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Copper-supported pillared clay catalysts for the wet hydrogen peroxide catalytic oxidation of model pollutant tyrosol
Applied Catalysis A: General 349 (2008) 20–28
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Copper-supported pillared clay catalysts for the wet hydrogen peroxide catalytic oxidation of model pollutant tyrosol R. Ben Achma a, A. Ghorbel a, A. Dafinov b, F. Medina b,* a b
Laboratoire de Chimie des Mate´riaux et Catalyse, De´partement de Chimie, Faculte´ des Sciences de Tunis. Campus Universitaire, 2092 El Manar, Tunis, Tunisia Departament d’Enginyeria Quı´mica, Universitat Rovira i Virgili, Av. Paı¨sos Catalans 26, 43007 Tarragona, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 March 2008 Received in revised form 25 June 2008 Accepted 10 July 2008 Available online 25 July 2008
The catalytic wet hydrogen peroxide oxidation of tyrosol, a major compound of the polyphenolic fraction present in olive oil mill wastewaters, was studied in batch and continuous reactors using coppersupported pillared clay catalysts under mild conditions. The catalysts were prepared by the solid-state reaction of Al-pillared clay synthesized with copper nitrate. The resultant materials were then calcined at 300 8C for 3 h under helium or oxygen. The catalytic activity of Cu-supported pillared clays was found to depend on the calcination method. The effects of several operating conditions were also studied. Our experimental results indicate that, under mild conditions and a stoichiometric amount of oxidant, the calcined material under helium (CuNHe catalyst) showed higher activity and stability (TOC abatement of roughly 80% and total elimination of tyrosol after 1 h reaction, without significant leaching of copper ions) than the calcined material under oxygen (CuNOxy). To better understand the catalytic behaviour of copper-supported pillared clay solids, fresh and used catalysts were characterized by X-ray diffraction, nitrogen adsorption, chemical analysis, temperature-programmed reduction (TPR), UV–visible diffuse reflectance spectroscopy, and transmission electron microscopy (TEM). ß 2008 Elsevier B.V. All rights reserved.
Keywords: Copper Pillared clay Wet hydrogen peroxide catalytic oxidation 2-(4-Hydroxyphenyl)ethanol
1. Introduction The Mediterranean countries are the main producers of olive oil. During olive oil extraction, large quantities (roughly 7 million tonnes per year) of strongly polluted water, known as olive oil mill wastewaters (OMW), are generated [1]. Because of its content (14– 15%) of organic substances and phenols (up to 10 g L1), OMW is thought to be one of the most polluting effluents produced by the agrofood industries, which are characterized by high specific COD (chemical oxygen demand) [2–4]. The untreated release of OMW onto land is a danger for the environment. Several studies have shown that simple OMW phenolic compounds with low molecular weights are responsible for toxicity on seed germination [5], aquatic organisms [6–8], and bacteria [9]. Recovering these phenolic compounds, which have interesting antioxidant properties, could provide an interesting alternative source of biologically active polyphenols [10]. Several researchers have evaluated the feasibility and economic processes involved in the recovery of these phenolic compounds from OMW or solid wastes [11,12]. After these processes, however, further treatments are needed to
* Corresponding author. E-mail address: [email protected] (F. Medina). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.021
eliminate the toxicity of the effluents before they are discharged into the natural stream. For these reasons, research has focused on developing efficient treatment technologies, including physical, chemical and biological processes [13]. Oxidation systems have often been used as pretreatments to decrease OMW toxicity and allow biological degradation. Since around 1980, attention has been paid to the development of an effective oxidation process called wet oxidation. In catalytic wet oxidation the reaction conditions are milder than in uncatalysed wet oxidation, but high temperature (over 150 8C) and high pressure (1–5 MPa) are still required. These severe operating conditions lead to high installation costs, which limit the practical applications of the process. Catalytic wet peroxide oxidation (CWPO) could be a more efficient process because the oxidizing properties of hydrogen peroxide are stronger than those of molecular oxygen. Moreover, the reaction conditions when hydrogen peroxide is involved as oxidant are close to the ambient ones (0.1 MPa, T < 80 8C). This enables a large amount of wastewater to be treated with the minimum consumption of energy. Heterogeneous catalysis would be preferred to homogeneous catalysis if a stable and active catalyst under operation conditions were found. Several materials have been proposed as catalysts for the oxidation of organic compounds in water [14,15]. However, deactivation was often observed with the solubilisation
R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28
of the active phase. To get round this problem, stable catalysts have recently been developed. Transition-metal exchanged zeolites [16,17], Cu2+-containing pillared montmorillonite [18,19], and Fe3+- or Cu2+-pillared clays [20–24] have been proposed as active catalysts for the oxidation of organic compounds. These catalysts show a higher rate of conversion of the pollutants and a lower sensitivity to pH than iron ions in solution at the same reaction conditions. However, some of them exhibit partial leaching of the cations, probably because the metal is not bound efficiently in the solid, for example in the pillars for the clays [21,25,26]. Tatiboue¨t and co-workers suggested that the high resistance to leaching and the good catalytic performance of the pillared clay catalysts could be attributed to a strong interaction between the metal species and the catalytic support [27]. In this context we have developed a simple, new protocol for Al–Cu-pillared clay preparation and studied the activity and stability of the catalysts obtained for the catalytic wet peroxide oxidation (CWPO) of tyrosol. p-Hydroxyphenylethanol (tyrosol) was chosen as the model compound because, like hydroxytyrosol, which can be produced during the oxidation of tyrosol, it is one of the most significant phenols in OMW. Also, fewer studies in the literature are related to the oxidation of tyrosol than to the oxidation of phenol, p-coumaric acid or p-hydroxybenzoic acid. 2. Experimental 2.1. Materials Tyrosol, 2-(p-Hydroxyphenyl)-ethanol 98% from Aldrich Co. was used in this study. Commercial Wyoming montmorillonite from Comptoir des Mine´raux (France) was used as starting clay. Al and Cu nitrate (Aldrich Co.) were used as precursors for the pillaring solution and solid reaction, respectively. 2.2. Catalysts preparation 2.2.1. Starting material The Wyoming montmorillonite was sieved to obtain an extracted fraction with a particle size of less than 2 mm. Its cation exchange capacity, determined by the adsorption of copper ethylenediamine complex, was 100 mequiv./100 g (ignited) clay, and its BET surface area was 29 m2/g. The starting material for the pillaring procedure was the sodium form of this montmorillonite, which was obtained by treatment with 1 mol L1 NaCl solution (three times exchanged), followed by washing and dialysis until the result of the reaction for the presence of Cl was negative. The product was then air dried at 60 8C. 2.2.2. Catalysts synthesis Alumina-pillared clay was prepared by slowly adding Alpillaring solution into an aqueous suspension of montmorillonite. The pillaring solution was prepared by dissolving 0.2 mol L1 Alnitrate in 0.45 mol L1 NaOH solution. The hydrolysis molar ratio OH/Al was kept at 2.25. The pH of the solution was about 3.8 and the solution remained clear. The diluted montmorillonite suspension (10 g L1) was prepared by adding the montmorillonite powder into the distilled water. The pillaring reaction was carried out under continuous vigorous stirring by dropwise adding the pillaring solution into the montmorillonite suspension. After centrifugation, the solid fraction was washed by dialysis in distilled water and dried at room temperature. The sample was then calcined for 5 h at 500 8C under airflow. The temperature was raised to 500 8C at a rate of 1 8C/min and this temperature was maintained for 5 h.
