T Rey

J.-B Le Cam

G Chagnon

D Favier

M Rebouah

F Razan

E Robin

P Didier

L Heller

S Faure

K Janouchova

Jean-Benoît Le Cam
email: jean-benoit.lecam@univ-rennes1.fr

An original architectured NiTi silicone rubber structure for biomedical applications

Keywords: Adhesion, interface, composite, NiTi, filled silicone rubber, biomedical applications, architectured structure

This paper deals with composite structures for biomedical applications. For this purpose, an architectured tubular structure composed of Nickel Titanium (NiTi) Shape Memory Alloy (SMA) and silicone rubber was fabricated. One of the main interest of such structures is to ensure a good adhesion between its two constitutive materials. A previous study of the authors (Rey et al., 2014) has shown that the adhesion between NiTi and silicone rubber can be improved by an adhesion promoter or plasma treatment. However, adhesion promoters are often not biocompatible. Hence, plasma treatment is in a favour to be used in the present study. Three different gases were tested; air, argon and oxygen. The effects of these treatments on the maximum force required to pull-out a NiTi wire from the silicone rubber matrix were investigated by means of pull-out tests carried out with a self-developed device. Among the three gases, a higher maximum force was obtained for argon gas in the plasma treatment. A tube shaped architectured NiTi/silicone rubber structure was then produced using this treatment. The composite was tested by means of a bulge test. Results open a new way of investigations for architectured NiTi-silicone structures for biomechanical applications.

Introduction

Smart materials and structures are widely used to solve biomedical problems. For example, stents are commonly used for several years. Some of the applications using a stent needed a tubular structure with a small variation in length during radial or bulge solicitations. This shows the interest of such architectured NiTi silicone rubber structures for biomedical applications. Some of the studies reported focus on NiTi associated with polymers [START_REF] Neuking | Polymer/NiTi-composites: Fundamental aspects, processing and properties[END_REF][START_REF] De Araùjo | Fabrication and static characterization of carbon-fiber-reinforced polymers with embedded NiTi shape memory wire actuators[END_REF]. To the best knowledge of the authors, the work by Heller et al. [START_REF] Heller | 3D flexible NiTi-braided elastomer composites for smart structure applications[END_REF] is the only one dealing with elastomer combined with NiTi. They proposed NiTi-braided elastomer composites for smart structure applications. Each of the two constitutive materials are well known for their typical behavior: NiTi exhibits superelasticity and shape memory behavior, and silicone rubber exhibits hyperelasticity allowing large deformations without permanent set. The use of NiTi was investigated for many applications during the last few years, especially in the medical field [START_REF] Duerig | An overview of nitinol medical applications[END_REF][START_REF] Morgan | Medical shape memory alloy applications-the market and its products[END_REF][START_REF] Petrini | Biomedical applications of shape memory alloys[END_REF][START_REF] Nespoli | Design and thermo-mechanical analysis of a new NiTi shape memory alloy fixing clip[END_REF] due to its biocompatibility [START_REF] Berger-Gorbet | Biocompatibility testing of NiTi screws using immunohistochemistry on sections containing metallic implants[END_REF][START_REF] Es-Souni | Assessing the biocompatibility of NiTi shape memory alloys used for medical applications[END_REF][START_REF] Khalil-Allafi | Biocompatibility and corrosion behavior of the shape memory NiTi alloy in the physiological environments simulated with body fluids for medical applications[END_REF][START_REF] Jani | A review of shape memory alloy research, applications and opportunities[END_REF][START_REF] Bansiddhi | Porous NiTi for bone implants[END_REF]. Also, similarly to SMA, elastomers are often considered to make composites [START_REF] Oppenheim | Fabrication and characterisation of protein fibril-elastomer composites[END_REF][START_REF] Thein-Han | Superior in vitro biological response and mechanical properties of an implantable nanostructured biomaterial: Nanohydroxyapatite-silicone rubber composite[END_REF]. For applications where the composite requires a good mechanical resistance, the efficiency strongly depends on the interface between the NiTi and the polymer matrix. Some investigations to improve their adhesion have already been reported in the literature, highlighting that numerous treatments can change the quality of the interface. Indeed, the adhesion between the two materials can be improved by mechanical [START_REF] Jonnalagadda | Local displacements and load transfer in shape memory alloy composites[END_REF][START_REF] Lau | Morphological study on twisted NiTi wires for smart composite systems[END_REF] or chemical [START_REF] Smith | Improved adhesion between nickel-titanium shape memory alloy and a polymer matrix via silane coupling agents[END_REF] treatments, or by a combination of mechanical, physical and chemical surface treatments [START_REF] Neuking | Surface engineering of shape memory alloy/polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys[END_REF]. Each treatment presents its own disadvantages: the mechanical treatments are not applicable for every geometries and chemical treatments such as adhesion promoters are often unsuitable for biocompatibility. For these reasons, the use of plasma treatment seems to benefit both of the interface resistance [START_REF] Neuking | Surface engineering of shape memory alloy/polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys[END_REF][START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF] and the interface biocompatibility [START_REF] Sui | DLC films fabricated by plasma immersion ion implantation and deposition on the NiTi alloys for improving their corrosion resistance and biocompatibility[END_REF]. It was also used in polymeric materials to improve the efficacy of voice prosthesis [START_REF] Everaert | In vitro and in vivo microbial adhesion and growth on argon plasma-treated silicone rubber voice prostheses[END_REF] or to fulfill the requirements medical in medical applications [START_REF] Oehr | Plasma surface modification of polymers for biomedical use[END_REF].

The aim of the present study is to fabricate and to characterize the mechanical response of a smart tubular structure, by assembling NiTi wires with silicone rubber whose interface resistance is improved by plasma treatment. Section 2 presents the materials, and the different self-developed devices to test the interface, to produce and to test the composite. Results obtained from the tests performed are given and discussed in section 3. Finally, some concluding remarks close the paper.

Experimental setup

Materials

Commercial pseudoelastic NiTi (50.8 at. % of Ni) SMA wires of 0.5 mm diameter were used for the tests of the interface. The Austenite to Rhombohedral phase (A to R) transformation occurs during cooling at a temperature equal to 0 • C and the reverse transformation (R to A) occurs at a temperature equal to 10 • C. The Rhombohedral to Martensite (R to M) transformation was not observable by means of DSC, because it occurred at a temperature lower than -90 • C, which is the limit of the DSC system used. Thinner NiTi wires with dimensions of 0.1 mm diameter were used to fabricate the composite. Characteristic temperatures were previously identified by DSC [START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF]. The austenite to rhombohedral (A to R) transformation occurs during cooling at a temperature equal to 30 • C and the reverse transformation (R to A) occurs at a temperature equal to 34 • C. Like previously, the rhombohedral to martensite (R to M) transformation was not observable on the DSC.

