Zeolite Imidazole Frameworks Membranes for CO2/CH4 Separation from Natural Gas: A Review

Reaching Quantum Accuracy in Predicting Adsorption Properties for Ethane/Ethene in Zeolitic Imidazolate Framework-8 at Low Pressure Regime

Nanoporous materials in the form of metal-organic frameworks such as zeolitic imidazolate framework-8 (ZIF-8) are promising membrane materials for the separation of hydrocarbon mixtures. To compute the adsorption isotherms in such adsorbents, grand canonical Monte Carlo simulations have proven to be very useful. The quality of these isotherms depends on the accuracy of adsorbate-adsorbent interactions, which are mostly described using force fields owing to their low computational cost. However, force field predictions of adsorption uptake often show discrepancies from experiments at low pressures, providing the need for methods that are more accurate. Hence, in this work, we propose and validate two novel methodologies for the ZIF-8/ethane and ethene systems; a benchmarking methodology to evaluate the performance of any given force field in describing adsorption in the low-pressure regime and a refinement procedure to rescale the parameters of a force field to better describe the host-guest interactions and provide for simulation isotherms with close agreement to experimental isotherms. Both methodologies were developed based on a reference Henry coefficient, computed with the PBE-MBD functional using the importance sampling technique. The force field rankings predicted by the benchmarking methodology involve the comparison of force field derived Henry coefficients with the reference Henry coefficients and ranking the force fields based on the disparities between these Henry coefficients. The ranking from this methodology matches the rankings made based on uptake disparities by comparing force field derived simulation isotherms to experimental isotherms in the low-pressure regime. The force field rescaling methodology was proven to refine even the worst performing force field in UFF/TraPPE. The uptake disparities of UFF/TraPPE improved from 197% and 194% to 11% and 21% for ethane and ethene, respectively. The proposed methodology is applicable to predict adsorption across nanoporous materials and allows for rescaled force fields to reach quantum accuracy without the need for experimental input.

Unveiling the mechanisms behind high CO2 adsorption by the selection of suitable ionic liquids incorporated into a ZIF-8 metal organic framework: A computational approach

There is growing focus on the crucial task of effectively capturing carbon dioxide (CO2) from the atmosphere to mitigate environmental consequences. Metal-organic frameworks (MOFs) have been used to replace many conventional materials in gas separation, and the incorporation of ionic liquids (ILs) into porous MOFs has shown promise as a new technique for improving CO2 capture and separation. However, the driving force underlying the electronic modulation of MOF nanostructures and the mechanisms behind their high CO2 adsorption remain unclear. This study reports the effect of encapsulating different imidazolium ILs in porous ZIF-8, to clarify the adsorption mechanism of CO2 using density functional theory (DFT)-based approaches. For this purpose, a range of anions, including bis(trifluoromethylsulfonyl)imide [NTf2], methanesulfonate [MeSO3], and acetate [AC], were combined with the 1-ethyl-3-methylimidazolium [EMIM]+ cation. [EMIM]+-based ILs@ZIF-8 composites were computationally investigated to identify suitable materials for CO2 capture. First, the intermolecular and intramolecular interactions between [EMIM]+ and different anions were examined in detail, and their effects on CO2 adsorption were explored. Subsequently, the integration of these ILs into the ZIF-8 solid structure was studied to reveal how their interactions influenced the CO2 adsorption behavior. Our results demonstrate that the incorporation of ILs strongly affects the adsorption capability of CO2, which is highly dependent on the nature of the ILs inside the ZIF-8 framework. DFT simulations further confirmed that the incorporation of ILs into ZIF-8 led to superior CO2 capture compared to isolated ILs and pristine ZIF-8. This improvement was attributed to the mutual interactions between the ILs and ZIF-8, which effectively fine-tuned CO2 adsorption within the composite structure. This understanding may act as a general guide for gaining more insight into the interfacial interactions between ILs and ZIFs structures and how these molecular-level interactions can help predict the selection of ILs for CO2 adsorption and separation, thereby addressing environmental challenges with greater precision and effectiveness.

Material and Process Tests of Heterogeneous Membranes Containing ZIF-8, SiO2 and POSS-Ph

Heterogeneous membranes made of a polymer matrix and containing nano-metric fillers in their structure may present improved physicochemical and process properties compared to homogeneous membranes made only of polymer materials. Membranes made of a PEBAX^®2533 block copolymer were tested with fillers such as ZIF-8, SiO₂ and POSS-Ph being dosed to them. The material analysis and process tests indicate that these nanomaterials can be used as fillers for heterogeneous membranes. Chemometric analyses determined the influence of individual fillers on selected physicochemical properties of the materials which were used to produce the membranes. For specific concentrations of these fillers, improvement in the permeability and selectivity of the membranes, or at least in one of these parameters, was achieved. The greatest increase in permeability against the homogeneous membrane was obtained for membranes containing 10 wt% ZIF-8 (for CO₂, an increase of 2.07 times; for CH₄, 2.36 times; for N₂, 3.08 times). In turn, the greatest increase in selectivity was obtained for the CO₂/CH₄ mixture for the membrane containing 5 wt% SiO₂ (1.15 times), and for the CO₂/N₂ mixture for the membrane containing 2 wt% POSS-Ph (1.21 times).

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.