Dry glue water heating method for synthesizing sheet type ZSM-5 zeolite

Abstract

The invention belongs to the technical field of petrochemical catalysis, and relates to a dry glue water heating method for synthesizing sheet type ZSM-5 zeolite. The preparation method comprises the following steps: 1, preparing urea-containing aluminosilicate hydrogel by taking fumed silica as a main silicon source, sodium silicate as an auxiliary silicon source and pseudo-boehmite as an aluminum source, and taking the urea-containing aluminosilicate hydrogel as a precursor for synthesizing sheet type ZSM-5 zeolite; 2, carrying out low-temperature drying treatment on the aluminosilicate hydrogel precursor containing urea to remove free water in the aluminosilicate hydrogel precursor, and converting the aluminosilicate hydrogel precursor containing urea into an aluminosilicate dry glue intermediate containing urea to be used as a direct raw material for synthesizing the sheet type ZSM-5 zeolite; and 3, carrying out hydrothermal crystallization treatment on the urea-containing dry glue intermediate in a high-pressure kettle under a hydrothermal condition lower than the melting point temperature of urea, and converting the urea-containing dry glue intermediate into the sheet type ZSM-5 zeolite.

Description

Technical Field

[0001] The invention belongs to the technical field of petrochemical catalysis and relates to a dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite. Background Art

[0002] ZSM-5 zeolite is an aluminosilicate zeolite with a high silicon-aluminum ratio and is the most important member of the MFI zeolite family. ZSM-5 zeolite has a unique three-dimensional crystal structure and a ten-membered ring cross-pore system. Its straight pores are parallel to the b-axis and have an opening of 0.53×0.56nm; the Z-shaped curved pores are parallel to the a-axis and have an opening of 0.51×0.55nm. ZSM-5 zeolite has three extra-framework countercation sites, which are located in the pore intersection, straight pores and Z-shaped curved pores. The above countercations (usually sodium ions) can be replaced by hydrogen protons and other metal ions to produce The surface acidity of ZSM-5 zeolite can be adjusted to meet the needs of specific reactions by selecting an appropriate silicon-aluminum ratio during hydrothermal synthesis (the framework silicon-aluminum ratio can be selected arbitrarily between 10 and ∞) and by employing appropriate post-modification treatments (such as steam passivation and ion modification) during catalyst preparation. Therefore, ZSM-5 zeolite has a wide range of applications.

[0003] Among the known uses of ZSM-5 zeolite, the largest is as a catalyst promoter in gasoline producers' catalytic cracking units, primarily for increasing propylene production and improving gasoline octane rating. In addition, ZSM-5 zeolite is used as a catalyst for refinery distillate dewaxing, methanol-to-gasoline (MTG), methanol-to-propylene (MTP), ethylbenzene synthesis (Mobil-Badger process), aromatization of liquefied petroleum gas (LPG) and light naphtha, cyclohexene hydration, and shape-selective direct synthesis of para-dialkylbenzenes (para-xylene, para-diethylbenzene, and para-methylethylbenzene). It is also used as a catalyst for the synthesis of fine chemicals such as 3,5-lutidine and triethylenediamine.

[0004] Recent research has demonstrated that ZSM-5 zeolite exhibits unique catalytic properties in biomass conversion and waste plastic degradation. Therefore, as an environmentally friendly solid acid catalyst, ZSM-5 zeolite not only plays an important role in traditional oil refining, petrochemicals, and coal chemical industries, but also holds promising prospects in the production of biomass-based fuels and chemicals, which may become emerging industries in the future, as well as in the recycling of waste resources such as waste plastics.

[0005] It is well known that the grain size and crystal morphology of ZSM-5 zeolite have a significant impact on its catalytic activity, selectivity, and resistance to coking and deactivation. This is because changes in grain size and morphology can significantly alter the textural properties of ZSM-5 zeolite, affecting the length of its pores (such as the straight pores along the b-axis), thereby affecting the diffusion properties of its micropores. Furthermore, changes in grain size and morphology also affect the number of pores per unit weight on the outer surface of ZSM-5 zeolite crystals and the porosity of the interstitial pores (secondary pores) between the grains. All of these factors affect the performance of ZSM-5 zeolite catalysts, such as coking capacity and resistance to coking and deactivation.

[0006] Those familiar with the art know that synthesizing nano ZSM-5 zeolite (particle size ≤ 100 nm) or thin-plate ZSM-5 zeolite can significantly improve the micropore diffusion performance and carbon holding capacity of ZSM-5 zeolite. Nano ZSM-5 zeolite particles generally exhibit a quasi-spherical morphology, with their straight and zigzag pores being of comparable length and short. However, nano ZSM-5 zeolite typically exists as aggregates, making complete dispersion difficult even under external forces (such as ultrasonic vibration). In contrast, thin-plate ZSM-5 zeolite typically exhibits a very short b-axis and relatively long a and c-axes. This type of zeolite typically exhibits no agglomeration, resulting in monodisperse crystals and no twinning. Because the straight pores of ZSM-5 zeolite run parallel to the b-axis, thin-plate ZSM-5 zeolite, also known as short-b-axis ZSM-5 zeolite, is effectively a single crystal ZSM-5 zeolite with relatively short straight pores. Although the specific range of the b-axis length for thin-plate ZSM-5 zeolite has not been determined in the literature, published literature indicates that the straight-channel diffusion performance of thin-plate ZSM-5 zeolite significantly improves when the b-axis length is less than 50 nm (less than 100 nm, entering the nanomaterial category) (CRChimie 19 (2016) 183e191; J. Am. Chem. Soc. 2021, 143, 4, 1993–2004). Thus, nano ZSM-5 zeolite and thin-plate ZSM-5 zeolite are two types of ZSM-5 zeolite with different crystal characteristics. Both have similar effects in improving the catalytic activity and resistance to coking deactivation of ZSM-5 zeolite catalysts. However, the strong hydrophilicity and agglomeration tendency of nano ZSM-5 zeolite surface increase the difficulty and cost of catalyst preparation. Therefore, thin-plate ZSM-5 zeolite is expected to become an ideal alternative to nano ZSM-5 zeolite.

[0007] To date, several methods have been proposed for synthesizing thin-plate ZSM-5 zeolites, specifically those capable of suppressing b-axis growth. These methods include template methods, fluoride methods, urea methods, ammonium salt methods, pyrrolidone and guanidine compound methods, and auxiliary methods such as alcohol solvents, seed crystals, and microwave heating. Among these, reports involving template methods, fluoride methods, and urea methods are more common.

[0008] To the best of our knowledge, the following organic templates have been used to synthesize thin-plate ZSM-5 zeolites to date:

[0009] First, tetrapropylammonium cations (TPA) and their dimers and trimers are templates. Publications such as Science 2003, 300, 456, Chem. Mater. 2004, 16, 5697-5705, and Angew. Chem. Int. Ed. 2006, 45, 1154–1158 report that, in studies of seeded, oriented-growth all-silica MFI zeolite (S-1), TPA cations and their dimers and trimers have different regulatory effects on the oriented growth of S-1 zeolite along the three coordinate axes (i.e., the a, b, and c axes, where the b axis is the direction of the straight pores and the a axis is the direction of the zigzag pores). When TPA cations are used as templates, the crystal morphology of the synthesized S-1 zeolite is usually characterized by a longer c-axis and a shorter b-axis (Lc>La>Lb); however, when dimers and trimers of TPA cations are used as templates, S-1 zeolites with a or b-axis as the long axis can be synthesized. In TPA dimers and trimers, the bridge between the two quaternary ammonium ions is a normal hydrocarbon chain with 6 carbon atoms ((C3H7)3N + C3H6-C3H6N +(C3H7)3), which can be represented as dC6. Studies have shown that TPA dimer templates tend to fill straight pores, with quaternary ammonium nitrogen atoms located at pore intersections; whereas TPA trimers tend to fill zigzag pores. The effect of TPA oligomer templates on the oriented growth of S-1 zeolite is related to their different filling patterns within the zeolite pores. The oriented growth synthesis of S-1 zeolite can be used to prepare ultrathin, high-flux separation membranes. Our literature search revealed that the open literature (doi:10.1038 / nature21421(2017)) mentions a TPA-like dimer template (i.e., bis-1,5(tripropyl ammonium)pentamethylene diiodide, where the hydrocarbon chain bridge between the two quaternary ammonium ions has 5 carbon atoms) with a dC5 bridge segment. With the help of a carefully synthesized nano-MFI seed (S-1, particle size of 30nm), this dC5-type TPA dimer template can be used to synthesize S-1 nanosheets with a b-axis length of only 5nm. The nano-MFI seed is obtained by using TPAOH as a template and undergoing a two-step variable temperature synthesis process (50°C for 6 days and then 100°C for 3 days). Similarly, the public document Sci.Adv.8,eabm8162 (2022) also involves the synthesis research of the above-mentioned dC5-type TPA dimer template and S-1 nanosheets with a b-axis length of 5nm. The difference is that the nano-S-1 seeds used in this study have a nanosheet morphology.

[0010] The second is the surfactant C22-6-6 (C 22 H 45 -N + (CH3)2-C6H 12 -N + (CH3)2-C6H 13) template. Publicly available literature, including Nature 461, 246-249 (2009), J. AM. CHEM. SOC. 2010, 132, 4169–4177, and Chem. Mater. 2011, 23, 5131-5137, reports methods for preparing this template and using it to hydrothermally synthesize ZSM-5 zeolite nanosheets (SiO2 / Al2O3 = 100) with a b-axis length of only 2 nm (equivalent to the unit cell size of an MFI zeolite). The C22-6-6 template, with a hydrocarbon chain bridge between the two quaternary ammonium ions at its hydrophilic end having 6 carbon atoms (similar to a TPA dimer), has excellent structural guidance for the formation of the MFI zeolite structure. Its long carbon tail (22 carbon atoms) is hydrophobic and can form a micellar layer, acting as an isolation zone for the growth of ZSM-5 zeolite crystals along the b-axis. The synthesized nanosheet ZSM-5 zeolite can be used as a solid acid catalyst, and is particularly suitable for catalyzing organic matter conversion reactions in which the catalytic reactant molecules are large and the reaction rate is severely affected by micropore diffusion, or in which the carbon deposition rate is high. The nanosheet ZSM-5 zeolite catalyst exhibits extremely high reactivity for the former type of reaction, and extremely high reaction stability (or extremely high resistance to carbon deposition and deactivation) for the latter type of reaction. In addition, a diquaternary ammonium ion surfactant C18-6-6Br2 (C 18 H 37 -N(CH3)2-C6H 12 -N(CH3)2-C6H 13 ]Br2) template, and a method for synthesizing ZSM-5 zeolite nanosheets (Si / Al=39) using the template. The main difference between the template and C22-6-6 is that the long tail carbon chain at the water-increasing end has slightly fewer carbon atoms, namely 18 carbon atoms.

