Cerium-Modified Phosphomolybdenum Heteropoly Acid for Selective Oxidation of Methylacrolein to Methacrylic Acid

Abstract

Selective oxidation of methylacrolein (MAL) to methacrylic acid (MAA) is a crucial step in the production of methyl methacrylate from isobutene. Herein, a kind of NH₄⁺-containing Keggin-type phosphomolybdenum heteropoly acid was modified by Ce⁴⁺ as a countercation through a one-pot method for selective oxidation of MAL to MAA. Multiple characterizations including FT-IR, XRD, physical N₂ adsorption and desorption, H₂-TPR, NH₃-TPD, O₂-TPD-MS, and XPS were employed for systematical analysis of physicochemical properties. The effects of cerium content and calcination temperature on the catalytic performance of the as-prepared phosphomolybdenum heteropoly acid were demonstrated in combination with the comprehensive characterization results. It was found that the cerium was doped to replace the H⁺ in phosphomolybdenum heteropoly acid, leading to the modulation of acid properties (both Brønsted and Lewis acid site densities) and redox behavior of molybdenum species. Specifically, the MAA yield and selectivity showed a linear relationship with the reduction capability of Mo⁶⁺ to Mo⁵⁺ and strong acid site density, respectively. As a result, the MAA yield could reach up to 50% with MAA selectivity of 61% on the modified catalyst, which was enhanced by 14% and 15%, respectively, compared with the pristine one.