ITSA20130011A1 - PHOTOCATALYTIC POLYMERIC AIRPLANES AND PROCESSES FOR THEIR ACHIEVEMENT. - Google Patents

Description

DESCRIPTION

accompanying a patent application for an industrial invention entitled:

PHOTOCATALYTIC POLYMER AIRGROUNDS AND PROCESSES FOR THEIR OBTAINMENT

DETAILED DESCRIPTION

The invention presented concerns polymeric airgels containing micrometric and nanometric particles with photocatalytic properties, homogeneously distributed. The preparation of these airgels allows to effectively disperse the photocatalytic particles, avoiding their aggregation and thus maximizing their photoactive surface.

By photocatalysis we mean the acceleration of a chemical reaction activated by light in the presence of a catalyst. Typically the process involves a solid semiconductor. The absorption of a quantity of light energy greater than or equal to that corresponding to the band jump of the semiconductor E), gives rise to a transfer of electrons from the valence band to the conduction band and to the consequent generation of electronic holes (h <+ >) in the valence band. The electron-hole pair can react with an acceptor or donor species of electrons adsorbed on the surface of the semiconductor giving rise to reduction or oxidation reactions respectively. These processes are very aggressive and can lead to the destruction of bacteria or viruses or oxidize organic molecules forming C02 and H20. In the last decade, for example, the use of titanium dioxide as a photocatalyst has become the focal point of numerous studies due to its use in the treatment of pollutants in both liquid and gaseous streams.

The most widely used semiconductor catalyst in photoinduced processes is titanium dioxide (Ti02) or titania in anatase form. The use of solar radiation significantly reduces the energy costs related to photocatalytic processes, but the high band-gap of titanium oxide (3.2 eV for anatase) nevertheless restricts the fraction of solar radiation capable of cause the formation of the electron-hole pair. Only 4-5% of sunlight that reaches the earth's surface has a wavelength in the 300-400 nm range (near UV); consequently, since titania can only be activated through UV radiation, this natural source is generally not sufficient. For this reason, there are numerous studies on this catalyst that aim to modify its structural and compositional characteristics (through various types of doping and synthesis) in order to minimize this intrinsic problem of titanium dioxide. Through doping with transition metals or with eternatomes, the titania absorption threshold shifts from the ultraviolet to the visible region.

In order to achieve high photocatalytic activities, in addition to the formulation of the catalyst, an adequate reactor configuration is also required. In fact, when a photocatalytic reaction is carried out, optimal irradiation of the photocatalyst and good contact between catalyst and reactants are required. For this purpose, different reactors have been used (T. Van Gerven, G. Mul, J.

Moulijn, A. Stankiewicz, Chem. Eng. Process, 46 (2007) 78J); among the most cited are slurry reactors (liquid-solid-gas) with complete mixing, which entail a high probability of collision between the scattered photons and the photocatalyst particles dispersed in the liquid phase. However, these reactors have the disadvantage of having to provide for expensive processes for separating the catalyst from the purified liquid phase. Furthermore, the recirculating pumps typically present in such reactor configurations are subject to high wear when suspensions containing nanoparticles circulate in the liquid phase.

A proposed solution for this technical problem is to realize heterogeneous fixed bed reactors in which the photocatalyst is supported in granular or thin film form on both organic materials (P. Lei, F. Wanga, X. Gaoa, Y. Dinga, S. Zhanga, J. Zhàob, S. Liuc, M. Yanga, Journal of Hazardous Materials 227-228 (2012) 185; S. Singh, H. Mahalingam, P. Kumar Singh, Applied Catalysis A: General 462- 463 ( 2013) 178) and inorganic. In this case, however, phenomena of aggregation between the catalyst particles are induced which are reflected in a worsening of the photocatalytic properties (O. Sacco, M. Stoller, V. Vaiano, P. Ciambelli, A. Chianese, D. Sennino, International Journal of Photoenergy (2012) Volume 2012, Artide ID 626759, 8 pages doi: 10.1 155/2012/626759; L. Rizzo, D. Satinino, V. Vaiano, O. Sacco, A. Scarpa, D. Pietro giacomi, Applied Catalysis B: Environmental 144 (2014) 369). It is therefore essential to fix the catalyst on a support and at the same time minimize the aggregation phenomena between the particles of the semiconductor and thus keep the surface area of the photocatalyst high. A possible technical solution to the problem consists in the production of silica-based monolithic inorganic aerogels, of the type SÌO2-TÌO2 or Si02-W03. (Cao, S .; Yao, N .; Yeung, K.L. Journal of Sol-Gel Science & Technology 2008 , 46, 322-323. Liu, J; Bai, L .; Shi, F. CN102876147 A20130116, 2013). However, these inorganic aerogels have the drawback of a limited VOC absorption capacity and, consequently, reduced photo-catalytic activities.

