Production of ionic liquids through metathesis and melt crystallization

Abstract

Establece las condiciones de dinámica de fluido y termodinámicas adecuadas en un sistema de par salino recíproco multicomponente, el cual, vía reacción de metátesis, logre una eficiente conversión a líquidos iónicos (LIs) de alto valor. Se estudia las características de cristalización del sistema y se aplican procedimientos adicionales de separación ‘downstream’ —destilación y centrifugación— para lograr una alta concentración de los productos. Con esto se buscar llevar a cabo una comparación de procesos alternativos y proponer un procedimiento de producción y concentración basado en cristalización fraccionada para obtener LIs por metátesis vía diálisis en membrana y/o columna de intercambio iónico. La reacción por síntesis directa es la principal vía de producción de LIs. Para la producción de LIs adicionales específicos, sin embargo, no todas las combinaciones anión–catión pueden ser obtenidas a un rendimiento o costo aceptables por esta vía. El intercambio iónico por metátesis representa una vía complementaria, donde el diseño conceptual implicó el uso de eductos de bajo costo junto con un LI precursor. El proceso, acoplado a una etapa de separación vía destilación y cristalización por fundido, representa una alternativa para obtener LIs concentrados. La metátesis fue llevada a cabo por : (1) generación de un sistema de par salino recíproco mediante mezcla de dos eductos, conduciendo a la formación de un sistema ternario o cuaternario a partir del cual el producto puro fue obtenido por cristalización; (2) diálisis Donnan de los eductos mediante sistema de membrana, con el gradiente como única fuerza motriz necesaria para la reacción; (3) electrodiálisis, con la fuerza motriz de intercambio iónico basada en el potencial eléctrico; y (4) intercambio iónico en columna. Se comprobó así la factibilidad técnica de obtener LIs como sal o hidróxido, por sustitución aniónica, incluyéndose un breve análisis económico comparativo.

Ionic liquids (ILs) are a very important class of innovative compounds consisting entirely of ions and which are normally liquid at or near room temperature. They are currently becoming an important segment in the chemical research an industry due to their interesting properties, such as, among others, their ability to be applied as practically non-volatile solvents. A wide variety of anion-cation combinations of ILs can be theoretically produced, with a specific application in each case, as solvents in (bio)-catalysis or as electrolytes among other current and potential uses. A reaction process of direct synthesis is essentially the way through which the first series of ILs have been and are still produced nowadays. This basically involves first the quaternization of molecules that can easily complex to various metal ions (as organometallic agents), like amine, phosphine or heterocyclic nitrogen rings, with the corresponding cation as the intermediary product, which ultimately reacts with an alkylation agent forming the IL. A number of important ILs has thus been generated from established firms in the chemical industry. For the production of further specifically tailored and highly valuable ILs, however, direct synthesis alone may not always be enough, since not all the anion - cation combinations needed for each particular application can actually be obtained at a high yield and/or at a low cost through this method. There exists, though, a complementary path for generating ILs, and it is based on the ion exchange principle; highly valuable ILs can potentially be produced by this reaction path known as metathesis, a classic type of reaction that basically implies the exchange of ions from two educts being put into contact. This concept, as applied in the presented work, implies the use of a readily available, low-cost educts, with a low-value ‘precursor’ IL itself being one of these educts. This process, when coupled with a final separation or concentration enrichment of the resulting IL by means of distillation and fractional (melt) crystallization, represents an interesting and potentially feasible alternative for obtaining concentrated or high-purity tailored compounds of this kind. Metathesis here can be accomplished by several approaches; each one of them was studied in detail in this work. A first approach in the metathesis reaction was the generation of a reciprocal salt pair system through the mixture of two educts; this led to the formation of a ternary or quaternary system, from where the desired product can be obtained in high purity degree by final crystallization. A second approach in this work was the Donnan dialysis of the educts through a membrane system. The advantage of this method lies in the gradient being the only real driving force for the reaction as well as the feasibility of performing it at mild temperature conditions. A third approach was by means of electrodialysis. Here, the driving force for ion exchange is based on the electrical potential. Finally, a fourth approach was the ion exchange through a column. Through these different approaches, all of them based on the principle of metathesis, it was technically possible to obtain tailored ILs in the form of salts or as hydroxides, by anion substitution. Moreover, provided there is a feasible production of ILs by means of anion substitution, the potential application of the ion exchange principle to tailor even more IL salts by means of cation substitution will then also be a strong issue.