TiO2 catalysts in the photocatalytic cyclohexane oxidative dehydrogenation by a fluidized bed photoreactor

Applied Catalysis A: General 394 (2011) 71–78

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Avoiding the deactivation of sulphated MoOx /TiO2 catalysts in the photocatalytic cyclohexane oxidative dehydrogenation by a fluidized bed photoreactor D. Sannino a,∗ , V. Vaiano a , P. Ciambelli a , P. Eloy b , E.M. Gaigneaux b a b

Department of Chemical and Food Engineering, University of Salerno, Via Ponte Don Melillo, 84084 Fisciano, Salerno, Italy Unité de catalyse et chimie des matériaux divisés, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium

a r t i c l e

i n f o

Article history: Received 18 October 2010 Received in revised form 15 December 2010 Accepted 19 December 2010 Available online 24 December 2010 Keywords: Photocatalytic fluidized bed reactor Photocatalytic fixed bed reactor MoOx /TiO2 Catalyst deactivation Sulphate

a b s t r a c t Since the relevance of deactivation phenomena in heterogeneous gas–solid photocatalytic processes, a study on the deactivation of MoOx /TiO2 catalysts in the oxidative photodehydrogenation of cyclohexane has been carried out in a fixed bed reactor and compared to the behaviour of catalysts used in a twodimensional fluidized bed photoreactor. Superior photocatalytic performances were obtained in the fluidized bed reactor with respect to the fixed bed reactor both in terms of cyclohexane conversion and benzene yield. At the opposite of the fixed bed, in the fluidized bed reactor, catalyst did not deactivate. Characterization performed on catalysts after photocatalytic tests evidenced that the deactivation of the catalysts is correlated to the accumulation of carbonaceous species on catalyst surface and the sulphur disappearance. The catalysts used in fluidized conditions remained active as they maintained their initial sulphur content at the surface. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Photocatalytic oxidation has been widely used for processes like the decontamination of water and air [1–3]. A drawback of these processes is the catalyst deactivation phenomena which mainly occur in heterogeneous gas–solid systems. A literature summary of catalyst deactivation in gas–solid photocatalysis was reported by Sauer and Ollis [4]. They showed that deactivation is a very commonly observed phenomenon, especially for single pass flow reactor because a part of the products remain adsorbed on the surface. It must be also noted that deactivated photocatalysts appear brownished, as found by Einaga et al. [5]. In particular they identified the occurrence of carbon deposits on TiO2 surface during the heterogeneous photocatalytic decomposition of benzene, toluene, cyclohexane and cyclohexene. In such system, it also appeared that deactivated TiO2 catalysts can be photochemically regenerated in the presence of water vapour as the carbon deposits oxidized to COx . Selective oxidation of hydrocarbons by O2 is an important goal for economic, environmental, and scientific reasons [6–8]. The most studied photoreaction in liquid phase is the cyclohexane oxygenation which is an important commercial reaction, as the resultant products, alcohol and ketones, are precursors in the syntheses of adipic acid, which is in turn an intermediate in the production of nylon [9–11].

∗ Corresponding author. Tel.: +39 089964147; fax: +39 089964057. E-mail address: [email protected] (D. Sannino). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.12.025

Conversion of cyclohexane to benzene is, in the same nature of idea, of industrial interest because of the variety of applications of benzene with respect to the few applications of cyclohexane [12]. To this purpose, molybdena based catalysts have been applied to cyclohexane oxidative dehydrogenation at temperature above 280 ◦ C [13–17]. Starting on deep observations performed on Mo-exchanged ferrierites [18], we found that cyclohexane is also selectively photooxidized to benzene on MoOx /TiO2 catalysts in the presence of gaseous oxygen at temperature of 35 ◦ C under UV illumination [19]. Further studies evidenced that benzene selectivity is a function of the sulphate content of the support [20]. Several experiments were performed in a gas–solid fixed bed reactor showing the occurrence of a deactivation of the catalyst as function of time on stream [19,20]. In addition, we showed the improving in the benzene production from cyclohexane on MoOx based catalysts by using a fluidized bed reactor [21,22]. Fluidized bed gives advantage of an easy temperature control and an enhanced mass transfer to and from the photocatalysts with respect to slurry reactors or fixed bed reactors with immobilized TiO2 . Moreover, the photoactivity increase in fluidized bed photoreactor could be partially associated with higher light absorption due to utilization of scattered light by the catalysts particles and to photocatalyst recirculation [23]. In the present work, a study on the deactivation of MoOx /TiO2 catalysts in the oxidative photodehydrogenation of cyclohexane performed in a fixed bed reactor has been affected in comparison to a two-dimensional fluidized bed photoreactor.

