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# Innovative synthesis of YF3-TiO2 acid-base catalysts with high surface area by embedding yttrium trifluoride nanoparticles in TiO2 metallogel

Shashank Mishra, Roua Ben Salem, Marion Eternot, Bhagyesh Purohit, Pascal Bargiela, Nadine Essayem

► To cite this version:

Shashank Mishra, Roua Ben Salem, Marion Eternot, Bhagyesh Purohit, Pascal Bargiela, et al. Innovative synthesis of YF3-TiO2 acid-base catalysts with high surface area by embedding yttrium trifluoride nanoparticles in TiO2 metallogel. ChemCatChem, 2024, pp.e202401530. 10.1002/cctc.202401530. hal-04787859

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Submitted on 18 Nov 2024

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# Innovative Synthesis of YF₃-TiO₂ Acid-Base Catalysts with High Surface Area by Embedding Yttrium Trifluoride Nanoparticles in TiO₂ Metallogel

Roua Ben Salem[a], Marion Eternot[a], Bhagyesh Purohit[a], Pascal Bargiela[a], Nadine Essayem*[a] and Shashank Mishra* [a]

[a] Dr R. Ben Salem, Dr M. Eternot, Dr B. Purohit, P. Bargiela, Dr N. Essayem, Dr. S. Mishra

Université Claude Bernard Lyon 1, CNRS, Institut de Recherches sur la Catalyse et l'Environnement de Lyon (IRCELYON)

2 avenue Albert Einstein, 69626 Villeurbanne (France)

E-mail: nadine.essayem@ircelyon.univ-lyon1.fr, shashank.mishra@ircelyon.univ-lyon1.fr

**Abstract:** Combining a water-tolerant oxide such as TiO₂ with yttrium fluoride is expected to provide catalysts with enhanced acid-base properties for catalytic applications in water. We present here a new strategy of incorporating pre-formed YF₃ nanoparticles (NPs) in a TiO₂-based metallogel, followed by its soft drying at room temperature to produce YF₃-TiO₂ xerogel with high surface area. The as-synthesized YF₃-TiO₂ materials were calcinated at 300 and 400 and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), ¹⁹F NMR and N₂ physisorption, calorimetry of NH₃ adsorption, and FT-IR of pyridine adsorption. These studies indicate that the fluorine is present under a stable form of YF₃ for the catalysts calcined at 300 °C. The acid-base properties of these YF₃-TiO₂ catalysts were investigated in a model reaction, i.e., dihydroxyacetone (DHA) conversion in water, and compared with the blank TiO₂ and YF₃ NPs alone. The incorporation of YF₃ in the TiO₂ matrix leads to enhanced initial rate of DHA dehydration into pyruvaldehyde, which is slowly converted to lactic acid as the reaction progresses. This suggests that the Brønsted acidity was boosted by the presence of YF₃ species via water adsorption in a dissociative form.

## Introduction

Titania (TiO₂) based materials are among the most efficient heterogeneous oxide catalysts employed so far for the catalytic transformation of dihydroxyacetone (DHA) into lactic acid (LA) in water. This transformation is often considered as an interesting model reaction for designing and testing acid catalysts for the production of commodity chemicals from lignocellulosic biomass in water.[¹⁻⁴] In addition to its intrinsic catalytic properties, TiO₂ has other advantages, such as being water-tolerant, cost-efficient, non-toxic and chemically stable. Thus, TiO₂ is also considered as a support of choice for the preparation of supported noble metals for the aqueous phase transformations. A catalyst's performance can further be enhanced by a careful choice of the synthetic methods as well as its composition to bring in the desired properties (specific active sites, high surface area, dispersibility, stability, etc.). One possible way to enhance the Lewis acidity of a catalytic system is to incorporate a Lewis acid species. For instance, it was recently shown that the introduction of yttrium species in the β-zeolite catalyst can enhance its acid properties, leading to the selective production of lactic acid during the cellulose conversion.[⁵]

Lanthanide fluorides (LnF₃) are interesting Lewis acids which have been employed as heterogenous catalysts for different organic reactions.[⁶] Kang and co-workers synthesized several lanthanide fluorides NPs (e.g., LaF₃, NdF₃ and YF₃) with hollow mesoporous morphology having crystal defects which acted as Lewis acid catalytic sites.[⁷] As a result, these hollow NPs showed 800 times more activity than the commercial LaF₃ without pores for the cyanosilylation reaction of benzaldehyde using trimethylsilyl cyanide. The Ru-supported catalysts Ru/LaF₃ and Ru/NdF₃ were also extremely active for the hydrogenation of benzene to cyclohexane as well as levulinic acid conversion into g-valerolactone. Yu et al. synthesized scandium-based fluorides NaScF₄ and KSc₂F₇ using thermal decomposition of metal trifluoroacetates and employed them as Lewis acid catalysts in the silylcyanation reaction of aromatic aldehydes with trimethylsilyl cyanide in CH₃CN.[⁸] Besides showing an excellent catalytic activity (complete conversion of aromatic aldehyde by loading only 5% catalyst at room temperature within 24 h), these catalysts also offered a remarkable stability and recyclability even after 10 cycles. Astruc et al. synthesized a mixed barium-lanthanum fluoride NPs (Ba₁ₓLaₓF₂+ₓ), either by co-thermolysis of metal trifluoroacetates or precipitation methods, for catalytic fluorination of 2-chloropyridine by Cl/F exchange in the presence of HF.[⁹] Li et al. synthesized nanoflowers-like yttrium fluoride (YF₃) hydrothermally and applied them as an electrocatalyst for the nitrogen reduction reaction for the synthesis of ammonia (NH₃) under ambient conditions.[¹⁰]

