Preparation method and application of MgO supported boron-doped modified MCM-22 molecular sieve

Abstract

The application relates to a preparation method and application of MgO-loaded boron-doped modified MCM-22 molecular sieves and belongs to the field of chemical catalysis technology. In the application, silica sol, sodium metaaluminate, boric acid, sodium hydroxide and hexamethylene imine are mixed according to a certain molar ratio, MCM-22 crystal seeds are added, water bath aging, oven crystallization, washing, drying, calcination and other steps are carried out to obtain H-type xB-MCM-22 molecular sieves, and then a certain amount of MgO is loaded to obtain the xB-MCM-22 molecular sieves; the obtained molecular sieves are doped with boron atoms to realize the regulation and control of Al site distribution, the Al sites are more inclined to the inside of the molecular sieves, the external surface acid sites of the molecular sieves are reduced, and the internal active sites are increased; the amount of external surface acid and pore internal acid sites is covered by loading MgO, so that the finally obtained molecular sieves have more internal active sites and lower external surface acidity, and the molecular sieves exhibit higher catalytic activity and product selectivity in the reaction of toluene alkylation to synthesize p-xylene.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of preparation and application of modified molecular sieve catalysts, in particular to a preparation method of MgO supported boron doped modified MCM-22 molecular sieve and its application in selective synthesis of p-xylene by toluene alkylation. BACKGROUND

[0002] Para-xylene (PX) is an important organic chemical raw material, which can be used to produce terephthalic acid (PTA) and polyester products (PET), and is widely used in the synthesis of resins, medicines, plastics and many other fields. PX production processes mainly include aromatic complex production, toluene disproportionation and transalkylation, selective toluene disproportionation, xylene isomerization, etc., but most of them have problems such as difficult separation, high operation cost, and many side reactions. In recent years, toluene alkylation process has become a research hotspot due to its short process flow, simple operation, and economic efficiency.

[0003] MCM-22 molecular sieve is a commonly used catalyst for toluene alkylation due to its special pore system and good shape selection effect. Although MCM-22 molecular sieve has excellent activity and stability, the acid sites on its outer surface will cause the generated PX to undergo isomerization reaction during alkylation, thereby reducing the para-selectivity performance of the catalyst. Therefore, it is often necessary to eliminate the surface acid sites of MCM-22 through corresponding modification methods to improve the selectivity of PX. The common methods for modifying MCM-22 at present include gas phase silicon deposition, liquid phase silicon deposition, pre-carbon deposition and oxide modification, etc., the purpose of which is to eliminate the acid sites on the outer surface of the molecular sieve and reduce the pore size. However, due to the high proportion of acid sites on the surface of MCM-22 molecular sieve, the activity of the catalyst after modification is often significantly reduced.

[0004] Therefore, it is necessary to develop a modification method that can reduce the acid sites on the outer surface of the molecular sieve while improving the internal active sites, so as to obtain a MCM-22 molecular sieve catalyst with high activity and high selectivity for toluene alkylation reaction. SUMMARY

[0005] The present application is aimed at the problem of high proportion of acid sites on the outer surface of MCM-22 molecular sieve and low activity after modification, and provides a method for reducing the acid sites on the outer surface and improving the internal active sites, so as to obtain a MCM-22 catalyst with high activity and high selectivity for toluene alkylation reaction.

[0006] To solve the above technical problems, the method for preparing a boron doped modified MCM-22 molecular sieve shape selective catalyst according to the present application is carried out according to the following steps:

[0007] (1)Silica sol, sodium aluminate, boric acid, sodium hydroxide and hexamethylene imine are dissolved in distilled water according to the following molar ratio n(SiO2):n(Al2O3):n(H3BO3):n(NaOH):n(HMI):n(H2O)=1:0.033:0.033-0.167:0.25:0.35:35 to obtain a white gel;

[0008] (2)Seed crystals of MCM-22 corresponding to 1% of the mass of SiO2 are added to the white gel obtained in step (1), and the mixture is aged in a water bath at 25°C for 24 h, then moved to a hydrothermal reactor and placed in an oven at 1150°C for static crystallization for 7 days, and then washed and dried to obtain xB-MCM-22(P) without demoulding;

[0009] (3)The xB-MCM-22(P) without demoulding obtained in step (2) is calcined in a muffle furnace at 550°C for 8 h to obtain Na-type xB-MCM-22 molecular sieve;

[0010] (4)The Na-type xB-MCM-22 molecular sieve obtained in step (3) is added to 1 mol·L -1 of NH4Cl solution, and stirred at 80°C for 2 h, then dried by suction filtration and calcined in a muffle furnace at 550°C for 4 h to obtain H-type xB-MCM-22 molecular sieve;

[0011] (5)The H-type xB-MCM-22 molecular sieve obtained in step (4) is placed in a beaker, and a certain concentration of magnesium nitrate solution is added to the beaker by a pipette, and after absorption, it is left to stand for 2 h, dried in an oven at 80°C overnight, and then calcined in a muffle furnace at 550°C for 3-5 h to obtain the MgO-loaded boron-doped modified MCM-22 molecular sieve, denoted as yMgO / xB-MCM-22.

