A method for photocatalytic conversion of methane to formic acid

Abstract

The present application relates to a kind of photocatalytic methane conversion preparation formic acid method, the method includes the following steps: ultrasonic uniform mixing palladium supported photocatalyst with solvent, then oxygen and methane are successively introduced, and methanol, formaldehyde, methyl hydroperoxide and the formic acid obtained by reaction under irradiation;The palladium supported photocatalyst includes palladium active component and tungsten trioxide carrier.The present application uses methane as raw material, oxygen as oxidant, under the joint action of illumination and palladium supported photocatalyst, methane is converted into formic acid, and the obtained formic acid has higher yield and selectivity;The reaction condition of photocatalytic methane conversion system into formic acid is mild, process is simple and green environmental protection, suitable for large-scale industrial production.

Description

TECHNICAL FIELD

[0001] The application relates to the field of catalytic synthesis, in particular to a method for preparing formic acid by photocatalytic conversion of methane. BACKGROUND

[0002] With the gradual depletion of coal and oil resources, the discovery of large natural gas fields and shale gas fields, and the gradual maturity of natural gas exploitation technology, natural gas with methane as the main component has become one of the most promising clean fossil energy and chemical raw materials. Methane usually exists in gaseous form and is flammable and explosive, which is not suitable for long-distance transportation, thus limiting the application of methane. It is a more feasible way to convert methane into liquid with higher added value, such as formic acid, methanol or formaldehyde, which is easy to transport.

[0003] Methane is a stable inert molecule with a symmetrical tetrahedral structure of four equal carbon-hydrogen bonds, which is difficult to activate and convert. At present, the conversion of methane in industry adopts a synthesis gas route, which converts methane into synthesis gas through methane wet reforming, and is used in downstream processes such as Fischer-Tropsch synthesis or methanol synthesis. This process generally needs to be carried out at high temperature (700-1100℃) and high pressure (3-5 MPa), and the reaction conditions are relatively harsh, and a large amount of carbon dioxide will be discharged. In recent years, photocatalytic conversion of methane to products with higher added value is considered as a very promising route. Among them, the conversion of methane to formic acid has important significance. Formic acid is an ideal hydrogen storage liquid with low toxicity, high volumetric hydrogen density and good chemical stability, and is expected to become a hydrogen source in fuel cells. Therefore, the study of photocatalytic preparation of formic acid from methane has important research significance, and provides a new idea for the utilization of methane.

[0004] Wu Mingtian et al. (Angew. Chem. Int. Ed. 2021, 60, 8889-8895) used FeN x / C as a catalyst and hydrogen peroxide as an oxidant to realize photocatalytic preparation of formic acid from methane under mild reaction conditions. The yield of formic acid in the reaction system is 5.49 μmol, and the selectivity of formic acid is 90%. However, hydrogen peroxide as an oxidant has a high cost, so this method cannot be applied on a large scale.

[0005] Wang Chunling et al. (J. Mater. Chem. A, 2021, 9, 1713-1719) used HSiMo / TiO2 to realize the photooxidation of methane to formic acid. When the reaction temperature increases from 50℃ to 150℃, the yield of formic acid increases from 2.8 μmol to 27.2 μmol, and the selectivity of formic acid is only 46%. The photocatalytic conversion of methane to formic acid also has the problems of low yield and poor selectivity, and the reaction conditions are relatively severe.

[0006] CN 112094173A discloses a method for photocatalytic reaction of CH4 and O2 to generate organic chemicals, belonging to the technical field of advanced material chemistry. The catalyst used is a Ni nanocluster with precise nanostructure. The invention utilizes the special energy level structure of the Ni nanocluster to selectively convert CH4 and O2 into organic chemicals using light energy. However, the preparation of the catalyst used in this method is relatively complex, and the product still needs to be further separated, which adversely affects the purity.

[0007] In view of the deficiencies of the prior art, there is an urgent need to provide a method for converting methane to formic acid with high yield and high selectivity. SUMMARY

[0008] The purpose of the present application is to provide a method for photocatalytic conversion of methane to formic acid, which uses a palladium-loaded tungsten trioxide carrier as a photocatalyst. By controlling the content of the palladium active component, the photocatalytic preparation process parameters and the wavelength of the light, the yield and selectivity of formic acid in the photocatalytic conversion of methane to formic acid system are relatively high. At the same time, the catalytic system has the advantages of mild reaction conditions, simple operation and low environmental pollution.

