TECHNICAL FIELD

[0001] The invention relates generally to zeolite catalysts, their synthesis and use.

BACKGROUND

[0002] Zeolites are crystalline microporous aluminosilicates, which are built up from corner- sharing Si0 ₄ and A10 ₄ tetrahedra. The aluminosilicate crystalline structure creates a framework with pores and channels that provide sites where small molecules and ions can reside, making them useful as catalysts for producing various compounds. Zeolites are extensively employed in the petrochemical industry for cracking crude oil fractions into fuels and chemical feedstocks for other industrial processes as heterogeneous catalysts with shape/size- selective characteristics.

[0003] One of the limitations for the use of zeolites as catalysts is the limited access to the active sites within the individual crystals. This attribute serves as the material's strength in that it provides shape selectively, but also creates a weakness due to diffusion or mass transport limitations.

[0004] Accordingly, a need exists to overcome the limitations of diffusion caused by the small pore size of the zeolites while also providing active sites that are readily accessible for facilitating desired reactions.

SUMMARY

[0005] A catalyst composition comprises a catalyst core material of a mesoporous zeolite having both mesopores and micropores. The mesopores of the catalyst core material have at least one of a metal and a metal oxide deposited therein. A catalyst shell surrounds the catalyst core material, with the catalyst shell being a porous structure having an average pore size that is smaller than the average pore size of the mesopores of the catalyst core.

[0006] In particular embodiments, the catalyst shell is a microporous structure and the catalyst shell is a mesoporous structure. The mesopores of the catalyst core material may have an average pore size of from 2 nm to 50 nm. Further, the micropores of the catalyst core material have an average pore size of less than 2 nm.

[0007] In certain instance, the metal or metal oxide may be at least one of platinum (Pt), gold (Au), palladium (Pd), copper (Cu), nickel (Ni), iron (Fe), cobalt (Co), ruthenium (Ru), antimony (Sb), bismuth (Bi), Fe ₂ 0 ₃ , CuO, Cu0 ₂ , NiO, Co ₂ 0 ₃ , ZnO, Ru0 ₂ , Ce0 ₂ , Os0 ₄ , Er ₂ 0 ₃ , PdO, Cr ₂ 0 ₃ , Mo0 ₃ , V ₂ 0 ₅ , W0 ₃ , Ti0 ₂ , A1 ₂ 0 ₃ , Ni ₂ 0 ₃ , Co ₃ 0 ₄ , Zr0 ₂ , CaO, BaO, MgO, SrO, FeO, CdO, In0 ₂ , and mixtures thereof.

[0008] In specific embodiments, the mesoporous zeolite is at least one of a TS-1 zeolite, silicalite, silicalite-1, zeolite Beta, ZSM-5 zeolite, A1PO-5, MCM-41 and SAPO. The catalyst shell may include at least one of a microporous zeolite material, a metal organic framework (MOF) material, a zeolitic imidazolate framework (ZIF) material, a covalent organic framework (COF) material, a porous silica material, an alumina material, a titanium oxide material, zirconium dioxide, and a carbon material.

[0009] In particular embodiments, the catalyst core material is functionalized with a silane reagent.

[0010] A method of forming a catalyst is also accomplished by forming a catalyst core material of a mesoporous zeolite having both mesopores and micropores. The mesoporous zeolite is treated with at least one of a metal and a metal oxide so that the mesopores of the catalyst core material having at least one of the metal and the metal oxide deposited therein. The treated catalyst core material is surrounded with a catalyst shell surrounding the catalyst core material. The catalyst shell is a porous structure having an average pore size that is smaller than the average pore size of the mesopores of the catalyst core.

[0011] In particular embodiments, the catalyst shell is a microporous structure and the catalyst shell is a mesoporous structure. The mesopores of the catalyst core material may have an average pore size of from 2 nm to 50 nm. Further, the micropores of the catalyst core material have an average pore size of less than 2 nm.

