MOF-5上甲醇、乙醛和丙酮吸附机理的密度泛函理论研究

采用密度泛函理论对甲醇、乙醛和丙酮3种VOCs气体在MOF-5中的吸附机理进行了系统的理论研究,结果表明:3种气体分子在MOF-5中的吸附位置主要有4种,分别为大胞腔中的金属簇Zn₄O、苯环BDC位置和小胞腔中的金属簇Zn₄O、苯环BDC位置,其中在金属簇Zn₄O位置吸附能皆为最大。对比甲醇、乙醛和丙酮的结构特性和在MOF-5中的吸附能,结果预测:在有机配体苯环中引入吸电子基团有利于甲醇这类含有羟基基团的有机分子在MOF-5上的吸附,而引入给电子基团或更多含氧官能团,则有利于增强乙醛和丙酮这类含有羰基基团的有机分子与MOF-5的吸附作用。     

温度对MOF-74(Ni)吸附分离丙烯丙烷机理和选择性的影响

采用水热法成功制备了MOF-74(Ni),使用PXRD、孔径分析对材料进行了表征,测定了材料在不同温度下的C₃H₆和C₃H₈吸附等温线,应用程序升温脱附技术估算了脱附活化能,并使用IAST理论预测了材料对C₃H₆/C₃H₈二元体系的吸附选择性。讨论了温度对吸附机理和吸附选择性的影响。结果显示,MOF-74(Ni)的BET比表面积高达1306 m²·g^-1。在298 K下,C₃H₆的吸附量高达7.4 mmol·g^-1。随着温度升高,C₃H₈的吸附量大幅降低,而C₃H₆的吸附量下降程度较小,导致材料对C₃H₆/C₃H₈吸附选择性升高。当温度为328K时,MOF-74(Ni)对C₃H₆/C₃H₈二元气体混合物的吸附选择接近12。程序升温脱附的实验结果显示,C₃H₆在MOF-74(Ni)上的脱附活化能大于C₃H₈,分别为68.92 kJ·mol^-1和50.80 kJ·mol^-1。C₃H₆是通过与MOF-74(Ni)的不饱和金属位点Ni²⁺以p络合作用方式吸附,作用力较强,而C₃H₈与Ni²⁺之间的作用力较弱。根据吸附机理不同的特点,适当提高温度,将有助于提高MOF-74(Ni)吸附分离C₃H₆/C₃H₈混合物体系的吸附选择性。     

A Computational Study of the Adsorptive Removal of H<Subscript>2</Subscript>S by MOF-199

Metal–organic framework material MOF-199 is a new type of adsorption material for removal toxic H₂S. In this work, the effects of temperature and pressure on the performance of H₂S adsorption in MOF-199 were studied by using the grand canonical Monte Carlo (GCMC) simulation; the interaction mechanism between framework atoms of MOF-199 and guest H₂S molecules were further discussed through density functional theory (DFT) calculations. It is found that the MOF-199 adsorption capacity towards H₂S decreases with increasing temperature and increases with increasing pressure. At low pressures, the frameworks containing the binding sites of copper dimers and trimesic acid are the main factor affecting the adsorption performance of MOF-199. While at high pressures, the free volume of MOF-199 contributes to the adsorption capacity as well. The adsorptive interactions between H₂S and the organic ligand are weak (>??14.469 kJ/mol). When H₂S adsorption on the Cu–Cu bridge, the binding energies of the modes where hydrogen is put inward of the copper dimer are generally smaller than that where hydrogen is outward, whereas the adsorption on the top of copper ion shows the smallest BEs value (<??50 kJ/mol) due to its tendency of forming a saturated six-coordinated configuration.     

