Man Yang et al. / Chinese Journal of Catalysis 39 (2018) 1027–1037
1035
more experimental studies to optimize the reaction conditions
over the optimal catalyst (Pt/2%Nb‐WOx) are being undertak‐
en.
4. Conclusions
In summary, mesoporous Nb‐doped WOx oxides were suc‐
cessfully synthesized by a solvothermal method in ethanol with
a uniform pore size of ca. 4.0 nm and high surface area of
130–144 m2/g. The morphology of WOx substantially evolved
from 1D needle‐stack structures to 2D flake‐stacks and 3D
sphere‐stack clusters when the Nb doping amount increased to
2 and 5 wt%, respectively. Highly dispersed Pt catalysts were
prepared on the obtained Nb‐WOx supports. During the selec‐
tive hydrogenolysis of glycerol to 1,3‐PD, Nb introduction can
successfully inhibit the over‐reduction of active W species at
high H2 pressures. An optimal catalyst was obtained when the
Nb doping was 2 wt% and the optimized catalyst demonstrated
promising performance in a wide operating window (1 to 5
MPa H2), which guarantees superior stability towards H2 pres‐
sure. The unique features brought about by Nb‐doping may
also enlarge the applications of Pt/WOx catalysts in other hy‐
drogen‐involved reactions, particularly under more severe
conditions. Moreover, the possibility to reconcile the conflicts
in H2 usage is soundly validated by modifying catalyst supports,
although the 1,3‐PD yield is far from satisfactory currently.
Instead of pursuing high performance, we took a quite different
approach in this regard and manipulated the supports to en‐
dow the catalyst with longevity with respect to H2 pressure; we
strongly believe that this will provide novel ideas and shed light
on the design of highly stable catalysts, which is desirable for
practical applications.
Fig. 11. NH3‐TPD (m/e = 16) curves of WOx and 2%Nb‐WOx.
and 3) under mild conditions. The discrepancy between hy‐
drogenolysis performance and catalyst acidity suggested that
the active acid sites might not be the measured surface acid
sites, but in‐situ Brönsted acids formed with the assistance of
H2 [29].
From a mechanistic point of view, an optimum balance be‐
tween acid and metal sites is necessary for efficient dehydra‐
tion (despite the aqueous reaction system), followed by fast
hydrogenation to stable products. Because dehydration is more
likely to be the rate‐determining step, a high H2‐pressure de‐
pendence seems unnecessary in this step, unless it is used to
obtain active acid sites. In combination with the aforemen‐
tioned results, we tentatively assumed that a high H2 pressure
is required to provide active in‐situ formed acid sites in the
rate‐determining dehydration step, rather than for fast hydro‐
genation. Consequently, the outstanding low‐pressure perfor‐
mances of Pt/WOx [6] systems can be explained by their supe‐
rior H2 dissociation over atomically dispersed Pt species to
in‐situ generated Brönsted acid sites. However, even with im‐
proved dispersion and stabilization of metal components, the
defect‐rich WOx supports were easily over‐reduced, thus re‐
sulting in a sharp activity loss as H2 pressure increased. In con‐
trast to conventional volcano‐profiled performance towards H2,
Pt/Nb‐WOx, in this regard, demonstrated a terrace‐profiled
performance as a function of H2 pressure, indicating that a high
H2 pressure is not indispensable or beneficial when H2 can be
sufficiently dissociated to provide active acid sites. It should be
admitted that in this work, the doping of Nb, to some extent,
might impair the capacity of WOx to dissociate H2 and generate
in‐situ acid sites, thus leading to a decrease in the activity as the
Nb doping amount increased. Though 2% Nb was found opti‐
mal for WOx doping for the selective hydrogenolysis of glycerol,
References
[1] W. H. Yu, P. P. Wang, C. H. Zhou, H. B. Zhao, D. S. Tong, H. Zhang, H.
M. Yang, S. F. Ji, H. Wang, Chin. J. Catal., 2017, 38, 1087–1100.
[2] H. D. Zheng, J. Z. Ou, M. S. Strano, R. B. Kaner, A. Mitchell, K. Kalan‐
tar‐zadeh, Adv. Funct. Mater., 2011, 21, 2175–2196.
[3] C. Di Valentin, G. Pacchioni, Acc. Chem. Res., 2014, 47, 3233–3241.
[4] H. Bai, W. C. Yi, J. Y. Liu, Q. Lv, Q. Zhang, Q. Ma, H. F. Yang, G. C. Xi,
Nanoscale, 2016, 8, 13545–13551.
[5] X. X. Guo, X. Y. Qin, Z. J. Xue, C. B. Zhang, X. H. Sun, J. B. Hou, T.
Wang, RSC Adv., 2016, 6, 48537–48542.
[6] J. Wang, X. C. Zhao, N. Lei, L. Li, L. L. Zhang, S. T. Xu, S. Miao, X. L.
Pan, A. Q. Wang, T. Zhang, ChemSusChem, 2016, 9, 784–790.
[7] Z. F. Huang, J. J. Song, L. Pan, F. L. Lv, Q. F. Wang, J. J. Zou, X. W.
Zhang, L. Wang, Chem. Commun., 2014, 50, 10959–10962.
[8] J. J. Song, Z. F. Huang, L. Pan, J. J. Zou, X. W. Zhang, L. Wang, ACS
Catal., 2015, 5, 6594–6599.
[9] C. W. Liu, C. H. Zhang, S. L. Hao, S. K. Sun, K. K. Liu, J. Xu, Y. L. Zhu, Y.
W. Li, Catal. Today, 2016, 261, 116–127.
Table 3
Hydrolysis of cellobiose over WOx and Nb‐WOx.
[10] J. Y. Zhang, B. L. Hou, A. Q. Wang, Z. L. Li, H. Wang, T. Zhang, AICHE
J., 2015, 61, 224–238.
Catalysts
WOx
Con. (%)
46.0
Glucose Sel. (%)
>99.0
[11] Y. B. Huang, L. Yan, M. Y. Chen, Q. X. Guo, Y. Fu, Green Chem., 2015,
17, 3010–3017.
2%Nb‐WOx
59.6
>99.0
Reaction conditions: 20 mL autoclave, catalyst 50 mg, 2 g of 5 wt%
cellobiose aqueous solution, 800 r/min, 120 °C for 1 h.
[12] K. Fabicovicova, M. Lucas, P. Claus, Green Chem., 2015, 17,
3075–3083.