Fluidized-Bed Reactor for the Intensification of Gas-Phase

Ind. Eng. Chem. Res. 2010, 49, 10279–10286

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Fluidized-Bed Reactor for the Intensification of Gas-Phase Photocatalytic Oxidative Dehydrogenation of Cyclohexane Vincenzo Palma,* Diana Sannino, Vincenzo Vaiano, and Paolo Ciambelli Department of Chemical and Food Engineering, UniVersity of Salerno, Fisciano, Italy

The behavior of a two-dimensional laboratory-scale fluidized-bed catalytic reactor for the photocatalytic oxidative dehydrogenation of cyclohexane to benzene has been studied, employing a molybdenum-based catalyst and a titania-alumina mixed oxide as a support. A preliminary mathematical model is proposed for the evaluation of the effect of the main factors that affect the overall reactor performances. Experimental data evinced that the reaction rate is not dependent on the oxygen concentration; instead, it depends on cyclohexane concentration, according to Langmuir-Hinshelwood (LH)-type kinetics. Taking into account that the interaction between photoexcited molybdate and adsorbed cyclohexane is the rate-limiting step, the functionality in light intensity was found. The mathematical model describes the performance of the photocatalytic fluidized-bed reactor well for all operating conditions examined. 1. Introduction Because of the important role of benzene as an intermediate in petrochemistry, the study of chemical reactions to convert low-value hydrocarbons to benzene, such as the aromatization of light alkanes, seems to be a very interesting aspect.1 Benzene can be produced by cyclohexane oxidative dehydrogenation. Several catalysts, such as vanadate2 and zeolite,3 have been studied in fixed-bed reactors. Moreover, molybdenum-based catalysts also have been employed in cyclohexane oxidative dehydrogenation at temperatures above 280 °C.4-6 Very recently, Ciambelli and co-workers showed that cyclohexane is selectively oxidized to benzene on MoOx/TiO2 catalysts in the presence of gaseous oxygen at temperature of 35 °C under ultraviolet (UV) illumination in a gas-solid fixed-bed reactor.7,8 The same authors showed that it is possible to realize the same reaction on MoOx/TiO2 catalysts, using a photocatalytic fluidized-bed reactor.9,10 To test these catalysts, to obtain good fluidization, experiments that involved physical mixtures with R-Al2O3 (Sauter average diameter of 50 µm and density equal to 3970 kg/m3) at different percentages of Mo-titania catalysts were performed.11,12 An alternative to the physical mixing between titania-based catalysts and R-Al2O3 is to realize TiO2-Al2O3 mixed-oxide catalytic supports. MoOx/TiO2-Al2O3 catalysts showed an improved synthesis of benzene, with respect to MoOx/TiO2-alumina physical mixtures.13 In the present work, the parameters that influence the kinetics of photocatalytic oxidative dehydrogenation of cyclohexane to benzene have been studied in a laboratory-scale gas-solid fluidized-bed photoreactor on a molybdenum-based catalyst, using a titania-alumina mixed oxide as a support. The effects of light intensity, as well as hydrocarbon and oxygen concentration, were experimentally analyzed and a preliminary mathematical model was developed. 2. Experimental Section 2.1. Catalyst Preparation. Anatase-phase TiO2, with a specific surface area of 87 m2/g and containing 1.2 wt % SO3 (PC100, Millemium Inorganic Chemicals), was used. A titania-alumina support was prepared by dispersing PC100 * To whom correspondence should be addressed. E-mail: [email protected].

powder in a bohemite sol (10 wt % of Condea Pural in doubledistilled water, pH 99%). A similar trend was shown by cyclohexene concentration; however, the values are very much lower, with respect to benzene. No deactivation of the catalyst was observed during photocatalytic tests.

Figure 2. Outlet reactor concentration (a.u.) of cyclohexane (m/z ) 84), oxygen (m/z ) 32), benzene (m/z ) 78), and cyclohexene (m/z ) 67), as a function of run time. Initial cyclohexane concentration ) 1000 ppm; oxygen/cyclohexane ratio ) 1.5; incident light intensity ) 85 mW/cm2.

