CO2 Capture Using Semi-Clathrates of Quaternary Ammonium Salt

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Article 2

CO Capture Using Semi-Clathrates of Quaternary Ammonium Salt: Structure Change Induced by CO and N Enclathration 2

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Bertrand Chazallon, Michael Ziskind, Yvain Carpentier, and Cristian Focsa J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 11 Sep 2014 Downloaded from http://pubs.acs.org on September 30, 2014

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The Journal of Physical Chemistry

CO2 Capture Using Semi-Clathrates of Quaternary Ammonium Salt: Structure Change Induced by CO2 and N2 Enclathration

Bertrand Chazallon*‡, Michael Ziskind‡, Yvain Carpentier‡, Cristian Focsa‡

Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, University Lille1 Bat. P5, F-59655 Villeneuve d’Ascq (France)

ACS Paragon Plus Environment

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ABSTRACT Semi-clathrates of tetrabutylammonium bromide (TBAB) are investigated for their potential application in the CO2 capture context based on hydrate technology. The three-phase lines of semi-clathrates of CO2-TBAB-H2O and N2-TBAB-H2O are established simultaneously with the structure using in-situ Raman scattering performed at high pressure. The preferred crystal phase obtained at ambient pressure from solution of 5 wt% and 40 wt% TBAB initial concentrations is shown to change upon enclathration of CO2 or N2, or by applying a higher pressure on the system. Deep in the stability field, metastable hydrate phases are occurring at the onset of the formation and correspond to the ones expected at ambient pressure conditions. Depending on pressure, they progressively transformed into the most stable ones on approaching equilibrium and dissociation points. Besides, it is shown that a 5 wt% TBAB original solution forms preferentially a mixed structure of both type B and type A at low gas pressure with CO2 as guestgas. A new structure is spectroscopically characterized at pressure higher than ~ 2MPa CO2. Type A is demonstrated to be stable at 5 wt% initial TBAB concentration with N2 as guest molecule and pressure between 8 and 12 MPa. These structural data address new insights on the relationship between the hydrophilic-anion and hydrophobic-cation intercalation with a guest gas producing hydrophobic interaction in a distorted water lattice.

KEYWORDS: Semi-clathrates, CO2, Raman, vibrational spectroscopy, high pressure


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1. INTRODUCTION

The global greenhouse effect of CO2 has attracted increased attention as a worldwide problem1 as annual global emissions of CO2 as escalated by approximately 80% between 1970 and 2004.2 This drastic rise has been attributed to an increasing dependence on the combustion of fossil fuels which account for 86% of anthropogenic greenhouse gas emissions, the remainder originating from land use change and chemical processing.3 CO2 capture and sequestration (CCS) for industry is a great challenge that is susceptible to reduce carbon emission. Exhausting gases from industry require treating important quantities of CO2, in the range of several cubic meters per second.4 In post combustion technology, which can be adapted to existing plants (in contrast to pre-combustion), CO2 concentration to be treated is generally of 15-20% for power plants, but it can be higher in steelmaking plants (up to 40%), or also in some cases for natural gas production.5 In this context, gas hydrates are currently getting a significant attention as a potential CO2 storage technology because they can well fulfils the requirements of a low cost process with respect to the specific compositions and operative pressure and temperature conditions. The principle of exploiting hydrates for the selective removal of CO2 from a multi-component gaseous stream to provide a CO2 depleted gaseous stream was first proposed by Spencer, 1997.6 On the other hand, the use of additives (alkyl-onium salt) in water was found to operate as a new gas separation method in which small gas molecules can be incorporated into the empty cavities of a crystalline semi-clathrate lattice.7 Further, this method allows a considerable reduction of the hardness of the formation conditions of hydrates. In recent years, intensive investigations have shown a noticeable alleviating effect of peralkyl ammonium salts on the pressure and


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temperature required to their formation8,9,10 and inspires applications in refrigeration,11,12,13,14 airconditioning industries,15 hydrogen storage for in-place or mobile application.16 Semi-clathrates of quaternary ammonium salts originally found by Fowler et al.17 have been known to have some unusual properties in comparison to ordinary clathrates. The most stricking features are that (1) they are easily formed by cooling their aqueous solutions at atmospheric pressure, and the concentration range from which the hydrate is separated out is very wide (2) they are very stable, i.e. they have high melting points as compared to ordinary clathrates (3) their formation is possible for salts having a variety of anions. It turns out that the hydrophilic anions (for e.g. Br−) form a part of the clathrate host framework substituting water molecules at the vertices of the cavities, whereas the hydrophobic cations usually made of alkyl groups branched to an ammonium (or a phosphonium or sulphonium) occupy the centre of four large cavities as guest. This contrasts to the usual structure of clathrate hydrates in which the host lattice is composed only from water molecules. Although it is known that the shape and size of the hydrophobic part of the guest determine the clathrate-forming ability of the guest, the anion often considered as secondary importance in semi-clathrates was however proven to play a role in the stability of the compounds.18 For e.g., fluoride ions are considered to have closest similarity to water in size and in the ability to form H-bonds with length of ~2.8 Å. On the other hand, larger ions such as bromide or chloride introduce additional distortions into the framework (with H-bonds length of 3.1 Å and 3.4 Å respectively) which makes the hydrates less stable and decrease their melting point. This tendency is even observed when a guest gas is incorporated. For instance, semi-clathrate of TBAF containing fluoride ions and incorporating hydrogen molecules in the vacant dodecahedral cavities are known to be more stable than the one


