Order In Hydrogen Pressure Research Articles

Enantioselective Hydrogenation of α-Methylcinnamic Acid Over Pd/Al2O3: A Kinetic Study of Solvent, Temperature and Pressure Effects

The enantioselective hydrogenation of α-methylcinnamic acid (MCA) over cinchonidine modified 5 wt% Pd/Al2O3 was studied in a liquid batch reactor. Both the reaction activity and enantioselectivity towards the (R) product are strongly solvent dependent. The reaction is zero order in hydrogen pressure and first order in MCA. The presence of cinchonidine does not affect the reaction order with respect to either acid or H2 pressure, but has a significant inhibiting effect on the reaction rate. The catalyst exhibits stable activity during the reaction with no sign of deactivation.

Hydrogen selective thin palladium–copper composite membranes on alumina supports

Abstract Thin and defect-free Pd–Cu composite membranes with high hydrogen permeances and selectivities were prepared by electroless plating of palladium and copper on porous alumina supports with pore sizes of 5 and 100 nm coated with intermediate layers. The intermediate layers on the 100 nm supports were prepared by the deposition of boehmite sols of different particle sizes, and provided a graded, uniform substrate for the formation of defect-free, ultra-thin palladium composite layers. The dependence of hydrogen flux on pressure difference was studied to understand the dominant mechanism of hydrogen transport through a Pd–Cu composite membrane plated on an alumina support with a pore size of 5 nm. The order in hydrogen pressure was 0.98, and indicated that bulk diffusion through the Pd–Cu layer was fast and the overall process was limited by external mass-transfer or a surface process. Scanning electron microscopy (SEM) images of the Pd–Cu composite membrane showed a uniform substrate created after depositing one intermediate layer on top of the alumina support and a dense Pd–Cu composite layer with no visible defects. Cross-sectional views of the membrane showed that the Pd–Cu composite layer had a top layer thickness of 160 nm (0.16 μm), which is much thinner than previously reported.

Hydrocarbon Conversion on Palladium Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.

Hydrocarbon conversion on palladium catalysts

The reaction pathways for acetylene trimerization and hydrogenation, and ethylene hydrogenation, catalyzed by palladium, are explored using a range of surface-sensitive techniques. Reflection–absorption infrared spectroscopy (RAIRS) and low-energy electron diffraction (LEED) show that ethylene is di-σ-bonded on clean Pd(1 1 1), but forms a π-bonded species on a hydrogen pre-covered surface, where the transformation is induced by sub-surface hydrogen. Catalytic ethylene hydrogenation proceeds on an ethylidyne-covered Pd(1 1 1) surface, and it is found that ethylene can still adsorb onto palladium in spite of the presence of an ethylidyne overlayer, and is still in a di-σ-configuration. The rates of ethylidyne formation, and removal by hydrogen are measured independently, where the latter rate is found to be first order in hydrogen pressure. The ethylidyne coverage is measured under reaction conditions as a function of P(H 2)/ P(C 2H 4) and found to decrease to ∼1/3 of saturation. Benzene is formed from acetylene on clean Pd(1 1 1) via a metallocyclic C 4H 4 intermediate. This further reacts with a third acetylene to form benzene. However, catalytic cyclotrimerization proceeds in the presence of a carbonaceous layer, which consists of vinylidene species (CH 2 C ). Thus, at high pressures, benzene is formed by reaction between acetylene adsorbed on the vinylidene-covered palladium surface and adsorbed vinylidene itself. The addition of high pressures of hydrogen to the reaction mixture forms a more open surface covered by a mixture of ethylidyne and vinylidene species, rationalizing the observed increase in the benzene formation rate with the addition of hydrogen.

Hydroisomerization and hydrocracking of long chain n-alkanes on Pt/amorphous SiO 2–Al 2O 3 catalyst

