A core-shell Co-based all-silicon molecular sieve catalyst, preparation method and application thereof

Abstract

The present invention provides a core-shell Co-based all-silicon molecular sieve catalyst, wherein the active component is Co and the carrier is Silicalite-1 molecular sieve; the CoO synthesized by the present invention x @Silicalite‑1 molecular sieve catalyst can regulate CoO x The particle size is small and sintering is inhibited during the reaction, while the interaction with the carrier Silicliate-1 is enhanced, thereby showing better propane dehydrogenation catalytic performance and propylene selectivity; the present invention adopts a method of first impregnation and then hydrothermal to synthesize efficient and stable core-shell CoO x @Silicalite‑1 molecular sieve propane dehydrogenation catalyst is more conducive to the active species CoO compared with the one-step hydrothermal synthesis method. x During the synthesis process, it is confined in the framework structure of the Silicalite-1 molecular sieve, making the active sites highly dispersed, reducing the deep dehydrogenation of propylene at the active sites and inhibiting the occurrence of side reactions such as carbon deposition, and has good stability.

Description

Technical Field

[0001] The invention belongs to the field of molecular sieve catalysts, and in particular relates to a core-shell Co-based all-silicon molecular sieve catalyst, a preparation method and an application thereof. Background Art

[0002] As one of the organic products of petrochemical industry, propylene can be used to synthesize downstream chemical products such as acrylonitrile, polypropylene and propylene oxide. It is also the main raw material for the three major synthetic materials of plastics, rubber and fibers. It is widely used in various aspects such as medical treatment, pharmaceuticals and daily life. Its demand is expected to continue to expand globally.

[0003] The traditional propylene industry chain is mainly produced through catalytic cracking or fluidized catalytic cracking of naphtha, diesel and other petroleum products. However, due to the shortage of petroleum resources, the supply is insufficient to meet the demand. The industry is in urgent need of seeking new process routes. Compared with the traditional oil-to-propylene production route, the propane dehydrogenation process has the advantages of high propylene selectivity, abundant raw material sources, simple product composition and easy separation. In particular, the development of shale gas has gradually attracted widespread attention from chemical companies at home and abroad. The commercial catalysts for propane dehydrogenation are mainly Pt-based catalysts and Cr-based catalysts. Pt-based catalysts have strong activation ability for CH bonds and high propylene selectivity, but they are expensive and have high industrial costs. Cr-based catalysts have strong toxicity and are not easy to protect the environment and sustainable development. Therefore, the development of alternative new catalysts is urgent. Compared with traditional metal catalysts, green and cheap metal oxides also show good dehydrogenation performance in PDH reactions, but they have the disadvantages of easy sintering or carbon deposition deactivation. Therefore, it is particularly important to select a suitable carrier to support active metals to improve the activity and stability of the catalyst.

[0004] Molecular sieves are used as carriers, and their pores and framework structures are used to confine active metals in the pores or T sites of the framework to control the particle size and dispersion of active species. In addition, by developing different molecular sieve synthesis methods, the molecular sieve particle size, crystal surface, etc. are regulated to achieve better confinement, which is of great significance to improving the activity and stability of the catalyst. Patent CN 114984999 A provides a method for preparing Co@Silicalite-1 catalyst by impregnating a cobalt source on a Silicalite-1 molecular sieve and then hydrothermally synthesizing it. The reaction pressure is 0.05MPa, the reaction temperature is 580℃, and the WHSV of propane gas feed is 8h -1Under the conditions of , the propane conversion rate is 36.2% and the selectivity is 88.3%, but the invention requires secondary crystallization of Silicalite-1 molecular sieve, and the process is relatively complicated. Patent CN 114984999A and patent CN113318774 B introduce second metal additives such as Mg, Zr, and Ca on the basis of Co-based all-silicon molecular sieve for modification. Although the above patent solutions improve the catalytic effect to a certain extent, they still have the problem of rapid deactivation and the activity needs to be further improved.

