In situ FTIR investigation of the solubility and swelling of model epoxides in supercritical CO2
Graphical abstract
Highlights
► An efficient in-situ FTIR method for the mutual solubility of epoxides and CO2. ► The solubility of propylene, styrene and cyclohexene oxide in supercritical CO2. ► CO2sorption and the resulting swelling of the epoxide rich phase. ► The pressure-composition phase diagram of these epoxide/CO2 mixtures are reported.
Introduction
The use of carbon dioxide both as a substitute to harmful solvent systems and as an environmentally friendly chemical reactant has been the subject of a particular attention in the last years [1], [2]. Indeed, CO2 has the advantage to be non-toxic, non-flammable, chemically inert, inexpensive and easily available. A number of reactions using CO2 as a raw material have been demonstrated and CO2 is especially useful as a phosgene substitute [3]. In view of green chemistry and atom economy, synthesis of cyclic carbonates via the coupling of CO2 with epoxides is one of the most promising methodologies in this area since CO2 can be incorporated without forming any co-products [4], [5]. In this context, we have recently investigated the nature and strength of the molecular interactions occurring between epoxides and CO2 by combining infrared spectroscopy with quantum chemistry calculations and we have shown that the Lewis acid–Lewis base interactions occurring between epoxides and CO2 were mainly governed by the partial charge on the oxygen atom of the epoxide [6]. In order to take place, the reaction between epoxides and CO2 must be catalyzed [7], [8], [9], [10], [11], [12], [13], [14], [15] and performed at high temperature. Through these literature data, it appears that for a given catalyst, yields obtained can be highly variable depending on the nature of the epoxide, the temperature–pressure conditions and the solvent. We emphasise that the reactions are often carried out under solvent-free conditions as it provides an environmentally benign and economically valuable process for the synthesis of cyclic carbonates. Furthermore, the use of traditional solvents is not always effective for the reaction due to the low solubility of CO2 in conventional solvents. An alternative is to use an ionic liquid as both solvent and catalyst exploiting the high solubility of CO2 in ionic liquids. Thus, several studies have been done using these systems, wherein, higher yields of cyclic carbonates were obtained. Finally works have been introduced in supercritical CO2 environment proving that it gives higher yields [8], [10], [16]. In this case, a single-phase mixture is obtained and composed of supercritical carbon dioxide, an epoxide, and a catalyst designed to exhibit carbon dioxide solubility. Supercritical carbon dioxide, present in excess, serves as the solvent and as a reactant. However, supercritical carbon dioxide and epoxides are not completely miscible at all conditions and, depending on the pressure–temperature conditions, the mixture can be in one or two phases. It is expected that the yields of the reaction strongly depend upon the phase behavior as one phase conditions would allow better mass transport and higher efficiency of the catalyst. Therefore, for the successful development of such a process, phase behavior plays a significant role. Unfortunately, they are very few data available currently on the thermodynamic behavior of epoxide–CO2 binary mixtures. Only few literature source reported on the phase equilibrium of propylene oxide–CO2 [17], [18] and 1,2-epoxycyclohexane–CO2 [19], [20] binary system in a limited temperature–pressure range.
These considerations prompted us to investigate the phase behavior of several epoxide molecules–CO2 binary mixtures. To this aim, we have chosen to use in situ IR transmission spectroscopy for a quantitative determination of the concentration of epoxides and CO2 in both the epoxide rich phase and the CO2-rich phase. The method, described in detail below, has been successfully applied previously by our group and others for the determination of the CO2 sorption and swelling of polymers by scCO2 [21], [22]. In particular, we remember that molar absorption coefficients of CH stretching vibrational modes and combination bands are expected to exhibit little sensitivity upon temperature and pressure conditions [21], [23], [24]. For example, Buback et al. [25] have shown that the molar absorption coefficient of combination bands of CO2 were almost independent of the CO2 density. Therefore, IR spectroscopy allows to determine the concentration of a given specie in a mixture with a statistical error lower than 10%. Besides, the use of in situ IR spectroscopy makes it possible to obtain information on the nature of the solute–CO2 intermolecular interactions [26], [27] and to detect impurities which could be present in the mixture, inducing potentially a non negligible effect on the phase behavior of the binary system.
Using this methodology, we have investigated three molecules widely used in the synthesis of cyclic carbonates: propylene oxide (PO), styrene oxide (SO) and cyclohexene oxide (CO). We present in this paper the change, as a function of pressure and temperature, of the epoxide and CO2 concentrations in the two phases and the two-phase boundaries of the investigated carbon dioxide–epoxide mixtures at 40, 70 and 100 °C for a range of pressures between 2 and 20 MPa.
Section snippets
Chemicals
We have investigated three epoxide molecules commercially available from the “Aldrich” company that are widely used in the cyclic carbonate synthesis: propylene oxide (PO) (purity: ≥99%), styrene oxide (SO) (purity: 97%) and cyclohexene oxide (CO) (purity: 98%). Carbon dioxide N45 (purity 99.95%) was supplied by Air Liquide.
Infrared set-up
The infrared absorption measurements were performed on a Biorad interferometer (type FTS-60A) equipped with a dual source capability (a globar and a tungsten halogen
Infrared absorption spectra
The infrared spectra of the epoxide/CO2 mixtures were recorded at three different temperatures (40,70 and 100 °C) for various pressures ranging between 0.5 and 20 MPa for both the epoxide-rich phase and the CO2-rich phase. Fig. 2 illustrates (for the case of SO) the spectral changes in the CO2-rich phase that occur with an increase of CO2 pressure. A number of significant peaks associated with combination bands of CO2 and fundamental modes of SO can be observed. Concerning CO2, one can detect
Conclusion
The phase behavior of carbon dioxide/epoxide mixtures has been investigated for three different temperatures (40, 70 and 100 °C) for pressures ranging between 0.1 and 20 MPa. The measurements have been performed using an efficient in situ FTIR method that allows us determining, as a function of temperature and pressure, the change of the concentration of each component in the two phases (CO2-rich phase and epoxide-rich phase). Thus, we have determined the solubility of propylene oxide (PO),
Acknowledgment
The authors acknowledge the “Agence Nationale de la Recherche” (ANR-09-CP2D-15-04) for the PhD fellowship of S. Foltran.
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