1

STRUCTURAL AND PHYSICO-CHEMICAL STUDIES OF IONIC

CLATHRATE HYDRATES OF TETRABUTYL– AND

TETRAISOAMYLAMMONIUM SALTS

Andrey Manakov

1, Tatyana Rodionova, Irina Terekhova,

Vladislav Komarov, Alexander Burdin, Artem Sizikov

Nikolaev Institute of Inorganic Chemistry SB RAS

3, Acad. Lavrentieva ave., Novosibirsk, 630090, RUSSIAN FEDERATION

ABSTRACT

This contribution presents short review of resent results of structural and physico-

chemical studies of ionic clathrate hydrates of tetrabutylammonium fluoride, chloride,

bromide, as well as of linear, cross-linked tetrabutylammonium and

tetraisoamylammonium polyacrylates and some double hydrates of these compounds with

methane.

Keywords: ionic clathrate hydrate, tetrabutylammonium salt, tetraisoamylammonium salt,

structure, enthalpy of fusion

1 Corresponding author: Phone: +7 383 316 5346 Fax +7 383 330 9489 E-mail: manakov@niic.nsc.ru

NOMENCLATURE

CS-I – cubic structure I

SCS-I – superstructure of CS-I

CRPac – cross-linked polyacrylate anion

D cavity – pentagonal dodecahedron (512

)

HS-I – hexagonal structure I

Pac – polyacrylate anion

P cavity - pentakaidecahedron (512

63)

T cavity – tetrakaidecahedron (512

62)

TBA – tetrabutylammonium cation

TiAA – tetraisoamylammonium cation

TS-I – tetragonal structure I

Hf, – enthalpy of fusion

INTRODUCTION

In the crystal structures of ionic clathrate hydrates

of TBA and TiAA halides water molecules and

halide anions form a polyhedral hydrogen bonded

water-anion framework. In the ionic clathrate

hydrates of tetraalkylammonium salts with

carboxylate anions carboxyl group is included in

water framework through its oxygen atoms to form

an edge of a polyhedron whereas hydrophobic part

of the anion occupies a cavity of the water

framework. Butyl and isoamyl groups of the

cations are arranged in polyhedral cavities of this

water-anion host lattice; nitrogen atom of cations

replaces water molecule situated in vertex

common of four neighboring cavities. Host lattices

of ionic clathrate hydrates are structurally related

to those of gas hydrates and can be described as

combination of face-sharing D, T, and P

polyhedra. Hydrocarbon groups of the cations are

located in the T and P cavities, D cavities are

empty [1, 2] (in some cases part of D-cages can be

filled with “guest” water molecules [3-7]). The

structures that are most common of ionic clathrate

hydrates structures are tetragonal structure-I (TS-

I), hexagonal structure-I (HS-I), superstructure of

cubic structure-I (SCS-I) with unit cell

stoichiometry 4P.16T

.10D

.172H2O, 2P

.2T

.3D

.40H2O,

48T.16D

.368H2O, respectively.

It is well known that participation of auxiliary

guest component in hydrate formation in many

cases results in formation of double hydrate with a

higher decomposition temperature [8]. This

method was used for increasing the decomposition

temperature of hydrogen hydrate with use of

auxiliary guest component tetrahydrofuran[9,10].

Practical use of auxiliary guest components faces

some additional requirements. Auxiliary

component must not be lost during formation –

decomposition of the hydrate, it must be produced

in industrial scale, it must be relatively

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011),

Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

2

inexpensive, etc. Tetraalkylammonium salts are

prospective for use as auxiliary guest component

because they at least partially meet the above

challenge and can serve as promoters of gas

hydrate formation process. At present, ionic

clathrate hydrates of TBA and TiAA salts are the

subject of extensive research stimulated by diverse

possible applications. TBABr polyhydrates were

suggested as cold storage and transportation

material [11,12]. Crystal structures of ionic

clathrate hydrates contain vacant voids suitable for

inclusion of molecules of appropriate sizes, thus

revealing potential for storage, transportation and

separation of gases. It has been found that double

polyhydrates H2-TBAF [13, 14], H2-TBACl [15],

and H2-TBABr [14, 16-18] are formed, which

have better thermal stability (provided equal

pressure) than hydrogen hydrate [19,20] or double

hydrate H2 -THF [9, 16]. It was demonstrated

recently that clathrate hydrate of TBA borohydride

represents a hybrid hydrogen storage material (H2

is produced not only on decomposition of the

clathrate hydrate but also through hydrolysis of the

borohydride ion) [21]. Ionic clathrate hydrates

TBAF [22], TBABr [18, 22-24] are able to form

double hydrates with CO2. It was found quite

recently that ionic clathrate hydrates of TBACl,

TBANO3, and (C4H9)4PBr also form double

hydrates with СО2, which decompose at higher

temperatures (for a given pressure) than pure СО2

hydrate [25]. It was demonstrated that double

hydrates are formed by TBABr with such gases as

methane [18, 26-28], nitrogen [18, 24, 27], and

hydrogen sulfide [27, 29], as well as by TBACl

with methane and nitrogen [15]. Additionally,

hydrates of TBABr appeared to be suitable for

separation of gas mixtures [24, 26, 27, 29, 30].

