# Molecular layer deposition of alucone and its conversion into gas selective alumina-based membranes

Lucie Badouric, Martin Drobek, Jim Cartier, Maged Bekheet, Christophe

Charmette, Bertrand Rebière, Anne Julbe, Mikhael Bechelany

► To cite this version:

Lucie Badouric, Martin Drobek, Jim Cartier, Maged Bekheet, Christophe Charmette, et al.. Molecular layer deposition of alucone and its conversion into gas selective alumina-based membranes. Journal of Environmental Chemical Engineering, 2025, 13 (3), pp.116455. 10.1016/j.jece.2025.116455. hal-05222150

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# Molecular layer deposition of alucone and its conversion into gas selective alumina-based membranes

Lucie Badouric¹, Martin Drobek¹, Jim Cartier¹, Maged F. Bekheet², Christophe Charmette¹, Bertrand Rebière³, Anne Julbe¹, Mikhael Bechelany¹,⁴*

¹Institut Européen des Membranes, IEM, UMR-5635, Univ Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France

²Technische Universität Berlin, Faculty III Process Sciences, Institute of Materials Science and Technology, Chair of Advanced Ceramic Materials, Straße des 17. Juni 135, 10623 Berlin, Germany

³UAR Plateforme d'Analyses et Caractérisations (PAC) Chimie Balard Montpellier, Univ Montpellier, CNRS, ENSCM, Place Eugène Bataillon, 34095 Montpellier, France

⁴Functional Materials Group, Gulf University for Science and Technology (GUST), Mubarak Al-Abdullah 32093, Kuwait

**Keywords:** Alucone, Molecular Layer Deposition, Ceramic membrane, Hydrogen separation, Molecular sieving

## Graphical Abstract

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

## Abstract

Hydrogen (H₂) is emerging as a crucial energy alternative for the coming decades. Focusing on its extraction from gas mixtures, this work reports on a novel strategy for manufacturing H₂-selective
ultramicroporous carbon-doped alumina membranes using Molecular Layer Deposition (MLD). The membranes were synthesized on commercial γ-alumina tubular supports using trimethylaluminum and ethylene glycol, forming a conformal hybrid organic-inorganic layer of alucone. The final porous structure of the membrane was induced and tuned via a thermal treatment, which converted the alucone into porous C-doped alumina. The influence of the thermal treatment conditions on the physicochemical characteristics and performance of the as-formed alumina membrane layer was investigated in details. Gas permeation measurements revealed an ultramicroporous membrane network, characterized by a dominant molecular sieving mechanism. This led to separation factors of 242 and 150 for the H₂/CH₄ and H₂/N₂ gas pairs respectively, measured at 126°C in quaternary gas mixtures containing CO₂. Furthermore, the microporous nature of the membrane was confirmed by calculating the activation energies for each tested gas. The results presented demonstrate the potential of MLD in synthesizing H₂-selective porous alumina membranes with competitive performance at relatively low operating temperatures. Additionally, this strategy could be applied to create membranes selective for other gas mixtures by carefully tuning the material porous structure. These microporous thin films could also serve as protective barriers against poisoning species or as selective layers to enhance gas sensors selectivity.

## Introduction

Meeting the growing energy demands has become a significant challenge. Hydrogen is gaining interest as a potential primary energy vector due to its ability to address issues like greenhouse gas emissions and limited natural resources. Efficient hydrogen production [1], storage [2], purification, transport, and use [3] are key. The most effective methods for producing hydrogen include steam methane reforming (SMR) and other processes based on water, biomass, or fossil fuels [4]. However, with the exception of water splitting, these methods often produce hydrogen with contaminants such as CO₂, CO, CH₄, H₂S, SOₓ, NOₓ, and NH₃. Therefore, additional purification steps, like cryogenic distillation, PSA/TSA, or membrane separation, are usually required [5], [6]. In comparison with other purification techniques, membrane separation features lower energy consumption, higher compacity and continuous operation. The main criteria for efficient gas separation membranes are both high permeability and selectivity alongside sufficient chemical and physical resistance under the applied operating conditions.

The most developed membranes for H₂ purification are polymeric membranes and dense metal palladium-based membranes, applicable at low and intermediate temperatures, respectively. Polymeric membranes are thin films that allow selective transport based on the “solution-diffusion” mechanism [7]. Glassy polymers such as polysulfone, polyimide, polyamide, and cellulose acetate have been extensively studied for hydrogen separation [8]. Because metallic membranes are generally altered by CO and sulfur compounds [9], [10], ceramic membranes, inert to these poisoning gases, are often considered a relevant alternative for H₂ purification [11], [12], [13], [14], [15], [16], [17].

Hydrogen-selective inorganic membranes are often prepared in the form of thin layers deposited on ceramic supports. A typical example of a widely studied support is mesoporous alumina deposited on the upper surface of a robust macroporous structure. Indeed, the former provides a smooth support surface suitable for the deposition of thin continuous layers (gas-selective membranes) deposited by liquid phase or gas phase methods [18].

