Autothermal partial oxidation of butanol isomers

Article abstract of DOI:10.1016/j.apcata.2011.10.023

The four isomers of butanol offer an interesting platform from which to study the reaction pathways of alcohols in an autothermal partial oxidation system, as they comprise one tertiary, one secondary, and two primary alcohols with the same number of carbon atoms. We demonstrate high yields of syngas or unsaturated molecules at contact times on the order of 10 ms, and investigate the reaction pathways of each isomer over Rh, RhCe, Pt, and PtCe catalysts for a range of carbon-to-oxygen (C/O) ratios. For each isomer, conversion to equilibrium syngas products is essentially complete at C/O = 0.8. As C/O ratio increases, the major product from the primary and secondary butanols switches to the corresponding carbonyl, producing butyraldehyde, isobutyraldehyde, and butanone from 1-butanol, isobutanol and 2-butanol, respectively. Selectivity to the carbonyls approached 30-50 as C/O approached 2.0. Dehydration to the corresponding butenes is relatively minor in comparison, representing less than 20 selectivity at C/O = 2.0. tert-butanol reacted differently, selecting mainly for the dehydration product isobutene. Acetone was the main carbonyl product from tert-Butanol, but selectivity to acetone was always 10. Global mechanisms in an autothermal reactor, based on pyrolysis, combustion and surface science literature, are proposed for each alcohol. Surface chemistry likely accounts for much of the syngas formation and heat generation, while the carbonyls and alkenes may be formed primarily through homogeneous routes.

Full text of DOI:10.1016/j.apcata.2011.10.023

Applied Catalysis A: General 411–412 (2012) 87–94
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Autothermal partial oxidation of butanol isomers
Jacob S. Kruger1, Reetam Chakrabarti1, Richard J. Hermann, Lanny D. Schmidt∗
University of Minnesota, 421 Washington Ave. SE, Minneapolis, MN 55455, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 August 2011
Received in revised form 12 October 2011
Accepted 16 October 2011
Available online 21 October 2011
The four isomers of butanol offer an interesting platform from which to study the reaction pathways of
alcohols in an autothermal partial oxidation system, as they comprise one tertiary, one secondary, and
two primary alcohols with the same number of carbon atoms. We demonstrate high yields of syngas or
unsaturated molecules at contact times on the order of 10 ms, and investigate the reaction pathways of
each isomer over Rh, RhCe, Pt, and PtCe catalysts for a range of carbon-to-oxygen (C/O) ratios.
For each isomer, conversion to equilibrium syngas products is essentially complete at C/O = 0.8. As C/O
ratio increases, the major product from the primary and secondary butanols switches to the correspond-
ing carbonyl, producing butyraldehyde, isobutyraldehyde, and butanone from 1-butanol, isobutanol and
2-butanol, respectively. Selectivity to the carbonyls approached 30–50% as C/O approached 2.0. Dehydra-
tion to the corresponding butenes is relatively minor in comparison, representing less than 20% selectivity
at C/O = 2.0. tert-butanol reacted differently, selecting mainly for the dehydration product isobutene. Ace-
tone was the main carbonyl product from tert-Butanol, but selectivity to acetone was always ≤10%. Global
mechanisms in an autothermal reactor, based on pyrolysis, combustion and surface science literature, are
proposed for each alcohol. Surface chemistry likely accounts for much of the syngas formation and heat
generation, while the carbonyls and alkenes may be formed primarily through homogeneous routes.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Butanol
Catalytic partial oxidation
Rhodium
Platinum
Cerium
1. Introduction
simple reactors that are appropriate for utilizing diffuse renewable
resources.
Significant research effort in recent years has focused on the
development of renewable liquid fuels in order to decrease the
economic and environmental impacts of fossil fuel consumption.
Four-carbon alcohols have received particular attention because
they can be produced in significant quantities from a variety of
renewable feedstocks [1–7] and have several advantages over
short-chain alcohols. In particular, the butanols have many prop-
erties similar to currently consumed fossil fuels, including high
energy density, low hygroscopicity, and low corrosivity [8,9].
