Fongarland

Dr Laura Reyes

Dr Léa Vilcocq
email: lea.vilcocq@cnrs.fr

Marie-Line Zanota

Dr Valérie Meille
email: valerie.meille@ircelyon.univ-lyon1.fr

Pr Clémence P Nikitine

Pascal Fongarland

Dr M.-L Zanota

TiO2-WOx Coatings on Aluminum Open Cell Foams as Potential Packing Material for Esterification of Glycolic Acid with Ethanol

Keywords: Open cell foams, Aluminium foams, Structured catalyst, Process intensification, Mixed oxides

TiO2-WOx Coatings on Aluminum Open Cell Foams as Potential Packing Material for Esterification of Glycolic

Acid with Ethanol

Introduction

Nowadays, industry and academic research must shift to environmentally friendly practices and processes, consequently green chemistry and engineering have become not desirable but mandatory. [1] The overall processes must be optimized looking for cleaner, safer, environmentally compatible and more efficient energy systems. Process intensification has been developed as a concept to meet this purposes. This can be approached in two key aspects: process intensifying methods and equipment. The first one makes reference to the integration of multiple unit operations (multifunctional reactors) or using alternative energy sources, and the second one to the optimization of critical parameters such as heat and mass transfer. [2,3] Structured catalysts and reactors are enlisted as process intensification equipment. The most interesting features are related to pressure drop and geometric surface area. Compared to conventional packed bed, structured reactors have very important surface area, between 1.5-4 times higher per reactor volume, with lower pressure drops, between 1 and 2 orders of magnitude in single and two phase flow. [2] Moreover, structured catalysts have inherent short diffusion paths granted by thin coated layers hence can provide outstanding catalytic efficiency, especially favorable in systems with selectivity limited by mass transfer resistance. [4] The most studied and applied alternative of structured catalyst in industrial processes are honeycomb monoliths. [4] However, in these structures the liquid flows through parallel straight channels, thus flow patterns similar to laminar flow regime are induced due to low turbulence. This results in poor gas to liquid mass transfer limiting the range of application in terms of reaction type and reactor selection. [5,6] Open cell foams may help to overcome this problem. These 3D macro structures inherently increase turbulence in the liquid flow due to the interconnected channels with high tortuosity. [7,8] Foams can be manufactured from ceramic (silicon carbide, cordierite and alumina) or metallic (stainless steel, chromium, cooper, aluminum) materials. [9] The benefits of using foams as packing for structured reactors are a reduce pressure drop per bed height, an improved hydrodynamic behavior and efficient heat and mass transfers, all this granted by greater geometrical surface area and important void fraction. [7,10] Furthermore, significant catalyst loadings can be reached as thin coating layers in the overall foam surface area. Foams as structured catalyst were successfully studied on a laboratory scale and proved to be highly effective for multiphase systems. [8,9,11,12] The versatility in terms of material and geometric characteristics selection encourages the field of application of foams to be in continuous growth. Foams can be merely appreciated as geometric supports to be employed as catalytic carrier, easing the recovery of the catalyst from the products. [13,14] Also, these structures are readily available in different metallic or ceramic materials, among which some are traditionally known and currently applied as heterogeneous catalysts or catalyst supports. Hence, the foam material can be carefully selected to promote synergistic effects between the coating and the support. [15] Besides the accurate choice of material, adequate specific surface may enable control of the exothermic nature of some reactions. [16] Furthermore, if the reaction requirements, catalytic performance and/or thermal conductivity, can not be optimized with a single type of foam, customized arrangements of foams of different materials can be tailored. [17] Beyond these examples of applications, mass transfer limitations and operating regimes in open cell foams have also been investigated, which is crucial for continued research and development of open-cell foams in largescale applications. [18] Catalytic foams can be applied on continuous processes such as fixed bed, trickle bed reactor and even multicomponent units such as reactive distillation. [19][20] All this makes open-cell foams one of the most promising tools and approaches for process intensification with regard to structured catalysts. [18] This study aims to develop a catalytic structure using aluminum foams as structured support and titanium doped with tungsten (TiO2-WOx) as active phase. Maximum catalytic loading, stability and reactivity towards a model reaction were targeted. For this, the geometrical and structural features were studied and the catalytic slurry parameters and the coating protocol itself were optimized.

