Direct hydrogenation of biomass-derived butyric acid to n-butanol over a ruthenium-tin bimetallic catalyst

Article abstract of DOI:10.1002/cssc.201402311

Catalytic hydrogenation of organic carboxylic acids and their esters, for example, cellulosic ethanol from fermentation of acetic acid and hydrogenation of ethyl acetate is a promising possibility for future biorefinery concepts. A hybrid conversion process based on selective hydrogenation of butyric acid combined with fermentation of glucose has been developed for producing biobutanol. ZnO-supported Ru-Sn bimetallic catalysts exhibits unprecedentedly superior performance in the vapor-phase hydrogenation of biomass-derived butyric acid to n-butanol (>98% yield) for 3500 h without deactivation.

Full text of DOI:10.1002/cssc.201402311

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DOI: 10.1002/cssc.201402311
Direct Hydrogenation of Biomass-Derived Butyric Acid to
n-Butanol over a Ruthenium–Tin Bimetallic Catalyst
[a]
[a]
[a, b]
[a]
[a]
Jong-Min Lee, Pravin P. Upare, Jong-San Chang,*
Young Kyu Hwang, Jeong Ho Lee,
[a]
[a]
[a]
[a, b]
Dong Won Hwang, Do-Young Hong, Seung Hwan Lee, Myung-Geun Jeong,
[a, b]
[b]
Young Dok Kim,
and Young-Uk Kwon
Catalytic hydrogenation of organic carboxylic acids and their
esters, for example, cellulosic ethanol from fermentation of
acetic acid and hydrogenation of ethyl acetate is a promising
possibility for future biorefinery concepts. A hybrid conversion
process based on selective hydrogenation of butyric acid com-
bined with fermentation of glucose has been developed for
producing biobutanol. ZnO-supported Ru–Sn bimetallic cata-
lysts exhibits unprecedentedly superior performance in the
vapor-phase hydrogenation of biomass-derived butyric acid to
n-butanol (>98% yield) for 3500 h without deactivation.
erated by fermentation could be an alternative intermediate in
the production of biobutanol because fermentation by anaero-
À1
bic bacteria provides a higher butyric acid titer (>60 gL ) and
[5,7]
hydrogen is co-produced, as determined in other works .
In this work, we propose an alternative method to produce
biobutanol by the so-called “hybrid conversion” process, that
is, selective hydrogenation of butyric acid combined with fer-
mentation of glucose or biomass feedstock. Scheme 1 depicts
the concept of the new hybrid conversion process for produc-
ing biobutanol, which may compete with direct fermentation
to biobutanol. A key to the success of this process is the devel-
opment of a selective catalyst that is stable and robust for the
direct hydrogenation of monocarboxylic acids to their corre-
sponding alcohols.
Renewable carbon resources, such as biomass, have recently
attracted much attention because of the depletion of fossil re-
sources and global efforts to reduce greenhouse gas emis-
sions, which promotes new strategies for the sustainable pro-
duction of fuels and chemicals from renewable raw materi-
Catalytic hydrogenation of organic carboxylic acids and their
esters, for example, cellulosic ethanol from fermentation of
acetic acid and hydrogenation of ethyl acetate by ZeaChem
[
1–3]
[8]
als.
Inc., is a promising possibility for future biorefinery concepts;
Biobased organic acids, such as lactic acid, 3-hydroxypro-
pionic acid, succinic acid, and others, obtained from biomass
carbohydrates are important platform chemicals because they
can be transformed into a variety of valuable biofuels and bio-
it is an important and widely applied technique for defunction-
[1,2]
alization in a biorefinery.
Only a few reports have demon-
strated that ruthenium-based catalysts are stable for the hy-
[10]
drogenation of carboxylic acid in the liquid phase or acetic
[
3,4]
[11]
based chemicals. Butyric acid is a short-chain monocarboxyl-
ic acid that can be generated via anaerobic fermentation of
acid in continuous-flow reactors without any metal leaching.
