Ultramicroporous Metal Organic Framework Research Articles

Spatially Confined π-Complexation within Pore-Space-Partitioned Metal-Organic Frameworks for Enhanced Light Hydrocarbon Separation and Purification.

Ultramicroporous metal-organic frameworks (MOFs) are demonstrated to be advantageous for the separation and purification of light hydrocarbons such as C2H2, C2H4, and CH4. The introduction of transition metal sites with strong π-complexation affinity into MOFs is more effective than other adsorption sites for the selective adsorption of π-electron-rich unsaturated hydrocarbon gases from their mixtures. However, lower coordination numbers make it challenging to produce robust MOFs directly utilizing metal ions with π-coordination activity, such as Cu+, Ag+, and Pd2+. Herein, a series of novel π-complexing MOFs (SNNU-33s) with a pore size of 4.6Å are precisely constructed by cleverly introducing symmetrically matched C3-type [Cu(pyz)3] (pyz = pyrazine) coordinated fragments into 1D hexagonal channels of MIL-88 prototype frameworks. Benifit from the spatial confinement combined with π-complex-active Cu+ of [Cu(pyz)3], pore-space-partitioned SNNU-33 MOFs all present excellent C2H2/CH4, C2H4/CH4, and CO2/CH4 separation ability. Notably, the optimized SNNU-33b adsorbent demonstrates top-level IAST selectivity values for C2H2/CH4 (597.4) and C2H4/CH4 (69.8), as well as excellent breakthrough performance. Theoretical calculations further reveal that such benchmark light hydrocarbon separation and purification ability is mainly ascribed to the extra-strong binding affinity between Cu+ and π-electron donor molecules via a spatially confined π-complexation process.

Maximizing Electrostatic Interaction in Ultramicroporous Metal-Organic Frameworks for the One-Step Purification of Acetylene from Ternary Mixture.

Developing efficient adsorbents for acetylene purification from multicomponent mixtures is of critical significance in the chemical industry, but the trade-off between regenerability and selectivity significantly restricts practical industrial applications. Here, we report ultramicroporous metal-organic frameworks with acetylene-affinity channels to enhance electrostatic interaction between C2H2 and frameworks for the efficient one-step purification of C2H2 from C2H2/CO2/C2H4 mixtures, in which the electrostatic interaction led to high regenerability. The obtained SNNU-277 exhibits significantly higher adsorption capacity for C2H2 than that for both C2H4 and CO2 at 298 K and 0.1 bar, while an ultrahigh selectivity of C2H2/C2H4 (100.6 at 298 K) and C2H2/CO2 (32.8 at 298 K) were achieved at 1 bar. Breakthrough experiments validated that SNNU-277 can efficiently separate C2H2 from C2H2/C2H4/CO2 mixtures. CO2 and C2H4 broke through the adsorption column at 4 and 14.8 min g-1, whereas C2H2 was detected until 177.6 min g-1 at 298 K. Theoretical calculations suggest that the framework is electrostatically compatible with C2H2 and electrostatically repels C2H4 and CO2 in the mixed components. This work highlights the importance of rational pore engineering for maximizing the electrostatic effect with the preferentially absorbed guest molecule for efficient multicomponent separation.

Increased CO2/N2 selectivity by stepwise fluorination in isoreticular ultramicroporous metal-organic frameworks

Exploration of porous adsorbents with high CO2/N2 selectivity is of great significance for reducing CO2 content in the atmosphere. In this study, a series of isoreticular ultramicroporous fluorinated metal-organic frameworks...

Rapid synthesis of ultra-microporous metal-organic framework tubular Ni-LAP membranes for H2 separation by linker salt method