[0012] In certain instance, the metal or metal oxide may be at least one of platinum (Pt), gold (Au), palladium (Pd), copper (Cu), nickel (Ni), iron (Fe), cobalt (Co), ruthenium (Ru), antimony (Sb), bismuth (Bi), Fe ₂ 0 ₃ , CuO, Cu0 ₂ , NiO, Co ₂ 0 ₃ , ZnO, Ru0 ₂ , Ce0 ₂ , Os0 ₄ , lr ₂ 0 ₃ , PdO, Cr ₂ 0 , Mo0 ₃ , V ₂ 0 ₅ , W0 ₃ , Ti0 ₂ , A1 ₂ 0 ₃ , Ni ₂ 0 ₃ , Co ₃ 0 ₄ , Zr0 ₂ , CaO, BaO, MgO, SrO, FeO, CdO, In0 ₂ , and mixtures thereof.

[0013] In specific embodiments, the mesoporous zeolite is at least one of a TS-1 zeolite, silicalite, silicalite-1, zeolite Beta, ZSM-5 zeolite, A1PO-5, MCM-41 and SAPO. The catalyst shell may include at least one of a microporous zeolite material, a metal organic framework (MOF) material, a zeolitic imidazolate framework (ZIF) material, a covalent organic framework (COF) material, a porous silica material, an alumina material, a titanium oxide material, zirconium dioxide, and a carbon material. [0014] In particular embodiments, the catalyst core material is functionalized with a silane reagent. The silane reagent may include at least one of triethoxyvinylsilane, (3- aminopropyl)triethoxy silane, (3 -Glycidyloxypropyl)trimethoxy silane, allyltrimethoxysilane, allyltriethoxysilane, trimethoxymethylsilane, 1H, lH,2H,2H-perfluorodecyltriethoxysilane, ethynyltrimethylsilane, 3-(trimethoxysilyl)propyl methacrylate, and tetraethyl orthosilicate.

[0015] In a method of forming a reaction product, a catalyst composition is contacted with a reaction feed under reaction conditions suitable for producing the reaction product. The catalyst composition is that comprising a catalyst core material of a mesoporous zeolite having both mesopores and micropores. The mesopores of the catalyst core material have at least one of a metal and a metal oxide deposited therein. A catalyst shell surrounds the catalyst core material, the catalyst shell being a porous structure having an average pore size that is smaller than the average pore size of the mesopores of the catalyst core.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a more complete understanding of the disclosure, reference is now made to the following descriptions taken in conjunction with the accompanying figures, in which:

[0017] FIG. 1 is a schematic of a hierarchical zeolite catalyst having active nanoparticles within mesopores of the hierarchical zeolite core, which is enclosed in a porous shell; and

[0018] FIG. 2 is a schematic representation of exemplary process steps used in forming an encapsulated mesoporous zeolite catalyst.

DETAILED DESCRIPTION

[0019] Hierarchical zeolites, which may also be referred to as mesoporous zeolites or mesostructured zeolites, are those zeolites that contain two or more types of pores of different sizes. Zeolites are typically microporous materials. As used herein, micropores are those pores with pore sizes of less than 2 nm, whereas mesopores are those pores with a pore size of from 2 nm to 50 nm. By creating a zeolite catalyst having both micropores and mesopores integrated into the microporous zeolite, this circumvents the diffusion limitations imposed by the smaller pore size of the microporous zeolite. This makes them useful for the catalysis of oil and other compounds that contain bulky molecules.

[0020] The mesoporous zeolite can be any type of zeolite, which may be synthetic or naturally occurring. There are more than 150 types of synthetic zeolites and approximately 40 natural zeolites. Non-limiting examples of suitable hierarchical zeolites include TS-1 zeolite, silicalite, silicalite-1, zeolite Beta, ZSM-5 zeolite, A1PO-5, MCM-41, and SAPO, and their combinations. The mesoporous zeolites may be formed from different methods. These may include those that are formed from treatments wherein an existing zeolite is modified to provide the mesoporous structure. This may include the desilication or dealumination of the zeolites or zeolitization of mesoporous materials or the hierarchical assembly of nanozeolites.

[0021] The mesoporous zeolite may also be formed using direct synthesis methods. When formed by a direct synthesis method both template and template-free methods may be used. In many applications, template-based formation of the hierarchical zeolite may be used. This may involve using a large organic polymer as the mesopore structuring agent along with a structure-directing agent (SDA) for the template. Such polymer templates may be surfactant or non-surfactant polymer templates.

