Characterization of a Mixture of CO2 Adsorption Products in

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Characterization of a Mixture of CO Adsorption Products in Hyperbranched Aminosilica Adsorbents by C Solid-State NMR 13

Jeremy K Moore, Miles A. Sakwa-Novak, Watcharop Chaikittisilp, Anil K. Mehta, Mark S. Conradi, Christopher W Jones, and Sophia Eugenie Hayes Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02930 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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Environmental Science & Technology

1

Characterization of a Mixture of CO2 Adsorption

2

Products in Hyperbranched Aminosilica Adsorbents

3

by 13C Solid-State NMR

4

Jeremy K. Moore†, Miles Sakwa-Novak∟, Watcharop Chaikittisilp∟, Anil K. Mehta§, Mark S.

5

Conradi‡, †, Christopher W. Jones∟, Sophia E. Hayes†,*

6 †

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Department of Chemistry, Washington University, One Brookings Drive, Saint Louis,

8

Missouri, 63130, United States ‡

9

Department of Physics, Washington University, One Brookings Drive, Saint Louis, Missouri,

10

63130, United States ∟

11

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst

12

Drive, Georgia 30332, United States §

13

Department of Chemistry, Emory University, Georgia 30322, United States

14 15

*

Corresponding author

16

ACS Paragon Plus Environment


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17

ABSTRACT:

18

Hyperbranched amine polymers (HAS) grown from mesoporous silica (SBA-15) exhibit large

19

capacities for CO2 adsorption (hereafter “SBA-15-HAS”). We have used static in situ and

20

magic-angle spinning (MAS) ex situ

21

adsorption of CO2 by SBA-15-HAS.

22

the chemisorbed species. HAS polymers possess primary, secondary, and tertiary amines,

23

leading to multiple chemisorption reaction outcomes, including carbamate (RnNCOO–),

24

carbamic acid (RnNCOOH), and bicarbonate (HCO3–) moieties. Carbamates and bicarbonate

25

fall within a small

26

including carbamic acid and carbamate, the former disappearing upon evacuation of the sample.

27

By examining the

28

polarization MAS NMR, carbamate is confirmed through splitting of the

29

species that is either bicarbonate or a second carbamate is evident from bimodal T2 decay times

30

of the ~163 ppm peak, indicating the presence of two species comprising that single resonance.

31

The mixture of products suggests: 1) the presence of amines and water leads to bicarbonate being

32

present, and/or 2) the multiple types of amine sites in HAS permit formation of chemically

33

distinct carbamates.

13

13

13

C nuclear magnetic resonance (NMR) to examine

13

C NMR distinguishes the gas-phase

13

CO2 signal from

C chemical shift range (162-166 ppm), and a mixture was observed

C-14N dipolar coupling through low field (B0=3 T)

34 35 36 37 38


13

C{1H} cross-

13

C resonance. A 3rd

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For Table of Contents only

40 41

42 43

Keywords:

44 45

13

C CPMAS NMR, 13C-14N dipolar coupling, hyperbranched aminosilicate, mesoporous silica SBA-15, in situ NMR, CO2 adsorption, carbon capture

46



47

INTRODUCTION:

48

The rising concentration of atmospheric CO2 has caused many to envision improvements to

49

current carbon capture and sequestration strategies.1–3 Existing CO2 capture technologies for

50

fossil fuel-based power plants focus on aqueous amine solutions, such as monoethanolamine

51

(MEA),2 owing to its high heat of adsorption, -84 kJ mol-1, and fast reaction kinetics.4,5 There

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are many drawbacks to these solutions including chemical degradation, low loading capacities,

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and high energy consumption for regeneration.1 Solid supported amines have been proposed as

54

candidates that can overcome some of these challenges and lead to more efficient CO2 capture.3

55

The amine adsorbents have been categorized into three classes.4 Class 1 adsorbents are

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characterized by support structures containing a physically adsorbed amine-containing molecule

57

or polymer.6 Class 2 adsorbents are defined by amine-containing small molecules that are

58

covalently bonded to the support structure.7,8 Class 3 amine adsorbents are characterized by

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structures with an amine-containing polymer covalently bonded to the support surface, typically

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prepared by in situ polymerization, which affords a combination of class 1 and class 2

61

properties.9,10 The amine-containing polymer structure allows class 1 and class 3 materials to

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have a high amine density inside the silica support. The covalent bonds between the support

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structure and amine-containing molecule in class 2 and class 3 materials increase the physical

64

stability of these adsorbing materials.4

65

In this study, a hyperbranched aminopolymer, created by in situ polymerization of aziridine

66

monomers, is covalently bonded onto a mesoporous silica SBA-15 support to form a class 3

67

adsorbent referred to as hyperbranched aminosilica (HAS).9 This class of materials is able to

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adsorb CO2 at low temperatures (ambient temperature and up to 120 oC) compared to other

69

sorbents, with a strong CO2 heat of adsorption that is comparable to other amine chemisorption


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materials (-45 to -96 kJ mol-1).3,4,11 This strong chemisorption interaction, coupled with the

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hyperbranched structure, which can potentially fill the whole pore volume of the silica support

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with primary, secondary, and tertiary adsorbing amine groups, yields a high CO2 uptake

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capacity. The CO2 capacities of the materials, described in the Supporting Information Table S1,

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are comparable to those of other supported amine adsorbents reported in the literature. Advanced

75

design of both the host support and amine can lead to improved adsorption performance,

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approaching the state of the art in CO2 adsorption. The SBA/HAS material here is meant for use

77

as a model for the study of CO2/amine interactions, rather than the design of the highest

78

performing material.

79

A detailed understanding of the CO2/amine chemistry, and new methods for studying the

80

interaction, are important for the conceptual design of optimal amine structures for a given CO2

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separation application. For example, if the mixture of CO2/amine adsorption products varied with

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the nature of the amine (primary, secondary, tertiary), or the spacing of amines relative to one

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and other or the surface of the material to which they are bound, new amine polymers or

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molecules could be designed and developed to target a particular product mixture that might

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require a lower regeneration energy, or be less prone to a particular degradative mechanism.

