The slurry is hard to pump and filter. Separation of the solid and liquid phases is expensive, time-consuming, and labor-intensive.

This was the situation that BASF faced during its production of diethoxyphenyl-phosphine, a photo-initiator intermediate prepared by reaction of dichlorophenyl-phosphine with ethanol. To avoid an unwanted side reaction of the product, HCl had to be removed from the reaction mixture. Since an aqueous extraction phase would have led to hydrolysis of the desired product, the reaction had to be carried out in a non-aqueous medium in the presence of equimolar quantities of triethylamine. The resulting mixture was the viscous slurry described previously. The challenge to improve this unfavorable process generated an interesting question, i.e., if an acid must be scavenged with a base and the formation of a salt cannot be avoided, why not form a liquid salt?

Instead of triethylamine, BASF decided to use 1-methylimidazole as the acid scavenger and immediately realized excellent results. 1-methylimidazole is a “reactive solvent” in this example. The ionic liquid formed in the reaction was methylimidazolium chloride that, with a melting point of 75oC, was liquid at the 80oC reaction temperature. Upon reaction, two clear liquid phases formed that were easily separated. Since the upper phase was pure diethoxyphenyl-phosphine product, no reaction solvent was needed. The lower phase was pure ionic liquid, which after separation from the diethoxyphenyl-phoshine could be recycled via a deprotonation with sodium hydroxide, thereby regenerating the 1-methylimidazole. Greater than 90% of the 1-methlimidazole is recovered because there is no degradation of solvent or accumulation of impurities.

In the very first lab trials, BASF found that 1-methylimidazole was doing an outstanding job of scavenging the byproduct acid. Of greater importance, we found that the ionic liquid that formed functioned as a nucleophilic catalyst. The result of this combined effect was a tremendous increase in the reaction yield per unit of volume and time from 8 to 690.000 kilograms per cubic meter per hour. This improvement enabled BASF to carry out the reaction, which previously required a 20 m3 batch vessel, in a jet reactor the size of a human thumb. This tiny reactor is now part of a continuously operated production plant that has a capacity of more than 1,000 tons per year.

Operation began in the third quarter of 2004 at BASF’s Ludwigshafen, Germany, site. An eco-efficiency analysis of this process clearly demonstrates that ionic liquids can make chemical processes much more sustainable.

The so-called BASIL acid scavenging technology has also been applied successfully for esterifications, acylations, silylations, phosphorylations, sulfurylations, eliminations, and deprotonations as well as for acid removals in general in lab trials. 

Potential applications

BASF has also developed, at pilot plant scale, the chlorination of alcohols with gaseous HCl in the presence of methylimidazolium chloride. Although the chlorination reaction is usually carried out using phosgene or thionylchloride as the chlorinating agent, it has been found that in the presence of the ionic liquid, HCl is a much stronger nucleophile, making it possible to carry out the chlorinations in an easier, safer, and more cost-efficient manner.

BASF offers 23 different BASIONICs ionic liquids, based mainly on imidazolium cations. Quantities are available in gram lab scale through industrial scale quantities. Ionic liquids are classified into categories according to characteristics. Some examples of categories include: acidic, basic, liquid at room temperature, and viscosity. This portfolio provides a broad range of basic properties and supports a wide range of possible applications. BASF also offers tailor-made ionic liquids engineered to satisfy each customer’s unique requirements precisely.

Each application requires clear definition of the optimal set of property specifications for the ionic liquid. For example, although an ionic liquid used as a non-aqueous electrolyte in a galvanic process can be colored, the same ionic liquid used for dye-sensitized solar cells must be colorless.
Matching the ionic liquid to an application is a multi-step process. The first step is to identify an existing ionic liquid that roughly meets the process or application needs of the customer. Next, the supplier and the customer define in detail the set of specification parameters such as purity, byproducts, halogen content, color, viscosity, etc., needed for commercial success. Finally, a process is designed for commercial scale production of the specific ionic liquid.

Regulatory support

From the beginning, BASF realized the toxicological and eco-toxicological characteristics of its ionic liquids are important. As for any new chemical, European Union regulations require a set of toxicological data and notification prior to sale or commercial use. Because of their nature, most ionic liquids, excluding halogens, are amenable to disposal by thermal oxidation.

Dr. Uwe Vagt is a Global Business Development Manager with BASF in Ludwigshafen, Germany; email him at uwe.vagt@basf-ag.de
Dr. Calvin Emanuel is a Business Development Manager with BASF in Florham Park, N.J; email him at emanuec@basf-corp.com.

Figures: IOLITEC Ionic Liquids Technologies GmbH & Co. KG