1606-08-2

The catalytic activity of nickel and cobalt phosphides, with a metal loading of 5 wt.%, supported on silica was investigated in the hydrodeoxygenation reaction (HDO) of dibenzofuran (DBF) as a model oxygenated compound at different contact times, H2 pressures and H2/DBF molar ratios. The aim of the study was to understand the mechanism of the reaction and to study the impact of H2 pressure and H2/DBF molar ratio on the reaction. The catalysts were characterized by N2 adsorption-desorption isotherm measurement at -196°C, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO chemisorption, NH3 Temperature-Programmed Desorption (NH3-TPD), IR spectroscopy and H2 Temperature-Programmed Desorption (H2-TPD). The prepared catalysts were tested in the HDO reaction of DBF in a continuous-flow fixed-bed stainless steel catalytic reactor at pressures ranging from 1-30 bar at 275°C. The results obtained indicate that the Ni2P catalyst is more active than the CoP catalyst, converting more than 90% of DBF at the highest contact time into oxygen-free products. The activity of both catalysts increases with increased contact time. At low contact times, the intermediates tetrahydrodibenzofuran (THDBF) and hexahydrodibenzofuran (HHDBF) are observed as products, while an increment in the contact time led to the transformation of THDBF and HHDBF into O-free compounds, mainly bicyclohexane (BCH), indicating that the HDO of DBF follows the path: DBF → HHDBF → THDBF → 2-CHP → BCH. Further, both Ni2P and CoP catalysts are active at medium pressures with HDO degrees similar to those obtained at 30 bar. Ni2P is less affected by the changes in H2/DBF ratio than CoP and the catalysts are more active at high H2/DBF molar ratios.

The reactions of di-2-pyridyl ketone (dpk), 2-benzoylpyridine (zpy) and 2-hydroxybenzophenone (Hbp) with [Re(CO)5Cl] (A) and trans-[ReOX 3(PPh3)2] (B, X = Cl, Br) were studied. The complexfac-[Re(CO)3 (dpk·OCH3)] was isolated from the reaction of A with dpk in methanol. The monoanionic tridentate chelate dpk·OCH3 was formed by the nucleophilic attack of methanol at the carbonylic carbon atom of dpk. A similar attack of water on dpk was observed in the compound cis-[ReOBr2(dpk·OH)]·2(dpkH +Br), which was formed from dpk and [ReOBr3(PPh 3)2] in acetone. The reaction of zpy with B in acetonitrile produced the complexes [ReIIIX3(zpy)(PPh 3)], but in methanol as solvent the compounds [ReOX 2(zpyH)(PPh3)] were isolated, where zpyH coordinates bidentately as the monoanionic ligand [C6H5(HC-O)C 5H4N]. With A as starting material the complexfac-[Re(CO)3(zpy)Cl] was isolated. The complexes cis-[ReOX2(bp)(PPh3)] were the products of the reaction of Hbp with B in acetonitrile; however, in methanol cis-[ReIIIBr 2 (bp)(PPh3)2] was isolated. All these complexes were characterized by conductance measurements, elemental analyses, UV-Vis, IR and NMR spectroscopy and by single crystal X-ray diffraction. DFT calculations regarding the electronic ground states show single states for all the complexes, except for the rhe-nium(III) complexes [ReIIIX 3(zpy)(PPh3)] and [ReBr2(bp)(PPh 3)2], in which the states are triplet. The DFT and experimental results are in agreement in all cases, especially the anisotropy of the Re-N bond length offac-[Re(CO)3(dpk·OCH3)] and exact O(1)-Re-O(3) angles for [ReOX2(bp)(PPh3)].

Mercury-photosensitized dehydrodimerization in the vapor phase can be made synthetically useful by taking advantage of a simple reflux apparatus (Figure 1), in which the products promptly condense and are protected from further conversion.This vapor pressure selectivity gives high chemical selectivity even at high conversion and on a multigram scale.Mercury absorbs 254-nm light to give the 3P1 excited state (Hg*), which homolyses a C-H bond of the substrate with a 3o>2o>1o selectivity.Quantitative prediction of product mixtures in alkane dimerization and in alkane-alkane cross-dimerizations is discussed.Radical disproportionation gives alkene, but this intermediate is recycled back into the radical pool via H atom attack, which is beneficial both for yield and selectivity.The method is very efficient at constructing C-C bonds between highly substituted carbon atoms, yet the method fails if a dimer has four sets of obligatory 1,3-syn methyl-methyl steric repulsions, as in the unknown 2,3,4,4,5,5,6,7-octamethyloctane.We have extended the range of substrates susceptible to the reaction, for example to higher alcohols, ethers, silanes, partially fluorinated alcohols, and partially fluorinated ethers.We see selectivity for dimers involving C-H bonds α to O or N and for S-H over C-H.An important advantage of our experimental conditions in the case of alcohols is that the aldehyde or ketone disproportionation product (which is not subject to H. attack) is swept out of the system by the stream of H2 also produced, so it does not remain and inhibit the rate and lower the selectivity. kdis/krec is estimated for a number of radicals studied.The very hindered 3o 1,4-dimethylcyclohex-1-yl radical is notable in having a kdis/krec as high as 7.1.

Unimolecular H2 elimination from alkanes was investigated in cyclopentane-cyclohexane, n-hexane-cyclohexane and cyclohexane-cyclooctane mixtures during radiolysis and 7.6 eV photolysis.During the radiolysis of all system, and when the fluorescence shift law allowed it, during the photolysis as well, inhibited H2 detachment was observed from the first component and sensitized hydrogen molecule elimination from the second.It has been concluded that the same excited state (the lowest singlet, S1) is responsible for the H2 elimination during radiolysis and photolysis and this is that one that gives rise to fluorescence in the experiments of other authors.The H2 and H elimination from alkanes generally have different excited precursors.The direct population of S1 by γ-irradiation is of limited importance and this intermediate is mainly produced in "charge neutralization" processes.