A highly selective synthesis of secondary aldehydes

Abstract

The present application relates to a kind of high selectivity synthesis method of secondary aldehyde.The synthesis method includes the following steps: adding catalyst, ligand, solvent into reaction device, and importing olefin and synthesis gas, so that the reaction system is under heating to carry out first stage reaction;After first stage reaction is carried out for 10-300 minutes, continue to import synthesis gas into reaction device, so that the pressure of reaction system is increased, and second stage reaction is carried out, to obtain secondary aldehyde;The pressure of reaction system during first stage reaction is 0.1-0.5 MPa;Olefin is terminal olefin or mixture containing terminal olefin.The present application first carries out low-pressure isomerization, then high-pressure hydroformylation, cooperates with specific kind of big steric hindrance monodentate phosphite ligand, controls the molar ratio of ligand and catalyst, and the three produce synergies, so that terminal olefin or terminal olefin in mixture material is first isomerized in situ, then together with original internal olefin is converted into target secondary aldehyde product with high selectivity.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis technology, and specifically relates to a highly selective synthesis method for secondary aldehydes. Background Technology

[0002] Hydroformylation is a key chemical process in which olefins react with syngas to form aldehydes under the action of a transition metal catalyst. The resulting aldehydes typically have both straight-chain and branched configurations. Secondary aldehydes such as 2-methylbutanal or 2-methylpentanal are important branched aliphatic aldehydes with wide applications in food flavorings, perfumes, pharmaceutical intermediates, and organic synthesis.

[0003] In the modern petrochemical industry, naphtha steam cracking or fluidized catalytic cracking produces a large number of byproducts. After the extraction of butadiene and isobutene, the remaining material is called raffinate C4. Raffinate C4 is a bulk and inexpensive chemical raw material, whose main components typically include 50% to 60% 2-butene (cis and trans) and 10% to 15% 1-butene.

[0004] If the raffinate C4 is directly subjected to hydroformylation, the existing homogeneous hydroformylation process requires extremely high purity of the raw materials. Since the raw materials contain 1-butene, the traditional catalytic system will promote the conversion of a large amount of 1-butene into low-value-added linear n-pentanal. As a result, the branch selectivity of the product in the entire reaction system is often less than 10%-20%, and the subsequent product separation is extremely difficult, making it difficult to obtain branched aldehyde products such as 2-methylbutanal.

[0005] Alternatively, the raffinate C4 can be separated and purified to obtain high-purity 2-butene, removing the 1-butene component, and then the relatively pure 2-butene can be used for hydroformylation to obtain the target branched secondary aldehyde. However, 1-butene and 2-butene have extremely similar boiling points, and using traditional superdistillation or cryogenic separation techniques to obtain high-purity 1-butene or pure 2-butene presents technical bottlenecks such as extremely high energy consumption and high equipment investment.

[0006] Chinese patent CN113385235B discloses a method for synthesizing 2-methylbutanal using 1-butene as a raw material and a composite ligand of phosphite and triphenylphosphine and a rhodium catalyst. Although this method can obtain the target branched aldehyde from 1-butene, i.e., terminal olefin, through hydroformylation, the selectivity of this method for the branched product 2-methylbutanal is only between 40% and 60%, the reaction selectivity of the branched target product is low, and the use of mixed ligands makes the entire operation process cumbersome.

[0007] In the hydroformylation reaction of C4 olefin feedstock, there are problems such as high energy consumption for feedstock separation, low selectivity of branched products in the hydroformylation reaction, and difficulty in separating the target product. It is difficult to achieve high selectivity and high yield of branched secondary aldehyde target products under relatively simple process conditions.

[0008] The challenge of achieving efficient, high-purity, and highly selective synthesis of branched secondary aldehydes from bulk mixed C4 feedstocks without requiring energy-intensive pre-separation steps is the hydroformylation reaction of C4 olefins. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide an improved method for synthesizing secondary aldehydes, which addresses the shortcomings and deficiencies of the prior art. This method can achieve high selectivity and high purity synthesis of target branched secondary aldehydes by hydroformylation reaction using terminal olefins or mixtures containing terminal olefins as raw materials. Moreover, the method is simple and does not require pre-separation of raw materials.

[0010] To solve the above technical problems, the present invention adopts the following technical solution: A method for synthesizing secondary aldehydes, the method comprising the following steps: A catalyst, ligand, and solvent are added to the reaction apparatus, and olefins and synthesis gas are introduced to allow the reaction system to undergo the first stage reaction under heating. After the first stage reaction has been carried out for 10 to 300 minutes, synthesis gas is continued to be introduced into the reaction device to increase the pressure of the reaction system and carry out the second stage reaction to obtain the secondary aldehyde; The pressure of the reaction system during the first stage of the reaction is 0.1~0.5 MPa; The olefin is a terminal olefin or a mixture containing terminal olefins.

