Adsorption ketone hydrogenation

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

The Baeyer-Villiger rearrangement of cyclohexanone and acetophenone with TS-I/H2O2 proved to be poorly selective [117]. Notably, Ti-P and Sn-P have different chemoselectivities in the oxidation of unsaturated ketones, leading selectively to corresponding epoxides and lactones, respectively [118]. The different oxidation pathways were attributed to the preferential adsorption of hydrogen peroxide on Ti-sites and of the carbonyl group on Sn-sites. 

In the cases of oxiranes, the formation of alcohols and oxo compounds (ketones and aldehydes) are the result of primary processes.3,274-286 Hydrogenation of oxo compounds does not occur on surfaces covered with oxirane. The adsorption of oxiranes on metal surfaces is irreversible during adsorption, ring-opening takes place also. As the temperature is raised, the proportions of the oxo compounds increase. 

Surface modification of skeletal nickel with tartaric acid produced catalysts capable of enantiose-lective hydrogenation [85-89], The modification was carried out after the formation of the skeletal nickel catalyst and involved adsorption of tartaric acid on the surface of the nickel. Reaction conditions strongly influenced the enantioselectivity of the catalyst. Both Ni° and Ni2+ have been detected on the modified surface [89]. This technique has already been expanded to other modified skeletal catalysts for example, modification with oxazaborolidine compounds for reduction of ketones to chiral alcohols [90],... 

A number of contradictory views have been published concerning the structure of adsorbed alcohols and the nature of adsorption sites (for review see ref. 69). Experimental evidence from IR investigations has shown that, on alumina, alcohols form several surface complexes of very different chemical natures (e.g. refs. 31, 32, 117, 133—137) (i) alcohol molecules weakly bonded to the surface, very probably by hydrogen bonds (I) (such complexes are sometimes denoted as physically sorbed alcohols) (ii) surface alkoxides (alcoholates) (II) (iii) surface carboxy-lates (III). Less certain is the existence of species with partial double bonds or of ketone-like species. The formation of the various surface complexes is dependent on the structure of the alcohol. For examples, weakly bonded species (I) have been found with all alcohols, alkoxides (II) mostly with primary alcohols, sometimes also with secondary alcohols, but have never been reported for tertiary alcohols. 

In neutral medium A16-, A17(20)-, and A20-olefins are hydrogenated over palladium in preference to the double bonds of A4-3-ketones.67 The double bonds of A4-3-ketones and A16-20-ketones are reduced in preference to A5-,93 A7-48 155 and A9(I -olefins.2 169 The double bond of a A14-16-ketone is saturated before a 5-ene.70,163 In basic medium the carbonyl conjugated double bond is preferentially saturated in all cases92 presumably because of the strong adsorption of the enolate anion (see section II-C). Debromination (9a-bromo 11-ketone) occurs before saturation of the A4-3-keto double bond14 over Raney nickel but hydrogenation of benzyl ethers takes place concurrently with the hydrogenation of this double bond over palladium in neutral medium.96... 

CARBON SKELETON. The technique of precolumn catalytic hydrogenation can be applied to reduce certain unsaturated compounds to their parent hydrocarbons. Compounds analyzed by this technique include esters, ketones, aldehydes, amines, epoxides, nitriles, halides, sulfides, and fatty acids. Fatty acids usually give a hydrocarbon that, is the next lower homolag than the parent acid. For most systems utilizing hydrogenation, hydrogen is also used as the carrier gas. Usually 1% palladium or platinum on a non-adsorptive porous support such as AW-Chromosorb P is used as the catalytic packing material. 

Janot et al. [90] described the increased hydrogen adsorption of ball-milled Mg2Ni alloys and interpreted their findings with the removal of oxide layers. A Pd-Mg catalyst precursor used for the synthesis of methyl isobutyl ketone was prepared by milling PdO with Mg [91]. Dehydration and dehydrogenation reactions of 4-methylpentan-2-ol over ball-mined catalysts such as CuM (M = Ti, Zr, Hf) have also been investigated [92]. 

In contrast to what is observed in both neutral and acidic solutions, in basic medium there is no change in product distribution when the hydrogen availability is varied. This, and the fact that a,/3-unsaturated ketones are hydrogenated in preference to olefins in base but not in neutral media (18) indicate that a strongly adsorbed species, such as an enolate anion, is present in basic solutions of unsaturated ketones. If it is assumed that the enolate anion is involved in the base-promoted hydrogenation of unsaturated ketones and also that the initial adsorption of this species is the product-determining step in the reaction, a reasonable mechanistic hypothesis, based on that initially proposed by Wilds et al. (19) (see Scheme 3), can be put forth for this reaction. Such a process involves a hydride ion transfer from the catalyst to an adsorbed enolate... 

This mechanistic rationale can also be used to explain the product stereochemistry data from the base-promoted hydrogenation of other a,(3-unsaturated ketones as well. For example, if an angular methyl group were present in the heteroannular enolate depicted in Fig. 8, trans adsorption would be greatly favored. Thus, on hydride ion transfer from the catalyst and protonation from the solution, cis-product formation would predominate, as observed (9,10). 

The pyridone surface species has a C=0 stretching band at 1634 cm-1,3 Hydrogen gas has been detected by mass spectrometry (210), and the formation of this surface compound has been established by chemical methods by Boehm (215). This surface reaction points to the existence of strongly basic OH" ions held to certain sites on alumina surfaces, their number being of the order of magnitude of 1013/cm2 (121). Additional evidence for the existence of these reactive and strongly basic OH" ions on aluminas comes from surface reactions observed on adsorption of nitriles and ketones (see Section IV.F) and of carbon dioxide (see Section IV.G). These reactions may, thus, be valuable for the detection of the corresponding sites that most probably have to be considered as acid-base pair sites. 

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