Upgrading of Ethanol to n-Butanol via a Ruthenium Catalyst in Aqueous Solution

Tarah A. DiBenedetto and William D. Jones*

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Upgrading of Ethanol to n‑Butanol via a Ruthenium Catalyst in

Aqueous Solution

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sı Supporting Information

ABSTRACT: The upgrading of ethanol to n-butanol via the Guerbet reaction in

aqueous media has been developed. This system allows for the upgrading of ethanol

to n-butanol in up to a 28% yield and 57% selectivity, at only 80 °C. This system is

also able to tolerate the same feedstock ratio found in fermentation broth (water to

ethanol ratio of 90:10), albeit a decrease in yield and selectivity (20% yield of n-

butanol, 48% selectivity). Smaller amounts of longer-chain alcohols are also formed.

INTRODUCTION

Scheme 1. Guerbet Reaction Mechanism

■

Ethanol is a renewable biofuel derived from the fermentation

of biomass (corn, sorghum, barley, sugar cane, trees, corn cobs,

and rice straw). There are several drawbacks to ethanol as a

drop-in fuel. Ethanol has only 70% of the energy density of

gasoline, leading to reduced fuel eﬃciency. It is also miscible

with water, leading to issues with separation and dilution in

storage tanks. Additionally, it is corrosive to current engine

technology and fuel infrastructure, which is why high ethanol

fuel blends are less common. An alternative fuel additive is n-

butanol. It has 90% the energy density of gasoline, is

noncorrosive, has limited miscibility with water, and can be

blended with gasoline at higher concentrations (16%) to be

used without any modiﬁcations in current engines. Addition-

ally, butanol has the potential to be blended in diesel fuel,

which would reduce soot and NO emissions.

Today, most butanol is produced synthetically through the

hydroformylation and subsequent hydrogenation of propy-

lene. Although an eﬃcient method of synthesis, the synthetic

at elevated temperatures with a maximum observed yield of

,3,12−20

38% n-butanol in homogeneous systems (Table 1).

production of butanol is tied to the price of crude oil, which is

With few exceptions, the catalysts used in these trans-

formations are sensitive to the water expelled by the aldol

condensation, leading to catalyst decomposition. Notably, in

2014, Mu and co-workers developed a system run under

a sensitive and volatile market. Alternatively, butanol can be

produced through the fermentation of renewable biomass

feedstocks, either directly through the ABE fermentation

process or indirectly by upgrading ethanol to butanol through

6−8

aqueous conditions. They were able to achieve a 52%

conversion of ethanol and 26% yield of butanol with 81%

selectivity for all alcohols formed (50% selectivity for n-

butanol, 16% higher alcohols formed) using an iridium catalyst,

phenanthroline-based ligand, and a mixture of KOH and

the Guerbet reaction.

In the Guerbet reaction, to produce

n-butanol, ethanol is dehydrogenated by a transition metal

catalyst to generate acetaldehyde, which then undergoes an

aldol condensation, typically catalyzed by base, to form

crotonaldehyde through the loss of water. The crotonaldehyde

is then rehydrogenated to form n-butanol (Scheme 1).

Received: April 7, 2021

Published: June 10, 2021

Ideally, the Guerbet system would utilize ethanol from a

fermentation broth, which is typically composed of 90% water

and 10% ethanol. Converting the ethanol to n-butanol in the

fermentation broth without prior distillation would make the

0,11

Guerbet reaction a more industrially viable process.

Unfortunately, typical Guerbet systems are run in neat ethanol

2021 American Chemical Society

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Table 1. Guerbet Systems

author ref

cat. (%)

time, h

base (%)

1 conver. (%)

2 yield (%)

selectivity (%)

Ishii

[Ir] (0.01)

NaOEt (5)

Tp’NiOH (5)

NaOEt (5)

NaOEt (4)

KOH (100)

Wass

[Ru] (0.1)

[Ir] (0.2)

Jones

Szymczak

[Ru] (0.1)

[Ru] (0.02)

Milstein

[Ir] (0.1)

Reactions at 150 °C in neat EtOH unless noted otherwise. Reaction at 120 °C. Reaction run in 5% EtOH/H O. Also used NaOAc (100%).

NaOAc at 150 °C (Table 1). Feng and co-workers have also

Table 2. Base Screen for the Upgrading of Ethanol

developed an aqueous Guerbet system, though with less

success, and Himeda has reviewed related water-soluble CO

hydrogenation catalysts.