In the case of pure alumina membranes, liquid phase methods (such as the sol-gel process) are poorly suited to the deposition of ultramicroporous H₂-selective membranes. Indeed, a heat treatment required for the consolidation of the alumina structure leads to the generation of pores generally larger than 1 nm, leading to gas transport mechanisms dominated by Knudsen diffusion [19]. As a result, research efforts have been directed more towards vapor phase methods (e.g., chemical vapor
deposition - CVD, PECVD- or atomic layer deposition -ALD), which tend to be more suitable for the preparation of ultramicroporous H₂-selective alumina membranes [20].

In this area, special attention is now devoted to the ALD technique, producing uniform and high-quality thin films on various substrates with excellent aspect ratio (>10,000:1). More detailed information on ALD can be found elsewhere [21], [22], [23], [24]. ALD of amorphous alumina layers could be carried out conventionally using trimethylaluminum (TMA) and water in thermal ALD (< 150 °C) [25]. ALD-alumina films are classically used to modify the surface chemistry of membranes for protecting materials/membranes from poisoning species [26]. They can thus limit corrosion or transport of steam across grain boundaries [27]. ALD-alumina has also been used to prepare gas separation molecular sieve membranes. A recent example was reported by Liu et al. [28] by depositing an ultra-thin layer of amorphous alumina by ALD at 50°C on small Anotec disks. High H₂ permeance values (~3 × 10⁻⁷ mol·s⁻¹·m⁻²·Pa⁻¹) and surprisingly high separation factors have been reported at room temperature for H₂/CO₂, H₂/O₂, H₂/N₂, and H₂/CH₄ gas mixtures (SF= 5148.2, 120.7, 161.5 and 219.5, respectively), in a Wicke-Kallenbach permeation cell (no transmembrane pressure applied). However, according to our knowledge, the method has not been applied to more robust supports such as industrially applicable membrane tubes capable of withstanding realistic separation conditions (pressure and temperature).

In order to address this drawback, another strategy was applied in the present work, based on the deposition of a dense hybrid alucone material by Molecular Layer Deposition (MLD) and its controlled conversion to C-doped alumina by thermal treatment. Similar to ALD, MLD allows ultra-thin conformal layers to be deposited on high aspect ratio substrates and requires only a small amount of precursors. Various approaches have been reported in the literature for the deposition of alucone layers, using different vapor phase organic precursors [29], [8], and various post-processing procedures to form the porosity either by plasma [30], chemical etching, pyrolysis or calcination [31], [10], [32], [33]. The conversion of such hybrid organic-inorganic metal alkoxide polymer films to ceramic could actually be compared to the fabrication of ceramics by the polymer-derived ceramic route (wet chemistry route classically used for Si-based ceramics [34]). For example, Wang et al. studied the influence of the curing and pyrolysis temperature of PCS polymers to obtain highly selective molecular sieve SiC-based membranes with H₂/N₂ and H₂/C₃H₈ selectivities of 31 and 1740 at 500°C, respectively [35]. However, compared with the above strategy, the thickness of MLD layers can be controlled much better, forming conformal ultrathin layers on a large diversity of substrates.

In general, alucone films can be used as protective layers [36], for drug delivery or to improve system performance in various applications [37], [38] with special attention given to selective gas separation [11] or detection [39]. For example, Yu et al. [40] used alucone to generate Al₂O₃/SAPO-34 zeolite composite membranes with attractive H₂ separation performance. The porous alumina layer obtained after calcination of the alucone reduces the pore entry of the SAPO-34 zeolite and thus increases the separation factor up to 1040 for an H₂/N₂ gas mixture compared to only 8 for the pristine SAPO-34 membrane. However, to the best of our knowledge, the use of MLD-alucone to synthesize all-alumina tubular membranes (support and filtration layer) for H₂ separation has never been reported in the literature.

To fill this gap, this paper describes a novel way to synthesize an H₂-selective molecular sieve alumina membrane derived from alucone films obtained by MLD on robust tubular commercial supports. The layer was formed by the stacking of –O-Al-(O-CH₂-CH₂-O)₂– groups resulting from a gas phase self-limited surface reaction of trimethylaluminum (TMA) with ethylene glycol (EG) [9]. Then, the coated support was heat-treated under controlled conditions to remove the majority of the -CH₂-CH₂- groups, forming a porous network of Al-O-C species and yielding a thin ultramicroporous C-doped alumina membrane. The deposition parameters and post-processing conditions (thermal treatment
temperature, atmosphere and heating rate) were optimized to avoid undesirable material shrinkage during the release of organic species, preserve membrane integrity and generate a uniform membrane network with controlled pore size. Single gas permeance measurements and gas mixture separations were carried out to monitor and evaluate the membrane microstructure and its performance.

# Experimental section

## 1. Membrane support and chemicals for MLD

The tubular ceramic membrane supports (OD/ID= 10/7mm) used for the MLD of alucone were supplied by Pall-Exekia (France). The supports have an asymmetric porous structure divided into 4 layers: a mesoporous inner γ-alumina top-layer deposited on 3 macroporous α-alumina layers with different pore sizes. Therefore, the mean pore sizes from the inner to the outer part of the tubes were ~8 nm, 0.2 µm, 0.8 µm and 10 µm, respectively. All the tested supports were 5 cm long and sealed at both ends (1 cm length, in/out) with a commercial enamel to avoid any gas leakage during permeation tests. The chemicals used for the MLD of alucone were trimethylaluminum (TMA) (Sigma-Aldrich, CAS Number 75-24-1, 97%) with ethylene glycol (EG) (Sigma-Aldrich Supelco EMSURE, CAS Number 107-21-1, ≥ 99.5%) as co-reactant. The choice of precursors is discussed in details in the SI (S1). They were used as received without any further purification. Both precursors were contained in stainless steel bubblers connected to the ALD reactor. The EG was heated to 90°C to ensure sufficient vapor pressure for its dosage in the ALD chamber. The TMA was kept at room temperature without any additional heating. Argon (Linde Gas, ≥ 99.9999%) was applied to purge the ALD chamber during deposition cycles.

