In Situ Diffuse Reflectance Infrared Fourier Transform Study Of Propylene (amm) Oxidation

Selective Oxidation of hydrocarbon is one of the most important reaction in industry. The target product usually is oxidation intermediates in this kind of reaction, and excess side reactions make it difficult to purify and separate the product. Especially, deep oxidation will bring environment pollution and waste raw materials. So there is lots of industrial significance and economy profit in the research of this system. In recent years, more and more investigation focus on the in-situ DRIFT technique to study surface reaction mechanism, and here this technique is applied to probe the mechanism of propylene ammoxidation.At first, some parameters such as the resolution, scanning time, catalyst particle size and gas velocity are fixed at best values, and then it is systemically investigated for the propylene selective oxidation over multi-component bismuth molybdate catalyst, including surface reaction and gas reaction behavior. The selective oxidation of propylene over catalyst surface effectively controls deep oxidation products of propylene such as carbon dioxide and carbon monoxide. The centers of active sites of selective oxidation are surface lattice oxygens. The nature of adsorbed intermediates detected by IR are:( i )a Ï€-allylic intermediate;(ii )an enolic type compound formed in the presence of Bronsted acid sites;(iii)a Ï€-boned propylene species interacting with Lewis acid sites.During propylene ammoxidation, the ammonia molecules can be bound to the oxide surface by either( i )a hydrogen-bond, NH …… O^2-, formed by ammonia atom with a surface oxygen anion or with the oxygen of a surface hydroxyl group;(ii )a hydrogen-bond between the nitrogen atom and a surface hydroxyl atom;(iii)a coordination bond, formed by ammonia atom with a surface cation (Lewis acid site). Moreover, the complete proton transfer can occur to produce an NH₄⁺ ion, or alternatively ammonia adsorption-decomposition can also take place to form the surface NH₂ and OH groups. The coadsorption of other molecules willchange the nature of ammonia chemisorbed species .The optimum temperature for propylene ammoxidation will be 400 °C that favors the detection of the intermediates. The results show that acrolein appears at the beginning of the reaction, but acrylonitrile appears earlier when the temperature is higher which indicates that the formation of acrylonitrile takes some time.