ES2946207B2 - Photocatalyst for the treatment of fluids containing organic matter - Google Patents

Description

DESCRIPTION

Photocatalyst for the treatment of fluids containing organic matter

Technique field:

The present invention belongs to the field of catalysts, specifically to heterogeneous photocatalysts based on semiconductors, useful in the photocatalytic oxidation of organic matter present in fluids.

Background of the invention:

Water pollution is one of the most relevant problems facing societies today. The availability of water is limited, and therefore, its

Decontamination and reuse is essential to preserve public health and conserve ecosystems. Wastewater can be composed of different types of contaminants: heavy metals, pesticides, dyes, drugs, pathogens, feces, salts, plastics, etc. These waters are largely produced by anthropogenic activities (urban waste, industry, agriculture) [Ajibade, F. O., Adelodun, B., Lasisi, K. H., Fadare, O. O., Ajibade, T. F., Nwogwu, N. A., Sulaymon, I. D., Ugya, A.Y., Wang, H.C., Wang, A., Woodhead Publ. Ser. Food Sci. Technol. Nutr., 321, (2021)].

Traditional wastewater treatment methods are classified as physical, biological and chemical, which use decanters, bacterial beds and chlorine baths, respectively. However, these techniques can produce bacteriological contamination and dangerous by-products (halogenated compounds) [Von Gunten, U., Eviron. Sci. Technol., 52, 5062 (2018)]. Furthermore, they are insufficient to eliminate biologically toxic compounds and persistent organic compounds, such as phenol, a carcinogenic aromatic molecule. These contaminants are usually present in wastewater in concentrations that range, depending on the case, from parts per million (ppm) to parts per billion (ppb), which makes their elimination difficult [Rivera-Utrilla, J., López-Ramón, M. V., Sánchez -Polo, M., Álvarez, M. Á., Velo-Gala, I., Catalysts, 10 (2020)]. However, advanced oxidation processes (AOPs) have been effective for the mineralization of persistent organic compounds present in wastewater. AOPs are characterized by the generation of highly reactive species, such as the hydroxyl radical ('OH), capable of oxidizing organic matter until it is mineralized, transforming it into carbon dioxide and water. Heterogeneous photocatalysis belongs to the family of AOPs and offers high mineralization efficiency, high degradation rates, reusability and low energy demand [Ma, D., Yi, H., Lai, C., Liu, X. Huo, X., An, Z., Li, L., Fu, Y., Li, B., Zhang, M., Qin, L., Liu, S., Yang, L., Chemosphere, 130104 (2021)].

Titanium dioxide (TiO2) is one of the semiconductors most used as a photocatalyst due to its high resistance to photocorrosion, its low cost and its ability to absorb solar radiation, its crystalline phase being anatase, the polymorph with the most photocatalytic activity [Weng , B., Qi, M.-Y., Han, C., Tang, Z.-R., Xu, Y. J., ACS Catal., 9, 5, 4642 (2019); Etacheri, V., Di Valentin, C., Schneider, J., Bahnemann, D., Pillai, S. C., J. Photochem. Photobiol. C Photochem. Rev., 25, 1 (2015)]. When TiO2 is excited with light (UV-A radiation) of energy equal to or greater than its bandgap (3.2 eV), an electron-hole pair is generated on its surface, which is capable of reducing oxygen to the superoxide radical anion. (O2-) and oxidizing water to the hydroxyl radical ('OH), respectively [Herrmann, J.-M., Catal. Today, 53, 1, 115 (1999)]. These highly reactive species are responsible for the oxidation of organic compounds. However, in TiO2 a high recombination efficiency of photogenerated electrons and holes is observed. This problem can be solved by reducing the particle size and increasing the surface area since the transport of electron-hole pairs from the core of the material to its surface is shortened [Li, T.; Abdelhaleem, A., Chu, W., Pu, S., Qi, F., Zou, J., Chem. Eng. J., 411, 128450 (2021)]. The reference commercial TiO2 is called Degussa P25. This material appears as a white powder composed of nanoparticles with an average size of 21 nm, with a crystalline phase ratio of 80:20 anatase:rutile and a BET surface area of 55 m2g<-1>[Ohtani, B., Prieto-Mahaney, O. O., Li, D., Abe, R., J. Photochem. Photobiol. A Chem., 216, 2, 179 (2010)]. The high photocatalytic efficiency exhibited by P25 explains its use in fluidized bed photoreactors where contaminated water is treated with suspended photocatalyst. However, the use of nanometric photocatalysts leads to technical problems such as particle agglomeration, with the consequent loss of photoactive area, or the need for centrifugation or ultrafiltration processes, which means greater use of energy and resources for industries. For these reasons, the immobilization of photocatalysts is of great interest for industrial scaling [Oblak, R., Kete, M., Stangar, U. L., Tasbihi, M., J. Water Process Eng., 23, 142 (2018) ].

