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Synthesis of stable Cu-supported pillared clays for wet tyrosol oxidation with H2O2
Journal of Physics and Chemistry of Solids 73 (2012) 1524–1529
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis of stable Cu-supported pillared clays for wet tyrosol oxidation with H2O2 R. Ben Achma a,n, 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, Tunisie 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
Available online 4 May 2012
In the present paper the synthesis of stable Cu-containing pillared clays catalysts (Cu-PILCs) is described. These catalysts were prepared by the solid-state reaction of Al-pillared clay with copper nitrate. The resultant materials were then treated with an oxalic acid solution in order to improve the dispersion of metal species on the alumina pillars. Characterization studies were performed by use of X-ray diffraction, nitrogen adsorption at 77 K, chemical analysis, temperature-programmed reduction (H2-TPR) and transmission electron microscopy (TEM). The texture of the copper catalysts showed a high specific surface area and microporosity (197 m2 g 1 and 0.081 cm3 g 1 respectively for the catalyst Cu5.6Al-PILC), with high loading of copper (up to 4.72Cu wt%). TPRprofiles of catalysts show the presence of isolated copper species with high interaction with the alumina pillars. The properties of copper-based catalysts have been studied in the wet hydrogen peroxide oxidation of an aqueous solution of tyrosol, a molecule which constitutes more than half of the light fraction of the olive mill wastewater. The reaction was conducted in batch and flow reactors. Performance of catalysts remained quite excellent; total removal of tyrosol with high TOC abatement (around 65%) was obtained after 2 h reaction. Furthermore, the Cu5.6Al-PILC(t) catalyst showed very high stability and performance in the flow reactor: 90% of tyrosol conversion and 78% of TOC removal are obtained in 192 h on stream, without the detection of any change in the activity. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Chemical synthesis C. X-ray diffraction D. Surface properties
1. Introduction Olive mill wastewater (OMW) arises from the production of olive oil in olive mills. It is produced seasonally by a large number of small olive mills scattered throughout the olive oil-producing countries. This liquid effluent is characterized by a high polluting load, mainly explained by the presence of chemical compounds, phenols in particular, with biostatic and phytotoxic activity [1]. Treatment and disposal of OMW represents one of the main problems for olive oil-producing countries of the Mediterranean basin. Every year, a large amount of this wastewater is stored in ponds to precipitate solid organic compounds, until further production. The disposal of this effluent is usually made in local rivers or agricultural soils, representing a major environmental problem because of its potential to compromise the quality of groundwater and surface freshwater resources and soil functions as well. Thus, it poses a serious risk to aquatic and terrestrial biota, and subsequently to the health of corresponding ecosystems. Hence, the great risk of producing irreversible environmental
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Corresponding author. Tel.: þ216872 600; fax: þ216885 008. E-mail address: [email protected] (R. Ben Achma).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.04.010
problems justifies the development of appropriate technologies to reduce toxic compounds, especially the phenolic content of this industrial residue. Phenolics present in OMW at high concentrations (1–10 g l 1) are considered as the major recalcitrant compounds, which have antimicrobial properties, and are difficult to be biologically degraded [2]. Therefore, it is not surprising that research efforts have been directed towards the development of efficient treatment technologies, including several physical, chemical and biological processes as well as various combinations of them [3]. Advanced technologies for treatment of OMW consider mainly the use of solid catalysts in processes that can be operated at room conditions. Wet hydrogen peroxide catalytic oxidation (WHPCO) appears to be a promising method for oxidation of toxic organic pollutants in aqueous solutions under mild temperature and pressure conditions. This process is based on the presence of hydrogen peroxide as a source of highly reactive radicals generated on transition metal cation sites within employed catalysts [4,5]. One of the promising support metal catalysts for organic pollutants oxidation is layered clays intercalated by polymeric metal-containing inorganic oxocations due to the material’s high resistance and stability, the development of surface area and microporosity with the presence of acid sites (Bronsted and Lewis sites) [6–9].
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Different Cu-containing PILCs [10–16] have been proposed as active catalysts for the oxidation WHPCO of model phenolic pollutants (p-coumaric acid, p-hydroxybenzoic acid [10,11], toluene, xylene [12] and phenol [15,16]). These catalysts are synthesized by ion exchange reaction using an aqueous solution of the copper salt [10] or by impregnation [12,13]. The use of mixed Cu and Al pillars was tried to improve the catalytic activity [11,14–16]. In this context, we have developed in a previous work [17] a simple and new protocol for the synthesis of Cu-supportedPILCS by solid exchange reaction which allows obtaining stable catalysts with high content of copper. The catalysts were active for WHPCO of a model pollutant, p-hydroxyphenylethanol (tyrosol), one of the most significant phenols in OMW. The results showed that isolated copper located on the alumina pillars was the active site in this reaction. The aim of the present work was to improve the catalytic activity and stability of supported-copper catalysts, first by the modification of the pillar state surface by reducing the temperature of calcination of the clay support and second by treating the resulting solids by oxalic acid. Indeed, the reduction of the calcination temperature would increase the density of hydroxyl groups [18–23] which would be at the origin of the retention of the copper by pillars, improving the metal loading and its dispersion and consequently its catalytic activity.
