A method for synthesizing low silica-to-alumina ratio ZSM-48 molecular sieves in a dual-alkali source system and its application.

Abstract

This invention belongs to the field of molecular sieve synthesis technology, and discloses a method for synthesizing low silica-to-alumina ratio ZSM-48 molecular sieves in a dual-alkali source system and its application. Using pyrrolidine as a template agent and sodium hydroxide and potassium hydroxide as dual alkali sources, the steps are as follows: The alkali source, silica source, and deionized water are mixed uniformly at 40-80℃ to obtain mixture A; the aluminum source, template agent, and deionized water are mixed uniformly and slowly added to mixture A to obtain an initial gel mixture; the initial gel is subjected to hydrothermal crystallization treatment, followed by filtration, washing, drying, and calcination to obtain ZSM-48 molecular sieves. The ZSM-48 molecular sieves synthesized by the method proposed in this invention have regular morphology, uniform particles, and high crystallinity. The silica-to-alumina ratio, calculated according to SiO2 / Al2O3, ranges from 20 to 100. The ZSM-48 molecular sieves exhibit high selectivity and yield for the hydroisomerization reaction of n-hexadecane.

Description

Technical Field

[0001] This invention belongs to the field of molecular sieve synthesis technology, and relates to a method for synthesizing ZSM-48 molecular sieve with a low silica-to-alumina ratio and its application in hydroisomerization reaction. Background Technology

[0002] ZSM-48 molecular sieve is a high-silica zeolite, initially discovered by Schlenker et al. (Nature, 1981, 294: 340-342) during the synthesis of ZSM-39 high-silica molecular sieve. Its morphology exhibits fibrous rod-like crystals. The pore structure of ZSM-48 molecular sieve is a one-dimensional linear channel with ten-membered ring pores, belonging to the *MRE topology, with a pore size of 0.56 × 0.56 nm. Therefore, ZSM-48 molecular sieve possesses molecular shape selectivity for specific sizes, and its crystals grow along the one-dimensional straight channel direction, exhibiting needle-like crystals or nanorod-like aggregates of nanoneedle-like crystals, making it an ideal support for the hydroisomerization of straight-chain alkanes. Hydroisomerization of straight-chain alkanes is an important oil processing technology, playing a crucial role in the high-value conversion of long-chain alkanes derived from clean energy sources such as biomass oils, Fischer-Tropsch synthesis oils, and waste plastic pyrolysis oils. It is widely used in the upgrading processes of high-end essential oils such as sustainable aviation fuels and lubricating oil base oils. Therefore, the rapid synthesis of ZSM-48 molecular sieves with high crystallinity, low silica-alumina ratio, and high acid density is of great significance.

[0003] Currently, the commonly used template agents for synthesizing ZSM-48 molecular sieves can be divided into: (1) linear organic alkylamines synergistic tetramethylammonium ions (US4397827, US4448675), (2) linear organic diamines (US8003074B2, US4423021, US5961951), (3) bisquaternary ammonium salt ions (US6923949B1, US7482300B2, US7625478B2, US9433935B2), and (4) novel bifunctional surfactants (CN103803576A, CN104003413B, Chem. Mater., 2018, 30: 1839-1843), etc.

[0004] In the early 1980s, Mobil first disclosed a patent (US4397827) for the synthesis of ZSM-48 molecular sieves using linear organic alkylamines (C2-C12 alkylamines, preferably C3-C5 alkylamines) in conjunction with tetramethylammonium hydroxide as a template agent. Subsequently, the company synthesized ZSM-48 molecular sieves using mono- or bi-molecular N-methylpyridine as a template agent (US4585747), but the initial gel-to-silica-to-alumina ratio required to be greater than 250, and the crystallization time greater than 5 days. The synthesis of ZSM-48 molecular sieves with low silica-to-alumina ratios has been a continuous research goal. US5961951 discloses a method for synthesizing ZSM-48 molecular sieves using ethylenediamine as a template agent, which can reduce the initial gel-to-silica-to-alumina ratio to 200 and shorten the crystallization cycle to less than 3 days.

[0005] As a zeolite derived from a high-silica gel system, ZSM-48 molecular sieves are often obtained from initial gels with a high silica-to-alumina ratio (SiO2 / Al2O3 > 150) and low basicity (Na2O / SiO2 = 0.02-0.08). Although the silica-to-alumina ratio of the molecular sieve can be reduced to some extent by decreasing the feed silica-to-alumina ratio, it is difficult to obtain ZSM-48 in a lower silica-to-alumina ratio range (SiO2 / Al2O3 < 100). Low silica-to-alumina ratio zeolites such as FAU, SOD, and LTA usually need to be crystallized from initial gels with high basicity due to the high concentration of OH-. - Preferential dissolution of silicon species allows aluminum species to enter the molecular sieve framework. However, the crystallization of ZSM-48 molecular sieves requires a low alkalinity gel system, which contradicts the requirement for a high alkalinity gel system for the crystallization of low silica-to-alumina ratio zeolites. It has been reported that pyrrolidine can be used as a template agent for the synthesis of ZSM-48 molecular sieves (Zeolites, 1991, 11: 573-576; Journal of Catalysis, 1999, 185: 435-444). Both reports first used a high-concentration alkaline solution to fully dissociate the silicon source (Na₂O / SiO₂ = 0.19), then mixed it with an aqueous solution containing concentrated sulfuric acid and an aluminum source, neutralizing excess OH⁻ through the concentrated sulfuric acid. - To achieve the low alkalinity required for ZSM-48 molecular sieve crystallization, the synthesized ZSM-48 molecular sieve still had a silica-to-alumina ratio greater than 150. Similarly, the synthesis of ZSM-48 molecular sieves containing heteroatoms Sn using pyrrolidine as a template agent also adopted a similar approach (Studies in Surface Science and Catalysis, 1997, 105:357-364): first, a high-concentration alkaline solution was used to dissociate the silicon source, and then concentrated sulfuric acid was used to reduce the alkalinity of the gel system to meet the alkalinity range required for ZSM-48 molecular sieve crystallization.

[0006] After 2005, Mobil successively disclosed patents (US6923949B1, US7482300B2, US7625478B2, US9433935B2) for synthesizing ZSM-48 molecular sieves with a silica-to-alumina ratio as low as 100 using hexamethyldiamine chloride as a template agent. The crystallization time can be shortened to 2 days, but the template agent cost is relatively high. Domestic patents also employ various template agents to synthesize low silica-to-alumina ratio ZSM-48 molecular sieves, such as allyltrimethylammonium chloride (CN110127719B), 12-crown ether-4 (CN103803576A), N,N-diethylhexamethyleneimine quaternary ammonium base (CN102040231B), diazinonoxane dibromosalt (CN106608635B), 1,n-bis(N-methylpyrrolidine)hexane bromide (n=1-10) (CN104003413B), and organic mixed template agents (dibromohexane, trimethylamine, and ethanol) (CN102910642B), etc.

[0007] In summary, simple and low-cost organic amines (such as n-octylamine synergistically with tetramethylammonium bromide, 1,6-hexanediamine, and pyrrolidine) are commonly used to synthesize ZSM-48 molecular sieves with a silica-to-alumina ratio higher than 150. However, the weak acid density and strength of these products severely limit the application of ZSM-48 molecular sieves in acid-catalyzed reactions. Since the discovery of ZSM-48 molecular sieves, although molecular sieve synthesis technology has continuously improved, and some progress has been made in synthesizing ZSM-48 molecular sieves with a silica-to-alumina ratio lower than 150, high-cost and complex template agents are often required, making large-scale production of ZSM-48 molecular sieves difficult. Therefore, how to efficiently and rapidly synthesize highly crystalline ZSM-48 molecular sieves with a low silica-to-alumina ratio using novel, simple, and inexpensive template agents remains a current research hotspot and challenge. Summary of the Invention

[0008] This invention selects a structurally simple and relatively low-cost organic amine as a template agent, providing a method for synthesizing highly crystalline, low silica-to-alumina ratio ZSM-48 molecular sieves in a dual-base source system. By controlling the ratio of two strong bases, NaOH and KOH, to form a dual-base source, and under stirring conditions of 40-80 °C, with the aid of OH... - The mineralization process first dissolves the silicon source, fully dissociating silicon species and making it easier for aluminum species to enter the framework of ZSM-48 molecular sieves. This method can efficiently and rapidly synthesize low-silicon-to-aluminum ratio ZSM-48 molecular sieves (SiO2 / Al2O3, 20-100), and simultaneously use this molecular sieve as a support to prepare catalysts for the hydroisomerization reaction of straight-chain alkanes.

