Exploring Defect-Engineered Metal-Organic Frameworks with 1,2,4-Triazolyl Isophthalate and Benzoate Linkers.

Synthesis and characterization of DEMOFs (defect-engineered metal-organic frameworks) with coordinatively unsaturated sites (CUSs) for gas adsorption, catalysis, and separation are reported. We use the mixed-linker approach to introduce defects in Cu2-paddle wheel units of MOFs [Cu2(Me-trz-ia)2] by replacing up to 7% of the 3-methyl-triazolyl isophthalate linker (1L2-) with the "defective linker" 3-methyl-triazolyl m-benzoate (2L-), causing uncoordinated equatorial sites. PXRD of DEMOFs shows broadened reflections; IR and Raman analysis demonstrates only marginal changes as compared to the regular MOF (ReMOF, without a defective linker). The concentration of the integrated defective linker in DEMOFs is determined by 1H NMR and HPLC, while PXRD patterns reveal that DEMOFs maintain phase purity and crystallinity. Combined XPS (X-ray photoelectron spectroscopy) and cw EPR (continuous wave electron paramagnetic resonance) spectroscopy analyses provide insights into the local structure of defective sites and charge balance, suggesting the presence of two types of defects. Notably, an increase in CuI concentration is observed with incorporation of defective linkers, correlating with the elevated isosteric heat of adsorption (ΔHads). Overall, this approach offers valuable insights into the creation and evolution of CUSs within MOFs through the integration of defective linkers.

Zirconium-Modified Medium-Entropy Alloy (TiVNb)85Cr15 for Hydrogen Storage.

In this study, we investigate the effect of small amounts of zirconium alloying the medium-entropy alloy (TiVNb)85Cr15, a promising material for hydrogen storage. Alloys with 1, 4, and 7 at.% of Zr were prepared by arc melting and found to be multiphase, comprising at least three phases, indicating that Zr addition does not stabilize a single-phase solid solution. The dominant BCC phase (HEA1) is the primary hydrogen absorber, while the minor phases HEA2 and HEA3 play a crucial role in hydrogen absorption/desorption. Among the studied alloys, Zr4 (TiVNb)81Cr15Zr4 shows the highest hydrogen storage capacity, ease of activation, and reversibly retrievable hydrogen. This alloy can absorb hydrogen at room temperature without additional processing, with a reversible capacity of up to 0.74 wt.%, corresponding to hydrogen-to-metal ratio H/M = 0.46. The study emphasizes the significant role of minor elemental additions in alloy properties, stressing the importance of tailored compositions for hydrogen storage applications. It suggests a direction for further research in metal hydride alloys for effective and safe hydrogen storage.

Temperature Evolution of Sorbonorit-4 Methane-Induced Deformation through the Eyes of Classical Density Functional Theory.

Activated carbons are widely used industrial adsorbents due to their attractive sorption properties. Although extensive research on activated carbon has been carried out for several centuries, some aspects of the adsorption-induced deformation of activated carbon remain unclear. The puzzling temperature dependence of the methane-induced deformation of activated carbon is investigated in the present work. Several experimental studies have shown that an increase in temperature leads to a reversal of the sign of adsorption strain at low pressures, i.e., the contraction turns into an expansion. Here we suggest a possible explanation for this effect by applying classical density functional theory to the adsorption isotherms of nitrogen, carbon dioxide, and methane as well as to methane-induced deformation isotherms. Our calculations show that the adsorption stress generated in the smallest pores predominates at higher temperatures and leads to material swelling. Lowering the temperature, on the other hand, leads to a predominance of larger pores and compression of the activated carbon material. We also investigated the possibility of determining the pore size distribution from methane-induced deformation and adsorption data and the predictive capabilities of our theoretical approach.

Hydrocarbon Sorption in Flexible MOFs-Part III: Modulation of Gas Separation Mechanisms.

