Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite

Article abstract of DOI:10.1016/j.jcat.2014.12.035

Kinetics and isotope labeling experiments were used to investigate the reaction pathways of n-butane on H-mordenite and Pt/H-mordenite at atmospheric pressure and temperatures of 543-583 K. Butenes, either formed on the catalyst or present in the feed, controlled the relative rates of mono- and bimolecular reaction pathways. The true activation energy for isobutane formation was found to be 120-134 kJ/mol. The reaction order for isobutane formation with respect to n-butene on Pt/H-mordenite was 1.0-1.2, consistent with a predominately monomolecular route of formation. An order close to 2 for disproportionation products indicated a bimolecular route of formation. An increase of the butene concentration from less than 20 ppm to about 120 ppm greatly increased the rate of bimolecular skeletal isomerization, as determined from conversion of 1,4-¹³C₂-n-butane. The findings explain how reaction conditions affect product selectivity and clarify the controversy around butane isomerization on solid acids.

Full text of DOI:10.1016/j.jcat.2014.12.035

Journal of Catalysis xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Mechanism of n-butane skeletal isomerization on H-mordenite
and Pt/H-mordenite
⇑
Matthew J. Wulfers, Friederike C. Jentoft
School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, OK 73019-1004, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxxx
Kinetics and isotope labeling experiments were used to investigate the reaction pathways of n-butane on
H-mordenite and Pt/H-mordenite at atmospheric pressure and temperatures of 543–583 K. Butenes,
either formed on the catalyst or present in the feed, controlled the relative rates of mono- and bimolec-
ular reaction pathways. The true activation energy for isobutane formation was found to be 120–134 kJ/
mol. The reaction order for isobutane formation with respect to n-butene on Pt/H-mordenite was 1.0–1.2,
consistent with a predominately monomolecular route of formation. An order close to 2 for dispropor-
tionation products indicated a bimolecular route of formation. An increase of the butene concentration
from less than 20 ppm to about 120 ppm greatly increased the rate of bimolecular skeletal isomerization,
as determined from conversion of 1,4-13C2-n-butane. The findings explain how reaction conditions affect
product selectivity and clarify the controversy around butane isomerization on solid acids.
Ó 2015 Elsevier Inc. All rights reserved.
Keywords:
Zeolite
MOR
Intramolecular
Beta-scission
Binomial distribution
Equilibrium
Dual-functional
Isotopic scrambling
Olefins
1. Introduction
reaction pathway is responsible, which would involve formation of
C8 intermediates from two C4 entities, rearrangement of the C8
Skeletal isomerization of n-butane is of interest, both with
respect to commercial application and with respect to the funda-
mental understanding of reaction mechanisms in acid catalysis.
The isomerization product, isobutane, is used in large quantities
for C4 alkylation and production of methyl tert-butyl ether and
ethyl tert-butyl ether via isobutene [1,2]. Isobutane can be pro-
duced from its less valuable skeletal isomer, n-butane, via acid-cat-
alyzed skeletal isomerization. The mechanistic details of this
reaction have been the subject of intense debate in the open liter-
ature [3–5].
In contrast to n-butane isomerization, skeletal isomerization of
paraffins with five or more carbon atoms is well understood and is
known to occur via a monomolecular reaction pathway [6,7].
Butane isomerization is thought to be a special case, and it has been
claimed that monomolecular skeletal isomerization of butane on
solid acids does not occur [8,9], or occurs very slowly [10] or with
great difficulty [11], or is not the main isomerization pathway
[12,13]. However, numerous solid acids catalyze butane skeletal
isomerization at significant rates, implying that either the above
claims are incorrect or that an alternative reaction pathway exists.
Thus far, many have claimed that a bimolecular (i.e., intermolecular)
intermediate, beta-scission of the C8 intermediate, and further
hydride transfer or hydrogenation to form either isobutane or side
products [12].
Arguments against the monomolecular butane isomerization
pathway originate in the mineral acid catalysis literature. Brouwer
and Hogeveen [14] found that the skeletal isomer of n-butane was
not formed in the superacid HF-SbF5 under conditions in which the
skeletal isomer of n-pentane formed readily. Intramolecular carbon
scrambling in n-butane, however, occurred at rates similar to those
of n-pentane skeletal isomerization [14,15]. The contrasting behav-
ior can be explained by the postulated transition states and inter-
mediates. n-Alkanes with at least five carbon atoms isomerize via a
classical cyclopropane ring structure that opens to a secondary car-
benium ion of the skeletal isomer. This monomolecular pathway is
accepted for both homogenous and heterogeneous catalysts [7]. In
the case of butane or butene conversion in mineral acids, opening
the cyclopropane ring to form the skeletal isomer results in a pri-
mary carbenium ion [16], which is the main argument against a
monomolecular skeletal isomerization pathway for these mole-
cules [13]. Hence, the butane isomerization activity of sulfated zir-
conia, chlorided alumina, heteropolyacids, and zeolites [7] is often
(but not always) ascribed to bimolecular dimerization–cracking
pathways, which circumvent the prohibitive energetic require-
ments of the monomolecular pathway. One large drawback of such
bimolecular pathways is that they are not selective to isobutane;
⇑
Corresponding author at: Department of Chemical Engineering, University of
Massachusetts, Amherst, USA.
E-mail addresses: fcjentoft@ou.edu, fcjentoft@umass.edu (F.C. Jentoft).
http://dx.doi.org/10.1016/j.jcat.2014.12.035
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
(2015), http://dx.doi.org/10.1016/j.jcat.2014.12.035

2
M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
the disproportionation products, propane and pentanes, are also
formed at significant rates [17] through cleavage of C8
intermediates.
similar. Isotope distributions in products of a bimolecular pathway
should be binomial, whereas the possibility exists for isobutane to
retain the same number of isotopes as the feed if it is formed
through a monomolecular pathway.
The zeolite H-mordenite has been investigated extensively as a
butane isomerization catalyst, yet no consensus has evolved on the
contributions of mono- and bimolecular reaction pathways to the
final product distribution. Experimental evidence suggesting dom-
inance of bimolecular pathways includes the following: intermo-
lecular carbon-13 scrambling in products formed from 1-13C-n-
butane [18] and 1-13C-isobutane [10], production of propane and
pentane side products [13,6], isobutane reaction orders with
respect to n-butane of close to 2 [8,6], and a product distribution
similar to that from isooctane [19]. Evidence of a significant contri-
2. Materials and methods
2.1. Materials
NH4-mordenite (Lot 32125-99) and Pt/H-mordenite (Lot 33407-
1, not pre-reduced by the manufacturer and Lot 33436-24, pre-
reduced) zeolites were received from UOP, a Honeywell company.
The Si/Al ratio of all samples was 9.1, specified by ICP-AES per-
formed by the manufacturer. Platinum had been introduced by
the incipient wetness method using hexachloroplatinic acid as
the precursor; the platinum content was 0.328 wt%, also specified
by ICP-AES. Calcination of the NH4-mordenite was performed in a
horizontal furnace. A quartz boat containing 1.5 g of catalyst was
placed inside a 2.3 cm ID quartz tube, which was placed inside
the furnace under a 100 ml minꢀ1 flow of air (Airgas, zero grade).
The temperature was first increased at 2 K minꢀ1 to 423 K, held
for 1 h, and then increased at 5 K minꢀ1 to 873 K and held for 2 h.
Reactant gases were of the following purities: n-butane (Math-
eson, 99.99%) containing 14 ppm isobutane and <1 ppm propene
impurities, and 1,4-13C2-n-butane (Isotec Inc., specified as 99.7%
gas purity and 99% isotope purity) with 5200 ppm ethane and
1200 ppm 1-butene impurities as quantified by in-house GC anal-
ysis. Other gases were N2 (Airgas, UHP 99.999%), O2 (Airgas, UHP
99.994%), H2 (Airgas, UHP 99.999%), and helium (Airgas, UHP
99.999%). All inorganic gases were passed through moisture traps
(Agilent, MT400-2 for N2, O2, and H2; Restek for helium), and N2
was also passed through an O2 trap (Chromres, Model 1000).
bution of
a monomolecular skeletal isomerization pathway
includes the following: isobutane reaction orders with respect to
n-butane of 1.17 [20] and 1 [6], and the inability to model isobu-
tane formation using only a first- or second-order kinetic model
[21]. Additionally, some investigators reported that selectivity to
isomerization or disproportionation products is affected by process
conditions such as temperature [13], concentration of acid sites
[13,20], conversion [13], reactant partial pressure [13,21], and
presence of H2 [6]; however, a consistent explanation for these
dependencies does not exist.
