Surface modification of mesoporous zeolite Y without pore clogging

Abstract

The invention provides a selective post-synthesis surface modification method of mesoporous Y zeolite (Meso-Y). The method produces a thin silica, alumina or aluminosilicate cover layer on the exterior surface without causing significant pore clogging that would otherwise lead to the absence of surfactant. The method relies on the cetyltrimethyl ammonium bromide (CTAB) surfactant enclosed in the Meso-Y as it after synthesis as a soft template, which protects internal microporosity and mesoporosity during the synthesis of the inorganic capping layer by directing its deposition to selectively occur on the external surface. The method is carried out under dry conditions.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application was filed on February 5, 2024 as a PCT International Application 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 hereby incorporated by reference in its entirety. Background Art

[0003] Zeolites are a class of crystalline microporous materials that have been widely used as catalysts for the production of valuable chemicals. Zeolite Y, in particular, has been used for decades in fluid catalytic cracking (FCC) and hydrocracking of petroleum-derived molecules. Although modern variants such as ultrastable Y (USY) zeolite have demonstrated excellent performance in catalytic applications, one of the significant challenges remains the severe diffusion limitations typically caused by the small micropores of zeolite materials. In this regard, researchers have developed several post-synthetic methods to introduce intracrystalline mesopores into zeolites.

[0004] For example, CBV-720 zeolite is a commercial USY zeolite (Si / Al=15) with a FAU topology and consists of irregular intracrystalline mesopores synthesized by steam treatment. This USY zeolite has been used as a starting material in previously reported surfactant templated processes, including hierarchical mesoporous Y zeolite (Meso-Y). The latter contains a uniform distribution of intracrystalline mesopores and is typically synthesized by treating commercially available USY zeolite CBV-720 with an alkali metal hydroxide solution containing hexadecyltrimethylammonium bromide (CTAB) surfactant. Atomic force microscopy (AFM) characterization data clearly demonstrates the formation of mesopores near the external surface during surfactant templated. The resulting enclosed surfactant is typically removed from the as-prepared material by combustion during calcination to synthesize mesopores. The sizes of these generated mesopores are consistent with those of the bulk measured by nitrogen physical adsorption and three-dimensional tomography based on transmission electron microscopy (TEM) - all of which are the result of CTAB surfactant assembly. These mesopores enable easy molecular transport and serve as pathways for the rapid diffusion of reactants and products to and from catalytically active sites within the zeolite material.

[0005] However, although hierarchical zeolite structures have recently attracted considerable attention due to their improved transport properties, new challenges have arisen in these and related hierarchical mesoporous zeolite materials related to surface diffusion barriers (which lead to increased diffusion time constants). This surface barrier is attributed to the large free energy differences between the gas / liquid phase, the zeolite exterior surface, and the zeolite interior, and has been demonstrated for conventional mullite zeolites by microimaging. To overcome these surface barriers, various post-synthesis surface modifications by chemical liquid and vapor deposition have been proposed.

[0006] A continuing synthetic challenge in this regard is to achieve spatially selective modification of the zeolite exterior surface with an inorganic oxide (e.g., amorphous aluminosilicate) capping layer without blocking the zeolite interior porosity. Answering this challenge could provide important applications in catalysis as well as electrostatic adsorption. Summary of the Invention

[0007] A method for selective post-synthesis surface modification of hierarchical mesoporous Y zeolite (Meso-Y) is provided, resulting in thin silica, alumina, or aluminosilicate coatings on the exterior surface without causing significant pore clogging, which would otherwise result from the absence of surfactant. The method relies on the encapsulated CTAB surfactant in the as-synthesized Meso-Y to act as a soft template, which protects the internal microporosity and mesoporosity during the synthesis of the inorganic coating by directing its deposition to occur selectively on the exterior surface. It has been found to be important to perform the surface modification under dry conditions during the reaction of the oxide molecular precursors with the as-synthesized Meso-Y. If dry conditions are not employed, pore clogging and thicker silicate coatings, as well as phase separation for the alumina coating, can occur, which can be observed by SEM. In contrast, the present process allows the synthesis of uniform silica / alumina nanoscale coatings under dry conditions, with no evidence of separate 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 crucial for applications involving adsorption and catalysis.

[0008] Among other factors, it has been found that the combination of CTAB surfactant and dry deposition conditions can successfully synthesize a uniform silica, alumina or aluminosilicate shell on the surface of mesoporous Y zeolite while retaining both mesoporosity and microporosity after coating. The presence of the surfactant CTAB in the as-synthesized Meso-Y is important for achieving selective modification of the external surface of Meso-Y without pore clogging. This is achieved through a mechanism based on soft protection by the CTAB surfactant. Given the trend of using inorganic oxides to surface-modify zeolites in functional applications, the resulting surface-modified zeolite materials are promising new surface-modified zeolite materials for functional applications involving adsorption and catalysis, where the surface charge (zeta potential) of the surface can be precisely controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings (incorporated into and forming a part of this specification) illustrate several aspects of the present disclosure. A brief description of the drawings is as follows:

[0010] Figure 1APowder X-ray patterns of samples of as-synthesized (as) deposited silica obtained under wet conditions are shown: (a) CBV-720, (b) Meso-Y-as, (c) Meso-Y-as@wet-4wt.%SiO2, (d) Meso-Y-as@wet-11wt.%SiO2.