However, most supported metal catalysts, even those including
noble metals, are not stable during direct hydrogenation of or-
ganic monocarboxylic acids because the acids are corrosive,
which generally causes leaching of metal and destruction of
the catalyst structures, leading to serious catalyst deactiva-
[
5]
biomass feedstocks. The anaerobic bacterium Clostridium ty-
robutyricum is known to produce butyric acid, acetic acid, hy-
[5]
drogen, and carbon dioxide via fermentation of glucose. Bio-
butanol is considered as both a next-generation biofuel and
[
6]
[9]
a chemical solvent for polymerization. However, there are
major hurdles to resolve before commercial production by fer-
tion. Moreover, among carbonyl group-containing com-
pounds, carboxylic acids are more difficult to hydrogenate be-
cause their carbonyl groups are less active than those of ke-
À1
mentation is viable, including the low butanol titer (<20 gL ),
[12]
the toxicity of butanol towards microorganisms, and the availa-
tones and aldehydes. For these reasons, robust catalysts for
the hydrogenation of organic acids have thus far been rarely
[
6]
bility of compatible feedstocks. Accordingly, butyric acid gen-
[10]
reported.
Among metal catalysts, tin-promoted ruthenium
+
+
[
a] Dr. J.-M. Lee, Dr. P. P. Upare, Prof. J.-S. Chang, Dr. Y. K. Hwang, J. H. Lee,
Dr. D. W. Hwang, Dr. D.-Y. Hong, Dr. S. H. Lee, M.-G. Jeong, Prof. Y. D. Kim
Biorefinery Research Group & Industrial Biochemical Research Group
Korea Research Institute of Chemical Technology (KRICT)
Jang-dong 100, Yuseong, Daejeon 305-600 (Korea)
(Ru–Sn) catalysts are well known to be particularly selective for
E-mail: jschang@krict.re.kr
[b] Prof. J.-S. Chang, M.-G. Jeong, Prof. Y. D. Kim, Prof. Y.-U. Kwon
Department of Chemistry
Sungkyunkwan University
Suwon 440-476 (Korea)
E-mail: jschang020@skku.ac.kr
+
[
] These authors contributed equally to this work.
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402311.
Scheme 1. A hybrid conversion process for producing bio-butanol by fer-
mentation followed by catalytic hydrogenation.
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[
13,14]
carbonyl-group hydrogenation.
This encouraged us to
focus on developing ruthenium-based hydrogenation catalysts
for organic carboxylic acids that are selective and stable.
To overcome the limitation of the hydrogenation catalysts
mentioned above, we report herein zinc oxide-supported Ru–
Sn bimetallic catalysts that are very powerful and exhibit long-
term performance for the vapor-phase hydrogenation of mon-
ocarboxylic acids to their corresponding alcohols. In particular,
biobased butyric acid derived from the fermentation of glu-
cose was used as a reactant.
ZnO-supported Ru (Ru/ZnO) and Ru–Sn (1Ru-2Sn/ZnO) cata-
lysts with 1.4 wt% ruthenium and a Ru/Sn molar ratio of 0.5
were prepared using a sequential coprecipitation-deposition
.
.
method with aqueous solutions of Zn(NO ) 6H O, SnCl 5H O,
3
2
2
4
2
.
and RuCl xH O at pH 7.2–7.5, followed by drying at 1208C and
Figure 1. Long-term activity of 1Ru-2Sn/ZnO catalyst as a function of time
for the hydrogenation of butyric acid. Reaction conditions: same as those in-
dicated in Table 1. Symbols: square (black), butyric acid conversion; circle
3
2
temperature-programmed reduction with 5% H at 4208C (see
Supporting Information).