RESULTS AND DISCUSSION

Our initial investigation began with the examination of a

■

ruthenium catalyst [(p-cymene)Ru(6,6′-dihydroxybipy)Cl]Cl

entry

base

selectivity

(

[Ru(bipy )]) with a 2,2-bipyridine ligand scaﬀold, which is

KOH

100

0.8

0.5

0.4

known in the literature to perform transfer hydrogenations in

NaOMe

NaOEt

NaO-t-Bu

NaOAc

NaOH

LiOH

aqueous media. Utilizing catalyst [Ru(bipy )] in deionized

water led to unforeseen diﬃculties in the analysis of the

catalytic reactions. The ruthenium catalyst is highly soluble in

basic aqueous media, making it very diﬃcult to remove from

the reaction after completion. Eﬀorts were made to develop an

extraction method, which included the acidiﬁcation of the

reaction and extraction with various organic solvents.

Unfortunately, ethanol’s high solubility in water complicated

this extraction method, and we were unable to quantify ethanol

conversion. Various ﬁltration materials were used in an eﬀort

to remove the catalyst (hydrophobic and hydrophilic syringe

ﬁlters, silica, basic alumina, acidic alumina, celite, glass ﬁber

ﬁlters, and cotton plug), as well as vacuum transfer, though

unfortunately they were unsuccessful due to loss of some

alcohol(s). Ultimately, a sequential ﬁltration through neutral

alumina followed by C¹⁸(reverse phase) silica allowed for

removal of both the base and the catalyst, allowing the reaction

to be analyzed by gas chromatography (GC) with reproduci-

bility of yields of 1 −2% (for full details on the workup method

please see the Supporting Information (SI)).

KO-t-Bu

All reactions conducted on a 1 mmol scale, ethanol (1 mmol, 1

equiv). Amount of ethanol recovered, GC yield (%) vs n-propanol as

the internal standard. % yields are based on the maximum possible =

00%; i.e., 1 mmol EtOH giving 0.5 mmol BuOH is 100% yield; for

hexanols, 0.33 mmol is 100% yield. GC yield (%) vs n-propanol as

the internal standard. Selectivity (%) is reported as the moles of n-

butanol formed vs moles of ethanol converted.

16%, although lower than the yield with potassium hydroxide

(Table 2, entries 1−3). Moving to a much stronger and more

sterically bulky base, catalysis with sodium tert-butoxide gave a

21% yield of n-butanol (Table 2, entry 4), outperforming the

reaction with potassium hydroxide as a base. Investigating the

counterion of the tert-butoxide base, potassium tert-butoxide

gave comparable results to sodium tert-butoxide, although with

better mass balance (Table 2, entry 8). We also examined the

counterion eﬀect with hydroxide bases. Sodium hydroxide gave

comparable yields to potassium hydroxide, with a slight

decrease in the yield of n-butanol, while lithium hydroxide

proved to be detrimental to catalysis, likely due to decreased

basicity (Table 2, entries 6−7). While all of these bases

Initial catalysis investigation began with the use of potassium

hydroxide as the base in deionized water with [Ru(bipy )] as

the catalyst (84:16 (v:v), H O:EtOH). Under optimal

conditions (vide infra), we were able to achieve a 19% yield

of n-butanol with a 56% selectivity (for full details on

optimization please see the SI). We did observe the formation

of higher chain alcohols (2-ethyl-1-butanol, 3% yield; 1-

hexanol, 0.8% yield) as well as a small amount of a white solid.

Note that the white solid was observed in all reactions using

strong bases (Table 2, entries 1−4, 6−8). It was insoluble and

unreactive to a butane ﬂame. An IR of the solid resulted in a

−

produce OH in water, their reactivities are clearly slightly

diﬀerent.

Of note, weaker bases such as sodium acetate did not show

any formation of alcohols or dehydrogenation products, nor

did sodium phenoxide. Organic amine bases were not

productive for catalysis. 1,8-Diazabicycloundec-7-ene (DBU),

N,N-diisopropylethylamine (Hunig’s base), and triethyl amine

did not form any alcohols, though sodium phenoxide and the

organic bases required workup via an acidic extraction, so the

conversion of ethanol could not be determined (see the SI for

GC chromatograms).

