Alternative pathways in the ruthenium catalysed hydrogenation of CO to alcohols

Article abstract of DOI:10.1039/c3cy00423f

CO hydrogenation in [PBu₄]Br in the presence of [Ru ₃(CO)₁₂] gives predominantly methanol, ethanol and propanol with small amounts of 1,2-ethanediol. Using RuO₂ as the catalyst precursor, the same products are f

Full text of DOI:10.1039/c3cy00423f

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Alternative pathways in the ruthenium catalysed
hydrogenation of CO to alcohols†
Cite this: Catal. Sci. Technol., 2014,
4, 218
a
b
b
a
*
Jan H. Blank, Robert Hembre, James Ponasik and David J. Cole-Hamilton
Received 20th June 2013,
Accepted 8th November 2013
CO hydrogenation in [PBu4]Br in the presence of [Ru3(CO)12] gives predominantly methanol, ethanol and
propanol with small amounts of 1,2-ethanediol. Using RuO2 as the catalyst precursor, the same products are
formed along with higher alcohols (1-butanol –1-heptanol). Reactions carried out using added 13CH3OH or
13CO show that ethanol and propanol come from homologation reactions of methanol and ethanol
respectively, but that the higher alcohols are not formed through the lower alcohols as intermediates.
DOI: 10.1039/c3cy00423f
www.rsc.org/catalysis
in which we discovered that [Bu3PH]Br, sometimes pre-
sent as an adventitious impurity in the solvent acts as a
Introduction
The hydrogenation of carbon monoxide using heterogeneous
catalysts, the Fischer Tropsch (FT) process, has been well-
developed and fine-tuned on a huge scale for the production
of alkanes and alkenes from coal or natural gas.1–3 In these
processes all of the oxygen is discarded as water so oxygen
functionalised molecules are minor side-products. With the
significant promoter of the reaction by releasing HBr into
the system.
The main products were methanol, ethanol and propanol
in declining amounts. Significant amounts of ethylene glycol
were also formed. We now report that the product distribu-
tion obtained when using RuO2 as the ruthenium precursor
is significantly different from that obtained when using
[Ru3(CO)12] and use 13C labelling studies to examine the
mechanisms of formation of the various products.
rising scarcity of oil, the production of oxygenates such as
4–7
alcohols from coal, natural gas or CO2
(C1 feedstocks) has
become more attractive, but despite this, following a flurry of
activity in the 70's and 80's the conversion of CO into higher
alcohols is still underdeveloped, Homogeneous CO hydrogena-
tion may potentially give better selectivity and control over prod-
uct formation and especially selectivity towards oxygenates.8
Homogeneous carbon monoxide hydrogenation was
pioneered by Gresham and Schweizer using cobalt based sys-
tems at high temperatures and pressures.9 Lower pressure
operation was possible using rhodium10,11 or ruthenium,
mainly [Ru3(CO)12],12–14 based catalysts with the major prod-
ucts being methanol and 1,2-ethanediol.
Experimental
Unless stated otherwise, all chemicals where obtained from
Sigma Aldrich and used as received. Air-sensitive compounds
where handled under N2 using standard Schlenk techniques.
NMR spectra where recorded on Varian 300 NMR or Bruker
AM 300/400 NMR spectrometers. The chemical shifts where
referenced to the solvent, which were in turn referenced to a
TMS standard. IR spectra were recorded by pressing a sample
of the liquid product between two KBr plates in a holder. The
samples were recorded on a Thermo Nicolet Avatar FTIR
spectrometer with a nitrogen cooled MCT detector. For quan-
titation Gas Chromatography analysis was performed using
a Supelcowax-10 capillary column (60 m × 0.32 mm × 1.0 μm
film thickness) using an Agilent 6890 N Network GC system
equipped with a flame ionisation detector. For identification
a HP 6890 series GC system equipped with a HP 5973 mass
selective detector was used. Both machines used the same
column using 1 ml min−1 helium carrier gas flow and 250 °C
injector and detector temperatures. The temperature
programmes was as follows: 50 °C, hold 3 minutes, ramp
20 °C min−1, 150 °C, hold 5 min, ramp 20 °C min−1, 220 °C,
hold 13 minutes. Split ratio: 1 : 100.
Improvements were obtained by adding halides to the
ruthenium system and Knifton and co-workers introduced15–17
phosphonium bromides as the solvent as well as RuO2 as an
alternative catalyst precursor.15 In these cases, ethanol also
became a significant product.
