A COF-TpBpy@VNbC composite photocatalyst and its preparation method

Abstract

The present invention belongs to the field of semiconductor photocatalysis technology, specifically a COF-TpBpy@VNbC composite photocatalyst and a preparation method thereof. The present invention comprises: etching and peeling VNbAlC to obtain high-quality single-layer / few-layer VNbC nanosheets, and modifying the surface of the VNbC nanosheets by hydroxylation modification; in situ growing a COF-TpBpy layer on the modified VNbC nanosheets through a Schiff base condensation reaction between the aldehyde group and the amino group of the monomer to obtain a heterogeneous structure composite catalyst, denoted as COF-TpBpy@VNbC; by adjusting the amount of VNbC added, COF-TpBpy@VNbC composite catalysts of different proportions are obtained. The catalyst exhibits high selectivity and high activity in a photocatalytic H2O2 synthesis and selective styrene epoxidation coupling reaction system. The present invention has a simple process, low cost, and readily available reactants; and has industrial application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor photocatalysis, and specifically relates to a COF-TpBpy@VNbC composite photocatalyst and a preparation method thereof. Background Art

[0002] Olefin epoxidation is an important industrial process for the production of a range of high-value-added fine chemicals, and significant efforts have been made in this field over the past few decades. Traditional epoxidation processes generally use highly corrosive and hazardous oxidants and high reaction temperatures, resulting in high costs and severe environmental pollution. In recent years, directly utilizing solar energy to promote olefin epoxidation to produce high-value-added chemicals has become a promising strategy. Olefin epoxidation through photocatalytic cleavage of C=C bonds is a sustainable and environmentally friendly strategy. In photocatalytic olefin epoxidation, hydrogen peroxide exhibits significant advantages over traditional oxidants, such as environmental friendliness and cost-effectiveness. However, currently, over 95% of hydrogen peroxide production uses the anthraquinone method, which has the disadvantages of expensive raw materials, high consumption, and severe environmental pollution. Therefore, it is necessary to explore new hydrogen peroxide synthesis methods and couple them with olefin epoxidation to build a new generation of olefin epoxidation technology.

[0003] Two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) have been widely used in many fields due to their high electrical conductivity, large surface area, and adjustable surface functional groups. In recent years, studies have found that double transition metal solid solution MXenes have more unique electronic structures, stronger metal affinity, and higher mechanical strength. Therefore, the development of double transition metal solid solution MXenes with excellent performance is very promising. At the same time, the metal sites (such as V and Nb) in double transition metal MXenes can be used to catalyze the activation of H2O2 and the epoxidation of olefins. However, many current catalytic systems require the additional addition of H2O2 or other organic peroxides. Therefore, there is an urgent need to establish a composite catalyst that couples olefin epoxidation with H2O2 synthesis.

[0004] Covalent organic frameworks (COFs) are excellent photocatalytic platforms with great potential for H2O2 photosynthesis. Our research has shown that two-dimensional crystalline COFs can be covalently assembled onto MXene nanosheets to form heterostructures. However, the application of such COF@MXene composites in photocatalytic olefin epoxidation and H2O2 photosynthesis has rarely been reported. Therefore, it is crucial to design a novel COF@MXene composite that meets the requirements of a photocatalytic coupling system to achieve efficient photocatalytic H2O2 synthesis and selective olefin epoxidative coupling reactions. Summary of the Invention

[0005] The object of the present invention is to provide a COF@MXene composite photocatalyst with high selectivity and high activity and a preparation method thereof.

[0006] The COF@MXene composite photocatalyst provided by the present invention is obtained by modifying the surface of VNbC nanosheets with hydroxyl groups and then in situ growing a COF-TpBpy layer on the modified VNbC nanosheets, which is recorded as COF-TpBpy@VNbC.

[0007] The preparation method of the COF-TpBpy@VNbC composite catalyst proposed in the present invention includes: etching and peeling VNbAlC to obtain high-quality single-layer / few-layer VNbC nanosheets, and modifying the surface of the VNbC nanosheets by hydroxylation; in situ growing a COF-TpBpy layer on the modified VNbC nanosheets through a Schiff base condensation reaction between the aldehyde group and the amino group of the monomer to obtain a heterogeneous structure composite catalyst, denoted as COF-TpBpy@VNbC; and by adjusting the amount of VNbC added, COF-TpBpy@VNbC composite catalysts with different proportions are obtained.

