Knoevenagel condensation reaction in supercritical carbon dioxide medium using a Zn(II) coordination polymer as catalyst

Abstract

The replacement of conventional organic solvents by the so-called “green solvents” is a usual approach to more sustainable chemical processes. Supercritical carbon dioxide (scCO2) has been successfully regarded as a candidate for such a purpose. Moreover, it possesses moderate critical pressure and temperature conditions and may be easily separated from the catalytic system by a simple depressurization. In the present work, the new Zn(II) coordination polymer [Zn(L1)(NMeF)]n·n(NMeF) (Zn-CP 1) is reported, being prepared by the solvothermal reaction of 5-{(pyren-4-ylmethyl)amino}isophthalic acid (H2L1) with Zn(NO3)2·6H2O in N-methylformamide (NMeF). The SCXRD analysis revealed that Zn-CP 1 has a 1D double chain type of structure. Its potential as catalyst was studied in the Knoevenagel condensation of benzaldehyde and malononitrile as a model reaction under mild conditions, in scCO2 medium. An increasing trend was observed in the reaction yield as we moved from aprotic (THF) to protic (EtOH and H2O) polar co-solvents, reaching the full conversion in the case of water. It was found that scCO2, in the absence of a protic co-solvent, is not the most suitable medium for this reaction. The catalyst can be recycled without a considerable loss of activity; SEM, PXRD, FT-IR and TGA analyses indicate its high stability throughout the process.