分子催化最新文献

Depending on the experimental conditions either monoperoxo or diperoxo vanadium complexes are formed in acid aqueous solutions by addition of H₂O₂ to NH₄VO₃. The ⁵¹V-NMR spectra of both species have been registered at different pH values. While the signal of the monoperoxo derivative is unchanged in the pH range 0–4, the chemical shift of the diperoxo species shows a continuous variation in the same interval thus suggesting the occurrence of a protonation equilibrium leading to a neutral diperoxo vanadium complex. In the presence of bidentate ligands L, e.g., picolinic or pyrazinic acid, under otherwise identical experimental conditions, complexed peroxo vanadium species are formed. The association equilibria can be conveniently studied by ⁵¹V-NMR spectroscopy. Thus, different species, e.g., VO(O₂)L, [VO(O₂)L₂]⁻, [VO(O₂)₂L]²⁻ as a function of the pH has been detected and the corresponding association constants have been measured.

The effect of palladium dispersion, on the selectivity for phenylacetylene hydrogenation was studied. Low dispersed palladium catalysts showed to be more selective to partial hydrogenation than high dispersed ones. The initial selectivity to styrene is increased with time on stream. Carbonaceous deposits were assumed to be responsible for this phenomenon. Selectivity was also modified by ammonia and sulfur addition: ammonia increased the styrene selectivity while sulfur had a negative effect. The poisoning effects were explained through an electronic effect. Ammonia is an electron donor modifier whereas sulfur is an electron acceptor modifier.

Effects of temperature on both the progress curves and reaction rates of the separate steps of triolein hydrolysis: decomposition of tri-, di- and monoolein as well as production of oleic acid by lipase (Candida cylindracea) in AOT/isooctane reversed micellar systems were investigated. The activation energy determined from the Arrhenius plot of fatty acid production was 22.3 kJ/mol.

The hydrodesulfurization of thiophene on a commercial CoMo/alumina catalyst was studied in a Berty type CSTR reactor at 350°C, with partial pressures of hydrogen sulfide in the range of 0.2 to 5.5 bar. A kinetic model with interconversion of active sites is superior to models based on a fixed number of active sites. The proposed mechanism for the interconversion of active sites predicts that the hydrogenation and hydrogenolysis activities of a HDS catalyst can be substantially varied by hydrogen sulfide. The model also explains the controversial hydrogen sulfide effects observed in various types of reactions on CoMo catalysts.

Hydrolysis of surcose by protonated Y-zeolites was shown to be strongly accelerate by delumination. Two or less protons per unit cell provide a maximal rate constant per acid site. The rate constant is the same compared to diluted homogenous acid solution. Hydrophobic interactions essentially contribute to the adsorption of the sucrose molecule but not to the energy of the activated transition state. It was proved that the zeolite pores are able to include a sucrose molecule in addition to at least one water molecule. Because of the limited space inside the zeolite pore, a change of conformation compared with the state in solution is reqired.

TiO₂ gel obtained by hydrolysis of titanium tetra-n-butoxide was heated at various temperatures, providing poorly crystallized TiO₂, disordered anatase and a well crystallized anatase—rutile mixture. Platinum dispersion was obtained on these TiO₂ supports by the reduction of platinum—allyl species with H₂ at 278 K. These catalysts and a Pt/SiO₂-gel sample were used for propene hydrogenation at various temperatures and H₂/propene molecular ratios. Fresh and run catalysts were subjected to X-ray diffraction analysis for the study of extant phases and determination of average size of platinum particles. In all cases, results obtained for catalysts as prepared or after H₂ treatment at 493 K indicated that the ordinary hydrogenation mechanism was maintained but that important changes of specific catalytic activity were observed, depending on H₂ treatment and nature of the support. TiO₂ morphology was assumed to be responsible for the specific activity decrease in Pt on the rutile—anatase mixture and for the opposite effect found for platinum on anatase.

Copolymers of styrene and chloromethylstyrene containing 2% ammonium salt groups have been grafted onto montmorillonite interlayers by cation exchange process. The remaining chloromethyl groups were modified to produce phosphonium salts. The catalytic activities of the phosphonium moieties supported on polystyrene-montmorillonite have been investigated in thiocyanate and nitrite nucleophilic reactions. The effectiveness of these catalysts has been examined under the effects of several factors governing the reactions rates such as the chemical composition of polymers backbone, chemical nature of the active moiety, degree of loading, nature of the solvent, and reaction temperature.

Mechanistic aspects of the higher alcohol synthesis (HAS) over a ZnCrO sample are investigated by flow-microreactor experiments of 1-propanol. Results indicate that the catalyst can promote a number of chemical functions including hydrogenation/dehydrogenation, ‘normal’ and ‘reversal’ aldolic-type condensation, ketonization, α-addition, dehydration. On comparing the data with those obtained over a K-promoted sample it appears that the alkali dopant does not affect the reactivity of ZnCrO towards hydrogenation/dehydrogenation and ‘normal’ aldolic-type condensation, is essential for ‘reversal’ aldolic-type condensation and α-addition, favors ketonization and essentially suppresses the dehydration function. Thus the alkali promoter, besides playing a crucial role in the C₁→C₂ step, significantly affects the chain growth mechanisms involving C₂₊ oxygenates and effectively preserves oxygenate molecules towards the production of undesired hydrocarbons by dehydration.

Temperature-programmed desorption (TPD) studies of allene on reduced TiO₂ (001) surfaces were undertaken to compare the chemistry of allene with simple alkynes (in particular, its isomer methylacetylene). The principal product of the reaction of allene was the hydrogenation product propylene; three dimerization products, dimethylene cyclobutane, benzene, and an open-chain C₆H₁₀ dimer, were produced in significantly smaller amounts. Conversion of allene was around 70% on the most active (most highly reduced) surfaces, with propylene production accounting for about two-thirds of the reactant converted on a carbon-content basis. Adsorption of allene was greatest on the most reduced surfaces, and decreased dramatically on less reduced surfaces prepared by prior annealing at 650 K and above. All dimer products were extinguished on surfaces annealed to over 650 K prior to adsorption, while smaller but non-zero amounts of propylene continued to be produced on surfaces annealed in this temperature range. All three dimeric products track the population of Ti( + 2) cations on the surface; this site requirement implies that these reactions involve a surface intermediate whose formation requires a two-electron oxidation of surface cations. A metallacyclopentane intermediate is proposed to account for the formation of allene dimerization products. This intermediate is similar to the metallacyclopentadiene involved in alkyne dimerization and cyclotrimerization on reduced TiO₂ surfaces. Although two of the products (propylene and the C₆H₁₀ dimer) from allene TPD are common to methylacetylene also, no formation of trimethylbenzene (nor of any trimer product) was observed from allene, in sharp contrast to the behavior of methylacetylene on reduced TiO₂ surfaces. Except for the novel production of benzene from allene in this study, analogies may be found for the distinct reaction pathways of both allene and methylacetylene in the chemistry of transition metal complexes.