[0008] The specific steps of preparation are:

[0009] (1) VNbAlC was added to HF solution and stirred continuously for reaction. The obtained product was intercalated with tetramethylammonium hydroxide and centrifuged to obtain single-layer / few-layer VNbC nanosheets.

[0010] (2) VNbC was dispersed in a mixed solution of ethanol and deionized water, a small amount of NaOH was added for hydroxylation, and the product was collected by freeze drying;

[0011] (3) The hydroxyl-functionalized VNbC was added to a mixed solution containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine, and the mixture was reacted in an oven at a constant temperature for a period of time. The COF-TpBpy@VNbC product was obtained by washing and drying.

[0012] Further:

[0013] In step (1), the ratio of the volume (mL) of the HF solution to the amount (g) of VNbAlC added is (20-100):1; the reaction temperature is 10-70° C.; and the reaction time is 2-48 h.

[0014] In step (2), the reaction temperature is 10-60° C. and the reaction time is 1-12 h;

[0015] In step (3), the ratio of the added amount of COF-TpBpy to VNbC is 1 to 20:1; the molar ratio of 1,3,5-triformylphloroglucinol to 2,2'-bipyridine-5,5'-diamine ligand is (0.5 to 3):1; the reaction temperature is 20 to 120° C.; and the holding time is 8 to 72 hours.

[0016] Furthermore, the method for preparing the COF-TpBpy@VNbC composite catalyst proposed in the present invention has the following specific operation process:

[0017] (1) Slowly add VNbAlC powder to HF solution, controlling the ratio to be 20-100:1, and continuously stir the mixture at 10-70°C for 2-48 hours. After the reaction, wash it with deionized water several times until the pH is close to 6-8. The sediment after centrifugation is intercalated with tetramethylammonium hydroxide and then washed with deionized water until the pH is close to 6-8. Ultrasonicate under inert gas atmosphere for 1-24 hours, and then centrifuge to separate the single-layer / few-layer VNbC material.

[0018] (2) functionalizing the VNbC obtained in step (1) with a hydroxylation reagent; dispersing the VNbC in a mixed solution of ethanol / deionized water, then adding a small amount of NaOH, and stirring the suspension at 10-60° C. for 1-12 hours; at the end of the reaction, washing with ethanol several times, and the product was dispersed in deionized water and collected by freeze drying;

[0019] (3) A certain amount of hydroxyl-functionalized VNbC obtained in step (2) is added to a mixed solution containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine, and the ratio of the two is controlled to be 1-20:1; the mixture is kept in an oven at 20-120°C for 8-72 hours, and after washing and drying, a COF-TpBpy@VNbC product is obtained.

[0020] This invention utilizes a simple synthetic method to prepare single-layer or few-layer VNbC. Hydroxyl groups are modified on the VNbC surface, and a COF-TpBpy layer is in situ grown on the modified VNbC nanosheets to produce a novel COF-TpBpy@VNbC composite photocatalyst. This photocatalyst is used for photocatalytic H2O2 synthesis and selective olefin epoxidative coupling reactions, significantly improving reaction performance. The preparation method is simple, low-cost, and readily available reactants.

[0021] The specific meanings of VNbC and COF-TpBpy in the present invention are as follows:

[0022] VNbC: It is a solid solution MXene material based on double transition metals V and Nb. It is a layered material synthesized by selectively etching the Al element in vanadium niobium aluminum carbon (VNbAlC).

[0023] COF-TpBpy: a covalent organic framework material (COF) obtained by Schiff base condensation reaction of 1,3,5-triformylphloroglucinol (Tp) and 2,2'-bipyridine-5,5'-diamine (Bpy), abbreviated as COF-TpBpy.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) The COF-TpBpy@VNbC composite photocatalyst exhibits excellent photocatalytic H2O2 synthesis and selective olefin epoxidative coupling performance, outperforming most reported photocatalysts;

[0026] (2) COF-TpBpy combines with VNbC to form a heterojunction, which effectively promotes the migration of photogenerated carriers and improves the catalytic activity and selectivity;

[0027] (3) The COF-TpBpy@VNbC catalyst can achieve up to 95% selectivity for epoxy products and 99% conversion rate; this study developed a new strategy for the selective photocatalytic H2O2 synthesis and selective olefin epoxidative coupling reaction system, highlighting its potential industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 XRD patterns of VNbAlC, VNbC, COF-TpBpy, and COF-TpBpy@VNbC.