[0012] As a limitation of the present application, in steps (2), (3), (4) and (5) of the present application, x in the xB-MCM-22 molecular sieve is the molar ratio of B / Al, and its value is 0.5-2.5; in step (5), y in yMgO / xB-MCM-22 is the percentage of the mass of loaded MgO to the mass of xB-MCM-22 molecular sieve, and its value is 5%-15%.

[0013] As a further limitation of the present application, the MgO loaded boron-doped modified MCM-22 molecular sieve described in the present application is applied as a shape-selective catalyst in the reaction of toluene alkylation to synthesize p-xylene. The specific application method is as follows: 1.0 g of modified MCM-22 molecular sieve with a particle size of 20-30 mesh is placed in a fixed bed reactor, the vaporization chamber temperature is raised to 150 DEG C, the reactor is raised to 360-420 DEG C, under the protection of inert gas Ar, N2, after the temperature reaches the set value, the raw materials toluene and dimethyl carbonate are pumped into the reactor at a molar ratio of 4:1 by a micro constant flow pump, the mass space velocity is 5 h -1 , the reaction liquid is obtained after condensation, and the toluene conversion rate and p-xylene selectivity are quantitatively analyzed by gas chromatography after 6 h of reaction.

[0014] After the above technical solution is adopted, compared with the prior art, the present application has the following beneficial effects:

[0015] By doping boron atoms, the present application realizes the regulation of the distribution of Al sites in the MCM-22 molecular sieve, so that the Al sites are more inclined to be located in the interior of the molecular sieve, thereby reducing the external surface acid sites of the molecular sieve and increasing the internal active sites. At the same time, by loading metal oxide MgO, the external surface acid sites are covered, and finally the obtained molecular sieve can have the superposition of both, i.e. more internal active sites and lower external surface acidity, so as to exhibit higher activity and selectivity in the reaction of toluene alkylation to synthesize p-xylene, and the modified catalyst remains stable and does not appear to be deactivated after 20 h of toluene alkylation reaction. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is the XRD pattern of the catalyst obtained in Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 8, it can be seen that the catalyst obtained after boron doping and MgO loading does not have obvious characteristic peaks of impurities, indicating that boron doping and MgO modification do not affect the crystal structure of the zeolite.

[0017] Figure 2 is the NH3-TPD pattern of the catalyst obtained in Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 8, wherein there are two obvious NH3 desorption peaks at about 200 DEG C and 400 DEG C, respectively corresponding to weak acid sites and strong acid sites, it can be seen that the desorption peak of the strong acid center basically disappears after boron doping and MgO loading, and the desorption temperature of the weak acid center shifts to the low temperature direction, indicating that the acid strength of the catalyst is obviously reduced after boron doping and MgO modification, which may be due to the positioning of boron atoms or the entry of magnesium nitrate molecules into the molecular sieve channels when the boron atoms or the magnesium nitrate solution are introduced, thereby covering the acid in the channels and leading to the reduction of acid strength.

[0018] Figure 3is the reaction stability chart of Example 2, it can be seen that the catalyst treated by both boron doping and MgO loading remains stable and does not show deactivation phenomenon after 20h reaction in toluene alkylation reaction. DETAILED DESCRIPTION

[0019] The present application will be further described with the following examples, but it should be understood that these examples are only illustrative and should not be interpreted as limiting the implementation of the present application.

[0020] Example 1

[0021] (1) Dissolve silica sol, sodium aluminates, boric acid, sodium hydroxide and hexamethylene imine in distilled water according to the following molar ratio n(SiO2):n(Al2O3):n(H3BO3):n(NaOH):n(HMI):n(H2O) = 1:0.033:0.033:0.25:0.35:35 to obtain a white gel;

[0022] (2) Add MCM-22 seeds equivalent to 1% of the mass of SiO2 to the white gel obtained in step (1), and then age in a water bath at 25℃ for 24h, and then move to a hydrothermal reaction kettle and place in an oven at 150℃ for static crystallization for 7 days, and then extract, wash, dry to obtain 0.5B-MCM-22(P) without template removal;