Autothermal partial oxidation over noble metal catalysts has
shown promise as a technique to convert a variety of feedstocks
(both renewable and fossil fuel-based) into syngas (a mixture
of H2 and CO) and longer-chain unsaturated molecules, both of
which are important intermediates in the synthesis of a wide
range of molecules [10–16]. Additionally, these reactions are
carried out at high temperatures with heat generated in situ and
occur on millisecond time scales, allowing for the use of small and
Recently, St. Clair and Lee [17] studied the autothermal partial
oxidation of isobutanol over Rh and ꢀ-Al2O3 catalysts and found
that Rh gave high yields of syngas while ꢀ-Al2O3 selected primar-
ily for the dehydration product isobutene. We have recently shown
that the addition of Ce to Rh catalysts improves yields of hydro-
gen from several fuels [10,11,15,18], and that autothermal short
contact time (SCT) reactors can also give high yields to nonequi-
librium products from light oxygenates [16,19]. Additionally, we
proposed that the behavior of a model compound of different func-
tional group classes (e.g. C2 alcohols, aldehydes, and acids) in an
autothermal SCT reactor could be extrapolated to other molecules
in the same class [16].
In this work we apply autothermal partial oxidation of the four
butanol isomers over Rh, RhCe, Pt, and PtCe catalysts to show that,
depending upon operation parameters, high selectivity to syngas or
nonequilibrium products may be obtained. We note that although
the formation of the nonequilibrium products may be primarily
homogeneous, the exothermic reactions that occur mainly on the
catalyst surface are required to provide heat for gas-phase dehy-
drogenation and dehydration reactions. Additionally, the behavior
of the primary butanols during autothermal processing appears to
be similar to ethanol [16], supporting the hypothesis that mem-
bers of a functional group family behave similarly in autothermal
SCT reactors.
∗
Corresponding author. Tel: +1 612 625 9391; fax: +1 612 626 7246.
E-mail addresses: schmi001@umn.edu,
schmi001@maroon.tc.umn.edu (L.D. Schmidt).
1
Both authors contributed equally to this work..
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.10.023

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J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
2. Experimental
Experiments were carried out with a 19 mm ID quartz tube as
shown in Fig. 1. Liquid fuel was fed through a stainless steel neb-
ulizer at the top of the reactor at 1 mL min−1 and was atomized
with 0.7 SLPM N2. Remaining N2 and O2 were fed around the nebu-
lizer, and total N2 and O2 were maintained at air stoichiometry. Gas
flow rates were varied to obtain different carbon-to-oxygen (C/O)
ratios, defined as the molar ratio of carbon in the feed molecule
to atomic oxygen in the O2 feed. The catalyst bed was positioned
30 cm below the nebulizer and reactor walls between the nebu-
lizer and catalyst were maintained at 175 ◦C to vaporize the fuel.
An 80 pores-per-linear-inch (ppi) ˛-Al2O3 foam monolith (Süd-
chemie) was positioned 3.5 cm upstream of the catalyst bed to mix
reactants. The catalyst bed was comprised of 3.25 g of 1.3 mm cat-
alytic spheres. The spheres (St. Gobain-Norpro) consisted of 1 wt%
M or 1 wt% M-1 wt% Ce supported on ˛-Al2O3, where M = Rh or Pt.
A 65 ppi ˛-Al2O3 foam monolith served as a back heat shield and
to hold thermocouples in place. Type K thermocouples (Omega)
were located at the upstream and downstream edges of the sphere
bed, and the entire bed was wrapped with aluminosilicate cloth
insulation to prevent heat loss and gas bypass during the experi-
ment. Spheres were prepared by incipient wetness impregnation of
aqueous catalyst precursor salts (Rh(NO3)3, H2PtCl6, and Ce(NO3)3)
followed by drying under vacuum. Rh and RhCe catalysts were pro-
duced in multiple cycles of deposition-dry-calcination at 600 ◦C,
where the calcination duration was ten minutes (to decompose the
nitrate salt), followed by a final calcination at 600 ◦C for 6 h. Pt and
Fig. 1. Reactor configuration for the autothermal partial oxidation of the butanol
isomers.
