Preparation and application of carbon dioxide hydrogenation catalyst directly modified by hydroxyl of polysaccharide

Abstract

The invention discloses preparation and application of a polysaccharide direct hydroxyl modified carbon dioxide hydrogenation catalyst, and belongs to the field of carbon dioxide catalysis. In the process of preparing the catalyst by a coprecipitation method, guar gum is utilized to directly introduce surface hydroxyl groups to the iron-based catalyst, and under the condition of high-temperature water bath, rich hydroxyl groups in guar gum are successfully grafted to the iron-based catalyst by utilizing strong interaction caused by high viscosity of guar gum, so that the iron-based catalyst is prepared. And the catalyst is kept unchanged in the process of preparing low-carbon olefin through carbon dioxide hydrogenation. The modified surface hydroxyl group obviously enhances the CO2 adsorption capacity and oxidation resistance of the iron-based catalyst. According to the method provided by the invention, doping of any other metal is not introduced, the preparation method is simple and green, and the catalyst has excellent reaction performance for preparing low-carbon olefin through CO2 catalytic hydrogenation and good stability.

Description

Technical Field

[0001] The present invention belongs to the field of carbon dioxide catalysis, and relates to the preparation and application of a carbon dioxide hydrogenation catalyst directly modified by hydroxyl groups of polysaccharides, and specifically relates to a hydroxyl-modified metal catalyst and its application to the method for hydrogenating carbon dioxide to synthesize light olefins (C 2 ＝ -C 4 ＝ ). Background Art

[0002] Nowadays, the rapid development of mankind has led to a continuous increase in the content of CO 2 in the atmosphere and the ocean. However, excessive CO 2 causes a series of environmental problems, such as the greenhouse effect and ocean acidification. At the same time, fossil fuels, as a non-renewable resource, are continuously consumed in industrial production.

[0003] In recent years, CO 2 hydrogenation has received increasing attention because it can efficiently convert CO 2 into various high-value-added products. Among them, light olefins (C 2 ＝ -C 4 ＝ ) are one of the organic intermediate chemicals with the highest production, and occupy an important position in the chemical industry. Generally speaking, the production of light olefins by CO 2 hydrogenation can be achieved through two routes: methanol-intermediate or Fischer-Tropsch synthesis (FTS). The methanol-intermediate route can break the Anderson-Schulz-Flory (ASF) distribution and exhibit higher selectivity for target products. However, due to the limitation of thermodynamic equilibrium, its CO 2 conversion rate is low and the CO selectivity is high, making it difficult to meet the requirements of industrial production. Therefore, designing a reasonable FTS catalyst is an urgent need for the application of CO 2 hydrogenation.

[0004] Since iron-based catalysts have excellent activities in both the reverse water-gas shift (RWGS) and chain growth reactions, they are widely used in the FTS route of CO 2 hydrogenation. However, unmodified iron catalysts usually exhibit ordinary catalytic performance. Therefore, researchers have been committed to developing various strategies to improve the performance of iron-based catalysts. Alkali metals such as K or Na are added to iron as modifiers. Higher catalytic performance and light olefin selectivity are obtained by enhancing CO 2 adsorption and inhibiting H dissociation. Another method is to construct a multi-metal catalytic system by introducing additional active metals. For example, compared with pure iron carbide, the formation of Fe-CO alloy carbide is more favorable for CO 2Conversion and light olefin production. In addition, the electron transfer effect between metal phases such as Fe / Zn and Fe / Mg also significantly improves the performance of iron-based catalysts.

[0005] Although these metal doping strategies have been successfully developed for modifying iron-based catalysts, the additional metals inevitably complicate the preparation process and increase the cost. At the same time, metal ions that are difficult to remove also cause environmental pollution. From another perspective, previous studies have shown that surface hydroxyl groups play a crucial role in improving CO 2 adsorption and stabilizing reaction intermediates. Therefore, modifying iron-based catalysts by introducing metal-free hydroxyl groups may provide a green and convenient route for CO 2 hydrogenation. However, to our knowledge, most of the studies on hydroxyl groups during the CO 2 hydrogenation process have focused on the surface hydroxyl groups brought by supports (Al 2 O 3 , TiO 2 , SiC, SiO 2 , LDH, etc.), and there are almost no reports on stable and direct methods for hydroxyl group modification of active sites, especially for iron-based catalysts. Therefore, developing an efficient hydroxyl group modification strategy is of great significance for CO 2 hydrogenation and even other reactions. SUMMARY OF THE INVENTION

