Calorimetric and Structural Studies of Tetrabutylammonium Chloride

Aug 24, 2010 - from (C4H9)4NCl solutions with approximate concentrations. 20 wt % (s1), 25 wt % (s2) ...... Pattern Matching Analysis. Abstracts of th...
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J. Phys. Chem. B 2010, 114, 11838–11846

Calorimetric and Structural Studies of Tetrabutylammonium Chloride Ionic Clathrate Hydrates T. Rodionova,* V. Komarov, G. Villevald, L. Aladko, T. Karpova, and A. Manakov NikolaeV Institute of Inorganic Chemistry SB RAS, 3, Acad. LaVrentieVa AVenue, NoVosibirsk, 630090 ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: August 3, 2010

Three ionic clathrate hydrates of different water content are formed in the binary system tetrabutylammonium chloride-water. The data on heats of fusion of these hydrates measured by DSC are presented. Single-crystal and powder X-ray diffraction studies of ionic clathrate hydrates of tetrabutylammonium chloride have been carried out. Structural justification of the occurrence of three different ionic clathrate hydrates derived from tetragonal structure-I in the (C4H9)4NCl-H2O system has been found for the first time. A novel mode of hydrophilic inclusion of the halide anion with displacement of two hydrogen-bonded host water molecules has been revealed. Structural data obtained are indicative of the possibility of location of tetrabutylammonium cations in combined cavities formed with participation of D-voids. Introduction In the crystal structures of ionic clathrate hydrates of tetrabutyl- and tetraisoamylammonium halides, water molecules and halide anions form a polyhedral water-anion framework through hydrogen bonding. Butyl and isoamyl groups are arranged in polyhedral cavities of this water-anion host lattice. These polyhydrates are structurally related to gas hydrates. The host lattices usually can be described as face-sharing polyhedra: pentagonal dodecahedron (512, D), tetrakaidecahedron (512 62, T), and pentakaidecahedron (512 63, P).1,2 Hydrocarbon groups of the cations are located in the T and P cavities, and D cavities are empty (although X-ray structural studies of some ionic clathrate hydrates revealed that part of D-cages can be filled with “guest” water molecules3–7). The most common structures of ionic clathrate hydrates are tetragonal structure-I (TS-I), hexagonal structure-I (HS-I), and cubic superstructure-I (CSSI) with unit cell stoichiometries 4P · 16T · 10D · 172H2O, 2P · 2T · 3D · 40H2O, and 48T · 16D · 368H2O, respectively. The last structure virtually is the generalized gas hydrate cubic structure-I (CS-I) with 8-fold unit cell. At present, ionic clathrate hydrates of tetrabutyl- and tetraisoamylonium salts are the subject of extensive research stimulated by diverse possible applications. (C4H9)4NBr polyhydrates were suggested as cold storage and transportation material since their latent heat of phase transition is sufficient for storage of cold; their formation/decomposition temperatures lie within 0-12 °C (in dependence on salt concentration); and in addition, they occur as a slurry that can be transported directly through a pipeline.8 Crystal structures of ionic clathrate hydrates have vacant voids suitable for inclusion of molecules of appropriate sizes, thus revealing potential for storage, transportation, and separation of gases. In the context of a highly topical quest for hydrogen storage materials, the studies of water solutions of tetrabutylammonium fluoride (C4H9)4NF, tetrabutylammonium chloride (C4H9)4NCl, and tetrabutylammonium bromide (C4H9)4NBr under hydrogen pressure are of importance. It has been found that double polyhydrates H2-(C4H9)4NF,9,10 H2-(C4H9)4NCl,11 and H2-(C4H9)4NBr10,12–14 are formed, which have better thermal stability (provided equal pressure) than * Corresponding author. E-mail: [email protected].