[0022] In certain embodiments, non- surfactant polymer templates may be used. Unlike surfactants, non- surfactant polymers do not self-assemble to form the mesostructure, but favor crystallization of the zeolite structures to produce 3-D continuous zeolitic frameworks with highly connected intracrystalline mesopores with increased structural integrity. Such non-surfactant templates act as dual function templates. The polymers may contain functional groups, such as quaternary ammoniums, that direct the formation of the zeolite structure. The second function is provided by the polymer itself, which acts as a porogen rather than an SDA. Examples of such dual template polymers include

N^methyl-N ⁶ , N ⁶ , N ⁶ -tripropylhexane-l,6-diamonium bromide) (PDAMAB-TPHAB) and N ⁶ , N ⁶ -trimethylhexane-l,6-diamonium bromide) (PDAMAB-TMHAB), [Ci ₈ H ₃₇ Me ₂ N (CH ₂ ) ₆ ]Br, [Ci ₈ H ₃₇ Me ₂ N(CH ₂ ) ₆ NPr ₃ ]Br, etc. Such polymers and resulting mesoporous zeolites and their synthesis are described in Tian, C. et al., Beyond Creation of Mesoporosity: The Advantages of Polymer-Based Dual-Function Templates for Fabricating Hierarchical Zeolites, Adv. Funct. Mater., 2016, pp. 1881-1891, 26, American Chemical Society, which is incorporated herein in its entirety for all purposes (hereinafter "Tian, C. et al.").

[0023] For those mesoporous zeolites synthesized with a polymer template, the polymer template may be removed by calcination. Calcinations as described herein may be conducted at temperatures of 400 °C or higher.

[0024] The mesoporous zeolites, formed from either preexisting zeolites or synthesized, provide good chemical stability, high thermal stability, high mass transport capability and high adsorption capacity when used for catalysts. The mesoporous zeolites are used as catalyst cores that are further treated and modified, as is described later on. In certain applications, such mesoporous zeolites used as catalyst cores may have a particle size of from 10 nm to 100 μιη, more particularly from 100 nm to 10 μιη. The pore volume of the mesoporous zeolite attributed by the micropores and mesopores may vary. As an example, the micropore volume may make up from 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50% or less of the total pore volume of the mesoporous zeolite. In certain embodiments, the micropore volume of the mesoporous zeolite may range from 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45% to 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90% of the total pore volume of the mesoporous zeolite.

[0025] It should be noted in the description, if a numerical value, concentration or range is presented, each numerical value should be read once as modified by the term "about" (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the description, it should be understood that an amount range listed or described as being useful, suitable, or the like, is intended that any and every value within the range, including the end points, is to be considered as having been stated. For example, "a range of from 1 to 10" is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific points within the range, or even no point within the range, are explicitly identified or refer to, it is to be understood that the inventor appreciates and understands that any and all points within the range are to be considered to have been specified, and that inventor possesses the entire range and all points within the range.

[0026] When selecting a particular hierarchical zeolite for use as a catalyst core material, the effects of the zeolite's acidic strength and pore confinement should be taken into account to provide the desired reaction activity, shape selectivity, and reaction mechanism. For example, in MFI zeolite materials containing dual mesopores and micropores, the external Br0nsted acidity has been shown to have an impact on parallel alkylation and etherification reactions. Higher acidity (i.e., lower Si/Al ratio) in mesoporous MCM-41 zeolites has also been shown to be responsible for cracking of heavy hydrocarbons and the formation of heavier aromatic products, while lower acidity (i.e., higher Si/Al ratio) was shown to induce a higher selectivity towards gasoline-range products (isoparaffins and olefins).

[0027] The mesoporous zeolite may be further functionalized to change the surface characteristics of the zeolite or to modify the pore size of the mesoporous zeolite. This may occur prior to or after the incorporation of any active metal or nanoparticle, as is discussed later on. Such functionalization may include modifying the surface so that its affinity to adsorb or bind compounds or ions is changed. This may include making the surface hydrophilic or hydrophobic. Additionally, the surface of the zeolite may be modified with organic functional groups. These may react with other organic or inorganic compound to tune adsorption ability. Pore size may also be modified through functionalization. Particularly useful for functionalization are silane reagents. These may include, but are not limited to, triethoxyvinylsilane, (3-aminopropyl)triethoxysilane, (3- glycidyloxypropyl)trimethoxysilane, allyltrimethoxysilane, allyltriethoxysilane, trimethoxymethylsilane, 1H, lH,2H,2H-perfluorodecyltriethoxy silane, ethynyltrimethylsilane, 3-(trimethoxysilyl) propyl methacrylate, and tetraethyl orthosilicate, and combinations of these. After treatment with such silane treating agents, the treated zeolite material is calcined so that the silane materials are converted to silica.