[0029] Figure 2 SEM image of COF-TpBpy@VNbC composite catalyst. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to specific examples, which are intended to explain the present invention rather than to limit it.

[0031] Example 1

[0032] (1) Preparation of single-layer / few-layer VNbC: 1 g of VNbAlC powder was slowly added to 30 mL of HF solution, and the mixture was stirred at 50°C for 48 h. After the reaction, the mixture was washed several times with deionized water until the pH was > 6. The sediment after centrifugation was intercalated with tetramethylammonium hydroxide and then washed with deionized water until the pH was 6-8. The obtained product solution was ultrasonicated in an inert gas atmosphere for 60 min, and then the single-layer / few-layer VNbC material was separated by centrifugation.

[0033] (2) Hydroxylation modification of VNbC. The specific process is as follows: the VNbC obtained in step (1) is dispersed in a mixed solution of ethanol / deionized water (v:v = 9:1), a small amount of NaOH solution is added, and the suspension is stirred and reacted at 25±2°C for 12 hours. After the reaction is completed, it is washed with ethanol several times, and the product is dispersed in deionized water and collected by freeze drying.

[0034] (3) The COF-TpBpy@VNbC composite catalyst was prepared by a solvothermal method. The specific process is as follows: the hydroxyl-functionalized VNbC (25 mg) obtained in step (2) was added to a mixed solution containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine (molar ratio of 1:3). The reaction was carried out in an oven at 60°C for 72 h. After washing and drying, the COF-TpBpy@VNbC product was obtained. 20 mg of the above catalyst (1#) was weighed and used to carry out photocatalytic H2O2 synthesis and selective olefin epoxidation experiments.

[0035] Example 2

[0036] (1) The preparation process of single-layer / few-layer VNbC is the same as that in Example 1.

[0037] (2) The hydroxylation modification process of VNbC was the same as that in Example 1.

[0038] (3) The COF-TpBpy@VNbC composite catalyst was prepared by a solvothermal method. The specific process was as follows: 37.5 mg of the hydroxyl-functionalized VNbC obtained in step (2) was added to a suspension containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine (molar ratio of 2:3). The reaction was carried out in an oven at 40°C for 48 h. After washing and drying, the COF-TpBpy@VNbC product was obtained. 20 mg of the above catalyst (2#) was weighed and used to carry out photocatalytic H2O2 synthesis and selective olefin epoxidation experiments.

[0039] Example 3

[0040] (1) The preparation process of single-layer / few-layer VNbC is the same as that in Example 1.

[0041] (2) The hydroxylation modification process of VNbC was the same as that in Example 1.

[0042] (3) The COF-TpBpy@VNbC composite catalyst was prepared by a solvothermal method. The specific process was as follows: the hydroxyl-functionalized VNbC (50 mg) obtained in step (2) was added to a suspension containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine (molar ratio of 2:3). The reaction was carried out in an oven at 120°C for 72 h. After washing and drying, the COF-TpBpy@VNbC product was obtained. An appropriate amount of the above catalyst (3#) was weighed and the photocatalytic H2O2 synthesis and selective olefin epoxidation experiments were carried out.

[0043] Example 4

[0044] (1) The preparation process of single-layer / few-layer VNbC is the same as that in Example 1.

[0045] (2) The hydroxylation modification process of VNbC was the same as that in Example 1.

[0046] (3) The COF-TpBpy@VNbC composite catalyst was prepared by a solvothermal method. The specific process was as follows: the hydroxyl-functionalized VNbC (62.5 mg) obtained in step (2) was added to a suspension containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine (molar ratio of 2:3). The reaction was carried out in an oven at 80°C for 4 h. After washing and drying, the COF-TpBpy@VNbC product was obtained. An appropriate amount of the above catalyst (4#) was weighed and the photocatalytic H2O2 synthesis and selective olefin epoxidation experiments were carried out.

[0047] Example 5

[0048] (1) The preparation process of single-layer / few-layer VNbC is the same as that in Example 1.

[0049] (2) The hydroxylation modification process of VNbC was the same as that in Example 1.

