CN113398912B - Catalyst for synthesizing dimethyl carbonate by alcoholysis of methyl carbamate - Google Patents
Catalyst for synthesizing dimethyl carbonate by alcoholysis of methyl carbamate Download PDFInfo
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Abstract
The invention relates to a catalyst for synthesizing dimethyl carbonate by alcoholysis of methyl carbamate. The catalyst is a ternary composite metal oxide catalyst, and is prepared by high-temperature roasting of ternary layered composite metal hydroxide containing transition metal and rare earth metal elements. The catalyst has more surface acid-base active sites. The rare earth metal element doped in the layered composite metal hydroxide can improve the dispersibility of the catalytic active metal, surface vacancies and defects are easy to form after high-temperature roasting, and the catalyst contains 4f orbits which are not full of electrons, can be used as an electron transfer path of a catalytic reaction, and can reduce the dissolution of the active metal in a reaction liquid in the catalytic reaction process, so that the activity and stability of the catalyst for catalyzing the alcoholysis of methyl carbamate to synthesize the methyl carbonate are improved. The catalyst has good application prospect in the field of synthesizing the dimethyl carbonate by the urea alcoholysis method, has cheap raw materials, simple preparation method and operation and is suitable for large-scale production.
Description
Technical Field
The invention belongs to the field of synthesizing dimethyl carbonate by a methyl carbamate alcoholysis method, and particularly relates to a catalyst for synthesizing dimethyl carbonate by methyl carbamate alcoholysis.
Technical Field
Dimethyl carbonate (DMC) is a good reagent for methylation, carbonylation and methoxylation because it contains methyl, methoxy, and carbonyl functionalities in its molecule. DMC as chemical raw material can synthesize a plurality of organic compounds with high added value, which is hopeful to replace phosgene (COCl) 2 ) The organic synthesis raw materials of the highly toxic dimethyl sulfate (DMS) and the like are therefore important organic synthesis intermediates. Meanwhile, the chemical has good physical and chemical properties and low toxicity, can be applied to various fields, and is a clean and green chemical. In recent years, the demand for DMC has been increasing, and DMC prices have also been increasing.
The existing DMC synthesis method mainly comprises phosgene method, transesterification method and CO 2 And direct synthesis of methanol and alcoholysis of urea. The process flow of the phosgene method is mature, but the method is eliminated because the raw material is highly toxic phosgene and the product contains HCl which is corrosive; the DMC yield and selectivity prepared by the transesterification method are higher,however, the process flow is complex, and the raw materials are expensive, so that the method has less application in industry; CO 2 Direct synthesis of methanol is due to raw material CO 2 The three methods have the defects that the activity is low, water is generated in the product, and a methanol-DMC-water ternary system which is difficult to separate is formed. The raw materials used in the urea alcoholysis method are cheap urea and methanol, and the characteristics of simple process, environment-friendly product, no water in the product, easy separation and the like are always research hot spots.
The DMC reaction system synthesized by the urea alcoholysis method comprises two steps: the first step is to react urea with methanol to form an intermediate Methyl Carbamate (MC), which can also obtain higher MC selectivity in the absence of a catalyst; the second step is that MC further reacts with methanol to generate DMC, and the reaction is a speed control step of synthesizing DMC system by urea alcoholysis method, and a catalyst with better performance is needed to participate. In the second reaction step, more side reactions occur in the system, such as self-decomposition of MC and further reaction of the generated DMC with MC to generate N-methyl carbamate (NMMC), so that there has been a great interest in finding a catalyst with better performance and simple preparation method.
CN200810101782.2 reports as γ -Al 2 O 3 The supported catalyst which is a carrier and is added with a surfactant to adjust the size of active grains has higher stability for catalyzing the alcoholysis of urea to synthesize DMC and higher yield of DMC; CN200410012504.1 is prepared from activated carbon and gamma-Al 2 O 3 And the catalyst is prepared by loading active components such as alkali metal and the like as a carrier, and is applied to a catalytic rectification reactor to prepare DMC, so that the yield of DMC can be further improved, the DMC has higher reaction activity and stability, and fewer byproducts are produced; CN201310499274.5 discloses a catalyst for preparing DMC from supported urea and methanol, which has higher activity and stability in synthesizing DMC by catalyzing urea and methanol; CN201110099602.3 adopts molecular sieve to load Fe 2 O 3 The catalyst is simple to prepare and easy to separate from a reaction system, no cocatalyst or cocatalyst is needed to be added, the yield of DMC can reach 36.7 percent, and the selectivity can also reachTo 97.4%.
