Further investigation of the effect of framework catenation on hydrogen uptake in metal-organic frameworks

Hydrogen-sorption studies have been carried out for the catenation isomer pairs of PCN-6 and PCN-6' (both have the formula of Cu(3)(TATB)(2), where TATB represents 4,4',4'-s-triazine-2,4,6-triyl-tribenzoate with a formula of C(24)H(12)N(3)O(6)). Inelastic neutron scattering (INS) studies reveal that the initial sites occupied by adsorbed H(2) are the open Cu centers of the paddlewheel units with comparable interaction energies in the two isomers. At high H(2) loadings, where the H(2) molecules adsorb mainly on or around the organic linkers, the interaction is found to be substantially stronger in catenated PCN-6 than in noncatenated PCN-6', leading to much higher H(2) uptake in the isomer with catenation. Hydrogen sorption measurements at pressures up to 50 bar demonstrate that framework catenation can be favorable for the enhancement of hydrogen adsorption. For example, the excess hydrogen uptake of PCN-6 is 72 mg/g (6.7 wt %) at 77 K/50 bar or 9.3 mg/g (0.92 wt %) at 298 K/50 bar, respectively, and that for PCN-6' is 42 mg/g (4.0 wt %) at 77 K/50 bar or 4.0 mg/g (0.40 wt %) at 298 K/50 bar. Importantly, PCN-6 exhibits a total hydrogen uptake of 95 mg/g (8.7 wt %) (corresponding to a total volumetric value of 53.0 g/L, estimated based on crystallographic density) at 77 K/50 bar and 15 mg/g (1.5 wt %) at 298 K/50 bar. Significantly, the expected usable capacity of PCN-6 is as high as 75 mg/g (or 41.9 g/L) at 77 K, if a recharging pressure of 1.5 bar is assumed.     

Mechanistic Elucidations of Highly Dispersed Metalloporphyrin Metal-Organic Framework Catalysts for CO2 Electroreduction

Immobilization of porphyrin complexes into crystalline metal–organic frameworks (MOFs) enables high exposure of porphyrin active sites for CO₂ electroreduction. Herein, well-dispersed iron-porphyrin-based MOF (PCN-222(Fe)) on carbon-based electrodes revealed optimal turnover frequencies for CO₂ electroreduction to CO at 1 wt.% catalyst loading, beyond which the intrinsic catalyst activity declined due to CO₂ mass transport limitations. In situ Raman suggested that PCN-222(Fe) maintained its structure under electrochemical bias, permitting mechanistic investigations. These revealed a stepwise electron transfer-proton transfer mechanism for CO₂ electroreduction on PCN-222(Fe) electrodes, which followed a shift from a rate-limiting electron transfer to CO₂ mass transfer as the potential increased from −0.6 V to −1.0 V vs. RHE. Our results demonstrate how intrinsic catalytic investigations and in situ spectroscopy are needed to elucidate CO₂ electroreduction mechanisms on PCN-222(Fe) MOFs.     

Metal-organic framework from an anthracene derivative containing nanoscopic cages exhibiting high methane uptake

A microporous metal-organic framework, PCN-14, based on an anthracene derivative, 5,5'-(9,10-anthracenediyl)di-isophthalate (H4adip), was synthesized under solvothermal reaction conditions. X-ray single crystal analysis revealed that PCN-14 consists of nanoscopic cages suitable for gas storage. N2-adsorption studies of PCN-14 at 77 K reveal a Langmuir surface area of 2176 m2/g and a pore volume of 0.87 cm3/g. Methane adsorption studies at 290 K and 35 bar show that PCN-14 exhibits an absolute methane-adsorption capacity of 230 v/v, 28% higher than the DOE target (180 v/v) for methane storage.     

Gas adsorption process in activated carbon over a wide temperature range above the critical point. Part 1: modified Dubinin-Astakhov model

Simulations of the thermal effects during adsorption cycles are valuable tools for the design of efficient adsorption-based systems such as gas storage, gas separation and adsorption-based heat pumps. An analytical representation of the measured adsorption data over the wide operating pressure and temperature swing of the system is necessary for the calculation of complete mass and energy conservation equations. In Part 1, the Dubinin-Astakhov (D-A) model is adapted to model hydrogen, nitrogen, and methane adsorption isotherms on activated carbon at high pressures and supercritical temperatures assuming a constant microporous adsorption volume. The five parameter D-A type adsorption model is shown to fit the experimental data for hydrogen (30 to 293 K, up to 6 MPa), nitrogen (93 to 298 K, up to 6 MPa), and for methane (243 to 333 K, up to 9 MPa). The quality of the fit of the multiple experimental adsorption isotherms is excellent over the large temperature and pressure ranges involved. The model’s parameters could be determined as well from only the 77 K and 298 K hydrogen isotherms without much reducing the quality of the fit.     

