Preparation method of a molecular sieve-confined Pt-based bimetallic catalyst and its application in propane dehydrogenation to propylene

Abstract

A method for preparing a molecular sieve-confined Pt-based bimetallic catalyst and its application in propane dehydrogenation to propylene, using silicon source and precious metal platinum as raw materials, adopting an in-situ encapsulation method, mixing and aging the silicon source, organic base, Pt precursor, auxiliary element precursor, etc., and crystallizing at low temperature under the action of a mineralizer, and then washing and drying to obtain a Pt-based molecular sieve confined catalyst. During the synthesis process, a mineralizer is used to treat the aging process, and a large number of mesoporous structures are formed on the basis of repairing the defects of the all-silicon molecular sieve, thereby improving the carbon deposition of gas reactants on the catalyst and the mass transfer resistance in the pores. In addition, Pt-based bimetallic nanoparticles are effectively anchored at specific sites in the molecular sieve pores, which is beneficial to the dispersion and confinement of active sites, and remains stable for a long time at high temperatures, significantly improving the performance of the catalyst for direct dehydrogenation of propane to propylene. The catalyst is particularly suitable for direct dehydrogenation of industrial propane to produce propylene and by-product hydrogen, and has good economic and environmental benefits.

Description

Technical Field

[0001] The present invention relates to the field of catalysis, and in particular to a method for preparing a molecular sieve-confined Pt-based bimetallic catalyst and an application of the catalyst in catalyzing direct dehydrogenation of propane to produce propylene. Background Art

[0002] Propylene is an important raw material for the production of various high-value chemicals or intermediates such as polypropylene, propylene oxide, acrylonitrile, acrolein, acetone, etc. Propane dehydrogenation is an industrial technology for the direct production of propylene that has received widespread attention in recent years. The direct dehydrogenation of propane to propylene under anaerobic conditions has the advantages of simple process, high economic benefits, and by-product hydrogen energy, and is therefore of great application value.

[0003] Pt-based catalysts are widely used in propane dehydrogenation conversion processes due to their excellent CH bond activation ability. However, the propane dehydrogenation process is a highly endothermic reaction. Due to thermodynamic limitations, it is difficult to increase the single-pass conversion rate, resulting in an increase in side reactions and low propylene selectivity. During the reaction, carbon deposits will cover the active sites of Pt, causing the catalyst to deactivate rapidly, which requires frequent catalyst regeneration. However, the regeneration process not only generates additional energy consumption and costs, but the regenerated catalyst often cannot reach its initial activity. This is because the Pt active sites cannot be firmly bonded to the carrier, and will migrate and undergo irreversible sintering at high temperatures. Therefore, enhancing the interaction between Pt and the carrier is crucial to improving the activity of the propane dehydrogenation reaction and the stability of the catalyst.

[0004] In order to achieve high selectivity for propylene in the propane dehydrogenation reaction, a second component is introduced to modify the surface of metal Pt to inhibit the undesirable CC bond dissociation. Sn has been shown to be an effective promoting element for Pt-catalyzed direct dehydrogenation of propane. However, most of the supported Pt-Sn catalysts form large particles on the outer surface of the catalyst after impregnation and calcination, which makes the utilization rate of precious metal Pt very low and increases the cost of catalyst raw materials. In addition to the commonly used Al 2 O 3 、SiO 2 In addition to other carriers, some literature currently tends to use the characteristics of molecular sieve channels to stabilize Pt-Sn. However, the impregnation method is an uneven loading method. After actual impregnation and calcination, Pt-Sn cannot be completely evenly distributed in the channels and is more likely to sinter and deactivate at high temperatures. Therefore, for efficient propane dehydrogenation catalysts used in industrial production, it is necessary to consider finding a suitable method to limit the particle size and position of Pt-Sn to improve the utilization rate of precious metal Pt.

[0005] At present, the domestic industry mainly uses the Oleflex process of UOP in the United States to produce propylene by dehydrogenation of propane using Pt-based catalysts. There is still a lot of work to be done in developing independent production processes for dehydrogenation of propane to produce propylene. So far, the Oleflex process has not solved the defects of easy coking during the reaction process and the inability to balance conversion rate and stability. In summary, under this background, the development of a molecular sieve confined catalyst with low Pt dosage and high dispersion is of great value and significance for long-term and efficient catalytic dehydrogenation of propane to produce propylene. Summary of the invention

[0006] The purpose of the present invention is to solve the above-mentioned problems in the prior art and provide a method for preparing a molecular sieve confined catalyst with low Pt-Sn dosage and high dispersion. The catalyst prepared by the present invention has high conversion rate, selectivity and stability in the oxygen-free dehydrogenation of propane to propylene.