Results and Discussion

Aluminum foam. It is important to correctly characterize the geometric and structural features of the foams, since they are correlated to the potential catalyst loading on the surface as well as to the hydrodynamic behavior. The main structural (pore and cell diameter) and geometric (void fraction and surface area) features are presented in ESI. The void fraction as well as the specific surface area values are in agreement with those reported in the literature for aluminum foams for similar ppi number. [21,22] Monoliths in similar range of surface to volume ratio (600-1000 m 2 .m -3 ) present lower void fraction (0.67-0.73) compared to the Al-foams. [8] Other open crossflow structured packing such as layered steel sheets, corrugated or not, may also show some drawbacks compared to foams. Compared to commercial packing such as Sulzer 500Y and BX with surface areas close to 500 m 2 .m -3 and void fractions of 0.9, a 20 ppi foam seems to be more advantageous as it presents higher values for both parameters. Moreover, for a similar value of void fraction (0.9), the 30 ppi foam doubles the surface area of both 500Y and BX. Flexypac 2 or Sulzer 125Y might present higher void fractions (0.95-0.98) than 20 and 30 ppi foams, nevertheless, their surface area is highly reduced (233-125 m 2 .m -3 ). [8] Structured packing can also be catalytically activated by filling up the channels formed by the parallel sheets with catalyst particles as in the case of Katapak; however, the void fractions are then considerably reduced. Depending on the catalytic loadings, these structures have a void fraction ranging between 0.6 and 0.8 with surfaces areas of 120-300m 2 .m -3 . [8,[START_REF] Viva | Experimental Analysis and Modelling of Novel Catalytic Structured Packings for Reactive Separation Processes[END_REF][START_REF] Brunazzi | IChemE Symp[END_REF][START_REF] Fard | [END_REF] These values are considerably lower than the foams presented here, confirming the interest in working with foams. Slurry concentration. Traditionally, the slurry (or suspension) formulation for dip coating methods contains the catalyst (in powder form), sometimes a dispersing agent (usually an acid or a surfactant) and a solvent. The proportion of each of these compounds directly impacts the stability and the rheological properties of the suspension. [26] Taking into account that the selected solid is an already made catalyst, which has shown an important activity in the esterification of carboxylic acids, it is in our best interest to preserve its tungsten loading and acid properties. This in turn reduces the freedom degrees in the formulation of the suspension. The addition of a binder as well as a dispersant usually generates a homogeneous stable slurry and improves the rheological properties. [26] However, in the case of coating ready-made catalyst, the use of an acid or binder agent is more likely to cause unwanted effects, especially for metal catalysts. In structured metallic catalyst, binders may irreversibly cover the metal phase of the solid (catalyst) generating activity losses. [4] Additionally, in some cases the acid added to the suspension can trigger important structural and chemical transformations as well as metal loss. [27] A wide range of organic solvents has been studied to achieve good slurry stability. However, the selection of the solvent is linked to the type of catalyst to be coated and deionized water is preferred. [9] The advantages of using water as a solvent are given by its ease of handling, vast availability, low cost and non-toxicity. Notably, this solvent is likely to be inert for the selected catalyst (TiO2-WOx). Conventional values of H2O/catalyst proportion are in between 1.5-4 g.g -1 while the total amount of solids can be ranged between 40 and 50 wt%. [9,26] After the selection of the solvent (water), the only remaining parameter to be optimized is the amount of catalyst powder in the suspension, this was varied between 125-666 g.L -1 (11-40 wt%). In parallel, the influence of the foam macroporosity on the coating load was studied for the 3 different aluminum foams (10, 20 and 30 ppi) (Figure 1). The mass uptake per foam after the coating procedure was estimated as the weight difference between the bare structured solid support and the catalytic structure after thermal treatment. Regarding the impact of the macroporosity of the structure, a similar trend is observed in the overall range of suspension concentrations studied. A higher mass uptake was observed on the 30 ppi foam followed by 20 and 10 ppi, as observed in Figure 3. Moreover, for the two highest concentrations, it was observed that the amount of catalyst loaded aver 20 ppi foam is much closer to the load of 30 ppi than to 10 ppi. During the withdrawal stage of the dip coating method, two types of forces coexist on the fluid film: viscous forces that prevent the liquid from slipping off the structure and gravitational forces pushing the liquid film downwards. For a constant viscosity (constant concentration in our case), complex geometries such as foams, especially with high cellular density, may cause capillary forces. [9] This phenomenon inhibits extra fluid to slide out of the substrates cells under influence of gravity. Thus, a much more important quantity of slurry remains trapped in the narrow sections of the channels and cells of the foam when increasing pore density. The main factor influencing coating results is viscosity. In our case, this parameter will be only correlated to the amount of solids, i.e. catalyst powder, in the solution. Not only an increase in suspension viscosity was observed with concentrated suspensions but also more stable suspensions were obtained with increasing catalyst concentration. The instability of the suspensions at low concentration, the tendency of the solids to segregate, gave rise to thin, dispersed or non-homogeneous coatings. Thus, it was reflected in the low loading of catalyst deposited on the structures. The results of the viscosity measurements of the most diluted solution (125 g.L -1 ) showed that this suspension cannot be classified as a Newtonian fluid, which is highly common for dip coating slurries. The nature of the 125 g.L -1 suspension rheology is more likely to be characterized as a Bingham fluid. For values of shear stress greater than 0.3 Pa, the fluid shows a Newtonian behavior with a plastic viscosity of 8.6 Pa.s. This type of rheological behavior has already been described before for similar systems, titanium oxide and water, by Rodriguez et al. [28] However, the values obtained for the yield stress are an order of magnitude lower for a double concentration (250 g.L -1 ). This can be explained by the fact that the nature of the titanium oxide is not the same, but also that in our case it has been doped with tungsten, which potentially modifies the surface interactions in the suspension. Slurries with higher solid concentration result as well in a non-Newtonian behavior profiles. However in this case the accurate measurements were not feasible with our equipment as values lacked reproducibility. At a slurry concentration of 375 g.L -1 , it was possible to observe a homogeneous coating on the overall structure. Favoring the highest catalyst loading in the minimum number of layers, the suspension with the highest amount of solid (666 g.L -1 ) was chosen from the concentration range studied. The figure 2 shows the SEM images obtained for a 20 ppi foam through the coating process. In the first image (A), the initial surface of the support is displayed. In the next stage (B), it is observed that the surface has been modified, a slight roughness is observed, due to the thermal treatment of the foam prior to coating. Finally, in images C and D, the figure shows the catalytic layer resulting from the dip coating process with a suspension concentration of 666 g.L -1 at different scales. The SEM images obtained showed that the washcoated TiO2-WOx layer is homogenous and presents aggregates of various sizes evenly distributed on the surface. Similar results were obtained for 30 and 10 ppi foams (ESI).

