A process for the synthesis of alpha-olefins using a composite shaping catalyst

Abstract

The application provides a method for synthesizing alpha-olefins, which comprises the following steps: reducing and pretreating an iron-based catalyst modified by rare earth elements with hydrogen; wherein the rare earth elements comprise one or a combination of two or more of La, Ce and Pr, and the content of the rare earth elements is 0.5-3.0 mol%; feeding raw material gas containing hydrogen, carbon monoxide and carbon dioxide through the pretreated catalyst to perform a synthesis reaction, and obtaining alpha-olefins; wherein the molar content of carbon dioxide, hydrogen and carbon monoxide in the raw material gas is 2.0-8.0 mol%, 49-86 mol% and 6-49 mol% respectively. The method of the application ensures that the iron active component in the iron catalyst maintains a suitable Fe5C2 / Fe3O4 ratio during the reaction process, reduces the generation of heavy hydrocarbons and the deposition of the heavy hydrocarbons on the catalyst surface, and maintains the activity of the catalyst and the stability of the reaction system.

Description

Technical Field

[0001] The present invention relates to the technical field of olefin preparation, and in particular to a method for synthesizing alpha-olefins by using a composite shaped catalyst. Background Art

[0002] Synthesis gas to α-olefins, a key process for producing α-olefins from non-petroleum resources, has attracted considerable attention. The most researched route for this process is the Fischer-Tropsch synthesis of α-olefins from synthesis gas. However, the distribution of the hydrocarbon products is influenced by the Amderson-Schulz-Flory (ASF) distribution, resulting in a broad carbon number distribution of the olefin products and low selectivity for both low-carbon and high-carbon olefins. Improving the carbon number distribution of the hydrocarbon products from Fischer-Tropsch synthesis and enhancing the distribution of the target olefin products are current research goals for the synthesis gas to α-olefin process, and catalyst development has been one of the primary approaches to achieving this goal.

[0003] Among the many catalysts studied, iron-based catalysts, as the most widely studied and cheapest Fischer-Tropsch catalysts, are naturally ideal candidates for developing syngas-to-α-olefin catalysts. Recent studies have shown improvements in the conversion and selectivity of iron-based catalysts for syngas-to-α-olefins. For example, CN109865515A discloses a method for preparing a syngas-to-α-olefin catalyst, CN114369002A discloses a two-stage composite catalyst for syngas-to-α-olefins, and CN114369003A discloses a pretreatment method for a synthetic α-olefin catalyst. All of these have contributed to improving the selectivity of products in the syngas-to-α-olefin process. However, for the syngas-to-α-olefin reaction system, the system stability is still insufficient, and improving the stability of the reaction system remains a challenging problem in this research.

[0004] CN114369002A discloses a method for synthesizing linear α-olefins from syngas, comprising the following steps: at 200-380°C and 0.5-5.0 MPa, passing syngas over a pretreated composite catalyst to synthesize linear α-olefins with a carbon number of 6-15; wherein the composite catalyst comprises a two-stage catalyst: an iron-based catalyst having iron oxide and / or iron-containing carbide as an active component; and a nickel-based catalyst or a cobalt-based catalyst having nickel oxide as an active component, respectively. CN109865515A discloses a method for preparing a catalyst for synthesizing linear α-olefins from syngas. This technology uses one or more alkali metal elements such as K, Mg, and Ca as electron promoters for the iron catalyst to prepare a highly efficient syngas conversion catalyst, enhance carbon chain growth capacity, and improve linear α-olefin product selectivity. However, the stability of the syngas-to-α-olefin reaction systems studied in these two technologies needs to be further improved.

[0005] CN114369003A discloses a pretreatment method for a catalyst for synthesizing α-olefins, which comprises the following steps: reduction, carbonylation, and re-reduction in sequence, wherein the reduction and re-reduction conditions are the same or different, wherein the reduction and re-reduction are carried out under a reducing atmosphere containing hydrogen at 250-500°C, 0.01-5.0 MPa, and a reducing atmosphere space velocity of 1000-50000 ml·h -1 gCat -1 The catalyst is reduced under the conditions of 2-30 hours; the carbonylation refers to the process of reducing the catalyst under the conditions of 250-450°C, 0.01-5.0 MPa and gas space velocity of 500-50000 ml·h in an atmosphere containing CO. -1 gCat -1 Although the purpose of this technology is to solve the problems of low α-olefin selectivity and poor catalyst stability in the traditional synthesis gas conversion process, the stability of the reaction system studied in this technology still needs to be further improved. Summary of the Invention

[0006] In order to solve the above problems, the object of the present invention is to provide a method for synthesizing α-olefins, which can ensure the stability of the reaction system for synthesizing α-olefins.