2. Experimental 2.1. Catalysts synthesis and characterization Commercial Wyoming montmorillonite, provided by Comptoir des Mine´raux (France) was used in this work. 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 L 1 NaCl solution, followed by washing and dialysis until the result of the reaction for the presence of Cl was negative. Aluminum and copper nitrates (Aldrich Co.) were used, respectively, as precursors for the pillaring solution and the copper exchange reaction. First, alumina-pillared clay was prepared by slow addition of the pillaring solution (0.2 mol L 1 Al nitrate and 0.45 mol L 1 NaOH, pH¼ 3.8) into the montmorillonite suspension (10 g L 1). After centrifugation, the solid fraction was washed by dialysis in distilled water, dried at room temperature, and calcined for 5 h at 350 1C. Two copper-supported Al-PILCs were then prepared by the solid ion exchange method. Typically, 3 g of the Al-pillared clay already synthesized was intimately mixed with desired percentage (2.8 or 5.6 wt%) of copper nitrate [Cu(NO3)3.9H2O] in agate mortar for 10 min, followed by heating at 300 1C for 3 h under helium flow. The temperature was raised at the rate of 1 1C min 1. The two solids were then cooled to the ambient temperature, washed six times with distilled water, dried at 80 1C. These two catalysts both calcined under helium flow are referred to as CuxAl-PILC (x¼2.8 or 5.6 is the percentage of copper introduced). Secondly, copper-supported catalysts (Cu2.8Al-PILC or Cu5.6Al-PILC), already synthesized, are treated with 10 2 M oxalic acid solution under continuous stirring for 4 h at 25 1C. After centrifugation, the solid fractions were washed five times with distilled water before being dried and then calcined for 3 h at 300 1C under oxygen flow. These samples are referred to as CuxAl-PILC(t), where x represents the percentage of copper introduced and (t) the treatment with the oxalic acid solution.
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The X-ray powder diffraction (XRD) patterns, quantitative chemical analysis of the copper in the modified clays, nitrogen adsorption (at 77 K) experiments, temperature-programmed reduction (TPR) measurements and transmission electron microscopy (TEM) observations are carried out as described in [17]. 2.2. Oxidation of tyrosol The batch 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 at 25 1C temperature and a pH of 5.6 (the natural pH of the solution) using a 500 ppm tyrosol (3.6 mmol L 1) aqueous solution in contact with 0.5 g L 1catalyst loading under continuous stirring. After 5 min of stirring, which allows tyrosol adsorption equilibrium, a hydrogen peroxide solution was added (time zero of the reaction) once. The H2O2/tyrosol molar ratio was 19/1(corresponding to [H2O2]¼6.8 10 2 M), which is the stoichiometric quantity needed to totally transform the tyrosol into CO2 [17]. In order to study the stability of the catalyst the oxidation reaction was performed using a flow reactor. The feed solution was similar to that for the batch reactor, using a flow of 30 mL h 1 and 1 g of catalyst. The reaction was realized at 60 1C (to study the possibility of some deactivation during the reaction) under atmospheric pressure. The products were analyzed by HPLC coupled with DAD and using a C18 column [17]. Samples withdrawn at regular times were immediately filtered and analyzed for organic compounds in solution by a high performance liquid chromatography (Dionex HPLC) equipped with a C18 column (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 carbon TOC of the solution (in mg of carbon per litre of solution) was measured with a Shimadzu Model 5050 TOCanalyzer.
3. Results and discussion 3.1. Characterization of catalysts The amount of copper in the studied samples, their specific surface area after calcinations, and their micropore volume are reported in Table 1. This table shows that the Al-pillaring leads to the expected increase of textural parameters. The BET surface area and micropore volume (of the starting clay) increases from 29 m2/ g and 0.010 cm3/g to 217 m2/g and 0.086 cm3/g respectively. Furthermore, when copper is introduced in the Al-PILCs, the BET surface area of the samples decreases. Cu2.8Al-PILC and Cu5.6Al-PILC samples showed the highest decrease of the BET Table 1 Textural properties and copper content of the catalysts. Sample
SBET (m2/g)
Vp (cm3/g)
Vmp (cm3/g)
Pore diameter (nm)
Cu (wt%)
Al-PILC (350) Al-PILC (500) Cu2.8Al-PILC Cu5.6Al-PILC Cu2.8Al-PILC(t) Cu5.6Al-PILC(t)
217 204 149 161 190 197
0.126 0.120 0.123 0.129 0.125 0.128
0.086 0.080 0.060 0.068 0.079 0.081
3.90 3.85 3.73 3.67 3.80 3.83
0 0 2.45 4.72 2.43 4.70
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 (Vlp), and the total pore volume at P/P0 ¼ 0.99 (Vp).
