ACS Catalysis
Research Article
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enough acidity to promote the dehydration of cyclohexanol
formed to cyclohexene, followed by the formation of benzene.
Thus, benzene might be formed by hydrogenation of the
aromatic ring as well as by hydrogenation of the carbonyl group
over Pd/t-ZrO2. In the case of Pd/m-ZrO2 and Pd/m,t-ZrO2
catalysts, the dehydration activity of the support is low, and this
reaction pathway does not occur. Therefore, the formation of
benzene likely takes place by an alternative route that involves
the tautomerization of phenol to a cyclohexadienone
intermediate.
Pd/ZrO2 catalysts significantly deactivated with time. The
dehydrogenation of cyclohexane and dehydration of cyclo-
hexanol showed that the Pd particle size increased and the
density of acid sites decreased during the HDO of the phenol
reaction. In addition, the DRIFTS spectra under reaction
conditions demonstrated that phenoxy and intermediate
species accumulated during the reaction. Losses in the Pd-
support interface responsible for the selective pathway to
benzene occurred, resulting in a steadily increasing inventory of
intermediates on the oxide surface with time on stream. This in
turn resulted in a covering of oxophilic sites by the
intermediates as a result of the narrowing bottleneck of the
selective route. These species thus remained adsorbed on the
oxophilic sites, blocking those sites and inhibiting further
reactant adsorption. Therefore, the steady growth of Pd
particles is likely responsible for the losses in the metal−
support interface with TOS, leading to catalyst deactivation and
a decrease in the density of available oxophilic sites.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
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AUTHOR INFORMATION
Corresponding Author
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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Support from the National Science Foundation (EP-
SCoR0814361), US Department of Energy (DE-
FG36GO88064), Oklahoma Secretary of Energy, and the
Oklahoma Bioenergy Center are greatly appreciated. Priscilla
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M. de Souza thanks CAPES (Coordenaca̧ o de Aperfeico̧ amento
̃
de Pessoal de Ensino Superior) for financial support. The group
thanks the LNLS (Brazilian Synchrotron Light Laboratory) for
the assigned beamtime at XAFS-1 and for the valuable support
to perform the XAFS studies. The authors thank Saint-Gobain
NorPro for providing the zirconia materials. CAER researchers
acknowledge support from the Commonwealth of Kentucky.
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