Phenol-promoted guanidine hydrochloride/lactic acid deep eutectic solvent for bamboo pretreatment to achieve...

The efficient valorization of lignin is hindered by the irreversible cleavage of the predominant β-O-4 linkages and competing condensation side-reactions. These processes yield a recalcitrant, condensed lignin structure with dark color. In this study, a novel ternary phenol-promoted guanidine hydrochloride/lactic acid (GH/LA) deep eutectic solvent (DES) was developed to achieve the efficiency fractionation and lignin protection. Unlike traditional phenol-assisted processes which typically rely on alkaline media, this work leverages the synergy between an acidic DES and a phenolic capping agent. The GH/LA system framework provides potent solvent power for biomass deconstruction, while phenol simultaneously suppresses acid-induced lignin condensation by quenching reactive intermediates. The pretreatment could achieve 92.0% delignification at 130 °C and 30 min while retain 88.6% cellulose and achieve a glucose yield of 89.7%. Recovered lignin exhibited less condensed structure with preserved β-O-4 linkage and light color, which provided 42.1% monomer yield from catalytic hydrogenolysis. The mechanism study suggested that phenol was kinetically more favorable than lignin fragment to react with the benzyl carbocation intermediate, therefore could suppress the condensation and prevent the cleavage of β-O-4 bond. The recyclability results showed that the prepared phenol/guanidine hydrochloride/lactic acid DES system could result in 75.6% delignification and 72.1% glucose yield after three pretreatment cycles. The findings from this study could provide an effective approach for biomass refining.