A boron-containing ZSM-48 molecular sieve and its preparation method and application

Abstract

The present invention discloses a boron-containing ZSM-48 molecular sieve, its preparation method, and application. The boron-containing ZSM-48 molecular sieve has a silicon-boron ratio (SiO2 / B2O3) of 10 to 80; a framework aluminum substitution rate of 40% to 70%; and a weak acid content of 45% to 80% of the total acid content. The boron-containing ZSM-48 molecular sieve provided by the present invention is used in the toluene-methanol methylation reaction and exhibits high conversion, high C8 selectivity, and long life.

Description

Technical Field

[0001] The present invention belongs to the technical field of molecular sieve materials, and in particular relates to a boron-containing ZSM-48 molecular sieve and a preparation method and application thereof. Background Art

[0002] As a raw material for a variety of fine chemical products, China's demand for paraxylene (PX) continues to grow. To meet market demand, toluene disproportionation and toluene methylation production processes have emerged. However, due to the large demand gap, large quantities of paraxylene are still imported annually. The toluene-methanol alkylation reaction can convert toluene into paraxylene, which has higher application value.

[0003] A variety of catalytic materials have been explored for the toluene-methanol alkylation reaction. Modified molecular sieves, such as X, Y, ZSM-5, and ZSM-48, as well as basic oxides such as magnesium and calcium, have all demonstrated promising catalytic performance. However, low xylene selectivity and catalyst life limit the industrial application of the toluene-methanol alkylation reaction. Therefore, modified catalysts are essential and urgently needed.

[0004] While the selectivity for C8 aromatics in methanol-toluene alkylation catalysts disclosed in existing literature is relatively good, the conversion of methanol and toluene still has significant room for improvement. Therefore, the development of high-conversion, highly stable molecular sieve catalysts for the alkylation of toluene with methanol has broad market demand and promising application prospects.

[0005] Currently, the method for introducing boron into the synthesis and modification of ZSM-35 molecular sieves generally involves impregnation with inorganic boron. Common impregnation modifiers include boric acid (CN201510700341.4, CN201610988624.8), boron phosphate (CN201510442870.9), boron oxide (CN201510045326.0), and ammonium borate (CN201410342698.5). Conventional impregnation methods can neutralize and modulate the surface acidity of the molecular sieve, but these treatments significantly reduce catalyst activity. Even at high temperatures, the conversion rate remains low. Furthermore, the easy loss of boron during high-temperature reactions is a drawback of impregnation methods.

[0006] CN 107265478A discloses a boron-containing zeolite molecular sieve catalyst, a preparation method, and an application thereof. The catalyst is characterized in that boron-containing sodium zeolite molecular sieve raw powder is ion-exchanged with an ammonium salt aqueous solution or dilute hydrochloric acid to obtain a hydrogen-type molecular sieve raw powder. During the ion exchange process, boron atoms are lost to varying degrees, resulting in a decrease in the ability of the molecular sieve to control the internal acidity.

[0007] CN 103191776A discloses a method for preparing a ZSM-5 molecular sieve catalyst, which is characterized in that a directly synthesized boron-containing molecular sieve B-ZSM-5 is further subjected to boron impregnation modification. This operation causes loss and waste of boron source during the direct synthesis of the boron-containing molecular sieve, and the efficiency of boron introduction is very low, resulting in a reduction in the ability of boron atoms to replace aluminum atoms, thereby reducing the catalytic ability of the molecular sieve catalyst.

[0008] CN201310301780.9 discloses a method for preparing a microporous molecular sieve catalyst modified by borate. This method uses borate with a larger molecular size as a modifier to prepare a boron-containing microporous molecular sieve catalyst. However, there are the following problems: borate is sensitive to water and will decompose when exposed to water during the preparation process, thereby causing the molecular sieve pores to be blocked; during the drying process, the borate will volatilize, resulting in the loss of the modifier and other problems.

[0009] The method disclosed in CN200910048322.2 uses an inexpensive cyclic amine, hexamethyleneimine (HMI) or piperidine (PI), or a mixture of the two, as an organic base template, followed by a directing agent synthesis process to produce a directing agent. A certain amount of the directing agent is then added, followed by hydrothermal crystallization, filtration, washing, drying, and calcination to produce a boron-containing molecular sieve. This preparation method is complex and can result in uneven dispersion of the boron oxide, which in turn reduces the catalytic performance of the catalyst.

