Separation of gas mixtures using a range of zeolite membranes: a molecular-dynamics study

Gas separation efficiencies of three zeolite membranes (Faujasite, MFI, and Chabazite) have been examined using the method of molecular dynamics. Our investigation has allowed us to study the effects of pore size and structure, state conditions, and compositions on the permeation of two binary gas mixtures, O(2)N(2) and CO(2)N(2). We have found that for the mixture components with similar sizes and adsorption characteristics, such as O(2)N(2), small-pore zeolites are not suited for separations, and this result is explicable at the molecular level. For mixture components with differing adsorption behavior, such as CO(2)N(2), separation is mainly governed by adsorption and small-pore zeolites separate such gases quite efficiently. When selective adsorption takes place, we have found that, for species with low adsorption, the permeation rate is low, even if the diffusion rate is quite high. Our results further indicate that loading (adsorption) dominates the separation of gas mixtures in small-pore zeolites, such as MFI and Chabazite. For larger-pore zeolites such as Faujasite, diffusion rates do have some effect on gas mixture separation, although adsorption continues to be important. Finally, our simulations using existing intermolecular potential models have replicated all known experimental results for these systems. This shows that molecular simulations could serve as a useful screening tool to determine the suitability of a membrane for potential separation applications.     

Atomistic simulations of CO2 and N2 within cage-type silica zeolites

The behavior of CO(2) and N(2), both as single components and as binary mixtures, in two cage-type silica zeolites was studied using atomistic simulations. The zeolites considered, ITQ-3 and paradigm cage-type zeolite ZK4 (the all-silica analog of LTA), were chosen so that the principles illustrated can be generalized to other adsorbent/adsorbate systems with similar topology and types of interactions. N(2) was chosen both because of the potential uses of N(2)/CO(2) separations and because it differs from CO(2) most significantly in the magnitude of its Coulombic interactions with zeolites. Despite similarities between N(2) and CO(2) diffusion in other materials, we show here that the diffusion of CO(2) within cage-type zeolites is dominated by an energy barrier to diffusion located at the entrance to the narrow channels connecting larger cages. This barrier originates in Coulombic interactions between zeolites and CO(2)'s quadrupole and results in well-defined orientations for the diffusing molecules. Furthermore, CO(2)'s favorable electrostatic interactions with the zeolite framework result in preferential binding in the windows between cages. N(2)'s behavior, in contrast, is more consistent with that of molecules previously studied. Our analysis suggests that CO(2)'s behavior might be common for adsorbates with quadrupoles that interact strongly with a material that has narrow windows between cages.     

定向含硼MFI型沸石膜对纯气体的渗透性以及对乙醇/水体系的分离选择性

研究定向含硼MFI型沸石膜和原多孔玻璃基材对纯气体的渗透性以及对乙醇/水体系的分离选择性.原多孔玻璃基材对纯气体的透过表现出诺森扩散选择性,表明气体以诺森扩散通过.纯气体透过焙烧后的B-A1-ZSM-5沸石膜,H₂,He,Ne,Ar,O₂,CO₂对N₂的理想选择性分别为16.8,15.6,12.6,9.41,8.60,5.32;CO和SO₂对N₂的理想选择性分别为0.135和0.0179;O₂对CO和SO₂的理想选择性分别为63.7和480.2.这表明该类沸石膜对纯气体的透过不仅具有良好的理想选择性,而且可能为新型防毒面具提供一种很好的可选材料.渗透汽化实验表明,在测定温度范围内原多孔玻璃基材对三种不同浓度的乙醇/水体系几乎没有分离性能.焙烧后的B-A1-ZSM-5沸石膜对5 wt%,50wt%和95wt%乙醇/水体系的分离,水的最大分离系数分别为28.2,135.7和518.5,且温度均为303 K.表明该MFI型沸石膜具有强的亲水性.     

