Format And Type Fonts


 

 CCHHEEMMIICCAALL  EENNGGIINNEEEERRIINNGG  TTRRAANNSSAACCTTIIOONNSS  
 

VOL. 39, 2014 

A publication of 

 
The Italian Association 

of Chemical Engineering 

www.aidic.it/cet 
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong  

Copyright © 2014, AIDIC Servizi S.r.l., 

ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI: 10.3303/CET1439095 

 

Please cite this article as: Vaiano V., Sannino D., Ciambelli P., 2014, Sustainable gas phase selective photocatalytic 

oxidation of cyclohexane on MoOx/TiO2/SiO2 catalysts, Chemical Engineering Transactions, 39, 565-570  

DOI:10.3303/CET1439095 

565 

Sustainable Gas Phase Selective Photocatalytic Oxidation 

of Cyclohexane on MoOx/TiO2/SiO2 catalysts 

Vincenzo Vaiano*, Diana Sannino, Paolo Ciambelli 

University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, 132, 84084, Fisciano (SA), Italy. 

vvaiano@unisa.it 

Gas phase selective photocatalytic oxidation of cyclohexane was studied in a two dimensional fluidized 

bed reactor on MoOx catalysts supported on grafted TiO2/SiO2. The influence of sulphate and Mo loading 

was evaluated. Photocatalytic tests showed that benzene was the main reaction product formed during 

irradiation with together CO2 and cyclohexene as by products. 

Depending on the TiO2, Mo and sulphate loading, an enhancement of the activity and catalysts stability 

occurred during irradiation. MoOx/TiO2/SiO2 samples showed a higher photocatalytic activity with respect 

to MoOx/TiO2-Al2O3. This behaviour was attributed to the better transmission of UV light through SiO2 than 

through Al2O3 and the lower absorption of the radiation. 

1. Introduction 

Heterogeneous photocatalysis can be an effective alternative to removal bacteria (Rizzo et al., 2014) and 

organic pollutants such as methyl-ethyl ketone (Hajaghazadeh et al., 2014), cyclohexane (Murcia et al., 

2013), methylene blue (Sannino et al., 2013a) and NOx (Sannino et al., 2013b) or obtain partial oxidation 

products in mild conditions such as styrene (Sannino et al., 2013c), acetaldehyde (Sannino et al., 2013d) 

and cyclohexanol or cyclohexanone (Sannino et al., 2013e). Ciambelli et al. showed that cyclohexane is 

selectively photoxidised on MoOx/TiO2 catalysts in the presence of gaseous oxygen at 35 °C under UV 

illumination in gas-solid fixed bed reactor (Ciambelli et al., 2008). Significant enhancement of catalytic 

activity was achieved with a fluidized bed photoreactor (Palma et al., 2010).  

Titania is typically a solid of group C in the particle distribution proposed by Geldart (1972). This type of 

particles exhibit cohesive tendencies and as the gas flow is further increased, bubbling regime is 

developed and the photoreactor acts as a “ratholing”; the gas opens bypass channels that extend from the 

gas distributor to the upper edge of the fluidized bed. If channels are not formed, the whole bed expands 

and lifts up as a piston. At higher velocities or with mechanical agitation or vibration, this type of particle 

will be fluidized but with the appearance of clumps or large clusters of particles. These phenomenons play 

a negative role on particle light exposition. 

For testing Mo-titania catalyst, in order to obtain good fluidization, physical mixtures with -Al2O3 was used 

to enhance the catalyst fluidizability (Ciambelli et al., 2007), since Al2O3 is a solid of group A. When gas 

is passed upward through a bed of particles of groups A, B, or D, friction causes a pressure drop and the 

fluidized bed is expanded. As the gas velocity is increased, the pressure drop increases until it equals the 

weight of the bed divided by the cross-sectional area. This velocity is called minimum fluidizing velocity, 

Umf. When this point is reached, the bed of group A particles will expand uniformly until at some higher 

velocity gas bubbles will form (minimum bubbling velocity, Umb). This process can avoid the appearance of 

clumps or clusters of titania particles and improve light exposition. 

As alternative, mixed oxides, such as TiO2-Al2O3, are useful supports for Mo-species, (Palma et al., 2010). 

