Hydrogen transfer reactions relevant to Guerbet coupling of alcohols over hydroxyapatite and magnesium oxide catalysts

Article abstract of DOI:10.1039/c7cy01393k

Hydrogenation and dehydrogenation reactions were performed over hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAP) and magnesia (MgO) to explore their role in the reaction network for the Guerbet coupling of ethanol to butanol. In particular, the dehydrogenation of benzyl alcohol at 633 K and the hydrogenation of ethene and acetone at 473 K using both H₂ and ethanol as a hydrogen source were studied. The H₂-D₂ exchange reaction at room temperature and the Guerbet coupling of ethanol at 613-673 K in the presence of D₂ were also performed. Although there was no consequence of adding D₂ to the Guerbet coupling of ethanol in terms of rate or selectivity, incorporation of deuterium into product butanol was only observed over MgO. This was attributed to the rapid exchange of H₂-D₂ that can occur over MgO but not over HAP. Hydrogenation of acetone occurred with ethanol as a sacrificial hydrogen donor via an MPV-like reaction whereas hydrogenation with H₂ was not observed. Hydrogenation of ethene with H₂ or ethanol was not observed above background. Comparing the rate of benzyl alcohol dehydrogenation to the rate of ethanol coupling over HAP and MgO suggests that the MPV-like hydrogen transfer reaction over HAP is mostly responsible for generating intermediate acetaldehyde during the Guerbet reaction instead of direct dehydrogenation.

Full text of DOI:10.1039/c7cy01393k

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Hydrogen Transfer Reactions Relevant to
Guerbet Coupling of Alcohols over
Hydroxyapatite and Magnesium Oxide
Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
Zachary D. Young and Robert J. Davis
www.rsc.org/
Hydrogenation and dehydrogenation reactions were performed over
4 6 2
hydroxyapatite (Ca10(PO ) (OH) , HAP) and magnesia (MgO) to explore their
role in the reaction network for the Guerbet coupling of ethanol to butanol.
In particular, the dehydrogenation of benzyl alcohol at 633 K and the
hydrogenation of ethene and acetone at 473 K using both H
a hydrogen source were studied. The H -D exchange reaction at room
temperature and the Guerbet coupling of ethanol at 613-673 K in the
presence of D were also performed. Although there was no consequence of
adding D to the Guerbet coupling of ethanol in terms of rate or selectivity,
incorporation of deuterium into product butanol was only observed over
MgO. This was attributed to the rapid exchange of H -D that can occur over
MgO but not over HAP. Hydrogenation of acetone occurred with ethanol as
sacrificial hydrogen donor via an MPV-like reaction whereas
hydrogenation with H was not observed. Hydrogenation of ethene with H
2
and ethanol as
2
2
2
2
2
2
a
2
2
or ethanol was not observed above background. Comparing the rate of
benzyl alcohol dehydrogenation to the rate of ethanol coupling over HAP
and MgO suggests that the MPV-like hydrogen transfer reaction over HAP is
mostly responsible for generating intermediate acetaldehyde during the
Guerbet reaction instead of direct dehydrogenation.
1
. Introduction
Catalytic reactions involving hydrogen have been well
1
studied for many years. Olefin hydrogenation , butene
–5
6
,7
8
isomerization , and H
commonly studied reactions involving hydrogen. The
mechanism for H activation is known to be different over
metal catalysts compared to basic metal oxides. While H is
generally accepted to dissociate into two hydrogen atoms on
2 2
-D exchange are all examples of
2
2
9
metal catalysts,
2
H activation either does not occur on basic
metal oxides such as MgO, or is heterolytically dissociated to a
7
hydride and a proton over surface acid and base sites,
respectively.
Many catalytic reactions involving hydrogen transfer can
be important to the rate and desired product selectivity in
more complex reaction networks. One example is the Guerbet
coupling of alcohols. The Guerbet reaction consists of coupling
two short chain alcohols to form a longer chain alcohol and
water. The Guerbet coupling reaction has experienced a surge
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in recent interest because it is a potential method to upgrade was used as a hydrogen source the formation of 1-butanol
biorenewable ethanol to butanol as shown in Scheme 1. For from crotonaldehyde via hydrogen transfer was observed.
more information on the Guerbet coupling reaction see the
The observation that H
2
does not change the rate or
1
recent reviews by Kozlowski and Davis and Gabriels et al.
