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 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 – A₃₂₀/A₂₈₀. 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(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 apparent-M 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 M products become evident. Linear regression of SI₃₂₀(M) mean values (r² = 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 (ca. 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-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.