Recently, the development of pillar-layered metal-organic frameworks (MOFs) is receiving more and more attention because of their pore surface chemistry tunability and designability. In this work, we firstly synthesized various sizes of high-quality Ni-LAP [Ni2(L-asp)2(pz)] (L-asp = l-aspartic acid, pz = pyrazine) crystals key for secondary growth by manipulating the crystal thermal dynamics and kinetics of Ni-LAP synthesis solution simply through choosing the Ni(OAc)2 as metal source and pretreatment at 40 °C, followed by the fabrication of defect-free Ni-LAP membranes as a model in a short time of 4 h using the linker salt method. It is found the pretreatment of Ni(OAc)2 precursor solution at 40 °C with continuous stirring for 6 h can effectively promote deprotonation to form 250 nm crystals. The linker salt method enables a greener and more rapid synthesis of Ni-LAP membranes, with continuous and compact Ni-LAP membranes, where soluble metal precursors and organic linking salts provide the synthesis solution a higher degree of supersaturation, both of which play an important role in the rapid preparation of Ni-LAP membranes. The preparation parameters of the Ni-LAP membranes were investigated in detail, and the results from the Ni-LAP membrane gas separation performance showed that the preferential (111) orientation membrane (M9) exhibited the best H2/CO2 separation performance with separation factor of 23.8 and H2 permeance of 2.11 × 10−7 mol m−2 s−1 Pa−1 compared to the preferential (002) orientations (M3), which may attributed to the preferential (111) orientation being at an angle to the optimal a-axis aperture perpendicular to the support thereby reducing the mass transfer resistance. Furthermore, the generation of corrosive nitric acid during the preparation process can be greatly reduced and replaced by eco-friendly potassium nitrate. The superiority of the pore structure and the remarkable stability of these MOF membranes show the potential for H2 purification. Moreover, our preparation route demonstrates the universality of preparing other isomorphs, such as Ni-LAB and Co-LAP membranes.

Hydrogen Adsorption in Ultramicroporous Metal-Organic Frameworks Featuring Silent Open Metal Sites.

In this study, we utilized an ultramicroporous metal-organic framework (MOF) named [Ni3(pzdc)2(ade)2(H2O)4]·2.18H2O (where H3pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H2) adsorption. Upon activation, [Ni3(pzdc)2(ade)2] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m2/g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H2 uptake saturation at 2.11 mmol/g at ∼0.15 bar. The affinity of this MOF for H2 was determined to be 9.7 ± 1.0 kJ/mol. In situ H2 loading experiments supported by molecular simulations confirmed that H2 does not bind to the open Ni(II) sites of [Ni3(pzdc)2(ade)2], and the high affinity of the MOF for H2 is attributed to the interplay of pore size, shape, and functionality.

A low cost and energy consumption Ultra-microporous CPM-20(Co) for CO2/N2 and CO2/CH4 separation

It is of vital importance to research carbon dioxide (CO2) removal from flue gas and natural gas. Metal-organic frameworks have been widely used in the adsorption field because of their high specific surface area, high porosity and special chemical properties. With the exception of separation performance, synthesis cost and energy consumption are also be considered in actual industry. Here, an ultra-microporous metal-organic framework CPM-20(Co) with mixed ligand was synthesized using cheap and convenient ligands (1,4-dicarboxybenzene and isonicotinic acid). The calculation shows the adsorption heat of CO2 was 20.16 kJ/mol resulting low regenerative energy. This material exhibits high selectivities of equimolar mixture CO2/N2 (350) and CO2/CH4 (45) at 298 K and 1 bar, which is better than other MOFs reported. Breakthrough experiments confirmed that CPM-20(Co) can separate CO2 from flue gas and natural gas under environmental conditions. It is speculated that the framework has suitable pore size and higher polarizability of CO2 are the key factor for CPM-20(Co) to achieve the adsorption separation of CO2/N2 and CO2/CH4. CPM-20(Co) has low regenerative energy and the material was easily obtained, which has a better application prospect in industry.

Simultaneous extraction of C3H8 and C2H6 from ternary C3H8/C2H6/CH4 mixtures in an ultra-microporous metal–organic framework