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surface area (from 217 m2/g to 149 and 161, respectively). We notice however, that the increase of the copper loading improves slightly the textural properties of the sample. Besides, the coppersupported samples treated with the oxalic acid solution showed an increase in the BET surface area with respect to the untreated samples (from 149 and 161 m2/g to 190 and 197 m2/g, respectively). An increase of the pore diameter of the samples was also observed. These facts could indicate a better dispersion of the copper species after the treatment with oxalic acid. The quantitative chemical analysis of the copper shows for all catalysts the very important contents of metal: 2.45 wt% for Cu2.8Al-PILC (corresponding to 95% efficiency) and 4.72 wt% for the sample Cu5.6Al-PILC (corresponding to 91% efficiency). After the treatment with oxalic acid the amount of copper in the samples remains practically constant. It is important to note that the reduction of the calcination temperature of Al-PILC support from 500 1C to 350 1C increases the BET surface area of copper catalysts (from 123 to 149 m2/g) for Cu2.8Al-PILC and (from 115 to 161 m2/g) for Cu5.6Al-PILC. Further, we observe for these catalysts an increase in the copper loading (2.45 instead of 1.93 wt%) for Cu2.8Al-PILC and (4.72 instead of 3.36 wt%) for Cu5.6Al-PILC (this work is compared with [24]). These results are in agreement with our proposals. Fig. 1 shows the XRD standards of parent Al-PILC calcined at 350 1C and copper-supported samples. The value of d(001) in Al-PILC was estimated as 1.8 nm; this value is maintained for CuxAl-PILC samples which indicates that adding copper does not affect the XRD pattern of the initial pillared clay. No copper oxide phases were observed in the XRD patterns of the samples, which indicate a high dispersion of the CuO phase on the pillars of the Al-PILC. TPR measurements were carried out on CuxAl-PILC catalysts to get information about the dispersion of the Cu species on the Al-PILC and also about the interaction between these metal species and the support. The TPR profiles of the two samples are shown in Fig. 2. For the Cu2.8Al-PILC sample, a main reduction peak at around 350 1C is observed. This peak shows a shoulder at a lower reduction temperature (around 260 1C). Besides, a small peak at a higher reduction temperature (around 580 1C) is also observed. It has been reported that isolated Cu2 þ ions are reduced to Cu0 by hydrogen in two steps: first, the reduction of Cu2 þ to Cu þ at lower temperature and then the reduction of Cu þ to Cu0 at higher reduction temperature [25–29]. On the other hand, CuO aggregates are reduced directly in one step to Cu0, between 350 and 400 1C. Taking into account these considerations, it can be considered that the main reduction peak can be attributed to the reduction of CuO aggregates. The presence of these CuO
Fig. 2. Temperature-programmed reduction (TPR) curves of CuxAl-PILC samples.
aggregates, probably in the entrance of the pores, produces a decrease of the BET surface area of the sample with respect to the Al-PILC support. On the other hand, the shoulder observed at lower reduction temperature and the small peak at higher reduction temperature could be attributed to the reduction of isolated copper species on the pillars of the Al-PILC. The TPR curve of the Cu5.6Al-PILC sample shows the presence of two broad reduction peaks at 280 1C and 560 1C. The amounts of hydrogen uptake for these two reduction peaks are quite similar. This could indicate that isolated copper species are predominant in this sample. The higher BET surface area of this sample with respect to the Cu2.8Al-PILC sample could confirm this assumption. A representative micrograph of the Cu5.6Al-PILC sample is shown in Fig. 3. The Cu5.6Al-PILC sample contains small particles of copper species with particle size in the 1–3 nm range. EDX analysis taken on different zones shows a high homogeneity of the copper on the support, which suggests that the metal is uniformly dispersed on the Al-PILC. With the aim of improving the dispersion of the copper species on the alumina-pillared clay, the two samples (Cu2.8Al-PILC and Cu5.6Al-PILC) were treated with a solution of oxalic acid as described in the experimental section (catalysts synthesis). For both treated samples it is very hard to obtain micrographs of copper species, indicating a very good dispersion of this copper oxide species. This fact is in correlation with the increase in the BET surface area observed for these samples. The results of the TPR measurements are shown in Fig. 4. The TPR profiles of the treated Cu2.8Al-PILC(t) sample show the disappearance of the main reduction peak detected at lower reduction temperature. A broad reduction band between 400 and 800 1C, with two representative peaks at around 580 1C and 700 1C, is observed. Furthermore, the amounts of the copper species detected by chemical analysis as well as by H2 consumption are quite similar to that obtained for the sample before the oxalic acid treatment. This fact indicates that after the oxalic acid treatment Fig. 5 a dispersion of the copper species of the sample has occured . However, the TPR profile of the Cu5.6AlPILC(t) sample is quite similar to that of the untreated Cu5.6AlPILC sample, although a shift towards slightly higher reduction temperature is observed. These TPR peaks for the samples treated with oxalic acid are attributed to the reduction in two steps of isolated Cu2 þ with stronger interaction with the support, showing a higher dispersion of the copper grafted on the alumina pillars. 3.2. WHPC oxidation of tyrosol
Fig. 1. X-ray diffraction (XRD) patterns of Al-PILC and CuxAl-PILC samples.
Preliminary tests were made to check the reactivity of tyrosol in the presence of H2O2 without the catalyst and to check the
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Fig. 3. TEM image of Cu5.6Al-PILC and corresponding EDX analysis: general view (A) and zoom on the zone (B).
Fig. 5. pH profile of reaction mixture containing Cu2.8Al-PILC(t) and Cu5.6AlPILC(t) catalysts ([tyrosol]¼ 500 ppm, [catalyst] ¼0.5 g L 1, [H2O2] ¼ 6.8.10 2 M, T¼25 1C).
