1 research outputs found
Understanding Hydrocarbon Adsorption in the UiO-66 Metal–Organic Framework: Separation of (Un)saturated Linear, Branched, Cyclic Adsorbates, Including Stereoisomers
The low coverage adsorption properties
of alkanes, alkenes, and
aromatics of the linear, branched, and cyclic type (ca. 70 molecules)
were studied using inverse pulse gas chromatography at zero coverage
on the zirconium metal–organic framework UiO-66 and its functionalized
analogues UiO-66-Me, UiO-66-NO<sub>2</sub>, UiO-66-Me<sub>2</sub> in
the temperature range 433–573 K. In our study, we determined
and analyzed the adsorption enthalpy, Henry constants, and entropic
factors. Preferential adsorption of bulky molecules is observed with
specific adsorbate and cage size effects, yielding very specific,
preferential adsorption. Remarkably high adsorption selectivity factors
(up to 14) for cyclo- compared to <i>n-</i>alkanes were
found. The presence of additional groups (methyl, nitro) on the linkers
in the framework influences adsorption properties significantly, mainly
by reducing the effective pore size. Whereas increased selectivity
is observed for UiO-66-Me, this effect decreases again upon addition
of a second methyl group, UiO-66-Me<sub>2</sub>. The latter allows
for tuning confinement factors inside the pores, thus adsorption properties
of the metal–organic framework. The selective adsorption results
from the interaction in the smallest octahedral cage. The extreme
confinement in the tetrahedral cage allows for stereoselective separation
of disubstituted cycloalkanes and <i>cis</i>/<i>trans</i> alkenes. Monte Carlo simulations were performed for the unfunctionalized
UiO-66 framework. First, a comparative study between the force fields
Dreiding and UFF is performed with <i>n-</i>alkanes to obtain
accurate and reproducible values. The simulations show adsorbate molecular
size–adsorbent cage size effects similar to window/cage effects
reported for zeolites (e.g., silicalite). Second, adsorption properties
were simulated for selected cases, including stereoisomers. Careful
analysis of the adsorbate’s molecular positioning in the framework
confirms the experimental data. The framework’s selectivity
results from adsorption in the tetrahedral cage at zero coverage.
Furthermore, simulations show important contributions of entropic
factors to the observed adsorption selectivity