分子催化最新文献

The interaction of carbon monoxide with nickel—iron alloys is studied on Ni(111)-Fe surfaces by means of ellipsometry and Auger electron spectroscopy, and by means of infrared spectroscopy on γ-Al₂O₃-supported Ni_xFe catalysts. The nickel-to-iron ratio of the alloy strongly affects the interaction with CO. On alloys with an iron atom fraction of 0.3 or less the adsorption of CO is molecular, whereas at fractions higher than 0.4 it is initially dissociative. Surfaces contaminated with carbon and oxygen exhibit a lower heat of adsorption. The presence of pre-adsorbed hydrogen lowers the amount of CO which can be adsorbed on Ni(111)-Fe surfaces, whereas on supported catalysts it leads to the formation of formate species already at room temperature.

A selection of topics dealing with instrumentation and applications of NMR under high pressure of gases is presented, with emphasis on some recent advances in mechanistic studies of homogeneous hydrogenation and carbonylation reactions carried out in the laboratory of the author.

Ni(salen) (salen = bis-(salicylidene)ethylenediamine) complex was encapsulated in zeolite Y and hydrogenations of cyclohexene, cyclooctene, 1-hexene and benzene were carried out with Ni(salen) encapsulated zeolite at moderately high temperature (40°C) and pressure (60 atm of H₂).

GeBu₄ reacts at 250°C with the external silanols of two different mordenites (Si/Al = 11 or 40) to give the surface organometallic complex SiOGeBu₃ 1 which has been characterized by ¹³C CP-MAS NMR; 1 is able to discriminate sterically different alkanes such as n-hexane, 2-methyl-pentane, 2,3-dimethyl butane, 2,2,4-trimethyl pentane. Such discrimination is parallel to that already observed for SiOSnBu₃ grafted on the same mordenites. Interestingly and in contrast to the behaviour of the grafted tin complex, 1 even after treatment at 400°C is still able to separate n-hexane from isooctane, a property ascribed to a steric molecular control of the Ge(Bu)₃ fragment which has an exceptional thermal stability.

Using ⁵¹V NMR, it was shown that peroxo complex VO(O₂) (Pic) (H₂O)₂ (Pic = picolinic acid) exists in CH₃CN, MeOH and H₂O in the forms of VO(O₂) (Pic) H₂O·CH₃CN, VO(O₂) (Pic) H₂O·MeOH and VO(O₂) (Pic) (H₂O)₂, respectively. To elucidate the nature of reactivity of vanadium peroxo complexes Of VO(O₂) (Pic) H₂O·S family towards hydrocarbons their interactions with spin trap 3,3,5,5-tetramethyl-pyrroline-N-oxide (TMPO) were investigated by EPR. Only the peroxo complex VO(O₂) (Pic) H₂O·CH₃CN was reactive towards the hydrocarbons and the spin trap. This complex can abstract the β-hydrogen atom from TMPO (V^V reduces to V^IV by this reaction). Such hydrogen atom abstraction was not observed for the other radical species studied (free radicals OH•(HO₂•), RO•(RO₂•) and superoxo complex Pd₃(OAc)₅O₂•). The latter species form with TMPO a spin adduct by addition to the double bond. The data provide evidence, that the diradical species V^IVOO• are active particles of oxidation by VO(O₂) (Pic) H₂O·CH₃CN.

Deactivation of VO(O₂) (Pic) H₂O·CH₃CN upon substitution of CH₃CN by stronger σ-donor ligands (e.g., H₂O, MeOH, DMF) may be caused by prevention of electron transfer from the peroxo group to the metal to give V^IVOO•. Vanadium(V) superoxo complex V^V(O₂^.−) detected by EPR in a solution of VO(O₂) (Pic) (H₂O)₂ in 30% H₂O₂was inert towards alkenes and spin traps.

Enzyme kinetics of pectin hydrolysis reaction catalysed by an endo-polygalacturonase from Rhizopus have been analysed experimentally in a stirred batch reactor at pH=4.5 and T=34°C using a sodium salt of polygalacturonic acid as substrate. The reaction products, consisting of oligomers of galacturonic acid, were classified in four classes of oligomeric fractions employing three hollow fibre ultrafiltration units characterised by different molecular weight cut-off (MW_co). On the basis of the time-concentration data, a kinetic model of the reacting system has been proposed in terms of a series-side reaction pattern. Identification of kinetic parameters was carried out by a non-linear regression optimization procedure based on the Powell's algorithm of conjugated directions. Finally, the kinetic model was used to predict the best reactor configuration able to control the product distribution for a given enzyme/substrate concentration ratio.

Chromium(VI) oxide induces a quasi-quantitative transfer of one oxygen atom from t-BuOOH to a benzylic methylene group when a high ratio of substrate/hydroperoxide is employed, the corresponding carbonyl compound being the major oxidation compound. Hydroxylation and tert-butyl-peroxidation are the main reactions observed for a tertiary benzylic carbon. The interference of free benzylic radicals under these conditions is a minor reactive pathway.

Homogeneous oxygenated aqueous solutions of chlorophenols, undergo photodegradation (λ > 320 nm) to CO₂ and Cl⁻ by polyoxometalates W₁₀O⁴⁻₃₂, PW₁₂O³⁻₄₀, and SiW₁₂O⁴⁻₄₀. Mineralization takes place, also, in the absence of dioxygen but the process is more than an order of magnitude slower. In presence of dioxygen the effectiveness of these catalysts compares with TiO₂ suspension.

(Rh + Nb)/SiO₂ catalysts were obtained by anchoring of nanometer size Rh particles on surface low-valent Nb^{n +} ions. These materials exhibit the following specific features compared to Rh/SiO₂: a low Rh surface coverage by chemisorbed hydrogen and carbon oxide; a high-frequency shift of linear CO absorption band and appearance of two absorption bands of a C- and O-bonded CO at 1704 and 1652 cm⁻¹. On exposure to H₂, the stability of surface carbonyls increases in the sequence: Rh-CO-Nb^{n +} < Rh-CO ≤ Rh₂(CO) < Rh⁺(CO)₂. The possible role of C- and O-bonded CO complexes in the conversion of syngas on supported rhodium catalysts promoted by transition element oxides is discussed.

Isobutane was alkylated with 2-butene, in batchwise conditions, using liquid 1-butyl-3-methylimidazolium chloride—aluminium chloride molten salts as the acidic catalyst. The effect of the operating variables on the product composition has been investigated. The control of the acidity of the catalyst has made possible the production of a high quality alkylate.