We have recently reported18 studies on [Ru3(CO)12]
catalysed CO hydrogenation in [Bu4P]Br, under milder con-
ditions (250 bar, 200 °C), than those studied by Knifton
a EaStCHEM, School of Chemistry, University of St. Andrews, St. Andrews, Fife,
KY16 9ST, Scotland, UK. E-mail: djc@st-and.ac.uk; Fax: +44 (0) 1334 463808;
Tel: +44 (0) 1334 463805
b Eastman Chemical Company, Kingsport, TN, USA
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00423f
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Full details of the methods used to analyse the positions
and amounts of 13C incorporated into the products are
provided in the ESI.†
very accurate at low pressures. The autoclave was closed and
subsequently heated to 200 °C under constant stirring. The
work-up and analysis were as described above. In this case
the distillate was analysed by GC–MS to assess the compound
individual masses and fragmentation patterns.
A similar reaction and work-up procedure was carried
out using [PBu4]Br (15 g, 44.2 mmol), [Ru3(CO)12] (0.5 g,
2.34 mmol), 13CO (17 bar) and H2 (pressure to 33 bar) before
pressurising to 180 bar.
Catalytic experiments
General procedure. A mixture of the solvent and catalyst
precursor was purged of air using syngas (CO : H2 1 : 1) by
repeatedly pressurising up to 10 barg and venting. The
system was then tested for leaks at 170 bar before being
heated up to 200 °C with the vessel sealed and under
continuous stirring. At 200 °C the pressure was adjusted to
250 bar and the reaction mixture was allowed to react for 4 h
at constant pressure, with CO/H2 (1 : 1) being fed from a
ballast vessel. The heating was switched off and the reactor
swiftly cooled to room temperature. The excess gas was
vented and the liquid product was collected for analysis.
When employing 13CO the reactor was not kept at constant
pressure (no feed from the ballast vessel) in order to prevent
unnecessary dilution of the 13C label.
For instance: in an unlabelled control experiment (used as
a reference for GCMS analysis) [Ru3(CO)12] (0.5 g, 2.3 mmol Ru)
and [PBu4]Br (15 g, 44.2 mmol) were added to the autoclave.
The autoclave was screwed onto the holder and purged by
pressuring to 11 bar and venting to ambient pressure 6 times
using CO/H2 1 : 1 v/v before pressurising to over 170 bar.
A leak test was performed and no pressure drop was
observed overnight other than that resulting from cooling
the autoclave. The following morning the heating jacket was
mounted, and switched on, the stirrer was switched on.
When the temperature reached 200 °C, the pressure was
adjusted to 250 bar and the stirrer was set to a fixed power
input. The temperature and pressure were held constant
until the heating and stirring were switched off after 4 h.
Taps to the autoclave were closed to prevent gas flowing into
the reactor because of the cooling. When the autoclave tem-
perature reached below 30 °C the autoclave was vented and
opened. The product mixture was usually a red liquid of
which a small sample was stored and was often analysed
using NMR or IR. The remainder of the liquid was transferred
to a flask and stripped of volatiles by vacuum distillation
using temperatures up to 250 °C and a liquid N2 cold trap.
The volatiles that where collected were diluted using acetoni-
trile/NMP stock solution (2 mL of 5% (v/v)) and analysed
using GC. The total product amounts were calculated using
the NMP and acetonitrile peaks as internal references.
Reactions using 13CH3OH. Reactions using 13CH3OH were
carried out similarly except that 13CH3OH (1 cm3) was added
to the autoclave with the [PBu4]Br and the Ru source and the
autoclave was pressurised with CO/H2 (1 : 1, 180 bar). CO/H2
were fed continuously from a ballast vessel to maintain
constant pressure and to make up for CO/H2 used in the
reaction: [PBu4]Br (15 g (44.2 mmol)), RuO2 (0.3 g (1.6 mmol)),
and methanol (99% 13C, 12% 18O, 0.5 ml, 12.35 mmol) were
added to the autoclave. The system was purged using 1 : 1
syngas and then pressurised to 180 bar. The autoclave was
closed, heated to 200 °C and stirred for 4 h. The autoclave
was cooled to 30 °C and the work up was performed as
described above.
A similar reaction and work-up procedure was carried
out using [PBu4]Br (15 g, 44.2 mmol), [Ru3(CO)12] (0.5 g,
2.34 mmol) and methanol (99% 13C, 12% 18O, 0.5 ml).
