Π-complexation Interactions Research Articles

Oriented Ultrathin π-complexation MOF Membrane for Ethylene/Ethane and Flue Gas Separations.

Rational design and engineering of high-performance molecular sieve membranes towards C2 H4 /C2 H6 and flue gas separations remain a grand challenge to date. In this study, through combining pore micro-environment engineering with meso-structure manipulation, highly c-oriented sub-100 nm-thick Cu@NH2 -MIL-125 membrane was successfully prepared. Coordinatively unsaturated Cu ions immobilized in the NH2 -MIL-125 framework enabled high-affinity π-complexation interactions with C2 H4 , resulting in an C2 H4 /C2 H6 selectivity approaching 13.6, which was 9.4 times higher than that of pristine NH2 -MIL-125 membrane; moreover, benefiting from π-complexation interactions between CO2 and Cu(I) sites, our membrane displayed superior CO2 /N2 selectivity of 43.2 with CO2 permeance of 696 GPU, which far surpassed the benchmark of other pure MOF membranes. The above multi-scale structure optimization strategy is anticipated to present opportunities for significantly enhancing the separation performance of diverse molecular sieve membranes.

Synergistic desulfurization over Ni(II) and Fe sites in adsorbent Ni@MIL-100(Fe)

SOx released from the combustion of sulfur-containing compounds in fuels pollute the environment and have a serious impact on human health. Adsorption desulfurization is an effective and environmentally friendly technique for removing sulfur-containing compounds from fuels. Metal-organic frameworks (MOFs) have high specific surface areas and can be easily modified and functionalized, making them excellent candidates for adsorption desulfurization in fuels. In this work, the precursor MIL-100(Fe)-SA was prepared using the hydrothermal method, and the adsorbent Ni@MIL-100(Fe)-SA was obtained by doping Ni2+ into the resulting MIL-100(Fe)-SA using the impregnation method. FT-IR, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and nitrogen adsorption-desorption isotherms were used to analyze the samples. The results indicated that Ni2+ was successfully loaded onto MIL-100(Fe)-SA, and the MOF's structure remained stable. The adsorption performance of benzothiophene and thiophene was evaluated through batch adsorption experiments. The adsorption capacities of Ni@MIL-100(Fe)-SA for thiophene and benzothiophene were 23.68 mg S/g and 49.58 mg S/g, respectively. These observations could be ascribed to the π-complexation interactions between Ni2+ of the framework and thiophenic compounds. The regeneration experiments demonstrated that the spent 11Ni@MIL-100(Fe)-SA-1.5 reach 86% of the adsorption capacity of fresh adsorbent after four cycles. Additionally, the regenerated adsorbent maintained good crystallinity after recycling.

Interplay between different metal–ligand binding modes in tin(II) complexes with pyridine

In this work, the π-complexation interaction between simple tin(II) molecules SnX2 (X = H, F, Cl, Br, I) and the π-cloud of pyridine (py) in a series of their model complexes SnX2·py was characterized using a variety of quantum chemical methods. Moreover, the coexistence of this interaction and the well-known σ-complexation interaction between the lone pair on the N atom of py and another SnX2 molecule was studied in the corresponding series of model complexes SnX2·py·SnX2. The interplay between the two metal–ligand binding modes was established in terms of structural changes, interaction energies, interaction energy components and topological electron density parameters. The interaction between SnX2 and py in their SnX2·py π-complexes is weak and its strength is further diminished by the σ-complexation of another SnX2 molecule, forming the SnX2·py·SnX2 complexes. The weakening is in fact mutual yet the strength of the σ-complexation interaction is reduced only marginally.

Photo-Induced Construction and Recovery of Cu+ Sites in Metal-Organic Frameworks.