86

Nearly all the published insights related to CO2/amine interactions at the molecular level in solid

87

adsorbents has been performed using FTIR. NMR offers a powerful, but underutilized, tool for

88

studying the CO2/amine interactions on these solid adsorbents materials.

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There are multiple possible chemisorption reactions--examples for primary amine sites are seen

90

in Scheme 1--that lead to different adsorption products.3–5,12–16 The structure of these products

91

along with typical 13C solution-state NMR isotropic chemical shifts indicated for the carbonyl

92

carbons are in Scheme 1.17 The asterisks indicate the position of the isotopic label provided



93

through the introduction of enriched 13CO2 gas.14,18 (The R groups represent the rest of the

94

organic polymer or molecule.)

95

Scheme 1: Primary amine and CO2 reaction pathways.

96 97

Characterization of these products is complicated by multiple factors including the lack of

98

crystallinity, which prevents some analytical techniques (like powder X-ray diffraction) from

99

being employed.19 The types of species formed can be influenced by the CO2 partial pressure as

100

well as the humidity level. .

101

In situ 13C static nuclear magnetic resonance (NMR) is a spectroscopic method capable of

102

monitoring the adsorption process through identification and quantification of reactants and

103

products. Recently, we developed hardware that permits this process to be observed in situ in a

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batch reactor system capable of detecting 13C in solid, liquid, gaseous, and supercritical phases.20

105

13

C NMR is well suited to study these reactions because NMR is a non-destructive technique that


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can detect the chemical identity of the 13C species, even in amorphous or disordered samples.

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This technique is ideal for these chemical systems because NMR is sensitive to both local

108

bonding and morphology of the species. The in situ static 13C NMR is able to detect the 13CO2

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gas as it is introduced to an adsorbent material, as well as the chemisorption of the 13CO2 by the

110

amine-containing polymer.

111

proton decoupling (denoted 13C{1H} CPMAS NMR) is performed in a separate set of ex situ

112

experiments, given the need for spinning the sample rapidly in an NMR rotor (sample holder) at

113

the “magic-angle” to remove dipolar coupling interactions.

114

characterize the solid-state reaction products after CO2 adsorption has occurred. Ex situ studies

115

provide additional information that aids in identifying the chemisorption products from variants

116

of the 13C{1H} CPMAS experiment, including detection of the 13C-14N dipolar coupling and

117

measuring T2 (transverse) relaxation time.

13

C cross-polarization magic-angle spinning (CPMAS) NMR with

13

C{1H} CPMAS NMR is able to

118 119

MATERIALS AND METHODS:

120

The material studied and presented here is a hyperbranched amine polymer, covalently bonded to

121

a SBA-15 mesoporous silica support.9,21 This material is synthesized by reacting aziridine

122

monomers with the silica surface; more details of the synthesis and characterization are provided

123

in the Supporting Information. The final SBA-15-HAS adsorbent contains a high amine density

124

(nitrogen content is 5.6 mmol g-1 as a mixture of primary, secondary, and tertiary amines) that

125

chemically adsorb CO2 in the pores of the high surface area support structure.

126

Prior to the NMR experiments, the SBA-15-HAS sample was heated under vacuum at

127

approximately 95 oC for 8 hours to remove adsorbed gases bound to the amine polymer. After



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128

heating, the oven was cooled and backfilled with nitrogen gas. Next, the sample was placed

129

(with minimal atmospheric exposure) either into the in situ NMR sample space,20 or packed into

130

a NMR rotor (without the cap inserted) and placed in a separate reaction chamber. Samples were

131

then evacuated of N2 and loaded with 99% enriched 13CO2 gas to 1 atm. For samples that were

132

evacuated after the 13CO2 reaction, the samples were subsequently placed under vacuum at 20 to

133

50 µTorr.

134

NMR experiments have been performed in external magnetic field, B0=14 T and 3 T

135

superconducting NMR magnets at 13C frequencies of 148.344 MHz and 32.238 MHz,

136

respectively. The in situ, static 13C NMR was conducted on a home-built single-channel NMR

137

probe in the 14 T magnet. Data were typically acquired using a ଶ pulse width of 24 µs, a recycle

138

delay of 100 s, and recording 240 transients. At 3 T, 13C-14N dipolar coupling was observed in

139

the 13C{1H} cross-polarization magic-angle spinning (CPMAS) spectrum. A modified

140

commercial HX Chemagnetics probe was used with typical conditions of a proton ଶ pulse width

141

of 4.3 µs (resonance frequency for 1H of 128.195 MHz at 3 T), a contact time of 1.5 ms, a

142

recycle delay of 6 s, and recording between 1,024 and 72,000 transients. (The latter for post-

143

reaction evacuated samples with low 13C signal-to-noise ratios).

144

13

145

Bruker Avance 600 spectrometer.

146

of 10 kHz and 1.75 ms 1H-13C cross-polarization time with 13C radio frequency field at 50 kHz

147

and 1H ramped from 50 kHz to 70 kHz followed by a 4 µs Hahn-echo π-pulse centered 2 rotor

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cycles after the CP pulse, 128 kHz SPINAL6422 1H (600.133 MHz) decoupling, and a recycle

గ

గ

C{1H} CPMAS spectra were collected on a Bruker 4 mm HCN biosolids MAS probe and a 13

C (150.925 MHz) spectra were collected with a MAS speed


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13

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delay of 9.4 s.

C chemical shifts were referenced externally to TMS by setting the methylene

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of adamantane to 38.48 ppm as a secondary reference.23

151

13

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refocusing-pulses every rotor period. To compensate for pulse imperfections, xy8 phase

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cycling24 of the 4 µs rotor-synchronized 13C π-pulses and EXORCYCLE phase cycling25,26 of the

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final 13C Hahn-echo refocusing pulse were applied with 128 kHz SPINAL64 1H decoupling to

155

minimize the effects of RF inhomogeneity.26,27

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refocusing times (54.4 ms, corresponding to ~544 13C π-pulses of 4 µs) and choosing the power

157

level that corresponded to the maximum signal intensity.28

158

the center band and attendant spinning sidebands.

