Do Manufacturing Techniques Affect Performance of Ultrastable Zeolite FCC Octane Catalysts?
The legal allowable level of lead in U.S. gasoline was reduced to 0.1 gram/ gallon on January 1, 1986. This "lead phase down" left many refiners short on overall octane in their gasoline pool. Since the time refiners began planning to compensate for the lead reduction, the use of ultrastable Y zeolite FCC catalyst has become recognized as an effective means to increase the octane of the FCC gasoline contribution to the pool. Today over 65 percent of the tonnage of FCC catalyst sold in the USA contains this ultrastable Y (or USY) zeolite. Every manufacturer of FCC catalyst markets at least one family of USY catalysts to serve the octane market. Manufacturers have developed different techniques for reducing the aluminum content in Y zeolites to make them ultrastable. These manufacturing differences can affect the performance of fresh catalysts, but have little affect on their performance after they have been hydrothermally aged in an FCC regenerator.
Description of USY Zeolite
The most active component in cracking catalysts is Y zeolite. Zeolite crystals have a regular network of very small diameter pores. This internal micropore network provides a measured surface area of about 700 square meters per gram. The acid sites that catalyze cracking reactions are distributed on the internal and external surface area of the zeolite. The building blocks for zeolites are silicon or aluminum atoms tetrahedrally surrounded by four oxygen atoms. Each of the four oxygen atoms is shared by two tetrahedra, so it contributes one negative charge to each tetrahedron. Silicon is in a +4 oxidation state, so a tetrahedron containing silicon is neutral in charge. In contrast, aluminum has one less electron and is in a +3 oxidation state. This means each tetrahedron containing aluminum has a net charge of -1 which must be balanced by a positive ion such as sodium or a proton. Protons that balance the negative charge of aluminum tetrahedra have strong acidity which is known to catalyze cracking reactions.
The Y zeolite in cracking catalysts is a crystalline framework of these silicon and aluminum tetrahedra connected by shared oxygen atoms. The silicon content exceeds the aluminum content by a ratio of about 2.5 to 1 in conventional Y zeolite. Each aluminum atom is separated from the next aluminum atom by at least one silicon tetrahedron.
Ultrastable Y zeolite is relatively poorer in aluminum atoms and enriched in silicon atoms. Its silicon to aluminum ratio is 4 or more. This means that the aluminum atom density and therefore the acid site density is reduced. This aluminum deficient, or dealuminated, Y zeolite has higher thermal and hydrothermal stability than conventional Y zeolite. The added stability is the reason it is called "ultrastable zeolite".
The increased isolation of the aluminum acid sites enhances their acidity and reduces their ability to catalyze reactions involving two or more molecules. These isolated sites give USY zeolite its characteristic ability to increase octane and olefin yield by reducing the effects of a bimolecular reaction called hydrogen transfer. This hydrogen transfer reaction saturates olefins that contribute to the octane potential of the gasoline.
Manufacture of USY
No zeolite crystallization conditions are known that will economically produce USY zeolite directly. Instead, catalyst manufacturers crystallize conventional zeolite Y. Following this, a high silica, dealuminated USY is made by removing aluminum atoms from the crystalline zeolite framework and replacing them with silicon atoms. There are two basic approaches to dealumination of the zeolite framework: hydrothermal and chemical treatments.
The hydrothermal approach to dealumination involes treating the zeolite with steam at elevated temperatures (greater than 1000°F). The aluminum-oxygen bonds are broken by the steam and the aluminum atom is expelled from the zeolite framework. The resulting hole is healed by a process that is said to involve insertion of Si(OH)4 into the site vacancy left by the departing aluminum. If the vacancy is not healed it can reduce zeolite stability. Commercial hydrothermal dealumination processes are operated at conditions that minimize zeolite destruction.
Various reagents have been used to affect chemical dealumination. Unless the reagent fills the vacancy left by dealumination it also weakens the zeolite structure. Ammonium silicon hexafluoride, (NH4)2SiF6, was first applied to zeolites in cracking catalysts in the early 1970's(Ref. 1). At that time the fluoride ion was believed to impart stability. More recent work indicates that silicon from this reagent can fill the vacancies left by dealumination(Ref. 2). Apparently this reagent can give low to moderate dealumination with very little, if any, zeolite loss.
Physical Characteristics of Dealuminated Catalysts
Experiments at BASF and at independent laboratories demonstrate that both chemical and hydrothermal dealumination are capable of providing USY catalyst with essentially equivalent silicon to aluminum ratios and sodium content. A major difference is the absence of non-framework aluminum in (NH4)2SiF6 chemically dealuminated zeolites. This chemical dealumination process removes extra framework aluminum as a soluble salt.
