Catalytic Upgrading of Biomass-Derived Methyl Ketones to Liquid Transportation Fuel Precursors by an Organocatalytic Approach

Article abstract of DOI:10.1002/anie.201412470

A highly efficient water-tolerant, solid-base catalyst for the self-condensation of biomass-derived methyl ketones to jet-diesel fuel precursors was developed by grafting site-isolated secondary amines on silica-alumina supports. It is shown that apart from the nature and density of amine groups and the spatial separation of the acidic and basic sites, the acidity of the support material plays a critical role in defining the catalytic activity. It is also found that a combination of weakly acidic silanol/aluminol with secondary amine groups can mimic proline catalysts and are more effective in catalyzing the selective dimerization reaction than the combination of amines with organic acids. In situ FTIR measurements demonstrate that acidic groups activate methyl ketones through their carbonyl groups leading to a favorable C - C bond formation step involving an enamine intermediate. DFT analysis of the reaction pathway confirms that C - C bond formation is the rate-limiting step.

Full text of DOI:10.1002/anie.201412470

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201412470
German Edition: DOI: 10.1002/ange.201412470
Bifunctional Catalysts
Catalytic Upgrading of Biomass-Derived Methyl Ketones to Liquid
Transportation Fuel Precursors by an Organocatalytic Approach**
Shylesh Sankaranarayanapillai, Sanil Sreekumar, Joseph Gomes, Adam Grippo,
George E. Arab, Martin Head-Gordon, F. Dean Toste, and Alexis T. Bell*
Abstract: A highly efficient water-tolerant, solid-base catalyst
for the self-condensation of biomass-derived methyl ketones to
jet-diesel fuel precursors was developed by grafting site-
isolated secondary amines on silica-alumina supports. It is
shown that apart from the nature and density of amine groups
and the spatial separation of the acidic and basic sites, the
acidity of the support material plays a critical role in defining
the catalytic activity. It is also found that a combination of
weakly acidic silanol/aluminol with secondary amine groups
can mimic proline catalysts and are more effective in catalyzing
the selective dimerization reaction than the combination of
amines with organic acids. In situ FTIR measurements dem-
onstrate that acidic groups activate methyl ketones through
their carbonyl groups leading to a favorable CÀC bond
and 2,6-dimethyl furan, respectively. Additionally, butan-2-
one can be produced in high yields by an acid-catalyzed
dehydration reaction of 2,3-butanediol (2,3-BDO) sugars or
by decarboxylation of levulinic acids. However, due to their
low carbon numbers and high O/C ratios, there is a strong
thrust to develop catalytic methods for the self-condensation
of methyl ketones that are fuel components in the gasoline,
jet, and diesel range.
[
4]
Aldol-type condensation provides an excellent pathway to
increase the carbon number and decrease the O/C ratio of
biomass-derived molecules. This approach requires the devel-
opment of multicatalytic functions that cooperatively pro-
[5,6]
mote efficient one-pot reaction sequences.
For example, in
organisms, aldol condensations are catalyzed by aldolases that
activate donor ketones using the amino groups of a con-
formation step involving an enamine intermediate. DFT
[
7]
analysis of the reaction pathway confirms that CÀC bond
strained lysine to give enamines. With precisely positioned
formation is the rate-limiting step.
active sites, enzymes catalyze selective CÀC bond-forming
reactions by synergistic interactions. Site isolation is thus
considered as one of the paradigms in heterogeneous
catalysis, especially in the separation of mutually incompat-
ible groups, such as acids and bases. Among the various
heterogeneous catalysts screened for aldol-type condensation
reactions, organic amines (basic sites) supported on silica
A
s the worldꢀs accessible fossil reservoirs are gradually
depleted, it is crucial to develop sustainable, long-term
strategies based on the utilization of renewable feed
[1]
stocks. Biomass-derived molecules are inherently oxygen-
rich; consequently, the excess oxygen must be removed to
raise the energy density of the products and make them
suitable as transportation fuels. It has recently been
reported that acetone/butanol/ethanol (ABE) mixtures pro-
duced by Clostridial fermentation of sugars can be catalyti-
surfaces (acidic sites) have shown outstanding catalytic
[
2]
[8–10]
performance.
