CN115448322A - Hierarchical-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst and preparation method and application thereof - Google Patents

Hierarchical-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst and preparation method and application thereof Download PDF

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CN115448322A
CN115448322A CN202110636241.5A CN202110636241A CN115448322A CN 115448322 A CN115448322 A CN 115448322A CN 202110636241 A CN202110636241 A CN 202110636241A CN 115448322 A CN115448322 A CN 115448322A
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李令豪
华子乐
施剑林
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Shanghai Institute of Ceramics of CAS
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Abstract

The invention relates to a preparation method of a hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst, a preparation method and application thereof, wherein the preparation method comprises the following steps: dripping a titanium source into a mixed solution containing a silicon source, a microporous structure directing agent solution and water at a low temperature, and stirring to obtain a precursor solution; the temperature for adding the titanium source at the low temperature is 2-10 ℃, and preferably 2-4 ℃; the dropping speed is 0.1 to 1.4 grams per minute; continuously stirring the obtained precursor solution to obtain precursor gel, freeze-drying at-90 to-5 ℃, and grinding to obtain dry gel powder; placing the obtained dry gel powder in a reaction kettle, and performing auxiliary crystallization under the steam condition; calcining to obtain the multistage nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst; the temperature of the low-temperature titanium source is 2-10 ℃, and preferably 2-4 ℃.

Description

Hierarchical-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst and preparation method and application thereof
Technical Field
The invention belongs to the field of inorganic material synthesis and catalysis, and particularly relates to a preparation method and application of a high-performance Oxidative Desulfurization (ODS) catalyst, in particular to a hierarchical-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst (ultra-small nanoscale particles TS-1 titanium silicalite molecular sieve aggregate catalyst), and a preparation method and application thereof.
Background
The combustion of organic sulfides in fossil energy such as coal and petroleum can generate a large amount of SO x The discharge to the atmosphere causes environmental problems such as acid rain. On the other hand, the presence of sulfur-containing substances may also cause poisoning of the denitration catalyst, promoting NO x And (4) discharging gas. In order to reduce and avoid the problem of environmental pollution caused by oil products, the national emission standard of the pollutant of motor vehicles at the sixth stage of the state issued in 2018 requires that the sulfur content of fuel oil of road vehicles must be lower than 10ppm. In the emerging Fuel Cell industry, solid Oxide Fuel Cells (SOFC) and ion Exchange Membrane Fuel cells (PEMFC) require the sulfur content of the Fuel to be less than 5ppm and 0.1ppm, respectively.
The organic sulfur-containing compounds in the fuel oil are mainly mercaptan (RSH) and thioether (R) 2 S), thiophene (Th), benzothiophenes and alkyl substituted sulfides thereof. Most oil refining enterprises currently use Hydrodesulfurization (HDS) to remove sulfur compounds from fuel oil, and the process needs to increase the hydrogen pressure and reaction temperature to stabilize the sulfur content of fuel oil to below 50 ppm. If the sulfur content of the system is further reduced, the risk factor of the HDS reaction process increases significantly, approaching the limits of commercial operation. Therefore, it is of great practical industrial value to develop a desulfurization catalyst with low energy consumption and high efficiency.
TS-1 titanium silicalite is a microporous material synthesized by Taramasso et al in 1983. A large number of researches show that due to the existence of high-activity four-coordination Ti species in a TS-1 titanium silicalite molecular sieve framework, TS-1 shows good catalytic oxidation desulfurization effect on thiophene in an 'ultrastable' thiophene compound. However, the active sites of TS-1 are mainly located in abundant microporous pores, and have poor catalytic effects on Benzothiophene (BT), dibenzothiophene (DBT) and corresponding derivatives thereof with larger molecular sizes. And for pure phase microporous zeolite, the single microporous structure can limit the transmission speed of reactants and products in pore channels, so that more side reactions of intermediates or products are initiated, carbon deposit is easily formed, and the service life of the catalyst is shortened.
In order to solve the problems of size limitation and diffusion faced by the traditional zeolite molecular sieve material in the actual reaction process, different solving strategies have been developed. If mesopores are introduced into the zeolite molecular sieve by a top-down etching method or a bottom-up template method, the etching method causes the problem of active site loss by sacrificing partial micropores and skeletons to form mesopores, and the template method generates pollution waste gas in the calcining and removing process of the template agent. The reduction of the grain size is another solution, and the short-distance diffusion channel of the nano-scale zeolite molecular sieve is beneficial to solving the problem of diffusion of larger molecules in microporous pore channels. However, the nanoparticles have common disadvantages, i.e., poor stability, difficult separation, low yield, etc. of the nanocrystalline materials.
Disclosure of Invention
In order to solve the problems, the invention aims to provide a hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst, and a preparation method and application thereof.
In a first aspect, the invention provides a preparation method of a hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst. The method comprises the following steps: dripping a titanium source into a mixed solution containing a silicon source, a microporous structure directing agent solution and water at a low temperature, and stirring to obtain a precursor solution; the temperature for adding the titanium source at the low temperature is 2-10 ℃, and preferably 2-4 ℃; the dropping speed is 0.1 to 1.4 grams per minute; continuously stirring the obtained precursor solution to obtain precursor gel, freeze-drying at the temperature of between 90 ℃ below zero and 5 ℃ below zero, and grinding to obtain dry gel powder; placing the obtained dry gel powder in a reaction kettle, and performing auxiliary crystallization under the steam condition; and calcining to obtain the multistage-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst. The temperature of the low-temperature titanium source is 2-10 ℃, and preferably 2-4 ℃.
