Biphasic Acid Scavenging Utilizing Ionic Liquids: The First

Chapter 10

Biphasic Acid Scavenging Utilizing Ionic Liquids: The First Commercial Process with Ionic Liquids Downloaded by NORTH CAROLINA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: March 15, 2005 | doi: 10.1021/bk-2005-0902.ch010

Matthias Maase and Klemens Massonne BASF A G , Chemicals Research and Engineering, GCI/P-M 311, D-67056 Ludwigshafen, Germany

Since the end of the 90s ionic liquids have faced a tremendous interest by the scientific community (1). They turned out to be novel materials providing unique new properties that simply had not been available before. Surprisingly no successful industrial application had been reported for quite a long time. In 2003 BASF revealed that they have teen running the first commercial process (BASIL™) that makes use of ionic liquids in a dedicated way since beginning of 2002 (2). BASIL™ stands for Biphasic Acid Scavenging utilizing Ionic Liquids. Since the end of 2003 BASIL™ is ready for licensing.

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© 2005 American Chemical Society

In Ionic Liquids IIIB: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction The handling of solids is a true challenge in large scale industrial processes although it might not be obvious to academic groups. Perhaps it is even more challenging than the handling of volatile or highly reactive species since the problems related to solids especially to suspensions often do not occur in a reliable way. But what is wrong with suspensions?

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They often make reaction mixtures much more viscous, leading to insufficient mixing of the reactants. Heat transfer in large vessels containing suspensions is often less than satisfactory, so hot spots may occur. This in turn favours side reactions and lowers the yield of the desired product. It is difficult to transport suspensions through piping or to store them in tanks. In both cases high flow rates or agitation is necessary to prevent the suspension from settling and plugging the system. Separation can be an important issue as well. If a solid is formed during a reaction as an unwanted by-product it has to be removed afterwards. Reactors that are designed to deal with suspensions are usually more complex and expensive than those suitable for pure liquids. Finally if a solid is generated during a reaction scale up is by far not easy. One can hardly predict how the solid looks like when it is prepared in a 10 m reactor rather than in a 100 ml flask. This can easily cause serious problems, for example if the solid that could be filtered off in a minute in the lab now takes two days on the large scale just because particles are several orders of magnitude smaller than expected. 3

Acid Scavenging One example in which industrial chemists have to deal with suspensions is acid scavenging. Browsing organic textbooks one will find numerous reactions which liberate acids that have to be scavenged in order to prevent decomposition of the product or other side reactions. Some examples are esterifications, silylations or phosphorylations of alcohols. Hydrochloric acid (HC1) is the acid most commonly formed. Usually, tertiary amines such as


128 triethylamine are added to scavenge the acid. With the acid, these substances form solid salts which turn the reaction mixture into a suspension.


The Synthesis of Alkoxyphenylphosphines Alkoxyphenylphosphines are important raw materials in the production of BASF's Lucirines®, substances that are used as photoinitiators to cure coatings and printing inks by exposure to UV light. HC1 is formed during the synthesis of diethoxyphenylphosphine (Scheme 1).

OR

CI

+ 2 ROH CI

+

2

+ 2

R N 3

R N*HCI 3

OR

Scheme 1: Synthesis of Dialkoxyphenylphosphines

Scavenging with a tertiary amine results in a thick, non-stirrable slurry (Figure 1). The problems mentioned earlier significantly lower the yield and capacity of the process.

Figure I: Slurry that is formed when a tertiary amine is used as an acid scavenger © BASF Aktiengesellschaft 2002.


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Ionic Liquids (Dis)solve the problem If an acid has to be scavenged with a base, the formation of a salt cannot be avoided, but why not form a liquid salt - a so-called ionic liquid? The properties of ionic liquids are:


• they are salts, consisting 100% of ions • they are liquid at temperatures below 100 °C. Since they are rather polar materials they often do not mix with solvents or nonpolar product molecules. Using the BASF product 1-methylimidazole as an acid scavenger, an ionic liquid is formed: 1 -methyl-imidazolium chloride (Hmim CI), which has a melting point of about 75 °C (Scheme 2).

Scheme 2:1-methylimidazole as acid scavenger

After the reaction two clear liquid phases occur (Figure 2) that can easily be separated. The upper phase is the pure product - no solvent is needed anymore - the lower the pure ionic liquid (3). Hmim CI as an ionic liquid has a great advantage over the classical dialkylated systems: they can be switched on and off just by protonation and deprotonation. This is crucial when recycling and purification of the ionic liquids is considered. To distinguish the switchable ionic liquids - the "Hmims" - from the conventional ones Michael Freemantle used the term "smart ionic liquids" in his C&EN paper on the BASIL™ process (2).

Lessons Learned: Why have Ionic Liquids been successful with BASIL™ ? There are several success factors being responsible that BASIL™ was established in routine production so quickly.



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Figure 2: The BASIL™ process. After the reaction two clear liquid phases are obtained - the upper being the pure product the lower being the ionic liquid Hmim CI © BASFAktiengesellschaft 2002


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First of all there was an existing problem - the formation of unwanted solids - to which the unique properties of ionic liquids offered a tailor-made solution. The lead structures of ionic liquids - the imidazoles - have been known from academic research already so industry had not to reinvent the wheel. The material (1-methylimidazole) was available in large quantities within the company since it is an existing BASF product. The process has been improved dramatically. There were clear economical benefits from BASIL™ being higher chemical and higher space-time-yields.

Finally one should not forget to mention that luckily the chemistry worked as well. Further investigations revealed that BASIL™ is not restricted to phosphorylation chemistry but is a general solution to all kinds of acid scavenging.

BASIL™ is more than just Acid Scavenging Methylimidazole is doing a perfect job by scavenging the acid. Looking closer at it one will find that methylimidazole also helps in setting the acid free. In other words: it acts as an nucleophilic catalyst (4). We found that the phosphorylation reaction is complete in less than a second. Having eliminated the formation of any solids and having increased the reaction rate new reactor concepts were possible. We were now able to do the same reaction that has been done in a large vessel in a little jet reactor that has the size of your thumb. Doing so the productivity of the process has been rised by a factor of 8 10 to 690000 k g r n h \ 4

3

This progress can be attributed to the existence of a new class of novel, promising materials: ionic liquids.

References: 1. Freemantle, M . Chem. Eng. News 1998, 76, 32; Green Industrial Applications of Ionic Liquids', Rogers, R.D., Seddon K . R., Volkov, S., Eds., Kluwer: Dordrecht, 2002; P. Wasserscheid, W. Keim, Angew.


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2. 3. 4.

Chem. Int. Ed. 39, 2000, 3772-3789; Ionic Liquids in Synthesis; Wasserscheid, P.; Welton T., Eds., VCH-Wiley: Weinheim, 2002; Ionic Liquids: Industrial Applications to Green Chemistry; Rogers R. D.; Seddon, K . R., Eds.; ACS Symposium Series 818, American Chemical Society: Washington D.C., 2002; Ionic Liquids as Green Solvents: Progress and Prospects; Rogers R. D.; Seddon, K. R., Eds.; ACS Symposium Series 856, American Chemical Society: Washington D.C., 2003. Freemantle, M . Chem. Eng. News 2003, 81, 9; WO 03/062171 (BASF AG). Seddon, K.R. Nature Mater. 2003, 2, 363; Rogers, R.D.; Seddon, K.R. Science, 2003, 302, 792. Chojnowski, J.; Cypryk, M . ; Fortuniak W. Heteroatom. Chem. 1991, 2, 63.


Biphasic Acid Scavenging Utilizing Ionic Liquids: The First

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