TY - JOUR
T1 - Structure sensitivity of hydrogenolytic cleavage of endocyclic and exocyclic C-C bonds in methylcyclohexane over supported iridium particles
AU - Shi, Hui
AU - Gutiérrez, Oliver Y.
AU - Haller, Gary L.
AU - Mei, Donghai
AU - Rousseau, Roger
AU - Lercher, Johannes A.
N1 - Funding Information:
Hui Shi thanks the Elitenetzwerk Bayern NanoCat for a Ph.D. grant and financial support. Prof. Dr. Xuebing Li (Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, China) is gratefully acknowledged for his help in technical aspects. The authors are indebted to Dipl.-Ing. Xaver Hecht for the help with construction of the reactor setup and for conducting N 2 physisorption and H 2 chemisorption measurements. The theoretical part of this work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under Contract DE-AC05-76RL01830. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. The computing time is provided by the user project from EMSL, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Part of computing time is also provided by National Energy Research Scientific Computing Center (NERSC) at Berkeley National Laboratory.
PY - 2013/1
Y1 - 2013/1
N2 - Structure sensitivities, H2 pressure effects, and temperature dependencies for rates and selectivities of endo- and exocyclic C-C bond cleavage in methylcyclohexane were studied over supported Ir catalysts. The rate of endocyclic C-C bond cleavage first decreased and then increased with declining Ir dispersion from 0.65 to 0.035. The ring opening (RO) product distribution remained unchanged with varying H2 pressure on small Ir particles, while further shifting to methylhexanes with increasing H2 pressure on large particles. In contrast, the rate and selectivity of exocyclic C-C bond cleavage decreased monotonically with increasing H2 pressure and decreasing Ir particle size. The distinct dependencies of endocyclic and exocyclic C-C bond cleavage pathways on Ir dispersion and H 2 pressure suggest that they are mediated by surface species with different ensemble size requirements. DFT calculations were performed on an Ir50 cluster and an Ir(1 1 1) surface, with or without pre-adsorbed hydrogen atoms, to provide insight into the observed effects of particle size and H2 pressure on RO pathways. On small Ir particles, the calculated dehydrogenation enthalpies for all endocyclic bonds were similar and affected to similar extents by H2 pressure; on large particles, the selectivity to n-heptane (via substituted C-C bond cleavage) was even lower than on small particles as a result of the least favorable adsorption and dehydrogenation energetics for hindered bonds.
AB - Structure sensitivities, H2 pressure effects, and temperature dependencies for rates and selectivities of endo- and exocyclic C-C bond cleavage in methylcyclohexane were studied over supported Ir catalysts. The rate of endocyclic C-C bond cleavage first decreased and then increased with declining Ir dispersion from 0.65 to 0.035. The ring opening (RO) product distribution remained unchanged with varying H2 pressure on small Ir particles, while further shifting to methylhexanes with increasing H2 pressure on large particles. In contrast, the rate and selectivity of exocyclic C-C bond cleavage decreased monotonically with increasing H2 pressure and decreasing Ir particle size. The distinct dependencies of endocyclic and exocyclic C-C bond cleavage pathways on Ir dispersion and H 2 pressure suggest that they are mediated by surface species with different ensemble size requirements. DFT calculations were performed on an Ir50 cluster and an Ir(1 1 1) surface, with or without pre-adsorbed hydrogen atoms, to provide insight into the observed effects of particle size and H2 pressure on RO pathways. On small Ir particles, the calculated dehydrogenation enthalpies for all endocyclic bonds were similar and affected to similar extents by H2 pressure; on large particles, the selectivity to n-heptane (via substituted C-C bond cleavage) was even lower than on small particles as a result of the least favorable adsorption and dehydrogenation energetics for hindered bonds.
KW - Demethylation
KW - Hydrogenolysis
KW - Iridium
KW - Methylcyclohexane
KW - Ring opening
KW - Structure sensitivity
UR - http://www.scopus.com/inward/record.url?scp=84870871607&partnerID=8YFLogxK
U2 - 10.1016/j.jcat.2012.09.018
DO - 10.1016/j.jcat.2012.09.018
M3 - Article
AN - SCOPUS:84870871607
SN - 0021-9517
VL - 297
SP - 70
EP - 78
JO - Journal of Catalysis
JF - Journal of Catalysis
ER -