Base free, one-pot synthesis of lactic acid from glycerol using a bifunctional Pt/Sn-MFI catalyst

Article abstract of DOI:10.1039/c4gc00723a

Under base free and mild reaction conditions, 80.5% selectivity of lactic acid (LA) was achieved at 89.8% conversion of glycerol using a bifunctional Pt/Sn-MFI catalyst. In the tandem reaction pathway, selective oxidation of glycerol to glyceraldehyde (GLA) and dihydroxyacetone (DHA) was cascaded with Lewis acid catalyzed isomerization of GLA/DHA into LA. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00723a

Volume 16 Number 7 July 2014 Pages 3381–3644
Green
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Wei Fan et al.
Base free, one-pot synthesis of lactic acid from glycerol using a
bifunctional Pt/Sn-MFI catalyst

Green Chemistry
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Base free, one-pot synthesis of lactic acid from
glycerol using a bifunctional Pt/Sn-MFI catalyst†
Cite this: Green Chem., 2014, 16,
428
3
Hong Je Cho, Chun-Chih Chang and Wei Fan*
Received 22nd April 2014,
Accepted 7th May 2014
DOI: 10.1039/c4gc00723a
www.rsc.org/greenchem
Under base free and mild reaction conditions, 80.5% selectivity of used as a precursor to synthesize polylactate or polylactic acid
lactic acid (LA) was achieved at 89.8% conversion of glycerol using (PLA), the second most manufactured bioplastic in the
1
4
a bifunctional Pt/Sn-MFI catalyst. In the tandem reaction pathway, world. LA can be produced from petroleum feedstocks in
selective oxidation of glycerol to glyceraldehyde (GLA) and di- high yields using mineral acid catalysts. However, the use of
hydroxyacetone (DHA) was cascaded with Lewis acid catalyzed toxic chemicals (e.g. hydrogen cyanide) and the high cost for
isomerization of GLA/DHA into LA.
product separation could lead to significant environmental
issues. Therefore, currently LA is mainly manufactured by the
fermentation of carbohydrates. However, the efficiency and
productivity of the fermentation process are still low and need
substantial improvements because of the high cost of enzyme
The need to replace fossil fuels with sustainable, eco-friendly
resources has driven research towards the discovery of new
catalytic pathways for converting renewable and waste
resources into value-added platform chemicals and fuels.
Glycerol is an inevitable by-product from biodiesel production
by the trans-esterification of vegetable oils. The rapid develop-
ment of oleochemistry leads to a dramatic increase in glycerol
production from about 600 000 tons in 1992 to 1.1–1.5 million
tons in 2010 with an average increase of 3.75% per year. Gly-
1
,2
catalysts and the need for precise control of operating
15–17
conditions.
There is thus a persistent need to develop
3
efficient thermochemical pathways for the production of LA
18,19
from new starting materials.
Although much research efforts have been devoted to tailor
the selectivity in glycerol oxidation, only a few reaction path-
ways can selectively convert glycerol into LA. Kishida et al.
reported that LA can be directly produced from the hydrother-
4
cerol can also be potentially obtained from non-edible
biomass, such as microalgae, cellulose and its derivatives, in
1
,5,6
the near future.
For these reasons, the conversion of gly-
20
mal reaction of glycerol in the presence of NaOH at 300 °C.
cerol to high-value chemicals has attracted much attention
from both academia and industry. Compared to hydrocar-
During the hydrogenolysis of glycerol in alkaline solutions at
3
21,22
2
00 °C and 4.0 MPa H , LA has also been detected.
These
2
bons, glycerol is a highly functionalized molecule consisting of
three hydroxyl groups. Selective hydrogenolysis, oxidation and
dehydration of the hydroxyl group of glycerol have been exten-
sively studied to synthesize valuable chemicals or
studies highlighted the potential of the production of LA from
glycerol although the efficiencies or harsh reaction conditions
of the reaction pathways may present significant hurdles to
their commercialization. Recently, a tandem reaction pathway
has been developed to selectively transform glycerol into LA.
