Comparative Adsorption of CO2 by Mono-, Di-, and Triamino

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Comparative adsorption of CO by mono-, di- and triamino- organofunctionalized magnesium phyllosilicates Karine Oliveira Moura, and Heloise Oliveira Pastore Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es402877p • Publication Date (Web): 30 Sep 2013 Downloaded from http://pubs.acs.org on October 4, 2013

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Environmental Science & Technology

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Comparative adsorption of CO2 by mono-, di- and

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triamino- organofunctionalized magnesium

3

phyllosilicates

4

Karine O. Moura and Heloise O. Pastore*

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Micro and Mesoporous Molecular Sieves Groups, Institute of Chemistry, University of

6

Campinas. Rua Monteiro Lobato, 270, 13083-861, Campinas – São Paulo, Brazil.

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Corresponding Author

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* E-mail: [email protected]. Phone number: 55 19 35213095

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11 12 13 14 15 16

ACS Paragon Plus Environment

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ABSTRACT

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Carbon dioxide adsorbents, constituted by organofunctionalized magnesium phyllosilicates,

19

were produced using 3-aminopropyltriethoxysilane (AMPTS), N-[3-(trimethoxysilyl)propyl]-

20

ethylenediamine (TMSPEDA), N-[3-(trimethoxysilyl)propyl]-diethylenetriamine (TMSPETA)

21

and tetraethoxyorthosilane (TEOS) as silicon sources with N/Si ratios of 1, 0.75, 0.5 and 0.25, by

22

conventional and microwave heating. Adsorption studies were performed using TGA and

23

temperature programmed desorption (TPD) methods. The results showed that the best

24

temperatures for adsorption were 41, 45 and 90 °C, when magnesium phyllosilicate

25

functionalized with TMSPETA, TMSPEDA and AMPTS respectively, were used as adsorbents.

26

Using TPD technique, the maximum efficiency was found to be between 0.285 and 0.899 for

27

100% AMPTS and 33.33% TMSPETA, obtained by conventional heating. Adsorption efficiency

28

of the materials prepared by conventional is higher than the ones obtained using microwave as

29

heating source, except for 100% AMPTS. Desorption kinetics of CO2, described using Avrami’s

30

model, show that the CO2 desorption rate constant is in the range from 0.130 to 0.178 min-1,

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similar to the values for CO2 desorption from monoetamolamine-functionalized TiO2 and

32

Li4SiO4 but in a narrower range of values.

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34 35 36 37 38


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1. INTRODUCTION

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Carbon dioxide has been considered the most important greenhouse gas. The major

41

contributors to CO2 emissions are fossil fuel power plants (including coal, natural gas and oil)

42

and several sectors of the industry, mainly chemicals, petrochemicals, iron and steel, cement,

43

paper, and metals production.1 In this landscape, the concentration of this gas in the atmosphere

44

has accelerated from less than 1 ppm year-1 before to 1970 to more than 2 ppm year-1 in the last

45

years.2

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In the literature the main options to reduce the CO2 concentration are energy saving, carbon to

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non-carbon energy switching and the use of CO2 capture and storage,3 the latter being a

48

technically feasible method for reducing of CO2 emissions in the shortest period of time. In

49

general, cryogen distillation, membrane purification, absorption with liquids and adsorption

50

using solids are the main approaches to capture CO2.4,5

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CO2 capture using cryogen techniques is based on the difference between the phase transition

52

properties of the components involved, but it requires low temperatures, with the consequent

53

high power consumption and blocking of pipes and heaters. The use of the membrane is known

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to be promising due of the low energy involved, but it works in limited scale and the CO2

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recovered is of low purity.4 Liquid amines are used to absorb CO2; regeneration of the solvent by

56

heating provides highly pure CO2. Although a method already established in the industry, its use

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demands

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evaporation/decomposition of the amines.6 Adsorption on solids has been considered a potential

59

option for CO2 capture, due to the low of energy consumption involved, the cost advantages of

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the process and the ease of use on a wide range of temperatures and pressures.7 The use of the

high

energy

for

regeneration,

causes

corrosion

of

equipment

and


3


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appropriated adsorbents can potentially decrease the cost associated with the CO2 uptake in

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capture and storage technology.

