Patent Application
20240367987
Surfactant-templating approaches have been widely used in mesoporous silica synthesis as exemplified by the successful synthesis of ordered mesoporous molecular sieves (MCM-41) with pore sizes ranging from 2 to 10 nm by using cetyltrimethylammonium bromide (CTAB) as the template under basic conditions. It was shown early on that these materials are amorphous—not crystalline—based on a Raman spectroscopic analysis. This approach provided a rational method of controlling mesopore diameter by increasing surfactant alkyl chain length. In addition, swelling agents, such as 1,3,5-trimethylbenzene (TMB), 1,3,5-triisopropylbenzene, isopropylbenzene, tridecane and para-xylene (p-xylene) proved to be efficient at expanding the pore diameter of MCM-41 materials up to 20 nm, while also modifying the textural properties of the resulting mesoporous silicates. In particular, both TMB and p-xylene have been extensively used as swelling agents to increase the mesopore diameter of micelle-templated silica (MTS). These two swelling agents have been compared in MTS syntheses employing C16TAB surfactant assemblies. With TMB as a swelling agent, an increasing MTS mesopore size from 3.9 nm to 9.0 nm was reported when using TMB/C16TAB ratios increasing in the range from 0 to 13 (with a plateau observed at 8.5 nm with a TMB/C16TAB ratio of 5). With p-xylene as a swelling agent, the same authors report a mesopore size of 3.9 nm at a p-xylene/C16TAB ratio of 2, and, upon increasing this ratio in the range of 5-20, there was growth of an additional mesopore, thereby creating a bimodal mesopore size distribution, in the range of 8.0 nm to 16.5 nm.
Following the success of MCM-41S-type materials syntheses, surfactant templating approaches have been extended to the synthesis of other materials, such as mesoporous polymers, mesoporous metal oxides, and mesoporous metal organic frameworks (MOFs). Among them, the synthesis of mesoporous zeolites has garnered great attention due to the reason that the surfactant templating method provides an approach for mesoporosity introduction within a zeolite, while preserving strong acidity and high hydrothermal stability of the zeolite. Notably, Garcia-Martinez et al. reported on the synthesis of mesoporous Y zeolites using a surfactant-templating method starting from Y zeolites (Zeolyst CBV720 with Si/Al=15) with NH4OH (or other bases such as NaOH, Na2CO3 or tetrapropylammonium hydroxide (TPAOH)), combined with the formation of a mesoporous phase via trimethylalkylammonium bromide (CnTAB, n=16 and 18) as surfactant. Their results demonstrate that a mesopore size around 3.8 nm is produced when using C16TAB as the surfactant, which can be increased by enlarging the carbon length of the alkyl chain (C8TAB, C10TAB, C12TAB, C14TAB, C16TAB, and C18TAB) in the surfactants. Using this approach, the mesopore diameter of mesoporous Y zeolites has been increased up to 5.3 nm when using C22TAB as the surfactant. Similar approaches of tailoring surfactant size have also been used to control mesoporosity in ZSM-5 zeolites, when using (CH3O)3SiC3H6N+(CH3)2CnH2n+1 as the surfactant.
The effect of the NaOH/SiO2 ratios on the mesoporosity of synthesized samples using octadecyltrimethylammonium bromide (C18TAB) as surfactant has also been studied. The data demonstrate an amorphous wall when using a NaOH/SiO2 ratio range of 0.125 to 0.25 in the mesoporous Y zeolite synthesis. It was also shown that upon increasing the NaOH/SiO2 ratio from 0.05 to 0.25, the mesopore size decreased from 4.30 nm to 3.91 nm. This indicates that NaOH concentration has little effect on the mesopore size, but it results in larger mesopore volumes, smaller zeolitic nanodomains, lower acidity and lower micropore volumes.
