Relative Rates Of Hydrogenation Research Articles

Encapsulation of metal clusters within MFI via interzeolite transformations and direct hydrothermal syntheses and catalytic consequences of their confinement.

The encapsulation of metal clusters (Pt, Ru, Rh) within MFI was achieved by exchanging cationic metal precursors into a parent zeolite (BEA, FAU), reducing them with H2 to form metal clusters, and transforming these zeolites into daughter structures of higher framework density (MFI) under hydrothermal conditions. These transformations required MFI seeds or organic templates for FAU parent zeolites, but not for BEA, and occurred with the retention of encapsulated clusters. Clusters uniform in size (1.3-1.7 nm) and exposing clean and accessible surfaces formed in BEA and FAU zeolites; their size remained essentially unchanged upon transformation into MFI. Encapsulation selectivities, determined from the relative hydrogenation rates of small (toluene) and large (alkyl arenes) molecules and defined as the ratio of the surface areas of all the clusters in the sample to that of external clusters, were very high (8.1-40.9) for both parent and daughter zeolites. Encapsulation into MFI via direct hydrothermal syntheses was unsuccessful because metal precursors precipitated prematurely at the pH and temperatures required for MFI synthesis. Delayed introduction of metal precursors and F(-) (instead of OH(-)) as the mineralizing agent in hydrothermal syntheses increased encapsulation selectivities, but they remained lower than those achieved via interzeolite transformations. These interconversions provide a general and robust strategy for encapsulation of metals when precursors can be introduced via exchange into a zeolite that can be transformed into target daughter zeolites with higher framework densities, whether spontaneously or by using seeds or structure-directing agents (SDA).

The effect of heat treatment on the performance and structure of carbon-supported Au–Pd catalysts for the direct synthesis of hydrogen peroxide

The direct synthesis of hydrogen peroxide using supported gold palladium catalysts prepared by incipient wetness impregnation is described and discussed. The effect of an acid pre-treatment step on the activated carbon support prior to the deposition of the metals, together with the effect of the calcination temperature, has been investigated. The acid pre-treated samples all show superior activity to those materials prepared with the omission of this acid pre-treatment stage. The calcination temperature affects both the re-usability and hydrogenation activity of the catalysts. Detailed characterisation using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy is described. The enhanced activity is associated with a higher surface concentration of palladium in the acid pre-treated samples which is principally present as Pd2+. Calcination of the catalysts at 400°C is required to achieve re-usable and stable catalysts, and this is associated with the morphology and dispersion of the metal nanoparticles. The surface ratio of Pd0/Pd2+ is found to be an important factor controlling the hydrogenation of hydrogen peroxide, and a series of controlled reduction and re-oxidation of a sample show how the Pd0/Pd2+ surface ratio can influence the relative rates of hydrogen peroxide synthesis and hydrogenation.

ChemInform Abstract: Homogeneous Catalytic Hydrogenation of Aromatic Hydrocarbons and Heteroaromatic Nitrogen Compounds: Synthetic and Mechanistic Aspects

The synthetic and mechanistic aspects of the homogeneous catalytic hydrogenation of mono and polynuclear aromatic hydrocarbons and the corresponding heteroaromatic nitrogen compounds will be reviewed. A comparison of the regioselectivities under various hydrogenation conditions for both classes of compounds will be discussed for a wide variety of transition-metal complexes with regard to substrate binding at the metal center and the role of free radical intermediates in metal carbonyl hydride reactions, where substrate does not bind to the metal center prior to hydrogen transfer. The polynuclear heteroaromatic nitrogen compounds appear to hydrogenate more readily, under similar reaction conditions, than the polynuclear aromatic hydrocarbons. The relative rates of hydrogenation of a variety of heteroaromatic nitrogen compounds as well as compounds that inhibit and enhance selective hydrogenation of the nitrogen-containing ring will be addressed. A relatively new spectroscopic technique, high pressure nuclear magnetic resonance spectroscopy, will be shown to be a powerful tool to elucidate the mechanisms of the regioselective hydrogenation of polynuclear heteroaromatic nitrogen compounds.

