Wang et al.
FULL PAPER
dried and calcined at 600 ℃ for 2 h to yield the ultimate
Results and discussion
catalysts.
Catalyst characterization
Characterization
Three binder-free catalysts with different framework
SAR, MD1, MD2 and MD3 were studied in this work.
Meanwhile, a binder-containing catalyst MD5, which is
a precursor one of MD3 without treatment by binder
transformation, was set as a contrast with the
binder-free catalysts. Table 1 lists the data of the com-
positions of the catalysts. The framework SAR values,
measured with XRF and calibrated by Al MAS NMR,
were 435, 92.4, 32.6 and 24.4, respectively for MD1,
MD2, MD3 and MD5. Figure 1 presents the XRD pat-
terns of the catalysts. No obvious differences exist in the
patterns of the catalysts, except that the relative crystal-
linity of MD5 is about 80% in comparison with that of
MD3 due to the amorphous binder in the bulk catalyst.
As shown in Figure 2, evident hysteresis loops appear in
XRD patterns were collected on a Riguka D-MAX/II
A X-ray diffractometer in a scanning range of 5°—35°
(
2θ) at the rate of 8 (°)/min with Cu Kα radiation, 30
kV/20 mA.
Chemical compositions were determined by X-ray
fluorescence scattering spectroscopy (XRF,
Bruker-AXS S4 EXPLORER) with rhodium target.
2
7
27
Spectra of Al magic-angle spinning (MAS) nuclear
magnetic resonance (NMR) were recorded on a Bruker
MSL 300 spectrometer operated at 78.205 MHz. The
width of the spectrum was δ 200, and the rotor was spun
at 3.0 kHz. The radio-frequency field was 51.0 kHz, the
reversing angle was 18°, and the recycle time was 500
2
7
3 2
ms. AlCl •6H O was used as a reference for the Al
2
7
chemical shift. The samples investigated with Al MAS
NMR were kept over a saturated solution of ammonium
chloride at room temperature for at least 12 h.
the isotherms of these catalysts in p/p
0
=0.45—1.0, in-
dicating that pore size distributions in mesopore range
exist in these catalysts. The obvious rising slope and the
largest hysteresis loop are present in the lowest adsorp-
tion isotherm for MD5 (see Figure 2a). The fact proved
that a significant adsorption in mesopore occurred at
2
The isotherms of N adsorption and desorption were
measured using a Micromeritric ASAP 2010 instrument
at -196 ℃. The zeolite samples tested were dehy-
drated at 300 ℃ for 6 h before determination.
p/p
0
>0.4, and implied a wide distribution of pore size
Temperature-programmed desorption of NH
3
in the binder-containing catalyst. Figure 3 shows the
pore size distributions in these catalysts, in which the
binder-free catalyst possesses the major average pore
size at ca. 6.8 nm. In comparison, the binder-containing
catalyst gives a wider pore size distribution consistent
with the result of the adsorption isotherms (see Figure
(
3
NH -TPD) was performed in a stainless steel U-type
tube connected with a thermal conductivity detector
TCD) in GC equipment at atmospheric pressure. Prior
(
to measurement, the samples were pre-hearted at 350 ℃
3
in He flow for 1 h. Pure gas of NH was then injected
1
1
into the tube till the samples reached adsorption satura-
tion in the He flow at 120 ℃. The temperature was then
raised up with a rate of 10 ℃/min from 120 to 600 ℃
3a). Based on the patent, sesbania powder, one kind of
forming assistant, was added into the mixture of a
binder and the raw zeolite powder to prepare the column
form particles of the catalyst with extrusion molding.
After calcinations, the sesbania powder in the catalyst
was burnt up, and some open space between zeolite
crystallites appeared. This may result in the mesoporous
structures. In catalyst MD5, some binder plugs the
mesopores. For this reason the diameter of the
mesopores decreases and the distribution of the pore
size becomes wider.
Table 2 summarizes the surface and porous pro-
perties of the catalysts calculated from the adsorption
and desorption isotherms above. The adsorption proper-
ties of the catalysts MD1, MD2 and MD3 are similar
with each other. Whereas, the surface area and the pore
volume of MD5 are much smaller than thoes of the
binder-free catalysts. The reason is that the existing
3
for desorbing NH .
Catalysis reaction
Catalytic dehydration of methanol on the catalyst
was carried out in a stainless steel fix-bed reactor at at-
mospheric pressure. In each run, 5.0 mL of catalyst was
loaded, and the reactor was kept at a fixed reaction
temperature in the range of (180—300)±1 ℃. The feed
of crude methanol (90.3%, purchased from Shanghai
Coking Co., Ltd.) was injected into the reactor by a mi-
cro-electron metering pump. The compositions of the
reaction products were analyzed online by an Agilent
6820 GC equipped with an FID detector. The gas lines
were kept at 200 ℃ for preventing the condensation of
the reactants and the products.
Table 1 Chemical compositions of catalysts
2
7
Al MAS NMR
Extra-framework
Sample
MD5
SAR measured by XRF
23.0
Framework SAR
24.4
Na
2
O/wt%
Framework
94.2%
5.8%
0.06
MD1
MD2
MD3
435
92.4
31.0
100%
100%
95.1%
435
92.4
32.6
—
—
4.9%
0.02
1
84
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Chin. J. Chem. 2010, 28, 183— 188