One-pot production of hydrocarbon oil from poly(3-hydroxybutyrate)

Article abstract of DOI:10.1039/c4ra00892h

Poly(3-hydroxybutyrate) (PHB) is an energy storage material of many microbial species, and has been found to be an effective feedstock for production of renewable hydrocarbon oils. A high oil yield (up to 38.2 wt%) was obtained in a phosphoric acid (H₃PO₄) solution at mild temperatures (165-240 °C). PHB and crotonic acid (C₄H ₆O₂), a dominant thermal degradation product of PHB, were deoxygenated mainly via decarboxylation, generating similar liquid and gaseous products. Carbon dioxide and propylene were the major products in gas phase with little CO formation. The hydrocarbon oil (C4-C16) is a mixture of alkanes, alkenes, benzenes and naphthalenes. Aromatics (C10-C15) were the major hydrocarbons in a 100 wt% H₃PO₄ solution, while alkenes and alkanes (C4-C9) were favored in diluted solutions (50 wt% to 85 wt% H ₃PO₄). The concentration of H₃PO₄ was a key factor that affected the oil composition and yield. A highly efficient decarboxylation of crotonic acid at 220 °C for 3 hours resulted in 70.8 wt% of oxygen being removed as CO₂ and 57.0 wt% of carbon being recovered as hydrocarbon oil. The H₃PO₄ solution can be repeatedly used for high yield oil production. This work shows that a type of new biological feedstock can be used to produce renewable hydrocarbon oil in an efficient one-pot reaction. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00892h

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One-pot production of hydrocarbon oil from
poly(3-hydroxybutyrate)†
Cite this: RSC Adv., 2014, 4, 14320
*
Shimin Kang and Jian Yu
Poly(3-hydroxybutyrate) (PHB) is an energy storage material of many microbial species, and has been found
to be an effective feedstock for production of renewable hydrocarbon oils. A high oil yield (up to 38.2 wt%)
was obtained in a phosphoric acid (H3PO4) solution at mild temperatures (165–240 ꢀC). PHB and crotonic
acid (C4H6O2), a dominant thermal degradation product of PHB, were deoxygenated mainly via
decarboxylation, generating similar liquid and gaseous products. Carbon dioxide and propylene were the
major products in gas phase with little CO formation. The hydrocarbon oil (C4–C16) is a mixture of
alkanes, alkenes, benzenes and naphthalenes. Aromatics (C10–C15) were the major hydrocarbons in a
100 wt% H3PO4 solution, while alkenes and alkanes (C4–C9) were favored in diluted solutions (50 wt%
to 85 wt% H3PO4). The concentration of H3PO4 was a key factor that affected the oil composition and
yield. A highly efficient decarboxylation of crotonic acid at 220 ꢀC for 3 hours resulted in 70.8 wt% of
oxygen being removed as CO2 and 57.0 wt% of carbon being recovered as hydrocarbon oil. The H3PO4
solution can be repeatedly used for high yield oil production. This work shows that a type of new
biological feedstock can be used to produce renewable hydrocarbon oil in an efficient one-pot reaction.