hydrogen hydrate15 or double hydrate H2-THF.12,16 It was demonstrated recently that clathrate hydrate of tetrabutylammonium borohydride includes hydrogen molecules in vacant cavities of its crystal structure and, hence, represents a hybrid hydrogen storage material (H2 is produced not only on decomposition of the clathrate hydrate but also through hydrolysis of the borohydride ion17). Another essential challenge is CO2 emission. It is accepted that the global warming and climate changes are fundamentally induced by emission of CO2 to the atmosphere, and the search for methods of separation and sequestration of CO2 is of importance. There are a number of studies confirming the ability of ionic clathrate hydrates (C4H9)4NF18 and (C4H9)4NBr14,18–20 to absorb carbon dioxide by virtue of formation of double hydrates. It was found quite recently that ionic clathrate hydrates of (C4H9)4NCl, (C4H9)4NNO3, and (C4H9)4PBr also form double hydrates with CO2, which decompose at higher temperatures (for a given pressure) than pure CO2 hydrate.21 It was demonstrated that double hydrates are formed by (C4H9)4NBr with gases such as methane,14,22–24 nitrogen,14,20,23 and hydrogen sulfide,23,25 as well as by (C4H9)4NCl with methane and nitrogen.11 Additionally, hydrates of (C4H9)4NBr appeared to be suitable for separation of gas mixtures.20,22,23,25,26 Single-crystal X-ray structure analysis of hydrate of tetraisoamylammonium bromide (i-C5H11)4NBr with krypton and methane confirmed that the gas molecules are located in the D cavities; these hydrates are characterized with high thermal stability.27 Apparently, the number of hydrates in a system, their structures and properties, thermal stability, and fusion heats are fundamental characteristics not only from the academic viewpoint but also for possible applications. However, it is noteworthy that, despite a large number of studies undertaken, the values of the above characteristics obtained by different researchers are often nonconsistent. Our early studies were devoted to structural6 and calorimetric28 examination of ionic clathrate hydrates of tetrabutylammonium fluoride. Further, we proceeded to structural and calorimetric studies of ionic clathrates of tetrabutylammonium chloride. The data on the number and the compositions of hydrates existing in the tetrabutylammonium chloride-water system show some discrepancy. References 29–31 report the formation

10.1021/jp103939q  2010 American Chemical Society Published on Web 08/24/2010

Tetrabutylammonium Chloride Ionic Clathrate Hydrates

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Figure 1. (a) T,X phase diagram of the (C4H9)4NCl-H2O binary system in the clathrate formation region:33 dashed lines denote metastable liquidus lines. (b) The solubility isotherm (8 °C) of the (C4H9)4NCl-NH4Cl-H2O ternary system in the clathrate formation field:33 O, solution composition; 0, wet residue composition; b, hydrate composition. Symbols h32, h30, and h24 correspond to (C4H9)4NCl · 32.2H2O, (C4H9)4NCl · 29.4H2O, and (C4H9)4NCl · 24.1H2O, respectively.

of a single hydrate with composition (C4H9)4NCl · 30H2O (the hydrate number was formulated as “around 30” by the author). Hydrate compositions (C4H9)4NCl · 32.1H2O (derived from analytical data for the solid) and (C4H9)4NCl · 33.8H2O (calculated from unit cell dimensions and density data) are given in ref 32. The T,X phase diagram of the (C4H9)4NCl-H2O binary system and the solubility isotherm (8 °C) of the (C4H9)4NCl-NH4Cl-H2O ternary system have been studied in our laboratory by differential thermal analysis and Schreinemakers’ method, respectively33 (Figure 1). It was shown that three ionic clathrate hydrates (C4H9)4NCl · 32.2H2O, (C4H9)4NCl · 29.4H2O, and (C4H9)4NCl · 24.1H2O exist in the clathrate formation field. Thus, even the number of phases of ionic clathrate hydrates of tetrabutylammonium chloride cannot be unambiguously determined yet from the available data. In this study, we report the results of structural and calorimetric examination of ionic clathrate hydrates of tetrabutylammonium chloride. Novel structural data obtained reveal the possibility of existence of three different hydrate phases belonging to the TS-I and elucidate the origin of phase multiplicity. Experimental Section An aqueous solution of tetrabutylammonium chloride was obtained by neutralizing a tetrabutylammonium hydroxide solution (10% solution from “Chemapol”, “pure” grade) with hydrochloric acid (∼2 M solution, “chemically pure” grade) using bromthymol blue as an indicator. Precipitation of crystals of hydrated tetrabutylammonium chloride was achieved by cooling to 0 °C, and then the material was promptly collected on a filter and washed with cold (0 °C) water. The material was recrystallized twice following the same procedure. Further, the obtained hydrate crystals were melted at room temperature, and the solution was concentrated to ∼90 mass % of tetrabutylammonium chloride over ascarite in a vacuum desiccator. In further experiments, this salt was used for preparation of the tetrabutylammonium chloride solutions of different concentra-

