Production of liquid hydrocarbon fuels with acetoin and platform molecules derived from lignocellulose

Article abstract of DOI:10.1039/c5gc02414e

Acetoin, a novel C₄ platform molecule derived from new ABE (acetoin-butanol-ethanol) type fermentation via metabolic engineering, was used for the first time as a bio-based building block for the production of liquid hydrocarbon fuels. A series of diesel or jet fuel range C₉-C₁₄ straight, branched, or cyclic alkanes were produced in excellent yields by means of C-C coupling followed by hydrodeoxygenation reactions. Hydroxyalkylation/alkylation of acetoin with 2-methylfuran was investigated over a series of solid acid catalysts. Among the investigated candidates, zirconia supported trifluoromethanesulfonic acid showed the highest activity and stability. In the aldol condensation step, a basic ionic liquid [H₃N⁺-CH₂-CH₂-OH][CH₃COO^-] was identified as an efficient and recyclable catalyst for the reactions of acetoin with furan based aldehydes. The scope of the process has also been studied by reacting acetoin with other aldehydes, and it was found that abnormal condensation products were formed from the reactions of acetoin with aromatic aldehydes through an aldol condensation-pinacol rearrangement route when amorphous aluminium phosphate was used as a catalyst. And the final hydrodeoxygenation step could be achieved by using a simple and handy Pd/C + H-beta zeolite system, and no or a negligible amount of oxygenates was observed after the reaction. Excellent selectivity was also observed using the present system, and the clean formation of hydrocarbons with a narrow distribution of alkanes occurred in most cases.

Full text of DOI:10.1039/c5gc02414e

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DOI: 10.1039/C5GC02414E
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Production of liquid hydrocarbon fuels with acetoin and platform
molecules derived from lignocellulose
Received 00th January 20xx,
Accepted 00th January 20xx
Chenjie Zhu,a,b Tao Shen,a,b Dong Liu,a,b Jinglan Wu,a,b Yong Chen,a,b Linfeng Wang,c Kai Guo,*a
Hanjie Ying*a,b and Pingkai Ouyanga,b
DOI: 10.1039/x0xx00000x
Acetoin, a novel C4 platform molecule derived from new ABE (acetoin-butanol-ethanol) type fermentation via metabolic
engineering, was used for the first time as a bio-based building block for the production of liquid hydrocarbon fuels. A
series of diesel or jet fuel range C9-C14 straight, branched, or cyclic alkanes were produced in excellent yields by means of
C-C coupling followed by hydrodeoxygenation reactions. The hydroxyalkylation/alkylation of acetoin with 2-methylfuran
www.rsc.org/
was investigated over
a series of solid acid catalysts. Among the investigated candidates, zirconia supported
trifluoromethanesulfonic acid showed the highest activity and stability. In the aldol condensation step, basic ionic liquid
[H3N+-CH2-CH2-OH][CH3COO-] was identified as an efficient and recyclable catalyst for the reactions of acetoin with furan
based aldehydes. The scope of the process has also been studied by reacting acetoin with other aldehydes, and it was
found that abnormal condensation products were formed from the reactions of acetoin with aromatic aldehydes through
aldol condensation - pinacol rearrangement route when amorphous aluminium phosphate was used as a catalyst. And the
final hydrodeoxygenation step could be achieved by using simple and handy Pd/C + H-beta zeolite system, no or a
negligible amount of oxygenates were observed after the reaction. Excellent selectivity was also observed under the
present system, the clean formation of hydrocarbons with a narrow distribution of alkanes was occurred for most cases.
