Selective Catalytic Oxidation of Benzyl Alcohol to Benzaldehyde by Nitrates

In this paper, ferric nitrate was used to oxidize benzyl alcohol in a mild condition and demonstrated its better performance compared to HNO₃. In the reaction, the conversion rate and product selectivity could be both as high as 95% in N₂ atmosphere, while the benzaldehyde yield also reached 85% in air. Similar to Fe(NO₃)₃·9H₂O, the other metallic nitrates such as Al(NO₃)₃·9H₂O and Cu(NO₃)₂·3H₂O could also oxidize the benzyl alcohol with high activity. The applicability of Fe(NO₃)₃·9H₂O for other benzylic alcohol was also investigated, and the reaction condition was optimized at the same time. The results showed the Fe(NO₃)₃·9H₂O would be more conducive in oxidizing benzyl alcohol under the anaerobic condition. The experiments in N₂ or O₂ atmospheres were conducted separately to study the catalytic mechanism of Fe(NO₃)₃. The results showed the co-existence of Fe³⁺ and $$ will generate high activity, while either was with negligible oxidation property. The cyclic transformation of Fe³⁺ and Fe²⁺ provided the catalytic action to the benzyl alcohol oxidation. The role of $$ was also an oxidant, by providing HNO₂ in anaerobic condition, while $$ would be regenerated from NO in aerobic condition. O₂ did not oxidize the benzyl alcohol conversion directly, while it could still be beneficial to the procedure by eliminating the unwelcome NO and simultaneously reinforcing the circulation of Fe²⁺ and Fe³⁺, which therefore forms a green cyclic oxidation. Hence, the benzyl alcohol oxidation was suggested in an air atmosphere for efficiency and the need of green synthesis.