[0007] To achieve the above object, the present invention provides a method for synthesizing α-olefins, which comprises the following steps:

[0008] A reduction pretreatment is performed on an iron-based catalyst modified with a rare earth element using hydrogen; wherein the rare earth element comprises one or a combination of two or more of La, Ce, and Pr, and the content of the rare earth element is 0.5-3.0 mol% based on the total molar amount of the metal elements in the iron-based catalyst;

[0009] A raw gas containing hydrogen, carbon monoxide and carbon dioxide is passed through a pre-reduction catalyst to perform a synthesis reaction to obtain α-olefins; wherein the molar content of carbon dioxide in the raw gas is 2.0-8.0 mol%, the molar content of hydrogen is 49-86 mol%, and the molar content of carbon monoxide is 6-49 mol%.

[0010] According to a specific embodiment of the present invention, preferably, the rare earth element is La.

[0011] In view of the problem that the iron-based catalyst for synthesizing alpha-olefins from synthesis gas is unstable and easy to be deactivated by carbon deposition, the present application adds a proper amount of rare earth metal elements to the iron-based catalyst, and controls the content of CO2 in the raw material gas, so as to ensure that the ratio of Fe5C2 / Fe3O4 of the iron active component in the iron catalyst is kept proper during the reaction, and the stability of the reaction system is maintained.

[0012] The main reason for the deactivation of the olefin synthesis catalyst is the carbon deposition (heavy hydrocarbon) on the catalyst surface. In the synthesis reaction of the present application, the iron phase of the iron-based catalyst mainly exists in the forms of alpha-Fe, chi-Fe5C2 and Fe3O4. In the initial stage of the synthesis reaction, the alpha-Fe component formed by high-purity hydrogen reduction activation reacts with CO2 and CO in the raw material gas to form iron carbide (chi-Fe5C2), and the chi-Fe5C2 is the active phase for the synthesis of olefins from synthesis gas and other raw material gas, which can catalyze the Fischer-Tropsch reaction to produce olefins. With the progress of the reaction, the olefins in the reaction atmosphere can in turn promote the formation of iron carbide and inhibit the generation of heavy hydrocarbons, thereby reducing the amount of heavy hydrocarbon carbon deposition on the catalyst and preventing the deactivation of the catalyst.

[0013] By adding the rare earth elements such as La, Ce and Pr to the iron-based catalyst, the present application can maintain the ratio of chi-Fe5C2 / Fe3O4 of the active species of the catalyst reaction at a high level, and thus maintain the stability of the reaction system. In addition, the addition of the rare earth elements can also promote the uniform dispersion of the iron elements in the catalyst. Meanwhile, by adding CO2 to the raw material gas, on the one hand, the combination of CO2 and high-pressure H2 can promote the generation of the iron carbide active component and inhibit the generation of heavy hydrocarbons; on the other hand, it can effectively reduce the probability of secondary reaction of the generated olefins. Therefore, the olefin production method provided by the present application has less carbon deposition than the prior art in the same reaction time, and the reaction stability of the catalyst is significantly improved.

[0014] According to the specific embodiment of the present application, preferably, the content of carbon dioxide in the raw material gas is 2.0-5.0 mol%.

[0015] According to the specific embodiment of the present application, preferably, the molar content of hydrogen in the raw material gas is 60-76 mol%, and the molar content of carbon monoxide is 18-32 mol%.

[0016] According to the specific embodiment of the present application, preferably, the content of carbon dioxide in the raw material gas is 2.0-5.0 mol%, the molar content of hydrogen is 60-76 mol%, and the molar content of carbon monoxide is 18-32 mol%.

[0017] According to the specific embodiment of the present application, preferably, the volume ratio of hydrogen to carbon monoxide in the raw material gas is 2-4.

[0018] According to the specific embodiment of the present application, preferably, the raw gas is a synthesis gas.

[0019] According to the specific embodiment of the present application, preferably, the temperature of the synthesis reaction is 250-350℃, and the pressure is 1.0-4.0MPa, preferably 2.0-3.0MPa.

[0020] According to the specific embodiment of the present application, preferably, the space velocity of the raw gas is 1000-6000ml·gCat -1 ·h -1 , preferably 2000-5000ml·gCat -1 ·h -1 .

[0021] According to the specific embodiment of the present application, preferably, the iron-based catalyst modified by rare earth elements contains iron oxide, which includes triiron tetroxide and / or diiron trioxide.

[0022] According to the specific embodiment of the present application, preferably, the iron-based catalyst modified by rare earth elements is prepared by co-precipitation or impregnation.

[0023] According to the specific embodiment of the present application, preferably, when the co-precipitation is used, the preparation method of the iron-based catalyst modified by rare earth elements is as follows: adding an alkali solution dropwise into a salt solution containing iron elements and rare earth elements to obtain a precipitate, separating the precipitate and drying and calcining to obtain the iron-based catalyst modified by rare earth elements.

[0024] According to the specific embodiment of the present application, preferably, when the impregnation is used, the preparation method of the iron-based catalyst modified by rare earth elements is as follows: adding an alkali solution dropwise into a salt solution containing iron elements, separating the precipitate, washing, drying, and calcining to obtain an iron-based catalyst, impregnating the iron-based catalyst into a salt solution containing rare earth elements, taking out the solid after the impregnation is completed and drying and calcining to obtain the iron-based catalyst modified by rare earth elements.