100%
80%
60%
40%
20%
0%
1200
1100
1000
900
100%
80%
60%
40%
20%
0%
1200
1100
1000
900
1-Butanol
i-Butanol
Rh
RhCe
Pt
Rh
RhCe
Pt
PtCe
PtCe
800
800
700
700
600
600
500
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(a) 1-butanol
(b) isobutanol
100%
80%
60%
40%
20%
0%
1200
1100
1000
900
100%
80%
60%
40%
20%
0%
1200
1100
1000
900
2-Butanol
Rh
Rh
RhCe
Pt
RhCe
Pt
PtCe
PtCe
t-Butanol
800
800
700
700
600
600
500
500
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(c) 2-butanol
(d) t -butanol
Fig. 2. Conversion and catalyst backface temperature of the four butanol isomers for each catalyst as a function of C/O ratio. (a) 1-Butanol, (b) isobutanol, (c) 2-butanol and
(d) t-butanol.

J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
89
100%
80%
60%
40%
20%
0%
100%
1-Butanol
i-Butanol
Rh
80%
RhCe
Rh
RhCe
Pt
Pt
PtCe
PtCe
60%
40%
20%
0%
H2
H2
H2O
H2O
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(a) 1-butanol
(b) isobutanol
100%
80%
60%
40%
20%
0%
100%
80%
60%
40%
20%
0%
2-Butanol
t-Butanol
Rh
Rh
RhCe
Pt
RhCe
Pt
PtCe
PtCe
H2
H2
H2O
H2O
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(c) 2-butanol
(d) t -butanol
Fig. 3. Selectivities to H2 and H2O from each isomer as a function of C/O ratio over all four catalysts. (a) 1-Butanol, (b) isobutanol, (c) 2-butanol and (d) t-butanol.
PtCe catalysts were prepared without the brief calcination steps
and were reduced under flowing H2 and N2 at 600 ◦C for 6 h. The
multistep calcination procedure was adopted for the Rh-based cat-
alysts to avoid loss of catalyst metal through delaminating bubbles
that formed when a concentrated precursor solution was used. For
the Pt-based catalysts, the reduction procedure was employed to
mitigate metal loss through volatile intermediates in the decom-
position of the precursor salt.
Analysis of products was performed on an HP6890 gas chro-
matograph equipped with thermal conductivity (TCD) and flame
ionization detectors (FID). The column was 30 m with a PLOT-Q sta-
tionary phase. N2 from the feed was used as an internal standard.
H2, O2, CO and CO2 were quantified with the TCD, while all others
were quantified with the FID, except H2O, which was calculated
by difference from an oxygen atom balance. Carbon and hydrogen
atom balances typically closed within 10%. Each data point shown
represents the average of three runs, and 95% confidence intervals
were generally 5–10% absolute. Products up to C6 were analyzed,
but no products > C4 were detected in selectivity >1%. Each catalyst
was run for at least 12 h without any observable deactivation.
Conversion and back face temperature data are shown in Fig. 2. At
the temperatures observed here, residence times ranged from 6 to
20 ms, calculated at the temperatures reported in Fig. 2.
Conversion was similar for 1-butanol across all four catalysts
and was essentially complete at C/O ≤ 1.0, as shown in Fig. 2. The Pt
catalysts yielded a higher temperature at the downstream edge of
the catalyst bed than their Rh counterparts by 50–100 ◦C. 2-Butanol
was similar to 1-butanol in that conversion was generally similar
across catalysts, and temperatures were higher over Pt-based than
Rh-based catalysts. Conversion of tert-butanol was generally higher
than the other butanols (Fig. 2), possibly due to its greater tendency
toward homogeneous pyrolysis. Like 1-butanol and 2-butanol, the
Pt catalysts gave a higher back face temperature that corresponded
to a lower selectivity to CO and H2 (Fig. 3). Conversion of isobu-
tanol fell off faster than the other isomers as C/O ratio increased
(Fig. 2). For isobutanol, the Pt catalyst operated roughly 100 ◦C
higher than PtCe over the entire C/O range; for the other three iso-
mers, PtCe gave temperatures comparable to or higher than the
Pt catalyst.
