Secondary shape selectivity

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. 

Another example of secondary shape selectivity is shown by John and co-workers (77,73). They found that the hydroisomerization/hydroeracking of n-hexane over Pt/H-mordenite is significantly inhibited by the presence of benzene. They also found a correlation between the aromatic size relative to zeolite pore size on the inhibition of the hexane reaction and the changes in isomer selectivities. Figure 4 illustrates the relation between the various aromatics co-fed and the n-hexane isomerization rates on H-mordenite. From Figure 4, it is shown that as the kinetic diameter of the aromatics is increased, the isomer formation rate appears to pass through a minimum. This result can be explained by considering the size of the zeolite pore and the kinetic... 

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. 

For the classic types of molecular shape-selectivity in zeolites, the reader is referred to the excellent review papers in literature [18-25]. In this paper we elaborate on the recently proposed distinction between primary and secondary shape-selectivity [26], and on the more or less abused concept of cage and window effects in cracl g and hydrocracking. In addition, some evidence available in literature for the speculative mechanism of pore mouth catalysis is presented. 

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. 

Namba et al. studied the cracking of octane on H-ZSM-5 in the presence of other alkanes [27]. The reaction conditions were such that the conversion of octane obeyed first order kinetics and that the coverage of the active sites was low. The octane conversion was not affected by the presence of 3-methylheptane or 2,2,4-trimethylpentane in the feed. 3-Methylheptane is expected to diffuse rapidly through the ZSM-5 pores, while 2,2,4-trimethylpentane is excluded from the pores. Secondary shape-selectivity does not occur with these two molecules. However, the octane conversion dropped sharply with increasing partial pressures of 2,2-dimethylbutane in the feed. This strong inhibition caimot be the result of primaiy shape-selectivity, since the competing 2,2-dimethylbutane molecule should not be selectively adsorbed over octane. The explanation is that the slowly diffusing 2,2-dimethylbutane molecules retard the diffusion and, consequently, the conversion of the octane molecules. 

In the conversion of a mixture of n-alkanes in absence of secondary shape-selectivity, the molecules with the largest carbon number inhibit the reaction of the less strongly adsorbed shorter molecules. Dauns and Weitkamp observed this phenomenon in the conversion of decane and dodecane on LaY type zeolite catalysts [28]. In this 12-MR zeolite, the conversion of dodecane is not affected by the presence of the lower boiling decane (it decreases from 64 to 60%), while the decane conversion drops from 47 to 20% (Table 1). Santilli and Zones studied the conversion of hexane and hexadecane on I M-5 and SSZ-16 zeolites [26]. The 10-MR zeolite ZSM-5 converts hexane and hexadecane to a similar extent in separate experiments (Table 1). In mixtures, the presence of hexane has little effect on the hexadecane conversion, but the latter inhibits the hexane conversion. However, on the 8-MR zeolite SSZ-16, hexadecane causes a slight increase in the hexane conversion, and hexane keeps hexadecane from reacting (Table 1). In the original work [26], no explanation for this secondary shape-selectivity effect has been... 

Quantitative treatment of shape-selectivity based on reaction/diffusion analysis has been applied to several reactions, including alkylation of toluene with methanol, isomerisation of xylenes and disproportionation of toluene [16,30]. It should be stressed that this diffusion/reaction analysis does not take into account eventual competition effects (secondary shape-selectivity) that may occur when reactants are mixed. 

Secondary shape selectivity occurs when one reagent affects the conversion of... 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

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. 

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). 

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. 

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. 

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl-pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly... 

Spatioselectivity plays a significant role in the formation of the heavy secondary reaction products responsible for deactivation ( coke ). Indeed, coke formation involves various bimolecular steps (condensation, hydrogen transfer) that, as indicated above, are very sensitive to steric constraints. Therefore, the rate of coke formation will greatly depend on the size and shape of cages, channels and their intersections. However, as discussed earlier in 1.3.2, coke formation involves not only chemical steps (involving spatioselectivity) but also physical retention in the zeolite pores due to steric blockage (reverse shape selectivity), at least at high reaction temperatures (43, 44). 

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) ... 

Anisole acetylation, which was one of the main reactions investigated, was first shown to be catalysed by zeolite ten years ago by Bayer (13), which was confirmed by Harvey et al. (14), then by Rhodia (15). Large pore zeolites and especially those with a tridimensional pore structure such as HBEA and HFAU were found to be the most active at 80°C, in a batch reactor with an anisole/acetic anhydride molar ratio of 5 and after 6 hours reaction, the yield in methoxyacetophenone (MAP) was close to 70% with HBEA and HFAU zeolites, to 30% with HMOR and 12% with HMFI. With all the zeolites and also with clays and heteropolyacids, the selectivity to the para-isomer was greater than 98%, which indicates that this high selectivity is not due to shape selective effects but rather to the reaction mechanism (electrophilic substitution). The lower conversion observed with HMOR can be related to the monodimensional pore system of this zeolite which is very sensitive to blockage by heavy secondary products. Furthermore, limitations in the desorption of methoxyacetophenone from the narrow pores of HMFI are probably responsible for the low activity of this intermediate pore size zeolite. 

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. 

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 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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