To assess the effect of light intensity and catalyst weight on the photo-oxidative dehydrogenation of cyclohexane, the experiments were performed with a light intensity within the range of 0-142 mW/cm2 and with two different catalyst weights (14 and 20 g). The results are plotted in Figure 3. Cyclohexane was unconverted in the absence of light, and its reaction rate conversion increased up to ∼29 µmol h-1 g-1, in correspondence of a light intensity equal to 111 mW/cm2 for a catalyst weight of 20 g. In all cases, the selectivity to benzene was >99%. The results reported in Figure 3 evidenced that photocatalytic activity increased by increasing the catalyst amount loaded

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

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Figure 5. Effect of initial oxygen concentration on cyclohexane consumption rate. Catalyst weight ) 20 g, initial cyclohexane concentration ) 1000 ppm, incident light intensity ) 85 mW/cm2. Figure 3. Cyclohexane consumption rate as a function of light intensity and catalyst weight. Initial cyclohexane concentration ) 1000 ppm; oxygen/ cyclohexane ratio ) 1.5.

In the absence of oxygen, no conversion was observed. The comparison of the catalytic performances obtained in stoichiometric and overstoichiometric initial ratios for oxidative dehydrogenation to benzene evidences a weak effect of oxygen concentration, according to our previous results19 and to other studies regarding the oxidation of cyclohexane with molecular oxygen,20 in which a negligible influence on conversion was reported at high oxygen partial pressure. 4. Mathematical Modeling

Figure 4. Effect of initial cyclohexane concentration on cyclohexane consumption rate and benzene formation rate. Catalyst weight ) 20 g, oxygen/cyclohexane ratio ) 1.5, incident light intensity ) 85 mW/cm2.

into the reactor, as expected. In fact, since the superficial gas velocity and apparent density of fluidized catalyst did not change, the height of the expanded bed was 0.4 and 0.57 dm for catalyst weights of 14 and 20 g, respectively. Therefore, the difference in the cyclohexane consumption rates is a consequence of the different conversions obtained operating with different bed heights. In Figure 4, the comparison of photocatalytic performances in the steady state of 10MoPC100, as a function of the initial cyclohexane concentration, is reported. Both cyclohexane reaction rate and benzene formation rate increased with the initial cyclohexane concentration, reaching a value of ∼21.7 and ∼21.4 µmol h-1 g-1, respectively, for an initial cyclohexane concentration of 6000 ppm. Moreover, there was not a significant increment of catalytic activity for an initial cyclohexane concentration of >3000 ppm. The results reported in Figure 4 suggest that the behavior can be well-described by the Langmuir-Hinshelwood (LH)type kinetic model widely used to take into account the dependence of the photocatalytic reaction rates on the concentration of organic reagents.17,18 To understand how the reaction rate depends on the oxygen concentration, photocatalytic tests were performed at a fixed cyclohexane concentration (1000 ppm) and changing oxygen partial pressures. The results are reported in Figure 5.

A mass balance for cyclohexane is performed, as a function of bed height, using the LH-type kinetic model. The model is able to represent the evolution of the photocatalytic oxidative dehydrogenation of cyclohexane to benzene when catalyst particles in contact with reactants are activated by UV radiation. No mass-transport limitations are assumed for the reactants and products in the gas phase; no homogeneous oxidation is considered within the gas phase; good mixing conditions result from the fluidized-bed operation; reaction heat is negligible, hence, isothermal behavior can be safely assumed; monochromatic radiation travels only perpendicular to the surfaces; no light scattering phenomena; and, finally, radiation is absorbed only by the photocatalyst. The elementary fluidized-bed volume in Figure 6 is selected, considering an orthogonal monometric system x-y-z with the plan Π xy coinciding with the cross section of the reactor and z-axis with reactor height. Photons enter into the reactor through the surfaces at x ) 0 and x ) L (where L is the reactor thickness). Since the value of incident light intensity is the same along the z-axis, it is possible to assume that the radiant flux value changes only with the x-axis. The cyclohexane concentration at any given x and z results from the hydrocarbon mass balance in the elementary volume dV ) B dx dz (see Figure 6), considering the diffusion effects to be negligible.

( RTP ) d(V B dx C (x,z)) ) -r(PC (x,z), PC (x,z), E )B dx dz F g

1

1

2

Φ

app

(1) where Vg is the gas superficial velocity (given in units of dm/h), B the reactor width (in units of dm), L the reactor thickness (also in units of dm), C1 the cyclohexane volume fraction, C2 the oxygen volume fraction, P the total pressure (P ) 1 atm), Fapp the apparent density of the fluidized catalyst (Fapp ) 875 g/dm3), EΦ a function that is dependent on the radiant flux φ0,21 R the universal

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Figure 6. Schematic picture of the elementary fluidized-bed volume.

