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

在873 K条件下，二氧化硅负载的硅钼酸（SMA）对甲烷直接部分氧化成甲醛表现出优异的活性。然而，当温度高于773 K时，根据下式，SMA可以分解为二氧化硅和氧化钼；H4SiMo12O40 =二氧化硅+ 12 moo3 + 2水。在反应过程中，进料中过量水蒸气的作用是促进逆反应，使二氧化硅表面的SMA再生。因此，催化活性组分SMA存在于二氧化硅表面，在分解和再生之间保持着脆弱的平衡，因此实验数据的再现并不容易。目前，我们还没有建立很好地再现优异催化活性的关键技术，之前得到的研究如下：1)水蒸气供给对催化性能脉动效应的研究：由于反应是从甲烷活化开始的，更多的是吸附在SMA上的H^+，因此有理由认为，在催化剂上持续的水蒸气供给会抑制H^+与甲烷的相互作用，导致催化活性的损失。我们在这项工作中的结果表明，水蒸气以“60秒开，15秒关饲料”的方式脉动，使我们的甲烷转化率最高（25%）。甲醛的选择性仅为10%，主要产物为CO和CO2。众所周知，甲烷部分氧化的产物CO来自于甲醛的分解，因此必须开发抑制甲烷部分氧化过程中甲醛分解的技术。2)反应过程中催化剂中Mo的损失研究：在873 K的反应过程中，由于SMA分解产生的MoO3的升华，催化剂中Mo的损失在一定程度上是不可避免的。反应过程中Mo的损失量很大程度上取决于SMA从MoO3、SiO2和水蒸气中再生的速度。活性优异的SMA催化剂反应过程中Mo损失较小；350小时试验损耗40%。研究发现，为了改善反应过程中Mo的损失，必须对二氧化硅载体的孔结构进行设计。少

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