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Review Article
Oligomerization of glycerol a critical review
Andreas Martin and Manfred Richter
Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock,
Rostock, Germany
The oligomerization of glycerol to preferentially di- and
triglycerol is reviewed, with primary focus on the
use of heterogeneous acidic and basic catalysts. Low
molecular-weight oligomers have found a wide field
of applications in cosmetics, food industry and polymer
production. The growing market intensified
research work on the selective catalytic oligomerization of
glycerol. Performing the reaction of glycerol in
the presence of microporous and mesoporous solid catalysts aims
at exerting shape-selective effects on
the reaction, suppressing the abundant formation of cyclic
isomers and cutting further polymerization of
the target products. Enhanced selectivity to diglycerol is
observed over some type of catalysts, but the
solids suffer from leaching of active alkaline cations from the
solid, severe deterioration of crystallinity of
zeolites and even dissolution of the solids in the hot glycerol
during batch reaction at temperatures in the
range of 2402608C. In those cases it is difficult to separate
homogeneous and heterogenous reactionroutes, and the
shape-selective effects are levelled off. The oligomerization is a
consecutive reaction, and
complete conversion of glycerol favours formation of
highmolecular-weight glycerol oligo- and polymers.
To achieve maximum yield of diglycerol, the reaction has to be
interrupted at glycerol conversions of
ca. 50%. Alternative reaction engineering is required to
overcome the inherent disadvantages of a
batch reaction. Examples will be given for a selective glycerol
oligomerization under reduced
pressure in a so-called fall-film reactor using super-acidic
polymers as catalysts.
Keywords: Catalyst stability / Diglycerol / Glycerol
oligomerization / Heterogeneous catalysts / Shape selectivity /
Reaction mechanism
Received: June 16, 2010 / Revised: August 16, 2010 / Accepted:
August 26, 2010
DOI: 10.1002/ejlt.201000386
1 Introduction
Glycerol (propane-1,2,3-triol, C3H8O3) occurs as backbone
in triglycerides which are the main constituents of all veg-
etable and animal fats and oils [13]. Processing of fats and
oils into soap has been the chief source of glycerol until
the
midst of the 20th century [3]. During World War I microbial
fermentation was used commercially for glycerol production
[4]. The first synthetic glycerol from petroleum feedstock,
using propylene and chlorine, was produced in 1943 by I.G.
Farben in Oppau andHeydebreck (Germany) and in 1948 by
Shell in Houston, Texas (USA.). This method became avail-
able once the high-temperature chlorination of propene to
allyl chloride could be controlled properly. The allyl
chloride
produced is oxidized with hypochlorite to dichlorohydrin,
which is converted without isolation to epichlorohydrin by
ring closure with calcium or sodium hydroxide. Hydrolysis to
glycerol is carried out with sodium hydroxide or sodium
carbonate [5].
As the manufacture of biodiesel fuel by transesterification
of seed oils with methanol appeared as an alternative to
preserve the oil resources, a large surplus of glycerol was
created as by-product. The use of glycerol as a raw
material,
even for the production of epichlorohydrin itself has become
attractive [6]. Biodiesel is obtained by transesterification
of
the triglycerides found in vegetable oils and animal fats
with
an excess of a primary alcohol (most commonly methanol) in
the presence of a homogeneous or heterogeneous catalyst.
Glycerol is coproduced in this process [7]. Production of
one
ton of biodiesel accumulates 100 kg of crude glycerol. In
its
raw state crude glycerol has a high salt and free fatty acid
content and a substantial colour (yellow to dark brown).
Consequently, crude glycerol has few direct uses, and its
fuel
value is also marginal. An economic solution for the purifi-
cation of crude glycerol streams combines electrodialysis
and
nanofiltration, affording a colourless liquid with low salt
content, equivalent to technical grade purity [4].
Monographs devoted to the glycerol issue were published
Correspondence: Dr. habil Andreas Martin,
Forschungsbereichsleiter,
Heterogen-katalytische Verfahren, Leibniz-Institut fur Katalyse
e.V.,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
E-mail: [email protected]
Fax: 0381 1281 [email protected]
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in 1953 [8] and 1991 [2]. An excellent overview on proper-
ties, production and traditional commercial applications of
glycerol can be found in ref. 3.
Glycerol in its pure chemical form is a versatile base
chemical that has been used for several decades to manufac-
ture a variety of products [2, 3, 8]. The market for
glycerol
and distribution over the variety of product classes is given
in
Fig. 1.
Further application fields are the admixture of glycerol to
animal food, where it reduces the emission of dust and keeps
the food in a moist state. It has some beneficial influence
on
the taste and promotes the food intake.
Besides this, glycerol is a green feedstock because of its
bioavailability and is today a base chemical for production
of
plenty of value-added products in chemical industries, where
a large portfolio of reaction types is applicable including
selective oxidation, selective hydrogenolysis to propylene
glycol, dehydration to acrolein, pyrolysis and gasification,
reforming to syngas, selective transesterification,
etherifica-
tion to fuel-oxygenates, fermentation to propane-1,3-diol,
oligomerization/polymerization, conversion of glycerol into
glycerol carbonate and synthesis of epichlorohydrin [3, 9
13]. Accompanied by the intense research on glycerol as it
becomes ubiquitary available with the biodiesel boom the
focus stretched also on the selective production of
oligomers
or polymers by reacting glycerol with itself.
2 Diglycerol basics
Whereas the different reaction classes of glycerol are
treated
in several excellent reviews and books [3, 10, 1113] no
exclusive review is assigned to the catalytic
oligomerization
of glycerol to diglycerol and triglycerol, although a
concise
chapter on the application of solid catalyst is contained in
Ref. 12, 13. The present review will focus exclusively on
the
reaction of glycerol to oligomers, where themain interest is
on
the evaluation of catalytic results, obtained preferentially
by
application of solid catalysts. Lab-scale stoichiometric
syn-
theses of di- and triglycerol will briefly be summarized.
The trivial names glycerol, di-, tri-, tetraglycerol etc.
are
used internationally and will also be used throughout this
article. Furthermore, it is common to denote a mixture con-
sisting of components with different glycerol condensation
degrees as polyglycerol. The reaction category leading from
glycerol to polyglycerol is not uniquely applied. In several
articles in the past, see e.g. [1417], the conversion of
glycerol
to oligomers is referred to as etherification, but, among
the
numerous conversion ways of glycerol, the reaction between
glycerol and isobutene or tertiary alcohols is most often
understood as etherification, and, to avoid confusion, the
reaction pathways of glycerol with itself to form oligomers
and polymers is designated as oligomerization or polymeri-
zation throughout this review. Often, oligomers with 24
glycerol units are viewed as polyglycerols without a strict
differentiation where the oligomers end and the polyglycerol
begins, and bearing the inherent possibility of confusion
with
the high-molecular weight branched polyglycerol produced
by anionic polymerization (see next paragraph).
Selectivity referring to diglycerol and triglycerol is a
fur-
ther term often not understood in a strict sense, simply
expressing the distribution of the isomers (in wt% or
mol%) in the liquid product. Actually, the selectivity of
diglycerol (in %) is given by Eq. (1) [18],
S nDGn0GnG
nGnDG
100 (1)
where nDG means the moles of linear diglycerol, n0G and nG
the moles of glycerol at the beginning of the reaction and
at
reaction time t, respectively, with nG and nDG as stoichio-
metric coefficients of the reaction according to Eq. (2).
