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

在873 K下，以二氧化硅为载体的硅钼酸（SMA）对甲烷直接部分氧化为甲醛具有良好的催化活性。然而，当在高于773 K的温度下加热时，SMA容易分解成二氧化硅和氧化钼，根据以下等式：H4 SiMo 12 O 40 =SiO2+ 12 MoO 3 + 2 H2O。进料中过量的水蒸气的作用是促进逆反应，以便在反应期间在二氧化硅表面上再生SMA。因此，催化活性组分SMA存在于二氧化硅表面上，分解和再生之间的平衡脆弱，因此实验数据的再现不容易。目前，我们还没有建立起充分发挥催化剂优良活性的关键技术，我们研究了以下几个方面：1）水蒸气供应对催化剂性能的脉动影响：由于反应是从甲烷活化开始的，因此，水蒸气供应的脉动对催化剂性能的影响很小，而水蒸气供应的脉动对催化剂性能的影响很小，因此，水蒸气供应的脉动对催化剂性能的影响很小，水蒸气 ...更多信息 因此，有理由认为水蒸气的连续通入会抑制H^+与甲烷的相互作用，从而导致催化剂活性的降低。我们在这项工作中的结果表明，以“60秒开和15秒关进料”的方式的水蒸气脉动使我们获得最大的甲烷转化率（25%）。然而，甲醛的选择性小至10%，并且主要产物是CO和CO2。众所周知，甲烷部分氧化反应中的产物CO来自甲醛的分解，因此我们必须开发抑制甲烷部分氧化反应中产生的甲醛分解的技术。2）反应过程中催化剂中Mo损失的研究：在873 K时，由于SMA分解生成的MoO 3在反应过程中发生升华，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的损失量主要取决于SMA从MoO 3、SiO2和水蒸气中再生的速率。对于显示出优异活性的SMA催化剂，反应期间的Mo损失小; 350小时持续时间测试的损失为40%。研究发现，我们必须设计二氧化硅载体的孔结构，以改善反应过程中的钼损失。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