Production of carbon-containing crystalline titanium oxide aerogels with large surface and large pore volume

Abstract

The invention relates to a sol-gel-based synthesis path for producing an aerogel supercritically dried in CO2 and containing titanium oxide and carbon (Ti approx. 24at.%, C approx. 11at.% and a molar ratio of C to Ti in the range of 0.16 to 0.8), and to corresponding titanium oxide aerogels.

Description

[0001] Preparation of carbon-containing crystalline titanium oxide aerogels with high surface area and large pore volume

[0002] The invention relates to a sol-gel based synthesis route for the production of a CO2 supercritically dried aerogel containing titanium oxide and carbon (Ti approx. 24at.%, C approx. 10at.% and a molar ratio of C to Ti in the range of 0.16 to 0.8) as well as corresponding titanium oxide aerogels.

[0003] Titanium oxide aerogels are three-dimensional, open-pore, amorphous materials produced using a sol-gel process through hydrolysis and condensation of various precursors (e.g., TiCH or titanium isopropoxide) followed by supercritical drying in CO2. Unique properties of titanium oxide aerogels, such as high surface areas and high pore volumes, as well as their typical electronic properties, make them promising semiconductor materials for applications in photocatalysis, such as photocatalytic hydrogen generation. In other applications, such as white pigments in non-transparent paints or in transparent, highly porous coatings, aerogels, unlike TiO2 nanoparticles, can be more easily applied in non-agglomerated form.

[0004] Titanium oxide aerogels are typically amorphous. Isolated anatase crystals have been described in the literature in predominantly amorphous aerogels supercritically dried in CO2 (SCHNEIDER, M. and BAIKER, A.: Titania-based aerogels. Catalysis Today, Vol. 35, 1997, pp. 339–365. ScienceDirect (online). DOI: httDs: / / doi.orq / 10.1016 / 50920-5861(96)00164-2 ). Schneider et al. describe the preparation of calcined titanium dioxide aerogels by supercritical drying followed by heat treatment.

[0005] US 2002 / 0 035 162 A1 relates to a method for enhancing the oxidation of air contaminants on a UV-light-accessible ultra-low density aerogel photocatalyst. The method comprises the steps of providing a photocatalytic reactor system, which broadly comprises a photocatalytic reactor cell, a UV light source, and a pump to force the contaminated air stream through the photocatalytic reactor cell. The photocatalytic reactor cell comprises a glass cell. Within the glass cell is a catalyst bed made of a titanium dioxide aerogel, whereby a high proportion of the titanium dioxide aerogel is accessible to UV light and gas. The catalyst bed is exposed to UV light from the UV light source, and a contaminated air stream is introduced into the photocatalytic reactor cell such that the air stream flows through the catalyst bed and causes oxidation of the contaminants in the air stream.This is due to the slightly increased tendency to crystallize compared to silica aerogels. Quantification of this low crystalline fraction has rarely been performed in the literature. In both literature, temperature and the supercritical fluid used during drying play a central role in crystallization. At the usual low temperatures (up to approximately 70 °C, SCCO2), predominantly amorphous aerogels could be produced. If higher temperatures are applied (up to 200 °C or 300 °C), similar to solvo / hydrothermal processes, supercritical solvents such as supercritical isopropanol or ethanol are typically used. These aerogels possess higher crystallinity due to the elevated temperature during the drying process. This leads to a reduction in the pore volume and the high surface area of ​​the aerogels.High crystalline content can also be achieved through subsequent heat treatment at 300 °C to 500 °C. The surface area and pore volume are significantly reduced depending on the selected temperature and duration, as sintering processes partially destroy the highly porous aerogel structure. The advantage of high porosity and large pore volume is thus lost. The resulting aerogels usually exhibit anatase as the crystalline phase at these calcination temperatures.