The use of such supported organocatalysts
could eliminate the need for large quantities of caustic base,
enabling condensation reactions to be carried out under
solvent-less reaction conditions with little or no sensitivity
[
3]
cally converted to aliphatic ketones. In this process, the
monoalkylation of acetone with ethanol or butanol produces
pentan-2-one or heptan-2-one, respectively. Alternatively,
C –C methyl ketones (pentan-2-one, hexan-2-one) can be
[11]
towards water.
We report here the use of silica–alumina (Si–Al) sup-
ported organoamine catalysts for the selective dimerization of
biomass-derived methyl ketones to produce liquid trans-
portation fuel precursors. Since weakly acidic silanols (SiÀ
5
6
produced by ring opening hydrogenolysis of 2-methyl furan
[
+]
OH) activate the electrophilic reactants through hydrogen
bonding during carbon–carbon bond forming reactions,
introducing stronger acidic groups (Si(OH)Al) onto the
support surface was hypothesized to enhance the cooperative
[
*] Dr. S. Sankaranarayanapillai, J. Gomes, Prof. A. T. Bell
Energy Bioscience Institute & Department of Chemical and
Biomolecular Engineering
University of California, Berkeley, CA-94720 (USA)
E-mail: alexbell@berkeley.edu
[12]
effect. Experiments were conducted to elucidate the nature
[+]
Dr. S. Sreekumar, A. Grippo, G. E. Arab, Prof. F. D. Toste
Energy Bioscience Institute and Department of Chemistry
University of California, Berkeley, CA-94720 (USA)
of grafted amine groups, their density and the influence of
spatial separation between the acid and base groups on the
selective dimerization of methyl ketones. To the best of our
knowledge, this is the first experimental and theoretical effort
aimed at demonstrating the enhanced catalytic activity of
supported amine catalysts for the selective dimerization of
biomass-derived methyl ketones (C –C ).
Prof. M. Head-Gordon
Department of Chemistry
University of California, Berkeley, CA-94720 (USA)
+
[
] These authors contributed equally to this work.
4
15
[
**] This work was supported by the Energy Bioscience Institute. We
gratefully acknowledge the contributions of S. Krishna, and Dr. G.
Ingle to the experimental section of the manuscript.
Details regarding the materials used, catalyst synthesis
and characterization methods, and catalyst testing can be
found in the Supporting Information (SI). A series of primary,
secondary, and tertiary amine catalysts were prepared with
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201412470.
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Control reactions carried out with a homogeneous amine
catalyst (N-methylbutylamine) did not furnish the desired
products. A combination of Si–Al and a homogeneous base
(N-methylbutylamine) demonstrated an intermediate level of
conversion (27%), albeit significantly lower than that
obtained from the acid–base bifunctional Si–Al-NHR cata-
lyst. These results illustrate the necessity of having the
immobilized acid and base groups in close proximity to
realize cooperative catalytic enhancement. Further support
for this conclusion was obtained by noting the absence of
Scheme 1. Amine catalysts screened for their activity in the dimeriza-
tion reaction of methyl ketones. Reaction conditions: heptan-2-one
activity for Si–Al-C -NHR, a catalyst in which the linker for
1
[16]
the amine group contains only one carbon atom. Therefore,
(
2 mmol, 0.228 g), catalyst (ꢀ0.1 g), t=12 h, T=443 K.
the amine groups must be flexible enough to work cooper-
atively with the acidic hydroxy groups but spaced far enough
from these groups to avoid mutual neutralization of the acid
and base groups. Blocking the silanol/aluminol groups by
silylation using trimethoxysilane decreased the catalyst activ-
similar amine loading so that a direct comparison is feasible
with similar textural qualities (Scheme 1). Briefly, the silica–
alumina support was first pretreated at 773 K and then
contacted with an ethanol solution of the amine precursor
under reflux for 16 h to functionalize the precursor onto the
[
17]
ity by five times. This observation confirms that acidic sites
must work cooperatively with the supported amines to
catalyze the selective dimerization of methyl ketones.
The acid–base pair effect was verified further by hetero-
genizing amines in combination with various alkyl-tethered
13
support. Solid-state C magic-angle spinning (MAS) NMR
spectroscopy confirmed the successful grafting of the respec-
tive organosilanes onto the Si–Al surfaces (SI). The observed
chemical shifts of the organic groups agree well with those of
the corresponding organosilane precursors measured in
organic acids like sulfonic acids (pK = À2), phosphoric acid
a
[
18]
(pK = 3), and carboxylic acid (pK = 5).