[0011] The ligand is ; R1, R2 and R3 are each independently selected from H, C1-C6 alkyl, and C1-C6 alkoxy, and at least two of R1, R2 and R3 are not H; The molar ratio of the ligand to the catalyst is 7-28.

[0012] In this invention, terminal olefins refer to olefins whose double bonds are located between the two terminal carbon atoms, while internal olefins, i.e., non-terminal olefins, are those whose double bonds are not located between the two terminal carbon atoms. For example, 1-butene is a terminal olefin, while 2-butene is an internal olefin. Similarly, 1-pentene is a terminal olefin, while 2-pentene is an internal olefin.

[0013] In this invention, the first-stage reaction is carried out at a lower pressure, while the second-stage reaction is carried out at a higher pressure. During the second-stage reaction, the pressure of the reaction system is increased by continuously introducing syngas into the reaction apparatus. In this invention, the pressure is gauge pressure. In this invention, the molar ratio of ligand to catalyst can be, for example, 7, 8, 9, 10, 12, 15, 18, or 20.

[0014] In some embodiments, the pressure of the reaction system during the second stage reaction is 0.05 to 3 MPa higher than the pressure of the reaction system during the first stage reaction.

[0015] In some embodiments, the pressure of the reaction system during the second stage reaction is 0.7 to 1.5 MPa higher than the pressure of the reaction system during the first stage reaction.

[0016] In some embodiments, the pressure of the reaction system during the second stage reaction is 0.55~3 MPa.

[0017] In some embodiments, the pressure of the reaction system during the first stage reaction is 0.1~0.3 MPa, and the pressure of the reaction system during the second stage reaction is 1.0~1.5 MPa.

[0018] In some embodiments, after the first stage reaction has been carried out for 30 to 60 minutes, synthesis gas is continued to be introduced into the reaction apparatus.

[0019] In some implementations, the second stage reaction time is 30 to 300 minutes.

[0020] In some embodiments, the second-stage reaction time is 60-120 minutes. This second-stage reaction time is also relatively short, resulting in high inventive efficiency.

[0021] In some embodiments, R1, R2 and R3 are each independently selected from H, methyl, tert-butyl, and 2-isobutyl.

[0022] In some embodiments, the molar ratio of the ligand to the catalyst is 9-20.

[0023] In some embodiments, the ligand is selected from one or more combinations of the following structural formulas: , , , .

[0024] In some embodiments, the catalyst is a rhodium-based catalyst and is selected from one or more combinations of RhCl(CO)(PPh3)2, RhH(CO)(PPh3)3, RhCl(PPh3)3, RhHCl(PPh3)3, [RhCl(COD)]2, or Rh(acac)(CO)2. Wherein, CO refers to carbonyl, PPh3 refers to triphenylphosphine, COD refers to 1,5-cyclooctadiene, and acac refers to acetylacetone.

[0025] In some embodiments, the mass of rhodium in the catalyst accounts for 10 to 300 ppm of the total mass of the catalyst, ligand, solvent, and olefin; preferably 150 to 285 ppm.

[0026] In some embodiments, the terminal olefin is selected from one or more combinations of 1-butene, 1-pentene, 1-hexene, and 1-heptene.

[0027] In some embodiments, the mixture further includes an internal olefin with the same number of carbon atoms as the terminal olefin and an alkane with the same number of carbon atoms as the terminal olefin; the internal olefin is an olefin in which the double bond is not located between the two terminal carbon atoms.

[0028] In some embodiments, the mixture comprises, by mass percentage, 10%-20% of terminal olefins, 45%-60% of internal olefins with the same number of carbon atoms as the terminal olefins, and 20%-35% of alkanes with the same number of carbon atoms as the terminal olefins, wherein the sum of the mass percentages of the components in the mixture is 100%.

[0029] In some embodiments, the mixture comprises, by mass percentage, 10%-20% 1-butene, 45%-60% 2-butene, and 20%-35% butane.

[0030] In some embodiments, the mixture is the raffinate C4 fraction after butadiene and isobutene have been extracted.

[0031] In some embodiments, the secondary aldehyde is selected from one or more combinations of 2-methylbutanal, 2-methylpentanal, 2-methylhexanal, and 2-methylheptanal.

[0032] In some embodiments, the first stage reaction and the second stage reaction are at the same temperature, both being 70~150°C.

[0033] In some embodiments, the first stage reaction and the second stage reaction are at the same temperature, which is 80~100°C.

[0034] In some implementations, the first stage reaction lasts for 30 to 60 minutes. This reaction time is very short, and the reaction efficiency is very high.

[0035] In some embodiments, the solvent is selected from one or more combinations of toluene, xylene, n-pentanal, 2-methylbutanal, hexanal, and 2-methylpentanal.

