Photocatalytic dissolution of cellulose for hydrogen and nanofiber production: unveiling crucial factors via experiments and informatics

The efficient utilization of biomass resources and solar energy is necessary for next-generation sustainable carbon-neutral societies. Although cellulose is the most abundant biomass on Earth, its utilization as a carbon resource is hampered by its strongly stabilized polymer-bundled structure. In this study, a new photoredox cascade catalyst (PRCC) conversion system was developed by combining dual-dye-sensitized Pt-cocatalyst-loaded TiO₂ nanoparticle photocatalysts (DDSPs) and a 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) oxidation catalyst for the production of cellulose nanofibers and hydrogen from various cellulose substrates (powder, paper, sponge, and wood pellets) under blue light irradiation without the use of strong acids/bases. UV-vis absorption and emission spectroscopy revealed that the loaded amount of the Ru(II) dye on the TiO₂ surface was successfully controlled in the range of 353–667 nmol/1 mg TiO₂, and the immobilization order of two Ru(II) dyes significantly affected the energy- and electron-transfer behaviors between the Ru(II) dyes and TiO₂ nanoparticles. Our systematic evaluation of the photocatalytic activity and machine learning analysis of 12 different DDSPs revealed that the immobilization order of the two Ru(II) dyes, full coverage of the TiO₂ nanoparticle surface with suitable Ru(II) dye molecules, and Zr⁴⁺ cation loading are crucial factors for achieving a high apparent quantum yield for the hydrogen-evolving PRCC conversion of cellulose to nanofibers (max. 1.62% at 467 nm excitation for the initial 1 h of reaction in a 0.3 M cellulose aqueous dispersion). The findings contribute to the development of an environmentally benign photocatalytic approach for the conversion of cellulosic biomass as a carbon resource into valuable organic products.