Received 31st January 2014
Accepted 7th March 2014
DOI: 10.1039/c4ra00892h
www.rsc.org/advances
Hydrotreating under high pressure hydrogen is a typical
method to obtain hydrocarbons from bio-oils, but consumes a
large amount of hydrogen.13,14 Thermal (catalytic) pyrolysis is an
1 Introduction
Poly(3-hydroxybutyrate) (PHB) is a biopolyester produced by
many bacterial species as an energy storage material.1–3 It has
been produced on large scales through industrial fermentation
of renewable carbohydrates.4,5 PHB is an environmentally
friendly bioplastic and has similar material properties to
polypropylene for a variety of applications.6–8 PHB is also an
energy-rich polymer and, similar to bio-oil, may be a renewable
feedstock for production of liquid hydrocarbons. Little work,
however, has been done to convert PHB into hydrocarbons as a
“drop in” transportation fuel. Crotonic acid (trans-2 butenoic
acid) is the major monomeric product formed from thermal
degradation of PHB.9,10 Because of the presence of carboxyl
groups, PHB deoxygenation is necessary in order to produce
hydrocarbons. It has been reported that at high temperatures
(>300 ꢀC) propylene is formed from PHB decarboxylation in
hydrothermal reforming.11 Lewis-acid may play a catalytic role
in decarboxylation of unsaturated carboxylic acids at high
alternative processes for decarboxylation/decarbonylation of
organic acids, which is usually performed at high temperatures
(e.g., $300 ꢀC).14–19 Thermogravimetric analysis has shown
complete gasication of PHB at high temperatures (>280
ꢀC),9,20,21 which implies that multiple catalytic reactors are
needed to produce liquid hydrocarbons from a solid PHB-con-
taining feedstock. Due to the worldwide demand for bio-based
“drop in” transportation fuels, new renewable feedstocks such
as PHB and processing technology should be explored. Specif-
ically, a novel method for PHB deoxygenation at low tempera-
ture without hydrogen is meaningful.
Phosphoric acid (H3PO4) is usually considered as an inde-
composable, non-volatile, non-oxidizing, mild acid, and has
been used as a catalyst or solvent in many applications.22–26 In
this work, we used H3PO4 solution in one-pot conversion of PHB
into liquid hydrocarbon oil. It is rst time demonstrated that a
high oil yield is achieved at relatively low temperatures (165 to
temperature (365 C).12 In general, hydrotreating and thermal
ꢀ
ꢀ
240 C) in the absence of hydrogen.
(catalytic) pyrolysis are two of the most studied processes
for deoxygenation of carboxylic compounds in biomass.
2 Experimental section
2.1 Materials
Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii
96822, USA. E-mail: jianyu@hawaii.edu
† Electronic supplementary information (ESI) available: FT-IR spectra of CO2, CO,
propylene, and various status of H3PO4 solution; GC-MS analysis of PHB derived
oils produced at various H3PO4 concentrations and temperatures; 31P-NMR
analysis of fresh 100 wt% H3PO4 and the H3PO4 solution aer reaction. See
DOI: 10.1039/c4ra00892h
Crotonic acid (98 wt%) and polyphosphoric acid (115% H3PO4
basis) were purchased from Sigma-Aldrich (St Louis, MO, USA).
Pre-determined concentrations (50 wt% to 100 wt%) of H3PO4
were prepared from polyphosphoric acid and distilled
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de-ionized water. Poly(3-hydroxybutyrate) (PHB 98 wt%) was was put in a 600 mL autoclave (Parr Instrument, IL, USA). The
obtained from Bio-on (Bologna, Italy). Its weight-average autoclave was then purged with nitrogen at least ten times of
molecular weight (138 000 Da) and number-average molecular the reactor volume to remove air. The reactor was sealed and
weight (53 100 Da) were measured with gel permeation chro- heated to a pre-determined temperature. When the temperature
matography (GPC) and calibrated with polystyrene standards. reached the setting value in about half an hour, the reaction
The polystyrene standards with narrow molecular weight time was set as zero, and thereaer recorded. Aer reaction, the
distribution were purchased from Sigma-Aldrich.
reactor was cooled down in ambient conditions and gas
samples were taken by using a FT-IR gas cell for qualitative
analysis. The quantitative determination of CO2 and propylene
was performed by using a gas chromatograph equipped with a
thermal conductivity detector (GC-TCD, Bruker 450-GC, FL,
USA) and a Carboxen-1006 Plot (30 m  0.53 mm) column. Both
CO2 and propylene were calibrated with pure gases against
helium.
