Pd/NbOPO4 multifunctional catalyst for the direct production of liquid alkanes from aldol adducts of furans

Article abstract of DOI:10.1002/anie.201403440

Great efforts have been made to convert renewable biomass into transportation fuels. Herein, we report the novel properties of NbO _x-based catalysts in the hydrodeoxygenation of furan-derived adducts to liquid alkanes. Excellent activity and st

Full text of DOI:10.1002/anie.201403440

Angewandte
Chemie
DOI: 10.1002/anie.201403440
Biomass Conversion
Pd/NbOPO4 Multifunctional Catalyst for the Direct Production of
Liquid Alkanes from Aldol Adducts of Furans**
Qi-Neng Xia, Qian Cuan, Xiao-Hui Liu, Xue-Qing Gong,* Guan-Zhong Lu, and
Yan-Qin Wang*
Abstract: Great efforts have been made to convert renewable
biomass into transportation fuels. Herein, we report the novel
properties of NbOx-based catalysts in the hydrodeoxygenation
of furan-derived adducts to liquid alkanes. Excellent activity
and stability were observed with almost no decrease in octane
yield (> 90% throughout) in a 256 h time-on-stream test.
Experimental and theoretical studies showed that NbOx species
basis of different strategies and catalysts.[2–4] Among them, an
important strategy is to increase the length of the carbon
À
chain by C C coupling reactions, for example, first through
the base-catalyzed aldol condensation or dimerization of a-
angelica lactone to give angelica lactone dimer[2] or acid-
catalyzed hydroxyalkylation/alkylation,[3] followed by
removal of the oxygen atoms of the resulting oxygenates to
produce liquid alkanes by hydrodeoxygenation over various
noble-metal-based catalysts at relatively high temperature
and pressure. However, harsh conditions would lead to severe
À
play the key role in C O bond cleavage. As a multifunctional
catalyst, Pd/NbOPO4 plays three roles in the conversion of
aldol adducts into alkanes: 1) The noble metal (in this case Pd)
is the active center for hydrogenation; 2) NbOx species help to
À
C C bond cleavage and coke formation. It has been reported
À
cleave the C O bond, especially of the tetrahydrofuran ring;
that ReOx, MoOx, and WOx species can effectively promote
the activity of a Rh/SiO2 or Rh/C catalyst for the selective
hydrogenolysis of cyclic ethers and polyols to a,w-diols.[5]
Burch et al.[6] also confirmed the promotion effect of Re on
the Pt/TiO2 catalyst during the hydrogenolysis of N-methyl-
and 3) a niobium-based solid acid catalyzes the dehydration,
thus enabling the quantitative conversion of furan-derived
adducts into alkanes under mild conditions.
=
T
he overall goal of the conversion of biomass resources into
pyrrolidin-2-one to N-methylpyrrolidine through C O bond
transportation fuels is the removal of oxygen, and great effort
has been devoted worldwide to the development of efficient
conversion processes.[1] Herein, we report the novel activity of
a Pd/NbOPO4 multifunctional catalyst for the direct con-
version of furans into alkanes under mild conditions. The
aldol adduct of furfural with acetone was directly and almost
entirely converted into octane at 1708C and 2.0 MPa, and the
catalyst lasted for 256 h without deactivation. This excellent
performance was attributed to the unique ability of NbOx as
cleavage both theoretically and experimentally. Furthermore,
we found that CoOx species could effectively work together
with the noble metal platinum to open the furan ring of
furfural alcohol and 4-(2-furyl)-3-buten-2-one (furfural ace-
tone single aldol adduct) under mild conditions.[7] However,
we also noticed that CoOx species have weak acidity and are
not active enough for the dehydration of octanediols. There-
fore, we then used the metal/solid-acid bifunctional catalyst
Pt/NbOPO4 and found that under mild conditions (1758C,
2.5 MPa) it not only converted octanediols into octane, but
also partially converted the saturated furanic compound into
À
an early-transition-metal oxide to cleave the C O bond in the
furan ring and function as a solid acid catalyst for the
dehydration.
Several integrated processes have been proposed for
upgrading biomass platforms into liquid hydrocarbons on the
À
octane through C O bond cleavage in the tetrahydrofuran
ring.[7b] Thus, we speculated that besides its acidity, NbOx may
have the ability to break a C O bond in the furan ring.[8] Thus,
À
the niobium-based-solid-acid-supported noble metal can be
used as a multifunctional catalyst for the direct production of
targeted alkanes from aldol adducts of furfural and/or 5-
hydroxymethylfurfural (HMF) (Scheme 1).
