Influence of Metal Doping on the Lewis Acid Catalyzed Production of Butadiene from Ethanol Studied by using Modulated Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Mass Spectrometry

Article abstract of DOI:10.1002/cctc.201700698

As a result of the increasing gap between the supply and demand of butadiene, the catalytic coupling of ethanol has regained the attention of the scientific and industrial community as an “on-purpose” production route to butadiene. The most promising systems are based on bifunctional catalysts that comprise metal sites that can dehydrogenate ethanol to acetaldehyde and Lewis acid sites that catalyze the aldol condensation between two aldehyde molecules and the Meerwein–Ponndorf–Verley reduction of the intermediate crotonaldehyde. Here, we investigate the role of Ag in an established Ag-Zr-BEA catalyst using modulated operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry experiments. We obtain insights into the complex reaction network that involves several consecutive and parallel reactions. Based on our investigations, we formulate suggestions for catalyst optimization.

Full text of DOI:10.1002/cctc.201700698

Accepted Article
Title: Influence of Metal-Doping on the Lewis-Acid Catalyzed
Production of Butadiene from Ethanol Studied by Modulated
Operando DRIFTS-MS
Authors: Philipp Mueller, Shao-Chun Wang, Samuel Burt, and Ive
Hermans
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ChemCatChem
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Influence of Metal-Doping on the Lewis-Acid Catalyzed
Production of Butadiene from Ethanol Studied by Modulated
Operando DRIFTS-MS
[
a]
[a]
[b]
[a,b]
Philipp Müller, Shao-Chun Wang , Samuel P. Burt , and Ive Hermans*
Abstract: Due to an increasing gap between the demand and
supply, the catalytic coupling of ethanol has regained the attention of
the scientific and industrial community as an “on-purpose”
production route to butadiene. The most promising systems are
based on bifunctional catalysts, comprising of metal sites that can
dehydrogenate ethanol to acetaldehyde, and Lewis acid sites that
catalyze the aldol condensation between two aldehyde molecules,
as well as the Meerwein-Ponndorf-Verley (MPV) reduction of the
intermediate crotonaldehyde. Here, we investigate the role of Ag in
an established Ag-Zr-BEA catalyst using modulated operando
DRIFTS-MS experiments. We obtain insights into the complex
reaction network that involves several consecutive and parallel
reactions. Based on our investigations, we formulate suggestions for
catalyst optimization.
coupling has been sparked again and research efforts are
devoted to it. In order to make the technology (again)
economically attractive, we need to further improve the
selectivity. Fundamental understanding of molecular level
interactions of reactants and intermediates with the catalyst
surface using sophisticated operando spectroscopic tools might
help to further improve the performance and make this approach
economically viable.
BD can be produced from EtOH using two different approaches
that use different catalysts ranging from solid bases to solid
[
10-15]
Lewis acids.
In the one-step process, catalysts directly use
EtOH as a feedstock, while in the two-step process, AA needs to
be co-fed because Lewis acid sites are not able to
dehydrogenate EtOH to AA. It is thus possible to transform a
two-step catalyst into a one-step catalyst and to utilize EtOH
directly, by simply adding a dehydrogenation functionality. This
concept has been applied for Lewis acidic Zr-BEA and Ta-BEA
Introduction
[15-17]
zeolites that were doped with Ag and Cu,
traditional one-step SiO
but also the more
2
-MgO system has been doped with Ag,
The desire for more sustainable manufacturing technologies –
[18-21]
Cu, and Zn.
In both cases the metal addition lead to
preferably from renewable resources
– initiated significant
improved activity and selectivity toward BD as shown in Table 1
for some examples with silver.
research in the utilization of biomass-derived feedstock using
[
1-4]
heterogeneous catalysis.
,3-butadiene (BD) – an important polymer precursor with a
yearly production volume of >10 Mt – is a historically important
The coupling of ethanol (EtOH) to
1
Table 1. Catalyst performance for selected silver-doped catalysts in the
direct conversion of EtOH.
[
5-8]
example that is currently attracting renewed attention.
