The formation of ketones and aldehydes from carboxylic acids, structure-activity relationship for two competitive reactions

Four carboxylic acids with a number of a-hydrogen atoms ranging from three to zero were tested in the selective hydrogenation to aldehyde. The acids used were acetic, propanoic, isobutyric, and pivalic acid. The oxides of iron, vanadium, zirconium, and titanium were used as catalysts. It was found that by decreasing the number of a-hydrogen atoms the selectivity to the aldehyde increased, while the formation of the main by-product, ketone, was suppressed. It is suggested that this is due to the fact that the ketonisation proceeds via a ketene-like intermediate, the formation of which needs the presence of a-hydrogen. Furthermore, the reactions to aldehyde and ketone seem to be in competition with each other

Reactions of carboxylic acids on oxides:1. selective hydrogenation of acetic acid to acetaldehyde R. Pestman,

Acetic acid has been used as a model compound in the selective\u3cbr/\u3ehydrogenation of aliphatic acids, which contain ®-hydrogen atoms,\u3cbr/\u3eto their corresponding aldehydes. In contrast to what the literature\u3cbr/\u3epredicts, it appeared to be possible to produce acetaldehyde directly\u3cbr/\u3efrom acetic acid. The appropriate catalyst consists of an oxide with\u3cbr/\u3ean intermediate metal–oxygen bond strength. Addition of platinum\u3cbr/\u3eto the catalyst enhances selectivity and activity. A mechanism is\u3cbr/\u3eproposed, based on the involvement of lattice oxygen (viz., a Mars\u3cbr/\u3eand Van Krevelen mechanism) and the spill-over of activated hydrogen\u3cbr/\u3efrom the platinum to the oxide. The most important side\u3cbr/\u3ereaction is the formation of acetone from two molecules of acetic\u3cbr/\u3eacid (ketonization), but this reaction is suppressed completely by\u3cbr/\u3ethe addition of platinum to the catalyst

Reactions of carboxylic acids on oxides:2. bimolecular reaction of aliphatic acids to ketones

The reaction of aliphatic carboxylic acids over oxidic catalysts\u3cbr/\u3ehas been studied. Ketones are the main product in this reaction. Up\u3cbr/\u3eto now there has been no agreement in the literature concerning the\u3cbr/\u3emechanism of this ketonization reaction. In the case of acetic acid,\u3cbr/\u3eit appears that the ketone can be formed via two different routes.\u3cbr/\u3eOn oxides with a low lattice energy, bulk acetates are formed, decomposition of which leads to acetone. On oxides with a high lattice energy, the reaction to acetone takes place on the surface and leaves the bulk structure of the catalyst unaltered. The surface reaction to ketones probably proceeds via an intermediate that is oriented parallel to the surface and that has chemical interactions with the catalyst via both the carboxyl group and the α-carbon of the alkyl group. For the latter interaction abstraction of an α-hydrogen atom is required. The alkyl group of this intermediate can react with a neighboring carboxylate to give the ketone. The remaining carboxyl group forms CO2. The intermediate is very likely to be in pseudoequilibrium with the corresponding ketene

Hydrogen permeation through palladium membranes and inhibition by carbon monoxide, carbon dioxide, and steam

\u3cp\u3ePalladium membranes are being developed for the separation of hydrogen from syngas in industrial applications. However, syngas constituents carbon monoxide, carbon dioxide, and steam are known to adsorb at the membrane surface and inhibit the permeation of hydrogen. The current study combines an experimental study and modelling approach in order to investigate and quantify the inhibition effects. Experiments have been performed with a 2.8. μm thick palladium membrane (surface area 174cm2) on a tubular alumina support, including systematic variation of the concentrations of carbon monoxide, carbon dioxide, and steam at 22. bar total pressure and 350-450. °C. Carbon monoxide and steam inhibit hydrogen permeation. No significant effect has been found for carbon dioxide, except indirectly by carbon monoxide produced in situ from carbon dioxide. A constriction resistance model has been derived, explicitly relating the decrease in surface coverage by adsorbed hydrogen to the ensuing decrease in transmembrane flux. Very high surface coverages by inhibiting species θi>0.995 are predicted. The results highlight that inhibition effects are greatly reduced at high hydrogen partial pressures due to competitive adsorption. Due to the lateral diffusion of permeating hydrogen atoms in the metallic membrane, the thickness of the palladium membrane strongly determines the extent to which surface coverage by non-hydrogen species causes a decrease in hydrogen transmembrane flux. Depending on the operating conditions, membranes are predicted to have an optimal minimum thickness below which an increased intrinsic permeance is offset by an increased impact of inhibition.\u3c/p\u3

Gold stabilized by nanostructured ceria supports : nature of the active sites and catalytic performance

The interaction of gold atoms with CeO2 nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition–precipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2–6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O–H bond cleavage appears to be rate limiting, while this shifts to C–H bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria