A preparation method and application of single crystal layered ZSM-5 molecular sieve for VOCs purification

Abstract

The present invention relates to the field of molecular sieve adsorption technology, and more particularly to a method for preparing a single crystal layered ZSM-5 molecular sieve for VOCs purification and its application, including: tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are stirred in a certain proportion for a period of time to prepare a clear solution, which is then transferred to an autoclave for reaction for a period of time to obtain an initial zeolite seed; a certain proportion of silica sol, sodium aluminate, sodium hydroxide, an organic structure directing agent and deionized water are mixed into a gel, and then the initial zeolite seed is added, and the autoclave is loaded for crystallization for a few days, and the solid obtained by washing and drying and filtering is finally calcined in a muffle furnace for a period of time to obtain a single crystal layered ZSM-5 molecular sieve. The present invention is crystallized by amino acid auxiliary, so that the ZSM-5 molecular sieve has good hydrothermal stability and excellent adsorption and desorption performance; the molecular sieve prepared by it has good application prospects and economic benefits in the field of industrial VOCs flue gas purification.

Description

Technical Field

[0001] The present invention relates to the technical field of molecular sieve adsorption, and in particular to a preparation method and application of a single crystal layered ZSM-5 molecular sieve for VOCs purification. Background Art

[0002] Volatile organic compounds (VOCs) are commonly found in the exhaust gases of industries such as waste incineration, electric furnace steelmaking, iron ore sintering, metal smelting, coal burning and coking. They are one of the main drivers of complex air pollution phenomena such as urban haze, photochemical smog, and atmospheric toxicity, which have had a serious impact on people's lives and health. Adsorption plays a very important role in the field of VOCs combined purification. High-performance porous materials are the key to their efficient adsorption of concentrated VOCs. Among the many adsorption materials, zeolite molecular sieve is a microporous aluminosilicate mineral. It has become a commonly used adsorbent in VOCs purification projects because of its clear pore structure and its high attraction to molecules of certain sizes and shapes.

[0003] In the prior art, ZSM-5 used for conventional VOCs purification is difficult to use for adsorption, concentration and purification of unsaturated bond-containing VOCs such as styrene, acrylic acid, acrylonitrile, butadiene, etc. due to its moderate acidity. Based on the needs of actual engineering applications, the country and enterprises are currently in urgent need of breaking through the bottleneck of molecular sieve adsorption and purification technology for such VOCs. Therefore, this application uses nano-scale amorphous native zeolite with a zeolite embryo structure as a seed to construct a single crystal ZSM-5 molecular sieve with penetrating internal mesopores and a highly complete skeleton. Only a small amount of organic structure directing agent and a low crystallization temperature are required to ensure that the native zeolite is used as the main growth guide site to induce crystallization. Summary of the invention

[0004] The purpose of the present invention is to solve the shortcomings of the prior art and propose a preparation method and application of a single crystal layered ZSM-5 molecular sieve for VOCs purification. Through amino acid-assisted crystallization, the ZSM-5 molecular sieve has good hydrothermal stability and excellent adsorption and desorption performance; the molecular sieve prepared by the method has good application prospects and economic benefits in the field of industrial VOCs flue gas purification.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing a single crystal layered ZSM-5 molecular sieve for VOCs purification comprises the following steps:

[0007] Step 1, tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are stirred evenly in a certain proportion to prepare a clear solution;

[0008] Step 2, transferring the clarified solution in step 1 into an autoclave and reacting it at a certain temperature for a period of time to obtain an initial zeolite seed;

[0009] Step 3, mixing a certain proportion of silica sol, sodium aluminate, sodium hydroxide, an organic structure directing agent and deionized water into an initial gel, and then adding the initial zeolite seeds in step 2 and stirring for a period of time to form a final gel;

[0010] Step 4, the final gel in step 3 is placed in a high pressure reactor, and crystallized at a certain temperature for several days to obtain a precipitate;

[0011] Step 5: Wash the precipitate after the reaction in step 4, dry and filter the obtained solid, and finally calcine it in a muffle furnace for a period of time to obtain a single crystal layered ZSM-5 molecular sieve.

