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Photooxidation of aniline on alumina with sunlight and artificial UV light
Catalysis Communications 6 (2005) 159–165 www.elsevier.com/locate/catcom
Photooxidation of aniline on alumina with sunlight and artificial UV light Chockalingam Karunakaran *, Sambandam Senthilvelan Department of Chemistry, Annamalai University, Annamalainagar 608002, India Received 14 July 2004; revised 26 November 2004; accepted 26 November 2004 Available online 5 January 2005
Abstract The photooxidation of aniline to azobenzene on alumina in ethanol with natural sunlight and artificial UV light (365 nm) was studied as a function of [aniline], catalyst loading, airflow rate, solvent composition, etc. The metal oxide exhibits sustainable catalytic activity. The product formation is larger with illumination at 254 nm than at 365 nm. Electron donors like triphenylphosphine, diphenylamine and hydroquinone enhance the photocatalysis. Singlet oxygen quencher, azide ion does not suppress the catalysis. The photooxidation occurs in protic and aprotic solvents. The product formation is analyzed using a kinetic model. 2004 Elsevier B.V. All rights reserved. Keywords: Solar photooxidation; UV photooxidation; Aniline; Al2O3
1. Introduction
2. Experimental
Studies on semiconductor photocatalysis are numerous [1–3] but rare are those with insulators (non-reactive solids) such as alumina [4,5]. Air-equilibrated solution of aniline yields azobenzene on irradiation at 365 nm with benzophenone sensitizing the oxidation [6,7]. UVirradiated ZnO brings in the oxidation of aniline to azobenzene [8,9] but TiO2 immobilized on porous nickel photodegrade aniline [10]. Here, we report photooxidation of aniline on alumina with direct sunlight and artificial UV light; reports on solar photocatalysis are also a few and preliminary [11–13], the problem of variation of sunlight intensity even under clear sky is overcome by conducting set of experiments simultaneously and comparing the results.
2.1. Materials Al2O3 (Merck) was used as received and the BET surface area determined as 10.63 m2 g1. Using Easy particle sizer M1.2, Malvern Instruments (focal length 100 mm, beam length 2.0 mm, wet (methanol) presentation) the particle sizes were measured as 57.7, 49.8, 42.9, 11.4, 9.8, 8.5, 7.3, 3.5, 3.0 and 2.6 lm at 3.8%, 17.8%, 8.7%, 1.8%, 28.2%, 15.3%, 10.0%, 1.0%, 9.7% and 3.0%, respectively. Aniline, AR was distilled before use. Commercially available ethanol was distilled over calcium oxide; other organic solvents were of LR grade and distilled prior to use. 2.2. Solar photocatalysis
*
Corresponding author. Tel.: +91 4144 221820; fax: +91 4144 238145. E-mail address: karunakaranc@rediffmail.com (C. Karunakaran). 1566-7367/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.11.014
The solar photocatalyzed oxidations were made from 10.30 a.m. to 12.30 p.m. during summer (March–July) under clear sky. The intensity of solar radiation was measured using Global pyranometer, MCPT, supplied
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by Industrial Meters, Bombay. Fresh solutions of aniline of desired concentrations were taken in wide cylindrical glass vessels of uniform diameter and appropriate height; Al2O3 covered the entire bottom of the vessel. Air was bubbled using a micro pump without disturbing the Al2O3 bed. The volume of the reaction solution was kept as 25 ml and the loss of solvent due to evaporation was compensated periodically. One millilitre of the reaction solution was withdrawn at regular intervals, diluted five times and the absorbance measured at 375 nm using Hitachi U-2001 or Jasco UVIDEC-340 UV–Vis spectrophotometer. 2.3. Photocatalysis with UV light Photocatalytic studies with UV light were carried out in a multilamp photoreactor (HML MP88, supplied by Heber Scientific) fitted with eight 8 W mercury UV lamps of wavelength 365 nm (Sankyo Denki, Japan) and highly polished anodized aluminum reflector; the sample was placed at the centre. Four cooling fans at the bottom of the reactor dissipate the generated heat. The reaction vessel was borosilicate glass tube of 15mm inner diameter. Photooxidation was also carried out in a Heber micro photoreactor (HMI SL W6) fitted with a 6 W 254 nm low-pressure mercury lamp and a 6 W 365 nm mercury lamp. Quartz and borosilicate glass tubes were used for 254 and 365 nm lamps, respectively. The photon flux of the light source (I) was determined by ferrioxalate actinometry. The volume of the reaction solution was always maintained as 25 mL in the multilamp photoreactor and 10 mL in the micro reactor. Air was bubbled through the reaction solution that effectively stirs the solution and keeps the suspended catalyst under constant motion. The absorbance was measured at 375 nm after centrifuging Al2O3 and diluting the solution five times to keep the absorbance within the Beer–Lambert law limit. 2.4. Product analysis Solar photooxidation of aniline in ethanol on Al2O3 yields azobenzene as the only product. The GC–mass [m/z with relative intensities in parentheses 182 (13), 152 (5), 105 (16), 77 (100), 51 (44)], IR and UV–Vis spectra of the extracted solid product are identical with those of trans-azobenzene (Fluka).
