CN110420633B

CN110420633B - Carbon-supported H2O-WOxNanoparticle composite structures and methods of making the same

Info

Publication number: CN110420633B
Application number: CN201910587415.6A
Authority: CN
Inventors: 张超; 张梦锐; 张红端; 王冠
Original assignee: Henan University
Current assignee: Henan University
Priority date: 2019-06-27
Filing date: 2019-06-27
Publication date: 2021-08-24
Anticipated expiration: 2039-06-27
Also published as: CN110420633A

Abstract

The invention provides a WO capable of providing additional empty coordination sites_xBased heterogeneous catalyst, such WO being able to provide additional vacant coordination sites_xBased on a heterogeneous catalyst of H₂O‑WO_x@ C. In the catalytic epoxidation of olefins, complex H₂O will be from WO_xDropping off and leaving an unoccupied coordination site to increase catalytic activity, and after the catalytic reaction is completed, the vacant coordination active site will be restored to H₂O‑WO_x@ C, whereby the stability of the catalyst is improved.

Description

Carbon-supported H2O-WOxNanoparticle composite structures and their preparationMethod

Technical Field

The invention relates to a carbon-supported H₂O-WO_xA nano-particle composite structure and a preparation method thereof belong to the technical field of material preparation.

Background

At present, the preparation of tungsten oxide nano-materials has made great progress, and various preparation methods are developed, such as a vapor deposition method, a laser pyrolysis method, a magnetron sputtering method, a sol-gel method, a hydrothermal/solvothermal method, a template method, an ultrasonic chemical method and the like. In the morphology of the prepared tungsten oxide nano material sample, nano structures such as zero-dimensional quantum dots, one-dimensional nano rods, nano wires, two-dimensional nano sheets, three-dimensional nano flowers, hollow microspheres and the like are reported. However, the synthesis conditions of these tungsten oxide nano materials are generally harsh, which is not favorable for mass production of nano-scale tungsten oxide materials.

Meanwhile, the epoxy compound has important application value in the fields of medicine, materials, chemical engineering and the like. The current method of synthesizing epoxides is to oxidize olefins to the corresponding epoxides using a catalyst and in the presence of an oxidant. In consideration of environmental protection and other factors, people tend to use hydrogen peroxide as an oxidant more and more, and the final product is water after oxidation. While catalyst selection has generally focused on early transition metals such as molybdenum, tungsten, titanium, niobium, and the like. Tungsten ore resources are abundant in China, and the tungsten-based catalyst has a superior effect on olefin epoxidation, so that the tungsten-based catalyst is widely researched. A reasonable tungsten-based catalyst needs to expose at least one empty coordination site that can be manipulated, as needed to take full advantage of the electron affinity of tungsten. All the tungsten-based catalysts with empty coordination sites known at present have strong hydrolysis tendency. Hydrolysis occurs when water or moist air is encountered, and the tungsten-based catalyst originally having an empty coordination site is immediately hydrolyzed to form WO₃. Unfortunately, tungsten trioxide, the most stable species of tungsten oxide, has all coordination sites occupied by bridging oxygens and leaves no vacant coordination sites available for catalytic reactions. Although WO₃Also having a certain olefin epoxyCatalytic performance, but its catalytic ability derives only from WO₃The pH of the solution is increased if the WO is increased₃The more feasible method is to make WO as much as possible₃The nanoparticles are small and supported on a substrate with a synergistic catalytic effect. However, this approach does not alter WO₃The fact that the medium tungsten atom has no empty active sites greatly limits its catalytic activity.

The present invention provides a WO capable of providing additional vacant coordination sites_xThe heterogeneous catalyst is prepared by first supporting a metal W on a C to form W @ C, and oxidizing the metal W supported on the C to form a water-coordinated tungsten oxide on carbon H₂O-WO_x@C。 H₂O-WO_X@ C as catalyst for catalyzing coordination H in olefin epoxidation reaction₂O will be from WO_xIt is detached and leaves an unoccupied coordination site to increase the catalytic activity. After the catalysis is finished, the empty coordination active site can be recombined with water molecule and changed back to H₂O-WO_x@ C, thereby greatly improving the stability of the catalyst.

Disclosure of Invention

The object of the present invention is to propose a WO which is able to provide additional empty coordination sites_xBased heterogeneous catalyst, such WO being able to provide additional vacant coordination sites_xBased on a heterogeneous catalyst of H₂O-WO_x@ C. In the catalytic epoxidation of olefins, complex H₂O will be from WO_xIt is detached and leaves an unoccupied coordination site to increase the catalytic activity. After the catalytic process is finished, the empty coordination active site is restored to H₂O-WO_x@ C to improve the stability of the catalyst.