H2 separation using defect-free, inorganic composite membranes

Defect-free, microporous Al(2)O(3)/SAPO-34 zeolite composite membranes were prepared by coating hydrothermally grown zeolite membranes with microporous alumina using molecular layer deposition. These inorganic composite membranes are highly efficient for H(2) separation: their highest H(2)/N(2) mixture selectivity was 1040, in contrast with selectivities of 8 for SAPO-34 membranes. The composite membranes were selective for H(2) for temperatures up to at least 473 K and feed pressures up to at least 1.5 MPa; at 473 K and 1.5 MPa, the H(2)/N(2) separation selectivity was 750. The H(2)/CO(2) separation selectivity was lower than the H(2)/N(2) selectivity and decreased slightly with increasing pressure; the selectivity was 20 at 473 K and 1.5 MPa. The high H(2) selectivity resulted either because most of the pores in the Al(2)O(3) layer were slightly smaller than 0.36 nm (the kinetic diameter of N(2)) or because the Al(2)O(3) layer slightly narrowed the SAPO-34 pore entrance. These composite membranes may represent a new class of inorganic membranes for gas separation.     

Computational Study of Adsorption and Separation of CO(2), CH(4), and N(2) by an rht-Type Metal-Organic Framework

In this work, a computational study is performed to evaluate the adsorption-based separation of CO(2) from flue gas (mixtures of CO(2) and N(2)) and natural gas (mixtures of CO(2) and CH(4)) using microporous metal organic framework Cu-TDPAT as a sorbent material. The results show that electrostatic interactions can greatly enhance the separation efficiency of this MOF for gas mixtures of different components. Furthermore, the study also suggests that Cu-TDPAT is a promising material for the separation of CO(2) from N(2) and CH(4), and its macroscopic separation behavior can be elucidated on a molecular level to give insight into the underlying mechanisms. On the basis of the single-component CO(2), N(2), and CH(4) isotherms, binary mixture adsorption (CO(2)/N(2) and CO(2)/CH(4)) and ternary mixture adsorption (CO(2)/N(2)/CH(4)) were predicted using the ideal adsorbed solution theory (IAST). The effect of H(2)O vapor on the CO(2) adsorption selectivity and capacity was also examined. The applicability of IAST to this system was validated by performing GCMC simulations for both single-component and mixture adsorption processes.     

Combination of MOFs and zeolites for mixed-matrix membranes

Mixed-matrix membranes (MMMs) were prepared by combinations of two different kinds of porous fillers [metal-organic frameworks (MOFs) HKUST-1 and ZIF-8, and zeolite silicalite-1] and polysulfone. In the search for filler synergy, the MMMs were applied to the separation of CO(2)/N(2), CO(2)/CH(4), O(2)/N(2), and H(2)/CH(4) mixtures and we found important selectivity improvements with the HKUST-1-silicalite-1 system (CO(2)/CH(4) and CO(2)/N(2) separation factors of 22.4 and 38.0 with CO(2) permeabilities of 8.9 and 8.4 Barrer, respectively).     

Unconventional, highly selective CO2 adsorption in zeolite SSZ-13

Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an 8-ring window. Under ideal conditions for industrial separations of CO(2) from N(2), the ideal adsorbed solution theory selectivity is >70 in each compound. For low gas coverage, the isosteric heat of adsorption for CO(2) was found to be 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, respectively. From in situ neutron powder diffraction measurements, we ascribe the CO(2) over N(2) selectivity to differences in binding sites for the two gases, where the primary CO(2) binding site is located in the center of the 8-membered-ring pore window. This CO(2) binding mode, which has important implications for use of zeolites in separations, has not been observed before and is rationalized and discussed relative to the high selectivity for CO(2) over N(2) in SSZ-13 and other zeolites containing 8-ring windows.     

在多孔γ-Al2O3氧化铝载体上微波合成NaA分子筛膜及气体渗透分离性能

The paper presented novel synthesis of NaA zeolite membrane with good performance using microwave heating. The method involved two steps, prior seeding 120 nm of LTA crystals on substrate and then employing a secondary hydrothermal synthesis. Effects of seeding times, synthesis time and synthesis times were investigated in this work. The quality evaluation of membranes respectively used single component gases (HE and N2) and H2/N2 (equivalent volume) mixture. The ideal H2,/N2 selectivity increased from 1.90 of the substrate to 6.37 of the three-stage synthesized membrane, which was distinctly higher than the corresponding Knudsen diffusion selectivity of 3.74. However, the real H2/N2 selectivity of the three-stage synthesis was much lower than the corresponding ideal selectivity and close to the corresponding Knudsen diffusion selectivity of 3.74.     

