Selective photocatalytic conversion of glycerol to hydroxyacetaldehyde in aqueous solution on facet tuned TiO2-based catalysts

Article abstract of DOI:10.1039/c3cc46515b

Glycerol is selectively converted to hydroxyacetaldehyde (HAA) and H ₂ in aqueous solution on TiO₂-based photocatalysts. The product selectivity was verified to be strongly dependent on the facets of TiO₂. Rutile with high percentage of {110} facets results in over 90% superior selectivity of HAA, while anatase with {001} or {101} facets gives only 16% and 49% selectivity for HAA, respectively.

Full text of DOI:10.1039/c3cc46515b

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Selective photocatalytic conversion of glycerol to
hydroxyacetaldehyde in aqueous solution on facet
Cite this: Chem. Commun., 2014,
0, 165
tuned TiO -based catalysts†
5
2
Received 26th August 2013,
Accepted 25th October 2013
ab
a
a
a
a
a
Ruifeng Chong,‡ Jun Li,‡ Xin Zhou, Yi Ma, Jingxiu Yang, Lei Huang,
a
a
a
Hongxian Han, Fuxiang Zhang and Can Li*
DOI: 10.1039/c3cc46515b
www.rsc.org/chemcomm
Glycerol is selectively converted to hydroxyacetaldehyde (HAA) and tailor-designing the surface structure of TiO to avoid overoxidation
2
8
H
2
in aqueous solution on TiO
selectivity was verified to be strongly dependent on the facets of
TiO . Rutile with high percentage of {110} facets results in over 90% polyols. Overall, it was found that aqueous glycerol can be converted
superior selectivity of HAA, while anatase with {001} or {101} facets to HAA, H and HCOOH through C–C bond cleavage on TiO -based
gives only 16% and 49% selectivity for HAA, respectively. photocatalysts under anaerobic conditions. However, the selectivity
of HAA production is strongly dependent on the dominant facets of
Biomass is an abundant and renewable resource in nature. The TiO . Rutile with a high percentage of {110} facets results in over
2
-based photocatalysts. The product of the products.
Herein, glycerol was chosen as a model compound of higher
2
2
2
2
utilization of biomass as feedstock to produce fuels and valuable 90% selectivity of HAA, while anatase with dominant {001} or {101}
chemicals is promising for the development of sustainable facets can only give a selectivity of HAA less than 20% and 50%,
1
chemical industry. Various kinds of attractive platform chemicals, respectively. More interestingly, EPR results show that a kind of
such as glucose, hydroxymethylfurfural (HMF) and polyols, have peroxo species was generated on rutile with {110} facets, which is
2
ꢀ
been obtained from biomass. Among these, the biomass-derived quite different from the reactive oxygen species OH generated on
polyols are particularly interesting, because they can be further anatase with dominant {001} or {101} facets, implying that different
converted into valuable chemicals, such as ethylene glycol and glycerol oxidation mechanisms result in dramatically different pro-
propylene glycol, etc., which are important precursors for pro- duct selectivity. Thus, this work provides a new strategy for selective
3
ducing polymers, resins and fuels.
conversion of biomass to valuable chemicals and H2.
The TiO photocatalysts with different dominant facets, including
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Photocatalysis is an attractive methodology for biomass con-
2
version, as it can use solar energy as the energy source. TiO is a anatase with {001} and {101} (denoted as A{001} and A{101},
2
frequently employed model semiconductor photocatalyst for explor- respectively) and rutile with {110} (denoted as R{110}), were synthe-
ing new reactions and understanding the fundamental processes of sized as shown in the ESI.† TEM and high-resolution TEM images
4
photocatalysis. And it has been already demonstrated that H
2
can (Fig. 1b–g) show that the morphology of A{001} is sheet-shaped,
2
-based photocatalysts under A{101} is bipyramid-like shaped, and R{110} is rod-like shaped along
be produced from polyols on TiO
anaerobic conditions. Partial photocatalytic oxidation of glycerol the [001] direction. The percentage of the exposed {001} facets in
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to valuable carbonyl compounds on TiO has also been reported, A{001} is estimated to be ca. 80%, the {101} facets in A{101} is
2
6
but the total selectivity was less than 20% at a conversion of 35%.
