Triacetin

The products of glycerol esterification with acetic acid are monoacetins (1-monoacetin, 2-monoacetin), diacetins (1,3-diacetin, 1,2-diacetin) and triacetin (Figure 1.2.2.1.1), which have great industrial applications.

The monoacetin is used as a food additive and in manufacturing explosives and smokeless powder (Nebel et al., 2008), also is valuable in pharmacochemistry and preparation of a specific antidote (Bernasconi, 1969).

The diacetin has been utilized as a cocoa butter blooming agent or as an intermediate in the synthesis of structural lipids (Watanabe et al., 2005), also is used for plasticizercoating and foodstuffs (Sastry et al., 1998; Baur, 1954).

The mixture of monoacetin and diacetin, have applications in cryogenics and biodegradable polyesters (Taguchi et al., 2000), chemical products in the food (Lal et al., 2006) and cosmetic industries (Baumann et al., 1988).

In terms of triacetin, apart from its use as fuel additives for increasing the octane number in gasoline (Delfort et al., 2005), it is reported to function as a cosmetic biocide, plasticizer, and solvent in cosmetic formulations.

It is a commonly used carrier for flavours and fragrances (Ogawa et al., 1992), and was affirmed as a generally recognized, as safe human food ingredient by the Food and Drug Administration.

Figure 1.2.2.1.1 Products of glycerol esterification by acetic acid

The conversion of glycerine to triacetin is a process that exists but that should increase its performance to be able to treat big quantities of glycerine in the most environmental friendly way (Galan et al., 2009).

Triacetin is commonly prepared by esterification of glycerol with acetic anhydride or acetic acid, or by reacting ketene with glycerol, or by the oxidation of allyl acetate in the presence of acetic acid. Crude triacetin typically contains acetic acid, acetic anhydride and smaller quantities of other impurities. Volatile impurities such as acetic anhydride and acetic acid are usually removed by distillation. The remaining triacetin is then usually

distilled to remove nonvolatile impurities and to eliminate color and odor. However, distillation generally requires relatively high temperatures that initiate additional reaction products, and thus even after distillation, triacetin typically has an odor and a yellow color, both of which must be eliminated for the triacetin to qualify as pure triacetin (Khramov et al., 1998).

The most common way of triacetin manufacture is the acid-catalyzed reaction of glycerol with acetic acid (Figure 1.2.2.1.2). However the selectivity to triacetin is normally limited.

The normal boiling points of monoacetin, diacetin and triacetin are BpMA=258 °C, BpDA=259 °C, BpTA=258-260 °C respectively (Dunbar et al., 1956), therefore the separation of triacetin from mono- and diacetins by distillation is practically impossible.

That is the reason why it is required to find a triacetin synthesis having the highest selectivity.

Figure 1.2.2.1.2 Reaction scheme for esterification of glycerol with acetic acid The presence of water influences the equilibrium and weakens the acidity of the used catalyst. This may also effect the consecutive acetylation of the hydroxyl groups (Silva et al., 2010).

In the esterification reactions the homogeneous catalysts are very effective but require additional handling steps. It is imperative to remove catalysts before distillation, therefore, higher production costs have to be expected (Lotero et al., 2005). Appropriate fixed-bed catalysts could be incorporated into a packed-bed, continuous flow reactor, simplifying the product separation and purification, but also reducing the waste formation.

In esterification reactions of carboxylic acids with alcohols it is standard to use homogeneous catalysts like sulfuric acid, methane sulfonic acid, hydrofluoric acid or p-toluene-sulfonic acid. Usually, such homogeneous catalysts have to be removed by additional treatment steps during the process.

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Therefore, heterogeneous catalysts are becoming more and more in the focus of the industry. The literature is containing numerous publications describing heterogeneous acid catalysts for esterification processes.

