In 2002 BASF established the first dedicated industrial-scale ionic liquid based process. The BASIL™ process (BASIL = Biphasic Acid Scavenging utilizing Ionic Liquids) is used for the synthesis of alkoxyphenylphosphines which are important raw materials in the production of BASF’s Lucirines® (Scheme 1), products that are used as photoinitiators to cure coatings and printing inks by exposure to UV light.
Scheme 1: Synthesis of Lucirin® TPO-L
HCl is formed during the synthesis of diethoxyphenylphosphine (Scheme 2).
Scheme 2: Synthesis of Dialkoxyphenylphosphines
Scavenging with a tertiary amine results in a thick, non-stirrable slurry. The problems mentioned earlier significantly lower the yield and capacity of the process. In order to provide a minimum of mixing and heat transfer of the exothermal reaction usually a solvent has to be added.
How can 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 instead? From the previous academic work the imidazolium cation has been known as a key building block of ionic liquids. Using the BASF product 1-methylimidazole as an acid scavenger, an ionic liquid is formed: 1-methyl-imidazolium chloride (HMIM Cl), which has a melting point of about 75°C (Scheme 3).
Scheme 3: 1-methylimidazole as an acid scavenger
After the reaction two clear liquid phases occur that can easily be separated. The upper phase is the pure product – no solvent is needed anymore - the lower phase the pure ionic liquid.
HMIM Cl as an ionic liquid has a great advantage over the classical dialkylated systems: HMIM can be switched on and off just by protonation and deprotonation. This is crucial when recycling and purification of the ionic liquids is considered.
Further investigations revealed that BASIL™ is not restricted to phosphorylation chemistry but is a general solution to all kinds of acid scavenging (Table 1). Acylations and silylations have been run succesfully as well as an elimination reaction. BASIL™ also is applicable to extractive acid removal from organic phases for example for the purpose of purification.
Acylation
Elimination
Phosphorylation
Sulfurylation
Silylation
Deprotonation
Table 1: Set of reactions suitable for the BASIL™ process
Figure 1: Today the BASIL™ process is run in a little jet stream reactor which has a capacity of 690000 kg m-3 h-1.
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. BASF 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. BASF was 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 a thumb (Figure 1). Doing so the productivity of the process has been rised by a factor of 8 104 to 690000 kg m-3 h-1. End of 2004 BASF has successfully started a dedicated BASIL™ plant using this jet stream reactor technology.
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