Biobased n-Butanol Prepared from Poly-3-hydroxybutyrate: Optimization of the Reduction of n-Butyl Crotonate to n-Butanol

Article abstract of DOI:10.1021/op500156b

Using metabolic engineering approaches, the biopolymer poly-3-hydroxybutyrate (P3HB) can be overproduced in organisms such as bacteria and plants. Thermolysis of P3HB, either in isolated form or within biomass, yields crotonic acid, a potential bioderived platform chemical. Reduction of crotonic acid provides n-butanol, which has value as a fuel and as a commodity chemical. Herein, we report optimization work on the hydrogenation of the n-butyl ester of crotonic acid to n-butanol and the potential of this chemistry to be incorporated into the production of bio-n-butanol from P3HB containing biomass.

Full text of DOI:10.1021/op500156b

Article
pubs.acs.org/OPRD
Biobased n‑Butanol Prepared from Poly-3-hydroxybutyrate:
Optimization of the Reduction of n‑Butyl Crotonate to n‑Butanol
Dirk Schweitzer,† Charles A. Mullen,*,‡ Akwasi A. Boateng,‡ and Kristi D. Snell†
†Metabolix, 21 Erie St., Cambridge, Massachusetts 02139, United States
‡Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 E. Mermaid Lane, Wyndmoor,
Pennsylvania 19038, United States
S
* Supporting Information
ABSTRACT: Using metabolic engineering approaches, the biopolymer poly-3-hydroxybutyrate (P3HB) can be overproduced in
organisms such as bacteria and plants. Thermolysis of P3HB, either in isolated form or within biomass, yields crotonic acid, a
potential bioderived platform chemical. Reduction of crotonic acid provides n-butanol, which has value as a fuel and as a
commodity chemical. Herein, we report optimization work on the hydrogenation of the n-butyl ester of crotonic acid to n-
butanol and the potential of this chemistry to be incorporated into the production of bio-n-butanol from P3HB containing
biomass.
catalysts.22 n-Butyl crotonate is the preferred hydrogenation
INTRODUCTION
■
substrate, producing two equivalents of n-butanol, one of which
n-Butanol is a widely used commodity and platform chemical,1
can be recycled to prepare new n-butyl crotonate (Scheme 1).
which also possesses physical properties desirable for its use as
The high-yielding esterification of crotonic acid to n-butyl
a liquid fuel.2,3 Since fossil-based starting materials (oil, coal,
crotonate employing a heterogeneous catalyst is well-known,24
natural gas) within the Earth’s crust are finite resources not
as is the n-butylation of n-butyric acid.25
renewable within a human time scale,4 the production of
Since the hydrogenation of an alkene is known to be more
versatile building blocks, which serve as the starting materials to
feasible than the hydrogenation of a carboxylate,26 we expected
prepare fuels and chemicals from renewable resources, is
the initial/fast formation of n-butyl n-butyrate, as an unisolated
desirable. One such versatile target is n-butanol.1 Indeed, the
intermediate, in the hydrogenation reaction of n-butyl
fermentation to produce a biobased n-butanol was first
crotonate. This intermediate is then, in a second slower/more
patented nearly a century ago.5 Due to n-butanol’s superior
difficult process, reduced to two equivalents of n-butanol. The
fuel properties compared to ethanol,2,3 multiple organizations
experimental results reported herein confirm this prediction.
are pursuing pathways6−12 to biobased n-butanol,13 often via
The vapor-phase continuous hydrogenation of n-butyl n-
the fermentation of bacteria or yeast.
butyrate over a commercial CuO + ZnO/Al2O3 catalyst has
Herein, we report proof-of-concept work on the production
been previously studied.27,28 In contrast, this study investigates
of the commodity chemical n-butanol from n-butyl crotonate, a
the liquid phase continuous hydrogenation.
