Catalytic Transformation of Aliphatic Alcohols to Corresponding Esters in O₂ under Neutral Conditions Using Visible-Light Irradiation

Abstract

Selective oxidation of aliphatic alcohols under mild and base-free conditions is a challenging process for organic synthesis. Herein, we report a one-pot process for the direct oxidative esterification of aliphatic alcohols that is significantly enhanced by visible-light irradiation at ambient temperatures. The new methodology uses heterogenerous photocatalysts of gold-palladium alloy nanoparticles on a phosphate-modified hydrotalcite support and molecular oxygen as a benign oxidant. The alloy photocatalysts can absorb incident light, and the light-excited metal electrons on the surface of metal nanoparticles can activate the adsorbed reactant molecules. Tuning the light intensity and wavelength of the irradiation can remarkably change the reaction activity. Shorter wavelength light (<550 nm) drives the reaction more efficiently than light of longer wavelength (e.g., 620 nm), especially at low temperatures. The phosphate-exchanged hydrotalcite support provides sufficient basicity (and buffer) for the catalytic reactions; thus, the addition of base is not required. The photocatalysts are efficient and readily recyclable. The findings reveal the first example of using "green" oxidants and light energy to drive direct oxidative esterification of aliphatic alcohols under base-free, mild conditions.

Chemical Preparation of Supported Bimetallic Catalysts. Gold-Based Bimetallic, a Case Study

“…Co-deposition-reduction can also be performed. Au-Pd nanoparticles were prepared on phosphate-modified hydrotalcite by co-impregnation with a solution of HAuCl 4 and PdCl 2 followed by chemical reduction obtained by addition of lysine (pH 8-9) then of NaBH 4 , washed and dried at 60˝C [46]. Nanoparticles of 4-5 nm with a d-spacing attesting the formation of Au-Pd alloy were obtained.…”

Improving the rates of Pd-catalyzed reactions by exciting the surface plasmons of AuPd bimetallic nanotriangles

“…Also, heterogeneous Pd catalysts can be easily recovered from the reaction mixture and reused for several reaction cycles. Recently, by using plasmonic nanoparticles, the rates of Pd-catalyzed reactions such as crosscoupling, 16,17 hydrogenation, [18][19][20][21] and oxidation reactions [22][23][24] have been improved, and it has been found that bimetallic nanoparticles show superior activity compared to the pure metallic nanoparticles. 25,26 The most studied bimetallic catalysts in plasmon-enhanced Pd-catalyzed reactions are AuPd bimetallic nanoparticles.…”

Metal/Semiconductor Hybrid Nanocrystals and Synergistic Photocatalysis Applications

“…Moreover, noble metal has plasmonic effect and induces the organic reaction under visible light and enhances the conversion efficiency [140]. Combining with semiconductor nanocrystals, metal/semiconductor hybrid nanocrystals have potential advantages in organic photosynthesis: higher conversion efficiency, higher selectivity, and lower by-products [141][142][143].…”

“…Beside, metal/semiconductor hybrid nanocrystals, such as Au/ZrO 2 , can be used for other organic reaction: reduction of nitroaromatic compound, Suzuki reaction, ester reaction. and so on [140,[142][143]. In summary, metal/semiconductor photocatalysts have shown their potential application on hydrogen evolution, photoreduction for conversing CO 2 into fuel and organic photosynthesis.…”