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Concepts in the ionothermal synthesis of zeolites and metal organic frameworks
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
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Concepts in the ionothermal synthesis of zeolites and metal organic frameworks Russell E. Morris EaStChem School of Chemistry, University of St Andrews, Purdie Building, St Andrews, KY16 9ST, UK
Abstract Ionothermal synthesis is the use of ionic liquids as the solvent and also as the structure directing agent in the synthesis of materials. The change of solvent chemistry from molecular (as in traditional hydrothermal or solvothermal synthesis) to ionic produces marked effects on the process. This change in chemistry has produced some new concepts that can be used in the synthesis of porous materials. The vanishingly low vapour pressure leads to zeolite synthesis at ambient pressure even at high temperatures, which may have some applications in the manufacture of zeolite coatings. Ionic liquids containing low concentrations of water behave rather differently to other solvents with similar moisture levels. This can lead to lower hydrolysis in the synthesis and to product materials with different chemical compositions to those normally prepared in hydrothermal synthesis. Finally, the properties of ionic liquids are extremely dependent on the nature of the anion present. This adds an extra degree control and can even lead to very unusual effects in zeolite or metal organic framework synthesis, such as the induction of chiral structures in materials comprising only achiral building units. Keywords: Ionic liquids, zeolites, metal organic frameworks, anion control
1. Introduction Innovation in zeolite synthesis remains an important aspect in the search for new framework materials with potential applications. The driver for the search for new zeolites and related solids is not only the need to provide materials for new and emerging applications [1] but is also the desire to understand how these fascinating materials are made, and ultimately how to control their architectures. Given that the applications of zeolites (and other porous solids such as metal organic frameworks) are intimately connected with their architecture new synthetic methods that aim to understand how structure can be controlled are very important. The core strategy that has been exercised over recent years has been the development of new organic compounds that can be used as structure directing agents (SDAs or templates). Simply preparing new SDAs has led to a significant increase in the numbers of zeolite structures over recent years. This is still a method that produces some remarkable new materials, such as IM-12 [2] However, other more innovative methods have also made their impact, both in terms of finding new ways to recycle templates [3] and in completely new synthesis concepts. The use of fluoride as a mineralizing agent to improve solubility of the starting reagents and to catalyse the formation of bonds in the framework. This method has been exploited by several groups to produce several new materials over recent years [4,5,6] Charge density mismatch solutions, developed by workers at UOP, have also provided routes to new solids [7]. In this process stable solutions of the inorganic starting materials are prepared by using organic cations that
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do not make good SDAs because their charge density does not match that of the chemical composition of the inorganic framework which will be formed. Crystallisation of the framework is then initiated by addition of another SDA, often in quite small amounts. High throughput methods have also been applied with some distinct success, particularly by Corma’s group in Valencia [8]. In our laboratory we have pioneered the use of ionic liquids as both the solvent and SDA simultaneously [9]. The change from a molecular solvent, such as water or organic molecules, to an ionic one changes the chemistry of the system markedly. We have given the name ionothermal synthesis to this method to delineate it from hydrothermal or solvothermal synthesis. Originally we applied this new synthetic method to the synthesis of aluminophosphate zeolites, and we have prepared several new ones in this work [10,11,12]. Since then however we have expanded our work to include other types of material, including coordination polymers and metal organic frameworks [13]. During the course of this work we have developed several new concepts, based on the particular properties of ionic liquids. These include ambient pressure synthesis, water deactivation, anion control and chiral induction. In this paper I will explain what ionic liquids are, why their properties are good for zeolite synthesis and how the above concepts can be applied in zeolite or coordination polymer synthesis.
2. Ionic liquids and ionothermal synthesis Near room temperature ionic liquids (ILs) are commonly defined as salts that melt under approximately 100oC. For applications we are interested in we tend to expand this definition to include salts that melt under about 200oC, as these are temperatures commonly used in zeolite and materials synthesis. There are several different classes of ILs, but we have concentrated on two main ones during our work, binary ILs based on medium-sized organic cations such as N,N’-dialkylimidazolium and N-alkyl pyridinium cations, in combination with halide or other anions and ternary (or supramolecular ILs) based on deep eutectic solvents (DES). Figure 1 shows some of the more common cations and anions used in binary ILs, and it is clear that the chemical structure of the cations matches quite well the types of structure used as structure directing agents in zeolite synthesis, which suggests that these ILs may make good SDAs.
Figure 1. Cations and anions commonly used in binary ionic liquids.
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Concepts in ionothermal synthesis
Deep eutectic solvents are a different class of IL based on mixing two compounds together to produce a new material with a much lower melting point than the starting materials individually [14]. For example urea (mp 133oC) and choline chloride (mp 302oC) can be mixed together in a 1:2 ratio to liquid with a melting point of 12oC (Figure 2).
