Shape Selectivity reactant

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

The critical sizes of the reactant molecules were estimated and are shown in Figure 5, where the figures for 2-hexanol, isopropylacetate, sec-butylacetate and cyclohexylacetate are estimated by MM2 from Pauling s atomic radius and molecular model [18]. Therefore, the unique catalysis of Cs2.2 is understood if one assumes that it is active only for small molecules. In other words, this catalyst exhibits "reactant shape selectivity", where the catalyst differentiates the reactants according to their size. 

There are four widely accepted theories of shape selectivity reactant shape selectivity (RSS), product shape selectivity (PSS), transition state selectivity (TSS) (Figure 12.2), and concentration effect all of them are based on the hypothesis that the reactions occur within the zeolite micropores only. As indicated earlier, this hypothesis is often verified, the external surface area of the commonly used zeolites being much lower (one to two orders of magnitude) than their internal surface area. ... 

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

Figure 11. Reactant shape selectivity in the oxidation of cC6 and cC12 on FePcY as a function of the size of the exchanged cations [69].

Reactant shape-selective catalysis only molecules with dimensions less than a critical... 

Reactant shape-selective catalysis is demonstrated in the dehydration of butanols. If butan-l-ol (rt-butanol) and butan-2-ol (rso-butanol) are dehydrated... 

This system demonstrates reactant shape-selective catalysis. The branched hydrocarbons are too bulky to pass through the pore openings in the catalyst. 

Both H2O2 and hydroperoxides are industrially important oxidants. An accurate evaluation of advantages and disadvantages requires an accurate analysis of every specific case, in view of the different technical problems and economic constraints that the use of one or the other entails. The reactivity of H202 is so high that it can easily oxidize many primary reaction products, and these reactions become more likely as the reaction temperature is increased. Some of these reactions are influenced by reactant shape selectivity and by restricted transition-state shape selectivity. 

However, the reactivities of primary alcohols are much lower than the reactivities of secondary alcohols. While an increase in reactivity of 2-alcohols with increasing chain length can be expected on the basis of chemical reactivity, the decrease beyond C8 must have another origin, which may be reactant shape selectivity in the TS-1 catalyst. The 2-alcohol generally react faster than the 3-alcohol (Van der Pol et al., 1993b). 

Reactant shape selectivity effects related to the dimensions of reactant molecules and catalyst pores, including restricted transition-state shape-selectivity effects as well as chemical and stereochemical selectivity. 

Cyclo-paraffins react more readily with REHY, presumably as a result of pore size differences between the 10 ring ZSM-5 and the 12 ring faujasite, resulting in size discrimination (reactant shape selectivity). Over both zeolites, C9-C10 cyclo-olefins are completely converted so that size discrimination is not observed. However, where conversion of cyclo-olefins is not complete (i.e. C6-C8) there is clear evidence for discrimination between the two zeolites (Figure 7). Presumably this high reactivity of C9/C10 cyclo-olefins over either zeolite can be explained by initial facile attack at outer surfaces. 

Shape selective reactions are typically carried out over zeolites, molecular sieves and other porous materials. There are three major classifications of shape selectivity including (1) reactant shape selectivity where reactants of sizes less than the pore size of the support are allowed to enter the pores to react over active sites, (2) product shape selectivity where products of sizes smaller than the pore dimensions can leave the catalyst and (3) transition state shape selectivity where sizes of pores can influence the types of transition states that may form. Other materials like porphyrins, vesicles, micelles, cryptands and cage complexes have been shown to control product selectivities by shape selective processes. 

Primary Shape Selectivity. There are several types of shape and size selectivity in zeolites. First, the reactant molecules may be too large to enter the cavities. A particularly good illustration of this behavior is given by Weisz and co-workers (5). Zeolites A and X were ion exchanged with calcium salts to create acid sites within the zeolite. These acid sites are formed as the water of hydration around the calcium ions hydrolyzes. When these zeolites are contacted with primary and secondary alcohols in the vapor phase, both alcohols dehydrate on CaX but only the primary one reacts on CaA. Since the secondary alcohol is too large to diffuse through the pores of CaA, it can not reach the active sites within the CaA crystals. This kind of selectivity is called reactant shape selectivity and is illustrated in Figure 3. 

Separation of molecules with different sizes can be achieved by a proper choice of zeolites (nature of the zeolite and adjustment of the pore architecture, especially the pore size). The simplest forms of shape selectivity come from the impossibility of certain molecules in a reactant mixture entering the zeolite pores (reactant shape selectivity) or of certain product molecules (formed inside the pore network) exiting from these pores (product shape selectivity). In practice, reactant and product shape selectivities are observed not only when the size of molecules is larger than the size of the pore apertures (size exclusion) but also when their diffusion rate is significantly lower than that of the other molecules. Differences of diffiisivities by 2 orders of magnitude are required to produce significant selectivities between reactant species (35). 

