Preparation method and application of low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry glue

Abstract

The invention belongs to the technical field of catalysts and application thereof, and particularly relates to a preparation method and application of a low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry glue. According to the method, a TS-1 titanium silicalite molecular sieve precursor dry glue containing an original zeolite unit is used as a seed crystal, and the small-grain TS-1 titanium silicalite molecular sieve with high framework titanium content is synthesized by using cheap raw materials. The grain size of the prepared molecular sieve is less than 200 nanometers, the prepared molecular sieve is a b-axis oriented single crystal, and the titanium content of a tetra-coordinated framework accounts for more than 90% of the total titanium content in percentage by mass. In the initial crystallization stage, the TS-1 titanium silicalite molecular sieve precursor dry glue is dissolved in an alkaline environment and recrystallized to form a large number of zeolite seed crystals, and as the crystallization time is prolonged, silicon-titanium nutrient substances in the cheap raw materials are supplied to the zeolite seed crystals to grow to form the TS-1 titanium silicalite molecular sieve. According to the method, tetrapropylammonium bromide is used for replacing tetrapropylammonium hydroxide to serve as a template agent, and various titanium sources, water glass and other silicon sources are adopted, so that the cost is remarkably reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts and their application, and specifically relates to a preparation method and application of a low-cost small-grain TS-1 molecular sieve based on a molecular sieve precursor dry gel. Background Art

[0002] In 1983, Taramasso first reported in patent US4410501 a method for synthesizing TS-1 titanium silicalite by hydrothermal crystallization. This method is regarded as a classic method for synthesizing TS-1 titanium silicalite (abbreviated as TS-1). The specific process is as follows: under nitrogen protection and CO2-free atmosphere, tetrapropylammonium hydroxide aqueous solution is slowly added to ethyl orthosilicate, and then tetraethyl titanate is slowly added, stirred for 1 hour, and then heated, alcohol removed, and water is added to obtain a synthetic glue, and crystallized at 175°C autogenous pressure for 10 days. After separation, washing, drying and roasting, the TS-1 product can be obtained. Based on the classic synthesis method, Thangaraj et al. proposed an improved method in the journal "Zeolites" (Volume 12, pages 934-950) in 1992. The method selects tetrabutyl titanate with weaker hydrolysis activity as a titanium source, disperses it in isopropanol, and then mixes it with a hydrolyzed silicon source to match the hydrolysis rates of the titanium source and the silicon source, thereby increasing the content of skeleton titanium in TS-1.

[0003] However, the hydrolysis and nucleation conditions of the classical method and its improved method are difficult to control, resulting in low catalytic activity and poor repeatability of the synthesized molecular sieves. In addition, the above methods have high requirements for raw material purity, especially the organic template agent tetrapropylammonium hydroxide, and large dosage, resulting in high molecular sieve costs. Therefore, reducing the synthesis cost of titanium silicalite molecular sieves has always been a research hotspot in this field.

[0004] In the past two decades, a lot of research has been conducted at home and abroad on reducing the cost of TS-1 synthesis, hoping to develop a low-cost, energy-saving and environmentally friendly TS-1 synthesis method. A large amount of tetrapropylammonium hydroxide is used in the classic synthesis method of TS-1, but in fact only a part of it plays a template role in the synthesis of molecular sieves. A large amount of tetrapropylammonium hydroxide is used to adjust the alkalinity of the reaction system and provide OH- ions. Patent EP Appl.0543247 discloses a method of using tetrapropylammonium bromide as a template, ammonia water as an alkali source, colloidal silica as a silicon source, and tetrabutyl titanate as a titanium source after complexing with hydrogen peroxide to obtain TS-1 titanium silicon molecular sieve through hydrothermal synthesis. Patent CN1060411C discloses a method for synthesizing titanium silicon molecular sieves, using silica gel and tetrabutyl titanate as silicon sources and titanium sources, and a mixture of tetraethylammonium hydroxide and tetrabutylammonium hydroxide as a template. Patent CN100344375C discloses a method for synthesizing titanium silicon molecular sieve using silica sol as silicon source, organic titanate as titanium source, hexamethyleneimine and piperidine as templates. However, the grain size of TS-1 synthesized by these methods is greater than 10 microns.