Choline Chloride
Urea
Eutectic mixture
Figure 2. Mixing two solids, choline chloride and urea, leads to a liquid suitable for use as a solvent in ionothermal synthesis. Given that ILs are ionic in nature, they are intrinsically polar liquids and so many make good solvents for inorganic reagents. Amongst the other properties are a low (often almost negligible) vapour pressure, good microwave absorption and controllable hydrophobicity/hydrophilicity. In the following sections I will explain how these properties lead to new concepts in porous materials synthesis.
3. Ambient pressure synthesis Perhaps the most striking feature of ionic liquids is their very low vapour pressure. This means that, unlike molecular solvents like water, the ILs can be heated to relatively high temperatures without the production of autogenous pressure. This means that high temperature reactions do not have be completed inside pressure vessels such as Teflonlined steel autoclaves but can be undertaken in simple containers such as round bottomed flasks. The absence of autogeneous pressure at high temperature also makes microwave heating a safer prospect as hot spots in the liquid should not cause excessive increases in pressure with their associated risk of explosion, assuming of course that the IL is stable and does not breakdown into smaller components during heating. Figure 3 shows the measured pressure during the synthesis of an aluminophosphate molecular sieve (SIZ-4) using a microwave heating experiment. The left hand panel is the pure ionic liquid solvent, and it is clear that no autogenous pressure is produced. The right
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hand panel, however, shows that even when only small amounts of water are added to the system that significant pressures are evolved. Pressure / bar
Time / mins
Time / mins
Figure 3. The evolution of pressure (in bar) in the microwave synthesis of aluminophosphate SIZ-4 from (left) a pure ionic liquid solvent with no water added and (right) the same solvent system with 0.018 ml of water added. One of the most interesting potential uses of ambient pressure synthesis of zeolite coatings for anti-corrosion applications. Yushan Yan has shown that ionothermallyprepared zeolite films (Figure 4) make excellent anti-corrosion coatings for several different types of alloys [15,16]. Given that current coatings technology is based on the use of environmentally-unfriendly chromium coatings there is interest in finding more acceptable alternatives. Sealed zeolites are one alternative. However, hydrothermal synthesis of zeolites inside sealed vessels in impractical for large, oddly shaped and cut pieces of metal. Yan contends that ambient pressure ionothermal synthesis eliminates the need for unwieldy sealed vessels, and given the excellent coatings that can be prepared using this approach, offers an interesting and potentially important alternative technology.
Figure 4. Ambient pressure ionothermal synthesis of zeolite coatings offers great potential in anti-corrosion technologies.
4. Water deactivation Another very interesting property of ionic liquids is the so called ‘water deactivation’ that occurs when small amounts of water are added. When present in small amounts,
Concepts in ionothermal synthesis
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water acts quite differently than it does when present in the same amounts in other molecular solvents. In essence, a ‘wet’ ionic liquid acts as if it is drier than one thinks. For example, PCl3, which is extremely sensitive to moisture and hydrolyses very violently on contact with water, is stable even in ILs with significant levels of water present [17]. In contrast, when contacted with a molecular solvent like THF that contains similar amounts of water there is a violent reaction as the PCl3 is hydrolysed to phosphorus oxides. The reason for this ‘water deactivation’ probably stems from the molecular structure of water dissolved in ILs. IR and molecular dynamics simulations indicate that molecules of water bind relatively strongly to the anions in many ILs, and are present as isolated molecules. As the concentration of water increases clusters of water molecules and eventually hydrogen bonded networks form when the concentration of water is large enough. When present as isolated molecules or small clusters the water is effectively shielded from reaction by the IL anions. In contrast, in many organic molecular solvents water, even at low concentrations, tends to be present as much larger hydrogen bonded clusters. In materials synthesis this property of water deactivation may play an important role in determining which phases are possible products of ionothermal preparations. Water, like fluoride, can be thought of as a mineralising agent when in relatively low concentrations, and can also act in a similar manner to fluoride by catalyzing the inorganic bond breaking – bond making processes required to make the product framework. The upshot of water deactivation in ILs is that it may be possible to prepare unusual materials that are unlikely to be prepared in aqueous or molecular solvents. This is most easily noticed in certain materials made at relatively low temperature from deep eutectic solvents containing chloride ions. A particular example of unusual chemistry is the structure of the layered material SIZ-13 (Figure 5), which is a cobalt aluminophosphate solid containing ordered cobalt ions and Co-Cl bonds [18]. Such chemistry is unusual in AlPO chemistry, and it is very unlikely that this material could be made using triditonal solvothermal methods as the Co-Cl bond is highly