Liquid- and vapor-phase hydrogenation of 1-epoxy-3-butene using self-assembled monolayer coated palladium and platinum catalysts

Article abstract of DOI:10.1016/j.apcata.2012.08.017

Alkanethiol self-assembled monolayers (SAMs) have recently been shown to be effective catalyst modifiers for increasing the selectivity of the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane in the gas phase. In the results reported here, we demonstrate that SAM coatings can similarly be applied to other supported metals (Pt) and in liquid-phase reaction environments. Coating a Pt/Al₂O₃ catalyst with n-octadecanethiol resulted in a large improvement in selectivity during vapor-phase EpB hydrogenation, similar to that observed for supported Pd. The liquid phase hydrogenation of EpB using SAM-coated catalysts showed similar selectivity trends in some cases, but interactions of the solvent with the SAM were also important in controlling selectivity. In particular, using a heptane solvent, epoxybutane selectivity increased from 36% with an uncoated Pd/Al ₂O₃ catalyst to 74% with a thioglycerol SAM-coated catalyst. SAM quality was shown to have a strong impact on the rate of reaction but little effect on selectivity. The results generally indicated that selectivity modification with thiol SAMs is extendable to other supported metals and a variety of reaction environments.

Full text of DOI:10.1016/j.apcata.2012.08.017

Applied Catalysis A: General 445–446 (2012) 102–106
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Liquid- and vapor-phase hydrogenation of 1-epoxy-3-butene using
self-assembled monolayer coated palladium and platinum catalysts
∗
Karl R. Kahsar, Daniel K. Schwartz, J. Will Medlin
Department of Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO 80309-0596, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2012
Received in revised form 9 August 2012
Accepted 10 August 2012
Available online 25 August 2012
Alkanethiol self-assembled monolayers (SAMs) have recently been shown to be effective catalyst modi-
fiers for increasing the selectivity of the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane in
the gas phase. In the results reported here, we demonstrate that SAM coatings can similarly be applied to
other supported metals (Pt) and in liquid-phase reaction environments. Coating a Pt/Al2O3 catalyst with
n-octadecanethiol resulted in a large improvement in selectivity during vapor-phase EpB hydrogenation,
similar to that observed for supported Pd. The liquid phase hydrogenation of EpB using SAM-coated cat-
alysts showed similar selectivity trends in some cases, but interactions of the solvent with the SAM were
also important in controlling selectivity. In particular, using a heptane solvent, epoxybutane selectivity
increased from 36% with an uncoated Pd/Al2O3 catalyst to 74% with a thioglycerol SAM-coated catalyst.
SAM quality was shown to have a strong impact on the rate of reaction but little effect on selectivity. The
results generally indicated that selectivity modification with thiol SAMs is extendable to other supported
metals and a variety of reaction environments.
Keywords:
Selective hydrogenation
Alkanethiols
Surface modification
Supported metal catalysts
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
During deposition (Scheme 1), organic thiols adsorb dissociatively
through cleavage of the S H bond. Though the adsorbed monolayer
is characterized by covalent bonds with the surface and has not
been observed to form ionic metal states, the S–metal attachment
does exert a significant electronic effect on the metal surface [3,4,8].
The thiols were found in surface science experiments to strongly
suppress epoxide C O scission while still permitting adsorption of
the olefin function in a reactive pi-bound state [4].
Methods for controlling the chemoselectivity of reactions are
desirable for industrial catalysts because they reduce separations
costs and increase productivity [1,2]. Recently, it was shown that
the coating of a Pd/Al O3 catalyst with alkanethiol self-assembled
monolayers (SAMs) increased the selectivity for vapor-phase
hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane from
2
<
20% on an uncoated catalyst to >90% on a SAM-coated catalyst
Various “tail group” ligands were found to have little effect on
selectivity; however, they played an important role in controlling
activity, with longer alkyl tails being associated with higher rates. It
is known that SAMs comprised of precursors with longer tail func-
tions generally result in more ordered (e.g. “crystalline”) films as
indicated by infrared spectroscopy measurements [3,8–10].
