Imidazole-catalyzed esterification of cellulose in ionic liquid/molecular solvents: A multi-technique approach to probe effects of the medium

Abstract

Observed rate constants (kobs) for the imidazole-catalyzed acylation of microcrystalline cellulose by carboxylic acid anhydrides (ethanoic to hexanoic) were calculated from conductivity data. The reaction media consisted of the ionic liquid, IL, 1-allyl-3-methylimidazolium chloride and the molecular solvents, MSs, N,N-dimethylacetamide, DMAc, dimethylsulfoxide, DMSO, and sulfolane, Sulfn. The latter two carry the same dipole (S=O) and have some similar properties. Using FTIR, we showed that the acylating agent is N-acylimidazole. The reaction third order rate constants (k3) depend slightly on [IL] because the reactant is cellulose-IL complex. For ethanoic anhydride, values of k3 decrease in the order: IL-DMSO>IL-DMAc>IL-Sulfn. We analyze this dependence on the nature of the MS by considering the reaction activation parameters; some macroscopic and microscopic properties of the binary solvent mixtures and cellulose solutions therein, and the results of molecular dynamics simulations. Favorable activation enthalpy; lower solution viscosity; larger empirical polarity and basicity, and stronger biopolymer-solvent interactions are responsible for the faster reaction in IL-DMSO, relative to that in IL-Sulfn. The dependence of the values of (k3) on the number of carbon atoms in the acyl moiety showed a complex behavior (decrease from C2 to C4 and then increase), similar to that observed before, probably because of enthalpy/entropy compensation.