Fig. 1. Solar photooxidation of aniline in ethanol on Al2O3. The UV– Vis spectra of the reaction solution diluted five times and recorded at 0, 30, 60, 90 and 120 min (›); [aniline] = 0.113 M, Al2O3 bed = 12.5 cm2, weight of Al2O3 = 1.0 g, airflow rate = 4.75 ml s1, volume of reaction solution = 25 ml.
cis- and trans-azobenzenes during the course of the reaction and the unstable cis form (Z) transforms to the trans form (E) slowly on standing. The UV–Vis spectrum of the irradiated reaction solution but allowed to stand for a few days in dark is identical with that of the authentic trans-azobenzene confirming the slow transformation of the unstable cis form to trans form. For a solution of cis and trans-azobenzenes ½E ¼ ðabs281 eZð433Þ abs433 eZð281Þ Þ =ðeEð281Þ eZð433Þ eZð281Þ eEð433Þ Þ and ½Z ¼ ðabs281 eEð433Þ abs433 eEð281Þ Þ =ðeZð281Þ eEð433Þ eZð433Þ eEð281Þ Þ; where e is the corresponding molar extinction coefficient. Calculation of the ratio [E]/[Z] using the above equations, the experimentally determined eE(433) and eE(281), the reported eZ(433) and eZ(281) and the measured absorbance of the reaction solution at 433 and 281 nm at different periods of the reaction shows that the ratio remains practically the same (1.72) during the course of the photooxidation followed. The total concentration of azobenzene, ([E] + [Z]) = abs375{1 + ([E]/[Z])}/ {eZ(375) + eE(375)([E]/[Z])}; eE(375) was determined experimentally and eZ(375) calculated from the measured abs375; abs375 = {eZ(375) + eE(375)([E]/[Z])}[Z]. 3. Results and discussion
2.5. Product estimation
3.1. Obtaining solar oxidation results
In both solar and UV photocatalysis, the UV–Vis spectra of the reaction solution recorded during the progress of the reaction are similar (Fig. 1; kmax = 375 nm) but not identical with that of the extracted product (kmax = 434 nm). This is because of formation of both
The measurement of solar radiation shows fluctuation of sunlight intensity (530 ± 40 W m2) during the course of the photooxidation even under clear sky. Now, for the first time, identical sunlight intensity was maintained for a set of photooxidation experiments of
C. Karunakaran, S. Senthilvelan / Catalysis Communications 6 (2005) 159–165
0.15
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Benzene-I
Ethanol-II
Benzene-II
Azobenzene formed / mM
Ethanol-I
Absorbance
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Solar-II
UV-I
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0 60 90 Time / min
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Fig. 2. Solar photooxidation of aniline in ethanol and benzene on Al2O3. Absorbance–time plots (experiments in each set conducted simultaneously and sets I and II on different days; the reaction solution diluted five times prior to absorbance measurements); [aniline] = 0.113 M, Al2O3 bed = 12.5 cm2, weight of Al2O3 = 1.0 g, airflow rate = 4.75 ml s1, volume of reaction solution = 25 ml.
different reaction conditions by carrying out the experiments simultaneously, thus making possible the comparison of the solar results. The solar photooxidation results are reproducible. For example, Fig. 2 is the linear increase of the absorbance of the reaction solution with the reaction time, one set of experiments conducted in ethanol and benzene side by side on one day and the other set similarly on another day. The ratio of the slopes of the absorbance-time profiles of the reactions in ethanol and benzene remains the same (1.1) although the experiments were conducted on different days, obviously under different sunlight intensities. This reproducibility is not surprising as the fluctuation of sunlight intensity is identical in test and control (standard) experiments and the ratio turns out to be independent of sunlight intensity. Further, the results of a pair of experiments performed simultaneously confirm the reproducibility of the rates of solar photocatalysis. Fig. 3 presents the solar photoformation of azobenzene in ethanol under identical conditions and carried out simultaneously. The ratio of the rates obtained from the linear plots is unity (1.01). 3.2. Factors influencing solar photocatalysis
0
50
100
150
Time / min Fig. 3. Photoformation of azobenzene in ethanol on Al2O3; [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
conducted simultaneously. The least squares slope of the linear plot of [azobenzene] versus time (e.g., Fig. 3) yields the rate of formation of azobenzene. Experiments at different [aniline] shows that the reaction rate increases with [aniline] (Fig. 4) and the variation conforms to the Langmuir–Hinshelwood model. The double reciprocal plot of rate versus [aniline] yields a straight line with a positive y-intercept.The variation of the amount of Al2O3 spread at the bottom of the reaction vessel
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Azobenzene formation / nM s
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[Aniline] / mM
The influence of various factors on the solar photocatalysis in ethanol was examined by carrying out the given set of experiments simultaneously; the data in each figure correspond to a set of photocatalytic experiments
Fig. 4. Azobenzene formation in ethanol on Al2O3 at different [aniline]; weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
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200 -1
Solar UV 100
0 0
1 2 Catalyst loaded / g
3
Fig. 5. Azobenzene formation in ethanol at different amounts of Al2O3 loading; [aniline] = 0.113 M, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
(catalytic bed) does not lead to any appreciable change in the photocatalysis rate (Fig. 5); the bottom of the cylindrical reaction vessel was fully covered by Al2O3 in all the cases and the increase of the amount of Al2O3 does not lead to increase of the area of the catalyst bed but only results in increased thickness of the Al2O3 bed. In the absence of the photocatalyst the reaction is an uncatalyzed one and hence a small rate. The photoformation of azobenzene increases linearly with the apparent area of the catalyst bed (Fig. 6). Study of the photooxidation as a function of airflow rate reveals enhancement of photocatalysis by oxygen (Fig. 7). The variation of reaction rate with the airflow rate indicates Langmuir–Hinshelwood kinetics and the linear double reciprocal plot of reaction rate versus airflow rate confirms the same. The reaction was also studied without bubbling air but the solution was not deaerated. The dissolved oxygen itself brings in the oxidation but the pho-
Azobenzene formation / nM s
Azobenzene formation / nM s
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Fig. 7. Azobenzene formation in ethanol on Al2O3 at different airflow rates: [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
tocatalysis is weak. The reaction does not occur in dark. Al2O3 used does not lose its activity on repeated use; reuse of Al2O3 yields identical results. Presence of water in the reaction medium slows down the reaction (Fig. 8). Addition of electron donors like triphenylphosphine (TPP), hydroquinone (HQ) and diphenylamine (DPA) enhances the photoformation of azobenzene. The variation of the enhanced photoformation with [TPP], [HQ] and [DPA] (Fig. 9) reveals Langmuir–Hinshelwood kinetics and the linear double reciprocal plots of the enhanced rate versus [TPP], [HQ] and [DPA] confirm the same. However, triethylamine (TEA) fails to facilitate the oxidation; addition of TEA (0.287 M) to the reac-
Azobenzene formation / nM s
-1
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Azobenzene formation / nM s
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Solar
100
0
Solar UV 100
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25 2 Catalyst bed / cm
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Fig. 6. Azobenzene formation (solar) in ethanol at varying areas of Al2O3 bed; [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 ml s1.
