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. 2022 Sep 14;144(36):16433-16446.
doi: 10.1021/jacs.2c04905. Epub 2022 Sep 1.

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light

Houeida Issa Hamoud  1 , Patrick Damacet  2 , Dong Fan  3 , Nisrine Assaad  2 , Oleg I Lebedev  4 , Anna Krystianiak  5 , Abdelaziz Gouda  6 , Olivier Heintz  5 , Marco Daturi  1 , Guillaume Maurin  3 , Mohamad Hmadeh  2 , Mohamad El-Roz  1
Affiliations

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light

Houeida Issa Hamoud et al. J Am Chem Soc. .

Abstract

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)2-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time via operando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·gcat-1·h-1) under ambient conditions. These performance indicators make UiO-66(COOH)2-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)2-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)2-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)2-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
(A) Structure of UiO-66-(COOH)2 showing the possible anchoring sites for Cu (Zr open metal sites created by missing linkers and free carboxylic functions). (B) SEM images of UiO-66-(COOH)2 crystals before and after copper metalation. (C) PXRD patterns of UiO-66-(COOH)2 and its metalated form. (D) EDX mapping of UiO-66-(COOH)2-Cu showing the distributions of Cu and Zr in the MOF crystals.
Figure 2
Figure 2
Evolution of (A) the formic acid conversion, (B) the 13CO2, 12CO2, and H2 quantities during HCOOH reforming (inset: zoomed-in view on the first minutes of reaction; the arrow shows the light-on time), and (C) the FTIR spectra of the reaction gas phase during the first minutes (4 min/spectrum) of the reaction. (D) FTIR spectra of the UiO-66-(COOH)2-Cu sample at steady state (a) in the dark and (b) during the reforming of HCOOH (labeled with 13C at 99% (H2O < 5%)) under visible-light irradiation. Reaction conditions: total flow rate = 25 cm3·min–1; [HCOOH-13C] = 2400 ppm (0.24%) in Ar; T = 25 °C; 150 W Xe lamp with a visible-light-pass filter (λ > 390 nm); irradiance = 71 mW·cm–2; mcat = 20 mg (self-supported pellet with a surface area of 1.6 cm2).
Figure 3
Figure 3
Evolution of the HCOOH conversion and the corresponding gas-phase products during three cycles/days. The regions designated by arrows correspond to the dark stages. Reaction conditions: total flow rate = 25 cm3·min–1; [HCOOH-13C] = 2400 ppm (0.24%) in Ar; T = 25 °C; 150 W Xe lamp with a visible-light-pass filter (λ > 390 nm); irradiance = 71 mW·cm–2; mcat = 20 mg (self-supported pellet with a surface area of 1.6 cm2).
Figure 4
Figure 4
(A) Effect of the temperature on FAc reforming in the dark and under visible-light irradiation. (B) Corresponding IR spectra of the reaction gas phase at the steady state at the studied temperatures between 25 °C (top) and 150 °C (bottom). Reaction conditions: total flow rate = 25 cm3·min–1; [FAc-13C] = 2400 ppm (0.24%) in Ar; 150 W Xe lamp with a visible-light-pass filter (λ > 390 nm); irradiance = 71 mW·cm–2; mcat = 20 mg (self-supported pellet with a surface area of 1.6 cm2).
Figure 5
Figure 5
(A) FTIR spectra of the UiO-66-(COO)2-Cu surface during the reforming of formic acid-13C under visible-light irradiation. (B) Evolution of the corresponding normalized intensities of the gas-phase products and the band area at 1855 cm–1 of the surface versus the irradiation time. Reaction conditions: total flow rate = 25 cm3·min–1; [formic acid-13C] = 2400 ppm (0.24%) in Ar; T = 25 °C; 150 W Xe lamp with a visible-light-pass filter (λ > 390 nm); irradiance = 71 mW·cm–2; mcat = 20 mg (self-supported pellet with surface are of 1.6 cm2). The arrow in (B) corresponds to the light-on time.
