Quasi-HKUST Prepared via Postsynthetic Defect Engineering for Highly Improved Catalytic Conversion of 4-Nitrophenol

Abstract

HKUST-1 [Cu₃(BTC)₂(H₂O)₃]_n·nH₂OMeOH was submitted to thermolysis under controlled conditions at temperatures between 100 and 300 °C. This treatment resulted in partial ligand decarboxylation, generating coordinatively unsaturated Cu²⁺ sites with extra porosity on the way to the transformation of the initial HKUST-1 framework to CuO. The obtained materials retaining in part the HKUST-1 original crystal structure (quasi-MOFs) were used to promote 4-nitrophenol conversion to 4-aminophenol. Because of the partial linker decomposition, the quasi-MOF treated at 240 °C contains coordinatively unsaturated Cu²⁺ ions distributed throughout the Q-HKUST lattice together with micro- and mesopores. These defects explain the excellent catalytic performance of QH-240 with an apparent rate constant of 1.02 × 10^-2 s^-1 in excess of NaBH₄ and an activity factor and half-life time of 51 s^-1g^-1 and 68 s, respectively, which is much better than that of the HKUST parent. Also, the induction period decreases from the order of minutes to seconds in the presence of the HKUST and QH-240 catalysts, respectively. Kinetic studies fit with the Langmuir-Hinshelwood theory in which both 4-nitrophenol and BH₄^- should be adsorbed onto the catalyst surface. The values of the true rate constant (k), the adsorption constants of 4-nitrophenol and BH₄^- (K_4-NP and K_BH4^-), as well as the activation energy are in agreement with a rate-determining step involving the reduction of 4-nitrophenol by the surface-bound hydrogen species.

Keywords: 4-nitrophenol reduction; defect engineering; heterogeneous catalysis; metal−organic frameworks; partial ligand removal.

Figure 1

Illustration of the details of the HKUST-1 structure. Color code: O, red; C, black; and Cu, blue.

Figure 2

PXRD patterns of the HKUST and QH-x samples.

Figure 3

N₂ isotherm collected at 77 K and 1 bar and pore size distribution for HKUST, QH-200, QH-240, and QH-260.

Figure 4

XP spectra of the Cu 2p peaks for the (a) HKUST-1 and (b) QH-240 samples and their best deconvolution to individual components (SS = shake-up satellite).

Figure 5

TEM images of (a) HKUST-1 and (b) QH-240. While HKUST shows a clean, smooth surface, QH-260 shows nanoparticles as well as the presence of macropores.

Figure 6

UV–vis spectra of 4-NP and NaBH₄ solution after 1.0 mg of QH-240 catalyst was added.

Figure 7

NH₃-TPD analyses of (a) HKUST and (b) QH-240 catalyst.

Figure 8

FT-IR spectra in the characteristic C≡N stretching region after adsorbing CD₃CN on HKUST-1 (a), QH-240 (b), QH-260 (c), QH-300 (d), and QH-400 (e). The peak at 2334 cm^–1 is associated with the C≡N interaction with Lewis acid sites of moderate strength, while the broad band at 2265 cm^–1 corresponds to physisorbed acetonitrile.

Figure 9

Dependence of the apparent rate constant (k_app) on 4-NP and BH₄^– concentrations at various temperatures. The black solid lines are the best fit of the experimental data to the LH model with the calculated surface area of QH-240 catalyst in solution of about 1.28 m² L ^–1.

Scheme 1. Mechanistic Proposal Based on the LH Model for the Reduction of 4-NP by NaBH₄^– in the Presence of QH-240 Catalyst

Figure 10

(a) 4-NP reduction and 4-AP conversion over QH-240 catalyst for four consecutive reuses. Experimental conditions: m_cat = 0.2 mg, [4-NP] = 0.05 mM, [NaBH₄] = 10 mM at room temperature for a reaction time of 6 min. (b) XRD pattern of QH-240 catalyst after four catalytic cycles.