80
100
%EtOH (v/v) Fig. 8. Azobenzene formation on Al2O3 in aq. EtOH; [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
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200 HQ DPA TPP
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Enhanced rate / nM s-1
Azobenzene formation / nM s
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300
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uv uv
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UV
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0 0
0 0
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[ED] / mM Fig. 9. Azobenzene formation in ethanol on Al2O3 in presence of electron donors (ED); [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
tion solution does not alter the reaction rate. Anionic and cationic surfactants enhance the photocatalysis; addition of aerosol OT (sodium bis-2-ethylhexyl sulfosuccinate, 0.0225 M), sodium lauryl sulfate (SLS, 0.0347 M) and cetyltrimethylammonium bromide (CTAB, 0.0274 M) to the reaction solution (conditions as in Fig. 9) increases the photoformation rate by 28%, 68% and 50%, respectively. Vinyl monomers like acrylonitrile (0.608 M) and acrylamide (0.141 M) neither suppress the photocatalysis nor undergo polymerization indicating the absence of free radicals in the reaction solution during the course of photocatalysis. 3.3. Factors influencing photocatalysis with UV light The photocatalyzed oxidation of aniline in ethanol in the presence of air on Al2O3 was studied using a multilamp photoreactor with mercury UV lamps of wavelength 365 nm. The linear increase of [azobenzene] with illumination time yields the photochemical formation rate (e.g., Fig. 3) and the rates are reproducible to ±6%. Rate measurements at different [aniline] show increase of the oxidation rate with [aniline] (Fig. 4) and the increase is according to Langmuir–Hinshelwood kinetics. The increase of the amount of Al2O3 suspended in the reaction medium does not significantly enhance azobenzene formation (Fig. 5). Study of the photooxidation as a function of airflow rate reveals enhancement of photocatalysis by oxygen and the variation of the reaction rate with flow rate conforms to the Langmuir–Hinshelwood kinetics (Fig. 7). The reaction was also studied without bubbling air but the solution was not deoxygenated. The dissolved oxygen itself brings in the oxidation but the photocatalysis is slow. The photooxidation was
25 6
50 -1
10 Intensity / einstein l s
-1
Fig. 10. Azobenzene formation in ethanol on Al2O3 at different light intensities; [aniline] = 0.113 M, Al2O3 suspended = 1.0 g, airflow rate = 7.8 ml s1, k = 365 nm, volume of reaction solution = 25 ml.
examined as function of light intensity. The oxidation was carried out with two, four and eight lamps, the angles sustained by the adjacent lamps at the sample are 180, 90 and 45, respectively. Fig. 10 presents the variation of rate with the light intensity. The reaction does not occur in dark. Investigation of the photocatalysis using a 6 W 365 nm mercury lamp (I = 1.81 · 105 einstein l1 s1) and a 6 W 254 nm low-pressure mercury lamp (I = 5.22 · 106 einstein l1 s1) separately in the micro reactor under identical conditions reveals that high energy radiation is more effective in bringing out the photocatalysis. Azobenzene formed in 10 min on illumination at 365 and 254 nm are 132 and 436 lM, respectively, and the corresponding quantum yields are 0.012 and 0.14 ([aniline] = 0.113 M, Al2O3 suspended = 0.2 g, airflow rate = 7.8 ml s1, volume of reaction solution = 10 ml). Al2O3 employed does not lose its activity on illumination. Reuse of the Al2O3 reveals sustainable photocatalytic efficiency. Water in the reaction medium suppresses the photocatalysis (Fig. 8). Addition of electron donors like TPP, HQ, DPA and TEA enhances azobenzene formation. The variation of the enhanced photocatalysis rate with [TPP], [HQ] and [DPA] suggests Langmuir–Hinshelwood kinetics (Fig. 9). TEA (0.287 M) in the reaction solution increases the rate of azobenzene formation by 22%. While aerosol OT (0.0225 M) fails to influence the catalysis SLS (0.0347 M) and CTAB (0.0274 M) enhance the reaction by 22% and 16%, respectively (conditions as in Fig. 9). Also, vinyl monomers like acrylonitrile (0.608 M) and acrylamide (0.141 M) fail to inhibit the photocatalysis. Nor do they polymerize. Azide ion (0.154 M), a singlet oxygen quencher, fails to suppress the formation of azobenzene indicating the absence of involvement of singlet oxygen in the photocatalysis.
