Towards the upgrading of fermentation broths to advanced biofuels: A water tolerant catalyst for the conversion of ethanol to isobutanol

Article abstract of DOI:10.1039/c7cy01553d

Isobutanol is an ideal gasoline replacement due to its high energy density, suitable octane number and compatibility with current engine technology. It can be formed by the Guerbet reaction in which (bio)ethanol and methanol mixtures are converted to this higher alcohol in the presence of a suitable catalyst under basic conditions. A possible limitation of this process is the catalyst's water tolerance; a twofold problem given that water is produced as a by-product of the Guerbet reaction but also due to the need to use anhydrous alcoholic feedstocks, which contributes significantly to the cost of advanced biofuel production. Isobutanol formation with pre-catalyst trans-[RuCl₂(dppm)₂] (1) has been shown to be tolerant to the addition of water to the system, achieving an isobutanol yield of 36% at 78% selectivity with water concentrations typical of that of a crude fermentation broth. Key to this success is both the catalyst's tolerance to water itself and the use of a hydroxide rather than an alkoxide base; other catalysts explored are less effective with hydroxides. Alcoholic drinks have also been used as surrogates for the fermentation broth: the use of lager as the ethanol source yielded 29% isobutanol at 85% selectivity in the liquid phase.

Full text of DOI:10.1039/c7cy01553d

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Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
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Towards the Upgrading of Fermentation Broths to Advanced
Biofuels: A Water Tolerant Catalyst for the Conversion of Ethanol
to Isobutanol
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
Katy J. Pellow, Richard L. Wingad and Duncan F Wass*
DOI: 10.1039/x0xx00000x
Isobutanol is an ideal gasoline replacement due to its high energy density, suitable octane number and compatability with
current engine technology. It can be formed by the Guerbet reaction in which (bio)ethanol and methanol mixtures are
converted to this higher alcohol in the presence of a ruthenium catalyst and under basic conditions. A possible limitation
of this process is catalyst water tolerance; a twofold problem given that water is produced as a by-product of the Guerbet
reaction but also due to the need to use anhydrous alcoholic feedstocks, contributing significantly to the cost of advanced
www.rsc.org/
2 2
biofuel production. Isobutanol formation with pre-catalyst trans-[RuCl (dppm) ] (1) has been shown to be tolerant to the
addition of water to the system, achieving an isobutanol yield of 36% at 78% selectivity with water concentrations typical
of that of a crude fermentation broth. Key to this success is both catalyst tolerance to water itself but also to the use of a
hydroxide rather than an alkoxide base; other catalysts explored are less effective with hydroxides. Alcoholic drinks have
also been used as surrogates for a fermentation broth, the use of lager as the ethanol source yielding 29% isobutanol at
8
5% selectivity in the liquid phase.
Introduction
The need to find sustainable alternatives to liquid fossil
fuels for transportation is crucial both from an environmental
perspective as well as to ensure energy security for the
1
future. The most widely used alternative to gasoline is
2
bioethanol, derived from the fermentation of biomass. But
bioethanol is by no means the ideal gasoline replacement; it is
corrosive to engine technology, has a significantly lower
energy density than gasoline and its hygroscopic nature leads
3
to storage problems. By contrast, butanols have fuel
Scheme 1 Co-condensation of ethanol and methanol via Guerbet chemistry.
characteristics much closer to that of conventional gasoline
and are often considered ‘advanced biofuels’ because of this
mixtures to isobutanol with high ethanol conversions to
Guerbet alcohol products (>75%) and excellent selectivity for
isobutanol formation (>99%), the first homogenous catalysts
4
superior performance. Biosustainable routes to butanol, such
as the ABE (acetone-butanol-ethanol) process, remain
5
challenging. Our group has investigated converting readily
9
for this reaction. Heterogeneous systems have been known
1
since 1990 and very recently a homogenous ruthenium(II)
0
available (bio)ethanol to n-butanol using homogeneous
ruthenium catalysts with either P-P bidentate or mixed donor
P-N ligands, achieving excellent selectivity at good conversion
1
1
bis(phosphino-phosphinine) pre-catalyst has been reported.
Our interest in forming isobutanol lies in the fact that it offers
6
improved fuel properties over n-butanol, such as a higher
1
octane number and energy density (98% of that of gasoline).
for this transformation. There have been several other recent
2
examples of the conversion of ethanol to n-butanol using
A Guerbet-type mechanism was proposed (Scheme 1) for the
formation of isobutanol; this “borrowed hydrogen” process
requires two cycles (ethanol/methanol to propanol,
propanol/methanol to isobutanol) and consequently two
equivalents of water are produced as the only by-product.
