Mahmood Nickkho-Amiry

Catalytic reactions of chlorofluoroethanes at fluorinated alumina and chromia aerogels and xerogels: A comparison of reaction pathways in alumina- and chromia-based catalysts

Abstract The differences in the dynamic behaviour of 1,1,2-trichlorotrifluoroethane over fluorinated alumina or chromia catalysts at moderate temperatures are significant. Alumina-based catalysts favour the isomerization of CCl 2 FCClF 2 followed by the dismutation of CCl 3 CF 3 so formed. Fluorinated chromias are less selective since halogen exchange and isomerization reactions both occur. An explanation in terms of the differences in AlX, X=Cl or F, bond energies compared with their CrX counterparts is suggested. The identity of the catalyst precursor has little effect in the chromium case and no effect in the case of aluminium.

The use of multiple probe molecules for the study of the acid–base properties of aluminium hydroxyfluoride having the hexagonal tungsten bronze structure: FTIR and [36Cl] radiotracer studies

The combination of several probe molecules has enabled the construction of a detailed picture of the surface of aluminium hydroxyl fluoride, AlF(2.6)(OH)(0.4), which has the hexagonal tungsten bronze (HTB) structure. Using pyridine as a probe leads to features at 1628 cm(-1), ascribed to very strong Lewis acid sites, and at 1620-1623 cm(-1), which is the result of several different types of Lewis sites. This heterogeneity is indicated also from CO adsorption at 100 K; the presence of five different types of Lewis site is deduced and is suggested to arise from the hydroxylated environment. Brønsted acid sites of medium strength are indicated by adsorption of lutidine and CO. Adsorption of lutidine occurs at OH groups, which are exposed at the surface and CO reveals that these OH groups have a single environment that can be correlated with their specific location inside the bulk, assuming that the surface OH group may reflect the bulk OH periodicity. A correlation between the data obtained from CO and pyridine molecules has been established using co-adsorption experiments, which also highlight the inductive effect produced by pyridine. Adsorption of the strong Brønsted acid, anhydrous hydrogen chloride, detected by monitoring the beta(-) emission of [(36)Cl]-HCl at the surface, indicates that surface hydroxyl groups can behave also as a Brønsted base and that H(2)O-HCl interactions, either within the hexagonal channels or at the surface are possible. Finally, the formation of strongly bound H(36)Cl as a result of the room temperature dehydrochlorination of [(36)Cl]-labelled tert-butyl chloride provides additional evidence that HTB-AlF(2.6)(OH)(0.4) can behave as a Lewis acid.

The properties of platinum or palladium supported on β-aluminium trifluoride or magnesium difluoride: catalysts for the hydrodechlorination of 1,1-dichlorotetrafluoroethane

Palladium or platinum supported on β-aluminium(III) fluoride or magnesium fluoride are effective catalysts for the conversion of the ozone-depleting compound, CCl2FCF3, to the environmentally acceptable refrigerant, CH2FCF3 (HFC-134a). The gas phase hydrodechlorination reaction has been studied in the temperature range 373–773 K under steady flow conditions, under which the fluorides were shown not to catalyse any isomerization or dismutation reaction that would lead to unwanted by-products. Increased catalytic activity was found for both metals with increasing metal content of the catalysts. The temperature dependencies revealed higher yields with increased temperature. The selectivity to the desired product, CH2FCF3, is significantly higher in the case of Pd on both supports and is slightly higher for β-AlF3 than for MgF2. The results of temperature programmed reduction (TPR) of the catalyst precursors and hydrogen chemisorption and transmission electron microscopy (TEM) studies on the catalysts are discussed in relation to the surface hydrolysis of the support materials, for which evidence has been obtained from [36Cl] measurements. It is proposed that water desorbed from the hexagonal channels of β-AlF3 has an influence on the metal dispersions of the catalysts.

Dynamic behaviour of chlorofluoroethanes at fluorinated chromia aerogels and fluorinated zinc(II) or magnesium(II) doped chromia aerogels

The preparation and characterisation of two series of fluorinated chromia aerogel materials, lightly doped with zinc(II) or magnesium(II), are described. They behave as heterogeneous catalysts for transformations of 1,1,2-trichlorotrifluoroethane under HF-free conditions and at moderate temperatures. Product distributions depend critically on the contact time. When the latter is very long (static conditions) the surface becomes chlorinated, notwithstanding its fluorinated nature. Rather surprisingly, in view of previous work, the catalytic behaviour of the materials is almost identical to that of the undoped fluorinated chromia aerogel. The nature of reactions occurring under HF-free conditions and low levels of doping achievable are discussed as possible factors in determining catalytic behaviour.

The behaviour of chlorofluoroethanes on β-aluminium(III) fluoride: a [36Cl] radiotracer study

Abstract A [ 36 Cl ] radiotracer study of the behaviour of 1,1,2-trichlorotrifluoroethane on β-aluminium(III) fluoride at elevated temperature indicates that the isomerisation of CCl2FCClF2 to CCl3CF3 occurs by an intramolecular process. Isomerisation is followed by dismutation of CCl3CF3 to give CCl2FCF3 and CCl3CClF2. In neither reaction, surface Al–Cl groups are formed. The compound CCl3CClF2 undergoes further reaction, readily, apparently also via dismutation processes.