Zeolite is widely used as a catalyst, an adsorbent, a molecular sieve, an ion exchanger or the like because it has a peculiar three-dimensional crystal structure of aluminosilicate, and has large pore and excellent ion-exchange performance compared to other aluminosilicate crystals. The use of natural zeolite is limited because of its structural restrictions, but the use of synthetic zeolite is gradually enlarged. In order to expand the use of zeolite, it is required to arbitrarily control the crystal size, particle size distribution, and form of zeolite to efficiently synthesize zeolite.
ZSM-5 zeolite forms three-dimensional pores defined by 10 tetrahedron rings, and its size is equal to that of zeolite A or is in the middle between zeolite X and zeolite Y. Further, ZSM-5 zeolite is a kind of pentasil zeolite which is a shape-selective catalyst exhibiting peculiar adsorption and diffusion characteristics, and generally has high thermal stability and has hydrophobicity because it has high ratio of SiO2/Al2O3 (silica/alumina). Further, ZSM-5 zeolite has both Lewis and Brønsted acid sites. In particular, ZSM-5 zeolite is used to directly obtain gasoline fraction having a high octane number from methanol by an MTG process, and is known to have excellent selectivity of gasoline fraction.
After ZSM-5 having a high content of silica was first developed by Mobil™ Oil Corporation in the early 1970's, research into this material has been variously made due to peculiar catalytic activity and shape selectivity resulting from the molecular sieve effect of this material. Unlike aluminosilicate zeolite, various kinds of organic materials have been used as structure inducing substances for forming a structure to prepare ZSM-5.
The conventional ZSM-5 catalysts are prepared by using binder material to form shaped-bodies, such as using spray-drying or extrusion to obtain the final formed catalyst, i.e. microspheres or extrudates, respectively. Due to the requirements on binding strength and other components, limited amount of zeolite crystals can be incorporated in the shaped catalyst form.
In the preparation of ZSM-5 zeolite microspheres, typically microspheres are formed first, and then the zeolite component crystals are combined in a separate step with the microspheres. In another technique, i.e., in situ crystallization technique, microspheres are first formed then the zeolite component is crystallized in situ within microspheres, to provide microspheres containing zeolite component.
In many catalytic processes, such as fluid catalytic cracking (FCC) processes, useful zeolite microspheres must be attrition-resistant as well as sufficiently porous. Generally, one of these qualities is achieved at the expense of the other. For example, as a microsphere of given chemical composition is formulated to be highly porous, the hardness usually decreases. Generally, in microsphere additives that contain higher than 25% ZSM-5 levels, the attrition resistance of the microspheres becomes an issue.
Kaolin alone or with a zeolitic molecular sieve can form coherent bodies such as microspheres which, when calcined, are further hardened. Kaolin is used as an ingredient for making microspheres for both economic and efficiency reasons. However, certain conversion processes, such as the conversion of methane to benzene performs poorly in the presence of kaolin microspheres, due to the fact that such microspheres are not inert to the conversion.
Further, for conventional ZSM-5 catalysts, seeds are often required to induce crystallization to form ZSM-5 zeolite within the microspheres. The ZSM-5 seeds incur additional cost in the zeolite synthesis by the in situ method.
There have been numerous attempts to either reduce the use of kaolin microspheres, or minimize ZSM-5 seeds in the in situ production of ZSM-5 catalysts.
Several ExxonMobil® U.S. patents (U.S. Pat. Nos. 6,831,203; 6,699,811; 6,198,013; 6,150,293; 6,111,157; 6,040,259; 6,039,864; 5,993,642) have shown that the first zeolite crystals can be coated and bound by the second inter-grown zeolite crystals crystallized from silica binder that binds the first zeolite crystals. The final zeolite catalyst (including both first and second zeolite crystals) contains less than 5% non-zeolite binder. The second zeolite as binder includes MFI crystals (U.S. Pat. No. 6,150,293). U.S. Pat. No. 6,150,293 teaches a preparation method for making zeolite bound by MFI structure type zeolite.
Chinese patent application CN 103785449A to Yuanyuan discloses a preparation method of no binder ZSM-5 catalyst consisting of mixing ZSM-5 crystals with solid silica fine particles of less than 10 wt. % and promoters of phosphorus and lanthanum elements in ranges of 0.1-10 wt. % and 0.01-5 wt. %, respectively. The mixture was extruded into extrudates that are further crystallized in a steam environment containing organic template molecules in gas phase. The fine silica particles were then converted to in-situ ZSM-5 nanoparticles (<1 μm) on top of existing large ZSM-5 crystals to provide the extrudates with mechanical strength.
US 2013/0225397 to Ma teaches mixing ZSM-5, 0.1-20 wt. % of an oxide or hydroxide, an aluminum compound and silica, molding and drying to obtain a molded catalyst precursor mix I, then crystallizing the mix I at 100-200° C. in water vapor or template vapor, drying and calcining the catalyst precursor to obtain a binderless molecular sieve catalyst.
U.S. Pat. No. 5,672,331 to Verduijn teaches MFI zeolite crystals of uniform and controllable size is produced by mixing a source of particulate silica, seeds of an MFI zeolite in the form of a colloidal suspension, an organic structure directing agent, and a source of fluorine or an alkali metal to form an aqueous synthesis mixture, and allowing the synthesis mixture to crystallize.
U.S. Pat. No. 7,601,330 to Wang teaches a process for producing a binder-free homogeneously crystallized and shaped ZSM-5 zeolite crystals comprising: mixing, a silica source, an aluminum source, ZSM-5 seeds, and an extrusion aid together with a silica sol or water glass then converting to binder-free ZSM-5 shaped zeolites by vapor-solid phase crystallization with organic amine and water vapor.
CN 102372286 to Wang teaches a method for the synthesis of small crystal ZSM-5 zeolite, comprising: mixing a directing agent of silica source that is formed by spray-drying having a resulting particle size of 10-200 am, with a base, a templating agent, and water and mix to obtain a raw material mixture, and the mixture was heated and dried to obtain crystal ZSM-5 zeolite.
U.S. Pat. No. 8,398,955 to Lai teaches a method of preparing a molecular sieve composition having at least one crystalline molecular sieve comprising the steps of: providing a reaction mixture of at least one source of ions of tetravalent element Y such as silicon or germanium, at least one source of alkali metal hydroxide, water, optionally at least one seed crystal, and optionally at least one source of ions of trivalent element X, wherein said reaction mixture is substantially free of a crystalline molecular sieve, extruding said reaction mixture to form a pre-formed extrudate; and crystallizing said pre-formed extrudate in a liquid medium comprising water under liquid phase conditions to form said molecular sieve composition having the crystalline molecular sieve.
CN 102583434 to Li teaches a ZSM-5 zeolite microsphere preparation of the following steps: mixing siloxanes and alcohol, an aluminum source, sodium hydroxide, organic amines, TEOS, and water; wash, heat then calcine to obtain ZSM-5 zeolite microspheres.
However, the mentioned prior art either require ZSM-5 seeds, or express no attrition resistance rate or high zeolite content for the resulted zeolitic microsphere. Accordingly it would be desirable to produce high attrition resistant ZSM-5 zeolite catalyst in-situ with substantially no clay base microspheres and/or ZSM-5.
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