Product-selective catalysis

Product selective catalysis only products of the appropriate size leave the cavity. Different products have different diffusion rates out of cavihes. 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The interest and success of the enzyme-catalyzed reactions in this kind of media is due to several advantages such as (i) solubilization of hydrophobic substrates (ii) ease of recovery of some products (iii) catalysis of reactions that are unfavorable in water (e.g. reversal of hydrolysis reactions in favor of synthesis) (iv) ease of recovery of insoluble biocatalysts (v) increased biocatalyst thermostability (vi) suppression of water-induced side reactions. Furthermore, as already said, enzyme selectivity can be markedly influenced, and even reversed, by the solvent. 

Apart from the cyclopropanation reaction, only one example has been published of the application of ionic liquids as reaction media for enantio-selective catalysis with bis(oxazoline) ligands. In this case, the complex 6b-ZnCl2 was used as a catalyst for the Diels-Alder reaction between cyclopen-tadiene and N-crotonyloxazolidin-2-one in dibutyUmidazoUiun tetrafluorob-orate (Scheme 9) [48]. Compared with the same process in CH2CI2, the reaction was faster and both the endofexo selectivity and the enantioselectivity in the endo product were excellent. However, experiments aimed at recovering the catalysts were not carried out. 

Reverse Osmosis Technology Applications for High-Purity-Water Production, edited by Bipin S. Parekh Shape Selective Catalysis in Industrial Applications,... 

Note that although the production of carbon nanotubes does not lend itself to an easy scale-up, the tunability of the carbon nanotube radii and the perfection of its structure could be important for their use as a template for the growth of inorganic nanotubes with a controlled radius. This property can be rather important for the selective catalysis of certain reactions, where either the reaction precursor or the product must diffuse through the (inorganic) nanotube inner core. 

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). 

A second use of this type of analysis has been presented by Stewart and Benkovic (1995). They showed that the observed rate accelerations for some 60 antibody-catalysed processes can be predicted from the ratio of equilibrium binding constants to the catalytic antibodies for the reaction substrate, Km, and for the TSA used to raise the antibody, Kt. In particular, this approach supports a rationalization of product selectivity shown by many antibody catalysts for disfavoured reactions (Section 6) and predictions of the extent of rate accelerations that may be ultimately achieved by abzymes. They also used the analysis to highlight some differences between mechanism of catalysis by enzymes and abzymes (Stewart and Benkovic, 1995). It is interesting to note that the data plotted (Fig. 17) show a high degree of scatter with a correlation coefficient for the linear fit of only 0.6 and with a slope of 0.46, very different from the theoretical slope of unity. Perhaps of greatest significance are the... 

For the production of chemicals, food additives and pharmaceutical products, homogeneous catalysis offers some attractive features such as a high selectivity and activity, e.g. in asymmetric synthesis. However, since most homogeneous catalysts are relatively expensive, their current industrial application is limited [3]. On the other hand, heterogeneous catalysts can easily be separated from the products and can be recycled efficiently. Membrane separations with emphasis on nanofil-tration and ultrafiltration will allow for a similar recyclability of homogeneous catalysts, which is important both from an environmental as well as a commercial... 

Figure 8. Comparison of phenol conversion and ortho products selectivity performance of Cul-xCoxFe204 catalysts at 3250C, TOS = 3h (right panel) and atomic ratio of Cu/(Co+Fe) (or Co/Fe) on Cu-containing (Cu-less) catalysts (left panel). Note the large production of desired 2,6-xylenol and Cu/(Co+Fe) = 0.9 at x = 0.5 composition on spent catalyst. Reprinted from Journal of Catalysis, 210, Mathew T., et al., 2002, 405-417 with permission from Elsevier.