C T2 data were collected with 13C{1H} CPMAS as above with 4 µs rotor synchronized 13C π

13

C π-pulse power level was arrayed in the long

13

C intensity is the sum of the area of

159 160

RESULTS AND DISCUSSION:

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The SBA-15 supported HAS sample captures CO2 by chemisorption,9 which can be observed by

162

static in situ 13C NMR. There are four important reaction pathways between CO2 and the amine

163

groups in SBA-15-HAS, each shown in Scheme 1.4,14–16,29–31 Here, the introduction of enriched

164

13

165

Figure 1 shows a representative in situ static 13C NMR spectrum of SBA-15-HAS with 1 atm

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overpressure of 13CO2 gas at 22 oC, after 2 hours of exposure to the gas.

CO2 gas leads to a chemisorbed species with a carbonyl carbon that is isotopically-labelled.



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C in situ NMR at 14 T of SBA-15-HAS reacted with

13

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Figure 1: A) Static

CO2(g). B)

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Expanded scale (x17) showing the broad chemisorbed spectrum. 240 transients were recorded.

170 171

The sharp resonance is free 13CO2 gas (at ~125 ppm), and the very broad signal (see Figure 1B)

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indicates that the 13CO2 has reacted to form a new solid carbon-containing species during the

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adsorption process. This latter resonance is broadened by multiple interactions in the solid phase

174

such as dipolar coupling to 1H, 13C, and 14N, chemical shift anisotropy (CSA), and potentially a

175

distribution of chemical environments.32 Therefore, the isotropic chemical shift(s) for the one or

176

more resonances comprising the broad chemisorbed signal cannot be determined by the center of

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mass of the resonance in Figure 1B, but requires either peak fitting (to a structural model) or

178

magic-angle spinning (MAS) NMR to narrow the signal and reveal the isotropic chemical


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shift(s). We note that the typical singularities and discontinuities of “textbook” CSA patterns are

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not evident in Figure 1B, since a combination of site heterogeneity and 1H—13C, 13C—13C, and

181

13

182

resonance is indicative of a new chemical moiety after 13CO2 exposure via in situ NMR, but to

183

determine the structure of the chemisorbed product requires NMR techniques with greater

184

resolution than that offered by static NMR experiments. For these ex situ experiments, the gas

185

signal at 125 ppm is not present because free 13CO2 gas is eliminated during transfer from the in

186

situ NMR probe to the rotors used for ex situ NMR, and any trapped gas will have a vanishingly

187

small signal. Hence, only the CO2-reacted solids were examined ex situ by solid-state 13C{1H}

188

CPMAS NMR to characterize the broad resonance of Figure 1.

189

Figure 2 shows ex situ solid-state NMR, using 13C{1H} CPMAS at 14 T, from the chemisorbed

190

reaction product seen in Figure 1. In contrast to the broad line shape seen in the in situ spectrum

191

(Figure 1), now a relatively narrow line shape is observed (Figure 2) with multiple peaks; a

192

resonance seen at 164.3 ppm with a width of 3.4 ppm, and a second resonance at 160.3 ppm with

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a width of 4.2 ppm. The urea reaction product has been shown to form when the reaction is

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heated to 135 oC under dry conditions,33 but this species has been excluded from consideration in

195

our analyses since the experimental temperature is kept well below this threshold.

C—14N dipolar interactions and

13

C-14N quadrupolar interactions are present. This broad



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C{1H} CPMAS NMR recorded ex situ at 14 T of reacted SBA-15-HAS solids

197

Figure 2:

198

collected from the in situ reactor (sample rotation, νR= 10 kHz). 416 transients were recorded.

199 200

The resonance at 164.3 ppm lies near the chemical shift values expected for typical bicarbonate

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(163 ppm)17 and carbamate (164 ppm)17 species that are listed in Scheme 1, and notably, the full-

202

width half-maximum (FWHM), 3.4 ppm, of the resonance is larger than the chemical shift

203

difference (~1ppm) between the two products. Therefore, an assignment is difficult to make

204

based on the 13C chemical shift of this spectrum alone. The resonance at 160.3 ppm that appears

205

as a weaker shoulder is assigned to carbamic acid. Once the sample was allowed to “age” in the

206

NMR rotor (under normal conditions at STP and in room air for a period of 4 days), the 160.3

207

ppm resonance disappeared. Carbamic acid is known to be a less stable product, as shown by

208

Pinto and coworkers,14 who studied the effects of evacuation following the adsorption of CO2 on

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supported amine structures. The amine-containing species was 3-aminopropyl-triethoxysilane

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(APTES) grafted on a porous clay heterostructure (PCH) support (“class 2”), consisting of only

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primary amines. The PCH-APTES adsorbent was reacted with CO2, forming carbamate and


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carbamic acid. Upon evacuation, the carbamate was seen to persist while the carbamic acid was

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desorbed, and its resonance disappeared in a manner similar to what we have observed.

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A reacted sample was also measured by 13C{1H} CPMAS at Bo = 7 T for reference, shown in the

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Supporting Information, Figure S2. The poorer resolution at lower field prevents the carbamic

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acid peak (at 160.3ppm) from being resolved. Also seen in the Figure S2 is a very weak

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resonance (between 40 - 50 ppm) from the sp3 carbons present in the HAS polymer at natural

218

abundance.

219

13

220

The spectrum in Figure 2 indicates that the CO2 reacted SBA-15-HAS sample forms a mixture

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that includes two or more species as chemisorption products that are spectroscopically

222

distinguished with 13C{1H} CPMAS NMR. The resonance at 164.3 ppm is more challenging to

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assign. The width of the peak is such that it could be carbamate, bicarbonate, or a mixture of

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both—since the chemical shift separation is not sufficient to resolve these, even at 14 T,

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requiring further experiments to fully identify the reaction products. Signals from 13C{1H}

226

CPMAS experiments are reliant on dipolar coupling to nearby protons; hence proton distances to

227

the 13C species are relevant in the analysis. However, all of the possible products, including both

228

of the overlapping resonances of carbamate and bicarbonate, have no directly-bonded protons

229

and have similarly-distant protons such that 1H-13C dipolar coupling based techniques (i.e.

230

CPMAS, dipolar dephasing34) are not able to easily distinguish between the structures effectively

231

by their 1H-13C dipolar couplings.

232

13

233

sample (one in which the shoulder had disappeared, and where only the larger peak persisted).

