SiO2 Catalyst

The optimum temperature for the formation of more N2 gas was between 200 and 210 °C; below this temperature range, more NH4+ ions were formed, and ...
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Ind. Eng. Chem. Res. 2005, 44, 7320-7328

Insight into Wet Oxidation of Aqueous Aniline over a Ru/SiO2 Catalyst Guntaka R. Reddy and Vijaykumar V. Mahajani* Chemical Engineering Division, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai-400019, India

Wet oxidation of aniline over a 5% Ru/SiO2 catalyst in the temperature range of 175-220 °C, oxygen partial pressure range of 0.34-1.38 MPa, and catalyst loading of 0.066-1.33 kg m-3 was studied. This heterogeneous catalyst was found to be very effective in the complete degradation of aniline and also to be active in the conversion of the -NH2 group in aniline into N2 gas. The optimum temperature for the formation of more N2 gas was between 200 and 210 °C; below this temperature range, more NH4+ ions were formed, and above this range, more NO3- ions were found. The highest conversion of -NH2 to N2 was ∼78%. The kinetic data were modeled using the power law rate expression in terms of chemical oxygen demand (COD) and also in terms of total organic carbon (TOC). The experimental data could be best correlated by the Langmuir-Hinshelwood type reaction model involving a single-site dissociative adsorption of O2. The addition of the free radical promoter, hydroquinone, during wet oxidation of aniline resulted in increased conversion of -NH2 to N2. On the other hand, the free radical scavenger, gallic acid, resulted in decreased conversion of -NH2 to N2. 1. Introduction Wet oxidation, also known as wet air oxidation, is a powerful environmental engineering process for the mineralization of toxic aqueous effluents, which either cannot be biotreated or are too dilute to incinerate.1 It is a subcritical mineralization of pollutants via wet oxidation. The free radical reaction proceeds through the formation of various intermediates before total mineralization takes place. The low-molecular-weight acids are the stable intermediates formed. Many times, their oxidation to CO2 and H2O is the rate-controlling step. Depending upon the substrate exhibiting chemical oxygen demand (COD), the temperature used during the wet oxidation process varies between 150 and 300 °C and the partial pressure of oxygen varies between 0.5 and 5 MPa. The system pressure will be higher depending upon the temperature of operation and, hence, the prevailing water vapor pressure. To reduce the severity of operating conditions and to achieve the highest mineralization, homogeneous as well as heterogeneous catalysts have been used.1-5 The use of homogeneous catalysts necessitates the catalyst recovery system as a downstream operation after the wet oxidation. On the other hand, in the heterogeneously catalyzed wet oxidation reaction, the catalyst might lose the activity because of the following reasons. The dissolved inorganic solids in the effluent stream might cause catalyst deactivation. Also depending upon the residence time, there might be deposits of carbonaceous matter on the catalyst surface, if aromatics or polynuclear aromatics are there in the waste (and also otherwise). If enough residence time is given, the carbon deposits will eventually be converted into CO2 by surface oxidation. The most important factor is the leaching of the catalyst element by low-molecular-weight acids, say, acetic acid * To whom correspondence should be addressed. Tel.: 9122-24145616. Fax: 91-22-24145614. E-mail: [email protected].

and propionic acid, formed as intermediates during wet oxidation. Aniline and its derivatives are used in the preparation of numerous synthetic organic chemicals such as agrochemicals, dyestuffs, and pharmaceuticals.6 Considerable concerns exist over the loss of aniline and other aromatic amines to the environment during production processes or incomplete treatment of industrial waste streams. These organic amines are priority pollutants, so complete mineralization is necessary. From a process engineering point of view, mineralization of the N atom to N2 gas is desirable. The dissolved ammonia, in the form of salts of acids if complete mineralization is not achieved, is also not desired in the waste stream because there exists a specification for ammonical nitrogen in the effluent. The conversion of N to NO3-/NO2- is undesirable because these ions may cause stress corrosion at a very high temperature and under the O2 environment which prevails in wet oxidation. Particularly, weld joints are more vulnerable. Therefore, when nitrogen bearing compounds are treated via wet oxidation, attention needs to be focused on two main issues, namely, complete destruction of toxic amines and maximum conversion of N in amines to N2 gas (minimization of NH3/NH4+, NO3-, and NO2-). There is scanty information available on the catalytic wet oxidation (CWO) of aniline in the published literature. It has been reported that ammonium (NH4+), nitrate (NO3-), and nitrite (NO2-) ions are the most refractory byproducts in the CWO of aniline.7,8 Wet oxidation of other nitrogen-containing organic compounds such as some aliphatic amines,9 nitrobenzene,7 nitrophenol,7 and TNT red water10 (dinitrotoluene sulfonates) results in the formation of NH4+ and NO3- ions. In addition, the mineralization of aniline by other advanced oxidation processes like anodic oxidation,11 photoelectro-Fenton,11 and ozonation catalyzed with Fe2+ 12 also results in the formation of NH3 and, hence, NH4+ ions in aqueous solution. So NH4+, NO3-, and