[0012] Preferably, in step 1, the molar ratio of tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine is 1:0.03:0.036:0.45:0.4.

[0013] Preferably, in step 2, the reaction temperature of the autoclave is 90° C. and the reaction time is 22 h.

[0014] Preferably, in step 3, the organic structure directing agent is any one or more of tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, tetraethylammonium hydroxide or 15-crown ether-5.

[0015] Preferably, in step 3, the molar ratio of the silica sol, sodium aluminate, sodium hydroxide, organic structure directing agent and deionized water is 1:0.003:0.32:(0.15-0.32):15.

[0016] Preferably, in step 3, the molar number of tetraethyl silicate in the initial zeolite seeds accounts for 5-13% of the molar number of silica in the silica sol in the initial gel.

[0017] Preferably, in step 4, the reaction temperature of the autoclave is 120° C., and the reaction time is 48 to 72 hours.

[0018] Preferably, in step 5, the calcination heating rate is 1-10°C.

[0019] Preferably, in step 5, the constant temperature of calcination is maintained at 500-600° C., and the maintenance time of the constant temperature stage of calcination is 5-6 hours.

[0020] By adopting the above technical solution: using nano-scale amorphous native zeolite with zeolite embryo structure as a seed, a single-crystal ZSM-5 molecular sieve with penetrating internal mesopores and a highly complete skeleton is constructed. Only a small amount of organic structure directing agent and a lower crystallization temperature are required to ensure that the native zeolite serves as the main growth guiding site to induce crystallization.

[0021] The present invention also provides an application of the single crystal layered ZSM-5 molecular sieve prepared by the above preparation method in VOCs purification, wherein the VOCs are mainly volatile organic compounds containing unsaturated bonds such as acrylic acid, acrylonitrile, butadiene, etc.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The present invention synthesizes single crystal layered ZSM-5 molecular sieve by hydrothermal synthesis of initial zeolite seeds, the raw materials are cheap and easily available, the process operation is simple, and the production cost is saved.

[0024] 2. The ZSM-5 molecular sieve prepared by the present invention exhibits more excellent VOCs purification performance.

[0025] 3. The ZSM-5 molecular sieve prepared by the present invention has good application prospects and economic benefits in the field of industrial VOCs flue gas purification. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the XRD diagram of the single crystal layered ZSM-5 molecular sieve prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the advantages and features of the present invention, thereby making a clearer definition of the protection scope of the present invention. The embodiments described in the present invention are only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0028] Embodiment 1:

[0029] Step 1, tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are mixed and stirred in a molar ratio of (1:0.03:0.036:0.45:0.4) to obtain a uniform clear solution;

[0030] Step 2, transferring the clarified solution in step 1 into an autoclave and reacting at 90° C. for 22 hours to obtain an initial zeolite seed;

[0031] Step 3, mixing silica sol, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and deionized water in a molar ratio of (1:0.003:0.32:0.2:15) to form an initial gel, adding 13% of the initial zeolite seeds in step 2, and stirring for a period of time to form a final gel;

[0032] Step 4, transferring the final gel in step 3 into an autoclave and reacting at 120° C. for 72 hours to obtain a precipitate containing single crystal layered ZSM-5 molecular sieve;

[0033] Step 5: Wash the precipitate after the reaction in step 4, dry and filter the solid, raise the temperature to 550°C at a rate of 2°C / min in a muffle furnace, and maintain this temperature for 6 hours. The resulting product is a single crystal layered ZSM-5 molecular sieve.

[0034] Figure 1 This is the XRD diagram of the single crystal layered ZSM-5 molecular sieve prepared in Example 1. Characterization shows that the prepared molecular sieve is a ZSM-5 molecular sieve.

[0035] The single crystal layered ZSM-5 molecular sieve prepared in Example 1 was used for acrylic acid adsorption and desorption, and a high adsorption and desorption activity was obtained. The results of this embodiment show that after 50 times of saturated adsorption of acrylic acid and then thermal desorption at 300°C, no obvious carbon deposition occurred on the surface of this type of ZSM-5 molecular sieve, indicating that this type of ZSM-5 has a high ability to resist the carbon deposition of acrylic acid on its surface and in its pores.

