Conversion of CCl2F2 (CFC-12) in the presence and absence of H2 on sol–gel derived Pd/Al2O3 catalysts

Conversion of CCl₂F₂ in the presence (hydrogenolysis) and absence of hydrogen was investigated on Al₂O₃, AlF₃ and Pd/Al₂O₃ xerogel and aerogel catalysts. CCl₂F₂ was found to form CClF₃ and CCl₃F on Al₂O₃ and AlF₃ in the presence and absence of hydrogen as well as on the Pd/Al₂O₃ catalysts in the absence of hydrogen. Overall activity increased during the hydrogenolysis reactions at 230°C as a function of time which was paralleled by a significant increase in the yield of CClF₃ formed through a Cl/F-exchange reaction. X-ray diffraction patterns of the spent catalyst recovered after 3 h of hydrogenolysis confirmed the presence of Pd(C) (Pd–carbon solid solution) and AlF₃ phases on Pd/Al₂O₃ catalysts indicated that the carbon incorporation into the Pd lattice and the transformation of Al₂O₃ to AlF₃ starts at the initial stage of the reaction. It was concluded that AlF₃ is responsible for the Cl/F-exchange reactions. CH₄, a complete hydrogenation product, is formed during hydrogenolysis. Another route for its formation is the reaction between hydrogen in the gas phase and the interstitial carbon.     

Combustion of methane over palladium/zirconia: effect of Pd-particle size and role of lattice oxygen

The catalytic combustion of methane over palladium/zirconia catalyst prepared by oxidation and subsequent reduction of Pd/Zr glassy metals has been investigated with special emphasis on the influence of the catalyst structure (particle size and specific surface area) on the catalytic performance and the impact of a redox mechanism on the product formation. The reaction of 1% methane and 4% oxygen (balance of He) in a fixed-bed microreactor was found to be strongly dependent on the particle size of palladium, which was controlled by appropriate reduction before catalytic testing. Pre-reduction of PdO resulted in an enhanced activity compared to the unreduced catalysts. The structural changes induced upon reduction were accompanied by altered physico-chemical properties, evidenced by different behaviors in the decomposition of PdO and the reduction of PdO by methane. The correlation established between catalytic performance and the rate of reduction of PdO by methane led to the postulation of a redox mechanism, involving the reaction of methane with the active palladium oxide phase and subsequent reoxidation of metallic Pd by oxygen, which was independently confirmed by the use of ¹⁸O labeled catalysts and pulse methods.     

Selectivity enhancement in heterogeneous catalysis induced by reaction modifiers

The application range of solid catalysts can be greatly extended by reaction or process modifiers, that is by simple addition of an inorganic or organic compound to the reaction mixture. The modifier, used in catalytic amounts, ideally interacts strongly with the active sites in a fashion which induces favorable changes in the outcome of the reaction. Evolution of the actual modified metal catalyst during reaction and the importance of in situ characterization in understanding these processes are illustrated using the examples of promotion by metal ions and nitrogen-containing bases. The major part of the review describes the advantages and limitations of employing N-base modifiers for tuning the performance of solid catalysts. Reactions discussed include chemo-, stereo-, enantio- and diastereoselective hydrogenations over metal catalysts, aerobic oxidation of alcohols with Pt and Pd, and epoxidation of allylic alcohols with titania–silica mixed oxides and alkyl hydroperoxides.     

Titania–silica epoxidation catalysts modified by acetoxypropyl groups

Mesoporous titania–silica aerogels with TiO₂SiO₂ = 1:9 weight ratio and varying amounts of covalently bound 3‐acetoxypropyl groups were prepared from 3‐acetoxypropyltrimethoxysilane, tetramethoxysilane and titanbisacetylacetonatdiisopropoxide. The textural properties of the hybrid aerogels were strongly influenced by the concentration of acetoxypropyl groups. The BET surface area and specific pore volume were significantly lowered upon introduction of the polar organic modification. Despite these less favorable textural properties, all modified aerogels were remarkably more active epoxidation catalysts, and in the epoxidation of cyclohexenol also more selective, than the unmodified titania–silica aerogel. For example, incorporation of only 2 mol% acetoxypropyl groups lowered the BET surface area from 813 to 339 m² g^-1, whereas the selectivity to 2,3‐epoxy‐cyclohexenol increased from 76 to 90%, and the reaction time (required to achieve 70% TBHP conversion) decreased from 46 to 5 min. The higher epoxide selectivity and the small enhancement in cis/trans ratio in the epoxide may indicate an electron pair donor interaction between the ester carbonyl group and the Ti active site. This revised version was published online in July 2006 with corrections to the Cover Date.     