molecule that can be derived from crotonic acid produced via
the direct pyrolysis of microbial or plant biomass containing the
biopolymer poly-3-hydroxybutyrate (P3HB) (Scheme 1).14−19
Crotonic acid itself is currently a specialty chemical with limited
markets. Production of crotonic acid from P3HB producing
plants provides the opportunity to create it as a cost-advantaged
feedstock and platform chemical for conversion to a range of
commodity and fine chemicals, such as n-butanol, fumaric acid,
maleic anhydride, propylene, and acrylic acid/acrylates, which
would otherwise be difficult to produce from plants.14 The
hydrogenation work reported herein, in combination with
previously demonstrated P3HB production in switchgrass
biomass14,20 and thermolysis of switchgrass biomass containing
P3HB to crotonic acid,15,18 provides proof of concept for
obtaining n-butanol from a renewable genetically engineered
switchgrass feedstock.
EXPERIMENTAL SECTION
■
All experiments were conducted in a custom-built continuous
flow, fixed-bed hydrogenation apparatus containing a 30 cm3
catalyst bed. The reactor itself had a volume of ∼100 cm3. The
catalyst was centered in the reactor with the area above and
below the catalyst filled with 5 mm glass beads. The standard
H2 gas space velocity used was 6000 h−1, with the exception of
experiment number 20 in which it was 8000 h−1, and the liquid
space velocity (calculated based on ambient temperature flows)
of the starting material n-butyl crotonate was 0.5 or 1.0 h−1
(0.25 or 0.5 cm3/min). Reactions were performed at 200, 250,
or 300 °C and at H2 pressures of 200, 400, or 600 psi. The flow
of H2 into the system was controlled with a mass flow meter
(Brooks), and its pressure was adjusted using a back pressure
To optimize the hydrogenation reaction, an alkyl crotonate
was chosen as the substrate since the hydrogenation of an alkyl
ester is known to be more feasible than that of the
corresponding carboxylic acid.21−23 In addition, free carboxylate
groups can leach base-metal ions from heterogeneous
Special Issue: Sustainable Chemistry
Received: May 16, 2014
© XXXX American Chemical Society
A
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Organic Process Research & Development
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Scheme 1. Synthesis of n-butanol from P3HB
Table 1. Catalysts used for the hydrogenation of n-butyl crotonate to n-butanol
ID
A
catalyst
composition
particle size (mm)
JM-PRICAT CZ 40-18 T
51/32/5/12
CuO/ZnO/CuCO3/Al2O3,C
6 × 5
B
C
D
E
F
JM-PRICAT CZ 30-18 T
JM-KATALCO 83-3
39/45/16
5 × 3
4 × 3
4 × 2
4 × 2
4 × 1
CuO/ZnO/Al2O3,C
50/30/20
CuO/ZnO/Al2O3,C
BASF-Cu 1986 T 1/8″
BASF-X 540 T 1/8″
65/20/15
Cr2CuO4/CuO/MnO2,SiO2,C
60/10/30
CuO/Al2CuO4/MnO2,Al2O3
BASF-Cu 0860 E 1/16″ 3F
60/40
CuO/SiO2,CaO
regulator on the outlet of the system. The n-butyl crotonate
liquid substrate was fed into the top of the packed reactor from
a reservoir using an HPLC pump (Lab Alliance). Cooling of the
product was achieved through a countercurrent heat exchange
with the feed material line, providing preheating for the liquid
feed. Collection of the products was achieved using a gas−
liquid separator. The product composition was determined via
quantitative GC-MS of 2−3 samples which were collected
following the system reaching a steady state (60 min). The
yield of formed n-butanol was calculated via (100[mol n-
butanol]out)/(2[mol n-butyl crotonate]in). Commercial n-butyl
crotonate (TCI America) was used as received. Evaluation
samples of six industrial heterogeneous base-metal hydro-
genation catalysts were provided by their commercial suppliers:
PRICAT CZ 40-18 T, PRICAT CZ 30-18 T, and KATALCO
83-3 from Johnson-Matthey as well as Cu 1986 T 1/8″, X 540
T 1/8″, and Cu 0860 E 1/16″ 3F from BASF. Catalysts were
cylindrical in shape. Approximate compositions of these
catalysts derived from their MSDS forms are listed in Table
1. Detailed experimental parameters and all results of all
performed experiments are provided in the Supporting
Information.
