An assessment of density functionals for predicting CO adsorption in diamine-functionalized metal-organic frameworks.

Diamine-functionalized M(dobpdc) (M = Mg, Mn, Fe, Co, Zn) metal-organic frameworks (MOFs) are among a growing class of crystalline solids currently being intensively investigated for carbon capture as they exhibit a novel cooperative and selective CO adsorption mechanism and a step-shaped isotherm. To understand their CO adsorption behavior, ab initio calculations with near-chemical accuracy (∼6 kJ/mol, an average experimental error) are required. Here, we present density functional theory (DFT) calculations of CO adsorption in m-2-m-Zn(dobpdc) (m-2-m = N,N'-dimethylethyle-nediamine and dobpdc = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) with different exchange-correlation functionals, including semilocal functionals [Perdew-Burke-Ernzerhof (PBE) and two revised PBE functionals], semiempirical pairwise corrections (D3 and Tkatchenko-Scheffler), nonlocal van der Waals (vdW) correlation functionals-vdW-optB88 (or vdW-DF-optB88), vdW-DF1, vdW-DF2, vdW-DF2-B86R (or rev-vdW-DF2), vdW-DF-cx (and vdW-DF-cx0), and revised VV10-and the strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (GGA). Overall, we find that revPBE+D3 and RPBE+D3 show the best balance of performance for both the lattice parameters and the CO binding enthalpy of m-2-m-Zn(dobpdc). revPBE+D3 and RPBE+D3 predict the m-2-m-Zn(dobpdc) lattice parameters to within 1.4% of experiment and predict CO binding enthalpies of -68 kJ/mol, which compare reasonably well with the experiment (-57 kJ/mol). Although PBE (-57.7 kJ/mol), vdW-DF1 (-49.6 kJ/mol), and vdW-DF2 (-44.3 kJ/mol) are also found to predict the CO binding enthalpy with good accuracy, they overestimate lattice parameters and bond lengths. The other functionals considered predict the lattice parameters with the same accuracy as revPBE+D3 and RPBE+D3, but they overbind CO by around 26-50 kJ/mol. We find that the superior performance of revPBE+D3 and RPBE+D3 is sustained for the formation enthalpy and the lattice parameters of ammonium carbamate, a primary product of the cooperative CO insertion in diamine-functionalized M(dobpdc) MOFs. Moreover, we find that their performance is derived from their larger repulsive exchange contributions to the CO binding enthalpy than the other functionals at the relevant range of the reduced density gradient value for the energetics of CO adsorption in the m-2-m-Zn(dobpdc) MOF. A broader examination of the performance of RPBE+D3 for the structural parameters and CO binding enthalpies of 13 diamine-functionalized Mg(dobpdc) MOFs further demonstrates that RPBE+D3 successfully reproduces experimental CO binding enthalpies and reveals a logarithmic relationship between the step pressure and the CO binding enthalpy of the diamine-functionalized Mg(dobpdc) MOFs, consistent with experiments where available. The results of our benchmarking study can help guide the further development of versatile vdW-corrected DFT methods with predictive accuracy.