CN110694679A - EMT/FAU core-shell molecular sieve catalyst and preparation method and application thereof - Google Patents

Abstract

The invention relates to an EMT/FAU core-shell molecular sieve catalyst and a preparation method and application thereof. The catalyst has a core-shell structure, uses an EMT molecular sieve as a core phase, and uses a FAU molecular sieve as a shell layer, and the surface of the EMT molecular sieve is treated with a weak alkaline solution. After treatment and addition of cocatalysts, the prepared EMT/FAU core-shell molecular sieve catalyst was used to catalyze the production of methyl acetate in the evaluation device. Compared with the existing technology, the EMT/FAU core-shell molecular sieve uses EMT as the active component, and on the basis of maintaining high selectivity and stability, it improves the conversion rate of dimethyl ether, reduces the cost, and has obvious technical advantages. Advantage.

Description

A kind of EMT/FAU core-shell molecular sieve catalyst and its preparation method and application

technical field

The invention relates to the field of catalysts, in particular to an EMT/FAU core-shell molecular sieve catalyst and a preparation method and application thereof.

Background technique

As an important clean energy, ethanol can be used directly as a liquid fuel or mixed with gasoline to reduce the emission of carbon monoxide and hydrocarbons in vehicle exhaust. It is of great significance to solve the problem of air pollution and achieve sustainable development in my country. Based on my country's energy structure of "poor oil, little gas, and relatively abundant coal resources" and the current situation of rising dependence on petroleum, the development of a new process for synthesizing ethanol from coal or biomass-based syngas is expected to significantly reduce my country's dependence on petroleum. , to promote my country's energy diversification reform.

Dimethyl ether (DME) carbonylation to synthesize methyl acetate, methyl acetate is further hydrogenated to obtain the target product ethanol, which is a new route to prepare ethanol in recent years. This route has high atom economy, wide sources of raw material CO, mild reaction conditions and good selectivity of target products. Compared with other production processes (such as fermentation method, direct synthesis method, etc.), the hydrogenation process of methyl acetate avoids the formation of ethanol-water azeotrope, and greatly saves the investment in equipment and energy consumption caused by separation. At present, the key technical difficulty of this process route is the development of high-efficiency and high-stability catalysts, and its related research will definitely promote the development of the entire ethanol industry.

In recent years, various catalysts for the carbonylation of dimethyl ether to methyl acetate have been reported. 2006 Berkeley's Enrique Iglesia research group (Angew. Chem, Int. Ed. 45 (2006) 10, 1617-1620, J. Catal. 245 (2007) 110, J. Am. Chem. Soc. 129 (2007) 4919 ) found that the carbonylation reaction of dimethyl ether can be carried out in Mordenite (mordenite) and Ferrierite (ferrierite) with an 8-membered ring molecular sieve system, and the selectivity of methyl acetate is very good, reaching 99%, but the dimethyl ether carbonyl chemical activity is very low. (Catal. Lett. 2010, 139: 33-37) comparatively studied the difference in the reactivity of methyl acetate for the carbonylation of dimethyl ether on MOR and ZSM-35 catalysts, and found that ZSM-35 molecular sieve had better reaction Stability and product selectivity, at 250℃, 1MPa, DME/CO/N ₂ /He=5/50/2.5/42.5～12.5ml/min reaction conditions, the conversion rate of dimethyl ether reaches 11%, methyl acetate The selectivity reaches 96%. Then CN106365995B reported that methyl acetate was obtained by carbonylation reaction on the catalyst of acidic EMT molecular sieve. It was found that the method was carried out in the presence of acidic EMT molecular sieve as a catalyst, and the reaction activity was high and the stability was remarkably improved. Patent CN 106890665A reported that the use of EMT molecular sieve as the active component can greatly improve the selectivity and stability of methyl acetate. After 100 hours of continuous reaction on the fixed bed, the selectivity of methyl acetate can still be maintained between 91% and 98.3%. between. However, the conversion rate of dimethyl ether is relatively low, ranging from 15% to 30%, and the relevant results cannot meet the needs of industrial production.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a kind of EMT/FAU core-shell molecular sieve catalyst and its preparation method and application in order to solve the above-mentioned problems. The EMT/FAU core-shell molecular sieve of the present invention has great advantages in improving the conversion rate of dimethyl ether. FAU core-shell molecular sieve has both the advantages of EMT and FAU molecular sieve. FAU molecular sieve has strong adsorption capacity for polar molecules, EMT molecular sieve has strong acidity and more acid content, and has excellent isomerization, alkane and alkane in petroleum processing. The performance of radicalization and aromatization, using the adsorption of FAU molecular sieve to dimethyl ether molecules and the acidic catalytic effect of EMT molecular sieve, on the one hand, the concentration of dimethyl ether molecules on the surface of the catalyst is increased, and on the other hand, the external surface acidity of the zeolite catalyst is improved. , can improve the conversion rate of dimethyl ether.

