阅读说明：本技术 一种四氯化硅同晶置换改性分子筛的制备方法 (Preparation method of silicon tetrachloride isomorphous replacement modified molecular sieve ) 是由 刘荣升 刘中民 于政锡 王莹利 于 2019-08-22 设计创作，主要内容包括：本申请公开了一种改性分子筛,所述改性分子筛最大的孔道为十元环孔道；相较于改性前的分子筛,所述改性分子筛的孔道内酸性位的摩尔含量的下降低于10％；所述改性分子筛的外表面酸性位的摩尔含量的降低大于15％。以及改性分子筛的制备方法,所述方法包括以下步骤：1)将分子筛通过钠离子交换得到钠型分子筛；2)对所述钠型分子筛进行活化,然后采用化学气相沉积方法进行四氯化硅吸附,然后吹扫干净；3)加热至发生同晶置换反应的温度T_R进行反应,反应结束后吹扫干净,水洗,通过氨离子交换得到氢型分子筛,焙烧,得到所述改性分子筛。从而达到有选择性的永久消除最大孔道为十元环孔道的分子筛外表面酸性位的目的。(The application discloses a modified molecular sieve, wherein the largest pore channel of the modified molecular sieve is a ten-membered ring pore channel; compared with the molecular sieve before modification, the mol content of the acid sites in the pore channels of the modified molecular sieve is reduced by less than 10%; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 15%. And a process for preparing a modified molecular sieve, the process comprising the steps of: 1) exchanging the molecular sieve with sodium ions to obtain a sodium type molecular sieve; 2) activating the sodium type molecular sieve, then adopting a chemical vapor deposition method to adsorb silicon tetrachloride, and then purging; 3) heating to the temperature T at which isomorphous replacement reaction occurs R Reaction is carried out, and purging is carried out after the reaction is finishedAnd (3) cleaning, washing with water, exchanging with ammonia ions to obtain a hydrogen type molecular sieve, and roasting to obtain the modified molecular sieve. Thereby achieving the purpose of selectively and permanently eliminating the acid sites on the outer surface of the molecular sieve with the largest pore channel being a ten-membered ring pore channel.)

1. The modified molecular sieve is characterized in that the largest pore channel of the modified molecular sieve is a ten-membered ring pore channel;

compared with the molecular sieve before modification, the mol content of the acid sites in the pore channels of the modified molecular sieve is reduced by less than 10%; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 15%.

2. The modified molecular sieve of claim 1, wherein the modified molecular sieve has a reduction in the molar content of acid sites within the channels of less than 10% as compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 90%.

3. A process for the preparation of a modified molecular sieve according to claim 1 or 2, characterized in that it comprises the following steps:

1) exchanging the molecular sieve with sodium ions to obtain a sodium type molecular sieve;

2) activating the sodium type molecular sieve, then adsorbing silicon tetrachloride by adopting a chemical vapor deposition method, and purging after the silicon tetrachloride is adsorbed and saturated;

3) heating to the temperature T at which isomorphous replacement reaction occurs_RThe reaction is carried out, and the reaction is completely purged after the reaction is finishedWashing with water, exchanging with ammonia ion to obtain hydrogen type molecular sieve, and calcining to obtain the modified molecular sieve.

4. The method of claim 3, wherein in step 1), the sodium ion exchange comprises:

(1) putting the molecular sieve into a soluble sodium salt solution, wherein the liquid-solid weight ratio is 2-10;

(2) stirring for 1-4 hours at 40-90 ℃;

(3) after solid-liquid separation, washing the molecular sieve with water until the solution is neutral;

(4) drying at 70-120 ℃, and then roasting at 450-600 ℃ for 2-6 hours to obtain the sodium type molecular sieve.

5. The method of claim 4, wherein the sodium salt in the soluble sodium salt solution is selected from at least one of sodium nitrate, sodium chloride, sodium acetate, and sodium carbonate.

6. The method of claim 3, wherein step 2) comprises:

and introducing an atmosphere I into the reactor loaded with the molecular sieve for activation, then introducing an atmosphere II carrying silicon tetrachloride for reaction and adsorption, and switching an atmosphere III to purge after the silicon tetrachloride is adsorbed to saturation.

7. The method of claim 6, wherein atmosphere I is selected from at least one of nitrogen and helium;

atmosphere II also includes at least one of an inert atmosphere; the volume concentration of the silicon tetrachloride in the atmosphere II is 1-50%;

atmosphere III is selected from at least one of non-reactive atmospheres;

the inert atmosphere is at least one selected from nitrogen, helium, neon and argon.

