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Lignin from lignocellulosic biomass is a promising source of energy, fuels, and chemicals. The conversion of the polymeric lignin to fuels and chemicals can be achieved by catalytic and noncatalytic pyrolysis. The influence of nonporous silica and zeolite catalysts, such as silicalite, HZSM5, and HUSY, on the radical and volatile product formation during lignin pyrolysis was studied by in situ high-temperature electron paramagnetic resonance spectroscopy (HTEPR) as well as GC-MS. Higher radical concentrations were observed in the samples containing zeolite compared to the sample containing only lignin, which suggests that there is a stabilizing effect by the inorganic surfaces on the formed radical fragments. This effect was observed for nonporous silica as well as for HUSY, HZSM5, and silicalite zeolite catalysts. However, the effect is far larger for the zeolites owing to their higher specific surface area. The zeolites also showed an effect on the volatile product yield and the product distribution within the volatile phase. Although silicalite showed no effect on the product selectivity, the acidic zeolites such as HZSM5 or HUSY increased the formation of deoxygenated products such as benzene, toluene, xylene (BTX), and naphthalene. [ABSTRACT FROM AUTHOR]

This study reveals that catalytic and non-catalytic fast pyrolysis of lignin is influenced by the lignin separation method. Lignin of the same soft- and hardwood source was prepared by the Dioxane-, Klason- and Organsolv-methods and pyrolyzed at different temperatures using H-USY zeolite. The product selectivity demonstrated temperature dependence, changing from phenol alkoxy ketone products to phenols at high temperatures. Hardwoods produced more monophenols with carbonyl groups, and softwoods generated more phenols at high temperature. Relative yields and selectivities varied between each lignin type. The yield in liquid products and gases depends on the overall bond structure in the lignin. The presence of aryl ether bonds increases the conversion above 600 °C, but has the inverse effect at lower temperatures. High selectivity for one group of products could be achieved under optimized conditions. Commercial Kraft lignin demonstrated the smallest change in yield and selectivity with temperature. [ABSTRACT FROM AUTHOR]

The radical concentrations and gfactors of stableorganic radicals in different lignin preparations were determinedby X-band EPR at 9 GHz. We observed that the gfactorsof these radicals are largely determined by the extraction processand not by the botanical origin of the lignin. The parameter mostlyinfluencing the gfactor is the pH value during ligninextraction. This effect was studied in depth using high-field EPRspectroscopy at 263 GHz. We were able to determine the gxx, gyy, and gzzcomponentsof the gtensor of the stable organic radicals in lignin.With the enhanced resolution of high-field EPR, distinct radical speciescould be found in this complex polymer. The radical species are assignedto substituted o-semiquinone radicals and can existin different protonation states SH3+, SH2, SH1-, and S2-. The proposed model structuresare supported by DFT calculations. The gprincipalvalues of the proposed structure were all in reasonable agreementwith the experiments. [ABSTRACT FROM AUTHOR]

Lignin pyrolysis is a promising method for the sustainable production of phenolic compounds from biomass. However, detailed knowledge about the radicals involved in this process and their influence on the molecular products is missing. Herein, we report on the pyrolysis of hard- and softwood Klason lignins under inert gas conditions in a temperature range between 350-550 °C. During the pyrolysis process, the formed radicals were detected by in situ high-temperature electron paramagnetic resonance spectroscopy. The overall formation of volatile products during lignin pyrolysis was determined using thermogravimetric analysis. The volatile molecular products were characterized and quantified using GC-MS analysis. Major differences were observed between hardwood and softwood lignins. Hardwood lignins form more radicals and volatile products than softwood lignins at temperatures from 350 to 450 °C. In the late stages of the pyrolysis process at 550 °C radical quenching reactions become dominant in hardwood lignins. We identified the disproportionation of two semiquinone radicals to quinone and hydroquinone species as the most likely quenching reaction. Our results show that both the pyrolysis temperature and the type of lignin source have a major influence on radical formation and the molecular products during the depolymerization of lignin. [ABSTRACT FROM AUTHOR]