Kinetics of Aqueous-Phase Hydrogenation of Organic Acids and Their Mixtures over Carbon Supported Ruthenium Catalyst

Aqueous-phase hydrogenation of lactic acid (2-hydroxypropanoic acid) and propionic (propanoic) acid over 5 wt % Ru/C catalyst was performed in a three-phase stirred batch reactor. Kinetic data were collected for reactions at 343−423 K, 3.4−10.3 MPa hydrogen pressure, and 0.05−5 M acid feed concentrations. Adsorption and reaction of individual acids, acid mixtures, and combinations of acids with their alcohol products were investigated to characterize relative hydrogenation rates of the two acids and the extent to which the presence of one species influences the reactivity of another. Mass transfer analysis showed that acid conversion rates were not limited by mass transport resistances over the reaction conditions studied. A two-site Langmuir−Hinshelwood (L−H) kinetic model with a single set of rate and adsorption constants fits the conversion kinetics of both individual and mixed acid hydrogenations. Competitive adsorption of acids and their alcohol products strongly affects hydrogenation rates.

Hydrogen-assisted 1,2,3-trichloropropane dechlorination on supported Pt–Sn catalysts

The chemical kinetics behavior of Pt–Sn/C catalysts for hydrogen-assisted 1,2,3-trichloropropane dechlorination is understood in terms of a modification of Pt by both Sn and Cl. Catalysts with Pt:Sn atomic ratios ranging from 9:1 to 1:6 exhibit a 10–20h time-on-stream (TOS) transient period characterized by a 2–3 fold decrease in activity. This transient behavior is followed by a ‘steady state’ period during which the activity remains essentially constant (5–7% change in conversion during 30–60h TOS); however, the relative rates of hydrogenation and hydrodechlorination decrease. For catalysts with a Pt:Sn atomic ratio ≥3, the decrease in relative rates manifests itself as an increase in propene selectivity at the expense of propane. In contrast, allyl chloride selectivity increases at the expense of propene for the catalysts with Pt:Sn ratio ≤1. Steady state turnover frequencies (TOFs) decrease with decreasing Pt:Sn atomic ratio to a minimum for the catalyst with the Pt:Sn=1. Further decreasing the Pt:Sn ratio results in a TOF increase, with the Pt1Sn6 catalyst TOF exceeding that of monometallic Pt/C.

Liquid-Phase Hydrogenation of Citral over Pt/SiO2 Catalysts: II. Hydrogenation of Reaction Intermediate Compounds

Liquid-phase hydrogenation of the four principal reaction intermediates formed during citral hydrogenation, i.e., nerol, geraniol, citronellal, and citronellol, was studied at 298 and 373 K under 20 atm H2 at concentrations of 0.5 to 1.0 M in hexane. A decrease in the initial reaction rate as temperature increased from 298 to 373 K was exhibited during the hydrogenation of all four compounds, just as reported earlier for citral; however, the decrease in rate at 373 K was only one-half for citronellal whereas it was orders of magnitude greater for nerol and geraniol. Furthermore, simultaneous hydrogenation of citronellal and geraniol at 298 K resulted in a continuous decrease in the rate of citronellal disappearance in contrast to the nearly constant rate of disappearance observed during hydrogenation of citronellal alone. Competitive hydrogenation of citral with either geraniol or citronellal showed that geraniol hydrogenation to citronellol is kinetically insignificant during citral hydrogenation at 373 K. The initial activity for hydrogenation of the intermediates at 298 K follows the following trend: geraniol (1.9 s−1) > nerol (1.5 s−1) > citronellol (0.21 s−1) >E-citral (0.13 s−1), citronellal (0.13 s−1) >Z-citral (0.06 s−1). Based on the relative hydrogenation rates of the intermediate alone versus its hydrogenation in the presence of other reactants, the relative size of the adsorption equilibrium constants for the various organic compounds appears to be as follows: Kcitral >Kcitronellal >Kgeraniol, Knerol >Kcitronellol >K3,7-dimethyloctanol. This study indicates that activation of the CO bond should be performed at higher reaction temperatures to maximize selectivity to the unsaturated alcohols.