2.2 Formation and analysis of hydrocarbon oil
In a typical experiment, 0.5 g PHB or crotonic acid and 10 mL of
H3PO4 solution were put into a 20 mL polytetrauoroethylene
(PTFE) reactor, and the reactor was purged with N2 for about 10
minutes. The PTFE reactor was then sealed and le for a pre-
determined reaction time in a thermostat oven that was main-
ꢀ
tained at a desired temperature (165 to 240 C). Aer reaction,
the PTFE reactor was quickly cooled down in tap water. The
reaction solution consisted of a top layer of oil products and a
bottom layer of aqueous phosphoric acid solution. The oil was
recovered by extraction with methylene chloride, and the water
moisture of solvent solution was removed with anhydrous
magnesium sulfate. Aer evaporation of methylene chloride at
40 ꢀC, the oil was weighted to calculate the yield from the initial
amount of PHB or crotonic acid, excluding the residual crotonic
acid. During the evaporation of methylene chloride in the
extracted oil samples, a control of pure methylene chloride with
the same volume was put in the same condition at the same
time. Aer the pure methylene chloride was completely evapo-
rated, the extracted oil samples was kept in the evaporation
condition for a little longer time (1–2 minutes) to make sure that
the solvent methylene chloride was also completely evaporated.
Duplicates or triplicates were performed to get an average yield
and error range. In the experiment of phosphoric acid reuse,
fresh PHB was added into the used H3PO4 solution to conduct
the reaction under the same conditions as described above.
In order to determine the residual crotonic acid aer reaction,
the methylene chloride solution and the H3PO4 solution aer
solvent extraction were analyzed by using a gas chromatograph
equipped with a ame ionization detector (GC-FID, Bruker 450-GC,
CA, USA) and a high performance liquid chromatograph (HPLC,
Shimadzu, Japan), respectively. The oil products in methylene
chloride were analyzed with a gas chromatograph-mass spec-
trometer (GC-MS, Bruker 436-GC, CA, USA), a Fourier transform
infrared spectrophotometer (FT-IR, Avatar 370, ThermoNicolet, FL,
USA), and a carbon-13 nuclear magnetic resonance spectrometer
(13C-NMR, Varian Unity Inova 400 MHz), respectively. Deuterated
chloroform was used as the solvent in 13C-NMR analysis. The
residual chemicals le in the H3PO4 solution before and aer
extraction were also analyzed with FT-IR, and phosphorus-31
nuclear magnetic resonance spectroscopy (31P-NMR, Varian Unity
Inova 500 MHz), and a total organic carbon (TOC) analyzer. The
TOC results were used for carbon recovery analysis along with the
determination of major gas products as shown below.
3 Results and discussion
3.1 Oil products analysis
The PHB- and crotonic acid-derived oils produced in typical
reaction conditions (100 wt% H3PO4, 220 ꢀC, 3 hours) were
analyzed by 13C-NMR (Fig. 1), FT-IR (Fig. 2), and GC-MS (Fig. 3),
respectively. The analysis indicates that the oil products derived
from both crotonic acid and PHB are almost the same.
Comparing the 13C-NMR spectra of the raw materials (crotonic
acid and PHB) with that of their oil products indicates that the
carboxyl groups were almost completely removed (Fig. 1), and
aromatic, alkene and alkane groups were formed in the oil
products. This is conrmed with FTIR analysis (Fig. 2). The
huge absorption peak of C]O at 1700 cmÀ1 for crotonic acid
and 1720 cmÀ1 for PHB disappeared in the oil products. The
peaks (3100 to 2800 cmÀ1, 1458 cmÀ1, 1380 cmÀ1, and
870 cmÀ1) of oil products indicate the presence of methyl,
methylene, and aromatic groups as the major groups. There is a
very small peak around 1710 cmÀ1 in the FT-IR spectra of both
crotonic acid and PHB derived oils, which may come from some
aldehyde and/or ketone compounds. With the GC-MS analysis,
a few aldehydes and ketones (e.g., retention time of 28.318 and
30.614, in Fig. 3) were detected. The GC-MS analysis also
conrms that the carboxylic group of organic acid and ester
were almost completely removed, and aromatics are the main
products. These analytical results consistently indicate that
PHB was to a great extent deoxygenated at a quite low temper-
ꢀ
ature (220 C) with formation of various hydrocarbons in one
pot reaction in the absence of hydrogen. Compared with the
conventional deoxygenation methods (e.g., pyrolysis and
hydrotreating), this reaction system has some unique advan-
tages in low reaction temperature and absence of hydrogen.
