Received: August 28, 2013 | Accepted: October 11, 2013 | Web Released: January 5, 2014
CL-130798
Comparative Study on the Photocatalytic Oxidation of Glycerol Using ZnO and TiO2
Natanael A. Hermes, André Corsetti, and Marla A. Lansarin*
Department of Chemical Engineering, Federal University of Rio Grande do Sul, Rua Luiz Englert s/N°,
90040-040, Porto Alegre, RS, Brazil
(
E-mail: marla@enq.ufrgs.br)
Herein, we have compared the behavior of ZnO and TiO in
30 mL, consisting of an aqueous solution of 1 mM glycerol
2
¹
1
¹2
the photocatalytic oxidation of glycerol through detection of the
main products. ZnO showed higher selectivity for the products
where the carbon chain was preserved, such as glyceraldehyde
with 1 g L catalyst. UV irradiation (3 mW cm , 365 nm) was
performed by an adapted Hg lamp (Philips, 125 W). The reaction
medium was aerated using a 15-W compressor, magnetically
stirred, and maintained at 30 °C. The period for adsorption
equilibrium was 30 min. The samples (1 mL) were centrifuged
and filtered to remove the catalyst.
(
GAD) and dihydroxyacetone (DHA). TiO2 generated more
products from the cleavage of the glycerol molecule, such as
formaldehyde (FORM) and glycolaldehyde (GCOL). ZnO
yielded about 16 times more GAD and 2.5 times more DHA
than did TiO2.
Identification and quantification of the products in the
reaction samples were carried out by high-performance liquid
chromatography (HPLC) using a Perkin-Elmer Series 200
chromatograph, using an ion-exchange column (Phenomenex
Rezex RHM, 300 © 7.8 mm) at 40 °C. The mobile phase was
ultrapurified water only (18.2 M³ cm), and the flow rate was
Glycerol is the main by-product of the biodiesel production
process. Typically, for each ton of biodiesel, 100 kg of glycerol
1
¹1
is obtained as the by-product. As the role of biodiesel has
0.5 mL min . Detection was performed by a refractive index
progressively gained importance in the world energy matrix,
there has also been a significant increase in the glycerol supply,
causing a saturation of this product on the market, thus
detector. The areas of partially overlapped peaks were deter-
mined by peak deconvolution using the software Fityk, version
0.9.8.
2
13
necessitating efforts to find new applications for glycerol. Thus,
the chemical transformation of glycerol into organic compounds
with higher added value is an alternative that comes up
naturally.
Figure 1 shows a comparison between the chromatograms
from the reaction runs using ZnO and TiO2. It is clear that ZnO
and TiO2 are selective to different products. ZnO exhibited
higher selectivity to primary oxidation products such as DHA
Oxidation is an attractive process for glycerol chemical
transformation because it can generate a large variety of
products, most notably slightly oxidized products. The main
routes to glycerol oxidation are catalytic oxidation and micro-
biological processes. The former is still at laboratory scale and
involves the use of noble metal catalysts such as Au, Pt, and
3
,4
Pd,
which are fairly expensive. Commercial applications
already exist for the latter, mainly for the production of
dihydroxyacetone (DHA),5 but it is a process with high
operating costs and relatively low productivity.6 Further, the
increased concentration of this product causes inhibition of
,7
8
bacterial growth.
The oxidation of glycerol by photocatalysis has recently
emerged as a potential alternative to these processes, and it is
worthy of investigation. However, in the scientific literature,
information on this subject is very scarce, since no more than
five papers have been found.6
,912
Moreover, none of these have
discussed the performance of ZnO in this reaction.
Therefore, the objective of this work was to fulfill part of the
gap on this subject, investigating the behavior of ZnO in the
photocatalytic oxidation of glycerol (POG) by identifying the
main products and their selectivity and comparing these results
with those obtained with TiO2.
The catalysts used in this work were commercial samples of
ZnO (Merck) and TiO2 (P25-Degussa). The glycerol used in the
photocatalytic tests was of analytical grade, and the water was
ultrapurified. Product identification was performed by compar-
ing commercial samples.
Retention time/min
The POG experiments were carried out in a batch slurry
reactor placed in a water-cooled bath. The reaction volume was
Figure 1. Chromatograms of photocatalytic oxidation glycerol
runs (2 h).
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