Reactant shape selectivity was the basis of the Selectoforming process previously mentioned. The n-alkanes of light gasoline (essentially n-pentane, n-hexane) enter the pores of the erionite catalysts and are transformed into propane and n-butane, whereas the branched alkanes are excluded from the pores and do not react (Figure 1.5a). 

There are numerous examples of reactant shape selectivity in the hydrogenation of olefins over noble-metal loaded zeolites (49). This can be important to remove impurities from olefin feedstocks, or as a criterion to assess the location of the noble metal, at the outer or inner surface of the zeolite. However, shape selectivity is also increasingly used in reductive conversion of (poly)functional molecules. 

Figure 5 schematically illustrates the concepts of reactant shape selectivity. Only linear paraffins that are able to diffuse and are adsorbed inside the pores can undergo a chemical transformation, e.g., acid catalyzed cracking. The property is exploited in some chemical processes, such as the dewaxing of lubes and middle distillates, through the selective cracking or isomerization of the linear paraffin fraction. 

However, since epoxidation occurs within pores of cross-section comparable to that of the olefin, steric restrictions generally prevail over inductive effects, leading to anomalous reactivity orders. They result from restrictions to diffusion in the pores (reactant shape selectivity) and to the approach of the double bond to the active species (transition state shape selectivity). The first is sufficient to explain... 

One of the most in ortant properties of zeolites is their ability to cany out shqie selective reactions [5]. These can be cl sified as, firstfy, product shape selective reactions in which the only products formed are those which can diffiise out of e pores of die zeolite, second, reactant shape selective reactions which occur when some of the molecules in a reactant mbcture are too large to diffiise through the catalyst pores, and, thirdfy, restricted transition-state selective reactions in which the only reactions which occur are those in which qiace exists in the pores or cavities to allow the formation of the activated transition state con lex. In some cases where the zeolite is three dimensional the gze of the channel intersections will also be a determining ictor. This unique catalytic property is related to the pore size of the zeolite and has led to the synthesis of zeohtes with a very w e range of pore gzes. 

In acid catalyzed reactions reactant shape selectivity reverses the usual order of carbocation reaction rates. Acid catalyzed reactivities of primary, secondary, and tertiary carbons differ. Tertiary carbon atoms form the most stable carbocations therefore, they react much faster than secondary carbon atoms. Primary carbon atoms do not form carbocations under ordinary conditions and therefore do not react. Only secondary carbocations can form on normal paraffins whereas tertiary carbocations form on singly branched isoparaffins. Therefore, in most cases, isoparaffins crack and isomerize much faster than normal paraffins. This order is reversed in most shape selective acid catalysis, that is, normal paraffins react faster than branched ones, which sometimes do not react at all. This is the essence of many applications of reactant or product type shape selective acid catalysis. 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

We can t see the organic molecules as they wiggle in and out of these channels. But we can measure how quickly or slowly they diffuse into ZSM-5. From diffusion studies (Ref. 11), we know that only straight chain and mono-methyl paraffins and olefins, certain one-ring aromatic and naphthenic molecules diffuse at useful rates through ZSM-5. The less bulky the molecule, the faster the diffusion rate. Larger molecules either diffuse in slowly, and react at a lower rate, or they are completely excluded. We call this reactant shape selectivity. 

Once admitted, the paraffins are catalytically cracked into smaller gasoline-type hydrocarbons, which escape easily. This gives us the ability to crack unwanted paraffins out of our liquid products, and, as a bonus, to make a little extra gasoline. We ve developed four commercial processes based on the reactant shape selectivity of ZSM-5 two for cracking out low octane paraffins in gasoline, and two for removing waxy paraffins from distillate fuels and lubricating oils (Refs. 12, 13). 

Reactant shape selectivity. The most obvious shape selectivity, and the first to be exploited in practice, stems from the fact that the molecules of some reactants can enter the catalyst... 

FIGURE 10.4 Reactant shape selectivity for zeolite catalysts. 

Control of the selectivity in catalytic reactions is one of the challenging topics. For example, the shape selectivity in catalytic reactions was described for the first time by Mobil research workers [3]. At the present, some categories of shape selectivity have been described in the literatures, i.e. reactant shape selectivity, product shape selectivity and transition state shape selectivity [4,5]. Zeolites such as ZSM-5 show the promising catalytic performance for the shape selectivity of reactants or products in the alkylation of aromatics and in the catalytic cracking of hydrocarbons [6]. The shape selectivity by zeolite catalysts... 

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