Gate-opening Pressure Research Articles

Breakthrough analysis for parameter estimation of CO2 adsorption on pelletized flexible metal–organic framework

Pellets of elastic layer-structured metal–organic framework-11 (ELM-11), one of the flexible metal–organic frameworks (MOFs), were prepared by mixing ELM-11 powder with a cellulose binder, and the CO2 adsorption behavior of ELM-11 pellets in a fixed-bed adsorber was analyzed to estimate equilibrium and kinetic parameters. Adsorption breakthrough tests were performed under various conditions for the simulated flue gas (20 % CO2–80 % N2) of a blast furnace gas in addition to CO2 isotherm measurements. Experimental CO2 adsorption isotherms on ELM-11 followed a weighted dual-site Langmuir equilibrium model well. The breakthrough analysis results by numerical calculation using the equilibrium model suggested that the gate-opening pressure of ELM-11 shifted to a lower pressure through repeated breakthrough tests. The CO2 adsorption kinetics on ELM-11 pellets agreed well with the linear-driving-force model. Furthermore, CO2 adsorption behavior under various conditions was described well by a mathematical model using a constant value of the mass transfer coefficient under the condition of this study, which was validated against additional experimental data. In addition, it was found that the mass transfer becomes slow under certain conditions that the driving force of the phase transition of ELM-11 is smaller than a threshold, where the amount of CO2 adsorbed on the breakthrough tests did not reach the equilibrium capacity of CO2, which may because the rate-limiting step may be dependent on the CO2 partial pressure and temperature.

Metal cation substitution can tune CO2, H2O and CH4 switching pressure in transiently porous coordination networks.

Compared to rigid physisorbents, switching coordination networks that reversibly transform between closed (non-porous) and open (porous) phases offer promise for gas/vapour storage and separation owing to their improved working capacity and desirable thermal management properties. We recently introduced a coordination network, X-dmp-1-Co, which exhibits switching enabled by transient porosity. The resulting "open" phases are generated at threshold pressures even though they are conventionally non-porous. Herein, we report that X-dmp-1-Co is the parent member of a family of transiently porous coordination networks [X-dmp-1-M] (M = Co, Zn and Cd) and that each exhibits transient porosity but switching events occur at different threshold pressures for CO2 (0.8, 2.1 and 15 mbar, for Co, Zn and Cd, respectively, at 195 K), H2O (10, 70 and 75% RH, for Co, Zn and Cd, respectively, at 300 K) and CH4 (<2, 10 and 25 bar, for Co, Zn and Cd, respectively, at 298 K). Insight into the phase changes is provided through in situ SCXRD and in situ PXRD. We attribute the tuning of gate-opening pressure to differences and changes in the metal coordination spheres and how they impact dpt ligand rotation. X-dmp-1-Zn and X-dmp-1-Cd join a small number of coordination networks (<10) that exhibit reversible switching for CH4 between 5 and 35 bar, a key requirement for adsorbed natural gas storage.

Facile Defect Engineering of Zeolitic Imidazolate Frameworks towards Enhanced C3H6/C3H8 Separation Performance

AbstractMembrane‐based olefin/paraffin separations can be an attractive alternative to the current energy‐intensive cryogenic distillation for their high energy efficiency. This paper reports an amine modulator‐induced defect engineering of zeolitic imidazolate framework‐8 (AZIF8) nanoparticles (60 nm) for enhanced molecular sieving ability and good compatibility with various polymer matrices. It is revealed that an initial suppression of nuclei formation during synthesis enables amine modulators to deprotonate the neutral organic ligand as well as coordinate with Zn2+ ions. The resulting AZIF8s exhibit more rigid characteristics compared to the conventional ZIF‐8 counterparts as demonstrated by its delayed gate opening pressure. Molecular dynamics simulations also suggest that AZIF8s exhibit a narrower pore size distribution with a smaller opening compared to the conventional ZIF‐8 analogues, which makes them more suitable for size‐discrimination between C3H6 and C3H8. Furthermore, the coordination of alkyl amines into frameworks allows excellent compatibility with various commercial polymer matrices. The highly concentrated 6FDA‐DAM/AZIF8 (50/50 w/w) mixed matrix membrane (MMM) exhibits superior C3H6/C3H8 separation performance (C3H6 permeability of 79.38 Barrer and C3H6/C3H8 selectivity of 39.8 at single gas condition) to most existing MMMs. The structural engineering proposed herein can be applied to other metal–organic frameworks for optimizing their performance in molecular separations.

Tuning Gate-Opening of a Flexible Metal-Organic Framework for Ternary Gas Sieving Separation.

