Unprecedented reductive esterification of carboxylic acids under hydrogen by reusable heterogeneous platinum catalysts

Article abstract of DOI:10.1002/adsc.201401172

Supported metal catalysts have been tested for an unprecedented reductive dimerization of carboxylic acids to esters under 8 bar hydrogen in solvent-free conditions. Among various metal-loaded tin oxide catalysts, platinum-loaded tin dioxide (Pt/SnO₂) shows the highest ester yield for the reaction of dodecanoic acid. Among Pt catalysts on various supports, Lewis acidic oxides, especially SnO₂, show high activity. The most active catalyst, 5 wt% Pt/SnO₂ reduced at 100°C, is effective for the reductive esterification of various carboxylic acids, and the catalyst is reusable for nine cycles, demonstrating the first successful example for the title reaction. Infrared (IR) studies of a model compound (formic acid) on some metal oxides indicate a strong Lewis acid-base interaction between SnO₂ and the carbonyl oxygen. For Pt/SnO₂ catalysts with different Pt particle sizes, the activity increases with decreasing size of Pt metal. A cooperative catalysis of the Pt metal nanoparticles and the Sn⁴⁺ Lewis acid sites is proposed.

Full text of DOI:10.1002/adsc.201401172

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
DOI: 10.1002/adsc.201401172
Unprecedented Reductive Esterification of Carboxylic Acids
under Hydrogen by Reusable Heterogeneous Platinum Catalysts
Abeda S. Touchy,a Kenichi Kon,a Wataru Onodera,a and Ken-ichi Shimizua,b,*
a
Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
Fax: (+81)-11-706-9163; e-mail: kshimizu@cat.hokudai.ac.jp
Elements Strategy Initiative for Catalysis and Batteries, Kyoto University, Katsura, Kyoto 615-8520, Japan
b
Received: December 17, 2014; Revised: March 2, 2015; Published online: April 29, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201401172.
Abstract: Supported metal catalysts have been tested first successful example for the title reaction. Infra-
for an unprecedented reductive dimerization of car- red (IR) studies of a model compound (formic acid)
boxylic acids to esters under 8 bar hydrogen in sol- on some metal oxides indicate a strong Lewis acid-
vent-free conditions. Among various metal-loaded base interaction between SnO2 and the carbonyl
tin oxide catalysts, platinum-loaded tin dioxide (Pt/ oxygen. For Pt/SnO2 catalysts with different Pt parti-
SnO2) shows the highest ester yield for the reaction cle sizes, the activity increases with decreasing size of
of dodecanoic acid. Among Pt catalysts on various Pt metal. A cooperative catalysis of the Pt metal
supports, Lewis acidic oxides, especially SnO2, show nanoparticles and the Sn4+ Lewis acid sites is pro-
high activity. The most active catalyst, 5 wt% Pt/ posed.
SnO2 reduced at 1008C, is effective for the reductive
esterification of various carboxylic acids, and the cat- Keywords: carboxylic acids; esters; heterogeneous
alyst is reusable for nine cycles, demonstrating the catalysis; hydrogenation; platinum; tin
Introduction
backs such as need for expensive reductant (Et3SiH)
and acidic conditions, high catalyst loading (5 mol%)
The synthesis of esters plays a major role in organic and difficulties in catalyst/product separation and cat-
synthesis in academic research and industrial produc- alyst reuse. Reductive esterification of carboxylic
tion of chemicals.[1] Conventionally, esters are pro- acids by H2 as an ideal method is difficult, because
duced by condensation of a carboxylic acid or activat- carboxylic acids are among the most difficult carbonyl
ed acid derivatives (acid chlorides or anhydrides) with substrates to hydrogenate due to the low electrophi-
excess amounts of an alcohol in the presence of an licity of the carbonyl carbon.[8] Goldberg et al. report-
acid catalyst or a dehydrating agent, producing large ed the selective hydrogenation of glacial acetic acid to
amount of wastes. Environmentally benign synthetic ethyl acetate under 27 bar H2 by homogeneous Ir or
methods such as condensation of an equimolar Rh catalysts,[9a] but the yields of ethyl acetate were
amount of carboxylic acid and alcohol,[2] esterification low, and the system was limited to acetic acid. Previ-
of aldehydes with alcohols,[3] dimerization of two alde- ous reports on the formation of esters by hydrogena-
hydes (Tishchenko reaction)[4] and dehydrogenative tion of carboxylic acid by homogenous Ru catalysts
dimerization of two primary alcohols[5,6] were report- suffer from low conversion and low selectivity to the
ed. In response to the increasing demand for simple esters.[9b,c] Hydrogenation of carboxylic acids by heter-
and environmentally benign procedures, development ogeneous catalysts generally gives alcohols[10] or alka-
of a novel direct route to esters from stable and readi- nes.[11a] In the course of our studies on support-con-
ly available substrates still remains a major challenge. trolled selective reductive transformations of carbox-
Potentially, esters can be synthesized by reduction of ylic acids by heterogeneous Pt catalysts,[11] we have
carboxylic acids as stable and readily available start- discovered the first successful example of the reduc-
ing materials. Sakai et al. reported the first example tive esterification of various carboxylic acids by H2
of reductive esterification of carboxylic acids with (8 bar) under additive-free and solvent-free condi-
3 equiv. of Et3SiH catalyzed by InBr3 with 0.1 equiv. tions by SnO2-supported Pt (Pt/SnO2) as a reusable
of H2SO4.[7] However, this system suffers from draw- heterogeneous catalyst. We report herein the catalytic
Adv. Synth. Catal. 2015, 357, 1499 – 1506
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1499

