Formylation with “Supercritical” CO2: Efficient Ruthenium-Catalyzed Synthesis of N-Formylmorpholine

Formylation of morpholine with supercritical CO₂ using the bidentate ruthenium catalyst RuCl₂(dppe)₂ affords high N-formylmorpholine production rate at almost 100% selectivity. The solventless reaction could be an interesting alternative to the present N-formylmorpholine synthesis routes, which are based on environmentally harmful formylation agents. Video monitoring of the reaction mixture during reaction revealed a complex phase behavior, including the formation of solid carbamate, which was most prominent at low conversion and gradually disappeared with higher conversion, i.e., with the formation of N-formylmorpholine and water as a by-product. Addition of water as an additive suppressed solid carbamate formation and enhanced the reaction rate. Infrared spectroscopy was applied to follow the changes in the liquid and dense gas phase. The ruthenium catalyst was confined to the liquid phase, containing morpholine, N-formylmorpholine, water, dissolved carbon dioxide and hydrogen. Although the solubility of the ruthenium catalyst in the liquid phase decreased with increasing conversion, good performance in the synthesis of N-formylmorpholine could be achieved.     

Determination of intraparticle diffusion coefficients in catalyst pellets—a comparative study of measuring methods

Effective intraparticle diffusion coefficients in different industrial catalysts were determined with three different measuring methods. On the one hand by steady state counterdiffusion measurements and dynamic pulse response measurements on single pellets in a diffusion cell, on the other hand by the gas chromatographic method employing the “single pellet string reactor” (S.P.S.R.). In general the steady state method and the dynamic pulse response measurements on single pellets yielded similar values for the effective diffusion coefficients. Within limits, the S.P.S.R. gave comparable diffusion coefficients, however, a statistical consideration indicates that with this method experimental uncertainties may lead to significantly broader confidence limits for the diffusion coefficients than with measurements on single pellets in a diffusion cell. Different pore models are employed for the prediction of the intraparticle diffusivities in the commercial catalyst pellets. The diffusivities predicted with the random pore model are about 1.5 times larger than the experimentally determined values. With measurements carried out on single pressed pellets in the diffusion cell no axial anisotropy effects caused by pellet pressing could be observed.     

Highly Efficient Platinum-Catalyzed Enantioselective Hydrogenation of Trifluoroacetoacetates in Acidic Solvents

The hydrogenation of methyl-, ethyl- and isopropyl-4,4,4-trifluoroacetoacetates to the corresponding chiral trifluoromethyl alcohols has been investigated over Pt/A1₂O₃ modified by O-methyl-cinchonidine. Up to 96% ee and 1850 h^-1 average TOF have been achieved in the synthesis of this important chiral building block. The special role of reaction medium and particularly the impact of water and acids on enantiodiscrimination are discussed.     

Catalyst potential measurement: a valuable tool for understanding and controlling liquid phase redox reactions

The experimental technique and possible applications of the steady state catalyst potential measurement during Pt metal catalyzed hydrogenation and oxidation reactions in slurry reactors has been reviewed. The value of this in situ method is illustrated by many examples, including the separation of consecutive reaction steps, determination of the oxidation state of platinum metal catalyst and promoter during reaction, elucidation of the role of surface impurities, and controlling the rate of alcohol oxidation by optimizing the surface concentration of the oxidizing species. This revised version was published online in August 2006 with corrections to the Cover Date.     