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The copper-supported Al-pillared clays were prepared by the solid ion exchange method. Typically, 5 g of the already synthesized Al-pillared clay were intimately mixed with the desired percentage of copper nitrate [Cu(NO3)39H2O] in an agate mortar for 10 min. This was followed by heating at 300 8C for 3 h under helium. The solid was then cooled to ambient temperature, washed six times with distilled water, dried at 80 8C for 24 h, and then calcined at 300 8C for 3 h under helium flow for the first (referenced as CuNHe) and under oxygen flow for the second (referenced as CuNOxy). CuN stands for copper nitrate precursor. 2.3. Characterizations The X-ray powder diffraction (XRD) patterns were measured on a Philips PW 1730/10 diffractometer CuKa (l = 1.54184 A˚) radiation. The position of the d0 0 1 ray in the XRD is related to the interlayer distance and, therefore, to the pillar height. Quantitative chemical analysis of the copper in the modified clays was determined by X-ray fluorescence using SFX Siemens SRS 330, and the amount of leached metal was determined by atomic absorption spectrometry using a PerkinElmer 400 spectrophotometer. Nitrogen adsorption experiments, for samples previously degassed at 120 8C for 12 h, were performed at 196 8C using a Micromeritics ASAP 2000 instrument. Specific surface area was calculated by the BET method. Temperature-programmed reduction (TPR) measurements were performed using a Thermofinnigan TPDRO 1100 system. A mixture of H2 (5%) in argon with a flow rate of 20 mL min1 was used to reduce the samples (0.12 g), which were placed in an oven heated from 40 to 990 8C at a heating rate of 10 8C min1 and maintained at 990 8C for 20 min. The reduction of CuO to metallic copper was used to calibrate the TPR apparatus for H2 consumption. UV–vis diffuse reflectance (UV–vis-DR) spectra were recorded in air on powdered samples using a Jasco V-570 spectrometer equipped with an integrating sphere for solid samples. The reference was BaSO4. Transmission electron microscopy (TEM) observations were carried out on a JEOL 2010 microscope (200 kV, resolution 0.19 nm). Samples for direct examination were prepared by suspending the powder in EtOH, and a drop of the suspension was allowed to dry on a copper grid. When the contrast was low, extractive replica were used. A drop of the suspension was deposited on freshly cleaved mica. After drying, the dispersed powder was covered by a carbon film. The mica was plunged into a solution containing a mixture of water, acetone and hydrofluoric acid to dissolve the support without dissolving the metal particles. These remained stuck to the carbon film, which was collected on a copper grid. 2.4. Oxidation of tyrosol The catalytic oxidation of tyrosol was carried out in a stirred (stirring rates of between 500 and 1200 rpm) and thermostated Pyrex well-mixed slurry batch reactor of 250 mL. The reaction was performed in the 25–80 8C temperature range and at a pH of 5.6 (the natural pH of the solution) using a 500 ppm tyrosol (3.6 mmol L1) aqueous solution in contact with different catalysts loading under continuous stirring. After 5 min of stirring, a hydrogen peroxide solution was added (time zero of the reaction) in one time. The H2O2/tyrosol molar ratio was 19/1, which is the stoichiometric quantity needed to totally transform the tyrosol into CO2 according to the reaction: C8 H10 O2 þ 19H2 O2 ! 8CO2 þ 24H2 O
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R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28 Table 1 Textural Properties of fresh and used catalysts (calcined at different temperatures) derived from the nitrogen adsorption/desorption isotherms at 77 K Catalysts
SBET (m2/g)
Sext (m2/g)
Sint (m2/g)
Vp (cm3/g)
Vmp (cm3/g)
Cu (wt.%)
Al-PILC CuNHe CuNOxy Used CuNHe Used CuNOxy
204 123 52 27 12
67 23 32 19 7
137 100 20 8 5
0.120 0.115 0.065 0.061 0.035
0.080 0.048 0.029 0.019 0.002
0 1.93 1.93
Total surface area obtained from the BET equation (SBET), micropore area obtained from the t-plot method (Sint), and mesopore area (Sext). Micropore volume obtained from the t-plot method (Vmp), and the total pore volume at P/P0 = 0.99 (Vp).
Fig. 1. X-ray diffraction (XRD) patterns of copper-supported samples. The Al-PILC pattern (a) was added at reference.
Samples withdrawn at regular times were immediately filtered and analysed for organic compounds and residual H2O2 in solution by a high performance liquid chromatograph (Dionex HPLC) equipped with a C18column (4.6 mm 250 mm; Shimpack VPODS). Identifications and quantifications of the aromatic compounds and organic acids were achieved by comparing with standards. The total organic content TOC of the solution (in mg of carbon per litre of solution) was measured with a Shimadzu Model 5050 TOC-analyser.
this difference: a decrease in the interlayer distance during doping or filling and/or the blocking of pores by copper species. X-ray investigations have shown that the interlayer distances are not significantly affected by the doping process [28]. This indicates that the mechanism responsible for the reduction of surface areas and pore volumes may be the blocking of pores by copper species. As we can see from Table 1, a large contribution of mesoporous is also observed in this sample, which suggests an important relation between the textural properties of the doped samples and the preparation method. It is likely, therefore, that during the ionexchange process part of the copper species was deposited outside the interlayer space. Upon calcination, these species produce external mesoporous copper particles that could significantly increase the external surface area while blocking the internal surface [29]. Bearing these results in mind, we took temperature-programmed reduction (TPR) measurements. TPR can be used to characterize the dispersion of metal ions in catalysts prepared via different routes. The reduction profile provides information about the dispersion of the metal species on the support and about the interaction between these metal species and the support. It has been reported that isolated Cu2+ ions can be reduced to Cu0 by hydrogen in two steps: first, the reduction of Cu2+ to Cu+; and second, the reduction of the Cu+ to Cu0 [30–34]. However, copper oxide aggregates (CuO) are reduced directly in one step to Cu0. The
3. Results and discussion 3.1. Characterization of the fresh catalysts Fig. 1 shows the X-ray diffraction (XRD) patterns for the parent Al-PILC calcined at 500 8C and for copper-supported samples. Taking the position of the peak assigned to the (0 0 1) reflection in the 2u axis and using the equation of Bragg, the value of the d(0 0 1) in Al-PILC was estimated to be 1.82 nm, and for the natural clay it was about 1 nm. Adding copper to the pillared clay for the CuNHe sample did not affect the XRD pattern of the initial pillared clay. However, the reflection of the basal (0 0 1) plane for the CuNOxy sample was broader. This indicates that some distortion in the pillared clay is produced by adding copper to the CuNOxy sample. No copper oxide phases were observed in the XRD patterns of the doped catalysts, which indicates a high dispersion of copper in the two samples prepared by the two calcination methods (the amount of copper in the two samples was around 2.0 wt.%). It also appears that the sample calcined under helium was more crystalline than the one calcined under oxygen. Table 1 summarizes the textural characteristics of Al-PILC and fresh and used Cu-supported pillared clay catalysts. Doping Al-PILC with copper affects the textural properties of the resulting material. When the sample was calcined under oxygen, there was a significant micropore surface area loss compared with the sample calcined under helium. There are two potential reasons for
Fig. 2. Temperature-programmed reduction (TPR) curves for Al-PILC and coppersupported samples.
R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28
Fig. 3. UV–vis diffuse reflectance spectra of copper-supported samples.
total amounts of hydrogen uptake and the H2/Cu ratio could provide information about the reduction of the copper species [34]. The TPR profiles for the CuNOxy, CuNHe and the support (Al-PILC) are shown in Fig. 2. The Al-PILC support showed a broad peak at a higher reduction temperature (between 500 and 700 8C), which can probably be attributed to the reduction of some iron species in the initial Al-PILC material [32]. However, the amount of iron is less than 0.12 wt.%. Adding copper to the clay structure (around 2.0 wt.%) results in new reduction peaks detected at lower temperatures. For the first sample (CuNOxy), we observed two reduction peaks. The first one, centered at 326 8C (which represents about 60% of the total copper species), was attributed to the reduction of copper oxide located on the surface of the support, probably in the entrance of the pores. The presence of these copper species led to a reduction in pore volume and BET values due to the filling of the interlayer [33]. The second peak, which was less pronounced and about 390 8C, may be related to copper oxide species grafted onto the pillars of the clay. For the second catalyst (CuNHe), the TPR curve shows an overlapping of these two peaks with a net decrease in the first peak (35% as opposed to 60% for the CuNOxy catalyst) related to the reduction in the copper adsorbed on the surface of the clay. The second peak, which may be related to the copper oxide species located in the pillars, is the most abundant. The H2/Cu ratio for the catalysts was around 0.90 and 0.92 for CuNOxy and CuNHe samples, respectively. This indicates that the main species detected is copper oxide, which is reduced to Cu0 in one step [34]. These two reduction peaks could be accounted for by CuO aggregates with different particle sizes and dispersion or by a different interaction between the CuO aggregates and the support (the surface of the clay or the pillars).
