Molecular sieve catalyst for selective hydrogenation of carbon dioxide to prepare low-carbon alcohols, preparation method and application thereof

Abstract

The present invention relates to a molecular sieve catalyst for preparing lower alcohols by selective hydrogenation of carbon dioxide, a preparation method and an application thereof, specifically the construction of an M-silicon-aluminum molecular sieve system and its application in the selective hydrogenation of acetylene, a hydrothermal method for synthesizing an M (copper, chromium, etc.)-molecular sieve catalytic system and applying it to the reaction of preparing lower alcohols by selective hydrogenation of carbon dioxide, which can solve the problems of high transition metal content, complex structure, high production cost and high energy consumption of the process flow in the catalytic system in the conventional process. The Cu-molecular sieve one-step hydrothermal synthesis method of the present invention is prepared and applied to the selective hydrogenation of carbon dioxide. The catalyst is cheap and readily available, has a simple preparation process, high catalytic activity and good stability. It is applied to the process of preparing lower alcohols by selective hydrogenation of carbon dioxide under relatively mild conditions, showing excellent catalytic activity and selectivity, and can be used for the industrial production of preparing lower alcohols (such as methanol) by selective hydrogenation of carbon dioxide.

Description

Technical Field

[0001] The present invention relates to a molecular sieve catalyst for the selective hydrogenation of carbon dioxide to produce lower alcohols, as well as a preparation method and application thereof. Specifically, the invention relates to the construction of an M-silica-alumina molecular sieve system and its application in the selective hydrogenation of acetylene. The M (copper, chromium, etc.)-molecular sieve catalytic system is synthesized by a hydrothermal method and applied to the reaction of selective hydrogenation of carbon dioxide to produce lower alcohols. Background Art

[0002] CO2, a naturally abundant and renewable carbon source, is an ideal energy source. Efficient recycling of CO2 is a crucial challenge facing humanity. Using clean energy sources like H2 to hydrogenate CO2 to produce low-carbon alcohols like methanol and ethanol is a viable strategy for achieving sustainable carbon resource utilization.

[0003] The hydrogenation of CO₂ to methanol or ethanol is a current research hotspot in catalysis, but the design and construction of catalyst systems still face numerous difficulties and challenges. On the one hand, the chemical inertness of CO₂ molecules makes it difficult to effectively activate them. On the other hand, CO₂ hydrogenation to methanol / ethanol is often accompanied by side reactions such as CO₂ methanation and reverse water-gas shift (RWGS), which limit the selectivity of the target products. To address the challenges of efficient CO₂ activation and highly selective conversion, a series of catalysts and corresponding preparation strategies have been developed. The most widely used method is to dope metal promoters to manipulate the electronic properties of active sites, such as the surface charge and d-band center, as is the case with most Cu-based catalysts. Furthermore, constructing highly active metal oxide interfaces and leveraging their synergistic catalytic effects is also a proven strategy for methanol synthesis, such as the commercially available CuZnAl catalyst (primarily composed of Cu₂, Zn₂, and Al₂O₃). Although the relevant technology has achieved a relatively high yield of low-carbon alcohols, it inevitably faces some technical defects: for example, the transition metal content in CuZnAl is relatively high (Cu content is above 60%), and the cost of the catalyst is relatively high, which affects the economic benefits of the entire catalytic process; in order to increase the yield of low-carbon alcohols during the production process, a higher reaction temperature (above 300°C) is often used, which greatly increases the production cost. Summary of the Invention

[0004] The present invention aims to provide a molecular sieve catalyst for the selective hydrogenation of carbon dioxide to produce lower alcohols, as well as a preparation method and application thereof. This catalyst addresses the high cost and energy consumption issues associated with CuZnAl composite catalysts in conventional production processes. The catalyst provided by the present invention is low-cost and exhibits extremely high mono-Cu or mono-Cr dispersion and excellent stability. In CO2 hydrogenation reactions, the catalyst, under the action of an M-molecular sieve system, achieves excellent catalytic activity and selectivity for lower alcohols.

[0005] The molecular sieve catalyst for preparing low-carbon alcohols by selective hydrogenation of carbon dioxide provided by the present invention comprises Cu or Cr as the main active ingredient, a composite alkali metal and a silicon-aluminum molecular sieve as a carrier, wherein the loading amount of Cu or Cr is 0.5-15% of the catalyst mass; and the loading amount of alkali metal is 0.8-8% of the catalyst mass.