[0009] To achieve this purpose, the present application adopts the following technical solutions:

[0010] The present application provides a method for photocatalytic conversion of methane to formic acid, which comprises the following steps:

[0011] The palladium-loaded photocatalyst and the solvent are uniformly mixed by ultrasonic, and then oxygen and methane are introduced in turn. Methanol, formaldehyde, methyl hydroperoxide and formic acid are obtained by reaction under light irradiation.

[0012] The palladium-loaded photocatalyst comprises a palladium active component and a tungsten trioxide carrier.

[0013] The method for photocatalytic conversion of methane to formic acid provided by the present application uses methane as raw material and oxygen as oxidant. Under the combined action of light and photocatalyst, methane is converted to formic acid. By controlling the appropriate amount of photocatalyst, the amount of oxidant and the wavelength of light, the main product formic acid has high yield and selectivity. The reaction conditions are mild, the catalytic reaction process is simple and green. The palladium-loaded tungsten trioxide carrier is used as a photocatalyst, which can be used for photocatalytic conversion under ultraviolet light or visible light. The loading of palladium can promote the activation of oxidation and produce active oxygen species such as hydroxyl and superoxide. The hydroxyl group can promote the activation and conversion of methane on the surface of the photocatalyst, and the methane-methanol-formaldehyde-formic acid reaction path realizes the efficient conversion of methane to formic acid.

[0014] Preferably, the mass of the palladium active component is 0.1-1wt% of the mass of the tungsten trioxide carrier, for example, it can be 0.1wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.5wt%, 0.8wt% or 1wt%, but is not limited to the listed values, other values not listed in the value range are also applicable, preferably 0.2-0.3wt%.

[0015] The proportion of the palladium active component is within a reasonable range, the yield and selectivity of the target product are high, and the photocatalytic effect is good. If the proportion of the palladium active component is too low, the active component is insufficient, resulting in a low yield of the product; if the proportion is too high, the particle size of palladium increases, resulting in a decrease in the yield of the product.

[0016] Preferably, the solid-liquid ratio of the palladium supported photocatalyst to the solvent is (5-20)mg:180mL, for example, it can be 5mg:180mL, 8mg:180mL, 10mg:180mL, 15mg:180mL or 20mg:180mL, but is not limited to the listed values, other values not listed in the value range are also applicable.

[0017] The amount of the palladium supported photocatalyst needs to be controlled within a reasonable range, which can efficiently photocatalyze the conversion of methane to formic acid. If the amount is too small, the photocatalytic reaction is slow, which further leads to a decrease in the yield of the product; if the amount is too large, the catalytic effect on the photocatalytic reaction is no longer improved, and resources are also wasted.

[0018] Preferably, the solvent comprises deionized water.

[0019] Preferably, the reaction is carried out in a high-pressure reaction kettle.

[0020] The high-pressure reaction kettle has a light-transmitting window at the top.

[0021] Preferably, the high-pressure reaction kettle is placed in a cooling pool for reaction.

[0022] Preferably, the air in the high-pressure reaction kettle is replaced with the oxygen.

[0023] Preferably, the partial pressure of the oxygen is 5-14bar, for example, it can be 5bar, 8ar, 10bar, 12bar or 14bar, but is not limited to the listed values, other values not listed in the value range are also applicable.

[0024] If the partial pressure of the oxygen is too small, the yield and selectivity of the target product are low; if the partial pressure is too large, the target product is easily overoxidized to carbon dioxide.

[0025] Preferably, the partial pressure of the methane is 20bar.

[0026] Preferably, the reaction time is 1-7h, for example, it can be 1h, 3h, 5h, 6h or 7h, but is not limited to the listed values, and other unlisted values within the value range are also applicable.

[0027] Preferably, the reaction temperature is 25℃.

[0028] Preferably, the light source used for the light irradiation comprises a xenon lamp.

[0029] Preferably, the wavelength of the xenon lamp is 300-780nm, for example, it can be 300-400nm, 402-438nm, 422-458nm, 470-510nm or 400-780nm, but is not limited to the listed values, and other unlisted values within the value range are also applicable.

[0030] The wavelength of the xenon lamp is 300-780nm, and the light intensity is 2.292W.

