Surface modification of mesoporous zeolite Y without pore blockage

Abstract

We present a post-synthesis selective surface modification method for mesoporous Y-type zeolite (Meso-Y). This method results in a thin silica, alumina, or aluminosilicate coating on the exterior surface without the significant pore blockage that occurs in the absence of surfactants. The technique is based on the encapsulated CTAB surfactant contained in the as-synthesized Meso-Y, which acts as a soft template. This soft template protects the interior microporosity and mesoporosity during the synthesis of the inorganic coating by directing the deposition of the inorganic coating to occur selectively on the exterior surface. The method is performed under dry conditions.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application was filed as a PCT international application on February 5, 2024, and claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 483,373, filed on February 6, 2023, the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] Zeolites are a type of crystalline microporous material that have been widely used as catalysts for the production of valuable chemical products. Zeolite Y, in particular, has been used for decades in fluid catalytic cracking (FCC) and hydrocracking of oil-derived molecules. While current variants such as ultrastable Y (USY) zeolites exhibit excellent performance in catalytic applications, one key issue remains the severe diffusion limitation typically caused by the small micropores in zeolite materials. In this regard, researchers have developed several post-synthetic methods to introduce intracrystalline mesoporosity into zeolites.

[0003] For example, CBV-720 zeolite is a commercially available USY-type zeolite (Si / Al = 15) with an FAU topology, composed of irregular intracrystalline mesopores synthesized by steam treatment. This USY-type zeolite has been used as the starting material in a previously reported surfactant templating method, which produces hierarchical mesoporous Y-type zeolites (Meso-Y). The latter contain homogeneously distributed intracrystalline mesopores and are typically synthesized by treating the commercial USY-type zeolite CBV-720 with an alkaline hydroxide solution containing the surfactant cetyltrimethylammonium bromide (CTAB). Atomic force microscopy (AFM) characterization data clearly demonstrate the formation of mesopores near the outer surface during the surfactant templating method. The resulting encapsulated surfactant is typically removed from the as-prepared material by combustion during calcination, allowing the mesopores to be synthesized. The sizes of these generated mesopores are comparable to those measured in bulk by nitrogen physisorption and transmission electron microscopy (TEM)-based three-dimensional tomography, all of which result from the assembly of the CTAB surfactant. These mesopores allow for facile molecular transport and act as pathways for the rapid diffusion of reactants and products to and from catalytically active sites within the zeolite material.

[0004] However, while hierarchical zeolite structures have attracted much attention in recent years due to their improved transport properties, new challenges have been raised regarding surface diffusion barriers that cause increased diffusion time constants in these and related hierarchical mesoporous zeolite materials. These surface barriers arise from the large free energy difference between the gas / liquid phase, the zeolite exterior surface, and the zeolite interior, and have been demonstrated by microimaging of conventional mordenite zeolites. To overcome these surface barriers, various post-synthesis surface modifications by chemical liquid deposition and chemical vapor deposition have been proposed.

[0005] In this regard, a current synthetic challenge is to achieve spatially selective modification of the zeolite exterior surface with inorganic oxide (e.g., amorphous aluminosilicate) coatings without blocking the zeolite's interior porosity. Meeting this challenge could lead to important applications in catalysis and electrostatic adsorption. Summary of the Invention

[0006] A post-synthesis selective surface modification method for hierarchical mesoporous Y-type zeolite (Meso-Y) is presented, which results in a thin silica, alumina, or aluminosilicate coating on the outer surface without the significant pore blockage that occurs in the absence of surfactants. This approach is based on the encapsulated CTAB surfactant contained in as-synthesized Meso-Y, which acts as a soft template. This soft template protects the interior microporosity and mesoporosity during the synthesis of the inorganic coating by directing the deposition of the inorganic coating selectively on the outer surface. It was found that performing the surface modification under dry conditions during the reaction of the oxide molecular precursor with as-synthesized Meso-Y is crucial. If dry conditions are not used, pore blockage and thickening of the silicate coating, as well as phase separation of the alumina coating, occur, which can be observed by SEM. In contrast, this method allows the synthesis of uniform silica / alumina nanoscale coatings under dry conditions, with no evidence of distinct phases, as demonstrated by TEM / SEM microscopy and zeta potential measurements. These uniform coatings control the surface charge of the Y zeolite, which can be important for applications involving adsorption and catalysis.

[0007] Among other factors, we found that the combination of CTAB surfactant and dry deposition conditions successfully synthesized uniform silica, alumina, or aluminosilicate shells on mesoporous Y-type zeolite surfaces while preserving mesoporosity and microporosity after coating. The presence of the surfactant CTAB in the as-synthesized Meso-Y-as zeolite was crucial for achieving this selective modification of the outer surface of the Meso-Y-as zeolite without pore blockage. This is achieved through a mechanism based on soft protection by the CTAB surfactant. Given the trend toward using inorganic oxide-surface-modified zeolites in functional applications, the resulting surface-modified zeolite materials represent a promising new class of surface-modified zeolite materials for functional applications involving adsorption and catalysis, where the surface charge (zeta potential) of the surface can be precisely controlled.

[0008] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several aspects of the present disclosure. A brief description of the drawings follows. [Brief explanation of the drawings]

[0009] [Figure 1A] Figure 1A shows the powder X-ray patterns of as-synthesized (as) silica deposition samples obtained under wet conditions: (a) CBV-720, (b) Meso-Y-as, (c) Meso-Y-as@wet-4 wt% SiO2, and (d) Meso-Y-as@wet-11 wt% SiO2.