[0006] The synthesis method of the molecular sieve catalyst provided by the present invention mainly comprises the following steps: using a soluble copper salt (or chromium salt), an organic amine ligand, an alkali source, an aluminum source and a silicon source as raw materials for a one-step hydrothermal synthesis; washing the product with water until it is neutral, drying it and then calcining it; placing the calcined sample in an alkali metal nitrate solution for ion exchange; then filtering and washing it, drying it and calcining it.

[0007] The soluble copper salt described in the present invention is copper nitrate or copper acetate (or soluble chromium salt chromium nitrate); the organic amine complexing agent is a small molecule organic amine such as ethylenediamine, diethylenetriamine, tetraethylenepentamine, and a siloxane amine compound; preferably, the siloxane amine compound is, for example, 3-aminopropyltriethoxysilane.

[0008] Optionally, the alkali source is sodium hydroxide; the silicon source is any one of silica sol, ethyl orthosilicate, silica aerosol, amorphous silica powder or silicate; and the aluminum source is any one of aluminum sol, aluminum isopropoxide, metaaluminate, aluminate, and pseudo-boehmite.

[0009] Optionally, the alkali metal M is lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium or barium; the silica-alumina molecular sieve includes structures such as FAU, MOR, CHA, and MFI.

[0010] The method for preparing a molecular sieve catalyst for selective hydrogenation of carbon dioxide to produce lower alcohols provided by the present invention specifically comprises the following steps:

[0011] 1) Dissolve the soluble copper salt in water according to the metered amount to obtain a copper salt aqueous solution, then fully stir it with the organic amine complexing agent for 10-40 minutes, and then add the aluminum source, alkali source and silicon source in the ratio of H20:SiO2=10-1000, Cu (or Cr):SiO2=0.002-0.2, NaOH:SiO2=0.5-10, Al:SiO2=0.05-0.5 in sequence and fully stir for 30-300 minutes to obtain the initial gel.

[0012] 2) Add the initial gel to a high-pressure reactor and statically crystallize for 6-168 hours. Cool to room temperature, filter and wash the product until it is neutral, dry it in a 50-200°C oven for 12-24 hours, and then calcine it in a muffle furnace at 300-600°C for 2-8 hours.

[0013] 3) The sample calcined in step 2) is placed in a 0.1-1 mol / L alkali metal (M) nitrate solution, and ion exchange is performed in a water bath at 10-90° C. for 1-60 hours. The sample is dried and calcined to obtain an M-molecular sieve catalyst.

[0014] The present invention provides an application method of a molecular sieve catalyst for selective hydrogenation of carbon dioxide to prepare lower alcohols, comprising the following steps:

[0015] 1) Add 0.1-0.5g of catalyst to a fixed-bed reactor at atmospheric pressure, pretreat the catalyst with hydrogen at 200-400°C for 1h, then cool to room temperature. Then, introduce hydrogen and CO2 gases at a molar ratio of hydrogen to CO2 of 2-5, a total space velocity of 10-100 ml / min, and a pressure of 0.5-4.0 MPa.

[0016] 2) Heat the reactor to 100-350°C to obtain the product.

[0017] 3) The products were analyzed using a gas chromatograph directly connected to the fixed bed.

[0018] The M-molecular sieve catalyst provided by the present invention has high yield, low cost, direct application, simple and easy preparation process, and stable structure and performance. The catalyst exhibits excellent catalytic activity and high methanol or ethanol yield in the CO2 selective hydrogenation reaction, has a long catalyst life, and can be recycled multiple times without significant decrease in catalytic activity. The catalytic process is environmentally friendly and pollution-free, and can be widely promoted and applied in the reaction of selective hydrogenation of CO2 to produce low-carbon alcohols. In summary, the present invention has high activity and methanol selectivity, and therefore has prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Neutron diffraction and structural analysis diagram of the catalyst prepared by the present invention.

[0020] Figure 2 This is a performance comparison chart of the Cu@FAU catalyst prepared in the present invention and traditional typical catalysts. Implementation Method

[0021] The present invention will be further described in detail and completely below with reference to specific embodiments.

