CN101934233B - Preparation method of catalyst Cu-ZnO/HZSM-5 for directly synthesizing dimethyl ether by using synthesis gas - Google Patents

Abstract

The invention is a preparation _method of catalyst _Cu - _ZnO /HZSM-5 _for directly synthesizing dimethyl ether from synthesis gas. Si/Al=22) molecular sieve is used as raw material and prepared by chemical homogeneous precipitation method. The raw materials used in the present invention are nitrates and urea, which are cheap and easy to obtain, and no impurity ions are generated in the reaction, and there is no need for cumbersome filter cake washing. After high-temperature roasting, a pure catalyst with uniform particles and good dispersion can be obtained. The process of the present invention is simple and the method Novel, in the reaction of direct hydrogenation of carbon monoxide to dimethyl ether, the catalyst has high activity and stability at a low temperature of 220-240 °C.

Description

Preparation method of catalyst Cu-ZnO/HZSM-5 for direct synthesis of dimethyl ether from synthesis gas

technical field

The invention relates to a preparation method of a catalyst Cu-ZnO/HZSM-5 for directly synthesizing dimethyl ether from synthesis gas.

Background technique

Dimethyl ether (molecular formula: CH ₃ OCH ₃ , molecular weight: 46) is an excellent solvent, which can be used as insecticide, polishing agent, antirust agent, alkylating agent, and can also be used in aerosol, hair spray, Air fresheners, civil fuels, etc. Due to the high cetane number (55~60) of dimethyl ether, good combustion performance, and low exhaust pollution during combustion, dimethyl ether has outstanding advantages as an alternative fuel for diesel engines. At the same time, dimethyl ether is also a very good environment-friendly refrigerant. Compared with traditional chlorofluorocarbons, it has no damage to the ozone layer and has a small greenhouse effect coefficient. Therefore, dimethyl ether is known as a clean energy source in the 21st century. .

At present, there are two main processes for the preparation of dimethyl ether, one adopts the methanol dehydration process, and the methanol dehydration process can be divided into liquid phase methanol dehydration and gas phase methanol dehydration:

2CH ₃ OH → CH ₃ OCH ₃ + H ₂ O

The traditional liquid-phase methanol dehydration uses concentrated sulfuric acid as a catalyst, which has the advantages of low reaction temperature and high conversion rate, but it is severely corroded, requires high equipment, and the residual liquid and waste water seriously pollute the environment, operating conditions are harsh, and product post-processing is difficult. At present, this law has basically been eliminated.

Gas-phase methanol dehydration generally uses activated alumina or aluminum silicate as a catalyst. Methanol vapor passes through a solid catalyst, and a heterogeneous reaction occurs to generate dimethyl ether. The reaction temperature is generally 330~400°C and the pressure is 15~20MPa, but this The reaction temperature of this process is relatively high, and the requirements for the reaction device are relatively strict.

Another production process for the preparation of DME is the direct synthesis of synthesis gas (CO/H ₂ ). The process uses a dual-functional catalyst to directly produce dimethyl ether from synthesis gas in a reactor. Relatively speaking, this method has attracted more and more attention from the industry because it is thermodynamically favorable and economically reasonable. The main reaction process of the one-step process is as follows:

4H ₂ + 2CO → 2CH ₃ OH

2CH ₃ OH → CH ₃ OCH ₃ + H ₂ O

CO + H ₂ O → CO ₂ + H ₂

It can be seen that the one-step synthesis of DME from synthesis gas requires a dual-functional catalyst capable of hydrogenation of carbon monoxide and dehydration of methanol. Among them, the catalyst for methanol synthesis is mainly copper-based catalysts, and the catalyst for methanol dehydration is mainly γ - Alumina, HZSM-5, kaolin and other solid acids.