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An appropriate adsorbent should combine several attributes, such as high CO2 adsorption

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capacity, fast kinetics of adsorption/desorption, high CO2 selectivity, mild conditions for

65

regeneration, stability during extensive adsorption/desorption cycling, tolerance to the presence

66

of moisture and other impurities in the feed, and low cost.2,8 According to this, the main

67

adsorbent classes are composed of zeolites, active carbon, calcium oxides, hydrotalcites, metal-

68

organic frame materials, amino-polymers and organic-inorganic hybrid materials.5,9 Zeolites,

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mesoporous carbon and organic-inorganic hybrid materials are operated at low temperatures,

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being the first and the last the ones that present higher CO2 adsorption capacity. However,

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zeolites require higher desorption energy than the organic-inorganic hybrid materials.10 Due to

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this difference, the use of the organic-inorganic hybrid materials as CO2 adsorbent has emerged

73

as a process close to application.

74

According to the literature, several types of alkylamine on silica supports have been used as

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CO2 adsorbent.2,5,8 Support materials can be functionalized by two methods: directly in the

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synthesis and by post-synthesis process. Post-synthesis procedures, also known as grafting, are

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the most common and are based on bonding of an organotrialcoxysilane onto a surface, through

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the reaction of silanol groups of the host material and the modifying agent, in a single step.

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However, this process has disadvantages such as the reduction of pore size of the support, it is

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limited to the number of accessible surface silanol groups, and is more time consuming. Due to

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these questions, direct synthesis, based on the introduction of functional groups during the sol-

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gel process, has been currently explored. The chemical modifications of the surface are

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generated from a co-condensation reaction between hydrolyzed organotrialkoxysilane and a


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silica source, producing support materials with organo-functional groups that are more likely to

85

be uniformly distributed in the support.11

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Recently, we have reported the direct synthesis of organo-functionalized magnesium

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phyllosilicate under conventional12 and microwave13 synthesis conditions. The syntheses of

88

organo-functionalized magnesium phyllosilicates using several different organosiloxanes14–19

89

and the application of these materials in catalysis,18,19 antimicrobial activity14 and adsorption of

90

metals20 and dyes16 have been studied. However, to the best of our knowledge, it has not been

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used as an adsorbent in carbon dioxide adsorption.

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In this work, organo-functionalized magnesium phyllosilicate obtained by direct synthesis,

93

using 3-aminopropyltriethoxysilane (AMPTS), N-[3-(trimethoxysilyl)propyl]-ethylenediamine

94

(TMSPEDA),

95

tetraethoxyorthosilane (TEOS) as silicon sources, in several molar proportions of the amines

96

relative to total silicon, and conventional or microwave heating, will be used as a carbon dioxide

97

adsorbents. The direct syntheses of these solids were recently reported.12,13 Factors such as molar

98

proportions of the amines presented in the solid, heating type used in the synthesis of the solids

99

and kinetic of desorption will be discussed here.

N-[3-(trimethoxysilyl)propyl]-diethylenetriamine

(TMSPETA)

and

100 101

2. EXPERIMENTAL SECTION

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2.1 Materials

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Sodium hydroxide (NaOH, Merck), magnesium nitrate hexahydrate (Mg(NO3)2.6H2O, Ecibra),

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tetraethoxyorthosilane (TEOS, Acros), 3-aminopropyltriethoxysilane (AMPTS, Sigma Aldrich),

105

N-[3-(trimethoxysilyl)propyl]-ethylenediamine

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(trimethoxysilyl)propyl]-diethylenetriamine (TMSPETA, Sigma Aldrich) were used as received.