C-14N Interactions in Low Field 13C CPMAS NMR

C NMR T2 (transverse) relaxation data was acquired at 14 T on a 13CO2 reacted SBA-15-HAS



234

Figure 3 shows the T2 decay of NMR intensity, taken from the area of the resonance at 164.3

235

ppm, on a log scale as a function of time. It is evident that this resonance has a non-linear trend

236

in the T2 relaxation curve, which indicates multiple T2 values, and two linear regions have been

237

fit as shown with T2 times of 14.7 ms and 76.9 ms. The T2 relaxation times of nitrogen-bearing

238

carbons have been measured to be 3-4 ms,35 whereas that of bicarbonate is significantly longer,

239

between 150 ms – 1s in solution-phase experiments.36,37 These significantly different relaxation

240

rates would suggest that one component is carbamate, and the longer T2 relaxation time

241

corresponds to bicarbonate. The plot is bimodal, indicating that more than one 13C-chemisorption

242

product is present within the sample at the broad 164.3 ppm resonance. The value for T2 is

243

dependent on the chemical environment of the nuclei, such that a highly abundant (and

244

quadrupolar) 14N that is bonded to the carbon in carbamate is expected to shorten T2 relative to

245

the predominantly 16O (nuclear spin, I=0, and therefore “silent” in coupling to its 13C neighbor)

246

bonds in carbamate and bicarbonate.

247


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Figure 3: Cross-polarization Hahn spin-echo measurements of T2 (transverse) relaxation time

249

curve of the resonance at 164.3 ppm in the

250

plotted as the natural log of the spin echo intensity. Lines are fit to linear regions of the semi-log

251

T2 plot with slopes of 14.7 ms for the data between 0 and 10 ms and 76.9 ms for the data from 35

252

to 70 ms. Each point represents 416 transients.

13

CO2 reacted SBA-15-HAS sample. The y-axis is

253 254

In order to identify the components that constitute the mixture of adsorption products, the

255

differences between products need to be exploited. A key difference between the carbamate and

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bicarbonate products is the coordination environment at the carbonyl carbon. Carbamate has a

257

nitrogen directly bonded to this 13C-enriched carbon, while bicarbonate does not, leading to an

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opportunity to probe 13C-14N dipolar interactions.38–42 The 14N is 99.6% naturally abundant,

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making the carbon-nitrogen pair a good candidate for probing these contacts, since nearly every

260

NMR-detected C-N pair is 13C-14N and thus will be dipolar coupled. The dipolar interaction

261

between the nuclear spin, I=

262

quadrupolar effects to be transferred to the 13C.38,39 The 13C-14N dipolar coupling is not averaged

263

to zero by MAS (as it is for 13C coupling to any spin-1/2, like another 13C) because the

264

quantization axis of the 14N is partially aligned with the magnetic field axis, which is fixed

265

relative to the lab frame, and partially with the electric field gradient (EFG) tensor, which is

266

fixed relative to the spinning sample. Therefore, as the sample rotates due to MAS, the

267

quantization axis wobbles, preventing the 13C-14N dipole-dipole interaction from averaging to

268

zero. Instead the NMR resonance from 13C is split into two peaks due to the dipolar coupling.

269

Importantly, this interaction is inversely proportional to the magnetic field, so that the signal

ଵ 13 C ଶ

nucleus and the I= 1 14N nucleus allows the 14N second-order



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splitting is only observable at low magnetic fields (typically < 4.7 T or a “200 MHz” NMR).38,39

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A single resonance will be observed at higher magnetic fields, like the 14 T spectrum seen in

272

Figure 2, with the 13C-14N interaction manifesting as a broadened line. The extent of splitting

273

also varies with the strength of the dipolar interaction (which is dependent on the 13C-14N

274

internuclear distance), the quadrupole asymmetry parameter (ηQ), and the orientation of the 14N

275

EFG tensor with respect to the 13C-14N dipolar tensor (characterized by the Euler angles, α and

276

β).38,39,41 For an identical EFG/dipolar tensor orientation, short distances with large dipolar

277

coupling can have two peaks that are easily resolved in the NMR spectrum, but for longer

278

distances with smaller dipolar couplings, the resolution may only appear as a broadening of the

279

coupled resonance.

280

An identifying characteristic of this dipolar splitting that can confirm the multiple peaks as a

281

single chemical species is the relative intensities of the peaks split by the 13C-14N interaction.

282

The center of mass of the dipolar-split resonance will lie at its isotropic chemical shift, and the

283

peaks will have a 2:1 area ratio.38 Figure S3 shows an example of this 13C-14N splitting at Bo=3T

284

for a reference sample of ammonium carbamate (NH4)(H2NCO2).

285

of the 13CO2-reacted SBA-15-HAS sample is informative because the low-field spectrum will

286

show the effects of the 13C-14N dipolar coupling if one or more species is carbamate. Notably,

287

however, if the product is bicarbonate, a single unsplit resonance is expected.

288

Figure 4 shows the 13C{1H} CPMAS NMR spectrum of the 13CO2-reacted sample, which

289

exhibits a complex lineshape. The spectrum at 3 T is dominated by a resonance centered at 163.6

290

ppm with a FWHM of 5.2 ppm. Upon close inspection it is evident that there are shoulders to

291

either side of the central peak that can not be within the single Gaussian fit. (Carbamic acid at

292

160.3 ppm has been ruled out in part because this sample had been allowed to “age” as noted


13

C{1H} CPMAS NMR at 3T

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previously, where the resonance at 160.3 ppm disappears before running this experiment.) Line

294

fitting of the spectrum is shown in Figure 4, where the central resonance is fit by a single

295

Gaussian centered at 163.7 ppm, and shoulders of the central peak are evident (addressed below).

296

297 13

C{1H} CPMAS NMR (νR= 3.8 kHz) at 3 T of the reacted SBA-15-HAS. The

298

Figure 4:

299

chemisorbed product resonance has a FWHM of 5.2 ppm. 1024 transients were recorded.

