Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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15
N Solid State NMR Spectroscopic Study of Surface Amine Groups for Carbon Capture: 3‑Aminopropylsilyl Grafted to SBA-15 Mesoporous Silica
Daphna Shimon,† Chia-Hsin Chen,† Jason J. Lee,‡ Stephanie A. Didas,‡ Carsten Sievers,‡ Christopher W. Jones,‡ and Sophia E. Hayes*,† †
Department of Chemistry, Washington University in St. Louis, St. Louis, Missouri 63130, United States School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
‡
S Supporting Information *
ABSTRACT: Materials composed of high-porosity solid supports, such as SBA-15, containing amine-bearing moieties inside the pores, such as 3-aminopropylsilane (APS), are envisioned for carbon dioxide capture; solid-state 15N NMR can be highly informative for studying chemisorption reactions. Two 15N-enriched samples with different APS loadings were studied to probe the identity of the pendant molecules and structure of the chemisorbed CO2 species. 15N crosspolarization magic-angle spinning NMR provides unique information about the amines, whether they are rigid or dynamic, by measuring contact time curves and rotating frame, T1ρ(15N), relaxation. Both carbamate and carbamic acid are formed; carbamic acid is shown to be less stable than carbamate. After desorption, a steady state for the chemisorbed reaction product is reached, leaving behind carbamate. 15 N NMR monitors the evolution of the species over time. During desorption, APS is regenerated, but the ammonium propylsilane intensity does not change, leading us to conclude that carbamic acid desorbs, while carbamate (to which ammonium propylsilane is ion paired) persists. A secondary ditehtered amine present does not react with CO2, and we posit this may be due to its rigidity. These findings demonstrate the versatility of solid-state NMR to provide information about these complex CO2 reactions with solid amine sorbents.
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INTRODUCTION
In amine solutions, the reactions between the CO2 gas and the amines have been extensively investigated.1,19−21 However, the mechanisms of CO2 adsorption and capture in the solidsupported materials are not yet fully understood, with the support potentially restricting the mobility of the amines relative to their states in aqueous solution. There have been relatively few mechanistic studies of the reactions of CO2 and supported amines at the molecular level, most of them using Fourier transform IR (FTIR) spectroscopy, and more recently nuclear magnetic resonance (NMR) spectroscopy.1,18,22−35 Understanding the routes for CO2 adsorption and characterizing the different surface species formed are critical to designing more efficient materials that can be put to widespread use. In the early literature, the mechanism of CO2 chemisorption and product formation was assumed to be essentially identical to that found in amine solutions, i.e., the formation of carbamate, carbamic acid, and bicarbonate. However, more work on the subject has shown that in solids things are more complex, and
Carbon dioxide (CO2) is a byproduct of fuel combustion, and is released from many sources. The negative consequences of accumulating CO2 in the atmosphere are well established, prompting worldwide research on CO2 mitigation. One of the most intriguing proposals to address such mitigation is to trap CO2 at its exit point, a method called carbon capture and storage (CCS).1 The CO2 can then be stored or potentially used as a feedstock chemical, ultimately to reduce the amount released into the atmosphere. Currently, CCS is conducted by passing power plant flue gas through large columns of aqueous amine solutions that capture the CO2 and let other gases pass through.1−5 However, this method is inefficient due to the high energy required for amine regeneration (CO2 desorption), high costs of operation and corrosiveness of the amine solutions.1,2,6,7 In an effort to circumvent these problems, it has been proposed that instead of using amine solutions, it may be advantageous to use solid sorbents.1 One class of proposed materials for this application are high porosity supports containing amine-bearing moieties inside the pores.1,8−18 These supported amines act as adsorbents for CO2, and can require less energy to remove the CO2 than in the aqueous amine solutions. © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
