Hydrothermal stability and catalytic cracking

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

In petrochemical and oil refining operations, the zeolite is primarily responsible for the catalyst s activity, selectivity and stability (catalytic, thermal and hydrothermal). The fluid catalytic cracking process (FCC) is the most widely used of the oil refining process and is characterized by the use of a finely divided catalyst, which is moved through the processing unit. The catalyst particles are of such a size (about 70 pm) that when aerated with air or hydrocarbon vapor, the catalyst behaves like a liquid and can be moved easily through pipes. 

The modification of zeolites mainly relies on secondary synthesis methods. The aim of modification is to reprocess the zeolites using suitable techniques to improve the properties and functions such as (1) acidity, (2) thermal and hydrothermal stability, (3) catalytic performance such as redox catalytic and coordination catalytic properties, etc., (4) channel structures, (5) surface properties and microporous frameworks and charge-balancing ions. Modification is also called secondary synthesis and can lead to new properties that cannot be achieved through direct synthesis. Let us consider the case of faujasite (FAU), the main component of the cracking catalyst, and its catalytic performance (represented by the catalytic activity K/K Std for n-hexane cracking). From Table 6.1 it is seen that the secondary synthesis affects the catalytic performance to a considerable degree. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

Different procedures can be used in practice to activate the zeolite, and the choice of a particular method will depend on the catalytic characteristics desired. If the main objective is to prepare a very active cracking catalyst, then a considerable percentage of the sodium is exchanged by rare earth cations. On the other hand, if the main purpose is to obtain gasoline with a high RON, ultrastable Y zeolites (USY) with very low Na content are prepared. Then a small amount of rare earth cations is exchanged, but a controlled steam deactivation step has to be introduced in the activation procedure to obtain a controlled dealumination of the zeolite. This procedure achieves a high thermal and hydrothermal stability of the zeolite, provided that silicon is inserted in the vacancies left by extraction of A1 from the framework (1). The commercial catalysts so obtained have framework Si/Al ratios in the... 

Recent literature shows a growing trend to include free alumina in the formulation of fluid catalytic cracking (FCC) products. Over the last dozen years, FCC catalysts containing free alumina have been cited in the open and patent literature for benefits including (1) enhanced catalyst reactivity and selectivity (1-3). (2) more robust operation in the presence of metals in the petroleum feedstock (4-7). (3) improved attrition resistance (8.9). (4) improved hydrothermal stability against steam deactivation during regeneration (2.8). (5) increased pore volume and decreased bulk density (8), and (6) reduction of SOx emissions (10). 

Several attempts were made to prepare pillared smectites with sufficient hydrothermal stability for use as active components in catalysts for catalytic cracking of heavy oil fractions. Although improvements were made, none of the attempts resulted in pillared materials stable enough to withstand the hydrothermal conditions found in the regenerator of a commercial FCC. One type of materials studied, i.e. alumina-montmorillonites, may be attractive alternatives to the active matrices, often alumina, currently used in FCC-catalysts designed for cracking of heavy oils. The alumina-montmorillonites can, perhaps, not be considered to be bona fide pillared smectites as they have considerably larger pores and a wider pore-size distribution than what is characteristic for pillared smectites. 

Hydrothermally dealuminated Y type zeolites (HDY s) possess a number of characteristics which make them very useful catalyst components—very high activity for acid-catalyzed reactions and outstanding thermal stability Such zeolites were thus rapidly incorporated into catalysts for two major petroleum refining processes, catalytic cracking [lp2] and hydrocracking, [3,4] which operate at high temperatures, and for which resistance to process upsets and the ability to withstand oxidative regeneration are both important. 

Y zeolites exchanged with rare earth cations are widely used as the active component for cracking catalysts in petroleum industry. Such cations improve the catalytic activity and the gasoline yield, lowering the gas production and the coke formation. They also promote the thermal and hydrothermal stability of the catalyst. 

Blasco, T., Corma, A., and Martinez-Triguero, J. (2006) Hydrothermal stabilization of ZSM-5 catalytic-cracking additives by phosphorus addition. J. Catal, 237,267 277. 

The firr t zeolite used in catalytic cracking was the rare earth exchanged form of X-type zeolite. This was soon replaced by Y-type zeolite which had improved thermal and hydrothermal stability because of a higher silica/alumina ratio. The basic Y zeolite structure has since been chemically modified to produce HY (a hydrogen form of Y zeolite) and ultra-stable Y (USY) which is a stabilized form of HY (Yanik et al., 1985). 

In summary, catalytic fast pyrolysis is a promising technology once the main drawbacks, such as severe coking, deep cracking, and hydrothermal stability of the zeolite are solved. 

He et al. [ 116] and Wan and Shu [117] reported on the influence of calcination and hydrothermal treatment on compositional characteristics and thermal stability of rare earth containing Y zeolites and their performance in catalytic cracking. The alkaline and hydrothermal stability of Y zeolites dealuminated via hydrothermal treatment and by the SiCl4 technique was studied by Lutz et al. [118]. Hydrothermal treatment was found to increase the chemical resistance of Y zeoHte to superheated water at 200°C as well as to alkaline solutions due to the formation of a protective layer of extra-lattice oxidic aluminum species on the external surface of the zeoHte crystals. The removal of this layer by acid leaching resulted in significantly less stable products. 

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