CN113198520B - One-pot preparation method of molecular sieve supported palladium carbon catalyst and application of molecular sieve supported palladium carbon catalyst in synthesis of dimethyl carbonate by gas phase method - Google Patents
One-pot preparation method of molecular sieve supported palladium carbon catalyst and application of molecular sieve supported palladium carbon catalyst in synthesis of dimethyl carbonate by gas phase method Download PDFInfo
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Abstract
The invention belongs to the technical field of catalyst preparation, and particularly relates to one-pot preparation of a molecular sieve supported palladium carbon catalyst and application of the molecular sieve supported palladium carbon catalyst in synthesis of dimethyl carbonate by a gas phase method. The catalyst utilizes a cheap organic polydentate ligand and a palladium complex to encapsulate palladium into a molecular sieve by a one-pot method, and then the molecular sieve supported palladium-carbon catalyst is prepared by directly taking the organic polydentate ligand as a carbon source through in-situ carbonization, wherein the mass fraction of palladium in the catalyst is 0.1-2.5%, and the mass fraction of carbon in the catalyst is 0.01-1.5%. The catalyst is applied to the reaction of synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite in a low-pressure gas phase method, solves the problems of equipment corrosion and easy inactivation caused by the traditional chlorine-containing catalyst, and has the advantages of high stability, high selectivity, high conversion rate and no reaction
Acid, chlorine-free high-performance catalyst, has certain industrial application prospect.
Description
Technical Field
The invention belongs to the technical field of catalyst preparation, and particularly relates to one-pot preparation of a molecular sieve supported palladium carbon catalyst and application of the molecular sieve supported palladium carbon catalyst in synthesis of dimethyl carbonate by a gas phase method.
Background
Dimethyl carbonate (DMC) is an environmentally-friendly chemical raw material with wide application, has been listed as a non-toxic chemical by various countries in Europe in the early 90 s, and is one of 50 fine chemicals which are intensively developed in the nine-five period of China. As an important organic chemical intermediate, the product can be used for producing products such as polycarbonate, medicines, pesticides and the like, replaces phosgene, halogenated methane and dimethyl sulfate to be used as carbonylation and methylation reagents, is used as a solvent in lithium battery electrolyte and paint coating industries (accounting for more than 50 percent of the total DMC consumption in China), and is expected to replace toxic methyl tert-butyl ether (MTBE) to be used as a gasoline and diesel additive. According to statistical data, the market demand of DMC in China is exponentially growing from 2007, and the demand of DMC in 2019 reaches 81 ten thousand tons per year. Therefore, DMC has huge potential market and wide application prospect.
The current methods for synthesizing dimethyl carbonate can be mainly divided into: phosgene method, oxidative carbonylation method, nitrite carbonylation method, ester exchange method, methanol/carbon dioxide one-step synthesis method and urea alcoholysis method. The phosgene method is being gradually eliminated due to the defects of high toxicity of raw materials, poor safety, serious environmental pollution and the like; although the ester exchange method has the conditions of simple operation, mild reaction conditions and the like, the separation and purification of the target product are relatively difficult, and the cost is high; the problem of difficult subsequent separation is also encountered by the urea alcoholysis method due to the use of a homogeneous catalyst; although the one-step synthesis method of methanol and carbon dioxide has the advantage of simple process method, the catalyst developed at the present stage has low catalytic activity because the carbon dioxide in the raw material is not easy to be activated. The methanol conversion rate of the one-step synthesis method of methanol and carbon dioxide reported in Chinese patent CN110479287A is only 11.2%, the selectivity of dimethyl carbonate is 75.6%, and the yield is only 8.5%.
In contrast, the process for synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite through a low-pressure gas phase method has attracted extensive attention due to the advantages of no pollution, environmental protection, no subsequent separation and the like in the production process. The catalyst for synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite through low-pressure gas phase method mainly includes chloric Pd-Cu/oxide and chloric Pd/molecular sieve catalyst system. Chlorine-containing catalysts require the continuous addition of chlorine (e.g., 100ppm HCl) to the feed, which can lead to severe corrosion of equipment, poor DMC product purity, and the like. For example, U.S. Pat. No. 5,5688984 discloses a spinel supported catalyst for dimethyl carbonate synthesis, which requires the addition of chlorine-supplementing agent hydrogen chloride or methyl chloroformate, which causes corrosion of equipment and is expensive. Therefore, the development of chlorine-free catalysts for the synthesis of dimethyl carbonate is a future development trend. Chinese patent application publication No. CN106423289A discloses a catalyst for synthesizing dimethyl carbonate and a preparation method thereof, copper and potassium are used as an auxiliary agent of a molecular sieve supported Pd catalyst Pd/molecular sieve, the space-time yield is 690g/(L h), while the selectivity of dimethyl carbonate based on methyl nitrite is only 45% -51%, and the catalyst has low selectivity. . Yamamoto et al, Japan, manufactured a molecular sieve-supported Pd catalyst having a selectivity of dimethyl carbonate based on methyl nitrite of 75%, but a CO conversion rate decreased even to 75% of the initial conversion rate after 150 hours of operation, and found that the molecular sieve-supported Pd catalyst manufactured by this company had poor stability (Catalysis and Catalysis of Pd/NaY for dimethyl carbonate synthesis from methyl nitrate and CO, Yamamoto et al, J.chem.Soc.Farady train, 1997, Vol. 93, p. 3721). In conclusion, the introduction of chlorine in the chlorine-containing catalyst can cause the problems of serious corrosion of equipment, low purity of DMC products and the like, and the chlorine-free Pd/molecular sieve catalyst has low conversion rate and selectivity and poor stability and still has a larger promotion space.