September 4, 2017 November 19, 2017 December 19, 2017 December 19, 2017 DOI: 10.1021/acs.est.7b04555 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
3-aminopropylsilane via postgrafting amination. Two samples with different surface concentrations of 15N-APS were synthesized. Both loading levels of APS possess an additional 15 N species that was introduced as a consequence of the synthetic route, identified as a ditethered secondary amine.35 Samples. Two 15N-APS SBA-15 samples were used in this study: low loading (LL) containing ∼1 mmol N/g and high loading (HL), containing ∼1.5 mmol N/g, which will be referred to as LL-15N-APS SBA-15 and HL-15N-APS SBA-15, respectively. Sample Characterization. Nitrogen gas physisorption isotherms for SBA-15, the HL-15N-APS SBA-15 sample and the LL-15N-APS SBA-15 sample are shown in the Supporting Information (SI). Elemental analysis results of the HL-15N-APS SBA-15 sample and the LL-15N-APS SBA-15 sample, determining the weight percent of carbon, nitrogen, hydrogen, and bromine are also shown in the SI.35 FTIR Spectroscopy. FTIR spectra of the HL-15N-APS SBA-15 sample comparing it to a nonisotopically enriched APS SBA-15 sample have been previously published in Chen et al.35 Additional FTIR spectra of the HL-15N-APS SBA-15 sample after activation and before and after loading with CO2 are shown in the SI. Samples Activation and Exposure to 13CO2. HL-15N-APS SBA-15 (high-loading): The samples were activated by heating at 110 °C under vacuum at 40 mTorr for ∼4 h in a roundbottom flask using a Schlenk line. The samples were then cooled to room temperature, and transferred to a continuous-flow nitrogen-purged glovebag, where they were packed into zirconia MAS NMR rotors. For 13CO2 (Sigma-Aldrich, 99% 13CO2) loading, the sample-filled rotor was placed in a glass tube and connected to a gas manifold, the sample was then evacuated to 40 mTorr, subsequently dosed with 1 atm 13CO2, and allowed to adsorb the gas for 10 h before removal from the glass tube and insertion of the rotor top-cap, quickly, in ambient air. In one case a rotor spacer with an O-ring was inserted before inserting the rotor top-cap. This will be discussed later below. For the quantitative spectrum, the sample was not activated. The material was stored in an inert environment, under continuousflow nitrogen-purged glovebag before packing into a rotor and performing the experiment. LL-15N-APS SBA-15 (low-loading): These samples were not activated or intentionally loaded with 13CO2. The material was stored in a continuous-flow nitrogen-purged glovebag, and as a result had limited contact to CO2 in the air. The sample was packed into a zirconia MAS NMR rotor under ambient air, and thus exposed to the CO2 in the air during the packing and experiments. Despite this, no measurable chemisorbed products were detected. Note that activation at a higher temperature (>130 °C) can result in urea formation, which we avoided by working at 110 °C.41,42 Also note, all samples almost certainly possess a small amount of residual water that cannot be removed without risking damage to the sorbent. NMR Experiments. All NMR experiments were performed on an Oxford 89 mm bore 6.93 T superconducting magnet with a Tecmag (Houston, TX) Apollo spectrometer equipped with either a 4 mm HXY or a 5 mm HXY MAS Chemagnetics probe operating at 15N, 13C, and 1H frequencies of ∼29.90 MHz, ∼ 74.17 MHz, and ∼294.97 MHz, respectively. The HL-15NAPS SBA-15 experiments were recorded with the 4 mm probe, and the LL-15N-APS SBA-15 experiments with the 5 mm probe.
