OBTAINING OF ZIRCONIUM SILICATE MATERIALS OF AEROGEL TYPE FROM AQUEOUS SOLUTIONS OF Na2SiO3 AND ZrOCl2 SALTS

Aerogels are gels in which the liquid phase is completely replaced by a gaseous one. They form a new class of solids with a very low density, a large specific surface area and high porosity, which opens up wide possibilities for their practical application. ZrO2-SiO2 aerogels, due to the strong binding energy of Zr-O-Si, very low thermal conductivity, and the presence of acid and basic centers, exhibit excellent properties like aerospace heat insulators, selective sorbents, catalysts, and catalyst supports for high-temperature reactions. The traditional way of obtaining aerogels is formation of a branched, three-dimensional, irregular network of wet gels by sol-gel synthesis from alkoxides of elements, aging of gels, replacement of intermicillar water by organic liquids with low surface tension and careful removal of the solvent by drying under supercritical conditions or under ambient pressure. But alkoxides of the elements are expensive and toxic, drying at elevated pressures requires special equipment and is also expensive and energy-intensive, and drying at atmospheric pressure requires complicated and lengthy gel modifications. In this work, the task was posed, on the basis of the direct method of large-scale sol-gel synthesis of highly porous, nanostructured, spherically granulated zirconium silicates from aqueous solutions of cheap, accessible salts, to obtain materials of aerogel type, without resorting to lengthy equipmently and chemically complex methods of processing the resulting hydrogels. The gels were prepared by forming a zirconium carbonate complex from aqueous solutions of ZrOCl2 and K2CO3 and its subsequent interaction with a Na2SiO3 solution by their coagulation in a drop. The obtained strong spherical granules of ZrO2 SiO2∙nH2O hydrogel were thoroughly washed from impurities with distilled water and subjected to hydrothermal treatment for various times, followed by decantation with ethanol mixed with benzine or alkothermal treatment in this mixture in tightly closed containers at supercritical temperature for ethanol. The heat resistance of the samples was determined by calcining them in air at high temperature. Using SEM, XRD, and N2 adsorption / desorption methods, it has been found that the amorphous Zr-Si materials obtained containing 45 wt. % ZrO2 have a specific surface area of more than 500 m2/g, pore volume > 2 cm3/g, average pore diameter of ~ 18 nm, wide mesopores with a diameter of ~ 28 nm, bulk density less than 0.3 g/cm3, which typical for aerogels based on oxides of metal and silicon, and exhibit high thermal stability.


INTRODUCTION
The aerogels are gels in which the liquid phase is completely replaced by a gaseous one. They form a new class of solids having a very low density, from 0.004 to 0.5 g/cm 3 , large specific surface area and high porosity, with cavities, occupying more than 50 % of the bulk. In addition, their state is thermodynamically metastable. These unique properties of aerogels open up wide possibilities for their practical applications as heat and sound insulators, heat and energy accumulators, cosmic dust absorbers and radioactive particles, components of electrochemical systems, glasses, sorbents, ion exchangers, heterogeneous catalysts, photocatalysts, etc. [1][2][3][4][5][6].
By their chemical nature, aerogels are classified as organic, inorganic and hybrid. Among them, ZrO 2 aerogel is one of the most important materials, due to its very high melting point, the lowest thermal conductivity among metal oxides, and the presence of both acidic and basic active centers [7]. But heating of a pure single-component ZrO 2 aerogel leads to the degradation of its nanopores and a significant decrease in surface area due to the ongoing phase modification of ZrO 2 , from amorphous to tetragonal and monoclinic [8]. The stabilization of the structure of the ZrO 2 aerogel by introducing silicon dioxide into its composition allows us to create ZrO 2 -SiO 2 materials showing excellent properties as aerospace heat insulators [9] and carriers for high temperature reaction catalysts [10].
The first and key step in the preparation of aerogels is the formation of a three-dimensional, highly porous, irregular, branched network of wet gels, which usually forms during the condensation of colloidal particles. Most aerogel syntheses follow this sol-gel route, with two options: a) hydrolysis and condensation reactions are initiated in aqueous solutions of ionic precursors; b) precursor molecules in an organic solvent are transformed into species capable of undergoing condensation [1]. The first silicate aerogels were obtained on the basis of an aqueous solution of water-glass -Na 2 SiO 3 , by its interaction with hydrochloric acid to form a gel of silicic acid, followed by the replacement of water with ethanol and drying at supercritical temperature [11]. However, this method was lengthy and also required thorough washing of the gels from foreign ions of the resulting NaCl salt. Hydrolysis and condensation of alkoxides of elements in alcohol-water solutions turned out to be relatively simple and controlled processes with the rapid formation of gels without the need for extraction, filtration and water replacement [12], which made it possible to reduce the time of production and processing of aerogels from week to day. Therefore, this method of producing oxide nanocomposites is widely distributed and is most often used [2]. Since the reactivity in the hydrolysis reactions of metal alkoxides is much higher than that of alkoxysilanes, the processes of their joint gel formation must be carefully monitored. This is achieved by the use of modifying or ligandforming agents and acidic and / or basic catalysts [1][2][3][4][5]. Under their action, gels with a uniform distribution of components, a large specific surface area and an adjustable pore size distribution are formed. At this stage, there the basic properties of the final material.
But metal alkoxides, and in particular zirconium, are usually expensive and toxic. Therefore, their replacement with aqueousalcoholic solutions of inorganic salts -ZrO(NO 3 ) 2 [13,14], ZrOCl 2 and ZrCl 4 [15,16], in the process of sol-gel synthesis together with silicon alkoxides, is considered a progressive method for producing ZrO 2 -SiO 2 aerogels. This method allows for inexpensive, easy to handle procedures with a short gelation time and to obtain products with a high surface area.
The second stage of preparation of aerogels is the aging of the obtained wet gels in the mother liquor while controlling the pH, temperature and time [17][18][19]. During this lengthy process, the unreacted small oxide groups dissolve and precipitate between the larger particles of the polymer chain. This strengthens the structure of the gel and prepares it for the next critical stage of obtaining aerogel -the drying process [3].
The capillary compressive forces of the gel during drying are determined by the bond strength of the particles in the gel framework and the surface tension of the pore fluid, which shows the highest values for water. Therefore, the pore water of the gel should be completely replaced with an organic liquid with a low surface tension. This is followed by the stage of high-temperature or low-temperature supercritical drying [3,5,20,21]. In the first case, the gel with the solvent is placed in the autoclave and the temperature and pressure in it are increased above the critical point of the solvent, while the liquid passes into a gaseous state and then is discharged from the autoclave without violating the integrity of the gel structure. In the second case, the liquid is displaced from the pore space by means by means of CO 2 , the compressed to its critical liquid state. However, these drying methods are energy-intensive and expensive due to the need to use high pressure and related equipment.
Therefore, in recent years, the method of producing aerogels using drying at atmospheric pressure has been intensively developed. This method consists in the phased replacement of the gel pore liquid by a liquid with an ever lower surface tension and drying at their subcritical temperature and ambient pressure. Especially often, this treatment is used in the development of more advanced methods for producing aerogels from cheap and affordable sodium silicate instead of expensive, toxic and flammable alkoxysilanes, which cannot be used in mass commercial production. However, the necessity of using cation exchange, coprecursors from organo-silanes and additional silylation of the gel makes these methods quite expensive and complicated [22][23][24].
In this work, the task was posed, on the basis of the previously developed method of direct large-scale sol-gel synthesis of highly porous, nanostructured, spherically granulated zirconium silicates from aqueous solutions of cheap, available salts -sodium silicate and zirconium oxychloride [25], to try to obtain materials such as aerogels without resorting to lengthy apparatly and chemically complex processing methods for the resulting hydrogels.

EXPERIMENTAL PART
The method used to obtain the mixed oxide ZrO 2 -SiO 2 , according to the patent [25], is based on conducting of the following reactions: It is known [26] that, in aqueous solutions, the addition of Zr(IV) salts to an excess of alkali metal or ammonium carbonates leads to the formation of metastable anionic carbonate complexes of Zr(IV), which can serve as precursors for the synthesis of new zirconium based materials. Treatment of such complexes with an aqueous solution of sodium metasilicate causes gel formation [27].
An additional application of droplet coagulation technology [28] ensured the production of spherically granulated ZrO 2 -SiO 2 hydrogel [25]. The process was carried out as follows.
Under stirring, a 2M ZrOCl 2 solution was gradually added to a 5M K 2 CO 3 solution with the formation of a zirconium carbonate complex. Its solution at a rate of 164 ml/min. and a sodium metasilicate solution, with a concentration necessary to obtain a Zr/Si ratio of 0.6, was fed at a rate of 100 ml/min into a flow reactor in which the solutions were mixed to form a mixed sol of zirconium oxyhydrate and silicic acid. A jet of sol was directed into a column filled with two layers of liquids. The top layer is undecane, and the bottom is running tap water. During the passage of the undecane layer (1.0-1.5 m), the sol stream was broken into separate drops, which in 3-5 s turned into durable spherical granules of Zr-Si hydrogel. They were transferred with water and first fell on a sieve, and then into a container, where they were washed with distilled water from alkalis, salts and traces of undecane, treated with a solution of NH 4 OH and washed again with water until neutral.
