Shape zeolite

Suzuki, H. (1987) Composite membrane having a surface layer of an ultrathin film of cage-shaped zeolite and processes for production thereof US Patent 4,699,892. 

So, heu of "ctemtcai iwccnn (STM and related apparatus) we daim to e(fea particular stereo-chcrncal syntheses by using specially shaped zeolites. But it is stated that these specially shaped zeoliics arc < synthesized—without chemical tweezers". How The answer is not as difficult as it may seem. 

H. Suzuki, Composite membrane having a single layer of an ultra thin film of cage shaped zeolite and process for production thereof. US patent 5.069.794. 

Large coffin-shaped zeolite H-ZSM-5 crystals have been chosen as suitable model systems to study the structural motifs and related intergrowth structure, which are also present in smaller powdered zeolite materials used in industrial catalysis. The advantage of large model crystals, which can be prepared with adjustable Si/Al ratio and size, is that the catalytic properties of each growth unit of a zeolite can be studied separately by means of optical microscopy methods, such as UV-vis microspectroscopy and confocal fluorescence microscopy [114, 122]. As both microspectroscopy methods obey the optical Abbe diffraction limit, features can be studied in the size of approximately X/2. The resolution d of an optical microscope is given by... 

Zeolites are crystalline aluminosiUcates characterized by a structure comprising a three-dimensional pore system and regular framework formed by linked TO4 tetrahedral (T = Si, Al) with different morphological and physico-chemical properties. Due to their impressive selectivity and uniform pore structure, they have very efficient molecular sieving properties, and are able to separate molecules based on size and shape. Zeolite powders, films and membranes are widely used in catalysis, adsorption and separation applications (McLeary et al, 2006 Pina et al., 2011). Zeolites are cheap and widely available due to their abundance in both natural and synthetic forms. The application of zeolites in the membrane field is growing very fast, and has been the subject of increased research focus during the last few decades (McLeary et al., 2006). 

An interesting point is that infrared absorptions that are symmetry-forbidden and hence that do not appear in the spectrum of the gaseous molecule may appear when that molecule is adsorbed. Thus Sheppard and Yates [74] found that normally forbidden bands could be detected in the case of methane and hydrogen adsorbed on glass this meant that there was a decrease in molecular symmetry. In the case of the methane, it appeared from the band shapes that some reduction in rotational degrees of freedom had occurred. Figure XVII-16 shows the IR spectrum for a physisorbed H2 system, and Refs. 69 and 75 give the IR spectra for adsorbed N2 (on Ni) and O2 (in a zeolite), respectively. 

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-... 

C2.7.6.7 SHAPE-SELECTIVE HYDROCARBON REACTIONS CATALYSED BY ZEOLITES... 

Haag W O, Lago R M and Weisz P B 1982 Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite Faraday Disouss. Chem. Soo. 72 317-30... 

Ti, B, Ni, Cr, Fe, Co, Mn) has been described, as was the synthesis of nonsiliceous materials such as oxides of W, Fe, Pb, Mo, and Sb [18]. Although these materials do not represent tme zeolites, they are highly interesting materials which are commonly covered in the zeolite literature with great potential for shape-selective catalysis of bulky molecules. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Figure C2.12.10. Different manifestations of shape-selectivity in zeolite catalysis. Reactant selectivity (top), product selectivity (middle) and transition state selectivity (bottom).

Only a very few selected examples have been discussed. The number of processes based on shape-selective catalysis by zeolites is ever increasing, particularly in the field of speciality and fine chemicals and quite a few have been... 

Weitkamp J, Ernst S and Puppe L 1999 Shape-selective catalysis in zeolites Cataiysis and Zeoiites, Fundamentais and Appiications ed J Weitkamp and L Puppe (Berlin Springer) pp 327-76... 

The powders of zeolites of various trademarks are used to produce petroleum-refining catalysts. In this connection, it is very important to have complete information concerning not only chemical composition and distribution of impurity elements, but also shape, surface, stmcture and sizes of particles. It allows a more detailed analysis of the physical-chemical characteristics of catalysts, affecting their activity at different stages of technological process. One prospective for solving these tasks is X-ray microanalysis with an electron probe (EPMA). 

The EPMA technique is developed for individual particles of fine-dispersed zeolite powder of various trademarks ZSM-5, ZSM-12, MOR, BEA. The phase and chemical composition of zeolite powder, the surface, shape of particles, stmcture and their distribution in terms of the size were studied using Superprobe-733 device. 

The detailed study of chemical composition, stmcture, surface, shape and sizes of particles of fine-dispersed zeolite powder by EPMA will provide useful recommendations to improve the technology of producing alkanes and alkyl benzenes catalysts. 

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

To improve the yield of mono- and dimethylamines, a shape selective catalyst has been tried. Carhogenic sieves are microporous materials (similar to zeolites), which have catalytic as well as shape selective properties. Comhining the amorphous aluminum silicate catalyst (used for producing the amines) with carhogenic sieves gave higher yeilds of the more valuable MMA and DMA. ... 

The important property of ZSM-5 and similar zeolites is the intercrystalline catalyst sites, which allow one type of reactant molecule to diffuse, while denying diffusion to others. This property, which is based on the shape and size of the reactant molecules as well as the pore sizes of the catalyst, is called shape selectivity. Chen and Garwood document investigations regarding the various aspects of ZSM-5 shape selectivity in relation to its intercrystalline and pore structure. 

ZSM-5 is Mobil Oil s proprietary shape-selective zeolite that has a different pore structure from that of Y-zeolite. The pore size of ZSM-5 is smaller than that of Y-zeolite (5.1°A to 5.6°A versus 8°A to 9 A). In addition, the pore arrangement of ZSM-5 is different from Y-zeolite, as shown in Figure 3-16. The shape selectivity of ZSM-5 allows... 

Molecular Sieve is a term applied to zeolite. Zeolite exhibits shape selectivity and hydrocarbon absorptions. 

At the low-molecular-weight end of the spectrum, a process newly commercialized by Mobil for converting methanol into gasoline has significantly expanded opportunities in C-1 chemistry— the upgrading of one-carbon molectrles to mrrlticarbon products. The process involves the use of ZSM-5, a shape-selective zeolite catalyst. (See "Zeolite and Shape-Selective Catalysts" in Chapter 9.)... 

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