Gas-phase catalysts

Heterogeneous reactions involve two or more phases. Examples are gas-liquid reactions, solid catalyst-gas phase reactions and products, and reactions between two immiscible liquids. Catalytic reactions as illustrated in Chapter 1 involve a component or species that participates in various elementary reaction steps, but does not appear in the overall reaction. In heterogeneous systems, mass is transferred across the phase. 

Table 5.12 Heat-transfer coefficient catalyst-gas phase.

The chlorination of alkyl aromatics by sulfuryl chloride promoted by free-radical initiators, which was originally discovered by Kharasch and Brown990, can be modified by incorporation of transition metal complexes. Matsumoto and coworkers have observed that, upon addition of Pd(PPh3)4, in place of a radical initiator, the side-chain monochlorination of toluene is substantially more selective991. Davis and his colleagues992 have extended this study and report that Pt(0) and Pd(0) are effective initiators for side-chain chlorination of toluene by sulfuryl chloride and dichlorine. Mn, Re, Mo and Fe complexes, on the other hand, behave more like Friedel-Crafts catalysts. Gas-phase chlorination of olefins to allyl chlorides is catalyzed by PdCl2 or by PtCl2993. 

The function/(c<) will in general contain the concentration of products as well as species introduced by the experimenter. Its functional form will be determined by the catalyst-gas phase interaction. 

Trichloroacetal-dehyde /H2O2/HF Trifluoro- acetaldehyde (hemiacetal) Cr catalyst Gas phase 31... 

When using conventional homogeneous Lewis or Br0nsted acidic catalysts only liquid-phase reactions are applicable. With heterogeneous catalysts gas-phase reactions, which are readily performed continuously, can also be realized. The product is readily separated from the catalyst and higher efficiency is usually achieved (space-time yield). The rearrangement of styrene oxides in the gas phase described later in this section [8,15,16] is an example of the improvement of yields by changing the reactor concept from liquid- to gas-phase. 

Here, we compare three different catalytic systems used to carry out the catalytic oxidation of ammonia gas-phase oxidation over transition-metal heterogeneous catalysts, gas-phase oxidation within zeolites and electrocatalytic oxidation of ammonia. 

Keywords metallocene catalyst, Ziegler-Natta catalyst, olefin polymerization, polyolefins, homogeneous catalysts, supported catalysts, stereoregularity, molecular weight distribution (MWD), chemical composition distribution, Unipol , Novolen , stereoselectivity, single site catalyst, multiple site catalyst, gas phase process, slurry process, homopolymerization, copolymerization. 

Figure 3. Ethylene polymerization activity in gas-phase and slurry reactors. [SMHNT catalyst gas-phase, 90 C, Pcair =0.4 MPa slurry phase, 70 C, Po,f =0.1 MPa].

Bielahski, A., Malecka-Lubahska, A., Micek-Ilnicka, A., and Pozniczek, J. The role of protons in acid base type reactions on heteropolyacid catalysts Gas phase MTBE synthesis on H4SiWi2O40. Top. Catal. 2000,11—12, 43-53. 

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 ... 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

Catalytic cracking is a key refining process along with catalytic reforming and alkylation for the production of gasoline. Operating at low pressure and in the gas phase, it uses the catalyst as a solid heat transfer medium. The reaction temperature is 500-540°C and residence time is on the order of one second. 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

As with the other surface reactions discussed above, the steps m a catalytic reaction (neglecting diffiision) are as follows the adsorption of reactant molecules or atoms to fomi bound surface species, the reaction of these surface species with gas phase species or other surface species and subsequent product desorption. The global reaction rate is governed by the slowest of these elementary steps, called the rate-detemiming or rate-limiting step. In many cases, it has been found that either the adsorption or desorption steps are rate detemiining. It is not surprising, then, that the surface stmcture of the catalyst, which is a variable that can influence adsorption and desorption rates, can sometimes affect the overall conversion and selectivity. 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Unsaturated nitriles are formed by the reaction of ethylene or propylene with Pd(CN)2[252]. The synthesis of unsaturated nitriles by a gas-phase reaction of alkenes. HCN, and oxygen was carried out by use of a Pd catalyst supported on active carbon. Acrylonitrile is formed from ethylene. Methacrylonitrile and crotononitrile are obtained from propylene[253]. Vinyl chloride is obtained in a high yield from ethylene and PdCl2 using highly polar solvents such as DMF. The reaction can be made catalytic by the use of chloranil[254]. 

M ass Transfer. Mass transfer in a fluidized bed can occur in several ways. Bed-to-surface mass transfer is important in plating appHcations. Transfer from the soHd surface to the gas phase is important in drying, sublimation, and desorption processes. Mass transfer can be the limiting step in a chemical reaction system. In most instances, gas from bubbles, gas voids, or the conveying gas reacts with a soHd reactant or catalyst. In catalytic systems, the surface area of a catalyst can be enormous. Eor Group A particles, surface areas of 5 to over 1000 m /g are possible. 

Boron trifluoride catalyst is used under a great variety of conditions either alone in the gas phase or in the presence of many types of promoters. Many boron trifluoride coordination compounds are also used. 

Formic acid can decompose either by dehydration, HCOOH — H2O + CO (AG° = —30.1 kJ/mol AH° = 10.5 kJ/mol) or by dehydrogenation, HCOOH H2 + CO2 (AG° = —58.6 kJ/mol AH° = —31.0 kJ/mol). The kinetics of these reactions have been extensively studied (19). In the gas phase metallic catalysts favor dehydrogenation, whereas oxide catalysts favor dehydration. Dehydration is the predominant mode of decomposition ia the Hquid phase, and is cataly2ed by strong acids. The mechanism is beheved to be as follows (19) ... 

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