[*] Q. N. Xia,[+] Q. Cuan,[+] X. H. Liu, Prof. X. Q. Gong, Prof. G. Z. Lu,
Prof. Y. Q. Wang
Key Laboratory for Advanced Materials
Research Institute of Industrial Catalysis
East China University of Science and Technology
Meilong Road 130, Shanghai 200237 (China)
E-mail: xgong@ecust.edu.cn
The aldol condensation of furfural and/or HMF with
acetone can be carried out with a liquid or solid base. In this
study, NaOH was used to enable the complete conversion of
furfural and/or HMF during the aldol condensation and
enhance the selectivity toward double adducts (a strong base
favors double-condensation products at a specific ketone-to-
furfural ratio; see the Supporting Information for details of
the reaction and product distributions).[2a,b] After aldol
condensation, the single adduct of furfural with acetone,
furfural acetone [4-(2-furanyl)-3-butene-2-one, FA], was fed
through a fixed-bed reactor filled with the Pd/NbOPO4
catalyst. The 256 h time-on-stream result is presented in
Figure 1. There were no oxygen-containing products detected
in the reaction effluents by GC–MS (see Figure S11), and the
wangyanqin@ecust.edu.cn
[+] These authors contributed equally.
[**] This project was supported financially by the National Basic
Research Program of China (No. 2010CB732306), the National
Natural Science Foundation of China (No. 21101063, 21273071),
the Commission of Science and Technology of Shanghai Munici-
pality (13520711400, 13JC1401902), and the Fundamental Research
Funds for the Central Universities. We also thank Prof. Yahong
Zhang at Fudan University for non-aqueous titration.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403440.
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various oxide-supported Pd cata-
lysts in FA hydrodeoxygenation, as
well as the BET surface area of
these supports, is summarized in
Table 1. The final products of the
reactions over these oxide-sup-
ported Pd catalysts were drasti-
cally different. Over the Pd/
NbOPO4 catalyst, FA was quanti-
tatively converted into alkanes,
with an octane yield of 94%.
However, on the common Pd/
Al2O3 catalyst, FA was only con-
verted into the hydrogenated
products 4-(2-tetrahydrofuryl)-2-
butanol (THFA) and 3-(5-methyl-
2-tetrahydrofuryl)-1-propanol
Scheme 1. Reactions involved in the conversion of biomass-derived platform chemicals into liquid
alkanes.
(MTHFA), without any deoxyge-
nation or ring-opening products.
Interestingly, the mechanical blending of NbOPO4 and Pd/
Al2O3 gave a much better catalytic activity than Pd/Al2O3
alone, although the hydrodeoxygenation was not complete
(51% yield of octane), probably owing to the weak inter-
action between Pd and NbOPO4. This result suggests that the
NbOPO4 support played a vital role in ring opening and
hydrodeoxygenation. Moreover, on the Pd/H-ZSM-5 catalyst,
octane was produced in only 25% yield along with oxygen-
containing products in over 69% yield. Considering that the
BET surface area and acidic properties of H-ZSM-5 were
similar to those of NbOPO4 (Table 1; see also Figure S6),
although the high number of acidic sites and large BET
surface area might have played important catalytic roles, they
may not be the main reason for high hydrodeoxygenation
activity. This hypothesis was confirmed by the reaction with
the supported Pd catalyst Pd/Nb2O5, which despite having the
smallest BET surface area and the lowest acid-site density
exhibited excellent catalytic performance (96% octane yield;
see Figure S12 for GC–MS results). Therefore, we propose
that NbOx species can actually break or assist in breaking the
Figure 1. Production of octane by the direct hydrodeoxygenation of
a solution of FA (3 wt%) in cyclohexane in a fixed-bed reactor over
a Pd/NbOPO4 catalyst. Reaction conditions: 1708C, 2 MPa, weight
hourly space velocity: 1.2 hÀ1, gas-flow rate: 20 mLminÀ1
.
À
main product was octane (> 90%), accompanied by small
C O bond and thus promote the high-yielding formation of
octane.
À
amounts of C1–C7 (produced by C C cleavage) and C16
alkanes (probably produced by the hydrodeoxygenation of
the intermolecular aldol adducts of C8 intermediates). Sur-
prisingly, the catalyst showed excellent activity and stability
with almost no decrease in octane yield (> 90% throughout).
