The catalytic properties of sulfated hematite in toluene disproportionation

Abstract

Iron oxides are important compounds in science and technology. Due to their structural, electronic, magnetic and catalytic properties they are useful for several applications. Among them, sulfated hematite (SO4 2- /αFe2O3) is especially interesting since it shows unusual acidic properties and then behaves as a superacid solid. It is catalytically active in a large variety of reactions, such as alkane isomerization, Friedel–Crafts acylation of toluene, polymerization of ethers, benzoylation of toluene with benzoyl chloride, esterification of alcohol with acetic acid and benzylation of toluene with benzyl chloride, among others. Due to its acidic properties, sulfated hematite is also expected to be active catalyst in toluene disproportionation, an important route to obtain xylenes and benzene, high value chemicals. In order to find new catalysts for toluene disproportionation, the preparation of sulfated hematite was studied in this work. Two precursors were prepared to get the samples: iron (III) hydroxide and iron (III) hydroxoacetate, which were further heated under nitrogen. The samples were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, acidity measurements by temperature programmed desorption of ammonia, sulfur and carbon analyses, specific surface area measurements and Mössbauer spectroscopy. The catalysts were evaluated in toluene disproportionation at 470 °C and 1 atm, using a reaction mixture with 98% benzene and 2% 1,2,4-trimethylbenzene whose composition is close to an industrial stream, coming from naphtha reforming or naphtha pyrolysis. Different phases of iron oxides were produced, depending on the precursor. The sulfate species increased the specific surface areas and the acidity of hematite, regardless the phases produced. The iron (III) hydroxoacetate precursor was found to be the most convenient one to produce the most acid hematite with the highest specific surface area. All catalysts were active in toluene disproportionation and were selective to benzene, ethylbenzene and xylenes isomers. The sulfated catalyst prepared with iron (III) hydroxoacetate is the most selective to xylenes while both sulfate solids were selective to benzene. Therefore, iron (III) hydroxoacetate is a more efficient precursor, as compared to the traditional iron (III) hydroxide precursor, to prepare iron oxide with high specific surface areas and with more and stronger acid sites. This solid can be conveniently used to produce xylenes and benzene, high value products in chemical and petrochemical industries.