Figure 6
Figure 6
(A–D) Evolution of (A, B) gas-phase products and (C, D) adsorbed species on UiO-66-(COO)2-Cu versus time in the FAc-13C/FAc-12C SSITKA experiment (t = 0 corresponds to the start of irradiation, and the dotted line corresponds to the FAc-13C/FAc-12C). (E, F) Relative evolution of the IR intensities from lower (blue color) to higher (red color) for (E) the reaction gas phase and (F) the photocatalyst surface. Reaction conditions: total flow rate = 25 cm3·min–1; [FAc-13C] = [FAc-12C] = 2400 ppm (0.24%) in Ar; T = 25 °C; 150 W Xe lamp with a visible-light-pass filter (λ > 390 nm); irradiance = 71 mW·cm–2; mcat = 20 mg (self-supported pellet with a surface are of 1.6 cm2).
Figure 7
Figure 7
Evolution of (A) the hydrogen isotopes and (B) the anhydride vibration bands during the photocatalytic dehydrogenation of DCOOH under visible light. The inset in (A) is a zoomed-in view of the first 40 min of the reaction. Reaction conditions: total flow rate = 25 cm3·min–1; [DCOOH] = 2400 ppm (0.24%) in Ar; T = 25 °C; 150 W Xe lamp with a visible-light-pass filter (λ > 390 nm); irradiance = 71 mW·cm–2; mcat = 20 mg (self-supported pellet with a surface area of 1.6 cm2).
Figure 8
Figure 8
(A) Cu 2p3/2 and (B) Zr 3d levels of UiO-66-(COO)2-Cu (a) before and (b) after reaction.
Figure 9
Figure 9
(a) HAADF-STEM image of UiO-66-(COOH)2-Cu material and (b) corresponding EDX-STEM elemental mappings for Zr L (green), Cu K (red), and O K (blue) and the overlaid color image before reaction. (c) HAADF-STEM image of UiO-66-(COO)2-Cu obtained after reaction with the corresponding ring SAED pattern indexed based on the cubic Pn3̅m structure of Cu2O. (d) High-resolution HAADF-STEM image of Cu-based nanoparticles formed after the reaction and assigned to Cu2O. The inset shows the corresponding [110] FT pattern indexed based on the cubic Cu2O cubic structure, and the simulated [110] HAADF-STEM image in the white box shows a good fit to the experimental image. (e) Bright-field HRTEM image of the edge of a UiO-66-(COO)2-Cu nanoparticle after reaction. The Cu2O NPs exhibit black contrast, marked with white arrows. The Cu-free near surface region of UiO-66-(COO)2-Cu nanoparticles should be noticed. (f) EDX-STEM elemental mappings of UiO-66-(COO)2-Cu particles after reaction for Zr L (green), Cu K (red), and O K (blue) and the overlaid color image showing the diffusion of the copper element inside the UiO-66-(COO)2-Cu framework.
Figure 10
Figure 10
(A) DFT-derived minimum-energy pathway for the dehydrogenation of formic acid by UiO-66-(COO)2-Cu and (B) corresponding illustrative snapshots of the different intermediate species. The energy barriers (Enthalpy, expressed in eV) for the three transition states (TSs) are also shown in the figure. Color codes for the MOF: C, gray; Cu, blue; O, red; H, white; Zr, green. Color codes for the adsorbed molecules: C, light blue; O, orange; H, white. The total free energy of the UiO-66-COO)2-Cu structure with a gas-phase HCOOH molecule was set to zero in the Gibbs free energy profile.
Scheme 1
Scheme 1. Plausible Restructuring Pathways of UiO-66-(COOH)2-Cu during FAc Reforming under Visible Light Leading to the Formation of the Supported and Stabilized Cu(0)/Cu(I) Binary System on UiO-66-(COO)2
See this image and copyright information in PMC

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