TABLE 1 relevant catalyst literature and patents
The activity of the catalyst is mainly related to the number and the type of active sites on the surface of the catalyst, hydrotalcite-like compounds (LDHs) are used as precursors, and the composite metal oxide (LDO) catalyst obtained by roasting can better meet the conditions. The prepared ZnAlLa-LDO catalyst has better catalytic effect, and the phenomenon shows that Zn, al and La have synergistic effect in the catalytic reaction process, and the catalyst exposes more acidic and alkaline active sites due to partial dissolution of Zn in the reaction process.
TABLE 2 comparison of the number of active sites on ZnAlLa-LDO and ZnAl-LDO catalyst surfaces
Aiming at solving the problem of more side reactions in a MC and methanol reaction system, rare earth metal elements are doped into layered composite hydroxide with hydrotalcite-like structures through reasonable design optimization, and the hydrotalcite-like base composite metal oxide catalyst is prepared through further roasting. With the participation of this catalyst, DMC selectivity is higher than that of the catalyst systems reported previously. Since the rare earth metal element contains a 4f orbit which is not filled with electrons, the rare earth metal element can be used as an electron transfer path of a catalytic reaction. The related research shows that the rare earth metal element doped in the catalyst can improve the dispersity of the catalytic active metal, and surface vacancies and defects are easy to form.
Disclosure of Invention
The invention solves the technical problem of providing a catalyst for synthesizing dimethyl carbonate by alcoholysis of methyl carbamate through a designed catalyst preparation scheme. The composite catalyst has the advantages that the rare earth metal elements are doped, so that the structure of the catalyst is stabilized, and more electron transfer paths are provided. In addition, the catalyst shows better catalytic activity when applied to a reaction system for synthesizing the dimethyl carbonate by alcoholysis of methyl carbamate. Meanwhile, the preparation process flow of the catalyst is simple and convenient, no toxic or harmful substances are generated, and the catalyst is suitable for large-scale industrial production.
The invention adopts the technical scheme that:
a catalyst for synthesizing dimethyl carbonate by alcoholysis of methyl carbamate, the catalyst characterized by:
(1) The catalyst is a composite metal oxide, and the active metal elements comprise transition metal elements and rare earth metal elements;
(2) The catalyst precursor is of a layered structure, the layered structure partially collapses after high-temperature roasting, and more acidic and alkaline active sites are exposed on the surface of the composite metal oxide catalyst.
In the catalyst, in the characteristic (1), the transition metal element can be Zn or Fe.
In the catalyst, in the feature (1), the rare earth metal element may be La.
The catalyst is characterized in that in the step (1), the composite metal oxide is prepared by oxidizing layered composite metal hydroxide under the high-temperature roasting condition, and the collapse degree of the laminate structure can be controlled by adjusting the roasting temperature and the roasting time.
The preparation method of the catalyst is characterized by comprising the following steps:
(1) Dissolving metal nitrate in deionized water and performing ultrasonic treatment to form a mixed solution, adding the mixed solution of nitrate into urea solution, stirring and refluxing the mixed solution at a certain temperature, ageing at room temperature, and performing suction filtration, washing and drying to obtain a catalyst precursor.
(2) Grinding the catalyst precursor into powder, and then placing the powder into a muffle furnace for high-temperature roasting for a certain time to finally obtain the catalyst.
In the preparation method, in the step (1), the molar ratio of the rare earth metal to the transition metal element is 0-4.
In the preparation method, in the step (1), the molar ratio of urea to nitrate is 1-4.
In the preparation method, in the step (1), the stirring speed is 500-1200rpm, the reaction temperature is 85-100 ℃, and the reaction time is 6-10h.
In the preparation method, in the step (2), the roasting temperature is 100-800 ℃ and the roasting time is 2-10h.
The ratio of the reactant methanol to MC molar ratio of the catalytic reaction is 0-40.
The use of the catalyst in an amount of 0.1 to 10wt.%.
The invention has the following characteristics:
the catalyst is prepared by doping rare earth metal elements into hydrotalcite-like layered hydroxide and further calcining at high temperature. The catalyst has more reactive sites. The preparation method of the catalyst is low in cost, simple to operate and suitable for large-scale production.
The catalyst is applied to the catalytic reaction of synthesizing the dimethyl carbonate by alcoholysis of the methyl carbamate, and has the following advantages:
(1) Since the catalyst contains rare earth metal elements, the rare earth metal elements contain 4f orbitals which are not filled with electrons and can be used as an electron transfer path of catalytic reaction, so that the catalytic activity of the catalyst is greatly improved compared with other types of catalysts.