[0006] In contrast to porous aerogels, titanium oxide nanoparticles can also be produced using a sol-gel process. The resulting particle suspension is usually dried or calcined in air at temperatures between 25 °C and 100 °C or at high temperatures above 300 °C, which triggers the crystallization process. Crystallization occurs primarily during the drying process. The resulting nanoparticles exhibit various phase compositions, such as anatase, brookite, and / or rutile, depending on the pH of the alcoholic solution during synthesis. The crystalline nanoparticles have low surface areas of less than 100 m². 2 / g and a negligible pore volume (e.g., P25 TiO2). Numerous patents are applied for the production and applications of titanium oxide nanoparticles. These are predominantly produced by hydrothermal or sol-gel synthesis. However, these are compact particles and not porous, chemically cross-linked systems.

[0007] The production and applications of titanium oxide aerogels and titanium oxide composite aerogels are also numerously patented.

[0008] US 7,943,116 B1 describes a process for producing crystalline TiO2 brookite nanoparticles using titanium isopropoxide and isopropanol.

[0009] CN 000110918008 A describes aerogel polymer composites, including TiO2 as a component, whose production is based on the sol-gel process and supercritical drying.

[0010] R.0 0 122 840 Bl describes a TiCh aerogel synthesis based on sol-gel, titanium isopropoxide, ethanol and HNO3.

[0011] US 2007 / 0 119 344 A1 describes a TiCh-SiCh aerogel and TiCh-SiO2 aerogel monolith with a regular and predeterminable shape and ordered mesoporosity, and a process for producing the aerogel and the aerogel monolith using surfactants to prepare the SiCh sol prior to mixing with the TiCh sol. The aerogel obtained by this method has a specific surface area of ​​more than 400 m 2 / g and a pore volume of more than 0.5 cm 3 / g. US Pat. No. 5,958,363 A describes transparent, monolithic metal oxide aerogels of varying densities, prepared by a process in which a metal alkoxide solution and a catalyst solution are prepared separately and reacted. The resulting hydrolyzed-condensed colloidal solution gels, and the moist gel is stored in a sealed but gas-permeable containment vessel during supercritical solvent extraction. The containment vessel is enclosed in an aqueous atmosphere above the supercritical temperature and pressure of the solvent in the metal alkoxide solution.

[0012] US 2019 / 0 077 675 A1 comprises a titanium oxide aerogel particle made of a metal compound having a metal atom and a hydrocarbon group, wherein the metal compound is bonded to a surface of the aerogel particle via an oxygen atom. The titanium oxide aerogel particle has a specific BET surface area of ​​120 m². 2 / g up to 1000 m 2 / g and has an absorption at wavelengths of 450 nm and 750 nm.

[0013] Titanium oxide aerogels produced by conventional sol-gel synthesis and supercritical drying typically exhibit a predominantly amorphous character. To produce crystalline materials, drying at various temperatures ranging from 25 °C to 100 °C and ambient pressure, or at high temperatures and high pressures, is typically used. However, these conditions result in the loss of the large pore volume and surface area. The object of the present invention is to optimize the crystallinity of titanium oxide aerogels directly during synthesis, without subsequent heat treatment or high synthesis temperatures, and to influence the phase composition through the novel synthesis route. In particular, the focus is on preserving the surface area and pore volume of the material.

[0014] To achieve this objective, the present invention provides a novel synthesis route, which involves the production of a (semi-)crystalline titanium oxide aerogel with a simultaneously large surface area and pore volume. A further objective of the invention is to adjust the phase composition between the amorphous portion, anatase, and brookite (and rutile) using defined synthesis parameters.

