Heterogeneous zinc

One of the earliest useful methods for asymmetric opening of meso-epoxides with sulfur-centered nucleophiles was reported by Yamashita and Mukaiyama, who employed a heterogeneous zinc tartrate catalyst (Scheme 7.10) [20]. Epoxides other than cydohexene oxide were not investigated, and the enantioselectivity depended strongly on the identity of the thiol. 

Carbon dioxide can itself be used as a feedstock as well as a solvent for the synthesis of aliphatic polycarbonates by precipitation polymerization. Propylene oxide [39] and 1,2-cyclohexene oxide [40] can both be polymerized with CO2 using a heterogeneous zinc catalyst (Scheme 10.21). 

Wilson, K., Renson, A. and Clark, J. H. Catalysis Letters 61 (1-2) (1999) 51-55. Novel heterogeneous zinc triflate catalysts for the rearrangement of alpha-pinene oxide. 

Thus far, the discussion of polymerizations conducted in carbon dixiode has centered on systems where CO2 acts only as a solvent for the polymerization. However, there are also examples of polymerization systems where CO2 acts as a comonomer. Most notable among these in the context of this chapter is the coploymerization of CO2 and epoxides. The copolymerization of propylene oxide and carbon dioxide was conducted in SCCO2 using a heterogeneous zinc catalyst [142]. Additionally, Beckman and co-workers have shown that a soluble, fluorinated ZnO-based catalyst can be effectively utilized to promote the copolymerization of CO2 and cyclohexene oxide [143]. These examples indicate that supercritical carbon dioxide can be viable as both a solvent comonomer in polymerization reactions. 

A number of metal-catalyzed polymerizations have utilized CO2 as both a solvent and as a reagent in the reactions. Precipitation copolymerization of either propylene oxide (83) or cyclohexene oxide (84) with CO2 in SCCO2 has been catalyzed using heterogeneous zinc catalysts. Copolymerizations of CO2 and propylene oxide formed PCs with a molecular weight of about 10 g/mol and incorporation of CO2 at greater than 90% (eq. (7)). A small percentage of propylene carbonate by-product was also observed. 

The inline conversion of nitrate to nitrite and subsequent spectrophotometric determination after diazotization and coupling reactions is the basis of an important set of analytical methods for the simultaneous determination of both ions. Either homogeneous (titanium(III) chloride, hydrazinium sulfate, etc.) or heterogeneous (zinc, cadmium, amalgamated cadmium, copperized cadmium, etc.) reduction agents, photochemical or enzymatic reactions have been used [157]. The nitrite so produced is determined through spectrophotometric detection of the reddish purple azo dye formed as a result of the modihed Griess chemistry. 

Klaus, S. Lehenmeier, M.W. Herdtweck, E. Deghnaim, P Ott, A.K. Rieger, B. Mechanistic Insights into Heterogeneous Zinc Dicarboxylates and Theoretical Considerations for CO -Epoxide Copolymerization. J. Am. Chem. Soc. 2011, 133, 13151-13161. 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

Dehydration of 1-pentanol or 2-pentanol to the corresponding olefins has been accompHshed, in high purity and yields, by vapor-phase heterogeneous catalyzed processes using a variety of catalysts including neutral gamma —Al Og catalyst doped with an alkah metal (23), zinc aluminate (24,25), hthiated clays (26), Ca2(P0 2 montmorillonite clays (28). Dehydration of 2-methyl-1-butanol occurs over zinc aluminate catalyst at... 

Recently, in extending the reaction to aliphatic glycols, Ghera observed that p-nitrobenzoate esters gave better results than acetates, and that zinc acetate could be substituted for activated zinc. He has proposed that the rearrangement is a heterogeneous catalytic process, with the zinc acting as a Lewis acid ... 

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

The mechanism of inhibition by the salts of the long chain fatty acids has been examined . It was concluded that, in the case of the lead salts, metallic lead was first deposited at certain points and that at these points oxygen reduction proceeded more easily, consequently the current density was kept sufficiently high to maintain ferric film formation in addition, any hydrogen peroxide present may assist in keeping the iron ions in the oxide film in the ferric condition, consequently the air-formed film is thickened until it becomes impervious to iron ions. The zinc, calcium and sodium salts are not as efficient inhibitors as the lead salts and recent work has indicated that inhibition is due to the formation of ferric azelate, which repairs weak spots in the air-formed film. This conclusion has been confirmed by the use of C labelled azelaic acid, which was found to be distributed over the surface of the mild steel in a very heterogeneous manner. ... 

Two mixtures. On the left is brass, a homogeneous mixture of copper and zinc. On the right is a piece of granite, a heterogeneous mixture that contains discrete regions of different minerals (feldspar, mica, and quartz). 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

The application of surface treatments to mbbers should produce improved wettability, creation of polar moieties able to react with the adhesive, cracks and heterogeneities should be formed to facilitate the mechanical interlocking with the adhesive, and an efficient removal of antiadherend moieties (zinc stearate, paraffin wax, and processing oils) have to be reached. Several types of surface preparation involving solvent wiping, mechanical and chemical treatments, and primers have been proposed to improve the adhesion of vulcanized SBR soles. However, chlorination with solutions of trichloroisocyanuric acid (TCI) in different solvents is by far the most common surface preparation for mbbers. 

Metals are insoluble in common liquid solvents but can dissolve in each other (like dissolves like). A mixture of substances with metallic properties is called an alloy. Some alloys are true solutions, but microscopic views show that others are heterogeneous mixtures. Brass, for instance, is a homogeneous solution of copper (20 to 97%) and zinc (80 to 3%), but common plumber s solder is a heterogeneous alloy of lead (67%) and tin (33%). When solder is examined under a microscope, separate regions of solid lead and solid tin can be seen. When brass is examined, no such regions can be detected. 

The underlying problem in testing the validity of the additivity principle in corrosion, mineral extraction, and electroless plating is that the electrode metal itself forms part of one of the half-reactions involved, e.g., zinc in equation (5) and copper in equations (8) and (12). A much better test system is provided by the interaction of two couples at an inert metal electrode that does not form a chemical part of either couple. A good example is the heterogeneous catalysis by platinum or a similar inert metal of the reaction... 

In this Section, we consider examples of application of semiconductor sensors in investigation of heterogeneous catalytic reactions of dehydration of isopropyl alcohol and dissociation of hydrosine on zinc oxide. 

In conclusion, we note that the appearance of hydrogen atoms in the gas volume in catalytic reaction of dehydration of alcohol at low pressures observed in [25] by the sensor technique confirms that dehydration of alcohol on the surface of the zinc oxide catalyzer yields hydrogen atoms. In other words, this heterogeneous reaction does not result in production of hydrogen molecules through the process... 

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