[0009] One objective of this invention is to provide a gel composition system that differs from existing methods for synthesizing ZSM-48 molecular sieves; another objective is to provide an efficient and cost-effective method for synthesizing ZSM-48 molecular sieves; and a third objective is to provide a method for synthesizing ZSM-48 molecular sieves with specific morphology and a low silica-alumina ratio.

[0010] The technical solution of the present invention:

[0011] A method for synthesizing low silica-to-alumina ratio ZSM-48 molecular sieves in a dual-alkali source system, comprising the following steps:

[0012] (1) First, place the mixture of alkali source, silicon source and deionized water in 40-80 ℃ and stir vigorously to obtain mixture A;

[0013] (2) After mixing the aluminum source, template agent and deionized water evenly, slowly add it to the mixture A in step (1) and stir thoroughly to obtain the initial gel mixture;

[0014] (3) Transfer the above initial gel mixture into a crystallization kettle for hydrothermal crystallization treatment. The crystallization temperature is 130-210 ℃ and the crystallization time is 12-72 h; either static crystallization or dynamic crystallization.

[0015] (4) The solid product obtained by the above hydrothermal crystallization treatment is filtered, washed, dried and calcined to obtain ZSM-48 molecular sieve.

[0016] In this invention, pyrrolidine is used as a template agent, and sodium hydroxide and potassium hydroxide are used as dual alkali sources. According to the specific addition order described in steps (1) and (2), silicon source, aluminum source, template agent, alkali source and deionized water are added in a molar ratio of 1:(0.01-0.05):(0.05-1.2):(0.02-0.12):(18-50) to form an initial gel. The gel is thoroughly stirred and mixed for 2-6 hours, preferably 3-4 hours. The mixture is then transferred to a crystallization kettle and rapidly heated to a crystallization temperature of 130-210 °C, preferably 155-195 °C. The crystallization time is 12-72 hours, preferably 24-48 hours. The crystallization is performed statically or dynamically at a speed of 20-1000 RPM. The preferred molar ratio of the initial gel mixture is 1:(0.01-0.05):(0.2-0.8):(0.03-0.08):(25-40).

[0017] In this invention, the pH range of the initial gel is controlled to be 12.3-13.8, preferably 12.6-13.4, wherein the silicon source, aluminum source, and alkali source mentioned in steps (1) and (2) are calculated as SiO2, Al2O3, and Me2O, respectively. The specific addition order is as follows: the mixture prepared by aluminum source, template agent, and deionized water in step (2) is slowly added to the mixture A prepared by alkali source, silicon source, and deionized water in step (1). The mixing and stirring time of alkali source, silicon source, and deionized water in step (1) is 0.5-4 h, preferably 2-3 h, and the stirring temperature is 40-80 ℃; the mixing and stirring time of aluminum source, template agent, and deionized water in step (2) is 0.1-1 h, preferably 0.25-0.5 h, and the stirring is carried out at room temperature.

[0018] In this invention, the template agent is pyrrolidine. The aluminum source is one or a mixture of two or more of aluminum nitrate, aluminum sulfate, sodium aluminate, and aluminum isopropoxide, preferably aluminum nitrate or aluminum sulfate. The silicon source is one or a mixture of two or more of fumed silica, silica sol, tetraethyl orthosilicate, and water glass, preferably fumed silica or silica sol.

[0019] The alkali source is a mixture of sodium hydroxide and potassium hydroxide, with a molar ratio of Na2O:K2O = 0.2-8.0.

[0020] In this invention, the molecular sieve solid product obtained by hydrothermal crystallization is filtered, washed, dried and calcined to obtain a low silicon-to-aluminum ratio ZSM-48 molecular sieve with a silicon-to-aluminum ratio (SiO2 / Al2O3) ranging from 20 to 100.

[0021] In this invention, the processes of hydrothermal crystallization, filtration, washing, drying, and calcination are all commonly used technical means and conditions in the synthesis of molecular sieves.

[0022] In this invention, the morphology of the low silicon-to-aluminum ratio ZSM-48 molecular sieve is a spindle shape formed by the aggregation of nanoneedles, and its silicon-to-aluminum ratio ranges from 20 to 100. The synthesized ZSM-48 molecular sieve has the following morphology.

[0023] In this invention, the template agent is pyrrolidine, and the dual alkali sources are sodium hydroxide and potassium hydroxide, which differs from the gel composition system used in existing ZSM-48 molecular sieve synthesis. The synthesis method for ZSM-48 molecular sieve described herein is highly efficient and rapid, producing products with high crystallinity, and the template agent has a simple structure and relatively low cost. The ZSM-48 molecular sieve synthesized according to this method exhibits a specific crystal morphology, specifically spindle-shaped crystals formed by the aggregation of nanoneedles. The ZSM-48 molecular sieve synthesized according to this method has a low silica-to-alumina ratio and high acid density, with a silica-to-alumina ratio ranging from 20 to 100.

[0024] In this invention, the ZSM-48 molecular sieve obtained according to the synthesis method is used as a support, and the catalyst prepared by impregnation method with metal is applied to the hydroisomerization reaction, which can obtain a high yield of isomer products. The metal is one or a mixture of two or more of Pt, Pd, Ni, and Mo.

[0025] In previous reports, when using simple organic amines as templates to synthesize ZSM-48 molecular sieves, the initial gel's silica-to-alumina ratio (S / A ratio) was typically higher than 200. Furthermore, the hydrothermal reaction process was prone to generating impurities due to the competitive growth of structural units such as cristobalite. While ZSM-48 molecular sieve nucleation and crystallization require low alkalinity, low S / A ratio zeolite molecular sieves require higher alkalinity. Therefore, achieving efficient ZSM-48 synthesis while maintaining a low S / A ratio is contradictory. The key challenge in synthesizing low S / A ratio ZSM-48 molecular sieves lies in ensuring aluminum species enter the sieve framework while synthesizing ZSM-48 in a low-alkalinity initial gel. Although some reports have described strategies using high-concentration alkali to dissociate the silicon source and then reducing the gel system's alkalinity with concentrated sulfuric acid to promote silicon dissociation and aluminum species entry into the sieve framework, the resulting ZSM-48 molecular sieves still had an S / A ratio greater than 150.

[0026] Based on extensive experimental research, this invention reveals that the entry of aluminum species into the molecular sieve framework requires the complete dissociation of silicon species in the initial gel under the influence of an alkaline source. Therefore, this invention utilizes pyrrolidine, a simple and inexpensive template agent, to synthesize ZSM-48 molecular sieves with a low silica-to-aluminum ratio, regular morphology, uniform particle size, and high crystallinity by selecting appropriate dual alkaline sources, controlling the mixing order of materials, and rationally adjusting the molar ratio of each material in the gel system. Sodium hydroxide or potassium hydroxide as a single alkaline source is insufficient for sufficient mineralization and dissociation of silicon species. This invention selects sodium hydroxide and potassium hydroxide in a specific molar ratio to form a dual alkaline source. Under a stirring temperature of 40-80 °C, the silicon species are first fully dissociated using the dual alkaline source, and then an aqueous solution composed of an aluminum source and a template agent is added. This allows aluminum species to more effectively enter the spaces between silicon species, forming an aluminum-rich molecular sieve framework, thereby efficiently synthesizing low silica-to-aluminum ratio ZSM-48 molecular sieves. Since potassium has a slightly stronger metallic character than sodium, potassium hydroxide more readily ionizes into hydroxide ions (OH-) in aqueous solution. - Potassium hydroxide and sodium hydroxide, when used together as an alkali source, can increase the OH- concentration. - The degree of dissociation of the silicon source is crucial. Since the nucleation and crystallization of ZSM-48 molecular sieves requires high precision in basicity and a high degree of dissociation of the silicon source, the slight difference between a dual-alkali source (potassium hydroxide and sodium hydroxide) and a single alkali source is essential for the efficient synthesis of low-silicon-aluminum-ratio ZSM-48 molecular sieves.