One variable that has been investigated to some extent, but has
not been linked to specific reaction pathways or product selectivi-
ties, is the concentration of alkenes. Pines and Wackher [3], who
worked with an AlCl3/HCl catalyst, reported a promoting effect of
alkenes on n-butane isomerization. Butane conversion on the zeo-
lite H-mordenite can also be promoted by addition of alkenes;
Fogash et al. observed that H-mordenite was completely inactive
for n-butane [22] or isobutane [23] conversion in the absence of
butene in the feed, but was very active when as little as 55 ppm
of butene was co-fed. The effect of alkenes is not specific to butene;
Engelhardt [24] found that n-butane conversion on H-mordenite
could be enhanced by addition of ethene, propene, or isobutene
to the feed. The data show an increase in selectivity to dispropor-
tionation products upon addition of any alkene, but this effect was
not discussed by the author. Similar observations have been made
with sulfated zirconia; alkenes reportedly promote conversion of
n-butane [25–29] and deactivation [22,26]. Intermolecular isotope
scrambling in isobutane formed from 1,4-13C2-n-butane can be
controlled by inclusion of platinum on sulfated zirconia and H2
in the feed [30,31]. Inclusion of platinum and H2 caused isobutane
to retain the same number of isotopes as the feed, which is indic-
ative of a monomolecular isomerization pathway, whereas the
absence of platinum and H2 resulted in significant intermolecular
isotope scrambling. The authors hypothesized that the butene con-
centration was low in experiments with platinum and H2, which
was considered as not conducive to the bimolecular pathway, but
the butene concentration was not reported. In fact, effluent alkene
concentrations are typically not documented; a rare exception is
the work by Weisz and Swegler on hexane isomerization [32].
The goal of this investigation was to determine the contribu-
tions of monomolecular and bimolecular isomerization and dispro-
portionation pathways on H-mordenite and to identify the process
variable(s) that control the relative rates. This knowledge would
allow one to predict changes in rate and selectivity with changing
reaction conditions and possibly design processes in which the
selectivity to a desired product can be tuned based on a physical
understanding of the reaction chemistry. In theory, the products
of monomolecular and bimolecular reaction pathways should be
distinguishable by kinetics parameters, such as reaction order
and activation energy, and the extent of intermolecular isotope
scrambling. If propane, pentanes, and isobutane are all produced
through similar bimolecular pathways, their kinetics parameters
and experimentally determined isotope distributions may be
2.2. Test apparatus and pretreatment conditions
All catalytic experiments were performed at atmospheric pres-
sure in down-flow packed-bed reactors. Zeolite powder was placed
in a borosilicate glass tube (5.5 mm ID) between two pieces of
quartz wool. The tube was heated by an electrically powered fur-
nace, and the temperature was controlled by an Omega CN3251
controller using a K-type thermocouple placed in a well at the cen-
ter of the furnace’s isothermal zone. The pressure drop through the
catalyst bed was monitored and was less than 0.5 psi during all
experiments. Quantification of products was performed by a gas
chromatograph (Varian 3800 GC) using a Fused Silica PLOT column
(Chrompack, 0.32 mm ID ꢁ 60 m) for separation and both thermal
conductivity and flame ionization detectors connected in series. In
experiments with 1,4-13C2-n-butane as reactant, a gas chromato-
graph equipped with a mass spectrometer detector (GC–MS, Agi-
lent 5975E) was used to determine the isotope distribution in
individual products. The electron impact ionization source in the
GC–MS was operated at 70 eV, and products were separated using
a GS-Gaspro PLOT column (Agilent, 0.32 mm ID ꢁ 60 m). Splitless
injections were performed on the GC–MS in all cases except when
Pt/H-mordenite was used without an alkene trap, in which case a
split ratio of 5:1 was used. Mass flow controllers (Bronkhorst) were
used to control gas flow rates, and gas transfer lines were heated to
a temperature of 343 K.
2.3. Pretreatment of catalysts
Pretreatment of H-mordenite began by raising the temperature
at 5 K minꢀ1 to 673 K in a 30 ml minꢀ1 (NTP) flow of synthetic air
and continued with sequential 0.5-h treatments in synthetic air,
N2, and H2. Pretreatment of Pt/H-mordenite began by heating at
5 K minꢀ1 to 403 K in a 30 ml minꢀ1 flow of N2 and holding the
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
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M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
3
temperature for 4 h. The temperature was then increased at
5 K minꢀ1 to the final reduction temperature in a flow of H2 and
held for 4 h. The final reduction temperature and H2 flow rate were
560 K and 30 ml minꢀ1, except in experiments with varying space
time, in which a final reduction temperature and H2 flow rate of
573 K and 50 ml minꢀ1 were used.
specified range of temperatures. The temperature was then low-
ered to 543 K, and four measurements were taken at the initial
set of conditions to determine whether deactivation had occurred.
The partial pressure of n-butane was then increased by 3.3 kPa
while holding the partial pressure of H2 constant and the
procedure was repeated. After taking measurements at the four
n-butane partial pressures of interest, the partial pressure of H2
was lowered by 20 kPa and the procedure was repeated. This pro-
tocol was used to gather rate data under all conditions of interest
or until deactivation was apparent from time on stream data. An
example of the data generated with increasing temperature is
shown in Fig. S1 in the supporting information.
2.4. Conversion of n-butane on H-mordenite with controlled
concentration of n-butenes in the feed
An experiment was performed with two reactors placed in ser-
ies, whereby the first reactor was used to control the concentration
of n-butenes in the feed to the second reactor. The first reactor was
an HVC-VUV environmental chamber from Harrick Scientific
(which is commonly used for in situ diffuse reflectance spectros-
copy) and contained 70 mg of a 1 wt% Pt/SiO2 catalyst (preparation
details and characterization of the Pt/SiO2 can be found in Ref.
[33]). The Pt/SiO2 catalyst was reduced in the environmental
chamber at a temperature of 548 K in H2 flowing at 30 ml minꢀ1
for 1.5 h and was then cooled to the desired operating tempera-
ture. The flow to the chamber consisted of 3 ml minꢀ1 of n-butane
and 12 ml minꢀ1 of helium, and was admixed with 15 ml minꢀ1 H2
between the first and the second reactor. A temperature of 323 K
was used in the first reactor to keep the concentration of butenes
below the detection limit of the GC, which was about 1 ppm, and
a temperature of 433 K was used to control the butene concentra-
tion at 18 ppm. The second reactor was the apparatus described in
Section 2.2 and held 87 mg of H-mordenite.
2.6. Measurement of product formation rates on Pt/H-mordenite at
various space times
Experiments were performed using the apparatus described in
Section 2.2. A minor deviation was that Pt/H-mordenite powder
(Lot 33436-24) was mixed with 1.0-mm-diameter quartz beads
in a 3:2 mass ratio and supported in a quartz tube (12 mm ID)
by a quartz frit. Four catalyst loadings between 0.10 g and 1.00 g
were used. Reactions were performed at a temperature of 573 K
applying a total gas flow rate of 50 ml minꢀ1, with the gas mixture
composed of 10 kPa n-butane, 50 kPa H2, and helium for balance.
2.7. Collection of data during conversion of 1,4-13C2-n-butane
Conversion of 1,4-13C2-n-butane was performed on 81–87 mg of
H-mordenite or Pt/H-mordenite (Lot 33407-1) either with or with-
out purification of the feed. For experiments in which the feed was
purified, the feed stream passed through a separate quartz tube
containing a bed of HY zeolite (190–200 mg, Zeolyst, Si/Al = 2.5),
which served as an alkene trap, before reaching the reactor that
contained mordenite. The HY zeolite was pretreated by heating
to a temperature of 473 K in the starting gas flow (either synthetic
air for H-mordenite or N2 for Pt/H-mordenite) and was held at this
temperature for 1 h before heating of the mordenite began. At the
end of the mordenite pretreatment, the HY zeolite was cooled to
ambient temperature and the temperature of the reactor contain-
ing the mordenite was set to 573 K. The reactant gas mixture
was then admitted to the reactor. A flow of 21 ml minꢀ1 H2,
6 ml minꢀ1 helium, and 3 ml minꢀ1 1,4-13C2-n-butane was used
as a standard composition except for the experiment employing
Pt/H-mordenite without feed purification, in which case the gas
mixture was composed of 27 ml minꢀ1 H2 and 3 ml minꢀ1
1,4-13C2-n-butane. In experiments without feed purification, mea-
surements by online GC-FID and GC–MS were started 3 min after
hydrocarbon flow to the reactor had begun. In experiments with
feed purification, the first injection was taken 6.5 min after hydro-
carbon flow to the reactor had begun. Conversion of n-butane to
gas phase products was less than 4% in all cases.
2.5. Measurement of product formation rates on H-mordenite and Pt/
H-mordenite over a range of temperatures, n-butane partial pressures,
and H2 partial pressures
Experiments were performed using the apparatus described in
Section 2.2. Either 82 mg of H-mordenite or 80 mg of Pt/H-
mordenite (Lot 33407-1) was used as the catalyst. Product forma-
tion rates were measured over a range of temperatures (543–
583 K, 10 K increments), n-butane partial pressures (3.3–13.3 kPa,
3.3 kPa increments), and H2 partial pressures (30–90 kPa, 20 kPa
increments). The total conversion of n-butane was in the range of
what is considered to be differential, not exceeding 1.7% on
H-mordenite or 2.6% on Pt/H-mordenite. Because conversions were
low, a product-based conversion X defined in Eq. (1) was used
throughout the manuscript.
FC;prod
FC;total
X% ¼
ꢁ 100
ð1Þ
where FC,prod is the flow rate of carbon atoms from product
molecules in the reactor effluent stream (i.e., everything except
n-butane), and FC,total is the total flow rate of carbon atoms in the
reactor effluent stream (i.e., includes n-butane). Selectivities are
reported as molar selectivities and were calculated according to
Eq. (2)
3. Results
Fx
Ft
Sx% ¼ ꢁ 100
ð2Þ
3.1. Effect of n-butenes in the feed on product formation rates
Sx is the selectivity for an individual product in percent, Fx is the
molar flow rate of the individual product, and Ft is molar flow rate
of all gas phase products.
