Crystal engineering of hybrid ultramicroporous materials for study of direct air capture of carbon dioxide

Global atmospheric CO2 levels are currently 409 ppm, an increase of 130 ppm since the pre-industrial era. Efficient mitigation strategies combined with advanced carbon capture technologies are required to address this global threat. Direct air capture (DAC) offers an attractive proposition that would facilitate onsite technologies that use CO2 as a feedstock, eliminating the need for storage and transportation. Currently, CO2 scrubbers based on aqueous alkanolamine solutions and amine grafted mesoporous materials are being used for DAC, but they suffer from high regeneration energy. CO2 selective physisorbents have the potential to reduce energy costs of DAC but have until recently not exhibited appropriate selectivity and hydrolytic stability. Crystal engineering, defined as “the field of chemistry that studies the design, properties and application of crystals” has recently enabled the design of a new generation of physisorbents with the pore size and pore chemistry suited for DAC. Specifically, hybrid ultramicroporous materials (HUMs) with inorganic anion pillars that offer strong electrostatics and tight binding sites for CO2 can offer precise control over pore size/chemistry to afford order of magnitude improvement in the carbon capture performance of physisorbents. A pyrazine based HUM, (Zn(pyrazine)2SiF6)n, SIFSIX-3-Zn, reported in 2013 was found to exhibit a new benchmark for CO2/N2 selectivity (> 1800). The primary objective of this study is to prepare and characterise a platform of related HUMs by systematically varying the metal node or the inorganic pillar in order to develop HUMs with the following characteristics: a) high thermal and hydrolytic stability, b) better DAC performance and c) cost-effective synthesis (high yield/low waste). A secondary objective is to gain insight into the reasons for the exceptional carbon capture performance of HUMs

Metal-organic frameworks as regeneration optimized sorbents for atmospheric water harvesting

As the freshwater crisis looms, metal-organic frameworks (MOFs) with stepped isotherms lie at the forefront of desiccant development for atmospheric water harvesting (AWH). Despite numerous studies on water sorption kinetics in MOF desiccants, the kinetics of AWH sorbents  are a challenge to quantify. Here, we report that the AWH kinetics of  seven known MOFs and the industry-standard desiccant Syloid are limited  by diffusion  to the sorbent bed surface. A quantitative model that exploits isotherm  shape enables simulation of sorption cycling to evaluate sorbent  performance through productivity contour plots  (“heatmaps”). These heatmaps reveal two key findings: steady-state  oscillation around partial loading optimizes productivity, and dense  ultramicroporous MOFs with a step at low relative humidity afford superior volumetric  performance under practically relevant temperature swing conditions  (27°C, 30% relative humidity [RH] − 60°C, 5.4% RH). Cellulose-desiccant  composites of two such regeneration optimized sorbents retain the  kinetics of powders, producing up to 7.3 L/kg/day of water under these  conditions.</p

The effect of centred versus offset interpenetration on C2H2 sorption in hybrid ultramicroporous materials

Fine-tuning of hybrid ultramicroporous materials (HUMs) can significantly impact their gas sorption performance. This study reveals that offset interpenetration can be antagonistic with respect to C2H2 separation from C2H2/C2H4 gas mixtures

A coordination network that reversibly switches between two nonporous polymorphs and a high surface area porous phase

We report a 2-fold interpenetrated primitive cubic (pcu) network X-pcu-5-Zn, [Zn2(DMTDC)2(dpe)] (H2DMTDC = 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid, dpe = 1,2-di(4-pyridyl)ethylene), that exhibits reversible switching between an as-synthesized “open” phase, X-pcu-5-Zn-α, and two nonporous or “closed” polymorphs, X-pcu-5-Zn-β and X-pcu-5-Zn-γ. There are two unusual features of X-pcu-5-Zn. The first relates to its sorption properties, which reveal that the α form exhibits high CO2 uptake (ca. 255 cm3/g at 195 K) via reversible closed-to-open switching (type F-IV isotherm) of the type desirable for gas and vapor storage; there are only three other reports of porous materials that combine these two features. Second, we could only isolate the β form by activation of the CO2 loaded α form and it persists through multiple CO2 adsorption/desorption cycles. We are unaware of a new polymorph having been isolated in such a manner. That the observed phase changes of X-pcu-5-Zn-α occur in single-crystal-to-single-crystal fashion enabled structural characterization of the three forms; γ is a coordination isomer of α and β, both of which are based upon “paddlewheel” clusters

Controlling the uptake and regulating the release of nitric oxide in microporous solids

Representative compounds from three classes of microporous solids, namely metal-organic frameworks (MOFs), hybrid ultramicroporous materials (HUMs) and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH2 when compared to its non-amine functionalized parent. The HUMs demonstrated reversible physisorption within the low pressure regime but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH2 and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium

Ultramicropore engineering by dehydration to enable molecular sieving of H2 by calcium trimesate

The high energy footprint of commodity gas purification and ever-increasing demand for gases require new approaches to gas separation. Kinetic separation of gas mixtures through molecular sieving can enable “ideal” separation through molecular size or shape exclusion. Physisorbents must exhibit just the right pore diameter to enable such ideal separation, but the 0.3-0.4 nm range relevant to small gas molecules is hard to control with precision. Herein, we report that dehydration of the ultramicroporous metal-organic framework Ca-trimesate, Ca(HBTC).H2O (H3BTC = trimesic acid), bnn-1-Ca-H2O, affords a narrow pore variant, Ca(HBTC), bnn-1-Ca. Whereas bnn-1-Ca-H2O (pore diameter 0.34 nm) exhibits ultra-high CO2/N2, CO2/CH4 and C2H2/C2H4 binary selectivities, bnn-1-Ca (pore diameter 0.31 nm) offers ideal selectivities for H 2 /CO 2 and H2/N2 under cryogenic conditions. Ca-trimesate, the first physisorbent to exhibit H2 sieving under cryogenic conditions, could be prototypal for a potentially general approach to exert precise control over pore diameter in physisorbents