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

Abstract

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 NH 3 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/TiO 2 ) for ammoxidation reactions at room temperature utilizing oxygen at atmospheric pressure, NH 3 as the nitrogen source, and NH 4 Br 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 TiO 2 with Fe(III) facilitates the formation of active • O 2 − radicals and increases the adsorption of NH 3 and amino intermediates on the catalyst, accelerating the ammoxidation to yield nitriles. The additive NH 4 Br impressively improves the catalytic efficiency via the formation of bromine radicals (Br • ) from Br − , which works synergistically with • O 2 − to capture H • from C α -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 • from the Br − additive consumes the photogenerated holes and OH • radicals to prevent over-oxidation, significantly improving the selectivity toward nitriles. This amalgamation of function and synergy of the Fe(III)-doped TiO 2 and NH 4 Br reveals new opportunities for developing semiconductor-based photocatalytic systems for fine chemical synthesis.

Carbon-regulated titania crystallization to construct a comprehensive palette of anatase/rutile mixed phases for advanced photocatalysis

“…58,59 Among the current available technologies, titania has been widely utilized to harvest solar light at ambient temperature for photocatalytic elimination of VOCs due to its high oxidation capacity, non-toxicity, low cost, and chemical stability. [59][60][61][62] It is generally believed that anatase is more active than rutile, and yet the majority of current research efforts are devoted to studying the RF effect rather than the anatase size effect on the photocatalytic performance of ARM-phase titania, [15][16][17][18]63,64 which may be due to the fact that in most cases the anatase sizes, rutile sizes, and RFs of ARM-phase titania can be hardly changed independently without interference with each other. In the present work, as shown in Fig.…”

Recent Advances in Nonprecious Metal Catalysis

“…12 In a recent contribution, Zhu and Zhang prepared an iron-modified TiO 2 catalyst that selectively converts both benzylic and aliphatic alcohols to the corresponding nitriles in the presence of oxygen, ammonia, and 365 nm light (Scheme 3). 13 The results obtained with iron-doped TiO 2 catalyst could be improved by using NH 4 Br as an additive. The method provided good to high yields (up to ∼98%) of relatively simple substrates bearing alkyl, ether, and ester functionalities.…”

Enzyme-mimicking single atoms enable selectivity control in visible-light-driven oxidation/ammoxidation to afford bio-based nitriles

“…22 Recently, Zhang et al reported the photocatalytic conversion of aromatic alcohols to arylnitriles with satisfactory yields over Fe-TiO 2 via aldehyde-imide intermediates under ultraviolet irradiation. 23 Encouraged by these limited examples, accessing arylnitriles with improved selectivity and universality under visible-light irradiation from biomassderived alcohols would be feasible, and it is greatly necessary to develop novel heterogeneous photocatalysts of high efficiency for generating desired reactive oxygen species to implement the sequential conversion process. Single-atom catalysts (SACs) are a class of promising heterogeneous materials for the high-efficiency utilization of metal species or active sites.…”