Aldol Condensation of Citral and Acetone on Mg,Al-mixed oxides - Study of
reaction conditions and catalyst reutilization
Raquel Bastiani1, Cristiane Assumpção Henriques2, José Luiz Fontes Monteiro1
1
NUCAT-COPPE/UFRJ, PO Box 68502, CEP: 21945-970, Rio de Janeiro, Brazil.
2
IQ/UERJ, Rua São Francisco Xavier, 524, CEP: 20559-900, Rio de Janeiro, Brazil.
Introduction
Pseudoionone (PSI) is an unsaturated ketone that is a key starting material for the
synthesis of vitamin A and various carotenoids, flavors and fragrances. This product
usually is obtained from the aldol condensation of citral and acetone catalyzed by
aqueous or alcoholic solutions of bases. However, these homogeneous processes have
some disadvantages such as the occurrence of undesired side-reactions, laborious and
costly separation and purification steps, and environmental problems due to the use of
alkali metal hydroxides. So, the replacement of these conventional processes by a
heterogeneous one (using solid catalysts) would reduce the amount of toxic waste and
the number of steps involved. As reported by different authors [1-4], hydrotalcites
seem to be promising precursors for active catalysts on the aldol condensation
between citral and acetone in heterogeneous media. They are layered double
magnesium-aluminum hydroxides with general formula: [Mg8-xAlx(OH)16(CO32)x/2].nH2O, whose structure consists of brucite-like layers, where partial substitution
of Mg2+ by Al3+ cations originate an excess of positive charge which is compensated
by carbonate anions located with water molecules in the interlayer space [5]. Thermal
treatment of hydrotalcites up to 773 K induces dehydration, dehydroxilation and loss
of compensation anions, forming the Mg-Al-mixed oxides with basic properties and a
poor crystallized MgO-type structure [5,6]. These oxides present small particle size,
large specific surface area, and basic properties which have been attributed to
structural hydroxyl groups, O-2 - M+n acid-base pairs and O2- anions [7]. The present
work reports the on production of pseudoionones by aldol condensation of citral with
acetone catalyzed by a Mg,Al-mixed oxide derived from hydrotalcite. The influence
of reaction parameters, such as acetone/citral molar ratio, temperature, and time of
reaction, on catalytic activity and selectivity was evaluated. The reutilization of the
catalyst was also investigated.
Experimental
The Mg,Al-mixed oxide used as catalyst was obtained from an hydrotalcite precursor
with chemical formula Mg6,4Al1,6(CO3)0,8(OH)16⋅5,86H2O by calcination under dry air
at 723K for 10h. The reaction was studied in liquid phase using a 50mL stainless steel
batch reactor magnetically stirred and heated with an oil bath. The catalytic tests were
carried out under autogenous pressure in the presence of an inert atmosphere (N2) and
using 5wt-% of catalyst. Acetone/citral molar ratio (1 and 5), temperature (303-398K)
and reaction time (30min–10h) were the parameters evaluated. The reaction products
were analyzed by gas chromatography using a 50 m Carbowax 20 M capillary
column.
Results and Discussion
The main products formed from the aldol condensation between acetone and citral
catalysed by a Mg,Al-mixed oxide were the two isomers of pseudoionone (PSIA and
PSIB). Other compounds observed were those from the condensation between two
citral molecules (PCS) or between pseudoionone isomers and one acetone molecule.
The influence of acetone citral/molar ratio was evaluated at 398K. The results showed
that when the acetone/citral molar ratio was increased from 1 to 5 both citral
conversion and PSI selectivity increased and complete citral conversion was observed
after 30min. The influence of reaction temperature was studied using an acetone/citral
ratio of 5.
As can be seen in Figure 1, citral conversion
increased with the increase on both
temperature and time of reaction. Complete
conversion was attained for all temperatures
studied, for runs varying from 0.5h (398K)
to 10h (303K). The selectivity to PSI was
not significantly influenced by reaction
temperature but was strongly dependent on
citral conversion, values in the range 7095% being observed, the highest values
obtained for conversions close to 100%. So,
Figure 1. Influence of temperature on
these results indicated that PSI can be
citral conversion for acetone/citral
obtained from the aldol condensation of
molar ratio of 5.
citral and acetone in heterogeneous media
with high conversion and selectivity using
reaction conditions that are appropriate for the development of the process in a
commercial scale. The reutilization of the catalyst was evaluated using three different
procedures: (i) the catalyst was removed by centrifugation and then reused on a new
catalytic test; (ii) the catalyst removed by centrifugation was exhaustively washed
with acetone and reused on a new catalytic test; (iii) the catalyst was removed by
centrifugation, recalcined at 723K for 10h and then reused. The results for catalytic
tests at both 303K and 398K showed that regardless of the procedure used the
activities of the reused catalysts were similar but lower than that of the fresh one
(Mg,Al-mixed oxide). This indicates that catalyst deactivation cannot be simply
attributed to the adsorption of voluminous by-products in the surface of the catalyst,
but can be better explained considering the occurrence of irreversible modifications in
its texture and/or basicity.
References
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