Search

The enantioselective hydrogenation of ethyl benzoylformate over (−)-cinchonidine (CD)-modified Pt/Al2O3 catalyst in semi-batch and continuous fixed bed reactors was studied as a function of the modifier concentration and reaction temperature. The kinetic results from the semibatch reactor showed a higher enantioselectivity and lower initial rate as the amount of modifier was increased. The results from the fixed bed reactor demonstrates that continuous enantioselective hydrogenation is possible and that continuous feeding of (−)-CD is needed to maintain a high steady-state enantioselectivity.

A kinetic model was developed for the enantioselective hydrogenation of ethyl benzoylformate (EBF) on a modified Pt/Al2O3 catalyst. This model was based on the assumption of different numbers of si...

The enantioselective hydrogenation of ethyl benzoylformate (EBF) to (R)-ethyl mandelate over (−)-cinchonidine (CD)-modified Pt/Al2O3 catalyst in toluene in a semi-batch reactor was studied as a function of modifier concentration, reaction temperature, stirring rate and catalyst particle size. The kinetic results showed higher enantioselectivity and lower initial rates tending asymptotically to a constant value as the amount of modifier increased. The maximum enantiomeric excess (ee) and the initial hydrogenation rate or turnover frequency obtained under optimized conditions with an initial concentration of 5.6 mmol L−1 EBF at 25 °C over 5% (w/w) Pt/Al2O3 were 85% (modifier-to-surface Pt of 1.28) and 109 h−1 (modifier-to-surface Pt of 0.16), respectively. Additionally, catalyst deactivation due to the presence of impurities in the feed was prominent in some cases; therefore activated carbon was used as a cleaning agent of the raw material to remove impurities prior to catalyst addition.

Catalyst deactivation is traditionally described with semi-empirical models involving separable kinetics and time-dependent activity factors, but the rational approach is to treat catalyst deactivation as a chemisorption process among other surface processes. The catalytic process A → R along with surface poisoning P + ⁎ → P ⁎ (⁎ is an active site) was modelled for batch and continuous reactors. Simple semi-analytical mathematical solutions were obtained, which avoid the use of surface coverage in computations. Numerical simulations revealed the behaviour of the model in the parameter space. For batch reactors, the approach to a limit conversion as well as the increase of the apparent reaction order due to catalyst poisoning was illustrated. The model was applied to batchwise hydrogenation of sitosterol to sitostanol on a dispersed platinum catalyst and in the presence of a sulphurous poison. The behaviour of the system was successfully described with a simple, mechanistic model based on the adsorption-reaction-poisoning concept.

Solvent effects in the enantioselective hydrogenation of ethyl benzoylformate (EBF) to (R)-ethyl mandelate over (−)-cinchonidine (CD)-modified Pt/Al2O3 catalyst were studied in a semi-batch reactor. Solvents of different nature were used: protic (2-propanol, 1-propanol, 1-octanol and ethanol), aprotic polar (methyl acetate, acetone, ethyl acetate and tetrahydrofuran) and apolar solvents (methyl cyclohexane and toluene). The effects of pure solvents and binary solvent mixtures on hydrogenation rates and enantioselectivity were investigated. The highest enantiomeric excess (ee) of 72 % was obtained in ethyl acetate, decreasing nonlinearly with increasing dielectric constant (e) being close to 20 % in ethanol. The highest value of the initial hydrogenation rate was obtained in the apolar solvents (21 mmol dm−3 min−1g cat −1 ) while the lowest one was observed in tetrahydrofuran (2 mmol dm−3 min−1 g cat −1 ). A kinetic model was proposed for the enantioselectivity dependence on dielectric constant based on Kirkwood treatment. The non-linear dependence of ee on (e) was included in the model to describe quantitatively the variation of ee in different solvents. The results showed a good fit for ee as a function of e.

Kinetics of the hydrogenation of β-sitosterol to β-sitostanol is of industrial interest, since the desired product is used for suppressing cholesterol absorption in human body. The main drawback when using microporous Pd/C catalyst in this reaction is catalyst deactivation. In the current work the performance of microporous and mesoporous Pd catalysts in the hydrogenation of β-sitosterol was compared. The catalytic hydrogenations were performed in a shaking batch reactor in 1-propanol as a solvent. With larger amounts of catalyst less catalyst deactivation occurred due to the fact that the catalyst support acted also as an adsorbent. The mesoporous 4 wt.% Pd/C (Sibunit) catalyst showed higher sitosterol conversions and less catalyst deactivation compared to a microporous 5 wt.% Pd/C catalyst. The kinetics of the hydrogenation of β-sitosterol to β-sitostanol was studied over 4 wt.% Pd/C (Sibunit) catalyst at different temperatures between 60 °C and 80 °C and by reusing the catalyst. The origin for catalyst deactivation was poisoning by phosphorus and sulphur, as well as coking. In situ catalyst potential measurements showed that there is a correlation between catalyst deactivation and decreasing catalyst potential with increasing sitosterol conversion. A mechanistic kinetic model including a deactivation factor was successfully applied to this reaction and the kinetic parameters were determined.