Selective oxidation of glycerol to formic acid in highly concentrated aqueous solutions with molecular oxygen using V-substituted phosphomolybdic acids

Article abstract of DOI:10.1039/c4ra05424e

Formic acid is an important commodity chemical as well as a promising medium for hydrogen storage and hydrogen production. In this paper, we report that formic acid can be produced through selective oxidation of glycerol, a low-cost by-product of biodiesel, by using vanadium-substituted phosphomolybdic acids as catalysts and molecular oxygen as the oxidant. Significantly, this catalytic system allows for high-concentration conversions and thus leads to exceptional efficiency. Specifically, 3.64 g of formic acid was produced from 10 g of glycerol/water (50/50 in weight) solution.

Full text of DOI:10.1039/c4ra05424e

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Selective oxidation of glycerol to formic acid in
highly concentrated aqueous solutions with
molecular oxygen using V-substituted
phosphomolybdic acids
Cite this: RSC Adv., 2014, 4, 35463
Received 6th June 2014
Accepted 31st July 2014
a
b
a
*
Jizhe Zhang, Miao Sun and Yu Han
DOI: 10.1039/c4ra05424e
www.rsc.org/advances
Formic acid is an important commodity chemical as well as a prom-
The signicantly increased global demand for biodiesel
ising medium for hydrogen storage and hydrogen production. In this results in the large surplus of glycerol, the main byproduct in
paper, we report that formic acid can be produced through selective manufacturing biodiesel fuels by the triglyceride trans-
oxidation of glycerol, a low-cost by-product of biodiesel, by using esterication process. As a consequence, the price of crude
vanadium-substituted phosphomolybdic acids as catalysts and glycerol has dropped markedly in the last 10 years from ꢀUSD
molecular oxygen as the oxidant. Significantly, this catalytic system 0.25 per pound to ꢀUSD 0.05 per pound.22,23 This makes glycerol
allows for high-concentration conversions and thus leads to excep- a promising feedstock to be converted to various high-valued
tional efficiency. Specifically, 3.64 g of formic acid was produced from products.24 The current price of FA is comparable to that of the
10 g of glycerol/water (50/50 in weight) solution.
rened glycerol (approximately USD 0.30 per pound). Given that
the price of glycerol will certainly fall further along with the
increase of biodiesel production, it is desirable to develop a
route to selective conversion of glycerol to FA as a useful
commodity chemical as well as an intermediate compound for
hydrogen production. It is worth noting that the use of glycerol to
produce H2 would fully integrate biodiesel into the renewable fuel
concept, considering that in addition to itself, the H2 obtained
from its by-product is also a clean fuel (Scheme 1).8,25–28 Hydrogen
generation from glycerol has been investigated by means of
aqueous/gas phase reforming.25–31 These processes suffer
from low yields of H2 or high reaction temperatures,25,28 and they
can only be applied to very diluted (ꢀ1%) aqueous solutions.8,26
Processes that enable selective reforming of concentrated glycerol
solutions at low temperatures remain to be developed.
In the context of the future hydrogen economy, effective
production of hydrogen (H2) from readily available and
sustainable resources is of crucial importance.1,2 Currently, H2
is mainly produced from nonrenewable natural gases, petro-
leum and coal through reforming process, while technologies
for producing H2 from water by solar energy are not yet mature.3
The possibility of generating H2 from biomass that is both
abundant and ecologically sustainable has attracted substantial
research efforts in the last decade, and various technologies
have been developed including fermentation,4 enzymatic
conversion,5,6 gasication,7 and steam/aqueous reforming.8,9
However, high-yield, low-cost production of H2 from biomass
remains a challenge. As a traditional commodity chemical in
high demand in the chemical, pharmaceutical and agricultural
industries, formic acid (FA) has recently received particular
attention because it was demonstrated to be an efficient storage
medium for H2.10–12 With the development of new processes that
allow selective decomposition of FA into H2 and CO2,13–21 it is
also possible to consider FA as a precursor to hydrogen
production, although this is economically and ecologically
meaningful only when FA can be produced from renewable
feedstock using low-cost processes.
aAdvanced Membranes and Porous Materials Center, Physical Science and Engineering
Division, King Abdullah University of Science and Technology, Thuwal 23955-6900,
Saudi Arabia. E-mail: yu.han@kaust.edu.sa
Scheme 1 Schematic illustration of the renewable fuel concept based
on biomass conversion: (1) triglyceride transesterification; (2) photo-
synthesis; (3) aqueous/gas phase reforming; (4) catalytic oxidation; and
(5) dehydrogenation of FA.
