Nanomapping and speciation of C and Ca in thermally treated lignocellulosic cell walls using scanning transmission X-ray microscopy and K-edge XANES

Abstract

The carbon matrix in cell walls of lignocellulosic plants has high recalcitrance to chemical and biological decomposition. Thermal treatments, such as torrefaction and pyrolysis are therefore of interest to pre-process biomass. Reed canary grass (RCG) as biomass model was treated at 90, 300, 400 and 500°C in N2 atmosphere. The induced cell wall changes, especially for C and Ca and their speciation and nanomapping were studied using synchrotron based (1) scanning transmission X-ray microscopy (STXM) C 1s near edge X-ray absorption fine structure (NEXAFS) spectroscopy, (2) STXM Ca K-edge NEXAFS, and (3) Ca K-edge X-ray absorption of near edge structure (XANES) spectroscopy.Clusters of nano-based pixels having different spectral features were identified using principal component analysis of mosaics of C 1s NEXAFS images. At 300°C there were only minor changes and peaks related to lignin were only slight reduced. At 400°C, the chemical change was substantial and most resonances related to CO and CH functional groups declined considerably. Finally, at 500°C there were two clusters indicating that the structural integrity of the cell wall was lost and aromatic CC resonances related to quinones appeared. These results were confirmed by FT-IR. The nanomapping also indicated that Ca oxalate has a sub-micron particle size distribution, most sizes <200nm. These may act as seed particles during combustion. Ca speciation according to Ca K-edge XANES showed that oxalate (CaC2O4) was the primary phase of Ca and thermal treatment induced minor Ca phase transition into butschliite (K2Ca(CO3)2) and hydroxyapatite (Ca5(PO4)3OH).