[0006] The object of the present invention is to provide a preparation method and application of a highly efficient catalyst for CO 2 hydrogenation, which uses guar gum, a non-toxic, edible, and easily degradable polysaccharide, to directly introduce surface hydroxyl groups onto the iron-based catalyst during the preparation process. The modified surface hydroxyl groups significantly enhance the CO 2 adsorption capacity and antioxidant capacity of the iron-based catalyst. The method provided by the present invention does not introduce any doping of other metals, has a simple and green production method, and has excellent reaction performance and good stability for the catalytic hydrogenation of CO 2 to prepare light olefins.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] A preparation method of a polysaccharide direct hydroxyl-modified carbon dioxide hydrogenation catalyst, which is prepared by a co-precipitation method and includes the following steps:

[0009] Dissolve the Fe source in deionized water as solution A, and dissolve the precipitant in deionized water as solution B;

[0010] While dropping solution A and solution B into the guar gum solution respectively and stirring to obtain a suspension;

[0011] The suspension is successively freeze-dried and calcined to obtain a carbon dioxide hydrogenation catalyst directly modified with polysaccharide hydroxyl groups.

[0012] The Fe source is an acidic iron salt, preferably iron nitrate; the precipitating agent is a soluble alkali salt, preferably ammonium carbonate; in terms of mass ratio, Fe source: precipitating agent = 1:(1.5 - 2.0), preferably Fe source: precipitating agent = 1:2.0.

[0013] The concentration of the guar gum solution is 0.001M - 0.01M, preferably 0.005M;

[0014] When the addition of either solution A or solution B is completed, the reaction stops; at the end of the addition, the Fe source is completely precipitated; during the addition, the system pH is maintained at 7 - 9, and the stirring temperature is 70°C - 90°C.

[0015] The freeze-drying process is as follows: vacuum freeze-drying is adopted, and freeze-drying is carried out at -40°C to -50°C for 2 days to 3 days;

[0016] The calcination process is: calcination is carried out at 500°C to 700°C for 5h to 8h.

[0017] The application of the carbon dioxide hydrogenation catalyst directly modified with polysaccharide hydroxyl groups includes the following steps:

[0018] The catalyst is loaded into the constant temperature section of a fixed-bed reactor, and the catalyst is reduced with high-purity hydrogen;

[0019] The reaction gas is introduced to carry out the carbon dioxide hydrogenation reaction.

[0020] After the catalyst is tableted, crushed, and sieved, it is loaded into a fixed-bed reactor; the sieving is through a 20-mesh to 40-mesh sieve;

[0021] The purity of the hydrogen is ≥99.999%;

[0022] The reduction process is: the catalyst is reduced at 350°C to 450°C for 2h to 10h, the pressure during the reduction process is 0.1MPa to 1MPa, and the mass space velocity of hydrogen is 4000mLh -1 g -1 ~8000mLh -1 g -1 .

[0023] The reaction gas is a CO 2 and H 2 mixed gas, in terms of molar ratio, H 2 :CO 2 = 1 - 4;

[0024] During the hydrogenation of carbon dioxide, the reaction temperature is 300°C to 360°C, the reaction pressure is 2 MPa to 4 MPa, and the mass space velocity of the reaction gas is 3000 mLh -1 g -1 ~9000 mLh -1 g -1 。

[0025] The beneficial conditions of the present invention are as follows:

[0026] 1. The present invention changes the strategy of adding alkali metal promoters to the iron oxide precursor, introducing transition metal structural promoters or synthesizing multi-component ferrites in the past. By changing the traditional co-precipitation method, using the edible, non-toxic and easily degradable macromolecular polysaccharide guar gum, surface hydroxyl groups are directly introduced onto the iron-based catalyst during the process of preparing the catalyst by co-precipitation. Under the condition of high-temperature water bath, using the strong interaction induced by the high viscosity of guar gum, the abundant hydroxyl groups in guar gum are successfully grafted onto the iron-based catalyst and remain unchanged during the hydrogenation of carbon dioxide to prepare light olefins. In addition, the presence of hydroxyl groups promotes the formation of CH * intermediates, thus accelerating the chain growth reaction and inhibiting the formation of carbon oxides. Compared with the initial Fe-based catalyst, the light olefin selectivity and CO 2 conversion rate of G-Fe are increased from 7.7% and 19.6% to 33.4% and 40.5% respectively at most. At the same time, the modified G-Fe catalyst still maintains excellent stability during the long-term evaluation process and has good industrial application prospects.

[0027] 2. The present invention provides a new hydroxyl modification method without metal and carbon slag, which is more efficient and green compared with the prior art. At the same time, it clearly reveals the influence of hydroxyl groups on metal catalysts and CO 2 hydrogenation, and can be extended to other metal catalysts and other heterogeneous catalytic reactions, having broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the reaction performance of Fe modified with different concentrations of guar gum.