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2. Experimental 2.1. Catalyst preparation TiO2 -supported MoO3 catalysts, with nominal MoO3 content of 4.7 and 8 wt% were prepared by the incipient wetness impregnation method. Support material was titania in the anatase form containing 2 wt% sulphate (DT51, Rhone Poulenc). This support was impregnated with aqueous solutions of ammonium heptamolybdate (Aldrich). The resulting samples were dried at 120 ◦ C overnight and then calcined in air at 400 ◦ C for 3 h. In the following, catalysts are denoted as MoX, where X represents the wt% nominal MoO3 loading. 2.2. Catalyst characterization Chemical analysis was performed by inductive coupled plasmamass spectrometry (7500c ICP-MS, Agilent) after microwave digestion (Ethos Plus from Milestone) of catalysts in HNO3 /HCl and HF/HCl mixtures. Thermogravimetric analysis on catalyst before and after activity tests were carried out in air flow (100 cm3 /min STP) at atmospheric pressure in a thermobalance (Q600, TA Instruments), connected online with quadrupole mass detector (Quadstar 422, Pfeiffer Vacuum) in the range 20–1100 ◦ C, with an heating rate of 10 ◦ C/min. Temperature programmed desorption (TPD) experiments of the catalysts recovered after activity measurements were carried out in nitrogen flow (500 cm3 /min STP) at atmospheric pressure in a quartz flow reactor, connected online with CO, CO2 (Uras 10E Hartmann & Braun) and with on-line quadrupole mass detector (MD800, ThermoFinnigan) in the range 20–500 ◦ C, with an heating rate of 10 ◦ C/min. Catalysts specific surface area was evaluated by N2 adsorption–desorption isotherms at −196 ◦ C with a Costech Sorptometer 1040. Powder samples were treated at 180 ◦ C for 2 h in He flow (99.9990%) before testing. The binding energy of chemical elements on catalyst surface was characterized by XPS analyses performed with a Kratos Axis Ultra spectrometer (Kratos Analytical – Manchester, UK) equipped with a monochromatised aluminium X-ray source (powered at 10 mA and 15 kV). The sample powders were pressed into small stainless steel troughs mounted on a multi specimen holder. The pressure in the analysis chamber was around 10−6 Pa. The angle between the normal to the sample surface and the lens axis was 0◦ . The hybrid lens magnification mode was used with the slot aperture resulting in an analyzed area of 700 ␮m × 300 ␮m. The pass energy was set at 40 eV. In these conditions, the energy resolution gives a full width at half maximum (FWHM) of the Ag 3d5/2 peak of about 1.0 eV. Charge stabilization was achieved by using the Kratos Axis device. The following sequence of spectra was recorded: survey spectrum, C 1s, O 1s, Ti 2p, S 2p, Mo 3d and C 1s again to check the stability of charge compensation in function of time and the absence of degradation of the sample during the analyses. The binding energies were calculated with respect to the C–(C, H) component of the C 1 s peak fixed at 284.8 eV. The spectra were decomposed with the CasaXPS program (Casa Software Ltd., UK) with a Gaussian/Lorentzian (70/30) product function after subtraction of a linear baseline. Although Mo 3d3/2 and Mo 3d5/2 peaks overlap S 2s peak, it was taken into account that the theoretical distance between Mo 3d3/2 and Mo 3d5/2 peaks is 3.13 eV and the area ratio is 2/3. Also, thanks to the sample without Mo, constraints for the position (232.2 ± 0.2 eV) and the width (3.0 ± 0.2 eV) of S 2s peaks were determined and used for subtracting the S 2s contributions from each Mo peaks. A comparative analysis of • OH radicals formed on the photocatalysts after their use for photocatalytic tests was performed by 2-hydroxyterephthalic fluorescence measurement [24]. About 25 mg of each used photocatalyst was added to 25 mL of the 5 × 10−4 M terephthalic acid

