Hydrotalcite-like compounds as heterogeneous catalysts in liquid phase organic synthesis. II. Preparation of 4H-chromenes promoted by hydrotalcite doped with hydrous tin(IV) oxide

New heterogeneous catalysts for liquid phase organic synthesis have been obtained by precipitating MgAl-hydrotalcites like compounds according to the urea method in the presence of Sn(IV) and characterised for their composition, thermal behaviour, X-ray diffraction pattern and B.E.T. specific surface area. The anion exchange capacity of the solids has been determined to have information on the possible incorporation of Sn(IV) into the brucite layer of the hydrotalcites. The values obtained give support to the hypothesis that most part of Sn(IV) is present as co-precipitated hydrous oxide. The base strength surface sites was estimated with the method of adsorbed acid–base indicators and the samples have been used as heterogeneous catalysts in the preparation of 4H-chromenes. The results obtained showed that MgAl-hydrotalcite doped with Sn(IV) catalyses the tandem Knoevenagel–Michael reactions, giving good to excellent yields in short time and its performance has been compared with that of the corresponding MgAl-hydrotalcite. Moreover, the catalyst was recovered and reused several times without loss of activity.