(TMSPEDA,

Sigma

Aldrich)

and

N-[3-


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107 108

2.2 Synthesis of organo-functionalized magnesium phyllosilicate adsorbents

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Mg(NO3)2.6H2O (12 mmol) was dissolved in distilled water (100 mL). Silicon sources (TEOS,

110

AMPTS, TMSPEDA or TMSPETA) were added in volumes as indicated previously13 and

111

displayed in Supporting Material Table S1. NaOH (48 mL, 0.5 mol L-1) was added dropwise

112

under magnetic stirring and the resultant suspension was aged for 4 h, at 50 °C. The gel was

113

hydrothermally treated using conventional heating for 48 h, at 100 °C, according to Ferreira et

114

al.12, and microwave heating for 2 or 4 h, according to Moura et al.13 The product mixture was

115

dispersed in distilled water, left standing overnight, and centrifuged, washed with copious

116

amounts of distilled water, dried at room temperature and size sieved at 0.074 mm. The

117

organoalkoxysilane/total silicon molar ratios used, SiT/Sitotal, maintained the N:Si molar ratios

118

constant to 0.25, 0.50, 0.75 and 1 for all the silicon sources, by conventional and microwave

119

heating.

120 121

2.3 Determination of optimum temperatures of CO2 adsorption by TGA

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CO2 adsorption measurements were performed on TGA/DTA (Setaram Instrumentation Setsys

123

16/18 model). The mixture 5 vol. % CO2/95 vol. % He was used for the adsorption studies.

124

Approximately 10 mg of the magnesium organophyllosilicate, functionalized with AMPTS,

125

TMSPEDA and TMSPETA, at SiT/Sitotal molar ratios of 100, 37.5 and 33.33% (the samples with

126

maximum pending groups in each amine set), respectively, and prepared by conventional

127

heating, were heated from room temperature to 225 °C under N2 (16 mL min-1) at a heating rate

128

of 10 °C min-1, and were held in this temperature for 3 h to eliminate water and CO2 previously

129

adsorbed. The samples were then cooled to room temperature, with a cooling rate of 5 °C min-1


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under N2 (16 mL min-1) and were kept in this temperature for 30 min, for stabilization, before

131

adsorption. Then, the samples were heated from room temperature to 900 °C under CO2/He (16

132

mL min-1) at a heating rate of 5 °C min-1. Each of the temperatures used in this experiment were

133

determined by ordinary TGA scanning of the samples under N2, as described above. The

134

optimum temperatures for CO2 adsorption were determined based on the rapid weight gain of

135

adsorbents, under CO2, at these temperatures.

136 137

2.4 CO2 adsorption studies by TPD

138

CO2 adsorption capacities determined by TPD were obtained using a TPR/TPD, Quantachrome

139

ChemBET model. Approximately 100 mg of the magnesium organophyllosilicate functionalized

140

with AMPTS, TMSPEDA and TMSPETA, at the SiT/Sitotal molar ratios studied here and heating

141

sources used in this work, were placed in the reactor. The samples were treated as described for

142

surface cleaning at 225 °C, and then were cooled to the optimum adsorption temperatures, at

143

cooling rate of 10 °C min-1 under He (30 mL min-1). Then, the samples were held in this

144

temperature for 2 h under CO2/He (20 mL min-1). CO2 was replaced by He (20 mL min-1) and the

145

sample was cooled to room temperature and held for 1 h to remove CO2 adsorbed in solids.

146

Finally, the desorption process started: the sample was heated from room temperature to 150 °C

147

at a heating rate of 10 °C min-1, monitored by a thermal conductivity detector connected to the

148

output of the reactor. The amount of the carbon dioxide desorbed was determined by an external

149

titration method.

150 151 152


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3. RESULTS AND DISCUSSION

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3.1 Synthesis of organo-functionalized magnesium phyllosilicate

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The organo-functionalized magnesium phyllosilicates were obtained using microwave and

156

conventional oven, at different amine:TEOS molar ratios, in order to study the influence of the

157

variable quantity of organic groups linked to surface silicon atoms and the amount of nitrogen

158

atoms on carbon dioxide adsorption. The possible existence of diffusion restrictions was also of

159

interest. The success of the syntheses was confirmed by X-ray diffraction (XRD), Fourier

160

transformed infrared spectroscopy (FT-IR), 13C nuclear magnetic resonance spectra (13C-NMR),

161

elemental analysis (CHN) and thermal analysis (TGA). The results were discussed previously.13

162 163

3.2 Effect of temperature on adsorption

164

Initially the materials were heated at 225 °C for 3 h for treatment, and cooled to room