Fig. 1. Gas–solid photocatalytic fixed bed reactor.

solution in 2 × 10−3 M NaOH. The suspension was irradiated with UV light (at 365 nm) for 15 min. Terephthalic acid reacted proportionally with OH radicals giving 2-hydroxyterephthalic acid. After filtration through 0.45 ␮m membrane to separate the photocatalyst, the formed 2-hydroxyterephthalic acid was analyzed on a Perkin–Elmer fluorescence spectrophotometer, exciting at 315 nm and measuring the fluorescence at 425 nm. 2.3. Photocatalytic tests Figs. 1 and 2 report a schematic picture of the fixed bed and fluidized bed photoreactor used in this work. The annular section of the fixed bed photocatalytic reactor was realized with two axially mounted 500 mm long quartz tubes of 140 and 40 mm diameter, respectively. The reactor was equipped with

Fig. 2. Gas–solid photocatalytic fluidized bed reactor.

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Table 1 Characteristics of analyzed photocatalysts. Catalysts

Nominal MoO3 content (wt%)

MoO3 content by ICP analysis (wt%)

Specific surface area (BET) (m2 /g)

Theoretical MoOx surface coverage degree (wt%)

SO3 (wt%) (by TG–MS)

Mo4.7 Mo8

4.7 8.0

4.4 7.6

68 63

57 100

1.7 1.3

3. Results Nominal and effective metal loading, surface areas, and sulphate content are reported in Table 1. MoOx coverage calculated from nominal MoO3 loading and assuming a monolayer capacity of 0.12% (w/w) for MoO3 /m2 [25,26], is 57% for Mo4.7 and 100% for Mo8. Fig. 3 shows cyclohexane conversion as a function of irradiation time on Mo8 catalyst in the fluidized bed and the fixed bed reactors. In the fixed bed reactor, the cyclohexane conversion reached a maximum value (15.6%) after about 5 min, then the activity decreased approaching a steady state conversion (3%) after 20 min. The fast deactivation occurs within 15 min, and then the cyclohex-

cyclohexane conversion, %

18 16 14

fixed bed reactor

12

fluidized bed reactor

10 8 6 4 2 0 0

20

40

60

80

100

irradiation time, min Fig. 3. Cyclohexane conversion as a function of irradiation time on Mo8 catalyst using the fluidized bed and fixed bed reactors. Reaction conditions: total gas flow rate: 830 (STP)cm3 /min; inlet cyclohexane concentration: 1000 ppm; oxygen/cyclohexane ratio: 1.5; water/cyclohexane ratio: 1.6; reaction temperature: 70 ◦ C.

ane is converted at progressively slower rate during the time. The steady state conversion after 80 min was further lower, levelling at 2%. The behaviour is quite different when the fluidized bed reactor is used. In this case, when the lamps were switched on, the cyclohexane conversion immediately increased reaching a steady state value corresponding to 8% cyclohexane conversion after about 10 min. This level of conversion remained constant even after prolonged time on stream. This result evidences that the fluidization enhanced and stabilized the cyclohexane conversion. Fig. 4 compares the benzene outlet concentration on Mo8 in the fixed and fluidized bed photoreactor. By using fixed bed reactor, the outlet concentration of benzene progressively increased reaching a maximum value of 17 ppm after 90

