T.-S. Zhao et al. / Catalysis Today 149 (2010) 98–104
99
was quick enough under appropriate temperature and pressure
conditions.
(30 mL/min) from room temperature to the desired temperature in
a ramp rate of 5 K/min.
Our previous experiments demonstrated that potassium
carbonate can be a promoter for the two-step methanol synthesis
with potassium formate as an intermediate for the formation of
methyl formate at 443 K [11]. In this paper a binary catalyst
composed of homogeneous potassium formate and solid copper–
magnesia, and its activity for the slurry phase methanol synthesis
from CO/H2/CO2 at low-temperature conditions are described. The
possible reaction pathway for the salient catalytic performance is
also investigated.
Cu dispersion was analyzed as follows: 200 mg catalyst sample
was first reduced at 533 K by 5% H2 (30 mL/min) for 1 h and purged
by He (30 mL/min) at 363 K for 1 h. Then N2O was pulse injected
and the effluents N2O and N2 were synchronously analyzed on a
GC-950(TCD) until the reaction of N2O on the Cu surface finished.
Cu dispersion and surface areas were calculated based on the
consumed N2O.
Cu dispersion (%) = surface Cu atom number/total Cu atom
number
Cu surface area (m2/g) = 4
catalyst weight, where r presented Cu atom radius, 0.1278 nm.
p
r2 ꢁ surface Cu atom number/
2. Experiments
3. Results and discussion
Cu/MgO catalysts in different molar ratios were prepared by
coprecipitation method from nitrates with sodium carbonate as
precipitator. The precipitating pH and temperature were 10 and
338 K, respectively. The obtained precipitates were dried at 393 K
for 6 h, followed by calcinations in air at 623 K for 1 h and then
crushed into 20–40 meshes, reduced in a 5% H2 flow at 623 K for 6 h
and passivated by 1% O2. Activated carbon (AC) supported
palladium catalyst for the separate experiments was prepared
as follows: AC of 20–40 meshes was first vacuum pretreated at
393 K for 4 h, then impregnated with a Pd(NH3)2(NO2)2 aqueous
solution and dried at 393 K for 12 h. Then it was heated at 673 K for
6 h in N2 atmosphere, reduced in 5% H2 for 6 h and passivated by 1%
O2.
Activity evaluation of the catalysts was carried out in a flow-
type semi-batch autoclave reactor with an inner volume of 100 mL.
Typical reaction conditions were as follows unless specially
indicated: catalyst: Cu/MgO 2 g/HCOOK 2 g, ethanol solvent:
40 mL, charge gas: CO/H2/CO2/Ar = 31.34/63.69/1.99/2.98, W/
F = 75 g h/mol, temperature 423 K, pressure 3 MPa, stirring speed,
1000 rpm and time-on-stream, 12 h. Evaluation procedure was as
follows: catalysts were first ground in ethanol to slurry and
transferred into reactor. The reactor was then sealed, purged 3
times using charge gas and poured to the desired reaction pressure.
Once the desired reaction temperature was reached, the reaction
started. Effluent gas was analyzed on a GC-920 (TCD) connecting a
2 m AC separation column using argon as internal standard. After
the reaction, the reactor was cooled rapidly to ambient tempera-
ture. Liquid products were analyzed on a GC-16A (TCD) connecting
a 2 m GDX-203 separation column using 1-propanol as internal
standard. Result calculations were as follows:
3.1. Catalytic activity
Cu/MgO catalyst alone showed lower activity for methanol
formation from CO/H2/CO2 at 423 K in ethanol solvent as shown in
Table 1. Using Cu/MgO (1:1) only as the catalyst, the conversion of
CO and the total carbon conversion were 10.4% and 8.6%,
respectively. As the molar ratio of Cu/MgO increased to 3:1, the
conversion of CO and the total carbon conversion were 18.9% and
16.1%, respectively, and the selectivity of methanol was 99.4%. In
addition, the conversion of CO2 was negative, suggesting that CO2
was produced accompanying the synthesis reaction. Because of the
existence of trace amount of water in ethanol or by-product water,
the CO2 formation could be attributable to water gas shift reaction
(1) even at lower temperature condition.
H2O þ CO ! CO2 þ H2
(1)
In the presence of HCOOK only as the catalyst, CO showed very
low reaction while CO2 exhibited high conversion as shown in
Fig. 1. The product was merely ethyl formate and the reaction
terminated at ethyl formate. The possible reactions involved are:
HCOOK þ C2H5OH ! HCOOC2H5 þ KOH
CO þ KOH ! HCOOK
(2)
(3)
(4)
CO2 þ KOH ! KHCO3
Reaction (3) happened at a low extent while CO2 quickly reacted
with KOH and was consumed.
ðACOðinÞ=AArðinÞ ꢀ ACOðoutÞ=AArðoutÞÞ
CO conv: ð%Þ ¼
CO2 conv: ð%Þ ¼
ꢁ100
ACOðinÞ=AArðinÞ
=AArðinÞ ꢀ ACO
ðACO
=AArðoutÞÞ
2ðoutÞ
2ðinÞ
ꢁ100
A
=AArðinÞ
CO2ðinÞ
ACOðinÞ
ðACOðinÞ þ ACO ðinÞÞ
ACOðinÞ
Total carbon conv: ð%Þ ¼ CO conv: ꢁ
þCO2 conv: ꢁ
ðACOðinÞ þ ACO ðinÞÞ
2
2
X
Sel: ð%Þ ¼ pi ðmolÞ=
pi ðmolÞ ꢁ 100
The phase of the catalysts was analyzed on a Rigaku D/MAX-
2200PC X-ray diffractometer with Cu K radiation, 40 kV, 40 mA at
Table 2 shows the activity of Cu/MgO catalysts in different
compositions combining with HCOOK for the conversion of CO/H2/
CO2 at 423 K. It is clear that the conversion of CO and the total carbon
conversion changed as the molar ratio of Cu/MgO changed. The CO
conversion increased from 31.7% to 58.9% as the molar ratio of Cu/
MgO increased from 1:1 to 3:1, with the selectivity of methanol
above 96%. Side-products were found to be low amounts of ethyl
formate and ethyl acetate, similar to that using Cu/MnO as catalyst
[11]. CO2 was similarly formed in this case. As discussed later in the
reaction mechanism, HCOOK regeneration from CO2-related KHCO3
hydrogenation resulted in water formation and water might
accelerate CO2 formation by reaction (1) on Cu/MgO catalyst.
a
scanning speed of 88/min. The morphology was observed on a
KYKY-2008B scanning electron microscopy (SEM) instrument. BET
surface areas were determined on a Micromeritics ASAP-2010 M
N2 physioadsorption instrument. NMR analysis was carried out at
213 K on a JEOL JNM-A400 ALPHA FT NMR System equipped with a
JEOL Superconducting Magnet 400 MHz.
H2-TPR (temperature programmed reduction) was conducted
on a TP-5000 adsorption instrument: 50 mg unreduced catalyst
sample was enclosed in a quartz tube and pretreated at 623 K in He
(30 mL/min) atmosphere for 1 h and then reduced by 5% H2