[0011] Surfactants with multiple quaternary ammonium ion hydrophilic ends, C18-D-C6-D-C18 and C18-[D-C6]3-D-C18 templates, are also reported in the published literature Chem. Eur. J. 2014, 20, 11511–11521. These polyquaternary ammonium ion surfactant templates, C18-D-C6-D-C18 and C18-[D-C6]3-D-C18, contain triethylenediamine structural units, as well as a method for synthesizing ZSM-5 zeolite nanosheets (SiO2 / Al2O3 = 100) with a b-axis length of 10 nm using these templates. In these polyquaternary ammonium ion surfactant templates, "D" represents a triethylenediamine unit, each providing two quaternary ammonium ions. C6 indicates that the carbon number of the bridging hydrocarbon chain between the two triethylenediamine units is 6, and C18 indicates that the carbon number of the long tail carbon chain at the hydrophilic end of the surfactant is 18. The results show that the synthesized ZSM-5 zeolite nanosheets have extremely high catalytic activity for the acylation reaction of indole, the benzoylation reaction of resorcinol, the etherification reaction of benzoic acid and benzyl alcohol, the synthesis of Vitamin E, and the reductive amination reaction of aniline and methyl acrylate.

[0012] In addition, there are surfactants with a hydrophilic end on one side and only one quaternary ammonium ion group, a hydrophobic end on the other side and containing a biphenyl or naphthyl group capable of π-π stacking; and dumbbell-shaped surfactant templates with hydrophilic ends on both sides and two quaternary ammonium ion groups on each hydrophilic end, and a hydrophobic end in the middle and containing a biphenyl group. This type of template was reported in the public document NATURE COMMUNICATIONS|DOI:10.1038 / ncomms5262. The public document also reported a method for synthesizing ZSM-5 zeolite nanosheets (SiO2 / Al2O3=100) using this type of template. The surfactant with a hydrophilic end on one side and only one quaternary ammonium ion group, a hydrophobic end on the other side and containing a biphenyl or naphthyl group capable of π-π stacking, specifically C6H5–C6H4–O–C 10 H 20 –N + (CH3)2–C6H 13 (Br - ) (codenamed CPh–Ph-10-6) and C6H4–C4H3–O–C 10 H 20 –N + (CH3)2–C6H 13 (Br - ) (codenamed CNh-10-6); the hydrophilic end is on both sides and each hydrophilic end has two quaternary ammonium ion groups, and the hydrophobic end is in the middle and contains a biphenyl dumbbell-shaped surfactant, specifically C6H 13 –N +(CH3)2–C6H 12 –N + (CH3)2–(CH2) n –O–C6H4–C6H4–O–(CH2)n–N + (CH3)2–C6H 12 –N + (CH3)2–C6H 13 (4Br - )(codenamed BCPh-n-6-6).

[0013] From the above, it is clear that when synthesizing thin-plate ZSM-5 zeolites using a template method, particularly when using amphiphilic surfactants as templates, the ZSM-5 zeolite structure is primarily generated by the structural guidance of the hydrophilic end, while the long carbon tails of the hydrating end form a micellar layer, blocking crystal growth along the b-axis. Using these amphiphilic surfactant templates, ultrathin ZSM-5 zeolites with a b-axis length of ≤10 nm, known as nanosheets, can be synthesized. However, these amphiphilic surfactant templates are difficult to obtain, hindering their industrial application. Furthermore, ZSM-5 zeolite nanosheets with a b-axis thickness of only a few nanometers have poor structural stability, making them difficult to meet the industrial requirements for the longevity of ZSM-5 zeolite catalysts.

[0014] In contrast, the fluoride and urea methods are both very cost-effective methods for synthesizing thin-plate ZSM-5 zeolites. Furthermore, the b-axis length of thin-plate ZSM-5 zeolites synthesized using these two methods can generally be below 100 nm. With specialized techniques (for example, microwave heating or the addition of seed crystals with a particle size of only 20-30 nm), the b-axis length of thin-plate ZSM-5 zeolites synthesized using these two methods can even reach the ideal range of 30-50 nm.

[0015] The following patent documents and public documents involve a method for synthesizing thin plate type zeolite using fluoride as an auxiliary agent, wherein the zeolite is ZSM-5 or its homologous S-1 and TS-1 zeolites: Chinese invention patent CN105523569A (application date 2014.10.24), Chinese invention patent CN108275697A (application date 2018.03.13), Chinese invention patent CN116924429A (application date 2022.04.09 ... Chinese invention patent CN115490243A (application date 2022.09.27), Chinese invention patent CN117263204A (application date 2023.08.24), Chinese invention patent CN117566753A (application date 2023.10.23), Chinese invention patent CN118988388A (application date 2024.08.02), Chinese invention patent CN118976533A (application date 2024.09.11). Microporous and Mesoporous Materials 74(2004)171–178; Acta Phys.-Chim.Sin.2013,29(8),1809-1813; Chem.Mater.2014,26,4368-4376; J.Am.Chem.Soc.,Just Accepted Manuscript·DOI:10.1021 / ja5124013·Publication Date(Web):09Jan 2015;RSC Adv.,2015,5,61354;Angew.Chem.2018,130,1-6;J.Am.Chem.Soc.2021,143,1993-2004;:ACS Appl.Mater.Interfaces 2022,14,11415-11424; Chem.Mater.2022,34,3217-3226; Microporous and Mesoporous Materials 333(2022)111767; ACS Catal.2023,13,3794-3805; Materials Today Sustainability 22(2023)100364; Microporous and Mesoporous Materials 365(2024)112905; Adv.Funct.Mater.2025,2422074.

[0016] Studies have shown that fluoride ions (F -) can selectively adsorb on the ac plane (perpendicular to the b-axis) of MFI zeolites (including ZSM-5, S-1, and TS-1), thereby preventing crystal growth in the b-axis direction and achieving the purpose of assisting the synthesis of thin-plate MFI zeolites. It can be seen that the synthesis mechanism followed by the fluoride method in synthesizing thin-plate ZSM-5 zeolite is completely different from that of the template method. Generally speaking, there are usually two methods for synthesizing thin-plate zeolites using fluoride as an auxiliary agent. One is to synthesize thin-plate zeolites using fluoride as an auxiliary agent without adding seed crystals; the other is to synthesize thin-plate zeolites using fluoride as an auxiliary agent with adding seed crystals.

[0017] In short, the synthesis of thin-plate ZSM-5 zeolite using fluoride as a promoter without seeding is carried out in two stages. The first stage involves low-temperature pre-crystallization of the fluoride-free hydrogel precursor, thereby generating a large number of ZSM-5 zeolite nuclei within the hydrogel precursor. The second stage involves oriented growth of the ZSM-5 zeolite nuclei under the action of a fluoride promoter, thereby crystallizing the hydrogel into thin-plate ZSM-5 zeolite. Therefore, when synthesizing thin-plate zeolite using fluoride as a promoter without seeding, the fluoride promoter is generally added to the synthesis system after the induction period. This technical feature can be found in the following documents: Chinese invention patent CN108275697A (application date 2018.03.13), Chinese invention patent CN118976533A (application date 2024.09.11); Angew.Chem.2018,130,1-6; J.Am.Chem.Soc.2021,143,1993-2004; ACS Appl.Mater.Interfaces 2022,14,11415-11424; Microporous and Mesoporous Materials 333(2022)111767; Materials TodaySustainability 22(2023)100364; Microporous and Mesoporous Materials 365(2024)112905. The advantage of doing so is that it can avoid the inhibitory effect of the fluoride additive on the formation of crystal nuclei during the induction nucleation stage, which is conducive to the synthesis of ZSM-5 zeolite with smaller grain size and thinner b-axis direction.

[0018] Unlike the above approach, the fluoride method for synthesizing thin-plate ZSM-5 zeolite with seed crystals (typically S-1 seeds) eliminates the need for a pre-crystallization step. This technical feature is described in the following documents: Chinese Invention Patent CN117566753A (filing date: October 23, 2023), Chinese Invention Patent CN118988388A (filing date: August 2, 2024); Chem. Mater. 2022, 34, 3217-3226; ACS Catal. 2023, 13, 3794-3805; Adv. Funct. Mater. 2025, 2422074. Therefore, the synthesis process for synthesizing thin-plate ZSM-5 zeolite with seed crystals using the fluoride method is relatively simple. Not only that, this approach has a more important benefit, which is to adjust the thickness of the thin-plate ZSM-5 zeolite in the b-axis direction by changing the particle size of the seed crystal.

[0019] The following patent documents and public documents involve a method for synthesizing thin-plate zeolite using urea as an auxiliary agent, wherein the zeolite is ZSM-5 or its homologous S-1 and TS-1 zeolites: Chinese invention patent CN110857218A (application date 2018.08.23), Chinese invention patent CN112661168A (application date 2019.10.15), Chinese invention patent CN118341480A (application date 2024.04.15), and Chinese invention patent CN118719130A (application date 2024.07.24). Chem. Commun., 2011, 47, 1048–1050; ChemCatChem 2013, 5, 1517–1523; Ind. Eng. Chem. Res. 2019, 58, 12611-12622; Journal of Energy Chemistry https: / / doi.org / 10.1016 / j.jechem.2018.10.008; Natural Gas Chemical Industry-C1 Chemistry and Chemical Engineering, 2019, 44, 11-18; Journal of Industrial and Engineering Chemistry 88(2020)127–136; Acta Petrolei Sinica (Petroleum Processing), 2021, 37(2), 269-280; Catalysis Letters https: / / doi.org / 10.1007 / s10562-023-04352-9; ChemistrySelect 2023,8,e202203687; Journal of Solid State Chemistry 318(2023)123772; Journal of Energy Chemistry https: / / doi.org / 10.1016 / j.jechem.2024.08.048.