[0025] According to the specific embodiment of the present application, preferably, the calcination temperature in the co-precipitation or impregnation is 400-500℃.

[0026] According to the specific embodiment of the present application, preferably, in the pretreatment of the iron-based catalyst modified by rare earth elements, the reduction reaction temperature is 320-380℃, and the reaction time is 4-16h. More preferably, the reduction reaction temperature is 350℃, and the reaction time is 8h.

[0027] According to a specific embodiment of the present invention, preferably, the iron-based catalyst modified by rare earth elements catalyzes the synthesis reaction in the form of a composite shaped catalyst, and the components of the composite shaped catalyst include the iron-based catalyst modified by rare earth elements, alumina, and graphite, wherein the mass ratio of the iron-based catalyst modified by rare earth elements to alumina is 1-5:1, the ratio of graphite to alumina is 1:4-10, and the mass percentage of alumina in the composite shaped catalyst is 20-50%.

[0028] The Fischer-Tropsch synthesis of α-olefins from syngas is a highly exothermic reaction, placing high demands on the molding process for iron-based metal oxide catalysts. Iron-based catalysts have poor self-adhesiveness, making molding difficult, prone to pulverization after the reaction, and exhibiting poor strength after molding. Therefore, to address this issue, the present invention further prefers to catalyze the reaction using the iron-based catalyst modified with the rare earth element as a composite molded catalyst.

[0029] The composite shaped catalyst of the present invention incorporates appropriate amounts of alumina and graphite as shaping aids. During the shaping process, the distance between active components and the shaped particle size are controlled, preventing local overheating of the catalyst and facilitating the timely removal of reaction heat, thereby ensuring the catalyst's reactivity. Furthermore, the alumina and graphite forming aids interact strongly with the catalyst components, enhancing the strength of the composite shaped catalyst and preventing it from breaking or pulverizing.

[0030] In some specific embodiments of the present invention, the lateral pressure strength of the composite molded catalyst increases from 35 N / cm to 90 N / cm. A more obvious effect is that the lateral pressure strength of the catalyst increases from pulverized to 63 N / cm after 100 hours of reaction.

[0031] According to a specific embodiment of the present invention, preferably, the components of the composite shaped catalyst further include water, and the mass ratio of water to graphite is 1:10-50, more preferably 1:20-50.

[0032] According to a specific embodiment of the present invention, preferably, in the composite shaped catalyst, the purity of the graphite is ≥99.9%.

[0033] According to a specific embodiment of the present invention, preferably, in the composite shaped catalyst, the iron-based catalyst modified by rare earth elements and alumina are both granular, and the particle size of the two is 20-80 mesh, more preferably 40-80 mesh, and further preferably 60-80 mesh.

[0034] According to a specific embodiment of the present invention, preferably, the composite shaped catalyst is cylindrical, with a diameter of 3-6 mm, more preferably 3-4 mm, and a height of 3-8 mm, more preferably 3-5 mm.

[0035] According to a specific embodiment of the present invention, preferably, the method for preparing the composite formed catalyst comprises the following steps:

[0036] (1) dry granulating the iron-based catalyst modified by the rare earth element, and then crushing and screening the catalyst to obtain catalyst particles of a certain size;

[0037] (2) The catalyst particles are mixed with alumina particles and graphite, water is added, and then tablets are pressed to obtain the composite shaped catalyst.

[0038] According to a specific embodiment of the present invention, preferably, the particle size of the granules obtained by dry granulation is 2-5 mm.

[0039] In the catalyst forming process of the present invention, a rare earth element-modified iron-based catalyst is dry-granulated into a powdered form to a desired particle size. This powder is then crushed and screened with a sieve to obtain the catalyst of the desired mesh size for tableting. Dry granulation is used to form the catalyst powder into larger particles, which are then crushed and screened to obtain the catalyst of the target particle size. During the granulation process, particles of different particle sizes undergo different formation processes, resulting in different bonding strengths between the catalyst powders and varying crushing strengths and durations, all of which affect the catalyst's strength.

[0040] According to a specific embodiment of the present invention, preferably, the molding pressure is 0.5-3.0 MPa, more preferably 1.0-2.5 MPa.

[0041] The technical solution provided by the present invention has the following beneficial effects:

[0042] The present invention adds an appropriate amount of rare earth metal elements to the iron-based catalyst and controls the appropriate CO2 content in the raw gas to ensure that the iron active components in the iron catalyst maintain a suitable Fe5C2 / Fe3O4 ratio during the reaction operation, reduce the generation of heavy hydrocarbons and their deposition on the catalyst surface, and thus maintain the activity of the catalyst and the stability of the reaction system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is the Mössbauer spectrum of the catalyst after 500 h of reaction in Example 1;

[0044] Figure 2 This is the Mössbauer spectrum of the catalyst after 500 hours of reaction in Comparative Example 2;

[0045] Figure 3 It is a comparison chart of the reaction stability of the reaction systems of Example 8 and Comparative Examples 1-3. DETAILED DESCRIPTION

[0046] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be understood as limiting the scope of implementation of the present invention.