[0028] In addition to the catalytic properties of the mesoporous zeolite itself, both treated or untreated with functionalization agents (e.g., silane reagent), the hierarchical pore structure facilitates the formation of bifunctional or multifunctional catalysts. This may be achieved by incorporating active nanoparticles into the mesoporous zeolite. The nanoparticles may include metal and metal oxides. These may be provided from metals, metal oxides, metal salts, metal alloys, and combinations of these. The particular metal and metal oxide nanoparticles are selected to provide the desired catalyst characteristics. The metals and metal oxides may be incorporated using various techniques, which may be aqueous and nonaqueous treatments. The treatment may include slurry evaporation, incipient wetness, spray dry methods, solution or metal vapor deposition, etc. Solid-solid mixing of the mesoporous zeolite and metal compound may also be used, followed by bringing the mixture into contact with a suitable solvent, such as liquid water or water vapor. When so treated, the metal nanoparticles are dispersed within the mesopores of the mesoporous zeolite, with the size of the metal or metal oxide particles being limited by and thus defined by the size of the mesoporous channels themselves.

[0029] The amount of metals used may vary but typically the metals and metal oxides are used to provide a final metal or metal oxide nanoparticle content of from 0.05 wt.% to 50 wt.% based upon the total weight of the final catalyst, more particularly from 0.05 wt.% to 10 wt.% based upon the total weight of the final catalyst. Non-limiting examples of suitable metal and metal oxides include platinum (Pt), gold (Au), palladium (Pd), copper (Cu), nickel (Ni), iron (Fe), cobalt (Co), ruthenium (Ru), antimony (Sb), bismuth (Bi), Fe ₂ 0 ₃ , CuO, Cu0 ₂ , NiO, Co ₂ 0 ₃ , ZnO, Ru0 ₂ , Ce0 ₂ , Os0 ₄ , ¾<¼, PdO, Cr ₂ 0 ₃ , Mo0 ₃ , V ₂ 0 ₅ , W0 ₃ , Ti0 ₂ , A1 ₂ 0 ₃ , Ni ₂ 0 ₃ , Co ₃ 0 ₄ , Zr0 ₂ , CaO, BaO, MgO, SrO, FeO, CdO, In0 ₂ , and mixtures thereof. After metal deposition, the metal-treated mesoporous zeolite core material may be calcined. The calcination may occur prior to or after the formation of the porous shell structure.

[0030] Because the hierarchical zeolites have different active sites that may be spatially separated in the micropores and mesopores, the nanoparticles can be introduced into the mesopores to provide new active sites for catalytic activity. The microporous zeolite material surrounding the mesopores provides an excellent support material that stabilizes nanoparticles within the mesopores of the zeolite. The metal and metal oxide nanoparticles materials that are deposited into the mesopores of the hierarchical zeolite inherently have a large surface area. As a result, this increases the probability of contact with surrounding particles. When heated, such as during catalytic reactions, neighboring nanoparticles that contact one another will tend to aggregate and grow bigger. The microporous zeolite walls of the mesopores separate the active nanoparticles contained within the mesopores to prevent this aggregation.

[0031] Additionally, a porous shell structure can be provided that surrounds the mesoporous zeolite, with the nanoparticles contained within the mesopores, so that the mesoporous zeolite forms a catalyst core, as discussed previously. Such a shell structure further contains and isolates the nanoparticles within the mesoporous channels of the zeolite so that they are further separated and enclosed by the porous shell, and thus contact and aggregation of the nanoparticles is prevented even under extreme reaction conditions. This is illustrated in the catalyst particle schematic of FIG. 1.

