PULSATION EFFECTS OF WATER VAPOR SUPPLY UPON CATALYTIC PARTIAL OXIDATION OF METHANE OVER SILICA-SUPPORTED SIILICOMOLYBDIC ACID CATALYSTS

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Silicomolybclic acid (SMA) supported on silica showed excellent activity for partial oxidation of methane directly into formaldehyde in the presence of excess water vapor at 873 K. SMA is, however, ready to be decomposed into silica and molybdenum oxide when heated at the temperatures higher than 773 K according to the following equation ; H4SiMo12O40=SiO2+12MoO3+2H2O. The role of excess water vapor in feed is to enhance the reverse reaction in order to regenerate SMA on silica surface during the reaction. Thus, the catalytic active component, SMA, is present on silica surface with the fragile balance between decomposition and regeneration, and hence the reproduction of experimental data is not easy. At the moment, we do not establish the key-techniques to well reprpduce the excellent catalytic activity, previously obtained Following terms were studied;1)Study on the pulsation effects of water vapor supply upon catalytic performances : Since the reaction starts with activation of metha … More ne by H^+ adhered on SMA, it is reasonable to consider that the continuous feed of water vapor over the catalyst will depress the interaction between H^+ and methane and result in the loss of catalytic activity. Our results in this work showed that the pulsation of water vapor in the manner of "60 second ON and 15 second OFF of feed" gave us the maximum conversion of methane (25%). The selectivity of formaldehyde was, however, as small as 10% and the main products were CO and CO2. Since it was well known that the product CO, in the partial oxidation of methane, came from the decomposition of formaldehyde, we have to develop the technique to suppress the decomposition of formaldehyde, yielded during methane partial oxidation.2)Study on the loss of Mo in the catalysts during reaction : Because of the sublimation of MoO3, produced by the decomposition of SMA, during reaction at 873 K, the loss of Mo in the catalysts is inevitable to some extent. The amount of Mo loss during reaction strongly depends upon the rate of regeneration of SMA from MoO3,SiO2 and water vapor. For SMA catalysts showing excellent activity the Mo loss during reaction was small; 40% loss for 350 hour duration test. It is found that we have to design the pore structures of silica support in order to improve the Mo loss during reactions. Less