**2. Experimental**

### **2.1. Materials**

monoacetin + acetic acid diacetin + water « (2)

diacetin + acetic acid triacetin + water « (3)

Current glycerol esterification processes are carried out using mineral acids. However, these technologies are not environmentally friendly, and much attention has been put on the development of new techniques that use acidic heterogeneous catalysts. Recently, the synthesis of new supported materials containing immobilized sulfonic acid groups, which behave as active and selective catalysts for esterification, has been reported [4-6]. Other reports can also

Sulfated zirconia obtained by the sol-gel method, was evaluated in the esterification of glycerol with acetic acid at 328 K; however, leaching of sulfur occurred during the reaction [7]. Propylsulfonic and fluorosulfonic acid functionalized mesostructured silica (SBA-15) was synthesized and have demonstrated excellent catalytic behavior in the acetylation of glycerol with acetic acid [8]. Sulfonation of carbon-based materials also produced a highly active, and

A great attention has been devoted to the conversion of glycerol into oxygenated additives for liquid fuels. In this context, an industrially relevant route for the conversion of glycerol into oxygenated chemicals is the etherification to tert-butyl ethers. Tert-butyl ethers of glycerol with a high content of di-ethers are considered promising as oxygenated additives for diesel fuels (smoke suppressors and pour point depressants for diesel, biodiesel and their mixtures).

It is found however that mono-tert-butyl ethers of glycerol (MBGEs) have a low solubility in diesel fuel and they are soluble in water. However, if the etherification of glycerol produces mainly di- and tri-ethers, the product is readily blended in the fuel, and other restrictions related to the fuel properties controlled by quality standards can also be met. Thus, when diand tri-tertiary butyl ethers of glycerol are incorporated to standard 30–40% aromaticcontaining diesel fuel, emissions of particulate matter, hydrocarbons, carbon monoxide and

The alkylation of glycerol can be performed with many etherifying agents: isobutylene, tertbutyl alcohol and C4 olefinic petrochemical fractions. Tert-butyl alcohol avoids the need to use solvents to dissolve glycerol; however, water is formed as a by-product that may deactivate the heterogeneous catalysts used. When isobutylene is used, two phases might be present depending on the reaction conditions. The existence of multiple phases may lead to some

Many heterogeneous catalysts have been used in the alkylation of glycerol and reported in the scientific literature: acidic ion-exchange resins (mainly Amberlyst15 and 35), acid form wide pore zeolites (e.g., H-Y and H-Beta), sulfonic mesostructured silicas, sulfonated niobia and

Reported homogeneous catalysts for etherification of glycerol are the p-toluenesulfonic acid and sulfuric acid. Glycerol etherification with tert-butyl alcohol (TBA) is an acid catalyzed

be found that deal with zeolites, poly vinyl sulfonic resins and niobic acid.

stable solid acid catalyst for this reaction [9].

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unregulated aldehydes decrease significantly [10, 11].

problems of mass transfer in the reactor.

pillared clays.

An Amberlite 15W resin was supplied by Rohm & Haas and used as reference acid catalyst in most tests.

CNR 115 activated carbon (AC sample) was supplied by Norit. This activated carbon was sulfonated by three different procedures: (a) Immersion in hot (373 K) concentrated sulfuric acid (H2SO4 98%) for 10 h (ACa sample). (b) Immersion in aqua regia for 20 h at room tem‐ perature followed by rinsing with water until neutral pH and drying. Final sulfonation in hot concentrated sulfuric acid like in (a) (ACb sample). (c) Sulfonation with sulfuric acid and naphthalene. 0.3 g of naphthalene were dissolved in 20 ml of tetrahydrofuran at room temperature and 2 g of AC were immersed in this solution for 1 hour with gently stirring. Then the carbon was filtered and dried at 373 K. Finally the naphthalene doped AC sample was sulfonated with hot sulfuric acid like in (a) (ACc sample).

