in situ DRIFTS/MS studies of transesterification of methanol
and ethyl acetate over Mg/La catalyst
Adão S. Gonçalves,1,2 Deborah V. Cesar,2 Cristiane A. Henriques1,2,*
1
NUCAT/PEQ/COPPE/UFRJ, Rio de Janeiro, 21941-914, RJ, Brazil
PPGEQ/IQ/UERJ, Rio de Janeiro, 20550-013, RJ, Brazil
*Corresponding author: [email protected]
2
Keywords: Mg-La catalyst, DRIFTS-MS, methanol, ethyl acetate, transesterification
1. Introduction
Transesterification reactions play a significant role
in many areas of the chemical industry and can be
promoted by acid or basic catalysts, in both
homogeneous and heterogeneous medium [1-3].
Mg-La materials with different Mg:La molar ratios
(9:1, 1:1 and 1:9) were promising solid catalysts to
obtain methyl esters via transesterification of
soybean oil with methanol [3]. However, although
the adsorption of methanol on basic surfaces has
been well investigated using diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS)
[4,5,6], similar studies concerning in situ
transesterification followed by DRIFTS coupled to
mass spectrometry (MS) were less common
particularly over Mg-La catalysts. Thus, in this work,
in situ DRIFTS-MS was used to investigate the
interaction of methanol (MeOH) and ethyl acetate
(EA) (chosen as a model ester) with Mg-La catalyst
surface trying to provide some insights on the
transesterification route.
2. Experimental
The Mg-La catalyst with a molar ratio of 1:1 was
synthesized by the methodology described by
Santorio et al. [3] and characterized by XRF, XRD,
and N2 physisorption. In situ DRIFTS-MS analyses
were performed on a Thermo Nicolet spectrometer
(Nexus 470), with an MCT-A detector, resolution of
4 cm-1 and a high-temperature chamber (SpectraTech) with ZnSe windows. The outlet line of the
chamber was coupled to a Pfeiffer mass
spectrometer; model QMS Prisma, with CH-TRON
detector. The catalyst was pre-treated in situ under
He flow (40 mL min-1) from room temperature to
200 ºC and kept at this temperature for 1 h. Then,
the sample was cooled down to 30 ºC and a stream
of He saturated with MeOH and/or EA was
introduced into the chamber. After removing the
reversibly adsorbed model molecule using a flow of
He for 1 h, the catalyst was heated up to different
temperatures from 50 ºC to 450 ºC under He flow
(40 mL min-1). Spectra were acquired at each 50 ºC
of temperature stages. The spectrum of the sample
treated was used as background (150 scans). The
selected MS signals (m/z) were monitored
continuously with time and temperature: 44 (CO2);
17 (H2O); 2 (H2); 31 (methanol); 30 (formaldehyde);
14 (methane); 46 (ethanol); 26 (ethylene); 61 (ethyl
acetate); 58 (acetone).
3. Results and discussion
The 1Mg1La catalyst was formed by a lowcrystallinity lanthanum di-oxycarbonate phase
(La2CO5; JPCDS 23-0320) in which MgO is well
dispersed. Its BET specific area was 38 m2 g-1.
The infrared spectra of irreversibly adsorbed MeOH
showed bands of methoxy monodentate species
type I (1100cm-1), type II (sh 1080 cm-1) and Type H
(1030 cm-1), C-H stretching (2790 - 2950 cm-1)
(methoxy and/or methanol molecularly adsorbed)
and H2O (1660 cm-1) [4,5,6]. Upon heating, the
decomposition of methoxy species was observed
with the formation of bi- and monodentate
carbonates (1150 - 1550 cm-1, 843 cm-1). At 200 oC,
CO2 gas phase formation began, which could be
attributed to the reaction of methoxy species with
the interlayer water or with oxygen from the surface.
The methoxy decomposition also leads to the
formation of formate species (1750-1600 cm-1). The
carbonates were very stable at high temperature
(450 ºC) as well as the formate species. The gaseous
phase composition determined by on-line MS
showed that H2 formation began at 100 ºC, reaching
a maximum at 250 ºC and 300 ºC. It could be
associated to methanol dehydrogenation, which also
forms formaldehyde, and to methanol oxidation with
the formation of CO2 at temperatures higher than
150 ºC [7].