2C3H8O3 ! C6H14O5 H2O (2)
Selectivity values for triglycerol has correspondingly
to take into account the stoichiometry that from 1 mol of
glycerol 1/3 mol of triglycerol can be formed at maximum,
equivalent to a selectivity of 100%.
The general structural formula for oligoglycerol can be
sketched as
CH2OHCHOHCH2OCH2CHOHCH2OnCH2CHOHCH2OH;
where n 0 results in diglycerol, n 1 in triglycerol, n 2in
tetraglycerol etc. [19], including branched isomers formed
by reaction of secondary hydroxyls [11].
Physical data of oligomers up to n 2 and of a commer-cial
oligomeric product, polyglycerol-3 (Solvay Chemicals),
are summarized in Table 1 [1922]. The oligomer compo-
sition of polyglycerol-3 grade consists of ca. 29% of digyl-
cerol, 42% of triglycerol, 18% of tetraglycerol; the
remainder
comprises penta- to nonaglycerol [21, 22].
With increase of molecular weight the hydroxyl number
decreases (diglycerol has 4 hydroxyls, triglycerol 5,
Figure 1. Market for glycerol (volumes in per cent), and
industrial
uses [3].
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tetraglycerol 6 etc.). This changes the polarity of
oligomers,
e.g., low oligomers are more hydrophilic than higher ones
and they have a better solubility in polar solvents like
water. The viscosity increases with higher degree of
oligome-
rization, often accompanied by colour changes from water-
clear (glycerol) to dark yellow. Presumably, colouration
occurs due to dehydration side reactions. From glycidol
(2,3-Epoxy-1-propanol, C3H6O2) it is possible to produce
colour-free oligomeric products.
From physical data available in ref. 19 it is clear that
oligomers with n 1 are syrup-like high-boiling liquids
(tri-glycerol) or crystalline solids (tetraglycerol) [19, 20].
Three configurational isomers of linear diglycerol,
C6H14O5, are known so that the conversion of glycerol to
linear diglycerol can be formulated as given in Fig. 2.
Besides linear diglycerol, cyclic dimers, C6H12O4, can be
formed by further condensation [23, 24], or even acyclic
side
products like ketones, aldehydes or diols of the same mol-
ecular composition C6H12O4 as identified by Medeiros et al.
[25] who performed the oligomerization of glycerol
with H2SO4 in homogeneous phase (Fig. 3).
The actual product class of polyglycerol represents a
polymer with molecular weights between 1000 and
30 000 g/mol with own specific fields of application.
Polyglycerol is a highly branched polyol and is specifically
produced by either anionic [26] or cationic [27] polymeri-
zation of glycidol. A scheme of the synthesis route of
hyperbranched polyglycerols by anionic ring-opening multi-
branching polymerization is shown in Fig. 4 [28, 29].
Hyperbranched polyglycerol possesses an inert polyether
scaffold. Each branch ends in a hydroxyl function, which
renders hyperbranched polyglycerol a highly functional
material. This high functionality, in combination with the
reactivity of the hydroxyls, forms the basis for a variety
of
derivatives. Partial esterification of polyglycerol with
fatty
acids yields amphiphilic materials which behave as nanocap-
sules [28]. Such nanocapsules can, for example, incorporate
polar molecules as guests and solubilize them within a non-
polar environment.
Table 1. Physical data of diglycerol and oligomers [1922]
Name
Molecular formula/
weight (g/mol)
Refractivity
n20D (- )
Density
(g/cm3)
Boiling point
(8C)/(Pa)Hydroxyl numbera)
(mg KOH/g)
Glycerol C3H8O3 1.4720 1.2560 290 1830
92
Diglycerol C6H14O5 1.4897 1.2790 205/133 1352
166
Triglycerol C9H20O7 1.4901 1.2646 >250 /13.3 1169
240 (408C) (408C)Tetraglycerol C12H26O9 1.4940 1.2687 6973
(melting point) 1071
314 (408C) (408C)Polyglycerol-3b) 1.4910 1.2840
a) The hydroxyl number is defined as the mg of KOH equivalent to
the hydroxyl content of 1 g of sample. The method suggested in
DIN
53240-2 is based on the catalyzed acetylation of the hydroxyl
group. After hydrolysis of the intermediate, the remaining acetic
acid is titrated in
a non-aqueous medium with alcoholic KOH solution.b) Product
offered by Solvay Chemicals, see e.g. [21, 22].
Figure 2. Glycerol transformation to linear diglycerol.
Figure 3. Cyclic diglycerol components [23, 24], and acyclic
side-
products from acid-catalyzed glycerol oligomerization in
homoge-
neous phase [25], all with total formula C6H12O4.
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2.1 Application and uses
Diglycerol finds wide application in mainly the following
three areas: (i) cosmetics, (ii) food industry and (iii) in
poly-
mer and plastics industry.
In cosmetics, diglycerol is ingredient in personal care
formulations. It enhances fragrance and flavour impact and
longevity in products such as toothpastes, mouthwashes
and deodorant sticks [22, 28]. The rate of menthol evapor-
ation is reduced when dissolved in diglycerol instead of
glyc-
erol [22]. With the refractive index higher than glycerol,
diglycerol has additional benefits in the formulation of
clear
gels. Transparent emulsions are obtained when the aqueous
and oily phases have the same refractive indices. The use of
higher refractive index ingredients, such as diglycerol, in
the
aqueous phase allows the evaporation of more water, leading
to a reduction in cost. It also results in products with
better
optical clarity [26].
The food industry uses polyglycerol polyricinoleates as
emulsifiers in chocolate products. Diglycerol is also used
in
the production of fatty acid ester emulsifiers, and is part
of
food additives. Often, diglycerol is further processed to
derivatives. The abundant reaction is the esterification of
diglycerol with fatty acids to mono-, di-, tri- and
tetraester.
This reaction can be accomplished with or without basic
catalysts and leads in general to amixture of ester
compounds
[19]. These ester exhibit lipophilic/hydrophilic properties
and are favourably included as emulsifier in the food
industry,
specifically in bakery products and oleomargarine [19].
Current data on the market of glycerol oligomers or
polymers are hardly available. One estimation for
application
of polyglycerol in food industry 2005 is given in ref. 30.
Nearly 59% are allotted to bakery products, 29% to confec-
tionery, 5% to oleomargarine, 1.5% to chocolate, 1.5% to ice
cream and 4% to others.
In the polymer industry diglycerol is included in the
production of plasticizer in polyvinyl alcohol films or
starch-based biodegradable thermoplastic compositions,
and is used in the manufacture of polyurethanes and
polyesters.
3 Diglycerol synthesis
3.1 Laboratory-scale routes
For laboratory-scale production of pure diglycerol direct
syntheses routes were described by Wittcoff et al. [31, 32],
Behrens and Mieth [20], and Jakobson [19]. Figure 5 sum-
marizes the possibilities.
Favourably, diallyl ether 4 is used as primary reactant.
Diallyl ether is accessible by reaction between allyl chloride
2
and allyl alcohol 3 in inert solvents under HCl release.
