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CHEMICAL SCIENCES • An. Acad. Bras. Ciênc. 96 (suppl 3) • 2024 • https://doi.org/10.1590/0001-3765202420240096 linkcopy

Open-access Synthesis of TiO2 nanoparticles by the solvothermal method and application in the catalysis of esterification reactions

AuthorshipSCIMAGO INSTITUTIONS RANKINGS

Abstract

TiO2 nanoparticles have numerous applications, prompting extensive efforts to optimize their synthesis for various technologies. This study synthesized TiO2 nanoparticles via the solvothermal method, using a chemometric approach to vary time, temperature, and concentrations of both the precursor and crystallization agent, aiming to understand these variables’ effects on crystallite size. Post-synthesis, a sample underwent treatment for sulfate adsorption to alter surface properties from hydrophobic to hydrophilic. Characterization techniques included X-ray diffraction (XRD), infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results indicated the formation of crystalline anatase phase nanoparticles, with particle sizes ranging from 25 to 38 nm and crystallite sizes from 8 to 10 nm. Sulfation was confirmed via FTIR, and SEM revealed particle agglomeration. Catalytic performance was assessed through esterification reactions, with two analyses: one comparing nanoparticles with and without surface modification, and the other examining the effects of variables like time, catalyst amount, and temperature on product formation. Sulfated samples exhibited excellent catalytic performance, achieving 60% conversion after 2 hours at 50 °C. Pure TiO2 samples also showed good conversion rates when synthesized at higher temperatures and Ti4+ concentrations.

Key words
catalysis; esterification; nanoparticles; TiO2; titanium(IV) oxide

INTRODUCTION

Titanium(IV) oxide, TiO2, a compound widely studied in nanotechnology, is a stable material with photocatalytic properties in the UV region, being used in water purification due to its ability to degrade organic molecules, and also being applied in sensors and in solar filters due to the response of TiO2 to UV light. A very common application of TiO2 is in white pigments, being a material that is present in our daily lives (Gupta & Tripathi 2011, 2012, Haider et al. 2017, Vargas Urbano et al. 2011, Robby et al. 2023, Melnikova et al. 2023). TiO2 has three crystalline phases, namely: anatase (tetragonal arrangement), rutile (tetragonal arrangement) and brookite (orthorhombic arrangement) (Rzaij & Abass 2020, Haidry et al. 2024, Kumar Patnaik & Divya 2023). These are phases with different crystalline structures and arrangement of atoms. These phases are stable at certain particle sizes. Anatase is stable in particles smaller than 11 nm, brookite in particles from 11 to 35 nm and rutile in particles larger than 35 nm (Feltrin et al. 2013, Madras et al. 2007, Levchenko et al. 2006, Istiroyah et al. 2021, Nikolaev et al. 2020). The use of catalysts in chemical reactions allows compounds to be synthesized at a higher speed and at lower temperatures, due to the lower activation energy path provided to the reaction by the catalyst. This factor, combined with a good reaction yield, is of great interest to the industry (Yin & Alivisatos 2005, Chen & Mao 2007, Wei et al. 2015, Alves et al. 2014, Lott & Deutschmann 2023, Wang et al. 2023, Alves et al. 2021).

Metal oxide nanoparticles have a large specific surface area, which is responsible for the high adsorption performance of these nanoparticles (Monárrez-Cordero et al. 2014, Rajh et al. 2002, Gholizadeh et al. 2023, Gakis et al. 2023), hence why they are widely used as catalysts in organic reactions (Fan & Gao 2006, Gautam et al. 2020). Due to their high specific surface area, they are used in hydrolysis and esterification reactions, in which the performance of nanoparticles is associated with the presence of Lewis acid sites characteristic of their surfaces (Alves et al. 2014, Chen et al. 2023). Gold nanoparticles supported by metal oxides, such as TiO2, are also described as catalysts for oxidative esterification reactions (Wei et al. 2015, Farah et al. 2023, Mu et al. 2024). TiO2 is a very stable material, tending only to agglomerate due to its high surface energy, leading to some researchers using surface modification to counteract this issue, such as using surfactants (Sun et al. 2024, Alsheheri 2021, Qamar et al. 2023).

