Vapor phase catalysts reaction mechanism

Homogeneous catalysts. With a homogeneous catalyst, the reaction proceeds entirely in the vapor or liquid phase. The catalyst may modify the reaction mechanism by participation in the reaction but is regenerated in a subsequent step. The catalyst is then free to promote further reaction. An example of such a homogeneous catalytic reaction is the production of acetic anhydride. In the first stage of the process, acetic acid is pyrolyzed to ketene in the gas phase at TOO C ... 

In addition to production of simple monofunctional products in hydrocarbon oxidation there are many complex, multifimctional products that are produced by less weU-understood mechanisms. There are also important influences of reactor and reaction types (plug-flow or batch, back-mixed, vapor-phase, Hquid-phase, catalysts, etc). 

Since the discovery of the catalyst of Au over Ti02 support for vapor phase C3H6 epoxidation [1], great efforts have been made to understand the reaction mechanism in order to improve the catalyst performance [2,3]. Currraitly the Au catalyst suffers from low activity and fast deactivation, and is thus far from commercialization. Perhaps it is why at present no publication on the reaction kinetics can be found in the literature. 

For some applications, an adsorbent may be impregnated with a material that enhances its contaminant-removal ability. The improved effectiveness may be related to any of several mechanisms. The impregnating material may react with the vapor contaminant to form a compound or complex that remains on the adsorbent surface. Some impregnants react with the contaminant, or catalyze reactions of the contaminant with other gas constituents, to form less noxious vapor-phase substances. In some instances, the impregnant acts as a catalyst intermittently, for example, under regeneration conditions. In this case, the contaminant is adsorbed by physical adsorption and destroyed by a catalytic reaction during regeneration. 

Note that ethylbenzene is a derivative of two basic organic chemicals, ethylene and benzene. A vapor-phase method with boron trifluoride, phosphoric acid, or alumina-silica as catalysts has given away to a liquid-phase reaction with aluminum chloride at 90°C and atmospheric pressure. A new Mobil-Badger zeolite catalyst at 420°C and 175-300 psi in the gas phase may be the method of choice for future plants to avoid corrosion problems. The mechanism of the reaction involves complexation of the... 

Vapor-phase hydrogenation results and experimental evidence of this type lead to the conclusion that catalysts on basic supports are suitable for nonsplitting prehydrogenation-type reactions and that acidic supports are best used for splitting catalysts. Activated alumina was found to be the best support because of rapid reduction of tar acids. Especially, alumina precipitated from aluminum salts at constant pH was satisfactory and produced catalysts that could be formed into pellets of high mechanical strength. 

Catalytic Oxidation of Ethene to Acetaldehyde and Acetic Acid. -Evnin et al120 studied Pd-doped V2 Os catalysts for the vapor-phase oxidation of ethene to acetaldehyde in a heterogeneous type of Wacker process. From a mechanistic study they establish a redox mechanism with Pd both as the site of the ethene oxidation and of the reoxidation of the catalyst. On the basis of the role of the V4+ ions proposed by these authors, Forni and Gilardi121 substantiated this mechanism by adding tetra- and hexa-valent dopants to the V2 05 and studying the effects on the catalytic reaction. 

These isomerizations, rearrangements, and cleavages are best explained by a carbonium-ion mechanism. Vapor-phase dehydration of alcohols over aluminum oxide greatly reduces the tendency for isomerization and rearrangement. The alcohol vapors are passed over the catalyst at 300-420°. In this manner, pure 1-butene is prepared from re-butyl alcohol and t-butylethylene is obtained from methyl-/-butylcatbinol (54%). The relative rates of dehydration of the simpler alcohols over alumina have been studied. The main side reaction is dehydration to ethers (method 118). 

This volume consists of reviews devoted to a range of important subjects. Vadim Guliants and Moises Carreon (University of Cincinnati) review the selective oxidation of butane. This is an excellent example of a catalytic process designed to add value to an inexpensive raw material, and is the only vapor phase selective oxidation of an alkane that is practiced industrially. This process also avoids the use of benzene, which eliminates the risk of handling this carcinogenic compound. The authors review the synthesis, activation, and mechanism of this reaction on V-P-O catalysts. 

Catalytic reactions can be run over large, massive metal particles as well as the much smaller, dispersed metal crystallites. The massive metal catalysts can be the single crystal catalysts such as those shown in Fig. 3.2 or polycrystalline forms of bulk metal such as wires, foils or ribbons. These latter materials were used somewhat routinely in the early catalytic research efforts that were involved with developing the mechanisms of vapor phase catalytic processes. These materials were considered to be analogs of the supported catalysts in which the effect of the support, if any, was eliminated. 