Single crystal X-Ray structure analysis of hydrate

of TiAABr with krypton and methane confirmed

that the gas molecules are located in the D

cavities; these hydrates are characterized with high

thermal stability [31]. Development of methods of

practical use of tetraalkylammonium salts requires

information on their structures and thermodynamic

properties. However, at present these points are

studied rather poorly. Another interesting point is

formation of double hydrates of tetraalkyl-

ammonium salts with gases in which gas

molecules partially occupy large cavities. It is

necessary to increase gas content in these hydrates.

Unfortunately, no data concerning formation of

such hydrates is available. Thereby, the studies of

the composition and properties of double hydrates

of tetraalkylammonium salts with different gases

is a topical task.

Development of gas hydrate technologies

applicable for gas storage and separation of gas

mixtures is not possible without fast and energy-

sparing industrial-scale synthesis of these

compounds that, consequently, require creation of

large interface between liquid water and a gas. At

present energy-consuming procedures of water

and/or gas dispersion are used to accelerate

reaction of hydrate formation. A possible way to

solve this problem is to use clathrate hydrates that

do not change the aggregate state as they

decompose. The authors of the work [32] suggest

to use hydrate formation within slightly cross-

linked sodium polyacrylate particles swelled in

tetrahydrofuran – H2O solution for storage of H2

gas. Decomposition of clathrate hydrates in such

medium does not cause the change of aggregative

state of the system. Another way that is discussed

in this work consists in using water swellable

clathrate forming polymeric materials. These

materials are slightly cross-linked TBA and TiAA

polyacrylates (carboxylic cationites). The

formation of polyhydrates in swollen grains of

cross-linked TBA and TiAA polyacrylates with

low degree of cross-linking was first revealed in

[33]. Invariable aggregative state of these

compounds in the course of hydrate formation and

decomposition makes studies of these compounds

very interesting.

In this report we present the short review of our

recent structural and physic-chemical studies of

ionic clathrate hydrates of TBA and TiAA salts.

EXPERIMENTAL SECTION The aqueous solutions of TBAF, TBACl, and

TBABr were obtained by neutralizing the TBAOH

solution with corresponding acid and following

recrystallization of polyhydrates from aqueous

solutions. For preparation of cation-exchange

resins in TBA and TiAA forms the carboxylic

cation-exchange resins in the H-form were

neutralized with a 3-fold stoichiometric excess of

0.1N aqueous solutions of TBAOH and TiAAOH,

washed with distilled water and after that were

centrifuged at the same conditions. Aqueous

solution of TiAAOH was synthesized from TiAA

iodide by reaction with Ag2O in water suspension

and purified through the recrystallization of a

clathrate hydrate. Polyacrylic acid with average

molecular mass of 1800 daltons (Aldrich) without

further purification and tetraisoamylammonium

3

hydroxide solution were used to prepare TBA and

TiAA salt of the linear (unlinked) polymeric

anion. Methane gas with a purity not less than

99.9% and distilled water were used in

experiments. The content of TBA (TiAA) cations

was measured by potentiometric titration with a

sodium tetraphenylborate solution using an ion-

selective electrode. Samples of the double hydrates

of TBA salts with methane were prepared in

reaction of powdered crystals of TBA salts with

compressed methane in autoclave. The synthesis

was continued for 2-3 weeks to complete the

reaction.

Single crystals of ionic clathrate hydrates of

TBAF, TBACl, TBABr and TBAPac with

different hydrate numbers were prepared by

cooling and holding of aqueous solutions with

different concentrations in the home-made air

thermostat for a span varying from a few hours to

a few days. These crystals were used for

measurement of enthalpies of fusion and X-ray

diffraction experiments. The crystals were

separated from the solutions and quickly dried

between two sheets of filtering paper up to air-dry

state. All procedures were conducted in the air

thermostat at the temperature of the crystal

growth. A portion of the crystals was further

placed into flask for analytical determination of

the hydrate composition, the other portion was

simultaneously used for calorimetric and X-ray

experiments.

A differential scanning calorimeter DSC-111

(Setaram) was used for determination of enthalpies

of fusion. The samples of hydrates were sealed up

in 0.1 ml steel pans for calorimetric studies of

enthalpies of fusion. 5-8 measurements were

carried out for each of the hydrates. Samples

weight varied from 0.03 to 0.09 g. Measurements

were carried out at heating rate of 0.5oC/min. The

observed heats of fusion were normalized by

electric calibration provided by the manufacturer.

Single-crystal diffraction experiments were

conducted at 150±1 K on a Bruker X8 APEX CCD

diffractometer with MoK radiation [34]. X-Ray

powder diffraction studies were performed at –

20oC on a Bruker D8 Advance diffractometer

(CuK radiation, = 1.5418 Å) equipped with

Anton Paar refrigerating device. Phase diagrams

were investigated by DTA at high pressure; high-

pressure equipment has been described in [35].