0
11
selectivity of Guerbet coupling over HAP and the ability of an
Ethanol is a commonly used bio-renewable fuel additive, but it alcohol to hydrogenate crotonaldehyde over HAP suggests
has many drawbacks such as lower energy density and higher that hydrogen transfer occurs through a Meerwein-Ponndorf-
1
2
corrosivity when compared to gasoline . Guerbet coupling has Verley-like (MPV-like) mechanism. The MPV reaction consists
been studied over many different acid-base catalysts including of the hydrogenation of an aldehyde or ketone through direct
1
3–15
16,17
18
, hydrotalcites , hydrogen transfer from a sacrificial alcohol. In the case of
magnesium oxide
, mixed metal oxides
1
cation exchanged zeolites , and metal phosphates
9
20–24
.
Guerbet coupling, an MPV-like hydrogenation of adsorbed
Additionally, the Guerbet coupling reaction has been carried intermediate crotonaldehyde by ethanol would produce crotyl
out using a heterogeneous metal catalyst coupled with a alcohol and acetaldehyde. This type of hydrogen transfer
2
5,26
homogeneous base catalyst
HAP, Ca10(PO (OH) ) has been shown to be the most active Hall many years ago. In that work, 2-butanol-2d
catalyst with high selectivity toward butanol. with 3-pentanone to form 3-pentanol-3d at 423 K. Moteki and
.
Of these, hydroxyapatite reaction was shown to occur rapidly over HAP by Kibby and
4
3
(
4
)
6
2
1
was reacted
1
2
3
A commonly-accepted mechanism for Guerbet coupling, Flaherty used isotope experiments to show that MPV-like
shown in Scheme 2, consists of a sequence of hydrogen hydrogen transfer saturates the C=O bond of butanal over
transfer and carbon-carbon coupling steps. First, ethanol is HAP. Those authors claimed that the C=C bond of crotyl
dehydrogenated to acetaldehyde, which subsequently, alcohol was hydrogenated by surface hydrogen. Although
undergoes a self-aldol condensation to form crotonaldehyde there are excellent studies of hydrogen transfer reactions, it is
(2-butenal). Crotonaldehyde is then hydrogenated to form the still unclear if the sacrificial alcohol transfers hydrogen directly
saturated product 1-butanol. While this mechanism is to the carbonyl group, or if this hydrogen transfer is mediated
2
2–24,27
supported by many studies
mechanism whereby ethanol self-condenses or condenses
with an acetaldehyde molecule has also been the reaction is autocatalytic, during which butanol is produced
, a direct condensation by the catalyst surface.
Prior work on the Guerbet coupling of ethanol suggest that
13,15,19,28
proposed
.
in great excess beyond the small amount of acetaldehyde
2
2,23
Although many studies have been performed on the produced by dehydrogenation of ethanol to produce H
1
overall Guerbet coupling reaction
2
.
4,15,19–24,27
,
the key However, if ethanol provides H atoms to hydrogenate
elementary steps that comprise the reaction path need to be adsorbed intermediates produced from aldol condensation,
understood to facilitate catalyst design and optimization. In then intermediate acetaldehyde is produced in the
particular, aldol condensation has been studied over catalysts appropriate stoichiometric amount. The direct MPV-like
2
9–36
that have demonstrated activity for Guerbet coupling
. Our hydrogen transfer from ethanol to adsorbed crotonaldehyde
previous work on the aldol condensation of acetaldehyde over starts a reaction cascade that will result in the eventual
HAP and MgO showed that surface reactions such as enolate production of one molecule of 1-butanol together with the
formation are very rapid while the adsorption of acetaldehyde formation of two molecules of acetaldehyde that were
2
9
as well as product desorption are kinetically relevant . consumed in the aldol condensation reaction.
