Catalytic sieves

ShelP published information on the biodegradability of methyl-substituted surfactants. The Shell and P G research paved the way for new anionic surfactant design with controlled alkyl substitution on the hydrophobe and acceptable biodegradability. Selective isomerization of the linear olefins using recent advances in zeolite catalytic sieve technology are the key technology advancements for these new surfactant hydrophobes, which require only one additional step in the current Shell linear alcohol process as shown in Figure 6.14. 

Catalytic sieves make viable the controlled incorporation of methyl substitution on the parent linear olefin. The HSAS added efficiency and performance compensate for the outcome of an additional process step in the standard alcohol processing. 

Catalytic system addition of molecular sieves to "soak" up any water with 3A sieves, 5-10 mol % catalyst is used,... 

The technological appHcations of molecular sieves are as varied as their chemical makeup. Heterogeneous catalysis and adsorption processes make extensive use of molecular sieves. The utility of the latter materials Hes in their microstmctures, which allow access to large internal surfaces, and cavities that enhance catalytic activity and adsorptive capacity. 

ZeoHte-based materials are extremely versatile uses include detergent manufacture, ion-exchange resins (ie, water softeners), catalytic appHcations in the petroleum industry, separation processes (ie, molecular sieves), and as an adsorbent for water, carbon dioxide, mercaptans, and hydrogen sulfide. 

The special case involving the removal of a low (2—3 mol %) mole fraction impurity at high (>99 mol%) recovery is called purification separation. Purification separation typically results in one product of very high purity. It may or may not be desirable to recover the impurity in the other product. The separation methods appHcable to purification separation include equiUbrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this Hst as this method typically caimot achieve extremely high purities. Table 8 presents a Hst of the gas—vapor separation methods with their corresponding characteristic properties. The considerations for gas—vapor methods are as follows (26—44). 

There are numerous stmctures that are similar to 2eofites, such as aluminophosphate molecular sieves, AlPOs, but these have not found catalytic apphcations. Zeofites can be modified by incorporation of cations in the crystalline lattice which are not exchangeable ions, but can play catalytic roles. For example, sificahte, which has the stmcture of ZSM-5 but without Al, incorpora ting Ti in the lattice is a commercial catalyst for oxidation of phenol with H2O2 to give diphenols the catalytic sites may be isolated Ti cations (85). 

Experience in air separation plant operations and other ciyogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubihty limit. For this reason, the carbon dioxide content of the feed gas sub-jec t to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

A rather unexpected discovery was made in connection to these investigations [49]. When the 1,3-dipolar cycloaddition reaction of la with 19b mediated by catalyst 20 (X=I) was performed in the absence of MS 4 A a remarkable reversal of enantioselectivity was observed as the opposite enantiomer of ench-21 was obtained (Table 6.1, entries 1 and 2). This had not been observed for enantioselective catalytic reactions before and the role of molecular sieves cannot simply be ascribed to the removal of water by the MS, since the application of MS 4 A that were presaturated with water, also induced the reversal of enantioselectivity (Table 6.1, entries 3 and 4). Recently, Desimoni et al. also found that in addition to the presence of MS in the MgX2-Ph-BOX-catalyzed 1,3-dipolar addition shown in Scheme 6.17, the counter-ion for the magnesium catalyst also strongly affect the absolute stereoselectivity of the reac-... 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

To improve the yield of mono- and dimethylamines, a shape selective catalyst has been tried. Carhogenic sieves are microporous materials (similar to zeolites), which have catalytic as well as shape selective properties. Comhining the amorphous aluminum silicate catalyst (used for producing the amines) with carhogenic sieves gave higher yeilds of the more valuable MMA and DMA. ... 

The emergence of the powerful Sharpless asymmetric epoxida-tion (SAE) reaction in the 1980s has stimulated major advances in both academic and industrial organic synthesis.14 Through the action of an enantiomerically pure titanium/tartrate complex, a myriad of achiral and chiral allylic alcohols can be epoxidized with exceptional stereoselectivities (see Chapter 19 for a more detailed discussion). Interest in the SAE as a tool for industrial organic synthesis grew substantially after Sharpless et al. discovered that the asymmetric epoxidation process can be conducted with catalytic amounts of the enantiomerically pure titanium/tartrate complex simply by adding molecular sieves to the epoxidation reaction mix-... 

Although trityl perchlorate is used to accomplish the glycosidation of the C-8 hydroxyl in 44 with acetoxy glycoside 49, control experiments have demonstrated that no reaction takes place in the presence of 4 A molecular sieves or 2,6-di-terf-butylpyridine. This observation suggests that the actual catalyst is not trityl perchlorate, but perchloric acid. Consistent with this conclusion is the observation that catalytic amounts of a strong Brpnsted acid such as triflic or perchloric acid can catalyze the glycosidation of 44 with 49 in the absence of trityl perchlorate. 

Catalytic Adsorption. This method can reduce impurities, such as H2, O2, CO, and hydrocarbons, to less than 10 ppb. The catalyst converts these impurities into CO2, H2O, and other species that can then be removed by molecular sieves and cryogenic adsorption. 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

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