CO_2,CH_4在全硅MFI型沸石中吸附的分子模拟

采用巨正则蒙特卡洛(GCMC)分子模拟方法研究了CO2,CH4分子在全硅MFI型沸石中的吸附行为,计算得到了纯组分和两组分的混合物在不同温度时的吸附等温线;分析了全硅MFI型沸石对混合组分的选择性吸附;模拟还得到了CO2,CH4分子在全硅MFI型沸石中的吸附位。     

MER型沸石吸附分离CO2/CH4的分子模拟

MER型沸石在吸附分离CO₂/CH₄方面展现出良好的工业应用前景,受到广泛关注,但还缺乏理论基础数据。本文采用巨正则蒙特卡洛（GCMC）模拟方法,以全硅MER型沸石作为对照,模拟分析了CO₂/CH₄在Na⁺、K⁺、Cs⁺和Ca²⁺交换的MER型沸石中的吸附分离行为。结果表明：不同阳离子交换的MER型沸石对CO₂和CH₄的吸附符合Langmuir-Freundlich吸附等温线模型,平衡吸附量的大小顺序为：Ca-MER＞Na-MER＞K-MER＞Cs-MER,与沸石的自由体积和比表面积大小顺序一致,且近似成线性关系,选用高价阳离子MER型沸石可以提高吸附量;CO₂和CH₄主要分布在沸石的pau笼中,在d8R笼和ste笼中也有少量分布;骨架外阳离子与CO₂的强吸附作用和独特的八元环窗口孔径是MER型沸石对CO₂/CH₄混合组分表现出超高吸附选择性的原因,吸附选择性高达1000以上。综合吸附量、吸附热和吸附选择性分析指出,Na-MER和K-MER型沸石是优良的CO₂吸附剂。本研究为MER型沸石吸附分离CO₂/CH₄提供了理论依据和实验指导。     

孔活性炭储存CH4的分子模拟

吸附天然气被认为是未来最具有发展前景的CH₄储存方式之一,其中的关键是寻找合适的吸附剂,使其具有良好的CH₄吸脱附性能。以纳米多孔碳材料作为多孔活性炭的结构,采用巨正则蒙特卡洛（GCMC）和平衡分子动力学（EMD）的方法研究CH₄在纳米多孔碳材料中的吸附和扩散行为,通过考察不同因素的影响,期望获得高的CH₄吸附量和自扩散系数。其中,研究的主要因素有：构成多孔碳材料的石墨片微元的大小、多孔碳材料的不同密度、多孔碳材料的表面基团改性。结果表明：多孔碳材料对CH₄的吸附受石墨片微元种类和混合比例的影响不大;在所研究的多孔碳材料密度范围内,存在最优密度,能够获得最大的CH₄吸附量,此最优密度在0.50 g·cm^-3左右,但是多孔碳材料的表面经羟基修饰后,最佳材料密度发生了偏移,此时最优密度在0.72 g·cm^-3左右;此外,经表面改性后的多孔碳材料吸附量大小顺序为：CH₃ > OH > CO > COOH;对于CH₄在多孔碳材料中的扩散,总体规律是低的密度,并且经过COOH修饰,其CH₄自扩散系数最大。     

Y型沸石吸附噻吩的能量及结构异质性的分子模拟

采用GCMC方法计算了噻吩在全硅Y和NaY沸石上的吸附,通过变化不同的噻吩吸附量得到吸附能量的异质性,并基于主客体径向分布函数剖析了吸附能量的异质性。根据对噻吩分子与沸石之间能量的分析,噻吩分子与沸石之间的作用主要由短程的色散作用控制。NaY沸石上吸附表现出与全硅Y沸石明显不同的吸附位特征,钠离子的引入明显地增强了对噻吩的吸附作用。钠离子与噻吩之间的静电和色散作用是引起这两种不同化学组成沸石吸附热不同的重要因素。进一步对径向分布函数分析确定了NaY沸石中S—Na,CHsp2—Na之间存在独特的作用位。     

