# Generation of isolated Ti4+ sites via one-pot synthesis of mesoporous Ti-SBA-15 catalysts for high-performance oxidative desulfurization of dibenzothiophene

Adisak Guntida, Françoise Maugé, Cassandre Kouvatas, Luis-Jacobo

Aguilera, Oleg I Lebedev, Piyasan Praserthdam, Supareak Praserthdam,
Karine Thomas

► To cite this version:

Adisak Guntida, Françoise Maugé, Cassandre Kouvatas, Luis-Jacobo Aguilera, Oleg I Lebedev, et al. Generation of isolated Ti4+ sites via one-pot synthesis of mesoporous Ti-SBA-15 catalysts for high-performance oxidative desulfurization of dibenzothiophene. Materials Today Sustainability, 2025, 31, pp.101204. 10.1016/j.mtsust.2025.101204. hal-05353259

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# Generation of isolated Ti⁴⁺ sites via one-pot synthesis of mesoporous Ti-SBA-15 catalysts for high-performance oxidative desulfurization of dibenzothiophene

Adisak Guntida<sup>a</sup>, Françoise Maugé<sup>a</sup>, Cassandre Kouvatas<sup>a</sup>, Luis-Jacobo Aguilera<sup>a</sup>, Oleg I. Lebedev<sup>b</sup>, Piyasan Praserthdam<sup>c</sup>, Supareak Praserthdam<sup>d</sup>, Karine Thomas<sup>a,*</sup>

<figure><img src="image_2.png" /></figure>

## ARTICLE INFO

### Keywords:

Oxidative desulfurization

Ti-SBA-15

Dibenzothiophene

Infrared spectroscopy

NMR

## ABSTRACT

Ti-incorporated SBA-15 catalysts were synthesized via a one-pot hydrothermal method and tested in the liquid-phase oxidative desulfurization (ODS) process at 80 °C and atmospheric pressure for low-sulfur fuel production. These catalysts exhibited superior ODS performance compared to conventional impregnation-based catalysts. Characterization confirmed the successful incorporation of titanium ions into the SBA-15 framework, forming Si–O–Ti bonds with tetrahedrally coordinated Ti⁴⁺ species. Two types of Ti⁴⁺ sites—crystalline and isolated—were identified, with the isolated sites contributing more significantly to catalytic activity. The high accessibility of dibenzothiophene (DBT) to isolated Ti⁴⁺ sites, along with the minimal presence of crystalline TiO₂ phases, were key factors in the enhanced performance. Quantitative analysis showed that isolated Ti⁴⁺ sites exhibited a turnover frequency (TOF) approximately 20 times higher than that of crystalline Ti⁴⁺ sites, confirming their role as the primary active sites in the ODS reaction.

## 1. Introduction

The removal of sulfur from transportation fuels and petrochemicals has garnered increasing attention due to the growing recognition of the adverse effects of burning sulfur-containing oils on both human health and the environment. Sulfur-containing compounds commonly found in fuel oils—such as mercaptans, thiophenes (T), benzothiophenes (BT), and dibenzothiophenes (DBT)—generate sulfur oxides (SOₓ) during combustion. These SOₓ emissions are major contributors to acid rain and urban air pollution. Moreover, sulfur compounds can lead to corrosion and catalyst deactivation in desulfurization processes within the refining industry. Therefore, the effective removal of these compounds is essential for the production of environmentally friendly, low-sulfur fuels that meet the stringent sulfur content limits (10–15 ppm) set by the United States Environmental Protection Agency (USEPA). This transition is critical for mitigating the environmental impact of sulfur emissions [1–3].

The ODS process has attracted significant attention as a promising

alternative for sulfur removal, owing to its mild operating conditions, including low temperature and atmospheric pressure. Additionally, ODS offers enhanced selectivity for the removal of refractory aromatic sulfur compounds compared to conventional hydrodesulfurization methods. The selective oxidation of sulfur-containing compounds can be facilitated by transition metal-based catalysts (e.g., Mo, Ti, Fe, V, W) that possess high oxidation states, along with appropriate Lewis acidity and redox properties. Among solid catalysts, titanium-containing silicas (Ti–SiO₂) have emerged as some of the most effective materials for the ODS process due to their favorable catalytic characteristics [4–8].

Various materials have been investigated as selective oxidation catalysts for the ODS reaction, including microporous zeolitic Ti-silicates (e.g., TS-1, Ti-beta, Ti-MOR, Ti-ITQ-7, Ti-MWW), mesoporous Ti-silicas (e.g., Ti-MCM-41, Ti-HMS, Ti-SBA-15), and commercial amorphous titania–silica composites (TiO₂/SiO₂). Previous studies have demonstrated that the performance and stability of ODS catalysts are influenced by several factors, including the coordination environment of the titanium centers, structural and surface characteristics of the

<footnote><sup>a</sup> Université de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, 14000, Caen, France</footnote>

<footnote><sup>b</sup> Université de Caen Normandie, ENSICAEN, CNRS, CRISMAT, Laboratoire de Cristallographie et sciences des matériaux, 14000, Caen, France</footnote>

<footnote><sup>c</sup> Center of Excellence on Catalysis and Catalytic Reaction Engineering (CECC), Chulalongkorn University, Bangkok, 10330, Thailand</footnote>

<footnote><sup>d</sup> High-Performance Computing Unit (CECC-HCU), Chulalongkorn University, Bangkok, 10330, Thailand</footnote>

<footnote>* Corresponding author.
E-mail address: karine.thomas@ensicaen.fr (K. Thomas).</footnote>
support, the nature of the oxidant, reagent ratios, and reaction temperature. Among these materials, mesoporous Ti–SiO₂ catalysts exhibit higher activity and longer operational lifetimes than their microporous counterparts. This advantage is attributed to their larger pore diameters, which facilitate improved diffusion of bulky sulfur-containing molecules to the active sites, unlike the restricted access presented by the narrow pores of microporous materials. Furthermore, the catalytic performance is critically dependent on both the amount and the nature of the titanium species dispersed within the silica matrix. In particular, the presence of tetrahedrally coordinated Ti sites is essential, as the selective oxidation of sulfur compounds typically initiates with the activation of the oxidant—such as hydrogen peroxide (H₂O₂)—at a coordinatively unsaturated metal center [1–3,8].

SBA-15 is widely recognized as a promising support for the development of advanced catalytic materials, owing to its large pore size, uniform hexagonal mesostructure, and narrow pore size distribution. Its relatively thick amorphous silica walls also confer excellent hydrothermal stability [9]. Traditionally, titanium has been introduced onto SBA-15 via post-synthetic methods such as chemical grafting or incipient wetness impregnation, as reported by numerous research groups. However, a hydrothermal one-pot synthesis method—employed by only a limited number of studies—has demonstrated greater efficiency in incorporating titanium directly into the SBA-15 framework. This approach enables the insertion of Ti⁴⁺ cations into tetrahedral sites within the silica network, forming Si–O–Ti linkages and significantly altering the electronic environment of the titanium centers [10]. In titanosilicate-based catalysts, the ODS of DBT, a model sulfur compound, proceeds via a well-established radical mechanism. Initially, the titanium sites coordinate with the lone pair electrons on the oxygen atom of H₂O₂, forming a titanium–peroxo complex. This interaction leads to cleavage of the Si–O–Ti bond, generating Ti–OOH and Si–OH species. Subsequently, the titanium–peroxo complex undergoes nucleophilic attack by the sulfur atom in DBT, yielding dibenzothiophene sulfoxide. Further oxidation by an additional titanium–peroxo complex results in the formation of dibenzothiophene sulfone [11].