Direct
hydroxylation of 4 can be performed with peroxyformic acid,
CH2O3, or permanganate, at 408C under safety precautionsfor 4.5
h, but plenty of additional steps for neutralization,
filtration, derivatization and fractional distillation are
necess-
ary for isolation of the diglycerol.
Addition of hypochlorous acid to 4 yields dichlorohydrin
ether 5 that is converted with NaOH to diglycidyl ether
6, C6H10O3, (2-Epoxypropyl ether) in the presence of pow-
dered NaOH in anhydrous ether at 25308C. The reactionhas to be
continued under reflux conditions for 4 h. Isolation
of 6 comprised several steps of filtration, washing with
ether
Figure 4. Synthesis of hyperbranched polyglycerols by anionic
ring-opening multibranching polymerization. Anionic polymerization
of
glycidol to polyglycerol (from [29], with permission of the
American Chemical Society).
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and fractional distillation. Hydrolysis of 6 is performed
with
diluted sulphuric acid under reflux for 4.5 h. Isolation of
diglycerol 1 required neutralization with e.g. barium
hydrox-
ide solution, centrifugation to separate the solid, digestion
of
the product in absolute ethanol and fractional distillation
under reduced pressure. Further routes for manufacturing
of diglycerol proceed via glycerol derivatives [20, 30],
using
e.g. isopropylidene rests as protecting groups.With acetone
7,
glycerol 8 is transformed into 1,2-0-Isopropylidene glycerol
9
in a first step. The water formed during reaction has to be
withdrawn by addition of a water-distracting agent like
Na2SO4 to get conversion levels between 60 and 90%.
Further conversion of 9 with epichlorohydrin 10 in the pres-
ence of sodium/dioxane yields the glycidyl ether 11 of 9.
The
latter can directly be transformed into pure diglycerol by
splitting off the acetone protecting moiety by treating with
mineral acid. The epichlorohydrin route will be discussed in
Section 3.3.
All described possibilities so far had the disadvantage that
either the starting substances are difficult to get or the
syn-
thesis requires several intermediate steps, and the
conversion
produces great amounts of salts as by-products.
3.2 Thermal conversion of glycerol
In general, the thermal reaction is performed at a certain
temperature under inert protecting atmosphere. For a selec-
tive reaction, the glycerol should contain less than 1% of
water and should not contain organic impurities. Often,
before the use of the oligomeric products for further
reactions, a distillation is required to separate
nonconverted
glycerol. Reaction temperature, basicity and organic impur-
ities have paramount influence on the glycerol oligomeriza-
tion [20]. The temperature window at normal pressure is
limited. A purely thermal conversion without addition of a
catalyst sets in above 2008C; at a temperature of 2908C
dark,strongly smelling products are formed. At low temperature
(1808C) and in the presence of alkaline a minute formation
ofdiglycerol from glycerol is observed, but at a low conversion
degree of glycerol. Care must be taken during the reaction
to
exclude air from the system. Traces of oxygen form acrolein
and other condensation products which darken the final
product [33].
3.3 Industrial processes for converting glycerol todiglycerol
and oligomers
Industrially, the epichlorohydrin route [19] is applied. It
is
assumed that during basic hydrolysis of epichlorohydrin 10
(cf. Fig. 5) by NaOH intermediary glycidol 11 is formed
besides glycerol 8, and glycidol 11 reacts with nonconverted
10 or 8 to diglycerol 1. Further separation and
purifications
steps are necessary. The residual glycerol has to be
separated,
then water has to be removed from the raw diglycerol, and,
finally, the product has to be subjected to a fine
distillation.
The specification is given as 90% with some residual
glycerol
and triglycerol [21].
The reactions of glycidol or epichlorohydrin with
glycerol have in common that the coupling of OH groups
is not confined to the terminal positions but the middle OH
Figure 5. Laboratory-scale syntheses routes of diglycerol [19,
20, 3032].
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groups of glycerol can be involved as well. This leads to
the
formation of a,b- and b,b -diglycerol, besides a,a -digly-
cerol (cf. Fig. 2).
4 Liquid product analysis, separation
Separation of oligoglycerol mixtures into their components
is
difficult by direct distillation. But, it was recognized as
early
as 1947 by Wittcoff [31] that diisopropylidene diglycerol
could readily be removed by fractional distillation from an
acetonated polyglycerol mixture.
The diisopropylidene derivatives formed by reaction of
the oligoglycerol mixtures with acetone are colourless
liquids,
and the boiling points are lower by 1008C than those of thefree
oligomers. Thus, the derivatives can comfortably be
distilled under vacuum at ca. 1 mbar. Table 2 summarizes
structures of diisopropylidene derivatives of glycerol
oligomers, boiling points (B.p.), refractivity (n20D ) and
theoretical element composition (Comp.).
Recently, Deutsch et al. [34] could show a similar way to
separate the configurational diglycerol isomers via
acetaliza-
tion. This has been accomplished by reacting a diglycerol
mixture of initial distribution of a,a -diglycerol:
a,b-digly-
cerol: b,b -diglycerol 69: 27: 4 with a superacidic
polymercatalyst (Amberlyst-36) under reflux conditions using
meth-
ylene chloride as solvent to remove water as azeotropic mix-
ture from the batch diglycerol. After 3 h reaction time, 88%
of the a,a -diglyceroldiketal was found and 12% of a,b-
diglyceroldiketal, whereas no b,b -diglyceroldiketal was
formed (Fig. 6).
The observed preferential formation of a,a -diglycerol-
diketal as well as missing b,b -diglyceroldiketal,
specifically
at short reaction times, point to a more rapid reaction of
acetone with OH groups in 1,2 positions than with OH
Table 2. Data of diisopropylidene derivatives of glycerol
oligomers [19].
Name Structure B.p. (8C)/(Pa) nD20 (-) Comp. (%)
Diisopropylidene diglycerol, C12H22O5 88/40 1.4404 C 58.52
H 9.00
O 32.48
Diisopropylidene triglycerol, C15H28O7 157158 /67 1.4561 C
56.23
H 8.01
O 34.31
Diisopropylidene tetraglycerol, C18H34O9 220225 /67 1.4643 C
54.82
H 8.69
O 36.50
Figure 6. Separation of configurational linear diglycerol
isomers through acetalization under super-acidic conditions
[34].
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groups in 1,3 position (as in b,b -diglycerol). In other
words,
formation of five-ring ketals is sterically and kinetically
favoured in comparison to formation of six-ring ketals.
This is in accordance with earlier investigations on the
ketal-
ization of glycerol with acetone, where exclusively the
five-
ring ketal was obtained [35]. Therefore, the different rates
of
ketalization of configurational linear diglycerol isomers
allow
an enrichment of a,a -diglycerol in mixtures of the config-
urational linear diglycerol isomers. A vacuum distillation
of
the ketal mixture at 10 mbar using a Vigreux column yielded
a further enrichment of a,a -diglycerol up to 93%. To date,
no advantage in using a,a -diglycerol instead of a mixture
of
the three linear isomers is known, but for preparative pur-
poses, the described approach is a viable alternative to
other
preparative techniques.
For gaschromatographic analysis, silylation of the glycerol
oligomers is recommended. The method is described e.g. by
Sweeley et al. [36]. Weighed amounts of the liquid sample
were mixed with carefully dried pyridine until dissolution
is
complete, and then hexamethyldisilazane and trimethylchlor-
osilane are added. The mixture is heated to 708C for 1 h
tocomplete silylation. A separation of the oligomers is
possible
under GC conditions as described e.g. in ref. 37.
