CTAB-assisted sol-microwave method for fast synthesis of mesoporous TiO2 photocatalysts for photocatalytic conversion of glucose to value-added sugars

Article abstract of DOI:10.1016/j.materresbull.2017.08.016

Fabrication technique is an important factor for development of catalysts. Titanium dioxide (TiO₂) is one of efficient photocatalysts. In this work, we firstly report the fabrication of TiO₂ nanoparticles by sol-microwave method with cetyltrimethylammonium bromide (CTAB) surfactant. Absence of surfactant, microwave treatment significantly reduced the cluster sizes of TiO₂, but high aggregations of TiO₂ particles were observed. CTAB has great impact on morphology, cluster size and mesoporous structure of TiO₂. Therefore, surface area of TiO₂ synthesized by sol-microwave method with 0.108 M CTAB increased from 15.97 to 37.60 m²/g. Photocatalytic activity of TiO₂ was tested via the glucose conversion to produce value-added chemicals (gluconic acid, xylitol, arabinose and formic acid). It was found that surface area, mesoporous structure and pore size of TiO₂ are crucial properties for glucose conversion and product distribution. From the reaction test, 0.108 M CTAB/MW-TiO₂ achieved the highest glucose conversion (62.28%).

Full text of DOI:10.1016/j.materresbull.2017.08.016

Accepted Manuscript
Title: CTAB-Assisted Sol-Microwave Method for Fast
Synthesis of Mesoporous TiO Photocatalysts for
2
Photocatalytic Conversion of Glucose to Value-added Sugars
Authors: Jiraporn Payormhorm, Surawut Chuangchote,
Navadol Laosiripojana
PII:
S0025-5408(17)30845-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2017.08.016
MRB 9494
To appear in:
MRB
Received date:
Revised date:
Accepted date:
3-3-2017
8-7-2017
8-8-2017
Please cite this article as: Jiraporn Payormhorm, Surawut Chuangchote,
Navadol Laosiripojana, CTAB-Assisted Sol-Microwave Method for
Fast Synthesis of Mesoporous TiO2 Photocatalysts for Photocatalytic
Conversion of Glucose to Value-added Sugars, Materials Research
Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.08.016
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<AT>CTAB-Assisted Sol-Microwave Method for Fast Synthesis of Mesoporous TiO
2
Photocatalysts for Photocatalytic Conversion of Glucose to Value-added Sugars
a,b
c,d,
<
#
<
AU>Jiraporn Payormhorm , Surawut Chuangchote *
#Email##surawut.chu@kmutt.ac.th ##/Email## Navadol Laosiripojanaa,b
AU>
a
<
AFF> The Joint Graduate School of Energy and Environment, King Mongkut’s University
of Technology Thonburi, 126 Prachauthit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand
b
<
AFF> Centre of Excellence on Energy Technology and Environment, Science and
Technology Postgraduate Education and Research Development Office, Bangkok, Thailand
c
<
AFF> Department of Tool and Materials Engineering, King Mongkut’s University of
Technology Thonburi, 126 Prachauthit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand
d
<
AFF> Nanoscience and Nanotechnology Graduate Program, King Mongkut’s University of
Technology Thonburi, 126 Prachauthit Rd., Bangmod, Tungkru, Bangkok 10140, Thailand
*
<
PA> Corresponding author: Tel.: +66 2 470 9213; Fax: +66 2 872 9080.
<
<
ABS-Head><ABS-HEAD>Graphical abstract
ABS-P>
1
► 2► 3► 4► 5► 6► 7 Hightlight► TiO with different surface areas is obtained
2
from sol-microwave synthesis.► CTAB plays an important role in control of mesoporous
TiO structure.► Synergetic effect of CTAB and microwave results in small and uniform
2
pores in TiO
products.
