Kinetics of the Homogeneous and Heterogeneous Coupling of Furfural with Biomass-Derived Alcohols

Article abstract of DOI:10.1002/cctc.201701866

The tandem dehydrogenation and aldol condensation of butanol with furfural was investigated over homogeneous and heterogeneous catalysts using kinetics and isotope effects. In the homogeneous system, Ni(dppe)Cl₂ catalyzes the transfer dehydrogenation of butanol to the furfural, whereas the aldol condensation of butyraldehyde and furfural takes place over the basic K₂CO₃ cocatalyst. In the heterogeneous system, a transition-metal-free mixed Mg–Al oxide, both the transfer hydrogenation and aldol condensation take place over the basic sites of the catalyst, and the rate-determining step is the alpha-hydride transfer from the butanol to the furfural.

Full text of DOI:10.1002/cctc.201701866

Accepted Article
Title: Kinetics of the Homogeneous and Heterogeneous Coupling of
Furfural with Biomass-derived Alcohols
Authors: Konstantinos Antonios Goulas and Amit A Gokhale
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Kinetics of the Homogeneous and Heterogeneous Coupling of
Furfural with Biomass-derived Alcohols
Konstantinos A. Goulas*[a,b] Amit A. Gokhale[a,c]
Abstract: The tandem dehydrogenation and aldol condensation of
butanol with furfural was investigated over homogeneous and
heterogeneous catalysts using kinetics and isotope effects. In the
homogeneous system, Ni(dppe)Cl2 catalyzes the transfer
dehydrogenation of butanol to the furfural, while the aldol
condensation of butyraldehyde and furfural takes place over the basic
K2CO3 cocatalyst. In the heterogeneous system, a transition-metal-
free mixed Mg-Al oxide, both the transfer hydrogenation and aldol
condensation take place over the basic sites of the catalyst, with the
rate-determining step being the alpha-hydride transfer from the
butanol to the furfural.
use of acidic catalysts to form C-C bonds between alkylfurans and
aldehydes,[9] and the use of basic catalysts for the aldol
condensation of aldehydes and ketones to longer-chain
molecules.[10] Subsequent hydrodeoxygenation of these species
affords alkanes and cycloalkanes that can be directly blended with
petroleum-derived fuels.[11] Basic catalysts have also been used
to increase the carbon chain length, via aldol condensation
12]
reactions.
Among
other
molecules,
acetone,[6,
cyclopentanone[13] and angelica lactone[14] have been used to
provide acidic protons that can undergo enolization and
subsequent C-C bond formation to give long-chain alkanes after
hydrodeoxygenation.[15]
In the recent years, alkylation of furan and furfural, to produce
chemicals and fuels has also been an area of increased interest.
For example Koehle et al. reported on the acylation of furans with
acetic anhydride over acidic zeolites and the subsequent
hydrodeoxygenation of the products to form precursors to p-
methylstyrene and p-divinylbenzene.[16] Park et al. reported on the
synthesis of long-chain alkylfurans using lauric acid and
trifluoroacetic anhydride, and their subsequent sulfonation to form
renewable surfactants.[17] By employing homogeneous and
heterogeneous Ni-based catalysts, in a previous work, our group
showed that water-sensitive anhydrides are unnecessary in the
aldol condensation of furfural with biomass-derived alcohols to
form unsaturated long-chain furylaldehydes (Scheme 1).
Subsequent hydrodeoxygenation reactions form the desired
gasoline, jet and diesel-range blendstocks. Based on life-cycle
analysis, this approach was reported to achieve net greenhouse
gas decreases of 53% to 79%, compared to conventional fuels.[18]
Introduction
It has now been well established that the rise in atmospheric CO2
levels over the last two centuries have led to global warming. In
both, the developed and developing economies, the
transportation sector contributes significant proportion of the
overall greenhouse gas (GHG) emissions. Therefore, reduction
of GHG emissions, primarily CO2, from the transportation sector
is a cornerstone in our quest to mitigate the impacts of climate
change. To attain this goal, the incorporation of renewable fuels,
such as those derived from biomass, into the fuels value chain is
necessary as part of a move to a sustainable economy. Many of
the processes for conversion of biomass into fuels or chemicals
often employ chemical catalysis[1] or fermentation[2] as an
intermediate step to obtain platform molecules. Some platform
molecules that have garnered attention are the C5 and C6 sugar-
derived aldehydes such as furfural and 5-hydroxymethylfurfural,
O
OH
O
3]
which can be produced using homogeneous[1a,
or
O
O
O
+
+
2
OH
heterogeneous catalysis.[4] Similarly, fermentation of sugars can
yield short-chain alcohols,[5] ketones[6] and carboxylic acids[7]
which may also be used as platform molecules.
