A method for preparing olefins by dehydrogenating low-carbon alkanes at low temperature

Abstract

A method for preparing olefins by low-temperature dehydrogenation of low-carbon alkanes can be used to dehydrogenate C2-C4 alkane feed gas on a solid heterogeneous catalyst in the presence of water vapor in a light-heat synergistic manner: the water vapor concentration is 0.1% to 10%; the reaction temperature is 20 to 400°C, the light wavelength is 200 to 500nm, and the light intensity is 0.1 to 100W / cm 2 Solid heterogeneous catalysts based on TiO 2 The carrier is a single-atom Cu active metal component; ethane, propane, n-butane and isobutane can be highly selectively converted into ethylene, propylene, n-butene and isobutene and hydrogen in a photothermal synergistic catalytic device. The present invention provides a new method for preparing corresponding olefins and hydrogen from low-carbon alkanes under mild conditions.

Description

Technical Field

[0001] The present invention relates to a method for producing olefins by low-temperature dehydrogenation of low-carbon alkanes assisted by water vapor. The present invention proposes to produce hydrogen and active hydroxyl species under the conditions of photothermal synergistic catalysis using water vapor as a co-catalyst; wherein the active hydroxyl species can react with low-carbon alkanes to generate olefins and water, indirectly realizing the reaction route of dehydrogenating low-carbon alkanes to prepare corresponding olefins and hydrogen. The present invention proposes a new reaction route for dehydrogenating alkanes to produce olefins, which has outstanding advantages in terms of reaction temperature and is suitable for the dehydrogenation reaction of ethane, propane, n-butane and isobutane. Background Art

[0002] Light olefins are an important type of basic chemical raw materials that can be used to produce various chemical materials and key intermediates such as polyethylene, polypropylene, butadiene, etc. Its output, scale and technical process are one of the indicators of the development level of a country's petrochemical industry. At present, the source of light olefins is still mainly petroleum steam cracking. However, its operating conditions are harsh and the product distribution is limited, and new technical processes are urgently needed. With the large-scale development and utilization of shale gas and natural gas, the alkane dehydrogenation route has developed into an important part of the source of light olefins.

[0003] Taking propylene production as an example, direct propane dehydrogenation technology can produce propylene without relying on crude oil. It is a hot spot for the development of the propylene preparation industry and has been successfully industrialized. However, the bond energy of the CH bond in propane is as high as 401 kJ / mol. The direct dehydrogenation reaction of propane is a strong endothermic process. The reaction needs to be carried out under high temperature conditions, which can easily cause sintering or carbon deposition of the catalyst. In addition, the catalyst used in the direct dehydrogenation technology of propane is a supported Pt or Cr2O3 catalyst. Pt is expensive and Cr2O3 has a strong carcinogenic effect. This patent describes a method for the low-temperature dehydrogenation of low-carbon alkanes to produce olefins assisted by water vapor. Based on a new reaction route, low-carbon alkanes can be dehydrogenated at low temperature on a Cu / TiO2 catalyst under the synergistic conditions of light and heat to produce corresponding olefins and hydrogen. Summary of the invention

[0004] The present invention provides a method for producing olefins by dehydrogenating low-carbon alkanes at low temperatures. The method can realize the dehydrogenation of low-carbon alkanes to produce corresponding olefins and hydrogen in a relatively low temperature range under the conditions of light and heat synergy, thus solving the problem of high activation temperature in the existing direct dehydrogenation reaction of low-carbon alkanes.

[0005] In the low-temperature dehydrogenation method of low-carbon alkanes involved in the present invention, water vapor is used as a promoter, and the volume concentration of the water vapor is 0.1% to 10%.

[0006] The method for preparing olefins by low-carbon alkane dehydrogenation at low temperature of the present invention is specifically implemented by the following steps (taking propane dehydrogenation reaction as an example): photogenerated electrons (e) are generated on the surface of TiO2 under the condition of light excitation. -) and holes (h + ), the photogenerated electrons are used to reduce the H generated by water ionization + Generates hydrogen, holes and OH - It is used for the dehydrogenation reaction of propane to produce propylene and water, indirectly realizing the reaction route of dehydrogenation of low-carbon alkanes to produce corresponding olefins and hydrogen.

[0007] TiO2+hν→TiO2(e - +h + ) (1)

[0008] H2O→H + +OH - (2)

[0009] 2H + +2e - →H2(Photoreduction process) (3) C3H8+2OH - +2h + →C3H6+2H2O (photooxidation process) (4)

[0010] C3H8→C3H6+H2(overall reaction) (5)

[0011] The present invention uses water vapor as a co-catalyst to generate hydrogen and active hydroxyl species on a Cu / TiO2 catalyst under photothermal synergistic catalytic conditions. The obtained active hydroxyl species can react with low-carbon alkanes to generate olefins and water, indirectly realizing the reaction route of dehydrogenating low-carbon alkanes to prepare corresponding olefins and hydrogen.