Clearly, it is important that Guerbet catalysts demonstrate
good water tolerance so as not to be poisoned by this by-
product. Another important consideration in terms of
sustainable technology is the recovery of ‘dry’ bioethanol from
a typical fermentation broth (Figure 1). This is achieved by
homogeneous ruthenium or iridium catalysts which
7
demonstrate similar performance. This area, and related
heterogeneous approaches, have been extensively reviewed
8
over the last few years.
We recently reported that our family of ruthenium
catalysts are also capable of converting ethanol/methanol
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methanol/ethanol) without studying these systems with more
realistic alcohol/water mixtures, and probing the all-important
effect of water levels and base selection and concentration.
We now report in detail the effect of these parameters with
several ruthenium based catalysts, showing that surprisingly
good conversions can still be achieved with trans-
2 2
[RuCl (dppm)] (1) at high levels of water. In addition,
experiments with alcoholic beverages, used as surrogates for
fermentation broths, reveal that
impurities.
1 can also tolerate biogenic
Results and Discussion
Catalyst screening
We have previously reported the use of ruthenium pre-
catalysts and , which utilise P-P or P-N bidentate ligands, to
1
2
Figure 1 The drying stages involved in obtaining anhydrous ethanol for catalytic
upgrading.
upgrade ethanol/methanol mixtures to isobutanol in high
9
yield and excellent selectivity (Table 1, Entries 1 and 9). In
this paper we have investigated the impact of using wet
ethanol/methanol mixtures on these two catalysts and also
distillation, yielding typically 95.6 wt% ethanol when corn-
1
3
starch is used. Molecular sieves are then used in the final
drying process to yield >99% pure ethanol. This is a highly
energy intensive process, in fact during the production of first
generation bioethanol, typically over half of the energy is
pre-catalyst
Complex
3
which exploits a P-N-P tridentate ligand.
3
is a commercially available hydrogenation/
dehydrogenation pre-catalyst which shows excellent activity in
related alcohol conversions and therefore we felt it may
7
b
consumed in the drying procedures. It is clear that using a
fermentation broth rather than analytically pure ethanol as a
substrate in ethanol-to-butanol catalysis would be highly
desirable (Figure 1, Ideal Pathway), and could significantly
impact the economics and sustainability of such a process.
Removing the need to dry ethanol before upgrading to butanol
means that only one drying step is required in which the water
from the fermentation broth and formed during the Guerbet
cycle can be removed in a single purification step. The
water/isobutanol separation by distillation is also much easier
to achieve than ethanol/water distillation. Even using
approximately 95 wt% ethanol could lead to benefits in this
regard (Figure 1, Alternative Pathway). Unfortunately, many
Guerbet catalytic systems are reported to lose activity and
1
8
perform well in our system. We employed our standard
reaction conditions of sodium methoxide base, a methanol to
ethanol molar ratio of 14.4:1 (to prevent homocoupling of
ethanol), a catalyst loading of 0.1 mol% (relative to ethanol)
and a reaction temperature of 180 °C. Performance of catalyst
3
is comparable to that of catalyst
2 after two hours (Entry
1
2).‡ An increase in reacꢀon ꢀme led to a modest increase in
both yield and selectivity for isobutanol formation (Entry 13).
Accompanying the formation of liquid Guerbet products using
pre-catalysts 1-3 was the formation of solid products; sodium
formate, carbonate and acetate, and associated significant
hydrogen evolution. The origin of these products will be
discussed in the following section.
6
c, 7e, 14
selectivity in the presence of water.
Water can
Base investigation
potentially lead to catalyst decomposition or solubility issues,
and deactivation of alkoxide bases to often inactive
With a selection of catalysts in hand, we first considered
the choice of base before attempting to add water to the
system. Addition of water to the reaction mixture results in
conversion of sodium methoxide base to methanol and sodium
hydroxide. In our previous communication we reported that a
1
0b, 15
hydroxides.
Water may also promote the formation of
carboxylate salts and esters in competing reactions, lowering
7
d,
product selectivity and the basicity of the reaction mixture.