氢气在共价有机骨架材料中的吸附机理

共价有机骨架材料(COFs)是一种新型的储氢材料。采用密度泛函理论研究了COFs的7种主要团簇对H₂的吸附机理,计算中选用GGA/PBE泛函。结果表明,当H₂垂直接近时,团簇中心苯环为最优先吸附位,当H₂平行接近时,硼氧环的中心以及氧原子上为最优先的吸附位。有机链团簇C原子数目的增加有利于对氢气的吸附,多个H₂分子在COF-10和COF-105团簇的吸附主要属于协同吸附。对COFs进行碱金属(Li、Na、K)掺杂和官能团替代(甲基-CH₃、巯基-SH、氨基-NH₂)改性,发现H₂吸附能得到显著提高,说明改性是提高COFs材料H₂吸附能力的有效途径。     

锆基金属有机骨架材料用于氨吸附性能的研究

近年来,金属有机骨架材料（MOF）在气体吸附和储存领域得到了迅速发展,但由于结构的不稳定性,其在强腐蚀性气体氨（NH₃）的吸附方面并不令人满意。考虑到NH₃是唯一的无碳排放的氢能源载体,开发高效的储氨技术来载氢是有效的降低二氧化碳排放的手段。利用MOF材料具有的高比表面积和结构多样的特性,在NH₃的吸附和储存方面具有广阔的应用前景。而NH₃具有孤对电子,会攻击金属与配体之间形成的配位键,使MOF材料的结构遭到破坏。锆基金属有机骨架材料是公认结构稳定性较好的MOF材料,但其是否能胜任干燥NH₃及含水条件下的稳定性仍未深入考察,由此需探究该系列材料在NH₃吸附领域的适用性。在此,通过实验和计算模拟研究锆基系列的金属有机骨架UiO-66、NU-1000、MOF-801和 MOF-808的结构特征、稳定性和NH₃吸附性能。结果表明,UiO-66、NU-1000和MOF-808在纯NH₃环境下的稳定性较好,并且显示出高吸附量且可循环的氨吸附性能（13.04、6.38、9.65 mmol/g）。受限于水和氨对结构的协同破坏作用,NU-1000和MOF-801的结构均不能维持,而UiO-66和MOF-808的结构非常稳定,无论在干燥NH₃环境及含水NH₃环境下均能胜任而应用于NH₃吸附和储存。     

M-MOF-74吸附分离H2/He混合物的分子模拟研究

为了探究有机金属框架MOF-74能否作为一种优良的固体吸附剂,分离H₂/He混合物中H₂,并达到提纯He的目的,采用分子模拟的手段研究了H₂、He及H₂/He混合物在M-MOF-74（M=Mg、Co、Ni、Cu、Zn）上的吸附性能及吸附机理。结果表明,在1 bar（1 bar=10⁵ Pa）压力和25℃条件下,纯H₂对纯He在Ni-MOF-74上的选择性达6.58,而Mg-MOF-74对H₂的吸附量最大,其值为0.19 mmol·cm^-3,为He吸附量的6.46倍。当H₂/He混合物的浓度发生变化时,对其在M-MOF-74上的吸附分离因子没有较大影响,说明浓度变化不会影响M-MOF-74上吸附位点容纳H₂和He的能力。吸附位点和吸附热分析表明,MOF-74上的金属离子未饱和位点能够显著增强其对H₂的吸附能力。其结果对判断M-MOF-74是否具有分离H₂/He混合物的潜力,以及定量分析MOFs金属未饱和位点对H₂/He混合物分离的贡献提供了一定的理论基础。     

