Catalytic Performance of Nanoscopic, Aluminium Trifluoride‐Based Catalysts in the Synthesis of (all‐rac)‐α‐Tocopherol

Abstract

A novel nanoscopic, partly hydroxylated aluminium fluoride was prepared in a sol-gel fluorination synthesis and characterised by IR probe molecules. It was shown that this material has, in contrast to high surface area aluminium fluoride (HSAlF 3 ) which is one of the strongest Lewis acids, a low amount of strong Lewis acid sites combined with weak and medium strength Brønsted sites. Both materials were tested in the synthesis of (allrac)-a-tocopherol through the condensation of 2,3,6-trimethylhydroquinone (TMHQ) with isophytol (IP). The activity data indicate the partly hydroxylated aluminium fluoride to be a very efficient and highly selective catalyst for (all-rac)-a-tocopherol (> 99.9%, for an IP conversion of 100%) whereas high surface area aluminium fluoride is a poor catalyst for this reaction.

Sol–gel prepared nanoscopic metal fluorides – a new class of tunable acid–base catalysts

“…6. Thus, the simultaneous fluorolysis and hydrolysis reaction leads to partly hydroxylated aluminium or magnesium fluorides, respectively, as has been already demonstrated [9,23]. The amount of the hydroxyl groups can be adjusted to very small numbers (x < 0.1) confirming that the fluorolysis is the dominant reaction.…”

“…The amount of the hydroxyl groups can be adjusted to very small numbers (x < 0.1) confirming that the fluorolysis is the dominant reaction. However, as a big surprise these hydroxyl groups are Brønsted acidic in nature [9,23]. That is astonishing because we would normally expect the hydrolysis of an Mg-OCH 3 bond to result in a Brønsted basic Mg-OH group as is commonly known for magnesium hydroxide [20,21,24].…”

“…12 [40]), quinones (formed in the presence of traces of oxygen in the reaction system [9,41]) and chromene (formed through a quinone-chromene rearrangement equilibrium, in the presence of basic centers [42]). These by-products lead to a decreased yield of tocopherol, and a decreased atom economy of the reaction.…”

“…It has been shown that the Lewis acidity of HS-AlF 3 is too strong for several organic reactions resulting in the formation of by-products [9]. On the other hand, the Lewis acidity of HS-MgF 2 is too weak to provide sufficient activity [10].…”

Investigating acidity of metal fluoride surfaces by spectroscopic and chemical methods

“…In a very recent example [60], a series of materials was prepared by introducing different quantities of water into the non-aqueous sol-gel synthesis. Examination of the surface acidity of the resulting HS-AlF 3Àx (OH) x compounds using CO and lutidine (lut) as IR basic probes has led to the conclusion that this series has a relatively small proportion of strong Lewis sites coupled with higher proportions of Brønsted sites having weak or medium strength.…”