165

temperature. The weight loss observed during this temperature range was attributed to the loss of

166

absorbed water and CO2. The temperature effect on adsorption was evaluated by the change of

167

the amount of CO2 adsorbed with the increase of the temperature (Supporting Material Figure

168

S1): carbon dioxide begins to react with the adsorbent material even at low temperatures (23 °C),

169

showing the maximum weight gain at 41, 45 and 90 °C for TMSPETA, TMSPEDA and

170

AMPTS, respectively: these were considered the optimum temperatures of adsorption for each

171

material and will be used throughout this work. From these temperatures on to higher values of

172

temperature materials start to lose weight. Furthermore, there is no significant difference in the

173

curve profile relative to the optimum adsorption temperature between TMSPEDA and

174

TMSPETA-functionalized materials (see Supporting Material Figure S1). A different

175

dependence was observed for the sample where AMPTS was the silicon source. This is,


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probably, due to the fact that primary and secondary alkylamines have different temperatures for

177

reaction with carbon dioxide, as already shown in the literature.21 TMSPEDA and TMSPETA-

178

modified materials present similar behavior, since the adsorption sites in these solids are primary

179

and secondary alkylamines, while in AMPTS CO2 adsorbs only in primary alkylamine sites.

180 181

3.3 CO2 adsorption studies by TPD

182

The interaction of carbon dioxide and alkylamines is governed by different reaction

183

mechanisms depending on the type of alkylamine involved. Primary and secondary alkylamines

184

react directly with CO2 to produce carbamate throught zwitterionic intermediates.22,23 In this

185

case, a pair of eletrons in the primary, or in the secondary, alkylamine attacks the CO2 carbon

186

atom to form the zwitterion ion. After this, free base deprotonates the zwitterion to form

187

carbamate (Support Material Schemes S1 and S2). Tertiary alkylamines react with CO2 through a

188

different mechanism: they catalyze the formation of bicarbonate in the presence of water

189

(Support Material Scheme S3).9 Thus, in the absence of water, alkylammonium carbamate is

190

formed; in hydrated samples, alkylammonium bicarbonate (RNH3+HCO3−) and alkylammonium

191

carbonate ((RNH3+)2CO32−) salts are formed, Schemes 1a and 1b.

192

Recently, Danon et al.24 showed that in the absence of the water, ammonium carbamate ion

193

pairs and silylpropylcarbamate, corresponding to surface-bound carbamate, are formed on

194

sorption of CO2 on SBA-15 functionalized with different AMPTS proportions, and the relative

195

proportion of products formed are dependent on both the surface coverage of amine groups and

196

on the availability of silanol groups. In the same conditions, Bacsik et al.25 observed that three

197

products are formed during carbon dioxide adsorption on n-propylamines modified silica:

198

propylammoniumpropylcarbamate ion pairs, hydrogen-bonded propylcarbamic acids, and


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199

silylpropylcarbamates, Schemes 1c-1e. The authors observed that alkylammonium carbamate ion

200

pairs and alkylcarbamic acids are formed rapidly and at large amounts when a high density of

201

amines is used, but they are easily desorbed. On the other hand, silylpropylcarbamates are

202

formed in larger quantities in the presence of low density of alkylamines and by slow

203

condensation of carbamic acids with silanol groups, and are released in higher temperature.

204

Figures 1-3 displays the CO2-TPD profile of all materials obtained in this work. It can be

205

observed that the profiles show one, two or three desorption peaks. The curves were decomposed

206

using PeakFit deconvolution software, version 4.00. For the AMPTS-modified solid (Figures 1A

207

and B) one peak at 146 and 149 °C or two peaks at about 127-136 and 147-150 °C could be

208

observed. With longer pending groups, three peaks appear at 96-104, 120-133 and 149-150 °C.

209

According with Yue et al.,26 these peaks correspond to the decomposition of different kinds of

210

compounds and to CO2 diffusion limitations, since different carbamate types, with different

211

thermal stabilities, can be formed depending on the amount and type of functional groups present

212

in the surface of the solids used as support material.

