Zeolites as carriers

The steric structure of the matrix-carrier can make an additional contribution to the asymmetric reaction, especially if the carrier contains chiral characteristics. Thus, Smith et al. showed that o// / o-cyclodextrin as the host compound can induce asymmetric reactions in the reduction of acetophenone and Tanaka et al. showed that the pyridine-borane-cyclodex-trin system induces chirality with an ee of 91%. Pd supported on a copolymer of Z eto-cyclodextrin-epichlorhydrin hydrogenates 2-methylpent-2-enoic acid with an ee of 9.3% whereas Pt supported on Z to-cyclodextrin polymer produced an optical yield of up to 15.7% (Smith et al. 

Zeolites can be used as supports for the preparation of chiral heterogeneous catalysts in different ways 

Method b) consists of the encapsulation of voluminous complexes within the micropores of zeolite. It was used in the enantioselective epoxi-dation on a Mn(III)Salen complex-zeolite catalytic system 

Method c) is exemplified in the chiral modification of surfaces of zeolites with chiral compounds. Sundarababu et al. modified NaX and NaY zeolites with (-)-ephedrine and used them in the asymmetric photolysis of ketones with ee s above 10%. A strange behavior of the nature of carriers for the configuration of products in the latter reaction was observed the NaY-(-)-ephedrine zeolite system gave (+)-rotating products, while the NaX-(-)-ephedrine zeolite system gave (-)-rotating products. Zeolite H-Y modified with R)- or (5)-dithiane-l-oxide showed catalytic activity in the asymmetric decomposition of racemic 2-butanol (Hutchings) 

The first example of an enantioselective hydrogenation reaction in the gas phase was published byHutchings s group They accomplished the 

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Small metal particles are frequently expected (however, the evidence is sometimes questionable) to experience an electron transfer with the carrier, which modifies the adsorption and catalytic properties of the metal particles [sometimes called the Schwab effect (108-116)]. In other cases, by special conditions under preparations of the catalysts, a so-called strong metal support interaction effect (SMSI) (117-121) was evoked. In particular, with zeolites as carriers, there are pieces of experimental evidence reported (115, 116) in support of the existence of such transfer (for remarks on those conclusions, see 122, 123). 

Water has been shown to be an effective solvent in some chemical reactions such as free radical bromination. Supercritical fluids such as liquified carbon dioxide are already commonly used in coffee decaffeination and hops extraction. However, supercritical carbon dioxide can also be used as a replacement for organic solvents in polymerization reactions and surfactant production. Future work may involve solventless or neat reactions such as molten-state reactions, dry grind reactions, plasma-supported reactions, or solid materials-based reactions that use clay or zeolites as carriers. 

Several authors successfully demonstrated the potential of micro- and meso-porous silica materials as well as zeolites as carriers. Corma et al. anchored car-bapalladacycles on high surface area inorganic supports and converted aryl... 

In additional experiments, HZSM-5 was precoked by converting methanol alone (into hydrocarbons) at 400 °C. Afterwards the zeolite was exposed to the 2-methylnaphtha-lene/methanol mixture, under the usual reaction conditions. The initial yield of 1-methylnaphthalene was significantly reduced (1.5 % compared to 4 % for the fresh catalyst, cf. Fig. 4). Furthermore, the initial content of 2,6- + 2,7-dimethylnaphthalene in the dimethyl-naphthalene fraction was 84 % instead of 70 % for the fresh catalyst. In another run, HZSM-5 was loaded with 0.5 wt.-% of Pt, and H2 was used as carrier gas instead of N2. Under these conditions, the formation of coke was avoided or at least drastically diminished. In-line with our model, no changes in the product yields and in the distribution of the dimethylnaphthalene isomers were observed with time on stream. 

J. Lemberg Zeolites recognized as carriers of base exchange in soils equivalence of exchange of bases proved. 1876... 

The gas phase isomerisation of o-dichlorobenzene (odCB) was studied over protonic zeolites HZSM5, HMOR, HMAZ, HOFF, HBETA and a pillared clay HPILC. All the catalysts deactivate. The deactivation rate is the highest when dry air is used as carrier gas, and the lowest with nitrogen... 

