Coordination mode engineering in stacked-nanosheet metal–organic frameworks to enhance catalytic reactivity and structural robustness

Optimising the supported modes of atom or ion dispersal onto substrates, to synchronously integrate high reactivity and robust stability in catalytic conversion, is an important yet challenging area of research. Here, theoretical calculations first show that three-coordinated copper (Cu) sites have higher activity than four-, two- and one-coordinated sites. A site-selective etching method is then introduced to prepare a stacked-nanosheet metal–organic framework (MOF, CASFZU-1)-based catalyst with precisely controlled coordination number sites on its surface. The turnover frequency value of CASFZU-1 with three-coordinated Cu sites, for cycloaddition reaction of CO2 with epoxides, greatly exceed those of other catalysts reported to date. Five successive catalytic cycles reveal the superior stability of CASFZU-1 in the stacked-nanosheet structure. This study could form a basis for the rational design and construction of highly efficient and robust catalysts in the field of single-atom or ion catalysis.

times, respectively. At last, the orange powder of MIL-88-Fe was obtained after drying in a vacuum oven at 100 °C for 12 h. Cyanosilylation over pristine HKUST-1 and CASFZU-1: The as-prepared samples were activated in vacuum at 120 °C for 12 h. Then, the degassed 6 mg pristine HKUST-1 and CASFZU-1 were transferred into a flask, respectively, and 80 uL benzaldehyde (0.79 mmol), 220 uL trimethylsilylcyanide (TMSCN, 1.65 mmol), 4 mL heptane were added. The mixture was allowed to react at 60 °C for 48 h with stirring and the products were analyzed by GC-MS. [Benzaldehyde (Aladdin Industrial Inc., 99.5%, GC), trimethylsilylcyanide (Aladdin Industrial Inc., 97%, AR), heptane (Aladdin Industrial Inc., 99.5%, GC). Theoretical models and computational details. As the catalytic active site is copper, the HKUST-1 and CASFZU-1 surfaces were modelled by simple building blocks (a paddlewheel Cu 2 dimer surrounded by four coordinated formic acid molecules for HKUST-1, a paddlewheel Cu 2 dimer surrounded by three coordinated formic acid molecules for CASFZU-1, see Supplementary Figure 48) to achieve a best compromise between computational cost and accuracy of computational outcomes. Besides, the large tetrabutylammonium bromide (TBAB) was also simplified to tetramethylammonium bromide. Then, the intermediates are constructed by combining the reactants with Cu-site via adopting different orientations, and the lowest-energy one was adopted for each intermediate after initial geometric optimisation. All the geometries of the isolated reactants, products, intermediates, and transition states involved in the cycloaddition reaction have been fully optimised without any constraints via DFT calculations by using the B3PW91 density functional. 10,11 The Los Alamos double-zeta-type LANL2DZ and effective core potential (ECP) basis sets were used for the Cu and Br atoms, while the 6-311+G(d,p) split valence basis set was used for the other atoms for the geometric optimisation. Vibrational frequency calculations, from which the thermal corrections to Gibbs free energy were derived, have been performed for each optimised structure at the same level to identify the nature of all the stationary points (local minimum or first-order saddle point). The intrinsic reaction coordinate (IRC) 12 pathways have been traced at the 6-31+G(d)/LANL2DZ level in order to verify that each saddle point links two desired minima. Finally, the zero-point-corrected Gibbs free energies for the isolated reactants, products, intermediates, and transition states were calculated at 298 K on the basis of the optimised structures to obtain the potential energy surface profiles of the cycloaddition reaction. All calculations were carried out using the GAUSSIAN 09 software package. 13 To compare the electronic structures of HKUST-1 and CASFZU-1, larger models with benzyl acid molecules as ligands are considered (see Supplementary Figure 1). Their HOMO, LUMO, and electronic static potential (ESP) were generated by the GaussView program. 14 The wave functions produced by GAUSSIAN 09 software at the B3PW91/6-311+G(d,p)/LANL2DZ level were used as inputs for Multiwfn 3.3.7 software 15 to plot the total density of states (TDOS) and partial density of states (PDOS). To evaluate the distortion of framework for paddlewheel Cu 2 cluster with different coordination number, four large models without water molecules (Supplementary Figure 38) and two large models with water molecules (Supplementary Figure 39) have been optimised at the B3PW91/6-31G/LANL2DZ level by using the GAUSSIAN 09 software. Herein, we mainly focus on the change of on the surfaces of HKUST-1 and CASFZU-1, and thus the interior parts of the materials, namely these atoms marked by green colour in the models were frozen in the optimization progress. All the other parts of the molecular models have been fully optimised without any constraints and the nature of local minima in the potential energy surface were characterized by means of harmonic vibrational frequencies analysis. Figure 1. Schematic models of the paddlewheel Cu 2 clusters with four different kinds of coordination numbers for the simulation of electrostatic surface potential (ESP) maps and frontier orbital energy levels and molecular orbital (MO) diagrams. Colour scheme for chemical representation: cyan for Cu, red for O, grey for C and white for H. It's expected that a polymer with the ability to bind copper metal would provide a favorable support for growing a HKUST-1 layer. Numerous nitrogen atoms and oxygen atoms in the molecule structure would play crucial role for their ability to form stable chelates with a wide variety of metals.

