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Title: Investigation of Combined Capture–Destruction of Toluene over Pd/MIL-101 and TiO 2 /MIL-101 Dual Function Materials
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Energy & Fuels
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Isostructural Cr and Fe nanoporous MIL-101, synthesized without mineralizing agents, are investigated for styrene oxidation utilizing aqueous hydrogen peroxide to yield valuable oxygenates for chemical synthesis applications. Styrene conversion rates and oxygenate product distributions both depend on metal identity, as MIL-101(Fe) is more reactive for total styrene oxidation and is more pathway selective, preferring aldehyde (benzaldehyde) formation at the α-carbon to the aromatic ring, where MIL-101(Cr) sustains epoxide (styrene oxide) production at the same α-carbon. These pathways often involve hydrogen peroxide derived radical intermediates (O, –HOO˙, –HO − ˙) and metallocycle transition states. We postulate that the higher reactivity of one of these surface intermediates, Fe( iv )O relative to Cr( iv )O, leads to higher styrene oxidation rates for MIL-101(Fe), while higher electrophilicity of Cr( iii )–OOH intermediates translates to the higher styrene oxide selectivity observed for MIL-101(Cr). Secondary styrene oxide and benzaldehyde conversions are observed over both analogs, but the former is more prevalent over MIL-101(Fe) due to higher Lewis/Brønsted acid site density and strength compared to MIL-101(Cr). Recyclability experiments combined with characterization via XRD, SEM/EDXS, and FT-IR and UV-vis spectroscopies show that the nature of MIL-101(Fe) sites does not change significantly with each cycle, whereas MIL-101(Cr) suffers from metal leaching, which impacts styrene conversion rates and product distribution. Both catalysts require active site regeneration, though MIL-101(Fe) sites are more susceptible to reactivation, even under mild conditions. Finally, examination of styrene conversion for three unique synthesized phases of MIL-101(Cr) rationalizes that nodal defects are largely responsible for observed reactivity and selectivity but predispose the framework to metal leaching as a predominant deactivation mechanism. 
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  2. Precious metals have been shown to play a vital role in the selective hydrogenation of α,β-unsaturated aldehydes, but still suffer from challenges to control selectivity. Herein, we have advanced the design of catalysts made out of Pt–Co intermetallic nanoparticles (IMNs) supported on a MIL-101(Cr) MOF (3%Pt y %Co/MIL-101(Cr)), prepared by using a polyol reduction method, as an effective approach to enhance selectivity toward the production of α,β-unsaturated alcohol, the desired product. XRD, N 2 adsorption–desorption, FTIR spectroscopy, SEM, TEM, XPS, CO adsorption, NH 3 -TPD, XANES and EXAFS measurements were used to investigate the structure and surface properties of our 3%Pt y %Co/MIL-101(Cr) catalysts. It was found that the Co-modified 3%Pt y %Co/MIL-101(Cr) catalysts can indeed improve the hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL), reaching a higher selectivity under mild conditions than the monometallic Pt/MIL-101(Cr) catalysts: 95% conversion of CAL with 91% selectivity to COL can be reached with 3%Pt3%Co/MIL-101(Cr). Additionally, high conversion of furfural (97%) along with high selectivity to furfural alcohol (94%) was also attained with the 3%Pt3%Co/MIL-101(Cr) catalyst. The enhanced activity and selectivity toward the unsaturated alcohols are attributed to the electronic and geometric effects derived from the partial charge transfer between Co and Pt through the formation of uniformly dispersed Pt–Co IMNs. Moreover, various characterization results revealed that the addition of Co to the IMPs can promote the Lewis acid sites that facilitate the polarization of the charge-rich CO bonds and their adsorption via their oxygen atom, and also generate new interfacial acid sites. 
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  3. Nitro-functionalized metal–organic frameworks (MOFs), such as Al-MIL-53-NO 2 , have been widely used in quantitative hydrogen sulfide (H 2 S) detection based on the “turn-on” effect, where fluorescence enhancements were observed upon contact with H 2 S. This was believed to be caused by the fact that the electron-withdrawing –NO 2 groups in the initial non-luminescent MOFs were reduced to electron-donating –NH 2 groups in the sensing process. However, since most H 2 S detection is conducted in a suspension system consisting of MOFs and solvents, it is still unclear whether these –NH 2 groups are on MOFs or in the liquid. Using Al-MIL-53-NO 2 as a model MOF, this work aims to answer this question. Specifically, the supernatant and undissolved particles separated from the Al-MIL-53-NO 2 suspensions after being exposed to H 2 S were analyzed systematically. The results showed that it is the free BDC-NH 2 (2-aminobenzene-1,4-dicarboxylic acid) in the solution rather than the formation of Al-MIL-53-NH 2 that really caused the fluorescence enhancement. In particular, the formed BDC-NH 2 was reduced from the shedded BDC-NO 2 (2-nitrobenzene-1,4-dicarboxylic acid) during the decomposition of Al-MIL-53-NO 2 , which was attacked by OH − in the NaHS solution. We anticipate that this work will offer new ways of tracing fluorophores for MOF-based sensing applications in aqueous systems. 
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