Two complementary design strategies, isomorphous ligand replacement and heterocycle doping, have been applied to iteratively enhance the proton conductivity of a metal–organic framework, β-PCMOF2. ...The resulting materials, PCMOF21/2(Pz) and PCMOF21/2(Tz) (Pz = 1H-pyrazole, Tz = 1H-1,2,4-triazole), have their proton conduction raised almost 2 orders of magnitude compared to β-PCMOF2. The bulk conductivities of these materials are over 10–1 S cm–1 at 85 °C and 90% relative humidity (RH), while maintaining the parent MOF structure. A solid state synthetic route for doping 1-D channels is also presented.
A sulfonated indium (In) metal organic framework (MOF) is reported with an anionic layered structure incorporating hydrogen-bonded dimethylammonium cations and water molecules. The MOF becomes ...amorphous in >60% relative humidity; however, impedance analysis of pelletized powders revealed a proton conduction value of over 10–3 S cm–1 at 25 °C and 40% RH, a very high proton conduction value for low humidity and moderate temperature. Given the modest humidity stability of the MOF, triaxial impedance analyses on a single crystal was performed and confirmed bulk proton conductivity over 10–3 S cm–1 along two axes corroborating the data from the pellet.
Post-combustion CO2 capture from the flue gas is one of the key technology options to reduce greenhouse gases, because this can be potentially retrofitted to the existing fleet of coal-fired power ...stations. Adsorption processes using solid sorbents capable of capturing CO2 from flue gas streams have shown many potential advantages, compared to other conventional CO2 capture using aqueous amine solvents. In view of this, in the past few years, several research groups have been involved in the development of new solid sorbents for CO2 capture from flue gas with superior performance and desired economics. A variety of promising sorbents such as activated carbonaceous materials, microporous/mesoporous silica or zeolites, carbonates, and polymeric resins loaded with or without nitrogen functionality for the removal of CO2 from the flue gas streams have been reviewed. Different methods of impregnating functional groups, including grafting techniques and modifying the support materials, have been discussed to enhance the performance of the sorbents. The performance characteristics of the solid sorbents are assessed in terms of various desired attributes, such as their equilibrium adsorption capacity, selectivity, regeneration, multicycle durability, and adsorption/desorption kinetics. The potential of metal-organic frameworks (MOFs) is also recognized to determine whether these novel materials provide better CO2 adsorption capacity under low CO2 partial pressure. A comprehensive critical review and analysis of the literature on this subject has been carried out to update the recent progress in this arena. A comparison of different solid sorbents at different stages is made. It also includes a brief review on techno-economic analysis and design aspects of sorbent bed contactor configuration. Finally, a few recommendations have been proposed for further research efforts to progress post-combustion carbon capture.
Metal-organic frameworks (MOFs) are a class of porous solid, which have a variety of potential applications. Unfortunately, MOFs often lack hydrolytic stability, which hinders their use as viable ...materials for large scale applications. Though there have been an increasing number of reports proving water stability, this aspect is often ignored and negative results often remain unpublished. As a result, this report has been produced to offer common benchmarks for stability of MOFs to moisture. This will be done by discussing what water stability means - both with regards to the exposure methods and the means of assessing the MOF after exposure. Based on these two criteria, definitions are proposed in order to allow MOFs to be discussed more consistently. The purpose of this report is not to rank existing MOFs based on water stability or for potential application but to promote and facilitate discussion about hydrolytic stability of MOFs.
Water stability of MOFs is reviewed including exposure techniques, characterization methods, and ultimately more consistent definitions of water stability.
A new porous metal–organic framework (MOF), barium tetraethyl-1,3,6,8-pyrenetetraphosphonate (CALF-25), which contains a new phosphonate monoester ligand, was synthesized through a hydrothermal ...method. The MOF is a three-dimensional structure containing 4.6 Å × 3.9 Å rectangular one-dimensional pores lined with the ethyl ester groups from the ligand. The presence of the ethyl ester groups makes the pores hydrophobic in nature, as determined by the low heats of adsorption of CH4, CO2, and H2O (14.5, 23.9, and 45 kJ mol–1, respectively) despite the polar and acidic barium phosphonate ester backbone. The ethyl ester groups within the pores also protect CALF-25 from decomposition by water vapor, with crystallinity and porosity being retained after exposure to harsh humid conditions (90% relative humidity at 353 K). The use of phosphonate esters as linkers for the construction of MOFs provides a method to protect hydrolytically susceptible coordination backbones through kinetic blocking.
Overcoming the brittleness of metal–organic frameworks (MOFs) is a challenge for industrial applications. To increase the mechanical strength, MOFs have been blended with polymers to form composites. ...However, this also brings challenges, such as integration and integrity of MOF in the composite, which can hamper the selectivity of gas separations. In this report, an “all MOF” material with mechanical flexibility has been prepared by covalent cross-linking of metal–organic polyhedra (MOPs). The ubiquitous Cu24 isophthalate MOP has been decorated with a long alkyl chain having terminal alkene functionalities so that MOPs can be cross-linked via olefin metathesis using Grubbs second generation catalyst. Different degrees of cross-linked MOP materials have been obtained by varying the amount of catalyst in the reaction. Rheology of these structures with varying number of cross-links was performed to assess the cross-link density and its homogeneity throughout the sample. The mechanical properties were further investigated by the nanoindentation method, which showed increasing hardness with higher cross-link density. Thus, this strategy of cross-linking MOPs with covalent flexible units allows us to create MOFs of increasing mechanical strength while retaining the MOP cavities.
Metal-organic frameworks (MOFs) as solid sorbents for carbon dioxide (CO
) capture face the challenge of merging efficient capture with economical regeneration in a durable, scalable material. ...Zinc-based Calgary Framework 20 (CALF-20) physisorbs CO
with high capacity but is also selective over water. Competitive separations on structured CALF-20 show not just preferential CO
physisorption below 40% relative humidity but also suppression of water sorption by CO
, which was corroborated by computational modeling. CALF-20 has a low enthalpic regeneration penalty and shows durability to steam (>450,000 cycles) and wet acid gases. It can be prepared in one step, formed as composite materials, and its synthesis can be scaled to multikilogram batches.
Using the concept of isomorphous replacement applied to entire ligands, a C 3-symmetric trisulfonate ligand was substituted with a C 3-symmetric tris(hydrogen phosphonate) ligand in a proton ...conducting metal–organic framework (MOF). The resulting material, PCMOF21/2, has its proton conduction raised 1.5 orders of magnitude compared to the parent material, to 2.1 × 10–2 S cm–1 at 90% relative humidity and 85 °C, while maintaining the parent MOF structure.
Understanding the molecular details of CO₂-sorbent interactions is critical for the design of better carbon-capture systems. Here we report crystallographic resolution of CO₂ molecules and their ...binding domains in a metal-organic framework functionalized with amine groups. Accompanying computational studies that modeled the gas sorption isotherms, high heat of adsorption, and CO₂ lattice positions showed high agreement on all three fronts. The modeling apportioned specific binding interactions for each CO₂ molecule, including substantial cooperative binding effects among the guest molecules. The validation of the capacity of such simulations to accurately model molecular-scale binding bodes well for the theory-aided development of amine-based CO₂ sorbents. The analysis shows that the combination of appropriate pore size, strongly interacting amine functional groups, and the cooperative binding of CO₂ guest molecules is responsible for the low-pressure binding and large uptake of CO₂ in this sorbent material.
PDF
