39 INTRODUCTION The surface modification of layered silicates by organic compounds has been extensively studied since the forties

INTRODUCTION

The surface modification of layered silicates by organic compounds has been extensively studied since the forties ^1-5 . So-called “organoclays” are found in a wide variety of industrial and scientific applications going from rheological control agents in paints, inks and greases to the treatment of contaminated waste streams ⁵ . In the nineties, the demonstration of intercalation of monomers as well as polymer chains inside organoclays opened the way to their use as nanofillers ^6,7 . Research in this field of polymer/clay nanocomposites was and is still very largely covered in the scientific literature ^8-15 and in patents ¹⁶ ; due to the numerous properties that can be enhanced (mechanical, thermal, barrier, fire,…) and to the remaining challenge of clay exfoliation in non-polar polymers.

The most commonly used procedure to prepare organoclays for nanofillers consists in first dispersing sodium montmorillonite in water at 80°C, followed by the addition of quaternary ammonium salts ¹⁷ bearing one or several long alkyl chains (usually more than 10 carbons).

Tertiary or primary amines are also frequently employed ¹⁸ , requiring protonation by a strong acid added for this purpose to the reaction medium. The reaction product is then washed several times, filtered, dried and milled, resulting in organoclay powder. At laboratory scale, lyophilisation of an organoclay/water suspension is sometimes used to directly recover the organoclay in powder form. The cationized amine compound is usually added in stoechiometric proportion to that of the cation exchange capacity (CEC) of the clay.

The interlayer structure of organoclays was thoroughly investigated by several groups ^3,4,19-21 .

Early conventional models were based on assumption of a nearly all-trans conformation for

the long alkyl chains, packed in a layered structure (monolayer, bilayer, trilayer, etc.) ² . Vaia et

al. ³ first demonstrated by FTIR spectroscopy in conjunction with X-ray diffraction, that the

intercalated chains exist in states with varying degrees of order. In general, as the interlayer

packing density or the chain length decreases or the temperature increases, the intercalated

chains adopt a more disordered, liquid-like structure resulting from an increase in the

gauche/trans conformer ratio, in contrast with previous assumptions. Paul et al. ⁴ used

experimental and molecular simulation studies to show that the organoclay gallery height,

also called d-spacing, increases linearly with the intercalated surfactant to clay mass ratio. The

packing of the surfactant alkyls is disordered, but the density of the organic material in the

galleries is higher than that of a corresponding bulk liquid. Surfactants with hydroxyethyl units lead to an even more dense molecular packing, due to hydrogen bonding of the hydroxyl groups with the clay surface.

Besides the organoclay structure, the study of their thermal stability was also widely covered ^22-28 . Indeed, the preparation of polymer/clay nanocomposites by melt intercalation often requires working at temperatures close to that of onset degradation of intercalated ammonium surfactants, estimated between 160 and 210°C depending on the analysis technique and procedure. The initial degradation of quaternary ammonium cations generally proceeds by a Hofmann (β-elimination) process. However, when present in montmorillonite clay, additional mechanisms such as nucleophilic substitution are observed ^22,29 . Catalytic sites on the aluminosilicate layer reduce the thermal stability of a fraction of the surfactants by an average of 15-25°C ²² . Moreover, NMR studies confirm that degradation of the surfactant can occur during compounding ³⁰ .

Alternative solutions to conventional ammonium-montmorillonite have been proposed when thermal limitations are a concern. Among these, the most common is the use of more stable surfactants such imidazolium ^31-34 or phosphonium salts ^29,35-37 . However, the majority of these salts present a poor solubility in water, which may limit their use on a larger scale because of the necessary resort to organic solvents such as tetrahydrofuran or petroleum ether ³⁶ . In order to circumvent this problem, supercritical carbon dioxide (scCO 2 ) appears to be a very promising medium. This environmentally benign, inexpensive and non-flammable solvent has easily accessible critical parameters (31.1°C and 73.8 bar) and possesses the required high diffusivity to facilitate ionic exchange inside the clay layers ³⁸ . Moreover, as CO 2 is a gas at ambient conditions, a simple depressurization leaves a dry, ready-to-use clay powder.

