Guest Diffusion in Binderless High‐Performance NaX Molecular Sieves

The pulsed field gradient (PFG) NMR technique is applied for exploring the diffusion behavior of guest molecules in binderless NaX beads in comparison with the zeolite powder employed for their production. With both probe molecules applied (water, n‐hexane), the diffusivities in the powder and the beads are found to essentially coincide as long as the diffusion path lengths are sufficiently small in comparison with the extension of the individual particles (crystallites) of the powder. With increasing diffusion path lengths, characteristic deviations become observable that can be attributed to the differences in long‐range mass transfer through the intercrystalline void volume of the bed of crystallites and within the individual beads of the binderless molecular sieve, respectively. With these studies, PFG NMR demonstrates its potentials for simultaneously recording mass transfer phenomena in both the micro‐ and macropores of commercially produced binderless molecular sieves.     

Removing nitrosamines from mainstream smoke of cigarettes by zeolites

The adsorption of volatile nitrosamines in mainstream smoke of cigarettes by the use of zeolites and other porous materials has been investigated. Zeolites are shown to possess the ability of selectively adsorbing nitrosamines from the mainstream (MS) smoke, and among them, zeolites NaY and ZSM-5 exhibit excellent capacities. The impact of the porous structure and acidity of the adsorbent, the influence of co-adsorption of nitrogen oxides and the catalytic behavior of the zeolite for nitrosation are also explored. The use of zeolites as selective adsorbents for removing carcinogenic compounds from cigarette smoke would be an effective strategy to protect the environment and public health.     

Asymptotically exact driving force approximation for intraparticle mass transfer rate in diffusion and adsorption processes: nonlinear isotherm systems with macropore diffusion control

An asymptotically exact nonlinear driving force model of intra-particle mass-transfer rate for nonlinear isotherm systems with macropore diffusion control is presented. The obtained expression is compared with the solutions of the Fickian diffusion and adsorption model and excellent accuracy over the entire time (fractional uptake) domain is demonstrated. In the case of an irreversible isotherm the model reduces to the equations resulting from the shrinking core model a fact that guarantees its accuracy for highly nonlinear systems. The high accuracy of the model is further demonstrated by comparison with experimental data under various operational conditions.     

Molecular Sieve Membranes for Industrial Application: Problems,Progress, Solutions

Molecular sieve membranes are characterized by their high thermal and chemical resistance and by their monodisperse micropore system. Mixtures can be separated by their molecular size as well as by adsorptive interactions and differences in the diffusion coefficients. In this paper, techniques for a reproducible preparation of large area tubular zeolite membranes together with ways for their regeneration and repair are shown. The separation figures demonstrate the efficiency of the molecular sieve membranes developed. Potential application fields are discussed.     

Adsorption of carbon dioxide, ethane, and methane on titanosilicate type molecular sieves

The separation of carbon dioxide from light hydrocarbons is a vital step in multiple industrial processes that could be achieved by pressure swing adsorption (PSA), if appropriate adsorbents could be identified. To compare candidate PSA adsorbents, carbon dioxide, methane, and ethane adsorption isotherms were measured for cation exchanged forms of the titanosilicate molecular sieves ETS-10, ETS-4, and RPZ. Mixed cation forms, such as Ba/H-ETS-10, may offer appropriate stability, selectivity, and swing capacity to be utilized as adsorbents in CO₂/CH₄ PSA processes. Certain cation exchanged forms of ETS-4 were found to partially or completely exclude ethane by size, and equivalent RPZ materials were observed to exclude both methane and ethane, while allowing carbon dioxide to be substantially adsorbed. Adsorbents such as Ca/H-ETS-4 and Ca/H-RPZ are strong candidates for use in PSA separation processes for both CO₂/C₂H₆ and CO₂/CH₄, potentially replacing current amine scrubber systems.     

