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Swelling of Clays in N-Methyl-2- pyrrolidinone/Carbon Disulfide Mixed Solvents Pakorn Opaprakasit and Paul C. Painter* The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received December 2, 2003. Revised Manuscript Received July 30, 2004
The interaction of various solvents with some clays that are commonly found in coal has been examined. The mixed solvent N-methyl-2-pyrrolidinone/carbon disulfide can extract cations from various clays and apparently form complexes with them that strongly absorb light in the visible region. The individual solvents cannot form such complexes when placed in contact with the same clays. The ability to form such complexes with cations may be a key factor in the enhanced extraction yields that are obtained from certain coals using this mixed solvent system.
Introduction In a separate paper we obtained results that show that the N-methyl-2-pyrrolidinone/carbon disulfide (NMP/ CS2) mixed solvent system can form complexes with certain cations.1 Some of that work grew out of the study we will report here, an examination of clay/solvent interactions. It was the color changes observed in these systems, described below, that suggested that NMP/CS2 mixed solvents might form specific complexes. However, this work does not fit well into that paper, but remains important because many coals have a significant amount of mineral matter that could act as a source of the cations that can form ionomer-like junction zones or cross-links in the organic component of coal through π-cation interactions.2 For example, montmorillonite clays contain alkali-metal and alkaline-earth-metal cations in the interlayer spaces of their aluminosilicate sheets. These cations bind to the sheet layers by an ionic interaction. It therefore seemed intriguing to examine what fraction of these cations could be extracted by NMP and CS2, and whether the amount of extracted cation is enhanced by the 1:1 mixed solvent. Moreover, because Upper Freeport coal contains a relatively high content of mineral matter, mainly clays, it is possible that these play a significant role in the solution behavior of this coal. Accordingly, the information obtained from these clay experiments may be important in reaching an understanding of the coal dissolution process. Experimental Section Materials. Montmorillonite and organically modified montmorillonite clays were obtained from Nanoclay Co. under the brand names Cloisite Na+ and Cloisite 15A, respectively. Delaminated montmorillonite clays KSF and K10 were supplied by Aldrich. * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Opaprakasit, P.; Painter, P. C. Unpublished results. (2) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Energy Fuels 2002, 16, 543-551.
Study of the Clay Swelling Properties. The swelling ratio of Cloisite Na+ montmorillonite clay was examined by recording its volume change after 2 days of contact with various solvents, ranging from toluene to CS2, NMP, pyridine, water, and the NMP/CS2 binary mixture. The clay was placed in a glass test tube and submerged in an excess of solvent (see the Results and Discussion). The height of the compact clay submerged under the solvent was then recorded as an original dimension. After 2 days of contact, the change in the clay height was recorded, and the swelling ratio was calculated. Extractability and Color Change of Solvents in Contact with Montmorillonite Clays. The ability of solvents to extract cations from a montmorillonite clay was examined by placing 0.5 g of the Cloisite Na+ in the corresponding solvents, and ultrasonicating for 30 min. The clay was left in contact with the solvents for 2 days, and the mixture was then centrifuged at 3300 rpm for 10 min. The clear supernatant was collected, and the Na+ cation content was measured by a Perkin-Elmer model 703 atomic absorption spectrophotometer. Solvent extractability was calculated using a calibration curve generated from Na+ standard solutions. The color change experiments were conducted on kaolinite, Cloisite Na+, and delaminated montmorillonite clays KSF and K10. The change in color of the solvents in contact with these clays was recorded after 2 days of solvent/clay contact. Finally, the ability of NMP/CS2 binary solvents to extract or at least interact with clay interlayer organic surfactants was studied by examining the change in color of solvents in contact with an organically modified montmorillonite clay, Cloisite 15A.
Results and Discussion We will start by examining the swelling properties of the montmorillonite Cloisite Na+. This clay was placed in various solvents, and its volume change was recorded after 2 days of contact. The results, listed in Table 1, show that the nonpolar solvents used in this study are more effective swelling agents than their polar solvent counterparts. The swelling ratios observed in toluene and CS2 are almost identical at 2.1, while those in the more polar solvents pyridine and NMP are 1.9 and 1.3, respectively. The swelling ratio of this clay in water cannot be determined, because the clay forms a cloudy, dispersed, colloidal suspension in this liquid.