The adjustment of the valence state of metal ions is crucial for various applications because peculiar activity originates from metal ions with specific valence. Cu+ can interact with molecules possessing unsaturated bonds like CO via π-complexation, while Cu2+ doesn't have such ability. Meanwhile, Cu+ sites are easily oxidized to Cu2+ , leading to the loss of activity. Despite great efforts, the development of a facile method to construct and recover Cu+ sites remains a pronounced challenge. Here, for the first time a facile photo-induced strategy is reported to fabricate Cu+ sites in metal-organic frameworks (MOFs) and recover Cu+ after oxidation. The Cu2+ precursor was loaded on NH2 -MIL-125, a typical visible-light responsive Ti-based MOF. Visible light irradiation triggers the formation of Ti3+ from Ti4+ in framework, which reduces the supported Cu2+ in the absence of any additional reducing agent, thus simplifying the process for Cu+ generation significantly. Due to π-complexation interaction, the presence of Cu+ results in remarkably enhanced CO capture capacity (1.16mmolg-1 ) compared to NH2 -MIL-125 (0.49mmolg-1 ). More importantly, Cu+ can be recovered conveniently via re-irradiation when it is oxidized to Cu2+ , and the oxidation-recovery process is reversible.

Microwave-assisted rapid growth of corncob-like nano-Ag2O/ZIF-8 on PAN electrospinning nanofibers enabled highly efficient selective adsorption desulfurization

The higher energy consumption and processing cost greatly constricted the extensive application of the adsorption desulfurization technique. Herein, the nano-Ag2O/ZIF-8@ polyacrylonitrile (PAN) nanofiber membrane (NFM) was successfully made by using electrospinning and microwave-assisted synthesis. The characterizations demonstrated that the nano-Ag2O-dotted ZIF-8 with the exclusive corncob-like structure uniformly grew on the surface of PAN nanofiber. The in-situ fabrication of nano-Ag2O/ZIF-8 on the surface of PAN nanofiber was rapidly realized by microwave-assisted method, thus resulting in the less loss of the mechanical performance of NFM. The tests indicated that the tensile strength and young's modulus of Ag2O/ZIF-8@PAN NFM were 35 % and 185 % higher than PAN NFM, respectively. The fabrication of the multi-pore structure endowed the Ag2O/ZIF-8@PAN NFM to achieve the superior adsorption performance for thiophene (TH) from fuel oil, having the increase by 312 % and 175 % in the batch adsorption (maximum: 37.142 mg·g−1) and fixed-bed breakthrough adsorption (maximum: 35.423 mg·g−1) capacities compared to PAN NFM, respectively. Furthermore, the saturated adsorption capacity of Ag2O/ZIF-8@PAN NFM in fixed-bed adsorption experiment was higher than those of Cu2O-doped PAN NFM (4.571 mg·g−1) and Ag2O@HNTs-doped PAN NFM (31.982 mg·g−1) reported by our previous study. The adsorption mechanism of TH molecules on the Ag2O/ZIF-8@PAN NFM was explored by the kinetics, intra-particle diffusion, isotherms and thermodynamic models, which affirmed to exist the synergistic effect of physical adsorption, and chemical adsorption including π-complexation interaction and S-M interaction adsorption. Finally, this work provided a novel prospective adsorption desulfurization based on the membrane separation.

Tetranuclear CuII Cluster as the Ten Node Building Unit for the Construction of a Metal-Organic Framework for Efficient C2 H2 /CO2 Separation.

Rational design of high nuclear copper cluster-based metal-organic frameworks has not been established yet. Herein, we report a novel MOF (FJU-112) with the ten-connected tetranuclear copper cluster [Cu4 (PO3 )2 (μ2 -H2 O)2 (CO2 )4 ] as the node which was capped by the deprotonated organic ligand of H4 L (3,5-Dicarboxyphenylphosphonic acid). With BPE (1,2-Bis(4-pyridyl)ethane) as the pore partitioner, the pore spaces in the structure of FJU-112 were divided into several smaller cages and smaller windows for efficient gas adsorption and separation. FJU-112 exhibits a high separation performance for the C2 H2 /CO2 separation, which were established by the temperature-dependent sorption isotherms and further confirmed by the lab-scale dynamic breakthrough experiments. The grand canonical Monte Carlo simulations (GCMC) studies show that its high C2 H2 /CO2 separation performance is contributed to the strong π-complexation interactions between the C2 H2 molecules and framework pore surfaces, leading to its more C2 H2 uptakes over CO2 molecules.

