Vapor phase reactants

The first is a pyrolytic approach in which the heat dehvered by the laser breaks chemical bonds in vapor-phase reactants above the surface, allowing deposition of the reaction products only in the small heated area. The second is a direct photolytic breakup of a vapor-phase reactant. This approach requires a laser with proper wavelength to initiate the photochemical reaction. Often ultraviolet excimer lasers have been used. One example is the breakup of trimethyl aluminum [75-24-1] gas using an ultraviolet laser to produce free aluminum [7429-90-5], which deposits on the surface. Again, the deposition is only on the localized area which the beam strikes. 

Particular attention should be paid to both the stability in solution and the thermal stability. The condesation-hydrolysis equilibria of heteropolyanions in aqueous media are shown in Fig. 8. Each heteropolyanion is stable only at pH values lower than the corresponding solid line (55). Some solid heteropolyacids are thermally stable and applicable in reactions with vapor-phase reactants conducted at high temperatures. The thermal stability is measured mainly by X-ray diffraction (XRD), thermal gravimetric analysis, and different thermal analysis (TG-DTA) experiments. According to Yamazoe et al. (56), the decomposition temperatures of H3PM012O40 and its salts depend on the kinds of cations Ba2 +, Co2+ (673 K) < Cu2+, Ni2+ (683 K)[Pg.127]

There is, as is well known, a close similarity between the crystalline and porous structures of silicalite-1 and silicalite-2. The same similarity therefore exists between TS-1 and TS-2, and it appears logical that they should have very similar catalytic properties. TS-2 has been evaluated as a catalyst for many different reactions, such as Beckmann rearrangement of cyclohexanone oxime with vapor-phase reactants H202 oxidation of phenol, anisole, benzene, toluene, n-hexane, and cyclohexane and ammoximation of cyclohexanone. As described in detail in Section V.C.3, differences that had been claimed between the catalytic properties of TS-1 and those of TS-2 have not been substantiated. Later investigations have shown that, when all the relevant parameters are identical, the catalytic activities of TS-1 and TS-2 are also identical. The small differences in the crystalline structure between the two materials have no influence on their catalytic properties (Tuel et al., 1993a). 

Usually, the working electrode (W) is a porous metallic electrode in PEVD. Thus, reactant (B) in the vapor phase can reach the surface of the solid electrolyte for initial electrochemical reaction at a three-phase boundary of solid electrolyte (E), working electrode (W) and sink vapor phase (S) as shown in Eigure 3 (location II). All reactants for the sink side electrochemical reaction (1) or (2) are only available there. Subsequent reaction and deposition of the product (D) requires both electrons and ions to travel through product (D) to the surface to react with vapor phase reactant(s) electrochemically at location III in Eigure 3. 

The current, I, in a PEVD process can be recorded simultaneously by an ammeter in the external circuit to reveal the kinetics of the PEVD reactions. As discussed in the last section, solid-state reactant (A) needs to be transported as a combination of ionic and electronic species from the source to the sink side through the solid electrochemical cell to participate in a PEVD reaction with vapor phase reactant (B). The PEVD reaction rate, and subsequent product (D) formation rate, v(t), can be expressed as... 

As schematically shown in Figure 7a, initial PEVD reaction and product nucleation occurs at the three-phase boundary of solid electrolyte (E), working electrode (W) and the sink vapor phase (S) which contains vapor phase reactant (B). Only here are all reactants available for the half-cell electrochemical reaction at the sink side of a PEVD system. Although the ionic and electronic species can sometimes surface diffuse at elevated temperature to other sites to react with (B) in the vapor phase, the supply of the reactants continuously along the diffusion route is less feasible and the nuclei are too small to be stabilized under normal PEVD conditions. Only along the three phase boundary line are all the reactants available for further growth to stabilize the nuclei. Consequently, initial deposition in a PEVD process is restricted to certain areas on a substrate where all reactants for the sink electrochemical reaction are available. 

Chemical vapor infiltration (CVI) is widely used in advanced composites manufacturing to deposit carbon, silicon carbide, boron nitride and other refractory materials within porous fiber preforms. " Because vapor phase reactants are deposited on solid fiber surfaces, CVI is clearly a special case of chemical vapor deposition (CVD). The distinguishing feature of CVI is that reactant gases are intended to infiltrate a permeable medium, in part at least, prior to... 

Reaction conditions. Vapor-phase reactants in a once-through fixed-bed reaaor operated at atmospheric pressure. The molar ratio of hydrogen to toluene was 14.2, The molar ratio of hydrogen to hexane was 7.8. 

The related supported liquid-phase catalysts consist of traditional support materials such as y-AljOj having micropores filled with solvent and a dissolved catalyst. In small pores, because of the Kelvin effect, the vapor pressure of the solvent is small so that it will remain in the pore as a liquid, even when the catalyst is used at a high temperature in flowing vapor-phase reactants" . These catalysts are active for alkene hydroformy-lation the soluble catalyst can thus be used without the complications of corrosion and difficult separation from products—provided that it is stable (cf. 14.2.4). 

Ambient condition vapor phase reactants are stored in gas cylinders, generally in a compressed state. Subsequent to pressure regulation, their flows generally are measured with mass flow controllers which give high accuracy and permit microprocessor control of vapor phase flows. 

During catalytic reactions using supported ionic liquid-type catalysts gaseous or vapor-phase reactants diffuse through the residual pore space of the catalyst, dissolve in the liquid catalyst phase, and react at catalyst sites within the thin liquid catalyst film dispersed on the walls of the pores in the support material, as illustrated in Fig. 5.6-1. The products then diffuse back out of the catalyst phase into the void pore space and further out of the catalyst particle. 

The total helium flow (850 cc/min.) plus the vapor phase reactant and products maintained the bed in vigorous motion which, in turn, insured good temperature control. Runs were carried out at 1 WHSV based on the low EHI feed component, 410 C and atmospheric pressure. The catalyst was automatically oxidatively regenerated after each 10-20 minutes reaction interval. 

Paul et al. (25) observed that for polymer volume fractions less than 0.8, the functional dependence of the diffusion coefficients on the polymer volume fraction was, generally, in accordance with Equation 40. Muhr and Blanshard (26) provide additional supporting data on different polymers than those reported by Paul et al, Roucls and Ekerdt (27) measured the diffusion of cyclic hydrocarbons in benzene-swollen polystyrene beads their diffusion coefficients satisfy the general form of Equation 40. The effective dlffuslvltles of organic substrates in crossllnked polystyrene reported by Marconi and Ford (17) also follow trends predicted in Equation 40. In the absence of experimental data, it appears that Equation 40 provides a reasonable, and the simplest, means to estimate D for use in detailed modeling or in estimation methods such as Equation 38. Equation 40 was used by Dooley et al. (11) in their study of substrate diffusion and reaction in a macroreticular sulfonic acid resin which involved vapor phase reactants. 

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