Because of the same products formed from both PHB and cro-
tonic acid, crotonic acid may be the key intermediate of PHB
decarboxylation.
3.2 Gaseous products and analysis
The gas products formed from PHB and crotonic acid deoxy-
genation were analyzed with FT-IR (Fig. 4). In comparison
2.3 Gas formation and analysis
In a typical experiment, 3.6 g of PHB or crotonic acid and 72 mL with FT-IR spectra of pure CO2, CO and propylene (Fig. S1, in
of H3PO4 solution were added into a 180 mL pyro-beaker, which ESI†), it was found that CO2 was the major gas product
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Fig. 1 13C-NMR spectra of PHB, crotonic acid and their oils produced in 100 wt% H3PO4 at 220 ꢀC for 3 hours.
3.3 Carbon recovery analysis
Since crotonic acid is the key intermediate of PHB decarboxyl-
ation and generates the very similar liquid and gas products, we
used crotonic acid as the control reactant for carbon balance
analysis of the reaction system. As shown in Table 1, the total
carbon recovery from the initial carbon of crotonic acid was more
than 90.9 wt%, which included CO2 (18.0 wt%) and propylene
(4.9 wt%) in the gas phase, hydrocarbon oil (57.0 wt%) and
organic carbon residues (11.0 wt%) in the liquid phase. Very little
carbon (<2.1 wt%) was formed as char in solid phase. In addition
to the loss of solid carbon, a small amount of carbon in the gas
phase such as alkyl is also not included in the carbon balance,
even though they were detected by FT-IR (Fig. 4). Compared with
many biomass-to-liquid researches where only 25–45% of initial
carbon was recovered,27 this research has a quite good carbon
recovery. Importantly, the hydrocarbon oil contains 57.0 wt% of
initial carbon, which is miscible with methylene chloride and
hexane (data not shown here).
Fig. 2 FT-IR spectra of PHB, crotonic acid and their oils produced in
100 wt% H3PO4 at 220 ꢀC for 3 hours.
followed by propylene, with negligible amount of CO. In
another words, PHB was deoxygenated primarily via decar-
3.4 Effect of H3PO4 concentration
boxylation (CO2), instead of decarbonylation (CO). Based on Phosphoric acid is the catalyst in decarboxylation of PHB, and its
complete decarboxylation of a PHB monomer or crotonic acid concentration has a signicant effect on PHB conversion and oil
(C4H6O2 / C3H6 + CO2), the theoretical yield of CO2 is yield in typical reaction conditions (220 ꢀC, 3 hours) as shown in
51.2 wt% of initial PHB. Actually, the detected CO2 in gas Fig. 5. Crotonic acid was a major intermediate of PHB reaction in
phase accounts for 35.3 wt%, or 68.9% of maximum yield. It 50 wt% H3PO4. With increase of H3PO4 concentration, the
implies that at least 68.9% of oxygen in PHB was removed by residual crotonic acid content decreased, and little crotonic acid
decarboxylation. Some of oxygen might be removed by was remained in the 100 wt% H3PO4 and polyphosphoric acid
formation of water as discussed later. Similarly for crotonic (115% H3PO4 basis). The PHB-derived oil yield increased from
acid, 70.8% of oxygen was removed as CO2 via decarboxyl- 5.4 wt% to 37.3 wt% correspondingly with the increase of H3PO4
ation. Along with decarboxylation, propylene was the major concentration from 50 wt% to 100 wt%. Little char (<2 wt%) was
gaseous hydrocarbon formed from PHB or crotonic acid. A formed in these H3PO4 solutions. In polyphosphoric acid solu-
small amount of alkyl gases might also be formed, as indi- tion, however, a substantial amount of char (55.5 wt%) was
cated by the peaks at 3100, 2790, 1670, 1450, 910 cmÀ1 in formed, and the oil yield declined to 5.2 wt% of initial PHB.
