Mtt structure molecular sieve, preparation method and use thereof

Abstract

The application discloses a preparation method of MTT structure molecular sieve, which comprises the following steps: (1) uniformly mixing water, a template solution and a structure directing agent, then uniformly mixing lye, a silicon source and an aluminum source, and crystallizing at a certain temperature for a period of time; (2) performing water washing on the product of step (1) until neutralization, drying at 100-120 DEG C to obtain a solid; (3) calcining the solid obtained in step (2) at a certain temperature for a period of time to obtain potassium type MTT molecular sieve; (4) dispersing the potassium type MTT molecular sieve obtained in step (3) in 0.8-1.2 mol / L ammonium chloride aqueous solution, refluxing at a certain temperature for a period of time, and drying at 100-120 DEG C; and (5) calcining the solid obtained in step (4) at a certain temperature for a period of time to obtain hydrogen type MTT molecular sieve. The application further discloses the MTT structure molecular sieve and the use of the MTT structure molecular sieve as a catalyst carrier for simultaneously improving catalytic activity and isomerization selectivity in a long-chain normal alkane hydrogenation isomerization reaction.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of catalysts, and particularly relates to a MTT structure molecular sieve, a preparation method and application thereof. BACKGROUND

[0002] The normal alkane hydroisomerization catalyst is generally a bifunctional catalyst, the molecular sieve serves as a carrier and provides acid sites, and the noble metal is loaded to play a role of hydrogenation and dehydrogenation. The MTT structure molecular sieve has one-dimensional straight pores, and the pore size is 0.45 nm x 0.52 nm; these pore characteristics are suitable for the hydroisomerization reaction of long-chain normal alkane, and have high selectivity for isomerization products. However, the MTT structure molecular sieve crystals synthesized by the general method tend to form larger aggregates, which is not conducive to the diffusion of reactants, intermediates and products, and can cause more cracking. In addition, the acid amount in the micropores of the MTT structure molecular sieve is relatively large, the acid is relatively strong, and the proportion of the required accessible active sites is relatively small, so it is generally difficult to simultaneously improve the catalytic activity and isomerization selectivity of the isomerization catalyst.

[0003] Therefore, in order to simultaneously improve the catalytic activity and isomerization selectivity of the catalyst, promote the diffusion of reactants, intermediates and products, and at the same time inhibit the cracking reaction, the dispersion of the MTT structure molecular sieve crystals should be as high as possible, and the number of accessible acid sites of the molecular sieve should be appropriately increased, and the possibility of cracking should be reduced.

[0004] The synthesis of the molecular sieve is affected by many factors, and the growth of the crystal is carried out by the template or structure directing agent contained in the micropore channel of the molecular sieve crystal, so the template or structure directing agent plays a crucial role in the formation of the specific structure of the molecular sieve. The synthesis of the general molecular sieve crystal is completed under alkaline conditions, and the silicon-aluminum species is generally negatively charged under alkaline conditions, therefore, the selection of the positively charged auxiliary structure directing agent can effectively increase the interaction between the surfactant and the silicon-aluminum species. In order to improve the dispersion of the molecular sieve crystal, various in-situ regulating agents have been added to the crystallization liquid of the molecular sieve with different structure types in the past research. The in-situ regulating agent has various types, including surfactants, silane coupling agents, etc.

[0005] Patent CN 104787777 discloses a method for synthesizing high-dispersion small-grain ZSM-5. The ZSM-5 molecular sieve crystal grain size is uniform and the dispersion is good by stirring and degrading cellulose dissolved in a mixed solution of sodium hydroxide / urea / water, and then adding the mixed solution into a ZSM-5 molecular sieve synthesis solution. Rimer et al. (J. Am. Chem. Soc. 2013, 135(17), 6608-6617) proposed the concept of molecular sieve growth regulator, using various small molecular alcohols and amines to regulate the growth of L molecular sieve, and different length-diameter ratio of molecular sieve crystals can be obtained by adding different structure regulators. The growth of molecular sieve crystals can be regulated by the above methods and technologies to obtain high-dispersion molecular sieve crystals. However, in the past research, there is a lack of research on one-dimensional molecular sieve, and the molecular sieve commonly used for long-chain alkane hydrogenation isomerization is mostly one-dimensional molecular sieve. The existing technology for improving the molecular sieve crystal mostly involves molecular sieves with block crystals, such as ZSM-5, Y molecular sieve, beta molecular sieve, etc., and the research on one-dimensional pore needle-shaped crystals such as MTT is less, and such one-dimensional pore molecular sieve is most suitable as a carrier for long-chain n-alkane hydrogenation isomerization catalyst. Therefore, the regulation of the dispersion and acidity of MTT crystal is of research value.