1
The synthesis of high value-added fuels, such as diesel (C13-C26)
and jet fuel (C9-C14), from platform molecules requires the
formation of new C-C bonds, as most platform molecules are in
the C2-C6 range.[8] In these efforts, researchers have reported the
Scheme 1. Reaction of furfural with butanol to form furfuryl alcohol and the
unsaturated aldehyde 1.
While industrial scale-up of the heterogeneous systems perhaps
makes more sense, this is a unique system where both
homogeneous as well as heterogenous catalysis seem to work
quite well but most likely follow different mechanisms. In order to
compare the relative advantages of heterogeneous and
homogeneous catalysis, kinetic studies are quite useful and with
that motivation, we undertook this study to better understand the
condensation of furfural with alcohols. In this work, liquid-phase
and gas-phase kinetic experiments were carried out, in
combination with kinetic isotope effect investigations to propose a
mechanism for the homogeneous and heterogeneous furfural
condensation with aliphatic alcohols.
[a]
Dr. K. A. Goulas, Dr. A. A. Gokhale
Energy Biosciences Institute
University of California, Berkeley
Berkeley, CA 94720, USA
E-mail: goulas@udel.edu
Dr. K. A. Goulas,
Catalysis Center for Energy Innovation
University of Delaware
Newark, DE 19711, USA
Dr. A. A. Gokhale
[b]
[c]
BASF Corporation
Iselin, NJ 08830, USA
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201701866
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Results and Discussion
Table 1. Control experiments for the condensation of furfural with butanol
over Ni(dppe)Cl2 and K2CO3. 1 mmol C4 aliphatic oxygenates, 2 mmol
furfural, 413 K, toluene solvent.
First, the kinetics of the furfural-butanol coupling reaction over the
Ni(dppe)Cl2 catalyst and K2CO3 co-catalyst were investigated in
the liquid phase. This study revealed an induction period over the
first two hours of reaction (Figure 1). During this phase of the
reaction, no addition product 1 was formed, but butanol was
consumed. This suggests that some intermediate must be formed
during the induction phase. One possibility for the intermediate is
an active form of the catalyst obtained via replacement of Cl-
ligands with butoxide. Such a mechanism would consume 10% of
the butanol from our reaction mixture. However, we find that close
to 20% of butanol is consumed by the end of the induction period.
While this does not negate the possibility of Cl- ligand replacement,
it strongly suggests that the intermediate is a species along the
reaction pathway. Incidentally, this is also consistent with the
formation of butyraldehyde observed by GC during the induction
period.
Catalyst
Reactants
Yield 1
C4
(%)
conversion
Ni(dppe)Cl2, K2CO3
Ni(dppe)Cl2, K2CO3
Ni(dppe)Cl2, K2CO3
K2CO3
Butanol, furfural
79
81
78
77
10
88
Butyraldehyde, furfural
Butanol, furfural, BHT
Butyraldehyde, furfural
Butanol, furfural
100
85
100
22
K2CO3
The direct dehydrogenation of butanol to butyraldehyde and
hydrogen via metal hydride-mediated mechanism was
a
investigated. Such mechanisms have also been invoked as
intermediates for the dehydrogenation of alcohols to aldehydes[21]
and are the dominant mechanism of alcohol dehydrogenation
over heterogeneous catalysts. Moreover, Ni complexes are
known to form hydrides. In order to investigate the possibility of
Ni hydrides being intermediates of the reaction, the headspace of
the reaction was sampled and analyzed using GC/TCD. The
absence of hydrogen gas in this experiment is an indication that
a hydride mechanism is not active in this reaction. Instead of a Ni-
hydride mechanism, we propose that the Ni complex acts as a
Lewis acid[22] to coordinate the furfural, which receives a hydride
from a Ni n-butoxide intermediate. Such butoxides may be formed
by the deprotonation of alcohol by the base (K2CO3) and the
substitution of one or two chloride ligands on the Ni.
