Aqueous-phase depolymerization of waste cocoa pod husk lignin to produce aromatic monomers using terpyridine-based complex catalysts and advanced spectroscopic techniques

The catalytic depolymerization of lignin derived from cocoa pod husk (CPH) was studied under mild aqueous conditions to produce well-defined aromatic monomers. Three terpyridine-based catalytic systems were tested: a mononuclear Rh–terpyridine (complex 1), a binuclear Rh-Rh terpyridine (complex 2), and Cu–terpyridine (complex 3). Product distributions were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), gas chromatography–mass spectrometry (GC–MS), and nuclear magnetic resonance (NMR) spectroscopy. The mononuclear Rh–terpyridine system achieved the highest oil-phase yield, 71.2% of the initial lignin in the CPH, and selectively produced four main aromatic ketones: acetovanillone, propiovanillone, acetosyringone, and syringylpropanone. The binuclear Rh–Rh–terpyridine complex generated similar compounds with slightly lower efficiency. Conversely, the Cu–terpyridine catalyst produced only 29.4% oil and mainly guaiacyl-type monomers. MALDI-TOF MS spectra showed that Rh–catalysed reactions were dominated by the detected monomer peaks, while Cu–catalysed reactions displayed higher-mass signals at 332.4, 334.0, 336.0, and 374.2 Da, suggesting dimeric species. Proton NMR confirmed these trends, displaying strong aromatic and methyl ketone signals near 7.0 and 2.6 ppm, respectively, in the Rh-derived products, and much weaker signals in the Cu-derived oil. These results highlight the superior monomer selectivity and efficiency of Rh–terpyridine systems and suggest that Cu-based catalysts could serve as cost-effective alternatives for lignin valorisation.