[0022] Unless otherwise specified, the specific experimental methods and equipment involved in the following examples are all conventional methods or carried out according to the conditions recommended by the manufacturer's instructions, and the reagents involved are all commercially available. Example

[0023] A certain amount of soluble copper salt (copper nitrate) was dissolved in water to obtain a copper salt aqueous solution, which was then fully stirred with an organic amine (3-aminopropyltriethoxysilane) complexing agent for 30 minutes. Then, sodium aluminate, sodium hydroxide and silica sol were added in the ratio of H20:SiO2=20, Cu:SiO2=0.08, NaOH:SiO2=0.7, and Al:SiO2=0.1, and the mixture was fully stirred for 100 minutes to obtain an initial gel.

[0024] The initial gel was added to a high-pressure reactor and statically crystallized at 100°C for 100 h. The product was cooled to room temperature and filtered and washed until neutral. The product was dried in a 100°C oven for 12 h and then calcined in a muffle furnace at 550°C for 6 h.

[0025] The calcined samples were placed in a 1 mol / L sodium nitrate (or potassium nitrate) solution and subjected to ion exchange at 80°C for 12 hours in a water bath. After drying and calcination, the resulting Cu-Na-silicon-alumina molecular sieve (named Cu-Na-Z) and Cu-K-silicon-alumina molecular sieve (named Cu-KZ) were obtained. The catalysts had a silicon-alumina atomic ratio of 12, a Cu content of 3% by mass, and a Cu-to-alkali metal mass ratio of 2:1.

[0026] Applying the above catalyst to the selective hydrogenation reaction of carbon dioxide comprises the following steps:

[0027] 0.2 g of catalyst was added to a fixed-bed reactor at atmospheric pressure. The catalyst was pretreated with hydrogen at 300 °C for 1 h and then cooled to room temperature. Then, hydrogen and CO2 were introduced. The molar ratio of hydrogen to CO2 was 3, the total space velocity was 35 ml / min, and the pressure was 3 MPa.

[0028] The reaction product can be obtained by raising the reactor temperature to 250°C.

[0029] The products were analyzed by gas chromatography. The gas chromatograph used was a Tianmei 7900 gas chromatograph equipped with an FID detector and an Agilent HP-PLOT Q capillary column. The products were separated by programmed temperature increase, with the following heating steps: starting temperature was 60 o C, keep warm for 3 min, then o C / min to 200 o C, and kept warm for 5 min. The conversion rate of the raw material and the selectivity of the target product were calculated by normalization method. The results of the catalytic performance evaluation are shown in Table 1:

[0030] Table 1 Effects of different alkali metals on the selective hydrogenation of carbon dioxide

[0031] catalyst <![CDATA[CO2 conversion rate (mol%)]]> Methanol selectivity (%) Methanol yield (mmol / g / h) Cu-Na-Z 11.5 89.5 12.5 Cu-KZ 9.2 90.3 10.3

[0032] The results of catalytic performance evaluation show that there are certain differences in the CO2 conversion rate and methanol selectivity of catalyst samples containing different alkali metals M under the same reaction conditions. The Cu-Na-Z system catalyst shows the best catalytic activity in the selective hydrogenation of carbon dioxide, with a methanol yield of up to 12.5 mmol / g / h.

[0033] In the embodiment, chromium nitrate can be used instead of copper nitrate to carry out the experiment and obtain ideal results. Example

[0034] The catalyst used in this example is Cu-Na-Z. The effect of the continuous reaction time on the activity of the Cu-Na-Z catalyst in the selective hydrogenation of carbon dioxide was investigated. The catalytic performance evaluation is shown in Table 2:

[0035] Table 2 Effect of continuous reaction time on the activity of Cu-Na-Z in selective hydrogenation of carbon dioxide

[0036] Continuous reaction time <![CDATA[CO2 conversion rate (mol%)]]> Methanol selectivity (%) Methanol yield (mmol / g / h) 1 h 11.5 89.5 12.5 5 h 11.6 89.3 12.4 8 h 11.4 89.5 12.3 20 h 11.2 89.4 12.2 40 h 11.4 89.5 12.4

[0037] The results of catalytic performance evaluation show that within a certain time range, as the continuous reaction time increases, the conversion rate of CO2 alkyne and the selectivity of methanol can remain relatively stable. When the reaction time reaches 40h, the catalytic activity does not show a significant decrease, indicating that the Cu-Na-Z system catalyst has excellent catalytic stability.

