Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects.

Area of Science:

• Heterogeneous Catalysis
• Materials Science
• Inorganic Chemistry

Background:

• Keggin-type phosphomolybdic acid (PMo₁₂O₄₀) derivatives show promise in gas-phase oxidation catalysis.
• The precise origin of catalytic activity in modified polyoxometalates (POMs) like PyPMo-HT requires detailed investigation.
• Understanding structure-activity relationships is crucial for optimizing POM-based industrial catalysts.

Purpose of the Study:

• To elucidate the crystal structure of pyridine-treated phosphomolybdic acid (PyPMo-HT) after heat treatment.
• To identify the catalytically active species responsible for the observed oxidation activity.
• To correlate the structural transformation with enhanced catalytic performance in selective oxidation reactions.

Main Methods:

• Synthesis of PyPMo-HT via heat treatment of phosphomolybdic acid with pyridine under inert gas flow.
• Crystal structure determination of PyPMo-HT.
• Investigation of reaction mechanisms involving molecular oxygen and catalytic oxidation of propylene and methacrolein.

Main Results:

• PyPMo-HT forms a one-dimensional array of Keggin units and pyridinium cations with a specific HPy/Keggin ratio.
• Oxygen removal during transformation leads to localized electron density on Mo atoms, facilitating electron transfer.
• Electrophilic oxygen species formed bridge Keggin units, acting as the active sites for selective oxidation, yielding high methacrylic acid.

Conclusions:

• The structural modification of phosphomolybdic acid by pyridine treatment generates active electrophilic oxygen species.
• PyPMo-HT demonstrates superior catalytic activity for methacrolein oxidation compared to the parent compound.
• This study deepens the understanding of polyoxometalate-based catalysts for industrial oxidation processes.