Nicholas C. Burtch

Kinetic Water Stability of an Isostructural Family of Zinc-Based Pillared Metal–Organic Frameworks

The rational design of metal-organic frameworks (MOFs) with structural stability in the presence of humid conditions is critical to the commercialization of this class of materials. However, the systematic water stability studies required to develop design criteria for the construction of water-stable MOFs are still scarce. In this work, we show that by varying the functional groups on the 1,4-benzenedicarboxylic acid (BDC) linker of DMOF [Zn(BDC)(DABCO)(0.5)], we can systematically tune the kinetic water stability of this isostructural, pillared family of MOFs. To illustrate this concept, we have performed water adsorption studies on four novel, methyl-functionalized DMOF variations along with a number of already reported functionalized analogues containing polar (fluorine) and nonpolar (methyl) functional groups on the BDC ligand. These results are distinctly different from previous reports where the apparent water stability is improved through the inclusion of functional groups such as -CH(3), -C(2)H(5), and -CF(3) which only serve to prevent significant amounts of water from adsorbing into the pores. In this study, we present the first demonstration of tuning the inherent kinetic stability of MOF structures in the presence of large amounts of adsorbed water. Notably, we demonstrate that while the parent DMOF structure is unstable, the DMOF variation containing the tetramethyl BDC ligand remains fully stable after adsorbing large amounts of water vapor during cyclic water adsorption cycles. These trends cannot be rationalized in terms of hydrophobicity alone; experimental water isotherms show that MOFs containing the same number of methyl groups per unit cell will have different kinetic stabilities and that the precise placements of the methyl groups on the BDC ligand are therefore critically important in determining their stability in the presence of water. We present the water adsorption isotherms, PXRD (powder X-ray diffraction) patterns, and BET surface areas before and after water exposure to illustrate these trends. Furthermore, we shed light on the important distinction between kinetic and thermodynamic stability in MOFs. Molecular simulations are also used to provide insight into the structural characteristics governing these trends in kinetic water stability.

Molecular-level Insight into Unusual Low Pressure CO2 Affinity in Pillared Metal-Organic Frameworks

Fundamental insight into how low pressure adsorption properties are affected by chemical functionalization is critical to the development of next-generation porous materials for postcombustion CO2 capture. In this work, we present a systematic approach to understanding low pressure CO2 affinity in isostructural metal-organic frameworks (MOFs) using molecular simulations and apply it to obtain quantitative, molecular-level insight into interesting experimental low pressure adsorption trends in a series of pillared MOFs. Our experimental results show that increasing the number of nonpolar functional groups on the benzene dicarboxylate (BDC) linker in the pillared DMOF-1 [Zn2(BDC)2(DABCO)] structure is an effective way to tune the CO2 Henrys coefficient in this isostructural series. These findings are contrary to the common scenario where polar functional groups induce the greatest increase in low pressure affinity through polarization of the CO2 molecule. Instead, MOFs in this isostructural series containing nitro, hydroxyl, fluorine, chlorine, and bromine functional groups result in little increase to the low pressure CO2 affinity. Strong agreement between simulated and experimental Henrys coefficient values is obtained from simulations on representative structures, and a powerful yet simple approach involving the analysis of the simulated heats of adsorption, adsorbate density distributions, and minimum energy 0 K binding sites is presented to elucidate the intermolecular interactions governing these interesting trends. Through a combined experimental and simulation approach, we demonstrate how subtle, structure-specific differences in CO2 affinity induced by functionalization can be understood at the molecular-level through classical simulations. This work also illustrates how structure-property relationships resulting from chemical functionalization can be very specific to the topology and electrostatic environment in the structure of interest. Given the excellent agreement between experiments and simulation, predicted CO2 selectivities over N2, CH4, and CO are also investigated to demonstrate that methyl groups also provide the greatest increase in CO2 selectivity relative to the other functional groups. These results indicate that methyl ligand functionalization may be a promising approach for creating both water stable and CO2 selective variations of other MOFs for various industrial applications.

Modulating Adsorption and Stability Properties in Pillared Metal–Organic Frameworks: A Model System for Understanding Ligand Effects