[0015] A first embodiment of the present invention consists in a process for producing carbon-containing (semi-)crystalline titanium oxide aerogels, in particular with a crystallinity of 40 to >90% in a sol-gel process, in which

[0016] (a) a titanium oxide aerogel precursor dissolved in a solvent is first mixed with an aqueous solution of HCl,

[0017] (b) subsequently / particularly after a certain stirring time, water is added and, when gelation begins, transferred into a tightly sealable container, (c) the resulting alcogel is aged at an elevated temperature in the range of 40 °C to 60 °C, preferably for a period of at least 3 days to a maximum of 7 days,

[0018] (d) the solvent of the formed alcogel is exchanged several times and

[0019] (e) drying the alcogel at a further elevated temperature, in particular in the range of 35 °C to 60 °C and increased pressure to obtain the titanium oxide aerogels.

[0020] Crystalline, as used herein, encompasses semi-crystalline to virtually completely crystalline titanium oxide aerogels with a large surface area and large pore volume. The crystalline portion of the aerogel can be specifically adjusted using defined synthesis parameters.

[0021] Optionally, a step can be inserted between step (a) and step (b) in which the titanium oxide aerogel precursor solvent mixture, mixed with HCl, is left to cool for at least 15 minutes with stirring in order to achieve coordination of the Cl ions to the titanium and their equilibrium.

[0022] An advantage of the new synthesis route is its ease of implementation with few synthesis steps, without the addition of, for example, surfactants as templates. Since the high crystallinity is achieved through synthesis / aging, the step of calcination at high temperatures is omitted. Particularly preferred within the meaning of the present invention is titanium alkoxides, in particular titanium tetraisopropoxide, as the titanium oxide aerogel precursor. The solvent used for the titanium oxide aerogel precursor is, in particular, 2-propanol or ethanol, as well as mixtures thereof in a weight ratio of 1:1 to 1:9. By varying the solvent and their mixtures, the pore size distribution, the pore volume, and the phase composition can be adjusted. Using 2-propanol, predominantly anatase brookite aerogels are obtained, whereas using ethanol predominantly anatase aerogels.

[0023] While the precursors are typically treated with oxidizing acids to produce titanium oxide aerogels, the present invention utilizes an aqueous solution of HCl. Concentrated aqueous hydrochloric acid is preferred. The use of concentrated hydrochloric acid reduces the water content at the beginning of the reaction. This prevents the direct precipitation of TiCl or the hydroxide. When using HCl instead of oxidizing acids (e.g., HNO3), the formation of rutile has been observed after conventional drying methods, which is detrimental for photocatalytic purposes.

[0024] The aging temperature of alcogels is standard. Therefore, the alcogel is preferably aged at a temperature in the range of 40°C to 60°C. However, if the aging temperature is chosen higher, crystallization begins. The aging time varies in the literature from a few hours to several weeks or months at room temperature. Aging stabilizes the network. The temperature should be selected so that it is compatible with the boiling point of the solvent or mixture (ethanol approx. 78°C, isopropanol approx. 82°C). Furthermore, aging at different temperatures may alter the network, resulting in different conduction paths for electrical charges, based on percolation effects. Aging should take place over the course of several days.

[0025] The aging time, in combination with the slightly elevated temperature of approximately 50 °C, is crucial for the degree of crystallization, as crystallization takes place entirely in the wet gel. This was confirmed by in-situ Raman spectroscopy and in-situ / ex-situ synchrotron measurements. Crystallization begins at least 2-3 days after aging. With longer aging times, particularly up to 7 days, crystallinity can be steadily increased, up to a maximum crystallinity of approximately 80% for EtOH solvent-based gels or >90% for 2-propanol solvent-based gels. The crystallinity process can be stopped at any time by returning the temperature to approximately 20 °C (room temperature). This allows for different degrees of crystallinity to be precisely adjusted. The temperature and pressure of the supercritical drying process have no significant influence on the further crystallinity of the aerogels.

[0026] The solvent exchange rate is also within the usual range, so the solvent of the alcohol gel is preferably exchanged two to ten times. By exchanging the solvent, no precursors of the reaction are present in the gel, meaning there is no contamination by the precursor in the gel. Furthermore, the continuation of the reaction is largely prevented. Solvent exchange reduces the water content, which leads to better supercritical drying in CO2.