[0031] Preferably, the xenon lamp is further provided with a 365nm monochromatic filter, a 420nm monochromatic filter, a 440nm monochromatic filter and a 490nm monochromatic filter.

[0032] Preferably, the wavelength of the 365nm monochromatic filter is 347-383nm, and the light intensity is 0.2W.

[0033] Preferably, the wavelength of the 420nm monochromatic filter is 402-438nm, and the light intensity is 0.252W.

[0034] Preferably, the wavelength of the 440nm monochromatic filter is 422-458nm, and the light intensity is 0.28W.

[0035] Preferably, the wavelength of the 490nm monochromatic filter is 470-510nm, and the light intensity is 0.288W.

[0036] Preferably, the palladium-supported photocatalyst is prepared by the following preparation method:

[0037] (1) mix tungsten trioxide, palladium source and solvent according to the formula amount to obtain a mixed solution;

[0038] (2) ice bath the mixed solution obtained in step (1) with a reducing agent, and after solid-liquid separation, the solid phase is washed and dried to obtain the palladium-supported photocatalyst.

[0039] The preparation method of the palladium-supported photocatalyst loads noble metal palladium on the tungsten trioxide carrier by the method of ice bath assisted reduction of the reducing agent, and has the advantages of simple preparation method, mild reaction condition, good repeatability of synthesis and performance of the palladium-supported photocatalyst.

[0040] Preferably, the solid content of the mixed solution in step (1) is 0.4-0.6wt%, for example, it can be 0.4wt%, 0.45wt%, 0.5wt%, 0.55wt% or 0.6wt%, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0041] Preferably, the palladium source in step (1) comprises chloropalladic acid.

[0042] Preferably, the solvent in step (1) comprises deionized water.

[0043] Preferably, the temperature of the mixing in step (1) is 20-25℃, for example, it can be 20℃, 21℃, 23℃, 24℃ or 25℃, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0044] Preferably, the time of the mixing in step (1) is 5.5-6.5h, for example, it can be 5.5h, 5.8h, 6h, 6.2h or 6.5h, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0045] Preferably, the reducing agent in step (2) comprises sodium borohydride aqueous solution.

[0046] Preferably, the mass-volume ratio of sodium borohydride to deionized water in the sodium borohydride aqueous solution is (10-12)mg:1mL, for example, it can be 10mg:1mL, 10.5mg:1mL, 11mg:1mL, 11.5mg:1mL or 12mg:1mL, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0047] Preferably, the amount of deionized water used is 4.9-5.1mL, for example, it can be 4.9mL, 4.95mL, 5mL, 5.05mL or 5.1mL, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0048] Preferably, the time of the ice bath mixing in step (2) is 50-70min, for example, it can be 50min, 55min, 60min, 65min or 70min, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0049] Preferably, the temperature of the ice bath mixing in step (2) is 0℃.

[0050] The ice bath mixing is slow mixing.

[0051] Preferably, the solid-liquid separation in step (2) comprises centrifugal separation.

[0052] Preferably, the washing in step (2) is performed in deionized water.

[0053] Preferably, the washing in step (2) is performed 2-4 times, for example, 2 times, 3 times or 4 times.

[0054] Preferably, the drying in step (2) is performed at a temperature of 60-65℃, for example, 60℃, 61℃, 62℃, 63℃ or 65℃, but not limited to the listed values, other values not listed in the range of values are also applicable.

[0055] Preferably, the drying in step (2) is performed for 12-14h, for example, 12h, 12.5h, 13h, 13.5h or 14h, but not limited to the listed values, other values not listed in the range of values are also applicable.

[0056] As a preferred technical solution of the method of the present application, the method comprises the following steps:

[0057] The palladium-loaded photocatalyst and the solvent are uniformly mixed by ultrasonic, oxygen with a partial pressure of 5-14bar is introduced to replace the air in the high-pressure reactor, then methane with a partial pressure of 20bar is introduced, and the reaction is carried out under light irradiation at 25℃ in the high-pressure reactor for 1-7h to obtain methanol, formaldehyde, methyl hydroperoxide and the formic acid;

[0058] The palladium-loaded photocatalyst comprises a palladium active component and a tungsten trioxide carrier; the mass of the palladium active component is 0.1-1wt% of the mass of the tungsten trioxide carrier; and the solid-liquid ratio of the palladium-loaded photocatalyst and the solvent is (5-20)mg:180mL;