In the preparation of a chlorine-free Pd/molecular sieve catalyst, the Pd/NaY catalyst prepared by Dong et al by using a conventional ion exchange method has a certain catalytic activity in the reaction of synthesizing DMC by a methyl nitrite carbonylation method, but the prepared Pd/NaY catalyst contains a lot of Pd/NaY catalyst
Acids (Synthesis of Dimethyl Carbonate through Vapor-Phase Pd-Doped Zeolites: Interaction of Lewis Acidic Sites and Pd specifices, Yuanyan Dong et al, ChemCatchem, 2013, Vol. 5, p. 2174), while in Pd/NaY catalysts
The acid causes the decomposition of methyl nitrite as a reactant to produce methyl formate, dimethoxymethane and methanol as by-products, which greatly reduces the selectivity of DMC as a main product (Catalysis and Catalysis of Pd/NaY for dimethyl carbonate synthesis from methyl nitrate and CO, Yamamoto Y et al, J.chem.Soc., Faraday trains., 1997, Vol. 93, p. 3721), and thus it can be seen that Pd/NaY catalysts prepared by conventional ion exchange method contain much more Pd/NaY catalyst
Acid, which causes a large amount of decomposition of the raw material methyl nitrite, resulting in a low selectivity for dimethyl carbonate.
The preparation of carbide usually uses gas or organic compound as carbon source, when gas is used as carbon source, the required carbonization temperature is higher, metal sintering is easily caused while carbonization is carried out, organic compound is used as carbon source, metal is firstly coordinated with compound, carbonization causes rearrangement of metal and organic group formation of carbon species, the process is easy to control and uniform, and the carbide is easily formed when metal is closely connected with carbon atom. The method is mainly used for directly synthesizing carbide, and is rarely reported in the aspect of preparing molecular sieve catalysts, probably because organic compounds are difficult to enter molecular sieve channels, for example, the pore opening of an FAU type molecular sieve is only 0.74 nanometer, and the size of the organic compounds is often larger, so that the organic compounds are difficult to enter.
Disclosure of Invention
In view of the above problems in the prior art, the present invention aims to provide a catalyst with high stability, high selectivity, high conversion rate and no toxicity
The preparation method and application of the molecular sieve supported palladium carbon catalyst with adjustable acid, chlorine-free and Pd nano-particle sizes, in particular to a high-performance catalyst for the reaction of synthesizing dimethyl carbonate by using a low-pressure gas phase method of carbon monoxide and methyl nitrite.
In order to achieve the purpose, the invention adopts the following technical scheme:
a one-pot method preparation of a molecular sieve supported palladium carbon catalyst and application thereof in synthesizing dimethyl carbonate by a gas phase method are characterized in that: packaging palladium into a molecular sieve by using a cheap organic polydentate ligand and palladium complex in a one-pot method, and then directly using the organic polydentate ligand as a carbon source to prepare a molecular sieve-supported palladium-carbon catalyst through in-situ carbonization, wherein the mass fraction of palladium in the catalyst is 0.1-2.5%, the mass fraction of carbon in the catalyst is 0.01-1.5%, the type of the molecular sieve is FAU type, the average particle size of the molecular sieve is 0.1-4 micrometers, and the pore volume of the molecular sieve is 0.21-0.37 cm3(ii)/g, the specific surface area of the molecular sieve is 750-950 m2/g。
A one-pot method for preparing a molecular sieve supported palladium carbon catalyst comprises the following steps:
1) mixing silicon source, aluminum source, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.05-0.2: 0.005-0.02: 10-30 in a molar ratio, stirring for 1-3 hours, transferring the obtained solution into a hydrothermal reaction kettle, carrying out hydrothermal pre-crystallization for 6-24 hours at the temperature of 25-75 ℃, and then raising the temperature to 90-110 ℃ for hydrothermal crystallization for 24-72 hours to obtain a solution containing a molecular sieve structure directing agent;
2) mixing a palladium precursor and an organic polydentate ligand according to the molar ratio of Pd to the organic polydentate ligand of 1: 3-65, and performing ultrasonic dispersion to obtain a clear palladium complex solution;
3) mixing silicon source, aluminum source, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.01-0.05: 0.08-0.12: 10-30 in a molar ratio, and adding a certain amount of the mixture1) Stirring the obtained solution containing the molecular sieve structure directing agent for 1-3 hours, then adding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution into a reaction kettle, carrying out hydrothermal crystallization at 80-120 ℃ for 24-168 hours, filtering and washing the obtained sample, and drying at 25-100 ℃ for 1-48 hours to obtain the molecular sieve loaded with the palladium complex;
4) placing the molecular sieve loaded with the palladium complex obtained in the step 3) in flowing gas, raising the temperature from room temperature to 120-600 ℃ according to a certain heating rate, keeping the temperature for 0.5-6 hours, cooling to room temperature, and taking out to obtain the molecular sieve loaded palladium carbon catalyst.
