Synthesis of coumarins via Pechmann reaction catalyzed by 3-methyl-1-sulfonic acid imidazolium hydrogen sulfate as an efficient, halogen-free and reusable acidic ionic liquid

3-Methyl-1-sulfonic acid imidazolium hydrogen sulfate has been used as an acidic task-specific ionic liquid for Pechmann condensation reaction. The coumarin products could simply be separated and the ionic liquid could be recycled and reused several times without noticeable decrease in the catalytic activity.