Hydrocracking Catalysts Containing Stabilized Aggregates of Small Crystallites of Zeolite Y Associated Hydrocarbon Conversion Processes (ExxonMobil Research and Engineering Company)

CATEGORY: ZEOLITES
PATENT
Hydrocracking Catalysts Containing Stabilized Aggregates of Small Crystallites of Zeolite Y Associated Hydrocarbon Conversion Processes (ExxonMobil Research and Engineering Company)
United States Patent Application 20130026070
Abstract:
This invention relates to hydrocracking catalysts utilizing stabilized aggregates of small primary crystallites of zeolite Y that are clustered into larger secondary particles. At least 80% of the secondary particles may comprise at least 5 primary crystallites. The size of the primary crystallites may be at most about 0.5 micron, or at most about 0.3 micron, and the size of the secondary particles may be at least about 0.8 micron, or at least about 1.0 μm. The silica to alumina ratio of the resulting stabilized aggregated Y zeolite may be 4:1 or more. This invention also relates to the use of such catalysts in hydrocracking processes for the conversion of heavy oils into lighter fuel products. The invention is particularly suited for the selective production of diesel range products from gas oil range feedstock materials under hydrocracking conditions.
Inventors:
Oliveri, Christopher G. (Stewartsville, NJ, US)
Lai, Wenyih Frank (Bridgewater, NJ, US)
Dandekar, Ajit B. (Bridgewater, NJ, US)
Weigel, Scott J. (Allentown, PA, US)
Wu, Jason (Clinton, NJ, US)
Kay, Robert E. (Easton, PA, US)
Application Number:
13/412945
Publication Date:
01/31/2013
Assignee:
ExxonMobil Research and Engineering Company (Annandale, NJ, US)
FIELD
This invention relates to hydrocracking catalysts containing stabilized aggregates of small crystallites of zeolite Y and hydrocarbon conversion processes utilizing such catalysts. Small primary crystallites of zeolite Y can cluster into larger secondary particles, at least 80% of which may comprise at least 5 primary crystallites. When the outer surfaces of the secondary particles are viewed, e.g., in a SEM image, the average size (width/diameter) of the primary crystallites may be about 0.5 μm or less, or about 0.3 micron or less, though the average size of the secondary particles may be about 0.8 μm or more, or about 1.0 μm or more. The silica to alumina ratio of the resulting stabilize zeolite Y products may be 4:1 or more. Methods for stabilizing the aggregates of Y crystals as well as the catalyst synthesis using the aggregated stabilized Y crystals herein are also disclosed. Catalysts and the use of such catalysts in processes for hydrocracking of hydrocarbon feedstocks are also disclosed herein.
BACKGROUND
Zeolite Y, a member of the Faujasite family, is widely used in many catalytic processes such as fluid catalytic cracking (FCC), hydrocracking, aromatics alkylation, and aromatics transalkylation. A particular type of zeolite Y is known as ultra-stable Y zeolite (USY). Typical USY has crystal morphology of non-aggregated and submicrosized crystals and may contain intra-crystal mesopores after post-treatment involving high temperature steaming. The individual submicrosized crystals may have crystal defects which produce variously oriented crystal grains within an individual crystal particle. U.S. Pat. No. 6,284,218 states that such defects include stacking faults and screw defects.
When heavy molecules are catalytically processed, such as in catalytic cracking of heavy gas oil, transport properties (both intra-particle and inter-particle) of the catalyst are important, in order to operate outside of the diffusion limited regime that often leads to coking.
The article by K. Rajagopalan et al., “Influence of Zeolite Particle Size on Selectivity During Fluid Catalytic Cracking”, Applied Catalysis, 1986, 23, 69-80, reports that smaller particle size NaY zeolite fluid catalytic cracking (FCC) catalysts exhibited improved activity and selectivity to intermediate cracked products, such as gasoline and light cycle oil. Selectivity differences were said to be explained by considering the effect of diffusion resistance on the rate constants for cracking of gas oil and gasoline.
U.S. Pat. No. 5,620,590 reports that small crystal zeolite Y of less than 1 micron shows activity benefit in hydrocracking compared to larger crystals. However, small crystal zeolites often present problems in manufacturing (e.g., difficulties in filtration and formulation) due to their small particle sizes and low bulk density. Therefore, it is desirable to have zeolites that possess the performance advantages of small particles, while still maintaining the easy processability of large particles. Thus, one ideal zeolite morphology includes large secondary particles (often greater than 1 micron) formed by agglomeration of smaller primary crystallites (often less than, or even much less than, 1 micron). Furthermore, to improve mass transportation rates, zeolite crystals with small size or aggregated crystals containing inter-crystal mesopores can be desirable, e.g., for reducing diffusion limitations.
Conventional zeolite Y tends to have a crystal or primary crystallite size of much greater than 0.1 μm, even greater than 1 μm. Examples of such conventional forms of zeolite Y include U.S. Pat. Nos. 3,343,913, 3,690,823, and 3,808,326, for example. Small crystal size zeolite Y may be prepared by methods disclosed in U.S. Pat. Nos. 3,516,786 and 3,864,282.