Conversion of n-butane on both catalysts began at conditions of
543 K, 3.3 kPa n-butane, and 90 kPa H2. Helium was used to bal-
ance to atmospheric pressure. After taking four GC measurements
over a duration of about 1 h, the temperature was increased by
10 K and four measurements were again taken. This procedure
was repeated until measurements had been taken over the
Fig. 1 demonstrates the dramatic effect that n-butenes have on
the rate of n-butane conversion. A butene-free feed was used ini-
tially, and there was no conversion of n-butane to gas phase prod-
ucts. After 75 min on stream, 18 ppm of n-butenes were added to
the feed, resulting in significant conversion of n-butane. The con-
centration of products in the reactor effluent is shown in Fig. 1
instead of the rate to illustrate the magnitude of the effect of the
n-butenes on gas phase product formation. The isobutane concen-
tration reached 415 ppm, and the concentrations of propane and
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
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M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
500
400
300
200
100
0
15
13.3 kPa n-butane
10 kPa n-butane
isobutane
6.7 kPa n-butane
3.3 kPa n-butane
10
5
propane
pentanes
0
0
50
100
150
30
40
50
60
70
80
90
Time on Stream / min
P(H2) / kPa
Fig. 1. The concentration of gas phase products resulting from conversion of n-
butane (10 kPa partial pressure) on H-mordenite at a temperature of 473 K using
50 kPa H2 and 40 kPa helium as diluents. The concentration of n-butenes in the feed
was 0 ppm during the first 75 min on stream and 18 ppm after 75 min on stream.
Fig. 2. Measured concentrations and calculated equilibrium concentrations of
trans-2-butene at a temperature of 563 K over the range of n-butane and H2 partial
pressures. Solid circles: measured on Pt/H-mordenite. Open squares: measured on
H-mordenite. Lines: calculated equilibrium concentrations using the software
package Thermosolver (v. 1.0).
pentanes (n-pentane and 2-methylbutane) were between 60 and
80 ppm. The concentration of n-butenes in the effluent stream
was 1–3 ppm. A slight decrease in product concentrations with
time was observed starting at about 100 min on stream, that is,
soon after butenes were introduced and conversion was stimulated.
Initially unknown to the experimenters, the commercial
1,4-13C2-n-butane also contained butene, which was present as
an impurity in the as-purchased commercial product. Diluted to
a partial pressure of 10 kPa, the isotopically labeled feed contained
120 ppm of 1-butene. Thus, product formation rates and selectivi-
ties differed on catalysts, especially H-mordenite, used with and
without a bed of HY zeolite acting as a butene adsorbent (see rates
and selectivities of main products in Table 1 or all products in
Table S5 in the supporting information). Without the alkene trap
(that is, with 120 ppm of 1-butene in the feed), the rate of isobu-
the calculated equilibrium concentrations over the entire range
of conditions on Pt/H-mordenite and at high H2 partial pressures
on H-mordenite (see the concentration of trans-2-butene in Fig. 2
and an analysis over a wider range of reaction conditions in Section
S2 of the supporting information). When Pt/H-mordenite was the
catalyst, the effluent butene concentrations were close to equilib-
rium at H2 partial pressures of 50, 70, and 90 kPa. When H-mord-
enite was used, the concentration of n-butenes deviated
significantly from the calculated equilibrium concentration at
low H2 partial pressures, but was close to equilibrium at a H2 par-
tial pressure of 90 kPa. The three n-butene isomers were always
present in ratios expected from thermodynamics.
When isotopically labeled n-butane was used, the n-butene
concentration was slightly below the calculated equilibrium con-
centration in experiments on Pt/H-mordenite both with and with-
out the alkene trap, and was within 2 ppm of concentrations
measured with an unlabeled feed (Table 1). On H-mordenite, the
concentration of n-butenes was always higher than the calculated
equilibrium concentration. When an alkene trap was used, the con-
centration was 1 ppm higher, and when the alkene trap was not
used, the measured concentration of 32 ppm was twice the calcu-
lated equilibrium concentration of 16 ppm (Table 1).
tane formation on H-mordenite was 1770
isobutane selectivity was 60%. With an alkene trap, the rate of iso-
butane formation was 765
mol gꢀ1 hꢀ1 and the isobutane selec-
l
mol gꢀ1 hꢀ1 and the
l
tivity was 78%. Both rate measurements are higher than when an
unlabeled feed was used; considering that butenes increase the
rate of n-butane conversion, it is possible that the alkene trap
was only partially effective.
3.2. Comparison of n-butene effluent concentrations with equilibrium
concentrations
3.3. Trends in product formation rates and selectivities with changing
reaction conditions or space time
When unlabeled n-butane was used, the measured concentra-
tions of n-butenes (i.e., 1-butene, trans-2-butene, and cis-2-butene)
in the reactor effluent were within 10–34% (generally 10–20%) of
The product formed with the highest rate and highest selectiv-
ity under all reaction conditions was isobutane (Fig. 3 and Table 1).
Table 1
Rates and selectivities of major products produced from 1,4-13C2-n-butane or unlabeled n-butane at a temperature of 573 K and n-butane partial pressure of 10 kPa.
P(H2) (kPa)
Purified feed
Rate (
l
mol gꢀ1 hꢀ1
)
Selectivity (mol%)
n-Butene concentration (ppm)
Isobutane
Propane
Pentanes
Isobutane
Propane
Pentanes
Measured
Equilib.
Labeled feed
H-mordenite
Pt/H-mordenitea
H-mordenite
Pt/H-mordenite
70
90
70
70
No
No
Yes
Yes
1770
370
765
495
755
62
110
46
330
37
75
60
77
78
83
25
13
11
8
11
8
8
32
12
17
12
16
13
16
16
30
5
Unlabeled feed
H-mordenite
H-mordenite
Pt/H-mordenite
Pt/H-mordenite
70
90
70
90
n/a
n/a
n/a
n/a
555
498
537
422
55
45
40
24
43
33
33
14
81
82
84
86
8
7
6
5
6
5
5
3
12
10
14
11
16
13
16
13
a
Measured with GC–MS.
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M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
5
100
90
80
70
60
100
90
80
70
60
100
100
90
80
70
60
90
W/F
Temp.
P (n-butane)
P (H2)
80
70
60
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
Conversion / %
Coversion / %
Conversion / %
Conversion / %
15
10
5
15
10
5
15
10
5
15
10
5
W/F
P (n-butane)
P (H2)
Temp.
0
0
0
0
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
Conversion / %
Conversion / %
Conversion / %
Conversion / %
Fig. 3. Selectivities of isobutane (upper row) or pentanes (lower row) on Pt/H-mordenite with respect to conversion. Changes in conversion were caused by changes in
temperature (blue squares: 543–583 K, 10 kPa n-butane, 50 kPa H2, W/F = 0.17 h), H2 partial pressure (red circles: 573 K, 10 kPa n-butane, 30–90 kPa H2, W/F = 0.17 h), n-
butane partial pressure (magenta diamonds: 573 K, 3.3–13.3 kPa n-butane, 50 kPa H2, W/F = 0.13–0.51 h), or space time (green triangles: 573 K, 10 kPa n-butane, 50 kPa H2,
W/F = 0.13–1.29 h). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Propane and pentanes were the most significant side products,
with the propane to pentanes ratio being always larger than 1. Pen-
tanes are taken to be representative of the bimolecular pathway
throughout the manuscript because propane can also be produced
on platinum via hydrogenolysis (although even without platinum,
the propane/pentanes ratio was slightly greater than 1). Methane,
ethane, ethene, propene, and hexanes (i.e., n-hexane, 2-methylpen-
tane, and 3-methylpentane) were minor products (see Table S5 in
the supporting information for a complete product distribution).
Butenes (including some isobutene at higher conversions) were
detected in the product stream even when the unlabeled feed,
which did not contain any butene as an impurity, was used.
In kinetics experiments with unlabeled n-butane, the rate of
product formation increased with increasing temperature, increas-
ing n-butane partial pressure, and decreasing H2 partial pressure
(Fig. 3). Concurrent with this increase in overall rate (and thus con-
version), there was a decrease in selectivity to isobutane and an
increase in selectivity to side products, represented by pentanes
in Fig. 3. The only parameter that did not cause a drastic change
in selectivity was the space time, which caused a decrease in isobu-
tane selectivity of only 3.1% as conversion increased from 0.9% to
8.0%.
of propane and pentane formation were also lower with the puri-
fied feed. The selectivity to isobutane increased from 60% with
the non-purified feed to 78% with the purified feed. The concentra-
tion of n-butenes in the reactor effluent was 17 ppm.
Significant intermolecular isotope scrambling was seen in all
products when the feed was not purified. Isobutane molecules
with two carbon-13 atoms accounted for 49% of the isobutane
formed, whereas 37.5% is expected in a binomial distribution
(Table 2). 2-Methylbutane and n-pentane, which are products that
can only come from a bimolecular pathway, contained 2 or 3 car-
bon-13 isotopes in 34–36% of the product, whereas 37.5% consti-
tutes a binomial distribution.
Table 2
Isotope distributions in products produced from 1,4-13C2-n-butane.