[0027] The method of this invention requires a specific order of material addition: first, a dual-alkali source is used to dissociate silicon species, followed by the addition of an aqueous solution consisting of an aluminum source and a template agent. This specific order of material addition ensures the complete dissolution and dissociation of the silicon source by the dual-alkali source, effectively introducing aluminum species into the silicon source fragments and subsequently into the molecular sieve framework. This method uses a dual-alkali source to directly dissociate silicon species, with an optimal alkalinity (Na₂O / SiO₂) range of only 0.03-0.08. This avoids the use of excessive alkali and eliminates the need for additional concentrated sulfuric acid to adjust the initial gel pH, effectively reducing the amount of concentrated alkali and acid used.

[0028] In summary, the method of this invention employs a dual-alkali source to preferentially dissociate silicon species, which can both ensure the low alkalinity requirement of the ZSM-48 nucleation and crystallization process and effectively promote the entry of aluminum species into the silicon fragments, thus achieving the goal of synthesizing low silicon-to-aluminum ratio ZSM-48 molecular sieves from aluminum-rich and low-alkalinity initial gels.

[0029] The ZSM-48 molecular sieve synthesis technology described in this invention has the following advantages:

[0030] 1. The ZSM-48 molecular sieve synthesized in this invention has a regular morphology, uniform particles, and high crystallinity. The silicon-aluminum ratio (SiO2 / Al2O3) ranges from 20 to 100, and the acidity is adjustable. The synthesized ZSM-48 molecular sieve has strong acid properties, high acid density and acid strength, and can be used for reactions where acid catalysis is the rate-determining step, such as hydroisomerization.

[0031] 2. This invention uses sodium hydroxide and potassium hydroxide as dual alkali sources and pyrrolidine as a template agent, unlike existing gel composition systems for synthesizing ZSM-48 molecular sieves. Pyrrolidine is inexpensive and requires a low dosage, significantly reducing raw material costs. The gel system has a low H2O / SiO2 molar ratio and high single-reactor yield, facilitating the separation and recovery of solid molecular sieve products, thus significantly reducing synthesis costs. Simultaneously, it greatly reduces wastewater discharge and environmental pollution caused by the calcination process of the organic template agent.

[0032] 3. The ZSM-48 molecular sieve synthesis method described in this invention is simple to operate, requiring no multi-step crystallization, no addition of seed crystals, and no addition of concentrated sulfuric acid to adjust the pH. Low silica-to-alumina ratio ZSM-48 molecular sieves can be synthesized simply by selecting a suitable alkali source, controlling the mixing order of materials, and rationally adjusting the molar ratio of each material in the gel system. The crystallization cycle is short, the process is simple, and the operating cost is low. Attached Figure Description

[0033] Figure 1 The XRD diffraction pattern of ZSM-48 synthesized in Example 1 is shown.

[0034] Figure 2 This is a scanning electron microscope (SEM) image of ZSM-48 synthesized in Example 1.

[0035] Figure 3 The XRD diffraction pattern of ZSM-48 synthesized in Example 2 is shown.

[0036] Figure 4 This is a scanning electron microscope (SEM) image of ZSM-48 synthesized in Example 2.

[0037] Figure 5 The XRD diffraction pattern of ZSM-48 synthesized in Example 3 is shown.

[0038] Figure 6 This is a scanning electron microscope (SEM) image of ZSM-48 synthesized in Example 3.

[0039] Figure 7 The image shows the XRD diffraction pattern of the sample synthesized in Example 4.

[0040] Figure 8 The image shows the XRD diffraction pattern of the sample synthesized in Example 5.

[0041] Figure 9 The image shows the XRD diffraction pattern of the sample synthesized in Example 6.