[0012] Aiming at the problem of high reaction temperature in the dehydrogenation of low-carbon alkanes to olefins, the present invention proposes a reaction route of water-gas-assisted catalytic dehydrogenation, which has outstanding advantages in terms of reaction temperature and is suitable for the dehydrogenation of ethane, propane, n-butane and isobutane. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 .Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy image of the newly prepared Cu1 / TiO2 catalyst;

[0014] Figure 2 .The yields of different low-carbon olefins on Cu1 / TiO2 catalyst;

[0015] Figure 3 .Propylene and hydrogen yields in the light-switching experiment;

[0016] Figure 4 . Spherical aberration corrected high-angle annular dark-field scanning transmission electron microscopy image of the Cu1 / TiO2 catalyst after reaction;

[0017] Figure 5.Mass spectrometry signals of hydrogen and deuterium obtained by the reaction of deuterated propane with water vapor;

[0018] Figure 6 .The effect of different propane concentrations on propylene yield at 2% water vapor;

[0019] Figure 7 .The effect of different water concentrations on propylene yield at 30% propane;

[0020] Figure 8 .Effect of different Cu loadings on propylene yield;

[0021] Fig. 9 .Propane yields over Cu / TiO2-pre-impregnated and Cu / TiO2-post-impregnated catalysts;

[0022] Fig.10 .Propylene yield under different reaction conditions. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is not limited to the following specific implementation modes.

[0024] Example 1.

[0025] Add 0.76 mL of 20 mg / mL copper nitrate trihydrate solution to 20 mL of deionized water; add 200 mg of P25 powder and continue stirring, and irradiate with a 385 nm LED light for 10 minutes; after centrifugation, the above mixture is transferred to a 60 ° C oven and dried for 12 hours, and then ground to obtain Cu / TiO2. The ICP test results show that the actual mass content of Cu is 0.1%. Spherical aberration corrected high-angle annular dark field scanning transmission electron microscopy images show that Cu on the TiO2 surface exists in the form of single atoms, recorded as Cu / TiO2-photodeposition 1 (see Figure 1 ).

[0026] Example 2.

[0027] The process and conditions are the same as those of Example 1, except that the LED lamp is continuously illuminated for 2 minutes. The ICP test results show that the actual mass content of Cu is 0.02%. The spherical aberration corrected high-angle annular dark field scanning transmission electron micrograph shows that Cu on the TiO2 surface exists in the form of single atoms, recorded as Cu / TiO2-photodeposition 2.

[0028] Example 3.

[0029] The process and conditions are the same as those of Example 1, except that the LED lamp was continuously illuminated for 50 minutes. The ICP test results showed that the actual mass content of Cu was 0.8%. The spherical aberration corrected high-angle annular dark field scanning transmission electron micrograph showed that Cu nanoparticles appeared on the surface of TiO2, recorded as Cu / TiO2-photodeposition 3.

[0030] Example 4.

[0031] 1.71 mL of deionized water was added to 0.19 mL of 20 mg / mL copper nitrate trihydrate solution and mixed thoroughly; 1 g of P25 powder was added and stirred continuously until uniform; the mixture was transferred to a 45°C oven and heated continuously until the liquid was completely evaporated, then raised to 60°C and continued to dry for 12 hours; after grinding, calcined in air at 400°C for 2 hours, and a Cu / P25 catalyst with a theoretical mass content of Cu of 0.1% was obtained after grinding, which was recorded as Cu / TiO2-pre-impregnation.

[0032] Example 5.

[0033] Add 483.2 mg of copper nitrate trihydrate to 10 mL of deionized water, and continue stirring in an 80°C oil bath to obtain solution A; add 1.06 g of ascorbic acid to 10 mL of deionized water and stir until dissolved to obtain solution B; add solution B dropwise to solution A until a black colloidal solution is generated, then stop stirring and cool to room temperature for use.

[0034] Take 0.06 mL of the above colloidal solution, add 0.24 mL of deionized water and mix thoroughly; add 400 mg of P25 powder and continue stirring until uniform; transfer the above mixture to a 45°C oven, heat continuously until the liquid is completely evaporated, then raise the temperature to 60°C and continue drying for 12 hours; after grinding, calcine in air at 400°C for 2 hours, and after grinding, obtain a Cu / P25 catalyst with a theoretical mass content of Cu of 0.1%, recorded as Cu / TiO2-post-impregnation.

