Enhanced environment friendly surfactant production by the glycerolysis of castor oil using amino acid ionic liquid as a catalyst

Monoacylglycerol (MAG) is one kind of the oil-based nonionic surfactant, which has the excellent emulsifying property and biodegradability. In this work, several novel and green amino acid ionic liquids (AAILs) were developed as eco-friendly catalysts to promote the glycerolysis of castor ( Ricinus communis L.) oil for the green castor oil-based MAG production. Nine AAILs, consisting of an amino acid anion (arginine (Arg), lysine (Lys) and histidine (His)), and an organic hydroxide cation (choline hydroxide (ChOH), tetramethylhydroxide ammonium (TMAOH) and tetrabutylammonium hydroxide (TBAOH)), were synthesized by a simple and green chemical route and used as catalysts to catalyze the glycerolysis of castor oil. Effects of the structural composition of AAILs, reaction time (0-120 min), castor oil and glycerol molar ratio (1:2-1:10), AAIL load (4-12%), and temperature (60-120℃) on the glycerolysis were evaluated and optimized by Box–Behnken methodology. Particularly, the effect of free fatty acid content in castor oil (0-25%) was also investigated to analyze the fatty acid tolerance of AAILs. Among the tested AAILs, [Ch][Arg], [TMA][Arg], and [TBA][Arg] showed the outstanding catalysis performance. Especially, [Ch][Arg] as reaction catalyst for the glycerolysis of castor oil exhibited the best catalysis activity, and high selectivity for MAG formation. The maximum TAG conversion (99.6 ± 0.4%) and MAG content (83.3 ± 1.6%) were obtained under the optimized conditions: 93.7℃, 60.7 min, [Ch][Arg] load 12.3%, and substrate molar ratio 1:6.4. Besides, [Ch][Arg] still had 80% catalytic activity when the free fatty acid content was 20% in castor oil. Kinetic analysis of the glycrolysis was also evaluated. Accordingly, activation energy (Ea) and average reaction order (a) of the glycerolysis were obtained as 59.7 kJ/mol and 1.69, respectively.