Our group therefore proposes to use scCO 2 at the very first step of producing polymer/clay

nanocomposites, i.e. the organomodification of clays. This first chapter aims at investigating

the possibilities and limits of the process, in view of a large scale production of a new range

of organoclays.

EXPERIMENTAL SECTION

1 Materials

Sodium montmorillonite (MMT) labeled as Cloisite ^® Na ⁺ (cationic exchange capacity or CEC of 92.6 meq/100g) was supplied by Southern Clay Products (Rockwood Additives Ltd) and was dried at 80°C under vacuum for 8 hours before use. Carbon dioxide was obtained from Air Liquide Belgium (purity 99.998 %). Trihexyltetradecylphosphonium chloride (Cyphos IL101) and tetraoctylphosphonium bromide (Cyphos IL166) were kindly provided by Cytec.

The other surfactants were purchased from Sigma-Aldrich or ABCR (tetrabutylphosphonium iodide). All surfactants (Table 1) were used as received.

2 Modification of pristine clays

Typically, 2 g of dry Cloisite ^® Na ⁺ and a slight excess of onium salt (1.1 equivalents relative to CEC of the clay) were poured into a 40 ml high pressure reactor. To reach the supercritical state of CO _2, the vessel was heated at 40°C in an oil bath, followed by pressurization at 200 bar. After 2 hours of reaction under magnetic stirring, the vessel was slowly depressurized.

The quantity of onium salt introduced is calculated as follows:

g g

mol mol g cation of

M

2

100

1 . 1 10

. 26 . / 9

2

As dry clays are directly obtained with this method, the clay may still contain non-exchanged surfactant. To evaluate the yield of exchange, the resulting organoclays were washed by stirring 0.5 g of organoclay in 50 ml of water at room temperature for 10 min. After filtration, the procedure was repeated in 50 ml of acetone, followed by filtration and drying at 50°C under vacuum for one night before characterization. Variations of the exchange procedure include a higher temperature of reaction (105°C) or the addition of ethanol as a co-solvent (2.5 %vol).

3 Characterization

X-ray diffraction (XRD) was carried out in reflectance mode with a powder diffractometer

Siemens D5000 (Cu K radiation with = 0.15406 nm, 50 kV, 40 mA, Ni filter, step size =

0.05° and step time = 1 s) at room temperature, in order to investigate the interlayer distance

of the clays (before and after washings).

Thermogravimetric analysis (TGA) was used to determine the organic content of the prepared nanoclays after washings. A thermal analyser TGA Q500 from TA Instruments was used at a heating rate of 20K/min, typically from room temperature to 600°C, under N 2 flow. The organic content is measured by the weight loss between the onset of degradation (between 150 and 250°C) and 550°C, with a correction for the possible loss of water below 150°C.

RESULTS AND DISCUSSION

1 Ionic exchange with various ammonium, imidazolium and phosphonium salts

The choice of surfactants was based on their commercial availability, their size, the presence of functionalities (hydroxyl groups, carboxylic acid), the variation of counter-ion and the possibility to compare similar ammonium and phosphonium salts.

The solubility of these surfactants in scCO 2 was first evaluated qualitatively by the use of a high-pressure reactor equipped with sapphire windows. By visual observation of the remaining solid powder or liquid droplets, it appeared that none of the tested onium salts is completely soluble in this medium in the reaction conditions (a partial solubilisation is however possible but could not be confirmed with our equipment).

The reaction conditions were 40°C, during 2 hours and with 200 bar of CO _2. Clay powder recovered from the reactor was directly analysed by X-ray diffraction (Figure 1). In each case, the interlayer distance of the as-obtained clay is reported (Table 1). If d ₀₀₁ is significantly higher than that of the pristine montmorillonite clay (1.1 nm), we consider that intercalation of the surfactant between the clay layers has occurred.

