TY - JOUR
T1 - Characterization of Cr-MCM-41 and Al,Cr-MCM-41 mesoporous catalysts for gas-phase oxidative dehydrogenation of cyclohexane
AU - Lezanska, Maria
AU - Szymanski, Grzegorz S.
AU - Pietrzyk, Piotr
AU - Sojka, Zbigniew
AU - Lercher, Johannes A.
PY - 2007/2/1
Y1 - 2007/2/1
N2 - The effects of the Cr concentration in hydrothermally synthesized Cr-MCM-41 and the impact of various postsynthesis treatments including grafting with alumina were investigated. Using physicochemical characterization by XRD, diffuse reflectance UV-vis spectroscopy, EPR spectroscopy, 29Si MAS NMR spectroscopy, H2 TPR, and acidity measurements, formation of mono- and dichromate surface species as well as CrV, dispersed CrIII, and clustered CrIII was detected, confirming the high speciation and valence versatility of the surface chromium. The roles of each species in the preparation stage, calcination, leaching, and grafting are discussed. Leaching resulted in removal of all isolated CrIII species, suggesting that CrIII is attached to the MCM-41 surface via silanol groups. The resulting materials exhibited low acidity, with both Lewis and Brønsted acid sites being present. The oxidative dehydrogenation (ODH) of cyclohexane was catalyzed by Cr-MCM-41 with a conversion of up to 25.7% in the temperature range of 533-633 K. The CrIII sites have been concluded to be responsible for cyclohexane ODH, but total activity was influenced by the presence of easily reducible CrVI at the beginning of the reaction. During the reaction, high-valence Cr ions were reduced to clustered Cr2O3 species.
AB - The effects of the Cr concentration in hydrothermally synthesized Cr-MCM-41 and the impact of various postsynthesis treatments including grafting with alumina were investigated. Using physicochemical characterization by XRD, diffuse reflectance UV-vis spectroscopy, EPR spectroscopy, 29Si MAS NMR spectroscopy, H2 TPR, and acidity measurements, formation of mono- and dichromate surface species as well as CrV, dispersed CrIII, and clustered CrIII was detected, confirming the high speciation and valence versatility of the surface chromium. The roles of each species in the preparation stage, calcination, leaching, and grafting are discussed. Leaching resulted in removal of all isolated CrIII species, suggesting that CrIII is attached to the MCM-41 surface via silanol groups. The resulting materials exhibited low acidity, with both Lewis and Brønsted acid sites being present. The oxidative dehydrogenation (ODH) of cyclohexane was catalyzed by Cr-MCM-41 with a conversion of up to 25.7% in the temperature range of 533-633 K. The CrIII sites have been concluded to be responsible for cyclohexane ODH, but total activity was influenced by the presence of easily reducible CrVI at the beginning of the reaction. During the reaction, high-valence Cr ions were reduced to clustered Cr2O3 species.
UR - http://www.scopus.com/inward/record.url?scp=33847399608&partnerID=8YFLogxK
U2 - 10.1021/jp066498u
DO - 10.1021/jp066498u
M3 - Article
AN - SCOPUS:33847399608
SN - 1932-7447
VL - 111
SP - 1830
EP - 1839
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
IS - 4
ER -