(K,Mo)Y分子筛中钼组分的EXAFS研究

应用同步辐射Extended X-ray Absorption Fine Structure (EXAFS)技术研究固态法制备的KHMoY分子筛的氧化态和硫化态样品以及硫化态KHY/MoO3样品中钼组分的局域配位环境结构,并与KHMoY和KHY/MoO3样品催化加氢活性结果进行对照.结果表明,随原子比(K+2Mo)/Al的变化,钼原子周围的配位环境有显著的差异.当(K+2Mo)/Al＜1时,KHMoY和KHY/MoO3硫化后,钼组分主要以MoS2小原子簇分散在分子筛超笼中;(K+2Mo)/Al＞1时,钼组分则有两种存在环境,即分子筛超笼中的和分子筛外表面的钼组分.分子筛超笼中的MoS2原子簇的催化加氢合成醇选择性较高;分子筛外表面的MoS2微小颗粒的尺寸相对于超笼中的要大许多,其合成醇选择性较低.     

Evaluation of the impact of H2O, O2, and SO2 on postcombustion CO2 capture in metal-organic frameworks

Molecular modeling methods are used to estimate the influence of impurity species: water, O(2), and SO(2) in flue gas mixtures present in postcombustion CO(2) capture using a metal organic framework, HKUST-1, as a model sorbent material. Coordinated and uncoordinated water effects on CO(2) capture are analyzed. Increase of CO(2) adsorption is observed for both cases, which can be attributed to the enhanced binding energy between CO(2) and HKUST-1 due to the introduction of a small amount of water. Density functional theory calculations indicate that the binding energy between CO(2) and HKUST-1 with coordinated water is ~1 kcal/mol higher than that without coordinated water. It is found that the improvement of CO(2)/N(2) selectivity induced by coordinated water may mainly be attributed to the increased CO(2) adsorption on the hydrated HKUST-1. On the other hand, the enhanced selectivity induced by uncoordinated water in the flue gas mixture can be explained on the basis of the competition of adsorption sites between water and CO(2) (N(2)). At low pressures, a significant CO(2)/N(2) selectivity increase is due to the increase of CO(2) adsorption and decrease of N(2) adsorption as a consequence of competition of adsorption sites between water and N(2). However, with more water molecules adsorbed at higher pressures, the competition between water and CO(2) leads to the decrease of CO(2) adsorption capacity. Therefore, high pressure operation should be avoided in HKUST-1 sorbents for CO(2) capture. In addition, the effects of O(2) and SO(2) on CO(2) capture in HKUST-1 are investigated: The CO(2)/N(2) selectivity does not change much even with relatively high concentrations of O(2) in the flue gas (up to 8%). A slightly lower CO(2)/N(2) selectivity of a CO(2)/N(2)/H(2)O/SO(2) mixture is observed compared with that in a CO(2)/N(2)/H(2)O mixture, especially at high pressures, due to the strong SO(2) binding with HKUST-1.     

添加成膜促进剂合成致密ZSM-5分子筛膜

报道了一种添加氯化钠低温条件下合成出具有纳米尺寸的ZSM-5分子筛，并以 其为晶种预吸附在多孔氧化铝载体上，再通过添加成膜促进剂氯化钠二次水热合成 制备出高度致密ZSM-5分子筛膜的新方法。通过考察添加的钠离子浓度对成膜的影 响，认为在一定浓度范围内钠离子能够促进成膜。SEM结果也验证了这一结论，分 子筛膜主要是由孪生聚晶分子筛组成。渗透结果显示，当添加量为x = 100 (Al_2O_3:84SiO_2:10Na_2O:15TPABr:3500H_2O)时，在室温和0.1 MPa的条件下， H_2/C_3的理想选择性最大值23.7，温度提高到200 ℃时的选择性下降为9.4，但仍 然高于努森扩散值（4.69），表面该膜具有高度的完整性。     

Effect of air humidity on the removal of carbon tetrachloride from air using Cu-BTC metal-organic framework