[0010] CN201080050221.X discloses a borosilicate ZSM-48 molecular sieve synthesized using a novel structure-directing agent, wherein the molar ratio of silicon oxide to boron oxide is 40 to 400. This borosilicate ZSM-48 is used as a catalyst for the selective hydrogenation of heavy normal paraffins into lighter normal paraffin products while minimizing the formation of isoparaffins. However, this catalyst preparation method results in excessively long crystallization times and unsatisfactory molecular sieve crystallinity, increasing production costs.

[0011] As can be seen from the above review, although the method for synthesizing boron-containing molecular sieves is relatively mature, it is difficult to synthesize molecular sieves with high boron content. At the same time, when applied to the molecular sieve synthesis process, the production cost is high, and the application of boron-containing molecular sieve catalysts in the toluene methanol methylation reaction is rare and the catalytic efficiency is low, which greatly limits the application of boron-containing molecular sieves. Summary of the Invention

[0012] In order to solve the problems existing in the prior art of low boron content in boron-containing ZSM-48 molecular sieves and low catalytic efficiency in toluene-methanol methylation reactions, in particular low toluene conversion rate, low xylene (C8) yield, low para-xylene selectivity and poor molecular sieve stability, the present invention provides a boron-containing ZSM-48 molecular sieve and a preparation method and application thereof. The boron-containing ZSM-48 molecular sieve is used in toluene-methanol methylation reactions and has the characteristics of high conversion rate, high C8 selectivity and long service life.

[0013] The first aspect of the present invention provides a boron-containing ZSM-48 molecular sieve, wherein the silicon-boron ratio of the molecular sieve, calculated as SiO2 / B2O3, is 10 to 80; wherein the framework aluminum substitution rate in the boron-containing ZSM-48 molecular sieve is 40% to 70%, preferably 43 to 63%; and the weak acid content accounts for 45% to 80% of the total acid content, preferably 50% to 75%.

[0014] Furthermore, the molecular sieve has a silicon-boron ratio, calculated as SiO2 / B2O3, of 20 to 75.

[0015] Furthermore, the weak acid content of the boron-containing ZSM-48 molecular sieve of the present invention is increased by 10% to 50% compared with the corresponding ZSM-48 molecular sieve without boron.

[0016] Furthermore, the pore volume of the boron-containing ZSM-48 molecular sieve is 0.15 to 0.87 cm 3 / g, preferably 0.30 to 0.65 cm 3 / g, mesopore specific surface area 40~400 m 2 / g, preferably 170 to 350 m 2 / g, with a specific surface area of ​​250-500 m 2 / g.

[0017] Furthermore, the boron-containing ZSM-48 molecular sieve has a morphology of hexagonal prism crystals, and the crystal size is large, about 4 to 6 μm.

[0018] Furthermore, the relative crystallinity of the boron-containing ZSM-48 molecular sieve is 85% to 95%.

[0019] A second aspect of the present invention provides a method for preparing a boron-containing ZSM-48 molecular sieve, comprising the following steps:

[0020] (1) Preparation of glue solution: Mix the silicon source, aluminum source, template R, boron source and water, then add the complexing agent and alkali source to a pH of 11.5-14.0 to obtain glue solution;

[0021] (2) subjecting the gel described in step (1) to liquid crystallization to obtain a boron-containing ZSM-48 molecular sieve;

[0022] Wherein, the template agent R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and cetyltrimethylammonium bromide (CTAB);

[0023] The complexing agent is selected from at least one of EDTA, ethylene glycol, and chitosan, more preferably EDTA.

[0024] Furthermore, in step (1), the silicon source includes at least one of silica sol, sodium silicate, tetraethyl orthosilicate and white carbon black; the aluminum source includes at least one of sodium aluminate, aluminum sulfate and aluminum sol; the boron source is at least one of sodium borate, boric acid and ammonium borate; and the alkali source is selected from at least one of ammonia water, sodium hydroxide and potassium hydroxide.

[0025] Furthermore, in step (1), the silicon source calculated as SiO2, the boron source calculated as B2O3, the aluminum source calculated as Al2O3, and the - The molar ratios of alkali source, template agent R, complexing agent and water are as follows: SiO2 / B2O3 is 15-250, SiO2 / Al2O3 is 50-300, complexing agent / SiO2 is 0.02-0.2, R / SiO2 is 0.05-0.6, OH - / SiO2 is 0.05~0.3, and H2O / SiO2 is 10~40.