Compared to gas phase reactions, EpB hydrogenation in the
liquid phase presents new possibilities for tail group effects on
selectivity. For example, in some previous work, the solvent has
been shown to play a significant role in catalytic hydrogena-
tion reactions by affecting hydrogen solubility, thermodynamic
interactions of the solvent and reactant, and the adsorbed struc-
tures of reagents, thus affecting the reaction pathways [11–13].
The hydrophobicity of the solvent, as measured by the dielectric
constant, has also been shown to affect the enantioselectivity of
under equivalent conditions [3]. For this reaction, the deposition of
SAMs on the Pd/Al O surface increased the kinetic barrier to epox-
2
3
ide ring opening while the binding of the olefin functionality was
not significantly changed [4]. A similar thiol SAM coating technique
was also shown to increase the selectivity for the isomerization
of allyl alcohols to the corresponding carbonyl compounds using
a Pd nanoparticle catalyst [5]. SAM films produced from amines
have been applied to catalysts for the selective hydrogenation of
␣
,␤-unsaturated aldehydes such as cinnamaldehyde and citral [6].
More generally, chiral catalyst modifiers such as cinchonidine have
been used over platinum catalysts to steer enantioselectivity for
the selective hydrogenation of ethyl pyruvate [7].
In previous work, the improved selectivity of EpB hydrogenation
was largely attributed to the sulfur head group of the alkanethiol.
1
-phenyl-1,2-propanedione hydrogenation [14]. However, not all
hydrogenation reactions exhibit solvent effects. The hydrogenation
of ␣,␤-unsaturated aldehydes has been extensively studied in the
liquid phase, where solvent composition was shown to have an
∗ Corresponding author. Tel.: +1 303 492 2418.
E-mail address: Will.Medlin@colorado.edu (J.W. Medlin).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.08.017

K.R. Kahsar et al. / Applied Catalysis A: General 445–446 (2012) 102–106
103
Scheme 1. Covalent binding process of alkanethiols to the Pd/Al2O3 catalyst surface.
2800–3000 cm 1 was used to characterize the degree of order of the
aliphatic chains as well as thioglycerol adsorption to the catalyst
surface.
−
insignificant effect on the rate and the selectivity of the reaction
15–17].
SAMs may significantly alter solvent–adsorbate surface interac-
[
tions based on the fact that the presence of tail groups is expected
to modify the nature of the near-surface environment. In fact,
by selecting appropriate tail functionalities it may be possible to
control the degree of solvent exposure near the active catalytic
sites. For example, hydrophilic SAMs may be useful in excluding
hydrophobic solvents to promote selectivity. Here we report the
extension of alkanethiol coated catalysts for EpB hydrogenation to
include supported Pt and reactions conducted in the liquid phase.
Unlike n-alkanethiols, thioglycerol has not been extensively
characterized on metal surfaces. As shown in Fig. 1, thioglycerol
was analyzed by ATR and then compared to a thioglycerol coated
Pd/Al O3 sample by DRIFTS.
2
The methylene stretching mode was observed to shift from
−
1
−1
2931 cm
to 2928 cm upon adsorption. One wash of ethanol
to remove physisorbed thiols was found to have no effect on the
positions or the intensities of the DRIFTS spectra. ATR of pure thio-
−
1
glycerol exhibited an SH stretch at 2550 cm which disappeared
upon deposition of the thiol indicating that thioglycerol was bound
to the surface.