Metallogels are versatile soft materials having extended network of self-assembled low-molecular-weight species, which show significant advantages for advanced applications such as luminescence, templates for nanostructures, sensing, Magnetic Resonance Imaging (MRI) contrast agents, etc.[¹¹] These metallogels usually use organic ligands as gelators and are characterized by stable metal-ligand coordination network resulting in the formation of infinite, long-range and ordered one-, two-, or three-dimensional structures. Metallogels containing appropriately functionalized nanoparticles or clusters have been described as new exciting way of fabricating versatile materials for new applications, mainly for luminescent properties through cooperation between the NPs and the gel network.[¹²] In this article, we report embedding of yttrium trifluoride NPs in a transparent TiO₂ metallogel formed through controlled hydrolysis and condensation reactions and without using any organic gelator, as an innovative way of synthesizing new fluorinated
heterogeneous acid-base catalysts. Subsequent soft drying of this NPs-incorporated metallogel provided intimately mixed YF₃-TiO₂ catalysts with high specific surface area. The choice of yttrium fluoride NPs and its amount was based on the previous article.[5] These novel catalysts were thoroughly characterized in terms of phase, morphology and acidity using well established methods in gas phase i.e. pyridine or NH₃ adsorption monitored by FTIR and microcalorimetry. The model reaction of dihydroxyacetone (DHA) conversion in water was investigated to study these catalysts' acid/base properties in water.

## Results and Discussion

### Synthesis and physico-chemical characterizations of YF₃ nanoparticles, undoped TiO₂ and YF₃-embedded TiO₂ metallogels

#### Synthesis of [Y(TFA)₃(H₂O)₃] and its decomposition to YF₃ nanoparticles

The synthesis of [Y(TFA)₃(H₂O)₃] was achieved by reacting yttrium oxide (Y₂O₃) with trifluoroacetic acid (TFAH) in refluxing toluene for 24h. The hydrated product [Y(TFA)₃(H₂O)₃] was extracted as colorless solid from tetrahydrofuran.[13] The presence of TFA and H₂O groups in this precursor was confirmed by the FT-IR and TG-DTA studies,[13] the latter analysis being performed in argon atmosphere in the 20–600 °C temperature range. The TG-DTA curves of this precursor show a four-steps decomposition process (Figure 1A), the first 3 steps corresponding to loss of 3 coordinated water molecules in the temperature range 50-180 °C (11.9% exp. versus 11.2% calc.). This is further indicated by 3 endothermic peaks at 80, 123, and 178 °C. The last step (220–310 °C) corresponds to decomposition of the TFA ligands, as indicated by a strong exothermic peak at 305°C. The remaining weight of the residue is in a good agreement with the formation of YF₃ as the end-product (31.6% exp. vs. 30.3% calc.).

Taking a cue from the above decomposition pattern, yttrium trifluoride (YF₃) NPs were synthesized by thermal decomposition of yttrium trifluoroacetate [Y(TFA)₃(H₂O)₃] in 1-octadecene at 305 °C for 1h. We intentionally avoided using any surfactant such as oleic acid or oleyl amine, employed usually to control the particle size and their dispersibility, because the presence of these ligands on the surface of the NPs could potentially deactivate the catalytic sites. The XRD pattern of the as-prepared powder is shown in the Figure 1B, which exhibits well-crystallized orthorhombic YF₃ NPs (ICDD # 00-032-1431). An average nanocrystallite size of 10.6 nm was calculated using Rietveld refinement, whereas the BET surface area of the YF₃ NPs was found to be 82 m²/g.

### TiO₂ metallogel: synthesis and structural/textural analysis

To achieve robust gels, usually organic ligands with different functionalities are introduced into gel networks. However, presence of these ligands might deactivate the catalytic sites and, therefore, be detrimental for the catalysis. Fortunately, monolithic

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

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (A) TGA-DTA curves of [Y(TFA)₃(H₂O)₃] obtained with a heating rate of 5°C/min under argon atmosphere, and (B) Rietveld-refined XRD of as-synthesized YF₃ NPs.</figcaption></figure>

TiO₂ metallogels can be obtained via sol-gel process in a controlled acidic medium without using any organic ligand.[14,15] In the present case, we prepared transparent metallogel of TiO₂ by hydrolysing [Ti(OEt)₄] with HCl/H₂O solution containing a specific ratio of water (h = 2) and hydrochloric acid (R = 0.04), where h is the ratio of number of moles of water and titanium ethoxide, and R is the ratio of number of moles of HCl and titanium ethoxide.[16] A transparent monolithic gel was formed within 30 minutes. The undoped TiO₂ xerogel was obtained by leaving the metallogel in open air for 2 days. The as-dried metallogel was amorphous as indicated by its powder XRD. Its thermogravimetric analysis, performed in air in the range of 25°C to 600°C with a heating rate of 5°C/min (Figure 2A), shows one step dehydration process. This loss in the mass (25.9%) can be attributed to the removal of coordinated/weakly adsorbed surface water and alcohol molecules as well as condensation of the hydroxyl groups to TiO₂. No further mass loss was observed after 300°C. Subsequently, the as-dried undoped powder was calcined at different temperatures 300°C and 400°C (with a heating rate of 10°C/min for 2h, followed by natural cooling) to study the effect of calcination temperature on its crystallinity as well as surface properties. The powder XRD studies on these samples show that the xerogel starts crystallizing around 300 °C and is completely
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** (A) TG-DTG curve of the as-dried TiO₂ gel, and (B) Powder XRD patterns of TiO₂ xerogel calcined at 300°C and 400°C.</figcaption></figure>

crystallized at 400 °C (Figure 2B). All the XRD peaks are indexed to the ICDD database file number 04-001-7641, which corresponds to the anatase phase of TiO₂. The average particle size was calculated to be around 5 nm for TiO₂-300°C and 14 nm for TiO₂-400°C.