[0026] Furthermore, in step (1), the template agent R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and cetyltrimethylammonium bromide (CTAB), and the molar ratio is 1: (0.15-0.75): (0.4-1).

[0027] Furthermore, in step (1), the pH is preferably 10 to 13.5.

[0028] Furthermore, in step (2), the crystallization treatment conditions are: treatment at 160-185° C. for 24-180 h.

[0029] Furthermore, in step (2), after the crystallization step is completed, the product can be obtained from the obtained mixture by any conventional separation method and roasting treatment. As the separation method, for example, a method of filtering, washing and drying the obtained mixture can be cited. Here, the filtration, washing and drying can be carried out in any manner conventionally known in the art. Specifically, as the filtration, for example, the obtained product mixture can be simply filtered by suction. As the washing, for example, washing with deionized water and / or ethanol can be cited. As the drying temperature, for example, 40 to 250° C. can be cited, preferably 60 to 150° C., and as the drying time, for example, 8 to 30 hours can be cited, preferably 10 to 20 hours. The drying can be carried out under normal pressure or under reduced pressure. The roasting can be carried out in any manner conventionally known in the art, for example, the roasting temperature is generally 300 to 800° C., preferably 400 to 650° C., and the roasting time is generally 1 to 12 hours, preferably 2 to 6 hours. In addition, the calcination is generally performed in an oxygen-containing atmosphere, such as air or oxygen atmosphere.

[0030] Furthermore, the mass yield of B-ZSM-48 obtained by the preparation method of the present invention is 85% to 95%.

[0031] The third aspect of the present invention provides the use of the boron-containing ZSM-48 molecular sieve in the methylation of toluene and methanol to produce p-xylene.

[0032] Furthermore, in the reaction of toluene and methanol methylation to produce p-xylene, the raw materials are toluene and methanol, and the molar ratio of toluene / methanol is 1-3.

[0033] Furthermore, the toluene methanol methylation reaction to produce p-xylene is carried out in a fixed bed reactor at a reaction temperature of 380-500°C and a weight space velocity of 1-5 h -1 , the reaction pressure is 0.4~1MPa.

[0034] Compared with the prior art, the method of the present invention has the following advantages:

[0035] (1) The boron-containing ZSM-48 molecular sieve provided by the present invention has a special morphology, which is a hexagonal prism. The crystals have high crystallinity and large crystal size, about 4 to 6 μm. In addition, boron atoms replace the framework aluminum. Compared with conventional silicon-aluminum ZSM-48 molecular sieves, the framework aluminum substitution rate in the boron-containing molecular sieve reaches 40% to 70%, and the weak acid content is significantly improved.

[0036] (2) The method for preparing boron-containing ZSM-48 molecular sieves provided by the present invention uses an inexpensive template instead of an expensive template, thus having the advantage of low synthesis cost. Furthermore, the innovative addition of a complexing agent can adjust the morphology and crystallinity of the molecular sieve, resulting in large crystals. Furthermore, the complexing agent can interact with the boron source to increase the degree of boron substitution for aluminum in the molecular sieve framework, thereby reducing the silicon-boron ratio.

[0037] (3) The inventors found that acid strength is one of the key factors in the toluene-methanol alkylation reaction because toluene-methanol alkylation is an electrophilic substitution reaction. The strength of the acid required to catalyze the alkylation of toluene and methanol is weaker than the strength of the acid required to catalyze the disproportionation of toluene and the isomerization of xylene, but slightly stronger than the strength of the acid required for the methanol-to-olefins reaction. Therefore, it is necessary to modify the molecular sieve to regulate the acidity of the molecular sieve. The researchers found that the weak acid brought by boron to the molecular sieve can only catalyze methylation and alkylation reactions in the MTX reaction, and cannot catalyze hydrogen transfer, aromatization and other reactions, thereby inhibiting the occurrence of secondary reactions, reducing the carbon deposition deactivation rate, and extending the service life of the catalyst. During the reaction of boron-containing molecular sieves, boron can bond with the bridging hydroxyl groups in the molecular sieve, or replace part of the skeleton aluminum, resulting in a decrease in strong acid sites and an increase in weak acid sites. Therefore, it is hoped that by adding boron to regulate the content of weak acid in the molecular sieve, side reactions can be inhibited and xylene selectivity can be improved. The boron-containing ZSM-48 molecular sieve provided by the present invention is used as a catalyst for catalyzing the methylation reaction of toluene and methanol. The framework aluminum substitution rate in the boron-containing molecular sieve reaches 40% to 70%, and the weak acid content is significantly improved, which can effectively inhibit the occurrence of side reactions, extend the service life, and enhance the stability of the catalyst. At the same time, it has the effects of high toluene conversion rate, high C8 yield and high para-xylene selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is the XRD pattern of B-ZSM-48 synthesized in Example 1;