The multilevel structure refers to micropores of the zeolite molecular sieveAnd mesopores formed by stacking mutually small-sized nanocrystals. The chemical composition of the TS-1 titanium silicalite molecular sieve aggregate is SiO 2 And TiO 2 TS-1 (totally known as titanium silicalite-1) is a titanium silicalite molecular sieve with MFI topological structure and high activity framework Ti site. The multi-stage structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst is mainly used for catalytic oxidation reactions such as olefin epoxidation, aromatic hydrocarbon hydroxylation, ketone ammoximation, oxidative desulfurization and the like.
According to the invention, a silicon source, a micropore structure directing agent and deionized water are uniformly mixed, and the mixture is continuously stirred to fully hydrolyze the silicon source, so that a mixed solution is obtained. Then, dripping a titanium source into the mixed solution at a low temperature to avoid the rapid hydrolysis of the titanium source to form an inactive titanium species; and then the obtained precursor solution is continuously stirred to obtain precursor gel, and then the precursor gel is transferred to the temperature of minus 90 to minus 5 ℃ for freeze drying. "transferring the gel to-90 to-5 ℃ for freeze-drying" comprises the corresponding actions: firstly, reducing the water content of precursor gel to corresponding mass requires a certain time, in the process, various molecules in the precursor can slowly interact with each other and are mutually crosslinked to form a certain network structure, and the process can be called 'gelation'; secondly, as the freeze drying is a sublimation process of directly converting solid water into gaseous water, the water in the gel is not acted by capillary force, and the moisture content in the obtained xerogel is little. After the initial crystallization and nucleation, the xerogel can not freely migrate, rearrange and grow in a liquid phase, ostwald curing is hindered, and crystal nuclei grow in situ on a precursor to form a plurality of oriented crystal grain aggregates. Freeze-drying and grinding to obtain dry gel powder, placing the dry gel powder in steam condition for auxiliary crystallization treatment, washing, filtering and calcining the product to obtain the nano-crystalline TS-1 titanium silicalite molecular sieve aggregate catalyst with the multilevel structure. Because the material transmission in the dry gel precursor is blocked in the steam-assisted crystallization process, the rearrangement, fusion and growth of small grains generated in the initial stage of the crystallization process can be prevented, and thus the multi-stage structural aggregate consisting of ultrafine nano grains is formed.
Preferably, the silicon source is at least one of tetraethoxysilane, silicon dioxide nanospheres and mesoporous silicon dioxide nanospheres, the titanium source is tetrabutyl titanate or titanium isopropoxide or titanium tetrachloride, and the water is deionized water; the microporous structure directing agent is tetrapropylammonium hydroxide and/or tetrapropylammonium bromide.
Preferably, the mass fraction of the microporous structure directing agent in the microporous structure directing agent solution is 10-40 wt%; the molar ratio of the silicon source, the titanium source, the microporous structure directing agent and the water is (20-200): 1: (5-50): (1000-5000).
Preferably, the dropping rate is 0.2 to 0.5 g/min.
Preferably, the stirring temperature in the precursor solution obtained by stirring is 20-50 ℃, the time is 2-5 hours, and the speed is 300-500 r/min.
Preferably, the stirring temperature of the obtained precursor solution in the continuous stirring process is 30-60 ℃, and the speed is 300-500 r/min.
Preferably, the freeze-drying time is 5 to 36 hours.
Preferably, the ratio of the silicon source to the precursor gel to the xerogel powder is 1 mol: 100-130 g: 50 to 100 grams.
Preferably, the particle size of the xerogel powder obtained after grinding is 1-10 microns.
Preferably, the volume of the reaction kettle is 50-100 ml; the addition amount of the steam source is 0.1 to 2.7 g, preferably 0.3 to 1.7 g.
Preferably, the crystallization temperature is 100-200 ℃, and preferably 120-180 ℃; the crystallization time is 5 to 48 hours, preferably 10 to 17 hours.
Preferably, the calcining temperature is 400-600 ℃, preferably 450-550 ℃ and the time is 6-10 hours.
In a second aspect, the invention provides a hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst obtained by the preparation method, wherein the hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst is spherical particles with the diameter of 200-300 nanometers and is formed by aggregating primary particles with the size of 30-50 nanometers, and the primary particlesA stacking hole formed by stacking particles between the particles, and the specific surface area of the stacking hole is 400 to 600m 2 ·g -1 The pore volume is 0.50-0.90 cm 3 ·g -1 The aperture of the stacking hole is 20-40 nanometers.
When large-size substrate molecules are catalyzed in practical application, compared with the single crystal type molecular sieve, the agglomerate with the same size has the advantages that the size of primary particles is reduced, more accessible active sites are exposed, the large-size substrate molecules can be ensured to be diffused into the agglomerate due to the existence of stacking holes, and the catalytic reaction activity is jointly improved.
In a third aspect, the invention provides an application of the hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst in a fuel oil oxidation desulfurization process.
Preferably, the titanium silicalite molecular sieve agglomerate catalyst has more active sites available for contacting the reaction substrate during the reaction due to the reduced size of the primary particles. Therefore, the catalytic conversion efficiency of the catalyst on sulfur-containing organic matters (thiophene, benzothiophene and dibenzothiophene) can reach 100%, 70.3% and 79.1% respectively.
Has the advantages that:
(1) The preparation process is simple, the cost is low, the repeatability is good, and the method is easy to industrially popularize;
(2) The size of primary particle grains in the zeolite molecular sieve aggregate catalyst is further reduced, so that the zeolite molecular sieve aggregate catalyst has better catalytic performance;
(3) The submicron-grade aggregate formed by the accumulation of the primary particles reduces the difficulty of separation, recovery and reuse of the catalyst.
Drawings
FIG. 1 shows the XRD pattern (FIG. 1 a), the nitrogen adsorption-desorption isotherm (FIG. 1 b) and the pore size distribution (FIG. 1 c) of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 1.