In this route, glycerol is first selectively oxidized to dihydroxy-
acetone (DHA) and glyceraldehyde (GLA) on metal catalysts,
and then undergoes base-catalyzed dehydration and benzilic
acid rearrangement to produce LA. Shen et al. first reported
that 85.6% of LA selectivity was achieved from this reaction
pathway using the Au–Pt/TiO2 catalyst in the presence of
NaOH, and confirmed that DHA and GLA were formed during
3
,7–10
intermediates.
In particular, the oxidation of glycerol can
generate a series of reaction pathways capable of producing
various valuable oxygenated derivatives. However, due to the
complex nature of these reaction pathways, it is still a grand
challenge to control the selectivity to desired products.
Lactic acid (LA) and alkyl lactates have been widely used in
food, pharmaceutical and chemical industries as platform
1
1–13
chemicals and green solvents.
In particular, LA has been
2
3
the reaction as intermediates. Similar performances have
also been achieved on Au/CeO2 and Au–Pt/nanocrystalline
2
4,25
Department of Chemical Engineering, University of Massachusetts Amherst,
86 N. Pleasant St., 159 Goessmann Lab, Amherst, MA 01003, USA.
E-mail: wfan@ecs.umass.edu; Tel: +1 413 545 1750
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc00723a
CeO catalysts under basic conditions.
The use of a base in
2
6
the tandem reaction pathway is crucial because hydroxide ions
not only facilitate H-abstraction in the initial dehydrogenation
of the hydroxyl group of glycerol but also catalyze the reaction
†
3428 | Green Chem., 2014, 16, 3428–3433
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8
,23,26,27
of DHA/GLA to LA.
Although a high yield of LA can be
achieved from this reaction pathway, the use of base catalysts
causes the formation of acid salts (i.e. lactate). Additional reac-
tion and separation steps are required to produce LA, which
not only increase the cost of production but also produce a
stoichiometric amount of salt wastes. Development of a base-
free reaction pathway for the production of LA from glycerol
using heterogeneous catalysts is thus highly desired. In this
scenario, Xu and coworkers succeeded in attaining 66.6% of
LA selectivity at 13.6% of glycerol conversion by employing Au-
Pd/TiO2 in the presence of AlCl , and proposed that the
3
second step reaction of DHA/GLA to LA was catalyzed by
2
8
AlCl
3
.
Although no base catalyst was used in the reaction,
separation of the AlCl catalyst and catalyst poisoning by chlor-
3
ine were still issues for the catalytic process.
In order to overcome the challenges, herein we developed
an efficient base-free reaction pathway to selectively convert
glycerol into LA in the aqueous phase using a bifunctional Pt/
Sn-containing zeolite catalyst. In the reaction pathway, glycerol
is first oxidized to DHA/GLA with O on Pt nanoparticles under
2
mild oxidation conditions. The water tolerant Lewis acid cata-
lyst, Sn-MFI, then catalyzes the isomerization of DHA/GLA to
LA with high selectivity. Zeolites are microcrystalline materials
consisting of distinct pore dimensions, framework and compo-
sition. Because of their remarkable hydrothermal stability and
molecular sieving capability for various reactions, zeolites have
been widely used as catalysts for petrochemical and biomass
1
2,17
processing.
In this study, Sn containing zeolites were
selected due to their outstanding Lewis acidity in the aqueous
phase. Under optimized reaction conditions over 80% selecti-
vity to LA was achieved from the bifunctional catalyst.
Bifunctional Pt/Sn-containing zeolite catalysts were pre-
pared by the reduction of Pt precursors deposited on Sn-con-
taining zeolites with the MFI structure (Sn-MFI). Details of
syntheses of the catalysts are available in the ESI.† Sn-MFI was
chosen in this study because of its outstanding Lewis acidity Fig. 1 (a) Pyridine FT-IR spectrum, (b) Pt particle size distribution and
for catalyzing the isomerization of sugar molecules in the (c) TEM image for the Pt/Sn-MFI catalyst. The average Pt particle size
3
2
was estimated using the equation ∑d /∑d where d is the size of each
Pt particle.
