Selective oxidation of cellulose, mediated by N-hydroxyphthalimide, under a metal-free environment

Abstract

Three types of cellulosic samples, including microcrystalline, never-dried sulfite pulp, and Cellets, were selected to test the oxidative capacity of a new oxidation protocol involving the redoubtable N-hydroxyphthalimide (NHPI) catalyst. All reactions were performed under mild conditions, at room temperatures and at atmospheric pressure, in the presence of sodium hypochlorite and sodium bromide. The generation of the reactive radicalic species, i.e. phthalimide N-oxyl (PINO) from its parent hydroxylamine, NHPI, in the presence of sodium hypochlorite, has been firstly proved by using the UV-Vis technique, followed next by the reaction with each cellulose type sample for a designated period of time, varying the amount of the NaClO introduced in the reaction. The main finding of this new proposed protocol resides in the elimination of any metal cocatalyst required for the generation of the non-persistent free radical PINO, as has been previously used. The obtained amounts of the carboxylic groups after oxidation, determined using potentiometric titration, are satisfactory, whereas the degree of polymerization of the resulted products has still superior values, as compared to many other methods used today. All the oxidized samples exhibited an increased crystallinity compared to the unoxidized samples as revealed by X-ray diffraction experiments. This new approach not only avoids the introduction of an additional chemical reagent, but also provides an easy and convenient alternative to those existing today for the preparation of oxidized cellulose products having high amounts of carboxylic groups and remarkably, less depolymerized.