In comparison with the fast development of binary mixture separations, ternary mixture separations are significantly more difficult and have rarely been realized by a single material. Herein, a new strategy of tuning the gate-opening pressure of flexible MOFs is developed to tackle such a challenge. As demonstrated by a flexible framework NTU-65, the gate-opening pressure of ethylene (C2 H4 ), acetylene (C2 H2 ), and carbon dioxide (CO2 ) can be regulated by temperature. Therefore, efficient sieving separation of this ternary mixture was realized. Under optimized temperature, NTU-65 adsorbed a large amount of C2 H2 and CO2 through gate-opening and only negligible amount of C2 H4 . Breakthrough experiments demonstrated that this material can simultaneously capture C2 H2 and CO2 , yielding polymer-grade (>99.99 %) C2 H4 from single breakthrough separation.

High-throughput gas separation by flexible metal\u2013organic frameworks with fast gating and thermal management capabilities

Establishing new energy-saving systems for gas separation using porous materials is indispensable for ensuring a sustainable future. Herein, we show that ELM-11 ([Cu(BF4)2(4,4′-bipyridine)2]n), a member of flexible metal–organic frameworks (MOFs), exhibits rapid responsiveness to a gas feed and an ‘intrinsic thermal management’ capability originating from a structural deformation upon gas adsorption (gate-opening). These two characteristics are suitable for developing a pressure vacuum swing adsorption (PVSA) system with rapid operations. A combined experimental and theoretical study reveals that ELM-11 enables the high-throughput separation of CO2 from a CO2/CH4 gas mixture through adiabatic operations, which are extreme conditions in rapid pressure vacuum swing adsorption. We also propose an operational solution to the ‘slipping-off’ problem, which is that the flexible MOFs cannot adsorb target molecules when the partial pressure of the target gas decreases below the gate-opening pressure. Furthermore, the superiority of our proposed system over conventional systems is demonstrated.

Research on CO2-N2O separation using flexible metal organic frameworks

N2O and CO2 are two linear molecules, which have almost the same molecular size and similar physical properties (polarizability and quadrupole moment). Therefore, the separation of CO2 and N2O is extremely challenging. Flexible metal organic frameworks exhibit a gate-opening phenomenon for one type of gas molecule, but not for another, thus enabling excellent selectivity for twin-like molecules. In this study, the separation of CO2/N2O by flexible metal organic frameworks with different structures was studied systematically using a combination of adsorption and breakthrough tests. The results show that the gate-opening pressure of CO2 is lower than that of N2O on Cu(bpy)2(BF4)2 (ELM-11) and Cu(bpy)2(OTf)2 (ELM-12), which are two-dimensional layered flexible MOFs, whereas N2O has a lower gate-opening pressure than CO2 on MIL-53Al, a three-dimensional breathing structure with an adsorption capacity of up to 8 mmol/g was observed at 298 K and ~5 bar, indicating its good separation properties for N2O from CO2 under high pressure. Breakthrough experiments for N2O/CO2 mixtures confirmed that MIL-53Al is capable of separating N2O and CO2 under the optimized conditions (8 bar), making it a promising material for industrial application.

Tuning the Gate-Opening Pressure in a Switching pcu Coordination Network, X-pcu-5-Zn, by Pillar-Ligand Substitution.

Coordination networks that reversibly switch between closed and open phases are of topical interest since their stepped isotherms can offer higher working capacities for gas-storage applications than the related rigid porous coordination networks. To be of practical utility, the pressures at which switching occurs, the gate-opening and gate-closing pressures, must lie between the storage and delivery pressures. Here we study the effect of linker substitution to fine-tune gate-opening and gate-closing pressure. Specifically, three variants of a previously reported pcu-topology MOF, X-pcu-5-Zn, have been prepared: X-pcu-6-Zn, 6=1,2-bis(4-pyridyl)ethane (bpe), X-pcu-7-Zn, 7=1,2-bis(4-pyridyl)acetylene (bpa), and X-pcu-8-Zn, 8=4,4'-azopyridine (apy). Each exhibited switching isotherms but at different gate-opening pressures. The N2 , CO2 , C2 H2 , and C2 H4 adsorption isotherms consistently indicated that the most flexible dipyridyl organic linker, 6, afforded lower gate-opening and gate-closing pressures. This simple design principle enables a rational control of the switching behavior in adsorbent materials.

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.

Efficient Separation of n-Butene and iso-Butene by Flexible Ultramicroporous Metal-Organic Frameworks with Pocket-like Cavities.

