Large anisotropic negative thermal expansion in Cu-TDPAT metal-organic framework: A combined in situ X-ray diffraction and DRIFTS study

Cu-TDPAT (H₆TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine), a stable nanoporous metal-organic framework with rht topology, has sparked broad interest as an adsorbent for several chemical separation processes. In this work, in situ synchrotron diffraction experiments followed by sequential LeBail refinements reveal that Cu-TDPAT shows unusually large anisotropic negative thermal expansion (NTE). The PASCal crystallography tool, used to analyze the magnitude of the NTE, reveals an average volumetric thermal expansion coefficient α_V = -20.3 MK^-1. This value is significantly higher than the one reported for Cu-BTC (also known as HKUST-1), which contains the same Cu-paddlewheel building unit, α_V ≈ -12 MK^-1. In situ synchrotron single crystal X-ray diffraction and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed to shed light on the NTE mechanism. Using these two methods, we were able to elucidate the three main structural motions that are responsible for the NTE effect. The more pronounced NTE behavior of Cu-TDPAT is attributed to the lower symmetry combined with the more complex ligand structure when compared to Cu-BTC. The knowledge obtained in this work is important for understanding the behavior of the adsorbent under transient variable temperature conditions in fixed adsorption beds.