The influence of the precursor type in the synthesis of Mg/Al-hydrotalcite through a non-traditional method used for Claisen-Shmidt condensation

Abstract

Traditionally, the synthesis of MgAl-hydrotalcite is performed in the presence of inorganic alkalis-type precipitating agents using the co-precipitation method. As an viable alternative, the use of organic alkalis and the mechanochemical method can remove some of the issues that the traditional route generates, i.e. the possibility of final solid contamination with alkaline cations; a high energy consumption, relative to the amount of water used but also of electric current; the involvement of a large number of utensils necessary for the synthesis; time allocated to the solid preparation; etc. Changing the regular precursor type (i.e. nitrate) of the target cation with chloride or sulfate leads to obtaining hydrotalcites which have different basicities. The chlorides precursors lead to the synthesis of materials that show a better ability for the reconstruction of the layered structure. Moreover, TMAH acts both as a synthesis agent and as a templating agent. The tight pore distribution is characteristic of all materials obtained from chloride precursors, where the co-precipitation method generates pores of 97Å, while the mechanochemical method generates pores of 140Å. The material with the highest catalytic activity for Claisen-Schmidt condensation, conversion of 92%, is the one obtained by calcination of the hydrotalcite prepared from chloride precursors by the mechanochemical method, while the co-precipitation method leads to conversions of 90%. The novelty of this work is that these type of catalysts obtained from chlorides or sulfate precursors in presence of TMAH have not been used in Claisen-Schmidt condensation until now.