Catalyst
Trap
Number of 13C isotopes (%)
0
1
2
3
4
5
Isobutane
H-mordenite
Pt/H-mordenite
H-mordenite
Pt/H-mordenite
Binomial
No
No
Yes
Yes
8
3
7
9
19
14
9
3
25
49
72
70
81
21
10
13
7
4
0
1
0
3.4. Conversion of 1,4-13C2-n-butane on H-mordenite
6.2
37.5
25
6.2
Propane
H-mordenite
H-mordenite
Binomial
No
Yes
10
9
12.5
41
45
37.5
39
38
37.5
10
7
12.5
The rates of product formation on H-mordenite using
a
1,4-13C2-n-butane feed containing 120 ppm of 1-butene as an
impurity were significantly higher than the rates observed with
an unlabeled, butene-free feed (Table 1). The selectivity to isobu-
tane of 60% was also significantly lower than observed with the
unlabeled feed. The concentration of n-butenes in the effluent
stream was 32 ppm, which is higher than the equilibrium concen-
tration of 16 ppm.
2-Methylbutane
H-mordenite
H-mordenite
Binomiala
No
Yes
2
1
0
14
10
12.5
36
39
37.5
34
39
37.5
13
10
12.5
2
0
0
n-Pentane
H-mordenite
H-mordenite
Binomiala
No
Yes
1
14
9
12.5
36
40
37.5
35
43
37.5
13
11
12.5
2
0
0
When 1,4-13C2-n-butane was passed through an alkene trap
before reaching the catalyst bed, the rate of isobutane formation
ꢀ3
0
was 765
l
mol gꢀ1 hꢀ1
,
much lower than the rate of
a
Considering that C8 intermediates contain
isotopes.
a maximum of four carbon-13
1770
l
mol gꢀ1 hꢀ1 obtained with the non-purified feed. The rates
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6
M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
With a purified feed, 70% of isobutane molecules retained
the feed. However, addition of butene to the feed still caused an
increase in product formation rates and also a decrease in selec-
tivity to isobutane. The observation is exemplified by experi-
ments in which 1,4-13C2-n-butane was used as the reactant.
Diluted to a n-butane of 10 kPa, the feed contained 120 ppm of
1-butene. When H-mordenite was used without an alkene trap,
there was obviously an n-butene gradient from 120 to 32 ppm
through the catalyst bed. The n-butene concentration in the efflu-
ent was 17 ppm when the feed was purified, which is close to the
equilibrium value of 16 ppm and suggests that the trap removed a
considerable amount of butene from the feed. The two scenarios
of high and low butene concentration resulted in product forma-
tion rates and selectivities that were drastically different. With-
out the trap, isobutane formation rates were slightly more than
three times larger than rates measured when an alkene-free,
unlabeled n-butane feed was used. Pentane formation rates were
ꢂ8 times greater than when an unlabeled feed was used. Thus,
the rate of formation of disproportionation products was affected
more by butenes than the rate of formation of the isomerization
product, and the selectivity to isobutane of 60% with the labeled
feed was thus the lowest measured in any scenario. When the
alkene trap was used, the isobutane formation rate was only mar-
ginally higher than the rate with unlabeled feed, and the selectiv-
ity of 78% was only 3 percentage points lower than when an
unlabeled feed was used. One might argue that because butene
enhances product formation rates, secondary reactions of isobu-
tane will become prominent and alter the selectivity. However,
the space time data in Fig. 3 clarify that the product selectivity
is only a weak function of conversion, and the low selectivity to
isobutane cannot be attributed to secondary reactions. Thus,
butenes in the feed are linked to high product formation rates,
low selectivity to isobutane, and an increase in selectivity to the
disproportionation products propane and pentane.
two carbon-13 atoms (Table 2). The fractions of propane mole-
cules with one and two carbon-13 atoms were 45% and 38%,
respectively, whereas 37.5% is expected in a binomial distribu-
tion. Both n-pentane and 2-methylpentane also had a slightly
larger fraction of species with two and three carbon-13 atoms
than expected in a binomial distribution (i.e., 37.5%). One should
note that the concentrations of the disproportionation products
propane, n-pentane, and 2-methylbutane were low (115 ppm
for propane, <55 ppm for each pentane), and measurement of
their mass distribution relied on data near the detection limits
of the GC–MS.
3.5. Conversion of 1,4-13C2-n-butane on Pt/H-mordenite
Conversion of 1,4-13C2-n-butane with 120 ppm of 1-butene on
Pt/H-mordenite resulted in product formation rates, selectivities,
and isotope distributions that differed considerably from those
produced on the platinum-free H-mordenite. The rate of isobutane
formation was only 375 l
mol gꢀ1 hꢀ1, and the selectivity to isobu-
tane was 77%, which was 17% higher than on H-mordenite. The iso-
tope distribution in isobutane also changed considerably; the
fraction with two carbon-13 isotopes was 72%. The concentrations
of propane and pentane were too low to measure a reliable mass
spectrum by GC–MS.
In contrast to what was observed on H-mordenite, product for-
mation rates and selectivities on Pt/H-mordenite using purified
and non-purified 1,4-13C2-n-butane were similar. The selectivity
to isobutane of 83% on Pt/H-mordenite with the alkene trap was
the highest measured under any set of conditions, but was only
marginally higher than the 77% selectivity observed with a non-
purified feed. Purification of the feed also resulted in 81% of isobu-
tane molecules retaining two carbon-13 isotopes. This percentage
was the largest observed and coincided with the maximum mea-
sured selectivity to isobutane. The percentage of isobutane species
containing a number of carbon-13 atoms other than two did not
seem to follow a pattern. The low concentration of propane and
pentanes again prohibited an accurate measurement of their iso-
tope distributions.
4.2. Evidence for equilibration of n-butane, n-butene, and H2
Because alkenes have a dramatic effect on catalytic perfor-
mance, knowing their actual concentration in the catalyst bed is
compulsory. Moreover, it is important to realize that under condi-
tions of differential conversion of n-butane, there may still be a
considerable axial alkene gradient in a plug flow reactor that must
be accounted for in a kinetics analysis.
4. Discussion
H-form zeolites can catalyze hydrogenation and dehydrogena-
tion of alkanes and alkenes [34], but platinum is known to be more
effective [36]. Hence, it is conceivable that, especially on Pt/H-
mordenite, an equilibrium between n-butane, n-butenes, and H2
(Eq. (3)) may be reached inside the reactor.
4.1. Impact of butenes in the feed on product formation rates and
selectivities
Fig. 1 shows that H-mordenite did not convert n-butane at a
temperature of 473 K in the absence of butenes in the feed, but
became active after introduction of 18 ppm of n-butene. This
behavior indicates that n-butane cannot be protonated by the
Brønsted acid sites of H-mordenite (at a temperature of 473 K)
and is in agreement with the results of Fogash et al. [22]. The iso-
butane to butene ratio of ꢂ20 demonstrates that each butene ini-
tiates multiple turnovers; n-butane molecules are probably
activated through hydride transfer. A new observation is the net
consumption of n-butene, which was evident through an n-butene
concentration in the effluent stream of 1–3 ppm. Possible explana-
tions for the disappearance of butenes are (i) an approach to the
chemical equilibrium concentration, which would be 0.1 ppm of
n-butene under the applied conditions, through hydrogenation to
n-butane on acid sites [34], or (ii) formation of carbonaceous
deposits, consistent with the observed slight decline in conversion
also seen by others [22] and the reported propensity of alkenes to
react and form stable cyclic surface species [35].
n-C4H10ðgÞ ꢀ n-C4H8 ðgÞ þ H2ðgÞ
ð3Þ
All evidence points toward rapid equilibration on Pt/H-morde-
nite. A first indication is the measured butene concentrations,
shown in Fig. 2 for experiments with unlabeled n-butane and
Table 1 for 1,4-13C2-n-butane, which were generally within 10–
20% of the calculated equilibrium concentrations. Given the low
concentrations in the proximity of the detection limit of the GC
and variations in reported thermodynamic data [37–39], the agree-
ment between measured concentrations and calculated concentra-
tions is good. Additionally, changes in the measured concentration
closely mirror changes in the calculated equilibrium concentration
over the range of conditions. A criterion that must be satisfied if n-
butane, n-butene, and H2 are at equilibrium evolves from analysis
of what should be the apparent kinetics parameters for n-butenes,
reported in Table 3. If n-butene concentrations are dictated by the
chemical equilibrium, then the parameters will reflect thermody-
namic relationships pertaining to Eq. (3), that is, the law of mass
action given in Eq. (4):
At temperatures of about 523 K and above, the zeolite is able
to catalyze skeletal isomerization in the absence of alkenes in
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M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
7
Table 3
Comparison of experimentally determined apparent kinetic parameters and param-
eters expected at equilibrium for n-butenes on Pt/H-mordenite.
Parameter
Experimental
value
Equilibrium
value
n-Butane reaction order
H2 reaction order
1.0–1.2
1.0
ꢀ1.0 to ꢀ0.8
ꢀ1.0
Apparent activation energy
109–130 kJ molꢀ1
121 kJ molꢀ1
Scheme 1. Individual reactions involved in the monomolecular isomerization
pathway assuming that methyl shift (Step 3) is rate-limiting.