[0036] In some embodiments, the reaction apparatus is a batch reactor or a continuous reaction system; when the reaction apparatus is a batch reactor, the first stage reaction lasts for 10 to 120 minutes; when the reaction apparatus is a continuous reaction system, the first stage reaction lasts for 60 to 300 minutes.

[0037] In some embodiments, the synthesis method includes the following steps: 1) Add catalyst, ligand, and solvent to the batch reactor, and introduce olefins and synthesis gas to carry out the first stage reaction at 0.1~0.5 MPa and 70~150℃; 2) After the first stage reaction has proceeded for 30-60 minutes, synthesis gas is continuously introduced into the batch reactor to increase the pressure of the reaction system to 0.55-3 MPa, and the second stage reaction is carried out at the same temperature as the first stage reaction to obtain the secondary aldehyde. This method is a batch synthesis method.

[0038] In some embodiments, the continuous reaction system includes a first-stage reactor, a second-stage reactor, and an evaporator arranged in series; the first-stage reactor is used to carry out the first-stage reaction; and the second-stage reactor is used to carry out the second-stage reaction. The synthesis method includes the following steps: 1) A catalyst, ligand, and solvent are continuously added to the first-stage reactor, and olefins and synthesis gas are continuously introduced to continuously carry out the first-stage reaction at 0.1~0.5 MPa and 70~150℃. 2) After the reaction system stays in the first stage reactor for 60 to 300 minutes, it is continuously introduced into the second stage reactor, and synthesis gas is continuously introduced into the second stage reactor to increase the pressure of the reaction system to 0.55 to 3 MPa, and the second stage reaction is carried out at the same temperature as the first stage reaction. 3) The reaction system after the second-stage reaction is passed into the evaporator for evaporation to obtain the secondary aldehyde. The liquid containing the catalyst at the bottom of the evaporator is recycled back to the first-stage reactor to continue the first-stage reaction. This method is a continuous synthesis method. There can be one, two, or more second-stage reactors. The evaporator can be, for example, a falling film evaporator.

[0039] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: The synthesis method of the present invention can achieve highly selective and high-purity synthesis of target branched secondary aldehydes by using terminal olefins or mixtures containing terminal olefins as raw materials for hydroformylation reaction. Moreover, the method is simple and does not require pre-separation of raw materials.

[0040] This invention enables the direct utilization of low-value-added bulk raw materials without separation: it breaks through the dependence of traditional processes on high-purity raw materials. This invention can directly use inexpensive industrial-grade residual C4 or waste streams containing 1-butene as raw materials. It completely eliminates the high-energy-consuming and high-equipment-investment C4 olefin ultra-distillation separation process at the front end, greatly reducing overall production costs.

[0041] The synthesis method of this invention utilizes the same catalytic system (catalyst / ligand) to carry out a two-stage reaction under different pressures, overcoming the dual challenges of traditional ligands being easily deactivated at low pressures and having poor selectivity at high pressures. This method can first convert terminal olefins or terminal olefins in mixtures into their corresponding internal olefins in situ in the first stage, followed by a second-stage hydroformylation reaction, achieving a one-step, high-yield synthesis of high-value branched aldehydes with high branch selectivity. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the reaction system in Example 8. Detailed Implementation

[0043] In modern petrochemical industries, C4 olefins are usually obtained as a mixture. For example, after naphtha steam cracking or fluidized catalytic cracking, butadiene and isobutene are first extracted. The main components of the remaining material usually include 50% to 60% of 2-butene (cis and trans) and 10% to 15% of 1-butene. This remaining material is usually called raffinate C4.

[0044] If the raffinate C4 is directly subjected to hydroformylation, the existing homogeneous hydroformylation process requires extremely high purity of the raw materials. Since the raw materials contain 1-butene, the traditional catalytic system will promote the conversion of a large amount of 1-butene into low-value-added linear n-pentanal. As a result, the branch selectivity of the product in the entire reaction system is often less than 10%-20%, and the subsequent product separation is extremely difficult, making it difficult to obtain branched aldehyde products such as 2-methylbutanal.

[0045] Alternatively, the raffinate C4 can be separated and purified to obtain high-purity 2-butene, removing the 1-butene component, and then the relatively pure 2-butene can be used for hydroformylation to obtain the target branched secondary aldehyde. However, 1-butene and 2-butene have extremely similar boiling points, and using traditional superdistillation or cryogenic separation techniques to obtain high-purity 1-butene or pure 2-butene presents technical bottlenecks such as extremely high energy consumption and high equipment investment.

[0046] The current technology for obtaining high-purity single butene feedstock is too costly, while traditional hydroformylation processes cannot achieve highly selective synthesis of branched secondary aldehydes from mixed feedstocks containing 1-butene. The challenge of the hydroformylation reaction of C4 olefins lies in how to achieve efficient, high-purity, and highly selective synthesis of branched secondary aldehydes from large quantities of mixed C4 feedstocks without incurring energy-intensive pre-separation steps.

