rus
Issue #8/2016
K.Grafskaya, A.Piryazev, D.Anokhin, B.Zimka, D.Ivanov
Creation and study of organic zeolite-like materials
Creation and study of organic zeolite-like materials
The paper deals with the method of forming thermally stable cubic gyroid phase, which is organic
zeolite-like material.
zeolite-like material.
Zeolites as a class of microporous materials, are widely used for catalysis, adsorption separation and ion exchange [1]. In addition, zeolites are successfully applied in microelectronics, medicine, petroleum refineries and in the petrochemical industry [2]. High interest in such materials is caused by peculiarities of their composition and unique regular porous structure with pore sizes of 0.3 to 2.0 nm. The main properties of zeolites, due to their topology, morphology and chemical composition are: the huge specific surface; the possibility of separating the reactant and product; high adsorptive capacity; the possibility of changing the electronic properties of active centers in the presence of a strong electric field; the localization effects within the pores that causes preactivation of molecules.
Structures formed by organic molecules are the least studied porous systems, despite the huge potential for their use in various applications [3–5]. The concept of "organic zeolites" first appeared in connection with the study of the peculiarities of the physico-chemical behaviour of some coordination complexes [6, 7]. However, the composition of molecules of the first "organic zeolites" was not really organic and zeolite. The term organic zeolites was introduced in [7] for the definition of any solid substances of organic nature that is able to reversibly adsorb a large number of hydrophobic compounds [8]. In other words, this term was used in a narrow sense to refer to the porous nature of the new material (characteristic inherent to the true zeolites), while the definition of "organic" was used to emphasize the hydrophobic nature of the inner surface of the pores. The term began to be used in the 1980s and 1990s [9, 10], and the first review appeared in 1996 [11]. Zeolite behavior of the coordination complexes was considered in [12]. Later the reviews [13–15] were published, which related to the application of coordination polymers for the creation of organic zeolites.
Usual problem of the use of organic zeolites is that their structure is destroyed after the removal of solvent or change in external conditions, but there are interesting exceptions [16, 17]. Organic systems allow relatively easy modification of the molecular matrix by functionalization to obtain structural diversity with the one-dimensional, two-dimensional and three-dimensional supramolecular structures (Fig.1c) [18]. Development of organic zeolites requires solving two problems. The first one is the formation of the desired application-specific volume and geometry of pores. The second problem is to make a porous material quite stable under operating conditions.
The Laboratory of Material Science Engineering at the Faculty of fundamental physical and chemical engineering of MSU conducts a systematic study of the processes of self-organization of amphiphilic V-like compounds. In particular, the influence on the properties of their chemical structure (e.g. of length of the alkyl tail, the number of alkyl tails and nature of counter-ion) are studied. It is found that the humidity and temperature have most effective impact on the structure of the sample. When humidity increases, the formation in the sample of nanosized inner channels is observed. Temperature change influences the diameter of these channels (at temperature increase there is a reduction of diameter).
One of the most interesting amphiphilic compounds studied in the laboratory is a sector-shaped mesogen 2, 3, 4-tris(11’-acryloylundecyl-10-hydroxy)benzenesulfonate of sodium, A-Na (Fig.1a). This compound has three alkyl tails and sodium as a counter-ion. Using methods of high-angle x-ray diffraction with a moving beam the phase behavior of thin films of A-Na was investigated depending on the relative humidity of the atmosphere (Fig.2) [19, 20]. Under normal conditions the hexagonal columnar phase is a stable phase for this compound. But in conditions of high humidity, A-Na is able to absorb water with the formation of cubic bicontinuous phases, gyroid and diamond. These phases are an organic zeolites. Their main advantage is the occurrence of a network of continuous channels that can be used for various practical applications (ion-exchange membranes, separation systems, catalysis) [21]. It should be emphasized that two cubic phases coexist in the sample, which suggests their metastable nature. With decreasing humidity or increasing temperature (drying of the sample), the structure reverts to a columnar phase and a cubic phases are destroyed. However, the cubic phases of this compound can be chemically stabilized using photo-polymerization [22].
Another example of a system that forms bicontinuous cubic phases, is the pyridine salt 4’-[3’’, 4’’, 5’’-tris(octyloxy), benzyloxy]azobenzene-4-sulfonic acid (C8-Pyr) containing azo group in the hydrophilic part and an organic counter-ion of pyridine (Fig.1b). The dry film of this compound contains monoclinic columnar phase with one-dimensional channels oriented parallel to the substrate. Phase transition of monoclinic phase to hexagonal one is observed during heating up to 100°C, which is associated with the disordering of the linear alkyl chains and formation of columns with circular cross section [23]. This transition is reversible, and in a normal atmosphere during cooling the monoclinic phase is recovered. A different behavior is observed if to place C8-Pyr film into vapor of organic solvent, in particular of methanol.
