Examples of Molecular Sieving

Separation and purification can also be accomplished by using differences in diffusivities, that is, kinetic separation (discussed in Chapter 3). 4A zeolite has been used in PSA separation for producing nitrogen for inert purge applications 

---

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Examples of molecular sieve incidents other than the subentries Benzyl bromide, Molecular sieve, 2731 terf-Butyl hydroperoxide, Molecular sieves, 1692 Mercury(II) perchlorate. 6 (or 4)dimethyl sulfoxide, 4073 Nitromethane, Molecular sieve, 0455 Oxygen difluoride, Adsorbents, 4311 1,1,1-Trichloroethane, 0737... 

A striking example of molecular sieving in a stable, continuous b-oriented silicalile-1 film (having pores of about 0.55 nm) on an electrode was recently demonstrated with redox probe molecules of different sizes (Fig. 13).[100] Specifically, the smaller complex Ru(NII3)63+ with a diameter of ca. 0.5 nm was shown to travel through the film, while the larger complex Co(phen)32+ with a diameter of ca. 1.3 nm was completely excluded from the zeolite film and thus from redox processes. 

The use of zeolites can also be very helpful in removing a reaction product that unfavourably influences the yield of the desired product. Thus, in the manufacture of antibiotic cefoxitin, the amide acylation results in the generation of HCI, which can be removed by the addition of molecular sieve 3 A or 4 A, which has a large capacity for HCI (Weinstock, 1986). Other examples are reactions in which products like methanol or water retard the rate and prevent the reaction to reach the desired degree of completion. Molecular sieves capture methanol or water very well. 

This is the first example of a reaction for which the presence of a chelating ligand was observed to facilitate rather than retard metal-catalysed epoxidation (Gao et al., 1987). It was found that the use of molecular sieves greatly improves this process by removing minute amounts of water present in the reaction medium. Water was found to deactivate the catalyst. All these developments led to an improved catalytic version that allows a five-fold increased substrate concentration relative to the stoichiometric method. Sensitive water-soluble, optically active glycidols can be prepared in an efficient manner by an in situ derivatisation. This epoxidation method appears to be competitive with enzyme-catalysed processes and was applied in 1981 for the commercial production of the gypsy moth pheromone, (-1-) disparlure, used for insect control (Eqn. (25)). 

Apart from the preparation of radiotracers, microwave-assisted transformations have also been utilized to carry out simple hydrogen-deuterium exchange reactions. In the case of acetophenone, for example, simple treatment with deuterium oxide as solvent in the presence of molecular sieves at 180 °C for 30 min led to complete... 

These authors also found that the addition of excess MgO during the in situ preparation of allyltantalum species improved reaction outcomes, even allowing for the first example of allylation of imines derived from aliphatic amines and aliphatic aldehydes (prepared in situ at room temperature in the presence of molecular sieves) (Equation (18)). The selective addition to imines permitted three-component reactions. 

Maruoka and co-workers recently reported an example of a Zr-catalyzed cyanide addition to an aldehyde [64]. As is also illustrated in Scheme 6.20, the reaction does not proceed at all if 4 A molecular sieves are omitted from the reaction mixture. It has been proposed that the catalytic addition proceeds through a Meerwein—Ponndorf—Verley-type process (cf. the transition structure drawn) and that the crucial role of molecular sieves is related to facilitating the exchange of the product cyanohydrin oxygen with that of a reagent acetone cyanohydrin. The example shown is the only catalytic example reported to date the other reported transformations require stoichiometric amounts of the chiral ligand and Zr alkoxide. 

To a much smaller extent non-enzymic processes have also been used to catalyse the stereoselective acylation of alcohols. For example, a simple tripeptide has been used, in conjunction with acetic anhydride, to convert rram-2-acctylaminocyclohexanol into the (K),(R)-Qster and recovered (S),(S)-alcohol[17]. In another, related, example a chiral amine, in the presence of molecular sieve and the appropriate acylating agent, has been used as a catalyst in the conversion of cyclohexane-1(S), 2(/ )-diol into 2(S)-benzoyloxy-cyclohexan-1 f / j-ol1 IS]. Such alternative methods have not been extensively explored, though reports by Fu, Miller, Vedejs and co-workers on enantioselective esterifications, for example of 1-phenylethanol and other substrates using /. vo-propyl anhydride and a chiral phosphine catalyst will undoubtedly attract more attention to this area1191. 