coupling reactions (e.g. aldol condensation, hydroxyalkylation,
ketonization, oligomerization) with reactive intermediates.23,24 The
latter method is a very promising way to go, because it offers
1. Introduction
Due to the depleting fossil fuel reserves and increasing greenhouse
gas emission, the exploration of feasible pathways for the
conversion of abundant and renewable biomass into clean fuels to
supplement or gradually replace the petroleum-based industry is
highly desirable.1-7 In this sense, lignocellulosic biomass is a
promising candidate due to it is an abundant and carbon-neutral
energy resource.8-12 Generally, there are several major strategies to
convert lignocellulose into liquid hydrocarbon fuels: fast
pyrolysis,13-16 liquefaction,17-19 and gasification followed by Fischer-
Tropsch synthesis.20-22 These routes usually deal with whole
lignocellulose leading to upgradeable platforms such as bio-oil or
syngas. Another important route involves depolymerization of
lignocellulose to yield platform molecules such as furfural,
hydroxymethylfurfural (HMF), or levulinic acid, etc. Catalytic
transformation of platform molecules for the production of liquid
hydrocarbon fuels can be obtained by oxygen removal process (e.g.
dehydration, hydrogenation, hydrogenolysis, decarbonylation) or in
some cases along with increase the length of carbon chain via C-C
pathways for the desired chain extensions to meet the number of
carbon atoms for the aimed fuel range, such as diesel and jet fuels,
which are two types of fuel in greatest demand and currently
sourced mainly from petroleum. Various reactive intermediates
such as acetone,25-28 2-hexanone,29 hydroxyacetone or
dihydroxyacetone,30,31 mesityl oxide,32 methyl isobutyl ketone
(MIBK),33 propanal,34 butanal,35-37 and levulinic acid38 have been
employed as synthons for the synthesis of liquid hydrocarbon fuels.
This route is especially important and meaningful when the synthon
is derived from biomass.
On the other hand, recent advances in metabolic engineering
have enabled the biological production of transportation fuels via
non-fermentative pathway,39,40 however, these processes usually
suffer from low yields and titers. Alternatively, sugar molecules
derived from cellulose or hemicellulose can be fermented into
hydrocarbons by microorganisms or genetically altered
microorganisms.41-43 For example, acetone-butanol-ethanol (ABE)
fermentation is one of the oldest known industrial processes with a
history of more than 100 years. The typical mass ratio of acetone:
butanol: ethanol in the fermentation broth is 3:6:1 (2.3: 3.7: 1 in
molar ratio). However, the economics of this process was hampered
by a number of bottlenecks such as co-production of low value
product acetone and high cost of separation. Moreover, high
acetone levels in the fermentation broth result in serious mass loss
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Scheme 1 The protocol for the synthesis of diesel or jet range alkanes using
acetoin as a bio-based synthon.
2. Results and discussion
2.1 Hydroxyalkylation and alkylation of acetoin with 2-
methylfuran
Fig. 1 Metabolic flux distributions in the Clostridium acetobutylicum wild-type
strain and the genetically engineered strains. Flux for glucose uptake was set to
100 mol and the other fluxes were determined as relative molar flux normalized
to the flux for glucose uptake.
Inspired by the pioneering work of Corma et al.,35,36 initial
experiments were carried out using 2-methylfuran (2-MF) as a
model substrate, which can be obtained by the selective
hydrogenation of furfural.48 The HAA of acetoin and 2-MF was first
carried out using para-toluenesulfonic acid as a catalyst under
solvent-free condition, 1b was obtained as the main product in 82%
yield (Scheme 2 and Figure S1), 1a was not detected in the reaction
mixture, which can be explained by the much higher rate of the
alkylation reaction than the hydroxyalkylation reaction, this result
was consistent with that found using acetone as the substrate.49
Under the same conditions, only 47% yield of 1b was obtained by
using H2SO4 as a catalyst, besides, 18% yield of 1c was obtained due
to the trimerization of 2-MF, and which is also a suitable diesel
precursor.36
of acetone due to its high volatility, thus leading to a lower solvent
yield.44 Recently, we used metabolic engineering strategies to
replace the volatile C3 compound acetone with non-volatile value-
added C4 compound acetoin (3-hydroxy-2-butanone) during ABE
fermentation.45 By overexpressing the α-acetolactate decarboxylase
gene alsD in Clostridium acetobutylicum, the acetoin yield was
markedly increased while acetone formation was reduced.
