C.A. Ferretti et al. / Applied Catalysis A: General 399 (2011) 146–153
147
2.2. Catalyst characterization
CH2-OCO-R
CH2-OH
R
O
base
+
OH
OH
OH
+
CH-OH
CH-OCO-R + CH3OH
CH3
BET surface areas were determined by N2 physisorption at 77 K
in a NOVA-1000 Quantachrome sorptometer. Pore size distribu-
tions were estimated by the Barrett-Joyner-Halenda (BJH) method
O
CH2-OH
α-monoester
CH2-OH
β-monoester
FAME
Glycerol
Monoglycerides
˚
within the pore diameter range of ≈10–300 A; total pore volumes
base
+ FAME
were also measured during the analysis. Li content of the samples
was measured by atomic absorption spectrometry (AAS).
CH2-OCO-R
CH-OCO-R
CH2-OCO-R
CH2-OCO-R
CH-OCO-R
CH2-OCO-R
The structural properties of solid samples were determined by
X-ray diffraction (XRD) using a Shimadzu XD-D1 instrument.
Catalyst basic properties were measured by temperature-
programmed desorption (TPD) and infrared spectroscopy (IR) of
CO2. For the CO2 TPD experiments, samples were pretreated in situ
in a N2 flow at 773 K, cooled down to room temperature, and then
exposed to a flowing mixture of 3% of CO2 in N2 until surface satu-
ration was achieved (5 min). Weakly adsorbed CO2 was removed by
flushing with N2. Finally, the temperature was increased to 773 K
at a ramp rate of 10 K/min. The flow containing the desorbed CO2
was passed through a methanation reactor and converted to CH4
on a Ni/Kieselghur catalyst at 673 K; CH4 was then continuously
analyzed using a flame ionization detector (FID). Efficiency of the
CO2 conversion in CH4 was checked periodically by introducing
known amounts of CO2 and CH4 in the methanation reactor. The
same setup was used for the carbonate temperature-programmed
decomposition (TPDe) experiments. Samples were pretreated at
673–773 K, cooled down to room temperature, and then heated up
to 973 K at a ramp rate of 10 K/min in a N2 flow. The evolved CO2
was converted to CH4 in the methanation reactor and analyzed by
FID.
The chemical nature of adsorbed surface CO2 species was
determined by IR after CO2 adsorption at room temperature and
sequential evacuation at increasing temperatures. Samples were
pressed in a wafer and degassed in vacuum at 773 K for 1 h and
then cooled down to room temperature. The spectrum of the pre-
treated catalyst was then taken. After admission of 5 kPa of CO2 and
evacuation at 298, 373, 473 and 573 K, the CO2 adsorption spec-
tra were recorded at room temperature. Spectra of the adsorbed
species were obtained by subtracting the catalyst spectrum. Data
were collected in a Shimadzu FTIR Prestige-21 spectrometer using
an inverted T-shaped cell fitted with CaF2 windows containing the
sample pellet. The absorbance scale was normalized to 20 mg.
+ FAME
base
CH-OH
+ CH3OH
+
+ CH3OH
CH2-OCO-R
1,3-diester
CH2-OH
1,2-diester
Triglyceride
Diglycerides
Scheme 1. Monoglyceride synthesis via transesterification (glycerolysis) of a fatty
acid methyl ester (FAME) with glycerol and consecutive reactions to diglycerides
and triglycerides.
reaction media and discoloration followed by expensive molecular
distillation [5,7].
The heterogeneously catalyzed process for MG synthesis
presents environmental and practical advantages but its industrial
implementation to efficiently replace the current technology
using liquid bases is still a challenge. In previous work [8,9] we
studied the glycerolysis of an unsaturated FAME, methyl oleate
(C18:1), using solid catalysts with different acid–base properties.
We investigated the experimental conditions required to operate
in a kinetically controlled regime and discussed the optimization
of reaction parameters for improving MG yield [9]. Moreover,
the chemical nature and the acid-base requirements of the active
site for promoting the MG synthesis were investigated using
single oxide solid catalysts with different electronegativities. A
good correlation was found between the catalytic activity and the
base site number. In addition, we concluded that glycerolysis of
methyl oleate requires strong base sites such as the coordinatively
unsaturated oxygen anions present on the surface of MgO [8].
In this paper we continue our investigations on the hetero-
geneously catalyzed glycerolysis of methyl oleate. We focus on
MgO-based catalysts and study the effect of the addition of basic
promoters such as Li on the MgO structure and surface base proper-
ties. The Li-MgO catalysts were then tested in the FAME glycerolysis
reaction in order to correlate the activity and selectivity to MG with
the sample Li loading.
2.3. Catalytic tests
2. Material and methods
The transesterification of methyl oleate (oleic acid methyl ester,
Fluka, >60%, with 86% total C18 + C16 esters as determined by gas
chromatography), with glycerol (Aldrich, 99%,) was carried out at
493 K in a seven-necked cylindrical glass reactor with mechanical
stirring equipped with a condenser to remove the methanol gen-
erated during reaction. In the text FAME and Gly stand for methyl
oleate and glycerol, respectively.
A Gly/FAME molar ratio of 4.5 and a catalyst/FAME ratio
(Wcat/n0FAME) of 30 g/mol were used. The reactor was operated
in a batch regime at atmospheric pressure under flowing N2
(35 mL/min). First, the liquid reactants were introduced and the
reactor was flushed with nitrogen, then they were heated to reac-
tion temperature under stirring (700 RPM). Catalyst was pretreated
ex-situ at 773 K for 6 h to remove adsorbed water and carbon diox-
ide and kept overnight at 373 K in flowing N2 until used, then
quickly transferred to the reactor without exposing it to air to start
and 1,3-glyceryl dioleates (diglycerides, DG) and glyceryl trioleate
(triglyceride, TG) were analyzed by gas chromatography (GC) after
silylation to improve compound detectability [11]. Details are given
elsewhere [8,12]. Other reaction products were diglycerols (DGly)
2.1. Catalyst synthesis
Solids used were MgO (Carlo Erba 99%; 27 m2/g), Li(OH) (anhy-
drous, Johnson Matthey 99.3%) and Li2CO3 (Puratronic 99.99%).
with distilled water of low-surface area commercial MgO. Then the
resulting Mg(OH)2 was decomposed and stabilized in a N2 flow for
1 h at 373 K, then for 1 h at 623 K, and finally for 18 h at 773 K to
obtain a catalyst with a surface area 10 times higher than that of
the original MgO [10]. The catalyst was finally ground and sieved,
and the particles with average particle size of 177–250 m were
used for the catalytic experiments.
A set of lithium-promoted MgO samples was prepared by incip-
ient wetness impregnation. Lithium was added to the high surface
area MgO using a hydroxide aqueous solution containing the
required Li concentration to obtain catalysts with 0.09, 0.17, 0.30,
0.54 and 0.94 wt% of lithium. After incorporation of the Li promoter,
all samples were dried at 353 K and finally decomposed and stabi-
lized at 773 K during 18 h in flowing N2. The Li-doped MgO samples
were identified as Li/MgO-x (x = 09, 17, 30, 54 and 94).