Pressure was measured with precise gauge (0.25%

hydrate number,

(m.p.,oC)

Hf, kJ/mol hydrate

(kJ/mol H2O)

Ref.

TBAF hydrates

30.22 (25.0) 197.16 (6.52) [38]

30.0 (28.3) 184±4 [39]

32.44 (27.2) 203.0±2.3(6.26) [40]

28.94 174.3±3.1(6.03) [40]

TBACl hydrates

32.24 (15.0) 4.9 (5.56) [41]

29.74 (15.1) 156.9 .1 (5.28) [41]

30.0 (15.0) 156 [39,42]

30.0 (15.2) 164.1 (5.47) [43]

30.0 173.6 (5.79) [25]

24.83 (14.9) 127.9 .6 (5.16) [41]

TBABr hydrates

38.0 (9.9) 200.98±5.32 ( 5.29) [12]

38.13 (9.7) 219.4 .9 (5.76) [this work]

32.53 (12.0) 177.2 .5 (5.45) [this work]

26 (12.0) 152.76±6.74 (5.88) [12]

26 151.9 (5.84 ) [24]

26.46 (12.0) 150.7 .1 (5.71) [this work]

24.04 (12.3) 136.2 .1 (5.68) [this work]

Table 1. The stoichiometry, melting points, and

the enthalpies of fusion of the TBAF, TBACl and

TBABr polyhydrates.

error). Temperature was measured with a

Chromel-Alumel thermocouple with precision of

0.2oC. The release of gas from the studied samples

was monitored as a function of temperature by

collecting the gas into a calibrated burette placed

into a concentrated aqueous NaCl solution [36].

RESULTS AND DISCUSSION

Calorimetric studies of TBAF, TBACl, and

TBABr ionic clathrate hydrates. Our results of determination of the compositions

and enthalpies of fusion of TBAF, TBACl, TBABr

ionic clathrate hydrates are listed together with the

data available from the literature in Table 1.

Crystal structures

The crystal structure of TBAF.29.7H2O ionic

clathrate hydrate. The study of TBAF – H2O [37]

phase diagram revealed formation of two hydrates:

TBAF.32.4H2O (TS-I, [5]) and TBAF

.28.6H2O.

We have determined crystal structure of

TBAF.28.6H2O hydrate [6]. Idealized water

framework is isostructural with CS-I of gas

hydrates but with eight-fold unit cell (SCS-I, I-

43d, a=24.375(3)Å). Positions of fluoride-anions

4

Figure 1. The arrangement of TBA cation of

TBAF within 4T cavity; (b) the inclusion of

“guest” water molecule (O11) in distorted D

cavity.

Figure 2. (a) 4T combined cavity, (b) 3TP

combined cavity, (c) location of TBA of TBACl

cation within 3TP cavity.

Figure 3. Two modes of hydrophylic inclusion of

chloride anion of TBACl: (a) with displacement

of one hydrogen bonded water molecule of host

framework (oxygen atom O2 is substituted by

Cl2), and (b) with displacement of two water

molecules (pair O11-O12 is substituted by Cl1).

are not localized. Nitrogen atom of the cation

occupies the common vertex formed by four face-

sharing T polyhedra forming combined 4T cavity

(Fig.1a); hydrocarbon chains fill compartments of

this cavity. Inclusion of the cation results in

“pressed in” of some framework water molecules

in D-cavities which are partially filled by “guest”

water molecules with the filling degree 79.5 %

(Fig.1b). With this in mind the calculated hydrate

number 29.7 is greater than predicted by idealized

water framework 28.7.

The crystal structures of TBACl ionic clathrate

hydrates. T,X phase diagram of the TBACl – H2O

binary system and the solubility isotherm (8oC) of

TBACl – NH4Cl – H2O ternary system have been

studied in our laboratory by DTA and

Schreinemakers' method, respectively [44]. It was

shown that three ionic clathrate hydrates

TBACl.nH2O (n=32.2, 29.4, 24.1) form in this

system. We carried out the X-ray diffraction

experiments for four single crystals with the

stoichiometry TBACl*nН2О (n=·32.7(s1); n=30.4

(s2); n=·30.1(s3); n=

·27.4(s4)). It was shown that

all of them belong to space group P42/m

corresponding to idealized host water framework

TS-I, the unit cells dimentions, Å: a=23.571(11),

c= 12.4361(6); a=23.5818(12), c=12.4148(7);

a=23.5281(15), c=12.4918(7); a=23.4881(44),

c=12.5289(21), respectively. All structures

resemble the structure of ionic clathrate hydrate

TBAF.32.8H2O [5]. TBA cations are located in

four-compartment cavities of two types: 4T and

3TP (Fig.2). Chloride ions are included in the

hydrate framework, two inclusion modes being

found (Fig.3): (1) substitution of one water

molecule of the host framework; (2) substitution

of two hydrogen-bonded water molecules. Due to

distortion of the host framework induced by

inclusion of TBA cations, some of D-cavities have

three “pressed in” vertices and residual electron

density peaks within the cavities. This electron

density was refined as oxygen atoms of included

guest water molecule.