[0005] The grain size of TS-1 synthesized by using cheap templates (such as tetrapropylammonium bromide, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, hexamethyleneimine and piperidine) is larger than that of the classical TS-1 synthesis method, which is mainly due to the difference in OH- ion concentration in the synthesis system. When tetrapropylammonium bromide is used as a template, at least one of ammonia water, methylamine, ethylamine, ethylenediamine, diethylamine, triethylamine, propylamine, n-butylamine and hexamethylenediamine is needed as an alkali source to increase the OH- ion concentration in the synthesis solution. Since ammonia water and these organic amines can only partially dissociate, the OH- concentration in the reaction system is low. Even if a large amount of ammonia water or organic amines is added, the OH- concentration does not change much due to the existence of dissociation equilibrium. However, tetrapropylammonium hydroxide, as a strong base, can be completely dissociated in water, so the OH- concentration of the liquid phase in the classical TS-1 synthesis method is much higher than that of the above-mentioned cheap methods. In the inexpensive system using tetrapropylammonium bromide as the template, the lower OH- concentration leads to a lower concentration of silicate species in the liquid phase, and the apparent activation energy of the TS-1 crystallization process is higher, which ultimately makes the entire crystallization process proceed more slowly. The time required to achieve a higher degree of crystallinity is also much longer than the classical method, and the synthesized grain size is larger (greater than 10 microns).

[0006] In the process of synthesizing TS-1 titanium silicon molecular sieve in a cheap system, adding nano TS-1 titanium silicon molecular sieve seeds can promote the nucleation of the molecular sieve, reduce the induction time, increase the crystallization rate, and reduce the crystal size. Patent CN01145256.0 proposes a method for synthesizing small-grain titanium silicon molecular sieve in a cheap system. The method uses titanium tetrachloride as a titanium source and silica sol as a silicon source. In a tetrapropylammonium bromide-ammonia system, nano-scale titanium silicon molecular sieve is used as a seed to hydrothermally synthesize titanium silicon molecular sieve. Adding seeds can significantly accelerate the crystallization rate of TS-1 and shorten the crystallization time. The grain size obtained by this method is about 850 nanometers. Patent CN101913620A proposes a method for preparing small-grain titanium silicon molecular sieve by changing the way of adding seeds in a cheap synthesis system. The method uses silica sol as silicon source, titanium tetrachloride or tetrabutyl titanate as titanium source, tetrapropylammonium bromide as template, organic amine as alkali source, nano-scale TS-1 mother liquor is directly used as seed without separation, and hydrothermal synthesis is performed to obtain titanium silicon molecular sieve. The particle size of the molecular sieve is 600 nanometers. Patent CN112758951A proposes a low-cost method for preparing nano titanium silicon molecular sieve, which is characterized by mixing silicon source, titanium source and seed evenly, adding inorganic acid to adjust the pH value of the mixed solution to the range of 1 to 6, aging at a certain temperature for a certain time, and then filtering, washing and drying to obtain a molecular sieve crystallization precursor; mixing the titanium silicon mixed precursor with an alkaline aqueous solution containing a template, placing it in a closed container, crystallizing at 120 to 210°C for 0.5 to 3 days, filtering, separating, drying and calcining to obtain a titanium silicon molecular sieve. However, this method cannot obtain a highly dispersed TS-1 titanium silicon molecular sieve with a uniform grain size. Haihong Wu et al. proposed a method for synthesizing TS-1 titanium silicon molecular sieve using colloidal silicalite-1 as a seed and tetrapropylammonium bromide as a template in the journal Chinese Journal of Catalysis in 2015 (Volume 36, Pages 1928-1935). The method first uses tetraethyl orthosilicate as a silicon source and tetrapropylammonium hydroxide as an organic template, mixed in a specific ratio, and hydrothermally treated at 80°C for 72 hours to obtain colloidal silicalite-1. Then, tetrabutyl titanate and tetraethyl orthosilicate are hydrolyzed with ammonia water, and hydrogen peroxide is added to control the hydrolysis. The resulting mixture is heated at 60°C for 6 hours, followed by the addition of tetrapropylammonium bromide and colloidal silicalite-1, and crystallization is completed after hydrothermal treatment at 175°C for 3 days. The molecular sieve grains are spherical crystals or twins that grow alternately, which is not conducive to diffusion and mass transfer during the reaction.

[0007] The above patent uses cheap compounds to replace expensive tetrapropylammonium hydroxide as a template, and further reduces the molecular sieve grain size by adding fully crystallized nano-molecular sieves, nano-scale TS-1 mother liquor or colloidal silicalite-1 as seeds. However, the prepared molecular sieve still has problems such as large grains, uneven particle size distribution, and crystal morphology that is not conducive to mass transfer. Summary of the invention

[0008] The purpose of the present invention is to provide a method for preparing a low-cost small-grain TS-1 molecular sieve based on a molecular sieve precursor dry gel and its application, so as to solve the problems of the molecular sieves prepared in the prior art, such as large grains, uneven particle size distribution, and crystal morphology that is not conducive to mass transfer, and to improve its activity and stability in green catalytic oxidation reactions.

