The trade-off between physical adsorption capacity and selectivity of porous materials is a major barrier for efficient gas separation and purification through physisorption. We report control over pore chemistry and size in metal coordination networks with hexafluorosilicate and organic linkers for the purpose of preferential binding and orderly assembly of acetylene molecules through cooperative host-guest and/or guest-guest interactions. The specific binding sites for acetylene are validated by modeling and neutron powder diffraction studies. The energies associated with these binding interactions afford high adsorption capacity (2.1 millimoles per gram at 0.025 bar) and selectivity (39.7 to 44.8) for acetylene at ambient conditions. Their efficiency for the separation of acetylene/ethylene mixtures is demonstrated by experimental breakthrough curves (0.73 millimoles per gram from a 1/99 mixture).

A new metal-organic framework Zn2(H2O)(dobdc)·0.5(H2O) (UTSA-74, H4dobdc = 2,5-dioxido-1,4-benzenedicarboxylic acid), Zn-MOF-74/CPO-27-Zn isomer, has been synthesized and structurally characterized. It has a novel four coordinated fgl topology with one-dimensional channels of about 8.0 Å. Unlike metal sites in the well-established MOF-74 with a rod-packing structure in which each of them is in a five coordinate square pyramidal coordination geometry, there are two different Zn(2+) sites within the binuclear secondary building units in UTSA-74 in which one of them (Zn1) is in a tetrahedral while another (Zn2) in an octahedral coordination geometry. After activation, the two axial water molecules on Zn2 sites can be removed, generating UTSA-74a with two accessible gas binding sites per Zn2 ion. Accordingly, UTSA-74a takes up a moderately high and comparable amount of acetylene (145 cm(3)/cm(3)) to Zn-MOF-74. Interestingly, the accessible Zn(2+) sites in UTSA-74a are bridged by carbon dioxide molecules instead of being terminally bound in Zn-MOF-74, so UTSA-74a adsorbs a much smaller amount of carbon dioxide (90 cm(3)/cm(3)) than Zn-MOF-74 (146 cm(3)/cm(3)) at room temperature and 1 bar, leading to a superior MOF material for highly selective C2H2/CO2 separation. X-ray crystal structures, gas sorption isotherms, molecular modeling, and simulated and experimental breakthroughs comprehensively support this result.

As an alternative technology to energy intensive distillations, adsorptive separation by porous solids offers lower energy cost and higher efficiency. Herein we report a topology-directed design and synthesis of a series of Zr-based metal-organic frameworks with optimized pore structure for efficient separation of C6 alkane isomers, a critical step in the petroleum refining process to produce gasoline with high octane rating. Zr6O4(OH)4(bptc)3 adsorbs a large amount of n-hexane but excluding branched isomers. The n-hexane uptake is ~70% higher than that of a benchmark adsorbent, zeolite-5A. A derivative structure, Zr6O4(OH)8(H2O)4(abtc)2, is capable of discriminating all three C6 isomers and yielding a high separation factor for 3-methylpentane over 2,3-dimethylbutane. This property is critical for producing gasoline with further improved quality. Multicomponent breakthrough experiments provide a quantitative measure of the capability of these materials for separation of C6 alkane isomers. A detailed structural analysis reveals the unique topology, connectivity and relationship of these compounds.

Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during the electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial. Here, we prepared two Cu-based electrodes with mixed oxidation states, namely, HQ-Cu (containing Cu, Cu2O, CuO) and AN-Cu (containing Cu, Cu­(OH)2). We extracted an ultrathin specimen from the electrodes using a focused ion beam to investigate the distribution and evolution of various Cu species by electron microscopy and electron energy loss spectroscopy. We found that at the steady stage of the CO2RR, the electrodes have all been reduced to Cu0, regardless of the initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified this conclusion by control experiments in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to Cu0, and the pretreated electrodes showed even higher C2+ selectivity compared with their unpretreated counterparts. We observed that the oxide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nanosized irregular Cu grains under the applied negative potentials. Such a fragmentation process, which is the consequence of an oxidation–reduction cycle and does not occur in electropolished Cu, not only built an intricate network of grain boundaries but also exposed a variety of high-index facets. These two features greatly facilitated the C–C coupling, thus accounting for the enhanced C2+ selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the structural and chemical states of electrodes in unprecedented detail to gain new insights into a widely studied system.

in the production of polypropylene, the world's second-most widely produced synthetic plastic. The global demand for polypropylene has been rising continuously and its annual growth rate is expected to be 4-5% before 2020, resulting in increasing need for polymer-grade (>99.5%) propylene. [3] Nevertheless, the production of highly pure propylene represents a challenging and complicated process, which involves the separation of propylene from a propane/propylene mixture. Propane/ propylene mixtures are typically obtained by steam cracking of naphtha or during fluid catalytic cracking of gas oils in refineries, with a propylene purity of 50-60% for the former and 80-87% for the latter. Conventional separation of propane and propylene relies on cryogenic distillation, which is carried out at about 243 K and 0.3 MPa in a column containing over 100 trays. [4] Undoubtedly, this heat-driven process is highly energy-intensive.To lower the energy and operational cost and to suppress the carbon emissions associated with the propylene purification process through cryogenic distillation, several alternative technologies have been proposed and among them adsorptive separation, such as pressure/temperature swing adsorption, Adsorptive separation of olefin/paraffin mixtures by porous solids can greatly reduce the energy consumption associated with the currently employed cryogenic distillation technique. Here, the complete separation of propane and propylene by a designer microporous metal-organic framework material is reported. The compound, Y 6 (OH) 8 (abtc) 3 (H 2 O) 6 (DMA) 2 (Y-abtc, abtc = 3,3′,5,5′-azobenzene-tetracarboxylates; DMA = dimethylammonium), is rationally designed through topology-guided replacement of inorganic building units. Y-abtc is both thermally and hydrothermally robust, and possesses optimal pore window size for propane/propylene separation. It adsorbs propylene with fast kinetics under ambient temperature and pressure, but fully excludes propane, as a result of selective size exclusion. Multicomponent column breakthrough experiments confirm that polymer-grade propylene (99.5%) can be obtained by this process, demonstrating its true potential as an alternative sorbent for efficient separation of propane/propylene mixtures.