The solvothermal method involves a precursor, a solvent and heat treatment. With the use of organic solvents, it is possible to carry out the synthesis at higher temperatures, which accelerates the crystallization process, or even allows using precursor reagents that would otherwise react with water, such as alkoxides. The solvothermal method also allows for very homogeneous shape and size distribution of nanoparticles. A specific case of this synthesis, which was used in this work, involves an organometallic precursor and a solvent, while a surfactant may or may not be used. The precursor solution, containing the substances described previously, receives heat treatment and the reaction occurs continuously under heating until precipitation of nanoparticles is observed (Yin & Alivisatos 2005, Chen & Mao 2007, Riporto et al. 2024, Kløve et al. 2024, Zhang et al. 2023).

In heterogeneous catalysis, the catalyst and reactants are in different phases, and what favors catalysis in this case is the surface of the catalyst, as it acts through covalent interactions or adsorption, making it easier to separate the final products (Yin & Alivisatos 2005, Lott & Deutschmann 2023, Gupta et al. 2020, Chen et al. 2023). Some homogeneous catalysts are used in industry, such as sodium hydroxide, in the case of biodiesel production from transesterification reactions, which can generate major environmental problems, in addition to the unwanted formation of soap. Therefore, the use of heterogeneous catalysts is better in this regard (Shi & Li 2014, Melchiorre et al. 2021, Mandari & Devarai 2022, Maquirriain et al. 2021).

Nanoparticles are effective in heterogeneous chemical catalysis, since by obtaining a material with a very small particle size of the order of nanometers, a large ratio of exposed surface per mass of catalyst is also obtained, thus improving the catalytic activity compared to the same material on larger scales. It is important to highlight that, for these nanoparticles to perform well in catalytic activity, it is necessary to have a controlled shape and size distribution, aiming for homogeneity of the material so that it can then be analyzed and applied as catalysts for reactions (Raimondi et al. 2005, Munnik et al. 2015, Campelo et al. 2009, Liu & Corma 2023, Hirschbiegel et al. 2023).

Inorganic oxides are studied for catalysis of esterification reactions due to their acidic characteristics (Endalew et al. 2011, Borges & Díaz 2012, Otroshchenko et al. 2021, Abdullah et al. 2022, Kore et al. 2021, Han et al. 2020, Wang et al. 2022). Heterogeneous acid catalysts are of great interest because they do not cause problems, such as corrosion, which can occur with the use of homogeneous catalysts (Sajjadnejad et al. 2024). Therefore, the use of solid oxides becomes a good option. Furthermore, the use of homogeneous catalysts creates greater difficulty in separating them after use (Davies et al. 2001, Thomas & Raja 2006, Sani et al. 2014, Vilcocq et al. 2014, Carriello et al. 2023). When used as a heterogeneous catalyst, TiO2 can readily be recovered as it does not mix in the same phase as the reagents and is not consumed in the reaction, reducing negative impacts such as generating acidic or basic residues (Gupta et al. 2020).

Esterification reactions are very important for the world economy, due to organic esters having the most diverse applications, from solvents and lubricants to cosmetics and foods. A currently widely studied type of reaction is the esterification of fatty acids with alcohols to produce biodiesel. Its synthesis involves a carboxylic acid and an alcohol, which react through condensation (Alves et al. 2014, Endalew et al. 2011, Borges & Díaz 2012, Sani et al. 2014, Carriello et al. 2023, Nascimento et al. 2012, Ghedini et al. 2021, Khan et al. 2021).

