Method for synthesizing ZSM-48 molecular sieve with low silica-alumina ratio

Abstract

The invention provides a method for synthesizing a ZSM-48 molecular sieve with a low silica-alumina ratio. The molar ratio of SiO2 to Al2O3 in the molecular sieve is 20 to 100. The preparation method comprises the following steps: uniformly mixing a silicon source, an inert aluminum source, a template agent, water, alkali and a seed crystal to obtain mixed gel; the molar ratio of the effective components SiO2 to Al2O3 to the template agent to H2O to the alkali in the mixed gel is 1: (0.01-0.05): (0.08-0.6): (20-50): (0.08-0.4); and carrying out hydrothermal crystallization on the mixed gel, and filtering, washing, drying and roasting a crystallization product to obtain the ZSM-48 molecular sieve with the low silica-alumina ratio. The ZSM-48 molecular sieve prepared by the method is low in silica-alumina ratio, relatively high in acidity and crystallinity, uniform in morphology and rich in accumulated mesopores among crystal grains, the activity of a unit amount of catalyst is relatively high, quick contact between active sites of the catalyst and reactant molecules is facilitated, the diffusion performance is relatively good, and industrial application of the ZSM-48 molecular sieve is relatively facilitated.

Description

Technical Field

[0001] The present invention belongs to the technical field of molecular sieve synthesis, and particularly relates to a method for synthesizing ZSM-48 molecular sieve with a low silica-alumina ratio. Background Art

[0002] The heterocrystals of ZSM-48 grown epitaxially outside the ZSM-39 octahedron were first discovered, and then pure-phase ZSM-48 was synthesized in a system of silicon source, aluminum source, tetramethylammonium ion and n-propylamine. ZSM-48 is a type of high-silica zeolite molecular sieve, belonging to the orthorhombic crystal structure, with a molecular sieve having a ten-membered ring pore structure and a pore size of 0.53 nm × 0.56 nm. The ZSM-48 molecular sieve catalyst with a low silica-alumina ratio has good isomer selectivity in the hydroisomerization reaction and has received extensive attention.

[0003] The main advantage of the ZSM-48 molecular sieve is that its unique pore structure is an excellent molecular sieve for the hydroisomerization of n-alkanes, and it can also catalyze the isomerization reaction at a lower temperature, which can not only reduce energy consumption but also improve the conversion rate of the isomerization reaction. The defect is that the silica-alumina ratio of the ZSM-48 molecular sieve is too high, resulting in a small amount of acid, which greatly restricts its isomerization reaction performance. The synthesis of ZSM-48 with a low silica-alumina ratio is also a challenge. At present, the research focus of the ZSM-48 molecular sieve is still on its synthesis process, and the synthesis process of this molecular sieve has the following characteristics: the silica-alumina ratio of the synthesized molecular sieve is high. Even if an expensive template agent is used, the silica-alumina ratio of the generally synthesized molecular sieve is greater than 100; if a template agent with low cost is used, although the cost of synthesizing the molecular sieve is reduced, the silica-alumina ratio of the synthesized molecular sieve will be very high and it is easy to be accompanied by heterocrystals. The amount of acid contained in the unit molecular sieve of the ZSM-48 molecular sieve with a high silica-alumina ratio is small, which limits its wide application in catalytic reactions. Only by improving the synthesis method to synthesize the ZSM-48 molecular sieve with a low silica-alumina ratio and increasing the amount of acid in the unit molecular sieve can the bottleneck restricting the wide application of the ZSM-48 molecular sieve be broken through. Summary of the Invention

[0004] To solve the problem that it is difficult to synthesize ZSM-48 molecular sieve at a relatively low silica-alumina ratio, the present invention provides a method for synthesizing ZSM-48 molecular sieve with a low silica-alumina ratio. The present invention stirs and mixes a silicon source, an inert aluminum source, a template agent, an alkali, water and ZSM-48 seeds evenly, and obtains the ZSM-48 molecular sieve with a low silica-alumina ratio through hydrothermal crystallization.

