CN112076776A - Protonated carbon nitrides for selective photocatalytic oxidation of alcohols to esters and uses thereof - Google Patents

Abstract

The invention discloses a protonated carbon nitride and its application as a photocatalyst in the selective oxidation of alcohol to generate ester, and belongs to the technical field of photocatalytic organic synthesis. The protonated carbon nitride is prepared by first synthesizing bulk carbon nitride by thermal polymerization, then treating it with molten salt method to obtain molten salt carbon nitride, and then reacting molten salt carbon nitride with acid. Using the protonated carbon nitride as a photocatalyst, under the irradiation of visible light with a wavelength of 420 nm, oxygen can be used as an oxidant to efficiently catalyze the reaction of benzyl alcohol and methanol to generate methyl benzoate, which has important application prospects.

Description

Protonated carbon nitrides for selective photocatalytic oxidation of alcohols to esters and their applications

technical field

The invention belongs to the technical field of photocatalytic organic synthesis, in particular to a protonated carbon nitride and its application as a photocatalyst in selectively oxidizing alcohol to generate ester.

Background technique

Esters are a class of abundant and highly important chemicals that are widely used in polymers, natural products, pharmaceuticals, and other fine chemicals. Typically, esters are prepared by reacting carboxylic or active acid derivatives with alcohols, although this method is widely used in industry and scientific laboratories, this process is tedious and often results in the formation of large amounts of unwanted by-products . To overcome this limitation, scientists have developed many strategies for synthesizing esters in recent years. For example, the oxidative esterification of aldehydes is an efficient method for preparing esters. However, this method requires metal salts as oxidants or noble metals as catalysts ( Nat. Chem. 2010, 2, 61), which is not in line with the original intention of green chemistry. Therefore, it is very meaningful to develop an environment-friendly and low-cost preparation method for ester compounds.

Alcohol compounds can be produced from renewable biomass, are environmentally friendly and abundant, and are often used as solvents and substrates for various chemical reactions. Among various alcohol conversion reactions, the direct oxidation of alcohols to esters using oxygen is certainly attractive. To date, there are only few reports on the direct esterification of alcohols, most of which are based on homogeneous catalytic systems that require noble metal-based catalysts, such as palladium ( Org. Lett. 2013, 15, 5072), ruthenium ( Angew. Chem. , Int. Ed. 2012, 51, 5711), gold ( J. Am. Chem. Soc . 2010, 132, 15096 ), and iridium ( J. Org. Chem. 2011, 76, 2937 ). Furthermore, in order to obtain higher yields, these homogeneous catalytic systems require the addition of a base to neutralize the acid produced in the process, and usually require high temperatures and pressures. In general, heterogeneous catalysis has advantages over homogeneous catalysis because the catalyst is easily separated from the reaction mixture and can be reused efficiently. Therefore, the development of heterogeneous, inexpensive and readily available catalysts to efficiently catalyze the oxidation of alcohols to esters is both attractive and challenging.

As an important non-metallic material, polymer carbon nitride has attracted much attention due to its unique properties, especially its visible light response and excellent chemical stability ( Angew. Chem. Int. Ed. , 2019, 58, 6164 ). These properties make carbon nitride widely used in heterogeneous organic synthesis reactions. In the reported selective oxidation of alcohols involving carbon nitride, the products are usually aldehydes rather than esters ( Chem. Sci. , 2013, 4, 3244). So far, there is no report on the direct oxidation of alcohols to esters without the addition of metal carbon nitride catalysts. The present invention uses acid to treat carbon nitride to obtain protonated carbon nitride. It not only retains the good oxidation ability of molten salt carbon nitride, but also introduces acid sites on the surface. Under the conditions of oxygen and light, it can realize the efficient oxidation of benzyl alcohol and methanol to form methyl benzoate.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a protonated carbon nitride and a method for selectively oxidizing alcohol to generate ester by using it as a photocatalyst. The protonated carbon nitride prepared by the invention overcomes the disadvantage of weak oxidizing ability of traditional carbon nitride, and can realize the efficient oxidation of benzyl alcohol and methanol to generate methyl benzoate.

To achieve the above object, the present invention adopts the following technical solutions:

A protonated carbon nitride used for selective photocatalytic oxidation of alcohol to generate ester, which is firstly synthesized by thermal polymerization method to obtain bulk carbon nitride, then treated with molten salt method to obtain molten salt carbon nitride, and then The molten salt carbon nitride is reacted with acid to obtain the protonated carbon nitride. During this process, the surface of the protonated carbon nitride is weakened in basicity, enhanced in acidity, weakened in hydrophilicity, enhanced in hydrophobicity, and has the ability to activate oxygen. enhanced.