FIG. 2 is an SEM photograph (a in FIG. 2), DLS particle size distribution (b in FIG. 2) and TEM photograph (c in FIG. 2) of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 1 and a high resolution image thereof (d in FIG. 2).
FIG. 3 is a UV-vis spectrum of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 1.
FIG. 4 is an SEM photograph of the TS-1 titanium silicalite molecular sieve aggregate catalyst prepared in example 2; fig. 4 (a) shows that the molar ratio of the titanium source to the silicon source is 34:1, TS-1 titanium silicalite molecular sieve aggregate catalyst; fig. 4 (b) shows that the molar ratio of the titanium source to the silicon source is 70:1, TS-1 titanium silicalite molecular sieve aggregate catalyst.
FIG. 5 is a UV-vis spectrum of the TS-1 titanium silicalite molecular sieve agglomeration catalyst prepared in example 2.
FIG. 6 is an XRD pattern of the TS-1 titanium silicalite molecular sieve agglomerate catalyst made in example 3.
FIG. 7 is an SEM photograph of the TS-1 titanium silicalite molecular sieve agglomerate catalyst made in example 3.
FIG. 8 shows the nitrogen adsorption-desorption isotherms (a in FIG. 8) and pore size distribution (b in FIG. 8) of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 3.
FIG. 9 is a graph of thiophene (FIG. 9 a), benzothiophene (FIG. 9 b), and dibenzothiophene (FIG. 9 c) conversion versus reaction time for the TS-1 titanium silicalite molecular sieve agglomerate catalyst measured in example 4.
Fig. 10 is a digital photograph of the materials of example 1 (a in fig. 10) and comparative example 1 (b in fig. 10) after being ultrasonically dispersed in 5 ml of water and being taken out and left to stand for about 5 minutes.
FIG. 11 is the nitrogen adsorption-desorption isotherm (FIG. 11 a) and pore size distribution (FIG. 11 b) of the nano-sized TS-1 titanium silicalite molecular sieve prepared in comparative example 1.
FIG. 12 is a SEM photograph (a in FIG. 12) and a DLS particle size distribution plot (b in FIG. 12) of the nano-sized TS-1 titanium silicalite molecular sieve prepared in comparative example 1.
FIG. 13 is a graph showing the conversion of thiophene (FIG. 13 a), benzothiophene (FIG. 13 b), and dibenzothiophene (FIG. 13 c) with reaction time for the nano-sized TS-1 titanium silicalite molecular sieve prepared in comparative example 1.
FIG. 14 is an SEM photograph (a in FIG. 14) and a DLS particle size distribution plot (b in FIG. 14) of the submicron TS-1 titanium silicalite molecular sieve prepared in comparative example 2.
FIG. 15 is a graph showing the conversion of thiophene (FIG. 15 a), benzothiophene (FIG. 15 b), and dibenzothiophene (FIG. 15 c) in the submicron TS-1 titanium silicalite molecular sieve prepared in comparative example 2 as a function of reaction time.
FIG. 16 shows porous TiO prepared in comparative example 3 2 -SiO 2 XRD pattern of the catalyst.
FIG. 17 shows porous TiO prepared in comparative example 3 2 -SiO 2 UV-vis spectrum of the catalyst.
FIG. 18 shows porous TiO prepared in comparative example 3 2 -SiO 2 Thiophene (fig. 18 a), benzothiophene (fig. 18 b), and dibenzothiophene (fig. 18 c) conversion of the catalysts were plotted as a function of reaction time.
FIG. 19 is SEM photograph (a in FIG. 19) and DLS particle size distribution diagram (b in FIG. 19) of the TS-1 Ti-Si molecular sieve aggregate catalyst prepared in comparative example 4.
FIG. 20 is a graph showing the conversion of thiophene (FIG. 20 a), benzothiophene (FIG. 20 b), and dibenzothiophene (FIG. 20 c) in the micron TS-1 titanium silicalite agglomerated catalyst prepared in comparative example 4, as a function of reaction time.
Detailed Description
The following detailed description of the present invention is provided in connection with the accompanying drawings and examples. It is to be understood that the following drawings and examples are illustrative of the invention and are not to be construed as limiting the invention.
Uniformly mixing a silicon source, deionized water and a microporous structure directing agent at a certain temperature, and slowly dropwise adding a titanium source under a low-temperature condition; the quality of the gel obtained by full hydrolysis and gelation is controlled within a certain range; freeze-drying the gel to obtain a dry gel precursor; and carrying out steam-assisted crystallization treatment for a certain time to obtain the TS-1 titanium silicalite molecular sieve aggregate.
The invention first mixes silicon source, deionized water and micropore structure guiding agent evenly. Then adding a titanium source dissolved in isopropanol at low temperature and continuing stirring, thereby improving the dispersibility of the active sites and being beneficial to improving the catalytic performance of the material.
At present, freeze drying is not applied to the formation of zeolite molecular sieves, and the formation of the zeolite molecular sieves needs high-temperature crystallization. In the existing concentrated gel steam-assisted crystallization process, the water content of precursor gel needs to be reduced to 20% -30% at a certain temperature (40-100 ℃), but the process is usually accompanied with condensation of the precursor gel, and part of water cannot be removed from the gel due to the action of capillary force. The freeze drying in the invention has the following functions: the low-temperature drying avoids condensation of the precursor, and further reduces the water content on the basis of maintaining the structure of the precursor so as to facilitate the subsequent formation of nano crystals and aggregates.
The preparation method of the multistage structure TS-1 titanium silicalite molecular sieve aggregate catalyst provided by the invention is exemplarily illustrated as follows.
