Selective Oxidation of Methacrolein to Methacrylic Acid over Cs-Doped Sb 0.1 NH ₄ H 2.5 PMo ₁₁ VO ₄₀ Heteropolyacid

Selective oxidation of methacrolein (MAL) to methacrylic acid (MAA) is a key step in the industrial production of methyl methacrylate (MMA). In this study, a series of Cs-modified Sb-containing Keggin-type ammonium phosphomolybdovanadate heteropolyacid catalysts (Cs x Sb 0.1 NH 4 HPAV) were systematically investigated for MAL selective oxidation. Comprehensive physicochemical characterizations including physical N 2 adsorption–desorption isotherms, XRD, FT-IR, Raman spectroscopy, NH 3 -TPD, H 2 -TPR, XPS, SEM and in-situ DRIFTS were conducted to elucidate the structural and functional impact of Cs incorporation. The results demonstrated that Cs was progressively introduced into the secondary structure through the partial substitution of H + and NH 4 + species, while preserving the primary Keggin framework. Cs addition significantly decreased the total Brønsted acid site density, enhanced lattice oxygen content, and modulated the Mo 6+ /Mo 5+ redox equilibrium. A positive correlation was observed between the MAL conversion and lattice oxygen content, whereas MAA selectivity was governed by the balance between the reversible redox capacity and moderated acidity. In-situ DRIFTS results further revealed that the Cs incorporation accelerated MAL adsorption and promoted the formation of MAA-related surface species during selective oxidation. The optimal catalyst with Cs/P = 0.65 achieved the highest MAA selectivity of 68.8% at a MAL conversion of 71.5% in the absence of water. These findings highlight the critical role of acid–redox synergy regulated by Cs doping in improving selective oxidation performance.