[0042] Figure 10 The XRD diffraction pattern of ZSM-48 synthesized in Comparative Example 1 is shown.

[0043] Figure 11 The graph shows the trend of the conversion rate of n-hexadecane of the catalyst prepared in Example 1 as a function of reaction temperature.

[0044] Figure 12 The graph shows the trend of isohexadecane selectivity of the catalyst prepared in Example 1 as a function of the conversion rate of n-hexadecane.

[0045] Figure 13 The graph shows the trend of hexadecane yield as a function of reaction temperature for the catalyst prepared in Example 1. Detailed Implementation

[0046] The present invention will be described in detail below through embodiments, but the present invention is not limited to these embodiments.

[0047] Example 1

[0048] The molar ratio of silicon source, aluminum source, pyrrolidine, alkali source, and deionized water was 1:0.01:0.35:0.05:30, where the alkali source was a mixture of sodium hydroxide and potassium hydroxide with a Na / K molar ratio of 0.7. 4.00 g of aluminum sulfate octadechydrate, 120 g of silica sol (30 wt.%), 0.99 g of NaOH, 1.98 g of KOH, 14.94 g of pyrrolidine, and 240 g of deionized water were weighed out. First, the deionized water was divided into two portions. One portion was used to dissolve the alkali source and silica sol, and stirred for 1 h. Then, the aluminum source and pyrrolidine were dissolved in the other portion of deionized water, stirred for 0.25 h, and slowly added dropwise to the mixed solution of alkali source and silica sol to obtain an initial gel mixture. After vigorous stirring for another 3 h, the mixture was transferred to a 500 ml crystallization vessel, rapidly heated to 180 ℃, and statically crystallized for 2 days. The obtained solid product was filtered and washed, and the filter cake was dried in an oven at 100 ℃ for 12 h, followed by calcination at 550 ℃ for 6 h to obtain ZSM-48 molecular sieve. ICP analysis showed that the obtained ZSM-48 molecular sieve had a silica-to-alumina ratio of 91.

[0049] Example 2

[0050] The steps in Example 1 were repeated, except that the molar ratio of silicon source, aluminum source, pyrrolidine, alkali source, and deionized water was 1:0.0143:0.4:0.03:40, where the alkali source was a mixture of sodium hydroxide and potassium hydroxide with a Na / K molar ratio of 0.4. 1.90 g of aluminum sulfate octadechydrate, 40 g of silica sol (30 wt.%), 0.14 g of NaOH, 0.48 g of KOH, 5.69 g of pyrrolidine, and 116 g of deionized water were weighed out. The initial gel was transferred to a 200 ml crystallization vessel for hydrothermal treatment. The resulting solid product was then filtered, washed, and the filter cake was dried in a 100 °C oven for 12 h, followed by calcination at 550 °C for 6 h to obtain ZSM-48 molecular sieve. ICP analysis showed that the obtained ZSM-48 molecular sieve had a silica-to-alumina ratio of 67.

[0051] Example 3

[0052] The steps in Example 1 were repeated, except that the molar ratio of silicon source, aluminum source, pyrrolidine, alkali source, and deionized water was 1:0.05:0.35:0.076:40, where the alkali source was a mixture of sodium hydroxide and potassium hydroxide with a Na / K molar ratio of 0.2. 6.66 g of aluminum sulfate octadechydrate, 40 g of silica sol (30 wt.%), 0.20 g of NaOH, 1.42 g of KOH, 4.98 g of pyrrolidine, and 116 g of deionized water were weighed out. The initial gel was transferred to a 200 ml crystallization vessel for hydrothermal treatment. The resulting solid product was then filtered, washed, and the filter cake was dried in a 100 °C oven for 12 h, followed by calcination at 550 °C for 6 h to obtain ZSM-48 molecular sieve. ICP analysis showed that the obtained ZSM-48 molecular sieve had a silicon-to-aluminum ratio of 24.

[0053] Example 4

[0054] The steps in Example 1 were repeated, except that sodium hydroxide was used as the sole alkali source. The initial gel was transferred to a 200 ml crystallization vessel for hydrothermal treatment. The resulting solid product was then filtered, washed, and the filter cake was dried in a 100°C oven for 12 h. After calcination at 550°C for 6 h, a solid powder sample was obtained.

