Chemical stability of choline-based ionic liquids supported on carbon materials

[Chol][Pro], [Chol][Ser], [Chol][Ala], [Chol][MeSO ₃] and [Chol][2-MeBu] ionic liquids were synthesized and then supported on five mesoporous carbon materials. A commercially available mesoporous carbon (MB) was used as a starting material, and four additional supports were prepared from MB by heat treatment and oxygen plasma as well as chemical oxidations, bringing about materials with differences in both textural and surface chemical properties. The purpose of this work was to study the chemical stability of different carbon-supported choline-based ionic liquids as a previous step to their use as heterogeneous catalysts in liquid phase organic synthesis. To this end, the ILs were firstly immobilized on the carbon materials, subsequently de-immobilized at increasing periods of time and finally, analyzed by ¹H NMR. Their structure was compared to that of the corresponding unsupported ionic liquid. The results demonstrated that besides the textural and surface chemical properties of the carbon materials, the composition of the anion as well as the strength of the anion-cation bond of the choline-based ionic liquids are crucial factors influencing on their chemical stability after being supported. Thus, [Chol][Ala], [Chol][Ser] and [Chol][Pro] decompose quickly as a consequence of the interaction between the amino group of the anion and the oxygen groups on the surface of the carbon material. However, [Chol][MeSO ₃] and [Chol][2-MeBu] remain stable. A decrease of the surface area of the carbon material as well as a highly pure carbon surface significantly increases the chemical stability of the supported choline-based ionic liquids.