The catalyst
a
a
29
solution. Si MAS NMR measurements further confirmed
the presence of organic functional groups grafted on the Si–Al
support. Peaks at d = À110, À100, À90, À65, and À55 ppm
activity was found to decrease with decreasing pK of the
a
acid groups (Figure 1a). This interesting result suggests that
strong acids in close proximity to the basic sites can protonate
the nitrogen to lower its nucleophilicity and slow down the
formation of enamines. It was also observed that the
cooperative interactions of amines with weakly acidic sila-
4
3
were
assigned
to
Q (Si(OSi) ),
Q (Si(OH)(OSi) ),
4
3
2
3
2
Q (Si(OH) (OSi) ), T (SiR(OSi) ), and T (Si(OH)R(OSi) )
2
2
3
2
[
13]
sites, respectively.
The presence of peaks at À65 and
n
À55 ppm indicated the formation of
T
sites [RSi-
nols/aluminols (pK = 6–8) are much more effective than
a
3
2
29
(
OCH CH ) (OSi) ] and a higher T /T ratio from the Si
organic acids unless the interaction between the amine and
2
3
n
3Àn
[19]
MAS NMR confirmed a strong covalent linkage between the
the acid groups is constrained as in proline (Scheme 1).
organocatalysts and the Si–Al support (SI).
Nevertheless, it should be emphasized that the Si–Al-sup-
ported amine showed a three-fold higher conversion than the
corresponding silica-supported amine catalyst (SI). The
improved catalytic activity for the amine-grafted Si–Al
compared to that of amorphous silica suggests that the
Brønsted acidity of the support plays an important role in
enhancing the catalytic activity of the heterogenized amine
Screening of various amine catalysts under neat condi-
tions for the dimerization of heptan-2-one showed that Si–Al-
supported secondary amines are more active than primary
amines, whereas tertiary amines are less active (Scheme 1).
The only products observed under any of the conditions
investigated were C14 dimers. Interestingly, under equal
concentration of amine groups, Si–Al-supported secondary
amines (Si–Al-NHR) produced similar conversion to that of
supported proline. This result suggests that heterogenizing
secondary amines on silica–alumina can mimic the con-
strained site isolation occurring in proline. Changing the R
group in Si–Al-NHR from methyl to butyl did not change the
yield of the dimerized product. The catalyst prepared with
a diamine (-NHCH CH NH ) afforded slightly better activity
2
2
2
than the Si–Al-NHR catalyst, whereas heterogenizing a tri-
amine (-(NHCH CH ) NH ) catalyst resulted in approxi-
2
2
2
2
mately two times lower activity than that found for the
diamine catalyst. Supported tertiary amines, such as TBD and
DMAP, showed no activity, whereas supported imidazole or
piperazine exhibited moderate catalytic activity (Scheme 1).
The low catalytic activity of the tertiary amine catalysts
compared to the secondary amines suggests that enamines are
the key intermediate, since tertiary amines have difficulty for
forming enamines due to lack of hydrogen on the nitrogen
Figure 1. a) Effect of acid strength on the catalytic activity of the
supported amine catalysts and b) effect of amine loading on the yield
of dimer product. Reaction conditions: heptan-2-one (2 mmol,
À2
0
.228 g), Si–Al-NHCH CH NH catalyst (0.1 g, 0.4 aminesnm ),
2
2
2
[14,15]
atom.
t=12 h, T=443 K.
4
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samples. This result is corroborated by the strong upfield shift
in the terminal carbon position by adsorbing homogenous
[
20]
triethylamine (NEt ) on Si–Al than on silica. Remarkably,
3
1
3
C CP MAS NMR spectroscopy of the amine-tethered Si–Al
sample showed similar peak positions to that in the silica
sample, indicating the absence of strong interactions between
the Brønsted acid sites and the heterogenized amine groups
(SI). These results suggest that amine–acid site isolation is
maintained in Si–Al-supported amine catalyst because pro-
tonated amines are known to be less active for condensation
[
16]
reactions.
