Selective Hydrogenation of Carboxylic Acids to Alcohols or Alkanes Employing a Heterogeneous Catalyst

Article abstract of DOI:10.1021/acscatal.7b03484

The chemoselective hydrogenation of carboxylic acids to either alcohols or alkanes is reported, employing a heterogeneous bimetallic catalyst consisting of rhenium and palladium supported on graphite. α-Chiral carboxylic acids were hydrogenated without loss of optical purity. The catalyst displays a reverse order of reactivity upon hydrogenation of different carboxylic functions with esters being less reactive than amides and carboxylic acids. This allows for chemoselective hydrogenation of an acid in the presence of an ester or an amide function.

Full text of DOI:10.1021/acscatal.7b03484

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Letter
Selective Hydrogenation of Carboxylic Acids to Alcohols
or Alkanes Employing a Heterogeneous Catalyst
Johannes Ullrich, and Bernhard Breit
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03484 • Publication Date (Web): 20 Dec 2017
Downloaded from http://pubs.acs.org on December 20, 2017
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Selective Hydrogenation of Carboxylic Acids to
Alcohols or Alkanes Employing a Heterogeneous
Catalyst
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Johannes Ullrich and Bernhard Breit*
Institut für Organische Chemie
AlbertꢀLudwigs Universität Freiburg
Albertstraße 21, 79104 Freiburg i. Brg., Germany
Abstract: The chemoselective hydrogenation of carboxylic acids to either alcohols or alkanes is
reported employing a heterogeneous bimetallic catalyst consisting of rhenium and palladium
supported on graphite. αꢀChiral carboxylic acids were hydrogenated without loss of optical
purity. The catalyst displays a reverse order of reactivity upon hydrogenation of different
carboxylic functions with esters being less reactive than amides and carboxylic acids. This
allows for chemoselective hydrogenation of an acid in the presence of an ester or an amide
function.
Keywords: Hydrogenation • Heterogeneous • Carboxylic acid • Alcohol • Alkane
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Alcohols are important building blocks for the fine chemical industry and in laboratory
synthesis. Among the many ways of their synthesis reduction of carboxylic acids and their
derivatives employing stoichiometric amounts of metal hydrides is a reliable and common
procedure. However, disadvantages are safety issues as well as generation of stoichiometric
amounts of waste. A “greener“, more attractive solution to this problem would be a catalytic
reduction using molecular hydrogen. In particular, the catalytic hydrogenation of esters1 and
amides2 has seen a lot of progress in the past and a variety of efficient catalytic systems were
found to generate the respective alcohols or amines under convenient reaction conditions
(Scheme 1).
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Carboxylic acids are abundant building blocks available also from bio mass resources (e.g.
fatty acids) but their hydrogenation has been found to be more difficult. Nevertheless, several
homogeneous catalyst systems for acid to alcohol hydrogenation have been reported based on
different transition metals such as Cu, Cd, Ru, Rh/Mo, Ru/Mo, Ir, Re or Co.2i, 3 From a practical
point of view heterogeneous catalysts are advantageous, since they allow for easy substrate
separation through filtration. Indeed, research on heterogeneous catalytic hydrogenation of
carboxylic acids reaches back to the early 20th century when Norman and Schrauth reported
about Ni/Cu catalysts almost simultaneously.4,5 A number of subsequent reports mirrors the high
6hꢀk
interest in this field, with rhenium being a very frequent metal,
especially as part of
synergistic compositions of two or three metals.6aꢀg However, all reported catalyst systems share
the disadvantage for the need of harsh reaction conditions such as high reaction temperatures
and/or high pressure of hydrogen gas.
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We recently reported, that amides could be hydrogenated to the corresponding amines under
mild reaction conditions applying a heterogeneous catalyst consisting of 2 w% palladium and
10 w% rhenium supported on high surface area graphite (HSAG).7 We herein report that the
same catalyst allows for hydrogenation of carboxylic acids to give either alcohols or alkanes,
respectively.
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Previous work
O
O
O
Ru, Os, Fe...
H2
R
OR
NR2
OH
R
OH
NR2
OH
Fe, Ru, Rh/Re...
H2
+
NHR2
R
R
R
O
OH
Ru, Ir, Rh/Mo...
H2
R
R
O
R
NRR'
H2
R
OH
H2
Pd/Re/C
R
OH
R
NRR'
Reduction of Amides
This work
Scheme 1. Hydrogenation of carboxylic acids and its derivatives.
The catalyst was prepared by sequential incipient wetness impregnation of activated HSAG
with [Pd(NO3)2] and [Re2O7].8 The final catalyst composition was confirmed by EDX (energy
dispersive Xꢀray spectroscopy). The surface was measured by N2ꢀadsorption/desorption to be
19.6 m2/g according to BET. Following our interest in the oxidation state of the metals we
performed XPS (Xꢀray photoelectron spectroscopy) measurements of the catalyst before and
after treatment with hydrogen (Figure 1). For evaluation, the resulting binding energies (BE)
were compared with the literature. The spectrum of the native catalyst shows two distinct peaks
for the 3d3/2 and the 3d5/2 signals of Pd(II) and 4f5/2 and 4f7/2 of Re(VII), proving that both metals
are still oxidized. After hydrogenation, the Pd 3d5/2 signal shifted from 337.7 eV to 335.7 eV,
indicating that the palladium was reduced from Pd(II) to Pd(0). For the rhenium signals however,
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no shift could be observed, implying that the oxidated rhenium is the active species. It might
therefore act as a Lewis acid, though its exact role in the catalytic process cannot be elucidated.
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Figure 1. Binding Energies (BE) of Re and Pd in XPS before reaction (top line) and after
reaction (bottom line) with hydrogen.
Initial reactivity assays were performed with stearic acid. To our delight we obtained the
desired alcohol in promising yields (Scheme 2). However, while optimizing the reaction
conditions,8 we also observed significant amounts of the deoxygenated alkane, a sort of
defunctionalisation that has been reported by others as well.6e,9,10
O
Pd/Re/C, H2, (30 bar), MS
+
OH
OH
H
16
n-hexane, 130 °C, 18 h
16
16
71%
>99% conv.
20%
Scheme 2. Reduction of stearic acid (MS: molecular sieve, 4 Å).
To gain more insight into that process, we monitored the reaction kinetics of this
hydrogenation via GC (Figure 2).8 For a more convenient GCꢀanalysis (higher volatility of
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products) octanoic acid was chosen a substrate. The conversion was determined after
transforming the starting material to its methyl ester by addition of TMSꢀdiazomethane.
Temperature and pressure were selected such that the measurement could be performed in a
reasonable time frame. During the first two hours of the reaction the alcohol as the primary
product is formed with the starting material simultaneously decreasing. Interestingly, only when
all of the starting material (carboxylic acid) was consumed the hydrogenation of the alcohol to
furnish the corresponding alkane started to proceed. After six hours of reaction time the alcohol
was entirely consumed leaving only the alkane as the final product. This profile allows for two
conclusions: First, it shows that the alkane is generated from the alcohol and not in a parallel
reaction pathway. Secondly, it seems as that the presence of carboxylic acid substrate suppresses
hydrogenation of the alcohol to the alkane.11
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Figure 2. Hydrogenation profile of octanoic acid, conditions: 3 mmol octanoic acid, 2 mol% Pd/
7 mol% Re, nꢀhexane (0.2 M), 160 °C, H2 (30 bar).
We next explored the influence of reaction temperature, pressure and solvent (Table 1).8 Thus,
optimal yields for the alcohol reduction product were obtained at 130 °C and 20 bar hydrogen
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pressure (entry 2). Changing to ethereal solvents led to strong decrease of conversion (entries 6,
7), which has been reported for the heterogeneous hydrogenation of amides as well.2f The
superior solvation properties of aliphatic solvents for hydrogen could explain that observation.12
Using these conditions in a catalyst recycling experiment revealed a significant drop in
conversion to 36% after four runs (for more information see SI), which is an aspect of this
system, that still needs to be improved. During these optimization studies, we also performed
reactions with single metal catalysts containing either only supported palladium or rhenium,
respectively. While there was almost no conversion for Pd/C, the catalyst Re/C showed at least
some, therefore confirming the already reported hydrogenation capability of rheniumoxides. It
also emphasizes the synergistic effect of the bimetallic mixture, though the exact contribution of
each compound remains subject for further investigations. So far, we can state that palladium is
known for activating molecular hydrogen at low pressures und temperatures while obviously
being unsuitable for carboxylic acid activation, whereas Rhenium(oxide) on the other hand is in
principle able to do both. Hence, it can be assumed that its main role in the reaction is to activate
the acid while palladium provides the activated hydrogen.
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Table 1. Optimizing reaction conditions.a
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#
T [°C] p
[bar]
Solvent %
conv.