Fig. 4 and in comparison with pure propylene in Fig. S1 (in Carbonization of PHB or its derived hydrocarbons became the
ESI†). Interestingly, the FT-IR analysis also indicates that predominant reaction in polyphosphoric acid solution. A similar
crotonic acid and PHB have almost the same gas products effect of phosphoric acid on crotonic acid was also observed and
(Fig. 4).
showed in Fig. 5. We suspect that PHB carbonization occurs aer
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Fig. 3 GC-MS analysis of PHB- and crotonic acid-derived oils produced in 100 wt% H3PO4 at 220 ꢀC for 3 hours.
its thermal degradation into crotonic acid. This fact also supports unsaturated ketones and aldehydes implies a possible way for
our previous conclusion that crotonic acid is the key intermediate aromatics formation, which may occur by some transient
of PHB conversion into hydrocarbon oil.
intermediate ketones and aldehydes through acid catalytic aldol
The concentration of H3PO4 also affected the composition of condensation, dehydration and aromatization reactions.31–33 In
the PHB-derived oils as revealed by their GC-MS chromato- diluted phosphoric acid solutions (70–85 wt%), however, acyclic
grams (Fig. 3 and S2 in ESI†). Table 2 gives the relative peak area alkenes and alkanes or cyclo-alkenes and alkanes became the
(%) of the chemicals identied by GC-MS analysis. Specically, important oil compounds, which means aromatization reac-
aromatics (benzene and naphthalene derivatives) were the main tions should be somewhat inhibited. In 50 wt% H3PO4, the
oil products in 100 wt% H3PO4, and the naphthalene derivatives O-containing compounds were the main products, indicating
via ring condensation28 predominated in polyphosphoric acid that PHB deoxygenation was not completed yet. For conclusion,
solution. Formation of high C/H ratio hydrocarbons in high phosphoric acid concentration is a potential tool in controlling
phosphoric acid is in agreement with our previous observation oil yield, deoxygen extent, and the hydrocarbon composition.
on char formation in polyphosphoric acid. A few unsaturated
Furthermore, high concentration of H3PO4 was favorable to
ketones and aldehydes were detected by GC-MS analysis, which formation of hydrocarbons with high carbon numbers (Table 3).
may be produced together with CO2 and water by phosphoric C4–C9 compounds were the main products in 50 wt% H3PO4
acid catalytic deoxygenation of PHB/crotonic acid in consid- solution while C10–C15 became the main products in 100 wt%
ering of former reports.16,29,30 Furthermore, the existence of H3PO4 solution. Based on the carbon numbers and the content
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interesting catalyst solution for PHB deoxygenation. We further
investigated the inuence of reaction time and temperature on
PHB conversion and oil yield.
3.5 Effect of reaction temperature and time
Temperature had apparent effect on residual crotonic acid
ꢀ
content and oil yield from 165 to 220 C in both PHB and cro-
tonic acid reactions (Fig. 6). The residual crotonic acid content
decreased while the oil yields increased with the increase of
temperature. Little residual crotonic acid was detected at 220 ꢀC
or above, indicating that this mild temperature is high enough
for complete conversion of PHB and crotonic acid. The highest
oil yield (38.2 wt%) of PHB was obtained at 230 ꢀC, and further
increase of temperature didn't result in higher oil yield. This oil
yield is very high, compared with the highest hydrocarbon
oil yield (17.78 wt%) from catalytic co-deoxy-liquefaction of
biomass and vegetable oil at 350–500 ꢀC.35 Besides, the reaction
temperature is also much lower than those of conventional
biomass deoxygenation. Interestingly, the PHB derived oils
produced at different temperatures have very similar peak
distribution in their GC-MS chromatograms (Fig. S3, in ESI†).