Well known and very handy are polymeric matrix composites that include photo catalytic nano and micro particles (a review of the work done in the field is reported by Singh, SD .; Mahalingam, H .; Singh, P.K. Applied Catalysis A 2013, 462-463 , 178-195). However these <•> nanocomposites have the disadvantage of diffusivity of the reactant molecules (VOC) much lower than those observed in the slurry, which obviously limits the kinetics of the photocatalytic reactions. The diffusivity of the reacting molecules can be increased by using polymeric foams as a matrix for the photocatalyst particles (F. Magalhaes, R.M. Lago Solar Energy 2009, 83, 1521-1526; F. Magalhaes, F.C.C. Moura, R.M. Lago T1O2 / LDPE composites: A new floating photocatalyst for solar degradation of organic contaminants Desalination 2011, 276, 266-271). The use of polymeric foams also has the advantage of arriving at floating artifacts, useful for photocatalysis processes both for the high efficiency of use of light and for the ease of post-treatment recovery. However, these foams have the disadvantage of low surface areas, typically less than 5 m <2> / g, and therefore low capacity and slow VOC absorption kinetics.

A further technical solution proposed is to support the photo catalyst on carbon airgels with a high surface area, obtained by pyrolysis of polymeric airgels with chemical crosslinking based on benzoaxazine (Somiok, S .; Chaisuwan, T .; Wongkasemjit, S. PMSE Preprints 2009, 100, 215-216). However, these carbon aerogels have the disadvantage of being black and therefore opaque in the visible region, thus preventing an efficient use of solar radiation. The polymeric airgels which include nano and micro photocatalytic particles, object of the present invention, have the high manageability of the composites combined with the high diffusivity of the reactant molecules.

The polymeric airgels object of the present invention consist of transparent polymers in the visible spectral region, with absorbance lower than 0.1, contain an amount of photocatalytic nano or micro particles between 0.1 and 50% by weight, are characterized by an apparent density between 0.7 and 0.005 g / cm <3>, preferably between 0.3 and 0.01 g / cm <3> and have a surface area greater than 5 m <2> / g, preferably greater than 100 m <2> / g.

The airgels of the present invention are obtained starting from corresponding gels with solvents in which the catalytic particles are dispersed, by means of one of the processes known for the transformation of gels into airgels, such as extractions with carbon dioxide, liquid or in supercritical conditions or freeze-drying (" freeze-drying ").

Polymeric gels usable for the present invention are both chemical and physical gels and are characterized by a polymer concentration in the solvent comprised between 70% and 0.5% by weight, and preferably between 30% and T 1% by weight and a content of photo-catalytic particles between 0.1% and 50% by weight, with respect to the weight of the polymer. Such polymeric gels can be obtained with one of the traditional techniques for the production of chemical gels, such as sol-gel technologies, or for the production of physical gels, which involve the dissolution of crystallizable polymers in a suitable solvent, followed by sudden cooling.

Preferred formulations of the present invention include aerogels based on semi-crystalline polymers which are capable of forming physical gels and which are transparent not only in the visible but also in the ultraviolet region, such as for example polyethylene, isotactic polypropylene or polylactic acid.

Particularly favored formulations of the present invention provide for the use of polymeric airgels in which the crystalline phases constituting the physical nodes are nanoporous crystalline phases, i.e. phases whose density is lower than that of the corresponding amorphous phases, such as the crystalline forms δ and ε of syndiotactic polystyrene (s-PS) or the crystalline modifications of the poly (2,6-dimethyl-1,4-phenylene) oxide (PPO), and in which the degree of crystallinity is greater than 5% and preferably greater than 10%.