The use of TiO2 nanoparticles can overcome these problems for some reasons, such as: TiO2 nanoparticles have acidic characteristics; TiO2 is a stable and non-toxic inorganic oxide; its nanoparticles are solid and have a large specific surface area, which favors catalysis (Chen & Mao 2007, Al Taee & Al Shabander 2022, Mohammadi & Isazadeh 2019, Qiang et al. 2023, Rahman et al. 2023, Kowalkińska et al. 2023).

In this work, TiO2 nanoparticles were synthesized using the solvothermal route with benzyl alcohol as a solvent, investigating through a chemometric approach the influence of variables such as reaction time, temperature, and concentrations of titanium and hydrochloric acid in the formation of nanoparticles and their agglomeration. These nanoparticles were applied as catalysts for the esterification of oleic acid with ethanol, calculating the conversion rates of the reactions by using a calibration curve and the FTIR spectra of the products, leading to a simple and non-destructive method of determining the amount of formed product.

MATERIALS AND METHODS

Titanium(IV) oxide

Synthesis

The solvothermal route was used for the synthesis of the nanoparticles, using benzyl alcohol as a solvent, titanium(IV) isopropoxide as the Ti precursor and hydrochloric acid as a crystallizing agent (Bian et al. 2016). The experiments were organized using a 24-1 factorial design, whose variables and conditions are shown in Tables I and II. The levels of the variables were chosen arbitrarily.

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Table I
Variables for the synthesis of titanium(IV) oxide nanoparticles.
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Table II
24-1 factorial planning.

First, 20.0 mL of benzyl alcohol (98%, Synth, Brazil) were heated until reaching the desired synthesis temperature of 100 or 150 °C. Subsequently, titanium(IV) isopropoxide (97%, Sigma Aldrich, United States of America) and hydrochloric acid (37%, Synth, Brazil) were added, and the mixture was kept under heating for Δt, according to Tables I and II. After the solvothermal treatment, the suspension was centrifuged, and the nanoparticles were washed with tetrahydrofuran (99%, Synth, Brazil) three times. TiO2 crystallization occurs at very high temperatures, and the use of HCl as a crystallization agent allows TiO2 crystallization at lower temperatures, below 100 °C (Watson et al. 2004). Furthermore, the surface of the nanoparticles was modified through the sulfation of TiO2 by treatment in H2SO4 to verify whether the sulfate was adsorbed on its surface. For this, a solution of 10.0 mL of 1.0 mol L-1 H2SO4 was prepared and added to a beaker. Then, 0.1 g of TiO2 from sample 1 was added and left under stirring for 1 hour at room temperature. This sulfated sample was named Ti-9.

Characterizations

For X-ray diffractometry (XRD), a Rigaku diffractometer was used, with θ-2θ varying in the range between 5 and 75°, with an angular step of 0.02° and CuKα radiation of λ = 0.15406 nm. For Fourier transform infrared spectroscopy (FTIR), a Bruker model Equinox 55 spectrometer operating in diffuse reflectance mode was used. A total of 32 scans were performed between 400 and 4000 cm-1 with a resolution of 4 cm-1. The samples for analysis were prepared by mixing potassium bromide and the nanoparticle sample in a proportion of 100:1, approximately. Scanning electron microscopy (SEM) was done using a Zeiss Supra 35 model microscope equipped with a field emission gun, operated at 30 kV, was used. The samples were prepared by dispersing a small amount of nanoparticles in 900 μL of tetrahydrofuran until a cloudy suspension was obtained and then 20 μL of the suspension was deposited on a monocrystalline silicon plate with dimensions of approximately 0.5 x 0.5 cm.

Catalytic tests

Initially, three preliminary tests were carried out. The first with only ethanol (99,5%, Synth, Brazil) and oleic acid (90%, Sigma Aldrich, United States of America) with a proportion of 10:1. The second test was executed in the same manner with the Ti-1 catalyst, while the third used a modified catalyst (Ti-9). All of these three tests were carried out at 70 °C for one hour with stirring in beakers sealed with parafilm to reduce alcohol volatilization, with 1% in mass of catalyst in relation to oleic acid for the catalyzed reactions. The esterification reactions can be seen in Figure 1.

