Food Waste and Catering Waste; Focus on Valorisation of Used Cooking Oil and Recovered Triglycerides - The Economic Utilisation of Food Co-Products - page 122

OH

OAc OAc

OAc

O

T = 120 °C, 4 h

OH

+n

OAc OAc

+ OH

+

R OH

Cata. STARBON-Acids/
Sulphated Zirconia

OH

OAc OH

OH

d n 9 r 3 n g | 0

O

ACETIC
ACID

GLYCEROL ACETINS MIXTURE

Ac =

R

n = 1-3

R = C2, 15-C17

d y y f n n 3 .

Scheme 6.15 General scheme for the acetylation of glycerine.

Acetylated mixtures (Scheme 6.15) require little or no purification. Mixed
esters (acetic acid/long-chain free fatty acids) are also relevant because they
may have improved properties from the parent glycerine in terms of hydro-
phobicity, or plasticizing ability. 172 Our research interest has focused on the use
of heterogeneous catalysts for the direct production of acetins with either acetic
acid or acetic anhydride.

6.6.8.1 Selectivity and Mechanism
A number of heterogeneous catalysts have been used in esterifications of
glycerol with different lengths of fatty acids, with alkaline oxides and
hydroxides (MgO, Ca(OH) 2 , NaOH), 173 mineral (sulfuric acid), zeolites,
heteropolyacids, zirconias, organomodified silicas and carbonaceous materials
as main examples of catalysts. The influences on conversion and selectivity are
variable; the most relevant, however, involves the relative anity of the
substrate to the catalysts and the influence of temperature and high molar ratio
acid/glycerol and temperature on the extent of reaction and increasing degree
of substitution. This is shown in the acetylation of glycerol with hybrid orga-
nosulfonic acid silicas and mesoporous carbonaceous materials through
microwave activation. 174 In the first paper, selectivity patterns were described,
successfully developing a multidimensional model to optimise conditions for
the production of mixtures of acetylated glycerol.
There are also good examples of selective acetylation of glycerol in the
literature, using heteropolyacids. 175 Other examples of selective processes to
triacetin have been reported either using acetic anhydride, or in a combined
esterification/acetylation process. 176 The mechanisms for the esterification have
been found to vary according to conditions. Scheme 6.16 shows the mechanism
proposed by Jacobs and coworkers, when modified silicas are employed to
selectively produce monoglycerides. 177
Similar shape-dependent processes have been reported by Corma et al., 178
Je´ roˆ me et al. 179 and others 180 by using hybrid organo-modified silicas and
hydrotalcites amongst other heterogeneous catalysts, and bare fatty acids or
methyl esters as acylating agents. For these types of catalyst and conditions, the