二氧化硫在HZSM-5分子筛中吸附的分子模拟

通过实验测定了二氧化硫(SO2)在HZSM-5型分子筛中的吸附性能,采用巨正则系综Monte Carlo方法建立相应的吸附模型,模拟计算相应的吸附行为。在实验与模拟结果相吻合的基础上,进一步模拟计算了SO2在HZSM-5型分子筛中的吸附等温线、吸附微观构型及吸附热等性质。结果表明:随着硅铝比(摩尔比)的增大,分子筛对SO2的吸附量降低,当硅铝比大于191时,硅铝比对SO2的吸附量影响很小。SO2分子主要吸附在氢离子和Al原子周围。在Al原子周围,SO2分子比较集中,同时发现氢离子的存在会阻碍SO2的扩散。在吸附量相同的条件下,SO2在HZSM-5型分子筛的吸附热随着硅铝比的升高而降低。     

金属有机骨架材料在CO2/CH4吸附分离中的研究进展

CO₂/CH₄分离能耗高是生物甲烷过程核心难题之一。金属有机骨架材料（metal organic frameworks,MOFs）由于其优异的CO₂吸附分离性能,被视为最具潜力的CO₂分离捕集材料,近年来引起了广泛的关注。本文结合沼气的特点和MOFs研究的最新进展,对MOFs材料在CO₂/CH₄吸附分离过程的相关实验研究工作进行了综述。     

二甲醚产物中CO、CO2、CH4、C2H4的气相色谱分析

用填充柱气相色谱法分析了气态二甲醚产物中所含的CO、CO2、CH4、C2H4，用外标法对这些组分进行定量，得到了满意的结果，并在1．0万t／a二甲醚生产装置中得到应用。     

在MFI型高硅沸石上用一乙醇胺选择性合成三乙烯二胺

在常压和无载气条件下,使一乙醇胺(MEOA)与18%质量分数的NH3及50%质量分数的H2O共进料反应,用小型固定床反应器评价了一种MFI型高硅沸石催化剂TEDAMAX-01催化一乙醇胺合成三乙烯二胺(TEDA)的性能.研究表明当反应温度在330～380℃内变化时,TEDA MAX-01的活性和选择性随反应温度的提高而增加.当混合料液的质量空速(V WHS)在1～4 h-1区间内变化时,TEDA MAX-01的活性和选择性保持不变.在370℃和V WHS为2 h-1的条件下,TEDA MAX-01催化剂能使一乙醇胺转化率接近99%,三乙烯二胺选择性达到97%以上.按此条件连续运转266h,一乙醇胺转化率保持不降,三乙烯二胺选择性在前100h缓慢下降,之后维持在90%左右.对将分离母液循环作为进料氨源和水源的反应,以及催化剂经过反复4次高温焙烧再生之后的反应进行了评价,该催化剂都表现出同样高的活性、选择性和稳定性.     

用高压气体吸脱附-微量热联用法对CO2和CH4在煤上吸附热力学的研究

为了从热力学角度揭示CO_2和CH_4在煤上的竞争吸附实质,为驱替理论提供热力学参考,利用高压气体吸脱附-微量热联用仪,通过容量法测定30℃,40℃,50℃温度下CO_2和CH_4在煤上的吸附等温线,同时计算等量吸附热和极限吸附热,并测定了CO_2和CH_4在煤上的吸附热。结果表明:CO_2和CH_4吸附等温线均属于Ⅰ型吸附等温线,且均符合Langmuir吸附模型;温度升高,CO_2和CH_4在煤上的吸附量均减小;相同条件下,CO_2在煤上的吸附量明显高于CH_4的吸附量,表明CO_2更容易吸附于煤上。CO_2和CH_4在煤上的等量吸附反映出煤吸附CO_2和CH_4的过程均为物理吸附。同时CO_2在煤上的等量吸附热高于CH_4的等量吸附热,说明CO_2分子与煤分子之间作用力强于CH_4与煤分子之间的作用力,极限吸附热和实验测得的吸附热也显示了同样结果,从热力学角度阐释了CO_2和CH_4在煤体表面竞争吸附的实质。     