10.1021/ef034093s CCC: $27.50 © 2004 American Chemical Society Published on Web 10/08/2004
Swelling of Clays in NMP/CS2 Mixed Solvents
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Table 1. Degree of Swelling of Montmorillonite Cloisite Na+ Clay after 2 Days of Solvent Contact solvent
swelling ratio
solvent
swelling ratio
toluene CS2 pyridine
2.1 2.1 1.9
NMP NMP/CS2 water
1.3 1.3 N/Aa
a
The clay forms a suspension in water.
Interestingly, the swelling ratio observed in the NMP/ CS2 mixed solvent is similar to that observed in neat NMP (1.3), which is much lower than that in neat CS2. These results show that CS2 alone is an effective swelling agent. However, when mixed with NMP, this ability is diminished. The swelling of clays in organic solvents has been studied by a number of authors (see refs 3-8 and citations therein). Clays such as montmorillonite consist of negatively charged aluminosilicate layers held together by cations. Swelling can occur through various mechanisms. For example, intracrystalline dimensions (i.e., the separation of the layers) can change as a result of solvent interactions that modify the organization of water and cations in the interlayer regions,7 or as a result of the formation of interlayer complexes involving the solvent molecules themselves.9 However, much of the physical swelling of clays in organic solvents is a result of changes in the equilibrium distance between the clay particles. This, in turn, has been interpreted in terms of the dielectric constant of the medium and its effect on electrostatic forces,4-6 or in terms of regular solution theory.8 Many of these studies relied on a limited range of solvents, and it is interesting that the large swellings we observed with toluene and CS2 would not fit in the correlations reported in this work (e.g., swelling versus solubility parameter). However, we are more concerned in this study with the possibility of complex formation. The degree of swelling of clays in particular solvents is probably unrelated to the extraction yields obtained from coals, in that a low degree of bulk swelling does not necessarily mean that the solvent involved has not penetrated the interlayer region of the clay or is not strongly interacting with the surface of the clay particles. Accordingly, we next examined the ability of solvents to extract cations from the montmorillonite clay by measuring the amount of Na+ extracted from Cloisite Na+ after 2 days of clay/solvent contact. The Na+ cation was chosen to represent the clay cations, because of its abundance in this type of clay, and its extractability was calculated using a calibration curve generated from standard solutions. The results are summarized in Table 2. The extraction yield from CS2 and toluene is, as expected, below the detection limit of the instrument, indicating that although these nonpolar solvents swell the clay considerably, they are not capable of extracting (3) MacEwan, D. M. C. Nature 1948, 162, 935-936. (4) Murray, R. S.; Quirk, J. P. Soil Sci. Soc. Am. J. 1982, 865-868. (5) Chen, S.; Low, P. F.; Cushman, J. H.; Roth, C. B. Soil Sci. Soc. Am. J. 1982, 865-868. (6) Green, W. J.; Lee, F. G.; Jones, R. A.; Palit, T. Environ. Sci. Technol. 1983, 17, 278-282. (7) Nensen, E. J. M.; Smit, B. J. Phys. Chem. B 2002, 106, 1266412667. (8) Graber, E. R.; Mingelgrin, U. Environ. Sci. Technol. 1994, 28, 2360-2365. (9) Mortland, M. M. Adv. Agron. 1970, 22, 75-116.
Table 2. Amount of Extracted Na+ Cation from Montmorillonite Cloisite Na+ Clay after 2 Days of Clay/ Solvent Contact extraction yield sample
g of Na+/g of clay (× 105)
wt %a
toluene CS2 pyridine NMP NMP/CS2b NMP/CS2c
0 0 0.8 5.9 7.2 38.1
0 0 1 6 8 41
a Calculated on the basis of the total cation exchange capacity, CEC (92.6 × 10-5 g of Na+/g of clay). b After 2 days of clay/solvent contact. c After 30 days of clay/solvent contact.
this cation from the clay galleries or interlayers. The yield obtained from pyridine and NMP is 8 × 10-6 and 5.9 × 10-5 g of Na+/g of clay, respectively. This result indicates that NMP and pyridine can disrupt the clay/ cation interactions and extract Na+ species. NMP is roughly an order of magnitude more effective than pyridine. The corresponding result for an NMP/CS2 mixed solvent is somewhat greater than for pure NMP, 7.2 × 10-5 g/g of clay (2 days of extraction). The ability of this binary solvent to extract cations depends on the extraction time. When the clay is placed in the solvent for 30 days, the amount of extracted Na+ increases by a factor of almost 10, to 3.8 × 10-4 g/g of clay. Given that the clay has an original total Na+ content of 9.3 × 10-4 g/g (obtained from the cation exchange capacity, CEC), the extraction yield of Na+ in these various solvents is 6%, 8%, and 41% for NMP, NMP/CS2 (2 days), and NMP/CS2 (30 days), respectively. It is apparent that NMP is the most effective single solvent in extracting Na+ cations, but the addition of CS2 enhances the extraction yield considerably. It is well-known that pyridine can form strong hydrogen bonds with water in the interlayer regions as a result of the polarizing effect of metal cations.9 It is therefore not unreasonable to assume that NMP can do the same, but how this is enhanced by CS2 remains unknown. FTIR spectra of the supernatant solutions were examined, but were uninformative. However, significant color changes were observed in solvents placed in contact with various clays, as shown in Figures 1 and 2. Four clays were used in this particular study: kaolinite, a clay that contains only small amounts of interlayer cations, Cloisite Na+ (montmorillonite), a Na+-saturated clay, and montmorillonites KSF and K10, delaminated clays. These latter “delaminated” clays have been acid treated, resulting in a collapse in their layer structures. Figure 1 shows results obtained after 2 days of solvent/clay contact. The color of the CS2, pyridine, and NMP single-solvent supernatants remain unchanged in all the clays. Significant changes were observed using mixed solvents of NMP/CS2 and pyridine/CS2, however. The largest change was observed using mixed solvents in contact with delaminated KSF and K10 clays. The supernatants changed from transparent to black after 2 days, and the swollen clays also appeared to be black. In kaolinite, however, the color remained unchanged after 2 days, but gradually turned to yellow after 2 weeks. This indicates that a strong interaction and probably complex formation occur between the mixed
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Opaprakasit and Painter
Figure 1. Color changes of various solvents in kaolinite and delaminated montmorillonite KSF and K10 clays.
solvent and the species extracted from the clays, presumably cations. The large change observed in the delaminated clays is due to the fact that these clays are acid treated, which disrupts the cation/clay sheet interactions. As a result, the binding energy is smaller, resulting in an easier cation exchange with the binary solvents.10-12 On the other hand, kaolinite has a lower interlayer cation content, and the exchange process, as reflected by a color change, is slower and requires a longer time. Figure 2 shows the results obtained using cyclohexane/CS2 and toluene/CS2 mixed solvents. After 2 days of contact with the three clays, the color of the cyclohexane/CS2 supernatants remains unchanged. As ex(10) Pinnavaia, T. J.; Tzou, M. S.; Landau, S. D.; Raythatha, R. H. J. Mol. Catal. 1984, 27, 195-212. (11) Suzuki, K.; Mori, T.; Kawase, K.; Sakami, H.; Iida, S. Clays Clay Miner. 1988, 36, 147-152. (12) Occelli, M. L.; Landau, S. D.; Pinnavaia, T. J. J. Catal. 1984, 90, 261-269.
