Effects of partial oxidation on the microstructure of wood derived carbons

Abstract

The transformation of wood into advanced carbon materials relies on the partial preservation of its hierarchical structure after thermal treatments. Unlike isotropic materials, wood exhibits inherent anisotropy due to the alignment of cellulose fibrils along the tree’s growth direction within its lignocellulosic matrix. In this study, a pre-oxidation step was introduced before pyrolysis to investigate the effects of structural decomposition at five different temperatures: 250°C, 275°C, 287°C, 300°C and 325°C. These temperatures remain below the threshold for complete cellulose and lignin decomposition, selectively modifying the matrix by targeting hemicellulose, while aiming to preserve the alignment of cellulose fibers through subsequent pyrolysis at 800°C under two different heating rates (7°C/min, 1°C/min) and atmospheres (Nitrogen, Vacuum), and finally activation at 800°C with CO2 , ultimately leading to the formation nanoporous carbons with some anisotropy. To assess structural modifications, Wide-Angle X-ray Diffraction (WAXD) was used to study the crystallography of cellulose by their intensity curves, focused on the <110>, <11̅0> and <020> planes through all thermal steps. For the pyrolized samples, crystallite sizes from turbostratic nanocarbons arrangement were calculated for the in-plane (La) and out-of-plane (Lc) directions, according to <100> and <002> planes respectively. From the azimuthal intensity distribution of pyrolized samples a parameter 𝜂� was calculated as a degree of preferred orientation. Beyond WAXD, Small-Angle X-ray Scattering (SAXS) elucidated the density phase contrast difference between cellulose and lignocellulosic matrix for pre-oxidized samples, and details of pore structure for the pyrolized samples. Gas sorption measurements (N2 at 77k and CO2 at 273k) were conducted to investigate the specific pore volume (SPV), specific surface area (SSA) and pore size distribution (PSD) of the pyrolized and activated samples. As a main important outcome, samples that underwent partial oxidation at 287°C showed decomposition of hemicellulose while mostly preserving the crystalline cellulose fiber structure, indicating a controllable method to partially decompose the lignocellulosic matrix. Moreover, 287°C pre-oxidated wood followed by pyrolysis at 800°C with 1°C/min heating rate under vacuum demonstrated enhanced development of pore structure as well as the highest degree of preferred orientation with around 12% of the carbon being preferentially oriented along the tree’s growth direction. Final activation at 800°C with CO2 indicated pore structure development and high material yield for same pre-oxidated temperature at 287°C followed by pyrolysis at 800°C with 1°C/min heating rate. This highlights the potential to preserve anisotropic features to some extent after thermal treatments, suggesting pathways for tailoring wood nanocarbons for advanced functional applications.

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