Effect of Linker Structure and Functionalization on Secondary Gas Formation in Metal–Organic Frameworks

Rare-earth terephthalic acid (BDC)-based metal–organic
frameworks
(MOFs) are promising candidate materials for acid gas separation and
adsorption from flue gas streams. However, previous simulations have
shown that acid gases (H2O, NO2, and SO2) react with the hydroxyl on the BDC linkers to form protonated
acid gases as a potential degradation mechanism. Herein, gas-phase
computational approaches were used to identify the formation energies
of these secondary protonated acid gases across multiple BDC linker
molecules. Formation energies for secondary protonated acid gases
were evaluated using both density functional theory (DFT) and correlated
wave function methods for varying BDC–gas reaction mechanisms.
Upon validation of DFT to reproduce wave function calculation results,
rotated conformational linkers and chemically functionalized BDC linkers
with −OH, −NH2, and −SH were investigated.
The calculations show that the rotational conformation affects the
molecule stability. Double-functionalized BDC linkers, where two functional
groups are substituted onto BDC, showed varied reaction energies depending
on whether the functional groups donate or withdraw electrons from
the aromatic system. Based on these results, BDC linker design must
balance adsorption performance with degradation via linker dehydrogenation
for the design of stable MOFs for acid gas separations.