Competition for adsorption

The adsorption of contaminants on geosorbents also is affected by climatic conditions reflected in the subsurface temperature and moisture status. Calvet (1984) showed how the soil moisture content may affect adsorption of contaminants originating from agricultural practices. The moisture content determines the accessibility of the adsorption sites, and water affects the surface properties of the adsorbent. The competition for adsorption sites between water and, say, insecticides may explain this behavior. Preferential adsorption of the more polar water molecules by soil hinders... 

The hydration status of the clay or earth material may affect the adsorption capacity of nonpolar (or slightly polar) toxic chemicals. Continuing with parathion as a case study, Fig. 8.33 shows the increase adsorbed parathion on attapulgite from a hexane solution, as the adsorbed water on the clay surface decreases. This behavior may be explained by the competition for adsorption sites between the polar water and the slightly polar parathion. Possibly, however, the reduction in adsorption due to the presence of water is caused by the increased time required for parathion molecules to diffuse through the water film to the adsorption sites. 

The competition for adsorption sites is very important for the kinetics of a heterogeneous catalytic reaction. For this reason sites,, are included as a reactant in the kinetic model. As a site must be either free or occupied by one of the surface intermediates, there is a conservation law for the coverages... 

More complex rate expressions were observed when the reaction was carried out in the absence of solvents or when other solvents, such as H20 or ter/-butyl alcohol, were used. Complex rate expressions arise because a number of effects come into play, including preferential sorption in the hydrophobic zeolite, competition for adsorption at the titanium center, and competitive diffusion and reaction. It is therefore difficult to obtain unambiguous answers to mechanistic questions from kinetics. 

Competition for adsorption influence on reaction rate, stability... 

COMPETITION FOR ADSORPTION INFLUENCE ON REACTION RATE, STABILITY AND SELECTIVITY... 

The Fries rearrangement of PA over H-BEA zeolites, which is a simple reaction, was chosen to introduce the competition for adsorption on the zeolite catalysts and its role on the reaction rate. Ortho- and para-hydroxyacetophenones (o- and p-HAP), para-acetoxyacetophenone (p-AXAP) and phenol (P) are the main products o-HAP, P and p-AXAP, which are directly formed (primary products),... 

In these reactions, the competition for adsorption involves both reactant molecules and often solvent and product molecules, which makes the choice of optimal operating conditions (solvent, reactant concentrations, temperature) and catalysts much more difficult. The competition between product and reactants plays a major role because such molecules are often largely different in polarity and bulkiness, which was not the case in rearrangement reactions (see above). 

Competition between reactant, solvent and product molecules for adsorption within the zeolite micropores is demonstrated directly (adsorption experiments) and indirectly (effect of the framework Si/Al ratio on the activity, kinetic studies) to occur during Fine Chemical synthesis over molecular sieve catalysts. This competition, which is specific for molecular sieves (because of confinement effects within their micropores), adds up to the competition which exists over any catalyst for the chemisorption of reactant, solvent and product molecules on the active sites. Both types of competition could affect significantly the activity, stability and selectivity of the zeolite catalysts. Although the relative contributions of these two types of competition cannot be estimated, the large change in the activity of the acidic sites (TOF) with the zeolite polarity seems to indicate that the competition for adsorption within the zeolite micropores often plays the major role. 

The negative effect that this latter competition has can be limited or even avoided by an adequate choice or tailoring of the molecular sieve hydrophilic/hydrophobic properties. The optimization of the operating conditions is also indispensable. Increasing the reaction temperature and the ratio between the concentrations of the less and more polar reactants, as well as a proper choice of the solvent polarity, are simple and complementary solutions to limit the negative effect of competition for adsorption between reactant and product molecules within the zeolite micropores. 

So, on palladium catalysts, the role of Pd-S (presulfiding) is different from the role of sulfur-containing molecules in competition for adsorption with the unsaturated hydrocarbon. 

Finally, dilution of the ethanol feed with water reduced significantly the production of diethylether. However, at high water dilution, the yield in ETBE decreased markedly (Table 2) because of the strong competition for adsorption of water with ethanol on the acidic reaction sites. 

