George K. H. Shimizu

Direct Observation and Quantification of CO2 Binding Within an Amine-Functionalized Nanoporous Solid

Designing Carbon Dioxide Traps One widely discussed means of stemming the rise in atmospheric carbon dioxide concentration is to capture the gas prior to its emission and then bury it. The materials currently known to best adsorb CO2 for this purpose tend to involve amine groups; however, the precise molecular details of adsorption often remain murky, and rational improvement of sorbent properties by structural modification has been challenging. Vaidhyanathan et al. (p. 650; see the Perspective by Lastoskie) have crystallographically resolved the binding motifs of CO2 in an amine-bearing metal-organic framework solid. Accompanying theoretical simulations matched the experimental observations. Crystallographic resolution of bound carbon dioxide in a porous solid validates methods of theoretically predicting binding behavior. 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.

Anhydrous proton conduction at 150 °C in a crystalline metal–organic framework

Metal organic frameworks (MOFs) are particularly exciting materials that couple porosity, diversity and crystallinity. But although they have been investigated for a wide range of applications, MOF chemistry focuses almost exclusively on properties intrinsic to the empty frameworks; the use of guest molecules to control functions has been essentially unexamined. Here we report Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. Proton conduction in β-PCMOF2 was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores and reached 5 × 10(-4) S cm(-1) at 150 °C in anhydrous H(2), as confirmed by electrical measurements in H(2) and D(2), and by solid-state NMR spectroscopy. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)(0.45)) was also incorporated into a H(2)/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.

Facile Proton Conduction via Ordered Water Molecules in a Phosphonate Metal−Organic Framework

A new phosphonate metal-organic framework (MOF) with a layered motif but not that of the classical hybrid inorganic-organic solid is presented. Zn(3)(L)(H(2)O)(2)·2H(2)O (L = [1,3,5-benzenetriphosphonate](6-)), henceforth denoted as PCMOF-3, contains a polar interlayer lined with Zn-ligated water molecules and phosphonate oxygen atoms. These groups serve to anchor free water molecules into ordered chains, as observed by X-ray crystallography. The potential for proton conduction via the well-defined interlayer was studied by (2)H solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H(2) was measured as 3.5 × 10(-5) S cm(-1) at 25 °C and 98% relative humidity. More interestingly, an Arrhenius plot gave a low activation energy of 0.17 eV for proton transfer, corroborating the solid-state NMR data that showed exchange between all deuterium sites in the D(2)O analogue of PCMOF-3, even at -20 °C.

A Water-Stable Metal–Organic Framework with Highly Acidic Pores for Proton-Conducting Applications

Metal-organic framework (MOF) materials are a nontraditional route to ion conductors, but their crystallinity can give insight into molecular-level transport mechanisms. However, some MOFs can be structurally compromised in humid environments. A new 3D metal-organic framework, PCMOF-5, is reported which conducts protons above 10(-3) S/cm at 60 °C and 98% relative humidity. The MOF contains free phosphonic acid groups, shows high humidity stability, and resists swelling in the presence of hydration. Channels filled with crystallographically located water and acidic groups are also observed.

The supramolecular chemistry of the sulfonate group in extended solids

Abstract Organosulfonates (RSO 3 − ) are largely regarded as poor ligands by coordination chemists, and have typically been employed as ‘non coordinating’ anions in past synthetic and structural work. Indeed, the majority of transition metal aqua complexes with sulfonate counter anions show that the sulfonate group cannot readily displace water from the coordination sphere of the metal ion. There exists a strong structural analogy between the RSO 3 − and the phosphonate RPO 3 2− group, a functionality which has been employed with great success in the generation of functional extended architectures. The contrast lies in the fact that individual metal–oxygen interactions with the sulfonate group are weaker, when employed with suitably soft metal cations, but may be employed cooperatively, to still yield a stable solid. This review deals with some of the functional frameworks we have reported which take advantage of the inherent coordinative pliancy of the sulfonate group to generate extended networks. Features of the discussed frameworks include sponge-like guest sorption, anion exchange, and topotactic intercalation.

Proton Conduction with Metal-Organic Frameworks

Further development of microporous crystalline materials as proton conductors may lead to better electrolyte membranes for fuel cells. Proton-exchange membrane fuel cells (PEMFCs) generate electricity because the electrons generated by the reaction of hydrogen and oxygen must travel through an external circuit; the membrane electrolyte only transfers protons. The membrane materials of choice have been ionomeric polymers, such as sulfonated fluoropolymers (Nafion), that achieve proton conductivities of up to 1 S cm−1, but the requirement to keep these materials hydrated limits their operating temperature and efficiency. Metal-organic frameworks (MOFs), in which inorganic assemblies are joined by organic linkers, have inherent porosity that could be exploited for the development of proton-conducting membranes. Among recent studies of experimental proton-conducting MOFs [e.g., (1)], two general targets for PEMFC operation have emerged: developing better materials for operations under humid conditions (below 100°C), and developing efficient anhydrous proton conductors that could unlock the cost efficiencies enabled by humidity-independent operation above 100°C.

