Christopher S. John

The mechanisms of dimethylbutene isomerization over alumina

Reactions of dimethylbutenes over pure and chlorided γ-alumina have been studied in the presence of selective poisons and/or with various host molecules for deuterium label. The extent of H2S poisoning has been used to determine the relative roles of π-allylic species and alkyi carbonium ions in the isomerization of the 2,3-dimethylbutenes on alumina at 273 K. The absence of poisoning by H2S of skeletal isomerization of 3,3-dimethylbut-l-ene at 352 K is evidence that H2S does not inhibit reaction through carbonium ions whereas it is known to poison activity involving π-allylic species. The isomerization of 3,3-dimethylbut-l-ene in the presence of suitably labeled alkenes showed that vinylic and allylic hydrogen atoms can equilibrate with Bronsted-acid-type hydrogen on alumina at about 273 K. But adsorbed vinyl species have been found to retard skeletal isomerization, suggesting identity, or at least close proximity, of the sites for Bronsted acid reactions and vinylic CH exchange. Catalyst chloriding enhanced isomerization but retarded exchange of CH bonds with D. The influence of activation temperature and of impurities is discussed for reactions of dimethylbutenes over alumina.

Role of Brønsted acid centres for alkene double bond migration over alumina at temperatures above 450 K

Water was added to alumina samples following their activation in vacuo at both 723 and 1023 K. Reactions of [2H0]propene and of CD2CH—CH3 were then studied on these catalysts. Adsorption of water (2.7 × 1018 molecule m–2) at ∼300 K is molecular and serves only slightly to poison catalytic activity at ∼300 K. Adsorption of water at > 453 K occurs dissociatively and completely poisons those sites on the fresh catalyst surface which promote reactions at ∼300 K. Reaction of propene occurs on such a water poisoned catalyst at ∼483 K and proceeds with incorporation of D+(from D2O) through an associative mechanism involving carbonium ions, in contrast to a dissociative mechanism at a reaction temperature of 300 K on an unpoisoned catalyst. Water “added-back” to activated alumina at > 453 K is therefore effective in producing Brϕnsted acid centres on the surface and it is suggested that residual hydroxyls, those left following activation, can also participate in carbonium ion formation from propene at > 453 K. Therefore, not only is the activity of alumina strongly influenced by its activation temperature but it is also determined by the temperature used for subsequent alkene catalysis.

Determination of the active site and mechanism for alkene isomerization in CuII exchanged Y-type zeolite

Reactions of CD2CH—CH3, but-1-ene and 3,3-dimethylbut-1-ene have been studied over a Y-type zeolite containing ∼4 CuII per unit cell. Kinetic induction periods, associated with the generation of active sites by CuII reduction, were only observed for the deuterium redistribution reactions of CD2CH—CH3. Nevertheless, arguments are presented to show that such generation of active sites is central to alkene isomerization reactions over CuII exchanged X- and Y-type zeolites. The mechanism of alkene isomerization is shown to require carbonium ions, generated from protons produced following CuII reduction by the alkene.

Interaction of diatomic molecules with clean metal wire surfaces. Part 1.—Hydrogen on tungsten

Heats of adsorption, sticking probabilities and temperature programmed desorption spectra are reported for hydrogen on polycrystalline tungsten at 88 and 290 K. The heats were measured using a fine-wire calorimeter, as invented by J. K. Roberts, but with automatic recording. The initial heat of adsorption onto a clean surface at 290 K was 192 ± 8 kJ mol–1. Three desorption peaks were observed in the range 290–700 K. A discussion of the results is presented in terms of the crystal planes believed to be present at this polycrystalline surface. It is deduced that the heat of adsorption for hydrogen onto the (100) and (110) planes is 137 ± 5 kJ mol–1 and that on these planes there is a repulsive interaction energy between near neighbours of 5.25 ± 0.50 kJ (mol pair)–1. The importance of adatom interaction in this system is emphasised.