In Chinese invention patent ZL98107687.4, CuMnMx/carrier is prepared by co-precipitation impregnation method (Mx is one or two of V, Cr, Fe, Co, Ni, Zn, W or Mo, and the carrier is Al ₂ O ₃ , ZrO _2. One or a combination of MgO, SiO ₂ or molecular sieves) catalyst, under H ₂ /CO=3/2 (volume ratio), pressure 2.0MPa, reaction temperature 250~270℃ and space velocity 1500h ^- Under the condition of ¹ , the conversion rate of CO is 43-65%. In the Chinese invention patent ZL200410052571.6, methanol synthesis catalyst is first prepared by using ethanol as solvent and oxalate as precipitant, and then mechanically mixed with HZSM-5 molecular sieve to obtain dimethyl ether synthesis catalyst, using hydrogen-combined biomass gas as raw material, Under the conditions of pressure 3.0~3.5MPa, reaction temperature 239~284℃, and space velocity 2000~3000h ^-1 , the conversion rate of CO is 53.8~76.9%. In Chinese invention patent ZL200710185215.5, a dimethyl ether catalyst is prepared by mechanically mixing a methanol synthesis catalyst composed of Cu-Zn-Al(O) and a methanol dehydration catalyst such as γ-alumina and HZSM-5, at H ₂ /CO=1 (volume ratio), the pressure is 4.0MPa, and the reaction temperature is 280°C, the conversion rate of CO is 16.21~40.07%. In the Chinese invention patent application 200710139634.5, Cu-Mn-Zn/commercial molecular sieve is used as the _dimethyl ether catalyst, and the conversion rate of CO is 10.92~ 19.97%, and the selectivity of dimethyl ether is 60.1~80.9%. Chinese invention patent application _{200810202609.1} is a catalyst prepared by mixing acidic modified kaolin with a copper-based catalyst. The CO conversion rate is 40~60%. Chinese invention patent application 200810046592.5 uses alkali metal salt as a precipitant, and prepares Cu-Zn-Al-Mg(O)-γ-Al ₂ O ₃ by co-precipitation and impregnation method, at H ₂ /CO=1.5~2.5 (volume ratio) , the pressure is 4.0~6.0MPa, and the reaction temperature is 275~285℃, the CO conversion rate is about 80%, and the CO ₂ selectivity is as high as 28.53%.

It can be seen that the DME synthesis catalyst is generally prepared by coprecipitation impregnation method or mechanical mixing method. The raw material used is nitrate or acetate, and the precipitant is mostly alkali metal hydroxide, carbonate or oxalate. . The homogeneity and dispersion of the prepared catalyst are not high enough, and there are problems such as the influence of impurity ions. Therefore, the activity of the prepared catalyst is limited and the requirements for the reaction conditions are harsh. For example: in the above-mentioned patents, the pressure of direct synthesis of dimethyl ether from synthesis gas is mostly above 4.0MPa, the reaction temperature is generally higher than 250°C, the optimum working temperature is above 280°C or even higher, and the conversion rate of carbon monoxide in the reaction is low, The yield of dimethyl ether is low.

Contents of the invention

The purpose of the present invention is to provide a method for preparing a catalyst for the direct synthesis of dimethyl ether from synthesis gas with high activity, high selectivity and high stability. In the preparation method, the raw materials are cheap and easy to obtain, the process is simple and clear, and there is no discharge of any pollutants, which is very easy to promote large-scale industrial production.

In order to overcome many deficiencies of traditional catalysts and preparation methods, the present invention has made the following improvements and innovations:

1. Abandon the traditional co-precipitation method, and adopt a unique chemical uniform precipitation method to prepare catalysts. The purpose is to simplify the preparation process and obtain highly dispersed and highly active catalysts;

2. Choose nitrate (rather than organic salt) as the precursor, the purpose is to reduce the cost of raw materials and reduce environmental pollution;

3. Urea is selected as the slow precipitant, the purpose is that the thermal decomposition temperature is low and no impurities will be produced;

4. The open system of continuous heating and reflux (instead of the closed system of the traditional high-pressure reactor) and highly diluted reaction solution are adopted, the purpose is to reduce the formation of metal ammonium ions and reduce the waste and loss of the catalyst preparation process;

5. Use the highly acidic HZSM-5 (Si/Al=22) molecular sieve as the carrier to promote the forward progress of the chemical reaction process and increase the selectivity of the target product DME.

