ARTICLE pubs.acs.org/JPCC
Immobilization of Ionic Liquids in Layered Compounds via Mechanochemical Intercalation Hang Hu,†,‡ Jarett C. Martin,† Min Xiao,‡ Cara S. Southworth,† Yuezhong Meng,*,‡ and Luyi Sun*,† †
Department of Chemistry and Biochemistry & Materials Science and Engineering Program, Texas State University-San Marcos, San Macros, Texas 78666, United States ‡ The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China
bS Supporting Information ABSTRACT: A facile mechanochemical intercalation approach was adopted to immobilize 1-butyl-3-methylimidazolium chloride (BMIMCl) in layered R-zirconium phosphate (ZrP). The intercalation compounds with an expanded layer distance were confirmed by X-ray diffraction. Thermogravimetric characterization revealed that a portion of the BMIMCl was intercalated into the ZrP galleries, while a portion was adsorbed on the layer surface. The immobilized BMIMCl was evaluated for catalysis evaluation of the coupling reaction of CO2 and propylene oxide to synthesize propylene carbonate. The immobilized BMIMCl exhibited similar reactivity as free BMIMCl. Overall, the mechanochemical approach proves to be effective in immobilizing ionic liquids in layered compounds and thus may expand the applications of ionic liquids and, meanwhile, improve catalyst separation and recycling.
’ INTRODUCTION Ionic liquids (ILs) have attracted significant attention and have been extensively studied over the past decade.1-3 Because of their unique chemical and physical properties and the facile property tunability, ILs not only have been used as alternatives to classical molecular solvents in a wide range of applications but also have led to many new applications,4 such as catalysis,5-7 electrolytes,8 lubricants,9 biomass processing,10-13 energetic materials,14 etc. However, the use of ILs has two major issues: cost and viscosity. One of the most promising approaches to solve these two problems is to immobilize ILs on solid supports.6,15 In fact, immobilization of ILs can also increase efficiency, facilitate recycling, and bring about new applications.6 For example, for IL-catalyzed reactions, supported ILs can bring a number of advantages, including facilitating catalyst separation, increasing catalysis efficiency, minimizing product contamination, and opening the possibility to use fixed-bed reactor systems.6,16 In addition, immobilization of ILs can minimize the potential toxicity of ILs, which has been largely ignored but has recently begun to draw attention.17 Thus far, porous silica and zeolite have been the main solid supports of choice.6 Layered materials can be another group of ideal candidates for the immobilization of ILs. In addition to immobilize ILs on the surface of layered materials, ILs can also be immobilized within the galleries of layered materials. In this way, the immobilized ILs can be better protected, and the release of ILs from the interlayer space might also be controlled, which would be r 2011 American Chemical Society
very beneficial for certain applications. In addition, such a layered structure might be ideal for some specific applications, such as electrolytes. Layered R-zirconium phosphate (ZrP), Zr(HPO4)2 3 H2O, is a particularly ideal candidate as a support to immobilize ILs because of its high ion-exchange capacity, highly ordered structure, ease of synthesis, and ease of crystallinity and size control.18-25 It has been used as a model system for intercalation chemistry studies,23-27 as well as in a wide range of engineering applications.28-38 Direct intercalation of ILs into the gallery of ZrP in aqueous solution has been attempted, but not successful.39 ILs cannot be introduced into the gallery of ZrP until ZrP was preintercalated by butylamine.39 Although the two-step intercalation might be acceptable for some applications, the preintercalated amine is highly undesirable or unacceptable for many applications, such as catalysis. The unsuccessful intercalation of bulky ILs into layered compounds in solution systems is mainly owing to the dimensional mismatch. When the dimension of ILs is larger than the interlayer gap of layered compounds, simple stirring or ultrasonication is usually not sufficient to force the guest ILs into the host layered compounds. To solve the problem, we explored introducing ILs into layered materials via a mechanochemical approach. Received: December 7, 2010 Revised: January 25, 2011 Published: March 03, 2011 5509
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Mechanochemical reactions have been applied to several hostguest systems to achieve intercalation.40,41 Unlike a regular solution intercalation route, this route can be proceeded under ambient or high-temperature conditions via adsorption, a displacement or functional reaction, with prime benefits of not requiring solvent, higher production yield, and short reaction time (as short as a few minutes).40-44 Herein, we report the successful intercalation of ILs into the galleries of ZrP via a mechanochemical reaction and evaluation of such immobilized ILs for catalysis applications.