This fact indicates that temperature is not an important factor
in determining the oil composition.
Fig. 4 FT-IR spectra of PHB- and crotonic acid-derived gases.
Table 1 Carbon recovery of crotonic acid decarboxylation and oil
formationa
Phase
Gas
Products
Carbon recoveryb (wt%)
CO2
18.0
The inuence of reaction time on PHB conversion was tested
in 100 wt% H3PO4 at 220 ꢀC (Fig. 7). Three hours seemed
sufficient for complete conversion of PHB, as little crotonic acid
was detected in both PHB and crotonic acid reactions. Exten-
sion of reaction time (e.g., 6 or 9 hours) seems not benecial,
because the yield of PHB-derived oil declined to some extent
with extended reaction time. Some products such as char which
are not soluble in methylene chloride were formed in extended
reaction. As mentioned in the method of oil recovery, the
insoluble products are not accounted for hydrocarbon oil. On
the other hand, the inuence of reaction time on PHB conver-
sion showed quite similar results of crotonic acid conversion,
which implies again that crotonic acid is the key intermediate of
hydrocarbon oil production.
C3H6
Oil
4.9
57.0
Liquid
Solid
Residues in H3PO4 solutionc
11.0
Char
Total
<2.1%d
>90.9%
a
The reaction was performed in 100 wt% H3PO4 solution at 220 ꢀC for 3
hours. b The carbon mass percentage recovered from the initial carbon
of crotonic acid. c The residual organic carbons le in H3PO4 solution.
d
The carbon content of char is assumed 60 wt%.
of aromatics,34 the oil produced in 100 wt% H3PO4 could be a
‘drop in’ gasoline. According to the residual crotonic acid
content, oil yield and product distribution, 100 wt% H3PO4 is an
Fig. 5 The effect of H3PO4 concentration on residual crotonic acid content and oil yield at 220 ꢀC for 3 hours.
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Table 2 Chemical distributiona of PHB-derived oils generated in different phosphoric acid solutions at 220 ꢀC for 3 hours
Relative peak area (%)
O-containing compounds Acyclic alkanes Cycloalkanes Acyclic alkenes Cycloalkenes Benzenes Naphthalenes
50 wt% H3PO4
70 wt% H3PO4
85 wt% H3PO4
100 wt% H3PO4
Polyphosphoric acid
66.3
27.5
16.2
15.2
0
3.7
1.7
3.2
0.7
0
1.6
1.0
3.7
0.4
0
16.7
61.7
57.1
0.2
3.9
3.8
1.7
5.2
18.4
7.7
4.3
18.1
72.0
52.6
0
0
0
6.3
28.9
0
a
The chemical structures are identied with GC-MS.
Table 3 Carbon distributiona of PHB-derived oils formed in different phosphoric acid solutions at 220 ꢀC for 3 hours
Relative peak area (%)
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
50 wt% H3PO4
70 wt% H3PO4
85 wt% H3PO4
100 wt% H3PO4
Polyphosphoric acid
12.1
5.5
0.8
0
4.6
0
0
0
0
5.3
3.1
0.7
0.4
18.4
31.2
20
4.5
0
5.9
2.7
5.1
0.2
0
24.3
59.6
34.2
1.1
2.6
4.9
21.2
8.5
0.6
0
6.8
5.3
8
4.6
0.3
8.8
13.1
10
8.8
3.6
13.6
40.5
20.9
0
0
0
2.5
6.2
18.0
0
0
0
0
0.3
1.8
24.7
6.2
0
0
0.2
13.6
4.7
a
The carbon numbers distributions are based GC-MS analysis.
Fig. 6 The effect of reaction temperature on residual crotonic acid content and oil yield in 100 wt% H3PO4 for 3 hours.
yield declines a little bit with water formation in the reused
acid solution.