MWNT (multi-walled nanotubes) were supplied by Arkema (Lot number 6068). This MWNT were also sulfonated by three different procedures: (a) Immersion in hot (373 K) concentrated sulfuric acid (H2SO4 98%) for 10 h (MWNTa sample). (b) Immersion in aqua regia for 20 h at room temperature followed by rinsing with water until neutral pH and drying. Final sulfo‐ nation in hot concentrated sulfuric acid like in (a) (MWNTb sample). (c) Sulfonation in very hot concentrated sulfuric acid (503 K), H2SO4 98%) for 10 h (MWNTc sample).

Molybdophosphoric acid (H3PMo12O40.7H2O) (HPA) was supplied by Merck. HPA/AC was prepared by first treating AC with aqua regia (3 parts of HCl + 1 part HNO3 + 1 part water H2O), then rinsing with water and drying. 0.4 g of HPA were dissolved in 15 ml of water and 0.5 ml of HNO3. AC was added to a solution of 10 ml of water and 0.3 ml of HNO3. Then, the HPA solution was added to the AC solution while stirring gently. The solution was then kept at room temperature under constant stirring for 12 h. Then the carbon was washed repeatedly with hot water (373 K) and finally filtered and dried [12-14].

### **2.2. Characterization of the carbon-based catalysts**

The functionalized carbon materials were characterized by Raman spectroscopy, thermog‐ ravimetry, FT-IR spectroscopy and chemical titration.

Raman spectroscopy analysis of the solid samples were performed at room temperature with a Jobin–Yvon Horiba Labram II micro-Raman system with an excitation laser wavelength of 632 and 514 nm. The incident power was kept well below 3 mW to avoid sample damage or laser-induced heating. For each sample, spectra were acquired at three different spots and averaged, except when large variations were observed.

Thermogravimetric analysis (TGA) was carried out in a N2 atmosphere with a heating rate of 10 K min-1, from 25 ºC to 800 ºC (TA Instruments, Q500 TGA).

The samples were analyzed by infrared spectroscopy using a Varian 3100 FT-IR Spectrom‐ eter. Spectra were acquired by accumulating 100 scans at 4 cm−1 resolution in the range of 400–1200 cm−1.

The titration of the acidic sites was performed using 100 mg of catalyst and a back titration method. The sample was first immersed in 10 cm3 of a 0.1 M NaOH aqueous solution and stirred gently for 1 h. Then the resulting solution was titrated with a 0.1 molar HCl solution.

### **2.3. Catalytic tests**

*Esterification of glycerol:* the reagents were glycerol (99.5% purity, Sigma-Aldrich) and acetic acid (99.7% purity, Sigma-Aldrich). The reaction was carried out in liquid phase 20 at 80 ºC in a stainless steel PTFE lined autoclave. Typically, the mass composition of the reaction mixture was 2.5 g of glycerol, 10 g acetic acid, i.e. 6:1 acetic acid/glycerol molar ratio, and a constant catalyst mass of 0.1 g. Samples of the reacting mixture were analyzed by gas chromatography in a Varian 3900 chromatograph using a CP-SIL 8 CB column (30 m long, 0.25 mm ID, film thickness 0.25 µ) and a flame ionization detector.

*Etherification of glycerol:* the reagents were glycerol (99.5% purity, Sigma-Aldrich) and tert-butyl alcohol (99.7% purity, Sigma-Aldrich). The reaction was carried out in liquid phase in a stainless steel PTFE lined autoclave. The stirring rate was maintained at 1200 min-1 in order to limit the effects of external mass transfer phenomena. Experiments were performed under different reaction conditions, 70-90 ºC reaction temperature, 2-6 tert-butyl alcohol/glycerol ratio and 1-7 h reaction time. The catalyst concentration was constant, 5% with respect to the glycerol mass. The catalysts were dried before each catalytic test. In a typical run, 5 g of glycerol and 0.2 g of the dry catalyst were used. The reaction products were sampled periodically and analyzed off-line in a Shimadzu 2014 gas chromatograph equipped with a flame ionization detector and a capillary column (J&W INNOWax 19091N-213, 30 m length) using acetonitrile as internal standard.