WO provided by the invention and capable of providing additional empty coordination sites_xBased on a heterogeneous catalyst of H₂O-WO_xA process for the preparation of @ C, which comprises first supporting a metal W on C to form W @ C, and oxidizing the metal W supported on C to form a water-coordinated tungsten oxide on carbon H₂O-WO_x@C。

WO proposed by the invention capable of providing additional empty coordination sites_xRadical phase ofThe catalyst is H₂O-WO_xThe preparation method of @ C comprises the following steps and contents, as shown in figure 1:

step (1) Keggin-type polyoxometallate K₅[α-BW₁₂O₄₀]·11.4H₂O (noted as BW)₁₂) In situ packaging into ZIF-8 cavity to form BW₁₂@ZIF-8；

Step (2) pyrolysis of BW under Ar protection₁₂@ZIF-8，BW₁₂Reduced into metal tungsten nanoparticles (WNPs) fixed on a porous carbon carrier to form W @ C;

step (3) oxidizing the metal tungsten nanoparticles (W NPs) to obtain amorphous H₂O-coordinated tungsten oxide H₂O-WO_x@ C nanoparticles.

In step (1), the diameter is 0.9nm BW₁₂The anion can be loaded into the ZIF-8 pore channel with a pore size of 1.1nm in a "pre-assembly" synthesis step and it cannot leach out of the ZIF-8 maximum window (0.3 nm). Second, BW is guaranteed₁₂In BW₁₂The preparation process of @ ZIF-8 can be kept stable and BW₁₂Can be present in the encapsulation BW₁₂In adjacent cages of (b) such that BW₁₂The distribution is uniform.

Drawings

FIG. 1 is H₂O-WO_xSchematic representation of the synthesis process of @ C.

FIG. 2 is a UV spectrum of a sample and various materials.

FIG. 3 shows W @ C-18.7(18.7 represents the mass fraction of tungsten therein: 18.7%) and the result obtained after oxidation₂O-WO_xXRD contrast pattern of @ C-18.7(18.7 denotes a sample obtained by further oxidation treatment using W @ C-18.7) and H₂O-WO_x@ C-18.7 and WO₃Infrared test results of the mixture with water.

FIG. 4 is H₂O-WO_xThe XPS test results of @ C-18.7.

FIG. 5 is W @ C-18.7 and H₂O-WO_xThe TEM image of @ C-18.7, where FIG. 5a is the TEM image of W @ C-18.7 and FIG. 5b is H₂O-WO_x@C-18Transmission electron microscopy images of 7, inset corresponding particle size distribution.

FIG. 6 is N₂Adsorption isotherms, wherein FIG. 6a is W @ C-18.7 and FIG. b is H₂O-WO_x@ C-18.7, inset: corresponding pore size distribution (using BJH method).

Detailed Description

The technical solution of the present invention is further illustrated by the following examples.

WO proposed by the present invention capable of providing additional empty coordination sites_xHeterogeneous catalyst H_xO-WO_XA process for the preparation of @ C, characterised by: firstly, the metal W is loaded on C to form W @ C, and the metal W loaded on C is oxidized to form water coordinated tungsten oxide H supported on carbon₂O-WO_x@C。

WO one, capable of providing additional vacant coordination sites_xBased on a heterogeneous catalyst of H₂O-WO_XA preparation method of @ C comprises the following steps:

(1)BW₁₂the synthesis of (2):

100g of Na₂WO₄·2H₂O is dissolved in 100mL of distilled water, and 5g of H is added to the system until the sodium tungstate is completely dissolved₃BO₃Vigorously stirred, and then 60mL of 6 mol. L^-1A hydrochloric acid solution, wherein the pH of the mixed solution is 6, boiling for at least 1 hr while continuously supplying distilled water to the system to prevent water from evaporating, and adding Na as solid₁₀W₁₂O₄₁·xH₂O is filtered out by suction; the pH of the filtrate at this time was 6.72, using 6 mol. L^-1The pH was adjusted to 2 with HCl solution, then boiled for 30 minutes, and 20g of solid potassium chloride was added to give a white precipitate, which was filtered off with suction and washed repeatedly with diethyl ether to give about 60g of crude product. Followed by recrystallization in 50mL of water at 60 ℃ to give about 50g of the title compound.