As a preferable scheme, the aluminum source in step 1) and step 3) is one or a combination of more of aluminum powder, aluminum sulfate, aluminum hydroxide, aluminum triisopropoxide and sodium metaaluminate, and the silicon source in step 1) and step 3) is one or a combination of more of ethyl orthosilicate, butyl orthosilicate, methyl orthosilicate, white carbon black, silica gel, silica sol and water glass.
Preferably, the palladium precursor in step 2) is one or a combination of more of palladium nitrate, palladium acetate, palladium chloride, ammonium chloropalladate, potassium chloropalladate, tetraammine palladium chloride and tetraammine palladium nitrate, and the organic polydentate ligand in step 2) is one or a combination of more of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, N- [3- (trimethoxysilyl) propyl ] ethylenediamine, tetraethylammonium hydroxide, tetrabutylammonium hydroxide and tetrapropylammonium hydroxide.
Preferably, the flowing gas in the step 4) is one or a combination of several of nitrogen, argon, helium, ammonia, carbon dioxide, carbon monoxide, hydrogen, methane and ethylene, and the temperature rise rate is 0.1-2.0 ℃/min.
The molecular sieve supported palladium carbon catalyst is applied to the reaction of synthesizing dimethyl carbonate by using a low-pressure gas phase method of carbon monoxide and methyl nitrite.
Compared with the prior art, the one-pot preparation method of the molecular sieve supported palladium carbon catalyst and the application of the molecular sieve supported palladium carbon catalyst in the synthesis of dimethyl carbonate by a gas phase method have the following remarkable characteristics:
(1) with Pd (NH)3)4 2+The preparation of Pd/FAU molecular sieve for Pd precursor by ion exchange method is a common catalyst for synthesizing dimethyl carbonate by gas phase method, however, NH generated from ammonia water in the preparation process of the catalyst4 +With Na+Is easy to generate ion exchange, and forms more in the molecular sieve by roasting
Acids of these
The acid can cause the ineffective decomposition of methyl nitrite as a reactant, thereby greatly reducing the selectivity of dimethyl carbonate as a main product. The invention utilizes the cheap organic polydentate ligand and the complex of palladium to encapsulate the palladium into the molecular sieve by a one-pot method, thereby avoiding the generation of the complex in the traditional ion exchange process
Acid, the prepared catalyst shows better catalytic performance to the synthetic dimethyl carbonate, and the invention greatly reduces
The generation of acid is an advance of the present invention.
(2) The FAU molecular sieve is synthesized under a strong alkaline condition, but a conventional palladium precursor is difficult to stably exist in a strong alkaline solution, the palladium can be stabilized in the strong alkaline solution by the organic polydentate ligand and the palladium complex, and the selection of the appropriate organic polydentate ligand is a key control factor.
(3) Although carbonization by using a metal organic complex is a common method for preparing carbide, the conventional organic matter is difficult to enter the pore channel of the molecular sieve when being used as a carbon source, for example, the pore opening of the FAU type molecular sieve is only 0.74 nm, and the size of the organic matter is often large, so that the organic matter is difficult to enter; therefore, the method has the advantages that the palladium complex encapsulated by the molecular sieve is directly utilized for in-situ carbonization, so that the problems of extra carbon source, complicated steps and the like introduced in the traditional carbonization process are solved.
(4) The heating rate and the roasting carbonization temperature in the catalyst carbonization process are important factors for controlling the size distribution of the Pd particles, when the heating rate is too high and the roasting temperature is too high, the Pd is easy to sinter to form larger Pd particles without catalytic activity, and when the heating rate is too high and the roasting temperature is too low, the Pd complex compound is difficult to completely decompose, so that the pore passages of the molecular sieve are blocked, and the catalytic activity of the catalyst is reduced. Therefore, the invention can effectively regulate and control the interaction between the carbon species and the palladium species by controlling the factors such as the heating rate of the in-situ carbonization process, the composition of flowing gas and the like, in the process of forming the Pd cluster, part of carbon atoms tightly connected with the Pd cluster can automatically fall on the sub-surface (sub-surface carbon) of Pd which is the lowest energy position, meanwhile, the other part of carbon species can cover the surface of the Pd cluster or remain in the pore canal of the molecular sieve, and the carbon species can effectively prevent Pd from migrating to the edge of the molecular sieve particles, effectively prevent the sintering of palladium and regulate the valence state of the palladium species. The invention has the beneficial effects that: the invention utilizes the cheap organic polydentate ligand and the complex of palladium to encapsulate the palladium into the molecular sieve by a one-pot method, thereby avoiding the generation of the complex in the traditional ion exchange process
The palladium precursor is stabilized in a strong alkaline solution by adopting an organic polydentate ligand and a palladium complex, the palladium complex packaged by a molecular sieve is directly used for in-situ carbonization to prepare the palladium-carbon catalyst, and the interaction between a carbon species and a palladium species can be effectively regulated and controlled by controlling the factors such as the heating rate of the in-situ carbonization process, the composition of flowing gas and the like, so that the sintering of palladium is effectively prevented, and the valence state of the palladium species is regulated. The prepared palladium-carbon catalyst shows excellent catalytic performance in the reaction of synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite through a low-pressure gas phase method, and CO is converted intoThe conversion rate is more than 90%, the selectivity of dimethyl carbonate based on methyl nitrite is more than 85%, and the stable operation can be carried out for more than 300 hours.
