Mass transport reaction selectivity

The second set of reactions is more related to the fine chemicals and pharmaceutical industries, although some of them are carried out industrially on a very significant scale. Temperature-control in three-phase systems is easier, and is rarely a problem, but adequate mixing of the phases is essential to avoid mass-transport limitation. Selectivity here is more directed towards securing the desired product, which may be one of several closely related ones. 

In terms of the reaction parameter (M), two limiting regions of behavior may be identified. For (M) < 0.5, the reaction takes place in the bulk liquid phase and, while the rate of reaction may be affected by the mass transport, the selectivity is unaffected. For (M) > 2, the concentration of A is essentially depleted within the film and no reaction occurs in the bulk liquid phase. Whether there is a significant gradient of B within the film, and hence whether there is a significant effect on selectivity, depends entirely on the relative concentrations of A and B. For large values of the ratio, (Cb /Ca ) 3> the concentrations of B and C in the film... 

With TS-1 as the catalyst, the oxidation products of phenol are hydro-quinone and catechol (para- and ort/to-hydroxyphenol), with minor yields of water and tar formed as by-products. Numerous early papers are concerned with this reaction (218), and patents (219) have been iiled. In the reaction catalyzed by TS-1, the conversion of phenol and the selectivity to dihydroxy products are significandy higher than achievable by either radical-initiated oxidation or acidic catalysts. The catechol/hydroquinone molar ratio is within the range of 0.5—1.3 and depends on the solvent. When the reaction occurs in aqueous acetone, the ratio is close to 1.3. It is believed that the product ratio is the result of restricted transition-state selectivity as well as mass transport shape selectivity associated with the different diffusivities of the ortho and para products. Hydroxylation at the para-position of phenol should be less hindered relative to that at the ortho-position, and hydroqui-none has a smaller kinetic diameter than catechol, facilitating diffusion. Tuel and Taarit (220) proposed that catechol is mainly produced at the external surface of TS-1 crystals. Thus, the different catechol/hydroquinone ratios obtained when methanol or acetone is used as a solvent could be explained by either rapid or very slow poisoning of external sites by organic deposits, respectively. Accordingly, the authors were able to show that tars were easily dissolved by acetone (i.e., external sites for catechol formation remained available in this solvent) while they were insoluble in methanol. 

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

Rates and selectivities of soHd catalyzed reactions can also be influenced by mass transport resistance in the external fluid phase. Most reactions are not influenced by external-phase transport, but the rates of some very fast reactions, eg, ammonia oxidation, are deterrnined solely by the resistance to this transport. As the resistance to mass transport within the catalyst pores is larger than that in the external fluid phase, the effectiveness factor of a porous catalyst is expected to be less than unity whenever the external-phase mass transport resistance is significant, A practical catalyst that is used under such circumstances is the ammonia oxidation catalyst. It is a nonporous metal and consists of layers of wire woven into a mesh. 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

Worz et al. stress a gain in reaction selectivity as one main chemical benefits of micro-reactor operation [110] (see also [5]). They define criteria that allow one to select particularly suitable reactions for this - fast, exothermic (endothermic), complex and especially multi-phase. They even state that by reaching regimes so far not accessible, maximum selectivity can be obtained [110], Although not explicitly said, maximum refers to the intrinsic possibilities provided by the elemental reactions of a process under conditions defined as ideal this means exhibiting isothermicity and high mass transport. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

Therefore, criteria in the selection of an electrode reaction for mass-transfer studies are (1) sufficient difference between the standard electrode potential of the reaction that serves as a source or sink for mass transport and that of the succeeding reaction (e.g., hydrogen evolution following copper deposition in acidified solution), and (2) a sufficiently low surface overpotential and rate of increase of surface overpotential with current density, so that, as the current is increased, the potential will not reach the level required by the succeeding electrode process (e.g., H2 evolution) before the development of the limiting-current plateau is complete. 

Rieckmann and Volker fitted their kinetic and mass transport data with simultaneous evaluation of experiments under different reaction conditions according to the multivariate regression technique [116], The multivariate regression enforces the identity of kinetics and diffusivities for all experiments included in the evaluation. With this constraint, model selection is facilitated and the evaluation results in one set of parameters which are valid for all of the conditions investigated. Therefore, kinetic and mass transfer data determined by multivariate regression should provide a more reliable data basis for design and scale-up. 

It must be emphasized that the above considerations were made in the absence of reaction. Interfacial mass transfer followed by reaction requires further consideration. The Hatta regimes classify transfer-reaction situations into infinitely slow transport compared to reaction (Hatta category A) to infinitely fast transport compared to reaction (Hatta category H) [42]. In the first case all reaction occurs at the interface and in the second all reaction occurs in the bulk fluid. Homogenous catalytic hydrogenations, carbonylations etc. require consideration of this issue. An extreme example of the severity of mass transport effects on reactivity and selectivity in hydroformylation has been provided by Chaudari [43]. Further general discussions for homogeneous catalysis can be found elsewhere [39[. 

The objectives of using solvents are diverse, e.g., to dissolve a solid substrate, to limit catalyst deactivation, to improve selectivity, or to enhance mass-transport. The solvents are selected depending on the substrate and the desired effect. Hence, they range from water, alcohols, ethers, or low alkanes, to CO2. The effects of the solvent on phase-behaviour, viscosity, and density at different concentrations, temperatures and pressures can explain much about the effect of the solvent on the reaction. 

Double-potential-step chronoamperometry does not require precise control of potential. It is only necessary that the potential during each step be at a value for which the desired reaction occurs at the mass-transport-limited rate. Thus, it was not necessary to correct for solution iR drop in these experiments as it would have been if cyclic voltammetry had been selected as the method for quantitative evaluation of the rate constant. 

Different complications arise if the selective layer is a solid-state ionic conductor. At such an interface, a net electrochemical reaction, governed by Faraday s law, takes place and the mass transport of the electroactive ionic species within the contact region and formation of a depletion layer must be considered. In general, when the response of the sensor depends on the chemical modulation of the contact resistance by one of the above mechanisms it will be a strongly nonlinear function of concentration. Furthermore, because Rc is always dependent on the applied voltage, the optimization of the response must be done by examining the voltage-current characteristics of the contact. 

Several quantitative analyses of the effect of intraparticle heat and mass transport have been carried out for parallel, irreversible reactions [1]. Roberts and Lamb [2] have worked on the effect of reversibility on the selectivity of parallel reactions in a porous catalyst. The reaction selectivity of a kinetic model of two parallel, first order, irreversible reactions with a second order inhibition kinetic term in one of them has also been investigated [3]. 

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