purity) was purchased from the Shandong Plant Protection
Drug Factory (China), purified by recrys◦tallizing twice from
re-distilled water and then dried at 60–70 C until of constant
weight. All other chemicals were of analytical grade and used
without further purification. PE, p-toluene sulfonic acid and
activated charcoal were dried at 60 ◦C until of constant weight
before use. C5–C9 straight-chain monocarboxylic acids were
reaction temperature was also promptly determined by placing
a thermocouple into the reaction mixture when the reaction
had ceased. In the conventional heating procedure, the same
reactants, composite catalyst and activated charcoal as the MW
reaction were added into a 100 ml round-bottomed flask with
two anti-bumping zeolite granules. Then, the reaction mixture
was heated by an immersion heater for a set time at a set
temperature, and the water produced was continually distilled
out under a reduced pressure (46–63 KPa) until no further
water could be removed. The water produced was collected in a
recipient vessel.
˚
dehydrated by 4 A molecular sieves, and sulfuric acid was stored
under vacuum for some time before use.
Analytical methods
After cooling down to ambient temperature, the reaction
mixture was filtered. The filtrate was washed with 15 ml distilled
water, followed separately 2 or 3 times by 15 ml of a 10% sodium
hydroxide solution. The final repeated washing was with distilled
water until the organic layer was neutral. The organic layer was
dried over anhydrous sodium sulfate. The crude product was
purified by silica gel column chromatography using diethyl ether
as the eluent. The product structures were characterized by IR,
1H NMR, 13C NMR and mass spectroscopy.
Due to the molecular weights of products 4a–d and 5a–e
being higher, both their boiling points and viscosities are also
higher.18 Therefore, the products cannot be measured on a gas
chromatogram, and are also easily lost during separation and
purification. Therefore, the yield (Y) was calculated from the
ratio of the experimental (We) and theoretical (Wt) values of
the water produced (eqn (3)): 21
IR spectra were recorded on a Perkin-Elmer GX FT-IR spec-
trophotometer in KBr pellets. NMR spectra were measured on
a Bruker AV 300 spectrometer (300 MHz) in CDCl3. MS spectra
were obtained using a Bruker HCT Agilent Technologies 1200
Series instrument. The colors of the prepared esters were de-
termined by platinum-cobaltous colorimetry (GB/T3143-82).20
Refractive indices were measured on a ZWA-J refractometer at
25 ◦C. Product viscosity was measured at 40 0.05 ◦C using
an Ubbelohde viscometer in a temperature-equilibrated bath.
Re-distilled water was used as the calibrant.
General procedure for evaluating the microwave absorbance of
different materials
The MW reactor used (WP-650, Nanjing Lingjiang Science and
Technology Corporation) could process in distillation or reflux
modes, and the MW power was adjustable from 0 to 650 W.
Temperatures were measured by placing a thermocouple into
the sample immediately before, and again at the end of, each
irradiation.
C5–C8 carboxylic acids 1 (20 ml) or solid samples of 2 or 3
(5.0 g) were added into a 50 ml beaker. Then, the beaker was
placed into the MW oven. The samples were heated at 300 W
for 1 or 320 W for 2 and 3 for 10, 20, 30, 40, 50, 60, 70 or 80 s,
and the final temperatures were then measured. DT, the change
in temperature, was defined as in eqn (2):
Y(%) = 100 ¥ We/Wt
(3)
The experimental value (We) of water produced was determined
by weighing the empty recipient vessel (W1) and the recipient
vessel containing the produced water (W2). The product water
weight (We) was calculated as in eqn (4):
(4)
We = W2 - W1
Conclusions
(2)
DT = T2 - T1
This study presents the thermal behavior of starting materials
of different polarities under microwave irradiation. An efficient
composite catalyst consisting of sulfuric acid and p-toluene
sulfonic acid was screened. Optimization of the reaction condi-
tions was performed for di-PE and PE. The physical properties
of the polyol esters were investigated. From a green chemical
standpoint, the present method is a rapid, highly efficient and
eco-friendly synthetic approach for the preparation of polyol
esters for lubrication oils.
where T2 was the final temperature and T1 was the initial temper-
ature. Solid samples of 2 and 3 were stirred mechanically during
irradiation to ensure uniform heating. Before the temperatures
were measured, liquid samples were briefly stirred to create a
uniform overall temperature.
General procedure for MW-activated and conventional heating
reactions of 4a–d and 5a–e
di-PE 2 (10 mmol) or PE 3 (22 mmol), C5–C9 carboxylic acids
1 (80 mmol for di-PE 2 and 96 mmol for PE 3), composite
catalyst and activated charcoal (0.13 g) were added into a
100 ml round-bottomed flask with two anti-bumping zeolite
granules. The above mixture was MW-irradiated at a power of
300–320 W and the water produced was continually distilled
out under a reduced pressure (46–63 KPa) until no further
water was distillable and the mixture ceased to boil. At this
point, the reaction was considered “ceased”, and the reaction
time was determined using a stopwatch by counting from the
beginning to the ceasing of boiling of the reaction mixture. The
Acknowledgements
The authors gratefully acknowledge financial support from the
Chongqing Grand Natural Science Department in China (Grant
no. 6683). F. Z. thanks Dr Shi-Hong Chen for language revision
of the paper.
References
1 (a) C. S. Li, Sci. Technol. Chem. Ind., 2000, 8, 76; (b) B. Z. Zhang,
Synth. Lubricants, 1997, 24, 26; (c) Y. K. Luo and D. H. Hu, Synth.
Lubricants, 1994, 21, 36.
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 178–184 | 183
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