External moisture was removed from a part of the obtained hydrogel spheres and dried at 130 °C for further structural and elemental analysis of initial sample N 1 ( Table 1). The other four parts of the spherical hydrogel were placed in four tightly closed massive cylindrical forms with a volume of 40 cm 3 made of stainless steel, filling their volume almost completely. Then forms were heated in a muffle furnace in different liquid media at a gradual increase of its temperature during various period of time (samples 2-5, Table 2).
Sample N 2 was subjected to hydrothermal treatment (HTT) for 5 h at 270 °C. It was shown [29] that the continuous dissolution and reprecipitation of hydroxides during HTT leads to the production of materials with a more defect-free structure, which can withstand the loss of surface area upon exposure to high temperatures. Therefore, this treatment can replace the long stage of aging of the hydrogel.
Samples N 3 and N 4 were acted upon by the HTT for a shorter time -2 h. After that, the samples were removed from the molds and washed thoroughly -first with ethanol and then with anhydrous ethanol (N 3) and its mixture with 2 % benzine (N 4) and left in these liquids for 6 h. Hydrogel N 5 was not subjected to HTT, and after washing it in a similar way, it was left in ethanol-benzine medium for 12 h and then placed in a closed form and subjected to alkothermal treatment (ATT) in this mixture at 270 °C for 5 h. After the described treatment, all the samples were dried at 130 °C, their bulk density was measured by weighing and then they were calcined in air at 600 °C. The texture of the samples was determined before and after calcination.
The isotherms of adsorption / desorption of N 2 for the traditional analysis of the texture of the samples were obtained according to the standard procedure after vacuum treatment at 150 °C on an ASAP 2405N Micromeritics instrument. Pore size distribution was calculated using the density functional theory (DFT).
An analysis of the microstructure and elemental composition of the sample N 1 was determined by scanning electron microscopy (SEM) using a JEOL JSM 7001F scanning electron microscope with an microanalyzer system.
X-ray diffraction patterns of the studied samples were recorded on a DRON-4-07 diffractometer in the CuK  radiation of the anode line with a Ni filter in the reflected beam with Bragg -Brentano geometry.

RESULTS AND DISCUSSION
From the data given in Table 1 it is seen that the amount of zirconium dioxide included in the structure of the obtained gel is lower than it was in the synthesis ash and the real Zr/Si ratio in it is ~ 0.45. Spherical granules are well defined and have a size of 200-700 µm. Hydrothermal treatment and replacement of water in the pore space of a zirconium silicate hydrogel with "organic fluids" leads to a change in the form of adsorption isotherms of N 2 desorption (Fig. 1 a) and to the corresponding character of pore distribution (Fig. 1 b). The isotherm of the initial hydrogel, type IV, has an H2-type hysteresis loop [30] with a long horizontal section on the desorption branch characteristic of corpuscular systems of an indeterminate nature with narrow "throats" and various wider cavities. This structure is also confirmed by the calculation of the pore size distribution of the sample, showing the presence of pores with diameters of ~ 1.4, 2.7 and 4.8 nm, which is typical for micro\mesoporous systems. The HTT of the Zr-Si hydrogel within 5 h changes the shape of the hysteresis loop on its isotherm -the horizontal section on the desorption branch becomes shorter, which indicates a decrease in the difference between the diameters of the "throat" and "cavity" in the pore structure of the sample. In this case, the type of hysteresis loop Н2 transforms into Н1, which is typical for systems with a uniformly developed mesoporous structure. The calculation of the pore size distribution shows the presence of mesopores with a diameter of mainly 5.7 nm. After 2 h HTT, replacing intermicellar water in Zr-Si hydrogels with dehydrated ethanol or its mixture with benzine leads to a narrowing of the hysteresis loops on the isotherms of the samples, indicating an additional equalization of pore sizes, their diameter increases, remaining within the mesopore range, and volume significantly increased. Such structural changes reach the greatest degree after the ATT of gel (Table 2). At the same time, each successive stage of sample processing leads to a gradual decrease in the bulk density of the resulting material from 1.33 to 0.22 g/cm 3 . Comparison with the data of the scientific literature shows that, in the form of nitrogen adsorption/desorption isotherms, pore distribution, and texture parameters, the materials obtained are close to the published characteristics of aerogels. For example, in [13] ZrO 2 -SiO 2 aerogels with a zirconia content of 50 wt. % were prepared from tetraethoxysilane (TEOS) and zirconyl nitrate dihydrate salt by supercritical drying in ethanol. The values of texture parameters S BET , V pores and average D pores , given after calcination of the sample at 500 °C, are 547.9 m 2 /g, 1.67 cm 3 /g and 6.3 nm, respectively. The authors of [13] carried out drying of the gel in an autoclave as described in detail in [2]. A wet gel coated with an ethanol layer in a stainless steel liner was placed in a volume autoclave with additional ethanol outside In our work, when the ATT gel was in an ethanol-gasoline mixture, the liquid layer above the wet gel, which almost completely filled the volume of the metal form, did not exceed 0.5-1.0 cm, therefore the volume of the mixture could be insufficient for achieve the critical pressure of the mixture. But it was shown in [31] that SiO 2 aerogels with minimal shrinkage can be obtained under the pressures significantly lower than the critical pressure of the organic solvents used as pore liquids. The aerogel drying pressure can be reduced by increasing the initial gel strength. Heating silicate gels in organic solvents to their supercritical temperatures causes the gel network to rearrange, the surface of the aerogels is transesterified, becoming hydrophobic, more rigid and stable. In addition, the full filling of the drying container with wet gel [32] ensured a relatively fast ventilation of the supercritical fluid, preventing swelling and destruction of the gel monolith.