213

As indicate before, it is observed in Figures 1A (d) and 1B (d) that the profile of the TPD for

214

the magnesium phyllosilicate functionalized with 100% AMPTS shows only one peak.

215

Considering that the

216

desorption peak observed corresponds to the decomposition of the product of R-NH2 pending

217

groups with CO2, that is, the propylammonium propylcarbamate. However, in lower proportions

218

of AMPTS, two desorption peaks appear: the original one at 146-150 °C corresponding to the

219

alkylammonium alkylcarbamate and another one at 127-136 °C. In these cases, there are free

220

hydroxyl groups in the lamella, which allow formation of other compounds, such as hydrogen-

221

bonded propylcarbamic acids or silylpropylcarbamates. Since the last compound is more stable

29

Si-NMR indicated that only T-sites are present in this material, the


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222

than propylcarbamic acids,25 in the other proportions of AMPTS, silylpropylcarbamates and

223

alkylammonium carbamate are formed during the CO2 adsorption process and released at 127-

224

136 °C and 146-150 °C, respectively, showing, contrary to the literature,25 that alkylammonium

225

alkylcarbamate ion pairs are released at a higher temperature than silylpropylcarbamates.

226

In Figures 2 and 3, three peaks appear, independent of the TMSPEDA and TMSPETA

227

proportion used. In these cases, the temperature ranges 96-104 °C, 120-133 °C and 149-150 °C

228

probably correspond to the release of CO2 from hydrogen-bonded propylcarbamic acids (Scheme

229

1d), silylpropylcarbamates (Scheme 1e) and propylammonium propylcarbamate ion pairs

230

(Scheme 1c), respectively, concluded from the results observed for the previous materials.

231

Integration of the CO2-TPD profiles in Figures 1-3 leads to the curves in Supporting Material

232

Figures S2-S4 and values in Table 1. The profiles of the curves obtained for samples prepared

233

with the same silicon sources with the two heating devices are similar but the maximum capacity

234

of materials prepared by conventional heating is higher than the ones prepared by microwave

235

heating, except for 75% and 100% AMPTS, and increase with the increased concentration of the

236

alkylamine groups in the lamella of these materials. In these cases the concentration of N found

237

in the samples obtained by conventional heating is from 18 to 25% lower relative to ones

238

prepared using microwave heating, leading to a lower adsorption capacity. Smaller N

239

concentrations were also found for 25 and 50% AMPTS and 25% TMSPEDA but the values are

240

very close to the ones for samples prepared by microwave heating. The maximum capacity of

241

CO2 adsorption (Supporting Material Figures S2-S4 and Table 1), obtained by the integrated area

242

using the Origin Software, version 8.0, were in the range of 0.669 to 3.227 mmol CO2 g-1,

243

corresponding to materials with SiT/Sitotal molar ratios of 8.33% TMSPETA and 50% TMSPEDA

244

for the materials obtained using microwave as heating source, and from 0.894 to 3.642 mmol


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245

CO2 g-1, relative to materials with SiT/Sitotal molar ratios of 100% AMPTS and 33.33% TMSPTA

246

when conventional oven is used as a heating source.

247

The efficiency of the adsorption is defined as the number of moles of carbon dioxide captured

248

per number of moles of nitrogen in the interlayer space. As two surface primary or secondary

249

alkylamines are required for the formation of alkylammonium carbamate, the maximum

250

adsorption efficiency of a primary or of a secondary alkylamine is CO2/N molar ratio of 0.5

251

under dry conditions. For the tertiary alkylamine adsorbent, the maximum efficiency is 1.9,27

252

Considering the efficiency of adsorption of the materials, the results obtained are in the range

253

of 0.466 to 0.801, for amine/total Si molar ratios of 16.67% TMSPETA and 50% TMSPEDA for

254

the materials obtained using microwave as heating source, from 0.285 to 0.899 for amine/total Si

255

molar ratios of 100% AMPTS and 33.33% TMSPETA when a conventional oven is used as a

256

heating source. Adsorption efficiency of the materials prepared by conventional heating is higher

257

than the ones obtained using microwave as heating source, except for 100% AMPTS, and, in the

258

most of these results, the alkylamine efficiency found is higher than the maximum alkylamine

