# Ni-NbOx Bifunctional Catalysts for Selective Hydrodeoxygenation of m-Cresol to Toluene

Camila Abreu Teles, Carmen Ciotonea, Ruben Palacio, Diana Lopez,

Sébastien Royer, Frédéric Richard

► To cite this version:

Camila Abreu Teles, Carmen Ciotonea, Ruben Palacio, Diana Lopez, Sébastien Royer, et al. Ni-NbOx Bifunctional Catalysts for Selective Hydrodeoxygenation of m-Cresol to Toluene. Molecular Catalysis, 2026, 588, pp.115516. 10.1016/j.mcat.2025.115516. hal-05346898

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# Ni–NbOₓ Bifunctional Catalysts for Selective
Hydrodeoxygenation of m-Cresol to Toluene

Camila A. Teles<fnref n="1" />*, Carmen Ciotonea<fnref n="2" />, Ruben Palacio<fnref n="3" />, Diana Lopez<fnref n="3" />, Sébastien Royer<fnref n="2" />,
Frédéric Richard<fnref n="1" />

Submitted to Molecular Catalysis

August 2025

<footnote n="1">¹ Université de Poitiers, CNRS, Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285, rue Michel Brunet, BP633, 86022 Poitiers, France.</footnote>

<footnote n="2">² Université du Littoral Côte d'Opale, Unité de Chimie Environnementale et Interactions sur le Vivant-UCEIV, UR4492, 59140 Dunkerque, France.</footnote>

<footnote n="3">³ Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad deCiencias Exactas y Naturales, Universidad de Antioquia UdeA; Calle 70 No. 52-21, Medellín, Colombia.</footnote>

<footnote>* Corresponding author: camila.abreu.teles@univ-poitiers.fr</footnote>
**Abstract**

The catalytic performances of a series of Ni supported on mesoporous silica (SBA-15) and niobia, as well as Ni-NbOₓ dispersed on SBA-15 were evaluated for the hydrodeoxygenation (HDO) of m-cresol at 300 °C under atmospheric pressure. Under the reaction conditions, hydrogenation and C-C hydrogenolysis pathways yielding only oxygenated products dominated over the monometallic Ni catalyst. In contrast, the Direct DeOxygenation pathway (DDO) leading to toluene was significantly promoted when Ni was in contact with oxophilic NbOₓ surface. Tuning the Ni-Nb ratio on silica revealed a remarkable enhancement of the DDO rate constant. Indeed, the kinetic rate constant determined over 5Ni5Nb/SBA was about 11 times higher than that measured on the catalyst containing only the Ni phase. This enhanced performance can be attributed to the formation of well-dispersed Ni-NbOₓ interfacial sites, where the hydrogenation capability of Ni associate with the oxophilic character of Nb⁵⁺ /Nb⁴⁺ species allowing to a more efficient activation of the C-O bonding and promoting the DDO reaction pathway. These results offer valuable insights for the rational design of selective catalysts for the transformation of lignin-derived bio-oils into aromatic hydrocarbons.

**Keywords:** Biomass, hydrodeoxygenation; SBA-15; Ni/NbOₓ site interface
# 1. Introduction

Lignocellulosic biomass is nowadays one of the most important sources of renewable feedstock to the production of biofuels and bio-based products. Particularly, lignin fraction is a rich source of aromatics compounds that can be converted into a promising liquid fuel (bio-oil) for use in transport engines, through the fast pyrolysis process. However, this liquid cannot be directly used due to its poor quality related to the high content of oxygen [1,2]. Catalytic conversion of those oxygenated products, mostly phenolic compounds, can be achieved by the hydrodeoxygenation (HDO) process. Herein, the challenge is to develop suitable catalysts, able to promote the selective formation of deoxygenated products with a minimum consumption of hydrogen, hence aromatic compounds like BTX (Benzene, Toluene, Xylene) products.

Co-Mo and Ni-Mo sulphated catalyst, traditionally used for the hydrotreatment process, can efficiently reduce the oxygen content in the bio-oil [3,4]. However, due to their instability under reaction and the requirement of sulfur in the stream to preserve the active phase, alternative catalyst formulations have been proposed. Among these, bifunctional catalysts combining of metallic phase with oxophilic sites appears to efficiently promote deoxygenation of model phenolic compounds such as phenol, cresols and anisole [5–11]. The efficiency of such catalytic systems arises from the activation of the oxygen atom at the oxophilic sites located near the metal particles, followed by selective hydrogenation/hydrogenolysis activated by the metal, thus producing aromatic products. This has been reported to happen over a series of noble metals such as Pt, Pd, Rh, Ru on oxophilic support such as ZrO₂, TiO₂, Nb₂O₅ [9,12–19]. Among them, niobia-supported catalysts stand out with higher activity and selectivity toward deoxygenated products. Teles et al. have investigated a series of Pd supported on various oxides (SiO₂, CeO₂, ZrO₂, TiO₂ and Nb₂O₅)
on the HDO reaction of phenol, m-cresol and anisole at 300 °C under atmospheric pressure [13]. The product distribution significantly varied with the support type. While Pd supported on SiO₂ and CeO₂ favored the formation of the hydrogenated product (methyl)-cyclohexanone, supports like ZrO₂, TiO₂ and Nb₂O₅ promoted the formation of benzene/toluene, with selectivity following the order Pd/Nb₂O₅ > Pd/TiO₂ > Pd/ZrO₂ >> Pd/CeO₂ ~ Pd/SiO₂, regardless the model compound. The higher deoxygenation activity of Pd/Nb₂O₅ catalyst was explained by its superior oxophilicity, which strongly interact with the oxygen atom of the oxygenated organics, thus facilitating the C-O bond cleavage or selective hydrogenation of C=O bond. Rezende et al., in turn, investigated different metals (Pt, Rh and Ni) supported on the same Nb₂O₅ support in the HDO of phenol at 300°C and P₃H₂ = 1bar [20]. All of catalysts exhibited comparable selectivity to benzene (ranging from 92 to 97 %). However, when the NiO phase was fully reduced (activation at 500 °C), the HDO rate decreased, likely due to the strong metal-support (SMSI) effect, which led to partial coverage of the Ni metal particles. Surprisingly, a different behavior was observed when a Pt/Nb₂O₅/MgAl₂O₄ catalyst was evaluated in HDO of m-cresol at 350 °C under atmospheric pressure [21]. Indeed, after the catalyst reduction at 500 °C, Pt was encapsulated by the partially reduced NbOₓ species, leading to the improvement of activity (by 47.5 % instead of 10%) and selectivity to toluene (98% instead of 87%) compared to Pt on non-SMSI support (Pt/MgAl₂O₄). The promoting effect of NbOₓ species on the Pt based catalysts was also demonstrated by Chen et al. [22]. In that study, the activity of Pt/C and NbOₓ-modified Pt catalysts was evaluated for the HDO of m-cresol at 350 °C under atmospheric pressure. The NbOₓ-Pt/C catalyst exhibited an enhanced selectivity to toluene (95%), which was attributed to the interaction of the hydroxyl group of m-cresol with uncoordinated Nb cations. This adsorption mode, with the aromatic ring tilted away from the surface, favored the direct deoxygenation pathway. Moreover, interactions at the NbOₓ-Pt interface were proposed to
stabilize $NbO_x$ in its partially reduced state. A similar promoting effect on toluene selectivity has also been reported for $WO_x$-decorated Pt catalysts, where Pt stabilizes the $WO_x$ film while facilitating the generation of oxygen vacancies [23,24]. In this case, HDO of m-cresol proceeds via direct C–O bond cleavage of molecules adsorbed on the oxygen vacancies of the $WO_x$ species. These studies may imply that the active sites for deoxygenation, i.e., metal-oxophilic interfaces, are created when the $NbO_x$ overlayes on the metal surface. However, in the case where higher catalyst activation temperatures are needed, the extension of $NbO_x$ migration may needs to be controlled to avoid excessive loss of exposed metal surface area, thus decreasing the interface perimeter and consequent loss of HDO activity.

