Amrit Kumar

Direct Air Capture of CO2 by Physisorbent Materials

Sequestration of CO2, either from gas mixtures or directly from air (direct air capture, DAC), could mitigate carbon emissions. Here five materials are investigated for their ability to adsorb CO2 directly from air and other gas mixtures. The sorbents studied are benchmark materials that encompass four types of porous material, one chemisorbent, TEPA-SBA-15 (amine-modified mesoporous silica) and four physisorbents: Zeolite 13X (inorganic); HKUST-1 and Mg-MOF-74/Mg-dobdc (metal-organic frameworks, MOFs); SIFSIX-3-Ni, (hybrid ultramicroporous material). Temperature-programmed desorption (TPD) experiments afforded information about the contents of each sorbent under equilibrium conditions and their ease of recycling. Accelerated stability tests addressed projected shelf-life of the five sorbents. The four physisorbents were found to be capable of carbon capture from CO2-rich gas mixtures, but competition and reaction with atmospheric moisture significantly reduced their DAC performance.

Tuning Pore Size in Square‐Lattice Coordination Networks for Size‐Selective Sieving of CO2

Porous materials capable of selectively capturing CO2 from flue-gases or natural gas are of interest in terms of rising atmospheric CO2 levels and methane purification. Size-exclusive sieving of CO2 over CH4 and N2 has rarely been achieved. Herein we show that a crystal engineering approach to tuning of pore-size in a coordination network, [Cu(quinoline-5-carboxyate)2 ]n (Qc-5-Cu) ena+bles ultra-high selectivity for CO2 over N2 (SCN ≈40 000) and CH4 (SCM ≈3300). Qc-5-Cu-sql-β, a narrow pore polymorph of the square lattice (sql) coordination network Qc-5-Cu-sql-α, adsorbs CO2 while excluding both CH4 and N2 . Experimental measurements and molecular modeling validate and explain the performance. Qc-5-Cu-sql-β is stable to moisture and its separation performance is unaffected by humidity.

Hydrophobic pillared square grids for selective removal of CO2 from simulated flue gas

Capture of CO2 from flue gas is considered to be a feasible approach to mitigate the effects of anthropogenic emission of CO2. Herein we report that an isostructural family of metal organic materials (MOMs) of general formula [M(linker)2(pillar)], linker = pyrazine, pillar = hexaflourosilicate and M = Zn, Cu, Ni and Co exhibits highly selective removal of CO2 from dry and wet simulated flue gas. Two members of the family, M = Ni and Co, SIFSIX-3-Ni and SIFSIX-3-Co, respectively, are reported for the first time and compared with the previously reported Zn and Cu analogs.

Bimodal Functionality in a Porous Covalent Triazine Framework by Rational Integration of an Electron-Rich and -Deficient Pore Surface.

A porous covalent triazine framework (CTF) consisting of both an electron-deficient central triazine core and electron-rich aromatic building blocks is reported. Taking advantage of the dual nature of the pore surface, bimodal functionality has been achieved. The electron deficiency in the central core has been utilized to address one of the pertinent problems in chemical industries, namely separation of benzene from its cyclic saturated congener, that is, cyclohexane. Also, by virtue of the electron-rich aromatic rings with Lewis basic sites, aqueous-phase chemical sensing of a nitroaromatic compound of highly explosive nature (2,4,6-trinitrophenol; TNP) has been achieved. The present compound supersedes the performance of previously reported COFs in both the aspects. Notably, this reports the first example of pore-surface engineering leading to bimodal functionality in CTFs.

Flue-gas and direct-air capture of CO2 by porous metal-organic materials.

Sequestration of CO2, either from gas mixtures or directly from air (direct air capture), is a technological goal important to large-scale industrial processes such as gas purification and the mitigation of carbon emissions. Previously, we investigated five porous materials, three porous metal–organic materials (MOMs), a benchmark inorganic material, Zeolite 13X and a chemisorbent, TEPA-SBA-15, for their ability to adsorb CO2 directly from air and from simulated flue-gas. In this contribution, a further 10 physisorbent materials that exhibit strong interactions with CO2 have been evaluated by temperature-programmed desorption for their potential utility in carbon capture applications: four hybrid ultramicroporous materials, SIFSIX-3-Cu, DICRO-3-Ni-i, SIFSIX-2-Cu-i and MOOFOUR-1-Ni; five microporous MOMs, DMOF-1, ZIF-8, MIL-101, UiO-66 and UiO-66-NH2; an ultramicroporous MOM, Ni-4-PyC. The performance of these MOMs was found to be negatively impacted by moisture. Overall, we demonstrate that the incorporation of strong electrostatics from inorganic moieties combined with ultramicropores offers improved CO2 capture performance from even moist gas mixtures but not enough to compete with chemisorbents. This article is part of the themed issue ‘Coordination polymers and metal–organic frameworks: materials by design’.