sensitive to hydrolysis. Increasing the temperature of the synthesis tends to increase the rate of hydrolysis in the system from the small amount of water that is present leading to more commonly found zeolite-type structures such as SIZ-14, which has the LEV framework. Similar examples of this type of unusual chemistry can be found in the synthesis of porous coordination compounds, where normally quite hydrolytically sensitive chemical groups can be prepared. From the very earliest examples of ionothermal synthesis in zeolite synthesis it has been recognized that water can play an important reactant role in the synthesis method. The use of 1-methyl 3-ethyl imidazolium bromide (EMIM Br) as solvent and template in the synthesis of SIZ-3 (AEL) and SIZ-4 (CHA) indicated that it was extremely important in determining which phases were formed, and that as the concentration of water was raised to intermediate levels this prevented the formation of any zeolite phase. Recently we have carried out systematic high throughput studies of the effect of water content in the microwave synthesis of SIZ-3 and SIZ-4 using EMIM Br, as well as a parallel study of the synthesis of SIZ-4 using ethylpyridinium bromide. Both studies produced the same trends, which are summarized in Figure 6. The results show that at very low (essentially zero) water concentrations (measured using Karl Fischer titration) about 50% of reactions failed to produce any crystalline products, while the other 50% produce identifiable zeolitic phases, albeit often rather poorly crystalline. As the water content is increased from water:IL ratios of 0.1 to 0.7 zeolitic phases are by far the most common products. However, as the water content is increased still further towards water:IL ratios of 1 dense phases tend to become more prevalent. As the water content
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is increased still further the reactions become hydrothermal in nature as opposed to ionothermal. Dense phases remain prevalent although a significant proportion of zeolitic phases can be prepared. This study illustrates the important role water content can play in determining the success of ionothermal reactions. One of the interesting features of these high throughput experiments is that in this system it is actually easier to consistently make zeolite framework materials than it is in a similar hydrothermal system.
Figure 5. The structure of SIZ-13 showing the D4R like building unit similar to that seen in SSZ-51 (SFO) [19] containing ordered cobalt and containing Co-Cl bonds.
5. Anion Control Another extremely important feature of ILs is that many of their important physical and chemical properties are controlled by the anions in the liquid, almost irrespective of the nature of the cation. So while it is the cation that tends to be incorporated into the materials it is the anion that tends to control the chemistry of the systems. For example, the original ionothermal reactions were completed using EMIM-Br as the solvent. This IL has a melting point of 83oC. However, replacing the bromide counter anion with, for example, the bis(trifluoromethanesulfonyl)imide anion (Tf2N, See Figure 1) leads to an IL with completely different properties. EMIM-Br is a solid at room temperature but is highly hygroscopic – it is very difficult to keep completely dry. EMIM- Tf2N is, on the other hand, is a liquid at room temperature that is highly hydrophobic. The same aluminophosphate synthesis carried out in the two ILs leads to completely different products. In EMIM-Br we get the zeolitic phases SIZ-1, SIZ-3 and SIZ-4 where the IL cation is occluded in the final structure, while using EMIM-Tf2N the only product is a chain aluminophosphate where the IL cation is not occluded. This work illustrates two general features of ionothermal synthesis. As the anion is changed to produce a more hydrophobic IL the resultant products from ionothermal tend to be lower dimensional solids (i.e. chains and layers are more prevalent) and it becomes more difficult to occlude the IL cation in the final structure (i,e, templating gets less common). Perhaps more importantly the two ILs,
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Concepts in ionothermal synthesis
EMIM-Br and EMIM-Tf2N are miscible and so the properties of any IL can be tuned across a wide range by mixing the two ILs in various amounts. We have used this effect, which we have termed anion control of ionothermal synthesis, to great effect in the synthesis of coordination polymers [20]. For example, when preparing cobalt or nickel trimesate-based MOFs changing the anion has a marked effect. EMIM-Br, when used as the solvent produced one framework material (Figure 7a). Changing the anion for bis((trifluoromethyl)sulfonyl)amide (bistriflamide, Tf2N-) leads to the production of a different material (Figure 7b), and simply mixing the bromide and bistriflamide in a 1:1 ratio produces another material again (Figure 7c). A fourth material (Figure 7d) is produced if a coordinating ligand, such a 2,2’ bipyridine is added to the synthesis mixture. While it is clear that the anion is the controlling feature of the synthesis, it is important to note that in no case is the anion incorporated into the final structure.
100%
Product %
80% 60% 40% 20% 0% 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.9
Water/IL Ratio 100%
Product %
80% 60% 40% 20% 0% 1.1
1.3
2.2
2.4
3
3.5
4.1
9.8
19.7
Water/IL Ratio
Zeolitic phases
Dense phases
No crystalline product
Figure 6. The results of 94 ionothermal (top) and 46 hydrothermal (bottom) reactions using different amounts of water added to the ionic liquids.