[0005] The present invention adopts the following technical scheme:

[0006] A method for synthesizing ZSM-48 molecular sieve with a low silica-alumina ratio, comprising the following steps:

[0007] (1) Mix the ZSM-48 seed crystal, template agent, silicon source, inert aluminum source, alkali and water evenly to obtain a mixed gel; the template agent is hexamethonium bromide;

[0008] (2) Subject the mixed gel to hydrothermal crystallization, and the crystallization product is filtered, washed, dried and calcined to obtain a low silica-alumina ratio sodium type ZSM-48 molecular sieve;

[0009] The molar ratios of the active ingredients SiO2, Al2O3, template agent, H2O and alkali in the mixture prepared in step (1) are 1: 0.01-0.05: 0.1-0.6: 20-50: 0.08-0.4, where SiO2 is derived from the silicon source; the molar ratio of SiO2 to Al2O3 in the low silica-alumina ratio ZSM-48 molecular sieve is 20 to 100.

[0010] The inert aluminum source is high-purity pseudo-boehmite or aluminum hydroxide.

[0011] The addition amount of the ZSM-48 seed crystal is 0.012%-10% of the mass of SiO2.

[0012] The pH of the mixed gel is 9-10.

[0013] The molar ratios of the active ingredients SiO2, Al2O3, template agent, H2O and alkali in the mixture are 1: 0.0429-0.05: 0.1-0.6: 20-50: 0.08-0.4, and the molar ratio of SiO2 to Al2O3 in the low silica-alumina ratio sodium type ZSM-48 molecular sieve is 20 to 70.

[0014] The seed crystal ZSM-48 molecular sieve is a calcined sodium type molecular sieve or an uncalcined sodium type molecular sieve, and the molar ratio of silicon dioxide to alumina is between 68 and 98.

[0015] The mixing temperature in step (1) is 20-90 °C.

[0016] The hydrothermal crystallization temperature is 140-200 °C; the hydrothermal crystallization time is 1-8 days.

[0017] The alkali includes sodium hydroxide or potassium hydroxide; the silicon source includes silica sol, fumed silica or white carbon black.

[0018] The calcination temperature is programmed to 550-600 °C and calcined for 8-10 h.

[0019] The beneficial effects of the present invention are as follows:

[0020] Traditional methods generally can only synthesize ZSM-48 molecular sieves with a high silica-alumina ratio. The amount of acid contained in a single molecular sieve is relatively small, thus limiting its wide application in catalytic reactions. In the present invention, ZSM-48 molecular sieves with a low silica-alumina ratio are successfully obtained: the charge density and hydrophobicity of the template agent affect the assembly of silicon-aluminum species. In the present invention, hexamethonium bromide is used as the template agent. The strong orientation of the double quaternary ammonium salt can reduce the energy barrier for Al 3+ to enter the framework, while inhibiting the high-silica condensation rate and allowing aluminum to participate more fully in the framework construction. Each Al 3+ substituting Si 4+ in the ZSM-48 framework will introduce a negative charge, which needs to be neutralized by a cation (such as Na + ). As a charge compensator, sodium ions can promote more Al 3+ to enter the framework, thereby reducing the silica-alumina ratio; Na + may participate in the cooperative orientation of the template agent (such as organic amine), affecting the crystallization kinetics and crystal morphology. The pH of the obtained mixed gel is 9-10 (slightly lower than the pH of 11-12 of traditional ZSM-48), reducing the competitive coordination of OH - to Al 3+ and promoting the entry of Al into the framework. The inert aluminum source (such as pseudo-boehmite) slowly hydrolyzes under hydrothermal conditions, providing a controllable Al 3+ release rate, avoiding the formation of amorphous alumina or impurity phases (such as Al-rich impurity zeolites) caused by local supersaturation, and ensuring the uniform embedding of Al into the framework; the inert aluminum source can also provide aluminum-oxygen species in a specific polymerization state (such as Al(OH) 4- ), which matches the hydrolysis product (SiO2) of the silicon source (such as silica sol), promotes the orderly connection of Si-O-Al bonds, and improves the crystallinity. Through the above synergistic effects, ZSM-48 molecular sieves with high crystallinity and a low silica-alumina ratio are synthesized. The ZSM-48 molecular sieves with a low silica-alumina ratio prepared by this method have a higher amount of acid contained in a single molecular sieve, so the activity of the catalyst per unit amount is higher; in addition, the ZSM-48 molecular sieves with a low silica-alumina ratio also have high crystallinity, a uniform morphology, and abundant inter-granular packing mesopores, which are beneficial for the rapid contact between the active sites of the catalyst and the reactant molecules and have good diffusion performance. The synthesis process of the present invention is easy to repeat and scale up production, with good economic and environmental benefits, which is more conducive to the industrial application of the catalyst. The above advantages enable the catalyst to perform excellently in the hydroisomerization reaction and have high industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is an X-ray diffraction (XRD) test pattern of the samples prepared in Examples 1-4.

