# Exploring Stabilized Alumina Phase Coatings as a Catalyst Carrier for Oxalic Acid Hydrogenation

Limor Ben Neon, Issam Nciri, Martin Drobek, Mikhael Bechelany, Christophe

Coutanceau, Karine de Oliveira Vigier, Anne Julbe

► To cite this version:

Limor Ben Neon, Issam Nciri, Martin Drobek, Mikhael Bechelany, Christophe Coutanceau, et al. Exploring Stabilized Alumina Phase Coatings as a Catalyst Carrier for Oxalic Acid Hydrogenation. Chemistry–Methods, 2025, pp.e202500085. 10.1002/cmtd.202500085. hal-05325518

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Submitted on 22 Oct 2025

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# Exploring Stabilized Alumina Phase Coatings as a Catalyst Carrier for Oxalic Acid Hydrogenation

Limor Ben Neon<sup>a*</sup>, Issam Nciri<sup>b</sup>, Martin Drobek<sup>a</sup>, Mikhael Bechelany<sup>a,c</sup>, Christophe Coutanceau<sup>b</sup>, Karine De Oliveira Vigier<sup>b*</sup>, Anne Julbe<sup>a*</sup>

[a] Institut Européen des Membranes (IEM), UMR 5635, Univ Montpellier, CNRS, ENSCM; Place Eugène Bataillon; 34095 Montpellier; France

E-mail: limor.ben-neon@umontpellier.fr; anne.julbe@umontpellier.fr

[b] Institut de Chimie Des Milieux et Des Matériaux de Poitiers (IC2MP-UMR 7285); Université de Poitiers, CNRS; 1 Rue Marcel Doré; CEDEX 9, 86073 Poitiers; France;

E-mail: karine.vigier@univ-poitiers.fr

[c] Functional Materials Group, Gulf University for Science and Technology (GUST), Masjid Al Aqsa Street, Mubarak Al-Abdullah 32093, Kuwait

DOI: 10.1002/cmtd.202500085

Supporting information for this article is given via a link at the end of the document.

**Abstract:** The catalytic hydrogenation of oxalic acid to glycolic acid (GA) and/or ethylene glycol (EG) has been identified as a promising route for producing value-added chemicals and potential hydrogen carriers. GA serves as a precursor for plastics and cosmetics, while EG can function as a liquid organic hydrogen carrier. In this work, we present an eco-friendly and scalable bottom-up approach to fabricate a Ru-based hydrogenation catalyst on θ-alumina-coated SiC foam. Nitrogen doping of the MOF(Al)-based template enhances the mechanochemical stability of the resulting alumina coating by significantly modifying its surface texture and morphology. This phase-stabilization method enabled higher OA conversion compared to γ-alumina and yielded nearly equimolar amounts of GA and EG under continuous-flow conditions (120 °C, 50 bar). These findings demonstrate the potential of this synthesis approach for producing stabilized metal oxide phases with tailored morphologies that improve catalyst performance.

## Introduction

Oxalic acid (OA), discovered by Scheele in 1734, is an organic, dicarboxylic acid with the chemical formula $C_2H_2O_4$.<sup>1</sup> Initially identified in plants and microorganisms<sup>2</sup>, OA has since become a critical compound in various industrial sectors. Currently, the pharmaceutical industry is the largest consumer of OA, followed by its use in agriculture and the textile industry<sup>1</sup>. It serves various purposes, including as reactant in drug formulations, as well as a buffering, chelating and sterilization agent<sup>2,3</sup>. Given its industrial significance, numerous recent studies have focused on sustainable methods for OA production. These include biological strategies, such as scaling up production using microorganisms<sup>2</sup>, electrocatalytic conversion of $CO_2$ to OA<sup>1</sup>, and sugar oxidation<sup>4</sup>. Moreover, the optimization of OA's use as a reactant for the synthesis of derived compounds, such as glycolic acid (GA), has garnered a significant attention<sup>5</sup>. GA, in turn, is widely utilized in cosmetics for dermatological applications<sup>6</sup> and in petrochemistry for producing (bio)plastic<sup>7</sup>. One promising approach for converting OA to GA is through catalytic hydrogenation, typically performed in the presence of hydrogen gas ($H_2$) at pressures between 50 and 250 bar and temperatures ranging from 90 °C to 150 °C<sup>5,8</sup>. This reaction is commonly catalyzed by Platinum (Pt), Palladium (Pd), or Ruthenium (Ru)-based catalysts<sup>5,8</sup>. However, ethylene glycol (EG) is often produced as an unintended byproduct during this reaction. Due to the differences in their melting points (i.e., $T^{°GA} = 75$ °C; $T^{°EG} = -12.9$ °C), GA and EG can be separated via recrystallization, enabling the recovery of both compounds. Attractively, once EG recovered, it can be converted back into OA through an electrocatalytic dehydrogenation process that simultaneously releases $H_2$—highlighting the potential of this reversible system for sustainable hydrogen storage and transport. This closed-loop system, as presented in Figure 1, offers the potential for the continuous, low-cost production of OA and GA, with EG serving as a Liquid Organic Hydrogen Carrier (LOHC) for $H_2$ storage<sup>9</sup>. Such a system could significantly enhance the sustainability of OA and GA production while providing an efficient method for hydrogen storage.
<figure><img src="image_2.png" /><figcaption>**Figure 1:** Schematic principle of OA catalytic hydrogenation. The formed GA, can be directly used by the petrochemical and cosmetic industries, while the byproduct, EG, serving as Liquid Organic Hydrogen Carrier (LOHC) can attractively release H₂ upon electrocatalytic dehydrogenation.</figcaption></figure>

In this study, we focus on the first reaction step consisting in the hydrogenation of OA to GA and EG. The reaction pathway to form EG from OA goes through the formation of GA (Figure 2) ⁵.