’ EXPERIMENTAL SECTION Zirconyl chloride octahydrate (ZrOCl2 3 8H2O, 98%, Aldrich), phosphoric acid (85%, Aldrich), and 1-butyl-3-methylimidazolium chloride (BMIMCl, Aldrich) were used as received. Propylene oxide (PO) with a purity of 95.0% was pretreated by potassium hydroxide and refluxed over calcium hydride for 24 h. It was then distilled under dry nitrogen gas and stored over 4A molecular sieves prior to use. ZrP platelets with different lateral dimensions were synthesized according to the procedures described in an earlier report.22 Briefly, a sample of 10.0 g of ZrOCl2 3 8H2O was refluxed with 100.0 mL of 3.0 M H3PO4 at 100 °C for 24 h to synthesize ZrP(3M-RF). A sample of 6.0 g of ZrOCl2 3 8H2O was mixed with 60.0 mL of 6.0 M H3PO4 in a sealed Teflon-lined pressure vessel and reacted at 200 °C for 24 h to prepare ZrP(6M-HT). ZrP(3M-RF) and ZrP(6M-HT) were used as supports in this research. The following mechanochemical reaction procedures were strictly followed to intercalate BMIMCl into ZrP to prepare a series of intercalation compounds. In an agate mortar, a predetermined amount of ZrP was first ground for 3 min. BMIMCl was then added to the mortar and ground with ZrP for 10 min to generate the intercalation compound. Various amounts of BMIMCl were reacted with two ZrP hosts, which were formulated based on the molar ratio of available cations in the guest BMIMCl to the total cation-exchange capacity of the host. For example, ZrP(3M-RF)-50 refers to a sample formulated with an amount of BMIMCl that counts 50% of the total exchangeable cations in ZrP(3M-RF). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 diffractometer with Bragg-Brentano θ-2θ geometry (20 kV and 5 mA), using a graphite monochromator with Cu KR radiation. The thermal stability of the intercalation compounds was characterized by a thermogravimetric analyzer (TGA, TA Instruments model Q50) under an air atmosphere (40 mL/min) at a heating rate of 10 °C/min. The catalysis application of the immobilized BMIMCl was evaluated through a coupling reaction of carbon dioxide (99.99%) and PO45 in a 100 mL stainless steel autoclave equipped with a magnetic stirrer. For a typical reaction process, the immobilized BMIMCl and PO were charged into the reactor, which was pressurized with carbon dioxide at 1.5 MPa and reacted at 110 °C for 10 h. The reactor was then cooled to room temperature, and the resulting mixture was filtered. The unreacted PO was separated by the distillation of the filtrate under vacuum, and the product propylene carbonate was collected. ’ RESULTS AND DISCUSSION The unsuccessful direct intercalation of BMIMCl into ZrP in aqueous solution39 indicated that stirring and ultrasonication
Figure 1. XRD patterns of ZrP(3M-RF) and ZrP(6M-HT). The insets show the SEM images of ZrP(3M-RF) and ZrP(6M-HT).