[0007] In order to achieve the above object, the present invention adopts the following technical scheme:

[0008] The method for preparing the molecular sieve confined Pt-Sn bimetallic catalyst comprises the following steps:

[0009] A) mixing a silicon source, an organic base, a Pt precursor, a Sn precursor, and deionized water in a container;

[0010] B) sealing and stirring the mixed solution at a certain temperature;

[0011] C) adding an aqueous solution of ammonium fluoride to the above homogeneous liquid, and continuously heating and stirring in the open air until gelation;

[0012] D) transferring the gel to a stainless steel autoclave and placing it in an oven at a certain temperature for crystallization;

[0013] E) The product is filtered and washed with a washing solution, dried, ground and collected to finally obtain a molecular sieve-confined Pt-Sn bimetallic catalyst.

[0014] In step A), the silicon source is tetraethyl silicate, the organic base is tetrapropylammonium hydroxide, the Pt precursor is chloroplatinic acid, and the Sn precursor is tin chloride; the molar ratio of the silicon source, the organic base, Pt, Sn, and deionized water can be 1:0.1:(0.001-0.01):(0-0.03):12.

[0015] In step B), the temperature may be 90° C. to 110° C., and the stirring time may be 12 to 13 hours.

[0016] In step C), the molar ratio of the added ammonium fluoride, deionized water and the silicon source in step A) is (0-1.6):18:1; the heating temperature can be 90° C.-110° C., and the stirring time can be 12-14 hours.

[0017] In step D), the oven temperature is set at 90° C. to 110° C., and the holding time can be set at 3 to 4 days.

[0018] In step E), the washing solution is a water / acetone mixed solution, the mixing mass ratio can be less than 4:1, and the number of washing times is 4 to 5 times; the drying temperature can be 50 to 100° C., and the drying time is 10 h to 12 h.

[0019] The present invention provides a catalyst prepared by the method for preparing a molecular sieve-confined Pt-Sn bimetallic catalyst; the catalyst comprises Pt-Sn nanoparticles and a molecular sieve carrier, and the Pt-Sn nanoparticles are confined and grown in the catalyst carrier.

[0020] The catalyst is used in the reaction of producing propylene by dehydrogenation of propane without oxygen.

[0021] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0022] 1. The present invention adopts an in-situ encapsulation method to uniformly confine Pt-Sn nanoparticles in the pores of the molecular sieve, so that the Pt-Sn nanoparticles have ultra-high dispersion;

[0023] 2. The Pt-Sn particles prepared by this method are of sub-nanoscale, which enables the precious metal Pt to have high utilization rate at low dosage;

[0024] 3. The ammonium fluoride additive used in the synthesis of this method can form a large number of mesoporous structures to improve the mass transfer resistance on the catalyst;

[0025] 4. The fluoride ions in the pores of the molecular sieve prepared by this method effectively anchor the Pt-Sn nanoparticles, and the strong interaction between the two enables the catalyst to remain stable for a long time at high temperature.

[0026] 5. The synthesis process adopted in this method is relatively stable, and the prepared molecular sieve confined Pt-Sn bimetallic catalyst has excellent catalytic performance in the propane dehydrogenation reaction to produce propylene. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a high-angle annular dark field scanning transmission electron microscopy image (a) and corresponding element distribution diagrams (b-f) of the molecular sieve confined Pt-Sn bimetallic catalyst in Example 1 of the present invention.

[0028] Figure 2 It is a high-resolution transmission electron micrograph of the molecular sieve-confined Pt-Sn bimetallic catalyst (a) in Example 1 of the present invention and the molecular sieve-confined Pt-Sn bimetallic catalyst (b) in Comparative Example 1 without fluorine addition. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] Example 1

[0031] This embodiment provides a method for preparing a molecular sieve confined Pt-Sn bimetallic catalyst, comprising the following steps:

[0032] A) 16 g of tetraethyl silicate, 1.56 g of tetrapropylammonium hydroxide, 0.031 g of chloroplatinic acid, 0.06 g of tin chloride, and 16.6 g of deionized water were weighed and mixed in a container;

[0033] B) sealing and stirring the mixed solution at 90° C. for 12 h;

[0034] C) adding a mixed solution of 4.55 g of ammonium fluoride and 24.9 g of deionized water to the above homogeneous liquid, and stirring at 90° C. for 12 h to gel;

[0035] Tetraethyl silicate, molecular formula is Si(OC 2 H 5 ) 4 , molecular weight is 208.33, colorless transparent liquid, insoluble in water.