The catalytic performance of different noble-metal-
loaded NbOPO4 catalysts was also investigated. Pt/NbOPO4
and Pd/NbOPO4 showed the best performance with a total
yield of alkanes of over 99% and an octane yield of over 93%;
the remaining alkanes obtained in 6–7% yield were mainly n-
heptane and isohexadecane, consistent with the results
obtained in the fixed-bed reactor. Over a Ru/NbOPO4
catalyst, alkanes were obtained in a total yield of 88%,
along with a small amount of oxygenates. Surprisingly, even
the non-noble-metal-loaded NbOPO4 catalyst Ni/NbOPO4
showed moderate hydrodeoxygenation activity to produce
octane in 8% yield under these mild conditions. When the
temperature was increased to 2008C, the octane yield was
raised to 54%, with products still containing the tetrahydro-
furan ring produced in only 12% yield. The by-products were
mainly dioctyl ether (DOE) and 1-octanol. These results (see
Table S1 in the Supporting Information for details) indicate
that besides expensive Pt and Pd, much cheaper Ru or even
À
Owing to the mild reaction conditions, less C C cracking
occurred, and the amounts of CH4 (ca. 0.3–0.5%) and CO2
(ca. 0.1–0.3%) in the gas phase were very small. This result is
quite different from those obtained at higher temperature and
À
pressure, under which conditions severe C C cleavage always
occurs.[2a,b,3a] Hence, besides reducing the cost and energy
consumption of biorefining, the mild conditions in our process
À
can also improve the stability of the catalyst and restrain C C
cleavage, thus making the whole process more cost-effective
and energy-efficient.
To investigate the role of each component in the catalyst,
we carried out the reactions in batch reactors with various
oxide-supported Pd catalysts, as well as NbOPO4 combined
with different noble metals. The catalytic performance of
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Table 1: The BETsurface area and catalytic performance of different supported Pd catalysts for the direct
Pd, is a facile and fast process. The
content of the dehydroxylation
intermediates (MPTHF and
hydrodeoxygenation of FA.[a]
Catalyst[b]
Pd/NbOPO4
Pd/Al2O3
NbOPO4 &
Pd/H-ZSM-5
Pd/Nb2O5
[c]
Pd/Al2O3
BTHF) then increased and that of
the hydrogenated products (THFA
and MTHFA) decreased rapidly
between 0.5 and 4 h, thus indicating
that the hydrogenolysis of the hy-
droxy group (by acid-catalyzed
dehydration and palladium-cata-
lyzed hydrogenation) is also a fast
process. After 4 h, with the opening
of the tetrahydrofuran ring (cata-
lyzed by NbOx species), octanols
(mainly 1-octanol) and C16 ethers
(mainly DOE) first accumulated
and then diminished in quantity as
the reaction progressed. This plot
suggests that the sequence of the
SBET [m2 gÀ1
]
360
>99.9
>99.9
198
>99.9
0
ND[d]
>99.9
54
318
>99.9
25
86
>99.9
>99.9
Conversion [%]
Selectivity [%][e]
Yield [%]
octane
other alkanes
94
6
0
0
51
3
24
1
96
4
0
73
0
0
0
(THFA)
(MTHFA)
(BTHF)
0
0
0
27
0
0
0
0
0
0
2
17
37
Pd/NbOPO4-catalyzed
reactions
0
16
was as follows: 1) hydrogenation
of unsaturated bonds, 2) dehydra-
tion/hydrogenation of the secon-
dary hydroxy group, 3) tetrahydro-
furan-ring-opening reaction, and
(MPTHF)
octanols
dioctyl ether (DOE)
other oxygenates
0
0
0
0
0
0
8
20
0
10
5
6
0
0
0
[a] Reaction conditions: catalyst (0.1 g), FA (0.2 g), cyclohexane (6.46 g), 1708C, initial H2 pressure:
2.0 MPa, reaction time: 24 h. [b] Pd loading: 5 wt%. All Pd catalysts were reduced in situ. [c] The
4) dehydration/hydrogenation
of
reaction was carried out with NbOPO4 (0.1 g) and Pd/Al2O3 (0.1 g). [d] Not determined. [e] Selectivity the primary hydroxy group. This
for the formation of alkanes.
order is consistent with the relative
difficulty of each step in the reac-
tion pathway (see Figure S15).
the non-noble metal Ni loaded on NbOPO4 can be used as
multifunctional catalysts for the efficient hydrodeoxygenation
of biomass-derived oxygenates, owing to the significant
According to the product distribution versus reaction time,
we tentatively propose the reaction pathway shown in
Scheme 2.