Zeolite X, zeolite Y, and natural faujasite have identical structure types and differ only in the ratio of silica to alumina in the final crystal structure. For example, zeolite X is generally referred to as having a Si/Al2 molar ratio of 2-3, whereas zeolite Y is generally referred to as having a Si/Al2 molar ratio of 3-7.
U.S. Pat. Nos. 5,993,773 and 6,306,363 describe various forms of low-silica faujasite zeolite, referred to as LSX, having silica to alumina molar ratios of 1.9-2.1. These patents include SEM photographs showing LSX zeolite particle size and morphology.
In U.S. Pat. No. 6,306,363, it is stated that, when zeolites are observed by a SEM, they may be visible either (1) in the form of non-aggregated primary crystallites only, which are the smallest units of zeolite particles, or (2) in the form of secondary particles which are formed by agglomeration of a plurality of primary crystallites. Primary crystallites of zeolites may have their shapes predetermined, depending upon the type of zeolite. For example, A-type zeolite tends to have a cubic shape, and faujasite-type zeolite tends to have an octahedral shape or a polyhedral shape developed from a generally spherical shape with some angularity, as shown in FIG. 3 of this patent. However, it is possible for faujasite-type zeolites to have other shapes, such as elongated shapes (e.g., rod-like shapes).
Usually, particle sizes distributions of these particles are roughly symmetric about an average peak maximum. A method for obtaining an average particle size from particles having a distribution is described in detail, for example, at pages 1 to 31 of “Powder Engineering Theory”, Shigeo Miwa ed., 1981, Nikkan Kogyo Shinbun K. K. The primary crystallite size of the faujasite-type zeolite may be described as a number average particle size of the primary crystallite particle diameters (observed by SEM) as approximated to spheres, which is called the “projected area diameter” or “Heywood diameter”.
LSX in U.S. Pat. No. 6,306,363 is described as being of high purity and characterized in its primary crystallite size of at least 0.05 μm and less than 1 μm, which is said to be a fine (small) size, in comparison with previously known forms of LSX, e.g., where the primary crystallite size is from 3-5 μm, and even more generally where it is at least 1 μm. In this patent, it is stated that, when fine LSX of high purity is used, for example, as an adsorbent of various substances, diffusion into the interior will be facilitated, and improvement in various dynamic properties can be expected.
The LSX described in U.S. Pat. No. 5,993,773 is said to be characterized not only by high purity, but also a peculiar primary crystallite size distribution, wherein the primary crystallite size of a smaller set of particles is from 1-8 μm, the primary crystallite size of a larger set of particles is from 5-15 μm, and 90% or more of the particles are in the smaller set. The right hand portion of FIG. 2 of this patent illustrates a large single crystal or primary crystallite having a spherical polyhedral shape with angularity or edges developed.
SUMMARY
A stabilized aggregated form of zeolite Y utilized in the hydrocracking catalysts and associated hydrocarbon conversion processes disclosed herein comprises small primary crystallites and secondary particles of larger size. At least 80%, e.g., at least 90% or at least 95%, of the primary crystallites may be aggregated or clustered to form the secondary particles. The ratio of the average size (width/diameter) of the secondary particles to the average size (width/diameter) of the primary crystallites, when the outer (i.e., external) surfaces of the secondary particles are viewed, may be at least 3:1, for example at least 5:1 or at least 10:1. When the outer surfaces of the secondary particles are viewed, e.g., in an SEM image, the average size of the primary crystallites in a secondary particle may be about 0.5 μm or less, for example about 0.3 μm or less, about 0.2 μm or less, or about 0.1 μm or less, whereas the average size of the secondary particles may be about 0.8 μm or more, for example about 1.0 μm or more or about 2.0 μm or more. At least 80%, e.g., at least 90% or at least 95%, of the aggregated secondary particles may comprise at least 5, for example at least 10, primary crystallites. These primary crystallites and secondary particles as described herein may be observable, e.g., by an SEM under sufficient conditions including appropriate magnification and resolution.
The average sizes of the primary crystallites and secondary particles can be determined, for instance, by viewing one or more sufficient two-dimensional SEM images of the secondary particles and approximating the shape of the primary crystallites and secondary particles roughly as two-dimensional spherical projections (circles). When percentages (e.g., 80%, 90%, 95%, or the like) of primary crystallites and secondary particles are referred to herein, it should be understood that these percentages are based on numbers of these particles. Although SEM images referred to herein do not necessarily depict all of the particles in an entire batch of primary crystallites and secondary particles, it should also be understood that the SEM images referred to herein are viewed as representative of an entire batch of primary crystallites and secondary particles, including even those particles not specifically observed.
The aggregates of zeolite Y of the present invention can have enhanced stability, particularly enhanced thermal and/or hydrothermal stability, relative to the as-synthesized forms of these aggregates. The present stabilized aggregates of zeolite Y, which can advantageously have an alkali metal content less than 4 wt %, can further have different chemical compositions than the as-synthesized forms of these aggregates, which can typically comprise as much as 8 wt % or more alkali metal content.