[0029] Figure 2 is the CO 2 -TPD spectra of Fe and G-Fe catalysts.

[0030] Figure 3 is the XPS Fe2p spectra and fitting curves of different catalysts: among them, a) is Fe and b) is G-Fe.

[0031] Figure 4 is the viscosity of different concentrations of guar gum solution at 80°C.

[0032] Figure 5 Viscosities of 0.005 M guar gum solution, glucose solution, starch solution and cellulose solution at 80 °C.

[0033] Figure 6 Reaction performance of G-Fe catalyst for 100 h.

[0034] Figure 7 Transmission electron microscope images of the catalyst and elemental images of Fe and G-Fe catalysts: a) is Fe and b) is G-Fe.

[0035] Figure 8 Scanning electron microscope images of different catalysts; a) is Fe and b) is G-Fe-0.005M. Detailed implementation mode

[0036] The present invention will be specifically described below in conjunction with embodiments to facilitate the understanding of the present invention by those skilled in the art. It is necessary to specifically point out here that the embodiments are only used to further illustrate the present invention and cannot be understood as limiting the protection scope of the present invention. Those skilled in the art, based on the above-mentioned inventive content, make non-essential improvements and adjustments to the present invention, which should still fall within the protection scope of the present invention. At the same time, for the raw materials not specifically described below, they are all commercially available products; for the process steps or preparation methods not specifically mentioned, they are all process steps or preparation methods known to those skilled in the art.

[0037] The present invention adopts the co-precipitation method, uses acidic iron salt (preferably ferric nitrate) as the iron source, and soluble alkali salt (preferably ammonium carbonate) as the precipitating agent. The iron source is dissolved in deionized water as solution A, and the precipitating agent is dissolved in deionized water as solution B. Solution A and solution B are respectively added dropwise to a 0.001 M to 0.01 M, 100 mL to 200 mL guar gum solution, maintaining the pH of the reaction system at 7 to 9, and stirring at 70 °C to 90 °C. When the addition of any one of solution A and solution B is completed, the reaction stops. At this time, the iron source is completely precipitated to obtain a suspension; wherein, in terms of the mass ratio of solutes, the mass ratio of the iron source to the precipitating agent is 1:(1.5 - 2.0), preferably 1:2.0. Vacuum freeze-drying is adopted. The suspension is freeze-dried at -40 °C to -50 °C for 2 to 3 days, and then calcined at 500 °C to 700 °C for 5 to 8 h to obtain a carbon dioxide hydrogenation catalyst modified by direct hydroxyl groups of polysaccharides.

[0038] The carbon dioxide hydrogenation catalyst directly modified by hydroxyl groups prepared by the present invention is used for hydrogenating carbon dioxide to prepare light olefins, and its catalytic performance is tested: To evaluate the catalytic performance of the prepared catalyst, a fixed-bed reactor with an inner diameter of 6 mm is used. After the catalyst prepared by the present invention is tableted, crushed, and sieved (20 mesh to 40 mesh), it is filled in the isothermal section of the fixed-bed reactor; then the catalyst is reduced with high-purity H -1 g -1 ~8000 mLh -1 g -1 at a mass space velocity of 4000 mLh 2 and a reduction condition of 350°C to 450°C and a pressure of 0.1 MPa to 1 MPa for 2 h to 10 h. After reduction, the reactor is naturally cooled to room temperature. Then, CO 2 / H 2 reaction gas (H 2 / CO 2 molar ratio of 1 to 4, mass space velocity of 3000 mLh -1 g -1 ~9000 mLh -1 g -1 ) is introduced into the reactor, and the system temperature and pressure are gradually increased to 300°C to 360°C and 2.0 MPa to 4.0 MPa, respectively. W / F is 3.5 ghmol -1 (relative to the weight of G-Fe). To collect heavy hydrocarbons and eliminate the by-product water generated by the reaction, an ice trap is placed between the reactor and the back pressure valve, and 2 g of octane is added to the ice trap to absorb heavy hydrocarbons. After the reaction, the products in the ice trap are collected, and 0.2 g of dodecane is added to the oil phase as an internal standard. An off-line gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector (FID) and a DB-1 capillary column is used to detect the oil-phase and water-phase products. Two sets of on-line gas chromatographs (GL Sciences GC320 and Shimadzu GC-2014) are used to identify the gas-phase products: one set is equipped with a thermal conductivity detector (TCD, GC320) and an activated carbon column for analyzing Ar, CO, CH 4 and CO 2 ; the other set is equipped with a flame ionization detector (FID, GC-2014) and a GS-ALUMINA capillary column for analyzing light hydrocarbons.

[0039] The present invention is further described below with reference to the accompanying drawings and embodiments.