1
Indeed a recent review by Wang and Cao highlights that
4b, 16
the vast majority of bioethanol upgrading systems utilise
anhydrous feedstocks, with attempts to use “wet” ethanol
combination of
1 and co-catalyst NaOH was able to efficiently
9
convert ethanol/methanol mixtures to isobutanol even
though hydroxide has proven to be detrimental in other
1
7
showing only limited success. Xu and Mu et al. have
developed immobilised iridium catalysts capable of converting
either ethanol to n-butanol or mixtures of ethanol and
methanol to isobutanol in aqueous conditions with impressive
yields and selectivity; however, the system is limited by long
reaction times that are needed to ensure high ethanol
1
0b, 15
Guerbet systems.
hydroxide base, we felt this warranted further investigation
with other catalysts. Performing the catalysis with complex
Given the high activity of 1 with
2
and sodium hydroxide as the base did result in a decrease in
both yield and selectivity compared to when sodium
methoxide was the co-catalyst, however, a reasonable
isobutanol yield is still produced (Table 1, Entry 10).
Contrastingly, use of sodium hydroxide as the base with
7
b, 10d
conversion.
In our preliminary communication of homogeneous
9
methanol/ethanol to isobutanol catalysts we focused on our
standard catalyst screening conditions (using analytically pure
catalyst
3
resulted in only a trace amount of propanol being
2
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T
able 1. Conversion of ethanol/methanol to isobutanol.
c
Yield (Selectivity) (%)
a
Added
Ethanol
Entry
Catalyst
Base (mol%)
b
Water (mL) Conversion (%)
Isobutanol
n-Propanol
Other
d
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
2
2
2
3
3
3
3
NaOMe (200)
NaOH (200)
NaOH (200)
NaOH (150)
NaOH (100)
NaOH (50)
-
-
67
74
74
75
67
17
0.2
0.2
49
39
19
48
53
0.2
1.0
65 (98)
71 (96)
71 (97)
72 (96)
59 (90)
11 (68)
0.1 (42)
0.1 (57)
39 (86)
28 (74)
10 (55)
36 (82)
44 (89)
-
1.2 (1.8)
2.8 (3.8)
2.2 (3.0)
2.4 (3.2)
5.3 (8.2)
3.9 (25)
0.1 (58)
0.1 (43)
3.1 (6.8)
8.6 (23)
7.7 (41)
4.7 (11)
2.4 (4.8)
0.2 (100)
0.2 (20)
0.8 (0.7)
0.5 (0.3)
0.4 (0.3)
0.8 (0.5)
2.6 (2.0)
2.4 (7.7)
-
d
d
0.62
-
-
-
-
-
-
-
0.62
-
-
-
NaOC(O)H (200)
Na CO (200)
-
2
3
d
9
NaOMe (200)
NaOH (200)
NaOH (200)
NaOMe (200)
NaOMe (200)
NaOH (200)
NaOH (200)
6.6 (7.2)
2.5 (3.3)
1.5 (3.9)
7.1 (7.9)
6.7 (6.7)
-
1
1
1
1
1
1
0
1
2
3
4
5
e
0.62
0.3 (48)
0.5 (32)
a
b
Conditions: 1 mL (17.13 mmol) ethanol, 10 mL (247.13 mmol) methanol, 0.1 mol% [Ru], base (mol% as stated) (mol% based on ethanol substrate), 180 °C, 2 h.
Conversion of ethanol based on total amount of liquid products obtained as determined by GC analysis. Total yield and selectivity of Guerbet products in the liquid
c
d 9 e
fraction as determined by GC analysis. Previously reported by us. 20 h.
formed in the liquid fraction as the sole Guerbet product resulted in the formation of significant residual pressure in the
(Entry 14). As with the reaction with alkoxide base, hydrogen autoclave and the precipitation of sodium carbonate from the
was generated over the course of the reaction evidenced by a reaction mixture (see ESI, Section 4.1†). However, carrying out
pressure increase in the autoclave, concurrent with sodium the same experiment with sodium methoxide instead of
formate, carbonate and acetate precipitation from the sodium hydroxide base did not result in precipitation of solids
reaction mixture.§ Beller et al. have previously reported that from the reaction mixture nor any observable residual
catalyst
3 in an open system readily converts ethanol to ethyl pressure (See ESI, Section 4.2†).§§ These results, in line with
acetate, which, if formed in our system, may then be Beller’s earlier work, demonstrate that water/hydroxide is
converted to sodium acetate and ethanol by sodium essential for formate and carbonate formation. Rapid
1
8
hydroxide. The same group have also demonstrated that formation of water as a consequence of the Guerbet reaction
methanol undergoes water/hydroxide assisted in our standard isobutanol forming system using sodium
dehydrogenation to carbon dioxide (trapped as carbonate methoxide as base therefore results in these same products
under basic conditions) using catalyst , with formate being being formed, even in an initially hydroxide free system.
