互穿结构和静电作用影响二氧化碳与氢混合物在MOF材料中分离行为的分子模拟研究

In this work grand canonical Monte Carlo simulations were performed to study gas separation in three pairs of isoreticular metal-organic frameworks (IRMOFs) with and without catenation at room temperature. Mixture composed of CO2 and H2 was selected as the model system to separate. The results show that CO2 selectivity in catenated MOFs with multi-porous frameworks is much higher than their non-catenated counterparts. The simulations also show that the electrostatic interactions are very important for the selectivity, and the contributions of different electrostatic interactions are different, depending on pore size, pressure and mixture composition. In fact, changing the electrostatic interactions can even qualitatively change the adsorption behavior. A general conclu-sion is that the electrostatic interactions between adsorbate molecules and the framework atoms play a dominant role at low pressures, and these interactions in catenated MOFs have much more pronounced effects than those in their non-catenated counterparts, while the electrostatic interactions between adsorbate molecules become evident with increasing pressure, and eventually dominant.     

CO2/CH4 and CH4/N2 separation on isomeric metal organic frameworks☆

Two isomeric metal-organic frameworks(MOFs) with 2-dimensional(2D) and 3-dimensional(3D) topologies both comprised of Cu(Ⅱ) and OTf(OTf = trifluoromethanesulfonate) ions were synthesized and characterized.The CO_2,CH_4 and N_2 adsorption properties of the two isomeric MOFs were investigated from 263 K to 298 K at0.1 MPa.The results showed that the 2D MOF exhibited a higher selectivity for CO_2 from CO_2/CH_4 and CH_4from CH_4/N_2 compared to the 3D MOF,even though it possessed a lower surface area and pore volume.The higher adsorption heats of gases on the 2D MOF inferred the strong adsorption potential energy in the layered MOFs.Dynamic separation experiments using CO_2/CH_4 and CH_4/N_2 mixtures on the two MOFs proved that the2 D MOF had a longer elution time than the 3D MOF as well as better separation abilities.     

Synthesis,gas adsorption and reliable pore size estimation of zeolitic imidazolate framework-7 using CO2 and water adsorption

Reliable estimation of the pore size distribution(PSD) in porous materials such as metal–organic frameworks(MOFs) and zeolitic imidazolate frameworks(ZIFs) is crucial for accurately assessing adsorption capacity and corresponding selectivity. In this study, the so-called zeolitic imidazolate framework-7(ZIF-7) is successfully synthesized via relatively fast and convenient microwave technique. The morphology and structure of the obtained MOF were characterized by XRD, SEM and N_2 and CO_2adsorption/desorption isotherms at 77 K and0 °C respectively. Then, to determine the PSD of the fabricated MOF, carbon dioxide isotherms are experimentally measured at various temperatures up to atmospheric pressure. Afterward, the experimental CO_2 isotherms data are utilized in two recently proposed in-house algorithms of SHN1 and SHN2 to extract the true PSD of manufactured ZIF-7. The obtained results revealed that median pore diameter of the fabricated ZIF-7 is estimated around 0.404 nm and 0.370 nm by using CO_2 isotherms at 273 K and 298 K respectively. These values are in good agreement with the real pore diameter of 0.42 nm. Moreover, experimental data of water adsorption isotherms over four different MOFs, borrowed from literature, are employed to illustrate further effectiveness of the above algorithms on successful determination of the corresponding pore size distributions. All predicted PSDs are proved to be in good agreement with those obtained from independent methods such as topology and morphology studies.     

水合物法分离H2+CH4体系的模拟计算

This paper presents two novel conceptions in multi-stage hydrate separation technology for H2 CH4 system, i.e. the multi-stage equilibria adsorption and the reaction adsorption. It is assumed that there already exists clathrate structure before the hydration reaction, and the hydration reaction is taken as gaseous adsorption in the crystal structure of hydrate. During the simulation of multi-stage equilibria adsorption, gases and water interact on every equilibrium stage till establishing full equilibria, wherein the gases that just entered one stage are in equilibrium with the liquid phase of the previous stage, and the water that just entered one stage is in equilibrium with the gas phase of the previous stage as well. A kinetic model of hydrate growth for methane is introduced into the reaction adsorption so that this simulation is closer to the reality. As hydrogen doesn‘t react with water to form hydrate, the amount of hydrogen adsorption is calculated according to the proportion of methane and hydrogen adsorbed in the small cavities. Simultaneously, the plate column is employed as an example, where the gas-hydrate phase loads and hydrogen mole fraction are calculated by the multi-stage equilibria adsorption and reaction adsorption methods, and the results calculated by the two said methods are compared.     

