In-depth ESI-MS Investigations into the formation for Metal Organic Framework Materials (MOFs)

This project focuses on unravelling the elusive mechanism of formation of crystalline metal-organic framework materials (MOFs). MOFs are infinite networks consisting of metal centres linked by organic ligands. They are intensely studied for their wide variety of potential applications due to structural diversity. The number of potential applications associated with MOFs is huge, but the number of existing applications is low. There is a gap in the knowledge when it comes to how these materials self-assemble, and we believe this is key for realising true applications of MOFs. Addressing this gap for the design of new MOFs with built-in properties tailored for real applications is highly desirable. We aim to elucidate the mechanism of formation of MOFs gaining information to address the gap between their potential and actual applications, and to aid the synthesis of new MOFs. A series of MOF compounds that are particularly interesting are the isostructural microporous coordination polymers [M2(dhtp)] (CPO-27-M, M-MOF-74 or M2(dobdc), where dhtp/dobdc = (C8H2O6)4- and M = Co, Ni, Mg, Mn, Zn, og Cu). Their high specific surface area and concentration of coordinatively unsaturated metal sites, combined with a stable and rigid open framework structure renders them interesting for application; from gas storage and separation to heterogeneous catalysis. To be able to elucidate the mechanism and understand the self-assembly processes, in-situ analytical techniques are important. Currently, only a few reports mention the use of electrospray ionisation mass spectrometry (ESI-MS) analysis, and we believe it has great potential. It is important and challenging to ensure that we can compare and draw conclusions from the gas phase observed in the MS to the solution and solid state. To tackle these challenges other analytical techniques, along with data from the literature will be used to validate our results. Early laboratory work focussed on finding the optimal approach for studying such systems by ESI-MS methods, both on the experimental side as well as the suitable parameters for the instruments. CPO-27-M can be synthesised with different metal cations, which give us a broader basis to build our research upon. This makes it easy to change one significant parameter; the metal cation, and observe the effect this has on the self-assembly process. Our primary objective is to utilise the ESI-MS technique to investigate the fragmentation and formation of MOFs, particularly the CPO-27 series. Initially the study focussed on identifying species and secondary building units (SBUs). This was carried out by first adapting the synthetic conditions if required, to render the synthesis compatible with the ESI-MS technique. Secondly, comprehensive ESI-MS studies were carried out on the starting materials under the same parameters, to allow for correct comparison ground. Finally, we then set out to follow the complete reactions from the starting materials to the final product, resulting in a comprehensive study of the self-assembly mechanisms. For the first time, ESI-MS has been utilised to investigate the self-assembly processes governing the formation of the well-known MOF, CPO-27-M. Standard solvothermal conditions were applied in the synthesis of CPO-27-Ni and -Co, as well as crystallisation at room temperature. The reactions were followed by ESI-MS. Independent of the metal salt, the reaction conditions and the 1:1 or 2:1 mol ratio of metal salt to H4dhtp used, the elegantly simplistic building unit {M1(HXdhtp)1} was identified as key for the self-assembly of CPO-27-Ni and -Co. In addition, we have used time resolved powder X-ray diffraction analysis to follow the solvothermal synthesis of CPO-27-Ni in-situ, which confirm that no other crystalline products occur in the reaction mixture prior to the formation and crystallisation of CPO-27-Ni. The crystallisation at room temperature resulted in the expected CPO-27-M structure for the 2:1 mol ratio Co:H4dhtp reactions, whereas the 1:1 mol ratio Co:H4dhtp reactions as well as both 2:1 and 1:1 mol ratio Ni:H4dhtp reactions, resulted in the formation of a one-dimensional chain structure. The {M1(HXdhtp)1} building unit was identified as key for the formation of both CPO-27-M and the 1D chain structure. We have also used ESI-MS to gain insight into what occur during the self-assembly processes of CPO-27-M in the presence of a modulator. In 2009 Kitagawa et al. reported that it is possible to modulate the equilibria in a MOF synthesis by adding a modulator with the same chemical functionality as the organic linker inherent to the particular synthesis. This slows down the coordination interaction between the metal ions and linker, which generates a competitive situation that regulates the rate of framework extensions and crystal growth. Our ESI-MS results corroborate the competitive nature between benzoic acid and H4dhtp for the metal coordination sites.

This project focuses on unravelling the elusive mechanism of formation of crystalline metal-organic framework materials (MOFs). Their potential applications are huge, but the number of existing applications is low. There is also a gap in knowledge when it comes to how these materials self-assemble, and these two aspects are likely to be closely related. Addressing this gap for the design of new MOFs with built-in properties tailored for real applications is highly desirable. To be able to elucidate the m echanism, in-situ analytical techniques are important, and several methods have been used. Only a few reports mention the use of electrospray ionisation mass spectrometry (ESI-MS) analysis, and we think the potential for its use is underestimated. Our pri mary objective is to utilise the ESI-MS technique to investigate the fragmentation and formation of a selection of MOF materials. Initially the study will be focussed on identifying species and secondary building units (SBUs), resulting in a comprehensive study from which to be able to propose self-assembly mechanisms. The most critical R&D challenges are to be able to make the compounds soluble in solvents suitable for MS analysis. Subsequently, it is important, and challenging, to ensure that we can co mpare and draw conclusions from the gas phase observed in the MS to the solution and solid state. To tackle these challenges other analytical techniques, along with data from the literature will be used to validate our results. Major efforts will be made to to keep the reactions conditions and parameters the same across several batches of the sample compounds to ensure consistency. We aim to elucidate the mechanism of formation of MOFs gaining information to address the gap between their potential and a ctual applications, and aid the synthesis of new MOFs, where the desired end-product directs the design and synthesis; hence, the fabrication of materials tailored to specific applications can be achieved.