By syndiotactic polystyrene (sPS) we mean the polymer in which the syndiotactic structure is present for at least long stretches of the chain, in such a way as to allow crystallization in the nanoporous forms. This polymer is obtained, for example according to the method described in the European patent application No. 0271875- Himont Italy. Also included in the definition are copolymers of styrene, with a predominantly syndiotactic microstructure and crystallizable in nanoporous crystalline forms, with olefins CH2 <=> CH-R, in which R is an alkyl-aryl or a substituted aryl containing 6-20 carbon atoms , or with other copolymerizable ethylenically unsaturated monomers. The nanoporous crystalline form δ of the sPS, according to what is described in the Italian patent application No. RM94A000030, is characterized by an X-ray diffraction spectrum (CuKa) presenting reflections of greater intensity at 20cUK approximately equal to 8.4 °, 10.6 °, 13.3 °, 16.8 °, 20.7 °, 23.5 ° and with the ratio between the intensities of the two peaks I (8.4 °) / I (10.6 °) higher than 5. The form <:> a nanoporous crystalline ε of the SPS, according to what described in the Italian patent application N.SA2006A000022 and in the international patent PCT / IB2007 / 053332, is characterized by an X-ray diffraction spectrum presenting reflections of greater intensity at 20 (CuKa) approximately equal to 6.9 °, 8.2 °, 13.8 ° , 16.4 °, 20.5 °, 23 °.

Poly (2,6-dimethyl-1,4-phenylene) oxide (PPO) is a commercial thermoplastic polymer that has an amorphous phase characterized by a high free volume, also called ultra-impermeable, widely used as a material for membranes with high parameters of permeation, which is also capable of crystallizing in different crystalline modifications. The nanoporous crystalline modifications of PPO are described in detail in the literature, and are characterized by X-ray diffraction spectra presenting reflections of greater intensity at 2Θ (CuKa) listed for example in table 1 of the work Daniel, .C .; Longo, S .; Fasano, G .; Vitillo, J.G .; Guerra, G. Nanoporous Crystalline Phases of Poly (2,6-Dimethyl-l, 4-phenylene) oxide. Chemistry of Materials (2011), 23, 3195-3200.

The degree of crystallinity of the polymeric substrate is expressed in percent by weight of the crystalline phase and can be easily evaluated from the X-ray diffraction profiles, using the classical method of Hermans and Weidinger. According to this method, the degree of crystallinity is given by the ratio between the areas of the peaks due to the diffraction of the crystalline phase and the area of the total diffraction.

Airgels with nanoporous crystalline phases based on sPS and PPO, are obtained, preferably, by processes that include extraction with carbon dioxide,

supercritical gels based on a solvent which is a suitable "guest" of a co-crystalline form of the polymer ("host") and which contain the polymer in a concentration between 70% and 0.5% by weight, and preferably between 30% and Γ 1% by weight. These gels can be obtained with one of the traditional techniques for the production of physical gels, which involve the dissolution of the polymer in a suitable solvent and the dispersion of the photo-catalytic particles, followed by sudden cooling.

A typical process for the preparation of photocatalytic airgels, which constitutes a further object of the present invention, comprises the following stages:

A) dispersion of photo-catalytic micro or nano particles in a solvent, which is able to solubilize the polymer indicated in stage B, using an amount between 0.1% and 50% by weight, at a temperature between room temperature and 100 ° C, keeping the dispersion under constant stirring, for a time between 1 min and 10 h.

B) the polymer is dissolved in the dispersion prepared in A, in such a quantity that the polymer / photo catalyst weight ratio is included in the range 50/50 - 99.9 / 0.1 and that the solvent is not less than 30% by weight of the dispersion .

C) The mixture thus obtained is subjected to sonication with a vibration frequency greater than 10 Hz for a time greater than 1 min, at temperatures between room temperature and 100 ° C.

D) the dispersion is cooled to temperatures ranging from room temperature to -80 ° C and a uniform gel is obtained

E) the gel is extracted with supercritical liquid carbon dioxide p at pressures between 50 and 350 bar and at temperatures between 20 ° C and 70 ° C and preferably between 25 ° C and 60 ° C.