The main objective of the present study was to examine the performance of Ni-$NbO_x$ species interface in the gas phase HDO reaction of m-cresol. As the niobium oxide is a reducible support that promote the SMSI effect, a Ni-$Nb_2O_5$ system deposited on silica surface was proposed in order to maximize metal-oxophilic sites interface and minimize Ni surface coverage. SBA-15 was chosen as silica support owing to its high surface area and ordered channel-type mesopores, allowing the accommodation of reactant and the control of metal particle size [25,26]. A series of catalysts varying the content of Ni and Nb was prepared, characterized and evaluated in the HDO reaction of m-cresol at 300 °C under atmospheric pressure (fixed bed configuration).

## 2. Experimental Section

### 2.1. Support Preparation

For the preparation of the support SBA-15, P123, used as a structure-directing agent, was solubilized in a 1.6 M HCl solution at 40 °C with stirring. The silicon source (TEOS) was then added dropwise, and the solution was continuously stirred at 40 °C during 2 h. The resulting mixture was then submitted to a hydrotreatment in a Teflon-lined autoclave at 100
°C for 48 h. Afterward, the precipitated was recovered by filtration, washed with deionized water, and dried at 100 °C for 24 h. The resulting solid was then calcined at 550 °C for 6 h (heating ramp of 1.5 °C min⁻¹) to remove the organic template. The support was denoted as SBA-15. Nb₂O₅ support was obtained by calcination of niobic acid (CBMM) at 500 °C (1.5 °C min⁻¹) for 6 h.

## 2.2. Catalyst Preparation

Ni and/or NbOₓ supported on SBA-15 and Nb₂O₅ catalysts were prepared by the melt infiltration (MI) of Ni nitrate. In this procedure, the precursor Ni(NO₃)₂·6H₂O was ground with the supports at ambient conditions. The resulting powder was further treated in a teflon autoclave at the melting point of the precursor (57 °C) during 4 days. For the preparation of Ni-NbOₓ supported materials, niobium oxalate (NH₄[NbO(C₂O₄)₂(H₂O)₂]·nH₂O) and nickel nitrate were ground together with the respective supports and submitted to the same thermal treatment at 57 °C. The materials were further calcined in a muffle at 500 °C (1.5 °C min⁻¹) for 6 h to obtain the supported catalysts and afterwards, pelleted, crushed and sieved (200-315 µm). The catalysts were prepared by varying the content of nickel and niobium and were denoted by xNiyNb/SBA where x and y varied between 2.5 and 5 wt.%.

A 5 wt.% of Ni supported on Nb₂O₅ was also prepared by the same MI procedure, followed by calcination at the conditions previously described. This sample was denoted as 5Ni/Nb₂O₅.

## 2.3. Catalyst Characterization

Both Ni and Nb contents were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Analyses were performed on a SPECTRO ARCOS ICP-OES
instrument. Before analysis, the samples were digested using concentrated nitric acid and
hydrofluoric acid on a microwave heating system.

The textural properties of the materials were determined by N₂ physisorption at -196 °C using a Micromeritics Tristar II automated gas sorption system. Before analysis, the samples were outgassed under dynamic vacuum at 240 °C for 3 h. Specific surface areas were determined using the multipoint B.E.T. algorithm, in the 0.10 – 0.25 P/P₀ interval. The pore volume and mesopore size distribution were determined using the B.J.H. method from the desorption branch of the isotherms.

Small and wide-angle X-ray diffraction (XRD) analysis of the materials were obtained using a PAN-analytical EMPYREAN powder diffractometer in Bragg-Brentano configuration with Cu Kα radiation (λ = 0.154 nm, 45 kV, 40 mA) in the 10–80° 2θ range with a 0.02° 2θ step. Phase identification was performed by comparison with the ICDD database. The XRD patterns of the supported catalysts were obtained for the samples after *ex situ* activation at 500 °C, 5 °C min⁻¹, under pure hydrogen flow (50 ml min⁻¹) for 1 h.

The reducibility of the materials was studied by H₂-TPR. The analyses were performed on a Micromeritics AutoChem 2910 from room temperature to 1000 °C, using a heating ramp of 10 °C min⁻¹, under 10 vol.% of H₂ in Ar flow (flow rate of 30 mL min⁻¹). Prior to the TPR run, the samples were pretreated with pure oxygen (30 mL min⁻¹) at 400 °C during 1 h. The hydrogen consumption was monitored using a TCD detector.

The morphology of bare SBA-15 and dispersion of Ni and Nb species were analyzed by transmission electron microscopy (TEM) using a TITAN Themis 3007 S/TEM. The instrument was equipped with a high brightness Schottkey field emission gun, a probe aberration corrector, super-X detector system with four windowless silicon drift detectors for electron dispersive X-ray spectroscopy (EDX), and several annular dark field detectors. Experiments were conducted at 300 kV with semi-convergence angle of about 20 mrad, a
probe size of the order of 500 pm and a probe current at around 100 pA. For high-angle
annular dark field (HAADF) imaging, the camera length was adjusted to yield collection
angles between 50 and 200 mrad. The EDX mapping was performed in multi-frame spectrum
imaging mode with dwell time per pixel of about 15 µs and continuously scanning until a total
acquisition time of 15-20 minutes was achieved. The samples were reduced *ex-situ* under pure
H₂ flow (50 mL min⁻¹) at 500 °C (5 °C min⁻¹) during 1 h and then embedded in a polymeric
resin, sliced into 50 nm-thick sections using an ultramicrotome and further deposited on a
carbon grid for analysis. The software ImageJ version 1.8 was used to analyze the images and
to plot particle size histograms.

CO chemisorption analysis was performed in order to determine Ni dispersion. The samples were firstly reduced at 500 °C (5 °C min⁻¹), under pure hydrogen flow (30 mL min⁻¹) for 1 h. The reactor was then cooled down to the adsorption temperature (30 °C) under helium (30 mL min⁻¹). Successive pulses of pure CO (0.465 mL) were injected into the reactor each 3 min. The quantification of chemisorbed CO was carried out using a gas phase chromatograph equipped with a TCD detector and a Porapak Q column. A CO/Ni ratio of 1 was considered.

The acidity of the catalysts was measured by DRIFTS of adsorbed pyridine. Experiments
were conducted on a Nicolet Nexus instrument equipped with a DTGS detector and a quartz
cell with CaF₂ windows. The samples were first reduced at 500 °C under flow of pure
hydrogen for 1 h and then, cooled down to 150 °C, where a spectrum was registered as
reference. Pyridine adsorption was carried out during 5 min, with a further vacuum purge
during 1 h. Spectra were recorded between 150 and 450 °C at a resolution of 2 cm⁻¹ and
accumulating 64 scans bet.

Thermogravimetric analysis (ATG) of spent catalysts was performed in order to
investigated the deposition of carbonaceous species over the catalysts during the HDO
reaction. The measurements were carried out on a SDT Q600 TA Instruments apparatus. The
analysis was performed in the temperature range of 25 °C up to 1000 °C, 10°C min⁻¹, under flow of 100 mL min⁻¹ of synthetic air.