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Figure 1
Scheme for the esterification of oleic acid with ethanol catalyzed by TiO2.

After the characterization of the previous preliminary tests, the absence of alcohol and ethyl oleate was observed. Another test was carried out with the presence of catalyst (Ti-1) and at a reduced temperature of 50 °C for two hours. The absence of alcohol and ethyl oleate was then observed again. With this, three final preliminary tests were carried out. The first test had oleic acid and ethanol in a proportion of 1:100, with stirring and a temperature of 50 °C for 1 hour, the second test in the same way using a catalyst (Ti-1) and the third test using modified catalyst (Ti-9). With the ethanol to oleic acid proportion of 100:1 derived from the preliminary tests, along with the maximum temperature of 50 ºC, the definitive catalytic tests were performed, with all their variables described in Table VI.

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Table V
Primary effects on average particle size and average agglomeration size.
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Table VI
Catalytic test conditions and conversion percentages for all tests.

The influence of resting time on the percentage of ethyl oleate formed was also investigated, under the same conditions of test C-2. Test C-12 was analyzed immediately after the reaction, test C-13 with one day of rest and test C-14 after two days of rest. The composition of the bottom of the container with the supernatant was also evaluated, with two analyses being carried out (C-15 and C-16) under the same conditions as C-9. Fourier transform infrared spectroscopy was used to characterize and analyze the catalytic tests, according to the methodology described by Pegoraro et al. (2022). A calibration curve was obtained by preparing mixtures of oleic acid and ethyl oleate and varying their molar fraction from pure oleic acid to pure ethyl oleate in steps of 0.1 molar fraction, resulting in 11 mixtures. As the proportional amount of ethyl oleate increases in each mixture, the carbonyl peak centered on 1747 cm-1 in the FTIR spectra increases in area, while the carbonyl peak of oleic acid, centered on 1717 cm-1 in the FTIR spectra, decreases in total area. The ratios of these areas of reagent and product carbonyl peaks were plotted versus the molar fraction of ethyl oleate for each mixture, resulting in a linear relation, whose equation was used to determine the conversion rates in the catalytic tests.

RESULTS AND DISCUSSION

Synthesis

The diffractograms obtained for the eight synthesized samples are shown in Figure 2.

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Figure 2
X-ray diffraction patterns of nanoparticle samples synthesized by the solvothermal method according to the 24-1 factorial design described in Tables I and II, Ti-X, with X = 1 to 8, along with commercial TiO2.

Figure 2 shows the presence of peaks characteristic of the anatase phase in all synthesized samples, in accordance with literature (Wetchakun & Phanichphant 2008), which coincide with the peaks in crystallographic records (JCPDS-02-0387), in which well-defined peaks are characteristic of crystalline samples (Mishra 2019). All samples, even those treated for shorter periods of time and at lower temperatures, were crystalline. This result clearly shows the effectiveness of the proposed synthesis method, which allows obtaining crystalline oxides at comparatively low temperatures, in accordance with works already reported in the literature (Wetchakun & Phanichphant 2008, Bischoff & Anderson 1995, Imai & Hirashima 1999). Similar synthesis routes have already shown scalability aiming at market applications, resulting in over 20 grams of crystalline metal oxide nanoparticles in one large batch. By employing such temperatures in benzyl alcohol, the need for a calcination step is eliminated, offering cost reduction (Garnweitner et al. 2007).

Additionally, as seen in Figure 2, both commercial TiO2 samples presented the rutile phase. The lattice parameters can be calculated using the Miller indices present in the crystallographic sheet of anatase TiO2 (Antić et al. 2012, Pang et al. 2013). The results of Miller indices and average crystallite sizes (Dm) are shown in Table III after calculations using Bragg’s Law (Bragg 1912a, b, 1913) and Scherrer equation (Miranda & Sasaki 2018).