[0005] By using molecular sieves as carriers and utilizing their pores and framework structure to confine active metals in the pores or at the T sites of the framework, the particle size and dispersion of active species can be controlled. Therefore, developing a new molecular sieve to improve the activity and stability of the catalyst is a technical problem that needs to be solved urgently. Summary of the invention

[0006] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a core-shell Co-based all-silicon molecular sieve and a preparation method thereof which is green, environmentally friendly, low-cost, efficient and stable for catalyzing propane dehydrogenation. The pores and skeleton structure of the Silicalite-1 molecular sieve are used to confine the active metal Co in the pores to control the particle size and dispersion of the active species, thereby providing a new idea for the development of industrial catalysts for propane dehydrogenation.

[0007] The present invention provides a core-shell Co-based all-silicon molecular sieve catalyst, wherein the active component is Co and the carrier is Silicalite-1 molecular sieve; wherein the mass of the active component is calculated as metal elements, the mass of the carrier is calculated as SiO2, and the active component as an element is 0.5wt% to 5wt% of the weight of the carrier.

[0008] The present invention also provides a method for preparing the core-shell Co-based all-silicon molecular sieve catalyst as described above, using an impregnation and hydrothermal method to crystallize CoO x The precursor is confined in the pore structure of the molecular sieve, thereby preparing a catalyst with a core-shell structure.

[0009] Furthermore, the present invention provides a method for preparing the core-shell Co-based all-silicon molecular sieve catalyst as described above, comprising the following steps:

[0010] Step S1: drying the silica gel at 100° C. for 6 to 7 hours to fully remove moisture;

[0011] Step S2: Add 0.5wt%-5wt% of active component cobalt source to anhydrous ethanol and dissolve for 60 minutes, add silicon source to anhydrous ethanol and stir first and then ultrasonicate, the stirring speed is 350rpm, the ultrasonic intensity is 40KHZ; the stirring and ultrasonication time are both 30 minutes to make it evenly dispersed;

[0012] Step S3: impregnating the cobalt source on the silicon source by evaporation impregnation method, drying at 100° C. for 6-7 hours after impregnation, and then calcining at 350° C. for 3 hours to obtain a Co3O4 / SiO2 precursor;

[0013] Step S4: Mix the Co3O4 / SiO2 precursor with the template, NaOH and water at room temperature and stir for 12 hours to obtain a uniform turbid solution at a stirring speed of 300-400 rpm;

[0014] Step S5: pouring the stirred turbid liquid into a hydrothermal kettle for hydrothermal crystallization at 180° C. for 24 h;

[0015] Step S6: The crystallized product is centrifuged, washed, dried and calcined to obtain Na-type CoO x @Slicalite-1 molecular sieve;

[0016] Step S7: NH4Cl solution is heated at 90°C and the Na-type CoO x @Slicalite-1 molecular sieve ion exchange three times, then centrifugation washing, drying and calcination to obtain H-type CoO x @Slicalite-1 molecular sieve;

[0017] Step S8: Grind the calcined molecular sieve and then press it into tablets to obtain 40-60 mesh CoO x @Slicalite-1 molecular sieve propane dehydrogenation catalyst particles.

[0018] Furthermore, in step S2, a cobalt source with a mass of 0.0988g-0.9877g is weighed, and 2-3mL of anhydrous ethanol is added to every 0.0988g of the cobalt source for immersion and dissolution, and every 0.0988g of the cobalt source corresponds to 3-4g of the silicon source, and 40mL of ethanol is required for every 3-4g of the silicon source; the active components of the cobalt source and the silicon source in step S2 are cobalt nitrate hexahydrate and silica gel, respectively, and the template in step S4 is tetrapropylammonium hydroxide (TPAOH).

[0019] Furthermore, in step S3, the silicon source is first added into the oil bath, and then the cobalt source is added dropwise into the silicon source at a dropping rate of 2 drops / second; the hydrothermal crystallization for 24 hours in step S5 is specifically placed in an oven at 180°C for 24 hours.

[0020] Furthermore, in step S4, the molar ratio of Co3O4 / SiO2 precursor, template, NaOH and water is: 0.9-1.2: 0.3-0.4: 0.25-0.1: 7-8.