100
80
60
Yield (%)
BuOH remaining
40
20
0
0
5
10
15
20
Time (hr)
Figure 1. Liquid-phase reaction progress study of the addition of butanol to
furfural. 1 mmol butanol, 2 mmol furfural, 1 mL toluene, 413 K. 26.4 mg
Ni(dppe)Cl2, 49.6 mg K2CO3. Dashed lines represent qualitative trends.
Control experiments indicate that no reaction was observed with
butanol and furfural in the absence of the Ni complex, suggesting
that its primary role lies in the dehydrogenation of the butanol
(Table 1). Consistent with earlier reports,[19] the condensation of
the furfural with the butyraldehyde proceeded readily over K2CO3,
both in the presence and the absence of Ni.
It is known that radical mechanisms can be involved in nickel-
catalyzed reactions[20]. In order to test the possibility of radical
intermediates being involved in the butanol dehydrogenation
reaction, a radical inhibitor was added to the reaction (butylated
hydroxytoluene-BHT). A negligible reduction of the yield was
observed, compared to the reaction in the absence of BHT. This
observation suggests that a radical mechanism can be ruled out.
Scheme 2. Proposed mechanism of dehydrogenation of butanol to
butyraldehyde via hydride transfer to furfural.
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Scheme 3. Proposed mechanism of transfer hydrogenation over a basic catalyst.
No butanol was converted to butyraldehyde in the absence of
furfural. This, combined with the observation (Figure 2) that the
butanol consumption rate in the induction period was proportional
to the furfural concentration is further evidence of the existence of
Table 2. Textural properties of heterogeneous catalysts.
Catalyst
Surface Area
(m2 g-1)
Ni content (wt. %)
a
transfer hydrogenation mechanism. These observations
HT
221
212
248
216
0
suggest that the rate-determining step of the catalytic cycle, as
depicted in scheme 2, is the transfer of the hydride from the
butoxide to the furfural. This transfer is plausible, as it can occur
Ni10HT
Ni20HT
Ni40HT
9.6
5.6
2.8
via
a
favorable
six-member
transition
state.
Figure 3 shows the effects of the Ni loading on the reaction rates.
Remarkably, both the aldol condensation rate and the
dehydrogenation rate decreased with increasing Ni loading on the
catalyst, reducing by about 50% from 0% Ni to 9.6% Ni. This is
not due to changes in the surface area or the crystal structure of
the catalyst, as all catalysts have surface areas from 210 to 250
m2 g-1 (Table 2) and are mostly cubic periclase-like solids (Figure
4),[23] with minor contributions from a mostly amorphous Mg-Al
oxyhydroxide, as evidenced by the broad peak around 32°.[24]
Instead, this shows that in the heterogeneous system, Ni is not
required for the reaction to take place. On the contrary, the
presence of Ni seems to be detrimental to the reaction. The
reason for this could be that both hydrogenation and aldol
condensation reactions take place over basic sites, the strength
and number of which has been shown to be reduced upon
substitution of Mg2+ by Ni2+.[25] Basic solids are known to catalyze
the dehydrogenation of alcohols via a hydride transfer to a
carbonyl group and a mechanism like this could be in play here
as well.[26] Therefore, in order to probe this hypothesis, a set of
kinetic experiments was performed in the flow reactor.
3
2.5
2
1.5
1
0.5
0
0
1
2
Furfural concentration (M)
3
Figure 2. Rate of butanol dehydrogenation as
concentration. 0.05 mmol Ni(dppe)Cl2, 0.3 mmol K2CO3, 413 K, CBuOH = 1 M.
a function of furfural
Heterogeneous catalysts are generally preferred in the industry to
homogeneous ones, as they are generally more robust and easier
to handle. For this reason, the heterogenization of the reaction
was investigated by combining a Ni2+ material with a basic support,
by synthesizing a Ni-MgO-Al2O3 catalyst from a Ni-substituted
hydrotalcite precursor. Hydrotalcites and the oxides derived
thereof have found extensive use in many biomass-related
applications.[22] A gas-phase reaction system was selected to
probe the reaction, in order to obtain higher accuracy
measurements at differential conditions and also with the end goal
of using a two-bed reactor system to carry out the condensation
and the hydrodeoxygenation reactions. We synthesized a range
of materials with different Ni weight loadings, in order to probe the
effect of Ni and to optimize the catalyst.