3.2. Syngas and combustion products
3. Results
At C/O ≤ 1.2, CO and H2 were favored from all four isomers, as
shown in Figs. 3 and 4. Rh and RhCe generally gave higher con-
version to syngas than the Pt catalysts, and PtCe gave significantly
lower syngas yields than the other three catalysts for C/O ≥ 1.4. The
lower selectivity to syngas from the Pt-based catalysts is consistent
with the lower steam reforming activity of Pt [20,21], although the
selectivity to H2O was not significantly higher.
3.1. Conversion and temperature
Each of the butanols sustained autothermal operation for 0.8 ≤
C/O ≤ 2.0, and higher C/O ratios were not attempted. At C/O = 0.8,
conversion was ≥99% and selectivities closely reflect those pre-
dicted by thermodynamic equilibrium, which is primarily syngas.

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J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
100%
80%
60%
40%
20%
0%
100%
i-Butanol
PtCe
Rh
1-Butanol
Rh
RhCe
Pt
RhCe
Pt
80%
60%
40%
20%
0%
PtCe
PtCe
PtCe
CO
Other
Other
CO
Pt
Rh
CO2
CO2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(a) 1-butanol
(b) isobutanol
100%
80%
60%
40%
20%
0%
100%
80%
60%
40%
20%
0%
Rh
Rh
t-Butanol
PtCe
2-Butanol
PtCe
RhCe
Pt
RhCe
Pt
PtCe
PtCe
Other
CO
CO
Other
Rh
RhCe
CO2
CO2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(c) 2-butanol
(d) t -butanol
Fig. 4. Selectivities to CO and CO2 from each isomer as a function of C/O ratio over all four catalysts. ‘Other’ represents the sum of all carbonaceous products other than CO
and CO2. For clarity, only the minimum and maximum lines are shown. (a) 1-Butanol, (b) isobutanol, (c) 2-butanol and (d) t-butanol.
H2O and CO2 were present at 20–30% selectivity regardless of
catalyst or isomer; H2O was present in slightly higher selectivity
over the PtCe catalyst, although the magnitude of difference is near
the limit of experimental uncertainty.
product formed from tert-butanol, but represented ≤10% selectivity
at all C/O ratios investigated.
3.4. Other intermediates
3.3. C4 intermediates
The other species observed in significant amounts were gen-
erally only ethylene and propylene. Selectivities to these two
olefins were similar over 1-butanol at 5–15% for C/O ≥ 1.4, regard-
less of catalyst. Isobutanol, however, showed higher selectivity to
propylene at 10–20% for C/O ≥ 1.4 and only minor selectivity to
ethylene, at ≤5%. The difference in selectivity to propylene from
isobutanol across catalysts cannot be explained by temperature
alone. Pt and Rh, which displayed the highest and lowest back-
face temperatures, respectively (Pt was over 100 ◦C higher than
Rh at all C/O ratios), gave comparable selectivities to propylene,
while RhCe and PtCe give noticeably higher selectivity to propylene.
This observation suggests that either production or consumption
of propylene (or both) occurs to some extent on the catalyst sur-
face. 2-butanol and tert-butanol showed lower selectivities to these
two olefins, although 2-butanol produced significant selectivity to
each over PtCe. It is difficult to discern the role of the PtCe cat-
alyst in the formation of ethylene and propylene from 2-butanol
because the range over which their selectivities are significant cor-
responds to the range where PtCe temperature was nearly 100 ◦C
higher than the other catalysts. Their formation may therefore be
homogeneous, possibly from the thermal decomposition of 1- and
2-butenes, which is discussed below (Fig. 6).
1-Butanol, 2-butanol and isobutanol gave high selectivity to
carbonyl products as C/O ratio increased, with relatively minor
selectivity to C4 olefins as shown in Fig. 5. The trend for tert-butanol
was reversed, with isobutene as the major product.
From the primary butanols, selectivity to the aldehyde reached
20–30% as C/O approached 2.0, though the Rh catalyst showed
lower selectivity to isobutyraldehyde. Selectivity to butanone from
2-butanol reached 35–45% for the same C/O range. For 1-butanol
over all catalysts and for 2-butanol over the Pt-based catalysts,
selectivity to the carbonyl at C/O ≤ 1.4 was comparable to butene
selectivity (shown as a sum of 1-butene and both 2-butene iso-
mers), but at C/O = 2.0, the carbonyl was favored by a factor of 2–3.