Addition of a cosolvent has also been widely used to increase mesopore size in MTS systems. The physiochemical properties of cosolvent, such as its polarity and dielectric constant, as well as its amount were reported to influence d-spacing, mesopore size, mesophase transition, and morphology of mesoporous silica. Also, the effect of cosolvents in conjunction with surfactant templating strategies has been previously investigated in mesoporous zeolite syntheses involving Y zeolite, ZSM-5 zeolite, and NaX zeolite. These have all used C16TAB-surfactant templating using a bottom-up method (i.e., not postsynthetic modification of a zeolite or zeolite precursor), which proved that the cosolvent plays different roles including that of a self-assembly modulator, in addition to a mesopore size expander. However, reports of using cosolvents in the presence of swelling agents in surfactant templating syntheses remain rare.
Provided is a method of preparing mesopore Y zeolite catalysts with larger mesopore sizes. The method employs swelling agents, and optionally cosolvents. The larger mesopores improve accessibility for mass transport of bulky reactants.
In the preparation a swelling agent is used in combination with cetyltrimethylammonium bromide (CTAB). It has been found that such a combination of materials can increase the mesopore size of the Y material upwards of 6.8 nm, with a mesopore volume of up to 0.41 cm2/g. Cosolvents can also be used to affect a strong swelling effort.
The swelling agents can be any swelling agent that is suitable. In one embodiment, the swelling agents can comprise 1,3,5-trimethylbenzene (TMB) or para-exylene (p-xylene). In one embodiment a cosolvent is also used. The cosolvents can be any suitable cosolvent to help aid a strong swelling effect. The cosolvent in the present method is generally an alcohol. In one embodiment, the cosolvent can comprise ethanol or tert-butyl alcohol (TBA).
Among other factors, the use of swelling agents, optionally together with cosolvents such as alcohols, has been found to be able to expand on the mesopore size of CTAB-templated mesopore Y zeolites. The use of swelling agents in the synthesis of Y zeolites has been found to extend the mesopore size. In one embodiment, the use of TMB provides excellent results. The addition of a cosolvent, such as an alcohol cosolvent, can further expand the mesopore size. In one embodiment, TBA as a cosolvent provides excellent results.
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:
Before the present processes for achieving pore expansion in mesoporous materials are disclosed and described, it is to be understood that this disclosure is not limited to the particular structures, process steps, or materials disclosed herein, but is extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting. It must be noted that, as used in this specification, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a step” may include multiple steps, reference to “producing” or “products” of a reaction or treatment should not be taken to be all of the products of a reaction/treatment, and reference to “treating” may include reference to one or more of such treatment steps. As such, the step of treating can include multiple or repeated treatment of similar materials/streams to produce identified treatment products.
Numerical values with “about” include typical experimental variances. As used herein, the term “about” means within a statistically meaningful range of a value, such as a stated particle size, concentration range, time frame, molecular weight, temperature, or pH. Such a range can be within an order of magnitude, typically within 10%, and more typically within 5% of the indicated value or range. Sometimes, such a range can be within the experimental error typical of standard methods used for the measurement and/or determination of a given value or range. The allowable variation encompassed by the term “about” will depend upon the particular system under study, and can be readily appreciated by one of ordinary skill in the art. Whenever a range is recited within this application, every whole number integer within the range is also contemplated as an embodiment of the invention.
Provided is a method of preparing mesopore Y zeolite catalysts with larger mesopore sizes. The method employs swelling agents, and optionally cosolvents. The larger mesopores improve accessibility for mass transport of bulky reactants.
Reference will now be made in detail to exemplary aspects of the present disclosure that are illustrated in the accompanying drawings.
It has been found that swelling agents, cosolvents, and cosurfactants can impact the mesopore size distribution in C16TAB- and C18TAB-templated mesoporous Y syntheses. The present process expands pore size in mesoporous Y zeolites beyond what has heretofore been attainable. While surfactants with longer alkyl chain length produced slightly larger mesopore sizes, adding swelling agents extended the mesopore size further, with TMB exhibiting a stronger mesopore size expansion effect than p-xylene. A sample synthesized with a TMB/C18TAB ratio of 12 exhibited the largest mesopore size of 6.8 nm, with a mesopore volume of 0.41 cm3/g. The addition of cosolvents could also alter the mesopore size, with TBA generally showing a stronger effect at mesopore expansion compared to ethanol, but the effect of cosolvent addition became less pronounced as the TMB/CnTAB ratio increased.