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containing bromide.8 A similar behaviour is observed when CO2 is trapped into semi-clathrates containing bromide, chloride or fluoride.19 In this communication, we report on the structure and equilibrium properties of CO2- and N2semi-clathrates of tetra-butyl ammonium bromide (TBAB). The structure and composition of TBAB hydrates free of guest gases have been investigated in numerous studies under atmospheric pressure conditions.20,21,22,23,24,25 It appears that the hydration number (n) varies from n = 2.03 to n = 38 and thus corresponds to different crystal structures of the system (C4H9)4N+Br−·nH2O.20,21,23,26 However, some uncertainties still exist in the associated crystal structures corresponding to n = 24, 26, 32, 36 and 38. Further, it was shown that TBAB hydrates with n = 26 and n = 38 (renamed type A and type B, respectively) are preferentially formed and their stoichiometric composition is found respectively at 40 wt% and 32 wt% TBAB. 25,27 Further, Type A with the hydration number of n = 26 (determined at congruent melting point (mp) of 12°C) was found to have a crystal structure with tetragonal lattice and the following parameters: a = 23.41 Å and c = 12.56 Å.25,27 In contrast, a recent accurate re-investigation of the TBAB-H2O phase diagram by Sato et al.28 reveals a congruent melting point (mp) at 285.9 K (12.75°C) for type A with TBAB content in water comprised between 0.35 < xTBAB < 0.37 (xTBAB in unit of weighted fraction). This corresponds to n between 30 and 33. Thus, Sato et al.’s results28 may questioned the hydration number previously reported for type A (n = 26) at maximum mp.25 In his review, Davidson29 (based on McMullan & Jeffrey diffraction work20) reported the structure of TBAB-H2O as being tetragonal with space group P42/m, a mp of 12.5°C, a hydration number n ~ 32 and lattice parameters: a = 23.6 Å, c = 12.5 Å. A comparison with the mp and hydration number reported by Sato et al.28 reveal a good agreement between these studies.


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However, the structure and lattice parameters are close to the type A (with n=26) reported by Shimada et al..27 This apparent discrepancy makes the assignment of the type A and its associated hydration number uncertain. In other diffraction studies carried out by Dyadin & Udachin21 and Gaponenko et al.26, it is shown that two hydrates with tetragonal symmetry and n values of 26 and 32 can exist. They were observed to differ in their incongruent mp of 12.2°C and 11.6°C, respectively, and also in their lattice parameters: a= 23.9 Å, c = 50.8 Å, and a= 33.4 Å, c = 12.7 Å, respectively. Further, these authors reported a hydrate with a higher mp at 12.4°C and monoclinic symmetry (space group C2/m). It was recognized that this latter was especially difficult to obtain.23 In re-visiting the TBAB-H2O system, Rodionova et al.30 obtained tetragonal crystal structures that differ in the way of including the bromide anion and arranging the tetrabutylammonium cation. Accordingly, they established that distinct tetragonal TBAB ionic semi-clathrates exist and can be divided into three groups according to their unit cell parameters with (1) c = 12.6 Å, (2) c = 38 Å and (3) c = 50.5 Å. They established a compositional range of existence between 30 and 50 wt% TBAB, corresponding to hydration numbers between 24 and 32. Therefore, we believe that the tetragonal hydrate (type A) identified by Shimada et al.27 with lattice parameters: a = 23.41 Å and c = 12.56 Å belongs to the Rodionova’s group (1) and is analogous to the one reported by Mc Mullan & Jeffrey20 (with similar lattice parameters), but with an actual n value of 32. Further diffraction work is certainly needed to confirm this tendency. The determination of the structure and composition of type B hydrate is apparently less problematic. The structure was solved by Shimada et al.24 who determined a congruent mp close to 9.9°C and a crystal structure of orthorhombic symmetry (space group Pmma). It should be noted that this hydrate has a structure close to the TBAB 36H2O identified by Dyadin & Udachin


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(mp at 9.5°C), with a crystal of same symmetry but a distinct space group (Pmmm) and lattice parameters of comparable values (a = 21.3 Å, b = 12.9 Å, c = 12.1 Å). We believe that the compounds obtained in both independent studies are most likely the same. If it is recognized that type A hydrate is the preferable phase at high TBAB concentration (> 18 wt%)25 under atmospheric pressure conditions, the situation is less clear when a guest gas is incorporated. Indeed, the likely reason is because most of the studies conducted so far are based on classical p-V-T techniques or gas phase analysis and adopted the unjustified assumptions that no structural transition occurred when the guest gas is encaged. Only few studies have reported systematic approaches and spectroscopic viewpoints to verify the structure of the (guest-gas) + TBAB-H2O system using more sophisticated techniques. From the temperature-composition diagram measured with differential scanning calorimetry,31 the maximum dissociation temperature at pressure above 2 MPa is found at wTBAB ~ 0.25 for different gas(guest)+ TBAB semi-clathrates, thus implying higher hydration number than the one expected in the TBAB-H2O system free of guest gases. This view is consistent with a change of the stable structure associated with the enclathration of guest components. Besides, in a combined Raman and X-ray diffraction study, Jin & Nagao32 were able to observe a change in the stable crystal phase of TBAB hydrates enclosing xenon. Furthermore, Type B was demonstrated to be the preferred crystal phase in our preliminary Raman work on CO2 enclosed in semi-clathrates at initial composition of 40wt% TBAB.33 In contrast, no change of the TBAB semi-clathrate structure was observed when CH4 guest is incorporated into the 512 dodecahedral cages, as reported in a NMR spectroscopic study.34 It becomes then obvious that more details on the relation between guesthost interactions, the distribution and cage occupancies of the guest gases and the TBAB semiclathrate structures should be investigated. In addition, the efficiency of capture and separation


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of CO2 from flue gases or fuel gas mixtures appears to depend on the hydrate structures. For e.g., differences in the CO2 uptake and separation efficiency from simulated fuel gas mixture was observed in TBAB 40wt% system relative to the TBAB 5wt% one. This effect was attributed to the different semi-clathrate structures induced by the initial loading concentration of TBAB in water.35 Using the unit cell formulas for the idealized frameworks30, an upper limit for the maximal guest-gas content can be derived depending on the structure formed. The tetragonal type A will have the expected composition 5TBAB·10CO2·162H2O (i.e. a mass fraction of CO2 of 8.9 wt%) whereas a type B will have the idealized composition of TBAB·3CO2·38H2O, i.e. a maximal mass fraction of CO2 of 13.1 wt%. Therefore, the design of a process to capture CO2 on a hydrate-based technology requires a better understanding of the phase behaviour and structural properties of the semi-clathrate systems. This appears also to be the prerequisite for the development of reliable models to predict the phase equilibria and their application in several fields.36,37 The present work reports on the phase equilibrium and structure of the CO2- and N2-TBAB-H2O system at two TBAB concentration (5wt% and 40 wt%) and different gas pressure conditions between 0.6 MPa and 13 MPa. The structure and composition of all samples are examined via Raman spectroscopy concomitantly with p-V-T measurements.

