In situ UV–VIS-NIR spectrophotometric detection system as a research tool for environment-friendly chemical processes

Abstract We recently demonstrated the solid-state catalyst-mediated hydrothermal flow reactor system, SSCM-HF, for in situ ultraviolet–visible (UV–VIS) absorption spectrophotometric measurements of solid–liquid two-phase reactions within the second time-scale at high temperature. Successful results were obtained using SSCM-HF in the UV–VIS region, including the oxidation of alcohols to aldehyde with dissolved oxygen and the conversion of glucose into 5’-hydroxymethylfurfural. Here, the scope and detailed characteristics of the Hydrothermal Flow detection system for in situ absorption spectrophotometric measurements in the UV–VIS near-infrared (NIR) region (UVN region) (HF-UVN) were investigated. The optical and mechanical performance of the HF-UVN, which consists of a high pressure pump, automatic sample injector, high-temperature reactor with a detection coupler, quenching bath, and automatic pressure controller, were evaluated at high temperature and pressure. Our previous study implied that the difficulty of detection was due to the drift at high temperature and pressure since the surfaces of optical fibers with 0.2 mm core size and capillary surfaces with outer and inner diameters of 0.375 mm and 0.1 mm, respectively, were placed at right angles inside the heating blocks without a tight coupler. The optical characteristic of the combination of fused-silica capillary and optical fibers in the HF-UVN was evaluated, and a coupler was designed to fix the fused-silica capillary and optical fibers. The HF-UVN was applied to monitor the stability of chromogenic reagents at 300 °C and the oxidation of aldehyde to carboxylic acid. This study demonstrated that the HF-UVN can be applied for UV–VIS-NIR monitoring not only in the laboratory but also in practice for the optimization of chemical processes in the liquid phase at high temperature and pressure.