Facile fabrication of efficient Cu(I)–Y zeolite adsorbent towards the adsorption desulfurization

Herein, we fabricate cuprous oxides within the pores of zeolite Y by the ion exchange and liquid reduction (LR) strategy combined with the confined effect of micropores. First, Cu2+ ions are incorporated into the pores of zeolite Y through ion exchange, and then part of Cu2+ ions within the pores of zeolite Y are reduced to cuprous oxide by using sodium borohydride as reducing agent. As-fabricated cuprous oxides within the confined pore of zeolite Y (Cu(I)–Y) adsorbent exhibits excellent adsorption desulfurization performance such as large breakthrough adsorption sulfur capacity of DBT (20.0 mg S g−1) and superior regeneration (15 cycles). The outstanding adsorption desulfurization performance could be attributed to the unique pore structure, proper pore sizes and π-complexation interaction between Cu(I) sites and aromatic sulfur compounds such as DBT and TP. Besides, the confined effect of pore for Cu(I)–Y also plays an important role in stabilizing Cu(I) sites in the micropores. This facile fabrication strategy combined with the size and confined effect of micropore for Cu(I)-zeolite adsorbent provides a new way for the composite of porous materials with various active sites.

Adsorptive desulfurization using Cu+ modified UiO-66(Zr) via ethanol vapor reduction

Adsorptive desulfurization is one of the alternative technologies for the effective removal of sulfides from fuels. Functionalization of metal-organic frameworks (MOFs) with Cu+ is extremely desirable for adsorption separation via π-complexation interaction with adsorbed molecules. Here we report that Cu+ cation is significant and highly effective for adsorption desulfurization in a new type of Cu+- modified UiO-66(Zr). First, Cu2+ was introduced via ion-exchange with H+ of UiO-66-SO3H. Then, Cu+ cation was obtained from the reduction of Cu2+ by using vapor-induced reduction, in which vapors of reducing agents diffuse into the pores of MOFs at a lower temperature. The resulted UiO-66-SO3Cu(I)− 0.15 has a better adsorption capacity for thiophene compared to pure UiO-66. In the presence of toluene, the UiO-66-SO3Cu(I)− 0.15 also exhibits high adsorption capacity for thiophene. After recycling four times, UiO-66-SO3Cu(I)− 0.15 still remains a high adsorption capacity.

A metal-organic framework for C2H2/CO2 separation under highly humid conditions: Balanced hydrophilicity/hydrophobicity

Metal-organic frameworks (MOFs) intended for C2H2/CO2 separation should possess high C2H2/CO2 adsorption selectivity and large C2H2 adsorption capacity. Here we report the scalable synthesis of a cage-interconnected microporous MOF (JNU-2). Dynamic column breakthrough experiments on JNU-2 manifest a C2H2-capture capacity of 3.08 mmol g−1 for an equimolar C2H2/CO2 mixture at 298 K and 1 bar, exceeding the benchmark materials such as UTSA-74a and FeNi-M’MOF. Meanwhile, the selectivity factor was estimated to be ca. 1.5 for JNU-2 and consistent with the increasing flow rate. Theoretical calculations confirm that the surface of the JNU-2 framework enables preferential binding of C2H2 over CO2via multiple C–H···O, C–H···N, and π-complexation interactions. More importantly, JNU-2 retains its C2H2/CO2 separation capacity under highly humid conditions. Although the equilibrium adsorption of H2O is much higher than those of C2H2 and CO2, kinetic studies reveal that the diffusion of C2H2 and CO2 in JNU-2 is more than two orders of magnitude faster than that of H2O. A balanced hydrophobicity/hydrophilicity of the MOF framework surface might be the key to achieving large adsorption capacity for nonpolar gas molecule while minimizing the competition of water vapor adsorption.

