Covalent Triazine Frameworks Based on the First Pseudo-Octahedral Hexanitrile Monomer via Nitrile Trimerization: Synthesis, Porosity, and CO2 Gas Sorption Properties.

Herein, we report the first synthesis of covalent triazine-based frameworks (CTFs) based on a hexanitrile monomer, namely the novel pseudo-octahedral hexanitrile 1,4-bis(tris(4'-cyano-phenyl)methyl)benzene 1 using both ionothermal reaction conditions with ZnCl2 at 400 °C and the milder reaction conditions with the strong Brønsted acid trifluoromethanesulfonic acid (TFMS) at room temperature. Additionally, the hexanitrile was combined with different di-, tri-, and tetranitriles as a second linker based on recent work of mixed-linker CTFs, which showed enhanced carbon dioxide captures. The obtained framework structures were characterized via infrared (IR) spectroscopy, elemental analysis, scanning electron microscopy (SEM), and gas sorption measurements. Nitrogen adsorption measurements were performed at 77 K to determine the Brunauer-Emmett-Teller (BET) surface areas range from 493 m2/g to 1728 m2/g (p/p0 = 0.01-0.05). As expected, the framework CTF-hex6 synthesized from 1 with ZnCl2 possesses the highest surface area for nitrogen adsorption. On the other hand, the mixed framework structure CTF-hex4 formed from the hexanitrile 1 and 1,3,5 tricyanobenzene (4) shows the highest uptake of carbon dioxide and methane of 76.4 cm3/g and 26.6 cm3/g, respectively, at 273 K.

Capture and Separation of SO2 Traces in Metal-Organic Frameworks via Pre-Synthetic Pore Environment Tailoring by Methyl Groups.

Herein, we report a pre-synthetic pore environment design strategy to achieve stable methyl-functionalized metal-organic frameworks (MOFs) for preferential SO2 binding and thus enhanced low (partial) pressure SO2 adsorption and SO2 /CO2 separation. The enhanced sorption performance is for the first time attributed to an optimal pore size by increasing methyl group densities at the benzenedicarboxylate linker in [Ni2 (BDC-X)2 DABCO] (BDC-X=mono-, di-, and tetramethyl-1,4-benzenedicarboxylate/terephthalate; DABCO=1,4-diazabicyclo[2,2,2]octane). Monte Carlo simulations and first-principles density functional theory (DFT) calculations demonstrate the key role of methyl groups within the pore surface on the preferential SO2 affinity over the parent MOF. The SO2 separation potential by methyl-functionalized MOFs has been validated by gas sorption isotherms, ideal adsorbed solution theory calculations, simulated and experimental breakthrough curves, and DFT calculations.

Zirconium and Aluminum MOFs for Low-Pressure SO2 Adsorption and Potential Separation: Elucidating the Effect of Small Pores and NH2 Groups.

Finding new adsorbents for the desulfurization of flue gases is a challenging task but is of current interest, as even low SO2 emissions impair the environment and health. Four Zr- and eight Al-MOFs (Zr-Fum, DUT-67(Zr), NU-1000, MOF-808, Al-Fum, MIL-53(Al), NH2-MIL-53(Al), MIL-53(tdc)(Al), CAU-10-H, MIL-96(Al), MIL-100(Al), NH2-MIL-101(Al)) were examined toward their SO2 sorption capability. Pore sizes in the range of about 4-8 Å are optimal for SO2 uptake in the low-pressure range (up to 0.1 bar). Pore widths that are only slightly larger than the kinetic diameter of 4.1 Å of the SO2 molecules allow for multi-side-dispersive interactions, which translate into high affinity at low pressure. Frameworks NH2-MIL-53(Al) and NH2-MIL-101(Al) with an NH2-group at the linker tend to show enhanced SO2 affinity. Moreover, from single-gas adsorption isotherms, ideal adsorbed solution theory (IAST) selectivities toward binary SO2/CO2 gas mixtures were determined with selectivity values between 35 and 53 at a molar fraction of 0.01 SO2 (10.000 ppm) and 1 bar for the frameworks Zr-Fum, MOF-808, NH2-MIL-53(Al), and Al-Fum. Stability tests with exposure to dry SO2 during ≤10 h and humid SO2 during 5 h showed full retention of crystallinity and porosity for Zr-Fum and DUT-67(Zr). However, NU-1000, MOF-808, Al-Fum, MIL-53(tdc), CAU-10-H, and MIL-100(Al) exhibited ≥50-90% retained Brunauer-Emmett-Teller (BET)-surface area and pore volume; while NH2-MIL-100(Al) and MIL-96(Al) demonstrated a major loss of porosity under dry SO2 and MIL-53(Al) and NH2-MIL-53(Al) under humid SO2. SO2 binding sites were revealed by density functional theory (DFT) simulation calculations with adsorption energies of -40 to -50 kJ·mol-1 for Zr-Fum and Al-Fum and even above -50 kJ·mol-1 for NH2-MIL-53(Al), in agreement with the isosteric heat of adsorption near zero coverage (ΔHads0). The predominant, highest binding energy noncovalent binding modes in both Zr-Fum and Al-Fum feature µ-OHδ+···Î´-OSO hydrogen bonding interactions. The small pores of Al-Fum allow the interaction of two µ-OH bridges from opposite pore walls with the same SO2 molecule via OHδ+···Î´-OSOδ-···Î´+HO hydrogen bonds. For NH2-MIL-53(Al), the DFT high-energy binding sites involve NHδ+···Î´-OS together with the also present Al-µ-OHδ+···Î´-OS hydrogen bonding interactions and C6-πδ-···Î´+SO2, Nδ-···Î´+SO2 interactions.

