分子催化最新文献

The thermolysis of anisole and phenetole under flow and batch conditions was examined over zeolites (HZSM-5, P-HZSM-5, H-Theta-1, H-Y and H-mordenite). The major components of the product mixture were phenol, the rearranged products (cresols and ethylphenols) and alkylated substrates (methylanisoles and ethylphenetoles). All the zeolites showed ortho selectivity in cresols and ethylphenols while the medium-pore zeolites (HZSM-5, P-HZSM-5 and H-Theta-1) showed para selectivity in methylanisoles and ethylphenetoles in contrast to the large-pore zeolites (HY and HM) which showed ortho selectivity. The observed selectivities are thought to be due to both kinetic effects (diffusion) and transition state selectivity over these zeolites.

Strontium hydroxyapatite of various Sr/P ratios catalyzes oxidative dehydrogenation of methane to carbon oxides and hydrogen at a reaction temperature of 600°C. Although the selectivities are essentially independent of the composition of the catalyst, the activity decreases with decreasing concentration of strontium. The relatively high ratio of H₂/CO produced cannot be rationalized either through the oxidation of methanol on the catalyst or the water gas shift reaction, but appears to result primarily from the direct oxidation of methane, with the reaction to produce CO₂ and H₂ proceeding at a rate approximately twice that to CO. New active sites associated with strontium appear to be responsible for the catalytic properties of the hydroxyapatite.

The role of cobalt(II) and its complexes with 1-phenyl 1-hydrazonyl 2-oximinopropanedione (HPHOPD) as homogeneous and heterogeneous catalysts in the decomposition of hydrogen peroxide has been investigated by measuring evolution of oxygen at different time intervals in the temperature range 35–60°C. Enhanced activity is observed with Co(PHOPD)₂ used as heterogeneous catalyst and with cobalt:HPHOPD (1:1) complex heterogenized by adsorption on alumina, while in case of solutions of cobalt(II) chloride and cobalt:HPHOPD (1:1), used as homogeneous catalysts, the activity is comparatively low and negligible respectively. The reaction rate, in general, is found to increase with increase in temperature and quantity of the catalyst. Various thermodynamic parameters have been calculated. The adduct formation with a heterocyclic base like pyridine, 3-methyl pyridine, chloro pyridine or quinoline is found to inhibit the catalytic activity of Co(PHOPD)₂. The probable reaction mechanisms have been suggested for Co(PHOPD)₂ and alumina-Co^II-HPHOPD as catalysts.

The catalytic activity of ClCo(PPh₃)₃ in oligomerization of alkynes and reduction of carbonyl compounds has been investigated. Monosubstituted alkynes with an electron withdrawing group give quantitative yields of arene derivatives when reacted with 5 mol% of ClCo(PPh₃)₃. Cyclotrimerization of disubstituted alkynes with at least one electron withdrawing group requires the addition of NaBPh₄. In the oligomerization of phenylacetylene, cyclotrimerization and linear dimerization are in competition. However, when the reaction is run in the presence of 1.2 equiv. of NaOMe at room temperature, only linear dimerization is observed. Diethyl allylpropargylmalonate also cyclodimerizes or codimerizes with diphenylacetylene by catalytic amount of ClCo(PPh₃)₃ in the presence of NaOEt. Reaction of a variety of carbonyl compounds with 5 mol% of ClCo(PPh₃)₃ in i-propanol leads to the reduction of the compounds to the corresponding alcohols. The reaction requires the addition of NaH which produces (i-PrO)Na in situ. The role of alkoxides or NaBPh₄ in ClCo(PPh₃)₃ catalyzed oligomerization of alkynes or reduction of carbonyl compounds is discussed.

The impact of various k²-P⌢O chelate ligands on nickel catalysed ethylene oligomerization has been investigated. The ligands 4-8 and 14, 15 were reacted with bis(π-methallyl)nickel yielding seven new complexes, 9-13 and 16, 17 in which the k²-P⌢O chelate ring size was altered and in which methyl substituents were introduced into the k²-P⌢O chelate. These complexes served as precursor complexes leading to active ethylene oligomerization catalysts. Increasing the chelate ring size from a five-membered to a six or seven-membered ring diminishes catalytic activity dramatically. However, by introducing double bonds or an aromatic ring into the six-membered nickelaheterocycle yielded very active ethylene oligomerization systems. Introducing methyl groups into Ph₂PCH(CH₃)COOH (14) and Ph₂PC(CH₃)₂COOH (15) also leads to a significant increase in activity.

In the framework of an EC Stimulation Action Programme, alumina-supported mono- and bimetallic Ru, RuGe, RuSn and RuPb catalysts have been prepared from both organic and inorganic precursors. Herein are presented some basic properties of these materials obtained by TPR and TPO experiments, H₂ and CO chemisorption, TEM examination, XPS and EXAFS studies. The use of organic precursors led to very small Ru particles, with a coordination number between Ru atoms of 4.35 (Ru⁰-Ru⁰+Ru⁰-Ru^δ+). The size of the metallic particles was not modified by adding Ge, Sn or Pb by the controlled surface reaction technique. In very small aggregates, some Ru atoms (10–15%) remained partially oxidised even after reduction at 623 K in flowing hydrogen. During the coimpregnation of alumina with inorganic precursors, the modifier played the role of a nucleation center for Ru, thus allowing small particles to be formed. We believe that CO could modify the surface composition of small bimetallic RuGe particles during the chemisorption.

Extended Hückel calculations are performed to study how Na₂Bi₄, NaBi₄ and Bi₄ clusters and atomic Na react with HCl. We find that Bi₄ fragments act as “charge transfer catalysts” for the reaction of Na with HCl, weakening the HCl bond. Our results point to a higher reactivity of the Na₂Bi₄ cluster compared with NaBi₄, a repulsive state characterizing the reaction of Bi₄ with HCl and a “thermodynamic” impediment for the reaction of Na with HCl. These facts are in good agreement with experimental results.

The Wilkinson complex RhCl (PPh₃)₃, which is inactive for 2-propanol dehydrogenation, exhibited its catalytic activity by adding Et₃N, or by using a hydride complex, instead of chloride, for ketone hydrogenation as well. The elemental steps of the proposed dehydrogenation cycle was investigated with EHMO method.

The oxidation of cyclohexene with molecular oxygen catalyzed by solid Pd/C or Pd^II acetate—hydroquinone—iron phthalocyanine gives cyclohexane, benzene and oxygenated products. Oxygen pressure and solvent used influence significantly the distribution of the products and at the pressure above 2 atm no cyclohexane is formed. Over Pd/C catalyst in the absence of oxygen, disproportionation to cyclohexane and benzene (the ratio is nearly 2:1) proceeds exclusively. Under comparable conditions 2-cyclohexenol is disproportionated to cyclohexanol and phenol and some of it rearranges to cyclohexanone. The explanation for the disproportionation of cyclohexene under the Wacker conditions is that Pd⁰ centres intermediately formed after stoichiometric oxidation of cyclohexene by Pd^II are not completely reoxidized, but depending on the reaction conditions, they can partly aggregate and then, similarly to metallic surfaces, dehydrogenate cyclohexene to benzene. The hydrogen species formed migrate on the palladium surface and hydrogenate cyclohexene or at sufficient oxygen pressure they are oxidized to water.