Molecular sieves such as zeolite have also been used as carriers for low-molecular-weight amines. An amine, such as DETA, can be absorbed into the sieve prior to mixing with an epoxy resin. The sieve protects the amine from reacting with the epoxy resin, and a relatively long shelf life is possible. Release of the DETA is then initiated by heat or through displacement by atmospheric moisture. 

Zeolites as excellent stabilizing agents for Pd(II) or well dispersed Pd(0) were then used as a carrier for this reaction (35). It was shown that the activity of the catalyst depended chiefly on the ability of the transition metal ion to undergo easily redox cycles. In this respect, transition metal with quasi-permanent cationic state such as cobalt or with quasi permanent metallic state such as platinum exhibited the lowest activity. By contrast copper, rhodium, iridium and particularly palladium showed the highest activity though with varying selectivities. 

On the other hand, the catalytic activities of various H-zeolites used as carriers in Rh/H-zeolite catalysts for the cracking of thiophene and cumene were also examined at 400°C by use of a pulse reactor under helium stream. It was ascertained that USY zeolite showed the... 

This work is devoted to the synthesis of Zr02 by various methods, the synthesis of zirconium-containing pentasils and Zr02 - -zeolite based binary carriers. These materials were used as carriers of transition metal oxides (chromium, cobalt) and their catalytic properties were characterized in the selective reduction of NO by methane and propane-butane mixture, the acidic properties of the samples were investigated by thermoprogrammed desorption and IR-spectroscopy methods. 

The disproportionation of toluene was the dominant reaction on Mo exchanged ZSM-5 and mordenite (0.4 wt % Mo). Compared with H-ZSM-5 and H-mordenite, the Mo exchanged samples had a much better resistance to deactivation, and this efftect was more pronounced on Mo mordenite, especially when was used as carrier gas. The variation in the distribution of xylene isomers with the conversion is plotted in Fig. 2. The Mo zeolites enhanced the relative yield of para + meta xylenes. 

This paper deals with the asymmetric hydrogenation of ethyl pyruvate to ethyl lactate showing a high enantiomeric excess in favour of the R-enantiomer over (-)cinchonidine modified Pt/carrier catalysts. Due to their regular structures, zeolites in particular have been used as carrier materials. 

The filling of the distribution column contains a material called carrier, which is coated with a liquid absorbent. Zeolites or glass spheres are usually used as carriers whereas non-volatile liquids such as silicone oil or poly-ethylene glycol are used as the absorbents in distribution columns. 

Catalytic properties were examined in a fixed bed continuous flow microreactor at atmospheric pressure with Hg resp. N2 as carrier gases (flow rate 10 1 h applying the conversion of ethylbenzene and the isomerization of m-xylene as probe reactions on one gram of the binder-free zeolite (0.35 - 1.0 mm). The shape selectivity was tested as described elsewhere [7]. ... 

Phenols substituted by the 2-hydroxymethyl group represent the monomers of phenol/formaldehyde polymers and their regio specific synthesis has received extensive experimental study. A number of preparations are Used in Table 6.3 (refs.27-33). The alkyl methyl ether of o-methylsalicyl alcohol [2-(1 -methoxyethyl)phenol] has been formed in a yield of 12% from a mixture of phenol and 1,1-dimethoxyethane (1 10) by passage at 250 C with helium as carrier gas through a fixed bed catalyst of an H-ZSM-5 zeolite proton exchangeable) supported on glass wool with a contact time of 6.9 secs. (ref.34). 

Rocha, C., Ducso, L., Gonsalves, M. R, Teixeira, J. A. (2005). Spent-grains and zeolites as potencial carriers for trypsin immobilization, 4 Mercosur Congress on Process Systems Engineering Proceedings (CD-ROM), Costa Verde, Brasil. 

Reschetilowski et al. also gives a comparison of the efficacy of Pt-zeolite-Cnd catalysts in the hydrogenation of EtPy based on alumina enriched with zeolite Y or ZSM-35 and silica enriched Zeolite Beta or ZSM-5 as carriers (Fig.5.L). 

Zeolites can also be used as carriers for other metal catalysts or organometallic complexes. In such cases, hybrid catalysts result in which the properties of the... 

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