TiO2-Al2O3 supports for MoOx species showed good fluidization properties and higher cyclohexane 

conversion in presence of UV irradiation (Palma et al., 2010). To further increase the performance of the 

gas-phase photocatalytic fluidized bed reactor, TiO2 based catalysts could be directly supported on 

material transparent to ultraviolet radiation. Interesting examples are TiO2/SiO2 catalysts that are 

considered advanced support materials able to support pure TiO2 (Sannino et al., 2013f). 



 
566 

 
In this work gas phase photocatalytic selective oxidation of cyclohexane was studied with MoOx catalysts 

supported on grafted TiO2 on SiO2. The influence of TiO2 sulphate and Mo loading was evaluated. 

2. Experimental 

2.1 Catalysts preparation and characterization 

Supports of silica coated with TiO2 was prepared in a grafting procedure, starting from titanium 

isopropoxide dissolved in toluene and a commercial silica followed by calcination at 500 °C for 8 h 

(Cozzolino et al., 2007). TiO2/SiO2 samples were added to an aqueous solution of ammonium sulphate, 

dried at 120 °C and calcined in air at 300 °C for 3 h. A second impregnation with an aqueous solution of 

ammonium heptamolybdate was performed, followed by calcination at 400 °C for 3 h to obtain 

MoOx/TiO2/SiO2 catalysts. Catalysts were characterized with different techniques. Specific surface area 

was measured by N2 adsorption measurement at -196 °C with a Costech Sorptometer 1040 after 

pretreatment at 150 °C for 30 min in He flow (99.9990 %).  

Laser Raman spectra were obtained at room temperature with a Dispersive MicroRaman (Invia, 

Renishaw), equipped with 633 nm diode-laser, in the range 100-2,500 cm
-1

 Raman shift. 

Thermogravimetric analysis (TG-MS) of powder samples was carried out in air flow with a thermoanalyzer 

(Q600, TA) in the range 20–1,100 °C at 10 °C/min heating rate. 

UV-Vis reflectance spectra were recorded with a Perkin Elmer spectrometer Lambda 35. Equivalent band 

gap determinations were obtained from Kubelka-Munk theory (Ciambelli et al., 2007) by plotting 

[F(R)·h]
2
 vs h and calculating the x intercept of a line through 0.5 < F(R) < 0.8. X-ray diffraction 

(XRD) was carried out using an X-ray microdiffractometer Rigaku D-max-RAPID, using Cu-K radiation. 

2.2 Photocatalytic tests 
Photocatalytic tests were carried out feeding 830 Ncm

3
/min He stream containing 0.1 % cyclohexane, 0.15 

% oxygen and 0.16 % water. Photocatalytic fluidized bed reactor was illuminated by two UVA-LED 

modules, positioned in front of each Pyrex windows (40 x 50 mm). The light intensity was regulated at 100 

mW/cm
2
. Each UVA-LED array consisted of 20 pieces of UVA-LEDs (provided by Nichia Corporation) 

emitting at 365 nm. To control the reaction temperature at 120 °C, an electrical heater and a thermocouple 

were immersed within the catalytic bed. The catalyst weight was 7 g. 

The gas composition was continuously measured by an on-line quadrupole mass detector (TraceMS, 

ThermoElectron) and a continuous CO–CO2 NDIR analyser (Uras 10, ABB). UV light was switched on 

after the reaching of adsorption equilibrium of cyclohexane on the catalyst surface. 

3. Results and discussion 

3.1 Catalysts characterization 
The list of supports and catalysts with their TiO2, MoO3 and SO3 content are reported in Table 1. The 

values of specific surface area (SSA) and band-gap energy are also displayed. MoO3 and SO3 percentage 

were evaluated on the effective TiO2 loading in TiO2/SiO2 sample. 

