Catalyst turnovers, hydrocarbon oxidation

In the majority of the homogeneous oxidations of hydrocarbons by oxometal-based catalysts (including metalloporphyrins), there is appreciable decomposition of catalyst ligands by oxidation, and hence appreciable loss in activity after a few turnovers. A similar degradation of organic ligands, often hydrophobic long-chain carboxylates, is also observed in industrial processes of hydrocarbon... 

All the binary Cu/ZnO catalysts were found highly selective toward methanol without DME, methane, or higher alcohols and hydrocarbons detected in the product by sensitive gas chromatographic methods (59). Several of the composites were also found to be very active when subjected to a standard test with synthesis gas C0/C02/H2 = 24/6/70 at gas hourly space velocity of 5000 hr- pressure 75 atm, and temperature 250°C. The activities, expressed as carbon conversions and yields, are summarized in Table VIII. The end members of the series, pure copper and pure zinc oxide, were inactive under these testing conditions, and maximum activity was obtained for the composition Cu/ZnO = 30/70. The yields per unit weight, per unit area of the catalyst or the individual components, turnover rates per site titratable by irreversible oxygen and by irreversible carbon monoxide, are graphically... 

Abstract. Perhalogenated ruthenium porphyrins were found to be efficient catalysts for the oxygenation of hydrocarbons including secondary alkanes and benzene in the presence of 2,6-dichloropyridine A-oxide under mild conditions in aprotic media. Up to 15,000 turnovers and rates of 800 TO/min were obtained. A mechanism where Ru(III) - Ru(V) intermediates play an important role is proposed and discussed. 

Transformed rare earth and actinide intermetallic compounds are shown to be very active as catalysts for the synthesis of hydrocarbons from CO2 and hydrogen. Transformed LaNis and ThNis the most active of the materials studied they have a turnover number for CH formation of 2.7 and 4.7 X 10 sec at 205°C, respectively, compared with I X 10 sec for commercial silica-supported nickel catalysts. Nickel intermetallics and CeFe2 show high selectivity for CHj formation. ThFcs shows substantial formation of C2H6 (15%) as well as CHi,. The catalysts are transformed extensively during the experiment into transition metal supported on rare earth or actinide oxide. Those mixtures are much more active than supported catalysts formed by conventional wet chemical means. 

Applications of molybdenum-based catalysts for CO-H2 reactions have received much attention since workers at the U.S. Bureau of Mines (111) reported high rates of molybdenum catalysts for methanation, exceeded only by those of the most active group 8 metal catalysts (Fe, Co, Ni, and Ru). Moreover, they were relatively resistant to sulfur poisoning and could simultaneously catalyze the water gas shift reactions during hydrocarbon synthesis, thus allowing a CO-rich gas mixture to be used. Recently, studies have been extended to various molybdenum compounds and supported catalysts (45,49,106,112). The turnover rates based on CO chemisorption for supported and unsupported molybdenum carbides (0.04-0.131 s ) were higher than for corresponding Mo metal (0.02 s ) at 570 K and atmospheric pressure (95,113). Oxides, sulfides, and nitrides of molybdenum were reported to exhibit somewhat lower turnover rates than metal or carbide counterparts (112). These values are comparable to those of Ru (0.03-0.77 s ), Ni (0.066 s ), and Co (0.09 s ), which are known as the most active catalysts for these reactions (114). 

Clean tungsten carbides, a-WC and a-W C, form essentially only hydrocarbons from CO—H2 reactions. At 673 K and atmospheric pressure, the main products on WC, W2C, and W are methane, CO2, and H2O (121). Ethane and propane are also formed at lower temperatures. WC was substantially more active than W2C and W. The nature of the products can be modified by oxide promoters, as for the case of Rh or Pt, or by the carbon vacancies at the surface (122). At 573 K and 5 MPa with 2H2/CO, turnover rates (based on sites titrated by CO chemisorption) of 0.25-0.85 s were reported for hydrocarbon synthesis over bulk and Ti02-supported tungsten carbides. In addition, WC and WC/Ti02 produced alcohols and other oxygenates with 20-50% selectivity. However, W2C of more metallic character did not produce any oxygenates. Coexistence of carbidic and oxidic components on the catalyst surface appeared to be responsible for alcohol formation. 

A Ru(TPFPP)(CO) (4) complex has been prepared, and it was found that 4 is an efficient catalyst for the aerobic oxidation of alkanes using acetaldehyde [140]. Thus, the 4-catalyzed oxidation of cyclohexane with molecular oxygen in the presence of acetaldehyde gave cyclohexanone and cydohexanol in 62% yields based on acetaldehyde with high turnover numbers of 14 100 (Eq. (7.86)). It is worth to note that iron [139] and copper [141] catalysts are also efficient for the oxidation of non-activated hydrocarbons at room temperature under 1 atm of molecular oxygen. 

Another metallocene, namely, decamethylosmocene, (Mc5C5)20s (catalyst 1.2), turned out to be a good precatalyst in a very efficient oxidation of alkanes with hydrogen peroxide in acetonitrile at 20 — 60 °C [9]. The reaction proceeds with a substantial lag period that can be reduced by the addition of pyridine in a small concentration. Alkanes, RH, are oxidized primarily to the corresponding alkyl hydroperoxides, ROOH. TONs attain 51,000 in the case of cyclohexane (maximum turnover frequency was 6000 h ) and 3600 in the case of ethane. The oxidation of benzene and styrene afforded phenol and benzaldehyde, respectively. A kinetic study of cyclohexane oxidation catalyzed by 1.2 and selectivity parameters (measured in the oxidation of n-heptane, methylcyclohexane, isooctane, c -dimethylcyclohexane, and trans-dimethylcyclohexane) indicated that the oxidation of saturated, olefinic, and aromatic hydrocarbons proceeds with the participation of hydroxyl radicals. 

Work in this laboratory has shown also that the Ru(poip)(0)2 complexes (porp = TMP, TDCPP, and TDCPP-Clg) are practically inactive for thermal 02-oxygenation of saturated hydrocarbons . Some activity data for 0.2 mM Ru solutions in benzene under air at 25°C for optimum substrates such as adamantane and triphenylmethane at 6 mM did show selective formation of 1-adamantol and trityl alcohol, respectively, but with turnover numbers of only -0.2 per day the maximum turnover realized was -15 after 40 days for the TDCPP system Nevertheless, this was a non-radical catalytic processes there was < 10% decomposition of the Ru(TDCPP)(0)2, and a genuine O-atom transfer process was envisaged . Quite remarkably (and as mentioned briefly in Section 3.3), at the much lower concentration of 0.05 mM, Ru(TDCPP-Clg)(0)2 in neat cyclooctene gave effective oxidation. For example, at 90°C under 1 atm O2, an essentially linear oxidation rate over 55 h gave about -70% conversion of the olefin with - 80% selectivity to the epoxide however, the system was completely bleached after - 20 h and, as the activity was completely inhibited by addition of the radical inhibitor BHT, the catalysis is operating by a radical process, but in any case the conversion corresponds to a turnover of 110,000 As in related Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered to be very effective catalysts for the decomposition of hydroperoxides (eqs. 

---