2
.► Surface area and pore size of TiO affect photocatalytic selectivities of
2
<
ABS-HEAD>Abstract
<
ABS-P>Fabrication technique is an important factor for development of catalysts. Titanium
dioxide (TiO ) is one of efficient photocatalysts. In this work, we firstly report the fabrication
of TiO nanoparticles by sol-microwave method with cetyltrimethylammonium bromide
CTAB) surfactant. Absence of surfactant, microwave treatment significantly reduced the
cluster sizes of TiO , but high aggregations of TiO particles were observed. CTAB has great
impact on morphology, cluster size and mesoporous structure of TiO . Therefore, surface
area of TiO synthesized by sol-microwave method with 0.108 M CTAB increased from
5.97 to 37.60 m /g. Photocatalytic activity of TiO
produce value-added chemicals (gluconic acid, xylitol, arabinose and formic acid). It was
found that surface area, mesoporous structure and pore size of TiO are crucial properties for
glucose conversion and product distribution. From the reaction test, 0.108 M CTAB/MW-
TiO achieved the highest glucose conversion (62.28%).
KWD>Keywords: A. titanium dioxide; A. mesoporous TiO
2
2
(
2
2
2
2
2
1
2
was tested via the glucose conversion to
2
2
<
2
; B. surfactant; B. sol-
microwave method; D. photocatalysis
Category: A. = types of material, B. = preparation and processing, D. = properties and
phenomena)
(
<H1>1. Introduction
Biorefinery process is a biomass conversion which is considered as a sustainable process for
chemical, power and biofuel productions [1]. Green chemical technology is a sustainable
1

technology that focuses on approaches to develop the products, equipment and systems. It
emphasizes on reduction of nonrenewable resource uses and prevention of environmental
problems [2]. The integration of green chemistry into biorefinery has been widely
investigated for value-added chemical productions. Heterogeneous catalyst is one of crucial
materials in chemical productions, because it could be involved in many conversion steps. It
is also easily recovered and reused.
Photocatalysis is one of the promising processes for biorefinery and green chemical synthesis
because it can efficiently perform under solar irradiation, room temperature and mild
conditions. Most of the heterogeneous photocatalysts is solid semiconductors such as TiO
ZnO, NiO, ZrO , SnO , SiC, WO , SrTiO , Ge , g-C , Fe , CdS, ZnS, etc. [2-6].
Photocatalysts have been used in several applications depending on their features. Among
them, titanium dioxide (TiO ) is known as a high potential photocatalyst for green organic
synthesis [7-10]. Photocatalytic conversion of glucose by TiO produces varieties of products
such as gluconic acid, glucaric acid, arabitol, arabinose, erythrose, glyceraldehyde formic
acid, H , CO , etc. [11-13].
Development of catalyst fabrication is a key point of the improvement of morphology and its
catalytic properties. TiO photocatalyst has been fabricated by various methods such as sol-
2
,
2
2
3
3
3
N
4
3
N
4
3
O
4
2
2
2
2
2
gel, hydrothermal, solvothermal, microwave, electrodeposition, sonochemical and
electrospinning methods [14-17]. Sol-gel method (SG) has been widely used as a
conventional way to synthesize TiO
method needs long time and use high temperature [14]. In addition, this fabrication method
increases the average size of TiO particles and causes high aggregation of particles, which
2
. Nevertheless, the formation of particles from this
2
subsequently causes less material surface area. Alternatively, microwave irradiation during
sol-gel process (so called sol-microwave method) has been applied to assist in preparations of
various nanomaterials [18-20]. The major advantages of using microwave are easy technique,
rapid heat transfer, energy-saving method, shorter reaction time and convenient method [21-
2
4]. It can be also considered as a green method for photocatalyst synthesis [20]. The
homogenous heating of microwave leads to uniform nucleation of nanomaterials.
During synthesis, surfactants are used in a large number of inorganic material synthesis as a
stabilizer and template to control particle sizes and crystal growth, and reduce
aggregation/agglomeration. An extensive variety of materials with novel structure can be
obtained from the incorporation of surfactants into the inorganic networks during sol-gel
method [25-27]. Among several surfactants, cetyltrimethylammonium bromide (CTAB),
which is a cationic surfactant, has been widely employed in material synthesis to form
spherical particles and reduce the size of particles. CTAB can reduce the surface tension and
make high dispersion of TiO
Previously, some researchers have also reported the effect of CTAB in conventional sol-gel
and hydrothermal methods on the morphology and photocatalytic activity of TiO
photocatalysts. They reported that CTAB modified the surface area and morphology and
increased the charge separation rate of TiO [28, 29]. From opened literature, there is no work
2
precursor in solution during the sol-gel reaction [28].
