Mervyn A. Long

Mössbauer spectroscopy and catalytic studies of iron-exchanged, silicon-substituted Y-zeolite

Iron-exchanged zeolites with SiAl ratios ranging from 2.49 to 8.88 were characterized by Mossbauer spectroscopy and studied for the decomposition of N2O. Mossbauer spectra were collected at room temperature after pretreatments in H2 and O2 at 700 K, and after exposure of the samples to CO or H2O. Nitrous oxide decomposition activities were determined at 800 K under a N2O partial pressure of 3.5 kPa. Mossbauer spectra collected after pretreatment in H2 showed that silicon-substituted Fe-Y had a larger fraction of iron cations in sites I′, II′, and/or II than conventional Fe-Y, in which most iron cations were in site I. The silicon-substituted Fe-Y catalysts had higher turnover frequencies for the decomposition of N2O than conventional Fe-Y. Mossbauer spectra collected after pretreatment in O2 showed that a fraction of the iron cations in silicon-substituted Fe-Y was more difficult to oxidize to Fe3+ than the iron in conventional Fe-Y. This result suggests that pairs of iron cations in close proximity are needed for the oxidation process to occur readily. Finally, exposure of the zeolites to CO or H2O produced Mossbauer spectroscopy doublets due to iron cations associated with adsorbate molecules. These doublets indicate that ferrous cations at sites I′, II′, and/or II serve as adsorption centers for H2O, the adsorption process increasing the coordination of the ferrous cations, and that ferrous cations at site II can serve as adsorption Centers for CO as well.

Orientation in metal-catalyzed hydrogen exchange between alkanes, naphthalene, or biphenyl and deuterium or deuterium oxide

Abstract Hydrogen isotope exchange between deuterium gas and protium in hexane, pentane, 2-methylbutane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,4-dimethylpentane has been catalyzed by clean platinum films (70–100 °C). A selection of these reactions has been catalyzed by films of rhodium and iridium (typically −13 to −35 °C). In all cases, multiple exchange occurred. Product analysis by mass and proton NMR spectroscopy showed that exchange in methylene and methine groups was more rapid than that in methyl groups. A similar orientation effect was observed in reactions over platinum powder but not over platinum-alumina. For exchange between deuterium oxide and hexane catalyzed by platinum films at 200 °C, the rate of exchange in methyl groups exceeded that in methylene groups. It is proposed that preferential exchange in methylene and methine groups is normal behavior during alkane exchange with molecular deuterium over these platinum metals when their surfaces (i) are initially clean or (ii) contain several adjacent sites which are unperturbed by the presence of any electronegative adsorbed species. In these cases, multiple exchange may occur via the interconversion of CnH2n + x (ads) and CnH2n + x − 1(ads), where x is probably 1, but may be zero or negative. Preferential exchange in the methyl groups of hexane results from contamination of the surface by adsorbed D2O, OD, or O; this may be a geometric effect or an electronic effect, depending on the magnitude of the surface coverage of water, which is unknown. It is proposed that, under these circumstances, alkane is adsorbed as a charge-transfer state, and multiple exchange occurs via the interconversion of CnH2n + 2(ads) and CnH2n + 1(ads) in a manner analogous to that proposed earlier for catalysis by chloroplatinate ion in deuterated solvent. Orientation in the exchange between deuterium gas and naphthalene or biphenyl catalyzed by films of platinum and iridium does not differ from that observed in exchanges where the isotope source is deuterium oxide or deuterated solvent, but the M value calculated for exchange in naphthalene is higher than that previously reported.

Nematic-phase nuclear magnetic resonance investigation of rotational isomerism. Part 7. An investigation into the structure of 2,2′-bipyridyl using the proton and deuterium spectra of liquid crystalline solutions

A 220 MHz proton n.m.r. spectrum of 2,2′-bipyridyl in a nematic phase solution has been analysed to yield a set of dipolar coupling constants, Dij. The couplings Dij are not in complete agreement with a description of the molecule as a single conformation. The trans-form alone occurs in solid, 2,2′-bipyridyl, but although this is probably the predominant form in the liquid state, there does appear to be a difference between the molecular structure in the two phases.

Homogeneous and heterogeneous metal-catalysed hydrogen isotope exchange of nitrobenzene with tritium gas and tritiated water

Hydrogen isotope exchange of nitrobenzene with tritium gas and tritiated water is studied over heterogeneous and homogeneous forms of platinum, palladium and nickel. Exchange with water is facile where homogeneous platinum and palladium salts are the catalysts but this is not the case over heterogeneous forms of the metals. In contrast, exchange with tritium gas is facile over heterogeneous platinum, palladium and nickel. Exchange in nitrobenzene is thus an indicator of homogeneous, as distinct from heterogeneous, exchange as previously claimed, only where the isotope source is water and not elemental hydrogen. Detailed isotope orientation patterns, determined by 3H NMR, are interpreted in terms of the type of absorption intermediate on the metal, and compared with those of aniline.

