Fatty Acid Hydrogenation
Unprecedented Shape Selectivity in Hydrogenation of Triacylglycerol
Molecules with Pt/ZSM-5 Zeolite**
An Philippaerts, Sabine Paulussen, Annika Breesch, Stuart Turner, Oleg I. Lebedev,
Gustaaf Van Tendeloo, Bert Sels,* and Pierre Jacobs
Vegetable oils are made up of triacylglycerol molecule
mixtures, which comprise three fatty acids esterified to a
glycerol backbone. The fatty acids can differ in chain length,
number, and double-bond configuration. Common vegetable
oils contain fatty acid chains with 16 to 20 carbon atoms and 0
to 3 double bonds, all of which are in a cis configuration
(Table 1).Fatty acid chains are located at the central (sn-2)
or outer (sn-1,3) positions on the glycerol backbone. Thus
SOS refers to oleate at the sn-2 and stearate at the sn-1,3
Partial hydrogenation of highly unsaturated vegetable oils
with a nickel-based catalyst is used in the food industry to
stabilize the oils against autoxidation.Unfortunately, a
significant degree of cis/trans isomerization simultaneously
leads to formation of trans fatty acid chains, suchas E on some
of the triacylglycerols, which leads to a serious risk of
cardiovascular disease.Many unsuccessful efforts have
also been published concerning the double-bond reduction
of vegetable oils yielding semi-solid fat products with specific
melting properties for use as shortenings and bakery fats with
low levels of trans fatty acids.
We recently reported the preparation of a shape-selective
Pt/NaZSM-5 catalyst, which enables the bent cis chain in
methyl oleate (MO) and the linear trans chain in methyl
elaidate (ME) to be discriminated during room-temperature
adsorption and hydrogenation.Herein, unprecedented
shape selectivity on zeolites with MFI topology during
adsorption and on Pt/ZSM-5 during hydrogenation of model
triacylglycerols was demonstrated. Applied to vegetable oils,
this should allow the conversion of unsaturated oils into
autoxidatively stable products, which are useful as a basis for
dressing oils, shortenings, or bakery fats devoid of trans fatty
Results from room-temperature sorption experiments of
ME, MO, EEE (trielaidate), and OOO (trioleate) on
NaZSM-5/78 and reference g-alumina are given in Table 2.
Adsorption equilibrium constants K of ME and MO on g-
alumina show hardly any difference for the respective geo-
metric isomers. Compared to the trans isomers, the sorption is
even slightly in favor of the cis isomer, which is attributed to
its better access to the wall of a non-constrained pore. With
MFI zeolite, discrimination among cis and trans chains is
found for methyl esters as well as for triacylglycerols, with
sorption being in favor of the slimmer trans fatty acid chains.
The strongly reduced values of the sorption constants for
triacylglycerols compared to methyl esters point to the
inaccessibility of the intracrystalline zeolite space, with
sorption being limited to pore mouth penetration of only
one of the chains of the triacylglycerol.
As a measure of the hydrogenation rate, the first-order
rate constant k was divided by the chromatographically
determined adsorption equilibrium constant K. Indeed, under
conditions of substrate dilution and limited hydrogen sol-
ubility, a bimolecular Langmuir–Hinshelwood rate equation
transforms into an equation that is first-order in both
reactants. With the Pt/alumina catalyst (Table 3), the hydro-
Table 1: Description and notation of common C16/C18fatty acid chains in
Chain name Double bonds[a]
[a] n=number, conf.=configuration. [b] c=cis, t=trans.
Table 2: Room-temperature adsorption equilibrium constants K of
geometrically different mono-unsaturated methyl esters and triacylgly-
[a] Mono-unsaturatedmethyl esters: ME=methylelaidate,MO=methyl
oleate; triacylglycerols: EEE=trielaidate, OOO=trioleate.
[*] A. Philippaerts, Dr. Ir. S. Paulussen, A. Breesch, Prof. B. Sels,
Prof. P. Jacobs
Dept. M2S, K.U.Leuven
Kasteelpark Arenberg 23, 3001 Heverlee (Belgium)
Dr. S. Turner, Dr. O. I. Lebedev, Prof. G. Van Tendeloo
EMAT, University of Antwerp
Groenenborgerlaan 171, 2020 Antwerpen (Belgium)
[**] A.P. acknowledges the F.WO.-Vlaanderen (Research Foundation–
Flanders) for a doctoral fellowship. We acknowledge the Flemish
Government for long-term sponsoring (Methusalem, CASAS). We
are grateful to BELSPO for an IAP-PAI network.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2011, 50, 3947–3949 ? 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
genation rate of cis chains is favored, which is attributed to a
better contact between the platinum surface and the double
bond, and the values for the respective methyl esters and the
triacylglycerols are comparable. With the zeolite-based cata-
lyst, hydrogenation of trans-configured chains is faster in both
cases.The higherratesfor hydrogenationoftriestercompared
to monoester molecules is again in line with pore mouth
hydrogenation on occluded platinum particles compared to
reaction inside zeolite pores. TEM images of the Pt/ZSM-5
catalyst (Figure 1) indeed show homogeneously distributed
platinum clusters inside the crystals with a narrow particle
size distribution and a mean platinum cluster size of (1.7?
Hydrogenation of triacylglycerols with an unsaturated
chain at a different position on the glycerol backbone, namely
POP, PPO, and PEP, PPE, reveals the existence of position
selective preferences, indicating that double bonds in fatty
acid chains at position sn-1/3 or sn-2 react differently.
