Complex metal oxide catalysts for selective oxidation of propane and derivatives: II. The relationship among catalyst preparation, structure and catalytic properties

Abstract

The effects of the preparation methods on the structure and catalytic performance of the MVTeNb (M=Mo or W) complex metal oxides (CMO) in selective oxidation of propane to acrylic acid (AA) were investigated. Preparation methods can play as significant roles as the chemical composition in determining the structures and catalytic properties of a CMO catalyst. Undesirable preparation methods or conditions can lead to the formation of ineffective crystal phases in the resulting catalysts, leading to poor catalytic activity or selectivity. An effective MoVTeNb oxide catalyst for propane selective oxidation to acrylic acid could be obtained with a combination of a proper metal ratio and proper preparation procedures. Certain drying methods, such as freeze dry and heat evaporation, are undesirable because phase segregation tends to occur during such processes, which negatively affect the catalytic performance. The preferred drying method is the rotavap method, which favors the formation of an effective crystal phase and suppresses the formation of impurity phases. The preferred calcination atmosphere is an inert atmosphere while a calcination atmosphere containing oxygen leads to the formation of significant amounts of MO 3 in MVTeNb oxides (M=Mo or W), which is known to be inactive in propane oxidation. It is proposed that an effective MoVTeNb oxide catalyst for propane selective oxidation to acrylic acid should contain at least two major crystal Phases A and B. Based on studies of oxides of pure Phase A and those exclusively enriched with Phase B, X-ray diffraction (XRD) characteristics of the two major crystal phases (A and B) have been identified and their catalytic functions studied. It is proposed that Phase A is active in propane activation but relatively unselective for the acrylic acid formation, while Phase B is reasonably active for propane activation and fairly selective for acrylic acid formation. It has been shown that using MoVTeNb oxide catalyst with a proper ratio of Phases A and B and synergy between them, very high propane conversion (73%) and acrylic acid selectivity (58%) can be achieved at the same time.