Patent Application
20240262700
Zeolites are a class of crystalline microporous materials that have been widely used as catalysts for the production of valuable chemical products, and zeolite Y in particular has been used for the fluid catalytic cracking (FCC) and hydrocracking of oil-derived molecules for decades. Although modern variants such as ultra-stable Y (USY) zeolite exhibit excellent performance in catalytic applications, one of the significant problems continues to be the severe diffusional limitations caused by the small micropores of zeolitic materials in general. In this regard, researchers have developed several postsynthetic methods to introduce intracrystalline mesoporosity into zeolites.
For instance, CBV-720 zeolite is a commercial USY zeolite (Si/Al=15) possessing FAU topology, and consists of irregular intracrystalline mesopores that are synthesized via steam treatment. This USY zeolite has been used as a starting material in previously reported surfactant-templating processes, which include hierarchical mesoporous Y zeolite (Meso-Y). The latter comprises a homogeneous distribution of intracrystalline mesopores and is typically synthesized by treating commercially available USY zeolite CBV-720 with an alkali hydroxide solution containing cetyl trimethylammonium bromide (CTAB) surfactant. Atomic-force microscopy (AFM) characterization data clearly demonstrate the formation of mesopores close to the external surface during surfactant templating. The resulting occluded surfactant is typically removed from the as-made material via combustion during calcination, to synthesize mesopores. The size of these generated mesopores is commensurate to those measured in the bulk by nitrogen physisorption and three-dimensional tomography based on transmission electron microscopy (TEM)—all resulting as a consequence of the CTAB surfactant assembly. These mesopores enable facile molecular transport, and act as a conduit for rapid diffusion of reactants and products to and from the catalytically active sites within the zeolitic material.
However, although hierarchical zeolitic structures have drawn much recent attention because of their improved transport properties, new challenges relating to surface diffusional barriers have been raised in these and related hierarchical mesoporous zeolite materials, which are responsible for increasing diffusional time constants. Such surface barriers are attributed to the large free energy differences between the gas/liquid phase, zeolite external surface, and the zeolite interior, and have been demonstrated by microimaging for a conventional Mordenite zeolite. To overcome these surface barriers, various postsynthetic surface modifications by chemical liquid and vapor deposition have been proposed.
An ongoing synthetic challenge in this regard is achieving spatially selective modification of the zeolite external surface with an inorganic-oxide (e.g., amorphous aluminosilicate) overlayer, without blocking zeolite interior porosity. Answering such a challenge can provide important applications in catalysis, as well as electrostatic adsorption.
Provided is a selective postsynthetic surface modification process of hierarchical mesoporous Y zeolite (Meso-Y), which results in a thin silica, alumina or aluminosilicate overlayer on the external surface without causing significant pore blockage, which otherwise results in the absence of surfactant. The approach relies on occluded CTAB surfactant in as-synthesized Meso-Y acting as a soft template, which protects internal microporosity and mesoporosity during inorganic overlayer synthesis, by directing its deposition to selectively occur on the external surface. It has been found important to conduct the surface modification under dry conditions at the stage of reacting oxide molecular precursors with the as-synthesized Meso-Y. If dry conditions are not employed, pore blockage and thicker silicate overlayers as well as phase separation for alumina overlayers occurs, which is observable via SEM. In contrast, under the present process, uniform silica/alumina nanoscale overlayers can be synthesized under dry conditions with no evidence of a separate phase, as demonstrated via TEM/SEM microscopy as well as Zeta-potential measurements. These uniform overlayers control the surface charge of Y zeolites, which can be crucial for applications involving adsorption and catalysis.
Among other factors, it has been found that a combination of CTAB surfactant and dry deposition conditions can successfully synthesize a uniform silica, alumina, or aluminosilicate shell on a mesoporous Y zeolite surface, while retaining mesoporosity and microporosity after coating. The presence of surfactant CTAB in the as-synthesized Meso-Y-as is important to achieve this selective modification of the Meso-Y-as zeolite external surface without pore blockage. This is achieved by a mechanism based on soft protection by the CTAB surfactant. Given the propensity for using inorganic-oxide surface-modified zeolites in functional applications, the resulting surface-modified zeolite materials are a promising new type of surface-modified zeolitic material for functional applications involving adsorption and catalysis, in which the surface charge (Zeta potential) of the surface can be precisely controlled.