pected, the mixed solvents in contact with the lowinterlayer-cation-content clay, kaolinite, also remain visually unchanged. However, the supernatants in contact with KSF and K10 clays changed slightly to a light yellow color. Furthermore, the solutions above the clay appear to phase separate into two layers, with a bottom yellow layer corresponding to CS2 while the top transparent layer is predominantly cyclohexane. Corresponding results were also observed in toluene/CS2 mixed solvents. A control set of samples showed no significant color change, but the mixed solvent in contact with KSF changed to yellow. This binary solvent does not phase separate into two layers, however, probably because of a closer match of the solubility parameter of the two solvents. Given this result, it is apparent that CS2 is also affected by mixing with these clays, even though the results from atomic absorption spectroscopy indicate a lack of detectable extracting ability. Clearly, the color change observed in these solvents is a sensitive
Swelling of Clays in NMP/CS2 Mixed Solvents
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Figure 2. Color changes of nonpolar solvents in kaolinite and delaminated montmorillonite KSF and K10 clays.
Figure 3. Color changes of NMP, CS2, and the binary mixture in contact with the montmorillonite Cloisite Na+ and organically modified montmorillonite Cloisite 15A clays.
indicator of interactions with very small amounts of extracted material. Finally, the ability of NMP/CS2 binary solvents to extract or at least interact with clay interlayer organic surfactants was studied. Essentially, an organically modified montmorillonite clay, Cloisite 15A, whose interlayer Na+ is replaced by dimethyl-dihydrogenated tallow-quaternary ammonium, (CH3)2-N+-HT2, was used. This quaternary amine carries a single positive charge, but it has a much larger molecular size than Na+ ions, and its organic nature provides a different characteristic to the Cloisite Na+ sample. This modification results in an enlargement of the interlayer spacing of the clay. Figure 3 shows that the color of the pure NMP supernatant in contact with Cloisite 15A remains unchanged. This is equivalent to the results obtained with NMP in contact with Cloisite Na+. However, the color of the NMP/CS2 mixed solvent changes to dark
yellow, also similar to the results obtained with Cloisite Na+. This again reflects an interaction between extracted cations and NMP/CS2. The results indicate that NMP/CS2 mixtures have the ability to “break” or solvate an ionic interaction and extract surfactant molecules from a clay interlayer that the pure solvents do not. Moreover, a surprising result was observed in the CS2/ Cloisite 15A system. Recall that in CS2/Cloisite Na+ mixtures no color change is observed, but the clay swells by 200%. In the organically modified Cloisite 15A, CS2 effectively forms a permanent suspension with the clay particles. This is probably due to a combination of factors, possibly a stronger interaction of the highly polarizable CS2 with the quaternary amine and/or the fact that the modification to electrostatic interactions as a result of an increase in interlayer spacing is such that the particles are now easier to disperse. Given that the original clay sheets are not soluble in CS2, the
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addition of this surfactant apparently promotes a compatibility of this clay with CS2 that may have intriguing practical consequences. Conclusions We have demonstrated that the mixed solvent NMP/ CS2 can extract cations from various clays and apparently form complexes with them that strongly absorb light in the visible region. The individual solvents cannot form such complexes, but nevertheless can extract smaller amounts of the cations from the clays. We believe this ability to form complexes has ramifications in terms of coal solubility. Takahashi et al.13 observed that when certain salts are added to the NMP/ CS2 mixed solvent system extraction yields are enhanced to an extent that is proportional to the size of the anion involved. Similarly, Chen et al.14 demonstrated that an anion formed from tetracyanoethylene (13) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, 141-146.
Opaprakasit and Painter
also promotes solubility. These anions presumably associate with positively charged organic functional groups in certain coals or species such as cations that through strong interactions (e.g., π-cation) act as cross-links in these coals. The ability of NMP/CS2 mixed solvents to form complexes with cations, while the parent solvents cannot, could therefore be at the heart of their ability to dissolve larger fractions of certain coals than other solvents. Acknowledgment. We gratefully acknowledge the support of the Office of Chemical Sciences, U.S. Department of Energy, under Grant No. DE-FG02-86ER13537. P.O. is a recipient of, and partially supported by, the Development and Promotion of Science and Technology Talent Project (DPST), Thailand. EF034093S (14) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 11801183.