Traditionally, BD is obtained as by-product in ethylene
[
a]
[b]
[c]
a
Catalyst
Conversion
0.324
Selectivity
0.672
Yield
Reference
production via steam cracking. The trend towards the use of
lighter feedstock (i.e. shale gas) for steam cracking results,
however, in a lower BD production which affects its price
Ag-Zr-BEA
Ta-BEA
0.218
0.022
0.519
0.036
0.202
17
16
16
18
18
0.133
0.164
[
8]
volatility and the supply of BD. Hence, a flexible drop-in
process producing renewable BD would offer several
advantages. EtOH has already been recognized as a convenient
feedstock to produce BD in the early 20th century, i.e. prior to
the large-scale implementation of cracking technology. During
World War II, Union Carbide and Carbon Corporation produced
Ag-Ta-BEA
0.829
0.626
MgO-SiO
2
0.100
0.360
Ag-MgO-SiO
2
0.440
0.460
[
9]
up to 0.3 Mt of BD from EtOH per year. After World War II,
naphtha cracking outcompeted this route. However, with the
recent changes in cracking feedstock, as well as the increased
production of biomass-derived ethanol, interest in the EtOH
[a] defined as moles of EtOH consumed divided by moles of EtOH fed, [b]
defined as moles of BD produced per 2 moles of EtOH consumed [c]
defined as conversion multiplied by selectivity
The reaction mechanism is particularly complex because it
consists of many reaction steps that proceed over different
catalyst functionalities. The generally accepted pathway is
[
a]
b]
Philipp Müller, Shao-Chun Wang, Prof. Dr. Ive Hermans
Department of Chemistry
[
8,11]
displayed in Scheme 1.
University of Wisconsin - Madison
1101 University Ave, Madison, WI 53706 (USA)
E-mail: hermans@chem.wisc.edu
[
Samuel P. Burt, Prof. Dr. Ive Hermans
Department of Chemical and Biological Engineering
University of Wisconsin - Madison
1415 Engineering Dr, Madison, WI 53706 (USA)
Supporting information for this article is given via a link at the end of
the document
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ChemCatChem
10.1002/cctc.201700698
FULL PAPER
CA hydrogenation with H
2
, predominantly yielding crotyl alcohol
Hence, it is possible that the direct
[
23-25]
over butyraldehyde.
hydrogenation of CA takes place under reaction conditions over
metal-doped Lewis acid catalysts as well. The additional Ag
particles might also influence the formation of by-products, such
as the fully hydrogenated butanol that could be formed from
COH.
In the present work, we hence investigate the influence of Ag
addition to a Lewis acidic Zr-BEA catalyst. First, we characterize
the material thoroughly using Inductively Coupled Plasma -
Optical Emission Spectroscopy (ICP-OES), N
2
-physisorption,
NH Temperature Programmed Desorption (TPD), UV-vis
3
spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and
Fourier Transform Infrared spectroscopy (FTIR) of adsorbed
pyridine. Then, we perform an operando Diffuse Reflectance
Infrared Fourier Transform Spectroscopy - Mass Spectrometry
(
DRIFTS-MS) study to investigate the reaction mechanism with
a setup that allows to simultaneously monitor dynamic changes
on the catalyst surface as well as the product distribution in the
gas phase. To do so, we use a modulation approach that
simplifies our understanding of the origin of consecutive reaction
products compared to steady-state studies. Finally, we propose
a mechanism that rationalizes the influence of Ag addition to Zr-
BEA, which is likely generally valid for metal-doped Lewis acid
catalysts.
Scheme 1. Generally accepted reaction pathway for the coupling of ethanol to
1
,3-butadiene.
Results and Discussion
EtOH gets dehydrogenated to acetaldehyde (AA) that couples
with another AA molecule to give the instable intermediate
acetaldol (AOL) that is easily dehydrated to form crotonaldehyde
Material Synthesis and Characterization
(
CA). Then, CA is transformed into crotyl alcohol (COH) either
Throughout this study we investigate three different catalysts.
The Zr-BEA was prepared using a post-synthetic procedure,
starting from a commercially available Al-BEA zeolite that was
dealuminated and then subjected to a liquid phase impregnation
through a Meerwein-Ponndorf-Verley (MPV, right) reaction with
EtOH forming AA, or through direct hydrogenation of the
aldehyde (left).
A
final dehydration step produces BD.
[
26]
Alternatively, EtOH can get directly coupled with another EtOH
molecule to form the by-product diethyl ether (DEE), releasing
water. Other by-products such as ethylene, propylene, and
with ZrOCl
2
in DMSO.
After calcination, this sample was
impregnated with an appropriate amount of aqueous silver
nitrate solution (incipient wetness impregnation) giving the
sample denoted as Ag-Zr-BEA with loadings of 2.2 wt% Zr and
[
15]
trienes have also been observed.
While avoiding the
formation of such products is important to maximize the
selectivity, we focus on the major pathways in this manuscript.
0
.8 wt% Ag. For comparison, we furthermore impregnated silica
with Ag, resulting in the sample denoted as Ag-SiO with 1.0
2
wt% Ag. The key characteristics of these catalysts are
summarized in Table 2. The BET surface area slightly
decreases upon Ag incorporation, while leaving the microporous
We have recently studied the Lewis acidic two-step catalyst Ta-
BEA and put forward a molecular level description of the
[
22]
reaction mechanism.