[0023] (3) Calcine the 0.5B-MCM-22(P) without template removal in step (2) in a muffle furnace at 550℃ for 8h to obtain Na-type 0.5B-MCM-22 molecular sieve;

[0024] (4) Add the Na-type xB-MCM-22 molecular sieve obtained in step (3) to 1 mol·L -1 NH4Cl solution, stir at 80℃ for 2h, and then extract, dry, and then calcine in a muffle furnace at 550℃ for 4h to obtain H-type 0.5B-MCM-22 molecular sieve;

[0025] (5) Take the H-type 0.5B-MCM-22 molecular sieve in step (4) and place in a beaker, and then uniformly drop a certain concentration of magnesium nitrate solution into the beaker by using a pipette, and then stand for 2h after absorption, and then dry at 80℃ in an oven overnight, and then calcine in a muffle furnace at 550℃ for 3-5h to obtain the MgO loaded boron doped modified MCM-22 molecular sieve, wherein the loading amount of MgO is 5%, and the catalyst is recorded as 5MgO / 0.5B-MCM-22.

[0026] Example 2

[0027] Example 2 and the first four steps of Example 1 are the same, and the loading amount of MgO is changed to 10% in step (5) to obtain a catalyst recorded as 10MgO / 0.5B-MCM-22.

[0028] Example 3

[0029] Example 3 is the same as Example 1 up to the fourth step, in step (5) the loading of MgO is changed to 15%, the resulting catalyst is noted as 15MgO / 0.5B-MCM-22.

[0030] Example 4

[0031] Example 4 is the same as Example 1 up to the fourth step, in step (5) the loading of MgO is changed to 5%, the resulting catalyst is noted as 5MgO / 0.85B-MCM-22.

[0032] Example 5

[0033] Example 5 is the same as Example 4 up to the fourth step, in step (5) the loading of MgO is changed to 10%, the resulting catalyst is noted as 10MgO / 0.85B-MCM-22.

[0034] Example 6

[0035] Example 6 is the same as Example 4 up to the fourth step, in step (5) the loading of MgO is changed to 15%, the resulting catalyst is noted as 15MgO / 0.85B-MCM-22.

[0036] Example 7

[0037] Example 7 is the same as Example 1 up to the fourth step, in step (5) the loading of MgO is changed to 5%, the resulting catalyst is noted as 5MgO / 1.25B-MCM-22.

[0038] Example 8

[0039] Example 8 is the same as Example 7 up to the fourth step, in step (5) the loading of MgO is changed to 10%, the resulting catalyst is noted as 10MgO / 1.25B-MCM-22.

[0040] Example 9

[0041] Example 9 is the same as Example 7 up to the fourth step, in step (5) the loading of MgO is changed to 15%, the resulting catalyst is noted as 15MgO / 1.25B-MCM-22.

[0042] Example 10

[0043] Example 10 is the same as Example 1 up to the fourth step, in step (5) the loading of MgO is changed to 5%, the resulting catalyst is noted as 5MgO / 1.75B-MCM-22.

[0044] Example 11

[0045] Example 11 is the same as the first four steps of Example 10, except that in step (5) the loading of MgO is changed to 10%, and the resulting catalyst is designated 10MgO / 1.75B-MCM-22.

[0046] Example 12

[0047] Example 12 is the same as the first four steps of Example 10, except that in step (5) the loading of MgO is changed to 15%, and the resulting catalyst is designated 15MgO / 1.75B-MCM-22.

[0048] Example 13

[0049] Example 13 is the same as Example 1, except that the B / Al molar ratio is 2.5, and the loading of MgO is 5%, and the resulting catalyst is designated 5MgO / 2.5B-MCM-22.

[0050] Example 14

[0051] Example 14 is the same as the first four steps of Example 13, except that in step (5) the loading of MgO is changed to 10%, and the resulting catalyst is designated 10MgO / 2.5B-MCM-22.

[0052] Example 15

[0053] Example 15 is the same as the first four steps of Example 13, except that in step (5) the loading of MgO is changed to 15%, and the resulting catalyst is designated 15MgO / 2.5B-MCM-22.

[0054] Comparative Example 1

[0055] Comparative Example 1 is the same as Example 1, except that no boron is doped and no MgO is loaded, and the resulting catalyst is designated MCM-22.

[0056] Comparative Example 2

[0057] Comparative Example 2 is the same as Example 1, except that no MgO is loaded, and the resulting catalyst is designated 0.5B-MCM-22.

[0058] Comparative Example 3

[0059] Comparative Example 3 is the same as Example 4, except that no MgO is loaded, and the resulting catalyst is designated 0.85B-MCM-22.

