Understanding the effect of density functional choice and van der Waals treatment on predicting the binding configuration, loading, and stability of amine-grafted metal organic frameworks

Metal organic frameworks (MOFs) are crystalline, three-dimensional structures with high surface areas and tunable porosities. Made from metal nodes connected by organic linkers, the exact properties of a given MOF are determined by node and linker choice. MOFs hold promise for numerous applications, including gas capture and storage. M2(4,4'-dioxidobiphenyl-3,3'-dicarboxylate)-henceforth simply M2(dobpdc), with M = Mg, Mn, Fe, Co, Ni, Cu, or Zn-is regarded as one of the most promising structures for CO2 capture applications. Further modification of the MOF with diamines or tetramines can significantly boost gas species selectivity, a necessity for the ultra-dilute CO2 concentrations in the direct-air capture of CO2. There are countless potential diamines and tetramines, paving the way for a vast number of potential sorbents to be probed for CO2 adsorption properties. The number of amines and their configuration in the MOF pore are key drivers of CO2 adsorption capacity and kinetics, and so a validation of computational prediction of these quantities is required to suitably use computational methods in the discovery and screening of amine-functionalized sorbents. In this work, we study the predictive accuracy of density functional theory and related calculations on amine loading and configuration for one diamine and two tetramines. In particular, we explore the Perdew-Burke-Ernzerhof (PBE) functional and its formulation for solids (PBEsol) with and without the Grimme-D2 and Grimme-D3 pairwise corrections (PBE+D2/3 and PBEsol+D2/3), two revised PBE functionals with the Grimme-D2 and Grimme-D3 pairwise corrections (RPBE+D2/3 and revPBE+D2/3), and the nonlocal van der Waals correlation (vdW-DF2) functional. We also investigate a universal graph deep learning interatomic potential's (M3GNet) predictive accuracy for loading and configuration. These results allow us to identify a useful screening procedure for configuration prediction that has a coarse component for quick evaluation and a higher accuracy component for detailed analysis. Our general observation is that the neural network-based potential can be used as a high-level and rapid screening tool, whereas PBEsol+D3 gives a completely qualitatively predictive picture across all systems studied, and can thus be used for high accuracy motif predictions. We close by briefly exploring the predictions of relative thermal stability for the different functionals and dispersion corrections.

Entropic Effects on Diamine Dynamics and CO2 Capture in Diamine-Appended Mg2(dopbdc) Metal-Organic Frameworks

Recent measurements [Xu, J.; J. Phys. Chem. Lett. 2019, 10 (22), 7044-7049] have reported temperature-dependent rates of detachment of diamine from Mg sites in diamine-appended Mg2(dobpdc) [dobpdc4- = 4,4'-dihydroxy(1,1'-biphenyl)-3,3'-dicarboxylic] metal-organic frameworks, a process hypothesized to be a precursor for cooperative CO2 adsorption, leading to step-shaped isotherms or isobars. Here, we compute the rate of diamine exchange in this system for different diamines using metadynamics simulations based on a density functional theory-derived neural network potential. Reanalyzing recent measurements accounting for entropic effects, we find a positive correlation between the previously reported CO2 adsorption step pressure and the free energy at room temperature and show that the experiments and simulations are in good qualitative and quantitative agreement. The rates obtained here provide new insight into the chemical dynamics of CO2 adsorption in a class of materials that are promising for carbon capture and a lower bound on the time scale of cooperative adsorption.

Handling fluorinated gases as solid reagents using metal-organic frameworks

Fluorine is an increasingly common substituent in pharmaceuticals and agrochemicals because it improves the bioavailability and metabolic stability of organic molecules. Fluorinated gases represent intuitive building blocks for the late-stage installation of fluorinated groups, but they are generally overlooked because they require the use of specialized equipment. We report a general strategy for handling fluorinated gases as benchtop-stable solid reagents using metal-organic frameworks (MOFs). Gas-MOF reagents are prepared on gram-scale and used to facilitate fluorovinylation and fluoroalkylation reactions. Encapsulation of gas-MOF reagents within wax enables stable storage on the benchtop and controlled release into solution upon sonication, which represents a safer alternative to handling the gas directly. Furthermore, our approach enables high-throughput reaction development with these gases.

CO2 Capture by Diamines in Dry and Humid Conditions: A Theoretical Approach

This work is a mechanistic study of the CO2 reaction with diamines under both dry and wet conditions. All protic α,ω-diamines R1H1N1-(CH2)n-N2H2R2, with n = 1-5 and R1 and R2 = H and/or CH3, were investigated. Depending on the nature of the diamine, the reaction was found to follow one of two concerted asynchronous reaction mechanisms with a zwitterion hidden intermediate. Both mechanisms involved two processes. The first process consisted of a nucleophilic attack of the nitrogen N1 of the first amine group on the carbon of CO2, accompanied by the transfer of a hydrogen atom H1 from N1 to the nitrogen N2 of the second amine group, leading to the formation of a carbamate zwitterion. The subsequent process corresponds to the transfer of a hydrogen atom H2 from the second amine group N2 to an oxygen atom of CO2, thus ending the reaction by the formation of carbamic acid. The structure of the zwitterion hidden intermediate was determined using the reactive internal reaction coordinates (RIRC), a reaction pathway visualization tool, consisting of a 3D representation of the potential energy versus the internuclear distances N2-H1 and N2-H2, which correspond to the bond being formed and the bond being broken, respectively. The life span of the transitory species, i.e., the zwitterion, was found to depend on the nature of the second amine group. For primary amines, the life span of the zwitterion was "short", whereas for secondary amines, it was "long". The corresponding mechanisms were termed the "early" and "late" asynchronous mechanism, respectively. Regardless of the mechanism, the activation barriers were found to decrease with the length of the carbon chain linking the two amine groups, with an asymptotic behavior from n = 4. Involvement of a water molecule generates a significant catalytic effect for diamines with short carbon chains (n < 4), whereas for longer chain diamines, water has a slightly adverse effect.