Zeolite membrane interfaces

Organosilane coupling agents have been widely used to improve the adhesion at the zeolite/polymer interface of the mixed-matrix membranes ]62, 63, 65, 70]. 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

Fig. 3 One-dimensional loading profiles of benzene across a NaX zeolitic, single crystal membrane. The loading is the spatial average over planes perpendicular to the main diffusion direction (three-dimensional simulations are conducted periodic boundary conditions are employed in the transverse direction and Robin at the membrane interfaces exposed to the high- and low-pressure sides). The inset shows a schematic of the membrane. (View this art in color at www. dekker.com.)...

Tricoli V, Sefdk J, McCormic AV (1997) Synthesis of oriented zeolite membranes at the interface between two fluid phases. Langmuir 13 4193... 

In other respects, we can consider zeolite membranes as pertaining to the ceramic material category. Indeed, zeolites are classified for the most part as microporous, crystalline silicoaluminate structures with different aluminum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeolites are crystalline materials and they have a structural porosity very different from microporous amorphous silica [167]. Zeolite films can be grown as intergrown layers on porous metallic and ceramic membrane supports. These zeolite films constitute a special type of nanostructured interface capable of very specific interactions with individual molecules so that it can be used as membrane for the selective separation of molecular... 

Zeolite membranes have been applied for gas permeation and separation, and liquid pervaporation. A clear advantage of microscale zeolite membranes is the higher probability of obtaining a defect-free interface, since this probability increases for smaller membrane areas [41]. In zeolite MMRs, the zeolites are incorporated as a catalyst for reaction and a membrane for separation, as well as structural material of the reactors. Reactions conducted in MMRs include mainly Knoevenagel condensation [3, 42,43] and selective oxidation reactions [39]. Supra-equilibrium conversion may be obtained in the former, while the latter displays improved performance against catalyst deactivation. 

The resistivities for the membrane are found from the concentration and temperature on the /-side of the membrane. All values for resistivities inside the membrane are taken from Inzoli et al. (2008) see Fig. 18.5. These values are obtained for a system with -oriented silicalite-1, where the straight channels in the silicalite emerge on the gas-zeolite interface. All values in the work of Inzoli are obtained for such a system. Also, that model does not take defects, for example, pinholes or small fractures, into account. For practical applications, the zeolite membrane has to be fixed to some frame... 

PolycrystalHne membrane growth proceeds by initial formation of a gel layer on the surface of the support crystallization takes place at the interface between the bulk Hquid phase and the gel layer, resulting in deposition of zeolite nuclei and crystals formed [8]. Concurrently, the crystals deposited onto the support surface continue to grow, eventually resulting in a continuous membrane layer. Postsynthesis treatment is necessary when a template is used in synthesis to activate the zeolite and open the pores. Usually this is accomplished through calcination or burn out of the organic molecule. 

Most reported zeolite/polymer mixed-matrix membranes, however, have issues of aggregation of the zeolite particles in the polymer matrix and poor adhesion at the interface of zeolite particles and the polymer matrix. These issues resulted in mixed-matrix membranes with poor mechanical and processing properties and poor separation performance. Poor compatibility and poor adhesion between the polymer matrix and the zeolite particles in the mixed-matrix membranes resulted in voids and defects around the zeolite particles that are larger than the micropores of the zeolites. Mixed-matrix membranes with these voids and defects exhibited selectivity similar to or even lower than that of the continuous polymer matrix and could not match that predicted by Maxwell model [59, 60]. 

Most recently, significant research efforts have been focused on materials compatibility and adhesion at the zeoHte/polymer interface of the mixed-matrix membranes in order to achieve enhanced separation property relative to their corresponding polymer membranes. Modification of the surface of the zeolite particles or modification of the polymer chains to improve the interfacial adhesion provide new opportunity for making successful zeolite/polymer mixed-matrix membranes with significantly improved separation performance. 

