P. Hirunsit et al. / Applied Catalysis A: General 460–461 (2013) 99–105
101
60
(a)
12
10
8
Equilibrium
Experimental
No catalyst
50
40
30
20
10
0
DME
6
4
Methanol
Other C-containing
species
2
0
0.0
0.5
1.0
1.5
2.0
2.5
200
250
300
350
400
Temperature (oC)
S/C (mole ratio)
Fig. 5. The effect on S/C mole ratios on DME hydrolysis. Reaction tempera-
ture = 300 ◦C; GHSV = 4000 h−1
1.0
0.8
0.6
0.4
0.2
0.0
.
(b)
H2
CO
CH4
CO2
thermal conductivity detector (VARIAN, CP-4900). A PoraPLOT Q
column was used for separation of DME, CH3OH, and CO2 and a
molecular sieve 5A column for separation of H2, O2, N2, CH4, and
CO. DME conversion is defined as follows:
ꢀ
ꢁ
FDMEin − FDMEout
DME conversion (%) = 100
FDMEin
where FDMEin and FDMEout stand for the molar flow rate of DME into
and out from the conventional flow reactor, respectively.
2.3. Computational details
The simulation methodology employed fully periodic plane-
wave Density Functional Theory (DFT) calculations as implemented
with Projector Augmented Wavefuction (PAW) [30,31] method
for representing the non-valence core electrons. For all calcula-
tions reported herein, we have used 400 eV cutoff energy and the
Methfessel-Paxton smearing [32] of order 1 with a value of smear-
ing parameter of 0.2 eV.
200
250
300
350
400
Temperature (oC)
Fig. 4. Temperature dependence of reactants and products during DME hydrolysis
catalyzed by ␥-Al2O3: (a) effluent flow of DME, methanol, and C-containing gases,
(b) effluent flow of H2, CO, CH4 and CO2. The dotted lines (. . .) in (a) represent the
equilibrium data. Reaction conditions: S/C = 1.5; GHSV = 2200 h−1
.
catalysts. A sample was heated up to 500 ◦C and kept for 1 h in He
flow for degassing. The catalyst sample (200 mg) was cooled down
to 100 ◦C, followed by adsorption of NH3 in pure NH3 flow until sat-
uration. Consequently, NH3–TPD was initiated in a heating process
at a rate of 10 ◦C min−1 up to 800 ◦C. The NH3 desorbed from the
catalyst surface was monitored by an on-line mass spectrometer.
Temperature-programmed oxidation (TPO) was employed to ana-
lyze the carbon deposition on catalyst surfaces. The catalyst sample
(100 mg) was oxidized in 5% O2/He flow in a heating process at a
rate of 10 ◦C min−1. The product gases were monitored by on-line
mass spectrometer.
The ␥-Al2O3 bulk model structure was taken from Ref. [33,34]
shown in Fig. S1 in the supplementary material. The experimen-
tal results found the alumina to be in gamma crystalline phase
or partially in an amorphous form. Attempts to characterize the
structure of the alumina in the amorphous phase were however
unclear. Thus, the (1 0 0) and (1 1 0) crystalline surfaces were chosen
because the (1 0 0) and (1 1 0) dominates in ␥-alumina nanocrys-
(1 1 0) is similar to the predominant surface hydroxyl group type
found on an amorphous alumina surface obtained by the atom-
istic molecular dynamics simulation by Curtiss and co-workers
[35]. The slab models of ␥-Al2O3(1 0 0) and (1 1 0) surfaces con-
˚
2.2. Evaluation of catalytic performance
tain ten Al2O3 molecular units and ∼12 A of the vacuum region
excluding adsorbates. The side and top view of dry ␥-Al2O3(1 0 0)
and (1 1 0) surfaces are shown in Fig. 1a and b, respectively. For ␥-
Al2O3(1 0 0), the slab is composed of 9 atomic layers, where the top
5-layers are relaxed while the bottom fixed 4-layers were set to ␥-
Al2O3 bulk bond distances according to their optimized structural
model as determined from bulk calculations. For ␥-Al2O3(1 1 0),
the relaxed atoms are located at ∼45% of the top slab excluding
the vacuum region. The p (1 × 1) unit cell is applied for hydrox-
alted surface study and the bigger p (2 × 1) unit cell is appled when
DME and DME dissociation products adsorbed on hydroxylated
The evaluation of catalytic activity was carried out using a
conventional flow reactor under atmospheric pressure. Mass flow
controllers were used to adjust the gas feed rate and a low pressure
gradient pump was used to control the water feed rate. A mixture
of DME, steam, and nitrogen at a steam-to-carbon ratio (S/C) and a
gas hourly space velocity (GHSV) was supplied to the catalyst bed
at reaction temperature.
The analysis of influent and effluent gaseous compositions was
carried out using online gas chromatographs equipped with a