分子催化最新文献

In using 9-(t-butylperoxy)-fluorene as test substrate and various chromium compounds, it has been shown that the transformation to fluorenone requires the simultaneous presence of t-BuOOH to be efficient. This transformation is much less effective when using free t-BuO^. or t-BuOO^. radicals. A mechanism involving the homolytic scission of the peroxidic bond of the substrate is proposed.

A study of the hydrogenation of diphenylacetylene (tolane) over UV-irradiated metal/TiO₂ powders in alcoholic media has been performed at room temperature. The nature of the alcohol exerts a net influence on the activity and selectivity toward cis- and trans-diphenylethylene (stilbene) formation, presumably because of the influence of alcohol on the adsorption of tolane on the catalyst surface. The effect of the nature of deposited metal is closely related to the difference between the amount of hydrogen abstracted and that detected in the gas phase which reflects the photoreductive ability of metal toward hydrogenation of unsaturated compounds at the expense of the H₂-evolution reaction. Although the transfer hydrogenation of tolane proceeds at a lower rate compared with the conventional catalytic hydrogenation process because of a low availability of hydrogen photogenerated from an alcohol, good yields in the reduction of tolane and high selectivities toward olefin formation can be achieved by using in situ photogenerated hydrogen. The results obtained in this work show that the photoassisted transfer hydrogenation reaction can constitute a practical synthesis method, particularly by using Pd/TiO₂ which exhibits outstanding performance in tolane reduction.

The effect of chlorine using precursors like H₂PtCl₆ (0.55,2 and 10 wt.% Pt) and Pt(NH₃)₄(NO₃)₂ (2 wt.% Pt) and of the Nb₂O₅ loading on the structure of Pt/Al₂O₃ and its catalytic activity in methanol/deuterium exchange has been investigated. In presence of niobia the BET surface of the support increases but niobia itself remains amorphous and its presence in increasing quantities results in the easier reduction of both chlorine-containing and chlorine-free Pt samples measured by TPR. XPS provides evidence for the effect of chlorine and niobia on the reduction of Pt. After reduction Nb₂O₅ is converted into NbO_x species which may migrate on top of the Pt particles measured by XRD, CO chemisorption and O/H titration. The increased perimeter of the Pt/NbO_x/Al₂O₃ interface raises the deuterium availability across this interface, which in turn, gives rise to the interaction of methyl groups in the anchored species with Pt for multiple exchange as was assumed in Part I.

A new efficient hectorite-supported ruthenium catalyst showing molecular-sieve effects in the liquid-phase hydrogenation of alkylsubstituted benzenes to the corresponding cyclohexanes is described.

The hydrogen transfer reduction of alkyl aryl (heteroaryl) ketones readily occurs at 82°C in a solid—liquid biphasic system when using 2-propanol as hydrogen donor, potassium fluoride on alumina (50 wt.%) as a base, [Ir(COD)Cl]₂ as the metal catalyst and 18-crown-6 as the phase-transfer agent. The molar ratio of ketone : [Ir] : F⁻] : 18-crown-6 was 200:1:5:2. A number of alkyl aryl, and alkyl heteroaryl carbinols have been prepared in 73–84% isolated yield under HTR-PTC conditions.

A natural bentonite was pillared with aluminum, aluminum—iron and aluminum—ruthenium polyoxocations. The alkali cations of some clays were successively exchanged with iron. The prepared pillared clays were characterized by X-ray diffraction, DTA, TGA, ESCA and N₂ adsorption. The relative amount of Lewis and Brønsted acid sites was estimated from FT-IR spectra of adsorbed pyridine. All the clays resulted active catalysts for the vapour phase conversion of propene to acetone in the 150–350°C temperature range. The reaction appeared to occur via acid catalyzed propene hydration to isopropanol and successive oxidative dehydrogenation of the alcohol to acetone. The reactivity of all the prepared samples was correlated to the nature and number of the redox and acid sites.

