One-pot electrocatalytic oxidation of glycerol to DHA

Article abstract of DOI:10.1016/j.tetlet.2006.07.123

One-pot, waste-free oxidation of glycerol to 1,3-dihydroxyacetone (DHA) was achieved by simply applying a small electric potential (1.1 V vs Ag/AgCl) to a glycerol solution in water buffered at pH 9.1 in the presence of 15 mol % TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). Prolonging the reaction time affords comparable amounts of hydroxypyruvic acid.

Full text of DOI:10.1016/j.tetlet.2006.07.123

Tetrahedron Letters 47 (2006) 6993–6995
One-pot electrocatalytic oxidation of glycerol to DHA
Rosaria Ciriminna,a Giovanni Palmisano,b Cristina Della Pina,c
Michele Rossic and Mario Pagliaroa,d,
*
aIstituto per lo Studio dei Materiali Nanostrutturati, CNR, via Ugo La Malfa 153, 90146 Palermo, Italy
bDipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita´ degli Studi, viale delle Scienze, 90128 Palermo, Italy
cDipartimento di Chimica Inorganica, Universita´ degli Studi, via Venezian 21, 20133 Milano, Italy
dInstitute for Scientific Ethics and Methodology, CNR, via Ugo La Malfa 153, 90146 Palermo, Italy
Received 31 May 2006; revised 25 July 2006; accepted 25 July 2006
Available online 14 August 2006
Abstract—One-pot, waste-free oxidation of glycerol to 1,3-dihydroxyacetone (DHA) was achieved by simply applying a small elec-
tric potential (1.1 V vs Ag/AgCl) to a glycerol solution in water buffered at pH 9.1 in the presence of 15 mol % TEMPO (2,2,6,6-
tetramethylpiperidine-1-oxyl). Prolonging the reaction time affords comparable amounts of hydroxypyruvic acid.
ꢀ 2006 Elsevier Ltd. All rights reserved.
The selective oxidation of glycerin (glycerol, 1,2,3-pro-
panetriol) to valued oxygenates is an important objec-
tive of current chemical research due to rapidly
increasing surplus of glycerin obtained as a byproduct
(10% wt/wt) in manufacturing of biodiesel fuel.1
in the cosmetics industry as a tanning substance and also
in fungicides.4
In particular, DHA is the main active ingredient in all
sunless tanning skincare preparations, since it is the
most effective sun free tanning additive.
In practice, all the oxygenates obtained by selective oxi-
dation of glycerin are commercially relevant. Unfortu-
nately, however, the extensive functionalisation of the
triol glycerol molecule with hydroxy groups of similar
reactivity renders its selective conversion particularly
difficult.
This skin browning effect is nontoxic, and similar to
the Maillard reaction. DHA reacts with the amino
acid groups, which are part of the protein containing
keratin layer on the skin surface. Various amino acids
react differently to DHA, producing different pigments
(melanoidins) that are similar in colouration to
melanin, the natural substance in the deeper skin
layers which browns or ‘tans’ upon exposure to UV
rays.5
Moreover, glycerol is easily converted to formaldehyde,
formic acid and CO2 and in some cases mass balances as
low as 20% are observed.2 Good selectivities at high
glycerol conversion are difficult to achieve due to rapid
overoxidation, and high selectivities become more diffi-
cult as the compounds become more oxidised.
Its commercial sales are rapidly increasing as concerns
surrounding damage associated with UV tanning
options spurred further popularity of sunless tanning
products as an alternative to UV tanning.
1,3-Dihydroxyacetone (DHA) currently produced from
glycerol by microbial fermentation over Gluconobacter
oxydans,3 is one of such valued oxygenate products. In-
deed, dihydroxyacetone is an important chemical used
Improvements in the DHA manufacturing process,
resulted in products that produced a more natural
looking colour and better fading.
The G. oxydans is strongly product-inhibited, and for
high space-to-time yields and efficient downstream puri-
fication, multiple-stage reactor systems have been
developed.
*
Corresponding author. Tel.: +39 091 680 93 70; fax: +39 091 680 92
47; e-mail addresses: mario.pagliaro@ismn.cnr.it; mario.pagliaro@
gmail.com
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.123

