Technical lignins are increasingly available at industrial scale, offering opportunities for valorization, such as by (partial) depolymerization. Any downstream lignin application requires careful tailoring of structural properties, such as molecular weight or functional group density, properties that are difficult to control or predict given the structure variability and recalcitrance of technical lignins. Online insight into changes in molecular weight (Mw), to gauge the extent of lignin depolymerization and repolymerization, would be highly desired to improve such control, but cannot be readily provided by the standard ex‐situ techniques, such as size exclusion chromatography (SEC). Herein, operando attenuated total reflectance infrared (ATR‐IR) spectroscopy combined with chemometrics provided temporal changes in Mw during lignin depolymerization with high resolution. More specifically, ex‐situ SEC‐derived Mw and polydispersity data of kraft lignin subjected to aqueous phase reforming conditions could be well correlated with ATR‐IR spectra of the reaction mixture as a function of time. The developed method showed excellent regression results and relative error, comparable to the standard SEC method. The method developed has the potential to be translated to other lignin depolymerization processes.

Spectroscopic insight into aqueous phase catalytic reactions under realistic conditions aids detailed understanding and fosters further improvements. However, this is also very challenging, particularly in the liquid phase, given the typically elevated temperatures and pressures employed. Here, we report an operando Attenuated Total Reflectance‐Infrared (ATR‐IR) spectroscopic method to monitor the Aqueous Phase Reforming (APR) of Kraft lignin at 225 °C and 30 bar over a Pt/Al2O3 catalyst. This is a showcase reaction, while the presented method developed is more generic. In addition to the high temperature and pressures conditions, spectral interference from bulk water is a major challenge. Other challenges include self‐absorption of IR light, molecular interactions, high substrate loadings and the noise resulting from water vapor and CO2. We present a protocol utilizing the temperature‐dependent single‐beam method to obtain high‐quality ATR‐IR spectra virtually free from temperature, background and matrix effects. The protocol allowed for effective correction for the aqueous solvent as a function of reaction temperature. A comparison with conventional spectral acquisition and correction shows the advantages of this approach.

Invited for this month's cover is the group of Bert M. Weckhuysen at the Utrecht University (The Netherlands). The cover picture shows an attenuated Total Reflection Infrared spectroscopy (ATR‐IR) probe monitoring a chemical reaction in the liquid phase, particularly water, at demanding conditions. The light penetrates few microns deep in proximity of the ATR‐IR probe and thus any IR‐active entity in that range is probed. This research systematically addresses the challenges associated with acquiring operando ATR‐IR spectra for liquid/aqueous phase reactions, which is essential for better understanding of chemical reactions in the liquid phase. Read the full text of their Full Paper at 10.1002/cmtd.202100041.

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