Hetero-bimetallic paddlewheel complexes for enhanced CO2 reduction selectivity in MOFs: a first principles study.

The reduction of carbon dioxide (CO2) into value-added feedstock materials, fine chemicals, and fuels represents a crucial approach for meeting contemporary chemical demands while reducing dependence on petrochemical sources. Optimizing catalysts for the CO2 reduction reaction (CO2RR) can entail employing first principles methodology to identify catalysts possessing desirable attributes, including the ability to form diverse products or selectively produce a limited set of products, or exhibit favorable reaction kinetics. In this study, we investigate CO2RR on bimetallic Cu-based paddlewheel complexes, aiming to understand the impact metal substitution with Mn(II), Co(II), or Ni(II) has on bimetallic paddlewheel metal-organic frameworks. Substituting one of the Cu sites of the paddlewheel complex with Mn results in a more catalytically active Cu center, poised to produce substantial quantities of formic acid (HCOOH) and smaller quantities of methane (CH4) with a suppressed production of C2 products such as ethanol (CH3CH2OH) or ethylene (C2H4). Moreover, the presence of Mn significantly reduces the limiting potential for CO2 reduction from 2.22 eV on the homo-bimetallic Cu paddlewheel complex to 1.19 eV, thereby necessitating a smaller applied potential. Conversely, within the Co-substituted paddlewheel complex, the Co site emerges as the primary catalytic center, selectively yielding CH4 as the sole reduced CO2 product, with a limiting potential of 1.22 eV. Notably, the Co site faces significant competition from H2 production due to a lower limiting potential of 0.81 eV for hydrogen reduction. Our examination of the Cu-Ni paddlewheel complex, featuring a Ni substituent site, reveals two catalytically active centers, each promoting distinct reductive processes. Both the Ni and Cu sites exhibit a propensity for HCOOH formation, with the Ni site favoring further reduction to CH4, whereas the Cu site directs the reaction towards methanol (CH3OH) production. This study holds significance in informing and streamlining future experimental efforts for synthesizing and evaluating novel catalysts with superior capabilities for CO2 reduction.

Air-Stable Cu(I) Metal-Organic Framework for Hydrogen Storage.

Metal-organic frameworks (MOFs) that contain open metal sites have the potential for storing hydrogen (H2) at ambient temperatures. In particular, Cu(I)-based MOFs demonstrate very high isosteric heats of adsorption for hydrogen relative to other reported MOFs with open metal sites. However, most of these Cu(I)-based MOFs are not stable in ambient conditions since the Cu(I) species display sensitivity toward moisture and can rapidly oxidize in air. As a result, researchers have focused on the synthesis of new air-stable Cu(I)-based materials for H2 storage. Here, we have developed a de novo synthetic strategy to generate a robust Cu(I)-based MOF, denoted as NU-2100, using a mixture of Cu/Zn precursors in which zinc acts as a catalyst to transform an intermediate MOF into NU-2100 without getting incorporated into the final MOF structure. NU-2100 is air-stable and displays one of the initial highest isosteric heats of adsorption (32 kJ/mol) with good hydrogen storage capability under ambient conditions (10.4 g/L, 233 K/100 bar to 296 K/5 bar). We further elucidated the H2 storage performance of NU-2100 using a combination of spectroscopic analysis and computational modeling studies. Overall, this new synthetic route may enable the design of additional stable Cu(I)-MOFs for next-generation hydrogen storage adsorbents at ambient temperatures.

Noncryogenic Air Separation Using Aluminum Formate Al(HCOO)3 (ALF).