## 2.4. Catalytic transformation of m-cresol

Vapor phase HDO reaction of m-cresol was conducted in a fixed-bed flow reactor system, operating at 300 °C under atmospheric pressure. The carrier gas flow rate was 50 mL min⁻¹ of H₂ and a H₂/m-cresol molar ratio was fixed at 80. Before reaction, the catalysts were reduced with pure hydrogen (50 mL min⁻¹) at 500 °C for 1 h. The temperature was then cooled to the reaction temperature (300 °C) and the flow of hydrogen and m-cresol adjusted to different contact times in order to obtain m-cresol conversion around 10 % to ensure differential conditions. The reactant feedstock, introduced at the top of the reactor by using a push-syringe, was composed by m-cresol (7 mol %), decane (3 mol %; used as internal standard), and n-heptane as a solvent. The line at the bottom of the reactor was maintained at 5 °C using a Minichiller-Huber condenser, in order to condense any unconverted reactant and products. The liquid samples were collected at 0.5 h and analyzed by a Varian 430 chromatograph equipped with a DB-5 capillary column and a flame-ionization detector (FID). The conversion of m-cresol and product selectivity for each product were calculated using eqs. (1) and (2):

$$X (\text{in} \%) = \frac{n^0_{\text{m-cresol}} - n_{\text{m-cresol}}}{n^0_{\text{m-cresol}}} \times 100 \quad (1)$$

$$S_i (\text{in mol} \%) = \frac{n_i}{n^0_{\text{m-cresol}} - n_{\text{m-cresol}}} \times 100 \quad (2)$$

Where $n^0_{\text{m-cresol}}$ and $n_{\text{m-cresol}}$ are the number of moles of m-cresol in the beginning and after 0.5 h of reaction, respectively; $n_i$ is the number of moles of a given i product.
Assuming a pseudo-first-order reaction, the total kinetic rate constant ($k_{TOT}$ in mmol $g_{Ni}^{-1} h^{-1}$) and the kinetic rate constants for the different reaction pathways proposed were calculated using the equations (3) and (4):

$$k_{TOT} (\text{in mmol } g_{Ni}^{-1} h^{-1}) = \frac{-F \ln (1-X)}{w.(\%Ni)} \quad (3)$$

$$k_y (\text{in mmol } g^{-1} h^{-1}) = r_{TOT} \cdot S_y \quad (4)$$

Where $k_{TOT}$ is the total kinetic rate constant calculated for the global transformation of m-cresol and $k_y$ are the kinetic rate constants for the different reaction pathways; X is the m-cresol conversion; F is the m-cresol molar flow rate (in mmol $h^{-1}$); w is catalyst weight (in g), %Ni is the Ni content measured by ICP-OES and $S_y$ is the selectivity to the different products: toluene for the deoxygenation route (DDO), 3-methylcyclohexanone, 3-methylcyclohexanol, methylcyclohexene isomers and methylcyclohexane for the hydrogenation route (HYD), phenol and benzene for the hydrogenolysis route (HYG).

# 3. Results and Discussion

## 3.1. Nickel supported on SBA-15 and $Nb_2O_5$ catalysts

### 3.1.1 Characterization results

Small-angle XRD pattern of the SBA-15 and wide-angle XRD patterns of the Ni supported catalysts after reduction (500 °C, 1h) are shown in Figs. 1a and S1. The small-angle XRD pattern showed three main diffraction peaks corresponding to the (100), (110), and (200) reflections, which are characteristic of the highly ordered hexagonal 2D pore structure of SBA-15 with P6mm symmetry, thus confirming the successful synthesis of the material. Besides that, the presence of (210) and (300) reflections is a clear indication of a long-range mesopore ordering and the good textural uniformity of the SBA-15. The SBA-15 supported
catalyst exhibited a broad signal centered at 22.6°, typical of amorphous silica, and two diffraction lines at 44.5 and 51.9° corresponding to (111) and (200) planes of Ni metallic phase (PDF 04-0850). For the niobia supported catalyst, it is observed a set of intense diffraction lines, characteristic of pseudohexagonal TT-Nb₂O₅ (JCPDS: 28-0317). In addition, a weak diffraction peak at 44.5° is observed, corresponding to the metallic Ni(0) phase. Ni particles size was calculated using the Scherrer equation, being obtained approximately the same (around 8 nm) for both 5Ni/SBA and 5Ni/Nb₂O₅ samples.

The textural properties of the calcined samples were evaluated by N₂ physisorption. The adsorption/desorption isotherms (Fig. S2) for the bare SBA-15 supported and after NiO introduction, showed a type IVa isotherm, characteristic of mesoporous materials and a H1-type hysteresis, indicating the presence of cylindrical (tubular) pores as awaited for the SBA-15 material. For the niobia-supported catalyst, a type IV – type II mixed isotherm was observed, indicating the presence of mesopores, however without saturation plateau and resulting in wider pore size distribution. As expected, a high surface area was obtained for SBA-15 support (754 m² g⁻¹), while much lower value is obtained for Nb₂O₅ (60 m² g⁻¹). The surface area of niobium oxide strongly varies with calcination temperature and the crystallography phase formed. A low surface area is commonly reported in the literature for calcination at temperatures above 500 °C [27,28]. Upon introduction of NiO, both samples showed reasonable decrease in the surface area (< 14 and 32% for Ni/SBA and Ni/Nb₂O₅ respectively), which is associated to the denser NiO phase in the solid.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) XRD patterns of the reduced 5Ni/SBA and 5Ni/Nb<sub>2</sub>O<sub>5</sub> samples and b) H<sub>2</sub>-TPR profile of the respective calcined samples.</figcaption></figure>

H<sub>2</sub>-TPR analysis was performed to assess the reducibility of the samples (Fig. 1b). For 5Ni/SBA, two reduction phenomena are observed at around 420 and 560 °C, corresponding to the reduction of bulky NiO particles located at the external surface of SBA-15, and NiO mesopores confined NPs, respectively. In the latter case, the higher temperature (520 °C) may be likely to the presence of multiple chained particles in the same pore, thus reducing the reduction rate [25]. In the case of 5Ni/Nb<sub>2</sub>O<sub>5</sub> sample, a more complex reduction profile is observed. A first reduction peak centered at 500 °C may be associated to the reduction of NiO in strong interaction with Nb<sub>2</sub>O<sub>5</sub> support, as previously reported [5,29]. The hydrogen consumption observed at higher temperatures, 675 °C and 865 °C, may be assigned to the reduction surface and bulk Nb<sub>2</sub>O<sub>5</sub> to NbO<sub>2</sub>, respectively, facilitated by hydrogen activation and dissociation on Ni metallic sites, followed by hydrogen spillover to the support, which promotes niobia reduction. To verify this assumption, the degree of reduction was calculated.
When considering the total hydrogen consumption over the entire temperature range, or even only the events at 500 and 675 °C, values well above 100% were obtained. In contrast, a degree of reduction close to 100% was achieved when only the consumption at 500 °C was taken into account. This confirms the attribution of the 500 °C peak to the reduction of NiO to metallic Ni, while the higher-temperature events are likely associated with the partial reduction of Nb₂O₅. These reduction phenomena have been previously reported for a NiO/Nb₂O₅ catalyst, where the TPR profile exhibited a peak associated to NiO reduction between 230 a 430 °C, and two hydrogen consumption peaks above 700°C and 800 °C related to the reduction of Nb₂O₅ [30].

The dispersion of Ni particles on the catalysts was estimated by CO chemisorption (Table 1). Both samples presented same value of CO uptake (0.04 mol<sub>CO</sub> mol<sub>Ni</sub><sup>-1</sup>), which correspond to a dispersion of around 4% and thus, suggesting the presence of large Ni particles. However, these values do not correspond to the metal particles size derived from XRD analysis. One could speculate that for 5Ni/SBA sample, CO molecules may not assess Ni particles chained in a same pore, while for 5Ni/Nb₂O₅ sample, Ni particles may be partially covered by NbOₓ species which migrates to the surface of nickel during catalyst reduction, due to the strong metal-support interaction (SMSI) effect [31].