The present process comprises combining CTAB with a Y zeolite, a swelling agent and sodium hydroxide solution. The mixture is heated and stirred. When the reaction is complete and solids have precipitated, the mixture is cooled and the solids filtered. The solids can be washed with water to attain a neutral pH. The filtered solids can then be dried can calcined. The swelling agent, in one embodiment, is TMB or para-xylene, with TMB being the preferred swelling agent. The CTAB, in one embodiment, is either C16TAB or C18TAB. The combination of C18TAB and the selling agent TMB has been found quite advantageous. In one embodiment, the Y zeolite is CBV270.
In another embodiment, the process comprises first combining the sodium hydroxide solution with CTAB and the swelling agent. The resulting mixture is then heated as noted above. The heating can continue for 20-40 minutes at a temperature in the range of from 85-95° C., and then the Y zeolite is added to the mixture with the mixture being stirred until the reaction is complete. In one embodiment, as noted above, the CTAB is either C16TAB or C18TAB. In one embodiment, the swelling agent is TMB or para-xylene. And, as noted above, in one embodiment, the Y zeolite is CBV270.
The stirring of the reaction mixture contains for a time of 4-7 hours. In one embodiment, the stirring continues for about 6 hours. Once the reaction is complete and solids have precipitated, the mixture is cooled or allowed to cool, generally to room temperature. The precipitated solids are then filtered. In one embodiment, the solids are washed several times with water until the filtrate is of neutral pH. The neutral pH can be detected by pH paper.
The filtered solids, whether washed or not, are then calcined. The calcination can generally be conducted in the temperature range of from 550-650° C. In one embodiment, the temperature is about 580° C. The calcination will generally be conducted for several hours, e.g., 4-7 hours. In one embodiment, the calcination is conducted for about 5 hours. The calcination is also, in one embodiment, conducted in dry air. The mesopores of the Y zeolite are significantly larger on an average basis than those of the original or parent Y zeolite.
In the present process, the CTAB can be any suitable CTAB. In one embodiment, the CTAB is C16TAB or C18TAB. The larger alkyl chain in the CTAB surfactant generally provides enlarged mesopores. In one embodiment, C18 TAB is used in the process.
The swelling agent is generally TMB or para-xylene. Both have been found to improve and enhance the size of the mesopores in combination with CTAB. In one embodiment, TMB is used as the swelling agent. Using TMB in conjunction with C18TAB has been found to be quite beneficial and successful in expanding the mesopores. This is particularly true when the molar ratio of TMB to C18TAB is about 12.
The molar ratio of the TMB or par-xylene swelling agent to the CTAB surfactant is generally in the range of from 4 to 12. In one embodiment, the molar ratio is about 6, and in another embodiment is about 12.
In one embodiment, an alcohol cosolvent is also used together with the swelling agent. The alcohol cosolvent can be in the initial mixture of the CTAB, Y zeolite, swelling agent and sodium hydroxide solution. When the Y zeolite is added subsequently to creating an initial mixture comprising sodium hydroxide aqueous solution, CTAB and swelling agent, the cosolvent can then be added after the Y zeolite is added.
In this embodiment, the initial mixture is heated for 20-40 minutes, e.g., about 25 minutes, and then the Y zeolite is added to the mixture with stirring. The cosolvent can then be added 20-40 minutes later, e.g., about 30 minutes, after the addition of the Y zeolite. This later process has been found to work quite well when using a cosolvent.
The alcohol cosolvent can be any suitable cosolvent. In one embodiment, the alcohol cosolvent can be ethanol or TBA. The alcohol solvent is preferably a larger molecule alcohol, such as TBA. Improved expansion of the mesopores is realized when using a larger molecule alcohol such as TBA.
The Y zeolite can be any suitable Y zeolite. Generally, the Y zeolite comprises mesopores to begin with. CBV720 is such a Y zeolite. The present process works particularly well with CBV720.