that each one of these species can exist in several forms. Both FTIR and NMR studies have identified the formation of several types of carbamic acid (monomer, dimer, and surface bound) and at least two types of carbamate (stabilized by ion-pairing to either an ammonium species, or a surface silanol group).22−26,35 Moore and co-workers observed the formation of multiple chemisorption products on hyperbranched amine polymers impregnated in SBA-15 mesoporous silica, and it was concluded that the additional adsorption product could either be another type of carbamate (in addition to ammonium carbamate) or bicarbonate.17,22 Bicarbonate is known to form in the presence aqueous solutions that contain primary, secondary, or tertiary amines;1,17,21 however, its existence in supported samples containing primary amines and secondary amines (such as those studied by Moore et al.) has been difficult to verify. Didas provided evidence for the formation of bicarbonate materials containing low loadings of 3-aminopropyl groups on mesoporous silica by in situ FTIR spectroscopy.18 Very recently, Chen et al. found evidence that is consistent with bicarbonate formation in a material comprised of 3-aminopropylsilane grafted on SBA-15 mesoporous silica (a primary amine sample) using solid-state NMR distance measurements.35 Foo and co-workers used FTIR spectra of 3-aminopropylsilane or methylaminopropylsilane in SBA-15 to suggest that two types of carbamic acid exist (one monomer and one dimer) and that two types of carbmate also exist in these samples.25 They showed that in both cases the amounts of each carbamic acid and of each carbamate depend on whether the sample contains primary or secondary amines.25 In another recent paper, Mafra co-workers identified a third type of moisture-sensitive carbamic acid with the combination of 13C NMR and DFT calculations.24 This carbamic acid is only visible in the 13C spectrum under extremely dry conditions. Additionally, in that work they proposed, using experiments and a computer model, that the species previously assigned by NMR as carbamate (stabilized by ion-pair to an ammonium propylsilane) in multiple studies over the past decade,1,18,25−28,30−32,35−38 is in fact a “carbamate-like” hybrid between carbamate and carbamic acid, stabilized by both hydrogen bonds and partial charges. Other computational work from Hahn et al. confirmed the formation of carbamic acid and ammonium carbamate via DFT calculations.39 In this work, 15N NMR spectroscopy has been conducted on two 15N-enriched samples of 3-aminopropylsilane (APS) functionalized mesoporous SBA-15 silica (APS SBA-15) with different APS loadings to probe the structure of chemisorbed CO2 species. Solid-state 13C NMR spectroscopy on these samples revealed the presence of carbamate and carbamic acid.35 Solid-state 15N NMR spectroscopy helps reveal details about the CO2 adsorption that we and others have been unable to observe using 13 C NMR, such as the nature of the pendant molecules before and after CO2 binding. The 15N NMR results detailed here provide a window into the processes of CO2 adsorption and desorption, which gives us insight into the reactions taking place in the system.
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MATERIALS AND METHODS Synthesis. 15N-enriched 3-aminopropylsilane (APS) grafted to SBA-15 was synthesized according to the method described by Moschetta et al.40 and Chen et al.35 Briefly, after preparing the SBA-15, the organic pendant molecules were added by grafting 3-bromopropylsilanes to the SBA-15 surface and then reacting them with 15N-enriched ammonia gas (98% enriched, Cambridge Isotope Laboratories, Inc.) to form B
DOI: 10.1021/acs.est.7b04555 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Scheme 1. Possible 15N Species in Our Samples and Their 15 N Resonance Frequencya
Dry air was used for the bearing and drive of the magic-angle spinning (MAS). Unless otherwise noted, the experiments were performed using cross-polarization (CP) from 1H to 15N nuclei with heteronuclear decoupling, known as 15N{1H}-CPMAS. A 2 μs or a 5 μs π/2 1H excitation pulse was used, for the HL-15N-APS SBA-15 (4 mm probe) and the LL-15N-APS (5 mm probe), respectively. The 1H−15N CP Hartman-Hahn radio frequency (RF) field strength was ∼17−20 kHz. The contact time was 0.5 ms unless otherwise specified. Contact time (CT) curves were recorded by measuring the 15N signal and varying the length of the CT, tCT. The curves were fitted with 2 exponential functions, resulting in the CP buildup time, TIS, and the decay which is the 1H rotating-frame spin−lattice relaxation time, T1ρ(1H). The 15N rotating-frame spin−lattice relaxation time, T1ρ(15N), was recorded by measuring the 15N signal and varying the 15N spin-locking time, tSL after the CP step. Eightstep phase cycling and a recycle delay of 2 s were used in all CPMAS experiments. A single quantitative direct-detect 15 N-MAS NMR spectrum was recorded, with a 6.5 μs π/2 15 N excitation pulse. Four-step phase cycling and a recycle delay of 60 s were used, in order to ensure that the nuclei were fully relaxed between scans (T1’s of