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3.4. Kinetic analysis Al2O3 provides an ordered two-dimensional environment for effective electron transfer from the donor to the acceptor. The donor aniline molecule may undergo photoexcitation followed by electron transfer to oxygen molecule, both adsorbed on the non-reactive surface. The reaction of aniline radical-cation with superoxide radical-anion results in nitrosobenzene. Condensation of nitrosobenzene with aniline, present in large excess, yields azobenzene. This is in agreement with the reported photodegradation of aniline on titania immobilized on porous nickel when present at ppm level [10]. The kinetic law that governs heterogeneous photocatalyzed reaction in a continuously stirred tank reactor (CSTR) [5] is rate ¼ kK 1 K 2 SIC½PhNH2 c=ð1 þ K 1 ½PhNH2 Þð1 þ K 2 cÞ; where K1 and K2 are the adsorption coefficients of aniline and oxygen on Al2O3, k is the specific rate of oxidation of aniline, c is the airflow rate, S is the specific surface area of Al2O3, C is the amount of Al2O3 suspended per liter and I is the light intensity in einstein l1 s1. Linear double reciprocal plots of rate of azobenzene formation versus (i) [PhNH2] and (ii) airflow rate are in agreement with the kinetic law and afford the adsorption coefficients as K1 = 48 l mol1, K2 = 0.18 ml1 s, k = 42 lmol l m2 einstein1. The data fit to the curves (Figs. 4 and 7), governed by the above kinetic law and drawn using a computer program, support the rate expression. However, the rate of photocatalysis fails to increase linearly with the amount of Al2O3 suspended. This is because of high Al2O3 loading. At high Al2O3 loading, the surface area of Al2O3 exposed to illumination does not commensurate with the weight of Al2O3. The amount of Al2O3 employed is beyond the critical amount corresponding to the volume of the reaction solution and reaction vessel; the whole amount of Al2O3 is not exposed to illumination. The photocatalysis lacks linear dependence on illumination intensity; less than first power dependence of surface-photocatalysis rate on light intensity at high intensity is well known [14]. Generally, use of electron donors in semiconductor photocatalysis leads to hole trapping [2]. But, in the photooxidation on non-reactive surface these reagents act as sensitizers. They, on adsorption on alumina and on photoexcitation, gain electron from the adsorbed aniline molecule and generate aniline radical cation thereby enhancing the oxidation. The photonic efficiencies of the oxidation of aniline on alumina are 1.2% and 14% at 365 and 254 nm, respectively. And this is comparable to the reported quantum yield of photodegradation of aniline (1.89%) on titania immobilized on porous nickel using UV light of wavelength 365 nm [10]. However, a similar comparison with benzophenone sensitized [6,7] and ZnO catalyzed [8,9] photooxidations
of aniline could not be made, as the corresponding quantum yields are not reported. 3.5. Photocatalysis in protic and aprotic solvents Adsorption of aniline and oxygen on alumina and the concentration of dissolved oxygen in the reaction medium vary with the solvent. The oxidation of aniline on Al2O3 with sunlight and UV irradiation was carried out in eighteen solvents and the UV–Vis spectra reveal formation of azobenzene in all the solvents studied. The least-squares slopes of the linear absorbance-time traces of photocatalysis with UV light are 25.2, 20.0, 22.0, 42.6, 19.2, 26.3, 21.5, 16.7, 18.8, 15.0, 17.2, 13.0, 12.9, 9.4, 7.5, 11.8, 62.9 and 57.8 (in 106 s1) in ethanol, n-butanol, t-butanol, propane-1,2-diol, 2-butoxyethanol, ethyl methyl ketone, acetic acid, dimethylformamide, acetonitrile, ethyl acetate, 1,4-dioxane, benzene, toluene, chlorobenzene, nitrobenzene, n-hexane, chloroform and carbon tetrachloride, respectively ([PhNH2] = 0.113 M, Al2O3 suspended = 1.0 g, airflow rate = 7.8 ml s1, k = 365 nm, I = 2.46 · 105 einstein l1 s1, volume of reaction solution = 25 ml). The corresponding relative slopes of solar photocatalysis are: 1.00, 1.31, 1.13, 1.76, 1.73, 2.30, 2.96, 0.89, 0.94, 1.01, 1.78, 0.96, 1.00, 1.21, 0.80, 0.71, 3.16 and 3.30 ([PhNH2] = 0.113 M, weight of Al2O3 = 1.0 g, catalyst bed = 12.5 cm2, airflow rate = 4.75 ml s1, volume of reaction solution = 25 mL). Calculation of the photocatalytic oxidation rates in different solvents requires the molar extinction coefficients of cis- and trans-azobenzenes at appropriate wavelengths and the ratio at which cis- and trans-azobenzenes are formed in each solvent and hence could not be made. The relative slopes of solar photocatalysis do not conform to those with UV-light, as they are not the true rates.
4. Conclusions The effects of [aniline], airflow rate, solvent, electron donors, etc., on the rate of solar photocatalyzed oxidation of aniline on Al2O3 are similar to those with UV light; solar photocatalysis is smaller then that with UV light.
