Heterogeneity of Adsorption Sites and Adsorption Kinetics of n-Hexane on Metal-Organic Framework MIL-101(Cr)

abstract The heterogeneity of adsorption sites and adsorption kinetics of n-hexane on a chromium terephthalate-based metal-organic framework MIL-101(Cr) were studied by gravimetric method and temperatu...     

乙醇在MIL-101上的吸附相平衡及其吸附机理

主要研究了MIL-101材料对乙醇的吸附性能和吸附机理。采用水热合成法制备了MIL-101(Cr),并分别应用N₂静态吸附、X射线粉末衍射(PXRD)、傅里叶红外光谱(FTIR)等分析手段对MIL-101晶形结构、孔隙结构参数进行分析表征。应用静态吸附法测定乙醇和水蒸气在不同温度下的吸附等温线,并讨论乙醇吸附在MIL-101(Cr) 4种吸附位的机理,根据吸附等温线估算出乙醇和水蒸气在MIL-101上的等量吸附热,并测试了乙醇在MIL-101上的吸附循环性能。研究表明,在298 K下,MIL-101的乙醇吸附容量为20.3 mmol·g^-1,远高于传统吸附材料。在低压下MIL-101对乙醇的吸附量高于水蒸气的吸附量,这是由于乙醇的偶极矩和分子动力学直径均比水大,使得乙醇分子在孔道中受到更大吸附力场作用;在低吸附量范围,乙醇在MIL-101上的等量吸附热要高于水蒸气的等量吸附热。在较高吸附压力条件下,主要发生多层吸附或孔填充,受吸附剂的孔容限制效应,尺寸越大的分子被吸附的物质的量会越少,由于乙醇的动力学直径(0.45 nm)大于水分子的动力学直径(0.268 nm),所以在较高吸附压力下乙醇在MIL-101上吸附量要小于水蒸气的吸附量。多次吸附脱附等温线测试显示MIL-101具有良好的乙醇吸附循环性能。     

乙二胺改性金属有机骨架材料MIL-101(Cr)常压下吸附CO2

采用溶剂热法将乙二胺接枝到金属有机骨架材料MIL-101(Cr)上,用于常压下CO2的吸附,研究了乙二胺接枝量及温度对材料结构、形貌和CO2吸附性能的影响. 结果表明,乙二胺改性的MIL-101材料在常温常压下对CO2的吸附量可达2.43 mmol/g,比改性前提高14.6%,CO2/N2的吸附分离系数从11提高至17,比改性前提高55.6%. 改性后材料经80℃真空加热可完全脱附再生,具有很好的再生稳定性.     

氨基化MIL-101表面印迹聚合物的制备及其吸附行为

研究了金属有机骨架材料MIL-101表面印迹聚合物的制备方法及其吸附行为。以MIL-101为载体,先通过化学修饰氨基制备了ED-MIL-101材料,再以京尼平苷为模板,甲基丙烯酸为功能单体,二乙烯基苯为交联剂,表面接枝京尼平苷分子印迹聚合物制备了MIPs@MIL-101印迹聚合物。通过傅里叶红外光谱（FTIR）、X射线衍射光谱（XRD）、扫描电镜（SEM）对聚合物进行结构表征,测试了聚合物的等温吸附及吸附动力学性能,探讨了聚合物的固相萃取性能。红外光谱及XRD衍射分析表明了MIL-101氨基化修饰及表面接枝复合材料的成功制备。吸附动力学研究表明当分子印迹聚合物用于吸附京尼平苷时,可在270min内达到吸附平衡。当温度为298K、308K、318K、328K时,印迹聚合物对模板的吸附量分别为55.94mg/g、46.16mg/g、38.98mg/g、31.47mg/g。吸附热ΔH为26.997kJ/mol。分子印迹固相萃取杜仲提取物中的京尼平苷时,总回收率达95.0%。     

Selective Liquid Phase Adsorption and Separation of ortho-Xylene with the Microporous MIL-53(Al)

Metal-organic framework MIL-53(Al) pellets were tested for the selective adsorption and separation of xylene isomers, in liquid phase and using n-heptane as eluent. The objective of this work is to assess relevant data for a posterior xylene isomers separation process design. In order to complete this study, single and multi-component breakthrough experiments were performed at 313 K, in the presence of n-heptane. MIL-53(Al) presented a preference for o-xylene over m-xylene and p-xylene in all experiments. The selectivities of 2.0 were obtained for o-xylene over m-xylene and over p-xylene. It is concluded that MIL-53(Al) may be used for separating o-xylene from the other xylene isomers, using n-heptane as desorbent. Supplemental materials are available for this article. Go to the publisher's online edition of Separation Science & Technology to view the free supplemental file.     