Metal-organic frameworks (MOFs) are nanoporous materials with highly tunable properties that make them ideal for a wide array of adsorption applications. Through careful choice of metal and ligand precursors, one can target the specific functionality and pore characteristics desired for the application of interest. However, among the wide array of MOFs reported in the literature, there are varying trends in the effects that ligand identity has on the adsorption, chemical stability, and intrinsic framework dynamics of the material. This is largely due to ligand effects being strongly coupled with structural properties arising from the differing topologies among frameworks. Given the important role such properties play in dictating adsorbent performance, understanding these effects will be critical for the design of next generation functional materials. Pillared MOFs are ideal platforms for understanding how ligand properties can affect the adsorption, stability, and framework dynamics in MOFs. In this Account, we highlight our recent work demonstrating how experiment and simulation can be used to understand the important role ligand identity plays in governing the properties of isostructural MOFs containing interconnected layers pillared by bridging ligands. Changing the identity of the linear, ditopic ligand in either the 2-D layer or the pillaring third dimension allows targeted modulation of the chemical functionality, porosity, and interpenetration of the framework. We will discuss how these characteristics can have important consequences on the adsorption, chemical stability, and dynamic properties of pillared MOFs. The structures discussed in this Account comprise the greatest diversity of isostructural MOFs whose stability properties have been studied, allowing valuable insight into how ligand properties dictate the chemical stability of isostructural frameworks. We also discuss how functional groups can affect adsorbate energetics at their most favorable adsorption sites to elucidate how functional groups can affect the adsorptive performance of these materials in ways that are unexpected based on the isolated ligands properties. We then highlight a variety of simulation tools that not only can be used to understand the differing molecular-level behavior of the adsorbate and framework dynamics within these isostructural MOFs, but also can shed light on possible mechanisms that govern the differing chemical stability properties among these materials. Lastly, we provide perspective on the challenges and opportunities for utilizing the structure-property relationships arising from the ligand effects described in this Account for the design of further MOFs with enhanced chemical stability and adsorption properties.

Understanding DABCO Nanorotor Dynamics in Isostructural Metal-Organic Frameworks

Flexible framework dynamics present in the subset of metal-organic frameworks known as soft porous crystals give rise to interesting structural properties that are unique to this class of materials. In this work, we use experiments and molecular simulation to understand the highly dynamic nanorotor behavior of the 1,4-diazabicyclo[2.2.2]octane (DABCO) ligand in the pillared Zn-DMOF and Zn-DMOF-TM (TM = tetramethyl) structures. While DABCO is known to be displaced in the presence of water in the parent Zn-DMOF structure, the Zn-DMOF-TM variation is highly stable even after adsorbing significant amounts of water vapor. The dynamics of DABCO in the presence of water guest molecules is therefore also explored in the Zn-DMOF-TM structure via in situ NMR and IR experiments. This analysis shows that the rotational motion of the DABCO linkers is dependent on water content, but not a likely source of water instability because the dynamics are fast and largely unaffected by the presence of methyl functional groups.

Investigating water and framework dynamics in pillared MOFs

In this work we highlight the use of molecular simulation to study the behaviour of water inside isostructural Zn-DMOF structures. Among the Zn-DMOF structures, the parent DMOF, and the DMOF-DM and DMOF-TF variants are known to be less stable than the DMOF-A and DMOF-TM structures in the presence of water. We apply tools such as radial distribution functions, rotational auto-correlation functions and the visualisation of adsorbate density distributions to investigate the differences in water behaviour within these structures. We also study properties that are inherent to the frameworks themselves such as thermal expansion and ligand flexibility. Our results indicate that water is only able to get 0.5 Å closer to the metal hydrolysis site in the water unstable structures than in the more water stable structures. The results can be somewhat sensitive to the details of the modelling of the electrostatic potential energy surface and, for dynamical properties, modelling of framework flexibility.

Flexible Force Field Parameterization through Fitting on the Ab Initio-Derived Elastic Tensor

Constructing functional forms and their corresponding force field parameters for the metal–linker interface of metal–organic frameworks is challenging. We propose fitting these parameters on the elastic tensor, computed from ab initio density functional theory calculations. The advantage of this top-down approach is that it becomes evident if functional forms are missing when components of the elastic tensor are off. As a proof-of-concept, a new flexible force field for MIL-47(V) is derived. Negative thermal expansion is observed and framework flexibility has a negligible effect on adsorption and transport properties for small guest molecules. We believe that this force field parametrization approach can serve as a useful tool for developing accurate flexible force field models that capture the correct mechanical behavior of the full periodic structure.

Predicting Multicomponent Adsorption Isotherms in Open-Metal Site Materials Using Force Field Calculations Based on Energy Decomposed Density Functional Theory

For the design of adsorptive-separation units, knowledge is required of the multicomponent adsorption behavior. Ideal adsorbed solution theory (IAST) breaks down for olefin adsorption in open-metal site (OMS) materials due to non-ideal donor-acceptor interactions. Using a density-function-theory-based energy decomposition scheme, we develop a physically justifiable classical force field that incorporates the missing orbital interactions using an appropriate functional form. Our first-principles derived force field shows greatly improved quantitative agreement with the inflection points, initial uptake, saturation capacity, and enthalpies of adsorption obtained from our in-house adsorption experiments. While IAST fails to make accurate predictions, our improved force field model is able to correctly predict the multicomponent behavior. Our approach is also transferable to other OMS structures, allowing the accurate study of their separation performances for olefins/paraffins and further mixtures involving complex donor-acceptor interactions.