2
2
reported on effect of surfactant in sol-microwave method for TiO
of the obtained materials.
2
synthesis and applications
The main objective of this study is to prepare TiO particles composed with anatase and rutile
2
using sol-microwave method with and without the presence of CTAB as the surfactant.
Herein, the synergetic effect of CTAB concentration and microwave irradiation on size,
morphology and crystallization of TiO
2
were investigated. Moreover, the photocatalytic
activity of synthesized TiO for conversion of glucose was tested. Productions of gluconic
2
acid and xylitol (value-added sugars) as well as formation of arabinose and formic acid from
biomass were emphasized in this study. Both xylitol and gluconic acid are called platform
molecules from biomass conversion, which can be further synthesized for other chemicals.
2

They are currently approved for use in foods, pharmaceuticals and oral health products [12,
0].
3
<
H1>2. Experimental
<H2>2.1 Materials and methods
Titanium(IV) butoxide (≥97.0%, Fluka, USA, 17.1446 g), a TiO precursor, was mixed with
2
the same mole of acetylacetone (Sigma-Aldrich, USA) at room temperature to slowdown the
hydrolysis and condensation reactions. Subsequently, CTAB aqueous solution (Ajax
Finechem, Australia, 0-0.108 M, 133 mL) was slowly dropped into the precursor solution.
Isopropyl alcohol (QRëC, New Zealand, 67 mL) was added in the solution with continuous
stirring. For TiO
temperature for 24 h for gel formation. TiO
overnight. On another hand, for TiO fabricated by modified sol-microwave method (MW), it
2
synthesized by sol-gel method (SG), the mixed solution was kept at room
2
particles were dried in an oven at 80 °C
2
was synthesized in the same manner with the conventional sol-gel method, followed by
treatment with microwave irradiation (Whirlpool, 2.45 GHz, 970W) for 4 min. The yellow
precipitants were obtained and dried overnight at 80 °C. As-synthesized TiO
pulverized to powder, then they were finally calcined at 500 °C for 2 h. The samples
modified with CTAB are denoted as ``X CTAB/Y-TiO '', where X refers to the CTAB
2
samples were
2
concentration (0, 0.027, 0.054, 0.081 or 0.108 M) and Y refers to the synthesis method (SG
or MW).
<
H2>2.2 TiO
Surface area of samples was determined according to Brunauer-Emmett-Teller (BET) method
by N adsorption (Belsorp Mini II, Japan) at 77.0 K. The photocatalyst samples were
2
photocatalyst characterizations
2
degassed under vacuum at 110 ºC for 4 h before the analysis. The microstructure of TiO
particles was monitored by scanning electron microscopy (FESEM; JSM-7610F). The
2
crystalline structures of the TiO
2
photocatalysts were measured by X-ray diffraction (XRD;
RIGAKU RINT 2100) with Cu-Kα radiation (λ=0.15418 nm) at 40 kV and 30 mA. The
average crystal sizes were estimated from the line broadening of X-ray diffraction reflections
using Scherrer equation (1):
K
cos
D 
(1)

where D is crystallite size, K is a coefficient (0.94), λ is wavelength of X-ray radiation and β
is full-width at half-maximum intensity of peak, and θ is diffraction angle. The ratio of TiO
2
anatase to rutile phase can be calculated from equation (2):
1
A 
(2)
1
1.26(I / I )
R
A
where I
A
and I
R
are the diffraction intensities of anatase (1 0 1) and rutile (1 1 0) crystalline
phases at 2θ = 25.21° and 27.41°, respectively.
Band gap energies of samples were determined using UV-vis diffuse reflectance with a UV-
100 Shimadzu spectrophotometer equipped with an integrating sphere in the wavelength
range of 200-800 nm.