## 2. Membrane material synthesis

Alucone deposition was carried out using a custom-built Atomic Layer Deposition (ALD) reactor in our laboratory. The ALD chamber was heated to 130°C, while the chemical precursors were injected into the reactor via feed lines connected to bubblers and heated to 100°C. The exhaust was connected to a vacuum pump and heated to 100°C to prevent liquid condensation. The deposition chamber was maintained under vacuum (from 2.10⁻² to 5.10⁻² mbar), and the injection of the precursors was regulated by the aperture time controlled by automatic valves. Each MLD protocol consists of a defined number of pulse-exposure-purge sequences whose execution is custom-programmed using Labview software. The sequence of the alucone deposition cycle was as follows: 0.04s pulse of TMA + 20s exposure and 40s purge, followed by a 2s pulse of EG + 20s exposure and 90s of the purge. During the purge step, argon was introduced continuously while being pumped to ensure the removal of all by-products and unreacted species.

The heat treatment of the as-deposited alucone layer was carried out after the MLD process to form the final porous C-doped alumina membrane. The samples were annealed at temperatures ranging from 200°C to 400°C (2h, 1°C/min) under a flow of N₂ (200mL/min) in a tubular oven. This step proved crucial for controlled rearrangement, partial decomposition and removal of carbonaceous species from the hybrid organic-inorganic alucone layer while minimizing any damage to the inorganic alumina framework. Indeed, both the heating rate and the temperature during heat treatment are influencing the final microstructure and porosity of the membrane. Heating rates of 5°C/min and 3°C/min were also tested, but the resulting membranes had lower ideal selectivity (He/N₂) due to increased N₂ permeances. Annealing temperatures of 200°C and 400°C led respectively in either too low He permeance or too low He/N₂ ideal selectivity. Finally, only membranes annealed at 300°C with a slow heating rate of 1°C/min featured attractive permeance/selectivity properties and were further studied in this work. After thermal treatment, all membranes were stored under vacuum in a desiccator.
### 3. Physico-chemical characterizations

Scanning Electron Microscopy (SEM, Zeiss EVO HD15) was used to examine the morphology, thickness and homogeneity of MLD layers. The layer thickness was also measured with a Semilab GES5E ellipsometer (5eV-1.24eV, Xe source) for the as-deposited and heat-treated alucone layers on silicon wafers (100, p-doped) companion samples. The software for data processing was WinElli2. The Cauchy law was used as a fitting equation to calculate the layer thickness of MLD-alucone. A fitting model which simulates the porous C-doped alumina layer by coupling void and alumina optical data was used to calculate the final membrane thickness. Fourier Transformed Infrared spectra were recorded with FTIR Spectrometer (Nexus, ThermoFisher). X-ray photoelectron spectroscopy (XPS) was carried out using an ESCALAB 250 spectrometer (ThermoElectron). Thermogravimetric Analysis (TGA) on STA 409 PC LUXX (Netzsch, Germany) coupled with mass spectrometry OMNi Star GSD 320 (Pfeiffer Vacuum, Germany) was performed on γ-alumina beads (NorPro, Accu® spheres, 100 m²/g) coated with MLD-alucone at in nitrogen atmosphere with a heating rate of 10 °C/min. The latter analysis was conducted at the Technische Universität in Berlin, Germany.

### 4. Single gas permeance measurements and gas mixture separations

Single gas permeance and permselectivity values of the final C-doped alumina membranes were measured with N₂ (99.995%), CO₂ (99.992%), CH₄ (99.95%) and He (99.999%), provided by Linde Gas. The membranes were equipped with silicone o-rings to provide effective sealing within the tubular stainless steel permeation cell. The single gas flow through the membrane was measured from room temperature to 150°C with a bubble flowmeter connected to the atmosphere. A relatively high transmembrane pressures (ΔP) of 6 bar was applied in order to achieve measurable permeances. All membranes were systematically outgassed under vacuum for 15 minutes after each gas change to avoid cross-contamination. The accuracy of the permeance values was ensured by a sufficiently long stabilization time before each series of permeance measurements and their successive repetition. The permeance (Πᵢ) and permselectivity (αᵢⱼ*) values were obtained using the following equations:

$$ \Pi_i = \frac{D_i}{A \times \Delta P} \quad (1) $$

$$ \alpha_{i/j}^* = \frac{\Pi_i}{\Pi_j} \quad (2) $$

where Dᵢ is the flow of gas i measured experimentally (mol.s⁻¹), A is the membrane active surface area (m²), and ΔP is the applied transmembrane pressure (Pa).

