Alcohols, adsorption kinetics

The picture is different for the bulkier tert-butyl alcohol the kinetics of its adsorption at room temperature are markedly retarded with increasing crystallite sizes of HZSM-5 and they exhibit a t12 dependence (t is the time after adsorption) (see Fig. 3c of ref. 8d). This is symptomatic of a diffusion-influenced process. 

It has been concluded that, for most cases, catalysis over zeolites occurs within the intracrystalline voids. Strong supporting evidence for this was provided by Weisz (71), who compared the rates of dehydration of 2-butanol over Linde lOX and 5A zeolites at relatively high temperatures and low conversion. The rate constant per unit volume of 5A was 1/lOO-l/lOOOth that for lOX, a magnitude consistent with the ratio of available surface areas for the external area of 1-5/x-sized 5A crystals and for lOX, where the internal surface area was available to the alcohol. The strong driving force for occlusion within the intracrystalline zeolite voids is exemplified by the rapid adsorption kinetics and rectangular adsorption isotherms observed for molecules whose dimensions are not close to those of the entry pores. 

Comparing the adsorption kinetics of DHTDMAC-fatty alcohol on microcrystalline cellulose, terry towel, and short-napped cotton showed evidence that the high substantivity on cotton is due to its very large specific surface area, not to the presence of negative charges. The larger the surface area, the more quickly and completely the DHTDMAC deposits (Figure 12.9). Microcrystalline cellulose is at once saturated, while terry towels adsorb more quickly than short-napped cotton. 

Malysa et al. (1985) measured the retention time of foams in aqueous n-alcohol solutions. The surface activity of the surfactants and the adsorption kinetics determine the elasticity of the related adsorption layers. The dilational elasticity of the stabilisers is additionally influenced... 

Calculations show that the model of a non-equilibrium surface layer is an alternative to kinetic-controlled adsorption models. On the basis of the purely diffusion-controlled adsorption mechanism the proper consideration of a non-equilibrium diffusion layer leads to a satisfactory agreement between theory and experimental data for various studied systems, systematically demonstrated for the short-chain alcohols [132], The non-equilibrium model is applicable in the concentration range from 10 to 10 mol/cm at different values of the Langmuir constant at- For l < 10 mol/cm a consideration of non-equilibrium layer effects is not necessary. For ai > 10 mol/cm and large surfactant concentration the Ay values calculated from the proposed theory do not compensate the discrepancy to the experimental data so that other mechanisms have to be taken into account. An empirical formula also proposed in [132] for the estimation of the non-equilibrium surface layer thickness leads to a better agreement with experimental data, however this expression restricts the validity of the non-equilibrium surface layer model as alternative to non-diffusional adsorption kinetics. 

Krawczyk (44) investigated the influence of different demulsifiers on the stability of water-in-crude oil emulsions. He defined a partitioning coefficient, KP = Ca/cQ, where Cg refers to the demulsifier concentration in the aqueous phase, and cq to the concentration in the oil phase. He concluded that demulsifiers with K = 1 gives the best results. He also concluded that the interfacial activity and adsorption kinetics of the demulsifier are important parameters. The interfacial region can be expected to be more dynamic, and considerable interfacial fluctuations may occur in the presence of medium-chain alcohols. 

Sdehi, E., Madaeni, S. S., Rajabi, L., Vatanpour, V., Derakhshan, A. A., Zinadini, S., and Ahmadi Monfared, H. 2012. Novel chitosan/polyfvinyl) alcohol thin adsorptive membranes modified with amino functionalized multi-walled carbon nanotnbes for Cu(II) removal from water Preparation, characterization, adsorption kinetics and thermodynamics. Sep. Purif. Technol. 89 309-319. 

The kinetics of adsorption and the amount of polar (alcohols) and nonpolar molecules, Keggin type compounds, was extensively studied through IR spectroscopy and thermogravimetric analysis [16,31,32,34,35]. Misono and coworkers [31] established that the rate of alcohol adsorption depends on the size of the probe molecule, and that the amount of adsorbed molecules in H3PW12O40 is an integral multiple of the number of protons. The observation of the diffusion coefficients and adsorption of polar molecules into the bulk structure of the HPAs proved unequivocally the pseudo-liquid phase behavior of those compounds. 

Surface tension methods measure either static or dynamic surface tension. Static methods measure surface tension at equilibrium, if sufficient time is allowed for the measurement, and characterize the system. Dynamic surface tension methods provide information on adsorption kinetics of surfactants at the air-liquid interface or at a liquid-liquid interface. Dynamic surface tension can be measured in a timescale ranging from a few milliseconds to several minutes [315]. However, a demarkation line between static and dynamic methods is not very sharp because surfactant adsorption kinetics can also affect the results obtained by static methods. It has been argued [316] that in many industrial processes, sufficient time is not available for the surfactant molecules to attain equilibrium. In such situations, dynamic surface tension, dependent on the rate of interface formation, is more meaningful than the equilibrium surface tension. For example, peaked alcohol ethoxylates, because they are more water soluble, do not lower surface tension under static conditions as much as the conventional alcohol ethoxylates. Under dynamic conditions, however, peaked ethoxylates are equally or more effective than conventional ethoxylates in lowering surface tension [317]. 

We have further attempted to suggest a procedure which would make use of the advantages of the method of competitive reactions, i.e. its simplicity and little time demand, and at the same time would yield separately the absolute values of rate constants and adsorption coefficients also for reactions with a more complicated kinetics. Using the values of relative reactivities S from the method of competitive reactions, the adsorption coefficients, for example, of the alcohols (Kb) in the reesterification reaction described by Eq. (26) can be evaluated from the relation... 

