CN110479306B

CN110479306B - Preparation method of carbon-coated cobalt-silver alloy catalyst

Info

Publication number: CN110479306B
Application number: CN201910782431.0A
Authority: CN
Inventors: 陈超; 胡卓; 史顺利; 汪淑华; 赵丹; 丁顺民
Original assignee: Nanchang University
Current assignee: Nanchang University
Priority date: 2019-08-23
Filing date: 2019-08-23
Publication date: 2023-03-31
Anticipated expiration: 2039-08-23
Also published as: CN110479306A

Abstract

The invention discloses a preparation method of a carbon-coated cobalt-silver alloy catalyst, which comprises the following steps: s1, weighing a proper amount of organic ligand 2, 5-dihydroxy terephthalic acid, metal salt cobalt nitrate hexahydrate and silver nitrate, dissolving in a mixed solution of water, ethanol and N, N-dimethylformamide, and uniformly stirring; s2, pouring the mixed solution into a hydrothermal reaction kettle containing a polytetrafluoroethylene lining, and placing the hydrothermal reaction kettle in an electric heating constant-temperature air blast drying box for crystallization; s3, taking out the crystallized mixture, sequentially centrifugally washing the crystallized mixture by using N, N-dimethylformamide and methanol, drying the washed mixture at room temperature, and taking out the dried mixture to obtain a metal-organic framework compound precursor; and S4, adding the metal-organic framework compound precursor into a quartz tube, and calcining in the nitrogen atmosphere to obtain the mesoporous carbon coated cobalt-silver alloy catalyst. The catalyst precursor is simply and quickly obtained through a one-pot method, and the mesoporous carbon layer-coated confinement cobalt-silver alloy catalyst is obtained through high-temperature pyrolysis in-situ reduction, and has magnetism and is easy to recycle.

Description

Preparation method of carbon-coated cobalt-silver alloy catalyst

Technical Field

The invention relates to the technical field of chemical materials, in particular to a preparation method of a carbon-coated cobalt-silver alloy catalyst.

Background

Citral, also known as 3, 7-dimethyl-2, 6-octadienal, is a typical α, β -unsaturated aldehyde, mainly derived from citronella oil and litsea cubeba oil, and is paid attention to by people due to its abundant natural resource advantages and wide application range. There are three double bonds in citral that can be hydrogenated: isolated C = C double bonds, conjugated C = O and C = C double bonds, and a wide variety of products after hydrogenation, such as nerol, geraniol, citronellol, citronellal, 3, 7-dimethyloctanol, tetrahydrocitronellol, and the like. It is worth noting that nerol and geraniol (Z/E-3, 7-dimethyl-2, 6-octadien-1-ol) obtained by hydrogenation of the C = O double bond conjugated with citral are two valuable fragrance and pharmaceutical intermediates, with high antibacterial and antioxidant properties. However, thermodynamic studies have shown that C = C double bonds in the citral conjugated double bonds are easier to hydrogenate than C = O double bonds, and it is therefore desirable to develop selective catalysts for the hydrogenation of C = O double bonds to improve product yields. At present, most of research workers for hydrogenating citral select noble metal-based catalysts such as Ru, pt, pd and Ir, but the application is limited to a great extent due to high price.

The metal-organic framework materials, called MOFs for short, are crystalline pore channel structure materials formed by self-assembling inorganic metal ions and organic functional groups through covalent bonds. The zeolite molecular sieve has a crystal structure similar to that of a zeolite molecular sieve, and a pore channel has designability and cuttability, and a nanometer-sized pore channel cavity can be obtained through the directional design of a topological structure and the expansion of organic functional groups. Due to their diverse structures, MOFs have become important materials for the preparation of various catalyst precursors in recent years.