The first mode of deactivation is clearly shown with HZSM5, At low coke content, Vr/Va is close to 1 4 coke molecules are needed to deactivate one acid site, This weak deactivating effect can be explained by a competition for adsorption on the acid sites between the reactant and the coke molecules which are too weakly basic to be "irreversibly adsorbed at the reaction temperature. However limitations in the rate of diffusion of the reactant can also be responsible for deactivation. The size and the basicity of the coke molecules increase with the coke content, which causes an increase in the deactivating effect of the coke molecules. Beyond a certain size of the coke molecules the channel intersection is completely inaccessible to the reactant and to the adsorbates and Vr/Va can be lower than T This first mode of deactivation occurs also with USHY. However the deactivating effect of coke molecules is initially very high because coke molecules are formed on the strongest (hence the most active) acid sites. 

This instability can be avoided by adding a non-ionic surfactant to the surface of the latex, forming a hydrophilic layer (Triton x-405 of 30 units) on the surface of the latex [22]. In addition, this compound reduces the stacking effect by masking the hydrophobic domains (or properties) of the surface. Indeed, competition for adsorption between the ODN and the surfactant molecules can also lead to desorption. However, this effect was not observed in all reported studies, but it is in principle accessible by comparing the adsorption energies of ODN and the surfactant on the surface of the latex. 

Adsorption of methyl mercaptan in moist conditions was performed on numerous samples of activated cartons of various origins. Methyl mercaptan adsorption was tested by a dynamic method. The amount of products of surface reaction was evaluated using thermal analysis. The results revealed that the main product of oxidation, dimethyl disulfide, is adsorbed in pores smaller than SO A. There is apparent competition for adsorption sites between water (moist conditions) and dimethyl disulfi. The comp ition is won by the latter molecule due to its strong adsorption in the carbon pore system. Althou dimethyl disulfide has to compete with water for the adsorption sites it can not be formed in a significant quantity without water. Water facilitates dissociation of methyl mercaptan and thus ensures the efficient removal process. 

An objective of this paper it to describe the results of our further investigation of the competition for adsorption sites between water and dimethyl disulfide molecules during methyl mercaptan adsorption on activated carbons. Moreover, we attempt to indicate the apparent borderlines between the conditions of adsorption processes leading to different adsorption/oxidation paths. Those working conditions have a significant effect on the feasibility of methyl mercaptan removal. 

Table V shows that the amount of adsorption onto Silastic from plasma is significantly depressed below its saturation value measured in buflFer presumably because of competition from other components of the plasma. The diflFerence in adsorption from the two plasma pools may result from the increased fibrinogen concentration in one pool which would allow more eflFective competition for adsorption onto Silastic and result in enhanced adsorption. Since the adsorption of fibrinogen onto poly (HEMA)/Silastic from plasma is not so greatly depressed relative to adsorption from buflFer (see Table V), an increase in plasma fibrinogen concentration might not have so large an eflFect on adsorption onto poly-(HEMA)/Silastic as it apparently does on adsorption onto Silastic itself.

The SO2-CO2 system is a bit more interesting (Figure 1) here the competition for adsorption sites is more intense, and deviations from the pure-component curves is far more significant. With an increase of temperature, the selectivity for SO2 is greatly increased, so much so that around 30°C a reversal occurs and SO2 becomes the preferred species. 

As we have explained in the previous sections, the Langmuir model has been established on firm theoretical groimd for gas-solid adsorption, a case where there is no competition between the adsorbate and the mobile gas phase. On the contrary, in liquid-solid adsorption, there is competition for adsorption between the molecules of any component and those of the solvent. Although we can choose a convention canceling the apparent effect of this competition on the isotherm [30,36], the conditions of validity of Eq. 3.47 are not met. These conditions are (i) the solution is ideal (ii) the solute gives monolayer coverage (iii) the adsorption layer is ideal (iv) there are no solute-solute interactions in the monolayer (v) there are no solvent-solute interactions. These conditions cannot be valid in liquid-solid adsorption, especially at high concentrations. 

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