Performance of van der Waals Corrected Functionals for Guest Adsorption in the M-2(dobdc) Metal-Organic Frameworks

Small-molecule binding in metal-organic frameworks (MOFs) can be accurately studied both experimentally and computationally, provided the proper tools are employed. Herein, we compare and contrast properties associated with guest binding by means of density functional theory (DFT) calculations using nine different functionals for the M-2(dobdc) (dobdc(4-) = 2,5-dioxido,1,4-benzenedicarboxylate) series, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn. Additionally, we perform Quantum Monte Carlo (QMC) calculations for one system to determine if this method can be used to assess the performance of DFT. We also make comparisons with previously published experimental results for carbon dioxide and water and present new methane neutron powder diffraction (NPD) data for further comparison. All of the functionals are able to predict the experimental variation in the binding energy from one metal to the next; however, the interpretation of the performance of the functionals depends on which value is taken as the reference. On the one hand, if we compare against experimental values, we would conclude that the optB86b-vdW and optB88-vdW functionals systematically overestimate the binding strength, while the second generation of van der Waals (vdW) nonlocal functionals (vdw-DF2 and rev-vdW-DF2) correct for this providing a good description of binding energies. On the other hand, if the QMC calculation is taken as the reference then all of the nonlocal functionals yield results that fall just outside the error of the higher-level calculation. The empirically corrected vdW functionals are in reasonable agreement with experimental heat of adsorptions but under bind when compared with QMC, while Perdew-Burke-Ernzerh of fails by more than 20 kJ/mol regardless of which reference is employed. All of the functionals, with the exception of vdW-DF2, predict reasonable framework and guest binding geometries when compared with NPD measurements. The newest of the functionals considered, rev-vdW-DF2, should be used in place of vdW-DF2, as it yields improved bond distances with similar quality binding energies.

Gas-Phase Ozone Reactions with a Structurally Diverse Set of Molecules: Barrier Heights and Reaction Energies Evaluated by Coupled Cluster and Density Functional Theory Calculations

Jeremy Samuel Arey, Peter Rudolf Tentscher, Daniela Trogolo

Reactions with ozone transform organic and inorganic molecules in water treatment systems as well as in atmospheric chemistry, either in the aqueous phase, at gas/particle interfaces, or in the gas phase. Computed thermokinetic data can be used to estimate the reactivities of molecules toward ozone in cases where no experimental data are available. Although the gas-phase reactivity of olefins with ozone has been characterized extensively in the literature, this is not the case for the richer chemistry of ozone with polar molecules, which occurs in the aqueous phase or in microhydrated environments. Here, we selected a number of model reactions with small molecules (ethene, ethyne, hydrogen cyanide, hydrogen chloride, ammonia, bromide, and trimethylamine) to study the accuracy of different quantum chemical methods for describing the reactivities of these molecules with ozone. We calculated benchmark electronic energies of gas-phase reactions of these systems with single reference coupled cluster (CC) theory. These benchmark results for the binding energy in the van der Waals complex, the energy of the transition structure, and the reaction energy were estimated to be accurate within 1-2 kcal mol(-1). Singlet oxygen (O-1(2)) is a common product of ozone reactions. Coupled cluster calculations with up to perturbative quadruples (CCSDT(Q)) were needed to obtain reaction energies accurate within 1 kcal mol(-1) when this species was involved. In (micro)hydrated environments or at interfaces, coupled cluster methods are prohibitively expensive in most cases. We tested the suitability of some contemporary density functional theory (DFT) methods to reproduce the benchmark electronic energy differences. Range-separated functionals were found to be promising candidates to estimate forward barrier heights, with LC-omega PBE rivaling the accuracy of CCSD(T). For energies of reaction, however, DFT methods exhibited large systematic errors, depending on their fraction of orbital exchange. This was found to worsen when O-1(2) is a product, and no safe recommendation can be given for DFT reaction energies in such cases.

AMER CHEMICAL SOC2019

An In-Situ Neutron Diffraction and DFT Study of Hydrogen Adsorption in a Sodalite-Type Metal-Organic Framework, Cu-BTTri

Mehrdad Asgari, Safak Bulut, Michele Ceriotti, Ilia Kochetygov, Wendy Lee Queen, Rocio Semino Barbaresi, Olga Trukhina, Shuliang Yang

Herein we present a detailed study of the hydrogen adsorption properties of Cu‐BTTri, a robust crystalline metal–organic framework containing open metal‐coordination sites. Diffraction techniques, carried out on the activated framework, reveal a structure that is different from what was previously reported. Further, combining standard hydrogen adsorption measurements with in‐situ neutron diffraction techniques provides molecular level insight into the hydrogen adsorption process. The diffraction experiments unveil the location of four D2 adsorption sites in Cu‐BTTri and shed light on the structural features that promote hydrogen adsorption in this material. Density functional theory (DFT), used to predict the location and strength of binding sites, corroborate the experimental findings. By decomposing binding energies in different sites in various energetic contributions, we show that van der Waals interactions play a crucial role, suggesting a possible route to enhancing the binding energy around open metal coordination sites.

2019

Characterization of metal organic frameworks of interest for gas adsorption/separation applications

Mehrdad Asgari

Metal-Organic Frameworks (MOFs) are a class of porous materials that are applicable in many energy and environmentally relevant areas, due to their unique features including unprecedented internal surface areas and easy chemical tunability. Gas separation and storage are among the most important applications for which MOFs have been extensively studied. Considering the large number of potential MOFs that can be accessed, through a combination of numerous metal clusters and organic linkers, determining the relationship between their structural features (such as pore size and shape and chemical functionalities) and corresponding gas adsorption properties is of utmost importance. In this regard, in-situ characterization, especially diffraction, can be advantageous. MOFs can offer high crystallinity, and so their structure can readily be characterized via diffraction. In addition, due to the presence of metal ions, and predefined molecular building blocks, which exhibit multiple chemical moieties, the potential energy surface for a guest molecule within a MOF cavity possesses multiple minima that correspond to well-defined adsorption sites with varying binding energies. This makes diffraction the most direct way to probe static site-specific binding properties. Given this, the focus of this work is on understanding the structure-derived function of MOFs of interest in gas separation/storage applications. To do this, standard adsorption experiments are coupled with in-situ diffraction techniques. The latter is able to reveal the location and orientation of adsorbed guest species inside the framework, provide insights into the framework response to varying pressure, temperature, and atmosphere, and allow one to determine the relative differences in binding energy between neighboring adsorption sites. The aforementioned information can then be used to rationalize the relationship between different physiochemical features of a given framework and their performance in an adsorption process. In addition to establishing structure-property relations for the usage of MOFs, the obtained experimental results are used to corroborate those calculated by DFT methods, providing a stress test for computational methods aimed at predicting the structures and properties of hypothetical MOFs. The first chapter of this thesis, the introduction, offers a brief review of different characterization methods for studying gas adsorption/separation applications in MOFs. Although the focus of this chapter is on carbon dioxide capture as the application, these characterization methods are also used to study other adsorption processes. The next three chapters, chapters 2-4, present several prominent MOF families that are of interest in hydrogen storage and carbon dioxide capture applications. These case studies demonstrate how using in-situ diffraction techniques coupled with adsorption measurements and DFT calculations can be used to gain molecular level insight into how small molecules bind inside the selected MOF structures. Further, these structure-property correlation studies are able to help unveil how altering MOF building blocks, such as metals and ligands, gives rise to enhanced or diminished adsorption properties. The last chapter, chapter 5, is a study of the response of an activated MOF to varying temperature; in this chapter various structural motions, which are responsible for a large negative thermal expansion (NTE), are elucidated.