Subsequent disruption of adc gene effectively abolished acetone
formation and further increased acetoin yield (Fig. 1). Furthermore,
the removal and separation of ABE products from the fermentation
broth was achieved based on adsorption methodology using hyper-
cross-linked resin.46,47 Finally, the alsD-overexpressing adc mutant
generated butanol (13.8 g/L), ethanol (3.9 g/L), and acetoin (4.3 g/L),
but no acetone. When one glucose molecule is converted to the C4
compound acetoin (molecular weight, 88) instead of the C3
compound acetone (molecular weight, 58), both mass yield and
atom economy were improved. Butanol and ethanol produced in
the present system can be directly used as renewable
transportation fuels for blending with gasoline.
Acetoin, a novel product formed from the present new type of
ABE fermentation (Acetoin-Butanol-Ethanol) system, is amenable to
C-C bond formation with other platform molecules due to it
possesses α-hydrogen atoms. We were interested in the
exploration of using acetoin as a potential bio-based C4 building
block for the synthesis of renewable liquid hydrocarbon fuels. To
the best of our knowledge, there is no report on the production of
diesel or jet range alkanes that use acetoin as a platform molecule.
Herein, we report the highly efficient synthesis of C9-C14 straight,
branched, or cyclic alkanes from the reaction of acetoin with
lignocellulose derived platform molecules (Scheme 1).
Scheme 2 Pathway for the reaction between acetoin with 2-MF.
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However, taking into account the operation safety, equipment for the reaction, we were very pleased to find that noticeable
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corrosion, the cost of separation and waste disposal, solid acid is conversion and excellent selectivity was aDcOhiIe: v10e.d10i3n9/tCh5eGCc0a2s4e14oEf
preferable for use in the reaction. Therefore, a series of solid acid using TFA-ZrO2 as a catalyst (Figure S3), the desired product 1b was
catalysts were test for the reaction, which include zeolites, isolated in 93% yield (Table 1, entry 9). In the control experiments,
heteropolyacids, acid resins, and functionalized solid acids. Using there was no product formation when using ZrO2 as a catalyst
zeolite catalysts for the reaction were unsuccessful, the yields of 1b (Table 1, entry 10).
were either much lower or null (Table 1, entries 1-4), increased the
Encouraged by these results and in order to search for the
Si/Al ratio to enhance the acid strength of the acid sites in the H-Y optimum reaction conditions, we then screened a variety of
zeolite did not show any improvement (Table 1, entries 2 and 3).50 parameters of this reaction, such as different loading of TFA over
The use of heteropolyacids such as H4SiW12O40, H3PW12O40 or zirconia support (5 to 30 wt%, designated as TFA5~30-ZrO2), dosage
Cs2H2PMo11VO40 as catalysts was also unsatisfactory due to the of catalyst, reaction temperature and reaction time (Table S1).
poor reaction selectivity caused by over-alkylation (Table 1, entries Under optimum conditions, 100% conversion of acetoin and 96%
5-7).
yield of 1b were obtained over 0.1 g TFA20-ZrO2 after reacting at
In the previous work of Zhang’s group,31,32,37,49 Nafion-212 film 323 K for 1 h under solvent free condition. It is noteworthy that the
was found to be an efficient solid acid catalyst for the HAA reaction. reaction can also proceed smoothly even at room temperature,
However, the high cost and inconvenient operation of Nafion-212 albeit a longer reaction time was required (Table 1, entry 11). Under
film as well as recent significant advances in designing new solid the same reaction conditions, the yield of 1b was much lower when
acids for biomass conversion prompted us to develop alternative using Nafion-212 resin as a catalyst (Table 1, entry 12), which clearly
catalyst for this reaction.51 According to the previous research demonstrate the superior catalytic activity of TFA-ZrO2. According
results,52 the HAA reaction is sensitive to the acid strength of the to the yield of 1b over TFA-ZrO2 and Nafion-212 at low catalyst
catalyst, strong acid is more active than weak acid for this reaction. dosage (Fig. 2a) and the acid amount measured by NH3
Trifluoromethanesulfonic acid (CF3SO3H) is known to be a strong chemisorption (0.56 mmol g-1 for TFA-ZrO2) or provided by supplier
acid (H0 = -13),53 and has been used a homogeneous acid catalyst (1.1 mmol g-1 for Nafion-212), we also calculated the TONs and
for a variety of chemical transformations due to its extremely high TOFs for the conversion of 2-MF to 1b over these two catalysts (Fig.