The comparison of pairwise Rint(i, j) statistics

indicates that the intensity data arrays can be

divided in two groups: s1, s2 and s3, s4. The most

distinct differences between the groups s1, s2 and

s3, s4 are manifested in details of the structural

models (site occupation factors, positions and

values of residual electron density peaks) obtained

on structure solution. The major dissimilarities

appear in occupation factors of chloride-ion

positions and atomic displacement parameters of

oxygen atoms of the host framework. Significant

differences are also observed in positions of

residual electron density peaks. Two of them 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 TBA cations in combined cavities

involving D-voids. Correct structural model

describing disordering in these crystals is, most

likely, quite complicated and we have not

succeeded yet in elaborating it. The data obtained

suggest the formation of two phases of ionic

clathrate hydrates of tetrabutylammonium

chloride, both having TS-I but differing in

5

Figure 4. Powder diffraction patterns of

TBACl·nH2O in 2θ range 5 to 25o.

stoichiometry. The total of the data obtained on the

compositions of the hydrates indicates that the

stoichiometry of crystals s1, s2 is, most likely,

close to TBACl·32Н2О, while for s3, s4 - to

TBACl·30Н2О. The occurrence of only two

different hydrates in four experiments suggests the

absence of continuous solid-state solubility in the

studied concentration range.

In order to confirm the existence of hydrates with

stoichiometry close to TBACl·32Н2О,

TBACl·30Н2О, TBACl·24Н2О and unit cell

similar to that of TS-I, X-ray powder diffraction

study of three samples of hydrate phases has been

carried out. The chemical analysis of the

composition of the representative probes taken

from the samples revealed hydrate numbers

n=32.2, 29.7, 24.8 (TBACl·nH2O). In all three

cases the diffraction patterns obtained are quite

similar (Fig.4). We calculated positions of

diffraction peaks expected for a TS-I hydrate

possessing the highest symmetry P42 /m known for

this type of hydrates. All reflections observed in

this range can be interpreted in the framework of

TS-I structural model with P42/m symmetry. These

results support the hypothesis about the presence

of several hydrates having different compositions

but the same TS-I unit cell.

The crystal structure of TBABr ionic clathrate

hydrates. There are different data on the number of

ionic clathrate hydrates and their stoichiometry in

TBABr – H2O binary system [12,39,45,46]. Our

data on examination of T,X phase diagram of the

TBABr – H2O system and of solubility isotherms

of TBABr – third component – H2O systems

indicated the occurrence of four ionic clathrate

hydrates: TBABr·nH2O (n=36, 32, 26, 24) [46].

With regard to structural characteristics the only

Figure 5. The arrangement of pair of TBA cations

of TBABr within eight-compartment 4D4T

cavity.

a b

Figure 6. Br− in tetrahedral (a) and square

pyramidal (b) coordination .

structure of orthorhombic TBABr·38H2O is known

at present [47]. We carried out a single crystal X-

ray structure analysis of hydrate TBABr·24H2O.

The structure was solved by the direct method.

The structure of host water framework is TS-I,

space group is P-4 with unit cell dimensions

a=23.492(3)Å, c=38.021(8)Å, structural hydrate

number is 24.8÷25.7. Two manners of inclusion of

TBA cations were revealed: (1) arrangement of

one TBA cation in four compartment 3TP cavity,

and (2) insertion of two TBA cations in eight-

compartment 4D4T cavity (Fig.5). Probably, the

latter peculiarity is responsible for existence of

several hydrates of TS-I in the same system and

was not known until now. In addition, a previously

unknown way of hydrophilic inclusion of Brˉ in

the host water framework is found. Brˉ is

coordinated by four water molecules so that it is

situated on the top of a square pyramid (Fig.6). An

occurrence of superstructure (triple c parameter) is

apparently caused by minimization of distortions

of hydrogen bond network of water anion host

framework arising from formation of eight-

compartment cavities.

Phase diagrams, structural and calorimetric

studies of the polyhydrates of linear and cross-

linked TBA and TiAA polyacrylates

We studied phase diagrams of the binary systems

water – cross-linked TiAACRPac (0,5-3% cross-

linking) [48], water – TBACRPac (1% cross-

linking). The phase diagrams studies were also

6

n, (m.p.,oC),

Hf, kJ/mol hydrate

(kJ/mol H2O)

Ref.

