Basicity and catalytic activity of porous materials based on a (Si,Al)-N framework

• Porous basic catalysts containing nitrogen linked to silicon and aluminum have been prepared in the presence of ionic liquids.
• The strength of the basic sites increases with the aluminum content.
• These materials are efficient catalysts for the Michael addition reaction between chalcone and malononitrile.
• The basic strength of these materials is in the range 11 < pKa < 13.3.

Porous materials based on a framework containing T-N linkages, where T represents silicon or silicon and aluminium atoms, have been prepared by the ammonolysis at low temperature of the corresponding silicon and aluminium chlorides in an ionic liquid, both in the presence and in the absence of mineralizing agents such as pyrrolidine, ammonia and sodium amide. IR spectroscopy and XPS data are consistent with these materials having a framework based on T-NH-T groups (T = Si or Al), that contain also a large fraction of T-NH2 terminal groups. Moreover, XPS evidences the presence of Si-NH-Al groups whenever aluminium is present in the solid. The basic strength of the materials has been determined by pyrrole adsorption, and it has been found that the average basic strength increases with the population of T-NH-T groups and with the aluminium content of the framework. However, this strength is lower than that of a calcined hydrotalcite (Al/(Al + Mg) = 0.33) taken as a reference. These materials are active and very selective catalysts in the Michael addition reaction between chalcone [and malononitrile (pKa 11), but turned to be inactive when diethylmalonate having a weaker acidity (pKa 13.3) is used as donor.

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