Conversion of Methyl Radicals to Methanol and Formaldehyde over Molybdenum Oxide Catalysts

Abstract

Methyl radicals, formed by the thermal decomposition of azomethane, were allowed to react over both MoO[sub 3] and MoO[sub 3]/SiO[sub 2] catalysts. In the continuous flow mode, the dominant reaction was the homogeneous coupling of CH[sub 3] radicals to form C[sub 2]H[sub 6]; however, significant amounts of HCHO and CH[sub 3]OH were also observed. Over MoO[sub 3] at 300-[degree]C, CH[sub 3]OH was the principal oxygenate, but at 500[degree]C the CH[sub 3]OH was converted mainly to HCHO. At both temperatures, HCHO was the favored product over MoO[sub 3]/SiO[sub 2]. The addition of H[sub 2]O to the reactants enhanced the formation of CH[sub 3]OH at 300[degree]C. During temperature-programmed reaction studies, Ch[sub 3]-radicals were first allowed to react with MoO[sub 3]/SiO[sub 2] at 150[degree]C. Methanol began to appear in the gas phase at about 170[degree]C and was the main product up to 400[degree]C. As a result of adding H[sub 2]O continuously during the TPR experiment, CH[sub 3]OH was produced at 80[degree]C, and the integral amount of CH[sub 3]OH formed over the temperature range from 80 to 500[degree]C increased. These results are consistent with the view that CH[sub 3]-radicals react with MoO[sub 3] to form surface methoxide ions. The methoxide ions may either decomposemore » to form HCHO or they may react with surface protons or water to form CH[sub 3]OH. 24 refs., 3 figs., 4 tabs.« less