The potential of organic polymer-based hydrogen storage materials

The challenge of storing hydrogen at high volumetric and gravimetric density for automotive applications has prompted investigations into the potential of cryo-adsorption on the internal surface area of microporous organic polymers. A range of Polymers of Intrinsic Microporosity (PIMs) has been studied, the best PIM to date (a network-PIM incorporating a triptycene subunit) taking up 2.7% H(2) by mass at 10 bar/77 K. HyperCrosslinked Polymers (HCPs) also show promising performance as H(2) storage materials, particularly at pressures >10 bar. The N(2) and H(2) adsorption behaviour at 77 K of six PIMs and a HCP are compared. Surface areas based on Langmuir plots of H(2) adsorption at high pressure are shown to provide a useful guide to hydrogen capacity, but Langmuir plots based on low pressure data underestimate the potential H(2) uptake. The micropore distribution influences the form of the H(2) isotherm, a higher concentration of ultramicropores (pore size <0.7 nm) being associated with enhanced low pressure adsorption.     

Application of carbonized cellulose-based catalyst in nitrobenzene hydrogenation

Carbonized cellulose catalyst support was prepared and decorated with 5 wt% Pd nanoparticles using an impregnation method. According to the SEM images, the carbonized cellulose catalyst support kept its original fibrous structure with an average diameter of 200 nm, owing to the carbonization of the cellulose fibers. The surface of the formed carbon fibers is richly coated by palladium with even coverage. The particles can be divided into two groups within which the average diameter is either 5 nm, or 20–70 nm. TGA method was used to measure the amount of the remained carbon, which was 31.71 wt%. The FTIR spectrum shows the presence of oxygen containing functional groups on the surface of the support, which are hydroxyl groups. XRD method was used to determine the phases of Pd on the support where elemental Pd was detected which confirms the success of the activation step. The catalyst was tested in nitrobenzene hydrogenation in methanolic solution as a model reaction for nitroarene hydrogenation, meanwhile the temperature dependence of the reaction was also examined. Catalytic tests were carried out at four different temperatures (283–323 K) and constant hydrogen pressure (20 bar). The highest conversion (>99%) has been reached at 303 K and 20 bar. The corresponding activation energy was calculated by non-linear regression based on Arrhenius plot, and it was 24.16 ± 0.8 kJ/mol. All in all, the granulated cellulose beads are ideal starting points for carbon based catalyst supports.     

In situ Neutron Powder Diffraction on Intermediate Hydrides of MgPd3 in a Novel Sapphire Gas Pressure Cell

A novel gas pressure cell for in situ neutron powder diffraction has been developed. It is based on a single crystal sapphire tube as a sample holder, allows a 360° unobstructed access by the neutron beam and has little background contribution. This device was used to study the hydrogenation of α‐MgPd₃, which undergoes a hydrogen driven rearrangement from a ZrAl₃ to a AuCu₃ type structure. Deuterium could be located and a strong preference of [Pd₆] voids was found in α‐MgPd₃D_0.79 under 5 bar and in α‐MgPd₃D_0.94 under 20 bar deuterium pressure. The crystal structure may be described as a new defect superstructure variant of the NaCl type. In situ thermal analysis under 5 bar hydrogen pressure showed that both the hydrogen uptake of α‐MgPd₃, which is complete at temperatures below 450 K, and the transformation to the hydride of cubic β‐MgPd₃, starting around 550 K, are exothermic. This completion of the hydrogenation‐dehydrogenation series of MgPd₃ suggests, that the rearrangement of the metal structure proceeds by a hydrogen assisted gliding mechanism with a shift vector of [110]. This is also supported by quantum chemical calculations, which show a decohesion of the intermetallic structure upon hydrogenation accompanied by the appearance of Pd–H bonding interactions.     

Ferrocene-based hypercrosslinked polymers derived from phenolic polycondensation with unexpected H2 adsorption capacity

Metal-doped porous organic polymers often display unique properties for applications in gas uptake owing to the incorporation of the metal elements in the polymer networks. In this study, a series of novel ferrocene-based hypercrosslinked polymers were prepared by phenolic polycondensation (Fc-PR-HCPs). To generate the hypercrosslinked polymers, 1,1′-ferrocenedicarboxaldehyde (Fc(CHO)₂) and bisphenol A (BPA) were used as the building blocks. The maximum value of BET and micropore surface area is determined to be 1111.4 and 487.4 m²/g for Fc-PR-HCP3. A significant H₂ adsorption capacity of 3.11 wt% was achieved for Fc-PR-HCP3 at 77 K/1.0 bar, which was noted to be higher than the porous organic polymers with even higher BET surface area value. The high micropore surface area value and the adsorption sites (aromatic rings and metal ion-active sites) provided by two building blocks were used to explain the significant H₂ adsorption capacity successfully. Overall, the findings from this study indicate that Fc-PR-HCPs highlighted prospective applications in the field of H₂ capture.     