[0047] Example 1

[0048] This embodiment provides a method for synthesizing α-olefins, comprising the following steps:

[0049] (1) Preparation of rare earth element-modified iron-based catalysts

[0050] 63.78g Fe(NO3)3·9H2O, 94.50g Fe(NO3)2·6H2O and 4.55g La(NO3)3·6H2O were mixed with 500mL water to form an iron salt solution, and 15.30mL of 12.1mol / L HCl solution was added; 500ml of 0.5mol / L K2CO3 solution was added at a uniform rate under stirring conditions at 60°C; within about 2h, the acidic pH value of the solution was adjusted to a pH value of about 9; after the addition was completed, the solution was stirred at a constant temperature for 1h and cooled to room temperature; after the reaction was completed, the deposited product was separated by magnetic field adsorption, washed with deionized water to neutrality, dried, calcined at 450°C, pressed into tablets, and crushed to 20-40 mesh. The obtained catalyst sample was labeled La-Fe-1-Cat.

[0051] (2) Catalytic reaction

[0052] The catalyst La-Fe-1-Cat obtained in step (1) was filled into a reactor with a catalyst filling amount of 1 g, and subjected to reduction pretreatment at 350° C. for 8 h under pure hydrogen conditions; after the reduction was completed, the temperature of the reactor was adjusted to 300° C. at a rate of 1° C. / min, and the inlet gas was switched to a reaction raw gas, which was a mixture of H2 / CO / CO2 / N2, with the molar percentages of each gas being H2: 60.6 mol%, CO: 30.3 mol%, CO2: 5 mol%, and N2: 4 mol% (the volume ratio of H2 / CO / CO2 was 2 / 1 / 0.165, with N2 as an internal standard), and the back pressure was brought to a reaction pressure of 2.0 MPa. The raw gas was heated at 300° C., 2.0 MPa, and 5000 ml·gCat. -1 ·h -1 The synthesis reaction was carried out under the catalytic conditions of . The catalytic reaction results are shown in Table 1.

[0053] Example 2

[0054] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 1, except that: in step (1), 4.55g La(NO3)3·6H2O is replaced with 4.56g Ce(NO3)6·6H2O, and the obtained catalyst sample is labeled Ce-Fe-2-Cat; in step (2), the catalytic conditions are replaced with: 320°C, 3.0MPa, 3000ml·gCat -1 ·h -1 The catalytic reaction results are shown in Table 1.

[0055] Example 3

[0056] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 1, except that: in step (1), 4.55 g La(NO3)3·6H2O is replaced with 2.25 g La(NO3)6·6H2O, and the obtained catalyst sample is labeled La-Fe-3-Cat; the catalytic conditions in step (2) are replaced with: 300°C, 2.0 MPa, 2000 ml·gCat -1 ·h -1 The catalytic reaction results are shown in Table 1.

[0057] Example 4

[0058] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 1, except that: in step (1), 4.55 g La(NO3)3·6H2O is replaced with 6.55 g La(NO3)6·6H2O, and the obtained catalyst sample is labeled La-Fe-4-Cat; the catalytic conditions in step (2) are replaced with: 300°C, 3.0 MPa, 2000 ml·gCat -1 ·h -1 The catalytic reaction results are shown in Table 1.

[0059] Example 5

[0060] This embodiment provides a method for synthesizing α-olefins, comprising the following steps:

[0061] (1) Preparation of rare earth element-modified iron-based catalysts

[0062] An iron-based catalyst containing no rare earth elements was prepared, labeled as Fe-Cat. The preparation method was the same as step (1) of Example 1, except that La(NO3)3·6H2O was omitted. The obtained catalyst sample containing no rare earth elements was labeled as Fe-Cat.

[0063] La element was loaded by impregnation method: 1.55 g La(NO3)3·6H2O was dissolved in 50 ml water to form a La(NO3)3 solution. Then 25 mL of the formed La(NO3)3 solution was taken and impregnated with 15 g of the above-mentioned iron-based catalyst Fe-Cat at room temperature for 24 h. The sample was then taken out and dried at 100°C and calcined at 400°C. The obtained catalyst sample was labeled La-Fe-5-Cat.

[0064] (2) Catalytic reaction

[0065] The same as step (2) of Example 1, except that the catalytic conditions were replaced with: 310°C, 2.0 MPa, 4000 ml·gCat -1 ·h -1 The catalytic reaction results are shown in Table 1.

[0066] The evaluation conditions for the catalyst La-Fe-5-Cat prepared in Example 5 were the same as those in Example 1, except that the temperature in the evaluation conditions was replaced by 305°C. A 200-hour reaction stability test of the catalyst was carried out, and the reaction results are listed in Table 1.

[0067] Example 6

[0068] This embodiment provides a method for synthesizing α-olefins, which is the same as that of embodiment 1, except that the catalytic conditions in step (2) are replaced with: 300°C, 2.0 MPa, 2000 ml·gCat -1 ·h -1 The catalytic reaction results are shown in Table 1.