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Photooxidation of aniline on alumina with sunlight and artificial UV light Chockalingam Karunakaran *, Sambandam Senthilvelan Department of Chemistry, Annamalai University, Annamalainagar 608002, India Received 14 July 2004; revised 26 November 2004; accepted 26 November 2004 Available online 5 January 2005
Abstract The photooxidation of aniline to azobenzene on alumina in ethanol with natural sunlight and artificial UV light (365 nm) was studied as a function of [aniline], catalyst loading, airflow rate, solvent composition, etc. The metal oxide exhibits sustainable catalytic activity. The product formation is larger with illumination at 254 nm than at 365 nm. Electron donors like triphenylphosphine, diphenylamine and hydroquinone enhance the photocatalysis. Singlet oxygen quencher, azide ion does not suppress the catalysis. The photooxidation occurs in protic and aprotic solvents. The product formation is analyzed using a kinetic model. 2004 Elsevier B.V. All rights reserved. Keywords: Solar photooxidation; UV photooxidation; Aniline; Al2O3
1. Introduction
2. Experimental
Studies on semiconductor photocatalysis are numerous [1–3] but rare are those with insulators (non-reactive solids) such as alumina [4,5]. Air-equilibrated solution of aniline yields azobenzene on irradiation at 365 nm with benzophenone sensitizing the oxidation [6,7]. UVirradiated ZnO brings in the oxidation of aniline to azobenzene [8,9] but TiO2 immobilized on porous nickel photodegrade aniline [10]. Here, we report photooxidation of aniline on alumina with direct sunlight and artificial UV light; reports on solar photocatalysis are also a few and preliminary [11–13], the problem of variation of sunlight intensity even under clear sky is overcome by conducting set of experiments simultaneously and comparing the results.
2.1. Materials Al2O3 (Merck) was used as received and the BET surface area determined as 10.63 m2 g1. Using Easy particle sizer M1.2, Malvern Instruments (focal length 100 mm, beam length 2.0 mm, wet (methanol) presentation) the particle sizes were measured as 57.7, 49.8, 42.9, 11.4, 9.8, 8.5, 7.3, 3.5, 3.0 and 2.6 lm at 3.8%, 17.8%, 8.7%, 1.8%, 28.2%, 15.3%, 10.0%, 1.0%, 9.7% and 3.0%, respectively. Aniline, AR was distilled before use. Commercially available ethanol was distilled over calcium oxide; other organic solvents were of LR grade and distilled prior to use. 2.2. Solar photocatalysis
*
Corresponding author. Tel.: +91 4144 221820; fax: +91 4144 238145. E-mail address: karunakaranc@rediffmail.com (C. Karunakaran). 1566-7367/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.11.014
The solar photocatalyzed oxidations were made from 10.30 a.m. to 12.30 p.m. during summer (March–July) under clear sky. The intensity of solar radiation was measured using Global pyranometer, MCPT, supplied
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by Industrial Meters, Bombay. Fresh solutions of aniline of desired concentrations were taken in wide cylindrical glass vessels of uniform diameter and appropriate height; Al2O3 covered the entire bottom of the vessel. Air was bubbled using a micro pump without disturbing the Al2O3 bed. The volume of the reaction solution was kept as 25 ml and the loss of solvent due to evaporation was compensated periodically. One millilitre of the reaction solution was withdrawn at regular intervals, diluted five times and the absorbance measured at 375 nm using Hitachi U-2001 or Jasco UVIDEC-340 UV–Vis spectrophotometer. 2.3. Photocatalysis with UV light Photocatalytic studies with UV light were carried out in a multilamp photoreactor (HML MP88, supplied by Heber Scientific) fitted with eight 8 W mercury UV lamps of wavelength 365 nm (Sankyo Denki, Japan) and highly polished anodized aluminum reflector; the sample was placed at the centre. Four cooling fans at the bottom of the reactor dissipate the generated heat. The reaction vessel was borosilicate glass tube of 15mm inner diameter. Photooxidation was also carried out in a Heber micro photoreactor (HMI SL W6) fitted with a 6 W 254 nm low-pressure mercury lamp and a 6 W 365 nm mercury lamp. Quartz and borosilicate glass tubes were used for 254 and 365 nm lamps, respectively. The photon flux of the light source (I) was determined by ferrioxalate actinometry. The volume of the reaction solution was always maintained as 25 mL in the multilamp photoreactor and 10 mL in the micro reactor. Air was bubbled through the reaction solution that effectively stirs the solution and keeps the suspended catalyst under constant motion. The absorbance was measured at 375 nm after centrifuging Al2O3 and diluting the solution five times to keep the absorbance within the Beer–Lambert law limit. 2.4. Product analysis Solar photooxidation of aniline in ethanol on Al2O3 yields azobenzene as the only product. The GC–mass [m/z with relative intensities in parentheses 182 (13), 152 (5), 105 (16), 77 (100), 51 (44)], IR and UV–Vis spectra of the extracted solid product are identical with those of trans-azobenzene (Fluka).
Fig. 1. Solar photooxidation of aniline in ethanol on Al2O3. The UV– Vis spectra of the reaction solution diluted five times and recorded at 0, 30, 60, 90 and 120 min (›); [aniline] = 0.113 M, Al2O3 bed = 12.5 cm2, weight of Al2O3 = 1.0 g, airflow rate = 4.75 ml s1, volume of reaction solution = 25 ml.