Other differences include the appearance of pores and fissures in the hydrothermally dealuminated USY(Ref. 3,4). These holes and channels in the crystals give rise to a network of pores larger than the normal zeolite pore structure. This secondary pore system can allow larger molecules access to the interior of the zeolite crystal.
Catalytic Performance of Dealuminated Zeolites
In the unaged fresh state chemically dealuminated USY and hydrothermally dealuminated USY have different cracking selectivity. In studies where small molecule model compounds were cracked, the differences observed have been attributed to the presence of residual non-framework aluminum in the hydrothermal USY or, alternatively, to the presence of occluded non-framework aluminum fluoride species in chemically dealuminated USY(Ref. 5,6). In more realistic gas oil cracking studies(Ref. 7), fresh selectivites were also different as shown in Figure 1, which compares a hydrothermally dealuminated USY and chemically dealuminated USY with the same zeolite silicon to aluminum ratio. The hydrothermally dealuminated USY produces more gasoline and less gases than the chemically dealuminated USY. This has been attributed to the secondary pore system, which reduces the effective size of the zeolite crystals. Smaller zeolite crystals reduce the effects of diffusion and thus give rise to increased gasoline and diesel selectivity(Ref. 8).
A comparison of the same chemically dealuminated zeolites after hydrothermal aging, which simulates FCC regeneration conditions, shows essentially equivalent selectivities (Figure 2). This occurs because the hydrothermal aging process reduces any differences between the fresh zeolites. The aging process in a commercial FCC unit causes hydrothermal dealumination without any control to prevent zeolite destruction. As a result of this "in-unit" dealumination, pores and fissures develop in the chemically dealuminated USY. Thus the aging process tends to make the surface of the zeolite made by either process identical. Although the chemically dealuminated zeolite starts out without any secondary pore system, it soon develops one due to hydrothermal aging in a commercial unit .
Zeolite Framework Aluminum Versus Non-Framework Aluminum
Although the secondary pore network of hydrothermally dealuminated zeolites is the physical characteristic that appears to affect gasoline, diesel, and gas selectivities, there is evidence that the presence of non-framework aluminum may affect coke make(Ref.5,6,7). The commercial hydrothermal aging process increases this non-framework aluminum content in both chemically and hydrothermally dealuminated zeolite. Furthermore, there is evidence(Ref. 7,9) that the non-framework aluminum concentrates in the secondary pore system and on the zeolite exterior, where it is readily exposed to the gas oil reactants. Thus a large bulk concentration of non-framework aluminum may not be necessary to affect selectivites.
Figure 3 shows how a relatively large amount of non-framework aluminum may be generated from chemically dealuminated zeolite during commercial use. The figure follows alumination of Y zeolite through the manufacturing process as well as the commercial aging process.
Conventional Y zeolite with a silicon to aluminum ratio of 2.4 would have 52 aluminum atoms per unit cell. Its unit cell size, which is an X-ray measurement that correlates with zeolite aluminum content, is 24.70. Either hydrothermal or chemical dealumination can reduce the unit cell size to 24.43, where only 23 aluminum atoms remain in the framework. Further dealumination by (NH4)2SiF6 has not been reported in the literature. Even if further dealumination were possible, it might not be desirable because eliminating more aluminum acid sites would depress the catalyst activity.
Subsequent deactivation in a commercial unit reduces the number of framework aluminum atoms to 6 (24.29 Angstrom unit cell size) in a typical USY equilibrium catalyst. Thus in the chemically dealuminated USY catalyst example, 17 out of a total of 23 zeolitic aluminum atoms are non-framework, even though the fresh chemically dealuminated zeolite had no non-framework aluminum. These non-framework aluminums could represent up to 74% of the zeolite cracking sites. Because they are concentrated on the external zeolite surface, they can have an important influence on selectivity and activity for gas oil cracking. The net effect is that the zeolite external surfaces of chemically dealuminated and hydrothermally dealuminated USY zeolite appear similar to large gas oil molecules.
Although it is possible to affect fresh catalyst selectivity by using different USY manufacturing processes, the commercial aging process tends to eliminate these differences. As a result, the zeolite in all USY catalysts has similar selectivity regardless of the dealumination process. Thus, factors other than the dealumination process, such as the zeolite sodium and rare earth cation concentrations, are dominant in determining USY zeolite selectivity.
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2. U.S. Patent 4,503,023 (1985) to Union Carbide.
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