4A分子筛吸附净化异丙醇中微量水的研究

对异丙醇中微量水在4A分子筛上的吸附性能进行了研究。对该体系的吸附平衡数据进行了测定,并用Langmuir、Freundlich模型进行拟合且吻合较好;采用Crank单孔模型对测定的吸附动力学数据进行拟合,得到了290K时的有效扩散系数De=3.539×10-8cm2/s;采用固定床测定了不同床层高度、不同流量及不同初始浓度下的动态透过曲线;这些结果为吸附工艺设计提供了基础数据。     

Adsorption of lactic acid from fermentation broth and aqueous solutions on Zeolite molecular sieves

The recovery of lactic acid from fermentation broth and aqueous solutions was studied by adsorption on Silicalite molecular sieves. Batch experiments were used to measure the adsorption isotherms of the lactic acid on Silicalite. A linear correlation was found for both solutions. Silicalite showed a higher adsorptive capacity in the case of the aqueous solution than that of the fermentation broth. Henry's constants were estimated as and for the aqueous and broth solutions, respectively. The effect of temperature on adsorption was also studied in batch mode. Henry's constant dependency on temperature was derived from Van’t Hoff's equation. The heat of adsorption was calculated as (29±17) kJ/mol. The kinetics of adsorption was investigated in column studies where the breakthrough and elution curves were measured. The adsorption process was controlled by the internal diffusion in the Silicalite pellets rather than the diffusion through the fluid film around the pellet. The fluid phase resistance was estimated as 21% of the overall resistance. The glucose presence in the fermentation broth had a negligible effect on lactic acid breakthrough curve in the studied range.     

Representation of Adsorption Data for the Isopropanol‐Water System using Neural Network Techniques

Molecular sieves and palm stone, a newly developed bio‐based adsorbent, were used to break an azeotropic isopropanol‐water system via an adsorptive distillation process. Equilibrium data at different inlet water contents are presented. The data were obtained with a fixed bed adsorptive distillation process using Type 3A and Type 4A molecular sieves and palm stone. An artificial neural network (ANN) technique was used to represent the isotherm equilibrium data of this azeotropic system. The ANN prediction results were compared with the Guggenheim‐Anderson‐de Boer (GAB) isotherm model. It was possible to break the isopropanol‐water azeotrope using this separation process with the adsorbents used in this work. Water uptake increases as the water content in the feed decreases from 16 % to 10 %. Although the GAB isotherm model was found to be applicable to the water vapor sorption data on the adsorbents examined, the ANN model fitted the equilibrium data more efficiently.     

Rational design of shape selective separation and catalysis—I: Concepts and analysis

A molecule can enter a zeolite channel when it fits the size and shape of the opening. Zeolites have been used commercially for their ability to admit selectively molecule A but not molecule B. The search and design of zeolites for separation has been based on finding a channel with a diameter that is larger than molecule A, but smaller than molecule B. This method is not sufficiently accurate when the molecule is not a sphere or the channel is not a circle. We present here first a more accurate screening method based on comparing both the major and minor diameters of the molecule projection, called the “footprint”, with the diameters of the channel. Then we present an even better method that is based on the activation energy required to strain and distort a molecule to fit a given channel, thus leading to a lowering of its Boltzmann concentration in this channel. This paper compiles the activation energies encountered among thirty-eight (38) molecules and two hundred and seventeen (217) zeolite channels into a database, and shows how one can use this database to identify the most promising zeolites for the separation of a set of molecules of interest. Such a screening method would help to identify a zeolite that has much lower activation energy for molecule A than for molecule B. The appropriate temperature range for separation would be centered around the optimal temperature T^*, where the degree of selectivity is at a maximum. The separation of the three pentane molecules was selected as an illustration.     

Effects of carbonisation on pore evolution and gas permeation properties of carbon membranes from Kapton polyimide

Carbon membranes were prepared by carbonisation of Kapton^® polyimide at different temperatures under vacuum and nitrogen flow. Pore structure development of the membranes during carbonisation was studied. Carbonisation temperature was critical in the modification of membrane structure. At the same temperature, the carbon membranes fabricated under nitrogen atmosphere had higher gas permeances than those fabricated under vacuum. During heat treatment, the value of d-spacing for the carbon membranes decreased with increasing temperature, however, vacuum and nitrogen atmosphere had different influences on the changes in the d-spacing. CO₂ adsorption showed that the carbon membranes prepared at 1273 K under vacuum had the highest micropore volume whilst the membranes prepared at 1073 K under vacuum had the highest characteristic adsorption energy. N₂ adsorption showed that the samples obtained at 873 K under vacuum had the highest nitrogen uptake. Mesopores were deemed to be connected through micropores and narrow channels between meso- and/or micropores were supposedly present. The micropores predominantly controlled the transport properties of the carbon membranes. The membrane samples obtained at 1173 K under vacuum yielded ideal separation factors of 558.27, 60.87, 19.69 and 138.53 for He/N₂, CO₂/N₂, O₂/N₂ and CO₂/CH₄, respectively, with permeances of 7.26, 0.79, 0.26, 0.13 and 0.006 mol/(m² s Pa) for He, CO₂, O₂, N₂ and CH₄, respectively.     

