A method for improving the dispersion of nano-gold particles of titanium silicon molecular sieve immobilized gold catalyst and its application

Abstract

The present invention belongs to the technical field of catalysts, and provides a method for improving the dispersion of nano-gold particles of titanium silicon molecular sieve solid-supported gold catalysts. The method uses freeze-dried uncalcined titanium silicon molecular sieve as a carrier, mixes a gold precursor solution, a precipitant and the freeze-dried uncalcined titanium silicon molecular sieve, and then dries and activates the mixture to obtain a titanium silicon molecular sieve solid-supported gold catalyst with highly dispersed nano-gold particles. Compared with the prior art, the freeze-drying technology can significantly reduce the agglomeration degree of titanium silicon molecular sieve particles, and the nano-gold particles on the prepared titanium silicon molecular sieve solid-supported gold catalyst also have a higher dispersion. The gold catalyst prepared above is used in the reaction of propylene hydrogenation to prepare propylene oxide, and the catalyst exhibits a higher catalytic activity.

Description

Technical Field

[0001] The invention belongs to the technical field of catalysts, and in particular relates to a method for improving the dispersion of nano-gold particles of a titanium silicon molecular sieve immobilized gold catalyst and its application. Background Art

[0002] Propylene oxide (PO) is the second largest propylene derivative after polypropylene and is mainly used to produce chemical products such as polyether polyols and propylene glycol. Compared with the current industrial production of PO by chlorohydrin method, co-oxidation method and direct oxidation method with hydrogen peroxide, propylene in H 2 / O 2 The direct epoxidation synthesis of PO in atmosphere, namely the HOPO process, has unique advantages such as cheap and readily available raw materials, simple process, and green and environmentally friendly process. It has long been regarded as an ideal process for producing PO.

[0003] At present, the catalyst used to catalyze the hydrogenation of propylene to produce PO is a nano-gold catalyst immobilized on a titanium silicate molecular sieve. This titanium silicate molecular sieve supported gold catalyst is an Au-Ti bifunctional catalyst: the Au site catalyzes H 2 With O 2 Hydroperoxide species are generated, which then diffuse to the Ti site to form Ti-OOH active intermediates, and propylene reacts with the active intermediates to form PO.

[0004] A large number of studies have found that the particle size of gold nanoparticles is a key factor affecting the performance of Au-Ti bifunctional catalysts. Generally, gold nanoparticles with smaller particle sizes expose more corner sites, which helps promote the adsorption of oxygen, thereby increasing the rate of generation of hydroperoxide species on the gold nanoparticles, and ultimately showing higher catalytic activity. The properties of titanium silicate molecular sieves have an important influence on the particle size of gold nanoparticles. Generally speaking, increasing the dispersion of titanium silicate molecular sieve particles is beneficial to promoting the uniform loading of gold precursors on the surface of the molecular sieve during the catalyst preparation process, thereby obtaining a higher dispersion of gold nanoparticles.

[0005] According to literature reports, the drying method of nanomolecular sieves has a significant effect on the degree of agglomeration of nanomolecular sieve particles. Generally speaking, due to the very high surface tension of water, the water adsorbed on the nanomolecular sieve particles is easy to evaporate when the nanomolecular sieve sample is dried at high temperature, causing the nanomolecular sieve particles to aggregate into a large number of larger agglomerates. Therefore, if the surface tension of water molecules adsorbed on the surface of nanomolecular sieve particles can be effectively reduced, it will help to improve the dispersion of titanium silicon molecular sieve particles, thereby promoting the uniform loading of gold precursors on the surface of titanium silicon molecular sieves, thereby improving the dispersion of nano-gold particles, and ultimately improving the performance of titanium silicon molecular sieve immobilized gold catalysts in propylene hydroxyl epoxidation reactions. At present, the effect of the drying method of titanium silicon molecular sieves on the performance of supported gold catalysts in propylene hydroxyl epoxidation reactions has not been reported. Summary of the invention

[0006] In view of this, the present invention provides a method for improving the dispersion of nano-gold particles of titanium silicalite immobilized gold catalyst, by using freeze-dried uncalcined titanium silicalite immobilized gold to improve the dispersion of nano-gold particles, and the prepared titanium silicalite immobilized gold catalyst is used in the reaction of propylene hydrogenation to produce PO, which shows higher catalytic activity.