[0035] Example 6.

[0036] The Cu / TiO2-photodeposition 1 obtained in Example 1 was used as a catalyst to study the photothermal synergistic catalytic water vapor-assisted low-temperature dehydrogenation of ethane to produce ethylene and hydrogen. The reaction and test conditions were: a top-illuminated sleeve photothermal synergistic reactor (patent number: 201911170165.2) was used; the xenon lamp energy density was 0.5W / cm 2 , wavelength of 320-400nm; the tubular heating furnace maintained the reaction temperature at 80°C; the raw gas composition (volume ratio) was 30% C2H6, 2% H2O, 68% N2, and the raw gas space velocity was 9000mL·g -1 ·h -1The gas phase products of the reaction were analyzed online by gas chromatography (Agilent, 7890B) equipped with a Porapak Q column and TCD to determine the yields of olefins and hydrogen. The yield of ethylene was 446 μmol g cat -1 h -1 (See Figure 2 ).

[0037] Example 7.

[0038] The difference from Example 6 is that propane is used instead of ethane, and the Cu / TiO2-photodeposition 1 obtained in Example 1 is used as a catalyst to study the photothermal synergistic catalytic water vapor assisted propane low-temperature dehydrogenation to produce propylene and hydrogen. The reaction and test conditions are the same as those in Example 6, and the yield of propylene obtained is 1201 μmol g cat -1 h -1 (See Figure 2 ).

[0039] Example 8.

[0040] The difference from Example 7 is that n-butane is used instead of propane, and the Cu / TiO2-photodeposition 1 obtained in Example 1 is used as a catalyst to study the photothermal synergistic catalytic water vapor-assisted low-temperature dehydrogenation of n-butane to produce n-butene and hydrogen. The reaction and test conditions are the same as those in Example 5, and the yield of n-butene obtained is 649 μmol g cat -1 h -1 (See Figure 2 ).

[0041] Example 9.

[0042] The difference from Example 8 is that isobutane is used instead of n-butane, and the Cu / TiO2-photodeposition 1 obtained in Example 1 is used as a catalyst to study the photothermal synergistic catalytic water vapor-assisted low-temperature dehydrogenation of isobutane to produce isobutylene and hydrogen. The reaction and test conditions are the same as those in Example 6, and the yield of isobutylene is 645 μmol g cat -1 h -1 (See Figure 2 ).

[0043] Under the conditions of photothermal synergistic catalysis, the corresponding olefins and hydrogen can be obtained in Examples 6, 7, 8, and 9 by using water vapor to assist the dehydrogenation of low carbon alkanes. Among them, the activity of propane dehydrogenation to produce propylene in Example 7 is the highest, reaching 1201 μmol g cat -1 h -1 .

[0044] Example 10.

[0045] The Cu / TiO2-photodeposition 1 obtained in Example 1 was used as a catalyst to study the photothermal synergistic catalytic water vapor assisted propane low temperature dehydrogenation to produce propylene and hydrogen. The difference is that the illumination condition is adjusted to 0.3W / cm 2 , the heating temperature was adjusted to 50°C, and other reaction and test conditions were the same as those in Example 5. The light was turned on for 0.5 h to carry out the photothermal synergistic catalytic reaction, and after sampling and analysis, the light was turned off for 0.5 h to carry out the thermal catalytic reaction, and after sampling and analysis, the light was turned on again for 0.5 h to carry out the photothermal synergistic catalytic reaction, and this was carried out alternately.

[0046] Under light conditions, propane dehydrogenation produces propylene and hydrogen in a molar ratio of about 1:1. Under light-off conditions, no propylene and hydrogen are produced, which proves the occurrence of propane dehydrogenation and the important role of light in the reaction process. In the on-off light experiment for 5.5 hours, the yields of propylene and hydrogen did not decrease significantly, indicating that the reaction has good stability (see Figure 3 ), and no obvious agglomeration of single-atom Cu on the surface of Cu / TiO2-photodeposition 1 catalyst was found (see Figure 4 ).

[0047] Example 11.

[0048] The process and conditions are the same as those of Example 10, except that deuterated propane (C3D8) is used instead of propane, and detection and analysis are performed by online mass spectrometry. The Cu / TiO2-photodeposition obtained in Example 1 is used as a catalyst, and other reactions and test conditions are the same as those of Example 5.

[0049] like Figure 5 As shown, as the reaction proceeds, the signal of hydrogen detected in the mass spectrometry signal increases significantly, indicating that the hydrogen in the water-assisted light alkane dehydrogenation reaction comes from water rather than light alkane.