[0036] Embodiment 2:

[0037] Step 1, tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are mixed and stirred in a molar ratio of (1:0.03:0.036:0.45:0.4) to obtain a uniform clear solution;

[0038] Step 2, transferring the clarified solution in step 1 into an autoclave and reacting at 90° C. for 22 hours to obtain an initial zeolite seed;

[0039] Step 3, mixing silica sol, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and deionized water in a molar ratio of (1:0.002:0.32:0.2:15) to form an initial gel, adding 13% of the initial zeolite seeds in step 2, and stirring for a period of time to form a final gel;

[0040] Step 4, transferring the final gel in step 3 into an autoclave and reacting at 120° C. for 72 hours to obtain a precipitate containing single crystal layered ZSM-5 molecular sieve;

[0041] Step 5: Wash the precipitate after the reaction in step 4, dry and filter the solid, heat it to 550°C at a heating rate of 2°C / min in a muffle furnace, and maintain this temperature for 6 hours. The resulting product is a single crystal layered ZSM-5 molecular sieve.

[0042] The single crystal layered ZSM-5 molecular sieve prepared in Example 2 was used for acrylonitrile adsorption and desorption, and a high adsorption and desorption activity was obtained. The results of this embodiment showed that after 50 times of saturated adsorption of styrene and then thermal desorption at 300°C, no obvious carbon deposition occurred on the surface of this type of ZSM-5 molecular sieve, indicating that this type of ZSM-5 has a high ability to resist the carbon deposition of acrylonitrile on its surface and in its pores.

[0043] Embodiment 3:

[0044] Step 1, tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are mixed and stirred in a molar ratio of (1:0.03:0.036:0.45:0.4) to obtain a uniform clear solution;

[0045] Step 2, transferring the clarified solution in step 1 into an autoclave and reacting at 90° C. for 22 hours to obtain an initial zeolite seed;

[0046] Step 3, mixing silica sol, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and deionized water in a molar ratio of (1:0.003:0.32:0.2:15) to form an initial gel, adding 13% of the initial zeolite seeds in step 2, and stirring for a period of time to form a final gel;

[0047] Step 4, transferring the final gel in step 3 into an autoclave and reacting at 120° C. for 72 hours to obtain a precipitate containing single crystal layered ZSM-5 molecular sieve;

[0048] Step 5: Wash the precipitate after the reaction in step 4, dry and filter the solid, heat it to 550°C at a heating rate of 2°C / min in a muffle furnace, and maintain this temperature for 6 hours. The resulting product is a single crystal layered ZSM-5 molecular sieve.

[0049] The single crystal layered ZSM-5 molecular sieve prepared in Example 3 was used for butadiene adsorption and desorption, and a high adsorption and desorption activity was obtained. The results of this embodiment showed that after 50 times of saturated adsorption of acrylonitrile and then thermal desorption at 300°C, no obvious carbon deposition occurred on the surface of this type of ZSM-5 molecular sieve, indicating that this type of ZSM-5 has a high ability to resist the polymerization and carbon deposition of acrylonitrile on its surface and in its pores.

[0050] Embodiment 4:

[0051] Step 1, tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are mixed and stirred in a molar ratio of (1:0.03:0.036:0.45:0.4) to obtain a uniform clear solution;

[0052] Step 2, transferring the clarified solution in step 1 into an autoclave and reacting at 90° C. for 22 hours to obtain an initial zeolite seed;

[0053] Step 3, mixing silica sol, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and deionized water in a molar ratio of (1:0.001:0.32:0.2:15) to form an initial gel, adding 13% of the initial zeolite seeds in step 2, and stirring for a period of time to form a final gel;

[0054] Step 4, transferring the final gel in step 3 into an autoclave and reacting at 120° C. for 72 hours to obtain a precipitate containing single crystal layered ZSM-5 molecular sieve;

[0055] Step 5: Wash the precipitate after the reaction in step 4, dry and filter the solid, heat it to 550°C at a heating rate of 2°C / min in a muffle furnace, and maintain this temperature for 6 hours. The resulting product is a single crystal layered ZSM-5 molecular sieve.