cannot supply sufficient energy to force the ILs into the gallery of ZrP in the solution state. To provide additional energy to help ILs overcome the intercalation energy barrier, we explored a mechanochemical reaction approach. Two ZrP hosts were synthesized for this project following a previous report with an attempt to obtain two model platelets with different lateral dimensions and levels of crystallinity, and thus different intercalation barriers, for comparison.22 Figure 1 shows the XRD patterns and SEM images of the two ZrP samples. The patterns confirm the formation of ZrP.19,20,22,23 The two samples exhibit significantly different levels of crystallinity, as evidenced by different peak widths and signal-to-noise ratios, but both have an average interlayer distance of 7.6 Å. Furthermore, SEM images clearly show that both of the two ZrP samples exhibit a platelike structure, with a lateral dimension of approximately 80-100 and 800-1000 nm for ZrP(3MRF) and ZrP(6M-HT), respectively. The broadened peaks for ZrP(3M-RF) are mainly owing to its less ordered layer stacking. It is hoped that such a stacking disorder in ZrP(3M-RF), together with its relatively low lateral dimension, will lower the overall intercalation energy barrier compared to ZrP(6MHT)23,25,27 thus will be beneficial for the mechanochemical intercalation. It was observed that, after initial grinding, the ZrP (both ZrP(3M-RF) and ZrP(6M-HT)) and BMIMCl mixture formed a wet paste. This is understandable because, under the grinding pressure, BMIMCl turns to a liquid phase. Upon mixing with ZrP powders, they form a paste. As grinding proceeds, the paste gradually became drier and drier. For the samples with a low concentration of BMIMCl, the final products turned out to be dry powders (as shown in Figure 2), whereas the ones containing a high concentration of BMIMCl turned out to be a highly viscous suspension. This phase transition indicated that, at the earlier stage, a simple powder/liquid mixture formed. As the reaction proceeded, BMIMCl was gradually intercalated into the gallery and resulted in dry powder samples. This phenomenon indicates that the mechanochemical reaction can effectively enable BMIMCl to be intercalated into layered ZrP. Multiple samples with different hosts and various formulation ratios were prepared and are summarized in Table 1. To verify the above hypothesis, the prepared samples were characterized by XRD. Figure 3 presents the XRD patterns of 5510
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Figure 3. XRD patterns of ZrP(3M-RF)/BMIMCl intercalation compounds with various BMIMCl loadings.
Figure 2. Digital pictures showing gradual reaction progress between 0.600 g of ZrP(3M-RF) and 0.348 g of BMIMCl (1:1 molar ratio): (A) before grinding, (B) wet paste formed after initial grinding for ca. 1 min, (C) drier paste formed after ca. 2 min of grinding, and (D) after ca. 5 min of grinding; the mixture turned out to be dry powders.
Table 1. Formulation and Appearance of ZrP/BMIMCl Intercalation Compounds BMIMCl BMIMþ/
weight
exchangeable
percentage
product
sample
cation
(wt %)
appearance
ZrP(3M-RF)-25
0.25:1
22.5
powder
ZrP(3M-RF)-50
0.50:1
36.7
powder
ZrP(3M-RF)-100
1.00:1
53.7
paste
ZrP(6M-HT)-25
0.25:1
22.5
powder
ZrP(6M-HT)-50
0.50:1
36.7
powder
ZrP(6M-HT)-
1.00:1
53.7
paste
100
ZrP(3M-RF)/BMIMCl intercalation compounds, which clearly show an increased interlayer distance after mechanochemical reaction. To be noted, even at a low formulation ratio of 25% (ZrP(3M-RF)-25), the peak at 7.6 Å corresponding to the pristine ZrP completely disappeared, while a new intensive peak located at 12.8 Å was observed on the pattern. This indicated that a new intercalation compound formed with no pristine ZrP left. At a formulation ratio of 100% (ZrP(3M-RF)-100), the reduced peak intensity indicates that a portion of the BMIMCl may not be intercalated into the gallery but instead may be adsorbed on the surface. This is also consistent with the paste appearance of the sample, as summarized in Table 1. The reason BMIM cations cannot be 100% intercalated into ZrP is believed to be owing to the high density of cation-exchange sites (hydroxyl groups) in ZrP, while the BMIM cation has a dimension larger than the
Figure 4. XRD patterns of ZrP(6M-HT)/BMIMCl intercalation compounds with various BMIMCl loadings.
distance between neighboring hydroxyl groups. The steric hindrance prevents 100% intercalation. Similar mechanochemical intercalation results were achieved in ZrP(6M-HT), as shown in Figure 4. However, when ZrP (6M-HT) was used as the host, the pristine ZrP phase cannot be completely removed. Although a higher BMIMCl loading led to a lower concentration of the pristine ZrP phase, even at a formulation ratio of 100%, a tiny amount of pristine ZrP still remained in the intercalation compound. This is believed to be mainly owing to the much larger lateral dimension of ZrP (6M-HT) (800-1000 nm) compared with ZrP(3M-HT) (80-100 nm).22,25 Because the intercalation of BMIM cations into the gallery progresses from the edge to the center, it is more difficult for BMIM cations to diffuse throughout the layer in larger platelets.25 Besides, the more ordered structure (higher crystallinity) of ZrP(6M-HT) compared with ZrP(3M-RF), and, therefore, higher Van Der Waals forces between the layers, should be another factor for the different intercalation behaviors.25 5511
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Figure 5. Dimension of a BMIM cation under minimized energy (Chem3D Pro 10.0).