The advantage of using aerogels with nanoporous crystalline phases, as described for example in the patent "Microporous and nanoporous artifacts based on syndiotactic polystyrene and processes for their obtaining" (Brev.lt N.SA2003A13, WO 2005012402, EP1646687, US2006 / 0229373A1) , consists in the ability to absorb many of the VOCs, such as, for example, halogenated compounds (chloroform, methylene chloride, carbon tetrachloride, dichloroethane, trichlorethylene, tetrachlorethylene, dibromoethane, methyl iodide, etc.) aromatic compounds (benzene, toluene, styrene, etc.) cyclic compounds (cyclohexane, tetrahydrofuran, etc.). This absorption occurs very efficiently even when the concentration of VOC in the chemical environment is very low (of the order of a few ppm or ppb). This feature makes it possible to reach concentrations of halogenated, cyclic or aromatic VOCs in sPS that are many orders of magnitude higher (from 10 to 100,000) than those of the surrounding chemical environment. The good dispersion of the photo catalyst in the airgel and the high abilities and kinetics of VOC absorption by the nano-porous crystalline phases, lead to improved catalytic performance;

The photocatalytic nano and micro particles dispersed in the airgels can consist of titanium dioxide and titanium dioxide doped with metals such as Fe, W, Cr, Ru, V, Mo, Nb, Au, Pt, Pd, Ag, Sb, Co, Mn, Ni, Zn, Zr or with eternatoms such as fluorine, sulfur, carbon, phosphorus, boron, iodine, chlorine, bromine, nitrogen.

Preferred formulations include particles of titania, in anatase crystalline form, also doped with eternatomes, preferably nitrogen, prepared upstream of the dispersion phase in the polymeric solution.

Particularly favored formulations include nitrogen-doped titanium dioxide photocatalyst particles (NDT) which is characterized by a molar ratio N / Ti in the range 4-25, and more preferably 12-20.

Samples of titania doped with eternatoms and metals are for example prepared by the sol-gel method using precursor salts (preferably organic) of titania and aqueous solutions containing precursors of heteroatoms and metals.

Other photocatalysts that can be dispersed in the polymeric matrix are different types of semiconductors such as ZnO, WO3, SrTi03, Fe203, ZnS.

The photocatalytic nano and micro particles can also consist of semiconductors such as ZnO, WO3, SrTi03, Fe203, ZnS, titanium dioxide preferably in anatase or doped titanium dioxide form, supported on phosphorescent particles (phosphors), in order to maximize the exploitation of the light radiation from the supported photocatalyst.

The main advantage of the new manufactured articles is that they have comparable and / or improved photocatalic properties with respect to those of photocatalytic fine powders and, at the same time, better handling. In fact, the preparation of polymeric airgels allows to obtain macroscopic photocatalytic products, for example in the form of granules, or monolithic filters and membranes, or in any case in the macroscopic form most suitable for specific application requirements. The use of macroscopic airgels has the additional advantage of greatly reducing the risks of contamination or inhalation of nano and micro particles.

Another advantage of using photocatalytic polymeric airgels, of the present invention, is to avoid the expensive processes of separating the catalyst from the purified liquid phase, such as sedimentation and filtration units. In general, therefore, it is possible to avoid the use of recirculating pumps which would be subject to high wear when nanoparticulate suspensions circulate in the liquid phase.

A further advantage of using photocatalytic polymeric aeogels is to exploit their buoyancy. In this way they can use solar radiation directly, minimizing the phenomena of attenuation of the radiation due to the liquid matrix containing the contaminants. Furthermore, the buoyancy of the photocatalytic polymeric airgels eliminates possible problems of 02 transfer from the gas phase.

Some illustrative and non-limiting examples of the invention are described below.

EXAMPLES

Example 1

It is used as a photocatalyst of nitrogen-doped titanium dioxide (NDT), prepared by carrying out the hydrolysis reaction between titanium tetraisopropoxide and an aqueous solution containing ammonia. The preparation is carried out at a temperature of 0 ° C to limit the heating of the system caused by the exothermicity of the hydrolysis reaction. The system is then kept stirred until a gel is formed. The mixture obtained is centrifuged and washed with double distilled water and then placed in a muffle at 450 ° C for 30 minutes. The molar ratio N / Ti is included in the range 12-20. The NDT samples thus prepared were characterized by using different analytical techniques whose results are reported in 0. Sacco, M. Stoller, V. Vaiano, P. Ciambelli, A. Chianese, D. Sannino, International Journal of Photoenergy (2012) Volume 2012, Artide ID 626759, 8 pages doi: l 0.1155 / 2012/626759 and in L. Rizzo, D. Sannino, V. Vaiano, O. Sacco, A. Scarpa, D. Pietrogiacomi, Applied Catalysis B : Environmental 144 (2014) 369. In summary, the micro-Raman and X-ray diffraction analysis showed the presence of titanium dioxide signals in anatase form. In addition, the NDT samples have an average crystallite size in the 10-25 nm range. The diffuse reflectance spectra, on the other hand, highlight the absorption capacity of light radiation in the visible region as demonstrated by the band gap energy values which are in the 2-2.5 eV range.