Scheme 4. Proposed mechanism of dehydrogenation of butanol to
butyraldehyde via hydride transfer to furfural.
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Scheme 5. Proposed mechanism of aldol condensation of butyraldehyde and furfural.
Figure 5 shows the selectivity of the reaction as a function of the
butanol conversion (controlled by the residence time). At low
butanol conversion values, the main product was butyraldehyde.
At higher butanol conversion values, selectivity shifts towards the
butyraldehyde-furfural adduct. These observations suggest that
butanol is first dehydrogenated to the reactive butyraldehyde,
which in turn reacts with furfural to form the adduct.
outweighed by the lower cost and abundance of these materials,
compared to Ru and Hf.
Ni10HT
Ni20HT
1600
Dehydrogenation
1400
Ni40HT
HT
Aldol Condensation (x10)
1200
1000
800
600
400
200
0
20
30
40
50
60
70
2θ, degrees
Figure 4. X-ray diffractograms of calcined NiHT catalysts.
0
2
4
6
%wt Ni
8
10
12
Figure
6 shows the dependence of the rate of butanol
Figure 3. Effects of Ni loading on dehydrogenation and aldol condensation rates.
Dashed lines indicate qualitative trends. 413 K, 0.8 kPa furfural, 0.4 kPa butanol.
dehydrogenation on furfural and butanol pressures. As was the
case in the liquid phase Ni(dppe)Cl2-catalyzed reaction, the
dehydrogenation rate is first-order in furfural. On the other hand,
the dependence on butanol pressure shows
a first-order
Comparing the two catalysts under identical reaction conditions
(liquid phase, 413 K, Figure 1 and Figure S 1) and a conversion
below 25%, we can calculate the turnover frequency over both.
Assuming that the (100) plane is the dominant one on the surface
of the mixed oxide and using a lattice constant of a = 4.214 Å,
there are two oxygen atoms every 17.76 Å2. Given the surface
area of the mixed oxide being equal to 221 m2 g-1, the number of
active sites is 2.3 mmol/g. Based on the reaction rate in the
heterogeneous system, the turnover frequency (rate normalized
by surface oxygen atoms) is equal to 0.78 h-1, compared to 1.6 h-
1 in the homogeneous system. This is also comparable to systems
dependence at low butanol pressures and is independent of
butanol pressure at the higher pressure regime. This behavior
can be explained by the change of surface coverage from a clean
surface to a butanol covered surface, probably in the form of
butoxide, as basic solids are known to dissociatively adsorb
alcohols on their surface, forming alkoxides, as has been shown
in past XPS[24] and FT-IR investigations.[25]
These observations suggest that furfural participates in the rate-
limiting step of the butanol dehydrogenation reaction. This is
consistent with a transfer hydrogenation, in which a butoxide on
the surface transfers a hydride atom to a molecule of furfural,
forming furfuryl alcohol and butyraldehyde. In order to test this
hypothesis, we performed kinetic isotope effect experiments,
using 1,1-d2-butanol and butanol-OD. No kinetic isotope effect
was detected in the case of butanol deuterated in the OH group.
However, in the case of the C1 di-deuterated butanol, we
measured a KIE equal to 2.0. This is consistent with a primary
reported in the literature[27], such as a Ru/RuOx/C system (3.6 h-
1
)
[28] used in the context of furfural hydrodeoxygenation. On a per-
site basis, the turnover rates reported herein are lower than those
reported over Hf-BEA Lewis acidic zeolites (480 h-1).[29] However,
the rate disadvantage of the Mg-Al oxide catalytic system is
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kinetic isotope effect and is suggestive of a C1-H bond scission in
the rate-determining step. The combination of kinetics and
isotope effects supports the hypothesis that the rate-determining
step of this sequence is the hydride transfer from the butoxide to
the furfural. The reaction sequence for the transfer hydrogenation
is shown in scheme 3 and the proposed transition state is shown
in scheme 4. These results show that the dehydrogenation of
butanol to the reactive butyraldehyde takes place via a similar
hydride transfer step from the C1 carbon of the butanol to that of
the furfural.
