Zinc oxide reactions catalyzed

Krishnakumar, V., Kumar, K.M., Mandal, B.K. and Nawaz Khan, F-R. 2012. Zinc oxide nanoparticles catalyzed condensation reaction of isocoumarins and 1,7-heptadiamine in the formation of bis-isoquinolinones. Sci. World J. 2012, ID 619080. 

The low temperature shift catalyst functions 100-200°C below the temperature of the high temperature system and is operated between 210 and 240°C. The catalysts used in this process are copper/zinc oxide/alumina materials. The composition and method of preparation of the copper-based precursors are crucial in determining the final properties of the catalyst. The typical composition of a commercial copper/zinc oxide/alumina catalyst may be 33% CuO, 34% ZnO, and 33% AI2O3. Although the ability of copper/zinc oxides to catalyze the WGS reaction was recognized in the late 1920s, routine usage in commercial plants did not occur until 1963. Until that time, the susceptibility of the catalyst to... 

High pressure processes P > 150 atm) are catalyzed by copper chromite catalysts. The most widely used process, however, is the low pressure methanol process that is conducted at 503—523 K, 5—10 MPa (50—100 atm), space velocities of 20, 000-60,000 h , and H2-to-CO ratios of 3. The reaction is catalyzed by a copper—zinc oxide catalyst using promoters such as alumina (31,32). This catalyst is more easily poisoned than the older copper chromite catalysts and requites the use of sulfiir-free synthesis gas. 

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. 

Zinc-tartrate complexes were applied for reactions of both nitrones and nitrile oxides with allyl alcohol and for both reaction types selectivities of more than 90% ee were obtained. Whereas the reactions of nitrones required a stoichiometric amount of the catalyst the nitrile oxide reactions could be performed in the presence of 20 mol% of the catalyst. This is the only example on a metal-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrile oxides. It should however be no-... 

Synthesis of the remaining half of the molecule starts with the formation of the monomethyl ether (9) from orcinol (8). The carbon atom that is to serve as the bridge is introduced as an aldehyde by formylation with zinc cyanide and hydrochloric acid (10). The phenol is then protected as the acetate. Successive oxidation and treatment with thionyl chloride affords the protected acid chloride (11). Acylation of the free phenol group in 7 by means of 11 affords the ester, 12. The ester is then rearranged by an ortho-Fries reaction (catalyzed by either titanium... 

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... 

Experimental results clearly demonstrate that catalytic reaction of dehydration of alcohols on zinc oxide proceeds via formation of radicals. Emission of hydrogen atoms from the catalyzer surface may be associated with structure relaxation of the catalyzer surface excited during the reaction [26]. 

In homogeneous (15, 16) and heterogeneous (17) base catalyzed reactions, such species, with anionic character, are firmly established as intermediates. It seems clear that a similar species must be regarded as a prime candidate for an intermediate in isomerization over zinc oxide. Regardless of what the isomerization intermediate is, however, it is clearly different from that involved in isomerization over metals. 

These acid-catalyzed C-glycosylations were successfully extended to the D-ribofuranose series by Sorm and coworkers,148 who utilized the reaction in the first reported synthesis of showdomycin. Thus, treatment of 2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl bromide (81) with 1,2,5-trimethoxybenzene in the presence of zinc oxide gave 2,4,6-trimethoxy-l-(2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl)benzene (196). Ozonolysis of the corresponding acetate derivative, followed by esterification, gave the highly functionalized C-/3-I>ribofuranosyl derivative (197), which was used as a key intermediate in the synthesis of showdomycin (see Section III,l,b). 

In the case of the esterification of the diacid, the reaction is self-catalyzed as the terephthalic acid acts as its own acid catalyst. The reverse reaction, the formation of TPA and EG from BHET is catalytic with regard to the usual metal oxides used to make PET, but is enhanced by either the presence of hydroxyl groups or protons. In the case of transesterification of dimethyl terephthalate with ethylene glycol, the reaction is catalytic, with a metal oxide needed to bring the reaction rate to commercial potential. The catalysts used to produce BHET are the same as those needed to depolymerize both the polymer to BHET and BHET to its simpler esters. Typically, titanium, manganese and zinc oxides are used for catalysts. 

