Biphasic hydroformylation catalysis in ionic liquid media

Abstract

The ionic liquids 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF 4]), 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim][PF 6]) and 1,2-dimethyl-3-butyl-imidazolium hexafluorophospate ([bdmim][PF 6]) were investigated as potential media for hydroformylation catalysis in a liquid–liquid biphase reaction environment. The study used the known rhodium/tppti* (tppti* = tri( m-sulfonyl) triphenyl phosphine 1,2-dimethyl-3-butyl-imidazolium salt) system as the catalyst and hexene-1 as the substrate, which enabled the produced heptanal to be conveniently isolated from the ionic liquid by phase separation. All catalyst evaluations were carried out in high-purity ionic liquids to avoid any decomposition of the catalyst, substrate and/or ionic liquid. The activity of the biphasic ionic liquid system containing the Rh/tppti* catalyst was approximately one order of magnitude lower than that observed for the conventional Rh/PPh 3 catalyst in toluene. The Rh/tppti* catalyst system showed a decrease of activity in the semicontinuous hydroformylation of hexene-1 due to catalyst deactivation and/or metal loss. The n/ i ratio of the produced heptanal only slightly increased after changing the Rh/P ratio from 1:3 to 1:100 in [bmim][PF 6]. The loss of rhodium metal from the ionic liquid phases appears to be correlated to the solubility characteristics of the ionic liquid and the concentration of the aldehyde in the organic phase. In support of this theory, the ionic liquid [bmim][PF 6] gave the best results due to its very low miscibility with polar substances. A high-pressure NMR (HP-NMR) study revealed that the solution structure of HRh( 13CO)(tppts) 3 (tppts = tri( m-sulfonyl)triphenyl phosphine sodium salt) in [bmim][BF 4] was similar to that of HRh( 13CO)(PPh 3) 3 in toluene-d 8. The HP-NMR investigation at elevated syngas pressures showed that HRh( 13CO)(tppts) 3 lies in equilibrium with HRh( 13CO) 2(tppts) 2.