In this study, the ODS of DBT, a model sulfur compound, was investigated using titanium-incorporated SBA-15 catalysts synthesized via a one-pot hydrothermal method. The reaction was carried out in a batch reactor using H₂O₂ as the oxidant. Particular attention was given to assessing the influence of the Si/Ti molar ratio on catalytic activity. To gain a comprehensive understanding of the catalyst’s structure–activity relationship, a suite of advanced characterization techniques was employed, including Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray spectroscopy (EDX-STEM), ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR), and in situ Fourier-transform infrared (FTIR) spectroscopy using adsorbed pyridine, 2,6-dimethylpyridine (lutidine), and low-temperature carbon monoxide adsorption. Additionally, in situ attenuated total reflectance infrared (ATR-IR) spectroscopy was performed to monitor the adsorption of pyridine and acridine under liquid-phase conditions. These analyses collectively validate the morphology, structural integrity, and spatial distribution of titanium active sites within the SBA-15 framework, while also confirming the highly efficient liquid-phase conversion of DBT. By integrating both qualitative and quantitative data, this study provides critical insights into the structure–function relationship of titanium-incorporated SBA-15 catalysts for ODS applications.

## 2. Experimental

### 2.1. Materials and catalysts preparation

SBA-15 was synthesized hydrothermally following previously documented procedures [12]. The synthesis was carried out using an aqueous

solution of Pluronic P123 triblock copolymer (EO₂₀PO₇₀EO₂₀, MW = 5800, Sigma-Aldrich) and hydrochloric acid (HCl, Fisher Scientific, 37.5 %). Tetraethoxysilane (TEOS, Sigma-Aldrich, 98 %) was used as the silicon source in all cases. The molar composition of the starting reaction mixture was 0.022 mol of P123, 1.0 mol of TEOS, 0.7 mol of HCl, and 130 mol of H₂O. The synthesis procedure is as follows: 8.0 g of Pluronic P123 was dissolved in 146.25 g of distilled water along with 4.5 g of HCl under vigorous stirring at 35 °C for 6 h. After complete dissolution, 13.0 g of TEOS was added to the homogeneous solution all at once. The mixture was then stirred at 35 °C for an additional 24 h. Subsequently, the synthesis mixture was transferred to an oven and heated at 80 °C for 24 h under static conditions. The resulting silica product was filtered while hot and dried overnight at 140 °C. For template removal, the as-synthesized silica powder was calcined at 550 °C for 5 h.

Titanium-incorporated SBA-15 was synthesized using a one-step method, as described in previous studies [13–15]. A calculated amount of the titanium precursor, titanyl (IV) acetylacetonate (Sigma-Aldrich), was added to TEOS and stirred vigorously at 35 °C for 24 h. Catalysts with varying Si/Ti molar ratios (50, 20, 10, and 5) were prepared, and the titanium content was accurately determined by inducively coupled plasma optical emission spectroscopy (ICP-OES). The resulting Ti content was 0.5 %, 2 %, 6 %, and 12 % by weight, corresponding to the catalyst samples labeled 0.5Ti-SBA-15, 2Ti-SBA-15, 6Ti-SBA-15, and 12Ti-SBA-15, respectively. For comparison, a conventional impregnated catalyst with 2 % Ti content was synthesized and labeled 2Ti/SBA-15.

### 2.2. Catalyst characterization

The porosity and surface area of the solids were determined by nitrogen adsorption isotherms at –196 °C using a Micrometrics ASAP 2020 gas adsorption system. Measurements were performed at a relative pressure (P/P₀) ranging from 0.05 to 1. Prior to adsorption, 100 mg of each sample was outgassed under vacuum at 350 °C for 4 h.

XRD patterns were obtained at low and high angles using a Dynamic 500 Anton Paar diffractometer with CuKα radiation (λ = 0.15418 nm, 40 mA, 45 kV) and a step size of 0.00012°.

Raman spectroscopy was employed to analyze the phase nature of TiO₂ using a Jobin Yvon Labram 300 spectrometer equipped with a confocal microscope, an Nd:YAG laser (frequency doubled, 532 nm), and a CCD detector. Spectra were recorded at room temperature in the 250–1400 cm⁻¹ range.

UV–Vis spectroscopy was performed using a Varian Cary 4000 UV–Vis spectrophotometer over a wavelength range of 200–800 nm.

ICP-OES was performed using a Varian ICP-OES 720-ES. Approximately 50 mg of the sample was dissolved in 0.5 mL of aqua regia (HNO₃/HCl = 1:3 v/v) and 3 mL of hydrofluoric acid. After stirring at 110 °C for 1 h, 2 g of boric acid was added, and the solution was diluted to a final volume of 100 mL.

SEM images were obtained using a Tescan Mira I LMH microscope operating at 20 kV. HAADF-STEM was carried out using a JEM 200F cold field emission gun (FEG) microscope operating at 200 kV. The microscope was equipped with a CENTURIO XL EDX-STEM detector, and EDX-STEM elemental mapping data were simultaneously acquired. TEM samples were prepared by depositing an ethanolic suspension of the target phase onto a Cu holey carbon grid.

Solid-state MAS NMR spectra were acquired using an AVANCE III Bruker Ultrashield™ 500 Plus spectrometer, operating at 500.13 MHz and 99.36 MHz for ¹H and ²⁹Si, respectively, and equipped with a double-resonance 4 mm probe head. Samples were packed into 4 mm zirconia rotors and spun at 12 kHz. For ²⁹Si MAS NMR experiments, single-pulse excitation with a π/6 flip angle and a 20-s recycle delay were used. [¹H]–²⁹Si cross-polarization (CP) MAS experiments were conducted under Hartmann-Hahn conditions without proton decoupling, with a contact time of 6 ms and a recycle delay of 3 s.

In-situ FTIR experiments for gas-phase adsorption were conducted to
investigate the adsorption behavior of pyridine, lutidine, and low-
temperature carbon monoxide (CO). An in-situ IR cell was connected
to a vacuum system maintaining a dynamic vacuum level of better than
10<sup>-5</sup> mbar. FTIR measurements were performed using a Nicolet iZ10
FTIR spectrometer equipped with an MCT detector, covering the range
from 400 to 5500 cm<sup>-1</sup> with a spectral resolution of 4 cm<sup>-1</sup> and 128
scans in transmission mode. Prior to recording the spectra, a sample disk
(typically 15 mg and 2 cm<sup>2</sup>) was activated in the in-situ vacuum cell at
500 °C (ramping at 4 °C/min) for 2 h. Following activation, the sample
was cooled to room temperature under vacuum, and its IR spectrum was
recorded. For pyridine and lutidine adsorption, the respective substance
was introduced into the cell and saturated at 1.33 mbar. Physisorbed
molecules were subsequently removed by evacuation. FTIR spectra were
collected at various desorption temperatures, typically ranging from 75
to 300 °C. For CO adsorption, a low-temperature cell was modified to
adsorb CO at –196 °C. CO doses, ranging from 0.48 mbar (low coverage)
to 5.29 mbar (high coverage), were introduced via a calibrated volume.