The ratio of carbonyl to butene was even higher over the Rh cata-
lysts for 2-butanol, reaching a factor of 5–10 in preference of the
carbonyl. For isobutanol, selectivity at C/O = 2.0 favored the car-
bonyl over isobutene by a factor of 3–5.
Because tert-butanol lacks an ˛-H atom, formation of a carbonyl
cannot occur through simple hydrogen abstraction reactions. A
dehydration route to isobutene is thus predominant, with selec-
tivities to isobutene reaching 60–70% over the Pt catalysts and
30–50% over the Rh catalysts. Acetone was the main carbonyl

J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
91
70%
60%
50%
40%
30%
20%
10%
0%
70%
1-Butanol
i-Butanol
60%
50%
40%
30%
20%
10%
0%
Rh
Rh
RhCe
Pt
RhCe
Pt
PtCe
PtCe
i-C4H8O
i-C4H8O
Butenes
Butenes
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(a) 1-butanol
(b) isobutanol
70%
60%
50%
40%
30%
20%
10%
0%
2-Butanol
t-Butanol
70%
60%
50%
40%
30%
20%
10%
0%
Rh
Rh
RhCe
Pt
RhCe
Pt
PtCe
PtCe
Butanone
Butenes
Butenes
Acetone
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(c) 2-butanol
(d) t -butanol
Fig. 5. Selectivities to major carbonyl and C4 olefins from each isomer as a function of C/O ratio over all four catalysts. Butenes represents the sum of butene isomers. (a)
1-Butanol, (b) isobutanol, (c) 2-butanol and (d) t-butanol.
4. Discussion
of several intermediate species via demethylation and dehydration
reactions [24,26].
4.1. Chemistry of the isomers
Other than the lower selectivity to syngas species for PtCe, there
are few features in the selectivity data to aid in distinguishing
between the different catalyst surfaces for 1-butanol. Further dis-
cussion of potential surface reactions of 1-butanol will therefore
not be attempted here; potential surface mechanisms of alcohols
in general are discussed below.
Reaction schemes within a CPO reactor are complex networks
of homogeneous and heterogeneous reactions that are difficult
to untangle from analysis of integral data. However, the reaction
schemes shown below are not unprecedented in the literature, and
by observation of reaction intermediates and comparison between
catalysts, a qualitative formulation of the dominant reaction path-
ways is possible.
4.1.2. 2-Butanol
The major non-equilibrium product from 2-butanol for C/O ≥ 1.2
was butanone, which reached 35–45% selectivity as C/O increased
to 2.0. At C/O ≤ 1.4 over the Pt catalysts, dehydration reactions
appeared to be competitive with dehydrogenation, as selectivities
to the butenes and butanone were comparable. When the butenes
were produced in ≥ 2% selectivity, 1-butene was favored over either
isomer of 2-butene, with selectivities to 1-butene, cis-2-butene, and
trans-2-butene present generally in the ratio 2:1:1, respectively.
Negligible selectivity to acetaldehyde (<1%) and only minor selec-
tivity to propanal were observed in these experiments over any of
the catalysts, despite their prominence in the 2-butanol pyrolysis
literature [22,27]. We note here that O2 conversion was less than
100% under some reaction conditions (PtCe for C/O ≥ 1.2); in other
similar experiments, the presence of O2 significantly decreased
selectivity to acetaldehyde [16,28], and in 2-butanol combustion,
acetaldehyde and propanal were much less prominent [29]. These
observations likely mean that oxygen persists until near the down-
stream edge of the catalyst bed, at least for the PtCe catalyst. This
4.1.1. 1-Butanol
Alcohols generally decompose by either dehydrogenation to
produce a carbonyl or dehydration to produce an alkene [22,23].
For 1-butanol, dehydration and dehydrogenation routes are com-
petitive in the gas phase in the absence of O2 up to 500 ◦C, while
dehydrogenation predominates at higher temperatures [22,24].