thermal and chemical stability.54,55 However, the direct use of 2b). As shown in the Figure 2b, TFA-ZrO2 showed much higher
CF3SO3H as a catalyst in the present HAA process led to a violent catalytic efficiency for the alkylation of 2-MF with acetoin compared
reaction, the formation of black tar was observed during the to the Nafion-212 catalyst.
reaction (Figure S2). We envisaged that immobilization of CF3SO3H
The reusability of the catalyst TFA-ZrO2 was also investigated by
would properly address this problem by providing a readily the HAA of acetoin with 2-MF. As can be seen in Figure S9, the
recyclable catalyst with reasonably catalytic activity. Recently, catalyst could be recycled for 5 times with no apparent loss of
zirconium compounds have drawn considerable attention for activity, however, the acetoin conversion decreased by 14% and the
various organic transformations, and the search for zirconium oxide yield of 1b decreased by 22% for the recycle runs 6, the decrease of
as supports for different catalysts has also received keen interest in the activity of TFA-ZrO2 may be due to the adsorption of reactants
the
past
decade.56,57
Thus,
zirconia
supported and HAA products on the strong acid sites of the catalyst. After
trifluoromethanesulfonic acid (TFA-ZrO2) was prepared and tested washed by MeOH and calcinated at 473 K for 6 h, the acetoin
conversion and the yield of 1b returned to the original levels (Figure
Table 1 Results of the HAA of acetoin with 2-MF a
S9, run 7). The recovery of the activity means that the irreversible
poisoning did not take place in the catalyst. Elemental analysis
showed that the sulfur content of the catalyst fell only a little from
4.88 to 4.79 after seven reaction cycles. According to the high
activity and stability of TFA-ZrO2, it can be considered as a
promising catalyst for further study.
Product selectivityc (%)
Entry
Catalyst
Conv.(%)b
1b
1c
--
other
1
H-β (Si/Al = 40)
H-Y(Si/Al = 5)
H-Y(Si/Al = 20)
H-ZSM-5 (Si/Al = 38)
H3PW12O40
4
30
34
13
87
82
90
92
100
--
--
100
22
37
43
73
66
64
12
5
2
33
28
57
19
23
20
78
95
--
45
35
--
3
4
5
8
6
H4SiW12O40
11
16
10
--
7
Cs2H2PMo11VO40
Amberlyst-15
TFA-ZrO2
8
9
10
11d
ZrO2
--
--
TFA-ZrO2
100
74
91
80
--
9
12
a
Nafion-212
6
14
Reactions were performed by using 2-MF (22 mmol), acetoin (10 mmol),
catalyst (0.1 g) under solvent-free condition at 333K for 2 h unless otherwise
Fig. 2 a) Conversion of acetoin and yield of 1b under the catalysis of Nafion-212
or TFA-ZrO2 at low catalyst dosage. Reaction conditions: 2-MF (22 mmol), acetoin
(10 mmol), catalyst (0.015 g) under solvent-free condition at 323K for 1 h; b)
TONs and TOFs for the conversion of acetoin to 1b calculated based on the yield
of 1b and the acid amounts of catalysts.
b
c
noted.
Conversion with respect to the limiting reactant acetoin.
Determined by GC. d The reaction was performed at room temperature for 6
h.