A uniaxial tensile test was performed at room temperature on two NiTi wires. Results are presented in Figure 1. Both wires exhibit large deformations (for a metallic material) with a low residual strain, and a typical superelastic behavior: an (austenitic) elastic part is first observed, followed by a plateau corresponding to a phase transformation (A to M), and a second (martensitic) elastic part. The different values of the plateau during loading and unloading is induced by a mechanical hysteresis. A biocompatible silicone rubber (Bluestar RTV 4404) was used as polymer matrix. The elaboration process of the silicone rubber is the following: its two components were mixed, this mixture was then degassed, and finally cured for 1 hour at 80 • C. The silicone rubber is subjected to cyclic tensile test for four cycles at four increasing maximum strains at room temperature. Results obtained are presented in Figure 1, in terms of the nominal stress (i.e. first Piola-Kirchhoff stress, defined as the ratio of the current force by the initial surface) versus strain.

Here, several classical phenomena in rubber can be observed. Firstly, rubber undergoes large deformations upon loading with low residual strain after unloading (less than 8% for a maximum strain equal to 150%). Secondly, mechanical hysteresis was observed, i.e. difference of stress between loading and unloading. Thirdly, the stress response for subsequent loadings was different. Stress softening or 'Mullins effect' [START_REF] Mullins | Effect of stretching on the properties of rubber[END_REF] was mainly observed during the first loading.

Interface characterization 2.2.1. Samples preparation

The adhesion between NiTi and silicone rubber was performed by using a plasma treatment without any chemical substances [START_REF] Neuking | Surface engineering of shape memory alloy/polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys[END_REF][START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF]: a gas under low pressure is subjected to a high frequency oscillating electromagnetic field and the accelerated ions in the gas collide with the gas molecules, ionizing them and forming plasma. The ionized gas particles in the plasma interact with solid surfaces placed in the same environment by removing organic contamination from surfaces and modifying or enhancing the physical and chemical surfaces characteristics [START_REF] Winters | Surface science aspects of etching reactions[END_REF]. The type of interaction between plasma and surface depends on parameters such as the intrinsic properties sample or gas(es) composition and flux, electrical power, exposure time, pressure [START_REF] Rossnagel | Handbook of plasma processing technology : fundamentals, etching, deposition, and surface interactions[END_REF]... The treatments were carried out with different gases to study their influence on the mechanical response of the NiTi/silicone rubber structure. The plasma treatments using argon or oxygen as working gases were performed using a RIE 300, whereas the plasma treatment with air as working gas was performed using a Harrick Plasma Cleaner. In both cases, the power and exposure time were fixed at 30 W and 720 s respectively. The treatment was performed under vacuum conditions, with operation pressure set equal to 10 -1 kPa and 2.10 -2 kPa for the RIE 300 and the Harrick Plasma Cleaner, respectively. Once the wire surface was treated by plasma, it was placed into a self-developed mold, already used in a previous paper of the authors [START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF]. The mold was then closed, and the silicone was blended to a curing agent and degassed, before injected into the mold using a medical syringe. Finally, the mold was placed into an oven for 1 hour at 80 • C to cure the silicone. The diameter of the cured disk of silicone is 30 mm and the thickness is 5.4 mm.

Pull-out tests

In order to investigate the adhesion between silicone rubber matrix and embedded NiTi wire, pull-out tests were performed. This type of test is classically used in the literature [START_REF] Lau | Morphological study on twisted NiTi wires for smart composite systems[END_REF][START_REF] Smith | Improved adhesion between nickel-titanium shape memory alloy and a polymer matrix via silane coupling agents[END_REF][START_REF] Neuking | Surface engineering of shape memory alloy/polymer-composites: Improvement of the adhesion between polymers and pseudoelastic shape memory alloys[END_REF][START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF][START_REF] Sadrnezhaad | Improved adhesion of NiTi wire to silicone matrix for smart composite medical applications[END_REF] to evaluate the mechanical resistance of the interfaces. For this test, the increase of the adhesion strength is defined by the increase of the maximum required force to pull-out the NiTi wire from the polymer matrix. The pull-out tests were performed using an Instron 5543 testing machine, which load cell capacity is 1 kN. The device is illustrated in a previous paper of the authors [START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF]. During the test, the sample was kept in the mold to avoid any slipping of the silicone rubber. The mold was mounted in a self-developed device. The wire was placed into the jaw, and was then pulled from the silicone disk in the vertical direction at room temperature and at a displacement rate of 0.6mm/s.

In order to check the interface stability over time, pull out tests were performed at different times after molding, between 0 and 7 days.

Additional analyses

Measurements of the wire deformation were carried out by marks tracking method in order to check that no transformation occurred in the SMA wires during the pull-out tests. Moreover, Scanning Electron Microscopy (SEM) observations were performed with a JEOL JSM 6301F in order to observe surfaces of the pulled-out wires. In order to analyze the effects induced by the plasma treatment, several observations were made before and after the treatment.

Roughness evaluation of a NiTi plate was performed before and after the more efficient plasma treatment. The composition of the NiTi plate is the same than those of the NiTi wires (50.8 at. % of Ni), and its length, width and thickness are 75, 15 and 3 mm, respectively. This analysis was carried out with a surface profiler (Alpha-Step IQ, KLA Tencor). Measurements were performed by means of a diamond tip moving on the specimen surface. For the present measurements, the length, speed and frequency were chosen equal to 500 µm, 20 µm/s and 200 Hz, respectively. The roughness was calculated as the arithmetic average between successive minimum and maximum values of the height (called Ra).

Wettability tests were also carried out on a similar NiTi plate. A liquid droplet was put at the surface of the specimen under study and the diameter and height of this droplet were measured. By this way, the contact angle between the droplet and the specimen can be calculated. The droplet volume is equal to 3 mm 3 . Two liquids were used: water and diiodomethane. The surface energy was then calculated, using the method proposed in [START_REF] Owens | Estimation of the surface free energy of polymers[END_REF].