259

efficiency, which is CO2/N molar ratio of 0.5 under dry condition, as indicated earlier for the

260

formation of carbamate only. This occurs, probably, because besides alkylammonium carbamate,

261

other compounds are also formed, as discussed before and detected by TPD adsorption

262

technique, that does not involve a binary alkylaminic system, allowing to obtain an amine

263

efficiency higher than 0.5, this seems more meaningful for the materials obtained by

264

conventional heating. In the case of 100% AMPTS, the adsorption efficiency found for materials

265

obtained by conventional heating is much lower one measured for the sample obtained by

266

microwave probably because of the larger presence of alkylamine groups on the solid surface


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267

that can cause the interaction between neighbors alkylamine groups, thus reducing the adsorption

268

sites of CO2.

269

The adsorption efficiencies of the materials prepared here are higher than the ones found in the

270

literature. In 2012, Wang et al.21 studied the adsorption capacity of the layered double hydroxide

271

modified with 3-aminopropyl triethoxysilane, N-2-aminoethyl-3-aminopropyl trimethoxysilane

272

and 3-[2-(2-Aminoethylamino) ethylamino]propyl-trimethoxysilane, the similar amine sources

273

used in the present work, in the temperature range from 25 to 80 °C. The results obtained

274

presented the maximum capacity in the range (0.41-0.48), (0.15-0.24) e (0.12-0.29) mol CO2

275

mol-1 N, for 3-aminopropyl triethoxysilane, N-2-aminoethyl-3-aminopropyl trimethoxysilane and

276

3-[2-(2-Aminoethylamino) ethylamino]propyl-trimethoxysilane respectively. Other results in this

277

range of efficiency and using similar amine sources can be found in the literature: adsorption

278

efficiencies of 0.15, 0.21 and 0.10 for CO2 on AMPTS functionalized MCM-41, SBA-15 and

279

large pore SBA-15, respectively;27 0.24 for CO2 adsorbed on TMSPEDA functionalized SBA-

280

16;28 and 0.336 for CO2 adsorbed on TMSPETA functionalized AMS (amine-functionalized

281

mesoporous silica).29

282

As mentioned in the Introduction, the fast adsorption/desorption kinetics is one of the desired

283

features for a good adsorbent. To evaluate adsorption kinetics, the literature provides a large

284

number of models,30 being Lagergen’s pseudo-first order and Ho and McKay’s pseudo-second

285

order the most common models. However, certain kinetic parameters, such as changes of the

286

desorption rate as a function of the initial concentration or contact time, or even the fractional

287

kinetic order, are outside pseudo-first and second order models.31 To solve this limitation,

288

Avrami’s exponential function, a case of the kinetic thermal decomposition model, has been

289

successfully used as an optimal model in studies of solution/solid31 and gas/solid21,32 adsorption.


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290

In 2008, Kinefuchi et. al33 investigated the thermal decomposition of ultrathin oxide layers on

291

silicon surface by temperature programmed desorption and described the desorption rate by

292

Avrami kinetics.

293

According to the literature,21,31–34 Equation 1 describes the general Avrami’s model.

294 295

1

(Eq. 1).

296 297

In Eq. 1 is the Avrami kinetic rate constant of CO2 desorption, in min-1, t is the reaction time,

298

in min, n is the Avrami exponent (dimensionless), which is related to the CO2 desorption order

299

and to the changes of interaction mechanism, and α corresponds to the extent of reaction

300

calculated as shown in Eq. 2

301 302

(Eq. 2)

303 304

is the amount of CO2 release at time , and is the maximum amount of CO2 released.

305

Replacing eq. 2 in the eq.1 provides Eq. 3.

306 307

1

(Eq. 3)

308 309

To determine the validity of the Avrami kinetic model and to ensure the reliability of the

310

calculated parameters and validate their applicability, the function based on the normalized

311

standard deviation (Eq. 4) was calculated.

312


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313


∑ %

!". $%&. '/!". ) $+

*

100

(Eq. 4)

314 315

In Eq. 4 % is the standard deviation function, !". is the experimental data of the amount

316

of CO2 desorbed at time t, %&. is the amount desorbed as calculated by the Avrami model and

317

n is the total number of the experimental points. In this work, Avrami’s model was applied to the

318

thermodesorption curves shown in Supporting Material Figures S2-S4 by non-linear regression.

