Full conversion of lignocellulose using polyoxometalate catalysts with redox sites and antagonistic acidity/basicity

The full utilization of lignocellulose involves two distinct catalytic routes: i) oxidative depolymerization of lignin and ii) acid/alkaline hydrolysis of hemicellulose and cellulose. To improve efficiency and reduce costs, constructing a single-cluster catalyst represents a desirable yet challenging strategy. Herein, triple-functional molecular polyoxometalates (POMs), NLL_nH_6-nV₂Mo₁₈O₆₂ (n = 1-6) were fabricated using N-lauroyl-l-lysine (NLL) and H₆V₂Mo₁₈O₆₂ as precursors. Besides its amphiphilicity to form nano-micelles with polyanion uniformly dispersed outside and NLL inside, NLL also provided basic sites to H⁺/redox POMs to compensate the loss of acidity and enabled spatial separation of antagonistic acid/base sites within a single POM molecule. Density Functional Theory, Molecular Dynamics simulations and experiments were employed to analyze these processes. The adsorption of -OH in 2-phenoxy-1-phenylethanol (pp-ol) was achieved by interacting with polyanion and extra with NH and C = O groups in NLL. These synergistic effects resulted in concentrating and confining pp-ol and reactive oxygen species around polyanion, which turnover frequency increased by 0.066 h^-1 compared to homogeneous H₆V₂Mo₁₈O₆₂. Full conversion of various soft and hard lignocellulose was achieved using NLLH₅V₂Mo₁₈O₆₂ catalyst under gradually increasing temperature. During the conversion process, the lignin was oxidized mainly through β-O-4 bond cleavage without addition of NaOH, and the degradations of hemicellulose and cellulose were realized through acidic hydrolysis. The characteristics of this triple POMs allowed it to show higher activity than homogeneous H₆V₂Mo₁₈O₆₂ and previous BetH₅V₂Mo₁₈O₆₂ (Bet, i.e. betaine), which provided an alternative to developing new surfactant-type POMs in biomass conversion. The temperature-controlled properties in NLLH₅V₂Mo₁₈O₆₂ allowed easy separation and regeneration.