p-Sulfonic acid calix[4]arene-functionalized alkyl-bridged organosilica in esterification reactions

Two new p-sulfonic acid calix[4]arene-and p-sulfonic acid calix[6]arene-functionalized organosilica have been synthesized using a sol-gel method and applied as heterogeneous catalysts in esterification reactions. The catalytic performance was evaluated using the esterification of carboxylic acids with ethanol, and good catalytic activity (i.e., 55-88%) was observed under the optimum reaction conditions. This study reports the first promising example of the successful employment of calix[n]arenes as a heterogeneous catalyst for catalytic esterification. The catalyst was easily separated by filtration and reused five times without any significant loss of activity.