Method for synthesizing urea through photocatalytic coupling of carbon monoxide and ammonia

Abstract

The invention discloses a method for synthesizing urea by photocatalytic coupling of carbon monoxide and ammonia, which comprises the following steps: in a reaction atmosphere containing carbon monoxide, mixing a photocatalyst and ammonia water, and carrying out illumination reaction to prepare the urea. Compared with an existing system for synthesizing urea through photocatalytic reduction, the method has good selectivity and catalytic performance, liquid-phase by-products are prevented from being generated, and the purification and separation cost of urea is reduced. The invention also finds that when oxygen with the volume concentration of 20% is introduced, carbon monoxide and ammonia can be oxidized and coupled into urea with high selectivity, the catalytic performance of the method keeps the urea generation rate of 9290.67 [mu] gh <-1 > gcat <-1 > within 4 hours, and the selectivity is 100%.

Description

Technical Field

[0001] The invention belongs to the technical field of photochemical urea synthesis, and in particular relates to a method for synthesizing urea by photocatalytic coupling of carbon monoxide and ammonia. Background Art

[0002] Urea, as a nitrogen-containing fertilizer with a nitrogen content of up to 46%, not only plays a pivotal role in agricultural production, but is also used in the fields of plastics, medicine, and chemicals. At present, the industrial production of urea mainly adopts a two-step synthesis process. First, nitrogen and hydrogen are synthesized into ammonia through the Haber-Bosher method, and then ammonia and carbon dioxide are synthesized into urea through the Bosher-Meiser method. As we all know, this process is a high-energy and high-pollution process, which consumes nearly 2% of the world's energy consumption and emits a large amount of greenhouse gases under harsh conditions (150-200°C, 150-250bar). Therefore, it is of great significance to develop a sustainable, green and efficient CN coupling technology for environmentally friendly urea synthesis.

[0003] At present, electrocatalytic or photocatalytic synthesis of urea can utilize renewable clean energy to convert CO2 and nitrogen-containing compounds (N2, NO2 - , NO3 - However, the current photo- or electrocatalytic CN coupling reaction involves a complex multi-electron proton reduction process, which leads to many by-products and low reaction selectivity. In addition, the reduction reaction is usually severely limited by the thermodynamic oxygen reduction reaction. Therefore, the reaction device needs to be sealed to prevent oxygen contamination, which increases the reaction cost and limits industrial production. Summary of the invention

[0004] In order to improve the above technical problems, the present invention provides an application of a photocatalyst in the preparation of urea by photocatalytic coupling of carbon monoxide and ammonia;

[0005] The photocatalyst is selected from at least one of rutile phase titanium dioxide, anatase phase titanium dioxide, brookite phase titanium dioxide, P25 type titanium dioxide, graphite phase carbon nitride, indium trioxide, cadmium sulfide and oxygen vacancy titanium dioxide.

[0006] Preferably, the photocatalyst is oxygen vacancy titanium dioxide.

[0007] According to an embodiment of the present invention, the preparation method of the oxygen vacancy titanium dioxide is:

[0008] The titanium dioxide substrate is calcined in the presence of an inert gas to obtain the oxygen vacancy titanium dioxide.

[0009] According to an embodiment of the present invention, the titanium dioxide substrate can be commercially available, for example, rutile titanium dioxide, anatase titanium dioxide, brookite titanium dioxide or P25 titanium dioxide.

[0010] According to an embodiment of the present invention, the inert gas is, for example, argon; preferably, the flow rate of the inert gas is 30 to 50 mL min -1 .

[0011] According to an embodiment of the present invention, the calcination temperature is 300-600°C, preferably 300-450°C, and the calcination time may be 1-8h, preferably 1-6h, such as 1h, 2h, 3h, 4h, 5h, 6h.

[0012] According to an embodiment of the present invention, the preparation of oxygen vacancy titanium dioxide further includes post-treatment steps such as water washing and centrifugation.

[0013] As an exemplary embodiment of the present invention, the preparation method of the oxygen vacancy titanium dioxide is:

[0014] The titanium dioxide substrate is spread flat in an Al2O3 ceramic boat, calcined in an argon atmosphere, washed with water after calcination, and the precipitate is collected by centrifugation to prepare the oxygen vacancy titanium dioxide.

[0015] The present invention also provides a method for synthesizing urea by photocatalytic coupling of carbon monoxide and ammonia, the method comprising:

[0016] In a reaction atmosphere containing carbon monoxide, the photocatalyst and ammonia water are mixed, and urea is prepared after light irradiation reaction.