[0006] The molecular sieve crystal synthesized according to the general method tends to form larger aggregates, which can lead to smaller intercrystalline stacking pores, and is not conducive to the diffusion of reactants, intermediates and product molecules. It is difficult to simultaneously improve the catalytic activity and isomerization selectivity of the catalyst. For the hydrogenation isomerization reaction of long-chain n-alkane, the acid sites are mainly distributed in the micropore channel, and when the reaction activity is high, the isomerization selectivity will be reduced. Therefore, in order to improve the performance of the long-chain n-alkane hydrogenation isomerization catalyst, it is necessary to improve the dispersion of the carrier molecular sieve crystal and appropriately increase the B acid sites of the outer surface accessibility. SUMMARY

[0007] The present application aims to provide a method for in-situ regulation and preparation of high-dispersion MTT structure molecular sieve, and the obtained high-dispersion MTT structure molecular sieve is used for the hydrogenation isomerization reaction of long-chain n-alkane, and the catalytic activity and isomerization selectivity of the catalyst are simultaneously improved. The present application regulates the synthesis of MTT structure molecular sieve by in-situ adding a structure-directing agent 1-n carbon alkyl-3-isopropyl imidazolium salt (IPIM) to improve the dispersion of the molecular sieve crystal, and appropriately increase the B acid sites of the outer surface accessibility, and further improve the catalytic activity and isomerization selectivity of the catalyst for the hydrogenation isomerization reaction of long-chain n-alkane. n IPIM + ) to improve the dispersion of the molecular sieve crystal, and appropriately increase the B acid sites of the outer surface accessibility, and further improve the catalytic activity and isomerization selectivity of the catalyst for the hydrogenation isomerization reaction of long-chain n-alkane.

[0008] The technical scheme of the present application is as follows:

[0009] The first aspect of the present application discloses a preparation method of MTT structure molecular sieve, comprising the following steps:

[0010] (1) mixing water, template solution and structure directing agent uniformly, then adding alkali liquor, silicon source and aluminum source to mix uniformly, and crystallizing at a certain temperature for a period of time;

[0011] (2) separating the product of step (1) by washing with water to neutral, and drying at 100-120 ℃ to obtain solid;

[0012] (3) calcining the solid obtained in step (2) at a certain temperature for a period of time to obtain potassium type MTT molecular sieve;

[0013] (4) dispersing the potassium type MTT molecular sieve obtained in step (3) in 0.8-1.2 mol / L ammonium chloride aqueous solution, refluxing at a certain temperature for a period of time, and drying at 100-120 ℃;

[0014] (5) calcining the solid obtained in step (4) at a certain temperature for a period of time to obtain hydrogen type MTT molecular sieve.

[0015] Preferably, the template is 1,3-diisopropylimidazolium salt (anion can be chloride, bromide or OH-), the structure directing agent is 1-n carbon alkyl-3-isopropylimidazolium salt (anion can be chloride, bromide or OH-), the silicon source is silica sol with SiO2 content of 30wt%-40wt%, or silicon gas sol, and the aluminum source is aluminum sol or aluminum oxide; the molar ratio of each substance is as follows:

[0016] Scope SiO2 / Al2O3 15-100 H2O / SiO2 20-300 OH - / SiO2]] 0.02-0.5 Q / SiO2 0.02-0.5 C n IPIM + / Q]]> 1-5

[0017] wherein Q represents the template, C n IPIM + represents the structure directing agent 1-n carbon alkyl-3-isopropylimidazolium salt.