benzene concentration, ppm

seven 40 W UV fluorescent lamps providing photons with wavelengths in the range from 300 to 425 nm, with primary peak at 365 nm. One lamp (UVA Cleo Performance 40 W, Philips) was centred inside the inner tube while the others (R-UVA TLK 40 W/10R flood lamp, Philips) were located symmetrically around the reactor. Both photoreactor and lamps were covered with reflecting aluminium foils. In order to avoid temperature gradients in the reactor caused by irradiation, the temperature was controlled to 70 ± 2 ◦ C by cooling fans. The catalytic reactor bed was prepared in situ, by coating quartz flakes previously loaded in the annular section of a quartz continuous flow reactor with aqueous slurry of catalysts powder. The coated flakes were dried at 120 ◦ C for 24 h in order to remove the excess of physisorbed water. This treatment resulted in uniform coating well adhering to the surface of each quartz flake. The amount of deposited catalyst, evaluated by weighing the reactor before and after the coating treatment was 20 g. The fluidized bed reactor had a 40 mm × 10 mm cross section. A bronze filter (5 ␮m size) was used for gas feeding with a uniform distribution. The walls of the reactor, 230 mm in height, were made of 2 mm thick pyrex-glass. An expanding section was located on the top of reactor to minimise the elutriation phenomenon. The reactor was illuminated by four UV light sources (EYE MERCURY LAMP, 125 W) in a dark box. In order to control the reaction temperature, a PID controller connected to a heater system was installed near the reactor. The reaction temperature was 70 ◦ C. To improve the fluidization properties of Mo–titania catalysts ␣-Al2 O3 with specific surface area of 10 m2 /g (Aldrich), Sauter average diameter of 50 ␮m and density equal to 3970 kg/m3 was used. An optimal fluidization of Mo–titania catalysts powder was found realizing physical mixtures of 14 g of catalyst and 63 g of ␣-Al2 O3 and used in all the tests. Catalytic tests were carried out feeding 830 (STP)cm3 /min N2 stream containing 1000 ppm cyclohexane, 1500 ppm oxygen and adding 1600 ppm of water. Nitrogen was the carrier gas for cyclohexane and water vaporized from two temperature controlled saturators. The gas flow rates were measured and controlled by mass flow controllers (Brooks Instrument). The reactor inlet or outlet gases were analyzed by an online quadrupole mass detector (MD800, ThermoFinnigan) and a continuous CO–CO2 NDIR analyser (Uras 10, Hartmann & Braun). UV sources were switched on after complete adsorption of cyclohexane on catalyst surface. Blank tests both with fixed bed and fluidized bed reactor were performed following the experimental procedure reported in [19,22].

80 70 60

fixed bed reactor

50

fluidized bed reactor 40 30 20 10 0

0

20

40

60

80

100

irradiation time,min Fig. 4. Comparison of benzene outlet concentration on Mo8 in the fixed bed and fluidized bed photoreactors. Reaction conditions: total gas flow rate: 830 (STP)cm3 /min; inlet cyclohexane concentration: 1000 ppm; oxygen/cyclohexane ratio: 1.5; water/cyclohexane ratio: 1.6; reaction temperature: 70 ◦ C.

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Table 2 Comparison between performances of Mo4.7 and Mo8 catalysts in the fixed and fluidized bed reactor.a Sample

Cyclohexane conversion (%)

Mo4.7 fixed bed Mo4.7 fluidized bed Mo8 fixed bed Mo8 fluidized bed

Benzene selectivity (%)

4 10 2 8

Carbon dioxide selectivity (%)

27 67 65 99

Cyclohexene selectivity (%)

8 32.2 5 0

0.7 0.8 1.5 1.0

a Reaction conditions: total gas flow rate: 830 (STP)cm3 /min; inlet cyclohexane concentration: 1000 ppm; oxygen/cyclohexane ratio: 1.5; water/cyclohexane ratio: 1.6; reaction temperature: 70 ◦ C.

100

90 fluidized bed reactor fixed bed reactor

80

a

70 0

20

40

60

80

irradiation time, min

Table 3 Surface atomic ratios.

