Enantioselective hydrogenation of pyruvate esters over cinchona alkaloid modified platinum catalysts

The enantioselective hydrogenation of pyravate esters has been investigated over cinchona alkaloid modified platinum catalysts. Three topics have been examined: (i) A1MCM-41 as a novel support for the platinum catalysed enantioselective hydrogenation of ethyl pyruvate. (ii) The enantioselective hydrogenation of pyruvate esters over cinchonidine pre-modified PtAy-AbQa catalysts in a continuous flow reactor. (iii) An investigation into the modifier concentration dependence on the sense of enantioselectivity in ethyl pyruvate hydrogenation exhibited by derivatives of hydroquinidine and hydroquinine. A series of Pt-A1MCM-41 catalysts were synthesised using both CI 6- and CI 8-alkyl chain surfactant templates with platinum loadings approaching 2 wt/%. High resolution transmission electron microscopy (HRTEM) data confirmed retention of the mesoporous structure in the active catalysts and gave information on the platinum particle size and their location within the mesopore. High resolution magic angle spinning (HRMAS) 27 Al NMR was used to study the movement of aluminium within the mesoporous structure during the different preparation stages. The resultant platinum catalysts afforded enantioselectivities comparable to that obtained with the standard reference catalyst EUROPT-1 (6.3% Pt/silica), at rates moderated by an order of magnitude by mass transfer limitations. This is the first report of an enantioselective hydrogenation occurring in a mesopore. The hydrogenation of pyruvate esters over cinchonidine pre-modified catalysts in a continuous flow reactor operating at ambient temperature and pressure (0.25 bar/g) has been investigated. With dichloromethane as a solvent sustained enantiomeric excess of > 70% was maintained at very low cinchonidine/platinum ratios and are comparable to the enantioselectivity achieved when the reaction is conducted in the same solvent at autogeneous pressures (50 bar). When dichloromethane is used as a reaction solvent conversion is seen to decrease with time on line and is considered to be the result of polymer formation on the catalyst surface. The pre-treatment of the catalyst bed with (R)-ethyl lactate prior to the introduction of methyl pyruvate is detrimental to the catalytic system with an 18 % drop in enantioselectivity, potentially the result of an unfavourable interaction between lactate and modifier. In the enantioselective hydrogenation of ethyl pyruvate over hydroquinidine-4-chlorobenzoate modified Pt/v-AbQ} the sense of enantioselectivity is a function of modifier concentration. At low concentration (S)-ethyl lactate is preferentially formed while at high concentration (K)-ethyl lactate is the preferred enantiomer with an enantiomeric swing of 40% across the modifier range tested. The opposite trend with a similar magnitude was observed with hydroquinine-4-chlorobenzoate. The effect is sensitive to the way in which the Pt/y-Al203 catalyst is prepared with reduction temperature determining if inversion of enantioselectivity was observed. The same experiments conducted over platinum silica and platinum graphite catalysts failed to demonstrate the same modifier concentration dependence on the sense of enantioselectivity.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Enantioselective hydrogenation of pyruvate esters over cinchona alkaloid modified platinum catalysts

