Zinc catalysts formation

Chen, L.-W. and Chen, J.-W., Kinetics of diethylene glycol formation from bishydroxyethyl terephthalate with zinc catalyst in the preparation of polyethylene terephthalate),./. Appl. Polym. Sci, 75, 1229-1234 (2000). 

In situ spectroscopic studies have identified a variety of species, such as formate, dioxymethylene, carbonate, and methoxide, to coexist under methanol synthesis conditions on Cu/ZnO-based catalysts [22, 23], Fourier transform infrared spectroscopy studies of CuZn-based catalysts under H2/C02 identified the presence of formate bound to both Cu and ZnO, whereas methoxide was found on ZnO only. Carbonates were found to form via C02 adsorption on ZnO [24] and partially oxidized Cu [23], and were quickly converted into formate via Cu-activated hydrogen. Upon exposure to CO mixtures, only zinc-bound formate was observed [22], The hydrogenation of these formates to methoxide is thought to be rate determining in methanol synthesis. 

The reaction of hydrogen with CO forms the basis of an industrial process for making methanol carried out at high pressure over a mixed copper-zinc catalyst. Though CO is insoluble in, and unrcactive with, water at ordinary pressures, formic acid is produced at very high pressures. Under similar conditions CO and aqueous NaOH combine to give sodium formate. 

For copper-zinc catalysts if the same preparation technique [carbonates] or [oxalates] is used a nearly linear correlation can be established between the copper surface area, the amount of formates and the methanol yield. But if the preparation technique is changed the formates and methanol formation, related to a unit of copper surface area, is not maintained and the observed correlation is restricted to catalysts prepared by the same technique as shown in figure 7. 

Support for Methanol Synthesis research involving Raney copper-zinc catalysts and alkyl formates was provided under the National Energy Research Development and Demonstration Program administered by the Commonwealth Department of National Development. 

Mixed copper/zinc catalysts with high copper-to-zinc ratios are widely used as catalysts for low-pressure methanol production and for low-temperature shift reaction [2, 31], see also Chapter 15. These catalysts are commonly made by coprecipitating mixed-metal nitrate solutions by addition of alkali. Li and Inui [32] showed that apart from chemical composition, pH and temperature are key process parameters. Catalyst precursors were prepared by mixing aqueous solutions of copper, zinc, and aluminum nitrates (total concentration 1 mol/1) and a solution of sodium carbonate (1 mol/1). pH was kept at the desired level by adjusting the relative flow rate of the two liquids. After precipitation was complete, the slurry was aged for at least 0.5 h. When the precipitation was conducted at pH 7.0, the precipitate consisted of a malachite-like phase (Cu,Zn)C03(0H)2 and the resulting catalysts were very active, while at pH < 6 the formation of hydroxynitrates was favored, which led to catalysts less active than those prepared at pH 7.0 (Figure 7.8). 

Hydrosilylation of ketones in high ee has been achieved under ambient conditions in toluene using a silane and a zinc catalyst in which the metal ion is chelated simultaneously by a chiral 1,2-diamine and a chiral 1,2-diol. Although the catalyst could not be isolated in crystalline form, a combination of diethylzinc, diamine and diol shows evidence for it by H-NMR, and addition of the silane results in a new peak at 5 = 4.50 ppm, consistent with Zn-H this peak decreases on addition of ketone. CD spectra are also reported, and extensive DFT calculations support the hydride formation, as well as preorganization of substrate and catalyst via an N-H---0=C hydrogen bond. 

Mannitol, CH,0H(CH0Hi4CH40H, is a hexahydric alcohol obtained by the reduction of mannose. Since ring formation does not occur in mannitol, the hexacetyl derivative can exist in only one form, and therefore either zinc chloride or sodium acetate can be used as a catalyst for the acetylation. 

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

The zinc oxide component of the catalyst serves to maintain the activity and surface area of the copper sites, and additionally helps to reduce light ends by-product formation. Selectivity is better than 99%, with typical impurities being ethers, esters, aldehydes, ketones, higher alcohols, and waxes. The alumina portion of the catalyst primarily serves as a support. 

Nitrile Process. Fatty nitriles are readily prepared via batch, Hquid-phase, or continuous gas-phase processes from fatty acids and ammonia. Nitrile formation is carried out at an elevated temperature (usually >250° C) with catalyst. An ammonia soap which initially forms, readily dehydrates at temperatures above 150°C to form an amide. In the presence of catalyst, zinc (ZnO) for batch and bauxite for continuous processes, and temperatures >250° C, dehydration of the amide occurs to produce nitrile. Removal of water drives the reaction to completion. 

Organic amines, eg, pyridine and piperidine, have also been used successfully as catalysts in the reactions of organosilanes with alcohols and silanols. The reactions of organosilanes with organosilanols lead to formation of siloxane bonds. Nickel, zinc, and tin also exhibit a catalytic effect. 

Copper—cadmium and zinc—chromium oxides seem to provide most selectivity (38—42). Copper chromite catalysts are not selective. Reduction of red oil-grade oleic acid has been accompHshed in 60—70% yield and with high selectivity with Cr—Zn—Cd, Cr—Zn—Cd—Al, or Zn—Cd—A1 oxides (43). The reduction may be a homogeneously catalyzed reaction as the result of the formation of copper or cadmium soaps (44). 

W. B. Williamson and co-workers. Catalyst Deactivation Due to Glac Formation from Oil-Derived Phosphorus and Zinc, SAE 841406, Society of Automotive Engineers, Warrendale, Pa., 1984. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

Diols yield acetonides, even in the presence of a 17oc-hydroxylgroup. Reaction with acetone in the presence of zinc chloride as catalyst leads to the formation of diacetone alcohol acetal as a by-product. ... 

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