Selectivities in porous catalysts

The best way to improve the selectivity of a catalytic process is to find a more selective catalyst. Even so, loss of selectivity may occur caused by concentration gradients resulting from internal diffusion limitations. 

An important selectivity problem arises with the following reaction scheme  

The undesired reactions leading to X or X can be suppressed by using a sufficient excess of B. The desired reaction is so rapid, that A has disappear at a short distance from the external particle surface ((p 3, Ti ) Since there is an excess of B, it will penetrate further into the porous catalyst where it may undergo one of the undesired reactions leading to T or K. The best way to avoid this is to use smaller catalyst particles, or a coated catalyst, and choose conditions such that the effectiveness factor for reactant A is close to 1. 

In section 5.3.1.2 we discussed the case of the chemical conversion of a massive solid that is not soluble, so that the reaction takes place at the solid surface. This is a type of reaction of which there are very many examples, particularly in inorganic chemistry (e.g., in hydrometallurgy). In many cases, however, the solid is porous. This case will be treated here. It is to be distinguished from the case where the solid is slightly soluble and reacts in the fluid phase (see sections 53.4.2 and 5.4.2J, p. 153). 

The insoluble solid (B) is present in the form of porous particles, that are attacked by another chemical (A), that is present in excess in the continuous phase. The surface reaction is described by eq. (5.11). We consider isothermal conditions. Often two distinct phases in the process can be recognized  

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The affect of diffusion on catalyst selectivity in porous catalysts operating under non-isothermal conditions has been examined by a number of workers. The mathematical problem has been comprehensively stated in a paper [21] which also takes into account the affect of surface diffusion on selectivity. For consecutive first-order exothermic reactions, the selectivity increases with an increase in Thiele modulus when the parameter A (the difference between the activation energy for reaction... 

Wheeler<26) considered the problem of chemical selectivity in porous catalysts. Although he employed a cylindrical pore model and restricted his conclusions to the effect of pore size on selectivity, the following discussion will be based on the simple geometrical model of the catalyst pellet introduced earlier (see Fig. 3.2 and Section 3.3.1). 

Adsorption/desorption and pore-transport are key parameters influencing the activity, effectiveness factors, and product selectivity in porous catalysts. With conventional reaction media (either gas or liqnid phase), one of these parameters is generally favorable while the other is not. For instance, while desorption of heavy hydrocarbons from the catalyst is nsnally the rate-limiting step (and therefore detrimental to catalyst performance) in gas-phase reactions, transport of the reactants/products is the limiting step in liquid-phase reaction media. Furthermore, with conventional media, it is usually difficult to achieve the desired combination of flnid properties for optimum system performance. In contrast, density and transport properties can be continuously pressure-tuned in the near-critical region to obtain unique fluid properties (eg, gas-like transport properties yet liquid-like solvent power and heat capacities). 

The reactions are still most often carried out in batch and semi-batch reactors, which implies that time-dependent, dynamic models are required to obtain a realistic description of the process. Diffusion and reaction in porous catalyst layers play a central role. The ultimate goal of the modehng based on the principles of chemical reaction engineering is the intensification of the process by maximizing the yields and selectivities of the desired products and optimizing the conditions for mass transfer. 

Mass transfer (continued) in monolith, 27 89 in porous catalyst, 27 60-63, 68 in tubular reactor, 27 79, 82, 87 Mass transport processes, 30 312-318 convective, 30 312-313 diffusive, 30 313-315 selectivity, 30 316... 

Wheeler s treatment of the intraparticle diffusion problem invokes reaction in single pores and may be applied to relatively simple porous structures (such as a straight non-intersecting cylindrical pore model) with moderate success. An alternative approach is to assume that the porous structure is characterised by means of the effective diffusivity. (referred to in Sect. 2.1) which can be measured for a given gaseous component. In order to develop the principles relating to the effects of diffusion on reaction selectivity, selectivity in isothermal catalyst pellets will be discussed. 