So for instance, good results were reported using ion-exchange resins like Amberlyst-15 or Nafion in esterification processes (Chen et al., 1999; Heidekum et al., 1999). Generally speaking, the catalytic activity of organic resins is strongly depending on their swelling properties. Resin swelling capacity is fundamental since it controls substrate accessibility to the acid sites and, therefore, affects its overall reactivity. Once swelled, the resin pores usually become macropores. This means that big molecules with long hydrocarbon chains show no diffusion limitations and can readily access the acid sites in the bulk.

There are some studies in the literature searching for catalysts for the reaction of glycerol and acetic acid, for instance Lu and Ma (1991) get 87% triacetin using acidic ion exchange resin and MgSO4 at room temperature for 72 h. Yang and Lu (1996) use SO4

2-/ZrO^2-TiO2, referring the best activity of the catalyst at 450 °C and Wu et al. (2007) get a yield of 93.6% at a reaction temperature of 130 °C with the same catalyst. Hou et al.

(1998) use aminosulfonic acid with a yield higher than 90%, Zhang (1999) determines the optimal conditions for the ratio glycerol/acetic acid/catalyst (SnCl4.5H2O/C) obtaining over 96% yield. Zhang and Yuan (2001) use phosphotungstic acid as the catalyst and with the optimum reaction conditions of mass ratio of catalyst to reactant 3.8%, reaction temperature 135–155 °C and reaction time of 7 h, get up to 84.6%.

For Ding et al. (2003), using H3PW12O40 as catalyst, the product yield and purity reached >98% and >99%, respectively. Dong and Guo (2003) use solid sulfated Fe2O3/TiO2, easily recovered and reused. Melero et al. (2007) obtain the best performances (up to 90% of glycerol conversion) using sulfonic acids catalysts within 4 h reaction time, Liu et al. (2007) use p-toluenesulfonic acid/C reaching a product yield of 92%, Li et al.

(2007) use ionic liquids ([HSO3-pmim][PTSA]) and the optimum conditions were obtained at a glycerol/acetic acid ratio of 1:8, catalyst amount 10.5% the total mass of reactants, reaction time 6 h and reaction temperature 120 °C.

Deng et al. (2001) investigated ionic liquid 1-butylpyridinium chloride–

aluminium(III) chloride as green reaction medium for esterification of glycerol with acetic acid. The yields of total monoacetins and triacetin reached a minimum (17.1%) and maximum (24%), respectively, at ca. 75 °C for ionic liquid as catalyst in case of

glycerol/acetic acid ratio of 1:3. The outstanding advantage is that the resultant esters may not dissolved in the ionic liquid and therefore they could be isolated easily.

Liao et al. (2009) carried out esterification of glycerol with acetic acid over resin and zeolites. After 4 hours operation time under optimal reaction condition (at a temperature of 105 °C and an acetic acid to glycerol molar ratio of 9:1), the selectivity of triacetin reaches almost 100% in 15 min by adding thereto acid anhydride.

Several patents propose various approaches for triacetin synthesis in the presence of catalysts, such as Bremus et al. (1981), Gawrikow et al. (1982), Pechenev et al. (1995), Mitsuya and Ogawa (1996), Mhaskar and Kulkarni (2002).

Some of the strategies to favour the formation of triacetin using the Le Chatelier-Braun principle are: to use a large excess of acetic acid, eliminate the water by reaction with acetic anhydride generating acetic acid or eliminate the water by simple distillation (Galan et al., 2009).

Figure 1.2.2.1.3 Preparation of Gliperol

The pursuit of a biofuel that integrates the glycerol is currently a target of high interest. Gliperol is a relatively new biofuel, which consists of a mixture of three molecules of FAME and a molecule of triacetin (Kijeński et al., 2004). It can be obtained after the transesterification of a mole of TG with three moles of methyl acetate (see Figure 1.2.2.1.3) using lipase as a catalyst (Xu et al., 2003; Xu et al., 2005; Du et al, 2004;

Kijeński et al., 2004; Kijeński et al., 2007).

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Triglyceride Methyl acetate Triacetin Fatty acid methyl esters