[0040] Example 1

[0041] 1. Preparation of Fe catalyst

[0042] Take 10 g of Fe(NO 3 )3 ·9H₂O 2 O is dissolved in 100 mL of deionized water to form solution A; simultaneously, 20 g of (NH 4 )₂ 2 CO₃ 3 is dissolved in 100 mL of deionized water in another beaker to form solution B. Subsequently, the two solutions are separately dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum is 0 M. During the stirring, the pH of the system is maintained at 8 and the temperature is 80 °C. When either solution is exhausted, the reaction stops. The obtained suspension is freeze-dried and then calcined at 600 °C for 6 hours to finally obtain a catalyst denoted as Fe.

[0043] Application of Fe catalyst in the hydrogenation of CO to prepare light olefins: 2

[0044] After the Fe catalyst is tableted, crushed, and passed through a 20 - 40 mesh sieve, it is filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst is first reduced with hydrogen. The reduction conditions are: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mL·h⁻¹ -1 ⁻¹ -1 ; then carbon dioxide and hydrogen are introduced for reaction. The reaction conditions are: temperature 320 °C, pressure 3.0 MPa, and the molar ratio of H₂ / CO₂ is 3, and the mass space velocity is 6000 mL·h⁻¹ 2 / CO₂ 2 ⁻¹, and the reaction results are shown in Table 1 and -1 ⁻¹ -1 , Figure 1 .

[0045] Example 2

[0046] 1. Preparation of G-Fe-0.001 catalyst

[0047] Take 10 g of Fe(NO₃)₃ 3 ·9H₂O 3 ·9H₂O 2 is dissolved in 100 mL of deionized water to form solution A; simultaneously, 20 g of (NH₄)₂ 4 CO₃ 2 CO₃ 3 is dissolved in 100 mL of deionized water in another beaker to form solution B. Subsequently, the two solutions are separately dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum is 0.001 M. During the stirring, the pH of the system is maintained at 8 and the temperature is 80 °C. When either solution is exhausted, the reaction stops. The obtained suspension is freeze-dried and then calcined at 600 °C for 6 hours to finally obtain a catalyst denoted as G-Fe-0.001.

[0048] 2. Application of G-Fe-0.001 catalyst in CO2 Application in the hydrogenation for preparing light olefins:

[0049] After the G-Fe-0.001 catalyst was tableted, crushed, and sieved through a 20-40 mesh sieve, it was filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst was first reduced with hydrogen. The reduction conditions were: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mLh -1 g -1 ; then carbon dioxide and hydrogen were introduced for the reaction. The reaction conditions were: temperature 320 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2 was 3, and the mass space velocity was 6000 mLh -1 g -1 , and the reaction results are shown in Table 1 and Figure 1 .

[0050] Example 3

[0051] 1. Preparation of the G-Fe-0.005 catalyst

[0052] Take 10 g of Fe(NO 3 ) 3 ·9H 2 O and dissolve it in 100 mL of deionized water as solution A; meanwhile, dissolve 20 g of (NH 4 ) 2 CO 3 in 100 mL of deionized water in another beaker as solution B. Subsequently, the two solutions were respectively dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum was 0.005 M. During the stirring, the pH of the system was maintained at 8 and the temperature was 80 °C. When either solution was exhausted, the reaction stopped. The obtained suspension was freeze-dried and then calcined at 600 °C for 6 hours to finally obtain the catalyst denoted as G-Fe-0.005 (or G-Fe).

[0053] 2. Application of the G-Fe-0.005 catalyst in the hydrogenation of CO 2 for preparing light olefins:

[0054] After the G-Fe-0.005 catalyst was tableted, crushed, and sieved through a 20-40 mesh sieve, it was filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst was first reduced with hydrogen. The reduction conditions were: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mLh -1 g -1 ; then carbon dioxide and hydrogen were introduced for the reaction. The reaction conditions were: temperature 320 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2The molar ratio is 3 and the mass space velocity is 6000 mL / h -1 g -1 , and the reaction results are shown in Table 1 and Figure 1 .

[0055] Example 4

[0056] 1. Preparation of G-Fe-0.01 catalyst

[0057] Take 10 g of Fe(NO 3 ) 3 ·9H 2 O and dissolve it in 100 mL of deionized water as solution A; at the same time, dissolve 20 g of (NH 4 ) 2 CO 3 in 100 mL of deionized water in another beaker as solution B. Subsequently, the two solutions are respectively dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum is 0.01 M. During the stirring, the pH of the system is maintained at 8 and the temperature is 80 °C. When any one of the solutions is exhausted, the reaction stops. The obtained suspension is freeze-dried and then calcined at 600 °C for 6 hours. Finally, the obtained catalyst is denoted as G-Fe-0.01.