3.6 Reuse of H3PO4 solution
As shown in Fig. 8, the PHB-derived oil yield in a 100 wt% H3PO4
was consistently higher than 32% in repeated use for 6 times. We
compared the FT-IR spectra of the fresh and used acid solution
3.7 Reaction mechanism
(Fig. S4 in ESI†), and found no visible change. We also compared As shown above, crotonic acid is the key intermediate in
the P-NMR spectra of the acid solutions (Fig. S5, in ESI†) and conversion of PHB into hydrocarbon oil. Research on PHB
found that H4P2O7 in fresh acid solution disappeared in the thermal degradation has revealed that two neighboring mono-
reused acid solutions. Most likely, A self-dissociation reaction of meric units of a PHB backbone form a transient structure of a
H4P2O7 occurred in the solution (2H3PO4 $ H4P2O7 + H2O)25,36,37 six-member ring, resulting in one unsaturated end and one
and the water was provided from PHB deoxygenation. In another carboxylic acid end.38,39 Sequential or simultaneous degradation
words, some oxygen was removed as H2O, forming high C/H of the oligomers nally generates crotonic acid. It is widely
hydrocarbons or even char. We also demonstrated that oil yield accepted that this transient structure occurs predominantly in
declined in diluted acid solutions. This may explain why the oil thermal degradation of PHB, particularly at the melting point
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Fig. 7 The effect of reaction time on residual crotonic acid content and oil yield in 100 wt% at 220 ꢀC.
that propylene is not a key intermediate but a byproduct of
deoxygenation of crotonic acid and PHB. A very active inter-
mediate might be formed in decarboxylation of crotonic acid,
and hydrocarbons could be formed directly from the interme-
diate rather than from propylene. It should be pointed out that
propylene can also be converted into hydrocarbon products
with phosphoric acid catalysts.24,40–42 Further investigation is
needed on the reaction mechanism, especially on what happens
aer crotonic acid is formed from PHB degradation.
4 Conclusions
PHB is a type of new renewable feedstock from which hydro-
carbon oil is effectively produced in a simple one-pot reaction.
Compared to conventional biomass processing, PHB processing
demonstrated in this work has some technical advantages such
Fig. 8 The yield of PHB-derived hydrocarbon oil in a reused H3PO4
(100 wt%) solution at 220 ꢀC for 3 hours.
ꢀ
as mild reaction temperature (around 220 C), cheap reusable
catalyst (80–100 wt% H3PO4), and high oil yield (38.2 wt%)
without using hydrogen.
(around 180 ꢀC) or above because the PHB backbone has a great
exibility to form the planar six-membered ring structure.
Decarboxylation of crotonic acid (C4H6O4 / CO2 + C3H6)
occurs because of conjugation of the unsaturated bond
(–C]C–) and the carboxy bond (C]O). The promotion effect of
the conjugation effect is conrmed when butanoic acid, a
saturated C4 acid, was treated in the same conditions, but
generating little deoxygenated products (data not shown here).
Obviously, propylene is formed along with decarboxylation of
crotonic acid. We originally believed that propylene undergoes
radical oligomerization to generate a complex mixture of
hydrocarbons (>C6). However, when propylene was used as a
gas reactant, the hydrocarbons formed were not identical with
those formed from crotonic acid and PHB (Fig. 3). Specically,
we observed two major products derived from both crotonic
acid and PHB and their retention times in GC-MS chromato-
grams are 32.520 min and 41.909 min, respectively. They are
tentatively identied as a 1,2,3,4-tetramethyl-5-(1-methylethyl)-
benzene and a 3,3,4,5,7-pentamethyl-1-indanone based on their
high match probability ($890&) of MS spectra (Fig. 3). The lack
of these two major products from propylene reaction implies
Acknowledgements
The work is supported by the Office of Naval Research, USA
(Award N00014-13-1-0463).
Notes and references
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