Diffusion-enhanced reaction

A suggestion was made that A add of the 2-hydroxy-2-propyl radicals of DAR to MA (Schemes 12.1 and 12.3) decreases with the viscosity increase,which would be a sign of a diffusion-controlled or a diffusion-enhanced reaction.In fact, an increase in viscosity 1200 times leads to a decrease in A add only 5 times and not by orders of magnitude, which does not allow the classification of the addition as almost a diffusion-controlled (diffusion-enhanced) reaction. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Selected entries from Methods in Enzymology [vol, page(s)] Analysis of GTP-binding/GTPase cycle of G protein, 237, 411-412 applications, 240, 216-217, 247 246, 301-302 [diffusion rates, 246, 303 distance of closest approach, 246, 303 DNA (Holliday junctions, 246, 325-326 hybridization, 246, 324 structure, 246, 322-324) dye development, 246, 303, 328 reaction kinetics, 246, 18, 302-303, 322] computer programs for testing, 240, 243-247 conformational distribution determination, 240, 247-253 decay evaluation [donor fluorescence decay, 240, 230-234, 249-250, 252 exponential approximation of exact theoretical decay, 240, 222-229 linked systems, 240, 234-237, 249-253 randomly distributed fluorophores, 240, 237-243] diffusion coefficient determination, 240, 248, 250-251 diffusion-enhanced FRET, 246, 326-328 distance measurement [accuracy, 246, 330 effect of dye orientation, 246, 305, 312-313 limitations, 246,... 

Figure 6.3a shows the idealized sketch of concentration profiles near the interface by the 1 latta model, for the case of gas absorption with a very rapid second-order reaction. The gas component A, when absorbed at the interface, diffuses to the reaction zone where it reacts with B, which is derived from the bulk of liquid by diffusion. Ihe reaction is so rapid that it is completed within a very thin reaction zone this can be regarded as a plane parallel to the interface. The reaction product diffuses to the liquid main body. The absorption of CO2 into a strong aqueous KOH solution is close to such a case. Equation 6.21 provides the enhancement... 

Silicon Dioxide. Si02 layers produced by PECVD are useful for intermetal dielectric layers and mechanical or chemical protection and as diffusion masks and gate oxides on compound-semiconductor devices. The films are generally formed by the plasma-enhanced reaction of SiH4 at 200-300 °C with nitrous oxide (N20), but CO, C02, or 02 have also been used (238-241). Other silicon sources including tetramethoxysilane, methyl dimethoxysilane, and tetramethylsilane have also been investigated (202). Diborane or phosphine can be added to the deposition atmosphere to form doped oxide layers. 

Enhanced Mass Transfer, Diffusivity Supercritical fluids share many of the advantages of gases, including lower viscosities and higher diffusivities relative to liquid solvents, thereby potentially providing the opportunity for faster rates, particularly for diffusion-limited reactions. 

In this connection, it is helpful to look first at the reactivity of the anions. There is no generally acceptable measure of nucleophilic reactivity since both the scale and order of relative reactivities depend on the electrophilic centre being attacked (Ritchie, 1972). However, in the present reaction, the similarity in the reactivity of the different anions is remarkable. Thus, the Swain and Scott n-values (cf. Hine, 1962) indicate that the iodide ion should be 100 times more reactive than the chloride ion in nucleophilic attack on methyl bromide in aqueous acetone. In the present reaction, the ratio of the rate coefficients for iodide ions and chloride ions is 1.4. This similarity led to the suggestion that these reactions are near the diffusion-controlled limit (Ridd, 1961). If, from the results in Table 5, we take this limit to correspond to a rate coefficient (eqn 19) of 2500 mol-2 s 1 dm6 then, from the value of ken for aqueous solutions at 0° (3.4 x 109 mol-1 s 1 dm3 Table 1), it follows that the equilibrium constant for the formation of the electrophile must be ca. 7.3 x 10 7 mol-1 dm3. This is very similar to the equilibrium constant reported for the formation of the nitrosonium ion (p. 19). The agreement is improved if allowance is made for the electrostatic enhancement of the diffusion-controlled reaction by a factor of ca. 3 (p. 8) the equilibrium constant for the electrophile then comes to be ca. 2.4 x 10-7. 

The characteristics of liquid reaction with various values for M are listed in Table 7.1. It can be seen that, in the last two cases, i.e. Vm" <0.3, the processes are controlled by reaction kinetics and so enhancement of transfer becomes of no use in the medium case of 1, both diffusion and reaction kinetics affect the overall rate of the process, a measure of enhancing transfer may have a certain positive effect while in the former two cases, i.e. 4m >3, the reaction(s) in liquid proceed fast, the global processes are controlled by diffusion, and thus the measure of enhancing transfer will play a positive key role. 