Competitive Hydrogenation of Benzene and Toluene on Palladium and Platinum Catalysts

This study examines the influence of the support on the relative hydrogenation rates of benzene and toluene in mixtures, and it also analyzes the influence of the chosen reaction model on the interpretation of fitted rate parameters. The relative hydrogenation rates,RT/B, over Pd catalysts at an equimolar feed composition of 50% toluene and 50% benzene were 0.65±0.10 and had little dependence on the support, the pretreatment temperature, or the reaction temperature. A supported Pt catalyst was also studied, and the threefold higherRT/Bvalue for Pt indicates that toluene hydrogenation was more favored over Pt compared to Pd. The variations in the ratio of adsorption equilibrium constants,KT/B, which have been attributed previously to changes in the electron deficiency of the metal based on a simple dual-site kinetic model, have been reexamined. Some internal inconsistencies which existed when the kinetic parameters were evaluated at different reaction temperatures were removed when a more comprehensive model invoking formation of hydrogen-deficient surface species was utilized. These apparentKT/Bratios can include additional kinetic dehydrogenation parameters, and analysis of the results with the latter model revealed that theseKT/Bratios can also reflect the relative formation of hydrogen-deficient species like phenyl or tolyl groups. Consequently, the assumption that variations inKT/Bvalues represent only changes in relative adsorbate bond strength must be viewed with caution because the propensity of the metal surface to form hydrogen-deficient surface species under reaction conditions may also alter these ratios.

Legislation & Regulation

The direct synthesis of hydrogen peroxide using supported gold palladium catalysts prepared by incipient wetness impregnation is described and discussed. The effect of an acid pre-treatment step on the activated carbon support prior to the deposition of the metals, together with the effect of the calcination temperature, has been investigated. The acid pre-treated samples all show superior activity to those materials prepared with the omission of this acid pre-treatment stage. The calcination temperature affects both the re-usability and hydrogenation activity of the catalysts. Detailed characterisation using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy is described. The enhanced activity is associated with a higher surface concentration of palladium in the acid pre-treated samples which is principally present as Pd2+. Calcination of the catalysts at 400 °C is required to achieve re-usable and stable catalysts, and this is associated with the morphology and dispersion of the metal nanoparticles. The surface ratio of Pd0/Pd2+ is found to be an important factor controlling the hydrogenation of hydrogen peroxide, and a series of controlled reduction and re-oxidation of a sample show how the Pd0/Pd2+ surface ratio can influence the relative rates of hydrogen peroxide synthesis and hydrogenation.

Selective hydrogenation and isomerization of pentadiene over molybdenum hexacarbonyl encaged in a NaY zeolite

Highly stereo-selective hydrogenation of trans-1,3-pentadiene to cis-2-pentene and isomerization of 1,4-pentadiene to cis-1,3-pentadiene were found to take place thermally at 423 K over Mo(CO)6/NaY. Relative hydrogenation rates of pentadiene isomers indicated that s-cis conformation of diene was crucial for the selective hydrogenation to occur. This was substantiated by an IR study.