[0069] Example 7

[0070] This embodiment provides a method for synthesizing α-olefins, which is the same as that of embodiment 1, except that the catalytic conditions in step (2) are: 300°C, 2.0 MPa, 2000 ml·gCat -1 ·h -1 The molar ratio of each gas in the raw gas (a mixture of H2 / CO / CO2 / N2) was: H2: 62.66 mol%, CO: 31.33 mol%, CO2: 2 mol%, and N2: 4 mol% (the volume ratio of H2 / CO / CO2 was 2 / 1 / 0.165). The catalytic reaction results are shown in Table 1.

[0071] Table 1 Catalytic reaction results of Examples 1-7

[0072]

[0073] It can be seen from the results in Table 1 that the catalyst prepared in the examples of the present invention exhibits excellent performance in the production of α-olefins, and the reaction system can maintain stability.

[0074] Comparative Example 1

[0075] This comparative example provides a method for synthesizing α-olefins, which is the same as Example 1, except that the catalyst used in this comparative example is the Fe-Cat catalyst prepared in Example 5. The catalytic reaction results are shown in Table 2.

[0076] Comparative Example 2

[0077] This comparative example provides a method for synthesizing α-olefins, which is the same as Example 1, except that the catalyst used in this comparative example is the Fe-Cat catalyst prepared in Example 5 (which does not contain rare earth elements), and the feed gas composition is H2: 64 mol%, CO: 32 mol%, and N2: 4 mol% (H2 / CO volume ratio of 2 / 1), with no CO2. The catalytic reaction results are shown in Table 2.

[0078] Comparative Example 3

[0079] This comparative example provides a method for synthesizing α-olefins, which is the same as Example 1, except that there is no CO2 in the feed gas in step (2).

[0080] The catalytic reaction results of Comparative Examples 1-3 are shown in Table 2.

[0081] Table 2 Catalytic reaction results of Comparative Examples 1-3

[0082]

[0083]

[0084] Wherein, α-olefin / olefin (C%) refers to the ratio of α-olefin to the total amount of olefins in terms of carbon number; in Table 2, C 6-15 ＝ is an alkene with a carbon number of 6-15, C 16-18 ＝ It is an olefin having a carbon number of 16-18.

[0085] Comparing the catalytic results of Example 1 with Comparative Examples 1 and 2, Tables 1 and 2 show that the reaction system catalyzed by the iron-based catalyst modified with rare earth elements is more stable than the unmodified Fe-Cat catalyst. Comparing the catalytic results of Example 1 with Comparative Examples 2 and 3 shows that the inclusion of an appropriate amount of CO2 in the feed gas significantly improves the stability of the catalyst and the reaction system compared to feed gas without CO2.

[0086] Figure 1 and Figure 2 The Mössbauer spectra of the catalysts after 500 h of reaction in Example 1 and Comparative Example 2 are shown respectively. Figure 1 and Figure 2Corresponding data.

[0087] Table 3 Mössbauer spectroscopy data of the catalyst after 500 h of reaction in Example 1

[0088]

[0089] Table 4 Mössbauer spectroscopy data of the catalyst after 500 h of reaction in Comparative Example 2

[0090]

[0091]

[0092] From the data, it can be calculated that the Fe5C2 / Fe3O4 ratio (calculated as iron) in the catalyst after 500 hours of reaction in Example 1 is 7.43, while the Fe5C2 / Fe3O4 ratio (calculated as iron) in the catalyst after 500 hours of reaction in Comparative Example 2 is 3.65. This demonstrates that a reaction system with an appropriate CO2 content in the feed gas and a rare earth element-modified catalyst can maintain the Fe5C2 / Fe3O4 ratio of the active species in the catalytic reaction, thereby maintaining the stability of the reaction system.

[0093] In general, the reaction system of the synthesis method of the present invention has excellent reaction stability, which is closely related to the presence of an appropriate amount of CO2 in the feed gas and the rare earth element modification of the catalyst system.

[0094] Example 8

[0095] This example provides a method for synthesizing α-olefins and conducts reaction stability testing. The method for synthesizing α-olefins in this example is identical to that in Example 1, differing only in that the catalytic reaction temperature is changed from 300°C to 305°C. The reaction stability test lasted 200 hours, and the results are shown in Table 5.

[0096] Table 5 Test results of the stability of the La-Fe-1-Cat catalyst in the synthesis of α-olefins in Example 8

[0097] Reaction time (h) 5 50 100 150 200 CO conversion rate (mol%) 92.8 92.7 92.6 92.4 92.6 Hydrocarbon selectivity (mol%) 60.2 60.4 60.1 60.3 60.0 Olefins / hydrocarbons (C%) 70.0 70.2 70.1 69.9 70.0 α-olefin / olefin (C%) 75.3 75.2 75.1 75.2 75.0

[0098] Figure 3 The reaction stability comparison diagram of the reaction system of Example 8 and Comparative Examples 1-3 is shown. Figure 3 The results show that in the method for synthesizing α-olefins of the present invention, the catalyst maintains good activity and the reaction system also has good stability.