<figure><img src="image_3.png" /><figcaption>**Figure 2:** Reaction pathway of the catalytic hydrogenation of OA to EG.</figcaption></figure>

For this kind of reaction, it is known that Ru-based catalysts are the most active hydrogenation catalysts compared to Pd and Pt ⁸. Schuler *et al.* demonstrated that the catalyst Ru (5 wt%) supported on γ-alumina could induce the selective formation of GA from OA at low temperatures (75 °C) and high hydrogen pressures (80 bar) without its further conversion to EG ⁵. Harth *et al.* have studied the catalytic hydrogenation of GA to EG using Ru-based catalysts supported on γ-alumina and other types of carriers such as TiO₂ and carbon at a higher reaction temperatures of 150 °C and at lower hydrogen pressure of 40 bar in a batch reactor ⁸. They observed a significant variation in conversion rates per Ru loading and selectivity toward EG formation depending on the applied support material. Specifically, in the work of Harth *et al.*, titanium dioxide (TiO₂) emerged as the most active and selective catalyst, while carbon also exhibited relatively high conversions (~60%) under comparable batch conditions (150 °C, 40 bar H₂), although their selectivity toward EG remained limited. More recently, Schuler *et al.* have demonstrated that the use of carbon-based supports (Ru/C, RuSn/C) as well as MXene- and mixed-metal carbon supports enabled much higher OA conversion rates (up to >90%) with GA selectivity exceeding 95% ⁵,¹⁰. The authors suggested that the observed differences in catalytic performance could be ascribed to varying interaction strengths between Ru and the support, which influenced Ru particle sizes and the interaction force between the active site and reactants. However, the chemical stability of the support should not be neglected under these harsh acidic conditions as it might lead to lower conversion rates and diminish selectivity as observed for γ-alumina. While MXenes and mixed-metal carbon-based supports show excellent performance for OA hydrogenation, their synthesis often involves multiple steps, costly precursors, or hazardous reagents (e.g., Hydrofluoric acid etching for MXenes), which may limit scalability. In contrast, alumina is abundant, inexpensive, and industrially well-established. However, remains a challenge, as dissolution under low pH conditions leads to catalyst deterioration ¹¹. Developing stabilized theta-alumina phases would offer a comparatively simpler, more environmentally benign, and potentially more cost-effective route for fabricating robust catalyst supports.

Unfortunately, improving the acid resistance of alumina often involves thermal treatment at elevated temperatures to produce theta (θ) and alpha (α) alumina phases ¹². These phases, while more resistant to low pH, have low specific surface areas and limited hydroxyl groups, which are essential for anchoring Ru during impregnation. Consequently, increasing the surface area of θ/α-alumina while maintaining phase stability remains a significant challenge, often requiring the use of additives, metal salts, and solvents during synthesis ¹²,¹³. Furthermore, the involved multi-step synthesis procedures can be time-consuming and complex, making them less suitable for industrial applications.
To mitigate these limitations, an innovative, scalable, and eco-friendly synthesis procedure for supported metal-organic frameworks (MOFs) was developed previously in our research group ¹⁴. This method converts alumina-coated silicon carbide (SiC) foams into MOF(Al) via a microwave (MW)-assisted hydrothermal reaction. The key innovation of this procedure lies in the combination of the thermally dissipative SiC substrate under MW irradiation with the utilization of atomic layer deposited (ALD) coating as a metal precursor, which (i) promotes the growth of large, epitaxially aligned MOF(Al) crystals in just 90 minutes, and (ii) prevents competition between MOF growth and alumina facet rearrangement due to the amorphous nature of the ALD-alumina coating ¹⁵. This approach saves time and energy, minimizes the use of organic solvents like dimethylformamide (DMF), promotes the use of water and eliminates the need for metal salts. In addition, the as-prepared supported MOF(Al) can serve as a template for producing highly porous θ-alumina ($S_{BET}$ = 227 m²/g) on SiC foams ¹⁴. Using this material as a catalyst support for the hydrogenation of OA to EG and GA could significantly enhance the conversion rates, selectivity, and stability of Ru-based catalysts. Furthermore, synthesizing Ru-based catalysts via a bottom-up approach on SiC foams offers the advantage of easy handling and manufacturing. The open-cell structure of SiC foams also improves mass transfer during reaction, making it a promising candidate for use in a fixed bed reactor operating under liquid flow conditions ¹⁶.

In this work, we report for the first time the successful hydrogenation of OA to EG and GA in a fixed bed reactor utilizing a novel Ru-based catalyst featuring a unique root-like θ-Al₂O₃ morphology supported on a ceramic foam substrate. This catalyst consists of 5 wt% Ru dispersed on a porous θ-alumina-coated SiC foam with an open-cell structure via a MW-assisted polyol method. This method avoids the presence of chlorine, a known inhibitor for hydrogenation ¹⁷. Besides, we emphasize the significance of the formed MOF template. Specifically, we demonstrate that incorporating an amino group into the organic linker terephthalic acid structure induces morphological, structural, and textural modifications of the formed MOF, which subsequently have a profound impact on the generated porous alumina after calcination. Microscopic analysis and adhesion tests show that using N-doped MOF(Al) as a template significantly enhances the mechanical and chemical stability of the derived alumina in strong acidic media due to its particular morphology. After optimizing the organic linker's nature and calcination temperature, the most stable alumina carrier was selected for Ru deposition. The resulting catalyst maintained a stable activity for up to 90 minutes under operating conditions (120 °C, 50 bar), achieving conversion rates of 30 % with a near-equimolar selectivity between EG and GA.

## Experimental

### Materials

SEDEX-10 PPI SiC foams were purchased from EPMF-France. Oxalic acid (OA, 99%), organic linkers terephthalic acid (98 %) and amino-terephthalic acid (98 %), Ru/Al₂O₃ as well as solvents N,N-dimethylformamide (DMF, 99.8 %), ethanol (99 %) and ethylene glycol (EG, 98 %), were obtained from Sigma Aldrich. NaHCO₃ (> 99 %) and RuCl₃ (min. 40 wt% Ru) were provided by TCI.