It was noted that the interlayer distance for both ZrP(3MRF)- and ZrP(6M-HT)-based intercalation compounds only enhanced slightly with increasing concentration of BMIMCl in the formulation and saturated at ca. 12.9 and 12.4 Å, respectively. This is different from the solution intercalation results, which typically exhibit an enhanced interlayer distance with increasing intercalation ratio.23,25 The reason for such a different result is possibly owing to the nature of the reaction. During solution intercalation, ultrasonication is typically adopted to promote the insertion of guests into the host layers. The guest molecule vibration and localized heating generated by ultrasonication might effectively expand the host layer gradually, which allows more guest molecules to be intercalated into the gallery and then tilted. For mechanochemical intercalation, the mechanical force can only promote the insertion of guest molecules into the gaps of the layered compounds, but it was not able to effectively expand the interlayer distance. This is consistent with some earlier mechanochemical intercalation results, in which the interlayer distance of the montmorillonite/octadecylamine intercalation compound was independent of the concentration of octadecylamine.46 Heating the intercalated compounds prepared via the mechanochemical reaction did lead to further expansion of the interlayer distance (Figure S1 in the Supporting Information), which supports the above conjecture. The slight difference in the interlayer distance between the two series of samples is believed to be owing to the different levels of crystallinity between the two hosts. ZrP(3M-RF) is of lower crystallinity and less ordered23,25 and thus can be more easily intercalated, and thus the BMIM cations might also be slightly tilted. ZrP(6M-HT) is of much higher crystallinity and larger size, in which BMIM cations were almost perfectly parallel to the layers. Considering that the BMIM cation has a thickness of ca. 2.9 Å (Figure 5), the layer thickness of ZrP is ca. 6.3 Å,47,48 and ZrP(6M-HT)/BMIMCl intercalation compounds have an interlayer distance of ca. 12.4 Å, this indicates that a bilayer of BMIM cations stay virtually parallel to ZrP(6M-HT) layers, whereas a bilayer of BMIM cations with a small tilt angel should exist in ZrP(3M-RF) galleries. The thermal stability of the intercalation compounds was investigated by TGA. Prior to each test, the samples were
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Figure 6. TGA thermograms of pristine ZrP(3M-RF), BMIMCl, and ZrP(3M-RF)/BMIMCl intercalation compounds with various BMIMCl loadings.
isothermed at 90 °C for 30 min in an air flow to remove absorbed moisture, then cooled down to 50 °C to start the test. The TGA thermograms of ZrP(3M-RF)/BMIMCl intercalation compounds are shown in Figure 6, with BMIMCl and neat ZrP(3M-RF) as the controls. The sharp weight loss of BMIMCl started from ca. 210 °C, and it lost all the weight at ca. 315 °C. ZrP exhibits a two-step degradation at ca. 100-170 and 450580 °C, corresponding to the removal of hydration water and condensation water, respectively.27 The ZrP(3M-RF)/BMIMCl intercalation compounds mainly exhibited three weight losses at ca. 220-320, 340-410, and 450-580 °C. The first step of weight loss agrees well with the degradation of the BMIMCl control sample, but slightly delayed. This step of weight loss is owing to the degradation of BMIMCl adsorbed on ZrP surface. The increasing amount of adsorbed BMIMCl from sample ZrP(3M-RF)-25 to ZrP(3M-RF)-100 is also very consistent with the formulation. The second step of degradation can be attributed to the degradation of intercalated BMIMCl in the ZrP gallery. Because of the protection from the inorganic layers, and the electrostatic bonding with the layers, the degradation of this part of BMIMCl was delayed until ca. 340-410 °C. The delayed degradation indirectly supports that BMIM cations were intercalated into the interlayer space via the mechanochemical reaction. The third step of degradation corresponding to the removal of condensation water in ZrP was not clearly seen in Figure 6 but can be observed on the derivative curve (not shown). This is mainly because of the lowered weight concentration of ZrP in the intercalation compounds. Similar TGA results were obtained for ZrP(6M-HT)/BMIMCl intercalation compounds (Figure S2 in the Supporting Information). The immobilized BMIMCl in ZrP was evaluated for catalysis applications using the following reaction.45