[0099] Example 9

[0100] This embodiment provides a method for synthesizing α-olefins, wherein the catalyst used is a composite formed catalyst, and the method comprises the following steps:

[0101] (1) Preparation of catalyst

[0102] 2371.5g of FeCl3·6H2O, 940.5g of FeCl2·4H2O, and 135.2g of La(NO3)3·6H2O were mixed with 7.5L of water to form an iron salt solution, and 382.5mL of 12.1mol / L HCl solution was added. At 60°C with stirring, approximately 27.5L of 5% NH3·H2O solution was added at a uniform rate. The acidic pH value of the solution was adjusted to approximately pH 9 within approximately 2 hours. After the addition was complete, the solution was stirred at a constant temperature for 1 hour and cooled to room temperature. After the reaction was completed, the deposited product was separated by magnetic field adsorption, washed with deionized water until neutral, dried, and calcined at 450°C for 4 hours to obtain an iron-based catalyst modified with a rare earth element, i.e., the catalyst body.

[0103] (2) Catalyst molding

[0104] The catalyst body obtained in step (1) is dry-granulated to obtain 2 mm catalyst particles, and the obtained catalyst particles are crushed and screened to obtain 60-80 mesh catalyst particles; the 60-80 mesh catalyst particles, alumina particles (whose particle size range is the same as that of the catalyst particles), and graphite are mixed, and after mixing, an appropriate amount of water is added by mist spraying, wherein the mass ratio of graphite: water: alumina: catalyst body is 50:1:200:200, and the graphite is high-purity graphite with a purity of ≥99.9%; and then tableting is performed at a molding pressure of 2.5 MPa to obtain a cylindrical catalyst with a diameter of 4 mm and a height of 5 mm (Φ4×5 mm), i.e., a composite molded catalyst;

[0105] (3) Catalytic reaction

[0106] The composite shaped catalyst obtained in step (2) is filled in a reactor with a catalyst filling amount of 30g, and the composite shaped catalyst is pretreated with a mixture of hydrogen and nitrogen, the volume concentration of hydrogen gradually increasing from 10% to 100%, and the reduction pretreatment is carried out at 350°C for 12h; after the reduction is completed, the temperature of the reactor is adjusted to 280°C at a rate of 1°C / min, and the air intake is switched to a reaction raw gas, the raw gas being a mixture of H2, CO, CO2, and N2, the molar percentage of each gas being H2:60.6mol%, CO:30.3mol%, CO2:5mol%, and N2:4mol% (the volume ratio of H2 / CO / CO2 is 2 / 1 / 0.165, with N2 as an internal standard), and the back pressure is brought to a reaction pressure of 2.0MPa, and the raw gas is heated at 280-310°C (the catalyst bed has a temperature gradient with height), 2.0MPa, 2000ml·gCat -1 ·h -1The synthesis reaction was carried out under the catalytic conditions of . The catalytic reaction results are shown in Table 2.

[0107] Example 10

[0108] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 9, except that:

[0109] In step (2), the mass ratio of graphite: water: alumina: catalyst body is 20:1:200:300;

[0110] The size of the composite catalyst after tableting is Φ3×3mm.

[0111] Example 11

[0112] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 9, except that:

[0113] The catalyst particles obtained by dry granulation in step (2) have a particle size of 5 mm;

[0114] The mass ratio of graphite: water: alumina: catalyst body is 25:1:100:400.

[0115] Example 12

[0116] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 9, except that:

[0117] In step (2), the catalyst particles of 5 mm are obtained by dry granulation, and the catalyst particles of 40-60 mesh are obtained by crushing and screening;

[0118] The mass ratio of graphite: water: alumina: catalyst body is 50:1:100:400;

[0119] The size of the composite catalyst after tableting is Φ3×3mm.

[0120] Example 13

[0121] This embodiment provides a method for synthesizing α-olefins, which is the same as that of Example 9, except that:

[0122] In step (2), the catalyst particles of 4 mm are obtained by dry granulation, and the catalyst particles of 40-60 mesh are obtained by crushing and screening;

[0123] The size of the composite catalyst after tableting is Φ3×3mm.

[0124] Example 14

[0125] The present example provides a method for synthesizing α-olefins, which is the same as example 9, except that the tablet forming pressure in step (2) is 1.0 MPa.

[0126] Comparative Example 4

[0127] The present comparative example provides a method for synthesizing α-olefins, which is the same as example 9, except that:

[0128] In step (2), the catalyst particles of 20-40 mesh are obtained by crushing and screening;

[0129] The mass ratio of graphite: water: alumina: catalyst body is 50: 1: 100: 400;

[0130] The tablet forming pressure is 1.0 MPa.

[0131] Comparative Example 5

[0132] The present comparative example provides a method for synthesizing α-olefins, which is the same as example 9, except that:

[0133] In step (2), the catalyst particles of 20-40 mesh are obtained by crushing and screening; then without mixing with other raw materials, the catalyst particles of 20-40 mesh are directly tablet formed to obtain the shaped catalyst.

[0134] The catalyst tablet formed in the present comparative example is not good for demolding.