(2)BW₁₂Preparation of @ ZIF-x composite material:

adding Zn (NO)₃)₂And BW₁₂Dissolved in 15mL of deionized water, and another 2-methylimidazole was dissolved in 80mL of deionized water. Mixing the two solutions, and placing them in a chamberStir at room temperature for 1 h. The resulting white precipitate was centrifuged and washed with distilled water until no polyacid peak was present in the wash (detected by uv spectroscopy), confirming clean washing. And drying the obtained precipitate at 75 ℃ by using a vacuum drying oven to obtain the composite material. By changing BW₁₂And Zn (NO)₃)₂The molar ratio of (A) to (B) to obtain three kinds of BW respectively₁₂@ ZIF-x, x stands for BW₁₂X wt% W in @ ZIF-8, BW₁₂@ ZIF-8.4, yield: 0.71g, 70.5%; BW (Bandwidth)₁₂@ ZIF-3.5, yield: 0.59g, 62.7%; BW (Bandwidth)₁₂@ ZIF-2.4, yield: 0.43g, 46.2% of BW₁₂The content of (B) was determined by ICP-AES.

(3) Preparation of W @ C-x:

BW 0.30g₁₂The @ ZIF-x sample is placed in a high-temperature tube furnace, calcined under the argon atmosphere, heated and stayed for at least 2 hours, then heated to a specified temperature and carbonized, naturally cooled to room temperature, collected in black powder, weighed, and the yield: 0.075-0.09g, 25.00% -30.00%.

(4)H₂O-WO_xPreparation of @ C-x: dispersing a certain mass of W @ C-x in deionized water, and adding hydrogen peroxide while stirring. The mixed solution was then heated until all solvents evaporated, yielding a black powder, washed with water, dried using a vacuum oven, yield: 0.21g-0.26g, 105% -130% (according to the mass calculation yield before and after the reaction).

Preferably, in step (2), Zn (NO) is added₃)₂And BW₁₂Dissolved in 15mL of deionized water, BW₁₂And Zn (NO)₃)₂The molar ratio of (A) to (B) is 1: 20; dissolving 2-methylimidazole (2-mIm) in 80mL of deionized water; mixing the two solutions and stirring vigorously for 1 h; the resulting white precipitate was centrifuged, washed with distilled water and examined by ultraviolet spectroscopy until no polyacid peak was present in the wash. The obtained precipitate was dried at 75 ℃ using a vacuum drying oven to obtain BW₁₂@ ZIF-18.7 composite, yield: 0.87g, 73.10%.

Preferably, in step (3), 0.30g of BW is added₁₂And (2) putting the sample of @ ZIF-18.7 into a high-temperature tube furnace, calcining in an argon atmosphere, heating to 200 ℃ at first, staying for at least 2 hours, then heating to 900 ℃ and carbonizing for 3 hours, naturally cooling to room temperature, and collecting black powder, namely W @ C-18.7.

Preferably, in the step (4), a certain mass of W @ C-18.7 is dispersed in deionized water, and H is added under stirring₂O₂. Heating the mixed solution until all the solvent is volatilized to obtain black powder, washing with water, and drying with vacuum drying oven to obtain H₂O-WO_x@C-18.7。

Second, comparative sample preparation

(1) Synthesis of ZIF-8:

an appropriate amount of 2-methylimidazole (2-mIm) was dissolved in deionized water. Taking Zn (NO) additionally₃)₂Dissolving in deionized water, pouring into 2-methylimidazole solution under vigorous stirring, stirring at room temperature for 1h to produce white precipitate, centrifuging, washing with distilled water at least 5 times, drying at 75 ℃ using a vacuum drying oven, yield: 0.53g, 59.14%. Zn (NO) in the course of synthesis₃)₂And the molar ratio of 2 to mIm is constant.

(2) Pyrolyzing ZIF-8: 0.30g of ZIF-8 is weighed and placed in a porcelain boat, then the porcelain boat is transferred into a high-temperature tube furnace, and the porcelain boat is calcined under the argon atmosphere, firstly heated and kept for a plurality of hours, then heated to the specified temperature and carbonized, and naturally cooled to the room temperature. The black powder was collected and weighed. (yield: 0.12g, 40.00%)

(3)H₂O-WO_xThe preparation of (1): dispersing tungsten powder in deionized water, adding H under stirring₂O₂. The mixed solution was then heated to the indicated temperature until all the solvent was evaporated, yielding a beige powder.

(4)SCN-(H₂O-WO_xPreparation of @ C-18.7): h is to be₂O-WO_x@ C-18.7 and NH₄SCN was dispersed in deionized water, vigorously stirred, then transferred to an autoclave, heated in an oven for several hours, and allowed to cool to room temperature. Obtaining black powder, washing with water, drying,yield: 0.17g, 85%.