The most widely used support for immobilization of TÍO2 is silicon dioxide (SÍO2). This substrate has a low cost, a well-studied surface chemistry, high adsorption capacity, high thermal and chemical stability, and does not absorb ultraviolet light [Axinte, E., Mater. Des., 32, 4, 1717 (2011); Ullah, S., Ferreira-Neto, E. P., Pasa, A. A., Alcántara, C. C. J., Acuña, J. J. S., Bilmes, S. A., Martínez Ricci, M. L., Landers, R., Fermino, T. Z., Rodrigues-Filho, U. P., Appl. Catal. B Environ., 179, 333 (2015)]. On the one hand, methods have been used to deposit different amounts of TiO2 nanoparticles on SiO2 glass materials. Among them, it is worth highlighting the formation of polyurethane sponges which increase the surface of the glass which is subsequently impregnated with TiO2 by immersion in a suspension of Degussa P25. In this case, the photocatalytic activity of the material was evaluated in the photocatalytic reduction of Cr (VI) in the presence of EDTA [Loffler, F. B.; Altermann, F.J.; Bucharsky, E. C.; Schell, K.G.; Vera, M.L.; Traid, H.; Dwojak, A.; Litter, M. I., Int. J. Appl. Ceram. Technol., 17, 4, 1930 (2020)]. However, direct immobilization of P25 on glass substrates does not result in robust and durable coatings with sufficient mechanical stability, and TiO2 coatings can be removed by simple exfoliation. Therefore, titanium (IV) tetraisopropoxide (TTIP) and tetraethyl orthosilicate (TEOS) have been used as precursors of TiO2 and SiO2, respectively, to synthesize more robust materials. One of them has been the preparation of a gel to which P25 was subsequently added. The resulting mixture was supported on a microscope slide glass by immersion. The material was used for the batch photodegradation of 2-methylisoborneol and Geosmin, organic compounds responsible for odor and taste in water [Yaparatne, S.; Tripp, C. P.; Amirbahman, A., J. Hazard. Mater., 346, 208 (2018)], however, the thickness of the layer of P25 nanoparticles deposited on the glass was not evaluated, in addition to the fact that the support did not present any porosity or adequate ergonomics for a continuous process. Submicron glass fibers have also been used to prepare TiO2 photocatalysts using sol-gel and electrospinning techniques. For example, this condensed SiO2 core was coated with a layer of mesoporous SiO2 using the modified Stober method to give rise to a core@shell structure. Finally, a large layer of nanostructured TiO2 was adhered to this material using titanium (IV) chloride (TiCU) as a titanium precursor in an autoclave. The resulting material acquired a core@double-shell configuration and was tested in the photodegradation of rhodamine B [Ma, Z.; Chen, W.; Hu, Z.; Pan, X.; Peng, M.; Dong, G.; Zhou, S.; Zhang, Q.; Yang, Z.; Qiu, J., ACS Appl. Mater. Interfaces, 5, 15, 7527 (2013)].

This material turned out to be very efficient because it had a large photoactive surface, but neither the crystal size, which according to many studies is a TiO2 size between 10 and 25 nm, nor the thickness of the titanium layer had been optimized to obtain maximum photocatalytic efficiency [Topcu, S.; Jodhani; G.; Gouma, P. I., Front. Mater. 3, 35 (2016)]. Furthermore, due to the arrangement of the nanostructures on the surface of the photocatalyst, it is very likely that titanium dioxide leaching will occur. On the other hand, the submicron size of its support, even flexible and porous, would produce overpressures if it were used in continuous flow. Micrometric glass wool fibers covered by a TiO2 film synthesized by liquid phase deposition with a titanium precursor of (NH<4>)2TiF<6> have also been used. The characterization by scanning electron microscopy (SEM) of the material showed fractures in the TiO2 film deposited on the glass wool, which compromises its mechanical stability and flexibility [Kishimoto, H.; Takahama, K.; Hashimoto, N.; Aoi, Y.; Deki, S., J. Mater. Chem., 8, 9, 2019 (1998)]. Furthermore, in this case, the TiO2 layer was also micrometric, very far from the optimal photocatalytic thickness.

In other studies, submicron SiO2 spheres coated with different layers of TiO2 have been used, but in this type of materials there is only one case where the coating of TiO2 nanoparticles has been optimized, but at a crystal size of 5 nm, far from the range optimal, although they still observed that a layer of about 25-30 nm of these nanocrystals produced good photocatalytic results [Ullah, S.; Ferreira-Neto, E.P.; Pasa, A.A.; Alcántara, C.C.J.; Acuña, J.J.S.; Bilmes, S.A.; Martínez Ricci, M.L.; Landers, R.; Fermino, T.Z.; Rodrigues-Filho, U.P., Appl. Catal. B, 179, 333 (2015)]. However, this type of submicron material, in addition to being able to leach titanium dioxide due to being synthesized by the hydrothermal method, continues to present the same difficulties for its separation from the treated water as the P25 nanoparticles themselves, that is, they require centrifugation or filtration to their elimination and cannot be used in continuous flow because they generate overpressures due to their size.

