  Metal-organic frameworks (MOFs) have emerged as promising materials for
carbon capture applications due to their high CO2 capacities and tunable
properties. Amongst the many possible MOFs, metal-substituted compounds based
on M-DOBDC and M-HKUST-1 have demonstrated amongst the highest CO2 capacities
at the low pressures typical of flue gasses. Here we explore the possibility
for additional performance tuning of these compounds by computationally
screening 36 metal-substituted variants (M = Be, Mg, Ca, Sr, Sc, Ti, V, Cr, Mn,
Fe, Co, Ni, Cu, Zn, Mo, W, Sn, and Pb) with respect to their CO2 adsorption
enthalpy, $\Delta$H (T=300K). Supercell calculations based on van der Waals
density functional theory (vdW-DF) yield enthalpies in good agreement with
experimental measurements, out-performing semi-empirical (DFT-D2) and
conventional (LDA and GGA) functionals. Our screening identifies 13 compounds
having $\Delta$H values within the targeted thermodynamic window 40 $\leq$
$\Delta$H $\leq$ 75 kJ/mol: 8 are based on M-DODBC (M=Mg, Ca, Sr, Sc, Ti, V,
Mo, and W), and 5 on M-HKUST-1 (M= Be, Mg, Ca, Sr and Sc). Variations in the
electronic structure and the geometry of the structural building unit are
examined and used to rationalize trends in CO2 affinity. In particular, the
partial charge on the coordinatively unsaturated metal sites is found to
correlate with $\Delta$H, suggesting that this property may be used as a simple
performance descriptor. The ability to rapidly distinguish promising MOFs from
those that are "thermodynamic dead-ends" will be helpful in guiding synthesis
efforts towards promising compounds.