Increase of network hydrophilicity from sql to lvt supramolecular isomers of Cu-MOFs with the bifunctional 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate linker.

The bifunctional linker 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H2mpba) was used for the synthesis of new (square lattice) sql 2D metal-organic frameworks (MOFs) [Cu(Hmpba)2]·L (L = DMF or ACN) in a solvent-mixture of dimethylformamide/water and acetonitrile/water. These sql 2D MOFs are supramolecular isomers of the lvt 3D network [Cu(Hmpba)2]·4MeOH·1H2O (lvt-MeOH) that was synthesized previously by Richardson and co-workers. All these frameworks are potentially porous structures with solvent molecules included in the channels of the as synthesized materials. After activation all three materials showed good CO2 adsorption capacity, demonstrated here for lvt-MeOH for the first time, with a saturation uptake of 113 cm3 g-1 (lvt-MeOH-act.), 111 cm3 g-1 (sql-DMF-act.) and 90 cm3 g-1 (sql-ACN-act.) at 195 K. The flexibility of the lvt-MeOH-act. network is evidenced by a gate-opening effect seen in the CO2 measurement at 195 K and under gravimetric high-pressure CO2 adsorption. According to the water and ethanol sorption measurements the new sql frameworks can be categorized as hydrophobic materials in contrast to the hydrophilic lvt framework. In the lvt-MeOH structure the crystal solvent can be replaced with water to yield the structurally authenticated water-only network lvt-H2O containing 3D arrays of S4-symmetric (H2O)20 clusters.

A practical guide to calculate the isosteric heat/enthalpy of adsorption via adsorption isotherms in metal-organic frameworks, MOFs.

Porous materials such as MOFs are interesting candidates for gas separation and storage. An important parameter to gain deeper insights to the adsorption process of an adsorptive on an adsorbent is the isosteric enthalpy of adsorption, ΔHads which is defined as the heat to be released/required when an adsorptive binds to/detaches from the solid surface of an adsorbent. Two or three adsorption isotherms at different but close temperatures with ΔT ≤ 20 K for two and ΔT ≈ 10 K for three isotherms are the basis to derive the isosteric enthalpy of adsorption through the Clausius-Clapeyron approach or the virial analysis. This Perspective presents the procedure of the common (dual-site) Freundlich-Langmuir fit/Clausius-Clapeyron approach and the virial fit of the isotherms with usable Excel sheets and Origin files for the subsequent derivation of ΔHads. Exemplary adsorption isotherms of CO2, SO2 and H2 at two temperatures on MOFs are analyzed. The detailed computational description and comparison of the Clausius-Clapeyron approach and the virial analysis to determine ΔHads outlines the limitations of the two methods with respect to the available experimental data, especially at low pressure/low uptake values. It is emphasized that no extrapolation beyond the experimental data range should be done. The quality of the important and underlying isotherm fits must be checked and ensured with logarithmic-scale n/p isotherm plots for the (dual-site) Freundlich-Langmuir fit in the low-pressure region and through low standard deviations for the coefficients in the virial analysis.

Encapsulation of a Porous Organic Cage into the Pores of a Metal-Organic Framework for Enhanced CO2 Separation.

We present a facile approach to encapsulate functional porous organic cages (POCs) into a robust MOF by an incipient-wetness impregnation method. Porous cucurbit[6]uril (CB6) cages with high CO2 affinity were successfully encapsulated into the nanospace of Cr-based MIL-101 while retaining the crystal framework, morphology, and high stability of MIL-101. The encapsulated CB6 amount is controllable. Importantly, as the CB6 molecule with intrinsic micropores is smaller than the inner mesopores of MIL-101, more affinity sites for CO2 are created in the resulting CB6@MIL-101 composites, leading to enhanced CO2 uptake capacity and CO2 /N2 , CO2 /CH4 separation performance at low pressures. This POC@MOF encapsulation strategy provides a facile route to introduce functional POCs into stable MOFs for various potential applications.