Fig. 4. Temperature-programmed reduction (TPR) curves of CuxAl-PILC samples before and after treatment with oxalic acid: Cu2.8Al-PILC (A) and Cu5.6Al-PILC (B).
decomposition of hydrogen peroxide in the absence of pollutant with and without the catalyst. When working without catalyst at 25 1C and at a H2O2/tyrosol molar ratio of 19/1, the conversion of tyrosol was negligible (less than 3% after 24 h). Without the catalyst and pollutant, decomposition of hydrogen peroxide was roughly 2% after 24 h at 25 1C. 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 of adsorption phenomena on the catalyst and the results were negligible (less than 1% of adsorbed amount of pollutant after 24 h). Moreover, during the reaction, the catalyst was used in the form of very fine powder ( o2 mm) to avoid problems of interphase diffusion. Figs. 6 and 7 show the tyrosol and TOC conversion curves for the four catalysts synthesized (Cu2.8Al-PILC, Cu2.8Al-PILC(t), Cu5.6Al-PILC and Cu5.6Al-PILC(t)). It is clear that all catalysts reach 100% of tyrosol conversion (or percentages greater than 98%) after 2 h reaction. However, in order to compare the catalytic activity between the catalysts it is necessary to consider the conversion percentages at lower reaction times where the most significant differences in conversion take place. In fact, in the first
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Fig. 6. Catalytic activity for Cu2.8Al-PILC and Cu2.8Al-PILC(t) ([tyrosol]¼ 500 ppm, [catalyst] ¼ 0.5 g L 1, [H2O2] ¼6.8 10 2 M, T¼ 25 1C): (a) Conversion of tyrosol and (b) TOC conversion.
Fig.7. Catalytic activity for Cu5.6Al-PILC and Cu5.6Al-PILC(t) ([tyrosol]¼ 500 ppm, [catalyst] ¼ 0.5 g L 1, [H2O2] ¼6.8.10 2 M and T¼ 25 1C): (a) total conversion of tyrosol and (b) TOC conversion.
Fig. 8. Stability of Cu5.6Al-PILC(t) catalyst using a flow reactor (flow ¼30 mL h 1, [catalyst] ¼ 1 g, T¼60 1C): (a) conversion of tyrosol and (b) TOC abatement.
hour of the reaction the catalysts that reached the highest percentage of tyrosol conversion were Cu2.8Al-PILC(t) which achieved pollutant conversion greater than 75% and Cu5.6AlPILC(t) with conversion above 95%. Also the TOC abatement is more important for these catalysts. Indeed, after 2 h reaction the TOC removal is 22% and 31% respectively for Cu2.8Al-PILC and Cu5.6Al-PILC catalysts. This conversion reaches 52% and 65% when these two samples were treated by the oxalic acid. This result is in agreement with those obtained by TPR which showed
for samples treated by the oxalic acid the dominance of isolated copper Cu2 þ , indicating that this species is the main sites responsible for the catalytic activity. It is important to note that during the first 6 h of reaction the concentration of oxalic acid, which is the main intermediate product detected by HPLC, increases with time. After this an important reduction of this acid is observed; we note then an increase of the pH solution indicating the disappearance of this organic acid (see Fig. 5). At the end of 8 h reaction, the TOC removal reaches 90% for the Cu5.6Al-PILC(t) catalyst. The high visible activity of this catalyst can be directly related to higher concentration of active Cu2 þ isolated species. This means that the active phase is strongly linked to the support, so the solid could be very stable. The stability of the most active catalyst (Cu5.6Al-PILC(t)) was investigated in the oxidation of tyrosol reaction using a flow reactor. The concentration of the feed solution was similar to that for the batch reactor using a flow rate of 30 mL h 1 and 1 g of catalyst. The reaction temperature was 60 1C under atmospheric pressure. Fig. 8 plots tyrosol and TOC concentrations with time on stream under operational conditions described above. The obtained results show a very high stability of the catalyst. Indeed, 90% of tyrosol conversion and 78% of TOC removal are obtained for 192 h on stream, without the detection of any change in the activity. Thus, we retain that the treatment by the oxalic acid, lead to the formation of Cu2 þ isolated species, as active phase, better linked to the clay support so the generated solids have better catalytic potential. Furthermore, no leaching of copper was observed, indicating the high stability of the catalyst.
4. Conclusion A new methodology of synthesis of rich Cu-supported Al-pillared clays systems by solid exchange reaction was developed, in which the calcination temperature of the clay support
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was reduced from 500 1C to 350 1C and the supported-copper catalysts were treated with oxalic acid solution. The results of the characterizations carried out for the synthesized solids indicate the following. First, the reduction of calcination temperature of aluminapillared clay allowed the retention of more important copper loading on the alumina pillars. Secondly, the treatment of copper catalysts by the oxalic acid solution generated Cu2 þ isolated species, uniformly dispersed and with a strong interaction with the support. In addition, the catalytic results show for Cu5.6AlPILC(t) sample a total conversion of tyrosol after 2 h reaction at 25 1C with a TOC removal of 65% which reaches 90% after 8 h. Furthermore, this catalyst exhibits a high stability in a flow reactor with conversions of tyrosol and TOC which remain very high even after 192 h on stream at 60 1C and under atmospheric pressure without copper leaching or deactivation. These encouraging preliminary results relative to the oxidation of the model molecule allow envisaging the effective use of these catalysts in the treatment of real effluent from OMW. Acknowledgments The authors are grateful to the AECI Project from the Minister˜ a and the ior de Asuntos Exteriores y Cooperacio´n de Espan Tunisian Ministry of High Education, Scientific Research and Technology. References [1] R. Casa, A. D’Annibale, S.R. Pieruccetti Stazi, S.G. Giovannozzi, B. Locascio, Chemosphere 50 (2003) 959–966. [2] M. Ahmadi, F. Vahabzadeh, B. Bonakdarpour, E. Mofarrah, M. Mehranian, J. Harzard. Mater. B123 (2005) 187–195.
