**1. Introduction**

The last years, a dramatic increase in the installed capacity for biodiesel fuel has taken place. This is a technically mature biofuel replacement of petrodiesel, with improved properties of cetane number, lubricity, biodegradability and flash point. This increasing biodiesel production has resulted in an excess production of the glycerol by-product (stoichiometrically 10 wt% of the product of a biodiesel plant). The transformation of bioglycerol into glycerol-ethers and glycerol-esters via etherification and esterification reactions is considered to be a convenient alternative for glycerol utilization. These value-added chemicals have potential uses in many industrial applications. Particularly acetylation of bio-glycerol with acetic acid into glycerol-esters can produce di- and triacetin that have potential for vast quantity utilization as valuable biodiesel and petro fuel additives. In the case of the addition to biodiesel, the in-factory utilization of the product is quite advanta‐ geous. In addition, di- and triacetin are used as fuel additives for viscosity reduction. Triacetin meets the specifications of flash point (>374 K) and oxidation stability (6 h at 383 K) required by the standards EN 14214 and ASTM D6751 [1-3].

The esterification of glycerol with acetic acid produces mono-, di- and tri-acetates of glycerol. The mono- and di-acetates are known as monoacetin (2-monoacetyl-1, 3-propanediol or 3 monoacetyl-1, 2-propanediol, MAG) and diacetin (1, 2-diacetyl-3-propanol or 1, 3-diacetylpropanol, DAG). The scheme of reaction is depicted below:

glycerol + acetic acid monoacetin + water « (1)

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$$\text{\textbullet monoacetic} + \text{acetic acid} \leftrightarrow \text{diacetic} + \text{water} \tag{2}$$

$$\text{diameter} \newline + \text{acetic acid} \newline \leftrightarrow \text{triacetic} \newline + \text{water} \newline \tag{3}$$

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

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 stable solid acid catalyst for this reaction [9].

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 unregulated aldehydes decrease significantly [10, 11].

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 problems of mass transfer in the reactor.

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 pillared clays.

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 reaction, resulting in a mixture of mono-tert-butyl-glycerol (MTBG), di-tert-butyl-glycerol (DTBG) and tri-tert-butyl-glycerol (TTBG). Some unwanted by-products can also be formed that are mainly a result of polymerization reactions.
