Specific Photocatalytic C–C Coupling of Benzyl Alcohol to Deoxybenzoin or Benzoin by Precise Control of C_α–H Bond Activation or O–H Bond Activation by Adjusting the Adsorption Orientation of Hydrobenzoin Intermediates

Benzyl alcohol (BA) is a major biomass derivative and can be further converted into deoxybenzoin (DOB) and benzoin (BZ) as high-value products for industrial applications through photocatalytic C–C coupling reaction. The photocatalytic process contains two reaction steps, which are (1) the C–C coupling of BA to hydrobenzoin (HB) intermediates and (2) either dehydration of HB to DOB or dehydrogenation of HB to BZ. We found that generation of DOB or BZ is mainly determined by the activation of Cα–H or O–H bonds in HB. In this study, phase junction CdS photocatalysts and Ni/CdS photocatalysts were elaborately designed to precisely control the activation of Cα–H or O–H bonds in HB by adjusting the adsorption orientation of HB on the photocatalyst surfaces. After orienting the Cα–H groups in HB on the CdS surfaces, the Cα–H bond dissociation energy (BDE) at 1.39 eV is lower than the BDE of the O–H bond at 2.69 eV, therefore improving the selectivity of the DOB. Conversely, on Ni/CdS photocatalysts, the O–H groups in HB orient toward the photocatalyst surfaces. The BDE of the O–H bonds is 1.11 eV to form BZ, which is lower than the BDE of the Cα–H bonds to the DOB (1.33 eV), thereby enhancing the selectivity of BZ. As a result, CdS photocatalysts can achieve complete conversion of BA to 80.4% of the DOB after 9 h of visible light irradiation, while 0.3% Ni/CdS photocatalysts promote complete conversion of BA to 81.5% of BZ after only 5 h. This work provides a promising strategy in selective conversion of BA to either DOB or BZ through delicate design of photocatalysts.