Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation

Abstract

The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal-organic materials (MOMs). Here we report that a crystal engineering or reticular chemistry strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF(2-)(6)) anions enables a 'sweet spot' of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

A Fine-Tuned Fluorinated MOF Addresses the Needs for Trace CO2 Removal and Air Capture Using Physisorption

“…mol -1 , F…F dist.= 6.483(1) Å) revealed that the CO 2 is more strongly adsorbed at lower concentrations when the size of the square-shaped channel is further contracted, as a result of the shortened distance between the pendant fluorine moieties from diagonally adjacent (SiF 6 ) 2-pillars. 17,30,31 Considerately, in addition to enabling the MOF pore system characteristics, contracted squareshaped channels with a periodically aligned proximal fluorine moieties, two other parameters were considered for the design and construction of the looked-for hydrolytically stable MOF for carbon capture. Specifically, we elected: i) to employ the Ni(II) as the octahedrally-coordinated metal node since the SIFSIX-3-Ni offers a shorter M-F bond distance (d M-F = 1.99(5) Å) 32 then the SIFSIX-3-Cu (d M-F = 2.12(1) Å) 17 and thus prone to express the requisite water vapor tolerance, and ii) to substitute the (SiF 6 ) 2-pillar with an appropriate fluorinated inorganic pillar, offering a relatively stronger coordination bonds and the adequate nucleophilicity that can preclude the observed phase change in the SIFSIX-3-Cu, associated with the water molecule introduction within the Cu(II) coordination sphere.…”

Low concentration CO2 capture using physical adsorbents: Are metal–organic frameworks becoming the new benchmark materials?

“…CO 2 selectivity is a product of a complex interplay between the porosity (pore size), kinetics and the charge density of the adsorbent. 10 The charge density is directly related to CO 2 interactions with the adsorbent framework. 11,12 (iv) Mild conditions for regeneration: The ability to regenerate the adsorbents is a key parameter in the selection of materials for CO 2 separation.…”

“…6 We focus our analysis and discussion on examples of materials (mainly physical adsorbents) that show the best compromise in terms of the intensity of CO 2 energetics 13 as well as the uniformity of interactions. 10 (v) High stability: The adsorbents lifetime, which determines the frequency of their replacement, has a direct impact on the economics of any commercial-scale operation. The stability of the material is a key property that should be considered from the early stage of synthesis and evaluation.…”

“…Several types of MOFs have been proposed for CO 2 capture, including (i) MOFs with open metal sites, [25][26][27][28][29][30][31][32][33][34][35][36][37] (ii) MOFs without open metals sites, [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] (iii) MOFs with narrow pore size via interpenetration 10,56,57 or shortening the size of the ligands, 10,41 and (iv) MOFs decorated with specific functional groups, including (NH 2 , OH, etc.). 23,51,[58][59][60][61][62][63][64] Functionalization of these types of MOFs may be carried out by post-synthetic modification (PSM) on the open metal sites, 47,[65][66]…”

A smart microelectromechanical sensor and switch triggered by gas

“…to traditional organics and polymers. [23][24][25][26][27] Drop-on-demand inkjet printer with a nozzle of 20 lm diameter is used to coat the microstructure. Fig.…”