Polydopamine modified heteropolyacids catalyst for methacrolein oxidation: Organic ammonia functionalized and regulation of morphology

The oxidation of methacrolein (MAL) to methacrylic acid (MAA) catalyzed by Keggin-type phosphomolybdovanadic acid catalyst belongs to the acid-catalyzed and oxygen-catalyzed co-promoted reactions. It is a challenge to design catalysts with suitable acidity and redox properties. In this paper, a strategy using organic ammonia to modify CsH3PMo11VO40 (CsPAV) was proposed. Core-shell polymerization precursors were synthesized by using polydopamine (PDA) microspheres as modifiers and templates, and regularly arranged PDA-CsPAV bowl-shaped catalysts were obtained via one-step calcination. The morphology, structure, thermal stability, acidity and redox properties of PDA-CsPAV catalysts were characterized by XRD, FT-IR, TG-DTA, XPS, SEM, H2-TPR and NH3-TPD, and the catalytic performance was evaluated by oxidation of MAL to MAA. The rich functional groups on the surface of PDA microspheres combine with H4PMo11VO40 (HPAV) through strong coordination bonds to form a catalyst film. The uniform spherical structure of PDA can adjust the dispersion and morphology of CsPAV. NH4+ was formed and rearranged with heteropoly anions during calcination, which positively regulated the acidity and redox property of the catalyst. The addition of PDA accelerated electron transfer and strengthened the self-reduction process of HPAV. Compared with CsPAV, PDA-CsPAV has more acid sites and oxidation catalytic active centers (VO2+), thus exhibiting better MAL oxidation performance and good thermal stability.