aqueous phase, in particular for C sugar derivatives, such as
3
DHA, GLA and pyruvaldehyde (PA). Sn-MFI was synthesized
2
9
according to a method published in the literature. Character-
istic XRD peaks corresponding to the MFI structure were
observed for the Sn-MFI catalyst (Fig. S1, ESI†), revealing that diffraction peak at 2θ = 39.8°, corresponding to the (111) plane
3
3
a highly crystalline sample with the MFI structure was syn- of Pt crystals with a cubic close-packed structure. It is evident
thesized without any other impurity phase. Lewis acidity of the that the formation of crystalline Pt nanoparticles on Sn-MFI
Sn-MFI catalyst was confirmed by FT-IR measurements of the does not influence the original crystalline structure of the Sn-
sample with adsorbed pyridine. As shown in Fig. 1a, Sn-MFI MFI catalyst. The TEM image confirms that the Pt nanoparti-
−1
provided a strong absorption band at 1454 cm , assigned to cles were well dispersed on the Sn-MFI catalyst surface
1
7,30
the pyridine bound with the Lewis acid Sn sites.
A very (Fig. 1c). Particle size distribution of Pt calculated from TEM
weak absorption feature at 1547 cm related to the Brønsted images was shown in Fig. 1b. The size of Pt nanoparticles
−
1
1
7,30
acid was also observed in the spectrum,
that in the conversion of the triose sugars into LA, Sn-MFI good agreement with the size of 7.9 nm determined by H
might be able to catalyze both dehydration and intramolecular chemisorption (Table S1, ESI†). Pt on siliceous MFI (silicalite-
demonstrating mainly lies between 4 and 7 nm with an average of 6.7 nm, in
2
1
7,31,32
1
,2 hydride shift after hydration of PA.
Pt precursors (H PtCl ·6H
O) were loaded on the Sn-MFI TiO
2
1), Sn containing zeolites with the BEA structure (Sn-BEA) and
were also synthesized using the method applied for
2
6
2
sample using an incipient wetness method, and subsequently Pt/Sn-MFI. The particle size of Pt on the samples is in the
reduced to Pt nanocrystals. The Pt/Sn-MFI sample possesses a range of 6.4 nm to 9.5 nm, similar to the Pt/Sn-MFI sample
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a
Table 1 Oxidation of glycerol and other substrates on the catalysts synthesized in the study
Selectivity (%)
b
c
Entry
1
Catalyst
Substrate
TOF (s−1)
Conv. (%)
DHA
GLA
PA
4.4
—
—
29.8
0.0
0.0
0.0
LA
PyA
AA
2.1
46.8
94.7
3.7
5.4
0.9
5.9
GlyA
TarA
Pt/Sn-MFI
Pt/Sn-MFI
Pt/Sn-MFI
Pt/Sn-BEA
Pt/silicalite-1
Pt/AC + Sn-MFI
Glycerol
LA
PyA
Glycerol
Glycerol
Glycerol
Glycerol
1.95
0.05
0.23
2.33
1.11
0.07
2.21
89.8
4.7
22.5
93.4
83.8
53.6
92.3
0.9
—
—
6.2
9.7
0.0
3.6
0.6
—
—
4.9
42.0
0.0
18.6
80.5
—
—
28.1
0.0
80.8
0.0
1.6
40.4
—
1.0
0.0
0.0
0.0
8.7
—
—
18.4
32.7
7.8
0.8
—
—
3.0
4.2
0.9
7.8
d
d
2
d
d
3
e
4
5
6
7
Pt/TiO
2
55.4
a
O, substrate/Pt (mol mol−1) = 350, substrate/Sn (mol mol−1) = 226, pO
Reaction conditions: 0.2 M substrate in H
2
2
= 0.62 MPa, 100 °C, at 24 h.
b
e
c
d
TOF = moles of a converted substrate divided by moles of Ptsurface per reaction time, at 0.5 h. Substrate conversion. Reaction time: 4 h.
−1
Substrate/Sn (mol mol ) = 211.