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Figure 7. Four coordination polymer (metal organic framework materials, a-d, top left to bottom right) prepared simply by changing the anions of io based on the EMIM cation. This illustrates the type of control that the anion exerts in this type of synthesis. Another potential application of anion control is to ‘match’ the chemistry of the solvent to the chemistry of the target framework. A good example of this is in the synthesis of metal perfluorosuccinate metal organic frameworks. The highly fluorinated organic linker causes problems in both hydrothermal and ionothermal synthesis using EMIM Br. However, by creating a more fluorous compatible IL by mixing EMIM-Br with EMIM- Tf2N. from this mixed IL solvent ionothermal becomes much easier and crystalline products can be obtained.
6. Chiral Induction A potentially important advantage of ionothermal synthesis involves strong structure directing ability of ionic liquids. The large Coulombic interactions in ionic liquids mean that the fluid structure shows relatively long range correlations and a three dimensional distribution that reflects the asymmetry of the ions. In the case of a chiral ionic liquid one might expect this asymmetric distribution to reflect the chirality of the ions present. This could have major implications for structure direction in ionothermal synthesis, as one can imagine more efficient transfer of chiral information from the ionic liquid to the solid than is the case in situations, for example, where the chirality of the ions are ‘diluted’ by non-chiral molecular solvents. The easiest way to make a chiral IL is to
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simply ion exchange the anion for a naturally obtained amino acid anion, such as Laspartate. This is a very straightforward process that produces enatiomerically pure chiral ILs that are liquid at room temperature. Use of the chiral ionic liquid 1-butyl 3-methyl imidazolium L-aspartate produces a new chiral metal organic framework (SIMOF-4) a homochiral material that crystallises in space group P43212 (Figure 8) [21]. A non-chiral ionic liquid produces a non-chiral MOF (SIMOF-1). This is a chiral induction effect in that the chiral anion is not incorporated into the structure of the material. This is the first such effect ever seen in materials synthesis of this kind.
Br +
N
N
O
NH2 O
HO N
+
N
O
Figure 8. An achiral ionic liquid BMIM-Br can be used to form an achiral MOF (top) while a chiral ionic liquid EMIM-L-Asp induces a chiral MOF (bottom). The cation is occluded in both cases (only shown in the top picture), but the chiral anion is not included into the structure.
7. Conclusion Ionothermal synthesis represents a novel method of preparing inorganic and inorganicorganic hybrid materials, including zeolites and metal organic frameworks. As yet however, we have only just scratched the surface of the possibilities associated with this new preparative method. Changing the solvent chemistry from molecular to ionic clearly makes a huge difference to the types of products that can be prepared. In this paper I have described several new concepts that have emerged out of ionothermal synthesis. The target of the next few years in the area will be to exploit these new concepts and show that ionothermal synthesis complements traditional methods of zeolite and metal organic framework synthesis. This will be best done by preparing new materials with interesting and different properties.
Acknowledgements R.E.M. thanks the EPSRC, the Leverhulme Trust and GEMI for funding.
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References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Davis, M. E., Nature 417 (2002) 813. Paillaud, J. L., Harbuzaru, B., Patarin, J. & Bats, N., Science 304 (2004) 990. Lee, H., Zones, S. I. & Davis, M. E., Nature 425 (2003) 385. Caullet, P., Paillaud, J. L., Simon-Masseron, A., Soulard, M. & Patarin, J., Comptes Rendus Chimie 8 (2005) 245. Villaescusa, L. A., Lightfoot, P. & Morris, R. E., Chem. Commun. (2002) 2220. Villaescusa, L. A., Wheatley, P. S., Bull, I., Lightfoot, P. & Morris, R. E., J. Am. Chem. Soc., 123 (2001) 8797. Blackwell, C. S., Broach, R. W., Gatter, M. G., Holmgren, J. S., Jan, D. Y., Lewis, G. J., Mezza, B. J., Mezza, T. M., Miller, M. A., Moscoso, J. G., Patton, R. L., Rohde, L. M., Schoonover, M. W., Sinkler, W., Wilson, B. A. & Wilson, S. T., Angew. Chem. Int. Ed., 42 (2003) 1737. Corma, A., Diaz-Cabanas, M. J., Jorda, J. L., Martinez, C. & Moliner, M., Nature ,443 (2006) 842. Cooper, E. R., Andrews, C. D., Wheatley, P. S., Webb, P. B., Wormald, P. & Morris, R. E., Nature, 430 (2004) 1012. Parnham, E. R., Wheatley, P. S. & Morris, R. E., Chem. Commun. (2006) 380. Parnham, E. R. & Morris, R. E., J. Am. Chem. Soc.,128 (2006) 2204. Parnham, E. R. & Morris, R. E., Chem. Mater., 18 (2006) 4882. Lin, Z. J., Wragg, D. S. & Morris, R. E., Chem. Commun. (2006) 2021 Parnham, E. R., Drylie, E. A., Wheatley, P. S., Slawin, A. M. Z. & Morris, R. E., Angew. Chem. Int. Ed., 45 (2006) 4962. Cai, R., Sun, M. W., Chen, Z. W., Munoz, R., O'Neill, C., Beving, D. E. & Yan, Y. S., Angew. Chem. Int. Ed., 47 (2008) 525. Morris, R. E., Angew. Chem. Int. Ed., 47 (2008) 442. Amigues, E., Hardacre, C., Keane, G., Migaud, M. & O'Neill, M., Chem. Commun. (2006) 72. Drylie, E. A., Wragg, D. S., Parnham, E. R., Wheatley, P. S., Slawin, A. M. Z., Warren, J. E. & Morris, R. E., Angew. Chem. Int. Ed., 46 (2007) 7839. Morris, R. E., Burton, A., Bull, L. M. & Zones, S. I., Chem. Mater., 16 (2004) 2844. Lin, Z. J., Wragg, D. S., Warren, J. E. & Morris, R. E., J. Am. Chem. Soc., 129 (2007) 10334. Lin, Z. J., Slawin, A. M. Z. & Morris, R. E., J. Am. Chem. Soc., 129 (2007) 4880.