300 301

We evacuated our 13CO2-reacted sample for 31 hours (to ~10 mTorr vacuum) at 23 oC prior to

302

recording NMR spectra at 3T, in an effort to observe the more stable species present within the

303

complex lineshape shown in Figure 4. (The sample was returned to atmospheric pressure by

304

backfilling with N2 gas.) Figure 5 shows the resulting 13C{1H} CPMAS spectrum at 3T of an

305

evacuated CO2-reacted SBA-15-HAS material. Conspicuously, the evacuation of the sample led

306

to significant diminution of the central resonance centered at 163.6 ppm (as evidenced by the

307

smaller signal-to-noise ratio, despite acquiring 70 times more transients). Close inspection of the

308

spectrum after evacuation still shows the presence of this central peak, but the split resonances

309

are now more prominent.



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C{1H} CPMAS NMR at 3 T of the reacted SBA-15-HAS after evacuating the

311

Figure 5:

312

reacted sample for 31 hours. 72,000 transients were recorded (νR= 3.8 kHz). The experimental

313

spectrum is in black (top), the overall fit (bottom) is in black, the peak fits for carbamate are

314

blue, and the peak fit for bicarbonate is red.

315 316

Line fitting of the spectrum is shown in Figure 5, where the central resonance is fit by a single

317

Gaussian centered at 165.8 ppm, and a pair of peaks separated by 7.8 ppm are fit as shoulders of

318

the central peak. Upon desorption, it is apparent that tightly bound adsorption products remain,30

319

and the shoulders that were present in the unevacuated spectrum (Figure 4) are more

320

pronounced. It is notable that the extent of splitting manifested in these shoulders of the SBA-

321

15-HAS reaction product is similar to the splitting of the reference compound, ammonium

322

carbamate, which suggests that both are carbamate moieties. The EFG/dipolar tensor orientation

323

in ammonium carbamate and in the HAS sample will nevertheless be slightly different and will

324

affect the absolute magnitude of the splitting.


Page 19 of 28


325

This middle, dominant resonance is more difficult to assign. This resonance is not clearly a

326

doublet, yet its linewidth is still broader (see Table 1 below) than that found at higher magnetic

327

field—which is the opposite trend expected in resonances whose linewidth is dominated by

328

chemical shift dispersion effects. We posit that the broadened and unsplit peak at the center of

329

each spectrum could be either bicarbonate or an additional carbamate species that exhibits a

330

smaller splitting due to a different angular relationship of the 14N quadrupolar tensor to the 13C-

331

14

332

The chemical shifts and widths of the peaks from the low field NMR measurements are

333

summarized in Table 1. (The central resonance may have shifted slightly due to differences in

334

line fitting.)

335

Table 1. 13C CPMAS NMR chemical shifts and peak widths at Bo=3 T.

N dipolar interaction, as has been shown experimentally43 and by simulation.41

Chemical species

Description

CO2-reacted HAS

central resonance

CO2-reacted and evacuated HAS

shoulder

Ammonium carbamate

13

C δiso (ppm) 163.7

FWHM (ppm) 4.9

170.7

3.1

shoulder

162.9

2.7

central resonance

165.8

4.4

168.8

5.4

162.8

4.9

split peak split peak

336



337

Literature reports suggest that carbamate is the favored product of reactions of CO2 with amines

338

under water-free conditions and can form on primary or secondary amine groups through a

339

zwitterionic mechanism.10 It has also been reported that a surface-bound carbamate species can

340

form in amine adsorbents.16,30 This carbamate species is very stable and can persist under

341

vacuum.

342

The bicarbonate species requires water (Scheme 1, Reaction 2) to provide a free base for the

343

reaction and can form on all three types of amines.10 SBA-15 and aminosilica materials are

344

typically very hydrophilic,44 so water is expected to be present on the surface of these solids

345

under most conditions (even with our routine evacuation of the sample at 95 oC prior to CO2

346

exposure), meaning that either carbamate or bicarbonate reactions pathways may conceivably be

347

accessible.

348

The SBA-15-HAS contains primary, secondary, and tertiary amine groups that can adsorb CO2

349

and can lead to a wider variety of products depending on the type of adsorbing amine. Notably,

350

mesoporous silica SBA-15 is known to retain water due to abundant -OH groups on the silicate

351

surfaces. Hence, the aminopolymer supported on SBA-15 has the requisite H2O present for

352

bicarbonate formation.

353

It is worthwhile to note that several previous studies on the reaction of CO2 and supported amine

354

materials indicate that the adsorption product forms primarily carbamate.14–16,29–31,45–48 These

355

studies each differed from ours in at least one notable way: a different mixture of amines were

356

present, 14–16,29–31,45,46 a lower partial pressure of CO2 was used,15,29 short CO2 exposure times

357

were used, and/or high vacuum (10-6 Pa) was applied to the reacted sample before taking an

358

analytical measurement,48 such as NEXAFS.


Page 20 of 28

Page 21 of 28


359

Specifically, the aminopolymer used in this study contains tertiary amines, which are known to

360

catalyze the formation of bicarbonate.5,49 Furthermore, bicarbonate is thought to form much

361

more slowly than carbamate,3 and evidence for bicarbonate formation at long exposure times of

362

humid CO2 was recently reported for an aminopropylsilane modified SBA-15 adsorbent by

363

FTIR.50 Much of the previous literature reports equilibration times on the order of several hours,

364

which may be too short an equilibration time to observe bicarbonate, even under humid

365

conditions and especially on materials with relatively high amine loadings, and our equilibration

366

times were on the order of 20 hours.16,51 Furthermore, bicarbonate and carbamate may have

367

different extinction coefficients in FTIR experiments, one reason why IR evidence for

368

bicarbonate formation has been less commonly reported to date, though the precedent for

369

bicarbonate has been shown recently by Didas et. al.50

370

We surmise that by not evacuating samples prior to our initial NMR analysis, we are able to

371

elucidate a mixture of adsorption products which likely includes carbamate, carbamic acid, and

372

an additional species that is either a second chemically-distinct carbamate or a bicarbonate

373

species. One possible scenario is that a low CO2 partial pressure allows carbamate to form

374

initially, and more weakly sorbed species, such as bicarbonate and carbamic acid, only form

375

once more CO2 is added and the time for reaction is increased. This process will shift the amine-

376

to-CO2 ratio from 2:1 at low CO2 partial pressures, where only carbamate is formed, towards1:1

377

where the bicarbonate and carbamic acid products are accessible.4,5

378

Another scenario is that multiple carbamate species are present as products. As mentioned

379

before, a surface-bound carbamate site could form in the amine-to-CO2 ratio of 1:1 to allow more

380

CO2 to adsorb on the surface when higher CO2 concentrations and longer exposure times are

381

used. The mixture of carbamates could arise from reactions with primary amines and secondary



Page 22 of 28

382

amines giving different NMR signals. These carbamate sites could give a distinct 13C NMR

383

resonance if the 14N EFG between the carbamate structures is sufficiently different.