The accompanying drawings, which are incorporated in and constitute a part of the description, illustrate several aspects of the present disclosure. A brief description of the drawings is as follows:
A variety of additional inventive aspects will be set forth in the description that follows. The inventive aspects can relate to individual features and to combinations of features. It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad inventive concepts upon which the embodiments disclosed herein are based.
Reference will now also be made in detail to exemplary aspects of the present disclosure that are illustrated in the accompanying drawings.
The present process utilizes a combination of CTAB surfactant and dry deposition conditions to successfully synthesize a uniform silica, alumina or aluminosilicate shell on a mesopores Y zeolite surface. The mesopores are generally within the size regime of >2 but <50 nm. This is accomplished while retaining mesoporosity and microporosity after the deposition of the coating.
The present process involves depositing a silica, alumina or aluminosilicate layer on a Meso-Y zeolite surface. The Meso-Y zeolite has been treated so that the mesopores of the Y zeolite contain CTAB surfactant. Such treatment can include any suitable, known treatment. For example, a suitable amount of CTAB surfactant can be dissolved in a basic aqueous solution. The solution is heated, and then the Y zeolite, e.g., CBV-270, is introduced into the solution. The solution is then kept at a suitable temperature, e.g., 90° C., for a length of time such as 5 or 6 hours while stirring. The resulting zeolite powder can be filtered, washed and dried to result in an as-synthesized Meso-Y-as material containing CTAB surfactant inside the newly formed mesopores. After calcination, e.g., at 580° C., a Meso-Y-cal material is obtained. It is preferred to sue the Meso-Y-as material for the process. After which calcination can occur. However, the process of depositing a silica, alumina, or aluminosilicate layer under dry conditions can also be used on Meso-Y-cal.
The Meso-Y-as zeolite material comprising the CTAB surfactant inside its mesopores is first dehydrated. The dehydration in one embodiment comprises heating the Meso-Y-as zeolite material under vacuum. In one embodiment, the heating under vacuum can comprise heating at a temperature in the range of 200-300° C., such as 250° C., under vacuum for 7-12 hours, in one embodiment 10 hours.
The dehydrated zeolite is then inserted into a reactor with a dry environment. The dry environment can be created by employing an inert gas atmosphere. In one embodiment, a nitrogen gas atmosphere is used.
Alumina, silica or aluminosilicate precursors are then mixed with a dry solvent to create a mixture. Any suitable solvent which does not pick up moisture can be used. In one embodiment, the solvent is tetrahydrofuran (THF). The solvent itself is also insured to be dry and free of water.
The process can involve any suitable alumina precursor should an alumina coating on the surface of the zeolite be desired. In one embodiment, the alumina precursor can be Al(O-i-Pr)3. Should a silica coating be desired, then any suitable silica precursor can be used. In one embodiment, Si(OEt)4 can be the silica precursor. If an aluminosilicate coating is desired, then both alumina and silica precursors are provided.
The mixture solution comprising the precursors is also inserted into the same reactor containing the dehydrated zeolite. The contents of the reactor are then refluxed to achieve a deposition reaction creating a layer of alumina, silica, or aluminosilicate on the surface of the Meso-Y-as zeolite. The refluxing generally occurs 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 evacuation. The Meso-Y-as zeolite recovered from the reactor can be calcined, either before or after removal of the solvent. In one embodiment, the calcination occurs after removal of the solvent. The calcination in one embodiment is at a temperature of about 580° C. for about 4 hours. The temperature and length can be varied.
In one embodiment, after removal of the solvent, the Meso-Y-as zeolite can be dried. In one embodiment, the drying can occur under vacuum. Calcination can then occur after the drying. In one embodiment, all calcinations occur in dry air.