### Materials synthesis

#### Synthesis of supported MOFs on SiC foams

To synthesize supported MOFs on SiC foams, a two-step synthesis was applied. First, an amorphous alumina layer was deposited via Atomic Layer Deposition (ALD) on 0.5 x 1.5 x 1.5 cm SiC foam pieces, as described in our previous work ¹⁴. Secondly, this layer was converted into an aluminum-based MOF via a MW-assisted approach using a 1 wt% solution of either terephthalic acid (H₂BDC) or amino-terephthalic acid (NH₂H₂BDC) ¹⁴,¹⁵. In short, the ALD-alumina coated SiC foam was placed in a Teflon™ pressure vessel containing 0.4 g of either H₂BDC or NH₂H₂BDC, dissolved in 40 mL of Milli-Q water (18.2 MΩ·cm). The mixture was heated for 1.5 h at 220 °C (P = 600 W, 44 °C/min) using a Microsynth Milestone microwave oven equipped with a HPR 1000/10 rotor. After the reaction, the MOF-decorated SiC foams were removed from the vessel and sequentially washed with Milli-Q water, DMF, and again with Milli-Q water to ensure complete removal of DMF. The samples were then dried in air at 120 °C for 12 hours.

The resulting samples were labeled MOF(Al)@SiC and NMOF(Al)@SiC for which H₂BDC and NH₂H₂BDC were used as organic linkers respectively (Figure 3).

#### Synthesis of supported porous alumina on SiC foams

To generate porous alumina, MOF-supported samples (MOF(Al)@SiC and NMOF(Al)@SiC) were thermally treated in static air at different temperatures (T = 550 °C and 1000 °C) for 3 h (heating rate: 5 °C/min) in a furnace. This treatment aimed to evaluate the impact on the alumina phase nature, surface texture and stability. The
thermally treated samples were labeled T-Al₂O₃@SiC and T-NAl₂O₃@SiC, where “T” denotes the calcination temperature of the MOF(Al)@SiC and NMOF(Al)@SiC samples respectively (Figure 3).

Each synthesis step included weighing the prepared samples using a precision balance to monitor weight changes through the process.

## MW-assisted deposition of Ru on 1000-NAl₂O₃@SiC

To deposit a weight percentage of 5 wt% of Ru on porous alumina-coated SiC foams, a MW-assisted polyol method was applied. The Ru loading was fixed at 5 wt% to enable comparison with commercial Ru/γ-Al₂O₃ catalyst. In a Teflon™ pressure vessel, 0.15 g of RuCl₃ was mixed with 30 mL EG under sonication for 5 min. Afterwards, 1000-NAl₂O₃@SiC was added to the mixture and heated using a Microsynth Milestone microwave oven equipped with a HPR 1000/10 rotor for 60 min at 180 °C (P = 300 W, 44 °C/min). The mixture was rapidly cooled with ice. The coated SiC foam was then washed with a 0.1 M NaHCO₃ solution, Milli-Q water, and ethanol. The obtained black-coated SiC foam was dried overnight under vacuum at 90 °C (Figure 3).

<figure><img src="image_4.png" /><figcaption>**Figure 3:** Schematic illustration of the synthesis procedure for (A.) T-Al₂O₃@SiC and (B.) T-NAl₂O₃@SiC, followed by the Ru deposition process on 1000-NAl₂O₃@SiC.</figcaption></figure>

## Physico-chemical characterization

The surface morphology of pristine SiC and MOF-modified foams (MOF(Al)@SiC and NMOF(Al)@SiC) before and after thermal treatment, as well as after Ru deposition on T-NAl₂O₃@SiC, was examined using Scanning Electron Microscopy (FESEM, HITACHI S-4800). The elemental distribution of Al, Si, N, O, C and Ru was determined by Energy Dispersive X-ray Spectroscopy (EDX) with a Zeiss Sigma 300 SEM equipped with an Oxford Instruments X-Max N SDD detector. For the microscopic analysis samples were metallized with platinum.

The porous structure of the supported MOF materials before and after thermal treatment was investigated by N₂ physisorption experiments. Samples were first outgassed under vacuum at 150 °C and 200 °C, respectively, for 12h. N₂ adsorption/desorption isotherms were recorded using a Micromeritics ASAP 2020 equipment. The specific
surface areas (S<sub>BET</sub>) were calculated by the multipoint Brunauer–Emmett–Teller (BET) method. The total pore volume (V<sub>p</sub>) was evaluated at a P/P<sub>0</sub> close to 0.995.

Since the surface of the SiC foam is not flat, X-ray diffraction analysis is not feasible. To determine the nature of the alumina phase as a function of the MOF(Al) template and calcination temperature, the ALD-alumina coated SiC —used as both the alumina source and substrate — was replaced with boehmite particles (CATAPAL C1 alumina, Sasol). Boehmite, with its AIOOH chemical structure, closely resembles the chemical nature of ALD-alumina. The same synthesis procedure was applied, except the reaction time was reduced to 5 min to avoid competition between MOF growth and boehmite facet rearrangement, as reported in our previous study<sup>15</sup> (see SI for detailed synthesis). Powder X-ray diffraction (PXRD) patterns of the thermally treated samples were recorded using a Malvern Panalytical X'Pert Powder S diffractometer at a scan rate of 0.021°/s. The precision in diffraction peak position is 0.04° (2θ).

## Mechanical and chemical stability of supported alumina

Since the designed system is intended for continuous-flow hydrogenation catalysis, strong adhesion of the porous alumina to the SiC foam surface is crucial. Sonication is a simple, quick, and reliable method to test the adhesion of thin films on corrugated supports, where standard adhesion tests like scratch-tape tests are not applicable<sup>14</sup>. In this study, 0.25 g of porous alumina-coated SiC foams (T-Al<sub>2</sub>O<sub>3</sub>@SiC and T-NAl<sub>2</sub>O<sub>3</sub>@SiC) treated at different temperatures were immersed in 25 mL of a 10 wt% oxalic acid solution and subjected to ultrasonic treatment. This test provides two key information: i) the mechanical adhesion of the coating to the SiC foam, simulated by ultrasonic stress, and ii) the chemical stability of the coating in the presence of oxalic acid. Sonication tests were conducted using a Fisher Bioblock 86480 sonicator at an applied power of 345 W for 15 and 60 minutes. Aluminum concentrations ([Al]) were determined via EDX to evaluate any potential loss of alumina during sonication. Additionally, SEM observations were conducted before and after sonication tests to visualize any change in morphology.