The efficient purification of methane from other light hydrocarbons is of prime importance in natural gas purification. However, the exploitation of stable adsorbents with both high selectivity and high adsorption capacity remains a challenge. Herein, an ultra-microporous Co-based metal–organic framework (Co-MOF) with abundant selective binding sites decorating the pore wall was applied for the highly efficient separation of ternary C3H8/C2H6/CH4 mixtures. The Co-MOF shows remarkable C3H8 and C2H6 adsorption capacities with uptakes of 59.4 cm3/g and 58.6 cm3/g, respectively, while only adsorbing a low amount of CH4 with an uptake of 16.6 cm3/g at 298 K and 100 kPa. The ideal adsorbed solution theory selectivities for C2H6/CH4 (v/v = 50/50) and C3H8/CH4 (v/v = 50/50) reach 26.0 and 290.0 at 298 K, respectively, surpassing most of the state-of-the-art MOF materials. The molecular simulation reveals that the excellent adsorption and separation performance of Co-MOF derives from its suitable pore size and functional pore surfaces. Furthermore, the breakthrough experiments reveal that the Co-MOF exhibits excellent separation performance of C3H8 and C2H6 from natural gas. Notably, the dynamic separation performance of Co-MOF can be well-maintained after five cycles, demonstrating its excellent cycling stability. The high stability and excellent separation performance make Co-MOF a promising adsorbent for natural gas purification.

(Invited) Metal Organic Frameworks for the Storage, Separation, and Purification of Gaseous Fuels

We have seen a surge of interest in metal organic frameworks (MOFs) over the past decades with more than 100,000 structures reported today. This interest is based on the modular building-block nature of their structure, which makes them tunable and widely applicable to many applications. Of particular interest is the storage, separation, and purification of gaseous fuels with MOFs as a much-needed alternative to the energy-intense processes currently in use. My talk will focus on results of a variety of MOFs that have been obtained by a close integration of synthesis, in situ vibrational spectroscopy, and ab initio materials modeling. In particular, we report on the development and characterization of four MOFs for the purpose of separating linear, mono, and dibranched alkane isomers through selective molecular sieving, which is of significant value in the petrochemical industry. The separation of such alkane isomers is a challenging and important industrial process for the production of premium gasoline, blending components with high research octane number. I will also show recent results that point towards non-standard kinetics in certain separation processes in MOFs as well as results for ultramicroporous MOFs that have the ability to effectively perform separations in the presence of water.

Exquisitely Constructing a Robust MOF with Dual Pore Sizes for Efficient CO2 Capture.

Developing metal-organic framework (MOF) adsorbents with excellent performance and robust stability is of critical importance to reduce CO2 emissions yet challenging. Herein, a robust ultra-microporous MOF, Cu(bpfb)(bdc), with mixed ligands of N, N'-(1,4-phenylene)diisonicotinamide (bpfb), and 1,4-dicarboxybenzene (bdc) was delicately constructed. Structurally, this material possesses double-interpenetrated frameworks formed by two staggered, independent frameworks, resulting in two types of narrow ultra-micropores of 3.4 × 5.0 and 4.2 × 12.8 Å2, respectively. The above structural properties make its highly selective separation at 273~298 K with a CO2 capacity of 71.0~86.2 mg/g. Its adsorption heat over CO2 and IAST selectivity were calculated to be 27 kJ/mol and 52.2, respectively. Remarkably, cyclic breakthrough experiments corroborate its impressive performance in CO2/N2 separation in not only dry but also 75% RH humid conditions. Molecular simulation reveals that C-H···OCO2 in the pores plays a pivotal role in the high selectivity of CO2 adsorption. These results point out the huge potential application of this material for CO2/N2 separation.

Reversed CO2/C2H2 separation with ultrahigh carbon dioxide adsorption capacity in Zn-based pillared metal-organic frameworks

The removal of CO2 impurities from CO2/C2H2 gas mixtures to obtain high-purity acetylene used for manufacturing of various commodity chemicals, is very important for the chemical purifying of acetylene. However, CO2 and C2H2 exhibit analogous physical properties as well as molecular sizes causing their separation to become an industrially challenging process, and developing energy-efficient CO2-selective adsorbents is a class of materials of vast value. In this work, a pillared-layer ultramicroporous metal-organic framework (MOF) constructed from zinc (Ⅱ) and triazolium planar layers with oxalic acid, which has been termed as CALF-20, can preferentially absorb CO2 from C2H2. Unique pore environment and electrostatic effects enable this material foremost capture CO2. CALF-20 shows higher affinities towards CO2 than C2H2 based on heats of adsorption computed from single-component adsorption isotherms. Grand Canonical Monte Carlo simulations confirm the affinity interactions and multiple host-guest interactions between CO2 and CALF-20. Sorption experiments revealed that CALF-20 has excellent trapping volume with extremely high CO2 uptake capacity (140.2 cm3 cm−3).