The N₂ adsorption-desorption isotherms of these TiO₂ samples, pre-treated for 2 h at 200 °C under 10⁻³ Torr (Figure 3) are typical of mesoporous materials containing also micropores, as indicated by the presence of hysteresis loops and significant N₂ uptake at low relative pressure. The measurements show a decrease of the surface area from 171 to 143 as the calcination temperature increases from 300 to 400 °C. These TiO₂ metallogels are delicate in nature and their surface properties, including BET surface area and pore size distribution, depend strongly on the methods of solvent removal as well as pre-treatment conditions.[17]

### YF₃ NPs-embedded TiO₂ metallogels: synthesis and structural/textural analysis.

For the preparation of NPs-embedded TiO₂ metallogels, YF₃ NPs were added in the initial 1M solution of titanium ethoxide [Ti(OEt)₄] resulting in near-transparent suspensions. The target amount of F in the final TiO₂ solid was 10 wt%. Gelation was then achieved using HCl solution, as mentioned above for the synthesis of pure TiO₂ metallogel. The obtained NPs-embedded gel was dried in

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Nitrogen adsorption-desorption isotherms of TiO₂-300°C and TiO₂-400°C.</figcaption></figure>

open air. The solids were finally calcined at different temperature (300 and 400 °C) to obtain YF₃-TiO₂ xerogels, which were characterized by a range of physico-chemical techniques.

#### ¹⁹F MAS NMR and XRD

¹⁹F MAS NMR spectra of the as-dried and calcined YF₃-TiO₂ xerogels are shown in the Figure 4A and compared with that of YF₃ NPs alone. The YF₃ NPs show two ¹⁹F signals at -50.7 and -61.7 ppm, which correspond to two fluorine sites in the structure of yttrium trifluoride. These values are slightly shifted as compared to the ¹⁹F peak positions found in bulk YF₃ (at about -56.4 and -67.6 ppm).[18-19] Irrespective of the calcination temperature, all the YF₃-TiO₂ samples show identical ¹⁹F peak positions and pattern, thus indicating that the YF₃ NPs are stable in the metallogel environment of TiO₂ and remain unchanged even when calcined at high temperatures. This is further confirmed by the XRD patterns of the YF₃-TiO₂ catalysts calcined at different temperature (Figure 4B and C). While TiO₂ in YF₃-TiO₂-300°C remains poorly crystallized, the YF₃-TiO₂-400°C sample shows well-crystalline XRD peaks characteristic of both YF₃ and anatase TiO₂. After Reitveld refinement, average nanocrystallite size of 8.8 nm and 9.3 nm were calculated for YF₃ NPs in YF₃-TiO₂ samples calcined at 300°C and 400°C, respectively.

#### N₂ adsorption-desorption

The N₂ adsorption-desorption isotherm of YF₃-TiO₂ calcined at 300°C, pre-treated for 2 h at 200 °C under 10⁻³ Torr, is compared to TiO₂-300 as shown in the Figure 5. The YF₃-TiO₂-300 catalyst has a BET surface area of 227 m²/g whereas the BET surface area of TiO₂-300 is 171 m²/g. Both samples present hysteresis loops which starts at relative pressures of 0.4. The hysteresis loop ends at a relative pressure of 0.7 for TiO₂-300°C and is extended at higher values for YF₃-TiO₂-300°C. This corresponds to the presences of mesopores with diameter of 5-10 nm for TiO₂-300 and 5-15 nm for the YF₃-TiO₂-300°C. The increase of nitrogen uptake at high relative pressure for YF₃-TiO₂-300°C indicates the presence of macropores as indicated by BJH plot (Fig. S1 and S2). These nitrogen isotherms also present a significant uptake at
<figure><img src="image_7.png" /><figcaption>**Figure 4.** ¹⁹F MAS-NMR spectra (A) of YF₃ NPs and YF₃-TiO₂ catalysts (as-dried and calcined at different temperatures. Spinning side bands ¹⁹F MAS-NMR spectra are represented by *. Rietveld refined powder X-ray diffraction patterns of YF₃-TiO₂ catalysts calcined at 300 °C (B) and 400 °C (C)</figcaption></figure>

low relative pressure, which is indicative of the presence of micropores (Fig. S3 and S4). The samples calcined at 400 °C show BET surface areas of 143 and 169 m²/g, respectively, for TiO₂-400°C and YF₃-TiO₂-400°C (Table 1). These results suggest that the presence of YF₃ NPs during gelation process influences the hydrolysis and condensation process of [Ti(OEt)₄] leading to TiO₂ xerogel with slightly different pore size and volume distribution and surface area.

<table><caption>**Table 1.** BET surface areas of YF₃ NPs, blank TiO₂ and YF₃-TiO₂ catalysts calcined at 300 and 400°C. pretreatment: 2 h at 200 °C under vacuum (heating rate: 5 °C.mn⁻¹).</caption><thead><tr><td>Catalysts</td><td>S<sub>BET</sub> (m²/g)</td></tr></thead><tbody><tr><td>YF<sub>3</sub></td><td>82</td></tr><tr><td>TiO<sub>2</sub>-300°C</td><td>171</td></tr><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-300°C</td><td>227</td></tr><tr><td>TiO<sub>2</sub>-400°C</td><td>143</td></tr><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-400°C</td><td>169</td></tr></tbody></table>

### XPS analysis

The chemical state of as-dried YF₃-TiO₂ sample (Figure S5) as well as those calcined at 300 and 400°C were evaluated by the core-level spectra of F 1s, Y 3d, C 1s, Ti 2p and O 1s (Figure 6). For as-dried YF₃-TiO₂ and YF₃-TiO₂-300°C samples, the Y3d spectra are composed of expected doublet at 161.9 and 159.8 eV due to 3d₃/₂ and 3d₅/₂, respectively, thus indicating the presence of YF₃.[20] For the sample calcined at 400 °C, the doublet is poorly resolved giving a broad peak at 162.3 eV. The F 1s region is composed of two contributions. For as-dried YF₃-TiO₂ and YF₃-TiO₂-300°C samples, the major peak is at 685.7 eV which is attributed to yttrium-bound fluoride.[20] These spectra also show a minor contribution at ~688 eV, assigned previously to TiO₂-xFx environment.[21] For the sample YF₃-TiO₂-400°C, the intensity of the peak at 685.8 eV is reduced considerably, indicating that thermal energy probably triggers more F⁻ in the TiO₂-xFx environment than in YF₃ NPs at the catalyst surface. The Ti 2p spectra of all samples exhibit binding energies at ~458 and ~464 eV, corresponding to Ti 2p₃/₂ and Ti 2p₁/₂, respectively, which

