Preparation method of ZSM-5 molecular sieve jointly modified by magnesium oxide and boron oxide

Abstract

The invention relates to a preparation method of a ZSM-5 molecular sieve jointly modified by magnesium oxide and boron oxide, and belongs to the field of preparation of molecular sieve porous materials. According to the preparation method, triethyl borate with relatively large molecular volume is selected as a precursor for loading B2O3, so that B2O3 tends to be loaded on the surface of a molecular sieve. Organic acid is further selected to be complexed with magnesium nitrate to form an anion complex with a large volume, magnesium is inhibited from entering the pore channels of the molecular sieve, and meanwhile, MgO is further controlled to selectively fall on the outer surface of the molecular sieve by utilizing the mutual attraction effect of the formed anion complex and the electricity-deficient boron sites on the surface of B2O3. According to the method, acid sites on the outer surface of the molecular sieve can be eliminated, meanwhile, acid sites in pore channels can be reserved, the diameter of the pore channels can be further reduced through oxides at the pore channels, and therefore the PX selectivity is improved, meanwhile, the high activity and high stability of the molecular sieve are maintained, and the molecular sieve is widely applied to the fields of catalysis and adsorption.

Description

Technical Field

[0001] The invention relates to a method for preparing a ZSM-5 molecular sieve porous material, and belongs to the field of preparation of molecular sieve porous materials. Background Art

[0002] ZSM-5 molecular sieve is a porous material that is used in the fields of adsorption treatment of industrial waste gas, acid catalytic reaction, etc. For example, it can be used in: adsorption of volatile organic compounds (VOCs), selective adsorption of small molecule gases, separation and purification of products in the petroleum industry, industrial catalytic reaction of aromatic compounds, etc.

[0003] Among them, in terms of adsorption performance, ZSM-5 has great advantages among many adsorbents due to its non-flammability, adsorption stability and low cost. It is often used for the adsorption of VOCs and the selective adsorption of specific gases. 12 O Mixed CO2 and air will decompose due to overheating in industrial applications. Possible products include CO, CF4, C2F4, C6F 14 , C3F7H and other gases with certain toxicity; due to C6F 12 The molecular volume of O is larger than that of the decomposition product and slightly larger than the pore size of ZSM-5 (about 0.51-0.56nm), so ZSM-5 molecular sieve can be used as a catalyst for C6F 12 O decomposed gas adsorption treatment of suitable porous materials, so as to avoid toxic gases causing environmental pollution or personnel poisoning. For example, BTEX (benzene, toluene, ethylbenzene, xylene) as an atmospheric pollutant, its selective adsorption during environmental monitoring requires a reusable adsorbent, ZSM-5 is the best choice because of its pore size close to the BTEX molecular size.

[0004] In addition, ZSM-5 molecular sieves can also be modified to reduce the pore size to enhance the selective adsorption performance of molecular sieves. For example, the main component of new energy natural gas is CH4 and the natural gas reservoir contains a large amount of CO2 and N2. After ZSM-5 is modified, it can be used as a low-cost recyclable adsorbent in the CO2 and N2 removal process; similarly, ZSM-5 can also be modified and applied to the selective adsorption of benzene alone or benzene and toluene in the four gases of BTEX.

[0005] Modified molecular sieves are also used in catalytic reactions: for example, paraxylene (PX) is an important high-value-added organic chemical product, and its demand has gradually increased in recent years. Since the pores of ZSM-5 are comparable to the kinetic size of PX, PX can diffuse smoothly in the pores while inhibiting the diffusion of products such as o-xylene, m-xylene and trimethylbenzene, so it is an ideal catalyst for the alkylation reaction of toluene to produce PX. However, the unmodified ZSM-5 molecular sieve has many acid sites on the outer surface, and the PX generated by the reaction is easy to further react on the outer surface, resulting in a decrease in the selectivity of PX. The molecular sieve needs to be further modified to eliminate the acid sites on the outer surface to meet the demand for high PX selectivity. The commonly used method is to modify ZSM-5 by loading a variety of oxides such as P2O5, SiO2, and MgO. This method can reduce the acidity of the outer surface and significantly improve the selectivity of PX, but the oxides are easy to block the pores while eliminating the acid sites on the outer surface, resulting in the loss of effective active sites, thereby reducing the activity and stability of the molecular sieve. Therefore, it is necessary to develop a molecular sieve modification method that can maintain the high activity and stability of the molecular sieve porous material while improving the PX selectivity. Summary of the invention

[0006] In view of the problems that the conventional ZSM-5 modification method easily results in poor stability of the produced molecular sieve and low toluene conversion rate, a modification method is proposed to improve the PX selectivity while maintaining the high activity and stability of the molecular sieve.

