Design And In-situ FTIR Studies Of Catalysts For Propane Ammoxidation To Acrylonitrile

In this paper, a series of vanadium-antimonate catalysts for propane ammoxidation to acrylonitrilc (ACN) were designed and prepared based on the suppositional reaction mechanism by guide of the adaptability theory of catalyst surface group-structure. The catalysts were tested for performance, and some of them gave good activities and ACN selectivities. From the characterization of surface area test, XRD analysis, TEM analysis and in-situ FTIR technique, the active phase and surface reaction mechanism were obtained. Although the process of propane ammoxidation to acrylonitrile is new, attention has been paid greatly to selective oxidation of paraffins, especially with low carbon numbers. In the past decade, a series of catalysts for the process were developed, but their activities and ACN selectivities were too low because of the absence of effective catalyst design method or the use of inadaptable reaction mechnism in the catalyst design. In this work, the adaptability theory of catalyst surface group-structure and in-situ technique were introduced to explain the important influences of surface species on the active phase before or after activation on the surface reaction mechanism, and activities or ACN selectivities furtherly.At first, the suppositional mechanism for propane ammoxidation was suggested based on the adaptability theory of catalyst surface group-structure. It showed that the catalysts for design with good ACN yield should possess the adaptable surface groups constituted with dehydrogenation centers, oxygen-supplying centers and cyaniding centers with adaptable activity and number corresponding to the required mechanism of reaction. According to the adaptability theory, the rate control step in the ammoxidation mechanism is dehydrogenation and oxygen-supplying, so the adaptability of these surface group-structure is more important. Propane dehydrogenates difficultly, propylene, acyclic propylene and acrolein do easily, and oxygen supplies mildly, therefore V₂O₃, Sb₂O₃, V₂O₅, Sb₂O₅ were chosen as the main constituents of catalysts for design which formed from excessive Sb₂O₃ reacting with NH₄VO₃. After addition of auxiliary catalyts such as W, K, Ce, P, Sn, Fe, Co, Ni, Cr, Ag and catalyst carriers such as SiO₂ and Al₂O₃, the numbers, activity and relative ratio of dehydrogenation centers and oxygen-supplying centers were adjusted to adaptation. Finally, a series of vanadium-antimonate catalysts were prepared and tested for their performance and the influences of reaction temperature, contact time, type of carriers and activation conditions on the catalytic performance were studied. The results showed the good fitness between theoretical catalyst design and the experiment, especially for PAC-34 catalyst which gave the best propane conversion of 63.7%and ACN selectivity of 43.7%. In general, these values are still not high enough from industrial point of view.Secondly, the catalysts were characterized by surface area test, XRD analysis and TEM analysis. Results of surface area test showed that the surface area of the catalysts was larger than 80m²/g. XRD analysis showed that there was active phase Sb₂O₄ + VSbO₄ in the catalyst. TEM analysis...