On the other hand, the growing concern about environmental pollution is known, which may be due to the presence of dissolved and/or suspended organic matter, usually as a result of anthropogenic processes. In particular, volatile organic compounds (VOCs) are pollutants present in gaseous fluids normally produced by emissions from vehicles and industrial processes. Some VOCs such as benzene, toluene or formaldehyde are carcinogenic compounds and harmful to living beings [Mustafa, M. F., Lui, Y., Duan, Z., Guo, H., Xu, S., Wang, H., Lu,W.,J. Hazard. Mater. 327, 35, (2017)]. A widely used strategy is the elimination of VOCs through photoactive coatings, which are capable of carrying out advanced oxidation processes with great efficiency in the elimination of this type of contaminants [Hodgson, A. T., Destaillats, H., Sullivan, D. , Fisk, W. J., Indoor Air, 17, 305, (2007)]. In fact, the use of photoactive coatings for decontamination and/or self-cleaning of surfaces (such as windows, cement, etc.) has been widely studied and implemented in some commercial products [Ollis, D. F., C. R. Acad. Sci. Paris-Chem, 3, 405, (2000); Strini, A., Cassese, S., Schiavi, L., Appl. Catal. B, 61, 90 (2005); Puzenat, E., Pichat, P., J. Photochem. Photobiol., A, 160, 127, (2003)]. In this specific example, the photoactive TiO2 particles were supported on materials capable of resisting mechanical abrasion or wear due to the passage of time. In fact, three key requirements are usually considered for the manufacture of photocatalysts in order to produce the elimination of VOCs: (1) that the photocatalyst has a structure that facilitates the diffusion of VOCs towards the active sites of the photocatalyst and the production products. oxidation towards the reaction medium, (2) that the photocatalyst increases the effective residence time of the VOCs near it and finally, (3) that all the active sites of the photocatalyst are exposed to radiation [Kibanova, D ., Cervini-Silva, J., Destaillats, H., Environ. Sci. Technol. 43, 1500, (2009)].

Some other examples contemplate the invention of materials and/or systems for the elimination of VOCs:

1) Document ES1279076 describes a continuous photoreactor, for the treatment of air contaminated by pathogens and volatile organic compounds, composed of transparent quartz tubes coated with two or more semiconductors: TiO2, ZrO2 and SiO2, the latter acting as a filling material for create greater friction and contact surface. The irradiation source used is composed of rows of 8 W germicidal UV lamps.

2) Document ES2871201A1 reports a device and a procedure for treating air contaminated by VOCs and microorganisms (bacteria, viruses). The procedure consists of a double treatment composed of atmospheric plasma and photocatalysis. The device consists of a chamber where two electrodes create a potential difference, which causes the dielectric breakdown of air, which in turn produces a plasma discharge that eliminates part of the VOCs and microorganisms from the air. The air flow resulting from the first treatment flows to another chamber that houses titanium dioxide mesh barriers irradiated by ultraviolet light lamps, where the second gradual elimination of VOCs and microorganisms, as well as plasma byproducts carried by the flow, is carried out. of air from the first treatment.

3) Document ES2339081A1 exhibits equipment for purifying toxic gases of organic origin for application in chimneys or industrial extractor hoods. The catalyst is prepared from a mixture of TiO2 with an aqueous solution of CuSO<4>. The resulting material, CuTiO2, is calcined at 400 °C for 5 h. This material is then impregnated into the walls of a reactor. The VOC treatment system is based on flowing an air flow that drags the vapors of a pollutant solution to the reactor with the impregnated photocatalyst. Next, a UV irradiation source is applied to the reactor and the gas output from the system is monitored by gas chromatography.

The present invention aims to overcome the drawbacks of photocatalysts supported with TiO2 for use in the purification of fluids in continuous flow, also achieving high photocatalytic efficiency. The new photocatalyst combines in its support the flexibility and porosity properties of glass wool fibers with the decoration of SiO2 spheres that give the support a greater surface area. On the other hand, the TiO2 layer that homogeneously covers the glass wool fibers and the SiO2 spheres is photocatalytically active. In addition, it improves the photocatalytic efficiency of most of the materials described since it has a TiO2 crystalline layer with optimized thicknesses formed by crystalline structures covalently linked to each other. Thus, the material composed of the covalent union of spheres and glass wool formed by SiO2 and TiO2, which is described as a new photocatalyst, represents a qualitative technical leap with respect to the aforementioned state of the art. Furthermore, due to the hydrolysis and condensation reactions in the synthesis method, the bond between SiO2 and TiO2 is covalent, which prevents the leaching of titanium dioxide during fluid treatment (such as wastewater). Thus, this novel material has improved photocatalytic efficiency compared to all the materials described so far with supported TiO2 since the contact surface and the thickness of the TiO2 layer have been optimized in a material transparent to UVA light. It also has a great capacity for reuse for oxidation processes of organic matter and a high capacity for total mineralization (when necessary, mineralization values higher than 99%) of organic matter, such as contaminating organic compounds in fluids (such as for example methylene blue and phenol present in wastewater or methyl tert-butyl ether (MTBE) in gas streams).