3
3

<
H2>2.3 Photocatalytic conversion of glucose
Glucose (Carlo Erba Reagents, France) solution (1.0 g/L) was prepared in 10/90 v/v of mixed
solution of water and CH CN (Honeywell Burdick and Jackson, USA). The prepared solution
was transferred in a double-walled borosilicate glass reactor. TiO powder photocatalyst
0.15 g) was suspended in the glucose solution with continuous stirring. The glucose solution
was stirred in dark condition for 30 min for completed adsorption of glucose on TiO surface.
3
2
(
2
A mercury lamp (λmax=365 nm) was employed as a light source for irradiation. Cooling water
was circulated through a cylindrical jacket located around the reactor to control a reaction
temperature at 30 °C. Sample solutions were taken at specific times (15 and 30 min) and were
then filtered through 0.22 µm-nylon filters before analysis. Glucose conversion and product
yield were monitored by a high-performance liquid chromatography (HPLC, Shimudzu, LC-
2
0AD pump) equipped with a refractive index detector (Shimudzu RID-10A). Separation of
converted products was performed via an Aminex HPX-87H column (300 x 7.8 mm) at 65 °C
using 5 mM sulfuric acid as the mobile phase.
<H1>3. Results and discussion
<H2>3.1 Characterizations
<H3>3.1.1 Morphological appearances
Fig. 1 shows FESEM images of clusters of TiO
presence of CTAB during fabrication at different concentrations. For CTAB-absent cases, it
was found that the clusters of TiO particles synthesized by SG and MW consisted of small
particles (diameter = 8-40 nm). Spherical clusters with relatively smoother surface were
2
particles obtained from the absence and
2
observed in TiO
2
2
synthesized by SG, but irregular spherical clusters with rougher surface
samples synthesized by MW. This is because, during MW synthesis, the
appeared in TiO
rotation, friction and collision of solvent molecules (water and isopropyl alcohol) and
titanium(IV) butoxide molecules occurred during nucleation and TiO particle formation
under electric field of microwave which resulted in irregular aggregation of spherical clusters
31]. Obviously, the size of TiO clusters significantly decreased when TiO was synthesized
with MW, because MW inhibited further aggregation of TiO particles due to large amount of
nuclei was generated under increasing temperature in a very short time.
A mechanism of TiO particle formation is illustrated in Fig. 2. Titanium(IV) butoxide
2
[
2
2
2
2
molecules are hydrolyzed with four molecules of water. The hydroxyl terminated titanium is
then formed. Condensation reaction occurs after the hydrolysis step to form polymeric Ti(IV)
hydroxide which is further precipitated to be TiO
the chains of Ti(IV) hydroxide polymers could interact with other chains by inter-chain
hydrogen bonding, so aggregation of TiO particles and TiO clusters can be formed. The
2
particles [32]. In case of without CTAB,
2
2
4

reduction of TiO
2
aggregation occurs by the use of CTAB surfactant because the hydrogen
atoms of hydroxyl groups (-OH groups) in the polymeric Ti(IV) hydroxide can be interacted
+
with (C16
These head groups of micelles are initial sites for crystallizations of TiO
TiO synthesized by MW with CTAB, similar with no-CTAB case mentioned above, the
vibration of Ti(IV) hydroxide polymers and micelles under microwave irradiation caused
more irregular spherical shape of TiO clusters (compared with TiO synthesized by SG).
H33)(CH
3
)
3
N cationic head groups (hydrophilic groups) of CTAB micelles [33, 34].
2
particles. In case of
2
2
2
<H3>3.1.2 Pore type and shape
N
2
adsorption/desorption isotherms of TiO
2
photocatalysts synthesized by SG and MW with
was basically
different concentrations of CTAB are illustrated in Fig. 3. Isotherm of SG-TiO
2
a type III isotherm according to the IUPAC classification because it did not show any
hysteresis loop. This isotherm referred to a non-porous or macroporous (> 50 nm) material
which was obtained from weak interaction of SG-TiO
The isotherm of MW-TiO could be classified as a type V isotherm that normally occurs from
a mesoporous (2-50 nm) material with very weak adsorbent-adsorbate interaction. This shows
2
and N
2
molecules.
2
the benefit of MW synthesis that can prepare mesoporous TiO
significantly shorter time compared with conventional SG.
2
photocatalysts in a
The strong adsorbent-adsorbate interaction was observed in TiO
2
synthesized with CTAB.
