The Nature of Stable Char Radicals: An ESR and DFT Study of Structural and Hydrogen Bonding Requirements

Abstract

Pyrolysis is a promising way to convert biomass into fuels and chemicals. This reaction is complex and inevitably involves a cascade of radical reactions that lead to char formation, in which some radicals become trapped and stabilized. Their nature is difficult to characterize, and in this respect computational chemistry can be a strong supplementary tool to electron spin resonance spectroscopy and other experimental methods. Here biomass char radicals and oxidation reactivity are studied experimentally, and density functional theory is used to predict the thermodynamic stability and g‐values of carbon‐ and oxygen‐centered radicals of polyaromatic char models including defect structures. Hydroxylated and especially certain dihydroxylated structures provide exceptional stabilization of oxygen‐centered radicals. Hydrogen bonding plays a crucial role, and it is proposed that hydrogen atom transfer couples radical localizations. This is a new proposal on the structural requirements for stabilization of char radicals, which impacts our understanding of pyrolysis mechanisms and char reactivity.

Effect of Microwave and Thermal Co-pyrolysis of Low-Rank Coal and Pine Wood on Product Distributions and Char Structure

“…The overlapping between nitrogen-containing and oxygen-containing radicals with a g -value of 2.0031 could indicate the presence of nitrogen radicals in raw coal since it shows a g -value greater than 2.0030. Figure b also shows that the PAH-based defect structures, which usually have a g -value less than 2.0040, were not present in all measured samples . This indicates that the biomass, coal, and mixed chars mainly contained radicals, which are stable hydroxylated PAH-based structures.…”

“…Figure 7b also shows that the PAH-based defect structures, which usually have a g-value less than 2.0040, were not present in all measured samples. 67 This indicates that the biomass, coal, and mixed chars mainly contained radicals, which are stable hydroxylated PAH-based structures. The chars prepared under microwave and thermal pyrolysis conditions contained spectra with g-values between 2.0027 and 2.0029, suggesting the presence of carbon-centered radicals, which is between gvalues of 2.0025 and 2.0030.…”

Structural and Thermal Characterization of Novel Organosolv Lignins from Wood and Herbaceous Sources

“…The results have shown that lignins richer in guaiacol unit are less reactive than herbaceous lignins containing additionally H-and S-type phenolics, as shown in Figure 2b,c. This could be explained by the formation of large condensed and polyaromatic structures from guaiacol during heating of softwood lignin, whereas the HGS and GS lignins contain a high number of methoxy groups leading to less thermal stability on the basis of an altered reactivity [58,59]. On the basis of the array of lignins studied in here it is not possible, however, to identify points in the curves that would clearly reflect differences in phenolic OH-group content or the ratio between aliphatic and aromatic OH-groups.…”

Atomistic Modeling of Lignocellulosic and Carbonaceous Fuels and Their Pyrolysis Reactions: A Review

“…This understanding is vital for unraveling the role they play in the mechanisms of biochar formation and oxidation processes. 106 Traditionally, coal/char models are constructed based on characterization data. However, an alternative approach, as discussed in Sections 3.3 and 4.2.1 involves building a model that incorporates the solid products from the pyrolysis process.…”

“…However, due to their reactivity, most of these free radicals strive for stability by reacting with nearby reactive species. This process of stabilization, also known as condensation from the larger fragments, ultimately leads to the formation of biochar and tar. , There are two competitive mechanisms: the rupture of the molecules due to the thermal decomposition to generate mainly volatile compounds and cross-linking reactions to form carbon-network structures . Several authors have presented the main reactions depending on the temperatures. ,, The following sections highlights the fundamental discoveries associated with temperatures.…”