ca. 90%, and the {110} facets in R{110} is ca. 96%. Their XRD and
The low selectivity is mainly due to the existence of different active XPS are provided in the ESI† (Fig. S1 and S2).
sites on the catalyst surface for different reactions and the strong
The photocatalytic reactions of aqueous glycerol conversion were
loaded with some metal nanoparticles by an
ꢀ
reactive oxygen species ( OH) generated during the course of performed on TiO
2
7
photocatalysis. Therefore, it is expected that the selectivity of the in situ photodeposition method. Preliminary results show that no
photocatalytic conversion of glycerol can be further improved by reaction takes place when light or catalyst is absent (Table S1, ESI†).
Fig. 1a shows the selectivity of HAA production (Fig. S3 and S4, ESI†)
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy,
at a glycerol conversion of ca. 20%. The yields of the products from
glycerol are listed in Table S2 (ESI†). It can be seen that the selectivity
of HAA is remarkably varied with the variation of the dominant
facets of TiO . With Rh as the reduction metal cocatalyst, R{110}
2
exhibits the highest HAA selectivity of up to 96%, which is 2 times
that of A{101} and 6 times that of A{001}. This demonstrates that the
4
57 Zhongshan Road, Dalian, 116023, China. E-mail: canli@dicp.ac.cn;
Web: http://www.canli.dicp.ac.cn; Fax: +86-411-84694447
b
Graduate University of Chinese Academy of Sciences, Beijing, 100049, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc46515b
These authors contributed equally to this work.
†
‡
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Scheme 1 The possible reaction pathways of photocatalytic conversion
2
of glycerol in aqueous solution on TiO .
2
Fig. 1 Photocatalytic performances, TEM and HRTEM images of TiO with
different dominant facets. (a) The conversion of glycerol and the selecti-
vities of HAA on different TiO -based photocatalysts. And TEM and HRTEM
images of A{001} (b and e), A{101} (c and f), R{110} (d and g). The images
clearly show the size and morphology of TiO . Fourier transform (FFT)
2
2
patterns show that A{001} and A{101} are in a pure anatase phase and
R{110} is in a pure rutile phase. Reaction conditions for (a) catalyst, 0.1 g;
À1
co-catalyst, 0.1 wt% Rh; 0.02 mol L aqueous glycerol solution, 100 mL;
light source, Xe-lamp (300 W); reaction temperature, 353 K. Reaction time,
for A{001} and A {101}, 4 h; R{110}, 2 h.
2 2
Fig. 2 The initial H production rates of glycerol, HAA, FA on TiO catalysts
with different dominant exposed facets. Reaction conditions: catalyst, 0.1 g;
À1
Co-catalyst, 0.1 wt% Rh; 0.1 mol L aqueous reactant solution, 100 mL; light
source, Xe-lamp (300 W); reaction temperature, 353 K; reaction time, 1 h.
selectivity of HAA appears to be determined by the exposed facets of
TiO
other metals (Pt, Pd, Ru, Au, Cu, and Ni) loaded on R{110} were also as the reagent, Table S4, ESI†). And FA can be further decom-
tested (Table S3, ESI†). In all cases, when a metal cocatalyst is loaded posed to CO and H
on R{110}, the glycerol conversion efficiency is greatly improved and It is possible to hypothesize that the reactivity of glycerol, HAA and
the yield of H2 production is also significantly increased. The FA on the different facets of TiO would be different. To verify this
2
. To exclude the influence of Rh nanoparticles on the selectivity,
2
2
.
2
2
selectivities of HAA are over 90% for all of the photocatalysts though hypothesis, the initial H production rates obtained from aqueous
loaded with different cocatalysts, while the selectivities of glycer- solution of glycerol, HAA and FA were investigated on A{001}, A{101}
aldehyde (GA) are lower than 5%. This suggests that the main roles and R{110}, respectively. As shown in Fig. 2, for the same catalyst, the
of the metal cocatalysts are to enhance the photogenerated charge
separation, and catalyze H production. In other words, the selecti- reactant. For example, the reactivity of glycerol is more than twice that
vity of HAA is mainly dependent on the facets of TiO
deposited metal cocatalysts.
