Monitoring Aqueous Phase Reactions by Operando ATR-IR Spectroscopy at High Temperature and Pressure: A Biomass Conversion Showcase

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.