Elaboration of the composite

In order to obtain a tubular shaped composite, NiTi wires were knitted and then a shape setting was performed at 450 • C (see Figure 2a). The tube obtained was threaded on a metallic tube and fixed to two holding parts in order to avoid any displacement during the injection of the silicone rubber. Then, this tube was treated by plasma for 720 s at 30 W with the working gas giving the best results with the pull-out tests. The treated NiTi tube was then put in a self-developed mold, to be embedded in the polymer matrix, which is injected by the way of a medical syringe. Finally, the mold was placed into an oven at 80 • C for 1 hour. The obtained composite is presented in Figure 2b, with the two holding parts at its extremities. Its length, internal and external diameters are 30, 8 and 12 mm, respectively. Several tubes were fabricated in order to check the reproducibility of the process.

Bulge test

A bulge test was performed by injecting water into the composite. Figure 3 gives an overview of the experimental setup. The composite was suspended by one of the holding parts, and a water injection pipe was connected at this part. The other extremity was closed by a cork. It is worth noting that even though a low amount of force (0.54 N) was imposed (by the weight of the cork and the holding part), the displacements of the inferior part were not blocked. Water was injected at a constant volume rate by the way of a medical syringe. The pressure in the tube was measured with a manometer.

During the bulge test, the strain field at the composite surface was measured by means of a Stereo Digital Image Correlation (SDIC) system. The CDD sensor of the two cameras used features 1028 X 1028 connected pixels. A speckle pattern was done using a special powder coating on the tube surface for the SDIC, and appropriate lighting has been set up. A marks tracking method was also used to follow the displacement of four crosses at the tube surface and to obtain quantitative values of displacements. Two crosses were marked at the top and bottom of the tube in order to measure length variations, whereas the two others were marked at the center in order to observe the circumference variations.

Results and discussion

Pull-out tests

Results of the pull-out tests are presented in Figure 4 a). Tests were found to be repeatable. Hence, only one curve is plotted for each treatment. The force was found to increase with a plasma treatment regardless the type of gas considered: 33%, 48% and 159% for oxygen, air and argon, respectively. It is to note that for several applications, thickness of the embedding polymer matrix is not of the same order as the pull-out tests (i.e. at least ten times superior than the wire diameter). For this reason, the same tests were carried out with a thickness of the silicone pancake equal to 2 mm. The results obtained are presented in Figure 4 b). This Figure shows that even though the forces reached are different, the trend remains the same as that observed for the classical pull-out tests: the highest maximum forces were obtained for argon, with an increase of maximum force of 158% (14.05 N versus 5.45 N without any treatment). This shows that the efficacy of the treatment and the classification of the treatment do not depend on the thickness of the silicone pancake in the range of thickness tested. Additional pull out tests were carried out using both argon and oxygen (same proportion of each gas), and results showed once again that it is less efficient than using argon only. A comparison with the same tests performed in ref. [START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF] with another filled silicone rubber (Bluestar RTV 3428) shows that the efficacy of the plasma treatment strongly depends on the nature of the considered polymer. Indeed, in the present study, the treatment with air gas is less efficient than in [START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF]. In the present study, the ratio between the maximum forces reached with treated and untreated wires was inferior to 2, while it was superior to 7 in [START_REF] Rey | Mechanical characterization and comparison of different NiTi/silicone rubber interfaces[END_REF]. The results of the wires deformation measurements showed that the local deformations observed on the wires were too low to induce any transformation phenomenon.

Results of the pull-out tests performed several days after molding showed a low increase of the maximum force measured. Even if these results show that the mechanical properties of the interface remained constant at the time scale of several days, additional tests have to be carried out for higher post-molding times, depending on the considered application.

Post-mortem analysis of the interface zone

SEM observations are presented in Figure 5, for an untreated wire (a), and for wires treated with plasma using oxygen (b), air (c), and argon (d) as working gas. Without any surface treatment, no silicone rubber was observed on the surface of the pulled-out wire, whereas a few parts of silicone were observed for a wire treated with oxygen and air. As shown in Figure 5d), the surface treated with argon gas exhibited large silicone rubber patches which indicates that significant cohesive failures took place in the material before any adhesive failure. This confirms the best efficacy of the plasma treatment with argon. For each condition, no modifications on the surface of the wire were observed for areas without silicone. Results obtained for the roughness analyses showed that the roughness remained constant before and after the argon gas plasma treatment (Ra equal to 0.706 µm before and 0.695 µm after the treatment). Hence, the plasma treatment does not affect the surface of the NiTi wire, confirming the SEM observations on the pulled-out wire.

Results of the wettability analyzes show that the wettability is increased by plasma treatment with argon gas. Indeed, the contact angle measured for water and diiodomethane were equal to 56 • and 37 • against 17 • and 39 • before treatment respectively, giving an increase of the surface energy from 50 mJ/m 2 to 70 mJ/m 2 .

Bulge tests

As mentioned earlier, the composite was bulged by water injection. Several cycles were carried out in order to investigate the possible irreversibilities in the mechanical response. Figure 6 presents the pressure variation during 10 cycles with an injected water volume of 5 cm 3 . The measurements were not performed during the first cycle, as it is a cycle of setting up of the system. Thus the cycles presented in the figure are from the 2 nd to the 11 th . It is observed on this curve that a small amount of permanent set existed between each cycle. Therefore, the mechanical response was considered to be similar throughout the test. Variation of the tube dimensions were measured during the test by means of marks tracking method. During the test, the tube was kept in a circular shape. So, an increase of the distance between the two points in the radial direction corresponds to an increase of the diameter. It was observed that the diameter of the center of the composite increased during the injection of water. At the same time, its length decreased during injection. It should be noted that the composite rotated slightly during the test. These three effects were induced due to the response of the knitted NiTi tube: the mesh get tighten itself, inducing a length reduction because stitches are closer, and rotation because the knitting was carried out by a helical method.

In order to analyze the displacements and deformations at the tube surface, SDIC analyses were also performed during the test. The error on the strain measurements was equal to 0.1%. Figure 7 presents the distribution of the stretches in the diameter and length directions (λ 1 and λ 2 respectively), at a pressure slightly lower than 60 kPa. The stereo-correlation results confirmed that the composite diameter increased whereas the length decreased. Here, measurements made close to the holding parts are not taken into account in the analysis, as the 3D reconstruction was distorted due to the clipping of the tube. It can be observed that for a pressure slightly lower than 60 kPa, the strain reached in the diameter direction is equal to 7% in tension whereas it is equal to 3% in compression in the length direction.