[0007] In order to solve the above technical problems, the modification method of ZSM-5 molecular sieve modified by boron oxide and magnesium oxide according to the present invention is carried out according to the following steps:

[0008] (1) adding 2 g of H-ZSM-5 molecular sieve to a mixed solution of 0.07-1.18 g of triethyl borate and 2-20 g of ethanol;

[0009] (2) stirring the solution obtained in step (1) at room temperature for 1 h and then allowing to stand for 6 h;

[0010] (3) After drying the solution obtained in step (2), the solid was calcined in a muffle furnace at 550° C. for 4 h to obtain B / ZSM-5;

[0011] (4) Dissolve 0.05-1g of organic acid and 0.1-2g of Mg(NO3)2·6H2O in 0.2-2g of water, add B / ZSM-5, squeeze and stir evenly, and let stand for 12h, which is a solid to be dried;

[0012] (5) After drying the solid obtained in step (4), calcining it in a muffle furnace at 550° C. for 4 h to obtain a ZSM-5 molecular sieve modified with magnesium oxide and boron oxide.

[0013] As a limitation of the present invention, the H-ZSM-5 molecular sieve described in the above step (1) of the present invention is a homemade molecular sieve, and the specific preparation steps are as follows:

[0014] (1) Dissolve 0.2-2 g NaAlO2 in deionized water, slowly add tetrapropylammonium hydroxide solution (TPAOH, 25 wt% aqueous solution), and stir the solution in a water bath at 35°C until the solution becomes clear and transparent;

[0015] (2) Slowly add 10-100 g of tetraethyl orthosilicate (TEOS) to the above solution and continue stirring for 6 h to obtain a mixed solution with the following components: n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=1:0.01-0.08:0.25-0.55:45;

[0016] (3) transferring the mixture solution obtained in step (2) to a crystallization kettle lined with polytetrafluoroethylene, placing it in an oven at 170-200° C. for crystallization for 2 days, filtering, washing, and drying to obtain a white powder without template removal;

[0017] (4) placing the white powder obtained in step (3) in a muffle furnace, heating to 550° C. and calcining for 8 h to obtain a Na-ZSM-5 molecular sieve;

[0018] (5) Na-ZSM-5 molecular sieve was precipitated with 1 mol·L -1 The NH4Cl solution was ion exchanged three times at 80°C, and after drying, it was placed in a muffle furnace, heated to 550°C, and calcined for 4 hours to obtain the H-ZSM-5 molecular sieve.

[0019] As a limitation of the present invention, in the modification method of ZSM-5 molecular sieve modified with boron oxide and magnesium oxide described in the present invention, the organic acid in step (4) is one of citric acid or malonic acid; and in the modification step (4), the mass ratio of B / ZSM-5 to the solution is 0.5 to 5.

[0020] The modified ZSM-5 molecular sieve of the present invention can be used for the alkylation of toluene and dimethyl carbonate to synthesize PX. The specific application method is: 1.0g of ZSM-5 molecular sieve with a particle size of 30-60 mesh is placed in a fixed bed reactor, the reaction liquid is vaporized at 150°C in a vaporization chamber, the reactor temperature is 400°C, and the temperature is raised under the protection of inert gas He, Ar or N2. After the temperature reaches the set value, the raw material (prepared by toluene and dimethyl carbonate, with a molar ratio of 2-5:1) is pumped into the reactor by a micro-constant flow pump, and the mass space velocity is 5h -1 The gas mixture flows through the bed to react with the molecular sieve, and the condensed reaction liquid is quantitatively analyzed by gas chromatography, and the toluene conversion rate and PX selectivity of the reaction are further calculated.