[0011] Figure 1B Shown are powder X-ray patterns of samples of calcined deposited silica obtained under wet conditions: (a) CBV-720, (b) Meso-Y-cal, (c) Meso-Y-cal@wet-4wt.%SiO2, and (d) Meso-Y-cal@wet-11wt.%SiO2.

[0012] Figure 2 SEM images of (a and b) CBV-720, (c) Meso-Y-as, (d) Meso-Y-as@wet-4wt.%SiO2, (e) Meso-Y-as@wet-11wt.%SiO2, (f) Meso-Y-cal and (g) Meso-Y-cal@wet-4wt.%SiO2 and (h) Meso-Y-cal@wet-11wt.%SiO2 are shown.

[0013] Figure 3A N2 adsorption-desorption isotherms of CBV-720, Meso-Y-cal, Meso-Y-cal@wet-4wt.%SiO2, Meso-Y-cal@wet-11wt.%SiO2, and Meso-Y-as@wet-11wt.%SiO2 are shown.

[0014] Figure 3B The mesopore size distributions of CBV-720, Meso-Y-cal, Meso-Y-cal@wet-4wt.%SiO2, and Meso-Y-cal@wet-11wt.%SiO2 are shown.

[0015] Figure 3C N2 adsorption-desorption isotherms of CBV-720, Meso-Y-cal, Meso-Y-as@wet-4wt.%SiO2, and Meso-Y-as@wet-11wt.%SiO2 are shown.

[0016] Figure 3D The mesopore size distributions of CBV-720, Meso-Y-cal, Meso-Y-as@wet-4wt.%SiO2, and Meso-Y-as@wet-11wt.%SiO2 are shown.

[0017] Figure 4 SEM images of (a) Meso-Y-as@wet-0.9wt.%Al2O3, (b) Meso-Y-as@wet-2.2wt.%Al2O3, and (c) Meso-Y-as@wet-4.6wt.%Al2O3 are shown.

[0018] Figure 5 Powder X-ray patterns of samples of deposited silica obtained under dry conditions are shown: (a) Meso-Y-as, (b) Meso-Y-as@dry-4wt.%SiO2, (c) Meso-Y-as@dry-11wt.%SiO2 and (d) Meso-Y-as@dry-15.4wt.%SiO2.

[0019] Figure 6A N2 adsorption-desorption isotherms of Meso-Y-cal, Meso-Y-as@dry-4wt.%SiO2, Meso-Y-as@dry-11wt.%SiO2 and Meso-Y-as@dry-15.4wt.%SiO2 are shown.

[0020] Figure 6B The mesopore size distribution of Meso-Y-cal, Meso-Y-as@dry-4wt.%SiO2, Meso-Y-as@dry-11wt.%SiO2 and Meso-Y-as@dry-15.4wt.%SiO2 are shown.

[0021] Figure 7 SEM images of (a) Meso-Y-as@dry-4wt.%SiO2, (b) Meso-Y-as@dry-11wt.%SiO2, and (c) Meso-Y-as@dry-15.4wt.%SiO2 are shown.

[0022] Figure 8 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 are shown.

[0023] Figure 9Powder X-ray patterns of deposited alumina samples obtained under dry conditions are shown: (a) Meso-Y-as, (b) Meso-Y-as@dry-0.9wt.%Al2O3, (c) Meso-Y-as@dry-2.2wt.%Al2O3 and (d) Meso-Y-as@dry-4.6wt.%Al2O3.

[0024] Figure 10 SEM images of (a) Meso-Y-as@dry-0.9wt.%Al2O3, (b) Meso-Y-as@dry-2.2wt.%Al2O3, and (c) Meso-Y-as@dry-4.6wt.%Al2O3 are shown.

[0025] Figure 11A The N2 adsorption-desorption isotherms of Meso-Y-cal, Meso-Y-as@dry-0.9wt.%Al2O3, Meso-Y-as@dry-2.2wt.%Al2O3 and Meso-Y-as@dry-4.6wt.%Al2O3 are shown.

[0026] Figure 11B The mesopore size distribution of Meso-Y-cal, Meso-Y-as@dry-0.9wt.%Al2O3, Meso-Y-as@dry-2.2wt.%Al2O3 and Meso-Y-as@dry-4.6wt.%Al2O3 are shown.

[0027] Figure 12A Shown are the zeta potential changes of Meso-Y-cal with different silica loadings.

[0028] Figure 12B Shown are the zeta potential changes of Meso-Y-cal with different alumina loadings.

[0029] Figure 13 is a schematic description of the deposition process for preparing Meso-Y-as under wet and dry conditions. DETAILED DESCRIPTION

[0030] Various additional inventive aspects will be described in the following description. Inventive aspects may relate to individual features as well as combinations of features. It should be understood that the foregoing general description and the following detailed description are merely exemplary and explanatory and do not limit the broad inventive concepts on which the embodiments disclosed herein are based.

[0031] Exemplary aspects of the present disclosure will now also be described in detail with reference to the accompanying drawings.

[0032] This process utilizes a combination of CTAB surfactant and dry deposition conditions to successfully synthesize a uniform silica, alumina, or aluminosilicate shell on the surface of mesoporous Y zeolite. The mesopores are typically in the size range of >2 but <50 nm. This is achieved while retaining both mesoporosity and microporosity after coating deposition.