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Table III
Data on the experimental conditions of the eight experiments and respective lattice parameters and average crystallite size.

It is observed that experiments carried out at higher temperatures of 150 °C resulted in larger crystallite size. This behavior is expected because with higher temperatures the energy within the system will be greater and more shocks will occur between particles. As nanoparticles have a very large specific surface area, there is a tendency for particles to come together after this collision, generating larger particles (Lehtinen & Zachariah 2002). There was no noticeable influence by other variables on particle size, as the first sample was satisfactory with respect to average crystallite size and lattice parameters. Therefore, this sample was selected for subsequent tests, as it is a sample synthesized at a lower temperature, time and concentration of reagents. The Ti-9 sample, obtained by modifying the surface of the Ti-1 sample through the adsorption of sulfate by treatment in 1 mol L-1 sulfuric acid for 1 hour, was analyzed by FTIR, shown in Figure 3.

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Figure 3
FTIR spectra for samples Ti-1 to Ti-9, which was treated with sulfuric acid.

The most important information to be extracted from these spectra is the incorporation of sulfate on the surface of the nanoparticles, which can be verified by the appearance of the sulfate band indicated in Figure 3. Even with a shorter sulfation time than described in the literature (Srinivasan et al. 2006, Gómez et al. 2003, López et al. 2000, Murcia et al. 2015, Fadilah et al. 2023, Wijaya et al. 2023), it was possible to obtain nanoparticles with sulfate adsorbed on their surface. Hydrophilic behavior is expected from nanoparticles with sulfate adsorbed on the surface. For application in the catalysis of esterification reactions, hydrophilic nanoparticles are important for good dispersion in alcoholic medium. Furthermore, sulfated oxides may perform better in the catalysis in question (Wu et al. 2008). Little presence of bands related to organic compounds can be observed in both samples, indicating that the washing process was effective in removing residues. The presence of -OH bands may be observed in case water is adsorbed on a specific sample, as is evident in the case of Ti-4.

Scanning electron microscopy analysis allows observing aspects related to particle size and dispersion. The formation of large agglomerates by the nanoparticles was observed. In some cases, it is possible to see some dispersed particles, but they tended to agglomerate in all nine samples. Figure 4 shows the SEM images for samples Ti-1 and T-9.

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Figure 4
Scanning electron microscopy of samples Ti-1 and Ti-9.

Using the images of the 9 samples, an average calculation was performed by selecting five particles from each sample at a magnitude of 50.00 KX and five clusters from each sample at a magnitude of 10.00 kV. The results are seen in Table IV.

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Table IV
Average particle and agglomerate size in sampling of 5 measurements.

A relatively high value is observed for the average size of the nanoparticles. With the absence of surfactant, nanoparticles tend to grow and agglomerate due to their high surface energy (Sanchez-Martinez et al. 2018, Santacruz-Chávez et al. 2015). Even so, it is a small and somewhat controlled size, necessary for subsequent application in catalysis of esterification reactions.

Furthermore, using the chemometric planning described in Table II and the results shown in Table IV, it is possible to calculate the primary and secondary effects of each variable on the average particle size and the average agglomerate size. The effect of each variable was calculated by subtracting the sum of the responses at the lower levels from the sum of the responses at the higher levels of that variable. The results are shown in Table V.

It can be seen in Table V that time does not significantly influence the average particle size. However, the concentration of titanium(IV) ions negatively influences the size, that is, the higher the concentration, the smaller the size of the nanoparticles. This was also observed with hydrochloric acid, but with a lesser effect. Finally, temperature has the opposite effect. The higher the temperature, the larger the average particle size.

Regarding the size of the cluster, Table V shows that the concentration of titanium(IV) ions and hydrochloric acid are negligible. Temperature has the opposite effect, the higher the temperature, the smaller the size of the cluster. Finally, time has a positive effect, since longer times resulted in larger cluster size.