The enantioselective hydrogenation of pyravate esters has been investigated over cinchona alkaloid modified platinum catalysts. Three topics have been examined: (i) A1MCM-41 as a novel support for the platinum catalysed enantioselective hydrogenation of ethyl pyruvate. (ii) The enantioselective hydrogenation of pyruvate esters over cinchonidine pre-modified PtAy-AbQa catalysts in a continuous flow reactor. (iii) An investigation into the modifier concentration dependence on the sense of enantioselectivity in ethyl pyruvate hydrogenation exhibited by derivatives of hydroquinidine and hydroquinine. A series of Pt-A1MCM-41 catalysts were synthesised using both CI 6- and CI 8-alkyl chain surfactant templates with platinum loadings approaching 2 wt/%. High resolution transmission electron microscopy (HRTEM) data confirmed retention of the mesoporous structure in the active catalysts and gave information on the platinum particle size and their location within the mesopore. High resolution magic angle spinning (HRMAS) 27 Al NMR was used to study the movement of aluminium within the mesoporous structure during the different preparation stages. The resultant platinum catalysts afforded enantioselectivities comparable to that obtained with the standard reference catalyst EUROPT-1 (6.3% Pt/silica), at rates moderated by an order of magnitude by mass transfer limitations. This is the first report of an enantioselective hydrogenation occurring in a mesopore. The hydrogenation of pyruvate esters over cinchonidine pre-modified catalysts in a continuous flow reactor operating at ambient temperature and pressure (0.25 bar/g) has been investigated. With dichloromethane as a solvent sustained enantiomeric excess of > 70% was maintained at very low cinchonidine/platinum ratios and are comparable to the enantioselectivity achieved when the reaction is conducted in the same solvent at autogeneous pressures (50 bar). When dichloromethane is used as a reaction solvent conversion is seen to decrease with time on line and is considered to be the result of polymer formation on the catalyst surface. The pre-treatment of the catalyst bed with (R)-ethyl lactate prior to the introduction of methyl pyruvate is detrimental to the catalytic system with an 18 % drop in enantioselectivity, potentially the result of an unfavourable interaction between lactate and modifier. In the enantioselective hydrogenation of ethyl pyruvate over hydroquinidine-4-chlorobenzoate modified Pt/v-AbQ} the sense of enantioselectivity is a function of modifier concentration. At low concentration (S)-ethyl lactate is preferentially formed while at high concentration (K)-ethyl lactate is the preferred enantiomer with an enantiomeric swing of 40% across the modifier range tested. The opposite trend with a similar magnitude was observed with hydroquinine-4-chlorobenzoate. The effect is sensitive to the way in which the Pt/y-Al203 catalyst is prepared with reduction temperature determining if inversion of enantioselectivity was observed. The same experiments conducted over platinum silica and platinum graphite catalysts failed to demonstrate the same modifier concentration dependence on the sense of enantioselectivity.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Enantioselective hydrogenation of pyruvate esters over cinchona alkaloid modified platinum catalysts

The enantioselective hydrogenation of pyravate esters has been investigated over cinchona alkaloid modified platinum catalysts. Three topics have been examined: (i) A1MCM-41 as a novel support for the platinum catalysed enantioselective hydrogenation of ethyl pyruvate. (ii) The enantioselective hydrogenation of pyruvate esters over cinchonidine pre-modified PtAy-AbQa catalysts in a continuous flow reactor. (iii) An investigation into the modifier concentration dependence on the sense of enantioselectivity in ethyl pyruvate hydrogenation exhibited by derivatives of hydroquinidine and hydroquinine. A series of Pt-A1MCM-41 catalysts were synthesised using both CI 6- and CI 8-alkyl chain surfactant templates with platinum loadings approaching 2 wt/%. High resolution transmission electron microscopy (HRTEM) data confirmed retention of the mesoporous structure in the active catalysts and gave information on the platinum particle size and their location within the mesopore. High resolution magic angle spinning (HRMAS) 27 Al NMR was used to study the movement of aluminium within the mesoporous structure during the different preparation stages. The resultant platinum catalysts afforded enantioselectivities comparable to that obtained with the standard reference catalyst EUROPT-1 (6.3% Pt/silica), at rates moderated by an order of magnitude by mass transfer limitations. This is the first report of an enantioselective hydrogenation occurring in a mesopore. The hydrogenation of pyruvate esters over cinchonidine pre-modified catalysts in a continuous flow reactor operating at ambient temperature and pressure (0.25 bar/g) has been investigated. With dichloromethane as a solvent sustained enantiomeric excess of > 70% was maintained at very low cinchonidine/platinum ratios and are comparable to the enantioselectivity achieved when the reaction is conducted in the same solvent at autogeneous pressures (50 bar). When dichloromethane is used as a reaction solvent conversion is seen to decrease with time on line and is considered to be the result of polymer formation on the catalyst surface. The pre-treatment of the catalyst bed with (R)-ethyl lactate prior to the introduction of methyl pyruvate is detrimental to the catalytic system with an 18 % drop in enantioselectivity, potentially the result of an unfavourable interaction between lactate and modifier. In the enantioselective hydrogenation of ethyl pyruvate over hydroquinidine-4-chlorobenzoate modified Pt/v-AbQ} the sense of enantioselectivity is a function of modifier concentration. At low concentration (S)-ethyl lactate is preferentially formed while at high concentration (K)-ethyl lactate is the preferred enantiomer with an enantiomeric swing of 40% across the modifier range tested. The opposite trend with a similar magnitude was observed with hydroquinine-4-chlorobenzoate. The effect is sensitive to the way in which the Pt/y-Al203 catalyst is prepared with reduction temperature determining if inversion of enantioselectivity was observed. The same experiments conducted over platinum silica and platinum graphite catalysts failed to demonstrate the same modifier concentration dependence on the sense of enantioselectivity