Figure 1. XRD patterns of various onium-modified clays (a: successful intercalation, b: no

intercalation)

Table 1. Surfactants used for the organomodification of MMT in scCO ₂ and interlayer distance (d ₀₀₁ ) measured as recovered from reactor

Code Organic salt T _f (°C) d ₀₀₁ (nm)

TBA Cl tetrabutylammonium chloride 41 1.87

TBA Br tetrabutylammonium bromide 103 - ^a

TBA I tetrabutylammonium iodide 144 1.22

TMPA Cl trimethylphenylammonium chloride 246 - ^a TMTDA Br trimethyltetradecylammonium bromide 245 1.20 DDDMA Br didodecyldimethylammonium bromide 172 0.98

TOA Br tetraoctylammonium bromide 98 1.00

TBP Cl tetrabutylphosphonium chloride 62 1.76

TBP Br tetrabutylphosphonium bromide 100 1.78

TBP I tetrabutylphosphonium iodide 141 1.90

TBTDP Cl tributyltetradecylphosphonium chloride 45 2.10 THTDP Cl trihexyltetradecylphosphonium chloride < 20 2.63 THTDP Br trihexyltetradecylphosphonium bromide < 20 2.72

TOP Br tetraoctylphosphonium bromide 38 2.59

HETPP Br 2-hydroxyethyltriphenyl-phosphonium bromide 217 1.23 CBTPP Br 4-carboxybutyltriphenyl-phosphonium bromide 206 - ^a HMI Cl 1-hexyl-3-methylimidazolium chloride < 20 1.38 MOI Cl 1-methyl-3-octylimidazolium chloride <20 1.40

a ill-defined diffraction peak

The interlayer distance of pristine montmorillonite clay was significantly expanded with the

introduction of an ammonium salt (TBA Cl), several phosphonium salts (TBP, TBTDP,

THTDP, TOP) and imidazolium salts (HMI, MOI) in supercritical carbon dioxide. Indeed,

d 001 of as-obtained organoclays varies from 1.4 to 2.7 nm, depending upon the organic salt

(Figure 1a). In other cases, the obtained X-ray spectrum does not present a well-defined

intercalation peak (TBA Br, TMPA, CBTPP), or only that of the pristine clay (TBA I,

TMTDA), with the addition of very thin peaks due to the crystalline salts, indicating that the

surfactant was not intercalated (Figure 1b). It should also be noted that the aspect of the clay

collected from the reactor is already an indication of the success of exchange: successful

intercalation leads to dry homogenous clay while in case of lack of intercalation, non-

exchanged surfactants are usually still visible in the recovered mixture.

A closer analysis of Table 1 first reveals an influence of the melting temperature (T _f ) of the onium salt, that is nicely illustrated in the case of the series of tetrabutylammonium cation (TBA) counterbalanced by chloride, bromide or iodide. In this series, only the ammonium bearing the chloride anion leads to a successful intercalation. It might be interpreted in terms of temperature of fusion; since only the chloride salt was molten at the exchange temperature.

Some other ammonium salts studied here (with a phenyl group – TMPA Cl, one, two or four long alkyl chains – TMTDA Br, DDDMA Br, TOA Br), that are solid at 40°C, are not intercalated in clay layers at this reaction temperature. This observation can, to some extent, be related to the work of Thompson et al. ³⁹ , who studied the effects of scCO 2 on different organoclays (with alkyl quaternary ammonium surfactants) and showed that the physical state (solid or molten) of the intercalated surfactant is of critical importance for a basal spacing change. Although they contacted an already organomodified clay with scCO 2 , their conclusion seems to apply for the contact between ammonium salt, pristine clay and scCO ₂ .