observed as an intermediate. The use of hydroxide base in Reduction of the sodium hydroxide base loading to 150
our system presumably promotes similar processes, with the mol% with pre-catalyst maintains activity (Entry 4) but
formation of these products being favoured over Guerbet reducing the base loading further has a detrimental impact; for
3
1
9
1
coupling with pre-catalyst
3
.
example, a reduction in sodium hydroxide to 50 mol% resulted
in only an 11% isobutanol yield at 68% selectivity over other
In comparison the use of hydroxide base with catalyst
1
actually led to a small increase in isobutanol yield compared to liquid products (Entry 6). It has previously been proposed that
using sodium methoxide and high selectivity was maintained stoichiometric alkoxide base loadings are needed due to base
9
Entry 2). Significant levels of hydrogen, sodium carbonate degradation over time by the water produced, forming
(
1
0b
and formate products were still produced over the course of catalytically-inactive hydroxide. Clearly this cannot be the
the reaction but compared to catalyst it appears these case when using catalyst since sodium hydroxide is still an
3
1
processes do not dominate over Guerbet-type reactions.§ effective base. However, we do observe the further reaction of
Heating methanol only (to prevent Guerbet coupling), catalyst sodium hydroxide to sodium carbonate during catalysis, via
1
and sodium hydroxide under our standard conditions sodium formate. Crucially, neither sodium formate nor
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concentration in the heterogeneous rhodium-catalysed
conversion of n-butanol to 2-ethyl-1-hexanol, the rate of
product formation significantly decreased. A reaction was also
Isobutanol Yield (%)
Isobutanol Selectivity (%)
carried out using catalyst
1 with air-saturated solvents and 5
1
00
mL of water (Table S1†). Although a further drop in activity
was observed compared to a nitrogen saturated system, a
good isobutanol yield (27%) and selectivity (82%) was still
achieved.
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Analysis of the solid side-products
Solid side-products are often ignored in this area of
catalysis, with selectivity being defined as selectivity within the
liquid fraction. Clearly, as well as giving misleadingly high
figures of merit, this ignores products that can give insight into
competing reaction mechanisms. Analysis of the solid side-
products formed in our system confirmed them to be
comprised of predominantly sodium formate, carbonate and
hydroxide, with small amounts of sodium acetate and
methoxide formed (Table 2, ESI Table S4). The production of
significant amounts of sodium hydroxide when using sodium
methoxide as the base (Entry 1) is consistent with the high
levels of water produced in the Guerbet reaction, converting
the methoxide base to hydroxide. Comparison of entries 1 and
3
4.42
111.02
(2)
277.55
(5)
555.09 1110.19
(10) (20)
(
0.62)
Water Addition, mmol (mL)
carbonate are effective bases in our system (Table 1, Entries 7
and 8), a major issue being their insolubility in the reaction
a
mixture as well as the lower pK . So, a high concentration of
2
shows that a similar composition of solids is produced on use
sodium methoxide (or hydroxide) is required at the beginning
of each reaction to ensure appreciable amounts of active
alkoxide (or hydroxide) remain during the reaction before this
is degraded to inactive carbonate.
of either sodium hydroxide or methoxide base. With increased
addition of water to the system, there is a reduction in sodium
formate and carbonate formation. This is consistent with the
lower residual hydrogen pressure in the autoclave observed
with increased addition of water. For example, entry 2
produced a residual pressure of 8 bar, whilst entry 5 resulted
in a residual pressure of 5 bar. This trend in reduction in side-
product formation with increased addition of water follows
that of isobutanol formation, demonstrating the decrease in
catalyst activity with increasing water addition.
Water tolerance investigations
The water tolerance of catalysts
1
and
2
was examined. We
is tolerant to the
have previously reported that catalyst
1
addition of low levels of water (0.62 mL, 2 equivalents relative
9
to ethanol) at the beginning of the reaction (Table 1, Entry 3).
Addition of water in the same way when using catalyst
2 had a
more detrimental effect on the activity and selectivity for
isobutanol formation (Table 1, compare entries 10 and 11).