[0010] [Figure 1B] Figure 1B shows the powder X-ray patterns of calcined silica deposition samples obtained under wet conditions: (a) CBV-720, (b) Meso-Y-cal, (c) Meso-Y-cal@wet-4 wt% SiO2, and (d) Meso-Y-cal@wet-11 wt% SiO2.

[0011] [Figure 2]Figure 2 shows SEM images of (a and b) CBV-720, (c) Meso-Y-as, (d) Meso-Y-as@wet-4 wt% SiO2, (e) Meso-Y-as@wet-11 wt% SiO2, (f) Meso-Y-cal, (g) Meso-Y-cal@wet-4 wt% SiO2, and (h) Meso-Y-cal@wet-11 wt% SiO2.

[0012] [Figure 3A] Figure 3A shows the N adsorption–desorption isotherms of CBV-720, Meso-Y-cal, Meso-Y-cal@wet-4 wt% SiO2, Meso-Y-cal@wet-11 wt% SiO2, and Meso-Y-as@wet-11 wt% SiO2.

[0013] [Figure 3B] Figure 3B shows the mesopore size distributions of CBV-720, Meso-Y-cal, Meso-Y-cal@wet-4 wt% SiO2, and Meso-Y-cal@wet-11 wt% SiO2.

[0014] [Figure 3C] Figure 3C shows the N adsorption–desorption isotherms of CBV-720, Meso-Y-cal, Meso-Y-as@wet-4 wt% SiO2, and Meso-Y-as@wet-11 wt% SiO2.

[0015] [Figure 3D] Figure 3D shows the mesopore size distributions of CBV-720, Meso-Y-cal, Meso-Y-as@wet-4 wt% SiO2, and Meso-Y-as@wet-11 wt% SiO2.

[0016] [Figure 4] Figure 4 shows SEM images of (a) Meso-Y-as@wet-0.9 wt% Al2O3, (b) Meso-Y-as@wet-2.2 wt% Al2O3, and (c) Meso-Y-as@wet-4.6 wt% Al2O3.

[0017] [Figure 5] Figure 5 shows the powder X-ray patterns of silica deposition samples obtained under dry conditions: (a) Meso-Y-as, (b) Meso-Y-as@dry-4 wt% SiO2, (c) Meso-Y-as@dry-11 wt% SiO2, and (d) Meso-Y-as@dry-15.4 wt% SiO2.

[0018] [Figure 6A] Figure 6A shows the N adsorption–desorption isotherms of Meso-Y-cal, Meso-Y-as@dry-4 wt% SiO2, Meso-Y-as@dry-11 wt% SiO2, and Meso-Y-as@dry-15.4 wt% SiO2.

[0019] [Figure 6B] Figure 6B shows the mesopore size distributions of Meso-Y-cal, Meso-Y-as@dry-4 wt% SiO2, Meso-Y-as@dry-11 wt% SiO2, and Meso-Y-as@dry-15.4 wt% SiO2.

[0020] [Figure 7] Figure 7 shows SEM images of (a) Meso-Y-as@dry-4 wt% SiO2, (b) Meso-Y-as@dry-11 wt% SiO2, and (c) Meso-Y-as@dry-15.4 wt% SiO2.

[0021] [Figure 8] Figure 8 shows TEM images of (a, b) Meso-Y-cal, (c, d) Meso-Y-as@dry-11%SiO2, (e, f) Meso-Y-as@dry-15.4%SiO2, (g, h) Meso-Y-as@dry-2.2%Al2O3, and (i, j) Meso-Y-as@dry-4.6%Al2O3.

[0022] [Figure 9]Figure 9 shows the powder X-ray patterns of alumina deposition samples obtained under dry conditions: (a) Meso-Y-as, (b) Meso-Y-as@dry-0.9 wt% Al2O3, (c) Meso-Y-as@dry-2.2 wt% Al2O3, and (d) Meso-Y-as@dry-4.6 wt% Al2O3.

[0023] [Figure 10] Figure 10 shows SEM images of (a) Meso-Y-as@dry-0.9 wt%Al2O3, (b) Meso-Y-as@dry-2.2 wt%Al2O3, and (c) Meso-Y-as@dry-4.6 wt%Al2O3.

[0024] [Figure 11A] Figure 11A shows the N2 adsorption–desorption isotherms of Meso-Y-cal, Meso-Y-as@dry-0.9 wt%Al2O3, Meso-Y-as@dry-2.2 wt%Al2O3, and Meso-Y-as@dry-4.6 wt%Al2O3.

[0025] [Figure 11B] Figure 11B shows the mesopore size distributions of Meso-Y-cal, Meso-Y-as@dry-0.9 wt% Al2O3, Meso-Y-as@dry-2.2 wt% Al2O3, and Meso-Y-as@dry-4.6 wt% Al2O3.

[0026] [Figure 12A] FIG. 12A shows the zeta potential variation of Meso-Y-cal with various silica loadings.

[0027] [Figure 12B] FIG. 12B shows the zeta potential variation of Meso-Y-cal with various alumina loadings.

[0028] [Figure 13] Figure 13 is a schematic illustration of the deposition methods for preparing Meso-Y-as under wet and dry conditions. DETAILED DESCRIPTION OF THE INVENTION

[0029] Various additional aspects of the invention are set forth in the following description. Aspects of the invention can relate to individual features and combinations of features. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not intended to limit the broad inventive concepts underlying the embodiments disclosed herein.