[0021] Furthermore, in step S6, the product is washed by centrifugation with deionized water for 3 times, each time at a centrifugal speed of 10000 rpm for 5 minutes, and each time the amount of deionized water used is 40-50 g; the calcination conditions of step S6 and step S7 are 550° C. for 6 hours.

[0022] Furthermore, in step S7, ion exchange is performed three times using a 1 mol / L ammonium chloride solution, each time for 2 hours, and the amount of ammonium chloride used is 40 mL; centrifugal washing is the same as step S6.

[0023] The present invention also provides an application of the core-shell Co-based all-silicon molecular sieve catalyst as described above in a propane dehydrogenation reaction.

[0024] Compared with the prior art, the advantages and positive effects of the present invention are:

[0025] 1) Core-shell CoO synthesized by the present invention x @Silicalite-1 molecular sieve catalyst and traditional supported CoO x Compared with / Silicalite-1, the catalyst of the present invention can regulate CoO x On the other hand, it can make CoO x The active species are highly dispersed and sintering is inhibited during the reaction, while the interaction with the carrier Silicliate-1 is enhanced, thereby showing better propane dehydrogenation catalytic performance and propylene selectivity. The traditional supported catalyst is cobalt oxide particles directly supported on the surface of S-1 molecular sieve, which has problems of uneven distribution and uneven particle size. The core-shell CoO x @Silicalite-1 catalyst is first formed into a certain size of CoO by impregnation method x Then, during the hydrothermal crystallization process, the carrier SiO2 dissolves to form S-1 molecular sieve in situ to convert CoO x Confined in the pores, CoO x The species distribution is more uniform; based on the core-shell structure, CoO x The core-shell structure of @Silicalite-1 molecular sieve catalyst converts CoO x The particles are confined to suppress the active species CoO x The particles agglomerated and sintered at high temperatures, and from the TEM results, CoO x It is in a highly dispersed state, and then through the loaded CoO x / Silicalite-1, the dehydrogenation performance is significantly improved, indicating that the interaction between the active species and Silicalite-1 in the core-shell structure is stronger.

[0026] 2) The present invention adopts the method of first impregnation and then hydrothermal to synthesize efficient and stable core-shell CoO x @Silicalite-1 molecular sieve propane dehydrogenation catalyst, firstly, different loading amounts of cobalt source are impregnated on SiO2 and then calcined to obtain highly dispersed CoO x Precursor, and then dissolve SiO2 as a silicon source for in-situ crystallization in hydrothermal synthesis, which is more conducive to the crystallization of CoO x During the synthesis process, the active species are confined in the framework structure of the Silicalite-1 molecular sieve, making the active sites highly dispersed, reducing the deep dehydrogenation of propylene at the active sites and inhibiting the occurrence of side reactions such as carbon deposition, and having good stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the XRD pattern of the catalyst prepared in Example 1;

[0028] Figure 2 TEM and EDS comparison diagrams of the catalysts prepared in Example 1(c), Comparative Example 1(b) and Comparative Example 2(a);

[0029] Figure 3 The propane dehydrogenation reaction performance diagram of the catalyst prepared in Example 1 and Comparative Example 1 and Comparative Example 2, and the activity data diagram of (a) conversion rate, (b) selectivity, and (c) propylene production rate of all-silicon molecular sieve-based catalysts with different Co contents;

[0030] Figure 4 It is a comparison chart of the dehydrogenation performance of the catalysts of Example 1, Comparative Example 1 and Comparative Example 2;

[0031] Figure 5 This is a graph showing the stability test results of the catalyst prepared in Example 1. DETAILED DESCRIPTION

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0033] The technical solution of the present invention is further explained below in conjunction with implementation cases.