<table><caption>**Table 1.** Physical-chemical properties of Ni-based catalysts.</caption><thead><tr><th></th><th>Ni<br>wt<sup>a</sup>.%</th><th>D<sup>b</sup> Ni<br>(nm)</th><th>S<sub>BET</sub><sup>c</sup><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>Degree of red.<sup>d</sup><br>(%)</th><th>CO uptake<sup>e</sup><br>(mol<sub>CO</sub> mol<sub>Ni</sub><sup>-1</sup>)</th></tr></thead><tbody><tr><td>SBA-15</td><td>-</td><td>-</td><td>754</td><td>-</td><td>-</td></tr><tr><td>5Ni/SBA</td><td>5.0</td><td>8 (7*)</td><td>650</td><td>98</td><td>0.04</td></tr><tr><td>Nb<sub>2</sub>O<sub>5</sub></td><td>-</td><td>-</td><td>60</td><td>-</td><td>-</td></tr><tr><td>5Ni/Nb<sub>2</sub>O<sub>5</sub></td><td>2.9</td><td>8</td><td>41</td><td>100</td><td>0.04</td></tr></tbody></table>

<footnote><sup>a</sup> Ni loading (wt.%) measured from ICP-EOS analysis;</footnote>
### 3.1.2 Catalytic properties for m-cresol transformation

The products distribution and the constant reaction rates for HDO of m-cresol measured at low conversion (around 10%) are listed in Tables 2 and 3, respectively. The bare supports were found to be inactive under the reaction conditions used (300 °C, atmospheric pressure). In contrast, Ni-supported catalysts exhibited notable differences in both activity and selectivity. For the 5Ni/SBA catalyst, the main product formed was 3-methylcyclohexanone (51%), as shown in Table 2. In the case of 5Ni/Nb<sub>2</sub>O<sub>5</sub>, toluene was the only product observed. However, this catalyst exhibited a very poor activity since its total reaction rate constant being about eight times lower than that of the silica-based catalyst, with a DDO rate 1.7 times lower (Table 3).

<table><caption>**Table 2.** Product distribution obtained in the HDO of m-cresol at low conversion (T<sub>reaction</sub> = 300 °C, 1 atm, 0.5 h on stream).</caption><thead><tr><th rowspan="2">Catalyst</th><th rowspan="2">W/F<br>(g h mol<sup>-1</sup>)</th><th rowspan="2">Conversion<br>(%)</th><th colspan="5">Selectivity (mol %)</th></tr><tr><th>Cyclohexane</th><th>Di-n-butylamine</th><th>Di-n-butylamine + cyclohexane</th><th>Di-n-butylamine + cyclohexane + cyclohexanone</th><th>Di-n-butylamine + cyclohexanone</th></tr></thead><tbody><tr><td>5Ni/SBA</td><td>21</td><td>9</td><td>21</td><td>51</td><td>18</td><td>2</td><td>7</td></tr><tr><td>5Ni/Nb<sub>2</sub>O<sub>5</sub></td><td>214</td><td>11</td><td>100</td><td>-</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

<caption>**Table 3.** Reaction rate constant (in mmol g<sub>Ni</sub><sup>-1</sup> h<sup>-1</sup>) calculated for the global transformation of m-cresol and for each reaction pathway over the Ni-based catalysts (T<sub>reaction</sub> = 300 °C, 1 atm, 0.5 h on stream).</caption>

<footnote><sup>b</sup> Ni metal particle size calculated from XRD of reduced samples; *metal particle size calculated from TEM images;</footnote>

<footnote><sup>c</sup> BET surface area measured by N<sub>2</sub> physisorption analysis;</footnote>

<footnote><sup>d</sup> Degree of reduction for NiO species calculated from the TPR results (considering the peaks at the temperature below 600 °C) and considering the metal content from the ICP-OES analysis;</footnote>

<footnote><sup>e</sup> Content of CO uptake estimated by CO chemisorption analysis.</footnote>
<table><thead><tr><th>Catalyst</th><th>r<sub>TOT</sub></th><th>r<sub>DDO</sub></th><th>r<sub>HYD</sub></th><th>r<sub>HYG</sub></th></tr></thead><tbody><tr><td>5Ni/SBA</td><td>82</td><td>17</td><td>58</td><td>7</td></tr><tr><td>5Ni/Nb<sub>2</sub>O<sub>5</sub></td><td>10</td><td>10</td><td>-</td><td>-</td></tr></tbody></table>

Barrios et al. investigated Pd supported on commercial SiO₂ and Nb₂O₅ for the HDO of phenol under experimental reaction conditions similar to those used in this study (300°C, atmospheric pressure). Reduction steps were between 300 and 500 °C [5]. After the reduction step carried out at 300 °C, the deoxygenated product (benzene) obtained from the DDO route of phenol was mainly produced over Pd supported on niobia (80%) while cyclohexanone (the main product obtained from the HYD route of phenol) was preferentially formed over Pd supported on silica (92%). Trends are then consistent with those reported here with Ni-catalysts (Table 2). Interestingly, the DDO reaction rate was approximately 90 times higher for Pd/Nb₂O₅ compared to Pd/SiO₂ [5]. When the Pd based catalysts were reduced at higher temperatures (until 500 °C), the selectivity into benzene (DDO route) was maintained, however, the DDO rate significantly decreased (by up to 19-fold). The authors attributed the decrease in activity not only to the growth of Pd particles but also to the partial coverage by NbOₓ species formed during reduction (an effect similar to the one observed with titania, through the SMSI phenomena). Similar result was reported by Resend et al. with Rh and Ni supported on niobia where authors varied the reduction temperature of the catalysts [20]. In the present study, the constant reaction rate of the 5Ni/Nb₂O₅ catalyst was significantly lower than that of the 5Ni/SBA catalyst, despite both having similar Ni particle sizes and the 5Ni/Nb₂O₅ catalyst exhibiting higher selectivity toward toluene. This suggests that the markedly reduced activity of the 5Ni/Nb₂O₅ catalyst may be attributed to the coverage of Ni particles, a hypothesis supported by the low CO uptake measured for this sample (Table 1).

### 3.2. Ni-NbOₓ dispersed on SBA-15 catalysts: Influence of the Ni/Nb ratio
In an attempt to maximize Ni-NbOₓ interface sites while minimizing Ni particle coverage, a series of Ni-NbOₓ supported on SBA-15 samples was prepared by varying the Ni and Nb contents. The measured amounts of Ni and Nb in the samples (Table 4) closely matched the expected values, ranging from 2.9 to 5.0 wt% for Ni and from 2.8 to 9.0 wt% for Nb.