After assessing the integrity of the synthetized membranes using single-gas permeation measurements, gas separation experiments were conducted on a separate experimental setup. The gas mixture, supplied by Linde Gas contained 70.04% N₂, 10.03% CO₂, 9.97% H₂, and 9.96% CH₄. The applied transmembrane pressure (ΔP) was 3 bar, and the measurements were performed at temperatures ranging from 25°C to 155°C with a heating rate of 2°C/min. The choice of experimental conditions was based on protocols described in the literature for similar types of membrane. Limitations of the experimental setup such as detection limits and safety concerns, were also taken into account. The gas mixture was preheated before introduction into the membrane module and He was used as a sweep gas on the permeate side of the membrane. The latter was selected based on gas chromatograph calibration to avoid peak saturation. It is important to note that the applied transmembrane pressure of 3 bar prevents the sweep gas (He) from diffusing back to the feed side.
For measurements made above 150°C, graphite o-ring were used to seal the membrane in the permeation cell. Gas concentrations in the permeate was determined using a TCD-GC (PerkinElmer-Clarus 400) equipped with a shincarbon ST 100/120 column and a sampling loop. All tested membranes were kept inside the module under N₂ flow between measurements. The separation factors were calculated as follows:

$$F_{i/j} = \frac{x_{i \text{ downstream}}}{x_{i \text{ upstream}}} \frac{x_{j \text{ upstream}}}{x_{j \text{ downstream}}} \quad (3)$$

where Fᵢⱼ is the separation factor of gas i relative to gas j, and xᵢ and xⱼ are the gas concentrations of i and j after passing through the membrane (downstream) and before passing through the membrane (upstream).

## Results and discussion

### 1. Fabrication of porous C-doped alumina membranes

Membrane synthesis is a two-step process comprising the deposition of alucone followed by its thermal conversion in C-doped alumina. MLD was used to cast the hybrid organic/inorganic alucone layers onto tubular alumina support with a γ-alumina (γ-Al₂O₃) inner layer, while the post-processing heat treatment led to partial removal of carbon chains, thereby producing a C-doped alumina network on/in the inner γ-Al₂O₃ top layer.

The applied MLD process was based on a self-limited surface reaction governed by the alternating pulses of TMA and EG. EG was chosen due to its good reactivity with TMA [9] and its short carbon chain, likely leading to small pore sizes of the alumina material [33] generated during the final thermal treatment [41]. Indeed, like ALD, MLD is a particularly well-suited coating method since the precursor molecules could easily penetrate the small pores of γ-Al₂O₃ (~ 8nm). In addition, unlike Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD), which primarily coat the upper surface of the substrate through the deposition of target atoms or vapor phase chemical reaction, respectively, MLD enables highly conformal deposits throughout the entire volume of substrate by infiltrating precursors into its porous structure. Considering that, the as-deposited alucone is expected to form a continuous non-porous layer. The MLD conditions were assessed by the quasi-linear growth of the alucone layer with the number of MLD cycles (Figure S2). Gas tightness measurements have been carried out (before the thermal treatment step) to validate the quality of the MLD coatings. It was demonstrated that for the selected γ-Al₂O₃ support, 500 MLD cycles were necessary to obtain a gas-tight membrane barrier with He permeance lower than the detection limit of the device, i.e. ~10⁻¹² mol.s⁻¹m⁻²Pa⁻¹).

After the MLD process and gas tightness validation, the samples were thermally treated at different temperatures between 200°C and 400°C under an inert atmosphere (N₂ ~ 200mL/min). The determination of the optimal number of MLD cycles required for efficient alumina membrane preparation was based on ideal selectivity measurements between He and N₂, after the thermal treatment of the deposited alucone. It was found that applying 500 MLD cycles of alucone generated C-doped alumina membranes exhibiting the highest He/N₂ ideal selectivities (up to 93 at ΔP=6 bar and 25°C) and the protocol was therefore implemented for the rest of the study.

The single gas permeance values of the derived membranes are reported in Table 1.
<table><caption>**Table 1.** Single gas permeance values (Πi) of He and N₂ at ΔP=6 bar and 25°C, through an alucone/alumina membrane thermally treated at different temperatures. Ideal selectivity values α*(He/N₂) are also reported.</caption><thead><tr><th></th><th>200°C</th><th>300°C</th><th>400°C</th></tr></thead><tbody><tr><td>Π<sub>N2</sub> (GPU)</td><td>&lt; detection limit</td><td>0.6</td><td>1</td></tr><tr><td>Π<sub>He</sub> (GPU)</td><td>0.4</td><td>56</td><td>22</td></tr><tr><td>α*<sub>(He/N2)</sub></td><td>n.a.</td><td>93</td><td>19</td></tr></tbody></table>

The thermal treatment of alucone at 200°C proved insufficient to remove the organic part from the alucone layers and thus form a porous alumina network with adequate porosity. Increasing the heat-treatment temperature resulted in higher helium permeance while keeping the membrane nearly nitrogen-tight. Further increase in treatment temperature (above 300°C) did not lead to higher permeance or permselectivity values, which instead tended to decrease, suggesting the undesired transformation of the C-doped alumina porous network. Interestingly, it was also observed that the heating rate has a significant influence on the membrane permselectivity. In general, the slower the heating rate, the higher the final permselectivity, with maximum values obtained for thermal treatments carried out with a heating rate of 1°C/min. To some extent, this observation is in good agreement with the work of Ghazaryan et al. [42], who reported that a slow heating rate ensures less collapse of the layer structure and leads to higher porosity. Hence, in the remainder of this work, only membranes thermally treated at 300°C for 2h (1°C/min) under nitrogen were considered to study their potential for hydrogen separation.