intermediate n-butyl n-butyrate being present in the product
stream is preferable compared to overreaction of n-butanol into
other products, even if a higher yield of n-butanol is achieved in
the latter case. This is because n-butyl n-butyrate can be
recycled, whereas side product formation constitutes an
irreversible loss of material. Under most of the conditions
used in this study (200−400 psi of H2 above the liquid phase
and at a temperature of, or below, 250 °C) both the substrate n-
butyl crotonate and the intermediate n-butyl n-butyrate as well
as most of the n-butanol product are expected to be in the
liquid phase.29
First, the reduction of n-butyl crotonate to 2 mol of n-
butanol promoted by each one of the six hydrogenation
catalysts was evaluated under the identical conditions of 400 psi
H2 pressure, 250 °C, and a substrate liquid space velocity of 1
h−1 through the hydrogenation apparatus (Table 2, Experi-
ments 1−7). The highest yields of n-butanol were obtained
using the PRICAT CZ 40 catalyst [A] (75−86%, experiments 1
and 2), whereas the Cu 0860 catalyst provided the lowest
amount of n-butanol (58%, experiment 7) in the product
stream.
Each of the catalysts was also screened (Table 2, Experiments
8−15) at a 50 K lower temperature (200 °C), while keeping all
other parameters constant, to determine the effect of
temperature onto the hydrogenation reaction. In general, the
lower temperature resulted in an increased amount of n-butanol
being present in the product stream and a decreased formation
of other materials. This suggests that pathways converting n-
butanol to the observed side products, vide infra, are more
populated at higher temperatures. Under these 200 °C
conditions, BASF catalyst Cu 1986 [D] showed the most
RESULTS AND DISCUSSION
■
Catalyst Screen. To increase the economic viability of the
hydrogenation process, we choose to focus this study on base-
metal heterogeneous hydrogenation catalysts and avoided the
use of catalysts containing a precious metal. Using six industrial
catalysts (Table 1), efforts were focused on maximizing the
yield of n-butanol while minimizing the amount of product
decomposition. An incomplete reaction resulting in the
B
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a
Table 2. Comparison of catalysts at two different
temperatures
Table 4. Effect of an increased H2 pressure
a
(mol %)
250 °C (mol %)
200 °C (mol %)
p(H2)
T
yield
estimated yield
others
catalyst
no.
17
(psi)
(°C)
n-butanol
estimated
estimated
yield
others
yield
yield
yield
n-butanol
A
200
400
200
400
200
400
200
200
250
250
200
200
200
200
250
250
54
23
catalyst no. n-butanol
others
no.
8, 9
A
8, 9
18
75, 69
77
10, 26
3
A
B
C
D
E
F
1, 2
3
86, 75
72
6, 18
24
75, 69
71
10, 26
26
A
10
A
1,2
19
86, 75
63
6, 18
19
4
71
26
11
83
12
D
D
D
D
D
D
5
67
31
12, 13
14
90, 86
84
5, 13
15
12, 13
20
90, 86
80
5, 13
19
b
6
73
25
400
7
58
39
15
67
18
21
600
200
400
71
28
a
H2 pressure = 400 psi, liquid (n-butyl crotonate) space velocity = 1
22
85
7
h−1.
5
67
31
a
b
Liquid (n-butyl crotonate) space velocity = 1 h−1. Gas (H2) space
velocity = 8000 h−1, all other experiments used 6000 h−1.
potential, providing up to a 90% yield of n-butanol (experi-
ments 12 and 13).
