Infinite dilution activity coefficient measurements of organic solutes in fluorinated ionic liquids by gas-liquid chromatography and the inert gas stripping method

2.1.4. Potential applications of ionic liquids in the chemical industry

The properties of a given ionic liquid are determined by the nature of its ions (Rogers and Seddon 2003). Any desired set of properties can be imparted to an ionic liquid by a proper selection of the anion-cation pair. This tunability of ionic liquids` properties allows a very wide range of applications as shown in the table below.

Table 2-1: Potential applications of ionic liquids in the chemical industry.
(Plechkova and Seddon 2008)

Chemical engineering Electrochemistry Biochemistry

Mass separating agents Electrolyte batteries Biomass processing

Catalyst Metal plating Drug delivery

Coatings Solar panels Personal care

Lubricants Fuel cells Biocides

Plasticizers Electro-optics. Embalming

Dispersing agents

2.1.5. Commercial applications of ionic liquids

Many companies have recently invested considerably in the development of ionic liquids-based commercial processes. Examples of publicly reported achievements regarding industrial uses of ionic liquids, as reviewed by Plechkova and Seddon (2008) are briefly discussed in this section.

2.1.5.1. BASIL Process

The Biphasic Acid Scavenging utilizing Ionic Liquids (BASIL) process is the most popular industrial application of ILs. It has been used by the German company BASF in the production of alkophenyl phosphines. 1-methylimidazolium chloride is used to scavenge the acid formed during the reaction. Results are better than in the original process where triethylamine was used.

2.1.5.2. Isobutene alkylation

Petro China performs isobutene alkylation with an ionic liquid. This is the largest commercial use of ionic liquids.

2.1.5.3. DIMERSOL Process

In this process, IFP (Institut Français du Pétrole) utilizes chloroaluminate (III) ionic liquids for alkenes dimerisation.

2.1.5.4. Other commercial uses

The following success stories have also been reported:

a) Pharmaceutical intermediate production by Central glass Co, Ltd in Japan;

b) 2, 5-dihydrofuran production by Eastman Chemical Company from 1996 to 2004;

c) Production of lithium ion rechargeable battery by Pionics in Japan.

2.1.5.5. Advanced projects

Besides the very small number of established industrial processes, one can list many promising projects, some at pilot plant stage. The most known ones include:

a) Metathesis and olefin trimerisation by SASOL in South Africa;

b) Azeotrope-breaking for water -ethanol and water -tetrahydrofuran with reduced; costs of solvent recovery by BASF;

c) Cellulose dissolution by BASF;

d) Aluminum plating by BASF;

e) Phosgene replacement by an ionic liquid in 1, 4-dichlorobutane production, achieved by BASF.

2.1.6. Use of ionic liquids as solvents in separation processes 2.1.6.1. Ionic liquids versus molecular solvents

Due to properties stated in section 2.1.3, ionic liquids attracted much attention in recent years as potential replacements to conventional molecular organic solvents.

With hundreds of thousands of ion combinations, it is possible to design an ionic liquid with the desired properties to suit a particular application by a proper anion and cation selection. One can adjust and tune ionic liquids to provide a specific property, including density, melting point, viscosity, hydrophobicity, miscibility and selectivity in separation processes. The tunability of ionic liquids provides flexibility for both reaction and combined reaction/separation schemes. And, since thermodynamics and kinetics of reactions taking place in ionic liquids differ from those in conventional solvents, it is clear that ionic liquids open new opportunities for both separation and combined reaction/separation processes. In ionic liquids, ions are held together by wide range interactions such as high coulombic forces and in some cases, hydrogen bondings resulting in near-zero vapour pressure and therefore no emission to the atmosphere. This nonvolatile nature of ionic liquids allows the design and development of new environmentally-friendly separation processes. Another benefit is of course the reduced losses of solvents used in such processes. In addition, ionic liquids being generally non flammable, they should lead to safety benefits compared to molecular solvents commonly used in chemical engineering nowadays. Table 2-2 highlights more advantages of ionic liquids over molecular solvents that have enhanced their popularity.

Table 2-2: Ionic liquids versus molecular solvents (Plechkova and Seddon 2008).