[0017] According to an embodiment of the present invention, oxygen may also be added to the reaction atmosphere.

[0018] According to an embodiment of the present invention, the volume ratio of carbon monoxide to oxygen may be (1-0.5):(0-0.5), preferably (0.99-0.5):(0.01-0.5), for example 1:0, 0.9:0.1, 0.8:0.2, 0.7:0.3, 0.6:0.4 or 0.5:0.5.

[0019] According to an embodiment of the present invention, the carbon monoxide purity is 99.99%.

[0020] According to an embodiment of the present invention, the oxygen purity is 99.99%.

[0021] According to an embodiment of the present invention, the pressure of the reaction atmosphere may be 0.05-0.2 MPa, for example, 0.05 MPa, 0.1 MPa, 0.15 MPa or 0.2 MPa.

[0022] According to an embodiment of the present invention, the mass volume ratio of the photocatalyst to aqueous ammonia is 1 mg:(4-12) mL, preferably 1 mg:(4-10) mL.

[0023] According to an embodiment of the present invention, the amount of the photocatalyst may be 5-20 mg, such as 5 mg, 10 mg, 15 mg or 20 mg.

[0024] According to an embodiment of the present invention, the concentration of the aqueous ammonia solution may be 0.5-3 M, such as 0.5 M, 1 M, 2 M or 3 M. Preferably, the volume of the aqueous ammonia solution may be 20-60 mL, such as 20 mL, 30 mL, 40 mL, 50 mL or 60 mL.

[0025] According to an embodiment of the present invention, the light irradiation reaction is carried out in a photoreactor, which is a sealable quartz reactor with a circulating water glass jacket.

[0026] According to an embodiment of the present invention, the photoreaction is carried out under the irradiation of a xenon lamp; the light intensity of the xenon lamp may be 300-600 mW cm -2 , e.g. 300 mW cm -2 , 400mW cm -2 , 500mW cm -2 or 600mW -2 cm.

[0029] According to an embodiment of the present invention, the reaction time of the light irradiation reaction is more than 1 hour, for example, 1-16 hours.

[0030] According to an embodiment of the present invention, the light irradiation reaction is carried out under stirring, preferably, the stirring speed is 500-1000 rpm, preferably 700 rpm.

[0031] According to an embodiment of the present invention, the pH of the reaction system is 7-11.

[0032] According to an embodiment of the present invention, the main product of the reaction is urea.

[0033] As an exemplary embodiment of the present invention, the method for synthesizing urea specifically comprises the following steps:

[0034] The photocatalyst was added into a quartz reactor with a circulating water glass jacket filled with ammonia water, and ultrasonicated in an ultrasonic machine to form a uniform suspension. A mixed gas of carbon monoxide and oxygen (CO:O2=80:20) was introduced thereinto, with an initial pressure of 0.1 MPa. After 30 minutes, the solution reached a state of gas dissolution saturation, and then a xenon lamp was used for irradiation reaction to prepare urea.

[0035] Beneficial Effects

[0036] 1) Compared with the existing photocatalytic reduction system for synthesizing urea, the method of the present invention has better selectivity and catalytic performance, avoids the generation of liquid phase byproducts, and reduces the cost of urea purification and separation. The present invention also found that when 20% volume concentration of oxygen is introduced, the method of the present invention can selectively couple carbon monoxide and ammonia to urea, and its catalytic performance remains at 9290.67μgh within 4 hours. -1 g cat -1 The urea production rate is 100% and the selectivity is 100%;

[0037] 2) The photocatalyst used in the method of the present invention is universal. The photocatalyst can be rutile phase titanium dioxide, anatase phase titanium dioxide, brookite phase titanium dioxide, P25 type titanium dioxide, graphite phase carbon nitride, indium trioxide or cadmium sulfide, etc., which to a certain extent broadens the way for the development of photocatalytic synthesis of urea.

[0038] 3) The present invention can separate urea from a volatile ammonia solution by heating and distillation, which reduces the steps of purifying and separating the liquid phase product and provides a new idea for synthesizing urea using photocatalysis.

[0039] 4) The oxygen vacancy titanium dioxide photocatalyst provided by the present invention has excellent activity in the application of the above-mentioned photocatalytic oxidation synthesis of urea. The preparation method of the oxygen vacancy titanium dioxide photocatalyst is relatively simple and low in cost, and is of great significance in the field of urea synthesis.