tions. The concentration of tetrabutylammonium chloride was determined by measuring the content of tetrabutylammonium cations and chloride anions. The content of tetrabutylammonium cations was measured by potentiometric titration with a sodium tetraphenylborate solution using an ion-selective electrode. The amount of chloride anions was determined by titration with 0.03 N solution of Hg(NO3)2 in aqueous ethanol, with diphenylcarbazone as indicator. The results obtained agreed well within the limits of accuracy of the analyses. For measurement of enthalpies of fusion, the crystals of ionic clathrate hydrates of tetrabutylammonium chloride with different hydrate numbers were prepared by cooling ∼15% solution to 5-7 °C (1), ∼25% solution to 8-10 °C (2), and ∼55% solution to 7-9 °C (3). The solutions were held in a specially constructed constant-temperature chamber for a span varying from a few hours to a few days, until a solid phase was formed. The crystals were separated from the above solutions and quickly dried between two sheets of filtering paper up to air-dry state. All procedures were conducted in the same constant-temperature chamber at the temperature of the experiment. A portion of the crystals was further placed into flasks for analytical determination of the hydrate composition as described above, and the other portion was simultaneously sealed up in 0.1 mL steel pans for calorimetric studies of enthalpies of fusion. Five to eight measurements were carried out for each of the hydrates. The stoichiometries of the compounds corresponded to the compositions (C4H9)4NCl · (32.2 ( 0.4)H2O (h32), (C4H9)4NCl · (29.7 ( 0.4)H2O (h30), and (C4H9)4NCl · (24.8 ( 0.3)H2O (h24) for the experiments (1), (2), and (3), respectively. A differential scanning calorimeter DSC-111 (Setaram) was used for determination of enthalpies of fusion. Sample weights varied from 0.03 to 0.09 g. An empty 0.1 mL steel pan was used as a reference. Measurements were carried out at heating rate of 0.5 °C per minute. The observed heats of fusion were normalized by electric calibration provided by the manufacturer. X-ray diffraction experiments were carried out for four single crystals further denoted as s1-s4. Single crystals were picked from crystalline precipitates formed at 5-10 °C

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TABLE 1: Crystal Data and Structure Refinement Parameters crystal size unit cell dimensions a, Å c, Å µ(Mo KR), mm-1 reflections measured independent Rint Rσ θmin, θmax hmin, hmax kmin, kmax lmin, lmax number of parameters goodness of fit, S R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data) mean residual shifts/su largest diffraction peak and hole, e · Å-3

s1

s2

s3

s4

0.4 × 0.4 × 0.4 23.5711(11) 12.4361(6) 0.134 34994, 5593 0.0252 0.0173 2.04, 23.82 -26, 26 -26, 18 -14, 13 210 1.061 0.1024 0.2853 0.1167 0.2982 0.000 0.975, -0.539

0.4 × 0.4 × 0.4 23.5818(12) 12.4148(7) 0.134 33658, 5583 0.0302 0.0215 2.05, 23.81 -26, 16 -26, 26 -14, 14 210 1.084 0.1003 0.2784 0.1121 0.2880 0.000 1.075, -0.470