## 2. Physico-chemical characterization of membrane layers

### 2.1. Morphology of the MLD alucone layer

SEM observations of the membrane cross-section are shown in Figure 1. The asymmetric structure of the virgin tubular alumina support is shown in Figure 1(a), with its three α-alumina layers and the γ-alumina top-layer. Figure 1(b) shows a conformal coating of 500 MLD cycles of alucone on the alumina support surface after the thermal treatment at 300°C. In fact, the MLD technique allowed homogeneous coverage of the alumina grains thought-out the entire porous structure of the alumina support, including the inner γ-Al₂O₃ top-layer. A more compact MLD layer was observed at the very top of the γ-Al₂O₃ layer, assuming that its pores are completely filled with alucone. Indeed, once the pore diameter of the support is reduced beyond its critical value, the diffusion of alucone precursors is significantly hampered, leading to their preferential deposition in the upper part of γ-Al₂O₃ top layer.

Ellipsometry measurements on the accompanying Si wafer samples revealed an alucone growth rate ranging from 0.3 to 0.6 nm/cycle (GPC). For a layer prepared with 500 MLD cycles, a thickness of ~ 100 nm was measured after thermal treatment at 300°C for 2h under N₂ (with a heating rate of 1°C/min). Considering the limitations of both ellipsometry and SEM techniques, this result aligns well with the ~ 110 nm thickness measured from SEM observations on alumina support.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Cross-section views of (a) the virgin porous alumina tubular support (Pall-Exekia), (b) the C-doped alumina layer obtained after heat treatment of MLD-alucone at 300°C under N₂ atmosphere.</figcaption></figure>

## 2.2. Chemical composition of alucone and final C-doped alumina layers

To study the evolution of alucone during heat treatment, alucone films were deposited on alumina beads and examined by thermogravimetry. These alumina beads with γ-Al₂O₃ on their surface simulate the material of the membrane support and are more suitable for analyzes due to the higher amount of available material. The beads were coated with alucone using the same 500 MLD cycles used for the tubular membrane support with a γ-Al₂O₃ top layer. The sample was heated up to 600°C under nitrogen with a heating rate of 10°C/min. In addition to recording the mass loss due to gas release upon heating, the chemical composition of the released gases was analyzed by mass spectrometry (mass to charge ratio (m/z)). The results are shown in Figure 2.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Thermogravimetric and gas mass spectrometry analysis of alucone deposited on alumina beads.</figcaption></figure>

During the thermogravimetric analysis of alucone, we observed a first mass loss between 100°C and 300°C, then a second one between 300°C and 450°C. Using both thermogravimetric analysis and gas mass spectrometry, we confirmed that only gases containing carbon, oxygen and hydrogen were released. No aluminum compounds were detected, indicating that the Al-O network is not altered by heating from room temperature to 600°C.

The low weight loss (~1.5 wt.%) below 200°C is due to the release of adsorbed water and CO₂ from the membrane surface, confirmed by mass spectra detecting H₂O (m/z = 17 and 18), carbon (m/z = 12), and CO₂ (m/z = 44). FT-IR and XPS analyses (Figures 3 and 4, respectively) show that these gas releases correspond to the breakage of C-O and C-C bonds in the alucone layer[43], [44], [45]. As temperature increases, the C-O and C-C absorption bands in the FT-IR spectra decrease, and the XPS spectra show a reduction in the C-O peak intensity after heating to 300°C. This indicates that the decomposition of the alucone layer releases CO₂ and carbonaceous gases, likely from the cleavage of the C-C and C-O bonds.

As the temperature rises above 300°C, the weight loss continues to increase, reaching approximately 4 wt.% at 600 °C. This weight loss is associated with the release of gases such as H₂ (m/z = 12) C (m/z = 12), CO₂ (m/z = 44), indicating the partial decomposition of methyl (-CH₃) and/or ethylene glycol groups in the alucone layer [46]. The release of H₂ can originate from the decomposition of C-C polyromantic rings, which typically condense between 250°C and 400°C [44]. The layer structure may rearrange into aromatic C-C cycles [47], as suggested by the prominent absorption bands of C=C and -COOH groups in the FT-IR spectra.
Additionally, the energy band gap for C=C sp² is around 284.1 eV, which might overlap with the C-C sp³ peak at 284.8 eV in the XPS spectra shown in Figure 4. This suggests the presence of an intermediate structure around 300°C containing C=C, C=O, -COOH, C-O, and C-C bonds.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Infrared absorbance spectra of alucone layer deposited on silicon wafer and thermally treated at different temperatures.</figcaption></figure>