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NO2- are the main byproducts formed during the wet oxidation of aniline and other nitrogen-containing organic compounds. It was, therefore, thought desirable to study the effectiveness of the wet oxidation of aqueous aniline to make the stream ecofriendly by minimizing the COD of the original feed stream and also minimizing byproducts such as ammonium, nitrate, and nitrite ions by selectively converting the N atom in aniline into N2 gas by using a suitable catalyst. Recently, Barbier et al.13 also studied the wet oxidation of aniline and then ammonia over Ru/CeO2, and they reported that 200 °C was an optimum temperature for the formation of more N2 gas; at this temperature, complete degradation of aniline occurred. In the present research, catalyst screening was done mainly based upon the percentage conversion of nitrogen (from aniline degradation) into N2 gas. In other words, selectivity toward N2 gas was the barometer for the selection of a catalyst. It has been reported that Ru is not leached away in acetic acid, formed during wet oxidation.14 It was, therefore, thought desirable to concentrate on Ru alone as an active catalyst element. To provide better insight into waste treatment, we have investigated the kinetics in terms of COD and total organic carbon (TOC) destruction.

Figure 1. Scanning electron micrograph of the 5% Ru/SiO2 catalyst.

2. Experimental Section 2.1. Chemicals. Aniline and all other reagents used for chemical oxygen demand (COD) analysis were of analytical reagent grade, purchased from s.d. Fine Chemicals, Mumbai, India. Oxygen from a gas cylinder with a minimum stated purity of 99.5% was obtained from Inox Air Products, Ltd., India. Ruthenium trichloride trihydrate (RuCl3‚3H2O) was obtained from SISCO Research Laboratories Pvt., Ltd., Mumbai, India. High surface area silica (Degussa, Germany) was used as a support. All the solutions were prepared in distilled water, and for catalyst preparation, deionized water was used. 2.2. Catalyst Preparation. The supported ruthenium catalysts were prepared by the incipient impregnation of ruthenium trichloride trihydrate on the support. To prepare 5% Ru/SiO2 catalyst, an aqueous solution containing an appropriate amount of ruthenium trichloride trihydrate (RuCl3‚3H2O) in the deionized water was rapidly contacted with the finely divided silica support and the slurry was stirred for 5 h at 30 °C. This provides the degree of intimate contact between the species that was desired. The slurry was then contacted with only a stoichiometric amount of ammonia solution to precipitate ruthenium as hydroxide. It was then stirred again and heated at 60 °C for an hour. After settling, the top aqueous layer was separated from the precipitate below. Fresh deionized water was then added to the precipitate, and this solution was allowed to digest at 80 °C for 1 h to provide homogeneity to the catalyst. After ∼1 h, formaldehyde and a very small amount of ammonia solution were added to the mixture to ensure the completion of reduction, and heating at 80 °C was continued for another hour. Cooling and washing followed this step. Washing was done very fast by using a centrifuge rather than the conventional filtration. Washing was done repeatedly until the absence of Cl- and NH4+ ions was ensured in the decanted liquid, because Cl- and NH4+ ions interfere in the reaction as well as in the analysis on the high-

Figure 2. Schematic diagram of the experimental setup for the wet oxidation: PI, pressure indicator; TI, temperature indicator; SI, speed indicator; R, reaction vessel/autoclave; T, thermocouple; H, electric heater; RD, rupture disk; I, impeller; GS, gas sparger; SC, sample condenser; CY, gas cylinder; C, cooling coil; CWin, cooling water inlet; CWout, cooling water outlet; and S, solenoid valve. Table 1. Characteristics of the Heterogeneous Catalyst 5% Ru/SiO2 property

value

BET surface area external surface area micropore volume average pore diameter particle size range mean diameter