[0058] 2. Application of G-Fe-0.01 catalyst in the hydrogenation of CO 2 to prepare light olefins:

[0059] After the G-Fe-0.01 catalyst is tableted, crushed, and passed through a 20-40 mesh sieve, it is filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst is first reduced with hydrogen. The reduction conditions are: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity is 6000 mL / h -1 g -1 ; then carbon dioxide and hydrogen are introduced for reaction. The reaction conditions are: temperature 320 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2 is 3, and the mass space velocity is 6000 mL / h -1 g -1 , and the reaction results are shown in Table 1 and Figure 1 .

[0060] The catalytic performance of the catalysts prepared in Examples 1-4 was tested, and the results are shown in Table 1:

[0061] Table 1 Influence of different guar gum concentrations on the reaction performance of the catalyst

[0062]

[0063] From Table 1 and Figure 1As shown, for Examples 2, 3, and 4 after hydroxyl modification with guar gum, compared with Example 1, both the carbon dioxide conversion rate and the selectivity of light olefins are significantly improved. Among them, Example 3 exhibits the highest CO 2 conversion rate (40.5%) and light olefin selectivity (33.4%), far higher than the catalytic performance (19.6% and 7.7%) in Example 1. Through SEM analysis of Example 1 and Example 3 (as shown in Figure 7 and Figure 8 ), according to the elemental images of the two examples, it can be clearly found that Fe and O elements are evenly dispersed on the surface of the catalyst, proving the successful preparation of the catalyst. At the same time, according to the SEM images of the two examples, it can be found that as the viscosity of the guar gum solution increases, the precipitated Fe particles show a layered structure after being wrapped and pulled, enabling the catalyst to have a larger contact area when facing the reaction gas. At the same time, CO 2 -TPD and XPS analyses were carried out on Examples 1 and 3 (as shown in Figure 2 and Figure 3 ), and it can be clearly found that the catalyst G-Fe-0.005 in Example 3 has a stronger CO 2 adsorption capacity and a higher content of carbon-iron active phase compared with the catalyst Fe in Example 1. In addition, the change in the catalytic performance of Examples 2-4 is due to the fact that when the concentration of guar gum is too low, the small amount of hydroxyl content cannot fully modify the iron-based catalyst, while when the concentration of guar gum is too high, the high viscosity will hinder the stirring during the catalyst preparation process, thereby reducing the dispersion performance and catalytic performance of the catalyst. The viscosities of different concentrations of guar gum solutions are as shown in Figure 4 .

[0064] Example 5

[0065] 1. Preparation of G-Fe-0.005 catalyst

[0066] Take 10 g of Fe(NO 3 ) 3 ·9H 2 O and dissolve it in 100 mL of deionized water as Solution A; at the same time, dissolve 20 g of (NH 4 ) 2 CO 3 in 100 mL of deionized water in another beaker as Solution B. Subsequently, the two solutions are respectively dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum is 0.005 M. During the stirring, the pH of the system is maintained at 8 and the temperature is 80 °C. When either solution is exhausted, the reaction stops. The obtained suspension is freeze-dried and then calcined at 600 °C for 6 hours to finally obtain the catalyst denoted as G-Fe-0.005.

[0067] 2. Application of G-Fe-0.005 catalyst in the hydrogenation of CO 2 to prepare light olefins:

[0068] After the G-Fe-0.005 catalyst is tableted, crushed, and sieved through a 20-40 mesh sieve, it is filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst is first reduced with hydrogen. The reduction conditions are: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mLh -1 g -1 ; then carbon dioxide and hydrogen are introduced for reaction. The reaction conditions are: temperature 290 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2 is 3, and the mass space velocity is 6000 mLh -1 g -1 . The reaction results are shown in Table 2.

[0069] Example 6

[0070] 1. Preparation of G-Fe-0.005 catalyst

[0071] Take 10 g of Fe(NO 3 ) 3 ·9H 2 O and dissolve it in 100 mL of deionized water as solution A; at the same time, dissolve 20 g of (NH 4 ) 2 CO 3 in 100 mL of deionized water in another beaker as solution B. Subsequently, the two solutions are respectively dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum is 0.005 M. During the stirring, the pH of the system is maintained at 8 and the temperature is 80 °C. When either solution is exhausted, the reaction stops. The obtained suspension is freeze-dried and then calcined at 600 °C for 6 hours to finally obtain the catalyst denoted as G-Fe-0.005.