Thus, without being bound by theory, it is believed that the chemical composition change may be a reason for the increased stability. As such, aggregates of zeolite Y may attain increased stability through one or more of the following treatments: by exchanging alkali metal (e.g., sodium) atoms from the as-synthesized form of the aggregates with an ammonium salt and by calcining the ammonium exchanged aggregates under conditions sufficient to decompose ammonium; by steaming the calcined, ammonium exchanged form of the aggregates under steaming conditions sufficient, e.g., to remove framework aluminum from the zeolite Y crystallites; and/or by contacting (washing) steamed aggregates with an aqueous acid, e.g., to remove non-framework aluminum from the zeolite Y aggregates.
In other preferred embodiments, the aggregates of zeolite Y herein are incorporated into a catalyst by the use of a suitable binder material or mixtures of suitable binder materials. Suitable binder materials include materials selected from metal oxides, zeolites, aluminum phosphates, polymers, carbons, and clays. Most preferable, the binder is comprised of at least one metal oxide, preferably selected from silica, alumina, silica-alumina, amorphous aluminosilicates, boron, titania, and zirconia. Preferably, the binder is selected from silica, alumina, and silica-alumina. In a preferred embodiment, the binder is comprised of pseudoboehmite alumina.
A significant advantage of the present invention over the prior art, is that in the prior art, the zeolite crystals typically require a significant amount of binder material in order to get obtain sufficient mesoporosity which is defined herein as pore diameters from 2 to 30 nm (20 to 300 angstroms, A) for optimum hydrocracking of hydrocarbon feedstocks. In the present invention, the zeolite is aggregated into a structure containing a high relative mesoporosity and as such, the hydrocracking catalyst can made with very low concentration levels of binders and still maintain the necessary overall mesoporosity of the hydrocracking catalyst. While the catalysts of invention can contain from 0 to 99 wt % binder materials, due to the high mesoporosity of the aggregate Y zeolite, in preferred embodiments, the binders levels can be about 0 to about 80 wt %, more preferably, from about 5 to 50 wt %, or even from about 5 to about 25 wt % of the overall final hydrocracking catalyst. In other preferred embodiments, the hydrocracking catalyst can be less than 50 wt %, more preferably less than 25 wt %, and most preferably less than 10 wt % binder materials. As noted, in embodiments, the aggregated Y zeolites of the present invention may be used as the final catalyst without any binder materials.
The high relative mesoporosity of the catalysts of invention are indicated by the high Relative External Surface Areas of the catalysts. The Relative External Surface Area is defined herein as:
RelativeExternalSurfaceArea=BETExternalSurfaceAreaBETTotalSurfaceArea[1]
In preferred embodiments of the hydrocracking catalysts of invention herein, the catalyst has a Relative External Surface Area of at least 0.35, more preferably at least 0.50. In preferred embodiments of the hydrocracking catalysts of invention herein, the catalyst has a low BET surface area of less than 600 m2/g; more preferably less than 500 m2/g, and most preferably less than 450 m2/g. In other preferred embodiments of the catalysts of invention herein, the average pore diameter of the catalyst is at least 7.0 nanometers (nm), preferably at least 7.5 nm, more preferably at least 8.0 nm, and most preferably at least 9.0 nm.
In other preferred embodiments, the catalyst may contain additional zeolites or molecular sieves. In a preferred embodiment, the catalyst further comprises at least one of the following molecular sieves: beta, ZSM-5, ZSM-11, ZSM-57, MCM-22, MCM-49, MCM-56, ITQ-7, ITQ-27, ZSM-48, mordenite, zeolite L, ferrierite, ZSM-23, MCM-68, SSZ-26/-33, CIT-1, SAPO-37, ZSM-12, ZSM-18, and EMT faujasites. In more preferred embodiments, the catalyst comprises at least one of the following molecular sieves: beta, ZSM-5, ZSM-48, mordenite, and zeolite L. The molecular sieves listed above can be present in the as-synthesized form, or alternatively, can be post-modified chemically, thermally, or mechanically to create a stabilized form of the material.
The aggregates of zeolite Y, binder and additional components may be extruded, spray dried, or otherwise shaped into a catalyst particle for use in hydroconversion processes described herein. In preferred embodiments of the hydrocracking catalysts herein, the final catalyst contains an active Group VIA and/or Group VIIIA metal. In a preferred embodiment, the hydrocracking catalyst is comprised of at least one Group VIA metal selected from Mo and W, and at least one Group VIIIA metal selected from Ni and Co. In another preferred embodiment, the hydrocracking catalyst is comprised at least one Group VIIIA metal selected from Pt, Pd, Rh and Ru. In another preferred embodiment, the hydrocracking catalyst is comprised at least one Group VIIIA metal selected from Pt and Pd. In a preferred embodiment, the Group VIA metal is Mo and the Group VIIIA metal is Co. In another preferred embodiment, the hydrocracking catalyst is comprised of Pt. The active Group VIA or Group VIIIA metals may be incorporated into the catalyst by any technique known in the art. A preferred technique for active metal incorporation into the catalyst herein is the incipient wetness technique.
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