沸石ZSM-5吸附回收低浓度煤层气中CH4

利用高硅疏水性沸石ZSM-5吸附回收低浓度煤层气中的甲烷,对其吸附平衡、吸附动力学以及真空变压吸附分离过程进行了理论和实验研究。通过重量法和穿透曲线法测定了CH₄/N₂单组分及双组分的竞争吸附平衡数据,并采用Multisite Langmuir吸附等温线模型对其进行拟合。结合CH₄和N₂稀释穿透曲线实验数据和等温无动量损失的双分散二级孔结构扩散模型,获得CH₄和N₂在沸石ZSM-5上的微孔扩散系数。建立并求解包含质量、动量及能量传递的固定床吸附分离模型方程,预测了CH₄和N₂在沸石ZSM-5上的竞争吸附穿透曲线。进一步采用ZSM-5吸附剂填充床单柱四步真空变压吸附实验考察了进料浓度、进料流速、进料时间以及吹扫比对分离效果的影响。结果发现沸石ZSM-5对CH₄具有较好的选择性,沸石晶粒内的微孔扩散为吸附速率控制步骤,真空变压吸附工艺可将模拟煤层气中20%的CH₄提纯至31%～41%,回收率为93%～98%。     

2,2-Dimethylbutane adsorption and diffusion in MFI zeolite

Adsorption rates of 2,2-dimethylbutane (DMB) from the liquid phase into MFI zeolite crystals were measured by a volumetric/gravimetric method. Desorption rates in the presence of liquid n-hexane were measured by a non-adsorbing solvent (isooctane) method. Dimethylbutane desorbed, and was replaced by n-hexane, approximately three orders of magnitude faster than it adsorbed. Adsorption was slow and reached only 40% of saturation loading in 6-μm crystals after 45 days at 295 K, but this DMB loading desorbed into n-hexane in less than 1 h. Desorption may be faster due to slight expansion of the MFI unit cell by n-hexane adsorption. Although DMB diffused slower than n-hexane in MFI crystals, its transient time through a silicalite-1 membrane was an order of magnitude shorter than the n-hexane time at 313 K because DMB diffused through defects, whereas n-hexane diffused mainly through zeolite pores. The n-hexane transient time was longer because n-hexane: (1) expanded the MFI crystal size slightly and shrank non-zeolitic pores, and (2) adsorbed in the zeolite as it moved through the remaining non-zeolitic pores. A silicalite-1 membrane saturated with DMB at 423 K was sealed so effectively, and DMB desorbed slowly enough, that the helium permeation flux at 313 K was below the detection limit.     

Molecular dynamic simulation of carbon dioxide,methane, and nitrogen adsorption on Faujasite zeolite

Removing impurities such as carbon dioxide and nitrogen from natural gas is a technical challenge and one of the major concerns in natural gas treatment process. In this study, adsorption of CH₄, N₂, and CO₂ on the Faujasite(FAU) zeolite has been studied using molecular dynamics simulation at temperatures of 293, 308, and 323 K and pressures up to 1 MPa. COMPASS force field was used to model the interactions between zeolite and guest molecules. Ewald and atom-bas...     

Diffusion of hydrocarbon mixtures in MFI zeolite: Influence of intersection blocking

Branched and cyclic hydrocarbons such as iso-butane, 2-methylpentane, 3-methylpentane, 2,2-dimethyl-butane and benzene are preferentially adsorbed at the intersections of the channels of MFI zeolite; they serve as bottlenecks for molecular traffic in mixtures with linear C1–C6 alkanes. Molecular dynamics simulations show that as the loadings of tardier iC4, 2MP, 3MP, 22DMB, and Bz is progressively increased to four molecules per unit cell the diffusivity of the more mobile linear alkane reduces nearly to zero. The reduction in the n-alkane diffusivity is quantitatively similar irrespective of the branched/cyclic hydrocarbon.     