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Concepts in the ionothermal synthesis of zeolites and metal organic frameworks Russell E. Morris EaStChem School of Chemistry, University of St Andrews, Purdie Building, St Andrews, KY16 9ST, UK
Abstract Ionothermal synthesis is the use of ionic liquids as the solvent and also as the structure directing agent in the synthesis of materials. The change of solvent chemistry from molecular (as in traditional hydrothermal or solvothermal synthesis) to ionic produces marked effects on the process. This change in chemistry has produced some new concepts that can be used in the synthesis of porous materials. The vanishingly low vapour pressure leads to zeolite synthesis at ambient pressure even at high temperatures, which may have some applications in the manufacture of zeolite coatings. Ionic liquids containing low concentrations of water behave rather differently to other solvents with similar moisture levels. This can lead to lower hydrolysis in the synthesis and to product materials with different chemical compositions to those normally prepared in hydrothermal synthesis. Finally, the properties of ionic liquids are extremely dependent on the nature of the anion present. This adds an extra degree control and can even lead to very unusual effects in zeolite or metal organic framework synthesis, such as the induction of chiral structures in materials comprising only achiral building units. Keywords: Ionic liquids, zeolites, metal organic frameworks, anion control
1. Introduction Innovation in zeolite synthesis remains an important aspect in the search for new framework materials with potential applications. The driver for the search for new zeolites and related solids is not only the need to provide materials for new and emerging applications [1] but is also the desire to understand how these fascinating materials are made, and ultimately how to control their architectures. Given that the applications of zeolites (and other porous solids such as metal organic frameworks) are intimately connected with their architecture new synthetic methods that aim to understand how structure can be controlled are very important. The core strategy that has been exercised over recent years has been the development of new organic compounds that can be used as structure directing agents (SDAs or templates). Simply preparing new SDAs has led to a significant increase in the numbers of zeolite structures over recent years. This is still a method that produces some remarkable new materials, such as IM-12 [2] However, other more innovative methods have also made their impact, both in terms of finding new ways to recycle templates [3] and in completely new synthesis concepts. The use of fluoride as a mineralizing agent to improve solubility of the starting reagents and to catalyse the formation of bonds in the framework. This method has been exploited by several groups to produce several new materials over recent years [4,5,6] Charge density mismatch solutions, developed by workers at UOP, have also provided routes to new solids [7]. In this process stable solutions of the inorganic starting materials are prepared by using organic cations that
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do not make good SDAs because their charge density does not match that of the chemical composition of the inorganic framework which will be formed. Crystallisation of the framework is then initiated by addition of another SDA, often in quite small amounts. High throughput methods have also been applied with some distinct success, particularly by Corma’s group in Valencia [8]. In our laboratory we have pioneered the use of ionic liquids as both the solvent and SDA simultaneously [9]. The change from a molecular solvent, such as water or organic molecules, to an ionic one changes the chemistry of the system markedly. We have given the name ionothermal synthesis to this method to delineate it from hydrothermal or solvothermal synthesis. Originally we applied this new synthetic method to the synthesis of aluminophosphate zeolites, and we have prepared several new ones in this work [10,11,12]. Since then however we have expanded our work to include other types of material, including coordination polymers and metal organic frameworks [13]. During the course of this work we have developed several new concepts, based on the particular properties of ionic liquids. These include ambient pressure synthesis, water deactivation, anion control and chiral induction. In this paper I will explain what ionic liquids are, why their properties are good for zeolite synthesis and how the above concepts can be applied in zeolite or coordination polymer synthesis.