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
Abeda S. Touchy et al.
Table 2. Reductive esterification of dodecanoic acid.[a]
properties of this new catalytic system. We also study
the structure-activity relationship to address the influ-
ences of the electronic structure of the supported
metals, Lewis acidic nature of the support oxides, size
and phase of Pt on the activity.
Entry
Solvent
Conv. [%]
Yield [%][b]
1
2
3
4
5
6
no solvent
o-xylene
diglyme
mesitylene
n-octane
H2O
95
93
91
94
92
41
90 (3)
88 (2)
84 (4)
83 (3)
79 (3)
27 (9)
Results and Discussion
[a]
Optimization of Catalysts and Conditions
1 mol% Pt/SnO2, 1 mmol dodecanoic acid, 0 or 1 g sol-
vent, 2008C, 24 h.
GC yields. Yields of 1-dodecanol are in the parentheses.
[b]
As summarized in Table 1, we screened 18 types of
the supported transition metal catalysts, pre-reduced
in H2 at 1008C for 0.5 h, for hydrogenation of dodeca-
noic acid in solvent-free conditions under 8 bar H2 at CeO2, HZSM5 zeolite, SiO2, C). Pt/SnO2 (entry 1)
2008C for 24 h. First, we tested various transition showed a higher yield of the ester (91%) than the
metal (Pt, Rh, Ru, Pd, Ir, Re, Ni, Cu, Co, Ag) cata- other Pt-loaded catalysts. Pt/ZrO2 and Pt/TiO2 (en-
lysts supported on SnO2 (entries 1–10). Among the tries 12 and 13) gave moderate yields (45–63%), and
catalysts tested, Pt/SnO2 showed the highest yield the Pt loaded on Al2O3, CeO2, HZSM5 zeolite and
(90%) of the corresponding ester, dodecyl dodeca- SiO2 (entries 14–17) showed low yields (1–17%). A
noate. 1-Dodecanol was observed as a by-product in commercially available carbon-supported Pt as a con-
3% yield. Rh/SnO2 also showed high yield (85%), and ventional Pt catalyst was inactive (entry18). From
Ru- and Re-loaded SnO2 showed moderate yields these results, Pt/SnO2 is found to be the most effective
(50–54%). Entries 12–18 show the results of Pt cata- catalyst among the catalysts tested in Table 1. Taking
lysts loaded on other supports (ZrO2, TiO2, Al2O3, into account that SnO2 (entry 11) and Pt nanoparti-
cles loaded on an inert support (Pt/C) are inactive for
the hydrogenation of dodecanoic acid to the ester, it
is suggested that the high activity of Pt/SnO2 is due to
a synergistic effect of SnO2 and Pt. It should be noted
that Pt/SnO2 is also effective for the acceptor-less de-
hydrogenative dimerization of two primary alcohols
to esters as reported in our previous study.[5a]
Table 1. Catalyst screening for reductive esterification of do-
decanoic acid.
Next, we optimized the reaction conditions for the
hydrogenation of dodecanoic acid to the ester by
1 mol% of Pt/SnO2. Table 2 shows the effect of sol-
vent on the yield of the ester and 1-dodecanol as
a by-product for the reaction under 8 bar H2 at
2008C. The reaction under the solvent-free conditions
gave the highest yield of the ester (90%). Reactions
in various organic solvent (entries 2–5) were also suc-
cessful, which gave 79–88% yield of the ester. The re-
action in water gave the lowest yield (27%). Under
the solvent-free conditions, the reaction at a lower
temperature (1808C) resulted in lower yield (84%)
than that at 2008C (91%). The reaction under lower
H2 pressure (5 bar) at 2008C resulted in lower yield
(80%) than that under standard H2 pressure (8 bar).
A decrease in the catalyst amount from 1 mol% to
0.5 mol% decreased the ester yield from 90% to 59%.
The time course of the reaction under the standard
conditions (Figure 1) shows that the reaction time of
24 h is enough to obtain a high yield of the ester. Fur-
ther reduction of the ester to ether was not observed.
An interesting feature in Figure 1 is the fact that the
carboxylic acid is more reactive than the ester (dode-
cyl dodecanoate), because normally esters are more
reactive than carboxylic acids in hydrogenation reac-
Entry
Catalyst
Conv. [%]
Yield [%][a]
1
2
3
4
5
6
7
8
Pt/SnO2
Rh/SnO2
Ru/SnO2
Re/SnO2
Pd/SnO2
Ir/SnO2
95
90
61
53
58
37
3
1
0
0
0
71
59
56
52
11
7
90 (3)[b]
85 (2)[b]
54
50
38
30
3
1
0
0
Ni/SnO2
Cu/SnO2
Co/SnO2
Ag/SnO2
9
10
11
12
13
14
15
16
17
18
19
[c]
SnO2
0
Pt/ZrO2
Pt/TiO2
Pt/Al2O3
Pt/CeO2
Pt/HZSM5
Pt/SiO2
Pt/C
63
45 (2)[d]
17
12 (1)[d]
3 (2)[d]
1
0
0
0
0
blank
[a]
GC yields.
[b]
[c]
[d]
Yield of 1-dodecanol.
Catalyst amount was 39 mg.
Yield of n-dodecane.
1500
asc.wiley-vch.de
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 1499 – 1506