MIL-101(Cr)开式吸附性能实验

采用水热合成法制备水热稳定金属有机骨架MIL-101(Cr),基于太阳能吸附式空气取水选取不同的实验工况,将MIL-101(Cr)、细孔硅胶作为研究对象,相对湿度控制在50%、温度范围5～45℃条件下,测试并对比了MIL-101(Cr)与细孔硅胶的吸附性能。实验表明,35℃、50%RH条件下,吸附过程进行1000min,MIL-101(Cr)水吸附量为22.05g/100g,其吸附量相比细孔硅胶提高93%左右;当系统平衡时,MIL-101(Cr)有效平均吸附速率相比细孔硅胶提高120%左右。此外,在相对湿度（RH）50%、温度范围5～45℃条件下,MIL-101(Cr)的平衡吸附量在11.40～23.47g/100g之间。在所控温度下,MIL-101(Cr)在25℃时平衡吸附量最大,在5℃时平衡吸附量最小,25℃时MIL-101(Cr)的平衡吸附量相比5℃时提高106%左右。该实验可以为四季工况不同温度下MIL-101(Cr)用于太阳能吸附式空气取水提供基础数据。     

金属骨架材料MIL-53(Cr)合成及其吸附效果研究

采用新配比(n(Cr3+):n(H2BDC):n(H2O)=1:2:280),水热法合成MOFs材料MIL-53(Cr).材料后处理过程采用溶剂置换法.通过XRD、TG、SEM、IGA的表征手段对不同条件下合成后MIL-53(Cr)产物进行结构表征;通过N2、CO2、CH4、C2 H4、C2 H6单组份气体吸附实验对样品的吸附性能进行测试.结果显示:新的制备方案在制备过程中能够更稳定地合成出高纯度的MIL-53(Cr)产物,溶剂置换的后处理方法能够有效的将孔道内H2BDC除去,经过后处理MIL-53(Cr)比表面积为1355.9 m2?g?1,通过初步的气体吸附实验表明该材料具有广阔的应用前景.     

Diffusion kinetics of CO2 in amine-impregnated MIL-101, alumina,and silica adsorbents

CO₂ sorption kinetics of poly(ethylenimine) (PEI)-impregnated MIL-101, γ-alumina, and UVM-7 silica were investigated by the zero-length column technique for the purpose of understanding the effect of amine content, adsorbent porosity, and adsorption temperature on CO₂ sorption rates. Each of the adsorbents was impregnated with three different amine contents (20, 35, and 50 wt%) and the effective diffusion time constants were determined at 25°C. For each respective adsorbent, it was found that increasing the amine content results in diminished diffusion rates. Additionally, it was found that the pore size of the support has a profound effect on diffusional kinetics, where microporous MIL-101 yielded substantially slow desorption rates upon amine-functionalization compared to mesoporous γ-alumina. PEI-impregnated UVM-7 silica was further investigated at 50 and 75°C in order to provide insight into the effect of temperature on sorption kinetics. The results indicated that PEI-impregnated UVM-7 exhibited faster sorption kinetics at higher temperatures. Upon desorption, PEI-UVM-7 silica exhibited two distinct regions of mass-transfer control that occur at different sorption times. This is best explained by first the occurrence of surface diffusion followed by diffusion out of the bulky PEI polymer chains. The findings of this study provide novel kinetic characterizations on promising amino-adsorbents for carbon capture applications.     