384

of the polymer can afford the opportunity to discriminate between bicarbamate and carbamate, or

385

if a mixture of carbamates is present. Regardless of the scenario in which the mixture of

386

products is formed, upon evacuation more loosely bound chemisorbed CO2 products are

387

desorbed. This process is analogous to the vacuum swing desorption process that has been

388

previously studied in chemisorption and physisorption systems.4,33,52–58 Only more tightly bound

389

products remain after evacuation, which favors carbamate products, such as the surface bound

390

carbamate.

391

Mixtures of chemisorption products offer challenges for their characterization. The products of

392

13

393

observing both the gas and the chemisorbed solid, and by 13C{1H} CPMAS NMR at low

394

magnetic fields. We find that the reaction of CO2 and amine groups in SBA-15-HAS forms a

395

mixture of products, which likely contains carbamate, carbamic acid, and an additional

396

resonance that is either a different carbamate or bicarbonate. The 13C-14N dipolar coupling splits

397

the 13C NMR resonance of carbamate at low magnetic fields (3 T), due to its direct carbon-

398

nitrogen bond, which permits products to be distinguished. Evacuation of the reacted SBA-15-

399

HAS leads to a sample with the carbamate signal enhanced relative to the unsplit central peak,

400

suggesting it is the more stable species under those conditions. This mixture of adsorption

401

products can form because multiple reaction pathways are accessible for reaction with CO2; the

402

presence of primary, secondary, and tertiary amines enables multiple reactions. Also, these

403

results suggest that water must be present, to allow the proposed bicarbonate reactions. Since the

404

SBA-15-HAS polymer is hydrophilic the requisite water may exist within the pores. The longer

15

N labeling

CO2 chemisorbed on SBA-15-HAS have been studied with 13C NMR in an in situ NMR probe,


Page 23 of 28


405

CO2 exposure time allows species other than the previously identified surface-bound carbamate

406

to form. These findings show that the SBA-15-HAS sample undergoes a chemisorption reaction

407

with CO2 that forms a mixture of products. More work is needed to fully assign and quantify

408

these products via a combination of 13C{1H} CPMAS NMR techniques, especially low-field

409

13

C{1H} CPMAS NMR.

410 411 412 413

Acknowledgments: The authors acknowledge the National Science Foundation grant number CBET- 1403298 and

414

CBET- 1403239 for funding.

The researchers from Washington University acknowledge

415

funding from the Consortium for Clean Coal Utilization. MSN and CWJ acknowledge Corning

416

Inc. for support, and WC and CWJ acknowledge Global Thermostat, LLC for support.



417

REFERENCES

418 419

(1)

Bhown, A. S.; Freeman, B. C. Analysis and Status of Post-Combustion Carbon Dioxide Capture Technologies. Environ. Sci. Technol. 2011, 45, 8624–8632.

420 421

(2)

Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-Combustion CO2 Capture Using Solid Sorbents : A Review. Ind. Eng. Chem. Res. 2012, 51, 1438–1463.

422 423

(3)

Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources. ChemSusChem 2009, 2, 796–854.

424 425

(4)

Bollini, P.; Didas, S. A.; Jones, C. W. Amine-Oxide Hybrid Materials for Acid Gas Separations. J. Mater. Chem. 2011, 21, 15100–15120.

426 427

(5)

Vaidya, P. D.; Kenig, E. Y. CO2-Alkanolamine Reaction Kinetics: A Review of Recent Studies. Chem. Eng. Technol. 2007, 30, 1467–1474.

428 429 430

(6)

Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Novel PolyethylenimineModified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO 2 Capture. Energy & Fuels 2002, 16, 1463–1469.

431 432

(7)

Tsuda, T.; Fujiwara, T. Polyethyleneimine and Macrocyclic Polyamine Silica Gels Acting as Carbon Dioxide Absorbents. J. Chem. Soc. Chem. Commun. 1992, 1659–1661.

433 434

(8)

Tsuda, T.; Fujiwara, T.; Taketani, Y.; Saegusa, T. Amino Silica Gels Acting as a Carbon Dioxide Adsorbent. Chem. Lett. 1992, 21, 2161–2164.

435 436 437

(9)

Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. Designing Adsorbents for CO2 Capture from Flue Gas-Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. J. Am. Chem. Soc. 2008, 130, 2902–2903.

438 439 440

(10)

Drese, J. H.; Choi, S.; Lively, R. P.; Koros, W. J.; Fauth, D. J.; Gray, M. L.; Jones, C. W. Synthesis-Structure-Property Relationships for Hyperbranched Aminosilica CO2 Adsorbents. Adv. Funct. Mater. 2009, 19, 3821–3832.

441 442 443

(11)

Sumida, K.; Rogow, D. L.; Mason, J. A.; Mcdonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.; Long, J. R. Carbon Dioxide Capture in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 724–781.

444 445

(12)

Ma, X.; Wang, X.; Song, C. “Molecular Basket” Sorbents for Separation of CO2 and H2S From Various Gas Streams. J. Am. Chem. Soc. 2009, 131, 5777–5783.

446 447 448

(13)

Da Silva, Eirik, F.; Svendsen, H. F. Computational Chemistry Study of Reactions, Equilibrium and Kinetics of Chemical CO2 Absorption. Int. J. Greenh. Gas Control 2007, 1, 151–157.


Page 24 of 28

Page 25 of 28


449 450 451

(14)

Pinto, M. L.; Mafra, L.; Guil, J. M.; Pires, J.; Rocha, J. Adsorption and Activation of CO2 by Amine-Modified Nanoporous Materials Studied by Solid-State NMR and 13CO2 Adsorption. Chem. Mater. 2011, 23, 1387–1395.