Secondly, we may compare the results obtained with ammonium surfactants to those with phosphonium surfactants. We show that TBP Cl, TBP Br and TBP I even though not molten at 40°C, can be successfully intercalated. The ionic exchange in scCO ₂ seems therefore to be facilitated with phosphonium salts compared to their ammonium equivalents. In order to explain this phenomenon, we can refer to a recent interesting work on the evaluation of the melting points depression of onium salts measured in scCO ₂ . The melting point of TBP Br was shown to be decreased by 42°C at 150 bar while a depression of 23°C only was reported for TBA Br ⁴⁰ . The larger melting point depression experienced by phosphonium salts compared to analogous ammonium salts in scCO 2 might thus favour their intercalation.

Another hypothesis is that the longer ionic radius of phosphorus (212 pm) compared to that of nitrogen (171 pm) ⁴¹ enables a better contact with the anionic charge of the clay. All the other alkyl phosphonium salts and imidazolium salts tested in this work are in the liquid state at the temperature of reaction and are readily intercalated. These salts with low melting temperature are considered as ionic liquids and appear to be excellent candidates for the organomodification in scCO 2 .

Thirdly, two functional phosphonium salts tested (HETPP and CBTPP) are not intercalated,

probably due to their high melting temperature, linked with their higher rigidity.

2 Ionic exchange at higher temperature

To verify the hypothesis that the physical state is a predominant factor, ionic exchanges for TBA Br and TOA Br have been carried out at 105°C, i.e. a temperature superior to their melting points (note that as the maximum allowed temperature of the reactor is 120°C, this test could not be done for the other non-intercalated surfactants).

In these conditions, the interlayer distance of MMT increased to 1.91 and 2.35 nm with TBA and TOA respectively. These two surfactants are thus successfully intercalated when molten, which confirms our hypothesis. This could be explained by the fact that, in the solid state, these onium salts present a certain crystallinity which impedes their dissolution or fine dispersion in scCO 2 .

3 Ionic exchange with addition of a co-solvent

For high melting temperature salts, a co-solvent in the amount of 2.5 vol% compared to reactor volume, was added in order to facilitate the ionic exchange. Different co-solvents, acetone, tetrahydrofuran (THF), ethanol, dimethylformamide (DMF), were first tested with 2- hydroxyethyltriphenylphosphonium bromide salt (T _f = 217°C) as a model surfactant (Figure 2). In the conditions of exchange (addition of 1 ml of co-solvent for 0.8 g of salt), the salt is only partially soluble in each of the co-solvents.

Figure 2. XRD patterns of 2-hydroxyethyltriphenylphosphonium-modified clays, with different co-solvents (2.5 vol%)

No intercalation occurs with acetone and tetrahydrofuran while intercalation occurs with

ethanol and dimethylformamide, the last one being the most polar solvent. A co-solvent with

a high polarity seems thus necessary and efficient to facilitate the ionic exchange in scCO ₂ .

Ethanol was chosen, because of its non-toxicity compared to dimethylformamide, and it was used with the surfactants that showed no intercalation with scCO ₂ alone.

With the addition of as few as 2.5 vol% of a polar co-solvent, all of the onium chlorides and bromides tested could be intercalated inside the clay layers (interlayer distances are reported in Table 2), confirming the versatility of the scCO 2 process. Only with the iodide counter-ion, no significant diffraction peak related to the clay is detected. The effect of counter-ion is not clear and should be further investigated. One hypothesis is the lower lattice enthalpy of NaI (705 kJ/mol) compared to that of NaCl (787 kJ/mol) and NaBr (752 kJ/mol) ⁴¹ . Indeed, a higher negative value of lattice enthalpy favors the formation of the inorganic salt; which may be considered as a driving force of the ionic exchange in scCO 2 , as detailed below.

4 Determination of yield of exchange

In order to determine the efficiency of ionic exchange in scCO _2, it is necessary to eliminate the possible fraction of non-exchanged surfactant in the clay. In a first approach, organomodified clays were thoroughly washed with acetone, a fairly good solvent of the onium salts.

