Lignin Catalytic Depolymerization for Phenolic Monomers: Boosting the Selective Cleavage of β-O-4 Bonds of Lignin by Mo═O and Al(IV)–O–BO₂ Interfacial Sites in B–Mo/Sepiolite

Abstract

Selective breaking of β-O-4 bonds is a challenge for lignin depolymerization. Herein, boron (B) was utilized to construct superficial MoO and subsurface Al(IV)–O–BO2 interfacial sites of Mo/sepiolite (SEP), which acted as Lewis acid sites to promote β-O-4 bond cleavage of lignin. Characterizations and density functional theory (DFT) calculations showed that B could interact with the quadridentate Al(IV) atoms of SEP to generate Al(IV)–O–BO2 and did not affect the incipient MoO of Mo/SEP. The experimental results demonstrated that the B content altered the catalytic performance of Mo/SEP, and B–Mo/SEP achieved 99.3% lignin liquefaction and the highest selectivity of phenol (48.5%) and ethoxyphenol (27.4%) monomers. Combined with DFT simulation, the results confirmed that the antibonding orbital of MoO accepted electrons of alcohol oxygen, while the B bonding orbital in Al(IV)–O–B–O2 received electrons of benzyl oxygen, promoting the formation of C+ intermediates. Simultaneously, oxygen vacancies in SEP activated the ethanol medium to facilitate enol intermediate hydrogenation for phenolic compounds.

Hydrogen‐transfer strategy in lignin refinery: Towards sustainable and versatile value‐added biochemicals

“…[91,127] Chen et al investigated the cleavage of a βÀ O-4 model compound catalyzed by a boron-doped Mo/SEP in supercritical ethanol. [128] The integration of boron into the catalyst modulated the electron distribution over the active metallic sites, thus enhancing electron transfer between the substrate and the metal and improving the adsorption of the substrate. The computational analysis clarified the kinetics and mechanisms influencing the model compound's conversion process facilitated by the catalyst (Figure 3e).…”

“…Adapted with permission from ref. [128] Copyright 2023, American Chemical Society. hydroxyl group were not adequate for the complete dissociation of the ether bond, resulting in a partially degraded intermediate marked by a modified γÀ OH group.…”

“…e) Adsorption configuration and adsorption energy barrier of βÀ O-4 model compound and ethanol by the interface of Mo/SEP and BÀ Mo/SEP catalysts. Adapted with permission from ref [128]. Copyright 2023, American Chemical Society.…”

A Comparative Analysis of Bio-Oil Collected Using an Electrostatic Precipitator from the Pyrolysis of Douglas Fir, Eucalyptus, and Poplar Biomass

“…Although the lignin content of Douglas fir (Table 1), a softwood [39], was higher than that of poplar, its bio-oil contained the lowest level of lignin-derived compounds (42.29%) compared to that of eucalyptus (60.62%) and poplar (67.04%). The discrepancy in the concentration of the lignin-derived compounds in the bio-oil and the lignin content of the respective biomass can be explained by the number of easily breakable β-O-4 linkages present in the biomass [40,41]. For instance, for the same mass of Douglas fir and poplar, Yoo et al [31] found 87 β-O-4 linkages in poplar, which was greater than the number of β-O-4 bonds present in Douglas fir (total linkages = 68).…”

Oxidative cleavage of lignin model substrates with Co(salen) catalyst: an experimental study on the effect of different reaction parameters in batch and continuous flow

“…1) Most literature research dedicated to lignin depolymerization, aims to effectively cleave these ether bonds yielding various aromatic compounds. 2–6 These compounds can then undergo hydrodeoxygenation to obtain a high-quality bio-oil. 7…”