355.3 m2 g-1 203.7 m2 g-1 0.088 cm3 g-1 141.5 Å 43-68 µm 53.6 µm

performance ion chromatograph (HPIC). So the washing is done thoroughly with deionized water, and the absence of Cl- and NH4+ ions is checked on the HPIC. After washing, the catalyst is dried and stored in a desiccator. The characteristics of 5% Ru/SiO2 catalyst are presented in Table 1. The characterization was conducted in a BET (Brunauer-Emmett-Teller) apparatus (Micromeritics model ASAP 2010). A typical scanning electron micrograph (SEM) of this catalyst is shown in Figure 1. 2.3. Experimental Setup and Procedure. Figure 2 depicts the schematic diagram of the experimental setup. The wet oxidation studies were carried out in a 1 dm3 capacity SS-316 Parr high-pressure reactor (Parr Instrument Company, U.S.). The reactor was equipped with a turbine agitator, a variable speed magnetic drive,

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and an electrically heated jacket with proper insulation. The temperature and speed of agitation were controlled by means of a Parr 4842 controller. The gas inlet and off-gases release valve, cooling water feed line with solenoid valve, pressure gauge, and rupture disk were situated on the top of the reactor vessel. The liquid sample line and thermocouple well were immersed in the reaction mixture. The reactor was also provided with a cooling coil. A chilled water condenser was fitted to the sample valve exit line to avoid flashing during sampling. A series of absorbers having sintered disks for sparging gas were connected to the off-gases release line for analysis of the reactor off-gases. The reactor was first charged with 0.5 dm3 of an aqueous solution of a known concentration of aniline and a predetermined amount of catalyst. The reactor was then purged with nitrogen prior to the start of the experiment to ensure an inert atmosphere inside the reactor and to check the leaks. All the lines were closed. The speed of agitation was adjusted to a predetermined value. The reaction temperature was set. The reactor contents were heated to the desired temperature, and a sample was withdrawn. This was considered zero time for the reaction. Oxygen from the cylinder was then sparged into the liquid phase directly beneath the impeller to attain the desired partial pressure of oxygen. The wet oxidation reaction being exothermic, the temperature increased, and it was maintained constant by means of cooling water controlled by a solenoid valve. The amount of oxygen was far in excess of the theoretical amount required. Samples were withdrawn periodically after sufficient flushing of the sample line. The consumption of oxygen due to the reaction led to a decrease in the total pressure, as indicated on the pressure gauge. So more oxygen was charged intermittently from the cylinder through a manually operated controlled valve to make up for that consumed during the reaction, thus maintaining constant total pressure. The entire system was in semibatch mode. The reaction was allowed to proceed for a prescribed reaction time, after which the autoclave was allowed to cool up to 30 °C using cooling water. Then the reactor off-gases were absorbed in a series of absorbers containing dilute H2SO4 solution for trapping all the ammonia as ammonium sulfate. 2.4. Analytical Techniques. All the samples collected during the course of the experiments were analyzed for their chemical oxygen demand (COD) content by standard dichromate reflux method.15 The total organic carbon (TOC) was measured with the help of an Anatoc Series II TOC analyzer, SGE, Australia. The residual aniline in aqueous solution was analyzed by gas chromatography (Chemito 8610, Chemito Technologies Pvt., Ltd.). A stainless steel column (Tenax, length ) 1.3 m and diameter ) 6.3 mm) was used with a flame ionization detector (FID). The same column was used with a thermionic ionization detector (TID) for the detection of nitrogen-containing organic compounds. Reactor off-gases were absorbed in a series of absorbers containing dilute H2SO4 for the detection of ammonia in the gas phase of the reactor at the end of the reaction. The excess acid was titrated to find the ammonia. The ammonium ions in aqueous solution were determined with the help of a high-performance ion chromatograph (HPIC of Dionex, U.S.) using the CS 14-A column with electrochemical detector (ED 50) in con-

ductivity mode. The eluent used for the detection of NH4+ ions was 10 mM methane sulfonic acid at 0.1 mL/min. Nitrate and nitrite ions were also analyzed on the same unit using the AS 11 column. The eluent used was 10mM NaOH. The eluents used for HPIC were prepared in deionized water. The total material balance of nitrogen was done to find out the amount of N2 gas released. That was the difference between the amount of input nitrogen (in the form of aniline) and the amount of nitrogen in various forms (N-organic, NH4+, NO3-, and NO2-) after reaction. The error in all experimental and analytical measurements was 210 °C) and irrespective of catalyst loading, the formed ammonium ions were oxidized into nitrates. So, the oxidation of formed ammonium ions was an important step in the wet oxidation of aniline for the formation of more N2 gas, and that requires the optimum temperature and catalyst loading. From all these results, the optimum temperature for the formation of more N2 gas in the wet oxidation of aniline was between 200 and 210 °C. The catalyst loading depends on the input aniline concentration (input COD). The optimum catalyst loading was between 0.66 and 1.33 kg m-3 at 500 ppm of input aniline concentration. The reaction pathway of the formation of N2 gas, NO3-, and NO2- was also found, which was via the oxidation of