This scheme is consistent with the product spectrum observed
here, as temperatures were well above 500 ◦C and selectivity to the
primary products at high C/O ratios (where secondary reactions
are less favored) demonstrates 2–3-fold difference in selectivity
in favor of the aldehyde over the alkenes. As C/O decreases (and
temperature increases), these primary products can further decom-
pose. The butyraldehyde intermediate can react by decarbonylation
to yield CO, H2 and C3H6, or by demethylation to yield propanal and
a CHx radical; the butenes may decompose to produce propylene
and a CHx radical [22,24,25]. Ethylene may be formed by reactions

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J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
50%
40%
30%
20%
10%
0%
50%
1-Butanol
i-Butanol
Rh
40%
Rh
RhCe
Pt
RhCe
Pt
PtCe
PtCe
30%
C3H6
C3H6
20%
10%
0%
C2H4
C2H4
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(a) 1-butanol
(b) isobutanol
50%
40%
30%
20%
10%
0%
50%
40%
30%
20%
10%
0%
2-Butanol
t-Butanol
Rh
Rh
RhCe
Pt
RhCe
Pt
PtCe
PtCe
C3H6
C2H4
C3H6
C2H4
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
C/O Ratio
C/O Ratio
(c) 2-butanol
(d) t -butanol
Fig. 6. Selectivities to ethylene and propylene as a function of C/O ratio over all four catalysts. (a) 1-Butanol, (b) isobutanol, (c) 2-butanol and (d) t-butanol.
hypothesis is also consistent with the very small amounts of hydro-
carbon products observed both here and in 2-butanol combustion
[29].
surface reactions as well. We note that selectivity to isobutene from
tert-butanol across catalysts (lowest on RhCe, highest on Pt and
PtCe) more closely matches the conversion than the temperature
trends. Thus, at least some of the isobutene may come from surface
reactions; although selectivity to isobutene may also be related to
the ability of each catalyst to reform this intermediate.
It is also of interest that the activity of the catalysts toward C–O
bond scission may be a factor in determining the conversion and
selectivity trends of each catalyst. The Pt-containing catalysts may
be more active for C–O bond scission [32], resulting in a higher con-
version of tert-butanol. However, some of the initial dehydration
of tert-butanol may also be homogeneous; the lower temperatures
observed over the Rh-based catalysts (likely due to greater activity
for endothermic reforming) may also result in lower initial conver-
sion of the incoming fuel.
PtCe showed a higher selectivity to the ethylene and propylene
than the other three catalysts, despite the observed O2 break-
through. As discussed above, the range of C/O ratios over which
these two alkenes show significant selectivity corresponds to sig-
nificantly higher temperatures over PtCe; we thus surmise that
they may be produced by homogeneous decomposition of either
butanone (via unstable ketene intermediates [30]) or the butenes
[25,31]. Indeed, CH4 (not shown) was produced in nearly equal
quantities as C3H6, which is expected from the pyrolysis of 1- and
2-butene; the additional C2H4 may have been produced by oxida-
tive [23] or nonoxidative [30] decomposition of butanone. CH4 can
also be produced by pyrolysis of butanone by reactions that also
produce acetone and propanal [22], but the negligible selectivity to
acetone and propanal suggests that these pathways are minor.
The other nonequilibrium product was acetone, which may have
been produced in the gas phase by subsequent H and CH3 abstrac-
tions [26]. We also note that the negligible selectivity to CH4, C2H4
and C3H6 suggests that homogeneous decomposition of isobutene
is not significant [33,25,34].
4.2. tert-Butanol
Other than the four equilibrium species, tert-butanol yielded
only one main product, isobutene, which approached 70% selec-
tivity over PtCe at C/O = 2.0. Other light olefins were never present
in ≥1% selectivity, and acetone, the primary carbonyl-containing
product, was always ≤ 10% selectivity. Over PtCe, selectivity to
propanal (not shown) was comparable to acetone.
