Novel visualization studies of lignocellulosic oxidation chemistry by application of C-near edge X-ray absorption fine structure spectroscopy

Abstract

The research presented herein is the first attempt to probe the chemical nature of lignocellulosic samples by the application of carbon near edge X-ray absorption fine structure spectroscopy (C-NEXAFS). C-NEXAFS is a soft X-ray technique that principally provides selective interrogation of discrete atomic moieties using photoelectrons of variable energies. The X1A beam line of the National Synchrotron Light Source was employed for the specific purpose of observing carboxylic acid moieties that display a signature absorption band centered at 289 eV. This study caps a larger effort to support the mechanistic basis for lignocellulosic fiber chemical degradation induced by the disproportionation of hydrogen peroxide during fiber bleaching trials. It is shown that fibers that have been bleached with a hydrogen peroxide phase without removal of resident pendant metals (Mn, Cu, Fe) sustain significant macroscopic damage likely via classical Fenton-type radical reactions, as evidenced by a tensile reduction by over 30%. We present X-ray absorption spectra obtained using a scanning transmission X-ray microscope (STXM) at the end of a 2.5 GeV electron synchrotron that provided 1s →π^* contrast-enhanced micrographs illustrating a random distribution of acid functionalities that were principally located on fiber surfaces. Control studies using non-bleached fibers demonstrated that very little signature carboxylic acid absorption patterns were present in the fibers, suggesting that these groups are an incriminating fingerprint for macroscopic fiber strength damage during non-radical suppressed bleaching trials.