In-situ ATR-IR experiments with pyridine and acridine in the liquid phase were conducted to assess the accessibility of molecules across active sites. The cell setup includes a dome-like device on the ATR crystal, allowing gas or liquid flow via a 3-way valve. A Specac Golden Gate ATR with a diamond crystal was used in a Nicolet iS50 FTIR spectrometer with nitrogen-cooled MCT detection and CO₂-free air. Pyridine or acridine solutions and pure solvent were delivered through a multi-channel peristaltic pump, switched via a 3-way valve. N-hexane was used as the solvent, with test solutions of 1 wt% pyridine or 0.5 wt% acridine. Background spectra were recorded with an empty cell under a nitrogen flow of 50 mL/min. A solid suspension droplet was dried on the ATR crystal, pre-treated in nitrogen at 200 °C for 2 h, then cooled to room temperature and saturated with n-hexane for baseline spectra. Finally, pyridine or acridine solution was flowed over the sample to saturation, and non-adsorbed molecules were desorbed with n-hexane flow.

2.3. Activity test

The substrates used in this study included dibenzothiophene (DBT;
≥99 %, Sigma-Aldrich), hydrogen peroxide (H₂O₂; 30 wt% in H₂O;
Sigma-Aldrich) as the oxidizing reagent, n-octane (≥99 %, Sigma-
Aldrich) as the model fuel, and tetradecane (C₁₄H₃₀; ≥99 %, Sigma-
Aldrich) as the internal standard. DBT was initially dissolved in n-oc-
tane to create a model fuel with a sulfur content of 10,000 ppm [16,17].
The ODS reaction was performed in a 25 mL two-necked flask equipped
with a magnetic stirrer and a reflux condenser. A total of 250 mg of the
catalyst and 10 mL of the model fuel were added to the reactor, which
was placed in a thermostatically controlled water bath set at 80 °C. The
oxidation reaction was initiated by adding H₂O₂, maintaining a
H₂O₂/DBT molar ratio of 4:1. The mixture was vigorously stirred at a
constant speed using the magnetic stirrer. The concentration of DBT in
the n-octane phase was monitored at fixed time intervals using a gas
chromatograph (Shimadzu GC-2010) equipped with a flame ionization
detector (FID). After completion of the reaction, the model fuel was
extracted with acetonitrile to confirm the formation of the DBT sulfone
product, which was analyzed using GC-FID.

**3. Results and discussion**

**3.1. Structural and textural characterization of Ti-containing catalysts**

3.1.1. *Structural properties*

Fig. S1 presents the small- and wide-angle XRD patterns of both SBA-
15 and Ti-containing SBA-15 samples. In the small-angle region
(Fig. S1–a), three peaks are observed: one intense peak and two weaker
peaks at 2θ = 0.8–2.1°, corresponding to the (100), (110), and (200)
reflections of the hexagonal space group p6mm. These peaks indicate
the successful formation of the characteristic hexagonal structure of

SBA-15 throughout the synthesis process. Upon closer inspection of the dominant peak in the Ti-incorporated SBA-15 samples, it is apparent that the position of the (100) reflection remains almost unchanged. This suggests that the insertion of titanium into the framework does not significantly affect the unit cell parameter of the mesoporous structure. Similarly, the Ti-impregnated catalyst exhibited no significant change in unit cell dimensions, indicating that titanium introduction by either method preserves the structural integrity of the SBA-15 framework. However, the intensity of the signal, which is related to the ordering of the mesoporous structure, decreases with increasing titanium loading. Specifically, at 6% and 12% titanium loading, the mesoporous ordering is notably disrupted [18]. These results confirm that well-ordered Ti-incorporated SBA-15 catalysts were successfully synthesized through a one-pot method, particularly at moderate titanium loadings. In the wide-angle region (Fig. S1–b), both siliceous SBA-15 and Ti-containing SBA-15 catalysts exhibit a characteristic band between 2θ = 20° and 35°, which is attributed to the silica framework. The anatase phase of TiO₂ typically shows diffraction peaks at 2θ values of 24.8°, 37.3°, 47.6°, 53.5°, 55.1°, and 62.2° [JCPDS Card No. 21–1272] [19]. At low titanium loadings, these anatase TiO₂ peaks are absent, suggesting that crystalline TiO₂ is either not present on the catalyst surface or is present in amounts too small to be detected by XRD. However, the appearance of the anatase TiO₂ phase becomes evident in Ti-incorporated SBA-15 catalysts with higher titanium content, particularly at 6% and 12% titanium loading. At these higher loadings, larger aggregates of anatase TiO₂ are observed [20,21].

Raman spectroscopy is an effective tool for analyzing the crystal-
linearity of TiO₂ within Ti-SBA-15, as it is sensitive to vibrational modes
associated with ordered structures (Fig. S2). The characteristic Raman
bands for anatase TiO₂ appear at 399, 513, and 638 cm⁻¹. In the Ti-
incorporated SBA-15 catalysts, the intensity of these bands corre-
sponding to the anatase phase of TiO₂ increased with higher titanium
loading. However, no distinct anatase TiO₂ bands were observed in the
Ti-impregnated SBA-15 catalyst. These observations align with the XRD
results. For the SBA-15 silica framework, the typical Si–O–Si bending
vibration band appears at 490 cm⁻¹. This band appears to be diminished
upon titanium impregnation [22].

3.1.2. *Textural properties*

N₂ adsorption-desorption measurements were conducted to analyze
the changes in surface area, pore size distribution, and pore volume after
the incorporation of titanium into SBA-15. Fig. S3–a shows the N₂
adsorption-desorption isotherms for each sample. A typical type IV
isotherm is observed, characterized by large H1-type hysteresis loops at
high relative pressures (P/P₀ = 0.4–0.9). The sharpness of the steps
further indicates the typical ordered mesoporous nature of the material,
with larger pore sizes and a narrow size distribution. The pore size
distribution curve of each sample is also presented in Fig. S3–b. It is
 evident that the substitution of titanium did not significantly alter the
textural properties of the SBA-15 support. However, the relative pres-
sure range becomes significantly wider with increasing titanium con-
tent, particularly at 6 % and 12 %. This suggests a slight reduction in
pore size, as indicated in Table S1 [23]. As titanium content increases,
the surface area and pore size of the catalysts decrease, from 804 to 763
m²/g and 5.19 to 4.70 nm, respectively. This observation suggests that
the one-pot synthesis method successfully incorporated titanium within
the framework. However, at higher titanium loading, there is a possi-
bility that titanium oxide may form on the internal surface of the mes-
oporous samples [15]. This finding is consistent with the identification
of a crystalline TiO₂ anatase phase using XRD analysis.