From the results of this kinetic study and from the values of the adsorption coefficients listed in Table IX, it can be judged that both reactions of crotonaldehyde as well as the reaction of butyraldehyde proceed on identical sites of the catalytic surface. The hydrogenation of crotyl alcohol and its isomerization, which follow different kinetics, most likely proceed on other sites of the surface. From the form of the integral experimental dependences in Fig. 9 it may be assumed, for similar reasons as in the hy-drodemethylation of xylenes (p. 31) or in the hydrogenation of phenol, that the adsorption or desorption of the reaction components are most likely faster processes than surface reactions. 

It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and (2) (hydrogenation of crotonaldehyde to butyr-aldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-1)- This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. 

Subsequent to the adsorption onto a surface, surfactants, especially long chain fatty acids and alcohols tend to undergo alterations such as two-dimensional associations in the adsorbed layer, presumably at rates kinetically independent of preliminary steps. These intra-layer reactions have been shown to be very slow. 

Fig. 3. Correlation of the slopes p for the dehydration of secondary alcohols on various catalysts (series 3-6) with independently measured heats of adsorption of water and diethyl ether, sensitivity to pyridine poisoning (41), and deuterium kinetic isotope effects (68). [Reprinted with permission from Berdnek and Kraus (13, p. 294). Courtesy Elsevier Scientific Company.]...

Calculated adsorption equilibrium constants indicate the Schiffs base is adsorbed more favorably on the catalyst surface than the aldehyde. This observation is consistent with situation kinetics occurring during the initial stage of the hydrogenation. The apparent rate constant shows that the product C formation is much faster than the alcohol formation. 

Investigation of template poly condensation kinetics has only been studied within a very narrow scope. Polymerization of dimethyl tartrate with hexamethylene diamine was found to be enhanced by using as a template poly(vinyl pyrrolidone), poly(2-vinyl pyridine), or polysaccharides and poly(vinyl alcohol), poly(4-vinyl pyridine). In this case, the template can be treated as a catalyst. No information exists on the influence of the template on the order of reaction. The increase in molecular weight of the polymerization product by the template can be induced by a shift of equilibrium or by an increase in the reaction rate. A similar increase in the reaction rate was observed when poly(4-vi-nyl pyridine) was used in the synthesis of poly(terephtalamides) activated by triphenyl phosphite.The authors suggested that a high molecular weight template was involved in the increase of the local concentration of the substrate (terephthalic acid) by adsorption and activation via N-phosphonium salt of poly(4- vinyl pyridine). 

Different approaches to the kinetics of alcohol dehydration were attempted by two groups of authors [118,119]. In one case, it has been assumed that the active surface of alumina is formed either by free hydroxyl groups or by surface alkoxyl groups. The rate equation was then derived on the basis of the steady-state assumption a good fit to the experimental data was obtained [1118]. The second model was based on the fact that water influences the adsorption of an alcohol and diminishes the available surface. The surface concentrations of tert-butanol and water were taken from independent adsorption measurements and put into the first-order rate equation a good description of integral conversion data was achieved [119]. 

The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p, of the Taft equation for the dehydration of secondary alcohols on A1203 + NaOH, Zr02, Ti02 and Si02 (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p reflects the mechanism being... 

Fig. 5. Correlation of the Taft reaction parameter for the dehydration of secondary alcohols (see Table 4) on four different oxide catalysts with the heat of adsorption, A//ads> °f water and diethylether, with the sensitivity of the rate to pyridine poisoning 7> [55] and with the value of the deuterium kinetic isotope effect [123] for the same catalysts.

As appears from the examination of the equations (giving the best fit to the rate data) in Table 21, no relation between the form of the kinetic equation and the type of catalyst can be found. It seems likely that the equations are really semi-empirical expressions and it is risky to draw any conclusion about the actual reaction mechanism from the kinetic model. In spite of the formalism of the reported studies, two observations should be mentioned. Maatman et al. [410] calculated from the rate coefficients for the esterification of acetic acid with 1-propanol on silica gel, the site density of the catalyst using a method reported previously [418]. They found a relatively high site density, which justifies the identification of active sites of silica gel with the surface silanol groups made by Fricke and Alpeter [411]. The same authors [411] also estimated the values of the standard enthalpy and entropy changes on adsorption of propanol from kinetic data from the relatively low values they presume that propanol is weakly adsorbed on the surface, retaining much of the character of the liquid alcohol. 

Herrman [446] established an order-of-magnitude agreement between the values of the adsorption coefficients obtained by direct measurements of adsorption of alcohol and water vapour and those evaluated from kinetic data as KB and KR, which, in the author s opinion, supported the physical meaning of these constants. 

The structure of the reactants can affect the relative adsorptivities of ester and alcohol and perhaps, according to the view of Setmek and Rodriquez [435], also the kinetic mechanism, as already discussed in Sect. 4.1.3.(a) [see eqns. (26) and (28)]. 

Most liquid chromatographic experiments performed with PAD employ alkaline mobile phases or use postcolumn addition of base to get the electrode at the appropriate pH for the formation of the oxide. The exceptions to this are the detection of carbohydrates and alcohols in acidic media and the detection of sulfur compounds. The oxidation of carbohydrates and alcohols is not oxide catalyzed, and since they exhibit a stronger adsorption to piatinum than gold, they can be determined under acidic conditions. Sulfur compounds are adsorbed at oxide-free surfaces, and the kinetics for detection are favorable even at pH values below 7. 

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