High Nitrile Yields of Aerobic Ammoxidation of Alcohols Achieved by Generating •O2- and Br• Radicals over Iron-Modified TiO2 Photocatalysts

Catalytic ammoxidation of alcohols into nitriles is an essential reaction in organic synthesis. While highly desirable, conducting the synthesis at room temperature is challenging, using NH3 as the nitrogen source, O-2 as the oxidant, and a catalyst without noble metals. Herein, we report robust photocatalysts consisting of Fe(III)-modified titanium dioxide (Fe/TiO2) for ammoxidation reactions at room temperature utilizing oxygen at atmospheric pressure, NH3 as the nitrogen source, and NH4Br as an additive. To the best of our knowledge, this is the first example of catalytic ammoxidation of alcohols over a photocatalyst using such cheap and benign materials. Various (hetero) aromatic nitriles were synthesized at high yields, and aliphatic alcohols could also be transformed into corresponding nitriles at considerable yields. The modification of TiO2 with Fe(III) facilitates the formation of active O-center dot(2)- radicals and increases the adsorption of NH3 and amino intermediates on the catalyst, accelerating the ammoxidation to yield nitriles. The additive NH4Br impressively improves the catalytic efficiency via the formation of bromine radicals (Br-center dot) from Br-, which works synergistically with O-center dot(2)-- to capture H-center dot from C-alpha-H, which is present in benzyl alcohol and the intermediate aldimine (RCH=NH), to generate the active carbon-centered radicals. Further, the generation of Br-center dot from the Br- additive consumes the photogenerated holes and OH center dot radicals to prevent over-oxidation, significantly improving the selectivity toward nitriles. This amalgamation of function and synergy of the Fe(III)-doped TiO2 and NH4Br reveals new opportunities for developing semiconductor-based photocatalytic systems for fine chemical synthesis.

Automated summary

This study reports the use of Fe(III)-modified titanium dioxide (Fe/TiO2) photocatalysts for the ammoxidation of alcohols at room temperature. NH3 is used as the nitrogen source, O2 as the oxidant, and NH4Br as an additive. The Fe(III)-modified TiO2 catalyst shows high stability and activity, enabling the synthesis of various aromatic nitriles and aliphatic nitriles in high yields. The Fe(III) modification promotes the formation of active O•2- radicals and increases the adsorption of NH3 and amino intermediates on the catalyst, speeding up the ammoxidation reaction.