High-capacity dynamic exclusion for highly efficient dilute C2H2 separation from CO2 and multicomponent mixture in robust ultramicroporous MOF by topology regulation

Acquiring high C2H2 adsorption capacity and effectively separating it from the most difficult-to-separate byproduct CO2 in a dilute state are crucial for obtaining high productivity/purity monomer for polymers. Here, we address this challenge through topology regulation of fluorinated ultramicroporous metal–organic framework, producing a new hydrolytically stable adsorbent, ZUL-500. Uncovered by X-ray diffraction and modeling, a more favorable spatial distribution of GeF62- anions for C2H2 recognition is realized in sql topological ZUL-500 compare to ZU-32 with the same anion and metal node but pcu topology, offering remarkably increased C2H2 affinity and amplified C2H2/CO2 affinity difference that affords a dynamic exclusion of CO2 but excellent C2H2 adsorption in fixed-bed separation even for industrial-related mixtures (up to 8 components). Excellent values were simultaneously obtained for C2H2 uptake capacity, C2H2/CO2 separation factor and C2H2 productivity, coupled with good mass transfer and recyclability, providing exceptional productivity (2.92 ∼ 3.48 mmol g−1) of high purity (≥99.6%) C2H2 from mixtures.

Effective separation of acetylene from carbon dioxide via commensurate adsorption in a microporous metal-organic framework

Separating acetylene and carbon dioxide is a significant challenge due to their similar physicochemical properties. Herein, we reported an ultramicroporous metal–organic framework, PCN-200, which possesses narrow zig-zag one-dimensional channels with a size of about 4.5 Å. The pore surface of PCN-200 features numerous uncoordinated oxygen polar sites that are symmetrically distributed, facilitating the strong and commensurate binding of acetylene molecules (57.0 kJ mol−1). At 298 K and 1 bar, PCN-200 demonstrates a high selectivity (6.4) towards acetylene over carbon dioxide, and this separation performance is validated through dynamic breakthrough experiments. Simulation studies were conducted to gain insight into the adsorption mechanism for acetylene and carbon dioxide molecules in PCN-200.

In-situ construction of ionic ultramicroporous metal–organic frameworks for high-efficiency CO2/CH4 separation

The high-efficiency capture of CO2 from biogas by adsorption is highly desirable, which, yet faces the challenges regards of the unsatisfactory adsorption efficiency, selectivity, and regeneration performance. Herein, ionic ultramicroporous metal organic frameworks (IUM-MOFs) with high CO2 affinity are constructed by one-step in-situ coordination strategy and used for CO2/CH4 separation. The phenoxylate anion-functionalized ionic liquid (FIL) serves as competitive ligands in the formation of IUM-MOFs. The oxyl group exhibits high affinity with CO2 as the Lewis basic center, while the rigid counterion occupies the adsorption space of CH4 in the cavities of IUM-MOFs, thus bringing about superior CO2 adsorption capacity of 149 cm3 g−1 (150% higher than that of Cu-BTC) and high IAST predicted selectivity with CH4 of 22 under atmospheric condition. Mechanism analysis demonstrated that the formation of hydrogen bonds between CO2 with coordinated IL and H2O and IL-enhanced polarity of MOFs benefit to the CO2 adsorption. In addition, the IUM-MOFs could be facilely regenerated at 120 °C with favorable recycling stability. This work provides facile strategy for the rational design of IUM-MOFs and lays useful insights for structural tuning to promote their applications.

Cold, Hot, Dry, and Wet: Locations and Dynamics of CO2 and H2O Co-Adsorbed in an Ultramicroporous MOF.