The object of the present invention is achieved through the following technical solutions:

An EMT/FAU core-shell molecular sieve catalyst, which has a core-shell structure, uses an EMT molecular sieve as a core, and uses a FAU molecular sieve as a shell layer, the EMT molecular sieve is treated with a weak alkaline solution, and a co-catalyst is added.

EMT molecular sieves and FAU molecular sieves are composed of the same cage-shaped structural units (double 6-membered ring and β cage) connected in different ways. The similarity in structure provides good conditions for the preparation of EMT/FAU core-shell molecular sieves. Both molecular sieves have a 3-dimensional 12-membered ring channel system, which has low material diffusion resistance and excellent channel connectivity, which is more conducive to the adsorption of reactant molecules and the diffusion of product molecules, and can give full play to EMT molecular sieves and FAU molecular sieves. The synergistic effect in the ether carbonylation reaction improves the stability and conversion rate of the catalyst, and at the same time enables the catalyst to have good regeneration performance in the dimethyl ether carbonylation reaction.

Preferably, the cocatalyst is introduced into the EMT molecular sieve by in-situ synthesis, metal ion exchange or impregnation support, and the cocatalyst is selected from one or more of copper, iron, gallium or silver, based on the metal element , the content is 0.05 to 1.0 wt%. The acid site of EMT molecular sieve has the function of isomerization, and the metal promoter has the function of hydrogenation and dehydrogenation. Adding a promoter can improve the selectivity and stability of the catalyst. With the assistance of the promoter, the olefin is hydrogenated into alkane, which greatly increases the The formation of oligomers and coke precursors is reduced.

Preferably, the molar ratio of SiO ₂ to Al ₂ O ₃ in the FAU molecular sieve is 2.0-6.0, and preferably, the molar ratio of SiO ₂ to Al ₂ O ₃ in the FAU molecular sieve is 2.6-3.0. The small molar ratio of FAU molecular sieve makes it have strong polarity, and it is easy to adsorb polar and polar molecules. The appropriate mass ratio of EMT molecular sieve to synthetic gel enables FAU molecular sieve to grow on EMT molecular sieve better, avoiding the synthesis of pure crystalline FAU molecular sieve.

Preferably, the grain size of the EMT molecular sieve is 1-3 μm. EMT molecular sieves with a particle size ranging from 1 to 3 μm have higher utilization rates and are easier to synthesize.

Preferably, the FAU molecular sieve is coated on the surface of the EMT molecular sieve by a vapor phase inversion method, and the thickness is 0.1-1 μm. Thinner shell molecular sieves allow the reactants to have a shorter diffusion path and speed up the catalytic rate.

A preparation method of an EMT/FAU core-shell molecular sieve catalyst, comprising the following steps:

(1) The EMT molecular sieve is surface-treated with a weak base solution, and then a co-catalyst is added;

(2) Mixing of silicon source, aluminum source, alkali and water with molecular molar ratio of (3-10) SiO ₂ : 1Al ₂ O ₃ : (3-24) Na ₂ O: (200-1322) H ₂ O The liquid is used as the synthetic liquid of FAU molecular sieve;

(3) adding the FAU type molecular sieve synthesis solution to the dried EMT molecular sieve powder after the treatment in step (1), stirring evenly, aging treatment, adopting the vapor phase synthesis method to crystallize in the reactor, and then washing, drying and roasting , to obtain EMT/FAU core-shell molecular sieves.

In step (1), the weak base treatment can increase the surface roughness, so that the FAU molecular sieve is easy to grow on the EMT molecular sieve; on the other hand, the weak base treatment can improve the reactivity of the carbonylation of dimethyl ether.