2
. Experimental
2.1. Catalyst preparation
2.3. Reaction system
The 5 wt% Pd/Al O3 or Pt/Al O3 catalyst (Sigma Aldrich) was
loaded into a glass reactor tube. A mass of 500 mg of catalyst was
2
2
◦
Liquid phase hydrogenation was carried out at 30 C using a
oxidized under flow conditions of 100 sccm of 20:80 O :He at 573 K
100 mL Parr batch reactor. The system was prepared with 48 mL
of solvent, 5 mL of THF internal standard, and 1 mL of reactant. Pre-
pared catalyst, 1–50 mg, was added to the reactor, which was sealed
and preheated to the desired reaction temperature. Pressurization
with hydrogen started the reaction after which 1 mL samples were
released from the reaction chamber through a catalyst filter that
was replaced after each sample. Therefore, the catalyst to liquid
ratio was considered constant over the course of the reaction. Sam-
ples were taken at increasing intervals from zero to 90 min to allow
the system to reach 100% conversion. Catalyst recycling was per-
formed by filtering the final reaction mixture and allowing the
remaining catalyst to dry in air before being reused.
2
for 3 h, followed by reduction under 100 sccm of 20:80 H :He at
2
4
73 K for 2 h. The catalyst was then cooled and added to a stirred
beaker containing 40 mL of ethanol solvent with sufficient amounts
of thiol dissolved to achieve concentrations of 1 mM for octade-
canethiol and 10 mM for other thiol SAM precursors. All thiols were
obtained from Sigma Aldrich and were >97% purity. Thiols were sol-
vated in 200 proof anhydrous ethanol obtained from Sigma Aldrich.
The catalyst was incubated in the solution for 12–36 h and allowed
to settle out of the solution which was subsequently poured off.
The catalyst was rinsed once in ethanol, again allowing the catalyst
to settle out of solution (3–4 h) before pouring off the supernatant
ethanol. The remaining catalyst slurry was dried in a desiccator
under vacuum. All catalysts were stored under desiccant and used
within two days of preparation.
Gas phase hydrogenation was performed in a flow reactor. EpB
was vaporized in the feed stream by bubbling it with 10sccm helium
Poor quality SAMs were obtained by allowing the alkanethiol-
coated catalysts to sit for longer than two weeks prior to use.
During this time, oxidation of the Pd S bonds is known to result
in weakly bound sulfur species that are readily removed [18]. This
process deteriorated the quality of the SAM by creating defects in
the alkanethiol monolayer [9,19]. This process results in a relatively
disordered monolayer described here and by others as a poor or
disordered SAM [9,18,20].
2.2. Catalyst characterization
CO chemisorption was performed on a Quantachrome Autosorb-
1
to determine the active surface area of the Pd/Al O3 catalysts.
2
The uncoated commercial 5 wt% Pd/Al O was determined to have
2
3
2
an active surface area of 3.72 m /g. The surface area measured by
CO chemisorption decreased 94% to 0.22 m /g for a thioglycerol
2
modified catalyst, similar to results previously reported [3]. The
uncoated commercial 5 wt% Pt/Al O was determined to have an
2
3
2
active surface area of 2.87 m /g. Catalysts were also characterized
using a Thermo Scientific Nicolet 6700 FT-IR with an accessory
for diffuse reflectance infrared Fourier transform spectroscopy
Fig. 1. ATR-FTIR of pure thioglycerol and DRIFTS of thioglycerol SAMs deposited on
(
DRIFTS) of the dry powder catalyst. The C H stretching region of
Pd/Al2O3.

1
04
K.R. Kahsar et al. / Applied Catalysis A: General 445–446 (2012) 102–106
over 90 min as shown in Fig. 2. The epoxybutane selectivity was
observed to depend very weakly on conversion regardless of the
solvent or SAM used, indicating that epoxybutane is a non-reactive
product in the liquid-phase. This was confirmed in separate experi-
ments (not shown) where epoxybutane was fed as a reactant under
identical conditions to those used in Fig. 2, with no detectable con-
version. This contrasts with the gas-phase reaction studies where
selectivity decreased with increasing conversion [3]. Side prod-
ucts included crotonaldehyde, butyraldehyde, crotyl alcohol, and
C4 alcohols and are shown in Fig. 2.
Scheme 2. Reaction pathway for EpB hydrogenation.