And preparing a mixed solution. And uniformly mixing a silicon source, deionized water and the microporous structure directing agent to obtain a mixed solution. As an example, a silicon source, deionized water and a microporous structure directing agent are stirred at 20-60 ℃ for 1-8 hours, so that the silicon source is fully hydrolyzed to obtain a mixed solution. Wherein, the silicon source can be at least one of tetraethoxysilane, silicon dioxide nanospheres and mesoporous silicon dioxide nanospheres. The particle size of the silica nanospheres and mesoporous silica nanospheres can be 20-500 nanometers, preferably 40-60 nanometers. The titanium source may be at least one of tetrabutyl titanate and titanium tetrachloride. The molar ratio of the silicon source, the microporous structure directing agent and the water (deionized water) can be (1-40): 1: (20 to 1000), preferably (3 to 30): 1: (50-800). The mass fraction of the microporous structure directing agent in the microporous structure directing agent solution can be 10-40 wt%.
And preparing a precursor solution. And (3) cooling the mixed solution, dripping a titanium source into the mixed solution containing the silicon source, the micropore structure guiding agent and water at low temperature, and stirring to obtain a precursor solution. The low-temperature dropping is favorable for uniformly dispersing the titanium source and performing primary hydrolysis (the hydrolysis speed is slowed down by reducing the temperature, unstable Ti (OH) is formed during hydrolysis) 4 Si (OH) easily formed by TEOS hydrolysis 4 Combine to form Ti-O-SiO 3 The low-polymer state makes the titanium atom enter the zeolite framework more easily in the subsequent crystallization process, and further plays the role of an active site. Avoiding the titanium source from being hydrolyzed completely and generating zeolite which can not enterInactive TiO of skeleton 2 Problem(s). As an example, a titanium source dissolved in isopropyl alcohol may be dropped into the resulting mixed solution at a low temperature of 2 to 10 ℃ and continuously stirred at that temperature for 2 to 5 hours. The molar ratio of the titanium source, isopropanol, and the aforementioned silicon source may be 1: (30-200): (20-200). The mass fraction of titanium in the titanium source solution can be 0.2-7 wt%. The dropping rate is controlled to be between 0.1 and 1.4 g/min (preferably 0.2 and 0.5 g/min). When the total mass of the solution is 2-7 g, the dropping process is controlled within 5-20 minutes. The temperature during stirring can be 20-50 ℃, and the stirring speed can be 300-500 r/min.
And preparing precursor gel. And continuously stirring the obtained precursor solution to obtain precursor gel. As an example, the obtained precursor solution is placed in a water bath device at a certain temperature (30-60 ℃) and is continuously stirred until the water content of the gel is reduced to a certain quality. Based on the proportion of 1 mol of silicon source, the mass of the obtained gel is 100-130 g. The dropping rate may be 0.1 to 1.4 g/min, preferably 0.2 to 0.5 g/min. The temperature of the continuous stirring can be 30-60 ℃, and the speed can be 300-500 r/min.
A xerogel powder is prepared. And then freeze-drying the gel, and grinding the gel after drying to obtain dry gel powder. As an example, the gel is transferred to-90 to-5 ℃ for freeze drying for 5 to 36 hours, and the dry gel powder is obtained after grinding according to the proportion of 1 mol of silicon source, wherein the weight of the dry gel powder is 50 to 100 grams.
Obtaining the titanium silicalite molecular sieve aggregate with a multilevel structure TS-1. And (3) placing the dry gel powder in a reaction kettle, carrying out steam assisted crystallization treatment in a steam condition, washing, and fully drying to obtain a solid product. And then calcining the obtained solid product in a high-temperature air atmosphere to remove organic molecules, thereby obtaining the TS-1 titanium silicalite molecular sieve aggregate with the multilevel structure. At the initial stage of crystallization, steam is condensed and adsorbed on the precursor silicon-titanium oxide dry gel, the local pH value is higher, and the gel is gradually dissolved and depolymerized. After depolymerization to low polymerization degree silicate species, in the structure directing agent TPA + Under the action of the catalyst, crystal nuclei are quickly formed. If crystallization or moisture content is carried out under hydrothermal conditionsThe higher gel steam assists crystallization, and the formed crystal nucleus can freely migrate, rearrange and grow in a liquid phase, thereby successfully finishing Ostwald ripening and growth to form the single-crystal TS-1 titanium silicalite molecular sieve. However, the moisture content of the xerogel is very low, and the formed crystal nucleus can only grow on the precursor in situ, so that a plurality of oriented crystal grain aggregates are formed. As an example, the milled xerogel powder is placed in a crucible, the crucible containing the xerogel powder is placed into a polytetrafluoroethylene liner, and deionized water is added to the bottom of the liner to keep the water out of contact with the xerogel. Then the polytetrafluoroethylene lining is put into a stainless steel hydrothermal kettle for sealing, crystallized for a certain time at a set temperature, taken out and put into ice water for cooling. The volume of the polytetrafluoroethylene lining can be 50-100 ml, and the addition amount of water for steam assisted crystallization treatment can be 0.1-2.7 g, preferably 0.3-1.7 g. The temperature of the steam assisted crystallization treatment can be 100 to 180 ℃, preferably 150 to 160 ℃. The time of the steam assisted crystallization treatment may be 5 to 48 hours, preferably 10 to 17 hours. The drying process can be carried out in an oven at 80 ℃, and the drying time can be 12 hours. The temperature of calcination may be 400 to 600 c, preferably 450 to 550 c, and the calcination time may be 6 to 10 hours.