8. The method for preparing the modified molecular sieve as claimed in claim 3 or 6, wherein the temperature for silicon tetrachloride adsorption is 100-280 ℃, and the time for silicon tetrachloride adsorption is 0.5-4 hours.

9. The method of claim 3, wherein in step 2), the purging time is 1-6 hours.

10. The process for preparing modified molecular sieve according to claim 3 or 6, wherein in step 3), the temperature T of the isomorphous metathesis reaction_RThe temperature is 300-;

preferably, in step 3), after the isomorphous replacement reaction is finished, the solution is washed with water until no silicon tetrachloride remains, and then ammonia ion exchange is performed.

Technical Field

The invention belongs to the field of zeolite molecular sieves, and particularly relates to a preparation method of a molecular sieve modified by isomorphous replacement of silicon tetrachloride.

Background

The molecular sieve has wide application in petrochemical process and chemical synthesis due to unique acid property and pore channel structure, is mainly applied to two aspects of shape-selective catalysis and adsorption separation, and the shape-selective catalysis performance of the molecular sieve is mainly influenced by surface acid sites and the pore channel structure; on the other hand, the reactions occurring on the outer surface of the molecular sieve can also form carbon deposit species to be accumulated at the pore opening, and the inactivation of the molecular sieve is accelerated. Therefore, in order to increase the selectivity of the target reaction and the useful life of the molecular sieve catalyst, it is necessary to modify the microporous molecular sieve to reduce the effect of its external surface acidity sites on the reaction.

Common modification methods are chemical vapor silicon deposition, chemical liquid silicon deposition, pre-deposition of carbon, modification of metal oxides, and the like. The silicon deposition method can effectively improve the shape selective effect of the molecular sieve catalyst, but because the acting force between the hydroxyl on the surface of the molecular sieve and the deposition species is weaker, the ideal effect can be achieved by depositing for many times, so the operation is complicated, and the energy consumption is higher. The pre-carbon deposition can also effectively improve the shape-selective performance of the molecular sieve catalyst, but the catalyst after regeneration must be subjected to carbon deposition again, which is also a tedious operation. The method for covering the acid sites on the outer side surface of the molecular sieve by using metal oxide modification is simple and easy to operate, and can achieve good covering effect, but the method can reduce the acid sites on the outer surface and can also cause the reduction of the acid sites in the pore channels of the molecular sieve.

Disclosure of Invention

According to one aspect of the application, a modified molecular sieve is provided, wherein the modification is carried out through isomorphous replacement of silicon tetrachloride, so that acid sites on the outer surface of the molecular sieve are selectively eliminated, and the acid sites in the pore channels of the molecular sieve are reserved.

The modified molecular sieve is characterized in that the largest pore channel of the modified molecular sieve is a ten-membered ring pore channel;

compared with the molecular sieve before modification, the mol content of the acid sites in the pore channels of the modified molecular sieve is reduced by less than 10%; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 15%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 5% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 50%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 10% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 50%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 10% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 70%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 10% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 90%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 10% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 94%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 2% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 94%.

Optionally, the molar content of acid sites in the channels of the modified molecular sieve is reduced by less than 0.9% compared to the molecular sieve before modification; the reduction in the molar content of the external surface acid sites of the modified molecular sieve is greater than 93%.

Optionally, the modified molecular sieve is ZSM-5 or ZSM-22.

According to one aspect of the application, a preparation method of a molecular sieve modified by isomorphous replacement of silicon tetrachloride is provided, so that the aim of selectively and permanently eliminating the acid sites on the outer surface of the molecular sieve with the largest channel being a ten-membered ring channel is fulfilled.

The preparation method of the modified molecular sieve is characterized by comprising the following steps:

1) exchanging the molecular sieve with sodium ions to obtain a sodium type molecular sieve;

2) activating the sodium type molecular sieve, then adsorbing silicon tetrachloride by adopting a chemical vapor deposition method, and purging after the silicon tetrachloride is adsorbed and saturated;

3) heating to the temperature T at which isomorphous replacement reaction occurs_RAnd (3) carrying out reaction, blowing and cleaning after the reaction is finished, washing with water, obtaining the hydrogen type molecular sieve through ammonia ion exchange, and roasting to obtain the modified molecular sieve.