[0030] Reference will now be made in detail to the exemplary embodiments of the present disclosure, which are illustrated in the accompanying drawings.

[0031] This method utilizes a combination of CTAB surfactant and dry deposition conditions to successfully synthesize uniform silica, alumina, or aluminosilicate shells on mesoporous Y-type zeolite surfaces. The mesopores generally fall within the size range of greater than 2 nm to less than 50 nm. This is achieved while retaining mesoporosity and microporosity after coating deposition.

[0032] This method involves depositing a silica, alumina, or aluminosilicate layer on the surface of Meso-Y zeolite. The Meso-Y zeolite has been treated to incorporate a CTAB surfactant into the mesopores of the Y zeolite. Such treatment may involve any suitable known process. For example, an appropriate amount of CTAB surfactant can be dissolved in a basic aqueous solution. The solution is heated, and then a Y zeolite, such as CBV-270, is introduced into the solution. The solution is then held at an appropriate temperature, such as 90°C, with stirring for a period of time, such as 5 or 6 hours. The resulting zeolite powder is filtered, washed, and dried to obtain the as-synthesized Meso-Y-as material, which contains the CTAB surfactant within the newly formed mesopores. After calcination, for example at 580°C, the Meso-Y-cal material is obtained. It is preferable to use the Meso-Y-as material for this method. Calcination may then be performed. However, methods for depositing silica, alumina, or aluminosilicate layers under dry conditions can also be used with Meso-Y-cal.

[0033] The Meso-Y-as zeolitic material containing the CTAB surfactant within its mesopores is first dehydrated. In one embodiment, the dehydration comprises heating the Meso-Y-as zeolitic material under vacuum. In one embodiment, heating under vacuum may comprise heating at a temperature in the range of 200-300°C, e.g., 250°C, under vacuum for 7-12 hours, in one embodiment, 10 hours.

[0034] The dehydrated zeolite is then placed in a reactor in a dry environment, which can be created by using an inert gas atmosphere. In one embodiment, a nitrogen gas atmosphere is used.

[0035] The alumina precursor, silica precursor, or aluminosilicate precursor is then mixed with a dry solvent to form a mixture. Any suitable solvent that does not contain water can be used. In one embodiment, the solvent is tetrahydrofuran (THF). The solvent itself is also ensured to be dry and water-free.

[0036] If an alumina coating on the surface of the zeolite is desired, the method can involve any suitable alumina precursor. In one embodiment, the alumina precursor can be Al(Oi-Pr)3. If a silica coating is desired, any suitable silica precursor can be used. In one embodiment, Si(OEt)4 can be the silica precursor. If an aluminosilicate coating is desired, both an alumina precursor and a silica precursor are provided.

[0037] The precursor mixture solution is also placed in the same reactor containing the dehydrated zeolite. The contents of the reactor are then refluxed to achieve a deposition reaction that produces a layer of alumina, silica, or aluminosilicate on the surface of the Meso-Y-as zeolite. Refluxing is typically performed with stirring. The Meso-Y-as zeolite is recovered from the reactor, and any remaining solvent is removed. The solvent can be removed by any suitable method. In one embodiment, the solvent is removed by vacuuming. The Meso-Y-as zeolite recovered from the reactor can be calcined either before or after solvent removal. In one embodiment, calcination is performed after solvent removal. In one embodiment, calcination is performed at a temperature of about 580°C for about 4 hours. The temperature and time can be varied.

[0038] In one embodiment, the Meso-Y-as zeolite can be dried after removing the solvent. In one embodiment, the drying can be performed under vacuum. Then, calcination can be performed after drying. In one embodiment, all calcinations are performed in dry air.

[0039] The presence of CTAB in the as-synthesized Meso-Y-as is crucial for achieving selective modification of the outer surface of Meso-Y-as without pore blockage. This is achieved through a mechanism based on soft protection by the CTAB surfactant. It was also found that the use of dry deposition conditions, as described above, during the reaction of the oxide molecular precursor with Meso-Y-as is crucial. If dry conditions are not used, pore blockage and thickening of the silicate coating and phase separation of the alumina coating occur.

[0040] The following examples are given to illustrate, but not to limit, the method and products of the present invention.

[0041] Material Synthesis Preparation of mesoporous zeolite Y (Meso-Y) based on commercial Y zeolite.

[0042] CBV-720 was obtained from Zeolyst and ammonium ion-exchanged before use. The synthesis of Meso-Y was carried out according to a previously reported surfactant templating method. In a typical synthesis, 0.5 g of CTAB surfactant was dissolved in 20 mL of 0.09 M NaOH aqueous solution. The mixed solution was heated to 90 °C using an oil bath. After stirring at 90 °C for 30 min, 1 g of CBV-720 was added and then maintained at 90 °C for 6 h with stirring. The resulting zeolite powder was filtered, washed with deionized water, and dried at 80 °C for 12 h to obtain the as-synthesized intermediate material Meso-Y-as, containing the CTAB surfactant encapsulated within the newly formed mesopores. After further calcination at 580 °C in air, the as-prepared hierarchical Meso-Y-cal was obtained.

[0043] Deposition of silica on Meso-Y-cal and deposition of silica, alumina, or aluminosilicate on Meso-Y-as.