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis of stable Cu-supported pillared clays for wet tyrosol oxidation with H2O2 R. Ben Achma a,n, 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, Tunisie 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
Available online 4 May 2012
In the present paper the synthesis of stable Cu-containing pillared clays catalysts (Cu-PILCs) is described. These catalysts were prepared by the solid-state reaction of Al-pillared clay with copper nitrate. The resultant materials were then treated with an oxalic acid solution in order to improve the dispersion of metal species on the alumina pillars. Characterization studies were performed by use of X-ray diffraction, nitrogen adsorption at 77 K, chemical analysis, temperature-programmed reduction (H2-TPR) and transmission electron microscopy (TEM). The texture of the copper catalysts showed a high specific surface area and microporosity (197 m2 g 1 and 0.081 cm3 g 1 respectively for the catalyst Cu5.6Al-PILC), with high loading of copper (up to 4.72Cu wt%). TPRprofiles of catalysts show the presence of isolated copper species with high interaction with the alumina pillars. The properties of copper-based catalysts have been studied in the wet hydrogen peroxide oxidation of an aqueous solution of tyrosol, a molecule which constitutes more than half of the light fraction of the olive mill wastewater. The reaction was conducted in batch and flow reactors. Performance of catalysts remained quite excellent; total removal of tyrosol with high TOC abatement (around 65%) was obtained after 2 h reaction. Furthermore, the Cu5.6Al-PILC(t) catalyst showed very high stability and performance in the flow reactor: 90% of tyrosol conversion and 78% of TOC removal are obtained in 192 h on stream, without the detection of any change in the activity. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Chemical synthesis C. X-ray diffraction D. Surface properties
1. Introduction Olive mill wastewater (OMW) arises from the production of olive oil in olive mills. It is produced seasonally by a large number of small olive mills scattered throughout the olive oil-producing countries. This liquid effluent is characterized by a high polluting load, mainly explained by the presence of chemical compounds, phenols in particular, with biostatic and phytotoxic activity [1]. Treatment and disposal of OMW represents one of the main problems for olive oil-producing countries of the Mediterranean basin. Every year, a large amount of this wastewater is stored in ponds to precipitate solid organic compounds, until further production. The disposal of this effluent is usually made in local rivers or agricultural soils, representing a major environmental problem because of its potential to compromise the quality of groundwater and surface freshwater resources and soil functions as well. Thus, it poses a serious risk to aquatic and terrestrial biota, and subsequently to the health of corresponding ecosystems. Hence, the great risk of producing irreversible environmental
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problems justifies the development of appropriate technologies to reduce toxic compounds, especially the phenolic content of this industrial residue. Phenolics present in OMW at high concentrations (1–10 g l 1) are considered as the major recalcitrant compounds, which have antimicrobial properties, and are difficult to be biologically degraded [2]. Therefore, it is not surprising that research efforts have been directed towards the development of efficient treatment technologies, including several physical, chemical and biological processes as well as various combinations of them [3]. Advanced technologies for treatment of OMW consider mainly the use of solid catalysts in processes that can be operated at room conditions. Wet hydrogen peroxide catalytic oxidation (WHPCO) appears to be a promising method for oxidation of toxic organic pollutants in aqueous solutions under mild temperature and pressure conditions. This process is based on the presence of hydrogen peroxide as a source of highly reactive radicals generated on transition metal cation sites within employed catalysts [4,5]. One of the promising support metal catalysts for organic pollutants oxidation is layered clays intercalated by polymeric metal-containing inorganic oxocations due to the material’s high resistance and stability, the development of surface area and microporosity with the presence of acid sites (Bronsted and Lewis sites) [6–9].
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Different Cu-containing PILCs [10–16] have been proposed as active catalysts for the oxidation WHPCO of model phenolic pollutants (p-coumaric acid, p-hydroxybenzoic acid [10,11], toluene, xylene [12] and phenol [15,16]). These catalysts are synthesized by ion exchange reaction using an aqueous solution of the copper salt [10] or by impregnation [12,13]. The use of mixed Cu and Al pillars was tried to improve the catalytic activity [11,14–16]. In this context, we have developed in a previous work [17] a simple and new protocol for the synthesis of Cu-supportedPILCS by solid exchange reaction which allows obtaining stable catalysts with high content of copper. The catalysts were active for WHPCO of a model pollutant, p-hydroxyphenylethanol (tyrosol), one of the most significant phenols in OMW. The results showed that isolated copper located on the alumina pillars was the active site in this reaction. The aim of the present work was to improve the catalytic activity and stability of supported-copper catalysts, first by the modification of the pillar state surface by reducing the temperature of calcination of the clay support and second by treating the resulting solids by oxalic acid. Indeed, the reduction of the calcination temperature would increase the density of hydroxyl groups [18–23] which would be at the origin of the retention of the copper by pillars, improving the metal loading and its dispersion and consequently its catalytic activity.