As a detailed example of the preparation method of the high-performance oxidative desulfurization catalyst (the TS-1 titanium silicalite molecular sieve aggregate catalyst with a multi-stage structure), the preparation method mainly comprises the following steps:
firstly, uniformly mixing a silicon source, deionized water and a microporous structure directing agent in a water bath at 30-60 ℃, wherein the molar ratio of the silicon source to the microporous structure directing agent is 2-20; secondly, slowly dripping a titanium source solution into the mixed solution under the ice-water bath condition of 2-10 ℃, wherein the molar ratio of a silicon source to the titanium source can be 20-200, and the dripping process can be controlled within 5-20 minutes; the dropping rate is controlled to be 0.1 to 1.4 g/min, preferably 0.2 to 0.5 g/min. Stirring is continued for 2 to 5 hours at this temperature to allow preliminary hydrolysis and reaction with Si (OH) 4 And (4) polymerizing. Then continuously stirring in water bath at 30-60 ℃ until the silicon source and the titanium source are completely hydrolyzed, clarifying the precursor solution and continuously performing water bath at the temperature to obtain gel with certain water content (water content: 10-30%). Gel transfer toFreeze drying at-90 deg.c to-5 deg.c for 5-36 hr to obtain dry gel powder of 50-100 g. Crystallizing for 5-48 hours under the steam auxiliary condition of 100-180 ℃. And repeatedly washing the crystallized mixture, fully drying, and calcining for 6-10 hours at 400-600 ℃ to remove organic molecules such as a micropore structure directing agent and the like, thereby obtaining the TS-1 titanium silicalite molecular sieve aggregate catalyst with a multilevel structure.
The following example illustrates the use of a high performance multi-stage structure TS-1 titanium silicalite molecular sieve aggregate catalyst for oxidative desulfurization: 20-500 mg of catalyst is added into 10 ml of simulated fuel oil (using n-octane or n-heptane as simulated fuel oil, and sulfur content is 400-1000 ppm), and then methanol, acetonitrile or water with the same volume is added as solvent. As an example, the reaction is carried out in a three-neck flask, an electric heating jacket is heated to 40-90 ℃, the bottom is magnetically stirred, the rotating speed is 500-900 r/min, a thermometer is arranged under the liquid level for measuring the temperature, and three ports are respectively connected with the thermometer, a condenser tube and a rubber plug.
The preparation process is simple, the synthesis cost is low, and the obtained catalyst can be used for fuel oil oxidation desulfurization application. For the actual oxidation desulfurization reaction, under the conditions that the reaction temperature is 40-90 ℃ and the rotating speed is 500-900 r/min, the catalytic conversion efficiency of the catalyst on sulfur-containing organic matters (thiophene, benzothiophene and dibenzothiophene) can reach 100%, 70.3% and 79.1% respectively. And has the advantages of high reaction rate, excellent cycle stability, easy separation and the like.
The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also merely one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values exemplified below.
Example 1
The specific operation steps are as follows:
a) 10.4165 g of tetraethylorthosilicate, 18 g of deionized water, and 10.98 g of aqueous tetrapropylammonium hydroxide (25 wt%) were stirred in a water bath at 40 ℃ for 4 hours (rate 400 rpm) until mixed well. Then transferred to an ice bath to be cooled to below 4 ℃ and kept stirring continuously.
b) 0.34 g of tetrabutyl titanate is dissolved in 6 g of isopropanol and cooled to below 4 ℃, the solution obtained in a) is added dropwise at a rate of 1 g/min, the main components of the solution are oligomer (formed by the mutual crosslinking of units such as-Ti-O-Si-, -Si-O-Si-and-Ti-O-Ti-), structure directing agent and water, and the solution is stirred for 4 hours at the temperature.
c) The temperature of the bath was increased to 40 c and the precursor solution was stirred continuously (at a rate of 400 rpm) to obtain a gel, which was stirred continuously at this temperature to initially reduce the water content until it reached a mass of 6.4 grams.
d) The gel was transferred to-84 ℃ and freeze-dried for 24 hours to give a xerogel, which was ground into a powder using an agate mortar and then transferred to a crucible. And (3) putting the crucible filled with the dry gel powder into a polytetrafluoroethylene lining of 80 ml, adding 0.7 g of deionized water at the bottom of the lining as a steam source, wherein the water is not in contact with the dry gel, and then putting the polytetrafluoroethylene lining into a stainless steel hydrothermal kettle for sealing.
e) And (3) putting the hydrothermal kettle into an oven at 180 ℃ for crystallization for 10 hours, taking out, and putting into ice water for cooling.
f) And washing the crystallized sample for 3 times by using deionized water, and then putting the sample into an oven at 80 ℃ for drying for 12 hours. And then calcining for 6 hours in a muffle furnace at 550 ℃ under the air atmosphere, wherein the heating rate is 2 ℃/min, and thus the TS-1 titanium silicalite molecular sieve aggregate catalyst with the multilevel structure is obtained.
Table 1 shows the pore structure parameters of the samples obtained in example 1:
Figure BDA0003105333280000091
note: s BET Characterised by the total specific surface area, S micro Is characterized by a micropore ratioSurface area, S ext Characterised by the area of the outer surface, V total Characterised by the total pore volume, V micro Characterized by a micropore volume, V meso Characterized by mesoporous pore volume.
FIG. 1 shows the XRD pattern (FIG. 1 a), the nitrogen adsorption-desorption isotherm (FIG. 1 b) and the pore size distribution (FIG. 1 c) of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 1. As can be seen from the XRD patterns, the catalyst is a zeolite with MFI-type topology and has high crystallinity. Typical type IV isotherms and high pressure region H3 hysteresis loops can be seen from the nitrogen adsorption-desorption isotherms, which indicate the stacking pore structure existing in the aggregate zeolite catalyst, and the pore size distribution diagram can also see the stacking pores of 20-40 nm.
