Assessing Negative Thermal Expansion in Mesoporous Metal-Organic Frameworks by Molecular Simulation

Abstract

Most conventional materials display expansion upon heating, so there is considerable interest in identifying materials that display the opposite behavior, negative thermal expansion (NTE). In the current study, seven mesoporous metal-organic frameworks (MOFs) of varying topology and composition, which exhibit outstanding porosity, were investigated using molecular simulation for temperature-induced contraction. We find exceptional NTE for the most porous MOFs and a correlation between the coefficient of NTE and porosity. The large molecular subunits of the MOFs were further studied to find they intrinsically display NTE, corresponding to terahertz vibrational modes. As a result, NTE has a considerable effect on the mechanical properties of these MOFs and is an important consideration for understanding the mechanical stability of new extremely porous materials.

Guest-responsive thermal expansion in the Zr-porphyrin metal-organic framework PCN-222

“…4,31 The values agree with the correlation between mesopore diameter and volumetric expansivity predicted by Evans and coworkers. 20 At intermediate T , hydrated PCN-222 shows a small PTE along a and NTE along c, whereas the dehydrated samples exhibit near zero expansion along both axes. Thus, the heat-treatment has a smaller effect in this intermediate temperature range.…”

“…5,13 Less experimental attention has been devoted to mesoporous MOFs, although molecular simulations predict a strong correlation between the pore diameter and the NTE magnitude. 20 Since larger pore volumes allow for greater uptake of guests, stronger guest effects can be expected. Furthermore, the study of mesoporous MOFs at nonambient conditions helps evaluate their structural staa) Electronic mail: h.bostroem@fkf.mpg.de b) Electronic mail: b.lotsch@fkf.mpg.de bility and propensity for phase transitions during thermal fluctuations, which is relevant for future applications.…”

Thermal Stability of Methyl-Functionalized MOF-5

“…Similarly, MD simulations have been used to elucidate the NTE of MOF-5. 19,20 A recent work by Evans et al 21 highlighted the effect of NTE behavior of several MOFs on their mechanical properties that can govern their practical working temperature. However, in all these works, their implemented flexible force field does not permit bond breaking at high temperatures that would cause a structural collapse.…”

Reduced thermal expansion by surface-mounted nanoparticles in a pillared-layered metal-organic framework

“…Additionally, certain network topologies can lead to pronounced anisotropic TE behavior and temperature-induced phase transitions, of which MOFs with a wine-rack type network such as MIL-53(Al), abovementioned Zn 2 (fu-bdc) 2 dabco, and [Ag(en)]NO 3 (en=ethylenediamine) are prototypical examples 17,[22][23][24] . These insights are relevant for future MOF applications, because even single-percentage temperature-induced framework contraction can lead to considerable softening and greatly impact mechanical stability 25 . In addition to framework modification, external factors affecting porosity such as interstitial guest molecules can similarly impact TE 26 .…”