CN111420686B - Preparation of F, S, Zr, Al co-doped TiO2 photocatalyst and its photocatalytic degradation efficiency of acrylonitrile industrial wastewater - Google Patents

Abstract

The invention relates to a preparation method of an F, S, Zr, Al co-doped modified titanium dioxide composite oxide catalyst that meets national discharge standards for photocatalytic degradation of acrylonitrile actual wastewater COD under simulated and natural sunlight conditions. The present invention firstly provides an optimized sol-gel method for preparing F-doped _TiO2 catalysts, that is, using zirconium nitrate pentahydrate and thiourea as precursors for S, Zr, and Al co-doping. F, S, Zr, Al co-doped modified TiO ₂ catalyst has obvious synergistic advantages, the photoresponse range of the catalyst is broadened, the acid site strength on the sample surface is strong, and the addition of silica gel forms a rough appearance The light scattering caused by the pore structure is conducive to light absorption, which plays an important role in the improvement of photocatalytic activity. Among them, the addition of Zr and Al can significantly improve the stability of the entire catalyst system. The most suitable ratio of Ti, F, S, Zr and Al is 1:0.5-5%:1-10%:0.1-2%, and the suitable calcination temperature and calcination time are 350-500°C and 1-3h respectively. The catalyst was repeated 4 times, and the COD could still drop from about 89 to below 42mg/L.

Description

Preparation of F, S, Zr, Al Co-doped TiO2 Photocatalyst and Solar Photocatalytic Degradation of Propylene Nitrile Industrial Wastewater Efficiency

technical field

The invention relates to a composite oxide catalyst for the photocatalytic degradation of acrylonitrile actual wastewater, specifically, it relates to the photocatalytic degradation of acrylonitrile industrial wastewater in the water phase, and uses silica gel to disperse fluorine, sulfur, zirconium and aluminum co-modified titanium dioxide A photocatalyst and a preparation method thereof belong to the technical field of environmental protection.

Background technique

Acrylonitrile (acrylonitrile, CH ₂ =CH-CN, AN for short) is an important raw material (ABS) resin for the production of acrylic fiber, nitrile rubber, acrylamide, and acrylonitrile-butadiene styrene. The global acrylonitrile production capacity exceeds 5 million tons per year. China and the United States are the two largest producers of acrylonitrile in the world. Unfortunately, the current acrylonitrile production process generates at least one ton of waste water when one ton of acrylonitrile is produced. What's more serious is that the acrylonitrile production wastewater is composed of a large number of highly toxic compounds, and the acrylonitrile wastewater not only destroys the aquatic ecosystem, but also has great harm to human health. However, due to the huge demand for acrylonitrile in the market, the application of acrylonitrile is inevitable in a short time, and a plan to effectively treat acrylonitrile wastewater is urgently needed. At present, the treatment of acrylonitrile mainly includes adsorption, incineration, recycling, activated sludge and so on. The above method not only has high cost, but also has insufficient removal and harsh working conditions, making it difficult to effectively control the pollution of acrylonitrile wastewater. The most difficult pollutants to treat in acrylonitrile wastewater are polymers. It mainly comes from low molecular polymers or copolymers of nitrile substances. These polymers are usually present in water in colloidal or dissolved form, are difficult to hydrolyze and are used by microorganisms, and cannot be practically removed. Acrylonitrile wastewater is generally considered to be the most difficult organic wastewater to treat.

Photocatalyst oxidation is that the catalyst is irradiated by light, absorbs light energy, undergoes electronic transition, generates electron-hole pairs, directly oxidizes and reduces the pollutants adsorbed on the surface, generates strong oxidizing hydroxyl radicals, and oxidizes pollutants. Due to its abundant resources and low-carbon production, solar energy is one of the best alternative energy sources. At present, great progress has been made in the research and development of solar energy utilization systems. The most researched semiconductor catalytic material is TiO ₂ . In the ultraviolet range of 300nm to 390nm, TiO ₂ has a high photocatalytic activity, and in many It can still maintain a high photocatalytic _activity after repeated use. TiO ₂ can completely mineralize the target pollutants into water and carbon dioxide, and will not cause secondary pollution to the environment. Performance and other aspects have great stability, so it has been widely used in the field of environmental purification.