Sorption of hydrogen by palladium and palladium/silver alloy wires

Heats of adsorption q(θ), sticking probabilities and temperature programmed desorption spectra are reported for the adsorption of H2 onto clean wire surfaces of Pd, all as functions of coverage θ. Data, principally from temperature programmed desorption, have also been obtained for the inter action of H2 with two Pd/Ag alloys.The low temperature (104 K) sorption properties of Pd were dominated by the occurrence of absorption which limited the applicability of the technique used to determine q(θ). Certain sites on the Pd surface had to be occupied by strongly bound, β, adsorbed hydrogen before absorption could occur at 104 K, and the probability of their occupation depended on the mode of preadsorption. This is interpreted as evidence for the occurrence of metastable surface precursor states to absorption for which q was 30–60 kJ mol–1.Sorption by both of the Pd/Ag alloys studied was activated, its rate decreasing as temperature decreased. A model is proposed for the absorbed state of H which rationalises why the forms of the H2 desorption spectra from Pd and Pd/Ag alloys are dependent on metal sample size and which emphasises that large proportion of H2 sorbed by these alloys (PH2 < 5 mPa) may be absorbed even at room temperature. The possible role of absorbed hydrogen in catalytic reactions using H2 and Pd or Pd/Ag alloys is briefly considered.

The Mechanism of Olefin Isomerization on Different Forms of Chromia Investigated by Microwave Spectroscopy

The isomerlzation of CD2=CH-CH3 on amorphous and micro-crystalline chromia has been followed by combined microwave and mass spectroscopies. On both catalysts the isomerization is primarily intramolecular. The measured isotope effect kH/kD for intramolecular isomerization is 4 and 5 and the activation energies 9.2 and 14 kcal/mol, respectively, on amorphous and microcrystalline chromia. The intramolecular isomerization on amorphous chromia occurs through an allylic carbanion. The intermediate on micro-crystalline chromia is about 5-6 kcal/mol less stable than the allylic carbanion and is probably a covalently bonded σ-or π-allyl.

Hydrodesulphurization of ethanethiol and related reactions over CoO-MoO3-Al2O3 and molybdenum disulphide catalysts

Hydrodesulphurization (h.d.s.) of ethanethiol, hydrogenation of ethene, h.d.s. of diethylsulphide and some relevant exchange reactions with deuterium have been examined on CoO-MoO3-Al2O3(CMA) and related catalysts. In the presence of sulphur-containing reactants, CMA undergoes partial sulphiding and the catalytic properties become closely similar to those of molybdenum disulphide.The differences in the catalytic behaviour of the oxide form of CMA and the sulphided form are discussed and the relative rates of the various reactions on sulphided-CMA and MoS2 are compared in order to establish in outline a mechanistic scheme.

Use of labelled propene to distinguish between an associative, a dissociative and a concerted mechanism for the double bond shift reaction of alkenes

The specifically labelled propene, CD2CH—CH3, has been used as a model alkene for investigations of catalytic double bond shift reactions. Microwave spectroscopy has been used to analyse deuteropropene mixtures produced in this reaction. It is shown how in principle the positions of the labelling D atoms after reaction can be used to deduce whether an associative, a dissociative or a concerted mechanism has operated. Specifically, the data for Ω-zeolite are consistent with an associative mechanism, presumably on Bronsted acid sites, and arguments are presented to show that this is a general mechanism for alkene double bond shift reactions on this catalyst.

Use of simple test reactions to characterise the catalytic activity of a commercial CoO-MoO3-Al2O3 catalyst

Exchange of thiophene with D2 and reactions of deuterium-labelled propene and isobutene, followed using a combination of mass spectrometry and microwave spectroscopy, have been used to characterise the catalytic nature of a commercial CoO-MoO3-Al2O3(CMA) catalyst. The results indicate that the activity of partially sulphided CMA, produced by exposure to H2S or by hydrodesulphurization (h.d.s.) of ethanethiol, closely resembles that of unsupported MoS2. Thus, sulphided-CMA shows high selectivity/activity for exchange of the α hydrogens in thiophene and catalyses double bond migration in propene through half-hydrogenated intermediates, activity characteristic of MoS2, whereas oxide-CMA is much less active for α exchange and shows Lewis acid type activity for the latter reaction. It appears that a CMA catalyst during h.d.s. may usefully be described as “dual-functional”, with MoS2 supported on, and stabilised by, an acidic support.

Brønsted acidity of rutile developed during alcohol dehydration as shown by simultaneous alkene isomerization: microwave spectroscopic analyses

Microwave spectroscopic analyses of deuteriopropenes produced from CD2CH–CH3, and of deuterioisobutenes from CD2C(CH3)2, indicate conclusively that on rutile in the presence of water and/or alcohol carbonium ion intermediates are involved in alkene isomerization; in contrast, comparison of rates of reaction of CD2CH–CH3 and CD2C(CH3)2 implies that π-allylic species with partial carbonium ion character are the intermediates in isomerization in the absence of water or alcohol.