[0007] The technical solution of the present invention is to provide a method for improving the dispersion of nano-gold particles of titanium silicate molecular sieve immobilized gold catalyst, which uses freeze-dried uncalcined titanium silicate molecular sieve as a carrier, mixes a gold precursor solution, a precipitant and the freeze-dried uncalcined titanium silicate molecular sieve, and then performs drying and activation treatment to obtain the titanium silicate molecular sieve immobilized gold catalyst.

[0008] The present invention is further configured such that the preparation process of the titanium silicon molecular sieve immobilized gold catalyst is to add the freeze-dried uncalcined titanium silicon molecular sieve to a mixed solution containing water, a gold precursor solution, and a precipitant, and after uniform mixing, the obtained catalyst precursor is placed at room temperature and vacuum dried, and finally the dried catalyst precursor is activated to obtain an Au-Ti bifunctional catalyst.

[0009] It should be noted that the uncalcined titanium silicon molecular sieve subjected to freeze-drying treatment in the present invention refers to an uncalcined titanium silicon molecular sieve prepared by a conventional hydrothermal method and obtained by solid-liquid separation after the hydrothermal crystallization process is completed.

[0010] The present invention is further configured that the uncalcined titanium silicon molecular sieve includes one or more of Ti-SBA-15, Ti-MCM-41, Ti-MCM-48, Ti-MCM-36, Ti-MWW, Ti-MOR, Ti-Beta, Ti-TUD-1, TS-1, multi-level pore TS-1, TS-2 and multi-level pore TS-2.

[0011] The present invention is further configured that the freeze-drying temperature of the uncalcined titanium silicon molecular sieve is -80-0°C, preferably -50-0°C.

[0012] The present invention is further configured such that the freeze-drying time of the uncalcined titanium silicon molecular sieve is 24-96 hours, preferably 36-48 hours.

[0013] The present invention is further configured that the uncalcined titanium silicon molecular sieve needs to be pre-frozen at -10-0°C for 2-8 hours before freeze-drying.

[0014] The present invention is further configured such that the gold precursor in the gold precursor solution is selected from one or more of chloroauric acid, sodium chloroaurate, potassium chloroaurate, cesium chloroaurate, lithium chloroaurate or ammonium chloroaurate.

[0015] The present invention is further configured that the precipitant is selected from MOH, M 2 CO 3 , urea and ammonia water, wherein M is Li, Na, K, Rb or Cs.

[0016] The present invention is further configured such that the activation treatment is carried out in the presence of an activation atmosphere, which is air, a reducing atmosphere or an inert atmosphere; the activation temperature is 150-400°C, preferably 200-350°C, and more preferably 300-320°C.

[0017] The second aspect of the present invention provides a titanium silicon molecular sieve immobilized gold catalyst prepared by the method described above.

[0018] The present invention is further configured such that the particle size of the nano-gold particles on the titanium silicon molecular sieve immobilized gold catalyst is less than 5 nm, preferably less than 2.5 nm.

[0019] The third aspect of the present invention provides an application of the titanium silicalite immobilized gold catalyst in the propylene hydroxoepoxidation reaction to produce PO.

[0020] The present invention has the following beneficial effects: compared with the traditional drying method, the agglomeration degree of the nanoparticles of the titanium silicalite is significantly reduced by low-temperature freeze-drying the uncalcined titanium silicalite, the particle size of the nano-gold particles on the gold catalyst immobilized on the freeze-dried uncalcined titanium silicalite as a carrier is significantly smaller, and the prepared gold catalyst exhibits higher catalytic activity in the reaction of hydrogenation of propylene to produce PO. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the SEM image of the uncalcined titanium silicon molecular sieve sample prepared in Example 1.