U.S. Pat. No. 4,4100180 discloses a PtO/Zn/Fe composite catalyst used for selective hydrogenation of citral, and when the conversion rate reaches 70%, the selectivity of nerol and geraniol reaches 85.5%. However, 25ppm of Fe and Zn compounds were detected in the reaction product, indicating that the catalytic reaction was accompanied by a loss of the metal active sites. Chinese patent CN106824182A discloses a modified Ir-Ru/C catalyst, which takes isopropanol as a solvent, and the conversion rate of citral reaches 85.4 percent and the total selectivity of nerol and geraniol reaches 83.9 percent after the reaction is carried out for 2 hours at 80 ℃ and under 1.5Mpa of hydrogen. However, nitric acid and sodium hydroxide are used in the preparation process of the catalyst, the synthesis steps are complicated, and the catalyst needs hydrogen for reduction. Chinese patent CN1422835 discloses a Ru/Fe-C suspension catalyst, which is continuously carried out at 80 ℃ under 2.2MPa of hydrogen, and the total selectivity of nerol and geraniol can reach more than 90%. However, the catalyst is difficult to recycle quickly after use, and the catalyst can be separated only by adopting multiple steps of sedimentation, centrifugation, cross-flow filtration and the like, and the hydrogen reduction is also needed. Chinese patent CN107866249A discloses a non-noble metal molybdenum carbide catalyst, which reacts for 5h in a high-pressure kettle at 100 ℃ and 3MPa of hydrogen, the conversion rate of citral reaches 89%, and the total selectivity of nerol and geraniol is 37%. However, the reaction conditions are severe and the yield is significantly lower than that of the noble metal catalyst. It can be seen that it is very necessary to develop a catalyst with simple synthesis method, stability and high efficiency.

Disclosure of Invention

The invention aims to provide a preparation method of a carbon-coated cobalt-silver alloy catalyst, which is characterized in that a catalyst precursor is simply and quickly obtained through a one-pot method, and then the mesoporous carbon layer-coated limited cobalt-silver alloy catalyst is obtained through high-temperature pyrolysis in-situ reduction, and has magnetism and is easy to recycle.

The technical scheme provided by the invention is as follows:

a preparation method of a carbon-coated cobalt-silver alloy catalyst comprises the following steps:

s1, weighing a proper amount of organic ligand 2, 5-dihydroxyterephthalic acid, metal salt cobalt nitrate hexahydrate and silver nitrate, dissolving the organic ligand, the metal salt cobalt nitrate hexahydrate and the silver nitrate into a mixed solution of water, ethanol and N, N-dimethylformamide, and uniformly stirring, wherein the volume ratio of the water to the ethanol to the N, N-dimethylformamide is 1;

s2, pouring the mixed solution into a hydrothermal reaction kettle containing a polytetrafluoroethylene lining, and placing the hydrothermal reaction kettle in an electric heating constant-temperature air blast drying box for crystallization;

s3, taking out the crystallized mixture, sequentially centrifugally washing the crystallized mixture by using N, N-dimethylformamide and methanol, drying the washed mixture at room temperature, and taking out the dried mixture to obtain a metal-organic framework compound precursor;

and S4, adding the metal-organic framework compound precursor into a quartz tube, and calcining in the nitrogen atmosphere to obtain the mesoporous carbon coated cobalt-silver alloy catalyst.

Further preferably, in the step S1, the molar ratio of the organic ligand 2, 5-dihydroxyterephthalic acid to the metal salt cobalt nitrate hexahydrate is 1 to 10, and the silver nitrate is 1 to 20% of the total mass of the organic ligand 2, 5-dihydroxyterephthalic acid and the metal salt cobalt nitrate hexahydrate.

Further preferably, in the step S2, pouring the mixed solution into a hydrothermal reaction kettle containing a polytetrafluoroethylene lining, putting the hydrothermal reaction kettle into an electric heating constant temperature air blowing drying oven, heating to 100 to 140 ℃, and crystallizing for 12 to 36h.

Further preferably, in step S4, the calcination conditions are: the calcination temperature is 300 to 600 ℃, the heating rate is 1 to 10 ℃/min, the calcination time is 2 to 10h, and the gas flow rate is 20 to 100mL/min.

The invention also provides application of the carbon-coated cobalt-silver alloy catalyst in selective hydrogenation reaction of citral.