[0039] Figure 2 This is a low-magnification scanning electron microscope photograph of B-ZSM-48 synthesized in Example 1;

[0040] Figure 3 This is a high-magnification scanning electron microscope photograph of B-ZSM-48 synthesized in Example 1. DETAILED DESCRIPTION

[0041] The specific embodiments of the present invention are described in detail below. However, it should be noted that the protection scope of the present invention is not limited by these specific embodiments, but is determined by the claims.

[0042] All publications, patent applications, patents, and other references mentioned in this specification are incorporated herein by reference. Unless otherwise defined, all technical and scientific terms used in this specification have the meanings commonly understood by those skilled in the art. In the event of conflict, the definitions in this specification will prevail.

[0043] When this specification uses the prefix "well known to those skilled in the art", "prior art" or similar terms to introduce materials, substances, methods, steps, devices or components, the objects introduced by the prefix include those commonly used in the art when this application is filed, but also include those that are not commonly used at present but will become generally recognized in the art to be suitable for similar purposes.

[0044] It should be noted that two or more aspects (or implementation methods) disclosed in the context of this specification can be arbitrarily combined with each other, and the technical solutions (such as methods or systems) thus formed are part of the original disclosure of this specification and also fall within the scope of protection of the present invention.

[0045] Unless otherwise specified, all percentages, parts, ratios, etc. mentioned in this specification are based on weight, unless the weight basis does not conform to the general understanding of those skilled in the art.

[0046] In the context of this specification, including in the following examples and comparative examples, the X-ray powder diffractometer model of the sample is a Panalytical X PERPRO X-ray powder diffractometer, and the physical phase of the sample is analyzed using a CuKα ray source (λ = 1.54Å), a nickel filter, a 2θ scanning range of 2 to 50°, an operating voltage of 40 kV, a current of 40 mA, and a scanning rate of 10° / min.

[0047] In the context of this specification, including in the following examples and comparative examples, the scanning electron microscope (SEM) used for the samples is an S-4800II field emission scanning electron microscope. The crystal particle size of the sample is measured by observing the molecular sieve at a magnification of 10,000 times using the scanning electron microscope, randomly selecting an observation field, and calculating the average value of the sum of the particle sizes of all crystals in the observation field. This operation is repeated a total of 10 times. The average value of the sum of the 10 average values ​​is taken as the crystal particle size.

[0048] In the context of this specification, the relative crystallinity is calculated by comparing the sum of the XRD characteristic diffraction peak areas of the boron-containing ZSM-48 molecular sieve with a ZSM-48 molecular sieve standard sample to calculate the relative crystallinity.

[0049] In the context of this specification, the method for determining the framework aluminum substitution rate first determines the framework aluminum content (relative silicon content) of the corresponding boron-free ZSM-48 molecular sieve synthesized by the same method by nuclear magnetic resonance (Al-NMR), and then compares the framework aluminum content of the boron-containing ZSM-48 molecular sieve of the present invention. The framework aluminum substitution rate of the two types of molecular sieves is obtained by the difference method. Among them, the corresponding boron-free ZSM-48 molecular sieve refers to a ZSM-48 molecular sieve prepared by the method of the present invention, the only difference being that no boron source is added. For example, Example 3 of the present invention is a boron-containing ZSM-48 molecular sieve, and Comparative Example 3 is a corresponding boron-free ZSM-48 molecular sieve, and the difference between the two is whether a boron source is added.

[0050] In the context of this specification, the weak acid content and total acid content of the molecular sieve are determined by NH3-TPD. The high-temperature peak is a strong acid and the low-temperature peak is a weak acid. The strong acid and weak acid contents of the molecular sieve are obtained by integrating the high-temperature peak and the low-temperature peak area, and the total acid content is obtained by adding the two.

