Intrinsic reactivity, catalysts

As described above, the activity of a catalyst can be measured by mounting it in a plug flow reactor and measuring its intrinsic reactivity outside equilibrium, with well-defined gas mixtures and temperatures. This makes it possible to obtain data that can be compared with micro-kinetic modeling. A common problem with such experiments materializes when the rate becomes high. Operating dose to the limit of zero conversion can be achieved by diluting the catalyst with support material. 

Vesicle size was found to affect reaction kinetics for the alkaline hydrolysis and thiolysis of p-nitrophenyl octanoate, with small vesicles being more effective as catalysts, and it was concluded that this size dependence itself was brought about by differences in ion dissociation, substrate binding constants, and intrinsic reactiv-... 

Fig. 2. Reactant concentration as a function of distance from the center of a catalyst particle for fast mixing (A) and slow mixing (B). In both figures, (I) represents a reaction rate limited by intrinsic reactivity at the active site, (2) represents a reaction rate limited by mass transfer, and (3) represents a reaction rate limited by a combination of intraparticle diffusion and intrinsic reactivity. (Reprinted with permission from Ref.73). Copyright 1981 American Chemical Society)...

At one extreme diffusivity may be so low that chemical reaction takes place only at suface active sites. In that case p is equal to the fraction of active sites on the surface of the catalyst. Such a polymer-supported phase transfer catalyst would have extremely low activity. At the other extreme when diffusion is much faster than chemical reaction p = 1. In that case the observed reaction rate equals the intrinsic reaction rate. Between the extremes a combination of intraparticle diffusion rates and intrinsic rates controls the observed reaction rates as shown in Fig. 2, which profiles the reactant concentration as a function of distance from the center of a spherical catalyst particle located at the right axis, When both diffusion and intrinsic reactivity control overall reaction rates, there is a gradient of reactant concentration from CAS at the surface, to a lower concentration at the center of the particle. The reactant is consumed as it diffuses into the particle. With diffusional limitations the active sites nearest the surface have the highest turnover numbers. The overall process of simultaneous diffusion and chemical reaction in a spherical particle has been described mathematically for the cases of ion exchange catalysis,63 65) and catalysis by enzymes immobilized in gels 66-67). Many experimental parameters influence the balance between intraparticle diffusional and intrinsic reactivity control of reaction rates with polymer-supported phase transfer catalysts, as shown in Fig. 1. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

Intraparticle diffusion limits rates in triphase catalysis whenever the reaction is fast enough to prevent attaiment of an equilibrium distribution of reactant throughout the gel catalyst. Numerous experimental parameters affect intraparticle diffusion. If mass transfer is not rate-limiting, particle size effects on observed rates can be attributed entirely to intraparticle diffusion. Polymer % cross-linking (% CL), % ring substitution (% RS), swelling solvent, and the size of reactant molecule all can affect both intrinsic reactivity and intraparticle diffusion. Typical particle size effects on the... 

Variation in % CL of the catalyst support most likely affects intraparticle diffusion more than it affects intrinsic reactivity. Increased cross-linking causes decreased swelling of the polymer by good solvents. Thus the overall contents of the gel become more polystyrene-like and less solvent-like as the % CL is increased. Fig. 5 shows the... 

Substantial variations of the organic solvent used in triphase catalysis with polystyrene-bound onium ions have been reported only for the reactions of 1-bromo-octane with iodide ion (Eq. (4))74) and with cyanide ion (Eq. (3)) 73). In both cases observed rate constants increased with increasing solvent polarity from decane to toluene to o-dichlorobenzene or chlorobenzene. Since the swelling of the catalysts increased in the same order, and the experiments were performed under conditions of partial intraparticle diffusional control, it is not possible to determine how the solvents affected intrinsic reactivity. 