In one embodiment, a nonionic surfactant can be used in conjunction with the CTAB surfactant. It has been found that a nonionic surfactant together with the CTAB surfactant and TMB swelling agent can provide advantageous results.
The following examples are provided to further illustrate the present process and Y material with larger mesopores. The examples, however, are not meant to be limiting.
The water used in all experiments was deionized. The chemicals used for the experiments were hexadecyltrimethylammonium bromide (C16TAB, Sigma-Aldrich), trimethyloctadecylammonium bromide (C18TAB, Sigma-Aldrich), sodium hydroxide (Pellets, Neta Scientific Inc), parent Y zeolite CBV720 (ZE0290, Si/Al=15, Zeolyst), 1,3,5-trimethylbenzene, (TMB, Sigma-Aldrich), para-xylene (p-xylene, Sigma-Aldrich), 1, 4-diisopropylbenzene (DIPB, Sigma-Aldrich), ethanol (Sigma-Aldrich), tert-butyl alcohol (TBA, Sigma-Aldrich). NH4NO3 (Sigma-Aldrich, >99.5%), 2-methoxynaphthalene (2-MN, Sigma-Aldrich, 99%), acetic anhydride (Ac2O, Sigma-Aldrich, 99.5%), 1, 2-dichloroethane (DCE, Sigma-Aldrich, 99.8%), 1-acetyl-2-methoxynaphthalene (1, 2-AMN, TCI Chemicals, >98.0%), 2-acetyl-6-methoxynaphthalene (2,6-AMN, Sigma-Aldrich, 98.0%). All the chemicals, except Ac2O and DCE (which are dried with P2O5/K2CO3 and CaH2, respectively, and subsequently stored under N2 atmosphere), were used without further purification.
Mesoporous Y Synthesis Using CnTAB (n=16 and 18) as the Surfactant
Conventional surfactant-templated mesoporous Y zeolite was synthesized using the procedure described below, which is modified from the established procedures in the literature. Instead of adding all the reagents into a plastic container before heating as described in the literature, the parent zeolite CBV720 was introduced after the entire solution reached a set temperature. Thus, 0.5 g hexadecyltrimethylammonium bromide (C16TAB) and 20 mL NaOH solution (0.16 M unless specified (NaOH/SiO2 ratio of 0.19), which falls into the NaOH/SiO2 range of producing an amorphous wall via zeolite dissolution) were added to a plastic container and heated in a 90° C. oil bath for 25 min. Subsequently, 1.0 g of parent zeolite CBV 720 was added to the solution. The molar composition of the mixture were 1.00 SiO2:0.192 Na:0.082 CnTAB (n=16 and 18):66.7 H2O, and this mixture was stirred for 6 h. After cooling, the contents were poured onto a filter, and the precipitated solids were washed several times with water, until the filtrate became neutral as measured with pH paper. The sample was then dried at 60° C. overnight and was calcined at 580° C. for 5 h in dry air.
The synthetic approach for preparing mesoporous Y zeolite together with swelling agents was similar to the procedure of surfactant-templated mesoporous Y above, except that various amounts of swelling agents (1,3,5-trimethylbenzene (TMB); para-xylene (p-xylene); DIPB) were added after C16TAB or C18TAB, as shown in Scheme 1 below. In a typical synthesis, 0.5 g of C16TAB, 1.0 g TMB (0.87 g p-xylene; 1.35 g DIPB), and 20 mL NaOH solution (0.16 M) were added to the plastic bottle and heated in an oil batch at 90° C. for 25 min. Subsequently, 1.00 g of parent zeolite CBV 720 was added to the solution. The molar composition of the obtained mixture was 1.0 SiO2:0.19 Na:0.082 CnTAB (n=16 and 18):0.49 swelling agent and 66.7 H2O. If cosolvents were used, either ethanol or TBA was added into the solution 30 min later after the addition of the parent zeolite CBV 720, and the molar ratio of cosolvent to C11TAB (n=16 and 18) was 14.6:1. This mixture was stirred for 6 h. After cooling, the contents were filtered, and the solids were washed several times with water until the filtrate became neutral as measured with pH paper. The sample was then dried at 60° C. overnight, and was calcined at 580° C. for 5 h in dry air.