[0009] The technical solution of the present invention is:

[0010] A method for preparing a low-cost small-grain TS-1 molecular sieve based on a molecular sieve precursor dry gel, using a TS-1 titanium silicalite molecular sieve precursor dry gel containing an original zeolite unit as a seed crystal, and cheap raw materials: a template agent is tetrapropylammonium bromide, a titanium source is one or more of tetrabutyl titanate, tetraisopropyl titanate, titanium sulfate, titanium tetrachloride or titanium trichloride, a silicon source is at least one of water glass, silica gel, and silica sol, and an alkali source is an organic amine, including at least one of ammonia water, methylamine, ethylamine, ethylenediamine, diethylamine, triethylamine, propylamine, n-butylamine, and hexamethylenediamine, to synthesize a TS-1 titanium silicalite molecular sieve with small grains, b-axis orientation, and high skeleton titanium content; the prepared TS-1 titanium silicalite molecular sieve is a b-axis oriented flaky single crystal without inter-growth twins, and the crystal size is less than 200 nanometers.

[0011] The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel is as follows: in the initial crystallization stage of the reaction, the TS-1 titanium silicalite molecular sieve precursor dry gel containing the original zeolite unit is dissolved and recrystallized in an alkaline environment to form a large number of zeolite seed crystals in a short time; as the crystallization time increases, the silicon and titanium nutrients in the cheap raw materials supply the seed crystals to grow and form the TS-1 titanium silicalite molecular sieve.

[0012] The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel is a solid powder containing the original zeolite unit of TS-1 titanium silicalite molecular sieve.

[0013] The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry glue is as follows: first, deionized water and tetrapropylammonium hydroxide are mixed evenly, and then a silicon source is added until the mixed solution is clear and uniform; then, a pre-mixed titanium source is dropped into the above mixed solution and stirred until it is clear; then, condensation reflux is performed at 50 to 90°C for 3 to 24 hours for aging; finally, the aged molecular sieve precursor is dried to a water content of 10 to 30 wt.%, and ground into a powder with a particle size of 20 to 30 meshes; wherein the molar ratio of silicon source: titanium source: tetrapropylammonium hydroxide: deionized water is 1:0.1 to 0.01:0.1 to 0.5:10 to 50.

[0014] The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry glue is as follows: first, deionized water, tetrapropylammonium bromide and organic amine are mixed evenly, and then a silicon source is added dropwise, and stirring is continued until the solution is clarified and uniform, and then a titanium source is added and stirred into a clarified mixed solution; then, the molecular sieve precursor dry glue is added, and the mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner, and crystallized at 130-170°C for 12-48 hours; after the reactor is cooled to room temperature, the product is obtained by centrifugation, washing, drying and calcination; wherein the molar ratio of tetrapropylammonium bromide to silicon source is less than 0.05, and the amount of molecular sieve precursor dry glue added is less than 13wt%.

[0015] In the method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel, preferably, the molar ratio of tetrapropylammonium bromide to silicon source is 0.01-0.03, and the amount of molecular sieve precursor dry gel added is 5-10wt%.

[0016] In the method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel, preferably, the molar ratio of silicon source, titanium source, tetrapropylammonium bromide, organic amine and deionized water in the mixed solution is 1:0.01-0.05:0.01-0.05:0.5-1.0:20-40.

[0017] The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry glue is described. The mass percentage of the prepared TS-1 titanium silicon molecular sieve with four-coordinated framework titanium content in the total titanium content is greater than 90wt%, there is no anatase titanium dioxide, and the silicon-titanium atomic ratio is 20-200.

[0018] The application of the low-cost small-grain TS-1 molecular sieve based on the molecular sieve precursor dry gel, the TS-1 titanium silicon molecular sieve has good mass transfer performance and a high content of tetracoordinated framework titanium, which is beneficial to mass transfer and improves catalytic efficiency in olefin epoxidation, phenol oxidation, aromatic hydrocarbon hydroxylation, ketone ammoxidation, alkane partial oxidation or alcohol partial oxidation reactions.

[0019] The design concept of the present invention is:

[0020] The method of the present invention uses a TS-1 titanium silicalite precursor dry gel containing an original zeolite unit as a seed, and uses cheap raw materials to synthesize a small-grain TS-1 titanium silicalite with a high skeleton titanium content. In the initial crystallization stage, the TS-1 titanium silicalite precursor dry gel dissolves and recrystallizes in an alkaline environment, forming a large number of seed crystals in a short time. As the crystallization time increases, the silicon and titanium nutrients in the cheap raw materials supply the zeolite seed crystals to grow and form the TS-1 titanium silicalite. The method uses tetrapropylammonium bromide (TPABr) instead of tetrapropylammonium hydroxide (TPAOH) as a template, and uses a variety of titanium sources such as tetrabutyl titanate and silicon sources such as water glass, which significantly reduces the cost. More importantly, the TS-1 titanium silicalite precursor dry gel containing the original zeolite unit dissolves and recrystallizes in situ at the beginning of crystallization to form a large number of seed crystals, further reducing the amount of tetrapropylammonium bromide, shortening the synthesis time, and significantly reducing waste liquid discharge. Through the optimized preparation method, the TS-1 titanium silicalite prepared has a grain size of less than 200 nanometers and is a b-axis oriented single crystal. It also has good mass transfer performance and a high four-coordinated framework titanium content. It exhibits excellent catalytic efficiency in a variety of oxidation reactions and has broad industrial application prospects.