cis- and trans-azobenzenes during the course of the reaction and the unstable cis form (Z) transforms to the trans form (E) slowly on standing. The UV–Vis spectrum of the irradiated reaction solution but allowed to stand for a few days in dark is identical with that of the authentic trans-azobenzene confirming the slow transformation of the unstable cis form to trans form. For a solution of cis and trans-azobenzenes ½E ¼ ðabs281 eZð433Þ abs433 eZð281Þ Þ =ðeEð281Þ eZð433Þ eZð281Þ eEð433Þ Þ and ½Z ¼ ðabs281 eEð433Þ abs433 eEð281Þ Þ =ðeZð281Þ eEð433Þ eZð433Þ eEð281Þ Þ; where e is the corresponding molar extinction coefficient. Calculation of the ratio [E]/[Z] using the above equations, the experimentally determined eE(433) and eE(281), the reported eZ(433) and eZ(281) and the measured absorbance of the reaction solution at 433 and 281 nm at different periods of the reaction shows that the ratio remains practically the same (1.72) during the course of the photooxidation followed. The total concentration of azobenzene, ([E] + [Z]) = abs375{1 + ([E]/[Z])}/ {eZ(375) + eE(375)([E]/[Z])}; eE(375) was determined experimentally and eZ(375) calculated from the measured abs375; abs375 = {eZ(375) + eE(375)([E]/[Z])}[Z]. 3. Results and discussion
2.5. Product estimation
3.1. Obtaining solar oxidation results
In both solar and UV photocatalysis, the UV–Vis spectra of the reaction solution recorded during the progress of the reaction are similar (Fig. 1; kmax = 375 nm) but not identical with that of the extracted product (kmax = 434 nm). This is because of formation of both
The measurement of solar radiation shows fluctuation of sunlight intensity (530 ± 40 W m2) during the course of the photooxidation even under clear sky. Now, for the first time, identical sunlight intensity was maintained for a set of photooxidation experiments of
C. Karunakaran, S. Senthilvelan / Catalysis Communications 6 (2005) 159–165
0.15
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0.8
Benzene-I
Ethanol-II
Benzene-II
Azobenzene formed / mM
Ethanol-I
Absorbance
0.1
0.05
0.4
Solar-I
Solar-II
UV-I
0
0 60 90 Time / min
120
150
Fig. 2. Solar photooxidation of aniline in ethanol and benzene on Al2O3. Absorbance–time plots (experiments in each set conducted simultaneously and sets I and II on different days; the reaction solution diluted five times prior to absorbance measurements); [aniline] = 0.113 M, Al2O3 bed = 12.5 cm2, weight of Al2O3 = 1.0 g, airflow rate = 4.75 ml s1, volume of reaction solution = 25 ml.
different reaction conditions by carrying out the experiments simultaneously, thus making possible the comparison of the solar results. The solar photooxidation results are reproducible. For example, Fig. 2 is the linear increase of the absorbance of the reaction solution with the reaction time, one set of experiments conducted in ethanol and benzene side by side on one day and the other set similarly on another day. The ratio of the slopes of the absorbance-time profiles of the reactions in ethanol and benzene remains the same (1.1) although the experiments were conducted on different days, obviously under different sunlight intensities. This reproducibility is not surprising as the fluctuation of sunlight intensity is identical in test and control (standard) experiments and the ratio turns out to be independent of sunlight intensity. Further, the results of a pair of experiments performed simultaneously confirm the reproducibility of the rates of solar photocatalysis. Fig. 3 presents the solar photoformation of azobenzene in ethanol under identical conditions and carried out simultaneously. The ratio of the rates obtained from the linear plots is unity (1.01). 3.2. Factors influencing solar photocatalysis
0
50
100
150
Time / min Fig. 3. Photoformation of azobenzene in ethanol on Al2O3; [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
conducted simultaneously. The least squares slope of the linear plot of [azobenzene] versus time (e.g., Fig. 3) yields the rate of formation of azobenzene. Experiments at different [aniline] shows that the reaction rate increases with [aniline] (Fig. 4) and the variation conforms to the Langmuir–Hinshelwood model. The double reciprocal plot of rate versus [aniline] yields a straight line with a positive y-intercept.The variation of the amount of Al2O3 spread at the bottom of the reaction vessel
200 -1
30
Azobenzene formation / nM s
0
160
uv 120
Solar 80
40
0 0
50
100
150
200
250
[Aniline] / mM
The influence of various factors on the solar photocatalysis in ethanol was examined by carrying out the given set of experiments simultaneously; the data in each figure correspond to a set of photocatalytic experiments
Fig. 4. Azobenzene formation in ethanol on Al2O3 at different [aniline]; weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
C. Karunakaran, S. Senthilvelan / Catalysis Communications 6 (2005) 159–165 200
200 -1
Solar UV 100
0 0
1 2 Catalyst loaded / g
3
Fig. 5. Azobenzene formation in ethanol at different amounts of Al2O3 loading; [aniline] = 0.113 M, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
(catalytic bed) does not lead to any appreciable change in the photocatalysis rate (Fig. 5); the bottom of the cylindrical reaction vessel was fully covered by Al2O3 in all the cases and the increase of the amount of Al2O3 does not lead to increase of the area of the catalyst bed but only results in increased thickness of the Al2O3 bed. In the absence of the photocatalyst the reaction is an uncatalyzed one and hence a small rate. The photoformation of azobenzene increases linearly with the apparent area of the catalyst bed (Fig. 6). Study of the photooxidation as a function of airflow rate reveals enhancement of photocatalysis by oxygen (Fig. 7). The variation of reaction rate with the airflow rate indicates Langmuir–Hinshelwood kinetics and the linear double reciprocal plot of reaction rate versus airflow rate confirms the same. The reaction was also studied without bubbling air but the solution was not deaerated. The dissolved oxygen itself brings in the oxidation but the pho-
Azobenzene formation / nM s
Azobenzene formation / nM s
-1
162
160
uv
120
Solar
80
40
0
0
5
10
15
20
25
-1
Airflow rate / ml s
Fig. 7. Azobenzene formation in ethanol on Al2O3 at different airflow rates: [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
tocatalysis is weak. The reaction does not occur in dark. Al2O3 used does not lose its activity on repeated use; reuse of Al2O3 yields identical results. Presence of water in the reaction medium slows down the reaction (Fig. 8). Addition of electron donors like triphenylphosphine (TPP), hydroquinone (HQ) and diphenylamine (DPA) enhances the photoformation of azobenzene. The variation of the enhanced photoformation with [TPP], [HQ] and [DPA] (Fig. 9) reveals Langmuir–Hinshelwood kinetics and the linear double reciprocal plots of the enhanced rate versus [TPP], [HQ] and [DPA] confirm the same. However, triethylamine (TEA) fails to facilitate the oxidation; addition of TEA (0.287 M) to the reac-
Azobenzene formation / nM s
-1
150
Azobenzene formation / nM s
-1
200
Solar
100
0
Solar UV 100
50
0 60
0
25 2 Catalyst bed / cm
50
Fig. 6. Azobenzene formation (solar) in ethanol at varying areas of Al2O3 bed; [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 ml s1.