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

Many of the 60 known reactions catalyzed by monoclonal antibodies involve kinetically favored reactions e.g., ester hydrolysis), but abzymes can also speed up kinetically disfavored reactions. Stewart and Benkovic apphed transition-state theory to analyze the scope and limitations of antibody catalysis quantitatively. They found the observed rate accelerations can be predicted from the ratio of equilibrium binding constants of the reaction substrate and the transition-state analogue used to raise the antibody. This approach permitted them to rationalize product selectivity displayed in antibody catalysis of disfavored reactions, to predict the degree of rate acceleration that catalytic antibodies may ultimately afford, and to highlight some differences between the way that they and enzymes catalyze reactions. 

There is a wide range of possibilities for adjusting the solubility characteristics of ionic liquids, and this is one of their potential advantages for optimized performance in biphasic or multiphasic catalysis (/). Because of the generally weak coordinating ability of the anions, most catalysts can be isolated in the solvent in a stable state without loss of activity. The product selectivity can sometimes be improved as well by the phase isolation. Because the catalyst is concentrated in the ionic phase, the reaction volume can be much smaller than in classical... 

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

Three types of shape-selective catalysis are distinguished depending on whether pore size limits the entrance of reactant molecules, the departure of product molecules, or the formation of certain transition states [6]. The suitability of zeolite structure for the catalysis is essential for high shape-selectivity. Alkylation of biphenyl is also explained by sterlc control by pore size and shape of zeolite. HY, HL and HM have different pore structures... 

Supercritical fluids are benign alternatives to conventional organic solvents that may offer improvements in reaction rate, product selectivity, and product separation. We reported the first use of SCFs for phase-transfer catalysis (PTC), where these benign alternatives also offer greatly improved transport, product separation, catalyst recycle, and facile solvent removal (26-29). 

Zeolites are well known for shape-selective catalysis. Here the shape of the zeolite pores or cavities can control the shape of product. When catalytic reactions take place in channels of zeolites only those products that can be accommodated in the channels advance and emerge. Mobil s ZSM-5 is an example of a shape-selective catalyst. Many more zeolites with different pore sizes or large surface areas are being synthesized, extending the principle of shape-selective catalysis. Such developments are helpful for both existing industrial processes and environmental protection. 

Product shape-selective catalysis only products less than a certain dimension can... 

FIGURE 7.18 Shape-selective catalysis (a) reactant, (b) product, and (c) transition state. 

One of the industrial processes using ZSM-5 provides us with an example of product shape-selective catalysis the production of l,4-( ara- xylene. Para-xylene is used in the manufacture of terephthalic acid, the starting material for the production of polyester fibres such as Terylene . 

Whereas Pd-catalyzed asymmetric allylic substitution reactions, with carbon as well as with heteronucleophiles, are widespread in stereoselective catalysis, it seems unusual that sulfur nucleophiles are less commonly used. Therefore we tested our ligands in such a reaction. We employed ligands 2 and 3 successfully in the reaction of racemic 3-methoxycarbonyloxyhept-4-ene with lithium t-butylsulfinate in the presence of 1.5 mol% of Pd2dba3 and 4.5 mol% of the ligands. In all cases full conversion was achieved, but with marked differences in the product selectivities (Scheme 1.4.9, Table 1.4.7). 

Shape-selective catalysis in zeolite requires that the reactants diffuse inwards to the active sites located at the intracrystalline volume (pores), and that products counterdif-fuse after the reaction. At the active sites, presence of a high local electric field may direct the reaction according to steric requirements to yield specific products. Thus, shape-selectivity may be achieved by virtue of geometric factors, Coulombic field at the active sites and/or difference in diffusion rates. Accordingly, three different kinds of shape-selectivity are distinguished (Dwyer, 1984). If the geometric factors are such that... 

Figure 8.21 Relation between diffusion and shape-selective catalysis by zeolites (a) diffusion coefficients of -alkanes in potassium T-zeolite and (b) product distribution for the cracking of n-tricosane over H-erionite. (Following Gorring, 1973.)...

Zeolites are the most promising microporous materials for achieving highly shape-selective catalysis because their pores are uniformly distributed and have dimensions allowing both the organic reactants and products to enter, to react, and to leave.1... 

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