gas constant (expressed in units of atm dm3 mol-1 K-1), and T the reaction temperature (in Kelvin). To determine how the kinetic expression depends on radiant flux φ0, a photocatalytic test was performed on PC100Al. Only a small concentration of CO2 (30 ppm) was detected without the formation of dehydrogenated products, confirming that Mo surface species are responsible for the benzene production, according to the results reported in refs 7 and 19. In this cited literature, it was reported that the photoexcited octahedral polymolybdate species are able to initiate the oxidative dehydrogenation of adsorbed cyclohexene through hydrogen abstraction to form benzene. A similar role was previously reported for photoexcited decatungstate, which is able to initiate the selective oxidation of the cyclohexane to cyclohexanol and cyclohexanone through hydrogen abstraction in aqueous media, followed by reoxidation of the tungstate by O2.22 As a consequence, the photonic excitation of the molybdate appears as the initial step of the activation of the entire catalytic system (eq 2), whereas the interaction between photoexcited molybdate (MoOx)ex and adsorbed cyclohexane [C6H12]ads (eq 4) can be considered as the rate-limiting step. Since oxidative dehydrogenation reactions are free from thermodynamic limitations,23,24 the influence of water on the reaction rate can be considered negligible. k1

(MoOx)n + hγ 98 (MoOx)ex

(2)

The rate of the overall process is given by the following expression: r ) k3[MoOx]ex[C6H12]ads

(5)

Considering a steady-state approximation for the total concentration of photoexcited molybdate, at any instant, one has d[MoOx]ex ) 0 ) k1[MoOx]nφ0 - k2[MoOx]ex dt

(6)

[MoOx]tot ) [MoOx]ex + [MoOx]n

(7)

Since the total concentration of surface molybdate [MoOx]tot is the sum of photoexcited molybdate and nonphotoexcited molybdate (eq 7), it is possible to obtain the following expression for the concentration of photoexcited molybdate: [MoOx]ex ) [MoOx]tot

[

(k1 /k2)φ0 1 + (k1 /k2)φ0

]

(8)

Based on these last observations, and taking into account the results reported in Figures 4 and 5, eq 6 becomes

(

r ) k4

)(

k5φ0(x) bC1(x, z) 1 + k5φ0(x) 1 + bC1(x, z)

)

(9)

where k4 ) k3[MoOx]tot

k2

(MoOx)ex 98 (MoOx)n + energy

k3

(MoOx)ex + [C6H12]ads 98 (MoOx)reduced + R•

(3)

(4)

The photocatalytic activity can be reduced by the return of the molybdate to a nonphotoexcited state with simultaneous energy emission (see eq 3).

k5 )

k1 k2

and b is the adsorption coefficient of cyclohexane. To estimate the value of b, the amount of cyclohexane adsorbed on the catalyst surface in dark conditions has been measured as function of hydrocarbon concentration (see Table 1). Using nonlinear regression analysis of the experimental data to fit a Langmuir function for cyclohexane adsorption as a


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Table 1. Amount of Cyclohexane Adsorbed on Catalyst Surface initial cyclohexane concentration (ppm)

adsorbed cyclohexane (mol/g)

0 500 1000 3000 6000 10000

0 0.021 0.035 0.045 0.047 0.047

Table 2. Kinetic Parameters and Variables symbol

value

Fapp Vg b Rscat k4 k5

875 g/dm3 144 dm/h 2.22 × 103 atm-1 0.228 dm2/g 130.62 mol/(h g) 1.28 × 10-5 dm2/mW

Figure 7. Calculated radiation field profiles as a function of reactor thickness (x) for different incident radiant fluxes.

function of initial cyclohexane concentration, we obtain a b value of 2.22 × 103 atm-1. The nonlinear regression procedure was performed using the least-squares approach, based on the minimization of the sum of squared residuals. In the hypothesis of keeping the hydrocarbon concentration constant, the dependence of reaction rate on photonic flux (eq 9) is the same as that obtained by Giovanni Camera-Roda et al.,25 which presented a simple approach for the design of photocatalytic reactors, considering that photons can be treated as immaterial reactants. It is then possible to evaluate the radiation process in terms of parameters such as conversion, selectivity, and yield, which are common in conventional reaction engineering. By replacing eq 9 into eq 1 and rearranging it, the cyclohexane mass balance can be written as

( RT1 )V

g

(

)(

)

dC1(x, z) k5φ0(x) bC1(x, z) ) -k4 F dz 1 + k5φ0(x) 1 + bC1(x, z) app (10)

Figure 8. Comparison between model calculations and experimental data of the cyclohexane reaction rate, as a function of incident light intensity.