To study in real time of the structure formation processes in a changing external environment a special measuring cell was designed that allows to conduct x-ray diffraction analysis of the films at different temperatures in solvents vapors. On the diffraction pattern of the film, which is investigated in the vapor of methanol at room temperature, there is no noticeable changes compared to the initial dry film (Fig.3a). Heating to 100 °C leads to transformation into the hexagonal phase (Fig.3b). However, when the film is cooled in the presence of methanol, the diffraction patterns contain reflections that are characteristic of gyroid cubic phase (Fig.3c, 4). It is important to note that the heating and cooling of the dry films in water vapor doesn’t lead to the formation of cubic phase.
The cubic phase obtained during cooling shows a high stability. The gyroid phase persists after removal of methanol vapor, although the parameter of the cell is reduced by 5% (Fig.4). Dry film can be stored for several months without noticeable changes in the structure, the heating has also no significant effect on her. It is interesting to note that in the cubic phase the C8-Pyr film demonstrates the ability to effectively absorb water. For example, after a placement for a short time in a saturated moist atmosphere, the sample shows a strong increase of the lattice parameter corresponding to water content in the cubic phase in an amount of not less than 17% by volume (Fig.3e). The water content in the sample increases even more in the case of lowering the temperature below ambient (Fig.4). At 15 °C, there is a phase transition from the cubic to the lamellar phase (Fig.3f). This phase is probably thermodynamically stable in 100% humidity, and the transition is associated with condensation of water vapor on the film surface below the dew point. The transition of gyroid phase to the lamellar one was observed by us previously at prolonged storage of the sample in the atmosphere of saturated methanol vapor [24]. Thus, at room temperature the cubic phase is a non-equilibrium state of this compound. It is important to note that the recovering of monoclinic columnar phase that is characteristic of the dry sample is observed at subsequent heating of the film to room temperature.
Thus, the creation of organic zeolites can be realized using molecular self-assembly of amphiphilic mesogens having polar groups that form an ion channel and alkyl groups of different structure. The possibility of obtaining polymeric materials with thermally stable cubic gyroid phase, which is organic zeolite-like material, is of particular interest. The structure can be stabilized using chemical photopolymerization of alkyl groups or by physical local ordering. This structure can be used to create ion-conducting membranes, separation systems and in catalysis. ■
The project is executed at financial support of Russian Science Foundation (contract No. 16-13-10369).
Structures formed by organic molecules are the least studied porous systems, despite the huge potential for their use in various applications [3–5]. The concept of "organic zeolites" first appeared in connection with the study of the peculiarities of the physico-chemical behaviour of some coordination complexes [6, 7]. However, the composition of molecules of the first "organic zeolites" was not really organic and zeolite. The term organic zeolites was introduced in [7] for the definition of any solid substances of organic nature that is able to reversibly adsorb a large number of hydrophobic compounds [8]. In other words, this term was used in a narrow sense to refer to the porous nature of the new material (characteristic inherent to the true zeolites), while the definition of "organic" was used to emphasize the hydrophobic nature of the inner surface of the pores. The term began to be used in the 1980s and 1990s [9, 10], and the first review appeared in 1996 [11]. Zeolite behavior of the coordination complexes was considered in [12]. Later the reviews [13–15] were published, which related to the application of coordination polymers for the creation of organic zeolites.
Usual problem of the use of organic zeolites is that their structure is destroyed after the removal of solvent or change in external conditions, but there are interesting exceptions [16, 17]. Organic systems allow relatively easy modification of the molecular matrix by functionalization to obtain structural diversity with the one-dimensional, two-dimensional and three-dimensional supramolecular structures (Fig.1c) [18]. Development of organic zeolites requires solving two problems. The first one is the formation of the desired application-specific volume and geometry of pores. The second problem is to make a porous material quite stable under operating conditions.
The Laboratory of Material Science Engineering at the Faculty of fundamental physical and chemical engineering of MSU conducts a systematic study of the processes of self-organization of amphiphilic V-like compounds. In particular, the influence on the properties of their chemical structure (e.g. of length of the alkyl tail, the number of alkyl tails and nature of counter-ion) are studied. It is found that the humidity and temperature have most effective impact on the structure of the sample. When humidity increases, the formation in the sample of nanosized inner channels is observed. Temperature change influences the diameter of these channels (at temperature increase there is a reduction of diameter).
