Sulfonic acid-functionalized MIL-101 as a highly recyclable catalyst for esterification

The sulfoxy or sulfonic acid-functionalized MIL-101 catalyst was prepared by either post-functionalization or pre-modification of the organic linkers with the functionalized ligands, respectively, the latter is the so-called one-pot synthesis method. These catalysts were systemically characterized by XRD, N-2 adsorption, acid-base titration, and FT-IR techniques and applied in the liquid-phase esterification of monocarboxylic acids with monohydric alcohols. The results show that the sulfonic acid-functionalized MIL-101 (S-MIL-101) prepared by the one-pot synthesis method possesses a higher catalytic activity in the esterification than its counterpart the sulfoxy acid-functionalized MIL-101 (S/MIL-101) prepared by the post-functionalized method, due to the fact that S-MIL101 has a higher acid loading and a high utilization efficiency of the functionalized acid sites. Moreover, S-MIL-101 is stable to leaching, behaves as a true heterogeneous catalyst, can be easily recovered by filtration, and can be reused five times in succession without any loss of its catalytic activity.