(
Table S1, ESI†). The crystalline structure of these supports is tion rate slower than PyA oxidation under the same reaction
also well retained during the loading of Pt (Fig. S1, ESI†). conditions. Moreover, as shown in Table 1, the TOF value for
Table 1 shows the activities and selectivities of aerobic gly- the oxidation of LA is 39 times lower than that for the oxi-
cerol oxidation at 0.62 MPa of O and 100 °C on the bifunc- dation of glycerol, which allows us to optimize the reaction
tional catalysts prepared in this work. Pt/Sn-MFI catalyzed conditions to achieve a high selectivity to LA. Other by-pro-
2
−
1
glycerol oxidation to LA with an activity of 1.95 h and 80.5% ducts formed during the course of the reaction were glyceric
selectivity to LA at 89.8% conversion of glycerol. To the best of acid (GlyA) and tartronic acid (TarA), which are formed by the
3
4,35
our knowledge, it is the highest yield of LA achieved using het- sequential oxidation of GLA.
The overall carbon balance
erogeneous catalysts in the absence of a base. Concentration obtained from the analyses is above 94%.
profiles of reactants and various products as well as the selecti-
Although previous studies have suggested that Sn-BEA is a
vity to LA are shown in Fig. 2a and b, respectively (Fig. S2, highly active Lewis acid catalyst for the isomerization of triose
1
2,31,36,37
ESI†). At the beginning of the reaction, the main product is sugars in the aqueous phase,
the Pt/Sn-BEA catalyst
DHA formed by the oxidation of the secondary hydroxyl group provided much lower selectivity to LA (28.1%) compared to
of glycerol and the isomerization of GLA. GLA was also Pt/Sn-MFI (Table 1, entry 4). 29.8% selectivity to PA was
detected in the reaction with a low selectivity. With increasing observed at 93.4% conversion of glycerol, implying that the
glycerol conversion, the DHA concentration decreased concur- catalytic activity of the Pt/Sn-BEA catalyst is substantially lower
rently with an increase in the selectivity to LA, suggesting that than Pt/Sn-MFI for converting triose sugars into LA. Moreover,
DHA is an intermediate in the formation of LA. When the gly- a large amount of GlyA was detected at the end of the reaction,
cerol conversion is higher than 90%, the LA selectivity began indicating that the sequential oxidation of GLA significantly
to decrease with the formation of pyruvic acid (PyA) and acetic proceeded. To further understand the difference between the
acid (AA) which is due to further oxidation of the produced Pt/Sn-BEA and Pt/Sn-MFI catalysts, we studied the performance
3
4
LA. The reaction pathways were studied by starting the reac- of these catalysts in the conversion of PA to LA. The reactions
tions from LA and PyA on the Pt/Sn-MFI catalyst, respectively were first carried out over the Sn-BEA and Sn-MFI catalysts
(Table 1, entries 2 and 3). It was found that PyA is selectively without Pt. Although the BEA zeolite has larger pore sizes than
converted into AA. LA oxidation produced PyA and AA at a reac- MFI (Table S2, ESI†), Sn-MFI displays a 45% higher yield of LA
Fig. 2 (a) Concentration profile and (b) glycerol conversion and LA selectivity over Pt/Sn-MFI. Reaction conditions are as follows: 0.2 M glycerol in
O, glycerol/Pt (mol mol− ) = 350, glycerol/Sn (mol mol ) = 226, pO
1
−1
= 0.62 MPa and 100 °C. The error bars in the figures are from three repeated
H
2
2
reactions.
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a
Table 2 TOF values for the conversion of PA into LA
Sn-MFI
522
Pt/Sn-MFI
493
Sn-BEA
372
Pt/Sn-BEA
218
TOF (h−
1 b
)
a
O, PA/Sn (mol mol−1) = 226,
00 °C, batch reactor. TOF = moles of converted PA divided by moles
Reaction conditions: 0.2 M PA in H
2
b
1
of Sn per reaction time (h), at 15 min.
