The impact of crystal size and temperature on the adsorption-induced flexibility of the Zr-based metal–organic framework DUT-98

In this contribution we analyze the influence of adsorption cycling, crystal size, and temperature on the switching behavior of the flexible Zr-based metal–organic framework DUT-98. We observe a shift in the gate-opening pressure upon cycling of adsorption experiments for micrometer-sized crystals and assign this to a fragmentation of the crystals. In a series of samples, the average crystal size of DUT-98 crystals was varied from 120 µm to 50 nm and the obtained solids were characterized by X-ray diffraction, infrared spectroscopy, as well as scanning and transmission electron microscopy. We analyzed the adsorption behavior by nitrogen and water adsorption at 77 K and 298 K, respectively, and show that adsorption-induced flexibility is only observed for micrometer-sized crystals. Nanometer-sized crystals were found to exhibit reversible type I adsorption behavior upon adsorption of nitrogen and exhibit a crystal-size-dependent steep water uptake of up to 20 mmol g−1 at 0.5 p/p0 with potential for water harvesting and heat pump applications. We furthermore investigate the temperature-induced structural transition by in situ powder X-ray diffraction. At temperatures beyond 110 °C, the open-pore state of the nanometer-sized DUT-98 crystals is found to irreversibly transform to a closed-pore state. The connection of crystal fragmentation upon adsorption cycling and the crystal size dependence of the adsorption-induced flexibility is an important finding for evaluation of these materials in future adsorption-based applications. This work thus extends the limited amount of studies on crystal size effects in flexible MOFs and hopefully motivates further investigations in this field.


Materials and Methods
The following chemicals and methods were applied in the synthesis and analysis of  [S1]: ZrCl4 (Sigma Aldrich), N,N-dimethylformamide anhydrous (<150 ppm H2O, Sigma Aldrich), acetic acid (Fisher Scientific) and ethanol (abs.) (VWR Prolabo) were used as received. The Ligand H3CPCDC was used from a previous study [S1,S2].
Powder X-ray diffraction (PXRD) patterns were collected with a PANALYTICAL X'Pert Pro diffractometer operated at 40 kV and 30 mA with monochromatic Cu Kα1 (λ = 0.15405 nm) radiation. The samples were placed on a Si sample holder. Variable temperature PXRD was performed in a closed chamber under dynamic vacuum of 10 −4 mbar on a PANALYTICAL X'PERT PRO with λ = 0.15405 nm in Bragg-Brentano-geometry. The temperature was increased with 5 K min −1 steps starting at 300 K. For each selected temperature, PXRD patterns in the 2Ѳ-range from 4 to 22°, with a step size of 0.028° and 40 s exposure per step were recorded. Gas and vapor adsorption experiments were carried out on a Belsorp-max instrument (MicrotracBEL Corp., Japan). High purity gases were used (N2: 99.999%, He: 99.999%). Water adsorption isotherms were recorded at 298 K using a Hydrosorb instrument from Quantachrome Instruments. Thermogravimetric analysis (TGA) was carried out in air atmosphere using a Netzsch STA 409 thermal analyzer. Scanning electron microscopy (SEM) and Scanning electron transmission microscopy (STEM) images were taken with secondary electrons in a HITACHI SU8020 microscope. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded on a Bruker Vertex 70 spectrometer with 16 scans for each sample and a resolution of 2 cm −1 . Prior to the measurement 3-5 mg of sample were mixed with 60 mg of dry KBr and pressed in the sample holder. TEM images were recorded with a binning of 4 at 80 kV accelerating voltage using an FEI Titan G3 50-300 PICO microscope equipped with a 4096 x 4096 pixels Gatan Ultrascan 4000 UHS charge coupled device camera (CCD). The frame exposure time is 0.3 s.

Synthesis and activation procedure General synthesis of DUT-98 Synthesis and activation of DUT-98
Synthesis of DUT-98 samples with varying crystal size was performed as following:

Synthesis of DUT-98(1)
The synthesis was performed according to a previously reported procedure [S1].
The reaction mixture is kept at 120 °C for 2 d to form a white precipitate.
The reaction mixture is kept at 120 °C for 18 h to form a white gel.

Synthesis of DUT-98(4)
In a 100 mL Schott bottle ZrCl4 (370 mg, 1.6 mmol) and acetic acid (10 mL, 175 mmol) are dissolved in 45 mL of DMF. To the solution the ligand H3CPCDC (300 mg, 0.8 mmol) and water (5 mL, 280 mmol) are added and dissolved by ultrasonication. The reaction mixture is kept at 120 °C for 16 h to form a white gel.

Synthesis of DUT-98(Hf)
In a 20 mL Pyrex tube 256 mg (0.8 mmol) HfCl4 and acetic acid (0.5 mL, 8.75 mmol) are dissolved in 5 mL of DMF. To the solution the ligand H3CPCDC (60 mg, 0.16 mmol) and water (0.2 mL, 11.2 mmol) are added and dissolved by ultrasonication. The reaction mixture is kept at 120 °C for 24 h to form a white gel.

S4
An overview of the synthesis parameter is provided in the following table. S5 Table S2: BET surface area, specific pore volume and uptake of nitrogen of the series of investigated DUT-98 samples.