[0055] Example 5

[0056] The steps in Example 1 were repeated, except that potassium hydroxide was used as the sole alkali source. The initial gel was transferred to a 200 ml crystallization vessel for hydrothermal treatment. The resulting solid product was then filtered, washed, and the filter cake was dried in a 100°C oven for 12 h. After calcination at 550°C for 6 h, a solid powder sample was obtained.

[0057] Example 6

[0058] The steps in Example 1 were repeated, except that the specific order of raw material addition and mixing steps were not followed. Instead, the silicon source, aluminum source, pyrrolidine, alkali source, and deionized water were directly mixed evenly to form a gel. The initial gel was then transferred to a 200 ml crystallization vessel for hydrothermal treatment. The resulting solid product was then filtered, washed, and the filter cake was dried in a 100 °C oven for 12 h, followed by calcination at 550 °C for 6 h to obtain a solid powder sample.

[0059] Characterization tests by XRD, SEM, and ICP revealed that the ZSM-48 molecular sieves synthesized in Examples 1-3 had a silica-to-alumina ratio ranging from 20 to 100. Their morphology exhibited a spindle-shaped structure formed by aggregates of nanoneedle-like crystals, all being highly crystalline, uniformly distributed, and low silica-to-alumina ratio products. Analysis of Examples 4-5 showed that when sodium hydroxide or potassium hydroxide was used as a single alkali source, the provided OH...- The silicon source is difficult to fully dissociate, making it impossible to synthesize highly crystalline ZSM-48 products from the aluminum-rich initial gel. Analysis of Examples 1 and 6 shows that only by following the specific material addition sequence described in this invention can highly crystalline low-silicon-aluminum ratio ZSM-48 molecular sieves be synthesized. Directly mixing all raw materials to form a gel only yields amorphous silica products. This is mainly because the silicon source has not undergone OH treatment. - The silicon fragments are fully dissolved and dissociated, making it impossible to effectively introduce aluminum species into the framework. According to the method provided in this invention, a dual-alkali source is used to first fully dissociate the silicon source, and then introduce the aluminum source and template agent, thus synthesizing highly crystalline ZSM-48 molecular sieves with a low silicon-to-aluminum ratio.

[0060] Comparative Example 1 (using hexamethyl diammonium bromide (HMBr2) as a template agent)

[0061] According to the molar ratio of SiO2, Al2O3, HMBr2, Na2O, and H2O of 1:0.01:0.02:0.07:40, 0.33 g of sodium aluminate, 40 g of silica sol (30 wt.%), 0.89 g of NaOH, 1.45 g of HMBr2, 116 g of H2O, and 0.024 g of seed crystals were weighed out and subjected to hydrothermal crystallization treatment. The obtained solid product was filtered, washed, and dried in an oven at 100 ℃ for 12 h, followed by calcination at 550 ℃ for 6 h to obtain a low silica-to-alumina ratio ZSM-48 molecular sieve. ICP analysis showed that the silica-to-alumina ratio of the obtained ZSM-48 molecular sieve was 95.

[0062] Application Example 1

[0063] The ZSM-48 molecular sieves synthesized in Examples 1-3 and Comparative Example 1 were subjected to ammonium exchange to obtain an H-type ZSM-48 molecular sieve support. Catalysts with a Pt loading of 0.6 wt.% were prepared using an impregnation method, denoted as Pt / Example 1, Pt / Example 2, Pt / Example 3, and Pt / Comparative Example 1, respectively. A fixed-bed reactor was used to investigate the catalytic performance of the catalysts in the hydroisomerization of n-hexadecane. Reaction conditions: reaction temperature 250-360 ℃, reaction pressure 4 MPa, WHSV = 0.54 h⁻¹. -1 The hydrogen / hydrocarbon molecule volume ratio is 300.