80
100
%EtOH (v/v) Fig. 8. Azobenzene formation on Al2O3 in aq. EtOH; [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
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200 HQ DPA TPP
Solar 400
Enhanced rate / nM s-1
Azobenzene formation / nM s
-1
500
300
200
uv uv
100
uv
Solar
Solar
UV
100
0 0
0 0
80
160
240
320
400
[ED] / mM Fig. 9. Azobenzene formation in ethanol on Al2O3 in presence of electron donors (ED); [aniline] = 0.113 M, weight of Al2O3 = 1.0 g, volume of reaction solution = 25 ml, airflow rate = 4.75 (solar), 7.8 (UV) ml s1; solar: Al2O3 bed = 12.5 cm2; UV: k = 365 nm, I = 2.46 · 105 einstein l1 s1.
tion solution does not alter the reaction rate. Anionic and cationic surfactants enhance the photocatalysis; addition of aerosol OT (sodium bis-2-ethylhexyl sulfosuccinate, 0.0225 M), sodium lauryl sulfate (SLS, 0.0347 M) and cetyltrimethylammonium bromide (CTAB, 0.0274 M) to the reaction solution (conditions as in Fig. 9) increases the photoformation rate by 28%, 68% and 50%, respectively. Vinyl monomers like acrylonitrile (0.608 M) and acrylamide (0.141 M) neither suppress the photocatalysis nor undergo polymerization indicating the absence of free radicals in the reaction solution during the course of photocatalysis. 3.3. Factors influencing photocatalysis with UV light The photocatalyzed oxidation of aniline in ethanol in the presence of air on Al2O3 was studied using a multilamp photoreactor with mercury UV lamps of wavelength 365 nm. The linear increase of [azobenzene] with illumination time yields the photochemical formation rate (e.g., Fig. 3) and the rates are reproducible to ±6%. Rate measurements at different [aniline] show increase of the oxidation rate with [aniline] (Fig. 4) and the increase is according to Langmuir–Hinshelwood kinetics. The increase of the amount of Al2O3 suspended in the reaction medium does not significantly enhance azobenzene formation (Fig. 5). Study of the photooxidation as a function of airflow rate reveals enhancement of photocatalysis by oxygen and the variation of the reaction rate with flow rate conforms to the Langmuir–Hinshelwood kinetics (Fig. 7). The reaction was also studied without bubbling air but the solution was not deoxygenated. The dissolved oxygen itself brings in the oxidation but the photocatalysis is slow. The photooxidation was
25 6
50 -1
10 Intensity / einstein l s
-1
Fig. 10. Azobenzene formation in ethanol on Al2O3 at different light intensities; [aniline] = 0.113 M, Al2O3 suspended = 1.0 g, airflow rate = 7.8 ml s1, k = 365 nm, volume of reaction solution = 25 ml.
examined as function of light intensity. The oxidation was carried out with two, four and eight lamps, the angles sustained by the adjacent lamps at the sample are 180, 90 and 45, respectively. Fig. 10 presents the variation of rate with the light intensity. The reaction does not occur in dark. Investigation of the photocatalysis using a 6 W 365 nm mercury lamp (I = 1.81 · 105 einstein l1 s1) and a 6 W 254 nm low-pressure mercury lamp (I = 5.22 · 106 einstein l1 s1) separately in the micro reactor under identical conditions reveals that high energy radiation is more effective in bringing out the photocatalysis. Azobenzene formed in 10 min on illumination at 365 and 254 nm are 132 and 436 lM, respectively, and the corresponding quantum yields are 0.012 and 0.14 ([aniline] = 0.113 M, Al2O3 suspended = 0.2 g, airflow rate = 7.8 ml s1, volume of reaction solution = 10 ml). Al2O3 employed does not lose its activity on illumination. Reuse of the Al2O3 reveals sustainable photocatalytic efficiency. Water in the reaction medium suppresses the photocatalysis (Fig. 8). Addition of electron donors like TPP, HQ, DPA and TEA enhances azobenzene formation. The variation of the enhanced photocatalysis rate with [TPP], [HQ] and [DPA] suggests Langmuir–Hinshelwood kinetics (Fig. 9). TEA (0.287 M) in the reaction solution increases the rate of azobenzene formation by 22%. While aerosol OT (0.0225 M) fails to influence the catalysis SLS (0.0347 M) and CTAB (0.0274 M) enhance the reaction by 22% and 16%, respectively (conditions as in Fig. 9). Also, vinyl monomers like acrylonitrile (0.608 M) and acrylamide (0.141 M) fail to inhibit the photocatalysis. Nor do they polymerize. Azide ion (0.154 M), a singlet oxygen quencher, fails to suppress the formation of azobenzene indicating the absence of involvement of singlet oxygen in the photocatalysis.