The hydrovinylation of styrene and ring-substituted styrene molecules by the ionic nickel precursor trans-[Ni(2,4,6-Me₃C₆H₂) (CH₃CN) (P(CH₂Ph)₃)₂]BF₄ in THF solution has been investigated. The chemoselectivity towards hydrovinylation products is about 99%. For styrene the conversion is nearly complete and the regioselectivity towards the vinylation in the α-position in the conjugated aryl olefin derivatives is 100%. The yield of 3-phenyl-1-butene reaches 97% with minor amounts of the isomerization products cis- and trans-2-phenyl-2-butene. The turnover rate of the reaction at 25°C, 15 bar (initial pressure) of ethylene with a styrene/[Ni] ratio 1000/1 is 1915 h⁻¹. With styrene derivatives substituted in the vinyl fragment, the hydrovinylation reaction was very limited; however those substituted in the phenyl fragment (ArCH=CH₂; Ar:p- and m-Me(C₆H₄), p- and m-Et(C₆H₄), p-and m-vinyl(C₆H₄), p- and m-Cl(C₆H₄), p-MeO(C₆H₄) and m-NO₂(C₆H₄)) showed similar selectivity to styrene but different turnover rates. Activity decreases with the substituent in the sequence Cl∼OMe∼CH=CH₂∼H∼CH₂CH₃&>;Me&>;NO₂ related with the coordination ability of the olefin. Activity of the para derivatives is slightly higher than that of the meta analogues. The ionic compound [Ni(2,4,6-Me₃C₆H₂) (CH₃CN) ((PhCH₂)₂PCH₂CH₂P(CH₂Ph)₂)]BF₄ and the neutral trans-[NiBr(2,4,6-Me₃C₆H₂) (P(CH₂Ph)₃)₂] do not show any activity in the codimerization reaction of styrene and ethylene in the same conditions.

The transformation of cyclohexanone was carried out on PtHZSM5 catalysts under the following conditions: flow reactor, 473 K, pressures of cyclohexanone and hydrogen equal to 0.25 and 0.75 bar respectively. Six families of products were identified by GC or GC-MS analysis: C₆ cyclic hydrocarbons 1, C₁₂ bicyclic hydrocarbons 2 (e.g., cyclohexylcyclohexene), cyclohexenylcyclohexanone 3, cyclohexylcyclohexanone 4, phenylcyclohexanone 5, tricyclic ketones 6 (e.g., biscyclohexenylcyclohexanone). A reaction scheme is proposed to explain the formation of these products. Compounds 1 would result from the following steps: hydrogenation of cyclohexanone (probably in the enol form) on Pt sites, dehydration of cyclohexanol on the acid sites, hydrogenation or dehydrogenation of cyclohexene on Pt sites. Compounds 2 are mainly formed through successive transformations of 4: hydrogenation, dehydration…; 3 results from aldolisation of cyclohexanone followed by dehydration of the resulting alcohol, 4 from hydrogenation of 3, 5 from dehydrogenation of 3. The compounds 6 result from aldolisation of 3 with cyclohexanone followed by dehydration, hydrogenation and dehydrogenation steps. The dehydration of alcohols is much more rapid than aldolisation and hydrogenation—dehydrogenation steps. On a 0.2 PtHZSM5 catalyst with a platinum dispersion greater than 70%, aldolisation is slower than hydrogenation—dehydrogenation steps. The deactivation of the catalyst affects more the acid sites than the metallic ones.

Alkenes are converted to aldehydes under quite mild conditions (8.5 atm. CO, 105°C) by reaction with carbon monoxide and formic acid in the presence of catalytic amounts of 5% Rh/C and 1,3-bis(diphenylphosphino)propane (dppp) [250:4:1 ratio of reactant:dppp:Rh/C]. Good selectivity for the branched chain aldehydes resulted for the olefins RCHCH₂, where R = Ar, OAc, OCOPh, SPh, while the linear product was favored for R = SiMe₃ or SiPh₃.