天然气管网吸附调峰可行性的模拟研究

A lo9adleveling method through adsorption was presented to adjust the supply quantity according to the consumption rate of natural gas with time,An experimental simulation set up was designed and used to test the load-leveling function for a real pipline system.A storage tank filled with activated carbon together with a filter constitutes the major part of the load-leveling facility,Pressure and temperature of the system,as well as the real gas output of the storage tank were recorded.It is proven that load-leveling by adsorption in technically feasible even for low pipeline pressure of natural gas supply system.     

甲烷空气部分氧化制合成气喷动床反应器的研究

On the basis of hydrodynamic and scaling-up studies, a pilot-plant-scale thermal spouted bed reactor (50 mm in ID and 1500 mm in height) was designed and fabricated by scaling-down cold simulators. It was tested for making syngas via catalytic partial oxidation (CPO) of methane by air. The effects of various operating conditions such as operating pressure and temperature, feed composition, and gas flowrate etc. on the CPO process were investigated. CH4 conversion of 92.20％ and selectivity of 92.3% and 83.30/0 to CO and H2, respectively, were achieved at the pressure of 2.1 MPa. It was found that when the spouted bed reactor was operated within the stable spouting flow regime, the temperature profiles along the bed axis were much more uniform than those operated within the fixed-bed regime. The CH4 conversion and syngas selectivity were found to be close to thermodynamic equilibrium limits. The results of the present investigation showed that spouted bed could be considered as a potential type of chemical reactor for the CPO process of methane.     

Vacuum pressure swing adsorption process for coalbed methane enrichment

The enrichment of low concentration coalbed methane using adsorption process with activated carbon adsorbent was studied in this work.Adsorption isotherms of methane,nitrogen and carbon dioxide on activated carbon were measured by volumetric method,meanwhile a series of breakthrough tests with single component,binary components and three components feed mixture has been performed for exploring dynamic adsorption behaviors.Moreover,a rigorous mathematical model of adsorption bed containing mass,energy,and momentum conservation equation as well as dual-site Langmuir model with the Linear driving force model for gas-solid phase mass transfer has been proposed for numerical modeling and simulation of fixed bed breakthrough process and vacuum pressure swing adsorption process.Furthermore,the lumped mass transfer coefficient of methane,nitrogen and carbon dioxide on activated carbon adsorbent has been determined to be 0.3 s~(-1),1.0 s~(-1) and 0.06 s~(-1) by fitting the breakthrough curves using numerical calculation.Additionally,a six bed VPSA process with twelve step cycle sequence has been proposed and investigated for low concentration coalbed methane enrichment.Results demonstrated that the methane molar fraction in feed mixture ranged from 10% to 50% could be enriched to 32.15% to 88.75% methane in heavy product gas with a methane recovery higher than 83%under the adsorption pressure of 3 bar(1 bar=10~5 Pa) and desorption pressure of 0.1 bar.Energy consumption of this VPSA process was varied from 0.165 kW·h·m~(-3) CH_4 to 0.649 kW·h·m~(-3) CH_4.Finally,a dual-stage VPSA process has been successfully developed to upgrade a low concentration coalbed methane containing 20% methane to a target product gas with methane purity higher than 90%,meanwhile the total methane recovery was up to 98.71% with a total energy consumption of 0.504 kW·h·m~(-3) CH_4.     

Synthesis,adsorption and selectivity of inverse emulsion Cd(Ⅱ) imprinted polymers

Inverse emulsion polymerization was employed to synthesize inverse emulsion Cd(II) imprinted polymers(IEII P). The morphology and functional groups of IEIIP were characterized by SEM,FTIR and TG. Static adsorption experiments and competitive adsorption test were used to evaluate the adsorption ability of IEIIP. The adsorption capacity of polymers could reach 86.7 mg·g~(-1) under the optimal adsorption conditions. The pseudo second order kinetic model and Langmuir isotherm model could be used to analyze the experimental data well. The adsorption process of IEIIP was chemical adsorption process and monomolecular type. Thermodynamic parameters showed that the adsorption process was endothermic and could occur spontaneously. The selectivity coefficients k of Cd~(2+)/Pb~(2+), Cd~(2+)/Zn~(2+) and Cd~(2+)/Cu~(2+) were 2.4998, 1.2437 and 4.6882, respectively. The proposed method provides a new thought for removing Cd(II) in water samples.     