Results
During the course of our studies on CO hydrogenation in
melt systems, we discovered that the product distribution
was different when using RuO2 as the catalyst precursor from
that obtained with [Ru3(CO)12]. In both cases the major prod-
ucts were methanol, ethanol and propanol in declining
amounts together with 1,2-ethandiol (Fig. 1). However, when
using RuO2 as the catalyst precursor, significant amounts
of higher alcohols (1-butanol –1-heptanol, with an almost
Gaussian distribution peaking at 1-pentanol) were observed
as products (Fig. 2).
This unusual product distribution (the higher alcohols do
not follow the expected Schulz–Flory distribution adopted by
Reactions using 13CO. [PBu4]Br (15 g, 44.2 mmol) and
RuO2 (0.3 g, 1.6 mmol Ru) were added to the autoclave. The
system was purged using 1 : 1 syngas and brought to 185 bar
to test for leaks. When the system did not leak the autoclave
was vented to ambient pressure. Then the pressure was
raised to approximately 13 bar using 13C labelled CO and H2
was added to approximately 24 bar total pressure. Then the
pressure was increased to 180 bar using regular CO/H2 (1 : 1).
These pressures were measured using the Back Pressure
Regulator (BPR) which is accurate at higher pressures but not
Fig. 1 Product distribution from the homogeneous hydrogenation
of CO in [PBu4]Br using [Ru3(CO)12] (grey rectangles) or RuO2 (black
rectangles) as the catalyst precursor. CO/H2(1 : 1, 250 bar), 200 °C, 4 h.
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Table 1 The isotopic enrichmenta of the products when adding
13CH3OH at the start of the reaction
Precursor
RuO2
Methanol Ethanol Propanol Butanol Pentanol
22
40
34
33
34
2
22
0
7
Ru3(CO)12 14
a
The isotopic enrichment is calculated as 13C/(12C + 13C) × 100%
based on GC–MS isotopic patterns of the relevant peak groups.
The GC–MS pattern once again indicated exclusive labelling
at the β-position. Thus, in combination with the evidence
from NMR studies, we conclude that methanol carbonylation
is the major pathway to ethanol.
Fig. 2 Vertically expanded GC trace from CO hydrogenation in the
presence of RuO2, showing higher alcohol products; conditions as
in Fig. 1.
Interestingly, for propanol, the isotopic enrichment does
not occur exclusively at the γ-position. Instead, the label is
also found with almost equal abundance in the β-position
but not in the α-position (see Scheme 1). This indicates that
the ethyl moiety has time to switch carbons on the metal
centre, probably via reversible β-hydride elimination and H
addition steps before the CO insertion takes place.
Similar conclusions have been drawn by other workers
when adding 13CH3OH to CO hydrogenation systems (see
above) but the position of the label in propanol has not previ-
ously been discussed.
the lower alcohols) led us to believe that at least two different
mechanisms may operate in the formation of C–C coupled
products depending on the catalyst species present during the
reaction. One possibility was that the tetrabutylphosphonium
bromide solvent could act as a C4 source in the synthesis of
the higher alcohols, via solvent degradation. We were also
interested to discover whether ethanol and propanol in the
[Ru3(CO)12] and/or RuO2·H2O catalysed reactions might be
formed from intermediate methanol, as has previously been
reported.19–23 Knifton has shown that, when 13CH3OH is added
to a similar system ([Ru3(CO)12], [Bu4P]Br, 220 °C, CO/H2,
276 bar) the ethanol produced contains significant 13C in the
methyl group,21 whilst Ono et al. have shown similar results
when using [Ru3(CO)12] and [Co2(CO)8] in toluene containing
[heptylPPh3]Br and PPh3,23 or when using [(Ph3P)2 N]Cl.20
Using heterogeneous catalysts and 13CH3OH, 13C can be
incorporated into both the methylene and methyl groups
of ethanol.22 When we added some methanol at the start
of the reaction we found increased formation of ethanol.
Higher alcohol formation
In order to understand the mechanisms of formation of the
C4−6 alcohols in the two systems, we carried out studies using
13C-labelled CO with no added methanol.