The preparation method of the protonated carbon nitride specifically comprises the following steps:

(1) The amine-containing solid powder is placed in a crucible, and calcined in an air atmosphere at 300-600 °C for 10 hours to obtain bulk carbon nitride;

(2) placing the bulk carbon nitride and inorganic salt obtained in step (1) in a mortar and grinding at a mass ratio of 1:3-6 to obtain molten salt carbon nitride;

(3) calcining the molten salt carbon nitride obtained in step (2) in a nitrogen atmosphere at 400-650° C. for 12 hours to obtain a sample precursor;

(4) Wash off excess inorganic salts in the sample precursor obtained in step (3) with deionized water, place it in a beaker after drying, and add acid with a volume concentration of 20% at a mass-to-volume ratio of 20:1 mg/mL liquid, stirred overnight, centrifuged to remove the liquid, and dried again to obtain the protonated carbon nitride.

The amine-containing solid powder in step (1) is melamine, cyanamide or urea.

The inorganic salt in step (2) includes any one or more of sodium chloride, lithium chloride, potassium chloride, ammonium chloride and the like.

The acid solution used in step (4) is an aqueous solution of inorganic acids such as sulfuric acid, hydrochloric acid or phosphoric acid.

The protonated carbon nitride can be used for the selective photocatalytic oxidation of alcohol to generate ester, and specifically, the protonated carbon nitride is used as a photocatalyst, and under the irradiation of visible light with a wavelength of 420 nm, oxygen is used as an oxidant to selectively catalyze benzyl alcohol and methanol. The reaction produces methyl benzoate.

The significant advantages of the present invention are:

(1) The protonated carbon nitride prepared by the present invention is a new type of photocatalyst, and its oxygen activation ability is enhanced, which can realize the efficient oxidation of benzyl alcohol and methanol to generate methyl benzoate.

(2) The production process of the present invention is simple, easy to control, low in energy consumption, low in cost, meets actual production needs, and is conducive to large-scale promotion.

Description of drawings

FIG. 1 is an XRD pattern of the protonated carbon nitride obtained in Example 1. FIG.

FIG. 2 is an FT-IR chart of the protonated carbon nitride obtained in Example 1. FIG.

FIG. 3 is a SEM image of the protonated carbon nitride obtained in Example 1. FIG.

FIG. 4 is a TEM image of the protonated carbon nitride obtained in Example 1. FIG.

Figure 5 is an analysis diagram of the product components of the carbon nitride solid powder (a) and protonated carbon nitride (b) obtained in Example 1 for photocatalytic oxidation of benzyl alcohol and methanol.

6 is an analysis diagram of the product components of the photocatalytic oxidation of benzyl alcohol and methanol obtained by protonated carbon nitride obtained in Example 2.

7 is an analysis diagram of the product components of the photocatalytic oxidation of benzyl alcohol and methanol obtained by protonated carbon nitride obtained in Example 3.

Detailed ways

In order to make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below with reference to specific embodiments, but the present invention is not limited thereto.

Example 1

(1) 8 grams of melamine solid powder was weighed and placed in an alumina crucible, calcined in an air atmosphere at 410°C for 10 hours, taken out after natural cooling, and ground into powder to obtain bulk carbon nitride solid powder;

(2) Weigh 1 gram of bulk carbon nitride solid powder and 4.0 grams of lithium chloride, grind them evenly and place them in an alumina crucible, calcined in a nitrogen atmosphere at 600°C for 12 hours, take out after natural cooling, and grind into In powder form, wash with deionized water and remove the remaining lithium chloride by suction filtration, and then place the obtained sample in an oven to dry to obtain carbon nitride solid powder;

(3) Weigh 500 mg of carbon nitride solid powder into a beaker, add 20 ml of deionized water and 5 ml of concentrated hydrochloric acid, stir overnight, remove the liquid by centrifugation, and dry the centrifuged sample to obtain protonated carbon nitride of solid powder.

FIG. 1 is an XRD pattern of the protonated carbon nitride obtained in this example. It can be seen from the figure that the protonated carbon nitride solid powder has two obvious XRD diffraction peaks at 8° and 27.5°, which are attributed to the (100) and (002) crystal planes of the polymer carbon nitride, confirming the prepared The crystal structure of the carbon nitride material did not change significantly after protonation treatment.

FIG. 2 is an FT-IR image of the protonated carbon nitride obtained in this example.

3 and 4 are the SEM image and the TEM image of the protonated carbon nitride obtained in this example, respectively. It can be seen from the figure that the protonated carbon nitride samples have a sheet-like and stacked structure, and obvious lattice fringes can be observed.

Weigh 10 mg each of the obtained carbon nitride solid powder and protonated carbon nitride solid powder, place them in a 10-mL glass reactor, add 1 mL of methanol and 0.2 mmol (about 22 microliters) of benzyl alcohol, respectively, and pass Oxygen was added, and the reaction was carried out at 25° C. under the condition of 420 nm LED illumination for 24 hours, and the reaction products were analyzed by gas chromatography-mass spectrometry.

FIG. 5 is an analysis diagram of the product components of the carbon nitride solid powder (a) and protonated carbon nitride (b) obtained in this example by photocatalytic oxidation of benzyl alcohol and methanol. It can be seen from the figure that the carbon nitride solid powder can catalyze the conversion of benzyl alcohol (conversion rate 99%), but the main product is benzoic acid (selectivity 99%), and the selectivity of methyl benzoate is very low (less than 1 %); and protonated carbon nitride can not only catalyze the conversion of benzyl alcohol well (conversion rate of 99%), but also achieve high-efficiency selection of methyl p-benzoate (selectivity of 94%).