The mechanism over Zr-BEA, on the
network intact. The number of acid sites determined with NH
3
-
other hand, has not been studied in detail yet. Also, the
influence of the added metal in Lewis acidic one-step catalysts,
in our case Ag-Zr-BEA, on the reaction mechanism has not been
rigorously studied yet. Its primary role is most likely the
dehydrogenation of EtOH to AA. But the addition of Ag might
also open up other reaction pathways. For instance, it could be
possible to directly hydrogenate the aldehyde functional group in
CA instead of performing the MPV reaction with EtOH as shown
TPD decreases slightly when Ag is added. As expected, Ag-SiO
2
possesses a low number of weakly acidic sites. FTIR of
adsorbed pyridine was used to determine the ratio of Lewis to
Brønsted acid sites. The absence of a significant peak at 1545
-
1
cm upon adsorption of pyridine that is assigned to the
pyridinium ions that form over Brønsted acid sites, indicates that
in all samples mainly Lewis acid sites are present
(
Lewis/Brønsted ratios >10, see Figures S2-S4).
in the grey box in Scheme 1. It has been reported that Ag-SiO
can be used as catalyst for the hydrogenation of acrolein with H
2
2
,
predominantly forming allyl alcohol. Furthermore, Ag deposited
on other supports such as TiO have been used as catalysts for
2
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ChemCatChem
10.1002/cctc.201700698
FULL PAPER
carrier gas (Ag-Zr-BEA sH
in each spectrum.
2
). The spectra are normalized to the biggest signal
Table 2. Physicochemical characteristics of the synthesized catalysts.
[
a]
[a]
[b]
[c]
[d]
[e]
Catalyst
Ag-SiO
Ag
Zr
-
S
BET
V
micro
S
ext
N
acid
XPS was also used to study the oxidation state of Zr. The Zr 3d
photoelectron spectra are given in the supporting information
2
1.0
-
255
715
686
0.01
0.22
0.22
233
250
240
49
(
Figure S6). The calcined Zr-BEA shows a signal at 184.3 eV,
Zr-BEA
2.1
2.2
261
235
which is higher than previous literature spectra for hydrothermal
[
29]
Zr-BEA.
It is possible that the post-synthetic preparation
Ag-Zr-BEA
0.8
method leads to slightly different Zr sites as has been observed
[
30]
[
a] [wt%] as determined from ICP-OES, [b] BET surface area from N
2
with Sn-BEA zeolites before. The addition of Ag shifts the BE
downward to 184.0 eV. The spent catalysts are shifted even
further to 183.8 eV, indicating a slightly more reduced Zr species.
This shift could be induced by the formation of ethoxy/crotoxy
species on Zr-OH sites or coke, or a change in the electronic
structure of the Ag particles as will be discussed next. Upon
recalcination the signal shifts back to the initial position at 184.3
eV, potentially restoring Zr-OH by burning of organic species.
2
3
physisorption [m /g], [c] micropore volume [cm /g], [d] external surface
2
3
area [m /g], [e] number of acid sites from NH -TPD [µmol/g]
To study the electronic state of the silver particles in these
materials, we measured UV-vis spectra of our catalysts, as
shown in Figure 1. The band centered around 220 nm in the Zr-
BEA sample can be assigned to ligand-to-metal-charge-transfer
of neighboring oxygen atoms to isolated Zr(IV) centers and is
[
27]
indicative of a successful Zr incorporation.
added to this sample, much stronger bands centered around
45 nm superimpose the Zr band. Signals below ca. 230 nm can
When silver is
Operando DRIFTS-MS Modulation Experiments
2
To further investigate the reaction mechanism, we used an
be assigned to Ag(I), while bands up to ca. 320 nm are assigned
to small positively charged clusters. Absorbance above 320 nm
is indicative for metallic Ag(0) band transitions and plasmonic
operando DRIFTS-MS setup that is represented in Scheme 2
[
22]
and that we described in detail previously.
[
28]
resonances. Hence, mainly small clusters of slightly oxidized
Ag are present in our samples. Upon reduction, the Ag(I) band
gets smaller but does not completely disappear and an
additional broad peak in the Ag(0) region appears, indicating
non-complete reduction of Ag in the system. There are small
differences in the position and shape of the Ag(0) bands, but Ag
remains mainly metallic under reaction conditions irrespective of
the used carrier gas (Ag-Zr-BEA sHe for catalyst spent in He for
9
0 minutes and Ag-Zr-BEA sH
2 2
for catalyst spent in H for 90
minutes in Figure 1).
Scheme 2. Schematic overview over the DRIFTS-MS setup. Gases are
connected to two individual mass flow controllers. Two liquid pumps deliver
substrates into heated spiral, where evaporation takes place and the two-
position-four-way-valve selects substrate A or B to pass over the catalyst in
the DRIFTS accessory. An online quadrupole mass spectrometer continuously
collects spectra at the reactor outlet.
