Publication date: 15th February 2023
The MOF MIL-88 A was first described in 2004.[1] It consists of an trinuclear iron cluster and fumaric acid as linker and forms a porous structure with hexagonal pores. MIL-88 A crystals have a hexagonal bipyramidal shape, which is also described as diamond shape, spindle-like, or rod-like, depending on the length. The flexibility of MIL-88 A is known since 2005.[2] The volume of the unit cell can change by up to 85% depending on the pore loading with different solvents or as a function of temperature. Even though the system has been known for a long time, many questions remain unanswered about the dynamic behavior. One of these open questions is the study of flexibility and the influence of crystal size on the lattice deformation. Similar effects have already been observed in the flexible MOF DUT-49 systems.[3]
Here we show the synthesis of MIL-88 A crystals with different crystal size distributions by variating the reaction time and temperature of the solvothermal reaction. We present crystals from 5 up to 50 μm, as shown in the scanning electron microscopy (SEM) images.
We studied some of the larger single crystals by single crystal X-ray analysis (XRD), and refined the crystal structure. MIL-88 A crystallizes in the hexagonal space group P63/m with lattice parameters of a=b=12.69 Å, c=13.71 Å and a volume of V=1909.54 Å3. Upon solvent exchange, crystals decompose. We observed this behavior for various organic solvents.
In order to investigate the swelling behavior of the network, as a function of solvent, we attempt solvent exchange in the pores. We analyzed the observed changes in the unit cell parameters upon solvent exchange by powder x-ray diffraction (PXRD) and Pawley refinement and calculated volume ranges from 1804 cm3 (DMSO) to 2107 cm3 (Water). Analyzing the obtained information from various samples, we present a correlation between the crystal size and swelling behavior.
We would like to thank CRC 1333 and Carl Zeiss Stifftung for funding.
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