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Temperature-Induced Uptake of CO2 and Formation of Carbamates in Mesocaged Silica Modified with n-Propylamines
Stockholm University. (Department of Materials and Environmental Chemistry)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials. (Nanoteknologi och funktionella material)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials. (Nanoteknologi och funktionella material)
Stockholm University. (Department of Materials and Environmental Chemistry)
2010 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 26, no 12, 10013-10024 p.Article in journal (Refereed) Published
Abstract [en]

Adsorption-mediated CO2 separation can reduce the costs of carbon capture and storage. The reduction in cost requires adsorbents with high capacities for CO2 sorption and high CO2-over-N2 selectivity. Amine-modified sorbents are promising candidates for carbon capture. To investigate the details of CO2 adsorption in such materials, we studied mesocaged (cubic, Pm3n symmetry) silica adsorbents with tethered propylamines using Fourier transform infrared (FTIR) spectroscopy and volumetric uptake experiments. The degree of heterogeneity in these coatings was varied by either co-synthesizing or post-synthetically introducing the propylamine modification. In situ FTIR spectroscopy revealed the presence of both physisorbed and chemisorbed CO2 in the materials. We present the first direct molecular evidence for physisorption using FTIR spectroscopy in mesoporous silica sorbents modified with propylamines. Physisorption reduced the CO2-over-N2 selectivity in amine-rich sorbents. Samples with homogenous coatings showed typical CO2 adsorption trends and large quantities of IR-observable physisorbed CO2. The uptake of CO2 in mesocaged materials with heterogeneous propylamine coatings was higher at high temperatures than at low temperatures. At higher temperatures and low pressures, the post-synthetically modified materials adsorbed more CO2 than did the extracted ones, even though the surface area after modification was clearly reduced and the coverage of primary amine groups was lower. The principal mode of CO2 uptake in post-synthetically modified mesoporous silica was chemisorption. The chemisorbed moieties were present mainly as carbamate–ammonium ion pairs, resulting from the quantitative transformation of primary amine groups during CO2 adsorption as established by NIR spectroscopy. The heterogeneity in the coatings promoted the formation of these ion pairs. The average propylamine–propylamine distance must be small to allow the formation of carbamate–propylammonium ion pairs.

Place, publisher, year, edition, pages
2010. Vol. 26, no 12, 10013-10024 p.
Keyword [en]
carbon dioxide, propylamine, adsorption, IR, FTIR, silica, mesoporous
National Category
Materials Engineering
Research subject
Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-121987DOI: 10.1021/la1001495ISI: 000278427600114OAI: oai:DiVA.org:uu-121987DiVA: diva2:308065
Available from: 2010-04-06 Created: 2010-04-03 Last updated: 2016-04-18Bibliographically approved
In thesis
1. Novel Syntheses, Structures and Functions of Mesoporous Silica Materials
Open this publication in new window or tab >>Novel Syntheses, Structures and Functions of Mesoporous Silica Materials
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The field of mesoporous silica has been studied for about 20 years but it is still an area attracting a lot of attention. The use of novel templating molecules and several issues related to the synthesis and fine structural details are still poorly understood. These aspects are of special relevance to the theme of this thesis, which includes novel work on three fronts; the synthesis, characterization and applications of mesoporous materials.

The work described in this thesis aims to contribute to the mesoporous field by developing novel methods of mesoporous silica synthesis without relying on surfactant micelles as the templating agent but focusing instead on the stacking arrangement of aromatic molecules such as folic acid. The novel route presented here leads to 2D hexagonal structures with p6mm symmetry possessing high mesoporosity and large surface areas. The versatility of this route at various synthesis temperatures and using hydrothermal treatments has also been investigated.

A novel strategy is also proposed for the synthesis of mesocaged materials with Pm3n symmetry structures. The mechanism relies on the penetration of the neutral propylamino moiety of a co-structure directing agent into the hydrophobic core of the surfactant micelles. Beside these novel pathways, the effect of hydrothermal treatment (HT) at 100 oC on the 3D cubic Ia3d structure (AMS-6) over a long period of time was also examined, and the results show a phase transformation from a 3D cubic Ia3d to a 2D hexagonal p6mm structure and a return to the 3D cubic Ia3d structure at a later stage in the synthesis. This unexpected result is discussed.

In this work, the detailed structural characterization of mesoporous materials using electron microscopy techniques is an important task. In particular, to extend previous knowledge, the fine structural details of mesocaged materials possessing Pm3n symmetry prepared with various amphiphilic surfactants under acidic and alkaline conditions has been investigated using electron crystallography and sorption studies. The results show subtle fine structural differences with materials prepared under alkaline conditions exhibiting the largest mesocage sizes. The cage and window sizes are primarily determined by the charge density of the surfactant and the thickness of the hydration layer surrounding the surfactant micelles.

The relationship between the mesoporous structure and its function has been investigated by evaluating the rate of release of amphiphilic molecules, used as model molecules, from the internal pore structures of mesoporous materials with different pore geometries. In a similar study, the rate of proton diffusion from a liquid surrounding the mesoporous nanoparticles into the pore system of AMS-n was also assessed. The results show that the diffusion coefficients for the proton absorption process are higher than those for the release of the surfactant template molecules, with more complex 3D mesocaged particles showing the highest diffusion coefficients in both cases.

Finally, the quantity of CO2 adsorption was measured by modifying the internal surfaces of mesocaged material with n-propylamino groups. Results show that the cage-connecting window sizes limit the surface coverage of n-propylamino groups by pore blocking and affect the volume of CO2 adsorption. In addition, at the molecular level, CO2 adsorption shows physisorption or chemisorption depending on the localized distribution of n-propylamino groups, as studied by in-situ infrared spectroscopy.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 82 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 733
National Category
Materials Engineering
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-122289 (URN)978-91-554-7786-8 (ISBN)
Public defence
2010-05-21, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2010-04-28 Created: 2010-04-07 Last updated: 2010-05-18

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Atluri, RambabuGarcia-Bennett, Alfonso E.

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