At any given x, the cyclohexane concentration is only a function of the z-direction. The boundary condition for eq 10 is C1 ) C01

for z ) 0 and ∀x

where C10 is the initial cyclohexane concentration. ∀z is the average cyclohexane concentration in the cross section, calculated according to the following relationship:

∫

L

j1 ) C

0

C1 dx L

(11)

From eq 10, it is evident that the reaction rate is affected by the local photon “concentration” φ0, which, in turn, is derived from the Local Volumetric Rate of Photon Absorption (LVRPA), which requires solving the radiative transfer equation (RTE) in the reaction space. Thus, a key step is to know the radiant energy distribution inside the reactor volume.25 Many methods, mostly numerical, have been developed to solve the RTE and to get the radiant energy distribution.26 Other methods of solution, mainly based on Monte Carlo approaches, have been devised, but lengthy numerical computations are still required to obtain each particular solution.27 For this reason, interesting approximate radiation field models are developed, taking into account only the main factors that affect light distribution in photoreactors.25,28 Strongly simplified models can be obtained for fluidized-bed systems in the case of plane geometry and parallel

Figure 9. Comparison between model calculations and experimental data of the cyclohexane reaction rate, as a function of initial cyclohexane concentration.

irradiation. For the latter system, the predicted radiation field exhibits an exponential decrease while going deeper into the particle bed. In other words, it follows a “Lambert-Beer type” law with an extinction coefficient proportional to the mass concentration of particles, as shown by Brucato and Rizzuti.28 This simplified approach was used also for different reactor geometries, but always in the presence of fluidized photocatalysts.29-32 From all these observations, radiation transmission throughout fluidized photocatalysts, as a function of reactor thickness, can

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Figure 10. Schematic depiction of the modified fluidized-bed photoreactor: (a) reactor thickness ) 1 cm and (b) reactor thickness ) 0.6 cm.

be modeled using the Lambert-Beer type law,21,25 as represented by eq 12. Φ0(x) ) Φin exp(-RscatFappx)

(12)

where Φ0(x) is the radiant flux as a function of the x-direction (given in units of mW dm-2), Φin the incident radiant flux (in mW dm-2), Fapp the apparent density of fluidized catalyst (Fapp ) 875 g/dm3), and Rscat the specific extinction coefficient per unit catalyst mass (in dm2 g-1). Equation 10 was solved using the Eulero iterative method, coupled with eq 12, which gives the light intensity value at fixed x ) x*. Therefore, the model calculates the cyclohexane concentration as a function of reactor height z at fixed x ) x*: C1(z, x*). One of the main goals of the reactor simulation by mathematical model is to identify the constants k4, k5, and Rscat for the oxidative dehydrogenation of cyclohexane to benzene in a photocatalytic fluidized-bed reactor by fitting experimental data conversions reported in Figure 3, as a function of the light intensity for a catalyst weight of 14 g. Also, in this case, the fitting procedure was realized using the least-squares approach. The values of the apparent density of fluidized catalyst, the superficial gas velocity, and the cyclohexane adsorption coefficient used in the model, together with the calculated parameters, are reported in Table 2. The calculated radiation field profiles (see Figure 7) indicate a strong absorption for low values of x (∼0.2 cm) with a large fraction of reaction volume without UV irradiation.

Figure 8 shows a comparison between model calculations and experimental data of the cyclohexane reaction rate, as a function of incident light intensity. The accuracy of the developed model was tested by comparing the calculated cyclohexane consumption rate with that obtained from experimental tests, as a function of the incident light intensity for a catalyst weight of 20 g (see Figure 8) and as a function of the initial cyclohexane concentration (see Figure 9). The calculated values are in good agreement with the experimental data in both cases. The results obtained from the model and, in particular, the light intensity profiles, as a function of reactor thickness, suggest that the photoreactor performances could be enhanced by reducing the non-irradiated reaction volume. This can be realized by designing a reactor with a smaller thickness. For this purpose, the fluidized-bed photoreactor was modified: its thickness was decreased from 1 cm to 0.6 cm, while the remaining geometrical characteristics were unchanged (see Figure 10). Given the specific extinction coefficient value reported in Table 2 and the new thickness, the light intensity distribution into the reactor changed, as depicted in Figure 11. The two profiles are reported as functions of the dimensionless thickness x/L (where x is the axial coordinate along the reactor thickness and L is the reactor thickness). Clearly, the unirradiated fraction of the catalytic bed volume is decreased by reducing L from 1 cm to 0.6 cm. The cyclohexane conversion predicted by the model and experimental data, as a function of the catalytic bed height, for both thicknesses, is shown in Figure 12.