Traditional core-shell molecular sieves are synthesized by adding crystals to the growth mother liquor and using in-situ hydrothermal synthesis, which has many disadvantages. The mother liquor is only loaded on the external crystals in a small amount, and most of the mother liquid crystallizes to form isolated molecular sieves. Steps (3) This shortcoming can be overcome by the vapor phase synthesis method, which can be used for irregular carriers and can control the thickness of the shell layer; The formation of isolated molecular sieves is reduced, and the discharge of mother liquor is avoided to pollute the environment.

Preferably, the weak base is selected from sodium acetate, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, sodium fatty acid, sodium alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, hexadecylbenzene One of sodium sulfonate or basic aluminum chloride or a mixture thereof, and the weak base solution has a solubility of 0.05-2 mol/L.

Preferably, in step (1), the weak base solution is treated with the EMT molecular sieve at room temperature for 1-12 hours, washed with deionized water until the pH of the washing solution is about 10, and then dried. The weak alkali treatment conditions of 1-12h ensure that the EMT molecular sieve has a certain surface roughness.

Step (2) silicon source is methyl orthosilicate, ethyl orthosilicate, tetraethyl orthosilicate, silicic acid, sodium silicate, potassium silicate, calcium silicate, magnesium silicate, silica sol, water glass , silicon dioxide, silica, fly ash, or a mixture thereof; the aluminum source is one or a mixture of sodium aluminate, calcium aluminate, aluminum sol, alumina; the alkali is lithium oxide, oxide One or a mixture of sodium, potassium oxide, lithium hydroxide, sodium hydroxide, potassium hydroxide.

Preferably, the mass ratio of the EMT molecular sieve to the FAU type molecular sieve synthesis solution in step (3) is 1:0.1-5; the aging treatment conditions are, the temperature is 20-60°C, and the time is 2-24h; vapor phase crystallization The conditions are: crystallization at 95-110°C for 6-24h, washing and drying, and then calcining at 400-600°C for 2h to finally obtain EMT/FAU core-shell molecular sieve.

The catalyst is used for the carbonylation of dimethyl ether to synthesize methyl acetate, and is treated in a nitrogen atmosphere at 200-400° C. for 6 hours before use.

The experimental setup was one of a fixed bed reactor, a fluidized bed reactor or a moving bed reactor for evaluation. The specific experimental process is as follows: firstly, the catalyst is filled into the reaction, the temperature is 170-240°C, the pressure is 1.0-15.0MPa, the mass space velocity of the dimethyl ether feed is 0.1-2.5h ^-1 , and the carbon monoxide and dimethyl ether The evaluation was performed under the condition that the molar ratio of ether was 15:1 to 1:1.

Compared with the prior art, the present invention has the following technical characteristics:

(1) EMT/FAU core-shell molecular sieves can improve the conversion rate of catalysts. FAU type molecular sieves have adsorption effect on polar and easily polarized molecules, which can increase the concentration of reactants, thereby improving the conversion rate of reactants.

(2) The EMT/FAU core-shell molecular sieve can improve the stability of the catalyst, and the EMT/FAU core-shell structure can prevent the rapid deactivation caused by the strong acidity of the outer surface of the EMT molecular sieve.

(3) The EMT/FAU core-shell molecular sieve can improve the selectivity of the catalyst, and the EMT/FAU core-shell structure can reduce the by-products generated by the excessive acidity of the outer surface of the EMT molecular sieve.

Description of drawings

Fig. 1 is the surface SEM spectrum of EMT molecular sieve.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

1. Preparation of catalyst

Example 1

Treatment of EMT molecular sieve: Immerse EMT molecular sieve with a particle size of about 1 μm in 0.05mol/L sodium acetate solution at room temperature for 1h, wash and dry, and then soak in 0.01mol/L AgNO ₃ solution for 6h, wash and dry, as shown in Fig. 1 is the surface SEM spectrum of EMT molecular sieve.

Preparation of EMT/FAU core-shell molecular sieve: put 9.7g of NaOH solution in 80g of water, add 2.0g of NaAlO ₂ under strong stirring, mix and stir until a clear solution, cool for later use, and obtain solution A. Weigh 23 g of sodium silicate, dissolve it in 100 g of water, and stir for 0.5 h to obtain solution B. The solution A was slowly added dropwise to the solution B under stirring conditions until a uniform raw material molar ratio of 10SiO ₂ : Al ₂ O ₃ : 24Na ₂ O : 1322H ₂ O was formed into a synthetic solution, and 50 g of the synthetic solution was weighed and added 10g of EMT molecular sieve powder was mixed evenly and aged in an oven at 20°C for 24h, then the sol was placed on the upper layer of the crystallization kettle, 30g of water was added to the bottom of the kettle, and after crystallization at 95°C for 24h, EMT/FAU with a shell thickness of 0.9 μm was prepared Core-shell molecular sieve catalyst EF1.

