1D-2D-3D Transformation Synthesis of Hierarchical Metal–Organic Framework Adsorbent for Multicomponent Alkane Separation

A new
hierarchical MOF consisting of Cu­(II) centers connected by
benzene-tricarboxylates (BTC) is prepared by thermoinduced solid transformation
of a dense CuBTC precursor phase. The mechanism of the material formation
has been thoroughly elucidated and revealed a transformation of a
ribbon-like 1D building unit into 2D layers and finally a 3D network.
The new phase contains excess copper, charge compensated by systematic
hydroxyl groups, which leads to an open microporous framework with
tunable permanent mesoporosity. The new phase is particularly attractive
for molecular separation. Energy consumption of adsorptive separation
processes can be lowered by using adsorbents that discriminate molecules
based on adsorption entropy rather than enthalpy differences. In separation
of a 11-component mixture of C1–C6 alkanes,
the hierarchical phase outperforms the structurally related microporous
HKUST-1 as well as silicate-based hierarchical materials. Grand canonical
Monte Carlo (GCMC) simulation provides microscopic insight into the
structural host–guest interaction, confirming low adsorption
enthalpies and significant entropic contributions to the molecular
separation. The unique three-dimensional hierarchical structure as
well as the systematic presence of Cu­(II) unsaturated coordination
sites cause this exceptional behavior.