5 Recent developments in catalyzed glycerololigomerization
5.1 Glycerol oligomerization using acidichomogeneous
catalysts
A process for preparing oligoglycerol by heating glycerol
under reduced pressure (ca. 7 mbar) in the presence of sul-
phuric acid (ca. 0.3 wt%) and with addition of glycerol
acetate, C5H10O4, as promoter (ca. 7 wt%) is claimed in
ref. 38. The reaction was terminated by addition of NaOH
when the refractivity reached the characteristic value for
diglycerol. At this stage, 54.8% of the glycerol was found
to be converted. The composition of the reactionmixture was
given to consist of 45.2% glycerol, 29.1% linear diglycerol,
2.5% cyclic diglycerol, 12% linear triglycerol, 1.6% cyclic
triglycerol, 6% linear tetraglycerol, 2.2% linear
pentaglycerol
and 1.4% hexaglycerol.
A similar mixture comprising 50% residual glycerol,
33% diglycerol, 11% triglycerol and 4% tetraglycerol was
received performing the reaction under reduced pressure
at 1508C in the presence of dodecyl benzenesulphonic
acid[39].
Recently,Medeiros et al. [25] reported results on the acid-
catalyzed oligomerization of glycerol in homogeneous phase
using H2SO4 as catalyst. The reaction was performed at
2808C. Results showed that more than 90% of the glycerolhas been
consumed after 2 h reaction, but the product mix-
ture consists of mainly tri- and tetraglycerol. The
diglycerol
concentration went through amaximum between 1 and 1.5 h
reaction time but accounted for only ca. 15% of the mixture
at a glycerol conversion of 92.5%. The sum of products
including tri- and tetraglycerol amounted to approximately
20% only. This means that the remainder of 80% are non-
identified other products. This confirms the experience,
that
the homogeneous, acid-catalyzed reaction is fast but not
selective.
The disadvantage of acid-catalyzed glycerol conversion is
not primarily the missing selectivity but mainly the occur-
rence of secondary reactions (dehydration, oxidation) that
deteriorates the product quality by colouration.
The acid-catalyzed reaction mechanism is proposed to
proceed according to SN1 type organic reactions, starting
with protonation of one of the glycerol OH groups (Fig. 7)
[23, 40].
According to SN1, a carbocation should be formed by
splitting off water, followed by the nucleophilic attack of
a
hydroxyl group of another glycerol molecule. Finally, the
formed ether is deprotonated, yielding the respective digly-
cerol. This reaction can also obey a SN2 pathway through
a direct nucleophilic attack of the protonated glycerol by a
second one [17].
5.2 Glycerol oligomerization using acidicheterogeneous
catalysts
It is well known (see e.g. [4143]) that zeolites in their
protonic form exhibit acid strengths comparable to strong
mineral acids due to Brnsted acid sites formed by bridging
OH groups between tetrahedrally coordinated Al and Si in
crystalline aluminosilicates. An overview on the acidity of
different zeolites can be found in ref. 44.
Eshuis et al. [45] claimed a complete conversion of
glycerol at 2008C when adding acidic zeolites. In one
Figure 7. SN1 type mechanism of glycerol oligomerization in
acid-catalyzed homogeneous reaction [23, 40].
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example, the complete conversion of 200 g glycerol has
been achieved over 20 g of zeolite Beta (Si/Al molar ratio
50, Na2O content 0.1 wt%, crystal size 0.10.7 mm, surface
area 750 m2/g) at 2008C within 2 h. The yield was 120 g
ofpolymers where 30% consisted of linear diglycerol, C6H14O5,
(cf. Fig. 2) 30% of cyclic diglycerol, C6H12O4, (cf. Fig.
3),
30% of cyclic triglycerol isomers C9H18O6, and 10% of
higher oligomers.
Another example using acidic zeolite Beta (Si/Al 12)was given by
Cottin et al. [46]. Glycerol (50 g) was converted
in the presence of 4 wt% zeolite Beta at 2608C. A
glycerolconversion of 70% was achieved after 7 h, with a
diglycerol
selectivity of ca. 40%, and a triglycerol selectivity of ca.
30%.
Experimental results of Krisnandi (Y.K. Krisnandi, un-
published results) using acidic zeolite Beta (Si/Al 12.5)
areshown in Fig. 8.
It can be seen that glycerol is converted to ca. 80% after
12 h reaction at 2458C, but the maximum percentage oflinear
diglycerol amounts to only 20%, whereas higher
oligomers represent 50% of the mixture.
Figure 9 summarizes reported selectivity of glycerol
dimers (not distinguished between linear and cyclic) in
dependence on glycerol conversion for acidic Beta zeolite
catalysts.
Other examples are not well documented, as e.g. in Cottin
et al. [46] where glycerol conversions of 7 and 28% are
reported at 2608C using Y zeolites with Si/Al molar ratiosof 2.7
and 15, respectively, without specifying the reaction
time. Nevertheless, the higher conversion of glycerol on
zeo-
lite Y with lower concentration of Brnsted acid sites
(equiv-
alent to the higher Si/Al molar ratio of 15) is attributed
to
abundant formation of acrolein. This is caused by a higher
acid strength of the Brnsted acid sites. It is reported for
several zeolitic materials that diminution of acid site
concentration provokes an enhanced acid strength of the
residual Brnsted acid sites [43, 47]. Another reason is that
the solid becomes more hydrophobic with ongoing dealumi-
nation, and this might influence the interaction of glycerol
with the solid surface [46].
The same authors [46] included a mesoporous MCM-41
material with Si/Al 40 and studied the reaction of glycerolat
2208C. The MSM family where MCM-41 belongs to hasmesopores with
openings larger than 2 nm and should offer
good accessibility of the pore system by glycerol. Direct
comparison of performances cannot be made due to the
different reaction temperatures applied. The conversion of
glycerol over MCM-41 was lower due to the lower reaction
temperature. Comparing selectivity to diglycerol and trigly-
cerol at 50% glycerol conversion for both, zeolite Beta and
MCM-41, the diglycerol selectivity is better by ca. 10%
points for MCM-41, whereas no difference can be observed
for triglycerol selectivity.
Kraft [48] claimed the use of saponite catalysts for pre-
paring oligomers of glycerol. Saponite is amonoclinic
mineral
of the montmorillonite group having the general formula
Mg3[(OH)2(Si,Al)4O10] (Ca,Na)x(H2O)y. The Mg sapon-ite clay
catalyst was made acidic by ion exchange with
ammonium ions and calcination. Oligomerization in a batch
process at 2508Cusing 1.38 kg of glycerol and 2.5 wt% of
thecatalyst resulted in a glycerol conversion of 24% after 24
h,
with a composition of the mixture consisting of residual 76%
of glycerol, 17% of linear diglycerol and 7% of other
oligom-
ers where the latter comprised linear, branched and cyclic
components. The selectivity of the linear diglycerol at the
low
conversion degree of glycerol is as high as 78.5%. At 70% of
Figure 8. Reaction of glycerol on acidic zeolite Beta (Si/Al
12.5)in a stirred batchmode at 2458Cunder atmospheric pressure, 2
wt%of catalyst. Figure 9. Summary of glycerol conversion using
acidic zeolite Beta.