The obtained samples were designated as CnTAB (n=16 and 18)+x swelling agent (+14.6 cosolvent), where x is the molar ratio of swelling agent/CnTAB. The proton form materials were obtained via ion-exchange with 1 M NH4NO3 aqueous solution (liquid to zeolite weight ratio of 50:1) at 60° C. for 24 h followed by the same washing and drying procedure described above. This ion-exchange procedure was repeated for another two times before calcination according to the same procedure described above.
0.5 gas-made mesoporous Y material using C16TAB as the surfactant was mixed with 4.04 g TMB (TMB to C16TAB molar ratio of 70) to make a suspension, which was transferred into a Teflon-lined autoclave and heated to 160° C. for 16 h. After cooling, the contents were washed several times with either water or a mixture of water and ethanol followed by drying and calcination. The obtained samples were designated as C16TAB+70TMB postsynthesis and C16TAB+70TMB postsynthesis (water and ethanol).
N2 physisorption isotherms were measured at 77 K on a Micromeritics ASAP2020 adsorption instrument. Prior to sample analysis, the sample was degassed at 350° C. for 4 hours under vacuum to remove the residuals. The total surface area was calculated from the Brunauer-Emmett-Teller (BET) equation. The micropore volume, micropore surface area, and the external surface area were measured by the t-plot method. The mesopore volume was calculated from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method. X-Ray diffraction (XRD) patterns were collected on a Rigaku MiniFlex diffractometer using a Cu Kα radiation (40 kV, 15 mA) ranging from 5 to 500 with a step size of 0.01°. SEM was performed with a Zeiss Crossbeam 550 scanning electron microscope operated at 1 kV and 5.0 mm working distance.
Friedel Crafts Acylation (FCA) of 2-methoxynaphthalene (2-MN) using acetic anhydride (Ac2O) as the acylating agent was carried out to investigate the catalytic site accessibility of the synthesized materials. The as-made zeolite was calcined in a tube furnace and transferred moisture-free to an Ar-filled glovebox for reaction preparation. Solid 2-MN was brought to an Ar-filled glovebox for reaction preparation. For a typical run, 100 mg of selected material and 1.1 mmol of 2-MN were loaded into separate Schlenk tubes and sealed to maintain a moisture free environment. The tubes were placed in a N2 atmosphere, where 10 mL of dry DCE, 0.31 mL Ac2O (˜3.3 mmol) and 250 μL of dodecane (as an internal standard) were syringed in an air-free manner to the tube containing the organic solid. The molar ratio of 2-MN to Ac2O was 1 to 3. Then the catalyst and reagent tube were heated to the reaction temperature of 40° C. for 15 minutes, prior to introducing the reagent solution to the heated catalyst via cannula transfer where the reaction took place for 10 minutes. Upon reaction completion, the solution was syringed through a 0.2 μm PTFE filter, and was analyzed with an Agilent 6890 GC system equipped with a flame ionization detector (FID) and HP-1 column (50 m×0.32 mm×1.05 m). For all reactions, both the carbon balance and the mole balance reach near completion, with measured balances at >90% and >95% respectively. The main product formed was 1-acetyl-2-methoxynaphthalene (1, 2-AMN) with a selectivity higher than 97% for all experiments.
Conventional C16TAB-templated mesoporous Y exhibits a 3.5 nm mesopore which is absent in parent CBV720 zeolite. This mesopore size is similar to the size previously reported for related mesoporous Y materials in the literature. Compared with parent CBV720 zeolite, the total mesopore volume of conventional C16TAB-templated mesoporous Y is 2.5-fold higher. When mesoporous Y is synthesized at a higher NaOH concentration of 0.2 M instead of 0.16 M NaOH, (i) a nearly unchanged mesopore size; (ii) an increased mesopore volume (0.59 to 0.67 cm3/g)—a trend observed previously; and (iii) a slightly decreased recovered yield are observed. This suggests a higher base concentration does not enlarge mesopore size of conventional mesoporous Y.
