Shape-selective Separation Of C3 Hydrocarbons With Ionic Hybrid Porous Materials

The separation of complex hydrocarbon mixtures with similar structures is one of the important challenges in chemical separation,and is high energy and material consumption.Adsorption is considered as an energy-saving method without involving phase transition.The common separation mechanisms,like site selective effect,kinetic effect and molecular sieving,have already gained great achievement in separation.However,sometimes it is still unable to satisfy the high-purity separation of ultra-similar chemicals.Meanwhile,the current adsorbents have disadvantages,like poor selectivity and low capacity.Therefore,it is crucial to develop novel adsorbents and separation strategies.The key to design the highly-efficient adsorbents and develop separation mechanisms lies on the specific recognition between the porous material and the target guest molecule.The work provides the deep insight into the function of shape recognition in the specific molecular distinguishment and the efficient separation of structurally similar mixtures,benefiting from the precise pore size control,ordered pore structure and high density anions of ionic hybrid porous materials.Based on the multi-dimensional differences(e.g.molecular configurations,properties and etc)of the components in propylene mixtures(propylene,propane,propyne and propadiene),a series of ionic hybrid porous materials are tailor-made.The designed materials show efficient separation of C3 hydrocarbon mixtures with high selectivity and high capacity,providing theoretical and experimental basis for the development of novel separation methods and the design of highly-efficient adsorbents.The main research contents include:The deep removal of trace propyne is pivotal to propylene purification.According to the molecular size,shape and binding sites of propyne,the work revealed a single-molecule propyne trap(SIFSIX-3-Ni,[Ni(pyrazine)2(SiF6)]n)that achieved the removal of trace propyne(1000 ppm)from propylene mixtures via the control of pore structure,including functional sites distribution and pore size.The combination of experiment and simulation revealed the molecular-level adsorption behavior of propyne in different ionic hybrid materials and highlighted the effect on matching adsorption behavior caused by the pore size and anion distribution in confined space.The research showed that SIFSIX-3-Ni was able to achieve the specific recognition of propyne.The propyne uptake under ultra-low pressure(700 ppm)was up to 2.0 mmol g-1,and as the pressure reaching 3000 ppm,SIFSIX-3-Ni showed adsorption saturation with the uptake of 2.65 mmol g-1(one propyne molecule per unit),a record value under ultra-low pressure.The study highlights the advantage of single-molecule trap in trace components adsorption with high capacity.The propyne and propylene separation performance on flexible porous materials with stimuli-responsive behavior was further studied.The work synthesized several flexible ionic hybrid materials that were assembled with different anions(TiF62-,SiF62-,GeF62-).The rational control of the hydrogen bonding interactions between the anion and flexible organic linker(4,4'-azopyridine)realized the modulation of pore flexibility.The adsorption performance of propyne and propylene in different flexible ionic hybrid materials as well as the corresponding gate-opening pressure was investigated.Meanwhile,the propyne/propylene separation selectivity as well as the trace propyne adsorption ability,the dynamic propyne capacity and the cycling performance of different materials were also well studied.The dynamic changes of the framework accompanied with the accommodation of propyne was revealed by simulation,as well as the interaction way between the framework and the propyne.ZU-13(TIFSIX-14-Cu-i[Cu(4,4'-azobipyridine)2(TiF6)])with the sensitive propyne response showed the superior propyne/propylene separation performance with the selectivity of 355,which was significantly higher than the previous reported materials.The study shows that the flexible materials with stimuli-responsive behavior are promising in structurally similar mixtures separation with high capacity and selectivity,owing to their integrated recognition of the difference in both molecular size and properties.The one-step removal of propyne and propadiene can simplify the separation and purification process of propylene.The novel multisite adsorbent ZU-62(NbOFFIVE-2-Cu-i,[Cu(4,4'-bipyridine)2(NbOF5)]n)was designed by utilizing the asymmetric anion NbOF52-based on the coordination bond length difference between O/F and metal node.And for the first time,it achieved the one-step removal of propyne and propadiene from propylene mixtures,improving the separation efficiency.The work gave the detailed analysis about the crystal structure of NbOFFIVE-2-Cu-i,the stability and the adsorption performance of propyne,propylene and propadiene.The three-component mixed gas separation performance was also investigated.The experiment combined with simulation revealed the different preferential adsorption behavior of propyne and propadiene at the multiple sites of NbOFFIVE-2-Cu-i.The high density anions of NbOFFIVE-2-Cu-i realized the high-selective separation of propyne/propylene and propadiene/propylene with the corresponding selectivity of 48 and 34.The multiple adsorption sites with small pore size difference and staggered anion distribution provided the different preferential strong binding sites for propyne and propadiene.The capacity of propyne and propadiene at 298 K and 0.005 bar reached 1.87 and 1.74 mmol g-1,respectively.The study shows that tailor-made pore structure based on the comprehensive utilization of the small shape/property differences between different molecules can achieve efficient recognition and separation of multiple molecules and improve separation efficiency.The separation of propylene and propane is an important process for polymer grade propylene production.And the propane-selective adsorbents are promising in affording high purity propylene and simplifying the separation process.A polycatenated molecular cage was revealed within WOFOUR-1-Ni([Ni(bpe)2(WO4)],bpe-1,2-(4-pyridyl)ethene)and it showed preferential propane adsorption.Both the static adsorption isotherm and the dynamic adsorption curve of propylene and propane of WOFOUR-1-Ni were measured.And the breakthrough performance of WOFOUR-1-Ni for different proportions of propane/propylene mixtures as well as its recycling performance were also investigated.The deep insight via the simulation revealed the interaction way of the propane and propylene in the cage-like channel.WOFOUR-1-Ni exhibited higher propane affinity than propylene under low pressure,and the propane capacity reached 15 cm3 cm-3 at around 0.015 bar.The propane/propylene IAST selectivity was higher than 2.5 under the pressure of 0-0.1 bar.The study showed that the preferential propane adsorption behavior was attributed to the multiple C?-—H?+...C?-interactions between the aromatic surface and propane as well as the better shape match between the molecular cage and the oblate C3H8.The study shows that the shape recognition is able to realize the integrated recognition of molecular properties/configurations,and thus satify the specific recogtion of target molecules.In summary,the molecular shape recognition mechanism based on comprehensive molecular structure differences(such as size,shape,functional groups and etc.)can significantly improve the recognition ability of porous materials towards target guest molecules,thereby satisfying new separation requirements and meeting higher separation standards.The research also provides new ideas for the design of high-efficiency adsorbents and the development of new separation mechanisms.