Acid-catalyzed oxidation of levulinate derivatives to succinates under mild conditions

Article abstract of DOI:10.1002/cctc.201403014

Levulinate derivatives are an attractive platform for the production of renewable chemicals. Here we report on the oxidation of methyl levulinate into dimethyl succinate with peroxides under mild conditions using Br?nsted and Lewis acid catalysts. Selectivities to succinate and acetate derivatives of approximately 60 and 40 %, respectively, were obtained with strong Br?nsted acids in methanol. Although the molecular structure (i.e., carbon-chain length and branching around the C=O group) and the oxidant type affect the product distribution, solvent choice has the strongest impact on changing the location of oxygen insertion into the carbon backbone. Specifically, switching the solvent from methanol to heptane resulted in a decrease in the succinate/acetate ratio from 1.6 to 0.3. In contrast to Br?nsted acids, we demonstrate that the nature of the metal cation is responsible for changing the reaction selectivity of water-tolerant Lewis acidic triflate salts.

Full text of DOI:10.1002/cctc.201403014

DOI: 10.1002/cctc.201403014
Communications
Acid-catalyzed Oxidation of Levulinate Derivatives to
Succinates under Mild Conditions
Yuran Wang,[a] Ferdinand Vogelgsang,[a, b] and Yuriy Romꢀn-Leshkov*[a]
Levulinate derivatives are an attractive platform for the pro-
duction of renewable chemicals. Here we report on the oxida-
tion of methyl levulinate into dimethyl succinate with perox-
ides under mild conditions using Brønsted and Lewis acid cata-
lysts. Selectivities to succinate and acetate derivatives of ap-
proximately 60 and 40%, respectively, were obtained with
strong Brønsted acids in methanol. Although the molecular
structure (i.e., carbon-chain length and branching around the
C=O group) and the oxidant type affect the product distribu-
tion, solvent choice has the strongest impact on changing the
location of oxygen insertion into the carbon backbone. Specifi-
cally, switching the solvent from methanol to heptane resulted
in a decrease in the succinate/acetate ratio from 1.6 to 0.3. In
contrast to Brønsted acids, we demonstrate that the nature of
the metal cation is responsible for changing the reaction selec-
tivity of water-tolerant Lewis acidic triflate salts.
however, linear aliphatic ketones are generally difficult to oxi-
dize selectively. Consequently, only a few studies, mostly using
vanadium-, ruthenium-, and manganese-based catalysts, have
been reported on the oxidation of LA derivatives.[8] These cata-
lytic systems suffer from several challenges, including necessi-
tating high reaction temperatures, using toxic reagents, or re-
leasing a stoichiometric amount of CO2. Recently, Podolean
et al. demonstrated that Ru-based magnetic nanoparticles are
efficient catalysts for the oxidation of LA into SA under 10 bar
of O2 at 1508C.[1b] The authors hypothesized that strong
Brønsted acid sites are responsible for catalyzing the oxidation
of the LA carbon backbone via a Baeyer–Villiger (BV) mecha-
nism. Unfortunately, no systematic studies on the role of
Brønsted acids in the BV oxidation of LA into SA were reported.
This is particularly important given that Choudhary et al.
showed that strong Brønsted acids cannot catalyze the oxida-
tion of LA into SA with H2O2 in water.[1e]
Here, we investigate the oxidation of methyl levulinate into
dimethyl succinate under mild conditions using Brønsted and
Lewis acid catalysts. Given the low reactivity of LA in water,[1e]
we perform our reactivity studies in solvents other than water
to gain insight into the oxidation process. As shown in
Scheme 1, the BV oxidation mechanism is generally accepted
The conversion of renewable resources into commodity and
specialty chemicals through chemical or biological routes has
attracted considerable attention.[1] Thermocatalytic routes offer
an attractive alternative to biological routes, with less stringent
requirements for temperature and pH control, as well as po-
tentially less energy-intensive product separation and purifica-
tion processes.[1a,2] Succinic acid (SA) has been identified as
one of the top 12 building blocks from biomass by the US De-
partment of Energy.[3] The market for SA is expected to exceed
$1.1 billion in revenue by 2020.[4] Succinates, formed from the
esterification of SA with monoalcohols, are important plasticiz-
ers, lubricants, and chemical intermediates.[5] A thermocatalytic
route to produce SA and other succinate derivatives from re-
newable resources is highly desirable.