Specifically, the preparation method of the silicon tetrachloride isomorphous replacement modified molecular sieve comprises the following steps:

1) completely exchanging cations of the molecular sieve into sodium ions by an ion exchange method;

2) activating the molecular sieve obtained in the step 1) at 400-600 ℃ in an inert atmosphere, then adsorbing the molecular sieve in the step 1) with silicon tetrachloride by adopting a chemical vapor deposition method at a certain temperature, and blowing and cleaning after adsorption saturation;

3) increasing the reaction temperature to a temperature T required for the isomorphous displacement reaction to occur_RAnd after the reaction is finished, fully purging, washing with water, exchanging to a hydrogen form, and roasting in air to obtain the target molecular sieve.

In the step 1), the largest pore channel of the used molecular sieve is a ten-membered ring pore channel, and the size of the silicon tetrachloride molecule is larger than that of the ten-membered ring molecular sieve at low temperature, so that the silicon tetrachloride molecule can only be selectively adsorbed on the acid site on the outer surface of the molecular sieve.

Alternatively, in step 1), the largest channels of the molecular sieve are ten-membered ring channels.

Optionally, the molecular sieve is ZSM-5 or ZSM-22.

Optionally, in step 1), the sodium ion exchange comprises:

(1) putting the molecular sieve into a soluble sodium salt solution, wherein the liquid-solid weight ratio is 2-10;

(2) stirring for 1-4 hours at 40-90 ℃;

(3) after solid-liquid separation, washing the molecular sieve with water until the solution is neutral;

(4) drying at 70-120 ℃, and then roasting at 450-600 ℃ for 2-6 hours to obtain the sodium type molecular sieve.

Specifically, in step 1), the ion exchange method is prepared by the following steps:

(1) placing the molecular sieve sample in a soluble sodium salt solution with a certain concentration, wherein the liquid-solid ratio is 2-10 (weight ratio);

(2) stirring for 1-4 hours at 40-90 ℃;

(3) after solid-liquid separation, washing the molecular sieve sample by deionized water until the solution is neutral;

(4) drying at 70-120 ℃, and then roasting at 450-600 ℃ for 2-6 hours to obtain the pretreated molecular sieve.

In the step (1), the solution used for sodium ion exchange is one of sodium nitrate, sodium chloride, sodium acetate, sodium carbonate and other sodium ion-containing solutions with certain concentration.

Optionally, the sodium salt in the soluble sodium salt solution is selected from at least one of sodium nitrate, sodium chloride, sodium acetate and sodium carbonate.

Optionally, step 2) comprises:

and introducing an atmosphere I into the reactor loaded with the molecular sieve for activation, then introducing an atmosphere II carrying silicon tetrachloride for reaction and adsorption, and switching an atmosphere III to purge after the silicon tetrachloride is adsorbed to saturation.

Optionally, atmosphere I is selected from at least one of nitrogen, helium;

atmosphere II also includes at least one of an inert atmosphere; the volume concentration of the silicon tetrachloride in the atmosphere II is 1-50%;

atmosphere III is selected from at least one of non-reactive atmospheres;

the inert atmosphere is at least one selected from nitrogen, helium, neon and argon.

Optionally, the upper limit of the volume concentration of silicon tetrachloride in atmosphere II is selected from 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%; the lower limit is selected from 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40% or 45%.

Optionally, the temperature for adsorbing the silicon tetrachloride is 100-280 ℃, and the time for adsorbing the silicon tetrachloride is 0.5-4 hours.

Optionally, the upper limit of the temperature at which the silicon tetrachloride is adsorbed is selected from 150 ℃, 200 ℃ or 250 ℃; the lower limit is selected from 100 ℃, 150 ℃ or 200 ℃.

Alternatively, the upper limit of the time for silicon tetrachloride adsorption is selected from 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4 hours; the lower limit is selected from 0.5 hour, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or 3.5 hours.

Optionally, in step 2), the time of purging is 1 to 6 hours. And after the silicon tetrachloride is adsorbed and saturated, purging to completely remove the non-chemisorbed silicon tetrachloride.

Alternatively, in step 3), the temperature T of the isomorphous metathesis reaction_RThe temperature is 300 ℃ and 550 ℃, and the reaction time is 0.5-4 hours.