We considered four main mechanistic routes towards C–C
coupled products:
1. Chain growth at the metal centre with irreversible
release of the product alcohol
2. Chain growth through methanol and other alcohol
carbonylation
3. C–C coupling via aldol condensation from free aldehyde
intermediates
Ethanol and propanol formation
To confirm that methanol homologation was occurring for
the production of ethanol (and ethanol homologation for
propanol), we carried out the CO hydrogenation with
13CH3OH added at the start if the reaction.
4. Alcohol synthesis via solvent degradation
These mechanisms and their expected outcomes when
using RuO2 as the catalyst precursor and partially labelled
CO or added 13CH3OH are shown in Table 2.
GC–MS analysis of the higher alcohols produced when
using RuO2 as the catalyst and adding 13C labelled methanol
shows that no label is present in the higher alcohol products.
Hence, all the C atoms come from the gas phase and the
mechanism of formation of these longer chain alcohols is
different from that of the formation of ethanol and propanol,
thus ruling out mechanism 2.
When 99% 13C enriched methanol was added to reactions
using either [Ru3(CO)12] or RuO2 catalyst precursors. The
1H NMR spectra of the product mixtures showed large 13C
coupled satellites for the ethanol beta protons.‡ Interestingly,
the protons from the α-position did not show increased 13C
labelling in either case. Due to the crowded nature of the
1H NMR spectra only rough estimates of the isotopic abun-
dances for some compounds found in the products can be
made. Fortunately, the GC–MS data analysis (Table 1) showed
that ethanol contained 40% and 34% of the 13C isotope
for the reactions using RuO2 and [Ru3(CO)12], respectively.
‡ Unlabelled ethanol is also produced, presumably from methanol synthesised
during the reaction from CO and H2. Unlabelled methanol is also present (86
and 78% from [Ru3(CO)12] and RuO2·2H2O respectively).
Scheme 1 Ethanol and propanol synthesis via alcohol carbonylation.
The * indicates the position of 13C when starting with 13CH3OH.
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Table 2 Possible pathways to butanol from CO hydrogenation using RuO2 as the catalyst precursor, the predicted labelling patterns and
outcomes from CO hydrogenation using partially 13C labelled CO or 13CH3OH are shown. Those in bold are observed, but those in italics are not
Mechanism
Expected labelling pattern for butanol using RuO2
Using 13CO
Adding 13MeOH
All C entirely from gas
CO + H2 → BuOH
Methanol reincorporation
MeOH + CO + H2 → → BuOH
Acetaldehyde and aldol
MeOH + CO → CH3CHO → → BuOH
Solvent degradation
Up to 4 13C in butanol
Up to 4 13C in butanol
Up to 4 13C in butanol
No 13C in butanol
No 13C in butanol
A single 13C in butanol
Up to two 13CO in butanol in the β and δ positions
No 13C in butanol
[Bu4P]Br → BuOH
If the synthesis of higher alcohols occurs through the
synthesis of methanol, which is then carbonylated to ethanal
and undergoes subsequent aldol condensation we expect
to find some product doubly labelled exclusively at the β and
δ-position. Since there is no label in the higher products,
mechanism 3 is also eliminated.
More interestingly, the average 13C incorporation for all com-
pounds is of the same order, indicating that solvent degrada-
tion does not contribute significantly to the formation of the
longer chain alcohols.
For the reaction using [Ru3(CO)12] the average level of 13C
in the products was higher because the initial partial pres-
sure of 13CO used for the 13CO –12CO mixture was higher.§
Again, the GC–MS of the methanol, ethanol and propanol
show patterns consistent with extensive labelling in the prod-
ucts. The found % incorporations for the first three alcohols
are more or less the same at 11.5–12%. The amount of higher
alcohols formed when using [Ru3(CO)12] is not sufficient to
obtain meaningful information.
Using 13C enriched syngas and no added methanol we
would expect that 13C incorporation should take place uni-
formly throughout the products, regardless of the mechanism
except when product formation occurs through degradation
of the solvent, [Bu4P]Br.
It is possible to work out the average 13C percentage incor-
poration into each C position of the products using mass
spectrometry (see ESI†). If all the C atoms come from the CO,
the average % incorporation normalised for chain length
should be the same for all the products. If, however, the
C4+-alcohols display a lower 13C abundance than the C1−3
alcohols this would indicate that the higher alcohols were
made from degradation of the solvent, [Bu4P]Br.
We are not aware of any other studies that report labelling
experiments for the production of higher alcohols from
homogeneous CO hydrogenation.