[0059] The light source for the light irradiation comprises a xenon lamp with a wavelength of 300-780nm; the xenon lamp is further provided with a 365nm monochromatic filter with a wavelength of 347-383nm, a 420nm monochromatic filter with a wavelength of 402-438nm, a 440nm monochromatic filter with a wavelength of 422-458nm and a 490nm monochromatic filter with a wavelength of 470-510nm;

[0060] The palladium-loaded photocatalyst is prepared by the following preparation method:

[0061] (1) The tungsten trioxide, the palladium source and the solvent are mixed according to the formula amount at 20-25℃ for 5.5-6.5h to obtain a mixed solution; the solid content of the mixed solution is 0.4-0.6wt%;

[0062] (2) ice bath mixing reductant and the mixture obtained in step (1) at 0℃ for 50-70min, solid-liquid separation, washing the obtained solid phase with deionized water for 2-4 times, and drying at 60-65℃ for 12-14h to obtain the palladium loaded photocatalyst; the reductant includes sodium borohydride and deionized water, the mass-volume ratio of sodium borohydride aqueous solution is (10-12) mg:1mL, and the amount of deionized water is 4.9-5.1mL.

[0063] Compared with the prior art, the present application has the following beneficial effects:

[0064] The method for preparing formic acid by photocatalytic conversion of methane provided by the present application uses methane as raw material and oxygen as oxidant, and converts methane into formic acid under the joint action of light and palladium loaded photocatalyst. By controlling the amount of photocatalyst, the amount of oxidant and the wavelength of light, the yield of formic acid can reach 233.24μmol, the selectivity can reach 62.23%, and the quantum yield can reach 2.98% under monochromatic light irradiation of 420nm, indicating that the catalyst has good catalytic activity under visible light. The reaction conditions of the system for photocatalytic conversion of methane into formic acid are mild, the process is simple and green, and it is suitable for large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0065] Figure 1 is the NMR spectrum of the reaction product provided by application example 1 of the present application;

[0066] Figure 2 is the XRD spectrum of the palladium loaded photocatalyst provided by example 1 of the present application;

[0067] Figure 3 is the TEM diagram of the palladium loaded photocatalyst provided by example 1 of the present application;

[0068] Figure 4 is the elemental analysis diagram of the palladium loaded photocatalyst provided by example 1 of the present application. DETAILED DESCRIPTION

[0069] The technical solutions of the present application will be further described by specific embodiments. Those skilled in the art should understand that the embodiments are only to help understand the present application, and should not be regarded as specific limitations on the present application.

[0070] Example 1

[0071] The present embodiment provides a method for preparing formic acid by photocatalytic conversion of methane, which comprises the following steps:

[0072] The palladium supported photocatalyst is uniformly mixed with deionized water, oxygen with a partial pressure of 10 bar is introduced to replace the air in the high-pressure reactor, then methane with a partial pressure of 20 bar is introduced, and the reaction is carried out under light irradiation at 25℃ for 5h in the high-pressure reactor to obtain methanol, formaldehyde, methyl hydroperoxide and the formic acid, and the NMR spectrum is as shown in Figure 1 ;

[0073] The palladium supported photocatalyst comprises a palladium active component and a tungsten trioxide carrier; the mass of the palladium active component is 0.25wt% of the mass of the tungsten trioxide carrier; the solid-liquid ratio of the palladium supported photocatalyst and deionized water is 10mg:180mL; the light source used for light irradiation is a xenon lamp with a wavelength of 300-780nm and a light intensity of 2.292W;

[0074] The palladium supported photocatalyst is prepared by the following preparation method:

[0075] (1) The tungsten trioxide, palladium chloride acid and deionized water are mixed according to the formula amount at 23℃ for 6h to obtain a mixed solution; the solid content of the mixed solution is 0.5wt%;

[0076] (2) The sodium borohydride aqueous solution and the mixed solution obtained in step (1) are slowly mixed in an ice bath at 0℃ for 60min, and the solid phase obtained after centrifugal separation is washed with deionized water for 3 times and dried at 63℃ for 13h to obtain the palladium supported photocatalyst; the mass-volume ratio of sodium borohydride to deionized water in the sodium borohydride aqueous solution is 11mg:1mL; the amount of deionized water used is 5mL.