Property Organic solvents Ionic liquids

Number of solvents 1000 1 000 000

Applicability Single function Multifunction

Catalytic ability Rare Common and tuneable

Chirality Rare Common and tuneable

Vapour pressure Obeys the Clausius-Clapeyron Negligible vapour pressure

equation under normal conditions

Flammability Usually flammable Usually nonflammable

Solvation Weakly solvating Strongly solvating

Polarity Conventional polarity concepts Polarity concept questionable

Tuneability Limited range of solvents Virtually unlimited range means

available ==designer solvents«

Cost Normally cheap Typically between 2 and 100

times the cost of organic solvents

Recyclability Green imperative Economic imperative

Viscosity/ Cp 0.2 - 100 22- 40 000

Density/g cm³ 0.6- 1.7 0.8- 3.3

Refractive index 1.3- 1.6 1.5- 2.2

2.1.6.2. Recent scientific investigations

Investigations on the suitability of ionic liquids as separation solvents are reviewed by Heintz (2005). In the last four years, the list of ionic liquids investigated is in steady progression. A large number of data have been experimentally generated to deal with various separation problems (David 2004). Some recent examples are studies on the basis of experimental VLE (Zhao et al. 2006, and Zhang et al. 2009) and LLE (Deenadayalu et al. 2006b and Meindersma et al. 2006) data which are available in the literature. All available experimental infinite dilution activity coefficient data for organic solutes in fluorinated and non-fluorinated ionic liquids can be found in publications listed in appendix A. The chemical quantum approach has also been used in the open literature to deal with problems such as:

a) Olefins-paraffins separation (Lei et al. 2007);

b) Diesel desulphurization (Kumar and Banerjee 2009);

c) Alkanols/ olefins separation (Banerjee 2008).

Patents have been granted for these processes involving ionic liquids as mass separation agents for organic liquid mixtures:

a) Separation of ionic compounds (Roetteger et al. 2003);

b) Separation of close-boiling or azeotropic mixtures (Arlt et al. 2002);

c) Separation of dienes from olefins (Boudreau et al. 2002);

d) Removal of polarizable impurities from hydrocarbons and hydrocarbon mixtures (Wasserscheid et al. 2002);

e) Mercaptans removal from hydrocarbon streams (O` Rear et al. 2001);

f) Extraction of aromatic hydrocarbons from aromatic petroleum streams (Gmehling and Krummen 2001).

All these research works established that ionic liquids are potentially good alternatives to currently used organic molecular solvents. As illustrated by table 2-3, for the separation of aromatic from aliphatic compounds, represented by the n-hexane/benzene and cyclohexane/ benzene systems, many ionic liquids can lead to higher limiting selectivities, and sometimes higher capacities than commonly used molecular solvents such as sulfolane and NMP. Regarding the influence of structure on extraction capacity, it is worthy to mention that only imidazolium and ammonium-based ionic liquids have been widely investigated with respect to aromatic/aliphatic compounds separation problems. It has been found that selectivity for aromatic hydrocarbons/aliphatic hydrocarbons increases with decreasing length of the alkyl chain on the cation (Letcher et al. 2009). Favorable anions are those with small volume and sterical shielding around the charge centre. An investigation on olefins/ paraffins separation problem revealed the same trends (Lei et al. 2006, 2007b). More thermodynamic data is needed

for a better understanding of the effect of structure on the extracting capacities for other types of ionic liquids and separation problems.

Table 2-3: Literature selectivity and capacity data at infinite dilution of selected ionic

liquids, NMP and sulfolane for different separation problems at T = 313.15 K.

Reference

Laskowska (2009)

Marciniak (2008b)

Foco et al. (2006)

Heintz et al. (2001)

Kato and Gmehling

(2004)

Letcher et al. (2005a)

Krummen et al. (2002)

Kato and Gmehling

(2004)

and

Marciniak (2007)

Sumartschenkowa et al.

(2006)

Marciniak (2009)

Möllmann et al. (1997) Krummen et al. (2004) Kato and Gmehling (2005b)

Kato and Gmehling (2004)

Krummen et al. (2004) Krummen et al. (2004) Kato and Gmehling (2004)

Letcher and Reddy

(2007)

Letcher et al. (2009) David et al. (2003)

Deenadayalu et al.

