Silica‐immobilized acid ionic liquid: An efficient catalyst for pentanal self‐condensation

Abstract BACKGROUND Pentanal self‐condensation to 2‐propyl‐2‐heptenal is an important process of carbon chain extension for the industrial manufacture of 2‐propylheptanol, which is an important plasticizer alcohol. The existing industrial processes all use liquid alkali catalysts, so they suffer from problems such as difficult recovery and high costs for treatment of alkali‐containing wastewater. In the present work, three immobilized acid ionic liquids were prepared separately with SiO 2 , MCM‐41, and SBA‐15 as support for pentanal self‐condensation. RESULTS The structures of the as‐prepared immobilized acid ionic liquids were confirmed with the help of FT‐IR, TG, SEM‐EDS, and N 2 adsorption–desorption techniques. The characterization and activity evaluation results indicate that there are positive correlations among the number of hydroxyl groups on the support surface, ionic liquid loading, and the catalytic activity. Among the three immobilized acid ionic liquids, silica‐immobilized acid ionic liquids (SiO 2 ‐IL‐SO 3 H) showed an extraordinary catalytic performance for pentanal self‐condensation because of its higher ionic liquid loading. The yield and selectivity of 2‐propyl‐2‐heptenal could respectively reach 73.9% and 91.5% under suitable reaction conditions and a high turnover number (TON) of 179.1 was obtained. CONCLUSION Pyridine adsorption infrared spectroscopy and density functional theory (DFT) simulation were employed to investigate the structure and acidity of SiO 2 ‐IL‐SO 3 H. The optimized structures were obtained, and the calculation results indicate that the acidity of SiO 2 ‐IL‐SO 3 H is higher than the corresponding homogeneous ionic liquid system, which may be the main reason for its excellent catalytic performance and catalytic efficiency. © 2020 Society of Chemical Industry