High performance MIL-101(Cr)@6FDA-mPD and MOF-199@6FDA-mPD mixed-matrix membranes for CO2/CH4 separation.

Combination of the polyimide 6FDA-mPD (6FDA = 4,4'-hexafluoroisopropylidene diphthalic anhydride and mPD = m-phenylenediamine) and crystallites of the metal-organic frameworks (MOFs) MIL-101(Cr) or MOF-199 (HKUST-1, Cu-BTC) produces mixed-matrix membranes (MMMs) with excellent dispersion and compatibility of the MOF particles within the polymer matrix. Permeation tests of a binary CO2/CH4 (50/50) gas mixture showed a remarkable increase of CO2 permeabilities for MIL-101(Cr)@6FDA-mPD and significantly higher selectivities for MOF-199@6FDA-mPD. The CO2 permeability increased from 10 (neat polymer) to 50 Barrer for the 24 wt% MIL-101(Cr)@6FDA-mPD membrane (with essentially constant selectivity) due to the high pore volume of MIL-101(Cr). The CO2/CH4 selectivity increased from 54 to 89 from the neat 6FDA-mPD polymer to the 24 wt% MOF-199@6FDA-mPD membrane, apparently due to the high CO2 adsorption capacity of MOF-199.

Synthesis of Nano/Microsized MIL-101Cr Through Combination of Microwave Heating and Emulsion Technology for Mixed-Matrix Membranes.

Nano/microsized MIL-101Cr was synthesized by microwave heating of emulsions for the use as a composite with Matrimid mixed-matrix membranes (MMM) to enhance the performance of a mixed-gas-separation. As an example, we chose CO2/CH4 separation. Although the incorporation of MIL-101Cr in MMMs is well-known, the impact of nanosized MIL-101Cr in MMMs is new and shows an improvement compared to microsized MIL-101Cr under the same conditions and mixed-gas permeation. In order to reproducibly obtain nanoMIL-101Cr microwave heating was supplemented by carrying out the reaction of chromium nitrate and 1,4-benzenedicarboxylic acid in heptane-in-water emulsions with the anionic surfactant sodium oleate as emulsifier. The use of this emulsion with the phase inversion temperature (PIT) method offered controlled nucleation and growth of nanoMIL-101 particles to an average size of <100 nm within 70 min offering high apparent BET surface areas (2,900 m2 g-1) and yields of 45%. Concerning the CO2/CH4 separation, the best result was obtained with 24 wt.% of nanoMIL-101Cr@Matrimid, leading to 32 Barrer in CO2 permeability compared to six Barrer for the neat Matrimid polymer membrane and 21 Barrer for the maximum possible 20 wt.% of microMIL-101Cr@Matrimid. The nanosized filler allowed reaching a higher loading where the permeability significantly increased above the predictions from Maxwell and free-fractional-volume modeling. These improvements for MMMs based on nanosized MIL-101Cr are promising for other gas separations.

Synthesis and Characterization of Covalent Triazine Framework CTF-1@Polysulfone Mixed Matrix Membranes and Their Gas Separation Studies.

Covalent triazine framework CTF-1 and polysulfone (PSF) are used to form mixed-matrix membranes (MMMs) with 8, 16, and 24 wt% of the porous filler material CTF-1. Studies on permeability and selectivity are carried out concerning the gases O2, N2, CO2, and CH4. CO2 permeability of the synthesized MMMs increases by 5.4 Barrer in comparison to the pure PSF membrane. The selectivity remains unchanged for O2/N2 and CO2/CH4 but was found to be increased for CO2/N2. Further, comparisons to theoretical models for permeability prediction yield a permeability for CTF-1 which is about six times higher than the permeability of PSF. The inverse of the sum of the free fractional volumes (FFV) of the polymer and the filler correlate linearly to the logarithm of the permeabilities of the gases which conversely indicates that the porosity of the filler contributes to the gas transport through the membrane.

Metal-Organic Gels Based on a Bisamide Tetracarboxyl Ligand for Carbon Dioxide, Sulfur Dioxide, and Selective Dye Uptake.