[0072] 2. Application of G-Fe-0.005 catalyst in the hydrogenation of CO 2 to prepare light olefins:

[0073] After the G-Fe-0.005 catalyst is tableted, crushed, and sieved through a 20-40 mesh sieve, it is filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst is first reduced with hydrogen. The reduction conditions are: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mLh -1 g -1 ; then carbon dioxide and hydrogen are introduced for reaction. The reaction conditions are: temperature 360 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2The molar ratio is 3 and the mass space velocity is 6000 mL / h -1 g -1 , and the reaction results are shown in Table 2.

[0074] Example 7

[0075] 1. Preparation of G-Fe-0.005 catalyst

[0076] Take 10 g of Fe(NO 3 ) 3 ·9H 2 O and dissolve it in 100 mL of deionized water to obtain Solution A; at the same time, dissolve 20 g of (NH 4 ) 2 CO 3 in 100 mL of deionized water in another beaker to obtain Solution B. Subsequently, the two solutions are separately dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum is 0.005 M. During the stirring, the pH of the system is maintained at 8 and the temperature is 80 °C. When either solution is exhausted, the reaction stops. The obtained suspension is freeze-dried and then calcined at 600 °C for 6 hours to finally obtain the catalyst denoted as G-Fe-0.005.

[0077] 2. Application of G-Fe-0.005 catalyst in the hydrogenation of CO 2 to prepare light olefins:

[0078] After the G-Fe-0.005 catalyst is tableted, crushed, and passed through a 20-40 mesh sieve, it is filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst is first reduced with hydrogen. The reduction conditions are: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity is 6000 mL / h -1 g -1 ; then carbon dioxide and hydrogen are introduced for reaction. The reaction conditions are: temperature 390 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2 is 3, and the mass space velocity is 6000 mL / h -1 g -1 , and the reaction results are shown in Table 2.

[0079] The catalytic performance of the catalysts prepared in Examples 5-7 was tested, and the results are shown in Table 2:

[0080] Table 2 Catalytic performance of G-Fe-0.005M at different temperatures

[0081]

[0082] As shown in Table 2, with the increase of the reaction temperature, the G-Fe-0.005M catalyst for CO 2The selectivity increases from 30.1% to 43.2% as the reactant temperature rises. This is because at higher temperatures, the G-Fe-0.005 catalyst has higher reactivity. However, the higher reaction temperature weakens the catalyst's chain growth ability, and the excessive reaction temperature leads to the formation of a large amount of by-product methane. Therefore, the selectivity for light olefins shows a reaction trend of first increasing and then decreasing.

[0083] Example 8

[0084] 1. Preparation of G-Fe-0.005 catalyst

[0085] Take 10 g of Fe(NO 3 ) 3 ·9H 2 O and dissolve it in 100 mL of deionized water to obtain solution A. At the same time, dissolve 20 g of (NH 4 ) 2 CO 3 in another beaker containing 100 mL of deionized water to obtain solution B. Subsequently, drip the two solutions into 100 mL of guar gum solution and stir. The concentration of guar gum is 0.005 M. During stirring, maintain the system pH at 8 and the temperature at 80 °C. When either solution is exhausted, the reaction stops. Freeze-dry the obtained suspension and then calcine it at 600 °C for 6 hours to finally obtain the catalyst denoted as G-Fe-0.005.

[0086] 2. Application of G-Fe-0.005 catalyst in the hydrogenation of CO 2 to prepare light olefins:

[0087] After the G-Fe-0.005 catalyst is tableted, crushed, and passed through a 20-40 mesh sieve, it is filled in the isothermal section of a fixed-bed reactor. Before the reaction, first reduce the Fe catalyst with hydrogen. The reduction conditions are: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mLh -1 g -1 ; then introduce carbon dioxide and hydrogen for reaction. The reaction conditions are: temperature 320 °C, pressure 2.5 MPa, and the molar ratio of H 2 / CO 2 is 3, and the mass space velocity is 6000 mLh -1 g -1 , and the reaction results are shown in Table 3.

[0088] Example 9

[0089] 1. Preparation of G-Fe-0.005 catalyst

[0090] Take 10 g of Fe(NO 3 ) 3 ·9H2 O was dissolved in 100 mL of deionized water to form solution A; simultaneously, 20 g of (NH 4 ) 2 CO 3 was dissolved in 100 mL of deionized water in another beaker to form solution B. Subsequently, the two solutions were separately dropped into 100 mL of guar gum solution and stirred. The concentration of guar gum was 0.005 M. During the stirring, the pH of the system was maintained at 8 and the temperature was 80 °C. When either solution was exhausted, the reaction stopped. The obtained suspension was freeze-dried and then calcined at 600 °C for 6 hours to finally obtain the catalyst denoted as G-Fe-0.005.

