Productive adsorption

The problem posed by Eq. (6.22), without the additional complication of the O dependence, is a classical problem in heterogeneous catalysis. The usual approach it to use Langmuir isotherms to describe reactant (and sometimes product) adsorption. This leads to the well known Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetics.3 The advantage of this approach is... 

We also assume that adsorbed D and A are in equilibrium with gaseous D and A respectively (pD(g) = Po(ad), pA(g) = pA(ad)) and that product adsorption is relatively weak and their desorption fast, so that the reaction between adsorbed D and A is rate limiting. 

Like other heterogeneous chemical reactions, electrochemical reactions are always multistep reactions. Some intermediate steps may involve the adsorption or chemisorption of reactants, intermediates, or products. Adsorption processes as a rule have decisive influence on the rates of electrochemical processes. 

As stated earlier, the retardation term owing to the product adsorption or the constant K in the Langmuir-type equation, v = k/(l + /<[P]) (Equation 13.4), where P is the condensable product), is related to its adsorption equilibrium driven by the concentration term in solutions. Under boiling conditions, the reaction products are desorbed not only to the solution bulk but also into the bubble formed at the catalyst surface. Because the molecular translational entropy is far larger in the gas phase than in the liquid phase, the... 

Product detection via product adsorption and subsequent analysis has also become a standard technique for Stage I screening [28,37-39], In general, techniques, which are capable of indicating a physical change that can easily be detected like a change in color of the adsorbate, are used in accord with this method. The analysis of the adsorbate can also take place via optical methods in a parallel fashion. 

Another important factor which can significantly affect the shape and position of the voltammetric wave, and hence rate constants, is adsorption of product or reactant on the electrode surface. For a linear adsorption isotherm, if the reactant is adsorbed, a reduction wave will be shifted towards more negative potentials and if the product is adsorbed, towards more positive potentials [160], Non-linear adsorption isotherms give rise to pre-waves (product adsorption) and post-waves (reactant adsorption), a phenomenon which was first discussed by Brdidka at the DME and since then by many authors [161, 162]. At the RDE,... 

In conclusion, decrease in cyclohexanone oxime yield and caprolactam selectivity with time on stream is a major factor in the use of boria on alumina catalyst in the rearrangement reaction. Coke deposition and basic by-product adsorption have been suggested as a means of deactivation. In addition the conversion of water soluble boron, which is selective to lactam formation, to an amorphous water insoluble boron species is another factor that can account for the catalyst deactivation. 

This analysis assumes that no deactivation of catalyst occurs and that there is no competitive product adsorption [10]. 

Cons Throughout this investigation, the risks for data misinterpretation when HT experiments are carried out without satisfactory control of T P conditions were pointed out, possibly combined with physico-chemical side effects like support-induced reaction products adsorption, which may change markedly the output gas concentration and, therefore, the final ranking of performances. 

The use of continuous immobilized cell biofilm reactors eliminates downtime and hence results in superior reactor productivity (2,3). Adsorbed cell continuous biofilm reactors have been shown to favorably affect process economics (4). Application of these reactors reduces capital and operational cost, thus making the process simpler. Within these reactors, cells are immobilized by adsorption, which is a simpler technique than other techniques such as entrapment and covalent bonding (5). Adsorption is a simple technique and can be performed inside the reactors without the use of chemicals, whereas entrapment and covalent bonding are complicated techniques and require chemicals for bond formation. In anaerobic systems, such as butanol production, adsorption can be performed anaerobically within the reactor. An additional advantage of adsorption is that cells form uniform biofilm layers around the support, which lessens diffusion resistance compared to entrapped and covalently bonded cells. Hence, these reactors are called biofilm reactors. Because of reduction in diffusion resistance, the reaction rate is enhanced. For this reason, adsorption was chosen as the technique to be employed for Clostridium beijerinckii BA101 cell immobilization to produce butanol. In addition to being simple, it has the potential to be used in large-scale reactors. In the present study, clay brick was chosen as the cell adsorption support. It is available at a low cost and is easy to dispose of after use. 

Bimetallics prevent coke deposition and decrease unsaturated products adsorption. Re suppresses coke formation and cause coke redistribution on catalyst components.79 ... 

Since the adsorption of plant metabolite onto adsorbent is a strong function of pH, the effect of pH upon the adsorption capadty of a plant metabolite should be investigated to understand and quantify product adsorption. The pH dependence of indole alkaloids, yohimbine and ajmalicine, and berberine onto XAD-7 has been reported [ 16,20]. As a model system, XAD-7 was examined to quantify berberine adsorption, especially in terms of pH and concentration dependence, which were used for the in situ berberine separation in Thalictrum ru-gosum cultures [19, 20]. Dependence of berberine adsorption on pH and concentration was investigated as shown in Fig. 4. Equilibrium ratio (Q/C), an affinity of berberine, could be calculated by Eq. (1) ... 