**3.1.3. Morphology of the catalysts**

The morphology of the catalysts was examined using SEM, as shown
in Fig. S4. The SEM image of pure SBA-15 reveals a rod-like topography
with lengths exceeding 4 µm [24]. The Ti-incorporated SBA-15 catalysts
with low titanium loading also exhibit a rod-like shape, although the
length of the rods is shorter compared to the pure SBA-15 material. However, at higher titanium loadings (6 % and 12 %), the SEM images reveal an irregular shape, suggesting that the catalyst morphology is influenced by the titanium content during preparation. This is further supported by the presence of TiO₂ agglomerates in the anatase phase on the SBA-15 surface, as indicated by the XRD results. HAADF-STEM images provide additional insights into the structural characteristics of the catalysts. The hexagonal pore structure and orderly pore arrangement are clearly visible in the HAADF-STEM images of the Ti-impregnated (Fig. 1-a) and Ti-incorporated (Fig. 1-b) catalysts at 2 % loading, confirming the well-ordered mesoporous hexagonal structure of the SBA-15 matrix. These observations suggest that the structure of the original material is preserved after modification with titanium. Corresponding EDX-STEM elemental mapping clearly shows the distribution of titanium across each sample. In the 2Ti/SBA-15 sample, titanium is evenly distributed, while in the 2Ti-SBA-15 sample, some segregation of pure titanium is observed. Notably, high-resolution HAADF-STEM images and EDX-STEM mapping reveal that titanium is absent within the pores of SBA-15. In contrast, the long-range order and mesoporous structure of the 6Ti-SBA-15 catalyst are significantly disrupted, as evidenced by the presence of large titanium nanoparticle clusters on the surface (Fig. 1-c). The EDX-STEM elemental mapping further confirms the uneven distribution of titanium nanoparticles on the SBA-15 material, consistent with previous reports [25,26]. These findings are in agreement with the

earlier characterization results.

### 3.1.4. Evidence of titanium insertion into the SBA-15 framework

$^{29}$Si MAS-NMR was employed to confirm the insertion position of titanium within the silica framework of SBA-15, as previously reported [13]. The $^{29}$Si MAS NMR spectra of the samples are presented in Fig. 2. In all the samples, three peaks are observed at –92 ppm (Q$_{2}$), –101 ppm (Q$_{3}$), and –110 ppm (Q$_{4}$), which are attributed to Si(OSi)$_{2}$(OH)$_{2}$, Si(OSi)$_{3}$OH or Si(OSi)$_{3}$Ti, and Si(OSi)$_{4}$, respectively [27]. This was further corroborated by $^{[1]}$H-$^{29}$Si CPMAS experiments (Fig. S5), which showed an enhancement of the Q$_{2}$ and Q$_{3}$ signals relative to the Q$_{4}$ signals. It has been reported that -OH and -OTi groups have a similar effect on the chemical shift of the central Si atoms. Therefore, Ti-incorporated silicas lead to Q$_{3}$-like environments, where hydrogens in OH groups or silicons in Q$_{4}$ units are replaced by titanium [13,23,27]. Additionally, the areas of the three peaks were determined using Gaussian function fitting, and the ratio of relative intensity between Q$_{3}$ and Q$_{4}$ was calculated and is presented in Table S2. The increase in the Q$_{3}$/Q$_{4}$ ratio indicates an enrichment of Q$_{3}$ species, serving as a measure of Ti insertion into the silica framework. This increase is more pronounced in the Ti-incorporated catalysts compared to pure SBA-15, suggesting successful titanium ion incorporation into the SBA-15 framework via the one-pot synthesis method. The insertion of titanium follows the order: 2Ti-SBA-15 > 6Ti-SBA-15 > 0.5Ti-SBA-15 > 12Ti-SBA-15. In contrast,

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Low magnification and high resolution HAADF-STEM images, ADF image and simultaneously acquired EDX-STEM elemental mapping of Ti K image of the following catalysts materials: (a) 2Ti/SBA-15, (b) 2Ti-SBA-15 and (c) 6Ti-SBA-15.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** $^{29}$Si MAS NMR spectra of the catalysts.</figcaption></figure>

the Q₃/Q₄ ratio for the impregnated catalyst (2Ti/SBA-15) does not show a significant increase compared to pure SBA-15, indicating that titanium oxide nanoparticles are located on the surface of SBA-15.

ATR-IR spectroscopy provides direct evidence of Ti atom incorpo-
ration into the SBA-15 framework. This technique is particularly effec-
tive for analyzing chemical bonding changes on material surfaces,
including metal integration into silica matrices. Fig. S6 presents the
ATR-IR spectra of SBA-15 and Ti-containing SBA-15 samples in the
wavenumber range of 900–1000 cm⁻¹. In the SBA-15 support, a band at
972 cm⁻¹ is observed, corresponding to the stretching vibrations of
Si–O–Si linkages and surface hydroxyl groups [28]. Upon titanium
incorporation, these species react with Ti atoms to form Si–O–Ti bonds.
This interaction weakens the Si–O component, leading to a reduced vi-
bration frequency and a corresponding band shift to 954 cm⁻¹. This
redshift is widely recognized as an indicator of Ti incorporation and
suggests that titanium is chemically bonded to the silica framework,
rather than present as aggregated TiO₂ nanoparticles. These findings are
in good agreement with previous reports [21,23,27]. In contrast, the
impregnated 2Ti/SBA-15 catalyst exhibits a weakened 972 cm⁻¹ band,
and notably, no new band appears at 954 cm⁻¹, indicating the absence
of Si–O–Ti bonding. These observations imply that titanium is not in-
tegrated into the silica framework in the impregnated sample but rather
exists in a non-framework, likely surface-deposited form. Consistent
with the results from $^{29}$Si MAS NMR, the ATR-IR data further confirm
that Ti ions are successfully incorporated into the SBA-15 framework via
the one-pot synthesis approach.

Furthermore, the UV–Vis results provide valuable insight into the
coordination environment of Ti species (Fig. S7). An absorption band

observed around 210–225 nm is attributed to the ligand-to-metal charge
transfer (LMCT) of isolated tetrahedrally coordinated Ti⁴⁺ species
incorporated into the silica framework. This indicates that Ti has been
successfully substituted into the mesoporous SBA-15 structure. In
contrast, absorption bands appearing at wavelengths above 250 nm are
indicative of extra-framework Ti species, such as oligomeric or crystal-
line anatase-like TiO₂ clusters. These species typically exhibit octahedral
coordination and suggest partial segregation or agglomeration of Ti
outside the mesoporous walls [29]. Thus, UV–Vis analysis confirms the
presence of both framework (isolated) and non-framework (aggregated)
Ti species, with their relative proportions depending on the Ti loading in
the catalyst. As the Ti loading increases, the UV–Vis spectra exhibit both
a red shift in the absorption edge and an increase in absorbance in-
tensity. The red shift (i.e., absorption at longer wavelengths) indicates
the formation of non-framework Ti species, such as octahedrally coor-
dinated TiO₂, either as monolayers or aggregated crystalline domains.
These species exhibit lower-energy LMCT transitions compared to iso-
lated tetrahedrally coordinated Ti⁴⁺ in the silica framework. The
increased absorbance intensity reflects the higher concentration of Ti
species in the sample and may also suggest the formation of bulk-like
TiO₂ domains, especially at high loadings where crystalline anatase
phases may begin to form.

For the Ti/SBA-15 catalyst, CO-IR analysis revealed no evidence of isolated Ti⁴⁺ sites, but instead indicated the presence of crystalline TiO₂ species. This observation is consistent with XRD and TEM results, which showed the formation of very small, highly dispersed crystalline domains on the Ti/SBA-15 surface. Based on this combined evidence, it can be suggested that the TiO₂ exists predominantly in the form of monolayer crystalline domains rather than as isolated framework-incorporated species over Ti/SBA-15 catalyst. The LMCT bands of monolayer TiO₂ typically fall within the 230–260 nm range, which can overlap with or obscure the signals from true framework tetrahedral Ti⁴⁺. These monolayer domains are likely well-dispersed on the SBA-15 surface, yet retain a degree of crystalline order, as supported by the spectroscopic and microscopic analyses [30,31].