Climate change from anthropogenic carbon dioxide (CO2) emissions poses a severe threat to society. A variety of mitigation strategies currently include some form of CO2 capture. Metal-organic frameworks (MOFs) have shown great promise for carbon capture and storage, but several issues must be solved before feasible widespread adoption is possible. MOFs often exhibit reduced chemical stabilities and CO2 adsorption capacities in the presence of water, which is ubiquitous in nature and many practical settings. A comprehensive understanding of water influence on CO2 adsorption in MOFs is necessary. We have used multinuclear nuclear magnetic resonance (NMR) experiments at temperatures ranging from 173 to 373 K, along with complementary computational techniques, to investigate the co-adsorption of CO2 and water across various loading levels in the ultra-microporous ZnAtzOx MOF. This approach yields detailed information regarding the number of CO2 and water adsorption sites along with their locations, guest dynamics, and host-guest interactions. Guest adsorption and motional models proposed from NMR data are supported by computational results, including visualizations of adsorption locations and the spatial distribution of guests in different loading scenarios. The wide variety and depth of information presented demonstrates how this experimental methodology can be used to investigate humid carbon capture and storage applications in other MOFs.

One ethane-selective metal–organic framework with customized pore size and specific binding sites for efficient purification of ethylene

Removing small amount of ethane (C2H6) from ethylene (C2H4) to obtain high-purity C2H4 is of great significance in the industries. However, the construction of C2H6-selective metal–organic frameworks (MOFs) remains challenging, mainly because that C2H4 has smaller kinetic diameter and larger quadrupole moment. Herein, an ultra-microporous MOF (PCP-bdc) with customized pore size and specific binding sites is reported to efficiently purify C2H4 from C2H6/C2H4 mixture. The channel size of PCP-bdc is comparable to the size of C2H6 molecule, which can provide more accessible channel surfaces for C2H6. The channel surface of PCP-bdc is rich of low-polar aromatic groups and the uncoordinated O atoms can interact with C2H6 molecules strongly. Thus, PCP-bdc exhibits a remarkable C2H6/C2H4 (50/50) selectivity (2.82) and C2H6 uptake (53.8 cm3 g−1) at 298 K and 1 bar, which outperform those of many C2H6-selective adsorbents. Density functional theory calculations further reveal the formation of multiple synergies between this MOF and C2H6 enables the efficient separation for C2H6/C2H4 mixture. Breakthrough experiments demonstrate that PCP-bdc can purify C2H4 from C2H6/C2H4 mixture through one-step operation. These results indicate PCP-bdc is a promising adsorbent for C2H6/C2H4 separation.

Molecular Exclusion Separation of 1‐Butene Isomers by a Robust Metal–Organic Framework under Humid Conditions

AbstractSeparating n‐butene and i‐butene by adsorption is an energy‐efficient alternative, but designing porous adsorbents that distinguish the subtle differences between the isomers is extremely challenging. Currently, adsorbents that can sieve 1‐butene isomers and are stable enough to withstand humid gas mixtures are largely unmet. Herein, we propose a robust ultramicroporous metal–organic framework (MET‐Fe) that can separate 1‐butene isomers through molecular exclusion. The pore aperture size (4.6 Å) precisely matches the kinetic diameters of the isomers, as verified by static and kinetic adsorption experiments and theoretical calculations. Furthermore, dynamic breakthrough experiments confirmed the excellent separation performance, easy regeneration, and remarkable reusability of MET‐Fe in both dry and humid conditions. With its high selectivity, large breakthrough capacity, and outstanding stability, MET‐Fe provides an ideal platform for industrial butene isomers separation.

Molecular Exclusion Separation of 1-Butene Isomers by a Robust Metal-Organic Framework under Humid Conditions.

Separating n-butene and i-butene by adsorption is an energy-efficient alternative, but designing porous adsorbents that distinguish the subtle differences between the isomers is extremely challenging. Currently, adsorbents that can sieve 1-butene isomers and are stable enough to withstand humid gas mixtures are largely unmet. Herein, we propose a robust ultramicroporous metal-organic framework (MET-Fe) that can separate 1-butene isomers through molecular exclusion. The pore aperture size (4.6 Å) precisely matches the kinetic diameters of the isomers, as verified by static and kinetic adsorption experiments and theoretical calculations. Furthermore, dynamic breakthrough experiments confirmed the excellent separation performance, easy regeneration, and remarkable reusability of MET-Fe in both dry and humid conditions. With its high selectivity, large breakthrough capacity, and outstanding stability, MET-Fe provides an ideal platform for industrial butene isomers separation.

