Bridging Effect of Carbon Nitride with More Negative Conduction Potential and Halogens Promotes the Liquid-Phase Oxidation of Aromatic C–H Bonds

Abstract

The selective oxidation of benzyl C−H bonds of alkyl aromatic hydrocarbons under solvent-free conditions by using heterogeneous catalysis is a challenging task. In this work, we designed a carbon nitride photocatalyst with a high charge separation efficiency and a directed charge transfer path, which was doped with Ni and Br in the carbon nitride skeleton. Br was deposited directionally onto the electron-rich Ni surface traps to form a bond with Ni, which acted as a charge transfer bridge connecting CN and Br, resulting in a bridging effect. Photogenerated electrons were transferred from Ni target to Br, and electrons were aggregated to form a directional charge transfer path, thereby enhancing the photocatalytic performance of CN. The photocatalyst was utilized for the selective oxidation of ethylbenzene at room temperature, atmospheric pressure, and solvent-free conditions. Under batch conditions simulating solar irradiation, the conversion of ethylbenzene was 43.3% and the selectivity of the product acetophenone was up to 92.0%. With the continuous flow strategy, the conversion of ethylbenzene was increased to 52.4 and 48.1%, respectively, while the selectivity reached 92.7 and 91.0%, and the reaction time was reduced from 24 to 2.1 h. The catalyst was also found to be broadly applicable for the selective oxidation of C−H bonds in the benzyl position of alkyl aromatic hydrocarbons.

Solvent-free aerobic photocatalytic oxidation of C(sp3)–H and C(sp3)–OH to CO bonds

“…However, the peroxide can be stabilized by TEMPO and the corresponding ion peaks were obtained by HPLC-MS (Fig. S13†), 49 which further proved the presence of carbon radical (R˙) and peroxyl radical (ROO˙). In a continuous reaction, the generated alcohols formed in path (ii) can be further oxidized to ketones or aldehydes under the action of ˙O 2 − and 1 O 2 radicals.…”

Fabricating CdS/Ag/BiVO4 Z-heterojunction for solvent-free photocatalytic oxidation of amines

“…S7†) and H 2 O 2 . 45,46 The generation of H 2 O 2 was proved by the UV-vis absorption spectrum (Fig. S8†).…”

Highly Selective Photooxidation of Benzyl Alcohol to Benzaldehyde by Bi–Mo/g-C3N4 in Aqueous Solution

“…The photocatalytic mechanism is thus proposed as follows (Scheme ): (i) the corresponding photogenerated e – and h + are generated at conduction band (CB) and valence band (VB) on the surface of Bi–Mo/CN-2 under visible light; (ii) O 2 is subsequently photoreduced to Bi–OO• superoxo, Bi–OOH peroxo, and then H 2 O 2 at CB via H-abstraction from benzyl alcohol, resulting in selective photooxidation of benzyl alcohol to benzaldehyde; (iii) the reactive Mo sites on Bi–Mo/CN-2 further react with Bi–OOH peroxo or the produced H 2 O 2 to promote the formation of Mo–O 2 peroxo, which not only selectively oxidizes benzyl alcohol to benzaldehyde, but also effectively prevents the overoxidation of benzaldehyde during the photooxidation process. Meanwhile, the Mo–N bonds formed during pyrolysis are effective trapping centers for electrons, , thus accelerating the generation of Bi–OOH peroxo and Mo–O 2 peroxo species in O 2 activation. In this regard, the synergistic effect of Bi and Mo bimetallic sites results in the selective photooxidation of benzyl alcohol to benzaldehyde and effectively avoids the overoxidation of benzaldehyde.…”

“…29 As an ideal matrix, the N atoms in g-C 3 N 4 can act as alkaline sites 30 and favor the stabilization of the peroxo intermediates, thus facilitating the two-electron oxygen reduction reaction for H 2 O 2 production. 31 Moreover, the abundant lone pair of electrons of the N atoms in g-C 3 N 4 can be more readily coordinated with Mo to form the Mo−N bonds, 32,33 which are the effective trapping centers for electrons, thus enhancing the separation efficiencies of electron−hole pairs. 34,35 We then hypothesize that the Mo−N bonds not only enhance the transfer efficiencies of photogenerated charge carriers, which then accelerates the generation of �Bi−OOH peroxo in O 2 activation, but also promote the formation of the Mo−O 2 peroxo, thus achieving selective oxidation of benzyl alcohol to benzaldehyde.…”