[0050] (3) The COF-TpBpy@VNbC composite catalyst was prepared by a solvothermal method. The specific process was as follows: the hydroxyl-functionalized VNbC (75 mg) obtained in step (2) was added to a suspension containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine (molar ratio of 3:1). The reaction was carried out in an oven at 100°C for 12 h. After washing and drying, the COF-TpBpy@VNbC product was obtained. An appropriate amount of the above catalyst (5#) was weighed and the photocatalytic H2O2 synthesis and selective olefin epoxidation experiments were carried out.

[0051] Example 6

[0052] To compare the catalytic efficiency of the COF-TpBpy@VNbC composite photocatalyst, the present invention separately studied the photocatalytic H2O2 synthesis and selective olefin epoxidation activities of VNbC nanosheets. The specific preparation process was the same as that in Example 1. An appropriate amount of the above catalyst (6#) was weighed and the photocatalytic H2O2 synthesis and selective olefin epoxidation experiments were carried out.

[0053] Example 7

[0054] To compare the catalytic efficiency of the COF-TpBpy@VNbC composite photocatalyst, the present invention separately studied the photocatalytic H2O2 synthesis and selective olefin epoxidation activities of COF-TpBpy. The specific preparation process was the same as that in Example 1. An appropriate amount of the above catalyst (7#) was weighed and photocatalytic H2O2 synthesis and selective olefin epoxidation experiments were carried out.

[0055] The COF-TpBpy@VNbC composite photocatalyst provided by the present invention can be used for photocatalytic H2O2 synthesis and selective styrene epoxidation coupling reaction. The specific operation process is as follows: an appropriate amount of the prepared catalyst is dispersed into 5-20 mL of aqueous solution, adsorption equilibrium is carried out for 30-60 minutes, the system temperature is maintained at 30-60°C, a 100-500W xenon lamp is used as the irradiation light source, the organic liquid product is measured by gas chromatography, and the H2O2 yield is determined by iodine titration.

[0056] The results of using the catalysts prepared in Examples 1-7 for photocatalytic H2O2 synthesis and selective styrene epoxidative coupling reactions are shown in Table 1 below.

[0057] Table 1. Photocatalytic H2O2 synthesis and selective olefin epoxidation activity of COF-TpBpy@VNbC composite photocatalyst

[0058]

Claims

1. A method for preparing a COF-TpBpy@VNbC composite photocatalyst, characterized in that: include: VNbAlC was etched and peeled off to obtain high-quality single-layer / few-layer VNbC nanosheets, and the surface of the VNbC nanosheets was modified by hydroxylation. A COF-TpBpy layer was in situ grown on the modified VNbC nanosheets through a Schiff base condensation reaction between the aldehyde and amino groups of the monomer to obtain a heterogeneous structure composite photocatalyst, denoted as COF-TpBpy@VNbC. By adjusting the amount of VNbC added, COF-TpBpy@VNbC composite photocatalysts with different proportions were obtained.

2. The preparation method according to claim 1, characterized in that The specific steps are: (1) VNbAlC was added to HF solution and stirred continuously for reaction. The obtained product was intercalated with tetramethylammonium hydroxide and centrifuged to obtain single-layer / few-layer VNbC nanosheets. (2) VNbC was dispersed in a mixed solution of ethanol and deionized water, and NaOH was added for hydroxylation reaction. The product was collected by freeze drying; (3) The hydroxyl-functionalized VNbC was added to a mixed solution containing 1,3,5-triformylphloroglucinol and 2,2'-bipyridine-5,5'-diamine, and the mixture was reacted in an oven at a constant temperature for a period of time. The COF-TpBpy@VNbC product was obtained by washing and drying.

3. The preparation method according to claim 2, characterized in that In step (1), the volume ratio of the HF solution to the amount of VNbAlC added is 20-100 mL:1 g; the reaction temperature is 10-70 °C; and the reaction time is 2-48 h.

4. The preparation method according to claim 2, characterized in that In step (2), the reaction temperature is 10-60°C and the reaction time is 1-12 h.

5. The preparation method according to claim 2, characterized in that In step (3), the molar ratio of 1,3,5-triformylphloroglucinol to 2,2'-bipyridine-5,5'-diamine ligand is (0.5-3):1; the reaction temperature is 20-120 °C; and the holding time is 8-72 h.

6. A COF-TpBpy@VNbC composite photocatalyst obtained by the preparation method according to any one of claims 1 to 5.

7. Use of the COF-TpBpy@VNbC composite photocatalyst as claimed in claim 6 in photocatalytic H2O2 synthesis and selective olefin epoxidative coupling reaction.

Images