A process for the synthesis of alpha-olefins

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-olefin. 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, comprising 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 pretreated 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] To address the current problems of insufficient reaction stability and easy carbon deposition and deactivation of iron-based catalysts for synthesizing α-olefins from synthesis gas, 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 feed gas to ensure that the iron active components in the iron catalyst maintain an appropriate Fe5C2 / Fe3O4 ratio during the reaction process, thereby maintaining the stability of the reaction system.

[0011] The primary cause of olefin synthesis catalyst deactivation is carbon accumulation (heavy hydrocarbons) on the catalyst surface. In the synthesis reaction of the present invention, the iron phase of the iron-based catalyst primarily exists in the form of α-Fe, χ-Fe5C2, and Fe3O4. In the initial stages of the synthesis reaction, the α-Fe component formed through reduction and activation with high-purity hydrogen reacts with CO2 and CO in the feed gas to form iron carbide (χ-Fe5C2). χ-Fe5C2 is the active phase in the olefins-to-synthesis reaction from synthesis gas and other feed gases, catalyzing the Fischer-Tropsch reaction to produce olefins. As the reaction proceeds, the olefins in the reaction atmosphere in turn promote the formation of iron carbide, inhibiting the formation of heavy hydrocarbons, reducing the amount of heavy hydrocarbon carbon accumulation on the catalyst, and preventing catalyst deactivation.

[0012] By adding rare earth elements such as La, Ce, and Pr to the iron-based catalyst, the present invention can maintain a high ratio of the active species of the catalytic reaction, χ-Fe5C2 / Fe3O4, thereby maintaining the stability of the reaction system. Furthermore, the addition of rare earth elements promotes the uniform dispersion of the iron element in the catalyst. Furthermore, by adding CO2 to the feed gas, the combined action of CO2 and high-pressure H2 can promote the formation of the active component of iron carbide and inhibit the formation of heavy hydrocarbons. Furthermore, the probability of secondary reactions of the olefins produced by the reaction can be effectively reduced. Therefore, the olefin production method provided by the present invention produces less carbon deposits than the prior art within the same reaction time, and the reaction stability of the catalyst is significantly improved.

[0013] According to a specific embodiment of the present invention, preferably, the content of carbon dioxide in the raw gas is 2.0-5.0 mol%.

[0014] According to a specific embodiment of the present invention, preferably, in the feed gas, the molar content of hydrogen is 60-76 mol%, and the molar content of carbon monoxide is 18-32 mol%.

[0015] According to a specific embodiment of the present invention, preferably, in the raw gas, the carbon dioxide content is 2.0-5.0 mol%, the hydrogen content is 60-76 mol%, and the carbon monoxide content is 18-32 mol%.

[0016] According to a specific embodiment of the present invention, preferably, the volume ratio of hydrogen to carbon monoxide in the feed gas is 2-4.

[0017] According to a specific embodiment of the present invention, preferably, the raw gas is synthesis gas.

[0018] According to a specific embodiment of the present invention, preferably, the temperature of the synthesis reaction is 250-350° C., and the pressure is 1.0-4.0 MPa, preferably 2.0-3.0 MPa.

[0019] According to a specific embodiment of the present invention, preferably, the space velocity of the raw gas is 1000-6000 ml·gCat -1 ·h -1 , preferably 2000-5000ml·gCat -1 ·h -1 .

[0020] According to a specific embodiment of the present invention, preferably, the iron-based catalyst modified by rare earth elements contains iron-containing oxides, and the iron-containing oxides include ferrosoferric oxide and / or ferrous oxide.

[0021] According to a specific embodiment of the present invention, preferably, the iron-based catalyst modified by rare earth elements is prepared by a co-precipitation method or an impregnation method.

[0022] According to a specific embodiment of the present invention, preferably, when the co-precipitation method is adopted, the preparation method of the iron-based catalyst modified by the rare earth element is: adding an alkaline solution to a salt solution containing iron and rare earth elements to obtain a precipitate, separating the precipitate and drying and calcining it to obtain the iron-based catalyst modified by the rare earth element.

