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Spectroscopic Characterization of Adsorbed CO on 3Aminopropylsilyl-modified SBA15 Mesoporous Silica Chia-Hsin Chen, Daphna Shimon, Jason J. Lee, Stephanie A. Didas, Anil K. Mehta, Carsten Sievers, Christopher W Jones, and Sophia E. Hayes Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 3, 2017
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Environmental Science & Technology
Spectroscopic Characterization of Adsorbed 13CO2 on 3-Aminopropylsilyl-modified SBA15 Mesoporous Silica Chia-Hsin Chen, † Daphna Shimon, † Jason J. Lee, § Stephanie A. Didas, § Anil K. Mehta, ∟ Carsten Sievers, § Christopher W. Jones, § Sophia E. Hayes†* †
Department of Chemistry, Washington University in Saint Louis, One Brookings Drive, Saint Louis, Missouri 63130, United States
§
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States
∟
Solid-State NMR Center, Department of Chemistry, Emory University, Georgia 30322, United States
* Phone: 314-935-4624, Fax: 314-935-4481, Email:
[email protected] 1
Abstract
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Multiple chemisorption products are found from the interaction of CO2 with the solid-amine
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sorbent, 3-aminopropyl silane (APS) bound to mesoporous silica (SBA15) using solid-state NMR
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and FTIR spectroscopy. We have employed a combination of both 1
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N{13C} rotational-echo
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C{15N} REDOR to determine the chemical identity of
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double-resonance (REDOR) NMR and
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these products.
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distance of 1.45 Å. In contrast, both
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multiple 13C products. 13C CPMAS shows two neighboring resonances, whose chemical shifts are
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consistent with carbamate (at 165 ppm) and carbamic acid. The
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N{13C} REDOR measurements are consistent with a single 13
C{15N} REDOR and
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C-15N pair and
C CPMAS are consistent with
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C{15N} REDOR experiments
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resonant at 165 ppm show an incomplete buildup of the REDOR data to ~90% of the expected
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maximum. We conclude this 10% missing intensity corresponds to a
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resonates at the identical chemical shift, but that is not in dipolar contact with 15N. These data are
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consistent with the presence of bicarbonate, HCO3–, since it is commonly observed at ~165 ppm
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and lacks 15N for dipolar coupling.
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Keywords
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Introduction
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New solid-state materials for capture of CO2 from high-concentration sources, such as flue gases,
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is an ongoing, essential area of research and drives material and process design for CO2
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separations. Absorption using amine solutions is an available technology for carbon capture1;
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however, new materials, both liquid and solid, with improved performance (e.g., lower
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regeneration energy, less corrosive, lower cost) are actively being sought.
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C NMR species that
C{15N} REDOR, 15N{13C} REDOR, CO2 capture, solid amine sorbent, 15N CPMAS, carbamate
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Solid amine adsorbents are under development because of their low heat capacity and the
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relatively low threshold energy to desorb CO2.2 Amine modified mesoporous silica sorbents are
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promising materials being evaluated for the capture of CO2 released from high-concentration point 2
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sources such as power plants. Research targeted at maximizing the efficiency of CO2 uptake often
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focuses on the material itself, for example, tailoring different types of amines, varying the pore
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size of silica, and adjusting the amine density. Our goal is to study the interaction of CO2 with the
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sorbent material at the molecular level, determining chemisorption products between CO2 and
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amines to help maximize that efficiency.
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The mechanism of aqueous amines reacting with CO2 has been extensively studied.2–5 When
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CO2 becomes absorbed by primary and secondary amines in solution, a carbamate ion pair is
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formed.2,4 In contrast, bicarbonate is the chemisorption product formed by tertiary amines, but only
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when water is present.2,5 This second pathway to bicarbonates is also available to primary and
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secondary amines, yet it is much less favorable.2 Compared to reactions in solution, reactions of
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CO2 with solid amine adsorbents are more complicated as the solid-state host material’s reactivity
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is influenced by several factors, including humidity, partial pressure of CO2 and both the density
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and the nature of amine groups. These mesoporous systems are challenging to structurally
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characterize with conventional analytical techniques such as X-ray diffraction due to the lack of
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crystallinity. FTIR and solid-state NMR are two spectroscopies that can provide information about
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structure, especially the local coordination environment of chemical moieties present in CO2
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chemisorption products, even in amorphous and surface-bound sites.
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The identification of CO2 chemisorption products is complicated by multiple reaction products
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that can form, including two types of carbamate6 (an ion-paired species with an ammonium
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propylsilane moiety “ammonium carbamate ion pair” and a surface-coordinated carbamate
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“silylpropylcarbamate”) as well as bicarbonate.7 Bicarbonate was not found in several previous
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studies,6,8–11 and this species has only been previously observed when the amine groups are present
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at low concentrations (termed “low loading”) in parallel with ammonium carbamate ion pairs.7 In 3
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a recent study employing in-situ FTIR spectroscopy, multiple adsorbed CO2 species were
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postulated.12 The similarity of the vibrational stretching frequencies of the important C=O, NH3+,
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and COO– groups in FTIR spectra creates a situation where the experimental spectra are complex
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and difficult to interpret.
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Solid-state NMR is therefore a complementary tool, particularly well-suited for amorphous or
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disordered materials that can provide quantitative information about the adsorption products. For
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example, in a recent study of an amine-modified mesoporous silica sorbent deemed a
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hyperbranched aminosilica,13 two overlapping CO2 chemisorption products were distinguished by
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the multiple transverse relaxation times (T2) of a single carbon peak.14
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In this study, we used solid-state NMR to interrogate the chemisorbed products of 3-
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aminopropylsilane functionalized mesoporous SBA15 silica [(APS)-SBA15], which captures CO2
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by chemisorption, using three hypothesized chemisorption reactions, shown in Scheme 1.15
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Rotational-echo double-resonance (REDOR) experiments were used to probe the dipolar coupling
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between 13C and 15N enriched species.
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Experimental section
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To synthesize SBA15, Pluronic P-123 (24.0 g) was dissolved in concentrated HCl (120 mL) and distilled
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water (636 mL) in a 2 L Erlenmeyer flask. The solution was then stirred for 3 h at room temperature.
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Afterwards, tetraethyl orthosilicate (46.24 g) was added dropwise to the solution. The solution was then
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stirred continuously for 20 h at 40 °C. The mixture was then quenched and filtered with excess distilled
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water. The resulting white powder was then dried in an oven overnight at 75 °C. Afterwards, the sample
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was put into a calcination oven. To calcine the white powder, it was heated to 200 °C at 1.0 °C min−1, held
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at that temperature for 2 h, heated to 550 °C at 1.0 °C min−1, held at that temperature for 6 h, and finally
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cooled to room temperature.
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To graft 3-aminopropyltrimethoxysilane, SBA15 was evacuated on a Schlenk line at 120 °C at a pressure
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of
