Zeolites, electrostatic fields

Two important zeolite properties are (1) the intra-pore electrostatic field, and (2) its acid-base character. As discussed below post-synthetic modifications of many zeolites to fine-tune these properties are possible and provide a unique opportunity to influence reaction outcome. 

The Frei photooxygenations in Fig. 32 were all conducted by irradiation of the zeolite powder in the absence of solvent. Solvents shield the substrate from the electrostatic field of the cation and reduces the magnitude of the bathochromic shift.122 Kojima and coworkers reported that irradiation of traw.v-stilbenetajNaY in... 

Adsorbed water molecules on a zeolite adsorbent are polarizable due to a strong electrostatic field between the exchanged cations and alumina framework [26]. 

Oxidations initiated by thermally induced electron transfer in an oxygen-CT complex represent the thermal analog of the Frei photo-oxidation and are properly classified as hybrid type IlAOi-type IIaRH oxidations (Fig, 2), Such reactions require either zeolites with high electrostatic fields or substrates with low oxidation potentials. In addition, elevated temperatures are known to promote the thermally initiated electron-transfer step, although the possible intrusion of a classical free-radical initiation chain oxidation at higher temperatures must be considered. 

The efiect is thus not related to geometrical constraints induced on complexes anchored in mesoporous charmels (sometimes also called as confinement efiect, even if this definition is not properly correct), neither to shape-selectivity effects as possible in zeolites, since the size of mesoporous charmels is much larger than those of micro-porous materials. Instead, an effective modification on the characteristics of the fluids is observed due to the electrostatic field generated by the charmel walls. This is an enthalpic effect versus an entropic effect as observed when the modification is instead related to limitations in the translation modes of molecules. Recently, it was also demonstrated that wall curvature influence the molecular orientation of the... 

Not all zeolite catalysts are used in the decationized or acid form it is also quite common to replace the Na" ions with lanthanide ions such as La " or Ce ". These ions now place themselves so that they can best neutralize three separated negative charges on tetrahedral A1 in the framework. The separation of charges causes high electrostatic field gradients in the cavities which are sufficiently large to polarize... 

The replacement of Si4+ by Al3+ ions in the tetrahedra generates a deficit of one positive charge per aluminum ion, which must be compensated by the incorporation of extrinsic cations in the zeolite structure. The sodium or calcium ions which are most commonly found in natural or synthetic zeolites can be exchanged with other alkali, alkaline-earth, rare-earth, or transition metal ions. The zeolite open structure can accommodate not only the extraframework cations, but also various molecules provided that their size is smaller than the zeolite apertures. A key feature of cation-exchanged zeolites is the local electrostatic field associated with the cations. This has led to the view of zeolites as solid solvents (258 and references therein). 

An additional complexity arises from the dissociation of water molecules which occurs when alkaline-earth-exchanged zeolites are thermally activated since several modes of dehydroxylation are possible. This problem has been extensively investigated by IR spectroscopy, in particular by Ward (268,269) and Uytterhoeven et ai (270), and by X ray (271). They concluded that the electrostatic field associated with the cation causes dissociation of adsorbed water to produce acidic hydroxyl groups. The dissociation reaction may occur according to the following reactions ... 

A carbenoid-type mechanism with free or surface-bound species formed by a elimination from methanol promoted by the strong electrostatic field of zeolites was proposed first.433,456,457 Hydrocarbons then can be formed by the polymerization of methyl carbene, or by the insertion of a surface carbene (8) into a C-O bond453-455,458,459 (Scheme 3.2, route a). If surface methoxyl or methyloxonium species are also present, they may participate in methylation of carbene454,455,460,461 depicted here as a surface ylide (9) (Scheme 3.2, route b). A concerted mechanism with simultaneous a elimination and sp3 insertion into methanol or dimethyl ether was also suggested 433,454,457... 

O ynthetic zeolites have been used as catalysts for many reactions. Their catalytic activity depends strongly on the nature of exchangeable metal cations. Pickert and co-workers (1) proposed that the high catalytic activity of zeolites for carboniogenic reactions was caused by the strong electrostatic field near surface cations, resulting in polarization of reactant molecules. 