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Furthermore, the reactivity of acetoin, acetone, and butanal with the trans-geometry. Surprisingly, 1,2-diol compound 2c was not
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DOI: 10.1039/C5GC02414E
2-MF in the HAA reaction was compared under same conditions. As detected in the reaction mixture, this result is quite different from
can be seen from Scheme 3, the highest yield of the HAA product previously report about similar aldol condensation reaction
1b was observed when acetoin was used as the feedstock. This is between α-hydroxylated ketone with aldehyde, where 1,2-diol
quite different from previous reports,36,49 where an aldehyde compound was usually obtained as the major product.30,60,61 This
usually shows higher reactivity compared to a ketone in the HAA abnormal regioselectivity is probably due to this reaction is kinetic
reaction. The higher reactivity of acetoin could be explained by the control versus thermodynamic control. Meanwhile, no C13
enhanced electrophilicity of the carbonyl group by the electron- oxygenate was detected in the reaction mixture, which is generated
withdrawing effect of the hydroxyl group attached to one of the α- through the aldol condensation of one furfural molecule with two
carbon atoms. Furthermore, butanol needs to be partially oxidized acetoin molecules. Compared to the reaction of furfural with
or dehydrogenated to butanal before its use in the reaction, which acetoin, the condensation reaction between two acetoin molecules
will increase the cost of the process. Acetoin is one of the major is much slower, the steric hindrance and electron donating
products from the present new type of ABE fermentation and can properties of the attached hydroxyl groups prevent the formation
be directly used as carbonyl compound for the HAA reaction. This of reactive enolate intermediate, therefore, the self-condensation
can be considered as an advantage of acetoin route. Under the product of acetoin, 2,3,5,6-tetramethyl-1,4-dioxane-2,5-diol, was
same reaction conditions, an evidently higher yield of the HAA also not observed during this reaction due to this route is not
product (93 % of 1b vs. 82 % of 1d) can be achieved when acetoin thermodynamics favored. However, under such strong basic
was used to replace acetone as the carbonyl compound (Scheme 3). condition, other side reactions such as Cannizaro reaction, radical
polymerization, resinification of furfural, and Michael addition to
the formed enone may be happened during the reaction,62,63
resulting in generation of some unfavourable polymers. Thin layer
chromatography monitoring (Silica, 15% MeOH in CH2Cl2) of the
reaction showed the formation of 4-5 products in the reaction
mixture withdrawn after 0.5 h, and polymerization afterwards.
HPLC-MS also showed that a serious of high molecular weight
products (m/z > 400) was formed during the reaction.
Although the present NaOH catalytic system is practical due to it
is a cost effective base, it still have disadvantages such as low yield,
poor selectivity, and downstream processing issues. Thus,
numerous catalysts were tested for the reaction in order to solve
Scheme 3 The comparison of the reactivity of acetoin, acetone, and butanal with
the selectivity and environmental issue from using of such mineral
2-MF in the HAA. Reaction conditions: 2-MF (22 mmol), carbonyl compounds (10
base, which including heterogeneous catalysts, organocatalysts, and
mmol), TFA- ZrO2 (0.1 g), 333K, 2h.
task-specific ionic liquids. The conversions of the furfural, the yields
of 2b and others over different catalysts are shown in Figure 3. A
range of solid base catalysts were first tested for the reaction,
however, all of them were unsatisfactory except for hydrotalcite
2.2 Aldol condensation of acetoin with lignocellulose derived
aldehydes
Encouraged by the results obtained from HAA of acetoin with 2-
MF, we then wondered if furfural could be direct used for the
reaction with acetoin via Claisen-Schmidt reaction, due to it has
been produced for decades on an industrial scale by the hydrolysis-
dehydration of the hemicellulose part of lignocellulosic biomass.58,59
The aldol condensation of furfural and acetoin was first carried out
using NaOH as a catalyst, the results showed that the reaction of
furfural with acetoin at its methyl group which is connected to
carbonyl group to give 2b as the main product in 52% yield via
(rehydrated) and MgO-ZrO2 exhibited moderate catalytic activity.
Amine-based organocatalysts such as L-proline, tryptophan, and
1,8-diazabicycloundec-7-ene (DBU), which were previously efficient
organocatalysts for the aldol reaction,30,60,61,64 failing to catalyze this
reaction probably due to the steric reasons.
1
dehydration of 2a (Scheme 4 and Figure S10). H NMR spectrum of
2b showed two olefinic protons of the enone system at 6.93 and
7.49 ppm as two doublets with 16 Hz coupling constant, indicating
Fig. 3 Results of the reaction between acetoin with furfural over different
catalysts (For detailed experimental protocols, see the Supporting Information).
Scheme 4 Pathway for the reaction between acetoin with furfural.