[0033] The present process involves depositing a silica, alumina, or aluminosilicate layer on the surface of a Meso-Y zeolite. The Meso-Y zeolite has been treated so that the mesopores of the Y zeolite contain a CTAB surfactant. This treatment may include any suitable known treatment. For example, a suitable amount of CTAB surfactant may be dissolved in an alkaline aqueous solution. This solution is heated, and then the Y zeolite (e.g., CBV-270) is introduced into the solution. The solution is then maintained at a suitable temperature (e.g., 90°C) for a period of time (e.g., 5 or 6 hours) while stirring. The resulting zeolite powder is filtered, washed, and dried to obtain an as-synthesized Meso-Y-as material containing the CTAB surfactant within the newly formed mesopores. After calcination (e.g., at 580°C), a Meso-Y-cal material is obtained. The Meso-Y-as material is preferably used for this process. Calcination may then occur. However, processes that deposit a silica, alumina, or aluminosilicate layer under dry conditions on Meso-Y-cal may also be used.

[0034] A Meso-Y-as zeolite material (containing a CTAB surfactant within its mesopores) is first dehydrated. In one embodiment, dehydration comprises heating the Meso-Y-as zeolite 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.) for 7-12 hours, and in one embodiment, for 10 hours.

[0035] The dehydrated zeolite is then inserted into a reactor having a dry environment. A dry environment can be created by employing an inert gas atmosphere. In one embodiment, a nitrogen atmosphere is used.

[0036] The alumina, silica or aluminosilicate precursor is then mixed with a dry solvent to form a mixture. Any suitable solvent that does not absorb water can be used. In one embodiment, the solvent is tetrahydrofuran (THF). It is also ensured that the solvent itself is dry and does not contain water.

[0037] The process may involve any suitable alumina precursor if it is desired to have an alumina coating on the zeolite surface. In one embodiment, the alumina precursor may be Al(Oi-Pr)3. If it is desired to have a silica coating, any suitable silica precursor may be used. In one embodiment, Si(OEt)4 may be the silica precursor. If it is desired to have an aluminosilicate coating, both alumina and silica precursors are provided.

[0038] The mixture solution containing the precursor is also inserted into the same reactor containing the dehydrated zeolite. The contents of the reactor are then refluxed to achieve a deposition reaction, thereby forming an aluminum oxide, silicon dioxide or aluminosilicate layer on the surface of the Meso-Y-as zeolite. Reflux is typically performed under 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 evacuation. The Meso-Y-as zeolite recovered from the reactor can be calcined, either before or after the solvent is removed. In one embodiment, calcination occurs after the solvent is removed. In one embodiment, calcination is performed at a temperature of about 580°C for about 4 hours. The temperature and length can vary.

[0039] In one embodiment, after the solvent is removed, the Meso-Y-as zeolite may be dried. In one embodiment, the drying may be performed in a vacuum. Calcination may then be performed after drying. In one embodiment, all calcinations are performed in dry air.

[0040] The presence of CTAB in the as-synthesized Meso-Y-as is crucial for achieving selective modification of the Meso-Y-as outer surface without pore clogging. This is achieved through a mechanism based on soft protection by the CTAB surfactant. It was also found to be crucial to employ dry deposition conditions, as described above, during the reaction of the oxide molecular precursors with the Meso-Y-as. Failure to employ dry conditions results in pore clogging, a thick silicate overlayer, and phase separation of the aluminum oxide overlayer.

[0041] The following examples are provided to illustrate the present process and products but are not intended to be limiting.

[0042] Material synthesis

[0043] Mesoporous zeolite Y (Meso-Y) was prepared based on commercial Y zeolite.

[0044] CBV-720 was obtained from Zeolyst and subjected to ammonium ion exchange before use. The synthesis of Meso-Y was carried out according to a previously reported surfactant-templated 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 minutes, 1 g of CBV-720 was introduced and then maintained at 90°C for 6 hours with stirring. The resulting zeolite powder was filtered, washed with deionized water, and dried at 80°C for 12 hours to obtain the synthesized intermediate material Meso-Y-as, which contained enclosed CTAB surfactant within the newly formed mesopores. After further calcination at 580°C in air, the as-prepared graded Meso-Y-cal was obtained.

[0045] Silica is deposited on Meso-Y-cal and silica, alumina or aluminosilicate is deposited on Meso-Y-as.

[0046] Deposition of silica, alumina or aluminosilicate coatings was performed on both Meso-Y-as and Meso-Y-cal using post-synthesis surface modification techniques. Two different types of deposition conditions were employed, corresponding to wet and dry conditions for comparison. See e.g. Figure 13 .

[0047] In a typical wet deposition, the entire process is carried out while exposed to atmospheric moisture. 1 g of zeolite (Meso-Y-as or Meso-Y-cal) is dispersed in 25 mL of hexane. The required amount of tetraethyl orthosilicate (TEOS) or aluminum isopropoxide (Al(Oi-Pr)3) is introduced into the mixture and the deposition is carried out under reflux and stirring for 1 hour. Subsequently, the hexane is removed by vacuuming. The zeolite product is dried at 120°C under vacuum for 2 hours and calcined at 580°C in air for 4 hours. The final product is denoted as Meso-Y-as (Meso-Y-cal) @ wet-x wt.% SiO2 (or y wt.% Al2O3 at the end), where x and y represent the weight percentages of silica or alumina used for deposition, respectively. For example, for a product prepared from Meso-Y-as with 4 wt.% SiO2 deposited under wet conditions, it is represented as Meso-Y-as@wet-4 wt.% SiO2.