The kinetics of the heterogeneously catalysed vapor-phase oxidation of a,p-unsaturated aldehydes to the a,p-unsaturated acid has been investigated for a Mo-V-Cu-oxid catalyst. The reaction rates of aldehyde and oxygen consumption as function of the aldehyd, oxygen, acid and water partial pressure are described by a kinetic model based on a modified Mars-van Krevelen mechanism Also the rate of the a, p-unsaturated acid oxidation has been measured and described for varied partial pressures of acid, oxygen and water. 

Copper-containing mordenite catalysts have also been reported to be active for carbonylation of vapor-phase methanol [170]. Initially, the predominant reaction products were hydrocarbons resulting from methanol-to-gasoline chemistry, but after about 6 h on stream at 350 °C the selectivity of the catalyst changed to give acetic acid as the main product. A recent investigation was carried out with in situ IR and solid-state NMR spectroscopies to probe the mechanism by detecting surface-bound species. The rate of carbonylation was found to be enhanced by the presence of copper sites (compared to the metal-free system), and formation of methyl acetate was favored by preferential adsorption of CO and dimethyl ether on copper sites [171],... 

The fact that Weiss and Downs have been aide to isolate phenol in the products of their reactions with solid catalysts indicates a hydroxylation mechanism similar to that postulated in the case of vapor phase catalysis, in whidi the formation of the monohydroxylated derivative is the first step. The presence of the hydroxyl group as a substituent in the benzene molecule activates the para and ortho positions so that the introduction of a second oxygen molecule would be expected to result in the formation of quinol (C6H4(OH)2l 4) and catechol (C0H4(OH)21 2) with a preponderance of the former. Quinone which would result from the further oxidation of quinol has been found in the oxidation products from benzene for the case of the homogeneous catalytic reaction. 

Indeed, it has been found 70 that under the conditions of the experiments, the proportion of quinone to maleic acid remains fixed and is not materially altered by introduction of quinone with benzene. However, the proportion of quinone is never large in tire case where solid catalysts are used. The mechanism of the further oxidation of quinone to maleic anhydride is somewhat speculative since the isolation of any of the intermediate compounds in this step has not been reported in the vapor phase oxidation experiments. However, the formation of a poly-ketone by the following reactions seems a possibility since it may be assumed that a continuation of the hydroxylation process followed by the molecular rearrangement, would be expected. 

The catalytic fluorodecarboxylation of arylchloroformate to fluorobenzene and analogues has been achieved with high yield in an anhydrous hydrogen fluoride vapor phase flow reactor. This methodology can be successfully applied to various derivates, the main limitation being the stability of substituents under the reaction conditions. The best catalysts are chromium and aluminium oxyfluoride. The reaction proceeds between 300 and 400°C and occurs in a short space of time. The catalytic activity decreases by coking but can be fully recovered by an oxydative treatment at high temperature. An ionic mechanism is proposed. 

Although many solid-acid catalysts have been reported for the vapor-phase Beckmann rearrangement [2], their performance has been less than satisfactory from an industrial standpoint and the heterogeneously catalyzed Beckmann rearrangement has not yet been commercialized. In this chapter heterogeneous catalysis of the Beckmann rearrangement, its mechanism, and acid properties and reaction conditions suitable for the reaction will be reviewed. 

This rearrangement is well known in liquid phase and numerous mechanism studies of this [3,3] sigmatropic reaction on vanadates O = V(OR)3 appeared in the litterature [3,12]. But we found only four publications in heterogeneous vapor phase on that isomerisation, people from PUBLICKER [13] using phosphates catalysts obtained essentially methyl-2 butene-2 yne-3 3 (Scheme 2) like BERGMANN [9]. 

The vapor-phase ammonolysis of alcohol is much more complicated, because of side reactions. Although much information is available concerning operating conditions, effect of flow rates, and the effects of catalysts, there is little to be found in the literature on actual mechanism. 

Mechanisms of Vapor-phase Alkylations of Hydrocarbons. Paraffins can be alkylated in the absence of catalysts at sufficiently high temperatures, about 500°C, so that a small amount of the paraffins will decompose into free radicals. A free-radical mechanism for the alkylation seems probable, as is shown below for the reaction between propane and ethylene ... 

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