452 453 454

(15)

Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Carbon Dioxide Capture by Diamine-Grafted SBA-15 : A Combined Fourier Transform Infrared and Mass Spectrometry Study. Ind. Eng. Chem. Res. 2005, 44, 3702–3708.

455 456 457

(16)

Bacsik, Z.; Ahlsten, N.; Ziadi, A.; Zhao, G.; Garcia-Bennett, A. E.; Martín-Matute, B.; Hedin, N. Mechanisms and Kinetics for Sorption of CO2 on Bicontinuous Mesoporous Silica Modified with n-propylamine. Langmuir 2011, 27, 11118–11128.

458 459

(17)

Reich, H. J. C-13 Chemical Shifts http://www.chem.wisc.edu/areas/reich/handouts/nmrc13/cdata.htm.

460 461 462

(18)

Mani, F.; Peruzzini, M.; Stoppioni, P. CO2 Absorption by Aqueous NH3 Solutions: Speciation of Ammonium Carbamate, Bicarbonate and Carbonate by a 13C NMR Study. Green Chem. 2006, 8, 995–1000.

463

(19)

West, A. R. Solid State Chemistry and its Applications; Second Edi.; Wiley, 2014.

464 465 466

(20)

Surface, J. A.; Skemer, P.; Hayes, S. E.; Conradi, M. S. In Situ Measurement of Magnesium Carbonate Formation From CO2 Using Static High-Pressure and Temperature 13C NMR. Environ. Sci. Technol. 2013, 47, 119–125.

467 468 469

(21)

Chaikittisilp, W.; Didas, S. A.; Kim, H.; Jones, C. W. Vapor-Phase Transport as A Novel Route to Hyperbranched Polyamine-Oxide Hybrid Materials. Chem. Mater. 2013, 25, 613–622.

470 471

(22)

Fung, B. M.; Khitrin, A. K.; Ermolaev, K. An Improved Broadband Decoupling Sequence for Liquid Crystals and Solids. J. Magn. Reson. 2000, 142, 97–101.

472 473

(23)

Morcombe, C. R.; Zilm, K. W. Chemical Shift Referencing in MAS Solid State NMR. J. Magn. Reson. 2003, 162, 479–486.

474 475

(24)

Gullion, T.; Baker, D. B.; Conradi, M. S. New, Compensated Carr-Purcell Sequences. J. Magn. Reson. 1990, 89, 479–484.

476 477

(25)

Rance, M.; Byrd, R. A. Obtaining High-Fidelity Spin-1/2 Powder Spectra in Anisotropic Media: Phase-Cycled Hahn Echo Spectroscopy. J. Magn. Reson. 1983, 52, 221–240.

478 479 480

(26)

Sinha, N.; Schmidt-Rohr, K.; Hong, M. Compensation for Pulse Imperfections in Rotational-Echo Double-Resonance NMR by Composite Pulses and EXORCYCLE. J. Magn. Reson. 2004, 168, 358–365.



481 482

(27)

Weldeghiorghis, T. K.; Schaefer, J. Compensating for Pulse Imperfections in REDOR. J. Magn. Reson. 2003, 165, 230–236.

483 484 485

(28)

Ni, R.; Childers, W. S.; Hardcastle, K. I.; Mehta, A. K.; Lynn, D. G. Remodeling Cross-β Nanotube Surfaces with Peptide/Lipid Chimeras. Angew. Chem. Int. Ed. Engl. 2012, 51, 6635–6638.

486 487 488

(29)

Robinson, K.; McCluskey, A.; Attalla, M. I. An FTIR Spectroscopic Study on the Effect of Molecular Structural Variations on the CO2 Absorption Characteristics of Heterocyclic Amines. Chemphyschem 2011, 12, 1088–1099.

489 490

(30)

Danon, A.; Stair, P. C.; Weitz, E. FTIR Study of CO2 Adsorption on Amine-Grafted SBA-15 : Elucidation of Adsorbed Species. J. Phys. Chem. C 2011, 115, 11540–11549.

491 492 493 494

(31)

Knofel, C.; Martin, C.; Hornebecq, V.; Llewellyn, P. L. Study of Carbon Dioxide Adsorption on Mesoporous Aminopropylsilane-Functionalized Silica and Titania Combining Microcalorimetry and in Situ Infrared Spectroscopy. J. Phys. Chem. C 2009, 113, 21726–21734.

495 496

(32)

Schaefer, J.; Stejskal, E. O. Carbon-13 Nuclear Magnetic Resonance of Polymers Spinning at the Magic Angle. J. Am. Chem. Soc. 1976, 98, 1031.

497 498 499

(33)

Drage, T. C.; Arenillas, A.; Smith, K. M.; Snape, C. E. Thermal Stability of Polyethylenimine Based Carbon Dioxide Adsorbents and its Influence on Selection of Regeneration Strategies. Microporous Mesoporous Mater. 2008, 116, 504–512.

500 501

(34)

Duer, M. J. Introduction to Solid-State NMR Spectroscopy; Duer, M. J., Ed.; WileyBlackwell, 2005.

502 503 504 505

(35)

Su, Y.; Hong, M. Conformational Disorder of Membrane Peptides Investigated from Solid-State NMR Linewidths and Lineshapes Conformational Disorder of Membrane Peptides Investigated from Solid-State NMR Linewidths and Lineshapes Yongchao Su and Mei Hong *. J. Phys. Chem. B 2011, 115, 10758–10767.

506 507 508

(36)

Bewernitz, M. a.; Gebauer, D.; Long, J.; Cölfen, H.; Gower, L. B. A Metastable Liquid Precursor Phase of Calcium Carbonate and its Interactions with Polyaspartate. Faraday Discuss. 2012, 159, 291.

509 510 511

(37)

Miziorko, H. M.; Mildvan, a S. Electron Paramagnetic Resonance, 1-H, and 13C Nuclear Magnetic Resonance Studies of the Interaction of Manganese and Bicarbonate with Ribulose 1, 5-diphosphate Carboxylase. J. Biol. Chem. 1974, 249, 2743–2750.