[0047] In response, the inventors of this application discovered through research that when the raw materials contain terminal olefins, the reaction system can be controlled to favor either the isomerization reaction of terminal olefins or the traditional hydroformylation reaction by adjusting different pressures. The traditional hydroformylation reaction system for terminal olefins contains olefins, syngas reactants, catalysts, and ligands. Under the catalysis of these catalysts and ligands, the terminal olefins can also undergo isomerization to generate corresponding internal olefins. These internal olefins can then undergo further hydroformylation to generate the target secondary aldehyde. Although syngas is not a reactant in the isomerization process, it can activate the catalyst.

[0048] The inventors of this application discovered that isomerization and hydroformylation are competing reactions. When the reaction system is supplied with lower pressure via syngas, the system tends to undergo isomerization, while hydroformylation is almost nonexistent. Conversely, when the pressure of the reaction system is increased via syngas, the system tends to undergo hydroformylation, while isomerization is almost nonexistent. Therefore, this invention controls the first-stage reaction to proceed at a lower pressure, where it is almost entirely an isomerization reaction, allowing the terminal olefin to isomerize into the internal olefin. Subsequently, by increasing the pressure of the reaction system, the second-stage reaction is carried out, which is almost entirely a hydroformylation reaction, yielding the target secondary aldehyde.

[0049] This invention first isomerizes terminal olefins or mixtures containing terminal olefins under low pressure, converting the terminal olefins into internal olefins. Then, under the same catalytic system, by increasing the reaction pressure, a hydroformylation reaction can be carried out. Since the catalytic system for the isomerization of terminal olefins and the hydroformylation of internal olefins is the same, the two stages of the synthesis method of this invention can be carried out continuously without separation, and the catalyst can be added only at the beginning of the reaction. Furthermore, the syngas can serve as a catalyst promoter in the isomerization stage, a raw material for the hydroformylation reaction, and a source of reaction pressure. On the one hand, this simplifies the reaction system; on the other hand, the identical catalytic system and syngas in both stages do not introduce additional impurities or side reactions into the reaction system.

[0050] In the first stage of the isomerization reaction, the pressure of the reaction system is provided by syngas. In this stage, syngas acts as a catalyst promoter, activating the catalyst and simultaneously providing pressure to the reaction system. If syngas is not added in this stage, or if only one of CO or H2 from the syngas is added, the selectivity of the target product, the branched secondary aldehyde, will be significantly reduced. In the second stage of the hydroformylation reaction, the pressure of the reaction system is also provided by syngas. Since the pressure of the hydroformylation reaction is higher than that of the isomerization reaction, in the hydroformylation stage, in addition to continuing to use the syngas from the isomerization stage, additional syngas needs to be introduced to increase the pressure. Furthermore, in this stage, syngas is one of the raw materials for the hydroformylation reaction. Moreover, this invention, while employing the aforementioned "low-pressure isomerization-high-pressure hydroformylation" tandem reaction strategy, also incorporates a sterically hindered monodentate phosphite ligand with a specific structure, and a specific molar ratio of ligand to catalyst, enabling in-situ isomerization of terminal olefins or terminal olefins in the mixture, and highly selectively converting them, along with the original internal olefins, into the target secondary aldehyde product.

[0051] The two-stage reaction of this invention, combined with specific types of sterically hindered monodentate phosphite ligands and controlled molar ratios of ligands and catalysts, produces a synergistic effect, achieving a highly selective reaction of the target branched secondary aldehyde product. Therefore, the method of this invention can achieve efficient, high-purity, and highly selective synthesis of branched secondary aldehydes from large quantities of mixed C4 feedstocks without requiring energy-intensive pre-separation steps, solving the technical challenges of existing technologies.

[0052] The technical solutions of the present invention will be described in detail below with reference to specific embodiments, so that those skilled in the art can better understand and implement the technical solutions of the present invention, but the present invention is not limited to the scope of the examples described.

[0053] In the following examples, syngas refers to a mixture of CO and H2 in a molar ratio of 1:1.

[0054] Example 1 This example provides a method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of toluene as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L1. The catalyst composition was as follows: rhodium content in the catalyst was 250 ppm (by mass) of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 10. The reaction system was replaced with syngas, and the pressure was maintained at 0.2 MPa (gauge pressure). 56 g of 1-butene was added to the reactor, and the temperature was raised to 85°C. The mixture was stirred for 30 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.5 MPa (gauge pressure), and the reaction was continued at this pressure and 85°C for 60 minutes.

[0055] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-butene was 99.5% and the selectivity of 2-methylbutanal was 98.5%.