[0027] Drying of the alcohol gel is preferably carried out at temperatures in the range of 60 °C to 100 °C and a pressure of 100 to 150 bar. Crystallization begins in the range of 40 °C to 70 °C, and crystallization becomes more pronounced at elevated temperatures. The specified pressures and temperatures provide a safe process window for supercritical drying in CO2. They also ensure that the solvent-CO2 mixture remains supercritical during drying.

[0028] In particular, using the aforementioned process, carbon-containing crystalline titanium oxide aerogels are obtainable with a Ti content of approximately 24 at.%, C of approximately 10 at.%, and a C to Ti molar ratio in the range of 0.16 to 0.8. In particular, the aforementioned ranges include Ti of approximately 22 to 26 at.%, C of approximately 10 to 18 at.%, with a C to Ti molar ratio in the range of 0.16 to 0.8. Oxygen in the range of approximately 45 to 61 at.%, and small amounts of CI of approximately 1 to 3 at.% in the aerogels. The other components naturally include oxygen and chlorine. The carbon content originates from the originally used precursor, for example, an alkoxide.

[0029] The titanium oxide aerogels according to the invention have in particular a crystallinity of 40 to >90% and a surface area of ​​200 to 600 m 2 / g and a pore volume of 1.2 to 6 cm 3 / g. The crystallinity within the meaning of the present invention is determined by X-ray crystallographic quantitative phase analysis and Rietveld refinement using an internal standard.

[0030] The invention is based on the production of crystalline as well as meso- or macroporous TiCh aerogels using a sol-gel process with concentrated hydrochloric acid, in which the amount of concentrated hydrochloric acid in the reaction mixture was varied and increased during the synthesis. This can significantly improve the crystallinity of the titanium oxide aerogels and maintain the surface area and pore volume. The order of acid and water addition at a specific temperature is also of particular importance, as is the stirring time before adding the water. A low temperature in the range of -5 °C to 5 °C reduces the reaction rates and can lead to a more homogeneous aerogel. If the water is added after the acid addition, in contrast to the syntheses described in the literature, to prevent the direct precipitation of titanium oxide, transparent to translucent gels are obtained.In particular, HCl was chosen as the acid for the synthesis, although HNO3 is predominantly used in the literature, since HCl is said to favor the phase transition to rutile during the calcination of TiO2 nanoparticles. In the synthesis route according to the invention, no calcination step takes place; instead, the crystallinity and phase adjustment are achieved purely by the choice of solvent, the amount of hydrochloric acid added to the reaction mixture, and the aging of the wet alcogels, particularly between 3 and 7 days. The crystallinity of the aerogels increases with the proportion of hydrochloric acid. In addition to the aging time / temperature, crystallization is supported by the coordination of chloride ions to the titanium center, which promotes a rearrangement of the titanium octahedra into specific crystalline phases in the wet alcogel through steric and electrostatic effects. Ti-Cl bonds were confirmed by X-ray diffraction and X-ray spectroscopic investigations.The composition of the crystalline phases can be influenced by the choice of solvent and the amount of water used, in addition to the acid. When using 2-propanol, the phase composition changes with increasing hydrochloric acid content, so that at low acid contents, anatase is predominantly formed, while at high acid contents, a composition of 30 to 40 wt.% brookite and 60 to 70 wt.% anatase is achieved. When using ethanol, anatase is predominantly formed, whereas with increasing acid and water contents, brookite is also formed, along with anatase and small amounts of rutile (<5 wt.%). By simply varying the above-mentioned synthesis parameters, a desired phase composition and crystallinity can be achieved while maintaining the pore structure typical of aerogels, and adapted as required / application.