Extraction desulphurization of fuels using ZIF-8-based porous liquid

Porous ionic liquids (PILs), coupling the permanent porosity of porous solids with the good solubility and liquid fluidity of ionic liquids, have been successfully synthesized and offer great opportunities in the development of adsorption and separation. In this study, series of zeolitic imidazolate framework-8 based PILs (ZIF-8-PILs) are constructed and used for extractive desulfurization of fuels. Effects of extraction time, temperature, mass ratio of PILs to model oil and initial sulfur concentration on desulfurization are investigated. ZIF-8-based N-butylpyridinium bis(trifluoromethanesulfonyl)imide PILs (ZIF-8/[Bpy][NTf2]) exhibits encouragingly high performance with the removal efficiencies of thiophene (TH) and benzothiophene (BT) of 72.41% and 80.87% under optimum extraction conditions. After extraction, PILs can be facilely recovered and reused with steady removal efficiencies. The stable liquid porous structure of ZIF-8-PILs plays a dominant role for the excellent extractive desulfurization performance, and the synergistic effects of electrostatic interaction, hydrogen bonding and π-complexation interaction among ILs, ZIF-8 and sulfur compounds are also closely relevant to the high removal efficiency. The results of this work show that PILs are a new and promising extractant for fuel extraction desulfurization process.

Binding Free Energy Calculations of Bilayer Graphenes Using Molecular Dynamics.

Bilayer graphenes are dimeric assemblies of single graphene layers bound together by π-complexation interactions. Controlling these assemblies can be complicated, as the layered compounds disperse in solvents or aggregate into higher columnar configurations and clusters. One way to assess the interactions that contribute to the stability of the layered compounds is to use molecular simulation. We perform pulling molecular dynamics on bilayer graphenes with different sizes and obtain the normal and shear force profiles of dissociation. We generate pathways of dissociation along the two directions and calculate the binding free energies of the structures with umbrella sampling simulations. We show that the dissociation process is direction-dependent. Along the shear direction, we compute the same free energy for the different samples, which validates the consistency of our simulations. We notice that the dissociation is less adiabatic on the normal than the shear direction, having an entropic contribution to the Gibbs energy. This contribution is more enhanced for the larger bilayer graphenes.

Deep adsorptive desulfurization over ion-exchanged zeolites: Individual and simultaneous effect of aromatic and nitrogen compounds

The presence of aromatics and nitrogen compounds in petroleum fuel has a strong inhibiting impact on deep adsorptive desulfurization (ADS). This work aimed to clarify the individual and simultaneous effect of these compounds on ADS of the most refractory sulfur-containing polyaromatic compounds (dibenzothiophene and 4,6-dimethyldibenzothiophene) over Na–Y, Ni–Y, La–Y, Ce–Y and Ni–Ce–Y zeolites in a fixed bed system at ambient conditions. Polyaromatics (naphthalene) and nitrogen (quinoline and indole) compounds caused a marked decrease in the sulfur adsorption capacity, while the nitrogen compounds decreased the sulfur adsorption capacity more than naphthalene. The negative electrostatic potential of compounds might play an important role in the adsorptive selectivity on the pre-reduced Ni–Y zeolite. The results show that the adsorption sites of aromatics and nitrogen compounds on the adsorbents are the same as for the sulfur compounds. This competitive adsorption was very strong especially for the metal that enhanced π-complexation interaction. The reduced Ni2+ introduced into Na–Y zeolite increased resistance to aromatics and nitrogen compounds on ADS. For the spent adsorbent, the regeneration efficiency, in terms of the desorption amount, depended on the similarity of the polarity of the solvent (hexane and toluene) and adsorbate, where a high temperature and low flow rate of solvent favored the extraction of adsorbates leading to an increased regeneration efficiency. This study provides an insight into the interactions among these three kinds of components and adsorbents, which is useful for further development of the ADS process.

