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Luque et al. have also published research on a range of different Starbon acids, with sulphonic and carboxylic groups attached, as well as ZnCl₂ and BF₃ adsorbed onto the surface [29]. They characterised the nature of the acidity by pyridine titration and found that the sulphonic acid systems had both Brønsted and Lewis acidity. In contrast, the two Lewis acid‐doped materials had a greater Lewis acidity (as expected) but an unexpectedly low quantity of acid sites. The various materials were tested in the acetylation of 5‐acetyl methyl salicylate, and were found to be very active in the O‐acetylation, with Friedel Crafts acylation being absent despite the presence of Lewis acidic sites (Figure 3.8a). Alkylation of phenol with cyclohexene was also investigated (Figure 3.8b). While O‐alkylation was very dominant, especially after short reaction times, C alkylation (mainly in the ortho position) increased upon prolonged reaction. The reasons for this are not discussed in the paper, but similar behaviour is seen with some other catalysts. This may be due to a change in the nature of the catalyst with time, or more likely, to a slow Fries rearrangement reaction following a rapid O‐alkylation.

Further alkylations (of toluene and xylenes) with benzyl chloride have also been successfully carried out using Starbon‐400‐SO₃H under microwave conditions. High yields (70–95%) were achieved after 15 minutes for toluene and p‐xylene, while longer times were required for m‐xylene and, surprisingly, anisole [17].

Figure 3.8 Friedel Crafts reactions catalysed by a range of Starbon acids. (a) O‐Acylation of a phenol and (b) O‐alkylation of a phenol.