3.2. *The catalytic performance*

The ODS of DBT was carried out at 80 °C using H₂O₂ as the oxidant. As expected, the pure SBA-15 material exhibited no catalytic activity, confirming its inertness in the desulfurization process. However, upon incorporation of titanium into SBA-15, DBT conversion increased over time, clearly indicating that the observed ODS activity arises from the presence of titanium species within the Ti-containing SBA-15 catalysts. Under steady-state conditions, the catalytic activity followed the order: 2Ti-SBA-15 > 6Ti-SBA-15 > 0.5Ti-SBA-15 > 12Ti-SBA-15 > 2Ti/SBA-15 (Fig. S8). Notably, the Ti-impregnated catalyst (2Ti/SBA-15) exhibited relatively low activity, achieving only ~20 % DBT conversion, whereas all Ti-incorporated catalysts demonstrated substantially higher activity, ranging from 55 % to 100 %. Although 100 % conversion was achieved only with the 2Ti-SBA-15 catalyst, the possibility of partial deactivation over time cannot be excluded. The key objective is therefore to identify the surface sites formed on Ti-containing SBA-15 prepared by both impregnation and one-pot incorporation methods and to determine which of these sites are most catalytically active in the ODS reaction.

3.3. Surface sites of the Ti-containing catalysts

### 3.3.1. Hydroxyl groups

Fig. S9 shows the IR spectra of surface silanol groups on the catalysts after outgassing at 500 °C. A sharp peak at 3750 cm⁻¹ is attributed to the stretching vibration of isolated surface silanol groups (Fig. S9–a), while a shoulder around 3670 cm⁻¹ corresponds to terminal hydroxyl groups perturbed by titanium, either due to the formation of Si–O–Ti linkages or the presence of TiOₓ phases (Fig. S9–b) [32,33]. Notably, for the impregnated catalyst (2Ti/SBA-15), the intensity of the isolated silanol
peak at 3750 cm⁻¹ is significantly reduced compared to that of parent SBA-15, suggesting that Ti ions have condensed onto surface hydroxyl sites [24]. A similar reduction is observed for the high Ti-content incorporated catalyst (12Ti-SBA-15), likely due to the condensation of excess titanium species onto the silica framework's hydroxyl groups. In contrast, no significant decrease in the intensity of isolated silanol groups is observed in the Ti-incorporated SBA-15 catalysts with lower Ti loadings (0.5Ti-SBA-15 and 2Ti-SBA-15). This indicates that at higher loadings, TiO₂ particles—previously identified by Raman spectroscopy and XRD—are primarily deposited on the internal or external surface rather than incorporated within the SBA-15 framework. This interpretation is further corroborated by HAADF-STEM and EDX-STEM imaging results.

### 3.3.2. Acidity of the surface sites

#### 3.3.2.1. Gas-phase pyridine adsorption.
FTIR spectroscopy of adsorbed pyridine was employed to investigate the nature and strength of acid sites present on the catalysts (Fig. S10). The band at 1545 cm⁻¹ is assigned to pyridine protonated on Brønsted acid sites (BAS), while bands at 1448 cm⁻¹ and ~1608 cm⁻¹ correspond to pyridine coordinated to Lewis acid sites (LAS). The band at 1489 cm⁻¹ is associated with pyridine interacting with both BAS and LAS. The spectra clearly indicate that the Ti-containing SBA-15 catalysts predominantly exhibit Lewis acidity. This is attributed to the presence of tetrahedrally coordinated Ti⁴⁺ species within the mesoporous SBA-15 framework and, to a lesser extent, in the TiO₂ phase [14,34].

Fig. S12-a shows the variation in LAS concentration as a function of desorption temperature. A molar absorption coefficient of 2.2 cm/µmol was applied to quantify the pyridine adsorbed on LAS [35]. At 100 °C, the highest LAS concentration was observed for the 2Ti/SBA-15 catalyst. For the Ti-incorporated catalysts, LAS concentration increased proportionally with titanium loading. Upon evacuation at 200 °C and 300 °C, a sharp decrease in LAS was noted for the impregnated catalyst, whereas the Ti-incorporated catalysts exhibited a more gradual decline. This suggests that the LAS in the impregnated sample are weaker, likely due to the presence of surface TiO₂ rather than framework-incorporated Ti species. These results support the conclusion that the one-pot incorporation method promotes the formation of stronger and more thermally stable LAS through the formation of Si–O–Ti linkages.

#### 3.3.2.2. Gas-phase lutidine adsorption.
IR spectroscopy of lutidine adsorption was employed to probe the nature and strength of BAS in the Ti-containing catalysts [36]. Due to its higher basicity and significant steric hindrance from the methyl groups, lutidine is more selective for BAS and less likely to interact with LAS, making it a valuable probe for hydroxyl group characterization [37–39]. Fig. S11 presents the IR spectra of lutidine adsorbed on the catalyst surfaces, followed by desorption at increasing temperatures. Two prominent bands at 1648 and 1629 cm⁻¹, corresponding to the 8a and 8b vibrational modes of lutidinium ions, confirm the presence of acidic hydroxyl groups.

Fig. S12-b illustrates the evolution of BAS concentration as a function of desorption temperature. Quantification was performed using a molar absorption coefficient of 6.8 cm/µmol [39]. At low desorption temperatures (<100 °C), the Ti-impregnated catalyst exhibited a higher BAS concentration than the Ti-incorporated samples. This behavior is consistent with a higher density of surface-accessible Ti species, corroborated by pyridine adsorption data. However, at 125 °C, the BAS signal in the impregnated catalyst declined sharply, indicating weak acid strength incapable of retaining the lutidine molecule. In contrast, the Ti-incorporated catalysts retained lutidine at higher temperatures, suggesting the presence of stronger BAS. Additionally, the 8a band shifted from 1648 cm⁻¹ in the impregnated catalyst to 1653 cm⁻¹ in the incorporated samples, a shift indicative of increased acid strength [35]. These findings further support the superior incorporation of Ti into the

silica framework via one-pot synthesis, resulting in stronger and more thermally stable BAS [38].

#### 3.3.2.3. CO adsorption.
Low-temperature CO adsorption serves as a highly sensitive probe for detecting variations in the coordination and dispersion of titanium species at the atomic level. According to Hadjijianov et al. [40], distinct $\nu$(CO) stretching frequencies can differentiate between isolated Ti⁴⁺ sites and those associated with crystalline TiO₂ phases. Specifically, a band at 2181 cm⁻¹ corresponds to isolated tetrahedrally coordinated Ti⁴⁺ species ($Ti_i^{4+}$), while bands at 2190 and 2200 cm⁻¹ are associated with Ti⁴⁺ in anatase-type TiO₂ crystallites—denoted as $\beta$ and $\alpha$ crystalline sites ($Ti_i^{4+}$), respectively.