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Nitrogen adsorption-desorption isotherms of YF₃-TiO₂-300°C and pure TiO₂-300°C.</figcaption></figure>

indicate the presence of Ti⁴⁺.[22] The O1s spectra show contributions from Ti-O bond at ~530 eV as well as O-H and C-O groups at ~531 and ~533 eV, respectively, originating probably from the organic residues of the Ti(OEt)₄ precursor.[23] These organic residues of the precursor also contribute in the C 1s spectra which show peaks at 284-290 eV due to C-H, C-C, and C-O groups. The XPS quantitative analysis, which indicates that the atomic ratio Y/F of ~0.3 is maintained for all the samples (Table 2), provides further insight on the nature of F species at the surface together with their content. By combining these results with those of ¹⁹F MAS NMR and XRD, one can conclude that the fluorine is mostly under a stable form of YF₃ for the as-synthesized YF₃-TiO₂ and YF₃-TiO₂-300°C samples. The core-level F1s XPS spectra indicate a significant TiO₂-xFx environment on the surface through fluorine insertion in the titania network for the YF₃-TiO₂-400°C sample, even though the YF₃ NPs are preserved in the bulk as indicated by the XRD and ¹⁹F MAS-NMR studies as well as the fact that no significant fluorine loss is observed (Y/F= 0.31). From the XPS superficial elemental analysis, the superficial weight amount of F was calculated as 19 and 20 wt% for the non-calcined YF₃-TiO₂ and YF₃-TiO₂-300°C, respectively. These values are twice the target amount of 10wt% F indicating a possible superficial F enrichment.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Core-level XPS spectra of Y3d, F1s, Ti2p, O1s and C1s of YF₃-TiO₂-300°C and YF₃-TiO₂-400°C catalysts.</figcaption></figure>

<table><caption>**Table 2.** Atm % of F and atomic ratio Y/F determined by XPS analyses.</caption><thead><tr><th>Catalysts</th><th>Atm % F</th><th>Y/F atomic ratio</th></tr></thead><tbody><tr><td>YF<sub>3</sub>-TiO<sub>2</sub><sup>[a]</sup></td><td>26.1</td><td>0.28</td></tr><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-300°C</td><td>28.7</td><td>0.3</td></tr><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-400°C</td><td>22.0</td><td>0.31</td></tr></tbody></table>

[a] uncalcined.

## Catalysts acidity

### FTIR of pyridine adsorption

The blank TiO₂ and YF₃-incorporated TiO₂ catalysts calcined at 300 and 400°C were studied by FTIR analysis of pyridine adsorption to identify Lewis and/or Brønsted acid sites. The procedure involves the saturation of the catalyst's pellets with pyridine vapor pressure at ambient temperature, followed by desorption of the adsorbed pyridine species first at ambient temperature and then at 150°C. The different FTIR spectra obtained after pyridine adsorption on the two series of samples are shown in the Figure S6. First, we can note differences as regards to the global intensities of the pyridine absorption bands between 1400 and 1700 cm⁻¹ in the two series of samples without

any correlation with the calcination temperature. Second, for the samples calcined at 300°C, an abnormal increase in the intensity of all the absorption bands are observed after evacuation at 150°C as compared to evacuation at ambient temperature where a maximum number of physisorbed and chemisorbed Py are expected.

These abnormal data are probably due to the presence of micropores in the samples calcined at 300°C as indicated by N₂ physisorption. The relatively bulky Py might not penetrate easily through the micropores at ambient temperature but increasing the temperature to 150°C might help it to go deeper within the micropores and interact with more acid sites. Therefore, we will discuss here the results of pyridine adsorption qualitatively only. We can observe that all the catalysts exhibit bands at ~1445 and ~1610 cm⁻¹ which are characteristic of pyridine coordinated to the catalyst's Lewis acid sites.[24] The absorption at ~1490 cm⁻¹, due to the 19b vC-C vibration mode of Py, is common to pyridine coordinated to the Lewis acid sites and the pyridinium ions as well. A weaker band is observed at 1575 cm⁻¹ which has previously been ascribed to the pyridine retained to the surface by hydrogen bondings.[25] However, no peak at 1545 cm⁻¹ corresponding to the 19a vC-C vibration of pyridinium ions are clearly observed on bare TiO₂ or YF₃-TiO₂, irrespective of their calcination temperature or the evacuation temperature. From the present study, we can only conclude that bare TiO₂ possesses Lewis acidity, in agreement
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** Ammonia adsorption on YF₃-TiO₂-300, TiO₂-300 and YF₃ NPs. Isotherms (A) and calorimetric curves (B).</figcaption></figure>

with an anatase TiO₂ prepared by a simple sol-gel reaction of Ti(OPr)₄,[26] with no evidence of generation of Brønsted acidity upon YF₃ insertion.

### Microcalorimetry of NH₃ adsorption

The catalyst's total number of acid sites and their strength distribution can be determined by NH₃ adsorption monitored by microcalorimetry. The isotherms of NH₃ adsorption on YF₃-TiO₂-300°C and bare TiO₂-300°C were compared to that of YF₃ NPs (Figure 7A). The bare TiO₂ and YF₃-TiO₂ have around the same acid sites amount of 420 µmol. g⁻¹, whereas the YF₃ NPs have a much lower acid sites amount of 170 µmol. g⁻¹. Thus, if YF₃-TiO₂ exhibits the highest surface area among the three previous samples (227 m²·g⁻¹ for YF₃-TiO₂-300°C against 171 and 77 m²·g⁻¹ for TiO₂-300°C and YF₃ NPs, respectively), its total number of acid sites is equivalent to that of the bare TiO₂ sample calcined at the same temperature. The calorimetric curves (Figure 7B) are compared as a function of the acid site density (µmol. m⁻²). The materials are characterized by initial heat of ammonia adsorption of ~120 kJ·mol⁻¹. Few acid sites with this strongest heat of NH₃ adsorption are detected on the surface of YF₃ NPs (~0.5 µmol. m⁻²). Then a rapid decrease of the heat adsorption is recorded with