The presence of the CTAB in the as-synthesized Meso-Y-as is important to achieve selective modification of the Meso-Y-as external surface without pore blockage. This is achieved by a mechanism based on soft protection by the CTAB surfactant. It has also been found important to employ dry deposition conditions as discussed above at the stage of reacting the oxide molecular precursors with the Meso-Y-as. If dry conditions are not employed, pore blockage and thicker silicate overlayers occur, as well as phase separation for alumina overlayers.
The following examples are provided to illustrate the present process and product, but are not intended to be limiting.
Preparation of Mesoporous zeolite Y (Meso-Y) based on the commercial Y zeolite.
CBV-720 was obtained from Zeolyst and ammonium ion exchanged prior to use. Synthesis of Meso-Y was conducted according to a previously reported surfactant templating approach. 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. with an oil bath. After stirring for 30 min at 90° C., 1 g of CBV-720 was introduced and then kept at 90° C. for 6 h under stirring. The resulting zeolite powder was filtered, washed with deionized water, and dried at 80° C. for 12 h, resulting in the as-synthesized intermediate material Meso-Y-as, containing occluded CTAB surfactant inside the newly formed mesopores. After further calcination at 580° C. in air, the as-made hierarchical Meso-Y-cal was obtained.
Deposition of silica on Meso-Y-cal and deposition of silica, alumina or aluminosilicate on Meso-Y-as.
The deposition of silica, alumina or aluminosilicate overlayers was conducted on both Meso-Y-as and Meso-Y-cal, by using postsynthetic surface modification techniques. Two different kinds of deposition conditions were employed, corresponding to wet and dry conditions for comparison. See, for example,
In a typical wet deposition, the entire procedure was conducted upon exposure to atomospheric 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(O-i-Pr)3) was introduced into the mixture, and the deposition was carried out for 1 h under reflux and stirring. Subsequently, hexane was removed by evacuation. The zeolite products were dried at 120° C. for 2 h under vacuum and calcined at 580° C. for 4 h in air. The final products were denoted as Meso-Y-as(Meso-Y-cal)@wet-x wt. % SiO2 (or y wt. % Al2O3 at end), where x and y indicate the weight percentage of silica or alumina used for deposition, respectively. For example, for the product that was prepared from the Meso-Y-as with 4 wt. % SiO2 deposition under wet condition, it was denoted as Meso-Y-as@wet-4 wt. % SiO2.
The dry deposition experiments were performed under air-free conditions. For silica deposition, 1 g of the as-synthesized mesoporous zeolite (Meso-Y-as) was dehydrated at 250° C. under vacuum for 10 h to remove the residual water in the sample. After cooling down to room temperature, dry N2 was injected into reactor as a protective gas and then 40 mL of dry tetrahydrofuran (THF) and TEOS were introduced into the reactor (the latter corresponding to a SiO2 loading of 4-15.4 wt. %). The deposition was carried out under dry N2 atmosphere under reflux and stirring for 1 h. After reaction, the THF solvent was removed by evacuation. The obtained samples were further dried at 120° C. for 2 h under vacuum and subsequently calcined in air or dry air at 580° C. for 4 h.
For alumina deposition, the desired amount of Al(O-i-Pr)3 was introduced to a flask under Ar atmosphere in the glovebox (corresponding to a certain target weight percentage of Al2O3). Then dry THF solvent was mixed with Al(O-i-Pr)3 at 60° C. under stirring (approximately 100 mL of THF was used per g of Al(O-i-Pr)3). Sufficient THF solvent was used such that 80-90% of the Al(O-i-Pr)3 was dissolved. The resulting solution containing Al(O-i-Pr)3 was subjected to hot filtration under air-free conditions, to remove trace amounts of undissolved impurities. The filtrate was further mixed with the dry zeolite (Meso-Y-as). The mixture was allowed to react under N2 atmosphere under reflux and stirring for 1 h. After reaction, the THF solvent was removed by evacuation. The samples were dried at 120° C. for 2 h under vacuum and further calcined in air or dry air at 580° C. for 4 h. The obtained sample was denoted as Meso-Y-as@dry-x wt. % SiO2 or (y wt. % Al2O3 at end), where x and y were referred to as the weight percentage of deposited silica or alumina in the product, respectively. The deposition of an aluminosilicate overlayer is similar to that for the alumina deposition, except that a silica precursor is also introduced. Thus, both alumina and silica precursors are present.
