Lanzhou Greenchem ILs (Center for Greenchemistry and catalysis), LICP, CAS. Ionic Liquids

Whereas Eastman Chemical Company and BASF commercialised the first ionic liquid processes, IFP was the first to operate an ionic liquid pilot plant.96 The Dimersol process, based on traditional technology, consists of the dimerisation of alkenes, typically propene (Dimersol-G) and butenes (Dimersol-X) to the more valuable branched hexenes and octenes. This is an important industrial process, with thirty-five plants in operation worldwide, each plant producing between 20,000 and 90,000 tonnes per year of dimer, with a total annual production of 3,500,000 tonnes. The longer-chain olefins produced in the dimerisation process are usually hydroformylated to alcohols (e.g. isononanols): isononanols are then converted into dialkyl phthalates, which are used as poly(vinyl chloride) plasticisers.

The dimerisation reaction is catalysed by a cationic nickel complex of the general form [LNiCH2R9][AlCl4] (L = PR3) and is commonly operated without solvent. However, it has been found that the catalyst shows greater activity when it is dissolved in undesirable aromatic or halogenated hydrocarbons.

The use of chloroaluminate(III) ionic liquids as solvents for these nickel-catalysed dimerisation reactions has been developed and pioneered at IFP (France), especially by Nobel laureate Yves Chauvin and He´le`ne Olivier-Bourbigou. The reaction can be performed as a biphasic system between 215 ℃ and 5 ℃, as the products form a second layer that can be easily separated and the catalysts remains selectively dissolved in the ionic liquid phase. The activity of the catalyst is much higher than in both solvent-free and conventional solvent systems, and the selectivity for desirable dimers is enhanced. This process has been patented as the Difasol process and can be retro-fitted into existing Dimersol plants; it is operated by Axens, an IFP subsidiary, and is described in Chauvin’s Nobel lecture.

The combined Dimersol–Difasol process was described for dimerisation of 2-methylbut-2-ene under laboratory conditions:  the biphasic system is clearly superior. In comparison with the homogeneous Dimersol process, the advantages of the biphasic Difasol system are:

• a much better use of the catalyst and therefore a reduced catalyst disposal and cost
• a better dimer selectivity
• a higher yield into dimers can be achieved in a single step even with a low concentration alkene feed
• a possible extension of the Dimersol process to higher less reactive olefins
• from an engineering point of view, the reactor size is much smaller than in the homogeneous system

When the Difasol reactor, which involves the use of an ionic liquid, was added to the existing Dimersol reactor, the process became much more efficient. The process uses a biphasic mixture (cf. the BASIL process), which results in a much better use of the catalyst, and therefore a reduced catalyst disposal cost, and a better dimer selectivity. A higher yield into dimers can be achieved in a single step, even with a low-concentration alkene feed, and from an engineering point of view, the reactor size is much smaller than in the homogeneous system. Moreover, this new process can be possibly extended to higher, less reactive olefins.