<figure><img src="image_12.png" /><figcaption>**Figure 8.** DHA conversion catalyzed by YF₃, blank TiO₂ and YF₃-TiO₂ catalysts. **Conditions:** T= 90 °C, Pair =1 atm, Vwater = 20 mL, [DHA] = 0.1 mol. L⁻¹, [catalyst] = 10 g·L⁻¹.</figcaption></figure>

the NH₃ coverage. On the TiO₂-based samples, the strongest acids sites are more numerous, around 1 µmole·m⁻². Then, with the ammonia coverage, a slow and continuous decrease of the heat of NH₃ adsorption down to 80 kJ·mol⁻¹ is recorded on both samples. The decrease is more rapid over YF₃-TiO₂-300, indicating the presence of rather heterogeneous acid sites distribution in terms of acid strength. For this sample, there is indeed a contribution of the YF₃ on the total acid sites density (Q>80 kJ·mol⁻¹) which decreases from ~ 2.8 µmol. m⁻² for the TiO₂-300°C down to 2.1 µmol. m⁻² for the YF₃-TiO₂-300°C. The acid sites density of YF₃ NPs is around 1.5 µmol·m⁻². For this later sample, if the calorimetric curve shows a similar initial heat of NH₃ adsorption of ~120 kJ·mol⁻¹, by comparison to the TiO₂ based catalysts, the decrease of QdiffNH₃ with the NH₃ coverage is more rapid. This indicates a more heterogeneous acid sites distribution in terms of strength explained by different local environments of acid sites on the surface of YF₃-NPs. Note that the calorimetric curve of YF₃-TiO₂ has intermediate features between those of YF₃ and of bare TiO₂. The acid sites amount deduced from these experiments are summarized in the Table 3.

<table><caption>**Table 3.** Catalyst's acid sites amount measured by NH₃ adsorption monitored by microcalorimetry.</caption><thead><tr><th></th><th>Acid sites amount<br>(µmol. g<sup>-1</sup>)</th><th>Acid sites density<br>(µmol. m<sup>-2</sup>)</th></tr></thead><tbody><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-300</td><td>420</td><td>2.1</td></tr><tr><td>TiO<sub>2</sub>-300</td><td>420</td><td>2.8</td></tr><tr><td>YF<sub>3</sub></td><td>170</td><td>1.5</td></tr></tbody></table>

**Conditions:** The catalysts are pretreated at 250 °C under vacuum for 1 h, NH₃ adsorptions are performed at 80 °C.

### Model reaction of Dihydroxyacetone (DHA) conversion in water

Figure 8 shows the DHA conversion catalysed by YF₃ alone, blank TiO₂ and YF₃-TiO₂ catalysts as a function of time. The highest activity was obtained for YF₃-TiO₂-300 °C which showed
almost complete DHA conversion after 360 min. By contrast, YF₃ NPs were the least active with a DHA conversion of 8% only at the end of the reaction. The bare TiO₂ calcined at 300°C was the second most active catalyst with 72% of DHA conversion at the reaction end. The samples calcined at 400 °C, both bare TiO₂-400 °C as well as YF₃-TiO₂-400 °C, were less active, with a DHA conversion of 64 and 60%, respectively, after 360 min. The inactivity of YF₃ NPs, which have superficial total number of acid sites equals to ~40% of that of the most active samples YF₃-TiO₂-300 °C is not surprising given its poor dispersion in water due to fluoride-generated hydrophobicity. In contrast, YF₃-TiO₂ NPs does not present this difficulty. It is known that the existence of a superficial layer of hydration at the surface of an oxide favours its dispersion in water.[27] We reckon that YF₃ NPs, when dispersed within the TiO₂ matrix, are benefited from the easy hydration of the TiO₂ surface to become better dispersed in water. Apart from this problem of dispersion, the YF₃ NPs seem to be stable in water (with no formation of HF, etc.), because otherwise the released Brønsted acidity would have promoted the DHA dehydration into pyruvaldehyde readily.

On the other hand, the initial rates of DHA conversion for bare TiO₂ calcined at different temperatures can be correlated to the BET surface areas. This is indicated by initial rates of DHA conversion for TiO₂-300°C and TiO₂-400°C samples, measured as 0.05 mmolDHA·h⁻¹·m⁻² and 0.06 mmolDHA·h⁻¹·m⁻², respectively (Table 4). This is not surprising since both samples have anatase TiO₂ structure with only slight differences in morphology and, therefore, close superficial acid-base properties are expected. Accordingly, the evolutions of the DHA and product's distribution with time are also very close (Figure S7). Bare TiO₂ catalysts, calcined at 300 and 400°C, favored initially the formation of pyruvaldehyde (PA), which is produced through DHA dehydration and catalysed efficiently by Brønsted acid sites.[28] Hexoses are also formed initially in lower proportion by a base-catalyzed aldolic condensation as proposed previously (scheme 1).[4] The basic sites were proposed to be the O²⁻ ligands exposed at the TiO₂ surface.[4]

<figure><img src="image_13.png" /><figcaption>**Scheme 1:** Proposed base and acid-catalyzed DHA transformation pathways[4]</figcaption></figure>

*H+= Bronsted acid sites

L = Lewis acid sites

As the reaction progresses, lactic acid is formed as secondary product at the expense of PA by rehydration and isomerization over Lewis acid sites. The two acid-catalysed products are the major species all along the reaction with selectivities ranging from 34-40 C% for PA and 31-36 C% for LA at 40% DHA conversion (Table 4). For the base-catalysed hexose products, the selectivities remain close to 15-16 C% throughout the reaction (Figure S3). We can safely say that the calcination temperature doesn't have any significant effect on the evolution of the product's distribution which reflects similar surfaces in terms of acid-base properties of the bare TiO₂.

