Catalytic fast pyrolysis of beech wood lignin isolated by different biomass (pre)treatment processes: Organosolv, hydrothermal and enzymatic hydrolysis

Lignin is one of the three main structural components of lignocellulosic biomass and is considered as the most abundant source of aromatic and phenolic compounds. Lignin is produced as side-stream in the pulp/paper industry or as residue in the production of second-generation bioethanol. More recently, novel biomass fractionation processes in biorefineries have been developed aiming at the production of high quality/purity lignin towards its more efficient down-stream catalytic conversion to chemicals, monomers, and fuels. Within this context, in this work, we studied the thermal (non-catalytic) and catalytic fast pyrolysis on a Py/GC–MS system and a fixed-bed reactor unit of three types of lignin, all originating from the same biomass (beech wood sawdust) but with different isolation processes: organosolv lignin derived by the organosolv pretreatment/fractionation of biomass, surface extracted lignin derived by the mild Soxhlet extraction (with ethanol or acetone) from the hydrothermally (HT) in pure water pretreated biomass, and the enzymatic hydrolysis lignin derived as a lignin-rich solid residue from the enzymatic hydrolysis of the HT pretreated biomass. Conventional microporous ZSM-5 and mesoporous ZSM-5 zeolites (with intracrystal mesopores, ˜ 9 nm) were used as catalysts in the pyrolysis experiments. Both zeolites were very active in converting the initially produced via thermal pyrolysis methoxy-substituted phenols, benzenes and benzaldehydes mainly towards BTX mono-aromatics, such as 1,3-dimethylbenzene/p- and o-xylenes, toluene and trimethylbenzenes, as well as polycyclic aromatic hydrocarbons (PAHs, mainly naphthalenes). The mesoporous ZSM-5 induced higher dealkoxylation reactivity compared to the microporous ZSM-5 leading to higher concentration of BTX aromatics without a consequent increase of PAHs. The pronounced dealkoxylation/aromatization reactivity of both ZSM-5 zeolites resulted to lower yields (but highly aromatic) organic bio-oils (e.g. from 32−35 wt.% to 15−22 wt.%) compared to thermal pyrolysis, increased non-condensable gases (mainly CO, CO₂, methane, ethylene and propylene) and formation of relatively low amounts of coke on catalysts (e.g. 3−5 wt.% on lignin) in addition to thermal pyrolysis char. The observed moderate variations in the characteristics of the three types of beech wood lignin, i.e. molecular weight (GPC), S/G ratio, inter-unit linkages (2D HSQC NMR) and elemental composition, did not induce a systematic/substantial differentiation in the thermal and catalytic fast pyrolysis product yields and composition.