The conversion of CO2 to valuable chemicals has long been a challenge, and it has recently attracted particular interest owing to the climate issues related to CO2. One potential approach is the coupling reaction of CO2 and epoxides to synthesize cyclic 5512
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Table 2. Catalysis Evaluation Results for the Formation of Propylene Carbonate via the Coupling Reaction of CO2 and Propylene Oxides propylene amount of
amount
pressure
carbonate
catalyst (g)
of PO (mL)
(MPa)
yield (%)
BMIMCl
0.537
15.0
1.5
ZrP(3M-RF)-100
1.000
46.1
ZrP(6M-HT)-100
1.000
55.1
catalyst
55.9
carbonates, which has wide applications.45 Many catalytic systems have been developed for the coupling reaction, including ionic liquids,49,50 most of which suffer from serious issues, such as low catalytic activity and/or selectivity, low stability, requiring cosolvent, requiring high pressure/temperature, etc. Although ILs have been demonstrated to be effective in the chemical fixation of carbon dioxide,49,50 it is much more desirable to use immobilized ILs, as discussed above. During the evaluation of the immobilized BMIMCl, no cosolvent or cocatalyst was used. The detailed reaction conditions and evaluation results are summarized in Table 2. When 0.537 g of BMIMCl was used as the catalyst, the yield was 55.9%, whereas when 1.000 g of immobilized catalysts ZrP(3M-RF)-100 and ZrP(6M-HT)-100 (both containing 0.537 g of BMIMCl) was used, yields of 46.1 and 55.1% were achieved, respectively. The results showed that the immobilized BMIMCl can maintain a similar level of reactivity as free BMIMCl. Although ZrP(6M-HT)-100 exhibited a higher reactivity than ZrP(3M-RF)-100 during this catalysis evaluation, which is possibly owing to the more uniform intercalation and adsorption of BMIMCl on ZrP(6M-HT) layers, more detailed evaluations in different reaction systems are to be conducted to draw a conclusion. ILs have been widely considered as green solvents and used in many green chemistry applications.1-3 The facile mechanochemical reaction approach can effectively immobilize BMIMCl in layered compounds within minutes without using any solvent. Meanwhile, the immobilized ILs could perform more effectively and find promising applications, such as being used as catalysts for green chemical reactions, that is, the fixation of CO2. The mechanochemical reaction approach can thus be considered as a “green” approach (no solvent, low energy consumption, etc.), which renders ILs to be “greener” after immobilization. More detailed catalysis evaluation and optimization of such immobilized ILs and exploration of new applications of layered compound immobilized ILs are underway and will be reported later. Other chemicals, such as other ILs and Jeffamine M1000 amine (a solid-state polyoxyalkyleneamine from Huntsman Corporation; Figure S3 in the Supporting Information), have also been successfully intercalated into ZrP via the mechanochemical reaction. Meanwhile, BMIMCl has been successfully immobilized in other layered compounds, such as montmorillonite (Figure S4 in the Supporting Information). All these results show that the mechanochemical reaction can be adopted as a general approach to intercalate large molecules, which are difficult to be intercalated in solution state, into layered compounds.
’ CONCLUSIONS The mechanochemical reaction has been proved to be a facile and effective approach to immobilize ILs in layered compounds,
as evidenced by both the X-ray diffraction and TGA characterizations. Without using any solvent and requiring only a few minutes of the single-step reaction, the mechanochemical reaction serves as a “green” approach to immobilize “green” ILs to be “greener”, considering that the immobilized ILs could perform more effectively and efficiently for practical applications (such as catalysis). In addition, the mechanochemical reaction conducted in the lab using a mortar and pestle can be easily scaled up in industry using tools such as a ball-miller. Thus, it is expected that the mechanochemical reaction can be easily adopted for industrial applications.
’ ASSOCIATED CONTENT
bS
Supporting Information. XRD patterns and TGA thermograms of the intercalated compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: (512) 245-5563 (L.S.), (0086) 20-8411-4113 (Y.M.). Fax: (512) 245-2374 (L.S.), (0086) 20-8411-4113 (Y.M.). E- mail:
[email protected] (L.S.),
[email protected] (Y.M.).
