Abstract:
Compositions and methods for preparing mesostructured zeolites having improved hydrothermal stability. Such mesostructured zeolites can be prepared by subjecting a zeolite to rare earth ion exchange prior to and/or subsequent to introducing mesoporosity into the zeolite.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Patent Ser. No. 61/145,723 filed Jan. 19, 2009, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    One or more embodiments of the invention relate to mesostructured zeolites containing rare earth elements and having improved hydrothermal stability. 
         [0004]    2. Description of Related Art 
         [0005]    The hydrothermal stability of zeolites is a major concern in some important refining operations, such as Fluid Catalytic Cracking (FCC) where the catalyst is exposed at severe steaming conditions in the regenerator. This treatment produces the uncontrolled dealumination of the zeolite causing a dramatic decrease of activity and selectivity towards the desired fractions. 
       SUMMARY 
       [0006]    One embodiment of the invention concerns a material comprising at least one mesostructured zeolite one-phase hybrid single crystal having enhanced hydrothermal stability after regeneration, produced by the process of: (a) providing a fully-crystalline non-mesoporous zeolite having long-range crystallinity and comprising rare earth elements; and (b) forming a plurality of mesopores within the non-mesoporous zeolite to thereby form the mesostructured zeolite. 
         [0007]    Another embodiment of the invention concerns a method of forming a material comprising at least one mesostructured zeolite one-phase hybrid single crystal having long-range crystallinity and enhanced hydrothermal stability and microporosity retention after regeneration. The method of this embodiment comprises the steps of: (a) providing a fully-crystalline non-mesoporous zeolite having long-range crystallinity; (b) adding rare earth elements to the zeolite; (c) acid washing the zeolite; and (d) forming a plurality of mesopores within the acid-washed zeolite to thereby form said mesostructured zeolite. 
         [0008]    Yet another embodiment of the invention concerns a process for preparing a mesostructured zeolite comprising rare earth materials and a plurality of mesopores. The improvement of this process comprises: enhancing microporosity retention of a mesostructured zeolite after a hydrothermal regeneration by
       (a) ion exchanging rare earth materials into an initial zeolite to thereby form a rare earth exchanged zeolite; and   (b) forming a plurality of mesopores in the rare earth exchanged zeolite to thereby form the mesostructured zeolite.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0011]    Embodiments of the present invention are described herein with reference to the following drawing figures, wherein: 
           [0012]      FIG. 1  is a graph depicting a typical correlation between framework alumina content and unit cell size of zeolites; 
           [0013]      FIG. 2  is a process flow diagram depicting steps for preparing rare earth-containing mesostructured zeolites; 
           [0014]      FIG. 3  is a graph depicting the expected compromise between microporosity and mesoporosity in zeolite catalysts; 
           [0015]      FIG. 4  is a graph depicting the loss of microporosity and mesoporosity in a steamed zeolite catalyst; 
           [0016]      FIG. 5  is a graph depicting how both fresh and steamed porosities are not affected if the rare earth is exchanged on the CBV 500 zeolite after riving; 
           [0017]      FIG. 6  is a graph depicting how both fresh and steamed porosities are improved if the rare earth is exchanged on the CBV 500 zeolite prior to riving; and 
           [0018]      FIG. 7  is a graph showing how both fresh and steamed porosities are improved when Grace Davison&#39;s USY is used as the precursor. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    There are at least two main causes for non-reversible deactivation of FCC catalysts in the hydrothermal environment of an FCC regenerator. The first is the dealumination of the zeolite framework. Aluminum atoms, which provide needed acidity in the zeolite for catalytic cracking, are actually ejected from the framework during regeneration. As this occurs, the zeolite&#39;s unit cell size (UCS) contracts. A typical correlation between framework alumina content and UCS is shown in  FIG. 1 . 
         [0020]    Note that the Y-type zeolite used in FCC catalysts typically can have a UCS of 24.65-24.70 Angstroms as synthesized. It can be reduced down to 24.50-24.60 Angstroms in the manufacturing process (i.e., as the zeolite is transformed into ultrastable Y zeolite (using prior art processes)). This is called ultstrastabilization (USY) and is done by controlled hydrothermal calcination of the zeolite to carefully dealuminate it. Carefully bringing the UCS down in a controlled manner to ca. 24.55 Angstroms results in a more stable zeolite as it heads off to the harsh, uncontrolled hydrothermal environment in the fluid cracking catalyst unit (“FCCU”).  FIG. 1 . 
         [0021]    Equilibrium catalysts (ecat), the circulating inventory in an FCCU, have a range of UCS&#39;s dependent upon the catalyst properties and the severity of the FCCU environment. About 90% of the world&#39;s FCCUs operate at an ecat UCS of 24.24-24.32A.  FIG. 1 . 