FIG. 2 shows SEM (a in FIG. 2), DLS particle size distribution diagram (b in FIG. 2) and TEM photographs (c and d in FIG. 2) of the TS-1 titanium silicalite molecular sieve aggregate catalyst prepared in example 1. As can be seen from a and b in FIG. 2, the zeolite presents spherical particles with the diameter of 200-300 nanometers and has a loose surface, TEM photographs can show a large number of small-sized primary particles and stacked pores in the aggregate, and high-resolution images (d in FIG. 2) can show a plurality of oriented grains in the aggregate, which all indicate that the prepared material is the TS-1 titanium silicalite molecular sieve aggregate formed by stacking extremely small grains with the size of less than 50 nanometers.
FIG. 3 is a UV-vis spectrum of the TS-1 titanium silicalite molecular sieve aggregate catalyst prepared in example 1. As can be seen from FIG. 3, the catalyst has an absorption peak at 220 nm, which corresponds to the four-coordinate framework active Ti sites, and there is no absorption peak at longer wavelengths corresponding to inactive Ti, indicating that the prepared catalyst has active centers for catalytic oxidative desulfurization.
Example 2
The present embodiment is different from embodiment 1 in that: the molar ratio of the titanium source to the silicon source is adjusted to be 34:1 and 70: two titanium silicalite agglomerates of titanium source (tetrabutyl titanate) of 0.4254g and 0.2431g were prepared with the remainder as described in example 1.
Fig. 4 is SEM (a and b in fig. 4) photographs of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 2, and (a) in fig. 4 shows that the molar ratio of the titanium source to the silicon source is 34:1, TS-1 titanium silicalite molecular sieve aggregate catalyst; fig. 4 (b) shows that the molar ratio of the titanium source to the silicon source is 70:1 TS-1 titanium silicalite molecular sieve agglomerate catalyst. It can be seen from FIG. 4 that the zeolite exhibits spherical particles with a diameter of 200 to 300 nm and a loose surface.
FIG. 5 is a UV-vis spectrum of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 2. As can be seen from fig. 5, ti enters the skeleton in four coordinates and forms an active center, and the number of active bits varies correspondingly.
Example 3
The present embodiment differs from embodiment 1 in that: the amount of deionized water added to the reactor was adjusted to 0.3 g. The rest is as described in example 1.
FIG. 6 is an XRD pattern of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 3. It can be seen from figure 6 that under these conditions a zeolitic molecular sieve of MFI topology is likewise obtained.
FIG. 7 is an SEM photograph of the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 3. It can be seen that the agglomerate size is about 150 nm and the primary particles inside the agglomerate are about 50 nm.
FIG. 8 is a graph of the nitrogen adsorption-desorption isotherm (a in FIG. 8) and pore size distribution (b in FIG. 8) for the TS-1 titanium silicalite molecular sieve agglomerate catalyst prepared in example 3. The stacking holes increase due to the increase of the primary particles and the reduction of the agglomerate size.
Example 4
The reaction of this example was carried out in a 25 ml three-necked flask, heated by an electric jacket, magnetically stirred at the bottom, rotated at 700 rpm, with a thermometer placed below the liquid surface to measure temperature, three ports connected to a thermometer, a condenser and a rubber stopper, respectively. First, the catalyst powder prepared in example 1 was activated by vacuum treatment at 150 ℃ for 10 hours. 50 mg of the catalyst was charged into a reaction vessel, and 10 ml of a simulated fuel oil (sulfur-containing component: thiophene/benzothiophene/dibenzothiophene, sulfur content 500 ppm) was charged into the reaction vessel, followed by addition of 10 ml of acetonitrile as a solvent. Then stirring and heating are started, 64 microliter of hydrogen peroxide solution (30 wt%) is added after the temperature of the mixed solution is raised to 60 ℃, and the mixed solution is rapidly injected into a reaction system. At fixed time intervals, stirring is stopped, the mixture is kept stand for about 1 minute, 10 microliters of the upper oil phase is dissolved in 1 milliliter of ethanol, analysis is carried out through gas chromatography, and finally the conversion rate of the sulfur-containing substances is calculated.
FIG. 9 shows the thiophene (FIG. 9 a), benzothiophene (FIG. 9 b), and dibenzothiophene (FIG. 9 c) conversion as a function of reaction time for the TS-1 titanium silicalite molecular sieve agglomerate catalyst measured in example 4. As can be seen from fig. 9, the catalytic conversion rates of the catalyst prepared by the present invention for each sulfur-containing organic substance are: thiophene (100%), benzothiophene (70.3%) and dibenzothiophene (79.1%), and has high-efficiency fuel oil desulfurization effect.
Comparative example 1
The comparative example 1 material is a nano-scale TS-1 titanium silicalite molecular sieve, the single particle size is about 100nm, the Ti content of the catalyst prepared in the example 1 is the same, and the reaction conditions are the same as those of the example 4.
Table 2 shows the pore structure parameters of the samples obtained in comparative example 1:
Figure BDA0003105333280000111
as can be seen from the pore structure parameters of the catalysts prepared in example 1 and comparative example 1, the total specific surface area (519 m) of the TS-1 agglomerate catalyst prepared in example 1 is larger than 50 nm because the primary particles in the TS-1 agglomerate catalyst prepared in example 1 have a particle size of less than 50 nm 2 ·g -1 ) And outer surface area (223 m) 2 ·g -1 ) Are all larger than the nano-scale TS-1 titanium silicalite molecular sieve (the total specific surface area is 469 m) 2 ·g -1 Outer surface area 182m 2 ·g -1 )。
FIG. 10 is a digital photograph of the same mass (100 mg) of the materials of example 1 (a in FIG. 10) and comparative example 1 (b in FIG. 10) ultrasonically dispersed in 5 ml of water and allowed to stand for about 5 minutes. As can be seen from FIG. 10, due to the larger particle size of the agglomerates, the TS-1 titanium silicalite molecular sieve agglomerates catalyst can settle and separate by itself after the reaction is completed.