2. MATERIALS AND METHODS 2.1. High pressure stage


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The hydrates are synthesized in a high pressure optical reactor made of titanium (batch mode). A picture of the set-up is shown in Figure 1. The cell was designed at PhLAM and elaborated by Top Industrie S.A.. It is equipped with a sapphire window that has been conceived to stand pressures up to 20 MPa. The inner chamber where hydrates are formed has a small volume of ~ 1 cm3. A magnetic stirrer ensures agitation to facilitate homogenization of the liquid at the beginning of the reaction. A thermostated bath (Lauda) maintains the temperature of the stage at the desired value (± 0.1°C). The pressure is controlled using a high pressure transducer (0-20.00 ± 0.01 MPa, Keller) and the temperature is measured via a K-type thermocouple (± 0.2°C). Both pressure and temperature are recorded continuously on a data logger (Graphtec GL-200A).

2.2. In-situ Raman spectroscopy The Raman set-up (Figure 1) consists in an InVia Reflex Raman spectroscope (Renishaw) on which the reactor is mounted. The spectrometer has a 250 mm focal length allowing a spectral resolution of ~ 1.5 cm−1 with a holographic grating of 1800 grooves per mm and a Peltier cooled front illuminated CCD detector (576 × 400 pixels). The excitation radiation is produced by an Ar laser source (Modu-Laser) emitting at λ = 514.5 nm. An Olympus 50× objective (0.5 N.A.) provides a ~ 2 µm circular beam spot. The laser power at sample is less than 3 mW as measured by a Lasercheck power-meter (Coherent). The focus zone can be adjusted in the gas, in the liquid or in the solid hydrate phases. Spectral positions were calibrated against mercury lines that were simultaneously recorded with each hydrate spectra, yielding very accurate value for the spectral position of the guest gas contribution (less than 0.2 cm−1 uncertainty). All spectra are baseline


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corrected and the peak positions of the collected spectra were determined using a least square fitting procedure.

2.3. Semi-clathrate phase equilibrium Hydrate samples are prepared by injecting less than 1 ml of aqueous solution of TBAB (with 99.5% purity supplied by Sigma-Aldrich) into the reactor chamber. A detailed account for the production of hydrate samples has been given elsewhere38 and is briefly described below. The CO2 and N2 gases are supplied by Air-liquid and have a stated purity of 99.995%. The chamber and all the high pressure lines are first evacuated with a primary root (Scroll (Varian), dry-pump) to ensure that no residual air is present before the introduction of the gas. The gas is then stored into a reservoir (115 cm3) and the temperature is adjusted to reach the operational value chosen. It is kept constant during few minutes before the contact between the gas reservoir and the sample chamber is made. To avoid any co-existence with CO2- (or N2-) clathrates, the pressure and temperature conditions were chosen outside the stability field of the CO2- (or N2-) clathrates. The stirring is started immediately after pressurization of the system to facilitate homogenization and dissolution of the gas. Crystallization always requires that a certain sub-cooling (driving force) be reached in order to compensate the soft agitation. On cooling the system, a majority of ice and a small quantity of hydrates are formed. Much longer residence time is necessary to improve the conversion of ice into hydrates and this cannot be systematically monitored because the rate of conversion of water or ice into clathrates is too slow. Therefore, a multi cycle-mode of production is used to enable an efficient conversion of water into hydrates.31 Each cycle is composed of a cooling down followed by a heating to a temperature lower than the temperature


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of dissociation of the hydrates. A certain time (days to weeks) is allowed to let the system equilibrated until no pressure drop is observable. When the system is stabilized, Raman spectra of the hydrate and liquid phases are collected. Optical visualization indicates that at least three phases (vapor-liquid-hydrate) coexist as liquid aqueous solution of TBAB can be distinguished all around the hydrate slurry. Then, the temperature is increased by step of 0.3°C or 0.5°C during which the pressure increases due to hydrate dissociation. Time for equilibration is allowed during each step of heating and a new set of equilibrium data (liquid and hydrate phase) are collected by Raman. When all the hydrates have decomposed, the dissociation point is established. For TBAB semi-clathrates free of guest gases, the type A and type B are obtained using a cryostage (modified FDCS196 from Linkam) in which a 0.2 ml of aqueous solution of 40 wt% and 5wt% TBAB is respectively deposited. The samples are cooled down from room temperature at rate of 1 K/min to reach the stability zone of type A and type B according to the phase diagram established by Oyama et al..25 Raman spectra are collected at different temperatures between 268K and 283K for type A, and 268K and 280K for type B. A step heating procedure is applied until the complete dissociation of the hydrate is obtained.

