Beyond UV: Overcoming the Optical Bottleneck in Photodesulfurization of Diesel Oil Via Visible and Red-light...

Strict environmental regulations require the production of ultra-low sulfur diesel (ULSD); however, the conventional hydrodesulfurization (HDS) process remains highly energy-intensive. Photocatalytic oxidative desulfurization (PODS) at room conditions has emerged as a promising green alternative. However, industrial applications of UV-based PODS are severely hampered by the Inner Filter Effect (IFE), in which the aromatic-rich diesel matrix strongly absorbs UV radiation, leading to severe photon starvation within the liquid volume and strictly limiting the reaction to the interface. This critical review highlights a paradigm shift towards visible- and near-infrared (red and NIR) light-driven PODS to overcome this optical deadlock. Although red light has excellent penetrating power on diesel matrices, its low photon thermodynamic energy (2.0 eV) fundamentally hinders direct C-S bond breaking, which actually demands a much higher activation energy. To bridge this energy deficit, advanced nonlinear photon manipulation strategies are comprehensively evaluated. This review shows that simultaneous Two-Photon Absorption (TPA) is the most superior mechanistic pathway, fundamentally outperforming Second Harmonic Generation (SHG) and Triplet-Triplet Annihilation (TTA). While simultaneous TPA utilizing sensitizers with giant absorption cross-sections (> 1000 GM) provides direct access to highly excited states (S_n or Q₁), it fundamentally requires high-intensity laser excitation. Alternatively, defect-mediated Two-Step Two-Photon Absorption (TS-TPA) exploits real intermediate states to bridge the energy gap, triggering highly efficient photochemical degradation at highly accessible, low excitation densities (~ 0.1 W cm^- 2). Finally, future prospects emphasizing continuous-flow microreactors and advanced optical encapsulation strategies are discussed to realize commercially viable red-light PODS.