Influence of temperature and catalyst loading on the aqueous-phase catalytic oxidation of phenol

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

1769

Influence of temperature and catalyst loading on the aqueous-phase catalytic oxidation of phenol A. Santos, P. Yustos, B. Durbfin and F. Garcia-Ochoa Departamento Ingenieria Quimica. Facultad. CC. Quimicas. Universidad Complutense. 28040- Madrid. Spain. ABSTRACT The oxidation of phenol over a copper oxide-copper chromite catalyst has been studied in a basket stirred tank reactor in the temperature range 127-180~ with a catalyst loading range of 0-120 g/L and using an initial pH of 3.5 and 10. The phenol conversion and its mineralization to carbon dioxide have been measured. A higher oxidation rate of phenol was found when the initial pH was acid. At the lower temperatures studied (127 and 140~ an increase of both the phenol oxidation rate and the CO2/phenol conversion ratio was observed when the catalyst loading was increased. At the highest temperatures used (160 and 180~ the homogeneous mechanism is the main contribution to the phenol oxidation rate. 1. INTRODUCTION The problem of the pollution reduction of the aqueous streams containing phenolic compounds is of great importance in wastewaters from many industries (agroalimentary processes, pharmaceutical, fine chemical, petrochemical, etc). These effluents can not be treated through conventional processes of biological oxidation, because of their little biodegradability. The process currently used is the wet oxidation or supercritical oxidation, that requires high pressures and temperatures, and consequently high cost (1). An interesting alternative is the heterogeneous catalytic oxidation, because the catalyst permits a significant reduction of the temperature and pressure improving the economy of the process (2-4). Studies carried out about this topic show that the reaction involves a free radical mechanism, with an induction period before the steady-state regime is achieved, considering both homogeneous and heterogeneous reaction contributions. The influence of variables such as pH, catalyst loading and temperature on the induction period and steady-state rate are not yet clarified and results obtained from different authors often disagree (5-7). Most of the studies have been carried out in slurry reactors, with a maximum catalyst loading of 4-5 g/L. The few papers where the reaction is carried out in fixed bed (usually as trickle bed reactors) did not study the effect of different catalyst weights in the reactor. Hence the catalyst loading influence on the phenol conversion and its mineralization to carbon dioxide requires further studies. Furthermore, most of the papers do not consider the conversion to CO2 but only the conversion of phenol. However the phenol oxidation does not produce directly carbon dioxide but many intermediates have been detected in the oxidation route, being the main

1770 oxidation intermediates the dihidroxyphenols (cathecol and hydroquinone), the benzoquinones, the C-4 and C-2 dicarboxilic acids, the acetic acid, etc. The best catalyst should be as less selective as possible and promote the oxidation of these intermediates to carbon dioxide.

2. EXPERIMENTAL 2.1. Catalyst A commercial catalyst from Engelhard (Cu-0203) with copper as active component has been employed. The catalyst properties are summarized in Table 1. Leaches of metallic ions Cu and Cr have been measured after the catalyst has been in contact with an acid aqueous phenolic solution at 140~ and 16 atm of pressure during 10 hours. An almost negligible amount of those ions in solution was found, as can be seen in Table 1. Table 1. Properties of the Catalyst Engelhard Cu-0203

Chemical Composition ~

Compounds Copper oxide 67-77 Copper chromite 20-30 Synthetic graphite 1-3

~

Metals 60% Cu 10% Cr

Physical Properties

Metal leaches after 10 h (mg ion/g cat) T=140~ P=I 6 atm.

Sg m2/g

V cm3P/g

dp mm

Cu

Cr

10

0.10

3.175 (1/8")

0.241 0.0393 % Cuo

9.3.10 -4 9.10 -4 % Cro

2.2. Experimental Set-Up and Procedure A basket stirred tank reactor (BSTR) from Autoclave Engineers (500 ml Spectrum Reactor) was employed to carry out the experimental runs. A volume of 250 ml of the aqueous solution was introduced in the reactor after setting the pH to an initial value (pHo). The catalyst was boiled twice in distilled water before its use and placed in the basket of the reactor, and the rotating speed of the stirrer was fixed at 700 rpm. The reactor was pressurized at 16 atm. Steady state conditions for temperature and concentration of dissolved oxygen were achieved by means of a continuous flow of oxygen (0.2 L lnin -R at NTP conditions). Then, the volume of a loop containing a concentrated solution of phenol was injected being the initial concentration of phenol in the reactor liquid phase of 1200 ppm. Experimental runs were carried out at different temperatures (127 to 180~ catalyst loadings (0 to 120 g/L) and initial pH (3.5 and 10). Gas and liquid samples were taken periodically from the reactor and analyzed. Residual phenol concentration and some intermediates in the samples from the reactor were determined by HPLC with a Diode Array detector (HP G1315A). Other reaction intermediates were identified and quantified by means of a GC/MS technique (Hewlett Packard model 6890). Main intermediates identified at acid pH were 1,2 benzenediol, 1,4 benzenediol and 2,5-cyclohexadiene-l-4dione. Also maleic and oxalic acids were detected. Total organic carbon in the liquid phase values (TOC) have been

1771 obtained in a SGE analyzer. Copper and chromium leaches have been determined by means of a fotometer (Aqualytic AL282). The pH values of the liquid samples were also measured with time.

