Monolithic metal–organic frameworks for carbon dioxide separation†

Carbon dioxide (CO2) is both a primary contributor to global warming and a major industrial impurity. Traditional approaches to carbon capture involve corrosive and energy-intensive processes such as liquid amine absorption. Although adsorptive separation has long been a promising alternative to traditional processes, up to this point there has been a lack of appropriate adsorbents capable of capturing CO2 whilst maintaining low regeneration energies. In the context of CO2 capture, metal–organic frameworks (MOFs) have gained much attention in the past two decades as potential materials. Their tuneable nature allows for precise control over the pore size and chemistry, which allows for the tailoring of their properties for the selective adsorption of CO2. While many candidate materials exist, the amount of research into material shaping for use in industrial processes has been limited. Traditional shaping strategies such as pelletisation involve the use of binders and/or mechanical processes, which can have a detrimental impact on the adsorption properties of the resulting materials or can result in low-density structures with low volumetric adsorption capacities. Herein, we demonstrate the use of a series of monolithic MOFs (monoUiO-66, monoUiO-66-NH2 & monoHKUST-1) for use in gas separation processes

Highly selective trace ammonium removal from dairy wastewater streams by aluminosilicate materials

Water is a key solvent, fundamental to supporting life on earth. It is equally important in many industrial processes, particularly within agricultural and pharmaceutical industries, which are major drivers of the global economy. The results of water contamination by common activity in these industries is well known and EU Water Quality Directives and Associated Regulations mandate that NH4+ concentrations in effluent streams should not exceed 0.3 mg L−1, this has put immense pressure on organisations and individuals operating in these industries. As the environmental and financial costs associated with water purification begin to mount, there is a great need for novel processes and materials (particularly renewable) to transform the industry. Current solutions have evolved from combating toxic sludge to the use of membrane technology, but it is well known that the production of these membrane technologies creates a large environmental footprint. Zeolites could provide an answer; their pore size and chemistry enable efficient removal of aqueous based cations via simple ion exchange processes. Herein, we demonstrate efficient removal of NH4+ via both static and dynamic methodology for industrial application. Molecular modelling was used to determine the cation–framework interactions which will enable customisation and design of superior sorbents for NH4+ capture in wastewater

Enhanced Stability toward Humidity in a Family of Hybrid Ultramicroporous Materials Incorporating Cr₂O₇^2– Pillars

Dichromate (Cr₂O₇^2–) pillared <b>pcu</b> hybrid ultramicroporous materials, while previously shown to exhibit benchmark selectivity for small polarizable gases, sometimes suffer from poor stability when exposed to moisture, which could limit their potential application in gas separation systems. In attempting to improve their stability toward humidity, we have crystal engineered two new families of <b>DICRO-L-M-i</b> materials of formula [M­(L)₂(Cr₂O₇)]_<i>n</i> (M = Ni²⁺, Co²⁺; L = <b>5</b>: 1,4-bis­(4-pyridyl)­xylene; <b>6</b>: 1,4-bis­(4-pyridyl)­durene). Evaluating these materials in combination with a previously reported analogue, <b>DICRO-4-Ni-i</b>, in terms of their stability toward humidity has revealed a relationship between increasing the number of methyl groups on the dipyridyl organic linkers and a greater stability toward humidity

Immobilization of a polar sulfone moiety onto the pore surface of a humid stable MOF for highly efficient CO2 separation under dry and wet environment through direct CO2-sulfone interaction

The stability of microporous metal–organic frameworks (MOFs) in moist environments must be taken into consideration for their practical implementations, which has been largely ignored thus far. Herein, we synthesized a new moisture-stable Zn-MOF, {[Zn2(SDB)2(L)2]·2DMA}n, IITKGP-12, by utilizing a bent organic linker 4,4′-sulfonyldibenzoic acid (H2SDB) containing a polar sulfone group (−SO2) and a N, N-donor spacer (L) with a Brunauer–Emmett–Teller surface area of 216 m2 g–1. This material displays greater CO2 adsorption capacity over N2 and CH4 with high IAST selectivity, which is also validated by breakthrough experiments with longer breakthrough times for CO2. Most importantly, the separation performance is largely unaffected in the presence of moisture of simulated flue gas stream. Temperature-programmed desorption (TPD) analysis shows the ease of the regeneration process, and the performance was verified for multiple cycles. In order to understand the structure–function relationship at the atomistic level, grand canonical Monte Carlo (GCMC) calculation was performed, indicating that the primary binding site for CO2 is between the sulfone moieties in IITKGP-12. CO2 is attracted to the bonded structure (V-shape) of the sulfone moieties in a perpendicular fashion, where CCO2 is aligned with S, and the CO2 axis bisects the SO2 axis. Thus, the strategic approach to immobilize the polar sulfone moiety with a high number of inherent stronger M–N coordination and the absence of coordination unsaturation made this MOF potential toward practical CO2 separation applications

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

Metal–organic material polymer coatings for enhanced gas sorption performance and hydrolytic stability under humid conditions

Physisorbent metal–organic materials (MOMs) have shown benchmark performance for highly selective CO2 capture from bulk and trace gas mixtures. However, gas stream moisture can be detrimental to both adsorbent performance and hydrolytic stability. One of the most effective methods to solve this issue is to transform the adsorbent surface from hydrophilic to hydrophobic. Herein, we present a facile approach for coating MOMs with organic polymers to afford improved hydrophobicity and hydrolytic stability under humid conditions. The impact of gas stream moisture on CO2 capture for the composite materials was found to be negligible under both bulk and trace CO2 capture conditions with significant improvements in regeneration times and energy requirements

supporting information from 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, <b>Zeolite 13X</b> and a chemisorbent, <b>TEPA-SBA-15</b>, 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, <b>SIFSIX-3-Cu</b>, <b>DICRO-3-Ni-i</b>, <b>SIFSIX-2-Cu-i</b> and <b>MOOFOUR-1-Ni</b>; five microporous MOMs, <b>DMOF-1</b>, <b>ZIF-8</b>, <b>MIL-101</b>, <b>UiO-66</b> and <b>UiO-66-NH₂</b>; an ultramicroporous MOM, <b>Ni-4-PyC</b>. 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

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

Amino functionalised hybrid ultramicroporous materials that enable single-step ethylene purification from a ternary mixture

Pyrazine-linked hybrid ultramicroporous (pore size <7 Å) materials (HUMs) offer benchmark performance for trace carbon capture thanks to strong selectivity for CO2 over small gas molecules, including light hydrocarbons. That the prototypal pyrazine-linked HUMs are amenable to crystal engineering has enabled second generation HUMs to supersede the performance of the parent HUM, SIFSIX-3-Zn, mainly through substitution of the metal and/or the inorganic pillar. Herein, we report that two isostructural aminopyrazine-linked HUMs, MFSIX-17-Ni (17 = aminopyrazine; M = Si, Ti), which we had anticipated would offer even stronger affinity for CO2 than their pyrazine analogs, unexpectedly exhibit reduced CO2 affinity but enhanced C2H2 affinity. MFSIX-17-Ni are consequently the first physisorbents that enable single-step production of polymer grade (>99.95% for SIFSIX-17-Ni) ethylene from a ternary equimolar mixture of ethylene, acetylene and CO2 thanks to coadsorption of the latter two gases. We attribute this performance to the very different binding sites in MFSIX-17-Ni versus SIFSIX-3-Zn