Due to the low stability of the deposited layer against high mechanical impacts, such as those necessary to obtain a cross section of the coated foam, it was not possible to obtain an estimated value of coating thicknesses from SEM images. This value was estimated from the total mass uptake, the bulk powder density and the geometric features obtained during characterization of the support. Values of 32±1.9, 59±3 and 66±4 µm were obtained for the 30, 20 and 10 ppi foams, respectively. Thicker layers for foams with lower specific surface area were obtained. Please note that these values are only indicative since it was assumed that the TiO2 powder density is comparable to the density of the coated catalytic layer. [START_REF] Anval | Bulk Density Chart[END_REF] Consecutive coatings. In order to obtain higher catalyst loadings, the coating procedure can be repeated. However, the maximal amount of layers is limited by the adhesion affinity of the coated element. Additionally, for structured packing finding applications in multiphase reactors, the maximum coated thickness should not significantly reduce the final porosity of the packing. The coating of a new layer could be done either after drying or calcination step, the choice may affect the quantity of the catalyst loaded. If consecutive coatings are applied without intermediate calcination, the coated layer formed during the first dipping may partially dissolve. Trying to maximize the amount added with successive coatings, the procedure was resumed from the dip to the calcination of each additional layer. Figure 3 shows the mass uptake evolution of each type of foam as a function of the number of layers. Structures that exhibit higher cell density and surface area generally yield higher catalyst loadings for a defined slurry. However, in this case the 20 ppi foams presented values equal to or greater than the 30 ppi foam. This is explained by the fact that the 30 ppi foam, with a smaller pore size, is more prone to window sealing and pore blocking within the consecutive coating steps. Therefore, a higher air-blowing force must be applied to clear the pores, which has as a secondary effect a greater potential loss of catalytic slurry, hence a lower final coated load. This could also indicate that for this catalyst concentration in the slurry (666 g.L - 1 ), a saturation point is reached for 30 ppi foams in consecutive coatings. At the end of cycle 3 of successive coatings, a maximum average catalyst loading of 211±48, 235±56 and 171±28 kgcatalyst.m -3 were obtained for 30, 20 and 10 ppi respectively. Regarding the influence of the pore density on the estimated layer thickness, the trend of inverse correlation between these two variables was maintained. For increasing porosity values, lower thickness values were estimated: 470±78 µm for 10ppi, 405±96 µm for 20ppi and 221±30 µm for 30ppi. In order to obtain more accurate data of thickness distribution and porosity, the 3 times coated foams were analyzed by tomography and an image analysis program developed using Matlab software. [START_REF] Zanota | [END_REF] The image analysis showed a decrease in the width of the thickness distribution with the increase in cell density. Regarding the mean thickness, a trend similar to that previously estimated was observed, with values of 260 µm, 212 µm, and 192 µm for 10, 20, and 30 ppi, respectively. These values are lower than those previously estimated which could be attributed to the hypothesis considered for the calculation, namely the value of the coating density, yet they are in the same range of magnitude. As reasonable, the process of generating a catalytic layer on the surface has a direct impact on the final porosity of the structure. To obtain a more accurate comparison among structures, image analysis was performed to extract the respective porosity of the support (bare foam) and the final porosity with contributing of the catalyst layer for the same foam, as illustrated in the Figure 4. In general, a decrease in the void fraction of the structures was observed. As expected, this effect is slightly more important for foams with smaller pore sizes (20 ppi and 30 ppi), with mean values of 0.83 for both, compared to the 10 ppi foam (0.85). However, considering the catalytic load coated in the structures, a significant mean porosity is maintained at the end of the process. The graphs of porosity as a function of the height of the structure are shown in the ESI.