FIG. 11 is a nitrogen adsorption-desorption isotherm (FIG. 11 a) and pore size distribution plot (FIG. 11 b) for the nano-sized TS-1 titanium silicalite molecular sieve of comparative example 1. As can be seen in FIG. 11, the stacking pore size (60 nm) is larger than the stacking pore size (20-40 nm) of the TS-1 titanium silicalite molecular sieve agglomerates.
FIG. 12 is a SEM (a in FIG. 12) and DLS particle size distribution plot (b in FIG. 12) for the nano-sized TS-1 titanium silicalite molecular sieve of comparative example 1. As can be seen in FIG. 12, the nano-sized TS-1 titanium silicalite molecular sieve has a size of about 100nm, which is why the catalyst of this comparative example has a large bulk pore size.
FIG. 13 is a graph of thiophene (FIG. 13 a), benzothiophene (FIG. 13 b), and dibenzothiophene (FIG. 13 c) conversion as a function of reaction time for a nano-sized TS-1 titanium silicalite molecular sieve of comparative example 1. As can be seen from FIG. 13, the catalytic conversion rates of the nano-scale TS-1 titanium silicalite molecular sieve catalyst of comparative example 1 to sulfur-containing organic compounds are respectively: thiophene (100%), benzothiophene (61.0%), dibenzothiophene (56.7%), and it was difficult to effectively remove sulfur-containing organic substances having large molecular sizes under the same conditions.
Comparative example 2
This comparative example 2 is a single crystal sub-nanoscale TS-1 titanium silicalite molecular sieve. The catalyst is obtained by treating a precursor which is xerogel subjected to freeze drying treatment for 10 hours at 180 ℃ by using 10 g of deionized water as a steam source, and has the same Ti content as the catalyst in the example 1. The catalytic reaction conditions were the same as in example 4.
Fig. 14 is a SEM (a in fig. 14) and DLS particle size distribution diagram (b in fig. 14) of the catalyst obtained in comparative example 2. As can be seen from FIG. 14, the material obtained in comparative example 2 is a single-crystal sub-nano TS-1 titanium silicalite molecular sieve with a size of about 100-150 nm. Similarly, the amount of a steam source of a dry gel precursor obtained by freeze drying treatment is increased to 10 g, the liquid level of steam after the steam is condensed in the liner is higher than that of the dry gel, a hydrothermal process is carried out subsequently, a large amount of initial-nucleated nano crystals can be smoothly migrated and rearranged during crystallization, and the nano crystals grow into a single-crystal TS-1 titanium silicalite molecular sieve through Ostwald curing.
FIG. 15 shows the conversion of thiophene (FIG. 15 a), benzothiophene (FIG. 15 b) and dibenzothiophene (FIG. 15 c) in the catalyst obtained in comparative example 2 as a function of reaction time. As can be seen from fig. 15, the catalytic conversion rates of the submicron TS-1 titanium silicalite molecular sieve catalyst of the comparative example 2 to the sulfur-containing organic compounds are respectively: thiophene (100%), benzothiophene (40.8%), dibenzothiophene (46.5%). Because the single-crystal submicron TS-1 titanium silicalite molecular sieve only has micropores, the difference with the aggregate catalyst is shown in the reaction of removing sulfur-containing organic matters with larger sizes.
Comparative example 3
Comparative example 3 is TiO 2 -SiO 2 A porous titanium silicon catalyst. The catalyst is prepared by treating a precursor which is xerogel subjected to freeze drying treatment for 10 hours at 180 ℃ with 0 g of deionized water as a steam source, and has the same Ti content as the catalyst in the example 1. The catalytic reaction conditions were the same as in example 4.
Fig. 16 is an XRD pattern of the catalyst obtained in comparative example 3. It can be seen from fig. 16 that the catalyst obtained in this comparative example was an amorphous catalyst. The TS-1 molecular sieve is generated by crystallizing the precursor xerogel by a dissolution-nucleation-growth process, the water content of the xerogel is reduced to an extremely low level by freeze drying, and the xerogel can not be subjected to the dissolution-nucleation process and still maintains an amorphous state under the condition that no additional water is added in the crystallization process.
FIG. 17 is a UV-vis spectrum of the catalyst obtained in comparative example 3. As can be seen from FIG. 17, ti exists in a hexacoordinate or octadentate state.
FIG. 18 shows the conversion of thiophene (FIG. 18 a), benzothiophene (FIG. 18 b) and dibenzothiophene (FIG. 18 c) in the catalyst obtained in comparative example 3 as a function of reaction time. As can be seen from FIG. 18, amorphous porous TiSiO of comparative example 3 2 -SiO 2 The catalytic conversion rate of the catalyst on sulfur-containing organic matters is respectively as follows: thiophene (0%), benzothiophene (91.1%), dibenzothiophene (86.1%), and for benzothiophene and dibenzothiophene, alkyl substituents destroy the ultrastable "benzene-ring-like" structure of thiophene, and the apparent activation energy required for the reaction is lower than that of thiophene. Although amorphous porous titanium silicon TiO 2 -SiO 2 The catalyst has better effect on removing benzothiophene and dibenzothiophene, but has no activity on thiophene with higher content, because the amorphous porous titanium silicon TiO 2 -SiO 2 The catalyst does not have high-activity four-coordination framework Ti sites.
Comparative example 4
This comparative example 4 is a micron sized aggregate of TS-1 titanium silicalite molecular sieves. The catalyst is prepared by treating a precursor which is xerogel subjected to freeze drying treatment for 10 hours at 180 ℃ by using 5.4 g of deionized water as a steam source, and has the same Ti content as the catalyst in the example 1. The catalytic reaction conditions were the same as in example 4.