Effect of an Increased Temperature. The effects of
increasing the reaction temperature past 250 °C were also
determined using the PRICAT CZ 40 catalyst [A]. Notably, at
300 °C (Table 3, Experiment 16) a significantly lower amount
by the flow of liquid at ambient conditions. However,
qualitative observation of the liquid product collected in the
gas liquid separator after ∼30 min of liquid pumping through
the 100 cm3 of total reactor space at a 0.5 cm3/min flow rate
indicated that the actual liquid space velocity was approximately
6 times higher due to thermal expansion. The influence of the
substrate n-butyl crotonate liquid space velocity (flow rate
through the reactor bed) was investigated under two sets of
conditions, both using Cu 1986 as the catalyst (Table 5). Only
a small effect on the yield of n-butanol was observed. A reduced
liquid space velocity (substrate flow rate), i.e., increased
residence time in the reactor bed, resulted in less intermediate
n-butyl butyrate remaining but did not lead to an increase in the
amount of n-butanol present in the product stream. This
finding is consistent with the hypothesis that a lower flow rate
allows an increased formation of n-butanol, which, however,
now also has more time to further react, forming the observed
byproducts. In all tests the H2 flow was significantly higher than
the liquid flow, meaning in an industrial process provisions for
recycling of excess H2 must be made. The lack of gaseous
coproducts in this reaction results in a mostly pure H2 exhaust
stream which can be reutilized with little additional processing.
Further Reactions of n-Butanol. In several experiments,
in addition to the expected reduction intermediate n-butyl n-
butyrate and the desired reduction product n-butanol, several
other compounds were detected in the product stream. Their
formation can be rationalized via known reactions of n-butanol
(Scheme 2). The Guerbet reaction of n-butanol produces 2-
ethyl-hexanal. While we did not observe this particular
compound as verified using an authentic sample in a GC-MS
comparison, an isomeric compound of it with an identical
molecular weight, i.e., a saturated C8 ketone or aldehyde with a
molecular formula of C8H16O, was a major byproduct detected
via GC-MS in the product stream of several reactions. The
Guerbet reaction of n-butanol, and n-butanal formed from it via
a facile dehydrogenation, has been previously observed under
reaction conditions similar to ours.31−33 Additionally, at
temperatures above 150 °C acidic heterogeneous catalyst
supports such as Al2O3 and SiO2 are known to catalyze the
dehydration of n-butanol to 1-butene, which can then further
react to form products such as cis-2-butene, trans-2-butene, di-
n-butyl ether, and n-butyl-sec-butyl ether.34−38 All intermediate
reactions in the proposed pathway for the formation of 3-
methyl-heptane are known to occur under similar conditions to
the ones employed in these experiments.39−42
a
Table 3. Effect of an increased temperature
(mol %)
T
(°C)
yield n-
butanol
estimated yield
others
yield 3-methyl
heptane
no.
200
250
300
8, 9
1, 2
16
75, 69
86, 75
28
10, 26
6, 18
54
nd, nd
nd, trace
16
a
PRICAT CZ 40 catalyst [A], H2 pressure = 400 psi, liquid (n-butyl
crotonate) space velocity = 1 h−1, nd = not detected.
of n-butanol was found to be present in the product stream
compared to the experiments performed at lower temperatures.
It was accompanied by a large amount of hydrocarbons, the
most abundant of which was 3-methyl-heptane; however, C7
hydrocarbons and oxygenates were also observed. The
formation of 3-methyl-heptane is noteworthy, since it has
been suggested as a suitable replacement of conventional diesel
fuel due to its favorable oxidation kinetics.30
Effect of an Increased H2 Pressure. Catalysts PRICAT
CZ 40 [A] and Cu 1986 [D] were chosen to study the effect of
H2 pressure on the desired reduction of n-butyl crotonate to n-
butanol (Table 4). Further reactions of n-butanol to side
products were also found to be effected by pressure (Scheme
2). The effect of H2 pressure was variable and often dependent
on temperature. At 200 °C, an increase of the H2 pressure from
200 to 400 psi resulted in an increased amount of n-butanol
present in the product stream of both catalysts (experiments 17
versus 8 and 9 for PRICAT CZ 40 [A] and experiments 19
versus 12 and 13 for Cu 1986 [D]). Likely, a higher H2 pressure
suppresses follow-up reaction pathways of n-butanol, such as
shifting its dehydrogenation equilibrium to the left side, vide
infra (Scheme 2). This trend was not as apparent for the
reactions performed at 250 °C. The effect of increasing the gas
space velocity was also tested in one experiment by increasing
the H2 flow (experiment 20 versus 12 and 13). A small
reduction in n-butanol yield was noted in this case.
