Applications cracking, alkanes

Alkanes, which are the principal components of natural gas and crude oil, are still the preferred energy source of our society. In regard to the prime importance of alkanes as feedstock for the chemical industry, it appears a waste of resources simply to burn these precious raw materials. Unfortunately, attempts to transform alkanes into more valuable products are hampered by their low reactivity, as best illustrated by the use of alkanes as inert solvents. For example, the cracking process requires temperatures of about 1000 °C in order to convert long-chain alkanes into short-chain alkanes. Controlled conversion of hydrocarbons is difficult to achieve and limited to partial oxidations, such as the conversion of butane into acetic acid. It is obvious that processes that would enable efficient functionalization to occur at low temperature would have enormous potential application. Achievements towards this goal will almost certainly rely on the use of catalysts, which will have to activate the stable C-H bond (375-440 kf mol-1) in order to induce its scission. 

Another key step was the demonstration by P.B. Weisz and coworkers (3-5) of the shape selectivity of zeolite catalysts related to molecular sieving (1960). This initiated further research in the synthesis of new zeolites as well as industrial applications based on this property. The first commercial shape-selective process, Selectoforming, was developed by Mobil (1968) and allowed the selective cracking of the low octane (n-alkane) components of light gasoline over a natural zeolite (erionite) (6). 

The earliest applications of zeolites utilized the molecular sieving properties of small pore zeolites, e.g. zeolite A, in separation and purification processes such as drying and linear/branched alkane separation [33]. In 1962 Mobil Oil introduced the use of synthetic zeolite X, an FCC (fluid catalytic cracking) catalyst in oil refining. In the late sixties the W. R. Grace company introduced the "ultra-... 

In the next step dehydrogenation reactions give aromatic derivatives. The application of cracking catalyst not only lowers the cracking temperature and increases the reaction rate, but also results in higher production of iso-alkanes and aromatics. 

As catalysts, zeolites have found their most important application in petroleum refining processes. Their acid function is used in Fluid Catalytic Cracking (FCC), in hydroisomerisation of light alkane fraction as well as in ohgomerisation and isomerisation steps to upgrade the hquid fuels into gasohne and diesel. The combination of two different zeolites in the same industrial process is illustrated in the Shell-UOP TIP process an acidic zeolite, MOR, is used for isomerisation and the neutral LTA is used as molecular sieve for separation as shown in the scheme below (Figure 5.3). 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

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