The influence which the simultaneous transfer of heat and mass in porous catalysts has on the selectivity of first-order concurrent catalytic reactions has recently been investigated by 0stergaard(27). As shown previously, selectivity is not affected by any limitations due to mass transfer when the process corresponds to two concurrent first-order reactions ... 

Pore-diffusion limitations in porous catalysts affect catalyst and/or product selectivity. Two instances are considered here. The first involves a parallel reaction network in which the desired product is produced in one reaction and coke precursors form via the second reaction. Accumulation of the coke precursors in the catalyst leads to catalyst fouling and causes a continuous decline in catalyst activity. The second example is a series reaction network of the type A B C. Clearly, pore-diffiision limitations would hinder the removal of B (the desired product) from the catalyst, favoring further reaction to the ultimate product, C. Complementary experimental and theoretical investigations are presented below to show how pressure tuning with supercritical media may be exploited to stabilize catalyst activity and to enhance the selectivity of primary products. 

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. 

Selectivity of parallel and consecutive reactions and of reac tions in a porous catalyst... 

There are comparable incentives to develop new process-related materials that are more selective as catalysts, extractants, or separation membranes and more effective in controlling flow in porous media. In addition, the development of materials that are less energy intensive in terms of production and use is a goal equivalent to other means of energy conservation. 

In catalysis active sites are operative that allow for an alternative reaction path. For a satisfactory catalyst this alternative pathway leads to higher rates and higher selectivity. In heterogeneous catalysis reactant molecules adsorb at active sites on the catalyst surface at the surface sites reactions occur and products are desorbed subsequently. After desorption, active sites are again available for reactant molecules and the cycle is closed. In homogeneous catalysis the situation is essentially identical. Here complexation and decomplexation occur. A complication in heterogeneous catalysis is the need for mass transfer into and out of the catalyst particle, which is usually porous with the major part of the active sites at the interior surface. 

In any catalyst selection procedure the first step will be the search for an active phase, be it a. solid or complexes in a. solution. For heterogeneous catalysis the. second step is also deeisive for the success of process development the choice of the optimal particle morphology. The choice of catalyst morphology (size, shape, porous texture, activity distribution, etc.) depends on intrinsic reaction kinetics as well as on diffusion rates of reactants and products. The catalyst cannot be cho.sen independently of the reactor type, because different reactor types place different demands on the catalyst. For instance, fixed-bed reactors require relatively large particles to minimize the pressure drop, while in fluidized-bed reactors relatively small particles must be used. However, an optimal choice is possible within the limits set by the reactor type. 

When the effectiveness factors for both reactions approach unity, the selectivity for two independent simultaneous reactions is the ratio of the two intrinsic reaction-rate constants. However, at low values of both effectiveness factors, the selectivity of a porous catalyst may be greater than or less than that for a plane-catalyst surface. For a porous spherical catalyst at large values of the Thiele modulus s, the effectiveness factor becomes inversely proportional to (j>S9 as indicated by equation 12.3.68. In this situation, equation 12.3.133 becomes... 

This equation gives the differential yield of V for a porous catalyst at a point in a reactor. For equal combined diffusivities and the case where hT approaches zero (no diffusional limitations on the reaction rate), this equation reduces to equation 9.3.8, since the ratio of the hyperbolic tangent terms becomes y/k2 A/ki v As hT increases from about 0.3 to about 2.0, the selectivity of the catalyst falls off continuously. The selectivity remains essentially constant when both hyperbolic tangent terms approach unity. This situation corresponds td low effectiveness factors and, in tliis case, equation 12.3.149 becomes... 

The described method for the diagnostic of the activity and the transport hindrances in air gas-diffusion electrodes is very useful in the research of porous catalysts for air electrodes. The comparison of the activity and the transport hindrances of air electrodes with catalysts from various types of active carbon allow a proper selection to be accomplished. 