Catalytic tests. In order to verify if the coating treatment had an impact on the properties of the catalyst and its performance in the reaction, the catalytic structures were tested in a batch reactor for the glycolic acid esterification. The 20 ppi foam was selected as a reference and a first test was carried out with the uncoated foam as benchmark The esterification reaction of glycolic acid with ethanol is a slightly endothermic reaction that can be promoted by the acid itself, hence explaining the ester formation in the absence of solid catalyst. [31] The comparison in the reaction evolution between the system in the presence or not of uncoated Al-foam allowed ruling out catalytic activity linked to the bare foam. For the heterogeneous catalytic tests, different catalyst amounts were screened through the variation of the number of consecutive coatings. The test was run with a constant volume of catalytic structure (cylinder) and therefore different catalyst amounts. The evolution of the carboxylic acid conversion as a function of time for consecutive coatings from 1 to 3 is presented in the figure 5. As expected, an increase in conversion rate is observed with increasing the number of coatings and therefore catalyst mass in the system. The catalytic loading is 0.34, 1.26 and 2.29 wt% for 1, 2 and 3 consecutive layers (coatings) respectively. When comparing the benchmark (bare foam) with a catalytic foam with one layer, it is observed a slight increase in conversion, without being relatively significant, probably due to the low concentration of catalyst. However, as the catalyst mass increases, the difference from the reference point is easily appreciated. Moreover, comparing the structured catalyst with its equivalent in powder form (Figure 6) it is observed that for similar amounts of catalyst the evolution of the system is similar. Therefore, it can be concluded that the coating method does not affect the activity of the catalyst. It should also be noticed that for a particle size comprised between 200 and 700µm (powder form) the catalytic activity is comparable to the 3 successive coatings of the 20 ppi foam, suggesting no diffusional limitation in the case of the catalyst deposited on the foam. Additionally, the 20 ppi foam with three consecutive coatings was tested in three consecutive runs, without intermediate cleaning processes, each of 6 h for a total of 18 h. The conversion curves of glycolic acid are illustrated in Figure 7. The conversion curves overlap for the first two tests and a small loss in activity was observed for the third test. This can be associated with peeling of the catalyst from the structure (bare foam), which corresponded to approximately 3% loss at the end of the first run, and a total mass loss of 4% of the initial catalyst loading at the end of the third run. The mass loss only concerns the catalytic phase (TiO2-WOX) detaching from the aluminum foam surface; no other solids such as foam branches were present in the final reactant media. The catalyst that is lost from the structure either during installation or through the operation time remains in the reactor and is evacuated with the reaction medium at the end of the run (n), therefore the loss of mass (catalyst) is reflected in the run (n+1). The mass loss is rather attributed to mechanical stress (impact, bumping) applied over the catalytic structure when loading and unloading from the reactor than to instability of the coated layer when in contact to the reaction media or the stirring. Still, catalytic performance of the structure was maintained at the end of 18 h. Additionally, the strength and adhesion of the catalyst to the support were tested by ultrasonic exposure. The test was carried out for 5 min and ethanol was chosen as the solvent as it is the main compound in the system, which allows the adhesion to be evaluated under conditions representative of the application. Slightly higher losses were observed in structures with dense layers; however, they did not exceed 5%. Thicker layers are less stable and prone to present cracks weakening the coating, resulting in greater material losses.

