分子催化最新文献

The high-temperature acetone hydrogenation at atmospheric pressure over alumina and titania-supported nickel and cobalt catalysts has been studied. The results obtained during characterization have shown that the preparation steps produced a significant metal—support interaction in the alumina-supported precursors. On the other hand, the high-temperature (500°C) reduced titania-supported catalysts showed catalytic properties which resemble in some aspects those ascribed to the SMSI state. An interesting effect of the previous H₂ reduction temperature on the acetone hydrogenation selectivity has been found. 2-Propanol and methyl isobutyl ketone (MIBK) were the main reaction products. The MIBK selectivity of the alumina-supported catalysts decreased as the temperature of reduction was increased. However, whereas the low-temperature (300°C) reduced titania-supported catalysts did not give any MIBK, a remarkable activity to this product was developed when the temperature of reduction was increased to 500°C. The effect of the temperature of reduction on selectivity has been tentatively explained according to a general model which relates the activation of the carbonyl functional group with specific sites created at the metal—support interface under conditions favourable to the establishment of significant metal—support interactions.

Catalytic dehydrogenation of isopropyl alcohol was studied over Co₃O₄NiO/MgO catalysts containing 20 wt% of Co₃O₄ and NiO, or mixture of both oxides with different molar ratios. Catalysts were prepared from the corresponding nitrates (i.e. Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O) and supported on MgO. Catalyst precursors were examined by TG, DTG and DTA, and were calcined between 500–700°C for 5 h in air. The surface area of these samples were calculated using the BET method. Those catalysts calcined at 500°C were characterized by XRD analysis. The amount of excess surface oxygen for these catalysts was calculated iodometrically. Basicity measurements were estimated by the adsorption of acidic molecules using pulse technique method. The vapour-phase dehydrogenation of isopropyl alcohol (IP), has been carried out over the catalysts calcined between 500–700°C. All catalysts showed 99.5% selectivity towards acetone formation. A correlation between the catalytic activity and the basicity of these catalysts has been made. The activation energy and the order of the dehydrogenation reaction of IP over some catalysts are given.

Low-pressure on-stream conversion of cyclohexanone (CHN) was studied over HZSM-5 zeolites with various Si/ Al ratios (different number of strong acid sites). Each conversion run was followed by thermal decomposition of surface species which also were investigated using FTIR spectroscopy. It was found that, below 300°C, surface complexes consisting predominantly of CHN were held in the zeolites. The release of products, consisting of dehydrated CHN (C₆H₈), substituted aromatics and olefins, began above 250°C. The latter two compounds were formed via intermediates most probably of the aldol type (however, the dimerized methylcyclopentadiene intermediate cannot be excluded). The CHN dehydration to C₆H₈ predominated at 480°C. The yield of all products increased with the increasing number of zeolite strong acid sites. Ammonia, co-fed or preadsorbed, reacted with CHN to surface imines which hindered all other CHN reactions. The imine was strongly held in zeolites and decomposed at about 450°C to gaseous C₆H₈ and ammonia. Methanol partially methylated the reaction products but did not affect the main reaction routes, monomolecular dehydration and secondary intermolecular reactions.

Phosphorus-31 spin lattice relaxation times, T₁'s, for SAP hydroformylation catalysts derived from HRh (CO) (TPPTS)₃ and TPPTS (TPPTS = trisulfonated triphenyl phosphine trisodium salt) are consistent with a liquid-like character for the supported aqueous phase. Ale T₁ for solid TPPTS is 1150 s while for TPPTS in a supported aqueous phase T₁ varies from 220 to 4.9 s. A T₁ of 4.9 s is close to the value obtained for TPPTS in solution and is observed at a H₂O/P ratio of 24.9. At this level of water content previously reported SAP catalysts have shown maximum catalytic activity for the hydroformylation of olefins.

Asymmetric hydroformylation of styrene is performed using the bridge dithiolate chiral rhodium complexes [Rh₂(μ-(−)-DIOS)(cod)₂]_n n=2 (1); and n=1 (2); ((−)-DIOS: 2,3-O-isopropylidene-1,4-dithiO-L-threitol and cod: 1,5-cyclooctadiene) as catalyst precursors. The catalytic system 1 provides high conversion in the corresponding aldehydes (100% at 30 bar, 65°C) with a selectivity in 2-phenylpropanal of 64%. Addition of PPh₃ to the catalytic system 1 increases the selectivity in 2-phenylpropanal to 91 % in the same conditions. Although the ee's obtained using 1 were low (3–5%), the addition of a chiral phosphine, (+)-DIOP ((2S,3S)-2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis (diphenylphosphino)butane), improves the ee in (S)-2-phenylpropanal to 17%. However the addition of (−)-DIOP gives a lower ee (3% (S)). Comparative results were obtained using 2 as catalyst precursor indicating that similar species were formed during catalytic cycle.

The reduction pattern of PdCl₂ supported on oligomeric aramides has been investigated by temperature programmed reduction (TPR) measurements in the range of temperature −80 to 305°C. The influence both of chemical composition of the oligomeric matrix and Pd loading has been ascertained. A peculiar metal—support interaction greatly affects the reducibility of Pd, allowing its stabilisation in the oxidised form (i.e., Pd²⁺).

A study has been made of the liquid phase conversion of cyclohexene over metallic palladium and platinum catalysts under oxygen and nitrogen atmosphere. Special attention has been given to the effect of oxygen pressure, catalyst support, solvent and metal modifiers on the activity of metal catalysts and distribution of products. In every case the main reaction pathway is disproportionation of cyclohexene to benzene and cyclohexane. Formation of oxygenated products is low even at 2–6 atm of oxygen. Doping of Pd and Pt catalysts with metal modifiers strongly suppresses the conversion of cyclohexene and has no influence on the ratio of benzene/cyclohexane at experiments carried out in an inert atmosphere. An interpretation of these effects is presented.

The esterolytic activity of 5(6)-n-alkyl-2-methanethiolbenzimidazoles towards hydrophobic p-nitrophenyl esters has been investigated in the presence of cationic micelles (CTAB). The overall activity of the different pairs (thiol, ester) is characterized by an important variation during the time. The length of time from a very fast process (in comparison with the esterolytic rate constants obtained in a hydro-alcoholic medium) to a slow one depends notably upon the surfactant concentration but also on both thiol and surfactant chain lengths. In the case of a rapid process at pH close to neutrality, the high reactivity of these thiols is essentially due to a cooperative effect involving their neutral form.

As a part of converting coal to liquid fuels, the poisoning of commercial FeCuK catalyst in Fischer-Tropsch (FT) synthesis by carbonyl sulfide in synthesis gas was tested. Reduced coprecipitated FeCuK catalyst was investigated with synthesis gas containing 73.2, 184, 232.4, and 350.8 mg of sulfur as COS per cubic meter at 524 K under 2.31 MPa, with an hourly space velocity of 1854. The catalyst activity decreased rapidly in the first few hours of poisoning process, and finally decreased linearly with amount of sulfur fed to the catalyst until the relative activity was 0.01 to 0.05. The deactivation rates were different with the content of sulfur in synthesis gas. The selectivity to methane and gaseous C₂C₄ hydrocarbons increased when the catalyst was poisoned by COS. The weight content of C⁺₅ in product decreased significantly with increasing sulfur fed to catalyst. The water-gas shift reaction in FT synthesis was also discussed in this work.