Application of Polyoxometalate in Hydrogen Peroxide Bleaching under Acidic Conditions

The modified bleaching sequence OPAPPO from short-sequence bleaching OAP and OQP was studied in an effort to achieve higher quality straw pulp (with brightness 84.5% and acceptable viscosity 669 mL/g), which will be appropriate for more situations than straw pulp as presently produced. Though the OP and PO stages are recognized as the key processes used to increase the pulp’s brightness, addition of hydrogen peroxide in acid pretreatment with polyoxometalate (POM) as catalyst (AP stage) was mainly considered in this work. Phosphomolybdic acid was applied to improve straw pulp’s brightness, which was 4.8% ISO higher than the pulp treated without POM. The optimum conditions of the AP stage were: initial pH value 3, temperature 90 °C, H2O2 1.5%, and phosphomolybdic acid 1.0%. Comparison of the sequences OPQPO, OPAPO, and OPAPPO showed that the brightness of pulp bleached by OPAPPO was 2% and 4.7% higher than the same pulp subjected to OPAPO and OPQPO sequences, respectively. Four lignin samples (LOP, LOPA, LOPAP, LOPAPPO) were characterized by P NMR spectroscopy. The spectroscopic investigation showed that in LOPA and LOPAP, aliphatic hydroxyls, p-coumaryl units, and guaiacyl phenol moieties were degraded when compared with that in LOP. In LOPAPPO, all these aliphatic hydroxyls and guaiacyl phenols had been destroyed and carboxylic acid functionalities increased.