6994
R. Ciriminna et al. / Tetrahedron Letters 47 (2006) 6993–6995
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Clearly, a clean, efficient chemical process capable of
generating DHA from glycerol at lower cost is extremely
desirable.
Now we report a simple electrochemical process capable
of affording DHA in a yield comparable to that of the
fermentation process while furnishing a product of high
purity, and hence meeting an important requirement for
a chemical that finds use in the cosmetics and food
industries.
The process is electrocatalytic and does not require
traditional chemical oxidants (Scheme 1).6
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
By simple application of a low voltage (1.1 V vs Ag/
AgCl) to a solution of glycerol (0.05 M) buffered at
pH 9.1 with bicarbonate (0.2 M) in the presence of
catalytic TEMPO (0.0075 M), glycerol is selectively
converted to DHA at the anode. Prolonging the
reaction time results in the formation of hydroxypyruvic
acid (Scheme 2).
E (V)
Figure 1. Cyclic voltammetry recorded at 0.05 V sÀ1 over a glassy-
carbon electrode (1 cm2) of a solution containing glycerol (0.05 M) and
TEMPO (7.5 · 10À3 M) in the presence of 0.2 M NaHCO3.
Before the oxidation a cyclic voltammetry of the
solution was recorded (Fig. 1, where no redox peak of
TEMPO is observed due to the presence of reductant
glycerol), showing that potentials higher than 0.6 V are
necessary to perform the oxidation.
The reaction system was based on a single undivided cell
containing a glassy-carbon working electrode (1 cm2), a
Pt wire as counter electrode and a reference electrode of
Ag/AgCl.
After 20 h, an optimal 25% yield (calculated by calibra-
tion curves obtained using authentic reagents) of DHA
was obtained.7
The role of NaHCO3 here is to ensure both the required
basic pH (Scheme 1) and, as electrolyte, to enhance con-
ductivity in the electrolysis solution.
As mentioned above, extending the reaction time causes
overoxidation of the DHA and formation of hydroxy-
pyruvic acid (HPA). Hence, after 200 h a considerable
amount of the latter species (35%) was present in solu-
tion along with 30% of DHA.
R-CH2-OH
O
N+
R-CH(OH)2
1 e-
H2O
chemical
step
These results are truly surprising and markedly differ
from those observed in the TEMPO-mediated oxidation
of glycerol with NaOCl as auxiliary oxidant.8 It is well
established, in fact, that at alkaline pH the nitrosonium
ion TEMPO+ is a highly selective oxidant for primary
(vs secondary) alcohols.9
R-CHO
nitrosonium ion
H2O
R-COOH
electrochemical
step
H3O+
2 e-
OH
O
N
N
Accordingly, we reported the selective oxidation of
glycerol to ketomalonic acid mediated by the radical
TEMPO in the presence of catalytic BrÀ and NaOCl
at 2 ꢁC.8
hydroxylamine
electrochemical
step
nitroxyl radical
B-
BH
Scheme 1. General reaction scheme for electrochemical alcohol
oxidation mediated by TEMPO.
It is also surprising that no overoxidation to carb-
oxylic acid is observed even after prolonged reaction
times, showing that at the chosen potential (1.1 V) both
carbonyl products (DHA and HPA) are stable. Indeed,
in water at pH 9.1 hydration of aldehydes is usually fast
and in the presence of a chemical oxidant such as
hypochlorite, formation of the acid is similarly rapid.
O
OH
E (1.1 vs Ag/AgCl)
TEMPO, pH 9.1
HO
HO
OH
OH
1,3-dihydroxy-acetone
At this stage, it is not clear why in the electrochemical
oxidation, glycerol is preferentially oxidised at the sec-
ondary hydroxyl to yield DHA. We make the hypothesis
that the anodic surface of glassy-carbon may indeed
play a role in a concerted reaction mechanism involving
TEMPO. In fact, no reaction is observed in the absence
of added TEMPO.
O
HO
OH
O
hydroxypyruvic acid
Scheme 2. Electrochemical oxidation of glycerol mediated by TEMPO
in water affords DHA and, after longer reaction times, HPA.