[0021] After adopting the above technical solution, the present invention has achieved the following beneficial effects compared with other methods:

[0022] The present invention selects triethyl borate with a large molecular volume as a precursor for loading B2O3, so that B2O3 tends to be loaded on the surface of the molecular sieve. Further, organic acid complexing magnesium nitrate is selected to form a large complex anion, which inhibits the magnesium precursor from entering the molecular sieve pores, and at the same time, the formed anion complex can be attracted to the boron sites with electrical deficiencies on the surface of B2O3, and MgO is further controlled to selectively land on the outer surface of the molecular sieve to cover the acid sites on the outer surface of the molecular sieve. This method can achieve the elimination of the acid sites on the outer surface of the molecular sieve while retaining the acid sites in the pores, and the oxide at the pore mouth can further reduce the pore diameter, and finally obtain a highly active and highly stable modified ZSM-5 molecular sieve porous material. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a SEM image of the molecular sieve porous material prepared in Example 4. It can be seen from the image that the porous material is composed of grains of about 1 μm.

[0024] Figure 2 This is the stability test of Example 4 in the toluene alkylation reaction. It can be seen from the figure that the toluene conversion rate and PX selectivity of the molecular sieve in the reaction can be stable for more than 17 hours.

[0025] Figure 3 This is a stability test of Example 1 in the toluene alkylation reaction. By comparison, it can be seen that the stability of the molecular sieve of Example 4 is far superior to that of the conventional modification method. DETAILED DESCRIPTION

[0026] The present invention will be further described with reference to the following examples, but it should be understood that these examples are only for illustration and should not be construed as limitations of the present invention.

[0027] Example 1

[0028] The H-ZSM-5 molecular sieve with Si / Al=12.5 is prepared by the following steps: first, 1.639g NaAlO2 is dissolved in deionized water, 87.44g tetrapropylammonium hydroxide (TPAOH, 25wt% aqueous solution) solution is slowly added, and the solution is stirred in a water bath at 35°C until it becomes clear and transparent. Then 52.08g tetraethyl orthosilicate (TEOS) is slowly added, and stirring is continued for 6h to obtain a clear and transparent solution, and the proportion of each component in the transparent solution is controlled to be: n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=1:0.04:0.43:45; finally, the transparent solution is transferred to the lining of a crystallization kettle with a polytetrafluoroethylene lining, and the crystallization is allowed to stand in an oven at 180°C for 2 days, and a white powder without template removal is obtained after filtration, washing, and drying. The obtained white powder was placed in a muffle furnace, heated to 550°C, and calcined for 8 hours to obtain Na-ZSM-5 molecular sieve. Then, ion exchange was performed three times with 1M NH4Cl solution at 80°C, and after drying, it was placed in a muffle furnace, heated to 550°C, and calcined for 4 hours to finally obtain H-ZSM-5 molecular sieve.

[0029] Embodiment 2-10

[0030] First, 0-1.18g triethyl borate and 10g ethanol were added to a beaker in sequence, stirred until completely dissolved, and 2g H-ZSM-5 molecular sieve prepared in Example 1 was added, stirred at room temperature for 1h and then allowed to stand for 6h, then evaporated in an oil bath at 70℃, and then further dried in an oven at 110℃ for 2h, and the dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain xB / ZSM-5. 0.1-2g Mg(NO3)2·6H2O and 0.05-1g citric acid were dissolved in 1mL deionized water, and the solution was evenly dripped into xB / ZSM-5 and the molecular sieve was squeezed and stirred evenly, and allowed to stand at room temperature for 12h and then dried in an oven at 80℃ for 12h, and the dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain xB+yMg / Z5-C (x and y are the loading amounts of B2O3 and MgO, respectively: x% and y%).

[0031] Examples 11-12

[0032] First, 0.2517g triethyl borate and 10g ethanol were added to a beaker in sequence, stirred until completely dissolved, and 2g H-ZSM-5 molecular sieve prepared in Example 1 was added, stirred at room temperature for 1h and then allowed to stand for 6h, then evaporated in an oil bath at 70°C, and then further dried in an oven at 110°C for 2h, and the dried solid powder was roasted at 550°C in a muffle furnace for 4h to obtain 3B / ZSM-5. 0.64g Mg(NO3)2·6H2O and 0.261-1.042g citric acid were dissolved in 1mL deionized water, and the solution was evenly dropped into 3B / ZSM-5 and the molecular sieve was squeezed and stirred evenly, and allowed to stand at room temperature for 12h and then dried in an oven at 80°C for 12h, and the dried solid powder was roasted at 550°C in a muffle furnace for 4h to obtain 3B+5Mg / Z5-Cz (z=molar ratio of citric acid to magnesium nitrate).