[0022] Figure 2This is the SEM image of the uncalcined titanium silicon molecular sieve sample prepared in Comparative Example 1.

[0023] Figure 3 HAADF-STEM image and particle size distribution diagram of the catalyst prepared in Example 1.

[0024] Figure 4 HAADF-STEM image and particle size distribution diagram of the catalyst prepared in Comparative Example 1.

[0025] Figure 5 The catalytic activity results of the catalysts prepared in Example 1 and Comparative Example 1 in the propylene hydrooxyepoxidation reaction are compared. DETAILED DESCRIPTION

[0026] The technical scheme of the present invention is described clearly and completely below in conjunction with specific embodiments. It should be understood that the described embodiments are only part of the embodiments of the present invention, rather than 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 the present invention. The experimental methods in the following embodiments that do not specify specific conditions are usually carried out under conventional conditions or under conditions recommended by the manufacturer. Unless otherwise specified, room temperature refers to a temperature of 20-30°C.

[0027] Example 1

[0028] The Au-Ti bifunctional catalyst was prepared using urea as a precipitant, chloroauric acid as a gold precursor, and freeze-dried uncalcined TS-1 (i.e., TS-1-B) as a carrier, and the steps are as follows:

[0029] (1) The uncalcined titanium silicalite TS-1-B sample was placed in a freeze dryer, pre-frozen at -4°C for 2 h, and then freeze-dried at -50°C under vacuum (pressure 2 Pa) for 48 h.

[0030] (2) Take 1g of freeze-dried TS-1-B molecular sieve and place it in a 100ml beaker, add 39ml of water, 1ml of chloroauric acid aqueous solution, and 0.09g of urea, stir in a 90°C water bath in the dark for 6h, then transfer the suspension to a centrifuge tube and centrifuge it twice at 6000 rpm, and wash the catalyst sample with ultrapure water after each centrifugation. Place the obtained catalyst in a vacuum desiccator at room temperature and dry it in the dark overnight to obtain a catalyst precursor.

[0031] (3) The dried catalyst precursor was placed in a fixed bed reactor, and the temperature was raised from room temperature to 300° C. at a rate of 1° C. / min and maintained for 100 min under the conditions of hydrogen / nitrogen = 2:3 (volume ratio) and a gas flow rate of 50 mL / min to obtain a supported Au-Ti bifunctional catalyst with a gold loading of 0.09 wt%.

[0032] Comparative Example 1

[0033] The Au-Ti bifunctional catalyst was prepared using urea as a precipitant, chloroauric acid as a gold precursor, and uncalcined TS-1 (i.e., TS-1-B) dried in a high-temperature oven as a carrier. The steps are as follows:

[0034] (1) The uncalcined titanium silicalite TS-1-B sample was placed in an oven and then dried in static air at 60°C for 12 h.

[0035] (2) Take 1g of TS-1-B molecular sieve dried in a high temperature oven and place it in a 100ml beaker, add 39ml of water, 1ml of chloroauric acid aqueous solution, and 0.09g of urea, stir in a 90°C water bath in the dark for 6h, then transfer the suspension to a centrifuge tube and centrifuge it twice at 6000 rpm, and wash the catalyst sample with ultrapure water after each centrifugation. The obtained catalyst is placed in a room temperature vacuum desiccator and dried in the dark overnight to obtain a catalyst precursor.

[0036] (3) The dried catalyst precursor was placed in a fixed bed reactor, and the temperature was raised from room temperature to 300° C. at a rate of 1° C. / min and maintained for 100 min under the conditions of hydrogen / nitrogen = 2:3 (volume ratio) and a gas flow rate of 50 mL / min to obtain a supported Au-Ti bifunctional catalyst with a gold loading of 0.09 wt%.