Supplementary
In a similar way, for the (111) plane in FCC crystal, 3 bonds per atoms are broken, for the (110) plane in FCC crystal, 6 bonds per atoms are broken, Before etching, the membrane with pure HKUST-1 thin film was dried in vacuum at 100 °C for overnight, and then soaked in the mixed solvent for 5 min. Subsequently, the membrane was dried at 60 °C for 10 min to remove the mixed solvent on the surface of Nylon 66 membrane, and finally it was stored in relevant RH for a month. Actually, ethanol molecules in the mixed solvent are supposed to play a predominant role in stabilizing the framework with its hydrophobic alkane tails. Subsequently exploratory experiments with altered alcohols (methanol, n-propanol, isopropanol, n-butanol) were performed to test and verify the effect of alcohol. It was obviously observed that all the crystals exhibited nanosheet-like structure on the surface as the water/alcohol ratio was 1/1 at 25%RH, although the solvent polarity was slightly different (Supplementary table 1). Indeed, after extensive variation of synthesis conditions, we identified a synthesis window that produces CASFZU-1 via controllable etching. Before etching, the pure MOFs were dried in vacuum at 100 °C for overnight, and then soaked in the mixed solvent (H 2 O/ethanol, V water :V ethanol = 1:1) for 10 min. subsequently, the pure MOFs were dip coating on the membrane and the membrane was dried at 60 °C for 10 min to remove the mixed solvent on the surface of Nylon 66 membrane, and finally it was stored in relevant RH for a month.

