Evaluation of physical adsorbents for post-combustion CO2 capture

CO2 capture using physical adsorbents such as activated carbon, zeolites, and metal-organic frameworks (MOFs) have attracted a lot of attention because of the technical capability of adsorption technology using such separation agents to retrofit the energy demanding liquid amine scrubbing process. Nevertheless, this will be conceivable only if the separation agents (adsorbents) in question fulfil many unambiguous attributes in terms of porosity, CO2 affinity, kinetics, energetics, stability, throughout the right capture mechanism, in addition to the adsorbents cost. In this paper we report recent study about the evaluation of physical adsorbents for CO2 capture from simulated flue gas. Extensive CO2/N2:10/90 mixed gas adsorption at 298 and 328 K, in the presence of water vapor and other impurities, were carried out using both temperature and pressure swing regeneration modes. For comparison purpose, absorption in liquid amines and adsorption on 13X zeolites are considered as the reference technologies for post-combustion CO2 capture

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

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Made-to-order metal-organic frameworks for trace carbon dioxide removal and air capture

International audienceDirect air capture is regarded as a plausible alternate approach that, if economically practical, can mitigate the increasing carbon dioxide emissions associated with two of the main carbon polluting sources, namely stationary power plants and transportation. Here we show that metal-organic framework crystal chemistry permits the construction of an isostructural metal-organic framework (SIFSIX-3-Cu) based on pyrazine/copper(II) two-dimensional periodic 44 square grids pillared by silicon hexafluoride anions and thus allows further contraction of the pore system to 3.5 versus 3.84 Å for the parent zinc(II) derivative. This enhances the adsorption energetics and subsequently displays carbon dioxide uptake and selectivity at very low partial pressures relevant to air capture and trace carbon dioxide removal. The resultant SIFSIX-3-Cu exhibits uniformly distributed adsorption energetics and offers enhanced carbon dioxide physical adsorption properties, uptake and selectivity in highly diluted gas streams, a performance, to the best of our knowledge, unachievable with other classes of porous materials

Quest for Highly Connected MOFPlatforms: Rare-Earth Polynuclear Clusters Versatility Meets Net Topology Needs

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Imaging defects and their evolution in a metal–organic framework at sub-unit-cell resolution

© 2019, The Author(s), under exclusive licence to Springer Nature Limited. Defect engineering of metal–organic frameworks (MOFs) offers promising opportunities for tailoring their properties to specific functions and applications. However, determining the structures of defects in MOFs—either point defects or extended ones—has proved challenging owing to the difficulty of directly probing local structures in these typically fragile crystals. Here we report the real-space observation, with sub-unit-cell resolution, of structural defects in the catalytic MOF UiO-66 using a combination of low-dose transmission electron microscopy and electron crystallography. Ordered ‘missing linker’ and ‘missing cluster’ defects were found to coexist. The missing-linker defects, reconstructed three-dimensionally with high precision, were attributed to terminating formate groups. The crystallization of the MOF was found to undergo an Ostwald ripening process, during which the defects also evolve: on prolonged crystallization, only the missing-linker defects remained. These observations were rationalized through density functional theory calculations. Finally, the missing-cluster defects were shown to be more catalytically active than their missing-linker counterparts for the isomerization of glucose to fructose

Imaging defects and their evolution in a metal-organic framework at sub-unit-cell resolution

Defect engineering of metal–organic frameworks (MOFs) offers promising opportunities for tailoring their properties to specific functions and applications. However, determining the structures of defects in MOFs—either point defects or extended ones—has proved challenging owing to the difficulty of directly probing local structures in these typically fragile crystals. Here we report the real-space observation, with sub-unit-cell resolution, of structural defects in the catalytic MOF UiO-66 using a combination of low-dose transmission electron microscopy and electron crystallography. Ordered ‘missing linker’ and ‘missing cluster’ defects were found to coexist. The missing-linker defects, reconstructed three-dimensionally with high precision, were attributed to terminating formate groups. The crystallization of the MOF was found to undergo an Ostwald ripening process, during which the defects also evolve: on prolonged crystallization, only the missing-linker defects remained. These observations were rationalized through density functional theory calculations. Finally, the missing-cluster defects were shown to be more catalytically active than their missing-linker counterparts for the isomerization of glucose to fructose