[0135] Comparative Example 6

[0136] The present comparative example provides a method for synthesizing α-olefins, which is the same as example 9, except that:

[0137] In step (2), the catalyst particles of 20-40 mesh are obtained by crushing and screening;

[0138] The mass ratio of graphite: water: alumina: catalyst body is 10: 1: 100: 400;

[0139] The tablet forming pressure is 1.0 MPa.

[0140] The catalyst tablet formed in the present comparative example is not good for demolding.

[0141] Comparative Example 7

[0142] The present comparative example provides a method for synthesizing α-olefins, which is the same as example 9, except that:

[0143] In step (2), the catalyst particles of 20-40 mesh are obtained by crushing and screening;

[0144] The 20-40 mesh catalyst particles were mixed with graphite and water only without adding alumina, and the mass ratio of graphite:water:catalyst body was 10:1:400.

[0145] The lateral pressure strength of the composite shaped catalysts prepared in step (2) of the above Examples 9-14 and Comparative Examples 4-7 and the lateral pressure strength of the composite shaped catalysts after participating in the catalytic reaction in step (3) for 100 hours were tested.

[0146] The results are shown in Table 6.

[0147] Table 6 Side pressure strength of composite molded catalyst before and after reaction

[0148] Composite molding catalyst Strength before reaction, N / cm Strength after reaction, N / cm Example 9 90 63 Example 10 82 55 Example 11 83 57 Example 12 69 52 Example 13 86 65 Example 14 73 48 Comparative Example 4 52 36 Comparative Example 5 39 - Comparative Example 6 50 32 Comparative Example 7 35 -

[0149] The results of the catalytic synthesis reactions of Examples 9-14 and Comparative Examples 4-7 are shown in Table 7 and Table 8, respectively.

[0150] Table 7 Catalytic reaction results of Examples 9-14

[0151]

[0152] Table 8 Catalytic reaction results of Comparative Examples 4-7

[0153]

[0154] As can be seen from Tables 6 to 8, the present invention improves the strength of the catalyst and maintains good reaction activity stability by preparing the iron-based catalyst (catalyst body) modified by rare earth elements into a composite formed catalyst.

[0155] Compared with comparative examples 4-7, the catalyst strength and catalyst stability of Examples 9-14 were significantly increased, among which Example 9 showed the best strength index. The CO conversion rate of the catalysts of Examples 9, 10, 11, and 13 decreased by less than 1% after 100 hours of reaction, and the decrease in Examples 12 and 14 was also less than 3.5%, indicating that the catalyst prepared by the present invention has good reaction stability.

[0156] Compared with comparative example 4 using 20-40 mesh catalyst particles and a molding pressure of 1.0 MPa, the catalysts prepared in examples 9-14 of the present invention using 40-80 mesh catalyst particles and a molding pressure of 2.5 MPa have higher mechanical strength and reaction activity stability.

[0157] Comparative Example 5 uses 20-40 mesh catalyst particles and does not add other molding components. Although the molding pressure is 2.5 MPa, the strength is significantly worse than that of Example 9, the CO conversion rate decreases significantly after 100 hours of reaction, and the catalyst is difficult to remove from the mold after molding. The catalyst has been crushed after the activity stability test, indicating that the molding additives have a great influence on the strength, stability and demolding of the catalyst.

[0158] Comparative Example 6 uses 20-40 mesh catalyst particles, 1.0 MPa pressure, and reduces the amount of graphite added for molding. Compared with Example 9, the strength of the catalyst deteriorates, and the CO conversion rate decreases significantly after 100 hours of reaction.

[0159] Comparative Example 6, compared to Comparative Example 4, reduced the amount of graphite added. The catalyst strength decreased slightly compared to Comparative Example 4, and difficulty occurred during demolding after molding. This indicates that the addition of graphite can improve the catalyst strength, facilitate the molding process, and reduce equipment wear. The CO conversion rate of the catalyst prepared in Comparative Example 6 decreased to a certain extent at both 5 and 100 hours of reaction time compared to Comparative Example 4, indicating that the addition of graphite has a certain impact on the activity and stability of the catalyst.

[0160] In Comparative Example 7, no alumina was added. The strength of the catalyst was similar to that of Comparative Example 5, but the strength was lower and the catalyst was crushed after the reaction, indicating that alumina is an important component for improving the strength of the catalyst. The strength and stability of the catalyst prepared by adding alumina and graphite in the present invention are significantly improved, providing technical support for the further application of synthesis gas to α-olefin catalysts.

[0161] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as above in terms of a preferred embodiment, it is not intended to limit the present invention. Any technician familiar with this patent can make slight changes or modifications to equivalent embodiments using the above technical content without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the solution of the present invention.