(2) The catalyst has a higher number of active sites on the surface, which allows more reactants to be activated when participating in the catalytic reaction, and thus the catalyst has higher catalytic activity.
Drawings
FIG. 1 shows the X-ray single crystal diffraction pattern (XRD) of (3-1-0.9) ZnAlLa-LDO-700 and hydrotalcite-like precursor thereof.
FIG. 2 shows N of (3-1-0.9) ZnAlLa-LDO-700 2 Adsorption and desorption graph.
FIG. 3 shows the CO of (3-1-0.9) ZnAlLa-LDO-700 2 -TPD profile.
FIG. 4 shows NH of (3-1-0.9) ZnAlLa-LDO-700 3 -TPD profile.
Detailed description of the preferred embodiments
The invention will be further illustrated with reference to examples.
Example 1: a preparation method of a composite metal oxide catalyst ZnAlLa-LDO. Weighing a certain amount of urea, dissolving the urea in 100mL of deionized water, and marking the solution as solution A; zn (NO) was weighed out in a molar ratio of Zn, al, la of 3:1:0.9 3 ) 2 、Al(NO 3 ) 3 、La(NO 3 ) 3 Dissolve in 100mL deionized water, designated solution B, and sonicate solution a and solution B for 30min to allow for adequate dissolution. Then, the solution A and the solution B were transferred to a 500mL three-necked flask and heated with stirring at a temperature of 95℃for 8 hours, wherein the stirring speed was 600rad/min. And (3) carrying out suction filtration and separation on the solid-liquid mixture obtained after the reaction is finished, drying a solid sample at 85 ℃ for 12 hours, and grinding the solid sample into powder, wherein the obtained sample is marked as ZnAlLa-LDHs. And (3) placing the ZnAlLa-LDHs into a muffle furnace for roasting, wherein the roasting temperature is 700 ℃, and the roasting time is 6 hours, so that the composite metal hydroxide is oxidized into a composite metal oxide, and the obtained sample is recorded as (3-1-0.9) ZnAlLa-LDO-700.
Comparative example 1: the catalyst was commercial ZnO and was not further treated prior to use.
Comparative example 2: the preparation process is the same as in example 1, except that La (NO) is not added to solution B during the preparation of the catalyst 3 ) 3 The calcination temperature was 600℃and the obtained sample was designated (3-1) ZnAl-LDO-600.
Comparative example 3: the preparation method is the same as in example 1, except that in the preparation process of the catalyst, the solution B is a solution with the molar ratio of Zn, al and Fe of 3:1:0.5, the roasting temperature of a muffle furnace is 600 ℃, and the obtained sample is (3-1-0.5) ZnAlFe-LDO-600.
Example 2: the preparation method is the same as in example 1, except that in the preparation process of the catalyst, the solution B is a solution with the molar ratio of Zn, al and La of 3:1:0.5, the roasting temperature of a muffle furnace is 600 ℃, and the obtained sample is (3-1-0.5) ZnAlLa-LDO-600.
Example 3: the preparation method is the same as in example 1, except that in the preparation process of the catalyst, the roasting temperature of the muffle furnace is 600 ℃, and the obtained sample is (3-1-0.9) ZnAlLa-LDO-600.
Example 4: catalytic activity comparison experiments. The MC to methanol molar ratio of 1:15 was placed in a 100mL autoclave with mechanical agitation with a catalyst level of 1wt.%. After filling, a certain amount of nitrogen is introduced into the reaction kettle for leak detection, and then the nitrogen is discharged to maintain normal pressure in the reaction kettle. The reaction vessel was opened to heat and agitate, and the reaction vessel was allowed to warm to 180℃over 30 minutes with an agitation rate of 600rad/min. After reacting for 10 hours, the reaction kettle is placed in ice water bath to be cooled to room temperature, the products are collected for centrifugal separation, and gas chromatography qualitative and quantitative analysis is carried out on the liquid phase products. The conversion of MC and the selectivity of the main product DMC and the byproduct NMMC were calculated. The results are shown in Table 3.
TABLE 3 comparison of catalytic effects of comparative and example
Example 5: catalyst stability test. The experimental method is different from the catalytic activity comparison experiment in that for the (3-1-0.9) ZnAlLa-LDO-700 catalyst, the optimized molar ratio of MC to methanol is 1:20, the optimized catalyst dosage is 0.8wt%, the reaction temperature is 170 ℃, and the reaction time is 9 hours. The recovered catalyst was washed, dried and then supplemented to the original ratio to perform the catalytic reaction, and the catalytic stability results are shown in table 2.
