Solar-Driven C–N Coupling over Pd/TiO ₂ Enables High-Yield Synthesis of Methylamine Hydrochlorides from Methanol and Ammonium Chloride

Methylamine hydrochlorides (MACls) are important building blocks in organic synthesis, yet their conventional production requires energy-intensive, multistep processes. Herein, we report a photocatalytic strategy for the direct synthesis of MACls from methanol and ammonium chloride under mild conditions. A high MACls yield of 29.62 mmol·g^-1 is achieved within 4 h over Pd-decorated TiO₂ with oxygen vacancies, outperforming most previously reported photocatalytic C-N coupling systems. In situ spectroscopy, isotope effects, and theoretical studies reveal that methanol is initially oxidized to formaldehyde, a key intermediate that reacts with ammonium chloride via nucleophilic condensation and successive methylation to form C-N bonds, thereby yielding MACls. Photoexcitation enables both the thermodynamically and kinetically unfavorable methanol-to-formaldehyde conversion and drives stepwise methylation reactions, which are otherwise inactive in the dark. The cooperative interaction between Pd and oxygen vacancies promotes charge separation and methanol dehydrogenation, resulting in highly efficient MACls photosynthesis. This work advances mechanistic insights into photocatalytic C-N bond formation and presents a sustainable approach for producing value-added C-N compounds from simple C₁ and nitrogen feedstocks.