[0077] The XRD spectrum of the palladium supported photocatalyst is as shown in Figure 2 , which matches the tungsten trioxide standard card (JCPDS #43-1035) with monoclinic phase, and no diffraction peak of palladium is observed due to the low loading amount of palladium; the TEM image of the palladium supported photocatalyst is as shown in Figure 3 , from which it can be seen that the palladium nanoparticles are supported on the tungsten trioxide; the elemental analysis image of the palladium supported photocatalyst is as shown in Figure 4 , from which it can be clearly seen that the palladium nanoparticles are successfully supported on the tungsten trioxide.

[0078] Example 2

[0079] The present embodiment provides a method for photocatalytic conversion of methane to prepare formic acid, which comprises the following steps:

[0080] ultrasonic homogenate the palladium loaded photocatalyst with deionized water, oxygen with partial pressure of 10 bar is introduced and oxygen is used to replace the air in the high pressure reactor, then methane with partial pressure of 20 bar is introduced, the photoreaction is carried out in the high pressure reactor at 25℃ for 5h to obtain methanol, formaldehyde, methyl hydroperoxide and the formic acid;

[0081] The palladium loaded photocatalyst comprises a palladium active component and a tungsten trioxide carrier; the mass of the palladium active component is 0.1wt% of the mass of the tungsten trioxide carrier; the solid-liquid ratio of the palladium loaded photocatalyst and deionized water is 10mg:180mL; the light source used for the irradiation is a xenon lamp with wavelength of 300-780nm and light intensity of 2.292W;

[0082] The palladium loaded photocatalyst is prepared by the following preparation method:

[0083] (1) mix the tungsten trioxide, palladium chloride acid and deionized water according to the formula amount at 20℃ for 6.5h to obtain a mixed solution; the solid content of the mixed solution is 0.6wt%;

[0084] (2) slowly mix the sodium borohydride aqueous solution and the mixed solution obtained in step (1) under ice bath at 0℃ for 50min, the obtained solid phase is washed twice with deionized water and dried at 65℃ for 12h to obtain the palladium loaded photocatalyst; the mass-volume ratio of sodium borohydride and deionized water in the sodium borohydride aqueous solution is 10mg:1mL; the amount of deionized water used is 5mL.

[0085] Example 3

[0086] The embodiment provides a method for preparing formic acid by photocatalytic conversion of methane, which comprises the following steps:

[0087] ultrasonic homogenate the palladium loaded photocatalyst with deionized water, oxygen with partial pressure of 10 bar is introduced and oxygen is used to replace the air in the high pressure reactor, then methane with partial pressure of 20 bar is introduced, the photoreaction is carried out in the high pressure reactor at 25℃ for 5h to obtain methanol, formaldehyde, methyl hydroperoxide and the formic acid;

[0088] The palladium loaded photocatalyst comprises a palladium active component and a tungsten trioxide carrier; the mass of the palladium active component is 0.1wt% of the mass of the tungsten trioxide carrier; the solid-liquid ratio of the palladium loaded photocatalyst and deionized water is 10mg:180mL; the light source used for the irradiation is a xenon lamp with wavelength of 300-780nm and light intensity of 2.292W;

[0089] The palladium loaded photocatalyst is prepared by the following preparation method:

[0090] (1) 5.5 h at 25℃, mixed tungsten trioxide, chloropalladic acid and deionized water according to the formula amount, to obtain a mixed solution; the solid content of the mixed solution is 0.4wt%;

[0091] (2) 70 min at 0℃, slowly mixed sodium borohydride aqueous solution and the mixed solution obtained in step (1) in an ice bath, after centrifugal separation, the solid phase obtained was washed with deionized water for 4 times, and dried at 60℃ for 14 h, to obtain the palladium loaded photocatalyst; the mass-volume ratio of sodium borohydride to deionized water in the sodium borohydride aqueous solution is 12 mg: 1 mL; the amount of deionized water used is 5 mL.

[0092] Example 4

[0093] This embodiment provides a method for photocatalytic conversion of methane to formic acid, which is different from example 1 in that, in addition to adjusting the partial pressure of oxygen to 5 bar, the rest is the same as example 1.