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On the other hand, room-temperature ionic liquids (RTILs), as Scheme 6, 5-MF was an ideal substrate, the C10 fuel precursor 2d
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eco-friendly reaction media, have attracted increasing attention can be obtained with a high yield up to 87% (Figure S14). However,
due to their particular properties, such as the extremely low vapor the reaction of acetoin with HMF was not satisfactory (Figure S15),
pressure, high thermal stability, and excellent solubility. In addition, some unidentified polymers were formed during the reaction
RTILs have also been referred to as “designer catalysts” as their probably due to polymerization of HMF, which may be ascribed to
chemical and physical properties could be adjusted by careful the low stability of HMF under the present alkaline aqueous
choice of the cation or anion. Recent advances in ionic liquids conditions.38,70 When the substrate changed to aromatic aldehyde,
provided efficient routes to the preparation of task-specific ionic such as syringaldehyde, the results showed that no aldol
liquids (TSILs).65 Thus, two low-cost basic ionic liquid [H3N+-CH2-CH2- condensation reaction was occurred under the present system, and
OH][CH3COO-] (EAIL) and [H3N+-CH2-CH2-OH][CH3CH(OH)COO-] (LAIL) the corresponding condensation product 2f that could be expected
were designed and prepared using our previous method,66 and in analogy with furan aldehyde and acetoin was not detected after
tested as catalysts for this reaction. To our delight, the reaction was the reaction. After further screening of catalysts and optimizing the
performed with reasonable conversion and excellent selectivity, 83% procedure, we found that replacing the catalyst EAIL with acid-base
conversion of furfural and 78% yield of 2b were obtained when EAIL bifunctional catalyst amorphous aluminium phosphate (ALPO)71 and
was used as a catalyst (Fig. 3). LAIL shows comparable catalytic increasing the reaction temperature significantly promoted the
activity to EAIL and serves as another good catalyst to enhance the reaction (Scheme 7 and Figure S16), and the abnormal C11 fuel
reaction remarkably. For product separation and recycling of these precursor product 2g was identified as the exclusive product
TSIL catalysts, the protocol was indeed quite easy and simple. For through aldol condensation - pinacol rearrangement (Scheme 8 and
example, the EAIL is completely soluble in water, but insoluble in Figure S17).
EtOAc (Figure S11), this could be beneficial since it can provide an
easy route to separate the product and catalyst. After the reaction,
extraction of the mixture with EtOAc led to the separation between
the catalyst EAIL and products. Fresh substrates were then
recharged to the residual water layer which contains the catalyst
EAIL and the mixture was heated to react once again. It was found
that the catalyst EAIL could be recycled for five times with no
apparent loss of catalytic activity (Figure S12), and only 2.7% loss of
weight in EAIL was observed after five times recycling.
Although, so far, we cannot be certain of the actual role of EAIL in
this reaction, on the basis of the result obtained from the
comparative experiment using another basic ionic liquid 1-butyl-3-
methylimidazolium hydroxide [bmim][OH] as a catalyst (Fig. 3), we
believe that the reaction pathway was more than base-catalyzed
condensation, a synergistic catalysis effect might be occurred in this
reaction. A tentative mechanism is depicted in Scheme 5. The -NH3
and -OH group in the EAIL could form hydrogen-bonding network
with furfural and acetoin, respectively. Such hydrogen-bonding
network not only activates the furfural by increasing the positive
charge on the corresponding carbon atom, but also facilitates the
departure of α-H of acetoin to form the enolate intermediate.
With the aim to develop and define the scope and limitation of
the present novel catalytic system, this procedure was then
extended for the reaction of acetoin with other aldehydes, which
Scheme
6 The reaction of acetoin with other aldehydes derived from
lignocellulose catalyzed by EAIL.
including 5-hydroxymethylfurfural or 5-methylfurfural derived from Scheme 7 The reaction of lignin model compounds (p-hydroxy benzaldehyde,
cellulose,67,68 and syringaldehyde derived from lignin.69 As shown in
vanillin or syringaldehyde) with acetoin catalyzed by ALPO.
Scheme 5 Tentative mechanism for the reaction of furfural and acetoin catalyzed
Scheme 8 Tentative mechanism for the reaction of acetoin with syringaldehyde
by EAIL.
catalyzed by ALPO.