Efficient separation of n-butene (n-C4 H8 ) and iso-butene (iso-C4 H8 ) is of significance for the upgrading of C4 olefins to high-value end products but remains one of the major challenges in hydrocarbon purifications owing to their similar structures. Herein, we report a flexible metal-organic framework, MnINA (INA=isonicotinate), featuring one-dimensional pore channels with periodically large pocket-like cavities connected by narrow bottlenecks, for the first time for efficient n-/iso-C4 H8 separation. MnINA with smaller pore size (4.62 Å) compared with CuINA (4.84 Å), exhibits steep adsorption isotherms and high capacity of 1.79 mmol g-1 for n-C4 H8 (4.46 Å) through strong host-guest interactions via C-H⋅⋅⋅π bonding. The narrow bottlenecks exert barriers for the large molecules of iso-C4 H8 (4.84 Å) within the gate-opening pressure range of 0-0.1 bar. This gives rise to MnINA with excellent separation selectivity of 327.7 for n-/iso-C4 H8 mixture. The adsorption mechanism for n-C4 H8 and the gate-opening effect were investigated by dispersion-corrected density functional (DFT-D) theory, verifying the strong interactions between n-C4 H8 and the frameworks as well as the gate-opening effect derived from the rotation of organic linkers. The breakthrough tests confirmed MnINA and CuINA can be promising candidates for n-/iso-C4 H8 separation.

Cobalt substitution in a flexible metal-organic framework: modulating a soft paddle-wheel unit for tunable gate-opening adsorption.

Developing feasible ways to achieve tunable gate-opening pressure (Pgo) while minimizing the side effects on the adsorption capacity and enthalpy is greatly desired for flexible MOFs. In this work, we focused on solving this issue by cobalt substitution. We showed the successful modulation of the energy required for the reversible transformation of a soft paddle-wheel so that the whole framework presented a substitution-dependent Pgo for CO2 adsorption.

On the nanogate-opening pressures of copper-doped zeolitic imidazolate framework ZIF-8 for the adsorption of propane, propylene, isobutane, and n-butane

In this study, a zeolitic imidazolate framework-8 (ZIF-8) and its copper-doped variants (Cu-doped ZIF-8) were synthesized using a rapid microwave technique. The products were characterized by XRD, NanoSEM, FTIR, Raman spectra, TGA, XPS, and EDX. The gas adsorption properties of samples were performed using C3 and C4 hydrocarbons including propane, propylene, isobutane, and n-butane gases at 25 °C. Both equilibrium adsorption and kinetics were studied. It was found that ZIF-8 and Cu-doped ZIF-8 samples open their nanogates (i.e., six-membered rings) at some threshold pressures when adsorbing different gases, which it is denotes later as the gate-opening pressure (p0). The p0 value for each ZIF toward each gas was evaluated by fitting equilibrium adsorption data against a modified version of the Langmuir adsorption isotherm model. It was observed that the value of p0 differs significantly for each gas utilized, and with different extents for ZIF samples. The overall mass transfer coefficient values of adsorption process were estimated. Therefore, it is possible that the distinct values of p0 afford a unique chance to separate and purify these gases at mass production, and the best-studied adsorbent to for this purpose was Cu30%/ZIF-8.

Nanosheet Synthesis of Metal Organic Frameworks in a Sandwich-like Reaction Field for Enhanced Gate-Opening Pressures

Elastic layer-structured metal–organic frameworks (ELMs) are a family of flexible nanoporous metal organic frameworks (MOFs) showing gate-opening gas adsorption. The gate-opening pressure shifts to a higher value by crystal downsizing. However, the MOF nanoparticles and nanorods showing the gate-opening gas adsorption grow more than 50 nm even for their shortest sides. Here, we describe the synthesis and unique gas adsorption behavior of the first example of nanosheets of ELMs (ELM-NSs). The thickness and horizontal width of the ELM-NSs obtained from a new synthetic method using the inside the bilayers in hyperswollen lyotropic lamellar (HL) phases as sandwich-like reaction fields (SRFs) are a few nanometers and several hundreds of nanometers, respectively. The previously reported rationalization of the temperature dependence of the gate-opening pressures for ELMs enables us to discuss the size effects in terms of the adsorption-induced structural transitions and the Helmholtz free energy change of the host.

Tuning the gate opening pressure of a flexible doubly interpenetrated metal-organic framework through ligand functionalization.