0.4 × 0.3 × 0.3 23.5281(15) 12.4918(7) 0.134 35128, 5586 0.0388 0.0362 2.04, 23.81 -26, 26 -26, 26 -9, 14 210 2.052 0.1392 0.4385 0.1602 0.4687 0.045 1.247, -0.662

0.4 × 0.2 × 0.2 23.4881(44) 12.5289(21) 0.134 18468 5585 0.0289 0.0374 2.38, 23.81 -25, 26 -18, 26 -14, 13 210 1.047 0.1313 0.3536 0.1515 0.3719 0.050 1.082, -0.506

from (C4H9)4NCl solutions with approximate concentrations 20 wt % (s1), 25 wt % (s2), 30 wt % (s3), and 35 wt % (s4). For preparation of X-ray quality crystals, the solutions were kept at this temperature for several days without stirring. Composition of the crystalline precipitates was determined as described above. The stoichiometry corresponded to compositions (C4H9)4NCl · 32.7H2O, (C4H9)4NCl · 30.4H2O, (C4H9)4NCl · 30.1H2O, and (C4H9)4NCl · 27.4H2O for s1-s4, respectively. We have not succeeded in growing X-ray quality crystals from more concentrated solutions. Single-crystal diffraction experiments were conducted at 150 ( 1 K on a Bruker X8 APEX CCD diffractometer with Mo KR radiation using φ and ω scans.34 Complete-sphere integration was used. For the data set s1, the structural model was determined by the direct method of phasing of structure amplitudes.35 At the early stage of structure solution, the positional parameters of the nodes of the host framework were located, and all of them were initially refined as oxygen atoms. Further, the positions of atoms of the (C4H9)4N+, as well as positions of Cl-, in the nodes of the framework were determined. A node was considered as partially occupied with a chloride ion if the corresponding oxygen atom exhibited essentially smaller atomic displacement parameters as compared to the average over the framework. The sum of site occupation factors of alternative atoms in such positions was set to unity, while positional and displacement parameters were kept identical. The positions of the guest atoms were found from differential Fourier syntheses, two symmetrically nonequivalent positions being found for the tetrabutylammonium cations suffering orientational disorder. Besides, the splitting of oxygen positions of the framework nodes was occasionally observed (two alternative oxygen atom positions for the same node separated by 2σ(I)] s1 0.037, 0.042 s2 s s3 s i

Rint(i, s3) (all data), Rint(i, s3) [for I > 2σ(I)] 0.141, 0.157 0.119, 0.132 s

Rint(i, s4) (all data), Rint(i, s4) [for I > 2σ(I)] 0.129, 0.144 0.108, 0.120 0.038, 0.040

a Mutual coincidence parameters Rint(i, j) for i and j experimental data sets were calculated as

2 Rint(i, j) )

∑ |F

2 hkl(i)

2 - Fhkl (j)|

hkl

∑ |F

2 hkl(i)

2 + Fhkl (j)|

hkl

2 where Fhkl is averaged intensity of hkl equivalents.

TABLE 4: Occupancy (occ) and Atomic Displacement Parameters (adp), Most Different in s1-s2 and s3-s4 Series

Figure 3. Disordering of 4T cavity over two incompatible positions.

Figure 4. Two modes of hydrophilic inclusion of chloride anion: (a) with displacement of one hydrogen-bonded water molecule of the host framework (oxygen atom O2 is substituted by Cl2) and (b) with displacement of two water molecules (pair O11-O12 is substituted by Cl1).