Supplementary
Supplementary Figure 14. Moisture stability map of the ten MOFs discussed in this paper. The position for a given MOF represents its relative structural stability by SEM images.
From the result, it can be obtained that HKUST-1 would be slightly more stable with respect to reaction with water than MOF-5 and CuBDC. ZIF-8, UIO-66, ZIF-67, ZIF-7, MIL-110, MIL-53 (Al), MIL-53 (Cr), and MIL-88 are more stable than HKUST-1. These results are consistent with the previous reports. 16 Obviously, only HKUST-1 is suitable for controllable etching experiment at moderate moisture (25% RH). During the etching process, the structural integrity and crystallinity of MOF are well retained and no other crystals appeared.
Supplementary Figure 17. TG patterns for the materials. Thermal analysis revealed that the CASFZU-1 nanosheets remain stable up to 320 °C which is similar to the pristine HKUST-1. During the random-etching process, the main structural integrity and crystallinity of MOF are well and no other visible crystals appeared. However, in the high RH condition, there is major evidence of structural changes, and the transformation is thoroughly at the end. Comparison with pristine MOF, the CASFZU-1 has lower content in carbon, higher content in oxygen and copper. During the etching process of MOF, the reduced content percentage of carbon is about equal to the sum of increased content percentage of oxygen and copper. These results are in consistent with those in the corresponding evolution process of some pristine MOF to CASFZU-1 which has lost some ligands. While the MOF crystals structure collapsed in the process of etching, the content of oxygen is much higher than pristine MOF and the content of copper is lower than pristine MOF, which indicates that large amount of water molecules replace the organic ligands. The BET surface area for pristine HKUST-1 and CASFZU-1 are calculated to be 1550 and 1043 m 2 g -1 , respectively, suggesting that part of the inherent porous structure is blocked after some ligands fell off from the framework of MOFs. For random-etching MOFs, S BET = 637.78 m 2 g -1 , it indicated that the random-etching MOFs has lost half of its porosity. For collapsed MOFs S BET = 7.03 m 2 g -1 , it indicated that the collapsed MOFs has completely lost its porosity. From the XANES spectra, we can see that the Cu K-edge XANES is characterized by four peaks, including a pre-edge peak I at 8978 due to 1s→3d transition, a shoulder peak II at 8986 eV ascribed to 1s→4p dipolar shakedown transition, a white line peak III at 8998 eV and a resonance peak IV at 9042 eV. Upon dehydration, the Cu K-edge position does not show obvious change, which means an unaltered oxidation state for the Cu species; however, both an intensity decrease for the white line peak III and an intensity increase for the 1s→4p peak II indicate loss of axial ligands for the Cu cations, which is associated with the removal of water molecular. 19 Supplementary Figure 28. Fourier-transformed magnitude of Cu K-edge EXAFS spectrum for CuO reference. Measured and calculated spectra are well matched for all samples. The best-fit parameters are shown in Supplementary Table 3. From the heterogeneous catalysis result that cyanosilylation of benzaldehyde and trimethylsilylcyanide over the pristine HKUST-1 and CASFZU-1, it is clearly suggested that the Lewis-acid catalytic activity of CASFZU-1 nanosheets were greatly increased after three-coordinated copper sites exposed.

Supplementary Note 3. The amount of Carbon dioxide fixation
In present experimental conditions, 99% of 2-methyloxirane (25 mmol) has already reacted with pure CO 2 in the first 22 h; the corresponding amount of catalyst (CASFZU-1) is 5.5 mg: With the ligand lost, larger mesopore in MOFs was produced. The mesopore diameters are about 4.0 nm and 30.6 nm in CASFZU-1 which are much bigger than that in HKUST-1. We have solid reason to believe that CASFZU-1 with large mesopores will efficiently enhance its capture capacity of large molecule and will greatly improve the accessibility of target object into copper sites inside the pore. The mesopore diameters in random-etching MOFs are about 3.6 nm which is similar to the CASFZU-1. The PDOS of 4-coordinated Cu sites exhibited a very low tail at the conduction band minimum, whereas that in 3-coordinated Cu sites displayed obviously raised states density at the edge of conduction band. This greatly increased PDOS at the conduction band minimum of the 3-coordinated Cu sites could be ascribed to their exotic layered structure. Notably, the unfilled e g states of coordinately-unsaturated Cu sites in 3-coordinated Cu sites are less than those of fully-coordinated Cu sites in HKUST-1.  HKUST-1 exhibited CO 2 /N 2 adsorption selectivity of 13.6 while CASFZU-1 exhibited much higher CO 2 /N 2 adsorption selectivity of 18.9. This result demonstrates the stronger adsorption capacity of CASFZU-1 for the selective sorption of CO 2 from the low CO 2 concentration atmosphere, so it can have higher potential application for carbon fixation in nature. The TOF was 54.0 h −1 per CASFZU-1 for catalytic cycloaddition reaction of CO 2 with 2-methyloxirane. To our best knowledge, The CASFZU-1 has higher catalytic efficiency than most of the catalysts reported in catalytic cycloaddition reaction of CO 2 with 2-methyloxirane.