Continuous flow hydroformylation using supported ionic liquid phase catalysts with carbon dioxide as a carrier

Abstract

A supported ionic liquid phase (SILP) catalyst prepared from [PrMIM][Ph(2)P(3-C(6)H(4)SO(3))] (PrMIM = 1-propyl-3-methylimidazolium), [Rh(CO)(2)(acac)] (acacH = 2,4-pentanedione) [OctMIM]NTf(2) (OctMIM = 1-n-octyl-3-methylimidazolium, Tf = CF(3)SO(2)) and microporous silica has been used for the continuous flow hydroformylation of 1-octene in the presence of compressed CO(2). Statistical experimental design was used to show that the reaction rate is neither much affected by the film thickness (IL loading) nor by the syngas:substrate ratio. However, a factor-dependent interaction between the syngas:substrate ratio and film thickness on the reaction rate was revealed. Increasing the substrate flow led to increased reaction rates but lower overall yields. One of the most important parameters proved to be the phase behaviour of the mobile phase, which was studied by varying the reaction pressure. At low CO(2) pressures or when N(2) was used instead of CO(2) rates were low because of poor gas diffusion to the catalytic sites in the SILP. Furthermore, leaching of IL and Rh was high because the substrate is liquid and the IL had been designed to dissolve in it. As the CO(2) pressure was increased, the reaction rate increased and the IL and Rh leaching were reduced, because an expanded liquid phase developed. Due to its lower viscosity the expanded liquid allows better transport of gases to the catalyst and is a poorer solvent for the IL and the catalyst because of its reduced polarity. Above 100 bar (close to the transition to a single phase at 106 bar), the rate of reaction dropped again with increasing pressure because the flowing phase becomes a better and better solvent for the alkene, reducing its partitioning into the IL film. Under optimised conditions, the catalyst was shown to be stable over at least 40 h of continuous catalysis with a steady state turnover frequency (TOF, mol product (mol Rh)(-1)) of 500 h(-1) at low Rh leaching (0.2 ppm). The selectivity of the catalyst was not much affected by the variation of process parameters. The linear:branched (l:b) ratios were ca. 3, similar to that obtained using the very same catalyst in conventional organic solvents.

Biphasic and Flow Systems Involving Water or Supercritical Fluids

“…5) [11]. Pressure studies show [40] that the best reactivity is obtained at 100 bar, just below the critical pressure of the mixture (106 bar for 1-octene under the same flow conditions). This implies that a CO 2 expanded liquid is the best medium for carrying out such reactions.…”

Sustainable catalytic reaction engineering with gas-expanded liquids

“…leaching problem, the continuous hydroformylation of 1-octene using a SILP-supported catalyst was successfully carried out in a fixed bed reactor using non-polar, dense CO 2 as the mobile phase at conditions below the 'critical' point of the 1-octene/CO 2 mixture (the aldehyde products are likely to be less miscible), where a highly expanded liquid phase is still present [46]. However, increases in the CO 2 pressure are believed to have reduced the partitioning of the reactants into the SILP phase and led to decreases in the reaction rate.…”

Efficient and selective chemical transformations under flow conditions: The combination of supported catalysts and supercritical fluids

“…The system has been further improved through the use of different Xantphos-related ligands containing imidazolium subunits to favor the solubility in the IL phase [62]. A system involving the immobilization of the catalyst in a film of an IL supported on silica (SILP) has also been described with either scCO 2 [63] or in the gas phase [64]. The results obtained for the hydroformylation in SILP–scCO 2 revealed the achievement of higher rates, as compared with the bulk-IL–scCO 2 system, reaching TOF values of 800 h −1 and more than threefold improved STYs.…”