Effect of pore occupancy on the acoustic properties of zeolitic imidazolate framework (ZIF)-8: A Brillouin spectroscopic study at ambient and low temperatures

Anomalous Loading Rate Dependence of the Mechanical Properties of Metal-Organic Framework Crystals: Latent Heat Effects of the Pressure-Induced Local Phase Transition

The loading rate dependence of the mechanical properties of metal-organic framework (MOF) crystals is key in determining their performance in many engineering applications, which, however, remains almost unexplored. Here, in situ nanoindentation experiments were conducted to investigate the impact of loading rate on mechanical properties of HKUST-1, a classic MOF. The Young's modulus and hardness of crystalline HKUST-1 are found to stay stable or decline with decreasing loading rate by creeping when the loading rate is below a particular speed, but they significantly decrease as the loading rate grows when it has higher magnitudes. Our molecular dynamics simulations indicate that the anomalous loading rate dependence of mechanical properties is attributed to the competition between the release and transfer of latent heat from the pressure-induced amorphous HKUST-1 because the increase in local temperature at large loading rates could induce the softening of HKUST-1 and the increase in the volume of transformed materials.

Raman Evidence of Multiple Adsorption Sites and Structural Transformation in ZIF-4

The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10^-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N₂ and CO₂ around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm^-1). In contrast to a single vibrational mode generally reported for entrapped N₂, we show three Raman modes of adsorbed N₂ in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N₂ adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.

Mixed metal node effect in zeolitic imidazolate frameworks

We synthesized two series of bimetallic (zinc and cobalt) zeolitic imidazolate frameworks (ZIF-62) under different solvothermal conditions. It is found that the structure of the derived ZIF crystals is highly sensitive to synthesis conditions. One series possesses the standard ZIF-62 structure, whereas the other has a mixed structure composed of both the standard structure and an unknown one. The standard series exhibits a slight negative deviation from linearity of melting temperature (T _m) and glass transition temperature (T _g) with the substitution of Co for Zn. In contrast, the new series displays a stronger negative deviation. These negative deviations from linearity indicate the mixed metal node effect in bimetallic ZIF-62 due to the structural mismatch between Co²⁺ and Zn²⁺ and to the difference in their electronic configurations. The new series involves both cobalt-rich and zinc-rich phases, whereas the standard one shows one homogeneous phase. Density functional theory calculations predict that the substitution of Co for Zn increases the bulk modulus of the ZIF crystals. This work indicates that the structure, melting behaviour, and mechanical properties of ZIFs can be tuned by metal node substitution and by varying the synthetic conditions. Both series of ZIFs have higher glass forming abilities due to their higher T _g/T _m ratios (0.77-0.84) compared to most good glass formers.

Thermodynamic Evidence of Structural Transformations in CO2-Loaded Metal-Organic Framework Zn(MeIm)2 from Heat Capacity Measurements

Metal-organic frameworks are a class of porous compounds with potential applications in molecular sieving, gas sequestration, and catalysis. One family of MOFs, zeolitic imidizolate frameworks (ZIFs), is of particular interest for carbon dioxide sequestration. We have previously reported the heat capacity of the sodalite topology of the zinc 2-methylimidazolate framework (ZIF-8), and in this Article we present the first low-temperature heat capacity measurements of ZIF-8 with various amounts of sorbed CO₂. Molar heat capacities from 1.8 to 300 K are presented for samples containing up to 0.99 mol of CO₂ per mol of ZIF-8. Samples with at least 0.56 mol of CO₂ per mol of ZIF-8 display a large, broad anomaly from 70 to 220 K with a shoulder on the low-temperature side, suggesting sorption-induced structural transitions. We attribute the broad anomaly partially to a gate-opening transition, with the remainder resulting from CO₂ rearrangement and/or lattice expansion. The measurements also reveal a subtle anomaly from 0 to 70 K in all samples that does not exist in the sorbate-free material, which likely reflects new vibrational modes resulting from sorbate/ZIF-8 interactions. These results provide the first thermodynamic evidence of structural transitions induced by CO₂ sorption in the ZIF-8 framework.

A thermo-responsive adsorbent-heater-thermometer nanomaterial for controlled drug release: (ZIF-8,EuxTby)@AuNP core-shell

An adsorbent-heater-thermometer nanomaterial, (ZIF-8,Eu_xTb_y)@AuNP, based on ZIF-8 (adsorbent), containing Eu³⁺ and/or Tb³⁺ ions (thermometer) and gold nanoparticles (AuNPs, heater) was designed, synthetized, characterized, and applied to controlled drug release. These composite materials were characterized as core-shell nanocrystals with the AuNPs being the core, around which the crystalline ZIF-8 has grown (shell) and onto which the lanthanide ions have been incorporated or chemosorbed. This shell of ZIF-8 acts as adsorbent of the drugs, the AuNPs act as heaters, while the luminescence intensities of the ligand and the lanthanide ions are used for temperature monitoring. This thermo-responsive material can be activated by visible irradiation to release small molecules in a controlled manner as established for the model pharmaceutical compounds 5-fluorouracil and caffeine. Computer simulations and transition state theory calculations shown that the diffusion of small molecules between neighboring pores in ZIF-8 is severely restricted and involves high-energy barriers. These findings imply that these molecules are uploaded onto and released from the ZIF-8 surface instead of being inside the cavities. This is the first report of ZIF-8 nanocrystals (adsorbents) containing simultaneously lanthanide ions as sensitive nanothermometers and AuNPs as heaters for controlled drug release in a physiological temperature range. These results provide a proof-of-concept that can be applied to other classes of materials, and offer a novel perspective on the design of self-assembly multifunctional thermo-responsive adsorbing materials that are easily prepared and promptly controllable.

Zeolitic imidazolate framework-coated acoustic sensors for room temperature detection of carbon dioxide and methane

The integration of nanoporous materials such as metal organic frameworks (MOFs) with sensitive transducers can result in robust sensing platforms for monitoring gases and chemical vapors for a range of applications. Here, we report on an integration of the zeolitic imidazolate framework - 8 (ZIF-8) MOF with surface acoustic wave (SAW) and thickness shear mode quartz crystal microbalance (QCM) devices to monitor carbon dioxide (CO2) and methane (CH4) under ambient conditions. The MOF was directly coated on the Y-Z LiNbO3 SAW delay lines (operating frequency, f0 = 436 MHz) and AT-cut quartz TSM resonators (resonant frequency, f0 = 9 MHz) and the devices were tested for various gases in N2 under ambient conditions. The devices were able to detect the changes in CO2 or CH4 concentrations with relatively higher sensitivity to CO2, which was due to its higher adsorption potential and heavier molecular weight. The sensors showed full reversibility and repeatability which were attributed to the physisorption of the gases into the MOF and high stability of the devices. Both types of sensors showed linear responses relative to changes in the binary gas compositions thereby allowing to construct calibration curves which correlated well with the expected mass changes in the sorbent layer based on mixed-gas gravimetric adsorption isotherms measured on bulk samples. For 200 nm thick films, the SAW sensitivities to CO2 and CH4 were 1.44 × 10-6/vol% and 8 × 10-8/vol%, respectively, against the QCM sensitivities 0.24 × 10-6/vol% and 1 × 10-8/vol%, respectively, which were evaluated as the fractional change in the signal. The SAW sensors were also evaluated for 100 nm-300 nm thick films, the sensitivities of which were found to increase with the thickness due to the increased number of pores for the adsorption of a larger amount of gases. In addition, the MOF-coated SAW delay lines had a good response in wireless mode, demonstrating their potential to operate remotely for the detection of the gases at emission sites across the energy infrastructure.