The textural properties of the samples are displayed in Fig. 2, Fig. S3 and Table 4. All of NiNb/SBA samples presented isotherm type similar to that of the bare SBA-15 support, i.e., type IVa with H1-type hysteresis. This indicates that the mesoporous structure of the support was maintained after infiltration of nickel and niobium precursors followed by thermal treatment. However, discrete differences can be noticed. For the supported materials, the desorption branch is not congruent with the adsorption one, with a latency in its cloture, being more evident for the 2.5Ni5Nb/SBA sample. This is likely due to the filling of NiO and Nb₂O₅ species inside the tubular pores, leading to the partial porosity blockage and decreases in the surface area and pore volume, as evidenced in Table 4. The mesopore and micropore surface areas decreased after metal oxide infiltration, by approximately 25% and up to 65%, respectively. Assuming that NiO and Nb₂O₅ crystallites are confined within the tubular pores of SBA-15, their size should not exceed the pore diameter (~7 nm), which do not agree with the very large crystallites detected in the XRD patterns. This may indicate a heterogeneity in the particles size, with a fraction of particles likely both located within the pores and at the external surface of the silica grains.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** N₂ adsorption-desorption for the NiNb-supported on SBA calcined catalysts</figcaption></figure>
<table><caption>**Table 4.** Metal loading, textural and physical-chemical properties of the NiNb-based catalysts.</caption><thead><tr><th rowspan="2">Catalyst</th><th colspan="4">d<sup>b</sup> (nm)</th><th rowspan="2">S<sub>BET</sub><sup>c</sup><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th rowspan="2">S<sub>micro</sub><sup>d</sup><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th rowspan="2">V<sub>p</sub><sup>e</sup><br>(m<sup>3</sup> g<sup>-1</sup>)</th><th rowspan="2">V<sub>micro</sub><sup>f</sup><br>(m<sup>3</sup> g<sup>-1</sup>)</th><th rowspan="2">D<sub>p</sub><sup>g</sup><br>(nm)</th><th rowspan="2">Degree of<br>red.<sup>h</sup><br>(%)</th><th rowspan="2">CO uptake<sup>i</sup><br>(mol<sub>CO</sub> mol<sub>Ni</sub><sup>-1</sup>)</th><th rowspan="2">Total<br>acidity<sup>j</sup><br>(µmol g<sup>-1</sup>)</th></tr><tr><th>Ni</th><th>Nb</th><th>Ni</th><th>Nb<sub>2</sub>O<sub>5</sub></th></tr></thead><tbody><tr><td>5Ni2.5Nb/SBA</td><td>4.8</td><td>2.8</td><td>34</td><td>30</td><td>608</td><td>87</td><td>0.87</td><td>0.036</td><td>6.4</td><td>100</td><td>0.20</td><td>-</td></tr><tr><td>5Ni5Nb/SBA</td><td>5.0</td><td>8.5</td><td>38 (3.3*)</td><td>30</td><td>572</td><td>72</td><td>0.84</td><td>0.028</td><td>6.5</td><td>102</td><td>0.31</td><td>20</td></tr><tr><td>2.5Ni5Nb/SBA</td><td>2.9</td><td>4.6</td><td>31</td><td>27</td><td>511</td><td>157</td><td>0.59</td><td>0.069</td><td>6.0</td><td>99</td><td>0.18</td><td>14</td></tr><tr><td>5Nb/SBA</td><td>-</td><td>9.0</td><td>-</td><td>24</td><td>625</td><td>148</td><td>0.91</td><td>0.065</td><td>6.7</td><td>-</td><td>-</td><td>30</td></tr></tbody></table>

<footnote><sup>a</sup> Metal loading (wt.%) measured by ICP-EOS analysis;</footnote>

<footnote><sup>b</sup> Ni(0) and Nb(V) crystallite particle sizes calculated from XRD of reduced samples; *Ni(0) average particle sizes measured by TEM images from reduced samples;</footnote>

<footnote><sup>c</sup> BET surface area; <sup>d</sup> Micropore surface area; <sup>e</sup> Total pore volume; <sup>f</sup> Micropore volume; <sup>g</sup> BJH pore diameter calculated from the desorption branch.</footnote>

<footnote><sup>h</sup> Degree of reduction for the NiO species calculated from the TPR results (considering the peaks at the temperature range from 300 to 700 °C) and considering the metal content from the ICP-OES analysis;</footnote>

<footnote><sup>i</sup> Content of CO uptake estimated by CO chemisorption analysis;</footnote>

<footnote><sup>j</sup> Total acidity obtained from the pyridine adsorption-desorption DRIFTS experiments.</footnote>
Wide-angle XRD patterns of the catalysts reduced at 500 °C are presented in Fig. 3. In this series of Ni–Nb catalysts supported on SBA-15, narrow diffraction peaks are observed, indicating the presence of large Ni(0) and Nb(V) particles.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** XRD of the reduced NiNb-supported on SBA reduced catalysts.</figcaption></figure>

To obtain precise information on the location and dispersion of NiO and Nb<sub>2</sub>O<sub>5</sub> species on the support, HRTEM images of the reduced 5Ni/SBA (Fig. S4) and 5Ni5Nb/SBA (Fig. 4) catalysts were acquired. For the 5Ni/SBA catalyst, it is clearly observed Ni nanoparticles of around of 7 nm confined in the mesopores of the SBA-15, consistent with Ni(0) particle size calculated from XRD of the reduced sample. In the case of 5Ni5Nb/SBA, it is clearly observed the formation of small particles, appearing as agglomerates. Some of these particles may be confined within the mesopores, while others are agglomerated on the external surface of SBA-15. As a result, certain regions of the pore structure remain unfilled. However, from
Figs. 4(a,b), is also noticed in some regions, the formation of few larger particles (varying from 10 to 30 nm) located at the external surface of the SBA-15 grains, which can explain the sharp diffraction peaks of low intensity observed in the XRD pattern of the sample, as well as of those of all the NiNb-supported catalysts. From the histogram of 5Ni5NB/SBA sample (Fig. 4h), particles of size varying mainly from 2 to 4 nm, with an average of 3.3 nm are measured. Besides larger particles being present (> 10 nm), they are not representative in content. EDX elemental mapping of 5Ni5NB/SBA sample (Fig. 5) reveals that Nb element is uniformly dispersed onto the support, with a visible enrichment at the interface with Ni(0) particles confined in the mesopores. In other words, Nb<sub>2</sub>O<sub>5</sub> particles appear at the periphery of Ni particles, indicating close spatial contact between the two species.

Metallic sites accessibility was estimated by CO chemisorption analysis, Table 4. The CO uptake varied significantly with catalyst composition. As previously discussed, very low values of CO uptake (0.04 mol<sub>CO</sub> mol<sub>Ni</sub><sup>-1</sup>) were obtained for 5Ni/SBA and 5Ni/Nb<sub>2</sub>O<sub>5</sub> samples, reflecting the hard access of confined Ni particles or partial coverage by the NbO<sub>x</sub> species. For the NiNb/SBA samples, values of CO uptake were considerably higher (varying between 0.18 to 0.31 mol<sub>CO</sub> mol<sub>Ni</sub><sup>-1</sup>), but without any tendency respect to catalyst composition. Indeed, 5Ni5Nb/SBA exhibited a better Ni dispersion (around 30 %) which perfectly agree with the average particle size measured in the TEM images, thus showing that in this case, there is not partial coverage of Ni surface by NbO<sub>x</sub> species.
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Representative HAADF images of the 5Ni5Nb/SBA reduced catalyst (500 °C, pure H₂, 1h) and the respective particle size distribution.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 5.** EDXS elemental maps for the 5Ni5Nb/SBA reduced catalyst (500 °C, pure H₂, 1h).</figcaption></figure>

The H₂-TPR profiles of the NiNb/SBA samples are presented on Fig. 6. While the 5Nb/SBA sample shows no noticeable reduction, the NiNb/SBA samples exhibit a complex profile with multiple overlapping reduction peaks, complicating the interpretation. The multiple reduction events are likely associated with reduction of NiO particles of varying
sizes, distributed across different regions of the support, and with various interaction with
niobium oxide. All of Ni-Nb supported samples presented quite similar profile, with two main
reduction events, with maxima varying between 315-340 °C and 565-600 °C. These peaks are
related to the reduction of NiO particles located at the external surface and confined inside the
mesopores of the SBA-15, respectively, as also observed in the TPR profile of 5Ni/SBA
catalyst (Fig. 1b). A shoulder is also present between 415-435 °C, which may indicate the
reduction of Nb-containing NiO domains [32]. Smaller reduction event is detected starting
from around 800 °C for all of NiNb-containing samples, which is likely due to the partial
reduction of niobium oxide [30,32].