The DRIFT spectrum of irreversibly adsorbed EA
exhibited only low-intensity bands of EA
(3000-2600 cm-1, 1715 cm-1, 1245 cm-1, 1100 cm-1)
and band at 1600 cm-1 (acetate species adsorbed
through the oxygen of the carbonyl group) [8].
Increasing temperature promoted the decomposition
of the adsorbed species with the formation of
bidentate carbonates (1550-1000 cm-1) (T  150 ºC)
and CO2 either adsorbed (2465 cm-1) and in the gas
phase (2352 cm-1) (T > 200 ºC). At 300 oC, the
intensity of the band related to ethoxy species
(1040 cm-1) increased and the formation of
monodentate carbonate (840 cm-1) was noted. As
observed for MeOH adsorption, carbonate species
remained adsorbed even at 450oC. The MS profile
showed desorption of molecular EA (50-150 ºC)
followed by ethanol formation. At T > 250 ºC, the
formation of H2, methane, and acetone was detected.
Acetone was observed only in the MS and could
result from the dehydrogenation of ethoxy species
forming acetate, which produces acetone via
oxidation with the surface oxygen [9]. The acetate
adsorbed can also form carbonate and methane
according observed in DRIFTS and MS results,
respectively.
The spectrum of adsorbed MeOH + EA mixture
produced characteristic bands of both reagents.
Noticeable were the bands of adsorbed acetates
(1600 cm-1) and methoxy species type I (1100 cm-1)
and type H (1045 cm-1). Thermal treatment resulted
in the formation of mono- and bidentate carbonates.
At 350 ºC, bidentate carbonate disappeared
completely, and there was an increase in the
intensity of the bands related to ethoxy species
produced from the dissociative adsorption of ethyl
acetate. MS profiles showed that methanol and
traces of ethyl acetate desorb together at low
temperature (50-150 ºC) followed by ethanol. H2
formation occurred at 250-300 ºC through methanol
dehydrogenation and at T  400 ºC from the
decomposition of EA. Further, methoxy and ethoxy
adsorbed species also formed CO2, H2, and CH4 at
T  400 ºC. The formation of a small amount of
acetone via oxidation of ethoxy species that
remained adsorbed at 450 and 500 ºC was also
observed. The comparison of the relative intensity of
the bands associated with adsorbed MeOH and EA
suggests the adsorption of the alcohol occurs
preferentially on the basic sites of the 1Mg1La
catalyst, as proposed by Hattori et al. for basic
oxides [1].
The observed results support the mechanism
proposed in the literature for transesterification
reaction [1,2]. Moreover, they allowed us to suggest
a reaction scheme that takes into account not only
the formation of the main product but also of the
different by-products. Thus, as shown in Figure 1,
MeOH (as methoxy species) and EA (through the
oxygen of carbonyl group) are adsorbed on the
catalyst surface. Then, the oxygen of the methoxy
species interacts with the carbon of the carbonyl
group of EA forming a tetrahedral intermediate (1)
which rearranges leading to the formation of methyl
acetate and ethanol in the gas phase, along with the
formation of ethoxy species adsorbed on the catalyst
(2). The ethoxy species are oxidized by the oxygen
atoms of the catalyst surface forming acetone,
methane, and CO2 (3). Simultaneously, the
dehydrogenation of the methoxy species forms
formaldehyde and adsorbed formate species, which
decomposes to form H2 and CO2 (4).
Figure 1. Reactional scheme for the transesterification of
methanol and ethyl acetate.
4. Conclusions
The in situ DRIFTS-MS analyses made possible the
identification of the intermediates adsorbed on the
catalyst surface and to associate them with the gas
phase products. Thus, a reaction scheme that explain
not only the main reaction but also both the
formation the by-products could be proposed.
Acknowledgments
The authors thank
experimental support.
NUCAT/PEQ/COPPE/UFRJ
for
the
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