Alternatively, the temperature T of the isomorphous metathesis reaction_RThe upper limit is selected from 350 ℃, 400 ℃, 450 ℃, 500 ℃ or 550 ℃; the lower limit is selected from 300 deg.C, 350 deg.C, 400 deg.C, 450 deg.C or 500 deg.C.

Alternatively, the upper limit of the reaction time is selected from 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4 hours; the lower limit is selected from 0.5 hour, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or 3.5 hours.

Optionally, in step 3), after the isomorphous replacement reaction is finished, washing with water until no silicon tetrachloride remains, and then performing ion exchange.

Optionally, the ammonia ion exchange in step 3) comprises: placing the washed sample in ammonium salt solution with the molar concentration of 0.1-1mol/ml, wherein the liquid-solid weight ratio is 2-10;

stirring at 60-90 deg.C for 4-6 hr, and repeating for 2-4 times. Optionally, the roasting conditions in step 3) are as follows: roasting at 400-600 ℃ for 4-6 hours.

Optionally, the molecular sieve having the largest channels being ten-membered ring channels is ZSM-5 or ZSM-22.

According to the preparation method of the silicon tetrachloride isomorphous replacement modified molecular sieve, firstly, sodium ions of the molecular sieve are exchanged to a sodium type by ion exchange, and then SiCl is carried out on the sodium type molecular sieve at a low temperature by adopting a chemical vapor deposition method₄The chemical adsorption is carried out at the preferred adsorption temperature of 100 ℃ and 280 ℃ and the adsorption time of 0.5-4 h.

After the adsorption saturation, SiCl remained and physically adsorbed in the reaction tube is removed₄Removing completely, raising the reaction temperature to the temperature required by the isomorphous replacement reaction, and starting the reaction at the temperature T required by the isomorphous replacement reaction_RThe temperature is 300 ℃ and 550 ℃, and the reaction time is 0.5-4 h.

And after the reaction is finished, fully purging, purging impurities remained in the reaction tube and the molecular sieve, cooling, taking out the molecular sieve, washing the molecular sieve with deionized water for 6 times, and then exchanging ammonia ions to a hydrogen form to obtain the required molecular sieve catalyst.

In the present application, the molar content of the acid sites is the ratio of the number of moles of the acid sites to the weight of the molecular sieve; the acid site mole number is obtained by combining an infrared spectrometer with a probe molecule calculation test.

The beneficial effects that this application can produce include:

the molecular sieve is modified by adopting a silicon tetrachloride dealuminization and silicon supplementation method, so that the acid sites on the outer surface of the molecular sieve can be effectively eliminated, the acid sites in the pore canal of the molecular sieve can not be reduced, and the size of the pore canal opening of the molecular sieve is only slightly influenced.

Drawings

FIG. 1 Performance of the triisopropylbenzene cleavage reaction on ZSM-22 at different isomorphous metathesis temperatures;

FIG. 2 shows the performance of the tri-cumene cleavage reaction on ZSM-22 at different adsorption temperatures;

FIG. 3 shows the performance of the tri-isopropyl benzene cracking reaction on ZSM-5 at different isomorphous displacement reaction temperatures;

FIG. 4 shows the performance of the tri-isopropyl benzene cracking reaction on ZSM-5 at different adsorption temperatures.

Detailed Description

The present application will be described in detail with reference to examples, but the present application is not limited to these examples.

In the following implementation, the preparation of the molecular sieve is the existing mature technology, the Si/Al of the used ZSM-5 molecular sieve is 19, and the particle size is 100-200 nm; the Si/Al of the ZSM-22 molecular sieve is 35, and the grain diameter is 0.5-1.2 mu m.

The analysis method in the examples of the present application is as follows:

acid site assay analysis was performed using an infrared spectrometer from Bruker tenor 27 in conjunction with probe molecular adsorption.

Elemental analysis was performed using an Axios type fluorescence spectrometer (XRF) from PANalytical.