Stereochemical Studies of the Electrolytic Reactions of Organic Compounds. Part 18. Comparative Study of Hydrogenation on Metal-black Cathodes and Catalysts

Abstract Stereoselective hydrogenation on metal-black cathodes and catalysts, which were prepared by electrode-position under the same conditions, was comparatively investigated. Stereoisomeric ratios of alcohols formed from the ketones on the Pd- and Pt-black cathodes seemed to be almost equal to those on the corresponding metal-black catalysts, while the yields were quite different between the cathodic and catalytic hydrogenations. On the other hand, results of the hydrogenation of the acetylenes on the cathodes were also different, in not only the yield but also the isomeric ratio, from those on the catalysts. In addition to stereoselectivity, relative hydrogenation rates between cis- and trans-olefins were compared in both the cathodic and catalytic hydrogenations. It was also found that a mercury pool cathode gave quite different stereochemical results in the hydrogenation of the unsaturated compounds from those obtained with either the metal-black cathodes or the catalysts.

Relative hydrogenation rates of carbon monoxide, hydrocarbons, and oxygenates

This report considers the relative hydrogenation rates both of individual substrates and of substrate mixtures with carbon monoxide, over the Cu/ZnO catalyst for methanol synthesis. A pattern which emerges for the Cu/ZnO catalyst is then compared with those for nickel, cobalt, and rhodium to demonstrate some significant differences in the function of these catalysts.

Hydrogenation of nitroxides on silica gel-supported platinum

The hydrogenation of the nitroxide 2,2,6,6-tetramethylpiperidinyl-N-oxy (TMPNO) in ethylcyclohexane and other solvents has been studied from 0.6 to 35/sup 0/C on Pt/Si0/sub 2/. Hydrogenations were sandwiched between hydrogenations of cyclopentene using the techniques of Hyssey et al. Results were shown to be uninfluenced by mass transfer of hydrogen between gas and liquid phases and in the catalyst pores. Significant levels of poisons in the TMPNO appeared to have been absent. Rates of hydrogenation of TMPNO to the hydroxylamine were very fast. At 20/sup 0/C, turnover frequencies per second per surface atom of Pt for the loss of unsaturated molecules were approx.130 for TMPNO and approx.13 for cyclopentene, and E/sup a/'s were 21 and 34 kJ mol/sup -1/, respectively. The rates were zero order in TMPNO and about first order in hydrogen. In competitive hydrogenations of equimolar mixtures, TMPNO hydrogenated 3 times faster than cyclopentene. It appears that both TMPNO and cyclopentene adsorb strongly enough on Pt to form an essentially saturated adsorbed layer and that the rate of hydrogenation is that of the dissociative adsorption of hydrogen at gaps in the adsorbed layer of the unsaturated molecule. The relative rates of hydrogenation of such molecules are then proportional to themore » product (effective area of gaps) X (sticking coefficient of hydrogen).« less

The stoichiometric hydrogenation of substituted phenyl alkenes by hydridocobalt tetracarbonyl

The relative rates of hydrogenation of a series of styrenes, phenylpropenes, 1,1-diphenylethylenes, and 1,1-diphenylpropenes were measured. Compared to 1,1-diphenylethylene ( k 2 = 2.42 X 10 -2 I mol -1 sec -1), 1,1-diphenylpropene and styrene have relative rates of 0.0045 and 0.011 respectively. The effect of 4-chloro and 4-methoxy substituents on both styrene and diphenylethylene is slightly rate enhancing. An unusual kinetic dependence occurs with mixtures of alkenes.

Catalysis by supported group VIII metal compounds I. The interaction ofn-butene with hydrogen over silica-supported ruthenium carbonyl catalysts

The reaction of but-1-ene with hydrogen and with deuterium has been studied over the temperature range 353–415K by using catalysts prepared from silica-supported triruthenium dodecacarbonyl. Infrared and radiochemical studies show that the supported complex undergoes decomposition in a series of well defined stages, and that in the temperature range studied the active catalyst is a ruthenium subcarbonyl containing five carbonyl ligands per ruthenium triad. Kinetics and activation energies for hydrogenation and hydroisomerization are reported, together with the variations in product distributions with hydrogen uptake and the distributions of deuterium in the reaction products. But-1-ene isomerization occurs in the absence of molecular hydrogen. The relative rates of hydrogenation of then–butenes are in the order but-1-ene <cis-but-2-ene <trans-but-2-ene. The results are consistent with a mechanism in which hydrogenation occurs by the attack of molecular hydrogen on aπ-butene metal complex, whereas isomerization involves a hydrogen abstraction–addition mechanism and the formation of a 1-methyl-π-allyl intermediate.