Recently, we reported detailed descriptions of hydrocarbon chain growth on supported Ru catalysts (7,8) we showed that product distributions do not follow simple polymerization kinetics and proposed a diffusion-enhanced olefin readsorption model in order to account for such deviations (7,8). In this paper, we describe this model and show that it also applies to Co and Fe catalysts. Finally, we use this model to discuss a few examples from the literature where catalyst physical structure and reaction conditions markedly influence hydrocarbon product distributions. 

The trends in carbon number distribution and in a-olefin/paraffin ratio on Ru, Fe, and Co, three very different catalytic surfaces, are remarkably similar. All catalysts show a curved Flory plot and an a-olefin/paraffin ratio that decreases with increasing carbon number until only paraffins are observed at high carbon numbers. In each case, diffusion-enhanced olefin readsorption accounts for such trends. Its contribution depends on the catalytic surface, its physical structure, and reaction conditions. 

Concepts of local equilibrium and charged particle motion under - electrochemical potential gradients, and the description of high-temperature -> corrosion processes, - ambipolar conductivity, and diffusion-controlled reactions (see also -> chemical potential, -> Wagner equation, -> Wagner factor, and - Wagner enhancement factor). 

Bujan-Nunez MC, Lopez-Quintela MA. (2004) Enhancement of the recollision rate in diffusion-influenced reactions in an inhomogeneous medium. J Chem Phys 121 886-889. 

A pressure maximum, instead of minimum, inside the membrane could result from cases where both chemical reaction and surface diffusion are present [Sloot et al., 1992]. Thus the occurrence of a maximum or minimum local pressure inside the membrane depends on the reaction stoichiometry as well as the mobilities of the reaction species. It is assumed that only hydrogen sulfide adsorbs on the pore surface. Due to a higher transport rate of H2S enhanced by surface diffusion, the reaction zone is shifted toward the SO2 side of the membrane. In the reaction zone, larger amounts of the products are formed and higher molar fluxes of the products out of the membrane are expected so that the maxima of the mole fraction profiles of the products at the reaction zone can be sustained. 

The initial increase in C5+ selectivity as x increases arises from diffusion-enhanced readsorption of a-olefins. At higher values of CO transport restrictions lead to a decrease in C5+ selectivity. Because CO diffuses much faster than C3+ a-olefins through liquid hydrocarbons, the onset of reactant transport limitations occurs at larger and more reactive pellets (higher Ro, 0m) than for a-olefin readsorption reactions. CO transport limitations lead to low local CO concentrations and to high H2/CO ratios at catalytic sites. These conditions favor an increase in the chain termination probability (jSr, /Sh) and in the rate of secondary hydrogenation of a-olefins (j8s) and lead to lighter and more paraffinic products. 

The remarkably similar trends in carbon number distributions and chain termination kinetics on Fe, Co, and Ru catalysts demonstrate the important role of diffusion-enhanced secondary reactions, especially those that reverse... 

Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive a-olefins. These secondary reactions reverse chain termination steps that form these olefins and lead to heavier products, higher chain growth probabilities, and more paraffinic products. Diffusion-enhanced readsorption of a-olefins accounts for the non-Flory carbon number distributions frequently observed during FT synthesis on Co and Ru catalysts. Diffusion-limited reactant (H2/CO) arrival leads instead to lower selectivity to higher hydrocarbons. Consequently, intermediate levels of transport restrictions lead to highest selectiv-ities to C5+ products. A structural parameter containing the pellet diameter, the average pore size, and the density of metal sites within pellets, determines the severity of transport restrictions and the FT synthesis selectivity on supported Ru and Co catalysts. 

Mass Transfer Enhancement Low viscosity, high diffusivity, and very low surface tension of SCFs can improve the rates of diffusion-limited reactions (such as enzymatic reactions) by reducing mass transfer resistances (170). Mass... 

Diffusion at 1000-1100°C gives erratic results while causing damage to the Si surface. Random pitting results, suggesting enhanced reactions at surface defect sites. In the presence of low concentrations of O2, additional vapor phase reactions occur ... 

The activation energy for the photo enhanced reaction is 35 kJ/mole which is similar to the value obtained by Deutsch and Rathman. The authors note that this is close to the activation energy of atomic hydrogen diffusion on a tungsten surface (40 kJ/mole). Tsuzuku ct al. come to the following proposal for the reaction route ... 

Fluidization in the reactor s polymer bed is maintained by adequate recirculation of reacting gas. The reaction heat is removed from the recycle gas by a cooler, while the cooled gas is recycled back to the bottom of the gas-phase reactor for fluidization. This gas-phase reactor maintains a high degree of turbulence and enhances monomer diffusion and reaction rates, and ensures an efficient particle heat removal. 

In regime 2, the reaction is fast enongh to keep the bulk liquid phase concentration of the gaseous reactant essentially zero but not fast enough to occur substantially in the liquid film. There is no enhancement of mass transfer due to reaction. Diffusion and reaction take place in series fashion. 

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