One of the most interesting amphiphilic compounds studied in the laboratory is a sector-shaped mesogen 2, 3, 4-tris(11’-acryloylundecyl-10-hydroxy)benzenesulfonate of sodium, A-Na (Fig.1a). This compound has three alkyl tails and sodium as a counter-ion. Using methods of high-angle x-ray diffraction with a moving beam the phase behavior of thin films of A-Na was investigated depending on the relative humidity of the atmosphere (Fig.2) [19, 20]. Under normal conditions the hexagonal columnar phase is a stable phase for this compound. But in conditions of high humidity, A-Na is able to absorb water with the formation of cubic bicontinuous phases, gyroid and diamond. These phases are an organic zeolites. Their main advantage is the occurrence of a network of continuous channels that can be used for various practical applications (ion-exchange membranes, separation systems, catalysis) [21]. It should be emphasized that two cubic phases coexist in the sample, which suggests their metastable nature. With decreasing humidity or increasing temperature (drying of the sample), the structure reverts to a columnar phase and a cubic phases are destroyed. However, the cubic phases of this compound can be chemically stabilized using photo-polymerization [22].
Another example of a system that forms bicontinuous cubic phases, is the pyridine salt 4’-[3’’, 4’’, 5’’-tris(octyloxy), benzyloxy]azobenzene-4-sulfonic acid (C8-Pyr) containing azo group in the hydrophilic part and an organic counter-ion of pyridine (Fig.1b). The dry film of this compound contains monoclinic columnar phase with one-dimensional channels oriented parallel to the substrate. Phase transition of monoclinic phase to hexagonal one is observed during heating up to 100°C, which is associated with the disordering of the linear alkyl chains and formation of columns with circular cross section [23]. This transition is reversible, and in a normal atmosphere during cooling the monoclinic phase is recovered. A different behavior is observed if to place C8-Pyr film into vapor of organic solvent, in particular of methanol.
To study in real time of the structure formation processes in a changing external environment a special measuring cell was designed that allows to conduct x-ray diffraction analysis of the films at different temperatures in solvents vapors. On the diffraction pattern of the film, which is investigated in the vapor of methanol at room temperature, there is no noticeable changes compared to the initial dry film (Fig.3a). Heating to 100 °C leads to transformation into the hexagonal phase (Fig.3b). However, when the film is cooled in the presence of methanol, the diffraction patterns contain reflections that are characteristic of gyroid cubic phase (Fig.3c, 4). It is important to note that the heating and cooling of the dry films in water vapor doesn’t lead to the formation of cubic phase.
The cubic phase obtained during cooling shows a high stability. The gyroid phase persists after removal of methanol vapor, although the parameter of the cell is reduced by 5% (Fig.4). Dry film can be stored for several months without noticeable changes in the structure, the heating has also no significant effect on her. It is interesting to note that in the cubic phase the C8-Pyr film demonstrates the ability to effectively absorb water. For example, after a placement for a short time in a saturated moist atmosphere, the sample shows a strong increase of the lattice parameter corresponding to water content in the cubic phase in an amount of not less than 17% by volume (Fig.3e). The water content in the sample increases even more in the case of lowering the temperature below ambient (Fig.4). At 15 °C, there is a phase transition from the cubic to the lamellar phase (Fig.3f). This phase is probably thermodynamically stable in 100% humidity, and the transition is associated with condensation of water vapor on the film surface below the dew point. The transition of gyroid phase to the lamellar one was observed by us previously at prolonged storage of the sample in the atmosphere of saturated methanol vapor [24]. Thus, at room temperature the cubic phase is a non-equilibrium state of this compound. It is important to note that the recovering of monoclinic columnar phase that is characteristic of the dry sample is observed at subsequent heating of the film to room temperature.
Thus, the creation of organic zeolites can be realized using molecular self-assembly of amphiphilic mesogens having polar groups that form an ion channel and alkyl groups of different structure. The possibility of obtaining polymeric materials with thermally stable cubic gyroid phase, which is organic zeolite-like material, is of particular interest. The structure can be stabilized using chemical photopolymerization of alkyl groups or by physical local ordering. This structure can be used to create ion-conducting membranes, separation systems and in catalysis. ■
The project is executed at financial support of Russian Science Foundation (contract No. 16-13-10369).
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