[0037] The TS-1-B titanium silicon molecular sieve samples prepared in step (1) of the above-mentioned Example 1 and Comparative Example 1 were characterized by SEM, and the corresponding characterization results were as follows: Figure 1 and Figure 2 As shown, the agglomerates in Comparative Example 1 are significantly larger in size. Compared with Comparative Example 1, the agglomerates of the titanium silicate molecular sieve prepared by freeze-drying in Example 1 are significantly smaller than those of the titanium silicate molecular sieve dried at high temperature in a traditional oven, which means that the dispersion of the freeze-dried TS-1-B molecular sieve particles is significantly improved.

[0038] The gold catalysts prepared in Example 1 and Comparative Example 1 were characterized by HAADF-STEM, and the corresponding characterization results were as follows: Figure 3 and Figure 4 As shown, compared with Comparative Example 1, the catalyst prepared by using freeze-dried TS-1-B as a carrier to immobilize gold has a smaller nano-gold particle size.

[0039] Example 2

[0040] The catalytic performance of the gold catalyst prepared in Example 1 and Comparative Example 1 was evaluated in the reaction of preparing propylene oxide by propylene hydro-epoxidation. The propylene gas phase epoxidation reaction was carried out in a fixed bed reactor at atmospheric pressure. The reaction atmosphere composition was propylene: hydrogen: oxygen: nitrogen = 1:1:1:7 (volume ratio), and the space velocity was 14000 mL·h -1 ·g cat -1 The export product was analyzed by gas chromatography. The catalytic reaction results are shown in Table 1 and Figure 5 shown.

[0041] Table 1 Catalyst performance evaluation

[0042]

[0043] Combining Table 1 and Figure 5 It can be seen from the results that the freeze-dried uncalcined titanium silicalite TS-1-B supported gold catalyst prepared in Example 1 has a higher hydrogen utilization rate and a higher PO generation rate for the reaction of propylene hydrooxidation to prepare propylene oxide, compared with the uncalcined titanium silicalite TS-1-B supported gold catalyst prepared by the drying method in Comparative Example 1, that is, it has a higher catalytic activity.

Claims

1. A method for improving the dispersion of nano-gold particles on a titanium silicon molecular sieve immobilized gold catalyst, characterized in that: Using freeze-dried uncalcined titanium silicon molecular sieve as a carrier, a gold precursor solution, a precipitant and freeze-dried uncalcined titanium silicon molecular sieve are mixed and then dried and activated to obtain a titanium silicon molecular sieve immobilized gold catalyst; the freeze-drying temperature of the uncalcined titanium silicon molecular sieve is -80--50 o C, the freeze drying time of the uncalcined titanium silicon molecular sieve is 24-96h, and the uncalcined titanium silicon molecular sieve is heated to -10-0 o Prefreeze at 37°C for 2-8 hours; The uncalcined titanium silicon molecules are selected from one or more of Ti-SBA-15, Ti-MCM-41, Ti-MCM-48, Ti-MCM-36, Ti-MWW, Ti-MOR, Ti-Beta, Ti-TUD-1, TS-1, and TS-2; the gold precursor in the gold precursor solution is selected from one or more of chloroauric acid, sodium chloroaurate, potassium chloroaurate, cesium chloroaurate, lithium chloroaurate, or ammonium chloroaurate.

2. The method according to claim 1, characterized in that: The freeze-drying time of the uncalcined titanium silicon molecular sieve is 36-48 hours.

3. The method according to claim 1, characterized in that The precipitant is selected from one or more of MOH, M2CO3, urea and ammonia water, wherein M is Li, Na, K, Rb or Cs.

4. The method according to claim 1, characterized in that The activation treatment is carried out in the presence of an activation atmosphere, which is air, a reducing atmosphere or an inert atmosphere; the activation temperature is 150-400 o C.

5. The method according to claim 4, characterized in that The activation temperature is 200-350 o C.

6. The method according to claim 5, characterized in that The activation temperature is 300-320 o C.

7. An application of the method according to any one of claims 1 to 6, characterized in that: A titanium silicon molecular sieve immobilized gold catalyst is prepared according to the method according to any one of claims 1 to 6, and the titanium silicon molecular sieve immobilized gold catalyst is used in the reaction of propylene hydro-epoxidation to produce propylene oxide.

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