The hydroisomerization and hydrocracking of n-hexadecane, n-octacosane and n-hexatriacontane on a 0.3% platinum/amorphous silica–alumina (MSA/E) catalyst was investigated in a stirred microautoclave at 345, 360 and 380°C and between 2 and 13.1 MPa hydrogen pressure. For each n-paraffin, the reaction pathway and the kinetic parameters were determined. The results were used to elucidate the effect of chain length and operating conditions on isomerization and cracking selectivity. The conversion of the n-paraffins lead to the formation of a mixture of the respective isomers, as the main product, together with cracking products. At every temperature, the iso-alkane/ n-alkane ratio of cracking products increased considerably with increasing conversion degree. At the same conversion level, higher reaction temperatures lead to cracking products characterized by a lower iso-alkane/ n-alkane ratio. The conversion rate constants showed a considerable increase between n-C 16 and n-C 28, whereas a slight decrease between n-C 28 and n-C 36 was observed. The hydroisomerization selectivities showed a decrease as a function of chain length and with increasing conversion levels. The increase in reaction temperature leads to a small decrease in the isomerization selectivities only at low-medium conversion degrees and at the highest temperature investigated, while the effect of this parameter on the maximum yields achievable in iso-C16, iso-C28 and iso-C36 was negligible. The results indicate that the conversion of the n-paraffins follows a first-order kinetic in hydrocarbon while the order in hydrogen pressure was −1.1 ± 0.21 for n-C 16 and −0.66 ± 0.15 for n-C 28. Furthermore, an increase in hydroisomerization selectivity at higher hydrogen pressure for n-C 28 conversion was observed.

Kinetic studies on the selective hydrogenation of phenylacetylene catalyzed by [Rh(NBD)(PPh 3) 2]BF 4 (NBD=2,5-norbornadiene)

The complex [Rh(NBD)(PPh 3) 2]BF 4 catalyzes the selective hydrogenation of phenylacetylene to styrene. In dichloromethane solution at 25°C, selectivities close to 80% are achieved. The reaction is first order in catalyst and substrate, and second order in hydrogen pressure, while the addition of triphenylphosphine inhibits the reduction. The mechanism deduced for this hydrogenation, on the basis of the above mentioned kinetic results, suggests that the reduction of the diolefin of the catalyst does not occur during the hydrogenation of the alkyne.

Palladium-catalyzed hydrogenation without hydrogen: the hydrodechlorination of chlorofluorocarbons with solid state hydrogen over the palladium (111) crystal surface and its implications

The hydrogenation of chlorofluorocarbons (CFC's, 1,1- dichlorotetrafluoroethane, CCl2F-CF3) was found to be positive 1/2 order in hydrogen pressure over palladium foil while it was zero order in hydrogen pressure over palladium single crystals regardless of their orientation ((111), (100)). When hydrogen was completely removed from the gas phase, hydrogenation continued for about 104 turnovers over palladium single crystals which were pre-exposed to hydrogen. Solid state hydrogen, stored in the bulk of a palladium single crystal, was found to be responsible for the hydrodechlorination of CCl2F-CF3 in the absence of gas phase hydrogen. Lower reaction rates and increased selectivity for overhydrogenation to produce CH3-CF3 indicate that the nature of hydrogenation with solid state hydrogen in palladium is different from hydrogenation with gas phase hydrogen. The use of solid state hydrogen for various chemical processes instead of gas phase hydrogen has advantages that are enumerated.

Hydrogenolysis of adsorbed methylthiolate on the Pt(111) surface

A combination of temperature programmed reaction experiments and fluorescence yield near edge spectroscopy (FYNES) has been used to characterize hydrogen induced C-S bond activation in adsorbed methylthiolate on the Pt(111) surface. The low temperature methane peak observed at 310 K during temperature programmed decomposition of methanethiol clearly involves reaction with adsorbed hydrogen since deuterium incorporation is observed in the presence of coadsorbed deuterium. Some multiple deuteration is observed suggesting that a mixture of carbon containing species may be involved in the hydrogenolysis process. In situ FYNES above the carbon K edge has been used to characterize the kinetics of this hydrogenolysis process in more detail. Transient FYNES measurements were used to characterize changes in the total carbon concentration during the hydrogenolysis reaction. In situ temperature programmed reaction experiments performed in 0.02 Torr of hydrogen clearly indicate that hydrogenolysis begins near 290 K and all carbon containing species are removed by 360 K. In situ isothermal kinetic studies indicate that the hydrogenolysis reaction is first order in adsorbed thiolate coverage and approximately half order in hydrogen pressure. The apparent activation energy for the hydrogenolysis reaction is 18 kcal/mol in the presence of 0.02 Torr of gaseous hydrogen. This activation energy represents a lower bound for the hydrogenolysis activation energy since the apparent heat includes the heat of adsorption for hydrogen. These direct kinetic measurements of this hydrogenolysis reaction under reactive gaseous environments highlight the role that in situ methods can play in characterizing surface reaction processes.