In short, a two-position-four-way-valve allows for a quick change
in the gas-phase composition that flows over a heated catalyst
bed in the DRIFTS accessory. At the outlet of the accessory, an
online mass spectrometer continuously collects spectra. The
whole apparatus is heated to avoid condensation of liquid
products. Two independent mass-flow controllers and syringe
pumps enable evaporation of liquid substrates and precise
control over the gas-phase concentrations. By periodically
switching between two different flows, the influence of certain
components on the reaction mechanism can be followed in a
dynamic fashion. This approach, known as modulation excitation
spectroscopy (MES), has been used for heterogeneous liquid-
[
31]
phase and gas-phase catalysis before.
In a standard experiment, the catalyst is heated to reaction
Figure 1. UV-vis spectra of the three synthesized catalysts measured under
ambient condition. Ag-Zr-BEA was pre-treated for 30 minutes in a H flow (Ag-
Zr-BEA red), followed by an ethanol treatment under reaction conditions for 90
minutes (Ag-Zr-BEA sHe), and modified reaction conditions by using H as
temperature (320 °C) under a He flow and then pre-reduced for
2
30 minutes under H . The catalyst is then subjected to the initial
2
2
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flow A (given in the right hand side in the following MS figures)
until steady signals from the IR and the MS are achieved. Then,
the modulation experiment starts, by switching to flow B for 120
seconds (left hand side in the following MS figures) and back to
A for 120 seconds, while continuously acquiring IR and MS
spectra. This procedure is repeated several times until a quasi-
steady state is reached at which point the spectra of 3-12 of
these periods are collected and averaged to improve the signal-
to-noise ratio. After averaging the spectra, the first spectrum is
taken as reference and the relative changes compared to the
reference are shown.
XPS signals for the spent catalysts. As discussed earlier, the
change from a Zr-OH to a Zr-ethoxy or Zr-crotoxy species might
lead to such a subtle but significant shift.
Modulation Experiments with Zr-BEA
Figure 2 displays the results of an MES experiment for Zr-BEA.
The catalyst was first saturated with EtOH, followed by
periodically switching to a 3:1 mixture of EtOH/AA, a ratio that
has been shown to be optimal for BD production over Ta-
[
14]
BEA. The displayed m/z values have been chosen based on
their MS fragmentation patterns while avoiding contributions
from other compounds (see Table S1 for details). As expected
for a purely Lewis acidic catalyst, the addition of AA is required
for a significant production of BD; using pure EtOH mainly leads
Scheme 3. Proposed mechanism for the Meerwein-Ponndorf-Verley reaction
of CA and EtOH over Lewis acid catalysts.
[
14]
to the by-products DEE and ethylene (MS signals not shown).
When switching back from the mixture to pure EtOH, the BD
signal rapidly decreases and ultimately reaches zero. The
signals for the intermediates, CA and COH follow a similar
pattern as BD, indicating that they are part of the same reaction
pathway. The results of this experiment, together with additional
data given in the supporting information (see Figures S7-S9),
show very similar patterns to Ta-BEA and suggest that the
general pathway of BD formation over these different Lewis acid
catalysts is very similar. It has been suggested that the open site
in Zr-BEA, i.e. a Zr(IV) ion in tetrahedral framework position,
where one bond is hydrolyzed to form a Zr-OH group, is more
In the case of Zr-BEA (Figure 2 b), the IR signal for the crotoxy
-
1
-1
species is observed at 1693 cm instead of 1690 cm for Ta-
BEA, possibly due to differences in the acid strength of Ta and
Zr. The signals originating from C=O stretch vibrations of AA and
-
1
CA can be observed at higher wavenumbers (1710 cm - 1760
-
1
cm also see Table S2 for an assignment of important
vibrations). A stronger bond to the surface weakens the C=O
band shifting the frequency down, which is why the "crotoxy"
intermediate is expected to vibrate with a lower frequency than
gas-phase or physisorbed CA. The positive signals between
[
32]
active in the coupling of EtOH to BD. Together with the similar
spectroscopic patterns, it is hence likely that our proposed
mechanism for Ta-BEA that is based on a Ta-OH group is valid
for Zr-BEA as well. In this mechanism, an ethoxy group is
formed on the open site and then gets exchanged with an
incoming CA molecule to form a "crotoxy" intermediate with a
-
1
2
700 and 2850 cm can be assigned to C-H stretch vibrations
from adsorbed AA and CA. The negative signals above 2900
-
1
cm can be assigned to C-H stretch vibrations of COH and
predominantly EtOH that desorb or get consumed in the first
-
1
half-period. A weak negative signal around 3745 cm that can
be assigned to Si-OH vibrations appears to be more negative in
presence of AA, suggesting only a weak interaction of AA with
surface Si-OH (or Zr-OH) groups.
