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Synthesis of metal-organic frameworks from structure-directing iptycenyl lingands

University of British Columbia

Metal-organic frameworks (MOFs) are of great interest as hydrogen storage materials for use in vehicles. A series of triptycene-containing and pentiptycene-containing organic ligands was synthesized, characterized and used for MOF formation. A total of five unique triptycene-containing MOFs (TMOFs) and three pentiptycene-containing MOFs (PMOFs) were synthesized and examined using X-ray diffraction studies. Although the structure of each MOF was unique, structural adaptions to incorporate the rigid, bulky, structure-directing ligand into the framework were observed. For example, in some cases extended pseudo-[Zn₄O]⁶⁺ secondary building units (SBUs) were present in order to distort the framework to allow the ligand to coordinate. Reaction conditions also proved to be important in determining the dimensionality of MOF. It was shown that slight modifications of the reaction conditions using the same organic ligand could produce a 1-D, 2-D, and 3-D framework with different SBUs. The use of a ditopic bridging ligand, such as 4,4´-bipyridine, can also be used to increase the dimensionality of a structure. This protocol was used in converting 1-D chain structures and 2-D sheet structures into 2-D sheet structures and 3-D pillared structures, respectively. The overall functional groups in the ligand backbone can also have an adverse effect in the MOF structure. It was found that flexible linkages often led to collapse of the framework structure, whereas rigid linkages led to more robust structures. Extension along the carboxylate axis was also found to be important for iptycene incorporation. Unfortunately such extensions often led to close-packed or interpenetrated systems, which diminished porosity in the framework. Lastly, the long-range ordering of a MOF structure was predicted using modelling of powder X-ray diffraction (PXRD) peaks. When no single-crystal data were obtained, the PXRD data could suggest whether a material had a hexagonal structure or cubic structure. In all cases thermal stability studies were carried out and it was found that these TMOF and PMOF materials were thermally stable up to 400 °C. These iptycene-containing materials had been highlighted as potential hydrogen storage materials due to the potential for high aromatic surface areas and well-defined pore structure. As a result, nitrogen adsorption experiments were conducted on several of the robust frameworks to assess surface area. In each case, low surface areas were found.

Text

2015-08-31

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/50209

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/