Confinement effects in the hydrogen adsorption on paddle wheel containing metal-organic frameworks

The confinement effects upon hydrogen adsorption in Cu(II)-paddle wheel containing metal-organic frameworks (MOFs) were evaluated and rationalized in terms of the structural properties (cavity types and pore diameters) of PCN-12, HKUST-1, MOF-505, NOTT-103 and NOTT-112. First-principles calculations were employed to identify the strongest adsorption positions at the paddle wheel inorganic building unit (IBU). The adsorption centres due to confinement were located through analysis of 3D occupancy maps obtained from the hydrogen trajectories computed via molecular dynamics simulations. It was found that the confinement enhances the adsorption on the weakest adsorption centres around the IBU in regions close to the narrowest windows and promotes the formation of new adsorption regions into the small cavities. Our results indicate that at low pressure, the high H(2) uptake in these materials is partly due to the presence of small cavities (5.3-8.5 ?) or narrow windows where the long-range contribution to the adsorption becomes important. Conversely, confinement effects in cavities with diameters >12 ? were not observed.     

Influence of CO2 activation on hydrogen storage behaviors of platinum-loaded activated carbon nanotubes

In this work, platinum (Pt) metal loaded activated multi-walled carbon nanotubes (MWNTs) were prepared with different structural characteristics for hydrogen storage applications. The process was conducted by a gas phase CO₂ activation method at 1200 °C as a function of the CO₂ flow time. Pt-loaded activated MWNTs were also formulated to investigate the hydrogen storage characteristics. The microstructures of the Pt-loaded activated MWNTs were characterized by XRD and TEM measurements. The textural properties of the samples were analyzed using N₂ adsorption isotherms at 77 K. The BET, D-R, and BJH equations were used to observe the specific surface areas and the micropore and mesopore structures. The hydrogen storage capacity of the Pt-loaded activated MWNTs was measured at 298 K at a pressure of 100 bar. The hydrogen storage capacity was increased with CO₂ flow time. It was found that the micropore volume of the activated MWNTs plays a key role in the hydrogen storage capacity.     

Carbon nanotubes modified with 5,7-dinitro-8-quinolinol as potentially applicable tool for efficient removal of industrial wastewater pollutants

The environmental pollution due to the industrial wastewater of four different areas in the Gulf of Suez, Red Sea, Egypt, was studied. Adsorption capacities toward the concerned heavy metal ions Cu(II), Zn(II), Fe(II), and Pb(II) by multiwalled carbon nanotubes (MWCNTs) and modified-MWCNTs with 5,7-dinitro-8-quinolinol were investigated. MWCNTs as well as the modified-MWCNTs were characterized using Fourier transform infrared (FTIR), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). Adsorption of the studied divalent metal ions was measured by atomic absorption spectrometry (AAS). The effects of solution conditions such as pH, shaking time, metal ion concentration, ionic strength and adsorbent dosage on the adsorption process were also examined. The obtained results showed that removals of the heavy metal ions under consideration by MWCNTs are obviously dependent on the experimental conditions. The maximum adsorption capacities as calculated applying Langmuir equation to single ion adsorption isotherms were found to be 142.8 mg/g for Cu(II), 250 mg/g for Zn(II), 111.1 mg/g for Fe(II), and 200 mg/g for Pb(II) using MWCNTs; meanwhile, the modified-MWCNTs exhibited higher values of the respective maximum adsorption capacities as 333.3 mg/g for Cu(II), 500 mg/g for Zn(II), 200 mg/g for Fe(II), and 333.3 mg/g for Pb(II). Kinetic studies were also performed and the experimental data followed a pseudo-second order model of the adsorption process. The obtained results suggest that the tested adsorption systems of MWCNTs and modified-MWCNTs have suitable affinity toward the metal ion under consideration. Both systems could act as potentially applicable tool in environmental protection.     

