We concluded that complete rehydration needed to maximize the catalytic activity was associated with the efficient reincorporation of Al species into the HTC framework.