On the influence of synthetic parameters on the properties and aldol condensation performance of the MgAl mixed oxides and rehydrated hydrotalcites

Abstract

In recent decades, the high catalytic activity of MgAl mixed oxides (MOs) and rehydrated hydrotalcites (HTCs) has made them very promising basic catalysts for various organic reactions. The synthesis procedure as well as the mechanism of the formation of HTC structure has been well described. Despite this, little has been published on how synthetic parameters affect the physico-chemical properties, rehydration ability and performance of the hydrotalcite-derived catalysts. Here, a series of catalysts with the same molar ratio Mg/Al = 3 varied by the coprecipitation conditions (pH value, temperature, ageing duration) were synthesized and used as precursors for the preparation of the MgAl MOs and rehydrated HTCs. The set of methods (AAS, XRD, SEM, N2-physisorption, TGA and CO2-TPD) was used to establish a relation between the synthetic parameters and the physico-chemical properties of the prepared materials. Their catalytic performance was monitored in aldol condensation of furfural and acetone as a model reaction. Preparations at different pH values and temperature resulted in the formation of HTC samples with crystallite size evaluated by XRD in the range of 70-230 ?, (d003). Nevertheless, the crystallite size of the resulting MOs was similar in the range of 30-36 ?, (d003). The rehydration of the MOs for 5-120 min produced high-crystalline reconstructed HTC with a crystalline size in the range of 82-111 ?, (d003), i.e. the crystallite size of a large-size HTC precursor was not restored. Catalytic results revealed that the crystallite size of an as-prepared HTC precursor was a crucial parameter that affected the performance of both MOs and reconstructed HTCs in the condensation reaction. Furfural conversion over MOs and short-time rehydrated (5-10 min) materials in the reaction was strongly dependent on initial size of the as-prepared HTC crystallites. In contrast, the catalytic performance of the catalysts rehydrated for 120 min, was similar: furfural conversion was above 99% in all cases. Based on these results, the effect of the physico-chemical properties of the as-prepared HTCs on the catalytic performance of both MOs and reconstructed MgAl HTCs was established. A difference in the properties of the prepared samples was explained assuming the existence of extra-framework or partially framework Al-rich species that prevented the access of reactant molecules to catalytically active sites: the larger the crystallite size of the as-prepared precursor, the higher the excess of both structural defects and nonframework Al-rich species, and the lower the furfural conversion over these catalysts. Nevertheless, long rehydration time favored the involvement of the Al-rich species in the recrystallization of HTC structure as evidenced by SEM. As a consequence, furfural conversion increased over these catalysts. These results provide a new insight into the influence of synthetic parameters on the properties of MOs and rehydrated HTCs: the crystallite size of the as-prepared precursors determines the performance of the produced catalysts in aldol condensation by affecting the rehydration ability of the derived MOs.