Pt on the catalytic activity of Sn-BEA was more obvious and
41.5% decrease was observed. Overall, Pt/Sn-MFI provided a 2.3
times higher initial reaction rate than Pt/Sn-BEA for the conversion
of PA to LA. This indicates that the loading of Pt might par-
tially block the Lewis acid Sn sites in these two catalysts.
Corma et al. reported that the Sn framework in the Sn-BEA
sample can stabilize the nucleation of Pt nanoparticles inside
0
4+
41
the zeolite channels and form Pt –Sn sites. We believe that
a part of Pt precursors (H PtCl ·6H O) used in our synthesis
2
6
2
can diffuse into the zeolite BEA channels because of the large
0
4+
pore size of the BEA structure. The Pt –Sn sites might not be
able to catalyze the reaction of PA to LA. In addition, the Pt
nanoparticles within the zeolite channels could also block
diffusion pathways of the chemicals involved in the reactions.
In contrast, it is more difficult for the Pt precursors to diffuse
into the MFI structure due to a smaller pore size. The activity
of the Sn sites in Sn-MFI was affected by the loading of Pt to a
lesser extent. As a result, the remarkable catalytic performance
of Pt/Sn-MFI in the studied reactions can be attributed to the
accessible Sn site, open framework structure and hydrophilic
Fig. 3 Reaction profiles for the conversion of PA into LA over Pt/Sn-
MFI, Sn-MFI, Pt/Sn-BEA and Sn-BEA catalysts. Reaction conditions are as surface.
follows: 0.2 M PA in H
2
O, PA/Sn (mol mol− ) = 226, 100 °C, batch
1
To understand the reaction pathway and mechanism for
the conversion of glycerol into LA over the bifunctional cata-
lysts, additional control experiments were performed. It was
found that glycerol reaction does not occur at all on Sn-MFI
reactor.
than Sn-BEA (Fig. 3). The higher catalytic activity of Sn-MFI for and Sn-BEA catalysts in the absence of Pt. On the other hand,
PA conversion in water might be due to its relative hydrophilic Pt on siliceous MFI, Pt/silicalite-1, catalyzed the oxidation of
surface compared to the Sn-BEA catalyst. It has been known glycerol predominantly to GLA (42.0%) and GlyA (32.7%,
that zeolites synthesized in fluoride medium contain fewer Table 1, entry 5). The fact that PA and LA were not formed in
structural defects and silanol groups than the ones synthesized the absence of either a Lewis acid or Pt suggests that the com-
3
8,39
in caustic medium.
In this study, the Sn-MFI catalyst was bination of a selective oxidation catalyst and an isomerization
synthesized using a conventional approach without using flu- catalyst is indispensable for the tandem reaction pathway.
oride ions. In contrast, hydrofluoric acid (HF) had to be used Interestingly, oxidation of glycerol over Pt/silicalite-1 mainly
in the synthesis of the Sn-BEA catalyst. The smaller amounts of produced GLA and GlyA, not DHA. This result clearly indicates
defects and silanol groups in the Sn-BEA provide a more that Pt prefers to catalyze the oxidation of the primary hydroxyl
hydrophobic environment for the catalyst as shown in the pre- group of glycerol to produce GLA. DHA, a thermodynamically
3
9,40
vious literature.
This observation is also evidenced by stable isomer of GLA, is formed over the Lewis acid sites of the
FT-IR measurements of the samples in the OH stretch range of Pt/Sn-MFI catalyst. This finding is consistent with previous
800–3000 cm− (Fig. S3, ESI†). It is believed that different reports.