[0034] Example 1

[0035] A method for preparing a core-shell Co-based all-silicon molecular sieve catalyst, the specific steps are as follows:

[0036] Step S1: drying the silica gel at 100° C. for 6 h to fully remove moisture;

[0037] Step S2: 0.0988g, 0.1975g, 0.3951g, 0.6914g, 0.8889g, and 0.9877g of cobalt nitrate hexahydrate were weighed respectively and added to anhydrous ethanol, 2mL of anhydrous ethanol was required for every 0.0988g of cobalt nitrate hexahydrate, and the mixture was allowed to stand for 60min to dissolve, and 4g of dried SiO2 was required to be added with 40mL of anhydrous ethanol for every 0.0988g of cobalt nitrate hexahydrate, and the mixture was first stirred and then ultrasonically dispersed, wherein the stirring speed was 350rpm, the ultrasonic intensity was 40KHZ, and the stirring and ultrasonic time were 30min respectively;

[0038] Step S3: The six groups of SiO2 turbid solutions prepared in step S2 are transferred to an eggplant-shaped bottle and placed in an oil bath, and the corresponding dissolved cobalt nitrate solution is added dropwise to the turbid solution at a dropping speed of 2 drops / second. After impregnation, the solution is dried at 100°C for 6 hours, and then calcined at 350°C for 3 hours to obtain a Co3O4 / SiO2 precursor;

[0039] Step S4: Weigh 4.92g of tetrapropylammonium hydroxide solution and 5.94g of deionized water, mix and stir at room temperature for 0.5h, stirring at a speed of 350rpm, prepare 1mol / L NaOH solution at the same time, add 3mL of the prepared NaOH solution, and continue stirring for 0.5h. Finally, add the Co3O4 / SiO2 precursor prepared in step S3, continue stirring for 12h, and keep the stirring speed at 350rpm;

[0040] Step S5: pour the stirred turbid liquid into a high-temperature hydrothermal kettle, put it into a 180°C oven, and hydrothermally crystallize it for 24 hours;

[0041] Step S6: The crystallized product was washed with deionized water by centrifugation for 3 times, each time at a centrifugal speed of 10000 rpm for 5 min, and each time with a deionized water dosage of 40 g, dried at 100 ° C for 7 h, and then calcined at 550 ° C for 6 h to remove the template agent, and calcined to obtain Na-type CoO x @Slicalite-1 molecular sieve;

[0042] Step S7: 1 mol / L NH4Cl solution was added to the Na-type CoO at 90°C. x @Slicalite-1 molecular sieve ion exchange three times, each time the amount of ammonium chloride is 40mL, each time 2h, then centrifugal washing, the centrifugal speed is 10000rpm, centrifugal 5min, each time the amount of deionized water is 40g, and then calcined at 550℃ for 6h to obtain H-type CoO x @Slicalite-1 molecular sieve.

[0043] Comparative Example 1

[0044] Weigh 0.3457g of cobalt nitrate hexahydrate, add 14mL of anhydrous ethanol, stir and stand to dissolve, the stirring speed is 350rpm, and the standing time is 60min; add 2g of Silicalite-1 molecular sieve into 20mL of anhydrous ethanol, stir and ultrasonicate for 30min each, the stirring speed is 350rpm, and the ultrasonic intensity is 40KHZ; then transfer the Silicalite-1 emulsion to an eggplant-shaped bottle and place it in an oil bath, add the dissolved cobalt nitrate solution dropwise to the emulsion, the dropping speed is 2 drops / second, after the dropwise addition is completed, evaporate and soak at 90℃ for 6h, and then calcine at 550℃ for 6h to obtain Co3O4 / Silicalite-1 catalyst.

[0045] Comparative Example 2

[0046] Weigh 0.3457g of cobalt nitrate hexahydrate, add 14mL of anhydrous ethanol, stir and stand to dissolve, the stirring speed is 350rpm, and the standing time is 60min; add 2g of SiO2 into 20mL of anhydrous ethanol, stir and ultrasonicate for 30min each, the stirring speed is 350rpm, and the ultrasonic intensity is 40KHZ; then transfer the SiO2 turbid liquid to an eggplant-shaped bottle and place it in an oil bath, add the dissolved cobalt nitrate solution dropwise into the turbid liquid, the dropping speed is 2 drops / second, after the dropping is completed, evaporate and soak at 90℃ for 6h, and then calcine at 550℃ for 6h to obtain Co3O4 / SiO2 catalyst.

