Mass-Resolved UV–Vis–GPC Mapping Diagnoses Catalyst Ageing in RCF Lignin Streams

Abstract

Catalyst stability is central to the viability of lignin-first biorefineries, yet conventional characterisation often fails to detect the subtle deactivation processes that govern product quality. Here we show that ultraviolet–visible (UV–Vis) spectroscopy, when coupled to gel permeation chromatography (GPC), can serve as a sensitive diagnostic of hydrogenation performance decline in reductive catalytic fractionation (RCF). We introduce a concentration-independent spectral index (SI₃₂₀), derived from the absorbance ratio at 280 and 320 nm, given by SI₃₂₀ = 1 – A₃₂₀/A₂₈₀. Native-like lignins show negligible absorbance above 320 nm (SI₃₂₀ ≈ 1), whereas condensation, benzylic oxidation, and extended π-conjugation depress SI₃₂₀. As a ratio, SI₃₂₀ is concentration-independent within the Beer–Lambert regime and can be profiled across the chromatogram to yield SI₃₂₀(M) profiles, with M denoting apparent molar mass. SI₃₂₀(M) profiles report directly on the formation of chromophores associated with catalyst ageing across the lignin molar-mass distribution. Using post-consumer cardboard as a substrate, we tracked RCF over Raney® Ni across multiple recycling runs. Comparative analysis of fresh and recycled catalysts revealed systematic SI₃₂₀ downshifts in heavy oligomer fractions, signalling chromophore accumulation well before bulk yield changes were evident. Linear regression of mean SI₃₂₀(M) values (r² = 0.95) enables a practical catalyst-life estimate. Under our conditions, Raney® Ni suppresses lignin degradation for up to ~80 hours of operation, after which the chromophore density approaches that of organosolv lignin. Our findings reframe UV–Vis spectroscopy from a simple detection method for GPC analysis into a diagnostic platform of lignin-first catalysis. By funnelling a M resolved spectra into a simple index, GPC–UV-Vis enables rapid, non destructive monitoring of catalyst performance, supports optimisation of RCF conditions and recycling protocols, and highlights the stabilising action of hydrogen transfer catalysis. In the broader context, the approach is general to diverse feedstocks, catalysts, and lignin first modalities, offering a practical route to correlate catalyst ageing with product quality and to guide development of durable, robust catalysts for circular economy and lignin valorisation.