−

large peak around 990 cm , indicating that this material is a

silicate, formed via the etching of the glass reaction vessels (see

IR, Figure S8 ). With optimized conditions in hand, we next

turned our attention to screening various inorganic and organic

bases (Table 2). Sodium methoxide showed a 6% decrease in

yield from our standard conditions with potassium hydroxide,

but sodium ethoxide showed a higher yield of n-butanol at

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Table 3. Optimization of Reaction Conditions Using Potassium tert-Butoxide

entry

X (mol %)

Y (mol %)

H O:EtOH (v/v)

time, h

T °C

selectivity

TON

1.5

2.5

84:16

91:9

84:16

2.2

2.4

1.7

0.5

1.2

1.5

All reactions conducted on a 1 mmol scale, ethanol (1 mmol, 1 equiv). Amount of ethanol recovered, GC yield (%) vs n-propanol as the internal

standard. GC yield (%) vs n-propanol as the internal standard. % yields are based on the maximum possible = 100%; i.e., 1 mmol EtOH giving 0.5

mmol BuOH is 100% yield; for hexanols, 0.33 mmol is 100% yield. Selectivity (%) is reported for the formation of n-butanol vs conversion of

ethanol. TON based upon EtOH consumed. 1 equiv of ethanol added at the beginning of the reaction; after 24 h, the reaction was cooled and

another 1 equiv of ethanol was added and the reaction was allowed to react for another 24 h (total 48 h).

Moving forward, we explored the reactivity of sodium tert-

butoxide under more dilute conditions (see the SI for details).

chained alcohols compared to the reaction at 80 °C (Table 3,

entries 1 and 4). Reaction at 75 °C produced only 13% n-

butanol after 92 h. Increasing the loading of the catalyst and

base while running at the elevated temperature produced the

highest yield of n-butanol at 29% but also gave poor selectivity

(Table 3, entry 5). We next investigated the eﬀect of base

loading and reaction time at the higher catalyst loading of 2

mol %. When the reaction was run with 55 mol % of base, a

lower yield and more unselective reaction was seen at both 18

and 24 h compared to the use of 60 mol % base (Table 3,

entries 6−9). Notably we were able to achieve a 28% yield of

n-butanol after 18 h at 2 mol % catalyst and 60 mol % base

loading (Table 3, entry 8). Lower base loadings proved to

result in reduced yields. Note that compared to the earlier

studies in Table 1, the concentration of base is similarit is

due to the smaller percentage of ethanol in aqueous solution

Increasing the water content from 84:16 (v:v, H O:EtOH) to

1:9 (v:v, H O:EtOH) led to a drastic decrease in the yield of

n-butanol after 18 h, from 21% with 0.3 mL of water to 13%

with 0.58 mL of water. Not only did the yield of n-butanol

decrease, the yield of higher chain alcohols did as well, leading

to the hypothesis that the rate of the reaction was signiﬁcantly

decreased under more dilute conditions. Allowing the reaction

to run for 48 h instead of 18 h only increased the yield by 1%

(

see the SI for details). The concentration of the reaction

seems to be the most important factor for reactivity, and

increasing the time of the reaction gave diminishing returns.

Due to the higher mass balance and selectivity observed with

the use of potassium tert-butoxide (Table 2), we further

explored catalysis with the use of this base. Monitoring the

reaction for 24 h led to an increase in yield of n-butanol from

the above reported 19% (18 h) to 24% after 24 h (compare

Table 2, entry 8 and Table 3, entry 1). Further extending the

reaction beyond 24 h showed minimal eﬀect, increasing the

yield of n-butanol to only 25% at 30 h, along with an increase

in the higher alcohols formed (Table 3, entry 2). Extending the

reaction to 48 h led to a 5% decrease in yield of n-butanol and

a subsequent decrease in higher chain alcohols (Table 3, entry

that the % base appears high (as in Mu’s work ).

With our optimized conditions for the reaction in 0.3 mL

water (84:16 (v:v), H O:EtOH), we again looked at diluting

the reaction in 0.58 mL of water to achieve a 91:9 ratio (v:v) of

water to ethanol. Compared to the previous conditions (1.5

mol % catalyst, 50 mol % base) that gave a 14% yield of n-

butanol after 48 h (Table 3 entry 10), running the reaction

more dilute under our optimized conditions led to an increase

in yield to 19% of n-butanol (Table 3, entry 11), drastically

lower than under the more concentrated reaction conditions.