Having established supported diamine (-NHCH CH NH )
2
2
2
as the most active catalyst for heptan-2-one dimerization, we
examined the effect of varying the density of amines groups
on the Si–Al surfaces. As shown in Figure 1b, increasing the
amine content on the Si–Al surface decreased the catalytic
activity. Stated another way, at low amine loading, an increase
in cooperativity between basic sites and silanols/aluminols
was achieved by preventing amine aggregation on the Si–Al
Figure 2. Substrate scope of the Si–Al-NHCH CH NH catalyst in the
2
2
2
selective dimerization reaction of C
conditions: methyl ketone (2 mmol), Si–Al-NHCH CH NH catalyst
–C15 methyl ketones. Reaction
4
2
2
2
À2
(
0.1 g, 0.4 aminesnm ).
[
10]
support surface.
We also note that the Si–Al-supported diamine catalyst is
remarkably water tolerant and retains its catalytic activity
even when the reaction is performed in a mixture containing
5
0 wt% water (Figure S10). By contrast, calcined hydrotalcite
(
MgAlO ), completely loses its activity when less than 5 wt%
x
of water is added to the reaction mixture. This observation
clearly demonstrates the practical advantage of supported
organocatalysts in biomass upgrading reactions over conven-
[21]
tional inorganic solid-base catalysts, such as hydrotalcites.
The scope of the dimerization of methyl ketones catalyzed
by Si–Al-NHCH CH NH was examined for ketones con-
2
2
2
taining 4–15 carbon atoms. All substrates exclusively pro-
duced dimers of the ketones with yields generally greater than
6
0% (Figure 2). Only the C15 substrate showed a low product
yield, probably due to its long carbon chain and its difficulty
in diffusing into the narrow pores of the support. The
solid-supported organocatalysts were then used as catalysts
for continuous flow process. Catalytic activity of the most
active silica–alumina-supported amine catalyst (Si–Al-
NCH CH NH ) evaluated in the continuous, gas-phase self-
Figure 3. In situ FTIR spectra of the Si–Al-NHCH CH -NH catalyst
using butan-2-one as a probe molecule at different reaction temper-
atures.
2
2
2
can be related to the formation of iminium species, the
formation of which have been reported frequently for
2
2
2
condensation of butan-2-one showed excellent catalytic
activity and did not exhibit significant deactivation over
[
14]
homogenous secondary amine catalysts. The formation of
iminium/enamine groups observed over secondary amines is
in good accordance with previous reports that these catalysts
operate through an enamine mechanism. Increasing the
temperature from 298 K to 473 K produced a progressive
3
0 h on stream (Figure S11).
To obtain insights into the reaction mechanism, the Si–Al-
supported amine catalyst was characterized by in situ FTIR
spectroscopy using butan-2-one as probe molecule
Figure 3). The spectrum recorded at 298 K over Si–Al-
a
À1
(
decrease in the peak at 1700 cm and the appearance of
À1
À1
NHCH CH NH catalyst showed a band at 1700 cm fol-
lowed by the formation of a strong band at 1660 cm and
a weak band at 1630 cm . The 1700 cm peak is related to
the adsorption of methyl ketone on the Si(OH)Al groups
which showed ca. 15 cm downfield shift in carbonyl peaks
a new peak for carbon–carbon vibrations at 1595 cm . These
2
2
2
À1
observations show that enamines interact with the adsorbed
methyl ketones to produce the ketone dimer. To validate this
finding, in situ IR spectra of Si–Al-NHCH CH NH were
À1
À1
2
2
2
À1
acquired by first saturating the sample with pyridine prior to
exposure of the catalyst to butan-2-one. Saturation of the
sample with the pyridine prior to the addition of butan-2-one
from the gas phase whereas the sharp peak observed at
À1
1
)
660 cm can be assigned to the formation of imine (-HC=N-
[15,22]
13
À1
species.
The C CP MAS NMR spectrum of the Si–Al-
resulted in the absence of a peak at 1700 cm . This
NHCH CH NH sample after treatment with butan-2-one
observation shows that pyridine passivates the sites at which
2
2
2
showed a peak at d = 160 ppm (-N=CH-), and at d = 63 ppm
the ketone adsorbs and subsequently reacts to form CÀC
(
N-CH ), indicating the rapid formation of an imine inter-
bond. The IR results also support the need for a combination
of basic amine groups and acidic Si(OH)Al groups and
2
[12,13]
À1
mediate by dehydration (SI).
The shoulder at 1630 cm
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.
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explain why supported amines are more active than the
homogenous amine catalysts.