[0033] Embodiment 13

[0034] First, 0.2517g triethyl borate and 10g ethanol were added to a beaker in sequence, stirred until completely dissolved, and 2g H-ZSM-5 molecular sieve prepared in Example 1 was added. After stirring at room temperature for 1h, it was allowed to stand for 6h, and then evaporated in an oil bath at 70℃, and then further dried in an oven at 110℃ for 2h. The dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain 3B / ZSM-5. 0.64g Mg(NO3)2·6H2O and 0.26g malonic acid were dissolved in 1mL deionized water, and the solution was evenly dripped into 3B / ZSM-5 and the molecular sieve was squeezed and stirred evenly. After standing at room temperature for 12h, it was dried in an oven at 80℃ for 12h. The dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain 3B+5Mg / Z5-P.

[0035] The modified molecular sieve obtained in the above example is used in the process of toluene alkylation to synthesize PX. The typical reaction conditions are: reaction temperature 400°C, toluene to dimethyl carbonate molar ratio of 4:1, raw material mass space velocity of 5h -1 The catalytic performance of each molecular sieve obtained is shown in Table 1 below:

[0036] Table 1 Comparison of the activity of modified molecular sieves obtained by different preparation methods

[0037]

[0038]

[0039] Comparative Example 1

[0040] First, 0.2517g triethyl borate and 10g ethanol were added to a beaker in sequence, stirred until completely dissolved, and 2g ZSM-5 molecular sieve prepared in Example 1 was added. After stirring at room temperature for 1h, it was allowed to stand for 6h, and then evaporated in an oil bath at 70℃, and then further dried in an oven at 110℃ for 2h. The dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain 3B / ZSM-5. 0.64g Mg(NO3)2·6H2O was dissolved in 1mL deionized water, and the solution was evenly dripped into 3B / ZSM-5 and the molecular sieve was squeezed and stirred evenly. After standing at room temperature for 12h, it was dried in an oven at 80℃ for 12h. The dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain 3B+5Mg / Z5.

[0041] Comparative Example 2

[0042] First, 0.2517g triethyl borate and 10g ethanol were added to a beaker in sequence, stirred until completely dissolved, and 2g ZSM-5 molecular sieve prepared in Example 1 was added. After stirring at room temperature for 1h, it was allowed to stand for 6h, and then evaporated in an oil bath at 70℃, and then further dried in an oven at 110℃ for 2h. The dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain 3B / ZSM-5. 0.64g Mg(NO3)2·6H2O and 0.148g acetic acid were dissolved in 1mL deionized water, and the solution was evenly dripped into 3B / ZSM-5 and the molecular sieve was squeezed and stirred evenly. After standing at room temperature for 12h, it was dried in an oven at 80℃ for 12h. The dried solid powder was roasted at 550℃ in a muffle furnace for 4h to obtain 3B+5Mg / Z5-A.

[0043] Comparative Example 3

[0044] First, 0.64 g Mg(NO3)2·6H2O and 0.521 g citric acid monohydrate were dissolved in 1 mL deionized water, 2 g ZSM-5 molecular sieve prepared in Example 1 was taken, the solution was evenly dropped into the ZSM-5 molecular sieve and squeezed and stirred evenly, and the mixture was allowed to stand at room temperature for 12 h and then dried in an oven at 80°C for 12 h. The dried solid powder was calcined in a muffle furnace at 550°C for 4 h to obtain 5Mg / Z5-C; 0.2517 g triethyl borate and 10 g ethanol were added to a beaker in sequence and stirred until completely dissolved, 5Mg / Z5-C was added, the mixture was stirred at room temperature for 1 h and then allowed to stand for 6 h, then evaporated in an oil bath at 70°C, and then further dried in an oven at 110°C for 2 h. The dried solid powder was calcined in a muffle furnace at 550°C for 4 h to obtain 5Mg+3B / Z5-C.

