Brian D. Flockhart

Evidence for the redox nature of zeolite catalysts

Abstract Hydrogen Y-zeolite (HY), under suitable activation conditions, possesses well-marked surface redox properties similar to those described for alumina and for amorphous silica-aluminas of high alumina content. The reducing activity for tetracyanoethylene of sodium-form Y-zeolite 90% exchanged with ammonium ion is intermediate between that of alumina and a silica-alumina cracking catalyst, whereas its oxidizing activity for perylene is considerably greater than that of either. The electron donor sites on the surface of these zeolites may cover a wide spectrum of energies. Two different surface sites are responsible for the reducing properties of HY. At activation temperatures of around 250 °C the site responsible for the reduction of tetracyanoethylene appears to be a surface hydroxyl ion. At higher activation temperatures of around 660 °C dehydroxylation of the zeolite leads to the formation of electronegative and electropositive sites which may be the source of the redox activity. Present evidence suggests that site II (which projects slightly outwards from the unshared hexagonal face into the supercage of the zeolitic structure) is the lattice position associated with the electron transfer process.

Evidence for the redox nature of the surface of catalytic aluminas

Abstract Oxidation of polynuclear aromatic hydrocarbons to the radical-cation form occurs readily on partially dehydrated aluminas, the surface of which has previously been saturated with a polynitrobenzene. Conversely, reduction of mono- and polynitrobenzenes to the radical-anion form occurs on a surface previously saturated with anthracene or perylene. Different surface sites are therefore responsible for the oxidation and reduction processes. Direct electron transfer between the donor and acceptor molecules is not involved. Under certain conditions superimposed electron spin resonance spectra of the radical-cation and the radical-anion may be obtained from the same alumina sample. A marked enhancement of the radical-anion signal results when perylene is adsorbed on the alumina surface and similarly the radicalcation signal is reinforced by adsorption of trinitrobenzene. This indicates that the active sites responsible for oxidation and for reduction are to some degree mutually dependent. These results are interpreted using the Peri model for the surface of partially dehydrated catalytic aluminas.

Research and development topics in Analytical Chemistry

Summaries of seven of the papers presented at a Meeting of the Analytical Division held on July 13-14, 1993, in the University of Bradford.

Alkylation of toluene with propene using zeolite catalysts

The catalytic activity of Y-type zeolite for the alkylation reaction of toluene with propene is strongly dependent on the degree of ion-exchange and on the calcination temperature. The activity increases as the sodium ions in Na-form Y zeolite are replaced by hydrogen ions up to at least 90% cation exchange. Optimum activity occurs at activation temperatures around 400 °C when the Bronsted acidity is at a maximum. Ortho-para substitution always predominates, but at low activation temperatures (<400 °C) the p isomer is preferred. This preference is reduced with increasing activation temperature, accompanied by a marked increase in the proportion of the m isomer. Present results indicate that the alkylation proceeds by a carbonium ion mechanism involving Bronsted acid sites on the zeolite surface. There is no correlation between the catalytic activity of the zeolite and its electron-transfer properties. The isomeric composition of the products suggests, however, that the selectivity of the catalysts is a function of their Lewis acidity. These findings, therefore, provide further evidence for the dual nature of zeolite catalysts.

Electron-transfer at alumina surfaces: 4. Reduction of iodine

Abstract The reduction of iodine to iodide ion occurs readily on the surface of partially dehydrated catalytic aluminas and silica-aluminas. An increase in the alumina content of the latter results in an increase in reducing activity. Replacement of the hydroxyl ion on the alumina surface by the fluoride ion decreases the reducing power. Two different surface sites are responsible for the reducing properties. For alumina samples dehydrated at low temperatures (

The effect of base exchange on the oxidizing properties of silica-alumina catalysts

Abstract A quantitative study has been made of the effect of base exchange on the oxidizing (radical-forming) power of a silica-alumina catalyst. A single base-exchange-dehydration cycle causes a drop in radical-forming activity to about one-third and subsequent base-exchange-dehydration cycles result in a further stepwise reduction in oxidizing power. The sodium uptake and the decrease in surface acidity follow a similar stepwise pattern. These results are interpreted in terms of the Danforth formulation of the acid sites on a silica-alumina surface. They provide support for the view that oxidizing activity is associated with Lewis acid sites produced by elimination of the elements of water from hydroxyl groups surrounding aluminum ions in the surface. The role of silica is to facilitate the formation of these sites and to contribute to their stability. A study of the effect of activation temperature on the oxidizing properties of the unexchanged and exchanged catalysts supports this interpretation. An explanation of the results based on the blocking of active oxidizing centers by the hydration sheath surrounding metal cations on adjacent sites is also discussed.