The present invention also provides a method for preparing unsaturated alcohols using selective hydrogenation of citral, comprising:

s5, weighing a certain amount of the catalyst prepared by the preparation method of any one of claims 1 to 4, adding the catalyst into the lining of a high-pressure hydrogenation reaction kettle, sequentially adding a solvent ethanol, a reactant citral and an internal standard substance n-dodecane, uniformly mixing and stirring, and then taking a small amount of sample to pass through an oil film to prepare a blank sample;

s6, packaging the high-pressure hydrogenation reaction kettle, and introducing hydrogen to remove air in the kettle;

s7, introducing pure hydrogen with the pressure of 0.5-5 MPa, setting the reaction temperature to be 60-120 ℃, the reaction time to be 30-120min and the citral concentration in the reactants to be 1-100mmol/L, and starting mechanical stirring;

and S8, after the reaction is finished, removing hydrogen, taking out a sample, and making the sample through an oil film to obtain a reacted sample, thereby completing the preparation of the unsaturated alcohol.

In the preparation method of the carbon-coated cobalt-silver alloy catalyst, the metal-organic framework compound is used as a precursor to prepare the high-dispersion and high-stability metal active centerA good carbon-coated cobalt-silver alloy catalyst is prepared by firstly combining silver nitrate, cobalt nitrate hexahydrate and organic ligand 2, 5-dihydroxy terephthalic acid by a one-pot method to obtain a metal-organic framework compound precursor AgNO ₃ The catalyst is prepared by the following steps of @ Co-MOF, and then roasting under an inert atmosphere to obtain the xAg-Co @ C-T catalyst with different silver contents, wherein x represents the weight percentage of Ag in the catalyst, and T represents the roasting temperature.

The catalyst prepared by taking MOFs as a precursor can well disperse metal particles in a carrier (nano metal active centers in the catalyst are uniformly dispersed and have no agglomeration phenomenon); meanwhile, a carbon layer formed in the process of the MOFs pyrolysis can reduce most of metal into a simple substance, so that hydrogen reduction is not needed, and the process is simple and convenient; furthermore, the catalyst has good stability and magnetism and can be recycled for multiple times, the mesoporous carbon layer completely wraps the metal simple substance, the growth of metal particles is limited, the loss of metal active centers in the reaction process can be prevented, and the catalyst can be separated from the reaction through a simple magnet. The cheap noble metal silver is combined with the transition metal cobalt, the conversion rate of the citral can reach 99 percent under mild conditions in the selective hydrogenation reaction of the citral, the selectivity of the nerol and the geraniol reaches 67 percent, and the effect of the catalyst is equivalent to that of the common noble metal Ru, pd and Pt catalysts; after the catalyst is applied to the citral hydrogenation reaction and recycled for 5 times, the catalytic activity and selectivity are still not changed, and the catalyst has excellent cycle performance.

Drawings

The foregoing features, technical features, advantages and embodiments are further described in the following detailed description of the preferred embodiments, which is to be read in connection with the accompanying drawings.

Fig. 1 is a schematic flow chart of a preparation method of a carbon-coated cobalt-silver alloy catalyst in the invention.

Detailed Description

The essence of the invention is further illustrated below with reference to the figures and examples, but the invention is not limited thereto.

As shown in fig. 1, which is a schematic flow chart of a preparation method of a carbon-coated cobalt-silver alloy catalyst provided by the present invention, it can be seen from the figure that the preparation method comprises:

s1, weighing a proper amount of organic ligand 2, 5-dihydroxyterephthalic acid, metal salt cobalt nitrate hexahydrate and silver nitrate, dissolving the organic ligand, the metal salt cobalt nitrate hexahydrate and the silver nitrate in a mixed solution of water, ethanol and N, N-Dimethylformamide (DMF), and uniformly stirring to obtain a pink solution, wherein the volume ratio of the water to the ethanol to the N, N-dimethylformamide is 1; the molar ratio of the organic ligand 2, 5-dihydroxyterephthalic acid to the metal salt cobalt nitrate hexahydrate is 1-10, and the silver nitrate accounts for 1-20% of the total mass of the organic ligand 2, 5-dihydroxyterephthalic acid and the metal salt cobalt nitrate hexahydrate.