苯在改性活性炭上的脱附活化能

采用浸渍法制备了KH560/改性活性炭、1706/改性活性炭和A172/改性活性炭等3种活性炭,并利用程序升温脱附技术测定了苯在这3种改性活性炭上的脱附活化能. 结果表明,苯在经改性的活性炭上的脱附活化能均大于其在未改性活性炭上的脱附活化能,表明用有机硅烷改性活性炭可以提高其对苯的吸附能力. IGC分析结果表明,经硅烷改性的活性炭的特殊作用吸附自由能DGs均小于原始活性炭的DGs,这4种活性炭表面与苯的特殊作用吸附自由能DGs大小顺序与苯在这些活性炭上的脱附活化能大小顺序正好相反,这表明DGs越小,吸附质与活性炭表面形成的吸附越牢固,吸附质从其表面脱附所需的活化能越大.     

正庚烷在5A分子筛上的高温吸附/脱附及扩散性能

用智能重量分析仪(IGA-100)在较高温度下测定了5A分子筛(自制)对正庚烷吸附/脱附等温线及其滞后环;计算TIE庚烷在5A分子筛上的吸附热和吸附、脱附扩散系数,分析了正庚烷在5A分子筛微孔内吸附作用.结果表明:吸附温度升高,正庚烷吸附等温线优惠程度明显降低,有吸附滞后环;在523 K下,5A对正庚烷的平衡吸附量为0...     

Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe3+ Hydrolysis and Fe2+ Oxidation: Kinetics,Isotherms and Adsorption Mechanism

Good sorption properties and simple synthesis route make schwertmannite an increasingly popular adsorbent. In this work, the adsorption properties of synthetic schwertmannite towards Cr(VI) were investigated. This study aimed to compare the properties and sorption performance of adsorbents obtained by two methods: Fe³⁺ hydrolysis (SCH_A) and Fe²⁺ oxidation (SCH_B). To characterise the sorbents before and after Cr(VI) adsorption, specific surface area, particle size distribution, density, and zeta potential were determined. Additionally, optical micrographs, SEM, and FTIR analyses were performed. Adsorption experiments were performed in varying process conditions: pH, adsorbent dosage, contact time, and initial concentration. Adsorption isotherms were fitted by Freundlich, Langmuir, and Temkin models. Pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models were used to fit the kinetics data. Linear regression was used to estimate the parameters of isotherm and kinetic models. The maximum adsorption capacity resulting from the fitted Langmuir isotherm is 42.97 and 17.54 mg·g⁻¹ for SCH_A and SCH_B. Results show that the adsorption kinetics follows the pseudo-second-order kinetic model. Both iron-based adsorbents are suitable for removing Cr(VI) ions from aqueous solutions. Characterisation of the adsorbents after adsorption suggests that Cr(VI) adsorption can be mainly attributed to ion exchange with SO₄²⁻ groups.     

Effect of temperature on the removal of lead(II) by adsorption on China clay and wollastonite

The removal of Pb(II) from aqueous solution by adsorption on china clay and wollastonite is an attractive process. The amount of Pb(II) removed by adsorption is highly dependent on the temperature of the adsorbate solution and favours low temperature. The equilibrium times were noted, i.e. 90 min for china clay and 120 min for wollastonite. The various rate parameters of the adsorption process have been determined at different temperatures. The activation energies were determined and found to be ?5.345 kJ mol^?1 and ?8.730 kJ mol^?1 for Pb(II)-china clay and Pb(II)-wollastonite systems, respectively. The adsorption isotherm was measured experimentally at various temperatures. The negative values of enthalpy change (ΔH = ?77.95 kJ mol^?1 and ?16.40 kJ mol^?1 for china clay and wollastonite, respectively) indicate the exothermic nature of the adsorption processes for both systems. The isosteric heats of the adsorption process have been determined at various surface coverages of the adsorbents used. The various thermodynamic parameters have been calculated to elucidate the mechanism involved in the adsorption process.     