A)

<figure><img src="image_14.png" /></figure>

B)

<figure><img src="image_15.png" /><figcaption>**Figure 9.** Evolution of DHA and product's distribution with time over A) TiO₂-300°C, and B) YF₃-TiO₂-300°C.</figcaption></figure>

<table><caption>**Table 4.** Initial rate of DHA conversion (rate<sub>ini</sub> DHA), Turnover frequencies (TOF) and product's selectivities (at 40% DHA conversion),</caption><thead><tr><th>Catalysts</th><th>rate<sub>ini</sub>DHA<br>(mmolDHA·h<sup>-1</sup>·m<sup>-2</sup>)</th><th>TOF<sup>[a]</sup><br>(h<sup>-1</sup>)</th><th>Se PA<sup>[b]</sup><br>(C%)</th><th>Se LA<sup>[c]</sup><br>(C%)</th><th>Se Hexd<br>(C%)</th></tr></thead><tbody><tr><td>TiO<sub>2</sub>-<br>300°C</td><td>0.05</td><td>10</td><td>40</td><td>31</td><td>16</td></tr><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-<br>300°C</td><td>0.07</td><td>25</td><td>71</td><td>2</td><td>20</td></tr><tr><td>TiO<sub>2</sub>-<br>400°C</td><td>0.06</td><td>-</td><td>34</td><td>36</td><td>15</td></tr><tr><td>YF<sub>3</sub>-TiO<sub>2</sub>-<br>400°C</td><td>0.04</td><td>-</td><td>28</td><td>30</td><td>16</td></tr></tbody></table>

[a] initial rate of PA and LA formation per acid sites as determined by calorimetry, and selectivities in [b] pyruvaldehyde, [c] lactic acid, and [d] hexoses at 40% DHA conversion

Upon incorporation of YF₃ NPs into TiO₂, followed by calcination at 300°C, the catalyst turned the most active one and the DHA conversion became much more selective towards PA formation and its further transformation into LA was slow (Figure 9). It achieved 71 C% selectivity for the PA and only 2 C% for the LA at 40% DHA conversion (Table 4). Note that the hexoses formation wasn't strongly changed upon YF₃ incorporation. This suggests that YF₃ NPs dispersed within and at the surface of TiO₂
has a beneficial impact on the catalyst's superficial acidity, leading to the fast DHA dehydration into PA without successive transformation into LA. This suggests that YF₃ NPs insertion has increased the Brønsted / Lewis acid sites balance in the reaction medium.

On the other hand, over YF₃-TiO₂-400°C, YF₃ incorporation makes the catalyst surface less active as indicated by a reduced initial rate of DHA formation, 0.04 mmol DHA·h⁻¹·m⁻², as compared to those of bare TiO₂ or YF₃-TiO₂-300°C catalysts. We can also observe that the product's selectivities are close to those observed over bare TiO₂ catalysts (Table 4, Figure S7). Upon calcination of YF₃-TiO₂ at 400°C, while the YF₃ NPs are preserved in the bulk as shown by XRD and ¹⁹F MAS-NMR (Figure 4), most of the superficial F species are in a TiO₂-xFx environment according to the XPS studies. By contrast, the characteristics of the YF₃ species are found at the surface of YF₃-TiO₂-300°C sample with only a weak contribution of TiO₂-xFx environment (Figure 6). These structural differences suggest that YF₃ entities dispersed at the TiO₂ surface of YF₃-TiO₂-300°C would be at the origin of the activity enhancement and the selective formation of the PA. If the production of LA as secondary products can be well explained by the presence of water tolerant Lewis acidity of TiO₂,[26] the formation of PA as the main product during the whole reaction, as observed over YF₃-TiO₂-300°C, is not straightforward. Indeed, it is generally accepted that at low temperature, Brønsted acids catalyze efficiently the DHA dehydration into PA but these sites are not active to produce further the LA.[29] On the contrary, water tolerant Lewis acids such as Sc(OTf)₃ alone, catalyzes hardly this first step, although they are efficient to further convert PA to LA. Consequently, the kinetic profile obtained over Sc(OTf)₃ alone shows at low DHA conversion equivalent amounts of PA and LA.[29] This does not fit with the kinetic evolution of the products distribution observed in the present study over YF₃-TiO₂-300°C where PA is the major product all along the reaction (Figure 9). These observations suggest that the DHA transformation over YF₃-TiO₂-300°C proceeds via a bifunction Brønsted and Lewis acid catalyzed pathway. However, we do not detect intrinsic Brønsted acid sites on YF₃-TiO₂-300°C (Figure S6). In the case of α-pinene isomerization in hot water, selectivity changes due to a Brønsted acid-catalyzed pathway, as observed previously over TiO₂ free of intrinsic Brønsted acidity. The authors proposed that the Lewis acid site of TiO₂ might be converted into Brønsted ones by water dissociation leading to the stabilization of protons at the TiO₂ surface.[30] Such conversion of Lewis acid sites into Brønsted acid sites upon water adsorption was also reported over different acid materials such as ZrSO₄,[31] Nb₂O₅,[32] H-ZSM-5[33] or Sn-H-ZSM5[34]. In the present study, we think that, the generation of a Brønsted acidity at the surface of YF₃-TiO₂-300°C suspended in hot water might explain the selectivity changes, which are beneficial to pyruvaldehyde, and lead to increase the overall reaction rate.

By contrast to our expectation, the acidity tuning upon YF₃ incorporation favours formation of PA at the expense of LA. In other words, it appears that YF₃ incorporation into TiO₂ enhances the stabilisation of more abundant Brønsted acid sites on the catalyst surface. Presumably, the new Lewis acid sites brought in by the dispersed YF₃ entities might favour Brønsted acidity

generation at the catalyst surface in a hot water environment via polarization of water molecules adsorbed on YF₃ Lewis acid sites as proposed in Scheme 2.

<figure><img src="image_16.png" /><figcaption>**Scheme 2:** Schematic presentation of water adsorption at the YF₃-TiO₂-300°C with adsorbed water under molecular form or dissociative form on YF₃ entities.</figcaption></figure>

Can we better rationalize these selectivity changes as regards to the material's superficial structures and intrinsic acidity?