A metal-organic gel (metallogel) based on the new tetracarboxyl ligand N1, N4-(diterephthalic acid)terephthalamide in combination with chromium(III) has been converted into its xero- and aerogel and demonstrated to have excellent specific sorption properties for dyes in its metallogel state, where fuchsine is adsorbed faster than the two other dyes, calcein and disulfine blue, and for water, sulfur dioxide and carbon dioxide in its xero- and aerogel state. The metallogel showed very good shape retention and could be extruded from molds in designed shapes. In a rheology experiment, the storage modulus was determined to be 1440 Pa, and the metallogel is elastic up to 3 Hz, breaking at strains higher than 0.3%. Additional metallogels utilizing the same ligand with a wide range of metal ions (Al(III), Fe(III), Co(III), In(III), and Hg(II)) have also been synthesized, and the aluminum and mixed aluminum-chromium derivative were also converted into its aerogel. The highly porous Cr, Al, and AlCr metal-organic aerogels proved stable against water vapor in a physisorption experiment and were used to model breakthrough curves for SO2/CO2 gas mixtures with the idealized adsorbed solution theory from their physisorption isotherms. The breakthrough simulation utilized SO2/CO2 equivalencies from a real world application and showed effective retention of SO2 from the gas mixture. Furthermore, the materials in this work exhibit the highest SO2 uptake values for metal-organic aerogels so far (up to 116.8 cm3 g-1, or 23.4 wt %).

Metal-Organic Frameworks with Potential Application for SO2 Separation and Flue Gas Desulfurization.

Sulfur dioxide (SO2) is an acidic and toxic gas and its emission from utilizing energy from fossil fuels or in industrial processes harms human health and environment. Therefore, it is of great interest to find new materials for SO2 sorption to improve classic flue gas desulfurization. In this work, we present SO2 sorption studies for the three different metal-organic frameworks MOF-177, NH2-MIL-125(Ti), and MIL-160. MOF-177 revealed a new record high SO2 uptake (25.7 mmol·g-1 at 293 K and 1 bar). Both NH2-MIL-125(Ti) and MIL-160 show particular high SO2 uptakes at low pressures ( p < 0.01 bar) and thus are interesting candidates for the removal of remaining SO2 traces below 500 ppm from flue gas mixtures. The aluminum furandicarboxylate MOF MIL-160 is the most promising material, especially under application-orientated conditions, and features excellent ideal adsorbed solution theory selectivities (124-128 at 293 K, 1 bar; 79-95 at 353 K, 1 bar) and breakthrough performance with high onset time, combined with high stability under both humid and dry SO2 exposure. The outstanding sorption capability of MIL-160 could be explained by DFT simulation calculations and matching heat of adsorption for the binding sites Ofuran···SSO2 and OHAl-chain···OSO2 (both ∼40 kJ·mol-1) and Ofuran/carboxylate···SSO2 (∼55-60 kJ·mol-1).

Role of Filler Porosity and Filler/Polymer Interface Volume in Metal-Organic Framework/Polymer Mixed-Matrix Membranes for Gas Separation.

Metal-organic frameworks (MOFs) and inorganic fillers are frequently incorporated into mixed-matrix membranes (MMMs) to overcome the traditional trade-off in permeability ( P) and selectivity for pure organic polymer membranes. Therefore, it is of great interest to examine the influence of porous and nonporous fillers in MMMs with respect to the possible role of the polymer-filler interface, that is, the void volume. In this work, we compare the same MOF filler in a porous and nonporous state, so that artifacts from a different polymer-filler interface are excluded. MMMs with the porous MOF aluminum fumarate (Al-fum) and with a nonporous dimethyl sulfoxide solvent-filled aluminum fumarate (Al-fum(DMSO)), both with Matrimid as polymer, were prepared. Filler contents ranged from 4 to 24 wt %. Gas separation performances of both MMMs were studied by mixed gas measurements using a binary mixture of CO2/CH4 with gas permeation following the theoretical prediction by the Maxwell model for both porous and nonporous dispersed phase (filler). MMMs with the porous Al-fum filler showed increased CO2 and CH4 permeability with a moderate rise in selectivity upon increasing filler fraction. The MMMs with the nonporous Al-fum(DMSO) filler displayed a reduction in permeability while maintaining the selectivity of the neat polymer. A linear dependence of log P versus the reciprocal specific free fractional volume (sFFV) rules out a significant contribution from a void volume. The sFFV includes the free volume of the polymer and the MOF, but not the polymer-filler interface volume (so-called void volume). The sFFV for the MMM was calculated between 0.23 cm3/g for a 24 wt % Al-fum/Matrimid MMM and 0.12 cm3/g for a 24 wt % Al-fum(DMSO)/Matrimid MMM. The negligible effect of an interface volume is supported by a good matching of theoretical and experimental density of the Al-fum and Al-fum/(DMSO) MMMs which gave a specific void volume below 0.02 cm3/g, often even below 0.01 cm3/g.