2
H production activity is closely dependent on the structure of the
2
2
, not on the of FA and HAA on R{110}, which might be due to the distinct
difference in interactions between these molecules on the active sites
For R{110} loaded with different metal cocatalysts, the main of TiO . As for a given reactant, the reactivity is mainly determined by
2
products are H
as formic acid (FA), CO
ratio of H /HAA is about 2 and that of CO
all cases, indicating that majority of the glycerol is converted results indicate that the prominent reaction is different on the surface
into the products of HAA and H . Based on the above results, of TiO . Therefore, the reaction of glycerol can be promoted and the
2
and HAA, together with some byproducts such the exposed facets of TiO
, and GA (Table S3, ESI†). The molar pared to glycerol and FA on A{001}, FA shows the highest reactivity on
/H is below 0.07 in A{101}, and glycerol is the most reactive species on R{110}. These
2
. HAA exhibits the highest reactivity com-
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2
2
2
2
2
the possible photocatalytic reaction pathways can be drawn as successive reaction of HAA can be suppressed on R{110}. This is
shown in Scheme 1. Under photoirradiation, the major reaction crucial for the high selectivity of HAA production.
pathway is the reaction of glycerol with water to produce HAA,
In the reaction, water serves not only as a solvent but also as a
FA and H , while the minor reaction is the direct dehydrogena- main reactant. It plays a key role in the reaction of glycerol and the
2
tion of glycerol to GA. Once HAA and FA are formed, the successive reaction of HAA (Scheme 1). The reactive oxygen species
consecutive reactions can take place. HAA can be subsequently are mainly derived from the oxidation of water by a photo-
converted to formaldehyde, FA, and CO (verified by using HAA generated hole on the TiO surface. The EPR spin-trap technique
2 2
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Fig. S7 (ESI†), with the increase of the reaction time, the conver-
sion rate distinctly increases, while the selectivity of HAA slightly
decreases. Nevertheless, the selectivity still remains over 90% even
when the glycerol conversion reaches 40%. This demonstrates
that the photocatalytic conversion strategy is a viable means for
converting glycerol into valuable chemicals, such as HAA. The
intermediate HAA can be easily hydrogenated at room tempera-
ture using a commercial Ru/C catalyst to value-added chemicals,
such as ethylene glycol, which is an important chemical currently
produced in a large scale mainly from hydration of epoxide.
In conclusion, we found that photocatalytic conversion of glycerol
in aqueous solution can selectively produce HAA and H2 via a
photocatalytic reaction under anaerobic conditions. The selectivity
of HAA can be over 90% for a rutile-based catalyst. The selectivity was
Fig. 3 Typical EPR spectra of DMPO trapped radical species upon photo-
2
verified to be dependent on the facets of TiO . The reactive oxygen
catalytic oxidation of H
2 2
O on TiO -based catalysts with different dominant
À1 À1
species derived from H O plays a key role in selective conversion of
exposed facets. Concentrations: TiO
2
, 0.1 g L ; DMPO, 0.05 mol L ,
2
in argon. The characteristic quarter peaks for A{001} and A{101} are ascribed glycerol. The present work provides a new strategy for the conversion
to the DMPO-OH adduct, and the typical seven-line paramagnetic signal for
of bio-derived platform molecules to highly valuable chemicals.
R{110} is assigned to 5,5-dimethyl-1- pyrrolidone-2-oxyl (DMPO–X).
This work was financially supported by the National Natural
Science Foundation of China (NSFC Grant No. 21090341 and
(with DMPO as a trapping reagent) was employed to probe the 21061140361) and the Ministry of Science and Technology of
reactive oxygen species. As shown in Fig. 3, four characteristic peaks China (MOST Grant No.2014CB239400).
10
of DMPO–OH were obviously observed for A{001} and A{101}.
Notes and references
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(
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Chem. Commun., 2014, 50, 165--167 | 167