Catalytic Oxidation of Methane Over Alpo4-5 and Metal – Doped Alpo4-5

Abstract The catalytic oxidation of methane over AlPO4-5 and metal – doped AlPO4-5 has been studied over a range of temperatures (500–900°C) and O2 : CH4 ratios. The major products of the reaction were ethane, ethene, CO2 and H2O. The product distribution was found to be dependent on both temperature and O2 : CH4 Doping of AlPO4-5 with various metals (Pt, Pb, V, Ag, Co, Cu) altered the product distribution, with PbAlPO4-5 the most reactive and selective supported metal catalyst for methane oxidation.

Deuterium and tritium labelling of aromatic hydrocarbons by zeolite-catalysed exchange with perdeuteriobenzene, tritiated benzene, and [p-3H]toluene

Three hydrogen zeolites, HNaY, H-Mordenite, and HZSM-5, have been employed as catalysts for hydrogen isotope exchange reactions. The zeolites catalysed exchange between the isotope sources perdeuteriobenzene, tritiated benzene, and [p-3H]toluene, and a wide range of organic substrates, thus providing a useful labelling technique. The extent and orientation of exchange in reaction products were assessed through the techniques of radio-gas chromatography, mass spectrometry, and 3H n.m.r. spectroscopy. Substituted benzenoid compounds were non-specifically labelled in the aromatic centres at temperatures from 40 to 175 °C, but heterocyclic compounds and alkanes were not exchanged. A series of competitive exchange experiments gave an insight into the reasons for the lack of reactivity of some substrates. Inspection of the multiple exchange parameters and relative rates of exchange of substrates of different bulk suggested correlations between zeolite pore sizes, substrate reactivity, and labelling patterns. Consideration of these experimental results together with those from redistribution reactions with [p-3H]toluene led to interpretation of results in terms of acid exchange mechanisms, where these mechanisms are mediated by the constraints of zeolite pore geometry.

Tritium nuclear magnetic resonance studies of the specificity in plantinum-catalysed hydrogen isotope exchange of nitrogen heterocyclic compounds

The orientation of plantium-metal catalysed exchanged between tritiated water and pyridine, the picolines, 2,6-lutidine, and the quinolines has been carefully studied 3H n.m.r. Exchange at 130 °C occurs preferentially at a position adjacent to the nitrogen atom unless a methyl substituent on C-2 hinders exchange, when a position remote from the methyl substituent is preferred as in alkylaromatic exchange. The results are interpreted in terms of previously proposed π-complex exchange mechanisms.

Ruthenium metal-catalysed deuterium exchange between organosilanes and perdeuteriobenzene

Ruthenium metal prepared by NaBH4-reduction of RuO2 possesses unique activity in consistently catalysing hydrogen isotope exchange between organosilanes and C6D6., exchange occurring readily in alkul and aryl positions, particularly in trisubstituted silanes; a modification of the technique can also be used for the tritium labelling of these silanes.

Mechanistic studies of the hydrogen isotope exchange reaction catalysed by homogeneous platinum salts

Deuterium exchange between D2O–CH3CO2D solvent and a variety of aromatic compounds is catalysed by tetrachloroplatinate(II) ions. Rhodium and iridium salts are also shown to be homogeneous catalysts for these reactions. Multiple exchange is a consistent phenomenon of all systems. Conditions influencing the stability of platinum(II) in solution are discussed in detail and the basis for the very careful distinction between homogeneous catalysed exchange and the possibility of accompanying heterogeneous exchange by colloidal Pt0 is delineated. The exchange kinetics with benzene show inverse first-order dependence on Cl–. A complicated acid dependence on the reaction is found and interpreted in terms of the effect of acid on the stability of the platinum(II) in solution. These kinetic results are shown to be consistent with the predominant participation of a dissociative π-complex mechanism for the exchange process. The possible relevance of reaction intermediates proposed by other workers is also discussed.

Application of 3H n.m.r. Spectroscopy to metal catalysis. Orientation of incorporated isotope in halogenated benzenes and alkylbenzenes tritiated by heterogeneous platinum exchange

3 n H N.m.r. spectroscopy is shown to be a powerful tool for the mechanistic study of metal catalysed reactions; using the technique, data obtained from a re-investigation of platinum catalysed isotopic hydrogen exchange correct previously reported anomalous ortho effects in the halogenated benzenes and also, with the alkylbenzenes, provide additional orientation information which supports the predominant participation of a π-dissociative process for the heterogeneous system; steric and not electronic effects appear to determine the pattern for degree of exchange.