(kJ molꢀ1
)
[42]; this picture is consistent with the observation that OH groups
are consumed (as opposed to perturbed) upon adsorption of 1-pen-
tene on H-mordenite [35]. For the cases in which platinum is pres-
ent, a rate law that can be used to determine an activation energy
for isomerization must consider the dehydrogenation–hydrogena-
tion equilibrium that is preceding the isomerization, similar to the
approach taken by Chiang and Bhan [43] and Macht et al. [44] for
hexane isomerization. The fact that equilibrium concentrations of
butenes are observed in the gas phase suggests that adsorption
on the catalyst is reversible under the applied conditions, and the
adsorption equilibrium shown in Eq. (5), characterized by the con-
stant Kads, can be assumed:
KDH½n-C4H10
½H2ꢃ
ꢃ
½n-C4H8ꢃ ¼
ð4Þ
with KDH the equilibrium constant for Eq. (3), and brackets indicat-
ing activities of the respective species. If equilibrated, an experi-
ment designed to determine reaction orders of n-butene should
return values of 1 and ꢀ1 with respect to n-butane and H2, and
an experiment designed to determine the apparent activation
energy should return 121 kJ molꢀ1, which is the enthalpy of reac-
tion at temperatures of 543–583 K. Indeed, the values obtained in
these experiments were 1.0–1.2 for the reaction order with respect
to n-butane, ꢀ0.8 to ꢀ1.0 for the reaction order with respect to H2,
and 109–130 kJ molꢀ1 for the apparent activation energy. Thus, the
agreement between the parameters of the thermodynamic relation-
ship in Eq. (3) and the experimental values is good (Table 3), which
proves that n-butane, n-butenes, and H2 are at equilibrium at the
end of the catalyst bed.
Another criterion that must be satisfied if equilibrium is
achieved is the independence of the n-butenes concentration of
the space time, which was also satisfied, as shown in Fig. 4. Thus,
the butene gradient along a bed of Pt/H-mordenite is, under the
conditions applied, minimal. The situation is different for H-mord-
enite; equilibrium concentration was typically not achieved with
unlabeled n-butane (Fig. 2), and when 1,4-13C2-n-butane with
120 ppm of 1-butene in the feed was used, the effluent stream still
contained a quantity of n-butene greater than the equilibrium con-
centration. The rapid equilibration between butane, butenes, and
H2 on Pt/mordenite implies that the concentrations throughout
the bed are known, and hence the subsequent discussion of kinet-
ics will focus on this catalyst.
n-C4H8ðgÞ þ Sꢄ ꢀ n-C4H8 ꢀ S
ð5Þ
with S⁄ a vacant surface site. Once formed, the n-butyl alkoxide then
undergoes a monomolecular methyl shift to form an isobutyl alkox-
ide, which can be released into the gas phase as isobutane through
either hydride transfer with n-butane or desorption as isobutene
and hydrogenation on platinum.
If the methyl shift (Step 3 in Scheme 1) is assumed to be rate-
limiting, then the rate law for monomolecular isomerization at dif-
ferential conversion can be written as Eq. (6):
kmonoK1K2CT Pn-C4H10
ꢀ
ꢁ
rmono
¼
ð6Þ
K1K2Pn-C4H10
Pi-C4H10
PH2 1 þ
þ K
PH2
5K6PH2
where kmono is the monomolecular isomerization rate constant, K is
an equilibrium constant, P is a partial pressure, and CT is the total
number of active sites. Given that the butene concentration is only
a few ppm, a small fractional coverage can be assumed (implying
also that the number of vacant sites [S⁄] is constant) and the rate
law can be approximated with Eq. (7).
Platinum can also catalyze skeletal isomerization of butane
[40], but the rate data in Table 1 give no indication of significant
contributions, consistent with the low platinum loading and the
high activation energy for skeletal isomerization on platinum [36].
Pn-C4H10
PH2
rmono ¼ kmono
K
ð7Þ
4.3. Possible outcomes of kinetics and isotope labeling experiments
Thus, one would expect to measure a first-order dependence on the
n-butane partial pressure and a negative first-order dependence on
the H2 partial pressure. The energy barrier in the rate constant has
been investigated in DFT studies and is discussed in Section 4.4.
Additionally, because all carbon–carbon bond breaking or bond
forming events in the reaction mechanism are intramolecular, the
number of carbon-13 isotopes in the isomerized product should
be the same number as in the normal feed.
Discussion of two possible isomerization pathways, monomo-
lecular and bimolecular, can be found in the literature. Differentiat-
ing between the two isomerization pathways requires an
understanding of the experimental evidence that would prove their
existence. Disproportionation products such as propane and pen-
tane can only form through bimolecular pathways and provide a
standard for products formed through a known pathway.
The individual steps that occur in a proposed monomolecular
pathway are outlined in Scheme 1. The first surface species is
formed through an initiation reaction, which is likely protonation
of n-butene, because H-mordenite did not convert n-butane with-
out n-butene in the feed (Fig. 1). Hence, the concentration of active
surface species is controlled by the partial pressure of n-butene,
which presents a lower energetic barrier for protonation than n-
butane and also has a larger heat of adsorption [41,46]. The active
The kinetics parameters and isotope distributions for products
formed through bimolecular pathways are expected to be consid-
erably different than those for products formed through monomo-
lecular pathways. The individual reactions in
a proposed
bimolecular pathway leading to formation of the disproportion-
ation products propane and pentane are shown in Scheme 2. The
C4 surface intermediates are again formed from butene, but to ana-
lyze the kinetics of disproportionation, the reaction partner of the
butoxide must be identified, as both butane and butene have been
suggested [13]. Alkanes without tertiary carbon atoms (i.e., n-
butane) play only a minor role in alkylation [1], and a plot of the
pentanes formation rate vs the product of butene and n-butane
surface intermediate can be a
p-complex, an alkoxide, or an ion
pair complex. In the zeolite, an alkoxide is the most likely state
in the case of short alkyl chains according to DFT calculations
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8
M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
distribution of isotopes. If the C8 intermediate undergoes beta-
scission before a random distribution of isotopes is achieved, then
the disproportionation products will not have a binomial distribu-
tion of isotopes.
4.4. Evidence for monomolecular isobutane formation at low alkene
partial pressure
Scheme 2. Individual reactions involved in the bimolecular disproportionation
pathway assuming that beta-scission (Step 5) is rate-limiting.
Both kinetics experiments and isotope labeling experiments
uncovered evidence of a predominately monomolecular isomeriza-
tion pathway at low alkene partial pressure. The isotope distribu-
tions in isobutane derived from isotope labeling experiments can
be split into two groups: those in which isobutane contained two
carbon-13 isotopes in at least 70% of the product, and that in which
only 49% of isobutane contained two carbon-13 isotopes. The three
experiments in which at least 70% of isobutane retained two car-
bon-13 isotopes were characterized by a selectivity to isobutane
of 77% or greater, whereas the experiment in which only 49%
retained two carbon-13 labels delivered an isobutane selectivity
of 60%. The low amount of intermolecular isotope scrambling
and high selectivity to isobutane are linked, and both are evidence
of a significant contribution from a monomolecular isomerization
pathway.
As discussed in Section 4.3, it is possible for disproportionation
products to contain a non-binomial distribution of isotopes if a
random distribution of isotopes is not achieved in the C8 interme-
diate. In fact, Adeeva and Sachtler [45] questioned the assumption
that carbon-13 atoms in C8 intermediates are statistically scram-
bled during conversion of n-butane. When investigating the reac-
tion pathways of 1,4-13C2-n-butane on industrial chlorided
alumina catalysts, more isobutane with two carbon-13 isotopes
was detected than what is expected in a binomial distribution.
The authors used isobutane molecules with three and four car-
bon-13 atoms, which can only be formed through bimolecular
pathways, as a ‘‘control’’ to determine whether there was rapid
scrambling in the C8 intermediate. Because molecules with three
and four carbon-13 isotopes were not formed in a 4:1 ratio as
required in a binomial distribution, the authors dismissed a mono-
molecular isomerization pathway as cause of the high fraction of
concentrations consistently produced a series of curves. In con-
trast, a universal curve was obtained when plotting the pentanes
formation rate vs butene concentration (see Fig. S3 in the support-
ing information). If beta-scission of the resulting surface C8 inter-
mediate is the rate-limiting step, then the rate equation can be
written as Eq. (8):
2
kbiK1K2K3K4CT ðPnC4H10
Þ
ꢂ
ꢃ
ð8Þ
rbi
¼
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
2
2
PC4H10
PH2
PnC4H10
PnC4H10
PH2
K1K2 PiC5H12
ðPH2
Þ
1þK1K2
þK1K2K3
2 þK1K2K3K4
þ
PH2
K7
PH2
where rbi is the bimolecular rate and kbi is the bimolecular rate con-
stant. Under conditions at which the surface coverage is negligible,
the equation simplifies to Eq. (9).
ꢂ
ꢃ
2
P
nC4H10
rbi ¼ kbiK
ð9Þ
PH
2
Thus, the kinetic parameters of a product formed through a bimo-
lecular pathway are characterized by a reaction order with respect
to n-butene (or n-butane) of 2 and a reaction order with respect to
H2 of ꢀ2. The chemical reactions in the aforementioned rate equa-
tions are represented in Scheme 3 using structural formulae.
The isotope distribution in products of a bimolecular pathway
will depend on the extent of isotope scrambling in C8 intermedi-
ates. If skeletal rearrangement of the C8 species is facile and
carbon-13 isotopes are distributed randomly, then the gas phase
disproportionation products will also contain
a
binomial
Scheme 3. Possible reaction pathways of n-butane on Pt/H-mordenite.