[0050] Example 12.

[0051] The process and conditions are the same as those in Example 7, except that the volume concentration of water vapor is fixed at 2%, the volume concentration of propane is changed to 50%, 30%, 20%, 10% and 5% respectively, the balance gas is nitrogen, and other reaction and test conditions are the same as those in Example 7.

[0052] like Figure 6 As shown, with the decrease of propane concentration, the production of propylene first increases and then decreases, and the optimal reaction concentration of propane is 30%.

[0053] Example 13.

[0054] The process and conditions are the same as those in Example 10, except that the concentration of propane is fixed at 30%, and the concentration of water vapor is changed to 1%, 1.5%, 2%, 3% and 5%. Other reaction and test conditions are the same as those in Example 10.

[0055] With the increase of water vapor concentration, the yield of propylene increases first and then decreases. The optimal reaction concentration of propylene is 2% (see Figure 7 ).

[0056] Example 14.

[0057] The difference from Example 7 is that the Cu / TiO2-photodeposition 2 obtained in Example 2 is used as the catalyst for the test. The other reaction and test conditions are the same as those in Example 7. The yield of propylene is 230 μmol g cat -1 h -1 (See Figure 8 ).

[0058] Example 15.

[0059] The difference from Example 14 is that the Cu / TiO2-photodeposition 3 obtained in Example 3 is used as the catalyst for the test. The other reaction and test conditions are the same as those in Example 14. The yield of propylene is 960 μmol g cat -1 h -1 (See Figure 8 ).

[0060] Example 16.

[0061] The difference from Example 15 is that the Cu / TiO2-pre-impregnation obtained in Example 4 is used as the catalyst for testing. The other reaction and test conditions are the same as those in Example 15. The yield of propylene is 182 μmol g cat -1 h -1 (See Fig. 9 ).

[0062] Example 17.

[0063] The difference from Example 16 is that the Cu / TiO2-post-impregnation obtained in Example 5 is used as the catalyst for testing. The other reaction and test conditions are the same as those in Example 16. The yield of propylene is 49 μmol g cat -1 h -1 (See Fig. 9 ).

[0064] like Fig. 9As shown, when the Cu / TiO2-pre-impregnation and Cu / TiO2-post-impregnation catalysts obtained in Examples 2 and 3 were used for water-gas-assisted photothermal synergistic catalytic propane low-temperature dehydrogenation reaction, the propane yields were 182 μmol g cat -1 h -1 and 49 μmol g cat -1 h -1 , which is much lower than the 1201 μmol g of Cu / TiO2-photodeposition catalyst in Example 5. cat -1 h -1 , indicating that the Cu single-atom catalyst prepared by photodeposition has higher activity in this reaction.

[0065] Comparative Example 1.

[0066] The difference from Example 7 is that no water vapor is added, the volume concentration of propane in the feed gas remains unchanged, and other conditions are the same as those in Example 7 to test the feasibility of water vapor-assisted low-temperature dehydrogenation of light alkanes. The results show that the amount of propylene product is 127 μmol g cat -1 h -1 , will be significantly reduced. Therefore, water vapor assistance can greatly increase the reaction rate of dehydrogenation of low-carbon alkanes to olefins (see Fig.10 ).

[0067] Comparative Example 2.

[0068] The difference from Example 7 is that no light is added. When other conditions are the same as those in Example 7, the feasibility of water vapor-assisted low-temperature dehydrogenation of light alkanes is tested. The results show that no olefin products and hydrogen are detected when only heating is used. Therefore, the reaction process of water vapor-assisted dehydrogenation of light alkanes to olefins requires light to activate the reaction (see Fig.10 ).

[0069] Comparative Example 3.

[0070] The difference from Example 7 is that P25 (denoted as TiO2) is used to replace the Cu / TiO2-photodeposition catalyst. When other conditions are the same as those of Example 7, the effect of single-atom Cu in the reaction is tested. The results show that when single-atom Cu is not added to the catalyst, the water vapor-assisted propane dehydrogenation reaction cannot proceed, confirming that single-atom Cu plays an important role in the reaction (see Fig.10 ).

[0071] Comparative Example 4.

[0072] The difference from Example 1 is that without centrifugation: 0.76 mL of 20 mg / mL copper nitrate trihydrate solution was added to 20 mL of deionized water; 200 mg of P25 powder was added and stirred continuously, and irradiated with a 385 nm LED lamp for 10 minutes; the above mixture was directly transferred to a 60° C. oven for drying for 12 hours without centrifugation, and then ground to obtain Cu / TiO2, whose theoretical Cu loading was 2%. It is recorded as Cu / TiO2-photodeposition 4.