In the examples of the present application, the triisopropylbenzene conversion was calculated by:

examples 1 to 5

The ZSM-22 molecular sieve catalyst of this example was modified by the following procedure:

1) the 5g k-ZSM-22 molecular sieve and 30ml of sodium nitrate solution with the molar concentration of 0.6mol/ml are mixed evenly, stirred at the constant speed of 80 ℃ for 2h, and washed with deionized water for three times. After repeating the sodium ion exchange for three times, washing with deionized water, drying at 120 ℃ for 12h, and then roasting at 500 ℃ for 6h to obtain the Na-ZSM-22 molecular sieve;

2) adopting a chemical vapor deposition method to adsorb the Na-ZSM-22 molecular sieve by silicon tetrachloride: placing 4g of Na-ZSM-22 molecular sieve in a fixed bed quartz tube reactor, introducing nitrogen at 500 ℃ for activating for 2h, cooling to 250 ℃, then introducing a mixed gas of silicon tetrachloride and nitrogen, wherein the volume concentration of the silicon tetrachloride in the mixed gas is 5%, switching pure nitrogen to feed after adsorbing for 1h, purging for 5h, and purging the residual non-chemisorbed silicon tetrachloride in the reaction tube;

3) increasing the reaction temperature to a temperature T required for the isomorphous displacement reaction to occur_RIn example 1T_RIs 300 ℃; in example 2T_RIs 350 ℃; in example 3T_RIs 400 ℃; in example 4T_RIs 450 ℃; in example 5T_RThe temperature was 500 ℃. And after the reaction is finished, fully purging, washing, exchanging to a hydrogen form, and roasting in the air at 500 ℃ for 6 hours to obtain the target molecular sieve. The ammonia ion exchange operation is as follows: 3g of the obtained molecular sieve sample and 20ml of ammonium nitrate aqueous solution with the molar concentration of 0.5mol/ml are uniformly mixed, stirred at a constant speed for 3 hours at 80 ℃, washed with deionized water for three times and repeated for three times.

Characterization results for the molecular sieves of the examples.

The molecular size of the 2, 6-di-tert-butylpyridine is 1.07nm, while the ZSM-22 main channel is a one-dimensional ten-membered ring channel parallel to the [001] direction, the size is 0.46 multiplied by 0.57nm, and the maximum spherical size capable of being accommodated is 0.571 nm. Therefore, 2, 6-di-tert-butylpyridine molecules cannot diffuse into the pore channels of the ZSM-22 molecular sieve and can only be adsorbed on the outer surface, and 2, 6-di-tert-butylpyridine can generate specific signals on an infrared spectrogram after being adsorbed on the molecular sieve, so that the infrared of 2, 6-di-tert-butylpyridine is a good means for representing the number of acid sites on the inner and outer surfaces of the pore channels of the ZSM-22 molecular sieve. The characterization results are shown in Table 1. The specific test method comprises the following steps: and (3) characterizing the acid content of the outer surface of the ZSM-22 by using a 2, 6-di-tert-butylpyridine in-situ vacuum device and an infrared spectrometer. Firstly, preparing 10mg of ZSM-22 molecular sieve to be measured into sample pieces with the same mass, thickness and area, filling the sample pieces into a 2, 6-di-tert-butylpyridine in-situ vacuum device, activating for 1h at 450 ℃ in a vacuum environment, cooling to 50 ℃, starting to adsorb 2, 6-di-tert-butylpyridine, then desorbing for 1h at 150 ℃, removing the physically adsorbed 2, 6-di-tert-butylpyridine, measuring a spectrogram, and calculating according to the spectrogram to obtain the number of acid sites contained in the outer edge surface of the ZSM-22 molecular sieve.

The molecular size of the triisopropylbenzene is 0.85nm, so that the triisopropylbenzene molecules can not diffuse into ten-membered ring channels of the ZSM-22 molecular sieve and can only react on acid sites on the outer surface of the molecular sieve, and the triisopropylbenzene molecular sieve is very goodThe probe reaction of the external surface acid site of the molecular sieve. The specific test method comprises the following steps: filling 0.15g of molecular sieve to be evaluated (40-60 meshes) in a fixed bed reaction tube, activating for 1h at 500 ℃ under a nitrogen atmosphere, cooling to 360 ℃, introducing mixed gas of nitrogen and saturated steam of triisopropylbenzene, starting reaction, starting sampling by a gas chromatograph at 5min, and analyzing gas composition. As can be seen from FIG. 1, with T_RThe increase of the content of the triisopropylbenzene leads the conversion rate of the triisopropylbenzene cracking to be gradually reduced, and finally, the reaction hardly occurs, which shows that the acid sites on the outer surface of the ZSM-22 molecular sieve can be eliminated by using a method of silicon tetrachloride isomorphous replacement for dealuminization and silicon supplement.

The number of acid sites on the inner and outer surfaces of the pore channels after isomorphous replacement modification of the ZSM-22 molecular sieves of examples 1-5 is shown in Table 1.