[0048] Dry deposition experiments were carried out under airless conditions. For silica deposition, 1 g of the as-synthesized mesoporous zeolite (Meso-Y-as) was dehydrated at 250 ° C for 10 hours under vacuum 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 reflux and stirring in a dry N2 atmosphere for 1 hour. After the reaction, the THF solvent was removed by vacuum extraction. The obtained sample was further dried at 120 ° C for 2 hours under vacuum and then calcined at 580 ° C for 4 hours in air or dry air.

[0049] For alumina deposition, the required amount of Al(Oi-Pr)3 (corresponding to a certain target weight percentage of Al2O3) was introduced into a flask in a glove box under an Ar atmosphere. Dry THF solvent was then mixed with Al(Oi-Pr)3 under stirring at 60°C (approximately 100 mL of THF was used per gram of Al(Oi-Pr)3). Sufficient THF solvent was used to dissolve 80-90% of the Al(Oi-Pr)3. The obtained solution containing Al(Oi-Pr)3 was hot filtered under airless conditions to remove traces of undissolved impurities. The filtrate was further mixed with dry zeolite (Meso-Y-as). The mixture was reacted for 1 hour under reflux and stirring under N2 atmosphere. After the reaction, the THF solvent was removed by vacuuming. The sample was dried at 120°C under vacuum for 2 hours and further calcined at 580°C for 4 hours in air or dry air. The resulting samples are designated as Meso-Y-as@dry-x wt.% SiO2 or (y wt.% Al2O3 at the end), where x and y refer to the weight percentage of silica or alumina deposited in the product, respectively. The deposition of the aluminosilicate overlayer is similar to that of alumina, except that a silica precursor is also introduced. Therefore, both alumina and silica precursors are present.

[0050] Characterization

[0051] Using a CuKα radioactive source ( Powder X-ray diffraction patterns (PXRD) were collected on a Bruker D8 Advance diffractometer at 40 kV, 40 mA. Scanning electron microscopy (SEM) images were captured using a Hitachi S-5000 microscope. N adsorption isotherms were measured at 77 K and calculated using the BJH method using a Micrometrics ASAP 2020 instrument. Figure 3B ; Figure 3D ; Figure 6B ;and Figure 11B The mesopore size distribution shown in . Before measurement, the sample was preheated at 350 ° C under vacuum conditions for 4 hours. Transmission electron microscopy (TEM) images were acquired in low dose mode with a JEOL JEM 2010 microscope, which was operated at an accelerating voltage of 200 kV and equipped with a LaB6 electron gun. The sample was prepared by embedding the sample in epoxy resin and curing it, and then cutting thin sections (~30-50 nm) with a Leica EMUC7 ultramicrotome. The section was floated on a 300 mesh Cu grid with a thin (20-30 nm) porous carbon support film. Zeta potential measurements were performed using a Malvern Zetasizer Nano-Z (Malvern Instruments, Malvern, UK). Before measurement, the sample was dispersed in deionized water at 0.5 wt.%. After ultrasonic treatment for 5 minutes, at least 5 measurements were performed at room temperature to calculate the average value.

[0052] Results and Discussion, Conclusion

[0053] The first material used as a starting point for subsequent post-synthetic surface modification was a calcined mesoporous Y zeolite (Meso-Y-cal), which was synthesized from the parent CBV-720 (ammonium form) based on a previously reported surfactant templated 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 and include a combination of type I and type IV isotherms, shown by the filling of micropores at relative pressures below 0.1 and pore condensation in mesopores at higher relative pressures. Figure 3B The deconvolution of the mesopore size of the isotherm data in Figure 3 shows a narrow size distribution centered around 3.6 nm and further confirms the generation of uniform intracrystalline mesoporosity. Therefore, the Meso-Y-cal sample exhibits an enhanced total specific surface area (886 m 2 g -1 Compared to CBV-720's 760m 2 g -1 ), mesoporous surface area (525m 2 g -1 Compared to CBV-720's 180m 2 g -1 ) and mesopore volume (0.44 cm 3 g -1 Compared to CBV-720's 0.22cm 3 g -1 ), while the micropore surface area (362m 2 g -1 Compared to CBV-720's 580m 2 g-1 ) and micropore volume (0.14 cm 3 g -1 Compared to CBV-720's 0.23cm 3 g -1 These data indicate the development of mesoporosity in Meso-Y-cal, accompanied by a slight decrease in microporosity in CBV-720.

[0054] The subsequent post-synthesis surface modification under wet conditions can be viewed as a two-step process. The first step involves mixing the monomeric precursors of the oxide capping layer (i.e., Si(OEt)4 for silica or Al(Oi-Pr)3 for alumina) with the graded Meso-Y-cal zeolite under wet conditions (i.e., exposed to ambient air). During this process, the monomeric precursors diffuse into the Meso-Y, thereby infiltrating the pores near the external surface. It is believed that partial hydrolysis and condensation occur during this initial mixing, and in the second step, during the subsequent calcination, the organic shell is burned in air to synthesize steam and hydrolysis / condensation cross-linked inorganic oligomers, which lead to the formation of an inorganic capping layer near the external surface of the zeolite.