[0091] 2. Application of G-Fe-0.005 catalyst in the hydrogenation of CO 2 to prepare light olefins:

[0092] After the G-Fe-0.005 catalyst was tableted, crushed, and passed through a 20-40 mesh sieve, it was filled in the isothermal section of a fixed-bed reactor. Before the reaction, the Fe catalyst was first reduced with hydrogen. The reduction conditions were: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity 6000 mLh -1 g -1 ; then carbon dioxide and hydrogen were introduced for reaction. The reaction conditions were: temperature 390 °C, pressure 3.5 MPa, and the molar ratio of H 2 / CO 2 was 3, and the mass space velocity was 6000 mLh -1 g -1 . The reaction results are shown in Table 3.

[0093] The catalytic performance of the catalysts prepared in Examples 5-7 was detected, and the reaction results are shown in Table 3.

[0094] Table 3 Catalytic performance of G-Fe-0.005M under different pressures

[0095]

[0096] As shown in Table 3, when the system reaction pressure was 2.5 MPa, the lower reaction pressure decreased the intermolecular collision frequency, and it was difficult for the G-Fe-0.005M catalyst to have sufficient contact and effective collision with the reaction gas under this pressure. Therefore, a lower carbon dioxide conversion rate (28.1%) was exhibited. As the reaction pressure increased, both the carbon dioxide conversion rate and the light olefin selectivity showed a trend of first increasing and then decreasing. This was because the excessive reaction pressure squeezed the active sites of the catalyst, thereby reducing its catalytic activity. Secondly, from a thermodynamic perspective, although the excessive pressure would promote the forward reaction, under high-pressure conditions, the formation of by-product methane became easier.

[0097] Comparative Example 1

[0098] 1. Preparation of K-Fe catalyst

[0099] The catalyst Fe prepared in Example 1 was impregnated by the impregnation (IWI) method with K 2 CO 3 for 10 - 30 min to load 2% of K on the metal catalyst Fe, followed by stirring and ultrasonic treatment. Finally, it was calcined in a tubular furnace at 600 °C for 4 h. The obtained metal catalyst was named K-Fe.

[0100] 2. Reaction of CO 2 hydrogenation to prepare light olefins

[0101] The K-Fe catalyst was pelletized, crushed, and passed through a 20 - 40 mesh sieve, and then filled in the isothermal section of a fixed bed reactor. Before the reaction, the Fe catalyst was first reduced with hydrogen under the reduction conditions of: temperature 400 °C, time 10 h, pressure 0.1 MPa, and mass space velocity of 6000 mLh -1 g -1 ; then carbon dioxide and hydrogen were introduced for the reaction under the reaction conditions of: temperature 320 °C, pressure 3.0 MPa, and the molar ratio of H 2 / CO 2 being 3, and the mass space velocity of 6000 mLh -1 g -1 , and the reaction results are shown in Table 4.

[0102] Table 4 Influence of K modification on the reaction performance of the catalyst

[0103]

[0104] As shown in Table 4, compared with the K-Fe catalyst prepared by the traditional alkali metal impregnation method, although the two have relatively close light olefin selectivities, the K-Fe catalyst still shows worse performance in terms of CO 2 conversion and CO selectivity than the guar gum-modified G-Fe-0.005M catalyst. At the same time, compared with the guar gum modification, it is difficult for the modification with alkali metal potassium to reduce the alcohol selectivity (6.1% for K-Fe and 0.5% for G-Fe-0.005M). Therefore, compared with the traditional alkali metal loading modification method, the high-viscosity polysaccharide hydroxyl modification method not only has higher catalytic performance, but also simplifies the catalyst preparation difficulty, and at the same time, due to the absence of the use of alkali metal salts, the preparation process is more green and economical.

[0105] Comparative Example 2

[0106] In this comparative example, the preparation of the catalyst only changed the 0.005 M guar gum solution in the co-precipitation to a 0.005 M glucose solution, and all other preparation steps and reaction conditions were the same as those in Example 3. The catalyst finally prepared in this comparative example was named Glucose-Fe.CO 2 The reaction conditions for the hydrogenation to prepare light olefins were exactly the same as those in Example 3, and the reaction results are shown in Table 5.

[0107] Comparative Example 3

[0108] In this comparative example, the preparation of the catalyst only changed the 0.005 M guar gum solution in the co-precipitation to a 0.005 M soluble starch solution, and all other preparation steps and reaction conditions were the same as those in Example 3. The catalyst finally prepared in this comparative example was named Starch-Fe.CO 2 The reaction conditions for the hydrogenation to prepare light olefins were exactly the same as those in Example 3, and the reaction results are shown in Table 5.