Discussion
Fig. 3 shows the % of 13CO incorporated into each product
after reaction of 13C enriched syngas (H2: CO, 1 : 1) with
[Ru3(CO)12] or RuO2 in tetrabutylphosphonium bromide at
200 °C and at a starting pressure of 260 bar (at 200 °C) in a
closed batch system.
It has been proposed14,24–26 that methanol is formed on ruthe-
nium catalysts by a series of intermolecular hydride transfers
and protonations (Scheme 2a). The initial hydride transfer
Fig. 3 clearly shows that a reaction using RuO2 in 13C
enriched syngas yields isotopically enriched products. The
average 13CO incorporation found for this reaction was 8.5%.
Scheme 2 Possible mechanisms for the formation of a) methanol,
ethanol and propanol; b), c) higher alcohols from the hydrogenation of
CO using RuO2 as the catalyst precursor.
Fig. 3 The incorporation of 13CO into alcohol products during CO
hydrogenation using 13CO enriched synthesis gas, based on the
isotope pattern found in the GC-MS analysis for each product. The
values given are 13C/12C + 13C × 100%, normalised for chain length.
▲[Ru3(CO)12] as catalyst precursor; ■ RuO2 as catalyst precursor.
§ Different pressures were used because the cylinder containing the 13CO
depleted between the reactions.
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is believed to occur from a species such as [Ru3H(CO)11]− to
CO bound to another ruthenium centre, probably derived
from [Ru(CO)3Br3]− in this system. The reaction is proposed
to proceed via formyl and hydroxycarbene intermediates to
a hydroxymethyl group which can be protonated to give
methanol24 (see left side of Scheme 2a). Since we have shown
that ethanol is formed by homolgation of methanol, the mech-
anism probably involves reaction of methanol with HBr (HBr
is a promoter for this reaction) to give methyl bromide which
is attacked by the ruthenium centre to give a coordinated
methyl group. Methyl migration onto coordinated CO then
provides the C–C bond forming step (right side of Scheme 2a).
For the higher alcohols, a different mechanism clearly
operates and it must involve a different ruthenium centre
since it is only observed when using RuO2 not when using
[Ru3(CO)12] as the catalyst precursor. IR studies of post reac-
tion mixtures do not show major differences between the two
systems, so it is not possible to speculate on what the alterna-
tive ruthenium centre may be, although the production of
water during the formation of active species from RuO2 may
make the systems different.
Conclusions
CO hydrogenation using ruthenium based catalyst precursors
in [PBu4]Br under relatively mild conditions (250 bar, 200 °C)
gives mainly methanol ethanol and propanol together with
small amounts of ethylene glycol. Studies, in which 13CH3OH
is added at the start of the reaction, show that ethanol and
propanol are formed by homologation of methanol and ethanol
respectively. When using RuO2 as the ruthenium source, higher
alcohols (butanol–heptanol) are produced with a Gaussian
distribution centred on pentanol. Studies using either added
13CH3OH or 13CO and no added methanol show that these
higher alcohols are not formed by homologation but rather by
a chain growth mechanism occurring at the Ru centre.
Acknowledgements
We are greatly indebted to the Eastman Chemical Company
for financial support and to Mr. Robert Cathcart and Mr.
Peter Pogorzelec for invaluable technical support.
One possible chain growth mechanism is that the
hydroxymethyl intermediate in this alternative catalytic com-
plex is reduced to methyl and that a sequence of migrations
and reductions occurs (Scheme 2b). If this is the case, the
methyl complex formed on this catalytic centre by reduction
of the hydroxymethyl complex cannot be available from a
reaction of the related ruthenium centre with MeBr derived
from methanol as this would allow homologation. A further
alternative mechanistic possibility is that the hydroxymethyl
ligand migrates onto CO and the acyl is then reduced to
hydroxyethyl (Scheme 2c). Repeating this sequence leads to
chain growth. Interestingly, these two mechanisms leave the
hydroxide on opposite ends of the growing chain in the final
alcohol product, but our labelling studies cannot distinguish
between them.
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system, NMR and GCMS give no evidence for 13C incorporation
into the 1,2-ethanediol formed, although 13CO is incorporated
from the gas phase. The most logical initial steps in the forma-
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reductive elimination of 2-hydroxyethanal (glycolaldehyde)
from the hydroxyacetyl complex, followed by aldehyde hydro-
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in the Pichler–Schulz mechanism for chain growth in
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