The sirtuins (silent information regulator 2-related proteins class III HDACs) form a specific class of histone deacetylases. First, they do not share any sequence or structural homology with the other HDACs. Second, they do not require zinc for activity, but rather use the oxidized form of nicotinamide adenine dinucleotide (NAD ) as cofactor. The reaction catalyzed by these enzymes is the conversion of histones acetylated at specific lysine residues into deacetylated histones, the other products of the reaction being nicotinamide and the metabolite 2 -0-acetyl-adenosine diphosphate ribose (OAADPR) [51, 52]. As HATs and other HDACs, sirtuins not only use acetylated histones as substrates but can also deacetylate other proteins. Intriguingly, some sirtuins do not display any deacetylase activity but act as ADP-ribosyl transferases. 

Excerpt 4F is taken from an article written by Demko and Sharpless. (Barry Sharpless was a co-recipient of the Nobel Prize in Chemistry in 2001 for his work on chirally catalyzed oxidation reactions.) In this article, the authors propose a way to synthesize aromatic tetrazoles from nitriles in water, using only sodium azide and a zinc salt. Water, despite its obvious advantages (i.e., safe and inexpensive), rarely succeeds as a solvent in organic synthesis. Thus, a synthesis that uses water successfully is an important scientific accomplishment. 

The above dramatic dependence of regio- and stereoselectivity on the nature of the metal can be explained by the reaction mechanism shown in Scheme 11.49 (167). The nitrone cycloadditions of allylic alcohols are again magnesium-specific just like the nitrile oxide reactions described in Section 11.2.2. Magnesium ions accelerate the reaction through a metal ion-bound intramolecular cycloaddition path. On the other hand, zinc ions afford no such rate acceleration, but these ions catalyze the acetalization at the benzoyl carbonyl moiety of the nitrone to provide a hemiacetal intermediate. The subsequent intramolecular regio- and stereoselective cycloaddition reaction gives the observed products. 

Nonstoichiometric metal oxides are effective catalysts for a variety of oxidation-reduction reactions (as might be expected) since the variable valence of the constituent ions enables the oxide to act as a sort of electron bank. Nonstoichiometric metal oxides resemble metals in that they can also catalyze hydrogenation and alkene isomerization reactions. However, on zinc oxide, for instance, these two processes are independent, whereas hydrogen must be present for isomerization to occur on metals. 

In this connection, the dependence of the rate of the hydrogen-deuterium exchange reaction catalyzed by zinc oxide, upon the concentration of a foreign oxide in ZnO is interesting. This system was investigated by Molinari and Parravano (94). As shown in Fig. 16, the rate of... 

Another method to hydrogenate butadiene occurs during an oxidation—reduction reaction in which an alcohol is oxidized and butadiene is reduced. Thus copper—chromia or copper—zinc oxide catalyzes the transfer of hydrogen from 2-butanol or 2-propanol to butadiene at 90—130°C (87,88). 

Copper is an essential element to most life forms. In humans it is the third most abundant trace element only iron and zinc are present in higher quantity. Utilization of copper usually involves a protein active site which catalyzes a critical oxidation reaction, e.g., cytochrome oxidase, amine oxidases, superoxide dismutase, ferroxidases, dopamine-/ -hydrox-ylase, and tyrosinase. Accordingly, animals exhibit unique homeostatic mechanisms for the absorption, distribution, utilization, and excretion of copper (J). Moreover, at least two potentially lethal inherited diseases of copper metabolism are known Wilson s Disease and Menkes s Kinky Hair Syndrome (I). 

By far the most important synthesis gas reaction is its conversion into methanol, using copper/zinc oxide catalysts under relatively mild conditions (50 bar, 100-250°C). Methanol is further carbonylated to acetic acid (see Section 22-7), so that CH3C02H, methyl acetate, and acetic anhydride can all be made from simple CO and H2 feedstocks. Possible pathways to oxygenates in cobalt catalyzed reactions are shown in Fig. 22-6. 

Recently Chen et al. " reported that hydroxyl-free zinc oxide was prepared by solvothermal oxidation of zinc powders with two equivalents of trimethyl amine A-oxide or 4-picoUne A-oxide as the oxidant in organic solvent toluene, ethylenediamine, A,A,A,A-tetramethylethylenediamine at 180°C. The morphology of the product is affected by the oxidant, and trimethylamine A-oxide yields rod-like particles, while 4-picoline A-oxide produces spherical particles. Solvent affects the particle size of the prodnct and the smallest particles (24 nm, 4-picoline A-oxide) are obtained in less-polar tolnene. Chen et al. showed that a small amount of water in organic solvent catalyzes the reaction. 

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