R. Ciriminna et al. / Tetrahedron Letters 47 (2006) 6993–6995
6995
3. For a recent interesting study aimed at optimisation of the
biotechnological process, see: Bauer, R.; Hekmat, D.
Biotechnol. Prog. 2006, 22, 278.
Interestingly, similar pronounced alteration in the selec-
tivity of electrocatalytic oxidation of alcohols mediated
by TEMPO has recently been reported.10
4. DHA does not damage the skin as it does not penetrate
beyond the stratum corneum or dead skin surface layer. It
is considered a safe skin colouring agent and nutritional
supplement and has been approved for cosmetic use by
most regulating authorities. See: http://en.wikipedia.org/
wiki/Dihydroxyacetone.
Finally, it is of relevance to this report that the catalyst
TEMPO can be easily recovered (by extraction with
organic solvent) at the end of the reaction and thus
reused.
5. Once the darkening effect has occurred, the tan will not
sweat off or wash away with soap or water. It will fade
gradually over 3–10 days, in conjunction with the skin’s
normal exfoliation process. Exfoliation, prolonged water
submersion, or heavy sweating can lighten the tan, as these
all contribute to rapid dead skin cell exfoliation.
6. Typical experimental conditions of the electrooxidation: a
solution of glycerol (0.05 M) and TEMPO (0.0075 M) in
water (10 mL) kept under stirring in a single cell contain-
ing a glassy-carbon electrode (1 cm2), a Pt wire as counter
electrode and a reference electrode of Ag/AgCl was
electrolysed under a 1.1 potential (vs Ag/AgCl) by using
a CH Instruments (Austin, TX) potentiostat CHI630B
Electrochemical Workstation. The concentration of prod-
ucts was monitored by withdrawing reaction samples of
10 lL. All chemicals were purchased from Aldrich (St.
Louis, MO).
7. The HPLC column system (Alltech IOA 1000 for organic
acids) was connected in series with a UV–vis detector
(Shimadzu) set at 220 nm using 0.005 M H2SO4 as mobile
phase and keeping the oven containing the column at
75 ꢁC. Retention times are: DHA = 22.91 min and
HPA = 16.2 min. No other products could be detected
by HPLC. Both authentic DHA (as dimer) and HPA used
for the calibration curves were from Aldrich.
Facile scale-up can be envisaged by increasing the elec-
trode surface area and by use of a simple cation-
exchange resin membrane to remove (and recycle)
the bicarbonate buffer electrolyte.11 Under these condi-
tions, the high solubility of glycerol in water (1 kg/L at
25 ꢁC) would allow the use of much higher glycerol
concentrations.
In conclusion, a clean one-pot oxidation process capable
of affording reasonable yields of DHA from glycerol has
been discovered that might find soon commercial use as
an alternative to cumbersome biological conversion.
Acknowledgements
M.P. dedicates this work to (Senator) Professor Bruno
Di Maio in memory of a wonderful 1994 electoral
campaign.
References and notes
8. Ciriminna, R.; Pagliaro, M. Adv. Synth. Catal. 2003, 345,
383.
1. This is causing a worldwide shutdown of traditional
glycerin plants and the opening of other plants that use
glycerin as a raw material. McCoy, M. Chem. Eng. News
2006, 84, 7.
9. de Nooy, A. E. J.; Besemer, A. C.; van Bekkum, H.
Synthesis 1996, 1153.
10. Liaigre, D.; Breton, T.; Belgsir, E. M. Electrochem.
Commun. 2005, 7, 312.
2. McMorn, P.; Roberts, G.; Hutchings, J. Catal. Lett. 1999,
63, 193.
11. Scha¨mann, M.; Scha¨fer, H. J. Eur. J. Org. Chem. 2003,
351.