However, the role of hydrogenation and dehydrogenation
The role of surface hydrogen in hydrogenation reactions of
2
,5,23
reactions during Guerbet coupling over HAP and MgO are alkenes has also been studied
. For example, ethene
much less understood, despite being critical steps in the initial hydrogenation can occur on MgO that has been thermally
2
,4,5,44
dehydrogenation of ethanol to acetaldehyde as well as the treated at very high temperatures (>1000 K)
final hydrogenation reactions of crotonaldehyde to butanol. reaction occurs on sites capable of heterolytically dissociating
Indeed, the importance of hydrogen transfer reactions during that are exposed during the high temperature treatment. A
Guerbet coupling has been demonstrated by the promotion of surface hydride attacks the adsorbed olefin resulting in a
. The
H
2
3
7–39
activity by adding metal components to the catalyst
. The carbanion that is subsequently protonated by a surface
7
presence of the added metal is thought to increase the rate of proton . While this reaction can take place under
Guerbet coupling by facilitating the initial dehydrogenation of temperatures relevant to Guerbet coupling, it was also shown
ethanol.
that the introduction of water to the thermally-treated MgO
In the absence of metal promoters, ethanol destroys the sites that are active for heterolytic hydrogen
4
4
dehydrogenation on basic metal oxides typically takes place at dissociation .
relatively high temperatures and is thought to proceed Other reactions that involve surface hydrogen have also
through an E1cB mechanism . Moteki and Flaherty as well as been studied on basic metal oxides, namely 1-butene
4
0
23
4
1
2 2 2
Hanspal et al. have shown that co-feeding H during the isomerization and H -D exchange. The isomerization reaction
Guerbet coupling reaction over HAP has no effect on the rate is thought to occur through the abstraction of an allylic
or selectivity at 548 K and 613 K, respectively. Additionally, hydrogen resulting in an allyl anion. This anion is then
4
2
2
Ogo et al. have shown that H is unable to hydrogenate protonated by a surface proton resulting in a mixture of 1-
7
crotonaldehyde to butanol at 573 K. However, when 2-butanol butene, cis-2-butene, and trans-2-butene . Since crotyl alcohol
has a similar electronic structure to butene, it is feasible that
2
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on basic metal oxides double bond isomerization could
produce an enol, which would rapidly tautomerize to adsorbed BET adsorption method on a Micromeritics ASAP 2020 at liquid
butanal. The H -D exchange reaction was studied by Boudart temperature. Both HAP and MgO had a surface area of 35
2
The catalyst surface area was determined with N using the
2
2
N
2
8
2 -1
et al. who demonstrated exchange occurs at 78 K on MgO m g .
that was thermally-activated between 773 and 1173 K in
vacuum. The proposed mechanism for H
2
-D
2
exchange on MgO
with a surface
2
.3 Catalytic Reactions
involves reaction of molecularly-adsorbed D
2
hydroxyl group through a triangular transition state. Unlike the 2.3.1 Guerbet Coupling in the Presence of H
/D
2
2
case of olefin hydrogenation on MgO, hydride is not present
Guerbet coupling was carried out in the presence of both
H2 (GTS-Welco, 99.999%) and D2 (GTS-Welco, 99.7%) to
determine if gas phase or surface hydrogen could be
incorporated into the product. These isotopic studies were
carried out using a downward-flow, fixed-bed, ¼” O.D.
stainless steel reactor that was connected to an on-line mass
spectrometer and an on-line gas chromatograph (GC). The
catalyst bed (HAP or MgO) was supported in the reactor on
during the exchange reaction.
In an attempt to better understand the role of hydrogen
transfer reaction on the Guerbet coupling reaction, this paper
investigates relevant hydrogenation and dehydrogenation
reactions over HAP and MgO. In particular, benzyl alcohol
dehydrogenation was chosen as a probe reaction because it
has an α-hydrogen, which allows the dehydrogenation
reaction to occur, but it does not have a β-hydrogen, which
prevents aldol condensation of the aldehyde product.
Preventing aldol condensation is important because higher
levels of aldehyde formation would be measured if the alcohol
is being oxidized during an MPV-like reaction with the aldol
condensation product. Hydrogenation of acetone with either
3
-1
quartz wool and was thermally treated in 50 cm min flowing
argon (GTS-Welco, 99.999%) at 773 K prior to the reaction.
Ethanol (Sigma-Aldrich, 99.5%) was introduced to the feed by
flowing H2 through a saturator at room temperature. The
ethanol/H stream was then passed over the catalyst. After the
2
reaction came to steady-state, fifteen mass spectra of the
product stream were collected. The carrier gas flowing through
H
2
or ethanol was evaluated over HAP and MgO to explore the
relative importance of MPV-like hydrogen transfer on the
catalysts. In addition, the rate of the H -D exchange reaction
as well as the effect of H or D on the Guerbet coupling of
ethanol over HAP and MgO were evaluated.
the saturator was then switched to D and allowed to pass
2
2
2
over the catalyst for 1 h before reaction rates were measured
using an online GC equipped with a flame ionization detector
and a Restek MXT-Q-Bond column (0.53 mm i.d., 30 m length).