Water Vapor Sorption in Hybrid Pillared Square Grid Materials

We report water vapor sorption studies on four primitive cubic, pcu, pillared square grid materials: SIFSIX-1-Cu, SIFSIX-2-Cu-i, SIFSIX-3-Ni, and SIFSIX-14-Cu-i. SIFSIX-1-Cu, SIFSIX-3-Ni, and SIFSIX-14-Cu-i were observed to exhibit negative water vapor adsorption at ca. 40-50% relative humidity (RH). The negative adsorption is attributed to a water-induced phase transformation from a porous pcu topology to nonporous sql and sql-c* topologies. Whereas the phase transformation of SIFSIX-1-Cu was found to be irreversible, SIFSIX-3-Ni could be regenerated by heating and can therefore be recycled. In contrast, SIFSIX-2-Cu-i, which is isostructural with SIFSIX-14-Cu-i, exhibited a type V isotherm and no phase change. SIFSIX-2-Cu-i was observed to retain both structure and gas sorption properties after prolonged exposure to heat and humidity. The hydrolytic stability of SIFSIX-2-Cu-i in comparison to its structural counterparts is attributed to structural features and therefore offers insight into the design of hydrolytically stable porous materials.

Photochromism of novel chromenes constrained to be part of [2.2]paracyclophane: remarkable ‘phane’ effects on the colored o-quinonoid intermediates

The photochemistry of rationally designed chromenes that are constrained to be part of [2.2]paracyclophane, i.e., CP-H and CP-OMe, was investigated to examine the effect of through-space delocalization in the cyclophane core (phane effect) on the photochromic behavior. In contrast to the parent chromene, i.e., 2,2-diphenylbenzopyran CH, for which the photoinduced coloration is not observable at room temperature, the cyclophanochromenes CP-H and CP-OMe lend themselves to readily observable photochromism. The photogenerated o-quinonoid intermediates that are responsible for the observed color revert slowly to permit kinetic monitoring of their decays. The difference in the kinetic rate constants for reversion of the photogenerated o-quinonoid intermediates derived from CP-H and CP-OMe attests to remarkable through-space delocalization. To the best of our knowledge, the observed results constitute the first examples of the influence of phane effects on the stabilization or otherwise of the reactive intermediates.

Impact of partial interpenetration in a hybrid ultramicroporous material on C2H2/C2H4 separation performance

Phases of a 2-fold pcu hybrid ultramicroporous material (HUM), SIFSIX-14-Cu-i, exhibiting 99%, 93%, 89%, and 70% partial interpenetration have been obtained. 1 : 99 C2H2/C2H4 gas separation studies reveal that as the proportion of interpenetrated component decreases, so does the separation performance.

Efficient CO2 Removal for Ultra-Pure CO Production by Two Hybrid Ultramicroporous Materials

Removal of CO2 from CO gas mixtures is a necessary but challenging step during production of ultra-pure CO as processed from either steam reforming of hydrocarbons or CO2 reduction. Herein, two hybrid ultramicroporous materials (HUMs), SIFSIX-3-Ni and TIFSIX-2-Cu-i, which are known to exhibit strong affinity for CO2 , were examined with respect to their performance for this separation. The single-gas CO sorption isotherms of these HUMs were measured for the first time and are indicative of weak affinity for CO and benchmark CO2 /CO selectivity (>4000 for SIFSIX-3-Ni). This prompted us to conduct dynamic breakthrough experiments and compare performance with other porous materials. Ultra-pure CO (99.99 %) was thereby obtained from CO gas mixtures containing both trace (1 %) and bulk (50 %) levels of CO2 in a one-step physisorption-based separation process.

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 ultra-microporous 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.