M-MOF-74吸附分离H2/He混合物的分子模拟研究

为了探究有机金属框架MOF-74能否作为一种优良的固体吸附剂,分离H₂/He混合物中H₂,并达到提纯He的目的,采用分子模拟的手段研究了H₂、He及H₂/He混合物在M-MOF-74（M=Mg、Co、Ni、Cu、Zn）上的吸附性能及吸附机理。结果表明,在1 bar（1 bar=10⁵ Pa）压力和25℃条件下,纯H₂对纯He在Ni-MOF-74上的选择性达6.58,而Mg-MOF-74对H₂的吸附量最大,其值为0.19 mmol·cm^-3,为He吸附量的6.46倍。当H₂/He混合物的浓度发生变化时,对其在M-MOF-74上的吸附分离因子没有较大影响,说明浓度变化不会影响M-MOF-74上吸附位点容纳H₂和He的能力。吸附位点和吸附热分析表明,MOF-74上的金属离子未饱和位点能够显著增强其对H₂的吸附能力。其结果对判断M-MOF-74是否具有分离H₂/He混合物的潜力,以及定量分析MOFs金属未饱和位点对H₂/He混合物分离的贡献提供了一定的理论基础。     

水处理提高MOF-199催化苯羟基化反应性能

以一水合乙酸铜和均苯三酸为原料,采用常温搅拌法合成金属有机框架化合物MOF-199,用XRD、IR、BET对其表征.将MOF-199用水处理后作为催化剂用于苯羟基化反应,考察了催化剂的用量、反应温度、反应时间、双氧水添加量对催化反应的影响.较优反应条件为:MOF-199(140mg,0.23mmol)、乙腈4mL、苯1.1mL、H₂O₂3.3mL,在60℃的水浴中反应30min,所得苯的氧化物的产率为23.3%,苯酚的选择性为53.1%,转化频率(TOF值)为25.0h^-1.水处理显著提高了MOF-199的催化活性,并形成了一种新的氧化模式,在一定程度上保护了MOF-199的框架结构.     

Mechanism of benzene diffusion in MOF-5: A molecular dynamics investigation

In this contribution, the diffusion of benzene in the porous metal organic framework MOF-5 is investigated by molecular dynamics simulations. Previously, we have shown that by using a first principles derived fully flexible force field the experimentally determined self-diffusion coefficients D_self could be well reproduced [S. Amirjalayer, M. Tafipolsky, R. Schmid, Angew. Chem. Int. Ed. 46 (2007) 463]. Here, we use the same methodology to determine the loading dependence on the diffusion. It is found that diffusivity, which is in the range of liquid benzene, slightly increases up to a load of 32 molecules per unit cell and then falls off at higher load. Free energy maps reveal that additional sites appear at higher load due to attractive guest–guest interactions. The topology of these sites is very close to the experimentally determined locations of ferrocene molecules in MOF-5, which corroborates that attractive π–π interactions govern these systems. The site–site and site-phenylene distances are very similar to the first solvation radius of liquid benzene. For the very open MOF-5, the main barrier for diffusive transport is to overcome the attractive interaction in the binding pockets, which is in contrast to zeolitic microporous systems, where the barrier for diffusion is the hindrance of the pore window. Spatial free energy maps are used to investigate the diffusion pathway on a molecular level and the load dependence of the free energy barriers for these transport processes.     

Cluster model studies on oxygen sites at the (010) surfaces of V2O5 and MoO3

Cluster model studies have been performed to examine the electronic structure and adsorption properties near structurally different oxygen sites at the (010) surfaces of vanadium pentoxide, V₂O₅, and molybdenum trioxide, MoO₃. In addition, adsorption of hydrogen at the oxygen sites and desorption of OH groups has been studied in order to find site specific differences. The electronic properties and total energies of the clusters have been obtained from ab initio density functional theory (DFT) calculations. The surface oxygen sites are found to be ionic where bridging oxygens become more negative than terminal ones. Further, hydrogen adsorbs at all oxygen sites where binding is strongest at the bridge sites on the V₂O₅(010) surface whereas on MoO₃(010) the terminal sites are preferred. The latter difference can be understood by simple geometric arguments. Surface OH groups formed by H adsorption and involving terminal oxygens are strongly bound to the surface whereas those involving bridging oxygens are mobile and become available for subsequent reactions.     