However, upon XRD analysis of the washed products (Table 4, d ₀₀₁ aw1), we noticed an important reduction of d ₀₀₁ of certain organoclays. TGA analysis of corresponding organoclays indicated a strong reduction in the organic content. To explain this observation, we suspected that washing in acetone could reverse the ionic exchange. Indeed, the ionic exchange reaction can be expressed as the following equilibrium:

P ⁺ X ^- + M M T ^- N a ⁺ P ⁺ M M T ^- + N a ⁺ X ^-

Where P ⁺ stands for the phosphonium cation (or an ammonium cation), X ^- the halide counterion and MMT ^- the negatively charged clay nanolayer. Whereas during ionic exchange in water, the precipitation of the organoclay pushes the equilibrium to the right, we state that in scCO ₂ the equilibrium is displaced due to the insolubility of NaX. Indeed, the presence of the crystalline salt (NaCl or NaBr) can be observed on the X-ray spectra of the as-obtained organoclays (Figure 3).

During washing in acetone, the possible solubilisation of the inorganic salt can displace the

equilibrium back to the left, since MMT ^- Na ⁺ is known to precipitate or flocculate in organic

solvents. On X-ray spectra of washed organoclays (aw1), we still observe a characteristic

peak of NaCl for the modification with chloride salts (Figure 3a), whereas the peak of NaBr

disappears for the products of exchange with bromide salts (Figure 3b). Thus, experimentally the back exchange when washing with acetone was observed for NaBr but not for NaCl. To verify the proposed hypothesis, the solubility of sodium halides in acetone was evaluated by dispersing 0.5 10 ^-2 mol of NaCl and NaBr in 50 ml acetone (p.a., freshly opened bottle, H 2 O content < 0.2 wt%) during one night in a closed flask. The solutions were filtered and then evaporated. It appeared that 8.3 10 ^-5 mol of NaBr were solubilized versus 1.7 10 ^-5 mol of NaCl, which confirms the larger solubility of NaBr in acetone and hence the proposed mechanism.

Figure 3. XRD patterns of organoclays before and after washings (bw = before washing, aw1

= after washing with acetone, aw2 = after washing with water followed by acetone) a: with NaCl salt, b: with NaBr salt

In the examples with addition of a co-solvent, no reduction of d 001 is observed for the last three bromide salts (DDDMA, HETPP and CBTPP) after acetone washing. Conformingly, the peak of NaBr is still visible for two of them, which further indicates that no “back-exchange”

occurred in that case. As the intensity of NaBr peak is lower than that of NaCl, it is possible that it could not be detected in the last one. The fact that no back-exchange occurred in that case is possibly due to a poorer solubility in acetone of the organomodified clay.

Therefore, to avoid the back-formation of the sodium clay upon purification, washings were

first performed with water. During this process, the halide salt can be eliminated with, as a

consequence, the elimination of back-exchange capacity. A subsequent washing in a good

solvent of the surfactant such as acetone will therefore only eliminate the free onium salt. The

yield of exchange can then be determined as follows:

Yield = 100 ) (

%

) (

%

above see CEC on based organic

TGA washings after

organic

Table 2. Results of the organomodification of MMT in scCO 2 (bw = before washing, aw1 = after washing with acetone, aw2 = after washing with water followed by acetone)

d ₀₀₁ bw (nm)

d ₀₀₁ aw1 (nm)

Peak of salt aw1

d ₀₀₁ aw2 (nm)

Org. cont. aw2 (%)

Yield aw2 (%)

Without co-solvent

TBA Cl 1.87 1.67 yes 1.60 13.3 73

TBA Br ^b 1.91 1.40 no 1.51 14.5 79

TOA Br ^b 2.35 1.31 no 2.12 24.3 81

TBP Cl 1.76 1.73 yes 1.68 18.5 96

TBP Br 1.78 1.49 no 1.68 18.4 95

TBTDP Cl 2.10 1.91 yes 1.98 26.4 98

THTDP Cl 2.63 2.31 yes 2.22 28.4 92

THTDP Br 2.72 1.55 no 2.23 31.8 103

TOP Br 2.59 1.56 no 2.25 31.0 100

HMI Cl 1.38 1.56 yes 1.38 13.6 101

MOI Cl 1.40 1.41 yes 1.39 11.9 78

With co-solvent

TMPA Cl 1.50 1.49 yes 1.45 12.0 107

TMTDA Br 1.55 1.40 no 1.63 19.0 99

DDDMA Br 1.95 1.88 yes 1.90 23.2 89

HETPP Br 1.80 1.77 yes 1.80 24.8 110

CBTPP Br 1.86 1.82 no 1.77 21.1 84

b ionic exchange realized at 105°C

In general, we observe a slight decrease of interlayer distance after washings with water

