Ethanol as capping agent and formaldehyde scavenger for efficient depolymerization of lignin to aromatics

Abstract

ARTICLE This journal isObtaining renewable fuels and chemicals from lignin presents an important challenge to the use of lignocellulosic biomass to meet sustainability and energy goals. We report on a thermocatalytic process for the depolymerization of lignin in supercritical ethanol over a CuMgAlO x catalyst. Ethanol as solvent results in much higher monomer yields than methanol. In contrast to methanol, ethanol acts as a scavenger of formaldehyde derived from lignin decomposition. Studies with phenol and alkylated phenols evidence the critical role of the phenolic -OH groups and formaldehyde in undesired repolymerization reactions. O-alkylation and C-alkylation capping reactions with ethanol hinder repolymerization of the phenolic monomers formed during lignin disassembly. After reaction in ethanol at 380 °C for 8 h, this process delivers high yields of mainly alkylated mono-aromatics (60-86 wt%, depending on the lignin used) with a significant degree of deoxygenation. The oxygen-free aromatics can be used to replace reformate or can serve as base aromatic chemicals; the oxygenated aromatics can be used as low-sooting diesel fuel additives and as building blocks for polymers. ARTICLE This journal is ARTICLE This journal is ARTICLEThis journal is

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

“…[286] As will be discussed in the next sections,h igh temperatures (300-450 8 8C) and elevated H 2 pressures (MPa) are typically required for cracking the CÀCb onds in technical lignins to produce low M w products in heterogeneously catalysed processes. [287,288] Conversely,strategies based on ECCL benefit from the intrinsically high reactivity of native lignins compared to condensed and therefore recalcitrant technical lignins.I ndeed, from the solvolytically released lignin fragments,t he ECCL directly produces monophenols and small oligomers (M w 100-400 Da). [198,277,278] Notably,h emicellulose sugars released by solvolytic processes also play akey role in tuning the catalyst activity and selectivity,thus carrying major implications for the product distribution achieved by ECCL, as recently demonstrated for the process employing Raney Ni as acatalyst for H-transfer reactions.…”

“…When employing a1:1 methanol:ethanol solvent mixture,e thanol can furthermore "scavenge" any methanol-derived formaldehyde and prevent condensation reactions with aromatic species.T his effect is beneficial, as formaldehyde is also likely to form through elimination of the g-CH 2 OH groups on the propyl side-chain of lignin units.Building on this insight, further optimisation of the reaction conditions with actual Protobind lignin allowed the formation of 60 wt %a lkylated mono-aromatics from lignin at 380 8 8C. [288] 3.4.5. Catalyst Stability and Lignin Impurities Sufficient catalyst lifetime is essential for economically viable lignin downstream processing.…”

“…[397] However,as ignificant beneficial effect of using ethanol as asolvent was later discovered. [288,371] Indeed, ethanol may function as a" capping agent" by ethoxylating reactive phenol moieties and preventing subsequent recondensation. HSQC NMR analysis of aP rotobindderived lignin oil, depolymerised in ethanol over the same catalyst at 300 8 8C, demonstrated extensive alkylation, with phenol-O alkylation preceeding ring C-alkylation.…”

Thermal stability of low and high Mw fractions of bio-oil derived from lignin conversion in subcritical water

“…5. These were performed based on the 1 H- 13 C NMR assignments of the model lignin-derived compounds in DMSO-d 6 and some references [26,[36][37][38][39][40][41][42]. These crosspeaks are related to the main types of aromatic products confirmed by GC-MS measurement.…”

Decomposition of β-O-4 Linked Lignin Model Compound in Anhydrous Ethanol without any Added Catalyst

“…The main products, phenol and acetophenone, and the by-product, POPE, all have two more hydrogen atoms compared to the raw material of 2-PAP (Line II and III in Scheme 1). Solvents with hydrogen donating capability, such as methanol, ethanol, and formic acid, were employed in the decomposition of the lignin compound (Xu et al 2012;Song et al 2013;Huang et al 2015).…”