[0021] The present invention has the following advantages and beneficial effects:

[0022] 1. Low cost and environmental protection: Using cheap tetrapropylammonium bromide as a template to replace traditional tetrapropylammonium hydroxide significantly reduces the cost of raw materials. The use of organic templates is reduced during the synthesis process, and environmentally friendly raw materials and processes are used, which is more environmentally friendly.

[0023] 2. High-efficiency crystallization and high framework titanium content: Different from the method of using conventional liquid molecular sieve precursors or fully crystallized molecular sieves as seeds, TS-1 titanium silicon molecular sieve precursor dry gel containing original zeolite units is used as seeds to form a large number of zeolite seeds in a relatively short period of time, significantly increasing the framework titanium content, reducing the TS-1 zeolite grain size, reducing the amount of template used, shortening the crystallization time, and using a variety of cheap silicon sources and titanium sources to further reduce production costs.

[0024] 3. Excellent catalytic performance: The prepared TS-1 titanium silicon molecular sieve is a b-axis oriented flaky single crystal, the mass percentage of the tetracoordinate framework titanium content in the total titanium content is greater than 90wt%, there is no inter-growth twin, the crystal size is less than 200 nanometers, the particle size distribution is uniform, the mass transfer performance is good, and it shows excellent catalytic efficiency in a variety of oxidation reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 : (a) is a scanning electron microscope image of the TS-1 titanium silicalite synthesized in Example 1 of the present invention, (b) is a scanning electron microscope image of the TS-1 titanium silicalite synthesized in Comparative Example 1, (c) is a scanning electron microscope image of the TS-1 titanium silicalite synthesized in Comparative Example 2, and (d) is a scanning electron microscope image of the TS-1 titanium silicalite synthesized in Comparative Example 3.

[0026] Figure 2 The ultraviolet spectra of TS-1 titanium silicalite molecular sieves synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention.

[0027] Figure 3 This is the X-ray spectrum of the TS-1 titanium silicalite molecular sieve synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0028] Below, the present invention is further described in detail by examples.

[0029] Example 1

[0030] In this embodiment, the preparation method of TS-1 titanium silicalite molecular sieve mainly examines the physical and chemical properties of the final TS-1 titanium silicalite molecular sieve obtained by using molecular sieve precursor dry gel as a seed and adding cheap raw materials.

[0031] Preparation of molecular sieve precursor dry gel containing original zeolite:

[0032] After tetrapropylammonium hydroxide and deionized water are evenly mixed, ethyl orthosilicate is added dropwise. After ethyl orthosilicate is completely hydrolyzed until it is clear and uniform, tetrabutyl titanate is added and stirred to form a clear mixed solution. The molar ratio of ethyl orthosilicate, tetrapropylammonium hydroxide, tetrabutyl titanate and deionized water in the mixed solution is 1:0.32:0.03:19. After the above mixed solution is condensed and refluxed at 80°C for 8 hours, it is poured into a beaker and stirred at 50°C to remove part of the water to obtain a molecular sieve precursor dry gel with a water content of 20wt%. The dry gel is crushed into 20-30 mesh particles for standby use.

[0033] Cheap raw materials for synthesizing TS-1 titanium silicate molecular sieve:

[0034] First, deionized water, tetrapropylammonium bromide and ethylamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, titanium tetrachloride is added and stirred into a clear mixed solution. The molar ratio of silica sol, titanium tetrachloride, tetrapropylammonium bromide, ethylamine and deionized water in the mixed solution is 1:0.03:0.03:0.8:30. Then, molecular sieve precursor dry glue is added to the above mixed solution, and the amount of dry glue added is 8wt%. The mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 12 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and calcined to obtain the product.

[0035] Example 2

[0036] In this embodiment, the preparation method of TS-1 titanium silicalite mainly examines the effect of the water content of the molecular sieve precursor dry gel on the physical and chemical properties of the final TS-1 titanium silicalite molecular sieve after adding cheap raw materials using the molecular sieve precursor dry gel as a seed.

[0037] Preparation of molecular sieve precursor dry gel containing original zeolite:

[0038] After tetrapropylammonium hydroxide and deionized water are evenly mixed, ethyl orthosilicate is added dropwise. After ethyl orthosilicate is completely hydrolyzed until it is clear and uniform, tetrabutyl titanate is added and stirred to form a clear mixed solution. The molar ratio of ethyl orthosilicate, tetrapropylammonium hydroxide, tetrabutyl titanate and deionized water in the mixed solution is 1:0.32:0.04:19. After the above mixed solution is condensed and refluxed at 90°C for 12 hours, it is poured into a beaker and stirred at 50°C to remove part of the water to obtain a molecular sieve precursor dry gel with a water content of 25wt%. The dry gel is crushed into 20-30 mesh particles for standby use.