3. RESUTS AND DISCUSSION 3.1. Spectral features of the molecular components in the TBAB-H2O system: structure discrimination On figure 2, an example of crystals of TBAB type A and type B hydrates is displayed. Figure 2a shows the faceted tetragonal type A crystals formed at 268K and heated to 281K. In figure 2b,


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the type B appears as a polycrystalline-like surface, in agreement with previous observations.27 A comparison of the corresponding Raman spectra is provided on figure 3. The spectral range chosen 2700-4000 cm−1 covers the C-H stretching modes of the butyl groups (2700-3050 cm−1). In type A, it is assumed that a tetra-n-butyl cation (TBA+) is located at the center of 3 tetrakaidecahedrons and one pentakaidecahedron.27 In type B, the TBA+ cation is located at the center of two tetrakaidecahedrons and two pentakaidecahedrons.24 This distinct environment of the TBA+ cation in type A and type B causes a spectral shape of the bands corresponding to C-H modes proper to each crystal structure (Fig. 3) and then allows their direct identification. In the spectral range covering the O-H stretching modes of water molecules (3050-4000 cm−1), two broad bands can be deconvoluted at ~3185 cm−1 and 3420 cm−1. In the spectrum of pure water (supercooled), the low frequency band contribution appears at relatively higher frequencies (~3200 cm−1) whereas a less significant change is observed for the 3420 cm−1 band. These bands are respectively attributed in pure liquid water to the in-phase (highly polarized i.e. with a depolarization ratio close to zero) and out-of-phase O-H stretching of a water aggregates consisting of a central H2O molecules and its first and higher neighbors.39 The downshift observed in the solid hydrates is consistent with the formation of H-bonds of higher strength in comparison to those occurring in liquid water. The close resemblance of this spectral region in both hydrates to that of liquid water has been observed in the case of structure II (sII) clathrate hydrates of methane, or of mixed hydrocarbons.40 It was argued that the network of water molecules in structure II hydrates must have something in common with the network of water molecules in liquid water. It was suggested that it is the predominance of small cages (512) in sII that is responsible for the analogies between sII and liquid water spectra, especially due to Hbonded pentagons. In contrast, the predominance of large cavities (51262) with hexagonal faces in


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structure I (sI) and thus the associated higher ratio of H-bonded hexagonal rings relative to pentagonal ones in sI induce a vibrational behavior similar to that of ice Ih. On this basis, we infer that the ratio of pentagons to hexagons can be used to predict the vibrational behavior of the host lattice in clathrate compounds. Type B which is composed of 6512 + 451262 + 451262 cavities contains 10 times more pentagons than hexagons and thus the local structure of water is closer to that of (super-cooled) water in which pentamers are more dominant than into ice. Note that the pentagons to hexagons ratio is equal to 9 in type II clathrates (i.e. with a “water-like” O-H vibrational spectra) and 8 in type I (i.e. with a “ice-like” O-H vibrational spectra). The detailed structure of type A has not been determined yet, but assuming an ideal unit cell composed of 10512 + 1651262 + 451262 cavities29,30, the ratio of pentagons to hexagons is 9, thus, the host lattice O-H vibrational spectra of type A is also expected to be “water-like”. It is noticeable that an earlier study of Walrafen et al.41 is not in full agreement with these assumptions. The Raman O-H spectral region in sII clathrates of THF presents similarities to that of highly supercooled water (240 K), i.e. that the low frequency part of the O-H band in water (observed at 3190 cm−1)42 has a significant intensity maximum. This increase in intensity was attributed to an increased proton correlation due to the formation of polyhedral clathrate structures in liquid water. Enhanced proton correlation occurs as the water molecules combined to form networks of five and six (or higher)-membered rings. Lowering the temperature of water in the supercooled regime (down to 228 K) furthers its structural similarities with structure II clathrates, as the concentration of pentagonal rings, which are more favored energetically over hexagonal rings, increases as temperature decreases. Further, it was suggested that the supercooled water Raman spectrum would match the one of sII clathrates if the supercooling


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reaches 228K. However, Raman spectra have not yet been obtained from water under such extreme conditions. In comparison, it is to noticed that the main O-H contribution in hexagonal ice occurs at ~ 3134 cm−1 at 240 K (not shown here). Moreover, it should be noted that the low frequency band at 3220 cm−1 in water (room temperature) that is thought to arise from collective motions of small water clusters43 also results from the complications of intermolecular and intramolecular vibrational coupling that makes the interpretation of this portion of spectra difficult without using isotopic dilution. Furthermore, several improvements in the understanding on the structure and dynamics of this part of spectra for liquid water have been gained from theoretical approaches.44 In consequence, our data allow an easy distinction between the different crystal phases based on the spectral fingerprints of this spectral region alone. The occurrence of empty dodecahedral water cavities with pentameric ring structure in both semi-clathrate type A and B provide a vibrational signature close to that of supercooled water. It is noticeable that the TBAB powder does not present any contribution from water molecules. Other spectral regions between (700-1600 cm−1) and (100-600 cm−1) are also investigated to support the comparison (Figure 4). These results are in agreement with earlier Raman investigations.33,45 A splitting of the band at 1320 cm−1 and 1455 cm−1 is observed in type B relative to type A or TBAB powder. This peak separation is found to be very helpful to distinguished between the different hydrate structures. Such peak splitting may come from a change in the structural environment of the alkyl groups and can be attributed to CH2-bending modes in this spectral region.46


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The (90-600 cm−1) spectral range is featured by a sharp TBAB peak at ~260 cm−1 in type A and ~259 cm−1 in type B, also present in the TBAB powder (~259 cm−1). Further, a broad band (~25 cm−1 width) located at 193 cm−1 and ~204 cm−1 is observed in the spectra of type A and type B respectively (Figure 5). The assignment of these bands can be performed by comparing the spectrum of each hydrate with those of supercooled liquid water and ice obtained in the same conditions. The low frequency Raman intensity has been converted into reduced intensity Ir(ω) (with Bose-Einstein and quasi-elastic correction) in order to obtain low frequency Raman spectra free of band shape distortion due to thermal population effects. In supercooled water (268 K), a broad band occurs at ~ 189 cm−1. This well-known feature also present in aqueous solutions (not shown here) is attributed to collective intermolecular OH···O stretching vibration involving tetra-coordinated water molecules in pentameric structure.47 A lowering of temperature induces a shift toward higher frequencies consistent with H-bonds strengthening. In ice, an intense peak appears at 211 cm−1 in the translational region where water molecules undergo transverse-optic vibrations.48 In semi-clathrates, the band is shifted toward lower frequencies relative to the ice case which might indicate the existence of softer H-bonds, most probably caused by the deformation of the tetrahedral units. Thus, the trend in the strength of the H-bonds is thought to decrease from ice to the hydrates and then to super-cooled water. At deeper super-cooling the local water structure may be composed of H-bonds with strength much closer to that occurring in the hydrates. Further, in type A, the band is observed at lower frequencies than in type B, which is an indication that softer H-bonds occurred in this structure which are also closer to the H-bonds strength occurring in water. 3.2. Phase diagram and associated Raman spectra of the CO2-TBAB-H2O system