[0032] As shown in FIG. 1, a catalyst particle 10 having a core 12 of a hierarchical or mesoporous zeolite has mesopores 14. Active nanoparticles 16, such as metals or metal oxide particles are deposited and contained within the mesopores 14 of the mesoporous zeolite core 12. All or at least a portion of the core 12 is surrounded or encapsulated by a porous shell 18. This creates a catalyst particle having what may be described as having both core-shell and yoke-shell structural characteristics, or wherein the particle constitutes a "hybrid" core- shell/yoke- shell structure, having both core-shell portions and yoke-shell portions or substructures. Such particle may also constitute a nanorattle material.

[0033] With such a structure, the yoke-shell portions containing the nanoparticles constitute "nanoreactors." This is distinguished from catalysts where the metal or nanoparticle is merely embedded in a bulk support. With the shell, the nanoparticles are isolated within the yoke-shell portions of the catalyst core material so that a homogenous environment is created that surrounds the nanoparticles. Additionally, the interaction of the active nanoparticles with the underlying zeolite support material in such yoke-shell portions can lead to higher catalytic activity than when such nanoparticles are used with bulk supports in a conventional manner.

[0034] The shell structure may be formed from a porous material having an average pore size that is smaller than the average pore size of the mesopores of the catalyst core. The shell structure should be a porous material that allows the transfer of reactants through the shell to the underlying catalyst core, and conversely, allows the transfer of reaction products through the shell from the core. Furthermore, the porous shell material should have pores that are small enough to prevent the migration of the nanoparticles contained within the mesopores of the core material. Typically this will constitute a microporous material. In certain instances, however, the shell may also constitute a mesoporous material, wherein the mesoporous shell material has pores that are small enough to prevent or at least reduce the migration of the nanoparticles contained within the mesopores of the core material through the shell.

[0035] The shell structure may be formed from a variety of materials. These may include, but are not limited to, a zeolitic imidazolate framework (ZIF) material, a covalent organic framework (COF) material, a porous silica material, an alumina material, a titanium oxide material, zirconium dioxide, and a carbon material. The shell structure can be formed by in- situ growth or by secondary growth via seeding the catalyst core material with seeds of the shell material. Calcination and sol gel processing may also be used.

[0036] The shell formed may have a final shell thickness of from 1 nm to 1000 nm, more particularly from 10 nm to 500 nm.

[0037] FIG. 2 shows a schematic representation of exemplary process steps used in forming encapsulated mesoporous zeolites. As shown, the initial uncoated zeolite core material 12 has mesopores 14, as well as micropores (not shown). The core material 12 is combined with active metal nanoparticles 16 (M ⁿ⁺ ) according to Step 20 so that the nanoparticles 16 are deposited within the mesopores 14. The core material 12 with the active metal nanoparticles 16 is then coated or encapsulated to form a porous shell structure 18, according to Step 24. The formed catalyst particle 10 can then be used in catalysis for forming various compounds.

[0038] In use, the resulting encapsulated, metal-containing mesoporous zeolite catalyst composition is contacted with a reaction feed under reaction conditions suitable for producing the reaction product. The type of reactions may depend upon the loaded metals of the catalyst. In particular applications, the catalyst loaded with metals or metal oxide or mixture of them can be used in the production of methanol from methane and olefins from paraffins, or other applications as shown in the following exemplary reactions (1) and (2) below:

HOCH ₂ CH ₂ OH + MeOH + O2→ HOCH ₂ COOMe + 2H ₂ 0 ( 1 )

CH ₂ =CH ₂ + CH ₃ C0 ₂ H + ½0 ₂ → CH ₂ =CH0 ₂ CCH ₃ = H ₂ 0 (2)

[0039] The following examples serve to further illustrate various embodiments and applications. EXAMPLES EXAMPLE 1

[0040] An encapsulated mesoporous MCM-41 zeolite catalyst formed according to the process steps shown in FIG. 2. Purchased mesoporous MCM-41 zeolite (available from Sigma) core material 12 is combined with active metal nanoparticles 16, as shown in Step 20. In this example, platinum (Pt) is used as the active metal nanoparticle. The metal is deposited by mixing 0.5 g of calcined meso-MCM-41 powder with 10 mL of an aqueous K ₂ PtCl ₄ solution (0.01M) for 5 minutes and the treated particles area separated from the liquid by centrifugation. After adsorption of PtCl ₄ ^" , the treated meso-MCM-41 zeolite is calcined at 500 °C for 2 hours to remove the template as the Pt precursor is reduced to Pt metal nanoparticles to form the metal-treated particle 16, as shown in Step 22.