-
1
distinct IR signal at 1690 cm (Structure 3 in Scheme 3) that
[
22]
gets exchanged with EtOH again to form COH.
This
mechanism might also explain the observed shift in the Zr 3d
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Figure 2. Modulation experiment between EtOH in He and a 3:1 mixture of EtOH/AA over Zr-BEA at 320 °C. The figure on the left (a) shows the recorded MS
spectra of selected m/z signals and the figure on the right (b) shows the corresponding surface IR spectra. The black line corresponds to the time at which the gas
streams are switched.
Figure 3. Modulation experiment between EtOH in He and pure He over Ag-SiO
2
at 320 °C. The left figure shows the recorded MS spectra of selected m/z
signals and the right figure shows the corresponding surface IR spectra. The black line corresponds to the time at which the gas streams are switched.
-
1
of positive signals in the C-H stretch region at 2900-3000 cm it
can be concluded that mainly inactive physisorbed ethanol or
Modulation Experiments with Ag-SiO
2
[
33]
ethoxy species are formed.
The emergence of a broad IR
-
1
As shown in Figure 3, the adsorption of EtOH to Ag-SiO
2
band centered around 3250 cm can be interpreted as Si-OH
groups interacting with ethanol molecules physisorbed on the
enables the dehydrogenation of EtOH and forms AA. However,
the MS signal for crotonaldehyde is almost a flat line, indicating
insufficient coupling activity. Also, signals from COH and BD are
essentially zero, clearly showing that an acid functionality is
required to go further than AA in the reaction pathway. In the IR
spectra, no significant signals in the C=O stretch region could be
observed, indicating fast desorption of AA after its formation. A
2
surface. In an in situ transmission FTIR study on Ag-SiO , where
the catalyst was pressed into a wafer, exposed to EtOH vapor
for 30 seconds and then kept at 373 K for 5 minutes, the
production of AA could be observed by the emergence of a band
-
1 [34]
at 1726 cm .
The absence of this signal in our spectra
exemplifies the importance of performing spectroscopic studies
under reaction conditions. Neither in that study nor in our
spectra, could we find an indication for Ag-H species that would
-
1
strong negative signal at 3745 cm is indicative of consumption
of Si-OH groups, i.e. together with the simultaneous appearance
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-
1 [35]
[8, 11]
be expected around 1800 cm .
Thus AA and H
2
desorption
determining step is the dehydrogenation of EtOH.
However,
the fact that both EtOH and AA show significant signals might
indicate that both the dehydrogenation and the coupling of AA
are kinetically relevant steps in the reaction mechanism. The IR
are unlikely to be kinetically relevant steps.
Modulation Experiments with Ag-Zr-BEA
The previous modulation experiments showed that Zr-BEA
follows a reaction mechanism similar to Ta-BEA and enables the
spectra in Figure 5 show similar results to the spectra of Ag-SiO
2
(Figure 3). Positive signals in the C-H stretch region for EtOH
-
1
formation of BD from a mixture of EtOH and AA. Ag-SiO
2
is able
(2900-3000 cm ) and negative signals in the Si-OH region (3745
-
1
to dehydrogenate EtOH to AA, but the acid functionality is
missing to go further in the reaction mechanism. When, on the
other hand, Ag is incorporated in Zr-BEA (Figure 4), the reaction
goes to completion and produces BD. Interestingly, the BD
signal shows a maximum in the second half-period, indicating
hindered desorption of either an intermediate or BD when the
EtOH concentration is too high. As such behavior was not
observed in the case of Zr-BEA where a mixture of EtOH/AA
was fed, it is likely that the Ag is responsible for this. It might be
that intermediates and/or BD adsorb to the Ag surface and the
desorption only occurs at low EtOH pressures. The MS signals
of COH and CA are relatively low and are almost constant
throughout a modulation period. This is surprising as for the two-
step catalysts Ta-BEA and Zr-BEA, the MS signals for CA and
COH were usually coupled, showing distinct signals and similar
patterns. This result might be caused by a different rate-
determining step compared to the two-step catalysts. It has been
cm ) are visible and indicative of the formation of physisorbed
ethanol or ethoxy species. However, the C-H signals are more
intense compared to Ag-SiO
2
demonstrating the stronger binding
of the ethoxy species on Zr-BEA. In the C=O stretch region
-
1
around 1700 cm no significant signals were observed. However,
-
1
there is a small shoulder below 2900 cm that is assigned to
either CA or AA. Greenler's surface selection rule states that
infrared vibrations with an oscillating dipole moment parallel to
the metal surface for metal particles larger than 20 Å get
[
36]
suppressed.
