AQUEOUS PHASE CATALYTIC OXIDATION OF PHENOL IN A TRICKLE BED REACTOR: EFFECT OF THE ph

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1 PII: S (98) Wat. Res. Vol. 33, No. 4, pp. 1005±1013, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain /99/$ - see front matter AQUEOUS PHASE CATALYTIC OXIDATION OF PHENOL IN A TRICKLE BED REACTOR: EFFECT OF THE ph C. MIROÂ, A. ALEJANDRE, A. FORTUNY, C. BENGOA, J. FONT* and A. FABREGAT Departament d'enginyeria QuõÂ mica, Escola TeÁ cnica Superior d'enginyeria QuõÂ mica, Universitat Rovira i Virgili, Carretera de Salou, s/n, Tarragona, Spain (First received April 1998; accepted in revised form July 1998) AbstractÐThe catalytic oxidation of organic compounds in aqueous phase is a promising technique for waste water treatment. Obtaining e cient and durable catalysts and determining the optimal process conditions are the key to successfully implementing this treatment. Copper-based catalysts supported over either silica or g-alumina were prepared for this purpose. This research studies the in uence of the ph on the performance of these catalysts. Activity tests were conducted for nine days in a trickle bed reactor operating at 1408C using air as oxidant. The results show that the silica supported catalyst is very sensitive to the acidic medium which leads to very short lifetimes. At rst, the alumina supported catalyst also quickly losses activity but subsequently it stabilises with a residual phenol conversion several times higher than that of the silica supported catalyst. For both catalysts, a higher ph reduces the rate of catalyst deactivation by preventing the leaching of the copper oxides and lengthening their lifetime. The atmosphere (air or nitrogen) during calcination does not change their performance. # 1999 Elsevier Science Ltd. All rights reserved Key wordsðphenol oxidation, copper catalyst, ph, trickle bed regime, waste water, wet oxidation INTRODUCTION Disposal of waste water is acquiring increasing importance all over the world, due to the progressively more restrictive environmental constraints. This generates the need to develop e ective treatment technologies for di erent kinds of wastes. Phenol and its derivatives, which are present in many waste water streams, must be speci cally treated because of their extreme toxicity for aquatic life. Among the di erent methods to remove phenol from aqueous wastes, solvent extraction is only economically attractive if the phenol concentration is high enough, i.e. over 1% (Earhart et al., 1977; Greminger et al., 1982). Likewise, biological treatment is only appropriate for low phenol concentrations due to the bactericide properties of the phenolic compounds, even at concentrations as low as 0.001% (Wang, 1992; Lee and Carberry, 1992). Thus, chemical oxidation emerges as a promising route for phenol removal at intermediate concentrations. Oxidation techniques such as incineration (Lanouette, 1997; Wilhelmi and Knopp, 1979), ozonation (Singer and Gurol, 1983; Gurol and Nekouinaini, 1984; Gurol and Vatistas, 1987), wet air oxidation (Flynn and Flemington, 1979; Baillot *Author to whom all correspondence should be addressed. [Tel.: ; Fax: ; E- mail: jfont@etseq.urv.es]. et al., 1982; Dietrich et al., 1985), photocatalytic oxidation (Wei and Wan, 1991; Chatterjee et al., 1994), supercritical wet oxidation (Thornton and Savage, 1992a,b), aerobic coupling (Lim et al., 1983) or electrochemical oxidation (Comninellis, 1992; Comninellis and Pulgarin, 1993) have been used for phenol removal. However, the severe operation conditions and the investment needed in the most cases have prevented them from being widely implemented. Thus, the development of nonexpensive but e cient routes for phenol oxidation is still a challenge nowadays. In the last decades, catalytic wet oxidation (CWO) using air or pure oxygen has received great attention (Sadana and Katzer, 1974a,b; Pruden and Le, 1976; Ohta et al., 1980; Devlin and Harris, 1984; Levec, 1990; Joglekar et al., 1991; Pintar