À
promotional effect of NbOx species on C O cleavage.
To better understand the origin of the high activity of
niobium-based supports toward the ring opening of furans, we
also performed a first-principles simulation of the corre-
sponding reaction processes at characteristic NbOPO4 and
Nb2O5 surfaces. NbOPO4(100) and Nb2O5(001) surfaces,
which both exhibit well-ordered flat structures, were chosen
as the substrates. The bulk-truncated NbOPO4(100) surface
includes singly coordinated O atoms (Figure 3a), which were
therefore saturated with H atoms for our calculation, whereas
Nb2O5(001) (Figure 3e) took the original conformation with
doubly coordinated O (O2c) and pentacoordinated Nb (Nb5c).
Nevertheless, the NbOPO4(100) and Nb2O5(001) surfaces
both contain NbO chains (see the dashed black squares in
Figure 3a,e and corresponding local structures in Figure 3b,f)
with similar atomic structures.
Considering that the reaction could be too fast and that
some of the intermediates may not be detected at 1708C, we
conducted the direct conversion of FA under the catalysis of
Pd/NbOPO4 at a slightly lower temperature of 1508C and
a H2 pressure of 1.5 MPa. The determined reaction profile
(see Figure 2) shows that during the first 0.5 h, the main
products were THFA and MTHFA, and no unsaturated
products were found (see Figure S14 for GC–MS results),
thus suggesting that the hydrogenation, which is catalyzed by
For surface reactions, we first considered the adsorption of
furan at NbOPO4(100). In the calculated structure (Fig-
À
ure 3c), the C O bond of furan is actually broken, and
a bidentate adsorption configuration is observed with a calcu-
lated adsorption energy of 0.66 eV. At the same time, the
surface Nb-O-Nb chain at the adsorption site also opens up to
À
accommodate the adsorbate, and as a result, two Nb O
bonds, one between surface Nb and O atoms and the other
between surface a Nb atom and the O atom of furan, occur
with nearly identical distances of 1.90 and 1.96 ꢀ. We also
calculated the tetrahydrofuran adsorption and obtained
Figure 2. Product distribution from the direct conversion of FA over
Pd/NbOPO4 versus the reaction time at 1508C and 1.5 MPa.
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Scheme 2. Proposed reaction pathway for the direct conversion of FA into octane.
can see, the first hydrogen
atom favors bonding to O2c
À
beside the Nb O bond with
a negative adsorption energy
of À0.23 eV, which indicated
that this hydrogen atom is
unstable. By performing climb-
ing image nudged elastic band
(CI-NEB) calculations, we
located the transition state for
the diffusion of the adsorbed
H atom to bond with the
O atom of the Nb-O-C linkage
and found an energy barrier of
0.34 eV. A stable intermediate
state resulted in which the
H atom is located in the
middle of two Nb-O-C link-
ages and shared by the two
O atoms. Afterwards, the
Figure 3. Calculated structures (side view) of a) hydrogenated NbOPO4(100), e) clean Nb2O5(001), b,f) the
Nb-O-Nb chains highlighted in dashed squares on both surfaces, and c,d,g,h) the corresponding structures
with one adsorbed furan (c,g) or tetrahydrofuran molecule (d,h). The Nb atoms are shown in light blue, P
in pink, O in red, C in black, and H in white.
a similar bidentate structure (Figure 3d), but with a much
higher adsorption energy of 1.39 eV. For the Nb2O5(001)
surface (Figure 3e), the calculated structures of adsorbed
furan and tetrahydrofuran are illustrated in Figure 3g,h, and
the corresponding adsorption energies were estimated to be
0.38 and 1.02 eV, respectively. The calculations at both
surfaces gave consistent results regarding the adsorption
structures and energetics. In particular, owing to the inherent
large stress within the Nb-O-Nb chain at the surface (as seen
second hydrogen atom diffuses onto the O atom of the Nb-
O-C linkage; for this step, the energy barrier is only 0.06 eV.