[0020] Studies have shown that urea can also act like fluoride ions (F - ) selectively adsorbs on the ac planes of MFI zeolites (including ZSM-5, S-1, and TS-1), thereby preventing crystal growth along the b-axis and assisting in the synthesis of thin-plate MFI zeolites. Therefore, the synthesis mechanism followed by the urea method for the synthesis of thin-plate ZSM-5 zeolites is completely different from that of the template method, but is essentially the same as that of the fluoride method.

[0021] Although fluoride and urea have similar mechanisms of action in assisting the synthesis of thin-plate ZSM-5 zeolite, there are significant differences in the magnitude and strength of their auxiliary effects. In general, using fluoride as an auxiliary agent makes it easier to obtain thin-plate ZSM-5 zeolite with a thinner b-axis direction. For industrial applications, the problem with fluoride auxiliary agents is that they are highly corrosive to equipment and polluting to the environment. In particular, the harmless treatment of fluoride-containing wastewater is very difficult. The problem with urea auxiliary agents is that although they are not seriously corrosive to equipment and polluting to the environment, urea is prone to chemical changes when heated to temperatures above its melting point (131-135°C), generating byproducts such as biuret, ammonia, and cyanuric acid. These urea derivatives have a very adverse impact on equipment, products, safety, and the environment. Summary of the Invention

[0022] The invention provides a dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite.

[0023] Specifically, the present invention provides a dry-gel hydrothermal method for synthesizing thin-plate ZSM-5 zeolite in one step using urea as an auxiliary agent without adding seed crystals. The method provided by the present invention is essentially different from the known urea method.

[0024] We have found through research that if thin plate-type ZSM-5 zeolite is synthesized according to the known urea process, that is, according to the urea process reported in the patent documents and the open literature, thin plate-type ZSM-5 zeolite is synthesized. The urea process reported in the patent documents and the open literature includes the urea process for synthesizing thin plate-type ZSM-5 zeolite, S-1 zeolite and TS-1 zeolite. The common feature of the urea process is that the crystallization process of the thin plate-type zeolite is carried out at a high temperature of 150°C-180°C. However, when urea is used as an auxiliary agent to assist in the synthesis of thin plate-type zeolite at a temperature higher than the urea melting point (131-135°C), due to the combined effects of high temperature and strongly alkaline hydrothermal environment, a part of the urea auxiliary agent undergoes complex chemical changes during the zeolite synthesis process. In addition to generating biuret, ammonia and cyanuric acid, it also produces highly toxic and highly corrosive cyanic acid. Moreover, there is a chemical equilibrium relationship between biuret, cyanuric acid and cyanic acid, and they can generate each other. Due to the high volatility of cyanic acid and the easy crystallization (melting point 360°C) and sublimation characteristics of cyanic acid, cyanic acid will be enriched in the gas phase space of the hydrothermal synthesis reactor. On the one hand, it will cause serious corrosion of the stainless steel reactor wall and pipeline surface in contact with the gas phase. On the other hand, cyanuric acid crystals will be produced in the pipelines at the feeding port and pressure relief port, leading to production accidents such as pipeline blockage, pressure relief system failure, product contamination, and even production safety accidents.

[0025] We have further discovered through research that if thin plate-type ZSM-5 zeolite is synthesized according to the known urea method, that is, according to the urea method reported in patent documents and public documents, the urea method reported in the patent documents and public documents, including the urea method for synthesizing thin plate-type ZSM-5 zeolite, S-1 zeolite and TS-1 zeolite, has in common that the thin plate-type zeolite is obtained by direct crystallization of a hydrogel containing urea. In other words, the synthesis process of thin plate-type zeolite is a conventional hydrogel crystallization process (conventional hydrothermal method). Since in the hydrogel, the alkaline mineralizer (OH - Active substances such as ions, organic templates, and urea additives are all dissolved in a large amount of water at low concentrations, resulting in only a weak activation effect on the amorphous aluminosilicate framework in the hydrogel. This results in low reactivity of the amorphous aluminosilicate framework in the hydrogel, and its hydrothermal crystallization process (including the nucleation induction stage and crystal growth stage) can only proceed at higher temperatures. This is the fundamental reason why the urea method reported in the literature for synthesizing thin-plate ZSM-5 zeolite, and even the urea method for synthesizing S-1 and TS-1 zeolites, all conduct crystallization at high temperatures of 150°C-180°C, that is, temperatures above the melting point of urea (131-135°C). Our experience has shown that the known urea method for directly crystallizing hydrogels containing urea, when carried out in conventional industrial crystallization reactors for the purpose of synthesizing thin-plate ZSM-5 zeolite, cannot avoid the use of high-temperature crystallization conditions, and thus the problems caused by chemical degradation of urea at high temperatures. The problem is not only that some urea loses its additive function due to deterioration, but more importantly, some urea produces chemical substances such as cyanic acid and cyanuric acid due to deterioration, which have very adverse effects on equipment, products, safety and the environment.

[0026] After research, we were pleasantly surprised to find that if the urea-containing hydrogel is first subjected to low-temperature drying to remove the free water therein and converted into a dry gel intermediate containing urea, and then the dry gel intermediate containing urea is subjected to hydrothermal crystallization, the hydrothermal crystallization process of the dry gel intermediate containing urea can be carried out at a temperature below the melting point of urea (131-135°C). On the one hand, high-quality thin-plate ZSM-5 zeolite is obtained, while at the same time, urea deterioration and the various problems it brings are avoided. The reason why this new urea method for synthesizing thin-plate ZSM-5 zeolite based on dry gel intermediates can be carried out at a temperature below the melting point of urea (131-135°C), thereby avoiding urea deterioration and the various problems it brings, is mainly due to two aspects: on the one hand, in the dry gel intermediate, the alkaline mineralizer (OH -Active substances such as ions, organic templates and urea additives are highly concentrated in the rich irregular pores provided by the amorphous skeleton of aluminosilicate. The irregularity of the pores refers to the randomness of the pore shape and size. The irregular pores refer to some pores that are open and connected to the outside world, while other pores are closed and not connected to the outside world. During the hydrothermal crystallization of dry glue, the alkaline mineralizer (OH) present in the large number of irregular pores of amorphous aluminosilicate - ions), organic templates and urea additives and other active substances can promote the transformation of amorphous aluminosilicate skeleton into thin plate-type zeolite crystals (crystallization reaction) at a relatively low temperature by virtue of the chemical kinetic advantages generated by their high concentration; on the other hand, the process of low-temperature drying the hydrogel containing urea and preparing a dry gel intermediate is actually a process of low-temperature activation of the amorphous aluminosilicate skeleton. Moreover, the activation effect produced by this activation process is better than the activation effect that can be produced by low-temperature aging of the hydrogel in a closed system as is customary in the art. People familiar with the art know that low-temperature aging of hydrogels is a common practice for synthesizing various small crystals and nano-zeolite molecular sieves, and is mainly used to generate a large number of zeolite nuclei. Therefore, it is not difficult for people familiar with the art to understand that if the thin plate-type ZSM-5 zeolite is synthesized by the known urea method and the hydrogel containing urea is subjected to low-temperature aging pretreatment, then due to the alkaline mineralizer (OH) contained in the hydrogel - ions), organic templates, and urea additives are all diluted in a large amount of water, and the concentration of the active substances does not increase during the low-temperature aging process. Therefore, the active substances contained in the hydrogel can only activate the amorphous aluminosilicate skeleton in the hydrogel to a limited extent during the low-temperature aging process. In contrast, in the present invention, during the low-temperature drying treatment of the hydrogel containing urea and the removal of its free water to prepare the dry gel intermediate, the alkaline mineralizer (OH - ions), organic templates and urea additives, etc., the concentration of active substances increases with the reduction of free water, so the activation effect of these active substances on the amorphous skeleton of aluminosilicate is also increasing. When the hydrogel precursor is made into a dry gel intermediate, the alkaline mineralizer (OH - ions), organic templates and urea additives and other active substances also reached a relatively high level, and their activation of the amorphous skeleton of the aluminosilicate in the dry glue intermediate also reached the highest level simultaneously.

[0027] The present invention provides a dry gel hydrothermal method for synthesizing thin-plate ZSM-5 zeolite. Its technical features can be summarized in the following three steps: first, using fumed silica as a main silicon source, sodium silicate as an auxiliary silicon source, and pseudo-boehmite as an aluminum source to prepare an aluminosilicate hydrogel containing urea as a precursor for synthesizing thin-plate ZSM-5 zeolite; second, subjecting the aluminosilicate hydrogel precursor containing urea to low-temperature drying to remove free water therein, thereby converting it into an aluminosilicate dry gel intermediate containing urea, which is used as a direct raw material for synthesizing thin-plate ZSM-5 zeolite; and third, subjecting the dry gel intermediate containing urea to hydrothermal crystallization in an autoclave under hydrothermal conditions below the melting point of urea to convert it into thin-plate ZSM-5 zeolite.

[0028] The technical solutions of the present invention are as follows:

[0029] A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite, comprising the following steps:

[0030] Step 1: Preparation of aluminosilicate hydrogel precursor containing urea

[0031] In addition to urea, fumed silica (primary silicon source), sodium silicate (auxiliary silicon source), pseudoboehmite (aluminum source) and deionized water, tetrapropylammonium hydroxide (TPAOH) template is also required when preparing the urea-containing aluminosilicate hydrogel precursor.

[0032] The present invention requires that when preparing the aluminosilicate hydrogel precursor containing urea, the amounts of urea, tetrapropylammonium hydroxide (TPAOH) template, main silicon source, auxiliary silicon source, aluminum source and deionized water must meet the following conditions:

[0033] Urea-silicon ratio (calculated as the molar ratio of urea to SiO2): suitable range is 0.4-1.5; preferred range is 0.6-1.2; more preferred range is 0.8-1.0.

[0034] Mold-to-silicon ratio (calculated as the molar ratio of TPAOH to SiO2): suitable range is 0.10-0.36; preferred range is 0.12-0.30; more preferred range is 0.15-0.25.