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5wt% TBAB. We focused below on how CO2 affect the equilibrium and hydrate structure upon incorporation. Significant discrepancies exist in the literature concerning the determination of the three phase line (liquid-vapor) of the CO2-TBAB-H2O system.5,12,19,49,50,51 We report in figure 6 equilibrium lines given by different authors on experimental data obtained at the same TBAB concentration of 5wt%. A plot of the CO2-H2O system52 is also provided to highlight the higher thermal stability of the CO2-semi-clathrate (with TBAB) in comparison to that of the CO2clathrate (without TBAB). For the same initial TBAB concentration (5wt%), differences in the dissociation temperature of up to 2.5°C between literature values are observed. 5,19,50,51 This corresponds also to incertitude of ~0.6 MPa in pressure scale. Data obtained at 4.4 and 4.5 wt% TBAB concentration12,49 are equivalently dispersed among the experimental points obtained at 5wt%. However, thermodynamic models proposed to calculate the phase diagram of the CO2+TBAB semi-clathrates need accurate experimental dissociation data to succeed in predicting the equilibrium lines.36 Indeed, model parameters (e.g. parameters of square well cell potential for CO2) are adjusted on experimental dissociation data. Our micro-Raman set-up offers a unique opportunity to determine accurately the hydrate dissociation points by measuring the spectral response of the sample under in-situ conditions (see experimental part above) and detect very small quantity of hydrates very close to equilibrium. It can be seen (Fig.6, table 1) that our results are in good agreement with the most recent ones provided by Ye & Zhang,51 with less than 0.5% deviation. It is to notice that with the log scale chosen, the equilibrium line is not linear and appears to be slightly tilted when temperature increases at dissociation temperature values above ~10°C. This change of slope is more clearly seen on the data obtained by Li et al.19 but at ~12°C. A change of slope in the equilibrium line is generally representative of a structural


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phase change associated with a change in the dissociation enthalpy of the solid formed. According to the Clapeyron equation, one has: ௗ௟௡௉ భ ௗ( ) ೅

=−

୼ு೏೔ೞ ௭ோ

,

where ∆Hdis is the dissociation enthalpy of the hydrate phase in kJ/molgas, z is the compressibility factor. Such structural change has not been considered so far, as most of earlier studies have adopted the assumption that no structural transition occurred when CO2 is encaged. The use of indirect technique of characterization like p-V-T measurements5,12,19,49-51 makes the detection of structural transformation difficult. In contrast, several features of the vibrational spectra corresponding to the C-H modes and O-H···O hydrogen bonds will be affected. Coming first to a surprise, but now corroborated by us in a number of runs, the hydrate type A is found to co-exist with the stable type B at initial composition of 5wt% TBAB. In figure 7, we provide for the first time spectral evidence for this assertion at 7.9°C and 0.74 MPa and conditions close to dissociation of the CO2-semi-clathrate. Raman spectra covering the C-H stretching mode of TBAB molecules (Fig.7a) and collected at different locations onto the sample exhibit different features. The vibrational spectra of hydrates can be identified on small solid grains (typically of 20-50 µm sizes) dispersed in the liquid aqueous mixture of the sample. A comparison with Raman spectra obtained at ambient pressure (displayed in Fig.3) allows identifying type B and type A which are found to coexist on a same solid particles or on distinct remaining particles. The stars in figure 7a indicate that a small sample part of type B is irradiated by the laser during the analysis of type A.


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The vibrational fingerprint of the TBAB aqueous solution surrounding the solid particles can be distinguished from that of the hydrates, as it can be seen in figure 7a, with characteristic three intense broad bands in the C-H spectral region between 2850 and 3050 cm−1. Further, the coexistence of three phases (hydrate – liquid – vapor) corroborates the fact that the sample is close to equilibrium conditions. The presence of occluded CO2 can be recognized by two intense bands located at 1381 cm−1 and 1273.5 cm−1 (Fig.7b, table 2). They are caused by a Fermi resonance effect between the symmetric stretching mode (ν1) and the overtone of the bending mode (2ν2). In comparison, gaseous CO2 can be observed at 1388 and 1284 cm−1.38 Both bands represent a mixture of ν1 and 2ν2. Two minor bands at 1263 and 1408 cm−1 can only be observed in the gas phase and denoted as hot bands. In comparison to CO2 gas phase, the CO2 trapped in the hydrates (type A or B) exhibits broader bands (width of ~ 12 and 20 cm−1, respectively) which are downshifted at 1381 cm−1 and 1273.5 cm−1 (type B) and 1381.6 cm−1 and 1273.5 cm−1 (type A). This downshift is attributed to a perturbation of the local electrostatic fields produced by the water molecules forming the cages.53 In contrast, the bands of CO2 dissolved in the solution are less shifted and occurred at 1275.6 cm−1 and 1382.7 cm−1. Note that the detection of such distinct environment for CO2 opens the possibility to derive solubility data for CO2 in TBAB solutions, which is an important missing parameter in model.54 At low frequency, the O-H···O contribution appears to be slightly downshifted (~3 cm−1) in comparison to the hydrates free of CO2 (Fig. 3 and 8) which indicate softer H-bonds. The intermolecular H-bonds are softer because of the supplementary deformation of the tetrahedral units of water added by the presence of CO2 in the dodecahedral cages. On the other hand, the


