Polyoxometalate-Based Closed Systems for Oxidative Delignification of Wood Pulp Fibers

Polyoxometalates in Oxidative Delignification of Chemical Pulps: Effect on Lignin

“…They have been under investigation as promising alternative bleaching agents, which may provide a basis for closed-mill delignification technologies. These studies have included the application of POMs as regenerable redox reagents [13,14] or as catalysts in oxygen bleaching [15,16]. …”

“…In accordance with these criteria, POMs with a redox potential in the range of +0.7 to 0.8 V vs. NHE have been used for delignification [13,20]. In delignification performed under anaerobic conditions, lignin is oxidized by POMs (reaction 1) in one stage and reduced POMs are re-oxidized (reaction 2) by oxygen at elevated temperatures in a separate stage; that is, the two reactions of transferring electrons from lignin to oxygen are performed in different process stages [13,14]. In an aerobic process, by contrast, lignin is oxidized (reaction 1) and reduced POMs are re-oxidized (reaction 2) in one stage [16,20].…”

“…A two-step process using this POM produced delignified pulp that had properties comparable to the properties of the pulp delignifed in the CDE bleaching sequence, to the same kappa number of 4.6. New POMs, including [SiV 2 W 10 O 40 ] 6- and [AlVW 11 O 40 ] 6- , have also been synthesized to allow reuse of POMs in numerous cycles for achieving mill closure while maintaining effective delignification [ 13 , 14 , 21 ].…”

“…The corresponding bleached pulps delignified to very low kappa number (about 4) at the same level of refining energy were of similar tensile index, burst index, tear index, and zero span tensile index. However, the CDE-bleached pulp was still superior in viscosity (viscosity of unbleached kraft pulp 28.7 mPa℘s, DE-bleached pulp 23.1 mPa℘s, POM-E bleached pulp 19.5 mPa℘s) [ 14 ]. Similar results were obtained in the comparative analysis of the properties of bleached pulps obtained in DE and POM bleaching followed by alkaline extraction (POM-E) [ 23 ].…”

“…Alkaline extraction of residual lignin, however, successfully resulted in a decrease in carbohydrates in lignin [ 31 ]. For this reason, it is important to follow the POM treatment with an alkaline extraction step to break any potentially present alkali-labile lignin-carbohydrate associations and remove oxidized lignin fragments that are insoluble under acid conditions [ 14 , 25 , 31 , 34 , 35 ].…”