Fig. 3 presents the CO adsorption spectra on Ti-containing SBA-15 catalysts across varying coverages. Three characteristic bands emerge in the carbonyl stretching region: ~2181 cm⁻¹ (Ti⁴⁺–CO), ~2157 cm⁻¹ (CO bound to surface hydroxyl groups), and ~2138 cm⁻¹ (physisorbed CO) (Fig. 3-a) [41,42]. Fig. 3-b focuses on the Ti⁴⁺–CO region, while Fig. 3-c exclusively presents the Ti⁴⁺–CO region for both the initial and final doses of CO adsorption. The Ti-impregnated catalyst (2Ti/SBA-15) exhibits a single $\nu$(CO) band, initially at 2190 cm⁻¹, which shifts to 2184 cm⁻¹ with increasing CO coverage. This redshift reflects vibrational coupling among CO molecules adsorbed on densely packed Ti⁴⁺ $\beta$ sites, characteristic of crystalline anatase ($\beta$Ti⁴⁺) [40]. Conversely, Ti-incorporated catalysts display two distinct $\nu$(CO) bands at 2191 cm⁻¹ and 2181 cm⁻¹ at low CO coverage. The higher-wavenumber band (2191 cm⁻¹) shifts to 2187 cm⁻¹ upon saturation, indicative of $\beta$Ti⁴⁺ sites in proximity, while the persistent 2181 cm⁻¹ band is attributed to well-isolated $Ti_i^{4+}$ species embedded within the silica framework. In samples with high Ti content (6% and 12% Ti-SBA-15), an additional weak band appears at 2200 cm⁻¹, characteristic of highly acidic, undercoordinated $\alpha$Ti⁴⁺ sites, typically found at corners or edges of TiO₂ crystallites [40,43]. This observation aligns with STEM-EDX findings indicating the presence of crystalline TiO₂ nanoparticles in these samples. A minor band at 2173 cm⁻¹ is also detected, which may be assigned to CO adsorbed on Ti–OH sites, consistent with observations on TiO₂ surfaces [44,45]. These results further substantiate the structural and electronic differences between incorporated and impregnated Ti species and highlight the superior dispersion and framework integration achieved via the one-pot synthesis method.

In addition to qualitative analysis, quantitative evaluation was performed by integrating the areas of the CO stretching bands following spectral deconvolution (Fig. 4). A molar absorption coefficient of 2.6 cm/µmol was applied to quantify the CO adsorbed on Ti sites [46]. Notably, the Ti–OH-associated CO band shows a trend consistent with that observed for lutidine adsorption, suggesting a similar distribution of surface hydroxyl and BAS (Fig. 4-a). Furthermore, the total CO uptake at saturation for Ti–OH and $\beta$Ti⁴⁺ sites follows a comparable order: 2Ti/SBA-15 > 12Ti-SBA-15 > 6Ti-SBA-15 > 2Ti-SBA-15 ≥ 0.5Ti-SBA-15 (Fig. 4-b). This correlation highlights the fact that both bands originate from regions enriched with surface-accessible Ti species. For the impregnated catalyst (2Ti/SBA-15), the high concentration of exposed $\beta$Ti⁴⁺ and Ti–OH sites reflects effective dispersion of the titania phase across the SBA-15 surface. In contrast, Ti-incorporated catalysts with higher Ti loading exhibit fewer accessible crystalline Ti sites, attributed to the formation of small TiO₂ crystallites, as confirmed by STEM imaging. For the low-loading Ti-incorporated samples (0.5Ti-SBA-15 and 2Ti-SBA-15), titanium is predominantly embedded within the silica matrix, resulting in a minimal presence of crystalline TiO₂ domains. A focused analysis on isolated Ti sites ($Ti_i^{4+}$), as shown in (Fig. 4-c), reveals their exclusive presence in Ti-incorporated catalysts. The quantity of these isolated sites increases with Ti content up to 6 wt%, beyond which it declines significantly at 12 wt%. This reduction is ascribed to the coverage of surface-isolated Ti⁴⁺ sites by newly formed TiO₂ nanocrystallites, diminishing their accessibility.

These findings underscore the utility of low-temperature CO
<figure><img src="image_5.png" /><figcaption>Fig. 3. FTIR spectra of adsorbed CO at increasing coverages on the catalysts ranging from 0.48 (first dose) to 5.29 mbar (last dose) of CO exposure; overall spectra after CO exposure (a), Zoom Ti⁴⁺-CO region (b) and a closer look at Ti⁴⁺-CO region of the first CO dose (c).</figcaption></figure>

adsorption as a sensitive tool for probing the coordination environment and dispersion of titanium species. The combined spectroscopic evidence confirms the successful synthesis of Ti-incorporated SBA-15 materials via one-pot synthesis. These catalysts maintain the hexagonal mesostructure of SBA-15 and feature well-dispersed, tetrahedally coordinated Tiₗ⁴⁺ sites within the silica framework—sites identified as catalytically relevant for oxidation reactions such as ODS.

### 3.4. Probing surface sites by liquid phase adsorption

To assess the accessibility of DBT to titanium surface sites relevant

for the ODS reaction, the adsorption of acridine (C₁₃H₉N) was investigated. Acridine was chosen as a probe molecule due to its structural resemblance to DBT, in which the sulfur atom is replaced by a nitrogen atom. Similar to other nitrogen-containing aromatic bases such as pyridine and collidine [41,47], acridine is expected to interact with acid sites via its lone electron pair on nitrogen, making it a suitable probe for studying the accessibility and strength of acidic sites. Importantly, acridine's larger molecular size enables the evaluation of acid site accessibility for bulky substrates like DBT [48,49]. Due to its exceptionally low vapor pressure, acridine cannot be examined under conventional gas-phase adsorption conditions. Therefore, in-situ ATR-IR spectroscopy
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** The evolution of CO coverages adsorbed on the various Ti⁴⁺ sites: Ti-OH (a), crystalline Ti⁴⁺ (b), isolated Ti⁴⁺ (c).</figcaption></figure>

in the liquid phase was employed. This approach allows continuous flow and real-time monitoring of acridine adsorption under controlled conditions.

Before proceeding with acridine, the reliability and compatibility of liquid-phase adsorption data with those obtained from gas-phase experiments were validated. To this end, pyridine adsorption was first conducted in the liquid phase using the same in-situ ATR-IR setup, and the resulting spectra were compared with those from gas-phase pyridine adsorption discussed earlier. This comparative analysis confirmed that ATR-IR measurements in the liquid phase can provide consistent and complementary insights into acid site accessibility and strength, thus enabling a meaningful interpretation of acridine adsorption behavior on the Ti-containing catalysts.

### 3.4.1. Liquid-phase pyridine adsorption

Fig. 5-a displays the ATR-IR spectra obtained from (1) the pyridine solution in n-hexane (dotted line), (2) the steady-state adsorption of liquid-phase pyridine on the catalyst film (black solid line), and (3) the residual, strongly adsorbed species after flushing with n-hexane to remove weakly or physisorbed molecules (blue solid line). On the SBA-15 support, weak interactions between pyridine and surface silanol groups are evident from the residual bands at 1597 and 1446 cm⁻¹, corresponding to hydrogen-bonded pyridine species (Py-H). During the adsorption phase, additional bands at 1578 and 1438 cm⁻¹—associated with unbound pyridine in solution—are observed, but these disappear after rinsing, indicating that these species are not chemisorbed. No bands characteristic of pyridinium ions (typically observed near 1545 cm⁻¹) were detected on the Ti-containing SBA-15 catalysts, suggesting that pyridine, in its liquid-phase form, is not sufficiently basic to probe the relatively weak BAS present. In contrast, the persistent bands at 1597 and 1446 cm⁻¹ indicate the presence of LAS, with intensity increasing systematically with titanium content. Given the similarity in wavenumbers of the Py-H and Py-L bands in the liquid phase [47], the contribution of residual Py-H cannot be excluded. This is consistent with the generation of Ti⁴⁺-based Lewis acid centers, either as framework-incorporated Ti or TiO₂ domains.

