Zeolite crystal layer

Fig. 1 Schematic of the three types of zeolite membranes (A) a polycrystalline zeolite membrane (B) a zeolite matrix composite membrane and, (C) a zeolite crystal layer.

Another form of zeolite membranes is a zeolite crystal layer that consists of isolated crystals deposited on a solid substrate (Fig. 1C). The substrate can be a variety of materials such as metal, ceramic, or silicon wafer. Crystal layers have to be supported. There has been exciting fundamental research carried out in this area, however, demonstrated applications have been limited to sensors. The organic linker approach appears very promising for the preparation of these types of membranes. ... 

Thus zeolite ZSM-5 can be grown (ref. 15) onto a stainless steel metal gauze as shown in Figure 6. Presumably the zeolite crystals are chemically bonded to the (chromium-) oxide surface layer of the gauze. After template removal by calcination and ion exchange with Cu(II) a structured catalyst is obtained with excellent performance (ref. 15) in DeNOx reactions using ammonia as the reductant. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

Dwyer et al. (43) have also reported that dealumination of Y zeolites by a steam/acid leaching process produces a more uniform composition than dealumination by EDTA. The later method caused a depletion of Al in the outermost surface layer, producing a compositional gradient in the zeolite crystals. The conclusions reached by J. Dwyer in his studies of aluminum-deficient zeolites using the FABMS method are summarized in Table IV. 

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

Depending on the size of an incorporated dye, the angle of the transition dipole moment to the c axis lies between 0° for long molecules and 72° for smaller ones. Therefore, if a small molecule is inserted into the channels of zeolite L, part of the emission will be parallel to the c axis. Due to the flat and parallel ends of appropriately prepared zeolite crystals, one can envisage to arrange crystals between two mirrors or to add a reflecting layer on individual crystals. This might lead to a microlaser with a plane-parallel resonator. Apart from experimental difficulties, the realization of a dye-loaded zeolite L microlaser appears to be feasible. 

As the Beckmann rearrangement is believed to be a typical acid-catalysed reaction, many researchers have reported the relationship between the vapour phase reaction catalysis and the acidity of the catalysts tested on non-zeolitic catalysts - i2s- i3i. 318-334 and on zeolitic catalysts Another interesting point for the heterogeneous gas-phase Beckmann rearrangement is the location of the reaction on the catalyst and different studies have been published ° . The outer surface of the catalyst particle seems to be the most probable place for the Beckmann rearrangement supported by the traces of reagents, and notable amounts of by-products found only in the outer layers of the zeolite crystal. Development of new and more efficient catalysts have also been reported " . ... 

The CgQ surface coverage was determined to be 2.0 10 mol cm . The monolayer can be further modified with monomeric amine reagents, which demonstrates the potential of the self-assembly process for growing three-dimensional fullerene structures. Different surfaces such as quartz. Si-oxide [105] or ITO [102] were coated with multilayers of fullerene up to stacks of 9 layers. An imidirectional electron transfer is possible across the fullerene mulhlayers [102]. Not only can multiple layers of fullerenes be connected to a certain surface but amino-functionalized can also serve as a linker between two different surfaces. 3-Aminopropyl-tethered glass plates could be linked via a Cgg layer to 3-aminopropyl covered zeolite crystals [106]. 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

It is generally accepted that the ionic exchange process in zeolites is described by three steps [23-25] (see Figure 7.3) (1) interdiffusion in the adhering liquid thin layer (0 < t < /,) (2) intermediate step, where interdiffusion in the liquid thin layer and crystalline interdiffusion are both present (0 tb). 

Now, to explain the operation of the PFIEBR, it is proposed that the interdiffusion in the adhering liquid thin layer is the rate-determining step, then, it is possible to consider that n = 1 in Equation 7.33, since for this transport process the diffusion rate, k, is proportional to concentration [38], This approach is based on the assumption that states that the rate-determining process during the dynamic ionic exchange in zeolite columns determines the diffusion in the zeolite secondary porosity, that is, the transport process in the macro- and mesoporosities formed by the matrix inserted between zeolite crystals and the diffusion in the zeolite primary porosity, that is, in the cavities and channels which constitute the zeolite framework [38], This fact is experimentally justified later. With the help of Equations 7.33 through 7.35, we obtain ... 

