Self-solidification ionic liquids as heterogeneous catalysts for biodiesel production
Different from the common ionic liquids (ILs) as homogeneous catalysts, solid ionic liquids (SILs) as heterogeneous catalysts are simply fabricated herein via an efficient one-step reaction for biodiesel production. Specifically, low-cost aliphatic amines containing secondary and/or primary amine groups are selected as the starting materials followed by the reaction with 1,3-propane sultone under mild conditions to graft sulfonic acid (–SO3H) groups onto the final products. The –SO3H groups are divided into two types: the bonding –SO3H groups (zwitterion arising from the hydrogen bond between sulfonic acid groups and amine groups) make the products turn solid while the free –SO3H groups endow the products with catalytic activity. Such solid ionic liquids (SILs) present pronounced activity, excellent stability and reusability as heterogeneous catalysts in esterification of free fatty acids (FFAs), indicating great potential in biodiesel production because of their high performance and low cost.