High-Pressure Phase Behavior of Ternary Systems Containing Ionic Liquids + Carbon Dioxide + Organometallic Compounds

It is expected that future chemical engineering processes will use more environmentally benign solvents as reaction and separation media. For instance, a new method has been recently developed based on miscibility switch phenomena, in which reactions and separations are carried out efficiently in one step in presence of alternative solvents i.e., ionic liquids (ILs) and supercritical carbon dioxide (scCO₂)[1]. In this process, the reaction can be performed in a homogenous phase, which location depends on pressure, temperature and CO₂ concentration. After the completion of the reaction, changing the conditions results in a multi-phase system in which one of the phases is substantially free of IL from which the product can be recovered.

      It is our aim to apply the above-mentioned process set-up on the production of acetylferrocene (AcF), which can be synthesized from ferrocene (Fc) by the Friedel-Crafts reaction [2]. This reaction has been identified as playing a significant role in many applications of organometallic chemistry and materials science, and in asymmetric and electro-catalysis [3,4]. Conventionally, this reaction is carried out in toxic and volatile organic compounds such as dichloromethane or carbon disulfide, resulting in environmental and human health risks [5]. Moreover, corrosion issues and considerable quantities of harmful waste can be confronted depending on the Lewis acid used as catalyst. Recently, comprehensive investigation of the acetylation of Fc to AcF was conducted in the presence of ILs as solvent instead of the conventional organic solvents [6]. The promising results showed that up to 100% conversion and up to a 100% yield could be reached using imidazolium-based ILs as solvents with scandium triflate as the catalyst. Here, we intend to add scCO₂ to carry out the reaction at a higher rate in a homogeneous phase, and use the scCO₂ subsequently as separation medium.

In order to determine the operational conditions for applying the new process, accurate phase behaviour data are essential. Currently, phase behavior data for systems consisting of Fc or AcF in presence of ILs and/or scCO₂ are not available in the literature. The aim of this work is to measure the binary and ternary solubility data of the above-mentioned metal compounds in scCO₂ and ILs.

The phase behavior of the binary system consisting of Fc/AcF + scCO₂ was measured by using an analytical method with a quasi-flow apparatus at (308, 318, 328, 338 and 348) K and pressures between (8 and 24). High-performance liquid chromatography (HPLC) was applied through an online sampling to determine the solubility of Fc/AcF in the scCO₂ [7].

Ternary phase behavior data of systems containing Fc/AcF + [bmim][Tf₂N] + CO₂ were determined experimentally by a synthetic method using the Cailletet apparatus. This equipment allows measurements of phase equilibria within a pressure range of (0.1 to 15 MPa) and temperatures from (255 to 470 K) depending on the heat transferring fluid used. Using a Cailletet apparatus, it is possible to keep temperature (or pressure) at desired values, and the pressure (or the temperature) is varied until a phase change is visually observed for a sample with a constant overall composition [8].

The solubility of Fc and AcF at 308 K is investigated. The experimental molar solubility of the product AcF in scCO₂ ranged from 2.5 to 44.5 · 10^-4 at 308 K, which is sufficiently high for separation using the new process set-up. Moreover, it can be noticed that the solubility of the product AcF in scCO₂ is higher than the solubility of the reactant Fc in scCO₂, indicating that it is suitable solvent for separation. Results at other temperatures and the existence of a crossover area will be presented.

Phase behavior measurements of binary and ternary systems consisting of the organometallic compounds AcF and Fc in combination with ILs and CO₂ have been carried out. The solubility of AcF and Fc in CO₂ ranges from 2.5 to 44.5 · 10^-4, and from 8.9 to 15.6 · 10^-4, at 308 K, respectively. The results of the ternary Fc + [bmim][Tf₂N] + CO₂ system shows  crystallizing the Fc in lower temperatures. The results are used to find the optimal operating conditions for the reaction of Fc into AcF in IL + CO₂ systems and for the subsequent separation. References

[1] M. C. Kroon, et. al., International Patent WO 2006/088348 A1, (2006)

[2] R. Ojani, J. B. Raoof,B.  Norouzi, Electroanalysis, 20 (12), 1378-1382, (2008)

[3] R. Gomez, et. al., Angew. Chem. Int. Ed., 45, 7674-7715, (2006)

[4] Z. Gao, et. al., Appl. Organomet. Chem., 19, 1149-1154, (2005)

[5] R. J. Hu, et. al.,  Tetrahedron Letters., 49, 387-389, (2008)

[6] S. Berardi, et. al., J.  Org. Chem., 693, 3015–3020, (2008)

[7] H. Perrotin-Brunel, et. al., J. Supercrit. Fluids, 52, 6-10 (2010)

[8] S. Raeissi and Cor J. Peters, J. of Supercritical Fluids, 35, 10–17 (2005)