A comparison of hydrogen storage in activated carbons and a metal-organic framework (MOF-5)

A hydrogen storage study, on gravimetric and volumetric bases, has been carried out on three adsorbents having similar adsorption capacities: two activated carbons and a metal organic framework (MOF-5). These three samples have similar hydrogen storage on a gravimetric basis. However, expressed on a volumetric basis, both AC have higher hydrogen storage than MOF-5. Wrong conclusions could be avoided, when comparing these two types of adsorbents, if their hydrogen storage data were reported on a volumetric basis, using reliable measurable densities. Tap or packing densities are the most suitable. Crystal density, mostly used for reporting hydrogen storage on MOF, gives unrealistically high volumetric storage values.     

Hydrogen adsorption on metal-organic framework (MOF-5) synthesized by DMF approach

Metal-organic frameworks (MOFs), especially MOF-5, are believed to be promising new porous materials for hydrogen adsorption. A comparative study of material synthesis, characterization and hydrogen adsorption was performed to examine the effects of different synthesis conditions on crystal structure, pore textural property and hydrogen adsorption performance of MOF-5 materials. Three MOF-5 samples synthesized with dimethyl formamide (DFM) as solvent and slightly different procedures have shown similar phase structure and chemical composition, diverse crystal structures, varying pore textural properties and different hydrogen adsorption performance. It was established from the experimental results that higher order of crystallinity in the MOF-5 materials generates better adsorbents with larger crystal size, higher specific surface area, uniform pore size distribution (PSD), larger hydrogen adsorption capacity and faster hydrogen diffusion rate in MOF-5 adsorbents. The best MOF-5 sample synthesized in this work (MOF-5(γ)) has a Langmuir specific surface area of 1157 m²/g; it can adsorb 0.5 wt.% of hydrogen at 77 K and 800 mmHg; and results in hydrogen diffusivity inside MOF-5 crystal of 2.3 × 10⁻⁹ cm²/s. The density functional theory reasonably predicts the presence of mesopores and macropores in all three MOF-5 samples synthesized in this work.     

CO/N2/CO2在MOF-74(Ni)上吸附相平衡和选择性

主要研究了MOF-74(Ni)材料对CO/N₂/CO₂的吸附分离性能。应用水热法合成制备MOF-74(Ni),分别采用全自动表面积吸附仪、P-XRD、扫描电子显微镜对材料的孔隙结构和晶体形貌进行了表征,应用静态吸附法测定了CO、N₂和CO₂在MOF-74(Ni)上的吸附等温线,应用DSLF方程模拟了3种气体MOF-74(Ni)上的吸附等温线,依据IAST理论模型计算了MOF-74(Ni)对CO/N₂二元混合物和CO/CO₂二元混合物的吸附选择性。研究结果表明:在0.1 MPa和常温条件下,MOF-74(Ni)材料对CO吸附容量高达6.15 mmol·g^-1,而相同条件下N₂的吸附量只有0.86 mmol·g^-1。MOF-74(Ni)在低压下(0～40 kPa)对CO的吸附量明显高于其对CO₂的吸附量。应用IAST模型估算MOF-74(Ni)对二元混合物吸附选择性的结果表明:MOF-74(Ni)对CO/N₂混合物的吸附选择性在1000以上;MOF-74(Ni)对CO/CO₂的吸附选择性在4～9范围,在所研究的二元气体混合物吸附体系中,MOF-74(Ni)都能优先吸附CO。     