Intermediate Compounds Formed on Metal Surfaces during Catalysis

An idea of the formation of specific compounds over the metal surfaces has been applied to the metal surfaces during catalysis. One is the formation of c(2 × 2)-N overlayer on Pd (100). Rh(100) and Pt-Rh(100) surfaces during the reaction of NO with H2, that is, these metals make no bulk nitrides and are inactive for the adsorption of N2. These c(2×2)-N overlayers undergo the hydrogénation in the presence of H2, and the high resolution electron energy loss spectrum (HREELS) shows the formation of NHx species. Intensity of the energy loss peak for the N-H vibration depends on a half order in hydrogen pressure which indicates the establishment of an equilibration of N + 1/2H2⇄NH during the hydrogenation reaction. Another example is the formation of carbide overlayer on Ni(100) and Ni(111) surfaces. It was shown that the carbide overlayer on Ni(111) surface takes the same structure as that of the p4g carbide overlayer on Ni(100) surface, which may give a persuasive explanation to the structure insensitive methanation reaction on Ni(100) and Ni(111) surfaces. Finally, more direct evidence for the formation of surface compounds is given by the scanning tunneling microscopy (STM) works on Ag(110) surface. When Ag(110) surface is exposed to O2 at room temperature, rapid growth of AgO chain in parallel to the 〈001〉 direction was observed by STM, and the AgO chains are arranged by making p(n×1) ordered structures over the Ag(110) surface. The ordered phases of AgO chains make one-dimensional phase boundaries, and it was found that the boundary of the two phases necessarily makes a fluctuated structure. The atom model for the two-dimensional arrangement of the AgO chains proves the formation of energetically degenerated sites at the boundary, which is responsible to the fluctuated boundary structures.

Influence of preparation variables on the activity and selectivity of [formula omitted] catalysts

In this work we have studied the influence of various preparation variables on the activity and selectivity of Pd catalysts supported on aluminum orthophosphate in the liquid-phase reduction of benzylideneacetone with dihydrogen. Sodium tetrachloropalladiate, tetraamminepalladium(II) chloride, palladium(II) nitrate dihydrate, and palladium(II) acetylacetonate and acetate have been used as precursor salts. The catalysts have been obtained by calcination of the impregnated salts at 598 K and subsequent reduction either in a hydrogen stream or with α-phellandrene under reflux. The interaction between surface OH groups and strongly basic ligands gives rise to variable amounts of PdO upon calcination. The thermal decomposition of sodium tetrachloropalladiate and tetraamminepalladium(II) chloride yields palladium exclusively. Under our working conditions, the process is zero order in hydrogen pressure and benzylideneacetone concentration. Active sites of different coordination number are responsible for the competitive generation of 4-phenyl-2-butanone and 4-phenyl-2-butanol.

Titration of chemisorbed oxygen by hydrogen on Ru(001)

The titration of oxygen chemisorbed on Ru(001) by H 2 at low pressures is reported. For near-saturation doses of O 2, the subsequent H 2 titration is characterized by three distinct features: first, a long induction time, then a rapid reaction and, finally, a slow reaction. Electron spectroscopies, AES and XPS, were used to follow the decay of the oxygen signal. The first two features are accounted for by a model which involves dissociative adsorption of hydrogen and blockage of dissociation sites. The third feature is accounted for in terms of an inactive form of chemisorbed oxygen. All three regions are approximately first order in hydrogen pressure. The rate in the second region, at a fixed oxygen signal, is sensitive to the initial amount of oxygen present. This is ascribed to variations in the size and degree of perfection of (1 × 2) domains. With increasing temperature, the break between the second and third regions becomes less pronounced. This is accounted for in terms of an activated (6.5 ± 1.0 kcal mole −1) conversion of inactive to active oxygen. The induction times are very sensitive to small changes in surface oxygen concentration caused by electron stimulated desorption of oxygen.

The Catalytic Hydrogenation of Xylylenediamines with Ruthenium Catalysts.

Xylylene diamines were smoothly and selectively hydrogenated into the corresponding bis(aminomethyl)cyclohexanes in the presence of newly modified ruthenium catalysts. They were prepared by heating sodium ruthenate supported on active carbon under carbon dioxide atmosphere. Commercial ruthenium catalysts were not active enough to complete the hydrogenation in contrast with their full activity for aniline. The hydrogenation reaction of m-xylylenediamine is zero order in diamine concentration and first order in hydrogen pressure. Small quantities of m-methylbenzylamine, 1-aminomethyl-3-methylcyclohexane, and m-xylene were formed as byproducts presumably from the starting diamine through hydrogenolysis. Apparent activation energy for the formaton of 1, 3-bis(aminomethyl)cyclohexane was found to be 8.6 Kcal/mol. Though m-xylylenediamine gave predominantly cis-bis(aminomethyl)cyclohexane, almost 1 to 1 mixture of cis-and trans-isomers was obtained from p-xylylenediamine.