(2006)

Letcher et al. (2005b) Letcher et al. (2008)

Solvent

	for for Hexane/Benzene Cyclohex. /Benz.
[BMIM][SCN]	84.55	23.90	0.46
[EMIM][SCN]	78.41	28.37	0.29
[EMIM][BF4]	45.87	20.06	0.41
[BMPy][BF4]	36.88	-	0.61
[MMIM][CH3OC2H4SO4]	-	25.45	0.23
[BMIM][MDEGSO4]	33.66	15.35	0.50
[MMIM][BTI]	27.26	-	0.74
[MMIM][(CH3)2PO4]	-	13.13	0.28
[BMIM][TFA]	27.00	12.96	0.36
[EMIM][Et-SO4]	25.77	11.11	0.34
[Et3S][BTI]	25.10	12.76	0.90
Sulfolane	18.20	9.84	0.43
NMP+6 %(w/w)H2O	16.08	9.07	0.52
[BMPyrr][BTI]	15.47	-	1.16
[MMIM][Me-SO4]	14.33	17.55	0.15
NMP + 3 %(w/w)H2O	13.79	10.00	0.71
NMP	11.49	7.72	0.99
[Py][[C2H5OC2H4SO4]	11.18	8.47	0.26
[3C4C1P][Me-SO4]	9.90	5.50	1.00
[BMPyr][TOS]	9.65	5.54	0.25
[MOIM][Cl]	8.71	5.40	0.56
[MOIM][MDEGSO4]	8.69	4.99	0.73
[BMIM][Oc-SO4]	5.27	3.49	0.69
[3C6C14P][(C8H17)2PO2]	1.52	1.125	2.08

2.1.7. Barriers to the commercial use of ionic liquids

In December 2002, Chemical vision 2020 Technology partnership (Ford et al. 2004), an american industry-led organization appointed a task force to reflect on the barriers to the widespread use of ionic liquids and how to overcome them. Two years later, the vision 2020 ionic liquid task force (Ford et al. 2004) identified six barriers to the widespread use of ionic liquids in the chemical industry, including separation processes:

a) Lack of performance data under industrial conditions;

b) Lack of environmental and safety data for many ionic liquids;

c) Lack of economic benefit analyses;

d) Lack of fundamental understanding of compositional structure versus performance;

e) Ionic liquid manufacturing cost and scale-up;

f) Institutional issues such intellectual property and communication between researchers.

Seven years later, these obstacles still stand on the promising way leading to a wide scale incorporation of ionic liquids in chemical engineering processes. The present study falls under the fourth barrier.

2.1.8. Fluorinated ionic liquids (FILs)

For apparently unknown reasons, Fluoroanions and fluorocations ionic liquids are often respectively referred to as fluorous and fluorinated ionic liquids (Heitzman et al. 2006). Interesting reviews on fluorinated ionic liquids discuss specific features of this subset of ionic liquids (Xue et al. 2006 and Hagiwara and Ito 2000). Ionic liquids with fluorine-containing anions are the most commercially available and the most investigated as potential separation agents. Commonly found anions include [BF4]^-, [PF6] - , [CF3SO3]^-, [(CF3SO2)2N]^-, [CF3CO2] - and [SbF6]^-. Fluorine-containing ionic liquids are of great interest for South Africa. Its government launched recently the Fluorochemical Expansion Initiative aimed at researching and developing South Africa`s fluorinated products. The move is expected to put an end to the paradox pointed out by Prof. Deresh Ramjugernath<sup>1</sup>, the South Africa`s Research Chair in Fluorine process engineering and separation technology in these terms: South Africa possesses the second to largest supply of Fluorspar; it currently imports all its fluorinated products.? Synthesizing FILs at reasonable costs is therefore a possibility in this country. First direct measurements of IDACs in a fluorinated ionic liquid were reported by Heintz et al. (2001).

¹ http://www.caes.ukzn.ac.za/Collegeboastsnewresearchchairs, accessed 15 February 2009.