() H(Na)-Beta, Na/Al (w/w) 0.04, contains residual Na [37], (~)
H-Beta [37], T 2608C, 2 wt% catalyst, (*) H-Beta (Si/Al 12) [46],T
2608C, (~) H-Beta (Si/Al 50) [45], 20 wt% catalyst,T 2208C.
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glycerol conversion (achieved after 78 h reaction time), the
selectivity to linear diglycerol was still ca. 40%. This is
com-
parable with the diglycerol selectivity achievable with
zeolite
Beta at a glycerol conversion of 70% (cf. Fig. 9).
Super-acidic organic polymers represented another
catalyst class applied for glycerol oligomerization.
Preliminary results were given by Cottin et al. [46] who
used
Amberlyst 16. Amberlyst 16 is a large pore (average diameter
25 nm), sulphonic, acid-modified ion exchange resin, devel-
oped particularly for heterogeneous catalysis. It has,
however,
limited thermal stability. The maximum operating tempera-
ture recommended by the producer should be at 1308C.Datain ref.
46 were received at 2402608Cunder normal pressure,where thermal
degradation of the catalyst polymer can be
assumed to occur.
In order to accomplish conversion of glycerol over super-
acidic catalysts at low temperature, the operation pressure
has
to be reduced. This has been done in Ref. 23, 49 using
another type of ionomer with higher thermal stability, viz.
Nafion1.
Nafion1 is a perfluorinated ion exchange polymer with
sulphonic acid groups [50], possessing a molecular weight of
ca. 1070 g/mol. The acid strength of Nafion1 polymers has
a value of 12 on the Hammett acidity scale [51]. This is
equivalent to the strength of 100% sulphuric acid.
Nafion1 is commercially available in different forms [52,
53]. For the experiments, Nafion NM-112 foil was taken
andmounted on a supporting mesh wire. This wire was rolled
together to a multilayer tube-like cylinder and introduced
into the glass reaction tube for connecting it via a short
Vigreux column to the glycerol batch. Evacuation of the
reactor unit was accomplished by an oil pump near the top
of the unit where back condensation of glycerol vapour was
enforced by a cold-water dephlegmator finger, whereas water
condensation took place outside the unit in a cold trap on
the
way to the connected oil pump. A sketch of the unit is given
in
Fig. 10.
The foil was pre-swollen in glycerol and dried before
installation. The amount of catalyst was in the range of
0.55.0% of the starting amount of glycerol [34].
Experimental results obtained in the reactor setup
confirm (Fig. 11) that a high percentage of linear
diglycerol
(ca. 85%) can bemaintained at nearly complete conversion of
glycerol (>90%), whereas in batch reactor mode with per-
manent contact between catalyst and glycerol/liquid product
the diglycerol content permanently decreases with higher
glycerol conversion, reaching a value of 35 at 85%
conversion.
5.3 Glycerol oligomerization using basichomogeneous
catalysts
In the literature, hydrogen carbonates of alkali metals are
viewed as homogeneous catalysts because of their facile sol-
ubility in glycerol. Several bases have been tested as
catalysts
including hydroxides, carbonates and oxides of several
metals
(see e.g. ref. 33). The following order of activity has been
achieved using 2.5 mol% of the solid and performing the
reaction at 2608C for 4 h: K2CO3 > Li2CO3 > Na2CO3> KOH
> NaOH > CH3ONa > Ca(OH)2 > LiOH >
MgCO3 > MgO > CaO > CaCO3 ZnO.Although hydroxides are
stronger bases than
carbonates, K2CO3 was found to be a better catalyst than
KOH. From the solubility measurements and observations,
it was noticed that K2CO3 had a better solubility in
glycerol
and in the polymeric product than KOH at elevated
temperatures.
Figure 10. The catalytic fall-film reactor setup (adapted from
[23]).
Figure 11. Percentage of linear diglycerol dimerswithin the
product
mixture vs. glycerol conversion on catalyst Nafion NM112
under
reflux conditions with the catalyst foil as fall-film reactor
(,~,!),in comparison to batch reactor mode with permanent contact
between
catalyst and glycerol/liquid product (*) Temperature 1608C,
operatingpressure 2 mbar [23].
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Cottin [46] found (2608C, 49 h reaction time) thatCs2CO3 was
less active than Na2CO3 and K2CO3 and attrib-
uted this finding to the lower solubility of Cs2CO3 under
the
reaction conditions. This has been taken as evidence that
the
reaction is catalyzed by dissolved alkali in a principally
homo-
geneous reaction [46].
Table 3 summarizes relevant data on the use of alkaline
salts for glycerol conversion.
5.4 Glycerol oligomerization using basicheterogeneous
catalysts
Reactions performed with solid basic materials of limited
solubility should be viewed as heterogeneously catalyzed.
This is the case for earth alkaline metal compounds used
by Ruppert et al. [17], the more so, because the glycerol
oligomerization has been performed at a relatively low
temperature of 2208C. Other examples are basic-modifiedzeolites
and mesoporous materials, although solubility of the
solids occurred to a certain extent, and a leaching of con-
stituents of the solid into the liquid was observed. This will
be
discussed in Section 8.
Basic-catalyzed heterogeneous conversion of glycerol over
alkaline earth oxides was investigated by Weckhuysen et al.
[17, 54]. In ref. 55 glycerol conversion under batch con-
ditions at 2208C was reported for low surface area( CaO >
>MgO. Maximum
glycerol conversion of 80% has been achieved on BaO and
SrO at 2208C after 20 h, with a diglycerol selectivity of ca.40%
at this conversion. The conversion of glycerol could
nearly be doubled by applying a colloidal CaO with higher
surface area (54 m2/g). However, it turned out that the
selectivity-conversion profiles of all investigated
catalysts
were not significantly different, and the maximum percentage
of diglycerol is observed at low glycerol conversion. This
points to the consecutive character of the reaction, where
higher glycerol oligomers are formed via the dimers.
By DFT calculations [54], it could be shown that the
basicity of lattice oxygen atoms correlates with the
adsorption
energy of glycerol: BaO (- 3.02 eV) > SrO (- 2.85 eV)
>
CaO (- 2.05 eV) > MgO (- 1.35 eV). These interactions
have an exothermic character, that is, the glycerol
adsorption
process is more exothermic on alkaline earth metal oxides of
strong basicity. Thus, the dissociation of glycerol increases
in
the order: MgO (not dissociated) < CaO (partially
dissociated) < SrO (partially dissociated) < BaO (com-
pletely dissociated). The presence of defects is found to
play
a key role in the mechanism: glycerol interaction with a
stepped CaO surface presents the highest adsorption energy
(- 3.78 eV), and themolecule is found to dissociate at
stepped
surface regions [54].
Zeolites and mesoporous solids of the MSM family are of
interest because of the defined structure that is expected
to
have beneficial influence on the selectivity of the reaction,
and
the hope that they are not as easy dissolved by hot glycerol
as
alkali carbonates and hydroxides.
For alkali ion-exchanged zeolites, the type of alkali metal
used affects the basic strength of the resulting zeolites.
Effects
of the alkali ions on the basic strength of the modified
zeolites
are in the following order: Cs+ > Rb+ > K+ > Na+ >
Li+ .