[0035] Silicon-aluminum ratio (calculated as the molar ratio of SiO2 to Al2O3): suitable range is 50-500; preferred range is 60-450; more preferred range is 80-400.

[0036] The dosage of sodium silicate (calculated as the molar ratio of Na2O to SiO2) is in the range of 0.01-0.12, preferably 0.02-0.10, and more preferably 0.03-0.09.

[0037] Water-silicon ratio (calculated as the molar ratio of H2O to SiO2): suitable range is 12-50; preferred range is 15-40; more preferred range is 20-30.

[0038] The preparation of a urea-containing hydrogel precursor can be performed at room temperature. The basic steps are as follows: First, determine the amounts of sodium silicate, urea, TPAOH template solution, pseudo-boehmite, and deionized water based on the total amount of silica and the selected molar ratios of Na2O to SiO2, urea to SiO2, TPAOH to SiO2, SiO2 to Al2O3, and H2O to SiO2. Then, under stirring, prepare an aluminum source solution using the measured amounts of deionized water, TPAOH template, and pseudo-boehmite, and a secondary silicon source solution using the measured amounts of deionized water and sodium silicate. Next, add the secondary silicon source solution and the measured amounts of primary silicon source, fumed silica, to the aluminum source solution while stirring, and continue stirring until the hydrogel is uniform. Finally, add urea to the uniform hydrogel while stirring, and continue stirring for a period of time to obtain a urea-containing aluminosilicate hydrogel precursor.

[0039] The stirring time after adding the urea additive to the hydrogel is suitably in the range of 0.5-12 hours, preferably in the range of 1-6 hours, and more preferably in the range of 2-4 hours.

[0040] It should be further explained that the present invention does not utilize silicon sources such as tetraethyl orthosilicate, silica sol, or precipitated silica when preparing the urea-containing hydrogel precursor. Instead, it utilizes a dual-silicon source strategy, using fumed silica as the primary silicon source and sodium silicate as a supplementary silicon source. This is primarily due to the following reasons: Using tetraethyl orthosilicate as a silicon source is not only expensive but also requires alcohol removal during use, generating significant amounts of hazardous industrial waste. While other silicon sources, such as silica sol and precipitated silica, do not suffer from the same issues as tetraethyl orthosilicate, they generally suffer from high polymerization degrees and low reactivity, making them unsuitable for use under low-temperature crystallization conditions. In contrast, fumed silica and sodium silicate offer the advantages of both relative affordability and high reactivity. The reason for using a dual-silicon source strategy, using fumed silica as the primary silicon source and sodium silicate as a supplementary silicon source, rather than using either fumed silica or sodium silicate alone, is that sodium silicate can provide both highly reactive monomeric silicate and sodium ions. Sodium ions are essential countercations for balancing the negative charge of the framework aluminum in thin-plate ZSM-5 zeolite. However, using sodium silicate alone as the silicon source would hinder the synthesis of single crystal thin-plate ZSM-5 zeolite due to the excessive sodium ion content relative to the framework aluminum. Similarly, the use of pseudo-boehmite as the aluminum source in the present invention, rather than sodium metaaluminate, aluminum isopropoxide, aluminum nitrate, or aluminum sulfate, is based on similar considerations.

[0041] Step 2: Low-temperature drying of the aluminosilicate hydrogel precursor containing urea to remove free water and convert it into a dry gel intermediate containing urea.

[0042] The low-temperature drying treatment described in the present invention generally refers to various applicable low-temperature drying methods that can remove most of the free water in a hydrogel precursor containing urea and a tetrapropylammonium hydroxide template by heating and evaporating the water at a certain temperature without decomposing, converting or removing the urea and TPAOH template contained therein.

[0043] To provide a more detailed understanding of the low-temperature drying process and dry gel intermediate described herein, the present invention provides a typical laboratory method for preparing a small amount of a dry gel intermediate containing urea. The urea-containing hydrogel precursor is placed in a conventional electric oven at 80°C and dried to a constant weight to obtain a dry gel intermediate meeting the requirements of the present invention. Analytical data indicate that drying and dehydrating the hydrogel precursor in an 80°C conventional electric oven does not decompose, transform, or remove the urea and TPAOH template contained therein. Even after drying to a constant weight at 80°C, the dry gel intermediate still contains approximately 25 wt.% water, primarily bound water, with minimal free water. Thermogravimetric (TG) analysis revealed that the weight loss caused by the removal of free water from the hydrogel precursor occurred in the low-temperature zone of 30-130°C; the weight loss caused by the removal of bound water (bound by pores and hydroxyl pits) from the dry gel intermediate occurred in the medium-temperature zone of 130-166°C, the weight loss caused by the decomposition and removal of the TPAOH template occurred in the high-temperature zone above 166°C, and the destruction of urea occurred in the medium-temperature and high-temperature zones above 130°C.

[0044] Therefore, the low-temperature drying treatment of the present invention is preferably performed at a temperature between room temperature (30°C) and 130°C, preferably between 50°C and 120°C, and more preferably between 80°C and 110°C.

[0045] The low-temperature drying step is a crucial step for the present invention. On the surface, this step appears to simply remove free water from the urea-containing hydrogel precursor, converting it into a dry gel intermediate containing urea. However, its fundamental function is to maximize the activation of the aluminosilicate amorphous skeleton in the hydrogel precursor. Temperature is a key control parameter in the drying step of the urea-containing hydrogel precursor. At a selected temperature, by adjusting the pressure and time parameters, a dry gel intermediate meeting the requirements of the present invention can be produced. Research has found that if the low-temperature drying process can meet the present invention's requirements for the appearance of the dry gel intermediate, it also meets the present invention's essential requirement for the dry gel intermediate—maximum activation of its aluminosilicate amorphous skeleton. Therefore, engineers skilled in the art can predict whether a dry gel intermediate prepared by low-temperature drying meets the requirements of the present invention based on the following two aspects: first, negligible changes in the content, structure, and physicochemical properties of the urea and TPAOH template; and second, the removal of the vast majority of the free water in the hydrogel. In order to enable engineers in this field to quickly predict whether the prepared dry glue intermediate meets the requirements of the present invention in actual work, the present invention provides the following judgment method: (1) Judging whether the first characteristic is met by the appearance color of the dry glue intermediate. If the color of the dry glue intermediate is white, it means that urea and TPAOH have not decomposed and structurally changed during the drying process, and the first characteristic is met. If the color of the dry glue intermediate is light yellow or egg yolk, it means that urea and TPAOH templates have decomposed and structurally changed during the drying process, and the first characteristic is not met; (2) Take a sample of the dry glue intermediate for thermogravimetric analysis. If the dehydration weight loss rate of the sample in the low temperature range of 30-130°C is ≤10%, it indicates that the degree of dehydration during low temperature drying is appropriate. If the dehydration weight loss rate of the sample in the low temperature range of 30-130°C is >10%, it indicates that the degree of dehydration during low temperature drying is insufficient. If the dry glue intermediate that meets the requirements of the present invention is severely agglomerated, it needs to be post-processed by conventional grinding or pulverization methods and sealed for later use.

[0046] Engineers familiar with the art can flexibly select a specific low-temperature drying treatment method based on the preparation scale of the dry gel intermediate and the convenient conditions of the experiment and production site. For example, when preparing the dry gel intermediate on a laboratory scale, a common electric oven can be conveniently selected as a low-temperature drying device to carry out low-temperature drying of the hydrogel precursor. When preparing the dry gel intermediate on an industrial scale, a box furnace can be used as a low-temperature drying device, and a tunnel kiln can also be conveniently used as a low-temperature drying device. The drying process can be carried out in a flowing air atmosphere, in a static air atmosphere, or under conditions of a slight negative pressure. The present invention only limits the temperature range of the low-temperature drying process of the hydrogel precursor and the indicators of the dry gel intermediate. Other aspects, including parameters such as the drying equipment and drying pressure, atmosphere and time, are all selected by engineers familiar with the art as needed.

[0047] Step 3: Hydrothermal crystallization of the aluminosilicate dry colloid intermediate containing urea to convert it into thin plate-type ZSM-5 zeolite

[0048] The hydrothermal crystallization treatment of the dry glue intermediate is carried out in an ordinary autoclave. The general practice is to use deionized water as the crystallization liquid, and put the crystallization liquid and the dry glue intermediate into the autoclave according to a certain mass ratio, so that the dry glue intermediate is immersed in the crystallization liquid. Then, the dry glue hydrothermal system is subjected to a hydrothermal crystallization treatment at a certain temperature. The hydrothermal crystallization can be carried out in a dynamic manner under stirring, or in a static manner without stirring. The key to this step is to carry out the hydrothermal crystallization of the dry glue intermediate containing urea at a temperature not higher than 130°C, that is, below the melting point of urea, so that the chemical deterioration of urea itself and the various problems associated with it can be avoided in the process of synthesizing thin-plate type ZSM-5 zeolite. Therefore, the present invention requires that the hydrothermal crystallization conditions of the aluminosilicate dry glue intermediate containing urea are as follows:

[0049] The mass ratio of the crystallization liquid to the dry glue (liquid-solid ratio ml / g): the suitable range is: 1-10; the preferred range is: 1.5-8; the more preferred range is: 2-6.

[0050] Hydrothermal crystallization temperature: suitable range is: 80-130℃; preferred range is: 90-125℃; more preferred range is: 100-120℃.

[0051] Hydrothermal crystallization time: suitable range is: 12h-240h; preferred range is: 18h-120h; more preferred range is: 24h-96h.

[0052] After the hydrothermal crystallization treatment is completed, the autoclave is cooled and depressurized according to conventional hydrothermal synthesis of zeolite molecular sieves. When the temperature and pressure inside the autoclave approach the ambient temperature and pressure, the autoclave is opened and the thin plate-shaped ZSM-5 zeolite product is removed.