S2, pouring the mixed solution into a hydrothermal reaction kettle containing a polytetrafluoroethylene lining, and placing the hydrothermal reaction kettle in an electric heating constant-temperature air blast drying box for crystallization; specifically, the mixture is heated to 100 to 140 ℃ in an electric heating constant temperature air drying oven and crystallized for 12 to 36h.

And S3, taking out the crystallized mixture, sequentially centrifuging and washing the mixture by using DMF and methanol, drying the mixture at room temperature, and taking out the dried mixture to obtain the metal-organic framework compound precursor. For better results, 3 washes with N, N-dimethylformamide and methanol in succession are generally used.

And S4, adding the metal-organic framework compound precursor into a quartz tube, and calcining in the nitrogen atmosphere to obtain the mesoporous carbon coated cobalt-silver alloy catalyst. Specifically, the calcination conditions are as follows: the calcination temperature is 300 to 600 ℃, the heating rate is 1 to 10 ℃/min, the calcination time is 2 to 10h, and the gas flow rate is 20 to 100mL/min.

The application of the catalyst prepared by the method in the selective hydrogenation of citral comprises the following steps:

s5, weighing a certain amount of the catalyst prepared by the preparation method, adding the catalyst into the lining of the high-pressure hydrogenation reaction kettle, sequentially adding a solvent ethanol, a reactant citral and an internal standard substance n-dodecane, uniformly mixing and stirring, and then taking a small amount of sample to pass through an oil film to prepare a blank sample. Here, the mass of the catalyst is 40mg, ethanol is used as a solvent, and the molar ratio of the reactant citral to the internal standard n-dodecane is 5.

S6, packaging the high-pressure hydrogenation reaction kettle, and introducing hydrogen to remove air in the kettle; specifically, 1MPa of hydrogen was introduced and then discharged, and the reaction was repeated five times to remove the air in the autoclave.

s8, after the reaction is finished, removing hydrogen, taking out a sample, making the sample through an oil film to obtain a sample after the reaction, and detecting the change of substances in the sample before and after the reaction through gas chromatography.

The invention is described in further detail below by way of examples:

example 1:

1) Synthesizing a catalyst precursor by a hydrothermal method: dissolving 0.89g of 2, 5-dihydroxyterephthalic acid and 3.96g of cobalt nitrate hexahydrate in 300mL of a mixed solution of water, ethanol and DMF (the volume ratio of the water, the ethanol and the DMF is 1; then 0.24g of silver nitrate solid (5% silver by mass) is added, and stirring is continued until the silver nitrate is completely dissolved and the solution is pink emulsion. And then, equally dividing the uniformly mixed solution into four parts (in the practical application, the dosage is amplified by 4 times in the example because a trace amount of silver nitrate solid is difficult to accurately weigh), sequentially adding the four parts into 4100 mL hydrothermal reaction kettles containing polytetrafluoroethylene linings, packaging, putting the hydrothermal reaction kettles into an electrothermal constant-temperature air-blast drying oven, and crystallizing for 24 hours at 120 ℃. And after the reaction is finished, naturally cooling to room temperature, taking out the inner liner of the reaction kettle, washing the catalyst precursor with DMF (dimethyl formamide) and methanol for three times respectively, centrifugally separating, and drying in a vacuum drying oven at 60 ℃ overnight to obtain the catalyst precursor of 5Ag @ Co-MOF.

2) Adding the prepared catalyst precursor into a quartz tube, introducing nitrogen for 30min in advance to exhaust the air in the tube, then heating from room temperature of 25 ℃ in the atmosphere of 50mL/min of nitrogen flow rate, heating to 500 ℃ at 5 ℃/min, keeping for 360min, then naturally cooling to prepare the catalyst, and numbering the catalyst as 5Ag/Co @ C-500.