[0040] 5) The method of the present invention uses solar energy as a driving force, which reduces the reaction cost, avoids problems such as secondary pollution, and solves the problem of difficulty in urea synthesis. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Scanning electron microscope (SEM) images of oxygen vacancy titanium dioxide P25 (left) and P25-4h (right) of Example 1.

[0042] Figure 2 Transmission (upper left), element distribution (upper right and lower left), and high-resolution transmission electron microscopy (TEM, EDS Mapping, HRTEM) images of the oxygen vacancy titanium dioxide P25-4h of Example 1.

[0043] Figure 3 X-ray diffraction patterns (XRD) of oxygen vacancy titanium dioxide P25, P25-2h, P25-4h, and P25-6h of Example 1.

[0044] Figure 4Solid ultraviolet diffuse reflectance patterns (DRS) of oxygen vacancy titanium dioxide P25, P25-2h, P25-4h and P25-6h of Example 1.

[0045] Figure 5 This is a comparison chart of the yield and selectivity performance of the oxygen vacancy titanium dioxide photocatalyst P25-4h of Example 1 for synthesizing urea by coupling carbon monoxide and ammonia under different oxygen concentrations.

[0046] Figure 6 This is a performance comparison chart of different oxygen vacancy titanium dioxide photocatalysts in Example 1 (P25, P25-2h, P25-4h, Rutile-4h, Anatase-4h, Brookite-4h) and photocatalysts rutile phase TiO2 (Rutile), anatase phase TiO2 (Anatase), and brookite phase TiO2 (Brookite) in synthesizing urea by coupling carbon monoxide and ammonia in an oxygen atmosphere.

[0047] Figure 7 This is a performance comparison chart of graphite phase carbon nitride, indium oxide, and cadmium sulfide photocatalysts of Examples 3, 4, and 5 in synthesizing urea by coupling carbon monoxide and ammonia in oxygen-free and oxygen-containing atmospheres. DETAILED DESCRIPTION

[0048] The technical scheme of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only exemplary descriptions and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are included in the scope that the present invention is intended to protect.

[0049] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0050] Example 1

[0051] The preparation of an oxygen vacancy titanium dioxide photocatalyst is completed according to the following steps:

[0052] 1) Before use, the commercial P25 type TiO2 was washed and filtered with deionized water and ethanol respectively, and then vacuum dried at 60℃ for 12h to remove organic and inorganic impurities on the surface. First, weigh 500mg of P25 type TiO2 and place it in an Al2O3 ceramic boat, and then put it into a tube furnace. Use a vacuum pump to evacuate, Ar as the balance gas, and repeat the heating program three times. Heating program: heat from room temperature to 350℃ at a rate of 5℃ / min, and then keep at 350℃ for different times of 0h, 2h, 4h, and 6h. After cooling to room temperature, take out the sample, wash it with water, and filter it to obtain oxygen vacancy titanium dioxide, which are named P25, P25-2h, P25-4h, and P25-6h respectively; among them, P25 is the commercial P25 type TiO2.

[0053] 2) Based on the above method, rutile phase TiO2, anatase phase TiO2, and brookite phase TiO2 were calcined in the same way to prepare oxygen vacancy titanium dioxide, which were named Rutile-4h, Anatase-4h, and Brookite-4h, respectively.

[0054] Figure 1 The scanning electron microscope (SEM) images of P25 (left) and P25-4h (right) prepared in Example 1. From the comparison of the figures, it can be seen that the obtained P25-4h maintains the original nanoparticle shape of P25, and its diameter is about 20-50nm.

[0055] Figure 2 Transmission, element distribution and high-resolution transmission electron microscopy (TEM, EDS Mapping, HRTEM) images of P25-4h prepared in Example 1. Figure 2 It shows that Ti and O elements are evenly distributed on the catalyst surface.

[0056] Figure 3 X-ray diffraction analysis (XRD) diagrams of P25, P25-2h, P25-4h and P25-6h prepared in Example 1. Figure 3 It can be seen that after calcination under argon atmosphere, the crystal form of the synthesized P25 did not change, and the characteristic peaks corresponding to the standard cards of rutile and anatase phases were observed, indicating that the synthesis process did not affect the crystal structure of P25-4h.