[0036] Tetrapropylammonium hydroxide, molecular formula is (CH 3 CH 2 CH 2 ) 4 N(OH), with a molecular weight of 203.37, is generally present in the form of an aqueous solution.

[0037] Chloroplatinic acid, molecular formula is H 2 PtCl 6 , molecular weight is 409.818, appearance is orange-yellow powder, soluble in water, ethanol and acetone.

[0038] Tin chloride, molecular formula is SnCl 4 , molecular weight is 260.52, generally contains crystal water, and is a colorless to slightly yellow crystalline powder.

[0039] Ammonium fluoride, molecular formula is NH 4 F, molecular weight is 37.04, white crystalline powder, easily deliquescent, soluble in water and methanol, slightly soluble in ethanol, insoluble in acetone.

[0040] D) transferring the gel to a stainless steel autoclave and placing it in a 90° C. oven for 3 days to crystallize;

[0041] E) The product was filtered and washed 4 to 5 times with a water / acetone (4 / 1) mixed solution, dried at 50° C. for 12 h, and then ground and collected to finally obtain a molecular sieve-confined Pt-Sn bimetallic catalyst, which was recorded as “Example 1 Sample”.

[0042] like Figure 1 As shown in a to f, there is no obvious aggregation of large-sized or aggregated Pt-Sn particles in the synthesized catalyst. The element distribution diagram shows the low loading of Pt-Sn (ICP test results show that the Pt content in the catalyst is only 0.1wt%) and high dispersibility. Figure 2 As shown in a, the catalyst is in the form of an oblong sheet with a thickness of about 30 nm.

[0043] In this embodiment, the propane dehydrogenation performance of the molecular sieve-confined Pt-Sn bimetallic catalyst was investigated in a fixed bed reactor. The reaction conditions were: the mass ratio of quartz sand to catalyst was 3:1, the reaction gas was pure propane, the pressure was atmospheric pressure, and the reaction temperature was 600°C.

[0044] The specific operation steps of propane dehydrogenation to propylene are as follows: a hydrogen / argon mixed gas (hydrogen accounts for 10 vol%) is introduced into a fixed bed reactor equipped with a quartz sand diluted catalyst; the reactor is heated to 600°C at 15°C / min; after the reduction is completed, at a given reaction temperature, the bed is purged with inert high-purity argon gas for at least 30 minutes; finally, pure propane gas is switched, and after 15 minutes of introduction, continuous data collection and quantitative analysis of the components are started using gas chromatography.

[0045] In this example, when the propane dehydrogenation to propylene was carried out using the "Sample of Example 1", the propylene generation rate was 51.6 mol C3H6 ·g Pt -1 ·h -1 , the inactivation constant is 0.002h -1 , the specific results are shown in Table 1.

[0046] Example 2

[0047] On the basis of Example 1, the amount of chloroplatinic acid was increased to 0.31 g, and the amount of tin chloride was increased to 0.6 g. The obtained sample was recorded as "Example 2 Sample".

[0048] In this example, the reaction conditions and operation steps of propane dehydrogenation to propylene are the same as those in Example 1; the results show that when the propane dehydrogenation to propylene is carried out using the "Example 2 sample", the propylene generation rate is 53.4 mol C3H6 ·g Pt -1 ·h -1 , the inactivation constant is 0.0018h-1 , the specific results are shown in Table 1.

[0049] Comparative Example 1

[0050] On the basis of Example 1, the amount of ammonium fluoride was reduced to 0, and the obtained sample was recorded as "Comparative 1 Sample".

[0051] like Figure 2 As shown in b, the molecular sieve confined Pt-Sn bimetallic catalyst synthesized without fluorine addition is spherical.

[0052] In this comparative example, the propane dehydrogenation performance of the Pt-Sn bimetallic catalyst confined by fluorine-free molecular sieve was investigated. The reaction conditions and operation steps were the same as those in Example 1. The results showed that when the "Comparative 1 sample" was used for propane dehydrogenation to propylene, the propylene generation rate was 13.1 mol C3H6 ·g Pt -1 ·h -1 , the inactivation constant is 0.51h -1 , the specific results are shown in Table 1.

[0053] Comparative Example 2

[0054] On the basis of Example 1, the amount of tin chloride was reduced to 0, and the obtained sample was recorded as "Comparative 2 Sample".

