Molybdenum Isomorphously Substituted Decatungstates as Robust and Renewable Photocatalysts for Visible Light‐Driven Oxidation of Hydrocarbons by Molecular Oxygen

Abstract

This article reveals that molybdenum (Mo) heteroatoms isomorphously substituted decatungstates (Mo-DTs) can be conveniently synthesized via copolymerization of tungstate and molybdate in acidic solution. A series of characterizations support that the incorporated Mo heteroatoms play crucial roles in improving the redox capacity and visible light harvesting efficiency of Mo-DTs, especially the photo-excited probability and lifetimes, so that the novel Mo-DTs exhibit a remarkably enhanced photo-catalytic efficiency in the visiblelight-triggered selective oxidation of inert hydrocarbons by O 2 under mild conditions. In the photo-oxidation of cyclohexane under continuous visible light irradiation for 30 h, the best 1 % Mo-DT can achieve 44.8 % conversion and 89.7 % cyclohexanone selectivity in the presence of 2 M HCl, and the regenerated catalyst after simple hydrothermal treatment has excellent reusability. This is the first successful example of incorporating Mo atoms into the skeleton of non-lacunary DTs, which has special significance for expanding the application of DTs in visible-light-catalysis.

Mild and efficient photocatalytic coupled alcohol oxidative dehydrogenation and CO2 reduction driven by visible light

“…Three characteristic bands located near 960, 890 and 804 cm −1 belong to the stretching vibrations of WO t (O t : terminal oxygen), W–O c –W (O c : corner-sharing bridged oxygen) and W–O e –W (O e : edge-sharing bridged oxygen), respectively, reflecting the structural character of the DT anion. 25 c – e ,28,33 Several other characteristic bands at 2959, 2867, 1479 and 1382 cm −1 should be related to the tetraalkylammonium cations of the TBADT salt. 4% Al-DT exhibits very similar IR bands to TBADT, further supporting that the doping of Al 3+ species can well maintain the structural character of the DT anion.…”

“…Moreover, in the two secondary alcohols, α-PEA also has a lower reactivity than CyHA although the C-H bond energy of α-PEA is lower, 11.7 kcal mol −1 , than that of CyHA. This likely implies that the first-step ODH of alcohols does not dominate the current photocatalytic reaction owing to an unparalleled oxidation capacity of the photo-excited DT anions, 25,28,33,43 and the second-step ODH process of these low activity aromatic alcohols, as a rate-limiting step, likely encounters a greater impediment, as supported by the calculated BDE values for the O-H bonds of these alcohols and the O-H(R) bonds of their C-centered radicals (see Table 2). After one C-H bond of alcohols is cleaved to form the C-centered radicals, the BDE values for their O-H(R) bonds are drastically weakened by about 60-82 kcal mol −1 , suggesting that most of the alcohols with relatively low O-H(R) bond BDEs (24-37 kcal mol −1 ) can efficiently realize the second-ODH process, thus leading to the generation of blue H 2 DT as the key species of CO 2 reduction.…”