2. Experimental 2.1. Catalysts synthesis and characterization Commercial Wyoming montmorillonite, provided by Comptoir des Mine´raux (France) was used in this work. 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 L 1 NaCl solution, followed by washing and dialysis until the result of the reaction for the presence of Cl was negative. Aluminum and copper nitrates (Aldrich Co.) were used, respectively, as precursors for the pillaring solution and the copper exchange reaction. First, alumina-pillared clay was prepared by slow addition of the pillaring solution (0.2 mol L 1 Al nitrate and 0.45 mol L 1 NaOH, pH¼ 3.8) into the montmorillonite suspension (10 g L 1). After centrifugation, the solid fraction was washed by dialysis in distilled water, dried at room temperature, and calcined for 5 h at 350 1C. Two copper-supported Al-PILCs were then prepared by the solid ion exchange method. Typically, 3 g of the Al-pillared clay already synthesized was intimately mixed with desired percentage (2.8 or 5.6 wt%) of copper nitrate [Cu(NO3)3.9H2O] in agate mortar for 10 min, followed by heating at 300 1C for 3 h under helium flow. The temperature was raised at the rate of 1 1C min 1. The two solids were then cooled to the ambient temperature, washed six times with distilled water, dried at 80 1C. These two catalysts both calcined under helium flow are referred to as CuxAl-PILC (x¼2.8 or 5.6 is the percentage of copper introduced). Secondly, copper-supported catalysts (Cu2.8Al-PILC or Cu5.6Al-PILC), already synthesized, are treated with 10 2 M oxalic acid solution under continuous stirring for 4 h at 25 1C. After centrifugation, the solid fractions were washed five times with distilled water before being dried and then calcined for 3 h at 300 1C under oxygen flow. These samples are referred to as CuxAl-PILC(t), where x represents the percentage of copper introduced and (t) the treatment with the oxalic acid solution.
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The X-ray powder diffraction (XRD) patterns, quantitative chemical analysis of the copper in the modified clays, nitrogen adsorption (at 77 K) experiments, temperature-programmed reduction (TPR) measurements and transmission electron microscopy (TEM) observations are carried out as described in [17]. 2.2. Oxidation of tyrosol The batch 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 at 25 1C temperature and a pH of 5.6 (the natural pH of the solution) using a 500 ppm tyrosol (3.6 mmol L 1) aqueous solution in contact with 0.5 g L 1catalyst loading under continuous stirring. After 5 min of stirring, which allows tyrosol adsorption equilibrium, a hydrogen peroxide solution was added (time zero of the reaction) once. The H2O2/tyrosol molar ratio was 19/1(corresponding to [H2O2]¼6.8 10 2 M), which is the stoichiometric quantity needed to totally transform the tyrosol into CO2 [17]. In order to study the stability of the catalyst the oxidation reaction was performed using a flow reactor. The feed solution was similar to that for the batch reactor, using a flow of 30 mL h 1 and 1 g of catalyst. The reaction was realized at 60 1C (to study the possibility of some deactivation during the reaction) under atmospheric pressure. The products were analyzed by HPLC coupled with DAD and using a C18 column [17]. Samples withdrawn at regular times were immediately filtered and analyzed for organic compounds in solution by a high performance liquid chromatography (Dionex HPLC) equipped with a C18 column (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 carbon TOC of the solution (in mg of carbon per litre of solution) was measured with a Shimadzu Model 5050 TOCanalyzer.
3. Results and discussion 3.1. Characterization of catalysts The amount of copper in the studied samples, their specific surface area after calcinations, and their micropore volume are reported in Table 1. This table shows that the Al-pillaring leads to the expected increase of textural parameters. The BET surface area and micropore volume (of the starting clay) increases from 29 m2/ g and 0.010 cm3/g to 217 m2/g and 0.086 cm3/g respectively. Furthermore, when copper is introduced in the Al-PILCs, the BET surface area of the samples decreases. Cu2.8Al-PILC and Cu5.6Al-PILC samples showed the highest decrease of the BET Table 1 Textural properties and copper content of the catalysts. Sample
SBET (m2/g)
Vp (cm3/g)
Vmp (cm3/g)
Pore diameter (nm)
Cu (wt%)
Al-PILC (350) Al-PILC (500) Cu2.8Al-PILC Cu5.6Al-PILC Cu2.8Al-PILC(t) Cu5.6Al-PILC(t)
217 204 149 161 190 197
0.126 0.120 0.123 0.129 0.125 0.128
0.086 0.080 0.060 0.068 0.079 0.081
3.90 3.85 3.73 3.67 3.80 3.83
0 0 2.45 4.72 2.43 4.70
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 (Vlp), and the total pore volume at P/P0 ¼ 0.99 (Vp).