[0073] The difference from Example 7 is that the Cu / TiO2-photodeposition 4 obtained in Comparative Example 4 was used as the catalyst for the test. The other reaction and test conditions were the same as those in Example 7. The yield of propylene was 763 μmol g cat -1 h -1 , indicating that Cu nanoclusters and particles will appear in the Cu / TiO2-photodeposition 4 prepared without solid-liquid separation, resulting in a decrease in catalytic activity, proving the superiority of the preparation method in Example 1.

[0074] It can be seen from the above examples and comparative examples that under the conditions of photothermal synergistic catalysis, low-carbon alkanes can undergo dehydrogenation reaction on Cu / TiO2-photodeposition catalyst under water vapor-assisted conditions to generate corresponding olefins and hydrogen, wherein the hydrogen comes from water. This reaction route can realize the dehydrogenation reaction of low-carbon alkanes under relatively mild conditions, providing a new way for the low-temperature dehydrogenation of low-carbon alkanes to olefins.

Claims

1. A method for preparing olefins by low-temperature dehydrogenation of light alkanes, characterized in that: Using C1-C4 low-carbon alkanes as raw materials and water as a co-catalyst, the corresponding olefins and hydrogen are produced by photothermal synergistic catalysis in the presence of a solid heterogeneous catalyst. The reaction temperature is 20 ~ 400 o C; The wavelength of light is 200 ~ 500 nm; Light intensity is 0.1 ~100 W / cm 2 ; The solid heterogeneous catalyst uses TiO2 as a carrier, and the loaded metal active component is single-atom Cu. The single-atom Cu is loaded on the TiO2 carrier by photochemical deposition to obtain a Cu / TiO2 catalyst. The content of Cu in the catalyst is 0.01~2 wt%; The process of preparing Cu / TiO2 single-atom catalyst by photochemical deposition is as follows: add 0.1~2 mL of copper nitrate trihydrate solution with a concentration of 1~50 mg / mL to 5~50 mL of deionized water; add 10~1000 mg of TiO2 powder and continue stirring, and irradiate with 320~450 nm LED light for 3~30 min; after solid-liquid separation, transfer the solid to a 40~120℃ oven and dry it for 3~48h to obtain Cu / TiO2.

2. The method according to claim 1, characterized in that: The wavelength of light is 320 ~450 nm; Light intensity is 0.5 ~50 W / cm 2 ; The Cu content in the catalyst is 0.02~1 wt%.

3. The method according to claim 2, characterized in that: The wavelength of light is 350 ~ 400 nm; Light intensity is 1 ~10 W / cm 2 .

4. The method according to claim 1, characterized in that: The hydrogen in the reaction products comes directly from water rather than from lower alkanes.

5. The method according to claim 1, characterized in that: The alkane dehydrogenation reaction is a photothermal synergistic catalytic dehydrogenation reaction of one or more of ethane, propane, normal butane and isobutane to produce corresponding olefins and hydrogen.

6. The method according to claim 1, characterized in that: The volume concentration of water in the raw gas of the photothermal catalytic alkane dehydrogenation reaction is 0.1% to 10%; the volume concentration of low-carbon alkanes is above 1%, the volume ratio of low-carbon alkanes to water is 1 to 50, and the rest is inert gas; The inert gas is one or more of N2, He, and Ar.

7. The method according to claim 6, characterized in that: The volume concentration of water in the raw gas of the photothermal catalytic alkane dehydrogenation reaction is 0.5-6%; the volume concentration of low-carbon alkanes is 10%-80%, and the volume ratio of low-carbon alkanes to water is 5-30.

8. The method according to claim 7, characterized in that: The volume concentration of water in the raw gas of the photothermal catalytic alkane dehydrogenation reaction is 1-3%; the volume concentration of low-carbon alkanes is 20%-60%, and the volume ratio of low-carbon alkanes to water is 10-20.

9. The method according to claim 1, characterized in that: The reaction temperature is 30~200℃.

10. The method according to claim 9, characterized in that: The reaction temperature is 40~100℃.

11. The method according to claim 1, characterized in that: The catalyst is evenly spread on the fixed bed of the fixed bed reactor, and the catalyst bed thickness is 10~50 μm. The generation rate of olefins and hydrogen in the dehydrogenation reaction of light alkanes increases with the increase of light power density.

12. The method according to claim 1, characterized in that: The TiO2 carrier is one or more of anatase TiO2, rutile TiO2, and P25.

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