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
Unprecedented Reductive Esterification of Carboxylic Acids under Hydrogen
Figure 2. Reuse of Pt/SnO2 for reductive esterification of
dodecanoic acid under the conditions in Table 1.
Figure 1. Time-yield profile under the conditions in Table 1.
tions by organometallic catalysts.[8] This unusual trend was separated by centrifugation. For each successive
could be partly due to steric hindrance of the large use, the recovered catalyst was washed by 2-propanol
ester, which was supported by the finding that the hy- three times, followed by centrifugation and drying in
drogenation of a smaller ester (methyl dodecanoate) an oven at 1008C for 3 h, followed by H2 reduction at
resulted in a moderate yield of dodecyl dodecanoate 1008C for 0.5 h. The recovered catalyst was reused
as shown in the Eq. (1). However, the lower yield eight times without a marked loss of its catalytic ac-
(42%) in Eq. (1) than that in the standard hydrogena- tivity. After the first cycle, ICP-AES analysis of the
solution confirmed that the content of Pt in the solu-
tion was below the detection limit (10 ppb) and that
of Sn was quite low (8 ppm, 0.008% of Sn in the cata-
lyst used). These results indicate that Pt/SnO2 acts as
a reusable heterogeneous catalyst for this reaction.
Table 3 shows the general applicability of the pres-
ent catalytic system with 1 mol% of Pt/SnO2. After
the reactions, the catalyst was removed from the mix-
ture and the esters were isolated by column chroma-
tography, resulting in moderate to high isolated yields
(52–93%) of the corresponding esters. Linear,
tion of dodecanoic acid (90%) suggests that carboxyl- branched and cyclic aliphatic carboxylic acids (en-
ic acids are intrinsically more reactive than esters in tries 1–8) underwent selective reductive esterification,
this heterogeneous catalytic system.
producing the esters in moderate to high isolated
Another explanation might be that the carboxylic yields. Aliphatic carboxylic acids with aromatic func-
acid is converted first to its anhydride which then hy- tional groups (entries 9–11) and benzoic acid deriva-
drogenates faster than the ester. However, the follow- tives (entries 12 and 13) were also selectively convert-
ing results excluded this possibility; hydrogenation of ed to the corresponding esters, without undergoing
stearic acid anhydride in the standard conditions did hydrogenation of the aromatic nuclei.
not give any hydrogenation products and SnO2 did
not show any activity for the anhydride formation
from stearic acid at 2008C.
Possible Pathway
We discuss a possible pathway of the reaction. Reduc-
tive self-esterification of two carboxylic acids by H2
may occur via two possible pathways: (a) reduction of
Catalytic Properties of Pt/SnO2
Under the optimized conditions with the most effec- one carboxylic acid to an alcohol, followed by its re-
tive catalyst (Pt/SnO2), we studied the catalytic prop- action with another carboxylic acid to give an ester,
erties of this catalytic system. As shown in Figure 2,
Pt/SnO2 was reusable for the reductive esterification
of dodecanoic acid. After the first cycle, 2-propanol
(6 mL) was added to the reaction mixture and catalyst
Adv. Synth. Catal. 2015, 357, 1499 – 1506
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1501