Hexagonal, P-6m2, a=12.15, c =12.58 Å (100 K)

a=12.26, c =12.71 Å (276 K) [50]

40.8 (18.4),

=0%

218.8 4.5 (5.36)

[54]

Hexagonal, a=12.25, c=12.72 Å (276 K) [49]

37.7 (15.5),

=0.5%

193.0 2,3 (5.12)

[54]

37.6 (14.6),

=1.0%

189.1 2.3 (5.03)

[54]

33.5 (13.7),

=2.0%

158.8 2.3 (4.74)

[54]

29.7 (13.0),

=3.0%

123.9 6.2 (4.17) [54]

Table 2. Properties of the hydrates of linear and

cross-linked tetraisoamylammonium polyacrylates

with different degrees of cross-linking ( = 0 –

3%). n – hydrate number (for the hydrates of the

polymeric salts the hydration number given is the

number of water molecules per the elementary unit

of the polymeric chain, taken from Ref. [48]). m.p.

– melting point.

n, (m.p.,oC),

Hf, kJ/mol hydrate

(kJ/mol H2O)

Ref.

monoclinic, 2/m, a=23,456 Å, b =12.327 Å

c = 23,454 Å, = 90,040

(150 K), a=23,60 Å, c =12,40

Å (248 K)

33.23 0,27

(13.2), =0%

157,1 2.7 (4,74) this

work

tetragonal, a=23.471 Å, c=12.328 Å (150 K)

31.62 0,31

(15.0), =0%

this

work

tetragonal, a=23.436 Å, c=12.330 Å (150 K)

30.04 0,31

(15.0), =0%

this

work

tetragonal, a=23.425 Å, c=12.364 Å (150 K)

a=23.58 Å, c=12.41 Å (248 K)

28.43 0,09

(17.5), =0%

130,8 6.8 (4.60) this

work

tetragonal, a=23.56 Å, c=12.37 Å (248 K) 29.9 (8.5),

=1% 128,9 3.0 (4,31) this

work

tetragonal, a=23.61 Å, c=12.40 Å (248 K) 26.5 (8.5),

=1% 119,2 4.2 (4,50) this

work

Table 3. Properties of the hydrates of linear and

cross-linked tetrabutylammonium polyacrylates

with cross-linking = 1%. n – hydrate number (for the hydrates of the polymeric salts the

hydration number given is the number of water

molecules per the elementary unit of the polymeric

chain). m.p. – melting point.

Figure 7. Phase diagrams of the binary

systems: (a) water – cross-linked TBACRPac

(cross-linking 1%) and (b) water – cross-linked

TiAACRPac (cross-linking 1%) in the region of

clathrate hydrate formation.

carried out with salts of liniar polymers. Obtained

information concerning compositions, melting

temperatures, structures and enthalpies of fusion is

summarized in Tables 2,3. The graphical

representation of phase diagrams is given in the

Fig.7.

The results of X-ray powder diffraction studies of

1, 2 and 3% cross-linked TiAACRPac hydrates

[49] and those of cross- linked (1% cross-linking)

TBACRPac revealed the existence of a crystalline

hydrate phase in frozen swollen grains of the

studied resins. It was shown that all TiAA hydrates

are hexagonal with the unit cell parameters (Table

2) corresponding well to HS-I. Polyhydrates of

cross-linked TBACRPac are tetragonal with the

unit cell parameters (Table 3) that suit well to TS-

1. Both structures are characteristic of many

clathrate hydrates of TBA and TiAA salts [1]. The

hydrate crystallites inside the grains are relatively

large, up to 0.01 mm3; each grain contains 100-200

crystallites.

It is not possible to obtain single crystals of

hydrates of cross-linked polymeric carboxylates

hence precise determination of their crystal

structure is also impossible. Therefore, a hydrate

with linear polyacrylates of TBA and TiAA were

attempted as a first model approximation. The

validity of this approach was demonstrated by the

comparison of powder X-ray diffraction patterns

of the samples of the polyhydrates with linear

polymers with those of cross-linked TiAA and

TBA polymers that revealed the identity of the

crystal structures (Fig.8,9, Tables 2,3).

Phase diagram of the binary system water –

tetraisoamylammonium polyacrylate (un-cross-

linked) was determined using DTA method. Only

one compound forms in the studied concentration

range with the melting point of +18.4oC. The

compound was isolated as a crystalline product

7

Figure 8. Powder diffraction patterns of the

polyhydrates of linear (f) and cross-linked (1%,

2%, 3%) TiAACRPac (a-d), recorded at 3 10C

and those (e) recorded at room temperature (200C);

c,d – 3% degree of cross-linking when using teflon

(c) and lavsan (d) coating films.