TABLE 1 acid sites on the inner and outer surfaces of the channels of ZSM-22 molecular sieves of examples 1 to 5 after isomorphous replacement modification

Examples 6 to 8

The ZSM-22 molecular sieve catalyst of this example was modified by the following procedure:

1) the 5g k-ZSM-22 molecular sieve and 30ml of sodium nitrate solution with the molar concentration of 0.5mol/ml are mixed evenly, stirred at 50 ℃ for 4 hours at constant speed and washed with deionized water for three times. After repeating the sodium ion exchange for three times, washing with deionized water, drying at 100 ℃ for 12h, and then roasting at 500 ℃ for 6h to obtain the Na-ZSM-22 molecular sieve;

2) adopting a chemical vapor deposition method to adsorb the Na-ZSM-22 molecular sieve by silicon tetrachloride: placing 4g of Na-ZSM-22 molecular sieve in a fixed bed quartz tube reactor, introducing nitrogen at 500 ℃ for activation for 2h, and then cooling to the adsorption temperature, in example 6, cooling to 150 ℃; in example 7, the temperature was reduced to 200 ℃; in example 8, the temperature is reduced to 250 ℃, then a mixed gas of silicon tetrachloride and nitrogen is introduced, the volume concentration of the silicon tetrachloride in the mixed gas is 5%, pure nitrogen is switched to feed after 1h of adsorption, purging is carried out for 5h, and the residual non-chemisorbed silicon tetrachloride in the reaction tube is purged completely;

3) raising the reaction temperature to 450 ℃ required for the isomorphous replacement reaction, fully purging and cleaning after the reaction is finished, exchanging to a hydrogen form after washing, and roasting for 6 hours at 500 ℃ in the air to obtain the target molecular sieve. The ammonia ion exchange operation is as follows: 3g of the obtained molecular sieve sample and 20ml of ammonium nitrate aqueous solution with the molar concentration of 0.5mol/ml are uniformly mixed, stirred at a constant speed for 3 hours at 80 ℃, washed with deionized water for three times and repeated for three times.

Molecular sieve characterization results of the examples

As can be seen from fig. 2, the conversion of triisopropylbenzene cracking gradually decreased with the increase in adsorption temperature, and almost no reaction occurred at the end.

The number of acid sites on the inner and outer surfaces of the channels after isomorphous replacement modification of the ZSM-22 molecular sieves of examples 6-8 is shown in Table 2.

TABLE 2 acid sites on the inner and outer surfaces of the channels of ZSM-22 molecular sieves of examples 6 to 8 after isomorphous replacement modification

Examples 9 to 13

The ZSM-5 molecular sieve catalyst of the embodiment is modified by the following steps:

1) uniformly mixing 5g of sodium type ZSM-5 molecular sieve and 30ml of sodium nitrate solution with the molar concentration of 0.6mol/ml, uniformly stirring for 2 hours at 80 ℃, washing for three times by deionized water, drying for 12 hours at 120 ℃, and then roasting for 6 hours at 500 ℃ to obtain the Na-ZSM-5 molecular sieve;

2) adopting a chemical vapor deposition method to adsorb the Na-ZSM-5 molecular sieve by silicon tetrachloride: placing 4g of Na-ZSM-5 molecular sieve in a fixed bed quartz tube reactor, introducing nitrogen at 500 ℃ for activating for 2h, cooling to 250 ℃, then introducing a mixed gas of silicon tetrachloride and nitrogen, wherein the volume concentration of the silicon tetrachloride in the mixed gas is 5%, switching pure nitrogen to feed after adsorbing for 1h, purging for 5h, and purging the residual non-chemisorbed silicon tetrachloride in the reaction tube;

3) increase the reactionTemperature up to the temperature T required for the isomorphous metathesis to take place_RIn example 1T_RIs 300 ℃; in example 2T_RIs 350 ℃; in example 3T_RIs 400 ℃; in example 4T_RIs 450 ℃; in example 5T_RThe temperature was 500 ℃. And after the reaction is finished, fully purging, washing, exchanging to a hydrogen form, and roasting in the air at 500 ℃ for 6 hours to obtain the target molecular sieve. The ammonia ion exchange operation is as follows: 3g of the obtained molecular sieve sample and 20ml of ammonium nitrate aqueous solution with the molar concentration of 0.5mol/ml are uniformly mixed, stirred at a constant speed for 3 hours at 80 ℃, washed with deionized water for three times and repeated for three times.

