Diffusion of reactant molecules

Shape selective catalysis as typically demonstrated by zeolites is of great interest from scientific as well as industrial viewpoint [17], However, the application of zeolites to organic reactions in a liquid-solid system is very limited, because of insufficient acid strength and slow diffusion of reactant molecules in small pores. We reported preliminarily that the microporous Cs salts of H3PW12O40 exhibit shape selectivity in a liquid-solid system [18]. Here we studied in more detail the acidity, micropore structure and catal3rtic activity of the Cs salts and wish to report that the acidic Cs salts exhibit efficient shape selective catalysis toward decomposition of esters, dehydration of alcohol, and alkylation of aromatic compound in liquid-solid system. The results were discussed in relation to the shape selective adsorption and the acidic properties. 

The open three-dimensional nature of zeolite structures permits diffusion of reactant molecules into the interior voids in the crystal and accounts for the high effective surface area of these materials. Faujasitic zeolites have channels of about 8-A diameter connecting cavities of 13-A diameter (supercages) in a three-dimensional network. The zeolite mor-denite has parallel channels with a diameter of about 7-A. The intracrystalline surface of the zeolite is, therefore, accessible to molecules with kinetic diameters equal to or smaller than the channel diameters. 

The usefulness of mordenite-based catalysts will depend on the extent and character of adsorption and rates of diffusion of reactant molecules and products in the mordenite base. 

In Modes 1 and 2 the limitation or the blockage is due to chemical reasons i.e., the coke molecules are (1) reversibly or (2) quasi-irreversibly adsorbed on the acid sites (site poisoning or site coverage) and/or to steric reasons the diffusion of reactant molecules through the cavity or the channel intersection is (1) limited or (2) blocked. With these modes the toxicity of the coke molecules is low as only the sites located in the cavity or at the chaimel intersection, often only one site, are partially (Mode 1) or totally (Mode 2) deactivated. 

Diffusion of reactant molecules within the interfacial region. 

For non-porous catalyst pellets the reactants are chemisorbed on their external surface. However, for porous pellets the main surface area is distributed inside the pores of the catalyst pellets and the reactant molecules diffuse through these pores in order to reach the internal surface of these pellets. This process is usually called intraparticle diffusion of reactant molecules. The molecules are then chemisorbed on the internal surface of the catalyst pellets. The diffusion through the pores is usually described by Fickian diffusion models together with effective diffusivities that include porosity and tortuosity. Tortuosity accounts for the complex porous structure of the pellet. A more rigorous formulation for multicomponent systems is through the use of Stefan-Maxwell equations for multicomponent diffusion. Chemisorption is described through the net rate of adsorption (reaction with active sites) and desorption. Equilibrium adsorption isotherms are usually used to relate the gas phase concentrations to the solid surface concentrations. 

Absolutely. Bimolecular (two species) reactions taking place in the gas phase and in solutions are quite similar in that they involve random diffusion of reactant molecules until they collide, at which point a reaction may take place. For reactions involving solids, the surface of a solid is often reacting with another species that is present at the interface between the solid and a gas or liquid with which it is in contact. An example would be your car rusting to form oxides. 

Diffusion of reactant molecules through the fluid in the direction of the surface. 

Acetyl transfer between aspirin and sulfadiazine is a bimolecular reaction in which the translational diffusion of reactant molecules becomes rate determining when molecular mobility is limited in the solid state [33]. Therefore, it can offer a useful reaction model for understanding the ways in which chemical degradation rates in lyophilized formulations are affected by molecular mobility. Figure 17A shows the temperature dependence of the rate constant of acetyl transfer in lyophilized formulations containing dextran. Figure 17B shows the pseudo rate constant of aspirin hydrolysis that occurs in parallel with acetyl transfer in the presence of water. The rate constant of acetyl transfer ( t) and the pseudo rate constant of hydrolysis ( H> pseudo) are described by following equations ... 

Little attentions are paid on the reaction transport in the catalysts unless the pore size of catalyst is too small to geometrically confine the diffusion of reactant molecules in the pore. However, if the catalytic support consisted of active species, rather than traditional support materials, the new catalytic support could also have chemical effect with reactants. The influence of chemical effect of such active support on the transport mechanism of reactants is unknown. In recent years, some investigators proposed the unsupported catalyst for designing the new catalyst. Instead of being loaded on the catalytic support, active species are made into the mesoporous material to increase the highly catalytic activity. Though it obtains some progress, the unit catalytic performance of unsupported catalyst cannot show a proportional improvement compared with that of supported catalysts (Eijsbouts et al, 2007). 

Diffusion of reactant molecules from bulk gas stream to external surface of catalyst. 

Type II electrodes are two-phase composite media that consist of a nanoporous and electronically conductive medium filled with liquid electrolyte or ionic liquid. The electrochemically active interface forms at the boundary of the two phases. The electrolyte phase must provide pathways for diffusion and permeation of protons, water, and reactants. Flooded two-phase CLs could work well when they are made extremely thin, not significantly exceeding a thickness of Icl — 200 nm. Rates of diffusion of reactant molecules and protons in liquid water are then sufficient to provide uniform reaction rate distributions over the thickness of the layer. 

Bulk flow and diffusion of reactant molecules through large electrode pores. 

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