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Figure 9. Profiles of aniline, NH4+, NO3-, NO2-, and N2 gas at different times in the reaction (220 °C, O2 partial pressure 0.69 MPa, catalyst loading 1.33 kg m-3, initial aniline 500 ppm (75.3 ppm of nitrogen)): (b), aniline; (9), NH4+; (2), NO3-; (O), NO2-; (0), N2 gas.

ammonium ions that were formed because of the degradation of the -NH2 group in aniline. Barbier et al.13 also reported similar results. It was found that the pH of the aqueous solution after reaction was significantly decreased, which was due to the formation of low-molecular-weight acids such as formic, acetic, and propionic acids. Initially, the NH4+ formed might be associated with stable, low-molecularweight acids such as acetic acid (ammonium acetate), formic acid, or propionic acid. We have detected acetic acid as an intermediate. We postulate that the NH2 group from aniline might get detached to form NH3 and, hence, NH4+ ions, which were picked up by the acetic acid formed because of the free-radical-induced breakage of the aromatic ring. Ammonium acetate is then oxidized to CO2 and N2. Further, ammonia is partially converted into NO3- and NO2-. Both these ions are capable of oxidizing organic molecules (COD) and getting themselves reduced to N2. The overall mechanism is shown below.

3.5. Effect of Additives. The wet air oxidation is a free radical reaction. The generation or abstraction of free radicals from the reaction system is expected to have an influence on the rate of reaction. It was, therefore, thought desirable to externally add the free radical promoter (initiator) hydroquinone (CAS No. 12331-9) and a free radical scavenger, say, gallic acid (3,4,5trihydroxy benzoic acid, CAS No. 149-91-7), and see the course of reaction. The heterogeneous catalytic wet oxidation of aniline was studied in the presence of the free radical promoter, hydroquinone, at a concentration of 5 × 10-2 kg m-3 (50 ppm) at 200 °C and 0.66 kg m-3 of catalyst loading. The addition of hydroquinone even in such trace amounts enhanced the rate of reaction. In the presence of hydroquinone, complete degradation of aniline was achieved within 60 min; the hydroquinone also helped

Figure 10. Effect of additives on the formation of N2 gas (200 °C, O2 partial pressure 0.69 MPa, catalyst loading 0.66 kg m-3, initial aniline 500 ppm (75.3 ppm of nitrogen), reaction time 120 min): crossed vertical and horizontal lines, NH4+; solid black, NO3-; diagonal lines, NO2-; solid white, N2 gas.

in the degradation of the formed NH4+ ions into N2 gas. After 120 min of reaction at these conditions, ∼69% of input nitrogen was in the form of N2 gas, and without hydroquinone, it was 52%. The results are shown in Figure 10. To study the influence of the free radical scavenger gallic acid on the catalytic wet oxidation of aniline, an experiment was performed in the presence of 5 × 10-2 kg m-3 of gallic acid at 200 °C and 0.66 kg m-3 of catalyst loading. Even in the presence of the heterogeneous catalyst, the presence of gallic acid reduced the rate of degradation of aniline. After 120 min of reaction, 96% of aniline degradation was found, and without gallic acid, the aniline degradation was 100%. The % conversion of input nitrogen into N2 gas was also decreased significantly. From the above studies, it is seen that the free radical promoters would help in converting the amino group into N2 gas. It was interesting to note that the free radical promoter also increased NO3- and NO2-. This means free radicals assist in converting NH4+ ions into NO3- and NO2-. However, when the scavenger was added, more NH4+ was formed along with NO3-, but no NO2- was formed. The detailed study of the optimum concentration of both additives is out of the scope of this investigation. To find out the role of the metal wall of the reactor in the termination of free radicals, an experiment was conducted at 200 °C and 0.66 kg m-3 of catalyst loading, in a reactor having a Pyrex glass liner. Under these conditions with a Pyrex liner, the complete degradation of aniline was achieved within 90 min, while it took 120 min for the same result without the Pyrex liner (Figure 4). The enhanced rate of the reaction was, thus, observed in the Pyrex glass liner. The increase in the rate of reaction could possibly be due to the fact that, in the presence of the liner, the metal walls of the reactor (SS 316) do not destroy the free radicals (OH*). These free radicals, in turn, participated in the reaction, thereby enhancing the rate. However, it was observed that there was no significant change in the % conversion of the nitrogen of aniline into N2 gas in the presence of the Pyrex liner (Figure 10). 3.6. Reusability of the Catalyst. To study the reusability of this catalyst, four experiments were conducted on fresh aqueous solutions of aniline with the same catalyst reuse after separation from the solution by centrifuge. After 8 h of use of the catalyst, that is, 2 h in each reaction (fresh, 1st reuse, 2nd reuse, and 3rd

