Article pubs.acs.org/jced
Ultrasonic Properties of Hydrophobic Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids Gregory Chatel,†,‡ Laurent Leclerc,§ Emmanuel Naffrechoux,† Corine Bas,§ Nathalie Kardos,† Catherine Goux-Henry,‡ Bruno Andrioletti,*,‡ and Micheline Draye*,† †
Laboratoire de Chimie Moléculaire et Environnement, Université de Savoie, Campus scientifique, Le Bourget du Lac Cedex, 73376, France ‡ Institut de Chimie et Biochimie Moléculaire et Supramoléculaire (CASYEN), UMR-CNRS 5246, Université Claude Bernard Lyon 1, Bâtiment Curien (CPE), 43, boulevard du 11 Novembre 1918, Villeurbanne Cedex, 69622, France § LEPMI, UMR 5279, CNRSGrenoble INPUniversité de SavoieUniversité J. Fourier, Bât. IUT, Campus scientifique, Le Bourget du Lac Cedex, 73376, France S Supporting Information *
ABSTRACT: Energy conversion in sonochemistry is known to be an important factor for understanding the mechanism of chemical reactions. However, ultrasound characteristics are dependent on the physical properties of the liquid medium. This work describes specific heat capacities (cp) of five hydrophobic bis(trifluoromethylsulfonyl)imide based ionic liquids ([C 8 mpyrr][NTf 2 ], [C 4 mpyrr][NTf 2 ], [C 8 epip][NTf 2 ], [C8mim][NTf2], [C8mpy][NTf2]) and, for the first time, the acoustic power when they are submitted to ultrasound at two selected electric powers. The behavior of ultrasound in ionic liquids is then compared to the one in water. Despite the difference in cp values, the measured acoustic powers are very similar for both media. Indeed, ionic liquids heat up faster than water, and this can lead to interesting effects when using ionic liquids as solvents in organic chemistry.
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INTRODUCTION Ultrasound-promoted synthesis has attracted much attention during the past few decades. Indeed, ultrasound is known to enhance some processes through a physical phenomenon called acoustic cavitation which is the formation, growth, and collapse of micrometer-sized bubbles when a pressure wave of sufficient intensity propagates through an elastic liquid.1,2 By imploding, these bubbles create locally extreme temperatures and pressures that lead to high-energy radical reactions but also generate some interesting physical effects.3,4 The use of sonochemistry has grown up in the recent years, resulting in a significant body of empirical research. Indeed, the effect of ultrasound on the chemical reaction is dependent on a large number of sonochemical parameters.5 Masson and Cordemans de Meulenaer have given 10 recommendations/ steps for the optimization of ultrasonic processes.6 In reality, it is very difficult to reproduce ultrasound-promoted experiments reported in the literature because of the lack of precise data on ultrasonic experimental conditions. In most cases, the use of ultrasound is shown to improve reactions yield and kinetics, to reduce particle size, and so forth, but few or none acoustic experimental details are accurately given. Therefore, the involvement of ultrasound in these modifications is not systematically and rigorously demonstrated. Traditionally, in a typical protocol in silent conditions, information related to the © 2012 American Chemical Society
activation mode (stirring means) is not precisely described. However, to reproduce experiments, the use of ultrasound requires detailing the sonochemical parameters. In this work, we describe some important parameters of the sonochemistry, classically given in water. In the recent years, the use of ionic liquids has become a topic of much interest. Their application as reaction media for a wide variety of synthetic processes is an area of intense research, and new approaches are proposed for catalyst separation and recycling. Ionic liquids display many interesting properties which make them very attractive for catalysis.7−11 In these almost vaporless, air- and moisture-stable solvents, polar or ionic catalysts can be immobilized without any postfunctionalization, allowing easy recycling. We recently reported a smart epoxidation of olefins under ultrasound and in ionic liquid media.12 We then demonstrated the efficiency of their combination for enantioselective oxidation of olefins using a chiral manganese porphyrin.13 However, the study of the recycling of the catalytic system drove us to investigate the effects of ultrasound on ionic liquids. The literature displays only Suslick’s studies about sonochemistry and sonoluminesReceived: April 17, 2012 Accepted: October 11, 2012 Published: October 24, 2012 3385
dx.doi.org/10.1021/je300377a | J. Chem. Eng. Data 2012, 57, 3385−3390
Journal of Chemical & Engineering Data
Article
cence in an imidazolium-based chloride ionic liquid.14,15 However, the dynamics of cavity growth and implosion of bubbles under ultrasound are strongly dependent on local conditions, in particular physicochemical properties of the liquid medium.16 In this context, the calorimetric method, which consists in measuring the heat dissipated in a volume of liquid, takes into account the heat capacity of this liquid (Cp) by which the acoustical energy is absorbed.17,18 The main assumption of the method is that all absorbed acoustical energy is transformed into heat. To determine the efficiency of sonochemical processes under the acoustic wave, it is necessary to know the amount of acoustical energy introduced and absorbed in the volume of liquid. Indeed, very often the yield is not quantitative, in a range of (20 to 30) % for a low-frequency ultrasonic system, and can reach (60 to 70) % for high frequencies, according to the literature.17 In addition, it is usually very difficult to clearly determine the real effects that ultrasound has on a chemical reaction. In the recent literature, there are only a very few publications ascribing the increase of yield to a specific effect as it is most of the time arguable.4 To understand better the efficiency of sonochemical processes in a volume of five different hydrophobic bis(trifluoromethylsulfonyl)imide (NTf2−) based ionic liquids under the action of ultrasound, our work aims for the first time at a systematic description of the ultrasonic system and of the acoustical energy introduced and absorbed. A comparison with water behavior is also explored.
diameter, 100 mm in height) was used as in previous work,12 with a liquid sonified volume of 3 mL. Calorimetry experiments were carried out in dry ionic liquids (water content