2
(
red), selectivity to n-butanol; triangle (black), selectivity to butyraldehyde;
To evaluate the catalytic activities of Ru/ZnO and 1Ru-2Sn/
ZnO, vapor-phase hydrogenation of butyric acid was per-
formed using a fixed-bed reactor at elevated pressures, as de-
picted in Figure S1 (Supporting Information). The evaluation
conditions for catalyst screening were chosen via catalytic
measurements as a function of the reaction parameters, such
as pressure, temperature, and space velocity (Supporting Infor-
mation, Figures S2 and S3). The best reaction temperature was
inverted triangle (green), selectivity to butyl butyrate. Region A, which is
marked by the dashed lines, indicates the section that was tested for the re-
action using a reactant feed of bio-butyric acid obtained from fermentation
followed by purification (see Supporting Information).
for 3500 h (145 days) without any significant deactivation
(Figure 1). During the operation, the catalytic activity abruptly
decreased twice because of unexpected disconnection of the
2658C among temperatures tested between 230 and 2708C
(data not shown). The carbon mass balance of the liquid-phase
H supply at 580 and 1652 h, respectively. However, the activity
2
products was estimated to be more than 97%. The main prod-
ucts in the gas phase were n-butanol, butyl butyrate, and n-
butane. As can be seen in Table 1, the 1Ru-2Sn catalyst exhibits
very high selectivity to n-butanol (up to 99%) with almost
quickly recovered after restoring H supply to normal, thereby
2
proving the high catalyst stability. To the best of our knowl-
edge, the excellent performance observed here for the direct
hydrogenation of organic monocarboxylic acid without the
help of any organic solvent has not been reported in the litera-
ture.
1
00% butyric acid conversion at 2658C, 25 atm, and WHSV=
À1
0.9 h . In this case, less than 1.3% of butyl butyrate and butyr-
aldehyde byproducts are produced. If biobutanol is applied as
a biofuel, butyl butyrate can be used in the biofuel mixture
with n-butanol. In contrast, tin-free Ru/ZnO is much less active
Figure S4 (Supporting Information) shows powder X-ray dif-
fraction (XRD) patterns of the reduced Ru and Ru–Sn catalysts.
The XRD pattern of reduced Ru/ZnO indicates the presence of
metallic ruthenium nanoparticles on the catalyst surface; how-
ever, that of the reduced 1Ru-2Sn catalyst shows no evidence
of metallic ruthenium, but rather the formation of a Ru Sn
(62.4% conversion) with only a slightly lower n-butanol selec-
tivity (97.8%) than 1Ru-2Sn/ZnO. The effect of reaction condi-
tions and the molar ratio of Ru/Sn on catalytic activities will be
investigated in more detail in future publications.
3
7
alloy phase. This implies that Ru Sn , among the many inter-
3
7
Above all, the most important aspect in the hydrogenation
of butyric acid is catalyst stability. The long-term catalyst stabil-
ity of Ru-Sn/ZnO was assessed at 2658C and 25 atm. To our
surprise, 1Ru-2Sn/ZnO exhibits superior stability and durability
metallic alloys of Ru–Sn, is be formed as the main intermediate
phase of the catalyst.
As shown in Figure 2 and Figures S5 and S6 (Supporting In-
formation), transmission electron microscopy (TEM) images of
reduced 1Ru-2Sn/ZnO illustrate nanoparticles of the Ru–Sn
alloy-rich phase, possibly Ru Sn . The Ru–Sn nanoalloy particles
3
7
Table 1. Catalytic activities and selectivities of Ru/ZnO and 1Ru-2Sn/ZnO
for vapor-phase hydrogenation of butyric acid.
are distinctively larger than the metallic ruthenium nanoparti-
cles (3–5 nm) on reduced Ru/ZnO (Figure S6). From the high-
resolution TEM images (Figure S5) of 1Ru-2Sn/ZnO, we can ob-
serve well-defined lattice fringes with an Im-3m space group of
[
a]
[
a]
Catalyst
S
BET
2
BA conv.