## Catalytic test: Hydrogenation of oxalic acid to glycolic acid and ethylene glycol

Catalytic performance tests were carried out using Ru supported on 1000-NAl<sub>2</sub>O<sub>3</sub>@SiC (Ru/1000-NAl<sub>2</sub>O<sub>3</sub>@SiC) in a fixed-bed reactor operating under continuous flow conditions. Reactions were conducted at 120 °C for 3 h, with a hydrogen flow rate (Q<sub>H2</sub>) of 40 mL/min and a 10 wt% aqueous solution of oxalic acid (OA) introduced at a flow rate (Q<sub>OA</sub>) of 0.4 mL/min. An amount of 0.4 g of catalyst was used.

The liquid-phase hydrogenation products were analyzed using a Shimadzu high-performance liquid chromatography (HPLC) system equipped with a refractive index detector (RID, LC-20AD). Product separation was achieved using a Coragel IC SEP 107H column (300 × 7.8 mm), with an eluent composed of 745 µL of 98% H<sub>2</sub>SO<sub>4</sub> in 2 L of ultrapure water. The eluent flow rate was maintained at 0.8 mL/min, and the column temperature was set to 35 °C.

Product quantification was performed through external calibration using commercially available standards. For each compound, conversion (X), yield (Y), and selectivity (S) were calculated and expressed as percentages according to the following equations, where n<sub>0</sub> is the initial number of moles of OA, and n(X) is to the number of moles of the specific compound formed after the reaction:

$$ \text{Conversion (\%)} = \frac{n_0 - n(X)}{n_0} \times 100 $$

$$ \text{Yield (\%)} = \frac{n(X)}{n_0} \times 100 $$

$$ \text{Selectivity (\%)} = \frac{\text{Yield}}{\text{Conversion}} \times 100 $$

For comparison, commercial Ru/γ-Al<sub>2</sub>O<sub>3</sub> powder (Sigma-Aldrich) was tested under identical reaction conditions. Prior to the reaction, all catalysts were activated *in-situ* in the reactor using a N<sub>2</sub>/H<sub>2</sub> gas mixture (1:1 ratio) at a total flow rate of 20 mL/min. Activation was carried out at 200 °C for 4 h.

## Results and Discussion

The physicochemical characteristics of the prepared catalyst carriers—derived from two different MOF(Al) templates supported on SiC foam (MOF(Al)@SiC and NMOF(Al)@SiC), as well as their corresponding calcined forms (T-Al<sub>2</sub>O<sub>3</sub>@SiC and T-NAl<sub>2</sub>O<sub>3</sub>@SiC)—are presented first. Subsequently, their mechanochemical stability was evaluated in the presence of oxalic acid (OA). Based on these results, the most stable alumina carrier
supported on SiC foam was selected for Ru deposition. Finally, the optimized catalyst was tested for the thermal
hydrogenation of OA to glycolic acid (GA) and ethylene glycol (EG) in a fixed-bed reactor.

Physicochemical characterization of materials

The MW-assisted conversion of metal-oxide to MOF using either terephthalic acid or amino-terephthalic acid was successful, as confirmed by SEM images (Figure 4). As previously reported for MOF(Al)@SiC, the use of H₂BDC results in the formation of urchin-like crystals uniformly distributed across the SiC surface ¹⁴. When this organic ligand was replaced with its amine-functionalized analogue (NH₂H₂BDC), the crystal morphology of the resulting MOF changed significantly. Rod-like structures composed of numerous agglomerated, parallel-aligned fibers were observed, with an average length of 8 µm—approximately four times longer than the crystals formed with H₂BDC. A brown coloration of the foam was also observed for NMOF(Al)@SiC.

<figure><img src="image_5.png" /><figcaption>**Figure 4:** SEM images of (A.) pristine SiC foam, (B.) MOF(Al)@SiC synthesized using H₂BDC as the organic linker, and (C.) NMOF(Al)@SiC synthesized using NH₂H₂BDC as the organic linker.</figcaption></figure>
Therefore, we can infer that the introduction of an amine group into the structure of the organic linker promoted the elongation of NMOF(Al) crystals and likely influenced the overall MOF morphology due to the incorporation of nitrogen into the crystals structure, as evidenced by EDX mapping (Figure 5). Besides, based on N₂ sorption tests, the specific surface area nearly doubled, while pore volume remained unchanged (Table 1). Incorporating nitrogen into the MOF structure increased the specific surface area from 90 m²/g to 151 m²/g.

<figure><img src="image_6.png" /><figcaption>**Figure 5:** EDX-mapping of NMOF(Al)@SiC sample showing the elemental distribution of C, Al, O, Si, N.</figcaption></figure>

The striking morphological differences between MOF(Al)@SiC and NMOF(Al)@SiC remain observable even after thermal treatment. When these samples are thermally treated under air for 3 h at 550 °C and 1000 °C, respectively, two distinct alumina morphologies supported on SiC are obtained (Figure 6). For the calcined MOF(Al)@SiC, referred to as T-Al₂O₃@SiC, the formation of accordion-like rods at 550 °C, which shrink in length at 1000 °C is observed. In contrast, the calcination of NMOF(Al)@SiC, referred to as T-NAl₂O₃@SiC, generates a root-like network across the SiC surface remaining morphologically stable even at higher calcination temperatures of 1000 °C.