3) Weighing 40mg of catalyst into a polytetrafluoroethylene lining of a 50mL high-pressure reaction kettle, sequentially adding 300uL of citral as a reactant, 100uL of dodecane as an internal standard substance and 15mL of ethanol as a solvent, uniformly mixing, and taking a small amount of oil film to prepare a blank sample before reaction. Then packaging the high-pressure reaction kettle, introducing 1MPa hydrogen, then discharging, and repeating for five times to remove air in the kettle. And finally, injecting 1MPa pure hydrogen into the reaction kettle, setting the reaction temperature to be 80 ℃, setting the reaction time to be 60min, and starting mechanical stirring. And taking out the reacted sample after the reaction is finished, and preparing the reacted sample through an oil film.

4) Samples before and after the reaction were subjected to the measurement by Agilent GC-7820A gas chromatography, wherein the column was HP-5 (30 m.times.0.25 mm.times.0.25 um), the detector was a FID hydrogen ion flame detector, and the reaction results are shown in Table 1.

Example 2:

1) The same method is adopted to synthesize catalyst precursors with different silver contents, the synthesis steps are the same as the step (1) in the example 1, but the adding amount of silver nitrate is 0.48g, and the prepared catalyst precursor is 9Ag @ Co-MOF.

2) The procedure for preparing catalyst 9Ag/Co @ C-500 was the same as in step (2) of example 1.

3) The process of applying the catalyst to selective hydrogenation of citral to prepare nerol and geraniol is the same as in step (3) of example 1.

4) The samples before and after the reaction were tested in the same manner as in step (4) of example 1.

Example 3:

1) The same method is adopted to synthesize catalyst precursors with different silver contents, the synthesis steps are the same as the step (1) in the example 1, but the adding amount of silver nitrate is 0.05g, and the prepared catalyst precursor is 1Ag @ Co-MOF.

2) The procedure for preparing catalyst 1Ag/Co @ C-500 is the same as in step (2) of example 1.

Example 4:

1) The same method was used to prepare a silver-free catalyst precursor, and the synthesis procedure was the same as in (1) of example 1, except that silver nitrate was not added, and the catalyst precursor obtained was Co-MOF.

2) The procedure for preparing the catalyst Co @ C-500 was as in step (2) of example 1.

3) The application of the catalyst in the selective hydrogenation of citral to prepare nerol and geraniol is the same as in step (3) of example 1.

Example 5:

1) The same method was used to prepare a catalyst precursor of 5Ag @ Co-MOF, the synthesis procedure was the same as in (1) of example 1.

2) The procedure for preparing the catalyst 5Ag/Co @ C-300 was the same as in step (2) of example 1, except that the calcination temperature was changed to 300 ℃.

Example 6:

1) The same method is adopted to prepare a catalyst precursor of 5Ag @ Co-MOF, and the synthesis steps are the same as the step (1) in the example 1.

2) The procedure for preparing the catalyst 5Ag/Co @ C-400 was the same as in step (2) of example 1, except that the calcination temperature was changed to 400 ℃.

Example 7:

2) The procedure for preparing the catalyst 5Ag/Co @ C-600 was the same as in step (2) of example 1, except that the calcination temperature was changed to 600 ℃.

Example 8:

2) The procedure for preparing the catalyst 5Ag/Co @ C-600 was the same as in step (2) of example 1, except that the calcination rate was changed to 1 deg.C/min.

Example 9:

2) The procedure for preparing the catalyst 5Ag/Co @ C-600 was the same as in step (2) of example 1, except that the calcination rate was changed to 10 deg.C/min.

Example 10:

2) The procedure for preparing catalyst 5Ag/Co @ C-500 was the same as in step (2) of example 1, except that the calcination time was changed to 2 hours.

Example 11:

1) The same method was used to prepare the catalyst precursor 5Ag @ Co-MOF. The synthesis procedure was the same as in (1) of example 1.

2) The procedure for preparing catalyst 5Ag/Co @ C-500 was the same as in step (2) of example 1, except that the calcination time was changed to 10 hours.