[0053] Step 4: Post-treatment of thin plate ZSM-5 zeolite product

[0054] The post-treatment of the thin-plate ZSM-5 zeolite prepared by the method of the present invention includes filtration, deionized water washing, drying and calcination steps. Among them, the deionized water washing treatment should be washed until the pH value of the washing liquid is close to neutral. The main purpose of the drying treatment is to remove free moisture in the zeolite product. The main purpose of the calcination treatment is to remove organic templates and other organic substances, including urea and its derivatives, in the zeolite pores. Due to the high thermal stability and hydrothermal stability of ZSM-5 zeolite, the present invention does not specifically limit the drying temperature and time, as well as the calcination temperature and time. Engineers familiar with this field can perform the various post-treatment operations described according to common sense.

[0055] Beneficial effects of the present invention:

[0056] The present invention provides a dry gel hydrothermal method for synthesizing thin-plate ZSM-5 zeolite using urea as an auxiliary agent. The main technical innovations of the present invention are: (1) by means of low-temperature drying and dehydration treatment, a hydrogel precursor containing urea is first converted into a dry gel intermediate containing urea, which is then used to synthesize thin-plate ZSM-5 zeolite; (2) when preparing the hydrogel precursor containing urea, fumed silica is used as the main silicon source and sodium silicate is used as the auxiliary silicon source. The combination of (1) and (2) maximizes the reactivity of the amorphous skeleton of the aluminosilicate in the dry gel intermediate, thereby enabling the dry gel hydrothermal crystallization process for synthesizing thin-plate ZSM-5 zeolite to be carried out at a temperature not higher than 130°C, avoiding the problem caused by urea deterioration when synthesizing thin-plate ZSM-5 zeolite at high temperature using the known urea method, and is conducive to industrial application. Specifically, the method provided by the present invention avoids the problem of the conventional urea method, which suffers from the loss of the additive's effectiveness due to the deterioration of some urea. In particular, the problem of the deterioration of some urea generating derivatives, which can have significant adverse effects on equipment, products, safety, and the environment. Furthermore, compared with conventional methods, the dry-gel hydrothermal synthesis method provided by the present invention easily yields thin-plate ZSM-5 zeolite with a thinner b-axis. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is the XRD pattern of the thin plate type ZSM-5 zeolite sample (sb-ZSM-5-1) prepared in Example 1;

[0058] Figure 2 is a SEM photograph of the thin plate type ZSM-5 zeolite sample (sb-ZSM-5-1) prepared in Example 1;

[0059] Figure 3 is a SEM photograph of the thin plate type ZSM-5 zeolite sample (sb-ZSM-5-1-COM) prepared in Comparative Example 1;

[0060] Figure 4 is an SEM photograph of the thin plate-type ZSM-5 zeolite sample (crystallized at 120° C. for 72 h) prepared in Comparative Example 2;

[0061] Figure 5 is an SEM photograph of the thin plate-type ZSM-5 zeolite sample (crystallized at 120° C. for 96 h) prepared in Comparative Example 2;

[0062] Figure 6 3 is a SEM photograph of the thin plate-type ZSM-5 zeolite sample prepared in Comparative Example 2 (crystallized at 120° C. for 120 h). DETAILED DESCRIPTION

[0063] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0064] The effects of the present invention are mainly demonstrated by sampling the synthesized ZSM-5 zeolite product, analyzing its silicon-aluminum ratio composition, characterizing its crystallinity, observing its crystal morphology and measuring its b-axis thickness.

[0065] Among them, X-ray fluorescence spectroscopy (XRF) is used to analyze the silicon-aluminum ratio composition; X-ray polycrystalline powder diffraction (XRD) method is used to characterize its degree of crystallinity; and scanning electron microscopy (SEM) method is used to observe its crystal morphology and measure its b-axis thickness.

[0066] In addition, the thermogravimetric (TG) method can be used to analyze and characterize the effects of removing free water from the urea-containing hydrogel precursor through low-temperature drying and preparing the urea-containing dry gel intermediate.

[0067] The present invention will be further described below by way of examples, but the present invention is not limited by these examples.

[0068] Example 1: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as a supplementary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying to convert it into a urea-containing dry gel intermediate. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. The removal of free water from the urea-containing hydrogel precursor by low-temperature drying to form the urea-containing dry gel intermediate, as well as the use of fumed silica as the primary silicon source and sodium silicate as a supplementary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate framework in the dry gel intermediate, thereby enabling the crystallization of the dry gel intermediate to proceed at a temperature not exceeding 130°C. In addition, the present invention uses urea as an auxiliary agent to synthesize thin-plate ZSM-5 zeolite in a dry gel hydrothermal system, which is easier to reduce the b-axis thickness than the known urea method of directly synthesizing thin-plate ZSM-5 zeolite using hydrogel.

[0069] The first step is to prepare aluminosilicate hydrogel precursor containing urea

[0070] First, the amount of each raw material is calculated based on the total amount of silica being 12 g, the molar ratio of urine / SiO2 being 0.8, the molar ratio of TPAOH / SiO2 being 0.24, the molar ratio of SiO2 / Al2O3 being 100, the amount of sodium silicate auxiliary silicon source (calculated as Na2O / SiO2) being 0.03, and the molar ratio of H2O / SiO2 being 25.

[0071] Next, 39.04 g of a 25 wt.% tetrapropylammonium hydroxide (TPAOH) template solution was mixed with 40.06 g of deionized water under stirring to obtain a template solution. Next, 0.316 g of pseudoboehmite (64 wt.% Al2O3) was carefully added to the template solution and stirred for 2 hours to obtain a transparent aluminum source solution. 1.28 g of Na2SiO3·5H2O was dissolved in 20 g of deionized water to obtain an auxiliary silicon source solution. Next, the auxiliary silicon source solution and 11.66 g of a primary silicon source, fumed silica (100 wt.% dry matter), were added sequentially to the aluminum source solution under vigorous stirring and stirred for another 2 hours to produce a uniform aluminosilicate hydrogel. Finally, 9.6 g of urea was added to the aluminosilicate hydrogel under vigorous stirring and stirred for another 2 hours to uniformly disperse the urea throughout the hydrogel, thereby producing a urea-containing aluminosilicate hydrogel precursor.

[0072] The second step is to perform low-temperature drying on the hydrogel precursor containing urea to remove the free water and convert it into a dry gel intermediate containing urea.

[0073] 100g of the hydrogel precursor containing urea prepared in the first step was added to a crucible, and then the crucible was placed in an ordinary electric oven at 80°C and slowly dried and dehydrated under normal pressure and still air. After 12 hours, a constant weight state was reached, and about 35g of a dry gel intermediate containing urea was obtained. The dry gel intermediate was subjected to a weight loss analysis using a thermogravimetric analyzer (TG). It was found that the dry gel intermediate had a weight loss rate of about 8% caused by removing its free water on the thermogravimetric analyzer, which occurred in the low temperature range of 30-130°C. The dry gel intermediate was white in color and soft in texture. The changes in the content, structure, and physicochemical properties of the organic template TPAOH and urea therein were all negligible. After being gently ground into a powder with a mortar, it met the requirements of the present invention and was sealed for standby use.

[0074] Step 3: Hydrothermal crystallization of the aluminosilicate dry colloid intermediate containing urea to convert it into thin plate-type ZSM-5 zeolite

[0075] The hydrothermal crystallization process for synthesizing thin-plate ZSM-5 zeolite using a urea-containing dry gel intermediate was carried out in a small, 100ml laboratory-scale, high-pressure hydrothermal crystallization reactor lined with polytetrafluoroethylene. Specifically, 20g of the urea-containing dry gel intermediate powder was added to the crystallization reactor. Then, 60g of deionized water was added to the reactor at a liquid-to-solid ratio of 3, immersing the dry gel intermediate in the crystallization solution. The reactor was then sealed and placed in a conventional electric oven, heated to a constant temperature of 120°C, allowing the urea-containing dry gel intermediate to undergo hydrothermal crystallization in a static manner for 72 hours.

[0076] After the crystallization is completed, the autoclave is cooled according to the conventional hydrothermal method for synthesizing zeolites. When the temperature inside the autoclave approaches the ambient temperature, the autoclave is opened and the synthesized product is taken out.

[0077] Step 4: Post-treatment of thin plate ZSM-5 zeolite product

[0078] The synthesized product obtained in the third step was first filtered to remove the crystallization liquid and collect the solid product. The solid product was then repeatedly filtered and washed with deionized water until the pH of the filtrate approached 8. The washed solid product was then dried (at 110°C overnight) and calcined (at 540°C for 6 hours) to obtain a thin-plate ZSM-5 zeolite product, code-named sb-ZSM-5-1.

[0079] The silicon-aluminum ratio (SiO2 / Al2O3) of the sb-ZSM-5-1 sample was measured by XRF method and was about 96. The XRD pattern of the sb-ZSM-5-1 sample is shown in Figure 1See Appendix 2 for SEM photos. Appendix 1 shows that the sb-ZSM-5-1 sample is a well-crystallized ZSM-5 zeolite with no impurities. Appendix 2 shows that the sb-ZSM-5-1 sample is a thin-plate ZSM-5 zeolite with a grain size of approximately 340nm × 140nm × 50nm and a b-axis thickness of approximately 50nm.

[0080] Example 2: This example further illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as the secondary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying, converting it into a dry gel intermediate containing urea. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. Among them, the method of removing free water from the urea-containing hydrogel precursor by low-temperature drying to prepare it into a urea-containing dry gel intermediate, and the use of fumed silica as the main silicon source and sodium silicate as the auxiliary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate skeleton in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C, avoiding the problem caused by urea deterioration when synthesizing thin-plate ZSM-5 zeolite at high temperatures using the known urea method, thereby making the present invention more conducive to industrial application. In addition, the present invention uses urea as an auxiliary agent to synthesize thin-plate ZSM-5 zeolite in a dry gel hydrothermal system, which makes it easier to reduce the b-axis thickness than the known urea method directly using hydrogel to synthesize thin-plate ZSM-5 zeolite.