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(2015), http://dx.doi.org/10.1016/j.jcat.2014.12.035

M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
9
isobutane with two carbon-13 atoms. The ratio between molecules
with one and zero carbon-13 atoms, which should also be 4:1, was
rejected as a criterion because too many fragments contribute to
these lower m/z values. The authors hypothesized that a concerted
reaction pathway to the 2,4,4-trimethylpentenium ion exists and
beta-scission occurs before carbon-13 atoms scramble. It is note-
worthy that the reported internal rearrangement of carbon-13
atoms in n-butane of 6–25% was lower than the conversion to
gas phase products (excluding n-butane molecules with internally
scrambled isotopes) of 10–36% [45]. This result is consistent with a
lack of intramolecular scrambling; however, it was not cited as evi-
dence by the authors.
To assess whether the high fraction of isobutane molecules with
two carbon-13 isotopes in this report was the result of a monomo-
lecular mechanism or slow intramolecular scrambling of C8 sur-
face species, three criteria were applied: (i) the distribution of
isotopomers, as considered by Adeeva and Sachtler [45], (ii) the
extent of internal isotope scrambling in n-butane, and (iii) isotope
distributions in C3 and C5 side products, which can only be formed
via bimolecular pathways.
Very few or no isobutane molecules with four carbon-13 iso-
topes were detected in the three cases in which the isotope distri-
bution in isobutane was far from binomial (Table 2). Thus, the ratio
between molecules with three and four isotopes can be taken to be
greater than the ratio of four expected in a binomial distribution.
However, the ratio between species with one and zero isotopes
was very close to, or significantly less than, four. The most exten-
sive scrambling was observed when a non-purified feed was used
in combination with H-mordenite. In this case, the ratio between
isobutane species with three and four carbon-13 isotopes was
5.9, while the ratio between species with one and zero carbon-13
isotopes was 2.6. If both ratios are considered, the criterion gives
inconclusive results in all cases.
The internal isotope scrambling in n-butane was found to be
10–11% for all cases (Table 4), which is much higher than the
0.5–4.5% conversion to gas phase products. This result demon-
strates that carbon-13 isotopes are rapidly redistributed in C4 sur-
face species and suggests (but does not prove) that rapid
intramolecular scrambling is also likely in C8 species.
The isotope distribution in propane and pentanes also indicated
that there was extensive isotope scrambling in C8 intermediates.
For the following calculation, it was assumed that four carbon-13
isotopes were present in C8 intermediates, which disproportionate
to produce propane with 0–3 isotopes and pentanes with 1–4 iso-
topes. The fractions of pentane molecules with zero or five carbon-
13 isotopes, which deviated by up to 3 percentage points from
zero, can be taken as a measure of the general uncertainty of the
data and perhaps also of secondary reactions. It is obvious from
Table 2 that, within this uncertainty, the isotope distributions in
the pentanes are almost perfectly binomial, and the distribution
is also close to binomial in propane. Analysis of the isotope distri-
butions in the side products demonstrates that intramolecular
scrambling in C8 intermediates is fast, and a non-binomial isotope
distribution in isobutane cannot be the result of a bimolecular
pathway with limited isotope redistribution in C8 intermediates.
It follows that a small portion of the isobutane was produced via
a monomolecular pathway on H-mordenite when an unpurified
feed was used, and significant portions of isobutane were produced
by a monomolecular pathway in experiments with the three other
feed–catalyst combinations.
With the evidence from isotope labeling experiments that a
substantial portion of isobutane is produced via a monomolecular
pathway when the concentration of butene is sufficiently low, one
can evaluate the kinetics data collected from conversion of unla-
beled n-butane on Pt/H-mordenite. It should first be noted that
mass transport was eliminated as being the source of rate control
using calculations described in Sections S4 and S5 of the support-
ing information. In the kinetics experiments on Pt/H-mordenite,
equilibration between n-butane, n-butene, and H2 was rapid, and
the gas phase concentration of butene was thus known along the
length of the catalyst bed. The reaction order for isobutane with
respect to n-butane or n-butene was 1.0–1.2 over the range of con-
ditions, as illustrated in Fig. 5 and reported in detail in Table S7 of
the supporting information. The reaction order with respect to H2
was ꢀ0.8 to ꢀ1.1. Both reaction orders are those expected in the
case of a predominately monomolecular mechanism, whereas the
slight increase in reaction order with respect to n-butane with
decreasing partial pressure indicates increasing participation of
the bimolecular pathway. One must keep the butene partial pres-
sure in mind when reading Fig. 5; because of the facile equilibra-
tion of alkane, alkene, and H2, the butene partial pressure
increases as the H2 partial pressure is lowered, and one would thus
expect increasing contributions from bimolecular pathways that
are second order with respect to butene.
To compare these results with published reaction orders with
respect to n-butane, one must make assumptions regarding butene
concentrations in the reported experiments because they are gen-
erally not reported. In the absence of platinum (or any other suit-
able metal), equilibration according to Eq. (3) relies on the intrinsic
activity of the zeolite and is not likely. Measured activation ener-
gies for dehydrogenation on an H-form zeolite range from 189 to
204 kJ molꢀ1 [34]. If platinum is present, equilibration is attained
and Eq. (3) is valid. If H2 is co-fed in ratios as those used here, then
the formed H2 will change the overall H2 concentration only mar-
ginally and the butenes equilibrium concentration will be propor-
tional to the n-butane concentration. If n-butane is fed without H2,
then the equilibrium concentration of n-butene is proportional to
the square root of the n-butane concentration (and an order of 1
with respect to n-butane can be consistent with a bimolecular
mechanism).
8000
100
isobutane
80
60
6000
40
20
0
0.0
0.5
1.0
W/F (h)
1.5
4000
2000
0
propane
Table 4
pentanes
Percent of 1,4-13C2-n-butane with internally scrambled carbon-13 atoms after
linear butenes
conversion on H-mordenite or Pt/H-mordenite.
0.0
0.5
1.0
1.5
Catalyst
Trap
Internal scrambling (%)
W/F (h)
H-mordenite
H-mordenite
Pt/H-mordenite
Pt/H-mordenite
No
Yes
No
11.0
10.1
9.7
Fig. 4. Concentration of the main gas phase products produced from conversion of
n-butane on Pt/H-mordenite. Conditions: reaction temperature 573 K, 50 ml minꢀ1
total flow, 50 kPa H2, 40 kPa helium, W/F = 0.13–1.29 h.
Yes
10.9
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
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10
M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
The following literature data are for platinum-free mordenite,
enthalpies of adsorption from the literature with the experimen-
tally determined apparent activation energy of 20–24 kJ molꢀ1
making it difficult to judge what the butene concentration in the
reactor might have been. Apparent orders for isobutane formation
with respect to n-butane of 1 (with H2 added) [6] and 0–1.25, 1.17,
or 2 (no H2 added) [13,6,20] have been reported. Apparent orders
for pentanes formation of 2 with respect to isobutane (with H2
added) [8] or 1.9 with respect to n-butane (no H2 added) [20] have
been observed. This report clarifies that the reaction order of 1 for
isobutane is in fact linked to a monomolecular pathway and is not
the result of a pseudo-first-order bimolecular pathway. It also clar-
ifies that isobutane can be produced through both monomolecular
(first order with respect to n-butene) and bimolecular (second
order with respect to n-butene) pathways, and it is thus conceiv-
able that the choice of reaction parameters determines the appar-
ent order that is measured.
With the knowledge that isomerization rates are first order
with respect to n-butene, an apparent first-order rate constant
can be calculated if the change in butene concentration with tem-
perature (as the equilibrium shifts with temperature) is consid-
ered. The calculated concentrations of n-butene were used to
avoid the uncertainty inherent to the measured values. Arrhenius
plots using the apparent first-order rate constants (the product of
kisom and Kads) were constructed for a number of conditions,
whereby the R2 values were 0.993 or greater and the uncertainty
was 8 kJ molꢀ1. An example Arrhenius plot is shown in Fig. S4
in the supporting information. The resulting apparent activation
energy for isobutane formation was 20–24 kJ molꢀ1. The apparent
activation energy is related to the true activation energy by the
enthalpy of adsorption according to Eq. (10):
yields a true activation energy of 120–134 kJ molꢀ1
.
True activation energies have been obtained by DFT, which can
deliver energies for elementary steps such as the rearrangement
of the adsorbed butyl species. Boronat et al. [48] found the activa-
tion energy of the skeletal rearrangement step of n-butene on a
Theta-1 cluster to be 110–145 kJ molꢀ1 depending on the basis
set that was used. More recently, Wattanakit et al. [49] found
the activation energy of the same step to be 87 kJ molꢀ1 on H-fer-
rierite. Gleeson [50] predicted the activation energy of carbenium
ion and alkoxide-based mechanisms to be 103 and 143 kJ molꢀ1
,
respectively, on H-ferrierite. The framework type generally affects
the enthalpy of alkane adsorption and the apparent activation
energy of isomerization, but not the true activation energy [51].
Hence, the reported computational results and the experimen-
tally determined value for the true activation energy for butyl
rearrangement agree within their respective error margins. Skel-
etal rearrangement of the adsorbed C4 species must be the rate-
determining step, followed by facile hydride transfer from an
incoming reactant molecule to liberate the product and close
the catalytic cycle.
Previously reported apparent activation energies for isobu-
tane formation on Pt/H-mordenite were between 44 and
71 kJ molꢀ1 and were found to depend on the hydrogen to
hydrocarbon ratio [21]. Because fits of the deactivation profile
and extrapolation to zero time on stream were used to generate
the values, the authors stated that they were ‘‘to be taken with
care in mechanistic considerations.’’ To date, these values are
the only activation energies reported for butane isomerization
on a zeolite. Domokos et al. [52] measured the apparent activa-
tion energy of isobutene and isobutane formation from n-butene
on H-ferrierite and found them to be 59 and 28 kJ molꢀ1, respec-
tively. The authors hypothesized that desorption is the rate-lim-
iting step for isobutene formation, but did not comment on the
rate-limiting step for isobutane. The reported apparent activa-
tion energy for isobutane is close to the value of 20–24 kJ molꢀ1
found in this work.
