Catalytic bypass 244

The catalytic activity of the pure /3-palladium hydride has been studied under the appropriate temperature and pressure conditions. The palladium sample was converted into the hydride in a manner which bypassed the area of coexistence of the phases. This was achieved by suitably saturating the metal with hydrogen at 35 atm above the critical temperature and then subsequently cooling the sample to the required temperature and reducing the hydrogen pressure. This method of sample prepare tion allowed one to avoid cracking the palladium crystallites, which would... 

A lot of work is currently carried out to extend this idea to fully dispersed two-dimensional (on a YSZ surface) or three-dimensional (in a porous YSZ structure) metal catalysts. The main problems to be overcome is current bypass and internal mass transfer limitations due to the high catalytic activity of such fully dispersed Pt/YSZ catalyst systems. 

The emulsion phase approaches the performance of a CSTR with its inherent lower yield for most reactions. To make matters worse, mass transfer between the emulsion and bubble phases becomes limiting to the point that some of the entering gas completely bypasses the catalytic emulsion phase. The system behaves like the reactor in Example 11.5. 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

The recycle reactor is shown schematically in Figure 1. It consists of a catalytic or electrocatalytic reactor unit with a bypass loop, a recycle pump and a molecular sieve trap unit. The latter comprises one or two packed bed columns in parallel each containing 2-10 g of Linde 5A molecular sieve pellets. On line gas chromatography (Shimadzu 14A) was used for the analysis of CH4, O2, CO, CO2, C2H4 and C2H6 in the reactants and products. 

Two major pathways exist for this reaction, one bypassing hydrogen peroxide (first pathway) and the other involving intermediate peroxide formation via reaction (15.21) (second pathway). The peroxide formed is either electrochemically reduced to water via reaction (15.22) or decomposed catalytically on the electrode surface via reaction (15.23), in which case half of the oxygen consumed to form it reemerges [in both cases the overall reaction corresponds to Eq. (15.20)]. 

Use of a catalytic cocaine antibody may bypass this potential downfall of the active immunization approach. The enzymatically active catalytic cocaine antibody cleaves cocaine into two inactive metabolites ecognine methyl ester and benzoic acid. The two metabolites are released from the catalytic antibody rendering the antibody free to degrade more cocaine. Because the... 

Novel polyketides could also be generated by blocking certain catalytic domains of PKSs to allow bypass of the early steps in polyketide biosynthesis. In one example, the Cys729 —> Ala... 

A few intriguing developments in the area of tetrahydro-P-carboline synthetic methodology include the report of a catalytic asymmetric Pictet-Spengler reaction <06JACS1086> and an enantioselective Pd-catalyzed intramolecular allylic alkylation of indoles <06JACS1424>. A one-step synthesis of 1-substituted-P-carbolines from L-tryptophan has appeared that bypassed the tetrahydro intermediate <06T10900>. 

Unavoidable loss of gas is compensated via a feed valve for supplying virgin nitrogen into the circulation pipe. The exhaust gas of the process has to be bypassed for purification. After the separation of dust by a filter, the gas is heated to 400 °C for the catalytic combustion of the side products. The gas is then cooled down, and the excess oxygen is catalytically converted to water by using hydrogen. For economic reasons, the gas flow will recover the heat via a heat exchanger and then be cooled down by a gas cooler. 

To circumvent such problems, up to two-thirds of the acid gas has been removed from the flame and sent directly to the catalytic reactor, the so-called "long bypass" scheme. This strategy has worked with mixed success in natural gas plants, where hydrocarbons in the acid gas are C1-C3 paraffins, chiefly methane, and no olefins. Such a configuration has been put forward for use in coal gasification plants. In the writer s opinion it is unworkable if the bypassed acid gas contains even traces of olefins or aromatics these compounds react with SO2 to form tarry products which foul the catalyst and discolor the product sulfur. 

Catalytic activities for n-hexane cracking were performed using an isothermally operated flow reactor. The feed stream of nitrogen was saturated at 3°C with hexane. With the help of a bypass it was possible to determine both the reactor inlet and outlet concentration of hexane using a gas chromatograph (Varian Star 3400) with FID-detector. 

Zhou, B. L., Pata, J. D., and Steitz, T. A. (2001). Crystal structure of a DinB lesion bypass DNA polymerase catalytic fragment reveals a classic polymerase catalytic domain. Mol. CellS, 427-437. 

Implementation of the enzyme mechanism is the means of increasing selectivity of chemical catalysis, because in this case substrate is converted to product in one catalytic stage, bypassing the formation of substrate intermediates (mediators may be formed). 

In fact, the implementation of highly selective two-electron processes of catalytic oxidation with H202 provides for the development of a principally new technology for substrate conversion to target products bypassing the formation stages of the substrate intermediates, which initiate additional channels of substrate consumption for side product synthesis. 

The split-flow process is used when the H2S concentration in the gas stream is too low to obtain a stable combustion. In this process, one-third of the total gas stream is fed to the reaction furnace and the rest is bypassed and directly fed to the first catalytic converter. As a consequence, the production of elemental sulphur from the thermal step is less than in the straight-through process. The remaining part of the process is essentially the same as the straight-through process. 

The hydridorhodium(III) species HRh(NH3)52+ and HRh(CN)4-(H20)2 are known to react with 02 to give hydroperoxide species, Rh—OOH (102, 103, 104) the sulfoxide oxidation via such an intermediate could be accommodated by several mechanisms akin to those discussed already for olefinic substrates (cf. Reactions 17 and 18). The oxidation also could occur via liberated hydrogen peroxide, which oxidizes sulfoxide to sulfone and can be formed by treatment of Rh—OOH species with acid (104). Such a mechanism would amount to a novel catalytic conversion of H2/02 mixtures to in situ H202. The part of the enzymatic cycle of P 450 that utilizes 02 and two electrons (from a hydride source) can be bypassed by using hydrogen peroxide (16, 18). 

Karge s combined TGA/GC method can be applied to catalysts which were used (and coked) before in a separate catalytic unit. It can, of course, be employed as well to catalysts on which the catalytic reaction was studied before in the same thermobalance (even though, in many instances, such microbalances are not very suitable as catalytic reactors, because part of the feed gas stream tends to bypass the sample holder or basket, on/in which the solid catalyst rests). If the latter is done, the mass of coke deposited during the catalytic reaction can be monitored in-situ via the weight gain. It has been shown [36] that such a strategy enables the experimentalist to perform systematic studies on the amount of coke formed and its h/wc ratio in dependence of time-on-stream and reaction temperature. 

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