The results of using C16TAB together with p-xylene and TMB as swelling agents in the synthesis of mesoporous Y are shown in
When using p-xylene as a swelling agent in the present synthesis, the mesopore size maximum increases from 3.5 to 5.4 nm as the p-xylene/C16TAB ratio increases from 0 to 20. (
As shown in Table 1, compared with conventional C16TAB-templated mesoporous Y, samples comprising added TMB or p-xylene had decreased mesopore volumes and increased micropore volumes. The decrease in the former overwhelmed the increase in the latter in this comparison, leading to an overall decrease in total pore volume when synthesizing mesoporous Y without swelling agents. This result is contrary to literature MTS materials syntheses, where addition of swelling agents increased the mesopore size as well as the total pore volume. These contrasting results between the two systems can be ascribed to different pore shapes.
In summary, a mesoporous Y material was synthesized with a mesopore size centered at 6.0 nm using a TMB/C16TAB ratio of 12. This sample had the highest mesopore volume of 0.49 cm3/g among the C16TAB-templated samples synthesized using the swelling agents in Table 1. The addition of swelling agents controlled the mesopore volume and increased the mesopore size.
aSBET = BET surface area (total surface area);
bSmicro = micropore surface area, t-plot method;
cSexternal = external surface area;
dVmicro = micropore volume, t-plot method;
eVmeso = mesopore volume, BJH adsorption branch;
fYield = (weight of inorganics after final synthesis step/weight of inorganics originally into the synthesis) where inorganic refers to aluminosilicate zeolite.
The PXRD patterns of the samples synthesized using C16TAB as the surfactant and TMB or p-xylene as swelling agent show the characteristic peaks of FAU zeolite at 20 of 6.16, 10.13, 15.69, and 23.82 et al., including the parent CBV720 zeolite, and the sample synthesized with C16TAB at a higher NaOH concentration of 0.2 M instead of 0.16 M. Based on the data, it can be concluded that qualitatively, all of the samples exhibited a degree of crystallinity that is comparable to the parent CBV720 zeolite. This conclusion is supported by quantitative crystallinity studies in the literature, which show only slight decreases in crystallinity in mesoporous Y relative to precursor materials.
In addition, when DIPB was applied as a swelling agent with a DIPB/C16TAB ratio of 12, no evidence for the synthesis of larger or additional mesopores, compared with parent CBV720 zeolite was observed. The high hydrophobicity of DIPB seems to have caused it not to incorporate into the surfactant assembly, because of competitive adsorption of DIPB into the bulk of the zeolite Y. Such a result suggests an optimum hydrophobicity of a swelling agent, which is well represented in TMB, in which there is more steric bulk of alkyl substituents compared with p-xylene, but not so much as in DIPB such that adsorption into the zeolite interior pores rather than the mesoporous surfactant assembly dominates. Thus, in one embodiment, TMB is used as the swelling agent with great advantage.
Based on the best results above obtained with TMB as a swelling agent, TMB was combined with cosolvents consisting of either ethanol or TBA, in an attempt to further increase mesopore size in surfactant-templated mesoporous Y materials. The addition of ethanol did not change the mesopore size at a TMB/C16TAB ratio of 6 (5.0 nm both with and without ethanol addition), and decreased the mesopore size at a TMB/C16TAB ratio of 12 (from 6.0 nm without ethanol to 5.4 nm within ethanol in
The addition of cosolvents can control the solubility of the swelling agent within the surfactant assembly. Upon adding ethanol, the solubility of TMB in solution (i.e. outside of the micelle core) increased, resulting in less TMB incorporation in the surfactant assembly and smaller mesopores. This phenomenon is more pronounced at a higher TMB/C16TAB ratio as shown in
Both cosolvents investigated (ethanol and TBA) caused an increase in the mesopore volume at a TMB/C16TAB ratio of 6 and a decrease at a TMB/C16TAB ratio of 12, when comparing with materials in the absence of added cosolvents in Table 1. The best results are observed upon addition of TBA at a TMB/C16TAB ratio of 6, which increased the mesopore volume up to 0.51 cm3/g compared with 0.38 cm3/g for the control in the absence of TBA. The data in Table 2 (relative to controls in Table 1) also demonstrate slightly higher recovered yields upon cosolvent addition. According to PXRD, the crystallinity of samples prepared using addition of TBA and ethanol cosolvents result in similar characteristic peak intensities corresponding to FAU zeolite compared with control materials synthesized in the absence of cosolvents.