Figure 9. Powder diffraction patterns of the

polyhydrates of linear (lower pattern ) and cross-

linked (1%) TBA polyacrylates (upper pattern),

recorded at -250C.

and analyzed (six independent determinations) to

give the composition TiAAPac·(40.8 0.5)H2O

(Table 2). The single crystal X-ray analysis of the

hydrate of this compound [50] confirms its

clathrate nature. The hydrate crystal was

hexagonal; the unit cell dimensions refined using

all data were a = 12.150(1), c = 12.580(1) Å (at

100 K). In the crystal structure, the water

molecules form a 3-D hydrogen-bonded host

framework which is distorted variant of HS-I. The

Figure 10. (a) the layer of face-sharing 15-hedral

(P) polyhedra at the z~0 level and one of

combined 4-sectioned cavities (P2T2, outlined with

bold lines). (b) The layer of face-sharing 12-hedral

(D) polyhedra at the z~0.5 level.

space group is P-6m2. The large P polyhedra are

located in a layer at the z~0 level sharing their

hexagonal faces (each P polyhedron has three

hexagonal faces) and producing a honey-comb

system (Fig.10a). Each hole is filled with two T

polyhedra on both sides of the layer; the T cavities

stack on top of each other producing columns

along the c axis. The next layer, at z~0.5, is filled

with D polyhedra sharing their pentagonal faces

(Fig.10b). All the large cavities are occupied by

the hydrophobic alkyl groups of TiAA cation

which is included in 2P2T four-compartment

cavity. Similar inclusion mode was observed in

previously reported clathrate hydrates with this

cation[51,52] and is consistent with the ability of

the isoamyl fragment to fit only in a large cavity

but not in a small one. The cations are situated in

layers at z~0 level (Fig.10a). The crystal,

therefore, has a pseudo-layered organization with

respect to the location of guest. According to our

structural model the chains of polyacrylate anion

can only be located within the layer of D-

polyhedra at z~0.5 (see Fig.10b). The location of

the chains is impossible to find directly from the

experiment due to a great number of possible

positions that produces a multi-fold disorder from

the viewpoint of crystallography.

There is no doubt that the incorporation of the

carboxylate groups of the polymeric chain occurs

as a hydrophilic inclusion, as it was observed for

tetraalkylammonium salts of monomeric

carboxylates [3,7,51]. In other words, the two O

atoms of the carboxylate group replace two

adjacent O atoms in the polyhedron. At the same

time, the incorporation of the main hydrocarbon

chain of the polymer in the layer of undistorted D-

polyhedra is not feasible because the D polyhedra

do not have "windows" of sufficient size. One

could assume that the local microenvironment

around the chain is distorted in such a way that the

8

D-polyhedra form a channel. The formation of

such channels requires energetically unfavorable

distortion of the hydrate framework. The

parameters of the polyacrylate main chain should

be similar to those of polyethylene for which the

translational period of 2.54 Å is expected. Taking

the shortest dimension of the hydrate unit cell,

12.15 Å, at least five polyacrylate monomers

should fit in a single unit cell through which the

polymeric chain would run, producing a negative

charge far greater than it is necessary to balance

the 1+ charge of the cation (HS-I hydrates contain

1 cation per unit cell). Therefore, there is

excessive space for the accommodation of the

polyanion, most unit cells do not host the polymer

at all, and the concentration of local distortions

induced by the anion inclusion is relatively low. In

addition, there are several ways of how the chain

may enter and exit the unit cell (cf. Fig.10b) and

so the accommodation of the polyanion is very

favorable in terms of entropy. The described above

inclusion of the polyanion should create high local

concentrations of negative charge in the structure

which would be very unfavorable in terms of

enthalpy. Partially the loss is compensated by the

entropy term. However, the transfer of a proton

from the host hydrate framework to carboxylate

group is very probable here. The residual hydroxyl

anions may be located near the positively charged

nitrogen atoms of the cation. In conclusion, it

should be noted that the observed layered

organization of the structure explains the

possibility of inclusion in the crystal of cross-

linked polyacrylate. The availability of excessive

space within the D-layer and high freedom for the

polymeric chain accommodation facilitate the

inclusion. The chain can change direction at every

3D-intersection as evident from Fig.10b. The

accommodation would be more difficult within a

channel structure as one reported for TBAPac

[53]. According to the above results, most D-

cavities in the hydrates are vacant from the main

guest. This available cavity space may be utilized

for accommodating an auxiliary guest.

In order to characterize the hydrate phases

crystallized in the system water – TBAPac single-

crystal X-ray diffraction studies were performed

for four polyhydrates formed in the system

according to phase diagrams studies and analytical

determination of the compositions of the hydrates.

Unit cell parameters of all crystals studied are

similar and vary in the ranges

(a≈b=23.42÷23.48Å, c=12.33÷12.37Å), the angles

are close to 90° (Table 3). It was found in the

course of the statistical analysis of hklF – data that

the crystals of three studied polyhydrates have

Laue class 4/m, the values of Rint indices vary from

0.024 to 0.062, the symmetry of the diffraction

picture for the crystal of fourth hydrate is lower

and is characterized with Laue class 2/m

(Rint=0.057). The structural models obtained show

that the studied hydrates have the crystalline water

frameworks related to the TS-1. Because of the

significant disordering in the guest and partially in

the host subsystems the structure cannot be

described in detail. It can be said, however, that

the cations of the polymeric guest are included in

the combined 3TP и 4T cavities in the structure.

Besides, the presence of the electronic density in

all symmetrically independent D cavities of the

framework can be associated with the

accommodation of the polyacrylate anion inside

the cavities.