Hence, it is possible that the reason for the
absence of C=O stretch vibrations is the parallel adsorption
mode of CA and/or AA on the Ag particles. When adsorbing AA
to Ag-Zr-BEA, on the other hand, CA is formed but no COH (see
Figure S20). In the IR spectra, some C=O stretch vibrations are
-
1
visible, but no signal around 1690 cm that we assigned to the
crotoxy intermediate. This further demonstrates the importance
of EtOH in the formation of this intermediate, which seems to be
crucial for BD production.
2
reported that for one-step catalysts such as SiO -MgO, the rate-
Figure 4. Modulation experiment between EtOH in He and pure He over Ag-Zr-BEA at 320 °C. The figure on the left (a) shows the recorded MS spectra of
selected m/z signals and the figure on the right (b) shows the corresponding surface IR spectra. The black line corresponds to the time at which the gas streams
are switched.
To study the effect of silver addition in more detail, we went a
step further in the reaction pathway and investigated the
conversion of CA to COH and finally BD. As mentioned in the
introduction, due to the added Ag, it is possible to perform the
direct hydrogenation of CA to COH. Thermodynamically, the
alternative MPV reaction with EtOH and CA as reactant
producing COH and AA is neutral, while the direct hydrogenation
2
Ag-Zr-BEA under H flow, COH and BD can be observed in the
product stream (see Figure S25). When deuterium is used as
carrier gas, the Si-OH groups exchange to form Si-OD as can be
-
1
seen by following the decrease at 3745 cm and increase at ca.
-
1
2750 cm (see Figure S19). Upon the adsorption of CA under D
2
flow, the main products are now the singly and doubly
deuterated COH and consequently the singly deuterated BD
with a m/z = 55 (see Scheme 4). However, irrespective of the
carrier gas, the retro aldol condensation takes place as AA is
formed in significant amounts as well.
[
8, 37]
is unfavorable by about 35 kcal/mol.
possible to use Ag as hydrogenation catalyst for CA
Accordingly, when CA is adsorbed to
Nevertheless, it is
a
[
23]
2
hydrogenation with H .
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are available in close proximity to where COH is formed, such
that BD can readily be produced. Additionally, the MPV reaction
is thermodynamically favored, leading to higher COH yield. In
the IR spectra, negative signals in the C=O stretch region
indicate consumption of adsorbed CA. Positive peaks in the C-H
-
1
region above 2900 cm indicate the adsorption of ethanol and
formation of some ethoxy species on the catalyst surface. In the
Si-OH and Zr-OH region, a negative signal can be observed,
indicating consumption of these groups upon ethanol adsorption.
2
Similar spectra have been obtained when H was used as carrier
Scheme 4 Possible reaction pathways for the formation of COH from CA. On
the left, the pathway for the direct hydrogenation with deuterium is shown,
while on the right the MPV reaction pathway is displayed. The numbers next to
the molecule stand for their respective molecular masses.
gas (see Figure S27). In presence of deuterium as carrier gas, it
is now possible to detangle the contributions of COH production
from the direct hydrogenation and the MPV reaction. As shown
in Scheme 4, the direct hydrogenation product gives the doubly
deuterated crotyl alcohol (CODD) with a signal at m/z = 74. Due
to the presence of water, and easily exchangeable Si-OH and
Si-OD groups, it is likely that the singly deuterated crotyl alcohol
To get further insight into the formation of COH, we modulated
the EtOH concentration at a constant CA concentration over Ag-
Zr-BEA with different carrier gases as shown in Figure 6. With
He as carrier gas, the addition of EtOH leads to higher AA and
BD formation. However, the COH concentration shows a reverse
pattern, where the maximum is in the second half-period. Note
that under He flow, the Ag mainly stays reduced as the UV-vis
(
COD) is formed as well. Both of these compounds can then be
dehydrated, releasing either or HDO and singly
H
2
O
a
deuterated BD (BDD) with a signal at m/z = 55. Unfortunately,
butanol also shows a signal at this m/z value, which makes a
clear determination of the BDD production impossible. The non-
deuterated BD signal at m/z = 54, on the other hand, clearly
increases in presence of EtOH and decreases again under pure
CA flow, indicating that mainly the MPV pathway is responsible
for its production. The COH signal at m/z = 57 follows a similar
pattern as the BD signal, suggesting the same reaction pathway
for these compounds. The deuterated crotyl alcohol species at
m/z= 73 and m/z = 74, on the other hand, follow a similar pattern
as the gas phase CA signal, also indicating that the
appearances of these compounds are linked.
results suggested. Hence, the H
2
produced from the EtOH
dehydrogenation can be used for the direct hydrogenation of CA
as well.
