J. Tian et al.
AppliedCatalysisA,General544(2017)108–115
needs to be further improved.
2.3. Catalytic test
Herein, we modify the Cu-based catalyst with Al and then with the
alkaline earth metal component to achieve higher performance for the
hydrogenation of ethyl acetate to ethanol. Different molar ratios of Cu/
Al for Cu9-Alx were investigated. Under the optimal Cu/Al molar ratio
of 9:0.5, the conversion of ethyl acetate reached 68.8% with the se-
lectivity to ethanol over 98%. After addition of the alkaline earth metal
like Mg to Cu9-Al0.5, the Cu9-Al0.5-Mg1.5 catalyst showed greatly en-
hanced activity, selectivity and stability.
The catalytic test was performed using
a
fixed-bed reactor.
Typically, 1.0 g of catalyst (20–40 meshes) was loaded into a stainless
steel tubular reactor (with 55 cm length and i.d. of 12 mm) with a
thermocouple inserted into the catalyst bed for control of the actual
temperature. The catalyst was reduced under hydrogen atmosphere
(99.999% purity, 2 MPa, 60 ml/min) at 350 °C for 2 h firstly. Then, the
hydrogen flow rate was modulated using a mass flow controller and the
pressure was controlled by a regulator valve. The hydrogenation of
ethyl acetate began after ethyl acetate was pumped to the catalyst bed
at a tunable flow rate. Normally, 0.02 ml/min of ethyl acetate flow rate,
60 ml/min of hydrogen flow rate, 2 MPa of hydrogen pressure and
hydrogenation at 250 °C were applied for a standard catalytic test. For
optimization of reaction parameters, effects of ethyl acetate flow rate,
hydrogen flow rate, hydrogen pressure and reaction temperatures on
the hydrogenation of ethyl acetate were also investigated separately.
The products were collected every 2 h and the second sample col-
lected after 4 h time of stream was analyzed offline using GC with an
FID (TECHCOMP GC-7900 Plus, Tianmei Ltd. Co.) to compare the
catalytic performance of different catalysts. The response factor of each
component was calculated using standard samples and was used to
calculate the conversion and selectivity. For catalyst stability test, the
reaction was performed continuously and the sample was collected
every 2 or 3 h. The error bar for the catalytic testing is 5% based on
GC analysis.
2. Experimental
2.1. Catalyst preparation
The Cu9-Alx or Cu9-Alx-My catalysts were prepared by a deposition-
precipitation method.
A mixed aqueous solution containing Cu
(NO3)2∙3H2O and Al(NO3)2∙9H2O with/without Mg(NO3)2∙6H2O (or Ca
(NO3)2 or Ba(NO3)2 or Sr(NO3)2) (Sinopharm Chemical Reagent Co,
Ltd.) with different molar ratios of 9/x or 9/x/y was made and stirred
for about 30 min at room temperature. Then, the mixed solution was
heated to 80 °C. After that, an aqueous solution of 10 wt.% Na2CO3 was
added to keep the solution pH value of about 8, and the mixture was
stirred at 80 °C for additional 8 h. Finally, the resultant precipitate was
recovered by filtration, washed with deionized water, and dried at
100 °C for 8 h. After calcination at 350 °C in air for 2 h, the samples
were shaped and then sieved to 20–40 meshes for use. For comparison,
the catalyst precursor was also calcined at other temperatures in air
before reduced in hydrogen.
3. Results and discussion
3.1. Catalytic performance of Cu9-Alx catalysts
2.2. Catalyst characterization
Firstly, we investigated the binary Cu9-Alx catalysts for the hydro-
genation of ethyl acetate to ethanol. It is worth noting that the se-
lectivity to ethanol is over 98% regardless of the conversion of ethyl
acetate. Trace amount of ethylene and COx derived from decomposition
of ethyl acetate were also detected in the product. Nevertheless, the
conversion of ethyl acetate was significantly affected by the molar ratio
of Cu/Al. As shown in Fig. 1, the conversion of ethyl acetate increased
dramatically with Al amount firstly and reached a maximum when x
was equal to 0.5. When the Al amount x was further increased, the
conversion of ethyl acetate was decreased instead. As a result, the
highest conversion of 68.8% was obtained with the Cu9-Al0.5 catalyst.
In order to explain this phenomenon, the reduced Cu9-Alx catalysts
were characterized using XRD. As can be seen from Fig. 2, the bare Cu
The ex-situ XRD patterns of samples were recorded on a Bruker D8
Advance X-ray diffractometer using Ni-filtered Cu Kα radiation
(λ = 1.5406 Å) with a scanning angle (2θ) range of 5–80°, a scanning
speed of 60°/min with a voltage of 40 kV, and a current of 40 mA. The
samples were reduced in flowing hydrogen at 350 °C for 2 h prior to
taking XRD patterns, and no protective step was taken to avoid surface
re-oxidation. N2 physisorption of the reduced samples was conducted
on a Quantachrome Autosorb-1 system at liquid nitrogen temperature
(−196 °C) after the samples were outgassed at 300 °C under vacuum for
3 h to remove physically adsorbed species. The specific surface areas
were calculated by the Brunauer-Emmett-Teller (BET) equation. Total
pore volumes were evaluated at relative pressures (P/P0) close to unity.
Pore size distributions were estimated by the Barrett-Joyner-Halenda
(BJH) method according to the desorption branch of the isotherms. The
transmission electron microscopy (TEM) images were recorded with an
FEI Tecnai G2-TF30 electron microscope operated at an accelerating
voltage of 300 kV.
The H2-TPR profiles were conducted with
a Quantachrome
CHEMBET-3000 apparatus. During the experiments, each sample
(100 mg) was outgassed under flowing He (99.999%, 30 ml/min) at
200 °C for 30 min and then cooled to ambient temperature. The H2-TPR
profiles were obtained with a 10% H2/N2 flow (30 ml/min). The tem-
perature was increased from 30 °C to 600 °C with a ramping rate of
10 °C/min.
To correlate with the reaction results, the X-ray photoelectron
spectrum (XPS) of sample was recorded with ESCALAB 250 spectro-
meter after the sample was pretreated in situ in flowing hydrogen at
350 °C for 2 h. The Al Kα X-ray radiation source (hv = 1486.6 eV) was
operated at 14 kV and 20 mA. The carbonaceous C 1 s line (284.6 eV)
was used as the reference to calibrate the binding energies (BEs). The
spectra shown in the figures have been corrected by subtraction of a
Shirley background. Spectral fitting and peak integration was done
using the XPSPEAK software.
Fig 1. The conversion of ethyl acetate versus x plot with Cu9-Alx catalysts. Reaction
conditions: 1 g of Cu9-Alx catalyst, Calcination temp. = 350 °C (air), Reduction
temp. = 350 °C (2.0 MPa H2), Reaction temp. = 250 °C, EA = 0.02 ml/min,
H2 = 60 ml/min, GHSV = 2400, LHSV = 0.8, 2 MPa of H2 pressure, 4 h > 98% se-
lectivity to ethanol was obtained during the whole process.
109