[0044] Deposition of silica, alumina, or aluminosilicate coatings was performed on both Meso-Y-as and Meso-Y-cal using post-synthesis surface modification techniques. For comparison, two different types of deposition conditions were used, corresponding to wet and dry conditions. See, for example, Figure 13.

[0045] In a typical wet deposition, the entire procedure was performed with exposure to atmospheric moisture. 1 g of zeolite (Meso-Y-as or Meso-Y-cal) was dispersed in 25 mL of hexane. The desired amount of tetraethyl orthosilicate (TEOS) or aluminum isopropoxide (Al(Oi-Pr)3) was introduced into the mixture, and deposition was carried out for 1 h under reflux and stirring. The hexane was then removed by vacuum. The zeolite product was dried under vacuum at 120 °C for 2 h and calcined in air at 580 °C for 4 h. The final product is designated Meso-Y-as (Meso-Y-cal)@wet-x wt% SiO2 (or with the suffix y wt% Al2O3), where x and y indicate the weight percent of silica or alumina used in the deposition, respectively. For example, a product prepared from Meso-Y-as with 4 wt% SiO2 deposition under wet conditions was designated Meso-Y-as@wet-4 wt% SiO2.

[0046] Dry deposition experiments were carried out under air-free conditions. For silica deposition, 1 g of as-synthesized mesoporous zeolite (Meso-Y-as) was dehydrated under vacuum at 250 °C for 10 h to remove residual water in the sample. After cooling to room temperature, dry N2 was injected into the reactor as a protective gas, and then 40 mL of dry tetrahydrofuran (THF) and TEOS were introduced into the reactor (the latter corresponding to 4–15.4 wt% SiO2 loading). Deposition was carried out under a dry N2 atmosphere with reflux and stirring for 1 h. After the reaction, the THF solvent was removed by evacuation. The resulting sample was further dried under vacuum at 120 °C for 2 h and subsequently calcined in air or dry air at 580 °C for 4 h.

[0047] For alumina deposition, the desired amount of Al(Oi-Pr)3 was introduced into a flask under an Ar atmosphere in a glovebox (corresponding to a specific target weight percent of Al2O3). Dry THF solvent was then mixed with Al(Oi-Pr)3 at 60 °C with stirring (approximately 100 mL of THF was used per 1 g of Al(Oi-Pr)3). Sufficient THF solvent was used to ensure 80-90% dissolution of Al(Oi-Pr)3. The resulting solution containing Al(Oi-Pr)3 was hot-filtered under air-free conditions to remove traces of undissolved impurities. The filtrate was further mixed with dried zeolite (Meso-Y-as). The mixture was reacted under reflux and stirring for 1 h under a N2 atmosphere. After the reaction, the THF solvent was removed by evacuation. The sample was dried under vacuum at 120 °C for 2 h and further calcined in air or dry air at 580 °C for 4 h. The resulting samples were designated Meso-Y-as@dry-x wt% SiO or (suffix y wt% AlO), where x and y refer to the weight percent of deposited silica or alumina, respectively, in the product. The deposition of the aluminosilicate coating layer is similar to that of alumina, except that a silica precursor is also introduced. Thus, both alumina and silica precursors are present.

[0048] Characterization Powder X-ray diffraction patterns (PXRD) were collected using a Bruker D8 Advance diffractometer equipped with a CuKα radiation source (λ = 1.5418 Å, 40 kV, 40 mA). Scanning electron microscopy (SEM) images were acquired on a Hitachi S-5000 microscope. N2 adsorption isotherms were measured at 77 K, and mesopore size distributions, as shown in Figures 3B, 3D, 6B, and 11B, were calculated by the BJH method using a Micrometrics ASAP 2020 instrument. Prior to measurements, samples were preheated at 350 °C for 4 h under vacuum conditions. Transmission electron microscopy (TEM) images were acquired on a JEOL JEM 2010 microscope operating at an accelerating voltage of 200 kV and equipped with a LaB6 electron gun in low-dose mode. Samples were prepared by embedding in epoxy resin, allowing it to harden, and then cutting thin sections (approximately 30–50 nm) with a Leica EM UC7 ultramicrotome. The sections were placed on a 300-mesh Cu grid with a thin (20–30 nm) lacy carbon support film. Zeta potential measurements were performed using a Malvern Zetasizer Nano-Z (Malvern Instruments, Malvern, UK). Prior to measurement, the samples were dispersed in deionized water at a concentration of 0.5 wt %. After 5 min of sonication, at least five measurements were taken at room temperature, and the average value was calculated.

[0049] Results, Discussion, and Conclusion The initial material used as the starting point for subsequent post-synthetic surface modification was calcined mesoporous Y-type zeolite (Meso-Y-cal), which was synthesized from the parent CBV-720 (ammonium form) based on a previously reported surfactant templating approach in the literature. The porosity of the Meso-Y-cal material was characterized using N2 physical adsorption at 77 K. The adsorption isotherm is shown in Figure 3A, which contains a combination of Type I and Type IV isotherms, indicated by micropore filling at relative pressures below 0.1 and intrapore condensation within the mesopores at higher relative pressures. Deconvolution of the mesopore size data in Figure 3B shows a narrow pore size distribution centered around 3.6 nm, further supporting the development of uniform intracrystalline mesoporosity. As a result, compared to the parent material CBV-720, the Meso-Y-cal sample exhibited an increased total specific surface area (760 m for CBV-720). 2 g -1 886m 2 g -1 ), and the mesopore surface area increases (180 m for CBV-720). 2 g -1 while 525m 2 g -1 ), the mesopore volume increased (0.22 cm for CBV-720). 3 g -1 0.44cm 3 g -1 ), but the micropore surface area decreased (580 m for CBV-720). 2 g -1 362m 2 g -1 ), the micropore volume decreased (0.23 cm for CBV-720). 3 g -1 while 0.14cm 3 g -1 These data suggest that mesoporosity was developed in Meso-Y-cal, with a slight decrease in microporosity in CBV-720.