[0081] Example 1 was repeated, but in the first step, the aluminosilicate hydrogel precursor containing urea was prepared using a scaled-up, raw material ratio of 12 kg of silica, a urea / SiO2 molar ratio of 0.8, a TPAOH / SiO2 molar ratio of 0.24, a SiO2 / Al2O3 molar ratio of 100, a sodium silicate auxiliary silicon source amount (calculated as Na2O / SiO2) of 0.03, and a H2O / SiO2 molar ratio of 25. Specifically, 39.04 kg of a 25 wt.% tetrapropylammonium hydroxide (TPAOH) template solution was mixed with 40.06 kg of deionized water under stirring to obtain a template solution. Then, 0.316 kg of pseudoboehmite (64 wt.% Al2O3) was carefully added to the template solution, and stirring was continued for 2 hours to obtain a transparent aluminum source solution. 1.28 kg of Na₂SiO₃·5H₂O was dissolved in 20 kg of deionized water to obtain an auxiliary silicon source solution. Next, the auxiliary silicon source solution and 11.66 kg of primary silicon source fumed silica (100 wt.% dry basis) were sequentially added to the aluminum source solution under vigorous stirring, and stirring was continued for 2 hours to produce a uniform aluminosilicate hydrogel. Finally, 9.6 kg of urea was added to the aluminosilicate hydrogel under vigorous stirring, and stirring was continued for 2 hours to uniformly disperse the urea in the hydrogel, thereby producing approximately 122 kg of aluminosilicate hydrogel precursor containing urea.

[0082] In the second step, the hydrogel precursor containing urea is subjected to a low-temperature drying process to remove the free water therein and convert it into a dry gel intermediate containing urea. 100 kg of the hydrogel precursor containing urea prepared in the first step is taken and dispersed into 10 stainless steel square trays. The trays are then placed in an industrial box furnace, heated to 80°C, and slowly dried and dehydrated under normal pressure and flowing air. After 10 hours, a constant weight state is reached, and about 33 kg of dry gel intermediate containing urea is harvested. The dry gel intermediate is subjected to a weight loss analysis using a thermogravimetric analyzer (TG). It is found that the dry gel intermediate has a weight loss rate of about 9% caused by the removal of its free water on the thermogravimetric analyzer, and the weight loss occurs in the low temperature range of 30-130°C. The dry gel intermediate is white in color and soft in texture. The changes in the content, structure, and physicochemical properties of the organic template TPAOH and urea therein are all negligible. After being gently ground into a powder with a mortar, it meets the requirements of the present invention and is sealed for standby use.

[0083] In the third step, the hydrothermal crystallization of the urea-containing aluminosilicate dry gel intermediate was performed using a 100L, unlined, stainless steel, standard industrial high-pressure hydrothermal crystallization kettle. Specifically, 17.5kg of urea-containing dry gel intermediate powder was added to the crystallization kettle. Then, 52.5kg of deionized water was added to the kettle at a liquid-to-solid ratio of 3, submerging the dry gel intermediate in the crystallization solution. The kettle was then heated to a constant temperature of 120°C, where the urea-containing dry gel intermediate was dynamically crystallized under continuous stirring for 72 hours.

[0084] After crystallization, the autoclave was cooled, following the conventional hydrothermal zeolite synthesis process. Once the temperature inside the autoclave fell below 100°C, the pressure in the crystallization reactor was released. Finally, the product was expelled using compressed gas. The decompression and discharge process went smoothly, resulting in a white slurry.

[0085] In the fourth step, the thin-plate ZSM-5 zeolite product is post-processed. First, the synthetic product slurry obtained in the third step is centrifuged to remove the crystallization liquid and collect the white solid product. The solid product is then repeatedly centrifuged and washed with deionized water until the pH value of the filtrate approaches 8. Next, the washed solid product is dried (at 110°C overnight) and calcined (at 540°C for 6 hours) to obtain the thin-plate ZSM-5 zeolite product, code-named sb-ZSM-5-2.

[0086] XRF analysis revealed a silicon-to-aluminum ratio (SiO₂ / Al₂O₃) of approximately 95 for the sb-ZSM-5-2 sample, with an iron impurity content of 0.03 wt%. Scanning electron microscopy (SEM) confirmed that the sb-ZSM-5-2 sample was also a thin-plate ZSM-5 zeolite, with a grain size of approximately 340 nm × 140 nm × 52 nm and a b-axis thickness of 52 nm.

[0087] It should be noted that in this case, since the urea-containing dry gel intermediate was hydrothermally crystallized at 120°C, which is below the melting point of urea (131-135°C), the crystallization process did not cause urea deterioration or the generation of harmful derivatives such as cyanic acid (a strong acid that causes equipment corrosion) and cyanuric acid (crystallized products that clog pipes). Therefore, despite the lack of a polytetrafluoroethylene (PTFE) lining in the 100L industrial crystallization reactor used, no equipment corrosion occurred, and the synthesized product slurry was white. Furthermore, the pressure gauge in the crystallization reactor operated normally during the hydrothermal synthesis process. The flow of steam and slurry was continuous and smooth during both pressure relief and compressed air discharge, indicating that no cyanuric acid crystals were attached to the pipe openings exposed to the upper gas phase within the synthesis reactor.

[0088] Comparative Example 1: This example illustrates the synthesis of thin-plate ZSM-5 zeolite using the known urea process, i.e., the urea process reported in patents and published literature. These urea processes, including those used to synthesize thin-plate ZSM-5 zeolite, S-1 zeolite, and TS-1 zeolite, share a common characteristic: the thin-plate zeolite is obtained by direct crystallization of a urea-containing hydrogel. Due to the low reactivity of the amorphous aluminosilicate framework in the hydrogel, the crystallization of the thin-plate zeolite must be performed at temperatures above the melting point of urea (131-135°C), specifically, at temperatures between 150°C and 180°C. As a result, the resulting thin-plate ZSM-5 zeolite has larger crystals and a thicker b-axis. Furthermore, at high temperatures, the urea additive readily deteriorates, generating highly toxic, volatile, and corrosive cyanic acid and easily sublimated and crystallized cyanuric acid (melting point 360°C). Among them, cyanic acid will be enriched in the gas phase space of the hydrothermal synthesis reactor. On the one hand, it will cause acidic corrosion of the metal reactor wall and the oxide film on the surface of the pipeline exposed to the gas phase space. On the other hand, cyanuric acid crystals will be produced at the pipe openings of the feeding port and the pressure relief port, causing various hazards.

[0089] Example 2 was repeated, but after producing approximately 120 kg of aluminosilicate hydrogel precursor containing urea in the first step, the low-temperature drying and dehydration process in the second step was omitted, and the third step was performed directly. Specifically, 70 kg of the hydrogel precursor containing urea was subjected to hydrothermal crystallization. Hydrothermal crystallization was carried out in a 100 L, unlined, stainless steel, industrial, high-pressure hydrothermal crystallization reactor. The crystallization temperature was 170°C and the crystallization time was 24 hours. Continuous stirring was maintained during the crystallization process. After crystallization, the autoclave was first cooled, as is conventional practice for zeolite synthesis by the hydrothermal method. Once the temperature within the autoclave fell below 100°C, the pressure in the crystallization reactor was released. Finally, the synthesized product slurry was extruded using compressed gas.

[0090] The results show that the ZSM-5 zeolite synthesized according to the known urea method (codenamed sb-ZSM-5-1-COM, COM stands for "comparative example"), its SEM photo is shown in Figure 3 ), with a grain size of approximately 3 μm × 780 nm × 170 nm and a b-axis thickness of 170 nm, also a thin-plate ZSM-5 zeolite. Because the crystallization process is carried out at high temperature, it takes a relatively short time.

[0091] However, compared with the thin plate type ZSM-5 zeolite sample (sb-ZSM-5-1) synthesized by the method of the present invention, it can be seen that under the condition of using the same materials, the thin plate type ZSM-5 zeolite sample synthesized by the known urea method in this example has a larger grain size and a thicker b axis.

[0092] More importantly, in this example, the crystallization process of the thin-plate ZSM-5 zeolite synthesized according to the known urea method needs to be carried out at a temperature higher than the melting point of urea (131-135°C), specifically at a high temperature of 150-180°C, resulting in the deterioration of the urea additive. The problems caused by this include: (1) the synthesized thin-plate ZSM-5 zeolite sample is yellow in color. XRF analysis shows that the sample has been contaminated by iron oxide impurities, and the iron content in the sample is as high as 0.21wt.%, which is much higher than the normal value (see Example 2); (2) the pressure gauge reading is distorted during the synthesis process; (3) the pressure relief line is blocked at the end of the synthesis, making it difficult to release the pressure and exhaust; (4) when compressed air is added to the reactor to press out the synthetic product slurry, it is found that the compressed air line is blocked, making it difficult to discharge the synthetic product slurry. After the reactor was opened for inspection, it was found that all these problems were caused by the deterioration of urea. Specifically, during the high-temperature synthesis of thin-plate ZSM-5 zeolite using a urea-containing hydrogel, a portion of the urea undergoes a chemical reaction, producing products including highly toxic, volatile, and corrosive cyanic acid and easily sublimated and crystallized cyanuric acid. Cyanic acid is highly volatile and easily accumulates in the vapor phase of the hydrothermal synthesis reactor, causing acidic corrosion of the oxide film on the metal reactor walls and pipe surfaces exposed to the vapor phase, "polishing" the metal surfaces and allowing the generated soluble iron salts to flow back into the synthesis product slurry. This is the cause of zeolite product contamination. Furthermore, the trimerization of cyanic acid to form cyanuric acid and its crystallization at the orifices of various pipelines can cause blockages at pressure gauge inlets, compressed air outlets, and pressure relief line inlets, preventing normal operation.

[0093] Comparative Example 2: This example illustrates the synthesis of thin-plate ZSM-5 zeolite using the known urea method, i.e., the urea method reported in patents and published literature. These urea methods, including those used to synthesize thin-plate ZSM-5 zeolite, S-1 zeolite, and TS-1 zeolite, share a common characteristic: the thin-plate zeolite is obtained by direct crystallization of a urea-containing hydrogel. Due to the low reactivity of the amorphous aluminosilicate framework in the hydrogel, the crystallization process of the thin-plate zeolite must be carried out at a temperature above the melting point of urea (131-135°C), specifically, at a high temperature of 150-180°C. As a result, the synthesized thin-plate ZSM-5 zeolite has larger crystal size and a thicker b-axis. In particular, urea additives are prone to deterioration at high temperatures, generating highly toxic, volatile, and corrosive cyanic acid and cyanuric acid (melting point 360°C), which sublimes and crystallizes easily, creating related problems. However, if the crystallization temperature of hydrogels produced using conventional urea methods is lowered to no higher than 130°C, urea-containing hydrogels will have difficulty crystallizing.