O/Ti C/Ti Mo/Ti S/Ti Al/Ti Si/Ti

100

Mo4.7

Mo8

Mo8 after test in fixed bed

Mo8 after test in fluidized bed

2.35 0.78 0.10 0.036 –

2.57 0.65 0.12 0.036 –

2.51 0.86 0.15 – –

2.87 0.98 0.18 0.028 0.28

The carbon mass balance (evaluated by comparing the inlet carbon as cyclohexane and the outlet carbon as the sum of unconverted cyclohexane and outlet benzene, cyclohexene, carbon monoxide and carbon dioxide) as function of irradiation time during the catalytic tests in the fixed bed reactor on Mo4.7 and Mo8 catalysts has been reported in Fig. 5. While carbon mass balance was closed to about 99% for both catalysts after 10 min in the fluidized bed reactor, on Mo4.7 in the fixed bed reached only 92% even for longer time. To explain the different photocatalytic performances, XPS and TG–MS characterization were performed. XPS results on Mo4.7 and Mo8 before photocatalytic tests and on Mo8 after tests in fixed bed reactor and fluidized bed reactor are reported in Table 3. For both fresh samples, sulphur appears always as sulphate (∼169 eV) and never as sulphide (∼167 eV) while Mo 3d peaks can be decomposed into MoVI (∼80%) and MoV (∼20%) species (Figs. 6a, 6b and 7a). Concerning the sample recovered after photocatalytic tests, XPS spectra revealed that (i) in both cases, molybdenum is in a more oxidized state with respect to the fresh sample (MoV content decreases); (ii) sulphur disappearance is observed in the case of fixed bed photoreactor while the sample used in fluidized conditions remains active and maintains a major part of its sulphur content at the surface (Fig. 6c and 6d); (iii) C/Ti ratios increase after both photocatalytic test but as some contamination of carbon could be also introduced during the manipulation and the preparation of the samples, it could be hazardous to conclude with a definite deposition of C at the surface due to the test.

total carbon mass balance, %

total carbon mass balance, %

40 min. Then it decreases to a steady state value (about 14 ppm) reached after about 80 min. By contrast, in fluidized bed reactor, the benzene outlet concentration progressively increased reaching a steady state value of 79 ppm. This value is much higher than that obtained in the fixed bed reactor evidencing an increase in benzene production with the fluidized bed. Table 2 summarises the comparison of steady state photocatalytic performances at 70 ◦ C of Mo4.7 and Mo8 catalyst in the fixed and fluidized bed photoreactor, respectively. By using fixed bed reactor, cyclohexane conversion was higher at lower MoOx coverage, 4% on Mo4.7 and 2% on Mo8. On both catalysts, products selectivity significantly changed in the two reactor configurations. In the fixed bed photoreactor, on Mo4.7 the selectivities to benzene and carbon dioxide reached 27% and 8% while in the fluidized bed reactor 67% and 32.2% are obtained respectively. It is worthwhile to note that on Mo4.7 in the fixed bed, selectivities do not account to 100% indicating that carbon mass balance was not satisfied. On Mo8, although with lower cyclohexane conversion, higher product selectivity was evidenced. In fixed bed reactor selectivity to benzene was 65%, with selectivity to carbon dioxide of 5% while in fluidized bed reactor 99% selectivity to benzene was instead obtained without any formation of carbon dioxide. With both catalysts the presence of very low amounts of cyclohexene in the reaction products was detected, corresponding to selectivity of about 0.7% and 1.5% on Mo4.7 and Mo8, respectively. The behaviour was slightly different when the fluidized bed reactor was used, with selectivity to cyclohexene of 0.8% on Mo4.7 and 1% on Mo8. The results show that both in the fixed bed reactor and in the fluidized bed reactor, cyclohexane conversion and selectivity to carbon dioxide decreased with molybdenum content, the selectivity to benzene strongly increased while that to cyclohexene was merely affected. The comparison between the two photoreactors evidenced that superior catalytic performances are obtained in the fluidized bed reactor than those obtained in the fixed bed reactor. Interestingly, when the fluidized bed reactor was used, the trend of cyclohexane conversion and outlet benzene concentration showed that there was no catalyst deactivation. Selectivity data showed that total carbon mass balance was closed to 100% only by utilizing the fluidized bed reactor.

100

90 fluidized bed reactor fixed bed reactor

80

b

70 0

20

40

60

80

100

irradiation time, min

Fig. 5. Total carbon mass balance in the fixed bed and fluidized bed reactor as a function of irradiation time on Mo8 (a) and Mo4.7 (b).