The present invention provides a _method for preparing _a catalyst Cu- _ZnO /HZSM-5 _for directly synthesizing dimethyl ether from synthesis gas. -5 (Si/Al=22) molecular sieve is used as raw material, prepared by chemical uniform precipitation method, the steps are as follows:

(1) Weigh a certain amount of copper nitrate trihydrate and zinc nitrate hexahydrate, control the molar ratio of copper nitrate and zinc nitrate to 1:1~20:1, add a certain amount of water, stir to dissolve, and control the water and copper ions The molar ratio is 1700:1;

(2) Weigh a certain amount of urea and add it to the above mixed solution, stir and dissolve, and control the molar ratio of urea to copper nitrate plus zinc nitrate to be 2:1~4:1;

(3) Then add a certain amount of HZSM-5 (Si/Al=22), and stir vigorously to form a uniform suspension, and control the mass ratio of HZSM-5 molecular sieve to copper nitrate plus zinc nitrate to 1:10 ~10:1;

(4) Transfer the above suspension into a three-necked flask and continue stirring vigorously for 30 minutes;

(5) Place the three-necked flask in an oil bath, heat to 90-100 degrees Celsius, stir and reflux for 15-20 hours;

(6) When the pH of the suspension reaches about 7.0, stop heating, immediately cool the suspension to room temperature, filter under reduced pressure, wash with 200 ml of deionized water, and then transfer to an oven at 80 degrees Celsius to dry for 24 hours;

(7) Put the dried material into a muffle furnace, raise the temperature to 350 degrees Celsius at 2 degrees Celsius/min and bake for 4 hours. A highly active core-shell Cu-ZnO (shell)/HZSM-5 (core) catalyst was obtained.

The Cu-ZnO/HZSM-5 catalyst prepared by the present invention is used in the reaction of direct hydrogenation of carbon monoxide to synthesize dimethyl ether. The reaction pressure is 2.0 MPa, the temperature is between 220 and 280 ° C, and the reaction space velocity is 1500 h ^-1 Under the condition of H ₂ /CO=2 (volume ratio), the conversion rate of CO can reach up to 90.9%.

The raw materials used in the present invention are nitrates and urea, which are cheap and easy to obtain, and no impurity ions are generated in the reaction, and there is no need for cumbersome filter cake washing. After high-temperature roasting, a pure catalyst with uniform particles and good dispersion can be obtained. The process of the present invention is simple and the method Novel, in the reaction of direct hydrogenation of carbon monoxide to dimethyl ether, the catalyst has high activity and stability at a low temperature of 220-240 °C.

Detailed ways

The present invention will be further described with reference to the following examples:

Example 1

Weigh 9.418 grams of copper nitrate trihydrate and 5.801 grams of zinc nitrate hexahydrate in a beaker, add 1200 milliliters of deionized water, stir to dissolve, then weigh 8.784 grams of urea and add it to the above mixture. After the urea dissolves, add 4.690 grams of HZSM-5 molecular sieve, and vigorously stir to form a uniform suspension, transfer the above suspension into a three-necked flask, continue to stir for 30 minutes, place the three-necked flask in an oil bath, stir at 100 degrees Celsius and Heating at reflux for 15-20 hours. When the pH of the suspension reaches 7.0, stop heating, immediately cool the suspension to room temperature, then filter under reduced pressure, wash with 200 ml of deionized water, and then dry in an oven at 80 degrees Celsius for 24 hours. Put it into a muffle furnace, raise the temperature to 350 degrees Celsius at 2 degrees Celsius/minute and bake it for 4 hours to obtain a Cu-ZnO/HZSM-5 catalyst with a weight ratio of (copper oxide + zinc oxide)/HZSM-5 of 1:1.