Rational design of shape selective separation and catalysis—II: Mathematical model and computational studies

In our previous work [Gounaris, C.E., Floudas, C.A., Wei, J., 2006. Rational design of shape selective catalysis and separation: I. Concepts and analysis. Chemical Engineering Science, in press, doi: 10.1016/j.ces.2006.09.012], we introduced the concept of molecule projections, referred to as footprints, and briefly described rigorous criteria that define them. We also introduced the concept of strain index and showed how it can be used to identify portals that have the potential of being highly selective between two molecules. In this paper, we present in detail the mathematical formulations for the calculation of footprints and the complete mathematical model and algorithmic framework developed for the calculation of the strain index of any given molecule/portal pair. Thirty-eight molecules of interest, as well as a collection of 123 zeolite structures, involving 217 different windows, have been considered and selectivity results at ambient temperature are presented. A number of commercially interesting examples are studied in further detail, across a wide range of temperatures. It should be noted that the methodology is generic and can be applied for any pair of molecule and portal. The results indicate that there is a great potential in systems involving non-regular portals that are usually not considered in the selection process of a catalyst or molecular sieve.     

Impact of Zeolites on the Petroleum and Petrochemical Industry

The general features of zeolites that led to their widespread use in oil refining and petrochemistry are highlighted as well as the details of their impact on selected processes. The analysis of the catalyst market and the position of zeolites therein is a good indication of their strategic importance. Zeolites have brought many disruptive changes to these fields (e.g. FCC). They impacted also these industries in an equally important way, although more subtle, by incremental improvement of processes. The new and vast challenges facing oil refining and petrochemistry as well as the managed transition to sustainable environmental benign transport fuel industries and chemical industries will require creative science and technologies. Zeolites offer the basis of many of these technological solutions provided efficient and balanced cooperations between industry and academia are further developed.     

Zeolite-based materials for novel catalytic applications: Opportunities, perspectives and open problems

Zeolites and related materials (including a wide range of microporous and mesoporous materials with ordered pore structure) have been one of the areas in the field of materials and catalysis with the largest impact on science, technology and industrial processes. We discuss here some recent developments in this field, with particular references how to tailor and design zeolite and related material properties to control/enhance the catalytic performances. Four main topics have been addressed. (i) The recent progress and perspectives in the field of tailored syntheses, with selected examples showing the trend and prospects to develop new structures, control the location of active sites, and the crystal size and morphology, including nanoarchitecture of the final catalysts. (ii) The development and prospects of two-dimensional zeolites presenting an extended view/concept of zeolite structures integrating the classical 3D frameworks and the various lamellar forms. (iii) The progresses in the design and synthesis of hierarchical zeolites, with discussion on the still existing challenges related to the synthesis, characterization and catalytic application. (iv) Novel opportunities and needs in terms of zeolite multifunctional design for catalytic applications, with a discussion of the critical issues related to the use in the field of fine chemicals, organic industrial syntheses and biorefinery, and the prospects for the use in two novel challenging areas of the direct conversion of CO₂ to light olefins and methane to methanol.     

Adsorption of propane and propylene onto carbon molecular sieve

Equilibrium data for propane and propylene adsorption on a carbon molecular sieve (CMS) 4A from Takeda are presented in the temperature range 343-423 K and 0-300 kPa pressure. The pellet adsorption loading is 0.9 mol/kg for propane and 1.2 mol/kg for propylene at 100 kPa and 373 K. The equilibrium selectivity for propylene in the low-pressure range are 2.3 (343 K) and 1.7 (423 K). Experimental data were fitted with the Toth and Dubinin models. Zero length column (ZLC) technique has been used to determine the controlling mechanism and estimate the diffusivity parameters. Transport of both hydrocarbons in the pellets is controlled by micropore diffusion. Breakthrough curves were measured in the same temperature range and atmospheric pressure, at the low partial pressure of adsorbate (linear region of the isotherm). Simple models have been used in the simulation of breakthrough curves.