Effect of pretreatment conditions on the catalytic performance of Ni-Pt-W supported on amorphous silica-alumina catalysts
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Date
2008
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Abstract
Activation procedures of Ni-W-Pt based catalysts, prepared by means of a hybrid technique: sol–gel and
incipient wetness impregnation, were investigated in terms of their activity in reaction of n-hexane
isomerization in a continuous fixed-bed quartz reactor operating at atmospheric pressure, by changing
operating conditions such as time on stream, SiO2/Al2O3 and H2/n-hexane ratios. This study aims to
understand the effects of the pretreatment conditions, such as calcination and reduction temperature,
over some properties of these catalysts and to analyze the relationship between the metal content, the
acidity (strength) of the samples and their catalytic performance.
The chemical composition of the prepared solids have bas characterized by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES), their specific surface areas were measured using BET
method, while their reduction behavior was characterized by temperature-programmed reduction and
their acidity was assessed by means of ammonia temperature-programmed desorption. The collected
data revealed that both BET surface areas and catalytic activity as well as surface acidity of these solids are
strongly dependent on their pretreatment conditions. Besides, the reduction behavior of the prepared
materials was related to nickel and platinum amounts, the higher the concentrations of these species the
easier the reducibility of the samples. On the other hand, it was found that all (NixPty) catalysts deactivate
with time on stream, with the conversion remaining steady after 100 min. Moreover, the obtained results
on (Ni12Pt0,4)AC catalyst showed that the SiO2/Al2O3 ratio and H2 partial pressure affect isomerization
selectivity positively while opposite effect was observed for activity (conversion). In addition, the main
products in n-hexane hydroconversion were found to be monobranched isomers which dominate all
practical conversions and their formation increases together with that of multibranched ones. Besides,
the cracked products are not produced in significant amounts until about 30% conversion.