Synergistic Effects of Superbasic Catalysts on the Selective Formation of Acrylonitrile via Oxidative Methylation of Acetonitrile with Methane

Abstract

The oxidative methylation of acetonitrile with methane to acrylonitrile occurs more actively and selectively over bialkali promoted CaO catalysts than over any monoalkali promoted system. The most effective catalytic systems are obtained with Li A + Cs A, Na A + Cs A, or K A + Cs A (A = SO 2− 4, OH −, C1 −, CH 3COO −, CO 2− 3, or NO − 3) supported on CaO, containing total alkali loadings of 10 mol% with equal molar amounts of both alkalis. At 750°C, under atmospheric pressure, at CH 4 : O 2 : CH 3CN : He partial pressure ratios of 5.0 : 1.0 : 1.5 : 6.5, and at a space velocity of 15,000 cm 3 g −1 V −1, the highest selectivity to acrylonitrile (70.0 mol%) and yield (25%) are obtained over (5 mol% Na + + 5 mol% Cs +)/ CaO (prepared from the sulfate precursors). Any bialkali-promoted system containing Rb was less effective, whereas the Li-containing systems, though active initially, gradually lost the activity due to its volatility. The performances of the effective bialkali systems, after an initial increase, remained almost unchanged for a period of 60 h. In contrast, the stability of any monoalkali promoted system with time-on-stream was very low and the maximum initial yield of acrylonitrile was only 11.5% under the aforementioned conditions. The synergistic increase in the catalytic performance of the bialkali promoted CaO is reflected in the synergistic increase of the surface basicity (leading to superbasicity) caused by the high enrichment of the surface layer with the alkali ions. The relationship between the catalytic performances and the physicochemical characteristics of the catalysts revealed by XPS, AAS, and basicity measurements is explored.