Influence of surface heterogeneity on hydrogen adsorption on activated carbons

This study presents an experimental and theoretical analysis of the effect of surface heterogeneity on the capacity of 20 commercial activated carbons to adsorb hydrogen at 77 and 258 K and for maximum pressures of 20 bar. Some of the samples have been subjected to surface modification by impregnation or by surface oxidation prior to the hydrogen adsorption measurements. All the activated carbons have been analyzed by N₂ adsorption at 77 K using the thermodynamic isotherm presented in a previous study. The hydrogen adsorption capacity of the activated carbons has been well correlated to the micropore volume and the characteristic m₂ parameter of the thermodynamic isotherm accounting for the energy heterogeneity of the material. On the basis of the model presented here, we discuss how surface heterogeneity, in addition to the adsorption strength, might affect the ability of activated carbons and related materials to adsorb hydrogen.     

Structural Deterioration of Well-Faceted MOFs upon H2S Exposure and Its Effect in the Adsorption Performance

The structural deterioration of archetypical, well-faceted metal–organic frameworks (MOFs) has been evaluated upon exposure to an acidic environment (H₂S). Experimental results show that the structural damage highly depends on the nature of the hybrid network (e.g., softness of the metal ions, hydrophilic properties, among others) and the crystallographic orientation of the exposed facets. Microscopy images show that HKUST-1 with well-defined octahedral (111) facets is completely deteriorated, ZIF-8 with preferentially exposed (110) facets exhibits a large external deterioration with the development of holes or cavities in the mesoporous range, whereas UiO-66-NH₂ with (111) exposed facets, and PCN-250 with (100) facets does not reflect any sign of surface damage. Despite the selectivity in the external deterioration, X-ray diffraction and gas adsorption measurements confirm that indeed all MOFs suffer an important internal deterioration, these effects being more severe for MOFs based on softer cations (e.g., Cu-based HKUST-1 and Fe-based PCN-250). These structural changes have inevitable important effects in the final adsorption performance for CO₂ and CH₄ at low and high pressures.     

Hydrogen adsorption kinetics on Pd/Ce0.8Zr0.2O2

Hydrogen adsorption on Pd/Ce(0.8)Zr(0.2)O(2) was studied by temperature-programmed reduction, volumetric measurements and IR spectroscopy. Hydrogen uptake and reduction rate at 353 K are strongly dependent on the hydrogen pressure. At relatively high hydrogen partial pressure, reduction involves PdO, the surface and a significant fraction of the bulk of the ceria based oxide. Formation of oxygen vacancies even at low temperature (<373 K) is observed. The hydrogen adsorption process is mainly irreversible, as is shown by an increase in the (2)F(5/2)-->(2)F(7/2) electronic transition of Ce(3+) with hydrogen pressure and surface dehydroxylation. This "severe" reduction has a negative effect on the subsequent hydrogen adsorption capability. The decrease of hydrogen uptake capacity and rate during adsorption can be associated with the partial loss of superficial OH and the presence of Ce(3+), which deactivates Pd electronically.     

Gas sorption properties of microporous metal organic frameworks

A low-temperature gas sorption study has been carried out on four three-dimensional microporous metal organic framework (MMOF) structures and two two-dimensional layered structures. The pore characteristics are analyzed based on the argon adsorption-desorption isotherms at 87 K. The results from hydrogen sorption experiments conducted at 77 and 87 K show that all MMOFs have a relatively high hydrogen uptake, with adsorbed hydrogen densities falling in the range of liquid hydrogen. Isosteric heats of hydrogen adsorption data calculated based on the Clausius-Clapeyron equation are consistent with these observations, indicating strong sorbent-sorbate interactions.     

Tuning Hydrogen Sorption Properties of Metal–Organic Frameworks by Postsynthetic Covalent Modification

Postsynthetic modification is presented as a means to tune the hydrogen adsorption properties of a series of metal–organic frameworks (MOFs). IRMOF‐3 (isoreticular metal–organic framework), UMCM‐1‐NH₂ (University of Michigan crystalline material), and DMOF‐1‐NH₂ (DABCO metal–organic framework) have been covalently modified with a series of anhydrides or isocyanates and the hydrogen sorption properties have been studied. Both the storage capacities and isosteric heats of adsorption clearly show that covalent postsynthetic modification can significantly enhance the sorption affinity of MOFs with hydrogen and in some cases increase both gravimetric and volumetric uptake of the gas as much as 40 %. The significance of the present study is illustrated by: 1) the nature of the substituents introduced by postsynthetic modification result in different effects on the binding of hydrogen; 2) the covalent postsynthetic modification approach allows for systematic modulation of hydrogen sorption properties; and 3) the ease of postsynthetic modification of MOFs allows a direct evaluation of the interplay between MOF structure, hydrogen uptake, and heat of adsorption. The findings presented herein show that postsynthetic modification is a powerful method to manipulate and better understand the gas sorption properties of MOFs.     