Gavalas et al. [7] prepared ZSM-5 membranes onto porous a-alumina disks by in-situ hydrothermal synthesis at 175°C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. The films grown at the optimized conditions were about 10 pm thick and consisted of well-intergrown crystals of about 2 pm in size Pure gas permeation measurements of the best preparations yielded hydrogen isobutane and butane isobutane ratios of 151 and 18 at room temperature and of 54 and 31 at 185°C, respectively. 

FIGURE 11.5 Study of the relation between the flux J and AP, during the transport of H2 and CO2 through a porous ceramic membrane produced by thermal treatment of a natural zeolite. (From Roque-Malherbe, R., Applications of natural zeolites in pollution abatement and industry. In Nalwa, H.S., ed.. Handbook of Surfaces and Interfaces of Materials, Vol. 5, Academic Press, New York, 2001.)... 

In asymmetric supported membranes the use of permeability data can give rise to much confusion and erroneous conclusions for several reasons. In most cases the layer thickness is not precisely known and usually it is not known whether this layer is homogeneous or has property gradients (e.g. a "skin" and a more porous part). In many cases the material of the layer penetrates the support to some extent and so it is not possible to separate properties of separation layer and support without giving account of the interface effect. Finally, even if all these complications can be avoided, a comparison based on separation layer properties expressed in terms of permeabilities can give a completely wrong impression of the practical possibilities (as done in e.g. Ref. [109]). This is illustrated by comparison of hydrogen permeabilities of ultra-thin silica layers (see Tables 9.14-9.16) with other materials such as zeolites and metals. The "intrinsic" material properties of these silica layers are not impressive ... 

More recently, these systems have been elevated to a new level of biomimetic sophistication, namely by embedding the zeolite-encapsulated complex in a polydimethylsiloxane membrane their performance could be improved even further [81]. The hydrophobic membrane mimics the pho holipid membrane in which cytochrome P-450 resides and acts as an interface between two immiscible phases (cyclohexane and aqueous 70%TBHP). 

One of the first zeolite based membranes were composite membranes, obtained by dispersion of zeolite crystals in dense polymeric films in order to make zeolite filled polymeric membranes [59,60,61], These membranes have been developed at the end of the 80 s for both gas separation and pervaporation. The clogging of zeolite pores by the matrix and the quality of the interface between the zeolite crystals and the polymer matrix (non-selective diffusion pathways) were key points. 

In particular, for the synthesis of optically pure chemicals, several immobilization techniques have been shown to give stable and active chiral heterogeneous catalysts. A step further has been carried out by Choi et al. [342] who immobilized chiral Co(III) complexes on ZSM-5/Anodisc membranes for the hydrolytic kinetic resolution of terminal epoxides. The salen catalyst, loaded into the macroporous matrix of Anodise by impregnation under vacuum, must exit near the interface of ZSM-5 film to contact with both biphasic reactants such as epoxides and water. Furthermore, the loading of chiral catalyst remains constant during reaction because it cannot diffuse into the pore channel of ZSM-5 crystals and is insoluble in water. The catalytic zeolite composite membrane obtained acts as liquid-liquid contactor, which combines the chemical reaction with the continuous extraction of products simultaneously (see Figure 11.28) the... 

Since different compounds permeate the membranes with different rates, spectral interference can be reduced [60], The membrane materials, used for that purpose, include polypropylene, poly(tetrafluoroethylene), cellulose, silicone rubber, dimethylvinyl silicone, polyethylene, and zeolite. The membrane can be interfaced to the mass spectrometer in different configurations - directly or with the aid of a sweep gas. Using MIMS, the sampling probe can be taken away from the mass spectrometer to which it is connected by a tube. Therefore, sampling of various environmental matrices can be conducted in situ. In fact, portable MIMS systems can be used as monitors, providing vital information which is not offered by other technologies (e.g., fluorescence, infrared spectroscopy) [64]. 

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