Accelerated methane storage in clathrate hydrates using surfactantstabilized suspension with graphite nanoparticles

In this study, enhanced kinetics of methane hydrate formation in the sodium dodecyl sulfate(SDS) solution with different concentrations of suspended graphite nanoparticles(GNPs) were investigated at 6.1–9.0 MPa and 274.15 K. The GNPs with rough surfaces and excellent thermal conductivity not only provided a considerable number of microsites for hydrate nucleation but also facilitated the fast hydrate heat transfer in the suspension system. At a relatively low pressure of 6.1 MPa, the suspension with 0.4 wt% of GNPs exhibited the minimum induction time of 22 min and maximum methane uptake of 126.1 cm~3·cm~(-3). However, the methane storage performances of the suspensions with higher and lower concentrations of GNPs were not satisfactory. At the applied pressure, the temperature increase arising from the hydrate heat in the suspension system with the optimized concentration(0.4 wt%) of GNPs was more significant than that in the traditional SDS solution. Furthermore,compared with those of the system without GNPs, enhanced hydration rate and storage capacity were achieved in the suspensions with GNPs, and the storage capacities were increased by 3.9%–17.0%. The promotion effect of GNPs on gas hydrate formation at low pressure is much more obvious than that at high pressure.     

An improved theoretical procedure for the pore-size analysis of activated carbon by gas adsorption

Amorphous carbon materials play a vital role in adsorbed natural gas (ANG) storage.One of the key issues in themore prevalent use of ANG is the limited adsorption capacity,which is primarily determined by the porosity and surface characteristics of porous materials.To identify suitable adsorbents,we need a reliable computational tool for pore characterization and,subsequently,quantitative prediction of the adsorption behavior.Within the framework of adsorption integral equation (AIE),the pore-size distribution (PSD) is sensitive to the adopted theoretical models and numerical algorithms through isotherm fitting.In recent years,the classical density functional theory (DFT) has emerged as a common choice to describe adsorption isotherms for AIE kernel construction.However,rarely considered is the accuracy of the mean-field approximation (MFA) commonly used in commercial software.In this work,we calibrate four versions of DFT methods with grand canonical Monte Carlo (GCMC) molecular simulation for the adsorption of CH4 and CO2 gas in slit pores at 298 K with the pore width varying from 0.65 to 5.00 nm and pressure from 0.2 to 2.0 MPa.It is found that a weighted-density approximation proposed by Yu (WDA-Yu) is more accurate than MFA and other non-local DFT methods.In combination with the trapezoid discretization of AIE,the WDA-Yu method provides a faithful representation of experimental data,with the accuracy and stability improved by 90.0％ and 91.2％,respectively,in comparison with the corresponding results from MFA for fitting CO2 isotherms.In particular,those distributions in the feature pore width range (FPWR)are proved more representative for the pore-size analysis.The new theoretical procedure for pore characterization has also been tested with the methane adsorption capacity in seven activated carbon samples.     

多级孔金属有机骨架材料的合成及其在生物医药中的应用研究进展

金属有机骨架（metal-organic frameworks,MOFs）是多孔材料领域的研究热点之一。MOFs具有高比表面积和孔道均一等特点,但微孔MOFs在大分子应用领域受到限制。本文介绍了延长配体法、模板剂法和聚合物法等多种制备多级孔MOFs的方法,合成后的多级孔MOFs兼具微孔、介孔和大孔,能够参与大分子反应,同时具有水热稳定性和化学稳定性,在催化、气体吸附分离、储能材料等诸多领域表现出优异性能。本文重点介绍了多级孔MOFs在生物医药领域的研究进展,结果表明多级孔MOFs是一种孔道可调节、可在特定条件下分解的生物相容性材料,用于固定化酶和负载医药分子均表现出良好性能。最后讨论了多级孔MOFs材料制备和应用目前存在的问题与挑战,展望了多级孔MOFs材料作为一类新型功能化多孔材料的应用前景。     

Tuning the topology and functionality of metal-organic frameworks by ligand design

Metal-organic frameworks (MOFs)-highly crystalline hybrid materials that combine metal ions with rigid organic ligands-have emerged as an important class of porous materials. The organic ligands add flexibility and diversity to the chemical structures and functions of these materials. In this Account, we summarize our laboratory's experience in tuning the topology and functionality of MOFs by ligand design. These investigations have led to new materials with interesting properties. By using a ligand that can adopt different symmetry conformations through free internal bond rotation, we have obtained two MOFs that are supramolecular stereoisomers of each other at different reaction temperatures. In another case, where the dimerized ligands function as a D(3)-Piedfort unit spacer, we achieve chiral (10,3)-a networks. In the design of MOF-based materials for hydrogen and methane storage, we focused on increasing the gas affinity of frameworks by using ligands with different geometries to control the pore size and effectively introduce unsaturated metal centers (UMCs) into the framework. Framework interpenetration in PCN-6 (PCN stands for porous coordination network) can lead to higher hydrogen uptake. Because of the proper alignment of the UMCs, PCN-12 holds the record for uptake of hydrogen at 77 K/760 Torr. In the case of methane storage, PCN-14 with anthracene-derived ligand achieves breakthrough storage capacity, at a level 28% higher than the U.S. Department of Energy target. Selective gas adsorption requires a pore size comparable to that of the target gas molecules; therefore, we use bulky ligands and network interpenetration to reduce the pore size. In addition, with the help of an amphiphilic ligand, we were able to use temperature to continuously change pore size in a 2D layer MOF. Adding charge to an organic ligand can also stabilize frameworks. By ionizing the amine group within mesoMOF-1, the resulting electronic repulsion keeps the network from collapsing, giving rise to the first case of mesoporous MOF that demonstrates the type IV isotherm. We use dendritic hexacarboxylate ligands to synthesize an isoreticular series of MOFs with (3,24)-connected network topology. The cuboctahedral cages serve as building blocks that narrow the opening of the mesocavities into microwindows and stabilize these MOFs. The resulting materials have exceptionally high surface areas and hydrogen uptake capacities. Despite the many achievements in MOF development, there is still ample opportunity for further exploration. We will be continuing our efforts and look forward to contributing to this blossoming field in the next decade.     

Classical density functional theory for methane adsorption in metal‐organic framework materials

Natural gas is considered as a promising alternative to petroleum as the next generation of primary transportation fuel owing to relatively smaller carbon footprint and lower SO_x/NO_x emissions and to fast developments of shale gas in recent years. Since the volumetric energy density of methane amounts to only about 1% of that of gasoline at ambient conditions, natural gas storage represents one of the key challenges for prevalent deployment of natural gas vehicles. In this work, we present a molecular thermodynamic model potentially useful for high‐throughput screening of nanoporous materials for natural gas storage. We investigate methane adsorption in a large library of metal‐organic frameworks (MOFs) using four versions of classical density functional theory (DFT) and calibrate the theoretical predictions with extensive simulation data for total gas uptake and delivery capacity. In combination with an extended excess entropy scaling method, the classical DFT is also used to predict the self‐diffusion coefficients of the confined gas in several top‐ranked MOFs. The molecular thermodynamic model has been used to identify promising MOF materials and possible variations of operation parameters to meet the Advanced Research Projects Agency‐Energy (ARPA‐E) target set by the U.S. Department of Energy for natural gas storage. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3012–3021, 2015     

面向生物甲烷分离的不同金属配位金属-有机骨架材料的分子设计

采用巨正则Monte Carlo法（GCMC）,对CH₄ /CO₂混合气体体系基于金属-有机骨架材料（MOFs）的吸附分离进行了模拟研究。吸附分离材料涉及3个系列（M-MOF-74、M-MIL-53和[M（atz）（bdc）_0.5]）（M=Mg,Co,Ni,Zn,Al,Cr）不同金属配位的8种MOF材料。研究表明,Mg-MOF-74的CO₂吸附性能在高压下优于其他材料;在低压时,拥有大量氨基官能团的[Zn（atz）（bdc）_0.5]和[Co（atz）（bdc）_0.5]材料有更高效的CO₂分离性能。通过径向分布函数和CO₂吸附构型快照重叠图进一步分析发现,各个系列材料不同金属配位对CO₂吸附构型的影响造成了材料吸附分离性能有较大的不同。研究结果能够为实验上设计和开发新型高效CO₂和CH₄吸附分离MOFs材料提供启发。     