Isobutene may result mainly from homogenous dehydration,
either molecularly or through sequential H and OH radical abstrac-
tions [26]. It is also possible that isobutene could be formed from
4.3. Isobutanol
The main nonequilibrium products for isobutanol were isobu-
tyraldehyde, propylene, and isobutene. Gas phase mechanisms for
the production of these species are relatively straightforward, and
closely reflect the patterns of 1-butanol and 2-butanol. Isobutene
may have been produced by either molecular dehydration or by
abstraction of H and OH radicals [26,29]. Sequential H abstractions

J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
93
(a) 1-butanol
(b) isobutanol
(c) 2-butanol
(d) t -butanol
Fig. 7. Overall reaction pathways for butanol isomers. For brevity, only products with >5% selectivity are included. Arrow thickness indicates relative selectivity. Intermediate
species may also adsorb to surface and react further. (a) 1-Butanol, (b) isobutanol, (c) 2-butanol and (d) t-butanol.
may have produced isobutyraldehyde, which in turn may have
decomposed to propylene and syngas species [22]. Alternatively,
propylene may have been produced either by abstraction of a pri-
mary H radical from isobutanol, followed by loss of a CH2OH radical,
or by demethylation of the parent alcohol followed by abstraction
of an OH radical [26,29].
PtCe and RhCe showed relatively high selectivities to the alde-
hyde and propylene, Rh showed relatively low selectivities to both,
and Pt showed high selectivity for the aldehyde and low selec-
tivity for propylene. As discussed above, these features cannot
be explained solely by temperature effects, suggesting that some
isobutyraldehyde and propylene are either produced or consumed
on the catalyst surface.
have been found to be similar [35–39], and initial reactions in com-
bustion of the primary and secondary butanols lead primarily to
carbonyls that may react on the surface [23]. The actual situation
is likely a convolution of the alcohol reacting on the surface and
in the gas phase, vapor-phase intermediates reacting further in the
gas phase and on the surface, and surface intermediates reacting
further on the surface and desorbing to the gas phase, where they
may or may not react further. To avoid unnecessary speculation,
we have kept our proposed mechanisms as general as possible,
proposing only what appear to be the dominant overall pathways
based on observed products and intermediates with ≥5% selec-
tivity. Fig. 7 shows general reaction schemes for the CPO of each
isomer.
4.4. Surface chemistry
4.5. Effect of catalyst
Although the product spectra generally match a homogeneous
mechanism well, surface reactions are certainly contributing to the
observed products. Previous experiments have found that alcohols
typically adsorb onto Pt and Rh surfaces by an oxygen lone pair,
followed by O–H scission [35–42]. Surface reactions after this point
may diverge on Pt and Rh [35], and to our knowledge have not
been well studied on PtCe or RhCe. A feature shared by alcohol
decomposition pathways on these surfaces is the dehydrogenation
and decarbonylation of intermediate species, and desorption of H2
and CO from the surface. Under certain conditions, intermediates
may desorb [35,43], although we are unable to discern the relative
contribution to the observed product spectrum with the current
analytical setup.
Trends for each alcohol were similar across the four catalysts
studied, namely, high selectivity to CO and H2 at low C/O ratios,
steady selectivity to H2O and CO2 across C/O ratios, and increasing
selectivity to intermediate species (e.g. aldehydes and olefins) at
high C/O ratios. However, the absolute selectivities were different
across catalysts, with PtCe consistently exhibiting lower selectiv-
ity to CO and H2 than the other catalysts. PtCe was conversely
more selective for intermediate species than the other catalysts,
particularly for 2-butanol and iso-butanol.
The observed temperature and syngas selectivity trends across
catalysts appears to be related to endothermic reforming reactions
of the feed alcohols and intermediate carbonyls and alkenes to CO
and H2. The trend of selectivity for CO and H2 across catalysts, in
order of decreasing selectivity, was RhCe ≈ Rh > Pt > PtCe, while
the trend of catalyst backface temperature was generally PtCe > Pt
> Rh ≈ RhCe.
It is also possible that intermediate carbonyl and olefin species
formed in the gas phase are reacting on the catalyst surface. Under
different conditions, Pt and Rh surface mechanisms for carbonyls

94
J.S. Kruger et al. / Applied Catalysis A: General 411–412 (2012) 87–94
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Funding for this research was graciously provided by the Depart-
ment of Defense (DOD) through Fuel Cell Energy, Inc. in Danbury,
CT.
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