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The UV–vis diffuse reflectance spectra of CuNX catalysts are shown in Fig. 3. For the CuNHe catalyst, absorption in the 45,450– 40,000 cm1 (220–250 nm) range corresponds to the charge transfer transition (Cu2+ O2) of isolated Cu2+ ions in octahedral environment [35,36]. The maximum at roughly 29,702 cm1 (336 nm) can be attributed to charge-transfer between Cu2+ and oxygen in oligonuclear [Cu–O–Cu]n species, while the peak at roughly 21,902 cm1 (456 nm) characterizes the presence of Cu+ three-dimensional clusters in CuO [37,38]. For the second catalyst (CuNOxy), when oxygen was used for treatment the distortion of the octahedron decreased. In fact, the spectrum of the CuNHe solid shows Cu2+ species in Oh more or less tetragonally distorted (d–d transition at 14,301 cm1 (699 nm)), while in the CuNOxy, the band at 13,111 cm1 (762 nm) is assigned to Cu2+ in a nearly perfect Oh symmetry. Fig. 4 shows representative micrographs of the two samples. The CuNHe sample contains small particles of copper with a nicely homogeneous particle size in the 1.7–2.0 nm range (Fig. 4a). For the (CuNOxy) sample, however, these particles were larger, with a broad particle size distribution of between 3.0 and 12.0 nm (Fig. 4b). All these results indicate that the preparation method is crucial to the properties of the obtained materials. 3.2. Oxidation of tyrosol We did preliminary tests to check the reactivity of tyrosol in the presence of H2O2 without the catalyst and to check the decomposition of hydrogen peroxide in the absence of pollutant with and without the catalyst. When working at 25 8C and a H2O2/ tyrosol molar ratio of 20, the conversion of tyrosol was negligible (less than 3% after 24 h). Without the catalysts and pollutant, decomposition of hydrogen peroxide was roughly 2% after 24 h at 25 8C. When the catalyst was introduced during the same experiment, decomposition increased to 6%. These results indicate a very low decomposition of hydrogen peroxide in these reaction conditions. We also evaluated the contribution by adsorption phenomena on the catalyst and the results were negligible (less than 1% of adsorbed amount after 24 h). The consumption of hydrogen peroxide during the catalytic tests can therefore be attributed to tyrosol degradation. Moreover, during the catalytic tests, the catalyst was used in the form of very fine powder (<2 mm) to avoid problems of interphase diffusion. Two series of experiments were completed by varying the concentration of the catalyst (0.25–1.5 g L1) and the hydrogen peroxide (H2O2)/tyrosol molar ratio from 5 to 20, and working at a reaction temperature of 25 8C in order to see how these parameters affect tyrosol conversion. Fig. 5 shows that, as expected, the initial degradation rate increased as the concentration of catalyst (CuNHe)
Fig. 4. TEM images of (a) CuNHe and (b) CuNOxy.
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Fig. 5. Catalyst concentration effect on the initial tyrosol degradation rate. [Tyrosol] = 500 ppm, T = 298 K and [H2O2] = 6.8 102 M, stirring rate of 900 rpm.
Fig. 6. Effect of hydrogen peroxide concentration on tyrosol conversion. [Tyrosol] = 500 ppm [catalyst] = 0.5 g L1 and T = 298 K.
increased to 0.5 g L1. This figure also shows a slight decrease at the value of 1 g L1. Similar behaviour was observed for the CuNOxy catalyst. Results were also similar with stirring rates of between 500 and 1200 rpm, which indicates that no external mass transfer limitation occurs under the reaction conditions. However, the results for a catalyst concentration of over 0.5 g L1 indicate that some diffusion limitation occurs. If we take into account the diffusion limitation phenomenon (which decreases the accessible number of surface active sites), the optimum concentration for this catalyst could be situated around 0.5 g L1 for the given tyrosol concentration. On the other hand, the results in Fig. 6 show that in these conditions the reaction cannot take place for a H2O2/tyrosol molar ratio of less than or equal to 5. As the concentration of H2O2 increases, the degradation of tyrosol is higher because more hydroxyl radicals are formed. We also conducted several experiments to determine how different reaction temperatures influence the effectiveness of tyrosol degradation. Fig. 7a shows the influence of reaction temperatures of between 25 and 80 8C on tyrosol conversion and the concentration of residual hydrogen peroxide (Fig. 7b) when the reaction is performed in the presence of the CuNHe catalyst. Our results show practically total conversion
of tyrosol after 2 h at a reaction temperature of 25 8C. When the reaction temperature increased to 40 8C, only 1 h was needed to achieve total conversion of tyrosol. TOC removal at reaction temperatures of 25, 40, 60 and 80 8C are shown in Table 2. TOC removal was around 60% for the CuNHe catalyst after 24 h at a reaction temperature of 25 8C. However, we should point out that the residual concentration of hydrogen peroxide in the solution was around 40% of the initial concentration. This indicates practically total efficiency of the hydrogen peroxide at these reaction conditions. The initial pH value of the sample (around 5.6) decreased to around 3.4, which indicates that, for large reaction times, the products obtained are mainly formed by carboxylic acids that are very difficult to degrade. Oxalic acid is the main product detected by HPLC and represents more than 98% of the TOC after 24 h at a reaction temperature of 25 8C. However, for short reaction times (1–4 h), other products such as 3,4dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid and 3,4dihydroxymandelic acid were obtained, though these products always represented a small part of tyrosol conversion. For example, the largest amount of 3-4 dihydroxyphenylethanol, which, excluding oxalic acid, is the main product was obtained after 3 h of reaction and represented around 5% percent of tyrosol
Fig. 7. (a) Effect of the temperature on tyrosol conversion. [Tyrosol] = 500 ppm [CuNHe] = 0.5 g L1 and [H2O2] = 6.8 102 M. (b) Residual H2O2 (for CuNHe catalyst).
R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28
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Table 2 Change of TOC as a function of time and temperature of reaction of oxidation of tyrosol Time (h)
0 1 2 4 6 8 24 a
25 8Ca
40 8Ca
60 8Ca
80 8Ca
TOC (mg L1)
pH
TOC (mg L1)
pH
TOC (mg L1)
pH
TOC (mg L1)
pH
346.56 247.70 232.70 227.40 194.10 164.00 145.40
5.60 4.79 4.53 4.10 3.85 3.73 3.38
346.56 198.00 178.40 162.90 156.00
5.60 3.91 3.49 3.31 3.38
346.56 152.00 141.20 133.40 125.00
5.60 3.07 3.77 4.19 4.36
346.56 87.78 82.80 76.55 72.04
5.60 3.80 3.78 3.82 3.96
Temperature.
Fig. 8. (a) Effect of the temperature on tyrosol conversion. [Tyrosol] = 500 ppm [CuNOxy] = 0.5 g L1 and [H2O2] = 6.8 102 M. (b) Residual H2O2 (for CuNOxy catalyst).
conversion, which is very low compared to other results with Fepillared clays [22]. This could indicate that the Cu-pillared clay catalyst is more efficient than the Fe-pillared clay catalyst for the total mineralization of tyrosol. At higher reaction temperatures, there was an important increase in activity for this catalyst. After 1 h of reaction, TOC removal was around 40%, 57% and 75% for reaction temperatures of 40, 60 and 80 8C, respectively. This was accompanied by a decrease in pH. At higher reaction times (mainly at 80 8C), TOC removal remained practically constant, being around 75% after 1 h and around 79% after 6 h. This could be because practically all the hydrogen peroxide was consumed in the first hour of reaction (see Fig. 7b). New additions of hydrogen peroxide lead to the total removal of TOC. We should bear in mind, however, that even at this high reaction temperature (80 8C), the efficiency of the hydrogen peroxide is around 80%. We studied the CuNOxy catalyst under the same reaction conditions as for the CuNHe catalyst. The conversion of tyrosol over time and the residual concentration of hydrogen peroxide present during the reaction are depicted in Fig. 8a and b, respectively. These results show that the activity of the CuNOxy catalyst and the efficiency of hydrogen peroxide are lower than with the CuNHe catalyst. Therefore, the sample preparation method plays an important role in the activity of the catalysts. Reusability of the catalysts was investigated in the oxidation of tyrosol with three consecutive experiments using the same catalyst after separation from the solution. After a first run (24 h reaction at 25 8C), the catalyst was removed from the reactor, then
washed with water, dried at 100 8C and reused. Fig. 9 plots the results of tyrosol conversion and TOC abatement against reaction time for the CuNHe catalyst. Performance was excellent even after three runs, both in terms of tyrosol conversion and TOC removal. However, for the CuNOxy catalyst, almost total deactivation of the catalyst was observed after three runs. To explain this behaviour, we studied the effect of the leaching of copper in the solution for both catalysts. For the CuNOxy catalyst, the amount of copper dissolved in the reaction medium after the first run was around 1 ppm, which represents 2.5% of the initial copper in the sample. For the CuNHe catalyst, however, it was around 0.3 ppm. To quantify the copper effect dissolved in the medium (homogeneous catalysis), we evaluated the catalytic activity in a solution with 1 ppm of Cu2+. These experiments indicated that for both catalysts the contribution of this dissolved copper to the activity was negligible. The blank test using 1 ppm of copper at the same reaction conditions (25 8C) showed a conversion of tyrosol of around 4% after 24 h of reaction (similar to a blank test without dissolved copper or in the absence of catalyst). Also, the activities of the re-used and fresh CuNHe catalysts were similar, which indicates the absence of significant deactivation. We also studied the stability of the most active and stable catalyst (CuNHe) under reaction conditions using a continuous reactor. The concentration of the feed solution was similar to that for the batch reactor using a flow rate of 30 mL h1 and 1 g of catalyst. The reaction temperature was 60 8C under atmospheric pressure. Fig. 10 plots the tyrosol and TOC concentrations against time. The tyrosol and TOC conversions
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Fig. 10. TOC and Tyrosol concentration against time for CuNHe catalyst using the continuous reactor. The reaction temperature was 60 8C.