Molecular simulation of hydrogen adsorption in organic zeolite

The adsorption of molecular hydrogen on model zeolites has been simulated employing Grand Canonical Monte Carlo (GCMC) procedure. The effects of cation type, available volume, surface area, temperature, pressure and pre-adsorbed organics such as benzene on the hydrogen uptake are analyzed. The hydrogen adsorption can be affected mainly by the available volume and surface area per g-zeolite at the same temperature and pressure. Increase of temperature results in the decrease of sorption intensity and capacity. The adsorption capacity correlates well with the pressure with high linearity at room temperature. Adsorption is lowered by the pre-adsorbed benzene molecule. The orientation and the number of benzene molecule in zeolite affect the adsorption capacity. The organic zeolite with larger available volume shows larger adsorption capacity.     

Molecular simulation of CH4 adsorption behavior in slit nanopores: Verification of simulation methods and models

The aim of this study is to select an appropriate method for CH₄ adsorption in organic nanopores for shale-gas development. Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations were performed. Three comparison studies were included: (i) comparison of the adsorption behavior in kerogen nanopores using different schemes for dispersion correction, (ii) comparison of the adsorption behavior in graphite nanopores using MD and GCMC simulations, and (iii) comparison of the adsorption behavior in kerogen and graphite nanopores using MD simulations. The result was reliable when using a particle-mesh Ewald scheme or a cut-off ≥1.5 nm without dispersion correction. The simulation results were essentially identical for the MD and GCMC simulations. The free-gas CH₄ density inside the nanopores started to deviate from the bulk density at ~2 nm for the graphite model and at ~7–10 nm for the kerogen model, whereas the total CH₄ density deviates from the bulk density at ~20 nm.     

具有b取向壳层MFI/MFI核壳分子筛的合成与表征

为了研究MFI/MFI核壳分子筛壳层b取向的形成机理,以圆柱形ZSM-5为核相,用碱性溶液对其表面预处理后,水热晶化合成具有高硅b取向壳层的MFI/MFI核壳分子筛,采用SEM、XRD、TEM、XPS和NMR等表征核相表面与晶化产物的结构。结果表明,核壳分子筛的（020）晶面衍射峰强度明显增强,壳层覆盖度为84％。核相碱处理是壳层形成的关键,未处理核相的表面上壳层不能生长,碱处理后在核相上形成的表面非骨架铝对高硅壳层颗粒的形貌和取向影响较大。在低铝含量条件下成核方式可能从非均相成核转变为均相成核,溶液中形成片状晶体,后经表面Si-OH强作用自组装得到二维紧密堆积的b取向壳层。     

MFI沸石中Maxwell-Stefan扩散系数与覆盖率的关系

基于描述多孔介质中表面扩散的Maxwell-Stefan（M-S）方程,结合本文动力学Monte Carlo（KMC）模拟及文献中分子动力学（MD）模拟数据,提出了MFI沸石中M-S扩散系数与覆盖率依赖关系的新表达式.该关系式考虑了分子间相互作用以及饱和吸附覆盖量对扩散系数与覆盖率依赖关系的影响,用于预测强受限组分CF₄、弱受限组分CH₄以及中度受限组分Xe等7种单组分在MFI沸石中的M-S扩散系数时,结果较现有其他关系式更为合理.     

Enhanced compactness of oriented MFI zeolite films by heat treatment of seed layer

The achievement of compact and defect-free film structure is crucial for the application of b-oriented MFI zeolite film. In this work, a novel heat treatment technique was used to treat zeolite seeded substrates prior to secondary growth. The influence of the heat treatment parameters such as treat temperature and time on the final film morphology were systematically investigated. The relationship between film compactness and the parameters was established. Under the optimized treat temperature of 120?°C and treat time of 1?h, compact and uniform b-oriented MFI zeolite film was achieved. The applicability of the optimum heat treatment condition was validated by employing various film substrates.     

Reactions of ethylbenzene in the presence of toluene over external surface passivated MFI zeolite

The reactions of ethylbenzene in the presence of toluene were studied over external surface passivated MFI zeolite. With increase in extent of external surface passivation the para selectivity of both transalkylation and disproportionation reactions enhanced. The effects of mole ratio of ethylbenzene to toluene, reaction temperature and WHSV on the performance of the modified zeolite are discussed. It was observed that the high para selectivity feature of the modified zeolite is not influenced by changes in reaction conditions.