[0045] The modified molecular sieve obtained in the above comparative example was used in the process of toluene alkylation to synthesize PX. The typical reaction conditions were: reaction temperature 400°C, toluene to dimethyl carbonate molar ratio 4:1, raw material mass space velocity 5h -1The catalytic performance of each molecular sieve obtained is shown in Table 2 below:

[0046] Table 2 Comparison of molecular sieve activity between the examples and the comparative examples

[0047] Molecular sieve source Molecular sieve name Toluene conversion rate (%) PX selectivity (%) Example 4 3B+5Mg / Z5-C 21 94 Comparative Example 1 3B+5Mg / Z5 12 88 Comparative Example 2 3B+5Mg / Z5-A 13 88 Comparative Example 3 5Mg+3B / Z5-C 19 89

[0048] According to the data in Table 1 and Table 2 and the attached drawings, it can be seen that the molecular sieve modified with B2O3 and MgO prepared by the method of the present invention can achieve extremely high PX selectivity, while maintaining high toluene conversion rate and stability; this method can better eliminate the acid sites on the surface of the molecular sieve and reduce the size of the molecular sieve pores, so this type of molecular sieve can be widely used in the fields of catalysis and gas adsorption.

[0049] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing ZSM-5 molecular sieve modified by magnesium oxide and boron oxide, characterized in that The method is carried out according to the following steps: (1) adding 2 g of H-ZSM-5 molecular sieve to a mixed solution of 0.07-1.18 g of triethyl borate and 2-20 g of ethanol; (2) stirring the solution obtained in step (1) at room temperature for 1 h and then allowing to stand for 6 h; (3) After drying the solution obtained in step (2), the solid was calcined in a muffle furnace at 550° C. for 4 h to obtain B / ZSM-5; (4) Dissolve 0.05-1g of organic acid and 0.1-2g of Mg(NO3)2·6H2O in 0.2-2g of water, add the above B / ZSM-5, squeeze and stir evenly, and let stand for 12h, at which time it is a solid to be dried; (5) After drying the solid obtained in step (4), calcining it in a muffle furnace at 550° C. for 4 h to obtain a ZSM-5 molecular sieve modified with magnesium oxide and boron oxide.

2. The method for preparing a ZSM-5 molecular sieve modified by magnesium oxide and boron oxide according to claim 1, characterized in that The H-ZSM-5 molecular sieve described in step (1) is a homemade molecular sieve, which is specifically prepared according to the following steps: (1) Dissolve 0.2-2 g NaAlO2 in deionized water, slowly add tetrapropylammonium hydroxide solution (TPAOH, 25 wt% aqueous solution), and stir the solution in a water bath at 35°C until the solution becomes clear and transparent; (2) Slowly add 10-100 g of tetraethyl orthosilicate (TEOS) to the clear transparent solution of step (1), and continue stirring for 6 h to obtain a mixed solution with the following components in a ratio of n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=1:0.01-0.08:0.25-0.55:45; (3) transferring the mixed solution obtained in step (2) to a crystallization kettle lined with polytetrafluoroethylene, placing it in an oven at 170-200° C. for crystallization for 2 days, filtering, washing, and drying to obtain a white powder without template removal; (4) placing the white powder obtained in step (3) in a muffle furnace, heating to 550° C. and calcining for 8 h to obtain a Na-ZSM-5 molecular sieve; (5) Na-ZSM-5 molecular sieve was precipitated with 1 mol·L -1 The NH4Cl solution was ion exchanged three times at 80°C, and after drying, it was placed in a muffle furnace, heated to 550°C, and calcined for 4 hours to obtain the H-ZSM-5 molecular sieve.

3. The method for preparing a ZSM-5 molecular sieve modified by magnesium oxide and boron oxide according to claim 1, characterized in that The organic acid in step (4) is one of citric acid and malonic acid.

4. The method for preparing a ZSM-5 molecular sieve modified by magnesium oxide and boron oxide according to claim 1, characterized in that The molar ratio of the organic acid to Mg(NO3)2·6H2O in step (4) is 0.5:1 to 2:1.

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