Table 1: Catalysts and their characteristics 

Catalyst TiO2 wt % MoO3 wt % SO3 wt % SSA m
2
/g Ebg  eV 

TiSi1 10 0 0 380 3.5 

TiSi2 18.8 0 0 340 3.4 

8Mo2STiSi1 10 8 2 350  3.5 

16Mo4STiSi1 10 16 4 345 3.1 

8Mo2STiSi2 18.8 8 2 321 3.3 

16Mo4STiSi2 18.8 16 4 314 3.2 

 

The specific surface areas decreased with TiO2 loading for the samples TiSi1 and TiSi2. The same effect 

is obtained after the addition of SO3 and MoO3 on the support surface. No significant effect on the band 

gap energy values was found. XRD analysis (not reported) did not show signals related to molybdenum 

species, but only diffraction patterns related to TiO2 in anatase phase, underlining that a very good 

dispersion of MoOx species was obtained. Figure 1a and 1b show the Raman analysis of all the prepared 

photocatalysts. With respect to TiO2/SiO2 supports, the presence of Mox species caused Raman signals in 

the range 900–1,100 cm
−1

. Specifically, Raman spectra show main bands at 955 cm
-1

 on 8Mo2STiSi1 and 



 
567 

8Mo2STiSi2, and at about 963 cm
-1

 on 16Mo4STiSi1 and 16Mo4STiSi2. These bands are attributed to 

polymolybdate coordinated with the support surface and Polymerization degree of these species increases  

  

700 800 900 1000 1100

Raman shift (cm
-1

)

C
o

u
n

ts
 (

a
.u

.)

700 800 900 1000 1100

Raman shift (cm
-1

)

C
o

u
n

ts
 (

a
.u

.)

TiSi1 

8Mo2STiSi1 

16Mo4STiSi1 

TiSi2 

8Mo2STiSi2 

16Mo4STiSi2 

(a) (b) 

 

Figure 1: Raman spectra of photocatalysts 

with the Mo loading (Ciambelli et al., 2008). The absence of bulk molybdena bands at 819 and 995 cm
−1

 

(Ciambelli et al., 2008) for all Mo-based catalysts confirms the absence of MoO3 crystallites. 

3.2 Photocatalytic activity tests 

Preliminary tests carried out in absence of UV light evidenced that no reaction occurred in dark conditions. 

Figure 2 shows cyclohexane consumption rate with together benzene and CO2 formation rate obtained 

over all the photocatalysts under UV irradiation. On TiSi1 and TiSi2, CO2 was the only product detected in 

the gas phase (100 % selectivity). Photocatalytic tests on MoOx/TiO2/SiO2 catalysts evidenced that 

benzene was the main reaction product formed during irradiation with together CO2 and cyclohexene as by 

products. On 8Mo2STiSi2, the cyclohexane consumption rate reached a maximum value after about 5 

min, and then activity decreased during irradiation. A completely different behaviour was found on 

8Mo2STiSi1. In this case the cyclohexane consumption rate increased to steady state value without 

deactivation phenomena. Moreover, on 8Mo2STiSi2 the formation rate of benzene progressively increased 

reaching a maximum value of about 9 mol h
-1

 g
-1

 after 9 min and then decreased. Instead, on 

8Mo2STiSi1 benzene formation rate progressively increased reaching a steady state value of about 8.5 

mol h
-1

 g
-1

 (corresponding to a benzene selectivity of 96 %) after about 12 min.  

TG-MS analysis in air flow of 8Mo2STiSi2 after photocatalytic test (Figure 3) evidenced the presence of 

carbonaceous species adsorbed on the catalyst surface that could explain the loss of photoactivity 

observed during irradiation. Deactivation phenomena were not observed when MoO3 and SO3 loading 

were increased to 16 and 8 wt % respectively. In fact, photocatalytic performances were strongly 

enhanced on 16Mo4STiSi1 and the increase of Mo loading determined an increase of photocatalytic 

activity. In particular, on 16Mo4STiSi1, steady state cyclohexane consumption rate was about 42 mol h
-1

 

g
-1

 (97 % benzene selectivity), higher than that obtained (17 mol h
-1

 g
-1

) on an optimized formulation for 

MoOx/TiO2-Al2O3 catalysts, where the support was obtained by sol-gel technique (Palma et al., 2010). The 

higher photoactivity of 16Mo4STiSi1 with respect to 16Mo4STiSi2 is likely attributed to the superior SSA of 

the first one. This result confirms what found before (Ciambelli et al., 2009), where the improved activity 

was attributed  to the better transmission of UV light through SiO2 than through Al2O3. 



