Highly defective ultra-small tetravalent MOF nanocrystals

Abstract

The size and defects in crystalline inorganic materials are of importance in many applications, particularly catalysis, as it often results in enhanced/emerging properties. So far, applying the strategy of modulation chemistry has been unable to afford high-quality functional Metal-Organic Frameworks (MOFs) nanocrystals with minimized size while exhibiting maximized defects. We report here a general sustainable strategy for the design of highly defective and ultra-small tetravalent MOFs (Zr, Hf) crystals (ca. 35% missing linker, 4-6 nm). Advanced characterizations have been performed to shed light on the main factors governing the crystallization mechanism and to identify the nature of the defects. The ultra-small nanoMOFs showed exceptional performance in peptide hydrolysis reaction, including high reactivity, selectivity, diffusion, stability, and show emerging tailorable reactivity and selectivity towards peptide bond formation simply by changing the reaction solvent. Therefore, these highly defective ultra-small M(IV)-MOFs particles open new perspectives for the development of heterogeneous MOF catalysts with dual functions.

Fig. 1. Comparison of MOF nanoparticles synthesis approaches.

Scheme of (a) the conventional modulator-induced-defect approach (MIDA) for the size-defect tuning of MOFs, (b) our approach that produces ultra-small and highly defective tetravalent-MOFs nanoparticles, Red, green, and brown represent O, Metal(IV), and C atoms, respectively; yellow sphere and green lines within the purple circles indicate the metal nodes and organic ligand, respectively.

Fig. 2. Characterizations of the synthesized UiO-66 nanoparticles.

a Schematic diagram of our strategy, (b) powder X-ray diffraction (PXRD) (λ_Cu = 1.5406 Å) patterns of UiO-66 synthesized with different volumes of EtOH, the yellow lines evidence the diffraction peaks from the calculated pattern, (c) statistical mean size of the synthesized UiO-66, (d) TEM image of the 5 nm UiO-66 (obtained with 80 mL EtOH, highlighted using yellow circles), i) enlarged selected zone, ii) structure of UiO-66 viewed from (101) axis direction, (e) SAED pattern of the 5 nm UiO-66, (f) High-resolution Transmission Electron Microscope (HRTEM) images of HD-US-UiO-66 and their contrast intensity profiles, viewed along (i) (220) and (ii) (011) directions, scale bar = 5 nm, the green rectangles represent the selected regions for contrast analysis, (g) TGA under oxygen atmosphere (scan rate of 3 °C/min) of the UiO-66 with different sizes, inserted bar chart: linker to Zr₆ ratio of the different samples; (h) 77 K N₂ sorption isotherms of UiO-66 with different sizes (P/P₀ = 1 bar), adsorption and desorption are represented by filled spheres, and open spheres, respectively, i) pore size distribution for different sizes of UiO-66 (same color label as in (h)).

Fig. 3. Characterization of the accessible acid sites by in situ FTIR.

In situ FTIR spectra at 298 K of CD₃CN (red to gray, probe small doses to up to 10 torr equilibrium pressure) adsorbed on HD-US-UiO-66 (5 nm), the peaks position and peak area are shown in dotted lines and red area, respectively, the small doses and equilibrium indicate the increasing CD₃CN dosing from 0.2 torr to saturated 6.4 torr.

Fig. 4. In situ and ex-situ characterizations of the MOF growth.

a Hydrodynamic size of HD-US-UiO-66 colloids (T = 25 °C) determined by in situ time-dependent DLS (time resolution t_R = 120 s), and the (b) ex-situ HAADF-STEM (0 = i, 1 min = ii) and HRTEM (120 min = iii, 180 min = iv) images of HD-US-UiO-66 at different times.

Fig. 5. Catalytic performance evaluation of the HD-US-UiO-66-X.

a Illustration of peptide hydrolysis using HD-US-UiO-66, (b) Pseudo first order hydrolysis rate of glycylglycine (GG) to glycine (G) using HD-US-UiO-66-NH₂ and MI-US-UiO-66-NH₂ nanoMOFs, reference refers to the value reported in the previous studies under the same conditions, (c) Selectivity of hydrolysis by HD-US-UiO-66-NH₂ and HD-200-UiO-66-NH₂ in producing the desired product G, as the starting concentration of GG increases from 2 mM to 500 mM, (d) Recyclability of HD-US-UiO-66-NH₂ over 5 reaction cycles in comparison to best-performing MOFs to date, percentage compared to yield of cycle 1, (e) illustration of amide bond condensation using HD-US-UiO-66, in MeOH, (f) Amide bond formation yield with HD-US-UiO-66 and HD-US-UiO-66-NH₂ starting from G, GG, and AG.