TABLE 4 catalytic stability results
As shown in FIG. 2, (3-1-0.9) the XRD spectrum of the ZnAlLa-LDO-700 precursor shows characteristic diffraction peaks of hydrotalcite-like compounds, indicating that La can be doped into the lattice of ZnAl-LDHs to form ternary composite metal hydroxide; (3-1-0.9) XRD spectrum of ZnAlLa-LDO-700 does not show characteristic diffraction peak of hydrotalcite-like compound, showing that layered structure of hydrotalcite-like compound collapses under calcination at 700 deg.C, forming composite metal oxide. As shown in Table 3, when (3-1-0.9) ZnAlLa-LDO-700 was used as the catalyst, the conversion rate of MC and the DMC selectivity were both the highest, and the MC conversion rate was increased more than that when (3-1) ZnAl-LDO-600 was used as the catalyst, because the rare earth metal La was added to the hydrotalcite-like compound during the preparation of (3-1-0.9) ZnAlLa-LDO-700 catalyst, so that the number of empty electron orbitals in the catalyst was increased, facilitating the transfer of electrons during the reaction, and the (3-1-0.9) ZnAlLa-LDO-700 catalyst had more acid-base active sites on the surface than (3-1) ZnAl-LDO-600 catalyst, as shown in Table 2 and FIGS. 3 and 4. So that the catalyst has higher catalytic activity, and further improves the conversion rate of MC and the selectivity of DMC.
In terms of catalyst stability, when (3-1-0.9) ZnAlLa-LDO-700 is used as a catalyst, the catalyst stability is superior to that of (3-1) ZnAl-LDO-600, and the catalytic activity after the catalyst is repeatedly used for three times is still higher than that of a newly prepared (3-1) ZnAl-LDO-600 catalyst. As is known from inductively coupled plasma emission spectroscopy (ICP), the amount of Zn contained in the composite metal oxide was reduced after the reaction, which suggests that Zn was dissolved as an active metal in the reaction liquid during the catalytic reaction, and Zn element was detected in the reaction liquid (table 5), so that the activity of the catalyst was reduced with the increase in the number of repeated use due to the reduction of the Zn element content. The amount of Zn lost after the reaction of the (3-1-0.9) ZnAlLa-LDO-700 catalyst is less than that of the (3-1) ZnAl-LDO-600 catalyst, so that the addition of La element into the catalyst can effectively reduce the dissolution of Zn in a reaction system, thereby improving the catalytic reaction activity and the catalytic stability of the catalyst.
Table 5.Icp results
The foregoing description of the embodiments has been provided for the purpose of illustrating the general principles of the invention, and is not meant to limit the invention thereto, but to limit the invention thereto, and any modifications, equivalents, improvements and equivalents thereof may be made without departing from the spirit and principles of the invention.
Claims (8)
1. The application of a composite metal oxide catalyst in synthesizing dimethyl carbonate DMC by alcoholysis of methyl carbamate MC is characterized in that:
(1) The catalyst precursor is a rare earth metal modified hydrotalcite-like compound, and is a composite metal oxide catalyst formed after high-temperature roasting;
(2) The active metal elements of the composite metal oxide catalyst comprise rare earth metal elements La and transition metal elements Zn and Fe.
2. The use according to claim 1, wherein the composite metal oxide is prepared by oxidizing a layered composite metal hydroxide under high temperature firing conditions, and the degree of collapse of the laminate structure is controlled by adjusting the firing temperature and firing time.
3. The use according to claim 1, wherein: the preparation method of the composite metal oxide mainly comprises the following steps:
(1) Dissolving metal nitrate in deionized water and performing ultrasonic treatment to form a mixed solution, adding the mixed solution of nitrate into urea solution, stirring and refluxing the mixed solution at a certain temperature, ageing at room temperature, and performing suction filtration, washing and drying to obtain a catalyst precursor;
(2) Grinding the catalyst precursor into powder, and then placing the powder into a muffle furnace for high-temperature roasting for a certain time to finally obtain the catalyst.
4. The use according to claim 3, wherein in step (1), the molar ratio of rare earth metal to transition metal element is 0 to 5 excluding 0.
5. The method according to claim 3, wherein in step (2), the calcination temperature is 100 to 800℃and the calcination time is 2 to 10 hours.
6. The use according to claim 1, wherein the ratio of the reactant methanol to MC molar ratio of the catalytic reaction is 0-40, excluding 0.
7. The use according to claim 1, wherein the catalytic reaction is carried out at a reaction temperature of 100-240 ℃ for a reaction time of 1-24 hours.
8. The use according to claim 1, wherein the catalyst is used in an amount of 0.1 to 10wt.%.
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