Powder X-ray diffraction patterns (PXRD) were collected using a Bruker D8 Advance diffractometer equipped with a Cu Kα radiation source (λ=1.5418 Å, 40 kV, 40 mA). Scanning electron microscopy (SEM) images were captured with a Hitachi S-5000 microscope. N2 adsorption isotherms were measured at 77 K, and the mesopore size distribution as shown in
The material first used as a starting point for subsequent postsynthetic surface modification was calcined mesoporous Y zeolite (Meso-Y-cal), which was synthesized from parent CBV-720 (ammonium form) based on a previously reported literature surfactant-templating approach. The porosity of the Meso-Y-cal material was characterized using N2 physisorption at 77 K. The adsorption isotherm is shown in
Under wet conditions, the subsequent postsynthetic surface modification can be considered as a two-step process. The first step involves mixing a monomer precursor to the oxide overlayer (i.e. Si(OEt)4 for silica or Al(O-i-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, imbibing into pores close to the external surface. It is believed that partial hydrolysis and condensation occurs during this initial mixing, and during the second step, subsequent calcination, the organic shell combusts in air to synthesize steam and hydrolyzed/condensed crosslinked inorganic oligomers, which lead to the formation of an inorganic overlayer close to the zeolite external surface.
Under wet conditions, the as-made (Meso-Y-as) and calcined (Meso-Y-cal) samples were first silica coated, using 4 wt. % and 11 wt. % SiO2 loaded onto the zeolite, prior to its calcination to SiO2 in air. After SiO2 overlayer deposition, both samples exhibited well-resolved PXRD patterns that are typical for FAU (
SEM images of the parent samples (Meso-Y-as and Meso-Y-cal) and the postsynthetically modified samples following SiO2-overlayer deposition in
Based on the N2 physisorption isotherms in
a Surface area was calculated by BET-plot.
b Micropore volume and mesoporous surface area were calculated by t- plot method.
c Total pore volume was calculated at P/P0 = 0.98.
The Meso-Y-as rather than calcined Meso-Y-cal material was investigated as a support for silica surface modification under wet conditions. N2 physisorption data in
The Meso-Y-as sample comprises CTAB surfactant in the mesopores, while the corresponding calcined-form Meso-Y-cal consists of an open hierarchically porous channel system (organic surfactant component was removed prior during calcination). It appears the surfactant CTAB plays an important role as a soft template in protecting interior microporosity from surface modification, based on the lack of micropore blockage in the 11 wt. % SiO2 overlayer material when coating is performed on Meso-Y-as. This template guides the deposition of the silica overlayer in Meso-Y-as to occur only on mesopores rather than micropores. This suggests that the initial stage of calcination of silica surface-modified Meso-Y-as (i.e., after reaction with TEOS under wet conditions) causes partial decomposition of CTAB surfactant and creates space for silica deposition on the intracrystalline mesopore surface. During surfactant combustion, the partially decomposed CTAB surfactants can organize the formation of silica oligomers at the organic/inorganic interface through electrostatic interactions between positively charged CTAB and negatively charged silica species. This interaction would be expected to direct the deposition of the SiO2 overlayer selectively, as observed, on the mesopore surface. During this process, owing to mesopore blockage by fragments of the CTAB surfactant, the silica oligomers would hardly penetrate into the micropore channel system of Meso-Y-as during calcination. However, spatially selective surface-modification approach benefits from the presence of surfactant CTAB in the mesopores, and is reminiscent of explanations of selective silylation of as-synthesized mesoporous silica MCM-41, when using organosilane. This condensed organosilane was found mainly grafted at the external surface of as-synthesized materials due to the soft protection afforded by the CTAB. It is also reminiscent of the protective effect that CTAB is known to offer zeolite Y under basic aqueous conditions, where the surfactant protects the zeolite from dissolution, due to strong interactions with the framework. Based on this observed protecting role of the microporosity conferred to by CTAB during silica deposition under wet conditions above, below the focus is on Meso-Y-as rather than Meso-Y-cal as the optimal starting material for inorganic-oxide overlayer deposition.