[0050] Under wet conditions, the subsequent post-synthesis surface modification can be considered a two-step process. The first step involves mixing the monomer precursor of the oxide coating layer (i.e., Si(OEt)4 for silica or Al(Oi-Pr)3 for alumina) with the hierarchical Meso-Y-cal zeolite under wet conditions (i.e., exposure to ambient air). During this process, the monomer precursor diffuses into the Meso-Y and is absorbed into pores close to the outer surface. Partial hydrolysis and condensation occur during this initial mixing. During the second step, i.e., subsequent calcination, the organic shell is combusted in air to synthesize vapors and hydrolyzed / condensed crosslinked inorganic oligomers, which then form the inorganic coating layer close to the zeolite outer surface.

[0051] Under wet conditions, the as-prepared (Meso-Y-as) and calcined (Meso-Y-cal) samples were first coated with silica using 4 wt. % and 11 wt. % SiO2 added to the zeolite prior to calcination against SiO2 in air. After deposition of the SiO2 coating, both samples exhibited well-resolved PXRD patterns typical of FAU (Figure 1A). The wet deposition conditions did not significantly affect the zeolite crystal structure. Similar observations are shown in Figure 1B for the sample synthesized starting from Meso-Y-cal. These observations are similar to those previously reported when surface-modifying HZSM-531 and SAPO-34 zeolites.

[0052] SEM images of the parent samples (Meso-Y-as and Meso-Y-cal) and the post-synthetic modified samples after deposition of the SiO2 coating layer in Figure 2 show no phase separation in any of the samples. This is because similar crystal morphologies and crystal sizes are observed for the CBV-720 and the resulting Meso-Y materials. The deposition of the SiO2 coating layer must have occurred as a thin (nanoscale) shell either on the outer surface of each crystallite or within the crystallite. Because it is impossible to distinguish between these latter two possibilities based on the SEM data (Figure 2) alone, N2 physisorption data was used.

[0053] Based on the N2 physisorption isotherms in Figure 3A and the calculated porosity data in Table 1 below, deposition of 4 wt% SiO2 onto Meso-Y-cal under wet conditions reduced the mesopore surface area and volume (approximately 12%), while maintaining the microporosity of Meso-Y-cal (Table 1, No. 3). When the SiO2 loading on Meso-Y-cal was 11 wt%, the resulting material exhibited a significant reduction in the micropore and mesopore surface area (approximately 20%), as well as a significant reduction in the pore volume (approximately 14% to 36%) (Table 1, No. 4). This increased amount of SiO2 appears to not only occur at the mesopore surface of Meso-Y-cal, but also to cause partial internal micropore blockage. The results clearly show that the open structure of hierarchical Meso-Y-cal provides a large mesopore surface area for the deposition of silica coating layers, which interact with surface silanols within the zeolite mesopores throughout the crystal during calcination, ultimately leading to the blockage of micropores and mesopores, especially at high SiO2 loadings of 11 wt% under wet conditions. [Table 1]

[0054] We investigated the Meso-Y-as material, rather than the calcined Meso-Y-cal material, as a support for silica surface modification under wet conditions. The N physisorption data in Figure 3C and Table 1 (No. 6) show the same material with an 11 wt. % SiO coating on Meso-Y-as compared to the parent Meso-Y-cal (Table 1, No. 2). This similar amount of microporosity in the coated material with an 11 wt. % SiO coating on Meso-Y-as is in stark contrast to the observed decrease in microporosity when silica-coating Meso-Y-cal. Also consistent with this trend, a smaller decrease in mesopore volume, i.e., 23%, was observed when an 11 wt. % SiO coating was applied to the Meso-Y-as material (versus a 36% decrease for the corresponding material starting from Meso-Y-cal). Furthermore, deposition of 11 wt% SiO2 on Meso-Y-as results in a narrower mesopore size distribution, as shown by the data in Figure 3D, whereas the data in Figure 3B show that deposition of the same amount of SiO2 on Meso-Y-cal did not change the mesopore size distribution. This different effect of post-synthesis silica surface modification on mesopore size distribution suggests that the deposition of the SiO2 coating layer occurs at different locations when using either Meso-Y-as or Meso-Y-cal as the support material, with the latter promoting micropore blockage.