[0031] A further embodiment of the present invention includes the use of the aforementioned gels, for example for:

[0032] • Use as a catalyst for photocatalytic hydrogen production

[0033] • as a white pigment in non-transparent paints

[0034] • for photocatalytic applications

[0035] • as an additive for existing applications. Example:

[0036] Production of titanium oxide aerogels:

[0037] Preparation of the solution and gelation

[0038] 1.) First, 2.7 g of titanium tetraisopropoxide were weighed into a beaker and 11 g of solvent (2-propanol) were added. The solution was homogenized by briefly swirling. The solution was cooled to 0 °C while stirring (400 rpm / bar stir bar) and held at this temperature for 5 minutes. Hydrochloric acid was then added in a range of 43.6 to 216.4 pL (37 wt% solution), and the solution was stirred for another 15 minutes. A minimum amount of 43.6 pL was required to produce a translucent gel. The amount of acid was crucial for the crystallinity of the aerogels. (Amorphous to poorly crystalline aerogels consisting of anatase were obtained in the range of <87.2 pL.)

[0039] Semi-crystalline to crystalline aerogels consisting of anatase and brookite, with an anatase:brookite ratio of 60-70:30-40%, were obtained in the range of 87.2 to 216.4 pL. Then, 829 pL of distilled water in 6.13-14.7 g of 2-propanol were slowly added dropwise. The solution was stirred for a further 30 to 60 s and, upon gelation, transferred to tightly sealable containers (PP).

[0040] 2.) Alternatively, 2.7 g of titanium tetraisopropoxide were weighed into a beaker and mixed with 11.37 g of absolute ethanol. The solution was homogenized by briefly swirling. The solution was cooled to 0°C while stirring (400 rpm / bar stir bar) and held at this temperature for 5 minutes. Then, 87.2 to 301.7 pL of hydrochloric acid (37 wt% solution) were added, and the solution was stirred for another 15 minutes.

[0041] A minimum amount of 87.2 pL was required to produce a transparent gel. The amount of acid was crucial for the crystallinity of the aerogels. In the range of <87.2 pL, amorphous to slightly crystalline aerogels consisting of anatase were obtained. In the range of 87.2–216.4 pL, semi-crystalline to crystalline aerogels consisting of anatase were obtained. Then, 829 pL of distilled water was added slowly and dropwise. The solution was stirred for a further 30 to 60 s and, upon gelation, transferred to tightly sealable containers (PP). However, when 1735 to 2487 pL of distilled water were added, with acid amounts in the range of 172.4 to 216.4 pL, crystalline aerogels consisting of anatase and brookite were obtained, with an anatase:brookite ratio of 60 to 70:30 to 40%. In the range of 216.4-301.7 pL, crystalline aerogels consisting of anatase, brookite and small amounts of rutile (<5 wt%) were obtained.

[0042] Aging and drying:

[0043] The tightly sealed containers according to 1.) and 2.) were aged in an oven at 50 °C for 7 days.

[0044] The containers were then opened, and the wet alcogel was wrapped in several layers of paper towels and placed in a container containing pure 2-propanol for 24 hours at a temperature of 25 °C. The solvent was changed at least four times after each 24-hour period.

[0045] The washed alcogels were transferred to an autoclave and dried in supercritical CO2 at 60 °C and 115 bar.

[0046] Fig. 1 shows the wet gels with increasing acid content, as well as a supercritically dried aerogel prepared with medium acid content (87.2 pL).

[0047] Titanium oxide aerogels prepared with increasing acid content (top, from left to right: 0 pL, 43.6 pL, 87.2 pL, 130.3 pL, 173.4 pL, 216.4 pL). The lower image shows a translucent supercritically dried titanium oxide aerogel prepared with isopropanol and a medium amount of acid (87.2 pL).