The experimental observations can be justified by considering several factors. Firstly, the relatively high average size of the nanoparticles observed can be attributed to their tendency to grow and agglomerate due to their high surface energy, even without the presence of a surfactant (Chaudhuri & Paria 2011). However, this growth is somewhat controlled, resulting in a small size suitable for subsequent application in catalysis of esterification reactions. Moreover, time has a positive effect on cluster size, as longer reaction times lead to larger clusters (Stolarczyk et al. 2016, Jensen et al. 1999).

Catalytic tests

The catalytic tests consisted of a study of the influence of variables such as temperature, reaction time and percentage of catalyst on the catalytic performance of the nanoparticles by quantifying the percentage of conversion of oleic acid and ethanol into ethyl oleate. The preliminary tests indicated that the temperature of 70 ºC and the proportion of 10:1 of ethanol to oleic acid were not adequate to produce ethyl oleate, leading to the evaporation of the alcohol. With a reduced temperature of 50 °C and the increase in the proportion of reagents to 100:1, the carbonyl peak of ethyl oleate was identified and the definitive catalytic tests were carried out following the variables in Table VI, in which control tests are named “B”, commercial TiO2 tests are named “T”, tests catalyzed by synthesized samples are named “C” and reusability tests are called “R”.

Using the methodology for quantification described previously, a conversion of 22.7% (C-1) was obtained in 1 hour of reaction and 24.1% (C-2) in two hours of reaction at 25 °C, higher than for the same experiment carried out at 50 °C. However, this sample was kept at rest before analysis, indicating that the reaction continued even without stirring.

Four other tests were carried out (C-3, C-4, C-5 and C-6) to verify whether the amount of catalyst influences the percentage of ethyl oleate formed. The experiments were carried out with an increase in temperature from 25 °C to 50 °C and with an increase in the amount of catalyst from 1% to 2%. A large increase in the percentage of ethyl oleate formed was observed, ranging from 47.4% to 52.6%, with only C-5 being an outlier at 41.0%, likely resulting from ethanol escaping the reaction vessel and leading to a smaller conversion rate. This phenomenon may have been related to both the temperature and the amount of catalyst. Therefore, five other tests were carried out (C-7, C-8, C-9, 1 C-10 and C-11), with an increase in the mass percentage of the catalyst from 2% to 3% and varied reaction times. The temperature was maintained at 50 °C and the tests were analyzed after rest. Data from all samples are shown in Table VI.

It can be seen in Table VI that the increase in the conversion rate may be associated with the increase in the amount of catalyst, given the smaller conversion rates of non-catalyzed reactions (B-1 and B-2), which is an influencing factor for the esterification reaction under the conditions that were studied. Since the surface of the catalyst is responsible for the catalytic activity (Wen et al. 2015), the results are as expected, as with a greater amount of catalyst and a larger exposed surface of the nanoparticles, the reaction has a higher conversion rate (Liu et al. 2022, Ding et al. 2017).

Comparing the conversion of the C-9 test with the commercial TiO2 samples used under the same conditions (T-1 and T-2), a similar conversion rate was achieved with the material synthesized in this work, while the other commercial titanium oxide (T-2) did not perform as well.

Additionally, reusing the same catalyst in another catalysis reaction resulted in a lower conversion rate under the same reaction parameters, as can be seen in tests R-1 and R-2. This indicates that an additional reactivation step is required, washing the nanoparticles or even performing a calcination step (Sienkiewicz et al. 2021, Moosavi et al. 2020). These results indicate that the catalyst shows satisfactory performance only in the first cycle, with a significant reduction in catalytic activity in subsequent cycles. This can be attributed to the agglomeration of nanoparticles and the possible leaching of active sites during the recovery and reuse process (Li et al. 2010).