Conclusion

This study focused on the potential use of aluminum foams as catalytic structures for esterification. Since the selected catalyst was already optimized in terms of synthesis, few variables could be adjusted to maximize the amount of catalyst loaded into the foams. The influence of foam macroporosity, catalyst concentration in suspension, and application of numerous coatings was examined. Higher cell densities (30>20>10 ppi) are associated with a higher loading of coated catalyst; it was also determined that treatment of a higher proportion of solids in the suspension results in more stable suspensions and thus more uniform and thicker coatings. Successive coatings of the structures with the higher slurry concentration studied (666 g.L -1 ) were carried out. The 20 ppi foams with 2 and 3 consecutive dip coatings presented values equal to or higher than the respective ones for 30 ppi. This could indicate that a saturation point for 30 ppi foams is reached not only with increasing slurry concentration but with the number of consecutive coatings. The optimization of parameters allowed to obtain a catalytic packing of up to 234 kgcatalyst.m -3 support maintaining a void factor greater than 0.83, which could be highly appealing for developing continuous processes based on structured catalytic supports. Additionally, batch tests showed that the catalyst does not deactivate or lose its catalytic performance during the different coating stages. The structured catalyst appears to present the same behavior, conversion of ≈ 65% of the glycolic acid after 6 h, as in its original form of powder. Moreover, after 18 h, the catalytic performance seem to be kept. Though a slight loss of activity was observed (2.4% conversion decrease) this is more likely linked to a loss of catalyst due to mechanical stress than to an instability of the catalytic layer upon contact with the reaction system.