[0094] Example 1 was repeated, but after the aluminosilicate precursor containing urea was prepared in the first step, the low-temperature drying and dehydration process in the second step was omitted, and the third step was carried out directly. That is, 70 g of the hydrogel precursor containing urea was taken and subjected to hydrothermal crystallization treatment in a small laboratory high-pressure hydrothermal crystallization kettle with a volume of 100 ml and a polytetrafluoroethylene lining. The hydrothermal crystallization temperature was set to 120°C, and the hydrothermal crystallization time was set to 72h, 96h and 120h respectively. After the crystallization was completed, the synthetic product was post-treated according to the usual practice of synthesizing zeolite by conventional hydrothermal method, and samples were taken for XRD and SEM characterization. The results show that the hydrogel precursor containing urea is difficult to crystallize at 120°C. There are still many amorphous substances in the synthetic products with crystallization times of 72h, 96h and 120h, respectively, and their SEM photos are shown in FIG. Figure 4 、 5 and 6.

[0095] Comparative Example 3: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention, using fumed silica as the primary silicon source and sodium silicate as the auxiliary silicon source. A urea-containing aluminosilicate hydrogel precursor is first prepared, followed by low-temperature drying to remove free water from the hydrogel precursor and convert it into a urea-containing dry gel intermediate. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. The low-temperature drying to remove free water from the urea-containing hydrogel precursor to form the urea-containing dry gel intermediate, as well as the use of fumed silica as the primary silicon source and sodium silicate as the auxiliary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate framework in the dry gel intermediate, enabling crystallization of the dry gel intermediate at temperatures not exceeding 130°C, thereby avoiding various problems associated with crystallization at high temperatures due to deterioration of the urea auxiliary. If fumed silica is not used as the main silicon source and sodium silicate is not used as the auxiliary silicon source when preparing the urea-containing aluminosilicate precursor, but other silicon sources such as tetraethyl orthosilicate, silica sol and silica dry gel are used, the prepared urea-containing dry gel intermediate will also be difficult to synthesize thin-plate ZSM-5 zeolite by hydrothermal crystallization at a temperature not higher than 130°C.

[0096] Example 1 was repeated, but in the first step, to prepare the urea-containing aluminosilicate hydrogel precursor, all 12g of silica was provided by tetraethyl orthosilicate or silica sol (Model JN-30, SiO2 content 30 wt.%) or type C silica gel (Model, SiO2 content 99.3 wt.%). Sodium hydroxide was used instead of sodium silicate to provide the sodium ions. When tetraethyl orthosilicate was used to provide the 12g of silica, the amount of tetraethyl orthosilicate used was 41.67g. Prior to adding the urea additive, the aluminosilicate hydrogel was subjected to an alcohol distillation treatment. This alcohol distillation treatment was performed according to conventional procedures at a temperature of 80°C. When silica sol was used to provide the 12g of silica, the amount of silica sol used was 40g; when type C silica gel was used to provide the 12g of silica, the amount of type C silica gel used was 12.08g.

[0097] After hydrothermal crystallization of the urea-containing dry gel intermediates prepared using the three alternative silicon sources at 120°C for 72 hours, the synthesized products were mainly amorphous, and their relative crystallinity (based on the sb-ZSM-5-1 sample) was less than 50%.

[0098] Example 3: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as the secondary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying to convert it into a dry gel intermediate containing urea. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. During the second step of low-temperature drying to remove free water from the urea-containing aluminosilicate hydrogel precursor and convert it into the dry gel intermediate containing urea, the drying temperature can be varied within a certain range without altering the benefits of the present invention. The present invention has two main benefits: First, the method of removing free water from the urea-containing hydrogel precursor through low-temperature drying to produce a urea-containing dry gel intermediate, and the use of fumed silica as the primary silicon source and sodium silicate as the auxiliary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate skeleton in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C. Furthermore, the present invention synthesizes thin-plate ZSM-5 zeolite using urea as an auxiliary agent in a dry gel hydrothermal system, making it easier to reduce the b-axis thickness than the conventional urea method for directly synthesizing thin-plate ZSM-5 zeolite using hydrogel.

[0099] Example 1 was repeated, but in the second step, in which the free water in the urea-containing aluminosilicate hydrogel precursor was removed by low-temperature drying to convert it into a dry gel intermediate containing urea, the low-temperature drying temperatures were sequentially changed to: 30°C (dry air flowing in an oven) to constant weight, 40°C (dry air flowing in an oven) to constant weight, 50°C (dried under a slight negative pressure) to constant weight, 70°C to constant weight, 110°C for 6 hours, 120°C for 5 hours, and 130°C for 4 hours. The urea-containing dry gel intermediates prepared at different drying temperatures were then analyzed for weight loss using a thermogravimetric analyzer (TG). It was found that the weight loss rates of these dry gel intermediates due to free water removal on the TG were all ≤10%, more specifically, between 5-10%, with the weight loss occurring in the low-temperature range of 30-130°C. Moreover, the prepared dry glue intermediate containing urea is white in color and soft in texture, and the changes in the content, structure and physicochemical properties of the organic template TPAOH and urea therein are negligible, which meets the requirements of the present invention.

[0100] These urea-containing dry gel intermediates, prepared at different drying temperatures, were hydrothermally crystallized at 120°C for 72 hours. The resulting samples were characterized using XRD and SEM. The results showed that all seven samples were well-crystallized, thin-plate ZSM-5 zeolites free of impurities. Their b-axis thicknesses ranged from 50 nm to 70 nm.

[0101] Example 4: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as the secondary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying, converting it into a urea-containing dry gel intermediate. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. In the first step of preparing the urea-containing aluminosilicate hydrogel precursor, the urea-to-silicon ratio (urea / SiO2 molar ratio) can be varied within a certain range without altering the benefits of the present invention. This is because the present invention has two main benefits: First, the method of removing free water from the urea-containing hydrogel precursor by low-temperature drying to prepare it into a urea-containing dry gel intermediate, and the use of fumed silica as the main silicon source and sodium silicate as the auxiliary silicon source when preparing the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate skeleton in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C. In addition, the present invention uses urea as an auxiliary agent to synthesize thin-plate ZSM-5 zeolite in a dry gel hydrothermal system, which makes it easier to reduce the b-axis thickness than the known urea method for directly synthesizing thin-plate ZSM-5 zeolite using hydrogel. However, changing the urea-silicon ratio has an impact on the b-axis thickness of the thin-plate ZSM-5 zeolite.

[0102] Example 1 was repeated, but in the first step, the urea-containing aluminosilicate hydrogel precursors were prepared with urea-to-silicon ratios of 0.4, 0.6, 1.0, 1.2, and 1.5, respectively. These dry gel intermediates with varying urea contents were hydrothermally crystallized at 120°C for 72 hours, and the resulting samples were characterized using XRD and SEM. The results showed that all five samples were well-crystallized, thin-plate ZSM-5 zeolites free of impurities. Their b-axis thicknesses were 150, 80, 48, 45, and 37 nm, respectively.

[0103] Example 5: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as the secondary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying, converting it into a dry gel intermediate containing urea. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. In the first step of preparing the urea-containing aluminosilicate hydrogel precursor, the stirring time after the addition of urea can be varied within a certain range without altering the benefits of the present invention. The present invention has two main benefits: First, the method of removing free water from the urea-containing hydrogel precursor through low-temperature drying to produce a urea-containing dry gel intermediate, and the use of fumed silica as the primary silicon source and sodium silicate as the auxiliary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate skeleton in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C. Furthermore, the present invention synthesizes thin-plate ZSM-5 zeolite using urea as an auxiliary agent in a dry gel hydrothermal system, making it easier to reduce the b-axis thickness than the conventional urea method for directly synthesizing thin-plate ZSM-5 zeolite using hydrogel.

[0104] Example 1 was repeated, but in the first step, the aluminosilicate hydrogel precursor containing urea was prepared. The stirring time after adding urea was changed to 0.5 h, 1 h, 2 h, 4 h, 6 h, and 12 h, respectively. These dry gel intermediates, each stirred for different times after adding urea, were hydrothermally crystallized at 120°C for 72 h. The resulting samples were characterized using XRD and SEM. The results showed that all six samples were well-crystallized, thin-plate ZSM-5 zeolites free of impurities. Their b-axis thicknesses fluctuated between 50 nm and 65 nm.

[0105] Example 6: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as a supplementary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying, converting it into a dry gel intermediate containing urea. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. In the first step of preparing the urea-containing aluminosilicate hydrogel precursor, the water-to-silicon ratio can be varied within a certain range without altering the benefits of the present invention. The present invention has two main benefits: First, the method of removing free water from the urea-containing hydrogel precursor through low-temperature drying to produce a urea-containing dry gel intermediate, and the use of fumed silica as the primary silicon source and sodium silicate as the auxiliary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate skeleton in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C. Furthermore, the present invention synthesizes thin-plate ZSM-5 zeolite using urea as an auxiliary agent in a dry gel hydrothermal system, making it easier to reduce the b-axis thickness than the conventional urea method for directly synthesizing thin-plate ZSM-5 zeolite using hydrogel.

[0106] Example 1 was repeated, but in the first step, the aluminosilicate hydrogel precursor containing urea was prepared with water-to-silicon ratios of 12, 15, 20, 30, 40, and 50, respectively. These hydrogel intermediates with varying water-to-silicon ratios were hydrothermally crystallized at 120°C for 72 hours. The resulting samples were characterized using XRD and SEM. The results showed that all six samples were well-crystallized, thin-plate ZSM-5 zeolites free of impurities. Their b-axis thicknesses fluctuated between 50 and 70 nm.