Example 2

(1) Weigh 8 grams of cyanamide solid powder and place it in an alumina crucible, calcined in an air atmosphere at 300°C for 10 hours, take out after natural cooling, and grind into powder to obtain bulk carbon nitride solid powder;

(2) Weigh 1 gram of bulk carbon nitride solid powder and 5.0 grams of potassium chloride, grind them evenly and place them in an alumina crucible, calcined for 12 hours in a nitrogen atmosphere at 400 °C, take out after natural cooling, and grind into In powder form, wash with deionized water and remove the remaining potassium chloride by suction filtration, and then place the obtained sample in an oven to dry to obtain carbon nitride solid powder;

(3) Weigh 500 mg of carbon nitride solid powder into a beaker, add 20 ml of deionized water and 5 ml of concentrated sulfuric acid, stir overnight, remove the liquid by centrifugation, and dry the centrifuged sample to obtain protonated carbon nitride of solid powder.

6 is an analysis diagram of the product components of the photocatalytic oxidation of benzyl alcohol and methanol obtained by the protonated carbon nitride obtained in this example. It can be seen from the figure that the protonated carbon nitride can catalyze the conversion of benzyl alcohol well (conversion rate of 99%), and at the same time, methyl benzoate also has a high selectivity (selectivity of 85%).

Example 3

(1) 8 grams of urea solid powder was weighed and placed in an alumina crucible, calcined in an air atmosphere at 600°C for 10 hours, taken out after natural cooling, and ground into powder to obtain bulk carbon nitride solid powder;

(2) Weigh 1 gram of bulk carbon nitride solid powder and 6.0 grams of ammonium chloride, grind them evenly and place them in an alumina crucible, calcined in a nitrogen atmosphere at 650°C for 12 hours, take out after natural cooling, and grind into In powder form, wash with deionized water and remove the remaining ammonium chloride by suction filtration, and then place the obtained sample in an oven to dry to obtain carbon nitride solid powder;

(3) Weigh 500 mg of carbon nitride solid powder into a beaker, add 20 ml of deionized water and 5 ml of concentrated hydrochloric acid, stir overnight, remove the liquid by centrifugation, and dry the centrifuged sample to obtain protonated carbon nitride of solid powder.

7 is an analysis diagram of the product components of the photocatalytic oxidation of benzyl alcohol and methanol obtained by the protonated carbon nitride obtained in this example. It can be seen from the figure that the catalytic conversion of protonated carbon nitride to benzyl alcohol is about 60%, and the selectivity to methyl parabenzoate is 31%.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims (6)

1. A protonated carbon nitride for selective photocatalytic oxidation of an alcohol to form an ester, characterized by: the preparation method comprises the steps of synthesizing bulk-phase carbon nitride by a thermal polymerization method, treating the bulk-phase carbon nitride by a molten salt method to obtain molten salt carbon nitride, and reacting the molten salt carbon nitride with acid to obtain the protonated carbon nitride.

2. The protonated carbon nitride according to claim 1, wherein: the method specifically comprises the following steps:

(1) placing the amine-containing solid powder in a crucible, and calcining for 10 hours at the temperature of 600 ℃ in an air atmosphere to obtain bulk-phase carbon nitride;

(2) putting the bulk-phase carbon nitride obtained in the step (1) and inorganic salt into a mortar according to a certain proportion, and grinding to obtain molten salt carbon nitride;

(3) calcining the fused salt carbon nitride obtained in the step (2) for 12 hours at the temperature of 400-650 ℃ in a nitrogen atmosphere to obtain a sample precursor;

(4) and (4) washing off redundant inorganic salt in the sample precursor obtained in the step (3) by water, drying, adding acid liquor, stirring overnight, centrifuging to remove liquid, and drying again to obtain the protonated carbon nitride.

3. The protonated carbon nitride according to claim 1, wherein: in the step (1), the amine-containing solid powder is melamine, cyanamide or urea.

4. The protonated carbon nitride according to claim 1, wherein: the mass ratio of the bulk-phase carbon nitride to the inorganic salt used in the step (2) is 1: 3-6;

the inorganic salt comprises one or more of sodium chloride, lithium chloride, potassium chloride and ammonium chloride.

5. The protonated carbon nitride according to claim 1, wherein: the mass-to-volume ratio of the sample precursor to the acid liquor used in the step (4) is 20:1 mg/mL;

the acid solution used has a volume concentration of 20% and is an aqueous solution of sulfuric acid, hydrochloric acid or phosphoric acid.

6. Use of protonated carbon nitride according to claim 1 for selective photocatalytic oxidation of an alcohol to form an ester, wherein: and under the irradiation of visible light with the wavelength of 420nm, the protonated carbon nitride is used as a photocatalyst, and oxygen is used as an oxidant to selectively catalyze the reaction of the benzyl alcohol and the methanol to generate the methyl benzoate.

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