D. Sannino et al. / Applied Catalysis A: General 394 (2011) 71–78

135

a

180

75

b

Intensity (101 counts/s)

Intensity (101 counts/s)

130 125 120 115 110 105 100 95

170

160

150

140

130

174

173

172

171

170

169

168

167

166

165

174

173

172

Binding Energy (eV) 72

74

c Intensity (101 counts/s)

Intensity (101 counts/s)

70 68 66 64 62 60

170

169

168

167

166

165

167

166

165

d

72 70 68 66 64 62 60

58

174

171

Binding Energy (eV)

173

172

171

170

169

168

167

166

165

173

Binding Energy (eV)

172

171

170

169

168

Binding Energy (eV)

Fig. 6. XPS S 2p spectra of Mo4.7 (a) and Mo8 (b) catalysts before test and Mo8 after test in the fluidized bed reactor (c) and fixed bed reactor (d).

A further confirmation can be found in TG–MS experiment in air flow. Figs. 8 and 9 show TG–MS results of Mo4.7 and Mo8 catalyst before and after the reaction in the fixed bed photoreactor. For both samples before the reaction (Figs. 8a and 9a), two main complex stages of weight loss, respectively below and above 350 ◦ C, can be detected. The first main step (up to about 300 ◦ C) was associated to water and titania hydroxyls removal, while the second main step was due to decomposition of different kind of surface sulphates, as identified from their characteristic fragments m/z = 48 and 64. Thermogravimetric analysis on Mo8 and on Mo4.7 after photocatalytic test in the fixed bed reactor (Figs. 8b and 9b) showed the presence of organic compounds adsorbed on catalyst surface since their decomposition/oxidation to carbon dioxide in the range 200–430 ◦ C (as evidenced by carbon dioxide characteristic fragment, m/z = 44) that could explain the carbon deficit checked during the reaction in the fixed bed reactor on Mo4.7. Moreover it is evident the total absence of surface sulphates for both catalysts as evidenced by the absence of fragments m/z = 48 and 64 in the whole temperature range, thus confirming the results obtained from XPS analysis. By using the fixed bed reactor, tests performed on both catalysts evidenced that a rapid decay of activity occurs in the first minutes on stream to reach a stable value for conversion under steady state conditions. This initial decrease of activity could be due to a poisoning of the surface by carbonaceous species. It can be supposed that this effect could be due either to a strong adsorption of the reactant or the products (like as benzene, CO2 or cyclohexene), or of carboxylates and other carbonaceous species that would block some active surface sites because the slower desorption rate in the fixed bed.

In order to elucidate this last aspect, TPD coupled with Mass Spectrometer and CO, CO2 continuous analyzers, in nitrogen flow, of catalysts after activity measurement in the fixed bed reactor in the range 20–500 ◦ C were performed (Fig. 10). Desorption of cyclohexene (m/z = 67) from 100 ◦ C up to 210 ◦ C, of benzene (m/z = 78) from 160 ◦ C to 260 ◦ C, and of CO2 from about 130 ◦ C up to 500 ◦ C were observed. Therefore, part of products remained adsorbed on the surface, and could be related to the observed deactivation. In addition, the presence of carboxylated compounds could be deduced by the strong CO2 release. Coherently to the TG results, sulphate was not detected during TPD test. For comparison, TPD tests performed on the same catalyst (with the same conditions described above) after fluidized bed, showed the only desorption of SO2 (m/z = 64) from 270 ◦ C (Fig. 11) without any further release. These last results confirm the presence of sulphate on catalysts surface after photocatalytic tests in the fluidized bed reactor. The non-appearance of the reaction products on catalyst surface is the effect of the absence of catalyst deactivation observed in the fluidized bed reactor. 4. Discussion Many researchers have investigated the gas-phase photocatalytic reaction of hydrocarbons, alcohols, chlorohydrocarbons and amines on the UV-irradiated TiO2 at low temperature [5] and in the most of cases, catalyst deactivation has been ascribed to the accumulation of the intermediates on TiO2 surfaces [27]. Regeneration processes of deactivated TiO2 catalyst have been reached by either heat or UV treatment of sample recovered after photocatalytic test. For instance, it was shown that TiO2 activity

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55

a

80

Mo

70

45 40 35 30

Mo

b

VI

Intensity (102 counts/s)

Intensity (102 counts/s)

50

V

25 20 15

60 50 40 30 20

10 240

238

236

234

232

230

240

238

Binding Energy (eV) 55

45

c

45 40 35 30 25 20 15

232

230

d

35 30 25 20 15 10

10 240

234

40

Intensity (102 counts/s)