Description of the invention:

The present invention describes a new photocatalyst formed by SiO2-TiO2 spheres supported on glass fiber covered by TiO2, which aims to overcome, first of all, the disadvantages of conventional treatments, such as biological, physical and chemical treatments, of fluids contaminated by a large variety of organic molecules and secondly, the deficiencies of photocatalysts based on supported TiO2, such as, for example, i) the impossibility of their use in decontamination of fluids in continuous flow; ii) the leaching of TiO2; iii) the loss of photoactive area due to the micrometric nature of the TiO2 of the supports used and iv) the low photocatalytic efficiency of TiO2 due to the thickness of its layer, as well as its obtaining procedure and its use for the oxidation of organic matter in different fluids. .

Thus, the present invention refers to a new photocatalyst for the partial or total oxidation of any organic matter contained in fluids, in which said catalyst is a material composed of SiO2-TiO2 core@shell spheres where said spheres comprise the core (a ) of SiO2 covered by a shell (b) of TiO2. These spheres are covalently linked to a layer of TiO2(c) that covers the glass wool support (d) ((GW) and where the TiO2 shell (b) of the spheres and the TiO2 layer ( c) of the glass wool are in the photoactive phase (see Figure 1).

According to a particular embodiment, the photoactive phase is preferably anatase phase in at least 50% of the TiO2 crystalline phase, more preferably greater than 70%.

The SiO2-TiO2 core@shell type spheres of the present invention have a SiO2 core (a) of a diameter selected between 100 and 1000 nm (preferably between 300 and 800 nm and more preferably between 400 and 700 nm) and a shell (b) of TiO2 of a thickness preferably selected between 8 and 50 nm and more preferably between 10 and 30 nm.

According to a preferred embodiment, in the SÍO2-TÍO2 core@shell type spheres, the SiO2 core (a) and the TiO2 shell (b) are covalently linked.

Furthermore, according to another preferred embodiment, the layer (c) of TiO2 that covers the glass fiber support (d) is covalently linked to it.

The advantages of the covalent bond between SiO2 and TiO2 lie in the fact that the support (SiO2) and the photoactive layer (formed by TiO2 crystals) form a unique structure. The photocatalysis will only be heterogeneous since there will be no leaching of the TiO2 crystals into the homogeneous medium (fluid). Furthermore, in this way the recovery and subsequent reuse of the photocatalyst is favored.

This TiO2 layer (c) that covers the glass fiber support (d) has a thickness preferably selected between 8 and 50 nm, and more preferably between 10 and 30 nm.

According to the present invention, the glass fiber (d) can be any commercial glass fiber that preferably has a diameter preferably selected between 1 and 50 microns, and more preferably between 20 and 40 microns.

According to the present invention, in the photocatalyst described, the SiO2-TiO2 core@shell type spheres are covalently linked to the TiO2 layer (c), which, as previously mentioned, minimizes the leaching of the TiO2 crystals into the homogeneous medium ( fluid) also favoring the recovery and subsequent reuse of the photocatalyst.

The advantages of the thicknesses of the TiO2 layer of the covalent bond with SiO2, both in SiO2-TiO2 core@shell type spheres and in glass wool, are that most of the photoactive layer (formed by TiO2 crystals) takes advantage of the light to generate charge separation and consequently the formation of oxidizing species. The covalent union of spheres with glass wool, both coated with TiO2, has the advantage of greatly increasing the surface of the photocatalyst without leaching processes of the TiO2-coated spheres.

The present invention also refers to the procedure for obtaining the photocatalyst described above.

The preparation of the photocatalyst can be divided into different consecutive synthetic steps and comprises, at least:

- a first stage (stage 1) of preparation of SiO2-TiO2 core@shell type spheres,

- a second stage (stage 2) of preparation of the glass wool-TiO2 support, - a third stage (stage 3) in which the products obtained in stage 1 and stage 2 are reacted to obtain the photocatalyst.

It is in the third stage where the covalent union of the glass wool-TiO2 support with the SiO2-TiO2 core@shell spheres is carried out, thus obtaining the final photocatalyst that is the object of the present invention.

Thus, the preparation of SiO2-TiO2 core@shell type spheres, which is carried out in stage 1, includes at least the following steps:

to. add a SiO2 precursor, preferably tetraethyl orthosilicate (TEOS), to a cold hydroalcoholic solution of NH<4>OH and stir,

b. then stir at room temperature for at least 1 hour, isolate, wash and dry,

c. slowly add the TiO2 precursor to an alcoholic suspension of the isolated product in “b” at a temperature below the boiling point and stir,

d. pass a stream of diluted water through the mixture obtained.

