Properties of ionic liquid solvents for catalysis

https://doi.org/10.1016/j.molcata.2003.11.029Get rights and content

Abstract

Ionic liquids are good solvents for catalytic reactions. The rational selection of the appropriate ionic liquid solvent for a particular reaction requires general knowledge of the properties of ionic liquids, and the details of some properties of the specific ionic liquid solvents being considered. The solvent properties of ionic liquids that are relevant to catalysis are discussed, and sources of the values of those properties for ionic liquids are identified. A roadmap for the literature values of density, viscosity, melting and glass transition temperatures, thermal stability, empirical solvent parameters, absorption, toxicity, surface tension, heat capacity, and thermal conductivity is provided.

Ionic liquids are good solvents for catalytic reactions. The rational selection of the appropriate ionic liquid solvent for a particular reaction requires general knowledge of the properties of ionic liquids, and the details of some properties of the specific ionic liquid solvents being considered. The solvent properties of ionic liquids that are relevant to catalysis are discussed, and sources of the values of those properties for ionic liquids are identified.

  1. Download : Download full-size image

Introduction

The increased interest in ionic liquids by chemists and technologists clearly is due to the utility of ionic liquids as solvents for reaction chemistry, including catalytic reactions. The desire for “green” solvents for industrial processes is partially responsible for this, but also many chemists now realize that ionic liquids offer some unique properties as solvents. Furthermore, ionic liquid solvents have the prospect for custom design of the solvent to meet specific requirements for a particular reaction type. Some barriers to the use of ionic liquids are the bewildering variety of ionic liquids reported in the literature, and the lack of a handbook of these new solvents.

There are perhaps about five common types of cations, and a similar number of anions that comprise most ionic liquids. Salts built on the 1,3-dialkylimidazolium cation (imidazolium) and various polyatomic anions (e.g., AlCl4, BF4, PF6, (CF3SO2)N=Tf2N) are the most commonly used ionic liquids reported since an early paper on these types of materials in 1982 [1]. There is no generally accepted shorthand notation for ionic liquids. I will use the fairly common notation best understood by example: 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide as bmim[Tf2N]. Structural variations of those types of cations and anions permute to thousands of ionic liquids that can be readily prepared by simple procedures described in the literature. If one considers the prospect of binary and ternary mixtures, which are common in high temperature inorganic molten salts, there are trillions of permutations. Fortunately, a relatively small number of ionic liquids are commonly employed as reaction solvents. Even so, the few dozen of the common salts span a wide range of physical and solvent properties. Ideally we would like to have available the ionic liquid equivalent of the classic solvent handbook by Riddick et al. [2]. In this paper, I will identify the solvent properties particularly important to catalytic reaction chemistry, then point to sources for values of those properties. The reader will not find an exhaustive compilation of properties, but rather a roadmap to be used to find those properties in the literature. Only single salts (not binary or higher mixtures) are considered here, in order to reduce the complexity of the discussion.

The usefulness and scope of ionic liquids as solvents and in organic reactions and catalysis has been reviewed sufficiently recently [3], [4]. It is especially instructive to read the table of 51 different reactions in ionic liquid solvents compiled by Olivier-Bourbigou and Magna [4]. The large majority of reactions employed ionic liquids with the 1,3-dialkylimidazolium cation. Reichardt, in his book on solvents for organic reactions, states the generally accepted rule for selecting a solvent for a reaction, “… maximum working range between melting and boiling point, good solubility of reactants and products (or sometimes insolubility of the products), compatibility with the analytical methods employed, and usually high degree of purity” [5]. Other solvent properties are important as well, but it is clear that ionic liquids often meet the basic criteria for an appropriate solvent.

Section snippets

Properties of solvents for catalysis

Solvents may be classified in several ways. One finds solvents organized by chemical constitution, by acid–base behavior, by solute–solvent interactions, or by properties. This paper is organized by properties. Several well known compilations of solvent properties provide the complete list of solvent properties in Table 1 [2], [5], [6]. See the references within the table for details about those properties.

Not all of those properties are equally important to catalytic reactions, and some are

Prediction of properties

Even at this early stage of development of ionic liquids the task of measuring and compiling all properties of all ionic liquids is a daunting challenge. Fortunately it may be possible to predict some of the important properties of ionic liquids; either from first principles or by structure–property correlations. Katritzky and co-workers have developed the strategy and software for predicting properties of many materials based on quantitative structure–property relationships (QSPR). Recently

References (52)

  • H Olivier-Bourbigou et al.

    J. Mol. Catal.

    (2002)
  • J.S Wilkes et al.

    Inorg. Chem.

    (1982)
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of chemistry, Organic Solvents, Physical Properties and Methods of...
  • P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2003, pp....
  • C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, third ed., Wiley-VCH Verlag, Weinheim, 2003, p. 419....
  • A.J. Gordon, R.A. Ford, The Chemist’s Companion—A Handbook of Practical Data, Techniques, and References, Wiley, New...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • A.J. Gordon, R.A. Ford, The Chemist’s Companion—A Handbook of Practical Data, Techniques, and References, Wiley, New...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, third ed., Wiley-VCH Verlag, Weinheim, 2003, p. 62....
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • E. Meyer, Chemistry of Hazardous Materials, Prentice-Hall, Englewood Cliffs, 1977, pp....
  • E. Meyer, Chemistry of Hazardous Materials, Prentice-Hall, Englewood Cliffs, 1977, pp....
  • C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, third ed., Wiley-VCH Verlag, Weinheim, 2003, p. 500....
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • J.A. Riddick, W.B. Bunger, T.K. Sakano, Techniques of Chemistry, Organic Solvents, Physical Properties and Methods of...
  • Cited by (804)

    • Enzymatic preparation of mono- and diacylglycerols: A review

      2023, Grain and Oil Science and Technology
    • Biomedicine meets nanozyme catalytic chemistry

      2023, Coordination Chemistry Reviews
    View all citing articles on Scopus
    View full text