The combined evidence from kinetics and isotope experiments
suggests that a monomolecular isomerization pathway is the dom-
inant isomerization pathway under conditions at which the partial
pressure of alkene is sufficiently low. But, if the claim is correct,
then questions remain about the alleged formation of a primary
carbenium ion along the reaction coordinate. If one accepts that
isomerization is preceded by dehydrogenation, then the DFT calcu-
lations performed by Boronat et al. [53] concerning the skeletal
isomerization of linear butenes on a zeolite are pertinent. (In prin-
ciple, the two reaction sequences should differ only in the initia-
tion and termination steps; the rearrangement steps should be
identical.) The authors determined that the transition state for
monomolecular skeletal isomerization of butene comprises a
cyclopropane ring that maintains elongated C–O bonds with the
zeolite lattice and thus does not resemble a primary carbenium
ion. Upon opening of the cyclopropane ring, a primary alkoxide is
formed, which is lower in enthalpy than the cyclopropane ring,
and is a minimum on the potential energy surface. The transition
state in the skeletal isomerization pathway catalyzed by liquid
superacids is distinctly different; in this case, the transition state
is the species formed upon opening of the cyclopropane ring,
which results in formation of a primary carbenium ion [16]. This
difference between solid and liquid acids can be understood when
comparing the properties of the respective anions: the nucleophi-
licity of oxygen anions in the zeolite framework is suited for the
formation of an alkoxide, whereas the anions in superacids lack
any nucleophilicity, thus making possible the existence of free
carbenium ions.
Ea ¼ Eapp
ꢀ
D
Hads
ð10Þ
where Ea is the true activation energy, Eapp is the apparent activa-
tion energy, and Hads is the enthalpy of adsorption. Thus, if the
D
enthalpy of adsorption can be determined, then the true activation
energy can be calculated.
Fogash et al. [23] estimated the enthalpy of adsorption for
butene on H-mordenite to be ꢀ100 kJ molꢀ1 by using a correlation
between the enthalpy of adsorption and the gas phase proton
affinity. Lechert and Schweitzer [46] reported an experimentally
determined enthalpy of adsorption of ꢀ110 kJ molꢀ1 for 1-butene
on H-MFI at a temperature of 523 K. Gomes et al. [47] recently
corroborated the value with DFT calculations. Combining these
2.5
Pentanes
2.0
1.5
1.0
Isobutane
0.5
0.0
20
40
60
80
100
P (H2) / kPa
Fig. 5. Reaction orders with respect to n-butene for isobutane and pentanes on Pt/
H-mordenite at a temperature of 573 K, n-butane partial pressure of 10 kPa, and H2
partial pressures between 30 and 90 kPa.
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
(2015), http://dx.doi.org/10.1016/j.jcat.2014.12.035

M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
11
4.5. Evidence that bimolecular pathways dominate at ‘‘high’’ alkene
partial pressure
two isobutane molecules [57]. In light of the findings of de Lucas
et al [55] and an equilibrium content of 2,2,4-trimethylpentane
at 600 K of only 1.5 mol% [54], this preferential pathway is not a
likely scenario.
Although there is clear evidence that isobutane formation is in
large part monomolecular under conditions of low alkene partial
pressure, the significant amount of intermolecular isotope scram-
bling when butene was co-fed suggests that there are also bimolec-
ular isomerization pathways that are favored by high alkene partial
pressure. Thus, a discussion of the bimolecular pathway and for-
mation of disproportionation products is appropriate.
The data for propane and pentanes (Table 5) are largely consis-
tent with what one expects for a bimolecular dimerization–crack-
ing pathway. The isotope distributions for both products, when
they were formed in concentrations high enough to measure com-
plete mass spectra, were binomial within error. The result is in
good agreement with the observations by Guisnet et al. [10],
who converted 1-13C-n-butane on H-mordenite and found a bino-
mial distribution of carbon-13 in pentanes over a wide range of
conversions.
The pentane reaction order with respect to n-butene was 1.4–
2.2, with deviations from 2 at high temperatures and low H2 partial
pressures, that is, at higher butene concentrations. The obtained
fractional order of less than 2 could be an indication of increasing
surface coverage, which would invalidate the omission of the
denominator in a rate law derived using a Langmuir adsorption
model. If that is the case, then the reaction order for isobutane for-
mation with respect to butenes would also be affected by coverage,
and the participation of a bimolecular mechanism could be some-
what higher than suggested. Overall, the higher-than-1 order with
respect to n-butene for pentanes formation is consistent with a
bimolecular pathway, but the changing orders between 1.4 and
2.2 are not fully understood.
If beta-scission of the isomerized C8 surface species (Step 5 of
Scheme 2) is assumed to be the rate-limiting step, the apparent
activation energy can be extracted using the same method as for
isobutane. The energies for pentane covered a wider range than
those for isobutane, but were generally around ꢀ20 kJ molꢀ1. Using
an enthalpy of adsorption of a C8 alkene of ꢀ160 kJ molꢀ1, calcu-
lated using the method of Fogash et al. [23], a true activation
energy around 140 kJ molꢀ1 is calculated. This value is in reason-
able agreement with a reported value of 153 kJ molꢀ1 obtained
computationally by Mazar et al. [54] for beta-scission of 3,3-dim-
ethylhexoxide on H-ZSM-5.
The extent of branching in the surface C8 species greatly
impacts the cracking products, as shown in the inclusive list of
pathways in Scheme S1 in the supporting information. It is true
that 2,2,4-trimethylpentane produces isobutane with high selec-
tivity; Lugstein et al. [58] hydrocracked the species on Ni/H-mord-
enite and found an isobutane to (propane + pentanes) ratio of 21. A
ratio of only 1.4 was found when 2,5-dimethylhexane was hydro-
cracked [57]. n-Octane has been hydrocracked on various morde-
nites, and ratios of 1.1 [55,57] and 0.5 [59] have been found.
Simonetti et al. [60] converted triptene in the presence of dimethyl
ether on H-beta and found a high selectivity to isobutyl product,
presumably through methylation and subsequent cracking. Thus,
when the zeolite is presented with a triply-methylated C8 species,
the species does crack to selectively product isobutane. However,
given that triply-methylated C8 species are not the primary prod-
ucts of n-butene dimerization, and are not thermodynamically
favored (i.e., there is no driving force for the subsequent isomeriza-
tion reactions to form them), a dimerization–cracking pathway
starting from n-butane that produces isobutane with high selectiv-
ity seems unlikely.
With the understanding that dimerization–cracking reactions
are not favoring isobutane formation, and with the additional evi-
dence that a monomolecular isomerization pathway contributes
significantly to isobutane formation, we are in a position to explain
variations in selectivity with varying reaction conditions. Such vari-
ations were previously noted in the literature [6,13,21] but lacked a
consistent interpretation. Asuquo et al. [13] observed that the selec-
tivity to isobutane was higher at low temperatures, but claimed
that the activation energy of a monomolecular pathway was too
high for it to contribute significantly to the product distribution.
Tran et al. [6] diluted n-butane in either H2 or N2 and associated
variations in selectivity with the concentration of sec-butyl carbe-
nium ions. Specifically, they proposed that the bimolecular path-
way requires two carbenium ions and consequently should be
more sensitive to a decrease in the concentration of carbenium ions
caused by the presence of hydrogen.
The data shown in Fig. 3 clarify that butene concentration is the
variable that controls selectivity to isobutane or disproportionation
products. The data were collected on Pt/H-mordenite, so under all
conditions the butene concentration was controlled by chemical
equilibrium with butane and H2. Thus, while reading Fig. 3, one
must consider how the butene concentration changes with reac-
tion conditions. Any change in a reaction condition that caused
the butene concentration to increase (i.e., raising the temperature,
raising the n-butane partial pressure, or lowering the H2 partial
pressure) caused a dramatic drop in selectivity to isobutane and
a dramatic increase in selectivity to pentanes. The only reaction
parameter that caused an increase in conversion but did not affect
the butene concentration is the space time; when the space time
was increased, the drop in selectivity to isobutane was only 3%.
The changes in selectivity with changing butene concentration
are explained by their different reaction orders with respect to n-
butene.
Since the skeletal rearrangement of C8 species is facile, as indi-
cated by the binomial distribution of isotopes in disproportion-
ation products, a distribution of skeletal isomers that closely
resembles the distribution expected at chemical equilibrium can
be assumed. In the gas phase at a temperature of 600 K, the equi-
librium composition of C8 isomers with 0, 1, 2, and 3 methyl
branches is 10, 43, 37, and 3.4 mol%, respectively [55]. de Lucas
et al. [56] converted n-octane on Pt/H-mordenite at 603 K and
already at 50% conversion found the isomer distribution to consist
entirely of species with only one or two methyl branches, which is
largely consistent with the gas phase equilibrium composition. The
absence of species with three methyl branches suggests that there
is not a transition state selectivity favoring formation of these spe-
cies. An argument had been put forth that 2,2,4-trimethylpentane
is selectively formed from n-butane and cracks to eventually make
4.6. Reconciliation of observations with previous results from
conversion of carbon-13 labeled butane
Table 5
Apparent kinetics parameters on Pt/H-mordenite.
