Toshiaki Ui

Stability of iron in the Keggin anion of heteropoly acid catalysts for selective oxidation of isobutane

The thermal stability and isobutane oxidation activity of catalysts with Fe selectively placed in the Keggin anion have been studied. For the Cs 3 H 1 PMo 11 FeO 39 salt, Fe was released from the Keggin structure above 570 K, as observed by FT-IR spectroscopy. However, in the presence of NH 4 + as counter-cation, Fe was released from the Keggin anion at 470 K, simultaneously catalysing the elimination of NH 4 + . Fe-substituted catalysts with Fe contents of 0-1, where ammonium was removed during the heat pre-treatment, showed a negative influence of Fe on the selectivity to methacrylic acid (MAA) and on the isobutane conversion. The influence of the initial position of Fe, inside or outside the Keggin anion, was studied. A catalyst in which Fe was initially as counter-cation, Cs 1.5 Fe 0.5 (NH 4 ) 2.0 PMo 12 O 40 , presented a 21% selectivity to MAA at 633 K after 20 h in operation, against a 15% selectivity of a catalyst that had a similar composition but with Fe initially inside the Keggin anion, Cs 1.5 (NH 4 ) 2.0 PMo 11.5 Fe 0.5 O 39.5 . Both catalysts showed similar isobutane conversions of ca. 8%. The catalysts underwent changes during the first few hours in a reaction that led to an increase of the selectivity to MAA in both the cases. However, the active sites derived from the lacunary species generated after release of Fe from the Keggin anion were less selective than those derived from 12-molybdophosphoric units.

Kinetics of isobutane selective oxidation over Mo-V-P-As-Cs-Cu-O heteropoly acid catalyst

Abstract Isobutane oxidation was investigated in wide ranges of experimental conditions over Mo-V-P-As-Cs-Cu-O heteropoly acid catalyst. The kinetics could be described by an empirical set of first order equations. Methacrylic acid is produced by a series of consecutive reactions via isobutylene and methacrolein. Neither methacrolein nor methacrylic acid is formed directly from isobutane. Both overoxidation of methacrylic acid and total oxidation of isobutane and intermediates are responsible for the undesired formation of acetic acid and carbonoxides. CO and CO 2 are formed via different paths from methacrylic acid, but the formation of CO is associated with the formation of acetic acid and acrylic acid. The activation energies and rate constants for each step of the reaction were determined. Reaction rates of isobutylene and methacrolein are 500 and 120 times faster than the reaction rate of isobutane. Methacrylic acid oxidation is about two times faster than isobutane oxidation. While the presence of isobutane does not influence the reactivity of methacrolein, methacrylic acid oxidation is slowed down due to inhibition of the catalyst active site by isobutane.

Enhancing the productivity of isobutane selective oxidation over a Mo-V-P-As-Cs-Cu-O heteropoly acid catalyst

Ways of enhancing the productivity of isobutane selective oxidation over a Keggin-type heteropoly acid catalyst, Mo12V0.5P1.5As0.4Cu0.3Cs1.4Ox, have been studied. Productivity increased with increasing temperature and isobutane partial pressure, although was limited by the complete consumption of oxygen and the catalyst thermal stability. Productivity was higher at short contact times. Simultaneously raising the total pressure and the space velocity enhanced the productivity up to 6.4 mmol methacrolein+methacrylic acid/h/gcat, which would be acceptable for an industrial application of the process.