In diagnostic tests crushing of the particles will not always be conclusive. Egg-shell catalysts or other types, zeolites and washcoated monoliths are exceptions. In washcoated monoliths the layer thickness is generally already quite low (<50 (im) and crushing will not yield smaller sizes. Cracking catalysts consist often of zeo-litic crystals of /im dimensions and a binder yielding particles of about 30 /im. If diffusion limitations exist in the zeolitic crystals, crushing will not eliminate these. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

If the values of the effective self-diffusion coefficients, D rf [calculated from the complete xit) curves in TD NMR experiments, assuming diffusion-limited uptake (52)] are below the corresponding intracrystalline data, Dintra (measured directly by PFG NMR), the existence of additional mass transfer resistances in a layer near or on the outer surface of the zeolite crystals is indicated. 

It will be clear that in case of onedimensional zeolites the orientation of the zeolite crystals should be such that the channel direction is perpendicular to the membrane layer configuration. Figure 2 shows in a schematic way several configurations in which zeolites govern - or contribute to -membrane permeation. 

In type a., the separating zeolite layer is equipped with catalytic sites (Bronsted add sites, Lewis acid sites (cations, special Al-sites), metal clusters, catalytic complexes). In type b., the non-supported side of the zeolite layer serves as a support for catalytic entities, e.g. metal crystallites. In type c., zeolite crystals with catalytic power are embedded in a matrix, e.g. a polymer membrane. 

Since then, layers of grown-together zeolite crystals have been prepared on porous supports of stainless steel [93] or of porous alumina [69,72,94], showing very promising results (see Table 2). However, major steps still have to be taken in order to render these highly selective porous membranes reliable and cheap enough to be produced at an industrial scale. If these problems are solved, the porous IMR technology will probably make its way toward practical success. 

The most successful approach to control membrane formation involves segregation of the processes of crystal nucleation and growth [24]. The so-called ex situ or secondary (seeded) growth methods, unlike the direct synthesis procedures just discussed, include a first step in which a closely packed layer of colloidal zeolite crystals, synthesized homogenously, is deposited onto... 

In general, the properties and separation abilities of the resulting membranes depend on the synthesis procedure. The amount of zeolitic material, support composition, penetration and adhesion to the support, orientation of the zeolite crystals, the density and distribution of nonzeolitic pores (i.e., intercrystalline voids), crystal boundaries, and the thickness of the zeolite layer are the main variables which affect the quality of the obtained membrane. 

Zeolite membranes are formed by a polycrystalline layer, which means that the permeation is not only through the ordered nanopores of the zeolite crystal, intracrystalline pathways, but also through the spaces between the crystals, intercrystaUine pathways (see Eigure 10.20). Nomura et al. [116] proposed a model taking into account this consideration. 

Single-layer zinc-phosphate zeolite crystals were grown with more than 90% of their (111) faces oriented to a gold-coated silicon surface. Sudi oriented zeolite films might find application as membrane catalysts or as specific chemical sensors [66]. 

The use of a matrix to define the reaction space is an intrinsically attractive approach to the preparation of large amounts of material which could be deposited from solution or from the vapor phase. A number of matrices have been used including zeolites [80], layered solids [81], molecular sieves [82-84], micelles/ microemulsions [85-89], gels [90-92], polymers [93-97] and glasses [98]. The matrix provides a mesoscopic reaction chamber in which the crystal can only grow to a certain size. 

Defect-free zeolite membranes have so far only been produced for membranes of the MFI (silicalite type) with thicknesses of about 50 im on stainless steel supports and 3-10 pm on alumina and carbon supports. They are produced by in situ methods of zeolite crystals grown directly on the support system. There are some reports of formation of defective membranes with, e.g., zeolite A. Much more research is needed to widen the range of available zeolite membrane types especially small and wide pore systems. The permeance values of the defect-free membranes is lower than that of the amorphous membranes (see Chapter 6) and to improve this the layer thickness must be decreased together with improving the crystal quality (no impurities, no surface layers, high crystallinity, crystal orientation) and microstructure (grain boundary engineering). 

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