Kenji Sumida

Carbon Dioxide Capture in Metal–Organic Frameworks

Kenji Sumida, David L. Rogow, Jarad A. Mason, Thomas M. McDonald, Eric D. Bloch, Zoey R. Herm, Tae-Hyun Bae, Jeffrey R. Long

Synthesis and Hydrogen Storage Properties of Be12(OH)12(1,3,5-benzenetribenzoate)4

The first crystalline beryllium-based metal-organic framework has been synthesized and found to exhibit an exceptional surface area useful for hydrogen storage. Reaction of 1,3,5-benzenetribenzoic acid (H(3)BTB) and beryllium nitrate in a mixture of DMSO, DMF, and water at 130 degrees C for 10 days affords the solvated form of Be(12)(OH)(12)(1,3,5-benzenetribenzoate)(4) (1). Its highly porous framework structure consists of unprecedented saddle-shaped [Be(12)(OH)(12)](12+) rings connected through tritopic BTB(3-) ligands to generate a 3,12 net. Compound 1 exhibits a BET surface area of 4030 m(2)/g, the highest value yet reported for any main group metal-organic framework or covalent organic framework. At 77 K, the H(2) adsorption data for 1 indicate a fully reversible uptake of 1.6 wt % at 1 bar, with an initial isosteric heat of adsorption of -5.5 kJ/mol. At pressures up to 100 bar, the data show the compound to serve as an exceptional hydrogen storage material, reaching a total uptake of 9.2 wt % and 44 g/L at 77 K and of 2.3 wt % and 11 g/L at 298 K. It is expected that reaction conditions similar to those reported here may enable the synthesis of a broad new family of beryllium-based frameworks with extremely high surface areas.

Evaluation of cation-exchanged zeolite adsorbents for post-combustion carbon dioxide capture

A series of zeolite adsorbents has been evaluated for potential application in post-combustion CO2 capture using a new high-throughput gas adsorption instrument capable of measuring 28 samples in parallel. Among the zeolites tested, Ca-A exhibits the highest CO2 uptake (3.72 mmol g−1 and 5.63 mmol cm−3) together with an excellent CO2 selectivity over N2 under conditions relevant to capture from the dry flue gas stream of a coal-fired power plant. The large initial isosteric heat of adsorption of −58 kJ mol−1 indicates the presence of strong interactions between CO2 and the Ca-A framework. Neutron and X-ray powder diffraction studies reveal the precise location of the adsorption sites for CO2 in Ca-A and Mg-A. A detailed study of CO2 adsorption kinetics further shows that the performance of Ca-A is not limited by slow CO2 diffusion within the pores. Significantly, Ca-A exhibited a higher volumetric CO2 uptake and CO2/N2 selectivity than Mg2(dobdc) (dobdc4− = 1,4-dioxido-2,5-benzenedicarboxylate; Mg-MOF-74, CPO-27-Mg), one of the best performing adsorbents. The exceptional performance of Ca-A was maintained in CO2 breakthrough simulations.

Hydrogen storage properties and neutron scattering studies of Mg2(dobdc)—a metal–organic framework with open Mg2+ adsorption sites

The hydrogen storage properties of Mg(2)(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate), a metal-organic framework possessing hexagonal one-dimensional channels decorated with unsaturated Mg(2+) coordination sites, have been examined through low- and high-pressure adsorption experiments, infrared spectroscopy, and neutron scattering studies.

Impact of Metal and Anion Substitutions on the Hydrogen Storage Properties of M‑BTT Metal! Organic Frameworks

Microporous metal-organic frameworks are a class of materials being vigorously investigated for mobile hydrogen storage applications. For high-pressure storage at ambient temperatures, the M(3)[(M(4)Cl)(3)(BTT)(8)](2) (M-BTT; BTT(3-) = 1,3,5-benzenetristetrazolate) series of frameworks are of particular interest due to the high density of exposed metal cation sites on the pore surface. These sites give enhanced zero-coverage isosteric heats of adsorption (Q(st)) approaching the optimal value for ambient storage applications. However, the Q(st) parameter provides only a limited insight into the thermodynamics of the individual adsorption sites, the tuning of which is paramount for optimizing the storage performance. Here, we begin by performing variable-temperature infrared spectroscopy studies of Mn-, Fe-, and Cu-BTT, allowing the thermodynamics of H(2) adsorption to be probed experimentally. This is complemented by a detailed DFT study, in which molecular fragments representing the metal clusters within the extended solid are simulated to obtain a more thorough description of the structural and thermodynamic aspects of H(2) adsorption at the strongest binding sites. Then, the effect of substitutions at the metal cluster (metal ion and anion within the tetranuclear cluster) is discussed, showing that the configuration of this unit indeed plays an important role in determining the affinity of the framework toward H(2). Interestingly, the theoretical study has identified that the Zn-based analogs would be expected to facilitate enhanced adsorption profiles over the compounds synthesized experimentally, highlighting the importance of a combined experimental and theoretical approach to the design and synthesis of new frameworks for H(2) storage applications.

Emerging applications of metal–organic frameworks

Metal–organic frameworks are a unique class of materials well known for their crystallinity and ultra-high porosity. Since their first report over fifteen years ago, research in this area has sought to actively exploit these properties, especially in gas adsorption. In this article we canvass some emerging topics in the field of MOF research that show promise for new applications in areas such as biotechnology, catalysis, and microelectronics.