[0047] Propane dehydrogenation reaction performance test:

[0048] First, 0.05g of the catalyst of 40-60 purpose prepared by embodiment 1, comparative example 1 and comparative example 2 is mixed with 0.5g of quartz sand (quartz sand and catalyst mass ratio is 10), after uniform mixing, fill into stainless steel tube, then carry out device heating and catalyst pretreatment. Raw gas concentration is 40%C3H8 / N2 mixed gas, total flow is 20mL / min, reaction temperature is 550 ℃ of devices under N2 atmosphere (30mL / min) rises to 550 ℃ with a heating rate of 2 ℃ per minute, stabilizes 10min, then carrier gas switches to air (30mL / min) oxidation 30min, switches to N2 (30mL / min) purge 5min afterwards, then switches to 5%H2 / N2 (30mL / min) mixed gas reduction 30min, switches to N2 (12mL / min) purge 5min afterwards, finally passes into C3H8 (8mL / min) and carries out propane dehydrogenation reaction performance test. The composition of the reaction products was analyzed by gas chromatography using a GC-7806C gas chromatograph (Beijing Huasheng Puxin Instrument Co., Ltd.).

[0049] Figure 1 CoO with different Co contents prepared in Example 1 x The XRD pattern of @S-1 catalyst shows that all catalysts are of MFI configuration, indicating that the molecular sieve catalyst has been successfully synthesized. However, the peak intensity corresponding to the special diffraction peak of Co3O4 is relatively small, which is due to the low content of Co or the presence of CoO x The particles are highly dispersed.

[0050] Figure 2 TEM and EDS comparison of the catalysts prepared in Example 1 (c), Comparative Example 1 (b) and Comparative Example 2 (a). It can be clearly observed that the surface CoO x The particles are of different sizes and unevenly distributed, while the CoO x @CoO in S-1 catalyst x The particles are relatively uniform, about 3-4 nm, and are highly dispersed, indicating that the preparation method of the present invention can better disperse the active sites.

[0051] The catalyst evaluation results are shown in Table 1:

[0052] Table 1 Propane dehydrogenation reaction performance test of the core-shell Co-based all-silicon molecular sieve catalyst prepared in Example 1

[0053] Co content / wt% 0.5 1 2 3.5 4.5 5 Propane conversion rate / % 8.2 15.9 21.6 24.0 21.0 10.7 Propylene selectivity / % 98.1 98.2 98.8 97.9 99.1 99.1

[0054] See also Figure 3 As can be seen from Figure (a) and Figure (b), the synthesized catalyst still has a propylene selectivity of more than 98% when the catalyst dosage is only 0.05g, of which 3.5% CoO x The propane conversion rate of @S-1 catalyst is as high as 23.98%, and the propylene production rate is 1.76mmol·g -1 ·min -1 , is the catalyst with the best performance. From the results, it can be seen that the propylene production rate and the Co content show a "volcano-shaped" curve relationship that first increases and then decreases. When the Co content is lower than 3.5%, the dehydrogenation activity of the catalyst is improved due to the increase in Co content and the increase in active sites. When the Co content is greater than 3.5%, the dehydrogenation activity of the catalyst is improved, which may be due to the increase in CoO x As the particles grow larger, the intrinsic activity of the catalyst decreases, resulting in a decrease in propane conversion.

[0055] from Figure 4 The performance comparison results show that the comparative example 1CoO x / S-1 has a propylene production rate of only 10.7%, while the CoO x / SiO2 catalyst has no catalytic activity, indicating that the 3.5% CoO x The Silicalite-1 support in the @S-1 catalyst anchors CoO x Active species form a strong interaction between the metal support and make it highly dispersed, and CoO x / S-1 and CoO x / Compared with SiO2, it has better propane dehydrogenation activity.

[0056] Figure 5 3.5% CoO prepared in Example 1 x The stability test result of @S-1 is shown in the figure. Its propane conversion rate is about 33%. The reaction stability was tested for 9 hours at a condition close to the equilibrium conversion rate (41.2%). The results show that the propylene selectivity of the catalyst is stable at more than 92%, while the propane conversion rate only decreases by 2% to 3%, and the deactivation rate is 2.4×10 -3 min -1 , it can be seen that the core-shell structure catalyst in the present invention has good stability.