Single gas sorption experiments with the C4-hydrocarbons n-butane, iso-butane, 1-butene and iso-butene on the flexible MOFs Cu-IHMe-pw and Cu-IHEt-pw were carried out with both thermodynamic equilibrium and overall sorption kinetics. Subsequent static binary gas mixture experiments of n-butane and iso-butane unveil a complex dependence of the overall selectivity on sorption enthalpy, rate of structural transition as well as steric effects. A thermodynamic separation favoring iso-butane as well as kinetic separation favoring n-butane are possible within Cu-IHMe-pw while complete size exclusion of iso-butane is achieved in Cu-IHEt-pw. This proof-of-concept study shows that the structural flexibility offers additional levers for the precise modulation of the separation mechanisms for complex mixtures with similar chemical and physical properties with real selectivities of >10.

Hydrocarbon Sorption in Flexible MOFs-Part II: Understanding Adsorption Kinetics.

The rate of sorption of n-butane on the structurally flexible metal-organic framework [Cu2(H-Me-trz-ia)2], including its complete structural transition between a narrow-pore phase and a large-pore phase, was studied by sorption gravimetry, IR spectroscopy, and powder X-ray diffraction at close to ambient temperature (283, 298, and 313 K). The uptake curves reveal complex interactions of adsorption on the outer surface of MOF particles, structural transition, of which the overall rate depends on several factors, including pressure step, temperature, as well as particle size, and the subsequent diffusion into newly opened pores. With the aid of a kinetic model based on the linear driving force (LDF) approach, both rates of diffusion and structural transition were studied independently of each other. It is shown that temperature and applied pressure steps have a strong effect on the rate of structural transition and thus, the overall velocity of gas uptake. For pressure steps close to the upper boundary of the gate-opening, the rate of structural transition is drastically reduced. This feature enables a fine-tuning of the overall velocity of sorption, which can even turn into anti-Arrhenius behavior.

Bottom-Up Synthesis of De-Functionalized and Dispersible Carbon Spheres as Colloidal Adsorbent.

Recent innovative adsorption technologies for water purification rely on micrometer-sized activated carbon (AC) for ultrafast adsorption or in situ remediation. In this study, the bottom-up synthesis of tailored activated carbon spheres (aCS) from sucrose as renewable feedstock is demonstrated. The synthesis is based on a hydrothermal carbonization step followed by a targeted thermal activation of the raw material. This preserves its excellent colloid properties, i.e., narrow particle size distribution around 1 µm, ideal spherical shape and excellent aqueous dispersibility. We investigated the ageing of the freshly synthesized, highly de-functionalized AC surface in air and aqueous media under conditions relevant to the practice. A slow but significant ageing due to hydrolysis and oxidation reactions was observed for all carbon samples, leading to an increase of the oxygen contents with storage time. In this study, a tailored aCS product was generated within a single pyrolysis step with 3 vol.-% H2O in N2 in order to obtain the desired pore diameters and surface properties. Adsorption characteristics, including sorption isotherms and kinetics, were investigated with monochlorobenzene (MCB) and perfluorooctanoic acid (PFOA) as adsorbates. The product showed high sorption affinities up to log (KD/[L/kg]) of 7.3 ± 0.1 for MCB and 6.2 ± 0.1 for PFOA, respectively.

Hydrocarbon Sorption in Flexible MOFs-Part I: Thermodynamic Analysis with the Dubinin-Based Universal Adsorption Theory (D-UAT).

The analysis of empirical sorption equilibrium datasets is still vital to gain insights into material-property relationships as computational methods remain in development, especially for complex materials such as flexible MOFs. Therefore, the Dubinin-based universal adsorption theory (D-UAT) was revisited and evaluated as a simple visualization, analysis, and prediction tool for sorption equilibrium data. Within the theory, gas properties are normalized into corresponding states using the critical temperatures of the respective sorptives. The study shows theoretically and experimentally that the D-UAT is able to condense differences of sorption data visualized in reduced Dubinin plots to just three governing parameters: (a) the accessible pore volume, (b) the reduced enthalpy of sorption, and (c) the framework's reduced free energy differences (in case of flexible behavior). This makes the theory a fast visualization and analysis tool, the use as a prediction tool depends on rough assumptions, and thus is not recommended.

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.

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).

From the Laves Phase CaRh2 to the Perovskite CaRhH3-in Situ Investigation of Hydrogenation Intermediates CaRh2H x.