[0039] Cheap raw materials for synthesizing TS-1 titanium silicate molecular sieve:

[0040] First, deionized water, tetrapropylammonium bromide and ethylenediamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, tetrabutyl titanate is added and stirred into a clear mixed solution. The molar ratio of silica sol, tetrabutyl titanate, tetrapropylammonium bromide, ethylamine and deionized water in the mixed solution is 1:0.04:0.03:0.8:30. Then, molecular sieve precursor dry glue is added to the above mixed solution, and the amount of dry glue added is 6wt%. The mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 24 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and calcined to obtain the product.

[0041] The synthesized molecular sieve crystal size is about 120 nanometers, and the crystal morphology is a b-axis oriented lamellar single crystal, rather than an alternatingly grown twin crystal; the mass percentage of the tetracoordinated framework titanium content in the molecular sieve crystal is about 93wt% of the total titanium content; the relative crystallinity is 98%; the phenol conversion rate is 26%, and the by-product benzoquinone selectivity is 0.5%.

[0042] Example 3

[0043] In this embodiment, the preparation method of TS-1 titanium silicalite mainly examines the effect of the amount of molecular sieve precursor dry gel added on the physical and chemical properties of the final TS-1 titanium silicalite obtained after adding cheap raw materials using the molecular sieve precursor dry gel as a seed.

[0044] Preparation of molecular sieve precursor dry gel containing original zeolite:

[0045] After tetrapropylammonium hydroxide and deionized water are evenly mixed, ethyl orthosilicate is added dropwise. After ethyl orthosilicate is completely hydrolyzed until it is clear and uniform, tetrabutyl titanate is added and stirred to form a clear mixed solution. The molar ratio of ethyl orthosilicate, tetrapropylammonium hydroxide, tetrabutyl titanate and deionized water in the mixed solution is 1:0.32:0.03:19. After the above mixed solution is condensed and refluxed at 80°C for 8 hours, it is poured into a beaker and stirred at 50°C to remove part of the water to obtain a molecular sieve precursor dry gel with a water content of 20wt%. The dry gel is crushed into 20-30 mesh particles for standby use.

[0046] Cheap raw materials for synthesizing TS-1 titanium silicate molecular sieve:

[0047] First, deionized water, tetrapropylammonium bromide and ethylamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, titanium tetrachloride is added and stirred into a clear mixed solution. The molar ratio of silica sol, titanium tetrachloride, tetrapropylammonium bromide, ethylamine and deionized water in the mixed solution is 1:0.03:0.03:0.8:30. Then, molecular sieve precursor dry glue is added to the above mixed solution, and the amount of dry glue added is 10wt%. The mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 12 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and calcined to obtain the product.

[0048] The synthesized molecular sieve crystal has a size of about 140 nanometers, and the crystal morphology is a b-axis oriented lamellar single crystal, rather than an alternatingly grown twin crystal; the mass percentage of the tetracoordinated framework titanium content in the molecular sieve crystal is about 96wt% of the total titanium content; the relative crystallinity is 100%; the phenol conversion rate is 26.8%, and the by-product benzoquinone selectivity is 0.3%.

[0049] Example 4

[0050] In this embodiment, the preparation method of TS-1 titanium silicalite mainly examines the effect of the amount of tetrapropylammonium bromide added to the cheap raw materials on the physical and chemical properties of the final TS-1 titanium silicalite obtained by using the molecular sieve precursor dry gel as a seed.

[0051] Preparation of molecular sieve precursor dry gel containing original zeolite:

[0052] After tetrapropylammonium hydroxide and deionized water are evenly mixed, ethyl orthosilicate is added dropwise. After ethyl orthosilicate is completely hydrolyzed until it is clear and uniform, tetrabutyl titanate is added and stirred to form a clear mixed solution. The molar ratio of ethyl orthosilicate, tetrapropylammonium hydroxide, tetrabutyl titanate and deionized water in the mixed solution is 1:0.32:0.03:19. After the above mixed solution is condensed and refluxed at 80°C for 8 hours, it is poured into a beaker and stirred at 50°C to remove part of the water to obtain a molecular sieve precursor dry gel with a water content of 20wt%. The dry gel is crushed into 20-30 mesh particles for standby use.

[0053] Cheap raw materials for synthesizing TS-1 titanium silicate molecular sieve:

[0054] First, deionized water, tetrapropylammonium bromide and ethylamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, titanium tetrachloride is added and stirred into a clear mixed solution. The molar ratio of silica sol, titanium tetrachloride, tetrapropylammonium bromide, ethylamine and deionized water in the mixed solution is 1:0.03:0.02:0.8:30. Then, molecular sieve precursor dry glue is added to the above mixed solution, and the amount of dry glue added is 8wt%. The mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 12 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and calcined to obtain the product.

