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Dummer, Nicholas
Languages: English
Types: Doctoral thesis
Subjects: QD
The enantioselective hydrogenation of 2,3-butanedione has been studied in the liquid and gas phase over cinchonidine modified Pt/silica catalysts. Reactions were conducted in three different reactors a stirred autoclave, a fixed-bed trickle-bed reactor and a micro-flow gas phase reactor. Screening of eight powder and four granular (0.5-1.0 mm) 2.5 % Pt/silica catalysts was conducted in the autoclave to identify ideal support characteristics. The rate of 2,3- butanedione enantioselective hydrogenation was used as the means to determine these characteristics. The support characteristics used for the manufacture of extruded silica gel supports were: PV > 0.40 ml/g, PD > 50 A and a SA > 300 m2/g (< 600 m2/g). Optimum values of rate and enantiomeric excess were identified with (i) pre-reduction at 200 C for 2 h of catalyst samples, (ii) modifier concentration of 1.89 mM, (iii) toluene with 0.1 M acetic acid additive, (iv) hydrogen pressure was 30 bar and (v) stirring speed of 1000 rpm. Under these conditions the rate achieved 1104 mmol/h/gcat in DCM was comparable to the reference catalyst EUROPT-1. The granular catalysts were used in the trickle-bed reactor to identify optimum reaction conditions and support characteristics. Ultraviolet spectroscopy of cinchonidine modification protocols and cyclohexene hydrogenation were employed to facilitate optimisation. In situ and ex situ pre-modification of the bed with cinchonidine were investigated. Conversion of dione was ca. 10 % for catalysts which possessed an un- restrictive pore structure. The ee of (K)-hydroxybutanone achieved was ca. 12 %, however, was at its maximum initially after in situ pre-modification. Whereas, the enantiomeric excess tended to increase over an ex situ modified catalyst bed. Continual replacement of modifier was necessary to ensure an ee was maintained. The reaction conditions used for the study of the catalyst with extruded silica gel support were, (i) ex situ pre-modification with 16.98 mM cinchonidine in DCM, (ii) 1 ml/min of feedstock with 0.1 M reactant and 8.49 mM CD in DCM, (iii) hydrogen pressure of 0.5 barg (4800 /h), (iv) catalyst bed consisting of 2g of catalyst and SiC fines. Hydrogenation of methyl pyruvate under these conditions yielded an initial conversion of 80 % and an ee of 52%. Hydrogenation of 2,3-butanedione and methyl pyruvate over an extrudate catalyst bed under the optimised conditions achieved an ee of 12 % and 36 % respectively at low conversion. Bed pre-modification was determined to be essential to achieve an enantiomeric excess. Hydrogenation of 2,3-butanedione was attempted at the gas/solid interface over pre-modified platinum catalysts. The strength of adsorption of the dione prevented a sustained reaction. An ee of 15 % at 35 C was achieved with a saturator temperature of 0 C. The addition of cinchonidine to the catalyst proved beneficial for greater reaction times. The lower strength of adsorption pyruvate esters on platinum facilitated the identification of optimum reaction conditions. The highest values of enantiomeric excess of (ft)-lactate ester were achieved when (i) catalyst samples were pre-modified ex situ with 3.4 mM CD in DCM, (ii) reaction temperature of 40 C, (v) helium then helium/hydrogen passed over bed prior to reaction, (iii) hydrogen concentration of 25 % in helium with a total flow rate of 80 ml/min, (iv) methyl pyruvate at 20 C (0.066 g/h). Under these reaction conditions an ee of ca. 36 % CK)-lactate ester was achieved over a 2.5 % Pt/SiC>2 catalyst (0.025 g) at 100 % pyruvate conversion for 5 h. This was enhanced to ca. 43 % by pre-treatment of the bed with the ()-lactate for 100 minutes prior to the reaction. Methyl and ethyl pyruvate were used to investigate the enantioselective site on the Pt surface. The outcome indicated that the site is substrate specific after 100 minutes on-line. The enantioselective hydrogenation of 2,3-butanedione has been studied in the liquid and gas phase over cinchonidine modified Pt/silica catalysts. Reactions were conducted in three different reactors a stirred autoclave, a fixed-bed trickle-bed reactor and a micro-flow gas phase reactor. The rate of 2,3-butanedione hydrogenation in the autoclave was used to compared a number of platinum catalysts. The outcome of this screening program resulted in an ideal set of support characteristics for the manufacture of an extruded silica gel support for use in the trickle-bed reactor
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