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Chromophores in cellulosics, XVIII. Degradation of the cellulosic key chromophore 5,8-dihydroxy-[1,4]-naphthoquinone under conditions of chlorine dioxide pulp bleaching: a combined experimental and theoretical study
摘要： DHNQ is one of the key chromophores occurring in all types of aged cellulosics. This study investigates the degradation of DHNQ by chlorine dioxide at moderately acidic (pH 3) conditions, corresponding to the conditions of industrial bleaching ("D stage"). The degradation involves three major pathways. As initial reaction, a hydrogen transfer from DHNQ to chlorine dioxide via a PCET mechanism occurs to form a radical DHNQ(cid:2) and chlorous acid. DHNQ(cid:2) is then attacked by water to give a pentahydroxynaphthalene radical PHN(cid:2) that is stabilized by strong delocalization of the non-paired electron into its aromatic ring. PHN(cid:2) immediately disproportionates to give the observable intermediate 1,2,4,5,8-pentahydroxynapththalene (I), which was comprehensively con?rmed by NMR and MS (path A). In the presence of excess ClO2, I is immediately further oxidized into acetic acid, glycolic acid, oxalic acid and CO2 as the ?nal, stable, and non-colored products (path C). In the absence of excess ClO2, elimination of water from I regenerates DHNQ (path B), so that at roughly equimolar DHNQ/ClO2 ratios ClO2 is fully consumed while a major part of DHNQ is recovered. To avoid such DHNQ "recycling" under ClO2 consumption—and to completely degrade DHNQ to colorless degradation products instead—ClO2 must be applied in at least ?vefold molar excess relative to DHNQ.
关键词： Density functional theory (DFT),Ab initio calculations,Yellowing,Cellulose,Brightness,Chromophores,5,8-dihydroxy-[1,4]-naphthoquinone,Chlorine dioxide,Brightness reversion,Pulp bleaching
更新于2025-09-23 15:21:01
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European Microscopy Congress 2016: Proceedings || Laser induced yellowing of stonework: a combined TEM imaging and STEM-EELS study on model samples
摘要： Nd-YAG Q-Switched laser devices operating at 1064 nm have been considered in the 1990s as the most promising tool for cleaning stone sculptures, and more particularly eliminating indurated black gypsum crusts. However, the spreading of this laser technology has been undermined because of the yellow hue it occasionally conveys to the cleaned surfaces as seen on Figure 1. Especially in France, this yellowing effect is considered as a major esthetic issue by conservators and the laser technique has gradually disappeared from the restoration sites. This discoloration issue remains partly unexplained: a currently admitted hypothesis states that the iron containing compounds present in the black crusts would transform, upon laser irradiation, into yellow iron-rich nanophase(s) that would re-deposit on the cleaned substrate. To verify this hypothesis, model black crusts have been elaborated by mixing hematite α-Fe2O3 and gypsum CaSO4.2H2O in different proportions. The model crusts were irradiated using a Nd-YAG QS laser resulting in ablation of numerous particles in a visible smoke and the color of the samples shifted instantaneously from red to a bright yellow. Transmission electron microscopy (TEM) has been used to characterize the morphology of the nanostructures generated by the laser, both in the smoke and on the surface of the samples as observed in Figure 2. In addition, the chemical composition of the neo-formed nanophases was determined by aberration corrected scanning transmission electron microscopy in combination with electron energy loss spectroscopy (STEM-EELS) and high angle annular dark field imaging (HAADF) as seen on Figure 3. It was found that both the surface of the samples and the ablated micro-materials are covered by an irregular nano-film and by dispersed spherical nanoparticles, all containing iron and oxygen. These results ascertain the link between the yellowing effect and the presence of iron containing nanophases after irradiation.
关键词： EELS,TEM,STEM,nano materials,iron,yellowing,laser
更新于2025-09-11 14:15:04