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surface area (from 217 m2/g to 149 and 161, respectively). We notice however, that the increase of the copper loading improves slightly the textural properties of the sample. Besides, the coppersupported samples treated with the oxalic acid solution showed an increase in the BET surface area with respect to the untreated samples (from 149 and 161 m2/g to 190 and 197 m2/g, respectively). An increase of the pore diameter of the samples was also observed. These facts could indicate a better dispersion of the copper species after the treatment with oxalic acid. The quantitative chemical analysis of the copper shows for all catalysts the very important contents of metal: 2.45 wt% for Cu2.8Al-PILC (corresponding to 95% efficiency) and 4.72 wt% for the sample Cu5.6Al-PILC (corresponding to 91% efficiency). After the treatment with oxalic acid the amount of copper in the samples remains practically constant. It is important to note that the reduction of the calcination temperature of Al-PILC support from 500 1C to 350 1C increases the BET surface area of copper catalysts (from 123 to 149 m2/g) for Cu2.8Al-PILC and (from 115 to 161 m2/g) for Cu5.6Al-PILC. Further, we observe for these catalysts an increase in the copper loading (2.45 instead of 1.93 wt%) for Cu2.8Al-PILC and (4.72 instead of 3.36 wt%) for Cu5.6Al-PILC (this work is compared with [24]). These results are in agreement with our proposals. Fig. 1 shows the XRD standards of parent Al-PILC calcined at 350 1C and copper-supported samples. The value of d(001) in Al-PILC was estimated as 1.8 nm; this value is maintained for CuxAl-PILC samples which indicates that adding copper does not affect the XRD pattern of the initial pillared clay. No copper oxide phases were observed in the XRD patterns of the samples, which indicate a high dispersion of the CuO phase on the pillars of the Al-PILC. TPR measurements were carried out on CuxAl-PILC catalysts to get information about the dispersion of the Cu species on the Al-PILC and also about the interaction between these metal species and the support. The TPR profiles of the two samples are shown in Fig. 2. For the Cu2.8Al-PILC sample, a main reduction peak at around 350 1C is observed. This peak shows a shoulder at a lower reduction temperature (around 260 1C). Besides, a small peak at a higher reduction temperature (around 580 1C) is also observed. It has been reported that isolated Cu2 þ ions are reduced to Cu0 by hydrogen in two steps: first, the reduction of Cu2 þ to Cu þ at lower temperature and then the reduction of Cu þ to Cu0 at higher reduction temperature [25–29]. On the other hand, CuO aggregates are reduced directly in one step to Cu0, between 350 and 400 1C. Taking into account these considerations, it can be considered that the main reduction peak can be attributed to the reduction of CuO aggregates. The presence of these CuO
Fig. 2. Temperature-programmed reduction (TPR) curves of CuxAl-PILC samples.
aggregates, probably in the entrance of the pores, produces a decrease of the BET surface area of the sample with respect to the Al-PILC support. On the other hand, the shoulder observed at lower reduction temperature and the small peak at higher reduction temperature could be attributed to the reduction of isolated copper species on the pillars of the Al-PILC. The TPR curve of the Cu5.6Al-PILC sample shows the presence of two broad reduction peaks at 280 1C and 560 1C. The amounts of hydrogen uptake for these two reduction peaks are quite similar. This could indicate that isolated copper species are predominant in this sample. The higher BET surface area of this sample with respect to the Cu2.8Al-PILC sample could confirm this assumption. A representative micrograph of the Cu5.6Al-PILC sample is shown in Fig. 3. The Cu5.6Al-PILC sample contains small particles of copper species with particle size in the 1–3 nm range. EDX analysis taken on different zones shows a high homogeneity of the copper on the support, which suggests that the metal is uniformly dispersed on the Al-PILC. With the aim of improving the dispersion of the copper species on the alumina-pillared clay, the two samples (Cu2.8Al-PILC and Cu5.6Al-PILC) were treated with a solution of oxalic acid as described in the experimental section (catalysts synthesis). For both treated samples it is very hard to obtain micrographs of copper species, indicating a very good dispersion of this copper oxide species. This fact is in correlation with the increase in the BET surface area observed for these samples. The results of the TPR measurements are shown in Fig. 4. The TPR profiles of the treated Cu2.8Al-PILC(t) sample show the disappearance of the main reduction peak detected at lower reduction temperature. A broad reduction band between 400 and 800 1C, with two representative peaks at around 580 1C and 700 1C, is observed. Furthermore, the amounts of the copper species detected by chemical analysis as well as by H2 consumption are quite similar to that obtained for the sample before the oxalic acid treatment. This fact indicates that after the oxalic acid treatment Fig. 5 a dispersion of the copper species of the sample has occured . However, the TPR profile of the Cu5.6AlPILC(t) sample is quite similar to that of the untreated Cu5.6AlPILC sample, although a shift towards slightly higher reduction temperature is observed. These TPR peaks for the samples treated with oxalic acid are attributed to the reduction in two steps of isolated Cu2 þ with stronger interaction with the support, showing a higher dispersion of the copper grafted on the alumina pillars. 3.2. WHPC oxidation of tyrosol
Fig. 1. X-ray diffraction (XRD) patterns of Al-PILC and CuxAl-PILC samples.
Preliminary tests were made to check the reactivity of tyrosol in the presence of H2O2 without the catalyst and to check the
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Fig. 3. TEM image of Cu5.6Al-PILC and corresponding EDX analysis: general view (A) and zoom on the zone (B).
Fig. 5. pH profile of reaction mixture containing Cu2.8Al-PILC(t) and Cu5.6AlPILC(t) catalysts ([tyrosol]¼ 500 ppm, [catalyst] ¼0.5 g L 1, [H2O2] ¼ 6.8.10 2 M, T¼25 1C).
