乙醇溶液在憎水性分子筛内扩散系数的测定

提出了一种新的测定分子筛液相吸附扩散系数的方法。测定了低浓度乙醇溶液在憎水性分子筛(沸石)内的扩散系数,建立了低浓度液相在分子筛内扩散过程的数学模型,所得的扩散系数与Weisz建立的物质在固体内扩散系数与晶体孔径大小关系的模型相一致。     

乙醇溶液在憎水性分子筛内扩散系数的测定[摘要]

提出了一种新的测定分子筛相吸附扩散系数的方法和设备,取得了低浓度乙醇溶液在憎水性分子筛(沸石)内的扩散系数的测量结果。建立了低浓度液相在分子筛内扩散     

乙醇溶液在憎水性分子筛内扩散系数的测定

提出了一种新的测定分子筛液相吸附扩散系数的方法和设备,取得了低浓度乙醇溶液在憎水性分子筛(沸石)内的扩散系数的测量结果。建立了低浓度液相在分子筛内扩     

ZLC技术测定沸石晶内扩散系数研究进展

简介了测定沸石晶内扩散系数的各种方法,重点介绍了零长柱（ZLC）技术基本原理、实验方法和测定探针分子在沸石晶内扩散系数的研究进展,对采用ZLC技术表征探针分子在新型多孔材料中的扩散行为进行了展望。     

沸石分子筛成型技术进展

一、沸石分子筛科研生产动态合成沸石分子筛是五十年代以后发展起来的一种近代新型化工产晶,近年来,发展较快,目前世界上关于沸石分子筛的理论研究及其在生产与科研中的应用研究已经形成了一门新的学科—沸石化学。国际沸石分子筛会议每二年召开一次。英国并于一九七八年四月成立了沸石协会,以适应沸石研究及     

粉煤灰基沸石改性的研究现状

粉煤灰特殊的化学组成，使其能够合成不同类型的沸石分子筛，在离子交换、吸附领域表现出良好性能，被广泛应用于废水、废气处理领域。国内外许多学者对粉煤灰基沸石分子筛的合成方法、结构表征、实际应用均有研究，但由于分子筛自身的局限性，为了制备出具有实际应用价值的粉煤灰沸石，需要对其改性。介绍了不同类型粉煤灰基沸石分子筛的结构特点和合成条件，在此基础上，论述了沸石的多种改性原理和方法，并对粉煤灰基沸石分子筛在现实中的应用现状和发展前景进行展望。     

沸石分子筛的性能与应用研究

近年来,沸石分子筛由于具有独特的性能,已经在吸附分离、催化等领域取得了广泛的应用。介绍了沸石分子筛的性能及其在干燥和净化领域、吸附分离、催化领域的应用。对沸石分子筛的研究提出几点建议:1)研制价格低廉的沸石分子筛;2)研究沸石分子筛的合成及改性,提高其吸附容量和选择性;3)建立和完善评价吸附剂性能的定量指标。     

新型炭基-沸石复合膜材料的合成研究进展

综述了国内外对多孔炭材料和沸石分子筛相结合形成的炭基-沸石复合材料的研究进展.重点分析了以多孔炭材料为载体,制备新型炭基-沸石复合膜的合成、性能及应用.采用水热合成法,直接在炭材料载体上利用沸石生长制备炭基-沸石复合膜,沸石不易在惰性的炭材料表面成核生长、形成连续膜层,而引入沸石晶种再成膜是一种良好的炭基-沸石复合膜制备方法.分析了引入晶种不同方法成膜的特点,对炭基-沸石复合膜材料的应用和发展进行了展望.     

沸石分子筛在大气与水污染治理应用的研究进展

介绍了沸石分子筛结构、性质以及近年来国内外利用沸石分子筛在大气与水体污染治理领域中的研究与应用进展,阐述了该材料在大气净化及污染水体治理领域中应用的原理及优势,提出了针对应用对象有效调控孔径大小及吸附容量、可循环利用的高效吸附降解复合材料制备是沸石研究的发展趋势。沸石分子筛的合理利用能为国内日益加剧的大气与水污染治理提供支持。     

随机游动法研究沸石晶内扩散

用随机游动方法研究了在简化的二维网络中的扩散过程，考察了沸石晶内的两种孔堵塞分布和晶内次级孔道对扩散的影响。结果表明，无因次晶内扩散系数随孔堵塞率的增加而迅速下降。对均匀孔堵塞分布，这是由于孔堵塞破坏了晶内的连续性网络；而对边缘孔堵塞分布则是由于减少了整个网络对外界通道的开放程度。沸石晶内次级孔的存在可以大大改善扩散性能，次级孔百分数与扩散系数之间有良好的线性关系。     