According to the present invention the term "diluted water stream" refers to streams that are not 100% water. The degree of dilution will depend on the amount of product and operating conditions.

According to a particular embodiment, the product obtained in stage 1 can also be subjected to a process of isolating the core@shell spheres obtained, washing them and subsequent calcination.

According to the present invention, the calcination process refers in all cases and the different stages to heating the material between 400 and 600 °C for at least 3 hours.

According to another particular embodiment of the present invention, the formation of the core@shell type microspheres is carried out by the Stober method [Stober, W., Fink, A., J. Coll. Interf. Sci. 26 62 (1968)] and varying proportions of SiO2 precursor, NH<4>OH and temperature, the different sizes of micrometric SiO2 spheres are obtained. Subsequently, these SiO2 spheres are used as a spherical core on which a TiO2 shell is grown that is covalently bonded to the SiO2 core.

According to a particular embodiment, in stage 1 of core@shell SiO2-TiO2 sphere formation, a solution of TiO2 precursor (for example Ti(IV) tetraisopropoxide, TTIP) is dispersed in an alcohol (preferably ethanol or ethanol/2-propanol mixtures) and is added to an alcoholic suspension of submicron SiO2 spheres and the mixture is subsequently heated below the boiling point, preferably between 40 and 80 °C. The mixture is maintained at that temperature under stirring under a stream of dilute water for at least 1 hour, preferably between 10 and 40 h, and more preferably between 15 and 20 h. Afterwards, the spheres obtained are centrifuged at 3000 rpm for 15 min to 50 min, washed with H2O and calcined at a temperature necessary to obtain them in the photoactive phase, preferably between 400 and 600 °C for at least 3 hours. According to this particular embodiment, the concentration of TiO2 precursor (preferably TTIP) used can vary between 0.5 and 2 g TTIP/g SiO2 spheres, depending on the size of the SiO2 spheres and the thickness of the TiO2 shell that is to be obtained. in the SiO2-TiO2 spheres.

According to the present invention, stage 2 of the process for obtaining the photocatalyst comprises, at least, the following steps:

to. subjecting the glass wool support (d) to an acid pretreatment followed by washing and subsequent basic treatment and drying,

b. To an alcoholic mixture of the product obtained in a) the TiO2 precursor is added and heated below the boiling point while stirring the mixture, c. The mixture obtained in b) is passed through a stream of diluted water.

This stage 2 can be carried out in parallel with stage 1.

According to a particular embodiment, the product obtained in stage 2 can also be subjected to a process of isolating the product obtained, washing and subsequent calcination.

According to a particular embodiment of the present invention, in stage 2 for the preparation of the glass wool-TiO2 support, the glass wool must be subjected to an acid pretreatment followed by various washes until washing waters with a pH close to the neutrality. The glass wool is then subjected to a basic treatment, preferably with NH<4>OH, followed by various alcohol washes. The already pretreated glass wool is suspended in an alcohol (preferably ethanol or ethanol/2-propanol mixtures) previously heated below the boiling point, preferably between 40 and 80 °C, and a solution of TiO2 precursor is added (e.g. example, TTIP) in the same alcohol in a proportion preferably between 3 and 200 mg of TTIP/g glass wool. The resulting mixture is kept at that temperature under stirring for at least 1 hour to later add a stream of diluted water that is maintained for at least one hour, preferably between 10 and 40 and more preferably between 15 and 20 hours. Afterwards, a suspension of ethanol or ethanol/2-propanol mixtures with previously synthesized SiO2-TiO2 spheres is added to this mixture (between 3 and 350 mg of SiO2-TiO2 spheres/g initial glass wool) and the resulting mixture is maintained between 40 and 80 °C under stirring and with a stream of diluted water for at least one hour, preferably between 10 and 40 and more preferably between 15 and 20 hours. Subsequently, the micrometric material is filtered, washed with H2O and dried. Finally, it is calcined at temperatures between 400 and 600 °C for at least 3 hours.

According to the method for preparing the photocatalyst of the present invention, once stage 1 and stage 2 described above have been completed, stage 3 is carried out, which includes at least the following steps:

to. react the SiO2-TiO2 core@shell type spheres obtained in stage 1 with the product obtained in stage 2, heat below the boiling point and stir,

b. The mixture obtained in a) is passed through a stream of diluted water, c. isolate and calcine the product obtained.