Fig. 9. Reusability of CuNHe catalyst during recycling in oxidation of tyrosol.
were around 85% and 60%, respectively. These results are similar to those obtained with the batch reactor. There was also a direct correlation between catalyst activity and the pH of the final solution. As we mentioned earlier, the pH of the feed solution was 5.6. The pH profile showed a rapid decrease during the first hour of operation from 5.6 to 3.6, and then remained constant, which indicates that during the oxidation reaction the products obtained are mainly made up of carboxylic acids that are very difficult to degrade. Oxalic acid was the main product detected by HPLC during batch and continuous experiments. We also observed that both tyrosol and TOC conversions remained constant after a reaction period of 120 h using the continuous reactor. This indicates the high stability for this catalyst both in batch and in the continuous reactor. To better understand the catalytic behaviour of coppersupported pillared clay catalysts, we characterized the used catalysts by TPR, infrared spectroscopy, DRX and BET. Before characterization, the catalysts were calcined at several temperatures between 100 and 300 8C (static treatment under air). Fig. 11a and b shows the TPR results for the CuNOxy and CuNHe used
Fig. 11. (a) Temperature-programmed reduction (TPR) curves for used CuNOxy treated under oxidation conditions at different temperatures between 100 and 300 8C. the fresh sample was added at reference. (b) Temperature-programmed reduction (TPR) curves for used CuNHe treated under oxidation conditions at different temperatures between 100 and 300 8C. The fresh sample was added at reference.
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Fig. 13. UV–vis diffuse reflectance spectra of fresh and used catalyst CuNOxy calcined at 300 8C.
Fig. 12. Infrared spectra of the fresh and used catalysts.
catalysts, respectively. After the reaction, the first TPR peak observed for the fresh CuNOxy catalyst at around 326 8C (see Fig. 2) disappears, while the second peak at around 390 8C remains. This could be explained by considering that part of the copper species is dissolved due to the carboxylic acid produced during the reaction. However, the amount of copper detected by hydrogen consumption was similar to that obtained for the fresh sample. This could be because copper oxide species are transformed into copper organic salts such as copper oxalate on the surface of the catalyst [39]. These observations were confirmed by the presence of a small quantity of copper dissolved in the final solution (around 1 ppm) and by IR spectra of the used catalyst (Fig. 12), which showed representative peaks of copper(II) oxalate Cu(C2O4)nH2O [40]. For the CuNOxy used catalysts, a new reduction peak was observed at around 580 8C. When the CuNOxy used catalysts were treated under oxidation conditions by flowing air at temperatures between 100 and 300 8C—these two reduction peaks (at around 390 and 580 8C) remained. Moreover, the total amount of hydrogen consumed during the TPR experiments corresponds to around 97% of the fresh copper in the sample. For the samples calcined at 100 and 200 8C, the H2/Cu ratio for each reduction peak was around 0.5. However, for the sample calcined at 300 8C the second peak shifted to higher reduction temperatures (from 580 to 590 8C), while the H2/Cu ratio also increased from 0.5 to 0.6. The difference between the reduction temperatures of fresh and used CuNOxy catalysts could therefore be accounted for by a different dispersion of the copper species and therefore different interaction with the support. For the fresh catalyst, the CuO species were in aggregate form and the two reduction peaks observed could indicate the presence of different CuO particle sizes that are both reduced in one step [34]. However, for the used CuNOxy catalysts, two reduction peaks suggest a two-step reduction process of isolated Cu2+ species with a strong interaction with the support [30–34]. This isolated Cu2+ comes from the reaction of the copper oxide species (mainly the species from the TPR peak at 326 8C, which disappears) and the carboxylic acids formed during the oxidation of tyrosol (mainly oxalic acid). The presence of carboxylic acid therefore leads to a dispersion of the copper species. When the temperature of the oxidation process increases, the reduction peak observed at higher temperature also increases. This indicates that during this oxidation process part of the isolated copper located in the pillars interacts with the aluminium oxide of the pillars to produce a new
species of high interaction with the alumina. This is probably copper aluminum oxide, which is more difficult to reduce. When the oxidation temperature increases, the formation of these types of species increases. However, it is known that the formation of spinel phases, such as copper aluminium oxide, between copper oxide and alumina is produced at a higher temperature [41]. The presence of these copper aluminium oxide species at low oxidation temperatures (<300 8C) could be explained by considering that, during the oxidation reaction of tyrosol, some organic compounds (oxalates and probably polymers) are adsorbed on the surface of the catalyst. During the oxidation treatment these organic species are burned, thus creating hot spots on the surface of the catalysts. The heat released during this burning process could be responsible for the formation of these copper aluminium oxide species during the TPR experiment. This has been corroborated by UV–vis-DRS. For the used CuNOxy catalyst calcined at 300 8C, the presence in the UV region of new absorption peaks (see Fig. 13) around 33,777 and 27,611 cm1 was attributed to the CuAl2O4 spinel phase [42]. Fig. 11b shows the TPR results for the fresh and used CuNHe catalysts. The used catalyst shows the disappearance of the copper species reduced at lower temperature (around 330 8C), representing around 35% of the total copper oxide species for the fresh CuNHe catalyst. However, the TPR peak at around 390 8C, which corresponds to 65% of the copper species, was preserved. These copper species anchored in the pillars are therefore the most abundant for the fresh and used CuNHe catalysts. For the used catalyst calcined between 100 and 300 8C, new reduction peaks between 450 and 700 8C were detected in similar way to the results obtained for the CuNOxy used catalyst. On the other hand, the used catalysts showed a noticeable decrease in specific surface area. The BET surface area for the CuNHe and CuNOxy fresh catalysts were 123 and 52 m2/g, respectively. After the reaction, the BET surface area values were 27 and 12 m2/g, respectively. This decrease in surface area was attributed to the adsorption of some organic compounds on the surface of the catalysts. The deposition of copper oxalate and/or the adsorption of organic compounds on the surface of the catalyst may therefore be responsible for the decrease in surface area. After thermal treatment with flowing oxygen at 300 8C, the surface areas of the catalysts were 97 and 32 m2/g, respectively. This oxidation process leads to the combustion of the organic compounds and the decomposition of copper oxalate [39]. 4. Conclusions The catalytic wet hydrogen peroxide oxidation of tyrosol was studied in a batch reactor using copper-supported pillared clay
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catalysts under mild conditions. Catalysts were prepared by solidstate exchange of the Al-pillared clay synthesized with copper nitrate. The resultant materials were then calcined at 300 8C for 3 h under helium or oxygen. The catalytic activity and stability of Cusupported pillared clays depended on the calcination method. Our experimental results indicate that the catalyst prepared under helium (CuNHe catalyst) showed higher activity and stability than the catalyst calcined under oxygen. The higher activity and stability of the CuNHe catalyst can be attributed to a better link of the copper species on the pillars of the clay, which decreases the formation of copper oxalate and organic compounds on the surface of the catalysts. The CuNHe catalyst showed high activity and stability in the oxidation of tyrosol using both batch and continuous reactor and hydrogen peroxide as oxidant. TOC is almost totally removed when this catalyst is used. The high stability and good performance of the CuNHe catalyst in the catalytic wet hydrogen peroxide oxidation of tyrosol solutions at mild conditions, without significant catalyst leaching or deactivation, make this system a promising alternative to the Fenton homogeneous system. Acknowledgments The authors are grateful to the AECI ref. A/5927/06 Project from ˜ a and the Ministerio de Asuntos Exteriores y Cooperacion de Espan the Tunisian Ministry of High Education, Scientific Research and Technology. References [1] [2] [3] [4] [5] [6] [7] [8]
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Copper-supported pillared clay catalysts for the wet hydrogen peroxide catalytic oxidation of model pollutant tyrosol R. Ben Achma a, A. Ghorbel a, A. Dafinov b, F. Medina b,* a b
Laboratoire de Chimie des Mate´riaux et Catalyse, De´partement de Chimie, Faculte´ des Sciences de Tunis. Campus Universitaire, 2092 El Manar, Tunis, Tunisia Departament d’Enginyeria Quı´mica, Universitat Rovira i Virgili, Av. Paı¨sos Catalans 26, 43007 Tarragona, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 March 2008 Received in revised form 25 June 2008 Accepted 10 July 2008 Available online 25 July 2008
The catalytic wet hydrogen peroxide oxidation of tyrosol, a major compound of the polyphenolic fraction present in olive oil mill wastewaters, was studied in batch and continuous reactors using coppersupported pillared clay catalysts under mild conditions. The catalysts were prepared by the solid-state reaction of Al-pillared clay synthesized with copper nitrate. The resultant materials were then calcined at 300 8C for 3 h under helium or oxygen. The catalytic activity of Cu-supported pillared clays was found to depend on the calcination method. The effects of several operating conditions were also studied. Our experimental results indicate that, under mild conditions and a stoichiometric amount of oxidant, the calcined material under helium (CuNHe catalyst) showed higher activity and stability (TOC abatement of roughly 80% and total elimination of tyrosol after 1 h reaction, without significant leaching of copper ions) than the calcined material under oxygen (CuNOxy). To better understand the catalytic behaviour of copper-supported pillared clay solids, fresh and used catalysts were characterized by X-ray diffraction, nitrogen adsorption, chemical analysis, temperature-programmed reduction (TPR), UV–visible diffuse reflectance spectroscopy, and transmission electron microscopy (TEM). ß 2008 Elsevier B.V. All rights reserved.