This H atom can then transfer to the C atom of the same
À
linkage to help break the C O bond with almost no barrier;
the resulting surface species can be viewed as dissociative
adsorbed butanol. Afterwards, one H atom can transfer via
the O atom of the left Nb-O-C linkage to the C atom to
promote the formation and desorption of butane as the
desired product. Further hydrogenation of the residual
O atom would lead to water formation and desorption, with
the surface ending up in the original state to finish the
catalytic cycle. In general, as can be seen from the calculated
energy profile, these processes should be able to occur under
rather mild conditions.
À
from the large angle), the local Nb O bond tends to break to
release the stress. Correspondingly, the “free” Nb and
O atoms can bind with the O and C atoms of the adsorbing
(tetrahydro)furan and promote its ring opening. These results
can therefore explain the ring-opening activity of NbOx-based
catalysts, and indicate the reaction order as furan hydro-
genation by supported metal clusters, followed by ring
opening of tetrahydrofuran at the substrate. This reaction
order is consistent with our experimental findings reported
above.
To further confirm the role of NbOx and investigate the
detailed reaction mechanism of the hydrodeoxygenation,
a series of in situ DRIFTS experiment were conducted with
tetrahydrofuran (THF) as the model compound. The result-
ing spectra (see Figure S9) indicated the chemisorption of
THF on Pd/NbOPO4 as a bidentate intermediate, and then its
transformation into adsorbed butanol by the addition of H2, in
agreement with the result of the DFT calculation.
We then calculated the subsequent hydrogenation pro-
cesses of adsorbed tetrahydrofuran on the Nb2O5(001) sur-
face. The corresponding energy profile is illustrated in
Figure 4. We assumed that H2 can readily dissociate at Pt
and then transfer to the surface of the Nb2O5 support. As we
For the production of long-chain alkanes from biomass
without complicated separation, other aldol-condensation
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Figure 4. Calculated energy profile for hydrogen diffusion and the hydrogenation of tetrahydrofuran on the Nb2O5(001) surface. All of the
calculated elementary steps are illustrated with the energy differences in black, the energy barriers in red, and the reacting H atoms in blue.
products with similar structures to that of FA, such as
difurfural acetone (DFA), 5-hydroxymethylfurfural acetone
(HMFA), di-5-hydroxymethylfurfural acetone (DHMFA), or
mixtures of these compounds, were also considered as
candidates for hydrodeoxygenation. Long-chain alkanes
(> 90%) were obtained in very high yield in each case with
the surface tension of the oxide bonding network. Moreover,
the complete hydrogenation process has been calculated and
appears to be very facile. In general, Pd/NbOPO4 as a multi-
functional catalyst plays three roles in the conversion of aldol
adducts into alkanes: 1) The supported noble metal is the
À
active center for hydrogenation; 2) NbOx species aid in C O
À
less C C bond cleavage, thus indicating that this process
bond cleavage, especially of the tetrahydrofuran ring; and
3) the niobium-based solid acid catalyzes the dehydration.
provides an approach to the production of long-chain alkanes
that is not only applicable to FA, but also to other biomass-
derived ketones. The composition of the alkanes can be tuned
by simply changing the nature (e.g., furfural versus HMF) and
the molar ratio of the biomass-derived reactants (e.g., furfural
or HMF/acetone = 1:10–1.5:1; see Figure S17).
Received: March 18, 2014
Revised: June 20, 2014
Published online: July 18, 2014
À
Keywords: biomass · C O bond cleavage ·
hydrodeoxygenation · liquid alkanes · multifunctional catalysts
This process does consume a lot of hydrogen for the
production of alkanes. For example, 8 mol of hydrogen are
consumed for the hydrodeoxygenation of 1 mol of 4-(2-furyl)-
3-buten-2-one to octane. However, the heat of octane
combustion is 5518 kJmolÀ1: much higher than that of 8 mol
of H2 (285.8 ꢁ 8 = 2286 kJmolÀ1). Thus, with respect to the
final products, the transformation of all the reactants, includ-
ing H2, still gives rise to an increase in energy.
In summary, we have reported a direct and efficient
approach for the production of liquid alkanes from biomass-
derived aldol adducts (FA, HMFA, DFA, and DHMFA) over
a multifunctional Pd/NbOPO4 catalyst under mild conditions.
Octane is obtained in very high yield (94%) from the direct
conversion of FA, and the catalyst can be used for 256 h
without deactivation at 1708C and 2 MPa, which is the best
performance reported so far. Further studies verified that
.
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