Metal Organic Frameworks for Natural Gas Storage in Vehicles

With the increasing demand for alternative fuels the storage of natural gas (NG) in adsorbents like metal organic frameworks (MOFs) will become more important. In order to use MOFs as storage media in fuel delivery systems, the optimization of mass and energy transfer of the system is crucial. For rapid NG filling of a tank, molecules need to reach the adsorption sites within a reasonable time while the heat of adsorption should be dissipated to the environment. In this article, mass transfer in shaped bodies of MOFs was determined by permeability measurements and pulsed field gradient (PFG) NMR spectroscopy. The heat dissipation was also experimentally measured and both data sets were used to set up a theoretical density function theory model to predict the behavior of MOFs for NG storage.     

煤层气回收及CH4/N2分离PSA材料的研究进展

我国煤层气蕴藏丰富,在面临能源危机时代煤层气可作为天然气能源的有效补充。本文介绍了低浓煤层气回收即CH₄/N₂分离几种常见技术：低温技术、水合物技术、溶解技术、膜分离和变压吸附技术（PSA）的分离原理、技术开发和研究的现状,并分析了各项技术目前存在的问题。讨论了多孔材料,如活性炭、碳分子筛、沸石分子筛和新型金属有机骨架材料（MOFs）等对CH₄/N₂吸附分离效果的研究进展,由于MOFs材料的吸附性能随温度或压力的改变出现飞跃,预示了其在PSA领域广阔的应用前景。     

缺陷UiO-66对水中塑化剂的吸附

金属有机骨架材料（metal-organic frameworks,MOFs）是一种具有骨架结构的新型多孔材料。本研究合成了金属有机框架化合物UiO-66和有缺陷的UiO-66（UiO-66-1）,并研究了其作为吸附剂吸附塑化剂邻苯二甲酸二甲酯（DMP）的性能及其吸附动力学,最后通过拟合吸附等温线和吸附动力学模型拟合了吸附剂的最大吸附量和最快吸附时间。实验结果表明,UiO-66-1对DMP有更好的吸附性能,在5~10min内快速达到吸附平衡,pH在3~10吸附率仍可保持稳定;分析测试结果表明,引入缺陷的UiO-66-1的比表面积比UiO-66大,达到了1438m²/g,孔容为0.58cm³/g,晶体的尺寸也明显增大,吸附量增大了近一倍,最大吸附量可达到404mg/g,且循环利用率高;通过吸附等温线模型和吸附动力学模型的拟合研究表明,UiO-66-1的吸附过程比较符合Langmuir模型和拟二级动力学模型。     

金属有机骨架材料用于吸附分离CH4和N2的研究进展

非常规天然气的利用不仅可以有效缓解常规天然气不足带来的能源问题,而且可以降低其肆意排放带来的温室效应,无论是低浓度煤层气的提浓还是低品质天然气的提质都需要解决甲烷与氮气的分离难题。基于金属有机骨架（MOFs）材料结构和功能均呈多样化的特色,本文主要从CH₄选择型MOFs吸附材料和N₂选择型MOFs吸附材料两个方面,综述了近年来MOFs材料在CH₄与N₂吸附分离方面的研究进展,讨论了影响二者分离的影响因素,并对吸附与分离机理与MOFs结构和性能关联进行了详细的总结与分析,提出了CH₄与N₂选择性提升的方法,即需要合适的孔道尺寸与弱极性表面性质或有利骨架结构的协同作用,最后展望了MOFs材料在甲烷富集和纯化领域的应用前景和发展趋势。     

Renaissance of the Methane Adsorbents

Clean vehicles empowered by natural gas is an immediate solution to meet the demands of the cut off of CO₂ emission, energy crisis, and air-pollution control. To boost the on-board storage capacity, safety and efficiency, great efforts have been made in developing advanced porous materials as methane adsorbents to store more gas under lower pressure or in a more compact space so-called adsorbed natural gas (ANG) technique. A blooming of methane adsorbents and the competition of methane storage records have been therefore awakened by the reticular chemistry of metal-organic frameworks (MOFs), through which the adsorption sites, functionalities, and nano-confinement space can be then controlled and manipulated. In this personal account, we walk through the research paradigm shifts in MOFs for on board methane storage. Furthermore, inspiration and considerations are also discussed, which can be further transferred into the discovery of other methane adsorbents.