We have used interatomic potential-based simulations to study the removal of carbon tetrachloride from air at 298 K, using Cu-BTC metal organic framework. We have developed new sets of Lennard-Jones parameters that accurately describe the vapour-liquid equilibrium curves of carbon tetrachloride and the main components from air (oxygen, nitrogen, and argon). Using these parameters we performed Monte Carlo simulations for the following systems: (a) single component adsorption of carbon tetrachloride, oxygen, nitrogen, and argon molecules, (b) binary Ar/CCl(4), O(2)/CCl(4), and N(2)/CCl(4) mixtures with bulk gas compositions 99?:?1 and 99.9?:?0.1, (c) ternary O(2)/N(2)/Ar mixtures with both, equimolar and 21?:?78?:?1 bulk gas composition, (d) quaternary mixture formed by 0.1% of CCl(4) pollutant, 20.979% O(2), 77.922% N(2), and 0.999% Ar, and (e) five-component mixtures corresponding to 0.1% of CCl(4) pollutant in air with relative humidity ranging from 0 to 100%. The carbon tetrachloride adsorption selectivity and the self-diffusivity and preferential sitting of the different molecules in the structure are studied for all the systems.     

In silico screening of metal-organic frameworks in separation applications

Porous materials such as metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) offer considerable potential for separating a variety of mixtures such as those relevant for CO(2) capture (CO(2)/H(2), CO(2)/CH(4), CO(2)/N(2)), CH(4)/H(2), alkanes/alkenes, and hydrocarbon isomers. There are basically two different separation technologies that can be employed: (1) a pressure swing adsorption (PSA) unit with a fixed bed of adsorbent particles, and (2) a membrane device, wherein the mixture is allowed to permeate through a micro-porous crystalline layer. In view of the vast number of MOFs, and ZIFs that have been synthesized there is a need for a systematic screening of potential candidates for any given separation task. Also of importance is to investigate how MOFs and ZIFs stack up against the more traditional zeolites such as NaX and NaY with regard to their separation characteristics. This perspective highlights the potency of molecular simulations in determining the choice of the best MOF or ZIF for a given separation task. A variety of metrics that quantify the separation performance, such as adsorption selectivity, working capacity, diffusion selectivity, and membrane permeability, are determined from a combination of Configurational-Bias Monte Carlo (CBMC) and Molecular Dynamics (MD) simulations. The practical utility of the suggested screening methodology is demonstrated by comparison with available experimental data.     

EFFECTS OF SYNTHESIS PARAMETERS ON ZEOLITE ZSM-48 IN A SOLID REACTION MIXTURE SYSTEM

IntroduCtionReCently,zcoliteZSM-48hasbeenspehesizedinapurelysolidsystemll1.Thismethedusedtosynthesizezcolitescangndlyincreasetheyieldofpnductsperunitvolume,simplifythepr0cedresanddecreasethecnvironmentalpolluti0n.ThisincreasestheacualPOssibilityofsynthesisofhigh-silicazcolitesinchemicalindustry.MoreOver,itisalsohelpfulforstUdyingthezeolitecrystalliZai0nmechanism.ItiswellknownthatthesamewneofzeolitessynthesizedindifferentsystemshasdifferentcatalyticpropertiesduetothechangesoftheirfinestruC…     

Adsorption of nitrogen, oxygen, and argon in cobalt(II)-exchanged zeolite X

Adsorption of nitrogen, oxygen, and argon on cobalt(II)-exchanged zeolite X at 288.2 and 303.0 K was studied. The nitrogen and oxygen adsorption capacities increase upon cobalt ion exchange up to 71%, beyond which it shows a decreasing trend because of the partial degradation of the zeolite structure during the cation exchange and high-temperature vacuum dehydration processes. The magnitude of the increase in the adsorption capacities for nitrogen is much higher than that of oxygen. The nitrogen/oxygen as well as nitrogen/argon selectivities in the low-pressure region increase with an increase in cobalt exchange. Marginal oxygen selectivity over argon is observed for zeolite samples with higher cobalt exchange. The heats of adsorption values for nitrogen and oxygen increase and that for argon remain unaffected by cobalt exchange in zeolite X. The very high nitrogen adsorption capacity, selectivity, and heat of adsorption in the low-pressure region for cobalt-exchanged zeolite X compared to the parent sodium form of the zeolite show stronger interaction between nitrogen molecules with the extraframework cobalt cations of the zeolite. This stronger interaction has been explained in terms of the pi-complexation between nitrogen molecules and cobalt cations of the zeolites, as confirmed by diffuse reflectance infrared Fourier transform spectroscopy, wherein the N[triple bond]N stretching frequency at 2099 cm(-1) is observed for N2 molecules adsorbed in NaCoX.     