319

Table 2 shows the values of the kinetic parameters, along with the associated standard

320

deviations as calculated using Eq. 4. The reaction order calculated by this model for the majority

321

of the samples is in the range from 1.944 to 2.267, that is essentially 2, and 2.903 for 100%

322

AMPTS, and do not seem to display any relationship with the amount, or type, of the alkylamine

323

used. Exception is made for the CO2 desorption from 8.33% and 16.67% TMSPETA, by

324

microwave, and 100% AMPTS by conventional heating: in these cases a fractional order was

325

observed. According with the literature, the fractional order of this model arises, probably, from

326

the complexity of the reaction mechanism, the occurrence of more than one reaction pathway

327

during the desorption process32 or change of mechanisms during the desorption. In these

328

particular cases, it may be also due to the type of species formed in the solid surface. The 7th

329

column of Table 1 shows that the adsorption efficiencies from 8.33% and 16.67% TMSPETA,

330

by microwave are the closest to 0.5, which is the case where the adsorption reaction probably

331

favors the alkylammonium carbamate. The formation of other products in these materials is of

332

lower importance. This observation suggests that in desorption, the order of the reaction, taken as

333

the number of different desorption mechanisms, would be the smallest. Attention is also drawn to

334

25.0% TMSPEDA, 25% and 33.33% TMSPETA: the efficiencies are the same as 16.67%


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Page 16 of 30

335

TMSPETA, but the reaction orders are next to 2. In these cases other factors, probably different

336

mechanisms play a major role.

337

The values of the kav are in the range from 0.130 to 0.178 min-1; these are overall constants

338

representing various reaction steps described by the non unitary order (n) obtained.32 The results

339

are similar to the literature: kav in the range from 0.0232 to 0.2897 min-1 for CO2 adsorbed on

340

monoethanolamine functionalized TiO235 and 0.0149 to 0.1644 min-1 for CO2 desorption from

341

Li4SiO4.36 The calculated standard deviations between the experimental and modeled values are

342

within an acceptable degree of accuracy.32

343

Summarizing, the data presented in this work indicate that organofunctionalized magnesium

344

phyllosilicates are promising adsorbents for CO2. The adsorption results obtained using TGA

345

shows that the optimum temperatures of adsorption were 41, 45 and 90 °C for TMSPETA,

346

TMSPEDA and AMPTS, respectively. The difference found between the temperatures is due to

347

the fact that primary and secondary alkylamines react with CO2 in different temperatures. TPD

348

technique shows that CO2 adsorption capacity increase with the increased of the concentration of

349

the pending groups in the interlayer space but, due to the diffusional difficulties, this capacity is

350

reduced when 100% of the silicon atoms of the structure bear pending groups. The values

351

corresponding to maximum efficiency of all the materials used as adsorbents are in the range of

352

the 0.285 to 0.899 and, in most of these cases, were higher than 0.5, because other compounds

353

are also formed besides alkylammomium carbamate, such as silylpropylcarbamates and

354

hydrogen-bonded propylcarbamic acids. The materials prepared by conventional heating showed

355

higher adsorption efficiency than the ones obtained using microwave as heating source, except

356

for 100% AMPTS. Desorption kinetics of CO2 was described successfully using Avrami’s

357

model, with a standard deviation lower than 8%. The CO2 desorption rate constants are in the


16

Page 17 of 30


358

range from 0.130 to 0.178 min-1. The reaction order calculated by this model was basically 2, or

359

3, except when 8.33% and 16.67% TMSPETA obtained by microwave, and 100% AMPTS

360

obtained by conventional heating, where the fractional order was observed probably due to the

361

complexity of the reaction mechanism involved during the desorption process.

362 363

ACKNOWLEDGMENTS

364

The authors are indebted to PETROBRAS for the financial support and K.O.M. for the

365

fellowship. H.O.P. acknowledges Conselho Nacional de Desenvolvimento Científico e

366

Tecnológico (CNPq) for the fellowship.

