[en] An efficient protocol for the synthesis of 12-aryl or 12-alkyl-8,9,10,12-tetrahydro-benzo[a] xanthen-11-one derivatives has been developed via three-component reaction of aldehyde, 2-naphthol and 1,3-cyclohexadione or 5,5-dimethyl-1,3-cyclohexadione in the presence of 12-tungstophosphoric acid (H₃PW₁₂O₄₀) under solvent-free conditions. The present methodology offers several advantages such as high yields, simple procedure, low cost, short reaction times, and mild conditions. (author)

[en] Undoped Cu nanoparticles (NPs) generally show poor selectivity and activity for electroreduction of CO₂ to formate due to hard desorption of HCOO* intermediate on Cu site. Here we report a Cu/pyrenyl-graphdiyne (Pyr-GDY) composite catalyst, in which Cu₂O and CuO NPs were in-situ formed and embedded in the matrix of a two-dimensional (2D) Pyr-GDY, during the synthesis of 2D Pyr-GDY using monolayer graphene covered Cu foil as a template, and copper acetate as a coupling catalyst. Cu₂O and CuO NPs in Cu/Pyr-GDY can be electrochemically reduced to cubic metallic CuNPs to get Cu/Pyr-GDY-R electrocatalyst, with the average size of metallic Cu NPs being 42 nm. The Cu/Pyr-GDY-R on Cu foil can be directly used as a highly efficient electrocatalyst for CO₂-to-formate conversion in a CO₂-saturated 0.1 M KHCO₃ electrolyte, with a formate Faradaic efficiency (FE_formate) as high as 95% (at −1.2 V vs reversible hydrogen electrode), far superior to that of Pyr-GDY-free Cu NPs (with a FE_formate of only 29%). The key reaction intermediate of HCOO* during CO₂-to-formate conversion was identified by in situ Raman spectroscopy. The results of density functional theory calculations revealed that the Pyr-GDY support can decrease the reaction free energy for the adsorption of HCOO* on Cu site, due to the electron transfer from metallic Cu NPs to conjugated diacetylene groups in 2D Pyr-GDY support, which leads to the high selectivity for formate over hydrogen production. (paper)

[en] Highlights: • A ligand replacement approach was developed for the large-scale production of 2D monolayer MOLs from general 3D MOFs. • The resulted 2D monolayer MOLs were used as precursors for the preparation of C₃N₄-based single-atom photocatalysts. • The cheap photocatalysts exhibited excellent catalytic performance for photochemical CO2 reduction. • The structure-activity relationship was investigated and revealed experimentally. The photochemical reduction of carbon dioxide (CO₂) into valuable chemicals or feedstock is very meaningful for environmental and energy sustainability. Development of efficient, robust and low-cost catalysts is necessary and desirable for their practical application. In this communication, we exploited such a catalyst by anchoring single-Co(II) sites on g-C₃N₄, which was firstly achieved by the pyrolysis of ultrathin cobalt metal-organic framework (MOF) nanosheets (also called metal-organic layers; MOLs) during the process of g-C₃N₄ formation. Benefitting from the confinement effect of MOL matrix and the close contact between MOLs and g-C₃N₄ precursor, the Co(II) sites can be homogeneously and atomically dispersed on the surface of g-C₃N₄ during the process of g-C₃N₄ formation. Impressively, this photocatalyst possesses excellent catalytic performance for photochemical CO₂-to-CO conversion, with the CO evolution rate as high as 464.1 μmol g⁻¹ h⁻¹, 3 and 222 times higher than those of using bulky Co-MOF and CoCl₂ as the cobalt sources, respectively. This work paves a new way to develop the cost-effective photocatalysts containing single-atom sites for clean energy production.