[0055] The Meso-Y-as sample contains the CTAB surfactant within the mesopores, whereas the corresponding calcined form of Meso-Y-cal consists of an open hierarchical porous channel system (the organic surfactant component was removed during calcination). Based on the absence of micropore blockage in the 11 wt% SiO2 overlayer material when coated onto Meso-Y-as, the surfactant CTAB appears to play an important role as a soft template in protecting the interior microporosity from surface modification. This template guides the deposition of silica overlayers in Meso-Y-as to occur only in the mesopores, but not in the micropores. This suggests that the early stage of calcination of silica-surface-modified Meso-Y-as (i.e., after the reaction with TEOS under wet conditions) causes partial decomposition of the CTAB surfactant, creating space for silica deposition on the surface of the intracrystalline mesopores. During surfactant combustion, the partially decomposed CTAB surfactant may orchestrate the formation of silica oligomers at the organic / inorganic interface through electrostatic interactions between positively charged CTAB and negatively charged silica species. This interaction is expected to selectively lead to the deposition of SiO2 coatings on the mesopore surfaces, as observed. During this process, silica oligomers barely penetrate the micropore channel system of Meso-Y-as during calcination due to mesopore blockage by CTAB surfactant fragments. However, the spatially selective surface modification approach benefits from the presence of the surfactant CTAB within the mesopores, much like the description of the selective silylation of the as-synthesized mesoporous silica MCM-41 when using organosilanes. This condensed organosilane was found to be primarily grafted to the outer surface of the as-synthesized material due to the soft protection provided by CTAB. This is similar to the protective effect that CTAB exerts on zeolite Y under basic aqueous conditions, where this surfactant protects the zeolite from dissolution through strong interactions with the framework.Based on this observed protective role of the microporosity imparted by CTAB during silica deposition under the wet conditions described above, in the following we focus on Meso-Y-as, rather than Meso-Y-cal, as the optimal starting material for the deposition of inorganic oxide coating layers.

[0056] Surface modification with alumina coatings was also investigated under wet conditions, starting with Meso-Y-as materials. The resulting samples, Meso-Y-as@wet-0.9 wt% Al2O3, Meso-Y-as@wet-2.2 wt% Al2O3, and Meso-Y-as@wet-4.6 wt% Al2O3, exhibited XRD patterns nearly identical to those of the parent Meso-Y-as samples (Figure 1A). The three coated samples also exhibited comparable microporosity and mesoporosity based on N2 adsorption data (Table 1, Nos. 7–9). However, the SEM images in Figure 4 reveal several small particles on the crystal surfaces of Meso-Y-as@wet-0.9 wt% Al2O3 and Meso-Y-as@wet-2.2 wt% Al2O3 (see Figures 4a and 4b). The amount of such particles is directly related to the amount of AlO surface modification for the Meso-Y-as@wet-4.6 wt% AlO sample (see 4c). Surface modification of AlO under wet conditions by hydrolysis of Al(Oi-Pr) and condensation upon exposure to atmospheric moisture appears to result in the formation of alumina nanoparticles on the zeolite outer surface. The condensation product ultimately results in a mixture of Meso-Y and alumina nanoparticles on the zeolite outer surface, providing a method for synthesizing a separate phase of alumina on the Meso-Y outer surface, but with minimal interfacial contact between alumina and zeolite. This may prove useful in controlling this degree of contact or nanoscale proximity for catalyst synthesis.

[0057] The above deposition experiments demonstrate that wet conditions can work for depositing silica shells onto Meso-Y-as containing CTAB surfactant only at low SiO loading (4 wt%) to synthesize materials without severe micropore and mesopore blockage. At higher SiO loadings (11 wt%), wet conditions resulted in materials with reduced mesopore volume and mesopore size. Furthermore, wet conditions resulted in a separate alumina phase that did not form good interfacial contact with the zeolite.

[0058] These inconsistent results were disappointing. Therefore, the present method was developed as a more general and effective synthetic method for more uniform (homogeneous) modification of the crystalline surface of Meso-Y zeolite, resulting in a high degree of interfacial contact between the zeolite material and the coating layer without the formation of a separate amorphous phase on the outer surface. This method allows both the mesoporosity and microporosity of the Meso-Y zeolite material to be preserved after modification. For silica deposition, residual moisture in the wet deposition system appears to induce partial hydrolysis of TEOS and catalyze the condensation between surface silanols and partially hydrolyzed silica species even before calcination. This leads to a non-uniform distribution of silica species that favors regions close to the outer surface, resulting in a reduction in mesopore size during calcination.

[0059] However, the present surface modification method is carried out under strictly dry conditions: residual water can be removed by carrying out the deposition under air-free conditions, for example by heating the Meso-Y-as material at a temperature of 250°C under vacuum, before the next step of calcining it in either ambient or dry air.

[0060] As shown in Figure 5, deposition of a silica coating layer on as-synthesized Meso-Y-as under dry conditions yielded materials with high crystallinity, similar to the parent mesoporous zeolite Y. Furthermore, N adsorption-desorption isotherms (Figure 6A) and the corresponding calculated surface areas and pore volumes of the resulting calcined samples at SiO loadings ranging from 4 to 15.4 wt% (Table 2, Nos. 3–5, below) indicated that the micropores of the parent Meso-Y sample remained open, with no evidence of pore blockage. This latter result is similar to that observed under wet conditions, but in contrast to silica surface modification under wet conditions (Figure 3D), the mesopore size of the silica-coated material was maintained after SiO deposition under dry conditions (Figure 6B). The slight decrease in mesopore surface area in the surface-modified samples is the result of silica deposition on the outer surface, resulting in limited blockage of some mesopore entrances throughout the crystal (Table 2, Nos. 3–5). However, any such silica coating on the outer surface was thin, as no distinct phases could be observed in the SEM images of these two samples (Figure 7). The TEM images in Figure 8 demonstrate the thinness (less than 10 nm) of the silica coating on the outer surface of Meso-Y zeolite. Silica deposition under dry conditions was successfully achieved on the Meso-Y-as material with no micropore blockage and limited mesopore blockage, even at a high SiO loading of 15.4 wt% (Table 2, No. 5). This small amount of pore blockage could not be achieved under wet silica deposition conditions. Under dry conditions, the introduced silica source TEOS interacted with the partially decomposed surfactant CTAB within the intracrystalline mesopores during calcination, forming a much more uniform silica coating on the mesopore surfaces of Meso-Y-as.