Further increases of the catalyst and base loading achieved a

20% yield of n-butanol under dilute conditions in 24 h (Table

3, entry 12). Control reactions showed that there is no

formation of product without the use of both the catalyst and

base (Table 3, entries 14−15). The recovery was slightly lower

). Reactions run for extended periods of time led to the

unproductive consumption of starting materials and side

reactivity of the products, resulting in the lower overall yield

of n-butanol and higher chain alcohols.

Increasing the temperature of the reaction to 85 °C under

our standard reaction conditions (1.5 mol % catalyst, 50 mol %

base) led to a more unselective reaction, making more higher

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The Supporting Information is available free of charge at

Article

for ethanol in the control reaction with just the catalyst due to

the ether extraction and solubility of ethanol in water.

selectivity (57% selectivity based on recovered ethanol) (Table

3, entry 8). To the best of our knowledge, this is the lowest

reported temperature for this transformation. Improvements

are needed in % conversion, catalyst activity, and reduction of

the quantity of base. Also, our earlier studies showed that

hindered bases such as Tp’Ni(OH) and Cu(IPr)(OH) give

high selectivity for n-BuOH. Adaption of similar hindered

bases that are water soluble might improve selectivity with this

ruthenium catalyst.

Further exploring the reactivity of this system under dilute

conditions, we hypothesized that the low yield was more likely

attributed to low ethanol concentration rather than catalyst

degradation from extended reaction times. We have observed

that this system is highly concentration-dependent, and lower

ethanol concentrations lead to a decrease in reactivity. This

likely occurs at higher conversions of ethanol (50%), as the

ethanol concentration would continuously decrease over the

course of the reaction, making catalysis under already dilute

conditions diﬃcult. To probe this possibility, we set up a

reaction under our standard dilute conditions (standard dilute

conditions Table 3 entry 12), and after 24 h of catalysis, the

reaction was cooled to room temperature and an additional 1

equiv of ethanol was added (eﬀectively doubling the

theoretical yield of n-butanol). If the poor reactivity was

caused by catalyst degradation, we would expect to only see

EXPERIMENTAL SECTION

■

General. Reagents were used as received from the following

sources: dichloro(p-cymene)ruthenium(II) dimer, crotonaldehyde, 1-

hexanol, 2-hexanol, 2-ethyl-1-butanol, sodium methoxide, sodium

ethoxide, sodium phenoxide, sodium tert-butoxide, Hunigs base, DBU

(

Sigma-Aldrich); 6,6′-dihydroxy-2,2′-bipyridine (TCI); ethanol 200

proof, n-butanol, n-propanol, potassium hydroxide, sodium hydroxide,

lithium hydroxide, triethylamine, basic alumina (Fisher Scientiﬁc);

acetaldehyde, potassium tert-butoxide (Acros Organics); acidic

alumina (M. Woelm Eschwege, Germany); neutral alumina (Alfa

Aesar); and C₁₈silica (J.T. Baker).

0% of the n-butanol yield we observed under our standard

conditions (Table 3, entry 12). If the catalyst was completely

decomposed after 24 h, our maximum yield of n-butanol could

only be 10%. We observe a 17% yield of n-butanol, indicating

that butanol continues to be formed upon addition of more

ethanol, which supports our hypothesis that the reduced

reactivity under dilute conditions can mostly be attributed to

low ethanol concentration rather than catalyst degradation

GC spectra were recorded on a Shimadzu GC-2010 Plus

instrument equipped with an AOC-20i autoinjector and a DB-

WaxETR column (30 M × 0.25 mm ID × 0.5 μm). Column ﬂow = 2

mL/min, injection temperature 225 °C, split ratio 20:1, pressure 20.3

psi, temperature program: 5 min at 50 °C, then increase at 15 °C/min

to 155 °C and hold for 3 min.

General Reaction Setup and Workup. In a glovebox, a Schlenk

(

Table 3, entry 13).