% A %B
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2
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4
5
6
7
140
130
120
130
130
130
130
20
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10
30
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nꢀHex
nꢀHex
nꢀHex
nꢀHex
nꢀHex
DME
THF
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100
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70
87
34
56
71
0
20
Traces
Traces
69
Traces
100
0
20
0
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15
0
8b 130
H2O
72
0
a1 mmol substrate, 2 mol% Pd/7 mol% Re, solvent
b
(0.2 M), 18 h, 200 mg MS 4 Å, 56% ester from starting
material and product alcohol
With optimized reaction conditions in hand we set out to test the reduction of a series of
carboxylic acids to the alcohol (Table 2). Primary aliphatic acids could be reduced in good yields
(1ꢀ4) with the yield only dropping in the case of acetic acid 4, which is likely to happen during
depressurization due to the volatility of the product. Different secondary and tertiary acids with
cyclic or acyclic structures were smoothly hydrogenated (5ꢀ12). If the secondary center is part of
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a carbocycle, the yields increased from 4ꢀ to 6ꢀmembered rings (8ꢀ10). Heteroatoms within the
ring are tolerated (11 and 12), but seem to lower the yield. Interestingly tetrahydrofuranyl
alcohol 11, obtained from hydrogenation of enantiomerically enriched starting material, was
obtained without loss or optical purity. This structural motif 11 is prevalent in several
medicinally relevant inhibitors.13 A diacid such as adipic acid could be reduced to the diol 13 in
quantitative yield. Such diols are valuable monomers for polymer chemistry.14 Interestingly,
hydrogenation of monoꢀmethyl adipate furnished the alcohol 14 while the ester function
remained untouched. This represents another result, where the earlier stated reactivity order of
carboxylic acid derivatives in hydrogenation15 is questioned. Other examples have been reported
by different groups in homogeneous3k as well as heterogeneous hydrogenations. 6b, 6k
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Table 2. Hydrogenation of saturated carboxylic acidsa
O
Pd/Re/C, H2
R
OH
R
OH
OH
OH
OH
OH
OH
OH
6
4
2
1, 73%
2, 74%
3, 69%
4, 31%
5, 70%
OH
6, 83%
OH
OH
OH
7, 89%
8, 60%
9, 78%
10, 89%
O
O
OH
OH
12, 78%
OH
HO
OH
MeO
4
O
11, 64%b
13, quant.c,d
14, 83%b,d
er acid: 94:6
er alcohol: 92:8
a1 mmol substrate, 2 mol% Pd/ 7 mol% Re, nꢀhexane (0.2 M), 130 °C, 20 bar H2, 18 h, 200 mg
b
MS 4 Å, all yields are GCꢀyields using nꢀdecane or nꢀdodecane as internal standard, reaction in
THF, cH2O, 160 °C, 30 bar, disolated yield.
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Interested in this behaviour we had a closer look into reactivity differences with respect to
hydrogenation between carboxylic acids, esters and amide functions (Table 3). As test substrates
we chose stearic acid 15, its amide 16 derived from piperidine and its methylester 17. Each of
these compounds was subjected to the standard conditions from table 1 (cond. A) as well as more
harsh ones (cond. B). The crude mixtures were analysed for their conversion to the
alcohols/alkanes in the case of acid and ester or the respective amine in the case of the amide.
From table 3 it is evident that the substrate reactivity upon hydrogenation with the Pd/Re/C
catalyst decreases from the carboxylic acid over the amide to the ester (Scheme 3).
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Table 3. Hydrogenation of carboxylic acid derivatives.a
O
O
O
N
16
OH
OMe
Conv. for:
16
16
17
15
16
41
100
Cond. Ab
Cond. Bc
100
0
100
89
b
aConv. was determined by NMR with TMB as internal standard, Cond. A: 1 mmol substrate,
c
2 mol% Pd/ 7 mol% Re, nꢀhex (0.2 M), 130 °C, 20 bar H2, 18 h, 200 mg MS 4 Å, Cond. B: 1
mmol substrate, 2 mol% Pd/ 7 mol% Re, nꢀHex (0.2 M), 160 °C, 30 bar H2, 18 h, 200 mg MS
4 Å.
Usual reactivity order
O
O
O
O
O
R
NR2
R
OH
R
R
OR
OH
O
R
OR
R
NR2
Restructured reactivity order for Pd/Re/C
Scheme 3. Reactivity of carboxylic acid derivatives for hydrogenation with the Re/Pd/C catalyst
system.