Subsequently, fifteen mass spectra were collected. Finally, the
carrier gas was switched to He and the ethanol conversion
rates were measured. A small amount of methane was
introduced to the reactant stream to serve as an internal
standard.
2
2
2. Materials and Methods
2.1 Catalyst Synthesis
Hydroxyapatite was synthesized using a co-precipitation
4
5
method originally developed by Tsuchida et al. and described
Product selectivity and conversion were calculated as
2
4,29
3
. A 200 cm solution of 0.5 M calcium
in previous works
nitrate tetrahydrate (Ca(NO
ꢄ ꢆ
ꢅ
ꢅ
ꢀ
ꢁꢂꢁꢃ. =
3
)
2
•4H
M
2
O, Acros Organics) and a
∑
ꢄ ꢆꢅ
ꢅ
3
00 cm solution of 0.3
2
diammonium phosphate
3
∑ ꢄ ꢆꢅ
ꢅ
(
(NH ) HPO
4
2
4
, Aldrich, >99.99%) were slowly added to 100 cm
ꢇꢈꢆꢉ. =
2
ꢆꢊꢋꢌꢍ
of distilled, deionized (DDI) water at 353 K. The slurry was aged
over night before being filtered and washed three times with
DDI water. The solid was subsequently dried in air at 373 K
before being thermally treated at 673 K for 2 h in flowing air.
Commercially-available MgO was purchased from Ube
Material Industries (Lot No. 109071725) and was thermally
treated at 673 K for 2 h in flowing air. The particles of both
HAP and MgO were sized between 106-180 μm prior to
catalysis.
i i
where N is the number of carbons in the product, n is the
molar flow rate of the product, and nEtOH is the molar flow rate
of the reactant ethanol.
2
.3.2 H
The H
reactor as described above for the Guerbet coupling in the
presence of H /D . During these reactions, an equimolar
mixture of H and D
the equimolar H /D
2
-D
2
Exchange
2
2
-D exchange reaction was performed in the same
2
2
2
2
was diluted in Ar. The partial pressure of
2
reaction mixture was varied while the
2.2 Catalyst Characterization
2
Phase purity was determined using XRD with a PANalytical
total pressure was maintained at 1 atm. The catalyst bed
either HAP or MgO) was supported on quartz wool and was
X’Pert Pro diffractometer using Cu-Kα radiation. These
materials have been used in previous studies and XRD patterns
(
3
-1
pretreated at 773 K in 100 cm min Ar for 1 h prior to the
reaction. The H /D /Ar mixture was then passed over the
2
can be found elsewhere .
9
2
2
Elemental analysis of HAP was performed by Galbraith
Laboratories (Knoxville, TN) using ICP-OES. The catalyst was
found to be very nearly stoichiometric with a measured Ca/P =
catalyst at room temperature and the product stream was
analyzed using a mass spectrometer measuring m/z=2,3,4 and
4
0.
1.67.
2
.3.3. Benzyl Alcohol Dehydrogenation
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A downward-flow, fixed-bed, ¼” O.D. stainless steel reactor reports showing a negligible influence of H
2
on coupling
with an on-line GC was used for the benzyl alcohol kinetics. Moreover, there is no kinetic isotope effect involving
dehydrogenation reaction. The catalyst bed was supported on
H
2
and D
The mass spectra of butanol produced over HAP and MgO in
and D diluent gas are shown in Figure 1. For butanol
alcohol (Sigma-Aldrich, 99.8%) was introduced to the system production over HAP, the mass spectrum changes very little
with a syringe pump and was vaporized in a high temperature after the switch from H to D . In contrast, the intensity of the
mixing chamber together with flowing N . A small amount of butanol peaks at m/z = 57 and 58 increases significantly when
methane (1 cm min ) was added to the product stream MgO was used as the catalyst, indicating a fraction of D from
downstream of the reactor as an internal standard. The the gas phase incorporates into the butanol product, although
reaction products were analyzed with an on-line gas there is no kinetic influence of D on the reaction.
chromatograph using a PoraPLOT Q-HT column and a flame Incorporation of deuterium into butanol can result from direct
ionization detector. hydrogenation of C=C and/or C=O by D or exchange of
2
.