Using real particulate matter to evaluate combustion catalysts for direct regeneration of diesel soot filters

The particulate produced by internal combustion engines has a complex composition that includes a large proportion of porous soot within which condensed and adsorbed organic molecules are trapped. However, many studies of the catalytic combustion of particulate are based on the assumption that commercially produced carbon can be used as a reliable mimic of engine soot. Here we show that soot removed from a diesel particulate filter is rich in the polyaromatic molecules that are the precursors of the solid particulate. Through a combination of solvent extraction and evolved gas analysis, we have been able to track the release and transformation of these molecules in the absence and presence of combustion catalysts. Our results reveal that, although the rate of combustion of the elemental carbon in diesel soot is higher than that of graphite, deep oxidation of the polyaromatic molecules is a more demanding reaction. An active and stable Ag–K catalyst lowers the combustion temperature for elemental carbon by >200 °C, but has little effect on the condensed polyaromatic molecules. The addition of a secondary catalyst component, with aromatic-oxidation functionality is required to target these molecules. Although the combined catalyst would not enable a completely passive regeneration system for diesel passenger cars, it would improve the efficiency of existing active systems by reducing the amount of fuel-injection required for trap regeneration

A simple continuous flow microwave reactor

A new simple procedure for microwave-assisted organic synthesis under continuous flow processing has been developed for use in a monomodal microwave synthesizer with direct temperature control using the instrument's in-built IR sensor. This design makes optimum use of the standing wave cavity to improve the energy efficiency of microwave-assisted flow reactions

Reprint of: Oxidative dehydrogenation of cyclohexane and cyclohexene over supported gold, -palladium catalysts

Supported gold, palladium and gold–palladium catalysts have been used to oxidatively dehydrogenate cyclohexane and cyclohexenes to their aromatic counterpart. The supported metal nanoparticles decreased the activation temperature of the dehydrogenation reaction. We found that the order of reactivity was Pd ≥ Au–Pd > Au supported on TiO2. Attempts were made to lower the reaction temperature whilst retaining high selectivity. The space-time yield of benzene from cyclohexane at 473 K was determined to be 53.7 mol/kgcat/h rising to 87.3 mol/kgcat/h at 673 K for the Pd catalyst. Increasing the temperature in this case improved conversion at a detriment to the benzene selectivity. Oxidative dehydrogenation of cyclohexene over AuPd/TiO2 or Pd/TiO2 catalysts was found to be very effective (conversion >99% at 423 K). These results indicate that the first step in the reaction sequence of cyclohexane to cyclohexene is the slowest step. These initial results suggest that in a fixed-bed reactor the oxidative dehydrogenation in the presence of oxygen, palladium and gold–palladium catalysts are readily able to surpass current literature examples and with further modification should yield even higher performance

Epoxidation of propene with graphite AuPd-supported nanoparticles

The oxidation of propene has been studied with O2 as the terminal oxidant using a carbon-supported gold palladium catalyst at 90 °C. The reaction is carried out in acetonitrile as solvent in an autoclave. Propene oxidation is only observed in the presence of benzoyl peroxide acting as a radical initiator, but the presence of O2 is essential to observe oxidation. The addition of aldehydes enhances the formation of the epoxide

Solvent-free oxidation of dec-1-ene using gold/graphite catalyst using an in situ generated oxidant

The oxidation of dec-1-ene is investigated under solvent-free conditions using gold nanoparticles supported on graphite and in a batch reactor in the presence of a radical initiator using oxygen from air as the terminal oxidant. The evolution of the products with reaction time shows that there is an initial induction period and during this time very little epoxide is fomed and the products of allylic oxidation are dominant. Subsequently the epoxide becomes the major product prior to the diol being formed from hydrolysis due to the presence of by-product water formed from the selective oxidation reaction. It is considered that the allylic oxidation products are in part converted in situ into aldehydes which form peracids during the induction period; the peracid leads to epoxide formation as the major product as the conversion is increased. The effect of addition of a number of aldehydes is investigated, all leading to enhanced epoxide formation when added in small amounts. Molar enhancements of epoxide yield can approach twice the amount of aldehyde initially added. This behaviour is in contrast to earlier studies which utilise aldehydes in greater than stoichiometric amounts as sacrificial reactants. The importance of in situ aldehyde formation is also demonstrated by the addition of benzyl alcohol which under the reaction conditions rapidly gives benzaldehyde and enhanced epoxide formation. Possible mechanistic interpretations of the observations are discussed