The integration of elicitation, in situ product adsorption with XAD-7, and the immobilization of Catharanthus roseus cells lead to an increase in productivity and a significant increase in extracellular ajmalicine production [5]. The integration of in situ product separation by two-phase culture and immobilized plant cells could be feasible for continuous production in immobilized plant cell bioreactors requiring the repeated use of cells. 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

The opposite of the pre-wave is a post-wave, which is a consequence of the adsorption of reactant on the electrode surface Ox (Ox)aas + ne Red. As the reactant is stabilized by the adsorption, an additional energy is needed for its reduction and the adsorption wave appears at lower potentials than the main wave. After the saturation of the electrode surface, the main wave appears before the adsorption wave, so the latter is called the post-wave. The total response depends on the reactant concentration, the drop life-time and the parameters of the adsorption isotherm similarly as in the case of product adsorption. 

Fig. 15 Increase of yield by application of P-zeolite for in situ product adsorption (Csuc 146 mM, cglc ...

Simpler examples of product adsorption are given elsewhere (4,39). 

Gaseous isobutane purification process from C4 mixture by adsorption has been developed. For the purification, two adsorbents are applied. Zeolite A is used for the removal of paraffin components, and CMS or silver impregnated clay for the removal of C4 olefins. The continuous 3-bed process operated at ambient temperature was comprised of six steps, i.e., pressurization-1 by the cocurrent effluent gas from the other bed, pressurization-2 by isobutane product, adsorption, cocurrent depressurization, countercurrent blowdown, and low pressure purge by isobutane product. 

Problems with rubber materials include leaching of constituents (e.g., zinc) into the product, adsorption of active ingredients or antimicrobial preservatives, and coring of the rubber by repeated insertion of a needle. Coring produces rubber particulates that affect the quality and, potentially, the safety of the product. 

The highest propene oxide yields were obtained with both the Ti-SBA-15- and the Ti-silica-supported catalysts, although a higher reaction temperature was needed in comparison to the titania-supported catalyst. The deactivation for these catalysts was also considerably less. At lower temperatures (up to 423 K), all catalysts had an inhibition period for both propene oxide and water formation, which is explained by product adsorption on the support. The side products produced by all catalysts were similar. Primarily, carbon dioxide and acetaldehyde were produced as side products and, in smaller quantities, also propanal, acrolein, acetic acid, and formaldehyde. Propanol (both 1- and 2- as well as propanediol), acetone, carbon monoxide, and methanol were only observed in trace amounts. 

In Fig. 12.7, it can be seen that this simple model describes the observed activity pattern well. Only in the first half hour of the experiment, a discrepancy exists between the rate predicted by the model and the observed rate. This can be explained by product adsorption on the support as discussed previously [20], which was not included in this model. A more extensive version of this simplified deactivation model, which includes the occupancies of all surface species, is published elsewhere [21]. 

The surface states at the semiconductor electrolyte interface under illumination for the electrochemical redjgtion of carbon dioxide has been determined to be 10 cm. Surface states are induced by adsorbed ions and act as faradaic mediators for the photo-electrochemical reduction of carbon dioxide. It is shown that CO is adsorbed on platinum and adsorbed C0 is the intermediate radical. The rate determining step involves further reduction of CO to give the final products. Adsorption of NH, ions on p-GaP has been studied using FTIRRAS. At cathodic potentials adsorbed ammonium ions are reduced and the reduced ammonium radical desorbs. The structure of adsorbed ammonium is investigated. 

In both reactions (Fig. 2), different Pd and Ft catalysts (Fig. 3, 4) and the combination of vinca and cinchona modifiers (dihydrocinchonidine (DHCND) and dihydrocinchonine (DHCNN)) were tested (Fig. 5, 6, 7). The effect of hydrogen and product adsorption was investigated for this reason the hydrogenation was allowed to proceed to 15-30% conversion before addition of the modifier (Table 1, 2). 

A generalized Volmer-Heyrovsky-Tafel mechanism explained the problem of the intermediate and product adsorption with a nearly zero coverage at potentials approximately equal to that of the net hydrogen evolution process [51]. The electrochemical reaction rates of the individual processes are... 

The performance of immobilized and freely suspended affinity adsorbents was compared by calculating adsorption rates and selectivities for four different bead geometries. Simulation results indicate that the performance of finely ground adsorbent particles immobilized in hydrogel beads is superior compared to freely suspended adsorbents. The mathematical model was further used for simulation studies to investigate the effect of bead design parameters on product adsorption. 

The purpose of this article is to formulate a model which considers simultaneous diffusion and binding reaction within the immobilized adsorbent particles. The model has been developed for batch adsorption processes. It can however be easily modified to predict product adsorption in other reactor configurations. 

Adsorption rate of desired product Adsorption rate of undesired product... 

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