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The chemical nature of CO2 adsorption in zeolite A
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0003-3185-3535
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The climate changes are accelerated by increasing levels of carbon dioxide in the atmosphere connected to the fossil-fuel-based energy system. Substantial reforms of the system are needed immediately and could include the implementation of carbon capture and storage (CCS) technologies. Adsorption-driven CO2 capture is one of the most promising post-combustion CO2 capture techniques, which aim to remove CO2 from N2 in flue gas.

The nature of adsorption of CO2 can vary. The process can act as physisorption with intermolecular interactions of the van der Waals type or as chemisorption with a significantly perturbed electronic structure of CO2 and for example the formation of CO32- and HCO3- species. The molecular details were elucidated by MAS NMR and IR studies for a zeolite, and the placement of adsorbed molecules was revealed by in situ diffraction data analysis.

Adsorption-driven processes can be implemented only if highly functional adsorbent materials have been developed. Zeolite A seems to be a promising candidate. This thesis broadly discussed the potential enhancement of the selectivity of CO2 over N2 and CH4 by replacing Na+ with larger monovalent cation e.g. K+ in pore apertures of zeolite A. The positions of the extra-framework cations were analyzed by in situ X-ray diffraction using synchrotron light source. The cations were positioned at the 4- and 6-rings and the 8-ring apertures of the aluminosilicate framework of zeolite A. K+ was favored at the 8-ring sites, and this cation did also gradually substitute the 6-ring sites with and increasing x in |Na12-xKx|-A. Large cations did not fit the mirror plane of the 6-ring and were placed on both its sides. K+ at both positions, in 8-rings and 6-rings, seems to have tailored the size of pore openings.

The effective pore aperture size was shown to depend on the K+ content and to partition small CO2 molecules from large N2 and CH4 because of, likely, differences in diffusivities. Various compositions of |Na12-xKx|-A demonstrated gradual decrease of CO2 uptake with x and an exclusion of N2 and CH4 already for low x. Although already absorbed CO2 molecules were revealed by in situ neutron diffraction to be coordinated mainly by the 8-ring cation or bridging adjacent 8-ring sites. Adsorbed CO2 molecules displaced the cations into the a-cages and resulted in a slight contraction of the overall distribution of extra-framework cations upon the adsorption of CO2.

The kinetically-enhanced separation of CO2 from N2/CH4 seemed to be associated by a restrained diffusion also for the CO2 molecules. This is problematic for pressure swing adsorption processes. However, it could potentially be addressed by the reduction of size of zeolite crystals to increase the extent of accessible porous space over limited time.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University , 2019. , p. 78
Keywords [en]
CO2 adsorption, flue gas CO2 capture, biogas upgrading, zeolite |NaK|-A, physisorption, chemisorption, kinetic separation, in situ diffraction, synchrotron XRD, NPD, NMR, IR
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-162364ISBN: 978-91-7797-438-3 (print)ISBN: 978-91-7797-439-0 (electronic)OAI: oai:DiVA.org:su-162364DiVA, id: diva2:1266017
Public defence
2019-01-11, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: In press. Paper 5: Submitted.

Available from: 2018-12-19 Created: 2018-11-27 Last updated: 2018-12-14
List of papers
1. CO2-Induced Displacement of Na+ and K+ in Zeolite INaKI-A
Open this publication in new window or tab >>CO2-Induced Displacement of Na+ and K+ in Zeolite INaKI-A
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2018 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 30, p. 17211-17220Article in journal (Refereed) Published
Abstract [en]

