Primary shape selectivity

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. 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

Santilli and Zones proposed to make a distinction between Primary and Secondary shape-selectivity [26]. Primary shape-selectivity is the result of interactions between a molecule and a micropore system. For mixtures of reagents and conditions of primary shape-selectivity, the rules for competitive adsorption in a sterically unrestricted environment apply. These are preferential adsorption of the molecule with the highest boiling point, and of (a)polar molecules on (a)polar zeolites. Secondary effects are present when one reactant interferes with the conversion of a second one in a way which does not occur in an unrestricted environment. Secondary effects can thus be superimposed on primaiy shape-selectivity. The following examples were given by Santilli and Zones [26] to illustrate the concept of secondary shape- selectivity. 

DIFFUSION/REACnON KINETIC MODELS FOR PRIMARY SHAPE-SELECTIVITY... 

Primary shape-selective phenomena can be rationalized using diffusion/reaction kinetics [20,29]. For two catalytic reactions, in which molecules A and B are converted, separately, the observed selectivity (Sq j ) is defined as ... 

Anhydrous zinc phosphonate Zn(03PCH3) is unreactive with water but reacts with primary amines (chain length up to Cg) and ammonia to form intercalation compounds. The intercalation is shape selective as amines with branching at the a-carbon cannot coordinate to the zinc. Different structures are noted for even and odd carbon chain lengths with odd numbers of carbons in the alkyl chain forming a more efficiently packed interdigitated structure.420... 

It has also been shown that the selectivity features of para-selective catalysts can be readily understood from an interplay of catalytic reaction with mass transfer. This interaction is described by classical diffusion-reaction equations. Two catalyst properties, diffusion time and intrinsic activity, are sufficient to characterize the shape selectivity of a catalyst, both its primary product distribution and products at higher degrees of conversion. In the correlative model, the diffusion time used is that for o-xylene adsorption at... 

D. L. Wu, A. P. Wight, M. E. Davis, Shape Selective Oxidation of Primary Alcohols using Perruthenate-Containing Zeolites, Chem. Commun. 6 (2003) 758-759. 

In comparing the various test procedures, there is always a good agreement found for hydrophobic retention and selectivity as well as for shape selectivity. However, the characterization of silanophilic interaction is still a matter of discussion. In part, the differences are due to the selection of the basic analyte. Therefore, the outcome of every test is different. It has been shown, that the peak asymmetry—used for detection of silanophilic interactions—does not correlate to the pA" value of the basic test solute [64]. A closer look at these data leads to the assumption, that the differences are related to the structure of the basic solute, irrespective of whether a primary, secondary, or a tertiary amine is used. The presence of NH bonds seems to be more important in stationary-phase differentiation than the basicity expressed by the pA value. For comparable test procedures for silanophilic interactions further studies seem to be required. 

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). 

Whereas the acetylation of phenyl ethers over zeolite catalysts leads to the desired products, acetylation of 2-MN occurs generally at the very activated C-l position with formation of l-acetyl-2-methoxynaphthalene (l-AMN). A selectivity for l-AMN close to 100% can be obtained over silicoaluminate MCM-41 mesoporous molecular sieves[22] and FAU zeolites,133 341 whereas with other large pore zeolites with smaller pore size (BEA, MTW, ITQ-7), 2-AMN (and a small amount of l-acetyl-7-methoxynaphthalene, 3-AMN) also appears as a primary product. Average pore size zeolites, such as MFI, are much less active than large pore zeolites. These differences were related to shape selectivity effects and a great deal of research work was carried out over BEA zeolites in order to specify the origin of this shape selectivity the difference is either in the location for the formation of the bulkier (l-AMN) and linear (2-AMN) isomers (only on the outer surface for l-AMN, preferentially within the micropores for 2-AMN)[19 21 24 28 381 or more simply in the rates of desorption from the zeolite micropores.126 32 33 351... 

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

In this paper, we review primary and secondary shape selective acid catalysis with zeolites. Next, we discuss shape selectivity with metal containing zeolites.We conclude with a section that deals with future trends in shape selective catalysis. 