While quantitative determination of acid site densities by ATR-IR remains challenging—primarily due to uncertainties in the exact volume probed by the evanescent field—the similar bulk densities of the samples allow for semi-quantitative comparisons. Under this assumption, the relative amount of pyridine chemisorbed on LAS sites follows the order: 0.5Ti-SBA-15 < 2Ti-SBA-15 < 6Ti-SBA-15 < 12Ti-SBA-15 < 2Ti/SBA-15. This ranking mirrors the trends observed in gas-phase pyridine adsorption (Fig. S13), confirming the consistency between gas- and liquid-phase analyses and reinforcing the conclusion that LAS concentration scales with titanium loading, especially in the impregnated sample.

### 3.4.2. Liquid-phase acridine adsorption

The in-situ ATR-IR spectra of acridine adsorption are presented in Fig. 5-b. On the SBA-15 support, saturation with acridine leads to the appearance of characteristic vibrational bands [50]; however, these signals are almost entirely removed upon flushing with n-hexane, indicating only weak physisorption on surface silanol groups. A similar trend is observed for the Ti-impregnated catalyst (2Ti/SBA-15), where acridine bands vanish after desorption, suggesting negligible interaction with the LAS associated with crystalline TiO₂ domains. In contrast, the Ti-incorporated SBA-15 catalysts exhibit persistent acridine adsorption bands following hexane flushing. Notably, the 19a bands at 1519 and 1465 cm⁻¹, along with the 8a (1560 cm⁻¹) and 8b (1623 cm⁻¹) modes, remain detectable in the desorbed spectra. This retention confirms stronger interactions between acridine and the catalyst surface, attributed to isolated Ti⁴⁺ sites within the silica framework (denoted as Acri-L), consistent with the behavior previously observed for pyridine adsorption.

A comparative analysis of LAS accessibility probed by liquid-phase pyridine and acridine is shown in Fig. 6. As illustrated in Fig. 6-a, the intensity of the 1446 cm⁻¹ band—associated with pyridine adsorption—increases with titanium loading, indicating a progressive increase in accessible LAS across the Ti-incorporated series. However, the opposite trend is observed for acridine (Fig. 6-b), where the intensity of the 1465 cm⁻¹ band decreases with increasing Ti content. This divergence implies that acridine, due to its bulkier molecular structure relative to pyridine, encounters steric limitations that hinder its access to active Ti⁴⁺ sites, especially at higher titanium loadings. Semi-quantitative analysis reveals that the decrease in the intensity of the Acri-L band correlates with a reduction in the number of sites interacting with acridine. In contrast, the number of isolated Ti⁴⁺ sites, as probed by CO adsorption, increases. These observations suggest that the reduction in acridine accessibility is not due to a loss of isolated Ti⁴⁺ sites, but
<figure><img src="image_7.png" /><figcaption>Fig. 5. ATR-IR spectra of liquid-phase pyridine (a) and acridine (b) adsorption on catalysts: black lines indicate spectra at steady-state adsorption from pyridine or acridine solution; blue lines show spectra after desorption of weakly adsorbed pyridine or acridine by hexane flow; dotted lines represent the spectrum of liquid pyridine or acridine.</figcaption></figure>

rather a decrease in their accessibility. This reduced accessibility is likely caused by the presence of adjacent titanium crystallites, which could block or hinder the interaction between acridine and the isolated Ti⁴⁺ sites within the mesoporous structure. These findings highlight the critical role of titanium dispersion and framework incorporation in optimizing site accessibility for bulky aromatic substrates such as DBT in oxidative desulfurization processes.

### 3.5. Active sites for ODS reaction

Previous characterizations have shown that Ti-containing catalysts possess OH groups with very weak acidity, and two distinct types of Ti⁴⁺ sites have been identified: isolated and crystalline Ti⁴⁺ sites. On the impregnated catalyst, a dense phase of titania forms on the SBA-15 surface, leading to the development of crystalline Ti⁴⁺ sites. In contrast, the Ti-incorporated catalyst incorporates Ti into the SBA framework, promoting the formation of Si-O-Ti bonds and resulting in the emergence of isolated Ti⁴⁺ sites. At higher Ti loadings in the incorporated Ti catalyst, small titania particles form on the SBA surface alongside the isolated Ti⁴⁺ sites. ATR-IR experiments involving large molecules, such as acridine, reveal that the accessibility of isolated Ti⁴⁺ sites can be hindered by the presence of titania particles. Steric hindrance, due to the densely packed crystalline lattice, restricts access to the undercoordinated surface Ti atoms [51]. Since DBT and acridine molecules are similar in size, a comparable steric hindrance effect is expected for DBT. Consequently, the accessibility of DBT to isolated Ti⁴⁺ sites is likely reduced when these sites are surrounded by TiO₂ particles [52–54].

The literature underscores the role of LAS in the ODS reaction [14, 43, 55, 56]. A critical question is which specific LAS are the most active for this reaction. The impregnated catalyst, which contains a high amount of crystalline Ti⁴⁺ sites, exhibits the lowest ODS activity. On the other hand, Ti-incorporated catalysts, which feature low Ti loading and a smaller number of both isolated and crystalline Ti⁴⁺ sites, demonstrate

higher ODS activity than the impregnated catalyst. This suggests that isolated Ti⁴⁺ sites are more active for the ODS reaction than crystalline Ti⁴⁺ sites. However, the Ti-incorporated catalyst with the highest Ti loading shows a lower ODS activity, despite having a higher concentration of both isolated and crystalline Ti⁴⁺ sites. This result may appear counterintuitive at first, but liquid-phase acridine adsorption experiments provide insight: large molecules cannot access isolated Ti⁴⁺ sites when they are surrounded by titania particles, likely reducing the overall catalytic activity.

The previous results demonstrated that ODS activity depends on the contributions of two distinct types of LAS, which can be expressed by the following equation:

$$ \text{Converted DBT} = \text{TOF}_c \cdot n_{\text{crystalline}} + \text{TOF}_i \cdot n_{\text{isolated}} \quad (1) $$

Where $n_{\text{crystalline}}$ and $n_{\text{isolated}}$ represent the moles of crystalline and isolated Ti⁴⁺ sites, respectively, as determined by IR spectroscopy of CO adsorption.

