FT Raman and UV Visible Spectroscopic Studies of a Highly Selective Polyoxometalate Bleaching System, 1993 Pulping Conference Proceedings

Near-Infrared Fourier Transform (NIR FT) Raman spectroscopy and ultra-violet (UV) visible spectroscopy were used to observe chemical changes in residual lignin in softwood kraft pulp upon exposure to a vanadium-substituted polyoxometalate, which is representative of a new class of bleaching agents currently under investigation in our laboratory. In conventional Raman spectroscopy, using visible laser excitation, considerable fluorescence is normally excited when chromaphores are present. In FT Raman spectroscopy, however, using excitation in the NIR, the magnitude of the fluorescence is significantly reduced. After exposure of kraft pulp to solutions of the polyoxometalate a -Keggin-K5[SiVW11 0*40*], spectroscopic evidence for the oxidation of phenols to quinones and a-hydroxyl (benzyl alcohol) moieties to u-ketones was obtained. The quantification of residual lignin by FT Raman spectroscopy of solid pulp samples and transmission UV visible spectroscopy of dissolved pulp samples was demonstrated.