Shape-selective zeolites crystallization

These shape-selective zeolites can be converted into a catalytically active form by ion exchange since K has a deleterious effect on the catalytic properties of the zeolite. Peterson, Helferich and Blytas (9a) and Sherry (9b) found that with ion exchange below 300°C the K content of erionite could not be reduced below 1.95 weight % or 2 K ions per unit cell without impairing their crystal structures. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

The zeolite provides the environment for shape selective chemistry and is also a high surface area support on which to disperse platinum in a relatively confined environment. The small platinum crystals within the zeolite channels and the orientation effect of the channel window are responsible for the high efficiency of the Pt-KL catalyst to convert linear paraffin to aromatics. Zeolite KL also provides an electron rich environment to enhance stronger platinum-substrate interaction via stronger platinum-support interaction. A review on the subject can be found in the article written by Meriasdeau and Naccache [85]. 

Reglospeclflc functionalization of biphenyl is drawing attention as one of key steps in developing advanced materials such as liquid crystals and liquid crystal polymers [1-5]. Catalysis using zeolites is the most promising way to prepare sterlcally small molecules by differentiating between reactants, products, and/or intermediates according to their size and shape. Sterlc restrictions by zeolites Increase the formation of preferred products and prevent the formation of undesirable products [6]. We describe herein shape selective catalysis of 12-membered zeolites, H-mordenite (HM), HY and HL In the alkylation of biphenyl. 

Important properties of zeolite adsorbents for a fixed-bed application are adsorptive capacity and selectivity, adsorption-desorption rate, physical strength and attrition resistance, low catalytic activity, thermal-hydrothermal stability, chemical stability, and particle size and shape. Apparent bulk density of zeolite adsorbents is important because it is related to the adsorptive capacity per unit volume and also somewhat to rate of adsorption and desorption. However, more important properties related to the rates and therefore to the actual useful capacity would be the zeolite crystal size and the macropore size distribution. Although the ultimate basis in selecting a zeolite adsorbent for a specific application would be the performance, the price, and the projected service life of a product, these factors depend largely upon the above properties. 

Unlike ordinary zeolites, silicalite is organophilic and hydrophobic and can remove from water a variety of dissolved organic compounds. Both ZSM-5 and silicalite display remarkable shape selectivity because of the geometry of the channels only certain reactants may enter and diffuse through the crystals, and only certain products may diffuse out of the intracrystalline space. 

Primary Shape Selectivity. There are several types of shape and size selectivity in zeolites. First, the reactant molecules may be too large to enter the cavities. A particularly good illustration of this behavior is given by Weisz and co-workers (5). Zeolites A and X were ion exchanged with calcium salts to create acid sites within the zeolite. These acid sites are formed as the water of hydration around the calcium ions hydrolyzes. When these zeolites are contacted with primary and secondary alcohols in the vapor phase, both alcohols dehydrate on CaX but only the primary one reacts on CaA. Since the secondary alcohol is too large to diffuse through the pores of CaA, it can not reach the active sites within the CaA crystals. This kind of selectivity is called reactant shape selectivity and is illustrated in Figure 3. 

MF1 type zeolites are known to show enhanced selectivity to p-substituted products in alkylation and isomerization of aromatic molecules [e.g. 1,2,3,4]. This shape selectivity is more pronounced with larger zeolite crystals and can be further enhanced by modification of the parent zeolites through post-synthesis treatments like impregnation with basic oxides, metal salts or the deposition of silica or coke [3,5,6,7]. The gain in selectivity is, however, usually accompanied with loss in catalytic activity and in some cases more rapid deactivation [8,9]. Despite the large number of patents and reports in open literature, the reasons for the enhancement of shape selectivity of MFI zeolites by post-synthesis treatment and the limits of the severity of this treatment are not unequivocally explained to date. 

Shape-selective effects may occur whenever the pore size of a microporous catalyst is in the same range as the diameter of the molecules or transition states involved in the reacting system. Common microporous materials are zeolites and related materials (aluminophospates, pillared clays, etc.) which possess a regular crystal lattice together with a well defined pore size. According to Weisz [111] and Csicsery [27], shape-selective effects may be classified into three types (Fig. 25). 

The foregoing discussion refers solely to intraparticle diffusivity (micropore diffusion) as distinct from interparticle effects (macropore diffusion). Since a practical zeolite catalyst will consist of composite particles, each containing a large number of individual zeolite crystals, it is important to make a clear distinction between these two types of diffusion. In some cases macropore diffusion may be important in determining the overall reaction kinetics but will obviously not introduce or affect shape selectivity in any way. 

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl-pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly... 

When the hydrogenation function is embedded in the crystal voids of an MFI topology, the formation of trans-isomers is strongly reduced. After partial reduction of soy bean oil with such catalyst from an iodine value of 140 to 80, virtually no trans-isomers are obtained (56). This is the result of pore mouth catalysis combined with zeolite shape selectivity. Due to the bent character of the cts-isomer chains in triglycerides, trans-configured chains preferentially enter the pore mouths for hydrogenation. In this environment, metal-catalyzed cis-trans isomerization is restricted for steric reasons as multiple readsorption is minimal. 

The catalytic properties associated with the molecular shape-selectivity exhibited by ZSM-5 are now well known. Recent work by Martens et al. (1995) has revealed that the external surfaces of zeolite crystals have also to be considered as potential shape-selective environments. Thus, strong evidence has been obtained for a lock-and-key model, which involves a form of pore mouth catalysis with bulky long-chain molecules that cannot penetrate into the intracrystalline micropores. The proposed lock-and-key model for n-alkane isomerization over ZSM-22 zeolite (with tubular pore openings of 0.55 x 0.45 nm) seems likely to be valid for other catalytic reactions. 

The Dependency of Shape Selectivity on Diffusivitv. The methylation of toluene to produce xylene isomers using ZSM-5 (MFl type) zeolite catalyst is a typical reaction, in which the shape selectivity is observed. In this reaction, xylene isomers once produced inside the zeolite crystal diffuse to the outer surface of the crystal, and the shape selectivity is ascribed to the difference of the diffusivity among the isomers. 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

If the charge balancing cation in a zeolite is then the material is a solid acid that can reveal shape selective properties due to the confinement of the acidic proton within the zeolite pore architecture. An example of shape selective acid catalysis is provided in Figure 5.3.7. In this case, normal butanol and isobutanol were dehydrated over CaX and CaA zeolites that contained protons in the pore structure. Both the primary and secondary alcohols were dehydrated on the X zeolite whereas only the primary one reacted on the A zeolite. Since the secondary alcohol is too large to diffuse through the pores of CaA, it cannot reach the active sites within the CaA crystals. 

As mentioned above, it is difficult to separate the influence of the crystal size and of the external surface on the shape-selectivity of zeohtes rigorously. Increasing the crystal size of the zeolite means at the same time reducing the influence of the external sur ce. For the industrial production it is desirable to operate with small crystal sizes. 

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