Claims

1. A method for synthesizing α-olefins, comprising the steps of: The iron-based catalyst modified by rare earth elements is subjected to reduction pretreatment using hydrogen; wherein, The rare earth element includes one or a combination of two or more of La, Ce, and Pr, and the content of the rare earth element is 0.5-3.0 mol% based on the total molar amount of the metal elements in the iron-based catalyst; A feed gas containing hydrogen, carbon monoxide, and carbon dioxide is passed through a pre-reduction catalyst to perform a synthesis reaction to obtain α-olefins; wherein the molar content of carbon dioxide in the feed gas is 2.0-5.0 mol%, the molar content of hydrogen is 60-76 mol%, and the molar content of carbon monoxide is 18-32 mol%; and the volume ratio of hydrogen to carbon monoxide in the feed gas is 2-4; In which, the iron-based catalyst modified by rare earth elements catalyzes the synthesis reaction in the form of a composite shaped catalyst, and the components of the composite shaped catalyst include the iron-based catalyst modified by rare earth elements, alumina, and graphite. In which, the iron-based catalyst modified by rare earth elements contains iron-containing oxides, and the iron-containing oxides include ferroferric oxide and / or ferric oxide; the mass ratio of the iron-based catalyst modified by rare earth elements to alumina is 1-5:1, the mass ratio of graphite to alumina is 1:4-10, and the mass percentage of alumina in the composite shaped catalyst is 20-50%.

2. The method for synthesizing α-olefins according to claim 1, wherein The rare earth element is La.

3. The method for synthesizing α-olefins according to claim 1, wherein The temperature of the synthesis reaction is 250-350°C and the pressure is 1.0-4.0 MPa.

4. The method for synthesizing α-olefins according to claim 3, wherein The pressure of the synthesis reaction is 2.0-3.0 MPa.

5. The method for synthesizing α-olefins according to claim 1, wherein The space velocity of the raw gas is 1000-6000 mL·gCat -1 ·h -1 .

6. The method for synthesizing α-olefins according to claim 5, wherein: The space velocity of the raw gas is 2000-5000 mL·gCat -1 ·h -1 .

7. The method for synthesizing α-olefins according to claim 1, wherein: The iron-based catalyst modified by rare earth elements is prepared by a co-precipitation method or an impregnation method.

8. The method for synthesizing α-olefins according to claim 7, wherein: When the co-precipitation method is adopted, the preparation method of the iron-based catalyst modified by rare earth elements is as follows: an alkaline solution is added dropwise to a salt solution containing iron and rare earth elements to obtain a precipitate, the precipitate is separated, and the precipitate is washed, dried, and calcined to obtain the iron-based catalyst modified by rare earth elements.

9. The method for synthesizing α-olefins according to claim 7, wherein: When the impregnation method is adopted, the preparation method of the iron-based catalyst modified by rare earth elements is as follows: an alkaline solution is added dropwise to a salt solution containing iron elements, a precipitate is separated, and the precipitate is washed, dried, and calcined to obtain an iron-based catalyst, and the iron-based catalyst is impregnated into a salt solution containing rare earth elements. After the impregnation is completed, the solid is taken out and dried and calcined to obtain the iron-based catalyst modified by rare earth elements.

10. The method for synthesizing α-olefins according to claim 7, wherein: The calcination temperature in the co-precipitation method or the impregnation method is 400-500°C.

11. The method for synthesizing α-olefins according to claim 1, wherein: In the pretreatment of the iron-based catalyst modified by rare earth elements, the reduction reaction temperature is 320-380° C. and the reaction time is 4-16 hours.

12. The method for synthesizing α-olefins according to claim 1, wherein: In the composite shaped catalyst, the purity of the graphite is ≥99.9%.

13. The method for synthesizing α-olefins according to claim 1, wherein: The iron-based catalyst modified by rare earth elements and alumina are both in granular form, and the particle size of the two is 20-80 meshes.

14. The method for synthesizing α-olefins according to claim 13, wherein: The iron-based catalyst modified by rare earth elements and the alumina have a particle size of 40-80 meshes.

15. The method for synthesizing α-olefins according to claim 13, wherein: The iron-based catalyst modified by rare earth elements and the alumina have a particle size of 60-80 meshes.

16. The method for synthesizing α-olefins according to claim 1, wherein: The composite shaped catalyst is cylindrical, with a diameter of 3-6 mm and a height of 3-8 mm.

17. The method for synthesizing α-olefins according to claim 16, wherein: The composite shaped catalyst has a diameter of 3-4 mm and a height of 3-5 mm.

18. The method for synthesizing α-olefins according to claim 1, wherein: The preparation method of the composite formed catalyst comprises the following steps: (1) dry granulating the iron-based catalyst modified by the rare earth element, and then crushing and screening the catalyst to obtain catalyst particles of a certain size; (2) The catalyst particles are mixed with alumina particles and graphite, water is added, and then the mixture is pressed into tablets to obtain the composite catalyst.

19. The method for synthesizing α-olefins according to claim 18, wherein: The particle size of the granules obtained by dry granulation is 2-5 mm.

20. The method for synthesizing α-olefins according to claim 18, wherein: The mass ratio of water to graphite is 1:10-50.

21. The method for synthesizing α-olefins according to claim 20, wherein: The mass ratio of water to graphite is 1:20-50.

22. The method for synthesizing α-olefins according to claim 18, wherein: The molding pressure is 0.5-3.0MPa.

23. The method for synthesizing α-olefins according to claim 22, wherein: The molding pressure is 1.0-2.5MPa.

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