Analyzing tests from C-1 to C-11 in Table VI, an increase in the amount of ethyl oleate is also observed over longer reaction times, but a more concrete conclusion cannot be established due to the reaction carrying on as the samples rest.

The C-12, C-13 and C-14 test results showed an increase in the amount of ethyl oleate over rest time, with respective conversion values of 25.9% after 2 hours of reaction time, 32.9% after one day of rest and 39.6% after two days of rest. In this way, a large amount of product is formed even after decreasing the temperature and without stirring. Over time, the nanoparticles that act as a catalyst settle on the bottom of the container, where the oily part of the solution is located.

This is in accordance with the C-15 and C-16 tests, in which it was noted that there is a presence of oleic acid and ethyl oleate both at the bottom and in the supernatant in the container. The conversion values for products in these two phases are very close, and the difference may be due to the inhomogeneity of the solution, with a certain area of the solution having a greater amount of reactants and products than another. The FTIR spectra of the products from the C-15 and C-16 tests were analyzed to obtain information about the composition in relation to other substances present, in addition to the ethyl oleate and oleic acid peaks, which are shown in Figure 5.

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Figure 5
FTIR spectra for the C-15 and C-16 tests.

In the spectra of Figure 5, the presence of the absorption band referring to the OH group stretching is observed at approximately 3500 cm-1. This band is characteristic of alcohols and water, and its appearance may indicate the presence of ethanol, used in the esterification reaction (Tarhan et al. 2022), or also the formation of water as a product of the reaction, in both parts of the container. The presence of ethanol together with oleic acid and a large amount of settled nanoparticles at the bottom of the container may justify the continuity of the reaction over time, which was observed in C-12, C-13 and C-14 tests. As ethanol was used in excess, it can alter the reaction equilibrium towards product formation.

This test stage provided semiquantitative results, as there were observations such as the continuity of the reaction even without heating and stirring and the composition of the supernatant and bottom of the container. For more conclusive results, it is suggested to centrifuge the catalytic test samples to remove the catalyst and heat them to evaporate the alcohol immediately after the esterification reaction. In this way, the reaction would be interrupted due to the lack of reagent and catalyst. Furthermore, the catalysts can be retrieved and further employed in other catalytic tests. Literature shows that TiO2 catalysts can present reusability after isolation, washing and drying, while the sulfate adsorption may need to be repeated for better effectiveness, in a process called reactivation (Zhou et al. 2022, Verma et al. 2017, Berrones-Hernández et al. 2019).

CONCLUSIONS

It was possible to synthesize TiO2 nanoparticles using the solvothermal method, obtaining a particle diameter of approximately 10 nm. It can be concluded that reaction time did not significantly influence the average particle size, and that the concentration of titanium(IV) ions and hydrochloric acid had a negative influence on it. Temperature, in turn, had a positive influence. Regarding the average size of the cluster, the concentration of titanium(IV) ions and hydrochloric acid were negligible, while temperature had a negative effect and time had a positive effect. Nanoparticles with surface modification through sulfate adsorption did not show catalytic activity, indicating that the catalyst must have been dispersed in the organic part of the solution, which was consistent with the literature, as protonation of oleic acid occurs by the acid catalyst. The catalytic activity of TiO2 nanoparticles was observed in the esterification reaction, and factors such as temperature and amount of catalyst influence and increase the catalytic activity of the nanoparticles. A quantity of ethyl oleate of up to 63.5% was obtained with the use of 3% catalyst at 50 °C for 3 hours. A significant decrease in catalytic activity was observed after the first cycle, indicating the need to explore methods to stabilize the catalyst. It is suggested to investigate surface modification techniques to prolong the catalyst’s lifespan over multiple cycles.

ACKNOWLEDGMENTS

This study was financed in part by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

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Publication Dates

  • Publication in this collection
    02 Dec 2024
  • Date of issue
    2024

History

  • Received
    29 Jan 2024
  • Accepted
    10 Sept 2024
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