Effect of acidity of Bronsted acidic ionic liquids on reaction between glycerol and acetone

Bronsted acidic ionic liquids (BAILs) with an imidazole cation (A), such as A[HSO4], A[CF3SO3], A[CHSO3], and A[CH3COO], were synthesized. The acid properties of BAILs in acetone were studied by the conductometric method. The dissociation constants (pKa) were calculated from electrical conductivity data. It was found that the acidic strength of BAILs in acetone decreases in the following order: A[HSO4] ~ ACF3SO3] > ACH3SO3] > A[CHCOO]. This trend is an agreement with the proton affinity of conjugate acid and electrostatic theory. Catalytic properties of BAILs were investigated in the reaction between glycerol and acetone. This reaction can be of interest because of glycerol transformation to biodiesel additives, for example, five-membered solketal (2,2-dimethyl-l,3-dioxane-4-methanol, (II)), which improves the cold properties and lowers viscosity in biodiesel formulation. The catalytic properties of BAILs were investigated at 55 °C at an ace-tone/glycerol molar ratio of 2.0 and an amount of BAIL of 0.8 mol.% It was found that in the presence of BAILs, the main product was (II) with 97.7-98.7% selectivity. Selectivity towards six-membered acetal (2,2-dimethyl-dioxane-5-ol) was insignificant, which is typical for Bronsted acids. Analysis of acidic and catalytic properties of BAILs indicates that Bronsted acidity is a key factor for the adjustment of the reaction rate. The reaction rate and yield of (II) decrease with decreasing pKa. The high reaction rate and yield of (II) are achieved in the presence of A[HSO4] with high Bronsted acidity (pKa). This trend is an agreement with the reaction mechanism, which is based on the transformation of a short-lived cation formed by the activation of the oxygen atom of the carbonyl group of acetone on Bronsted active site of catalyst. Stability of A[HSO4] catalyst in the reaction mixture was investigated by a recycling test at an acetone/glycerol molar ratio of 4.1 and an A[HS04] amount of 0.8 mol.%, at 55 °C for 240 min. After each catalytic cycle, A[HS04] was separated from the reaction mixture by extraction of products by ethyl acetate. A[HS04] was desiccated under vacuum at 110 °C for 2 h. It was demonstrated that A[HS04] can be used repeatedly without significant loss in catalytic activity during at least three catalytic cycles. The yields of (II) in cycles after 240 min of reaction were 98.2, 97.9, and 97.5%, respectively.

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