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To check the substituent effects on the lignin model compound H2 pressure, solvent and reaction time. The Pd/C + H-beta zeolite
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conversion, vanillin and p-hydroxy benzaldehyde were also tested system under moderate conditions (473 K and 3 MPa H2 pressure)
under the present system (Scheme 7 and Figure S17-S19), the was finally selected for the HDO process.
results showed that the presence of the methoxy group slightly
By maintaining all of the key parameters of the present catalytic
decreased the reactivity of the substrate, probably due to the system, we then applied the procedure for other fuel precursors
electron-donating property of the methoxy group. In view of the (Table 2). As shown in Fig. 5, most substrates underwent smooth
fact that the aldol reaction of electron-rich aromatic aldehydes is transformation to afford their corresponding theoretical alkane
much more difficult than the reaction of electron-poor aromatic products in moderate to excellent yields, and the present catalytic
aldehydes,64 the results obtained with the present procedure are system showed high selectivity for the production of diesel range
very satisfactory.
alkanes. In all cases, these fuel precursors were completely
converted to alkanes and no or a negligible amount of oxygenates
were observed based on the results from the GC-MS. The carbon
balance was almost quantitative for most of the HDO reactions,
about 95% of the carbon was in the liquid organic products
according to the TOC analysis while the rest of the carbon
containing products were CO, CO2, CH4, and C4H10. It is noteworthy
that for most HDO system, the products are very complicated by
furan-ring opening, carbon chain fragmentation, rearrangement,
2.3 Hydrodeoxygenation of fuel precursors into alkanes
As the final aim of this work, the direct hydrodeoxygenation of
these fuel precursors into alkanes were performed in batch reactor.
The aldol condensation product of acetoin and 5-MF 2d was first
chosen as a model fuel precursor for this study. According to the
previous research results on the HDO reaction mechanism of
biomass derived oxygenates,72,73 the dehydration over acid sites
followed by the hydrogenation over metal sites is the major
pathway for the C-O cleavage reaction. Noble metal nanoparticles
supported on acidic solid carriers have proven to be effective and
recyclable catalysts in the HDO process.49,70,74,75 To investigate the
role of each component in the catalysts, commercially available
hydrogenation catalysts such as Pd/C and Pt/C combined with
different solid acids were tested for the reaction. After initial
catalyst screening, three solid acids NbOPO4, TaOPO4, and H-Beta
zeolite (Si/Al = 40) were chosen and used for the HDO process. In
most cases, Pd/C shows comparable catalytic activity to Pt/C in
removing the oxygen atoms in the substrate, however, Pt/C shows a
more strong cracking capacity of C-C bond compared to Pd/C. Thus,
Pd/C was chosen as hydrogenation catalyst for further studies.
Most excitingly, changing the solid acid such as NbOPO4 or TaOPO4,
which were traditional effective catalysts in the HDO process,70,74 to
commercial available H-beta zeolite, led to the highest alkane
selectivity of 96%, and the clean formation of C9H20 and C10H22
alkanes through this highly effective HDO process is remarkable
(Figure 4 and Figure S20). Encouraged by these results and in order
to search for the optimum reaction conditions, we screened a
variety of parameters of this reaction, such as reaction temperature,
and cyclization reactions, rendering
a wide distribution of
hydrocarbons. However, excellent selectivity was observed under
the present system, the clean formation of hydrocarbons with a
narrow distribution of alkanes was occurred for most cases (Figure
S20-S24). Cyclic alkanes can also be obtained in high yield though
this effective catalytic system (Figure S24), which is the second
most abundant component required in the commercial and military
jet fuels with the mass percent of 20-50 wt%.76,77 Compared to
straight alkanes, cyclic alkanes have higher densities and volumetric
heating values due to the strong ring strain.78,79
However, if we take a closer look into the product distribution
(Fig. 6), it was found that the carbon yield of diesel range alkanes is
lowest when 1b was used as a feedstock, and its corresponding
theoretical alkane product or other C14 isomer alkanes were not
Table 2 The structures of the fuel precursors and their theoretical HDO products
Fuel precursors
Theoretical alkane product
3a
C9H20 straight alkane
2b
2d
2e
3b
C10H22 straight alkane
3c
C10H22 straight alkane
3d
1b
2i
C14H30 branched alkane
3e
Fig. 4 GC-MS chromatogram of the organic-phase products produced by HDO of
2d with Pd/C + H-beta zeolite.
