Zeolites large molecule formation

Lower dilution levels did not allow sufficient depolymerlzatlon and higher dilution caused excessive depolymerlzatlon In the aged solutions. Pillared clays prepared from aged dilute solutions had an enhanced microstructure which showed an Increased activity for selectively cracking large molecules to the light cycle oil range. This microstructure Is lost In the presence of steam which also reduces the formation of catalytic coke. Addition of rare earth zeolite to pillared clay can partially overcome the effects of this loss of microstructure. 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

The pore width of large pore zeolites like HY, HM, H6, can be characterized by proper catalytic test reactions. The isopropylation of large molecules like meta DIPB (Diisopropylbenzene) is a reaction wherein the formation of isomeric DIPB s isomeric TIPB s (Triisopropylbenzene) signify the pore dimensions and the structural properties. Thus isomerization is the major reaction in H13, whereas isomerization and alkylation of DIPB with isopropanol compete in HY. However no activity is observed in HM. It is known that bulky 1,3 DIPB does not enter the pores of HM. Similarly the formation of 1,2,4 TIPB in HY and 1,3,5 TIPB in HI3 depend on the structural features. The acidic nature of zeolite does not influence the product formation. 

Anisole can be acylated with carboxylic acids over ZSM-5 catalyst. The reaction is carried out in a batch reactor under reflux for 2 d. Table 4.12 shows fhe resulfs obfained utilizing carboxylic acids with different chain length. Prom propanoic acid to stearic acid, the conversion decreases from 92% to 1%, showing a trend opposite to that observed with REY zeolites. This behavior can be ascribed to the small micropore size of ZSM-5, in which the formation of large molecules is difficulf (for penfanoic to octanoic acid) or impossible (for longer-chain carboxylic acids). The poor conversion of AAC can be affribufable to the low reflux temperature. 

Different synthetic approaches have been described for the formation of zeolite inclusion compounds, including direct adsorption, impregnation, ion-exchange from solutions or solid-state ion-exchange, ship-in-the-bottle synthesis, formation of large molecules (e.g., by polymerization reactions), and zeolite synthesis around the metal complex. Some examples of these approaches are given in the next sections. 

A specially formulated catalyst for maximizing the yield of light olefins can be tailored in the light of the feed properties and target products by optimizing the composition of a mixture of different natural zeolites. The pore size distribution of the matrix should be sueh as to allow access to the large molecules of the feedstock, whilst incorporation of large pore Y zeolite favors intermediate molecule formation and mesoporous ZSM-5 favors production... 

Catalysis zeolites possess acid sites that are catalytically active in many hydrocarbon reactions, as we shall discuss in Chapter 9. The pore system only allo vs molecules that are small enough to enter, hence it affects the selectivity of reactions by excluding both the participation and formation of molecules that are too large for the pores. 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

The metal complexes in an SIB catalyst are confined to separate supercages. Consequently, the formation of inactive dimers is no longer possible. Shape-selectivity is another feature of SIB catalysts that follows from the restricted space inside the zeolite pore system. This can be simply due to discrimination in size of the reactant molecules (a large reactant molecule is excluded from the zeolite) or to a constrained orientation of the reactant at the catalytic site (transition state selectivity). 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

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