Scheme 1. Acid-catalyzed oxidation of ketones using hydroperoxides via
a Criegee intermediate.
to proceed in two steps where first, a peroxide is added to
a C=O group forming a Criegee intermediate, followed by mi-
gration of an R1 group adjacent to the C=O group to insert an
oxygen atom into the CÀC bond in a concerted manner. Both
Brønsted and Lewis acids catalyze this process by interacting
with the carbonyl group, the peroxide and/or the Criegee in-
termediate to facilitate the addition or the migration step and
influence the final product distribution.[9]
Levulinic acid (LA) is a key platform molecule that can be
readily produced from lignocellulosic carbohydrates.[6] Upon
scale up, the estimated LA price is expected to decrease to
less than $1 kgÀ1 from the current commercial price of approx-
imately $3.5 kgÀ1.[7] The carbonyl group in LA and its esters
can be oxidized to produce SA and its esters, respectively;
As shown in Scheme 2, the oxidation of methyl levulinate (1)
can follow two different pathways. For a pathway involving
oxygen insertion between the d-carbon and the g-carbon, di-
methyl succinate (2) is produced. Alternatively, when the
oxygen insertion occurs between the g-carbon and the b-
carbon, methyl 3-acetoxypropanoate is generated, which readi-
ly hydrolyzes into methyl acetate (3) and 3-hydroxypropanoate
(4) in the presence of an acid catalyst. We note that the propa-
noate 4 may undergo further transformation to generate
methyl 3-methoxypropanoate (5), methyl 3,3-dimethoxypropa-
[a] Y. Wang, F. Vogelgsang, Prof. Y. Romꢀn-Leshkov
Department of Chemical Engineering
Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139 (USA)
E-mail: yroman@mit.edu
[b] F. Vogelgsang
Department of Chemistry, Catalysis Research Center
Technische Universitꢁt Mꢂnchen
Lichtenbergstraße 4, 85747 Garching (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201403014.
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moles of substrate converted/initial moles of substrateꢂ100%)
and selectivities (defined as moles of product generated/moles
of substrate convertedꢂ100%) obtained using p-TsOH,
MeSO3H, and H2SO4 are expected based on their similar Ham-
mett acidity functions H0 of ꢀ2.0 (measured in ethanol[10,11]).
For instance, p-TsOH converted 56% of methyl levulinate
1 after 6 h with a selectivity of 61% and 39% to dimethyl suc-
cinate 2 and methyl acetate 3, respectively (entry 1). Owing to
the nature of the reaction, a concomitant 22% yield (defined
as moles of product generated/initial moles of substrateꢂ
100%) of 4 is obtained. The propanoate 4 is further converted
into 6 and 7 resulting in final yields of 13 and 8%, respectively.
Similar amounts of CO2 were detected when analyzing the
headspace samples of reactions using p-TsOH and H2O2 in
methanol in the presence and in the absence of methyl levuli-
nate, indicating that although terminal hydroxyl groups are
readily oxidized, carbon-carbon cleavage is unlikely under the
reaction conditions investigated.[8d] Overall, the heterogeneous
acids generated lower levulinate conversions compared to
their homogeneous counterparts. Specifically, Amberlyst-15
and a polymer-supported p-TsOH material featured conversions
of 13 and 24%, respectively (entries 6 and 8). Grinding the
resins into powders did not increase these conversion values,
but increasing the amount of catalyst from a levulinate/acid
molar ratio of 10:1 to 1:1 resulted in similar conversions and
product yields as those obtained with the homogeneous acids
(entry 7), demonstrating that although the polymer-supported
acids have lower reactivity (likely due to changes of the inter-
action between the acid sites and their chemical environment
after immobilization),[12] the product selectivity is not affected.
Interestingly, the zeolite H-BEA generated a levulinate conver-
sion of 47% with a succinate selectivity of 22% (entry 9),
which is much lower than that obtained with the other Brønst-
ed acids tested.
Scheme 2. Products obtained from the acid-catalyzed oxidation of methyl
levulinate with H2O2 in methanol.
noate (6), and dimethyl malonate (7). Although both strong
Brønsted and Lewis acids are active for the oxidation of methyl
levulinate, we demonstrate that the product distribution
strongly depends on the solvent polarity and, for Lewis acidic
triflates, the metal cation of the triflate salt.