The hydrogenation properties of the cubic Laves phase CaRh2 and the formation of the perovskite CaRhH3 were studied by in situ thermal analysis (differential scanning calorimetry), sorption experiments, and in situ neutron powder diffraction. Three Laves phase hydrides are formed successively at room temperature and hydrogen gas pressures up to 5 MPa. Cubic α-CaRh2H0.05 is a stuffed cubic Laves phase with statistically distributed hydrogen atoms in tetrahedral [Ca2Rh2] voids (ZrCr2H3.08 type, Fd3̅ m, a = 7.5308(12) Å). Orthorhombic ß-CaRh2D3.93(5) (own structure type, Pnma, a = 6.0028(3) Å, b = 5.6065(3) Å, c = 8.1589(5) Å) and γ-CaRh2D3.20(10) (ß-CaRh2H3.9 type, Pnma, a = 5.9601(10) Å, b = 5.4912(2) Å, c = 8.0730(11) Å) are low-symmetry variants thereof with hydrogen occupying distorted tetrahedral [Ca2Rh2] and trigonal bipyramidal [Ca3Rh2] voids. Hydrogen sorption experiments show the hydrogenation to take place already at 0.1 MPa and to yield ß-CaRh2H3.8(2). At 560 K and 5 MPa hydrogen pressure the Laves phase hydride decomposes kinetically controlled to nanocrystalline rhodium and CaRhD2.93(2) (CaTiO3 type, Pm3̅ m, a = 3.6512(2) Å). The hydrogenation of CaRh2 provides a synthesis route to otherwise not accessible perovskite-type CaRhH3.

Continuous Separation of Light Olefin/Paraffin Mixtures on ZIF-4 by Pressure Swing Adsorption and Membrane Permeation.

In this study, two zeolitic imidazolate frameworks (ZIFs) called ZIF-4 and ZIF-zni (zni is the network topology) were characterized by sorption studies regarding their paraffin/olefin separation potential. In particular, equilibrated pure and mixed gas adsorption isotherms of ethane and ethene were recorded at 293 K up to 3 MPa. ZIF-4 exhibits selectivities for ethane in the range of 1.5-3, which is promising for continuous pressure swing adsorption (PSA). ZIF-4 shows high cycle stability with promising separation potential regarding ethane, which results in purification of the more industrial desired olefin. Furthermore, both ZIF materials were implemented in Matrimid to prepare a mixed matrix membrane (MMM) and were used in the continuous separation of a propane/propene mixture. The separation performance of the neat polymer is drastically increased after embedding porous ZIF-4 crystals in the Matrimid matrix, especially at higher feed pressures (3-5 barg). Due to the smaller kinetic diameter of the olefin, the permeability is higher compared to the paraffin.

A Series of Robust Copper-Based Triazolyl Isophthalate MOFs: Impact of Linker Functionalization on Gas Sorption and Catalytic Activity †.

The synthesis and characterization of an isomorphous series of copper-containing microporous metal-organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula [Cu4(µ3-OH)2(R¹-R²-trz-ia)3(H2O)x] are presented. Through size adjustment of the alkyl substituents R¹ and/or R² at the linker, the impact of linker functionalization on structure-property relationships was studied. Due to the arrangement of the substituents towards the cavities, the porosity (pore fraction 28%-39%), as well as the pore size can be adjusted by the size of the substituents of the triazole ring. Thermal analysis and temperature-dependent PXRD studies reveal a thermal stability of the MOFs up to 230 °C due to increasing framework stability through fine-tuning of the linker substitution pattern. Adsorption of CO2 (298 K) shows a decreasing maximum loading with increasing steric demand of the substituents of the triazole ring. Furthermore, the selective oxidation of cyclohexene with tert-butyl hydroperoxide (TBHP) is studied over the MOFs at 323 K in liquid chloroform. The catalytic activity increases with the steric demand of the substituents. Additionally, these isomorphous MOFs exhibit considerable robustness under oxidizing conditions confirmed by CO2 adsorption studies, as well as by the catalytic selective oxidation experiments.

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.

Hierarchical Porous Zeolite Structures for Pressure Swing Adsorption Applications.