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When attempting the reduction of benzoic acid 19 (Table 4), we observed reduction of the acid
commitend with hydrogenation of the phenyl ring. However, employing homologeous
phenylacetic acid 21, hydrogenation furnished the alcohol 22 with the phenylꢀsubstituent
untouched. Hydrogenation of cinnamic acid 23 led to alcohol 24. Thus, alkene and carboxylic
acid function were hydrogenated. This result indicates that the alkene might be reduced most
easily, which leaves the carboxylic acid separated from the phenyl substituent by an alkyl spacer
similar to 21. Furoic acids 25 and 27 were converted to the saturated alcohols 26 and 28,
respectively. Conversely, 2ꢀthiophene carboxylic acid 29 showed no reactivity at all. In a control
experiment, stearic acid 15 was attempted to hydrogenate in the presence of 29. However, none
of the two substrates reacted which may suggest a poisoning effect of sulfur for our catalyst.
Isolated alkene and alkyne functions were hydrogenated as well as shown in the
examples of 31 and 33.
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Table 4. Hydrogenation of unsaturated carboxylic acids.a
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O
Pd/Re/C, H2
4
R
OH
R
OH
5
6
7
8
Alcohol
Alcohol
Acid
O
Acid
O
O
OH
OH
20, 72%b
OH
OH
9
O
28, 72%b
19
27
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O
O
OH
OH
S
S
OH
O
OH
O
22, 73%b,c
21
29
30, no conv.
O
OH
OH
OH
OH
7
7
7
7
32, 77%d
24, 67%c,d
23
31
O
O
O
O
OH
OH
OH
OH
7
7
34, 72%d
26, 69%b,c
25
33
a1 mmol substrate, 2 mol% Pd/ 7 mol% Re, nꢀhexane (0.2 M), 130 °C, 20 bar H2, 18 h, 200 mg
MS 4 Å, bGCꢀyield using nꢀdecane as internal standard, cSolvent: EtOAc, disolated yield.
Long chain fatty acids constitute an attractive class of renewable feedstocks. Besides their
usage as carboxylates in soaps and cosmetics16 they are also further derivatized to alcohols or
alkanes which are of high value as lubricants and fuels.17,18 Regarding our initial hydrogenation
profile, we proposed, that it should be possible to generate both alcohols and alkanes
chemoselectively by choice of the proper reaction conditions. As illustrated in table 5 we could
obtain the alcohols with the general reaction conditions A in good yields. However, increasing
reaction temperature and pressure allowed us to obtain the respective alkanes as single products
in excellent yields. Hence, our catalyst allows the selective preparation of either the alcohol or
the alkane from a single feedstock.
Table 5. Selective hydrogenation of fatty acids to either alcohols or to the alkanes.
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O
n
Pd/Re/C
cond. Aa
Pd/Re/C
cond. Bb
OH
H
OH
n
n
6
7
8
OH/H
OH/H
OH/H
12
85%, 90%
16
87%, 98%
14
88%, 95%
9
OH/H
10
OH/H
8
10
11
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73%, 87%
65%, 91%
aCond. A: 1 mmol substrate, 2 mol% Pd/ 7 mol%Re, nꢀhexane (0.2 M), 130 °C, 20 bar H2, 18
h, 200 mg MS 4 Å, bCond. B: 1 mmol substrate, 2 mol% Pd/ 7 mol% Re, nꢀhexane (0.2 M), 160
°C, 30 bar H2, 18 h, 200 mg MS 4 Å, all yields are isolated.
In conclusion, we report on a bimetallic heterogeneous catalyst system consisting of rhenium
and palladium supported on HSAG that allows for the selective hydrogenation of carboxylic
acids under mild conditions. The reaction allows the inclusion of αꢀstereocenters without
racemization. Aromatic rings can be reduced or retained depending on their structural activation.
The pool of natural fatty acids was made accessible for the production of industrially relevant
alcohols or alkanes by choosing the appropriate reaction conditions.
AUTHOR INFORMATION
Corresponding Author
EꢀMail: bernhard.breit@chemie.uniꢀfreiburg.de
ASSOCIATED CONTENT
Supporting Information.
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Experimental procedures and analytical data.
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ACKNOWLEDGMENT
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The authors would like to thank the following persons for performing measurements: R.
Thomann for EDX as well as TEM, A. Warmbold for BET and D. Moessner for XPS.
Furthermore, we thank TIMCAL for the kind supply with HSAG.
REFERENCES
(1) Selected examples: a) Teunissen, H. T.; Elsevier, C. J. Chem. Commun. 1997, 667–668.
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TOCꢀGraphic:
5
O
Pd/Re/C, H2 (30 bar)
Pd/Re/C, H2 (20 bar)
6
7
R
H
R
OH
R
OH
160 °C
130 °C
highly chemoselective
8
9
transformation of natural feedstock
good to excellent yields
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