According to a particular embodiment of the present invention, the photocatalyst composed of the glass wool material covered with TiO2 and attached to SiO2-TiO2 spheres obtained in stage 3, can be subjected to additional decoration processes by repeating the coating processes with spheres of SiO2-TiO2to further increase the photoactive surface of the photocatalyst. In this case, a suspension of the material obtained with glass wool covered with TiO2 attached to SiO2-TiO2 spheres in ethanol or ethanol/2-propanol previously heated between 40 and 80 °C is prepared and a solution of TTIP in 2-propanol is added. (values between 3 and 200 mg of TTIP/g micrometric SiO2-TiO2 material are used depending on the thickness of the coverage desired). The resulting mixture is kept hot with stirring for at least 1 hour and is subsequently subjected to a stream of diluted water for at least one hour, preferably between 10 and 40 and more preferably between 15 and 20 hours. Then, to this reaction mixture is added a suspension of ethanol or ethanol/2-propanol previously heated between 40 and 80 °C with previously synthesized SiO2-TiO2 spheres (3 and 350 mg of SiO2-TiO2 spheres/g wool. initial glass). To the resulting mixture, maintaining the temperature and under stirring, a stream of diluted water is added for at least one hour, preferably between 10 and 40 and more preferably between 15 and 20 hours. Subsequently, the micrometric material is filtered, washed with H2O and dried and then calcined.

Different procedures have been published in the literature for the preparation of photocatalysts based on supported TiO2 [Loffler, F. B., Bcharsky, E. C., Schell, K. G., Vera, M. L., Traid, H., Dwojak A., Litter, M. I., Int. J. Appl. Ceram. Technol. 171930 (2020); Sudheera, Y., Carl P, T., Aria, A., J. Hazard. Mater. 346 208 (2017); Ma, Z., Chen, W., Hu, Z., Pan, X., Peng, M., Dong G., Zhou, S., Zhang, Q., Yang, Z., Qiu, J., ACS Appl. Mater. Interfaces 5 15 7527 (2013)]. However, the present invention describes a new photocatalyst that integrates two composite materials covalently linked to each other. Thus, the covalent bonding of TiO2 to the glass wool and to the SiO2 spheres prevents the leaching of TiO2, and the covalent bonding of the submicron SiO2-TiO2 spheres to the glass wool-TiO2 support prevents the release of the spheres into the medium, thus as the photoactive area of the final material increases.

Furthermore, the present invention refers to the use of the catalyst of the present invention and obtained according to the production procedure described above in continuous flow or batch type photoreactors, in processes for the partial or total oxidation of any type of organic matter present in fluids.

According to a particular embodiment, the photocatalyst of the present invention is suitable for the treatment of water contaminated by organic compounds. The micrometric SiO2-TiO2 photocatalyst, in the presence of H2O, oxygen and UV light, oxidizes the organic matter present in the water until reaching total oxidation (complete mineralization) thanks to its great capacity to form hydroxyl radicals ('OH), species characteristic of advanced oxidation processes.

According to another particular embodiment, the photocatalyst of the present invention is suitable for the treatment of volatile organic compounds (VOCs) such as toluene or xylenes, emitted when solvents are used in industrial environments.

According to another particular embodiment, the tests of the photocatalyst in a continuous flow cylindrical photoreactor made it possible to demonstrate its great photooxidant capacity since they achieved the total mineralization of the phenol dissolved in water (initial concentration: 10-5 M), a model organic contaminant, in just 3.5 minutes, residence time of the liquid phase in the continuous flow cylindrical photoreactor.

Throughout the description and claims the word "comprises" and its variants are not intended to exclude other technical characteristics, additives, components or steps. For those skilled in the art, other objects, advantages and features of the invention will emerge partly from the description and partly from the practice of the invention.

Brief description of the figures:

Figure 1. Schematic representation (not to scale) of the new photocatalyst in which each of the parts are specified: a) SiO2 core; (b) TiO2 shell of the core that together form the SiO2-TiO2 core@shell spheres; c) TiO2 layer that covers the micrometric glass wool support and (d) micrometric glass wool support.

Figure 2. TEM images of submicron SiO2(A) spheres and submicron core@shell SiO2-TiO2(B) spheres. Right column: Corresponding size distributions.

Figure 3. FESEM images of submicron SÍO2(A) spheres and submicron core@shell spheres of SiO2-TiO2(B).

Figure 4. X-ray diffractogram (A) and Raman spectrum (B) of the SiO2-TiO2 submicron core@shell spheres.

Figure 5. Focused ion beam microscopy (FIB Ga+) images of glass wool (GW) (A), of the surface of GW coated with TiO2(GW-TiO2) (B), of a trench made in the surface of the GW-TiO2(C) material and the micrometric SiO2-TiO2(D) photocatalyst.

Figure 6. Diffuse reflectance spectrum of the SiO2-TiO2 core@shell spheres (solid trace) and the photocatalyst (dashed trace), where F(R) is the Kubelka-Munk function.

Figure 7. Absorption spectra of MB at different irradiation times using a Luzchem photoreactor with UVA lamps (Amax ca. 352 nm) starting from a solution of MB (Co = 1.5 x 10<-5>M) and the presence of the photocatalyst (17.5 mg mL-1).