They could be classified as type IV isotherms which refer to mesoporous materials. The
appearance of hysteresis loop in the multilayer adsorption range is related to capillary
condensation in mesopore structure [35, 36]. It was observed that the increase of CTAB
concentration from 0.027 to 0.108 M in MW synthesis resulted in increase the relative
pressure of capillary condensation from 0.43 to 0.61. This affects increasing pore size of TiO
clusters which will be discussed in the next section.
2
It is well recognized that the pore shape of synthesized TiO
hysteresis loop of N adsorption/desorption isotherm. Remarkably, the hysteresis loops of all
fabricated TiO photocatalysts in this work were mostly in H2 type which correspond to the
ink-bottle pore or the voids obtained from close-packed spherical particles [36, 37]. In
addition, the small hysteresis loops were also observed in the isotherms of MW-TiO and
.027 M CTAB/MW-TiO (P/P > 0.9). These hysteresis loops derived from a secondary
condensation step of the inter-particle spaces or the porosity of the aggregation of TiO
nanocrystals [38, 39]. This result indicated that MW-TiO and 0.027 M CTAB/MW-TiO
composed of 2 pore shapes (ink-bottle pore and inter-particle space). However, increase of
CTAB concentrations in MW synthesis resulted in only ink-bottle pore in TiO clusters.
2
can be determined with the
2
2
2
0
2
0
2
2
2
2
<H3>3.1.3 Surface areas, pore volumes, and pore size distributions
Surface areas, pore volumes and pore sizes of synthesized TiO
2
are summarized in Table 1. It
2
was found that 0.027 M CTAB/SG-TiO
2
(25.79 m /g) had the surface area larger than SG-
2
TiO
2
(2.12 m /g). Differently, the surface area of 0.027 M CTAB/MW-TiO
2
was similar to
that of MW-TiO
2
. These results were attributed to the different formations of CTAB micelles
at different synthesis temperatures. The critical micelle concentration (CMC) of CTAB at
room temperature was reported at ~0.001 M [40, 41]. In this study, CTAB micelles could
form during SG synthesis at room temperature because CTAB concentration was 0.027 M. In
the case of MW, the structures of micelles are less stable under high temperature of MW [42,
4
3]. This resulted in the increase of CMC of CTAB, therefore the CTAB micelles could not
form during MW synthesis with 0.027 M CTAB. Eventually, the surface area of synthesized
TiO increased with increasing CTAB concentration, because of more stability of micelles. In
this study, the highest surface area of 37.60 m /g was obtained from 0.108 M CTAB/MW-
TiO photocatalyst.
2
2
2
5

Considering pore sizes and pore volumes of synthesized TiO
had the lowest pore volume (0.0282 cm /g) and the largest pore size (pore radius = 26.62
2
, without CTAB cases, SG-TiO
2
3
3
nm). The pore volume of synthesized TiO
the smaller pore size (pore radius = 22.76 nm) was also obtained. It was clearly seen that the
applied microwave irradiation resulted in the increase of pore volume of synthesized TiO but
it affected the reduction of pore sizes of TiO . The pore-size distribution plots obtained from
the Barrett-Joyner-Halenda method (BJH) are shown in Fig. 4. It was found that MW-TiO
illustrates a narrow pore size distribution at pore radius around 1-6 nm and a wide pore size
distribution at pore radius around 33-46 nm. Therefore, MW-TiO showed the non-uniform
pore size in TiO clusters. Smaller pore sizes were found in all TiO synthesized with CTAB
compared with the absent CTAB cases. The pore sizes of synthesized TiO decreased with
increasing the CTAB concentrations. From pore size distribution results, it was found that
TiO had the narrow ranges of the distributions which indicated the presence of the uniform
pore size in TiO clusters. The increase of CTAB concentration to 0.108 M in MW synthesis
2
increased to 0.1817 cm /g with MW synthesis and
2
2
2
2
2
2
2
2
2
resulted in the most uniform pore size at pore radius of 8.0 nm.
.
