A magnetic hybrid sol-gel ionic network catalyst for direct alcohol esterification under solvent-free conditions

A novel cross-linked organic-inorganic magnetic ionic network nanoparticle (MINN) comprising core-shell silica-coated iron oxide nanoparticles as the seeds and high loading immobilized imidazolium-based poly(ionic liquid) (PIL) linkers was prepared via a two-step microemulsion and radical polymerization method. Subsequently, by a simple counter-anion exchange method, the sulfonic acid anion was stabilized on the support to prepare a powerful solid acid catalyst. The material was characterized with various techniques including thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), N₂ adsorption-desorption, CHNS elemental analysis, transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). Optimization experiments demonstrated that the best results were achieved in the esterification of both primary and secondary alcohols with acetic acid (5-7 equivalents), using as little as 0.1 mol% of the catalyst at 85 °C under solvent-free conditions. Under these optimized conditions, the developed catalyst demonstrated exceptional catalytic activity, selectivity, water resistance, and durability in the direct esterification of primary and secondary benzylic, aliphatic, and cyclic alcohols, yielding the corresponding esters in excellent yields ranging from 78% to 99%. Notably, the catalyst could be recovered and reused for up to 10 cycles without any significant loss in its performance and magnetic susceptibility. The strong reactivity and selectivity of the developed catalyst can be attributed to the well-distributed acidic sites on the 3D PIL support, which offers accessible nano-ionic active sites. Additionally, the hydrophobic nature of the network catalysts facilitates the easy diffusion of starting materials and provides excellent water repellency, thereby enhancing the reaction yield.