[0060] Comparative Example 4

[0061] Comparative Example 4 was identical to Example 7 except that no MgO was loaded. The resulting catalyst was labeled 1.25B-MCM-22.

[0062] Comparative Example 5

[0063] Comparative Example 5 was identical to Example 10 except that no MgO was loaded. The resulting catalyst was labeled 1.75B-MCM-22.

[0064] Comparative Example 6

[0065] Comparative Example 6 was identical to Example 13 except that no MgO was loaded. The resulting catalyst was labeled 2.5B-MCM-22.

[0066] Comparative Example 7

[0067] Comparative Example 7 was identical to Example 1 except that no boron was doped. The resulting catalyst was labeled 5MgO / MCM-22.

[0068] Comparative Example 8

[0069] Comparative Example 8 was identical to Comparative Example 7 except that the loading of MgO was 10%. The resulting catalyst was labeled 10MgO / MCM-22.

[0070] Comparative Example 9

[0071] Comparative Example 9 was identical to Comparative Example 7 except that the loading of MgO was 15%. The resulting catalyst was labeled 15MgO / MCM-22.

[0072] The catalysts obtained in Examples 1-15 and Comparative Examples 1-9 above were used in the reaction of toluene and dimethyl carbonate to synthesize p-xylene. The specific steps were as follows: 1.0 g of the surface-modified MCM-22 molecular sieve shape-selective catalyst with a particle size of 20-30 mesh was placed in a fixed bed reactor, the vaporizing chamber was heated to 150°C, and the reactor was heated to 360-420°C. Under the protection of inert gas Ar or N2, after the temperature reached the set value, the raw materials toluene and dimethyl carbonate were pumped into the reactor at a molar ratio of 4:1 by a micro-constant flow pump, the mass space velocity was 5 h -1 , the reaction flowed through the bed, and the reaction liquid was obtained after condensation. After 6 h of reaction, the conversion of toluene and the selectivity of p-xylene were quantitatively analyzed by gas chromatography.

[0073] The reaction conditions and the conversion of toluene and the selectivity of p-xylene are shown in Table 1.

[0074] Table 1 Comparison of the catalytic activities of the modified MCM-22 molecular sieves obtained in Examples 1-15 and Comparative Examples 1-9

[0075]

[0076]

[0077] From Table 1, it can be seen that the MCM-22 molecular sieve catalyst modified by loading MgO and boron doping in the technical scheme of the application has obviously increased activity and selectivity of the product p-xylene in the reaction of synthesizing p-xylene from toluene alkylation, and is very suitable for further industrialization and popularization and application.

[0078] The above only describes the preferred embodiments of the present application, and it should be noted that those of ordinary skill in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered within the protection scope of the present application.

Claims

1. A method for preparing MgO-supported boron-doped modified MCM-22 molecular sieve, characterized in that... The method is performed according to the following steps: (1) Silica sol, sodium aluminate, boric acid, sodium hydroxide and hexamethyleneimine were dissolved in distilled water in the following molar ratio nSiO2:nAl2O3:nH3BO3:nNaOH:nHMI:nH2O=1:0.033:0.033~0.167:0.25:0.35:35 to obtain a white gel; (2) Add MCM-22 seed crystals equivalent to 1% of SiO2 mass to the white gel obtained in step (1), age it in a 25 °C water bath for 24 h, then transfer it to a hydrothermal reactor and place it in a 150 °C oven for static crystallization for 7 days. After filtration, washing and drying, xB-MCM-22 P without template agent is obtained. (3) The xB-MCM-22 P without demolding template agent described in step (2) was calcined in a muffle furnace at 550 °C for 8 h to obtain Na-type xB-MCM-22 molecular sieve; (4) Add the Na-type xB-MCM-22 molecular sieve obtained in step (3) to The solution was stirred at 80 °C for 2 h, filtered and dried, and then calcined in a muffle furnace at 550 °C for 4 h to obtain H-type xB-MCM-22 molecular sieve. (5) Take the H-type xB-MCM-22 molecular sieve from step (4) and place it in a beaker. Use a pipette to evenly add a certain concentration of magnesium nitrate solution to the beaker. After absorption, let it stand for 2 hours, dry it overnight in an oven at 80 °C, and then calcine it in a muffle furnace at 550 °C for 3-5 hours to obtain the MgO-supported boron-doped modified MCM-22 molecular sieve, denoted as yMgO / xB-MCM-22. In steps (2), (3), (4), and (5), x represents the B / Al molar ratio of xB-MCM-22 molecular sieve, with a value of 0.5 to 2.

5. In step (5), y represents the percentage of the mass of MgO loaded on xB-MCM-22 to the mass of xB-MCM-22 molecular sieve, with a value of 5% to 15%.

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