[0055] Under wet conditions, the as-prepared (Meso-Y-as) and calcined (Meso-Y-cal) samples were first silica-coated using 4 wt.% and 11 wt.% SiO2 loading onto the zeolite before calcining it to SiO2 in air. After deposition of the SiO2 capping layer, both samples showed well-resolved PXRD patterns typical of FAU ( Figure 1A ). The wet deposition conditions had no significant negative effect on the zeolite crystal structure. For samples synthesized starting from Meso-Y-cal, Figure 1B Similar observations are shown in . These observations are similar to those reported previously when surface-modifying HZSM-531 and SAPO-34 zeolites.

[0056] Figure 2 SEM images of the parent samples (Meso-Y-as and Meso-Y-cal) and the post-synthesis modified samples after deposition of the SiO2 capping layer indicate that there is no phase separation in all samples, as similar crystal morphology and size are observed for CBV-720 and the resulting Meso-Y material. The SiO2-capping layer must have been deposited as a thin (nanoscale) shell, either on the external surface of each crystallite or within the crystallite. Based solely on the SEM data ( Figure 2 ), it is not possible to distinguish between these latter two possibilities, therefore, N2 physisorption data were used.

[0057] based on Figure 3ABased on the N2 physical adsorption isotherms in Figure 1 and the calculated porosity data in Table 1 below, deposition of 4 wt.% SiO2 on Meso-Y-cal under wet conditions reduced the mesoporous surface area and volume (approximately 12%) while retaining the microporosity of Meso-Y-cal (Table 1, No. 3). At an 11 wt.% SiO2 loading on Meso-Y-cal, the resulting material showed significantly reduced microporous and mesoporous surface area (approximately 20%) and pore volume (approximately 14%-36%) (Table 1, No. 4). With this increased amount of SiO2, it appears that the deposition process not only occurs on the mesoporous surface of Meso-Y-cal, but it also causes partial blockage of the internal micropores. This result reveals that the open structure of hierarchical Meso-Y-cal provides a large mesoporous surface area for the deposition of the silica overlayer, which interacts with the surface silanols in the zeolite mesopores throughout the crystal during calcination, ultimately leading to blockage of the micropores and mesopores, especially at the high 11 wt.% SiO2 loading under wet conditions.

[0058] Table 1. Coating parameters and porosity properties of parent CBV-720, Meso-Y-cal, and resulting core-shell meso-Y catalysts.

[0059]

[0060]

[0061] a Specific surface area calculated from the BET curve. b Micropore volume and mesopore surface area calculated from the t-curve method. c Total pore volume calculated at P / P0 = 0.98.

[0062] Meso-Y-as, rather than calcined Meso-Y-cal materials, were investigated as supports for silica surface modification under wet conditions. Figure 3C and the N2 physical adsorption data in Table 1 (No. 6) show that the same materials contain an 11 wt.% SiO2 capping layer on Meso-Y-as compared to the parent Meso-Y-cal (Table 1, No. 2). This same amount of microporosity in the coated material containing the 11 wt.% SiO2 capping layer on Meso-Y-as is in stark contrast to the reduced microporosity observed when Meso-Y-cal is coated with silica. Furthermore, consistent with this trend, a lower reduction in mesopore volume of 23% was observed when the 11 wt.% SiO2 capping layer was applied to the Meso-Y-as material (relative to 36% for the corresponding material starting with the Meso-Y-cal material). Furthermore, depositing 11 wt.% SiO2 on Meso-Y-as causes the mesopore size distribution to become narrower, as shown in FIG. Figure 3D As shown in the data, Figure 3BThe data in Figure 3 show that depositing the same amount of SiO2 on Meso-Y-cal does not change the mesopore size distribution. This different effect of post-synthesis silica surface modification on the mesopore size distribution suggests that the deposition of the SiO2 capping layer occurs at different locations when Meso-Y-as or Meso-Y-cal is used as the support material, favoring the clogging of micropores in the latter.

[0063] The Meso-Y-as sample contains the CTAB surfactant within the mesopores, while the corresponding calcined form of Meso-Y-cal consists of an open, hierarchical porous channel system (the organic surfactant component was removed prior to calcination). Based on the lack of micropore clogging in the 11 wt.% SiO2 overlayer material when coated on Meso-Y-as, it appears that the surfactant CTAB plays an important role in protecting the internal microporosity from surface modification by acting as a soft template. This template directs the deposition of the silica overlayer in Meso-Y-as to occur only on the mesopores, rather than the micropores. This suggests that the initial stages of calcination of the silica-surface-modified Meso-Y-as (i.e., after reaction with TEOS under wet conditions) lead to partial decomposition of the CTAB surfactant and create space for silica deposition on the intracrystalline mesoporous surfaces. During surfactant combustion, the partially decomposed CTAB surfactant can prevent the formation of silica oligomers at the organic / inorganic interface through electrostatic interactions between the positively charged CTAB and the negatively charged silica species. This interaction would be expected to selectively direct the deposition of the SiO2 overlayer onto the mesoporous surfaces, as observed. During this process, silica oligomers barely penetrate the microporous channel system of Meso-Y-as during calcination, as fragments of the CTAB surfactant clog the mesopores. However, the spatially selective surface modification method benefits from the presence of the surfactant CTAB in the mesopores and is reminiscent of the explanation for the selective silanization of as-synthesized mesoporous silica MCM-41 using organosilanes. The condensed organosilane was found to be primarily grafted onto the external surface of the as-synthesized material, due to the soft protection provided by CTAB. This is also reminiscent of the protective effect CTAB is known to provide to zeolite Y under alkaline aqueous conditions, where the surfactant protects the zeolite from dissolution due to its strong interaction with the framework. Based on this observed protective effect of CTAB on microporosity during silica deposition under wet conditions, the following focus is on Meso-Y-as, rather than Meso-Y-cal, as the optimized starting material for inorganic oxide capping layer deposition.