Adsorption technologies offer opportunities to remove CO2 from gas mixtures, and zeolite A has good properties that include a high capacity for the adsorption of CO2 . It has been argued that its abilities to separate CO2 from N-2 in flue gas and CO2 from CH4 in raw biogas can be further enhanced by replacing Na+ with K+ in the controlling pore window apertures. In this study, several compositions of I Na12-xKxI-A were prepared and studied with respect to the adsorption of CO2 N-2, and CH4, and the detailed structural changes were induced by the adsorption of CO2. The adsorption of CO2 gradually decreased on an increasing content of K+, whereas the adsorption of N-2 and CH4 was completely nulled already at relatively small contents of K. Of the studied samples, INa9K3I-A exhibited the highest CO2 over N-2/CH4 selectivities, with a(CO2/N-2 ) > 21 000 and a(CO2/CH4) > 8000. For samples with and without adsorbed CO2 analyses of powder X-ray diffraction (PXRD) data revealed that K+ preferred to substitute Na+ at the eight-ring sites. The Na(+ )ions at the six-ring sites were gradually replaced by K+ on an increasing content, and these sites split into two positions on both sides of the six-ring mirror plane. It was observed that both the eight-ring and six-ring sites tailored the maximum adsorption capacity for CO2 and possibly also the diffusion of CO2 into the alpha-cavities of INa12-xKxI-A. The adsorption of CH4 and N-2 on the other hand appeared to be controlled by the K+ ions blocking the eight-ring windows. The in situ PXRD study revealed that the positions of the extra-framework cations were displaced into the a-cavities of INa12(_)x,KxI-A on the adsorption of CO2 . For samples with a low content of K+, the repositioning of the cations was consistent with a mutual attraction with the adsorbed CO(2 )molecules.

National Category
Chemical Sciences Nano Technology Materials Engineering
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-158906 (URN)10.1021/acs.jpcc.8b03899 (DOI)000440956200023 ()
Available from: 2018-08-20 Created: 2018-08-20 Last updated: 2018-11-27Bibliographically approved
2. Site-Specific Adsorption of CO2 in Zeolite NaK‑A
Open this publication in new window or tab >>Site-Specific Adsorption of CO2 in Zeolite NaK‑A
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2018 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 47, p. 27005-27015Article in journal (Refereed) Published
Abstract [en]

Zeolite |Na12|-A is a commercial adsorbent, and its CO2-over-N2(CH4) selectivity can be further enhanced kinetically by replacing Na+ in the 8-ring windows that control gas diffusion with large cations. In this study, samples of zeolite |Na12–xKx|-A with x = 0.0, 0.8, 2.0, and 3.0 were prepared, and the positions of adsorbed CO2 molecules were determined using in situ neutron powder diffraction through profile refinement. Adsorbed CO2 molecules were located at three different sites within the large α-cavities in the zeolite structure, revealing the interaction between the adsorbed CO2 and the host framework. The number of CO2 molecules at each site depends on CO2 pressure and follows site-specific CO2 isotherms described with a Langmuir model. Most of the CO2 molecules in zeolite |Na12–xKx|-A bridge two cations at neighboring 8-ring sites. These are relatively weakly physisorbed, and therefore, most of the working capacity of CO2 adsorption is related to this site. The CO2 molecules at the second most populated site are coordinated to a cation in the 8-ring plane. Some of them seemed to form chemical bonds with the O atoms of the framework as carbonate-like species and acted as chemisorption. The remaining minor fraction of CO2 is directly attracted by Na+ at the 6-rings. The different positioning of physisorbed CO2 and the presence of chemisorbed CO2 was confirmed by in situ infrared spectroscopy.

National Category
Physical Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-162362 (URN)10.1021/acs.jpcc.8b09405 (DOI)000451933400025 ()
Available from: 2018-11-27 Created: 2018-11-27 Last updated: 2019-01-03Bibliographically approved
3. The Nature of Chemisorbed CO2 in Zeolite A
Open this publication in new window or tab >>The Nature of Chemisorbed CO2 in Zeolite A
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(English)In: Article in journal (Refereed) Submitted
National Category
Physical Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-162363 (URN)
Available from: 2018-11-27 Created: 2018-11-27 Last updated: 2018-11-27Bibliographically approved
4. Upgrading of raw biogas into biomethane with structured nano-sized zeolite |NaK|-A adsorbents in a PVSA unit
Open this publication in new window or tab >>Upgrading of raw biogas into biomethane with structured nano-sized zeolite |NaK|-A adsorbents in a PVSA unit
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(English)In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102Article in journal (Refereed) In press
National Category
Materials Engineering
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-162364 (URN)
Available from: 2018-11-27 Created: 2018-11-27 Last updated: 2018-12-07
5. Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH4 in Zeolites |Na12|-A and|Na12-xKx|-A
Open this publication in new window or tab >>Intracrystalline Transport Barriers Affecting the Self-Diffusion of CH4 in Zeolites |Na12|-A and|Na12-xKx|-A
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(English)In: Article in journal (Refereed) Submitted
National Category
Physical Chemistry
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-162366 (URN)
Available from: 2018-11-27 Created: 2018-11-27 Last updated: 2018-12-12Bibliographically approved

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