[%]
Selectivity to
BuOH [%]
Selectivity to
BuBu [%]
À1 [b]
[
m g
]
Ru Sn7 nanocrystal (Joint Committee on Powder Diffraction
3
Ru/ZnO
5
32
62.4
99.9
97.8
98.6
0.9
0.7
1Ru-2Sn/ZnO
Standards (JCPDS) card number 26-0504). The d value of the
(
330) plane of the Ru Sn crystal is estimated to be ca. 2.3 ꢁ,
3 7
[
0
a] Typical reaction conditions: T=2658C, H
2
pressure 25 atm, WHSV=
À1
which is close to 2.20 ꢁ of the corresponding crystal planes of
a standard cubic Ru Sn crystal. This value matches well with
.9 h , H
2
/butyric acid=35 (mol/mol), Time on-stream=200 h. Abbrevia-
tions: BA, butyric acid; BuOH, n-butanol; BuBu, butyl butyrate. [b] SBET
:
3
7
the specific Brunauer–Emmett–Teller (BET) surface area estimated from N
physisorption at À1968C.
2
the lattice distance of a d330 reflection plane corresponding to
a 2q value of 40.98 from the XRD pattern.
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As mentioned earlier, supported metal catalysts generally
tend to undergo metal leaching and catalyst deactivation
during direct hydrogenation of organic monocarboxylic acids
[9]
to the corresponding alcohols. In most cases, these hydroge-
nation reactions are carried out in the liquid phase of organic
[19]
acids at high pressures (>50 atm).
In this work, we use
vapor-phase reactions at higher temperatures and lower pres-
sures to avoid direct contact of the metal surface with highly
concentrated acid solutions. Consequently, it is clear that 1Ru-
2Sn/ZnO catalyst developed in this work can be successfully
Figure 2. TEM images of 1Ru-2Sn/ZnO catalysts. The selected area in the left
applied to vapor-phase or pseudo vapor-phase hydrogenation
of butyric acid.
image indicates the presence of the Ru
shown in the right image.
3 7
Sn phase formed on ZnO surface as
As a next step, the catalytic performance of Ru–Sn/ZnO
under more practical conditions was investigated. To investi-
gate the use of biobased butyric acid as a reactant, we fer-
mented glucose with Clostridium tyrobutylicum ATCC 25755 fol-
lowed by solvent extraction with dichloromethane and distilla-
tion (see Supporting Information). Fermentation profiles yield-
Dumesic and co-workers reported selective hydrogenation
of levulinic acid to g-valerolactone on carbon-supported Ru–Sn
[
14]
(
Ru Sn /C) catalysts. They noted that tin does not significant-
x y
À1
À1
ly leach from the catalysts when it is part of a ruthenium inter-
metallic alloy, such as Ru Sn3 or Ru Sn . These intermediate
ed that 62.4 gL of ammonium butyrate and 10.7 gL of am-
monium acetate were formed after 72 h (Figure 3). Using this
method, we obtained 600 mL of butyric acid with 99.8%
purity, as confirmed by gas chromatography. To determine the
effect of using butyric acid as actual feedstock, the butyric acid
obtained from the fermented broth was fed to the reactor
packed with reduced 1Ru-2Sn/ZnO for long-term operation. As
expected, the use of fermented butyric acid does not affect
the activity and stability of the catalyst (Figure 1).
2
3
7
phases were not actually observed in Ru Sn /C; however, the
x
y
phase compositions of the Ru and Ru–Sn alloys were predicted
for Ru Sn /C from the binary alloy phase diagram of Ru–Sn at
x
y
1
508C. Nevertheless, the presence of the Ru–Sn alloy is consis-
tent with these results.
The role of Sn in the catalytic activity of Ru is known to
strongly depend on the organic substrate to be hydrogenat-
[
14,15]
ed.
However, explanations of the specific role of tin on se-
From a more practical perspective, the present results pro-
vide an alternative method to produce biobutanol. In particu-
lar, a high yield of n-butanol (>98%) in this process could
have a positive impact on energy consumption during the pu-
rification step. Recently, ZeaChem Inc. developed a hybrid pro-
cess based on a cellulose-based biorefinery platform capable
2
+
4+
lectivity have focused on the contributions of Sn or Sn
0
[14,15]
ions, rather than Sn in the Ru–Sn alloy,
and generally in-
volve a strong interaction between the C=O group and tin cat-
[14,15]
ions and electronic interactions with the ruthenium metal.
