Halloysite Nanoclay with High Content of Sulfonic Acid-Based Ionic Liquid: A Novel Catalyst for the Synthesis of Tetrahydrobenzo[b]pyrans

One of the main drawbacks of supported ionic liquids is their low loading and consequently, low activity of the resultant catalysts. To furnish a solution to this issue, a novel heterocyclic ligand with multi imine sites was introduced on the surface of amino-functionalized halloysite support via successive reactions with 2,4,6-trichloro-1,3,5-triazine and 2-aminopyrimidine. Subsequently, the imine sites were transformed to sulfonic acid-based ionic liquids via reaction with 1,4-butanesultone. Using this strategy, high loading of ionic liquid was loaded on halloysite nanoclay. The supported ionic liquid was then characterized with XRD, SEM, TEM, EDS, FTIR, BET, TGA and elemental mapping analysis and utilized as a metal-free Bronsted acid catalyst for promoting one-pot reaction of aldehydes, dimedone and malononitrile to furnish tetrahydrobenzo[b]pyrans. The catalytic tests confirmed high performance of the catalyst. Moreover, the catalyst was stable upon recycling.