Example 1 the phosphoroamidite coupling procedure leading to dinucleosides and employing a catalytic amount of 5-(p-nitrophenyl)-lff-tetrazole (NPT) is illustrated [12b]. Dinucleoside phosphates can be prepared by this procedure at a multigram scale in 92-98% yield. The catalyst NPT (5 mol%) is used in the presence of molecular sieves 13X in acetonitrile at 40 °C (step a). 

The following applications include the removal of straight-chain from branched-chain or cyclic molecules. For example, type 5A sieves will adsorb n-butyl alcohol but not its branched-chain isomers. Similarly, it separates n-tetradecane from benzene, or n-heptane from methylcyclohexane. A logical development is the use of molecular sieves as chromatographic columns for particular preparations. 

In many processes (e.g. polymerizations, catalytic reactions) even trace amounts (< 50 ppm) of water can cause problems and the only practical solution for dehydration of these liquids is the use of an appropriate zeolite. For example, Stannet et al. (4 ) report on the use of molecular sieves to dehydrate liquid vinyl monomers prior to radiation-induced ionic polymerization. ... 

Imines are formed by the reaction of a primary amine with aldehydes or ketones with the simultaneous removal of water, for example by azeotropic distillation,213 by the addition of anhydrous sodium sulphate,214 by the addition of molecular sieves,215 or by the use of titanium(iv) chloride.216 When one, or both, of the reactants is aromatic, the imine is quite stable and usually known as a Schiff base (see Section 6.5.5, p. 902). In the case of wholly aliphatic reactants the imines tend to decompose or polymerise in these cases their further reaction is carried out without delay. 

The particularly high resolving power of moderately concentrated gel media is to a large extent a consequence of molecular sieving acting as an additional separative factor. For example, blood serum can be separated into about 25 components in polyacrylamide gel, but only into 5 components on filter paper or by moving boundary electrophoresis. 

For the first time, catalytic activity of lanthanide isopropoxides was detected in the TBHP (tert-butylhydroperoxide) assisted oxidation of allylic alcohols to epoxyalcohols (Eq. 26) [231], For example epoxy geraniol was obtained in up to 96% yield by using YbfOiPr in the presence of molecular sieves (4 A) [232]. 

V has been incorporated in the framework of molecular sieves, for example, V-A1PO-5 and V-silicalites. With V-A1PO-5, epoxidation of allylic alcohols proceeds with excellent selectivity (41) ... 

Metal phthalocyanines are easily synthesized by vapor-phase condensation of four molecules of dicyanobenzene in the presence of molecular sieves such as faujasites or A1PO-5 (123-126). This results in direct entrapment of the macrocycle inside the molecular sieve s channels and cages. There are also reports of ship-in-a-bottle synthesis of porphyrins in zeolites, but since porphyrin synthesis requires a mixture of pyrrole and an aldehyde instead of a single compound, porphyrin synthesis is a much less clean process than phthalocyanine preparation (127). Alternatively, soluble porphyrins or phthalocyanines can be added to the synthesis gel of, for example, zeolite X. This also results in entrapped complexes (128). 

If the makeup gas is absolutely free of catalyst poisons, such as water and carbon dioxide (for example, after molecular sieve drying or liquid nitrogen wash), it can be fed directly to the synthesis converter (Fig. 77 A). After the gas leaves the synthesis converter, ammonia is condensed by cooling and the recycle gas is referred to the recycle compressor. This represents the most favorable arrangement from a minimum energy point of view. It results in the lowest ammonia content at the entrance to the converter and the highest ammonia concentration for condensation. 

---