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3.4. Kinetic analysis Al2O3 provides an ordered two-dimensional environment for effective electron transfer from the donor to the acceptor. The donor aniline molecule may undergo photoexcitation followed by electron transfer to oxygen molecule, both adsorbed on the non-reactive surface. The reaction of aniline radical-cation with superoxide radical-anion results in nitrosobenzene. Condensation of nitrosobenzene with aniline, present in large excess, yields azobenzene. This is in agreement with the reported photodegradation of aniline on titania immobilized on porous nickel when present at ppm level [10]. The kinetic law that governs heterogeneous photocatalyzed reaction in a continuously stirred tank reactor (CSTR) [5] is rate ¼ kK 1 K 2 SIC½PhNH2 c=ð1 þ K 1 ½PhNH2 Þð1 þ K 2 cÞ; where K1 and K2 are the adsorption coefficients of aniline and oxygen on Al2O3, k is the specific rate of oxidation of aniline, c is the airflow rate, S is the specific surface area of Al2O3, C is the amount of Al2O3 suspended per liter and I is the light intensity in einstein l1 s1. Linear double reciprocal plots of rate of azobenzene formation versus (i) [PhNH2] and (ii) airflow rate are in agreement with the kinetic law and afford the adsorption coefficients as K1 = 48 l mol1, K2 = 0.18 ml1 s, k = 42 lmol l m2 einstein1. The data fit to the curves (Figs. 4 and 7), governed by the above kinetic law and drawn using a computer program, support the rate expression. However, the rate of photocatalysis fails to increase linearly with the amount of Al2O3 suspended. This is because of high Al2O3 loading. At high Al2O3 loading, the surface area of Al2O3 exposed to illumination does not commensurate with the weight of Al2O3. The amount of Al2O3 employed is beyond the critical amount corresponding to the volume of the reaction solution and reaction vessel; the whole amount of Al2O3 is not exposed to illumination. The photocatalysis lacks linear dependence on illumination intensity; less than first power dependence of surface-photocatalysis rate on light intensity at high intensity is well known [14]. Generally, use of electron donors in semiconductor photocatalysis leads to hole trapping [2]. But, in the photooxidation on non-reactive surface these reagents act as sensitizers. They, on adsorption on alumina and on photoexcitation, gain electron from the adsorbed aniline molecule and generate aniline radical cation thereby enhancing the oxidation. The photonic efficiencies of the oxidation of aniline on alumina are 1.2% and 14% at 365 and 254 nm, respectively. And this is comparable to the reported quantum yield of photodegradation of aniline (1.89%) on titania immobilized on porous nickel using UV light of wavelength 365 nm [10]. However, a similar comparison with benzophenone sensitized [6,7] and ZnO catalyzed [8,9] photooxidations
of aniline could not be made, as the corresponding quantum yields are not reported. 3.5. Photocatalysis in protic and aprotic solvents Adsorption of aniline and oxygen on alumina and the concentration of dissolved oxygen in the reaction medium vary with the solvent. The oxidation of aniline on Al2O3 with sunlight and UV irradiation was carried out in eighteen solvents and the UV–Vis spectra reveal formation of azobenzene in all the solvents studied. The least-squares slopes of the linear absorbance-time traces of photocatalysis with UV light are 25.2, 20.0, 22.0, 42.6, 19.2, 26.3, 21.5, 16.7, 18.8, 15.0, 17.2, 13.0, 12.9, 9.4, 7.5, 11.8, 62.9 and 57.8 (in 106 s1) in ethanol, n-butanol, t-butanol, propane-1,2-diol, 2-butoxyethanol, ethyl methyl ketone, acetic acid, dimethylformamide, acetonitrile, ethyl acetate, 1,4-dioxane, benzene, toluene, chlorobenzene, nitrobenzene, n-hexane, chloroform and carbon tetrachloride, respectively ([PhNH2] = 0.113 M, Al2O3 suspended = 1.0 g, airflow rate = 7.8 ml s1, k = 365 nm, I = 2.46 · 105 einstein l1 s1, volume of reaction solution = 25 ml). The corresponding relative slopes of solar photocatalysis are: 1.00, 1.31, 1.13, 1.76, 1.73, 2.30, 2.96, 0.89, 0.94, 1.01, 1.78, 0.96, 1.00, 1.21, 0.80, 0.71, 3.16 and 3.30 ([PhNH2] = 0.113 M, weight of Al2O3 = 1.0 g, catalyst bed = 12.5 cm2, airflow rate = 4.75 ml s1, volume of reaction solution = 25 mL). Calculation of the photocatalytic oxidation rates in different solvents requires the molar extinction coefficients of cis- and trans-azobenzenes at appropriate wavelengths and the ratio at which cis- and trans-azobenzenes are formed in each solvent and hence could not be made. The relative slopes of solar photocatalysis do not conform to those with UV-light, as they are not the true rates.
4. Conclusions The effects of [aniline], airflow rate, solvent, electron donors, etc., on the rate of solar photocatalyzed oxidation of aniline on Al2O3 are similar to those with UV light; solar photocatalysis is smaller then that with UV light.
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