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Table 1 The catalytic performance of different catalysts on this
conditions for another 24 h. The product was recovered by
reactiona
ltration, reuxing in ethanol for 4 h thrice and then dried at
ꢀ
50 C for 6 h. The dried solid powder was kneaded vigorously
Catalyst
Conversionb (%)
Selectivityc (%)
with 5 mL of acidic silica sol (25 wt%), followed by molding into
a cylinder with diameters of 3–3.5 mm by use of an extruder.
Then, the sample cylinders were dried at 65 ꢀC for 12 h and cut
into small pieces with lengths of about 5 mm. These obtained
catalysts were denoted by S-1, S-2, S-3 and S-4, respectively.
S-1
S-2
S-3
S-4
28.7
44.7
35.1
31.1
56.2
58.1
57.6
59.9
a
Reaction conditions: temperature: 60 ꢀC; molar ratio of ammonia/
acetone: 1 : 6; gas hourly space velocity (GHSV): 10.4 hꢂ1. The each
b
2.3 Catalytic reaction
data point is an average of three or more runs. The conversion of
acetone. c The selectivity of triacetonamine.
The condensation of acetone and ammonia was carried out in a
tubular, xed-bed reactor with an inner diameter of 15 mm and
a length of 650 mm, which was charged with 40 mL catalysts.
Acetone was dosed into the reactor by a syringe pump. The
ammonia ow rate was set by an S49 33/MT mass gas ow
controller. The temperature in the reaction zone was measured
by a thermocouple located in the center of the tube and con-
nected to a proportion integration differentiation cascade
controller. The reaction mixture was analyzed by gas chroma-
tography (GC) with a 30 m SE-54 capillary column and a Flame
Ionization Detector (FID). Moreover, the components of the
reaction mixture were identied by a gas chromatography-mass
spectrometry (GCMS) equipped with an HP-5 capillary column
(30 m ꢁ 0.25 mm, 0.2 mm lm thickness) and an ion trap MS
detector.
selectivity of triacetonamine has persecuted the manufacturers
for a long time. In order to increase the selectivity of tri-
acetonamine, the improvement of catalysts is crucial. Today,
NH4NO3 is considered as a suitable catalyst for industrial
production and its nature is a Brønsted acid. So a strong
Brønsted acid may be efficient for the synthesis of triaceton-
amine. In addition, considering the excellent mechanical
strength and stability of silicone, we tried to immobilize
sulfonic acid on the silicone to yield catalysts S1, S2, S3 and S4.
Their catalytic performances were investigated and compared
with NH4NO3/g-Al2O3, NH4NO3/SiO2, kinds of sulfonic acid
resin and solid super acids, such as SO42ꢂ/ZrO2 and SO42ꢂ/TiO2.
The NH4NO3 supported catalysts and the solid super acids
exhibited shorter life due to the leaking of the active compo-
nents. The sulfonic acid resins normally suffer from easy
swelling, poor mechanical strength and low thermal stability,
which inhibited the application in continuous synthesis of tri-
acetonamine. However, the sulfonic acid-functionalized meso-
porous silicas displayed excellent mechanical strength and
thermal stability. Their catalytic performances were character-
ized and the obtained results were shown in Table 1. It was
found that with the increase of MPTMS content, the acetone
conversion increased from 28.7% to 44.7% and then decreased
to 31.1%. The triacetonamine selectivity remained essentially
unchanged. Catalyst S-2 exhibited better catalytic performance
than others. So it was speculated that not just the acidity, but
also the textual property of catalysts might have an important
effect on the catalytic performance.15–17 In order to verify this
speculation, catalysts, S-1, S-2 and S-3, were characterized by
TG, XPS, elemental analysis, acid–base titration and N2
adsorption and desorption experiments.
2.4 Catalyst characterization
Temperature–gravity properties of these catalysts, S-1, S-2 and
S-3, were measured with an STA 409PC thermo gravimetric (TG)
analyzer. The catalysts were heated from room temperature to
800 ꢀC at a rate of 10 ꢀC minꢂ1 in a stream of N2 (40 mL minꢂ1).
X-ray photoelectron spectroscopy (XPS) was recorded on a
PHI1600 spectrometer with an Mg Ka X-ray source for excita-
tion. Acid–base titrations were used to measure acid capacity of
the catalysts. Typically, 0.05 g catalyst was added to 10 mL 2 M
NaCl solution and stirred overnight. This solution was then
titrated with 0.01 M NaOH solution in the presence of phenol-
phthalein.13 Elemental analysis was carried out on a Vario Micro
cube element analyzer. N2 adsorption and desorption experi-
ments were performed in liquid nitrogen using a NOVA 2000e
analyzer (Quantachrome, US). The total surface area (SBET) was
calculated from the linear part of the Brunauer–Emmer–Teller
(BET). The micro pore volume (Vmicro) and the external surface
area (SEXT) were estimated by the t-plot. The pore-size distri-
butions were obtained using the method of Barrett–Joyner–
Halenda (BJH).
3.2 Catalyst characterization
3.2.1 TG. Thermogravimetric proles of the three catalysts
(Fig. 1) showed similar features, with two distinct weight losses
in the ranges of 25–100 ꢀC and 250–750 ꢀC, corresponding to the
desorption of physisorbed water and the degradation of the
3. Results and discussion
3.1 Catalyst selection
As mentioned above, the reaction of acetone and ammonia is immobilization fraction, respectively. The Fig. 1 clearly dis-
quite complicated. A number of reversible reactions proceed played that with the increase of MPTMS content, the extent of
competitively at the same time. Many studies have demon- functionalization increased. However, it was surprised that DTG
strated that several compounds, like triacetonamine, acetonine, proles of these catalysts exhibited two peaks in the range of 300
phorone, diacetone alcohol, diacetonamine and mesityl oxide, to 600 ꢀC. The reason may be attributed to the presence of non-
are co-existed in the reaction mixture.14 Therefore, the low oxidized thiol groups, which was further demonstrated by XPS.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 17860–17865 | 17861