Selectivity reactant

Reactant selectivity means that only starting materials of a certain size and shape can penetrate into the interior of the zeolite pores and undergo reaction at the cataly-tically active sites. Starting material molecules that are larger than the pore apertures can not react (Fig. 7-5 a). Hence the term molecular sieve is justified. 

Molecule Kinetic diameter [nm] Zeolite, pore size [nm]  

The catalytic characterization of zeolites is generally carried out with the aid of test reactions [8]. For example, the constraint index Cl (Table 7-3) compares the relative rate of cracking of a 1 1 mixture of n-hexane (molecular diameter 0.49 nm) and 3-methylpentane (molecular diameter 0.56 nm). 

The Cl is strongly dependent on the pore size of the zeolite. Small values between zero and two mean little or no shape selectivity (large-pore zeolites), values between two and 12 a medium selectivity (medium-pore zeolites), and values higher than 12 a high shape selectivity (smaU-pore zeolites). 

Thus erionite, with the smallest pore opening of 0.38-0.52 nm, has the highest shape selectivity. It was found that with certain zeolites, the linear alkane n-hexane is cracked 40-100 times faster than the branched isomer 3-methylpentane. This is exploited industrially in the Selectoforming process, in which erionite is added to the reforming catalyst. 

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

Ca.ta.lysts, Catalyst performance is the most important factor in the economics of an oxidation process. It is measured by activity (conversion of reactant), selectivity (conversion of reactant to desked product), rate of production (production of desked product per unit of reactor volume per unit of time), and catalyst life (effective time on-stream before significant loss of activity or selectivity). 

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. 

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

When a microporous material, e.g. a zeolite, is used as a catalyst, only those molecules whose diameters are small enough to enter or pass through the pores can react and leave the catalyst. This is the basis for so-called shape-selectivity (Fig. 3.40). Reactant selectivity is encountered when a fraction of the feed molecules can enter the zeolite, whereas the other fraction cannot. For the molecules produced in the interior the same reasoning applies. The favoured products are the less bulky ones, i.e. those with diameters smaller than the pores of the zeolites. For instance, in the zeolite represented in Fig. 3.40 the production of p-xylene is favoured over the production of o- and m-xylenes. Also the bulkiness of the transition state can lead to a different. selectivity, as shown in the last example in Fig 3.40. 

However, besides the SCR activity, the selectivity is an important parameter for the assessment of the catalysts. In case of the SCR reaction, the selectivity with respect to both the product nitrogen and the reactant ammonia has to be considered. The product selectivity is important, as side products such as N20 can be formed and the reactant selectivity is important, as ammonia can be converted to nitrogen not only in the SCR reaction but also by the selective catalytic oxidation with oxygen [54],... 

Conducting reactions in nanospace where the dimensions of the reaction vessel are comparable to those of the reactants provides a new tool that can be used to control the selectivity of chemical transformations.1 This dimensional aspect of nano-vessels has been referred to as shape selectivity.2 The effect of spatial confinement can potentially be exerted at all points on the reaction surface but its influence on three stationary points along the reaction coordinate (reactants, transition states, and products) deserve special attention.3,4 (1) Molecular sieving of the reactants, excluding substrates of the incorrect dimension from the reaction site can occur (reactant selectivity). (2) Enzyme-like size selection or shape stabilization of transition states can dramatically influence reaction pathways (transition state selectivity). (3) Finally, products can be selectively retained that are too large to be removed via the nano-vessel openings/pores (product selectivity). 

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). 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

We will highlight this system by first giving a brief overview of the architecture, followed by some practical examples that cover several common tasks in the drug discovery process. The goal is not to give a detailed account of the methods employed, but rather to illustrate how the system functions in practice. We will present as examples some of the most widely used chemoinformatics applications customized database access, similarity and substructure searching, reactant selection, and library design. 

Many library design methods require that the size (number of products) and configuration (numbers of reactants selected for each component) of the library are specified upfront. However, it is often difficult to determine optimum values a priori and usually there is a trade-off between these criteria and the other criteria to be optimized. Consider the design of a library where the aim is to maximize coverage of some cell-based chemistry space. It is clear that as more products are included in the library the chance of occupying more cells increases. Thus, an optimal library is likely to be one that represents a compromise in size and diversity. 

Zeolite A (Ca) shows reactant selectivity. The straight-chain /j hexane can pass through the windows and undergo reaction but the branched-chain 3-methylpentane is excluded. The selective cracking of straight-chain hydrocarbons in the presence... 

Reactant selectivity occurs when some of the molecules in a reaction mixture can enter the pores and react in the catalyst pores. However, the molecules that are too large to diffuse through the pores cannot react. 

If the chemistry involves two reactants, selectivity is affected by the concentrations of the reactants. For example, supposed that there are two parallel reactions in which C is the desired product and D is the undesired product ... 

Fig. 1.5 Schematic representation of shape selective effects a) Reactant selectivity Cracking of an n-iso C6 mixture, b) Product selectivity Disproportionation of toluene into para-xylene over a modified HFMI zeolite, c) Spatioselectivity Disproportionation of meta-xylene over HMOR. The diphenylmethane intermediate A in formation of 1,3,5 trimethylbenzene is too bulky to be accommodated in the pores, which is not the case for B...

Any of the compound selection methods that have been developed for reactant selection can also be applied to the product library in a process known as cherry picking. A subset library selected in this way is shown by the shaded elements of the matrix in figure 3. However, a subset of products selected in this way is very unlikely to be a combinatorial library (the compounds in a combinatorial library are the result of combining all of the reactants available in one pool with all of the reactants in all the other pools). Hence, cherry picking is combinatorially inefficient as shown in figure 3 where 7 reactants are required to make the 4 products shown. 

Selectivity is a measure similar to yield in that it tells how much desirable product is made from a reactant. Selectivity is particularly informative for parallel reactions involving the same key reactant. Here it measures what proportion of the converted reactant goes to useful products. In the production of vinyl acetate, for example, ethylene reacts in the main reaction as well as in the side reaction. The selectivity of ethylene to vinyl acetate is... 

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