2. Ionic liquids and ionothermal synthesis Near room temperature ionic liquids (ILs) are commonly defined as salts that melt under approximately 100oC. For applications we are interested in we tend to expand this definition to include salts that melt under about 200oC, as these are temperatures commonly used in zeolite and materials synthesis. There are several different classes of ILs, but we have concentrated on two main ones during our work, binary ILs based on medium-sized organic cations such as N,N’-dialkylimidazolium and N-alkyl pyridinium cations, in combination with halide or other anions and ternary (or supramolecular ILs) based on deep eutectic solvents (DES). Figure 1 shows some of the more common cations and anions used in binary ILs, and it is clear that the chemical structure of the cations matches quite well the types of structure used as structure directing agents in zeolite synthesis, which suggests that these ILs may make good SDAs.
Figure 1. Cations and anions commonly used in binary ionic liquids.
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Deep eutectic solvents are a different class of IL based on mixing two compounds together to produce a new material with a much lower melting point than the starting materials individually [14]. For example urea (mp 133oC) and choline chloride (mp 302oC) can be mixed together in a 1:2 ratio to liquid with a melting point of 12oC (Figure 2).
Choline Chloride
Urea
Eutectic mixture
Figure 2. Mixing two solids, choline chloride and urea, leads to a liquid suitable for use as a solvent in ionothermal synthesis. Given that ILs are ionic in nature, they are intrinsically polar liquids and so many make good solvents for inorganic reagents. Amongst the other properties are a low (often almost negligible) vapour pressure, good microwave absorption and controllable hydrophobicity/hydrophilicity. In the following sections I will explain how these properties lead to new concepts in porous materials synthesis.
3. Ambient pressure synthesis Perhaps the most striking feature of ionic liquids is their very low vapour pressure. This means that, unlike molecular solvents like water, the ILs can be heated to relatively high temperatures without the production of autogenous pressure. This means that high temperature reactions do not have be completed inside pressure vessels such as Teflonlined steel autoclaves but can be undertaken in simple containers such as round bottomed flasks. The absence of autogeneous pressure at high temperature also makes microwave heating a safer prospect as hot spots in the liquid should not cause excessive increases in pressure with their associated risk of explosion, assuming of course that the IL is stable and does not breakdown into smaller components during heating. Figure 3 shows the measured pressure during the synthesis of an aluminophosphate molecular sieve (SIZ-4) using a microwave heating experiment. The left hand panel is the pure ionic liquid solvent, and it is clear that no autogenous pressure is produced. The right
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hand panel, however, shows that even when only small amounts of water are added to the system that significant pressures are evolved. Pressure / bar
Time / mins
Time / mins
Figure 3. The evolution of pressure (in bar) in the microwave synthesis of aluminophosphate SIZ-4 from (left) a pure ionic liquid solvent with no water added and (right) the same solvent system with 0.018 ml of water added. One of the most interesting potential uses of ambient pressure synthesis of zeolite coatings for anti-corrosion applications. Yushan Yan has shown that ionothermallyprepared zeolite films (Figure 4) make excellent anti-corrosion coatings for several different types of alloys [15,16]. Given that current coatings technology is based on the use of environmentally-unfriendly chromium coatings there is interest in finding more acceptable alternatives. Sealed zeolites are one alternative. However, hydrothermal synthesis of zeolites inside sealed vessels in impractical for large, oddly shaped and cut pieces of metal. Yan contends that ambient pressure ionothermal synthesis eliminates the need for unwieldy sealed vessels, and given the excellent coatings that can be prepared using this approach, offers an interesting and potentially important alternative technology.
Figure 4. Ambient pressure ionothermal synthesis of zeolite coatings offers great potential in anti-corrosion technologies.