C11H22 cyclic alkane
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5GC02414E
Fig. 5 Carbon yields of different alkanes from the HDO of different precursors
over the Pd/C+H-β system. The diesel range alkanes, gasoline range alkanes and
light alkanes account for C9-C15, C5-C8 and C1-C4 alkanes, respectively.
Scheme 9 Products identified in the HDO of 1b and control experiment catalyzed
by H-beta zeolite.
identified after the reaction. As shown in Scheme 9 and Figure S22,
two main compounds 3f (C12H26) and 3g (C9H20) were observed after
the reaction with 3f as the dominant product. These two products
were generated from C-C bond cleavage adjacent to the quaternary
carbon atom in the middle of the undecane chain, and removal of a
C2 (ethyl) or C5 (pentyl) group from the main chain, respectively.
This phenomenon can be explained by the stability of carbocation,
which is an important intermediate during the HDO process.80 It is
well know that the stability of carbocation increases in the order of
primary > secondary > tertiary. The C-C cleavage of 1b at the carbon
atom adjacent to furan ring will generate tertiary carbocation, the
formation of the most stable tertiary carbocation led to the C-C
cleavage products. Similarly, the lower yield of theoretical alkane
when 2i was used as a feedstock is due to the formation of
secondary carbocation. However, for products 3f or 3g, either was
Fig. 6 Products distribution of alkanes from the HDO of the fuel precursors.
derived from C-C cleavage of 3d through the formation of tertiary reaction conditions, 3h and 3i were detected in 61% and 10% yields,
carbocation, in principle, the selectivity of these two products respectively (Scheme 9 and Figure S25 and 26). These results
should be about equal. Thus, we speculated that the fragmentation confirmed that acid induced cracking reaction plays a dominant role
does not occur at the final stage of the HDO, but earlier in the in the HDO process. A similar phenomenon has also been recently
reaction coordinate. To check this point, control experiment was observed for the acidic amorphous silica-alumina catalyzed cracking
carried out using only H-Beta zeolite as a catalyst under similar reaction of biomass derived oxygenate.36
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In conclusion, a novel C4 building block “acetoin”, which is derived
from lignocellulose by new ABE type fermentation, has been used
as a bio-based synthon for the production of liquid hydrocarbon
fuels. It was found that TFA-ZrO2, ionic liquid EAIL, and aluminium
phosphate were efficient and recyclable catalysts for the HAA
reaction and aldol condensation, respectively. And the HDO step
could be achieved by using simple and handy Pd/C + H-beta zeolite
system, which was found to have high activity in the
conversion of various furan- or aromatic- based oxygenates, a series
of well-defined diesel or jet fuel range C9-C14 straight, branched, or
cyclic alkanes could be produced in excellent yields under the
present catalytic system. This work not only provides a new bio-
based platform chemical for the direct synthesis of liquid alkanes,
but also offers new opportunities to upgrade biomass to clean fuels
and chemicals by bridging the biological and chemical catalysis
gap.81,82
25 G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic,
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Acknowledgements
The authors thank the financial support by the National Science
Fund for Distinguished Youth Scholars (Grant No.: 21025625);
Program for Changjiang Scholars and Innovative Research Team in
University (Grant No.: IRT_14R28); The Major Research Plan of the
National Natural Science Foundation of China (Grant No.:21390204);
National High-Tech Research and Development Plan of China (863
Program, Grant No.: 2012AA021201); The National Natural Science
Foundation of China (Grant No.: 21406110) and Jiangsu Province
Natural Science Foundation for Youths (Grant No.: BK20140938);
The State Key Laboratory of Motor Vehicle Biofuel Technology
(Grant No.: KFKT2013001).
34 W. Shen, G. A. Tompsett, K. D. Hammond, R. Xing, F. Dogan, C.
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