<figure><img src="image_7.png" /><figcaption>**Figure 6.** H₂-TPR profiles for the NiNb-supported on SBA-15 samples: (a) 5Ni2.5Nb/SBA, (b) 5Ni5Nb/SBA, (c) 2.5Ni5Nb/SBA and (d) 5Nb/SBA.</figcaption></figure>

The acidic properties of the reduced catalysts were investigated by adsorption-desorption of pyridine analysis (Fig. 7). From the IR spectra after outgassing pyridine at 150
°C (Fig. 7), very low intensity bands are observed for the 5Ni/SBA and 5Ni2.5Nb/SBA samples, indicating the absence or very low acidity for these catalysts. At the opposite, the spectra of the NiNb/SBA samples exhibited:

- well defined bands associated to pyridine adsorbed on Lewis acid sites at around 1450 and 1610 cm⁻¹;

- broad and low intensity band at around 1547 cm⁻¹ characteristic of Brønsted acid sites;

- band at 1490 cm⁻¹ that can be associated with both Lewis and Brønsted acid sites [30].

These results indicate that niobium oxide generates mainly Lewis acid sites, with quantity affected by the Nb₂ O₅ content. According to the literature, the type and concentration of acid sites on niobium oxide strongly depends on the temperature of activation, which reflects on its structure and textural properties. Indeed, treating niobium at elevated temperatures (≥ 500 °C) lead to the formation of a crystalline phase of low surface area, thus decreasing the density of exposed acid sites, and dehydroxylating the surface with consequent elimination of Brønsted acid sites [27].

<figure><img src="image_8.png" /><figcaption>**Figure 7.** IR spectra of adsorbed pyridine after outgassing at 150 °C.</figcaption></figure>
### 3.2.2 Catalytic transformation of m-cresol

The transformation of m-cresol over the NiNb/SBA catalysts was evaluated at 300 °C under atmosphere pressure. A comparison on the product distribution obtained at the same level of conversion (~10%) is presented in Table 5. Niobia supported on SBA without Ni (5Nb/SBA) showed no measurable conversion, even at higher W/F values, indicating that metallic sites are essential for activating molecular hydrogen and enabling hydrogenation and hydrogenolysis steps. For Ni-containing catalysts, product selectivity varies significantly with catalyst composition. As previously observed, 3-methylcyclohexanone is the major product obtained (51%) over the 5Ni/SBA catalyst. The introduction of 2.5 wt.% of niobium drastically modified the product selectivity, with toluene becoming the major product (56%). Selectivity toward the ketone and alcohol decreased, while the formation of the respective alkenes and alkane increased. As the niobium content increased further (to 5 wt.%), the selectivity toward the alkenes and alkane remained relatively constant, while toluene formation continued to rise and ketone/alcohol declined. In addition, phenol, produced by hydrogenolysis of m-cresol, was no longer detected at high Nb loading.

<table><caption>**Table 5.** Product distribution obtained in the HDO of m-cresol (low conversion) over the NiNb-supported on SBA catalysts ($T_{reaction}$ = 300 °C, 1 atm, 0.5 h of time on stream).</caption><thead><tr><th rowspan="2">Catalyst</th><th rowspan="2">W/F<br>(g h mol<sup>-1</sup>)</th><th rowspan="2">Conversion<br>(%)</th><th colspan="6">Selectivity (mol %)</th></tr><tr><th>Phenyl</th><th>Phenyl</th><th>Phenyl</th><th>Phenyl</th><th>Phenyl</th></tr></thead><tbody><tr><td>5Ni/SBA</td><td>21</td><td>9</td><td>21</td><td>51</td><td>18</td><td>2</td><td>-</td><td>7</td></tr><tr><td>5Ni2.5Nb/SBA</td><td>21</td><td>15</td><td>56</td><td>15</td><td>2</td><td>14</td><td>7</td><td>6</td></tr><tr><td>5Ni5Nb/SBA</td><td>12</td><td>15</td><td>75</td><td>7</td><td>-</td><td>12</td><td>6</td><td>-</td></tr><tr><td>2.5Ni5Nb/SBA</td><td>31</td><td>11</td><td>67</td><td>9</td><td>-</td><td>13</td><td>11</td><td>-</td></tr></tbody></table>
From previous investigations on the HDO reaction mechanism of cresol over metals supported on silica and niobia, as well as Ni-based catalysts, different reaction pathways have been proposed, as illustrated in Scheme 1: (i) the direct deoxygenation (DDO) involving the cleavage of the C<sub>Ar</sub>-O bond to produce toluene; (ii) hydrogenation of the aromatic ring (HYD) to produce methylcyclohexanone and methylcyclohexanol, which may be dehydrated to methylcyclohexene and further hydrogenated to methylcyclohexane; (iii) hydrogenolysis (HYG) of the methyl group of m-cresol to produce phenol and methane. An alternative mechanism with lower energy barrier is the tautomerization of m-cresol, in which a keto-tautomer intermediate can be considered as a common intermediate between the DDO and HYD pathways. Once formed, this instable intermediate may react via two different routes: (i) selective hydrogenation of the C=O bond to produce an unsaturated alcohol which is rapidly dehydrated to toluene or (ii) successive hydrogenation of the C=C bonds followed by same steps previously described for the HYD route. The preference for each reaction pathway is strongly dependent on the nature of the metal and support.

<figure><img src="image_9.png" /></figure>
**Scheme 1.** Reaction pathways proposed for the transformation of m-cresol over the different metal-based catalysts.

For the 5Ni/SBA catalyst, considering the non-acidic or non-oxophilic character of the SiO₂ support and Ni being a hydrogenating metal, the hydrogenation (HYD) pathway is preferred, thus explaining the formation of hydrogenated oxygenated compounds as main products. Besides that, as previously reported, larger Ni particles are also active for hydrogenolysis, a characteristic that can explain the formation of phenol observed in this work for Ni-catalyst [22,31].

Niobia has been identified as an acidic and oxophilic oxide capable of activating the C-O bond in phenolic compounds due to the strong interaction established between the Nb⁵⁺/Nb⁴⁺ cations and the oxygen atom [7,36,37]. Incorporating niobium oxide in the perimeter of Ni particles thus facilitates the adsorption and activation of the C-O and/or C=O bond (from the tautomer), thereby lowering the energy barrier for deoxygenation reaction. In order to compare the catalytic performance of monometallic 5Ni/SBA and bifunctional NiNb/SBA catalysts, as well as to evaluate the effect of m-cresol conversion on the product distribution, the W/F values were varied through modification of catalyst weight. Catalysts with lower niobia content, or even reduced Ni content in the presence of niobia, exhibited higher activity than 5Ni/SBA (as evidenced in Fig. 8), thus showing the promoting effect of niobia to the activity of Ni.
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Pseudo-first order plots obtained from the HDO of m-cresol over the Ni-Nb based catalysts at 300 °C, 1 atm, 30 min of TOS.</figcaption></figure>

The product selectivity as a function of m-cresol conversion for 5Ni/SBA, 5Ni2.5Nb/SBA and 2.5Ni5Nb/SBA catalysts are presented in Fig. 9. For the 5Ni/SBA catalyst, as m-cresol conversion increases, selectivity toward 3-methylcyclohexanone and 3-methylcyclohexanol decreases, while selectivity to toluene and phenol rises. This indicates a sequential reaction pathway: the ketone is hydrogenated to the alcohol, which is further converted into methylcyclohexane, and subsequently dehydrogenated to produce toluene. Herein, the formation of methylcyclohexane from the alcohol could proceed via either dehydration to methylcyclohexene followed by hydrogenation, or direct C-OH bond cleavage. Given the weak acidity of Ni/SBA, direct hydrogenolysis is the more likely route. The selectivity to phenol, produced by hydrogenolysis of the methyl group, also increases with conversion, suggesting a competition between hydrogenolysis (HYG) and hydrogenation (HYD) pathways.
For the catalysts containing niobium, regardless of its content, toluene consistently remains the main product across the entire range of conversion. In the case of 5Ni2.5Nb/SBA catalyst, notable differences can be noticed for the minor products with increasing conversion: the selectivity to 3-methylcyclohexanone and methylcyclohexene decreases, while methylcyclohexane and phenol increase. This suggest that 3-methylcyclohexanol undergoes acid-catalyzed dehydration to methylcylohexene, followed by hydrogenation to the corresponding alkane, driven by the acidic nature of niobium oxide. For the 2.5Ni5Nb/SBA catalyst, phenol is never observed even at higher conversion values.