included (“guest”) water. The inclusion of “guest” water in D-cavities is known, as noted above, for other structures.3–7 The comparison of pairwise Rint(i, j) statistics (Table 3) indicates that the intensity data arrays can be divided in two groups: s1, s2 and s3, s4. Similar division can be made on the basis of the unit cell parameters. The most distinct differences between the groups s1, s2 and s3, s4 are manifested in details of the structural models (site occupation factors and positions and values of residual electron density peaks, Table 4) obtained on structure solution. The major dissimilarities appear in occupation factors of chloride-ion positions: “rib” Cl1 and Cl2 and atomic displacement parameters of oxygen atoms O101, O102 of the host framework. Significant differences are also observed in positions of residual electron density peaks. For the structures s1, s2 there are seven and five peaks stronger

parameter

s1

s2

s3

s4

Cl1 occ adp Cl2 occ adp O101a occ adp O102a occ adp C211b adp C212b adp C213b adp

0.032(4) 0.039(12) 0.306(14) 0.038(1) 0.498(13) 0.052(3) 0.498(13) 0.069(3) 0.046(3) 0.043(3) 0.070(4)

0.023(4) 0.043(18) 0.345(14) 0.039(1) 0.508(13) 0.052(3) 0.496(13) 0.061(3) 0.042(3) 0.038(3) 0.065(3)

0.085(5) 0.039(5) 0.205(16) 0.048(2) 0.432(15) 0.049(4) 0.53(2) 0.123(6) 0.085(6) 0.082(6) 0.086(4)

0.097(6) 0.044(5) 0.248(18) 0.043(2) 0.437(16) 0.045(4) 0.51(2) 0.130(8) 0.079(6) 0.086(7) 0.079(5)

a Simultaneous refinement of occupancy and atomic displacement parameters of O101 and O102. b C211, C212, and C213 are, respectively, R, β, and γ carbon atoms of (C4H9)4N+ located in the 4T cavity.

than 0.50 e/A3, and all peaks are near the atoms of the host framework. Anisotropic refinement of the host framework atoms strongly diminishes the intensities of these peaks and gives R1 indices of 0.0824 and 0.0830 for s1 and s2 data sets, respectively. There are 29 and 15 peaks stronger than 0.50 e/A3 for s3, s4; most of them are near the atoms of the host framework and also can be significantly reduced by anisotropic refinement. Among the remaining peaks, two are inside the T-voids involved in formation of the 4T-cavities, and another two are inside the D-voids adjacent to these T-cavities. The last four electron density peaks are considerably separated from the host framework atoms and, apparently, indicate the inclusion of (C4H9)4N+ cations in combined cavities involving D-voids. A correct structural model describing disordering in these crystals is, most likely, quite complicated, and we have not succeeded yet in elaborating it. This fact, in particular, makes senseless calculations of stoichiometry of crystals s1-s4 from the structural data. We believe that the low quality of the structural data is mainly related to strong disordering of the guest molecules, and hardly

Tetrabutylammonium Chloride Ionic Clathrate Hydrates

Figure 5. Powder diffraction patterns of the samples h32, h30, and h24. Positions of diffraction peaks calculated for TS-I hydrate (P42/m, typical unit cell parameters) are shown by vertical bars. Silicon was used as an internal standard, and positions of the silicon reflections are marked by arrows.