A functional doubly interpenetrated MOF [(Zn4O)2(MDCPB)6DMF]·3DMF·3H2O (JLU-Liu33F, H2MDCPB = 5-methyl-1,3-di(4-carboxyphenyl)benzene, DMF = N,N-dimethylformamide) with pcu topology has been solvothermally synthesized. JLU-Liu33F (F = functionalization) is the isostructural analogue of JLU-Liu33, which is constructed by -CH3 functional V-shaped ligand and a classical Zn4O cluster. Similar to the adsorption property of JLU-Liu33, JLU-Liu33F also exhibits breathing behavior upon N2 and CO2 adsorption at 77 and 195 K, respectively. However, due to the introduction of -CH3 functional groups, the interpenetration degree of JLU-Liu33F become closer to that of JLU-Liu33, which results in higher gate opening pressures for both N2 and CO2 adsorption. Moreover, the breathing behavior of JLU-Liu33F for C2H6 and C3H8 adsorption disappeares. Moreover, JLU-Liu33F also exhibits commendable selectivity for C2H2 over CH4.

Efficient adsorptive separation of C3H6 over C3H8 on flexible and thermoresponsive CPL-1

The separation of olefin/paraffin mixtures is of great importance for the petrochemical industry, mainly achieved by using energy-intensive cryogenic distillation. We herein reported a flexible pillared-layer framework CPL-1 possessing a thermoresponsive gate opening behavior for C3H6 adsorption. We measured the C3H6 and C3H8 adsorption and desorption isotherms of CPL-1 at six different temperatures (273, 278, 283, 288, 293 and 298K), respectively. Results showed that the gate opening pressure of C3H6 adsorption for CPL-1 increased with an increase of the adsorption temperature from 273K to 288K. However, no gate opening effect was observed within the measured pressure (<100kPa) as the temperature increased to 293 and 298K. In contrast, C3H8 adsorption for CPL-1 only occurred surface adsorption without appreciable amount adsorbed due to the closed phase of pores under all measured conditions. Thus, we investigated the adsorptive separation of C3H6 over C3H8 for CPL-1 at 273K and developed the regeneration process at room temperature (298K), based on its unique thermoresponsive property. Breakthrough experiments demonstrated the superior C3H6/C3H8 separation performance of CPL-1. Static and dynamic regeneration experiments showed that CPL-1 had excellent stability and high desorption efficiency after five consecutive adsorption-desorption cycles. Particularly, CPL-1 can be easily regenerated at 298K since the desorption activation energy of C3H6 on CPL-1 was only 22.81kJ/mol. Furthermore, molecular dynamics (MD) simulation revealed that the hydrogen bonding was a key factor governing the selective adsorption of C3H6 over C3H8 in CPL-1. This investigation highlighted the central importance of flexibility of adsorbent, and suggested that CPL-1 could be an interesting material for C3H6/C3H8 separation and had excellent regeneration property.

Tuning the gate-opening pressure and particle size distribution of the switchable metal-organic framework DUT-8(Ni) by controlled nucleation in a micromixer.

Controlled nucleation in a micromixer and further crystal growth were used to synthesize Ni2(2,6-ndc)2dabco (2,6-ndc - 2,6-naphthalenedicarboxylate, dabco - 1,4-diazabicyclo[2.2.2]octane), also termed DUT-8(Ni) (DUT = Dresden University of Technology), with narrow particle size distribution in a range of a few nm to several μm. The crystal size was found to significantly affect the switching characteristics, in particular the gate opening pressure in nitrogen adsorption isotherms at 77 K for this highly porous and flexible network. Below a critical size of about 500 nm, a type Ia isotherm typical of rigid MOFs is observed, while above approximately 1000 nm a pronounced gating behaviour is detected, starting at p/p0 = 0.2. With increasing crystal size this transition gate becomes steeper indicating a more uniform distribution of activation energies within the crystal ensemble. At an intermediate size (500-1000 nm), the DUT-8(Ni) crystals close during activation but cannot be reopened by nitrogen at 77 K possibly indicating monodomain switching.

Crystal Dynamics in Multi-stimuli-Responsive Entangled Metal-Organic Frameworks.

An understanding of solid-state crystal dynamics or flexibility in metal-organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single-crystallinity. In this article, detailed experimental studies on a twofold entangled MOF (f-MOF-1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f-MOF-1, by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f-MOF-1 a and f-MOF-1 b, could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO2 adsorption profiles. DFT-based calculations provide an insight into the selective and gated adsorption behavior with CO2 of f-MOF-1 b. The gate-opening threshold pressure of CO2 adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (-CH2 -CH2 -) in f-MOF-1 to an azo (-N=N-) functionality in an analogous MOF, f-MOF-2. The modulation of functionality has an indirect influence on the gate-opening pressure owing to the difference in inter-net interaction. The framework of f-MOF-1 is highly responsive toward CO2 gas molecules, and these results are supported by DFT calculations.