A tube of silicone rubber was bulged by the same way in order to analyze the influence of the tubular NiTi structure. Figure 8 presents the results of the bulge tests for a silicone rubber tube and a NiTi silicone rubber tube. It can be observed in Fig. 8 a) that the addition of the knitted NiTi tube led to a significant increase of stiffness of the structure. Indeed, the measured pressure in NiTi silicone rubber tube was very higher than for silicone rubber tube (for example, for a volume of 5 mL, the pressure in the composite tube was equal to 57 kPa instead of 6.5 kPa for silicone rubber, i.e. almost 9 times higher). Fig. 8 b) shows that the stretch measured in the longitudinal direction was more important than the one in the radial direction for silicone rubber only, and positive logarithmic stretches were observed in the two directions. For the composite, the logarithmic stretch in the longitudinal direction was negative, and less important than the stretch in the radial direction.

Discussion

Results highlighted that the composite tube behavior was very different from a tube of silicone rubber. Indeed, as previously observed, the tube of silicone rubber increases its length and diameter during a bulge test. The increase in length is higher than the increase in diameter. It reaches 35% of the initial length, which is not compatible with biomedical applications. When a knitted NiTi structure is used, the length slightly decreased instead of increasing steeply. Moreover, in this case, the length of contraction is lower than the diameter expansion (7%). This is a major advantage offered by such NiTi silicone rubber composite. Several other advantages can also be pointed out.

Firstly, its mechanical behavior can be easily modified. Indeed, it is mainly determined by the mechanical response of the knitted NiTi, whose mechanical behavior can be altered through heat treatments. Secondly, the mechanical response of the knitted tube can be modified by changing the mesh size or by using two wires with different mechanical behaviors before the shape setting step. For instance, to induce a type of anisotropy similar to that observed in soft tissues. Thirdly, the behavior of a NiTi based composite can be driven by several ways: it is well known to react with mechanical, thermal and electrical solicitations. For thermal and electrical solicitations, the silicone rubber isolates the NiTi from the tissues. Finally, no detectable mechanical response changes were observed during cyclic loadings. This offers many possibilities and such a composite should strongly be considered for applications, typically for applications like artificial vessels or stents.

Conclusion

In this paper, a composite structure has been proposed to meet the specific needs in the biomedical field. The composite structure was fabricated by associating a knitted NiTi with a silicone rubber. The interface resistance between both materials was improved by an optimized plasma treatment without any adhesion promoter. A tubular shaped composite structure was fabricated using a self-developed device and submitting it to cyclic internal pressure during a bulge test. Results have shown that under pressure, the strain level is low. The tube contracts in the longitudinal direction and stretches in the radial direction. Consequently, this particular composite structure provides promising aspects for biomedical applications, especially to fabricate artificial vessels or stents.

Figure 1 :

 1 Figure 1: Tensile test on the NiTi wires (on the left) and on a RTV 4404 silicone rubber (on the right).

Figure 2 :

 2 Figure 2: Knitted tube of NiTi (a), tube embedded in a silicone rubber matrix (b).

Figure 3 :

 3 Figure 3: Overview of the bulge test.

Figure 4 :

 4 Figure 4: Results of the pull-out tests with different treatments, with a silicone pancake thickness of a) 5.4 mm and b) 2 mm.

Figure 5 :

 5 Figure 5: Observation of the pulled-out wires a) without any treatment, and treated by plasma with b) oxygen, c) air and d) argon as working gas.

Figure 6 :

 6 Figure 6: Bulge test of the architectured composite.

Figure 7 :

 7 Figure 7: Estimation of the stretches by means of stereo-correlation measurements in the diameter (1) and length (2) directions.

Figure 8 :

 8 Figure 8: Comparison between the bulge of a silicone rubber tube and a NiTi silicone rubber composite tube.

Acknowledgement

This work has been supported by the French National Research Agency Program n2010BLAN90201 ANiM (Architectured NiTi Materials). The authors acknowledge Mei Sze Loo for fruitful discussions and valuable suggestions.


Luisa Damaris

Ramos Riascos

Alfonso Enrique

Ramírez Sanabria

Gerardo Andrés

Torres Rodríguez

Alexander Sachse

Cristian David

Miranda Muñoz

Gerardo Andrés Torres Rodríguez

Sulfonated Reduced Graphene Oxide: An Acid Catalyst that Efficiently Promotes the Esterification of Glycerol

Keywords: Reduced sulfonated graphene oxide, Heterogeneous catalysis, Glycerol acetylation, Glycerol valorization

Introduction

The booming production of biodiesel has brought an overproduction of glycerol, as unavoidable synthetic by-product, which corresponds to approximately 10% of the products in weight [START_REF] Okoye | Synthesis of oxygenated fuel additives via glycerol esterification with acetic acid over bio-derived carbon catalyst[END_REF]. A consequence of this is the quest of sound glycerol valorization strategies [START_REF] Pagliaro | From glycerol to value-added products[END_REF]. An attractive possibility is the chemical transformation of glycerol towards oxygenated biofuel additives. This would be advantageous for the biodiesel industry through favoring the establishment of a circular, profitable and sustainable industry [START_REF] Reddy | Synthesis of bio-additives: acetylation of glycerol over zirconia-based solid acid catalysts[END_REF].

In this context, the glycerol acetylation is considered as an excellent alternative to obtain oxygenated biofuel additives [START_REF] Gonçalves | Acetylation of glycerol catalyzed by different solid acids[END_REF], as mono-, di-and triacetyled glycerol (MAG + DAG + TGA) can be obtained [START_REF] Aghbashlo | Exergy-based sustainability analysis of acetins synthesis through continuous esterification of glycerol in acetic acid using Amber-lyst®36 as catalyst[END_REF]. Specifically, triacetin (TGA) is promising as fuel additive [START_REF] Malaika | Glycerol conversion towards valuable fuel blending compounds with the assistance of SO 3 H-functionalized carbon xerogels and spheres[END_REF], allowing to improve the cold flow properties of conventional diesel fuel, increasing the cetane number and reducing the emission of harmful gases [START_REF] Calero | An overview on glycerol-free processes for the production of renewable liquid biofuels, applicable in diesel engines[END_REF]. However, triacetin is the most unfavored product using acetic acid as a reagent. Its production requires hence the use of a catalyst [START_REF] Manurung | Triacetin production using SiO 2 -H 3 PO 4 catalysts derived from bamboo leaf biomass waste for esterification reactions of glycerol and acetic acid[END_REF].

