Glycerol

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]

100 Eur. J. Lipid Sci. Technol. 2011, 113, 100117

2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.com

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].

Eur. J. Lipid Sci. Technol. 2011, 113, 100117 Glycerol oligomerization 101


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.

102 A. Martin and M. Richter Eur. J. Lipid Sci. Technol. 2011, 113, 100117


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).



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].



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].



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].



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.



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].



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.



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].



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].



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.



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].



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].



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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