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Product selectivity as a function of m-cresol conversion for the Ni-Nb based catalysts. Reaction performed at 300 °C, 1 atm, 30 min of TOS. (●) Toluene, (▲)</figcaption></figure>
methylcyclohexenes, (●) methylcyclohexane, (♦) 3-methylcyclohexanone, (■) 3-methylcyclohexanol, (◆) phenol.

Comparison of the results obtained in this work with those reported in the literature is not straightforward, as reaction conditions (temperature, pressure, and feedstock composition) often differ significantly, strongly influencing the reaction. In addition, variations in catalyst formulation, such as the type of metal and support, also play a crucial role in HDO performance. To address this point, we have selected a number of studies from the literature that, while using different catalysts, were carried out under reaction conditions comparable to those employed in our work. Table 6 summarizes some results selected from the literature on the HDO of m-cresol (at low conversion) performed at temperatures ranging from 250-350 °C and under atmospheric pressure. The product distribution is strongly influenced by the catalyst type. Metals such as Pd, Pt and Ni supported on inert materials such as silica, exhibit higher selectivity toward the hydrogenated product (3-methylcyclohexanone). In contrast, more oxophilic metals such as Ru and Fe or the use of oxophilic supports such as ZrO₂, TiO₂ and Nb₂O₅, favor the deoxygenation, leading to toluene. For Ni-based catalysts, the selectivity toward toluene can be significantly improved by controlling the Ni particle size (smaller particles are more oxophilic and thus promote the DDO pathway), and by combining Ni with oxophilic metals such as Fe and Mo. This observation is consistent with the results obtained in this work, where combining Ni with niobium notably improved deoxygenation. Our 5Ni5Nb/SBA catalyst, which showed the best performance, achieved higher toluene selectivity compared to Ni/SiO₂ catalysts reported in the literature, and exhibited performance comparable to Pd or Pt supported on oxophilic supports. However, it represents a more attractive option, since it relies on non-noble metals.
<table><caption>**Table 6.** Conversion and product distribution obtained for the HDO of m-cresol at gas phase over different supported metal catalysts reported in the literature.</caption><thead><tr><th rowspan="2">Catalyst</th><th rowspan="2">T (°C)</th><th rowspan="2">P (atm)</th><th rowspan="2">H<sub>2</sub>/m-cresol molar ratio</th><th rowspan="2">Conversion (%)</th><th colspan="4">Selectivity (%)</th><th rowspan="2">Ref.</th></tr><tr><th>75</th><th>7</th><th>-</th><th>6</th></tr></thead><tbody><tr><td>5Ni5Nb/SBA</td><td>300</td><td>1</td><td>80</td><td>15</td><td>75</td><td>7</td><td>-</td><td>6</td><td>This work</td></tr><tr><td>Pd/SiO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>8.2</td><td>3.8</td><td>90.3</td><td>5.9</td><td>-</td><td></td></tr><tr><td>Pd/ZrO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>14.7</td><td>80.0</td><td>18.4</td><td>1.2</td><td>0.4</td><td>[13]</td></tr><tr><td>Pd/TiO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>7.3</td><td>87.9</td><td>12.6</td><td>-</td><td>-</td><td></td></tr><tr><td>Pd/Nb<sub>2</sub>O<sub>5</sub></td><td>300</td><td>1</td><td>60</td><td>13.1</td><td>95.6</td><td>3.8</td><td>-</td><td>0.6</td><td></td></tr><tr><td>Pd/SBA-15</td><td>300</td><td>1</td><td>60</td><td>7.0</td><td>38.0</td><td>62.0</td><td>-</td><td>-</td><td>[38]</td></tr><tr><td>Pd/CeO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>10.3</td><td>19.1</td><td>70.5</td><td>10.4</td><td>-</td><td></td></tr><tr><td>Pd/10SiCeO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>15.9</td><td>46.8</td><td>44.2</td><td>9.0</td><td>-</td><td>[39]</td></tr><tr><td>Pd/Fe<sub>2</sub>O<sub>3</sub></td><td>300</td><td>1</td><td>-</td><td>10.0</td><td>~ 87</td><td>&lt; 1</td><td>-</td><td>-</td><td>[40]</td></tr><tr><td>Pt/TiO<sub>2</sub></td><td>350</td><td>1</td><td>60</td><td>~ 20</td><td>~ 100</td><td>-</td><td>-</td><td>-</td><td>[41]</td></tr><tr><td>Pt-Nb/C</td><td>350</td><td>1</td><td>-</td><td>~ 35</td><td>95</td><td>-</td><td>-</td><td>5</td><td>[22]</td></tr><tr><td>Pt/SiO<sub>2</sub></td><td>300</td><td>1</td><td>-</td><td>7.2</td><td>33.3</td><td>60.2</td><td>6.5</td><td>-</td><td></td></tr><tr><td>Ru/SiO<sub>2</sub></td><td>300</td><td>1</td><td>-</td><td>4.5</td><td>38.5</td><td>7.4</td><td>-</td><td>-</td><td>[16]</td></tr><tr><td>Ni/SiO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>9.3</td><td>46.2</td><td>15.1</td><td>3.2</td><td>-</td><td>[42]</td></tr><tr><td>Ni/SiO<sub>2</sub></td><td>250</td><td>1</td><td>60</td><td>2.3</td><td>6.1</td><td>52.2</td><td>33.0</td><td>-</td><td>[43]</td></tr><tr><td>Fe/SiO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>8.8</td><td>60.2</td><td>-</td><td>-</td><td>-</td><td></td></tr><tr><td>NiFe/SiO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>13.7</td><td>52.6</td><td>-</td><td>-</td><td>-</td><td>[10]</td></tr><tr><td>Ni/SiO<sub>2</sub></td><td>300</td><td>1</td><td>60</td><td>17.5</td><td>46.0</td><td>16.5</td><td>1.9</td><td>2.7</td><td></td></tr><tr><td>Ni(1nm)/SBA</td><td>300</td><td>1</td><td>90</td><td>22.3</td><td>85.7</td><td>5.5</td><td>0.8</td><td>0.6</td><td>[25]</td></tr><tr><td>1Ni1Mo/SiO<sub>2</sub></td><td>250</td><td>1</td><td>60</td><td>10.8</td><td>83.3</td><td>0.0</td><td>0.0</td><td>5.9</td><td>[44]</td></tr><tr><td>Ni0.5Cu/SiO<sub>2</sub></td><td>250</td><td>1</td><td>50</td><td>17.8</td><td>20.3</td><td>43.5</td><td>34.3</td><td>0.7</td><td>[45]</td></tr><tr><td>NiCu(2nm)/SBA</td><td>300</td><td>1</td><td>90</td><td>17</td><td>85</td><td>11</td><td>2</td><td>-</td><td>[46]</td></tr></tbody></table>
*Others products to complete 100 % selectivity includes compounds such as CH₄, C₂-C₅ hydrocarbons, transalkylation products, phenol, depending on the catalyst type.