(5)SCN-(WO₃) The preparation of (1): SCN- (WO)₃) The synthesis procedure of (1) and SCN- (H)₂O-WO_x@ C-18.7) but using WO₃Substitute for H₂O-WO_x@C-18.7。

Third, sample characterization

XRD test results as shown in figure 3 show that the characteristic peak of corresponding metal tungsten disappears after W @ C-18.7 is oxidized, and amorphous substance H is obtained₂O-WO_X@ C-18.7, and infrared results indicate H₂O-WO_XIn @ C-18.7, three absorption peaks were found, each of which can be assigned as W ═ O (967.64 cm)^-1)、W-O-W(896.81cm^-1) And H₂O-W(813.23cm^-1) The characteristic vibration peak of the bond, evidencing the presence of W coordinated to water. In contrast, even if WO is used₃Mixing with water, and can not reach 813.23cm^-1A characteristic vibrational peak was observed for W coordinated to water (fig. 3 b). H₂O-WO_xXPS test results for @ C-18.7 (FIG. 4a) indicate that the catalyst is composed of the element C, N, O, W; the high resolution C1 s spectrum (fig. 4b) shows the presence of one strong peak and two relatively weak peaks, assigned to the C-O peak 286.5eV, the C-N peak 284.9eV, and the C-C/C ═ C peak 284.4eV, respectively; high resolution O1 s spectra (FIG. 4c), showing H₂O-WO_xThere are two different O species in @ C-18.7, respectively assigned to μ₂-bridging oxygen W-O-W (530.3eV) and water coordinating oxygen H₂O-W (532.3eV), evidencing the presence of water coordinated to W; high resolution W4f spectra, as shown in FIG. 4d, show two intense peaks at 35.2eV and 37.4eV, respectively assigned to the hexavalent W4f_7/2And W4f_5/2The transformation of W from metallic W (0) to +6 valent W (VI) was confirmed, indicating that the oxidation process was complete. No peroxide (-O-O-) was observed in either XPS or IR spectra, indicating that no peroxide was formed in the catalyst. The above results show that H obtained by the oxidation treatment₂O-WO_XStructure of @ C-18.7 catalyst and WO₃Is different.

By comparing W @ C-18.7 with H₂O-WO_XTransmission Electron microscopy images of @ C-18.7, as shown in FIGS. 5a and 5b, confirm H₂O-WO_x@ C-18.7 atThe morphology remained after post-modification oxidation, and H was observed₂O-WO_xThe NPs are still uniformly supported in the carbon substrate, and the particle size of the NPs is hardly changed (W NPs 2.28nm vs H₂O-WO_x2.24 nm). Furthermore, for the W @ C-18.7 sample, if W @ C-18.7 is oxidized to H as shown in FIG. 6a₂O-WO_x@ C-18.7, its specific surface area is from 361.50m²·g^-1Reduced to 221.66m²·g^-1This is because the mass of the W nanoparticles increases after the oxidation treatment. However, the pore size was not changed and remained around 3.81nm, indicating that the microstructure of the carbon substrate was not destroyed by the oxidation treatment. In post-modification Oxidation Process, 30% H₂O₂Without breaking the covalent bond between W and C, H₂O-WO_XThe peak at 32.0eV on the W XPS spectrum of @ C-18.7 is assigned as W4f in the W-C bond_7/2As shown in fig. 4 d. From single nanoparticles (W NPs) to single nanoparticles (H) in an oxidation process₂O-WO_xNPs) is associated with the retention of the W-C covalent bond on the carbon substrate and the formation of pores.

Claims

1. Carbon-supported H₂O-WO_xNanoparticles, characterized by: the metal W supported on C is oxidized to form water coordinated tungsten oxide H supported on carbon₂O-WO_x@ C as WO providing additional empty coordination sites_xBased on heterogeneous catalysts.

2. Carbon-supported H₂O-WO_xThe preparation method of the nano-particles is characterized by comprising the following steps: firstly, the metal W is loaded on C to form W @ C, and the metal W loaded on C is oxidized to form water coordinated tungsten oxide H supported on carbon₂O-WO_x@C。

3. The carbon supported H of claim 2₂The preparation method of the O-WOx nano-particles is characterized by comprising the following steps: step (1) Keggin-type polyoxometallate K₅[α-BW₁₂O₄₀]·11.4H₂O, is denoted as BW₁₂In-situ encapsulated into ZIF-8 tunnels to form BW₁₂@ZIF-8；

Step (2) pyrolysis of BW under Ar protection₁₂@ZIF-8，BW₁₂Is reduced into metal tungsten nano particles which are fixed on a porous carbon carrier to form W @ C;

step (3) oxidizing the metal tungsten nano particles to obtain amorphous H₂O-coordinated tungsten oxide H₂O-WO_x@ C nanoparticles.