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H. Xue et al. / Catalysis Communications 37 (2013) 75–79
Fig. 1. X-ray diffraction patterns (a) and N2 adsorption–desorption isotherms (b) of the HMOR and HMOR-1023 samples.
calcined at 773 K for 6 h in air. The sample thus obtained was denoted as
HMOR-1023.
temperature for 1 h to remove the physisorbed H2O. After the catalyst
was cooled down to 473 K, a 5% DME/50% CO/2.5% N2/42.5% He mixture
was introduced through a mass flow controller at a gas hourly space
velocity of 1250 ml/(g · h) and the reactor was pressurized to 1.0 MPa.
The effluent was analyzed by an online gas chromatograph (Agilent
6890 N) equipped with a thermal conductivity detector and a flame
ionization detector.
2.2. Catalyst characterization
XRD patterns of the samples were recorded on a D/MAX 2500 X-Ray
diffractometer (Rigaku) using a Cu–Kα radiation that operated at 40 kV
and 200 mA. The relative crystallinities of the samples were estimated
by assuming that the crystallinity of the parent HMOR zeolite was 100%.
Solid-state MAS NMR experiments were carried out on a Varian
Infinityplus-400 spectrometer. 29Si MAS NMR spectra were obtained at
79.4 MHz using 7.5-mm MAS probe with a spinning rate of 10 kHz.
Chemical shifts were referenced to 4,4-dimethyl-4-silapentane sulfonate
sodium. 27Al MAS NMR spectra were recorded at a resonance frequency
of 104.2 MHz with a spinning rate of 10 kHz. Chemical shifts were
referenced to (NH4)Al(SO4)2 · 12H2O at −0.4 ppm as a secondary refer-
ence. 1H MAS NMR spectra were collected at 399.9 MHz with a 4 s recycle
delay, 80 scans and spinning rate of 10 kHz. Chemical shifts were
referenced to admantane. Spectra deconvolution was performed using
the Dmfit software based on Gaussian–Lorentzian line shapes [15].
Temperature-programmed desorption of ammonia (NH3-TPD) was
conducted with a U-shape quartz tube reactor using He as the carrier
gas. 0.1 g sample was heated to 773 K at a rate of 10 K/min and
maintained at that temperature for 1 h under He flow (30 ml/min).
The sample was purged with a 10% NH3/He mixture (30 ml/min) for
30 min at 473 K. The physisorbed ammonia was removed by purging
the sample with a 0.6% H2O/He mixture (30 ml/min) at 473 K for 1 h.
After cooling down to room temperature, the sample was heated to
873 K at a rate of 10 K/min under He flow (30 ml/min). The amount
of NH3 desorbed was monitored by a mass spectrometry (Omnistar
QMS200).
3. Results and discussion
3.1. Structural analysis
Fig. 1 shows the XRD patterns of the HMOR samples. They exhibited
the typical diffraction lines of mordenite-type zeolites, and the crystallin-
ity of the steam-treated sample had decreased slightly to 92%, indicating
that the high-temperature steam treatment had not significantly changed
the crystalline structure. N2 adsorption–desorption isotherms revealed
that the HMOR sample exhibited a Type H4 loop associated with narrow
slit-like pore. But the HMOR-1023 sample showed a Type H3 loop, associ-
ated with slit-shaped pores [16]. As shown in Table 1, the steam treat-
ment had no obvious effect on the surface area, but has considerably
reduced the microporous surface area and volume. The total pore volume
of HMOR-1023 was larger than that of the parent HMOR, due to the cre-
ation of inner mesopores during the steam-treatment [17]. This result in-
dicates that the high-temperature steam treatment has considerably
modified the microporous structures of the parent HMOR.
Fig. 2 shows the NMR spectra of the samples. The 29Si MAS NMR
spectra were deconvoluted to four Gaussian peaks at about −113,
−107, −102 and −99 ppm, which were assigned to Si(0Al), Si(1A1),
Si–OH and Si(2Al) units [18], respectively. Compared with the parent
HMOR, the relative intensity of the −107 ppm peak from Si(1A1) de-
creased significantly in the HMOR-1023 sample, while the contribution
from Si(2Al) sites became negligible. Accordingly, the (Si/Al)NMR ratio
increased from 8.5 to 15.5, implying that about half of the framework Al
was removed. In the 27Al MAS NMR spectra, the strong peak at 54 ppm
Fourier transform infrared spectra were recorded on a Bruker
Vector-77 instrument with a resolution of 2 cm–1. Before the mea-
surements, the samples were evacuated at 723 K for 5 h.
Temperature-programmed oxidation (TPO) measurements on the
used samples were conducted with a U-shape micro-reactor connected
to a mass spectrometer (QMS-200, Omnistar). 100 mg sample was heat-
ed from room temperature to 1073 K at a rate of 10 K/min in a flowing
20% O2/He mixture (40 ml/min). The effluent was monitored by the
mass spectrometer.
Table 1
Textural and structural properties of the samples.
Sample
SBET
/
Smicropore
/
Vtotal
/
Vmicropore
/
NH3 desorption
amount (mmol/g)
2.3. Catalytic test
m2 g−1 m2 g−1
ml g−1 ml g−1
Total 8MR 12MR
DME carbonylation was carried out in a continuous flow fixed-bed re-
actor. 600 mg sample (40–60 mesh) was loaded into the reactor, heated
to 773 K at a rate of 5 K/min under N2 flow (30 ml/min) and kept at that
HMOR
528
503
443
0.326
0.402
0.254
0.224
1.57
0.84
0.76
0.73
0.81
0.11
HMOR-1023 528