To further examine the promotional effect of niobium oxide over a Ni-based catalyst, the kinetic rate constants (normalized to Ni content) were calculated for each reaction pathway (Table 6). As can be seen, the DDO rate constant significantly increases with introduction of niobium reaching a maximum with the 5Ni5Nb/SBA catalyst (a 11-fold increase compared to 5Ni/SBA). In contrast, the HYD rate constant remains relatively constant, while the HYG pathway becomes less active and is completely suppressed when Ni and Nb are present in equal proportions. Note that for the HYD rate constant, all of products from the hydrogenation pathway were considered, including methylcyclohexenes and methylcyclohexane. Thus, despite being constant, the HYD rate constant also reflect the formation of deoxygenated aliphatic hydrocarbons. To enable a more meaningful comparison, a DDO factor, defined as the ratio kDDO/(kHYD+kHYG), was calculated in order to represent the relative preference for the DDO pathway over HYD and HYG during the transformation of m-cresol. This factor increased followed the order: Ni/SBA (0.3) < 5Ni2.5Nb/SBA (1.3) < 2.5Ni5Nb/SBA (2.1) < 5Ni5Nb/SBA (3.1). This trend highlights that combining Ni with niobium significantly enhances the deoxygenation activity compared to Ni/SBA, and suggests the existence of an optimum niobium content and/or Ni/Nb ratio to maximize the direct deoxygenation of m-cresol into toluene.

<table><caption>**Table 6.** Reaction rate constant (in mmol g<sub>Ni</sub><sup>-1</sup> h<sup>-1</sup>) calculated for the transformation of m-cresol and for each reaction pathway over the Ni-Nb supported on SBA catalysts (T<sub>reaction</sub> = 300 °C, 1 atm, 0.5 h of time on stream).</caption><thead><tr><th>Catalyst</th><th>r<sub>TOT</sub></th><th>r<sub>DDO</sub></th><th>r<sub>HYD</sub></th><th>r<sub>HYG</sub></th><th>DDO factor<sup>a</sup></th></tr></thead><tbody><tr><td>5Ni/SBA</td><td>82</td><td>17</td><td>58</td><td>7</td><td>0.3</td></tr><tr><td>5Ni2.5Nb/SBA</td><td>146</td><td>82</td><td>55</td><td>9</td><td>1.3</td></tr><tr><td>5Ni5Nb/SBA</td><td>249</td><td>188</td><td>61</td><td>0</td><td>3.1</td></tr></tbody></table>
<table><tr><td>2.5Ni5Nb/SBA</td><td>123</td><td>83</td><td>40</td><td>0</td><td>2.1</td></tr></table>

ªThe DDO factor was calculated by $k_{DDO}/(k_{HYD}+r_{HYG})$

These results highlight the advantage of combining metallic Ni with niobium oxide species for the deoxygenation of lignin-derived compounds. Nickel, as a transition metal capable of dissociating H₂, promotes hydrogenation of aromatic rings and/or hydrogenolysis of C–O bonds. However, when used alone, Ni tends to favor ring hydrogenation and C–C hydrogenolysis pathways, yielding mainly alcohols and alkanes rather than achieving direct deoxygenation (DDO) to aromatics. In contrast, niobium in its oxide form (Nb⁵⁺/Nb⁴⁺), owing to its strong oxophilic character, interacts effectively with oxygen atoms and activates oxygenated functional groups (–OH, –OCH₃, C=O). When coupled with metallic sites, these NbOₓ species facilitate selective C–O bond cleavage, thereby enhancing the DDO pathway.

Finally, the stability of the catalysts was evaluated during 4 h on stream (TOS) using 5Ni/SBA and 5Ni5Nb/SBA, the last one selected for their superior DDO activity. As shown in Fig. 10, the m-cresol conversion decreased over time for both catalysts, with a higher loss observed for 5Ni5Nb/SBA (31%) compared to 5Ni/SBA (18%), indicating a stronger deactivation of the former, likely linked to the presence of Nb. This trend is consistent with previous reports on Ni-based catalysts for HDO of phenolics under similar conditions (300 °C, 1 atm), where the catalyst deactivation was attributed to the blockage of active sites by coke deposition, intermediates adsorption, and/or Ni particle sintering [25]. To probe these effects, TGA and XRD analyses were performed on both spent catalysts. The TGA profiles (Fig. S5) showed two main losses: the first one below 150°C, attributed to water desorption, and the second between 300 and 600 °C, attributed to the oxidation of strongly adsorbed reaction intermediates (~290 °C) and combustion of condensed carbon deposits (400–600 °C) [25,47]. The second loss (between 300 and 600 °C) was much more significant in the case of 5Ni5Nb/SBA, in agreement with the stronger deactivation observed for this catalyst compared
to Ni/SBA (Fig. 10). This suggests that the Nb-promoted catalyst, despite maintaining higher
toluene selectivity, is more prone to deactivation by oxygenated species strongly bound to its
active sites or by coke formation. The diffractograms of the spent catalysts (Fig. S6) showed a
profile similar to that of the reduced samples before the reaction, with 5Ni/SBA maintaining a
particle size of around 7 nm. This indicates that metal sintering is not occurring and the strong
adsorption of intermediates species on catalyst surface would be the main cause of
deactivation. In terms of product distribution, distinct behaviors were observed for both
catalysts as already discussed above. Over 5Ni/SBA, toluene selectivity gradually declined
with a concomitant increase in methylcyclohexanone and phenol, while for 5Ni5Nb/SBA, the
selectivity into toluene, the main product, remained relatively stable.

<figure><img src="image_12.png" /><figcaption>**Figure 10.** Effect of time on stream on the m-cresol and product distribution over (a) 5Ni/SBA and (b) 5Ni5Nb/SBA catalysts. T: 300°C, P: 1 atm, TOS: 4h.</figcaption></figure>

**4. Conclusion**

This work evaluated the performance of Ni and Ni-NbOₓ interfaces over SBA-15 silica for the
gas phase HDO of m-cresol at atmospheric pressure. The results demonstrated that the
addition of niobium significantly altered both the activity and product distribution of Ni/SBA
catalyst. While the monometallic Ni catalyst predominantly produced hydrogenated
compounds and phenol (via C-C hydrogenolysis), the addition of niobium greatly enhanced
toluene formation. In fact, the DDO activity increased by 11-fold with the bifunctional NiNb-
supported catalyst, and 18-fold compared to the Ni/Nb<sub>2</sub>O<sub>5</sub> catalyst. Although toluene
selectivity was lower than that of Ni/Nb<sub>2</sub>O<sub>5</sub> catalyst (which achieved 100% of selectivity), the
co-dispersion of Ni and NbO<sub>x</sub> on silica surface promoted a greater gain in activity. These
findings highlight the importance of interfacial Ni-NbO<sub>x</sub> sites in promoting the direct
deoxygenation pathway. Further optimization of catalysts synthesis procedure to ensure
uniform distribution of metal particles size and maximize Ni-NbO<sub>x</sub> interfacial sites would
lead to highly efficient and cheap catalysts for bio-oil upgrading.

Acknowledgements

This work was performed in the frame of the PYRODEOX project granted by the ANR
(ANR-21-CE43-0006). Camila A. Teles and Frederic Richard acknowledge financial support
from the European Union (ERDF), "Region Nouvelle Aquitaine". The authors also thank the
University of Antioquia, projects Minciencias, ECOSNORD Colombia-Francia, and CODI
University of Antioquia 80740-031-2023, 2023-60336. The authors thank Maya Marinova for
TEM images. This work likewise pertains to the French government program
"Investissements d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010). The Chevreul
Institute is thanked for its help in the development of this work through the ARCHI-CM
project supported by the "Ministère de l'Enseignement Supérieur de la Recherche et de
l'Innovation", the region "Hauts-de-France", and the "Métropole Européenne de Lille.

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