The lower COH yield in the presence of EtOH can be explained
by competitive adsorption on Ag sites. When suddenly the EtOH
concentration is increased, the sites that used to produce COH
through
direct
hydrogenation
now
perform
EtOH
dehydrogenation. On the other hand, the Zr sites are able to
perform the MPV reaction and, unlike for the COH formed on the
Ag particles, the (weak) acid sites required for COH dehydration
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-
1
In the IR spectra, consumption of CA and crotoxy species can
be observed in the presence of EtOH by negative signals below
EtOH. At the same time, the Si-OH region below 3745 cm
exhibits positive signals indicating some formation of Si-OH.
However, these bands are broad, indicating they are likely
interacting with EtOH. The positive signals in the C-H stretch
-
1
-1
1
724 cm . A small positive peak around 1762 cm can also be
seen, indicative of the production of AA, either from EtOH
dehydrogenation, from retro aldol condensation of CA, or MPV
-
1
region between 2900 and 3000 cm originate from physisorbed
EtOH or the formation of ethoxy species.
-
1
reaction of CA and EtOH. In the region below 2750 cm ,
negative signals are observed that can be assigned to the
consumption of Si-OD (or Zr-OD) groups upon the adsorption of
Figure 5. Modulation experiment between a 3:1 mixture of EtOH/CA in He and CA in He over Ag-Zr-BEA at 320 °C. The figure on the left (a) for He as carrier gas,
c) for D as carrier gas) shows the recorded MS spectra of selected m/z signals and the figure on the right (b) for He as carrier gas, d) for D as carrier gas) shows
the corresponding surface IR spectra. The black line corresponds to the time at which the gas streams are switched.
2
2
Mechanistic Interpretation
to an available acid site (B), where the dehydration to BD and
O takes place. In the meantime, EtOH can adsorb to a silver
particle (C) where the dehydrogenation takes place and AA is
released. Hydrogen atoms either desorb in the form of H (as
H
2
In Scheme 5, we summarize the findings of this work for the
coupling of EtOH to BD over the one-step catalyst Ag-Zr-BEA.
When EtOH pressures are low (left side of the figure), Ag
particles are available for the hydrogenation of CA to COH (A).
Consequently, COH desorbs from the metal surface and diffuses
2
also observed in the MS spectra), or remain on the surface until
another CA molecule comes in and can be hydrogenated. As the
EtOH pressure is low, Zr-OH sites are also available for coupling
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10.1002/cctc.201700698
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of two AA molecules to form CA (D, see reference 21 for details
on the Ta-BEA mechanism). When, on the other hand, the EtOH
pressure is high (as it usually is under reaction conditions), Ag
particles are mostly covered with EtOH and perform the
dehydrogenation of EtOH to AA, leaving fewer sites for the direct
hydrogenation of CA (A). AA then easily desorbs, couples over
the acid site and produces CA (B). At this point there are two
reaction paths for CA to take: either it gets directly hydrogenated
over Ag, or it undergoes MPV reaction over the Zr site (C). At
high EtOH pressure, not only the EtOH coverage on Ag is high,
but also ethoxy species are likely present on the Zr group. We
proposed this to be an intermediate in the MPV mechanism over
Ta-BEA, meaning that having a high ethoxy coverage likely
leads to better productivity in Zr-BEA as well. As the
thermodynamics are more favorable for the MPV reaction, most
of the CA will react over the Zr site, releasing COH and AA. In a
is formed from via an adsorbed carbanion intermediate that
reacts with another adsorbed acetaldehyde molecule instead of
[
38]
an MPV process. They also showed that the reaction of COH
over MgO does not yield BD with high selectivities. This
suggests that the addition of a weak acid functionality such as
SiO
reaction step. However, in these acid-base systems, changing
one parameter such as the SiO loading might also influence the
2
gives the catalyst enough acidity to perform this last
[
39]
2
amount of basic sites and their strength, making rational catalyst
design highly challenging. Furthermore, the preparation method
also influences the type and amount of sites present in the final
[
40-42]
catalysts.
For metal-doped Lewis acid catalysts, on the
other hand, the addition of Ag, for instance, does not add or
remove significant amounts of acidity/basicity to the system and
is mainly responsible for dehydrogenation activity. Hence, if the
EtOH conversion is too low, adding more Ag to the catalyst
might be a solution to boost activity, whereas if more coupling
and dehydration activity is needed, the Zr loading should be
increased.
2
final step, BD and H O are produced from COH.
Interestingly, for Ta-BEA it was found that doping with Cu leads
[
16]
to higher BD productivity than doping with Ag or Zn. Hence, a
targeted catalyst design might be possible by tuning the acidity
of the catalyst and adding different loadings and types of metals
for dehydrogenation.