Glycerol acetylation is traditionally promoted by homogeneous acid catalysts such as sulfuric acid (H 2 SO 4 ), phosphoric acid (H 3 PO 4 ) and hydrochloric acid (HCl). Nevertheless, homogeneous acid catalysts present important issues related to the production of toxic compounds, reactor corrosion and product separation [START_REF] Khayoon | Improved production of fuel oxygenates via glycerol acetylation with acetic acid[END_REF]. Hence their replacement with heterogeneous catalysts is sought. Acid catalysis is widely used in the chemical industry as a large variety of chemical transformations requires the involvement of acids [START_REF] Testa | Sulfonic acid-functionalized inorganic materials as efficient catalysts in various applications: a minireview[END_REF]. For instance, acidic resins (e.g. Amberlyst® 15) [START_REF] Kale | Selective formation of triacetin by glycerol acetylation using acidic ion-exchange resins as catalyst and toluene as an entrainer[END_REF], zeolites [START_REF] Cahyono | Acetylation of glycerol for triacetin production using Zr-natural zeolite catalyst[END_REF] clays [START_REF] Venkatesha | Volume accessibility of acid sites in modified montmorillonite and triacetin selectivity in acetylation of glycerol[END_REF], functionalized metal oxides [START_REF] Appaturi | Selective synthesis of triacetyl glyceride biofuel additive via acetylation of glycerol over NiO-supported TiO 2 catalyst enhanced by nonmicrowave instant heating[END_REF] and sulfonic acid-functionalized carbons [START_REF] Khayoon | Acetylation of glycerol to biofuel additives over sulfated activated carbon catalyst[END_REF] are some examples of frequently employed aid catalyst that allow for achieving superior performances [START_REF] Perez | Producción de acetinas (aditivos para combustibles) a partir de glicerol[END_REF].

Graphene related materials are an emerging class of promising heterogeneous catalytic systems, due to their 2D structure, high stability and flexibility of functionalization [START_REF] Wang | Synthesis and characterization of sulfonated graphene as a highly active solid acid catalyst for the ester-exchange reaction[END_REF]. These often feature defined textures with uniform pore sizes, among other properties, that positively influence their adsorption behavior and their catalytic properties. As an example, sulfonated graphene oxide, obtained by functionalization of GO with -SO 3 H groups, proved as excellent solid acid catalyst in organic transformations [START_REF] Wang | Synthesis and characterization of sulfonated graphene as a highly active solid acid catalyst for the ester-exchange reaction[END_REF][START_REF] Oger | Practical and scalable synthesis of sulfonated graphene[END_REF][START_REF] Mirza-Aghayan | Sulfonated reduced graphene oxide as a highly efficient catalyst for direct amidation of carboxylic acids with amines using ultrasonic irradiation[END_REF], featuring waterproof properties [START_REF] Ji | Sulfonated graphene as watertolerant solid acid catalyst[END_REF] and considerable thermal stability [START_REF] Liu | Sulfated graphene as an efficient solid catalyst for acid-catalyzed liquid reactions[END_REF]. Recently some of us reported improved glycerol etherification using sulfonated graphene oxide and allowed for the superior production of poly-substituted ethers [START_REF] Miranda | Sulfonated graphenes: efficient solid acid catalyst for the glycerol valorization[END_REF].

In the present work, we firstly report the use of sulfonated graphene oxide in the glycerol acetylation. The results prove for a very active, selective and stable catalyst under the chosen reactions conditions.

Experimental

Materials

Sulfonated Graphene Synthesis

Graphene Oxide (GO) Synthesis

Graphene oxide (GO) was obtained by a modification of the Hummer's method [START_REF] Miranda | Sulfonated graphenes: efficient solid acid catalyst for the glycerol valorization[END_REF]. For the modified method, H 2 SO 4 (55 mL) was added to a mixture of graphite powder (2.1 g) and KMnO 4 (7.5 g) in an ice bath. The reaction was then heated to 35 °C and stirred for 2 h. 30% H 2 O 2 (6 mL) was diluted with in deionized water (80 mL) and poured onto ice (≈ 5 °C). Then, deionized water (150 mL) was added and stirred all night long at rt. For workup, the mixture was centrifuged (5000 rpm for 10 min), and the supernatant was decanted and the solid was dried at 60 °C during 24 h.

Graphene Oxide (GO) Reduction [(GO)R]

Graphene oxide (0.513 g) was added to deionized water (600 mL) and stirred to form a suspension. The suspension was sonicated 2 h and then ascorbic acid (164.1 g) was added and stirred for 2 h at 80 °C. The reaction was cooled to rt and then filtered through a nylon filter (Whatman 0.45 μm, 47 mm). The filtrate was then washed with 25 mL of deionized water, 40 mL of methanol and 40 mL of ethyl acetate (4 ×) [START_REF] Miranda | Sulfonated graphenes: efficient solid acid catalyst for the glycerol valorization[END_REF]. The solid (GO)R was then dried to 60 °C by 12 h.

Reduced Graphene Oxide [(GO)R] sulfonation [(GO) RS]

Sulfonation of (GO)R was carried out by in situ diazotization with sulfanilic acid. (GO)R (0.276 g) was added to deionized water (40 mL) and sonicated 2 h. A mixture of NaNO 3 (0.95 g) and C 6 H 7 NO 3 S (0.795 g) was incorporated.

The reaction was stirred at 25 °C for 24 h. The solid was filtered through a nylon filter (Whatman 0.45 μm, 47 mm).

The filtrate was then washed with 50 mL of 1.0 M HCl and 70 mL of acetone (3 ×) [START_REF] Miranda | Sulfonated graphenes: efficient solid acid catalyst for the glycerol valorization[END_REF]. The solid (GO)RS was then dried to 50 °C by 12 h.

Characterization Techniques

X-ray diffraction (XRD) analysis was performed using a Xpert PANalytical Empyrean Series II-Alpha 1with a Cu-K α as radiation source. The measurements were performed with step size of 0.05 and a scan rate of 1° min -1 from 20 to 60° 2θ. Sulfonic groups in surface were detected by FT-IR (Fourier transform infrared spectroscopy) in Attenuated Total Reflection (ATR) using a spectrophotometer Nicolet™ iS™.

Structural, morphological and textural properties were investigated by scanning electron microscopy (SEM). The images were obtained by using a microscope Zeiss EVO HD15. Samples were dispersed as a powder onto copper grids coated with carbon film.