[0061] Investigation of the alumina surface modification of Meso-Y-as materials under dry conditions using Al(Oi-Pr)3 as a precursor yielded interesting results. Based on the PXRD patterns shown in Figure 9, after the synthesis of 0.9–4.6 wt% Al2O3 coating layers, the resulting samples retained a highly crystalline FAU framework structure. However, in stark contrast to the results obtained under wet conditions in Figure 4, the SEM data in Figure 10 indicate that after alumina surface modification under dry conditions, no other phases were observed, and the crystals retained their morphology. TEM images in Figure 8(g–j) show a thin (<10 nm) alumina coating layer on the outer surface of the Meso-Y zeolite material. Dry conditions resulted in a homogeneous alumina coating, with a high degree of interfacial contact between the alumina and the zeolite material. Under dry conditions, hydrolysis of the Al source caused by the presence of residual water was completely avoided, and Al(Oi-Pr)3 grafting occurred only on the outer surface of the parent zeolite sample. Furthermore, based on the N adsorption-desorption data (Fig. 11A), it was observed that at low AlO loadings ranging from 0.9 to 2.2 wt%, the micropores were not affected by the AlO surface modification, as the obtained samples exhibited micropore surface areas and micropore volumes comparable to those of the parent Meso-Y-cal (Fig. 11B; Table 2, Nos. 6–7, below). [Table 2] At a relatively high Al2O3 loading of 4.6 wt%, there is a slight decrease in the micropore surface area (362 m for Meso-Y-cal). 2 g -1 318m 2 g -1 ), and a slight decrease in micropore volume (0.14 cm for Meso-Y-cal 3 g -1 while 0.12cm 3 g -1 ) was observed (Table 2, No. 8). In one embodiment, a limit of alumina coating (2.2 wt. % Al2O3) on the Meso-Y surface under dry conditions is preferred to avoid micropore blockage.

[0062] Strictly dry air was also used to calcinate intermediate samples obtained after mixing inorganic oxide coating sources (i.e., TEOS or Al(Oi-Pr)3) with Meso-Y-as under dry conditions. Virtually no change in porosity after coating with a silica or alumina shell was observed in both dry and humid air. This result indicates that maintaining a dry condition during the first step of mixing the deposited inorganic oxide coating source with Meso-Y-as is more important than maintaining a dry condition during the calcination step, which in both cases releases vapors during the combustion of organic matter. These vapors cause hydrolysis and condensation between the inorganic shell and the silanols on the zeolite surface.

[0063] Considering the lack of observed phase separation between the silica / alumina overcoat and the Meso-Y-cal support, as evidenced by the SEM in Figures 7 and 10, and the intimate interfacial contact between the overcoat and the support in the TEM image in Figure 8, it can be concluded that the present synthesis approach under dry conditions results in a uniform nanoscale coating of silica / alumina on the Meso-Y-cal support.

[0064] We also investigated how such nanoscale coating layers affect the zeta potential (or surface charge) of the surface-modified Meso-Y-cal supports. To address this, the zeta potential of all materials synthesized under dry conditions was measured in deionized water at pH = 7. Zeta potential is known to directly reflect the degree of protonation / dissociation of surface hydroxyl groups, which is controlled by the surface Si / Al composition. These data are shown in Table 3 below. [Table 3]

[0065] Uncoated calcined Meso-Y-cal zeolite had a negatively charged surface, corresponding to a zeta potential of -39.3 mV. After silica surface modification, the zeta potential of the coated material became more negative with increasing amounts of silica overlayer, consistent with greater dissociation of surface hydroxyl groups in silica compared to Meso-Y-cal. In contrast, alumina surface modification increased the zeta potential of the coated material in a manner directly dependent on the amount of alumina overlayer, as shown by the data in Table 3. The linear change in zeta potential with the amount of silica / alumina overcoat is shown in Figures 12A and 12B. This linearity is consistent with the previously observed lack of phase separation across all compositions. That is, if silica / alumina phase separation had begun at a certain high loading, this would manifest as a nonlinearity in Figure 12, but no such nonlinearity is observed. Note that alumina generally has a more positive zeta potential compared to silica at pH=7.

[0066] The observed uniformity of the silica / alumina as a nanoscale coating on the Meso-Y-cal support, rather than as phase-separated oxide particles away from the support, as observed here by SEM / TEM microscopy imaging and supported by zeta potential measurements, makes the synthesis approach described herein under dry conditions a useful method for intelligently controlling the surface charge of Meso-Y-cal zeolite materials while avoiding pore blockage. This synthetic control has clear applications for the nucleation of mono- and multi-metallic metal clusters on zeolite surfaces using techniques such as strong electrostatic adsorption, as well as for controlling the dispersibility of Meso-Y-cal particles during catalyst shaping, which typically involves concentrated aqueous suspensions and where it is generally desirable to avoid agglomeration of the zeolite particles themselves.

