Guo et al.
FULL PAPER
was carried out on a Bruker D8 Advance X-ray diffrac-
dration of Al powder. It is worthy to note that almost no
BET surface area (14 cm2/g) was determined for the
CoAl alloy catalyst, suggesting that there is no direct
relationship between the BET surface area of the alloy
and the activity of the catalyst. However, it is interesting
to find that, after the reaction, most of the alloyed Al
was leached (only about 5% alloyed Al was left after
one cycle reaction), and the BET surface area of the
catalyst reached about 80 m2/g, meaning that the leach-
ing process leads to the great enhancement of the BET
surface area. This result agrees well with that of the
change of BET surface area for NiAl alloy before and
after NaOH leaching treatment. Therefore, we can draw
a conclusion that it is the alloyed Al that leads to the
formation of active Co species in the present hydro-
genolysis reaction, which was also stabilized by the al-
loyed Al because of their difference in electronegativity.
Figure 1 gives the results of glycerol reactions over
CoAl alloy catalyst. It is clear that the glycerol conver-
sion as well as the total yield of the diols increased with
the reaction time, whereas the yield of diols decreases
upon further extending the reaction time. Based on the
distribution of the liquid products upon the variation of
reaction time, further C— C cleavage could occur when
extending the reaction time, i.e., 1,2-PDO can be further
degraded to EG.12 Therefore, a suitable reaction time is
necessary for the high yield of diols.
tometer using nickel filtered Cu Kα radiation at 40 kV
and 20 mA. A JEOL 2011 microscope operating at 200
kV equipped with an EDX unit [Si (Li) detector] was
used for the TEM experimets. The samples for electron
microscopy were prepared by grinding and subsequent
dispersing the powder in ethanol and applying a drop of
very dilute suspension on carbon-coated grids. The N2
physisorption was carried out at 77 K on a Micrometrit-
ics TriStar 3000 apparatus.
Activity tests
In a typical test, glycerol aqueous solution (30 mL,
10 wt%) and 1 g CoAl alloy catalyst were added into
the stainless steel autoclave (reactor volume, 300 mL).
After purging the reactor with H2, the reaction was car-
ried out at 473 K for 15 h at a stirring speed of 600
r/min. The temperature was monitored with a thermo-
couple that was inserted into the autoclave and con-
nected to the thermo-controller. After the reaction was
halted, the reactor was cooled to room temperature. The
gas phase products were collected in a gasbag and the
liquid-phase products were obtained. Internal standards
were used to determine the product amount and carbon
balance. These products were analyzed by a GC
equipped with an FID detector.
Results and discussion
Various reaction conditions have been investigated
for the hydrogenolysis of glycerol at optimized reaction
conditions. 100% conversion and 66.5% yield could be
obtained under optimal reaction conditions (4 MPa H2
and 473 K) over a commercial CoAl alloy powder
without any pre-treatments. In addition, trace amount of
ethanol, n-propanol, hydroxy acetone and propanoic
acid was detected in the liquid products. CO2, CH4,
C2H6 and C3H8 were detected in the gas products. To
investigate the effect between cobalt and aluminum,
nanosized Co powder and Al powder were used also as
catalyst. It is found that neither Co powder nor Al pow-
der shows satisfied activity in this reaction. When Co
powder was used, the conversion of glycerol was 31.9%;
however, almost no detectable conversion was obtained
over the Al powder catalyst although most of the Al
powder was consumed after the reaction. When a me-
chanical mixture of these two metal powders was used
as the catalyst, the catalytic performance was greatly
enhanced, and 46.7% yield of liquid products was ob-
tained. However, this result is still much lower than that
of the CoAl alloy catalyst. All these findings indicated
that there was strong synergistic effect between two
metals in the alloy. We think that the addition of Al
powder in the Co catalyst enhanced the dehydration of
glycerol to acetol, thus the chemical equilibrium of de-
hydration was shifted after the hydrogenation of acetol
over cobalt active sites. Also the Co species could be
well dispersed on the alumina oxide formed by the hy-
Figure 1 Effect of reaction time on the catalytic performance
over CoAl alloy catalyst.
Table 1 shows that higher conversion was obtained
at diluted glycerol concentrations over the CoAl alloy
catalyst. While it is found that more degraded products
would be produced when much lower glycerol concen-
trations were used. If a 10% aqueous solution of glyc-
erol was used, 100% glycerol conversion was obtained
while 66.5% yield of total liquid products could be
achieved. So all the experiments were carried out by
using 10% glycerol taking into the considerations of
industrial application. The conversion of glycerol de-
creased with increasing the concentration of glycerol,
but the selectivity of diols almost unchanged, 100%
conversion could also be obtained when properly ex-
tending the reaction time. For example, if 20% glycerol
1564
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 1563— 1566