Mechanism for isomerization of n-hexane over sulfated zirconia: role of
hydrogen
Jean-Claude Duchet,*a Denis Guillaume,a Agnès Monnier,a Jacob van Gestel,a Georges Szabo,b Pedro
Nascimentob and Sebastien Deckerb
a
Catalyse et Spectrochimie, UMR CNRS 6506, ISMRA-Université, 14050 Caen, France. E-mail: duchet@ismra.fr
CERT, Total Raffinage Distribution, 76700 Harfleur, France
b
Received (in Cambridge, UK) 20th July 1999, Accepted 9th August 1999
The complex influence of hydrogen on n-hexane isomeriza-
tion over sulfated zirconia enriched with platinum has been
kinetically modelled with a simple three parameter rate
equation derived from a mechanism involving Lewis sites
and hydride species.
Sulfated zirconias are acidic catalysts which are able to
isomerize linear alkanes at low temperature.1,2 In practice,
hydrogen is added to the isomerization unit in order to saturate
the aromatics contained in the C5–C6 fraction and to prevent
coking of the platinum enriched catalyst. Some authors attribute
participation of hydrogen in isomerization via activation on
platinum by creating either Brønsted acid sites3,4 or hydride
species accelerating the desorption of the carbenium ions.5,6 In
the literature, analysis of the influence of hydrogen has been
limited to low hydrogen pressures.6,7 Here we report the
influence of hydrogen over a wide range of partial pressures
Fig. 1 Isomerization of n-hexane at 423 K and 5 MPa as a function of
hydrogen pressure.
(0.5–4.5 MPa) independently of the hexane pressure (0.1–0.5
MPa). Insight into the isomerization mechanism is obtained
from the kinetic treatment of the data.
The catalyst was prepared as follows: zirconium hydroxide
bifunctional mechanism has been developed on platinum–
was precipitated from an aqueous solution of zirconium
zeolite catalysts operating at 523 K. The detrimental effect of
oxychloride by ammonium hydroxide. The dry material was
hydrogen was easily interpreted by the dehydrogenation step of
sulfated with 0.5 M H2SO4, and then crystallised at 923 K. The
the hydrocarbon into an alkene. However, further dehydrogena-
sulfur content amounted to 2.0 wt%. Platinum was loaded at 0.3
tion into a diene is required to account for a positive effect at
wt% by impregnation with an H2PtCl6 solution and the final
low hydrogen pressure, so yielding a maximum in the curves.8
catalyst was calcined at 753 K. The catalyst was activated in the
We found that the corresponding rate equation completely
reactor at 623 K for 2 h in a dry air stream and contacted with
failed to fit our data in the whole range of hexane pressure.
flowing hydrogen at 423 K during 1 h. Dry n-hexane (0.1, 0.3 or
Indeed, a classical bifunctional metal–acid mechanism is
improbable at 423 K. The alkene concentration is likely to be
0.5 MPa) was vaporised in a dry hydrogen–helium mixture at 5
MPa total pressure and 423 K. The hydrogen partial pressure
too low, and the catalyst more likely operates by an acidic
was varied from 0.5 to 4.5 MPa. Conversions were kept low to
mechanism. The acidity of sulfated zirconia is generally
calculate initial rates.
attributed to Brønsted sites9 and accordingly, carbenium ions
are formed via carbonium intermediates. However, reaction
sequences based on this first step did not yield satisfactory rate
The reaction was not observed without hydrogen owing to
rapid deactivation. With hydrogen, the catalyst was stable for
> 1 week and cracking was negligible. The distribution among
equations. On the other hand, Lewis sites are readily created
isomers showed that 2-methylpentane, 3-methylpentane and
during the activation of sulfated catalysts at 923 K.9 We propose
2,3-dimethylbutane were primary products formed in their
a mechanism involving Lewis sites in which hydride abstraction
thermodynamic ratio. The formation of 2,2-dimethylbutane was
from n-hexane (RH) on coordinatively unsaturated zirconium
atoms creates carbenium ions which adsorb on Lewis basic sites
(bridged oxygen atoms). The adsorbed carbenium ions are then
rapidly isomerized and finally desorbed by the hydride species
a consecutive reaction. The distribution was not influenced by
hydrogen partial pressure.
The variation of the isomerization rate with hydrogen
pressure is shown in Fig. 1. At a given hexane pressure, the rate
(Scheme 1). The isomerization step is not rate-limiting, as
strongly increases up to a maximum, then slowly decreases. The
shown by the thermodynamic distribution of primary isomers.
position of the maximum was shifted from 0.3 to 1.5 MPa
This scheme is fully consistent with the poisoning effect of
pressure of hydrogen with increasing hexane pressure in the
water adsorbed on Lewis sites. The closed sequence isomerizes
range 0.1–0.5 MPa. The reaction order with respect to n-hexane
was slightly lower than unity at low hydrogen pressure, and
reached unity beyond the maximum.
Any proposed mechanism should account for the change in
reaction order for hydrogen, from positive to negative, so
yielding a maximum isomerization rate. Kinetic modelling was
used to infer the reaction sequence.
The maximum activity with hydrogen pressure has scarcely
been reported in the literature dealing with isomerization of
light alkanes over acidic catalysts. A classical metal–acid
Scheme 1 Isomerization sequence.
Chem. Commun., 1999, 1819–1820
1819
Duchet, Jean-Claude