Conclusions
In this study, we present new insights into the mechanisms of
EtOH coupling over a silver-doped Zr-BEA catalyst. A thorough
catalyst characterization was performed where it was found that
Ag forms small nanoparticles that stay mostly reduced under
reaction conditions. The Zr(IV) sites are isolated and highly
Lewis acidic. A modulated operando DRIFTS-MS study revealed
that the reaction mechanism for the two-step process over Zr-
[
22]
BEA proceeds similarly to one proposed earlier for Ta-BEA,
explaining why the open site in Zr-BEA is more active in this
reaction. The addition of Ag enables the dehydrogenation of
EtOH to AA. Lewis acidity is required for the coupling and
dehydration steps. When combined in one catalyst (Ag-Zr-BEA),
it is possible to form the crucial intermediate COH in two ways.
Either CA gets directly hydrogenated over Ag, or it forms COH
through an exchange with EtOH in an MPV mechanism over the
Lewis acid site. At high EtOH pressures, which are present
under actual reaction conditions, the latter is more likely to take
place and is mainly responsible for the production of BD. Finally,
the use of metal doped Lewis acids as one-step catalysts has
the advantage of easier catalyst design, as the dehydrogenation
and coupling events take place on different active sites within
the catalyst.
Scheme 5 Tentative reaction scheme for the coupling of EtOH to BD over the
one-step catalyst Ag-Zr-BEA. On the left side, the mechanism for low EtOH
pressures is shown, while on the right side the case of high EtOH pressures is
presented.
Experimental Section
Contrary to other previously used one-step catalysts such as
2
MgO or mixed SiO -MgO catalysts, in metal-doped Lewis acid
Material Synthesis
catalysts the dehydrogenation functionality and the coupling and
dehydration functionalities are occurring on different active sites,
which brings along certain advantages concerning catalyst
design. Cavani et al. have studied the ethanol coupling over
basic MgO and proposed a new reaction mechanism, where BD
The Zr-BEA zeolite was synthesized following a post-synthetic literature
[
26]
procedure.
dealuminated by acid leaching (13 M HNO
leading to SiO /Al ratios >1500. The dealuminated zeolite was filtered
The parent zeolite (Zeolyst, SiO
2
/Al
2
O
3
= 25) was
-
1
3
, 20 mL g , 373 K, 20 h)
2
2 3
O
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ChemCatChem
10.1002/cctc.201700698
FULL PAPER
off, dried in an oven at 373 K overnight, followed by drying under vacuum
at 423 K for 8 h. 3 g of the sample was then added to a solution of 24 g
desired temperature for 15 minutes before a spectrum was taken. The
blank catalyst was subtracted and baseline correction was applied using
the OPUS software package (OPUS 7.2, Bruker Optik GmbH).
of ZrOCl
2 2
· 8 H O in 170 g of DMSO. This suspension was then heated to
4
03 K and held at this temperature for 12 h. After that, it was added to 1
L of deionized water, filtered off, rinsed with water and dried at 373 K
overnight. The catalyst was calcined under an air flow with a ramp of 3 K
The analysis of the spectra was performed using MATLAB codes that
were written in-house and Origin was used to generate the plots.
-
1
min to 473 K, where the temperature was held for 6 h, followed by
-
1
another ramp of 3 K min to 823 K, where the temperature was held for 6
h. The SiO was obtained from Evonik (Aerosil 300) and was used as
received. The silver was added in an incipient wetness impregnation from
an aqueous AgNO solution, followed by a calcination in air at 823 K for 6
2
Acknowledgements
3
h. The Ag-containing catalysts were activated by reduction under a
hydrogen flow for 30 minutes before the DRIFTS experiments.
Financial support from the University of Wisconsin - Madison, as
well as the Donors of the American Chemical Society Petroleum
Research Fund under Grant No. PRF 56136-ND5, is kindly
acknowledged.
Characterization
ICP-OES was used to determine the Ag and Zr content in the samples
after digesting the solids in HF. N
2
-physisorption measurements were
Keywords: butadiene • ethanol • operando spectroscopy •
performed on Micromeritics 3flex apparatus. The samples were
a
zeolite • Lewis acid
degassed prior to measurement for 4 h at 373 K. Adsorption isotherms
were collected at 77 K and analyzed using the BET method. DRUV-vis
analysis was performed with a Maya2000 Pro spectrometer (Ocean
Optics) equipped with a deuterium/halogen light source (DH-200-BAL
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ꢀ
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Philipp Müller, Shao-Chun Wang,
Samuel P. Burt, and Ive Hermans*
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Page No. – Page No.
Influence of Metal-Doping on the
Lewis-Acid Catalyzed Production of
Butadiene from Ethanol Studied by
Modulated Operando DRIFTS-MS
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