Shape-selectivity

3 Shape Selectivity. One of the most important features of zeolite catalysts is their ability to act as a molecular sieve because the channels have molecular dimensions. Three types of shape selectivity can be distinguished reactant, product, and restricted transition state selectivity, depending on whether reactants can enter, products can leave, or intermediates can be formed in the zeolite catalyst, respectively. Medium-pore zeolites have been shown to have excellent restricted transition state selectivity. The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape selectivity. Transition state selectivity and product selectivity have been observed directly in the methanol conversion on ZSM-5 by means of magic-angle-spinning NMR.  

Closely related to the shape selectivity on ZSM-5 is the effect of molecular traffic control. This results from the existence of two types of intersecting channels reactant molecules preferentially enter the catalyst through a given channel system while the products diffuse out through the other, so that counterdiffusion limitations are avoided. 

1 Influence of Space Time. Studies on the effect of space time on the methanol conversion and the hydrocarbon distribution clearly showed that Cj-Cg olefins are intermediates in the conversion of methanol to gasoline. This is illustrated in Table 8, which presents the yields of light olefins, aromatics and paraffins as a function of space time and methanol conversion. 

Dietrich, B. Guilhem, J. Lehn, J.-M., Pascard, C. and Sonveaux, E., Molecular recognition in anion coordination chemistry , Helv. Chim. Acta., 1984, 67, 91-104. 

Note that the tertiary bridgehead nitrogen atom is not protonated because its basicity is reduced dramatically as a consequence of its proximity to the protonated secondary amines in each case. 

The solution binding constants in water for halide binding by protonated l7w(tren) are (log K) 4.19, 

Since then chemists worldwide have prepared numerous tailor-made modified zeolites, and the synthetic potential for the production of organic intermediates and high-value fine chemicals is enormous. How can the success of this new class of catalysts in industry and academe be explained It is due to the outstanding catalytic properties of the zeolites. No other class of catalysts offers so much potential for variation and so many advantages in application. Their advantages over conventional catalysts can be summarized as follows  

Let us first take a closer look at the most important properties of the zeolites  

We have seen that the inner pore system of the zeolites represents a well-defined crystalline surface. The structure of the crystalline surface is predetermined by the composition and type of the zeohte and is clearly defined. Such conditions are otherwise foimd only with single-crystal surfaces. 

The accessibility of the pores for molecules is subject to definite geometric or steric restrictions. The shape selectivity of zeolites is based on the interaction of re- 

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

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

Weisz P B 1981 Molecular shape selective catalysis Proc. 7th Int. Congr. on Catalysis (Tokyo) 1 1... 

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... 

Csicsery S M 1976 Shape-selective catalysis Zeoiite Chemistry and Cataiysis ACS Monograph vol 171, ed J A Rabo (Washington, DC American Chemical Society) pp 680-713... 

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... 

Shape anisotropy Shape control Shape factors Shape-memory alloys Shape-selective catalysis Shape selectivity Sharpless catalyst Shaving cream Shaving creams... 

As shown in Figure 1, the equiHbrium concentration is affected slightly by temperature (11). The actual concentration is affected by the reaction rate and the initial concentration of each isomer. Deviations beyond equiHbrium can be achieved when zeoHtes are used, owing to shape selectivity (see Molecularsieves). The thermal isomerization of the three xylenes has been studied at 1000°C (12). Side reactions predominated, and only a small percentage of xylenes was interconverted. 

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Shape selective catalysts, such as ZeoHtes of the H-ZSM-5 type, are capable of directing alkyl groups preferentially to the para position (18). The ratio of the catalyst to the substrate also plays a role ia controlling the regiochemistry of the alkylations. For example, selective alkylation of anilines at the para position is achieved usiag alkylatiag ageats and AlCl ia equimolar ratio (19). 

N. Y. Chen, W. E. Garwood, and F. G. Dwyer, Shape Selective Catalysis in Industrial Applications, Marcel Dekker, Inc., New York, 1989. 

Activated diffusion of the adsorbate is of interest in many cases. As the size of the diffusing molecule approaches that of the zeohte channels, the interaction energy becomes increasingly important. If the aperture is small relative to the molecular size, then the repulsive interaction is dominant and the diffusing species needs a specific activation energy to pass through the aperture. Similar shape-selective effects are shown in both catalysis and ion exchange, two important appHcations of these materials (21). 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

In shape-selective catalysis, the pore size of the zeoHte is important. For example, the ZSM-5 framework contains 10-membered rings with 0.6-nm pore size. This material is used in xylene isomerization, ethylbenzene synthesis, dewaxing of lubricatius oils and light fuel oil, ie, diesel and jet fuel, and the conversion of methanol to Hquid hydrocarbon fuels (21). 

A modified ZSM-5 catalyst has a unique shape-selective property for producing -ethyltoluene [622-96-8] selectively by the alkylation of toluene [108-88-3] with ethylene (54). j )-Ethyltoluene is an intermediate in the production of poly -methylstyrene) [24936-41-2] (PPMS), which is reported to have... 

The selective alkylation of toluene with methanol to produce -xylene as a predominant isomer can be achieved over shape-selective catalysts (99—101). With a modified ZSM-5 zeoHte catalyst, more than 99% -xylene in xylene isomers can be produced at 550°C. This -xylene concentration exceeds the equiHbrium concentration of 23% (99). The selective synthesis of -xylene using relatively low cost toluene is economically attractive however, this technology was not commercialized as of 1991. 

Synthetic Fuels. Hydrocarbon Hquids made from nonpetroleum sources can be used in steam crackers to produce olefins. Fischer-Tropsch Hquids, oil-shale Hquids, and coal-Hquefaction products are examples (61) (see Fuels, synthetic). Work using Fischer-Tropsch catalysts indicates that olefins can be made directly from synthesis gas—carbon monoxide and hydrogen (62,63). Shape-selective molecular sieves (qv) also are being evaluated (64). 

Future development of SAM-based analytical technology requires expansion of the size and shape selectivity of template stmctures, as well as introduction of advanced chemical and optical gating mechanisms. An important contribution of SAMs is in miniaturization of analytical instmmentation. This use may in turn have considerable importance in the biomedical analytical area, where miniature analytical probes will be introduced into the body and target-specific organs or even cell clusters. Advances in high resolution spatial patterning of SAMs open the way for such technologies (268,352). 

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. ... 

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