Experimental section

Aluminium foams. The open porous aluminum foams (10, 20, 30 ppi) employed are commercially available and were purchased from Fraunhofer. Aluminum foams were cut in cylinders (d=29 mm h=20±1 mm) and employed later as supports for catalyst washcoating. The cutting of the aluminum foam sheets (250 x 300 x 20 mm) was performed by an external company (EDM précision, Villeurbanne, France).

The characterization of the geometric and structural features of these cellular materials was carried out by X-ray tomography accompanied by image processing with Imorph (https://technopole-marseille.com/imorph-la-3dhaute-resolution-pour-caracteriser-les-materiaux/) and Matlab software. The surface of the structures was explored via scanning electron microscopy and a digital microscope.

Catalytic coating

Materials. Titanium dioxide (TiO2) anatase pellets and tungstic acid (H2WO4) powder were purchased from Alfa Aesar, whereas hydrogen peroxide (H2O2) solution 30% was purchased from Honeywell-Fluka.

Catalyst preparation. The synthesis TiO2-WOx catalyst was carried out as described by Vilcocq et al. [32] TiO2 pellets were ball milled (Retsch Ball Mill PM100) and then oven dried for 2 hours at 100°C. Tungstic acid was dissolved in hydrogen peroxide to 0.25 M concentration. The ratio between the impregnation solution and the support was 5 mL per each gram of powder. Prior impregnation a volume of water equivalent to the volume of total impregnation solution (H2WO4/H2O2) dictated by the solution-powder ratio (5mL.g -1 ) was added to the TiO2 dried. The impregnation solution was added to the TiO2 /H2O suspension using a syringe pump at a flow rate of 1 mL.min -1 and stirred overnight. The next day the catalyst suspension was filtered (Buchner filtration) and dried overnight at 100°C. Finally, it was calcined for 3 hours at 600°C at a constant air flow of 1L.min -1 .gcatalyst -1 . The main textural and physical properties of the TiO2-WOx solid were already described [32] displayed in Table 1.

Table 1. Characteristics of the TiO2-WOx catalyst. [32] Specific surface area [m bob was used as stirring mobile to correctly homogenize the suspension and/or avoid particle settling. Coating procedure. Before catalyst coating, the structures were sonicated in acetone for 15 min and oven dried for 1h at 100°C to remove any organic contamination. The selected technique for the catalyst deposition was dip-coating. Each piece was dipped into the catalyst-water slurry for 30 seconds and then withdrawn. The foams were gently blown with air gun to drain the suspension excess. After this, the solids were oven dried for 2h at 100°C and then calcined in a muffle furnace under airflow ( 70L.h -1 ) at 550°C for 3h with a heating ramp of 4°C.min -1 (around 2h). In the case of consecutive coatings the procedure is repeated from the dipcoating to the calcination step. The overall coating process is summarized in the flow chart (Fig 8). Structured catalyst characterization Microscopy. The surface of the foams was explored by two different techniques. The digital microscope (VHX-6000) made it possible to follow the surface changes for the whole cylinders. Concerning SEM microscopy (Hitachi TM-1000), smaller parts of foams (maximal height 1cm), coated the same way than the whole cylinders, were examined. Tomography.X-ray tomography followed by image processing and 3D reconstruction was used to describe the samples. [START_REF] Zanota | [END_REF] Imorph was employed for cell sizes characterization and matlab for coating thicknesses. X-ray tomography was performed in MATEIS laboratory at INSA-Lyon (France) on v|tome|x s equipment manufactured by Phoenix X-ray. A 3D volume of 1500 x 1500 x 1300 voxels with a voxel size of 20 µm was obtained after 3D reconstruction.