[0048] The titanium oxide aerogels described here possess a large surface area and a large pore volume. The addition of a specific amount of concentrated hydrochloric acid, in combination with the aging process, results in crystalline aerogels. The crystallinity of the aerogels increases with the proportion of hydrochloric acid and longer aging time. The composition of the crystalline phases changes depending on the solvent, with increasing proportion of hydrochloric acid, so that at high acid contents, a composition of 30 to 40 wt.% brookite and 60 to 70 wt.% anatase is achieved. This is illustrated in Fig. 1. Fig. 1 below shows diffraction patterns of various titanium oxide aerogels based on 2-propanol (left) and ethanol (right) with increasing acid content and increasing crystallinity. Fig. 2 shows the properties (surface area and pore volume) in relation to crystallinity, in particular the properties and crystallinity of the titanium oxide aerogels with increasing acid content.

[0049] It is shown that although the surface area / pore volume decreases with increasing crystallinity, the produced aerogels still exhibit considerably large surface areas and pore volumes compared to the literature. It is important to note that high temperatures above 50 °C were not used during synthesis to achieve this level of crystallinity. It is shown that, within a certain range of hydrochloric acid concentrations, a crystallinity of 40 to <90% can be achieved with a surface area of ​​200 to 600 m². 2 / g and pore volumes from 1.2 to 6 cm 3 / g. The surface area, pore volume, and pore size distribution are determined by gas sorption measurements. The surface area of ​​the titanium oxide aerogels according to the invention is determined using the BET (Brunauer-Emmett-Teller) theory. The pore volume and pore size distribution of the titanium oxide aerogels according to the invention are determined using the BJH (Barrett-Joyner-Halenda) method.

[0050] Fig. 3 shows the pore size distribution of the 2-propanol solvent-based aerogels at different acid concentrations, particularly the pore size distribution of the titanium oxide aerogel with increasing acid concentration and its influence on the formation of meso- and macropores. Fig. 4 shows the microstructure of titanium oxide aerogel with low acid concentration (left) and high acid concentration (right).

[0051] Fig. 5 shows TEM images of TiCh aerogel with a crystallinity of 40% and a surface area of ​​550 m 2 / g. Darkfield TEM (top right) shows crystalline particles (lens aperture centered on the diffraction ring of the 101 anatase reflection).

Claims

Patent claims:

1. A process for the production of carbon-containing crystalline titanium oxide aerogels with a crystallinity of 40 to >90% in a sol-gel process, in which (a) a titanium oxide aerogel precursor dissolved in a solvent is first mixed with an aqueous solution of HCl, (b) then adding water and transferring it to a tightly sealable container when gelation begins, (c) the forming alcogel ages at elevated temperatures in the range of 40 to 60°C, (d) the solvent of the formed alcogel is exchanged several times and (e) drying the alcogel at further elevated temperature and pressure to obtain the titanium oxide aerogels.

2. Process according to claim 1, characterized in that titanium tetraalkoxides, in particular titanium tetraisopropoxide, are used as the titanium oxide aerogel precursor.

3. Process according to claim 1 or 2, characterized in that 2-propanol and / or ethanol is used as solvent for the titanium oxide aerogel precursor.

4. Process according to one of claims 1 to 3, characterized in that concentrated aqueous hydrochloric acid is used.

5. Process according to one of claims 1 to 4, characterized in that the solvent of the alcogel is exchanged 2 to 10 times.

6. Process according to one of claims 1 to 5, characterized in that the alcogel is dried at a temperature in the range of more than 60 °C to 100 °C and a pressure of 100 to 150 bar.

7. Carbon-containing crystalline titanium oxide aerogels obtainable by a sol-gel process with a Ti content of approximately 24 at.%, C of approximately 10 at.% and a molar ratio of C to Ti in the range of 0.16 to 0.

8.

8. Titanium oxide aerogels according to claim 7 having a surface area of ​​200 to 600 m 2 / g and a pore volume of 1.2 to 6 cm 3 / G.

9. Use of titanium oxide aerogels according to one of claims 1 to 8 as a catalyst for photocatalytic hydrogen production, as a white pigment in non-transparent paints, for photocatalytic applications and / or as an additive for applications known per se.

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