CO2／CH4分离膜及沸石填料影响渗透过程的研究

报导了甲基硅橡胶和纤维素（ＣＡ、ＣＴＡ、ＥＣ）膜对ＣＯ２、ＣＨ４的选择透气性能，并讨论了沸石作为填料所引起的分子筛作用的气体渗透过程。甲基硅橡胶的气体渗透系数最高，而选择性最低，且不受填料沸石的影响。纤维素膜的气体选择性较大，渗透系数可以通过沸石的加人而明显增加。特别是沸石１３Ｘ．沸石３Ａ、４Ａ、５Ａ在ＥＣ膜中对气体分子筛作用，改变了气体原有的渗透过程，提高了选择性。使用Ａｒｒｈｅｎｉｕｓ公式计算出ＥＣ－沸石３Ａ膜的气体渗透活化能。     

Effect of nickel deposition on hydrogen permeation behavior of mesoporous gamma-alumina composite membranes

Ni/alumina composite membranes were prepared and investigated for hydrogen separation at high temperature. alpha-Alumina-supported gamma-alumina composite membranes were prepared by soaking-rolling method. In order to improve H2 selectivity and permeance of the gamma-alumina membranes, Ni was deposited by a soaking process. As a result of a single gas permeation test of the Ni/alumina composite membranes, hydrogen permeance and H2/N2 selectivity at permeation temperature of 450 degrees C were 6.29 x 10(-7) mol/m2 s Pa and 5.2 which exceeded theoretical Knudsen selectivity. Contribution of surface diffusion was investigated by temperature dependence of H(2) permeance. The surface diffusion was observed at higher temperature above 250 degrees C. The Ni deposition on surface of the gamma-alumina composite membrane led to hydrogen permeation via Knudsen diffusion combined with surface diffusion, which gave high H2 selectivity exceeding the Knudsen diffusion mechanism.     

Novel synthesis of FAU-type zeolite membrane with high performance

FAU-type zeolite membranes were synthesized by the vapor phase transformation (VPT) methods with or without prior seeding on the substrate, and it was revealed that the CO(2)/N(2) selectivity of the seeded membrane is greater than that of the unseeded membrane.     

小分子无机气体在NaA分子筛膜上的渗透分离性能

采用多次合成的方法，在微波加热15min下合成出较为致密的A型分子筛膜。经过四次合成的分子筛膜上H2/N2和H2/CO的理想分离系数分别为4．33和4．64，高于努森扩散值(分别为3．74和3．74)，表明膜是连续致密的。     

Urea as a new and cheap nitrogen source for the synthesis of metal nitride clusterfullerenes: the role of decomposed products on the selectivity of fullerenes

By using urea as the new nitrogen source, for the first time, Sc-based metal nitride clusterfullerenes (NCFs), Sc(3)N@C(2n) (2n=80, 78, 70, 68), have been synthesized successfully. The optimum molar ratio of Sc(2)O(3)/CO(NH(2))(2)/C for the synthesis of Sc NCFs is 1:3:15. The yield of Sc(3)N@C(80)(I(h) +D(5h)) per gram of Sc(2)O(3), using CO(NH(2))(2) as the new nitrogen source, was quantitatively compared to those obtained when using the reported nitrogen sources, including N(2), NH(3), and guanidinium thiocyanate. We find that there is a clear difference on the selectivity of Sc-based NCFs within the extract mixture obtained from one rod and accumulative two rods. According to discharging experiments and XRD analysis, we conclude that NH(3) generated in situ from the decomposition of CO(NH(2))(2) is mainly responsible for the formation of Sc-based NCFs when using only one rod, whereas in the second rod CO(NH(2))(2) would decompose into melamine during discharging of the first rod. Thus, the selectivity of fullerenes is clearly dependent on the decomposed product of CO(NH(2))(2). Finally the difference in the decomposition behavior of CO(NH(2))(2) and melamine was studied in detail and a possible decomposition process of CO(NH(2))(2) during discharging was proposed. Accordingly, the difference in the selectivity and yield of Sc NCFs for CO(NH(2))(2) and melamine was interpreted.