followed by acetone. This decrease is more pronounced for large initial d 001 , while it is absent

for relatively small initial d ₀₀₁ . We can interestingly compare the d ₀₀₁ above with that of

organoclays that were prepared by other groups using a wet process. For example, Calderon et

al. ³⁶ , prepared organoclays in diethyl ether with THTDP Cl (2.52 nm), TOP Br (2.52 nm) and

TBP Cl (1.84 nm). The interlayer distance obtained by our scCO ₂ process is a little higher

before washing for the first two, probably due to a small excess of surfactant, while it is a

little smaller after washings. It may be related to a rearrangement in a more dense molecular

packing during washings.

Information about the yield of exchange is seldom mentioned in examples of organomodification in a wet process. Here, the information is interesting to prove that a large majority of surfactant remains bounded to the clay. The yields presented in Table 2 confirm indeed our hypothesis, as very high yields are obtained for all phosphonium cations exchanged and high yields with smaller ammonium cations. This corroborates our previous observation of easier exchanges with phosphonium surfactants. Yields above 100 % are explained by the 10 % excess of surfactant used compared to the cationic exchange capacity of the clay. In case of HETPP Br, it might also be due to the formation of hydrogen bonds with the clay layers ⁴ reinforcing the ionic bond and thus inhibiting washing of excess of surfactant.

5 Correlation of interlayer distance with organic content

In reference to the work of Paul et al. ⁴ , we plotted all the interlayer values of washed organoclays against the organic content after washings (determined by TGA with correction for loss of water) (Figure 4).

Figure 4. Correlation between organic content and interlayer distance of organomodified clays after washings

A good linear relationship is obtained, with most of the points well aligned. It is interesting to

note that points situated above the linear fit are typical for organoclays with a lower yield of

exchange (e. g. : TBA Cl and TOA Br), thus with lower organic content but with apparently

maximized interlayer distance. On the contrary, points situated below the linear fit must be

explained by a denser surfactant packing between the clay layers, which is the case with

HETPP Br. This is in accordance with Paul et al. who demonstrated the more dense molecular

packing of surfactants with hydroxyethyl groups, due to the formation of hydrogen bonds

with the clay surface.

6 Optimization of conditions

To evaluate the feasibility of our process on an industrial scale, different conditions were tested with a selected phosphonium salt. Trihexyltetradecylphosphonium chloride was chosen because of the high interlayer distance obtained for the corresponding organoclay and its moderate cost. The amount of surfactant at constant clay content was first varied.

6.1 Influence of quantity of surfactant

Different quantities of salt compared to the cationic exchange capacity of the clay were tested, while all the other conditions were kept constant.

Figure 5. XRD patterns of trihexyltetradecylphosphonium-modified clays, with different surfactant stoechiometries

Table 3. Organomodification with trihexyltetradecylphosphonium chloride, influence of surfactant quantity (bw = before washing, aw2 = after washing by water followed by acetone)

Conditions d 001 bw (nm) d 001 aw2 (nm) Yield aw2 (%)

2.0 equivalents 2.84 2.25 96

1.1 equivalents 2.63 2.22 92

1.0 equivalent 2.43 2.23 89

0.85 equivalent - ^a 2.24 84

a ill-defined peak

Analysis of the different samples by X-ray diffraction (Table 3 and Figure 5) shows that with

0.85 equivalent, a diffraction peak of poor intensity appears around 2θ = 3.5 (d = 2.5 nm)

while the diffraction peak of the pristine clay is still visible at 2θ = 7.5 (d = 1.1 nm). In

contrast, high intensities of the first diffraction peak are obtained from a stoechiometric quantity, with a little larger interlayer distance for the sample with 2 equivalents. The excess of surfactant is however efficiently removed by the washing procedure (10 min stirring in water followed by 10 min stirring in acetone), as shown by identical d 001 (2.2 nm) for all samples and highest yield of 96 %.