Epoxybutane selectivity was found to be dependent on the
solvent for both uncoated and SAM-coated catalysts. Using an
uncoated catalyst, the selectivity to epoxybutane was 36% in hep-
tane but only 21% in ethanol. Thus interactions between the surface
and solvent appeared to result in a significant change in selectiv-
ity while the rate was relatively unaffected. For ethanol solvated
reactions, the deposition of thioglycerol monolayers on the cat-
alyst surface resulted in only a moderate increase in selectivity.
However, for the heptane-solvated reaction system, the thioglyc-
erol coated catalyst was shown to increase the selectivity to 74%,
indicating the presence of a solvent–SAM interaction mechanism.
Interestingly, an octadecanethiol-coated catalyst did not result in
significant selectivity improvement in either solvent. This result is
in sharp contrast to vapor-phase studies, where selectivity was high
for a wide range of SAMs, including those comprised of thioglycerol
or octadecanethiol.
◦
at 25 C. The feed stream was supplemented with hydrogen at a
◦
ratio of 10:1 hydrogen to EpB. The reaction was carried out at 40 C.
Selectivity was measured at 5% conversion as was done previously
[
3,21].
Liquid and gas phase reaction samples were analyzed with
an Agilent 5890A gas chromatograph and quantified with a
flame ionization detector and Agilent software. The column
was an Agilent DB-FFAP capillary column with dimensions of
3
0 m × 0.32 mm × 0.50 ␮m.
Epoxybutene (EpB, >98%) was purchased from Alfa Aesar. Hep-
tane (anhydrous >99%) and ethanol (>99.5%) solvents as well as
THF were obtained from Sigma Aldrich. All gases used for catalyst
preparation and reaction were Airgas ultra-high purity.
3
. Results and discussion
One hypothesis to explain the dependence of selectivity on
the solvent and SAM coating is that the solubility of the thiol
precursor in the solvent controls the stability of the SAM layer;
i.e., competing hydrophobic (solvent) and hydrophilic (thioglyc-
erol) environments may stabilize the SAM on the catalyst surface.
Ethanol is known to be a good solvent for the thiol precursors used
in these studies. Schlenoff et al. have previously shown that alka-
nethiol SAMs desorb at a faster rate in ethanol than in hexane or
other typical solvents and that this spontaneous desorption leads
to an equilibrium surface coverage which is both solvent and tem-
perature dependent [21]. Equilibrium coverage of SAM coated Pt
in hexane was 80% of the initial thiol coverage while equilibrium
coverage of SAM coated Pt in ethanol was 70% of the initial thiol
coverage; however, their work monitored experiments conducted
for 125 h and showed that any thiol desorption would be well under
10% in both cases over the course of a 1 h reaction [21]. Small
amounts of SAM desorption could be significant at longer reaction
times, or a faster SAM desorption rate would be expected to leave
vacant sites on the catalyst surface and could account for the lack
of selectivity improvement for some of the alkanethiol coated reac-
tion cases such as those run in ethanol. In addition, alkanethiols are
even more susceptible to desorption from the surface in the pres-
ence of hydrogen, as under the hydrogenation conditions in these
experiments [8,21].
3
.1. Vapor-phase hydrogenation using supported Pd and Pt
catalysts
Under hydrogenation conditions, EpB has been shown to
undergo both hydrogenation and isomerization through the
pathway shown in Scheme 2. Epoxide ring opening is thermody-
namically favorable over hydrogenation of the olefin [22].
Table 1 shows the measured rates and epoxybutane selectiv-
ity for EpB hydrogenation in the vapor phase using uncoated and
SAM-coated Pt and Pd catalysts. Consistent with previous reports,
the selectivity on uncoated Pt catalysts (40%) was noticeably higher
than on uncoated Pd catalysts (20%) under equivalent conditions
[
23]. The measured reaction rate was much lower on Pt, however.