Enhancement of CO2 Adsorption and CO2/N2 Selectivity on ZIF‐8 via Postsynthetic Modification

Imidazolate framework ZIF‐8 is modified via postsynthetic method using etheylenediamine to improve its adsorption performance toward CO₂. Results show that the BET surface area of the modified ZIF‐8 (ED‐ZIF‐8) increases by 39%, and its adsorption capacity of CO₂ per surface area is almost two times of that on ZIF‐8 at 298 K and 25 bar. H₂O uptake on the ED‐ZIF‐8 become obviously lower compared to the ZIF‐8. The ED‐ZIF‐8 selectivity for CO₂/N₂ adsorption gets significantly improved, and is up to 23 and 13.9 separately at 0.1 and 0.5 bar, being almost twice of those of the ZIF‐8. The isosteric heat of CO₂ adsorption (Q_st) on the ED‐ZIF‐8 becomes higher, while Q_st of N₂ gets slightly lower compared to those on the ZIF‐8 Furthermore, it suggests that the postsynthetic modification of the ZIF‐8 not only improves its adsorption capacity of CO₂ greatly, but also enhances its adsorption selectivity for CO₂/N₂/H₂O significantly. ©2013 American Institute of Chemical Engineers AIChE J, 59: 2195–2206, 2013     

Experimental Evaluation of the Adsorption,Diffusion, and Separation of CH4/N2 and CH4/CO2 Mixtures on Al-BDC MOF

Adsorption equilibrium, thermodynamics, and kinetics of CH₄, N₂, and CO₂ were investigated by volumetric-chromatographic and inverse gas chromatographic (IGC) methods on the Al-BDC MOF. The binary adsorption data from the volumetric-chromatographic experiments represents that the Al-BDC MOF has a high CO₂/CH₄ selectivity ca. 11 and a CH₄/N₂ selectivity ca. 4.3 at 303 K, and appears to be a good candidate for the CH₄ separation. The initial adsorption heats of CH₄, N₂, and CO₂ on the Al-BDC MOF were determined to be 15.3, 11.5, and 32.2 kJmol^?1 by IGC method, respectively. Moreover, the micropore diffusivities of N₂, CH₄ and CO₂ in the Al-BDC MOF at 303 K were also estimated to be 1.58 × 10^?7 cm²/s, 7.04 × 10^?8 cm²/s, and 3.95 × 10^?9 cm²/s, respectively. The results indicate that micropores play a crucial role in the adsorptive separation of the CH₄/N₂ and CH₄/CO₂ mixtures, and the IGC method is a validity manner to estimate the thermodynamic and kinetic parameters of MOF adsorbents.     

Isotherm,Thermodynamics, and Kinetics of Methyl Orange Adsorption onto Magnetic Resin of Chitosan Microspheres

Severe environmental pollution problems arising from toxic dyestuffs (e.g., methyl orange) are receiving increasing attention. Therefore, dyes’ safe removal has become a research hotspot. Among the many physical–chemical removal techniques, adsorption using renewable biological resources has proved to be more advantageous over others due to its effectiveness and economy. Chitosan is a natural, renewable biopolymer obtained by deactivated chitin. Thus, the magnetic resin of chitosan microspheres (MRCM), prepared by reversed-phase suspension cross-linking polymerization, was used to remove methyl orange from a solution in a batch adsorption system. The main results are as follows: (1) The results of physical and swelling properties of MRCM indicated that MRCM was a type of black spherical, porous, water-absorbing, and weak alkali exchange resin, and it had the ability to adsorb methyl orange when it was applied in solutions above pH 2.0. (2) In batch adsorption studies, the maximum adsorption capacity was obtained at pH 5; the adsorption equilibrium time was 140 min; and the maximum adsorption was reached at 450 mg/L initial concentration. (3) Among the three isotherm adsorption models, Langmuir achieved the best fit for the adsorption of methyl orange onto MRCM. (4) The adsorption thermodynamics indicated that the adsorption was spontaneous, with increasing enthalpy, and was driven by the entropy. (5) The pseudo-second-order kinetics equation was most suitable to describe the adsorption kinetics, and the adsorption kinetics was also controlled by the liquid–film diffusion dynamics. Consequently, MRCM with relatively higher methyl orange adsorption exhibited the great efficiency for methyl orange removal as an environment-friendly sorbent. Thus, the findings are useful for methyl orange pollution control in real-life wastewater treatment applications.