Industrial application of Nafion-systems in rearrangement-aromatisation, transesterification, alkylation, and ring-closure reactions

Abstract

Nafion, a perfluorinated sulphonic acid ion-exchange polymer, is known to be a very strong Brønsted acid. Thus, Nafion NR 50 and Nafion/SiO 2 with 15 wt.% Nafion-loading were selected, in order to elucidate the potential for rearrangement-aromatisation of ketoisophorone (KIP) to 2,3,5-trimethylhydroquinone diacetate (TMHQ-DA) in presence of acetic acid anhydride as acylating agent, transesterification of TMHQ-DA to 2,3,6-trimethylhydroquinone monoacetate (TMHQ-1-MA) and reaction of isophytol (IP) with trimethylhydroquinone (TMHQ) to (all- rac)-α-tocopherol. For the rearrangement-aromatisation of KIP to TMHQ-DA supported Nafion/SiO 2 was markedly more active than the unsupported Nafion NR 50. Both Nafion-systems generally revealed remarkably high selectivity, which ranged up to 94 GC-a% TMHQ-DA at high conversion. The major side-product was 3,4,5-trimethylcatechol diacetate. In case of Nafion/SiO 2, pre-treatment under vacuum and especially grinding of the extrudates seemed to increase activity at comparably high selectivity. Recycling Nafion/SiO 2 after filtering and rinsing with acetic acid anhydride led to gradually decreasing activity. Without intermediate isolation of TMHQ-DA, Nafion/SiO 2 was active and selective for the formation of TMHQ-1-MA. Using the Nafion systems in the reaction of IP and TMHQ revealed that remarkably high conversion of IP (>95%) and compared with zinc chloride/Brønsted acid or BF 3-catalysed reaction good yields (≈92%) and selectivities were obtained. We found a strong dependency on the solvent polarity. In further experiments, the recovery of the catalyst was tested.