Dendritic polyamide-amine modified phosphomolybdovanadic hybrid microspheres as a catalyst for methacrolein to methacrylic acid

Abstract

The selective oxidation of methacrolein (MAL) to high value-added methacrylic acid (MAA) using green chemical technology is of high research value. However, the process remains challenging owing to the few redox active sites and weak acid sites on the surface of the heteropolyacid catalyst, as well as the high CO bond dissociation energy of MAL. In this study, an organic amine-modified heteropolyacid, dendritic polyamide-amine–CsH₃PMo₁₁VO₄₀ (PAMAM–CsPAV), was synthesized by a facile co-precipitation method, and was applied as an efficient catalyst for the MAL–MAA oxidation system. Preliminary phosphomolybdovanadic acid (HPAV) anchored with PAMAM through proton bonding and a hydrogen bonding network to generate mixed amino (protonated and free amino groups) modified dendritic HPAV (dendri-PAV) precursors. The PAMAM–CsPAV formed by calcination benefited from the positive “dendritic effect” of PAMAM, which induced the assembly of the compacted CsPAV precursor into uniformly dispersed nano–microspheres. In particular, the PAMAM–CsPAV, which was coordinated by mixed amino groups and strong hydrogen bond networks, promoted proton hydrogen-driven aldehyde double bond cleavage through achieving weak acid center elimination for the first time. Furthermore, more redox active sites (VO²⁺) were formed and migrated to the secondary structure to accelerate electron-driven O₂ reduction and lattice oxygen-driven carboxylic acid group generation. Therefore, the selectivity to MAA (97%) of PAMAM–CsPAV was significantly higher than that of CsPAV (74%) at the same MAL conversion (60%). In addition, PAMAM–CsPAV also exhibited stable catalytic activity in the long-term evaluation test.