and Levec, 1992a,b; Kochetkova et al., 1992, Tukac and Hanika, 1995). CWO is capable of destroying the phenolic compounds mainly yielding water and carbon dioxide or harmless products. As has been demonstrated in previous papers (Fortuny et al., 1995; Alejandre et al., 1998), this process can be carried out in a tubular packed-bed reactor, operating in trickle ow regime, using air as oxidant. In those conditions, phenol is oxidised over a solid catalyst in a three-phase system under mild conditions of oxygen partial pressure (0.6±1.2 MPa) and temperature (120±1608C). Nonetheless, it was also found that the catalyst undergoes a rapid fall in its 1005

2 1006 C. Miro et al. activity (Fortuny et al., 1995), which is probably due to the leaching of the active metal in the acidic reaction medium (Alejandre et al., 1998). This paper focuses on the in uence that the operating ph has on both the activity and durability of copper catalysts supported over g-alumina or silica. These catalysts were tested for the continuous oxidation of aqueous phenol solutions in a tubular packed bed reactor working in trickle ow regime using air as oxidant. EXPERIMENTAL Materials and catalyst preparation The phenol used as reagent was purchased from Aldrich (analytical grade). High purity synthetic air was used as oxidant. Ultrapure water obtained from a Millipore system was used to prepare the di erent aqueous solutions. Carborundum granules (SiC, Carlo Erba, Milan) were used in the tests with inert material. g-alumina (Norton SA-6275) or silica (Silica Gel, Grade 40, Aldrich) were used as supports for the copper. Two copper-based catalysts was prepared using g- alumina as support. The alumina, which was supplied as spheres of 2.9 mm diameter, was grounded, sieved and the 25±50 mesh (0.701±0.294 mm) fraction collected for further impregnation. This fraction was washed several times to remove all nes and dried overnight in an oven at 1108C. The catalysts were made with a copper oxide loading of 10% and prepared by the dry soaking (pore lling) method using aqueous solutions of copper nitrate (extra pure Merck 2752) for impregnating the support. Later, the catalysts were dried at 1258C for 6 h, followed by calcining at 4008C for 8 h. The atmosphere during calcination was varied by using air or nitrogen. Both calcination temperature and copper oxide loading were selected as the best conditions in accordance with a previous work (Alejandre et al., 1998). The same procedure was followed to prepare one catalyst supported over silica, also with a copper oxide loading of 10%. In this case, the catalyst was calcined at 4008C under nitrogen. Table 1 lists the main physical characteristics of the di erent catalysts prepared and the supports used. The characterisation was conducted in a B.E.T. apparatus (Micromeritics Model ASAP 2000). Experimental set-up A 5 L stirred tank containing the feed aqueous phenol solution (5 g/l) is connected to a high-pressure metering (Eldex AA-100-S2, Napa, CA). The liquid is pumped and then mixed with air from a high-pressure gas cylinder. This mixture down ows through a packed-bed reactor placed in a constant temperature (218C) convecting air oven. The exited stream is rapidly cooled and later separated in gas and liquid phases by means of two vessels, one of them also used as sampler. The gas ow rate is measured and controlled by a owmeter equipped with a needle valve. The reactor consists of a 19 cm 3 stainless steel tube (20 cm in length and 1.1 cm ID) lled with catalyst. A detailed scheme of the experimental apparatus is available elsewhere (Fortuny et al., 1995). Experimental procedure The operating temperature was always set at 1408C and the oxygen partial pressure was maintained at 0.9 MPa, i.e. a total system pressure of 4.7 MPa. The gas ow rate was 2.3 ml/s (measured at standard conditions), which assures an excess of oxygen, as the liquid ow rate was 35 ml/h corresponding to 2.4 h 1 of WHSV (weighted hourly space velocity), i.e. a space time of 25 min. The