The Role of Entropy in the Structural
Transitions in Zeolitic Imidazolate
Frameworks
Peter I. Ravikovitch
1 Introduction
Nanoporous crystalline materials such as zeolites, metal–organic frameworks
(MOFs) and ZIFs are the subject of intense research due to their potential applications for separations, catalysis, sensing, gas and energy storage [1–8]. ZIFs are a
subclass of MOFs. They are composed of metal nodes and imidazolate-based linkers,
and their topologies often resemble zeolites [9, 10]. ZIFs are flexible materials [11,
12], and their framework flexibility often leads to complex free energy landscapes
and the existence of polymorphs with different porosities and structures [13, 14].
Displacive structural transitions between polymorphs can be triggered by adsorption
of guest molecules [15–18], or by applying external stimuli, such as pressure [19–
22] and temperature [23, 24]. Reliable methods for predicting structural transitions
in MOFs are an active area of research [25–43]. One of the difficulties is that first
principles (e.g., DFT) simulations are challenging due to computing limitations, and
also because the standard PBE level of theory is not always adequate for systems in
which dispersion interactions play a crucial role [41, 44].
In our previous publications, we have uncovered that even in the absence of
any guest molecules, ZIF-7 undergoes a structural transition from a narrow ZIF7(np) to a large ZIF-7(lp) pore structure at unusually high temperature [23]. We
used classical MD simulations with a temperature ramp method to mimic the experimental structural transition . We predicted the temperature-induced phase transition
in ZIF-7 in almost quantitative agreement with X-ray diffraction and adsorption
microcalorimetry experiments, including the temperature of the transition and the
associated enthalpy change [23]. Recently, the temperature ramp method was used
P. I. Ravikovitch (B)
ExxonMobil Research and Engineering, 1545 Route 22 East, Annandale, NJ, USA
e-mail: peter.ravikovitch@exxonmobil.com
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
E. J. Maginn and J. Errington (eds.), Foundations of Molecular Modeling
and Simulation, Molecular Modeling and Simulation,
https://doi.org/10.1007/978-981-33-6639-8_2
25
Transitions in Zeolitic Imidazolate
Frameworks
Peter I. Ravikovitch
1 Introduction
Nanoporous crystalline materials such as zeolites, metal–organic frameworks
(MOFs) and ZIFs are the subject of intense research due to their potential applications for separations, catalysis, sensing, gas and energy storage [1–8]. ZIFs are a
subclass of MOFs. They are composed of metal nodes and imidazolate-based linkers,
and their topologies often resemble zeolites [9, 10]. ZIFs are flexible materials [11,
12], and their framework flexibility often leads to complex free energy landscapes
and the existence of polymorphs with different porosities and structures [13, 14].
Displacive structural transitions between polymorphs can be triggered by adsorption
of guest molecules [15–18], or by applying external stimuli, such as pressure [19–
22] and temperature [23, 24]. Reliable methods for predicting structural transitions
in MOFs are an active area of research [25–43]. One of the difficulties is that first
principles (e.g., DFT) simulations are challenging due to computing limitations, and
also because the standard PBE level of theory is not always adequate for systems in
which dispersion interactions play a crucial role [41, 44].
In our previous publications, we have uncovered that even in the absence of
any guest molecules, ZIF-7 undergoes a structural transition from a narrow ZIF7(np) to a large ZIF-7(lp) pore structure at unusually high temperature [23]. We
used classical MD simulations with a temperature ramp method to mimic the experimental structural transition . We predicted the temperature-induced phase transition
in ZIF-7 in almost quantitative agreement with X-ray diffraction and adsorption
microcalorimetry experiments, including the temperature of the transition and the
associated enthalpy change [23]. Recently, the temperature ramp method was used
P. I. Ravikovitch (B)
ExxonMobil Research and Engineering, 1545 Route 22 East, Annandale, NJ, USA
e-mail: peter.ravikovitch@exxonmobil.com
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
E. J. Maginn and J. Errington (eds.), Foundations of Molecular Modeling
and Simulation, Molecular Modeling and Simulation,
https://doi.org/10.1007/978-981-33-6639-8_2
25