[0051] In the context of this specification, including in the following examples and comparative examples, the model of the temperature-programmed desorber (NH3-TPD) of the molecular sieve is Altamira Instruments AMI-3300, and the specific test method is: first, the molecular sieve is pretreated in a helium atmosphere at 550°C for 1 hour, and then the NH3 / He mixture is adsorbed at 100°C, and then the temperature is raised from 100°C to 600°C for NH3 desorption, and the TCD peak is detected; the acid content of the molecular sieve is quantitatively calculated by the software, where the signal peak above 350°C can be considered as a strong acid, and the signal peak at 120-150°C is considered as a weak acid. Example 1

[0052] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3=35, SiO2 / Al2O3=100, OH - / SiO2=0.1, R / SiO2=0.05, R / SiO2=0.15, H2O / SiO2=15. Boric acid, sodium aluminate, sodium hydroxide, template R, and EDTA were dissolved in deionized water respectively. Template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.23:0.5. Then, silica sol is added under continuous stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. Boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and XRD shows Figure 1 , appearance see Figure 2 and Figure 3The B-ZSM-48 crystals are hexagonal prisms with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 20, and the yield of ZSM-48 is 95% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Example 2

[0053] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3=50, SiO2 / Al2O3=60, OH - / SiO2=0.3, R / SiO2=0.45, ethylene glycol / SiO2=0.25, H2O / SiO2=25. Aluminum sulfate, sodium hydroxide, ethylene glycol, sodium tetraborate, and template R are dissolved in deionized water respectively. Template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT), and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.45:0.45. Then white carbon black is added under constant stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. Boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and XRD and Figure 1 Similar in appearance to Figure 1 and 2 Similarly, B-ZSM-48 is a hexagonal prism crystal with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 25, and the yield of ZSM-48 is 92% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Example 3

[0054] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3=30, SiO2 / Al2O3=200, OH - / SiO2=0.45, R / SiO2=0.65, chitosan / SiO2=0.3, H2O / SiO2=20. Sodium aluminate, boric acid, chitosan, sodium hydroxide, and template R are dissolved in deionized water respectively, wherein the template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.65:0.4. Then sodium silicate is added under continuous stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. The boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and the XRD and Figure 1 Similar in appearance to Figure 1 and 2Similarly, the crystals are hexagonal prisms with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 35, and the yield of ZSM-48 is 86% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Example 4

[0055] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3 = 120, SiO2 / Al2O3 = 120, OH - / SiO2=0.45, R / SiO2=0.05, EDTA / SiO2=0.15, H2O / SiO2=15. Ammonium borate, aluminum sol, EDTA, sodium hydroxide, and template R are dissolved in deionized water respectively, wherein the template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.65:0.95. Then sodium silicate is added under continuous stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. The boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and the XRD and Figure 1 Similar in appearance to Figure 1 and 2 Similarly, B-ZSM-48 is a hexagonal prism crystal with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 59, and the yield of ZSM-48 is 87% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Example 5

[0056] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3=100, SiO2 / Al2O3=120, OH - / SiO2=0.2, R / SiO2=0.8, EDTA / SiO2=0.5, H2O / SiO2=30. Sodium tetraborate, aluminum sulfate, sodium hydroxide, template R, and EDTA are dissolved in deionized water respectively, wherein the template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.2:0.8. Then tetraethyl orthosilicate is added under continuous stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. The boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and the XRD and Figure 1 Similar in appearance to Figure 1 and 2Similarly, the crystals are hexagonal prisms with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 48, and the yield of ZSM-48 is 88% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Example 6

[0057] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3=380, SiO2 / Al2O3=80, OH - / SiO2=0.45, R / SiO2=0.75, ethylene glycol / SiO2=0.3, H2O / SiO2=25. Boric acid, sodium aluminate, sodium hydroxide, template R, and ethylene glycol are dissolved in deionized water respectively, wherein the template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.7:0.5. Then, silica sol is added under continuous stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. The boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and the XRD and Figure 1 Similar in appearance to Figure 1 and 2 Similarly, B-ZSM-48 is a hexagonal prism crystal with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 75, and the yield of ZSM-48 is 91% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Example 7