Textural analysis reveals that both samples maintain high specific surface areas—exceeding 200 m²/g—even after calcination at 1000 °C (Table 1). This finding is noteworthy, given that high surface areas in ceramics prepared at elevated temperatures are usually attained only through the incorporation of surfactants or structure-directing additives ¹³,¹⁸. Comparison of the textural properties shows that T-NAl₂O₃@SiC possesses a slightly higher specific surface area than T-Al₂O₃@SiC. This increase is probably caused by the removal of amine groups during thermal treatment, which may promote the development of additional porosity.
<figure><img src="image_7.png" /><figcaption>**Figure 6:** SEM images showing the surface morphology of thermally treated supported MOFs on SiC foams in air at 550 °C and 1000 °C. (A.) 550-Al₂O₃(Al)@SiC, (B.) 1000-Al₂O₃@SiC, (C.) 550-NAl₂O₃@SiC, and (D.) 1000-NAl₂O₃@SiC.</figcaption></figure>

<table><caption>**Table 1:** Impact of thermal treatment on MOF-supported SiC samples (3 h at 550 °C or 1000 °C – 5 °C/min in air) on their textural characteristics: (a) Δm = m(thermally treated sample) - m(foam), (b) wt% of Al₂O₃ determined by EDX, (c) Mean pore size values at maximal distribution intensity. *SBET and Vp were calculated by considering the wt% of the formed material (Al₂O₃) as determined by EDX. (An error of ~10% is estimated for both SBET and Vp).</caption><thead><tr><th>Sample Reference</th><th>Δm (wt%)<sup>(a)</sup></th><th>Al<sub>2</sub>O<sub>3</sub> (wt%)<sup>(b)</sup></th><th>S<sub>BET</sub> (m<sup>2</sup>/g)</th><th>V<sub>p</sub> (cm<sup>3</sup>/g)</th><th>Formed supported material</th><th>S<sub>BET</sub> of formed supported material (m<sup>2</sup>/g)</th><th>V<sub>p</sub> of formed supported material (cm<sup>3</sup>/g)</th><th>Mean pore sizes (nm)<sup>(c)</sup></th></tr></thead><tbody><tr><td>MOF(Al)@SiC</td><td>9.1%</td><td>-</td><td>7</td><td>0.019</td><td>MOF(Al)</td><td>80</td><td>0.200</td><td>-</td></tr><tr><td>550-Al<sub>2</sub>O<sub>3</sub>@SiC</td><td>5.3%</td><td>3.7%</td><td>13</td><td>0.032</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>395<br>353*</td><td>0.965<br>0.869*</td><td>3; 11</td></tr><tr><td>1000-Al<sub>2</sub>O<sub>3</sub>@SiC</td><td>2.7%</td><td>3.5%</td><td>8</td><td>0.024</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>298<br>227*</td><td>0.926<br>0.681*</td><td>3; 8; 20</td></tr><tr><td>NMOF@SiC</td><td>5.4%</td><td>-</td><td>8</td><td>0.015</td><td>NMOF(Al)</td><td>151</td><td>0.284</td><td>-</td></tr><tr><td>550-NAl<sub>2</sub>O<sub>3</sub>@SiC</td><td>4.4%</td><td>3.4%</td><td>15</td><td>0.024</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>344<br>440*</td><td>0.559<br>0.704*</td><td>6; 11; 28</td></tr><tr><td>1000-NAl<sub>2</sub>O<sub>3</sub>@SiC</td><td>4.5%</td><td>3.4%</td><td>8</td><td>0.018</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>178<br>233*</td><td>0.397<br>0.524*</td><td>3.4; 11; 20</td></tr></tbody></table>
Although the incorporation of an amino group into the organic linker significantly influences surface texture and morphology, it does not affect the nature of the resulting alumina phase. The latter is governed solely by the calcination temperature. Calcining NMOF(Al) and MOF(Al) supported on SiC foams at 550°C resulted in the formation of γ-Al₂O₃, as evidenced by the PXRD patterns in Figure 7. Increasing further the calcination temperature to 1000°C promotes the formation of θ-Al₂O₃, with trace amounts of α-Al₂O₃ detected at 43.3° and 57° (2θ).

<figure><img src="image_8.png" /><figcaption>**Figure 7:** XRD patterns of (a) θ-Al₂O₃ (simulated pattern- ICSD code 82504), and of as-calcined MOF(Al) and NMOF(Al) grown on boehmite at 550°C (b, d) and 1000°C (c, e), respectively.</figcaption></figure>

Chemical and mechanical stability test

The chemical and mechanical stability of porous alumina supported on SiC was evaluated under intense sonication in a 10 wt% oxalic acid solution. As described in the experimental section, sonication simulates mechanical stress, allowing to assess the coating's stability on SiC in the acidic reaction medium.

As shown in Table 2, the amount of aluminum lost differs significantly between T-Al₂O₃@SiC and T-NAl₂O₃@SiC samples. While Al₂O₃@SiC calcined at 550°C (550-Al₂O₃@SiC) lost almost completely its alumina coating, T-NAl₂O₃@SiC remained much more stable, losing only a negligible amount of aluminum. SEM images of 550-Al₂O₃@SiC (Figure S1) confirm the absence of alumina. Increasing the calcination temperature helped to preserve most of the alumina coating by transitioning from an unstable γ-alumina phase to a stabilized θ-phase (Figure 7). Yet the acidic treatment shortened alumina fibers length due to leaching, as reflected by the aluminum loss. Consequently, using an amine-doped MOF as a template for porous alumina improved the mechanochemical stability of the resulting supported alumina.

Raising the calcination temperature enhanced the stability of alumina by stabilizing its phases, thereby increasing their resistance to acidic environments. Moreover, the combination of the unique root-like network morphology with the θ-alumina phase contributed to preserving the carrier's structure even under severe conditions (Figure S1).