Transition-state selectivity is sometimes difficult to distinguish from product shape selectivity. A recent study by Kim et al. (8) shows that the high para-selectivity for the alkylation of ethylbenzene with ethanol in metallosilicates (MeZSM-5) is not due to product selectivity alone. They conclude that the primary product of the alkylation on ZSM-5 type metallosilicates is p-diethylbenzene which isomerizes further inside the cavity of ZSM-5 to other isomers. As the acid sites of zeolites becomes weaker (achieved by substituting different metals into the framework of the zeolite), the isomerization of the primarily produced p-isomer is suppressed. Although Kim et al. attribute this suppression of the isomerization activity to restricted transition-state selectivity, it is more likely that this suppression is due to the decrease in acid strength. 

For these simulations, the primary isomer distribution is chosen according to the thermodynamic equilibrium (sec Table 6). Such a situation would be encountered in practice when neither the reaction mechanism kineti-cally favors a particular isomer nor restricted transition state shape-selectivity effects occur. The disproportionation reaction is assumed to be unaffected by diffusion (i.e. y < 0.01). The effective diffusivities of the ortho and meta isomers are fixed, and assumed to be equal, but by a factor of Ro smaller than the effective diffusivity of the para isomer. 

Clear-cut examples of effects of zeolite pore architecture on the selectivity of Diels Alder reactions are not easily found. For instance, 4-vinylcyclohexene is formed with high selectivity from butadiene over a Cu -Y zeolite however, the selectivity is intrinsically due to the properties of Cu1, which can be stabilized by the zeolite, and not to the framework as such (30-31). A simple NaY has been used in the cycloaddition of cyclopentadiene and non-activated dienophiles such as stilbene. With such large primary reactants, formation of secondary products can be impeded by transition state shape selectivity. An exemplary reaction is the condensation of cyclopentadiene and cis-cyclooctene (32) ... 

If the charge balancing cation in a zeolite is then the material is a solid acid that can reveal shape selective properties due to the confinement of the acidic proton within the zeolite pore architecture. An example of shape selective acid catalysis is provided in Figure 5.3.7. In this case, normal butanol and isobutanol were dehydrated over CaX and CaA zeolites that contained protons in the pore structure. Both the primary and secondary alcohols were dehydrated on the X zeolite whereas only the primary one reacted on the A zeolite. Since the secondary alcohol is too large to diffuse through the pores of CaA, it cannot reach the active sites within the CaA crystals. 

The study of the textural properties of catalyst supports is of primary importance in terms of understanding the catalytic phenomena involved in petrochemical and refining industry processes. In fact, characteristics, such as the specific surface area, pore size or total porous volume will be useful in various stages of a catalyst s existence its preparation (deposition of active phases), its use in catalysis and its regeneration. They directly influence the physicochemical properties of the solid as well as surface reactivity, shape selectivity and hydrodynamic properties. 

The primary use for the titanium silicalites is as shape selective catalysts for hydrogen peroxide oxidations. " Propylene is converted to propylene oxide at greater than 98% selectivity and 99% peroxide conversion at 50°C over TS-1. 2,97 Butadiene is oxidized to the monoepoxide (Eqn. 10.26), also in high selectivity, and primary alcohols are oxidized to the aldehydes in all cases with selectivites greater than 80%.97... 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

In contrast to the lack of selectivity observed in the TS-1 catalyzed oxidation of 3-penten-2-ol (1) (Eqn. 21.5), the oxidation of 1 with tert-butyl hydroperoxide (TBHP) over Cr-PILC gave the unsaturated ketone, 3, in 82% yield (Eqn. 21.13)42 while the oxidation of 1 over a vanadium pillared montmorillonite (V-PILC) gave the epoxy alcohol, 2, in 94% yield.43 V-PILC, however, does promote the oxidation of primary benzyl alcohols to the acids with tert-butyl hydroperoxide. This reaction exhibits shape selectivity in that para-substituted benzyl alcohols are oxidized while the ortho- and meta- substituted species are essentially inert (Eqn. 21.14).44... 

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. 

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