weight of catalyst loaded in the packed-bed was calculated in accordance with the above WHSV, taking into account the apparent density of the catalyst used. In those conditions, the reactor operates in trickle ow regime (Gianetto and Specchia, 1992). Nine day activity test runs were conducted for each catalyst. The above conditions were selected in order to achieve phenol conversions in the range 20±80% according to previous results (Fortuny et al., 1995), so a better comparison of results can be done. Higher temperatures or lower liquid ow rates would give too high phenol conversions and, on the other hand, lower temperatures or higher liquid ow rates would yield very low phenol conversions. In addition, in the selected conditions, the water vapourised to saturate the gaseous ow is less than 2% of the liquid ow, and the possible stripping of phenol is completely negligible due to its very low vapour pressure. In addition, tests with inert support (carborundum), silica and g-alumina were conducted in order to check the importance of the homogeneous reaction and the catalytic activity of both supports. Taking into account the accuracy of the analytical method, these experiments showed no signi cant phenol consumption in any case (<2%). ph adjustment The ph of a 5 g/l phenol solution is slightly acid, about 5.9. However, in some tests the ph of the feed solution was adjusted by adding sodium hydroxide. Thus, four experiments were done with C-3 starting from the following phs: 5.9, 8.3, 9.0 and In addition, three tests using C-2 were carried out with the following starting phs: 5.9, 9.0 and In the experiments where the ph was kept constant throughout the reactor, a bicarbonate bu er solution was added to the feed solution. Thus, one more test was conducted for C-2 with a bu ered feed solution at a ph of 8.3. Finally, tests combining an initial period with unmodi ed feed solution (ph = 5.9) and later a bu ered feed solution (ph = 8.3) were performed. The length of the unbu ered period was also varied (6 or 24 h). Figure 1 compares the ph pro le obtained in each case and illustrates how the ph is actually controlled. Table 2 summarises the operating conditions for each test carried out with modi ed ph. Product analysis The phenol concentration from the reactor e uent was determined by high performance liquid chromatography (Beckman System Gold, San Ramon CA) using a UV detector at a wavelength of 254 nm. A C 18 reverse phase liquid chromatography column (Spherisorb ODS-2) was Table 1. Main preparation and physical characteristics of the catalysts and supports Catalyst S-1 S-2 C-1 C-2 C-3 Active phase (CuO) (%) Support S±1 S-1 S-2 Calcination atmosphere air N 2 N 2 Surface area (m 2 /g) Average pore diameter (nm) Pore volume (cm 3 )

3 Catalytic wet oxidation of phenol solutions 1007 Fig. 1. ph pro les from di erent feed conditions. used. A mixture of methanol (35%) and distilled water (65%) was used as mobile phase at a ow rate of 1 ml/ min. Chemical oxygen demand (COD) was determined by the standard open-type re ux method using potassium dichromate as oxidant (Norme Franc aise, 1988). If analysed, the amount of copper dissolved in the outlet samples was measured by atomic absorption. Phenol conversion, X Ph, will be used as the main parameter for comparing the results. X Ph is de ned as X Ph ˆ PhŠ in PhŠ out 100, PhŠ in where [Ph] in and [Ph] out denote the inlet and outlet reactor phenol concentration, respectively. Similarly, COD reduction, X COD, is expressed as X COD ˆ CODŠ in CODŠ out 100, CODŠ in where [COD] in and [COD] out are also the inlet and outlet reactor COD, respectively. RESULTS AND DISCUSSION The in uence of the calcination atmosphere on the catalyst activity is illustrated by Fig. 2, which compares the phenol conversion pro les for C-1 and C-2, both with 10% of copper oxide supported over g-alumina. These catalysts were calcined at 4008C with either air or nitrogen. Both catalysts Table 2. Experiments with modi ed ph Test run Catalyst Feed