[0018] Preferably, n in the structure directing agent 1-n carbon alkyl-3-isopropylimidazolium salt is the number of alkyl carbon atoms, and 10≤n≤20.

[0019] Preferably, the crystallization temperature in step (1) is 160-180 ℃, and the time is 3-20 days.

[0020] Preferably, the mass of the ammonium chloride aqueous solution in step (4) is 5-15 times the mass of the potassium type MTT molecular sieve; the reflux temperature is 70-85 ℃, and the reflux time is 2-6 h; step (4) needs to be carried out not less than three times.

[0021] Preferably, the calcination temperature of step (3) is 580-620℃, and the calcination time is 4-6h; the calcination temperature of step (5) is 500-600℃, and the calcination time is 2-6h.

[0022] The second aspect of the present application discloses the MTT structure molecular sieve prepared by the preparation method.

[0023] The third aspect of the present application discloses the use of the MTT structure molecular sieve as a catalyst carrier for simultaneously improving the catalytic activity and isomerization selectivity of long-chain normal alkane in a hydrogenation isomerization reaction.

[0024] Preferably, the long-chain normal alkane is n-hexadecane; the hydrogenation isomerization reaction step is: loading the catalyst in a fixed bed reactor, filling the two ends with silicon carbide, and the hydrogenation isomerization reaction conditions are: pressure 3MPa, space velocity 2h -1 , hydrogen / oil ratio 1000, and temperature 250-300℃.

[0025] Preferably, before the hydrogenation isomerization reaction starts, the catalyst is reduced in a hydrogen atmosphere at 400℃ for 4h.

[0026] The beneficial effects of the present application are:

[0027] The present application is based on the traditional MTT structure molecular sieve commonly used template 1,3-diisopropyl imidazolium salt, and uses a suitable positively charged and hydrophobic long-chain auxiliary structure directing agent 1-n carbon alkyl-3-isopropyl imidazolium salt (C n IPIM + ), to reduce the possibility of crystal homodimerization; at the same time, the number of catalyst carrier orifices is increased, the accessibility of the B acid site on the outer surface is improved, the isomerization selectivity of the catalyst is improved, and the reaction activity of the catalyst is improved. The present application controls the synthesis of the MTT structure molecular sieve by adding the auxiliary structure directing agent C n IPIM + in situ, improves the dispersity of the molecular sieve crystal, and appropriately increases the B acid site on the outer surface, thereby simultaneously improving the catalytic activity and isomerization selectivity of the long-chain normal alkane hydrogenation isomerization reaction of the catalyst with the molecular sieve as the carrier. The addition amount of C 16 IPIM + is preferably (1-5)∶1 to the molar ratio of the template; too much addition will affect the generation of the crystal, and too little addition will cause serious homodimerization of the crystal.

[0028] The auxiliary structure directing agent C n IPIM + used in the present application can be purchased on the market or synthesized by oneself; wherein 1-hexadecyl-3-isopropyl imidazolium (C 16 IPIM+ ) Good effect. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 XRD patterns of the molecular sieves obtained in Inventive Examples 1-3 and Comparative Example 1.

[0030] Figure 2 SEM images of the molecular sieves obtained in Inventive Examples 1-3 and Comparative Example 1. DETAILED DESCRIPTION

[0031] The following examples embody the processes described in the present application, but the present application is not limited to these examples.

[0032] Comparative Example 1: No auxiliary structure directing agent was added.