The adsorption of butan-2-one in the presence of the enamine
À1
resting state is exothermic (DH = À32 kJmol ) and is
A theoretical analysis of the aldol condensation reaction
pathway was carried out to verify the proposed reaction
mechanism, determine the rate-limiting step, and explain the
high selectivity to dimers shown by the SiÀNHCH CH NH
promoted by hydrogen bonding between the silanol group
and the carbonyl group of butan-2-one (8, butanone adsorp-
tion). The formation of the CÀC bond (TS9) proceeds through
protonation of the butan-2-one carbonyl group by the silanol
2
2
2
[
22]
catalyst. The changes in enthalpy (at the reaction temper-
ature, 343.15 K) along the reaction pathway of butan-2-one
group and CÀC bond formation between the enamine and the
alpha-carbon of butan-2-one in a concerted fashion to
condensation calculated at the wB97X-D/6-311 ++ G- produce an iminium species (10, iminium). The CÀC bond
(
(
3df,3pd) level of theory and the corresponding free-energy
343.15 K) changes are shown in the SI.
formation rate-determining step occurs much more readily in
the case of Si–Al than SiO due to the increased Brønsted
2
The adsorption of butan-2-one to the catalyst surface is
acidity of Si(OH)Al relative to Si-OH groups. The hydrolysis
(TS11) proceeds from the iminium by the attack of water on
the complex (12, carbinolamine II). This process leads to
a concerted proton transfer from the water molecule to the
deprotonated silanol group, leaving the final product, 3,4-
dimethyl-4-ol-hexan-2-one, in the chemisorbed state. The
final step of the reaction pathway is the desorption of 3,4-
À1
exothermic (DH = À45 kJmol ) and is characterized by
a hydrogen bonding interaction between butan-2-one and
the grafted amine (2, butanone adsorption, Scheme 2). The
formation of a carbinolamine species proceeds through
a concerted CÀN bond formation and proton transfer reaction
(
4, carbinolamine). The enamine state is produced by intra-
molecular water elimination. The desorption of water upon
enamine formation exhibits a favorable free energy change
due to the additional gain in entropy (7, enamine). The
dimethyl-4-ol-hexan-2-one, which produces the C dimer and
leaves the catalyst active site in its original form.
8
CÀC bond formation (TS9) is determined to be the rate-
À1
À30 kJmol change in free energy upon desorption of water
determining step of the dimerization reaction. The calculated
apparent activation energy, calculated as the change in
suggests that the enamine intermediate, which is the resting
state of the catalyst, is not significantly poisoned by water.
À1
enthalpy between intermediate 7 and TS9, is 90 kJmol , in
good agreement with the experimentally
determined activation energy of
À1
9
5 kJmol for butan-2-one condensa-
tion over the Si-NHCH CH NH cata-
2
2
2
lyst.
Theoretical analysis of the trimeri-
zation pathway was performed to under-
stand the high selectivity to the dimer
product relative to the trimer (SI).
Trimerization of butan-2-one occurs
when carbinolamine II (13) undergoes
dehydration (TS16) rather than desorp-
tion (TS14) to form a C enamine (21).
8
The dehydration and release of water to
form (21) is endothermic but exergonic
due to the gain in entropy upon releas-
ing two molecules of water. CÀC bond
formation (TS23) is found to be the rate-
determining step of the trimerization
reaction. The difference in selectivity
can be rationalized by examining the
difference in the Gibbs free energy of
°
activation (DDG ) between the dimeri-
zation (TS9) and trimerization (TS23)
CÀC bond formation transition states.
At the reaction temperature of
°
À1
3
43.15 K, DDG = 58 kJmol , which
suggests that the catalyst will selectively
dimerize butan-2-one except at high
temperature. The difference in the
entropy of activation between (TS23)
°
À1 À1
and (TS9), DDS = 6 Jmol K , is
small and positive; therefore an increase
in the reaction temperature will result in
Scheme 2. Proposed reaction mechanism of butan-2-one condensation reaction over Si–Al-
°
NHCH CH NH catalyst.
a less positive value of DDG between
2
2
2
4
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dimerization of biomass-derived methyl ketones. Secondary
amines in combination with Si(OH)Al groups provide the
best acid–base pair and the spacing between acid–base groups
must be controlled to maximize their cooperative action and
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organocatalysts · secondary amines · silica–alumina
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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