[0041] To prepare the shell structure, a mixture of tetrapropylammonium hydroxide (TPAOH 35 wt.% in water): Na ₂ 0; Si0 ₂ : H ₂ 0: and ethanol is formed in the following ratios, respectively, 9:0.1: 1.25;480: 100. The mixture is mixed in polypropylene bottles with strong mixing for 4 hours and then heated at 98 °C for 24 hours to form TPA-silicalite-1. The resulting TPA-silicalite-1 colloidal suspension is purified by centrifuge and washed with ethanol for three times.

[0042] The meso-MCM-41 loaded with Pt in an amount of 0.5 g is mixed with 10 ml of 0.5 wt.% aqueous solution of polycation agent (poly(diallydimethylammonium chloride) and then the negatively charged silicalite-1 gel seeds are adsorbed on the catalyst core substrate. Calcination of the pre-treated meso-MCM-41 zeolite at 873K for 8 hour provides a material with open pores and seeds firmly fixed to the surface of the Pt-treated meso-MCM-41 zeolite 12 to form the porous shell 18, as shown in Step 24.

EXAMPLE 2

Experimental

[0043] In the following example, the materials of NaAl(¾, NaOH, tetraethyl orthosilicate (TEOS), polydiallyldimethylammonium chloride (PDADMAC, 20wt%, Mw= 1.5xl0 ⁵ ), K ₂ PtCl ₄ , tetrapropylammonium hydroxide solution (TPAOH, 40% in H ₂ 0) and titanium(rV) isopropoxide (TIPO) used were those commericially available from Sigma Corporation. Poly(N ¹ ,N ¹ -diallyl-N ¹ -methyl-N ⁶ ,N ⁶ ,N ⁶ -trimethylhexane-l,6-diamoniumbromide)

(PDAMAB-TMHAB) was synthesized according to Tian, C. et al., previously discussed.

[0044] Scanning electron microscopic (SEM) images were taken using a Nova NanoSEM (FEI). Transmission electron microscope (TEM) pictures were obtained using a Tecnai Twin TEM (FEI) operating at 120 kV. Energy dispersive x-ray spectroscopy (EDX) was obtained using a Nova NanoSEM (FEI) operated at 10-15 kV. X-ray diffraction (XRD) patterns were recorded at room temperature on a powder PANalytical Empyrean diffractometer using CuKa radiation (λ = 1.54059 A) at 45 kV and 40 mA. Nitrogen physisorption isotherms were measured at 77 K on an automatic volumetric adsorption apparatus (Micromertics ASAP 2420). The samples were filled into glass ampoules and outgassed in high vacuum at 473 K for 24 h before the start of the sorption measurements.

Formation of Core Structure

[0045] A hierarchical ZSM-5 (HZSM-5) was synthesized to form a core structure, such as the core structure 12 of FIG. 2. The HZSM-5 was synthesized by following the procedure set forth in Tian, C. et al., as previously discussed. NaOH at 0.2 g, 0.06 g of NaAlCte, and 0.5 g of polyiN^N^diallyl-N^methyl-^j^j^-trimethylhexane-l^-diamonium bromide)

(PDAMAB-TMHAB) were mixed in 20 mL of H ₂ 0 by stirring. After 12 h, 4.5 mL of tetraethyl orthosilicate (TEOS) was added into the mixture and stirred for another 12 h. The obtained gel was loaded into a Teflon-lined stainless steel autoclave (45 ml) and heated at 160 °C for 48 h. The precipitate was collected by centrifugation and washed with distilled water and ethanol. After drying at 100 °C overnight the resulting powder was heated at 550 °C for 10 hours under air to remove the organic templates

[0046] FIGS. 3A and 3B show the SEM and TEM images of the prepared HZSM-5 zeolite particles. The XRD patterns of the HZSM-5 zeolite as synthesized and ZSM-5 zeolite simulation are shown in FIG. 3C. The two XRD patterns matched very well, which means the synthesized HZSM-5 had an MFI structure. In addition, the EDX data (FIG. 3D) shows the HZSM-5 zeolite consists of O, Si, and Al, which means the synthesized particles are a ZSM-5 zeolite. FIG. 3E shows the N ₂ adsorption-desorption isotherm at 77 K for the HZSM-5 zeolite. The BET surface area is around 486 m /g. The pore size distribution of HZSM-5 is shown in FIG. 3F, which covers from micropore (0.5 nm), mesopore (1.6 nm) to macropore (6.8 nm), showing the synthesized HZSM-5 zeolite is a hierarchical porous ZSM- 5 zeolite.