[0064] Surface modification with an alumina overlay was also investigated under wet conditions, starting from Meso-Y-as material. The resulting samples, Meso-Y-as@wet-0.9wt.%Al2O3, Meso-Y-as@wet-2.2wt.%Al2O3, and Meso-Y-as@wet-4.6wt.%Al2O3, showed essentially identical XRD patterns to the parent Meso-Y-as sample ( Figure 1A ). Based on the N2 adsorption data (Table 1, Nos. 7-9), the three coated samples also showed comparable microporosity and mesoporosity. However, Figure 4 The SEM images in Figure 4 show a few 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 4a and 4b). For the Meso-Y-as@wet-4.6 wt.% Al2O3 sample, the number of such particles directly correlates with the amount of Al2O3 surface modification (see 4c). It appears that when the Al2O3 surface modification is performed under wet conditions, alumina nanoparticles are formed on the external zeolite surface by hydrolysis and condensation of Al(Oi-Pr)3 upon exposure to atmospheric moisture. The condensation products ultimately result in a mixture of Meso-Y and alumina nanoparticles on the zeolite's external surface, providing a method for synthesizing a separate alumina phase on the Meso-Y external surface, minimizing interfacial contact between the alumina and zeolite. This could prove useful in controlling the extent of this contact, or nanoscale intimacy, for catalyst synthesis.

[0065] The above deposition experiments demonstrate that only at low SiO2 loadings (4 wt.%) are wet conditions suitable for silica shell deposition on Meso-Y-as with CTAB surfactant suitable for synthesizing materials without severe micropore and mesopore blockage. At high SiO2 loadings of 11 wt.%, wet conditions result in materials with reduced mesopore volume and mesopore size. Furthermore, wet conditions result in a separate alumina phase that lacks good interfacial contact with the zeolite.

[0066] These inconsistent results were frustrating, so the present process was developed as a more general and efficient synthetic method to more uniformly (homogeneously) modify the crystal surface of Meso-Y, resulting in a high degree of interfacial contact between the zeolite material and the overlayer without forming a separate external surface amorphous phase, and in which the mesoporosity and microporosity of the Meso-Y zeolite material can be maintained after modification. For silica deposition, residual moisture in the wet deposition system appears to induce partial hydrolysis of TEOS and catalyze condensation between surface silanols and partially hydrolyzed silica species even before calcination. This results in an uneven distribution of silica species, favoring regions close to the external surface and causing a narrowing of the mesopore size during calcination.

[0067] However, the present surface modification process is carried out under strictly dry conditions. Residual water can be removed, for example, by heating the Meso-Y-as material at 250°C in a vacuum and depositing it in free air, followed by a further calcination in ambient air or dry air.

[0068] like Figure 5 As shown, deposition of a silica capping layer on the as-synthesized Meso-Y-as under dry conditions yields a material with high crystallinity, as in the parent mesoporous zeolite Y. In addition, the N2 adsorption-desorption isotherms ( Figure 6A ) and the corresponding calculated surface areas and pore volumes (Table 2 below, Nos. 3-5) indicate that the micropores of the parent Meso-Y sample remain open without any evidence of pore clogging. This latter result is similar to that observed under wet conditions, but is different from the silica surface modification under wet conditions ( Figure 3D ), the mesopore size of the silica coating material was maintained after SiO2 deposition under dry conditions ( Figure 6B The slightly reduced mesoporous surface area in the surface-modified samples is the result of silica deposition on the external surfaces, leading to limited blockage of the entrances to a few mesopores throughout the crystal (Table 2, Nos. 3-5). However, any such silica coating on the external surfaces is thin, as can be seen in the SEM images ( Figure 7 ) No separate phases can be observed in these two samples. Figure 8The TEM images in Figure 2 demonstrate the thinness of the silica coating on the external surface of the Meso-Y zeolite (less than 10 nm). Even at a high SiO2 loading of 15.4 wt.%, silica deposition under dry conditions can be successfully performed on the Meso-Y-as material without micropore clogging and with limited mesopore clogging (Table 2, No. 5). This small amount of pore clogging cannot be achieved under wet silica deposition conditions. Under dry conditions, during calcination, the introduced silica source TEOS interacts with the partially decomposed surfactant CTAB in the intracrystalline mesopores and forms a much more uniform silica coating at the mesopore surfaces of the Meso-Y-as.