<table><caption>**Table 2:** Aluminum concentrations (mean weight percentage) measured by EDX before and after the chemical and mechanical stability test for 550-Al₂O₃@SiC, 1000-Al₂O₃@SiC, 550-NAl₂O₃@SiC and 1000-NAl₂O₃@SiC, after 1 h of sonication in a 10 wt% oxalic acid solution.</caption><thead><tr><th></th><th>550-Al<sub>2</sub>O<sub>3</sub>@SiC</th><th>1000-Al<sub>2</sub>O<sub>3</sub>@SiC</th><th>550-NAl<sub>2</sub>O<sub>3</sub>@SiC</th><th>1000-NAl<sub>2</sub>O<sub>3</sub>@SiC</th></tr></thead><tbody><tr><td>[Al]wt (%) before test</td><td>7.6 ± 0.5</td><td>7.5 ± 0.5</td><td>9.6 ± 0.5</td><td>8.8 ± 0.5</td></tr><tr><td>[Al]wt (%) after test</td><td>1.1 ± 0.5</td><td>4.5 ± 0.5</td><td>8.8 ± 0.5</td><td>8.8 ± 0.5</td></tr><tr><td>[Al]wt (%) loss</td><td>6.5</td><td>3.0</td><td>0.8</td><td>0.0</td></tr></tbody></table>
## MW-assisted deposition of Ru in ethylene glycol

Based on the obtained results, the supported porous alumina derived from NMOF@SiC demonstrates superior chemical suitability as a catalyst carrier for the hydrogenation of OA. Hence, Ru was deposited on $T-NAl_2O_3@SiC$, prepared by calcining NMOF@SiC at 550 °C and 1000 °C. To achieve efficient Ru dispersion, a MW-assisted method using EG and $RuCl_3$ was employed. This approach effectively prevents chlorine poisoning of the catalyst, which could otherwise inhibit the reaction ¹⁷.

Although 550-NAl₂O₃@SiC presented promising chemical and mechanical stability, the deposition of Ru under MW conditions altered the surface of the alumina coating (Figure S2). The γ-alumina phase present in 550-NAl₂O₃@SiC is not stable enough for this type of Ru deposition. In contrast, the θ-alumina phase obtained after calcination at 1000 °C is sufficiently stabilized to withstand a MW-assisted Ru-polyol deposition. EDX mapping highlights the homogeneous distribution of Ru across the surface of 1000-NAl₂O₃@SiC, while SEM images confirm that the root-like alumina network is preserved (Figure S3). An Ru loading of 5 wt% was quantified by EDX, and the absence of Cl was confirmed.

## Catalytic test: Hydrogenation of oxalic acid to glycolic acid and ethylene glycol

The hydrogenation of OA to GA and EG was investigated in a fixed bed reactor using Ru/1000-NAl₂O₃@SiC as a catalyst.

### Conversion of OA to GA and EG

Results show that conversion rates of 30 % remain stable for up to 90 min of reaction and afterwards drop to zero (Figure 8A). Consequently, after 90 min, the Ru/1000-NAl₂O₃@SiC becomes inactive. Compared to commercial Ru/γ-Al₂O₃, the as-prepared supported catalyst exhibits 15 % higher conversion rates and superior stability (Figure S5). This enhanced stability is attributed to the stabilized alumina phase, providing better resistance to the acidic reaction medium, potentially reinforced by its unique root-like morphology. However, this chemical resistance of the Ru/1000-NAl₂O₃@SiC catalyst appears to be limited under the selected—relatively harsh—operating conditions (120°C, $Q_{OA}$ = 0.4 mL/min, $Q_{H2}$ = 40 mL/min).

<figure><img src="image_9.png" /><figcaption>**Figure 8:** Catalytic test results for the hydrogenation of oxalic acid (OA) under continuous flow for 3 h reaction at 120°C with $H_2$ flow rate of 40 mL/min and OA flow rate of 0.4 mL/min. (A) Conversion rates of OA with Ru/1000-NAl₂O₃@SiC (blue) and commercial Ru/Al₂O₃ powder (grey). Dashed lines indicate the conversion trend. (B) Selectivity rates for the supported catalyst Ru/1000-NAl₂O₃@SiC.</figcaption></figure>

Specifically, 7.3 mg/L of aluminum was detected at the outlet of the fixed-bed reactor (Table 3), which is approximatively close to the total amount of θ-Al₂O₃ coating on the supported catalyst (10.1 mg/L). This suggests that the coating is peeled off after 90 min of reaction under acidic continuous flow, leading to the removal of Ru particles from the catalyst surface (Table 3) and, consequently, a loss of catalytic activity. In concordance, analysis of the spent catalyst's surface morphology after 90 min of reaction, revealed zones where the alumina coating was either partially or completely delaminated (Figure S4). These observations indicate that catalyst deactivation is primarily driven by leaching of the alumina layer under the selected operating conditions (120°C, $Q_{OA}$ = 0.4 mL/min, $Q_{H2}$ = 40 mL/min).
Additionally, a small amount of Si (0.4 mg/L) was detected by ICP-MS (Table 3). The silicon (Si) originates from the SiC foam substrate, which contains native silica generated at 1000 °C but also silica-based additives used as sintering binders. It is plausible that, once the alumina coating is leached away, these binders become exposed to the acidic medium and begin to dissolve. This process likely contributes to the progressive degradation of the structural backbone of the SiC-foam. This observation leads us to consider, for future work, operating under milder conditions—specifically, using a lower temperature (< 120°C, as reported in other works [5]), and/or reduce oxalic acid and/or H₂ flow rates—in order to minimize abrasion of the catalytic layer. Choosing SiC foams without silica-based sintering additives will also be necessary to ensure the long term stability of the foam structure.