For the preparation of photocatalytic airgels, syndiotactic polystyrene (sPS) produced by DOW Chemical under the brand name Questra 101 is used.

Monolithic airgels in the form of cylinders with a diameter of 5 mm have been obtained by means of processes which include stages A-E reported in the "detailed description" of this patent, in particular:

A) an NDT powder is dispersed in chloroform, using an amount by weight equal to 0.5%, at a temperature of 100 ° C and the dispersion is kept under constant stirring for 5 hours.

B) syndiotactic polystyrene powder is dissolved in the dispersion prepared in A, in a quantity such that the polymer / photocatalyst weight ratio is equal to 95/5

C) the mixture thus obtained is subjected to sonication at the vibration frequency of 35 Hz for three hours, at a temperature of 80 ° C and subsequently inserted into a cylindrical glass mold with a diameter of 5 mm.

D) the dispersion is cooled in an ice-water bath and a uniform gel is obtained E) the gel removed from the cylindrical mold is extracted with supercritical carbon dioxide at a temperature of 40 ° C and a pressure of 200 bar for 5 hours, removing completely the chloroform and obtaining monolithic parerogel.

The X-ray diffraction spectrum of the obtained product, shown in Figure 1, clearly shows the reflection 101 of the anatase crystalline form of titanium dioxide, that is the crystalline form which has photo-catalytic properties, clearly visible at 2θ ~ 25 °.

Figure 1. Comparison between the diffraction spectra of nitrogen-doped titanium dioxide (NDT), in the crystalline form of anatase, and of a photocatalytic airgel based on syndiotactic polystyrene (sPS) and NDT obtained with the procedure reported in example 1 .

The photolytic airfrost thus obtained has a density of 0.1 g / cm <3> and a surface area, measured with the BET method, of 222 m <2> / g.

A comparison between the activity of the NDT photocatalyst dispersed in solution and dispersed in the sPS-based polymeric airgel on the discoloration of methylene blue is shown in figure 2. The experiments were conducted with an initial concentration of methylene blue equal to 7.5 ppm in air flow equal to 150 Ncc / min at ambient temperature and pressure. The light source used consists of a LED strip with emission in the visible region (luminous flux incident on the reactor walls: 32 mW / cm <2>). The test was carried out with NDT dispersed in solution in an amount equal to 0.13g / L leading to a conversion of 16% after 180 minutes of illumination.

In the case of NDT-sPS (present in a quantity equal to 2.93 g / L with an NDT content equal to 5% by weight corresponding to an equivalent dosage of NDT equal to 0.13 g / L) the effects of the presence of the doped photocatalyst are evident. , which promotes the discoloration activity of the methylene blue solution up to a conversion of 30% after 180 min of illumination.

Figure 2. Comparison between the activity of the NDT photocatalyst dispersed in solution and the photocatalyst dispersed in the sPS-based polymeric airgel.

Example 2

An s-PS-based airgel was prepared with the same procedure described in example 1 but without the addition of photocatalytic particles. The performance of this airfreeze was compared with that of the photocatalytic airfreeze, which includes NDT particles, of example 1.

The performance of the methylene blue discoloration of the airgel consisting of the polymer only and of the airgel consisting of polymer and photo catalyst are compared in Figure 2. The experiments were conducted with an initial concentration of methylene blue equal to 7.5 ppm in flow d 'air equal to 150 Ncc / min at ambient temperature and pressure. The light source used is the one used in example 1.

In the first analysis, the degree of adsorption in the absence of light radiation was evaluated by monitoring the degree of discoloration for 2h and, as shown in figure 2, the substrate alone has an initial degree of adsorption equal to 12%. The light source was then turned on. The presence of light radiation leads to a partial and low discoloration of the solution, and is attributable only to the photolysis effect. On the other hand, the description is different in the case of the NDT-sPS product in which there is a first adsorption phase in the absence of light and a second phase in which the effects of the presence of the doped photocatalyst which promotes the decolorization activity of the solution are evident. methylene blue up to a conversion of 39% after 270 min of irradiation. This clearly indicates that the photocatalytic activity is only promoted by the presence of the NDT catalyst dispersed in the polymer matrix.