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band position corresponding to TBAB molecules remains mostly unchanged at ~260 cm−1. In the aqueous solution, the Raman bands of the intermolecular hydrogen bonds are weaker and slightly downshifted in comparison to neat water (~192 cm−1 at 8°C).55 The presence of the solute in the aqueous mixture promotes a shift toward low frequency as it is observed in a number of electrolyte solutions,56 which is often associated with a capability of bond breakage of the dissolved salts. Next, we focused on the structural changes induced by an increase of the gas pressure of CO2. A sample is formed at 1.8 MPa and slight undercooling before approaching the equilibrium line at 1.6 MPa and 10.4°C. Some remaining particles of solid hydrates present specific signatures in their Raman spectra. In the C-H stretching mode region, new peaks appeared at 2959.3 cm−1 and 2979.3 cm−1 whereas the weak band occurring in type B at 3018 cm−1 is shifted to 3020.5 cm−1 (see arrows in fig. 9). Also, the relative intensity of the bands at 2968 cm−1 and 2991.5 cm−1 in type B is changed in the “1.6 MPa” sample. We believe that these spectral modifications (band splitting and intensity change) correspond to the formation of a new hydrate phase. Note that this structural transition happens at similar conditions where a change of slope in the equilibrium line is observed on the 5wt% TBAB sample, i.e. at ~10°C and 1.6 MPa (see Fig.6). It should be noted that this attribution needs to be corroborated by diffraction work or other techniques. Further, on some other particles of the same sample, type B can be identified (Fig. 9) at slight undercooling condition of ~0.5°C on approaching the equilibrium line. This phase is not detected when a temperature increment of +0.3°C is added (i.e. at the closest condition of dissociation). For sample formed at 2.9 MPa, the C-H modes of TBAB show similar features with new peaks appearing in the massive C-H contribution, with however less intensity as in the “1.6 MPa”


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sample. Weak bands are observed at 2959 and 2979 cm−1 whereas the high frequency band is shifted to 3020 cm−1 (see arrows in Fig. 9). It seems likely that the new phase detected at 1.6 MPa and 10.4°C is also present at higher CO2 pressure of 2.9 MPa and temperature of 12.3°C. Nevertheless, the weak signal may indicate that part of the sample co-exists with type B. In the medium-frequency range, soft changes in the vibrational spectra are observed for the new phase. More specifically, a new band appears at ~1407 cm−1. Further, the relative intensity of the peaks at ~1307.5 cm−1 and 1326 cm−1 is modified in comparison to type B or the liquid aqueous solution, with an increase in intensity of the band at 1326 cm−1. At intermediate wavenumbers (1000-1100 cm−1), the weak band attributed to the C-C stretch of TBAB shifts from ~1060 cm−1 (type B) to ~1051 cm−1 (new phase). This shift of frequency indicates a structural transformation in which the environment of the carbon atom is modified and makes the carbon stretch softer. In the 100-600 cm−1 range, the O-H···O stretch position is located at distinct wavenumbers corresponding to the different hydrate structures, thus corroborating the previous attribution (Fig.9 and 10). As it is shown on figure 11, the intermolecular H-bond stretch of type B is spotted at ~200 cm−1, whereas that observed in the new phase is slightly downshifted to ~195 cm−1, that is, at slightly higher value than in type A (~191 cm−1). We believe that this feature is an additional insight for the occurrence of a distinct phase from the known type A or type B. 40 wt%TBAB. In the 40wt% TBAB runs, 4 hydrate samples are investigated. In comparison to 5wt% TBAB samples, there are less literature data available and then less dispersion for the dissociation lines. Our data are shown to be in line with those from Deschamps and


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Dalmazzone31 for the lowest pressure and with the Lee et al.34 data for the highest pressure investigated. A deviation of less than ~2% is observed with these data. It is worth mentioning that deep in the stability field (for e.g. left part of the “40wt%” curve, with ~2-3°C undercooling), CO2-semi-clathrate type A is first formed at ~2.8MPa (starting value) (Fig. 13, see spectra ‘type A metastable’). Few days later, only CO2 semi-clathrate type B can be detected, (Fig.13, ‘type B stable’) and remains stable up to the dissociation line. Note that the CO2 contribution (at ~1381 cm−1 and ~1273 cm−1) has a relatively weak contribution in the spectra “type A metastable” 2.4 MPa as the data are collected at the beginning of the run. Additionally, the metastability of type A is reproduced for the samples at 3.4 MPa (starting value) in a number of runs. Although type A is the expected stable phase under atmospheric pressure condition, type B seems preferentially stabilized by the incorporation of CO2 in the structure already at pressure above 1.4 MPa. This spectroscopic view point is in agreement with visual inspections of the TBAB-water system with and without CO2, that showed at least two different hydrate morphologies.51 For samples formed at 1.4-1.6 MPa, they undergo a phase change on approaching the dissociation line. Starting from type B, their spectra contain a mixed contribution of CO2 semiclathrate type A and type B (Fig. 13) close to dissociation. Several striking spectroscopic features of both structure types are observed in the “mixed spectra” and the C-H and O-H stretching region (1.4 MPa). First, several shoulders appear on both the low and high frequency side of the massive “C-H band” (see arrows Fig. 13 and 1.4 MPa sample). As well, small peaks at 1134 cm−1, 1155 cm−1 and 1172 cm−1 characteristic of type A can be easily seen. A shoulder (characteristic of type A) (versus a peak at 926 cm−1 (type B)) occurs at ~ 929 cm−1 that confirms


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the existence of type A in this compound. In the O-H stretching region, a clear loss of intensity for the broad “water band” at ~3190 cm−1 is observed (see arrows, Fig. 13 and discussion in section 3.1). From these measurements, we infer that Type B is the preferred structure above ~2.0 MPa with the initial 40 wt% TBAB system, whereas, it is most likely a new phase at pressure above ~1.6 MPa with the initial 5wt% TBAB system. As the theoretical CO2 content is higher in type B relative to type A, the hydrate type B is better stabilized especially at higher pressure which allows a higher loading in gas molecules. In other words, the Langmuir constant is expected to be higher for CO2 in type B. Our experimental results are in line with the predicted behaviour developed by Paricaud for the CO2-semiclathrates.36 A determination of the structural modification in the presence of more gas components is also required to get a closer representation of the working industrial fluids.