The type of zeolite matrix is also of importance as shown
in Fig. 12 where the same alkali cation, viz. Na, is ion-
exchanged into zeolites X, Y and Beta.
It can be seen that the activity at 2608C is in the orderNaX
> NaY > NaBeta. The concentration of Na changes in
the same order 4.9 mmol/g (NaX) > 2.3 mmol/g
(NaY) > 0.7 mmol/g (NaBeta) [37]. The concentrations
of the linear diglycerol are different, but, as followed
from
Fig. 13, the selectivityconversion profile is not
significantly
different, e.g., there is no beneficial influence of the
zeolite
structure on the selectivity of the linear dimers. The
Table 3. Results on basic homogeneous catalysts
Catalyst mCat. (wt%) T (8C) XGly (%) after t SDi (%) STri (%)
STetra (%) SDi,50 (%) Ref.
Na2CO3 2 240 76 (9 h) 46 34 13 75 [46]
NaOH 2 240 63 (9 h) 60 32 7 n.a. [46]
Na2CO3 2 260 96 (8 h) 24 35 22 75 [14, 30]
Na2CO3 2 260 94 (8 h) 27 31 21 n.a. [15, 24]
Na2CO3 2 260 80 (8 h) 31 28 17 n. a. [54]
Na2CO3 n.a. 220 80 (n.a.) 45 36 n. a. 75 [17]
NaHCO3 0.2 260 75 (8 h) 27 12 0 30 [37]
CsHCO3a) 0.4 260 64 (8 h) 23 9.5 2.5 75 [18]
Cs2CO3a) 0.7 71 (8 h) 39 19 6 75 [18]
CsOHa) 0.3 260 74 (8 h) 32 21 5 75 [18]
a) 1.85 mmol Cs/mol glycerol, n. a.: data not available. XGly:
glycerol conversion, S denotes selectivity data of the respective
diglycerol,
triglycerol, tetraglycerol, SDi,50 means the selectivity of
diglycerol at 50% glycerol conversion.
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maximum diglycerol content will simply be shifted to shorter
reaction times, if a reactive catalyst is applied.
Table 4 summarizes relevant data on the use of sodium
modified solid catalysts.
The influence of the amount of the impregnated alkali
element on a surfacerich mesoporous support has been
studied by Clacens et al. [15]. The activity increased with
the amount of Cs but the selectivity of glycerol conversion
to
di- and triglycerol decreased. The deterioration of
selectivity
was explained by the structural collapse of the porous
struc-
ture during reaction. Additionally, Cs leaching during
reaction was found to take place during the collapsing and
the Cs dissolved in the homogeneous phase switched the
process to a homogeneously catalyzed transformation with-
out any shape-selective effect. It seems to be possible to
stabilize the Cs on MCM-41 by combination with other
elements like e.g. La [57].
Figure 14 summarizes the influence of the Cs concen-
tration (independent of the other structural diversity) for
selected solid catalysts in glycerol conversion.
Independent of the support structure, the primary influ-
ence of the Cs concentration is obvious. The activity of the
catalysts increases nearly linearly up to a Cs concentration
of
ca. 5 mmol/mol of glycerol, but seems to approach a border
Table 4. Relevant results reported for the use of different
sodium modified solid catalysts.
Catalyst mCat (wt%) T (8C) XGly (%) after t SDi (%) STri (%)
STetra (%) SDi,50 Ref.
NaAa) 2.4 240 84 (22 h) 38.3 24.2 n.a. n.a. [56]
NaXb) 2.4 240 90.4 (22 h) 34.2 24.3 n.a. n.a. [56]
NaXc) 4 260 68.8 (9 h) 68 22 n.a. 80 [46]
Na mordenited) 4 260 38.6 (9 h) 73 22 n.a. n.a. [46]
NaXe) 2 260 100 (24 h) 25 26 29 40 [37]
NaYf) 2 260 79 (24 h) 47.5 18.5 8 38 [37]
NaBetag) 2 260 52.5 (24 h) 44.5 7.2 0 50 [37]
Naimpr.MCM-41h) 2 260 85 (16 h) 63 30 n.a. 86 [15, 16]
a) Na12[(AlO2)12(SiO2)12] 27 H2O, Si/Al 1,b)
Na86[(AlO2)86(SiO2)106] 264 H2O, Si/Al 1.2,c) Si/Al 1.2, surface
area 780 m2/g, 100% Na exchanged,d) Si/Al 5, surface area 330 m2/g,
100% Na exchanged,e) Si/Al 1.1, surface area 868 m2/g,f) Si/Al 2.3,
surface area 918 m2/g,g) Si/Al 12.9, surface area 655 m2/g,h) Si/Al
20, Na 2.5 mmol/g.n.a.: data not available. SDi,50 is the
diglycerol selectivity at 50% glycerol conversion.
Figure 12. Glycerol oligomerization over zeolite NaX (Si/Al
molar
ratio 1.1, surface area 868 m2/g,micropore volume 0.35 cm3/g,
NaY
(Si/Al molar ratio 2.3, surface area 918 m2/g, micropore
volume
0.35 cm3/g), NaBeta (Si/Al molar ratio 11, surface area 655
m2/g,
micropore volume 0.30 cm3/g, T 2608C, 2 wt% of catalyst,
Argonflushing 15 cm3/min, water continuously removed [37].
Figure 13. Glycerol oligomerization over zeolite NaX, NaY,
and
NaBeta at T 2608C. Selectivity vs. conversion profile.
Conditionsas in Fig. 13 [37].
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line value beyond a Cs content of ca. 10 mmol Cs/mol of
glycerol. The scattering of data is significant for
mesoporous
supports, but the achievable activity of CsMCM materials is
similar to CsX and CsY faujasites at best.
6 Shape selectivity in glycerol oligomerization
Shape selectivity [58] in glycerol conversion is understood
in
a twofold sense. First, formation of undesired cyclic
oligom-
ers should be suppressed, and, second, formation of
polyglycerol should be limited. Additionally, a shift of the
distribution of diglycerol configurational isomers, a,a
-digly-
cerol, a,b-diglycerol and b,b -diglycerol, in favour of the
small a,a -diglycerol can be viewed as a shape-selective
effect
if occurring on microporous solids.
Considering the sizes of glycerol and diglycerol (Fig. 15)
[59] it could be concluded that pore entrances should be
larger than 0.515 nm to allow access of the (neutral)
glycerol
to the interior of the microcrystalline zeolite structure, but
the
pore size should be even higher to enable diglycerol for-
mation, at least 0.753 nm for the large b,b -diglycerol.
Therefore, the use of zeolite A with a pore entrance of
0.38 nm would not allow the utilization of the internal
pores
by glycerol. Medium pore size zeolite ZSM-5 has pore open-
ings of 0.510.55 nm that should, for geometrical reasons,
not allow full utilization of the internal active sites.
Zeolite
Beta (0.66 nm pore size), and zeolite X or Y (0.7 nm pore
size) would be more suitable, where the FAU structure zeo-
lites X and Y are characterized by the presence of a
supercage
of 1.2 nm size, large enough, to allow the formation of
bulkier
cyclic isomers or linear higher oligomers.