The broad absorption band between 3700 and 3000 cm⁻¹ in Figure 3 corresponds to O-H stretching vibrations, indicating the presence of -OH groups on the alucone surface and water. The peak at 910 cm⁻¹ is assigned to Al-O groups and is observed in both as-deposited alucone or its heat-treated analogue at 200°C and 300°C. However, interpretation of the broad peak below 1000 cm⁻¹ for samples treated at 400°C is difficult. The peak at about 1100 cm⁻¹, associated with C-O and C-O-C bonds, is predominant in as-deposited alucone, but decreases at higher temperatures and almost disappears at 400°C. This suggests a polymer-like structure in membranes processed at 300°C and above, similar to PEG polymers [48]. The two characteristic peaks of C-H bonds at 2880 and 2950 cm⁻¹ also decrease with temperature, indicating the removal of organic species (-CH₂, C-C and C-O-C) at higher temperatures, confirming the conversion of alucone to porous alumina. The C=O peak and the peak at ~1615 cm⁻¹, assigned to -COOH and C=C bonds [44], are more intense at 300°C and almost absent at 200°C or 400°C. These findings, together with the TGA and XPS data, support the idea of C-O bond cleavage and the formation of a complex intermediate structure with -COOH, C=O and C=C groups. This aligns with the degradation of PEG between 200°C and 250°C, leading to a carbonyl absorption band [49]. Thus, a mixed complex structure is highly probable, possibly featuring interconnected C-C rings, as discussed in the work of Dumont et al. [47].
<figure><img src="image_6.png" /><figcaption>**Figure 4.** X-ray photoelectron spectra of as-deposited alucone on silicon wafer and alucone thermally treated at 300°C under N₂ atmosphere for 2h.</figcaption></figure>

XPS analysis (Figures 4(a)-4(d)) shows the deconvolution of O 1s, Al 2p, and C 1s peaks for alucone on a silicon wafer and the same sample processed at 300°C. The O 1s peak at 532.3 eV (Fig. 4(a)) remains at the same binding energy but increases in intensity after heat treatment (under N2, 300°C, 2h, 1°/min). Similarly, the Al 2p peak at 74.7 eV (Fig. 4(b)) shows an increased peak area after heating, indicating the formation of a porous alumina structure with more Al-O bonds. These sharp and intense peaks, indicating a strong bond between O and Al atoms, confirm the presence of alucone-alumina species in the layer.

Figure 4(c) shows the XPS data of as-deposited alucone. The deconvoluted C 1s peaks at 284.8 eV, 286.6 eV and 288.4 eV correspond to C-C, C-O and C=O bonds, respectively, which is consistent with previous studies [50], [51], [52]. Figure 4(d) shows the XPS data for alucone heated to 300°C. The very close peak positions of the annealed and as-deposited alucone correspond to the same chemical bond. However, in Figure 4(d), the C-O bond area decreases while the C=O bond area increases. This suggests that C-O bonds break, forming carbon-rich domains structured in aromatic rings, which is consistent with TGA and FT-IR data. While post-synthesis treatments like water etching or heating in air at ~500°C [41] can fully eliminate the organic component, thermal treatment at a lower temperature and under an inert atmosphere results in only partial carbon removal. This has been confirmed by XPS analysis, which detected the presence of C-C bonds in the sample processed at 300°C.

In summary, by combining TGA, FT-IR, and XPS data, we conclude that the hybrid organic-inorganic alucone layer deposited by MLD resembles the structure proposed by Dameron et al. [53]. Subsequent thermal treatment causes cleavage of C-O and C-C bonds, forming reactive free radicals. At higher
temperatures, carbonization with molecular rearrangement, thermal polymerization, and aromatic condensation are likely to occur [54]. However, at a lower temperature of ~300°C, the initial non-aromatic structure only undergoes the first aromatization step, gradually converting alucone to an inorganic C-doped alumina material through partial removal of organic chains. Additionally, the chemical rearrangement during heat treatment enhances the stability of the final layer. Aromatic rings are known to be more stable than the hydroxyl-rich groups present in the as-deposited alucone layer. Heating the membrane above 300°C induces further molecular rearrangement and condensation of the aromatic rings. Therefore, the stability of this membrane is ensured up to 300°C, but it also depends on the nature of the gaseous species, which could potentially poison the membrane, during gas separation experiments. To assess the aging behavior of the membrane material, additional XPS and FT-IR measurements are provided in the SI (Figure S3 and S4). The membrane material appears to maintain its chemical integrity even after more than 1 year of storage in a desiccator.

## 2.3. Membrane performance in single gas permeance measurements.

Various permeation tests were conducted to evaluate the membrane quality and its potential for hydrogen separation. Initially, single gas permeance measurements ensured the membrane integrity, confirming the absence of macro-defects and structural irregularities (viscous flow contribution). Helium permeance was measured at room temperature to optimize the number of MLD cycles for creating continuous, dense, and gas-tight alucone layers. The optimized barrier layer was then thermally treated to form a microporous network of C-doped alumina. Virtually zero viscous flow contribution was detected through the final membranes (no increase in permeance when increasing transmembrane pressure (Figure S5), indicating no damage to the γ-Al₂O₃ support layer during the sealing, MLD process, or final thermal treatment. Single gas permeance measurements with He, N₂, CH₄, and CO₂ were used to assess membrane reproducibility and potential selectivity for hydrogen. Due to the similar kinetic diameters of He and H₂ (0.26 and 0.29 nm, respectively), He can be advantageously used to predict membrane performance for H₂ transport in a simple experimental setup without any safety issues.