The photocatalytic activity of _TiO2 can be improved by adding silica gel to increase its specific surface area. In recent years, many literatures have reported the loading methods of titanium dioxide, such as activated carbon loading TiO ₂ , molecular sieve loading TiO ₂ , glass beads loading TiO ₂ , and silica loading TiO ₂ . Utilizing the adsorption properties of the carrier can improve the mineralization ability of the catalyst to degrade pollutants. Compared with the activity of unsupported titanium dioxide, the activity of the supported catalyst is greatly improved, because the synergistic effect can be formed between titanium dioxide and the support. Among them, silica has a large specific surface area due to its good adsorption performance. Silica-supported TiO ₂ is a supported catalyst with excellent performance. This catalyst not only has the thermal stability and mechanical stability of silica, but also is a transparent body of light, which can reduce light scattering, thereby effectively improving Catalyst degradation performance. Adding SiO ₂ can effectively control the growth of TiO ₂ crystal particles, effectively inhibit the agglomeration of the catalyst, and enable the catalyst to obtain a smaller particle size and a higher specific surface area. _TiO2 supported on silica gel also undergoes interfacial diffusion with silica to form Si-O-Ti bonds. The formation of Si-O-Ti bonds can inhibit the transformation of anatase _TiO2 to rutile _TiO2 . Jin Liang and others found that nano-titanium dioxide can effectively degrade acrylonitrile in distribution water in an open reactor under sufficient and stable light conditions. Pang DD et al. found that with HF as the F source, the F-doped SiO ₂ -supported TiO ₂ composite photocatalyst exhibited the highest activity for degrading acrylonitrile. In situ infrared and NH ₃ -TPD results show that F doping improves the surface acid sites and acid strength of SiO ₂ supported TiO ₂ composite photocatalysts. When the molar ratio of HF to Ti is 1:1 and the loading amount of _TiO2 is 36%, the removal rate of acrylonitrile can reach 66% in 6 min under the irradiation of simulated sunlight.

One of the advantages of using non-metal element doping modification to improve the activity of TiO ₂ is that it can expand the visible light catalytic activity of TiO ₂ without affecting the catalytic activity of the catalyst under ultraviolet light. The d orbital energy level of the metal ion Ti ⁴⁺ in the semiconductor TiO ₂ determines the energy level of its conduction band, while the energy level of the valence band is determined by the p orbital energy level of the non-metal ion O ^2- . Compared with lowering the potential of the conduction band, there is more room for increasing the potential of the valence band and it is easier to realize, so the valence band is generally modified. Compared with the 2p orbital of O, non-metallic elements such as B, C, N and S have p orbitals with higher energy. When these elements replace O atoms, the valence band potential of semiconductor titanium dioxide is increased to a certain extent, thereby reducing the semiconductor Bandgap width of _TiO2 . But in theory, only when the ions of the non-metal element are very close to the O ion radius, the non-metal element ions can replace the oxygen ions in the crystal lattice of TiO ₂ . Therefore, the non-metallic elements studied by scientists are mainly distributed near oxygen elements, such as C, N, S, B and halogen elements. Fluorine atoms have a smaller diameter than oxygen atoms, so in theory, fluorine atoms could replace oxygen atoms in titanium dioxide. In a large number of studies on fluorine-doped modified _TiO2 , fluorine can replace oxygen ions in the _TiO2 lattice, and can also be adsorbed on the surface of _TiO2 particles. Unlike nitrogen-doped _TiO2 , fluorine-doped modified _TiO2 does not significantly change the light absorption edge of the catalyst because the potential of the 2p orbital of fluorine is lower than the valence band potential of _TiO2 . Some researchers used the hydrothermal method to synthesize anatase TiO ₂ , and the visible light catalytic activity of the catalyst was very high. The author found that after fluorine doped TiO ₂ , part of Ti ⁴⁺ was converted into Ti ³⁺ , forming oxygen vacancies and trapping the surface of the catalyst. Oxygen molecules in the catalyst generate superoxide radicals, thereby improving the activity of the catalyst. Besides, F doping _TiO2 can also increase the surface acidity of _TiO2 . After F doping TiO ₂ , Lewis and Bronsted acidic sites appear on the surface of the sample, and Lewis and Bronsted acidic sites are good adsorption centers for oxygen molecules and molecules with lone pair electrons, so F doping can effectively improve the concentration of electrons with lone pair electrons. Degradation activity of organic matter.