An ethane-trapping MOF PCN-250 for highly selective adsorption of ethane over ethylene

Energy-saving separation of ethane/ethene (C2H6/C2H4) mixtures is of great importance and challenge. In this work, we reported an iron-based metal–organic framework PCN-250 as a C2H6-selective adsorbent for efficient C2H4 purification from C2H6/C2H4 mixtures. Adsorption isotherms of C2H6 and C2H4 were measured and their adsorption capacities were 5.21 and 4.22mmol/g at 298K and 100kPa, respectively. The adsorption selectivities of C2H6/C2H4 mixtures (1:15 and 1:1, v/v) were in the range of 1.9–4.0 on the basis of ideal adsorbed solution theory (IAST). Meanwhile, its separation performance was also corroborated by the breakthrough experiments, indicating the potential of C2H4 purification from C2H6/C2H4 mixtures, particularly for trapping C2H6 from typical cracked gas mixtures (C2H6/C2H4, 1:15). Compared with C2H4-selective adsorbents through π-complexation interactions, the isosteric adsorption heat of C2H6 on PCN-250 was much lower and thus could require less energy for desorption. Meanwhile, configurational-biased grand canonical Monte Carlo (CB-GCMC) method was employed to well simulate its adsorption behaviors in PCN-250. Simulation results showed that the favorable adsorption sites of C2H6 and C2H4 were distributed at four corners near benzene rings and around the N atoms of PCN-250. Both the pore effect and the van der Waals (vdW) adsorption energy governed the separation of C2H6 over C2H4 for PCN-250, especially at low pressures. Providing that PCN-250 is used as a C2H6-selective adsorbent for purifying C2H4 from C2H6/C2H4 mixtures, this high-performance material can hold great potential for trapping C2H6 from C2H6/C2H4 mixtures.

Online profiling of triacylglycerols in plant oils by two-dimensional liquid chromatography using a single column coupled with atmospheric pressure chemical ionization mass spectrometry

The complexity of natural triacylglycerols (TAGs) in various edible oils is high because of the hundreds of TAG compositions, which makes the profiling of TAGs quite difficult. In this investigation, a rapid and high-throughput method for online profiling of TAGs in plant oils by two-dimensional (2D) liquid chromatography using a single column coupled with atmospheric pressure chemical ionization (APCI) mass spectrometry was reported. A novel mixed-mode 2D chromatographic column packed with silver-ion-modified octyl and sulfonic co-bonded silica was employed in this online 2D separation system. This novel 2D column combined the features of C8 column and silver-ion. In comparison with the traditional C18 column and silver-ion column, which are the two main columns used for the separation of complex TAGs in natural oil samples, this novel 2D column, could provide hydrophobic interactions as well as π-complexation interactions. It exhibited much higher selectivity for the separation of TAGs, and the separation was rapid. This online 2D separation system was successful in the separation of a large number of TAG solutes, and the TAG structures were evaluated by analyzing their APCI mass spectra information. This system was applied for the profiling of TAGs in peanut oils, corn oils, and soybean oils. 30 TAGs in peanut oils, 18 TAGs in corn oils, and 21 TAGs in soybean oils were determined and quantified. The highest relative content of TAGs was LLL, which was found in corn oil with the relative content up to 45.43 (%, w/w), and the lowest relative content of TAGs was LLS and OSS, which was found in soybean oil and corn oil respectively, with the relative content only 0.01 (%, w/w). In addition, the TAG data were analyzed by principal component analysis (PCA). Results of PCA enabled a clear identification of different plant oils. This method provided an efficient and convenient chromatographic technology for the fast characterization and quantification of complex TAGs in plant oils at high selectivity. It has great potential as a routine analytical method for analysis of edible oil quality and authenticity control.