CH4/C2H6/C3H8在MOF-505@5GO上吸附相平衡和选择性

应用溶剂热法合成了不同氧化石墨烯（GO）负载量的MOF-505@GO复合材料,分别采用全自动表面积吸附仪、P-XRD、SEM和Raman对材料进行了性能表征,测定了CH₄、C₂H₆和C₃H₈在MOF-505@GO上的吸附等温线,并进行Langmuir-Freundlich方程拟合,依据IAST理论模型计算了C₂H₆/CH₄和C₃H₈/CH₄二元混合气在MOF-505@5GO上的吸附选择性。研究结果表明,随着GO负载量增大,MOF-505@GO复合材料的孔容及BET比表面积先增大后减小,当GO负载量为5%（质量）时,复合材料MOF-505@5GO的孔容及BET比表面积达到最大,当GO负载量进一步增大至8%（质量）和10%（质量）时,复合材料的孔容及BET比表面积逐渐降低。在0.1 MPa和298 K条件下,MOF-505@5GO对CH₄、C₂H₆和C₃H₈的吸附容量分别为0.88、4.81和5.17 mmol·g^-1,相比MOF-505分别提高了14.9%、30.7%和13.1%。MOF-505@5GO对C₂H₆/CH₄和C₃H₈/CH₄的吸附选择性分别为40.1和3056.1,其对C₂H₆/CH₄和C₃H₈/CH₄具有极高的吸附选择性。     

Probing the Structure, Stability and Hydrogen Adsorption of Lithium Functionalized Isoreticular MOF-5 (Fe, Cu, Co, Ni and Zn) by Density Functional Theory

Li adsorption on isoreticular MOFs with metal Fe, Cu, Co, Ni and Zn was studied using density function theory. Li functionalization shows a considerable structural change associated with a volume change in isoreticular MOF-5 except for the Zn metal center. Hydrogen binding energies on Li functionalized MOFs are seen to be in the range of 0.2 eV, which is the desired value for an ideal reversible storage system. This study has clearly shown that Li doping is possible only in Zn-based MOF-5, which would be better candidate to reversibly store hydrogen.     

功能型离子液体协同吸收NH3和CO2的密度泛函理论研究

离子液体（ILs）由于其独特的结构可调性,作为添加剂可有效抑制氨法碳捕集中NH₃的逃逸并同时促进CO₂的吸收。揭示其吸收NH₃和CO₂的作用机理对于构建特定的功能型ILs结构具有重要意义。本文采用密度泛函理论（DFT）,在B3LYP/6-31'++G（d,p）基组水平下对设计的五种功能型ILs进行了结构优化、频率计算以及原子电荷分析,获得了优化后的结构参数、振动频率以及原子电荷等数据。在此基础上对ILs吸收CO₂和NH₃进行了相互作用分析。计算结果表明：[HEBim][His]的稳定性最好,经过BSSE校正后的相互作用能为-415.73 kJ·mol^-1。通过静电势和电荷分析找到了设计的ILs与气体作用的最佳位点：NH₃主要与ILs阳离子的羟基形成 O—H…N型氢键,其中,[HEBim][His]吸收NH₃的能力最强,形成的氢键结合能为38.52 kJ·mol^-1,具有较强的氢键作用;CO₂主要与阴离子中的氨基形成C—N…C型氢键,[HEBim][Ala]吸收CO₂的能力最强,形成的氢键结合能为10.15 kJ·mol^-1,具有较弱的氢键作用。当ILs同时与NH₃和CO₂相互作用时,其吸收能力均有不同程度的下降,[HEBim][His]与[HEBim][Ala]的综合吸收效果最佳。     