The addition of an alkanethiol SAM was found to reduce catalyst
activity while markedly increasing selectivity on both the Pd and
Pt catalysts [3]. Interestingly, the selectivity improvement on Pt cat-
alysts was somewhat less than on Pd, though the rate of reaction
was reduced to a lesser extent on Pt by the SAM. The reasons for
these relatively subtle differences are not yet clear. Overall, how-
ever, it appears that the qualitative effects of SAM coating are quite
similar on Pd and Pt catalysts, suggesting that SAMs can be used as
selectivity modifiers across a range of metals.
One hypothetical method of maintaining thiol surface cover-
age is to introduce alkanethiols into the reaction mixture itself
to account for potential desorption. Accordingly, a hydrogenation
reaction was performed under all the same reaction conditions
described in the experimental section using an octadecanethiol
coated catalyst in ethanol solvent but where different concentra-
tions of octadecanethiol up to 1 mM were added to the bulk phase
reaction vessel. As shown in Table 2, the octadecanethiol coated
catalyst did not exhibit a significant increase in selectivity over
the uncoated catalyst in ethanol. However, the rate of reaction was
reduced strongly by the addition of the thiol to the reaction vessel
suggesting the formation of multilayers. Such octadecanethiol mul-
tilayer formation has been previously observed to occur on gold at
concentrations of 1 mM [24]. The results in Table 2 show that while
desorption of SAMs may be an issue in certain solvents, it is not pos-
sible to improve selectivity to epoxybutane by simply increasing
3
.2. Liquid-phase hydrogenation of EpB
Liquid phase hydrogenation of EpB was carried out in a batch
reactor and a typical experiment was run to nearly full conversion
Table 1
Reaction of EpB over Pd and Pt catalysts in the gas phase under identical conditions
at 5% conversion. Selectivity to epoxybutane and turnover frequency (measured
as EpB consumed) values reported here had maximum uncertainties of +/−5% and
+
/−20%, respectively.
Selectivity (%)
Turnoverfrequency(1/s)
Pd
Pt
Pd
Pt
Uncoated
Octadecanethiol coated
21
91
40
81
9.17
0.31
0.17
0.037

K.R. Kahsar et al. / Applied Catalysis A: General 445–446 (2012) 102–106
105
Fig. 2. Selective hydrogenation of EpB to epoxybutane in the liquid phase. (ꢀ) EpB, (᭹) epoxybutane, (ꢁ) butyraldehyde, (ꢂ) crotonaldehyde, and (ꢃ) C4 alcohols. Total molar
concentrations are normalized to one. (a) Hydrogenation in heptane solvent using 3.8 mg of uncoated Pd/Al2O3 catalyst; (b) hydrogenation in heptane solvent over 12.4 mg
of thioglycerol coated Pd/Al2O3 catalyst; (c) selectivity measured at 5% conversion of different reaction systems; (d) selectivity to epoxybutene versus conversion of EpB for
the uncoated and thiolglycerol coated cases shown in (a) and (b).
the reaction concentration of alkanethiols. This suggests that alka-
nethiol desorption was not responsible for the low selectivity
The lower increase in selectivity for the octadecanethiol coated
catalyst could be explained by penetration of the reaction sol-
vent heptane into the octadecanethiol monolayer. It has been
previously shown that an n-alkane solvent such as decane can
penetrate into an alkanethiol monolayer of longer length such
as docosanethiol; interactions between the solvent, reactant, and
SAM in such cases may have an effect on selectivity, though it
is unclear what the nature of this effect would be [21,25]. There
is also evidence that the structure of an octadecanethiol mono-
layer is highly dependent on the adjacent environment where
both polar and non-polar liquids shifted the asymmetric methy-
lene peak stretch from 2918 in air to 2921 in liquid, consistent
with a modest reduction in conformational order in the alkyl
chain [26].
observed for octadecanethiol coated Pd/Al O3 in comparison to
2
thioglycerol coated Pd/Al O .