[0023] According to a specific embodiment of the present invention, preferably, when the impregnation method is adopted, the preparation method of the iron-based catalyst modified by the rare earth element is as follows: an alkaline solution is added dropwise to a salt solution containing the iron element, the precipitate is separated, and the iron-based catalyst is obtained by washing, drying, and roasting. The iron-based catalyst is impregnated into a salt solution containing the rare earth element. After the impregnation is completed, the solid is taken out and dried and roasted to obtain the iron-based catalyst modified by the rare earth element.

[0024] According to a specific embodiment of the present invention, preferably, the calcination temperature in the preparation method of the iron-based catalyst modified by rare earth elements is 400-500°C.

[0025] According to a specific embodiment of the present invention, preferably, in the pretreatment of the iron-based catalyst modified with rare earth elements, the reduction reaction temperature is 320-380° C. and the reaction time is 4-16 hours. More preferably, the reduction reaction temperature is 350° C. and the reaction time is 8 hours.

[0026] The technical solution provided by the present invention has the following beneficial effects:

[0027] 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

[0028] Figure 1 This is the Mössbauer spectrum of the catalyst after 500 h of reaction in Example 1;

[0029] Figure 2 This is the Mössbauer spectrum of the catalyst after 500 hours of reaction in Comparative Example 2;

[0030] 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

[0031] 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.

[0032] Example 1

[0033] This embodiment provides a method for synthesizing α-olefins, comprising the following steps:

[0034] (1) Preparation of rare earth element-modified iron-based catalysts

[0035] 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.

[0036] (2) Catalytic reaction

[0037] 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.

[0038] Example 2

[0039] 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.

[0040] Example 3

[0041] 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.

[0042] Example 4

[0043] 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 -1The catalytic reaction results are shown in Table 1.

[0044] Example 5

[0045] This embodiment provides a method for synthesizing α-olefins, comprising the following steps:

[0046] (1) Preparation of rare earth element-modified iron-based catalysts

[0047] 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.

[0048] 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.

[0049] (2) Catalytic reaction

[0050] 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.

[0051] 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.

[0052] Example 6

[0053] 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.

[0054] Example 7

[0055] 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 -1The 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.

[0056] Table 1 Catalytic reaction results of Examples 1-7

[0057]

[0058]

[0059] 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.

[0060] Comparative Example 1

[0061] 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.

[0062] Comparative Example 2

[0063] 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.

[0064] Comparative Example 3

[0065] This comparative example provides a method for synthesizing α-olefins, which is the same as Example 1, except that the feed gas in step (2) does not contain CO2. The catalytic reaction results are shown in Table 2.

[0066] Table 2 Catalytic reaction results of Comparative Examples 1-3

[0067]

[0068]

[0069] 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.

[0070] 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.

[0071] 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 2 Corresponding data.

[0072] Table 3 Mössbauer spectroscopy data of the catalyst after 500 h of reaction in Example 1

[0073]

[0074] Table 4 Mössbauer spectroscopy data of the catalyst after 500 h of reaction in Comparative Example 2

[0075]

[0076] 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.

[0077] 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.

[0078] Example 8

[0079] 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.

[0080] Table 5 Test results of the stability of the La-Fe-1-Cat catalyst in the synthesis of α-olefins in Example 8

[0081] 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

[0082] 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.

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; the iron-based catalyst modified by the rare earth element contains iron-containing oxides, and the iron-containing oxides include ferroferric oxide and / or ferric oxide; A feed gas containing hydrogen, carbon monoxide and carbon dioxide is passed through a pretreated catalyst to carry out 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.

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 1000-6000ml·gCat -1 ·h -1 .

6. The method for synthesizing α-olefins according to claim 5, wherein: The space velocity of the raw gas is 2000-5000ml·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 8 or 9, wherein: The calcination temperature is 400-500℃.

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.

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