[0033] 1.50 g of template 1,3-diisopropylimidazolium chloride was prepared into a solution with a concentration of 0.8 M-1.0 M in a polytetrafluoroethylene liner, 22.50 g of high-purity water was added thereto and shaken well; then 12.0 g of potassium hydroxide solution (concentration 1 mol / L) was added, and stirred at room temperature for 30 min; 7.62 g of silica sol was added to the above alkaline solution, and continued to be stirred; then 2.50 g of 1056 sol was added to the above stirring solution, so that the molar ratio of SiO2 to Al2O3 was 50, and continued to be stirred; finally, isobutylamine 0.90 g (the addition of isobutylamine can make the crystal particles smaller, and also can reduce the amount of template; it is also possible not to add isobutylamine) was added; the reactor was charged, and the synthesis reaction was carried out at 170°C and a rotation speed of 110 rpm for 9 days; the obtained substance was washed with water to neutral, and after separation, the obtained solid was dried; then it was placed in a muffle furnace, heated to 600°C and calcined for 4 h to remove the template; then the powder was dispersed in 1 mol / L of aqueous ammonium chloride solution, the liquid-solid ratio was 10, and refluxed at 80°C for 4 h; it was suction filtered and dried in an oven at 120°C, and then repeated ion exchange twice; finally, the powder was calcined at 550°C for 4 h to convert the molecular sieve into hydrogen type; and noble metal Pt was loaded, and the loading amount was 0.5 wt%.

[0034] The obtained product was analyzed by XRD, and it was determined that the structure of the product was MTT molecular sieve, see Figure 1 ; the SEM image thereof is shown in Figure 2 , it can be seen that the crystal has obvious agglomeration phenomenon, the crystal forms a relatively dense aggregate, and the pore between the crystals is small, which will cause the molecular diffusion to be hindered. The silicon-aluminum ratio of the product is shown in Table 1.

[0035] Example 1: Auxiliary structure directing agent C 16 IPIM + The addition ratio was 1 times the molar amount of template 1,3-diisopropylimidazolium chloride.

[0036] The implementation conditions are similar to those of Comparative Example 1, except that after all other components of the synthesis solution are added and stirred thoroughly, the template 1,3-diisopropylimidazolium chloride is added in a 1-fold molar amount of the structure-directing agent C 16 IPIM + Afterwards, stirring is continued for 30 min. The obtained molecular sieve is determined to be a molecular sieve with MTT structure by XRD analysis, see Figure 1 The crystal dispersion is improved compared to the sample obtained in Comparative Example 1, but there is still obvious aggregation, see Figure 2 The external surface accessible B acid sites of the product are shown in Table 1, which are higher than those of Comparative Example 1.

[0037] Example 2: Structure-directing agent C 16 IPIM + is added in a 2-fold molar amount of the template 1,3-diisopropylimidazolium chloride.

[0038] The implementation conditions are similar to those of Example 1, except that the structure-directing agent C 16 IPIM + is added in a 2-fold molar amount of the template 1,3-diisopropylimidazolium chloride. The obtained molecular sieve is determined to be a molecular sieve with MTT structure by XRD analysis, see Figure 1 The crystal dispersion is improved compared to the sample obtained in Example 1, see Figure 2 The external surface accessible B acid sites of the product are shown in Table 1, which are higher than those of Comparative Example 1.

[0039] Example 3: Structure-directing agent C 16 IPIM + is added in a 3.5-fold molar amount of the template 1,3-diisopropylimidazolium chloride.

[0040] The implementation conditions are similar to those of Example 1, except that the structure-directing agent C 16 IPIMBr is added in a 3.5-fold molar amount of the template 1,3-diisopropylimidazolium chloride. The obtained molecular sieve is determined to be a molecular sieve with MTT structure by XRD analysis, see Figure 1 The crystal dispersion is improved compared to the sample obtained in Example 2, see Figure 2 The external surface accessible B acid sites of the product are shown in Table 1, which are higher than those of Comparative Example 1.

[0041] The above results show that the addition of the structure-directing agent C 16 IPIM + has a significant regulating effect on the crystal dispersion and the acidity of the molecular sieve with MTT structure, and a higher crystal dispersion and slightly higher external surface accessible B acid sites can be obtained.

[0042] Table 1 B acid sites of sample outer surface accessibility

[0043]

[0044] Example 4: Catalytic hydroisomerization of n-hexadecane.

[0045] The catalysts obtained in Comparative Example 1 and Examples 1-3 (supported noble metal Pt, loading amount of 0.5 wt%) were applied to the hydroisomerization of n-hexadecane: the catalysts were packed in a micro fixed bed reactor, both ends filled with silicon carbide. Before the start of the hydroisomerization, reduction was carried out at 400°C for 4h under a hydrogen atmosphere. The conditions of the hydroisomerization were: pressure 3 MPa, space velocity 2h -1 -1, hydrogen to oil ratio 1000, temperature 250-300°C.