[0055] In this comparative example, the propane dehydrogenation performance of the molecular sieve confined Pt catalyst without Sn assistance was investigated, and the reaction conditions and operation steps were the same as those in Example 1; the results showed that when the "Comparative 2 sample" was used for propane dehydrogenation to propylene, the propylene generation rate was 4.4 mol C3H6 ·g Pt -1 ·h -1 , the inactivation constant is 0.7h -1 , the specific results are shown in Table 1.

[0056] Table 1 Performance comparison of propane dehydrogenation to propylene using different catalysts

[0057] catalyst <![CDATA[Propylene production rate (mol C3H6 ·g Pt -1 ·h -1 )]]> <![CDATA[Inactivation constant (h -1 )]]> Example 1 Sample 51.6 0.002 Example 2 Sample 53.4 0.0018 Comparison 1 sample 13.1 0.51 Compare 2 samples 4.4 0.7

[0058] It can be seen from Table 1 that, whether the Pt content is only 0.1wt% or the Pt content is increased to 1.0wt%, even with pure propane as the reactant, the molecular sieve-confined Pt-Sn bimetallic catalyst still has a strong catalytic activity; especially in terms of catalyst stability, the ultra-low deactivation constant indicates that the catalyst can maintain long-term operation. In the absence of ammonium fluoride or Sn addition, the catalyst obviously shows lower catalytic activity and poorer stability. Therefore, the molecular sieve-confined Pt-Sn bimetallic catalyst prepared by the present invention exhibits excellent performance in the oxygen-free dehydrogenation of propane to propylene, and has broad application prospects.

[0059] In the synthesis process of the present invention, an appropriate amount of mineralizer is used to treat the aging process, and a large number of mesoporous structures are formed on the basis of repairing the defects of the all-silicon molecular sieve, thereby improving the carbon deposition of gas reactants on the catalyst and the mass transfer resistance in the pores. In addition, the specific sites of the molecular sieve pores can effectively anchor Pt-based bimetallic nanoparticles, which is conducive to the dispersion and confinement of active sites, and remains stable for a long time at high temperatures, which can significantly improve the performance of the catalyst for direct dehydrogenation of propane to propylene. The Pt-based molecular sieve confined catalyst is particularly suitable for direct dehydrogenation of industrial propane to produce propylene and by-product hydrogen, and has good economic and environmental benefits.

Claims

1. A method for preparing a molecular sieve-confined Pt-based bimetallic catalyst, characterized in that: The following steps are involved: 1) Mix silicon source, organic base, Pt precursor, Sn precursor and deionized water in a container; 2) sealing and stirring the mixed solution in step 1) at a certain temperature to obtain a homogeneous liquid; 3) adding an aqueous solution of ammonium fluoride to the homogeneous liquid in step 2), and continuing heating and stirring in the open air until gelation; 4) transferring the gel from step 3) to a stainless steel autoclave and placing it in an oven at a certain temperature for crystallization; 5) The product of step 4) is filtered and washed with a washing solution, dried, ground and collected to obtain a molecular sieve-confined Pt-based bimetallic catalyst; In step 1), the molar ratio of silicon source, organic base, Pt precursor, Sn precursor and deionized water is 1:0.1:(0.001~0.01):(0~0.03):12; In step 2), the certain temperature is 90°C to 110°C, and the stirring time is 12 to 13 hours; In step 3), the heating temperature is 90°C to 110°C, and the stirring time is 12 to 14 hours; In step 4), the temperature of the oven is 90°C and the holding time is 3 to 4 days; The silicon source is tetraethyl silicate, the organic base is tetrapropylammonium hydroxide, the Pt precursor is chloroplatinic acid, and the Sn precursor is tin chloride.

2. The method for preparing a molecular sieve-confined Pt-based bimetallic catalyst according to claim 1, characterized in that: In step 3), the molar ratio of the added ammonium fluoride, water and the silicon source in step 1) is (0-1.6):18:

1.

3. The method for preparing a molecular sieve-confined Pt-based bimetallic catalyst according to claim 1, characterized in that: In step 5), the washing solution is a mixed solution of water and acetone, the mixing mass ratio of water to acetone is 4:1, and the number of washing times is 4 to 5 times.

4. The catalyst prepared by any one of the preparation methods of claims 1 to 3, characterized in that: The catalyst comprises Pt-Sn nanoparticles and a molecular sieve carrier, and the Pt-Sn nanoparticles are confinedly grown in the catalyst carrier.

5. The use of a catalyst prepared by any one of the preparation methods of claims 1 to 3, characterized in that: The catalyst is used in the reaction of producing propylene by dehydrogenation of propane without oxygen.

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