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Figure 11. Calculated radiation field profiles for different reactor thickness. Incident light intensity ) 85 mW/cm2.

5. Conclusions Factors that affect the kinetics of the photocatalytic oxidative dehydrogenation of cyclohexane to benzene in a two-dimensional fluidized-bed reactor were analyzed. A mathematical modeling has been performed using a Langmuir-Hinshelwood (LH)-type kinetic model for cyclohexane concentration. Experimental data evidenced that the reaction rate is not dependent on the oxygen concentration. Taking into account that the interaction between photoexcited molybdate and adsorbed cyclohexane is the rate-limiting step, the functionality in light intensity was found. The mathematical model describes the performance of the photocatalytic fluidized-bed reactor well for all operating conditions examined. The ultraviolet (UV) light distribution into the reactor seems to be the dominant factor when considering the photocatalytic performances. Figure 12. Cyclohexane conversion as a function of bed height and for both thickness values (1 cm and 0.6 cm). Incident light intensity ) 85 mW/cm2.

The system was simulated with the kinetic parameters and variables reported in Table 2, together with the radiation fields reported in Figure 11. Experimental data at a reactor thickness of 0.6 cm were obtained by adjusting the gas flow rate to 30 L/h (STP) and catalyst weights of 8.4 and 16.8 g were used to have the same superficial gas velocity and contact time as that used in the reactor that had a thickness of 1 cm. With these operating conditions, the height of the expanded bed was, again, 0.4 and 0.8 dm, respectively. The cyclohexane conversion increased with bed height and was influenced by reactor thickness (see Figure 12). In particular, at a fixed bed height, it increases with decreasing thickness. To properly affect the comparison, the main element to consider is that the percentage of irradiated catalyst volume is different. For a thickness of 0.6 cm, it is larger than that calculated for a thickness of 1 cm. This is confirmed by the j0 ) average light intensity into the reactor (calculated as Φ ∫0LΦ0(x) dx)/L), which is 8.52 and 13.4 mW/cm2 for a reactor thickness of 1 and 0.6 cm, respectively, indicating that the attenuation of the available UV energy is a key parameter that influences the photocatalytic performances of the reactor as found by Chiovetta et al.33

Acknowledgment The authors thank Millenium Inorganic Chemicals for providing the titania PC100 used in this work. A special thank to Ph.D. Roberto S. Mazzei, for making the schematic drawings of the reactor. Literature Cited (1) Nishi, K.; Komai, S.; Inagaki, K.; Satsuma, A.; Hattori, T. Structure and catalytic properties of Ga-MFI in propane aromatization. Appl. Catal., A 2002, 223, 187. (2) Kung, M. C.; Kung, H. H. Oxidative dehydrogenation of cyclohexane over vanadate catalysts. J. Catal. 1991, 128, 287. (3) Panizza, M.; Resini, C.; Busca, G.; Lo`pez, E. F.; Escribano, V. S. A Study of the oxidative dehydrogenation of cyclohexane over oxide catalysts. Catal. Lett. 2004, 89, 199. (4) Hashida, T.; Uchijima, T.; Yoneda, Y. Linear free energy relations in heterogeneous catalysis. IX. Kinetic study of catalytic dehydrogenation of cyclohexanes by the pulse technique. J. Catal. 1970, 17, 287. (5) Sonnemans, J.; Mars, P. Mechanism of pyridine hydrogenolysis on molybdenum-containing catalysts. I. Monolayer molybdenum(VI) oxidealuminum oxide catalyst. Preparation and catalytic properties. J. Catal. 1973, 31, 209. (6) Maggiore, R.; Giordano, N.; Crisafulli, C.; Castelli, F.; Solarino, L.; Bart, J. C. The mechanism of dehydrogenation of cyclohexane on molybdenum(VI) oxide/aluminum oxide catalysts. J. Catal. 1979, 60, 193. (7) Ciambelli, P.; Sannino, D.; Palma, V.; Vaiano, V. Photocatalysed selective oxidation of cyclohexane to benzene on MoOx/TiO2. Catal. Today 2005, 99, 143.

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ReceiVed for reView March 8, 2010 ReVised manuscript receiVed July 19, 2010 Accepted July 20, 2010 IE1005383

Fluidized-Bed Reactor for the Intensification of Gas-Phase

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