Simple and effective preparation of highly efficient heterogeneous base catalysts

Abstract

The simple hydration of physically mixed Mg and Al oxides results in the formation of an LDH (layered double hydroxide) phase via the dissolution–precipitation–crystallization mechanism. Nevertheless, the change in the properties and catalytic performances of these materials with respect to the hydration conditions has not been well explained to date. Therefore, in this study, we hydrated physically mixed Mg and Al oxides at T = 25 °C or 75 °C within a time range of 5 minutes to 7 days. Subsequently, we characterized the properties of the resulting materials using an array of appropriate methods and assessed their performances in aldol condensation of furfural and acetone. An increase in the hydration time and temperature promoted the formation of the desired LDH phase. Nevertheless, the performance of the catalysts did not improve accordingly. We assumed that the surprisingly inferior performance of the LDH-rich catalysts was due to the deposition of Al species on their external solid surface instead of being incorporated in the LDH phase, and these species blocked the access of reactants to the catalytically active sites in the HTC phase, resulting in a decreased catalytic activity. Thus, the maximum activity of the catalysts was observed at a relatively low concentration of water-soluble Al species and, accordingly, at a relative content of LDH phase of 20–25%. These results are beneficial for understanding the relationship between the hydration conditions of MgO + Al₂O₃ physical mixtures and the properties of the derived LDH-phase containing solids and have potential in the design of green, efficient and stable solid base catalysts.