Separating oxygen from air to create oxygen-enriched gas streams is a process that is significant in both industrial and medical fields. However, the prominent technologies for creating oxygen-enriched gas streams are both energy and infrastructure intensive as they use cryogenic temperatures or materials that adsorb N2 from air. The latter method is less efficient than the methods that adsorb O2 directly. Herein, we show, via a combination of gas adsorption isotherms, gas breakthrough experiments, neutron and synchrotron X-ray powder diffraction, Raman spectroscopy, and computational studies, that the metal-organic framework, Al(HCOO)3 (ALF), which is easily prepared at low cost from commodity chemicals, exhibits substantial O2 adsorption and excellent time-dependent O2/N2 selectivity in a range of 50-125 near dry ice/solvent (≈190 K) temperatures. The effective O2 adsorption with ALF at ≈190 K and ≈0.21 bar (the partial pressure of O2 in air) is ≈1.7 mmol/g, and at ice/salt temperatures (≈250 K), it is ≈0.3 mmol/g. Though the kinetics for full adsorption of O2 near 190 K are slower than at temperatures nearer 250 K, the kinetics for initial O2 adsorption are fast, suggesting that O2 separation using ALF with rapid temperature swings at ambient pressures is a potentially viable choice for low-cost air separation applications. We also present synthetic strategies for improving the kinetics of this family of compounds, namely, via Al/Fe solid solutions. To the best of our knowledge, ALF has the highest O2/N2 sorption selectivity among MOF adsorbents without open metal sites as verified by co-adsorption experiments..

Hydrogen Storage with Aluminum Formate, ALF: Experimental, Computational, and Technoeconomic Studies.

Long-duration storage of hydrogen is necessary for coupling renewable H2 with stationary fuel cell power applications. In this work, aluminum formate (ALF), which adopts the ReO3-type structure, is shown to have remarkable H2 storage performance at non-cryogenic (>120 K) temperatures and low pressures. The most promising performance of ALF is found between 120 K and 160 K and at 10 bar to 20 bar. The study illustrates H2 adsorption performance of ALF over the 77 K to 296 K temperature range using gas isotherms, in situ neutron powder diffraction, and DFT calculations, as well as technoeconomic analysis (TEA), illustrating ALF's competitive performance for long-duration storage versus compressed hydrogen and leading metal-organic frameworks. In the TEA, it is shown that ALF's storage capacity, when combined with a temperature/pressure swing process, has advantages versus compressed H2 at a fraction of the pressure (15 bar versus 350 bar). Given ALF's performance in the 10 bar to 20 bar regime under moderate cooling, it is particularly promising for use in safe storage systems serving fuel cells.

Catalyst Engineering for the Selective Reduction of CO2 to CH4 : A First-Principles Study on X-MOF-74 (X=Mg, Mn, Fe, Co, Ni, Cu, Zn).

The conversion of carbon dioxide (CO2 ) into more valuable chemical compounds represents a critical objective for addressing environmental challenges and advancing sustainable energy sources. The CO2 reduction reaction (CO2 RR) holds promise for transforming CO2 into versatile feedstock materials and fuels. Leveraging first-principles methodologies provides a robust approach to evaluate catalysts and steer experimental efforts. In this study, we examine the CO2 RR process using a diverse array of representative cluster models derived from X-MOF-74 (where X encompasses Mg, Mn, Fe, Co, Ni, Cu, or Zn) through first-principles methods. Notably, our investigation highlights the Fe-MOF-74 cluster's unique attributes, including favorable CO2 binding and the lowest limiting potential of the studied clusters for converting CO2 to methane (CH4 ) at 0.32 eV. Our analysis identified critical factors driving the selective CO2 RR pathway, enabling the formation CH4 on the Fe-MOF-74 cluster. These factors involve less favorable reduction of hydrogen to H2 and strong binding affinities between the Fe open-metal site and reduction intermediates, effectively curtailing desorption processes of closed-shell intermediates such as formic acid (HCOOH), formaldehyde (CH2 O), and methanol (CH3 OH), to lead to selective CH4 formation.

Fine-Tuning a Robust Metal-Organic Framework toward Enhanced Clean Energy Gas Storage.