bCooperate Research and Development Center in King Abdullah University of Science
and Technology, Saudi Aramco, Thuwal 23955-6900, Saudi Arabia
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Here, we report that glycerol can be selectively oxidized to FA
in an aqueous solution with molecular oxygen by using vana-
dium-substituted phosphomolybdic acids as catalysts. Given
that many efficient catalytic systems have been developed for
dehydrogenating FA to generate H2, the selective conversion of
glycerol to FA offers an alternative route to the direct extraction
of H2 from glycerol (Scheme 1). In comparison with conven-
tional reforming processes, this catalytic system requires a
lower temperature (423 K) and leads to higher selectivity; more
importantly, it can work with highly concentrated aqueous
solutions of glycerol (up to 75 wt%) to give rise to much higher
conversion efficiencies, i.e., higher potential H2 yields per unit
volume of the reactor. The liquid product FA also holds
advantages over H2 for its ease of storage and transportation
and its readiness to release H2 when needed.
In our recent studies, we have demonstrated that many
Keggin-type heteropoly acids (HPAs) can effectively convert
biomass substrates under oxidative conditions due to their
Fig. 1 Selective oxidation of glycerol in an aqueous solution over the
H4PV1Mo11O40 catalyst. Reaction conditions: 10 g of glycerol solution
with a designated concentration; 0.1 mmol of catalyst; at 423 K and a
constant O2 pressure of 2 MPa for 3 h.
¨
strong Bronsted acidity, while the reaction pathway is largely
determined by the type of addenda atom in the HPA catalyst.32–34
For example, we reported the direct conversion of cellulose
to glycolic acid with high yields (ꢀ50%) using molecular oxygen
were obtained in the system that initially contained 50 wt% of
glycerol. These values are exceptionally high, considering that
conventional oxidative biomass conversion reactions are
usually performed in diluted solutions (<5 wt%).38–40 With the
initial glycerol concentration exceeding 50 wt%, the conversion
as well as the selectivity of FA dramatically decreased, giving rise
to lower absolute yields of FA (Fig. 1). When a 90 wt% glycerol
solution was used in the reaction, for example, the conversion
of glycerol was 25% and AA was the only detectable product in
the liquid phase (Fig. 1). These results suggest that higher water
content in the reaction system favors the formation of FA. We
note that glycerol has been converted through selective oxida-
tion to various high-value chemicals, such as glyceric acid,41
tartronic acid,42,43 ketomalonic acid,44 and dihydroxyacetone,45,46
and that the use of HPAs to catalyze the dehydration of glycerol
to acrolein47–49 or glycerol acetylation50 has also been reported.
However, to the best of our knowledge, conversion of glycerol to
FA with such a high selectivity and efficiency has never been
achieved prior to this study. Actually, the conversion of glycerol
to FA was not attractive before because of the comparable values
of the two compounds.51 With the slump of the glycerol's price,
this conversion route has drawn more attention and several
systems have been investigated very recently. For example, Shen
et al. reported the formation of formic acid from glycerol using a
hydrothermal reaction at 250 ꢁC with H2O2 as an oxidant;52 Liu
et al. reported the selective oxidation of glycerol to formic acid
catalyzed by Ru(OH)4/r-GO in the presence of FeCl3;53 the
conversion of glycerol to FA using a phosphomolybdic acid
catalyst was mentioned but not specically discussed in ref. 37.
In these previous studies, however, the reactions were all
carried out in highly diluted solution, resulting in much lower
conversion efficiency in comparison with our system. Moreover,
the yield of AA, another useful chemical with important appli-
cations, is also remarkable in our system.
in
a water medium in which a phosphomolybdic acid
(H3PMo12O40) acts as a bi-functional catalyst to catalyze both
the hydrolysis of cellulose and the subsequent oxidation reac-
tions. Further study indicated that the product selectivity of this
system changed from glycolic acid to FA if some molybdenum
(Mo) atoms in the H3PMo12O40 catalyst were substituted by
vanadium (V).34 Similar results were also reported by other
research groups35–37 and were attributed to the selective oxida-
tive cleavage of C–C bonds by the V atoms in HPA. Inspired by
these previous works, we attempted to use vanadium-
substituted phosphomolybdic acids (H3+nPVnMo12ÀnO40) as
catalysts to convert glycerol through oxidation. The reactions
were carried out at 423 K for 3 hours under 2–4 MPa O2 in a
Teon-lined stainless autoclave reactor (75 mL) that contained
10 g of an aqueous solution of glycerol with a designated
concentration and 0.1 mmol of catalyst.