[0107] Example 7: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention. Using fumed silica as the primary silicon source and sodium silicate as the secondary silicon source, a urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying, converting it into a urea-containing dry gel intermediate. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. In the first step of preparing the urea-containing aluminosilicate hydrogel precursor, the tantalum-silicon ratio (TPAOH / SiO2) can be varied within a certain range without altering the benefits of the present invention. The present invention has two main benefits: First, the method of removing free water from the urea-containing hydrogel precursor by low-temperature drying to produce a urea-containing dry gel intermediate, and the use of fumed silica as the primary silicon source and sodium silicate as the auxiliary silicon source in the preparation of the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate framework in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C. Furthermore, the present invention uses urea as an auxiliary agent to synthesize thin-plate ZSM-5 zeolite in a dry gel hydrothermal system, making it easier to reduce the b-axis thickness compared to the conventional urea method for directly synthesizing thin-plate ZSM-5 zeolite using hydrogel. However, varying the urea-to-silicon ratio does affect the b-axis thickness of the thin-plate ZSM-5 zeolite. However, increasing the urea-to-silicon ratio (TPAOH / SiO2) within a certain range can help reduce the crystallite size of the thin-plate ZSM-5 zeolite and shorten the crystallization time.

[0108] Example 1 was repeated, but in the first step, the aluminosilicate hydrogel precursor containing urea was prepared with different TPAOH / SiO2 ratios (TPAOH / SiO2) of 0.1, 0.12, 0.15, 0.30, and 0.36, respectively. Hydrothermal crystallization of these dry gel intermediates with varying TPAOH / SiO2 ratios at 120°C yielded well-crystallized, thin-plate ZSM-5 zeolites free of impurities, with b-axis thicknesses of 97, 84, 50, 48, and 45 nm, respectively. The crystallite size decreased with increasing TPAOH / SiO2 ratio. Furthermore, it was observed that increasing the TPAOH / SiO2 ratio accelerated the crystallization rate and shortened the crystallization time of the dry gel intermediates.

[0109] Example 8: This example illustrates a method for synthesizing thin-plate ZSM-5 zeolite according to the present invention, using fumed silica as the primary silicon source and sodium silicate as a supplementary silicon source. A urea-containing aluminosilicate hydrogel precursor is first prepared. Free water in the hydrogel precursor is then removed by low-temperature drying, converting it into a dry gel intermediate containing urea. The urea-containing dry gel intermediate is then hydrothermally crystallized at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite. In the first step of preparing the urea-containing aluminosilicate hydrogel precursor, the silicon-to-aluminum ratio and the amount of sodium silicate supplementary silicon source can be varied within a certain range. In the third step of hydrothermally crystallizing the urea-containing dry gel intermediate at a temperature not exceeding 130°C to synthesize thin-plate ZSM-5 zeolite, the temperature and time of the hydrothermal crystallization of the dry gel intermediate can also be varied without altering the benefits of the present invention. This is because the present invention has two main benefits: First, the method of removing free water from the urea-containing hydrogel precursor by low-temperature drying to prepare it into a urea-containing dry gel intermediate, and the use of fumed silica as the main silicon source and sodium silicate as the auxiliary silicon source when preparing the hydrogel precursor, maximize the reactivity of the amorphous aluminosilicate skeleton in the dry gel intermediate, thereby enabling the crystallization process of the dry gel intermediate to proceed at a temperature not exceeding 130°C. In addition, the present invention uses urea as an auxiliary agent to synthesize thin-plate ZSM-5 zeolite in a dry gel hydrothermal system, which makes it easier to reduce the b-axis thickness than the known urea method for directly synthesizing thin-plate ZSM-5 zeolite using hydrogel. However, changing the urea-silicon ratio has an impact on the b-axis thickness of the thin-plate ZSM-5 zeolite.

[0110] Example 1 was repeated, except that in the first step, when preparing the aluminosilicate hydrogel precursor containing urea, the silicon-to-aluminum ratio (SiO2 / Al2O3) was changed to 50, 60, 80, 300, 400, 450, and 500, respectively. Accordingly, the amount of sodium silicate auxiliary silicon source (calculated as Na2O / SiO2) was also changed to 0.12, 0.11, 0.09, 0.02, 0.02, 0.01, and 0.01, respectively. The hydrothermal crystallization conditions of the dry gel intermediate in the third step were changed to 130°C for 240 h, 130°C for 192 h, 125°C for 168 h, 100°C for 144 h, 100°C for 48 h, 90°C for 24 h, and 80°C for 12 h, respectively. Under the above conditions, dry gel intermediates with different silicon-aluminum ratios were hydrothermally crystallized to obtain well-crystallized, thin-plate ZSM-5 zeolites without impurities. The b-axis thickness varied between 40nm and 150nm, and tended to become thinner as the silicon-aluminum ratio of the sample increased.

Claims

1. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite, characterized in that: Here are the steps: Step 1: Preparation of aluminosilicate hydrogel precursor containing urea In addition to urea, fumed silica as the main silicon source, sodium silicate as the auxiliary silicon source, pseudo-boehmite as the aluminum source, and deionized water, tetrapropylammonium hydroxide (TPAOH) template is also required when preparing the aluminosilicate hydrogel precursor containing urea; When preparing aluminosilicate hydrogel precursor containing urea, the amounts of urea, tetrapropylammonium hydroxide (TPAOH) template, main silicon source, auxiliary silicon source, aluminum source, and deionized water must meet the following conditions: The urine-silicon ratio is calculated as the molar ratio of urea to SiO2 and ranges from 0.4 to 1.5; The mold-silicon ratio is calculated as the molar ratio of TPAOH to SiO2 and ranges from 0.10 to 0.36; The silicon-aluminum ratio is calculated as the molar ratio of SiO2 to Al2O3 and ranges from 50 to 500; The amount of sodium silicate used is calculated as the molar ratio of Na2O to SiO2, ranging from 0.01 to 0.12; The water-silicon ratio is calculated as the molar ratio of H2O to SiO2 and ranges from 12 to 50; The operation of preparing a hydrogel precursor containing urea is carried out at room temperature; the steps are as follows: first, according to the total amount of silica and the selected molar ratios of Na2O to SiO2, urea to SiO2, TPAOH to SiO2, SiO2 to Al2O3, and H2O to SiO2, the amounts of sodium silicate, urea, TPAOH template solution, pseudo-boehmite, and deionized water are determined; then, under stirring, an aluminum source solution is prepared using measured amounts of deionized water, TPAOH template, and pseudo-boehmite, and an auxiliary silicon source solution is prepared using measured amounts of deionized water and sodium silicate; next, under stirring, the auxiliary silicon source solution and the measured amount of main silicon source fumed silica are sequentially added to the aluminum source solution, and stirring is continued until the hydrogel is uniform; finally, urea is added to the uniform hydrogel under stirring, and stirring is continued for a period of time to obtain an aluminosilicate hydrogel precursor containing urea; Step 2: Low-temperature drying of the aluminosilicate hydrogel precursor containing urea to remove free water and convert it into a dry gel intermediate containing urea. The low temperature drying process has a temperature range of room temperature to 130°C; Step 3: Hydrothermal crystallization of the aluminosilicate dry colloid intermediate containing urea to convert it into thin plate-type ZSM-5 zeolite The hydrothermal crystallization of the dry glue intermediate is carried out in a conventional autoclave. Deionized water is used as the crystallization liquid. The crystallization liquid and the dry glue intermediate are added to the autoclave in a certain mass ratio so that the dry glue intermediate is immersed in the crystallization liquid. The dry glue hydrothermal system is then subjected to a hydrothermal crystallization treatment at a certain temperature. The hydrothermal crystallization is carried out in a dynamic manner with stirring or in a static manner without stirring. The mass ratio of the crystallization liquid to the dry glue is expressed as liquid-to-solid ratio ml / g, ranging from 1 to 10; The hydrothermal crystallization temperature range is: 80-130℃; The hydrothermal crystallization time range is: 12h-240h; After the hydrothermal crystallization treatment is completed, the autoclave is cooled and depressurized; when the temperature and pressure in the autoclave are close to the ambient temperature and pressure, the autoclave is opened and the thin plate-type ZSM-5 zeolite product is taken out; Step 4: Post-treatment of thin plate ZSM-5 zeolite product The method comprises the steps of filtering, washing with deionized water, drying and roasting.

2. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 1, characterized in that, In the first step, The urine-silicon ratio is calculated as the molar ratio of urea to SiO2 and ranges from 0.6 to 1.2; The mold-silicon ratio is calculated as the molar ratio of TPAOH to SiO2, ranging from 0.12-0.30 The silicon-aluminum ratio is calculated as the molar ratio of SiO2 to Al2O3 and ranges from 60 to 450; The amount of sodium silicate used is calculated as the molar ratio of Na2O to SiO2, ranging from 0.02 to 0.10; The water-silicon ratio is calculated as the molar ratio of H2O to SiO2 and ranges from 15 to 40.

3. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 2, characterized in that, In the first step, The urine-silicon ratio is calculated as the molar ratio of urea to SiO2 and ranges from 0.8 to 1.0; The mold-silicon ratio is calculated as the molar ratio of TPAOH to SiO2 and ranges from 0.15 to 0.25; The silicon-aluminum ratio is calculated as the molar ratio of SiO2 to Al2O3 and ranges from 80 to 400; The amount of sodium silicate used is calculated as the molar ratio of Na2O to SiO2, ranging from 0.03 to 0.09; The water-silicon ratio is calculated as the molar ratio of H2O to SiO2 and ranges from 20-30.

4. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 1, characterized in that, In the first step, the stirring time after adding the urea additive to the hydrogel is in the range of 0.5-12 h.

5. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 1, characterized in that, In the second step, the low temperature drying treatment is carried out at a temperature ranging from 50°C to 120°C.

6. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 5, characterized in that, In the second step, the low temperature drying treatment is carried out at a temperature ranging from 80 to 110°C.

7. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 6, characterized in that, In the third step, the mass ratio of the crystallization liquid to the dry glue is calculated as liquid-to-solid ratio ml / g, ranging from 1 to 10.

8. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 1, characterized in that, In the third step, the mass ratio of the crystallization liquid to the dry glue is calculated as liquid-to-solid ratio ml / g, ranging from 1.5 to 8.

9. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 8, characterized in that, In the third step, the mass ratio of the crystallization liquid to the dry glue is calculated as liquid-to-solid ratio ml / g, ranging from 2 to 6.

10. A dry glue hydrothermal method for synthesizing thin plate type ZSM-5 zeolite according to claim 1, characterized in that: In the third step, The hydrothermal crystallization temperature range is 90-125°C; the hydrothermal crystallization time range is 18h-120h.

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