any improvement can be achieved through perfecting the structural experiment, growth, and picking of crystals. The data obtained suggest the formation of two phases of ionic clathrate hydrates of tetrabutylammonium chloride, both having TS-I structure but differing in stoichiometry. The total of the data obtained on the compositions of the hydrates (phase diagrams, multiple analytical determinations of compositions of solid hydrate phases) indicates that the stoichiometry of crystals s1, s2 is, most likely, close to h32, while for s3, s4 it is close to h30. The fact that the total composition of the crystalline precipitate, from which crystal s2 was picked, was determined ˆ can be attributed, most probably, to as (C4H9)4NCl · 30.4H2O the fact that this experiment yielded mostly the metastable hydrate h30 and a small amount of the stable hydrate h32. We suppose that one of these single crystals of h32 could be selected for X-ray experiment. We had numerous observations that, even on stirring and especially without stirring (crystalline phases used for picking X-ray quality crystals were obtained exactly in this way), metastable phases of ionic clathrate hydrates can occur for a rather long time. This also explains the fact that we picked a crystal with hydrate number close to 30 from a crystalline precipitate of overall composition (C4H9)4NCl · 27.4H2O (apparently, a mixture of h30 and h24 was formed in this case). The occurrence of only two different hydrates in four experiments suggests the absence of continuous solid-state solubility in the studied concentration range. Powder Diffraction Studies. To confirm in the system under study the existence of several hydrates with stoichiometries close to h32, h30, and h24 and unit cell similar to that of TS-I, we carried out X-ray powder diffraction examination of three samples of hydrate phases. The analysis of the composition of the samples revealed that they correspond to (C4H9)4NCl · 32.2H2O, (C4H9)4NCl · 29.7H2O, and (C4H9)4NCl · 24.5H2O. The compositions of the samples are in good accord with those found on fusion enthalpy measurements and further are also denoted as h32, h30, and h24, respectively. In all three cases, the diffraction patterns obtained are quite similar, but the quality of the pattern is apparently worse for the sample h24 (Figure 5). We calculated positions of diffraction peaks expected for a TS-I hydrate possessing the highest symmetry P42/m known for this type of hydrates.1 Even in this case, the diffraction pattern exhibits strong overlapping of peaks at 2θ > 20-25°, thus hindering their reliable indexing and usage in refinement of unit cell parameters. It should be noted that the actual symmetry is expected to be lower, as the diffraction patterns of samples h30 and h24 exhibit reflections at 25.5-26° missing

J. Phys. Chem. B, Vol. 114, No. 36, 2010 11843

Figure 6. Powder diffraction patterns of the samples h32, h30, and h24 in the 2θ range 5-25°.

in the theoretical pattern. Therefore, only peaks with 2θ < 23° were used for refinement of the unit cell parameters. Diffraction images recorded in this range and results of their processing are given in Figure 6 and Table 5. All reflections observed in this range can be interpreted in the framework of TS-I structural model with P42/m symmetry. The parameters and volumes of unit cells of the samples h32 and h30 do not show significant differences, while the sample h24 does show distinctions, although subtle. These results support the hypothesis about the presence of several hydrates having different compositions but the same TS-I unit cell. As we have not succeeded in single-crystal X-ray diffraction analysis of hydrate h24 (stability field of h24 lays in the region of rather concentrated solutions, which are viscous, and this makes growing X-ray quality crystals problematic) and the quality of the diffraction pattern of hydrate h24 was not quite satisfactory, we undertook additional X-ray powder diffraction examination of another three samples of the hydrate h24 (Figure 7). Analytical determinations revealed that they corresponded to compositions (C4H9)4NCl · 24.2H2O, (C4H9)4NCl · 24.5H2O, and (C4H9)4NCl · 24.0H2O. It is evident that two diffraction patterns (hydrates (C4H9)4NCl · 24.2H2O and (C4H9)4NCl · 24.0H2O) have “additional” diffraction peaks absent in the patterns considered above. Refinement of unit cell parameters using diffraction peaks positively attributed to the TS-I hydrates in all cases gave values quite similar to those found for (C4H9)4NCl · 24.5H2O composition (Table 5). We concluded that the most probable origin of the additional diffraction peaks is partial decomposition of the hydrate (accompanied by water loss) during sample preparation and handling. To check this assumption, after data acquisition at -20 °C, the sample (C4H9)4NCl · 24.0H2O was warmed to 0 °C, and diffraction patterns were recorded after 20, 30, and 40 min (Figure 8). It is clear that under conditions allowing relatively fast evaporation of water the intensity of the “additional” peaks increases. Similar results were obtained on evacuation of the chamber at -20 °C. Thus, these “additional” peaks in the diffraction pattern correspond to a new phase resulting from decomposition of the TS-I hydrate. Corresponding peaks are marked in Figure 7. Attempts to index these peaks and determine crystal system and unit cell parameters of the new phase did not produce unambiguous results, and the most symmetric solution corresponded to the rhombic crystal system with parameters a ) 29.0, b ) 21.0, and c ) 11.8 Å. It should be stressed that we cannot determine the moment of appearance of this new phase in the samples prepared for powder diffraction studies, so it is unreasonable to discuss the presence/absence of a relationship