From all results in this section, it can be concluded that microwave irradiation significantly
affected the increasing surface area and pore volume of TiO photocatalyst. High pore size
2
distribution, however, was found with microwave irradiation. CTAB and increase of CTAB
concentration in MW enhanced the surface area and the uniform of pore size in mesoporous
TiO
2
.
<H3>3.1.4 Structure of TiO
2
X-Ray diffraction patterns of synthesized TiO
2
photocatalysts are shown in Fig.5. The prepared TiO
2
photocatalysts (except MW-TiO ) had the mixed phase of anatase and rutile. The high crystallinity of
2
anatase exhibits at 25.2°, 36.8°, 37.8°, 38.1°, 47.8°, 53.8° and 55.2° in diffraction lines which
represent the indices of (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5) and (2 1 1) planes,
respectively. The small peaks of rutile also observed at 27.41° and 36.11° which correspond to (1 1 0)
and (1 0 1) planes.
.
Table 2 reveals the approximated crystallite sizes and phase compositions of TiO
CTAB in synthesis, the anatase crystallite size of MW-TiO (26.5 nm) was smaller than that of SG-
TiO (30.3 nm). The reduction of anatase crystallite size was attributed to the rapid heating of MW
2
. In case of without
2
2
which induced faster crystallization, the anatase crystal growth was then restricted [44-46].
Effect of CTAB concentration on crystallite size was also investigated. It was found that the use of
CTAB in SG could reduce crystallite sizes of anatase and rutile. On the other hand, the presence of
CTAB in MW leaded to increase of anatase crystallite size. The largest crystallite size of anatase
(
84.8 nm) was obtained from 0.027 M CTAB/MW-TiO . However, the increase of CTAB
2
concentration reduced the sizes of anatase crystals whereas it increased the sizes of rutile crystals.
This is because high amount of CTAB molecules suppressed the anatase formation during synthesis
[
33]. Considering phase composition, the results indicate that the increase of CTAB concentration did
not affect the crystal phase compositions of synthesized TiO
TEM images of grinded and dispersed TiO samples (SG-TiO
and 0.027 M CTAB/MW-TiO ) are illustrated in Fig. 6 to show the TiO
TiO had various sizes of crystals in their particles. Small particle sizes of samples synthesized by
2
.
2
2
, 0.027 M CTAB/SG-TiO
2
, MW-TiO ,
2
2
2
crystal sizes. Synthesized
2
MW caused hardly broken aggregation of crystals after TEM sample preparation, leading relatively
hard investigation of crystallite size in some TEM images. Anyhow, crystallite sizes observed from
TEM analysis was in line with the crystallite size obtained from XRD analysis.
6

UV-vis absorption spectra of synthesized TiO
MW-TiO (blue line) exhibits the relatively lower photo-absorption in UV region than SG-
TiO (black dashed line). Similarly, 0.027 M CTAB/MW-TiO (red line) displays the
adsorption spectrum shifted to shorter wavelength, compared with 0.027 M CTAB/SG-TiO
black line). It is clearly seen that the applied microwave irradiation in synthesis impacted on
decrease of UV adsorption of TiO . All TiO synthesized with higher CTAB concentrations
had relatively similar absorption edges.
Table 2 shows band gaps of synthesized TiO
adsorption spectra. SG-TiO had a narrower band gap than MW-TiO
CTAB/SG-TiO had the band gap narrower than 0.027 M CTAB/MW-TiO
of all TiO synthesized with CTAB were in the range of 3.03-3.11 eV. Therefore, the
different CTAB concentrations in MW synthesis did not significantly affect the band gaps of
TiO
H2>3.2 Photocatalytic conversion of glucose
Glucose was converted to gluconic acid and xylitol via photocatalysis with synthesized TiO
2
are shown in Fig. 7. For CTAB-absent cases,
2
2
2
2
(
2
2
2
calculated from adsorption edges of UV-vis
. Similarly, 0.027 M
. The band gaps
2
2
2
2
2
2
.
<
2
.