[0058] The initial gel was prepared according to the following molar ratios: SiO2 / B2O3=330, SiO2 / Al2O3=90, OH - / SiO2=0.55, R / SiO2=0.15, chitosan / SiO2=0.4, H2O / SiO2=18. Ammonium borate, sodium aluminate, sodium hydroxide, template R, and chitosan are dissolved in deionized water respectively, wherein the template R is hexamethylenediamine (HDA), 12-crown ether-4 (EOCT) and hexadecyltrimethylammonium bromide (CTAB), and the molar ratio is 1:0.5:0.5. Then white carbon black is added under continuous stirring. The above mixture is then placed in a 100 ml crystallization kettle and reacted at 160 ° C for 168 hours. The obtained product is filtered, washed, dried, calcined, and calcined at 550 ° C for 4 h. The boron-containing ZSM-48 molecular sieve B-ZSM-48 can be obtained, and the XRD and Figure 1 Similar in appearance to Figure 1 and 2Similarly, B-ZSM-48 is a hexagonal prism crystal with a size of 4 to 6 μm. The silicon-boron ratio of B-ZSM-48 is 55, and the yield of ZSM-48 is 87% based on the weight of the silica fed. The properties of B-ZSM-48 are shown in Table 1. Comparative Example 1

[0059] Referring to Example 1, the step of adding the template R in Example 1 was omitted, and the other steps were carried out according to Example 1. The product was amorphous. Comparative Example 2

[0060] Referring to Example 2, the step of adding the complexing agent (ethylene glycol) in Example 2 was not performed, and the other steps were performed according to Example 2. Figure 1 Similar, the morphology is irregular spindle-shaped crystals with low crystallinity and crystal size of 1 to 2 μm. Comparative Example 3

[0061] Referring to Example 3, the step of adding a boron source (boric acid) in Example 2 was not performed, and the other steps were performed according to Example 3. Figure 1 Similarly, the morphology is particle agglomeration and the crystal size is about 2 μm. Example 8

[0062] The molecular sieves described in Examples 1-7 and Comparative Examples 1-3 were used in the toluene-methanol methylation reaction, and the evaluation conditions were as follows:

[0063] 5.0 g of B-ZSM-48 molecular sieve was loaded into a micro-insulated fixed-bed reactor with an inner diameter of 1.5 cm. Glass beads were filled on the upper and lower sides of the reaction bed. Hydrogen was then introduced to ensure that the gas flow evenly passed through the catalyst bed. The temperature was programmed to 460°C over 120 minutes. After maintaining the temperature for 2 hours, a metering pump was used to introduce a reaction raw material with a composition of toluene / methanol = 2 / 1 (mol / mol). The mass space velocity of the organic reaction raw material was maintained at 2.0 h -1 The reaction pressure was 0.5 MPa. The toluene conversion, methanol conversion, xylene selectivity, and service life of the reaction were calculated by measuring the oil and water phases in the catalytic reaction effluent and performing gas chromatography analysis. The specific results are shown in Table 2. The service life is the time from the start of the reaction to the time when the toluene conversion drops to 10%.

[0064] Table 1 Physical and chemical properties of ZSM-48 in Examples and Comparative Examples

[0065] Crystal structure of the product (XRD) <![CDATA[Pore volume of B-ZSM-48 (cm 3 / g)]]> <![CDATA[Specific surface area of B-ZSM-48 (m 2 / g)]]> <![CDATA[The mesoporous specific surface area of B-ZSM-48 (m 2 / g)]]> Silicon-boron ratio of B-ZSM-48 Yield of B-ZSM-48 Skeleton aluminum substitution rate The ratio of weak acid content to total acid content Example 1 ZSM-48 0.30 268 174 20 95% 51% 62% Example 2 ZSM-48 0.39 368 241 25 92% 46% 67% Example 3 ZSM-48 0.60 374 281 35 86% 63% 72% Example 4 ZSM-48 0.45 405 309 59 87% 44% 53% Example 5 ZSM-48 0.54 402 295 48 88% 55% 64% Example 6 ZSM-48 0.38 264 199 75 91% 43% 54% Example 7 ZSM-48 0.48 296 212 55 87% 45% 56% Comparative Example 1 Amorphous 0.17 151 62 422 - - - Comparative Example 2 ZSM-48 0.31 234 195 548 75% 31% 39% Comparative Example 3 ZSM-48 0.29 246 217 - 84% - 42%

[0066] Table 2 Evaluation results of toluene methanol alkylation reaction performance of catalysts in Examples and Comparative Examples