Hydrogenation of unsaturated polymers with diimide

AbstractWe have studied the homogeneous hydrogenation of a series of unsaturated polymeric substrates with diimide formed from the thermal decomposition of p‐toluenesulfonylhydrazide (TSH) at elevated temperatures (110 to 160°C) and in aromatic solvents. At a TSH/olefin ratio of 2.0, homopolymers of butadiene, polycyclohexadiene, and polyisoprene were quantitatively hydrogenated as were two styrene/butadiene copolymers. Alkyl branching at or near the residual polymer double bond, as in methyl rubber and poly‐2.5‐dimethyl‐2.4‐hexadiene, gave less than 50% hydrogenation. Polychloroprene was not hydrogenated by diimide (conversions less than 10%) which is in agreement with the behavior previously observed for low molecular weight olefins.The unsaturated polymers which were successfully hydrogenated were soluble and, hence, showed no excessive crosslinking. Polycyclohexadiene and polyisoprene, the only samples evaluated for molecular weight changes, exhibited no chain degradation.Some qualitative kinetic data were obtained using per cent conversion vs. time data. Under the conditions of toluene reflux and TSH/olefin ratios of 1.5, the relative rates of hydrogenation for cis, trans and vinyl butadiene units was kcis ≃ ktrans < kvinyl.Byproducts from the diimide generation resulted in the introduction of 0.2 to 1.0% of p‐CH3C6H4SO2‐groups into the polymer based upon 100% of the double bonds originally present.

Further studies on the homogeneous hydrogenation of olefins using tris(triphenylphosphine)halogenorhodium(I) catalysts

Studies on the homogeneous hydrogenation of olefinic hydrocarbons and other unsaturated substrates using tris(triphenylphosphine)halogenorhodium(I), RhX(PPh3)3(X = Cl or Br), as catalyst in benzene solution have been extended. The activation enthalpies and entropies for a number of olefins have been determined and, for other substrates, the relative rates of hydrogenation under standard conditions have been compared. The data can be broadly explained in terms of steric factors but electronic effects are clearly also important. Quantitative studies have been made of the kinetic isotope effect and the influence of the halide and of solvents.

Relative hydrogenation rates of normal and conjugated linolenic and linoleic acid glycerides

SummaryA mixture of 50% tung oil and 50% linseed oil was hydrogenated and samples withdrawn at approximately 10 I.V. intervals during the course of the reaction were analyzed for fatty acid composition. Reactivity ratios for the various fatty acids were calculated.Because of the limitations of the analytical methods, particularly the spectrophotometric methods used for linolein, conjugated linolein, and linolenin, and the arbitrary procedures of arriving at corrected values for these constituents, the composition data and reactivity ratios derived therefrom must be regarded as only approximations.However it seems safe to conclude that during the initial stages of the hydrogenation the eleostearin was many times more reactive than normal linolenin for equivalent concentrations but as the eleostearin was reduced in concentration, its reactivity decreased until, at a 1% to 2% level, it was only slightly more reactive than the normal linolenin, again for equivalent concentration. Likewise the conjugated linolein formed from the eleostearin was many times as reactive as the normal linolein when present in considerable amounts, but the reactivity ratio of conjugated linolein to normal linolein dropped markedly in the latter stages of the process. Normal linolenin and normal linolein had similar reactivities.It is suggested that the amount of conjugation and unsaturation in the glyceride rather than in the individual fatty acid may be the controlling factor in hydrogenation selectivity.