         [0022]    There are trade-offs as UCS decreases. While activity is lessened (less active alumina sites), depending upon the objectives of the FCCU, selectivity can be improved. For example, customers who desire maximum gasoline octane and maximum olefins look to equilibrate at low UCS. Customers who desire more gasoline yield will look to equilibrate at higher UCS. 
         [0023]    Exchange of FCC catalyst zeolites with rare earths is one of the main ways to control UCS. Rare earths retard the tendency to dealuminate, and hence can be used, for example, to favor gasoline production. Retarding dealumination also preserves the activity and change selectivity. FCC catalyst manufacturers incorporate a specific amount of rare earths on the zeolite dependent upon the FCCU objectives and constraints. 
         [0024]    The second main cause for non-reversible deactivation of FCC catalysts in the hydrothermal environment of an FCC regenerator is sintering, or destruction, of the zeolite The harsh environment of an FCCU can cause the zeolite structure to collapse. This is especially likely as the vanadium level in the FCCU feedstock increases and is even more pronounced in the presence of sodium, either from the unit feedstock or from residual sodium on the zeolite. Rare earths probably do not play a major role in mitigating this aspect of deactivation. Rather, control of sodium level and vanadium “traps” are employed. 
         [0025]    In addition to controlling UCS, rare earth ion exchange can be used to improve the hydrothermal stability of zeolites. Other methods can be used to improve the hydrothermal stability of zeolites including controlled steaming (ultrastable zeolite) and chemical Al extraction (dealuminated zeolites). However, rare earth ion exchange is especially beneficial because each rare-earth cation coordinates to three negatively-charged positions of the zeolite inhibiting the extraction of the aluminum from the framework (dealumination) during operating conditions. An additional benefit of the rare-earth ion exchange in zeolites is an increase in gasoline yield due to the preservation of a high concentration of Bronstead acid sites. However, rare-earth ion exchange favors the hydrogen transfer reactions during cracking, causing a decrease in octane and cetane indexes of the gasoline fraction and the olefin content in the Liquefied Petroleum Gas (LPG) fraction. 
         [0026]    The hydrothermal stability of mesostructured zeolites can be controlled by tuning the amount of the mesoporosity introduced without significantly affecting their original activity and selectivity. 
         [0027]    The incorporation of rare-earth cations in mesostructured zeolites by either (i) direct ion-exchange or by (ii) incorporation of mesoporosity into already rare-earth ion-exchanged zeolites (see  FIG. 2 ) is an additional tool to further increase the hydrothermal stability of this novel family of materials. Introduction of rare earths into a zeolite prior to the riving step (the incorporation of mesoporosity into a zeolite) can improve the retention of microporosity, mesoporosity and crystallinity. In some cases, additional rare earths can still be applied to the zeolite after riving and ultrastabilization. As in the prior art, application of additional rare earths after riving can help to preserve the UCS targeted during the ultrastabilization step. 
         [0028]    Since catalyst manufacturers want to typically reduce the UCS of the fresh zeolite to ca. 24.55 Angstroms, and because it is well known that rare earths retard UCS reduction, rare earths are normally applied in the catalyst manufacturing process after the ultrastabilization process. This allows ease of UCS reduction during manufacture and control of UCS reduction during end use. The rare earths can be applied either just before or even during the incorporation of the zeolite with the other ingredients that make up the composite catalyst formulation. 
         [0029]    It is for this reason that most rare earths are added at the tail end of the zeolite processing. However, it was thought that incorporating rare earths upstream, before the riving process, might better position them in the desired location in the zeolite framework so as to better control UCS reduction of the final rived catalyst during regeneration in the FCCU. It was a pleasant surprise to see that doing so better preserved crystallinity (and hence acidity and activity) through the riving process, since preserving acidity is usually desired. As mentioned, this was not an expected result and is not a normal function of rare earths. The compromise is that the process of hydrothermal calcinationldealumination used during the ultrastabilization step must be adjusted to compensate for the presence of the rare earths. In addition a nominal amount of expensive rare earths can be removed in the riving step. 
         [0030]    In the riving process, some of the zeolite&#39;s microporosity is sacrificed in order to introduce mesoporosity. This is done to obtain, at least for fluid catalytic cracking (FCC) applications, enhancement of the catalyst&#39;s selectivity (the yield of preferred products) at the likely sacrifice of catalyst activity.  FIG. 3 . 
         [0031]    We expect this trade-off to occur because microporosity is to a large extent a measure of crystallinity, and how much crystallinity is preserved will in large part dictate activity, while mesoporosity implies improved accessibility, and therefore a better product slate (more transportation fuels, less coke, etc.) 