Comparative Example 1

Treatment of EMT molecular sieve: The EMT molecular sieve with a particle size of about 1 μm was soaked in 0.01 mol/L AgNO ₃ solution for 6 h, washed and dried to obtain the Ag-loaded EMT molecular sieve DB1.

Comparative Example 2

Preparation of FAU molecular sieve: add 0.14g of NaOH solution to 20g of water, add 3.7g of NaAlO ₂ under vigorous stirring, mix and stir until a clear solution, cool for later use, and obtain solution A. Weigh 12.8 g of sodium silicate, dissolve it in 27 g of water, and stir for 0.5 h to obtain solution B. The solution A was slowly added dropwise to the solution B under stirring conditions until a uniform raw material molar ratio of 3SiO ₂ : Al ₂ O ₃ : 3Na ₂ O : 200H ₂ O was formed into a synthesis solution, and then the synthesis solution was placed in In the crystallization kettle, the FAU molecular sieve DB2 was prepared after crystallization at 95°C for 24 hours.

Example 2

Treatment of EMT molecular sieve: The EMT molecular sieve with a particle size of about 3 μm was soaked in a 2mol/L tetraethylammonium hydroxide solution at room temperature for 12 hours, washed and dried.

Preparation of EMT/FAU core-shell molecular sieve: put 0.14g of NaOH solution in 20g of water, add 3.7g of NaAlO ₂ under strong stirring, mix and stir until a clear solution, cool for later use, and obtain solution A. Weigh 12.8 g of sodium silicate, dissolve it in 27 g of water, and stir for 0.5 h to obtain solution B. The solution A was slowly added dropwise to the solution B under stirring conditions until a uniform raw material molar ratio of 3SiO ₂ : Al ₂ O ₃ : 3Na ₂ O : 200H ₂ O was formed into a synthesis solution, and 1 g of the synthesis solution was weighed and added 10g of EMT molecular sieve powder was mixed well and aged in an oven at 60°C for 2h, then the sol was placed on the upper layer of the crystallization kettle, 30g of water was added to the bottom of the kettle, and after crystallization at 110°C for 6h, EMT/FAU with a shell thickness of 0.2 μm was prepared Core-shell molecular sieve catalyst EF2.

In Examples 3-8, EMT/FAU core-shell molecular sieve catalysts EF3 to EF8 were synthesized according to the synthesis ratio and synthesis conditions of Table 1, according to the similar method and steps of Example 1.

Table 1 embodiment 3-8 synthesis ratio and synthesis condition

2. Catalyst performance test example

Example 9

After tableting 2g of EF1 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 35.6% and the selectivity is 98%.

Example 10

After 2g of DB1 molecular sieve catalyst was pressed into tablets, the particles with a particle size of 20-40 meshes were sieved. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 26.2% and the selectivity is 83%. Because the DB1 molecular sieve catalyst is less polar than the EF1 molecular sieve catalyst in Example 1, and the surface of the DB1 molecular sieve catalyst does not have a shape-selective catalytic effect, The methyl acetate conversion and selectivity are both lower than those of Example 1.

Example 11

After 2g of DB2 molecular sieve catalyst was pressed into tablets, the particles with a particle size of 20-40 meshes were sieved. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 0% and the selectivity is 0%. Since the DB2 molecular sieve catalyst does not have the performance of catalyzing the carbonylation of dimethyl ether, the conversion rate and selectivity of methyl acetate are both 0%. .

Example 12

After tableting 2g of EF2 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 37% and the selectivity is 98%.

Example 13

After tableting 2g of EF3 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 41% and the selectivity is 99%.

Example 14

After tableting 2g of EF4 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 31% and the selectivity is 97%.

Example 15

After tableting 2g of EF5 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 36% and the selectivity is 99%.

Example 16

After tableting 2g of EF6 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 42% and the selectivity is 98%.