Shown in Table 1 are the results of the BV oxidation of
methyl levulinate using both homogeneous and heterogene-
ous Brønsted acid catalysts with H2O2 as oxidant and methanol
as solvent. Our data indicate that acid strength has a drastic
effect on activity. Acetic acid, a weak Brønsted acid, was not
active even at a levulinate/acid molar ratio of 1:1 (Table 1,
entry 5). However, strong Brønsted acid catalysts, such as p-tol-
uenesulfonic acid (p-TsOH), methanesulfonic acid (MeSO3H),
sulfuric acid (H2SO4), and triflic acid (TfOH), all catalyzed the
levulinate oxidation effectively, generating comparable product
distributions (entries 1–4). Similar conversions (defined as
For the BV oxidation, the reported migratory aptitude of the
R groups generally follows the order: tertiary alkyl>secondary
alkyl>primary alkyl>methyl.[9a,c] However, our data show that
the selectivity to dimethyl succinate, the product of methyl
group migration, is higher than the selectivity to methyl ace-
tate. To rationalize this effect, we investigated the influence of
the substrate’s molecular structure (i.e., carbon-chain length
and branching) and reaction conditions (i.e., different oxidants
and solvents) on the product distribution using both a hetero-
geneous (i.e., Amberlyst-15) and a homogeneous (i.e., p-TsOH)
Brønsted acid catalyst. Figure 1 shows the product distribu-
tions obtained from reacting 2-pentanone (8a), 2-hexanone
(8b), and C6 methyl ketones with a branched secondary alkyl
or a tertiary alkyl group (8c and 8d, respectively). In methanol
with H2O2 as oxidant, both p-TsOH (red, Table S3) and Amber-
lyst-15 (A-15, black, Table S4) generated methyl ester 9/acetate
10 molar ratios higher than 1 for all four methyl ketones
tested. Furthermore, the data show that branching of the
carbon backbone also results in higher methyl ester 9/acetate
10 molar ratios. For instance, the 9d:10d molar ratio with Am-
berlyst-15 as the catalyst is 10.5, a value over six times as high
as the molar ratio observed for 9c:10c (1.6) and about nine
times as high as the one for 9b:10b (1.2). When tert-butyl hy-
Table 1. Results for the Brønsted acid-catalyzed oxidation of methyl
levulinate in methanol with aqueous H2O2.[a]
Entry
Catalyst
Conv. [%]
SM [%][b]
SA [%][c]
1
2
3
4
5[d]
6
7[d]
8[e]
9
p-TsOH
MeSO3H
H2SO4
56
48
54
49
<1
13
67
24
47
61
54
61
61
n.d.
60
61
55
22
39
42
37
29
n.d.
36
39
29
32
TfOH
CH3COOH
Amberlyst-15
Amberlyst-15
p-TsOH
Al-H-BEA
[a] Reaction conditions: methyl levulinate 61 mmolLÀ1, levulinate/perox-
ide molar ratio=1:2, levulinate/acid molar ratio=10:1, 808C, 6 h. [b] SM,
selectivity to succinate, moles of succinate generated/moles of levulinate
convertedꢂ100%. [c] SA, selectivity to acetate, moles of methyl acetate
generated/moles of levulinate convertedꢂ100%. [d] Levulinate/acid
molar ratio=1:1. [e] Polymer-supported p-TsOH, levulinate/acid molar
ratio=9:1.
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We further verified this effect by oxidizing methyl
levulinate in heptane with TBHP under the same re-
action conditions as those shown in Table 1. In
agreement with the product distribution changes
shown in Figure 1, a lower selectivity of 14% to di-
methyl succinate was obtained (Table S6, entry 5).
We observe that different oxidants also affect the
migratory preferences during BV oxidation. With Am-
berlyst-15 in methanol, aqueous H2O2 (black) gener-
ated higher methyl ester 9/acetate 10 ratios than
TBHP (blue), indicating the methyl migration is less
favorable with TBHP as oxidant. Hawthorne et al.
showed a higher differentiation of the migration
groups with peroxyacetic acid as oxidant when com-
pared to trifluoroperacetic acid for the BV oxidation
of cyclohexyl phenyl ketone.[14] It was hypothesized
that the impact of the migrating groups on the
energy barrier is larger for the less reactive inter-
mediate.[14] In our case, the different electronic and
steric effects between TBHP and H2O2 may be re-
sponsible for the different product selectivities ob-
served.
Figure 1. Product distribution for the oxidation of methyl ketones expressed as methyl
ester 9/acetate 10 molar ratios. Reaction conditions: substrate 61 mmolLÀ1, substrate/
peroxide molar ratio=1:2, substrate/acid molar ratio=1:1, 808C, 6 h. p-TsOH as catalyst,
50 wt% aqueous H2O2 as oxidant in methanol (red). Amberlyst-15 (A-15) as catalyst,
50 wt% aqueous H2O2 as oxidant in methanol (black). A-15 as catalyst, tert-butyl hydro-
peroxide (TBHP) in decane as oxidant in methanol (blue). A-15 as catalyst, TBHP in
decane as oxidant in heptane (yellow).
As a consequence, we observe that the molecular
structure of the ketones, i.e., carbon-chain length
and branching, also affects the migration preference.