Characterization results for the molecular sieves of the examples.

The molecular size of 2, 6-di-tert-butylpyridine is 1.07nm, while ZSM-5 has two ten-membered ring channels with sizes of 0.51 × 0.55nm and 0.53 × 0.56nm, respectively, and the largest spherical size that can be accommodated is 0.636 nm. Therefore, 2, 6-di-tert-butylpyridine molecules cannot diffuse into the pore channels of the ZSM-5 molecular sieve and can only be adsorbed on the outer surface, and 2, 6-di-tert-butylpyridine can generate specific signals on an infrared spectrogram after being adsorbed on the molecular sieve, so that the infrared of 2, 6-di-tert-butylpyridine is a good means for representing the number of acid sites on the inner and outer surfaces of the pore channels of the ZSM-5 molecular sieve. The characterization results are shown in Table 3.

The molecular size of the triisopropylbenzene is 0.85nm, so that triisopropylbenzene molecules can not diffuse into ten-membered ring channels of the ZSM-5 molecular sieve and can only react on an acid site on the outer surface of the ZSM-5 molecular sieve, and therefore, the triisopropylbenzene catalytic cracking reaction can also well represent a probe reaction of the acid site on the outer surface of the molecular sieve. As can be seen from FIG. 3, the conversion rate of the triisopropylbenzene cracking is gradually reduced with the increase of the adsorption temperature, and finally, almost no reaction occurs, which shows that the acid sites on the outer surface of the ZSM-5 molecular sieve can be eliminated by using a method of silicon tetrachloride isomorphous replacement for dealumination and silicon supplement.

The number of acid sites on the inner and outer surfaces of the channels after isomorphous replacement modification of the ZSM-5 molecular sieves of examples 9-13 is shown in Table 3.

TABLE 3 acid sites on the inner and outer surfaces of the channels of ZSM-5 molecular sieves of examples 9-13 after isomorphous replacement modification

Examples 14 to 16

The ZSM-5 molecular sieve catalyst of the embodiment is modified by the following steps:

1) uniformly mixing 5g of a sodium type ZSM-5 molecular sieve and 30ml of a sodium nitrate solution with the molar concentration of 0.5mol/ml, uniformly stirring for 4 hours at 50 ℃, washing with deionized water, drying for 12 hours at 120 ℃, and then roasting for 6 hours at 500 ℃ to obtain the Na-ZSM-5 molecular sieve;

2) adopting a chemical vapor deposition method to adsorb the Na-ZSM-5 molecular sieve by silicon tetrachloride: placing 4g of Na-ZSM-5 molecular sieve in a fixed bed quartz tube reactor, introducing nitrogen at 500 ℃ for activation for 2h, and then cooling to the adsorption temperature, in example 14, cooling to 150 ℃; in example 15, the temperature was reduced to 200 ℃; in example 16, the temperature was reduced to 250 ℃, then a mixed gas of silicon tetrachloride and nitrogen was introduced, the volume concentration of silicon tetrachloride in the mixed gas was 5%, the pure nitrogen feed was switched after 1h of adsorption, purging was carried out for 5h, and the residual non-chemisorbed silicon tetrachloride in the reaction tube was purged;

3) raising the reaction temperature to 450 ℃ required for the isomorphous replacement reaction, fully purging and cleaning after the reaction is finished, exchanging to a hydrogen form after washing, and roasting for 6 hours at 500 ℃ in the air to obtain the target molecular sieve. The ammonia ion exchange operation is as follows: 3g of the obtained molecular sieve sample and 20ml of ammonium nitrate aqueous solution with the molar concentration of 0.5mol/ml are uniformly mixed, stirred at a constant speed for 3 hours at 80 ℃, washed with deionized water for three times and repeated for three times.

Molecular sieve characterization results of the examples

As can be seen from fig. 4, the conversion of triisopropylbenzene cracking gradually decreased with the increase in adsorption temperature, and almost no reaction occurred at the end.

The number of acid sites on the inner and outer surfaces of the channels after isomorphous replacement modification of the ZSM-22 molecular sieves of examples 14-16 is shown in Table 4.

TABLE 4 acid sites on the inner and outer surfaces of the channels of ZSM-22 molecular sieves of examples 14 to 16 modified by isomorphous replacement

Although the present application has been described with reference to a few embodiments, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the application as defined by the appended claims.