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TABLE 5: Unit Cell Parameters of the Samples with Compositions (C4H9)4NCl · 32.2H2O (h32), (C4H9)4NCl · 29.7 H2O (h30), and (C4H9)4NCl · 24.5H2O (h24) and Observed/Calculated Positions of Diffraction Peaks at Respective Powder Diffraction Patterns h32

h30

h24

a ) 23.737 ( 0.011 Å c ) 12.492 ( 0.001 Å, V ) 7039 ( 7 Å3

a ) 23.733 ( 0.013 Å c ) 12.513 ( 0.001 Å, V ) 7048 ( 8 Å3

a ) 23.608 ( 0.028 Å c ) 12.561 ( 0.003 Å, V ) 7001 ( 17 Å3

hkl

2θobs

2θcalc

hkl

2θobs

2θcalc

hkl

2θobs

2θcalc

011 120 111 201 220 211 030 130 221 131 012 321 140 330 202 212 240 411 331 222 302 241 132 050 232 501 511 402 412 251 103 242 060 023

7.991 8.318 8.817 10.276 10.541 10.934 11.184 11.786 12.702 13.762 14.656 15.212 15.397 15.849 16.033 16.464 16.705 16.958 17.378 17.689 18.086 18.170 18.474 18.691 19.591 20.013 20.355 20.646 20.983 21.355 21.688 21.976 22.483 22.634

7.998 8.329 8.824 10.281 10.541 10.937 11.183 11.790 12.707 13.763 14.664 15.212 15.391 15.840 16.034 16.466 16.703 16.957 17.367 17.701 18.094 18.159 18.479 18.691 19.591 20.008 20.358 20.645 20.985 21.376 21.667 21.975 22.473 22.629

011 120 111 021 220 211 300 130 221 311 012 321 140 330 202 212

7.978 8.339 8.819 10.258 10.558 10.922 11.188 11.788 12.705 13.757 14.645 15.204 15.393 15.850 16.002 16.449

7.988 8.331 8.815 10.274 10.543 10.931 11.184 11.792 12.702 13.759 14.642 15.209 15.393 15.843 16.015 16.447

011 120 111 201 220 211 300 130 221 311 012

7.973 8.376 8.814 10.307 10.612 10.920 11.239 11.841 12.735 13.814 14.580

7.975 8.376 8.812 10.286 10.601 10.949 11.246 11.857 12.736 13.802 14.598

140 330

15.492 15.953

15.478 15.930

141 331 222 302

16.954 17.370 17.673 18.096

16.955 17.365 17.683 18.077

212 240 411

16.421 16.782 17.014

16.426 16.798 17.022

302

18.102

18.075

132 050 232 501 511 042 142 251 103 242 600 023

18.465 18.685 19.581 20.013 20.356 20.646 20.980 21.343 21.613 21.957 22.462 22.616

18.463 18.694 19.576 20.007 20.357 20.631 20.971 21.376 21.633 21.963 22.477 22.597

132 050 232 501

18.447 18.763 19.568 20.078

18.465 18.798 19.590 20.095

412 251 103 242

20.992 21.377 21.539 21.976

21.000 21.338 21.560 21.957

531

22.988

22.977

203 160 351

22.574 22.909 23.063

22.537 22.918 23.085

between the occurrence of this phase and overall composition of the samples. Conclusions The similarity of the structures of the three hydrates h32, h30, and h24 is confirmed both by single-crystal data and identity of

Figure 7. Powder diffraction patterns of the samples with experimentally determined compositions: (C4H9)4NCl · 24.2H2O, (C4H9)4NCl · 24.5H2O, (C4H9)4NCl · 24.0H2O. Positions of “additional” diffraction peaks corresponding to the unknown orthorhombic phase are marked by vertical bars.

