Multi-stage catalytic carboxylation of mercerized cellulose fibers

A method of producing fibrous carboxylated cellulose, the method comprising:

• obtaining mercerized cellulose fibers;
• carboxylating the mercerized cellulose fibers by catalytically carboxylating said fibers in an aqueous alkaline suspension in at least two catalytic carboxylation stages, with each of said stages carried out in one of at least two corresponding catalytic carboxylation reactors, in which said stages and reactors are in series and each later stage and reactor further carboxylates the fibers from the previous stage and reactor;
• wherein the first stage of the at least two catalytic carboxylation stages is carried out in a first reactor and includes adjusting the pH of the fibers to 8-11, followed by providing amounts of a precursor of an active oxoammonium salt catalyst and a secondary oxidant adapted to generate the active catalyst sufficient to effect carboxylation;
• wherein each subsequent stage of the at least two catalytic carboxylation stages is carried out in a subsequent reactor and a includes adjusting the pH of the fibers to 8-11, followed by providing additional amounts of the precursor and the secondary oxidant sufficient to effect further carboxylation;
• wherein the time in each of the at least two catalytic carboxylation stages is about 10 seconds to 5 minutes;
• and
• wherein the precursor is selected from the group consisting of heterocyclic nitroxides in which the carbon atoms adjacent the nitroxide nitrogen lack hydrogen substitution, their corresponding amines and hydroxylamines, and mixtures thereof, and is stable under aqueous alkaline conditions.