[0057] To explore the light absorption of the catalyst, solid UV-visible diffuse reflectance (DRS) test was further performed. Figure 4 is the solid ultraviolet diffuse reflectance graph (DRS) of P25, P25-2h, P25-4h and P25-6h of Example 1, Figure 4It can be seen that P25-4h shows a slight red shift in the absorption band edge compared with P25, which indicates that the introduction of oxygen vacancies makes P25-4h have stronger light absorption ability.

[0058] Example 2

[0059] The oxygen vacancy titanium dioxide photocatalyst is used for photocatalytic urea synthesis, comprising the following steps:

[0060] 1) Disperse 10 mg of the oxygen vacancy titanium dioxide photocatalysts prepared in Example 1 (including P25, P25-2h, P25-4h, Rutile-4h, Anatase-4h, Brookite-4h) and the photocatalysts rutile phase TiO2 (Rutile), anatase phase TiO2 (Anatase), and brookite phase TiO2 (Brookite) in 50 mL of 2M ammonia water, and after ultrasonic dispersion for 30 min, transfer them to a quartz reactor with a circulating water glass jacket.

[0061] 2) Before irradiation, the reactor was sealed and purged with ammonia solution at a flow rate of 50 mL / min for 30 minutes with a mixture of CO and O2 in different proportions (O2 concentrations were 0%, 10v%, 20v%, 40v%, 50v%; CO concentrations were 100v%, 90v%, 80v%, 70v%, 60v%, 50v%). The pressure of the reactor was 0.1 MPa, and the reaction system was irradiated with a xenon lamp at an illumination intensity of 300 mW / cm 2 A rotor was added to the reactor, the rotor stirring speed was 700 rpm, and the illumination time was 4 h. During the reaction, 2 mL of the reaction solution was collected at intervals (1 h) and filtered with a syringe filter to obtain a liquid product.

[0062] Product detection: The gaseous products were collected by injection needle and analyzed by gas chromatography. Urea was analyzed by diacetyl monoxime method, and other liquid products were detected by ion chromatography.

[0063] Figure 5 The yield and selectivity performance diagram of P25-4h in the room temperature photocatalytic coupling of CO and ammonia to synthesize urea under different O2 concentrations. Figure 5 It can be seen that when the O2 concentration is 20%, P25-4h can achieve excellent selectivity for urea (100%) and high urea production rate (9290.67μgh -1 g cat -1 ).

[0064] Figure 6The urea production graph of the photocatalysts prepared in Example 1 (P25, P25-2h, P25-4h, Rutile-4h, Anatase-4h, Brookite-4h) and the photocatalysts rutile phase TiO2 (Rutile), anatase phase TiO2 (Anatase), and brookite phase TiO2 (Brookite) in the photocatalytic coupling of CO and NH3 under 20% O2, the urea production rate is 3665.68μgh -1 g cat -1 (P25), 5809.44μgh -1 g cat -1 (P25-2h), 9290.67μgh -1 g cat -1 (P25-4h), 4388.36μgh -1 g cat -1 (Rutile-4h), 381.29μgh -1 g cat -1 (Anatase-4h), 165.57μgh -1 g cat -1 (Brookite-4h), it can be seen that P25-4h has a higher urea generation rate. When 20% volume concentration of oxygen is introduced, the method of the present invention can oxidatively couple carbon monoxide and ammonia to urea with high selectivity, and the selectivity of urea is 100%.

[0065] Example 3

[0066] The preparation of a graphite phase carbon nitride photocatalyst is completed according to the following steps:

[0067] Weigh 15g of melamine and place it in a muffle furnace, heat it from room temperature to 500℃ at a rate of 5℃ / min, and then keep it at 500℃ for 3h to obtain a bulk C3N4 precursor. Weigh the C3N4 precursor and place it in a mortar, then add 6.6g of potassium chloride and 5.4g of lithium chloride, grind it thoroughly, transfer it to an Al2O3 ceramic boat, and then place it in a tubular furnace, heat it to 550℃ at 3℃ / min under Ar conditions, keep it for 4h, then after it cools to room temperature, take out the sample, add boiling water to elute, and filter it to obtain graphite phase carbon nitride.

[0068] The photocatalyst is used for photocatalytic urea synthesis, which includes the following steps:

[0069] 1) 10 mg of the graphite phase carbon nitride photocatalyst prepared in Example 3 was dispersed in 50 mL of 2M ammonia water, and after ultrasonic dispersion for 30 min, it was transferred to a quartz reactor with a circulating water glass jacket.