[0094] Example 5

[0095] This embodiment provides a method for photocatalytic conversion of methane to formic acid, which is different from example 1 in that, in addition to adjusting the partial pressure of oxygen to 14 bar, the rest is the same as example 1.

[0096] Example 6

[0097] This embodiment provides a method for photocatalytic conversion of methane to formic acid, which is different from example 1 in that, in addition to adjusting the partial pressure of oxygen to 3 bar, the rest is the same as example 1.

[0098] Example 7

[0099] This embodiment provides a method for photocatalytic conversion of methane to formic acid, which is different from example 1 in that, in addition to adjusting the illumination reaction time to 1 h, the rest is the same as example 1.

[0100] Example 8

[0101] This embodiment provides a method for photocatalytic conversion of methane to formic acid, which is different from example 1 in that, in addition to adjusting the illumination reaction time to 3 h, the rest is the same as example 1.

[0102] Example 9

[0103] This embodiment provides a method for photocatalytic conversion of methane to formic acid, which is different from example 1 in that, in addition to adjusting the illumination reaction time to 7 h, the rest is the same as example 1.

[0104] Example 10

[0105] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the solid-liquid ratio of the palladium loaded photocatalyst and deionized water is adjusted to 5mg:180mL, the rest is the same as the embodiment 1.

[0106] Embodiment 11

[0107] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the solid-liquid ratio of the palladium loaded photocatalyst and deionized water is adjusted to 20mg:180mL, the rest is the same as the embodiment 1.

[0108] Embodiment 12

[0109] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the solid-liquid ratio of the palladium loaded photocatalyst and deionized water is adjusted to 2mg:180mL, the rest is the same as the embodiment 1.

[0110] Embodiment 13

[0111] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the light source used for the light irradiation is a xenon lamp provided with a 365nm monochromatic filter with a wavelength of 347-383nm and a light intensity of 0.22W, the rest is the same as the embodiment 1.

[0112] Embodiment 14

[0113] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the light source used for the light irradiation is a xenon lamp provided with a 420nm monochromatic filter with a wavelength of 402-438nm and a light intensity of 0.252W, the rest is the same as the embodiment 1.

[0114] Embodiment 15

[0115] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the light source used for the light irradiation is a xenon lamp provided with a 440nm monochromatic filter with a wavelength of 422-458nm and a light intensity of 0.28W, the rest is the same as the embodiment 1.

[0116] Embodiment 16

[0117] The embodiment provides a method for preparing formic acid through photocatalytic conversion of methane, which is different from the embodiment 1 in that, except that the light source used for the light irradiation is a xenon lamp provided with a 490nm monochromatic filter with a wavelength of 470-510nm and a light intensity of 0.288W, the rest is the same as the embodiment 1.

[0118] Embodiment 17

[0119] The embodiment provides a method for preparing formic acid through photocatalytic methane conversion, which is different from the embodiment 1 in that, in addition to adjusting the mass of the palladium active component to 0.5% of the mass of the tungsten trioxide carrier, the rest is the same as the embodiment 1.

[0120] Comparative example 1

[0121] The comparative example provides a method for preparing formic acid through photocatalytic methane conversion, which is different from the embodiment 1 in that, the palladium supported photocatalyst is replaced by a gold supported photocatalyst with the same mass, the gold supported photocatalyst is composed of the same loading amount of gold active component and tungsten trioxide, and the rest is the same as the embodiment 1.

[0122] Comparative example 2

[0123] The comparative example provides a method for preparing formic acid through photocatalytic methane conversion, which is different from the embodiment 1 in that, the palladium supported photocatalyst is replaced by a platinum supported photocatalyst with the same mass, the platinum supported photocatalyst is composed of the same loading amount of platinum active component and tungsten trioxide, and the rest is the same as the embodiment 1.

[0124] The method provided in the embodiment 1-17 and the comparative examples 1 and 2 is applied to prepare formic acid through photocatalytic methane conversion, and ultraviolet-visible absorption spectrometers (acetylacetone colorimetry) and 1 H-NMR characterization are used to obtain the yield of a reaction product and the selectivity of formic acid, as shown in Table 1.