Example 12:

2) The procedure for preparing the catalyst 5Ag/Co @ C-500 was as in step (2) of example 1, except that the gas flow rate of nitrogen was changed to 20mL/min.

Example 13:

2) The procedure for preparing the catalyst 5Ag/Co @ C-500 was as in step (2) of example 1, except that the gas flow rate of nitrogen was changed to 100mL/min.

Example 14:

1) The catalyst precursor 5Ag @ Co-MOF was prepared by the same method, and the synthesis procedure was the same as in (1) of example 1, except that the hydrothermal synthesis temperature was changed to 100 ℃.

2) The procedure for preparing catalyst 5Ag/Co @ C-500 is the same as in step (2) of example 1.

Example 15:

1) The catalyst precursor 5Ag @ Co-MOF was prepared by the same method as in (1) of example 1, except that the hydrothermal synthesis temperature was changed to 140 ℃.

2) The procedure for preparing the catalyst 5Ag/Co @ C-500 is as in step (2) of example 1.

Example 16:

1) The catalyst precursor 5Ag @ Co-MOF was prepared by the same method as in (1) of example 1, except that the crystallization time was changed to 12 hours.

Example 17:

1) The same method is adopted to prepare a catalyst precursor of 5Ag @ Co-MOF, the synthesis steps are the same as the step (1) in the example 1, but the crystallization time is changed to 36h.

Table 1: the catalysts prepared in examples 1 to 17 have catalytic performance for selective hydrogenation of citral:

it should be noted that the above embodiments can be freely combined as necessary. The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a carbon-coated cobalt-silver alloy catalyst is characterized by comprising the following steps:

s1, weighing a proper amount of organic ligand 2, 5-dihydroxyterephthalic acid, metal salt cobalt nitrate hexahydrate and silver nitrate, dissolving in a mixed solution of water, ethanol and N, N-dimethylformamide, and uniformly stirring, wherein the volume ratio of the water to the ethanol to the N, N-dimethylformamide is 1;

s3, taking out the crystallized mixture, sequentially carrying out centrifugal washing by using N, N-dimethylformamide and methanol, drying at room temperature, and taking out to obtain a metal-organic framework compound precursor;

s4, adding the metal-organic framework compound precursor into a quartz tube, and calcining in the nitrogen atmosphere to obtain a mesoporous carbon coated cobalt-silver alloy catalyst;

in the step S1, the molar ratio of the organic ligand 2, 5-dihydroxyterephthalic acid to the metal salt cobalt nitrate hexahydrate is 1-10, and the silver nitrate accounts for 1-20% of the total mass of the organic ligand 2, 5-dihydroxyterephthalic acid and the metal salt cobalt nitrate hexahydrate;

in the step S2, the mixed solution is poured into a hydrothermal reaction kettle containing a polytetrafluoroethylene lining, and the hydrothermal reaction kettle is placed in an electric heating constant-temperature air blast drying oven to be heated to 100-140 ℃, and crystallized for 12-36 hours;

in step S4, the conditions of calcination are: the calcination temperature is between 300 and 600 ℃, the heating rate is between 1 and 10 ℃/min, the calcination time is between 2 and 10 hours, and the gas flow rate is between 20 and 100mL/min.

2. The use of the cobalt-silver alloy catalyst prepared by the method of claim 1 in the selective hydrogenation of citral.

3. A method for producing unsaturated alcohols using selective hydrogenation of citral, comprising:

s5, weighing a certain amount of the catalyst prepared by the preparation method of claim 1, adding the catalyst into the lining of a high-pressure hydrogenation reaction kettle, sequentially adding a solvent ethanol, a reactant citral and an internal standard substance n-dodecane, uniformly mixing and stirring, taking a small amount of sample, and preparing a blank sample through an oil film;

s7, introducing pure hydrogen with the pressure of 0.5-5 MPa, setting the reaction temperature to be 60-120 ℃, the reaction time to be 30-120 min and the concentration of citral in reactants to be 1-100 mmol/L, and starting mechanical stirring;