功能型离子液体协同吸收NH3和CO2的密度泛函理论研究

离子液体（ILs）由于其独特的结构可调性,作为添加剂可有效抑制氨法碳捕集中NH₃的逃逸并同时促进CO₂的吸收。揭示其吸收NH₃和CO₂的作用机理对于构建特定的功能型ILs结构具有重要意义。本文采用密度泛函理论（DFT）,在B3LYP/6-31'++G（d,p）基组水平下对设计的五种功能型ILs进行了结构优化、频率计算以及原子电荷分析,获得了优化后的结构参数、振动频率以及原子电荷等数据。在此基础上对ILs吸收CO₂和NH₃进行了相互作用分析。计算结果表明：[HEBim][His]的稳定性最好,经过BSSE校正后的相互作用能为-415.73 kJ·mol^-1。通过静电势和电荷分析找到了设计的ILs与气体作用的最佳位点：NH₃主要与ILs阳离子的羟基形成 O—H…N型氢键,其中,[HEBim][His]吸收NH₃的能力最强,形成的氢键结合能为38.52 kJ·mol^-1,具有较强的氢键作用;CO₂主要与阴离子中的氨基形成C—N…C型氢键,[HEBim][Ala]吸收CO₂的能力最强,形成的氢键结合能为10.15 kJ·mol^-1,具有较弱的氢键作用。当ILs同时与NH₃和CO₂相互作用时,其吸收能力均有不同程度的下降,[HEBim][His]与[HEBim][Ala]的综合吸收效果最佳。     

基于分子模拟的低温煤焦油中酚类化合物的溶解特性

低温煤焦油中酚类化合物溶解行为及机理研究是其高效分离的基础。本文以焦油中的典型组分为研究对象，借助混合能参数，模拟计算各组分与酚类化合物的相溶性；在焦油模型中，借助相互作用能、径向分布函数计算及电子密度分析，探究酚类化合物的溶解机理。结果表明：①焦油中各组分物质苯环上取代基基团、苯环数量等结构特征是影响酚类化合物在焦油中溶解行为的关键；②酚类化合物在各组分中的溶解行为受到缔合作用的影响，缔合作用越大、空间位阻效应越小，酚类化合物与其他组分相溶性越强；③模型焦油中，苯酚与苯、苯并噻吩之间缔合作用力均为π-π堆积作用力，苯酚与正己烷、吲哚之间缔合作用力分别为CH-π堆积作用力和N—H···O氢键。     

负载型钯团簇上氢分子吸附和解离吸附的密度泛函理论

基于密度泛函理论考察勃姆石（AlOOH）和氧化锆（ZrO₂）负载的钯团簇表面氢分子吸附和解离吸附过程，通过分析一系列不同尺寸的钯团簇上氢吸附能以及氢分子-钯团簇-载体三者之间的电荷转移系统地研究了粒径效应和载体效应。结果表明，顶点位和面位分别是分子吸附和解离吸附的最佳吸附位置。随着钯团簇粒径的上升，氢分子吸附能和解离吸附能都下降，载体-钯团簇-被吸附氢三者之间的电荷转移量和方向也改变。氧化锆载体负载的钯团簇上分子吸附能和解离吸附能均大于勃姆石载体负载的钯团簇，说明载体对氢分子吸附过程有重要影响。     

银基活性炭对废轮胎热解油的吸附脱硫机理研究

废轮胎经热解制备得到热解油和热解炭,热解炭活化制得活性炭,并利用Ag⁺对活性炭进行改性制得Ag⁺改性活性炭（Ag/AC）,将Ag/AC用于热解油的吸附脱硫实验,并利用GC/MS对热解油中的含硫化合物进行了分析。研究结果表明：活性炭吸附脱硫的合适温度和时间分别为20℃和12 h,此时未改性活性炭的脱硫率为15.33%;而Ag/AC的脱硫率提高到了38.6%。GC/MS分析发现热解油中有机硫的主要存在形式为噻吩、2-甲基噻吩、苯并噻吩、二苯并噻吩和4,6-二甲基二苯并噻吩,其中二苯并噻吩（DBT）的GC含量最高,为2.57%。利用原位红外、核磁共振氢谱、ICP-OES和元素分析等检测手段,进一步探究了Ag⁺与二苯并噻吩模型化合物在溶液中的反应机理,研究发现：二苯并噻吩分子上存在S原子和苯环2个反应位点,当Ag⁺加入二苯并噻吩溶液后,Ag⁺与二苯并噻吩分子上的S原子或者苯环发生配位数为1的配位反应生成2种配合物,分子式分别为Ag（D_S）NO₃和Ag（D_C6H6）NO₃。