(59b) Hydroisomerization of n-Hexadecane Over Anion Modified Pt/HfO2 Catalysts

Abstract

Nowadays,
isomerization of n-paraffins
plays an important role in the petroleum industry [1].  The most widely applied isomerization
catalysts include toxic and corrosive liquid acids, such as H₂SO₄
and HF, metal halides, and metal oxides promoted with Cl
or F.  These reagents are highly
corrosive and hence, environmentally hazardous. 
Many solid acids, such as zeolites, modified
metal oxides, and clays, evade these difficulties, and offers to tune not only
the number of acid sites but also their strength [2-5].  Among the solid acid studied, ZrO₂
modified by anions such as tungstate, sulfate, or
phosphate, has attracted considerable attention.  ZrO₂ promoted with sulfate groups
have first been proposed by Holm and Bailey [6] which are active for isomerization of light straight-chain alkanes
at low temperatures (303-423 K). 
Further, the stability of sulfated zirconia
catalysts was improved by adding noble metals and other transition metal
oxides. 

Hino
and Arata [7,8], the pioneer
in this field reported that zirconia-tungstate can be
used as a strong solid acid catalysts. 
Although tungstated-zirconia catalysts are
markedly less active than sulfated zirconia, they
showed superior stability and selectivity towards isomerization
by promoted with platinum for larger alkanes such as
n-heptane [9,10].  The reason for the appearance of strong
acidity in tungstate-containing ZrO₂
remains unclear in many cases and continues to be discussed in literature [11].
Iglesia et al. [11,12]
proposed that platinum particles dissociate molecular hydrogen into hydrogen
atoms which spill over to acid sites on the catalyst surface increases n-alkane conversion rates and isomerization
selectivities.

In the
present work, the effect of hafnia on acidic and hydroisomerization activity of n-hexadecane was studied using
anions such as tungstate and sulfate promoted with Pt.
Diffuse FT-IR pyridine adsorption  technique
was employed to estimate the relative amount of Brӧnsted
and Lewis acids present in the catalysts. Activity and selectivity of hafnia catalysts were compared with zirconia-tungstate
catalysts.

References

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