Surface modification with an alumina overlayer was also investigated under wet conditions, starting with the Meso-Y-as material. 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 nearly identical XRD patterns to the parent Meso-Y-as sample (
The above deposition experiments indicate that a wet condition could work for the silica shell deposition on the Meso-Y-as with CTAB surfactant only at a low SiO2 loading (4 wt. %), in order to synthesize a material without severe micropore and mesopore blockage. At the high SiO2 loading of 11 wt. %, a wet condition led to a material with decreased mesopore volume and mesopore size. Moreover, the wet condition led to a separate alumina phase, which did not have good interfacial contact with the zeolite.
These inconsistent results are frustrating, thus the present process was developed as a more general and effective synthetic method for more uniformly (homogeneously) modifying the crystal surface of Meso-Y, in a way that leads to a high degree of interfacial contact between the zeolitic material and overlayer, without formation of a separate external-surface amorphous phase, and in which both mesoporosity and microporosity of the Meso-Y zeolitic material could be preserved after modification. For silica deposition, the residual moisture in the wet deposition system appeared to induce the partial hydrolysis of TEOS, and catalyze condensation between surface silanols and partially hydrolyzed silica species even before calcination. This would cause an inhomogeneous distribution of silica species favoring areas close to the external surface, and lead to mesopore size narrowing during calcination.
The present surface modification process, however, is conducted under rigorously dry conditions. Residual water can be removed, e.g., under vacuum, by heating the Meso-Y-as material at a temperature of 250° C., and conducting the deposition under air-free conditions, prior to the next step of calcination in either ambient air or dry air.
As shown in
The investigation of alumina surface modification of the Meso-Y-as material under dry conditions, using Al(O-i-Pr)3 as the precursor provided interesting results. Based on the PXRD patterns shown in
a Surface area was calculated by BET-plot.
b Micropore volume and mesoporous surface area were calculated by t-plot method.
c Total pore volume was calculated at P/P0 = 0.98
At a relatively high Al2O3 loading of 4.6 wt. %, slightly decreased micropore surface area (318 m2 g−1 versus 362 m2 g−1 for Meso-Y-cal) and micropore volume (0.12 cm3 g−1 versus 0.14 cm3 g−1 for Meso-Y-cal) were observed (Table 2, No. 8). In one embodiment, a limit (2.2 wt. % Al2O3) for the alumina coating on the Meso-Y surface under dry conditions is preferred, to avoid micropore blockage.
Rigorously dried air was also used to calcine the intermediate sample that was obtained after mixing the inorganic-oxide overlayer source (i.e. TEOS or Al(O-i-Pr)3) with Meso-Y-as under dry conditions. The same near lack of change in terms of porosity after coating with a silica or alumina shell was observed in both dry and humid air. This result indicated that keeping the first step of mixing the deposited inorganic-oxide overlayer source with Meso-Y-as dry was more crucial than to keep the calcination step dry, in which steam is liberated in any case during combustion of organic. This steam causes hydrolysis and condensation between the inorganic shell and silanols on the zeolite surface.
Given the lack of phase separation observed between the silica/alumina overcoats and the Meso-Y-cal support as evidenced by SEM in
How such nanoscale overlayers affect the Zeta-potential (or surface charge) of the surface-modified Meso-Y-cal support was also investigated. To address this, the Zeta-potential of all of materials synthesized under dry conditions at pH=7 in deionized water were measured. The Zeta-potential is known to directly reflect the degree of surface hydroxyl group protonation/dissociation, which is controlled by the surface Si/Al compositions. These data are shown Table 3 below.