For the gas permeation experiments carried out at higher temperatures (Figure 5(a)), it was shown that the He permeance gradually increased. This phenomenon could be attributed to the presence of micropores, suggesting microporous diffusion as a preferential gas transport mechanism that contributes to achieving a thermally activated transport of small gases (He) beyond ambient temperature.

Figure 5(b) depicts the permeances of the above single gases measured at 150°C with the C-doped alumina membrane prepared under optimal synthesis conditions (500 MLD cycles followed by thermal treatment at 300°C in nitrogen). At this point, relevant measurements were only possible at 150°C because the permeances of CO₂, N₂ and CH₄ remained below the detection limit of the equipment at lower temperatures. The highest permeances were observed for He, reaching values up to 1.2x10⁻⁸ mol.s⁻¹.m⁻².Pa⁻¹, while significantly lower values were measured for CO₂, N₂ and CH₄ (1.7x10⁻¹⁰ mol.s⁻¹.m⁻².Pa⁻¹, 7.5x10⁻¹¹ mol.s⁻¹.m⁻².Pa⁻¹, and 1.6x10⁻¹⁰ mol.s⁻¹.m⁻².Pa⁻¹, respectively). Considering the porous structure of the studied membrane and the results of the gas permeation tests, we suggest that molecular sieving is likely the primary separation mechanism. To support this hypothesis, activation energies were plotted versus the kinetic diameter of each gas (Figure S6). It was observed that the activation energy (Ea) of permeation increases with the larger kinetic diameters of the gases. For H₂, the activation energy (Ea) is the lowest, which may indicate an easier passage of H₂ through the membrane, featuring an average pore size between 2.89Å and 3.30Å. A similar approach to determine membrane pore size and the associated separation mechanism based on molecular sieving was used by Araùjo et al. [55] for cellulose-based carbon membranes and by Tanco et al. [56] for composite
carbon molecular sieve membranes. However, it should be noted that $E_α$ can also be influenced by factors such as pore connectivity [57], pore size distribution [58], and tortuosity [59], adding significant complexity to the comprehensive description of the membrane porous structure. The highest permselectivity measured at 150°C was obtained for the He and $N_2$ gas pair reaching $α^*(He/N_2)$ values up to 160, i.e. much higher than the ideal Knudsen selectivity values ($α_κ(He/N_2)= 2.65$).

However, molecular sieving is not the only separation mechanism as the gas permeance values do not follow exactly the order of kinetic diameters. Considering only size effect, the order of gas permeance should be: $Π(He) > Π(CO_2) > Π(N_2) > Π(CH_4)$. Since the order observed in Figure 5(b) is different ($Π(He) > Π(CO_2) \sim Π(CH_4) > Π(N_2)$), a specific interaction/adsorption of the gas molecules e.g. $CO_2$ and $CH_4$ on the C-doped membrane material seems to occur as well. Indeed, depending on the type of material, pore sizes, gas molecule, pressure and temperature, various mechanism can contribute to gas transport in porous membranes [60], [61]. In the present case, configurational diffusion [62] is considered to be the dominant transport mechanism, with a negligible contribution of Knudsen diffusion and no viscous flow.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Single gas permeance measurements through a microporous $C-Al_2O_3$ membrane at constant transmembrane pressure (ΔP=6 bar) a) Evolution of He permeance as a function of the operating temperature and b) Comparison of single gas permeance values for He, $CO_2$, $N_2$ and $CH_4$ at 150°C.</figcaption></figure>

## 2.4. Evaluation of membrane performance for gas mixture separation

After the screening permeation tests with single gases, the gas separation performance of the optimized C-doped alumina membranes was measured with quaternary gas mixtures containing $N_2$ (70.04%), $CO_2$ (10.03%), $H_2$ (9.97%) and $CH_4$ (9.96%). Results are reported in Figure 6. It should be noted that the influence of sulfur compounds was intentionally excluded from the scope of this initial study. Indeed, unlike palladium membranes, alumina-based membranes typically exhibit strong resistance to sulfur compounds, as highlighted elsewhere [63], [64].

In good agreement with the thermally activated transport assessed for helium in single gas permeation tests, similar behavior was also confirmed in gas mixtures for hydrogen featuring increased permeances at higher temperatures.
<figure><img src="image_8.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_9.png" /></figure>

**Figure 6.** Performance of a C-Al₂O₃ membrane for the separation of a quaternary gas mixture (N₂ (70.04%), CO₂ (10.03%), H₂ (9.97%) and CH₄ (9.96%) at ΔP=3 bar. Evolution of a) gas permeances and b) separation factors as a function of the operating temperature.