[0069] The study of aluminum oxide surface modification of Meso-Y-as materials using Al(Oi-Pr)3 as a precursor under dry conditions provided interesting results. Figure 9 The PXRD patterns shown in Figure 2 show that after synthesizing 0.9-4.6 wt.% Al2O3 capping layers, the obtained samples retained a highly crystalline FAU skeleton structure. Figure 4 In stark contrast to the results obtained under wet conditions, Figure 10 The SEM data in show that no other phases were observed after alumina surface modification under dry conditions, and the crystals retained their morphology. Figure 8 TEM images in (gj) show a thin (less than 10 nm) alumina coating on the external surface of the Meso-Y zeolite material. Dry conditions produce a uniform alumina coating with a high degree of interfacial contact between the alumina and the zeolite material. Under dry conditions, hydrolysis of the Al source due to the presence of residual water is completely avoided, resulting in Al(Oi-Pr)3 being grafted only at the external surface of the parent zeolite sample. In addition, based on N2 adsorption-desorption data ( Figure 11A ), it can be observed that at low Al2O3 loadings of 0.9–2.2 wt.%, the micropores are not affected by the Al2O3 surface modification, as the obtained samples show comparable micropore surface area and micropore volume to those of the parent Meso-Y-cal ( Figure 11B ; Table 2 below, numbers 6-7).

[0070] Table 2. Coating parameters and porosity properties of parent CBV-720, calcined meso-Y, and the resulting core-shell meso-Y catalysts (calcined in air).

[0071]

[0072]

[0073] a Surface area calculated from the BET curve. b Micropore volume and mesopore surface area calculated from the t-curve method. c Total pore volume calculated at P / P0 = 0.98.

[0074] At a relatively high Al2O3 loading of 4.6 wt.%, a slightly decreased micropore surface area (318 m 2 g -1 362m with Meso-Y-cal 2 g -1 ) and micropore volume (0.12 cm 3 g -1 0.14cm with Meso-Y-cal 3 g -1 )(Table 2, No. 8). In one embodiment, a limit of alumina coating on the Meso-Y surface under dry conditions (2.2 wt.% Al2O3) is preferred to avoid micropore clogging.

[0075] Strictly dry air was also used to calcine intermediate samples obtained after mixing the inorganic oxide capping source (i.e., TEOS or Al(Oi-Pr)3) with Meso-Y-as under dry conditions. Similar, almost no change in porosity was observed after coating with the silica or alumina shell in both dry and humid air. This result suggests that keeping the first step of mixing the deposited inorganic oxide capping source with Meso-Y-as dry is more important than keeping it dry during the calcination step, where vapors are released during combustion of the organic matter. This vapor causes hydrolysis and condensation between the silanols on the zeolite surface and the inorganic shell.

[0076] Given that Figure 7 and 10 No phase separation was observed between the silica / alumina outer coating and the Meso-Y-cal support as evidenced by SEM in Figure 8 Given the intimate interfacial contact between the overcoat layer and the support in the TEM images, one must conclude that the present synthesis method uniformly leads to nanoscale silica / alumina covering layers on the Meso-Y-cal support under dry conditions.

[0077] We also investigated how this nanoscale coating affects the zeta potential (or surface charge) of the surface-modified Meso-Y-cal supports. To address this question, we measured the zeta potential of all materials synthesized in deionized water at pH 7 under dry conditions. It is known that the zeta potential directly reflects 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.

[0078] Table 3. Zeta potential measurements of Meso-Y-cal and the resulting core-shell meso-Y catalysts (which were calcined in air).

[0079] Material Zeta potential (mV) Meso-Y-cal -39.3±0.3 <![CDATA[Meso-Y-as@dry-4wt.%SiO2]]> -40.6±0.5 <![CDATA[Meso-Y-as@dry-11wt.%SiO2]]> -41.2±0.3 <![CDATA[Meso-Y-as@dry-15.4wt.%SiO2]]> -41.6±0.3 <![CDATA[Meso-Y-as@dry-0.9wt.%Al2O3]]> -37.3±0.6 <![CDATA[Meso-Y-as@dry-2.2wt.%Al2O3]]> -36.0±0.4 <![CDATA[Meso-Y-as@dry-4.6wt.%Al2O3]]> -32.0±0.5

[0080] Uncoated calcined Meso-Y-cal zeolite has a negatively charged surface, corresponding to a zeta potential of -39.3 mV. After silica surface modification, the zeta potential of the coated material becomes more negative as the amount of silica overcoat increases, consistent with the greater degree of dissociation of surface hydroxyl groups in silica than in Meso-Y-cal. In contrast, alumina surface modification increases the zeta potential of the coated material in a manner that is directly dependent on the amount of alumina overcoat, as shown in the data in Table 3. The linear progression of zeta potential with the amount of silica / alumina overcoat is shown in Table 3. Figure 12A and 12B This linearity is consistent with the uniform lack of phase separation across all compositions described above. That is, if silica / alumina phase separation were to occur at a certain high loading, this would manifest as nonlinearity in Figure 12 , which is not observed. It should be noted that alumina generally has a more positive zeta potential at pH 7 than silica.

[0081] Due to the observed uniformity of silica / alumina as a nanoscale overlay on top of the Meso-Y-cal support, rather than phase-separated oxide particles away from it, as observed by SEM / TEM microscopy imaging and supported by zeta potential measurements, the synthetic method described herein under dry conditions will be a useful approach for controlling the surface charge of Meso-Y-cal zeolitic materials in a rational manner—while avoiding pore clogging. This synthetic control has clear applications for nucleating mono- and bimetallic metal clusters on zeolite surfaces using methods such as strong electrostatic adsorption, as well as for controlling the dispersibility of Meso-Y-cal particles during catalyst formation, which involves concentrating aqueous suspensions and where it is generally desirable to avoid agglomeration of zeolite particles with each other.