Hydrogenation with Pt/alumina is faster for the O-containing
triacylglycerols POP and PPO compared to corresponding E-
containing PEP and PPE (Table 4). This result confirms the
data obtained with other substrates (Table 3). Interestingly,
with the microporous zeolite catalyst the differences are
opposite, E-containing triacylglycerols being more reactive
than O-containing forms, which is in agreement with the data
in Table 3. Moreover, for both chain configurations (E and
O), it is striking that the chain at sn-2 is more reactive with the
zeolite catalyst than at positions sn-1,3, and the effect is much
more pronounced for E chains.
Studies of the conformation of triacylglycerols in the solid
state are numerous.A recent modeling analysis of PPP and
OOO shows that in the crystalline phase, conformations show
a decreasing concentration in the following order: tuning fork
(A), chair (B), and trident (C). In the fluid phase, the random
conformation (D) of the triglycerides dominates over specific
configurations.It is therefore expected that the random
conformation of POP, PPO, PEP, and PPE in solution will be
dominant. However, the preferred hydrogenation of E and of
O at position sn-2 (Table 4) points to a specific adsorption and
reaction mode of triacylglycerols on the surface of ZSM-5
crystals according to the tuning fork conformation, probably
with the central fatty acid chain protruding in pore apertures,
thus allowing contact of double bonds on this chain with
platinum metal particles in pore mouths.As the triacylgly-
cerol molecules are unable to enter the pores, chains at
positions sn-1,3 are expected to be adsorbed in an energeti-
cally favorable configuration on the external surface of the
Regioselective analysis of products after partial hydro-
genation of trilinoleate confirms the preferred reduction at
the central linoleate (L) chain with Pt/ZSM-5, pointing to
preferred adsorption of triacylglycerols at the crystal surface
in the tuning fork (A) rather than the chair (B) or random (D)
conformation (Figure 2, Figure 3). This regioselective hydro-
genation will lead to reduced SSS levels in hardened fats and
thus to more desirable physical properties.With the
alumina-based catalyst, such positional discrimination is
Autoxidative stabilization of vegetable oils by catalytic
hydrogenation with hydrogen requires selective removal of
triene chainsand for dietary reasons retention of other cis
unsaturates.Therefore, selective stepwise reduction of the
double bonds on a polyene chain should occur at a decreasing
rate. Platinum is known to be lack such properties,as
confirmed in the distribution of chains upon partial reduction
ofLLL with platinum/aluminacatalyst(Table 5). Surprisingly,
platinum in the micropores of ZSM-5 zeolite shows this
preferred stepwise reduction, as shown by much enhanced
Table 3: Hydrogenation rate k/K of mono-unsaturated fatty acid chains
of methyl esters and triacylglycerols.[a]
[a] 2.0 wt% substrate in n-octane at 1008 8C and 6 mPa of hydrogen;
k [h?1], the first-order rate constant and K, the adsorption equilibrium
constant were determined chromatographically under the same con-
ditions. [b] Catalyst/substrate weight ratio 0.1 wt%. [c] 0.01 wt%.
Figure 1. TEM image of 0.5 wt% Pt in ZSM-5. The ZSM-5 crystal is
imaged along the  zone axis orientation (shown by FFT pattern;
lower left). The Pt nanoparticles (dark contrast spots) are approx-
imately 1–3 nm in diameter. Inset (top left): Bragg-filtered image of the
region indicated by the white rectangle. The pore arrangement of ZSM-
5 appears and the Pt nanoparticles (dark contrast) are visible.
Table 4: Hydrogenation rate k/K of mono-unsaturated fatty acid chains
[a] For conditions, see Table 3. [b] 0.6 wt% in octane; 0.05 wt% catalyst
to substrate. [c] 0.01 wt% in octane; 0.001 wt% catalyst to substrate.
? 2011 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimAngew. Chem. Int. Ed. 2011, 50, 3947–3949
formation of monoene chains and reduction of saturated
chains (Table 5). The latter behavior should be correlated
with the tuning fork sorption mode of triacylglycerols on
zeolite combined with enhanced affinity of more polar chains
(trienes>dienes>monoenes) for the polar pore mouths of
MFI. It can also be seen in Table 5 that discrimination among
the cis monoenes (9 and 12) does not occur.
In conclusion, it is clear that shape-selective hydrogena-
tion of model triacylglycerols on a Pt/ZSM-5 catalyst prefers
trans over cis hydrogenation of the fatty acid chains. More-
over, the chain on the central position of the glycerol
backbone is preferably reduced, and indicates pore-mouth
adsorption in the tuning fork conformation. This adsorption
mode allows discrimination among central chains with differ-
ent polarities. The overall hydrogenation specificities will
allow the synthesis of more healthy fats/oils (low in trans, high
in oleic) with desirable physical properties (low SSS) from
real feedstocks. Experiments are planned to confirm this
Model compounds were obtained from Larodan and Sigma Aldrich.
Hydrogenation was carried out with hydrogen in a stirred batch
reactor. Further information about the experiments is given in the
Received: November 30, 2010
Published online: March 18, 2011
triacylglycerol · zeolites
Keywords: adsorption · hydrogenation · platinum ·
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Figure 2. Possible conformations of triacylglycerols: a) tuning fork,
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Figure 3. Conversion of double bonds at the central (sn-2; white bars)
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trilinoleate using Pt/alumina and Pt/ZSM-5.
Table 5: Distribution of hydrogenation products from trilinoleate at 24%
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Angew. Chem. Int. Ed. 2011, 50, 3947–3949 ? 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim