Hydrogen bonding effect on the thermal behavior of acidic ionic liquids

Abstract

A relationship between the thermal performance (melting and thermal decomposition) and the internal hydrogen bonds for the protic ionic liquids pyridinium nitrate ([H–Pyr]+[NO3]–), pyridinium hydrogen sulfate ([H–Pyr]+[HSO4]–), pyridinium dihydrogen phosphate ([H–Pyr]+[H2PO4]–) and 4-amino-1H-1,2,4-triazolium nitrate ([4-AT]+[NO3]–) is investigated in the current work. For that purpose, melting/decomposition points, non-covalent bonding and intermolecular interaction are studied by means of TGA/DSC, reduced density gradient method and a Hirshfeld surface analysis.Results established that strong hydrogen bonds, rather than van der Waals interactions, influence the thermal behavior of the aforementioned ionic liquids in a great extent. Melting points of the compounds under investigation are found to decrease in order [H–Pyr]+[NO3]– > [4-AT]+[NO3]– > [H–Pyr]+[H2PO4]– > [H–Pyr]+[HSO4]– due a various hydrogen bonding degree. Overall decomposition of the pyridinium-based ionic liquids includes simple pyridine evaporation and protic acid (HNO3, H2SO4 and H3PO4) decay. However, decomposition of 2-aminotriazolium fragment (except HNO3) is also detected during [4-AT]+[NO3]– decay upon heating.