[0055] The synthesized molecular sieve crystal size is about 160 nanometers, and the crystal morphology is a b-axis oriented lamellar single crystal, rather than an alternating twin crystal; the mass percentage of the tetracoordinate framework titanium content in the molecular sieve crystal is about 94wt% of the total titanium content; the relative crystallinity is 97%; the phenol conversion rate is 24.8%, and the by-product benzoquinone selectivity is 1.5%.

[0056] Comparative Example 1

[0057] In this embodiment, the preparation method of TS-1 titanium silicalite mainly examines the physical and chemical properties of the obtained TS-1 titanium silicalite without adding seed crystals to the cheap raw materials.

[0058] First, deionized water, tetrapropylammonium bromide and ethylamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, titanium tetrachloride is added and stirred into a clear mixed solution. At this time, the molar ratios of silica sol, titanium tetrachloride, tetrapropylammonium bromide, ethylamine and water in the mixed solution are 1:0.05:0.03:0.8:30, respectively. Then, the mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 12 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and roasted to finally obtain the product.

[0059] Comparative Example 2

[0060] In this embodiment, the preparation method of TS-1 titanium silicalite mainly examines the physical and chemical properties of the TS-1 titanium silicalite obtained by adding fully crystallized nano TS-1 titanium silicalite as seeds into cheap raw materials.

[0061] Preparation method of fully crystalline nano TS-1 titanium silicate molecular sieve:

[0062] After tetrapropylammonium hydroxide and deionized water are evenly mixed, ethyl orthosilicate is added dropwise. After ethyl orthosilicate is completely hydrolyzed until it is clear and uniform, tetrabutyl titanate is added and stirred to form a clear mixed solution. The molar ratio of ethyl orthosilicate, tetrapropylammonium hydroxide, tetrabutyl titanate and deionized water in the mixed solution is 1:0.32:0.03:19 respectively. After the above mixed solution is condensed and refluxed at 80°C for 8 hours, it is placed in a reactor and hydrothermally synthesized at 130°C for 6 hours to obtain a completely crystallized nano TS-1 titanium silicon molecular sieve.

[0063] Cheap raw materials for synthesizing TS-1 titanium silicate molecular sieve:

[0064] First, deionized water, tetrapropylammonium bromide and ethylamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, titanium tetrachloride is added and stirred into a clear mixed solution. At this time, the molar ratio of silica sol, titanium tetrachloride, tetrapropylammonium bromide, ethylamine and deionized water in the mixed solution is 1:0.03:0.03:0.8:30, respectively. Then, fully crystallized nano TS-1 titanium silicon molecular sieve is added to the mixed solution in an amount of 8wt%. The mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 12 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and roasted to finally obtain the product.

[0065] Comparative Example 3

[0066] In this embodiment, the preparation method of TS-1 titanium silicalite molecular sieve mainly examines the physical and chemical properties of the TS-1 titanium silicalite molecular sieve finally obtained after adding the molecular sieve precursor sol as a seed to the cheap raw material.

[0067] Preparation of molecular sieve precursor sol containing original zeolite:

[0068] After tetrapropylammonium hydroxide and deionized water are evenly mixed, ethyl orthosilicate is added dropwise. After ethyl orthosilicate is completely hydrolyzed until it is clear and uniform, tetrabutyl titanate is added and stirred to form a clear mixed solution. The molar ratio of ethyl orthosilicate, tetrapropylammonium hydroxide, tetrabutyl titanate and deionized water in the mixed solution is 1:0.32:0.03:19 respectively. After the above mixed solution is condensed and refluxed at 80°C for 8 hours, a molecular sieve precursor sol is obtained. The obtained sol is crushed into particles of 20 to 30 mesh for standby use.

[0069] Cheap raw materials for synthesizing TS-1 titanium silicate molecular sieve:

[0070] First, deionized water, tetrapropylammonium bromide and ethylamine are mixed evenly, and silica sol is slowly added dropwise. After stirring until the solution is clear and uniform, titanium tetrachloride is added and stirred into a clear mixed solution. At this time, the molar ratios of silica sol, titanium tetrachloride, tetrapropylammonium bromide, ethylamine and deionized water in the mixed solution are 1:0.03:0.03:0.8:30, respectively. Then, molecular sieve precursor sol is added to the above mixed solution, and the amount of sol added is 8wt%. The mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner and crystallized at 160°C for 12 hours. After the reaction is completed, the reactor is cooled to room temperature, centrifuged, washed, dried and roasted to finally obtain the product.

[0071] The catalytic performance of the TS-1 titanium silicalite synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention was evaluated by using phenol hydroxylation to produce hydroquinone as a probe reaction. The experimental steps are as follows:

[0072] The reaction was carried out in a 100 mL double-necked round-bottom flask equipped with a reflux condenser. Catalyst TS-1 (200 mg), deionized water (10 mL), and phenol (2 g) were added in sequence. The mixture was heated to 80°C in a constant temperature oil bath and maintained for a period of time. Then, H2O2 (720 μL; the molar ratio of phenol to H2O2 was 3:1) was added. After reacting for different times, the liquid product was aspirated and extracted with ethyl acetate. An appropriate amount of the upper organic extract phase was taken and analyzed using a gas chromatograph (GC-Agilent8890) equipped with a 30 m capillary column (HP-5) and a FID detector.