Catalyst stability, role of the base and mechanism
In order to investigate the stability of the transition metal
This was perhaps surprising given the better performance of in
6
complex to water, pre-catalyst
ethanol/methanol mixture without base under our standard
conditions. This led to the conversion of into two major
species identified as trans-[RuH(CO)(dppm) ]Cl ( ) and
1 was heated in an
situ
diphenylphosphino)
cymene)] /dppm in ethanol homocoupling to n-butanol in the
formed
pre-catalyst
[RuCl
2
(η -p-cymene)]
2
6
/2-
(
ethyl-amine
over
2
[RuCl (η -p-
1
2
6
c
4
2
presence of water. As the water concentration was
increased, to a level similar to that found in fermentation
broths, a decrease in both isobutanol yield and selectivity was
Table 2. Analysis of the solids obtained from the post-reaction mixture.
d
Composition of Solid (wt%)
observed with catalyst
1
(Figure 2, Table S1†), but acceptable
Water
a
Entry
Sodium
Sodium
Sodium
yields and selectivity were still achieved. For example, with the
addition of 5 mL of water to the system (equivalent to a molar
methanol:ethanol:water ratio of 14.4:1:16.2) an isobutanol
yield of 36% at high selectivity (78%) was obtained. This level
of water is typical of a fermentation broth; the ethanol content
of which may be as high as 15 wt% from corn starch
(mL)
b
c
Hydroxide
Formate
Carbonate
e
1
-
37
49
43
57
60
19
12
10
9
37
35
39
33
23
2
3
4
5
-
0.62
5
1
3a
20
15
feedstocks.
a
Analysis of the solid collected from the post reaction mixture. Conditions: 1 mL
The addition of water to the system will lower the (17.13 mmol) ethanol, 10 mL (247.13 mmol) methanol, 0.1 mol% 1, 200 mol%
b
NaOH (mol% based on ethanol substrate), water as stated, 180 °C, 2 h. Amount
concentration of the base. As we have shown that a lower of sodium formate formed calculated by NMR spectroscopy using DMSO as
c
standard. Amount of sodium carbonate formed calculated from microanalysis
d
initial base loading results in loss in activity, this same trend is for inorganic carbon. Other solid products are sodium acetate and methoxide
e
(
expected on addition of water. Burk et al. also observed this 200 mol% NaOMe
see ESI Table S4†) with the remaining solid presumed to be sodium hydroxide.
1
5b
effect, noticing that in lowering the initial alkoxide
4
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2
0
trans-[RuCl(CO)(dppm)
2
]Cl (
are
decarbonylation reactions of ethanol or methanol.
5) (see ESI, Secꢀon 5.1†). These the proposed mechanism highlights the importance of the
carbonyl
complexes
presumably
formed
by base in activation of the ruthenium catalyst, in mediating the
No aldol condensation and in regenerating off-cycle ruthenium
7
d, 21
Guerbet products are formed confirming the need for basic carbonyl species.
conditions and suggesting that the carbonyl species poison the
catalysis unless base is present to remove the carbonyl ligand.
Table 3. Using alcoholic drinks as the ethanol source
Milstein et al. have reported the formation of an off-cycle
iPr
carbonyl species, [RuH(CO) (PNP )], incorporating a de-
aromatised P-N-P pincer ligand, which can be converted to a Entry
coordinatively unsaturated active ethanol to n-butanol catalyst
2
Ethanol
Source
Ethanol
d
Yield (Selectivity)
a
Conversion
c
(%)
b
Isobutanol Other
(ABV)
7
by loss of CO under basic conditions. Likewise, Szymczak et 1
d
Raki (45)
79
72 (93)
64 (91)
7.1 (7.2)
6.9 (8.8)
al. proposed the deactivated catalyst in their ethanol to n-
2
Gin (41.2) 71
butanol system to take the form of a ruthenium-carbonyl
7
species. Carrying out the same reaction but with addition of
e
Brandy
67
3
61 (91)
44 (87)
6.4 (9.0)
6.9 (13)
(
36)
0.62 mL of water also resulted in the formation of 4 and 5 as
well as increased amounts of an unidentified cis-diphosphine 4
complex. The similarity in these experiments implies water
does not have a detrimental impact on the pre-catalyst, at 5
least in the absence of base. This stability is perhaps not
Port (20)
51
48
Sherry
41 (86)
39 (88)
40 (86)
7.2 (14)
5.9 (12)
6.8 (14)
(
17.5)
surprising given that the synthesis of pre-catalyst
1
is carried
out in aqueous conditions. A second experiment in which 6
pre-catalyst was heated in methanol with sodium hydroxide
ethanol omitted to prevent Guerbet coupling) revealed that
carbonyl complexes and
were not detected (See ESI, 7
White
Wine
(13.5)
22
45
47
1
(
Red Wine
4
5
(
13.5)
Lager
8.5)
Ale (5)
Section 4.1†). This suggests base depletion via the mechanisms
outlined previously can have the added detrimental effect of
leading to deactivation of the transition metal species to these
8
35
14
29 (85)
9.9 (74)
5.9 (16)
4.1 (26)
(
inactive carbonyl complexes. The same major transition metal 9
3
species, observed by P{ H} NMR spectroscopy, were formed
1
1
a
Conditions: Ethanol source (17.13 mmol), 10 mL (247.13 mmol) methanol, 0.1
mol% 1, 200 mol% NaOH (mol% based on ethanol substrate), 180 °C, 2 h. See
b
with or without the addition of water to the basic system (See ESI general experimental informaꢀon† for further details of alcoholic drinks used
c
in this study. Conversion of ethanol based on total amount of liquid Guerbet
d
ESI, Section 5.3†). The three major peaks (16.5, 22.1 and 35.7 products obtained as determined by GC analysis. Total yield and selectivity of
Guerbet products in the liquid fraction as determined by GC analysis.