 
568 

 

0

10

20

30

40

50

60

0 5 10 15 20 25

irradiation time (min)

c
y

c
lo

h
e

x
a

n
e

 c
o

n
s

u
m

p
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

TiSi1

8Mo2STiSi1

16Mo4STiSi1

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25

irradiation time (min)

b
e

n
z
e
n

e
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

0

100

200

300

400

C
O

2
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

benzene TiSi1

benzene 8Mo2STiSi1

benzene 16Mo4STiSi1

CO2 TiSi1

CO2 8Mo2STiSi1

CO2 16Mo4sTiSi1

0

5

10

15

20

25

30

35

0 10 20 30 40

irradiation time (min)

c
y

c
lo

h
e

x
a

n
e

 c
o

n
s

u
m

p
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

TiSi2

8Mo2STiSi2

16Mo4STiSi2

0

5

10

15

20

25

30

0 10 20 30 40

irradiation time (min)

b
e

n
z
e
n

e
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

0

50

100

150

200

C
O

2
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)benzene TiSi2
benzene 8Mo2STiSi2

benzene 16Mo4STiSi2

CO2 TiSi2

CO2 8Mo2STiSi2

CO2 16Mo4sTiSi2

 

0

10

20

30

40

50

60

0 5 10 15 20 25

irradiation time (min)

c
y

c
lo

h
e

x
a

n
e

 c
o

n
s

u
m

p
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

TiSi1

8Mo2STiSi1

16Mo4STiSi1

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25

irradiation time (min)

b
e

n
z
e

n
e

 f
o

rm
a

ti
o

n
 r

a
te

 

( 
m

o
l*

h
-1

*g
-1

)

0

100

200

300

400

C
O

2
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

benzene TiSi1

benzene 8Mo2STiSi1

benzene 16Mo4STiSi1

CO2 TiSi1

CO2 8Mo2STiSi1

CO2 16Mo4sTiSi1

0

5

10

15

20

25

30

35

0 10 20 30 40

irradiation time (min)

c
y

c
lo

h
e

x
a

n
e

 c
o

n
s

u
m

p
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

TiSi2

8Mo2STiSi2

16Mo4STiSi2

0

5

10

15

20

25

30

0 10 20 30 40

irradiation time (min)

b
e

n
z
e
n

e
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)

0

50

100

150

200

C
O

2
 f

o
rm

a
ti

o
n

 r
a

te
 

( 
m

o
l*

h
-1

*g
-1

)benzene TiSi2
benzene 8Mo2STiSi2

benzene 16Mo4STiSi2

CO2 TiSi2

CO2 8Mo2STiSi2

CO2 16Mo4sTiSi2

 

Figure 2: Cyclohexane consumption rate, benzene and CO2 formation rate as a function of irradiation time 

(a) 

(b) 

(c) 

(d) 



 
569 

0.0

0.2

0.4

0.6

0.8

1.0

D
e

ri
v
. 
W

e
ig

h
t 
(%

/m
in

)

88

90

92

94

96

98

100

W
e

ig
h

t 
(%

)

0 200 400 600 800 1000

Temperature (°C)

 

Catalyst:8Mo2STiSi2 after photocatalytic test

m/z=44 (CO2)

0.0

0.2

0.4

0.6

0.8

1.0

D
e

ri
v
. 
W

e
ig

h
t 
(%

/m
in

)

88

90

92

94

96

98

100

W
e

ig
h

t 
(%

)

0 200 400 600 800 1000

Temperature (°C)

 

0.0

0.2

0.4

0.6

0.8

1.0

D
e

ri
v
. 
W

e
ig

h
t 
(%

/m
in

)

88

90

92

94

96

98

100

W
e

ig
h

t 
(%

)

0 200 400 600 800 1000

Temperature (°C)

 

Catalyst:8Mo2STiSi2 after photocatalytic test

m/z=44 (CO2)

 

Figure 3: TG-MS of 8Mo2STiSi2 sample after photocatalytic test 

4. Conclusions 

Photocatalytic selective oxidation of cyclohexane to benzene was studied MoOx/TiO2/SiO2 in a fluidized 

bed reactor. Photocatalytic tests showed that benzene was the main reaction product formed during 

irradiation with together CO2 and cyclohexene as by products. Depending on the TiO2, Mo and sulphate 

loading, an enhancement of the activity and catalysts stability occurred during irradiation.  