<table><caption>**Table 3:** ICP-MS results of the reaction medium after 3h reaction of oxalic acid (OA) hydrogenation at 120°C in a fixed bed reactor with Ru/1000-NAl₂O₃@SiC (%RSD < 2%).</caption><thead><tr><th>Element</th><th>Concentration (mg/L)</th></tr></thead><tbody><tr><td>Al</td><td>7.3</td></tr><tr><td>Ru</td><td>6.2</td></tr><tr><td>Si</td><td>0.4</td></tr></tbody></table>

## Selectivity of Ru/1000-NAl₂O₃@SiC toward the formation of GA and EG from OA

When OA is converted to GA using the as-prepared Ru/1000-NAl₂O₃@SiC catalyst under operating conditions, EG and other secondary compounds—such as mesoxalic acid and tartaric acid—are also observed in the liquid phase. Not all byproducts could be identified due to polymerization reactions. As shown in Figure 8B, after 90 min, the product selectivity is approximately 45 % GA, 34 % EG, and 18 % other compounds, including formic acid (FA). Subsequently, the fraction of FA increases as the overall conversion declines. This increase in FA selectivity is attributed to the thermal decomposition of GA rather than to catalytic activity, as the catalyst becomes inactive after 120 min of reaction. Indeed, when the reaction is carried out under similar conditions but without a catalyst, 100 % FA is obtained at a conversion below 15 % ¹.

A similar thermal conversion trend of GA to FA is observed with the commercial Ru/γ-Al₂O₃ catalyst. However, this transformation occurs significantly earlier (approximately 60 min), attributed to the catalyst's limited stability under acidic conditions (Figure S4). Employing θ-Al₂O₃ as a catalyst carrier not only enhances the mechanochemical resistance of Ru-supported alumina catalysts but also influences the selectivity toward EG and GA. When γ-Al₂O₃ is used as the carrier, the hydrogenation of OA at 120 °C and 50 bar of H₂ predominantly yields GA with a selectivity of 80 %, while no EG is formed (Figure S5). When comparing both catalytic systems at a similar conversion level (approximately 25 %), the commercial Ru/γ-Al₂O₃ catalyst exhibits a high selectivity toward GA, reaching 80 %, although EG is also detected. In contrast, the Ru/1000-NAl₂O₃SiC catalyst shows a lower GA selectivity of 51 %, while the selectivity toward EG increases to 31 %. These results can be further contextualized by comparing them with literature data for Ru/γ-Al₂O₃ catalysts used in batch reactor (Table 4) ⁵, where GA selectivity was reported to be the highest at similar conversion levels. Under identical reaction conditions, the θ-Al₂O₃ phase appears to promote a nearly equimolar formation of EG and GA. This observation underscores the critical role of alumina phase engineering in designing robust and selective catalysts for the hydrogenation of oxalic acid.

<table><caption>**Table 4:** Performance of several Ru-based hydrogenation catalysts supported on Al₂O₃, summarizing reaction conditions, conversion and selectivity rates (for EG and GA), and synthesis methods. This comparison includes the catalysts prepared in this work and a commercial Ru/γ-Al₂O₃. *Selectivity and conversion rates are reported after initial stabilization.</caption><thead><tr><th rowspan="2">Hydrogenation<br>Catalyst</th><th rowspan="2">Initial<br>reactant</th><th colspan="3">Operation conditions</th><th rowspan="2">Conversion<br>* (%)</th><th colspan="2">Selectivity*<br>(%)</th><th rowspan="2">Synthesis<br>method</th><th rowspan="2">Ref.</th></tr><tr><th>T<br>(°C)</th><th>P(H₂)<br>(bar)</th><th>Reactor<br>type</th><th>EG</th><th>GA</th></tr></thead><tbody><tr><td>Ru(5wt%)/Al₂O₃</td><td>OA</td><td>75</td><td>80</td><td>Batch</td><td>23</td><td>-</td><td>15</td><td>Wet incipient<br>impregnation of γ-<br>Al₂O₃ powder</td><td>5</td></tr><tr><td>Ru (5 wt%)/<br>γ-Al₂O₃<br>commercial</td><td>OA</td><td>120</td><td>50</td><td>Fixed<br>bed<br>(H₂ 40<br>mL/min<br>OA 0.4<br>mL/min)</td><td>22</td><td>-</td><td>80</td><td>Commercial<br>powder catalyst<br>(Sigma Aldrich)</td><td>This<br>work</td></tr><tr><td>Ru (5 wt%)/<br>1000-<br>NAl₂O₃@SiC</td><td>OA</td><td>120</td><td>50</td><td>Fixed bed<br>(H₂ 40<br>mL/min)</td><td>30</td><td>31</td><td>51</td><td>Supported θ-<br>Al₂O₃<br>(See synthesis<br>section)</td><td>This<br>work</td></tr></tbody></table>
OA 0.4
mL/min)

**Conclusion**

In this work, a robust and scalable bottom-up approach for the synthesis of supported hydrogenation catalysts using MW technology has been successfully developed. This innovative method significantly reduces solvent consumption, encourages the use of water as a green solvent, eliminates the reliance on metal salts, and simplifies catalyst handling procedures. The optimized engineered catalyst comprises a θ-Al₂O₃-coated SiC foam dispersed with Ru (5 wt%). The unique root-like morphology of the Al₂O₃—derived from a templated N-doped MOF(Al) and stabilized in the θ-phase—provides enhanced mechanochemical resistance compared to conventional γ-Al₂O₃ under continuous flow conditions.

The key findings of this work demonstrate that tuning the alumina phase allows to modulate the reaction's selectivity and to achieve higher conversion rates of 30 % compared to commercial Ru/γ-Al₂O₃ and to as prepared alumina catalyst by Schuler *et al.* ⁵. Under continuous flow conditions (10 wt% OA with QOA = 0.4 mL/min; PH₂ = 50 bar, QH₂= 40 mL/min; and T = 120 °C), the catalyst delivered 31 % selectivity toward EG and 51% toward GA.

These results underscore the potential of this eco-friendly synthesis strategy to produce robust, substrate-supported hydrogenation catalysts with enhanced activity and tunable selectivity for oxalic acid hydrogenation. Although catalyst stability remains a challenge, the findings provide clear directions for future optimization toward long-term operation and broader applicability in hydrogen storage and sustainable chemical production.