[0064] The reaction results are as follows Figure 11-13The catalysts prepared using ZSM-48 molecular sieves synthesized in Examples 1-3 as supports exhibited superior catalytic performance compared to the catalyst prepared using Comparative Example 1 as a support. The ZSM-48 molecular sieve synthesized using pyrrolidine as a template agent showed better catalytic performance than ZSM-48 synthesized using hexamethyldiammonium bromide as a template agent; the sample with a silica-alumina ratio of 24 (Pt / Example 3) showed higher catalytic activity than Pt / Example 1 (silicon-alumina ratio of 91). In the hydroisomerization reaction of n-hexadecane, the selectivity of isohexadecane remained above 90 wt.% within the range of 20-90 wt.% for n-hexadecane conversion, maintaining a high overall selectivity. Table 1 lists the reaction temperatures and detailed results for each catalyst at the highest isohexadecane yield. Pt / Example 3 achieved the highest conversion (95 wt.%) while maintaining a high isohexadecane selectivity of 97 wt.%, with an isohexadecane yield reaching 92 wt.%, and at a relatively low reaction temperature (310 °C). The catalyst prepared using hexamethyl diammonium bromide as a template agent (Pt / Comparative Example 1) had the highest yield of isohexadecane, only 79 wt.%, and the reaction temperature was 320 °C.

[0065] Table 1: Results of the hydroisomerization reaction of n-hexadecane

[0066]

[0067] The above description is merely a preferred embodiment of the present invention, but is not intended to limit the scope of the substantive technical content of the present invention. The substantive technical content of the present invention is broadly defined within the scope of the claims. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the claims and technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, shall fall within the scope of protection claimed by the present invention.

Claims

1. A method for synthesizing low silica-to-alumina ratio ZSM-48 molecular sieves in a dual-alkali source system, characterized in that, The steps are as follows: (1) First, place the mixture of alkali source, silicon source and deionized water in 40-80 ℃ and stir vigorously to obtain mixture A; (2) After mixing the aluminum source, template agent and deionized water evenly, slowly add it to the mixture A in step (1) and stir thoroughly to obtain the initial gel mixture; (3) Transfer the above initial gel mixture into a crystallization kettle for hydrothermal crystallization treatment. The crystallization temperature is 130-210℃ and the crystallization time is 12-72 h; either static crystallization or dynamic crystallization. (4) The solid product obtained by the above hydrothermal crystallization treatment is filtered, washed, dried and calcined to obtain ZSM-48 molecular sieve. The template agent is pyrrolidine; The alkali source is a mixture of sodium hydroxide and potassium hydroxide; The silicon source, aluminum source, template agent, alkali source, and deionized water are mixed in a molar ratio of 1:(0.01-0.05):(0.05-1.2):(0.02-0.12):(18-50). The molar ratio of sodium hydroxide to potassium hydroxide is Na₂O:K₂O = 0.2-8.

0.

2. The method according to claim 1, characterized in that, The stirring time should be 2-6 hours. The crystallization temperature is 155-195 ℃, and the crystallization time is 24-48 h; The dynamic crystallization speed is 20-1000 RPM.

3. The method according to claim 2, characterized in that, The silicon source, aluminum source, template agent, alkali source and deionized water are in a molar ratio of 1:(0.01-0.05):(0.2-0.8):(0.03-0.08):(25-40).

4. The method according to claim 1, characterized in that, The pH range of the initial gel mixture was controlled to be 12.3-13.

8.

5. The method according to claim 1, characterized in that, The stirring time in step (1) is 0.5-4 h, and the stirring temperature is 40-80 ℃; In step (2), the aluminum source, template agent and deionized water are mixed and stirred for 0.1-1 h at room temperature.

6. The method according to claim 2, characterized in that, The aluminum source is one or a mixture of two or more of aluminum nitrate, aluminum sulfate, sodium aluminate, and aluminum isopropoxide; The silicon source is one or a mixture of two or more of the following: fumed silica, silica sol, tetraethyl orthosilicate, and water glass.

7. The method according to claim 2, characterized in that, The morphology of the low silicon-to-aluminum ratio ZSM-48 molecular sieve is a spindle shape formed by the aggregation of nanoneedles, and its silicon-to-aluminum ratio ranges from 20 to 100.

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