Zeolite shape selective catalysts

H2 to aromatic molecules or to high-octane-number gasoline. First, methanol and olefins are produced by the catalytic reactions of CO and H2, as discussed above. Then, using a zeolite shape-selective catalyst that is introduced along with the ruthenium or other metal catalyst in the same reaction chamber, methanol and the olefins are converted to aromatic molecules, cycloparaffins, and paraffins. The mechanism involves the dehydration of methanol to dimethyl ether. The light olefins that also form are alkylated by methanol and by the dimethyl ether [134] to produce higher-molecular-weight olefins and then the final cyclic and aromatic products. 

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. ... 

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.)... 

Castaneda, R., Corma, A., Eornes, V., Martinez-Triguero, J., and Valencia, S. (2006) Direct synthesis of a 9 x 10 member ring zeolite (Al-lTQ-13) a highly shape-selective catalyst for catalytic cracking. J. Catal., 238, 79-87. 

The use of zeolites can overcome many of these limitations and provide new controlled entries into these oxidized hydrocarbons and new materials. For example, some of the most valuable industrial intermediates are terminally oxidized hydrocarbons, snch as n-hexanol or adipic acid, that are not readily available in free-radical chain processes. The ability of zeolites to function as shape-selective catalysts can, in principle, be used to restrict access, by reactant or transition state selectivity, to sites not normally attacked by oxidants [3]. 

Because of their well-defined porous structures and spatial limitations, zeolites and mesoporous aluminosilicates are shape-selective catalysts. The availability of... 

Zeolites are well known for shape-selective catalysis. Here the shape of the zeolite pores or cavities can control the shape of product. When catalytic reactions take place in channels of zeolites only those products that can be accommodated in the channels advance and emerge. Mobil s ZSM-5 is an example of a shape-selective catalyst. Many more zeolites with different pore sizes or large surface areas are being synthesized, extending the principle of shape-selective catalysis. Such developments are helpful for both existing industrial processes and environmental protection. 

Zeolites, which are aluminosilicates that can be regarded as being derived from AI2O3 and SiC>2, function as acidic catalysts in much the same way (Section 7.3). In addition, they catalyze isomerization, cracking, alkylation, and other organic reactions. A structurally related class of micro-porous materials based on aluminum phosphate (AIPO4) has also been developed (Section 7.7) like zeolites, they have cavities and channels at the molecular level and can function as shape-selective catalysts. 

The use of zeolites as selective catalysts in organic syntheses is a field of growing importance. Zeolites are salts of solid silicoaluminic acids characterized by a strictly regular structure of their crystalline lattice (ref. 1) and by their high acidity and shape selectivity (ref. 2). Attention to the broad potential of zeolites in organic reactions was first drawn in the sixties by Venuto (ref. 3) and various applications of their catalytic properties have been recently reviewed (ref. 4). 

The types of shape selective catalysis that occur in zeolites and molecular sieves are reviewed. Specifically, primary and secondary acid catalyzed shape selectivity and encapsulated metal ion and zero valent metal particle catalyzed shape selectivity are discussed. Future trends in shape selective catalysis, such as the use of large pore zeolites and electro- and photo-chemically driven reactions, are outlined. Finally, the possibility of using zeolites as chiral shape selective catalysts is discussed. 

Molecular sieve science is growing rapidly. The uses of molecular sieves as shape selective catalysts for various chemical reactions continue to increase. Below, we illustrate several new trends in the use of zeolites and other molecular sieves as shape selective catalysts. 

N. Y. Chen, T. F. Degman, Jr., C. M. Smith, Molecular Transport and Reaction in Zeolites - Design and Application of Shape Selective Catalysts, VCH, New York, 1994. 

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. 

One of the most significant stages in the development of zeolite catalysts was the synthesis by Mobil scientists (U.S. Patent 3,702, 866) of the zeolite now universally known as ZSM-5 (i.e. Zeolite Socony Mobil-5). This was the first - and most important - member of a new class of shape selective catalysts, which have made viable the production of synthetic gasoline . In this process, high-octane gasoline is produced by the catalytic conversion of methanol to a mixture of aromatic and aliphatic hydrocarbons (Derouane, 1980). Because of its unique combination of chemical nature and pore structure, ZSM-5 is a highly effective dehydration, isomerization and polymerization catalyst. 