In methanol with H2O2 as oxidant, a higher degree
of branching in the carbon backbone resulted in
droperoxide (TBHP) was used as oxidant in methanol (blue
bars in Figure 1, Table S5), methyl group migration continued
to dominate the product distribution under the reaction condi-
tions investigated. However, the branched ketones yielded
lower methyl ester 9/acetate 10 molar ratios when compared
to the H2O2 system. For instance, the 9d:10d molar ratio de-
creased from 10.5 to 0.9 when switching to TBHP. Moreover,
when the solvent was switched to heptane (yellow bars in
Figure 1, Table S6), all the methyl ester 9/acetate 10 molar
ratios became lower than 1.
higher methyl ester 9/acetate 10 molar ratios (Figure 1, red
and black bars). Although in methanol with TBHP as oxidant
the trend was reversed (Figure 1, blue bars), the ratios of
9a:10a (C5) were higher than those of 9b:10b (C6) under all
the reaction conditions investigated. To our knowledge, virtual-
ly no studies exist for the BV oxidation of asymmetric ketones
in alcohol solvents. Our data indicate that the migratory apti-
tude trend obtained in methanol is opposite to that in hep-
tane. Mora-Diez et al. showed that the polarity of solvents af-
fects the reaction mechanism.[15] Hence, the available studies
on BV oxidation provide limited guidance to understand the
effects of oxidants, catalysts, and the molecular structure on
the reaction selectivity in alcohol solvents. Future work in our
group is centering on obtaining a fundamental understanding
of these phenomena by way of coupled theoretical and experi-
mental investigations.
Lewis acids are known to catalyze BV oxidations.[9] Some
metal triflates have been shown to maintain their Lewis acid
activity in the presence of water, which allows their use with
aqueous solutions of H2O2.[16] Table 2 shows the activity of
metal triflates for the oxidation of methyl levulinate. Compared
to Brønsted acids, metal triflates generate lower levulinate con-
versions and yield a wider spectrum of product selectivities.
Among the metal triflates tested, Zn(OTf)2 showed the lowest
conversion (7%) with no significant succinate formation after
6 h (Table 2, entry 11). Increasing the reaction time to 24 h re-
sulted in a dimethyl succinate yield of 14%. The highest levuli-
nate conversions were obtained with Hf(OTf)4 (36%), Hg(OTf)2
(40%), and Sc(OTf)3 (38%), generating selectivities to succi-
nates exceeding 47% with the rest of the carbon ending up in
Evidently, the solvent choice seems to critically impact the
product selectivity. With Amberlyst-15 as catalyst and TBHP in
decane as oxidant, all methyl ester 9/acetate 10 molar ratios
were below 1 when heptane (yellow), instead of methanol
(blue), was used as solvent. The migration ratios obtained in
heptane are consistent with the general migratory aptitude for
BV oxidation.[9a,c] Selectivity shifts during the BV oxidation of al-
dehydes in different solvents were investigated by Lehtinen
et al.[13] Specifically, solvents capable of forming hydrogen
bonds, such as methanol and 1-propanol, were shown to favor
the migration of hydrogen over branched alkyl groups, while
the migration of branched alkyl groups was preferentially ob-
served in non-hydrogen-bonding solvents, such as toluene and
CH2Cl2. Indeed, when 1-propanol and 1-butanol were used as
solvents for the oxidation of 8a and 8b, respectively (Table S3,
entries 5–6), the product distributions obtained were similar to
those obtained in methanol (Table S3, entries 1–2). Considering
the highly oxygenated nature of our reactive species, alcohol
solvents are likely to interact strongly with the Criegee inter-
mediate, possibly altering the stability of the transition state.
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between the metal triflate and triflic acid formed in situ that
impacts product selectivity. The apparent cooperative effect
between the Lewis acid centers and the generated Brønsted
acid species of metal triflates has often been reported to im-
prove the reaction activity and/or alter the selectivity in reac-
tions, such as the etherification of glycerol with short chain
alkyl alcohols and hydrothiolation of non-activated olefins.[18a,19]
Further investigations are currently underway to understand
the origins of this effect.
Table 2. Metal triflates for methyl levulinate oxidation in methanol with
aqueous H2O2 as oxidant.[a]
[b]
Entry Catalyst
Conv. [%] SM [%] SA [%] SM/SA pKh
WERC[c]
1
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hg(OTf)2
In(OTf)3
Sc(OTf)3
Y(OTf)3
36
25
15
<1
40
16
38
17
16
16
7
57
61
61
n.d.
47
26
50
39
44
43
39
38
39
n.d.