Porous adsorbents with hierarchical structured macropores ranging from 1 to 100 µm are prepared using a combination of freeze casting and additional sacrificial templating of polyurethane foams, with a zeolite 13X powder serving as adsorbent. The pore system of the prepared monoliths features micropores assigned to the zeolite 13X particle framework, interparticular pores of ∼1-2 µm, lamellar pores derived from freeze casting of ∼10 µm, and an interconnected pore network obtained from the sacrificial templates ranging from around 100 to 200 µm with a total porosity of 71%. Gas permeation measurements show an increase in intrinsic permeability by a factor of 14 for monoliths prepared with an additional sacrificial templated foam compared to monoliths solely providing freeze casting pores. Cyclic CO2 adsorption and desorption tests where pressure swings between 8 and 140 kPa reveal constant working capacities over multiple cycles. Furthermore, the monoliths feature a high volumetric working capacity of ∼1.34 mmol/cm(3) which is competitive to packed beds made of commercially available zeolite 13X beads (∼1.28 mmol/cm(3)). Combined with the faster CO2 uptake showing an adsorption of 50% within 5-8 s (beads ∼10 s), the monoliths show great potential for pressure swing adsorption applications, where high volumetric working capacities, fast uptakes, and low pressure drops are needed for a high system performance.

Multifunctional phosphate-based inorganic-organic hybrid nanoparticles.

Phosphate-based inorganic-organic hybrid nanoparticles (IOH-NPs) with the general composition [M](2+)[Rfunction(O)PO3](2-) (M = ZrO, Mg2O; R = functional organic group) show multipurpose and multifunctional properties. If [Rfunction(O)PO3](2-) is a fluorescent dye anion ([RdyeOPO3](2-)), the IOH-NPs show blue, green, red, and near-infrared fluorescence. This is shown for [ZrO](2+)[PUP](2-), [ZrO](2+)[MFP](2-), [ZrO](2+)[RRP](2-), and [ZrO](2+)[DUT](2-) (PUP = phenylumbelliferon phosphate, MFP = methylfluorescein phosphate, RRP = resorufin phosphate, DUT = Dyomics-647 uridine triphosphate). With pharmaceutical agents as functional anions ([RdrugOPO3](2-)), drug transport and release of anti-inflammatory ([ZrO](2+)[BMP](2-)) and antitumor agents ([ZrO](2+)[FdUMP](2-)) with an up to 80% load of active drug is possible (BMP = betamethason phosphate, FdUMP = 5'-fluoro-2'-deoxyuridine 5'-monophosphate). A combination of fluorescent dye and drug anions is possible as well and shown for [ZrO](2+)[BMP](2-)0.996[DUT](2-)0.004. Merging of functional anions, in general, results in [ZrO](2+)([RdrugOPO3]1-x[RdyeOPO3]x)(2-) nanoparticles and is highly relevant for theranostics. Amine-based functional anions in [MgO](2+)[RaminePO3](2-) IOH-NPs, finally, show CO2 sorption (up to 180 mg g(-1)) and can be used for CO2/N2 separation (selectivity up to α = 23). This includes aminomethyl phosphonate [AMP](2-), 1-aminoethyl phosphonate [1AEP](2-), 2-aminoethyl phosphonate [2AEP](2-), aminopropyl phosphonate [APP](2-), and aminobutyl phosphonate [ABP](2-). All [M](2+)[Rfunction(O)PO3](2-) IOH-NPs are prepared via noncomplex synthesis in water, which facilitates practical handling and which is optimal for biomedical application. In sum, all IOH-NPs have very similar chemical compositions but can address a variety of different functions, including fluorescence, drug delivery, and CO2 sorption.

Microporous La-metal-organic framework (MOF) with large surface area.

A microporous La-metal-organic framework (MOF) has been synthesized by the reaction of La(NO3 )3 ⋅6 H2 O with a ligand 4,4',4''-s-triazine-1,3,5-triyltri-p-aminobenzoate (TATAB) featuring three carboxylate groups. Crystal structure analysis confirms the formation of 3D MOF with hexagonal micropores, a Brunauer-Emmett-Teller (BET) surface area of 1074 m(2) g(-1) and high thermal and chemical stability. The CO2 adsorption capacities are 76.8 cm(3) g(-1) at 273 K and 34.6 cm(3) g(-1) at 293 K, a highest measured CO2 uptake for a Ln-MOFs.