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Figure 11. Effect of temperature on the COD reduction (O2 partial pressure 0.69 MPa, initial aniline 500 ppm (75.3 ppm of nitrogen), catalyst loading 1.33 kg m-3): (O), 175 °C; (9), 200 °C; (2), 210 °C; (b), 220 °C.

Figure 12. Effect of catalyst loading on initial rates of COD and TOC reduction (200 °C, O2 partial pressure 0.69 MPa, initial aniline 500 ppm (75.3 ppm of nitrogen)): (2), COD; (9), TOC.

reuse), the activity of the catalyst was decreased by 6% in terms of COD reduction. Of course, these results are indicative in nature. In reality, the activity studies need to be done with the help of a waste sample from the operating plant. 3.7. Reaction Kinetics. The wet oxidation of aniline, as seen previously, is a complex mechanism that proceeds through the formation of various reaction intermediates through numerous pathways. Modeling of each individual reaction would be impractical. Therefore, a lumped parameter system in terms of COD and TOC is proposed here as O2

COD(organic substrate) 98 O2

COD(low molecular wt. compounds) 98 CO2 + H2O (3) While studying kinetics, additives were not added. The kinetics and selectivity toward N2 in the presence of the free radical generator (additive hydroquinone) is excluded from the kinetics presented below. The effects of oxygen partial pressure, temperature, and catalyst loading on the rates of reaction were studied. In all experiments, COD and TOC were measured at various time intervals and values were plotted against time. The effect of temperature on COD and TOC reduction was studied in the range of 175-220 °C. The effect of temperature on the COD reduction is shown in Figure 11. These COD and TOC profiles with respect to time were fitted to a polynomial expression, and the initial rates of reaction were deduced. Experiments were conducted at 200 °C and 0.69 MPa oxygen partial pressure to study the dependence of the initial rates of reaction on the catalyst loading. As shown in Figure 12, the initial rate varied linearly with the catalyst concentration in the range 0.066-1.33 kg m-3. The effect of the oxygen partial pressure on the COD and TOC reductions was studied in the range of pressures 0.69-1.38 MPa at 200 °C using a catalyst loading of 0.66 kg m-3. The dependence of the initial rate on the oxygen partial pressure is shown in Figure 13. 3.7.1. Power Law Model. The traditional approach is to explain kinetics in terms of turn over frequency (TOF), provided we know the exact number of active centers on the catalyst surface. The active centers are always proportional to the amount of catalyst taken. Therefore, the process design engineer would always prefer the rate expressed as per unit weight of catalyst.

Figure 13. Effect of oxygen partial pressure on initial rates of COD and TOC reduction (200 °C, catalyst loading 0.66 kg m-3, initial aniline 500 ppm (75.3 ppm of nitrogen)): (2), COD; (9), TOC.