Embedding dopamine nanoaggregates into a poly(dimethylsiloxane) membrane to confer controlled interactions and free volume for enhanced separation performance

In this study, a series of hybrid membranes with high separation performance and superior swelling-resistance were fabricated by incorporating metal ion-chelated dopamine nanoaggregates into a poly(dimethylsiloxane) (PDMS) bulk matrix membrane. The concomitant hydrogen bond, metal-organic coordination and π-complexation interactions render the synergy of a favorable free volume property, reinforced chain rigidity and facilitated transport function within the membranes. The membranes displayed simultaneously enhanced permeation flux and enrichment factors when utilized for model gasoline separation. Especially, when the weight fraction of dopamine/Cu reached 5.0 wt%, the membrane displayed an optimum separation performance with a permeation flux of 7.42 kg m−2 h−1 (2.7 times as much as that of the PDMS control membrane) and an enrichment factor of 4.81 (11% more than that of the PDMS control membrane). Thanks to the elevated cohesive energy and the chain extension effect, the swelling degree of the PDMS-dopamine/Cu membranes decreased remarkably with the dopamine/Cu content. This study may provide a novel route to the design and fabrication of robust, high-performance hybrid membranes to meet diverse energy and environment-related application requirements.

Removal of C2H4 from a CO2 Stream by Adsorption: A Study in Combination of ab Initio Calculation and Experimental Approach

Removal of C2H4 from a CO2 stream is very important in ultrapurification of CO2 for its application and utilization. However, it is difficult to remove C2H4 completely from a CO2 stream via conventional cryogenic distillation because the volatility of C2H4 is similar to that of CO2. Another choice is the catalytic combustion method, but it is an energy-inefficient process, especially when it is coupled with the cryogenic distillation. Selective adsorption for removing C2H4 might be a promising alternative one. A great effort has been made in olefin/paraffin separation and recovery of olefin. There are, however, few reports about the removal of a trace of C2H4 from a CO2 stream. In this study, ab initio calculations have been conducted to screen the potential adsorbents. We found that AgNO3 exhibits a much higher adsorption affinity for C2H4 through a π-complexation interaction, indicating that the AgNO3-modified zeolite might be a promising adsorbent. Furthermore, the AgNO3-modified NaY zeolite was prepared, and its adsorption performance was tested. We showed that the AgNO3/NaY is able to remove C2H4 from a CO2 stream to less than 1 ppmv with an adsorption capacity of 13 mL/g at a concentration level of 1030 ppmv C2H4. The results also imply that the ab initio calculations and the developed adsorption models in the present study are useful for designing the novel adsorbents.

Porous ceramic membranes for propane–propylene separation via the π-complexation mechanism: unsupported systems

The purpose of this work is the utilization of the selective π-complexation interactions exhibited between certain metal cations and the double bond of the olefins, in order to develop membrane materials for a continuous propane–propylene separation process. Mesoporous γ-Al 2O 3 and microporous SiO 2 membranes are synthesized by the sol–gel method and subsequently modified with Ag + by impregnation from silver nitrate solutions. Gas separation experiments of equimolar undiluted propane–propylene mixtures resulted in maximum separation factors of 1.3 for the mesoporous systems. For the microporous systems the maximum separation factor achieved was 7.7, which is one of the highest reported in the literature for porous ceramic membrane systems.

Separation of Polycyclic Aromatic Compounds from Model Gasoline by Magnetic Alumina Sorbent Based on π-Complexation

Magnetic alumina sorbent was used in the separation of aromatic compounds from gasoline by a π-complexation mechanism. Magnetic alumina sorbents can effectively adsorb polycyclic aromatics and polycyclic aromatic sulfur compounds from model gasoline, and the amounts adsorbed were up to 0.12 mmol·(g of sorbent)-1. The interactions between various aromatics and sorbents of different acidities were studied. The results showed that the amounts adsorbed are related to the capability of forming π-complexation interactions, and the magnetic alumina sorbent used was found to have the following order of adsorptive ability for aromatics: pyrene > naphthalene > DBT ≈ fluorene. Al3+ and its oxide are Lewis π-acids, and aromatic compounds are Lewis π-bases; thus, they can form π-complexes with each other. The π-complexation was verified by fluorescence spectrum. The quenching of the fluorescence of pyrene by Al3+ was due to partial π-electron transfer from pyrene to the cation according to Lewis theory and fluorescen...