[0082] Based on the above results, a mixed aluminosilicate coating was also synthesized on Meso-Y-as—a coating that includes both silica and alumina components in the outer coating, rather than an outer coating based solely on silica or alumina in the shell, as in the previous examples described above. This aluminosilicate shell has the advantage of comprising a catalytically active covering layer because it possesses Al—O—Si sites, which are known to be active for acid catalysis. This method utilizes dry conditions. Using this method, the surface was simultaneously modified with silica (i.e., TEOS) and alumina (i.e., Al(Oi-Pr)3) precursors to synthesize coatings with average silica to alumina ratios (SAR) ranging from 14 to 150, at both 4 wt.% and 8 wt.% (weight percentages refer to the total amount of SiO2 and Al2O3). The data show that the zeolite products after aluminosilicate surface modification have comparable microporosity (e.g., micropore volume) to the calcined parent material, Meso-Y-as. These data support the above conclusions regarding the pure silica and alumina coatings by confirming that the aluminosilicate surface modification process also did not result in clogging of the zeolite pores. The uniformity of the aluminosilicate coatings was also verified by SEM and TEM (the aluminosilicate layer thickness was less than 30 nm). As in the case of the pure silica and alumina surface modifications, the zeta potential of the aluminosilicate modified materials was measured, and the data showed that the zeta potential was significantly lower than that of the parent Meso-Y-cal material, in a manner that was sensitively dependent on the silicon to alumina ratio of the coating.

[0083] In view of the propensity to surface-modify zeolites using inorganic oxides in functional applications, the resulting surface-modified Meso-Y zeolitic materials are a promising new class of zeolitic materials for functional applications involving adsorption and catalysis, where the surface charge (zeta potential) of the surface can be controlled.

[0084] As used in this disclosure, the terms "comprises" or "comprising" are intended as open transition words, meaning the inclusion of specified elements, but not necessarily the exclusion of other, unspecified elements. The phrases "consisting essentially of" or "consisting essentially of" are intended to mean the exclusion of other elements of any significance to the composition. The phrases "consisting of" or "consisting of" are intended as transition words, meaning the exclusion of all elements other than the recited elements, with the exception of only trace amounts of impurities.

[0085] All patents and publications cited herein are incorporated herein by reference to the extent that they will not be inconsistent with this article. It will be understood that some of the above-mentioned structures, functions and operations of the above-mentioned embodiments are not necessary for practicing the present invention, and are included in the description only for the sake of completeness of one or more exemplary embodiments. In addition, it will be understood that the specific structures, functions and operations of the above-mentioned referenced patents and publications can be used in combination with the present invention, but they are not necessary for the practice of the present invention. Therefore, it will be understood that the present invention can be practiced in a manner different from that specifically described without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A method for depositing a silica, alumina or aluminosilicate layer on a Meso-Y zeolite, comprising: (a) dehydrating a Meso-Y zeolite comprising a CTAB surfactant within its mesopores; (b) mixing an alumina, silica or aluminosilicate precursor with a dry solvent to form a mixture; (c) inserting the dehydrated Meso-Y zeolite from (a) into a reactor with a dry atmosphere and inserting the mixture from (b) into the same reactor; (d) refluxing the contents of the reactor to effect a deposition reaction, thereby forming 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 the reactor.

2. The method according to claim 1, wherein After the Meso-Y-as zeolite is recovered from the reactor, the solvent is removed from the zeolite.

3. The method according to claim 2, wherein: The solvent was removed by vacuum.

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

5. The method according to claim 2, wherein: After removal of the solvent the zeolite is dried.

6. The method according to claim 5, wherein: The drying is performed in vacuo.

7. The method according to claim 5, wherein: The dried zeolite is calcined.

8. The method according to claim 7, wherein: The calcination was carried out in dry air.

9. The method according to claim 1, wherein The reflux is carried out under stirring.

10. The method according to claim 1, wherein In (b), an alumina precursor is mixed.

11. The method according to claim 1, wherein In (b), a silica precursor is mixed.

12. The method according to claim 1, wherein In (b) silica and alumina precursors are mixed to produce an aluminosilicate layer on the surface of the Meso-Y zeolite.

13. The method according to claim 1, wherein Dry tetrahydrofuran (THF) was the dry solvent.

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

15. The method according to claim 1, wherein The dry atmosphere in the reactor consisted of nitrogen.

16. The method according to claim 1, wherein Dehydration of the Meso-Y zeolite in (a) involves heating in a vacuum.

17. The method according to claim 16, wherein The heating in vacuum includes heating at 250° C. in vacuum for 10 hours.

18. The method according to claim 1, wherein The deposition reaction in (d) was carried out under reflux and stirring in a dry N2 atmosphere for 1 hour.

19. The method according to claim 5, wherein: The drying was performed in vacuum at 120° C. for 2 hours.

20. The method according to claim 1, wherein The solution mixture from (b) was filtered in the absence of air to remove any undissolved solids.

21. The method according to claim 1, wherein Meso-Y zeolites include CBV-720.

22. The method according to claim 1, wherein The silicon dioxide precursor includes Si(OEt)4.

23. The method according to claim 7, wherein The alumina precursor includes Al(Oi-Pr)3.

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