Spacer chain catalysts 3, 4, and 19 have been investigated under carefully controlled conditions in which mass transfer is unimportant (Table 5)80). Activity increased as chain length increased. Fig. 7 shows that catalysts 3 and 4 were more active with 17-19% RS than with 7-9% RS for cyanide reaction with 1-bromooctane (Eq. (3)) but not for the slower cyanide reaction with 1-chlorooctane (Eq. (1)). The unusual behavior in the 1-bromooctane reactions must have been due to intraparticle diffusional effects, not to intrinsic reactivity effects. The aliphatic spacer chains made the catalyst more lipophilic, and caused ion transport to become a limiting factor in the case of the 7-9 % RS catalysts. At > 30 % RS organic reactant transport was a rate limiting factor in the 1-bromooctane reations80), In contrast, the rate constants for the 1 -chlorooctane reactions were so small that they were likely limited only by intrinsic reactivity. (The rate constants were even smaller than those for the analogous reactions of 1-bromooctane and of benzyl chloride catalyzed by polystyrene-bound benzyl-... 

Spacer chains affect intrinsic reactivity as well as intraparticle diffusion. Rates for Br-I exchange reactions with spacer-modified catalyst 41 were larger than those with catalyst 35 containing no spacer (Fig. 11). An aliphatic spacer makes the catalyst more lipophilic and the intrinsic reactivity of the active site larger, though the intraparticle diffusity of an inorganic reagent is reduced. It is not known at this time how intrinsic reactivity contributes to the rate increase. 

The activity of polymer-supported crown ethers depends on solvent. As shown in Fig. 11, rates for Br-I exchange reactions with catalysts 34 and 41 increased with a change in solvent from toluene to chlorobenzene. Since the reaction with catalyst 34 is limited substantially by intrinsic reactivity (Fig. 10), the rate increase must be due to an increase in intrinsic reactivity. The reaction with catalyst 41 is limited by both intrinsic reactivity and intraparticle diffusion (Fig. 10), and the rate increase from toluene to chlorobenzene corresponds with increases in both parameters. Solvent effects on rates with polymer-supported phase transfer catalysts differ from those with soluble phase transfer catalysts60. With the soluble catalysts rates increase (for a limited number of reactions) with decreased polarity of solvent60), while with the polymeric catalysts rates increase with increased polarity of solvent74). Solvents swell polymer-supported catalysts and influence the microenvironment of active sites as well as intraparticle diffusion. The microenvironment, especially hydration... 

Rates for Br-I exchange reactions were 1.5-fold higher with 10 % RS, 1 % CL catalyst 37 when the amount of KI was changed from 2.4 to 8.0 mmol in 0.75 ml of water146). Rates for the same reactions with 26-34 % RS, 2 % CL catalysts 35 and 41 hardly changed as the KI concentration was increased from 6.7 M to 10.0 M. Rates with 14-17 % RS 35 and 41, and with 7 % RS 35, increased by a factors of 1.5 and 2, respectively, with that increase in the KI concentration 149). Apparently the concentration of inorganic salts in the aqueous phase affects complexation constants and/or intrinsic reactivity, especially the hydration state of the active site. The activity of lower % RS catalysts depends more on the salt concentration than does the activity of higher % RS catalysts, because the former are more lipophilic. 

Experimental variables which alter the shape of the metal profiles are a manifestation of the intrinsic reactivity and transport characteristics of the metal-bearing species. The extent to which the catalyst pore volume is utilized to accommodate metal deposits in response to these changes is... 

Many other redox reactions are potentially amenable to antibody catalysis. For example, the chemistry of the P-450 cytochromes, including the hydroxylation of alkanes and the epoxidation of alkenes, can be mimicked with synthetic porphyrins. Incorporation of such molecules into antibody active sites could conceivably yield new catalysts that combine the intrinsic reactivity of the cofactor with the tailored selectivity of the binding pocket. Work is just beginning in this area, but preliminary studies with porphyrin haptens have yielded some interesting results.126-130 Novel redox chemistry can also be anticipated for antibodies containing metal ions, flavins, nicotinamide analogs, and other reactive moieties. 

The intrinsic reactivity of strained cycloalkenes such as norbornene and cyclobutene ensures that they react as desired, and simple homogeneous metal halide catalysts are often effective for this transformation. However for less strained cyclic substrates, manipulation of catalyst activity/selectivity by means of modifying ligands is required. This is where the well-defined alkylidene catalysts pioneered by Grubbs and Schrock have come to the fore. An interesting example illustrating the range of catalyst reactivity is provided by the... 

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