For the following results, unless specifically indicated, a stoechiometric quantity of phosphonium salt related to CEC was used to avoid the presence of excess surfactant in the ready-to-use clay.

6.2 Reproducibility of ionic exchange

In order to evaluate the reproducibility of the ionic exchange in scCO 2 , exactly the same conditions were applied to obtain six samples of pristine clay modified with THTDP.

Table 4. Organomodification with trihexyltetradecylphosphonium chloride, reproducibility (bw = before washing, aw2 = after washing by water followed by acetone)

Code d ₀₀₁ bw (nm) Yield aw2 (%)

THTDP Cl 1 2.43 89

THTDP Cl 2 2.59 93

THTDP Cl 3 2.60 93

THTDP Cl 4 2.43 95

THTDP Cl 5 2.51 94

THTDP Cl 6 2.63 92

All six ionic exchanges in scCO ₂ lead to very similar results and high yield of exchange after the washings. The process is thus highly reproducible. The next parameter studied is the amount of clay compared to the reactor volume.

6.3 Influence of clay content

A 40 ml reactor was used for this study. The maximum content (full filling) of this reactor with pristine montmorillonite clay corresponds to a quantity of 12 g. The clay content was varied from 0.5 g to 10 g while the surfactant ratio to CEC (1.0 equivalent) was kept constant.

The clay content is expressed relative to the volume of the reactor, with some indications

relative to the filling. Note that we avoid here to speak about volume of CO 2 since it varies

with the pressure. However, the pressure was kept constant (200 bar).

Figure 6. XRD patterns of trihexyltetradecylphosphonium-modified clays, with different clay contents

Table 5. Organomodification with trihexyltetradecylphosphonium chloride, influence of clay content (bw = before washing, aw2 = after washing by water followed by acetone)

Clay content (g/ml) d ₀₀₁ bw (nm) Yield aw2 (%)

0.013 2.41 100

0.025 2.39 100

0.05 2.43 89

0.10 (1/3 filled) 2.29 75

0.15 (1/2 filled) - ^a 69

0.20 (2/3 filled) - ^a 61

0.25 (almost full) - ^a 56

a ill-defined peak

The X-ray analysis of the obtained samples (Table 5 and Figure 6) reveals that the peak

diffraction intensity and area decreases with increasing clay content in the reactor. With small

ratios, a large peak corresponding to an interlayer distance of about 2.4 nm is obtained. With

intermediated ratios (0.05 and 0.1), a sharper peak is visible. These results are in good

accordance with the paper of Silva et al. dealing with the effect of clay/water ratio in the

study of the wet organomodification process ⁴² . They showed that a lower clay/water ratio

resulted in a more disorderly structure, established by the diffraction peak area, and that a

disordered structure is favourable for the formation of a well-dispersed clay/polymer

nanocomposite. With higher ratios (>0.15), no more peak is visible. The clay aspect becomes

heterogeneous, which might be related to an insufficient stirring (magnetic stirring) and exchange yields are poor. A low clay/reactor volume ratio is thus to be privileged.

6.4 Influence of the presence of water

For the previous results, non-modified clay was dried at 80°C under vacuum during 8 hours before ionic exchange. To examine the influence of water, a non-dried sample of clay was used. Ionic exchange in scCO 2 was also performed with the addition of various amounts of water (vol% corresponds to the volume of water compared to the reactor volume).