4. Water deactivation Another very interesting property of ionic liquids is the so called ‘water deactivation’ that occurs when small amounts of water are added. When present in small amounts,
Concepts in ionothermal synthesis
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water acts quite differently than it does when present in the same amounts in other molecular solvents. In essence, a ‘wet’ ionic liquid acts as if it is drier than one thinks. For example, PCl3, which is extremely sensitive to moisture and hydrolyses very violently on contact with water, is stable even in ILs with significant levels of water present [17]. In contrast, when contacted with a molecular solvent like THF that contains similar amounts of water there is a violent reaction as the PCl3 is hydrolysed to phosphorus oxides. The reason for this ‘water deactivation’ probably stems from the molecular structure of water dissolved in ILs. IR and molecular dynamics simulations indicate that molecules of water bind relatively strongly to the anions in many ILs, and are present as isolated molecules. As the concentration of water increases clusters of water molecules and eventually hydrogen bonded networks form when the concentration of water is large enough. When present as isolated molecules or small clusters the water is effectively shielded from reaction by the IL anions. In contrast, in many organic molecular solvents water, even at low concentrations, tends to be present as much larger hydrogen bonded clusters. In materials synthesis this property of water deactivation may play an important role in determining which phases are possible products of ionothermal preparations. Water, like fluoride, can be thought of as a mineralising agent when in relatively low concentrations, and can also act in a similar manner to fluoride by catalyzing the inorganic bond breaking – bond making processes required to make the product framework. The upshot of water deactivation in ILs is that it may be possible to prepare unusual materials that are unlikely to be prepared in aqueous or molecular solvents. This is most easily noticed in certain materials made at relatively low temperature from deep eutectic solvents containing chloride ions. A particular example of unusual chemistry is the structure of the layered material SIZ-13 (Figure 5), which is a cobalt aluminophosphate solid containing ordered cobalt ions and Co-Cl bonds [18]. Such chemistry is unusual in AlPO chemistry, and it is very unlikely that this material could be made using triditonal solvothermal methods as the Co-Cl bond is highly sensitive to hydrolysis. Increasing the temperature of the synthesis tends to increase the rate of hydrolysis in the system from the small amount of water that is present leading to more commonly found zeolite-type structures such as SIZ-14, which has the LEV framework. Similar examples of this type of unusual chemistry can be found in the synthesis of porous coordination compounds, where normally quite hydrolytically sensitive chemical groups can be prepared. From the very earliest examples of ionothermal synthesis in zeolite synthesis it has been recognized that water can play an important reactant role in the synthesis method. The use of 1-methyl 3-ethyl imidazolium bromide (EMIM Br) as solvent and template in the synthesis of SIZ-3 (AEL) and SIZ-4 (CHA) indicated that it was extremely important in determining which phases were formed, and that as the concentration of water was raised to intermediate levels this prevented the formation of any zeolite phase. Recently we have carried out systematic high throughput studies of the effect of water content in the microwave synthesis of SIZ-3 and SIZ-4 using EMIM Br, as well as a parallel study of the synthesis of SIZ-4 using ethylpyridinium bromide. Both studies produced the same trends, which are summarized in Figure 6. The results show that at very low (essentially zero) water concentrations (measured using Karl Fischer titration) about 50% of reactions failed to produce any crystalline products, while the other 50% produce identifiable zeolitic phases, albeit often rather poorly crystalline. As the water content is increased from water:IL ratios of 0.1 to 0.7 zeolitic phases are by far the most common products. However, as the water content is increased still further towards water:IL ratios of 1 dense phases tend to become more prevalent. As the water content
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is increased still further the reactions become hydrothermal in nature as opposed to ionothermal. Dense phases remain prevalent although a significant proportion of zeolitic phases can be prepared. This study illustrates the important role water content can play in determining the success of ionothermal reactions. One of the interesting features of these high throughput experiments is that in this system it is actually easier to consistently make zeolite framework materials than it is in a similar hydrothermal system.
Figure 5. The structure of SIZ-13 showing the D4R like building unit similar to that seen in SSZ-51 (SFO) [19] containing ordered cobalt and containing Co-Cl bonds.
