Thermoresponsive behaviour of Metal Organic Frameworks

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Abstract

In this thesis, we aim to investigate the thermoresponsive behaviour, especially negative thermal expansion (NTE), in metal dicarboxylate metal organic frameworks (MOFs) using X-ray diffraction techniques. Four materials with the UiO-66 topology [Zr6O4(OH)4(bdc)12], [Zr6O6(bdc)12], [Zr6O6(bpdc)12] and [Zr6O6(2,6-ndc)12] (bdc = 1,4-benzenedicarboxylate, bpdc = 4,4’-biphenyldicarboxylate and 2,6-ndc = 2,6-napthalenedicarboxylate) were investigated, all of which contain a zero-dimensional inorganic cluster. All four members show NTE behaviour over the observed temperature ranges as a result of the twisting motion of the carboxylate groups of the organic linkers. This twisting motion introduces a concerted rocking motion within the inorganic cluster which causes an apparent decrease in the size of the cluster and hence overall volume contraction. Alteration of the structure of the organic linker has an effect on the magnitude of the expansivity coefficient which is believed to be related to the existence of specific vibrational modes of that particular organic linker. Four members of the MIL-53 family [Al(OH)(bdc)], [AlF(bdc)], [Cr(OH)(bdc)] and [VO(bdc)] were studied. All four materials show elements of NTE behaviour related to a “wine rack” thermo-mechanical mechanism which is determined by the connectivity of the framework. The thermoresponsive behaviour in these materials is dominated by the changes in the plane of the pore opening. These changes result from a combination of three distinct types of motion of the bdc linker including the rotation of the bdc linker about the chain of the inorganic octahedra, the “knee cap” bending mode of the carboxylate groups about the O-O vector and possibly the transverse vibrations within the bdc linker. The latter motion was not evident in this work due to the limitations of the structure refinements. The former two motions appear to be correlated and depend on the rigidity of the metal-centred octahedra which is determined by the constituent metal cation and anion types. The rigidity of the octahedra is also found to play an important role in determining whether the material undergoes a “breathing” phase transition at low temperature. [Sc2(bdc)3] shows NTE behaviour over the observed temperature range which is partially driven by a “wine rack” thermo-mechanical mechanism, but with an opposite framework compression direction when compared to the MIL-53 types MOFs. This is due to the presence of an additional bdc connecting linker in the plane of the pore opening. This extra connection inverses the compression direction and also impedes the structural changes in the plane of the pore opening. The contraction of the chain of inorganic octahedra is the main contributor to the overall unit cell contraction and is caused by the twisting motion of the carboxylate groups of the bdc linker while the magnitude of this contraction is determined by the flexibility of the chain of inorganic octahedra.

Layman's Abstract

In this thesis, we aim to investigate the thermoresponsive behaviour, especially negative thermal expansion (NTE), in metal dicarboxylate metal organic frameworks (MOFs) using X-ray diffraction techniques. Four materials with the UiO-66 topology [Zr6O4(OH)4(bdc)12], [Zr6O6(bdc)12], [Zr6O6(bpdc)12] and [Zr6O6(2,6-ndc)12] (bdc = 1,4-benzenedicarboxylate, bpdc = 4,4’-biphenyldicarboxylate and 2,6-ndc = 2,6-napthalenedicarboxylate) were investigated, all of which contain a zero-dimensional inorganic cluster. All four members show NTE behaviour over the observed temperature ranges as a result of the twisting motion of the carboxylate groups of the organic linkers. This twisting motion introduces a concerted rocking motion within the inorganic cluster which causes an apparent decrease in the size of the cluster and hence overall volume contraction. Alteration of the structure of the organic linker has an effect on the magnitude of the expansivity coefficient which is believed to be related to the existence of specific vibrational modes of that particular organic linker. Four members of the MIL-53 family [Al(OH)(bdc)], [AlF(bdc)], [Cr(OH)(bdc)] and [VO(bdc)] were studied. All four materials show elements of NTE behaviour related to a “wine rack” thermo-mechanical mechanism which is determined by the connectivity of the framework. The thermoresponsive behaviour in these materials is dominated by the changes in the plane of the pore opening. These changes result from a combination of three distinct types of motion of the bdc linker including the rotation of the bdc linker about the chain of the inorganic octahedra, the “knee cap” bending mode of the carboxylate groups about the O-O vector and possibly the transverse vibrations within the bdc linker. The latter motion was not evident in this work due to the limitations of the structure refinements. The former two motions appear to be correlated and depend on the rigidity of the metal-centred octahedra which is determined by the constituent metal cation and anion types. The rigidity of the octahedra is also found to play an important role in determining whether the material undergoes a “breathing” phase transition at low temperature. [Sc2(bdc)3] shows NTE behaviour over the observed temperature range which is partially driven by a “wine rack” thermo-mechanical mechanism, but with an opposite framework compression direction when compared to the MIL-53 types MOFs. This is due to the presence of an additional bdc connecting linker in the plane of the pore opening. This extra connection inverses the compression direction and also impedes the structural changes in the plane of the pore opening. The contraction of the chain of inorganic octahedra is the main contributor to the overall unit cell contraction and is caused by the twisting motion of the carboxylate groups of the bdc linker while the magnitude of this contraction is determined by the flexibility of the chain of inorganic octahedra.

Keyword(s)

Metal Organic Frameworks; Negative thermal expansion; Thermoresponsive behaviour

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