ph 1 C C C C C C C C a 9 C (0±6 h), 8.3 (6±192 h) a 10 C (0±24 h), 8.3 (24±192 h) a a Bu ered. behave similarly, so one can infer that the calcination atmosphere does not have a signi cant e ect on the catalyst activity. As can be seen, three di erent activity zones can be distinguished. In the rst zone, high phenol conversion is found almost constant during the rst 48 h, thus forming a plateau around 80% of phenol conversion. Then, phenol conversion rapidly decreases from 80 to about 45% in just 36 h. In the third zone, phenol conversion stabilises around 45% until the end of the run. The evolution of the catalytic activity has been related to the presence of two di erent species of copper attached to the alumina surface. The rst would be more active but also less stable in the reaction conditions. On the other hand, the second species would be more stable and responsible for the residual activity of the catalyst. An extensive characterisation of the catalysts proved that these species are respectively ``free'' copper oxide and nonstochiometric copper aluminate (Alejandre et al., 1998). It is well-known that the most metal oxides, as copper oxide, dissolve in hot acidic media such as the one existing in the reactor. On the other hand, the treatment at high temperature, e.g. a calcination temperature of 4008C, of a mixture of copper and aluminum oxides forms copper aluminate (Susnitzky and Barry Carter, 1991) that is more resistant to the acidic medium but is less catalytically active than the ``free'' copper oxide (Alejandre et al., 1998). The attempt to modify the distribution of copper species in the catalyst by varying the atmosphere calcination was unsuccessful as discussed above. The loss in catalytic activity can be attributed to the leaching of copper from the catalyst during the process. This speculation is proved by Fig. 3, which shows the Cu 2+ concentration in the outlet stream throughout the activity test. It should be noted that the total amount of Cu 2+ that dissolves during the

4 1008 C. Miro et al. Fig. 2. In uence of the calcination atmosphere on the catalyst activity (10% CuO, calcined at 4008C). reaction time is about 30% of the copper oxide present in the fresh catalyst. At rst, the rate of dissolution of copper oxide rapidly increases until giving a maximum Cu 2+ concentration around 70 ppm at about 60 h. Later, the Cu 2+ concentration drops until it stabilises more or less at 25 ppm. This behaviour agrees with the presence of the two di erent species, the ``free'' copper oxide being easily dissolved during the rst hours. Figure 4 shows the reactor outlet ph evolution during the activity test for C-2. Note that the inlet ph is 5.9, which corresponds to a phenol solution of 5 g/l. As can be seen, the ph of the reaction e uent considerably varies throughout the test. It must be noted that, in the rst plateau, the ph is steady around 4 when the phenol conversion is about 80%. Likewise, in the second plateau, when the phenol conversion stabilises around 45%, the ph is close to 3 and remains almost constant. In addition, the rapid fall in phenol conversion coincides with the decrease in the ph, but also with the highest leaching of copper, which proves that a good correlation can be established between them. The decrease in the ph is due to the production of organic acids and carbon dioxide from the phenol oxidation. Figure 4 also compares the phenol conversion and the COD reduction for C-2. It must be noticed that both phenol conversion and COD reduction show a similar evolution. Note that, in case of all the phenol destroyed yields only carbon dioxide, both pro les must coincide. On the contrary, assuming that the phenol removed only produces dihydric phenols, i.e. the rst step in the phenol oxidation mechanism (Devlin and Harris, 1984), the COD reduction would range from 6 to 3%. Hence, a comparison between the actual COD Fig. 3. Copper concentration pro le in the reactor e uent for a test run (10% CuO, calcined at 4008C in nitrogen).

