Zeolites in catalytic cracking

The Future Role of Rare Earth Exchanged Zeolites in Catalytic Cracking ... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Introduction of zeolites into catalytic cracking improved the quality of the product and the efficiency of the process. It was estimated that this modification in catalyst composition in the United States alone saved over 200 million barrels of crude oil in 1977. The use of bimetallic catalysts in reforming of naphthas, a basic process for the production of high-octane gasoline and petrochemicals, resulted in great improvement in the catalytic performance of the process, and in considerable extension of catalyst life. New catalytic approaches to the development of synthetic fuels are being unveiled. 

In 1962 Mobil Oil introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst In 1969 Grace described the first modification chemistry based on steaming zeolite Y to form an ultrastable Y. In 1967-1969 Mobil Oil reported the synthesis of the high silica zeolites beta and ZSM-5. In 1974 Henkel introduced zeolite A in detergents as a replacement for the environmentally suspect phosphates. By 2008 industry-wide approximately 367 0001 of zeolite Y were in use in catalytic cracking [22]. In 1977 Union Carbide introduced zeolites for ion-exchange separations. 

Synthetic zeolites and other molecular sieves are important products to a number of companies in the catalysis and adsorption areas and numerous applications, both emerging and well-established, are encouraging the industrial synthesis of the materials. There are currently no more than a few dozen crystalline microporous structures that are widely manufactured for commercial use, in comparison to the hundreds of structures that have been made in the laboratory. See Chapter 2 for details on zeolite structures. The highest volume zeolites manufactured are two of the earliest-discovered materials zeolite A (used extensively as ion exchangers in powdered laundry detergents) and zeolite Y (used in catalytic cracking of gas oil). 

The thermal stability of NH4Y zeolite in which ammonium ions have been exchanged at various levels with La3+ ions was studied. The catalytic activity of these La zeolites in isooctane cracking was measured as a function of pretreatment temperature, and an IR study of the chemisorption of pyridine was used to determine the numbers of Bronsted and Lewis sites. The structural damage resulting from high temperature calcination was examined qualitatively. 

Tphe rate-limiting processes in catalytic reaction over zeolites remain A largely undefined, mainly because of the lack of information on counterdiffusion rates at reaction conditions. Thomas and Barmby (7), Chen et al. (2, 3), and Nace (4) speculate on possible diffusional limitations in catalytic cracking over zeolites, and Katzer (5) has shown that intracrystalline diffusional limitations do not exist in liquid-phase benzene alkylation with propene. Tan and Fuller (6) propose internal mass transfer limitations and rapid fouling in benzene alkylation with cyclohexene over Y zeolite, based on the occurrence of a maximum in the reaction rate at about 100 min in flow reaction studies. Venuto et al (7, 8, 9) report similar rate maxima for vapor- and liquid-phase alkylation of benzene and dehydro-... 

Octane Enhancement in Catalytic Cracking by Using High-Silica Zeolites... 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

The most important process involving a zeolite, Fluid Catalytic Cracking (FCC) uses a catalyst containing an acid FAU zeolite (Chapter 5). Other examples of processes using acid zeolite catalysts will be examined in this book, like Methanol to Olefins (Chapter 12), Acetylation (Chapter 14) etc. 

Zeolites find major applications in catalysis. A form of the zeolite FAU is, for example, an active catalyst component in catalytic cracking of heavy hydrocarbons to produce motor gasoline and diesel. The catalyst activity arises from its Bronsted acidity, which in turn comes from the presence in the stmcture of protons attached to bridging oxygen atoms. Protons can be introduced by ion exchange of anunonium cations, followed by calcination to remove NH3 and generate the acid form of the zeolite. The process is more complex... 

The pearl method is also used to measure the Si/Al ratio in zeolites and in catalytic cracking catalysts. 

One of the major applications of crystalline zeolites is as catalytic materials for petroleum refining processes. A prime use is in catalytic cracking, which is the major conversion process in today s refineries. Another growing use is in hydrocracking, such as the Unicracking-JHC process, which gives the refiner added flexibility in oils which can be processed and products which can be made. 

To date, crystalline zeolite catalysts have been most effective in catalyzing carbonium ion reactions such as catalytic cracking and hydrocracking. Other carbonium ion reactions such as alkylation and isomerization also are catalyzed by certain forms of zeolites. I expect to see these applications expand— provided suitable catalyst compositions are developed to allow economically viable processes. Although X- and Y-type faujasite can be used in catalytic cracking and hydrocracking, the Y-type is preferred for paraffin-olefin alkylation. Y-type faujasite is suitable for use in hydroisomerization catalysts, but synthetic mordenite is also a promising material. 

The enqihasis of the present paper is on recent applications of zeolites for the production of enes and aromatics and their conversion to certain petrochemic. Hence the focus win be on recent developments in the use of zeolites, firstfy, in catalytic cracking, which is a key process in producing the classical building blocks of the petrochemical industry, viz. light alkenes and aromatics, and, secondly, on the conversion of these into higher-vahie products, via ... 

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