experimental powder patterns. The achieved quality of structure solution provides grounds to postulate that the formation of different phases belonging to the same structural type of ionic clathrate hydrates arises from development of two types of defect positions in the hydrate framework: (1) inclusion of chloride anions in the

Figure 8. Powder diffraction patterns of the sample (C4H9)4NCl · 24.0H2O at 0 °C. The increase in intensity of “additional” diffraction peaks probably occurs due to transformation of (C4H9)4NCl · 24.0H2O to a hydrate with lower water content caused by water evaporation. Powder diffraction pattern of the sample h32 is given for comparison.

Tetrabutylammonium Chloride Ionic Clathrate Hydrates host framework with substitution of two hydrogen-bonded water molecules and (2) realization of a novel method of accommodation of the (C4H9)4N+ cation in combined cavities involving D-voids. It should be noted that the formation of a new phase of a clathrate hydrate by virtue of the filling of small cavities of the framework with a guest was observed earlier for hydrates formed at high pressures in the system sulfur hexafluoride-water.39 In this case, the filling of the small cavities with the guest was induced by pressure. Here, the increase of the guest concentration in the solution is the factor causing the filling of the small cavities. Structural evolution of the hydrates in response to increasing guest concentration is evidently different in the systems (C4H9)4NF-H2O and (C4H9)4NCl-H2O. In the fluoride system, the enrichment with the guest results in formation of a novel hydrate of another structure: tetragonal hydrate (C4H9)4NF · 32.8H2O is replaced by cubic (C4H9)4NF · 29.7H2O,6 while in the chloride system all hydrate phases are formed on the basis of the same TS-I host framework. On examination of fusion heats of the hydrates, one can note that the difference in the energies of water molecules of the host frameworks between hydrates h32 and h30 is larger than between h30 and h24. We suppose that this is due to the fact that formation of the defects of types (1) and (2) in the hydrate framework is disfavored. Notably, after appearance of a certain concentration of defect positions in the framework, further increase in the number of the defects does not result in significant changes in the energies of water molecules in the framework. It is noteworthy that, as all the hydrates melt virtually at the same temperature, the loss in the formation enthalpy of the hydrate is compensated by the entropy contribution; i.e., water-poor phases are stabilized by high entropy of the framework. We believe that this is the reason causing strong disordering in the frameworks of the compounds under consideration, which hinders structure determination. First of all, disordering of hydrocarbon radicals accommodated in the voids of the hydrate framework is obvious. Most likely, substitution of water molecule positions with chloride anions in the framework also has stochastic nature (further studies are needed to clarify this point). The extent of filling of the small cavities with the “guest” water molecules in the framework is a variable quantity. Finally, two new types of defect positions in the framework were considered above. All these factors seem to exclude the possibility to obtain structural data of satisfactory quality. The data obtained indicate multiple ways of structural variation of ionic clathrate hydrates. In fact, our data prompt the possibility of the competition between the help-gas molecules (hydrogen, methane, etc.) and the tetrabutylammonium cations for the small cavities. This may appear as a factor of importance on employment of solutions of tetrabutylammonium salts for gas storage, as well as for development of processes of gas mixture separation with ionic clathrate hydrates. Previously reported data33 and results of the present work are indicative of occurrence of three phases of ionic clathrate hydrates of tetrabutylammonium chloride. It has been shown for the first time that host water frameworks of all three hydrates are formed on the basis of TS-I. The structures exhibit high extent of disorder. Novel modes of hydrophilic inclusion of the anion and hydrophobic inclusion of the cation have been found: chloride anion is included in the host water lattice with displacement of two water molecules, and tetrabutylammonium cation is capable of incorporating in combined voids comprising large and small cavities. Measured fusion heats provide the basis for quantitative estimates of energies of water molecules in the ionic clathrate hydrates of tetrabutylammonium chloride.

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