From the literature, we have learnt that the water tolerance of solid Lewis acids such as TiO₂ and Nb₂O₅ allowed efficient LA formation from PA.[26,29] The Lewis acid sites capable to promote PA isomerization into LA were reported to be weak Lewis acid sites, which do not form stable “Lewis acid-water adducts” at the origin of the partial deactivation of intrinsic acidity of these oxides in water. The water tolerance was correlated to the local environment of the Lewis centers. Besides, the Lewis acidity of the surface of nanoanatase TiO₂ was investigated in details by FTIR of CO adsorption as a function of its hydrations state.[35] It was reported that the Lewis acidity of coordinatively unsaturated Ti⁴⁺ depends on its coordination states. Ti⁴⁺ located on corner or edges positions, with a lower coordination state, would present the highest Lewis acidity, with the highest polarizing power as compared to the sites located on planar surface with higher coordinative states. The former sites, with the highest Lewis acidity, were reportedly capable of adsorbing water under a dissociative form, while the later ones adsorb only the molecular form of the water.

As deduced from the calorimetric curves shown in the Figure 8B, the surfaces of sol-gel synthesized bare TiO₂ and YF₃-TiO₂ materials are very heterogeneous as regards to the strength of the Lewis acid sites and are composed of coordinatively unsaturated Ti⁴⁺ in different location at the surface of TiO₂ NPs and of YF₃ NPs. The calorimetric measurements have shown that YF₃ insertion within the TiO₂ matrix made the material's surface more heterogeneous in term of acid strength distribution with an enhanced proportion of stronger Lewis acid sites characterized by heat of NH₃ adsorption of 100-120 kJ·mol·NH₃⁻¹.

Considering the previous literature on the different forms of water adsorption as a function of the Lewis acid strength, we can assume that different types of Lewis acid centers coexist at the catalyst-water interface leading to different strengths or forms of water adsorption. Differences among the different forms and strength of water adsorption would tune the competitive adsorption of water and DHA on the surface-active sites as well as its first activation step assumed to be its protonation over YF₃-TiO₂ or its coordination to a 'water-tolerant' Lewis acid site Ti⁴⁺. The following distinct Lewis acid sites and interaction with water are assumed to coexist on the YF₃ and TiO₂-based surfaces:
- The strongest Lewis site capable of adsorbing water in a dissociative form and to provide hydroxonium ions at the TiO₂ surface.

- Strong Lewis acid sites poisoned by adsorption of water in its molecular form and not displaced by the oxygenated reactants or intermediates.

- Less reactive Lewis acid sites covered by water molecules in a molecular form in a reversible way, that is to say weakly adsorbed water molecules which can be displaced by the oxygenated organic molecules.

Considering the reactivity and selectivity changes upon YF₃ insertion into TiO₂, we think that YF₃ Lewis acid sites are most likely to be the most energetic ones, providing the sufficient pool of hydronium ions at the catalyst surface via water polarization.

## Conclusion

Embedding pre-formed YF₃ NPs in a TiO₂-based metallogel, followed by its soft drying at room temperature, affords a convenient and efficient synthesis of new YF₃-TiO₂ acid catalysts with high surface area and enhanced Brønsted acidity. The physico-chemical characterization methods gave proof that the original synthesis employed during this study affords YF₃ NPs stabilized and well-dispersed in and at the surface of TiO₂ matrix after calcination at 300°C. Contrary to our expectations, YF₃ insertion into the TiO₂ catalysts didn't favour lactic acid formation which requires water tolerant Lewis acid sites. DHA conversion into pyruvaldehyde was boosted over YF₃-TiO₂.300°C. These activity and selective changes were tentatively explained by the occurrence of water adsorption under a dissociative form over the strong Lewis acid sites brought in by the YF₃ NPs leading to a pool of active protons/hydronium ions at the water-solid interface. This pool of protonic sites is efficient to catalyze DHA dehydration into pyruvaldehyde. Upon further calcination of these catalysts at 400°C, most of the superficial F species are inserted in the titania network to provide a TiO₂-xFx environment leading to different superficial acidity. The YF₃-embedded-TiO₂ xerogel reported here represents an original way to design bifunctional Lewis-Brønsted heterogeneous catalysts which are efficient in water medium.

## Experimental

### Materials

The chemicals (in bracket their supplier) such as yttrium oxide (Strem Chemicals), trifluoroacetic acid (Sigma Aldrich), 1-octadecene (90%, Sigma Aldrich) and anhydrous ethanol (Carlo Erba) were used without any further purification, whereas titanium ethoxide (Strem Chemicals) was distilled prior to use.

### Synthesis of [Y(TFA)₃(H₂O)₃]

A reaction mixture containing 5 g (22 mmol) of Y₂O₃ and 10 ml trifluoroacetic acid (132 mmol) was refluxed in toluene (150 ml) at 350 rpm for 24 h at 125 °C. After cooling, the solvent was eliminated through cannula filtration. The obtained powder, which contained both unreacted Y₂O₃ and Y(TFA)₃(H₂O)₃, was stirred with tetrahydrofuran (100 ml) for 2 h and the solution obtained was

filtered. Removal of THF under vacuum from the filtrate afforded [Y(TFA)₃(H₂O)₃] as white powder.

### Synthesis of YF₃ nanoparticles

In a typical method, 1g of [Y(TFA)₃(H₂O)₃] was taken in 5 ml of 1-octadecene and heated at 120 °C for 1 h at 370 rpm to obtain a clear and homogeneous solution (solution A). In parallel, 5 ml of 1-octadecene was purged with argon and heated to 305 °C (solution B). When the temperature became constant at 305 °C for solution B, the pre-heated solution A (at 120 °C) was injected in it all at once. The reaction mixture was then stirred at 305 °C for 1 h. It was then gradually cooled down to room temperature under a constant flow of argon. To collect the nanoparticles, ethanol was added as a precipitating agent, followed by centrifugation at 10000 rpm for 15 min. The resulting powder was then re-washed twice with ethanol and once with hexane to remove any organic impurities and then air-dried at room temperature for 24 h.