600
500
400
300
200
100
0
a
1
2-(2-furylmethyl)-butyraldehyde
0
1
2
3
4
5
0.8
Butyraldehyde
Butanol pressure (kPa)
0.6
0.4
0.2
0
200
160
120
80
b
0
0.1
0.2 0.3
Butanol fractional conversion
0.4
0.5
Figure 5. Effects of residence time on the selectivity of the condensation of
butanol and furfural. 413 K, 0.8 kPa furfural, 0.4 kPa butanol. The residence
-1
time was varied from 0.5 to 4.3 ks kgcat molbutanol
40
0
In the condensation step, we propose that the reaction proceeds
through an aldol-type mechanism, in which an enolate (formed
from butyraldehyde) attacks the electrophilic carbonyl carbon of
the furfural, based on the regioselectivity of the product. The
condensation mechanism is shown in scheme 5.
0
0.1
0.2
0.3
0.4
0.5
Furfural pressure, kPa
Figure 6. Effects of butanol (a) and furfural pressure (b) on the butanol
dehydrogenation rate. (413 K, 0.8 kPa furfural or 1.1 kPa butanol).
To probe the aldol condensation step of the reaction, we co-fed
butyraldehyde and furfural over the catalyst. The rate of aldol
condensation was independent of the butyraldehyde pressure, as
can be seen in Figure 7. Since the reaction pathway requires the
participation of butyraldehyde in every step, this observation
shows that butyraldehyde participates in the kinetically relevant
step and, at the same time, that it is the most abundant surface
intermediate during condensation reactions. It is also noteworthy
that the pressure at which butyraldehyde covers the surface
completely is much lower (lower than 0.1 kPa) than that for
butanol (5 kPa). This is consistent with the observations of
Hanspal et al.,[30] who showed that during the ethanol Guerbet
reaction, the aldehyde species are bound more strongly to MgO
than the alcohol species.
We also observe that the aldol condensation rate showed a first-
order dependence on the furfural pressure (Figure S 2). This
observation enabled us to distinguish between a rate-determining
enolate formation step and an equilibrated enolate formation step.
In particular, the observation indicates that furfural participates in
the rate-determining step of the aldol condensation between
furfural and butyraldehyde, signifying an equilibrated enolate
formation. This is consistent with the kinetics reported in the
literature for the condensation of acetone with butyraldehyde and
acetaldehyde.[10c] In this, it was shown that the enolate formation
from the acetone is equilibrated with gas-phase acetone. In this
case, the observation that furfural participates in the rate-
determining step shows that the enolate formation from
butyraldehyde is equilibrated and that the C-C bond formation is
rate-determining for the condensation of furfural and
butyraldehyde.
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823 K for 4 h, with a ramp rate of 1 K/min. Catalysts prepared in this
manner are labeled NiXXHT, where XX is the Mg:Ni ratio in the catalyst.
80
60
40
20
0
The surface areas of the catalysts were measured by nitrogen
physisorption at 77 K, using a Micromeritics TriStar 3000 instrument. The
surface area was quantified using the instrument software by a fitting the
BET equation to the P/P0 data.
The crystalline phase of the catalysts was determined by X-ray diffraction,
using a Bruker D8 Advance X-ray diffractometer, equipped with a Cu Ka
source.
Liquid-phase kinetic experiments were performed in 12 mL batch reactors
(Qtube, Sigma-Aldrich). In these experiments, the reactants, furfural and
butanol were loaded with an appropriate amount of catalyst under air,
sealed and heater under stirring to the reaction temperature. After the
reaction was complete, the reactor was cooled, the pressure was released
and the contents diluted with toluene or THF. The solids were separated
and aliquots of the liquid were taken for analysis by GC-MS and GC-FID
(Varian 3800, VF-5MS capillary columns).
0
0.2
0.4
0.6
0.8
1
Butyraldehyde pressure (kPa)
Figure 7. Dependence of aldol condensation rate on butyraldehyde pressure.