Another effective way to increase catalysis and activity is to increase the number of acidic sites on the surface. It has been confirmed that the photocatalytic activity increases with the increase of acidic sites on the catalyst surface. Cui et al. reported in 1995 that _TiO2 could be doped with metal oxides to increase its surface acidity and photocatalytic activity. Wang et al. reported in 2006 that amorphous TiO ₂ and sulfur compounds can synthesize sulfur-doped TiO ₂ at high temperature, which can be used as a photocatalyst with acidic sites. However, there are few reports on the photocatalytic degradation of acrylonitrile using acidic and highly stable catalysts.

On the one hand, the doping of Zr has entered the TiO2 lattice, and since Zr itself is very stable, the overall stability has been improved. On the other hand, since Zr also affects the growth direction of Ti and acts as a force during the crystal growth process, it also improves the stability of the crystal structure, and it also has a fluorine fixation effect. While Al has the above two effects, and the former effect is stronger.

Contents of the invention

The above-mentioned photocatalytic process is carried out in the actual acrylonitrile wastewater, because there are a large number of various interfering ions in the actual wastewater, and the organic and inorganic components are also very complicated. In order to solve the above problems, the purpose of the present invention is to provide efficient and reusable photocatalytic A photocatalyst that catalyzes the degradation of acrylonitrile actual wastewater, using 100-200 mesh silica as a dispersant, hydrogen fluoride, zirconium nitrate, aluminum nitrate and thiourea as doping precursors, to prepare a photocatalyst with a large specific surface area with a mesoporous structure Catalyst, the compound doping of F and S in the catalyst improves the acidity intensity and light absorption ability of the catalyst surface, which improves the photocatalytic activity and anti-interference ability, while the doping of Zr and Al greatly improves the overall photocatalytic activity of the catalyst. Persistence.

The purpose of the present invention is also to provide a preparation method for the catalyst for the solar photocatalytic degradation of the above-mentioned acrylonitrile actual wastewater, and prepare a titanium dioxide composite oxide catalyst loaded with fluorine, sulfur, zirconium and aluminum doped on silica gel by using a sol-gel method.

The inventors of the present invention have found through research that the F-doped _TiO2 catalyst has shown a very good effect on the degradation of acrylonitrile actual wastewater, and the dark adsorption of the catalyst and organic matter is also relatively large compared with P25 _TiO2 . Analysis It shows that there is an adsorption between the acid center on the surface of the catalyst and the lone pair of electrons in the organic matter of acrylonitrile wastewater.

In order to further improve the photocatalytic activity of the catalyst, the composite oxide catalyst provided by the present invention is co-doped with fluorine, sulfur and zirconium. In the preparation process by the sol-gel method, hydrogen fluoride, zirconium nitrate pentahydrate, aluminum nitrate and thiourea solution are added to the catalyst as doping precursors, and then calcined at high temperature to form a fluorine-doped catalyst, which has more The acidic sites on the surface, the addition of silica gel to form the uneven appearance and light scattering caused by the pore structure, is conducive to light absorption, which plays an important role in the improvement of photocatalytic activity, and can greatly improve its photocatalytic activity. Especially the addition of Zr and Al greatly improved the stability of the catalyst.

The present invention also provides the preparation method of above-mentioned composite oxide photocatalyst, it comprises the following steps:

Add 2-8ml of butyl titanate dropwise into 10-16mL of absolute ethanol and mix with it to obtain a clear solution which is solution A.

Add 12-32mL absolute ethanol, 2.3-5.3mL glacial acetic acid, 15-35mg thiourea, 429-449mg zirconium nitrate pentahydrate, 13-18mg aluminum nitrate, 0.2-1.5mL hydrofluoric acid and 0.4- 1.9mL deionized water, after ultrasonication for 2min, is solution B.

Add solution B to solution A drop by drop, seal and stir for 1-3 hours, open the seal, continue to stir to form a uniform aqueous sol, continue to stir for a period of time until an oily sol is formed, then add 100-200 mesh silica gel until a gel is formed.

Aging the above gel at room temperature (20-30°C) for about 8-12h, then drying in a drying oven at 70-130°C, grinding the obtained dry powder catalyst precursor, and finally Calcined in the furnace at 350-500°C for 1-3h. That is, a TiO ₂ /SiO ₂ catalyst dispersed with doped modified SiO ₂ is obtained. Its composition ratio is Ti:F:S:Zr:Al=1:1:0.5-5%:1-10%:0.1-2%.