5. Anion Control Another extremely important feature of ILs is that many of their important physical and chemical properties are controlled by the anions in the liquid, almost irrespective of the nature of the cation. So while it is the cation that tends to be incorporated into the materials it is the anion that tends to control the chemistry of the systems. For example, the original ionothermal reactions were completed using EMIM-Br as the solvent. This IL has a melting point of 83oC. However, replacing the bromide counter anion with, for example, the bis(trifluoromethanesulfonyl)imide anion (Tf2N, See Figure 1) leads to an IL with completely different properties. EMIM-Br is a solid at room temperature but is highly hygroscopic – it is very difficult to keep completely dry. EMIM- Tf2N is, on the other hand, is a liquid at room temperature that is highly hydrophobic. The same aluminophosphate synthesis carried out in the two ILs leads to completely different products. In EMIM-Br we get the zeolitic phases SIZ-1, SIZ-3 and SIZ-4 where the IL cation is occluded in the final structure, while using EMIM-Tf2N the only product is a chain aluminophosphate where the IL cation is not occluded. This work illustrates two general features of ionothermal synthesis. As the anion is changed to produce a more hydrophobic IL the resultant products from ionothermal tend to be lower dimensional solids (i.e. chains and layers are more prevalent) and it becomes more difficult to occlude the IL cation in the final structure (i,e, templating gets less common). Perhaps more importantly the two ILs,
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EMIM-Br and EMIM-Tf2N are miscible and so the properties of any IL can be tuned across a wide range by mixing the two ILs in various amounts. We have used this effect, which we have termed anion control of ionothermal synthesis, to great effect in the synthesis of coordination polymers [20]. For example, when preparing cobalt or nickel trimesate-based MOFs changing the anion has a marked effect. EMIM-Br, when used as the solvent produced one framework material (Figure 7a). Changing the anion for bis((trifluoromethyl)sulfonyl)amide (bistriflamide, Tf2N-) leads to the production of a different material (Figure 7b), and simply mixing the bromide and bistriflamide in a 1:1 ratio produces another material again (Figure 7c). A fourth material (Figure 7d) is produced if a coordinating ligand, such a 2,2’ bipyridine is added to the synthesis mixture. While it is clear that the anion is the controlling feature of the synthesis, it is important to note that in no case is the anion incorporated into the final structure.
100%
Product %
80% 60% 40% 20% 0% 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
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Water/IL Ratio 100%
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80% 60% 40% 20% 0% 1.1
1.3
2.2
2.4
3
3.5
4.1
9.8
19.7
Water/IL Ratio
Zeolitic phases
Dense phases
No crystalline product
Figure 6. The results of 94 ionothermal (top) and 46 hydrothermal (bottom) reactions using different amounts of water added to the ionic liquids.
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Figure 7. Four coordination polymer (metal organic framework materials, a-d, top left to bottom right) prepared simply by changing the anions of io based on the EMIM cation. This illustrates the type of control that the anion exerts in this type of synthesis. Another potential application of anion control is to ‘match’ the chemistry of the solvent to the chemistry of the target framework. A good example of this is in the synthesis of metal perfluorosuccinate metal organic frameworks. The highly fluorinated organic linker causes problems in both hydrothermal and ionothermal synthesis using EMIM Br. However, by creating a more fluorous compatible IL by mixing EMIM-Br with EMIM- Tf2N. from this mixed IL solvent ionothermal becomes much easier and crystalline products can be obtained.
6. Chiral Induction A potentially important advantage of ionothermal synthesis involves strong structure directing ability of ionic liquids. The large Coulombic interactions in ionic liquids mean that the fluid structure shows relatively long range correlations and a three dimensional distribution that reflects the asymmetry of the ions. In the case of a chiral ionic liquid one might expect this asymmetric distribution to reflect the chirality of the ions present. This could have major implications for structure direction in ionothermal synthesis, as one can imagine more efficient transfer of chiral information from the ionic liquid to the solid than is the case in situations, for example, where the chirality of the ions are ‘diluted’ by non-chiral molecular solvents. The easiest way to make a chiral IL is to
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simply ion exchange the anion for a naturally obtained amino acid anion, such as Laspartate. This is a very straightforward process that produces enatiomerically pure chiral ILs that are liquid at room temperature. Use of the chiral ionic liquid 1-butyl 3-methyl imidazolium L-aspartate produces a new chiral metal organic framework (SIMOF-4) a homochiral material that crystallises in space group P43212 (Figure 8) [21]. A non-chiral ionic liquid produces a non-chiral MOF (SIMOF-1). This is a chiral induction effect in that the chiral anion is not incorporated into the structure of the material. This is the first such effect ever seen in materials synthesis of this kind.
Br +
N
N
O
NH2 O
HO N
+
N
O
Figure 8. An achiral ionic liquid BMIM-Br can be used to form an achiral MOF (top) while a chiral ionic liquid EMIM-L-Asp induces a chiral MOF (bottom). The cation is occluded in both cases (only shown in the top picture), but the chiral anion is not included into the structure.
7. Conclusion Ionothermal synthesis represents a novel method of preparing inorganic and inorganicorganic hybrid materials, including zeolites and metal organic frameworks. As yet however, we have only just scratched the surface of the possibilities associated with this new preparative method. Changing the solvent chemistry from molecular to ionic clearly makes a huge difference to the types of products that can be prepared. In this paper I have described several new concepts that have emerged out of ionothermal synthesis. The target of the next few years in the area will be to exploit these new concepts and show that ionothermal synthesis complements traditional methods of zeolite and metal organic framework synthesis. This will be best done by preparing new materials with interesting and different properties.
Acknowledgements R.E.M. thanks the EPSRC, the Leverhulme Trust and GEMI for funding.
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