Mechanism of skeletal isomerization of n-butane using 1,4-13C2-n-butane on solid strong acids

Article abstract of DOI:10.1246/cl.2000.470

Mechanism of skeletal isomerization of n-butane over Cs_2.5H_0.5PW₁₂O₄₀ as well as sulfated ZrO₂ was revealed at a temperature range 393 - 523 K by using 1,4-¹³C₂-n-butane. ¹³-Distributions of isobutane at 393 K were close to binomial distribution, indicating a bimolecular pathway. On the other hand, an intramolecular (monomolecular) rearrangement became significant over Cs_2.5H_0.5PW₁₂O₄₀ at 523 K.

Full text of DOI:10.1246/cl.2000.470

470
Chemistry Letters 2000
Mechanism of Skeletal Isomerization of n-Butane Using 1, 4-13C2-n-Butane on
Solid Strong Acids
Tetsuo Suzuki and Toshio Okuhara*
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810
(Received February 3, 2000; CL-000117)
Mechanism of skeletal isomerization of n-butane over
Cs2.5H0.5PW12O40 as well as sulfated ZrO2 was revealed at a
temperature range 393 - 523 K by using 1,4-13C2-n-butane.
13C-Distributions of isobutane at 393 K were close to binomial
distribution, indicating a bimolecular pathway. On the other
hand, an intramolecular (monomolecular) rearrangement
became significant over Cs2.5H0.5PW12O40 at 523 K.
Skeletal isomerization of n-butane is practically important,
because the product isobutane is a raw material for alkylation
with butenes to form clean gasoline (C8 branched alkanes), and
the dehydrogenation product, isobutylene can be transformed
into methyl tert-butyl ether (MTBE). There are many reports
about the catalysts active and selective for this reaction. Liquid
acids like HF are effective,1 but these have problems of envi-
ronmental protection, etc. As solid acids, sulfated ZrO2,2-4 a Cs
hydrogen salt of H3PW12O40, Cs2.5H0.5PW12O40,5-7 and their Pt-
promoted catalysts8,9 have been demonstrated to be promising
catalysts.
Since the selectivity to isobutane is closely related to the
reaction mechanism, elucidation of mechanism is required for
development of prominent catalysts. Matsuhashi et al.10 pro-
posed that monomolecular and bimolecular mechanisms operat-
ed on sulfated ZrO2 at the initial and latter stage of the reaction,
respectively. Recently, isotopic studies have intently been per-
formed on sulfated ZrO2,11,12 but there is a controversy on the
mechanism. Garin et al.11 claimed that this reaction occurred
through monomolecular mechanism at 523 K and Adeeva et
al.12 inferred that it proceeded through bimolecular mechanism
at 353 K, while these discussions are qualitative because of
strong influence of fragmentation in mass analysis.
In the present study, we chose Cs2.5H0.5PW12O40 as well as
sulfated ZrO2 as solid acids, since the former possesses pure
and very strong protonic acids available for this reaction,5 and
the latter is exceptionally active at low temperatures for this
reaction.2,4 Zeolites are active, but are non-selective.6
The isotopic composition was analyzed by Field-Ionization
Mass Spectrometry. By this, we obtained the parent peak pat-
tern which makes possible to discuss quantitatively. Here we
report first the reaction mechanism of n-butane isomerization
over Cs2.5H0.5PW12O40 as well as sulfated ZrO2.
In the monomolecular mechanism, the isomerization would
involve a protonated cyclopropane and a sequent primary car-
benium ion as intermediates to form isobutane with 100%-
selectivity.13 Reaction paths via the monomolecular mecha-
nism are shown in Scheme 1, when 1,4-13C2-n-butane was used
as a reactant. 13C2-Isobutane would be produced exclusively
(intramolecular rearrangement), together with 1,3-13C2-n-
butane as an isotopomer by self-isomerization. On the other
hand, the bimolecular mechanism (Scheme 1) is possible if
butenes and sec-butyl carbenium ion are formed on the catalyst
surface to form octyl cation.13 The β-scission of octyl cation
would give C4 moieties, together with C3 and C5 hydrocarbons
as byproducts. In this case, the intermolecular isotopic scram-
bling in isobutane is expected. Thus we can distinguish two
mechanisms by the 13C isotopic distribution, intramolecular or
intermolecular pattern.
The reaction was performed in a closed circulation system
(300 cm3) equipped with an on-line GC at 393—523 K. After
Cs2.5H0.5PW12O40 (110 m2 g-1, abbreviated as Cs2.5) and sulfat-
ed ZrO2 (90 m2 g-1, abbreviated as SZ) were pretreated in a vac-
uum at 573 and 673 K, respectively, 40 torr of 1,4-13C2-n-
butane (Isotec Inc., 13C: 99%) was introduced. The product
isobutane and reactant n-butane were separated to be analyzed
by Field Ionization Mass Spectrometry (FI-MASS, JEOL JMS-
SX102A) for 13C-distribution.
The selectivity to isobutane for Cs2.5 was changed from
91% to 87% when the reaction temperature was raised from
393 K to 523 K, while it decreased from 92% to 78% over SZ.
Figure 1 shows the isotopic distributions of isobutane over
Cs2.5, where the binomial patterns were optimized to fit the
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
471
the larger amount of strong acid sites on Cs2.5 than on SZ.14
The trend that SZ preferred bimolecular mechanism is probably
due to the oxidative dehydrogenation ability of SZ,3 by which
n-butane is dehydrogenated to butenes over SZ to change readi-
ly to butyl carbenium ion, and then it forms octyl cation inter-
mediate of bimolecular mechanism.
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12
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