We rst used mono-V-substituted phosphomolybdic acid
(H4PV1Mo11O40) to convert glycerol in aqueous solutions of
different concentrations ranging from 1 wt% to 90 wt%. Full
conversion of glycerol was achieved in the 1 wt% solution with
three major products detected in the liquid phase, including FA
(selectivity: 51.2%), acetic acid (selectivity: 8.3%), and formal-
dehyde (selectivity: 7.2%), while the only product detected in
the gas phase was CO2. When a 5 wt% glycerol solution was
used for conversion, 98% of the glycerol was converted with
nearly unchanged product selectivities. However, it is worth
noting that the absolute yields of the products, which are
dened as their weight percentages relative to the initial reac-
tion mixture, were markedly increased in this case due to the
ve times higher substrate concentration (Fig. 1). Further
increasing the concentration of the glycerol substrate resulted
in gradually decreased conversion, disappearance of formalde-
hyde in the products, and continuously increased absolute
yields of FA and acetic acid (AA). As shown in Fig. 1, the
maximum absolute yields (29.2 wt% for FA and 17.7 wt% for AA)
We found that the conversion efficiency of glycerol to FA can
be further enhanced by increasing the vanadium content in the
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Table 1 Oxidative conversions of various substrates catalyzed by V-
investigated the stability of FA in this catalytic system by using
FA (50 wt% in water) as the substrate, and found that only ꢀ5%
of FA was decomposed under the reaction conditions. This
result reveals that FA is rather stable in this system. Therefore,
the CO2 produced in this reaction is not from the dehydroge-
nation of FA, as supported by the observation that varying the
glycerol/catalyst ratio did not much change the selectivity of FA
(Fig. 1). In addition, we can conclude from this result that the
absence of FA in the products of high concentration (e.g.,
90 wt%) glycerol conversion is not due to its decomposition but
more likely associated with the low water content in the reaction
system.
substituted phosphomolybdic acidsa
Selectivity
(%)
Conc.
(%)
Conversion
(wt%)
Catalysts
Substrate
FA
AA
b
HPMoV1
HPMoV2
HPMoV3
HPMoV1
HPMoV2
HPMoV3
HPMoc
HPMoV1
HPMoV1
HPMoV1
HPMoV1
Glycerol
Glycerol
Glycerol
Glycerol
Glycerol
Glycerol
Glycerol
Lactic acid
Methanol
Formaldehyde
Formic acid
50
50
50
1
1
1
1
1
1
1
90.6
93.0
94.8
99.5
100
100
—
100
5.8
43.9
4.7
41.2
48.5
51.3
51.2
55.2
60.0
—
7.4
5.2
83.8
—
19.2
13.0
11.2
8.3
2.9
2.8
—
38.2
—
—
b
b
c
c
c
c
d
d
c
Conclusions
50
—
We demonstrated that V-substituted phosphomolybdic acid
catalysts enable the selective oxidation of glycerol to formic acid
in highly concentrated aqueous solutions using molecular
oxygen. The preliminary results show that the absolute yield of
formic acid could reach 36.4 wt% of the initial reaction mixture,
representing exceptionally high conversion efficiency. This
reaction provides an alternative route to the production of H2
from glycerol, given that formic acid can be readily and selec-
tively converted to H2. In comparison with conventional
reforming processes, this process requires lower energy input
while offering higher selectivity and yield. Taking advantage of
the large surplus of glycerol, it fully integrates biodiesel in the
renewable fuel concept. A noteworthy advantage of the HPA
catalysts used in this study is that they can be recovered in solid
form aer reactions by distilling the products and solvent out,
and their good reusability has been demonstrated in our
previous studies.34
a
Reaction conditions: 10 g aqueous solution of the substrate with a
designated concentration; 0.1 mmol of catalyst; for 3 h. Besides FA
and AA, trace amount of formaldehyde, glycolic acid and other
b
unidentied products are detected in the liquid phase. 423 K and 4
MPa O2. c 423 K and 2 MPa O2. 453 K and 2 MPa O2.
d
phosphovanadomolybdic catalyst. Under identical reaction
conditions, multi-V-substituted phosphomolybdic acid cata-
lysts (H5PV2W12O40 and H6PV3Mo9O40) gave rise to more FA and
less AA than their mono-substituted counterpart. The FA
selectivity increases in the following order: H6PV3Mo9O40
>
H5PV2W12O40 > H4PV1Mo11O40, and this trend applies to both
concentrated (50 wt%) and diluted (1 wt%) glycerol solutions
(Table 1). The highest absolute yield of FA (36.4 wt%), which
corresponds to a selectivity of 51.3% from the 50 wt% glycerol
solution, was achieved by H6PV3Mo9O40 (Table 1). The fact that
the V-free phosphomolybdic acid is catalytically inactive under
the same reaction conditions (Table 1) further demonstrates the
crucial role of vanadium in this reaction.
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