Activity tests

Reactor. The batch catalytic tests were carried out in a SpinChem® Vessel V2 reactor in borosilicate glass, with a height of 250 mm, a width of 140 mm and a maximal liquid volume of 300 mL. The system is equipped with a double wall through which a heat transfer fluid flows allowing temperature control. Baffles located inside the reactor and mechanical stirring (200 rpm) grants system homogenization; four ports of 34, 29, 24 and 19 mm of internal diameter allow the loading of the reactor as well as periodic sampling throughout the test. The foam, either coated or not, was fixed to the mechanical stirrer with a metallic wire. Materials. Without additional purification, synthetic pure glycolic acid (Alfa Aesar 98%), anhydrous ethanol (VWR Chemicals), and ethyl glycolate (Aldrich 98%) were employed for the catalytic tests. For the mobile phase, sulfuric acid (H2SO4) and acetonitrile (ACN) were purchased from Sigma Aldrich. Catalytic test. The coated foams were tested for the esterification reaction of glycolic acid with ethanol. In a standard test, 120 mL of reaction medium were prepared by dissolving glycolic acid in ethanol at a molar ratio of 1 to 4. To guarantee complete dilution of the acid in the alcohol, the solution was treated with ultrasound for 5 min. Once the temperature of the reactor was constant (70°C), the glycolic acid/ethanol solution was loaded and stirring was activated, this was established as the time zero. Samples of approximately 0.5 mL were periodically taken in order to follow the progress of the reaction. HPLC analysis. The evolution of the compounds in the reaction medium was followed by HPLC. The samples were filtered with syringe filters of 0.22 µm and diluted in a ratio of 1/10 in pre-cooled ACN. A Shimadzu HPLC equipped with a Phenomenex Kinetex XB-C18 column (250 x 4.6 mm), a DAD (diode-array detector) and refractive index detector (RID) was used for analysis. A mixture of acidified water (0.0025 N H2SO4) and ACN in a 90/10 (v/v%) ratio served as the mobile phase. Ethanol was determined by RID analysis while glycolic acid and glycolate ester are detectable in DAD at a wavelength of 210 nm. External standards, i.e. solutions with established concentrations, were employed to determine the compounds concentration through the correlation between concentration and peak area. No other by-products than ethyl glycolate were observed on chromatograms and the conversion, yield and carbon balance were calculated as described in ESI.

Figure 1 .

 1 Figure 1. Mass uptake as function of support macroporosity and suspension concentration.

Figure 2 .

 2 Figure 2. SEM images of the coating process of 20 ppi Al foams for a suspension concentration of 666 g.L -1 ; A. Initial support (Al-foam) x1.0k;B. Support after coating pretreatment (calcination) x1.0k; C. Catalytic layer coated in the support x1.0k;D.Catalytic layer coated in the support x10.0k.

Figure 3 .

 3 Figure 3. Mass uptake as function of number of coatings.

Figure 4 .

 4 Figure 4. Image analysis of the foams porosity for a 20 ppi Al foam coated 3 times: Contour detection support (left); contour detection catalytic foam (right)

Figure 5 .

 5 Figure 5. Evolution of the glycolic acid conversion during esterification with the number of successive coatings (20 ppi aluminum foam); Conditions: 1/4 glycolic acid/ethanol molar ratio, 250 rpm, 70°C.

Figure 6 .

 6 Figure 6. Evolution of glycolic acid conversion during esterification: 20ppi Al foam coated 3 times (•) (2.29 %wt); Powder TiO2-WOx (200-700µm particle size) (2%wt) (■); Conditions: 1/4 glycolic acid/ethanol molar ratio, 250 rpm, 70°C.

Figure 7 .

 7 Figure 7. Evolution of glycolic acid conversion during esterification, 3 consecutive runs: 20ppi Al foam coated 3 times (2.3 %wt) Conditions: 1/4 glycolic acid/ethanol molar ratio, 250 rpm, 70°C.

Figure 8 .

 8 Figure 8. Flowchart of coating procedure for Al foams.

Acknowledgements Laura Reyes thanks the Ecole Doctorale de Chimie de Lyon for her PhD grant. This work was supported by the LABEX iMUST of the University of Lyon (ANR-10-LABX-0064), created within the « Plan France 2030 » set up by the french government and managed by the French National Research Agency (ANR)