2
3
Based on this result, the efficacy of the thioglycerol SAM
for increasing hydrogenation selectivity may depend more on
solvent–SAM interactions near the surface than the effect of the sol-
vent in removing SAMs. This effect was verified by testing a range
of alkanethiol coatings which are shown in Table 3. Although the
thioglycerol coated catalyst still led to the greatest gain in selec-
tivity over the uncoated case, the octadecanethiol coated catalyst
exhibited a much lower gain in selectivity compared to its shorter
alkanethiol counterparts.
Table 2
Activity and selectivity for hydrogenation reactions with varying concentrations of
octadecanethiol added in the reaction mixture. Hydrogenation was run in ethanol
◦
solvent at 30 C and 6 bar H2. Octadecanethiol was originally deposited on the
Table 3
Pd/Al2O3 catalyst in 1 mM ethanol. Each experiment was conducted using approx-
imately 3.5 mg catalyst. The maximum uncertainties in selectivity and turnover
frequency are +/−6% and +/−30%, respectively.
Selectivity at 5% conversion and initial activity for heptane solvated EpB hydrogena-
tion over thiol coated Pd/Al2O3 catalysts. The maximum uncertainties in selectivity
and turnover frequency were +/−7% and +/−15%, respectively.
Concentration (mM)
Turnover frequency (1/s)
Selectivity (%)
Coating
Selectivity to EpBane (%)
Turnover frequency (1/s)
1
0.0
0.4
1.7
5.4
24.6
0.31
–
32
32
28
24
91
None
36
74
43
66
60
69
48.4
12.7
11.9
13.3
12.1
9.8
0
0
0
0
.1
.05
.01
Thioglycerol
Octadecanethiol
Dodecanethiol
Hexanethiol
Propanethiol
Vapor phase

1
06
K.R. Kahsar et al. / Applied Catalysis A: General 445–446 (2012) 102–106
Fig. 3. Hydrogenation of epoxybutene over thioglycerol coated Pd/Al2O3 catalyst. (a) Rate (turnover frequency) and selectivity (%) for different preparation methods. Turnover
frequency data was calculated as the rate of epoxybutene consumed. (b) Methylene stretching region of octadecanethiol and thioglycerol coated Pd/Al2O3 for well-ordered
and disordered SAMs.
3
.3. Effect of SAM quality on liquid-phase hydrogenation
although other factors such as hydrogen solubility play significant
roles as well. The quality of the SAMs was shown to be very spe-
cific to the deposition technique where poor SAMs had a negative
effect on the rate of the reaction but had little effect on selectivity.
The most effective catalyst system measured was the contrasting
polarity of a nonpolar solvent heptane with the polar SAM thioglyc-
erol which increased the selectivity of epoxybutene hydrogenation
from 36% to 74% between the uncoated and coated cases respec-
tively.
To further explore the effects of alkanethiol tail disorder in the
liquid-phase hydrogenation reaction, we studied catalysts coated
with “good” and “poor” quality SAMs [9]. As a measure of SAM
quality, we used DRIFTS to measure the asymmetric methylene
stretching frequency on SAM-coated Pd/Al O3 catalysts prepared
in different ways. It has been shown previously that the asymmetric
methylene stretch shifts from its disordered solution phase peak of
2
−1
−1
2
930 cm toward its solid phase well-ordered peak at 2920 cm
as the alkanethiol chain length increases [3,8]. This shift to lower
frequencies is due to a lower fraction of gauche defects and a higher
degree of order [8]. In our liquid phase hydrogenation system, we
were able to detect this characteristic SAM quality by changes in the
rate of reaction during the hydrogenation of EpB. Shown in Fig. 3a,
Acknowledgement
This research was supported by the Department of Energy,
Office of Science, Basic Energy Sciences Program, Chemical Sci-
ences, Geosciences, and Biosciences Division under grant number
DE-FG02-10ER16206.
well-ordered SAM coated Pd/Al O3 led to a higher rate of reaction
2
than disordered SAM coating.
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2
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