Oxidation of iodides by silica-alumina catalysts

Abstract Aqueous solutions of iodide ion liberate iodine in a continuing reaction in the presence of a silica-alumina catalyst, suitably activated by heat treatment, provided molecular oxygen is present. A similar reaction occurs in benzene solutions of tetramethylammonium iodide. Pure alumina and pure silica are both inactive for the oxidation. Hydrogen Y zeolite has an activity comparable with that of the most active of the amorphous silica-aluminas. The catalytic activity of both the amorphous and crystalline silica-aluminas for the iodide ion oxidation is related to their Bronsted acidity. In the presence of water, the Bronsted sites on the oxide surface are continuously regenerated as the oxidation proceeds. Although silica-aluminas are active for the oxidation of methyl iodide to iodine, the reaction occurs much more readily on the surface of pure alumina. Alumina samples dehydrated at about 630 °C show maximum oxidizing activity and maximum Lewis acid activity. The production of methyl peroxy radicals shows that basic sites are also involved. Dissociative adsorption of methyl iodide probably occurs on the alumina surface and is followed by association of the halogen atoms to produce molecular iodine. Bronsted acid sites are not involved.

Determination of polycyclic aromatic hydrocarbons by electron spin resonance spectrometry

Abstract The use of electron spin resonance spectrometry with a modern instrument is described for the determination of polynuclear aromatic hydrocarbons (PAHs) down to nanogram levels, after adsorption on calcined silica/alumina. A single PAH or the total number of moles of PAHs can be determined. Implication for liquid chromatography are discussed.

Electron transfer at alumina surfaces. Part 7.—Nature of the radical species formed from perylene on the surface of an activated alumina—cation or anion?

The preadsorption of Lewis acids or Lewis bases on the surface of an activated alumina leads to deactivation of the reducing or the oxidizing function of the catalyst with respect to the 1,3,5-trinitrobenzene (TNB) or the perylene couple. The incorporation of small amounts of fluoride ion into alumina decreases the electron-donor properties of the oxide, as shown by radical formation with TNB or tetracyanoethylene. With perylene, 9,10-dimethylanthracene or triphenylamine as the adsorbate, small additions of fluoride ion to the oxide have exactly the opposite effect, a dramatic increase in the radical-forming power of the catalyst being observed. The effect of adsorbed TNB or benzoic acid is to decrease the reducing power of alumina for iodine, but adsorbed perylene or triphenylamine produces a marked enhancement of this activity.These results prove the existence of two different electron-transfer sites on the surface of an activated alumina: a site that can generate TNB ion radicals and a site that can produce a free-radical form of perylene. Since the radical derived from TNB is negatively charged, present findings demonstrate beyond reasonable doubt that the species formed when perylene is adsorbed on an active alumina surface is the cation radical.

Electron transfer at alumina surfaces. Part 6.—Redox properties of fluorided aluminas

An e.s.r. study of adsorbed perylene and adsorbed tetracyanoethylene has shown that the incorporation into alumina of up to 6% by weight of fluorine enhances the oxidizing activity and simultaneously lowers the reducing activity of the catalyst surface, on subsequent activation in the temperature range 180–800°C. The increase in oxidizing activity with increase in fluoride content is paralleled by the development of and increase in Bronsted acidity. This and other evidence indicate that fluoridation of alumina produces a surface with redox activity qualitatively intermediate between that of alumina and a typical silica–alumina cracking catalyst. Replacement by fluoride of hydroxyl ions in the surface of alumina samples activated at low temperatures (< 400°C) and of oxide ions in samples heated at higher activation temperatures accounts for the decrease in the reducing activity on fluoridation. A third type of site has been postulated to account for the reducing properties of some fluorided aluminas.