Ammoxidation phase composition

Silica supported bismuth molybdate catalysts were examined by Dalin et al. (85) during extended steady-state ammoxidation of propylene. X-Ray diffraction measurements indicated that a significant alteration in the phase composition of supported catalysts occurred during operation. The a and /3 phases of bismuth molybdate, which were originally present, transformed into the y phase of bismuth molybdate as described by the equation... 

It is well known that the catalysts used for oxidation reactions such as those of propylene to acrolein, isobutene to methacrolein, or for ammoxidations (propylene to acrylonitrile, methyl-substituted benzenic rings to the corresponding aromatic nitriles) contain many components. This complexity in elemental composition is reflected by a complexity in phase composition. 

This article therefore seeks to examine in depth just one mixed oxide catalyst, tin-antimony oxide, which has been commercially developed (2-5) for the oxidation of propylene to acrolein as well as for the ammoxidation of propylene to acrylonitrile and the oxidative dehydrogenation of butenes to 1,3-butadiene. A recent book (6) and a subsequent review (7) have shown how little unanimity has been established about the fundamental properties of the material. In particular there seems to be much confusion as to the phase composition, the nature of the cationic oxidation states, the chemical environment of the cations, the charge compensation mechanism, the nature of the active sites, the distortion of the host tin(IV) oxide lattice by the dopant antimony atoms and whether any changes in the catalyst result from the adsorption and catalytic processes. 

An in-depth analysis of the solid-state chemistry of the Mo-V-Te-Nb-0 propane ammoxidation catalyst system reveals the details of the two primary phases designated as Ml and M2 (150,151). Correlations of catalytic activity and phase composition for this catalyst system establish the specific functions of the two catalytically active phases (152,153). Specifically, the Ml phase is the phase primarily responsible for propane activation and conversion to acrylonitrile via intermediate, adsorbed propylene. The M2 phase is essentially inactive for propane activation but is capable for conversion desorbed propylene intermediate to acrylonitrile. 

Aouine, M., Dubois, J.L., and MUlet, J.M.M. Crystal chemistry and phase composition of the MoVTeNbO catalysts for the ammoxidation of propane. Client. Commun. 2001,(13), 1180-1181. 

The fresh catalysts used for the ammoxidation of xylenes consisted essentially of V205, SbV04, and %-phase for a vanadium catalyst respectively. X-ray diffraction patterns for the catalysts showed that a vanadium catalyst consisted essentially of V2O4, while mixed catalysts retained the original composition even after prolonged use. 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

At one time the preferred catalyst for propylene ammoxidation was a uranium-antimony oxide composition whose active phase was USb3O2 Q. We have found that the partial substitution of certain tetravalent metals for the pentavalent antimony in this phase greatly increases catalytic activity. 

The catalytic activity of the uranium-antimony oxide catalyst for propylene ammoxidation has been increased an order of magnitude by modifying the catalytically active phase rather than by adding various promoters to the optimum uranium-antimony oxide composition. This modification was accomplished by substituting titanium, zirconium, or tin for antimony in compositions with the empirical formula USb3 M Oy. Titanium and zirconium replaced... 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

In the Al-Sb-V-W-0 system a mtile-type phase with the approximate composition Alo,iSbo.8Vo,7Wo.404 is active for propane ammoxidation. It is predominantly formed in situ during the ammoxidation. Compared with the mtiles in the other two systems, the Al-Sb-V-W-oxide has a lower Sb V ratio. 

The aim of the present study was to prepare mixed Mo-V-Nb-Te oxides using a surfactant route, and to check in which extend modification of the composition and of the atmosphere in which these oxides are activated may lead to a phase corresponding to active catalysts for ammoxidation of propane. 

The most effective, from the listed catalysts till date, for propane oxidation to acrylic acid is Mo-V-Te-Nb mixed oxides, patented by Ushikubo et al. [56,57] and Lin and Linsen [59], which give more than 40% yield of acrylic acid. The catalyst with the same elemental composition appears to be very active and selective for propane ammoxidation reaction (58% yield of acrylonitrile at 89% propane conversion) [68]. This indicates that propane oxidation and propane ammoxidation share some fundamental reaction steps and active crystalline phases. 

Moreover, elTorts were also made on producing 3CP and niacin/niacina-mide directly from 3P using Re-Bi catalysts by liquid-phase ammoxidation [e.g., 81]. Eventhough, these compositions based on Re-Sb (e.g., Re Sb ) or Re-Bi (RCjBij) are rare and new, they gave relatively poor performance compared to the state-of-the-art, that is, low conversion of 3P (40-60%) and low selectivities (-75%) and niacin/niacinamide (<15%). 

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