[0109] Comparative Example 4

[0110] In this example, the preparation of the catalyst only changed the 0.005 M guar gum solution in the co-precipitation to a 0.005 M cellulose solution, and all other preparation steps and reaction conditions were the same as those in Example 3. The catalyst finally prepared in this comparative example was named Cellulose-Fe.CO 2 The reaction conditions for the hydrogenation to prepare light olefins were exactly the same as those in Example 3, and the reaction results are shown in Table 5.

[0111] Table 5 Effects of catalysts modified with different types of polysaccharide solutions on reaction performance

[0112]

[0113] As shown in Table 5, using other hydroxyl-rich saccharides such as glucose, soluble starch, and cellulose to replace the guar gum solution at the same concentration and using the same preparation method to modify the iron-based catalyst, however, due to the lower viscosity (as Figure 5 shown), it is difficult for the hydroxyl groups of these saccharides to graft onto the catalyst Fe, resulting in significantly lower performance than the G-Fe-0.005 M catalyst in Example 3. This difference in catalytic performance also reveals the key role of viscosity in hydroxyl modification and CO 2 hydrogenation.

[0114] Stability is an important criterion for evaluating catalytic performance. Further evaluation was carried out on the stability of the G-Fe-0.005 M catalyst in Example 3 for the hydrogenation of CO 2 to light olefins. At a temperature of 320 °C, a pressure of 3 MPa, H 2 / CO 2The molar ratio is 3 and the mass space velocity is 6000 mL / h -1 g -1 The reaction was continued for 100 h under the reaction conditions, and the results are shown in Figure 6 . As shown by Figure 6 , in the 100-hour test, there was no obvious deactivation of the G-Fe-0.005M catalyst in terms of CO 2 conversion rate, CO selectivity or light olefin selectivity.

Claims

1. Preparation of a polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst, characterized in that: The coprecipitation method is used for preparation, comprising the following steps: The Fe source was dissolved in deionized water as solution A, and the precipitant was dissolved in deionized water as solution B; At the same time, solution A and solution B are respectively dropped into the guar gum solution and stirred to obtain a suspension; The suspension is freeze-dried and calcined in sequence to obtain a polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst.

2. The preparation of the polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst according to claim 1, characterized in that: The Fe source is an acidic iron salt, and the precipitant is a soluble alkali salt; in terms of mass ratio, the iron source:precipitant=1:(1.5-2.0).

3. The preparation of the carbon dioxide hydrogenation catalyst modified by direct hydroxyl group of polysaccharide according to claim 2, characterized in that: The Fe source is ferric nitrate, and the precipitant is ammonium carbonate; in terms of mass ratio, the iron source:precipitant=1:2.

0.

4. The preparation of the carbon dioxide hydrogenation catalyst modified by direct hydroxyl group of polysaccharide according to claim 1, characterized in that: The concentration of guar gum solution is 0.001M~0.01M; When the dropwise addition of either solution A or solution B is completed, the reaction stops; when the dropwise addition is finished, the iron source is completely precipitated; during the dropwise addition, the system pH is maintained at 7-9, and the stirring temperature is 70°C-90°C.

5. The preparation of the carbon dioxide hydrogenation catalyst modified by direct hydroxyl group of polysaccharide according to claim 1, characterized in that: The calcination process is: calcination at 500°C to 700°C for 5h to 8h.

6. Use of the carbon dioxide hydrogenation catalyst directly modified with hydroxyl groups of the polysaccharide according to any one of claims 1 to 5, characterized in that: The following steps are involved: The catalyst is loaded into the constant temperature section of the fixed reaction bed, and the catalyst is reduced by using high-purity hydrogen; The reaction gas is introduced to cause a carbon dioxide hydrogenation reaction.

7. The use of the polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst according to claim 6, characterized in that: The catalyst is tableted, crushed, and sieved before being loaded into a fixed reaction bed; the sieve is a 20-40 mesh sieve.

8. The use of the polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst according to claim 6, characterized in that: The purity of the hydrogen is ≥99.999%; The reduction process is as follows: the catalyst is reduced at 350℃~450℃ for 2h~10h, the pressure during the reduction process is 0.1MPa~1MPa, and the mass space velocity of hydrogen is 4000mLh -1 g -1 ~8000mLh -1 g -1 .

9. The use of the polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst according to claim 6, characterized in that: The reaction gas is a mixed gas of CO2 and H2, with a molar ratio of H2:CO2=1-4.

10. The use of the polysaccharide directly hydroxyl-modified carbon dioxide hydrogenation catalyst according to claim 6, characterized in that: During the carbon dioxide hydrogenation process, the reaction temperature is 300℃~360℃, the reaction pressure is 2MPa~4MPa, and the mass space velocity of the reaction gas is 3000mLh -1 g -1 ~9000mLh -1 g -1 .

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