During the last decades motor gasoline formulation has been forced by legislative requirements to make this still important transportation fuel more environmentally friendly. The effects of gasoline combustion on the environment are directly related to its properties and composition. Above all, the emissions of carbon monoxide, hydrocarbons, SOx, and NOx have to be reduced in the exhausts of vehicles. The clean air regulation in the E.U. and the U.S.A., concerning the contents of alkenes, sulfur, nitrogen and aromatics, especially of benzene, in the gasoline will become increasingly strict 1. Hence, the different blending compounds for gasoline must be reevaluated from an environmental point a view. The FCC gasoline (mainly aromatic and unsaturated HCs) and the catalytic reformate (mainly aromatic HCs) are still the most important blending components for petrol. Nevertheless, FCC gasoline is a major contributor of sulfur and olefins whereas reformate is the main source of benzene and aromatics. Substitutes, namely MTBE or other ethers, which were established in order to make the gasoline more environmentally benign, have been found to contaminate drinking water and will be removed at least in North America 1. Alcohols (e.g. ethanol) proposed as alternative oxygenates exhibit a very high blending vapour pressure when mixed into gasoline. Hence, this enhancement of vapour pressure reduces their broad usage in gasoline.
In contrast to FCC gasoline and reformate, alkylate shows considerable advantages as a blending component. Alkylate offers a high octane number, a low Reid vapour pressure (RVP) and low octane sensitivity (difference of research octane number (RON) and motor octane number (MON)). It contains no aromatics and alkenes and nearly no sulfur. So it is an ideal blending component for gasoline [3, 4]. The world-wide installed alkylation production capacity is approx. 102 million tons/year. This amount of alkylate only fulfills about 10 % of the total amount of gasoline required all over the world.
Although the products from alkylation are of high quality, the catalysts used up to now in the technical processes are not ideal. Currently, the catalysts industrially employed are mainly sulfuric acid or anhydrous hydrofluoric acid. Therefore, the wide application of the alkylation process is restricted because of the high toxicity and corrosiveness of both acids [5, 6]. Numerous alternative catalysts, most of which are solid acids (e.g. zeolites, sulfate-promoted metal oxides) have been investigated [7 -13]. However, they have not yet achieved technical application as alkylation catalysts because of rapid catalyst decay by coking [14 - 16]. Therefore, an extensive research was carried out during the past decades in the industry and in academic research laboratories to overcome this problem.
An important requirement for a new alkylation catalyst is a very strong acidity and thus so-called ionic liquids could be an attractive option.
During the last decades ionic liquids (ILs) were introduced as a new class of liquid material [17 - 20]. ILs are defined as salts melting below 100 °C. They have in general no (or a practically not measurable) vapour pressure and thus a loss by evaporisation is impossible. Besides of a lot of other very unique and valuable properties, ILs can be prepared to have a high acidity. Therefore, they are intensively discussed as environmentally friendly and less hazardous solvents or catalysts in industry [21 - 24].
Therefore, some acidic ionic liquids ([BMIM]Cl/AlCl3, [CnMIM]X/AlCl3, and Et3NHCl/AlCl3) were already investigated as a new source for environmentally friendly catalysts for the alkylation of iso-butane with butenes [25 - 28]. Furthermore, the acidity can be tuned in the range from low acidic to superacidic systems 29.
The published results showed that alkylation catalysed by highly acidic ILs can be performed successfully but with a lower selectivity and alkylate quality compared to the up to now industrially used catalysts (HF, H2SO4). The results indicate that ionic liquids possessing a high acidity would be suitable as alkylation catalysts. During the final preparation of this work, an industrial plant for alkylation based on superacidic ILs was started in 2006 in China which does show the high potential of ILs for this process 30.
Therefore, it was the subject of this work to study acidic ionic liquids as alternative catalysts in the alkylation reaction. The focus of the thesis was directed to the tuning of the required acidity concerning the product quality and the stability of the catalytic systems. In the ideal case only highly branched hydrocarbons with a boiling point in the gasoline range and thus with a high octane number should be formed.