[0046] The catalysts obtained in Examples 1-3 and Comparative Example 1 were used for the catalytic hydroisomerization of n-hexadecane, and the conversion rate, isomerization rate, isomerization selectivity, and the molar ratio of single branched product to multi branched product are shown in Table 2. As can be seen from Table 2, at the same reaction temperature, compared with Comparative Example 1, the conversion rate and isomerization selectivity of the catalysts of Examples 1-3 are improved synchronously. The conversion rate can be increased by 15.95% 【95.38%-79.43%=15.95%】 at the highest; the cracking rate (i.e. "≤C 15 / wt%") is reduced, and can be reduced by 5.03% 【22.91%-17.88%=5.03%】 at the highest; the isomerization yield (i.e. "C 16 / wt%") and isomerization selectivity are significantly improved, and the isomerization yield can be increased by 19.75% 【76.27%-56.52%=19.75%】 at the highest; the isomerization selectivity can be increased by 8.8% 【79.96%-71.16%=8.8%】 at the highest. At the same reaction temperature, the conversion rate, isomerization yield and isomerization selectivity of the catalyst obtained in Comparative Example 1 are obviously poor, and the cracking rate is high. Therefore, the addition of the structure-directing agent C 16 IPIM + The conversion rate and isomerization amount of the catalyst prepared are both greatly improved compared with the conversion rate and isomerization amount of Comparative Example 1. It is shown that the effect of the MTT structure molecular sieve catalyst prepared in the present application on simultaneously improving the catalytic activity and isomerization selectivity for the hydroisomerization of long chain n-alkanes is obvious.

[0047] Table 2 comparison of catalytic performance results

[0048]

[0049]

Note

[0050] The above embodiments only express several embodiments of the present application, which are described in a more specific and detailed manner, but cannot be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are all within the protection scope of the present application. Therefore, the protection scope of the present application patent should be subject to the appended claims.

Claims

1. A method for preparing an MTT-structured molecular sieve, characterized in that, Includes the following steps: (1) Mix water, template agent solution and structure guiding agent evenly, then add alkali solution, silicon source and aluminum source and mix evenly, then crystallize at a certain temperature for a period of time; (2) The product from step (1) is washed with water until neutral separation, and dried at 100-120 °C to obtain a solid. (3) The solid obtained in step (2) is calcined at a certain temperature for a period of time to obtain potassium-type MTT molecular sieve; (4) Disperse the potassium-type MTT molecular sieve obtained in step (3) in a 0.8-1.2 mol / L ammonium chloride aqueous solution, reflux it at a certain temperature for a period of time, and then dry it at 100-120 °C; (5) The solid obtained in step (4) is calcined at a certain temperature for a period of time to obtain hydrogen-type MTT molecular sieve; The template agent is 1,3-diisopropylimidazolium salt, the structure directing agent is 1-n-carbonyl-3-isopropylimidazolium salt, the silicon source is silica sol or silica aerosol with a SiO2 content of 30wt%–40wt%, and the aluminum source is aluminum sol or alumina; the molar ratio range of each substance is: Where Q represents the template agent, and C... n IPIM + The representative structure-directing agent is the 1-n-carbon alkyl-3-isopropylimidazolium salt, where n represents the number of alkyl carbon atoms, 10 ≤ n ≤ 20.

2. The preparation method according to claim 1, characterized in that, Step (1) Crystallization temperature is 160-180 ℃ and time is 3-20 days.

3. The preparation method according to claim 1, characterized in that, The mass of the ammonium chloride aqueous solution in step (4) is 5-15 times the mass of the potassium-type MTT molecular sieve; the reflux temperature is 70-85 ℃ and the reflux time is 2-6 h; step (4) needs to be performed no less than three times.

4. The preparation method according to claim 1, characterized in that, The roasting temperature in step (3) is 580-620 ℃ and the roasting time is 4-6 h; the roasting temperature in step (5) is 500-600 ℃ and the roasting time is 2-6 h.

Images