[0125] The method provided in the embodiment 13-16 is applied to prepare formic acid through photocatalytic methane conversion, and the calculation formula of a quantum yield is QE=(R (electron) ·N A ·100%) / (ISt / E λ ); R (electron) =n(CH3OOH)×2+n(CH3OH)×2+n(HCHO)×4+n(HCOOH)×6+n(CO2)×8, E λ =(h·c) / λ, N A is the Avogadro constant, I is the light intensity, S is the light radiation area, t is the reaction time, h is the Planck constant, c is the light speed, and λ is the light wavelength. The obtained results are shown in Table 2.

[0126] Table 1

[0127]

[0128]

[0129] Table 2

[0130] Quantum yield (%) Example 13 5.39 Example 14 2.98 Example 15 0.50 Example 16 0.32

[0131] As can be seen from Table 1, by using the method for preparing formic acid by photocatalytic conversion of methane provided in the application, the palladium supported photocatalyst has good catalytic activity, the reaction condition is mild, and the obtained product formic acid has high yield and selectivity.

[0132] As can be seen from the comparison of Example 1 with Examples 2, 3 and 17, the low proportion of the palladium active component leads to low yield of the product; the high proportion and the increased particle size of palladium lead to low yield of the product; as can be seen from the comparison of Example 1 with Examples 4-6, the too small partial pressure of oxygen leads to low yield and selectivity of the target product, and the too large partial pressure of oxygen leads to easy peroxidation of the target product to carbon dioxide; as can be seen from the comparison of Example 1 with Examples 10-12, the too large or too small amount of the palladium supported photocatalyst has certain adverse effects on the photocatalytic conversion system; as can be seen from the comparison of Example 1 with Examples 13-16, the use of the xenon lamp provided with a monochromatic wavelength filter for light irradiation has high quantum yield.

[0133] As can be seen from the comparison of Example 1 with Comparative Example 1, the use of the palladium supported photocatalyst provided in the application for photocatalytic conversion has better catalytic effect, and the yield and selectivity of formic acid are high, while the use of the gold supported photocatalyst has low yield and selectivity of formic acid; as can be seen from the comparison of Example 1 with Comparative Example 2, the use of the platinum supported photocatalyst easily leads to peroxidation product carbon dioxide.

[0134] In summary, the method for preparing formic acid by photocatalytic conversion of methane provided in the application has high yield and selectivity of formic acid, and the quantum yield can reach 2.98% under monochromatic light irradiation at 420 nm, by using methane as raw material and oxygen as oxidant, and under the combined action of light irradiation and the palladium supported photocatalyst, the methane is converted into formic acid by controlling the amount of the photocatalyst, the amount of the oxidant and the wavelength of light irradiation; the photocatalytic conversion system for converting methane into formic acid has mild reaction condition, simple process and green environmental protection, and is suitable for large-scale industrial production.

[0135] The above merely describes specific embodiments of the application, but the protection scope of the application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed in the application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the application.

Claims

1. A method for photocatalytic conversion of methane to formic acid, characterized in that, The method includes the following steps: Palladium-supported photocatalyst and solvent are ultrasonically mixed uniformly, and then oxygen and methane are sequentially introduced. The reaction is carried out under light irradiation to obtain methanol, formaldehyde, methyl hydrogen peroxide and formic acid. The light source used for the illumination includes a xenon lamp; The wavelength of the xenon lamp is 300-780nm; The palladium-supported photocatalyst comprises a palladium active component and a tungsten trioxide support; The palladium-supported photocatalyst was prepared using the following method: (1) Mix tungsten trioxide, palladium source and solvent according to the formula to obtain a mixture; The palladium source in step (1) includes chloropalladium acid; (2) Mix the reducing agent with the mixture obtained in step (1) in an ice bath. After solid-liquid separation, the resulting solid phase is washed and dried to obtain the palladium-supported photocatalyst. The reducing agent in step (2) includes an aqueous solution of sodium borohydride.

2. The method according to claim 1, characterized in that, The mass of the palladium active component is 0.1-1 wt% of the mass of the tungsten trioxide support.

3. The method according to claim 2, characterized in that, The mass of the palladium active component is 0.2-0.3 wt% of the mass of the tungsten trioxide support.

4. The method according to claim 1, characterized in that, The solid-liquid ratio of the palladium-supported photocatalyst to the solvent is (5-20) mg: 180 mL.

