Mechanism for the depolymerization of cellulose under alkaline conditions

The mechanism for the hydroxyl-radical-induced depolymerization of cellulose under alkaline conditions in air was investigated using density functional theory at the B3LYP/6-31+G(d,p) level as well as electron transfer theory. The pathway for the depolymerization of cellulose was obtained theoretically and H abstraction from the C(3) atom of the pyran ring during the cleavage of the glucosidic bond was found to be the rate-limiting step due to its high energy barrier (16.81 kcal/mol) and low reaction rate constant (4.623 × 10⁴ mol L^-1 s^-1). Calculations of the electron transfer between O₂ and the saccharide radical performed with the HARLEM software package revealed that following the H abstraction, the oxygen molecule approaches C(2) on the saccharide radical and obtains an electron from the radical, even though no bond forms between the oxygen molecule and the radical. The rate constant for electron transfer could be as high as 1.572 × 10¹¹ s^-1. Furthermore, an enol intermediate is obtained during the final stage of the depolymerization.