Example 17

After tableting 2g of EF7 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 36% and the selectivity is 96%.

Example 18

After tableting 2g of EF8 molecular sieve catalyst, sieve the particles with a particle size of 20-40 meshes. Loaded into a fixed-bed reactor with a tube inner diameter of 10 mm, the temperature was 180 ° C, the pressure was 1.5 MPa, the molar ratio of carbon monoxide and dimethyl ether was 5:1, and the dimethyl ether feed mass space velocity was 0.1h ^{- 1.} After the device runs for 100 hours, the conversion rate of methyl acetate is 30% and the selectivity is 96%.

The foregoing description of the embodiments is provided to facilitate understanding and use of the invention by those of ordinary skill in the art. It will be apparent to those skilled in the art that various modifications to these embodiments can be readily made, and the generic principles described herein can be applied to other embodiments without inventive step. Therefore, the present invention is not limited to the above-mentioned embodiments, and improvements and modifications made by those skilled in the art according to the disclosure of the present invention without departing from the scope of the present invention should all fall within the protection scope of the present invention.

Claims (10)

1. an EMT/FAU core-shell molecular sieve catalyst, is characterized in that, this catalyzer is a core-shell structure, takes EMT molecular sieve as inner core, takes FAU molecular sieve as shell layer, and described EMT molecular sieve is processed through weak base solution, and adds auxiliary catalyst. 2. a kind of EMT/FAU core-shell molecular sieve catalyst according to claim 1, is characterized in that, described co-catalyst is introduced into EMT molecular sieve by in-situ synthesis, metal ion exchange or impregnation loading, and described co-catalyst is selected as zeolite. The content of one or more of copper, iron, gallium or silver is 0.05-1.0 wt % in terms of metal element. 3 . The EMT/FAU core-shell molecular sieve catalyst according to claim 1 , wherein the molar ratio of SiO ₂ to Al ₂ O ₃ in the FAU molecular sieve is 2.0 to 6.0. 4 . 4 . The EMT/FAU core-shell molecular sieve catalyst according to claim 1 , wherein the grain size of the EMT molecular sieve is 1-3 μm. 5 . 5 . The EMT/FAU core-shell molecular sieve catalyst according to claim 4 , wherein the FAU molecular sieve is coated on the surface of the EMT molecular sieve by a vapor-phase transformation method, and the thickness is 0.1-1 μm. 6 . 6. the preparation method of a kind of EMT/FAU core-shell molecular sieve catalyst as claimed in claim 1, is characterized in that, comprises the steps: (1) The EMT molecular sieve is surface-treated with a weak base solution, and then a co-catalyst is added; (2) Mixing of silicon source, aluminum source, alkali and water with molecular molar ratio of (3-10) SiO ₂ : 1Al ₂ O ₃ : (3-24) Na ₂ O: (200-1322) H ₂ O The liquid is used as the synthetic liquid of FAU molecular sieve; (3) adding the FAU type molecular sieve synthesis solution to the dried EMT molecular sieve powder after the treatment in step (1), stirring evenly, aging treatment, adopting the vapor phase synthesis method to crystallize in the reactor, and then washing, drying and roasting , to obtain EMT/FAU core-shell molecular sieves. 7. the preparation method of a kind of EMT/FAU core-shell molecular sieve catalyst according to claim 6, is characterized in that, described weak base is selected from sodium acetate, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, fatty acid One of sodium, sodium alkyl benzene sulfonate, sodium dodecyl benzene sulfonate, sodium hexadecyl benzene sulfonate or basic aluminum chloride or a mixture thereof, the weak base solution concentration is 0.05-2mol /L. 8 . The preparation method of an EMT/FAU core-shell molecular sieve catalyst according to claim 7 , wherein the weak base solution treats the EMT molecular sieve for 1-12 hours at room temperature. 9 . 9. the preparation method of a kind of EMT/FAU core-shell molecular sieve catalyst according to claim 6, is characterized in that, the mass ratio of EMT molecular sieve described in step (3) and described FAU type molecular sieve synthesis solution is 1:0.1 ~5; The aging treatment conditions are as follows: the temperature is 20-60 °C, and the time is 2-24 hours; The application of a kind of EMT/FAU core-shell molecular sieve catalyst as claimed in claim 1, it is characterized in that, this catalyst is used for dimethyl ether carbonylation to synthesize methyl acetate.

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