Figure 7. XRD patterns of trihexyltetradecylphosphonium-modified clay, with different water contents

Table 6. Organomodification with trihexyltetradecylphosphonium chloride, influence of water (bw = before washing, aw2 = after washing by water followed by acetone)

Conditions water/clay ratio d 001 bw (nm) Yield aw2 (%)

dry clay <1 wt% 2.43 89

clay not dried 6 wt% 2.38 94

addition of 1 vol% H 2 O 20 wt% 2.25 89

addition of 2 vol% H ₂ O 40 wt% 2.24 87

addition of 5 vol% H ₂ O 100 wt% - 68

From the obtained XRD patterns (Figure 7), it appears that a small quantity of water, up to 2

vol% which is well above the solubility of H 2 O in CO 2 (< 2 mol% which corresponds to

approximately 0.65 vol% in the present conditions ⁴³ ), does not impede the feasibility of ionic

exchange. However, decreasing peak intensity is observed as the water content is raised and a

lower yield of exchange is obtained. Moreover, at 5 vol% of water compared to the volume of

reactor, no intercalation peak is observed. Such an amount of water is thus undesirable for the scCO ₂ process but clay drying does not seem to be necessary.

After the study of various parameters, the process was applied to a thousand times higher scale, in a scCO 2 pilot reactor.

7 Clay organomodification in pilot reactor

In the pilot reactor of a capacity of 50 l, 3 kg of non-dried clay were contacted with 0.9 equivalent of trihexyltetradecylphosphonium salt (THTDP Cl). A small default of organomodifier was used in order to be sure to obtain a ready-to-use homogeneous powder.

Pressures, temperatures, times of exchange and stirring have not been optimized yet. The first two parameters were maintained compared to small scale (200 bar and 40°C) whereas time of exchange was increased to 24 hours to let enough time for stabilization of the system and homogenization was insured by a mechanical stirrer (instead of magnetic bar). The X-ray diffractogram of the as-obtained organoclay was compared with that of an organoclay prepared in a small reactor (Figure 8).

A second phosphonium salt was tested, tetraoctylphosphonium bromide (TOP Br), with 500 g of non-dried clay (limitation due to availability of onium salt) and 0.95 equivalent of TOP, with the same conditions of pressure, temperature and time of reaction. Results are presented in Table 7.

Figure 8. XRD patterns of trihexyltetradecylphosphonium-modified clays, in reactors of 40

ml and 50 l

Table 7. Organomodification in pilot reactor (bw = before washing, aw2 = after washing with water followed by acetone)

Code M _w (g/mol) d ₀₀₁ bw (nm)

d ₀₀₁ aw2 (nm)

Yield aw2 (%)

THTDP Cl 519.31 2.40 2.25 82

TOP Br 563.76 2.30 2.19 83

Both organoclays prepared in the pilot reactor present similar interlayer distance and yields compared to the corresponding organoclay prepared in a small reactor. These organoclays will be referred to as MMT-P14 (trihexyltetradecylphosphonium chloride or THTDP Cl) and MMT-P8 (tetraoctylphosphonium bromide or TOP Br) in the next chapters.

CONCLUSIONS

The process of organomodification of clays in scCO 2 was studied in detail, to investigate its

possibilities and limits. Typical experimental conditions were a temperature of 40°C and a

pressure of 200 bar for 2 hours. It was shown that intercalation occurs when the organic salt is

in its liquid state at the reaction temperature, with the formation of insoluble sodium halide as

the driving force of exchange. Moreover, the addition of a polar co-solvent enabled the

intercalation of high melting temperature salts, increasing the versatility of the process. High

yields of exchange were obtained with a slight excess of surfactant and interlayer distances

corresponded to those of organoclays obtained by the wet method. We further underlined that

clay drying and excess of surfactant were not necessary to ensure the feasibility of exchange,

whereas the clay content has to be limited to facilitate intercalation. Finally, the

reproducibility of the process and its application in a pilot reactor were demonstrated, making

this environmentally-friendly process a real option to produce new organoclays at the

kilogram scale, for example from thermally-stable ionic liquids.

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