Immobilization of the Polar Group into an Ultramicroporous Metal-Organic Framework Enabling Benchmark Inverse Selective CO2/C2H2 Separation with Record C2H2 Production.

One-step harvest of high-purity light hydrocarbons without the desorption process represents an advanced and highly efficient strategy for the purification of target substances. The separation and purification of acetylene (C2H2) from carbon dioxide (CO2) by CO2-selective adsorbents are urgently demanded yet are very challenging owing to their similar physicochemical properties. Here, we employ the pore chemistry strategy to adjust the pore environment by immobilizing polar groups into an ultramicroporous metal-organic framework (MOF), achieving one-step manufacture of high-purity C2H2 from CO2/C2H2 mixtures. Embedding methyl groups into prototype stable MOF (Zn-ox-trz) not only changes the pore environment but also improves the discrimination of guest molecules. The methyl-functionalized Zn-ox-mtz thus exhibits the benchmark reverse CO2/C2H2 uptake ratio of 12.6 (123.32/9.79 cm3 cm-3) and an exceptionally high equimolar CO2/C2H2 selectivity of 1064.9 at ambient conditions. Molecular simulations reveal that the synergetic effect of pore confinement and surfaces decorated with methyl groups provides high recognition of CO2 molecules through multiple van der Waals interactions. The column breakthrough experiments suggest that Zn-ox-mtz dramatically achieved the one-step purification capacity of C2H2 from the CO2/C2H2 mixture with a record C2H2 productivity of 2091 mmol kg-1, surpassing all of the CO2-selective adsorbents reported so far. In addition, Zn-ox-mtz exhibits excellent chemical stability under different pH values of aqueous solutions (pH = 1-12). Moreover, the highly stable framework and excellent inverse selective CO2/C2H2 separation performance showcase its promising application as a C2H2 splitter for industrial manufacture. This work paves the way to developing reverse-selective adsorbents for the challenging gas separation process.

Amine-functionalized ultramicroporous metal-organic framework for efficient propylene/propane separation

The separation of propylene/propane (C3H6/C3H8) through an energy-efficient way is a crucial task in the petrochemical industry. However, it remains highly challenging because of their striking similarities between C3H6 and C3H8. Here we report an amine-Functionalized ultramicroporous metal-organic framework (MOF) Zn2(atz)2(ipa) with pH stability. In addition, Zn2(atz)2(ipa) possesses ability to absorb a large number of C3H6 at low pressure. The experience has shown that the high C3H6/C3H8 separation efficiency contributes to its suitable pore shape as well as firm bonding force with exposed –NH2 groups in the pore aperture. The strong affinity towards C3H6 and weak affinity towards C3H8 lead Zn2(atz)2(ipa) to high selectivity under ambient conditions. This work renders a fresh solution to product polymer-grade C3H6 through physical adsorption by MOFs.

Ultramicroporous Metal-Organic Framework with Inert Pore Surfaces for Inversed Separation of Ethylene from C2 Hydrocarbons Mixtures.

The achievement of direct C2H4 separation from C2 hydrocarbons is very challenging in the petrochemical industry due to their similar molecular sizes, boiling points, and physicochemical properties. In this work, a nonpolar/inert ultramicroporous metal-organic framework (MOF), [Co3(μ3-OH)(tipa)(bpy)1.5]·3DMF·6H2O (1), with stand-alone one-dimensional square tubular channels was successfully constructed, its pore enriched with plenty of aromatic rings causing nonpolar/inert pore surfaces. The MOF shows preferential adsorption of C2H6 compared to C2H4 and C2H2 in the low-pressure region, which is further verified by adsorption heats and selectivities. The C2H4 separation potential in one step for binary C2H6/C2H4 (50/50 and 10/90) and ternary C2H4/C2H6/C2H2 (89/10/1) is also examined by transient breakthrough simulations. Moreover, grand canonical Monte Carlo simulations demonstrate that the unique reversed adsorption mechanism is due to the shortest and most number of C-H···π interactions between C2H6 and the framework.