3.3 Phase diagram and associated Raman spectra of the N2-TBAB-H2O system Our study is here focused on the N2-TBAB-H2O system with initial TBAB concentration of 5wt%. Four runs corresponding to hydrate samples formed at different pressure have been investigated (Figure 14). Literature data are scarce for the 5wt% TBAB system with nitrogen.5,50,57 As previously mentioned in a number of earlier work, the TBAB equilibrium lines lie well below the ones of the N2-H2O system also represented in figure 14. This highlights the better stability of the semi-clathrate hydrates.


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Our results on N2-TBAB-H2O (with 5wt% initial TBAB) are found to differ from previous published data (specifically in the 6-13 MPa range) with more than 1 K deviation. Our equilibrium data are reported in Table 3. At present time we do not have any clear explanation for this discrepancy. Besides, an extrapolation of the data from Duc et al.5 shows a possible change of slope of the equilibrium line (see Figure 14), which may be indicative of a structure change above 1 MPa. Raman spectroscopy applied to our samples is displayed in Figure 15. The Raman signature of the enclosed nitrogen into the dodecahedral cavities of the semi-clathrates can be observed at ~2324 cm−1, a value that is slightly downshifted relative to the N2 free gas phase (2328 cm−1).

This comes in agreement with the observed value in ordinary nitrogen-clathrates.58 In all of our samples, the phase formed at the onset of each run is always found to be type B. On approaching dissociation, type B remains stable only at the lowest pressure investigated that is, for the 6 MPa sample. Further, a mixture of type A and type B is obtained at the highest pressure (13 MPa). In contrast, samples formed at pressure of 8MPa and ~115 MPa, always show the progressive transformation of type B into a type A on approaching the dissociation line. The change of structure from type B to type A reflects a balance between several effects. In his model, Paricaud36 argue that the van der Waals−Platteeuw term in his equation (18) becomes more and more negative as the pressure is increased due to an increase of the occupied fractions of cavities with gas, and stabilize the hydrate phase. In contrast at low gas pressure, other terms dominate and the stable structure is the one obtained at atmospheric pressure. This general behaviour seems not in full agreement with our claims as type A appears at higher N2 pressure although type B is the expected low (atmospheric) pressure phase. Therefore, one infers that there must be a “guest” specific behaviour related to guest-host interactions and to the cage filling of the structure that influence the chemical potential of the semi-clathrates. Moreover, an improvement


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of our understanding of the properties of semi-clathrates filled with gas will be obtained via combined modelling and experimental approaches using spectroscopic techniques.

4. CONCLUSION A detailed Raman spectroscopic study of semi-clathrate of TBAB-H2O with CO2 as guest molecule has been carried out at different pressure and for two distinct initial TBAB concentrations of 5wt% and 40 wt%. As well, a preliminary study of nitrogen semi-clathrates of TBAB has been performed on the TBAB-H2O system with 5wt% initial TBAB concentration. This investigation has been performed in-situ with conditions close to the equilibrium liquidvapor three phase line of the semi-clathrates. The dissociation points of the solid hydrates have been investigated by p-T measurements combined with optical microscopy and micro-Raman spectroscopy. The corresponding structure of the semi-clathrates has been established directly via Raman spectroscopy, using intra- and intermolecular (lattice) specific signatures. Our experimental results show that the enclosing of CO2 (or N2) into the 512 cages leads to a change in the semi-clathrates structures. Additionally, the gas pressure applied on the system is shown to promote a change of the stable crystal structure. The change of structures from type B to type A (or the reverse) seems to be specific to the guests and is likely related on guest-host interactions and gas filling. It should be stressed that theoretical modelling in terms of Langmuir constants and fugacities of the different guests is certainly a good first approximation, however, it is unlikely to hold exactly true, as structural changes affecting the relative and absolute cage sizes have to be taken into account. Further investigations are in progress to relate the Raman measurements to the mixed CO2-N2 hydrates for the development of the hydrate-based


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technology of CO2 capture. Results of this work will be of major concerns for the understanding of gas fractionation effects in semi-clathrates applied to CO2 capture and storage.


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Figure 1: Schematic diagram of the experimental set-up: high pressure optical reactor (sample control system, gas mixtures preparation) and Raman spectrometer


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Figure 2: TBAB hydrates of a) type A collected at 284K and b) type B collected at 268 K. Scale bar indicates 50 µm

a)

b)


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Figure 3: Comparison of Raman spectra of TBAB hydrates type collected at 268 K and ambient pressure, and TBAB powder. The vibrational O-H stretching region indicates structural analogy with liquid water or ice Ih (see text).


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Figure 4: Comparison of Raman spectra of different types of TBAB hydrate collected at 268 K and ambient pressure, and TBAB powder. Distinction between the hydrate structures can be performed using this spectral range (see text).


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Figure 5. Low frequency Raman spectra of Type A and type B (ambient pressure) compared to that of water (super-cooled at 268 K) and ice Ih. Reduced intensity (Ir(ω)) is represented.


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Figure 6. Phase diagram of the CO2-TBAB-H2O system at 5wt% TBAB. Literature data and our data are compared (see legend).The equilibrium line of the CO2-H2O system is shown for comparison.


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Figure 7. Raman spectra (C-H modes (top) and TBAB bending modes (bottom) region) of a CO2-TBAB-H2O sample with initial composition of 5 wt% TBAB. Three phases are seen to coexist and can be distinguished in in-situ conditions (indicated in the legend) depending on the position analysed on the sample. The collected data are taken close to the hydrates dissociation point (see blue point in Figure 6). Note the distinct vibrational features of type A, type B and liquid aqueous solution of TBAB. Stars indicate a small contamination with type B in the Raman spectra of type A. The CO2 contribution is observed at distinct wavenumbers according to the structure formed (see text for details).


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Figure 8 Low frequency Raman spectra of CO2-TBAB-H2O (5wt% TBAB initial). The collected data are taken close to the hydrates dissociation point (see blue point in figure 6). Three phases (hydrate-liquid-vapor) coexist and can be distinguished at conditions indicated in the legend, according to the position analysed on the sample. The vertical dotted line is a guide for the eyes to see the distinct vibrational contribution of water (broad band ~ 180-200 cm−1) depending on the structure of the sample analysed.