Intensity (102 counts/s)

50

236

Binding Energy (eV)

238

236

234

232

230

Binding Energy (eV)

240

238

236

234

232

230

Binding Energy (eV)

Fig. 7. XPS Mo 3d spectra of Mo4.7 (a) and Mo8 (b) catalysts before test and Mo8 after test in the fluidized bed reactor (c) and fixed bed reactor (d).

in the toluene photooxidation is completely recovered by heating the catalyst above 420 ◦ C [28]. Peral and Ollis have reported that deactivated titania used in the 1-butanol photooxidation is regenerated trough prolonged UV irradiation in air [29]. It was also studied the deactivation phenomenon during gas-phase photocatalytic oxidation of cyclohexane, cyclohexene, benzene and toluene evidencing that carbonaceous intermediates are decomposed to COx in humidified air [5]. It can be argued that CO2 detected during TG–MS and TPD on catalyst recovered after fixed bed reactor (Figs. 8–10 respectively) is formed by thermal decomposition of carbonaceous intermediate which are, indeed responsible, of photocatalyst deactivation, together with the adsorbed benzene and cyclohexene. It must be considered that the OH radicals generated on the fraction of catalyst surface exposed to UV irradiation, are the active species for the decomposition and/or oxidation of carbon deposits [30]. Thus, on the fraction of surface not adequately irradiated, OH radicals cannot be formed anymore, hence the amount of carbon deposits will increase progressively lowering the reaction rate. This commonly happens when a fixed bed photocatalytic reactor is used; photocatalyst particles scatter a part of incident light, not captured by the other catalytic particles because they have a fixed position in the catalytic bed; in a fluidized bed, the increase of photoactivity is associated to the better utilization of scattered light by the catalyst [31] and to the mixing conditions that permits the periodical exposition of catalyst at the transparent irradiated windows. Consequently, a higher concentration of OH radicals than those in the fixed bed is obtained. This last phenomenon helps to oxidize carbonaceous compounds and to eliminate the deactivation. An increased CO2 production has

to be thus observed in fluidized bed tests, as indeed found for Mo4.7. At higher Mo coverage on titania, formation of carboxylate compounds is already limited, and the positive effects of the increased light transfer results in a significantly higher conversion of cyclohexane due to the increased photoexcitation of surface polymolybdate species. In addition, it must be considered that the strong improvements in solid–gas mass transfer rates due to the extensive mixing in fluidized bed systems with respect to fixed bed, allowing to increase the desorption rate of reaction products from catalyst surface, with a consequently higher turnover of catalytic sites. The quantity of OH radicals (evaluated by the formation and fluorescence analysis of 2-hydroxyterephthalic acid) obtained on Mo8 after fixed bed was more than double that found after test in the fluidized bed reactor. This could be explained by considering that the surface of Mo8 after fixed bed is free of sulphate, as it was lost during the illumination, and it is replaced by OH hydroxyls, resulting in the formation of a larger amount of hydroxyl radicals under irradiation. However, the fluidized catalyst resulted more active, underlining the relevance of the phenomena of light scattering which are minimized by the photocatalyst mixing in the fluidized bed photoreactor, nullifying the effects of screening which are instead present in the fixed bed reactor, as reported below. With regard to the adsorbed oxidative dehydrogenation products (cyclohexene and benzene), this explains the absence of adsorbed cyclohexene and benzene on catalyst recovered after photocatalytic test in the fluidized bed reactor. TG–MS and TPD results suggest that the deactivation phenomenon in the fixed bed reactor could be related to the

D. Sannino et al. / Applied Catalysis A: General 394 (2011) 71–78

during oxidation of reduced polymolybdate. Sulphites should be then re-oxidized to sulphates or by UV irradiation or by oxygen contained in gas-phase [34]. This latter reaction may occur in the fluidized bed reactor because of the high mass transfer rates that assure the presence of oxygen on catalyst surface. At the opposite, in the fixed bed reactor where catalyst surface is probably poor oxygenated and less UV exposed, both sulphate and sulphite can act as oxidizing species of reduced molybdates that could be hydrogen ion donors to SO3 2− species to form HSO3 − , then H2 SO3 and successively SO2 which desorbs from catalyst surface. The removal 300

