1994 Serrao et al.
Asian J. Chem.
O
O
ZrO2 – Solid acid
ROH
H2O
+
+
OH
OR
O
O
Alcohol
Levulinic acid
Alkyl levulinate
Scheme-I: Esterification of levulinic acid with an alcohol over a solid acid catalyst
Based on the advantages of honeycomb monoliths as a
catalyst carrier, and industrial application of ethyl levulinate,
the work done on the synthesis of cordierite honeycomb coated
with solid acids such as ZrO2, Mo(VI)/ZrO2 and W(VI)/ZrO2,
its physico-chemical characterization and catalytic activity
study in liquid phase ethyl levulinate synthesis via esterification
of levulinic acid is reported. The reaction parameters including
nature of the catalyst, amount of catalyst, levulinic acid to ethanol
ratio and reaction time in levulinic acid esterification is studied
to obtain highest productivity of ethyl levulinate.
Characterization of catalyst:The catalytic materials were
characterized for their physico-chemical properties such as
specific surface area, crystallinity, surface acidity and morpho-
logy by BET, powder XRD, NH3-TPD/n-butyl amine back
titration and SEM techniques, respectively. The BET surface
area of the catalytic material was determined using nitrogen
as the absorbent in a NOVA-1000 high-speed gas sorption
analyzer. The total surface acidity was measured by NH3-TPD
method as well as n-butyl amine back titration method using
bromothymol blue indicator. The crystalline nature of catalytic
materials was determined by recording their powder XRD
patterns on a BRUKER eco-D8-ADVANCE system working
with CuKα radiation (1.5418 Å). The morphologies of catalysts
were characterized by scanning electron microscopy (SEM,
SU3500, HITACHI).
Catalytic activity: Catalytic activity studies were investi-
gated in the esterification of levulinic acid with ethanol under
reflux conditions. In a typical procedure, a mixture of levulinic
acid and ethanol (1:8 molar ratio, total volume = 30 mL) was
taken along with known amount of catalyst (either honeycomb
coated or powder forms of ZrO2, 5 % Mo(VI)/ZrO2 and 5 %
W(VI)/ZrO2) in the reactor and refluxed for a definite period
of time. The reaction mixture was cooled to room temperature
after a specified time and analyzed for the products using GC
[Netel Gas chromatograph with 2 m stainless steel column
packed with 10 %AT-1200 + 1 % H3PO4 onW-HP]. The percen-
tage yield of the products was calculated from the GC results
based on the relative response of the different compounds in
the reaction mixture with respect to levulinic acid. The catalytic
activity measurements were made by varying the reaction
conditions such as molar ratio of reactants, amount of catalyst,
nature of catalyst and time of reflux to get higher yields and
better selectivity of the product. Similarly, esterification reactions
of levulinic acid with n-butanol, n-pentanol, isoamyl alcohol
and cyclohexanol were carried out under a definite set of reaction
conditions.
EXPERIMENTAL
Cordierite honeycomb monoliths (height = 1.20 cm, dia-
meter = 2.50 cm and hole size = 0.2 cm) were used for the
present work were supplied by Shreya Ceramics, Baroda, India.
Zirconyl nitrate, ammonium molybdate and tungstic acid were
supplied by M/s. LOBA Chemie India Ltd., India. Levulinic
acid, isoamyl alcohol, n-butanol, n-pentanol and cyclohexanol
were supplied by Sigma-Aldrich Chemicals Pvt. Ltd. and Fischer
Scientific Pvt. Ltd., India.
Preparation of catalytic material in honeycomb coated
form: Solid acids such as ZrO2, 5 % Mo(VI)/ZrO2 (MZ) and 5 %
W(VI)/ZrO2 (WZ) were coated on different honeycomb mono-
liths by using 'dip and dry' method [16]. Before coating the
active catalysts, the honeycomb monolith was wash coated
with zirconia in order to increase the surface area and to have
a support material which has a better interaction with the active
catalyst [17]. Typically, for coating ZrO2 on bare monolith, a
dilute solution containing known amount of zirconyl nitrate
was prepared. The resulting solution was coated on a monolith
by dipping and drying in a furnace preheated at 400 ºC. The
dipping and drying steps were repeated for 8-10 times until
~0.2 g of the catalyst is coated on the monolith. Similarly, bare
monoliths were coated with 5 % Mo(VI)/ZrO2 and 5 % W(VI)/
ZrO2 by using dilute solutions containing 0.96 g of zirconyl
nitrate and 0.25 g of ammonium molybdate and 0.03 g tungstic
acid.
RESULTS AND DISCUSSION
The honeycomb coated with ZrO2, 5 % Mo(VI)/ZrO2 and
5 % W(VI)/ZrO2 were calcined at 550 ºC for 5 h in a muffle
furnace before its use as catalysts.
The values of BET surface areas for different catalytic
materials (powder form) used in this investigation are presented
in Table-1.Among different catalytic materials, pure ZrO2 was
found to be having least surface area, but when pure ZrO2 was
incorporated with Mo(VI) or W(VI) ions the surface area
increased to almost two fold. This can be explained based on
the formation of Mo-O-Zr andW-O-Zr linkages in 5 % Mo(VI)/
ZrO2 and 5 % W(VI)/ZrO2, respectively [18].
Preparation of catalytic material in powder form: ZrO2,
5 % Mo(VI)/ZrO2 and 5 % W(VI)/ZrO2 were also prepared in
powder forms by impregnation method using salts such as
zirconyl nitrate, ammonium molybdate and tungstic acid. Known
amounts of the required salts were taken in a china dish and
made a paste with small amount of water. The paste was then
dried in an air oven at 120 ºC for 12 h and calcined at 550 ºC
for 5 h in a muffle furnace.
Powder XRD patterns of catalytic materials are presented
in Fig. 1. From the powder XRD patterns of pure ZrO2, it is