The development of adsorbents with molecular precision offers a promising strategy to enhance storage of hydrogen and methane─considered the fuel of the future and a transitional fuel, respectively─and to realize a carbon-neutral energy cycle. Herein we employ a postsynthetic modification strategy on a robust metal-organic framework (MOF), MFU-4l, to boost its storage capacity toward these clean energy gases. MFU-4l-Li displays one of the best volumetric deliverable hydrogen capacities of 50.2 g L-1 under combined temperature and pressure swing conditions (77 K/100 bar → 160 K/5 bar) while maintaining a moderately high gravimetric capacity of 9.4 wt %. Moreover, MFU-4l-Li demonstrates impressive methane storage performance with a 5-100 bar usable capacity of 251 cm3 (STP) cm-3 (0.38 g g-1) and 220 cm3 (STP) cm-3 (0.30 g g-1) at 270 and 296 K, respectively. Notably, these hydrogen and methane storage capacities are significantly improved compared to those of its isoreticular analogue, MFU-4l, and place MFU-4l-Li among the best MOF-based materials for this application.

Fine Tuning of MOF-505 Analogues To Reduce Low-Pressure Methane Uptake and Enhance Methane Working Capacity.

We present a crystal engineering strategy to fine tune the pore chemistry and CH4 -storage performance of a family of isomorphic MOFs based upon PCN-14. These MOFs exhibit similar pore size, pore surface, and surface area (around 3000 m2 g-1 ) and were prepared with the goal to enhance CH4 working capacity. [Cu2 (L2)(H2 O)2 ]n (NJU-Bai 41: NJU-Bai for Nanjing University Bai's group), [Cu2 (L3)(H2 O)2 ]n (NJU-Bai 42), and [Cu2 (L4)(DMF)2 ]n (NJU-Bai 43) were prepared and we observed that the CH4 volumetric working capacity and volumetric uptake values are influenced by subtle changes in structure and chemistry. In particular, the CH4 working capacity of NJU-Bai 43 reaches 198 cm3 (STP: 273.15 K, 1 atm) cm-3 at 298 K and 65 bar, which is amongst the highest reported for MOFs under these conditions and is much higher than the corresponding value for PCN-14 (157 cm3 (STP) cm-3 ).

Novel microporous metal-organic framework exhibiting high acetylene and methane storage capacities.

A new organic hexacarboxylic acid, 5,5',5″-(9H-carbazole-3,6,9-triyl)triisophthalic acid (H6CTIA), was developed to construct its first microporous metal-organic framework (MOF), Cu6(CTIA)2 (ZJU-70). With open metal sites and suitable pore sizes, this MOF exhibits high acetylene and methane storage capacities at room temperature.

A porous metal-organic framework with dynamic pyrimidine groups exhibiting record high methane storage working capacity.

We have realized a new porous metal-organic framework UTSA-76a with pyrimidine groups on the linker, exhibiting high volumetric methane uptake of ~260 cm(3) (STP) cm(-3) at 298 K and 65 bar, and record high working capacity of ~200 cm(3) (STP) cm(-3) (between 5 and 65 bar). Such exceptionally high working capacity is attributed to the central "dynamic" pyrimidine groups within UTSA-76a, which are capable of adjusting their orientations to optimize the methane packing at high pressure, as revealed by computational studies and neutron scattering experiments.

Water-stable zirconium-based metal-organic framework material with high-surface area and gas-storage capacities.

We designed, synthesized, and characterized a new Zr-based metal-organic framework material, NU-1100, with a pore volume of 1.53 ccg(-1) and Brunauer-Emmett-Teller (BET) surface area of 4020 m(2) g(-1) ; to our knowledge, currently the highest published for Zr-based MOFs. CH4 /CO2 /H2 adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions. The total hydrogen adsorption at 65 bar and 77 K is 0.092 g g(-1) , which corresponds to 43 g L(-1) . The volumetric and gravimetric methane-storage capacities at 65 bar and 298 K are approximately 180 vSTP /v and 0.27 g g(-1) , respectively.

Defect creation by linker fragmentation in metal-organic frameworks and its effects on gas uptake properties.

We successfully demonstrate an approach based on linker fragmentation to create defects and tune the pore volumes and surface areas of two metal-organic frameworks, NU-125 and HKUST-1, both of which feature copper paddlewheel nodes. Depending on the linker fragment composition, the defect can be either a vacant site or a functional group that the original linker does not have. In the first case, we show that both surface area and pore volume increase, while in the second case they decrease. The effect of defects on the high-pressure gas uptake is also studied over a large temperature and pressure range for different gases. We found that despite an increase in pore volume and surface area in structures with vacant sites, the absolute adsorption for methane decreases for HKUST-1 and slightly increases for NU-125. However, the working capacity (deliverable amount between 65 and 5 bar) in both cases remains similar to parent frameworks due to lower uptakes at low pressures. In the case of NU-125, the effect of defects became more pronounced at lower temperatures, reflecting the greater surface areas and pore volumes of the altered forms.

Diels-Alder cycloaddition polymerization for porous poly-phenylenes with exceptional gas uptake properties.

We report the synthesis of two-dimensional and three-dimensional porous polyphenylenes (2D/3D-pPPs) via the Diels-Alder cycloaddition polymerization reaction. The resulting 2D and 3D-pPPs showed surface areas up to 1553 m2 g-1, pore volumes of 1.45 cm3 g-1 and very high H2 uptake capacities of 7.4 and 7.1 wt% at 77 K, respectively, along with a competitive high-pressure CO2 and CH4 uptake performance.

Unusual and highly tunable missing-linker defects in zirconium metal-organic framework UiO-66 and their important effects on gas adsorption.

UiO-66 is a highly important prototypical zirconium metal-organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missing-linker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm(3)/g and Brunauer-Emmett-Teller surface areas ranging from 1000 to 1600 m(2)/g, the largest values of which are ∼150% and ∼60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO2. Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO2) suggesting the beneficial effect of the -OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air-sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements, and the gas binding strength enhancement by the -OH group was further supported by our first-principles calculations.

Methane storage in metal-organic frameworks: current records, surprise findings, and challenges.

We have examined the methane uptake properties of six of the most promising metal organic framework (MOF) materials: PCN-14, UTSA-20, HKUST-1, Ni-MOF-74 (Ni-CPO-27), NU-111, and NU-125. We discovered that HKUST-1, a material that is commercially available in gram scale, exhibits a room-temperature volumetric methane uptake that exceeds any value reported to date. The total uptake is about 230 cc(STP)/cc at 35 bar and 270 cc(STP)/cc at 65 bar, which meets the new volumetric target recently set by the Department of Energy (DOE) if the packing efficiency loss is ignored. We emphasize that MOFs with high surface areas and pore volumes perform better overall. NU-111, for example, reaches ~75% of both the gravimetric and the volumetric targets. We find that values for gravimetric uptake, pore volume, and inverse density of the MOFs we studied scale essentially linearly with surface area. From this linear dependence, we estimate that a MOF with surface area 7500 m(2)/g and pore volume 3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g while simultaneously exhibiting around ~200 cc/cc volumetric uptake. We note that while values for volumetric uptake are based on ideal single crystal densities, in reality the packing densities of MOFs are much lower. Finally, we show that compacting HKUST-1 into wafer shapes partially collapses the framework, decreasing both volumetric and gravimetric uptake significantly. Hence, one of the important challenges going forward is to find ways to pack MOFs efficiently without serious damage or to synthesize MOFs that can withstand substantial mechanical pressure.

Expanded organic building units for the construction of highly porous metal-organic frameworks.

Two new organic building units that contain dicarboxylate sites for their self-assembly with paddlewheel [Cu2(CO2)4] units have been successfully developed to construct two isoreticular porous metal-organic frameworks (MOFs), ZJU-35 and ZJU-36, which have the same tbo topologies (Reticular Chemistry Structure Resource (RCSR) symbol) as HKUST-1. Because the organic linkers in ZJU-35 and ZJU-36 are systematically enlarged, the pores in these two new porous MOFs vary from 10.8 Šin HKUST-1 to 14.4 Šin ZJU-35 and 16.5 Šin ZJU-36, thus leading to their higher porosities with Brunauer-Emmett-Teller (BET) surface areas of 2899 and 4014 m(2) g(-1) for ZJU-35 and ZJU-36, respectively. High-pressure gas-sorption isotherms indicate that both ZJU-35 and ZJU-36 can take up large amounts of CH4 and CO2, and are among the few porous MOFs with the highest volumetric storage of CH4 under 60 bar and CO2 under 30 bar at room temperature. Their potential for high-pressure swing adsorption (PSA) hydrogen purification was also preliminarily examined and compared with several reported MOFs, thus indicating the potential of ZJU-35 and ZJU-36 for this important application. Studies show that most of the highly porous MOFs that can volumetrically take up the greatest amount of CH4 under 60 bar and CO2 under 30 bar at room temperature are those self-assembled from organic tetra- and hexacarboxylates that contain m-benzenedicarboxylate units with the [Cu2(CO2)4] units, because this series of MOFs can have balanced porosities, suitable pores, and framework densities to optimize their volumetric gas storage. The realization of the two new organic building units for their construction of highly porous MOFs through their self-assembly with [Cu2(CO2)4] units has provided great promise for the exploration of a large number of new tetra- and hexacarboxylate organic linkers based on these new organic building units in which different aromatic backbones can be readily incorporated into the frameworks to tune their porosities, pore structures, and framework densities, thus targeting some even better performing MOFs for very high gas storage and efficient gas separation under high pressure and at room temperature in the near future.

Metal-organic frameworks with exceptionally high methane uptake: where and how is methane stored?

Metal-organic frameworks (MOFs) are a novel family of physisorptive materials that have exhibited great promise for methane storage. So far, a detailed understanding of their methane adsorption mechanism is still scarce. Herein, we report a comprehensive mechanistic study of methane storage in three milestone MOF compounds (HKUST-1, PCN-11, and PCN-14) the CH(4) storage capacities of which are among the highest reported so far among all porous materials. The three MOFs consist of the same dicopper paddlewheel secondary building units, but contain different organic linkers, leading to cagelike pores with various sizes and geometries. From neutron powder diffraction experiments and accurate data analysis, assisted by grand canonical Monte Carlo (GCMC) simulations and DFT calculations, we unambiguously revealed the exact locations of the stored methane molecules in these MOF materials. We found that methane uptake takes place primarily at two types of strong adsorption site: 1) the open Cu coordination sites, which exhibit enhanced Coulomb attraction toward methane, and 2) the van der Waals potential pocket sites, in which the total dispersive interactions are enhanced due to the molecule being in contact with multiple "surfaces". Interestingly, the enhanced van der Waals sites are present exclusively in small cages and at the windows to these cages, whereas large cages with relatively flat pore surfaces bind very little methane. Our results suggest that further, rational development of new MOF compounds for methane storage applications should focus on enriching open metal sites, increasing the volume percentage of accessible small cages and channels, and minimizing the fraction of large pores.

Optimization of the Pore Structures of MOFs for Record High Hydrogen Volumetric Working Capacity.