3
1
1
ppm) are also observed in the P{ H} NMR spectrum of the
post-reaction mixture of a standard isobutanol forming
reaction. (See ESI, Fig. S8†). We have previously reported that
ruthenium-hydride complexes are the likely catalytic resting
Towards fermentation broths
Commercial alcoholic drinks provide a useful surrogate for
ethanolic fermentation broths, essentially being produced in
the same way and containing both water and other impurities
6
a, 6c
species in our systems.
such as unfermented sugars. The tolerance of catalyst
1 to a
variety of ethanol sources was tested (Table 3). The ethanol
content (ABV) varied from 45% to 5% but in each case the
same amount of ethanol (17.13 mmol) was added to the
reaction in place of pure ethanol. Remarkably, catalyst
1 was
tolerant to the use of these ethanol sources, the trend in terms
of both yield and selectivity simply following water content
Scheme 2 Proposed reaction pathways for formaldehyde generated during
catalysis
(See ESI, Figure S1 and Table S2†). This has significant
implications in that the other components such as sugars and
sulfites appear to have little effect on catalyst activity.
Scheme
2 illustrates proposed alternative reaction
pathways which the formaldehyde generated can follow;
either Guerbet condensation or a methanol dehydrogenation
pathway resulting in the formation of sodium formate and
carbonate. The consumption of base into Guerbet-inactive
sodium formate and carbonate base eventually leads to
deactivation of the system. Detailed mechanistic studies of
Xu and Mu et al. have reported that ferulic acid, a common
biogenic component in fermentation broth derived from
inulin, is detrimental to isobutanol forming activity using a
heterogeneous iridium catalyst, decreasing ethanol conversion
1
0d, 25
by 60%.
In contrast, addition of trans-ferulic acid to our
system did not cause any loss in catalyst activity (See ESI, Table
S1†).
methanol dehydrogenation involving catalyst
3
and derivatives
Presumably similar
and mechanistic studies are
1
9, 24
have been carried out elsewhere.
routes may be operating with
1
currently underway to further examine these competing
processes along with the Guerbet mechanism. Examination of
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ARTICLE
Journal Name
Conclusions
Notes and references
‡
2 h was chosen as the standard reaction time as production of
2 2
The pre-catalyst trans-[RuCl (dppm) ] (1) was found to be
isobutanol significantly slowed after this time in kinetic
experiments with complex (See ESI, Section 3 ).
After cooling the autoclave to room temperature, the residual
pressure, when using pre-catalyst and NaOH base, is 22 bar
it is 8 bar. This indicates higher
formate/carbonate formation with pre-catalyst
§ On removing the volatiles from the post-reaction mixture,
small amounts of formate and carbonate were observed by NMR
spectroscopy, presumably due to trace amounts of water from
the solvent.
unique amongst the catalysts screened in its ability to
transform aqueous ethanol/methanol mixtures to isobutanol
in good yield and excellent selectivity. Key to this catalysts
1
†
§
3
success is both the tolerance to water but also the ability to whereas with pre-catalyst
utilise hydroxide rather than alkoxide base. Catalysts and
1
3
.
2
3
§
incorporating P-N and P-N-P ligands respectively, lose both
yield and selectivity under these same conditions, a major
problem being the promotion of competing side reactions
resulting in the formation of inactive formate and carbonate 1.
salts. The tolerance of our system to both water and other
biogenic impurities is a step forward in converting crude
fermentation broth to an advanced biofuel, significantly
lowering the total cost of biofuel production.
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