3
quartz wool and was pretreated at 773 K for 1 h in 100 cm
-1
min
N
2
(GTS-Welco, 99.999%) prior to reaction. Pure benzyl
H
2
2
2
2
2
3
-1
2
2
2
2
.3.4 Hydrogenation of Acetone and Ethene
deuterium with surface bound hydrogen atoms that eventually
incorporate into butanol. From the mass spectrum in Figure
The same flow reactor and catalyst pretreatment
1B, we estimate that 31% of the butanol had no deuterium,
conditions described for the benzyl alcohol dehydrogenation
reaction were used for the hydrogenation reactions. When
acetone was hydrogenated using ethanol as a hydrogen
source, a 95 wt% ethanol, 5 wt% acetone (Sigma-Aldrich,
whereas 36%, 27%, and 6% had one D, two D, and three D
atoms substituted into butanol, respectively. The lack of
deuterium incorporation into the product butanol produced
over HAP suggests that direct hydrogenation of C=C or C=O
9
9.9%) solution was fed to the high temperature mixing
chamber together with flowing N
When ethene was hydrogenated using ethanol as a
hydrogen source, a N /ethene (ethene from GTS Welco 99.5 %,
kPa ethene, 1 atm total pressure) gas mixture was passed
2
bonds of intermediate crotonaldehyde using H as a hydrogen
2
.
source does not take place to any significant extent.
The significant difference in the isotopomer distribution of
butanol produced over HAP and MgO motivated a study of the
2
4
H
2
-D
2
exchange reaction as shown in Scheme 3. The MgO
through the high temperature mixing chamber with a 95 wt%
ethanol/5 wt% octane solution. Octane was used as an internal
standard.
catalyst is known to be very active for this reaction at low
8,16
temperatures . Indeed, we observed the H
2 2
-D exchange
reaction occurred readily at 298 K on MgO with a rate of 1.3 x
-2 -1
2 2
To explore the reaction of acetone with H , pure H carrier
-5
1
2
0 mol m s when the H partial pressure was 2 kPa. The
gas was passed through an acetone saturator that was
submerged in a dry ice/acetone bath, which should produce
reaction was so rapid that conversion was ~30 % at all
pressures tested, which is 83 % of the calculated equilibrium
0
.02 % acetone in H
the reactor as an internal standard. For ethene hydrogenation
with H a mixture of ethene/H feed (4 kPa ethene, 1 atm total
pressure) was passed over the catalyst.
2
. Methane was introduced downstream of
4
conversion for the reaction (feq = 36 %, Keq = 3.2) . The rate of
6
8
H
2
-D
for an MgO sample calcined at ~650 K can be extrapolated to
98 K and 2 kPa H using their reported activation energy. The
2
exchange reported by Boudart et al. measured at 78 K
2
2
2
2
8
-5
-2 -1
extrapolated rate from Boudart et al. of ~1.5 x 10 mol m s
is similar to our measured rate and this agreement suggests
that the MgO catalyst used in the current study exposes similar
3
. Results and Discussion
2 2
3.1 Guerbet Coupling in the Presence of H /D
active sites for H
2
-D
Dihydrogen is a byproduct that can be produced during the Boudart et al. They suggest the reaction proceeds through an
Guerbet coupling reaction by alcohol dehydrogenation. Eley-Rideal like mechanism in which a proton on the catalyst
Although H is not a product of the coupling of ethanol into surface reacts with a weakly adsorbed D molecule through a
butanol, one molecule of H
2
exchange as the catalyst prepared by
8
2
2
8
2
is produced for every molecule of triangular transition state to produce a surface deuteron and
acetaldehyde, which is a common side product during the gas phase HD. For our work on Guerbet coupling of ethanol,
Guerbet coupling reaction. Our group and others have shown we suggest the surface deuteron produced via surface
that adding gaseous H
2
2
has no effect on the rate of the ethanol exchange with the D could then add to a carbanion that is
coupling reaction or the selectivity towards butanol over formed during the coupling reaction sequence. In this way,
2
3,41,42
HAP
that H
. These observations do not preclude the possibility deuterium can incorporate into product butanol.
interacts with the catalyst surface and can incorporate
As no H -D exchange was observed over the HAP catalyst
2
2
2
into product molecules. To explore this, the Guerbet coupling at 298 K, it is not surprising that no deuterium is incorporated
of ethanol was performed in the presence of dideuterium gas. into butanol during Guerbet coupling of ethanol.