CO2

250

m/z = 78

MS signal, a.u.

disappearance of sulphate species. Considering that a higher state of oxidation was found on 8Mo catalyst after test both in the fluidized bed reactor and in the fixed bed reactor with respect to fresh sample, the deactivation of the catalyst appears to be correlated with the sulphur disappearance since the sample used in fluidized conditions remains active and maintains its sulphur content at the surface. Disappearing of sulphate is a very significant phenomenon. In a previous work we showed the effect of sulphate content on catalytic performance. At constant Mo surface density, the increase of sulphate loading was the responsible of the increasing in benzene yield and selectivity [32]. The importance of sulphate species in the photocatalytic oxidative dehydrogenation of cyclohexane was also confirmed on Mo/Al2 O3 catalysts in which the absence of sulphate induced the absence of photoactivity [22]. In another our recent paper we proposed a reaction mechanism of photo-oxidative dehydrogenation to benzene on MoOx /TiO2 [33]. This mechanism involves reaction between the active oxygen species of the catalyst surface and photo-excited octahedral polymolybdate species (Mo8 O26 4− ), with a subsequent reoxidation of the catalyst by the gas phase oxygen according to a redox mechanism [33]. Several hypotheses were formulated about the role of sulphate species in promoting the selectivity to benzene formation. Sulphate present on TiO2 surface (SO4 2− /TiO2 ) could participate to the activation step, facilitating hydrogen abstraction from an adsorbed molecule of cyclohexane or cyclohexene, owing its strongly acid properties, and/or it may participate in the re-oxidation step of the photoreduced polymolybdate surface species. In the latter view, surface sulphates could be reduced to surface sulphite species (SO3 2− /TiO2 )

Fig. 9. TG–MS results on Mo8 catalyst before (a) and after (b) reaction in the fixed bed reactor.

m/z = 67

200

CO 2, ppm

Fig. 8. TG–MS results on Mo4.7 catalyst before (a) and after (b) reaction in the fixed bed reactor.

77

150 100 50 0 30

130

230

330

430

Temperature, °C Fig. 10. Outlet reactor concentration (MS signal) of cyclohexene, and benzene and of CO2 (NDIR analyzer, ppm) vs. temperature on Mo4.7 after activity measurement in the fixed bed reactor.

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m/z= 64, a.u.

hexane when a photoactive titania support is used as well. In contrast, the sample used in fluidized conditions remains active and maintains its sulphur content at the surface. References

200

250

300

350

400

450

500

Temperature, °C Fig. 11. Outlet reactor concentration (MS signal) of SO2 as a function of temperature on Mo4.7 after activity measurements in the fluidized bed reactor.

of sulphate could live unselective titania sites free, or could inhibit the activation step of cyclohexane molecules leading to the photocatalytic performances decrease. Anyway, all these observations underline the participation of SO4 2− species in the studied reaction, their influence on the activity and reaction selectivity on titania based catalysts, and their relevance in the photocatalyst formulations. 5. Conclusions Photocatalytic performances of MoOx /TiO2 catalysts in the selective oxidation of cyclohexane to benzene were studied in a fixed bed reactor and in a two dimensional fluidized bed reactor. In particular, increasing molybdenum loading resulted in higher benzene selectivity. Cyclohexane conversion and selectivity to benzene were higher in the fluidized bed reactor with respect to fixed bed reactor. Activity improvement in fluidized bed photoreactor is partially associated with higher light absorption due to utilization of scattered light by the catalyst. At the opposite of the fixed bed, catalyst deactivation was not observed in the fluidized bed reactor. TG–MS and TPD tests performed on catalysts after photocatalytic tests both in the fixed and in the fluidized bed reactor showed the positive effects of the improving mass and light transfer phenomena in fluidized bed reactor, evidencing no modifications of photocatalyst composition and the absence of adsorbed reactant, products, intermediates or accumulated carbonaceous species. The deactivation of the catalyst in the fixed bed reactor was ascribed to the accumulation of carbonaceous species but also to the sulphur disappearance from catalysts surface, confirming the important role of surface sulphates in photocatalytic partial oxidation of cyclo-

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