The molecular sieve supported palladium-carbon catalyst prepared by the invention has high stability, high selectivity, high conversion rate and no reaction
An acid and chlorine-free catalyst, in particular to a high-performance catalyst for the reaction of synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite through a low-pressure gas phase method.
Drawings
FIG. 1 is an SEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 1;
FIG. 2 is an XRD pattern of a molecular sieve supported palladium on carbon catalyst prepared in example 1;
FIG. 3 is a TEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 1;
FIG. 4 is an SEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 2;
FIG. 5 is an XRD pattern of a molecular sieve supported palladium on carbon catalyst prepared in example 2;
FIG. 6 is a TEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 2;
FIG. 7 is an SEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 3;
FIG. 8 is an XRD pattern of a molecular sieve supported palladium on carbon catalyst prepared in example 3;
FIG. 9 is a TEM image of the molecular sieve-supported palladium on carbon catalyst prepared in example 3;
FIG. 10 is an SEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 4;
FIG. 11 is an XRD pattern of a molecular sieve supported palladium on carbon catalyst prepared in example 4;
FIG. 12 is a TEM image of a molecular sieve-supported palladium on carbon catalyst prepared in example 4;
FIG. 13 is an SEM image of a molecular sieve supported palladium on carbon catalyst prepared in example 5;
FIG. 14 is an XRD pattern of a molecular sieve supported palladium on carbon catalyst prepared in example 5;
FIG. 15 is a TEM image of the molecular sieve-supported palladium on carbon catalyst prepared in example 5;
FIG. 16 is an XRD pattern of the molecular sieve supported palladium on carbon catalyst prepared in comparative example 1;
FIG. 17 is a TEM image of the molecular sieve-supported palladium on carbon catalyst prepared in comparative example 1;
FIG. 18 is an XRD pattern of the molecular sieve supported palladium on carbon catalyst prepared in comparative example 2;
FIG. 19 is a TEM image of the molecular sieve-supported palladium on carbon catalyst prepared in comparative example 2;
FIG. 20 is a graph showing the stability of the molecular sieve-supported palladium on carbon catalyst prepared in example 3;
fig. 21 is a pyridine adsorption infrared diagram of the molecular sieve supported palladium on carbon catalysts prepared in example 3 and comparative example 1.
Detailed Description
For the purpose of facilitating an understanding of the contents of the present invention, the present invention will now be described in detail with reference to the following examples. The examples are only for the understanding of the present invention and should not be construed as specifically limiting the present invention. Since the present invention may be described and illustrated in other embodiments without departing from the technical features of the present invention, all changes that come within the scope of the invention or the range of equivalents thereof are intended to be embraced therein.
The invention is further illustrated below with reference to examples, comparative examples and application examples.
Example 1
1) Mixing ethyl orthosilicate, aluminum powder, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O with the molar ratio of 1:0.06:0.008:12, stirring for 1.5 hours, transferring the obtained solution into a hydrothermal reaction kettle, carrying out hydrothermal pre-crystallization at the temperature of 30 ℃ for 12 hours, and then raising the temperature to 95 ℃ for hydrothermal crystallization for 28 hours to obtain a solution containing a molecular sieve structure directing agent;
2) mixing potassium chloropalladate and diethylenetriamine according to the molar ratio of Pd to diethylenetriamine of 1:32, and performing ultrasonic dispersion to obtain a clear palladium complex solution;
3) mixing ethyl orthosilicate, aluminum powder, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.02:0.08:13 in a molar ratio, adding a certain amount of the solution containing the molecular sieve structure directing agent obtained in the step 1), stirring for 1 hour, adding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution into a reaction kettle, performing hydrothermal crystallization at 85 ℃ for 48 hours, filtering and washing the obtained sample, and drying at 30 ℃ for 40 hours to obtain the molecular sieve loaded with the palladium complex;
4) placing the molecular sieve of the supported palladium complex obtained in the step 3) in flowing carbon dioxide gas, raising the temperature to 130 ℃ from room temperature according to the heating rate of 0.2 ℃/minute, keeping the temperature for 1 hour, cooling to room temperature, and taking out to obtain the molecular sieve supported palladium carbon catalyst.