aSBET = BET surface area (total surface area);
bSmicro = micropore surface area, t-plot method;
cSexternal = external surface area;
dVmicro = micropore volume, t-plot method;
eVmeso = mesopore volume, BJH adsorption branch;
fYield = (weight of inorganics after final synthesis step/weight of inorganics originally into the synthesis) where inorganic refers to aluminosilicate zeolite.
When using the sterically bulkier C18TAB surfactant, the surfactant-only synthesis (NaOH/SiO2 ratio of 0.19; 90° C. for 16 h) led to a mesopore size of 4.3 nm, as shown in
The effect of surfactant alkyl chain carbon length (C16TAB and C18TAB) on the mesopore size when using TMB as the swelling agent is shown in
Upon addition of TMB, the mesopore volume of the samples decreased, as shown in Table 3. This is the same trend as observed above in the C16TAB-templated system. However, a larger mesopore volume is observed at a TMB/C18TAB ratio of 6 in the C18TAB system (0.51 cm3/g in Table 3), compared with a TMB/C16TAB ratio of 12 in C16TAB system (0.49 cm3/g in Table 1). It seems that in the C18TAB-templated system, the effect of TMB as a swelling agent is more pronounced than in the corresponding C16TAB-templated system. In addition, comparable recovered yields using different surfactants are observed, as shown by data in Tables 1 and 3. It is noteworthy that at a TMB/C18TAB ratio of 12, a mesoporous Y material possessing a mesopore size of 6.8 nm was synthesized, which is the largest mesopore size among all of the materials investigated. It seems that the application of C18TAB increases the mesopore size compared with what is attainable with C16TAB, and the use of TMB as a swelling agent can further increase the mesopore size.
aSBET = BET surface area (total surface area);
bSmicro = micropore surface area, t-plot method;
cSexternal = external surface area;
dVmicro = micropore volume, t-plot method;
eVmeso = mesopore volume, BJH adsorption branch;
fYield = (weight of inorganics after final synthesis step/weight of inorganics originally into the synthesis) where inorganic refers to aluminosilicate zeolite.
The PXRD patterns of the samples synthesized in the C18TAB-templated system show a similar relative crystallinity compared to parent CBV 720 zeolite. This trend is consistent with literature results for a C18TAB-templated mesoporous Y synthesis, albeit at a higher crystallization temperature. Others have previously observed significant crystallinity decreases relative to CBV720 upon synthesizing mesoporous Y using a C18TAB surfactant. Details in the synthesis conditions outside of the surfactant used may be responsible for different crystallinity outcomes.
Data in
Upon adding TBA, the C18TAB-templated sample synthesized with a TMB/C18TAB ratio of 6 resulted in a mesopore size of 6.2 nm, which is slightly higher than the material described above with the addition of ethanol.
As shown in Table 4 below, at a C18TAB/TMB ratio of 6, the addition of cosolvents (both ethanol and TBA) decreases the mesopore volume compared to the result in Table 3, in the absence of cosolvents. At a C18TAB/TMB ratio of 12, although the mesopore size distribution is unchanged, the addition of TBA also decreased the mesopore volume compared to the comparator with the absence of TBA (Table 3). The recovered yields were similar to syntheses in the absence of these cosolvents. The corresponding PXRD patterns exhibit similar relative crystallinity to the comparator without cosolvent, which is consistent with results in the C16TAB-templated system.
aSBET = BET surface area (total surface area);
bSmicro = micropore surface area, t-plot method;
cSexternal = external surface area;
dVmicro = micropore volume, t-plot method;
eVmeso = mesopore volume, BJH adsorption branch;
fYield = (weight of inorganics after final synthesis step/weight of inorganics originally into the synthesis) where inorganic refers to aluminosilicate zeolite.