’ ACKNOWLEDGMENT This research was sponsored by the Cottrell College Science Award from the Research Corporation for Science Advancement and a grant from The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province. L.S. would also like to acknowledge the start-up fund and the Research Enhancement Grant from Texas State University-San Marcos, and the National Science Foundation (NSF) Summer Research in Solid State Chemistry for Undergraduates & College Faculty for partial support for this research. H.H. acknowledges the China Scholarship Council for offering her a scholarship to conduct research at Texas State University-San Marcos. We are grateful to Dr. Abraham Clearfield at Texas A&M University for valuable discussions. ’ REFERENCES (1) Welton, T. Chem. Rev. 1999, 99, 2071–2083. (2) Rogers, R. D.; Voth, G. A. Acc. Chem. Res. 2007, 40, 1077–1078. (3) Smiglak, M.; Metlen, A.; Rogers, R. D. Acc. Chem. Res. 2007, 40, 1182–1192. (4) Greaves, T. L.; Drummond, C. J. Chem. Rev. 2007, 108, 206–237. (5) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667–3691. (6) Mehnert, C. P. Chem.—Eur. J. 2005, 11, 50–56. (7) Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157–189. (8) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Nat. Mater. 2009, 8, 621–629. (9) Zhou, F.; Liang, Y. M.; Liu, W. M. Chem. Soc. Rev. 2009, 38, 2590–2599. (10) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974–4975. (11) Turner, M. B.; Spear, S. K.; Holbrey, J. D.; Daly, D. T.; Rogers, R. D. Biomacromolecules 2005, 6, 2497–2502. (12) Sun, N.; Swatloski, R. P.; Maxim, M. L.; Rahman, M.; Harland, A. G.; Haque, A.; Spear, S. K.; Daly, D. T.; Rogers, R. D. J. Mater. Chem. 2008, 18, 283–290. 5513
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The Journal of Physical Chemistry C (13) Pinkert, A.; Marsh, K. N.; Pang, S.; Staiger, M. P. Chem. Rev. 2009, 109, 6712–6728. (14) Smiglak, M.; Reichert, W. M.; Holbrey, J. D.; Wilkes, J. S.; Sun, L.; Thrasher, J. S.; Kirichenko, K.; Singh, S.; Katritzky, A. R.; Rogers, R. D. Chem. Commun. 2006, 2554–2556. (15) Castillo, M. R.; Fousse, L.; Fraile, J. M.; Garcia, J. I.; Mayoral, J. A. Chem.—Eur. J. 2007, 13, 287–291. (16) Wang, G.; Yu, N.; Peng, L.; Tan, R.; Zhao, H.; Yin, D.; Qiu, H.; Fu, Z.; Yin, D. Catal. Lett. 2008, 123, 252–258. (17) Zhao, D.; Liao, Y.; Zhang, Z. Clean: Soil, Air, Water 2007, 35, 42–48. (18) Clearfield, A. J. Mol. Catal. 1984, 27, 251–262. (19) Clearfield, A. Annu. Rev. Mater. Sci. 1984, 14, 205–229. (20) Alberti, G. Acc. Chem. Res. 1978, 11, 163–170. (21) Kumar, C. V.; Bhambhani, A.; Hnatiuk, N. Layered Α-Zirconium Phosphates and Phosphonates. In Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc.: New York, 2004; pp 313-372. (22) Sun, L.; Boo, W. J.; Sue, H.-J.; Clearfield, A. New J. Chem. 2007, 31, 39–43. (23) Sun, L.; Boo, W. J.; Browning, R. L.; Sue, H.-J.; Clearfield, A. Chem. Mater. 2005, 17, 5606–5609. (24) Boo, W. J.; Sun, L.; Liu, J.; Clearfield, A.; Sue, H.