Processing Study on the Stability of Heteropoly Acid Catalyst in the Oxidation of Methacrolein to Methacrylic Acid

The oxidation of methacrolein to methacrylic acid over a heteropoly acid catalyst was studied in a pilot scale unit with tubular multisampling reactor. The catalyst performance was monitored at different aging phases during a total runtime of 195 days. Deactivation was accompanied by a loss of catalyst mass through evaporation of molybdenum oxide. With time on stream, deactivation expands from an area close to the inlet of the catalyst bed downstream to the outlet. Several causes for the deactivation were evaluated in processing experiments. Possible detrimental effects of acetic acid and carbon monoxide were excluded by cofeeding these byproducts in high concentrations. A critical parameter is the oxygen concentration because reoxidation of the catalysts is rate limiting. Low oxygen supply results in a depletion of lattice oxygen accompanied by a marked activity loss. It is suggested that continued preservation of the catalyst in the reduced state promotes structural disintegration.