3. RESULTS and DISCUSSION 3.1. Influence of the pH on phenol and TOC conversion

The results obtained at 140~ and using a catalyst loading of 0 and 60 g/L for both initial pH 3.5 and 10 are shown in Figures 1a and lb. As can be seen, the phenol conversion is much slower at basic pH, probably due to the free radical mechanism proposed for this reaction (5). Regarding to the mineralization of the phenol to CO2 similar curves with and without catalyst are obtained at each pH. However at basic pH the mineralization achieved for a given phenol conversion is slightly higher than the obtained at acid pH.

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--A-- pH,,=10 Ccat=60 g L~

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P=16 atm. a) Phenol conversion b) Phenol mineralization.

3.2. Influence of temperature

Figures 2a,b and 3a,b show the results obtained for the phenol and the TOC conversion with time at the two extremes catalyst loading employed. As can be seen, for both catalyst loadings in the reactor, 0 and 120 g/L, the phenol conversion is highly affected by the temperature until 160~ then the increase of this variable does not produce an increase of the phenol conversion rate. On the other hand an increase of the temperature increases the mineralization grade if no catalyst is added. When the reaction is carried out at 120 g/L of catalyst loading no effect of the temperature on the mineralization achieved was noticed.

1772

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Figure 3. Effect of the temperature on the mineralization of phenol, pHo=3,5, P=16 atm. a) Ccat-0 g/L b) Ccat=120 g/L

3.3. Effect of the catalyst loading The effect of the catalyst concentration on the phenol conversion and its mineralization to CO2 at the four temperatures studied is shown in Figures 4a,b,c and 5a,b,c. As can be seen in Figure 4c at temperatures higher than 160~ the addition of the catalyst does not improve the phenol conversion. Thus, the main contribution to the phenol oxidation rate is due to the homogeneous reaction. At 180~ a similar behavior for the mineralization was found with and without catalyst. On the contrary, at 160~ the addition of the catalyst yields a higher mineralization of the intermediates produced from the oxidation of phenol. At the lower temperatures used, 127 and 140~ a significant positive influence of the addition of catalyst on both phenol and TOC conversion was found, as can be seen in Figures 4a,b and 5a,b. At these temperatures no differences in the grade of mineralization of phenol were found for a catalyst loading over 30 g/L.

1773

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Figure 4. Effect of the catalyst loading on the phenol conversion, pHo=3,5, P=16atm.

Figure 5. Effect of the catalyst loading on the mineralization, pHo=3,5, P=I 6 atm

a) 127~ b) 140~ c) 160, 180~

a) 127~ b) 140~ c) 160, 180~

In Figures 6a,b,c the pH change with time is shown at the two extremes catalyst loading used (0 and 120 g/L) for the temperatures studied. It can be seen that the catalyst loading has a significant influence on the pH change. When the catalyst was added the pH decreased initially with the increase of the phenol conversion, achieving a minimum, and then increased. This can be explained considering that the acid compounds (mainly C-2 and C-4 dicarboxilic acids) are the last intermediates of the oxidation route before the mineralization is achieved. The catalyst produces a higher oxidation rate of these acid compounds. The higher

1774 oxidation of the acid compounds achieved with the catalyst can explain the differences in the mineralization grade observed in Figures 5a,b,c. When the temperature increases the results obtained with and without catalyst for the oxidation of those acid compounds are closer. For example, negligible differences in pH change with time were found with and without catalyst at 180~ and consequently the same TOC abatement vs. phenol conversion was also observed. )H

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b) 140~ c)160, 180~

ACKNOWLEDGEMENTS This work has been supported by CAM under contract no. 07M/0035/1998. The authors wish to thank Engelhardt who kindly supplied the catalyst. REFERENCES

1. T.D. Thornton and P.E. Savage, J. Supercritic. Fluids 3(1990) 240. 2. A. Pintar and J. Levec, Catal. Today. 24 (1995) 51. 3. Z. Ding, S.N. Aki and M.A. Abraham, Env. Sci. Tech. 29 (1995) 2748. 4. S.H. Lin and S.J. Ho, Appl. Catal. B: Env. 9 (1996) 133. 5. A. Sadana and J.R. Katzer, J. Catal. 35 (1974) 140. 6. H. Otha, S. Goto and H. Teshima, Ind. Eng. Chem. Fundam. 19 (1980) 180. 7. A. Pintar and J. Levec,, Ind. Eng. Chem. Res. 33 (1994) 3070