[0067] Based on the above results, we also synthesized mixed aluminosilicate coatings containing both silica and alumina components in the overcoat on Meso-Y-as, rather than overcoats based solely on either silica or alumina in the shell as in the previous examples. The advantage of such an aluminosilicate shell is that it contains a catalytically active overcoat layer, since it contains Al-O-Si sites known to be active in acid catalysis. This technique used dry conditions. Using this technique, we simultaneously modified the surface with both a silica precursor (i.e., TEOS) and an alumina precursor (i.e., Al(Oi-Pr)3) at 4 wt% and 8 wt% (weight percent refers to the total amount of SiO2 and Al2O3) to synthesize coatings with average silica-to-alumina ratios (SARs) ranging from 14 to 150. The data show that the zeolite products after aluminosilicate surface modification had comparable microporosity (e.g., micropore volume) compared to the calcined parent material, Meso-Y-as. These data confirm the above conclusions regarding pure silica and alumina coatings by confirming that the aluminosilicate surface modification process also did not cause blockage of the zeolite pores. The uniformity of the aluminosilicate coating was also confirmed by SEM and TEM (the aluminosilicate layer is less than 30 nm thick). As with the pure silica and alumina surface modifications, the zeta potential of the aluminosilicate-modified materials was measured, and the data show that the zeta potential is significantly less negative than that of the parent Meso-Y-cal material, in a manner that is sensitively dependent on the silicon-to-alumina ratio of the coating.

[0068] Given the trend toward using inorganic oxide surface-modified zeolites in functional applications, the resulting surface-modified Meso-Y zeolite materials represent a promising new class of zeolite materials for functional applications involving adsorption and catalysis, where the surface charge (zeta potential) of the surface can be controlled.

[0069] As used in this disclosure, the words "comprises" or "comprising" are intended as open-ended transitions to mean the inclusion of the specified elements, but not necessarily the exclusion of other elements not specified. The phrase "consists essentially of" or "consisting essentially of" is intended to mean the exclusion of other elements of some essential significance to the composition. The phrase "consisting of" or "consists of" is intended as a transition to mean the exclusion of everything other than the recited elements, except for only trace amounts of impurities.

[0070] All patents and publications referenced herein are incorporated by reference to the extent not inconsistent herewith. It will be understood that certain of the above-described structures, functions, and operations of the above-described embodiments are not necessary to practice the present invention, but are included in the description merely to complete the exemplary embodiment(s). In addition, it will be understood that certain structures, functions, and operations set forth in the above-referenced patents and publications can be practiced in conjunction with the present invention, but are not essential to the practice of the present invention. It will therefore be understood that the present invention may be practiced otherwise than as specifically described without actually departing from the spirit and scope of the present invention, as defined by the appended claims.

Claims

1. 1. A method for depositing a silica, alumina, or aluminosilicate layer on a Meso-Y zeolite, comprising: (a) dehydrating Meso-Y zeolite containing CTAB surfactant within its mesopores; (b) mixing an alumina precursor, a silica precursor, or an aluminosilicate precursor with a dry solvent to form a mixture; (c) placing the dehydrated Meso-Y zeolite from (a) in a reactor with a dry atmosphere and the mixture from (b) in the same reactor; (d) refluxing the contents of the reactor to effect a deposition reaction that produces a layer of alumina, silica, or aluminosilicate on the surface of the Meso-Y zeolite; and (e) recovering the Meso-Y-as zeolite product from said reactor. A method comprising:

2. 10. The method of claim 1, wherein the solvent is removed from the zeolite after recovering the Meso-Y-as zeolite from the reactor.

3. The method of claim 2 wherein the solvent is removed by vacuuming.

4. 3. The method of claim 2, further comprising calcining the Meso-Y-as zeolite after removing the solvent.

5. 3. The method of claim 2, wherein the zeolite is dried after removal of the solvent.

6. 6. The method of claim 5, wherein the drying is carried out under vacuum.

7. 6. The method of claim 5, wherein the dried zeolite is calcined.

8. 8. The method of claim 7, wherein the calcination is carried out in dry air.

9. 10. The method of claim 1, wherein the refluxing is carried out with stirring.

10. The method of claim 1 , wherein in (b) an alumina precursor is mixed.

11. The method of claim 1 , wherein in (b) a silica precursor is mixed.

12. 2. The method of claim 1, wherein a silica precursor and an alumina precursor that provide an aluminosilicate layer on the surface of the Meso-Y zeolite are mixed in (b).

13. 10. The method of claim 1, wherein dry tetrahydrofuran (THF) is the dry solvent.

14. The method of claim 1 , wherein the dry atmosphere in the reactor comprises an inert gas.

15. 10. The method of claim 1, wherein the dry atmosphere in the reactor consists of nitrogen gas.

16. 2. The method of claim 1, wherein the dehydration of the Meso-Y zeolite in (a) comprises heating under vacuum.

17. 17. The method of claim 16, wherein said heating under vacuum comprises heating at 250°C under vacuum for 10 hours.

18. (d) The deposition reaction is carried out using dry N ２ 10. The process of claim 1, wherein the process is carried out under reflux and stirring under atmosphere for 1 hour.

19. 6. The method of claim 5, wherein the drying is carried out under vacuum at 120°C for 2 hours.

20. 10. The method of claim 1, wherein the solution mixture in (b) is filtered under air-free conditions to remove any undissolved solids.

21. 2. The method of claim 1, wherein the Meso-Y zeolite comprises CBV-720.

22. The silica precursor is Si(OEt) ４ The method of claim 1 , comprising:

23. The alumina precursor is Al(Oi-Pr) ３ The method of claim 7, comprising:

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