A flexible metal-organic framework with double interpenetration for highly selective CO2 capture at room temperature

A flexible metal-organic framework of 1a Cu(FMA)(4,4′-Bpe)0.5 (FMA=fumarate; 4,4′-Bpe=trans-bis-(4-pyridyl)ethylene) that exhibits guest molecule-controlled gate-opening adsorption has been reported, in which the flexible pores can be enlarged by CO2 molecules rather than CH4 and N2 under a certain gate-opening pressure. The CO2 uptake can be sharply improved from 6.85 cm3 g–1 at 0.60 atm to 33.7 cm3 g–1 at 1 atm due to the gate-opening effect, thus resulting in the notably enhanced adsorption selectivities for CO2/CH4 (32:1, v/v) and CO2/N2 (48:1, v/v) separations at room temperature.

Paddle Wheel Based Triazolyl Isophthalate MOFs: Impact of Linker Modification on Crystal Structure and Gas Sorption Properties.

Syntheses and comprehensive characterization of two closely related series of isomorphous metal-organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula ∞(3)[M2(R(1)-R(2)-trz-ia)2] (M = Cu, Zn) are presented. Using solvothermal synthesis and synthesis of microcrystalline materials on the gram scale by refluxing a solution of the starting materials, 11 MOFs are readily available for a systematic investigation of structure-property relationships. The networks of the two series are assigned to rutile (rtl) (1-4) and α-PbO2 (apo) (5-9) topology, respectively. Due to the orientation of the triazole substituents toward the cavities, both the pore volume and the pore diameter can be adjusted by choice of the alkyl substituents. Compounds 1-9 exhibit pronounced microporosity with calculated porosities of 31-53% and show thermal stability up to 390 °C as confirmed by simultaneous thermal analysis. Systematic investigation of adsorption properties by CO2 (298 K) and N2 (77 K) adsorption studies reveal remarkable network flexibility induced by alkyl substituents on the linker. Fine-tuning of the gate opening pressure and of the hysteresis shape is possible by adjusting the substitution pattern and by choice of the metal ion.

A generalized adsorption-phase transition model to describe adsorption rates in flexible metal organic framework RPM3-Zn.

Flexible gate-opening metal organic frameworks (GO-MOFs) expand or contract to minimize the overall free energy of the system upon accommodation of an adsorbate. The thermodynamics of the GO process are well described by a number of models, but the kinetics of the process are relatively unexplored. A flexible GO-MOF, RPM3-Zn, exhibits a significant induction period for opening by N2 and Ar at low temperatures, both above and below the GO pressure. A similar induction period is not observed for H2 or O2 at comparable pressures and temperatures, suggesting the rate of opening is strongly influenced by the gas-surface interaction rather than an external stress. The induction period leads to severe mass transfer limitations for adsorption and over-prediction of the gate-opening pressure. After review of a number of existing adsorption rate models, we find that none adequately describe the experimental rate data and similar timescales for diffusion and opening invalidate prior reaction-diffusion models. Statistically, the rate data are best described by a compressed exponential function. The resulting fitted parameters exceed the expectations for adsorption but fall within those expected for phase transition. By treating adsorption as a phase transition, we generalize the Avrami theory of phase transition kinetics to describe adsorption in both rigid and flexible hosts. The generalized theory is consistent with observed experimental trends relating to induction period, temperature, pressure, and gas-substrate interaction.

Structural flexibilities and gas adsorption properties of one-dimensional copper(II) polymers with paddle-wheel units by modification of benzoate ligands.

CO2 and N2 gas adsorption/desorption properties of one-dimensional copper(II) polymers with paddle-wheel units [Cu(II)2(p-XBA)4(pyrazine)]∞ were successfully controlled through the tuning of interchain interactions by modification of para-substituent X groups on the benzoate (BA) ligands (X = Cl, Br, I, and OCH3). Although none of the four crystals had sufficient void space to integrate the crystallization solvents, gate-opening gas adsorption and desorption behaviors coupled with structural phase transitions were observed for CO2 (T = 195 K) and N2 (T = 77 K), with differences depending on the precise substituent. van der Waals interchain interactions, specifically π···π, halogen···π, and C-H···π contacts, were dominant in forming the crystal lattice; their magnitude was associated with gate-opening pressure and hysteresis behaviors. Both the type and magnitude of the interactions were evaluated by Hirshfeld surface analysis, which indicated that structural flexibility decreased as larger halogen atoms were included. Overall, weak interchain interaction and structural flexibility generated new void spaces to adsorb CO2 and N2 gases.