Enhancing proton conduction in a metal-organic framework by isomorphous ligand replacement.

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, PCMOF2½, 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.

A Water Stable Magnesium MOF That Conducts Protons over 10–2 S cm–1

From the outset of the study of MOFs as proton conductors, both conductivity and hydrolytic robustness of the materials have needed to be improved. Here, we report a layered magnesium carboxyphosphonate framework, PCMOF10, that shows an extremely high proton conductivity value of 3.55 × 10(-2) S·cm(-1) at 70 °C and 95% RH. Moreover, PCMOF10 is water stable owing to strong Mg phosphonate bonding. The 2,5-dicarboxy-1,4-benzenediphosphonic acid (H6L) linker anchors a robust backbone and has hydrogen phosphonate groups that interact with the lattice water to form an efficient proton transfer pathway.

Enhancing Water Stability of Metal–Organic Frameworks via Phosphonate Monoester Linkers

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 CH(4), CO(2), and H(2)O (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.

Competition and Cooperativity in Carbon Dioxide Sorption by Amine-Functionalized Metal–Organic Frameworks†

Alkylamines, such as monoethanolamine, are used to scrub CO2 molecules from flue gas streams, however, as they form strong chemical bonds (85–105 kJmol ), the post-capture recovery of the amine is energy-intensive (130–150 8C including heating the entire aqueous solution). Alternatively, the use of less-basic amines, such as aryl amines, could favor strong physisorption (30–50 kJmol ) with CO2, rather than chemisorption. This would mean a porous compound with such amine groups could give easy-on/easy-off reversible CO2 capture balanced with selectivity. To obtain high efficiency at lower partial pressures, the material, along with having strong CO2 binding sites, needs to have reasonable surface area for capacity. Metal–organic frameworks (MOFs) are widely studied for gas sorption owing to the ability to modify pore sizes, shapes, and surfaces. Functionalizing with specific interaction sites is being actively studied as a route to selective gas capture. Computational modeling can give tremendous insight to the sorption properties of a MOF. We recently reported a zinc aminotriazolato oxalate MOF, {Zn2(Atz)2(ox)} (2), exhibiting amine-lined pores and a high heat of adsorption for CO2 (ca. 40 kJmol ). Further studies showed that the CO2 binding sites could be located crystallographically. These data offered an exceptional opportunity to validate a suite of computational methods to model not only the CO2 isotherm, but also the locations of binding sites and role of specific interactions to the overall CO2 binding enthalpy. The present study applies these methods to understanding CO2 uptake in another MOF, {Zn3(Atz)3(PO4)} (1), that intuitively should give better CO2 capture properties. In comparison to {Zn2(Atz)2(ox)}, only two-thirds of the number of trianionic phosphate groups are required to charge compensate [Zn(Atz)] layers, so larger, amine-lined pores were anticipated and observed. Despite this, the CO2 uptake (at 273 K) and heat of adsorption do not exceed those of 2. The computational methods provide crucial insight to understanding these phenomena and demonstrate the wide spread applicability of such techniques to ascertain binding details in MOFs not directly accessible by experiment. Although the role of the amine functionalities in 1 is surprisingly diminished, the cooperative interactions between CO2 molecules are found to augment overall binding by over 7 kJmol , a significant result for CO2 capture in any porous material. Solvothermal reaction of basic ZnCO3 with 3-amino-1,2,4triazole, H3PO4, and NH4OH gave {Zn3Atz3(PO4)(H2O)3.5}, 1·(H2O)3.5, in both single-crystal and bulk phases (Supporting Information, Figure S1). The aminotriazole ligand has been employed to construct otherMOFs, including with Zn ions, but has not been extensively studied for CO2 capture excepting 2. 1·(H2O)3.5 is made up of cationic Zn–Atz layers pillared by PO4 anions to form a 3D porous network (Figure 1). The Zn(Atz) layers lie in the ac plane and contain three independent Zn ions and Atz ligands. No amine groups coordinate to Zn ions; ligation is exclusively through triazole nitrogen atoms. Pillaring of these layers by the phosphate ions results in a 3D network of pores (accounting for van der