756
P.-Q. Yuan et al. / Catalysis Communications 12 (2011) 753–756
samples were pre-treated in the ultra vacuum at 773 K to remove
thoroughly the physisorbed and chemisorbed water, leaving highly
polymerized WOx species of weak Lewis acidity on the catalyst surface.
As for NH3-TPD characterizations, before NH3 adsorption only
physisorbed water was removed at 473 K. The structure of WOx species
with reduced polymerization degree in sub-CW thus can be partially
maintained at an ambient condition, presenting enhanced acidity in the
NH3-TPD experiments. It should be noted that in TPD progress water
released from the polymerization of WOx species may interfere with the
TCD signal more or less. Nevertheless, the discrepancy between the
NH3-TPD and Py-FTIR results exactly confirms the essential role of
hydrothermal environments in the evolution of surface acid centers.
4. Conclusions
Hydration of cyclohexene in sub-CW can be run free from the liquid–
liquid phase mass transfer resistance to reactionkinetics. The active sites
of WOx–ZrO2 catalysts undergo transformation along with the variation
of hydrothermal environments, which makes it possible to obtain
controllable WOx configurations with the desired acid characteristic and
acidity. WOx–ZrO2 catalyzed hydration of cyclohexene provides a
promising approach for the continuous production of cyclohexanol.
Fig. 7. NH3-TPD profiles of WOx–ZrO2 catalysts; (A) fresh, (B) used.
fresh and used catalysts. The above results consistently confirm the
structural stability of WOx–ZrO2 catalysts in sub-CW.
According to the calculated rHex listed in Table 2, the catalytic activity
of WOx/ZrO2 catalysts can both be enhanced by elevating hydration
temperature and pressure. The positive influence of reaction temper-
ature on the hydration kinetics can be naturally understood in terms of
Arrhenius effect. However, the promotion from increasing water partial
pressure to rHex is just beyond the general knowledge. At 573 K, rHex can
be readily doubled by elevating system pressure from 15 to 25 MPa
during which the increase in the concentration of reactants in the liquid
phase is less than 3%. It is deduced that such a phenomenon should be
resulted from the evolution of active WOx species under varied
hydrothermal environments. Subsequent Py-FTIR and NH3-TPD
characterizations were performed to validate this deduction.
Acknowledgments
This work was supported by the Fundamental Research Funds for
the Central Universities and by Shanghai Science and Technology
Committee (Grant No. 09ZR1407500).
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