Self-assembled tetramethyl cucurbit[6]uril–polyoxometalate nanocubes as efficient and recyclable catalysts for the preparation of propyl gallate

Abstract

The development of cucurbit[n]urils–polyoxometalate (Q[n]–POM) hybrids with the same microshape and nanoscale features is highly desirable, yet remains a great challenge. Herein, we design and synthesize a class of Q[n]–POM hybrids and tetramethyl cucurbit[6]uril–phosphomolybdic acid (TMeQ[6]–PMA) nanocubes (NCs) via a facile one-step self-assembly method, as heterogeneous acid catalysts for greatly boosting the catalysts in term of activity and stability for the esterification of gallic acid and n-propanol to propyl gallate (PG). The Fourier transform infrared (FTIR) spectroscopy reveals that the self-assembled mechanism of TMeQ[6]–PMA NCs based on the outer-surface interaction of Q[n]s. The temperature programmed desorption experiments with ammonia and FTIR analysis of the pre-adsorbed pyridine results confirm that the coexistence of medium and strong acid sites and a larger number of Lewis acid sites other than Brønsted acidic sites on the catalyst surface. These new features make the as-prepared TMeQ[6]–PMA NCs exhibit a high PG conversion (95.6%) and excellent stability, which represents a better catalytic performance than other reported catalysts.