[NU-1100] TEMPO radically expedites the conversion of sulfides to sulfoxides by pyrene-based MOF photocatalysis

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Abstract:
The article presents a novel approach to enhancing the photocatalytic conversion of sulfides to sulfoxides using a pyrene-based metal-organic framework (MOF) NU-1100. The study demonstrates that the addition of a redox mediator, 4-carboxy-TEMPO, significantly accelerates the oxidation process under green light irradiation, achieving more than five times the conversion rate compared to using NU-1100 alone. The findings highlight the potential of integrating redox mediators with MOFs to improve visible light photocatalytic reactions.

Research Background:
1.Industry Challenge: The conversion of solar energy into chemical energy is a crucial step in achieving large-scale applications. Semiconductor photocatalysis, which uses solar energy to drive chemical reactions, has received considerable attention. However, MOFs often suffer from poor selectivity and inefficient activity in photocatalytic processes.
2.Existing Solutions: Previous studies have explored various strategies to enhance the photocatalytic performance of MOFs, including heterojunction construction, hierarchical catalyst conjunction, transition metal insertion, post-synthetic modification, and integration of other active species. The oxidation of sulfides to sulfoxides is a particularly appealing model organic reaction, but achieving high activity and selectivity under mild conditions remains a challenge.
3.Innovative Approach: The authors build on this foundation by incorporating a highly conjugated linker into the MOF NU-1100 to extend its visible light absorption. They further introduce a redox mediator, 4-carboxy-TEMPO, to mediate hole transfer and enhance charge separation and transfer efficiency. This approach significantly improves the photocatalytic activity and selectivity of the MOF under green light irradiation.

Experimental Section:
1.Synthesis of Pyrene-Based MOF NU-1100:
   -Steps: ZrOCl2·8H2O and 4-biphenylcarboxylic acid were dissolved in DMF and heated at 80 °C for 0.5 h. After cooling, the linker PTBA was added, and the mixture was sonicated and heated at 120 °C for 72 h. The resulting precipitate was washed and dried to obtain NU-1100.
   -Results: The synthesis yielded shiny orange-red crystals of NU-1100 with a specific surface area of 777 m²/g and a total pore volume of 0.44 cm³/g.
2.Characterizations:
   - PXRD confirmed the crystal structure of NU-1100.
   - FTIR spectra showed the coordination between Zr ions and PTBA.
   - TGA indicated thermal stability up to 300 °C.
   - N2 adsorption-desorption isotherms revealed type-I characteristics with a pore size distribution of 1-2 nm.
   - SEM and HRTEM images confirmed the cubic morphology and porous structure of NU-1100.
3.Photocatalytic Reactions:
   -Procedure: NU-1100, sulfide, 4-carboxy-TEMPO, and methanol were introduced into a reactor and irradiated with green LEDs. The products were analyzed by GC-FID and GC-MS.
   -Results: The addition of 4-carboxy-TEMPO resulted in a 5.2 times increase in the conversion of phenyl methyl sulfide compared to NU-1100 alone. The reaction followed zero-order kinetics, and NU-1100 demonstrated excellent recyclability over six cycles.

Analysis and Testing:
1.PXRD and FTIR:
   -Results: The experimental PXRD pattern matched the simulated pattern, confirming the structure of NU-1100. FTIR spectra showed the presence of coordination bonds between Zr ions and PTBA.
2.TGA:
   -Results: NU-1100 exhibited thermal stability up to 300 °C.
3.N2 Adsorption-Desorption Isotherms:
   -Results: NU-1100 had a specific surface area of 777 m²/g and a total pore volume of 0.44 cm³/g, with pore sizes ranging from 1-2 nm.
4.SEM and HRTEM:
   -Results: SEM images revealed the cubic morphology of NU-1100, while HRTEM images confirmed its porous structure.
5.UV-vis DRS and Photoluminescence:
   -Results: NU-1100 displayed broad absorption below 600 nm, with a bandgap of 2.12 eV. The addition of 4-carboxy-TEMPO enhanced the photocurrent and reduced the charge transfer resistance.
6.XPS and EPR:
   -Results: XPS confirmed the presence of C, N, O, and Zr in NU-1100 after the addition of 4-carboxy-TEMPO. EPR spectra indicated the adsorption of 4-carboxy-TEMPO on the exterior surface of NU-1100.

Summary:
The study demonstrates that the integration of a redox mediator, 4-carboxy-TEMPO, with a pyrene-based MOF NU-1100 significantly enhances the photocatalytic conversion of sulfides to sulfoxides under green light irradiation. The findings highlight the potential of combining redox mediators with MOFs to improve visible light photocatalytic reactions, offering a new perspective for the design and application of MOFs in solar energy conversion.

TEMPO Radically Expedites the Conversion of Sulfides to Sulfoxides by Pyrene-Based Metal-Organic Framework Photocatalysis
Authors: Bing Zeng, Fengwei Huang, Yuexin Wang, Kanghui Xiong, Xianjun Lang
DOI: 10.1016/S1872-2067(23)64601-7
Link:https://www.sciencedirect.com/science/article/pii/S1872206723646017

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