The uptake of carbon dioxide and nitrogen gas by zeolite NaKA was studied. A very high ideal CO(2)-over-N(2) selectivity and a high CO(2) capacity were observed at an optimal K(+) content of 17 at.%. NaKA is a very promising adsorbent for CO(2) separation from water-free flue gases.
Charge equilibration (Qeq) methods
can estimate the electrostatic
potential of molecules and periodic frameworks by assigning point
charges to each atom, using only a small fraction of the resources
needed to compute density functional (DFT)-derived charges. This makes
possible, for example, the computational screening of thousands of
microporous structures to assess their performance for the adsorption
of polar molecules. Recently, different variants of the original Qeq
scheme were proposed to improve the quality of the computed point
charges. One focus of this research was to improve the gas adsorption
predictions in metal–organic frameworks (MOFs), for which many
different structures are available. In this work, we review the evolution
of the method from the original Qeq scheme, understanding the role
of the different modifications on the final output. We evaluated the
result of combining different protocols and set of parameters, by
comparing the Qeq charges with high quality DFT-derived DDEC charges
for 2338 MOF structures. We focused on the systematic errors that
are attributable to specific atom types to quantify the final precision
that one can expect from Qeq methods in the context of gas adsorption
where the electrostatic potential plays a significant role, namely,
CO2 and H2S adsorption. In conclusion, both
the type of algorithm and the input parameters have a large impact
on the resulting charges, and we draw some guidelines to help the
user to choose the proper combination of the two for obtaining a meaningful
set of charges. We show that, considering this set of MOFs, the accuracy
of the original Qeq scheme is often still comparable with the most
recent variants, even if it clearly fails in the presence of certain
atom types, such as alkali metals.
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