The data presented in this contribution implicate the butene
concentration as the variable that most strongly impacts the skele-
tal isomerization pathways of n-butane (either mono- or bimolecu-
lar). Consequently, the butene concentration also affects the extent
of isotope scrambling in isobutane. Isotope scrambling in isobutane
Parameter
Isobutane
Pentanes
Reaction order with respect to n-butene
0.9–1.2
1.4–2.2
Reaction order with respect to H2
ꢀ0.8 to ꢀ1.1
ꢀ2.1 to ꢀ2.7
Apparent activation energy (kJ molꢀ1
)
20–24 kJ molꢀ1
ꢂꢀ20 kJ molꢀ1
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
(2015), http://dx.doi.org/10.1016/j.jcat.2014.12.035

12
M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
Table 6
Summary of reaction conditions employed in previous reports in which isotopically labeled butane was used to elucidate isomerization reaction mechanisms.
First author, year
Catalyst
Conversion
Temp. (K)
Reactant
Balance gas
14 torr Ar
Reactor type
Adeeva, 1994
Adeeva, 1995
Adeeva, 1997
Adeeva, 1997
Adeeva, 1997
Adeeva, 1997
Bearez, 1985
Garin, 1995
Garin, 1995
Liu, 1995
Fe, MnSZ
SZ
A
B
C
D
HMOR
SZ
Pt/SZ
Pt/SZ
-Estimate 60%
ꢂ40%, 1 h
35%, 10 min
25%, 15 min
32%, 10 min
10%, 60 min
With time
8–17%
9%
30 min
Range
Range
353
403
498
348
498
538
Not given
523
523
Commercial, 7 torr
Commercial, 30 torr
Commercial, 9 torr
Commercial, 9 torr
Commercial, 9 torr
Commercial, 9 torr
Homemade, 0.01 bar
Homemade, 5
Homemade, 5
Commercial, 1 torr
Commercial, 40 torr
Commercial, 40 torr
Recirculation
Recirculation
Recirculation
Recirculation
Recirculation
Recirculation
Recirculation
Flow
40 torr H2
40 torr H2
40 torr H2
40 torr H2
P = atm, unspecified bal gas
H2
l
l
l pulse
l pulse
H2
Flow
523
423–523
393–623
50 torr H2
200 torr H2
200 torr H2
Recirculation
Recirculation
Recirculation
Suzuki, 2001
Suzuki, 2004
Heteropoly, SZ, with and without Pt
Heteropoly, with and without Pt
increased with increasing butene concentration, whereas the iso-
tope distribution in propane and pentane was independent of the
butene concentration. With this knowledge, contradictory reports
in the literature can be reconciled and a unified model can be for-
mulated. A summary of previous investigations in which labeled
butanes were used to elucidate their skeletal isomerization mecha-
nism on solid acids is given in Table 6.
binomial at about 40% conversion. Another possible way to inter-
pret Sachtler’s data is that alkene impurities, if present in the feed,
promoted isotope scrambling as seen in the data reported in this
paper. In contrast to the reports by Sachtler, Garin et al. [63]
found far less intermolecular isotope scrambling on sulfated zir-
conia and Pt/sulfated zirconia. In this case, conversions were only
8–18%, and a carefully purified homemade feed was used. The
carefully purified feed and perhaps also the added H2 excluded
significant alkene concentrations in the reactor, and the low con-
version limited secondary reactions, thus allowing for isotopic
evidence of a monomolecular pathway to be collected.
Finally, the skeletal isomerization mechanism on sulfated zirco-
nia, WO3/ZrO2, and the heteropoly compound Cs2.5H0.5PW12O40
was found to depend on the presence of platinum and H2 by Oku-
hara and co-workers [31,30]. Platinum and H2, for unknown rea-
sons, promoted the monomolecular pathway, as determined by
the isotope distributions in gas phase products. The 1,4-13C2-n-
butane in these experiments was also from Isotec, Inc. (with the
impurities not reported), conversions were around 10%, and H2
was used only in combination with catalysts containing platinum.
The results mirror those presented in this paper; extensive isotope
scrambling was seen on catalysts lacking platinum, and far less
scrambling was seen on platinum-containing catalysts with H2 in
the feed, thus under conditions implying low butene concentra-
tions. As demonstrated in this work, the operating mechanism is
not directly impacted by the platinum on the catalyst or H2 in the
feed, but instead is indirectly impacted by their ability to control
the gas phase concentration of alkenes.
The first investigation using carbon-13 labeled butane on H-
mordenite was performed by Bearez et al. [10], and to our knowl-
edge is the only paper reporting full isotope distributions in all
products. Another paper reports only selected results [9]. Using a
recirculation reactor with 1-13C-isobutane as the reactant, the iso-
tope distribution in propane, n-butane, and isopentane products
was measured as a function of conversion. Because isotope scram-
bling was detected in the three main products, a disproportion-
ation mechanism involving formation of a C8 carbenium ion as
the rate-limiting step was proposed. However, the data clearly
show that the isotope composition of propane and isopentane
did not change over the range of conversion investigated (ꢅ15–
50%), while the isotope composition of the skeletal isomerization
product, n-butane, changed significantly with conversion. Less iso-
tope scrambling was observed at 15% conversion compared to the
binomial distribution that was reached at 40% conversion. In fact,
the isotope distribution in n-butane mirrored that of the isobutane
reactant, not that of the propane and isopentane disproportion-
ation products. The non-changing binomial isotope distribution
in propane and isopentane demonstrates that isotopes in C8 inter-
mediates were statistically scrambled. Thus, if n-butane was
formed through similar bimolecular pathways as the dispropor-
tionation products, the isotope distribution should also be bino-
mial over the entire range of conversion. An alternative
explanation of that data is that the main pathway of n-butane for-
mation at low conversion was probably monomolecular. At higher
conversions, when secondary reactions are more likely, the isotope
distribution of both isobutane and n-butane was affected by inter-
molecular scrambling, and the isotopic evidence of a monomolec-
ular pathway was slowly erased.
5. Conclusion
Reaction pathways of n-butane on H-mordenite are shown to
depend on the concentration of n-butenes in the catalyst bed. In
the presence of platinum on the catalyst, n-butenes are found in
concentrations indicating equilibrium with n-butane and H2. Iso-
butane formation has a first-order dependence on the n-butene
concentration over a wide range of conditions, suggesting isomer-
ization by a monomolecular mechanism. The apparent activation
energy for isobutane formation, considering a first-order reaction
in butene, is 20ꢀ24 kJ molꢀ1. The calculated true activation energy
of 120ꢀ134 kJ molꢀ1 is in agreement with activation energies pre-
viously obtained by DFT for n-butene isomerization, supporting the
hypothesis of a monomolecular mechanism. With minimized
butene levels in the reactor, as much as 81% of the isobutane prod-
uct from 1,4-13C2-n-butane contained 2 carbon-13 isotopes, consis-
tent with predominantly monomolecular isomerization. In
contrast, with only 120 ppm of 1-butene in the feed and a catalyst
with poor hydrogenation capability (H-mordenite without plati-
num), the majority of the isobutane product molecules contained
a number of carbon-13 isotopes not equal to two.
Sachtler and co-workers used 1,4-13C2-n-butane, like the one
in this work from Isotec Inc. (impurities not reported), in a recir-
culation reactor on a variety of industrial chlorided alumina cata-
lysts [45] and platinum-free sulfated zirconia catalysts [25,61,62].
In all cases, a conclusion was reached that the operating mecha-
nism of isobutane formation was predominately bimolecular.
When any form of sulfated zirconia was used, the isotope distri-
bution was reported at 40–60% conversion. In these experiments,
the high conversion may have erased any evidence that existed at
lower conversions for a monomolecular pathway. In fact, the
authors stated that the isobutane product contained exclusively
two carbon-13 atoms at low conversion. In the previously dis-
cussed report of Bearez et al. [10], the distribution of carbon-13
isotopes in the isomerization product became almost completely
Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
(2015), http://dx.doi.org/10.1016/j.jcat.2014.12.035

M.J. Wulfers, F.C. Jentoft / Journal of Catalysis xxx (2015) xxx–xxx
13
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ation products and isomerization product. Disproportionation
products have a higher order dependence on butene, and reaction
conditions that result in ‘‘high’’ butene concentrations favor bimo-
lecular pathways that are non-selective to the desired product, iso-
butane. Contributions of a bimolecular mechanism to isobutane
formation at higher butene concentrations are evident through
an increase in reaction order.
The observation that product formation rates, selectivities, and
isotope distributions all depend on the partial pressure of butene
forms a convincing argument that a relevant monomolecular skel-
etal isomerization pathway exists for n-butane on H-mordenite and
Pt/H-mordenite. Previous discrepancies in the literature relating to
isotope distributions in products obtained from carbon-13 labeled
butane can be rationalized as the result of varying alkene concen-
trations (produced in the reactor or present as impurity in the feed).
Furthermore, it appears that one of the main roles of platinum and
H2, which are ingredients in commercial processes, is to control the
gas phase and thus the surface alkene concentration.
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This work was, in part, supported by the NSF EPSCoR award EPS
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Please cite this article in press as: M.J. Wulfers, F.C. Jentoft, Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite, J. Catal.
(2015), http://dx.doi.org/10.1016/j.jcat.2014.12.035