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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