It is worth pointing out that the earlier studies by Papish

ﬂask was charged with [Ru(bipy )] (0.02 mmol, 2.0 mol %) and

potassium tert-butoxide (0.6 mmol, 0.6 equiv). The Schlenk ﬂask was

then capped with a septum and removed from the glovebox. Once

outside the glovebox, the vessel was opened and deionized water (0.3

mL) was added to the reaction. Ethanol (200 proof) (1.0 mmol, 1.0

equiv) was then added to the reaction via an airtight glass syringe and

the weight of ethanol was recorded. The Schlenk ﬂask was sealed with

a Teﬂon pin and was placed in a metal heating block at 80 °C for 18

h. After 18 h, the reaction was cooled to room temperature and

worked up (see the Supporting Information , p. 15−17, for the

optimized workup procedure). GC chromatograms were recorded

after 10 μL n-propanol was added to each sample as the internal

standard. Additional experimental details can be found in the

Supporting Information .

examined [Ru(bipy )] as well as catalysts with methoxy

groups in place of OH, with C Me in place of cymene, with

unsubstituted bipy, and with [(p-cymene)RuCl ] . While all

catalysts were active for complete transfer hydrogenation of

acetophenone in PrOH solvent, when the solvent was 90:10

water: PrOH, only [Ru(bipy )] gave full conversion, with the

others showing only 22−50% product. In addition, Himeda

reported the use of the bis-4-hydroxybipyridine ligand on p-

cymene ruthenium for CO₂hydrogenation. Only the

deprotonated ligand (i.e., high pH) was active in catalysis,

which was attributed to higher solubility and more electron-

donating ligands. For these reasons, we did not investigate

other bipy derivatives, as the bis-ortho-hydroxybipy was the

best by far.

ASSOCIATED CONTENT

sı Supporting Information

■

Another interesting observation with this system is that no

acetate is produced, a common byproduct in Guerbet

reactions. Examination of the aqueous phase following

acidiﬁcation and extraction of the organic products with

https://pubs.acs.org/doi/10.1021/acs.organomet.1c00217.

Details of the optimization of the Guerbet reaction

conditions, GC and/or GCMS data for each reaction,

and catalyst characterization and synthesis details (PDF)

ether showed no acetic acid by H NMR spectroscopy (see the

SI).

In conclusion, we have been able to run the Guerbet

reaction in water without additional organic cosolvents, and

the ethanol to water ratio approaches the aforementioned

fermentation feedstock ratio (10% ethanol to 90% water).

Upgrading ethanol in water removes the need for the prior

distillation of the fermentation broth, which is a requirement

for most other systems. It also makes this a more environ-

mentally friendly process, as it forgoes the use of additional

organic cosolvents. Fermentation broth would also contain

other materials that may be detrimental to catalysis and would

need to be evaluated further. Another important factor that

contributes to the overall cost of this process is the heat

required by the reaction. Typical Guerbet reactions run at

temperatures upward of 150 °C, while this system is able to

run at 80 °C, achieving up to a 28% yield of butanol albeit low

AUTHOR INFORMATION

■

Corresponding Author

William D. Jones − Department of Chemistry, University of

Rochester, Rochester, New York 14627, United States;

orcid.org/0000-0003-1932-0963; Email: jones@

chem.rochester.edu

Author

Tarah A. DiBenedetto − Department of Chemistry, University

of Rochester, Rochester, New York 14627, United States;

orcid.org/0000-0001-8861-0740

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.organomet.1c00217

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Tolerant Catalyst for the Conversion of Ethanol to Isobutanol. Cat.

Article

Notes

Sci. Technol. 2017, 7, 5128−5134.

The authors declare no competing ﬁnancial interest.

19) Kulkarni, N. V.; Brennessel, W. W.; Jones, W. D. Catalytic

Upgrading of Ethanol to n -Butanol via Manganese-Mediated Guerbet

ACKNOWLEDGMENTS

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Reaction. ACS Catal. 2018, 8, 997−1002.

The authors gratefully acknowledge funding from the U.S.

Department of Energy, Basic Energy Sciences (BES) Chemical

Sciences, Geosciences, and Biosciences (CSGB) Division (DE-

SC0020230).

(20) Fu, S.; Shao, Z.; Wang, Y.; Liu, Q. Manganese-Catalyzed

Upgrading of Ethanol into 1-Butanol. J. Am. Chem. Soc. 2017, 139,

11941−11948.

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Ashraf, N.; Guan, J.; Mu, X. Direct Self-Condensation of Bio-Alcohols

in the Aqueous Phase. Green Chem. 2014, 16, 3971−3977.

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