The calorimetric studies of the polyhydrates of

TBA and TiAA polyacrylates

The heats of fusion were measured for the

clathrate hydrates comprising linear and cross-

linked TBACRPac (cross-linking degree 1%) and

TiAACRPac (cross-linking degree 0,5-3%) [54]. It

should be noted that no data on clathrate formation

thermodynamics for any tetraalkylammonium salts

with cross-linked polymeric anions could be found

in the literature, while only some data on fusion

enthalpies of the hydrates with linear TBA and

TiAA polyacrylates have been reported by

Nakayama [55]. The results of the calorimetric

measurements are presented in Table 2,3. It is seen

from the calorimetric data that cross-links in

polymeric anion cause the decrease in the fusion

enthalpies of the polyhydrates with cross-linked

polymeric guests in comparison with those of the

polyhydrates with linear polymers. For the

polyhydrates of TiAA cross-linked polyacrylates it

was shown [54] that both the fusion enthalpy and

composition of the hydrates of TiAA polyacrylate

are linear functions of the degree of cross-linking

of the guest polymeric anion included in the

hydrate framework.

Double ionic clathrate hydrates with gases

TBABr – methane – water system. We studied

decomposition curves of double TBABr - methane

hydrate formed by aqueous solutions of TBABr

with 1:38 (31.7 mass.%) and 1:81.5 (18 mass.%)

TBABr to H2O molar ratio and excess of gaseous

9

Figure 11. Decomposition curves of HS-I hydrate

TBABr*38H2O, double hydrate TBABr*

38H2O+CH4 and hydrate phases formed by

aqueous solution of TBABr (1:81.5 TBABr:H2O

molar ratio) with methane.

a

b

Figure 12. Effect of pressure on the decomposition

points for (a) TBACRPac hydrate (0.5%): without

hydrogen (A); under pressure of hydrogen (B); (b)

TiAACRPac hydrate (2%): without hydrogen

addition (A); under pressure of hydrogen (B).

methane. Some experimental data which were

obtained in these experiments are shown in Fig.11.

1:38 TBABr solution corresponds to

stoichiometric ratio TBABr and water which was

expected for double hydrate with the HS-I. On the

basis of this information we attribute DTA effects

corresponding to this solution to decomposition of

double hydrate (Fig.11). In the case of 1:81.5

solution the situation was more complex (Fig.11).

In addition to DTA effects corresponding to

decomposition of double hydrate, 2 groups of

DTA effects occur. Low-temperature group of

weak effects at temperature close to 0oC

correspond to melting of aqueous eutectic.

Positions of the second group of DTA effects

coincide with decomposition curve of methane

hydrate. This hydrate was undoubtedly formed by

aqueous eutectic and excess of methane gas.

Temperature of methane hydrate decomposition in

this case was not changed because aqueous

eutectic in the TBABr-H2O system represent itself

almost pure water [40].

Double hydrates with cross-linked polyacrylates.

The P,T curves of hydrate decomposition in the

systems TBACRPac (0.5% cross-linking) – H2O

and TiAACRPac (2% cross-linking) – H2O, along

with the decomposition curves of these hydrates

under a pressure of hydrogen, are presented in

Fig.12. The melting curves for the hydrates of pure

ion-exchange resins contain fractures (P = 46 MPa

for TiAACRPac hydrate and P = 65 MPa for

TBACRPac hydrate), which probably suggest the

formation of new hydrate phases at these

pressures. A considerable increase in hydrate

decomposition temperatures is observed in both

cases upon imposition of hydrogen pressure over

the entire pressure range. Furthermore, no

characteristic fractures on the curves were

observed; the scatter of experimental points is

within the experimental error. A similar picture is

typical of the formation of double hydrates, i.e. the

structures of TBACRPac and TiAACRPac

hydrates contain vacant cavities; filling of the

latter with hydrogen molecules increases their

decomposition temperatures. It should also be

noted that the specimens had the form of solid

granules both before and after the experiment.

The experiments discussed above has shown that

(1) reaction of methane with 18 mass.% solution

of TBABr results in mixture of double hydrate and

gas hydrate of pure methane, (2) cross-linked

TiAACRPac and TBACRPac can form the double

10

Figure 13. Typical curves of gas emission. For

comments see the text

experiment V V1 V2

11 103.6 81.4 22.2

21 103.2 81.7 21.5

32 42.0 28.2 13.8

42 34.5 22.1 12.4

Table 4. Volumes of gas emitted at 1-st and 2-d

steps of decomposition of the samples of double

hydrates with methane. 1 double hydrate was

formed by18 mass.% solution of TBABr (1 : 81.5

TBABr :water molar ratio); 2 double hydrate was

formed by cross-linked polyacrilat of TBA, 80%

substitution of H+ to TBA+ cations; 78 mass.%

content of water

hydrates. We plan to perform the series of

experiments aimed in determination of

composition of these hydrates. Here we present the

first results of this work. In all of these

experiments we used row samples with TBA :

water molar ratio below that characteristic of

respective hydrates (Tables 1-3). We suppose that

row samples of this type are most appropriate to

obtain double hydrates with partial occupation of

large cavities with methane molecules and,

consequently, with high content of gas. Samples

for these experiments were synthesized in

autoclaves (see Experimental section). The

autoclaves were discharged at the liquid nitrogen

temperature; volume of gas emitted from the

samples in the temperature range from liquid

nitrogen to room temperature was measured as

function of temperature at atmospheric pressure.