As shown in Figure 6(a), the H₂ permeance increases more than 65 times (from 5.4x10⁻¹⁰ mol·s⁻¹·m⁻²·Pa⁻¹ to 3.6x10⁻⁸ mol·s⁻¹·m⁻²·Pa⁻¹) when we compare the values measured at room temperature and at 155°C, thus approaching the values obtained for He in single gas permeation tests. The permeances of other gases remain relatively stable and increase only very slightly with temperature. This highlights the microporous diffusion mechanism, which is thermally activated for H₂ through the microporous membrane network. The observed thermally activated transport mechanism indicates that adsorption is not the primary transport mechanism for this type of membrane. The slight increase in CO₂ permeance when temperature is increased (Figure 6) may be due to a minor enlargement of pore size, caused by the combined effects of temperature and exposure to a gas mixture containing H₂. For similar membranes, such as CMS, oxygens chemisorb onto the carbon active sites, reducing the space between the pore walls [65], [66]. Verma et al. [67] demonstrated that using hydrogen at 150°C can remove oxygenated groups from the pore surfaces, thereby restoring and slightly increasing the pore size of the membrane. Hence, as shown in Figure 6(b), promising separation factors for H₂/CH₄ and H₂/N₂ were obtained, reaching values of 242 and 150, respectively, for measurements carried out at 126°C. The H₂/CO₂ separation factor is an order of magnitude lower, not exceeding 20 over the entire temperature range tested. It should be noted that the separation factors measured for H₂/CH₄ and H₂/N₂ are in reverse order compared to the ideal selectivities for He/CH₄ and He/N₂ in figure 5(b). This result is not really surprising, as ideal selectivities measured with single gases often differ from the separation factors obtained for gas mixture [68]. Due to competitive adsorption/diffusion [69], the interactions between gas molecules and membrane material are modified in gas mixtures, thereby leading to more complex transport/separation mechanisms. It must be recognized that the reported performance does not exceed those of other types of porous alumina-based composite membranes reported in the literature for the separation of H₂ (e.g., palladium-based [70], or carbon-based composites [56]). However, it should be emphasized that the alucone-derived membranes present attractive separation performance in an intermediate temperature window (50-180°C) where most membranes are either not sufficiently permeable to H₂ (thermally activated H₂ transport requiring higher temperatures, e.g. [71]) or no more H₂ selective (adsorption-driven selectivity requiring lower temperatures, e.g. [72]). For example, in the 3 µm thick alumina-carbon molecular sieve composite membranes reported by Llosa Tanco et al. [56], the gas permeation is a combination of molecular sieving and adsorption diffusion, and the H₂/CH₄ separation performance of the membrane decreases
sharply at temperatures above 50°C. In comparison, CMS membranes synthetized by Ngamou et al. [73] on γ-Al₂O₃ tubular supports achieved an H₂ permeance of 1.1x10⁻⁶ mol.m⁻².s⁻¹.Pa⁻¹ and separation factors of 24, 130 and 228 for H₂/CO₂,H₂/N₂ and H₂/CH₄, respectively, at 200 °C. However, It should be noted that these membranes require both higher synthesis and operating temperatures compared to the C-doped alumina membranes presented in this work. In addition, compared to the other alumina-based composite membranes for gas separation, the proposed synthesis strategy offers promising prospects given both its scalability on tubular industrial supports associated with the relatively high separation efficiency expected under realistic operating conditions (high transmembrane pressures). Finally, it should also be taken into account that the developed MLD method allows depositing an inexpensive membrane material in a very controlled manner, thereby significantly reducing manufacturing costs. This work thus represents a valuable contribution to broaden the applicability of the widely studied alucone layers [42], [53], [74], which could be advantageously transformed into gas-selective porous membranes. Moreover, these novel C-doped porous alumina membranes demonstrated good stability in term of permeances after several hours at 125°C under a gas mixture flow (Figure S7).

In order to fully assess the potential of preparing such nanocomposite membranes as a relevant alternative to other gas-selective barriers, several parameters still need to be studied and optimized. For example, further improvement in membrane performance in terms of higher permeance should arise from the judicious adjustment of the membrane architecture, e.g., reduce the thickness of the mesoporous supporting layer and, consequently, the depth and location of the infiltrated alucone and, therefore of the final alumina molecular sieve.

## Conclusion

In this work, we report for the first time the synthesis of a porous hydrogen-selective alumina membrane prepared by controlled heat treatment of alucone layers deposited by MLD on commercial tubular supports. Physico-chemical characterization highlighted that converting the hybrid organic-inorganic alucone layers led to forming C-doped molecular sieving membranes with a microporous structure. Both the MLD protocol (number of deposition cycles) and the subsequent thermal treatment (temperature and heating rate) were optimized to obtain selective membrane layers exhibiting selective H₂ transport behavior, based on microporous diffusion. Attractively, high separation factors F(H₂/CH₄) ~ 242 and F(H₂/N₂) ~ 150 in quaternary gas mixtures were obtained at relatively low temperatures not exceeding 150°C. It should be emphasized that the presented proof of concept reporting the fabrication of low-cost hydrogen-selective alumina membranes represents a valuable improvement over similar alumina membrane materials reported in the literature, both in terms of robustness and gas selectivity. Moreover, the versatility of the proposed strategy (possible tuning of the membrane porous structure) could be attractively applied to the synthesis of selective membranes for specific gases. Finally, in addition to gas separation, this deposition methodology on supports with a high surface/volume ratio could be further extended to the design of various protective films (avoiding poisoning species) or selective barriers, improving the selectivity of gas sensors.

## Acknowledgments

Authors thank the French National Research Agency (ANR, program ALD4MEM-ANR-20-CE09-0008-01) and the Occitanie Region (FEDER React-Eu Défi-clé Hydrogène Vert, MEMbyALD) for funding this research.
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