**Supporting Information**

Supplementary material is supplied with this manuscript.

**Data availability**

Additional data will be made available on request.

**Acknowledgements**

This research was funded by the French National Research Agency (ANR project PEHY-0016-BHyoLOHC), France 2030. The authors thank Bertrand Rebiere (UAR Plateforme d'Analyses et Caractérisations (PAC) Chimie Balard Montpellier, Univ Montpellier, CNRS, ENSCM) for SEM-EDX analyses, Lilian Eloy (ICP analysis). The authors acknowledge financial support from the European Union (ERDF) and Région Nouvelle Aquitaine. This work pertains to the French government program "Investissements d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010).

**Keywords:** heterogeneous catalysis • microwave • MOF template • ceramics • hydrogen storage

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Supplementary information

Exploring Stabilized Alumina Phase Coatings as a Catalyst Carrier for
Oxalic Acid Hydrogenation

Limor Ben Neon<fnref n="1" />*, Issam Nciri<fnref n="2" />, Martin Drobek<fnref n="1" />, Mikhael Bechelany<fnref n="1" />,<fnref n="3" />, Christophe Coutanceau<fnref n="2" />, Karine De Oliveira Vigier<fnref n="2" />*, Anne Julbe<fnref n="1" />*

*corresponding authors: [anne.julbe@umontpellier.fr](mailto:anne.julbe@umontpellier.fr); [limor.ben-neon@umpontpellier.fr](mailto:limor.ben-neon@umpontpellier.fr)
[karine.vigier@univ-poitiers.fr](mailto:karine.vigier@univ-poitiers.fr)

1. Synthesis of porous alumina derived from aluminum-based MOFs grown directly on boehmite particles for PXRD analysis

1 g of boehmite (CATAPAL C1 alumina, Sasol, ALOOH) was introduced into a Teflon liner containing 40 mL of a 1 wt% solution of either 2-aminoterephthalic acid or terephthalic acid in Milli-Q, water (18 MΩ). The mixture was heated in a Microsynth Milestone microwave oven with an HPR 1000/10 rotor at 220°C for 5 min (heating rate: 44°C/min, power: 600 W).

After the microwave-assisted conversion of ALOOH to MOF(Al), the samples were collected, washed
with Milli-Q water and DMF (N,N-Dimethylformamide) using a Sigma 3-16P centrifuge (8000 rpm, 5
min), and dried overnight at 120°C. Finally, the synthesized samples were calcined at 550 °C and 1000
°C for 3h each (heating rate: 5 °C/min) under static air.

<footnote n="1">¹Institut Européen des Membranes (IEM), UMR 5635, Univ Montpellier, CNRS, ENSCM; Place Eugène Bataillon; 34095 Montpellier; France</footnote>

<footnote n="2">²Institut de Chimie Des Milieux et Des Matériaux de Poitiers (IC2MP-UMR 7285); Université de Poitiers, CNRS; 1 Rue Marcel Doré; CEDEX 9, 86073 Poitiers; France;</footnote>

<footnote n="3">³ Functional Materials Group, Gulf University for Science and Technology (GUST), Masjid Al Aqsa Street, Mubarak Al-Abdullah 32093, Kuwait</footnote>
## 2. Additional SEM-EDX analysis results

<figure><img src="image_10.png" /></figure>

Figure S1: SEM analysis showing the Impact of chemical and mechanical stability tests on the surface morphology of Al₂O₃@SiC calcined at (A) 550 °C; (B) 1000 °C for 3h (5 °C/min) under air, and N₄Al₂O₃@SiC calcined at (C) 550 °C and (D) 1000°C for 3h (5°C/min) under air.

<figure><img src="image_11.png" /></figure>

Figure S2: SEM image of 550-NAl₂O₃@SiC after Ru deposition via a MW-assisted solvothermal reaction using ethylene glycol and RuCl₃.
<figure><img src="image_12.png" /><figcaption>Figure S3: EDX mapping showing the elemental distribution of Al, O, Si, C and Ru for 1000-NAl₂O₃@SiC sample after Ru deposition. No traces of N were detected.</figcaption></figure>

### 3- Additional catalytic results- Post-analysis of spent Ru/1000-NAl₂O₃@SiC catalyst

<figure><img src="image_13.png" /><figcaption>Figure S4: SEM images of spent Ru/1000-NAl₂O₃@SiC catalyst after 90 min of reaction at 120 °C.</figcaption></figure>
## 4- Additional catalytic results- Hydrogenation of OA using a commercial Ru/γ-Al₂O₃ catalyst

<figure><img src="image_14.png" /><figcaption>Figure S5: Evolution of reaction selectivity versus time for the hydrogenation of oxalic acid using a commercial Ru/γ-Al₂O₃ catalyst (0.4 g) in a fixed bed reactor, under continuous flow of OA (40 mL/min) and H₂ (0.4 mL/min) at 120 °C.</figcaption></figure>

## 5- HPLC chromatograms recorded during OA hydrogenation test under continuous flow

### a. Retention times of reactants and products:

<table><thead><tr><td>Chemical compound</td><td>Retention time (min)</td></tr></thead><tbody><tr><td>Oxalic acid</td><td>~ 7.5</td></tr><tr><td>Glycolic acid</td><td>~ 11.5</td></tr><tr><td>Ethylene glycol</td><td>~ 13.5</td></tr></tbody></table>

### b. Chromatogram of as prepared catalyst Ru/1000-NAl₂O₃@SiC

<figure><img src="image_15.png" /><figcaption>Figure S6: HPLC chromatogram obtained at 60 minutes of reaction recorded during OA hydrogenation test.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Figure S7: HPLC chromatogram obtained at 150 minutes of reaction recorded during OA hydrogenation test.</figcaption></figure>
c. Chromatogram of commercial Ru/Al₂O₃

<figure><img src="image_17.png" /><figcaption>Figure S8: HPLC chromatogram obtained at 60 minutes of reaction recorded during OA hydrogenation test.</figcaption></figure>