1
17,42,43
The subsequent reaction of DHA to LA also
3
hydrophobic/hydrophilic properties of the two catalysts may occurs over the Lewis acid catalyst. A physical mixture of Pt
affect their adsorption and catalytic performances. Higher catalysts supported on activated carbon (Pt/AC, 5 wt% Pt,
catalytic activity of Sn-MFI in the reaction of PA to LA should Sigma Aldrich) and Sn-MFI was also examined for this reaction
be due to the synergism between the structure of Lewis acid Sn (Fig. S4, ESI†). The catalytic activity of the mixture (TOF = 0.07
−
1
−1
sites and adsorption properties. After Pt nanoparticles were
s ) is much lower than the Pt/Sn-MFI catalyst (TOF = 1.95 s )
loaded, the catalytic activity of both catalysts decreased as even though the surface area of Pt on AC is much larger than
shown in Table 2. In the case of loading Pt onto Sn-MFI, the that of Pt on Sn-MFI (Table S1, ESI†). After 20 h, the LA selecti-
reaction rate decreased by around 5.5%. The effect of loading vity from the physically mixed catalysts exceeded 80%, which
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is similar to that from Pt/Sn-MFI. However, the Pt/AC catalyst has also been shown over the Pt/TiO
2
catalyst for conversion of
23
was deactivated obviously faster than the Pt/Sn-MFI and was glycerol into lactate in the presence of a base. Further
only able to convert 54% of glycerol over 24 h. It has been improvement in the reusability of the catalysts will be achieved
4
4
known that TiO
2
is also an active Lewis acid catalyst. Au/ by using bimetallic Au–Pt on the Sn-MFI catalyst, which is an
TiO , Pt/TiO and Au–Pt/TiO have been used for glycerol oxi- on-going project in our group.
2
2
2
2
3
dation in the presence of NaOH. Under the reaction con-
ditions in this study, the Pt/TiO catalyst could not produce
2
any LA due to the weak Lewis acidity, and the main products Conclusions
were GLA, GlyA and TarA (Table 1, entry 7). The catalytic per-
formance of Pt/TiO is similar to Pt/silicalite-1, indicating that
2
We have developed a base free one-pot reaction pathway to
produce LA in high yields from the oxidation of glycerol over a
bifunctional Pt/Sn-MFI catalyst under mild conditions. In the
cascade reaction route, the selective oxidation of glycerol to
GLA proceeds on the Pt catalyst. Sn-MFI exhibits outstanding
Lewis acidity for converting GLA into DHA and DHA into the
final product LA. The superior performance is mainly due to
its microporous structure, accessible Sn sites and Lewis
acidity.
the Lewis acidity of TiO is not capable of activating the func-
2
tional groups of the intermediates, GLA and DHA, under the
reaction conditions. On the basis of the results, it can be con-
cluded that Sn-MFI acts not only as a Lewis acid catalyst but
also as a superior support for the Pt catalyst as compared to
2
the activated carbon and TiO .
Based on these reaction results, a tandem reaction pathway
for selective oxidation of glycerol to LA is proposed in
Scheme 1. In a base free environment, Pt nanoparticles pre-
dominantly activate the primary hydroxyl group over the sec-
ondary one in glycerol, leading to the formation of GLA. The
formed GLA can undergo sequential oxidations to GlyA and
TarA. In the presence of the Lewis acid Sn-MFI catalyst, GLA is
quickly converted into a thermodynamically more stable
isomer, DHA, via intramolecular 1,2 hydride shift reac-
Acknowledgements
Hong Je Cho was supported by the start-up fund from the Uni-
versity of Massachusetts, Amherst. Wei Fan and Chun-Chih
Chang were supported by the Catalysis Center for Energy Inno-
vation, an Energy Frontier Research Center funded by the US
Dept. of Energy, Office of Science, and Office of Basic Energy
Sciences under award number DE-SC0001004.
1
7,42,43
tion.
Previous studies have reported that weak Brønsted
1
7,32
acid catalyzed dehydration of DHA produces PA.
The
formed PA undergoes another intramolecular 1,2 hydride shift
after hydration of PA, to generate LA. The superior catalytic
activity of Sn-MFI can facilitate the reaction of GLA to LA even
if the competitive sequential oxidations of GLA can also occur.
Moreover, further oxidation of LA on the catalyst is 39 times
slower than oxidation of glycerol, which favors a cascade reac-
tion stopping at LA.
The spent catalysts were regenerated by washing with de-
ionized water, ethanol and acetone at room temperature.
Unfortunately, a gradual decrease in catalyst activity was
observed for the recycled catalysts (Fig. S5, ESI†). This trend
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