They exhibit strong acidity, which is usually of the Bronsted type, partly because of the influence of the strong internal electrostatic fields (ca. 106 Vcm 1) exerted on the interlamellar water (which generates protons by dissociation) or, because of the additional influence of certain hydrated interlamellar cations, notably Al3+. Cation hydrolysis, just as with strongly polarizing cations in zeolites, yields free protons, thus ... 

The decrease in a0 with increasing aluminum content is consistent with the lower acidity of X zeolite in comparison with the lower aluminum content Y zeolite. The observed acidity decrease with increasing aluminum content was interpreted as a self-inhibition of the acidic sites as their number increases. Such a phenomenon is also consistent with the electrostatic field calculations of Pickert et al. (202) and Dempsey (203), who found that for a cation at a given distance from a cation site the electric field is smaller for X zeolite than for Y. 

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

The solution is assumed ideal. It is incompressible all lattice sites are filled with some species of molecule. All species of molecules at the lattice sites are of equal (or nearly equal) size. For physical reasons, only one molecule can occupy each lattice site. Since there is a distribution of adsorptive energies within the zeolite, corresponding to the locally varying electrostatic field, the adsorption problem is approached from the standpoint of a superposition of several solutions - all the sites in each being identical. The number of solutions that must be considered equals the number of different adsorptive energy sites that are found within the zeolite. 

SAPO-37 produces more liquid distillates and less gases and coke than USY zeolites with 20 Al/u.c. (2.442 nm). Meanwhile the hydrogen transfer is lower in SAPO-37, than in USY samples with lower density of acid sites. The high ratio of cracking to hydrogen transfer is related to the less polar character (less pronounced electrostatic fields) of SAPO-37 with respect to USY zeolites. 

Electric fields at field emitter tips, as discussed before, are typically on the order of 10 V ran-1. This is in the same range as the electrostatic fields that are present in zeolite cages (see below) and at the interface of electrode and electrolyte interface. Since these fields are all of the same order as the fields inside atoms and molecules, they are strong enough to induce the rearrangement of electronic orbitals of atoms and molecules. It is therefore expected that it should be feasible to stimulate chemistry with such electric fields. 

A question that arises is whether one can modify the electrostatic fields (or their gradients) around active sites in zeolite cages or around the pores in an MOF, by applying an external field. As far as we know experiments of this kind have not been reported, but a possible experimental configuration to do so is a capacitor in which the zeolite is sandwiched between two... 

Nucleophilic photosubstitution reactions of benzylic chlorides have also been observed to occur with nucleophiles other than the alcohol solvents. n-Nucleophiles such as amine solvents147 and halide ions and acetate ions148, as well as 7r-nucleophiles such as toluene149 have been used. The latter, a photoalkylation reaction, was achieved by irradiation of benzyl chloride absorbed within zeolite micropores in a slurry in cyclohexane. In cyclohexane itself only products of PhCH2 are formed. This large medium effect is due to the strong electrostatic fields experienced in the zeolite cavities149. 

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion-exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field (e/r) exerted by the cation. 

The surface character of the AlPO molecular sieves differs from that of the silica molecular sieves even though both framework types are neutral with no extra-framework cations. The molecular sieve silicalite is hydrophobic and the AlPO molecular sieves are moderately hydrophilic. Zeolites are hydrophilic due to the interaction of the dipole of the Hz0 molecule with the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The hydrophilicity of the AlPOi, materials is apparently due to the difference in electronegativity between Al(1.5) and P(2.1). Neither mechanism is possible with silica molecular sieves. The AlPOi, molecular sieves do exhibit less affinity for HzO than the hydrophilic zeolites such as Type A and Type X. 

Similar computations were carried out by Beran (106) for Mg2+- and MgOH+-containing zeolites. The results were compared with those obtained for Ca-containing zeolites. The distinctions were mainly ascribed to the stronger electron-acceptor properties of magnesium as well as to stronger electrostatic fields in the Mg-containing zeolites. In both cases the water-cation interaction was predicted to be rather weak. 

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