Ramanathan Vaidhyanathan

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.

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

Phosphonate Monoesters as Carboxylate-like Linkers for Metal Organic Frameworks

Bidentate phosphonate monoesters are analogues of popular dicarboxylate linkers in MOFs, but with an alkoxy tether close to the coordinating site. Herein, we report 3-D MOF materials based upon phosphonate monoester linkers. Cu(1,4-benzenediphosphonate bis(monoalkyl ester), CuBDPR, with an ethyl tether is nonporous; however, the methyl tether generates an isomorphous framework that is porous and captures CO(2) with a high isosteric heat of adsorption of 45 kJ mol(-1). Computational modeling reveals that the CO(2) uptake is extremely sensitive both to the flexing of the structure and to the orientation of the alkyl tether.

Ultralow Parasitic Energy for Postcombustion CO2 Capture Realized in a Nickel Isonicotinate Metal–Organic Framework with Excellent Moisture Stability

Metal-organic frameworks (MOFs) have attracted significant attention as solid sorbents in gas separation processes for low-energy postcombustion CO2 capture. The parasitic energy (PE) has been put forward as a holistic parameter that measures how energy efficient (and therefore cost-effective) the CO2 capture process will be using the material. In this work, we present a nickel isonicotinate based ultramicroporous MOF, 1 [Ni-(4PyC)2·DMF], that has the lowest PE for postcombustion CO2 capture reported to date. We calculate a PE of 655 kJ/kg CO2, which is lower than that of the best performing material previously reported, Mg-MOF-74. Further, 1 exhibits exceptional hydrolytic stability with the CO2 adsorption isotherm being unchanged following 7 days of steam-treatment (>85% RH) or 6 months of exposure to the atmosphere. The diffusion coefficient of CO2 in 1 is also 2 orders of magnitude higher than in zeolites currently used in industrial scrubbers. Breakthrough experiments show that 1 only loses 7% of its maximum CO2 capacity under humid conditions.

Zn7O2(RCOO)10 clusters and nitro aromatic linkers in a porous metal-organic framework.

A new metal-organic framework, CALF-22 comprising Zn7O2(COO)10 secondary building units and 2-nitro-1,4-benzenedicarboxylate, is reported. The porosity and gas adsorption of N2, H2, CO2, and CH4 are studied, and CALF-22 has a surface area in excess of 1000 m(2)/g. The stability of the larger zinc cluster and the effect of the nitro group on gas sorption are also studied.

A triazine–resorcinol based porous polymer with polar pores and exceptional surface hydrophobicity showing CO2 uptake under humid conditions

Several applications including post-combustion carbon capture require capturing CO2 under humid conditions. To obtain a material capable of interacting more strongly with CO2 than water, surface hydrophobicity and polarizing pores have been incorporated simultaneously into an ultra-microporous Bakelite-type polymer comprising of triazine–triresorcinol building units. Being built from C–C bonds, it exhibits exceptional chemical stability (survives conc. HNO3(g) + SO3(g) without losing any porosity). Triazine–phenol lined channels enable adsorption of CO2 (2.8 mmol g−1 with a good selectivity of 120 : 1 (85% N2 : 15% CO2) at 303 K, 1 bar) and the inherent surface hydrophobicity amply minimizes the affinity for H2O. When the adsorption was carried out using a humid CO2 stream (∼50% RH) the material loses only about 5% of its capacity. In a steam-conditioning experiment, the sample was exposed to high humidity (∼75% RH) for a day, and without any further activation, was tested for CO2 adsorption. It retains more than 85% of its CO2 capacity. And this capacity was intact even after 48 h of steam conditioning. The role of phenol in contributing to the surface hydrophobicity is exemplified by the fact that a ∼17% lithiation of the phenolic sites nearly removes all of the surface hydrophobicity. The local structure of the polymer has been modeled using tight-binding DFT methods (Accelrys) and three low energy conformers were identified. Only the CO2 isotherm simulated using the lowest energy conformer matches the experimental isotherm quite well. The triazine–phenol polymer presented here has good hydrophilic–hydrophobic balance, where the basic triazine units and the phenol groups seem to co-operatively assist the CO2 capture under humid conditions. These properties along with its excellent acid stability make the material a suitable candidate for post-combustion CO2 capture. Also, the study presents a new approach for simultaneously introducing polarizing character and surface hydrophobicity into a porous material.

Super-hydrophobic covalent organic frameworks for chemical resistant coatings and hydrophobic paper and textile composites

Covalent organic frameworks are crystalline polymers with modular tunability and ordered pores. If made super-hydrophobic, owing to their flexibility, texture and organic nature, they can be of use in several applications that demand hydrophobic surfaces. Super-hydrophobic surfaces have been developed by introducing micro/nano-asperities on metal surfaces by laser-etching or by nano-structuring their morphologies. Many industrial applications demand super-hydrophobicity under chemically harsh environments, something which such metal-based metastable surfaces cannot guarantee. Evidently, the most abundant are metal-free fluorine based polymer surfaces, but considering long-term environmental benefits developing fluorine-free alternatives is important. Here, porous super-hydrophobic COFs with 2D and pseudo-3D frameworks have been utilized to make coatings with exceptional water-repelling characteristics assisted by their Cassie–Baxter state (contact angle = 163 ± 2°; tilt-angle = 2°, hysteresis = 4°). Importantly, the coatings maintain their super-hydrophobicity even under harsh acidic/basic conditions (pH = 1–14) and towards ice and hot water (80 °C), something where even a lotus leaf fails. Also, their organic nature and fibrous texture enable their facile compositing with paper and textiles. At a mere <5% loading, the COFs seem to pack very well within the cellulose strands of these materials providing a markedly hydrophobic coating to these otherwise completely hydrophilic materials.