Acidity was assessed by elemental analysis and calculated as reported by Oger [START_REF] Oger | Practical and scalable synthesis of sulfonated graphene[END_REF] using a CHNS elemental analyzer vario EL cube.

Catalyst Testing

The reaction was carried out in a glass autoclave reactor, provided with temperature control, a manometer and stirring control. For the acetylation studies molar ratios of 1:3, 1:6 and 1:9 glycerol:acetic acid were studied and a constant catalyst loading of 5% (with respect to the glycerol mass) was used in all experiments. The total reaction volume was adjusted to 14 mL. 0.3 mL of sample was taken directly from the reaction and subjected to centrifugation. The reaction was followed at 8, 17, 25, 33, 42, 50, 60, 90 and 120 min. Additionally, a set of temperatures was evaluated (70, 90 and 110 °C) under constant and continuous stirring (1100 rpm). Amberlyst® 15 (A15) was used as reference catalyst.

Analysis of Products

The qualitative analysis was performed by Gas Chromatography coupled to Mass Spectroscopy (GC-MS) using a chromatograph Thermo Scientific model Trace 1300, HP-5MS column (30 m, 0.25 mm, 0.25 μm), a MS model ISQL detector and ethylene glycol (99%, Sigma Aldrich) as internal standard. The quantitative analysis was performed by Gas Chromatography using a chromatograph model Shimadzu GC-14A, DB-WAX column (30 m, 0.25 mm, 0.25 μm), an FID detector and ethylene glycol (99%, Sigma Aldrich) as internal standard. The temperature program used consisted of an isotherm at 100 °C (1 min), an increase rate of 20 °C/min until reaching 240 °C, where there was another isotherm for 2 min. The samples for injection (0.1 μL) for analysis were prepared as a mixture (200 mg of sample and 20 mg of internal standard). The response factors of products (mono-, diand triacetin) were determined by calibration performed with standards.

Catalytic Properties

Activity

Catalytic activity was determined from the glycerol conversion

The molar yield was also calculated

Triacetin Selectivity

The response factors of products (mono-, di-and triacetin) were used to evaluate the selectivity.

Recyclability

After each catalytic cycle, the catalyst was separated by centrifugation using a centrifuge (Hermle Z206A), washed with deionized water and ethanol (Sigma-Aldrich, 96%) and was then dried at 60 °C for 20 h.

Results and Discussion

Characterization of Solids

X-ray Diffraction (XRD)

To verify that the used synthesis process allowed for the formation of the desired solids, the XRD patterns were recorded (Fig. 1). For the starting material, graphite, the XRD pattern shows a characteristic peak at 26.5° 2θ, corresponding to the (002) reflection, while the XRD pattern of GO shows a main diffraction peak of approximately 11.2° 2θ, attributed to the (001) diffraction plane. The absence of the (002) reflection in the GO sample indicates that the graphite oxidation was complete [START_REF] Marcano | Improved synthesis of graphene oxide[END_REF]. Using Bragg's law, the interplanar spacing was calculated, which amounts to 0.80 and 0.34 nm for GO and graphite, respectively. The increase in the d-spacing upon oxidation is due to the insertion of oxygenated functional groups generating electronic repulsion and therefore higher spacing between layers [START_REF] Johra | Facile and safe graphene preparation on solution based platform[END_REF], as well as a greater affinity for water molecules [START_REF] Pan | Li storage properties of disordered graphene nanosheets[END_REF], which was further corroborated by FT-IR spectroscopy (Fig. 2). For GO, a broad peak at approximately 2θ = 42.3° was observed, which can be ascribed to the (100) plane [START_REF] Sahoo | Synthesis and characterization of graphene oxide and graphene from coal[END_REF]. Indeed, Diffraction peaks in GO are due to the short-range order of stacked graphene layers [START_REF] Stobinski | Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods[END_REF]. After reduction of GO with ascorbic acid [(GO)R], the oxygen-containing functional groups are removed, resulting in a shift of the 001 reflection to 25.3° 2θ, indicating a reduction of the interplanar spacing to 0.36 nm and a faint band at 43.4° 2θ (100). The enlargement of the diffraction peaks suggest a degradation of the long range structural order during the exfoliation step [START_REF] Oger | Practical and scalable synthesis of sulfonated graphene[END_REF]. Finally, the sulfonation treatment has little impact on the structural properties, as the XRD pattern of (GO)RS resembles the one of (GO)R.

Fourier Transform Infrared Spectroscopy (FT-IR)

The FT-IR spectra (Fig. 2), allow to observe the structural changes in the materials from graphite to (GO)RS. GO exhibits a broad band at approximately 3115 cm -1 , corresponding to the stretching vibrations of the -OH bonds of the carboxyl groups (-COOH) and to the water molecules absorbed in the material. A band at 1711 cm -1 , associated with the vibrations of the carbonyl group (C=O) and a band at 851 cm -1 , related to the epoxide bonds (C-O-C) [START_REF] Guliani | Catalytic performance of sulfate-grafted graphene oxide for esterification of acetic acid with methanol[END_REF], can be further observed. Characteristic bands of the asymmetric and symmetric O=S=O stretching vibrations were also observed at 1115 cm -1 and 998 cm -1 , which were attributed to remnants of sulfuric acid product of the GO synthesis [START_REF] Estevez | Sulfonated carbons from olive stones as catalysts in the microwave-assisted etherification of glycerol with tert-butyl alcohol[END_REF]. The reduction of GO was verified by the disappearance of both the bands associated with the oxygenated groups and the signals corresponding to the sulfate group in the (GO)R spectrum. Finally, the (GO)RS exhibits characteristic bands associated with the sulfonic group at 1103 cm -1 and 996 cm -1 , indicating successful surface grafting. In addition, the presence of a broad band at 2926 cm -1 indicates an increase in the polarity of the material with respect to the original material (GO)R, related to the insertion of the sulfonic group and the gain in Brønsted acidity.

Scanning Electron Microscopy (SEM)

Scanning electron microscopy has been used to investigate the evolution of the morphology of the materials after each synthesis step. Figure 3 shows the SEM images of graphite, GO, (GO)R and (GO)RS. For graphite dispersed particles with rather uniform granular size and shape can be observed. The GO, (GO)R and (GO)RS materials are heterogeneous in both size and shape forming smaller clusters with porous structure. The surfaces of graphene sheets are not perfectly flat presenting surface wrinkles as previously reported by Chen et al., which is related to different levels of transparency [START_REF] Chen | Mass-production of highlycrystalline few-layer graphene sheets by arc discharge in various H 2 -inert gas mixtures[END_REF].