Claims

1. An application of a molecular sieve catalyst for selective hydrogenation of carbon dioxide to prepare lower alcohols, wherein: The low-carbon alcohol is methanol; the molecular sieve catalyst is composed of metal Cu or Cr as an active ingredient, a composite alkali metal and a silicon-aluminum molecular sieve as a carrier, wherein the loading amount of the metal Cu or Cr is 0.5-15% of the catalyst mass; the alkali metal is 0.8-8% of the catalyst mass. The specific preparation method comprises the following steps: 1) Dissolve a soluble copper salt or chromium salt in water according to the metered amount to obtain a copper salt aqueous solution, then fully stir it with an organic amine complexing agent for 10-40 minutes, and then add an aluminum source, an alkali source, and a silicon source in the ratio of H20:SiO2=10-1000, Cu or Cr:SiO2=0.002-0.2, NaOH:SiO2=0.5-10, and Al:SiO2=0.05-0.5, and fully stir for 30-300 minutes to obtain an initial gel; 2) Add the initial gel to a high-pressure reactor and statically crystallize for 6-168 hours. Cool to room temperature, filter and wash the product until it is neutral, dry it in a 50-200°C oven for 12-24 hours, and then calcine it in a muffle furnace at 300-600°C for 2-8 hours. 3) The sample calcined in step 2) is placed in a 0.1-1 mol / L alkali metal nitrate solution, and ion exchange is carried out in a water bath at 10-90° C. for 1-60 hours. The sample is dried and calcined to obtain an M-silica-alumina molecular sieve catalyst.

2. The use of the molecular sieve catalyst according to claim 1, characterized in that: The loading amount of the metal active component is 3% of the mass of the catalyst; the mass ratio of the metal active component to the alkali metal is 2:

1.

3. The use of the molecular sieve catalyst according to claim 1, characterized in that: The preparation method of the molecular sieve catalyst specifically comprises the following steps: 1) Dissolve a soluble copper salt or chromium salt in water according to the metered amount to obtain a copper salt aqueous solution, then fully stir with 3-aminopropyltriethoxysilane organic amine complexing agent for 10-40 minutes, and then add aluminum source, alkali source and silicon source in the ratio of H20:SiO2=10-1000, Cu or Cr:SiO2=0.002-0.2, NaOH:SiO2=0.5-10, Al:SiO2=0.05-0.5 in sequence and fully stir for 30-300 minutes to obtain an initial gel; 2) Add the initial gel to a high-pressure reactor and statically crystallize for 6-168 hours. Cool to room temperature, filter and wash the product until it is neutral, dry it in a 50-200°C oven for 12-24 hours, and then calcine it in a muffle furnace at 300-600°C for 2-8 hours. 3) The sample calcined in step 2) is placed in a 0.1-1 mol / L sodium nitrate or potassium nitrate solution, and ion exchange is carried out in a water bath at 10-90° C. for 1-60 hours. The sample is dried and calcined to obtain an M-silica-alumina molecular sieve catalyst.

4. The use of the molecular sieve catalyst according to claim 3, characterized in that: The soluble copper salt is copper nitrate or copper acetate; the soluble chromium salt is chromium nitrate.

5. The use of the molecular sieve catalyst according to claim 3, characterized in that: The alkali source is sodium hydroxide; the silicon source is any one of silica sol, ethyl orthosilicate, silica aerosol, amorphous silica powder or silicate; and the aluminum source is any one of aluminum sol, aluminum isopropoxide, metaaluminate, aluminate and pseudo-boehmite.

6. The use of the molecular sieve catalyst according to claim 3, characterized in that: The M-silica-alumina molecular sieve includes FAU, MOR, CHA and MFI structures.

7. The use of the molecular sieve catalyst according to claim 1, characterized in that: The specific application method includes the following steps: 1) Add 0.1-0.5g of catalyst to a fixed-bed reactor at atmospheric pressure, pretreat the catalyst with hydrogen at 200-400°C for 1h, then cool to room temperature, then introduce hydrogen and CO2 gas, with a hydrogen to CO2 molar ratio of 2-5, a total space velocity of 10-100ml / min, and a pressure of 0.5-4.0 MPa; 2) Raise the temperature of the reactor to 100-350°C to obtain the product; 3) The products were analyzed using a gas chromatograph directly connected to the fixed bed.

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