Selective oxidation of o-xylene to phthalic anhydride over vanadia/titania: A comparative study of integral and recycle reactor operation

Two vanadia/titania catalysts, containing 1 and 8 wt% vanadia on anatase, have been investigated for the selective oxidation of o-xylene to phthalic anhydride. In a comparative study, activity and selectivity in o-xylene oxidation were measured using an integral reactor and a reactor with external product recirculation, with the objective of studying the influence of backmixing in the latter on the reaction behaviour. Adsorbed surface species were investigated by means of in-situ diffuse reflectance FT-IR spectroscopy, using an environmental chamber which corresponds to an integral reactor. No difference in activity and selectivity in o-xylene oxidation was observed between the two catalysts. FT-IR studies showed adsorbed o-xylene, benzoate and a high concentration of phthalic anhydride on the surface of both catalysts. o-Xylene oxidation follows a sequential reaction path, with o-tolualdehyde as the first intermediate and phthalide, phthalic anhydride and maleic anhydride as sequential products. No evidence was found for a direct oxidation-path of o-xylene to phthalic anhydride. Total oxidation products CO and CO₂ are formed by direct oxidation of the o-xylene and by oxidation of the reaction products. Due to greatly reduced concentration and temperature gradients, the recycle reactor proved to be highly suitable for studying the extremely exothermic oxidation of o-xylene.     

Chiral nitrogen compounds as new modifiers for the enantioselective hydrogenation of ethyl pyruvate

The enantioselective hydrogenation of ethyl pyruvate to (R)- or (S)-ethyl lactate has been studied over alumina- and carbon-supported Pt-metal catalysts modified by various heterocyclic N-compounds and substituted amides. The reactions were carried out under mild conditions in acetic acid; other solvents had a detrimental effect on enantioselectivity. An enantiomeric excess (ee) of 67% and a rate acceleration by a factor of 6, compared to the unmodified catalyst, was observed with alumina-supported Pt modified by (R)-1-(1-naphthyl)ethylamine. In contrast, carbon-supported Pd, Ru and Rh were non-selective and only little active. The studies indicated that besides naphthyl or quinolyl groups, two separate phenyl groups or one phenyl group together with two amino groups can provide a suitable anchoring of the chiral modifier on the Pt surface. The nature of interaction between the modifiers and ethyl pyruvate is briefly discussed.     

Influence of the A-site cation in AMnO3+x and AFeO3+x (A = La,Pr, Nd and Gd) perovskite-type oxides on the catalytic activity for methane combustion

The effect of rare-earth ions (La, Pr, Nd and Gd) in AMnO_3+x and AFeO_3+x perovskites on the thermal behavior and on the catalytic activity for the deep oxidation of methane has been studied. AMnO_3+x perovskites showed after preparation an oxidative non-stoichiometry. Oxygen desorption analysis revealed for the four manganites different desorption steps occurring between 930 and 1370 K. Stoichiometry was reached after the first desorption step. Heating the samples at temperatures above 1300 K resulted in phase segregation to the simple oxides. AFeO_3+x perovskites were more stable towards thermal decomposition than the Mn-perovskites, showing no oxygen evolution up to 1400 K. The reducibility of these perovskites in hydrogen correlated inversely with the relative effective ionic radii of the trivalent rare-earth cations. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600–1200 K. The activities at 770 K, expressed as reaction rates referred to the BET surface area, varied between 1.4 × 10^–7 and 2.9 × 10^–7 mol s^–1 m^–7 for the AMnO_3+x, and between 1.1 × 10^–7 and 1.6 × 10^–7 mols^–1m^–2for the AFeO_3+x perovskites.     

Enantioselective Hydrogenation of a Methoxypyrone with Cinchona-Modified Palladium

Hydrogenation of 4-methoxy-6-methyl-2-pyrone 1 has been investigated over cinchona-modified Pd/TiO₂. The appropriate start-up procedure including a catalyst reduction—oxidation—reduction cycle and short pretreatment with the modifier in the absence of reactant can remarkably enhance the enantiomeric excess (ee) to the dihydropyrone 2. Another key parameter is the alkaloid/Pd_s molar ratio; the alkaloid concentration in the slurry or the alkaloid/reactant ratio is not crucial. Under the best conditions 94% ee and 95% chemoselectivity to 2 were achieved at 80% conversion of 1, in only 30 min reaction time under ambient conditions. The ee can be further increased by kinetic resolution of 2. In the second reaction step the diasteroselectivity to the cis-tetrahydropyrone 3 is about 99%.