Fig. 19 is a SEM (a in fig. 19) and DLS particle size distribution diagram (b in fig. 19) of the catalyst obtained in comparative example 4. As can be seen from FIG. 19, the material obtained in comparative example 4 is a micron-sized aggregate of TS-1 titanium silicalite molecular sieves, with a size of about 1 micron. The steam source is increased to 5.4 g, the concentration of the dissolved precursor is lower during crystallization, the nucleation driving force is reduced, and the nucleation amount is reduced. More un-nucleated precursor then acts on the crystal growth and the size of the agglomerates is thus increased to 1 micron.
Fig. 20 shows the conversion of thiophene (fig. 20 a), benzothiophene (fig. 20 b), and dibenzothiophene (fig. 20 c) with reaction time for the catalyst obtained in comparative example 4. As can be seen from fig. 20, the catalytic conversion rates of the micron TS-1 titanium silicalite molecular sieve aggregate catalyst of the comparative example 4 to the sulfur-containing organic compounds are respectively: thiophene (100%), benzothiophene (23.1%), dibenzothiophene (20.4%). Because the kinetic size of thiophene is smaller than the micropores in the catalyst, the catalyst still has 100% removal effect on thiophene. However, benzothiophene and dibenzothiophene cannot enter micropores, and only can contact active sites on the surfaces of crystal grains, and a large number of active sites in micron-sized crystal grains cannot be effectively utilized, so that the micron-sized aggregate has weak catalytic capability on benzothiophene and dibenzothiophene, and the desulfurization activity of the micron-sized aggregate is obviously lower than that of a nanometer-sized aggregate catalyst.
In conclusion, the TS-1 titanium silicalite molecular sieve aggregate catalyst prepared by the method is formed by stacking crystal grains with extremely small sizes (less than 50 nanometers), and then the aggregate which is easy to separate is formed. Not only the silicon-titanium ratio and the particle size are adjustable, but also the catalyst has the advantage of high activity in the oxidation desulfurization reaction of fuel oil, and is expected to realize industrial application.

Claims (10)

1. A preparation method of a hierarchical structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst is characterized by comprising the following steps:
dripping a titanium source into a mixed solution containing a silicon source, a microporous structure directing agent solution and water at a low temperature, and stirring to obtain a precursor solution; the temperature for adding the titanium source at the low temperature is 2-10 ℃, and preferably 2-4 ℃; the dropping speed is 0.1 to 1.4 grams per minute;
continuously stirring the obtained precursor solution to obtain precursor gel, freeze-drying at-90 to-5 ℃, and grinding to obtain dry gel powder;
placing the obtained dry gel powder in a reaction kettle, and performing auxiliary crystallization under the steam condition; calcining to obtain the multistage nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst; the temperature of the low-temperature titanium source is 2-10 ℃, and preferably 2-4 ℃.
2. The preparation method of claim 1, wherein the silicon source is at least one of tetraethoxysilane, silica nanospheres and mesoporous silica nanospheres, the titanium source is tetrabutyl titanate or titanium isopropoxide or titanium tetrachloride, and the water is deionized water; the microporous structure directing agent is tetrapropylammonium hydroxide and/or tetrapropylammonium bromide.
3. The preparation method according to claim 1 or 2, characterized in that the mass fraction of the micropore structure directing agent in the micropore structure directing agent solution is 10-40 wt%; the molar ratio of the silicon source, the titanium source, the microporous structure directing agent and the water is (20-200): 1: (5-50): (1000-5000).
4. The production method according to any one of claims 1 to 3, wherein the dropping rate is 0.2 to 0.5 g/min.
5. The method according to any one of claims 1 to 4, wherein the freeze-drying time is 5 to 36 hours.
6. The method according to any one of claims 1 to 5, wherein the silicon source, the precursor gel and the xerogel powder are in a ratio of 1 mole: 100-130 g: 50 to 100 grams.
7. The production method according to any one of claims 1 to 6, wherein the volume of the reaction kettle is 50 to 100 ml; the addition amount of the steam source is 0.1-2.7 g, preferably 0.3-1.7 g; the crystallization temperature is 100-200 ℃, preferably 120-180 ℃; the crystallization time is 5 to 48 hours, preferably 10 to 17 hours.
8. The method of any one of claims 1 to 7, wherein the calcination is carried out at a temperature of 400 to 600 ℃, preferably 450 to 550 ℃, for a time of 6 to 10 hours.
9. The multistage-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst obtained by the preparation method according to any one of claims 1 to 8, wherein the multistage-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst is spherical particles with the diameter of 200 to 300 nanometers, the catalyst is formed by aggregating primary particles with the size of 30 to 50 nanometers, stacked pores formed by stacking the primary particles are formed among the primary particles, and the specific surface area is 400 to 600m 2 ·g −1 The pore volume is 0.50-0.90 cm 3 ·g −1 The aperture of the stacking hole is 20-40 nanometers.
10. The application of the multistage-structure nanocrystalline TS-1 titanium silicalite molecular sieve aggregate catalyst of claim 1 or 2 in the fuel oil oxidation desulfurization process.
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CN102530986A (en) * 2012-01-10 2012-07-04 复旦大学 Method for producing MFI type zeolites
CN106629770A (en) * 2016-12-25 2017-05-10 复旦大学 Synthesis method of microporous/mesoporous zeolite molecular sieve based on dry gel preparation
CN111484034A (en) * 2019-01-28 2020-08-04 上海恩昆化工科技有限公司 Preparation method of hierarchical pore ZSM-5 molecular sieve

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102530986A (en) * 2012-01-10 2012-07-04 复旦大学 Method for producing MFI type zeolites
CN106629770A (en) * 2016-12-25 2017-05-10 复旦大学 Synthesis method of microporous/mesoporous zeolite molecular sieve based on dry gel preparation
CN111484034A (en) * 2019-01-28 2020-08-04 上海恩昆化工科技有限公司 Preparation method of hierarchical pore ZSM-5 molecular sieve

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