Study on the reaction selectivity and hazard of m-xylene nitration catalyzed by SO₃H-functionalized ionic liquids with different anions: Experimental and theoretical approaches

Five imidazole based SO₃H-functionalized ionic liquids (SFILs) with different anions were selected as catalysts to achieve efficient m-xylene nitration. The acidity and thermal stability of the ILs were characterized by UV–vis and thermogravimetry. The reaction selectivity of the m-xylene nitration catalyzed by five ILs was compared. [MIMBs]HSO₄ had the best catalytic activity with the maximum yield of 98.1 %. The effect of ILs on the exothermic behavior of m-xylene nitration was explored by reaction calorimeter. The reaction enthalpy (ΔH) and adiabatic temperature rise (ΔT_ad) were calculated to evaluate process hazards. The reaction order for the decomposition of different nitration products under adiabatic conditions was essentially close to 1. The process risk level for m-xylene nitration with the introduction of selected ILs was essentially downgraded from conditionally acceptable to acceptable, and the criticality was reduced from class 3 to 1. Furthermore, the acidic active sites of the ILs were predicted by the electrostatic potential. The solubility of m-xylene in the IL was investigated through the calculation of interaction energy. The roles of ILs as solvents and catalysts in nitration were resolved by density functional theory calculations. These findings could contribute to the understanding of the effect of ILs in modulating the process hazards and improving the selectivity of m-xylene nitration, and provide theoretical guidance for elucidating the catalytic mechanism of ILs in nitration.