48
70
33
22
21
31
41
1.5
1.6
1.6
–
1.0
0.4
1.5
1.8
2.1
1.4
–
0.25
0.25
0.25
0.25
3.4
4.0
4.3
7.7
7.7
–
–
–
–
2[d]
3[e]
4[f]
5
2.0ꢂ109
4.0ꢂ104
4.8ꢂ107
1.3ꢂ107
8.0ꢂ107
1.4ꢂ108
5.0ꢂ108
6
7
8
9
10
11
In summary, the oxidation of methyl levulinate into dimethyl
succinate was performed using peroxide-based oxidants. Both
Brønsted and Lewis acids catalyzed the reaction without signif-
icant CO2 formation. With strong Brønsted acids, a selectivity
of ꢀ60% to dimethyl succinate was obtained. Upon switching
from methanol to heptane as the solvent, the selectivity de-
creased to 14%, indicating the strong solvent effect on direct-
ing product selectivity. Lewis acidic metal triflates generated
a broader spectrum of product selectivities in methanol, which
is dictated by the nature of the metal cations in the triflate
salt.
Yb(OTf)3
Er(OTf)3
Zn(OTf)2
7.9
9.0
n.d.
[a] Reaction conditions: methyl levulinate 61 mmolLÀ1, levulinate/perox-
ide molar ratio=1:2, levulinate/cat. molar ratio=10:1, 808C, 6 h. SM, SA,
same as in Table 1. [b] pKh =Àlog(Kh), Kh is the hydrolysis constant of the
metal triflate.[16b] [c] WERC, water exchange rate constant of the metal tri-
flate.[16b] [d] DTBP/Hf(OTf)4 molar ratio=0.5. [e] DTBP/Hf(OTf)4 molar
ratio=1.4. [f] DTBP/Hf(OTf)4 molar ratio=4.0.
acetate-derived products (entries 1, 5, and 7). In comparison,
triflates based on Y, Yb, and Er showed lower levulinate con-
versions of ꢀ16% with succinate selectivities of ꢀ40% (en-
tries 8–10). Interestingly, In(OTf)3 showed the highest selectivity
to methyl acetate (70%) at a levulinate conversion of 16%
(entry 6). Moreover, Group IIIB and IVB metal cations showed
higher selectivities to dimethyl succinate than to methyl ace-
tate (entries 1–3, 7–10), whereas Group IIB and IIIA metal cat-
ions showed higher selectivities to methyl acetate (entries 5, 6,
and 11).
Acknowledgements
This work was funded by the Cooperative Agreement between
the Masdar Institute of Science and Technology (Masdar Insti-
tute), Abu Dhabi, UAE and the Massachusetts Institute of Technol-
ogy (MIT), Cambridge, MA, USA-Reference 02/MI/MI/CP/11/07633/
GEN/G/00.
Keywords: biomass
derivatives · oxidation · solvent effects
·
Brønsted acids
·
levulinic acid
The catalytic activity of metal triflates can be correlated with
their hydrolysis constant Kh and water exchange rate constant
(WERC, defined as the exchange rate for substitution of inner-
sphere water ligands). A lower pKh value translates to a stron-
ger hydrolysis tendency of the metal salt.[16b] As such, Zn(OTf)2
possessing the highest pKh value (9.0), showed the lowest levu-
linate conversion. Similarly, triflate salts of Yb, Er, and Y, featur-
ing pKh values ranging from 7.7 to 7.9, showed moderate activ-
ities. For salts with pKh lower than 4.3, including Hf(OTf)4,
Sc(OTf)3, and Hg(OTf)2, higher levulinate conversions were ob-
tained. Although In(OTf)3 has a pKh of 4.0, it converted only
16% of the substrate (Table 2, entry 6). Unlike other triflates,
In(OTf)3 has a WERC that is orders of magnitude smaller. Given
that the dissociation rate of the triflate ligand is proportional
to the WERC,[17] a smaller WERC value translates into a slower
dissociation rate of the triflate ligand into triflic acid. The
higher activity associated with metal triflates that possess
higher dissociation and exchange constants provides strong
evidence that the hydrolysis product, TfOH, is likely the true
catalyst. This hypothesis was further confirmed by adding 2,6-
di-tert-butylpyridine (DTBP), a steric hindered base,[18] to the re-
action vessel at different molar ratios (Table 2, entries 2–4). As
expected, addition of DTBP progressively suppressed reactivity,
up to complete deactivation upon reaching a DTBP/Hf(OTf)4
molar ratio of 4.
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Received: December 12, 2014
Revised: February 1, 2015
ChemCatChem 2015, 7, 916 – 920
www.chemcatchem.org
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