In the following discussions, we have used the rate expressed as per kg of the catalyst. A simple power law model was first used to correlate dependence between the rate and the reactant concentrations; thus, the rate of reaction can be expressed as

rc ) kc[A]m[COD]n

(4)

rt ) kt[A]m′[TOC]n′

(5)

The above eq 4 was used to fit the reaction kinetic data obtained from COD vs time profiles, and eq 5 was used to fit the reaction kinetic data obtained from TOC vs time. The oxygen concentration in the liquid phase [A] is correlated to the partial pressure PA via the solubility parameter HA. Equations 4 and 5, therefore, can also be written as

rc ) kc[HAPA]m[COD]n )kc′[PA]m[COD]n

(6)

rt ) kt[HAPA]m′[TOC]n′ ) kt′[PA]m′[TOC]n′

(7)

where kc′ ) kc[HA]m and kt′ ) kt[HA]m′. Eqs 6 and 7 are the process design engineer friendly form of the power law rate expression. The orders of the reaction, m and n, with respect to oxygen and COD were obtained using linear regression. The values of kc′, m, n, kt′, m′, and n′ at various temperatures are given in Table 2 along with 95% confidence limits. Thus, the reaction orders (m, n, m′, and n′) vary with temperature. It is clear that the reaction system was complex. It was, therefore, thought worthwhile to gain further insight into the reaction

Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7327 Table 2. Rate Constants and Orders in Power Law Model temp (°C)

kc′ × 103

m

n

kt′ × 103

m′

n′

175 200 210 220

17 ( 0.4 48 ( 0.8 73 ( 1.5 108 ( 2.9

0.51 ( 0.04 0.64 ( 0.09 0.69 ( 0.12 0.74 ( 0.18

0.92 ( 0.25 1.06 ( 0.29 1.12 ( 0.32 1.2 ( 0.39

14 ( 0.2 53 ( 0.8 83 ( 1.4 133 ( 3.2

0.46 ( 0.02 0.52 ( 0.07 0.61 ( 0.1 0.69 ( 0.14

0.98 ( 0.26 1.08 ( 0.31 1.19 ( 0.39 1.23 ( 0.42

rc ) kc′ [PA]m[COD]n (eq 6) and rt ) kt′[PA]m′[TOC]n′ (eq 7). Table 3. Parameters for the Sequential Langmuir-Hinshelwood Model (Eq 8) temp (°C)

ksc

175 12.33 ( 0.3 200 68.8 ( 3.3 210 164.5 ( 6.2 220 459.3 ( 20.5 ∆Had (kJ mol-1)

Table 4. Parameters for the Sequential Langmuir-Hinshelwood Model (Eq 9)

KAC × 103

KCOD × 103

39 ( 0.7 33 ( 0.5 28 ( 0.3 23 ( 0.3 21.3

22 ( 0.8 15 ( 0.5 12 ( 0.3 7 ( 0.06 43.3

KAC is inferred for KAC′ ) KAC [HA]0.5 and applying correction for HA.

mechanism by using Langmuir-Hinshelwood type kinetic models. 3.7.2. Langmuir-Hinshelwood Kinetics. For correlating observed data, models based on Eley-Rideal and Langmuir-Hinshelwood mechanisms were tried. The model based on the Eley-Rideal mechanism was found to be not acceptable after correlating the experimental data. Therefore, Langmuir-Hinshelwood type models were postulated for the dissociative and nondissociative adsorption of oxygen. The experimental data fitted well to the dissociative adsorption of oxygen, far better than the fit for nondissociative adsorption. In fact, the model parameters were negative for nondissociative adsorption. The power law model resulted in ∼0.5 order with respective to oxygen, thereby strengthening the inference of the dissociative adsorption of oxygen. The series of steps in the proposed mechanism are as follows: (i) reversible adsorption of oxygen by a dissociative mechanism, forming an oxidized site on the surface of the catalyst; (ii) reversible adsorption of organic substrates contributing to COD on the catalyst; (iii) formation of byproducts and intermediates during the surface reaction between adsorbed reactants (organic substrates contributing to COD and oxygen); and (iv) further reaction with more oxygen to give CO2 and H2O. The adsorption of reactants (organic substrates contributing to COD and oxygen) was assumed to be rapid, while surface reactions steps were assumed to be rate controlling. The following rate expression was derived using the stationary hypothesis.

rc )

kscKAC′KCOD[PA]0.5[COD] (1 + KAC′[PA]0.5 + KCOD[COD])2

temp (°C)

kst

175 9.4 ( 0.6 200 48.9 ( 3.3 210 97.5 ( 5.2 220 276.3 ( 14.5 ∆Had (kJ mol-1)