Figure 3. Comparison between the activity of only the sPS polymeric airfrost and the photocatalyst dispersed in the sPS polymeric substrate (NDT-sPS).

Example 3

Photocatalytic airgels were prepared with the same procedure described in example 1. The photo-catalytic performance of these s-PS-NDT airgels were studied through the discoloration of methylene blue in initial concentrations equal to 7.5 and 15 ppm (Figure 3) .

The experiments were conducted under the same conditions described in example 2.

Increasing the initial concentration of methylene blue increases the degree of adsorption as shown in figure 3, which is equal to 12% in the case of an initial concentration of 7.5ppm and 39% in the case of operating with an initial concentration of 15ppm. After the first phase carried out in the absence of light radiation, the light source was switched on which led to a conversion equal to 39% after 270 minutes of lighting in the case in which the initial concentration of 7.5ppm and the 56% after 290 minutes in the case of operating with an initial concentration equal to 15ppm.

Figure 4. Comparison between Γ activity of the NDT-sPS sample operating with different initial concentrations of methylene blue.

Example 4: Evaluation of the stability of the NDT-sPS system

The stability of the NDT-sPS system was studied in the visible light generated by a LED strip with emission in the visible region (luminous flux: 32 mW / cm <2>) by subjecting the sample to a cycle of 500 hours of illumination and monitoring the weight variation of the sample over time. No variations were recorded, indicating that the polymeric support has a high stability in the presence of the NDT photocatalyst dispersed therein.

The activity of the NDT-sPS system was also analyzed for 5 cycles of photo-oxidation of methylene blue with the same sample under the same experimental conditions reported in the previous examples. It was noted that the NDT-sPS artifact shows in all five cycles, a photocatalytic activity that differs shortly after 180 minutes of illumination as shown in Figure 5.

Figure 5. Comparison between the activities of the NDT-sPS photocatalytic airgel for 5 cycles of photooxidation

Example 5

Syndiotactic polystyrene (sPS) produced by DOW Chemical under the brand name Questra 101 is used. It is used as a photocatalyst of titanium dioxide P25 produced by Sigma Aldrich in the form of nano powder with particles of 21 nm.

Monolithic aerogels with porosity equal to 90%, in the form of cylinders with a diameter of 5 mm, were obtained starting from dispersions containing 90% by weight of solvent (chloroform) by means of the following steps:

A) a TiO2 powder is dispersed in chloroform, at a temperature of 100 <0> C and the dispersion is kept under constant stirring for 5 hours.

B) powdered syndiotactic polystyrene is dissolved in the dispersion prepared in A, in a quantity such that the polymer / photocatalyst weight ratio is equal to 95/5 or 90/10, or 99/1.

C) the mixtures thus obtained are subjected to sonication (vibration frequency 35KHz) for three hours, at a temperature of 80 ° C and subsequently inserted into cylindrical glass molds.

D) the dispersions are cooled in an ice-water bath and uniform gels are obtained. E) the gels removed from the glass cylinder are extracted with supercritical carbon dioxide at 40 ° C and 2900 psi for 5 hours, completely removing the chloroform, obtaining monolithic aerogels.

Figure 6. X-ray diffraction profiles of Ti02P25 as is and of the three prepared airgels

Figure 6 shows the X-ray diffraction profiles of Ti02P25 as it is and of the three aerogels

get ready. The maintenance of the anatase crystalline form is evident, as evidenced by the

presence of the corresponding peak 101. The density of the three aerogels is approximately equal to 0.1 g / cm <3>.

Example 6

Syndiotactic polystyrene (sPS) produced by DOW Chemical under the brand name Questra 101 and poly (2,6-dimethyl-1,4-phenylene) oxide (PPO) produced by Sigma Aldrich is used. The NDT photo catalyst prepared as described in example 1 is used as a nano load.

Monolithic aerogels with porosity equal to 90%, in the form of cylinders with a diameter of 5 mm, were obtained starting from dispersions containing 90% by weight of solvent (methyl benzoate) by means of the following steps:

A) a powder of NDT is dispersed in methyl benzoate, at a temperature of 100 <0> C and the dispersion is kept under constant stirring for 5 hours.

B) in the dispersion prepared in A, s-PS and PPO in powder are dissolved in a weight ratio equal to 10/90, in a quantity such that the polymer / photocatalyst weight ratio is equal to 95/5.