[0070] 2) Before irradiation, the reactor was sealed and purged with a mixture of CO and O2 (O2 concentration was 20v%; CO concentration was 80v%) or a mixture of CO and Ar (Ar concentration was 20v%; CO concentration was 80v%) at a flow rate of 50mL / min for 30 minutes. The pressure of the reactor was 0.1Mpa. The reaction system was irradiated with a xenon lamp at an illumination intensity of 300mW / cm 2 A rotor was added to the reactor, the rotor stirring speed was 700 rpm, and the illumination time was 4 h to prepare urea.

[0071] Figure 7 The urea yield of the photocatalyst prepared in Example 3 in the photocatalytic coupling of CO and NH3 under anaerobic and aerobic conditions is 353.09 μgh under anaerobic and aerobic conditions, respectively. -1 g cat -1 、11670.87μgh -1 g cat -1 .

[0072] Example 4

[0073] The preparation of an indium trioxide photocatalyst is completed according to the following steps:

[0074] Weigh 0.3gIn(NO3)3·4.5H2O and dissolve it in 30mL of ethylene glycol, slowly add 0.5g / mL NaOH aqueous solution, ultrasonicate for 30min, transfer the mixed solution to an oven, hydroheat at 200℃ for 24h, wait for the oven to cool naturally to room temperature, wash with water and ethanol three times, and then dry the solid product in a vacuum drying oven at 60℃ for more than 12h to obtain indium trioxide.

[0075] The preparation of the photocatalyst for photocatalytic urea synthesis is the same as in Example 3, except that indium trioxide is used to replace the graphite phase carbon nitride photocatalyst.

[0076] Figure 7 The urea yield of the photocatalyst prepared in Example 4 in the photocatalytic coupling of CO and NH3 under anaerobic and aerobic conditions is 434.61 μgh under anaerobic and aerobic conditions, respectively. -1 g cat -1 、16100.25μgh -1 g cat-1 .

[0077] Example 5

[0078] The preparation of a photocatalytic cadmium sulfide photocatalyst is completed according to the following steps:

[0079] Weigh 0.122g CdCl2 and 0.107g sulfur powder and dissolve them in 20mL diethylenetriamine at room temperature. Transfer the above solution to an oven and heat it at 80℃ for 3 hours, cool it to room temperature at a rate of 5℃ / min, and then wash it three times with deionized water. The obtained solid product is vacuum dried at 80℃ for more than 12 hours to obtain cadmium sulfide.

[0080] The preparation of the photocatalyst for photocatalytic urea synthesis is the same as in Example 3, except that cadmium sulfide is used to replace the graphite phase carbon nitride photocatalyst.

[0081] Figure 7 The urea yield of the photocatalyst prepared in Example 5 in the photocatalytic coupling of CO and NH3 under anaerobic and aerobic conditions is 461.15 μgh under anaerobic and aerobic conditions, respectively. -1 g cat -1 、17332.47μgh -1 g cat -1 .

[0082] The above is an exemplary description of the embodiments of the present invention. However, the protection scope of the present invention is not limited to the above embodiments. Any modification, equivalent substitution, improvement, etc. made by those skilled in the art within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. Use of a photocatalyst in the photocatalytic coupling of carbon monoxide and ammonia to prepare urea, characterized in that: The photocatalyst is selected from at least one of rutile phase titanium dioxide, anatase phase titanium dioxide, brookite phase titanium dioxide, P25 type titanium dioxide, graphite phase carbon nitride, indium trioxide, cadmium sulfide and oxygen vacancy titanium dioxide.

2. A method for synthesizing urea by photocatalytic coupling of carbon monoxide and ammonia, characterized in that: The method comprises: The photocatalyst according to claim 1 and ammonia water are mixed in a reaction atmosphere containing carbon monoxide, and the urea is prepared after light irradiation reaction.

3. The method according to claim 2, characterized in that Oxygen is also added to the reaction atmosphere.

4. The method according to claim 3, characterized in that The volume ratio of carbon monoxide to oxygen is (1-0.5):(0-0.5).

5. The method according to claim 2, characterized in that: The pressure of the reaction atmosphere is 0.05-0.2 MPa.

6. The method according to claim 2, characterized in that The mass volume ratio of the photocatalyst to ammonia water is 1 mg:(4-12) mL.

7. The method according to claim 2, characterized in that The light reaction is carried out under the irradiation of a xenon lamp; the light intensity of the xenon lamp is 300-600 mW cm -2 .

8. The method according to claim 2, characterized in that: The reaction time of the light irradiation reaction is more than 1 hour.

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