-J. J. Phys. Chem. C 2007, 111, 10377–10381. (25) Sun, L.; O’Reilly, J. Y.; Kong, D.; Su, J. Y.; Boo, W. J.; Sue, H. J.; Clearfield, A. J. Colloid Interface Sci. 2009, 333, 503–509. (26) Clearfield, A. Progress in Intercalation Research. In Progress in Intercalation Research; M€uller-Warmuth, W., Schollhorn, R., Eds.; Kluwer: Dordrecht, The Netherlands, 1994; pp 240-263. (27) Clearfield, A.; Costantino, U. Layered Metal Phosphates and Their Intercalation Chemistry. In Comprehensive Supramolecular Chemistry; Alberti, G., Bein, T., Eds.; Elsevier: Oxford, U.K., 1996; Vol. 7, pp 107-149. (28) Sue, H.-J.; Gam, K. T.; Bestaoui, N.; Spurr, N.; Clearfield, A. Chem. Mater. 2004, 16, 242–249. (29) Sue, H.-J.; Gam, K. T.; Bestaoui, N.; Clearfield, A.; Miyamoto, M.; Miyatake, N. Acta Mater. 2004, 52, 2239–2250. (30) Sun, L.; Boo, W. J.; Sun, D.; Clearfield, A.; Sue, H.-J. Chem. Mater. 2007, 19, 1749–1754. (31) Boo, W. J.; Sun, L.; Liu, J.; Clearfield, A.; Sue, H.-J.; Mullins, M. J.; Pham, H. Compos. Sci. Technol. 2007, 67, 262–269. (32) Boo, W. J.; Sun, L.; Liu, J.; Moghbelli, E.; Clearfield, A.; Sue, H.J.; Pham, H.; Verghese, N. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 1459–1469. (33) Boo, W. J.; Sun, L.; Warren, G. L.; Moghbelli, E.; Pham, H.; Clearfield, A.; Sue, H.-J. Polymer 2007, 48, 1075–1082. (34) Sun, L.; Boo, W. J.; Clearfield, A.; Sue, H.-J.; Pham, H. Q. J. Membr. Sci. 2008, 318, 129–136. (35) Sun, L.; Boo, W.-J.; Liu, J.; Clearfield, A.; Sue, H.-J.; Verghese, N. E.; Pham, H. Q.; Bicerano, J. Macromol. Mater. Eng. 2009, 294, 103–113. (36) Moghbelli, E.; Sun, L.; Jiang, H.; Boo, W. J.; Sue, H.-J. Polym. Eng. Sci. 2009, 49, 483–490. (37) Sun, L.; Liu, J.; Kirumakki, S. R.; Schwerdtfeger, E. D.; Howell, R. J.; Al-Bahily, K.; Miller, S. A.; Clearfield, A.; Sue, H.-J. Chem. Mater. 2009, 21, 1154–1161. (38) Sun, L.; Sue, H.-J. Permeation Properties of Epoxy Nanocomposites. In Barrier Properties of Polymer Clay Nanocomposites; Mittal, V., Ed.; Nova Science Publishers: New York, 2010; pp 73-93. (39) Wang, H.; Zou, M.; Li, N.; Li, K. J. Mater. Sci. 2007, 42, 7738–7744. (40) Ogawa, M.; Hashizume, T.; Kuroda, K.; Kato, C. Inorg. Chem. 1991, 30, 584. .; Kristof, J.; Horvath, E.; Vagv€olgyi, V. J. Colloid (41) Mako, E Interface Sci. 2009, 330, 367–373. (42) Ogawa, M.; Aono, T.; Kuroda, K.; Kato, C. Langmuir 1993, 9, 1529–1533. (43) Choy, J. H.; Lee, S. R.; Han, Y. S.; Park, M.; Park, G. S. Chem. Commun. 2003, 1922–1923.
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(44) Lee, S.-R.; Han, Y.-S.; Park, M.; Park, G.-S.; Choy, J.-H. Chem. Mater. 2003, 15, 4841–4845. (45) Xiao, L.-F.; Li, F.-W.; Peng, J.-J.; Xia, C.-G. J. Mol. Catal. A: Chem. 2006, 253, 265–269. (46) Bujdak, J.; Slosiarikova, H. Appl. Clay Sci. 1992, 7, 263–269. (47) Kim, H. N.; Keller, S. W.; Mallouk, T. E.; Schmitt, J.; Decher, G. Chem. Mater. 1997, 9, 1414–1421. (48) Alberti, G.; Costantino, U. Intercalation Chemistry of Acid Salts of Tetravalent Metals with Layered Structure and Related Materials. In Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press, Inc.: New York, 1982; pp 147-180. (49) Li, F.; Xiao, L.; Xia, C.; Hu, B. Tetrahedron Lett. 2004, 45, 8307–8310. (50) Calo, V.; Nacci, A.; Monopoli, A.; Fanizzi, A. Org. Lett. 2002, 4, 2561–2563.
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