Study of Controlling Secondary Building Units of Metal-organic Frameworks

Abstract

A class of metal-organic frameworks (MOFs) were constructed by polynuclear metal clusters (secondary building units; hereinafter, SBUs) and their coordination with organic linkers (i.e., ligands). Such MOFs have been widely used in the various applications fields such as gas storage and separation, chemical sensing, and catalysis due to their stable structures and possibility of having a diverse set of functions. To make such MOFs have specific functions applicable to the application fields, we need to synthesize MOFs with specific crystal structures by controlling the two determinants of the crystal structures – the SBUs per se and/or their coordination with ligands. In this thesis, we reported the two research on controlling SBUs of MOFs. One research deals with the transformation of SBUs in synthesizing a rarely reported ndc-based Zn MOF. The other research deals with the formation of SBUs in synthesizing bdc-based Zn MOFs. For each research, we identified the set of parameters that facilitate the synthesis of MOFs with specific SBUs.

First, we conducted the transformation of SBUs in synthesizing a rarely reported ndc-based Zn MOF. We transformed specific MOFs –Zn MOFs having four handed paddlewheel SBUs linked by N-donor pillars – into another MOF – MOF having 7-coordinated Zn4O(COO)7 SBUs. We found that the two parameters – the presence of Zn+-O- in solution and the basicity of ligands – affect the transformation of SBUs. First, the presence of Zn+-O- in solution facilitates the transformation of SBUs. We conducted the transformation of the two same Zn-MOFs in two different solutions – one solution contains Zn+-O- in solution and the other solution does not contain Zn+-O- in solution. We found that the Zn-MOF in a solution containing Zn+-O- were transformed into the MOF that we want to obtain (i.e., MOF having 7-coordinated Zn4O(COO)7 SBUs) but the Zn-MOF in the other solution did not. Second, low basicity of ligands facilitates the solid-state transformation. We synthesized three Zn-MOFs by making each of them have different basicity of ligands and found that a Zn-MOF with a low level of basicity of ligands was successfully transformed into the other MOF that we want to obtain (i.e., a MOF having 7-coordinated Zn4O(COO)7 SBUs).

We also conducted the formation of diverse SBUs in synthesizing bdc-based Zn MOFs (bdc = benzene-1,4-dicarboxylate). We found that the two parameters – the reaction temperature and the molar ratio of precursors (i.e., metal precursors and ligand precursors)– influence the determination of the SBUs of the bdc-based Zn-MOFs .We found that at lower temperature, a Zn-MOF having Zn3(COO)6 SBUs was successfully formed. Secondly, we found that molar ratio of metal precursors to ligand precursors affects the formation of bdc-based Zn-MOFs. At the molar ratio of Zn(NO3)2·6H2O : H2bdc = 1 : 2, a Zn-MOF having Zn3(COO)6 SBU, to which two terminal DMF solvents are coordinated, was obtained. At the molar ratio of Zn(NO3)2·6H2O : H2bdc = 1 : 4, the other Zn-MOF having Zn3(COO)6(COOH)2 SBU (Not surely determined), which is linked by bdc pillars, was obtained.

After studying how to synthesize MOFs, we further explored the possibility of using the MOF that is synthesized by the above-mentioned transformation of SBUs (i.e., 7C-MOF) for CO2 cycloaddition. 7C-MOF has been rarely reported in the academic field, so exploring the characteristics of this MOF that can be applicable into the “real” world may contribute to expanding the usage of 7C-MOF. We analyzed 7C-MOF using CO2 sorption isotherm and found that 7C-MOF is capable of adsorbing CO2 molecules. This is because 7C-MOF is an anionic framework and has cation complex [Zn(DMF)6]2+ in its pores. Thus, we expected that 7C-MOF can act as a Lewis catalyst for CO2 cycloaddition. By analyzing product yields (100[mole of cyclic carbonate] / [mole of cyclic carbonate and epoxide]) using H-nuclear magnetic resonance spectroscopy, we found that 7C-MOF can act as a Lewis acid catalyst which facilitates CO2 cycloaddition.