The rate of ethanol conversion using He, H , or D as a diluent 3.2 Hydrogenation Reaction
2 2
are shown in Table 1. The Guerbet reaction was carried out at
low conversion in all cases (≤ 5%). Both the rate and selectivity
Results described in the previous section indicate that
neither HAP nor MgO are able to directly hydrogenate C=C or
C=O bonds of reactive intermediates formed during Guerbet
were relatively unaffected by the presence of H
similarity in rate and selectivity among H2, , and He as a
diluent gas over each catalyst is consistent with previous
2 2
or D . This
D
2
2
coupling using gas phase H . Potential reactions of ethene and
4
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2
acetone in the presence of H were then examined to further that the excess acetaldehyde that was observed during the
probe the hydrogenating ability of the catalyst. No 2-propanol reaction over MgO is due to either background
was formed when H and acetone were co-fed over either dehydrogenation of ethanol to acetaldehyde or to the transfer
catalyst at 473 K. However, C products were observed at 473 hydrogenation of C carbonyl compounds produced by aldol
K due to the self-aldol condensation of acetone. When ethene condensation of acetone.
and H were passed through a blank reactor at 473 K When ethene and ethanol were passed over both HAP and
containing no catalyst, a small amount of ethane was observed MgO at 473 K, no ethane formation was observed, indicating
~3 % ethene conversion). This background hydrogenation C=C bonds were not readily hydrogenated by either ethanol or
reaction was most likely catalyzed by the stainless-steel under our reaction conditions. This observation suggests
2
6
6
2
(
H
2
reactor. When the reaction was carried out in the presence of that the hydrogenation of intermediate crotonaldehyde
HAP and MgO, no conversion of ethene above the background proceeds through MPV-like alcohol transfer hydrogenation of
was observed. The hydrogenation of ethene by H
2
over MgO the C=O bond to form crotyl alcohol, followed by a double
has been previously reported to occur over catalysts that had bond isomerization reaction and keto/enol tautomerization to
4
,5,44
been pretreated at very high temperatures (>1000 K)
Formation of strong basic sites that were capable of form the saturated alcohol 1-butanol. The double bond
heterolytically dissociating H was proposed to account for the isomerization of butene will take place at water-ice
.
form butanal, and final MPV-like C=O bond hydrogenation to
2
4
7
need of high temperature treatment. The hydride that was temperatures over MgO and we have shown that enolate
formed during the heterolytic dissociation then attacks the formation occurs rapidly during the aldol condensation of
2
olefin resulting in a carbanion that would be subsequently acetaldehyde indicating that both of these reactions will be
9
7
protonated . The milder pretreatment condition used to very rapid for temperatures relevant to Guerbet coupling. This
activate the MgO in the current study is apparently unable to sequence of transfer hydrogenations from ethanol will
form sites that are active for olefin hydrogenation. It should be produce two molecules of acetaldehyde that can undergo an
noted, however, the background hydrogenation rates that aldol condensation step to regenerate the crotonaldehyde that
were observed in our reactor are an order of magnitude was converted to butanol. The proposed reaction path is
greater than the hydrogenation rates that were reported in consistent with the autocatalytic mechanism that was
4
22
the previous studies . As such, it is impossible for us to proposed by Ho et al.