Figure 8. TOC monitoring of the total elimination of the organic load generated by phenol in water (10<-5>M) versus reaction time using the photocatalyst of example 1 in a continuous flow photoreactor.

DESCRIPTION OF IMPLEMENTATION MODES

Below, some embodiments of the invention are described corresponding to the preparation of a SiO2-TiO2 photocatalyst with its characterization and its use in the treatment of fluids containing model compounds of organic contaminants.

Example 1: Preparation of a micrometric SÍO2-TÍO2 composite photocatalyst

60 mL of tetraethylorthosilicate (TEOS) are added to a solution of 250 mL of 28% NH<4>OH in 1500 mL of ethanol at 0°C and stirred for 2 hours. The reaction medium is then tempered and the mixture is stirred at room temperature for an additional 24 hours. The reaction mixture is then centrifuged, washed with ethanol and dried under vacuum. 10 g of the isolated material (SiO2 spheres) are suspended in 900 mL of ethanol until complete dispersion by sonication, and heated to 75°C. Once this temperature is reached, 10 mL of a solution of TTIP stabilized in 80 mL of 2-propanol are added (drop by drop) to the reaction medium. Later, a flow of humid air (1.5 L min-1) is circulated for 20 hours under vigorous agitation. The final suspension is then centrifuged at 4000 rpm for 20 minutes, washed with ethanol and water, and dried under vacuum. Finally, the resulting material is calcined for 2 hours at 500°C to obtain the core@shell SiO2-TiO2 microspheres with a BET area of 27.3 m2g-1, loading of 19.72% (w/w) of TiO2 and crystalline phase. photoactive anatase.

In parallel, 30 g of glass wool are washed with 1000 mL of 6 M HCl for 12 hours. Afterwards, it is washed consecutively with water until the washing waters present pH 6. It is then treated with 500 mL of 1 M NH<4>OH, followed by ethanol (750 mL), and dried at 100°C for 24 hours. 25 g of the glass wool, with a BET area of 0.02 m2g-1, are suspended in 1000 mL of ethanol and heated to 75 °C. Next, 0.6 mL of TTIP stabilized in 16 mL of 2-propanol is added to the reaction medium and stirred for 2 hours. Later, a flow of humid air (1.5 L min-1) is circulated for 20 hours to obtain the glass wool-TiO2 material. Next, a suspension of 1.2 g of SiO2-TiO2 core@shell spheres in 500 mL of ethanol is sonicated for 1 hour and added to the glass wool-TiO2 reaction mixture. Next, a flow of humid air (1.5 L min-1) is circulated for an additional 20 hours. Finally, the final material is filtered, washed with H2O and ethanol, dried at 100°C for 8 hours and calcined at 500°C for 2 hours. The isolated material is subjected to a second TiO2 coating and decoration of SiO2-TiO2 core@shell spheres with the same procedure to finally obtain the micrometric SiO2-TiO2 composite photocatalyst with a BET area of 0.9 m2g-1.

Example 2: Batch photodegradation of methylene blue using the photocatalyst of example 1.

The photocatalytic activity of the invention (micrometric SiO2-TiO2 composite photocatalyst) was evaluated in the photodegradation of a model polluting organic compound present in industrial wastewater, methylene blue (MB). MB presents a broad absorption band (550-700 nm) in the visible spectrum, so its degradation was monitored by UV-vis spectroscopy following the disappearance of this absorption band. In this way, a solution of 4 mL of MB in Milli-Q water (initial concentration: 1.5x10<-5>M) was introduced into a borosilicate glass tube with an internal diameter of 1.5 cm. Next, 70 mg of the micrometric SiO2-TiO2 composite photocatalyst were introduced into the container and the reaction mixture was kept in the dark for 10 minutes. The reaction medium was then irradiated with 8 W halogen lamps emitting light centered at 350 nm for 2 hours. Figure 7 shows the disappearance of the MB absorption band over time during irradiation in the presence of the micrometric SiO2-TiO2 composite photocatalyst of example 1.

Example 3: Total elimination of the organic load generated by phenol in water using the SiO2-TiÜ2 photocatalyst of example 1 in a continuous flow photoreactor.

The mineralization capacity of the invention was evaluated by photodegradation of phenol, a carcinogenic organic compound present in wastewater.