5 Catalytic wet oxidation of phenol solutions 1009 Fig. 4. Evolution of phenol conversion, COD reduction and outlet ph in an activity test (10% CuO, calcined at 4008C in nitrogen). and the two extreme cases could indicate the oxidation level of the products. Thus, a COD reduction close to the phenol conversion means, that the products mainly belong to nal stages of the oxidation pathway (organic acids and carbon dioxide). In our case, the COD reduction behaviour is much more close to the complete oxidation of phenol to carbon dioxide than to just oxidation to dihydric phenols. Therefore, one can speculate that this catalyst posses a considerable selectivity towards the production of highly oxidised products and carbon dioxide. Moreover, it is worth noticing that the selectivity does not change very much throughout the activity test. Thus, it is expected that the most reaction products are organic acids which enhances the acidity of the reaction environment. It must also be noted that no polymeric intermediates have been detected in the chromatographic analyses. This may be directly related to the use of a trickle bed as reacting system. Trickle conditions favour the heterogeneous reactions rather than homogeneous reactions, which enables the formation of polymers, due to the greater contact surface (Pintar and Levec, 1994). The above results demonstrate the intrinsic relationship between the catalyst activity and the ph of the reactor e uent. Therefore, one new catalyst (C-3 in Table 2) was prepared using silica as support in order to nd out whether or not a di erent support for the copper oxide could improve the resistance against leaching. Run 1 in Fig. 5 shows the phenol conversion pro le for C-3 feeding an unmodi ed phenol solution (ph = 5.9). As can be seen, the catalyst shows very characteristic trends. C-3 Fig. 5. E ect of the feed solution ph on C-3 performance.

6 1010 C. Miro et al. has a short induction period in which the phenol conversion increases until it reaches a maximum conversion of 87%. Then, it loses its activity very fast and, after running for just 48 h, the phenol conversion is as low as 20% and progressively approaching zero. This di erent behaviour could be explained by the existence of only ``free'' copper oxide linked to the silica, which is easily and continuously dissolved during the activity test. In this case, copper cannot form aluminate and all the copper loading is present as copper oxide. Thus, due to the higher activity of this species, C-3 renders a higher peak of conversion but the leaching is substantially more important. In order to decrease the leaching of copper, various tests at higher phs were conducted for this catalyst. Figure 5 also displays the phenol conversion pro les for C-3 feeding these solutions of increasing ph. At a feed ph of 8.3 (run 3), the induction period almost disappears giving an initial phenol conversion close to 100%. Then, the conversion goes steadily down until it stabilises at 10%, which indicates that this ph does not completely prevent the copper oxide from being dissolved. The reason for this behaviour is that, although the inlet ph is basic, the outlet ph (see Fig. 1) is still acid. The induction period completely disappears for runs 4 and 5 in which the feed solution was xed at ph 9.0 and 10.0, respectively. It should be pointed out that both cases have almost constant phenol conversion throughout the test. However, there is a signi cant di erence between the conversions reached. At ph 9.0, the phenol conversion is nearly 25% while, at ph 10.0, the phenol conversion is higher, approximately 40%. Thus, the higher the ph, the higher the remaining phenol conversion, so the inlet ph clearly a ects the activity of the catalysts. This can be explained by the di erent rates of dissolution of the copper oxide in the aqueous solution. The solubility of any metal oxide is usually higher at low ph than at high ph. Hence, in run 1, the rate of dissolution for the copper oxide should be the highest. A visual inspection at the end of the test certainly showed an intense decolouration of the catalyst due to the loss of copper. This decolouration was less important as the ph increased, which proves that a high ph prevents the leaching of copper. Nonetheless, it is di cult to discern whether or not the di erent remaining conversion is only due to the di erent rate of catalyst deactivation or there is also some change in the mechanism reaction. It has been shown that phenolate ion is much more reactive than phenol in basic media so the reaction occurs faster and gives a better phenol conversion (Shibaeva et al., 1969a,b). However, both show similar reaction rates in acidic media because the phenolate concentration is very low. At ph above 10, the phenolate form predominates in comparison with the undissociated phenol. Therefore, the higher remaining conversion observed for run 5 could also be due to the higher reactivity of the phenolic species present. The in uence of the feed ph on the catalytic activity was also tested for C-2. Figure 6 illustrates the dependence of the phenol conversion upon the feed ph using C-2. As can be seen, the behaviour of this catalyst maintains the general trends given by C-1 or C-2, regardless of the ph, so two di erent activity plateaus are observed. In the rst plateau, the catalysts show high activity for a short period in which the phenol conversion ranges from 70 to 80%. Then, after a progressive fall, the phenol conversion remains steady and forms a second plateau. The loss in catalytic activity can be delayed by increasing the ph but, in turn, the residual conversion is lower. Thus, at ph 5.9, the residual phenol conversion is slightly higher than 40% and Fig. 6. E ect of the feed solution ph on C-2 performance.