Thus, the acidic properties of tin ions activate and polarize the
carbonyl group of the carboxylic acid. The electronic interac-
tion of tin with the ruthenium metal is thought to poison the
[8]
of producing advanced ethanol, fuels, and chemicals. This ap-
proach to cellulosic biomass bioprocessing for the production
of ethanol involves enzymatic saccharification, acetic acid fer-
mentation, and downstream chemical processing (that is, cata-
lytic esterification and hydrogenation) to convert acetic acid
[
16]
active sites responsible for hydrogen activation. This implies
that we also must consider the role of tin ions or tin oxide spe-
cies in the vicinity of the metallic Ru or Ru Sn alloy, although
3
7
the role of the Ru–Sn alloy is focused on in this study. The
electronic interactions of metallic tin and ruthenium in the Ru–
Sn alloy should also be further investigated.
In Ru–Sn/ZnO the precise role of ZnO as support is not yet
[
17]
clear. However, the characteristics of the support are gener-
ally known to dominate the extent of Sn–support and Ru–sup-
[
15,18]
port interactions.
Strong metal–support interactions in
supported Ru–Sn catalysts may hinder the formation of Ru–Sn
alloys. It is therefore assumed that ZnO, as a support material,
may provide moderate metal–support interactions to generate
the Ru Sn alloy, which is the robust and active phase for hy-
3
7
drogenation. Hydrogen dissociation on ZnO followed by hy-
drogen spillover has already been observed on a Cu/ZnO/Al O
2
3
[16]
catalyst that is commercially used for methanol synthesis.
Likewise, the second role of ZnO is supposedly hydrogen
supply through hydrogen dissociation on ZnO and hydrogen
0
spillover from ZnO to Ru in the Ru–Sn alloy or on isolated
Figure 3. Profiles of anaerobic fermentation of glucose by Clostridium tyrobu-
ruthenium metal surfaces.
À1
&
tylicum at pH 6.0 and 378C. Initial glucose concentration: 50 gL
: butyrate.
, : acetate,
*
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into ethanol via ethyl acetate. It is known that ZeaChem’s cel-
lulosic ethanol process provides technological and economic
advantages over traditional ethanol and other cellulosic etha-
Keywords: alcohols
hydrogenation · ruthenium
·
biomass
·
carboxylic acids
·
[
8]
nol processes. Therefore, if an efficient method is developed
to accomplish the recovery of butyric acid-containing acid mix-
tures from the fermented broth while ensuring highly produc-
tive fermentation, then this method could be used with the
present catalytic approach to develop a new method for the
co-production of bioalcohols for use as biofuels or bio-based
chemicals.
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based on selective hydrogenation of butyric acid combined
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tin Ru–Sn catalysts supported on zinc oxide exhibit unprece-
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Experimental Section
Catalytic measurements were performed in a fixed-bed down-flow
stainless steel (SUS 316) reactor under elevated pressure. The pre-
heating zone was maintained at 3008C to vaporize the butyric acid
feed, obtained from the fermentation as well as Sigma–Aldrich.
Catalyst (1.0 g) was placed in the middle of the reactor on a quartz
wool support. The reaction was performed at 2658C under the
specified conditions. The hydrogen pressure at 25 bar was moni-
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and a hydrogen gas line. Butyric acid was introduced into the reac-
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This work was supported by the Institutional Research Program
of KRICT (KK-1401-B0). The authors thank Dr. J. W. Ha for his ben-
eficial contribution and Dr. U.-H. Lee, Dr. G. M. Cho and Prof. Y. H.
Kim for helpful discussions.
Received: April 15, 2014
Revised: June 5, 2014
Published online on August 14, 2014
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