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          Institute: Fritz-Haber-Institut     Collection: Inorganic Chemistry     Display Documents

ID: 276377.0, Fritz-Haber-Institut / Inorganic Chemistry
Acid-Base Catalyzed Activation of n-Alkanes: Isomerization of n-Butane
Authors:Papp, Helmut; Breitkopf, Cornelia; Jentoft, Friederike C.; Lloyd, Rhys; Wrabetz, Sabine; Meinel, Klaus; Förster, Stefan; Schindler, Karl-Michael; Kulla, Ralf; Neddermeyer, Henning; Widdra, Wolf; Hofmann, Alexander; Sauer, Joachim; Lercher, Johannes; Li, Xuebing; Olindo, Roberta
Research Context:Project 1, Acid-base & redox properties of promoted sulfated zirconia
Name of Conference/Meeting:105. Hauptversammlung der Deutschen Bunsen-Gesellschaft für Physikalische
Place of Conference/Meeting:Erlangen, Germany
(Start) Date of Conference/Meeting
End Date of Conference/Meeting 
Audience:Experts Only
Abstract / Description:Due to its unique activity for skeletal isomerization of short
alkanes at low temperature, sulfated zirconia (SZ) is generally
recognized as the most promising alternative for the zeolite
based hydroisomerization catalysts. However, despite the large
amount of investigations, several important topics related to SZ
are still discussed controversially. Here we report on our detailed
investigation of the mechanism of butane skeletal isomerization
on SZ. Typically, SZ had an induction period followed by a
period of virtually constant activity. The selectivity to isobutene
was higher than 96%, the byproducts being propane and pentanes.
The induction period can be related to the formation and
accumulation of reactive intermediates on the catalyst surface.
We show that the alkane activation is initiated via stoichiometric
oxidative dehydrogenation of butane by sulphate species to
butane, water and SO2. For the first time, direct experimental
evidence is given for all reaction products formed by oxidative
dehydrogenation. In situ IR spectroscopy and density functional
calculations indicate that pyrosulfate or re-adsorbed SO3
species are the active species for the oxidation. Butene formed
interacts with Bronsted acid sites and forms sec-butoxy groups
which isomerize mono-molecularly to tert-butoxy groups, as
deduced from the 100% selectivity to isobutane at zero conversion.
The tert-butoxy group undergoes hydride transfer from
n-butane, forming a new sec-butoxy group and isobutane. The
lower selectivity to isobutane with increasing conversion is explained
by the higher isobutene concentration which triggers a
bimolecular pathway. Note that isobutane is kinetically a primary
product, while propane and pentanes are secondary products
formed in sequential reactions. The larger amount of propane
with respect to pentanes for conversion above 40% is attributed
to multiple alkylation reactions followed by cracking. Transient
experiments showed conclusively that the isomerization of
the carbenium ion is the rate-determining step in the chain sequence
and that hydride transfer is in quasi equilibrium. A kinetic
model for butane isomerization under differential conditions
is presented showing that the overall rate of butane conversion is
proportional to the rate constant of the monomolecular isomerization
of the carbenium ion, the concentration of Bronsted acid
sites, the partial pressure of the alkane and the concentration of
the labile sulfate-based redox sites. We show here that the key
to successful catalysts for skeletal isomerization does not lie in
high acid strength, but that a subtle balance between redox and
acid sites is necessary.
Comment of the Author/Creator:DFG-SPP1091-Abschlusskolloquim - 25.-27.5.2006 in Erlangen,
Last Change of the Resource (YYYY-MM-DD):2006-05-24
Document Type:Talk at Event
Communicated by:Robert Schlögl
Affiliations:Fritz-Haber-Institut/Inorganic Chemistry/Functional Characterization
External Affiliations:1 and 2) Universit¨at Leipzig, Institut f¨ur Technische Chemie,
Linnestr. 3, 04103 Leipzig
6-11) Martin-Luther-Universit¨at Halle-Wittenberg, FB Physik,
06099 Halle
12-13)Humboldt-Universit¨at Berlin, Institut f¨ur Chemie, 10099
14-16)Technische Universit¨at M¨unchen, Department of Chemistry,Lichtenbergstr. 4, 85747 Garching
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