7 Catalytic wet oxidation of phenol solutions 1011 Fig. 7. E ect of the feed solution ph during the induction period on the C-2 performance. decreases to 30% at ph 10. Note that this behaviour is opposite to what was found with C-3. As discussed above, the two plateaus can be explained because of the two di erent copper species over the alumina surface, both with di erent catalytic activity. The decrease in activity occurs when the most active copper oxide dissolves. Because of their characteristics, these oxides dissolve more slowly as ph increases so the rst plateau is longer in basic medium. However, the remaining conversion is also lower, which is opposite to what could be expected. A probable explanation for this lower conversion is that the basic medium interferes with the catalyst during the induction period, giving less active catalysts. In order to check this possibility, three additional tests (run 8 to 10) were conducted in which the ph during the rst hours was varied. In run 8, the ph of the feed solution was bu ered at 8.3 whereas in run 9 and 10, the ph was initially kept at 5.9 for 6 and 24 h, respectively and then bu ered at a ph of 8.3. The results are shown in Fig. 7. As can be seen, the presence of two di erent catalytic activity zones is not so clear. However, it should be pointed out that the ph during the induction period has a remarkable in uence on the residual conversion. Thus, the nal conversion was of 45% when the ph was bu ered during the induction period but increased to 55% when the ph was initially unbuffered for 24 h. This means that a low ph improves catalyst activation and gives higher residual conversions. Once the catalyst is activated, the higher ph provided by the bu er prevents the copper oxide from being rapidly dissolved so the residual conversion is larger. Finally, Fig. 8 compares the COD reduction pro- les using C-2 with unbu ered (run 2) or bu ered (run 9) feed. It should be noted that the ph notably Fig. 8. COD pro les for di erent feed solution ph using C-2.

8 1012 C. Miro et al. a ects the evolution of phenol degradation. When the feed solution is unbu ered, the pro le shows the two plateaus characteristic of di erent catalyst activity joined by a sharp fall. On the contrary, COD reduction steadily decreases when the feed is bu ered, although the nal COD reduction is almost the same for both cases. However, it should be noted that the high selectivity towards the production of end products (carbon dioxide and shortchain organic acids) is not very a ected by the ph, because both COD reduction pro les are very close to the respective phenol conversion evolutions. CONCLUSIONS Several copper-based catalysts supported on g- alumina or silica have been prepared and tested under di erent conditions. Activity tests were conducted for the aqueous phase phenol oxidation in a trickle bed reactor in periods of nine days. All the catalysts proved to be very selective for phenol oxidation towards organic acids and carbon dioxide. However, the catalysts showed a rapid deactivation by dissolving the copper oxide from the catalyst, which is due to the acidic operating conditions. Thus, the ph of the e uent plays a decisive role in the catalyst deactivation. When the ph is low, the catalysts show a rapid fall in activity regardless the support, however, after a variable period, the alumina-supported catalyst activity remains stable, despite the initial deactivation. In contrast, the silica-supported activity decreases sharply until the phenol conversion is negligible. These di erent trends are related to the presence of two active species in the aluminasupported catalyst, copper oxide and copper aluminate, whereas the silica-supported catalyst only has copper oxide. The calcination atmosphere does not a ect the performance of the catalysts. It has also been found that adding sodium hydroxide to increase the inlet ph, decreases the rate of deactivation for both catalysts. 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