Fig. 1 is an SEM image of the molecular sieve-supported palladium on carbon catalyst prepared in this example, and it can be seen that the average particle size of the molecular sieve is 1.0 μm. The pore volume of the catalyst was 0.22cm as measured by nitrogen adsorption desorption3Per g, specific surface area 760m2(ii) in terms of/g. Fig. 2 is an XRD pattern of the molecular sieve supported palladium on carbon catalyst prepared in this example, from which it can be seen that there is no diffraction peak of metallic palladium, indicating that Pd has a higher degree of dispersion. Measured by an inductively coupled plasma emission spectrometer, the mass content of Pd was 0.2%, and the carbon content was 1.5%. Fig. 3 is a TEM image of the molecular sieve supported palladium carbon catalyst prepared in this example after 12 hours of catalytic reaction, wherein the Pd particles are uniformly embedded in the molecular sieve lattice, and the average particle size of the Pd particles is 0.3 nm.
Example 2
1) Mixing n-butyl silicate, aluminum sulfate, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.1:0.01:15 molar ratio together, stirring for 3 hours, transferring the obtained solution into a hydrothermal reaction kettle,carrying out hydrothermal pre-crystallization for 24 hours at the temperature of 70 ℃, and then raising the temperature to 110 ℃ for carrying out hydrothermal crystallization for 64 hours to obtain a solution containing a molecular sieve structure guiding agent;
2) mixing palladium tetraammine nitrate and tetraethylenepentamine according to the molar ratio of Pd to tetraethylenepentamine of 1:4, and obtaining a clear palladium complex solution through ultrasonic dispersion;
3) mixing n-butyl silicate, aluminum sulfate, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.04:0.11:25 in a molar ratio, adding a certain amount of the solution containing the molecular sieve structure directing agent obtained in the step 1), stirring for 2.5 hours, adding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution into a reaction kettle, carrying out hydrothermal crystallization at 115 ℃ for 24 hours, filtering and washing the obtained sample, and drying at 90 ℃ for 48 hours to obtain the molecular sieve loaded with the palladium complex;
4) putting the molecular sieve of the supported palladium complex obtained in the step 3) into flowing carbon monoxide gas, raising the temperature to 550 ℃ from room temperature according to the heating rate of 1.0 ℃/minute, keeping the temperature for 5 hours, cooling to room temperature, and taking out the molecular sieve to obtain the molecular sieve supported palladium carbon catalyst.
Fig. 4 is an SEM image of the molecular sieve supported palladium on carbon catalyst prepared in this example, and it can be seen that the molecular sieve has an average particle size of 0.9 μm. The pore volume of the catalyst was 0.31cm as measured by nitrogen adsorption desorption3Per g, specific surface area 840m2(ii) in terms of/g. Fig. 5 is an XRD pattern of the molecular sieve supported palladium on carbon catalyst prepared in this example, from which it can be seen that there is no diffraction peak of metallic palladium, indicating that Pd has a higher degree of dispersion. Measured by an inductively coupled plasma emission spectrometer, the mass content of Pd was 2.2%, and the carbon content was 0.05%. FIG. 6 is a TEM image of the Pd on molecular sieve catalyst prepared in this example after 12 hours of catalytic reaction, wherein the Pd particles have an average diameter of 15.9 nm.
Example 3
1) Mixing water glass, aluminum triisopropoxide, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.2:0.015:30 in a molar ratio, stirring for 1.5 hours, transferring the obtained solution into a hydrothermal reaction kettle, carrying out hydrothermal pre-crystallization at the temperature of 50 ℃ for 18 hours, and then raising the temperature to 105 ℃ for hydrothermal crystallization for 50 hours to obtain a solution containing a molecular sieve structure directing agent;
2) mixing palladium acetate and tetraethyl ammonium hydroxide according to the molar ratio of Pd to tetraethyl ammonium hydroxide of 1:65, and performing ultrasonic dispersion to obtain a clear palladium complex solution;
3) mixing water glass, aluminum triisopropoxide, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.03:0.10:15 in a molar ratio, adding a certain amount of the solution containing the molecular sieve structure directing agent obtained in the step 1), stirring for 1.5 hours, adding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution into a reaction kettle, performing hydrothermal crystallization at 90 ℃ for 24 hours, filtering and washing the obtained sample, and drying at 60 ℃ for 2 hours to obtain the molecular sieve loaded with the palladium complex;
4) putting the molecular sieve of the supported palladium complex obtained in the step 3) into flowing ethylene gas, raising the temperature to 600 ℃ from room temperature according to the heating rate of 0.5 ℃/minute, keeping the temperature for 0.5 hour, cooling to room temperature, and taking out to obtain the molecular sieve supported palladium carbon catalyst.
Fig. 7 is an SEM image of the molecular sieve supported palladium on carbon catalyst prepared in this example, and it can be seen that the average particle size of the molecular sieve is 4.0 μm. The pore volume of the catalyst was 0.35cm as measured by nitrogen adsorption desorption3Per g, specific surface area of 940m2(ii) in terms of/g. Fig. 8 is an XRD pattern of the molecular sieve supported palladium on carbon catalyst prepared in this example, from which it can be seen that there is no diffraction peak of metallic palladium, indicating that Pd has a higher degree of dispersion. Measured by an inductively coupled plasma emission spectrometer, the mass content of Pd is 1.1%, and the carbon content is 0.01%. FIG. 9 is a TEM image of the Pd on molecular sieve catalyst prepared in this example after 12 hours of catalytic reaction, wherein the Pd particles have an average diameter of 4.8 nm.