[0022] Figure 2 It is an X-ray diffraction (XRD) test pattern of the samples prepared in Examples 1 and 5.

[0023] Figure 3 Scanning electron microscope (SEM) images of the samples prepared in Examples 1, 2, 4, and 5.

[0024] Figure 4 X-ray diffraction (XRD) test pattern of the sample prepared in Comparative Example 1. Detailed implementation manners

[0025] Unless otherwise specified, the raw materials in the examples of this application are all purchased through commercial channels. The ZSM-48 zeolite molecular sieve used as the seed is from a purchased commercial product, and its oxide silica-alumina ratio is 68.396.

[0026] Unless otherwise specified, the analysis methods in the examples adopt the conventional settings and conventional analysis methods of the instruments.

[0027] The analysis methods in the examples of this application are as follows:

[0028] X-ray diffraction (XRD) analysis is used to calculate the relative crystallinity and metal dispersion of the sample: the analysis instrument used is the DX-2700B model X-ray diffractometer produced by Dandong Haoyuan Instrument Co., Ltd. The measurement conditions are as follows: CuKα fluorescence radiation, tube voltage 40Kv, tube current 30mA, scanning step 0.02°, scanning range 2θ = 5 - 60°, scanning speed 6° / min.

[0029] Scanning electron microscopy (SEM) test of the sample: The analysis instrument is the JSM-7900F type scanning electron microscope produced by JEOL, Japan. The analysis conditions are: the acceleration voltage is 5kV, and the sample is ultrasonically dispersed in absolute ethanol before testing.

[0030] Physical adsorption characterization of nitrogen on the sample: The analysis instrument is the JW-TB400 four-station nitrogen physical adsorption instrument of Beijing Jingwei Gaobo Science and Technology Co., Ltd. The test method is as follows: 1) Pretreatment: Load 0.15g of the catalyst sample to be tested (powder samples need to be tableted) into a quartz tube and vacuum-treat it at 350°C for 2h to remove the moisture and impurities adsorbed by the Silicalite-1 molecular sieve material; 2) Conduct nitrogen adsorption / desorption experiments at -195.7°C; 3) Use the Brunauer-Emmett-Teller (BET) equation to calculate the micropore and mesopore specific surface area of the sample, and the t-plot method to calculate the internal specific surface area, external specific surface area, area, and pore volume of the Silicalite-1 molecular sieve sample. The total pore volume is calculated based on the nitrogen adsorption amount at the relative pressure P / P0 = 0.99.

[0031] Elemental analysis (XRF) of the sample: The relative contents of SiO2 and Al2O3 in the sample were analyzed using an X-ray fluorescence spectrometer of model S8 TICER from Bruker, Germany.

[0032] In the examples of the present application, the ZSM-48 seeds were dried at 110 °C and calcined at 550 °C for 8 hours to obtain ZSM-48 seeds with the template removed.

[0033] Example 1

[0034] This example provides a method for synthesizing a ZSM-48 molecular sieve with a low silica-alumina ratio. The specific steps are as follows:

[0035] (1) Under stirring, NaOH and high-purity pseudo-boehmite were added to deionized water. After complete dissolution, the template agent hexamethonium bromide was added, and then silica sol (30 wt%) was slowly added dropwise. After mixing evenly, the ZSM-48 seeds with the template removed were added, and the mixture was stirred evenly at room temperature to obtain a mixed gel; the molar ratio of the effective components SiO2, Al2O3, template agent, H2O, and alkali in the mixed gel was 1:0.01666:0.6:26:0.1, where SiO2 was derived from the silica sol, and the addition amount of the ZSM-48 seeds was 5% of the mass of SiO2.

[0036] (2) The mixed gel obtained in step (1) was placed in a hydrothermal autoclave, and then the hydrothermal autoclave was placed in an oven at 180 °C for hydrothermal crystallization treatment for 24 h;

[0037] (3) The crystallized product obtained in step (2) was filtered, washed, dried in an oven at 110 °C, and then placed in a muffle furnace. The temperature was programmed to rise to 550 °C and calcined in an air stream for 8 h to obtain a Na-type ZSM-48 molecular sieve.

[0038] The sample obtained in this example was denoted as P1#.

[0039] Example 2

[0040] The operation was the same as that in Example 1, except that the crystallization time was changed to 48 h, and other operations were the same.