[0056] The single crystal layered ZSM-5 molecular sieve prepared in Example 4 was used for the adsorption and desorption of a mixed VOC of acrylic acid, acrylonitrile and butadiene, and a high adsorption and desorption activity was obtained. The results of this embodiment show that after 50 times of saturated adsorption of a mixed VOC of acrylic acid, acrylonitrile and butadiene and then thermal desorption at 300°C, no obvious carbon deposition occurred on the surface of this type of ZSM-5 molecular sieve, indicating that this type of ZSM-5 has a high ability to resist carbon deposition on its surface and in the pores of mixed unsaturated VOCs such as acrylic acid, acrylonitrile and butadiene.

[0057] Comparative Example 1:

[0058] The activation method of commercial ZSM-5 molecular sieve comprises the following steps:

[0059] Step 1, put 200 mg of commercial ZSM-5 molecular sieve into a crucible and place it in a muffle furnace;

[0060] Step 2: Raise the temperature to 550°C at a rate of 2°C / min and maintain this temperature for 2 hours. After the reaction is completed, the resulting product is the activated ZSM-5 molecular sieve.

[0061] The commercial molecular sieve prepared in Control Example 1 was used for acrylic acid adsorption and desorption, and a high adsorption and desorption activity was obtained. The experimental results showed that after this type of commercial molecular sieve was saturated with acrylic acid once and then thermally desorbed at 300°C, the color of the molecular sieve changed from white to black, and obvious carbon deposition appeared on the surface, indicating that the pore size of the molecular sieve was blocked and could not be used any further.

[0062] The above-mentioned embodiments and control examples illustrate that the single crystal layered ZSM-5 molecular sieve prepared by the present invention has a very high ability to resist carbon deposition and pore blocking by VOCs containing unsaturated bonds, and has good application prospects and economic benefits in the field of industrial VOCs flue gas purification.

[0063] The description and practice disclosed in the present invention are easy to think and understand for ordinary technicians in the technical field, and several improvements and modifications can be made without departing from the principles of the present invention. Therefore, modifications or improvements made without departing from the spirit of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a single crystal layered ZSM-5 molecular sieve for VOCs purification, characterized in that: The following steps are involved: Step 1, tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine are stirred evenly in a certain proportion to prepare a clear solution; Step 2, transferring the clarified solution in step 1 into an autoclave and reacting it at a certain temperature for a period of time to obtain an initial zeolite seed; Step 3, mixing a certain proportion of silica sol, sodium aluminate, sodium hydroxide, an organic structure directing agent and deionized water into an initial gel, and then adding the initial zeolite seeds in step 2 and stirring for a period of time to form a final gel; Step 4, the final gel in step 3 is placed in a high pressure reactor, and crystallized at a certain temperature for several days to obtain a precipitate; Step 5, washing the precipitate after the reaction in step 4, drying and filtering the obtained solid, and finally calcining it in a muffle furnace for a period of time to obtain a single crystal layered ZSM-5 molecular sieve; In step 1, the molar ratio of tetraethyl silicate, sodium aluminate, sodium hydroxide, tetrapropylammonium hydroxide and L-lysine is 1:0.03:0.036:0.45:0.4; In step 2, the reaction temperature of the autoclave is 90°C and the reaction time is 22h; In step 3, the organic structure directing agent is any one or more of tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, tetraethylammonium hydroxide or 15-crown ether-5; In step 3, the molar ratio of the silica sol, sodium aluminate, sodium hydroxide, organic structure directing agent and deionized water is 1: 0.003: 0.32: (0.15-0.32): 15; In step 3, the molar number of tetraethyl silicate in the initial zeolite seed accounts for 5-13% of the molar number of silica in the silica sol in the initial gel; In step 4, the reaction temperature of the autoclave is 120°C, and the reaction time is 48-72h; In step 5, the calcination heating rate is 1-10 °C; In step 5, the calcination is maintained at a constant temperature of 500-600°C, and the calcination constant temperature stage is maintained for 5-6 hours.

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