During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

The adsorption sites of rare gases on metallic surfaces: a review : Surface dynamics, phonons, adsorbate vibrations and diffusion

Upgrading of the biomass-derived levulinic acid (LA) with the activation of carbonyl functional groups is a pivotal reaction in converting biomass into high-value-added products. Herein, we reporte...

Bimetallic NiCu Alloy Catalysts for Hydrogenation of Levulinic Acid

Flexible metal-organic frameworks (MOFs) are of high interest as smart programmable materials for gas sorption due to their unique structural changes triggered by external stimuli. Owing to this property, which leads to opportunities such as maximizing deliverable gas capacity, flexible MOFs sometimes offer more advantages in sorption applications compared to their more rigid counterparts. Herein, we elucidate the effect of transition metal identity of a series of isonicotinate-based flexible MOFs, M(4-PyC)2 [M═Mg, Mn, and Cu; 4-PyC = 4-pyridine carboxylic acid] on the structural dynamic response to different gases (C2H4, C2H6, Xe, Kr, and SO2). Isotherms at different temperatures show that C2H4, C2H6, and Xe can form sufficiently strong interactions with both Mg(4-PyC)2 and Mn(4-PyC)2 frameworks resulting in gate-opening behavior due to the rotation of the linker's pyridine ring, while Kr cannot induce this phenomenon for the two MOFs under the measured conditions. In contrast, the gate-opening behavior occurs for Cu(4-PyC)2 solely in the presence of C2H4, and no other measured gas, due to the open metal sites of Cu centers. In comparison, SO2, a strong polar molecule, triggers the gate-opening effect in all three MOFs. Interestingly, a shape memory effect is observed for Cu(4-PyC)2 during the second SO2 sorption cycle. When comparing the different gate-opening pressures of each gas, we observed that the structural flexibility of the three frameworks is highly sensitive to the chemical hardness of the Lewis acidic metal ions (Mg2+ > Mn2+ > Cu2+). As a result, the gate opening behavior is observed at lower pressures for the MOFs containing weaker M-N bonds (harder metal ions), with the exception of Cu(4-PyC)2 toward C2H4. These observations reveal that different transition metals can be used to finely control the structural flexibility of the frameworks.

Tuning the Structural Flexibility for Multi-Responsive Gas Sorption in Isonicotinate-Based Metal-Organic Frameworks.

Efficient Xe/Kr separation on two Metal-Organic frameworks with distinct pore shapes

Carbon dioxide capture in gallate-based metal-organic frameworks

Separation of Xe and Kr is one of the greatest challenges in the gas industries owing to their close molecular structure and similar properties. Energy-effective adsorption-based separation using chemically stable carbon adsorbents is a promising technology. We propose a strategy for Xe/Kr separation using MOF-derived metallic carbon adsorbents. M-Gallate (M=Ni, Co) were used as precursors to fabricate CoNi alloy nanoparticles embedded carbon adsorbents by one-step auto-reduction pyrolysis. The optimal NiCo@C-700 exhibits record-high IAST selectivity (24.1) and Henry's selectivity (20.1) of Xe/Kr among reported carbon adsorbents. DFT calculations, local density of states calculation, charge density difference, and Bader charge analysis reveal the great affinity with Xe benefits from the presence of Ni or CoNi nanoparticles as a result of more charge transfer from Xe than Kr to metal, thus providing higher binding energy. Breakthrough experiments further verify NiCo@C-700 a promising candidate for Xe/Kr separation.

CoNi Alloy Nanoparticles Embedded in Metal–Organic Framework-Derived Carbon for the Highly Efficient Separation of Xenon and Krypton via a Charge-Transfer Effect

The separation of light hydrocarbon mixtures and the extraction of C3H8 and C2H6 from natural gas are of significant importance in natural gas purification and upgrading. Microporous carbon adsorbents are promising in light hydrocarbon separation. However, the fabrication of uniform ultramicropores to boost separation selectivity remains challenging. Herein, we fabricated a series of poly(vinylidene chloride)-resin-derived ultramicroporous carbon adsorbents with a relatively uniform pore size (5.2–5.3 Å) by activator-free pyrolysis. The optimal C-PVDC-800 exhibited record-high ideal adsorption solution theory (IAST) selectivity for C3H8/CH4 (3387) and C2H6/CH4 (75) and Henry’s selectivity for C3H8/CH4 (369) among all reported adsorbents under ambient conditions, combined with ultrahigh uptake of C3H8 (3.9 mmol g−1, 0.05 bar) and C2H6 (2.67 mmol g−1, 0.10 bar) at low partial pressures and 298 K. More importantly, fast gas adsorption kinetics favorable for fixed-bed adsorption applications was realized. Breakthrough experiments and cycling tests further confirm the superb separation performance of the extraction of C3H8 and C2H6 from natural gas. 

/pdf/extraction-of-propane-and-ethane-from-natural-gas-on-2r0dufhj.pdf

Kaiqing Guo

Papers

CoNi Alloy Nanoparticles Embedded in Metal–Organic Framework-Derived Carbon for the Highly Efficient Separation of Xenon and Krypton via a Charge-Transfer Effect

Extraction of propane and ethane from natural gas on ultramicroporous carbon adsorbent with record selectivity