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.

New insights into UTSA-16.

Among the metal organic framework materials proposed for CO2 separation, UTSA-16 possesses the highest CO2 volumetric density explained on the basis of favourable interactions between CO2 and structural water molecules in the material, as revealed by neutron diffraction. In this study, UTSA-16 was synthesised and extensively characterised by XRD, TEM combined with EDX analysis and DR-UV-Vis, Raman and FTIR spectroscopies, as well as by TGA measurements. The synthesised material shows XRD patterns, surface area, CO2 capacity and isosteric heat coincident to the ones reported for UTSA-16 in the original papers but a higher thermal stability and a complete removal of water upon activation under mild conditions (363 K). On the basis of EDX and IR measurements, the formula of UTSA-16 used in the present study is proposed to be K2Co3(cit)2. Infrared spectroscopy clearly shows that UTSA-16 described in this work reversibly interacts with water vapor, CO and CO2. The interaction is attributed to K(+) species, which are present as counterions in the pores. At 1 bar and 298 K a fraction of K(+) sites adsorbs 2 CO2 molecules.

CO2 Capture in Dry and Wet Conditions in UTSA-16 Metal-Organic Framework.

Water is the strongest competitor to CO2 in the adsorption on microporous materials, affecting their performances as CO2 scrubbers in processes such as postcombustion carbon capture. The metal-organic framework (MOF) UTSA-16 is considered a promising material for its capacity to efficiently capture CO2 in large quantities, thanks to the presence of open metal sites (OMSs). It is here shown that UTSA-16 is also able to desorb fully water already at room temperature. This property is unique from all the other materials with OMSs reported so far. UTSA-16 retains indeed the 70% of its CO2 separation capacity after admittance of water in a test flow, created to simulate the emissions from a real postcombustion carbon-capture process. This important aspect not yet observed for any other amine-free material, associated with a high material stability-tested for 160 cycles-and a small temperature swing necessary for regeneration, places UTSA-16 in the restrict number of systems with a real technological future for CO2 separation.