Formation of Metal Nano-Particle/Zeolite Composite

[0047] Platinum nanoparticles were deposited in mesopores of the HZSM-5 zeolite material prepared as discussed above to form a Pt-HZSM-5 zeolite composite material, as in Steps 20 and 22 of FIG. 2. The HZSM-5 zeolite powder at 0.2 g was dispersed in 20 ml of water. To this was added 0.5 ml of polydiallyldimethylammonium chloride (PDADMAC, Sigma, 20wt %, Mw= 1.5 x 10 ⁵ ) and 0.02 of K ₂ PtCl ₄ (Sigma). The HZSM-5 zeolite loaded with K ₂ PtCl ₄ was collected via centrifugation and dried under vacuum at 70 °C overnight. The particles were then loaded into a tubular furnace and heated from room temperature to 500 °C at 4 °C/min under air and kept for 2.5 h. After the heating cycle was complete, the furnace was cooled down naturally to room temperature.

[0048] FIGS. 4A and 4B show the SEM and TEM images of the platinum-nanoparticle- containing HZSM-5 zeolite particles, respectively. From FIG. 4B, the many dark dots can be seen in the HZSM-5 zeolite particles, which can be attributed to platinum nanoparticles. FIG. 4C shows the EDX of the Pt-containing hierarchical ZSM-5 zeolite (Pt@HZSM-5), which consists of O, Si, Al and Pt. Furthermore, the XRD patterns (FIG. 4D) show Pt peaks appear in the Pt@HZSM-5 particles, which means a Pt@HZSM-5 zeolite composite was obtained.

Formation of Shell Structure

[0049] A TS-1 zeolite encapsulating or shell material was formed, as in Step 24 of FIG. 2, around the Pt@HZSM-5 zeolite particles, prepared as described above. To form the shell, 0.624 g of tetraethyl orthosilicate (TEOS, Sigma) was hydrolyzed in 15 mL of H ₂ 0 with 0.625 g of tetrapropylammonium hydroxide solution (TPAOH, 40% in H ₂ 0) under stirring overnight. The Pt@HZSM-5 zeolite composite material at 0.2 g was added into the above solution and dispersed by sonication. Titanium(IV) isopropoxide (TIPO, 0.052 g) was added to 1.5 mL of isopropanol to form a stable Ti source. The two solutions were mixed under stirring condition and heated to 95 °C to remove isopropanol. Subsequently, the required amount of distilled water was added to form a reaction solution having a total volume of 20 mL. The final solution was transferred to a Teflon-lined autoclave and crystallized at 170 °C under stirring conditions for 72 h. Finally, the obtained suspension was centrifuged and the solid was washed with water and dried at 100 °C overnight, and then calcined at 550 °C for 6 h to remove the template.

[0050] FIGS. 5A and 5B show the SEM and TEM images of the formed Pt@HZSM-5 zeolite with a TS-1 encapsulating material (Pt@HZSM-5@TS-l) particles. It can be seen that Pt@HZSM-5 particles are encapsulated by a lot of small crystals, which can be attributed to TS-1. Some large crystals also can be seen on the surface of Pt@HZSM-5 particles due to crystal growth during synthesis. In FIG. 5B, dark dots can be seen, which can be attributed to platinum nanoparticles. The XRD pattern (FIG. 5C) shows Pt characteristic peaks appear in the synthesized particles, which indicates the particles contain Pt. ZSM-5 and TS-1 have the same MFI structure, so the XRD has the same peaks when comparing Pt@HZSM-5 and Pt@HZSM-5@TS-l. It is also shown by the EDX of Pt@HZSM-5@TS-l (FIG. 5D) that the synthesized particles consist of O, Ti, Si, Al, and Pt.

[0051] While the invention has been shown in some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes and modifications without departing from the scope of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.