Investigation on specific adsorption of hydrogen on lithium-doped mesoporous silica

This paper reports the synthesis, structure, and hydrogen adsorption property of Li-doped mesoporous silica (MPS) with a 2D hexagonal structure. The Li-doping is achieved by impregnation of the cylindrical mesopores with an ethanol solution of lithium chloride followed by heat treatment. Detailed characterization by solid-state NMR, TG-MS, and FT-IR suggests that, during the heat treatment, lithium chloride reacts with surface ethoxy groups (≡Si-OEt) to form ≡SiOLi groups, while ethyl chloride is released into the gas phase. The hydrogen uptake at 77 K and 1 atm increases from 0.68 wt% for the undoped MPS to 0.81 wt% for Li-doped MPS (Li-MPS). The isosteric heat of adsorption is 4.8 kJ mol⁻¹, which is consistent with the quantum chemistry calculation result (5.12 kJ mol⁻¹). The specific hydrogen adsorption on Li-MPS would be explained by the frontier orbital interaction between HOMO of hydrogen molecules and LUMO of ≡SiOLi. These findings provide an important insight into the development of hydrogen storage materials with specific adsorption sites.     

Density Functional Theory Study of Hydrogen Adsorption in a Ti‐Decorated Mg‐Based Metal–Organic Framework‐74

The Ti‐binding energy and hydrogen adsorption energy of a Ti‐decorated Mg‐based metal–organic framework‐74 (Mg‐MOF‐74) were evaluated by using first‐principles calculations. Our results revealed that only three Ti adsorption sites were found to be stable. The adsorption site near the metal oxide unit is the most stable. To investigate the hydrogen‐adsorption properties of Ti‐functionalized Mg‐MOF‐74, the hydrogen‐binding energy was determined. For the most stable Ti adsorption site, we found that the hydrogen adsorption energy ranged from 0.26 to 0.48 eV H₂^?1. This is within the desirable range for practical hydrogen‐storage applications. Moreover, the hydrogen capacity was determined by using ab initio molecular dynamics simulations. Our results revealed that the hydrogen uptake by Ti‐decorated Mg‐MOF‐74 at temperatures of 77, 150, and 298 K and ambient pressure were 1.81, 1.74, and 1.29 H₂ wt %, respectively.     

Surface modification of date palm activated carbonaceous materials for heavy metal removal and CO2 adsorption

In this study, high surface area activated carbon (AC) was prepared from a local palm tree (Phoenix Dactylifera) using a variety of metal carbonates activators and finally achieved an excellent S_BET of 2700 m²/g when Cs₂CO₃ was used as an activating agent at a temperature of 600 °C. Surface modification of AC was carried out using various nitrogen transporting agents, resulting in N-doped ACs with nitrogen content varying from 4.0 to 11.4 %, depending on the functionalizing agents and activators used. The bimodal (presence of micro- as well as meso-porosity) ACs with such excellent surface properties were studied for their CO₂ uptake capacity at two different temperatures (0 and 25 °C) by isotherms recorded at pressure 1 bar and showed a remarkable uptake ability of 3.52 mmol/g (at 25 °C) and 5.6 mmol/g (at 0 °C), respectively. Also, batch experiments with variable pH, contact time, adsorbate concentrations, adsorbent dose, and temperatures were evaluated to understand the mechanism of sorption phenomena of Cr(VI) and Pb(II) achieving > 99.9 % removal capacity by the prepared ACs. Depending on the heavy metal ions being investigated, it was revealed that the pH of the solution and the amount of adsorbent had a direct impact on the total adsorption ability. Nitrogen atoms doped into the carbon frameworks were found to enhance the adsorption in the case of Pb(II) while the removal of Cr(VI) appeared to be unaffected. Maximum adsorption for Cr(VI) was observed at pH 2 and was determined to follow Freundlich isotherm while that of Pb(II) was observed at pH 7 and follows Langmuir isotherm. Best adsorption was found at an adsorbate concentration of 10 ppm and an adsorbent dose of 10 g/L. Kinetic modeling parameters showed the applicability of pseudo-second-order model perfectly.     

Ultramicroporous metal-organic framework based on 9,10-anthracenedicarboxylate for selective gas adsorption

An ultramicroporous metal-organic framework based on 9,10-anthracenedicarboxylate (PCN-13) has been synthesized and structurally characterized. The desolvated PCN-13 demonstrates selective adsorption of oxygen and hydrogen over nitrogen and carbon monoxide.