[0073] Comparative analysis of the physical and chemical properties of the TS-1 titanium silicalite molecular sieves synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention.

[0074] like Figure 1 (a), (b), (c) and (d) are scanning electron microscope images of TS-1 titanium silicon molecular sieve synthesized in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3. It can be seen from the images that the molecular sieve crystal size synthesized using the molecular sieve precursor dry gel as the seed is about 150 nanometers, and the crystal morphology is a single crystal with b-axis orientation, rather than an alternating growth of twins. Figure 1 (a); The crystal size of the molecular sieve synthesized without any seed crystal is about 10 microns, see Figure 1 (b); Adding fully crystallized nano-TS-1 titanium silicon molecular sieve as a seed crystal synthesized molecular sieve crystal size is about 650 nanometers, the crystal morphology is an alternating growth of twins, see Figure 1 (c); The size of the molecular sieve crystal synthesized by adding the molecular sieve precursor sol as a seed crystal is about 500 nanometers, and the crystal morphology is an alternating growth twin, see Figure 1 (d).

[0075] like Figure 2 As shown, the ultraviolet spectra of TS-1 titanium silicon molecular sieve synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3. It can be seen from the ultraviolet spectrum that the molecular sieve synthesized without any seed crystals contains a large amount of non-framework titanium and anatase titanium dioxide, and the mass percentage of the tetracoordinate framework titanium content to the total titanium content is only 30wt%. After adding the seed crystals, the tetracoordinate framework titanium content increases significantly. Among them, adding fully crystallized nano TS-1 titanium silicon molecular sieve as a seed crystal, the mass percentage of the tetracoordinate framework titanium content to the total titanium content is about 70%; adding the molecular sieve precursor sol as a seed crystal, the mass percentage of the tetracoordinate framework titanium content to the total titanium content is about 75%; adding the molecular sieve precursor dry gel as a seed crystal, the mass percentage of the tetracoordinate framework titanium content to the total titanium content is about 95%.

[0076] like Figure 3 As shown, the X-ray spectra of the TS-1 titanium silicon molecular sieve synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention. It can be seen from the X-ray spectrum that the relative crystallinity of the molecular sieve synthesized without any seed crystals is 65%. After adding the seed crystals, the content of tetracoordinate framework titanium increases significantly. Among them, adding fully crystallized nano TS-1 titanium silicon molecular sieve as a seed crystal has a relative crystallinity of 75%; adding the molecular sieve precursor sol as a seed crystal has a relative crystallinity of 78%; adding the molecular sieve precursor dry gel as a seed crystal has a relative crystallinity of 100%.

[0077] Table 1 is a comparison of the catalytic performance of the TS-1 titanium silicalite synthesized in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention in the hydroxylation of phenol to produce diphenol.

[0078] Table 1

[0079]

[0080] As can be seen from Table 1, the phenol conversion rate of the molecular sieve synthesized without any seed crystals is only 0.9%, and the by-product benzoquinone selectivity reaches 52%. After adding seed crystals, the phenol conversion rate is greatly improved, and the by-product benzoquinone yield is greatly reduced. Among them, adding fully crystallized nano TS-1 titanium silicon molecular sieve as a seed crystal, the phenol conversion rate is 8.9%, and the by-product benzoquinone selectivity is reduced to 11.9%; adding molecular sieve precursor sol as a seed crystal, the phenol conversion rate is 10.7%, and the by-product benzoquinone selectivity is reduced to 3.3%; adding molecular sieve precursor dry gel as a seed crystal, the phenol conversion rate reaches 25%, and the by-product benzoquinone selectivity is reduced to 1.0%.

[0081] In summary, without adding seed crystals, the molecular sieve crystal size will be larger, the utilization rate of framework titanium will be low, and the catalytic performance will be poor. Nano TS-1 titanium silicon molecular sieve and molecular sieve precursor sol as seed crystals have a certain effect on the control of crystal morphology and size, but the improvement of tetracoordinate framework titanium content and catalytic performance is not as significant as the precursor dry gel. Precursor dry gel as seed crystals can significantly improve the crystal quality of molecular sieves, the content of tetracoordinate framework titanium, relative crystallinity and catalytic performance, showing the best effect.

