Hydrogenation over zinc oxide

Nearly quantitative yields of acetonitrile can be obtained by passing mixtures of NH3 and acetylene over zircon at 400-500°C [225], over CviOy on Y-alumina at 360°C [226] or by passing mixtures of NH, acetylene and hydrogen at 400-420°C over a mixture of zinc and thorium oxides on silica [227] or at 300-450°C over zinc oxide or zinc sulfate or zinc chloride on silica [228, 229], In such reactions, the role of traces of water has often been questioned. However, acetonitrile could be obtained under rigorously anhydrous conditions, thus demonstrating the direct amination of acetylene with NH,. It was also reported that ethyUdeneimine can be obtained in up to 26% yield [225], However, in the Ught of more recent work [230, 231] the product was most probably 2,4,6-trimethyl-l,3,5-hexahydrotriazine. 

Over zinc oxide (8-14) and chromia (8), which are also hydrogenation catalysts, we find the following features ... 

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

Kokes and Dent by combining IR spectroscopy with hydrogen isotope techniques, and by applying kinetic and stereochemical considerations were able to determine the intermediate surface species in the hydrogenation and isomerization of simple olefins over zinc oxide. 

The double bond in indole and its homologs and derivatives is reduced easily and selectively by catalytic hydrogenation over platinum oxide in ethanol and fluoroboric acid [456], by sodium borohydride [457], by sodium cyanoborohydride [457], by borane [458,459], by sodium in ammonia [460], by lithium [461] and by zinc [462]. Reduction with sodium borohydride in acetic acid can result in alkylation on nitrogen giving JV-ethylindoline [457]. 

Quinoline homologs and derivatives, including those with double bonds in the side chains, were reduced selectively by catalytic hydrogenation over platinum oxide (side chain double bonds), and to dihydro- and tetrahydro-quinolines by sodium in butanol, by zinc and formic acid, and by triethylam-monium formate [319, 472]. Catalytic hydrogenation of quinoline and its derivatives has been thoroughly reviewed [439]. 

Hydrogenation and Isomerization over Zinc Oxide R. J. Kokes and A. L. Dent... 

Hydrogen, passing at red heat over zinc oxide, kept at this temperature, may transform it, with absorption of heat, into water vapor and zinc vapor inversely, this last mixture passing into the cooler re ons of the tube, is transformed anew, with liberation of heat, into hydrogen and zinc oxide which is deposited in the crvstallized state. 

Cobalt(II) complexes (such as cob(II)alamin, formerly called B,2,) can be obtained by several methods including controlled potential reduction [14,15], anaerobic photolysis of some organocobalt species [16] (see Section 5.2(b)), partial oxidation of cobalt(I) complexes, in some cases acidification of solutions of cobalt(I) complexes (see below) and chemical reduction. Chemical reduction of cobalt(III) cobalamins to the +2 oxidation state has been achieved with hydrogen over platinum oxide [16], with neutral and acidic solutions of vanadium(III) [17], with chromous acetate at pH 3 [18] and with amalgamated zinc in 0.1 M aqueous perchloric acid [19]. All such cobalt(II) species are probably 5-coordinate, and are low spin d systems containing an unpaired electron and thus displaying an ESR spectrum [20]. 

During ammonia synthesis, the major reactions of production and purification of synthesis gas and the synthesis of ammonia, all are carried out over different catalysts. At least eight kinds of catalysts are used in the whole process, where natural gas or naphtha is used as feedstock and steam reforming is used to produce synthesis gas. These catalysts are Co-Mo hydrogenation catalyst, zinc oxide desulfurizer, primary- and secondary-steam reforming catalysts, high- and low-temperature shift catalysts, methanation catalyst and ammonia synthesis catalyst etc (Table 1.1). 

Zinc oxide or zinc white is used in paints, but more preferable, because of its better covering power, is lithopone (a mixture of zinc sulphide and barium sulphate). Both paints have the advantage over white lead that they do not blacken in air (due to hydrogen sulphide). Zinc dust and also zinc chromate are constituents of... 

---