Arabinose and formic acid were also found in the product solution. The possible reaction pathway is
as follows. Firstly, glucose is photo-oxidized to gluconic acid with hydroxyl radicals (•OH) obtained
from photo-oxidation of H
2
O molecules by hole at valence band of TiO [47]. The gluconic acid is
2
then continuously converted to be arabinose and formic acid via photocatalytic decarboxylation or α-
scission of glucose [13]. Moreover, the glucose is possibly oxidized to be glucuronic acid which is
continuously converted to xylose and formic acid by decarboxylation [48]. Xylose is then converted
to xylitol via photocatalytic reduction [49]. However, glucuronic acid and xylose (the intermediate
molecules) were not observed in this work.
The photocatalytic conversions of glucose with TiO
MW with different concentrations of CTAB are shown in Fig. 8. It was found that MW-TiO
provided higher photocatalytic activity than SG-TiO , because it had higher surface area than
SG-TiO . Moreover, the presence of CTAB in all synthesis methods improved the
photocatalytic activity of TiO . However, 0.027 M CTAB/MW-TiO gave lower conversion
of glucose than 0.027 M CTAB/SG-TiO . This results related to lower surface area and
aggregation of 0.027 M CTAB/MW-TiO as discussed above. These indicate that increase of
CTAB concentration in MW synthesis gradually enhanced the conversion of glucose because
of increasing surface area of synthesized TiO . The highest photocatalytic conversion of
glucose in 120 min (62.28%) obtained from the use of 0.108 M CTAB/MW-TiO as the
photocatalyst. Notably, the surface area of MW-TiO and 0.027 M CTAB/MW-TiO were
similar, but the photocatalytic conversion of glucose of MW-TiO
CTAB/MW-TiO . This could be explained by weak glucose-TiO
pore of MW-TiO which reduced the adsorption of glucose on TiO
this result could be explained by relatively higher composition of rutile phase (2.7%) in
structure of 0.027 M CTAB/MW-TiO compared with MW-TiO (rutile 0%) that could
increase photo-oxidation efficiency. Likewise, higher ratio of rutile phase in other
synthesized TiO tended to provide higher conversion of glucose. Rutile phase has narrower
band gap (3.0 eV) than anatase phase (3.2 eV), therefore the increasing composition of rutile
phase in TiO structure can enhance the light absorption of TiO [50] (see also Fig. 7).
2
photocatalysts synthesized by SG and
2
2
2
2
2
2
2
2
2
2
2
2
was lower than 0.027 M
interaction in the large
2
2
2
2
surface. Additionally,
2
2
2
2
2
Moreover, the presence of both phases of anatase and rutile in structure of TiO
better electron-hole separation.
2
assists in
The yields of products obtained from photocatalytic conversion of glucose at 120 min are
shown in Fig. 9. SG-TiO provided lower yields of all products compared with MW-TiO
which corresponded to lower conversion of glucose. From all cases of photocatalytic
2
2
,
conversions of glucose, it was clearly observed that the yields of arabinose and formic acid
related to glucose conversion. Oppositely, the yields of gluconic acid relatively decreased
with increasing conversion of glucose. This was attributed to high surface area of TiO which
2
7

enhanced the conversion of gluconic acid to arabinose and formic acid in long time reaction.
In this work, the highest yield of gluconic acid (7.83%) at 120 min was obtained from 0.054
M CTAB/MW-TiO
concentration of CTAB in MW. Obviously, the highest yield of xylitol (4.72%) at 120 min
derived from the use of 0.027 M CTAB/SG-TiO in photocatalytic reaction. This is because
the fact that the small pore radius (6.99 nm) and small pore volume (0.0901 cm /g) of 0.027
M CTAB/SG-TiO influenced the increase of selectivity of xylitol by the reduction of re-
adsorption of xylitol in pore of TiO photocatalyst.
2
. Regarding the yield of xylitol, it slightly increased with increasing
2
3
2
2
<H1>4. Conclusions
Development of synthesis of TiO
microwave method. The microwave irradiation was applied during synthesis to increase the
surface area and pore volume of TiO . It also provided the small pore size and small clusters
of TiO . In the presence of CTAB during MW synthesis, CTAB micelles decreased the
aggregation of TiO particles, eventually the mesoporous TiO photocatalysts with uniform
pore size were obtained. The increase of adsorption surface area of TiO evidently related to
2
photocatalysts was succeeded via a surfactant-assisted sol-
2
2
2
2
2
the enhancement of photocatalytic conversion of glucose. Two value-added sugars, i.e.
gluconic acid and xylitol, were produced from photocatalytic reaction. Arabinose and formic
acid were also produced from this reaction. The decrease of gluconic acid can be observed in
the high surface area TiO
2
case, because of further conversion of gluconic acid to arabinose
and formic acid. Interestingly, the small pore size and pore volume of mesoporous TiO
2
affected the selectivity of xylitol production. Therefore, future development of adsorption site
of photocatalyst should be considered to promote the higher conversion of glucose and
selectivity of product.