Fig. 4. Temperature-programmed reduction (TPR) curves of CuxAl-PILC samples before and after treatment with oxalic acid: Cu2.8Al-PILC (A) and Cu5.6Al-PILC (B).
decomposition of hydrogen peroxide in the absence of pollutant with and without the catalyst. When working without catalyst at 25 1C and at a H2O2/tyrosol molar ratio of 19/1, the conversion of tyrosol was negligible (less than 3% after 24 h). Without the catalyst and pollutant, decomposition of hydrogen peroxide was roughly 2% after 24 h at 25 1C. 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 of adsorption phenomena on the catalyst and the results were negligible (less than 1% of adsorbed amount of pollutant after 24 h). Moreover, during the reaction, the catalyst was used in the form of very fine powder ( o2 mm) to avoid problems of interphase diffusion. Figs. 6 and 7 show the tyrosol and TOC conversion curves for the four catalysts synthesized (Cu2.8Al-PILC, Cu2.8Al-PILC(t), Cu5.6Al-PILC and Cu5.6Al-PILC(t)). It is clear that all catalysts reach 100% of tyrosol conversion (or percentages greater than 98%) after 2 h reaction. However, in order to compare the catalytic activity between the catalysts it is necessary to consider the conversion percentages at lower reaction times where the most significant differences in conversion take place. In fact, in the first
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Fig. 6. Catalytic activity for Cu2.8Al-PILC and Cu2.8Al-PILC(t) ([tyrosol]¼ 500 ppm, [catalyst] ¼ 0.5 g L 1, [H2O2] ¼6.8 10 2 M, T¼ 25 1C): (a) Conversion of tyrosol and (b) TOC conversion.
Fig.7. Catalytic activity for Cu5.6Al-PILC and Cu5.6Al-PILC(t) ([tyrosol]¼ 500 ppm, [catalyst] ¼ 0.5 g L 1, [H2O2] ¼6.8.10 2 M and T¼ 25 1C): (a) total conversion of tyrosol and (b) TOC conversion.
Fig. 8. Stability of Cu5.6Al-PILC(t) catalyst using a flow reactor (flow ¼30 mL h 1, [catalyst] ¼ 1 g, T¼60 1C): (a) conversion of tyrosol and (b) TOC abatement.
hour of the reaction the catalysts that reached the highest percentage of tyrosol conversion were Cu2.8Al-PILC(t) which achieved pollutant conversion greater than 75% and Cu5.6AlPILC(t) with conversion above 95%. Also the TOC abatement is more important for these catalysts. Indeed, after 2 h reaction the TOC removal is 22% and 31% respectively for Cu2.8Al-PILC and Cu5.6Al-PILC catalysts. This conversion reaches 52% and 65% when these two samples were treated by the oxalic acid. This result is in agreement with those obtained by TPR which showed
for samples treated by the oxalic acid the dominance of isolated copper Cu2 þ , indicating that this species is the main sites responsible for the catalytic activity. It is important to note that during the first 6 h of reaction the concentration of oxalic acid, which is the main intermediate product detected by HPLC, increases with time. After this an important reduction of this acid is observed; we note then an increase of the pH solution indicating the disappearance of this organic acid (see Fig. 5). At the end of 8 h reaction, the TOC removal reaches 90% for the Cu5.6Al-PILC(t) catalyst. The high visible activity of this catalyst can be directly related to higher concentration of active Cu2 þ isolated species. This means that the active phase is strongly linked to the support, so the solid could be very stable. The stability of the most active catalyst (Cu5.6Al-PILC(t)) was investigated in the oxidation of tyrosol reaction using a flow reactor. The concentration of the feed solution was similar to that for the batch reactor using a flow rate of 30 mL h 1 and 1 g of catalyst. The reaction temperature was 60 1C under atmospheric pressure. Fig. 8 plots tyrosol and TOC concentrations with time on stream under operational conditions described above. The obtained results show a very high stability of the catalyst. Indeed, 90% of tyrosol conversion and 78% of TOC removal are obtained for 192 h on stream, without the detection of any change in the activity. Thus, we retain that the treatment by the oxalic acid, lead to the formation of Cu2 þ isolated species, as active phase, better linked to the clay support so the generated solids have better catalytic potential. Furthermore, no leaching of copper was observed, indicating the high stability of the catalyst.
4. Conclusion A new methodology of synthesis of rich Cu-supported Al-pillared clays systems by solid exchange reaction was developed, in which the calcination temperature of the clay support
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was reduced from 500 1C to 350 1C and the supported-copper catalysts were treated with oxalic acid solution. The results of the characterizations carried out for the synthesized solids indicate the following. First, the reduction of calcination temperature of aluminapillared clay allowed the retention of more important copper loading on the alumina pillars. Secondly, the treatment of copper catalysts by the oxalic acid solution generated Cu2 þ isolated species, uniformly dispersed and with a strong interaction with the support. In addition, the catalytic results show for Cu5.6AlPILC(t) sample a total conversion of tyrosol after 2 h reaction at 25 1C with a TOC removal of 65% which reaches 90% after 8 h. Furthermore, this catalyst exhibits a high stability in a flow reactor with conversions of tyrosol and TOC which remain very high even after 192 h on stream at 60 1C and under atmospheric pressure without copper leaching or deactivation. These encouraging preliminary results relative to the oxidation of the model molecule allow envisaging the effective use of these catalysts in the treatment of real effluent from OMW. Acknowledgments The authors are grateful to the AECI Project from the Minister˜ a and the ior de Asuntos Exteriores y Cooperacio´n de Espan Tunisian Ministry of High Education, Scientific Research and Technology. References [1] R. Casa, A. D’Annibale, S.R. Pieruccetti Stazi, S.G. Giovannozzi, B. Locascio, Chemosphere 50 (2003) 959–966. [2] M. Ahmadi, F. Vahabzadeh, B. Bonakdarpour, E. Mofarrah, M. Mehranian, J. Harzard. Mater. B123 (2005) 187–195.
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