The analysis of the literature data shows that zeolites modified with nobel metals are among perspective catalysts for this process. The main drawbacks related to these catalysts are rather low efficiency and selectivity. The low efficiency is connected with intracrystalline diffusion limitations in zeolitic porous system. Thus, the effectiveness factor for transformation of n-alkanes over mordenite calculated basing on Thiele model pointed that only 30% of zeolitic pore system are involved in the catalytic reaction [1], On the other hand, lower selectivity in the case of longer alkanes is due to their easier cracking in comparison to shorter alkanes. 

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

We have overviewed some strategies for the surface-mediated fabrication of metal and alloy nanoscale wires and particles in mesoporous space, and their structural characterization and catalytic performances. Extension of the present approaches for metal/alloy nanowires may lead to the realization of the prospechve tailored design of super active, selective and stable catalysts applicable in industrial processes. The organometallic clusters and nanowires offer exciting and prospechve opportunities for the creahon of new catalysts for industry. Various metal/ alloy nanowires and nanoparhcles in the anisotropic arrangement in porous supports would help in understanding the unexpected electronic and optic properties due to the quantum effect, which are relevant to the rational design of advanced electronic and optic devices. 

In practice, of course, it is rare that the catalytic reactor employed for a particular process operates isothermally. More often than not, heat is generated by exothermic reactions (or absorbed by endothermic reactions) within the reactor. Consequently, it is necessary to consider what effect non-isothermal conditions have on catalytic selectivity. The influence which the simultaneous transfer of heat and mass has on the selectivity of catalytic reactions can be assessed from a mathematical model in which diffusion and chemical reactions of each component within the porous catalyst are represented by differential equations and in which heat released or absorbed by reaction is described by a heat balance equation. The boundary conditions ascribed to the problem depend on whether interparticle heat and mass transfer are considered important. To illustrate how the model is constructed, the case of two concurrent first-order reactions is considered. As pointed out in the last section, if conditions were isothermal, selectivity would not be affected by any change in diffusivity within the catalyst pellet. However, non-isothermal conditions do affect selectivity even when both competing reactions are of the same kinetic order. The conservation equations for each component are described by... 

Mass and heat transfer between the bulk fluid phase and the external catalyst surface can have an affect on reaction rates, and hence the selectivity, because of modified concentration and temperature driving forces. Such effects are unimportant for porous catalysts, but are significant for catalysis by non-porous metallic gauzes (for example, in NH3 oxidation referred to in Sect. 6.1.1). 

We have presented a general reaction-diffusion model for porous catalyst particles in stirred semibatch reactors applied to three-phase processes. The model was solved numerically for small and large catalyst particles to elucidate the role of internal and external mass transfer limitations. The case studies (citral and sugar hydrogenation) revealed that both internal and external resistances can considerably affect the rate and selectivity of the process. In order to obtain the best possible performance of industrial reactors, it is necessary to use this kind of simulation approach, which helps to optimize the process parameters, such as temperature, hydrogen pressure, catalyst particle size and the stirring conditions. 

Potassium loaded on a porous silicon nitride has been reported as a superbase catalyst. With appropriate pre-treatment, the catalyst was found to be highly active for 2,3-dimethylbut-l-ene isomerisation yielding 2,3-dimethylbut-2-ene in close to 100% selectivity. In order for high activity to be demonstrated, 30 wt% potassium amide was loaded by impregnation. A trace of Fc203 was also added on impregnation. A silicon nitride synthesised via a silicon diimide precursor which was pre-treated at 1000°C was found to be best and the catalyst was activated by heating in vacuo. The possibility that this resulted in active potassium nitride species via ... 

An interesting new concept of catalyst immobilization is the use of supported aqueous phase catalysts. Here, the catalyst is immobilized in a thin water layer adhered within the pores of a high-surface-area porous support. A new Rh catalyst of this class with ligand 11 is stable, recyclable, and highly selective in the hydroformylation of higher alkenes to linear aldehydes.236... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

Lee, C. Bailey, J. E. 1974 Diffusion waves and selectivity modifications in cyclic operation of a porous catalyst. Chem. Engng. Sci. 29,1157. 

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