Example 4
1) Mixing silica sol, aluminum hydroxide, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.15:0.006:11 in a molar ratio, stirring for 2 hours, transferring the obtained solution into a hydrothermal reaction kettle, carrying out hydrothermal pre-crystallization at the temperature of 30 ℃ for 24 hours, and then raising the temperature to 115 ℃ for hydrothermal crystallization for 50 hours to obtain a solution containing a molecular sieve structure directing agent;
2) mixing palladium chloride and tetrabutylammonium hydroxide according to the molar ratio of Pd to tetrabutylammonium hydroxide of 1:50, and performing ultrasonic dispersion to obtain a clear palladium complex solution;
3) mixing silica sol, aluminum hydroxide, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.035:0.12:30 in a molar ratio, adding a certain amount of the solution containing the molecular sieve structure directing agent obtained in the step 1), stirring for 3 hours, then adding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution into a reaction kettle, carrying out hydrothermal crystallization at 80 ℃ for 160 hours, filtering and washing the obtained sample, and drying at 30 ℃ for 40 hours to obtain the molecular sieve loaded with the palladium complex;
4) putting the molecular sieve of the supported palladium complex obtained in the step 3) into flowing helium gas, raising the temperature to 300 ℃ from room temperature according to the heating rate of 1.8 ℃/min, keeping the temperature for 2 hours, cooling to room temperature, and taking out to obtain the molecular sieve supported palladium carbon catalyst.
Fig. 10 is an SEM image of the molecular sieve supported palladium on carbon catalyst prepared in this example, and it can be seen that the molecular sieve has an average particle size of 3.5 μm. The pore volume of the catalyst was 0.31cm as measured by nitrogen adsorption-desorption3Per g, specific surface area 857m2(ii) in terms of/g. Fig. 11 is an XRD pattern of the molecular sieve supported palladium carbon catalyst prepared in this example, from which it can be seen that there is no diffraction peak of metallic palladium, indicating that Pd has a higher degree of dispersion. Measured by inductively coupled plasma emission spectrometer, the mass content of Pd is 1.5%, and the content of carbon is 1.5%The amount was 1.3%. Fig. 12 is a TEM image of the molecular sieve supported palladium carbon catalyst prepared in this example after 12 hours of catalytic reaction, from which it can be seen that the lattice of the FAU molecular sieve is obvious, Pd particles are uniformly embedded in the molecular sieve lattice, and the average particle size of the Pd particles is 0.3 nm.
Example 5
1) White carbon black, sodium metaaluminate, sodium hydroxide and water are mixed according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.06:0.02:13 in a molar ratio, stirring for 3 hours, transferring the obtained solution into a hydrothermal reaction kettle, carrying out hydrothermal pre-crystallization at the temperature of 30 ℃ for 6 hours, and then raising the temperature to 115 ℃ for hydrothermal crystallization for 72 hours to obtain a solution containing a molecular sieve structure directing agent;
2) mixing palladium nitrate and tetrabutylammonium hydroxide according to the molar ratio of Pd to tetrabutylammonium hydroxide of 1:50, and performing ultrasonic dispersion to obtain a clear palladium complex solution;
3) white carbon black, sodium metaaluminate, sodium hydroxide and water are mixed according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.05:0.09:11 in a molar ratio, adding a certain amount of the solution containing the molecular sieve structure directing agent obtained in the step 1), stirring for 2 hours, adding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution into a reaction kettle, performing hydrothermal crystallization at 100 ℃ for 96 hours, filtering and washing the obtained sample, and drying at 80 ℃ for 12 hours to obtain the molecular sieve loaded with the palladium complex;
4) putting the molecular sieve of the supported palladium complex obtained in the step 3) into flowing ethylene gas, raising the temperature to 450 ℃ from room temperature according to the heating rate of 1.6 ℃/min, keeping the temperature for 3 hours, cooling to room temperature, and taking out the molecular sieve to obtain the molecular sieve supported palladium carbon catalyst.
Fig. 13 is an SEM image of the molecular sieve supported palladium on carbon catalyst prepared in this example, and it can be seen that the molecular sieve has an average particle size of 0.5 μm. The pore volume of the catalyst was 0.32cm as measured by nitrogen adsorption desorption3Per g, specific surface area of 868m2(ii) in terms of/g. Fig. 14 is an XRD pattern of the molecular sieve supported palladium on carbon catalyst prepared in this example, from which it can be seen that there is no diffraction peak of metallic palladium, indicating that Pd has a higher degree of dispersion. Measured by an inductively coupled plasma emission spectrometer, the mass content of Pd was 2.5%, and the carbon content was 0.3%. Fig. 15 is a TEM image of the molecular sieve supported palladium on carbon catalyst prepared in this example after 12 hours of catalytic reaction, from which it can be seen that a distinct molecular sieve lattice, Pd particles are uniformly embedded in the molecular sieve lattice, and the average particle size of the Pd particles is 0.5 nm.
