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acidic sites. The zeolite catalyst was finely ground and pressed into
a self-supporting wafer (ca. 8–10 mg cmꢀ2), placed into quartz IR
CaF2 windows, pretreated in situ in a 30 mL minꢀ1 of He from room
temperature to 773 K with a ramp rate of 10 K minꢀ1, and then
evacuated at 773 K and 5 ꢁ 10ꢀ3 Pa for 90 min. The cell was cooled
to room temperature and saturated with pyridine. After removing
of the excess pyridine, the spectrum was recorded. Then the sam-
ple was evacuated at different temperatures (473 K, 573 K, and
673 K) for 30 min; the corresponding spectrum was recorded to
distinguish the acid sites with different strengths. The relation
intensities of vibration band of 1540 cmꢀ1 and 1450 cmꢀ1 were as-
signed to the relative concentration of Bronsted and Lewis acid
sites, respectively. TGA experiments were performed on a SDTA851
(Mettlter-Toledo, Switzerland) TGA instrument.
as 1H NMR, HPLC, and ESI–MS (see the results in Supplementary
material). ESI–MS indicates that dimethylanthraquinone and the
uncyclized 2-RBBA are the main side products, with 99.0% purity
of 2-MAQ obtainable via the purification process. From the charac-
terization results, the desired product, 2-MAQ can be synthesized
via a one-pot synthetic protocol of the present paper. We believe
that this developed simpler, cheaper, and ecologically safer one-
pot route of synthesizing 2-MAQ has great potential in industrial-
ized application. In the following sections, the effects of the zeolite
structure and pre-treatment conditions, reaction parameters, acyl-
ation reagent, zeolite particle size on the catalytic performance
are presented, as well as the recycled performance of nano-H-beta
was investigated.
2.3. Catalytic reactions
3.1. Effects of zeolite structure and pre-treatment conditions
The experiments for liquid-phase one-pot synthesis of 2-MAQ
by combining the acylation of toluene with PHA and consecutive
dehydration through dehydration was performed in the stainless
steel autoclave with electro-magnetic stirrer in a batch mode. In
a typical experiment, the PHA (PA), toluene, and zeolite catalyst
(activated at 773 K for 1 h before use) are added into autoclave
with a continuous magnetic stirring. The mixture was heated to
the designed temperature, and held for 1–9 h at this temperature.
The mixture was cooled to room temperature after the reaction
was accomplished; the zeolite catalyst was filtered off from the
mixture, and then fully washed with 1,4-dioxane. The product
mixture was obtained by combining the filtrate and the washing
liquid. The solid cake (products and unreacted PHA) was obtained
by evaporating the toluene (the excess raw material) and 1,4-diox-
ane from the above-mentioned mixture, as well as water produced
in the reaction, with a rotating evaporator. The solid cake was dried
in the vacuum, and then dissolved in the 1,4-dioxane. The sample
was analyzed on a liquid-chromatogram (Agilent 1100) equipped
with a ZOBAꢁSB-C18 (250 ꢁ 4.6 mm) column at room tempera-
ture. The mobile phase was a mixture of H2O and CH3OH with a
proper ratio, flowing at a rate of 1.0 mL minꢀ1, with HPLC collected
at UV 257 nm wavelength. The calibration curves were linear for
PHA (r = 0.9994) and 2-MAQ (r = 0.9963). Typical HPLC can be seen
in Supplementary material. The weights of PHA and 2-MAQ were
calculated based on their peak areas and the correction factors.
The conversion of PHA was calculated by weight percent of the
consumed PHA in the total PHA amount; the selectivity to 2-
MAQ was calculated by weight percent of the desired product in
total products. The yield included in this paper was the HPLC yield,
which was calculated based on the conversion of PHA and the
selectivity of the desired products. The separated solid was washed
with alkaline solution to remove the unreacted PHA, and then the
product was obtained by recrystallization. The separated product
was further characterized by employing 1H NMR and ESI–MS
(see Supplementary material), as well as the melting point
measurement.
3.1.1. Effects of zeolite types
The acylation of toluene with PHA has been performed over the
four H-form zeolites with different structures, H-beta, H-Y, H-MOR,
and H-ZSM-5, to produce 2-MAQ. The results are presented in
Table 1.
From Table 1, H-beta zeolite catalyst appears to be the best ac-
tive catalyst in terms of both conversion of PHA and selectivity to
2-MAQ. H-Y and H-MOR zeolite catalysts exhibit a slightly lower
conversion of PHA and a very low selectivity to 2-MAQ, while H-
ZSM-5 zeolite catalyst exhibits very poor catalytic properties in
both conversion of PHA and selectivity to 2-MAQ; the side prod-
ucts might be produced by isomerization, disproportionation,
cracking, and polymerization. These results may be attributed to
the different structure and surface acidic properties of the cata-
lysts. Due to the interconnected channel architecture, H-beta and
HY zeolites allow for an easier diffusion of the products than H-
MOR. However, compared with that of H-beta, the lower Si/Al ratio
of H-Y will lead to a higher hydrophilicity, which may absorb some
produced water. As a result, the H-Y zeolite is passivated by the ab-
sorbed water. The higher hydrophobic H-beta zeolite is favorable
for the reaction. For the H-ZSM-5, due to its tiny pore size, the reac-
tion probably occurred only on the external surface. Fig. 1 presents
the NH3-TPD profiles for four types of zeolites.
From Fig. 1, it can be observed that these profiles consist of two
peaks: one appears at a low temperature range around 523 K and
the other appears at a high temperature range around 673 K for
H-Y, H-beta, and H-ZSM-5, or 843 K for H-MOR catalysts. The
low and the high temperature regions can be assigned to the weak
and the strong acid sites, respectively. The zeolites (H-Y and H-
MOR) with lower molar ratio of SiO2/Al2O3 possess more and stron-
ger acid sites than H-beta zeolite catalyst, which may lead to more
side reactions; as a result, very poor selectivity of 2-MAQ appears
over both HY and HM. Although the H-ZSM-5 zeolite with higher
molar ratio of SiO2/Al2O3 has a similar surface acidic properties;
again, its tiny pore size leads to very low activity and selectivity.
Table 1
Effects of zeolite structure on the catalytic performance.a
3. Results and discussion
Catalystb
Pore size (nm) Conversion (%) Selectivity (%) Yield (%)c
The micro-sized and nano-sized H-form zeolite-catalyzed liquid-
phase one-pot synthetic route was employed to produce 2-MAQ
(Scheme 1). The present method could be a green and effectual ap-
proach to replace both the traditional two-step routes and the one-
pot gas-phase route. By using the proposed liquid-phase one-pot
cascade reaction route, the heavy effluent pollution and complex
handling could be eliminated. The products were separated from
the reaction mixture (see Section 2). Both separated product and
reaction mixture were characterized by various techniques such
H-beta (22)
H-Y (5.5)
H-MOR (10)
H-ZSM-5 (76) 0.56 ꢁ 0.53
0.75 ꢁ 0.67
0.74 ꢁ 0.74
0.65 ꢁ 0.70
52.6
44.3
41.1
15.4
86.2
25.8
29.5
3.6
45.3
11.5
12.1
0.6
a
Reaction conditions: 0.05 mol PHA, 3:1 Nrea (denoted as molar ratio of toluene
to PHA), 0.27 Wcat (denoted as dosage of catalyst, which is calculated with respect to
the weight ratio of catalyst to PHA), and 5 h tR (denoted as reaction time) at 523 K.
b
The number in parentheses is the molar ratio of SiO2 to Al2O3.
HPLC yield.
c