Redox-neutral depolymerization of lignin-derived aryl ethers catalyzed by Rh(III)-complexes: a mechanistic insight

Area of Science:

• Computational Chemistry
• Catalysis
• Biomass Conversion

Background:

• Lignin depolymerization is crucial for converting biomass into valuable chemicals.
• Aryl ether bonds are key linkages in lignin's complex structure.
• Rhodium complexes show promise for catalyzing lignin breakdown.

Purpose of the Study:

• To elucidate the mechanism of redox-neutral depolymerization of lignin-derived aryl ether dimers.
• To investigate the catalytic activity of rhodium-terpyridine ([Rh]) and binuclear rhodium ([2Rh]) complexes.
• To understand the role of hydrogen sources in the catalytic process.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• The TPSSh-D3(BJ)/def2-TZVP (SMD, water) level of theory was utilized.
• Analysis focused on the Cβ-O bond cleavage in β-O-4 model compounds.

Main Results:

• The mechanism involves dehydrogenation to a ketone intermediate, followed by reductive ether bond cleavage.
• Rhodium-hydroxide ([Rh]-OH) and rhodium-hydride ([Rh]-H) intermediates play key roles.
• The binuclear rhodium complex ([2Rh]-H) demonstrated high reactivity with a low energy barrier for Cβ-O bond breakage (35.3 kcal mol⁻¹).
• Lignin itself can serve as a hydrogen source for generating active rhodium-hydride species.

Conclusions:

• The catalytic system effectively cleaves aryl ether bonds in lignin model compounds.
• The efficiency of aromatic product formation is limited by the hydrogen supply from lignin.
• Further optimization is needed to enhance yields in practical applications.