[0041] The sample obtained in this example was denoted as P2#

[0042] Example 3

[0043] The operation was the same as that in Example 1, except that the crystallization time was changed to 72 h, and other operations were the same.

[0044] The sample obtained in this example was denoted as P3#.

[0045] Example 4

[0046] The operation was the same as that in Example 1, except that the crystallization time was changed to 96 h, and other operations were the same.

[0047] The sample obtained in this example was denoted as P4#.

[0048] Example 5

[0049] The operation was the same as that in Example 1, except that only the molar ratio of the initial gel was changed. The molar ratio of the active ingredients SiO2, Al2O3, template agent, H2O and alkali in the mixed gel was 1:0.05:0.6:26:0.1. The sample obtained in this example was denoted as P5#.

[0050] Comparative Example 1

[0051] The operation was the same as that in Example 1, except that only the aluminum source of the initial gel was changed to aluminum sulfate octadecahydrate. This active aluminum source could not successfully synthesize ZSM-48 under the condition of SiO2 / Al2O3 = 60. The sample obtained in this comparative example was denoted as D1#.

[0052] As shown in Table 1 (the reference sample was commercial ZSM-48 molecular sieve with a crystallinity of 100%) and Figures 1-4 as shown, high-crystallinity pure-phase ZSM-48 could be synthesized under the condition of crystallization for 1 - 4 days. The SiO2 / Al2O3 measured by XRF elemental analysis was about 60, which was close to the feed silicon-aluminum ratio. Moreover, the silicon-aluminum ratio could be adjusted in the range of 20 - 70, and the synthesized ZSM-48 with a low silicon-aluminum ratio still maintained high crystallinity and uniform particle size. However, when the active aluminum source aluminum sulfate octadecahydrate was used, ZSM-48 molecular sieve with SiO2 / Al2O3 = 60 could not be successfully synthesized under the same conditions. In the system of the present invention, an inert aluminum source could synthesize ZSM-48 molecular sieve with a low silicon-aluminum ratio.

[0053] Table 1 Physical adsorption, relative crystallinity and elemental analysis of each sample

[0054]

Claims

1. A method for synthesizing a low silicon-aluminum ratio ZSM-48 molecular sieve, characterized in that: The steps include: (1) uniformly mixing ZSM-48 seed crystals, a template, a silicon source, an inert aluminum source, a base and water to obtain a mixed gel; wherein the template is hexamethyldiammonium bromide; (2) subjecting the mixed gel to hydrothermal crystallization, filtering, washing, drying and calcining the crystallized product to obtain a low silicon-aluminum ratio ZSM-48 molecular sieve; The molar ratio of the active ingredients SiO2, Al2O3, template, H2O and alkali in the mixed gel of step (1) is 1: 0.01-0.05: 0.1-0.6: 20-50: 0.08-0.4, wherein SiO2 originates from a silicon source; and the molar ratio of SiO2 to Al2O3 in the low silicon-aluminum ratio ZSM-48 molecular sieve is 20-100.

2. A method for synthesizing a low silicon-to-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The inert aluminum source is high-purity pseudo-boehmite and / or aluminum hydroxide.

3. The method for synthesizing a low silicon-to-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The added amount of the ZSM-48 seed crystals is 0.012%-10% of the mass of the SiO2.

4. The method for synthesizing a low silicon-to-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The molar ratio of the effective components SiO2, Al2O3, template, H2O and alkali in the mixed gel is 1: 0.01429-0.05: 0.1-0.6: 20-50: 0.08-0.20; the molar ratio of SiO2 to Al2O3 in the low silicon-aluminum ratio ZSM-48 molecular sieve is 20-70.

5. The method for synthesizing a low silicon-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The seed ZSM-48 molecular sieve is a calcined sodium-type ZSM-48 molecular sieve or an uncalcined sodium-type ZSM-48 molecular sieve, and the molar ratio of SiO2 to Al2O3 is 68-98.

6. The method for synthesizing a low silicon-to-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The hydrothermal crystallization temperature is 140-200° C.; the hydrothermal crystallization time is 1-8 days.

7. The method for synthesizing a low silicon-to-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The alkali includes sodium hydroxide or potassium hydroxide; the silicon source includes silica sol, fumed silica or white carbon black.

8. The method for synthesizing a low silicon-to-aluminum ratio ZSM-48 molecular sieve according to claim 1, characterized in that: The calcination temperature is programmed to 550-600° C. and calcined for 8-10 hours.

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