It should be noted that the geometrical considerations on
basis of isolated molecules can only convey a simplified
picture because the strong association of
glycerol/diglycerol
in liquid phase is not adequately taken into account. In the
aqueous phase, glycerol is stabilized by a combination of
intramolecular hydrogen bonds and intermolecular solvation
of the hydroxyl groups. A highly branched network of mol-
ecules connected by hydrogen bonds exists in all phases and
at all temperatures [9]. This is certainly the case for
glycerol
oligomers as well.
In Refs. [30] and [56] comparisons of reactivity/selectivity
data are given for zeolites with different pore sizes: a
zeolite
NaA (CHA structure, composition Na12[(AlO2)12(SiO2)12]
27 H2O, Si/Al molar ratio 1, pore size 0.38 nm)
Figure 14. Summary of glycerol conversion on Cs modified
cata-
lysts from different sources but at comparable conditions
(T 2608C, catalyst weight 2 to 4 wt% of glycerol, 0.54 mol
gly-cerol). Squares: Cs ion-exchanged Na zeolites X, Y, and Beta
[37],
diamond: Cs(H)Y, Si/Al ratio of Y zeolite 2.7, Cs content
34%,exchange degree 65%, residual Brnsted acid sites present
[46],
full triangles: Cs impregnated AlMCM-41, Si/Al 20 [30], open
tri-angles: Cs impregnated on MCM-41, Si/Al 20 [15].
Figure 15. Sizes of (neutral) glycerol, anionic glycerol and the
linear dimer isomers from DFTcalculations. Black circles: C, red
circles: O,
grey circles: H [59].
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and zeolite NaX (FAU structure, composition
Na86[(AlO2)86(SiO2)106] 264 H2O, Si/Al molar ratio 1.2,pore size
0.7 nm). The reaction has been performed with
2.4% of the catalyst at 2408Cunder nitrogen for 22 h. Resultsfor
the two samples are shown in Fig. 16. Cylclic dimers were
not identified but are probably included in the remainder
higher oligomers.
The zeolites were found insoluble in the glycerol/product
mixture and could be removed at the end of the reaction.
NaX exhibits a slightly higher glycerol conversion (90%)
than
NaA (85%) at comparable Na concentration. This can be
understood considering that in case of zeolite A interior
active
sites are widely excluded from reaction due to restricted
access by glycerol. For the same reason, an influence of
the pore structure on the product distribution cannot be
expected for zeolite A. Even in case of zeolite NaX, active
sites on the external surface area of the crystals can level
of
any shift in product selectivity brought about by interior
active sites due to the long reaction time within the batch
reactor. One way to overcome this drawback could be a
specific passivation of external sites before reaction, e.g.
by
silylation, as is known to have an impact on activity and
selectivity of zeolite catalysts [60].
Averaging of shape-selectivity by interference of
external surface sites of microporous zeolites might be one
reason, why often salts without defined porosity yields com-
parable selectivity in converting glycerol to oligomers. For
example, sodium silicate was used in ref. 56 under the same
reaction conditions as zeolites NaA and NaX, and gave
32.5 wt% of diglycerol at 91.5% glycerol conversion, which
is only slightly lower than 38.3 wt% diglycerol observed for
NaA.
Barrault et al. [61] also stated that most often no signifi-
cant pore size effects of zeolite catalysts on the product
distribution are observable due to the preponderance of
reaction on external active sites, together with limited
acces-
sibility of interior sites in the pores. Thus, the conclusion
is
reasonable to use catalysts with larger pores, as e.g. meso-
porous structured solids, and to minimize, by synthesis, the
concentration of active site on the external surface. It could
be
made plausible, by comparison with homogeneous catalysts,
that CsmodifiedmesoporousMCM-41 solid catalysts had an
effect on the product distribution during the batch
conversion
of glycerol at 2608C.Moreover, an impact on the distributionof
the configurational isomers of linear diglycerol could be
observed [54] as shown in Fig. 17a.
The conclusion is in line with the molecular sizes of the
linear diglycerol configurational isomers given in Fig. 15,
where the formation of the smallest dimer, viz.
a,b-diglycerol
should be favoured on the expense of the isomers with larger
size.
The same effect on the distribution of the linear diglycerol
configurational isomers is confirmed for microporous zeolite
Y (Fig. 17b) [37].
Figure 16. Product composition (wt%) of glycerol conversion
over catalysts NaA and NaX (2.4 wt%) at 2408C after 22 h
reactiontime [56].
Figure 17. Shape selective effect of CsMCM-41 (a) [60], and
zeo-
lite NaY (b) [37] on the distribution of linear diglycerol
configurational
isomers. T 2608C, 2 wt% of catalyst, 0.54 mol of glycerol,
con-version level 50%. Reference: Na2CO3.
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7 Mechanism and kinetics
Mechanistically, the reaction with basic homogeneous cata-
lysts is proposed to follow a SN2 type route. Interaction of
the
base BOH with glycerol weakens one of the glycerol OH
bonds and enhances the nucleophilic character of the
hydroxyl oxygen. Attack of this polarized glycerol to a
carbon
of a second glycerol with simultaneous split off of water
results in diglycerol (Fig. 18).
On heterogeneous basic oxide, Ruppert et al. [17] pro-
posed a scheme involving Lewis acid sites represented by
coordinatively unsaturated surface metal ions. These Lewis
acid sites facilitate the hydroxyl leaving process in the
glycerol
oligomerization reaction (Fig. 19).
Kinetics of the reaction in homogeneous phase (thermal
or with addition of alkali) was studied by Zajic [62] and
Richter et al. [18].
By means of IR spectroscopy, Zajic found that in a first
stage glycidol is formed and combined readily with glycerol
(cf. Fig. 5). But, the author also considered the possibility of
a
free-radical propagation of glycerol coupling according to
the
reaction scheme
where A symbolizes glycerol, I the intermediate formed
fromglycerol (that might have a free radical character according
to
Zajic [62]), B diglycerol, C triglycerol and D
tetraglycerol.
The conversion of glycerol would obey a pseudo-mono-
molecular reaction with the pre-equilibrium step of initial
intermediate formation. By solving the corresponding kinetic
equations rate constants were determined for the purely
thermal glycerol oligomerization between 200 and 2608Cand for
separate experiments with different amounts of
Na2O added (0.1, 0.3 and 1 wt%).
A further kinetic treatment for the CsHCO3 catalyzed
liquid-phase oligomerization of glycerol was performed by
Richter et al. [18]. Concentration vs. time profiles of
glycerol
and oligomers up to tetramers were determined in a discon-
tinuous batch reactor at 2608C under normal pressure for0.1, 0.2
and 0.4 wt% CsHCO3 added. A maximum of linear
diglycerol was observed after intermediate reaction times of
8 h. Independent of the catalyst concentration, a unique
conversionselectivity profile was observed with 100% linear
diglycerol at low glycerol conversion, but only 10% at com-
plete glycerol conversion. The conversion of glycerol obeyed
a 1st order kinetics (Fig. 20).
Figure 21 shows a comparison of data from Zajic [62] and
Richter et al. [18] how the rate constant depends on the
concentration of Na and Cs, respectively. After translating
the rate constant of Zajic from 220 to 2608C using theactivation
energies given in ref. 62 (47.3 kJ/mol for
0.3 wt% Na2O), the dashed line in Fig. 22 allows a compari-
son of rate constants for Cs and Na catalyzed glycerol con-
version. It is confirmed, that the rate constant is higher for
Cs
than for Na, reflecting the different basic strengths of the
elements.