### Preparation of TiO₂ metallogels and YF₃-embedded TiO₂ metallogels

The transparent metallogel of TiO₂ was synthesized by preparing 1M solution of titanium ethoxide [Ti(OEt)₄] in anhydrous ethanol (EtOH). To 10 ml of this solution, 0.4 ml of 1M HCl solution was added to have a ratio R(HCl) = n[HCl] / n[Ti(OEt)₄] = 0.04 and h (H₂O) = n[H₂O] / n[Ti(OEt)₄] = 2. After 30 minutes of this addition, a transparent solid gel-like network is formed. The gel is left to dry at room temperature to obtain a powder. For the preparation of YF₃-embedded TiO₂ metallogel, we chose to have a final 10 wt% of fluoride in the matrix of TiO₂, so for 10 ml of the initial 1M solution of titanium ethoxide, we added 0.24 g of YF₃, and stirred the solution for 15 min before the gelation step as described above. The obtained YF₃-embedded TiO₂ metallogel was finally dried by exposing it to air for 2 days at room temperature.

### General Procedures

The TGA/DTA data of the starting yttrium trifluoroacetate precursor [Y(TFA)₃(H₂O)₃] was collected in argon on a Setaram 92 system with a thermal ramp of 5 °C/min/ The masse of the sample used in TG measurement was 4.87 mg.
X-ray diffraction (XRD) measurements were performed on the powdered samples in a Bruker D8 Advance A25 diffractometer apparatus using CuKα radiation (0.154184 nm) at 50 kV and 35 mA. Powdered samples were analyzed within a 2θ range of 4–80° at 0.02° per step and with an acquisition time of 96 s per step. ¹⁹F MAS-NMR spectra were recorded on a Bruker Avance III 500 WB spectrometer in a 2.5 mm triple H/X/Y probe with an applied rotation frequency of 15-17 kHz.

XPS characterization was performed on a Kratos Axis Ultra DLD spectrometer equipped with a hemispherical electron analyzer using monochromatic Al Kα X-ray radiation (1486.6 eV, 12 mA, and 15 kV). Survey spectra were recorded with a Pass Energy of 160 eV and 1 eV step. High resolution spectra were recorded with a 40 eV Pass Energy and 0.1 eV step in the F1s, O1s, C1s and Zr3d regions. The binding energy of the C1s peak at 284.8 eV
was taken as internal standard. The CasaXPS software version 2.3.25 was used to data treatment.

The textural properties were analysed via nitrogen physisorption at 77K in a Micromeritics ASAP 2020 instrument. The catalysts were degassed at 200°C for 2 h under vacuum (heating rate 5 °C/min). The BET method was used to determine the specific surface areas, whereas the size of the mesopores and micropores were evaluated using BJH method and the t-plot, respectively.

Calorimetry of NH₃ and CO₂ adsorption was used to study the original acid-base properties of the solids. Experimental set up consist of TianCalvet calorimeter connected to a vacuum glass volumetric equipment to measure the differential heat of adsorption of the successive small amounts of the probe molecule up to the surface coverage and, in parallel, the isotherm of NH₃/CO₂ adsorption. About 50 mg of sample was placed in a glass cell and further treated under vacuum at 250 °C for 2h. Subsequently, the glass cell, kept under vacuum, is place into the calorimeter, and small doses of ammonia or carbon dioxide were introduced into the glass cell till the equilibrium pressure of the probe molecule was reached. The CO₂ and the NH₃ adsorptions were performed at 30°C or 80°C respectively using the same methodology.

Fourier Transformed Infrared (FTIR) spectroscopy of adsorbed pyridine were carried out to probe the nature of the acid sites. A Bruker Vector 22 FT-IR spectrophotometer equipped with DTGS detector was used. Self-supporting pellets (50 mg of catalyst, 18 mm diameter) were used for pyridine adsorption studies. First, the pellet was vacuum treated at 150°C then exposed to pyridine vapor pressure at room temperature for 10 min. The desorption was performed by outgassing the samples for 30 min at RT, 150°C, 250°C for the catalysts calcinated at 300°C, plus 350°C for those calcinated at 400°C and up to 450°C for those calcinated at 500°C and the FTIR spectra of adsorbed pyridine were recorded at room temperature after each step. FTIR spectra were also obtained from solid catalysts samples diluted in KBr pellet (2%).

### Dihydroxyacetone (DHA) conversion in water

The reactions were conducted in a two-neck round bottom flask (25 ml) equipped with a water condenser, a temperature sensor and a rubber septum. At first, water and catalyst (10 g.L⁻¹) of a total volume of 20 ml were heated at 90 °C in a controlled temperature oil bath and agitated by a magnetic stirrer. When the temperature stabilizes, DHA (Aldrich) powder was added (t = 0) and the reactions were conducted during 6 hours. The DHA initial concentration was 0.1 mol. L⁻¹. The reactant/products analysis was done using high-performance liquid chromatography (HPLC) (Model Shimadzu Prominence) with a refractive index detector (RID) equipped with a COROGEL 107H column, at 40°C, and acidified water (H₂SO₄ 1.7 mM) was used as mobile phase (0.6 mL.min⁻¹).

### Acknowledgements

RBS thanks the French ministry of higher education, research and innovation for her PhD grant (doctoral school of chemistry, Lyon). We also thank N. Bonnet, Y. Aizac and C. Lorentz from the Ir@tech plateform of IRCELYON, for isotherms, XRD and ¹⁹F MAS-NMR analyses.

**Keywords:** nanoparticles • titania • NP-embedded gel • acid base catalysts • dihydroxyacetone conversion

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<figure><img src="image_17.png" /></figure>

Incorporation of pre-formed YF₃ nanoparticles (NPs) in a TiO₂-based metallogel, followed by its soft drying at room temperature, yield YF₃-TiO₂ xerogel with high surface area and enhanced acidity, which show better catalytic activity than the alone component YF₃ or TiO₂ NPs for the dihydroxyacetone conversion in water.