         [0032]      FIG. 4  depicts a fresh unsteamed USY type zeolite and a steamed USY type zeolite. In the laboratory, steam deactivation of the zeolite is used to mimic the hydrothermal deactivation that occurs in an FCCU. As  FIG. 4  shows, steaming results in the loss of microporosity and mesoporosity of the zeolite. Prior to riving, the processing of the zeolite is typically synthesis of the sodium form zeolite (NaY), exchange with an ammonium salt to reduce the sodium level, and hydrothermal calcination to produce the USY. 
         [0033]    The reduction of the hydrogen transfer reactions observed when mesoporosity is introduced in zeolites (probably due the ready exits of the products from the interior of the mesostructured zeolite), described in U.S. patent application Ser. No. 10/830,714, is likely to reduce the adverse effect that rare-earth ion exchange in zeolites produces in octane and cetane indexes of the gasoline and the olefin content in the LPG fraction. 
         [0034]    Still referring to  FIG. 4 , moving the fresh, and more importantly the steamed, curves up and to the right, is always desirable. More microporosity can result in more activity and more mesoporosity can result in better selectivity. 
         [0035]    As discussed above, most FCC catalyst manufacturers ion exchange rare earths on their zeolites at the tail end of zeolite processing to minimize the deactivation that occurs in the FCCU and to optimize selectivity. The mechanism by which the rare earth works, predominantly, is to inhibit dealumination of the zeolite framework in the hydrothermal environment of the FCCU regenerator. 
         [0036]    More recently, the potential benefit of incorporating the rare earths prior to the riving step has been explored. One can incorporate the rare earth 1) prior to the ultrastabilization step, 2) after the ultrastabilization but prior to the acid treatment that precedes riving, or 3) after the ultrastabilization and acid treatment and just prior to riving. 
         [0037]    The zeolite can be acid washed to increase the silicon to aluminum ratio of the zeolite. However, if the rare earths are added to the zeolite before the zeolite is acid washed, the acid washing can remove some of the expensive rare earth elements. Therefore, when the rare earths are added to the zeolite before the zeolite is acid washed, the acid washing can be adjusted so that it is strong enough to produce the desired silicon to aluminum ratio in the zeolite, but not so strong that it removes an excessive amount of expensive rare earths. 
         [0038]      FIGS. 5-7  show microporosity and mesoporosity retention for zeolites that were rare earth ion-exchanged after the ultrastabilization but prior to acid washing. This sequence of steps was used with USY type zeolite made by Zeolyst (CBV-500) as well as similar USYs made by Grace Davison. 
         [0039]      FIG. 5  shows how both fresh and steamed porosities are not affected if the rare earth is exchanged on the CBV 500 after riving. 
         [0040]      FIG. 6  shows how both fresh and steamed porosities are improved if the rare earth is exchanged on the CBV 500 prior to riving. 
         [0041]      FIG. 7  shows how both fresh and steamed porosities are improved when Grace Davison&#39;s USY is used as the precursor. 
         [0042]    Referring to  FIGS. 5-7 , the data points for the RE-USY rivings are better positioned than those of the RE-free USY rivings. This is good but somewhat surprising. The mechanism by which rare earths traditionally work is preventing dealumination at severe (about 1100° F. or greater) hydrothermal conditions. Contrast this with improvements in microporosity (crystallinity) retention at the relatively mild conditions (about less that 200° F.) of riving. The practical importance can be higher activity per unit of zeolite, which can be used for added performance or to mitigate cost by reducing the amount of zeolite required. 
       EXAMPLES 
     Example 1 
       [0043]    Ultrastable zeolite Y, produced by controlled steaming of an NH 4 NO 3  ion-exchanged zeolite NaY, was ion-exchanged with REC 1   3  (RE indicating rare-earth elements in any ratio including but not limited to Lanthanum, Yttrium, and Neodymium) at room temperature and pH=4, and then calcined at 600° C. for 8 h. 
         [0044]    This material was subsequently hydrothermally treated in the presence of a surfactant and pH controlling agent to introduce controlled mesoporosity as described elsewhere (U.S. patent application Ser. No. 10/830,714). 
       Example 2 
       [0045]    Ultrastable zeolite Y, produced by controlled steaming of an NH 4 NO 3  ion-exchanged zeolite NaY, is hydrothermally treated in the presence of a surfactant and pH controlling agent to introduce controlled mesoporosity as described elsewhere (U.S. patent application Ser. No. 10/830,714). 
         [0046]    This mesostructured zeolite was subsequently ion-exchanged with RECl 3  (RE indicating rare-earth elements in any ratio including but not limited to Lanthanum, Yttrium and Neodymium) at room temperature and pH=4, and then calcined at 600° C. for 8 h. 
         [0047]    Note: NH 4 Y can be also first rare-earth ion-exchanged and then steamed to produce Re-USY.