Keywords: Copper Pillared clay Wet hydrogen peroxide catalytic oxidation 2-(4-Hydroxyphenyl)ethanol
1. Introduction The Mediterranean countries are the main producers of olive oil. During olive oil extraction, large quantities (roughly 7 million tonnes per year) of strongly polluted water, known as olive oil mill wastewaters (OMW), are generated [1]. Because of its content (14– 15%) of organic substances and phenols (up to 10 g L1), OMW is thought to be one of the most polluting effluents produced by the agrofood industries, which are characterized by high specific COD (chemical oxygen demand) [2–4]. The untreated release of OMW onto land is a danger for the environment. Several studies have shown that simple OMW phenolic compounds with low molecular weights are responsible for toxicity on seed germination [5], aquatic organisms [6–8], and bacteria [9]. Recovering these phenolic compounds, which have interesting antioxidant properties, could provide an interesting alternative source of biologically active polyphenols [10]. Several researchers have evaluated the feasibility and economic processes involved in the recovery of these phenolic compounds from OMW or solid wastes [11,12]. After these processes, however, further treatments are needed to
* Corresponding author. E-mail address: [email protected] (F. Medina). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.021
eliminate the toxicity of the effluents before they are discharged into the natural stream. For these reasons, research has focused on developing efficient treatment technologies, including physical, chemical and biological processes [13]. Oxidation systems have often been used as pretreatments to decrease OMW toxicity and allow biological degradation. Since around 1980, attention has been paid to the development of an effective oxidation process called wet oxidation. In catalytic wet oxidation the reaction conditions are milder than in uncatalysed wet oxidation, but high temperature (over 150 8C) and high pressure (1–5 MPa) are still required. These severe operating conditions lead to high installation costs, which limit the practical applications of the process. Catalytic wet peroxide oxidation (CWPO) could be a more efficient process because the oxidizing properties of hydrogen peroxide are stronger than those of molecular oxygen. Moreover, the reaction conditions when hydrogen peroxide is involved as oxidant are close to the ambient ones (0.1 MPa, T < 80 8C). This enables a large amount of wastewater to be treated with the minimum consumption of energy. Heterogeneous catalysis would be preferred to homogeneous catalysis if a stable and active catalyst under operation conditions were found. Several materials have been proposed as catalysts for the oxidation of organic compounds in water [14,15]. However, deactivation was often observed with the solubilisation
R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28
of the active phase. To get round this problem, stable catalysts have recently been developed. Transition-metal exchanged zeolites [16,17], Cu2+-containing pillared montmorillonite [18,19], and Fe3+- or Cu2+-pillared clays [20–24] have been proposed as active catalysts for the oxidation of organic compounds. These catalysts show a higher rate of conversion of the pollutants and a lower sensitivity to pH than iron ions in solution at the same reaction conditions. However, some of them exhibit partial leaching of the cations, probably because the metal is not bound efficiently in the solid, for example in the pillars for the clays [21,25,26]. Tatiboue¨t and co-workers suggested that the high resistance to leaching and the good catalytic performance of the pillared clay catalysts could be attributed to a strong interaction between the metal species and the catalytic support [27]. In this context we have developed a simple, new protocol for Al–Cu-pillared clay preparation and studied the activity and stability of the catalysts obtained for the catalytic wet peroxide oxidation (CWPO) of tyrosol. p-Hydroxyphenylethanol (tyrosol) was chosen as the model compound because, like hydroxytyrosol, which can be produced during the oxidation of tyrosol, it is one of the most significant phenols in OMW. Also, fewer studies in the literature are related to the oxidation of tyrosol than to the oxidation of phenol, p-coumaric acid or p-hydroxybenzoic acid. 2. Experimental 2.1. Materials Tyrosol, 2-(p-Hydroxyphenyl)-ethanol 98% from Aldrich Co. was used in this study. Commercial Wyoming montmorillonite from Comptoir des Mine´raux (France) was used as starting clay. Al and Cu nitrate (Aldrich Co.) were used as precursors for the pillaring solution and solid reaction, respectively. 2.2. Catalysts preparation 2.2.1. Starting material The Wyoming montmorillonite was sieved to obtain an extracted fraction with a particle size of less than 2 mm. Its cation exchange capacity, determined by the adsorption of copper ethylenediamine complex, was 100 mequiv./100 g (ignited) clay, and its BET surface area was 29 m2/g. The starting material for the pillaring procedure was the sodium form of this montmorillonite, which was obtained by treatment with 1 mol L1 NaCl solution (three times exchanged), followed by washing and dialysis until the result of the reaction for the presence of Cl was negative. The product was then air dried at 60 8C. 2.2.2. Catalysts synthesis Alumina-pillared clay was prepared by slowly adding Alpillaring solution into an aqueous suspension of montmorillonite. The pillaring solution was prepared by dissolving 0.2 mol L1 Alnitrate in 0.45 mol L1 NaOH solution. The hydrolysis molar ratio OH/Al was kept at 2.25. The pH of the solution was about 3.8 and the solution remained clear. The diluted montmorillonite suspension (10 g L1) was prepared by adding the montmorillonite powder into the distilled water. The pillaring reaction was carried out under continuous vigorous stirring by dropwise adding the pillaring solution into the montmorillonite suspension. After centrifugation, the solid fraction was washed by dialysis in distilled water and dried at room temperature. The sample was then calcined for 5 h at 500 8C under airflow. The temperature was raised to 500 8C at a rate of 1 8C/min and this temperature was maintained for 5 h.