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
FULL PAPERS
Abeda S. Touchy et al.
Table 3. Reductive esterification of various carboxylic acids
by Pt/SnO2.
value is more than 4 times faster than the Pt/SnO2-
catalyzed reductive dimerization n-dodecanoic acid to
the ester (0.3 mmolhÀ1). This result is consistent with
the absence of 1-dodecanol in the initial period of the
time course (Figure 1), and thus supports the pathway
(a) shown in the Eq. (2). Pt/SnO2-catalyzed hydroge-
nation of one carboxylic acid to an alcohol is slow,
and its thermal esterification with another carboxylic
acid is fast.
Entry Product
Isolated yield [%]
1
2
3
4
5
6
7
82
55
86
87
93
59
86
Factors Affecting the Catalytic Activity
We studied the relationship between various structur-
al parameters and catalytic activity for the hydrogena-
tion of dodecanoic acid (in 8 bar H2 at 2008C) to dis-
cuss important factors affecting the catalytic activity
in the present system. First, we discuss a possible
reason why Pt/SnO2 gives a higher yield of the ester
than other transition metal-loaded SnO2 catalysts
(Table 1, entries 1–10). The yields of the ester are re-
plotted in Figure 3 as a function of the d-band center
(ed) relative to the Fermi energy (EF), ed – EF, for the
clean metal surface computed by Hammer and Nør-
skov,[12a] which has been used as a descriptor of activi-
ty trends in various transition metal surfaces.[12,13]
Clearly, the platinum group metal (Pt, Ir, Pd, Rh, Ru)
catalysts having intermediate ed – EF values show
higher catalytic activity than the metals with low ed –
EF values (Ag and Cu) and those with high ed – EF
values (Ni and Co). Taking into account the facts that
the bond strength between a metal surface (M) and
a hydrogen atom (H) is weaker for a metal with
lower ed – EF value[12a] and that the catalytic hydroge-
8
9
73
91
10
11
12
85
89
72
À
nation includes the formation and dissociation of M
H bonds, the result in Figure 3 suggests that the mod-
À
erate M H bond strength is favorable for this catalyt-
13
52
as shown in the Eq. (2), or (b) reduction of two car-
boxylic acids to aldehydes, followed by their dispro-
portionation (Tishchenko reaction) to give the ester.
In our previous study, we showed that the reaction
of 1 mmol of n-octanal in the presence of Pt/SnO2 re-
sulted in no formation of the corresponding ester,
which ruled out the contribution of the route (b).
Combined with the fact that 1-dodecanol is observed
as a by-product in the reductive esterification of n-do-
decanoic acid (Figure 1), the reaction can proceed via
the route (a). The esterification of 1 mmol n-dodeca-
noic acid with 1 mmol 1-dodecanol under N2 in the
absence of the catalyst at 2008C for 0.5 h gave the
corresponding ester in 65% yield, corresponding to
Figure 3. Effect of the d-band center of metals relative to
the Fermi energy (edÀEF)[12a] on the yield of the ester (from
Table 1) for the reductive esterification of dodecanoic acid
the thermal esterification rate of 1.3 mmolhÀ1. This by the SnO2-supported transition metals.
1502
asc.wiley-vch.de
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 1499 – 1506