Quantifying molecular surface barriers and intracrystalline diffusion in nanoporous materials by zero-length column

The zero-length column (ZLC) method has been frequently used to measure the effective diffusivity of guest molecules in nanoporous crystalline materials. Recent studies unveiled that the mass transport of guest molecules could originate from either intracrystalline diffusion and/or surface barriers. Directly quantifying the intracrystalline diffusivity and surface permeability of guest molecules. therefore, is essential for understanding the mass transfer and thus optimizing the design of nanoporous crystalline materials. In this work, we extended the ZLC method, based on a derived theoretical expression of the desorption rate, to decouple the surface barriers and intracrystalline diffusion from effective diffusion of guest molecules in nanoporous materials. The diffusivities of ethane, propane and n-pentane in SAPO-34 and Beta zeolites have been experimentally measured to verify the effectiveness of this method.     

An approach for predicting intracrystalline diffusivities and adsorption entropies in nanoporous crystalline materials

Confinement of molecules in nanoporous crystalline materials results in the unique and diverse characteristics of intracrystalline diffusion and adsorption, which can significantly affect the efficiency of gas separation and/or catalysis. However, understanding the interplay between confinement and intracrystalline diffusion and adsorption remains elusive at the quantitative level. In this work, it is found that the intracrystalline diffusion could be related to the hopping rate, which might be further connected to the translational and rotational motion of molecules and quantified by corresponding partition functions. Based on this analysis, the correlations capable of predicting the intracrystalline diffusivity and the adsorption entropy are developed. It is shown that the correlations can well capture the experimental and simulation results of more than 20 frameworks, including zeolites and MOFs, for a wide range of guest molecules. This approach can potentially serve as rapid screening tool for nanoporous crystalline materials in gas separation and catalysis.     

A chromatographic study of diffusion of single components and binary mixtures of gases in 4A and 5A zeolites

Results of a chromatographic study of diffusion of light hydrocarbons and other permanent gases in 4A and 5A zeolites are reported. The data for CF₄ and iC₄H_m, on 4A sieve (no penetration of the zeolite lattice) suggest that micropore diffusion occurs primarily by a Knudsen mechanism. The majority of the measurements were made with small particles of crushed commercial pelleted zeolites under conditions of intracrystalline diffusion control. The dominance of intracrystalline resistance is shown by the similarity in the time constants obtained with different sizes of particle and the results are generally consistent with earlier gravimetric data. The data for the binary systems containing two adsorbed species are consistent with the simple hypothesis that each species in the mixture diffuses independently with the same intrinsic mobility as for single component diffusion at the same temperature. However, the actual diffusivity of a species in the binary mixture may differ from the pure component diffusivity due to differences in the form of the quilibrium isotherms.     

零长柱技术用于分子筛晶内扩散的研究进展

分子筛晶内扩散系数对吸附分离和择形催化具有重要的指导作用。概述了重量法、核磁共振法和分子模拟等分子筛晶内扩散研究方法,并对零长柱技术用于分子筛晶内扩散的原理和应用等方面研究进行了详细阐述,包括两种进样方式（气相进样和液相进样）的单组分气相扩散、二元体系气相扩散和互扩散、液相扩散以及吸附扩散数学模型修正和拓展,还探讨了当前存在的一些问题,最后对零长柱法用于烃类分子在多孔材料中的扩散研究进行了展望。     

Evaluation of Equilibrium and Kinetic Parameters of Smaller Molecular Size Amino Acids on KX Zeolite Crystals via Liquid Chromatographic Techniques

Abstract

Liquid chromatographic techniques have been employed to evaluate the sorption and diffusion characteristics of smaller molecular size amino acids on large zeolite KX-type crystals. The three amino acids investigated in the present study were cysteine, threonine, and serine. All three amino acids adsorbed quite rapidly but there are significant differences in both the equilibrium constants and intracrystalline diffusivities. Combined with the kinetic and equilibrium data of the other three amino acids reported previously, it can be concluded that the larger molecules generally show a lower equilibrium constant. However, no obvious trend can be observed for intracrystalline diffusivity data.     