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Figure 9. Raman spectra (C-H modes) of CO2-semi-clathrates of TBAB (5wt% initial) collected at different pressures (see legend), close to the dissociation line. Arrows indicate the formation of a distinct phase in Raman spectra of the 1.6 MPa (orange point in figure 6) and 2.9 MPa (red point in figure 6) in comparison to pure type B at 1.7 MPa. The dotted vertical line is a guide for the eyes to highlight the frequency shift of the weak band at 3020 cm−1 in the samples containing the new phase.


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Figure 10. Raman spectra (bending modes) of CO2-semi-clathrates of TBAB (5wt% initial) collected at different pressure (see legend), close to the dissociation line. The arrows indicate the changes brought by the formation of a new phase at 1.6 MPa and 2.9 MPa.


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Figure 11. Raman spectra (low frequency modes) of CO2-semiclathrates of TBAB (5wt% initial) collected at different pressure (see legend), close to the dissociation line. The vertical dotted line at low frequency is a guide for the eyes to highlight the distinct wavenumbers of the O-H···O stretching in the different samples.


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Figure 12. Phase diagram of the CO2-TBAB-H2O system at 5wt% and 40 wt% TBAB. The equilibrium line of the CO2-H2O system is also shown for comparison. For clarity, only the data of Ye & Zhang,51 and our data are represented for the 5wt% TBAB line. Literature data and our data (see legend) are reported for the 40wt%TBAB.


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Figure 13. Raman spectra (C-H modes (top) and TBAB bending modes (bottom)) of CO2TBAB-H2O samples collected at different pressure with same initial composition of 40 wt% TBAB. Three structures (liquid, type A, type B) coexist close to the dissociation line and can be distinguished at conditions indicated in the legend according to the position analysed on the samples. Note the distinct vibrational features of type A, type B, mixed contribution of type A and type B on approaching the dissociation. At 2.4 MPa and deep in the stability field, the initial phase formed is type A (see spectra ‘Type A (metastable’), whereas close to the dissociation line the sample is transformed into type B (‘stable’). The arrows (top figure) indicate the soft transformation of type B into (most probably) type A on approaching the dissociation line for the 1.4 MPa sample. Presence of CO2 is easily observed with the vertical dotted lines (bottom figure).


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Figure 14. Phase diagram of the N2-TBAB-H2O system at 5wt% TBAB. Literature data and our data are compared (see legend). The equilibrium line of the N2-H2O system59 is shown for comparison.


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Figure 15. Raman spectra (C-H modes) of N2-TBAB-H2O samples collected at different pressure and temperature (see legend and table 3) with same initial composition of 5wt% TBAB. Type A or type B coexist close to the dissociation line and can be distinguished at conditions indicated in the legend according to the position analysed on the samples. The vertical dotted line indicates the position of the band corresponding to the nitrogen stretching mode in semiclathrates.


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Table 1. Equilibrium data points obtained by Raman spectroscopy for two different initial TBAB concentrations on the CO2-TBAB-H2O system T/°C

CO2-TBAB-H2O

P/MPa

TBAB, w = 5wt% 7.9

0.74

10.4

1.61

12.6

2.80 TBAB, w = 40wt%

14.8

1.45

14.9

1.55

16.5

2.43

17.5

3.05


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Table 2. Wavenumbers and assignments of the Raman bands of CO2 in the gas phase, in the aqueous solution of TBAB (5wt%), in the CO2-semiclathrate phase type A and type B

Gas

Aqueous solution

Semi-clathrates CO2 / ν (cm−1)

CO2 / ν (cm−1)

CO2 / ν (cm−1)

Type A

Type B

1284

1275.6

1273.5

1273.5

1388

1382.7

1381.6

1381


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Table 3. Equilibrium data points obtained by Raman spectroscopy for an initial TBAB concentration of 5wt% TBAB on the N2-TBAB-H2O system

T/°C

N2-TBAB-H2O

P/MPa

TBAB, w = 5wt% 10.5

5.8

11.4

7.7

13.4

11.8

13.8

13.2


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AUTHOR INFORMATION Corresponding Author * E-mail: Bertrand.Chazallon@ univ-lille1.fr, Tel : +33320336463, Fax : +33320336468

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol)

ACKNOWLEDGMENT The author (BC) wishes to thank the Agence Nationale de la Recherche (ANR) as part of the SECOHYA project for financial support.

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D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem. Int. Ed. Engl. 2010, 49, 6058–6082.

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Metz, B. Davidson, O. De Coninck, H. Loos, M, Meyer, L. IPCC Special Report on CARBON DIOXIDE CAPTURE AND STORAGE; IPCC, Inte.; Cambridge University Press, 2005.

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Chakma, A. Mehrotra, A.K. Nielsen, B. Comparison of Chemical Solvents for Mitigating CO2 Emissions From Coal-Fired Power Plants. Heat Recover. Syst. CHP 1995, 15, 231– 240.


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Duc, N. H.; Chauvy, F.; Herri, J.-M. CO2 Capture by Hydrate Crystallization – A Potential Solution for Gas Emission of Steelmaking Industry. Energy Convers. Manag. 2007, 48, 1313–1322.

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Spencer, D. F. Methods for Selectively Separating CO2 From a Multicomponent Gaseous Stream. U.S. Pat. 5700311 1997.

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Kamata, Y.; Oyama, H.; Shimada, W.; Ebinuma, T.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Gas Separation Method Using Tetra- N -Butyl Ammonium Bromide SemiClathrate Hydrate. Jpn. J. Appl. Phys. 2004, 43, 362–365.

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Semi-clathrates structure and phase diagram for CO2-TBAB-H2O and N2-TBAB-H2O

For Table of Contents Only


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CO2 Capture Using Semi-Clathrates of Quaternary Ammonium Salt

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