Acidity Calculated by Elemental Analysis

The results of the elemental analysis are shown in Table 1.

The sulfur content may be associated with the presence of sulfate groups or it may be related to the grafting of -SO 3 H groups.

Initially, the presence of remaining sulfate groups in the GO, associated with the use of sulfuric acid in the synthesis, is detected. The reduction process allows these groups to be removed from the surface and subsequently only sulfonic groups are grafted, which provide the Brønsted acidity in the catalyst.

Catalytic Evaluation

Acetylation of glycerol with acetic acid is illustrated in Scheme 1. The mono-, di-and tri-acetylated products MAG (two isomers), DAG (two isomers) and TAG are formed as well as water as a byproduct. The reversible character of the reaction allows for the evaluation of different parameters on the chemical equilibrium [START_REF] Ferreira | Acetylation of glycerol over heteropolyacids supported on activated carbon[END_REF].

To examine the parameters that influence the reaction, Amberlyst® 15 was used as the reference catalyst and the optimal conditions of temperature, glycerol/acetic acid molar ratio and reaction time were evaluated as a function of glycerol conversion. The suitable amount of catalyst was determined to be 5% by mass with respect to the total mass amount of glycerol. The stirring speed was set to 1100 rpm, where external mass transfer limitations are negligible [START_REF] Zhou | The acetylation of glycerol over amberlyst-15: kinetic and product distribution[END_REF].

Effect of Reaction Temperature

The effect of temperature was evaluated at 70, 90 and 110 °C, using a 1:9 molar ratio of glycerol:acetic acid, during two hours of reaction. The results shown in Fig. 4 suggest a temperature dependency of the glycerol acetylation reaction. The maximum conversion was obtained at 110 °C, which can be attributed to the faster formation of acylium ions with increasing temperature [START_REF] Sandesh | Glycerol acetins: fuel additive synthesis by acetylation and esterification of glycerol using cesium phosphotungstate catalyst[END_REF]. Furthermore, an increase in the reaction temperature favors collision of reactant molecules, increasing the reaction rate. As far as selectivity is concerned, a proportional increase of TAG selectivity with temperature was observed. According to Scheme 1, a successive transformation of MAG → DAG → TAG occurs, which is favored when equilibrium is reached, it is hence possible to produce more TAG, when increasing temperature of 110 °C. Hence, reaction temperature was set to 110 °C in further tests.

Effect of Glycerol/Acetic Acid Molar Ratio

The influence of the glycerol/acetic acid molar ratio was studied by setting the ratios to 1:3, 1:6 and 1:9. The temperature was adjusted to 110 °C, 5% catalyst loading (Amberlyst® 15), and the reaction was held for 2 h. A higher glycerol conversion and TAG yield can be observed with increasing glycerol/acetic acid molar ratio (Fig. 5). This is due to the increase in the amount of acetic acid molecules in the reaction medium and therefore provides more acetylating agent, hence facilitating the change in the distribution of esters towards more substituted species [START_REF] Zhou | The acetylation of glycerol over amberlyst-15: kinetic and product distribution[END_REF]. This correlates with what was reported by Khayoon et al. [START_REF] Khayoon | Acetylation of glycerol to biofuel additives over sulfated activated carbon catalyst[END_REF]. Finally, a molar ratio of 1:9 of glycerol/acetic acid was chosen as most suitable for the glycerol acetylation.

Effect of Reaction Time

The effect of reaction time on glycerol acetylation was studied using a glycerol/acetic acid ratio of 1:9, 110 °C, a catalyst loading of 5% (relative to the mass of glycerol) and stirring of 1100 rpm. Figure 6 shows the behavior of glycerol conversion and TAG selectivity during 7 h. As far as the conversion is concerned, a plateau is reached after two hours of reaction at 92%. For the selectivity a continuous formation of TAG is observed before reaching a plateau after 4 h at 26%. Indeed, glycerol acetylation follows a consecutive reaction mechanism, where TAG is formed by successive esterification of DAG present in the reaction medium [START_REF] Zhou | The acetylation of glycerol over amberlyst-15: kinetic and product distribution[END_REF].

Catalytic Properties

Catalytic Activity

After establishing the conditions under which maximal glycerol was observed using A15 as a catalyst, the catalytic behavior of the (GO)RS solid was evaluated and compared to the one of GO (Fig. 7). A blank experiment (without catalyst) allowed to observe a conversion of 50% after 2 h, suggesting an autocatalytic effect promoted by the excess of acetic acid, which provides a Brønsted-type acidity to the medium and to the elevated reaction temperature (110 °C), which could favor glycerol conversion (endothermic reaction) [START_REF] Liao | Theoretical elucidation of acetylating glycerol with acetic acid and acetic anhydride[END_REF]. Subsequently, a test using GO as catalyst was carried out, for which high catalytic activity was observed. The elemental analysis (Table 1), allows to deduce the presence of sulfur in the sample (residues from synthesis with sulfuric acid). These groups can leach into the reaction medium, generating a homogeneous process. This was verified by removing the GO catalyst from the reaction medium 5 min after the start of the reaction; Here for a similar conversion (~ 92%) was observed after 2 h as in the presence of the GO catalyst. In order to elaborate a truly heterogeneous catalyst GO reduction process is required, which eliminates the sulfate groups (Table 1). Successive grafting of the sulfonic groups by diazotization then allows for the design of the heterogeneous acid catalyst.

glycerol conversion obtained with the (GO)RS catalyst was the same as with A15 after two hours of reaction (92%). Yet a strong difference in the reaction rate and activity were observed. Both are significantly higher for the (GO) RS catalyst compared to A15 (Table 2).

These activity results indicate a buffer-like effect of the surface of the solid (GO)RS against the by-product water. Hence, despite A15 featuring a higher number of acidic sites (2.37 mmol H + g -1 ) compared to (GO)RS (1.68 mmol H + g -1 ), the oxygenated groups in the latter play a crucial role. (GO)RS presents approximately 20% of oxygen groups that can adsorb water formed during the reaction, hence the acid sites only little interact with H 2 O molecules. Indeed, about 38 mmol of water is formed at 92% glycerol conversion. Several reports in the literature present strong adsorption of highly polar molecules, such as water, on resin surface [START_REF] Ziyang | Determination of adsorption and kinetic parameters for methyl tert-butyl ether synthesis from tert-butyl alcohol and methanol[END_REF][START_REF] Ozbay | Activity comparison of different solid acid catalysts in etherification of glycerol with tert-butyl alcohol in flow and batch reactors[END_REF], which was not the case for sulfonated graphene oxide, for which catalytic activity was observed even in the presence of high amounts of water [START_REF] Ji | Sulfonated graphene as watertolerant solid acid catalyst[END_REF].