completely rule out the possibility that some C=C bonds of The MPV-like C=O bond hydrogen transfer reaction was
reactive intermediates produced during Guerbet coupling are also proposed by Ogo et al. and Moteki and Flaherty . In the
4
2
23
4
2
being hydrogenated by these catalysts. Based on the lack of study by Ogo et al. crotonaldehyde was reacted with both H
deuterium incorporation into butanol over HAP (Figure 1), it and 2-butanol present as a hydrogen source, and the authors
seems unlikely that a H /D heterolytic dissociation is found that H was unable to hydrogenate crotonaldehyde at
2
2
2
2
kinetically relevant. Additionally, the presence of water was 573 K over a strontium substituted HAP, while the reaction
shown previously to destroy the strongly basic sites required between 2-butanol and crotonaldehyde lead to similar
for heterolytic H
Guerbet coupling, it is unlikely that these sites would exist and Flaherty
under Guerbet coupling conditions. hydrogenation in the presence of fully deuterated 2-propanol
commonly-proposed path for C=O hydrogenation and t-butanol with four times as much t-butanol present as 2-
2
dissociation. Because water is a product of products as the overall Guerbet coupling of ethanol. Moteki
2
3
studied butanal and crotyl alcohol
A
2
2,23
involves an MPV-like mechanism
8
where hydrogen is propanol-d . In this way, the surface of the catalyst will be
transferred directly from a sacrificial alcohol to the C=O bond. populated in both hydrogen and deuterium due to the
Therefore, hydrogenation of C=O and C=C bonds using ethanol decomposition of adsorbed alcohols into alkoxides. The three
as a hydrogen source was also tested. In the case of ethanol to one ratio of unlabeled t-butanol to fully deuterated 2-
reacting with crotonaldehyde, this would result in the propanol will result in a surface that has a three to one ratio of
formation of acetaldehyde and crotyl alcohol. To explore this hydrogen to deuterium. If the hydrogenation reaction takes
type of transfer hydrogenation, the reaction of acetone to 2- place predominantly through direct hydrogen transfer from
propanol was performed over both HAP and MgO at 473 K deuterated 2-propanol the mass spectrum of the product will
using ethanol as the sacrificial alcohol as shown in Scheme 4. be shifted 1 amu. In contrast, if hydrogen originates from the
When excess ethanol was co-fed with acetone, the formation surface, there will not be a shift in the mass spectrum. During
of 2-propanol was observed over both HAP and MgO, and the reaction of butanal under these conditions there was a 1
Figure 2 shows the rate of 2-propanol formation at a constant amu shift in the butanol mass spectrum indicating that a
ratio of acetone to ethanol diluted in N
observed over both HAP and MgO, which makes a direct to butanal. Interestingly, the hydrogenation of crotyl alcohol
comparison of rates difficult, but it is clear that both catalysts with 2-propanol-d did not shift the mass spectrum of the
are able to hydrogenate acetone using ethanol as a hydrogen product butanol, which lead the authors to conclude that the
source. Over MgO, some C products were observed and more C=C bond of crotyl alcohol was hydrogenated by a surface
acetaldehyde was produced than 2-propanol over this catalyst. hydrogen atom. The conclusion that C=C is hydrogenated is
No C products were observed over HAP and a similar amount not supported by the results of the current work and
2 8
. Deactivation was deuterium atom was transferred directly from 2-propanol-d
8
6
6
of acetaldehyde and 2-propanol were observed. It is possible
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additional studies are underway to understand the nature of
this inconsistency.
4
. Conclusions
The addition of H
2
or D
2
to the Guerbet coupling of ethanol
The observation by Moteki and Flaherty that there is
almost no deuterium incorporation while hydrogenating the
two carbon atoms in the C=C of crotyl alcohol in the presence
of 20% D coverage on the surface is intriguing. Since the
overall hydrogenation reaction should involve the
simultaneous dehydrogenation of deuterated 2-propanol (t-
butanol cannot dehydrogenate), the lack of appearance of any
D in the product indicates there is significant H-D exchange
with the surface on the time scale of the reaction. If the crotyl
alcohol in the Moteki and Flaherty experiment undergoes the
double bond isomerization and keto-enol tautomerization as
proposed in our work, the final C=O hydrogenation reaction
may involve surface H atoms.
over HAP and MgO had no effect on rate or selectivity of the
reaction toward butanol. The partial incorporation of
deuterium into the product butanol produced over MgO is
attributed to the deuteration of the hydroxyls of MgO through
2 2 2
rapid exchange with D . Indeed, rapid H -D exchange occurred
on MgO at room temperature but was not observed on HAP.
Both MgO and HAP catalyzed the MPV-like hydrogen transfer
from ethanol to acetone at 473 K, but neither catalyst could
hydrogenation acetone with H
to ethane was not observed above background conversion at
73 K using H and ethanol as hydrogen sources. Evidently, all
2
. The hydrogenation of ethene
4
2
of the hydrogen transfer reactions during Guerbet coupling
involve MPV-like reactions between alcohols and carbonyls.