It has been found that swelling agents, cosolvents, and cosurfactants can impact the mesopore size distribution in C16TAB- and C18TAB-templated mesoporous Y syntheses. Although surfactants with longer alkyl chain length produced slightly larger mesopore sizes, adding swelling agents extended the mesopore size further, with TMB exhibiting a stronger mesopore size expansion effect than p-xylene. A sample synthesized with a TMB/C18TAB ratio of 12 exhibited the largest mesopore size of 6.8 nm, with a mesopore volume of 0.41 cm3/g. The addition of cosolvents could also alter the mesopore size, with TBA generally showing a stronger effect at mesopore expansion compared to ethanol, but the effect of cosolvent addition became less pronounced as the TMB/CnTAB ratio increased.
Acid-site catalytic accessibility in the mesoporous Y zeolite with the largest pore size obtained was shown in the acylation of 2-MN with acetic anhydride (Ac2O) as a probe reaction. Data in Table 5 demonstrate that both C18TAB-templated (pore size 4.3 nm) and C18TAB+12TMB-templated (pore size 6.8 nm) exhibit high selectivity to 1,2-AMN, which is consistent with the literature data, although much higher reaction temperatures were used in the literature (100° C. and 150° C.) than in the present process, e.g., 50° C. or less, 20-50° C., with 40° C. in one embodiment. A similar conversion was observed for both materials (18.3%-20.0%), suggesting that the present supramolecular approach-maintained acid-site integrity and accessibility found in conventional mesoporous Y zeolites. This is consistent with the gentle nature of the non-covalent interactions and organic additives used for mesopore expansion in the present method.
aReaction conditions: T = 40° C., atmospheric pressure, catalyst weight = 100 mg, AC2O: 2-MN = 3:1 mmol, 10 mL 1,2-dichloroethane (DCE) as solvent, reaction time = 10 min.
Larger pores in CnTAB-templated (n=16 and 18) mesoporous Y zeolites can be achieved via a non-covalent approach involving the addition of swelling agents and cosolvents during the present surfactant-assembly and materials synthesis process. TMB exhibited a stronger mesopore size expansion effect than both p-xylene and DIPB as swelling agents, indicating an optimum hydrophobicity of the swelling agent. A sample synthesized with a TMB/C18TAB ratio of 12 exhibited the largest mesopore size of 6.8 nm, with a mesopore volume of 0.41 cm3/g, and showed the same catalytic accessibility and rate as a conventional mesoporous Y zeolite. It was also found that the addition of cosolvents could alter the mesopore size, with TBA generally showing a stronger effect at mesopore expansion compared to ethanol, but the effect of cosolvent addition is less pronounced as the TMB/CnTAB ratio increases. The addition of swelling agents and cosolvents decreased the mesopore pore volume due to their effect on decreasing [OH−] activity, but this can be compensated by using a higher NaOH concentration, while maintaining the same enlarged mesopore size distribution afforded by the organic additives. When a large excess of TMB is reacted postsynthetically at high temperature with an as-made conventional mesoporous Y material, little change in the maximum of the pore size distribution was observed, and only a slight synthesis of larger mesopores in the 9.4-17.0 nm range was observed. These results emphasize the importance of adding organic additives during mesoporous Y synthesis, when the surfactant assembly has greater flexibility to swell.
In spite of the present efforts, it is difficult to synthesize mesoporous Y materials with mesopore sizes larger than 10 nm, and alternative routes for the synthesis of such larger mesopores are needed in mesoporous Y zeolite materials.
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 63/499,669 filed on May 2, 2023, the disclosure of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63499669 | May 2023 | US |