5. The method according to claim 1, characterized in that, The solvent includes deionized water.

6. The method according to claim 1, characterized in that, The reaction is carried out in a high-pressure reactor.

7. The method according to claim 6, characterized in that, The air in the high-pressure reactor is replaced with oxygen.

8. The method according to claim 1, characterized in that, The partial pressure of the oxygen is 5-14 bar.

9. The method according to claim 1, characterized in that, The partial pressure of the methane is 20 bar.

10. The method according to claim 1, characterized in that, The reaction time is 1-7 hours.

11. The method according to claim 1, characterized in that, The reaction was carried out at a temperature of 25°C.

12. The method according to claim 1, characterized in that, The xenon lamp is also equipped with a 365nm monochromatic filter, a 420nm monochromatic filter, a 440nm monochromatic filter, and a 490nm monochromatic filter.

13. The method according to claim 12, characterized in that, The wavelength of the 365nm monochromatic filter is 347-383nm.

14. The method according to claim 12, characterized in that, The wavelength of the 420nm monochromatic filter is 402-438nm.

15. The method according to claim 12, characterized in that, The wavelength of the 440nm monochromatic filter is 422-458nm.

16. The method according to claim 12, characterized in that, The wavelength of the 490nm monochromatic filter is 470-510nm.

17. The method according to claim 1, characterized in that, The solid content of the mixture in step (1) is 0.4-0.6 wt%.

18. The method according to claim 1, characterized in that, The solvent in step (1) includes deionized water.

19. The method according to claim 1, characterized in that, The mixing temperature in step (1) is 20-25℃.

20. The method according to claim 1, characterized in that, The mixing time in step (1) is 5.5-6.5 hours.

21. The method according to claim 1, characterized in that, The mass-to-volume ratio of sodium borohydride to deionized water in the sodium borohydride aqueous solution is (10-12) mg: 1 mL.

22. The method according to claim 21, characterized in that, The amount of deionized water used is 4.9-5.1 mL.

23. The method according to claim 1, characterized in that, The mixing time for the ice bath in step (2) is 50-70 minutes.

24. The method according to claim 1, characterized in that, The temperature of the ice bath mixture in step (2) is 0°C.

25. The method according to claim 1, characterized in that, The washing in step (2) is carried out in deionized water.

26. The method according to claim 1, characterized in that, The number of times the washing is performed in step (2) is 2-4.

27. The method according to claim 1, characterized in that, The drying temperature in step (2) is 60-65℃.

28. The method according to claim 1, characterized in that, The drying time in step (2) is 12-14 hours.

29. The method according to claim 1, characterized in that, The method includes the following steps: Palladium-supported photocatalyst and solvent are ultrasonically and uniformly mixed. Oxygen at a partial pressure of 5-14 bar is introduced to replace the air in the high-pressure reactor. Then, methane at a partial pressure of 20 bar is introduced. The reaction is carried out in the high-pressure reactor at 25°C under light irradiation for 1-7 hours to obtain methanol, formaldehyde, methyl hydrogen peroxide and formic acid. The palladium-supported photocatalyst comprises a palladium active component and a tungsten trioxide support; the mass of the palladium active component is 0.1-1 wt% of the mass of the tungsten trioxide support; the solid-liquid ratio of the palladium-supported photocatalyst to the solvent is (5-20) mg: 180 mL; The light source used for illumination includes a xenon lamp with a wavelength of 300-780nm; the xenon lamp is also equipped with a 365nm monochromatic filter with a wavelength of 347-383nm, a 420nm monochromatic filter with a wavelength of 402-438nm, a 440nm monochromatic filter with a wavelength of 422-458nm, and a 490nm monochromatic filter with a wavelength of 470-510nm; The palladium-supported photocatalyst was prepared using the following method: (1) Tungsten trioxide, palladium source and solvent are mixed at 20-25℃ according to the formula for 5.5-6.5h to obtain a mixture; the solid content of the mixture is 0.4-0.6wt%. (2) Mix the reducing agent with the mixture obtained in step (1) in an ice bath at 0℃ for 50-70 min. After solid-liquid separation, the resulting solid phase is washed with deionized water 2-4 times and dried at 60-65℃ for 12-14 h to obtain the palladium-supported photocatalyst. The reducing agent includes an aqueous solution of sodium borohydride with a mass-volume ratio of (10-12) mg:1 mL of deionized water. The amount of deionized water used is 4.9-5.1 mL.

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