[0056] Example 2 This example provides a method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of 2-methylbutyraldehyde as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L2. The catalyst composition was as follows: rhodium content in the catalyst was 200 ppm of the total mass of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 15. The reaction system was replaced with syngas, and the pressure was maintained at 0.1 MPa (gauge pressure). 56 g of 1-butene was added to the reactor, and the temperature was raised to 90°C. The mixture was stirred for 30 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.5 MPa (gauge pressure), and the reaction was continued at this pressure and 90°C for 80 minutes.

[0057] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-butene was 99.3% and the selectivity of 2-methylbutanal was 98.8%.

[0058] Example 3 This example provides a method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of toluene as solvent to the reaction vessel, then add hydrogenated carbonyl tris(triphenylphosphine)rhodium RhH(CO)(PPh3)3 and ligand L1. The catalyst composition was as follows: rhodium content in the catalyst was 80 ppm of the total mass of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 20. The reaction system was replaced with syngas, and the pressure was maintained at 0.2 MPa (gauge pressure). 56 g of 1-butene was added to the reactor, and the temperature was raised to 85°C. The mixture was stirred for 45 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.0 MPa (gauge pressure), and the reaction was continued at this pressure and 85°C for 100 minutes.

[0059] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-butene was 99.1% and the selectivity of 2-methylbutanal was 98.6%.

[0060] Example 4 This example provides a method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of n-pentanal as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L3. The catalyst composition was as follows: rhodium content in the catalyst was 150 ppm of the total mass of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 18. The reaction system was replaced with syngas, and the pressure was maintained at 0.15 MPa (gauge pressure). 56 g of 1-butene was added to the reactor, and the temperature was raised to 85°C. The mixture was stirred for 45 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.0 MPa (gauge pressure), and the reaction was continued at this pressure and 85°C for 80 minutes.

[0061] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-butene was 99.2% and the selectivity of 2-methylbutanal was 99.3%.

[0062] Example 5 This example provides a method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of n-pentanal as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L4. The catalyst composition was as follows: rhodium content in the catalyst was 100 ppm of the total mass of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 9. The reaction system was replaced with syngas, and the pressure was maintained at 0.15 MPa (gauge pressure). 56 g of 1-butene was added to the reactor, and the temperature was raised to 80°C. The mixture was stirred for 60 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.3 MPa (gauge pressure), and the reaction was continued at this pressure and 80°C for another 80 minutes.

[0063] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-butene was 99.0% and the selectivity of 2-methylbutanal was 99.4%.

[0064] Example 6 This example provides a method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of toluene as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L1. The catalyst composition was as follows: rhodium content in the catalyst was 250 ppm (by mass) of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 10. The reaction system was replaced with syngas, and the pressure was maintained at 0.2 MPa (gauge pressure). 70 g of 1-pentene was added to the reactor, and the temperature was raised to 85°C. The mixture was stirred for 30 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.5 MPa (gauge pressure), and the reaction was continued at this pressure and 85°C for 60 minutes.

[0065] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-pentene was 99.0% and the selectivity of 2-methylpentanal was 98.4%.

[0066] Example 7 This example provides a method for the hydroformylation of a mixture containing terminal olefins, as detailed below: Add 250 g of toluene as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L1. The catalyst composition is as follows: the mass percentage of rhodium in the catalyst is 250 ppm, which is the total mass of the catalyst, ligands, solvent, and olefins. The molar ratio of ligands to rhodium is 10. Based on the industrially used residual C4 components, a simulated mixture was prepared with the following mass percentage composition: 15% 1-butene, 55% 2-butene, and 30% inactive n-butane.

[0067] After purging the reaction system with syngas, maintain the pressure at 0.15 MPa (gauge pressure) with syngas, add 100 g of the prepared simulated C4 mixture to the reactor, heat the reaction system to 85°C, and stir for 45 minutes to carry out in-situ isomerization. Subsequently, without replacing the catalyst or separating the materials, continue to purge the reaction system with syngas until the pressure inside the reactor jumps to 1.5 MPa (gauge pressure), and maintain the temperature at 85°C to continue the reaction for 90 minutes.

[0068] After the reaction, gas chromatography (GC) was used for sampling and analysis. The results showed that n-butane in the mixture was an inert component and did not participate in the reaction; while the total conversion rate of the mixed butene components in the system (including the original 1-butene and 2-butene) reached 99.2%. Among the generated aldehyde products, the selectivity of 2-methylbutyraldehyde was as high as 98.8%.

[0069] As can be seen, the method of the present invention can achieve highly efficient and selective hydroformylation of C4 mixed olefins, with high selectivity for the target branched secondary aldehyde. That is, it can achieve high selectivity in obtaining the target branched secondary aldehyde without prior separation of the mixed olefins.