[0067] Toluene conversion rate (%) Methanol conversion rate (%) Xylene selectivity (%) Xylene yield (%) p-Xylene selectivity (%) Service life (h) Example 1 22.3 100 87.2 19.6 92 690 Example 2 26.1 100 85.8 27.1 87 500 Example 3 28.8 100 83.9 22.3 85 650 Example 4 30.2 100 82.2 32.1 90 670 Example 5 28.2 100 84.1 19.3 82 400 Example 6 28.1 100 84.3 28.6 86 420 Example 7 30.0 100 82.5 27.1 87 650 Comparative Example 1 - - - - - - Comparative Example 2 17.3 100 82.2 13.4 52 120 Comparative Example 3 13.8 100 67.9 20.6 70 290

Claims

1. A boron-containing ZSM-48 molecular sieve, wherein the silicon-boron ratio of the molecular sieve is 10 to 80, calculated as SiO2 / B2O3; wherein: The framework aluminum substitution rate in the boron-containing ZSM-48 molecular sieve is 40% to 70%; the weak acid content accounts for 45% to 80% of the total acid content.

2. The boron-containing ZSM-48 molecular sieve according to claim 1, characterized in that The weak acid content of the boron-containing ZSM-48 molecular sieve is increased by 10% to 50% compared with the corresponding boron-free ZSM-48 molecular sieve.

3. The boron-containing ZSM-48 molecular sieve according to claim 1, characterized in that The pore volume of the boron-containing ZSM-48 molecular sieve is 0.15 to 0.87 cm 3 / g, mesopore specific surface area 40~400 m 2 / g, with a specific surface area of ​​250-500 m 2 / g.

4. The boron-containing ZSM-48 molecular sieve according to claim 1, characterized in that The pore volume of the boron-containing ZSM-48 molecular sieve is 0.30 to 0.65 cm 3 / g, mesopore specific surface area 170~350 m 2 / g.

5. The boron-containing ZSM-48 molecular sieve according to claim 1, characterized in that The boron-containing ZSM-48 molecular sieve has a hexagonal prism crystal morphology, and the crystal size is 4 to 6 μm.

6. A method for preparing the boron-containing ZSM-48 molecular sieve according to any one of claims 1 to 5, comprising the following steps: (1) Preparation of glue solution: Mix the silicon source, aluminum source, template R, boron source and water, then add the complexing agent and alkali source to a pH of 11.5-14.0 to obtain glue solution; (2) subjecting the gel described in step (1) to liquid crystallization to obtain a boron-containing ZSM-48 molecular sieve; Wherein, the template agent R is hexamethylenediamine, 12-crown ether-4 and hexadecyltrimethylammonium bromide; The complexing agent is selected from at least one of EDTA, ethylene glycol and chitosan.

7. The preparation method according to claim 6, characterized in that In step (1), the silicon source includes at least one of silica sol, sodium silicate, tetraethyl orthosilicate and white carbon black; the aluminum source includes at least one of sodium aluminate, aluminum sulfate and aluminum sol; the boron source is at least one of sodium borate, boric acid and ammonium borate; and the alkali source is selected from at least one of ammonia water, sodium hydroxide and potassium hydroxide.

8. The preparation method according to claim 6, characterized in that In step (1), the silicon source is calculated as SiO2, the boron source is calculated as B2O3, the aluminum source is calculated as Al2O3, and the - The molar ratios of alkali source, template agent R, complexing agent and water are as follows: SiO2 / B2O3 is 15-250, SiO2 / Al2O3 is 50-300, complexing agent / SiO2 is 0.02-0.2, R / SiO2 is 0.05-0.6, OH - / SiO2 is 0.05~0.3, and H2O / SiO2 is 10~40.

9. The preparation method according to claim 6, characterized in that In step (1), the template R is hexamethylenediamine, 12-crown 4-ether and hexadecyltrimethylammonium bromide, and the molar ratio thereof is 1: (0.15-0.75): (0.4-1).

10. The preparation method according to claim 6, characterized in that In step (2), the crystallization treatment conditions are: treatment at 160-185° C. for 24-180 h.

11. Use of the boron-containing ZSM-48 molecular sieve according to any one of claims 1 to 5 or the boron-containing ZSM-48 molecular sieve obtained by the preparation method according to any one of claims 6 to 10 in the methylation of toluene and methanol to produce p-xylene.

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