Adsorption of I2 by macrocyclic polyazadithiophenolato complexes mediated by charge-transfer interactions.

The macrocyclic complex [Ni2(L)(OAc)]ClO4 (1) adsorbs up to 17 molar equivalents (>270 wt%) of iodine, although it does not exhibit permanent porosity. Vibrational spectroscopic and crystallographic studies reveal that two I2 molecules are captured by means of thiophenolate→I2 charge-transfer interactions, which enable the diffusion and sorption of further I2 molecules in a polyiodide-like network. The efficient sorption and desorption characteristics make this material suitable for accommodation, sensing, and slow release of I2.

Network flexibility: control of gate opening in an isostructural series of Ag-MOFs by linker substitution.

An isostructural series of 15 structurally flexible microporous silver metal-organic frameworks (MOFs) is presented. The compounds with a dinuclear silver core as secondary building unit (Ag2N4) can be obtained under solvothermal conditions from substituted triazolyl benzoate linkers and AgNO3 or Ag2SO4; they exhibit 2-fold network interpenetration with lvt topology. Besides the crystal structures, the calculated pore size distributions of the microporous MOFs are reported. Simultaneous thermal analyses confirm the stability of the compounds up to 250 °C. Interconnected pores result in a three-dimensional pore structure. Although the porosity of the novel coordination polymers is in the range of only 20-36%, this series can be regarded as a model system for investigation of network flexibility, since the pore diameters and volumes can be gradually adjusted by the substituents of the 3-(1,2,4-triazol-4-yl)-5-benzamidobenzoates. The pore volumes of selected materials are experimentally determined by nitrogen adsorption at 77 K and carbon dioxide adsorption at room temperature. On the basis of the flexible behavior of the linkers a reversible framework transformation of the 2-fold interpenetrated network is observed. The resulting adsorption isotherms with one or two hysteresis loops are interpreted by a gate-opening process. Due to external stimuli, namely, the adsorptive pressure, the materials undergo a phase transition confirming the structural flexibility of the porous coordination polymer.

An isomorphous series of cubic, copper-based triazolyl isophthalate MOFs: linker substitution and adsorption properties.

An isomorphous series of 10 microporous copper-based metal-organic frameworks (MOFs) with the general formulas (∞)(3)[{Cu(3)(µ(3)-OH)(X)}(4){Cu(2)(H(2)O)(2)}(3)(H-R-trz-ia)(12)] (R = H, CH(3), Ph; X(2-) = SO(4)(2-), SeO(4)(2-), 2 NO(3)(2-) (1-8)) and (∞)(3)[{Cu(3)(µ(3)-OH)(X)}(8){Cu(2)(H(2)O)(2)}(6)(H-3py-trz-ia)(24)Cu(6)]X(3) (R = 3py; X(2-) = SO(4)(2-), SeO(4)(2-) (9, 10)) is presented together with the closely related compounds (∞)(3)[Cu(6)(µ(4)-O)(µ(3)-OH)(2)(H-Metrz-ia)(4)][Cu(H(2)O)(6)](NO(3))(2)·10H(2)O (11) and (∞)(3)[Cu(2)(H-3py-trz-ia)(2)(H(2)O)(3)] (12(Cu)), which are obtained under similar reaction conditions. The porosity of the series of cubic MOFs with twf-d topology reaches up to 66%. While the diameters of the spherical pores remain unaffected, adsorption measurements show that the pore volume can be fine-tuned by the substituents of the triazolyl isophthalate ligand and choice of the respective copper salt, that is, copper sulfate, selenate, or nitrate.

Sorption and separation of CO2 via nanoscale AlO(OH) hollow spheres.

The CO(2) uptake on nanoscale AlO(OH) hollow spheres (260 mg g(-1)) as a new material is comparable to that on many metal-organic frameworks although their specific surface area is much lower (530 m(2) g(¬1)versus 1500-6000 m(2) g(¬1)). Suited temperature-pressure cycles allow for reversible storage and separation of CO(2) while the CO(2) uptake is 4.3-times higher as compared to N(2).