The TEM results (left image) show that the catalyst is about 30nm in diameter, and the enlarged right image more clearly shows that these particles are agglomerated by seemingly smaller particles, and there are also square particles.

The activity evaluation of F, S, Zr, Al co-doped modified TiO ₂ /SiO ₂ photocatalysts selected the organic pollutants in the actual wastewater of acrylonitrile as the target degradation products.

From the point of view of research comparison and practical application, 350w AHD350 spherical xenon lamp and sunlight are respectively selected to treat the actual wastewater of acrylonitrile.

Description of drawings

Figure 1 is a TEM image of the FS-Zr-Al-TiO ₂ /SiO ₂ sample.

Figure 2 is the COD curve of the actual wastewater treated with FS-Zr-Al-TiO ₂ /SiO ₂ after four reactions (three recovery and activation) with simulated sunlight time.

Fig. 3 is the COD curve of the actual wastewater treated with FS-Zr-Al-TiO ₂ /SiO ₂ for the first time with simulation and natural light time.

Detailed ways

The implementation and excellent effects of the present invention are described below through specific examples, but the implementation scope of the present invention should not be construed as any limitation.

Evaluation method of catalyst activity:

Specific steps: The activity evaluation of the photocatalyst selects the actual wastewater with low concentration of acrylonitrile as the target degradation product, pours 150mL of actual wastewater into the quartz reactor, adds 300mg of photocatalyst, then closes the reactor, and stirs in the dark for 35min to achieve adsorption and desorption. After the balance, turn on the xenon lamp and start to illuminate. The light source is a 350w AHD350 spherical xenon lamp light source. During the illumination process, two sets of 4mL parallel samples of wastewater suspension are taken out at 2h, 6h, 10h, and 14h respectively.

Detection method: The determination of COD adopts the Lianhua LH-5B-3B (V8) COD rapid tester. In COD error ±10%. Determination of sample COD: Immediately after sampling, filter it with a 0.45 μm pore size filter and measure it, which is the corresponding value of COD at this time.

With the prolongation of the light time, the decline curve of COD in the catalyst and solution is shown in Figure 2. After 14 hours of irradiation, the COD of the solution dropped from 89 to about 25mg/L. After 4 reactions, it can still reach about 42mg/L, demonstrating that the organic matter is basically completely degraded. and good catalyst stability.

Using the same method, we obtained that the actual wastewater of acrylonitrile was degraded to about COD=50mg/L under sunlight irradiation and through sufficient sunlight reaction.

Claims (2)

1. a kind of degrading acrylonitrile actual wastewater F, S, Zr, Al co-doping modified TiO ₂ /SiO ₂ The preparation method of photocatalyst, it is characterized in that, described preparation method comprises the following steps: Step 1. Add 2-8mL of butyl titanate dropwise to 10-16mL of absolute ethanol and mix with it to obtain a clear solution which is solution A; Step 2. Add 12-32mL of absolute ethanol, 2.3-5.3mL of glacial acetic acid, 15-35mg of thiourea, 429-449mg of zirconium nitrate pentahydrate, 13-18mg of aluminum nitrate, and 0.2-1.5mL of hydrofluoric acid in a 150mL beaker. and 0.4-1.9mL deionized water, after ultrasonication for 2min, it is solution B; Step 3. Add solution B to solution A drop by drop, seal and stir for 1-3 hours, open the seal, continue to stir to form a uniform aqueous sol, continue to stir for a period of time, and add 100-200 mesh silica gel until an oily sol is formed. form a gel; Step 4. Aging the gel at 20-30°C for 8-12h, then drying it in a drying oven at 70°C-130°C, and grinding the obtained dry powdery catalyst precursor with an agate mortar, Finally, calcining at 350°C-500°C for 1-3h in a tube furnace to obtain F, S, Zr, Al co-doped modified TiO ₂ /SiO ₂ photocatalyst; In the photocatalyst, Ti:F:S:Zr:Al=1:1:(0.5-5%):(1-10%):(0.1-2%). 2. The preparation method according to claim 1, wherein the precursor hydrofluoric acid of fluorine is commercially available 40% hydrogen fluoride aqueous solution, the precursor thiourea of sulfur is analytically pure thiourea, and the precursor of zirconium nitric acid pentahydrate Zirconium is analytically pure zirconium nitrate pentahydrate, and the precursor of aluminum, aluminum nitrate, is analytically pure aluminum nitrate nonahydrate.

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