Table 3. Zeta-potential measurements of the Meso-Y-cal and the resulting core-shell meso-Y catalysts, which was calcined in air.
The 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 materials became more negative with increasing amount of silica overlayer, consistent with a greater dissociation of surface hydroxyl groups in silica compared with Meso-Y-cal. In contrast, alumina surface modification increased the Zeta-potential of the coated materials, in a manner that directly depended on the amount of alumina overlayer, as shown by data in Table 3. The linear progression of the Zeta potential with amount of silica/alumina overcoat is shown in
Because of the observed uniformity of the silica/alumina as nanoscale overlayers on top of the Meso-Y-cal support rather than phase-separated oxide particles away from it, as observed here with SEM/TEM microscopy imaging as well as supported by Zeta-potential measurements, the synthetic approach described herein under dry conditions will be a useful method for controlling the surface charge of Meso-Y-cal zeolitic material in a rational fashion—all while avoiding pore plugging. This synthetic control has clear applications for the nucleation of single and bimetallic metal clusters on the zeolite surface using approaches such as strong-electrostatic adsorption, as well as controlling the dispersability of Meso-Y-cal particles during catalyst shaping, which involves concentrated aqueous suspensions and a general desire to avoid aggregation of zeolite particles to themselves.
Based on the results above, a mixed aluminosilicate coating was also synthesized on the Meso-Y-as—one that comprises both silica and alumina components together in the overcoat, rather than an overcoat based only on either silica or alumina in the shell, as in the previous examples above. The advantages to such an aluminosilicate shell are inclusion of a catalytically active overlayer, because of it possessing Al—O—Si sites, which are known to be active for acid catalysis. The approach used dry conditions. Using this approach, the surface was modified with both silica (i.e. TEOS) and alumina (i.e. Al(O-i-Pr)3) precursors simultaneously, to synthesize a coating with average silica-to-alumina ratios (SAR) ranging from 14 to 150, both at 4 wt. % and 8 wt. % (weight percentages refer to total amount of SiO2 and Al2O3). The data shows the zeolite products after aluminosilicate surface modification possessed comparable microporosity (e.g., the micropore volume) compared with the calcined parent material Meso-Y-cal. These data support the conclusions above regarding pure silica and alumina coatings, by confirming that the aluminosilicate surface modification process also did not cause the blockage of zeolite pores. The uniformity of the aluminosilicate coatings was also verified by SEM and TEM (aluminosilicate layer is below 30 nm thickness). As in the case of the pure silica and alumina surface modification, the Zeta potential of the aluminosilicate modified materials was measured, and the data show the Zeta potentials to be significantly less negative than the parent Meso-Y-cal material, in a manner that sensitively depended on the silicon-to-alumina ratio of the coating.
Given the propensity for using inorganic-oxide surface modified zeolites in functional applications, the resulting surface modified Meso-Y zeolite materials are a promising new type of zeolitic material for functional applications involving adsorption and catalysis, in which the surface charge (zeta potential) of the surface can be controlled.
As used in this disclosure the word “comprises” or “comprising” is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase “consists essentially of” or “consisting essentially of” is intended to mean the exclusion of other elements of any essential significance to the composition. The phrase “consisting of” or “consists of” is intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
All patents and publications referenced herein are hereby 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 and are included in the description simply for completeness of an exemplary embodiment or embodiments. In addition, it will be understood that specific structures, functions, and operations set forth in the above-described referenced patents and publications can be practiced in conjunction with the present invention, but they are not essential to its practice. It is therefore to be understood that the invention may be practiced otherwise that as specifically described without actually departing from the spirit and scope of the present invention as defined by the appended claims.
The present application claims priority to U.S. provisional patent application 63/483,373 filed on Feb. 6, 2023, the disclosure of which is hereby incorporated by references in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63483373 | Feb 2023 | US |