The Zeolite Micropore as a Unique Reaction Field for the Selective Catalytic Reduction of NO by Hydrocarbon

Zeolite micropores offer unique reaction fields for catalytic reactions based on narrow pores, a wide variety of channel structure, ion exchangeability and strong solid acids. As a negative effect, narrow pore and channels may suppress the diffusivity of reactant molecules and reduce the catalyst effectiveness factor. This article focuses on the influence of the intracrystalline unique reaction fields of zeolite on the selective catalytic reduction of NO by hydrocarbons (HC-SCR) under extreme conditions, i.e. high space-velocity ratio and low content of reactants in exhausts. Zeolite diffusivity in the HC-SCR is affected by two types of diffusion: geometry-limited diffusion, which is determined by the relative size of diffusing molecule and zeolite pore-opening, and adsorption-controlled diffusion, which is determined by the strong interaction between diffusing molecule and intracrystalline cation.     

Influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbon over Cu-MFI zeolite

The influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbons (propane, ethene) was investigated by using Cu-MFI catalysts of different zeolite crystal sizes. The influence of hydrocarbon as a reductant on the diffusion process was also investigated, employing kinetic and temperature-programmed desorption studies. In the NO–C₃H₈–O₂ reaction, the apparent reaction rate of NO conversion into N₂ did not depend on the zeolite crystal size, which indicates that the reaction is not controlled by intracrystalline diffusion. In the NO–C₂H₄–O₂ reaction, on the other hand, the apparent reaction rate evidently depended on the zeolite crystal size; the reaction rate over large crystal Cu-MFI (1.29 μm) was significantly less than that over a small one (0.09 μm), which indicates that the reaction is controlled by intracrystalline diffusion. The larger diffusion resistance in the NO–C₂H₄–O₂ reaction was attributed to the slower diffusion rate of ethene in zeolite channels than propane, which was due to much stronger interaction of ethene with Cu-MFI catalyst. Thus, the adsorption property of hydrocarbon on the Cu-MFI catalyst is revealed to play an important role in determining intracrystalline diffusivity and the diffusion influence on the selective reduction of NO over zeolite catalysts.     

Liquid-phase diffusivity of benzene within mesoporous materials measured by a laser Raman technique

The intracrystalline diffusivities of benzene within a series of porous materials in the liquid phase (cyclohexane was used as solvent) were measured by a constant volumetric method using Raman spectroscopy at a temperature range from 323 to 393 K. Silicalite-1, mono-dispersed mesoporous silica spheres (MMSS), silica gel, γ-Al₂O₃, and SiO₂–Al₂O₃ were used as adsorbents. The intracrystalline diffusivity was calculated by parameter fitting using theoretical equations and the experimental transient change of benzene concentrations with time in response to the adsorption. The intracrystalline diffusivities of benzene within mesoporous silicas were almost the same as that within the micropore of silicalite-1, though the pore diameters of silicalite-1 and mesoporous silicas were different from each other. It is considered that the pore walls as well as the solvent molecules of cyclohexane affect the diffusion of benzene molecules in the mesopore region.     

Limitations of the Zero‐Length Column Technique to Measure Diffusional Time Constants in Microporous Adsorbents

The zero‐length column (ZLC) technique has been developed to measure the intracrystalline diffusivity of strongly adsorbed species in large zeolite crystals above 50 μm in the Henry's law range of sorption equilibrium. The ZLC is a macroscopic technique, and there is a need of large crystallites or pellets to measure the intracrystalline diffusivity D_c of fast diffusion species or the macropore diffusivity D_P of weakly adsorbed species, respectively. Another limitation is that ZLC desorption curves produce similar concentration profiles (linear isotherms) in bidisperse adsorbents (pellets) under macropore or micropore diffusion control. Moreover, the forms of the response curves are very similar in both diffusion‐ and nonlinear equilibrium‐controlled processes, leading to some misinterpretations of ZLC experiments. In this work, two criteria are developed showing that, in order to macroscopically measure the micropore diffusion time constant  or the macropore diffusion time constant , the time of the ZLC experiments t should be higher than 7.0 × 10^?2 or 7.0 × 10^?2 , respectively. The interpretation of the ZLC response curve data is also checked in two completely different regimes, showing that a single ZLC response curve is not enough to conclude if a system is under a kinetic or an equilibrium regime.     

Is 1-hexene epoxidation in TS-1 diffusion limited in different solvents?

PFG NMR diffusion measurements were carried out to determine the effect of solvent on intracrystalline reactant diffusivities and on 1-hexene epoxidation rates in TS-1 catalyst. Using n-hexane in silicalite as a mimic for the TS-1 system, the self-diffusivity of n-hexane in silicalite was found to be 24% higher in methanol solvent than in acetonitrile solvent and 45% higher than in acetone solvent. The presence of trace Al did not affect n-hexane diffusivity. Based on analysis of the Weisz modulus for a slab morphology, the 1-hexene epoxidation reaction in TS-1 was found to be diffusion limited only if the crystal size is at least 38 μm in the methanol system.