In addition, ln(Co/C) vs. reaction time was plotted to examine the reaction rate dependence. In Fig. S1 a linear relationship between the glycerol consumption and time can be observe for both A-15 and (GO)RS. These results suggest that under the applied conditions, i.e. a glycerol/acetic acid molar ratio of 1:9, 5% catalyst loading and 110 °C, a first order reaction occurs. These results are consistent with those observed in the same reaction promoted by heterogenized tungstophosphoric acid catalysts [START_REF] Patel | A green and sustainable approach for esterification of glycerol using 12-tungstophosphoric acid anchored to different supports: kinetics and effect of support[END_REF].

Selectivity

Selectivity towards TAG with respect to glycerol conversion is shown in Fig. 8. Glycerol acetylation consists of a consecutive MAG → DAG → TAG transformation (Scheme 1), which, according to the obtained results, is favored when the maximum glycerol conversion is reached. When the glycerol conversion is highest, i.e. between 90 and 92%, a increased selectivity of TAG is observed for all catalysts. At isoconversion of glycerol (91.7%) A TAG selectivity of 25.0%, 19.6% and 13.5% is respectively observed using GO, (GO)RS and Amberlyst®15. As far as GO is concerned, we previously discussed that the process corresponds rather to a homogeneous process in which sulfate groups are leached in the reaction medium. With respect to (GO)RS, it reaches equilibrium before A15, thus favoring the successive transformation of the products.

Stability

The stability of the catalyst was evaluated by 5 recycling cycles (Fig. 9). The conversion of glycerol reduces slightly after the third reaction cycle and falls to ~ 55% after the fifth cycle. The reduction in conversion upon recycling can be explained by the loss of -SO 3 H groups, leading to a lower acidity of the recycled catalyst (Table 3). Indeed, water, which develops as unavoidable byproduct of the reaction, to the hydrolysis of the -SO 3 H groups on the catalyst surface, hence reducing acidity [START_REF] Costa | Influence of acidic properties of different solid acid catalysts for glycerol acetylation[END_REF].

Conclusion

A heterogeneous acid catalyst was successfully synthesized by sulfonation of reduced graphene oxide, allowing to graft -SO 3 H groups onto the GO surface with an acidity of 1.68 mmol g -1 . The grafting of -SO 3 H groups leads to significant catalytic activity in the glycerol acetylation compared to the commercial acid catalyst Amberlyst® 15. Interestingly it was found that the oxygenated groups in (GO)RS act as water scavengers, which allows for maintaining high catalyst activity, as water is an unavoidable by product of the glycerol acetylation. (GO)RS allowed for achieving superior selectivity towards the trisubstituded ester, which is the most desired product of the reaction. Slight reduction of catalytic activity of the (GO)RS in recycling experiments could be related to partial hydrolysis of the -SO 3 H groups.

  Glycerol conversion (% ) = Moles of reacted glycerol Moles of initial glycerol × 100 Activity = moles of reacted glycerol Time (h) × mmol H + Molar yield (% ) = Moles of obtained product Moles of initial glycerol × 100 Triacetin selectivity (% ) = Moles of obtained triacetin Total moles of products

Fig. 1 X

 1 Fig. 1 X-ray diffraction of graphite, GO, (GO)R and (GO)RS

Fig. 2

 2 Fig.2FT-IR spectra of graphite, GO, (GO)R and (GO)RS

Fig. 3

 3 Fig. 3 SEM images of a graphite, b GO, c (GO)R and d (GO)RS

Scheme 1 Fig. 4 Fig. 5

 145 Scheme 1 Glycerol esterification with acid acetic

Fig. 6 Fig. 7

 67 Fig. 6 Effect of reaction time. Temperature = 110 °C, glycerol:acetic acid molar ratio = 1:9, Catalyst loading: 5% by weight (relative to the mass of glycerol), stirring: 1100 rpm

Fig. 8 TAGFig. 9 (

 89 Fig.8TAG Selectivity vs. glycerol conversion using GO, (GO)RS and A15 as catalysts. Catalyst loading: 5% by weight (relative to the mass of glycerol), temperature = 110 °C, glycerol:acetic acid molar ratio = 1:9, stirring: 1100 rpm

Table 1

 1 Elemental analysis for (GO)R and (GO)RS in comparison toAmberlyst® 15

	Material	Elemental analysis (%) C H	N	S	Acidity (mmol H + g -1 )
	A-15	-	-	-	-	2.37*
	GO	21.36 + 1.01	1.45 + 0.50	-	11.00 + 0.29	3.43**
	(GO)R	78.74 + 0.03	0.88 + 0.01	0.02 + 0.00	0.11 + 0.01	0.03**
	(GO)RS	64.93 + 0.07	1.90 + 0.02	0.11 + 0.01	5.37 + 0.01	1.68***
	*Measured by Pyr-IR adsorption [22]
	**Remaining sulfate groups
	***Associated with -SO 3 H groups

Table 2

 2 Activity and reaction rate for A15 and (GO)RS

	Catalyst	Rate	Activity
		(mol of glycerol reacted h -1 )	(mol of glyc-erol reacted h -1 mmol H+ -1 )
	A-15	50.3	170
	(GO)RS	82.4

Table 3

 3 Elemental analysis for

(GO)RS in comparison to (GO) RS-reused (5th cycle) Material Elemental analysis (%) Acidity (mmol H + g -1 ) C H N S (GO)RS-Fresh 64.93 + 0.07 1.90 + 0.02 0.11 + 0.01 5.37 + 0.01 1.676 (GO)RS-reused (5th cycle) 62.80 + 0.07 2.01 + 0.02 0.16 + 0.01 4.21 + 0.01 1.312

Acknowledgements Authors express our acknowledges for the financial support obtained from VRI-UNICAUCA (Project ID 5526). A.S. acknowledges financial support from the European Union (ERDF) and "Région Nouvelle Aquitaine".

Supplementary Information

The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s11244-022-01629-y.

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