Two metal-organic framework (MOF) systems have been studied here for their thermal expansion and hydrogen (H2) storage properties. One is the Zr-MOFs (Zr-BDC and analogues) consisting of twelve-coordinate Zr6-oxo-clusters, and the other is the M(II)-THFTC frameworks (M = Mn, Co, Cu and Zn) that reveal bare metals sites upon dehydration.
Firstly, the coefficients of thermal expansion (CTE) were quantified for the Zr-MOFs using variable temperature synchrotron powder X-ray diffraction (VT-SPXRD) technique. Full Rietveld refinement was performed to extract structural information for probing the mechanism of thermal expansion of the framework. From the unit cell length a, the linear CTE of Zr-BDC was found to be -0.96(2) × 10-6 K-1 between 100 and 500 K, which is classified as zero thermal expansion (±2 × 10-6 K-1). The temperature-dependence of structural changes were also examined and compared to that of IRMOF-1. It was found that local vibrations of the Zr-cluster are hindered due to the constrained coordination geometry; while transverse ligand vibrations are present, they are decoupled from the local cluster vibrations. This leaves the apparent contraction of the carboxylate (O-C-O) group as the main contributor of NTE of the framework.
Secondly, thermal expansion behaviour was further modified toward ZTE through four different approaches: 1) metal-substitution, 2) ligand functionalisation, 3) variation of ligand length, and 4) Zr-cluster dehydroxylation. The results show that: 1) Hf-substitution does not appear to affect the NTE of the framework, 2) Amine and bromo substitution of the ligand benzene ring decreases the NTE behaviour of the frameworks, 3) increasing the ligand length by addition of phenyl ring increases the NTE of the framework, and 4) dehydroxylation increases the NTE of the framework through cluster distortion and the reduction of steric constraints. Computation modelling of the vibrational frequencies was also employed to verify the existence of NTE-causing modes in the Zr-MOFs.
Lastly, the hydrogen (H2) storage aspect of the research focuses on the effect of bare metal sites on the H2-binding energy of the frameworks. Both Zr-MOFs and M(II)-THFTCs contain bare metal sites upon dehydroxylation and dehydration respectively but have very different structures. Through neutron powder diffraction with deuterium (D2) loaded samples, D2 positions were found in both framework systems and that the D2-metal distances are shorter in Co-THFTC (2.08(3) Å) than in Zr-BDC (3.95(6) Å) at the highest D2 loading. This supports the higher heat of adsorption values found for the M(II)-THFTCs than the Zr-MOFs, suggesting the importance of the accessibility of bare metals sites for strong H2-binding interactions.