Z. Wang et al. / Catalysis Communications 43 (2014) 38–41
39
2
.2. Activity test
The reaction was carried out in a 316 L stainless steel tubular flow
3. Results and discussion
As shown in Fig. 1, the 4% Ir/SiO
2
exhibited low activity for the hy-
reactor described in literature [22]. Before the reaction, the catalysts
drogenation of acetic acid at 373 K and 6 MPa. Over it, low acetic acid
conversion (5.6%) and ethanol yield (1.2%) were achieved. After the
were reduced in-situ in the reactor by hydrogen (160 mL min−1) at
4
(
73 K and 6 MPa for 2 h. After cooling down to reaction temperature
373 K), the aqueous solution of carboxylic acid was feed into the reac-
tor from the bottom by a HPLC pump along with hydrogen at a flow rate
modification of 4% Ir/SiO
effect was observed. Over the 4% Ir-MoO
of 0.13, high acetic acid conversion (75.9%) and ethanol yield (47.2%)
were achieved. We also investigated the catalytic performances of
2
with small amount of Mo, evident promotion
/SiO with Mo/Ir atomic ratio
x
2
−
1
of 60 mL min . The products from the reactor passed through a gas–
liquid separator, and became two phases. The gaseous products flowed
through a back pressure regulator to maintain the pressure in reaction
system at 6 MPa and were analyzed on-line by an Agilent 6890N GC.
MoO
cilitate the comparison, the Ir and/or Mo content in these catalysts was
same as the one in 4% Ir-MoO /SiO (Mo/Ir atomic ratio = 0.13). From
x 2 x 2 2
/SiO and the physical mixture of MoO /SiO and 4% Ir/SiO . To fa-
x
2
CO
2
in the gaseous products was analyzed by a Thermal Conductivity
Fig. 1, we can see that both catalysts have low activity for the hydroge-
nation of acetic acid. From these results, there should be a synergy effect
between the closely contacted Ir and Mo species, which is responsible
Detector (TCD) equipped with an Alltech HAYESEP DB 100/120 packed
column (30 ft, 1/8 in. O.D., 2.0 mm I.D.). Alkanes in the gaseous prod-
ucts were analyzed by a Flame Ionized Detector (FID) equipped with
an Rt®-Q-BOND capillary column (30 m, 0.32 mm I.D., 10 μm film).
Liquid products were drained from the gas–liquid separator after
reaction for 6 h and analyzed by another Agilent 6890N GC equipped
with a HP-INNOWAX capillary column (30 m, 0.25 mm I.D., 0.5 μm
film) and a FID detector.
x 2
for the excellent catalytic performance of Ir-MoO /SiO . According to
literature [6,23], the modification of Ir catalysts with Mo may promote
the adsorption of carboxyl acid (by the interaction between Mo cations
and the lone electron pair of hydroxyl or carbonyl group oxygen),
which is very important for the hydrogenation of carboxylic acid. To un-
x 2
derstand the interaction between Ir and Mo species, the Ir-MoO /SiO
catalysts were characterized by a series of techniques (the detail infor-
mation was offered in Support information). According to the XRD,
TEM and CO-chemisorption results shown in Figs. S2, S3 and Table S1
of Support information, the Ir species was totally reduced to metallic
2 2
state after being pretreated in H at 473 K. The modification of Ir/SiO
with Mo has no evident effect on the dispersion (or average particle
sizes) of Ir species. Therefore, we can't attribute the promotion of Mo
to the change of Ir dispersion. From the HAADF/STEM, H -TPR, EXFAS
2
and DRIFTS for CO-adsorption illustrated in Figs. S4 to S8 and Table S2
of Support information, Mo species are attached on the Ir particle in
x
the form of partially reduced isolated MoO . The close contacting of Mo
8
6
4
2
0
0
0
0
0
a)
Acetic acid conversion (%)
Ethanol yield (%)
species with Ir particles promotes its reduction by the spillover of hydro-
gen. The synergism effect of Ir particles and the partially reduced isolated
MoO
excellent performance of Ir-MoO
Fig. 2 shows the influence of Mo/Ir atomic ratio on the catalytic
performance of 4% Ir-MoO /SiO . From Fig. 2, we can see that the
activity of 4% Ir-MoO /SiO increased with Mo content initially, reached
x
species attached on them may be the intrinsic reason for the
x
/SiO catalyst.
2
x
2
x
2
the maximum when Mo/Ir atomic ratio was about 0.13, and then
decreased with the further increase of Mo content.
2
2
2
2
/
SiO
/SiO
x
x
2
4
% Ir/SiO
MoO
4% Ir/SiO/SiO
x
To prove the beneficial role of Ir in Mo-based systems, we tested
4
% Ir-MoO
+ MoO
the catalytic performance of MoO
Mo content of 4%. From the results shown in Fig. S1, the MoO
is inactive for the hydrogenation of acetic acid. In contrast, the
% Ir-MoO /SiO catalyst with Mo content of 4% is active for the
x
/SiO
2
and 4% Ir-MoO
x
/SiO
2
at the
x
/SiO
2
b)
4
x
2
hydrogenation of acetic acid. The conversions of acetic acid
(42.0%) and ethanol yield (25.5%) over this catalyst are evidently
lower than the ones (75.9% and 47.2%) observed over the 4% Ir-
MoOx/SiO2(Mo/Ir = 0.13) catalyst. This result further proved that
Ir is the active sites for the hydrogenation of acetic acid under the
investigated conditions.
In previous work of Chaudhari et al. [18], evident promotion effect of
Co was observed for the hydrogenation of succinic acid over Ru/C cata-
lyst. Moreover, considering the similar chemical properties of Mo and W
1
0
5
0
(
same group in periodic table), we think that W may also have promo-
tion effect on the hydrogenation of acetic acid over Ir/SiO catalyst.
Therefore, we studied the performances of the W and Co doped 4%
Ir/SiO catalysts and compared them with that of 4% Ir-MoO /SiO
2
2
2
2
4% Ir/Si O/ SiO2
x
2
/
SiO
/SiO
x
x
4% Ir/SiO
MoO
4% Ir-MoO
2
.
2
x
+
MoO
To facilitate the comparison, the atomic ratios of promoter to Ir in
all these catalysts were fixed at 0.13. From the results illustrated in
Fig. 3, W also has promotion effect on the hydrogenation of acetic acid
Fig. 1. Acetic acid conversion, ethanol yield (a) and the turnover frequency (TOF) of acetic
acid to ethanol (b) over the 4% Ir/SiO , 4% Ir-MoO /SiO (Mo/Ir atomic ratio = 0.13), 0.26%
Mo/SiO , the physical mixture of 4% Ir/SiO and 0.26% Mo/SiO (at the mass ratio of 1:1).
Reaction conditions: 373 K, 6 MPa, 2.0 g catalyst, 10% acetic acid aqueous solution
2
x
2
2
2
2
2
over Ir/SiO catalyst. However, such a promotion effect is less evident
flow rate = 0.04 mL min− , and H
to ethanol were deduced according to the ethanol yields and the Ir dispersions of catalysts
see Table S1).
1
−1
than that of Mo. In contrast to Mo and W, the modification of Co only
2
flow rate = 60 mL min . The TOFs of acetic acid
led to a slight increase of ethanol yield (from 1.2% to 3.0%) over Ir/SiO
catalyst.
2
(