[0057] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. A method for preparing a core-shell Co-based all-silicon molecular sieve catalyst, characterized in that: The steps include: Step S1: drying the silica gel at 100 °C for 6-7 h; Step S2: adding anhydrous ethanol to which the active component cobalt source has a mass fraction of 0.5 wt%-5 wt% and allowing to stand for 60 min, adding anhydrous ethanol to which the silicon source is first stirred and then ultrasonicated, the stirring speed is 350 rpm, the ultrasonic intensity is 40 KHZ; the stirring and ultrasonication time are both 30 min; Step S3: impregnating the cobalt source on the silicon source by evaporation impregnation method, drying at 100 °C for 6-7 h after impregnation, and then calcining at 350 °C for 3 h to obtain Co3O4 / SiO2 precursor; Step S4: Mix the Co3O4 / SiO2 precursor with the template, NaOH and water at room temperature and stir for 12 h to obtain a uniform turbid solution at a stirring speed of 300-400 rpm; Step S5: pour the stirred turbid liquid into a hydrothermal kettle and perform hydrothermal crystallization at 180 °C for 24 h; Step S6: The crystallized product is centrifuged, washed, dried and calcined to obtain Na-type CoO x @Silicalite-1 molecular sieve; Step S7: NH4Cl solution is reacted with the above Na-type CoO at 90 °C. x @Silicalite-1 molecular sieve was ion exchanged three times, then centrifuged, washed, dried and calcined to obtain H-type CoO x @Silicalite-1 molecular sieve; Step S8: Grind the calcined molecular sieve and then press it into tablets to obtain 40-60 mesh CoO x @Silicalite-1 molecular sieve propane dehydrogenation catalyst particles.

2. The method for preparing a core-shell Co-based all-silicon molecular sieve catalyst according to claim 1, characterized in that: In step S2, a cobalt source with a mass of 0.0988 g-0.9877 g is weighed, and 2-3 mL of anhydrous ethanol is added to each 0.0988 g of the cobalt source for static dissolution. Each 0.0988 g of the cobalt source corresponds to 3-4 g of the silicon source, and 40 mL of ethanol is required for each 3-4 g of the silicon source. The active components of the cobalt source and the silicon source in step S2 are cobalt nitrate hexahydrate and silica gel, respectively, and the template in step S4 is tetrapropylammonium hydroxide.

3. The method for preparing a core-shell Co-based all-silicon molecular sieve catalyst according to claim 1, characterized in that: In step S3, the silicon source is first placed in an oil bath, and then the cobalt source is added dropwise to the silicon source at a dropping rate of 2 drops / second; the hydrothermal crystallization for 24 hours in step S5 is specifically placed in an oven at 180°C for 24 hours.

4. The method for preparing a core-shell Co-based all-silicon molecular sieve catalyst according to claim 1, characterized in that: In step S4, the molar ratio of Co3O4 / SiO2 precursor, template, NaOH and water is: 0.9~1.2: 0.3~0.4: 0.25~0.1: 7~8.

5. The method for preparing a core-shell Co-based all-silicon molecular sieve catalyst according to claim 1, characterized in that: In step S6, the mixture is washed by centrifugation for 3 times with deionized water, each time with a centrifugal speed of 10,000 rpm for 5 min, and the amount of deionized water used each time is 40-50 g; the calcination conditions of step S6 and step S7 are 550° C. for 6 h.

6. The method for preparing a core-shell Co-based all-silicon molecular sieve catalyst according to claim 1, characterized in that: In step S7, ion exchange is performed three times using a 1 mol / L ammonium chloride solution, each time for 2 hours, and the amount of ammonium chloride used each time is 40 mL; centrifugal washing is the same as step S6.

7. Use of the core-shell Co-based all-silicon molecular sieve catalyst prepared by the preparation method according to any one of claims 1 to 6 in propane dehydrogenation reaction.

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