In [33], an aerogel was obtained from TEOS by a two-stage catalytic process during drying under ambient pressure by long periods of aging of the gel in ethanol, washing with n-hexane, modification with trimethylchlorosilane, again washing with n-hexane and drying at 50 °C. Calculating the texture parameters of the obtained material from an isotherm similar to those shown in Fig. 1 a (curves 4 and 5), the authors give the following values of S BET , V pores and average D pores -434 m 2 /g, 1.96 cm 3 /g and 18 nm, very close to the data us obtained, but the bulk density of the obtained silicate aerogel -0.418 g/cm 3 -turned out to be higher.
The thermal stability of the samples obtained in this work was evaluated by changing their texture after calcining in air to 600 °C, the shape of their isotherms and the nature of the distribution of pores changed little, the calculated quantitative texture parameters are given in Table 2. As can be seen, calcination reduces the specific surface area of the dried Zr-Si gel by almost 45 %. A preliminary HTT within 5 h strengthens the gel structure and more than doubles its thermal stability -the S BET value decreases only by ~ 20 %. Additional gel decantation with ethanol, even with a shorter duration of the HTT action, within 2 h, leads to a significant increase in pore volume and at the same time increases the thermal stability of the sample. The addition of 2 % benzine to the decantation mixture improves these parameters and leads to a significant expansion of mesopores. The alkothermal treatment of the gel in a mixture of ethanol and benzine for 5 h at 270 °C makes it possible to obtain the most thermostable sample with the largest volume and pore diameter.

Fig. 2. X-ray diffraction patterns of ZrO 2 -SiO 2 gels
calcined at 600 °C in air: 1 -initial gel, 2gel subjected to preliminary ATT The X-ray diffraction patterns of the initial sample and the final one subjected to ATT before calcining at 600 °C show only an amorphous halo (Fig. 2), which is typical for aerogels [2], which are usually amorphous. The absence of reflections of tetragonal zirconia t-ZrO 2 in our case is also consistent with the data of [34], which showed that the amorphous SiO 2 matrix, with its content of ≥ 50 % in the binary oxide ZrO 2 -SiO 2 , contributes to the preservation of the amorphous state of ZrO 2 to the calcination temperature mixed oxide up to 800 °C.
Thus, the study shows that by combining the HTT, ATT and decantation with organic liquids, it is possible to obtain materials with a structure similar to aerogels without using expensive hardware and chemically complex long-term methods for processing synthesized hydrogels of metallosilicates.

CONCLUSIONS
It has been shown that the use of the direct one-step sol-gel synthesis method from aqueous solutions of cheap and affordable salts -sodium metasilicate, zirconium oxychloride and potassium carbonate, supplemented by drop coagulation technology, allows for a short time to obtain strong spherically granular zirconium silicates hydrogels that can serve as the basis for further preparation of aerogels. It has been found that strengthening the hydrogel structure by hydrothermal treatment promotes an increase in its surface area, pore volume and width, and more than doubles the thermostability of the sample upon calcination. Additional decantation with ethanol and, even more anhydrous ethanol with an admixture of gasoline, improves these parameters and also reduces the density of the samples obtained, even with a shorter duration of their preliminary hydrothermal treatment. Alkothermal treatment of the sample in ethanolbenzine mixture leads to the production of the most thermostable amorphous zirconium silicate material with a density of < 0.3 g/cm 3 , having a specific surface area of more than 500 m 2 /g, pore volume > 2 cm 3 /g, average pore diameter of ~ 18 nm, and mesopores diameters > 27 nm, characteristic of metal oxide silicate aerogels.