Effect of Substrate Liquid Space Velocity (Flow Rate).
The liquid space velocity used in these experiments was either
1.0 or 0.5 h−1 (0.50 or 0.25 cm3/min actual flow) as measured
C
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Organic Process Research & Development
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Scheme 2. Proposed pathways for the formation of observed byproducts
a
Table 5. Effect of substrate liquid space velocity (flow rate)
ASSOCIATED CONTENT
■
S
* Supporting Information
(mol %)
Detailed experimental procedures, apparatus description, and
sample product gas chromatograms. This material is available
free of charge via the Internet at http://pubs.acs.org.
BuCr liquid space
T
BuBu
yield
estimated
b
velocity (h−1
)
no.
23
(°C)
(wt %) n-butanol yield others
0.5
1.0
0.5
1.0
200
200
250
250
0.6
89
10
12, 13
24
4.9, 0.9
0.6
90, 86
65
5, 13
34
AUTHOR INFORMATION
■
5
1.7
67
31
Corresponding Author
a
b
Cu 1986 catalyst [D], H2 pressure = 400 psi. In our reactor, 0.5 h−1
= 0.25 cm3/min and 1.0 h−1 = 0.5 cm3/min.
*E-mail: charles.mullen@ars.usda.gov. Fax: +1 215 233 6559.
Tel.: +1 215 836 6916.
Notes
This report was prepared as an account of work sponsored by
an agency of the United States Government. Neither the
United States Government nor any agency thereof, nor any of
their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy,
completeness, or usefulness of any information, apparatus,
product, or process disclosed, or represents that its use would
not infringe privately owned rights. Reference herein to any
specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise does not necessarily
constitute or imply its endorsement, recommendation, or
favoring by the United States Government or any agency
thereof. The views and opinions of authors expressed herein do
not necessarily state or reflect those of the United States
Government or any agency thereof. Mention of trade names or
commercial products in this publication is solely for the
purpose of providing specific information and does not imply
recommendation or endorsement by the U.S. Department of
Agriculture. USDA is an equal opportunity provider and
employer.
CONCLUSIONS
■
In this study we demonstrate that n-butanol can be synthesized
from n-butyl crotonate. This reaction is a key component and a
step forward in the overall proof-of-concept efforts to convert
poly-3-hydroxybutyrate (P3HB) containing biomass, such as
P3HB containing switchgrass, to bio-n-butanol (Scheme 1). We
have previously demonstrated proof-of-concept work to
convert P3HB containing switchgrass biomass to crotonic
acid employing mild pyrolysis conditions15,18 and have made
significant advances towards the production of this biopolymer
in switchgrass with P3HB levels of up to 7.7% dwt achieved in
leaf tissue.14 The optimized conditions for the reduction of neat
n-butyl crotonate to n-butanol are a hydrogen pressure of 400
psi, a reaction temperature of 200 °C, a substrate liquid space
velocity of 1 h−1, and a CuO-containing catalyst, such as the
commercially available BASF-Cu 1986. These reaction
conditions provide a yield of n-butanol in the high 80s
percentage.
The authors declare the following competing financial
interest(s): D.S. and K.D.S. are employees of Metabolix, Inc.
D
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Organic Process Research & Development
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ACKNOWLEDGMENTS
■
The authors thank Mr. Craig Einfeldt and Dr. Christina Dorado
of ERRC for technical assistance. This research was supported
by a grant from the US Department of Energy’s (DOE) Office
of Energy Efficiency and Renewable Energy (EERE) to
Metabolix (grant DE-EE0004943).
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