413 K, 0.8 kPa furfural, balance He.
Gas-phase kinetic experiments were performed in a gas-phase flow
reactor setup. Helium and hydrogen gases (Praxair 99.999%) were
introduced into the system through mass flow controllers (Parker model
201). Liquid reactants, furfural (Sigma-Aldrich), butanol (Sigma-Aldrich)
and butyraldehyde (TCI America), were introduced into the gas stream
through syringe pumps (KD Scientific Legato 100). Transfer lines were
heated to above 403 K to prevent condensation of reactants and products.
The catalyst bed was supported on a borosilicate glass frit in a tubular
reactor with plug-flow hydrodynamics. The temperature of the reactor was
kept constant at 413-443 K by an electric furnace (APS PA), controlled by
a PID controller (Watlow). The reaction products were analyzed by online
gas chromatography (Shimadzu GC 2014). Prior to reaction, the catalyst
was treated in a 50% H2/He mixture at 523 K for 1h. Reaction rates were
calculated at differential conversion, i.e. lower than 5%. Details on the
calculations of the rates can be found in the Supporting Information. The
influence of internal and external mass transfer limitations was ruled out.
Details for the rate calculation formulae, as well as the calculations for the
Mears criterion and the Weisz-Prater criterion, can be found in the
Supporting Information.
Conclusions
In this work, we have shown that the coupling of furfural with
aliphatic alcohols proceeds via a transfer hydrogenation-aldol
condensation mechanism. The transfer of a hydride from an
alkoxide to the furfural is the rate-limiting step in both the
homogeneous and heterogeneous cases, as can be shown by
kinetic experiments and the kinetic isotope effects. In the
heterogeneous system, both the transfer hydrogenation and the
aldol condensation are catalyzed by the basic sites on the mixed
Mg-Al oxide. On the other hand, in the homogeneous system, the
K2CO3 base is not strong enough to catalyze the transfer
hydrogenation, necessitating the presence of the Ni complex that
acts as a Lewis acid to coordinate the furfural and the butoxide
and catalyze the hydride transfer.
Acknowledgements
Experimental Section
This work was funded by BP through the Energy Biosciences
Institute. The authors wish to thank Prof. F. Dean Toste and Dr.
Sanil Sreekumar for useful technical discussions, and Mr.
Matthew Gilkey and Ms. Huong Nguyen for their insightful edits
to the manuscript.
[1,3-Bis(diphenylphosphino)ethane]dichloronickel(II) [Hereafter referred to
as Ni(dppe)Cl2] was prepared as follows: NiCl2.6H2O (Spectrum Chemical)
was dissolved in 20 mL ethanol (Koptec) and was reacted at room
temperature
for
30
min
with
a
solution
of
1,3-
bis(diphenylphosphino)ethane (Sigma-Aldrich) in dichloromethane
(Spectrum Chemical). The orange-red solid was filtered out, washed with
ethanol and dried in the fume hood.
Keywords: Aldol condensation • furfural • kinetics • transfer
Preparation of Ni-substituted hydrotalcites was carried out by modifying a
method described by Climent et al.[31]. Briefly, layered hydroxycarbonates
were precipitated from a solution of Ni, Mg and Al nitrates (1.5 M total metal
concentration, 3:1 Mg:Al – purchased from Spectrum Chemical Co.) with
an equal volume of a NH4OH and (NH4)2CO3 solution (3.375 M and 1 M,
respectively – Spectrum Chemical Co.). After precipitation at 333 K, the
solids were filtered and washed with copious amounts of warm (323 K)
water. The solids were then dried at ambient air at 368 K and calcined at
hydrogenation • hydrotalcite
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10.1002/cctc.201701866
ChemCatChem
FULL PAPER
Entry for the Table of Contents
FULL PAPER
We investigate the tandem
Konstantinos A. Goulas,* Amit A.
dehydrogenation and aldol
Gokhale
condensation between furfural and
butanol to jet fuel precursors. We
establish that heterogeneous systems
are equally active with homogeneous
ones, with the rate-determining step
being a hydride transfer from butanol
to furfural.
Page No. – Page No.
Kinetics of the Homogeneous and
Heterogeneous Coupling of Furfural
with Biomass-derived Alcohols
This article is protected by copyright. All rights reserved.