<ACK>Acknowledgements
The authors would like to acknowledge the Royal Golden Jubilee Ph.D. Program (Grant No.
PHD/0110/2556), Thailand Research Fund (TRF), for financial supports. This work was also
supported in parts by TRF (TRG5780129 and RTA598006), National Research Council of
Thailand (Project 169927), and Japan Science and Technology Agency (JST) [Collaboration
Hubs for International Research Program (CHIRP) within the framework of the Strategic
International Collaborative Research Program (SICORP)]. The authors also wish to express
their thanks to Department of Materials and Metallurgical Engineering, Faculty of
Engineering, Rajamangala University of Technology Thanyaburi for providing
characterization instruments.
<
<
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</BIBL>
<
Figure>Fig. 1 FESEM images of TiO samples synthesized by SG and MW with different
2
concentrations of CTAB.
<
Figure>Fig. 2
Figure>Fig. 3
Schematic illustration of TiO synthesis with/without CTAB.
2
<
N
2
adsorption/desorption isotherms of TiO photocatalysts synthesized
2
by SG and MW with different concentrations of CTAB.
1
1

<Figure>Fig. 4
Pore size distributions of TiO photocatalysts synthesized by SG and
2
MW with different concentrations of CTAB
<Figure>Fig. 5X-ray diffraction patterns of TiO
2
photocatalysts synthesized by SG and MW
with different concentrations of CTAB
<
Figure>Fig. 6 TEM images of grinded and dispersed TiO
2
samples synthesized by SG and
MW with and without 0.027 M CTAB.
<Figure>Fig. 7 UV-vis diffuse reflectance spectra of TiO
2
photocatalysts synthesized by SG
and MW with different concentrations of CTAB.
<Figure>Fig. 8 Photocatalytic conversions of glucose with TiO
2
photocatalysts synthesized
by SG and MW with different concentrations of CTAB.
<
Figure>Fig. 9 Yields of products obtained from photocatalytic conversions of glucose at
20 min with TiO photocatalysts synthesized by SG and MW with different concentrations
of CTAB (0-0.108 M).
1
2
<Table>Table 1 Effect of CTAB concentration on BET surface area, average pore size and
pore volume of synthesized TiO
2
photocatalysts.
CTAB
concentration
S
BET
2
r
p
V
m
3
Photocatalyst
(
m /g)
(nm)
(cm /g)
(M)
-
2.12
26.62
6.99
0.0282
0.0901
0.1817
0.1105
0.1000
0.1011
SG-TiO
2
0
.027
-
25.79
15.97
15.60
31.53
28.97
22.76
14.17
6.35
0
0
0
.027
.054
.081
MW-TiO
2
6.98
1
2

0
.108
37.60
7.86
0.1477
<Table>Table 2 Effect of CTAB concentration on crystallite size, crystal phase composition
and energy gap of fabricated TiO photocatalysts.
2
XRD
Crystallite size (nm)
CTAB
concentratio
n (M)
Photocataly
Crystal phase (%)
Anatase Rutile
E
gap (ev)
st
Anatase
101)
Rutile
(110)
(
-
30.3
26.5
26.5
84.8
38.6
128.0
81.4
-
89.7
94.0
100
10.3
6.0
-
3.00
3.03
3.13
3.11
3.03
3.06
3.08
SG-TiO
2
0
0
.027
-
.027
90.0
89.8
97.2
96.7
85.3
93.3
2.8
3.3
14.7
6.7
MW-TiO
2
0.054
0
0
.081
.108
30.2
26.5
131.6
135.0
TDENDOFDOCTD
1
3