Palladium-catalysed enantioselective hydrogenation of alkenoic acids. Role of isomerization

The enantioselective hydrogenation of 2-ethyl-propenoic acid over Pd/alumina modified with cinchonidine has been studied. The reaction, carried out in a batch reactor at 1 bar hydrogen pressure and room temperature, revealed that the isomerization of the C=C double bond is a competing side reaction. Double-bond migration and the subsequent hydrogenation of the two isomer alkenoic acids lowered the enantioselectivity drastically due to the formation of opposite enantiomers.     

Catalytic Synthesis of Higher Aliphatic Amines from the Corresponding Alcohols

Abstract

Aliphatic amines are of considerable industrial importance and find application in almost every field of modern technology, agriculture, and medicine [1], Lower aliphatic amines (C₁ to C₆) are important intermediates for the chemical and pharmaceutical industries. A large number of drugs, herbicides, pesticides, dyes, and other chemicals contain amino pups which originate from reactions with such intermediates, Many important applications of higher aliphatic amines (fatty amines) and their derivatives (most important derivatives are quarternary ammonium compounds) are based on their cationic surface activity. Relatively small amounts of such compounds are usually required to achieve the desired changes in surface and colloidal properties. Thus, not surprising, one of the first applications of fatty amines was in the flotation separation of nonmetallic materials such as potash, feldspar, phosphate, and mica. Today, probably the biggest demand for fatty amines lies in the production of fabric softeners. There are other important applications for aliphatic amines in the plastics and protective coat industries as emulsion stabilizers, mold release agents, pigment dispersers, and flushing agents. They are used as catalysts for polyurethane production. For granular products, alkylamines are used as anticaking and antidusting agents. In the rubber industry they are used as oxidation inhibitors and catalysts for accelerating vulcanization. Aliphatic amines find also many applications in the petroleum industry, especially as corrosion inhibitors and as components of lubricating oils, greases, and fuel oil where they act as sludge dispersants and stabilizers. They are added to gasoline as corrosion inhibitors.     

Palladium catalyzed synthesis of methylamines from carbon dioxide, hydrogen and ammonia

The synthesis of methylamines from carbon dioxide, hydrogen, and ammonia has been studied over two palladium-alumina catalysts with palladium loadings of 2.8 and 7.6 wt%, respectively. Catalytic tests were performed in a fixed-bed microreactor at 0.6 MPa total pressure in the temperature range 473--573 K. The catalysts showed high activity for methylamine formation at low temperatures and produced monomethylamine with selectivities higher than 80%, besides of smaller amounts of di- and trimethylamine. Other carbon containing products observed were carbon monoxide, formed by the reverse water-gas shift reaction, and methane, which was the prevailing product above 573 K. In situ diffuse reflectance FTIR studies were performed to identify the species present on the catalyst surface under reaction conditions. Ammonia and methylamines are adsorbed on Brønsted and Lewis acid sites on the catalyst. Evidence is given that surface formate and isocyanate species are present on palladium-alumina under reaction conditions.     

Enantioselective hydrogenation of carbonyl compounds over Pd-Pt/alumina catalysts

A series of Pd-Pt/alumina catalysts were prepared by consecutive deposition of Pd onto supported Pt particles. Electrochemical model studies indicated that under very mild conditions in aqueous acetic acid the reduction of Pd²⁺ ions occurred partly via the ionization of hydrogen adsorbed on Pt and partly by the slow oxidation of acetic acid. There was only a moderate change in the surface Pd/Pt atomic ratio during heat treatment in flowing hydrogen at 400°C, as determined by XPS analysis. The catalytic performance of the bimetallic catalysts was tested in the enantioselective hydrogenation of ethyl pyruvate and ketopantolactone, in the presence of cinchonidine. Pd was virtually inactive and acted as a site blocker, which decreased the size of Pt ensembles and hindered the formation of the bulky transition complex between the reactant and the chiral modifier. The intrinsically low activity and selectivity of Pd are discussed in the light of H/D exchange studies in deuterated ethanol. This revised version was published online in July 2006 with corrections to the Cover Date.     

Space-Resolved Profiling Relevant in Heterogeneous Catalysis

Knowledge of gradients involved in chemical processes, such as heterogeneously catalyzed reactions, on molecular as well as reactor scale is of paramount importance for understanding and optimizing such processes. This review highlights and discusses recent advances in spatially resolved methods for the detection of chemical (structural) and temperature gradients, with particular focus on in situ methods.