[0082] In general, the use of molecular sieve precursor dry gel as seed can not only effectively control the crystal size and morphology of the molecular sieve, but also significantly improve its physical and chemical properties and catalytic performance. Since the TS-1 titanium silicalite molecular sieve precursor dry gel containing the original zeolite unit dissolves and recrystallizes to form a large number of seed crystals in a short period of time in the initial stage of the reaction, the crystallization time is shortened, the amount of template agent is saved, and the preparation cost is saved; the prepared TS-1 titanium silicalite molecular sieve has good reaction activity and mass transfer performance, and shows high activity and high stability in many green catalytic oxidation reactions. The TS-1 titanium silicalite molecular sieve has good mass transfer performance and a high content of tetracoordinated framework titanium, which is beneficial to mass transfer and improves catalytic efficiency in olefin epoxidation, phenol oxidation, aromatic hydroxylation, ketone ammoxidation, partial oxidation of alkanes or partial oxidation of alcohols.

Claims

1. A method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel, characterized in that: The TS-1 titanium silicalite precursor dry gel containing the original zeolite unit is used as a seed crystal, and cheap raw materials: the template agent is tetrapropylammonium bromide, the titanium source is one or more of tetrabutyl titanate, tetraisopropyl titanate, titanium sulfate, titanium tetrachloride or titanium trichloride, the silicon source is at least one of water glass, silica gel and silica sol, and the alkali source is an organic amine, including at least one of ammonia water, methylamine, ethylamine, ethylenediamine, diethylamine, triethylamine, propylamine, n-butylamine and hexamethylenediamine, to synthesize a TS-1 titanium silicalite with small crystals, b-axis orientation and high skeleton titanium content; the prepared TS-1 titanium silicalite is a b-axis oriented flaky single crystal, has no inter-growth twins, and has a crystal size of less than 200 nanometers.

2. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 1, characterized in that: In the initial crystallization stage of the reaction, the TS-1 titanium silicalite precursor dry gel containing the original zeolite unit is dissolved and recrystallized in an alkaline environment, forming a large number of zeolite seeds in a short time; As the crystallization time increases, the silicon and titanium nutrients in the cheap raw materials provide the seeds for growth to form the TS-1 titanium silicon molecular sieve.

3. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 1, characterized in that: The molecular sieve precursor dry gel is a solid powder containing the original zeolite unit of TS-1 titanium silicalite molecular sieve.

4. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 1, characterized in that: The preparation method of molecular sieve precursor dry gel is as follows: first, deionized water and tetrapropylammonium hydroxide are mixed evenly, and then a silicon source is added until the mixed solution is clear and uniform; then, a pre-mixed titanium source is added dropwise into the above mixed solution and stirred until it is clear; then, condensation reflux is performed at 50 to 90° C. for 3 to 24 hours for aging; finally, the aged molecular sieve precursor is dried to a water content of 10 to 30 wt.%, and ground into a powder with a particle size of 20 to 30 meshes; wherein the molar ratio of silicon source: titanium source: tetrapropylammonium hydroxide: deionized water is 1:0.1 to 0.01:0.1 to 0.5:10 to 50.

5. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 1, characterized in that: The method for preparing TS-1 titanium silicalite molecular sieve with cheap raw materials is as follows: first, deionized water, tetrapropylammonium bromide and organic amine are mixed evenly, and then a silicon source is added dropwise, and stirring is continued until the solution is clarified and uniform, and then a titanium source is added and stirred into a clarified mixed solution; then, a molecular sieve precursor dry gel is added, and the mixture is transferred to a high-pressure reactor with a polytetrafluoroethylene liner, and crystallized at 130 to 170° C. for 12 to 48 hours; after the reactor is cooled to room temperature, the product is obtained by centrifugation, washing, drying and calcination; wherein the molar ratio of tetrapropylammonium bromide to silicon source is less than 0.05, and the amount of molecular sieve precursor dry gel added is less than 13wt%.

6. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 5, characterized in that: Preferably, the molar ratio of tetrapropylammonium bromide to silicon source is 0.01 to 0.03, and the added amount of molecular sieve precursor dry gel is 5 to 10 wt %.

7. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 5, characterized in that: Preferably, the molar ratio of the silicon source, the titanium source, tetrapropylammonium bromide, the organic amine and the deionized water in the mixed solution is 1:0.01-0.05:0.01-0.05:0.5-1.0:20-40.

8. The method for preparing low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to claim 1, characterized in that: The prepared TS-1 titanium silicon molecular sieve has a mass percentage of tetra-coordinated framework titanium content in the total titanium content greater than 90wt%, contains no anatase titanium dioxide, and has a silicon-titanium atomic ratio of 20-200.

9. An application of the low-cost small-grain TS-1 molecular sieve based on molecular sieve precursor dry gel according to any one of claims 1 to 8, characterized in that: The TS-1 titanium silicalite molecular sieve has good mass transfer performance and a high content of tetracoordinated framework titanium, which is beneficial to mass transfer and improves catalytic efficiency in olefin epoxidation, phenol oxidation, aromatic hydrocarbon hydroxylation, ketone ammoxidation, alkane partial oxidation or alcohol partial oxidation reactions.

Images