First, 7.52 g of the micrometric SiO2-TiO2 composite photocatalyst was introduced into a 35 cm3 glass tubular photoreactor. Milli-Q water was then continuously circulated with a flow rate of 10 mL min-1 through the photoreactor for 45 minutes in the dark, until reaching steady state. Next, a 10-5 M phenol solution (720 ppb C) prepared in Milli-Q water was circulated through the water treatment system with a flow rate of 10 mL min-1, for 45 minutes in darkness. Next, maintaining the flow of 10-5 M phenol solution with a flow rate of 10 mL min-1, the LEDs, the lights with an emission band centered at 365 nm of the photoreactor, were turned on for 1 hour. The photoreaction between the micrometric SiO2-TiO2 photocatalyst and the phenol was continuously monitored by measuring the total organic carbon using the total organic carbon (TOC) system coupled to the continuous photoreactor. The water flow leaving the photoreactor was diluted with a 1:3 ratio before reaching the total organic carbon (TOC) detector. Thus, Figure 8 shows the evolution of the reaction. It can be seen that there is a TOC response delay time of about 30 min which is observed both when adding phenol to the water flow and when turning on the LED lights. Thus, taking this into account, it can be clearly observed that there is a complete mineralization of the phenol in the water at continuous flow, that is, all the organic matter that the contaminant has in the water is completely eliminated when passing through the illuminated area of the photoreactor. It should be noted that, under these reaction conditions, the residence time of the 10-5 M phenol solution in the illuminated zone of the photoreactor was only 3.5 minutes.

Claims (16)

1. Photocatalyst for partial or total oxidation of any organic matter contained in fluids, characterized in that it is a material composed of SiO2-TiO2 core@shell spheres covalently linked to a layer of TiO2(c) that covers a glass wool support (d ) and where the TiO2 shell (b) of the spheres and the TiO2 layer (c) of the glass wool are in the photoactive phase. 2. Photocatalyst according to claim 1, characterized in that the core@shell type spheres of SiO2-TiO2 are spheres where the core (a) is SiO2 and is covered by a shell (b) of TiO2. 3. Photocatalyst according to claim 2, characterized in that the TiO2 shell (b) of the SiO2-TiO2 core@shell type spheres has a thickness selected from <8>at 50 nm. 4. Photocatalyst according to any of the preceding claims, characterized in that the SiO2 core (a) of the SiO2-TiO2 core@shell type spheres have a diameter size selected between 1<0 0> and 1<0 0 0>nm. 5. Photocatalyst according to any of the preceding claims, characterized in that in the SiO2-TiO2 core@shell type spheres, the SiO2 core (a) and the TiO2 shell (b) are covalently linked. <6>. Photocatalyst according to claim 1, characterized in that the layer (c) of TiO2 that covers the glass fiber support (d) is covalently linked to it. 7. Photocatalyst according to claim <6>, characterized in that the layer (c) of TiO2 that covers the glass fiber support (d) has a thickness selected between <8> and 50 nm. <8>. Photocatalyst according to any of claims <6>and 7, characterized in that the glass fiber (d) has a diameter selected between 1 to 50 microns. 9. Photocatalyst according to any of the preceding claims, characterized in that the SiO2-TiO2 core@shell type spheres are covalently joined to the TiO2 layer (c). 10. Procedure for obtaining the photocatalyst according to claim 9, characterized in that it comprises at least the following steps: stage 1: preparation of SiO2-TiO2 core@shell type spheres, stage 2: preparation of the glass wool-TiO2 support, stage 3: the products obtained in stage 1 and in stage 2 to obtain the photocatalyst. 11. Procedure for obtaining the photocatalyst according to claim 10, characterized in that stage 1 comprises, at least, the following steps: a) add a silicon dioxide precursor to a cold hydroalcoholic solution of NH4OH and stir, b) stir at room temperature for at least 1 h, isolate, wash and dry, c) slowly add the TiO2 precursor to an alcoholic suspension of the product isolated in “b” at a temperature below the boiling point and stir, d. pass a stream of diluted water through the mixture obtained. 12. Procedure for obtaining the photocatalyst according to claim 11, characterized in that stage 1 also comprises isolating the spheres, washing them and calcining them. 13. Procedure for obtaining the photocatalyst according to claim 10, characterized in that stage 2 comprises, at least, the following steps: a) subject the glass wool support (d) to an acid pretreatment followed by washing and subsequent basic treatment and drying, b) to an alcoholic mixture of the product obtained in a) the TiO2 precursor is added and heated below the boiling point while stirring the mixture, c) a stream of diluted water is passed through the mixture obtained in b) . 14. Procedure for obtaining the photocatalyst according to claim 13, characterized in that stage 2 further comprises isolating the product obtained, washing it and calcining it. 15. Procedure for obtaining the photocatalyst according to any of claims 10 to 14, characterized in that stage 3 comprises, at least, the following steps: a) mix the SiO2-TiO2 core@shell type spheres obtained in stage 1 with the product obtained in stage 2, heat below the boiling point and stir. b) a stream of diluted water is passed through the mixture obtained in a). c) isolate and calcine the product obtained. 16. Use of the photocatalyst according to claims 1 to 9 obtained according to the procedure described in claims 10 to 15 in continuous flow or batch type photoreactors, in processes to partially or totally oxidize any organic matter present in fluids.