512 513

(38)

Harris, R. K.; Olivieri, A. C. Quadrupolar Effects Transferred to Spin-1/2 Magic-Angle Spinning Spectra of Solids. Prog. NMR Spectrosc. 1992, 24, 435–456.


Page 26 of 28

Page 27 of 28


514 515 516

(39)

Olivieri, A. C.; Frydman, L.; Diaz, L. E. A Simple Approach for Relating Molecular and Structural Information to the Dipolar Coupling 13C-14N in CPMAS NMR. J. Magn. Reson. 1987, 75, 50–62.

517 518

(40)

Hexem, J. G.; Frey, M. H.; Opella, S. J. Influence of 14N on 13C NMR Spectra of Solids. J. Am. Chem. Soc. 1981, 103, 224–226.

519 520 521

(41)

Hexem, J. G.; Frey, M. H.; Opella, S. J. Molecular and Structural Information From 14N– 13C Dipolar Couplings Manifested in High Resolution 13C NMR Spectra of Solids. J. Chem. Phys. 1982, 77, 3847–3856.

522 523 524

(42)

Naito, A.; Ganapathy, S.; McDowell, C. A. High Resolution Solid State 13C NMR Spectra of Carbons Bonded to Nitrogen in a Sample Spinning at the Magic Angle. J. Chem. Physics1 1981, 74, 5393–5397.

525 526

(43)

Naito, A.; Ganapathy, S.; McDowell, C. . 14N Quadrupole Effects in CP-MAS 13C NMR Spectra of Organic Compounds in the Solid State. J. Magn. Reson. 1982, 48, 367–381.

527 528 529

(44)

Didas, S. A.; Kulkarni, A. R.; Sholl, D. S.; Jones, C. W. Role of Amine Structure on Carbon Dioxide Adsorption from Ultradilute Gas Streams such as Ambient Air. ChemSusChem 2012, 5, 2058–2064.

530 531 532

(45)

Huang, S.-J.; Hung, C.; Zheng, A.; Lin, J.; Yang, C.; Chang, Y.; Deng, F.; Liu, S. Capturing the Local Adsorption Structures of Carbon Dioxide in Polyamine-Impregnated Mesoporous Silica Adsorbents. J. Phys. Chem. Lett. 2014, 5, 3183–3187.

533 534 535

(46)

Li, D.; Furukawa, H.; Deng, H.; Liu, C.; Yaghi, O. M.; Eisenberg, D. S. Designed Amyloid Fibers as Materials for Selective Carbon Dioxide Capture. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 191–196.

536 537 538 539

(47)

Wang, X.; Schwartz, V.; Clark, J. C.; Ma, X.; Overbury, S. H.; Xu, X.; Song, C. Infrared Study of CO 2 Sorption over “ Molecular Basket ” Sorbent Consisting of Polyethylenimine-Modified Mesoporous Molecular Sieve. J. Phys. Chem. C 2009, 113, 7260–7268.

540 541 542 543

(48)

Espinal, L.; Green, M. L.; Fischer, D. A.; Delongchamp, D. M.; Jaye, C.; Horn, J. C.; Sakwa-novak, M. A.; Chaikittisilp, W.; Brunelli, N. A.; Jones, C. W. Interrogating the Carbon and Oxygen K-Edge NEXAFS of a CO2-Dosed Hyperbranched Aminosilica. J. Phys. Chem. Lett. 2015, 6, 148–152.

544 545

(49)

Donaldson, T. L.; Nguyen, Y. N. Carbon Dioxide Reaction Kinetics and Transport in Aqueous Amine Membranes. Ind. Eng. Chem. Fundam. 1980, 19, 260–266.

546 547 548

(50)

Didas, S. A.; Sakwa-novak, M. A.; Foo, G. S.; Sievers, C.; Jones, C. W. Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species. J. Phys. Chem. Lett. 2014.



549 550

(51)

Hiyoshi, N.; Yogo, K.; Yashima, T. Adsorption Characteristics of Carbon Dioxide on Organically Functionalized SBA-15. Microporous Mesoporous Mater. 2005, 84, 357–365.

551 552 553

(52)

Li, W.; Choi, S.; Drese, J. H.; Hornbostel, M.; Krishnan, G.; Eisenberger, P. M.; Jones, C. W. Steam-Stripping for Regeneration of Supported Amine-Based CO2 Adsorbents. ChemSusChem 2010, 3, 899–903.

554 555 556

(53)

Chaffee, A. L.; Knowles, G. P.; Liang, Z.; Zhang, J.; Xiao, P.; Webley, P. A. CO2 Capture by Adsorption: Materials and Process Development. Int. J. Greenh. Gas Control 2007, 1, 11–18.

557 558

(54)

Chou, C.-T.; Chen, C.-Y. Carbon Dioxide Recovery by Vacuum Swing Adsorption. Sep. Purif. Technol. 2004, 39, 51–65.

559 560 561 562

(55)

Ishibashi, M.; Ota, H.; Akutsu, N.; Umeda, S.; Tajika, M.; Izumi, J.; Yasutake, A.; Kabata, T.; Kegeyama, Y. Technology for Removing Carbon Dioxide From Power Plant Flue Gas by the Physical Adsorption Method. Energy Convers. Manag. 1996, 37, 929– 933.

563 564 565

(56)

Liu, X.; Zhou, L.; Fu, X.; Sun, Y.; Su, W.; Zhou, Y. Adsorption and Regeneration Study of the Mesoporous Adsorbent SBA-15 Adapted to the Capture/Separation of CO2 and CH4. Chem. Eng. Sci. 2007, 62, 1101–1110.

566 567 568

(57)

Zhang, J.; Webley, P. A.; Xiao, P. Effect of Process Parameters on Power Requirements of Vacuum Swing Adsorption Technology for CO2 Capture From Flue Gas. Energy Convers. Manag. 2008, 49, 346–356.

569 570 571

(58)

Belmabkhout, Y.; Sayari, A. Isothermal versus Non-isothermal Adsorption - Desorption Cycling of Triamine-Grafted Pore-Expanded MCM-41 Mesoporous Silica for CO 2 Capture from Flue Gas. Energy & Fuels 2010, 24, 5273–5280.

572


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Characterization of a Mixture of CO2 Adsorption Products in

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