[0070] Example 8 This example provides a continuous hydroformylation method for terminal olefins, as shown in the schematic diagram. Figure 1 As shown, the details are as follows: The continuous reaction unit includes three continuously stirred tank reactors (No. 1, No. 2, and No. 3) configured in series, as well as a falling film evaporation separation system.

[0071] N2 was first introduced into all three reactors connected in series, followed by syngas to purge them. A pre-prepared catalyst solution (using 1 kg of 2-methylbutyraldehyde as solvent, containing Rh(acac)(CO)2 and ligand L1, with a rhodium concentration of 250 ppm and a L1 / Rh molar ratio of 10) was added to the first reactor (Unit 1). Syngas was continuously introduced into Unit 1 to maintain a pressure of 0.15 MPa and a constant temperature of 90°C. Liquid 1-butene substrate was continuously introduced into Unit 1 at a flow rate of 1 kg / h for in-situ isomerization. The residence time in Unit 1 was 3.5 h. Using a transfer pump, the catalyst-containing reactant flowing from the bottom of Unit 1 was continuously pumped into Unit 2, connected in series. The residence time in Unit 2 was 3.5 h. After the reaction in Unit 2 was complete, the reactant was also continuously pumped into Unit 3 using a transfer pump. The residence time in Unit 3 was 3.5 h. Synthesis gas was continuously forced into reactors #2 and #3, both maintained at a high pressure of 1.5 MPa and a temperature of 90°C for high-pressure hydroformylation. The reaction liquid discharged from the end of reactor #3 was directly introduced into a falling film evaporator. Under short-term heating and negative pressure conditions, the crude 2-methylbutyraldehyde was flash-evaporated from the top into a collection tank; the bottom heavy component liquid containing the complete set of active catalysts was continuously pumped back to reactor #1 for closed-loop reuse. After the system stabilized, gas chromatography (GC) analysis showed that the total conversion rate of 1-butene reached over 99.5%, and the selectivity of the target product 2-methylbutyraldehyde was approximately 99.1%.

[0072] It should be noted that the hydroformylation reaction can also be carried out in a single reactor #2 without the reactor #3 connected in series, and the reaction effect will be similar.

[0073] Comparative Example 1 This example provides a comparative method for the hydroformylation of terminal olefins, as detailed below: Add 250 g of toluene as a solvent to the reaction vessel, then add dicarbonyl (acetylacetone) rhodium Rh(acac)(CO)2 and ligand L1. The catalyst composition was as follows: rhodium content in the catalyst was 250 ppm (by mass) of the catalyst, ligands, solvent, and olefins, with a molar ratio of ligand to rhodium of 10. The reaction system was replaced with syngas, and the pressure was maintained at 1.0 MPa (gauge pressure). 56 g of 1-butene was added to the reactor, and the temperature was raised to 85°C. The mixture was stirred for 30 minutes to initiate the isomerization reaction. Subsequently, syngas was continuously introduced into the reactor until the pressure inside the reactor jumped to 1.5 MPa (gauge pressure), and the reaction was continued at this pressure and 85°C for 60 minutes.

[0074] The reaction system was tested by gas chromatography (GC), and the results showed that the conversion rate of 1-butene was 99.7% and the selectivity of 2-methylbutanal was 58.3%.

[0075] A comparison of Example 1 and Comparative Example 1 shows that excessive pressure in the reaction system during the isomerization reaction leads to a significant decrease in the selectivity of the target branched secondary aldehyde 2-methylbutyraldehyde during the hydroformylation stage. This is because under high syngas pressure, the alkylrhodium intermediate formed by the terminal olefin (1-butene) and the catalyst rapidly captures the high concentration of carbon monoxide in the system and directly undergoes a migration insertion reaction, generating a large amount of straight-chain n-pentanal along the default hydroformylation pathway. This high-pressure environment kinetically strongly inhibits the internal isomerization of the 1-butene double bond to 2-butene. Due to the lack of 2-butene as a key precursor, the reaction system cannot selectively generate branched 2-methylbutyraldehyde in subsequent stages. This invention achieves high selectivity for the target product 2-methylbutyraldehyde by controlling a suitable, lower reaction pressure in the isomerization stage. Furthermore, the lower reaction pressure reduces the equipment requirements of the reaction system, significantly reducing the overall cost of the method.

[0076] Comparative Example 2 This example provides a comparative method for the hydroformylation of terminal olefins, which is basically the same as in Example 1, except that ligand L1 is replaced with a strongly chelating bidentate ligand. The results showed a 1-butene conversion rate of 99.6% and a 2-methylbutyraldehyde selectivity of only 8.5%.