As the shortcomings of the traditional preparative methods outlined above became apparent, it was realized that alternative procedures were required to produce uniform or tailor-made adsorbents and shape-selective catalysts. As we saw in Chapter 11, one major route was opened up by the Linde synthesis in 1956 of the crystalline molecular sieve zeolite A. The search for new microporous crystalline materials has continued unremittingly and has resulted in the synthesis of novel zeolitic structures including the aluminophosphates, which are featured in this chapter. 

Related to their similar pore diameter and pore structure, unsurprisingly the Henry adsorption constants for linear alkanes are very close to each other on zeolite ZSM-22 and ZSM-23 (Table I). Somewhat higher constants are obtained for 2- and 3-methylbranched alkanes on ZSM-23 compared to zeolite ZSM-22. The adsorption constants of linear alkanes are obviously hi er than branched alkanes on the two cases. The separation power of a zeolite between a linear and a branched hydrocarbon may be given by the separation factor (a), which is the ratio of Henry consteints of linear and branched molecules at a certain temperature, a values at 523 K are given for both zeolites in Table 1. For comparison, values for ZSM-5 are also included, which is one of the most popular shape selective catalyst used in isomerization reactions. From this table it can be seen that both ZSM-22 and ZSM-23 have higher separation constants compared to ZSM-5. The zeolites can be listed in the following order with respect to their separation capacity between linear and 2- and 3-methylbranched alkanes ZSM-22 > ZSM-23 > ZSM-5. In narrow pore structures such as zeolites ZSM-22 and ZSM-23 it is very probable that linear alkanes with smaller kinetic diameters have more access to the available adsorption sites compared to the more bulky branched molecules. This may be regarded as the first... 

The formation of di- and tri-alkyl aromatics and b henyls from their aromatic precursor is a consecutive reaction, in A hich first the mono-substituted aromatic conopound is formed which subsequently reacts to form the di-alkyl compund. This was observed in the shape selective ethylation of bphenyl [91] isopropjdation of naphthalene [92] over H-Mordenite and the isopropylation of napthalene over HY [63], Therefore, if a dialkyl-isomer is the desired product, the reaction conditions (reaction time/readence time, partial pressures and tenperature) have to be optimized to obtain the the maximum yield of this de ed product. With shape selective catalysts the formation of poly-substituted aromatic compounds can be suppressed because they are not able to diffuse out of the channels of the zeolites [63]. 

The most important aspect concerning the catalytic application of zeolites is not this range of potential acid-base properties since that is also available with the amorphous aluminosilicates. Instead it is the presence in these crystalline materials of molecular sized cavities and pores that make the zeolites effective as shape selective catalysts for a wide range of reactions. >53-59... 

Most shape selective catalysts are molecular sieve zeolites. Aluminum or silicon occupies all framework tetrahedra in natural zeolites. B, Ga, Ge, Fe, Ti, V, P, and other heteroatoms may substitute aluminum or silicon in the framework of some synthetic molecular sieves. 

The new generations of zeolites and other microporous materials will start a new era for the petroleum processing, petrochemical, and chemical industries. These developments will also benefit our environment. Regenerable molecular sieves will replace corrosive and difficult-to-dispose-of catalysts. Shape selective processes can also generate less low-value byproducts and thus help us using our available resources more efficiently. Future shape selective catalysts and processes will be based on one or more of the foUowing ... 

Solid state ion exchange is a versatile tool for the fast and easy preparation of metal containing small pore (i. e., 8-membered ring) zeolites. Therefore it offers a valuable alternative to the crystallization inclusion method with its limited applicability. The introduction of noble metals into small pore zeolites via solid state ion exchange results in highly shape selective catalysts over which the hydrogenation of the linear alkene out of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) is strongly preferred. This indicates that the major part of the metal is located in the intracrystalline voids of the zeolites. Preliminary fUrther experiments in our laboratory surest that the new method is not restricted to noble metal chlorides, but also works with other salts, e. g., oxides and nitrates. 

Since the work of Lee et al. [37], zeolite mordenite continues to play an extraordinary role as a shape selective catalyst for the isopropylation of biphenyl. A high degree of dealumination [37, 38] and high pressure of propylene [39] seem to be advantageous to achieve high selectivities for 4,4 -diisopropylbiphenyl. Also, selective poisoning of the external sites with tributylphosphite [40] and the use of cerium exchanged sodium mordenites [41] are reported to suppress an undesired consecutive isomerization of 4,4 -diisopropylbiphenyl once formed in the pores. 

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