C) the mixture thus obtained is subjected to sonication (vibration frequency 35KHz) for three hours, at a temperature of 80 ° C and subsequently inserted into a cylindrical glass mold. D) the dispersion is cooled in an ice-water bath and a uniform gel is obtained E) the gel removed from the glass cylinder is extracted with supercritical carbon dioxide at 40 ° C and 2900 psi for 5 hours, completely removing the benzoate of methyl and obtaining the monolithic airgel.

Figure 7. X-ray diffraction profiles of the NDT photocatalyst and monolithic airgel based on PPO and containing sPS and NDT

Figure 7 shows the X-ray diffraction profiles of the NDT photocatalyst and the monolithic airgel based on PPO and containing sPS and NDT. The maintenance of the anatase crystalline form is evident, as evidenced by the presence of the corresponding peak 101. The diffraction figure also shows that the PPO is in the limit nanoporous crystalline modification with values of 2Θ higher observed, as reported in table 1 of the publication. Daniel, C .; Longo, S .; Vitillo, J.G .; Fasano, G .; Guerra, G. Chem. Mater. 2011, 23, 3195. The density of the airgel is approximately equal to 0.1 g / cm <3>.

Claims (14)

Claims 1. Airgels based on polymers with absorbance lower than 0.1 in the visible spectral region, characterized by homogeneous distribution of micrometric or nanometric particles with photòcatalytic properties in quantities between 0.1% and 50% by weight, such aerogels. having bulk density between 0.7 and 0.005 g / cm <3>, preferably between 0.3 and 0.01 g / cm <3>, and with a surface area greater than 5 m <2> / g, preferably greater than 100 m <2> / g. 2. Airgels, according to claim 1, based on polymers with absorbance lower than 0.1 in the visible and ultraviolet spectral regions, consisting of semi-crystalline polymers that are able to give rise to the formation of physical gels. 3. Airgels according to claims 1 and 2, based on polyethylene, preferably of high density. 4. Airgels according to claims 1 and 2, based on polylactic acid. 5. Airgels, according to claim 1, based on polymers capable of forming nano-porous crystalline phases, i.e. crystalline phases having a density lower than that of the corresponding amorphous phases, and in which the content of such phases is higher than 5% in weight and preferably higher than 10% by weight. 6. Airgels, according to claims 1 and 5, based on syndiotactic polystyrene. 7. Airgels, according to claims 1 and 5, based on the poly (2,6-dimethyl-1,4-phenylene) oxide. 8. Airgels, according to claims 1-7, wherein the photocatalytic particles are based on doped titanium oxide having dimensions lower than 25 nm. 9. Airgels according to claims 1-7, wherein the photocatalytic particles are based on semiconductor particles having a size lower than 25 nm. 10. Airgels according to claims 1-7, wherein the photocatalytic particles are based on semiconductors supported on phosphorescent particles (phosphors). 11. Airgels, according to claims 1-10, in the form of monolithic products usable as filters or membranes. 12. Processes for the preparation of airgels according to claims 1-11 which include i following stages: A) dispersion of photo catalytic micro or nano particles in a solvent, which is capable of solubilize the polymer indicated in step 2, using an amount comprised between 0. 1% and 50% by weight, at a temperature between ambient temperature and 100 ° C, keeping the dispersion under constant stirring, for a time between 1min and 10h. B) the polymer is dissolved in the prepared dispersion in A, in a quantity such that the polymer / photocatalyst weight ratio is in the range 50 / 50-99.9 / 0.1 and that the solvent is not less than 30% by weight of the dispersion. C) the dispersion thus obtained is subjected to sonication with a vibration frequency higher than 10 Hz for a time higher than 1 min, at temperatures between room temperature and 100 ° C. D) the dispersion is cooled to temperatures ranging from room temperature to -80 ° C and a uniform gel is obtained E) the gel is extracted with liquid or supercritical carbon dioxide at pressures between 50 and 350 bar and at temperatures between 20 ° C and 70 ° C and preferably between 25 <P> C and 60 ° C. 13. Processes for the photocatalytic degradation of organic contaminants from liquid or in wastewater gaseous streams, in the presence of UV and / or visible light, which use aerogels as from claims 1-11, at temperatures between 10 and 60 ° C and pressures between 1 and 10 ATM. 14. Processes according to claim 13, for which the organic pollutants are VOCs halogenated, cyclic, aromatic or organic dyes.