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The copper-supported Al-pillared clays were prepared by the solid ion exchange method. Typically, 5 g of the already synthesized Al-pillared clay were intimately mixed with the desired percentage of copper nitrate [Cu(NO3)39H2O] in an agate mortar for 10 min. This was followed by heating at 300 8C for 3 h under helium. The solid was then cooled to ambient temperature, washed six times with distilled water, dried at 80 8C for 24 h, and then calcined at 300 8C for 3 h under helium flow for the first (referenced as CuNHe) and under oxygen flow for the second (referenced as CuNOxy). CuN stands for copper nitrate precursor. 2.3. Characterizations The X-ray powder diffraction (XRD) patterns were measured on a Philips PW 1730/10 diffractometer CuKa (l = 1.54184 A˚) radiation. The position of the d0 0 1 ray in the XRD is related to the interlayer distance and, therefore, to the pillar height. Quantitative chemical analysis of the copper in the modified clays was determined by X-ray fluorescence using SFX Siemens SRS 330, and the amount of leached metal was determined by atomic absorption spectrometry using a PerkinElmer 400 spectrophotometer. Nitrogen adsorption experiments, for samples previously degassed at 120 8C for 12 h, were performed at 196 8C using a Micromeritics ASAP 2000 instrument. Specific surface area was calculated by the BET method. Temperature-programmed reduction (TPR) measurements were performed using a Thermofinnigan TPDRO 1100 system. A mixture of H2 (5%) in argon with a flow rate of 20 mL min1 was used to reduce the samples (0.12 g), which were placed in an oven heated from 40 to 990 8C at a heating rate of 10 8C min1 and maintained at 990 8C for 20 min. The reduction of CuO to metallic copper was used to calibrate the TPR apparatus for H2 consumption. UV–vis diffuse reflectance (UV–vis-DR) spectra were recorded in air on powdered samples using a Jasco V-570 spectrometer equipped with an integrating sphere for solid samples. The reference was BaSO4. Transmission electron microscopy (TEM) observations were carried out on a JEOL 2010 microscope (200 kV, resolution 0.19 nm). Samples for direct examination were prepared by suspending the powder in EtOH, and a drop of the suspension was allowed to dry on a copper grid. When the contrast was low, extractive replica were used. A drop of the suspension was deposited on freshly cleaved mica. After drying, the dispersed powder was covered by a carbon film. The mica was plunged into a solution containing a mixture of water, acetone and hydrofluoric acid to dissolve the support without dissolving the metal particles. These remained stuck to the carbon film, which was collected on a copper grid. 2.4. Oxidation of tyrosol The catalytic oxidation of tyrosol was carried out in a stirred (stirring rates of between 500 and 1200 rpm) and thermostated Pyrex well-mixed slurry batch reactor of 250 mL. The reaction was performed in the 25–80 8C temperature range and at a pH of 5.6 (the natural pH of the solution) using a 500 ppm tyrosol (3.6 mmol L1) aqueous solution in contact with different catalysts loading under continuous stirring. After 5 min of stirring, a hydrogen peroxide solution was added (time zero of the reaction) in one time. The H2O2/tyrosol molar ratio was 19/1, which is the stoichiometric quantity needed to totally transform the tyrosol into CO2 according to the reaction: C8 H10 O2 þ 19H2 O2 ! 8CO2 þ 24H2 O
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R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28 Table 1 Textural Properties of fresh and used catalysts (calcined at different temperatures) derived from the nitrogen adsorption/desorption isotherms at 77 K Catalysts
SBET (m2/g)
Sext (m2/g)
Sint (m2/g)
Vp (cm3/g)
Vmp (cm3/g)
Cu (wt.%)
Al-PILC CuNHe CuNOxy Used CuNHe Used CuNOxy
204 123 52 27 12
67 23 32 19 7
137 100 20 8 5
0.120 0.115 0.065 0.061 0.035
0.080 0.048 0.029 0.019 0.002
0 1.93 1.93
Total surface area obtained from the BET equation (SBET), micropore area obtained from the t-plot method (Sint), and mesopore area (Sext). Micropore volume obtained from the t-plot method (Vmp), and the total pore volume at P/P0 = 0.99 (Vp).
Fig. 1. X-ray diffraction (XRD) patterns of copper-supported samples. The Al-PILC pattern (a) was added at reference.
Samples withdrawn at regular times were immediately filtered and analysed for organic compounds and residual H2O2 in solution by a high performance liquid chromatograph (Dionex HPLC) equipped with a C18column (4.6 mm 250 mm; Shimpack VPODS). Identifications and quantifications of the aromatic compounds and organic acids were achieved by comparing with standards. The total organic content TOC of the solution (in mg of carbon per litre of solution) was measured with a Shimadzu Model 5050 TOC-analyser.
this difference: a decrease in the interlayer distance during doping or filling and/or the blocking of pores by copper species. X-ray investigations have shown that the interlayer distances are not significantly affected by the doping process [28]. This indicates that the mechanism responsible for the reduction of surface areas and pore volumes may be the blocking of pores by copper species. As we can see from Table 1, a large contribution of mesoporous is also observed in this sample, which suggests an important relation between the textural properties of the doped samples and the preparation method. It is likely, therefore, that during the ionexchange process part of the copper species was deposited outside the interlayer space. Upon calcination, these species produce external mesoporous copper particles that could significantly increase the external surface area while blocking the internal surface [29]. Bearing these results in mind, we took temperature-programmed reduction (TPR) measurements. TPR can be used to characterize the dispersion of metal ions in catalysts prepared via different routes. The reduction profile provides information about the dispersion of the metal species on the support and about the interaction between these metal species and the support. It has been reported that isolated Cu2+ ions can be reduced to Cu0 by hydrogen in two steps: first, the reduction of Cu2+ to Cu+; and second, the reduction of the Cu+ to Cu0 [30–34]. However, copper oxide aggregates (CuO) are reduced directly in one step to Cu0. The
3. Results and discussion 3.1. Characterization of the fresh catalysts Fig. 1 shows the X-ray diffraction (XRD) patterns for the parent Al-PILC calcined at 500 8C and for copper-supported samples. Taking the position of the peak assigned to the (0 0 1) reflection in the 2u axis and using the equation of Bragg, the value of the d(0 0 1) in Al-PILC was estimated to be 1.82 nm, and for the natural clay it was about 1 nm. Adding copper to the pillared clay for the CuNHe sample did not affect the XRD pattern of the initial pillared clay. However, the reflection of the basal (0 0 1) plane for the CuNOxy sample was broader. This indicates that some distortion in the pillared clay is produced by adding copper to the CuNOxy sample. No copper oxide phases were observed in the XRD patterns of the doped catalysts, which indicates a high dispersion of copper in the two samples prepared by the two calcination methods (the amount of copper in the two samples was around 2.0 wt.%). It also appears that the sample calcined under helium was more crystalline than the one calcined under oxygen. Table 1 summarizes the textural characteristics of Al-PILC and fresh and used Cu-supported pillared clay catalysts. Doping Al-PILC with copper affects the textural properties of the resulting material. When the sample was calcined under oxygen, there was a significant micropore surface area loss compared with the sample calcined under helium. There are two potential reasons for
Fig. 2. Temperature-programmed reduction (TPR) curves for Al-PILC and coppersupported samples.
R.B. Achma et al. / Applied Catalysis A: General 349 (2008) 20–28
Fig. 3. UV–vis diffuse reflectance spectra of copper-supported samples.
total amounts of hydrogen uptake and the H2/Cu ratio could provide information about the reduction of the copper species [34]. The TPR profiles for the CuNOxy, CuNHe and the support (Al-PILC) are shown in Fig. 2. The Al-PILC support showed a broad peak at a higher reduction temperature (between 500 and 700 8C), which can probably be attributed to the reduction of some iron species in the initial Al-PILC material [32]. However, the amount of iron is less than 0.12 wt.%. Adding copper to the clay structure (around 2.0 wt.%) results in new reduction peaks detected at lower temperatures. For the first sample (CuNOxy), we observed two reduction peaks. The first one, centered at 326 8C (which represents about 60% of the total copper species), was attributed to the reduction of copper oxide located on the surface of the support, probably in the entrance of the pores. The presence of these copper species led to a reduction in pore volume and BET values due to the filling of the interlayer [33]. The second peak, which was less pronounced and about 390 8C, may be related to copper oxide species grafted onto the pillars of the clay. For the second catalyst (CuNHe), the TPR curve shows an overlapping of these two peaks with a net decrease in the first peak (35% as opposed to 60% for the CuNOxy catalyst) related to the reduction in the copper adsorbed on the surface of the clay. The second peak, which may be related to the copper oxide species located in the pillars, is the most abundant. The H2/Cu ratio for the catalysts was around 0.90 and 0.92 for CuNOxy and CuNHe samples, respectively. This indicates that the main species detected is copper oxide, which is reduced to Cu0 in one step [34]. These two reduction peaks could be accounted for by CuO aggregates with different particle sizes and dispersion or by a different interaction between the CuO aggregates and the support (the surface of the clay or the pillars).