Figure 18. Basic catalyzed homogeneous glycerol dimerization
[17, 40].
Figure 19. Reaction scheme of glycerol dimerization to
diglycerol
over basic oxide catalysts by concerted action of basic sites
and
Lewis acid sites [17].
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The rate constant changes with concentration of Cs
or Na that points to some mechanistic modifications.
Experimentally, it is proven that the reaction does not obey
a strict consecutive formation of the oligomers by reaction
with glycerol, because diglycerol reacts preferentially with
itself to tetraglycerol (Fig. 22).
Analysis at zero time reflects the composition of the com-
mercial diglycerol applied, containing 94.2% of linear
digly-
cerol, 5.8% of triglycerol and no glycerol or cyclic
diglycerol.
Complete conversion of diglycerol has been achieved within
24 h, but dimerization of diglycerol to tetramers prevailed.
Triglycerol is not formed, indicating that back-scission of
diglycerol to glycerol did not occur under reaction
conditions.
Instead, clyclization of linear diglycerols is observed, and,
the
percentage of oligoglycerols higher than tetraglycerol is
sig-
nificantly enhanced [18].
8 Stability of solid catalysts
The good dissolution of salts in hot glycerol is one reason
that
solids like alkali carbonates or hydrogen carbonates are
dis-
solved and act as homogeneous catalysts. Alkaline or earth
alkaline elements loaded on microporous or mesoporous
aluminosilicate supports are partially leached during
reaction
[15, 29, 63]. Catalysts like CsX, NaX, CsY andNaY suffered
severe deterioration of crystallinity during the reaction of
glycerol under batch conditions at 2608C [37]. This struc-tural
damage occurred within the first 4 h of reaction as could
be seen from XRD analysis in dependence on reaction time.
Broad diffraction lines appearing at 2 u 11 and 218 pointedto
formation of an ill-defined new phase once the zeolite
structure had been destroyed. This phase could not unam-
biguously be identified, but might consist of some sodium
aluminate or sodium silicate phase, because elemental
analysis of NaX showed an increase in the Na content at
longer reaction times. It is possible that Na in solution is
precipitated on the remaining Si-poor solid forming sodium
aluminate [37].
Clacens et al. [15] and Charles et al. [30] impregnated a
mesoporousMCM-41 solid with different amounts of Cs and
characterized the modification of structural parameters and
the Cs content during impregnation and subsequent catalytic
test performed at 2608C with glycerol under batch con-ditions.
Preparation of CsMCM-41 has been performed by
agitating the separately synthesizedMCM-41 support (either
Figure 21. Dependence of pseudo-1st order rate constant (hS1)
on
the amount of CsHCO3 and Na2O, given in mmol Me/mol glycerol
(Me Cs, Na) for reaction at 2608C [18] and 2208C [62].
Dashedline: calculated values for 2608C from [62] applying the
activationenergies given (47.3 kJ/mol for 0.3 wt% Na2O).
Figure 22. Reactivity of linear diglycerol under batch
conditions and
normal pressure in the presence of 0.4 wt% CsHCO3 at 2608C
[18].
Figure 20. Nonlinear fitting of relative glycerol concentration
with
time of reaction using CsHCO3 as catalyst (4 wt%) at
2608C.Squares: experimental points, dashed line: 0th order
reaction, solid
line: 1st order reaction, dotted line: 2nd order reaction
[18].
114 A. Martin and M. Richter Eur. J. Lipid Sci. Technol. 2011,
113, 100117
2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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in a purely siliceous form or with Al to yield a Si/Al molar
ratio
of 20) [14] together with the calculated amount of Cs
acetate
in 50 g of methanol at ambient temperature during 2 h. The
solvent was evaporated under vacuum and the solid calcined
under air at 4508C overnight. The loading of the MCM-41material
with Cs was in the range of 6.4 \times 10- 4 to
40.7 \times 10- 4 mol/g (8.5 wt% to 54.1 wt% Cs). The
authors observed that the activity increases with the amount
of Cs but the selectivity decreases. For explanation it could
be
shown that a structural collapse of the mesoporous support
occurred. During reaction, caesium oxide had been dissolved
and the reaction assumed a predominantly homogeneous
character, thus exhibiting the higher activity seen for
homo-
geneous catalysts, however, loosing all shape-selective
effects
of the mesoporous structure. The structural instability of
modified MCM-41 was also observed for variants impreg-
nated by Li, La, Na andMg [63]. The problem of insufficient
stability of structured solids is not confined to zeolites
and
mesoporousMCM-41, but could also be observed for CaO in
colloidal form as catalyst for glycerol conversion at a
relatively
low reaction temperature (2208C). Ruppert et al. [17]reported
that up to 6% Ca were found in the liquid after
20 h reaction. Thus, a solid catalyst with high activity and
stability is still missing.
9 Conclusions and outlook
The glycerol chemistry is a subject to continuous change
striving for catalyzed selective conversion in value-added
products. For many applications in cosmetics and food
industry pure glycerol can favourably be replaced by its
oligomers, above all linear diglycerol and triglycerol.
Solid
catalysts with specific microporous (crystalline) structures
or
ordered mesoporous surface-rich silica or aluminosilicates
are applicable, besides dense oxides of alkaline earth
metals.
One drawback is recognized in the good solubility of the
solid
catalysts and leaching of active elements like Cs, leading to
a
superimposition of heterogeneously and homogeneously cat-
alyzed reaction pathways. This causes a loss of shape
selective
effects exerted by an intact micro- or mesostructured pore
system and, additionally, the dissolution of the solids in
the
liquid mixture renders difficult the separation of the
catalyst
after completion of the reaction. To overcome this problem,
a
rational design of catalysts with focus on only marginal
dis-
solution in hot glycerol is a challenge. One approach to
conserve the mesostructure of MCM-41 is tried by exchang-
ing protons of surface hydroxyls with Cs. Another approach
consists in using CaO in colloidal form at reduced tempera-
ture (e.g. 2208C), where only marginal leaching of Ca2+
wasobserved.
Alternatively, the reaction engineering can be modified.
Traditionally, the reaction is performed in batch mode,
need-
ing relatively long reaction times for a complete glycerol
conversion and a correspondingly long residence time of
the desired products in contact with the catalyst. Since the
glycerol oligomerization is a consecutive reaction, the
desired
diglycerol and triglycerol are further converted to higher
oligomers at longer reaction times and progressive glycerol
conversion. Using a reactor setup similar to reactive
distil-
lation, where glycerol is evaporated under reduced pressure
and back-condensed on the top of a reactive section contain-
ing a superacidic polymer, high selectivity to diglycerol
could
be achieved, even at nearly complete conversion of glycerol.
Further research work is needed to combine the benefits
from shape-selective solid catalysts offering a tuned pore
system, and high stability under reaction conditions, with
reaction engineering tools of process intensification thus
allowing high conversion degrees of glycerol at high
selectivity
of the desired products. This would make a sustainable high-
yield process to di- and triglycerol by glycerol
oligomerization
economically attractive, and would represent an environmen-
tally benign reaction route in comparison to the current
epichlorohydrin process.
The technical assistance of Dr. H.-L. Zubowa is gratefully
acknowledged. Mr. M. Cecinski is thanked for DFT
calculations.
The authors have declared no conflict of interest.
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