Design of Superhydrophobic Porous Coordination Polymers through the Introduction of External Surface Corrugation by the Use of an Aromatic Hydrocarbon Building Unit

We demonstrate a new approach to superhydrophobic porous coordination polymers by incorporating an anisotropic crystal morphology featuring a predominant surface that is highly corrugated and terminated by aromatic hydrocarbon moieties. The resulting low-energy surface provides particularly promising hydrophobic properties without the need for postsynthetic modifications or surface processing that would block the porosity of the framework. Consequently, hydrophobic organic molecules and water vapor are able to penetrate the surface and be densely accommodated within the pores, whereas bulk water is repelled as a result of the exterior surface corrugation derived from the aromatic surface groups. This study provides a new strategy for the design and development of superhydrophobic porous materials.

Hierarchical structuring of metal–organic framework thin-films on quartz crystal microbalance (QCM) substrates for selective adsorption applications

Continuous stepwise liquid-phase epitaxial (LPE) growth is one of the most effective procedures for structuring metal–organic frameworks (MOFs) as two-dimensional superstructures, such as thin-films. Alternation of the building block precursors between the individual LPE growth cycles (i.e. from one linker to the other) allows heterostructured MOF films consisting of two different MOFs with different structural or chemical properties to be synthesized with a precise control of the growth sequence. Here, we employ the LPE growth strategy for the preparation of highly functional, hierarchically structured core–shell architectures consisting of [Zn4O(3,5-dialkylcarboxypyrazolate)3]n-based frameworks. Specifically, the small-pore [Zn4O(3-methyl-5-isopropyl-4-carboxypyrazolate)3]n (Zn-MI) and [Zn4O(3,5-diethyl-4-carboxypyrazolate)3]n (Zn-DE) frameworks are respectively deposited as a size selective layer upon larger-pore [Zn4O(3,5-dimethyl-4-carboxypyrazolate)3]n (Zn-DM) and [Zn4O(3-methyl-5-ethyl-4-carboxypyrazolate)3]n (Zn-ME) layers. Direct growth of the MOF layers on the Au surfaces of quartz crystal microbalance (QCM) sensors allowed the adsorption properties of the heterostructures to be probed in real-time. Multiple-component adsorption experiments in an environment-controlled QCM apparatus revealed size selectivity with respect to the adsorption of alcohols, as well as the molecular recognition of methanol over water. These properties stem from the positioning of the small-pore Zn-MI (or Zn-DE) layer on the larger-pore Zn-DM (or Zn-ME) layer, facilitating attractive synergistic properties for adsorptive selectivity and providing a possibility for further development in MOF-based sensing applications.

Porphyrin complexes containing coordinated BOB groups: synthesis, chemical reactivity and the structure of [BOB(tpClpp)]2+

The reactions of boron halides with free base porphyrins under conditions where partial hydrolysis of the boron halides can occur give diboron porphyrin complexes containing BOB moieties in which each boron is bonded to two porphyrin nitrogen atoms. BF(3).OEt(2) with H(2)(por) gives B(2)OF(2)(por) (por = tpp, ttp, tpClpp, oep) which has an asymmetric structure in which one boron lies in the porphyrin plane (B(ip)) while the other lies above it (B(oop)). BCl(3).MeCN with H(2)(por) gives B(2)O(2)(BCl(3))(2)(por) which contains a four-membered B(2)O(2) ring and is stable only in the presence of excess BCl(3). BBr(3) with Li(2)(tpClpp) gives the dicationic complex [B(2)O(tpClpp)](2+) as its [BBr(4)](-) salt, and is the first example of a boron porphyrin containing three-coordinate boron to be structurally characterised. B(2)O(2)(BCl(3))(2)(por) can be chromatographed on basic alumina to give the hydroxyboron complex B(2)O(OH)(2)(por), which is deduced from its NMR spectra and DFT calculations to have a structure analogous to B(2)OF(2)(por). The OH protons are shifted upfield to near delta -4 (B(oop)-OH) and -10 (B(ip)-OH) by the diamagnetic porphyrin ring current. The reaction of either B(2)O(2)(BCl(3))(2)(por) or B(2)O(OH)(2)(por) (por = ttp, tpClpp) with alcohols (ROH, R = Et, 4-C(6)H(4)CH(3)) gives B(2)O(OR)(2)(por), which can in turn be converted to B(2)O(OR)(OH)(por) by repeated chromatography. The reaction of PhBCl(2) with H(2)(por) (por = ttp, tpClpp) gives B(2)O(Ph)(OH)(por) which has been characterised by spectroscopy in concert with DFT calculations. It is a further example of the B(2)OF(2)(por) structural type, in which the phenyl group is coordinated to the out-of-plane boron and the OH group to the in-plane boron, as are its derivatives B(2)O(Ph)(X)(tpClpp) (X = F, OEt). Steric drivers for the facile hydrolysis of haloboron porphyrins relative to their dipyrromethene and expanded porphyrin counterparts are discussed.

Impact of Molecular Clustering inside Nanopores on Desorption Processes

Understanding the sorption kinetics of nanoporous systems is crucial for the development and design of novel porous materials for practical applications. Here, using a porous coordination polymer/quartz crystal microbalance (PCP/QCM) hybrid device, we investigate the desorption of various vapor molecules featuring different degrees of intermolecular (hydrogen bonding) or molecule-framework interactions. Our findings reveal that strong intermolecular interactions lead to the desorption process proceeding via an unprecedented metastable state, wherein the guest molecules are clustered within the pores, causing the desorption rate to be temporarily slowed. The results demonstrate the considerable impact of the chemical nature of an adsorbate on the kinetics of desorption, which is also expected to influence the efficiency of certain processes, such as desorption by gas purge.