23
The UV–vis diffuse reflectance spectra of CuNX catalysts are shown in Fig. 3. For the CuNHe catalyst, absorption in the 45,450– 40,000 cm1 (220–250 nm) range corresponds to the charge transfer transition (Cu2+ O2) of isolated Cu2+ ions in octahedral environment [35,36]. The maximum at roughly 29,702 cm1 (336 nm) can be attributed to charge-transfer between Cu2+ and oxygen in oligonuclear [Cu–O–Cu]n species, while the peak at roughly 21,902 cm1 (456 nm) characterizes the presence of Cu+ three-dimensional clusters in CuO [37,38]. For the second catalyst (CuNOxy), when oxygen was used for treatment the distortion of the octahedron decreased. In fact, the spectrum of the CuNHe solid shows Cu2+ species in Oh more or less tetragonally distorted (d–d transition at 14,301 cm1 (699 nm)), while in the CuNOxy, the band at 13,111 cm1 (762 nm) is assigned to Cu2+ in a nearly perfect Oh symmetry. Fig. 4 shows representative micrographs of the two samples. The CuNHe sample contains small particles of copper with a nicely homogeneous particle size in the 1.7–2.0 nm range (Fig. 4a). For the (CuNOxy) sample, however, these particles were larger, with a broad particle size distribution of between 3.0 and 12.0 nm (Fig. 4b). All these results indicate that the preparation method is crucial to the properties of the obtained materials. 3.2. Oxidation of tyrosol We did preliminary tests to check the reactivity of tyrosol in the presence of H2O2 without the catalyst and to check the decomposition of hydrogen peroxide in the absence of pollutant with and without the catalyst. When working at 25 8C and a H2O2/ tyrosol molar ratio of 20, the conversion of tyrosol was negligible (less than 3% after 24 h). Without the catalysts and pollutant, decomposition of hydrogen peroxide was roughly 2% after 24 h at 25 8C. When the catalyst was introduced during the same experiment, decomposition increased to 6%. These results indicate a very low decomposition of hydrogen peroxide in these reaction conditions. We also evaluated the contribution by adsorption phenomena on the catalyst and the results were negligible (less than 1% of adsorbed amount after 24 h). The consumption of hydrogen peroxide during the catalytic tests can therefore be attributed to tyrosol degradation. Moreover, during the catalytic tests, the catalyst was used in the form of very fine powder (<2 mm) to avoid problems of interphase diffusion. Two series of experiments were completed by varying the concentration of the catalyst (0.25–1.5 g L1) and the hydrogen peroxide (H2O2)/tyrosol molar ratio from 5 to 20, and working at a reaction temperature of 25 8C in order to see how these parameters affect tyrosol conversion. Fig. 5 shows that, as expected, the initial degradation rate increased as the concentration of catalyst (CuNHe)
Fig. 4. TEM images of (a) CuNHe and (b) CuNOxy.
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Fig. 5. Catalyst concentration effect on the initial tyrosol degradation rate. [Tyrosol] = 500 ppm, T = 298 K and [H2O2] = 6.8 102 M, stirring rate of 900 rpm.
Fig. 6. Effect of hydrogen peroxide concentration on tyrosol conversion. [Tyrosol] = 500 ppm [catalyst] = 0.5 g L1 and T = 298 K.
increased to 0.5 g L1. This figure also shows a slight decrease at the value of 1 g L1. Similar behaviour was observed for the CuNOxy catalyst. Results were also similar with stirring rates of between 500 and 1200 rpm, which indicates that no external mass transfer limitation occurs under the reaction conditions. However, the results for a catalyst concentration of over 0.5 g L1 indicate that some diffusion limitation occurs. If we take into account the diffusion limitation phenomenon (which decreases the accessible number of surface active sites), the optimum concentration for this catalyst could be situated around 0.5 g L1 for the given tyrosol concentration. On the other hand, the results in Fig. 6 show that in these conditions the reaction cannot take place for a H2O2/tyrosol molar ratio of less than or equal to 5. As the concentration of H2O2 increases, the degradation of tyrosol is higher because more hydroxyl radicals are formed. We also conducted several experiments to determine how different reaction temperatures influence the effectiveness of tyrosol degradation. Fig. 7a shows the influence of reaction temperatures of between 25 and 80 8C on tyrosol conversion and the concentration of residual hydrogen peroxide (Fig. 7b) when the reaction is performed in the presence of the CuNHe catalyst. Our results show practically total conversion
of tyrosol after 2 h at a reaction temperature of 25 8C. When the reaction temperature increased to 40 8C, only 1 h was needed to achieve total conversion of tyrosol. TOC removal at reaction temperatures of 25, 40, 60 and 80 8C are shown in Table 2. TOC removal was around 60% for the CuNHe catalyst after 24 h at a reaction temperature of 25 8C. However, we should point out that the residual concentration of hydrogen peroxide in the solution was around 40% of the initial concentration. This indicates practically total efficiency of the hydrogen peroxide at these reaction conditions. The initial pH value of the sample (around 5.6) decreased to around 3.4, which indicates that, for large reaction times, the products obtained are mainly formed by carboxylic acids that are very difficult to degrade. Oxalic acid is the main product detected by HPLC and represents more than 98% of the TOC after 24 h at a reaction temperature of 25 8C. However, for short reaction times (1–4 h), other products such as 3,4dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid and 3,4dihydroxymandelic acid were obtained, though these products always represented a small part of tyrosol conversion. For example, the largest amount of 3-4 dihydroxyphenylethanol, which, excluding oxalic acid, is the main product was obtained after 3 h of reaction and represented around 5% percent of tyrosol
Fig. 7. (a) Effect of the temperature on tyrosol conversion. [Tyrosol] = 500 ppm [CuNHe] = 0.5 g L1 and [H2O2] = 6.8 102 M. (b) Residual H2O2 (for CuNHe catalyst).
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Table 2 Change of TOC as a function of time and temperature of reaction of oxidation of tyrosol Time (h)
0 1 2 4 6 8 24 a
25 8Ca
40 8Ca
60 8Ca
80 8Ca
TOC (mg L1)
pH
TOC (mg L1)
pH
TOC (mg L1)
pH
TOC (mg L1)
pH
346.56 247.70 232.70 227.40 194.10 164.00 145.40
5.60 4.79 4.53 4.10 3.85 3.73 3.38
346.56 198.00 178.40 162.90 156.00
5.60 3.91 3.49 3.31 3.38
346.56 152.00 141.20 133.40 125.00
5.60 3.07 3.77 4.19 4.36
346.56 87.78 82.80 76.55 72.04
5.60 3.80 3.78 3.82 3.96
Temperature.
Fig. 8. (a) Effect of the temperature on tyrosol conversion. [Tyrosol] = 500 ppm [CuNOxy] = 0.5 g L1 and [H2O2] = 6.8 102 M. (b) Residual H2O2 (for CuNOxy catalyst).
conversion, which is very low compared to other results with Fepillared clays [22]. This could indicate that the Cu-pillared clay catalyst is more efficient than the Fe-pillared clay catalyst for the total mineralization of tyrosol. At higher reaction temperatures, there was an important increase in activity for this catalyst. After 1 h of reaction, TOC removal was around 40%, 57% and 75% for reaction temperatures of 40, 60 and 80 8C, respectively. This was accompanied by a decrease in pH. At higher reaction times (mainly at 80 8C), TOC removal remained practically constant, being around 75% after 1 h and around 79% after 6 h. This could be because practically all the hydrogen peroxide was consumed in the first hour of reaction (see Fig. 7b). New additions of hydrogen peroxide lead to the total removal of TOC. We should bear in mind, however, that even at this high reaction temperature (80 8C), the efficiency of the hydrogen peroxide is around 80%. We studied the CuNOxy catalyst under the same reaction conditions as for the CuNHe catalyst. The conversion of tyrosol over time and the residual concentration of hydrogen peroxide present during the reaction are depicted in Fig. 8a and b, respectively. These results show that the activity of the CuNOxy catalyst and the efficiency of hydrogen peroxide are lower than with the CuNHe catalyst. Therefore, the sample preparation method plays an important role in the activity of the catalysts. Reusability of the catalysts was investigated in the oxidation of tyrosol with three consecutive experiments using the same catalyst after separation from the solution. After a first run (24 h reaction at 25 8C), the catalyst was removed from the reactor, then
washed with water, dried at 100 8C and reused. Fig. 9 plots the results of tyrosol conversion and TOC abatement against reaction time for the CuNHe catalyst. Performance was excellent even after three runs, both in terms of tyrosol conversion and TOC removal. However, for the CuNOxy catalyst, almost total deactivation of the catalyst was observed after three runs. To explain this behaviour, we studied the effect of the leaching of copper in the solution for both catalysts. For the CuNOxy catalyst, the amount of copper dissolved in the reaction medium after the first run was around 1 ppm, which represents 2.5% of the initial copper in the sample. For the CuNHe catalyst, however, it was around 0.3 ppm. To quantify the copper effect dissolved in the medium (homogeneous catalysis), we evaluated the catalytic activity in a solution with 1 ppm of Cu2+. These experiments indicated that for both catalysts the contribution of this dissolved copper to the activity was negligible. The blank test using 1 ppm of copper at the same reaction conditions (25 8C) showed a conversion of tyrosol of around 4% after 24 h of reaction (similar to a blank test without dissolved copper or in the absence of catalyst). Also, the activities of the re-used and fresh CuNHe catalysts were similar, which indicates the absence of significant deactivation. We also studied the stability of the most active and stable catalyst (CuNHe) under reaction conditions using a continuous reactor. The concentration of the feed solution was similar to that for the batch reactor using a flow rate of 30 mL h1 and 1 g of catalyst. The reaction temperature was 60 8C under atmospheric pressure. Fig. 10 plots the tyrosol and TOC concentrations against time. The tyrosol and TOC conversions
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Fig. 10. TOC and Tyrosol concentration against time for CuNHe catalyst using the continuous reactor. The reaction temperature was 60 8C.