367

SUPPORT INFORMATION AVAILABLE

368

This information is available free of charge via the internet at http://pubs.acs.org/.

369 370

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480

FIGURES CAPTIONS

481

Figure 1. Profile of temperature programmed desorption of CO2 by organomagnesium

482

phyllosilicates functionalized with AMPTS by conventional (A) and microwave oven (B), at

483

SiT/Sitotal molar ratios of (a) 25; (b) 50; (c) 75 and (d) 100 %.

484


485

phyllosilicates functionalized with TMSPEDA by conventional (A) and microwave oven (B), at

486

SiT/Sitotal molar ratios of (a) 12.5; (b) 25; (c) 37.5 and (d) 50 %.

487


488

phyllosilicates functionalized with TMSPETA by conventional (A) and microwave oven (B), at

489

SiT/Sitotal molar ratios of (a) 8.33; (b) 16.67; (c) 25 and (d) 33.33 %.

490

Scheme 1. Adsorbed species on the solids surfaces in hydrated samples ((a) ammonium

491

bicarbonate

492

propylammoniumpropylcarbamate ion pairs, (d) hydrogen-bonded propylcarbamic acids, and (e)

493

silylpropylcarbamates).

and

(b)

ammonium

carbonate)

and

in

dry

samples

((c)

494 495


23


Page 24 of 30

496 497

(A)

498 499

(B)

500

Figure 1.


24

Page 25 of 30


501 502

(A)

503 504

(B)

505

Figure 2.


25


Page 26 of 30

506 507

(A)

508 509

(B)

510

Figure 3.


26

Page 27 of 30


511 512

Scheme 1.

513 514


27


Page 28 of 30

Table 1. Adsorption capacity and efficiency of the materials. Conventional Heating Molar Proportions / %

Microwave Heating

Adsorption Capacity

N contents

Efficiency

Adsorption Capacity

N contents

Efficiency

mmol CO2 g-1

mmol N g-1

mol CO2 mol-1 N

mmol CO2 g-1

mmol N g-1

mol CO2 mol-1 N

25 AMPTS – 75 TEOS

0.906

1.228

0.738

0.854

1.286

0.664


1.601

1.986

0.806

1.413

2.157

0.655


2.287

2.564

0.892

2.383

3.430

0.697

100 AMPTS

0.894

3.136

0.285

2.019

3.843

0.620

12.5 TMSPEDA – 87.5 TEOS

1.474

1.900

0.776

0.981

1.486

0.660


2.029

2.507

0.809

1.221

2.600

0.470


2.845

3.368

0.845

2.087

3.021

0.686


*

*

*

3.227

4.028

0.801

8.33 TMSPETA – 91.67 TEOS

1.406

1.671

0.841

0.669

1.164

0.575


1.586

2.571

0.617

0.973

2.086

0.466


2.321

3.236

0.717

1.319

2.614

0.504


3.642

4.050

0.899

2.046

3.756

0.545

*sample not obtained.


28

Page 29 of 30


Table 2. Values of the kinetic model parameters for CO2 desorption. Kinetics parameters Molar Proportions / %

Conventional heating

Microwave heating

kav /min-1

n

SD** (%)

kav /min-1

n

SD** (%)


0.155

2.265

6.535

0.138

2.138

4.737


0.137

2.117

5.765

0.150

2.114

4.474


0.143

2.062

5.110

0.146

2.189

3.815

100 AMPTS

0.162

2.420

7.041

0.151

2.903

4.309


0.138

1.944

3.399

0.149

1.978

3.274


0.155

2.022

2.856

0.166

1.967

4.068


0.159

2.034

5.678

0.159

2.029

4.753


*

*

0.166

2.116

6.672


0.136

2.023

6.147

0.142

1.716

3.182


0.157

1.993

4.312

0.163

1.869

3.973


0.130

2.267

3.324

0.178

1.946

4.407


0.147

2.141

7.256

0.178

2.001

3.561

*sample not obtained; **SD = Standard deviation.


29


211x114mm (96 x 96 DPI)


Page 30 of 30

Comparative Adsorption of CO2 by Mono-, Di-, and Triamino

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