Metal-organic frameworks (MOFs) are promising materials for onboard hydrogen storage thanks to the tunable pore size, pore volume, and pore geometry. In consideration of pore structures, the correlation between the pore volume and hydrogen storage capacity is examined and two empirical equations are rationalized to predict the hydrogen storage capacity of MOFs with different pore geometries. The total hydrogen adsorption under 100 bar and 77 K is predicted as ntot = 0.085× Vp - 0.013× Vp 2 for cage-type MOFs and ntot = 0.076× Vp - 0.011× Vp 2 for channel-type MOFs, where Vp is the pore volume of corresponding MOFs. The predictions by these empirical equations are validated by several MOFs with an average deviation of 5.4%. Compared with a previous equation for activated carbon materials, the empirical equations demonstrate superior accuracy especially for MOFs with high surface area (i.e., SBET over ≈3000 m2 g-1 ). Guided by these empirical equations, a highly porous Zr-MOF NPF-200 (NPF: Nebraska Porous Framework) is examined to possess outstanding hydrogen total adsorption capacity (65.7 mmol g-1 ) at 77 K and record high volumetric working capacity of 37.2 g L-1 between 100 and 5 bar at 77 K.

Vertical two-dimensional layered fused aromatic ladder structure.

Planar two-dimensional (2D) layered materials such as graphene, metal-organic frameworks, and covalent-organic frameworks are attracting enormous interest in the scientific community because of their unique properties and potential applications. One common feature of these materials is that their building blocks (monomers) are flat and lie in planar 2D structures, with interlayer π-π stacking, parallel to the stacking direction. Due to layer-to-layer confinement, their segmental motion is very restricted, which affects their sorption/desorption kinetics when used as sorbent materials. Here, to minimize this confinement, a vertical 2D layered material was designed and synthesized, with a robust fused aromatic ladder (FAL) structure. Because of its unique structural nature, the vertical 2D layered FAL structure has excellent gas uptake performance under both low and high pressures, and also a high iodine (I2) uptake capacity with unusually fast kinetics, the fastest among reported porous organic materials to date.

A microporous metal-organic framework with naphthalene diimide groups for high methane storage.

We reported a microporous MOF FJU-101 with open naphthalene diimide functional groups for room temperature (RT) high methane storage. At RT and 65 bar, the total volumetric CH4 storage capacity of 212 cm3 (STP) cm-3 of FJU-101a is significantly higher than those of the isoreticular MFM-130a and UTSA-40a. The enhanced methane uptake in FJU-101a is attributed to the polar carbonyl sites, which can generate strong electrostatic interactions with CH4 molecules.

Structural Properties of Kerogens with Different Maturities.

The thermogenic transformation of kerogen into hydrocarbons accompanies the development of a pore network within the kerogen that serves as gas storage locations both in pore space and surface area for adsorbed gas within the source rock. Therefore, the successful recovery of gas from these rocks depends on the accessible surface area, surface properties, and interconnectivity of the pore system. These parameters can be difficult to determine because of the nanoscale of the structures within the rock. This study seeks to investigate the pore structure, surface heterogeneity, and composition of recovered kerogens isolated from source rocks with progressively increasing thermogenic maturities. Prompt gamma-ray activation analysis (PGAA), nitrogen and methane volumetric gas sorption, and small-angle neutron scattering (SANS) are combined to explore the relationship between the chemical composition, pore structure, surface roughness, surface heterogeneity, and maturity. PGAA results indicate that higher mature kerogens have lower hydrogen/carbon ratio. Nitrogen gas adsorption indicates that the pore volume and accessible specific surface area are higher for more mature kerogens. The methane isosteric heat at different methane uptake in the kerogens is determined by methane isotherms and shows that approximately two types of binding sites are present in low mature kerogens while the binding sites are relatively homogeneous in the most mature kerogen. The hysteresis effect of the structure during the adsorption and desorption process at different CD4 gas pressures are studied. An extended generalized Porod's scattering law method (GPSLM) is further developed here to analyze kerogens with fractal surfaces. This extended GPSLM quantifies the surface heterogeneity of the kerogens with a fractal surface and shows that kerogen with high maturity is chemically more homogeneous, consistent with the results from methane isosteric heat. SANS analysis also suggests a pronounced surface roughness in the more mature kerogens. A microporous region circling around the nanopores, which contributes to high surface roughness and methane storage, is shown to develop with maturity.