Fig. 9. Reusability of CuNHe catalyst during recycling in oxidation of tyrosol.
were around 85% and 60%, respectively. These results are similar to those obtained with the batch reactor. There was also a direct correlation between catalyst activity and the pH of the final solution. As we mentioned earlier, the pH of the feed solution was 5.6. The pH profile showed a rapid decrease during the first hour of operation from 5.6 to 3.6, and then remained constant, which indicates that during the oxidation reaction the products obtained are mainly made up of carboxylic acids that are very difficult to degrade. Oxalic acid was the main product detected by HPLC during batch and continuous experiments. We also observed that both tyrosol and TOC conversions remained constant after a reaction period of 120 h using the continuous reactor. This indicates the high stability for this catalyst both in batch and in the continuous reactor. To better understand the catalytic behaviour of coppersupported pillared clay catalysts, we characterized the used catalysts by TPR, infrared spectroscopy, DRX and BET. Before characterization, the catalysts were calcined at several temperatures between 100 and 300 8C (static treatment under air). Fig. 11a and b shows the TPR results for the CuNOxy and CuNHe used
Fig. 11. (a) Temperature-programmed reduction (TPR) curves for used CuNOxy treated under oxidation conditions at different temperatures between 100 and 300 8C. the fresh sample was added at reference. (b) Temperature-programmed reduction (TPR) curves for used CuNHe treated under oxidation conditions at different temperatures between 100 and 300 8C. The fresh sample was added at reference.
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Fig. 13. UV–vis diffuse reflectance spectra of fresh and used catalyst CuNOxy calcined at 300 8C.
Fig. 12. Infrared spectra of the fresh and used catalysts.
catalysts, respectively. After the reaction, the first TPR peak observed for the fresh CuNOxy catalyst at around 326 8C (see Fig. 2) disappears, while the second peak at around 390 8C remains. This could be explained by considering that part of the copper species is dissolved due to the carboxylic acid produced during the reaction. However, the amount of copper detected by hydrogen consumption was similar to that obtained for the fresh sample. This could be because copper oxide species are transformed into copper organic salts such as copper oxalate on the surface of the catalyst [39]. These observations were confirmed by the presence of a small quantity of copper dissolved in the final solution (around 1 ppm) and by IR spectra of the used catalyst (Fig. 12), which showed representative peaks of copper(II) oxalate Cu(C2O4)nH2O [40]. For the CuNOxy used catalysts, a new reduction peak was observed at around 580 8C. When the CuNOxy used catalysts were treated under oxidation conditions by flowing air at temperatures between 100 and 300 8C—these two reduction peaks (at around 390 and 580 8C) remained. Moreover, the total amount of hydrogen consumed during the TPR experiments corresponds to around 97% of the fresh copper in the sample. For the samples calcined at 100 and 200 8C, the H2/Cu ratio for each reduction peak was around 0.5. However, for the sample calcined at 300 8C the second peak shifted to higher reduction temperatures (from 580 to 590 8C), while the H2/Cu ratio also increased from 0.5 to 0.6. The difference between the reduction temperatures of fresh and used CuNOxy catalysts could therefore be accounted for by a different dispersion of the copper species and therefore different interaction with the support. For the fresh catalyst, the CuO species were in aggregate form and the two reduction peaks observed could indicate the presence of different CuO particle sizes that are both reduced in one step [34]. However, for the used CuNOxy catalysts, two reduction peaks suggest a two-step reduction process of isolated Cu2+ species with a strong interaction with the support [30–34]. This isolated Cu2+ comes from the reaction of the copper oxide species (mainly the species from the TPR peak at 326 8C, which disappears) and the carboxylic acids formed during the oxidation of tyrosol (mainly oxalic acid). The presence of carboxylic acid therefore leads to a dispersion of the copper species. When the temperature of the oxidation process increases, the reduction peak observed at higher temperature also increases. This indicates that during this oxidation process part of the isolated copper located in the pillars interacts with the aluminium oxide of the pillars to produce a new
species of high interaction with the alumina. This is probably copper aluminum oxide, which is more difficult to reduce. When the oxidation temperature increases, the formation of these types of species increases. However, it is known that the formation of spinel phases, such as copper aluminium oxide, between copper oxide and alumina is produced at a higher temperature [41]. The presence of these copper aluminium oxide species at low oxidation temperatures (<300 8C) could be explained by considering that, during the oxidation reaction of tyrosol, some organic compounds (oxalates and probably polymers) are adsorbed on the surface of the catalyst. During the oxidation treatment these organic species are burned, thus creating hot spots on the surface of the catalysts. The heat released during this burning process could be responsible for the formation of these copper aluminium oxide species during the TPR experiment. This has been corroborated by UV–vis-DRS. For the used CuNOxy catalyst calcined at 300 8C, the presence in the UV region of new absorption peaks (see Fig. 13) around 33,777 and 27,611 cm1 was attributed to the CuAl2O4 spinel phase [42]. Fig. 11b shows the TPR results for the fresh and used CuNHe catalysts. The used catalyst shows the disappearance of the copper species reduced at lower temperature (around 330 8C), representing around 35% of the total copper oxide species for the fresh CuNHe catalyst. However, the TPR peak at around 390 8C, which corresponds to 65% of the copper species, was preserved. These copper species anchored in the pillars are therefore the most abundant for the fresh and used CuNHe catalysts. For the used catalyst calcined between 100 and 300 8C, new reduction peaks between 450 and 700 8C were detected in similar way to the results obtained for the CuNOxy used catalyst. On the other hand, the used catalysts showed a noticeable decrease in specific surface area. The BET surface area for the CuNHe and CuNOxy fresh catalysts were 123 and 52 m2/g, respectively. After the reaction, the BET surface area values were 27 and 12 m2/g, respectively. This decrease in surface area was attributed to the adsorption of some organic compounds on the surface of the catalysts. The deposition of copper oxalate and/or the adsorption of organic compounds on the surface of the catalyst may therefore be responsible for the decrease in surface area. After thermal treatment with flowing oxygen at 300 8C, the surface areas of the catalysts were 97 and 32 m2/g, respectively. This oxidation process leads to the combustion of the organic compounds and the decomposition of copper oxalate [39]. 4. Conclusions The catalytic wet hydrogen peroxide oxidation of tyrosol was studied in a batch reactor using copper-supported pillared clay
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catalysts under mild conditions. Catalysts were prepared by solidstate exchange of the Al-pillared clay synthesized with copper nitrate. The resultant materials were then calcined at 300 8C for 3 h under helium or oxygen. The catalytic activity and stability of Cusupported pillared clays depended on the calcination method. Our experimental results indicate that the catalyst prepared under helium (CuNHe catalyst) showed higher activity and stability than the catalyst calcined under oxygen. The higher activity and stability of the CuNHe catalyst can be attributed to a better link of the copper species on the pillars of the clay, which decreases the formation of copper oxalate and organic compounds on the surface of the catalysts. The CuNHe catalyst showed high activity and stability in the oxidation of tyrosol using both batch and continuous reactor and hydrogen peroxide as oxidant. TOC is almost totally removed when this catalyst is used. The high stability and good performance of the CuNHe catalyst in the catalytic wet hydrogen peroxide oxidation of tyrosol solutions at mild conditions, without significant catalyst leaching or deactivation, make this system a promising alternative to the Fenton homogeneous system. Acknowledgments The authors are grateful to the AECI ref. A/5927/06 Project from ˜ a and the Ministerio de Asuntos Exteriores y Cooperacion de Espan the Tunisian Ministry of High Education, Scientific Research and Technology. References [1] [2] [3] [4] [5] [6] [7] [8]
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