Mass-resolved UV–Vis–GPC mapping diagnoses catalyst ageing in RCF lignin streams

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 demonstrate that ultraviolet-visible (UV-Vis) spectroscopy, when combined with gel permeation chromatography (GPC), can serve as a sensitive diagnostic tool for detecting catalyst 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 - <i>A</i>₃₂₀/<i>A</i>₂₈₀. Native-like lignins show negligible absorbance at 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(<i>M</i>) profiles, with <i>M</i> denoting apparent molar mass. SI₃₂₀(<i>M</i>) profiles report directly on the formation of chromophores associated with catalyst ageing across the lignin apparent-<i>M</i> distribution. Utilising post-consumer cardboard as a substrate, we tracked RCF over RANEY^® Ni across multiple recycling runs. A comparative analysis of fresh and recycled catalysts revealed systematic SI₃₂₀ downshifts in oligomer fractions, indicating chromophore accumulation well before changes in bulk yield of low <i>M</i> products become evident. Linear regression of SI₃₂₀(<i>M</i>) mean values (<i>r</i>² = 0.95) enables a practical estimate of catalyst life. Under our conditions, it is estimated that RANEY^® Ni can sustain lignin stabilisation for up to 15 runs of catalyst use (<i>ca.</i> 45 h 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 apparent-<i>M</i>-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.