No evidence for C=C hydrogenation was observed, which
3.3 Benzyl Alcohol Dehydrogenation
Although an autocatalytic cycle that converts ethanol to suggests hydrogenation via MPV-hydrogen transfer steps likely
butanol is consistent with Guerbet coupling kinetics, formation undergo a double bond shift to form an enol, which is
of acetaldehyde via dehydrogenation must also take place to tautomerized to a carbonyl. The rate of benzyl alcohol
initiate the autocatalytic sequence. To study this initiation dehydrogenation was lower than the rate of ethanol coupling
reaction, the dehydrogenation of benzyl alcohol, shown in over HAP, consistent with MPV-like conversion of ethanol to
Scheme 5, was performed. The dehydrogenation of benzyl acetaldehyde being a major source of the acetaldehyde
alcohol occurred readily over both HAP and MgO with little needed to form butanol.
deactivation and no observed side products. Figure 3 shows
the rate of reaction over both HAP and MgO at 633 K and
Conflicts of Interest
different alcohol partial pressures. The MgO catalyst was more
active on an area basis for alcohol dehydrogenation than HAP.
The rate of reaction is fit well by a Langmuir-Hinshelwood rate There are no conflicts of interest to declare.
equation derived from a two-step model consisting of
equilibrated dissociative adsorption of the alcohol followed by
a rate determining unimolecular surface reaction, which is Acknowledgements
most likely C-H bond activation of the adsorbed alkoxide
intermediate. This is consistent with an E1cB mechanism that
4
0
has been proposed elsewhere .
This work was supported by the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Sciences, Office of Science, U.S. Department of Energy, grant
no. DE-FG02-95ER14549.
Over MgO, the rate of benzyl alcohol dehydrogenation is
higher than the rate of Guerbet coupling of ethanol, despite
the benzyl alcohol reaction being carried out 40 K lower, which
may result from the presence of the aromatic ring. No Guerbet
coupling of ethanol was observed over MgO at 633 K with
similar contact times. Over HAP, the rate of benzyl alcohol
dehydrogenation (Figure 3) is lower than the overall rate of
ethanol Guerbet coupling shown in Table 1 despite the
dehydrogenation reaction being carried out at a temperature
Notes and references
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Scheme 1. The overall Guerbet coupling reaction of
ethanol to butanol and water.
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Scheme 2. The Guerbet coupling reaction network
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2 2
Scheme 3. The H -D exchange reaction.
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Scheme 4. The overall MPV-like hydrogen transfer
reaction from ethanol to acetone.
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Scheme 5. Benzyl alcohol dehydrogenation.
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Table 1. Influence of H or D on Rate and
2
2
Selectivity of Ethanol Conversion over HAP and
(
a)
MgO
Ethanol
Diluent Conversion
Butanol
Selectivity
(C%)
T
K)
Catalyst
(
Gas
Rate (mol
-
2 -1
m s )
-
8
8
8
8
8
8
HAP
HAP
HAP
MgO
MgO
MgO
613
613
613
673
673
673
He
5.0 x 10
6.4 x 10
7.2 x 10
2.5 x 10
2.5 x 10
2.7 x 10
65
61
63
17
23
24
-
-
-
-
-
H
D
2
2
He
H
D
2
2
(
a) Partial pressure of ethanol = 6 kPa,
remainder diluent gas to give 101 kPa total
pressure. All ethanol conversion levels were
<
5%. Catalyst loading = 0.05 g for HAP
3
-
and 0.2 g for MgO. Flow rate = 50 cm min
1
.
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Figure 1. Mass spectra of the butanol carbon backbone over A) HAP
and B) MgO during the Guerbet coupling reaction in the presence of
2 2
H or D carrier gas.
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Figure 2. The rate of MPV-like hydrogenation over HAP and MgO at
73 K. The reaction was carried out at atmospheric pressure with an
acetone to ethanol molar ratio of 0.07 at all acetone partial
4
3
-1
pressures. Catalyst loading = 0.2 g Flow rate = 25 cm min .
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Figure 3. The rate of benzaldehyde formation over HAP and MgO at
33 K. The reaction was carried out at atmospheric total pressure
with N as a diluent gas. Catalyst loading = 0.05 g. Total flow rates
were between 200 – 400 cm min .
6
2
3
-1
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Table of Contents
The rates of double bond hydrogenation, deuterium exchange, and benzyl alcohol dehydrogenation
were compared to those of ethanol coupling.