US EPA

1495333 

Journal Article 

Study of Ir/WO3/ZrO2-SiO2 ring-opening catalysts: Part II. Reaction network, kinetic studies and structure-activity correlation 

Lecarpentier, S; van Gestel, J; Thomas, K; Gilson, JP; Houalla, M 

2008 

Yes 

Journal of Catalysis
ISSN: 0021-9517
EISSN: 1090-2694 

254 

1 

49-63 

10.1016/j.jcat.2007.11.016 

WOS:000253646100005 

The present paper is the second part of a systematic study
of the influence of Wand Ir loading on the activity of Ir/WO3/ZrO2-SiO2 catalysts for the ring-
opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A
series of Si-stabilized tungstated zirconias, WOx/ZrO2-SiO2, Containing up to 3.5 atom W/nm(2),
was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these
solids. Characterization of the metal dispersion and catalyst acidity was described in a previous
article. The objective of the present study was to determine the best metal/acid balance for
optimal performance of Ir/WOx/ZrO2-SiO2 catalysts in the ring-opening reaction of MCH,
Monofunctional (acid WOx/ZrO2-SiO2 or metal Ir/ZrO2-SiO2) and bifunctional (Ir/WO3/ZrO2-SiO2)
catalysts were examined. Based on the analysis of the yields and products distributions, a
reaction network was proposed, and kinetic data (e.g., activation energies, initial rates) were
calculated. Correlations between characterization results obtained earlier (e.g., acidity,
dispersion) and catalytic performance are also reported. The monofunctional acid catalysts
WOx/ZrO2-SiO2 showed a low selectivity for ring opening. The ring-contraction activity developed
for W surface density above a threshold value of 1 atom W/nm(2). This was attributed to the
appearance and the development of a relatively strong Bronsted acidity monitored by infrared
measurements. MCH ring contraction and C5 naphthene ring opening occur according to a classic
acid mechanism. For low conversions, the monofunctional metal catalysts Ir/ZrO2-SiO2 exhibited
significant selectivity for ring opening that decreased with increasing conversion. Because of
the lack of ring-contraction products, the observed activity was attributed to the direct ring
opening of the MCH. Ring opening and cracking occur according to it dicarbene mechanism. The
study of MCH conversion on Ir/WOx/ZrO2-SiO2 catalysts indicated that MCH ring contraction to
alkylcyclopentanes occurs before ring opening. The best yields for ring opening were obtained
with the 1.2% Ir/WOx/ZrO2 (1.5 atom of W/nm(2)). Further increases in W surface density led to a
decrease in the indirect ring-opening yield, attributed to a decrease in Ir dispersion. For
bifunctional metal/acid catalysts, analysis of the mechanism is less straightforward. The
activation energy for C6 ring contraction and indirect C6 ring opening is a function of the
metal/acid ratio. For high ratios, indirect ring opening occurs essentially over metallic sites.
A decrease in the metal/acid ratio enhances the contribution of acid mechanism. (c) 2007 Elsevier
Inc. All rights reserved. 

tungstated zirconia; iridium; methylcyclohexane conversion; selective ring opening