Vol. 25, No. 5 (2013)
Supported Ionic Liquids as Catalysts for the Synthesis of Propylene Glycol Methyl Ether 2723
Generally, the mechanism is considered to be as follows15.
Basic catalysis:
Preparation of TMG/MCM-41: The MCM-41 material
was directly prepared by sol-gel method13. 10 g of MCM-41
was dispersed in 150 mL of aqueous solution containing 5.3 g
of [TMG][Lac] and stirred for ca. 6 h. Then the MCM-41 was
separated by filtration and treated with fresh [TMG][Lac] aqueous
solution again. The treated MCM-41 was filtered, washed with
a large amount of deionized water, dried at 105 ºC for 24 h
and named as TMG/MCM-41.
Acidic catalysis:
Preparation ofTMG/Silica-gel:A mixture of tetraethoxy-
orthosilicate (TEOS, 10 mL) and EtOH (7 mL) was heated to
60 ºC and then ionic liquids containing (4 g) of [TMG][Lac]
were immediately transferred into the TEOS.After the formation
of a clear and homogeneous liquid mixture, hydrochloric acid
(5 M, 5 mL) was added and the mixture gradually coagulated.
After aging at 60 ºC for 12 h, the resultant solid material was
dried in vacuum at 150 ºC for 3 h and 7 g of solid sample was
obtained and named TMG/silica-gel.
The epoxide ring of propylene oxide can open at either of
the C-O bonds, to form primary or secondary alcohols. Com-
pared to secondary alcohols, the primary alcohols (such as
2-methoxy-1-propanol) exhibit reproductive and developmental
toxicity16. Therefore, a high selectivity to secondary alcohols
is required for this process.An acid catalyst provides a mixture
of secondary and primary alcohols and the proportion of the
isomers depends on the acid strength. However, the ring of
propylene oxide might preferentially open at the least sterically
hindered position over a basic catalyst, leading to mostly the
secondary alcohol 1-methoxy-2-propanol. Therefore the base
catalysts are attracting much attention in the research and
application of these materials.
It is worth noting that a dry solid sample of silica-gel-
confined ionic liquid was obtained even with ionic liquid loading
of up to 53 wt %, indicating that the ionic liquid added could
be completely encapsulated in the sol-gel. Therefore, the loadings
of confined ionic liquids could be calculated according to the
weight of ionic liquids added originally.
In the past decades, a lot of homogenous base catalysts
were exploited and showed high selectivity to secondary
alcohols, but they had the drawbacks of separation, liquid waste
treatment and corrosion problems. Therefore solid bases were
developed to catalyze the synthesis of propylene glycol ether.
Those heterogeneous catalysts included anionic double
Preparation of TMG/Atta:Attapulgite was treated with
36.5 % hydrochloric acid for 24 h, followed by washing with
distilled water until the washing water was pH 6.5 then drying
in vacuum at 50 ºC for 24 h14. A 10.0 g sample of the purified
attapulgite was dispersed in a 150 mL aqueous solution conta-
ining 5.3 g of [TMG][Lac] with stirring for ca. 6 h. Then the
attapulgite was separated via centrifugation and treated with
fresh ionic liquid aqueous solution again. This procedure was
repeated for a third time. The attapulgite (Atta) treated with
aqueous solution of ionic liquid was filtered, washed with a
large amount of deionized water, dried at 105 ºC for 24 h and
named as TMG/Atta.
hydroxide clays17, basic zeolites18 and basic metal oxides19,20
.
In this work, ionic liquid 1,1,3,3-tetramethylguanidium
lactate ([TMG][Lac]) was immobilized on different solid supports,
i.e., MCM-41, Silica-gel and attapulgite. The as-prepared
materials were used as the basic heterogeneous catalysts for
preparing propylene glycol methyl ether from methanol and
propylene oxide. Good yields and isomer selectivities were
obtained over the three types of catalysts. The catalyses can
be separated from the products easily by centrifugation and
reused without a decrease in activity or selectivity.
Characterization: FT-IR spectra of the samples as KBr
discs were taken in the range of 4000-400 cm-1 on Nicolet-
360 FT-IR spectrometer. Thermal analysis experiments were
performed using a TGA apparatus operated in the conven-
tional TGA mode (SDT Q-600, TA Instrument) at the heating
rate of 10 K/min in a nitrogen atmosphere and the sample size
depended on each type of catalyst. The specific surface area,
total pore volume and average pore diameter were measured
by N2 adsorption-desorption method using Micromeritics
ASAP 2020 instrument. The samples were treated at 453 K
for 3 h under vacuum and then the adsorption-desorption was
conducted by passing nitrogen into the sample, which was
kept under liquid.
EXPERIMENTAL
1,1,3,3-Tetramethylguanidine was purchased from Baigui
Chemical Company (Shijiazhuang, China). Lactic acid was
procured from Shanghai Jingchun Reagent Plant (Shanghai,
China). Acetic acid, ethanol, methanol and propylene oxide,
C16H33N(CH3)3Br, sodium silicate (SiO2/Na2O = 3), CH3COOH,
NH3 25 %, tetraethoxyorthosilicate (TEOS) were from Beijing
Chemical Plants (Beijing, China).
Attapulgite (Atta) provided by Hongfei Company
(Jiangsu, China), the composition of the attapulgite clay was
61.23 % SiO2, 11.42 % Al2O3, 9.07 % MgO, 0.29 % Na2O,
1.32 % K2O, 0.34 % CaO, 7.2 % Fe2O3, 1.07 % TiO2, 0.023 %
P2O5 and 0.17 % FeO. Other reagents and solvents were analy-
tical grade and produced by Shanghai Chemical Reagents
Company. All the chemicals were used as received.
Catalytic performance: The catalytic performance of the
catalysts was evaluated in a 10 mL batch reactor with molar
ratio of methanol to propylene oxide ranging from 1:1 to 5:1.
After running at 323-373 K for the required time under
magnetic stirring, the reactor was cooled down to room tempe-
rature. After centrifugal separation from the catalyst, the
products were analyzed by GC (Fuli 9730) equipped with a
FID detector and a capillary column (PEG20M: length 30 m
× inner diameter 0.25 mm × film thickness 0.25 µm). 1-Propanol
was used as the internal standard to calculate the amount of
products. The yield of propylene glycol methyl ether (primary
Preparation of the catalysts
Preparation of [TMG][Lac] ionic liquid: [TMG][Lac]
ionic liquid was prepared directly by neutralization of 1,1,3,3-
tetramethylguanidine with lactic acid at room temperature12.