Weigh 2 grams of the catalyst prepared by the above method with a particle size of 20-40 mesh, and evaluate the hydrogenation activity of carbon monoxide in a fixed-bed reactor. Reaction conditions: H ₂ /CO=2 (volume ratio), pressure 2 MPa, reaction temperature 220~280°C, reactant space velocity 1500h ^-1 . The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 1:

Table 1 The activity evaluation result of the catalyst of embodiment 1

Example 2

Referring to the preparation method and steps of Example 1, the difference is that the amount of HZSM-5 is 3.126 grams. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 1.5:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 2:

Table 2 The activity evaluation result of the catalyst of embodiment 2

Example 3

Referring to the preparation method and steps of Example 1, the difference is that the amount of HZSM-5 is 2.680 grams. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 1.75:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 3:

Table 3 The activity evaluation result of the catalyst of embodiment 3

Example 4

Referring to the preparation method and steps of Example 1, the difference is that the amount of HZSM-5 is 2.345 grams. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 2:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 4:

Table 4 The activity evaluation result of the catalyst of embodiment 4

Example 5

Referring to the preparation method and steps of Example 1, the difference is that the amount of HZSM-5 is 1.876 grams. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 2.5:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 5:

The activity evaluation result of table 5 embodiment 5 catalyst

Example 6

Referring to the preparation method and steps of Example 2, the difference is that the temperature of the oil bath is adjusted to 95 degrees Celsius. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 1.5:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 6:

The activity evaluation result of table 6 embodiment 6 catalyst

Example 7

Referring to the preparation method and steps of Example 3, the difference is that the temperature of the oil bath is adjusted to 95 degrees Celsius. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 1.75:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 7:

The activity evaluation result of table 7 embodiment 7 catalyst

Example 8

Referring to the preparation method and steps of Example 4, the difference is that the temperature of the oil bath is adjusted to 95 degrees Celsius. A Cu-ZnO/HZSM-5 catalyst with a (copper oxide+zinc oxide)/HZSM-5 weight ratio of 2:1 was obtained.

The carbon monoxide hydrogenation activity of the catalyst was evaluated by the same method and conditions. The conversion rate of reactant under different temperatures and the selectivity of product are as shown in table 8:

The activity evaluation result of table 8 embodiment 8 catalyst

Claims (2)

1. A preparation method of a catalyst CuO-ZnO/HZSM-5 for directly synthesizing dimethyl ether from synthesis gas, characterized in that it is Cu(NO ₃ ) ₂ 3H ₂ O, Zn(NO ₃ ) ₂ 6H ₂ O, HZSM-5 molecular sieve with Si/Al=22 is used as raw material and prepared by chemical uniform precipitation method, the steps are as follows: (1) Weigh a certain amount of copper nitrate trihydrate and zinc nitrate hexahydrate, control the molar ratio of copper nitrate and zinc nitrate to 1:1 to 20:1, add a certain amount of water, stir to dissolve, and control the water and copper ions The molar ratio is 1700:1; (2) Take a certain amount of urea and join in the above-mentioned mixed solution, stir and dissolve, control the mol ratio of urea and copper nitrate plus zinc nitrate to be 2: 1～4: 1; (3) Add a certain amount of HZSM-5 with Si/Al=22, and vigorously stir to form a uniform suspension, and control the mass ratio of HZSM-5 molecular sieve to copper nitrate plus zinc nitrate to be 1: 10～ 10:1; (4) Transfer the above-mentioned suspension into a three-necked flask, and continue to stir vigorously for 30 minutes; (5) Place the three-necked flask in an oil bath, heat to 90-100 degrees Celsius, stir and reflux for 15-20 hours; (6) When the pH of the suspension reaches 7.0, stop heating, immediately cool the suspension to room temperature, filter under reduced pressure, wash with 200 milliliters of deionized water, and then dry in an oven at 80 degrees Celsius for 24 hours; (7) Put the dried material into a muffle furnace, raise the temperature to 350 degrees Celsius at 2 degrees Celsius/min and bake it for 4 hours to obtain a highly active core-shell CuO-ZnO/HZSM-5 catalyst. 2. The CuO-ZnO/HZSM-5 catalyst prepared as claimed in claim 1 is used for the reaction of direct hydrogenation of carbon monoxide to synthesize dimethyl ether, and it is characterized in that reaction condition is: reaction temperature is 220～280 ℃, and reaction pressure is 2.0MPa, The reaction space velocity is 1500h ^-1 and H ₂ /CO=2 (volume ratio), and the CO conversion rate can reach up to 90.9%.