1
34
S. Jin et al.
small pores of 2–3 nm in large amounts, consistent with its high
hydrogenation catalysis if this existed. Generally, the acid group
2
−1
−1
specific surface area (1314 m g ). The lack of large pores
> several 10 nm or even larger) in the AC support probably
amount of AC and O-CNT was ∼1.2 mL g . The mechani-
(
cal strength of the granulated Pt/CNT catalyst, measured by a
crushing machine, was much higher than that of Pt/AC and was
comparable to that of alumina powder.
hindered the mass transfer of H2 inside the Pt/AC catalyst, and
would be responsible for the low selectivity of AN and the rel-
atively low conversion of NB. From comparing the differences
between the conversion of NB and the selectivity of AN over the
two catalysts, the transfer of one H2 molecular to convert NB
into NSB would be relatively easy but the further conversion of
NSB would be difficult because of the rapid consumption of H2
inside the Pt/AC particles. Thus, this also gave an explanation
for why a high pressure operation was often used in an indus-
trial scale process using a Pt/AC catalyst (to increase the mass
transfer rate of hydrogen in the inner pores of the AC).
Catalyst (0.05 g) was added into a 100 mL quartz flask and
flushed with N2 for 20 min, then H2 for 10 min to remove air.
◦
The catalyst was calcined and reduced by H2 at 350 C for 2 h.
After cooling to ambient conditions, 50 mL ethanol containing
1% NB was added and H2 was introduced for the hydrogenation
◦
at 30 C and 1 atm, with continuous stirring. Liquid samples were
analyzed by GC-MS (Agilent 6890).
The catalysts were characterized by SEM (JSF7401F, 3–
10 kV).The pore structures of the different carbon supports were
characterized by a nitrogen adsorption instrument (ASAP2010,
BET).
ThelargeamountsofAZBfound, butundetectableamountsof
azoxylbenzene (with N(O)=N group) and phenylhydroxylamine
(
with NHOH group) (PHA) suggested thatAZB was mainly pro-
duced by the reaction of NSB with AN, and not with PHA and
the subsequent hydrogenation.Thus, it can be deduced that PHA,
once produced, can be quickly converted intoAN. Because PHA
has a larger molecular size than AN but is smaller than AZB, the
above results suggested that the mass transfer rate of the smaller
molecule was sufficient for further reaction inside the catalyst.
From this point of view, because H2 has the smallest molecular
size, a reason why it was present in insufficient amounts in the
small pores of the Pt/AC catalyst was also because there was a
mass transfer barrier from the bulk gas phase to the bulk liquid
phase and then to the liquid phase inside the catalyst. In con-
trast, H2 easily diffused into the large pores (50–100 nm) of the
Pt/CNT particles where the liquid phase inside the catalyst was
nearly the same as in the bulk liquid phase. Such tendency is
more significant as using the mono-dispersed Pt/CNT catalyst
considering the CNTs dispersed well in the bulk liquid phase.
Thus, the hydrogenation of NB would follow the most favourable
route to first produce NSB, then PHA, and finally AN. In addi-
tion, the sufficient supply of H2 also favoured the simultaneous
hydrogenation of any intermediate that existed (Fig. 3c). The
advantages of the structure of the CNT particles may be general
for any gas-liquid-solid heterogeneous catalytic processes and
this would imply many promising commercial applications.
In summary, we have shown that granulated CNTs as the
support of Pt catalysts are advantageous over a conventional
AC support in the hydrogenation of NB. The large pores of the
granulated CNTs give the advantage of a faster mass transfer
rate for the reactants to enhance the conversion of NB into AN
with high selectivity.
Acknowledgements
This work is supported by FANEDD (200548), NSFC (20606020,
20736007), NBRPC (2006CB932702).
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