[0077] As can be seen from the comparison between Example 1 and Comparative Example 2, the present invention, by employing a sterically hindered monodentate phosphite ligand, combined with the low-pressure isomerization reaction followed by a high-pressure hydroformylation reaction, can achieve high selectivity for the target product 2-methylbutyraldehyde. However, when using bidentate phosphite ligands, due to their strong chelating effect, a rigid dual-coordination structure (usually equatorial-equatorial coordination) is formed at the rhodium center. This specific large "bite angle" and confined spatial microenvironment exert a forced stereoguiding effect on the insertion direction of the olefin, greatly favoring the generation of straight-chain alkyl rhodium intermediates. As a result, the reaction is mostly directed towards the straight-chain product (n-pentanal), leading to very low selectivity for the target product 2-methylbutyraldehyde.

[0078] Comparative Example 3 This example provides a comparative method for the hydroformylation of terminal olefins, essentially the same as in Example 1, except that the molar ratio of ligand to rhodium is replaced with 2, i.e., a very low amount of ligand is used. The results show a 1-butene conversion of 75.2% and a 2-methylbutanal selectivity of 90.9%, indicating a certain degree of reduction in both.

[0079] Comparative Example 4 This example provides a comparative method for the hydroformylation of terminal olefins, essentially the same as in Example 1, except that the molar ratio of ligand to rhodium is replaced with 30, i.e., a very high amount of ligand is used. The result is a 1-butene conversion of 87.3% and a 2-methylbutanal selectivity of 80.3%, indicating a certain degree of reduction in both.

[0080] As can be seen from the comparison of Example 1 and Comparative Examples 3-4, the present invention can further improve the conversion rate of terminal olefins and the selectivity of the target product 2-methylbutyraldehyde by controlling the molar ratio of ligands and catalysts within a specific and suitable range. When the ligand ratio is too low, due to the severe lack of coordination protection of the active metal center, the unsaturated rhodium atoms are prone to aggregate, forming polynuclear metal clusters lacking catalytic activity or even directly precipitating, resulting in a large amount of catalyst deactivation. At the same time, the incomplete coordination environment will also exacerbate the occurrence of side reactions, leading to a decrease in conversion rate and target product selectivity. When the ligand ratio is too high, due to the large steric hindrance of the monodentate phosphite ligand used in the present invention, excessive ligand will cause the active metal center to be completely encapsulated, making it extremely difficult for carbon monoxide and olefin substrate molecules to approach and coordinate to the catalytic center in space. This not only significantly reduces the reaction rate but also destroys the highly selective catalytic microenvironment for inducing branched aldehyde formation, leading to a decrease in conversion rate and target product selectivity. In addition, excessive ligands will increase costs.

[0081] Comparative Example 5 This example provides a comparative method for the hydroformylation of terminal olefins, essentially the same as in Example 1, except that ligand L1 is replaced with the conventional triphenylphosphine TPP ligand. The results show a 1-butene conversion of 99.5% and a 2-methylbutyraldehyde selectivity of only 25.3%.

[0082] As can be seen from the comparison between Example 1 and Comparative Example 5, the present invention, based on the isomerization reaction under low pressure followed by hydroformylation under high pressure, can achieve highly selective yield of branched secondary aldehyde products during the hydroformylation reaction of terminal olefins by further selecting specific types of ligands. In contrast, traditional TPP ligands lack the strong π-acceptor effect to accelerate CO dissociation, and the reaction still follows the default pathway to generate low-value straight-chain aldehydes, resulting in very low selectivity for the target branched aldehyde product.

[0083] This invention first isomerizes terminal olefins or mixtures containing terminal olefins under low pressure, converting the terminal olefins into internal olefins. Then, under the same catalytic system, the reaction pressure is increased by reintroducing syngas, allowing for further hydroformylation. Simultaneously, this two-stage reaction, combined with specific types of sterically hindered monodentate phosphite ligands and controlled molar ratios of ligands and catalysts, produces a synergistic effect, achieving highly selective reaction of the target branched secondary aldehyde product. Therefore, the method of this invention can achieve efficient, high-purity, and highly selective synthesis of branched secondary aldehydes from large quantities of mixed C4 feedstocks without requiring energy-intensive pre-separation steps, solving the technical challenges of existing technologies.

[0084] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

[0085] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

Claims

1. A method for synthesizing secondary aldehydes, characterized in that: The synthesis method includes the following steps: A catalyst, ligand, and solvent are added to the reaction apparatus, and olefins and synthesis gas are introduced to allow the reaction system to undergo the first stage reaction under heating. After the first stage reaction has been carried out for 10 to 300 minutes, synthesis gas is continued to be introduced into the reaction device to increase the pressure of the reaction system and carry out the second stage reaction to obtain the secondary aldehyde; The pressure of the reaction system during the first stage of the reaction is 0.1~0.5 MPa; The olefin is a terminal olefin or a mixture containing terminal olefins; The ligand is ; R1, R2 and R3 are each independently selected from H, C1-C6 alkyl, and C1-C6 alkoxy, and at least two of R1, R2 and R3 are not H; The molar ratio of the ligand to the catalyst is 7-28.

2. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The pressure of the reaction system during the second stage of the reaction is 0.05~3 MPa higher than that during the first stage of the reaction.

3. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The pressure of the reaction system during the second stage of the reaction is 0.7 to 1.5 MPa higher than that during the first stage of the reaction.

4. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The pressure of the reaction system during the second stage of the reaction is 0.55~3 MPa.

5. The method for synthesizing secondary aldehydes according to claim 4, characterized in that: The pressure of the reaction system during the first stage of the reaction is 0.1~0.3 MPa, and the pressure of the reaction system during the second stage of the reaction is 1.0~1.5 MPa.

6. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: After the first stage reaction has proceeded for 30 to 60 minutes, synthesis gas is continued to be introduced into the reaction apparatus.

7. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The second stage reaction lasts for 30 to 300 minutes.

8. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: R1, R2 and R3 are each independently selected from H, methyl, tert-butyl, and 2-isobutyl.

9. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The molar ratio of the ligand to the catalyst is 9-20.

10. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The ligand is selected from one or more of the following structural formulas: 、 、 、 。 11. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The catalyst is a rhodium-based catalyst, and is selected from one or more combinations of RhCl(CO)(PPh3)2, RhH(CO)(PPh3)3, RhCl(PPh3)3, RhHCl(PPh3)3, [RhCl(COD)]2 or Rh(acac)(CO)2; and / or, the mass of rhodium in the catalyst accounts for 10 to 300 ppm of the total mass of the catalyst, ligand, solvent, and olefin.

12. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The terminal olefin is selected from one or more combinations of 1-butene, 1-pentene, 1-hexene, and 1-heptene; and / or, the mixture further includes an internal olefin with the same number of carbon atoms as the terminal olefin and an alkane with the same number of carbon atoms as the terminal olefin; the internal olefin is an olefin in which the double bond is not located between the two terminal carbon atoms.

13. The method for synthesizing secondary aldehydes according to claim 12, characterized in that: The mixture comprises, by mass percentage, 10%-20% terminal olefins, 45%-60% internal olefins with the same number of carbon atoms as the terminal olefins, and 20%-35% alkanes with the same number of carbon atoms as the terminal olefins, and the sum of the mass percentages of the components in the mixture is 100%.

14. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The secondary aldehyde is selected from one or more combinations of 2-methylbutyraldehyde, 2-methylpentanaldehyde, 2-methylhexanaldehyde, and 2-methylheptanaldehyde; and / or, the first stage reaction and the second stage reaction are at the same temperature, both being 70~150℃; and / or, the solvent is selected from one or more combinations of toluene, xylene, n-pentanaldehyde, 2-methylbutyraldehyde, hexanaldehyde, and 2-methylpentanaldehyde.

15. The method for synthesizing secondary aldehydes according to claim 1, characterized in that: The reaction apparatus is either a batch reactor or a continuous reaction system; when the reaction apparatus is a batch reactor, the first stage reaction lasts for 10 to 120 minutes; when the reaction apparatus is a continuous reaction system, the first stage reaction lasts for 60 to 300 minutes.

16. The method for synthesizing secondary aldehydes according to claim 15, characterized in that: The synthesis method includes the following steps: 1) Add catalyst, ligand, and solvent to the batch reactor, and introduce olefins and synthesis gas to carry out the first stage reaction at 0.1~0.5 MPa and 70~150℃; 2) After the first stage reaction has been carried out for 30 to 60 minutes, synthesis gas is continued to be introduced into the batch reactor to increase the pressure of the reaction system to 0.55 to 3 MPa, and the second stage reaction is carried out at the same temperature as the first stage reaction to obtain the secondary aldehyde.

17. The method for synthesizing secondary aldehydes according to claim 15, characterized in that: The continuous reaction system includes a first-stage reactor, a second-stage reactor, and an evaporator arranged in series; the first-stage reactor is used to carry out the first-stage reaction; the second-stage reactor is used to carry out the second-stage reaction. The synthesis method includes the following steps: 1) A catalyst, ligand, and solvent are continuously added to the first-stage reactor, and olefins and synthesis gas are continuously introduced to continuously carry out the first-stage reaction at 0.1~0.5 MPa and 70~150℃. 2) After the reaction system stays in the first stage reactor for 60 to 300 minutes, it is continuously introduced into the second stage reactor, and synthesis gas is continuously introduced into the second stage reactor to increase the pressure of the reaction system to 0.55 to 3 MPa, and the second stage reaction is carried out at the same temperature as the first stage reaction. 3) The reaction system after the second stage reaction is passed into the evaporator for evaporation to obtain the secondary aldehyde. The liquid containing the catalyst at the bottom of the evaporator is recycled to the first stage reaction vessel to continue the first stage reaction.

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