Amine-functionalized mesoporous silica: A material capable of CO2 adsorption and fast regeneration by microwave heating

The surface of ordered mesoporous (MCM-48) silica has been subjected to covalent grafting with silane molecules containing one to three amino groups. The dielectric properties of the materials were studied in detail, and the functionalized materials were used for CO2 adsorption at room temperature, followed by regeneration under either conventional heating or microwave irradiation. It has been found that, as the intensity of functionalization with amino groups increases (from mono- to tri-amino silanes) both the CO2 load and the dielectric response at microwave frequencies increase. In particular, functionalization with a tri-amino silane derivative gave the highest CO2 adsorption and the fastest microwave heating, resulting in a fourfold acceleration of adsorbent regeneration. The grafted material was fully stable for at least 20 adsorption-regeneration cycles, making it an ideal candidate for microwave-swing adsorption (MWSA) processes. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 547-555, 2016Financial support from the European Research Council ERC-Advanced Grant HECTOR is gratefully acknowledged. Hakan Nigar also acknowledges financial support from the Spanish Ministry of Education for the FPU grant (Formacion del Profesorado Universitario-FPU12/06864).Nigar, H.; García-Baños, B.; Penaranda-Foix, FL.; Catalá Civera, JM.; Mallada, R.; Santamaria, J. (2016). Amine-functionalized mesoporous silica: A material capable of CO2 adsorption and fast regeneration by microwave heating. AIChE Journal. 62(2):547-555. https://doi.org/10.1002/aic.15118S54755562

Performance of polyethyleneimine–silica adsorbent for post-combustion CO2 capture in a bubbling fluidized bed

The high performance of polyethyleneimine (PEI)-based solid adsorbent for CO2 capture has been well recognized in thermogravimetric analysis (TGA) and small-scale fixed bed reactors through the measurements of their equilibrium capacities but has not been really demonstrated on larger scales towards practical utilization. In the present study, a laboratory-scale bubbling fluidized bed reactor loaded with a few kg adsorbent is used to evaluate the adsorption performance of PEI–silica adsorbent under different working conditions including with/without the presence of moisture, different gas–solid contact times, initial bed temperatures, and CO2 partial pressures. The adsorption capacities have shown a clear degradation tendency under dry condition. However, they can be stabilized at a high level of 10.6–11.1% w/w over 60 cycles if moisture (ca. 8.8 vol%) is present in the gas flow during adsorption and desorption. Breakthrough capacities can be stabilized at the level of 7.6–8.2% w/w with the gas–solid contact time of 13 s. The adsorption capacities for the simulated flue gases containing 5% CO2 are only slightly lower than those for the simulated flue gases containing 15% CO2, indicating that the PEI–silica adsorbent is suitable for CO2 capture from flue gases of both coal-fired and natural gas-fired combined cycle power plants. The exothermal heat of adsorption is estimated by the energy balance in the fluidized bed reactor and found to be close (within 10%) to the measured value by TG-DSC. The regeneration heat for the as prepared PEI–silica adsorbent is found to be 2360 kJ/kgCO2 assuming 75% recovery of sensible heat which is well below the values of 3900–4500 kJ/kgCO2 for a typical MEA scrubbing process with 90% recovery of sensible heat

Differential guest location by host dynamics enhances propylene/propane separation in a metal-organic framework

Energy-efficient approaches to propylene/propane separation such as molecular sieving are of considerable importance for the petrochemical industry. The metal organic framework NbOFFIVE-1-Ni adsorbs propylene but not propane at room temperature and atmospheric pressure, whereas the isostructural SIFSIX-3-Ni does not exclude propane under the same conditions. The static dimensions of the pore openings of both materials are too small to admit either guest, signalling the importance of host dynamics for guest entrance to and transport through the channels. We use ab initio calculations together with crystallographic and adsorption data to show that the dynamics of the two framework-forming units, polyatomic anions and pyrazines, govern both diffusion and separation. The guest diffusion occurs by opening of the flexible window formed by four pyrazines. In NbOFFIVE-1-Ni, (NbOF5)2- anion reorientation locates propane away from the window, which enhances propylene/propane separation