Nitrate-Mediated Alcohol Oxidation on Cadmium Sulfide Photocatalysts

Abstract

Developing photocatalysts capable of organic oxidations enables the generation of value-added products from biomass feedstocks through visible light irradiation. Through a series of nonaqueous photocatalytic experiments, we have uncovered that CdS nanowires catalyze benzyl alcohol (BnOH) oxidation and 5-hydroxymethylfufural (HMF) oxidation. The rate can be improved by introducing nitrate salts that act as a redox mediator in solution. Specifically, nitrate salts of lithium, magnesium, calcium, and manganese promote the selective photooxidation of BnOH to benzaldehyde on CdS in 70–100% yields at rates up to 13.6 mM h–1, compared to 8% yield at 3 mM h–1 in the absence of a nitrate mediator. Kinetic analysis reveals that, in the absence of nitrate salts, the reaction is first-order with respect to BnOH, while in the presence of nitrate, the reaction is half-order in BnOH. This rate law disparity, along with radical trapping and kinetic isotope experiments, suggests that nitrate-mediated alcohol oxidations proceed through a mechanism involving the catalytic generation of a nitrate radical, NO3 •. The generation of this radical also enables the selective photooxidation of HMF to 2,5-diformylfuran at a rate of 2.6 mM h–1 using CdS nanowires.

Nitrate Radical Facilitates Indirect Benzyl Alcohol Oxidation on Bismuth(III) Vanadate Photoelectrodes

“…Previous work in our lab demonstrates that NO 3 − acts as a redox mediator (i. e. it is regenerated) in photocatalytic PhCH 2 OH oxidation on CdS nanowires . Photoelectrochemically, NO 3 − is not regenerated, representing an EC i mechanism.…”

“…Recently, a visible‐light excited state of Acr + ‐Mes dye was used to oxidize NO 3 − to NO 3 ⋅ for use as an oxidant in subsequent chemical reactions . Continuing with light‐harvesting examples, our lab demonstrated cooperativity between the NO 3 − /NO 3 ⋅ couple and valence‐band holes on semiconducting CdS nanowires toward indirect alcohol oxidation; this photocatalytic system oxidizes PhCH 2 OH to PhCHO with >99 % selectivity in acetonitrile …”

Innovative Protocols in the Catalytic Oxidation of 5‐Hydroxymethylfurfural

“…For instance, DiMeglio et al. used CdS nanowires as an excellent heterogeneous catalyst and deployed them for conversion of HMF to DFF under benign reaction conditions [67] . High DFF selectivity up to 99 % was obtained assisted by nitrate salts as mediator, which proceeds via a mechanism involving the catalytic generation of a nitrate radical NO 3 .…”

High‐performance photocatalysts for the selective oxidation of alcohols to carbonyl compounds

“…On a similar note, it has been recently revealed that the presence of redox mediator in the reaction mixture can improve the efficiency of CdS photocatalysts in oxidizing alcohols. DiMeglio and co‐workers [ 129 ] studied the effect of nitrate ions on the selective photocatalytic conversion of benzyl alcohol and 5‐hydroxymethyl furfural. By employing a facile‐hydrothermal method, CdS nanowires were produced and successfully employed to convert benzyl alcohol to benzaldehyde.…”

“…However, for a smooth electron transfer, the redox potential of the substrate must be more negative than the VB of the photocatalyst for oxidation reactions or it must be more positive than the CB of the photocatalyst to promote reduction reactions. Since in the case of DiMeglio et al's [ 129 ] work the standard redox potentials of Ca 2+ /Ca (−2.76 eV), Mg 2+ /Mg (−2.37 eV), Li + /Li (−3.05), and Mn 2+ /Mn (−1.18 eV) are all significantly more negative than the CB potential (~ −0.56 eV), these cations cannot react with the photogenerated holes or electrons, thereby having a negligible quenching role on the photocatalytic activity. Additionally, these metal cations can act as a bridge between the benzyl alcohol and CdS surface, causing improved adsorption and LMCT, which could explain the higher conversion rates observed.…”