Typical experimental curves are shown in Figure

13, numerical results of the experiments are

presented in Table 4. In all cases two steps of the

gas emission occurs. Temperature of the first step

(V1 in Table 4) roughly corresponds to

decomposition temperature expected for methane

hydrate at atmospheric pressure. The second step

we attribute to decomposition of double hydrates.

Assuming 100% transformation of TBA cation to

the double hydrate and that TBA : water molar

ratio in the double hydrates is equal to what in the

hydrates of pure TBA

salts we determined

compositions of the TBABr+CH4 double hydrate

as TBABr·(1.7-1.8)CH4·38H2O and TBACRPac+

CH4 double hydrate as TBAPac·(0.8-0.9)CH4·

26.5H2O. The compositions expected for 100%

occupation of small D-cavities with methane are

TBABr·3CH4·38H2O and TBAPac·2CH4·26.5H2O,

respectively. Taking into account that composition

of methane hydrate is close to CH4·6H2O we can

calculate quantity of the methane hydrate

decomposed at the first step of the process and,

consequently, to calculate quantity of non-reacted

water in each experiment. The samples with

TBABr contained about 5.5 % of the initial water

content was not transformed to hydrate. In the case

of cross-linked polyacrilates quantity of non-

reacted water was higher (40-45% of water

available for hydrate formation). molecules.

Concluding remarks

The most interesting and unexpected result of this

work is formation of several ionic clathrate

hydrates with the same water framework (TS-I),

almost undistinguishable parameters of the unit

cell but significantly different compositions.

Hydrates of this type occurred in the systems

water - TBABr, TBACl and TBAPac. We

speculate that formation of these hydrates is result

of the compromise between the tendencies to the

closest filling of crystal structure with atoms and

molecules, to formation of phases with highest

entropy, and to form hydrate framework with

undistorted hydrogen bonds. In the case of hydrate

11

formation in the diluted (in comparison with the

hydrate compositions) solutions of TBACl and

TBABr the hydrates with the smallest distortion of

hydrate framework form. In these hydrates

occupation degree of small D-cavities with guest

component is zero or minimal. Crystallization of

ionic clathrate hydrates from solutions with higher

guest concentration results in the formation of

hydrates with high content of occupied 3DT and

4T4D cavities and smaller hydrate numbers. In

this case more dense packing of the hydrate

framework is caused by partial occupation of small

D-cavities with alkyl radicals of TBA cation.

Formation of the hydrates with dense packing of

crystal structure requires distortion of hydrate

framework, e.g. in these frameworks coordination

of some anions with water molecules is pyramidal

instead of tetragonal. We believe that entropy of

these hydrates is higher in comparison with the

hydrates with lager hydrate numbers. The factors

considered above result in systematic decrease of

specific enthalpy of hydrate fusion with decrease

of hydrate number of the respective ionic clathrate

hydrates (Table 1). We suppose that the formation

of several TS-I hydrates by cross-linked TBA

polyacrylates is caused by the same peculiarities of

the crystal structures. Further work is necessary to

prove this idea. Some other peculiarities of the

studied hydrates should be mentioned. First of all

the formation of several hydrates in one system

with the same structural type but significantly

different compositions is possible only for TBA

salts. No examples of this type are known for

TiAA hydrates. Most probably, the size of butyl

group allows it to occupy small D-cavity with

minimal distortion of the hydrate framework that

is not possible for isoamyl group. The second

interesting observation is that hydrates of this type

form in the systems with the anion distorting

hydrate framework (Cl-, Br

- but not F

-) or even

destroying it (cross-linked polyacrylates).

Probably, weakening of hydrogen bonds in

hydrate framework makes possible structural

modernization of this framework without change

of the type of this framework. Finally, the results

concerning formation of double hydrates of

TBABr and cross-linked polyacrylates of TBA

with methane show that substitution of TBA cation

with methane is unlikely. In all cases the samples

obtained after reaction of the row samples with

methane contained not only double hydrate, but

also gas hydrate of pure methane. We speculate

that the formation of two types of gas hydrates is

more favorable energetically in comparison with

formation of double hydrates with partial

occupation of large cavities with methane.

ACKNOWLEDGMENTS

We acknowledge financial support from Russian

Foundation of Basic Research (Grant 09-03-

00367) and Siberian Branch of Russian Academy

of Science (Integration project “Fundamental

aspects of Physical Chemistry of Gas Hydrates:

Research for Practical Use”)

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