Comparative example 1
1) Adding a commercially available FAU type molecular sieve with the silica-alumina molar ratio of 2.5 into water, and stirring to form a molecular sieve suspension;
2) dissolving palladium chloride precursor by diluted ammonia water to form Pd (NH)3)4 2+A solution;
3) pd (NH)3)4 2+And gradually dropwise adding the solution into the molecular sieve suspension, stirring and reacting for 24 hours at the temperature of 50 ℃ to ensure that a palladium chloride precursor in the solution and cations in the molecular sieve are subjected to complete exchange, filtering and washing, and roasting for 2 hours at the temperature of 300 ℃ to obtain the palladium-loaded catalyst.
FIG. 16 is an XRD pattern of the palladium-supported catalyst prepared in comparative example 1, from which it can be seen that there is significant metallic palladium (Pd)0) The diffraction peak of (a) indicates that the dispersion degree of Pd is low. The mass content of Pd was 1.1% as measured by inductively coupled plasma emission spectrometer. Fig. 17 is a TEM image of the palladium-supported catalyst prepared in comparative example 1 after 12 hours of catalytic reaction, from which large Pd particles (circles in the figure) having an average particle diameter of 100 nm could be observed, indicating that sintering of metallic Pd occurred.
Comparative example 2
1) Step 1) as in comparative example 1;
2) same as step 2) of comparative example 1;
3) same as step 3 of comparative example 1);
4) and (3) adopting glucose as a carbon source, loading the glucose to the palladium-loaded catalyst obtained in the step (3) by an impregnation method, and carbonizing for 2 hours at 500 ℃ by adopting nitrogen gas to obtain the molecular sieve supported palladium carbon catalyst.
FIG. 18 is an XRD pattern of the modified molecular sieve supported palladium on carbon catalyst prepared in comparative example 2, from which it can be seen that there is significant metallic palladium (Pd)0) The diffraction peak of (a) indicates that the dispersion degree of Pd is low. Measured by an inductively coupled plasma emission spectrometer, the mass content of Pd was 0.6%, and the carbon content was 1.0%. Fig. 19 is a TEM image of the modified molecular sieve-supported palladium on carbon catalyst prepared in comparative example 2 after 12 hours of catalytic reaction, from which large Pd particles (circles in the figure) having an average particle size of 150 nm can be observed, indicating that sintering of metallic Pd has occurred.
Application example
The catalysts prepared in the above examples 1 to 5 and comparative example were subjected to catalytic activity evaluation on a continuous flow fixed bed reactor, the tubular reactor had a length of 36cm and an inner diameter of 8mm, the catalyst loading was 0.1g, carbon monoxide and methyl nitrite were used as raw materials in the reaction, nitrogen was used as a diluent gas, and the gas flow ratio was carbon monoxide: methyl nitrite: nitrogen (CO: CH)3ONO:N2) The reaction raw materials do not contain any chlorine element, the reaction temperature is 110 ℃, and the reaction pressure is low pressure. The resulting product was analyzed directly by on-line gas chromatography and included dimethyl carbonate (DMC), the main product, dimethyl oxalate (DMO), Methyl Formate (MF) and Dimethoxymethane (DMM). From this, the conversion X of carbon monoxide was calculatedCOMethyl nitrite-based selectivity S for dimethyl carbonateDMC/MNAnd selectivity S of each by-product based on methyl nitriteDMO/MN、SMF/MNAnd SDMM/MN。
As can be seen from Table 1, the CO conversion rate and DMC selectivity of the molecular sieve supported palladium-carbon catalysts prepared in examples 1-5 are obviously higher than those of the molecular sieve supported palladium-carbon catalysts prepared in comparative examples 1-4, which indicates that the molecular sieve supported palladium-carbon catalysts prepared in the application have higher selectivity and conversion rate.
TABLE 1 catalytic performances of catalysts of examples 1-5 and comparative examples
Comparative example 3 of the prior art shows that the CO conversion decreases to 0.75 times the initial CO conversion after 150 hours, indicating that the catalyst is less stable. Through stability evaluation, it is found that the molecular sieve-supported palladium carbon catalyst prepared in example 1 to example 5 can stably operate for more than 300 hours, and the CO conversion rate and DMC selectivity are basically kept unchanged, wherein fig. 20 is a catalytic stability test chart of the molecular sieve-supported palladium carbon catalyst prepared in example 3. After 300 hours of reaction, the Pd particles of examples 1 to 5 were found to have sizes of 0.5 nm, 16.0 nm, 5.3 nm, 0.4 nm and 0.8 nm, respectively, as characterized by TEM, which is substantially the same as the Pd particles at 12 hours of reaction. Therefore, the molecular sieve supported palladium carbon catalyst prepared by the method has higher stability.
FIG. 21 is an infrared image of pyridine adsorption of comparative example 1 and example 3, with example 3 being substantially free of
Acid, whereas comparative example 1 has much
Acid, example 3 utilizes a cheap organic polydentate ligand and palladium complex to encapsulate palladium into a molecular sieve in a one-pot method, avoiding the generation of ions in the traditional ion exchange process
Acid, while comparative example 1 was prepared by conventional ion exchange, the catalyst had much more on top
The acid, resulted in a lower selectivity of DMC of the catalyst of comparative example 1, only 50.0%, whereas the selectivity of DMC of the catalyst of example 3 of the present invention was as high as 90.0%.
From the results of the above examples and comparative examples, the following conclusions can be drawn: the molecular sieve supported palladium-carbon catalyst prepared by the invention has high stability, high selectivity and high yieldConversion rate, no
The catalyst with adjustable acid, chlorine-free and Pd nano-particle sizes provides a high-performance catalyst for the reaction of synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite through a low-pressure gas phase method.
Claims (4)
1. A one-pot method for preparing a molecular sieve supported palladium carbon catalyst is characterized by comprising the following steps: the method comprises the steps of packaging palladium into a molecular sieve by using a cheap organic polydentate ligand and palladium complex in a one-pot method, and then directly using the organic polydentate ligand as a carbon source to prepare a molecular sieve supported palladium carbon catalyst through in-situ carbonization, wherein the mass fraction of palladium in the catalyst is 0.1-2.5%, the mass fraction of carbon in the catalyst is 0.01-1.5%, the type of the molecular sieve is FAU type, the average particle size of the molecular sieve is 0.1-4 micrometers, and the pore volume of the molecular sieve is 0.21-0.37 cm3(ii)/g, the specific surface area of the molecular sieve is 750-950 m2/g；
The preparation method comprises the following steps:
1) mixing silicon source, aluminum source, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.05-0.2: 0.005-0.02: 10-30 in a molar ratio, stirring for 1-3 hours, transferring the obtained solution into a hydrothermal reaction kettle, carrying out hydrothermal pre-crystallization for 6-24 hours at the temperature of 25-75 ℃, and then raising the temperature to 90-110 ℃ for hydrothermal crystallization for 24-72 hours to obtain a solution containing a molecular sieve structure directing agent;
2) mixing a palladium precursor and an organic polydentate ligand according to the molar ratio of Pd to the organic polydentate ligand of 1: 3-65, and performing ultrasonic dispersion to obtain a clear palladium complex solution;
3) mixing silicon source, aluminum source, sodium hydroxide and water according to SiO2:Al2O3:Na2O:H2Mixing O-1: 0.01-0.05: 0.08-0.12: 10-30 in a molar ratio, adding a certain amount of the solution containing the molecular sieve structure directing agent obtained in the step 1), stirring for 1-3 hours, and then addingAdding a certain amount of the palladium complex solution obtained in the step 2), transferring the formed mixed solution to a reaction kettle, carrying out hydrothermal crystallization at 80-120 ℃ for 24-168 hours, filtering and washing the obtained sample, and drying at 25-100 ℃ for 1-48 hours to obtain the molecular sieve loaded with the palladium complex;
4) placing the molecular sieve loaded with the palladium complex obtained in the step 3) in flowing gas, raising the temperature from room temperature to 120-600 ℃ at a heating rate of 0.1-2.0 ℃/min, keeping the temperature for 0.5-6 hours, cooling to room temperature, and taking out to obtain the molecular sieve loaded palladium carbon catalyst;
the palladium precursor is one or a combination of more of palladium nitrate, palladium acetate, palladium chloride, ammonium chloropalladate, potassium chloropalladate, tetraammine palladium chloride and tetraammine palladium nitrate, and the organic polydentate ligand is one or a combination of more of diethylenetriamine, triethylene tetramine, tetraethylenepentamine, N- [3- (trimethoxysilyl) propyl ] ethylenediamine, tetraethylammonium hydroxide, tetrabutylammonium hydroxide and tetrapropylammonium hydroxide.
2. The one-pot preparation method of the molecular sieve supported palladium carbon catalyst according to claim 1, wherein the method comprises the following steps: the aluminum source in the steps 1) and 3) is one or a combination of more of aluminum powder, aluminum sulfate, aluminum hydroxide, aluminum triisopropoxide and sodium metaaluminate, and the silicon source in the steps 1) and 3) is one or a combination of more of ethyl orthosilicate, butyl orthosilicate, methyl orthosilicate, white carbon black, silica gel, silica sol and water glass.
3. The one-pot preparation method of the molecular sieve supported palladium carbon catalyst according to claim 1, wherein the method comprises the following steps: the flowing gas in the step 4) is one or a combination of several of nitrogen, argon, helium, carbon dioxide, carbon monoxide, methane and ethylene.
4. The application of the molecular sieve supported palladium carbon catalyst prepared by the preparation method of claim 1 is characterized in that: the molecular sieve supported palladium carbon catalyst is applied to the reaction of synthesizing dimethyl carbonate by using carbon monoxide and methyl nitrite through a low-pressure gas phase method.
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