Uk. Singh et Ma. Vannice, Influence of metal-support interactions on the kinetics of liquid-phase citral hydrogenation, J MOL CAT A, 163(1-2), 2000, pp. 233-250
The kinetics of liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octa
dienal) on PuTiO2 catalysts were studied between 298-423 K and 7-21 arm H-2
and compared to those reported earlier for Pt/SiO2 catalysts. The kinetic
data were shown to be free of transport limitations by application of the M
adon-Boudart test and the Weisz-Prater criterion. Near zero- and first-orde
r kinetics were observed for the initial rate of citral hydrogenation over
the Pt/TiO2-LTR (T-red = 473 K) and Pt/SiO2 catalysts with respect to citra
l concentration and hydrogen pressure, respectively. In contrast, each depe
ndency dropped by about one order with Pt/TiO2-HTR (T-red = 773 K) catalyst
s as they were negative first-order on citral concentration and near zero-o
rder on hydrogen pressure. Furthermore, the initial rates over Pt/TiO2-LTR
and Pt/SiO2 exhibited an activity minimum as temperature increased whereas
conventional Arrhenius behavior was exhibited by Pt/TiO2-HTR with an activa
tion energy of 18 kcal/mol. Pt/TiO2-LTR and HTR catalysts initially exhibit
ed 90% selectivity to the unsaturated alcohol as compared to 40% for hydrog
enation over Pt/SiO2. Metal-support interactions (MSI) resulted in a dramat
ic enhancement in specific activity at 373 K, 20 atm H-2 and 1M citral in h
exane as Pt/TiO2-HTR exhibited a turnover frequency of 1.0 compared to 0.02
s(-1) for Pt/TiO2-LTR and 0.004 s(-1) fur Pt/SiO2. The reaction kinetics w
ith Pt/TiO2-HTR in the differential conversion regime were described by a c
onventional Langmuir-Hinshelwood model assuming quasi-equilibrium for react
ant adsorption, competitive adsorption between citral and hydrogen, and add
ition of the first H atom as the rate determining step. The reaction rate a
t higher conversions was modeled by invoking a decarbonylation reaction sim
ilar to that proposed earlier for this reaction over PVSiO2 catalysts to ex
plain any observed deactivation. (C) 2000 Elsevier Science B.V. All rights
reserved.