KAT × 103

KTOC × 103

34 ( 1.8 31 ( 1.4 27 ( 1.1 21 ( 0.9 17.8

28 ( 1.1 20 ( 0.9 16 ( 0.6 10 ( 0.3 38.3

in Table 3. A similar mechanism was proposed in terms of TOC and the rate expression is shown below

rt )

kstKAT′KTOC[PA]0.5[TOC] (1 + KAT′[PA]0.5 + KTOD[TOC])2

(9)

where KAT′ ) KAT[HA]0.5. The parameters (kst, KAT, and KTOC) at different temperatures and heats of adsorption of oxygen and organic substrates contributing to TOC are shown in Table 4. The energy of activation for the surface reaction was found to be 133.3 kJ mol-1. The parity plot of predicted and observed rates with respect to COD is shown in Figure 14. Similar results were obtained for TOC as well. It was, thus, seen that, under the experimental conditions used, eqs 8 and 9 were adequate to model the kinetics of the wet oxidation of aqueous aniline over theRu/SiO2 catalyst. 4. Conclusions The heterogeneous catalyst Ru/SiO2 was found to be an effective catalyst in the wet oxidation of aqueous aniline. The catalyst was found to be very active in the complete degradation of aniline and also found to be good in the conversion of the -NH2 group in aniline into N2 gas. The highest selectivity toward N2 gas formation was observed in the optimum temperature range of 200-210 °C. Below the temperature 200 °C, more amounts of ammonium ions were formed, and at higher temperatures (>210 °C), more NO3- ions were formed because of the harsh oxidizing conditions. The observed

(8)

where KAC′ ) KAC[HA]0.5. This expression was found to satisfactorily explain the observed kinetic behavior. The various rate parameters in the expression were determined using nonlinear least-squares regression analysis. The experimental data could be fitted well to this model. The various parameters estimated at different temperatures are as shown in Table 3. From the temperature dependence of ksc, the activation energy for the surface reaction was found to be 140 kJ mol-1. The adsorption equilibrium constants KAC and KCOD were correlated to temperature using the Van’t Hoff isochore. The values of the heats of adsorption of oxygen and organic substrates contributing to COD are also shown

Figure 14. Comparison of predicted rates with experimental rates (with COD reduction): (s), predicted rate; (9), observed rate.

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rate was adequately explained by using LangmuirHinshelwood kinetics. A single-site mechanism with the dissociative adsorption of oxygen and the surface reaction as the rate-controlling step provided the best fit of the experimental data. The free radical initiator hydroquinone exhibited a marked increase in the rate of destruction of aniline as well as in the selectivity of N2 gas. Acknowledgment G.R.R. is grateful to the University Grants Commission, New Delhi, Government of India, for the financial support. Thanks are also given to Director, CIRCOT, Government of India, for the help in scanning electron microscopy. Nomenclature A ) oxygen [A] ) concentration of oxygen in the bulk liquid phase, kg m-3 Cs ) surface concentration of reactant, kg m-3 De ) effective diffusivity, m2/s HA ) Henry’s constant for oxygen, kg m-3 MPa-1 KAC′ ) adsorption equilibrium constant for A in eq 8 KAT′ ) adsorption equilibrium constant for A in eq 9 KCOD ) adsorption equilibrium constant for COD KTOC ) adsorption equilibrium constant for TOC kc ) reaction rate constant in eq 4, (m3)m+n kg1-m-n (kg of cat.)-1 min-1 kt ) reaction rate constant in eq 5, (m3)m+n kg1-m-n (kg of cat.)-1 min-1 kc′ ) rate constant in eq 6, (m3)n kg1-n (kg of cat.)-1 min-1 MPa-1 kt′ ) rate constant in eq 7, (m3)n kg1-n (kg of cat.)-1 min-1 MPa-1 ksc ) surface reaction rate constant in eq 8, m4.5 kg-0.5 (kg of cat.)-1 min-0.5 kst ) surface reaction rate constant in eq 9, m4.5 kg-0.5 (kg of cat.)-1 min-0.5 m ) order with respect to oxygen in eq 4 m′ ) order with respect to oxygen in eq 5 n ) order with respect to COD n′ ) order with respect to TOC PA ) partial pressure of oxygen, MPa R ) catalyst particle radius, m r ) overall rate of reaction, kg (kg of cat.)-1 min-1 rc ) overall rate of reaction (in terms of COD), kg (kg of cat.)-1 min-1 rt ) overall rate of reaction (in terms of TOC), kg (kg of cat.)-1 min-1 t ) time, min w ) catalyst concentration, kg m-3 η ) effectiveness factor Φ ) Thiele modulus

∆Had ) heat of adsorption, kJ mol-1

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Received for review April 12, 2005 Revised manuscript received June 29, 2005 Accepted July 7, 2005 IE050438D