The TOF of crystalline sites (TOF~c~) was calculated from the activity of the impregnated catalyst, which exclusively contains crystalline Ti⁴⁺ sites and no isolated ones. Using the fixed value of TOF~c~, the TOF~i~ corresponding to isolated sites was subsequently calculated by accounting for the total converted DBT and the number of both site types on Ti-incorporated catalysts. To minimize the effect of catalyst deactivation over time, the TOF values were determined based on catalytic activity measured after 15 min of reaction (as shown in Table 1). The results clearly indicate that TOF~c~ is significantly lower than TOF~i~. Specifically, TOF~i~ values for 0.5 % and 2 % Ti-SBA-15 are approximately 1.2 mmol g⁻¹ h⁻¹, while for 6 % and 12 % Ti-SBA-15, they drop sharply to ~0.2 mmol g⁻¹ h⁻¹. This decrease is attributed to the limited accessibility of isolated Ti⁴⁺ sites when they are surrounded by crystalline TiO₂ particles, as previously suggested by acridine adsorption studies. These findings lead to the conclusion that isolated Ti⁴⁺ sites are approximately 20 times more catalytically active than crystalline Ti⁴⁺ sites in the ODS of DBT, highlighting the critical role of site accessibility in catalyst
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Comparison of LAS intensity detected by the adsorption of liquid-phase pyridine at 1446 cm⁻¹ (a) and acridine at 1465 cm⁻¹ (b) against Ti loading for the Ti-incorporated SBA-15 catalysts.</figcaption></figure>

design.

**Scheme 1** presents the proposed surface model of Ti-incorporated SBA-15 during the ODS of DBT, highlighting the spatial distribution and functional relevance of titanium sites. In this model, isolated Ti⁴⁺ sites, which are tetrahedrally coordinated within the silica framework, are identified as the primary active sites responsible for the ODS reaction. These sites are readily accessible and exhibit high catalytic activity due to their ability to effectively adsorb and activate DBT molecules. As the titanium loading increases, a significant portion of Ti migrates to form crystalline TiO₂ domains, either as discrete particles or agglomerates on the SBA-15 surface. These crystalline regions are more densely packed and provide fewer active sites due to the lack of under-coordinated Ti⁴⁺ species essential for DBT activation. Moreover, the formation of crystalline TiO₂ near or around isolated Ti⁴⁺ sites introduces steric hindrance, impeding the access of large aromatic molecules such as DBT. As a result, the catalytic contribution of these isolated

sites is diminished, not due to loss of intrinsic activity, but rather due to reduced accessibility. Therefore, the surface model underscores that while both isolated and crystalline Ti⁴⁺ sites are present, the effectiveness of the catalyst is governed not only by the quantity but also by the spatial arrangement and accessibility of isolated Ti⁴⁺ sites. Maximizing the number of accessible isolated Ti⁴⁺ sites, and minimizing their encapsulation by crystalline TiO₂, is thus essential for achieving optimal ODS performance.

## 4. Conclusions

In summary, titanium-incorporated SBA-15 catalysts were successfully synthesized via a one-pot hydrothermal method, enabling the direct incorporation of titanium into the mesoporous silica framework. These materials were evaluated for their catalytic performance in the ODS of DBT. The results demonstrated that this catalytic approach enables efficient and, in some cases, complete removal of DBT under mild conditions. The incorporation of titanium into the SBA-15 framework was confirmed by solid-state ²⁹Si MAS NMR, which revealed the formation of Si–O–Ti bonds indicative of tetrahedrally coordinated Ti⁴⁺ species. This structural assignment was further corroborated by ATR-IR spectroscopy. Low-temperature FTIR spectroscopy using CO as a probe molecule allowed for the identification of two distinct types of Ti⁴⁺ Lewis acid sites: (i) isolated Ti⁴⁺ species incorporated into the silica

<figure><img src="image_9.png" /><figcaption>**Scheme 1.** Proposed surface model of Ti-incorporated SBA-15 during the ODS reaction of DBT.</figcaption></figure>

<table><caption>**Table 1**
TOF of crystalline and isolated sites for each catalyst.</caption><thead><tr><th rowspan="2">Catalyst</th><th>DBT conversion at 15 min</th><th>Converted DBT at 15 min</th><th>Mole of crystalline Ti<sup>4+</sup></th><th>Mole of isolated Ti<sup>4+</sup></th><th>TOF crystalline sites</th><th>TOF isolated sites</th></tr><tr><th>(%)</th><th>(mmol.g cat<sup>-1</sup>)</th><th>(µmol.g cat<sup>-1</sup>)<sup>a</sup></th><th>(µmol.g cat<sup>-1</sup>)<sup>a</sup></th><th>(mmol.g cat<sup>-1</sup>.h<sup>-1</sup>)</th><th>(mmol.g cat<sup>-1</sup>.h<sup>-1</sup>)<sup>b</sup></th></tr></thead><tbody><tr><td>2Ti/SBA-15</td><td>17</td><td>0.376</td><td>28.4</td><td>0</td><td>0.052</td><td>–</td></tr><tr><td>0.5Ti-SBA-15</td><td>34</td><td>0.734</td><td>2.7</td><td>2.1</td><td>–</td><td>1.30</td></tr><tr><td>2Ti-SBA-15</td><td>41</td><td>0.892</td><td>2.9</td><td>3.2</td><td>–</td><td>1.10</td></tr><tr><td>6Ti-SBA-15</td><td>48</td><td>1.050</td><td>20.8</td><td>20.4</td><td>–</td><td>0.15</td></tr><tr><td>12Ti-SBA-15</td><td>45</td><td>0.981</td><td>25.5</td><td>11.9</td><td>–</td><td>0.22</td></tr></tbody></table>

TOF crystalline = DBT conversion / mol of crystalline = 22/28.4 = 1.3.

a mole of isolated and crystalline sites was from CO adsorption results.

b TOF isolated = [DBT conversion – (TOF crystalline x n crystalline)]/n isolated.
matrix, and (ii) Ti⁴⁺ species associated with crystalline TiO₂ anatase.

Kinetic analysis based on turnover frequencies revealed that the isolated Ti⁴⁺ sites are approximately 20 times more active than their crystalline counterparts. Among the tested materials, the 2Ti-SBA-15 catalyst exhibited the highest catalytic activity, underscoring the crucial role of accessible, framework-incorporated Ti⁴⁺ sites in the ODS reaction. Catalysts that maintained a high proportion of isolated Ti⁴⁺ species and minimized TiO₂ crystallite formation demonstrated superior performance. However, increasing the titanium loading beyond an optimal threshold led to the formation of TiO₂ particles or agglomerates on the SBA-15 surface, which sterically hindered access to the isolated Ti⁴⁺ sites. This phenomenon was supported by ATR-IR studies of liquid-phase pyridine and acridine adsorption, where large probe molecules mimicking DBT showed reduced interaction with these active sites due to limited accessibility.

Overall, this study highlights the importance of both the nature and accessibility of active sites in determining catalytic performance. The findings provide key insights into the design of efficient, mesostructured Ti-based catalysts for sustainable desulfurization technologies and underscore the need to balance titanium dispersion and loading to optimize the number of accessible, high-activity Ti⁴⁺ sites.

### CRediT authorship contribution statement

**Adisak Guntida:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization. **Françoise Maugé:** Writing – review & editing, Validation, Supervision. **Cassandre Kouvatas:** Validation, Formal analysis. **Luis-Jacobo Aguilera:** Methodology. **Oleg I. Lebedev:** Validation, Formal analysis. **Piyasan Praserthdam:** Validation. **Supareak Praserthdam:** Validation. **Karine Thomas:** Writing – review & editing, Validation, Supervision, Methodology, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors would like to thank Dielix-Sarpi-Veolia and ADEME for supporting the UCOME2B100 project, as well as the Franco-Thai Mobility Program/PHC SIAM and LPI program of the Université de Caen Normandie and Carnot ESP for their financial support.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtsust.2025.101204.

### Data availability

Data will be made available on request.

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