MoOx/TiO2/SiO2 samples showed a photocatalytic activity higher than MoOx/TiO2-Al2O3. This behaviour 

was attributed to the better transmission of UV light through SiO2 than through Al2O3. 

Acknowledgements 

The authors wish to thank University of Salerno for funding the project “orsa111873; processi chimici 

catalitici per la produzione di energia sostenibile e l'ambiente”.  

References 

Ciambelli P., Sannino D., Palma V., Vaiano V., Bickley R.I., 2008. Reaction mechanism of cyclohexane 

selective photo-oxidation to benzene on molybdena/titania catalysts. Applied Catalysis A: General, 

349, 140-147. 

Ciambelli P., Sannino D., Palma V., Vaiano V., Eloy P., Dury F., Gaigneaux E.M., 2007. Tuning the 

selectivity of MoOx supported catalysts for cyclohexane photo oxidehydrogenation. Catalysis Today, 

128, 251-257. 

Ciambelli P., Sannino D., Palma V., Vaiano V., Mazzei R.S., 2009. A step forwards in ethanol selective 

photo-oxidation. Photochemical and Photobiological Sciences, 8, 699-704. 

Geldart D., 1972. The effect of particle size and size distribution on the behaviour of gas-fluidised beds. 

Powder Technology, 6, 201-215. 



 
570 

 
Hajaghazadeh M., Vaiano V., Sannino D., Kakooei H., Sotudeh-Gharebagh R., Ciambelli P. 2013. 

Heterogeneous photocatalytic oxidation of methyl ethyl ketone under UV-A light in an LED-fluidized 

bed reactor. Catalysis Today, 230, 79-84. 

Murcia J.J., Hidalgo M.C., Navío J.A., Vaiano V., Sannino D., Ciambelli P., 2013. Cyclohexane 

photocatalytic oxidation on Pt/TiO2 catalysts. Catalysis Today, 209, 164-169. 

Palma V., Sannino D., Vaiano V., Ciambelli P., 2010. Fluidized-bed reactor for the intensification of gas-

phase photocatalytic oxidative dehydrogenation of cyclohexane. Industrial and Engineering Chemistry 

Research, 49, 10279-10286. 

Rizzo L., Sannino D., Vaiano V., Sacco O., Scarpa A., Pietrogiacomi D., 2014. Effect of solar simulated N-

doped TiO2 photocatalysis on the inactivation and antibiotic resistance of an E. coli strain in biologically 

treated urban wastewater. Applied Catalysis B: Environmental, 144, 369-378. 

Sannino D., Vaiano V., Sacco O., Ciambelli P., 2013a. Mathematical modelling of photocatalytic 

degradation of methylene blue under visible light irradiation. Journal of Environmental Chemical 

Engineering, 1, 56-60. 

Sannino D., Vaiano V., Sarno G., Ciambelli, P., 2013b. Smart tiles for the preservation of indoor air quality. 

Chemical Engineering Transactions, 32, 355-360. 

Sannino D., Vaiano V., Ciambelli P., 2013c. A green route for selective synthesis of styrene from 

ethylbenzene by means of a photocatalytic system. Research on Chemical Intermediates, 39, 4145-

4157. 

Sannino D., Vaiano V., Ciambelli P., 2013d. Innovative structured VOx/TiO2 photocatalysts supported on 

phosphors for the selective photocatalytic oxidation of ethanol to acetaldehyde. Catalysis Today, 205, 

159-167. 

Sannino D., Vaiano V., Ciambelli P., Murcia J.J., Hidalgo M.C., Navío J.A., 2013e. Gas-phase 

photocatalytic partial oxidation of cyclohexane to cyclohexanol and cyclohexanone on Au/TiO2 

photocatalysts. Journal of Advanced Oxidation Technologies, 16, 71-82. 

Sannino D., Vaiano V., Ciambelli P., Carotenuto G., Di Serio M., Santacesaria E., 2013f. Enhanced 

performances of grafted VOx on titania/silica for the selective photocatalytic oxidation of ethanol to 

acetaldehyde. Catalysis Today, 209, 159-163.