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Oxidative Degradation of Phenol in Aqueous Medium Catalyzed by Lab Prepared Cobalt Oxide.

Byline: MUHAMMAD SAEED, MOHAMMAD ILYAS AND MOHSIN SIDDIQUE

Summary:

This work explores the preparation and characterization of cobalt oxide catalyst and investigation of its catalytic activities for oxidative degradation of phenol in aqueous medium. The catalyst was prepared by mechanochemical process in solid state at room temperature by reaction of Co(NO3)2.6H2O and NH4HCO3. The prepared catalyst was characterized by physical (surface area, particle size, XRD, SEM and FTIR analysis) and chemical (determination of oxygen content) techniques. The prepared cobalt oxide was used as catalyst for oxidative degradation of phenol in aqueous medium by taking 15 mL of 0.71 M phenol solution. The catalytic performance of cobalt oxide was explored in terms of effect of time, temperature and partial pressure of oxygen on degradation of phenol. The catalyst was separated from the reaction mixture by filtration.

Langmuir- Hinshelwood type of mechanism was followed in the reaction where adsorption of phenol and oxygen at the surface of catalyst was taking place according to Freundlich model. Linear and non- linear least square method of analysis was used for reaction kinetics.

Key Words: Phenol, cobalt oxide, XRD, FTIR, Freundlich.

Introduction

The pollution of the surface water with phenol is a highly important environmental problem, first of all because of the propagation of the pollution, and second because of its unfavorable consequences on the aquatic life. Being a basic structural unit for a variety of synthetic organic compounds, wastewater originating from many industries like paper and pulp, resin manufacturing, gas and coke manufacturing, tanning, textile, plastic, rubber, pharmaceutical, and petroleum contain phenol and substituted phenol [1]. Decay of vegetation also contributes phenols to water bodies. Phenols are considered as priority pollutants since they are harmful to organisms at low concentrations and many of them have been classified as hazardous pollutants because of their potential harm to human health. The ingestion of phenol-contaminated water in the human body causes protein degeneration, tissue erosion, and paralysis of the central nervous system and also damages the kidney, liver and pancreas [2].

The threshold value of phenol in water is 4000 ug/L. The majority of phenols are toxic substances and some are known or suspected carcinogens. It is important to remove phenols and aromatic compounds from contaminated industrial aqueous streams before discharged into any water body. The effective removal of these pollutants from wastewater is a problem of great importance and interest [3]. Therefore, great attention has been given to the different alternative for the removal of phenol and its derivative from water. Most of the processes, like biological oxidative degradation, adsorption at porous surfaces and air striping, have limitations and are not very promising on large scale. The wet oxidation (WO) process is one of the most economically and technologically viable solution for the removal of phenol and other dissolved organic compounds in wastewater.

Wet oxidation seems to be convenient process of destructive oxidation of phenol and substituted phenols over traditional biological process in which organic compounds can be converted to water and carbon dioxide [4]. Wet oxidation can be defined as the oxidation of both organic and inorganic substances in aqueous solution by means of oxygen at elevated temperatures (398-593 K) and pressures (0.5-20 MPa). Due to high temperature and pressure, high equipment and operational costs are inevitable, which can be lowered by adding a suitable catalyst either homogenous or heterogeneous type, referred as catalytic wet oxidation (CWO), resulting lower instrumental and operational costs. Several homogeneous catalysts exhibit good performances for different waste water treatments, however the metal ions left in the treated water is also a pollutant, which requires a costly separation process to remove them from the final effluent.

Heterogeneous catalyst can overcome this secondary contamination problem and hence catalytic wet oxidation of phenol and other organic pollutants in wastewater using heterogeneous catalyst has gained much attention. Heterogeneous catalytic wet oxidation is more promising process for aqueous waste treatment, which uses oxygen (air), ozone, hydrogen peroxide, or a combination as the oxidative agent and solid supported/unsupported metals/metal oxides as heterogeneous catalysts. Heterogeneous wet catalytic process of wastewater has many advantages like operation at milder conditions of temperature and pressure and easy recovery, regeneration and reuse of the catalyst [5-7]. Several heterogeneous catalysts have been attempted for mineralization of phenol in aqueous medium. For example, zeolite supported cerium-manganese catalyst, doped with potassium at temperature 383 K and 0.5 MPa pressure of oxygen has been used for mineralization of phenol.

Potassium was added to reduce the carbonaceous material and to improve oxidation of phenol and zeolite was added to improve the stability of the catalyst [4]. D. H. Bremner and his co-workers [8] studied heterogeneous sono-Fenton system (Fe2O3, H2O2 and ultrasonic radiation) for treatment of phenol in aqueous medium. They reported that the sono-Fenton system can degrade aqueous phenol solutions of concentrations ranging from 0.625 to 5 mM with high efficiency but more concentrated solutions, up to 10 mM, show a decreased reactivity. TiO2/activated carbon composite catalyst has been used as catalyst for removal of phenol from aqueous medium by C. Ngamsopasiriskun et al [9]. It was reported that the percentage removal of 100 ppm phenol by 0.4 g of TiO2/AC in 4 h was 68.03%, due to both adsorption (54.14%) and catalytic degradation (13.88%), with the highest rate constant being 0.1080 h-1. N. A. Laoufi et al [10] have also reported TiO2 as heterogeneous catalyst for degradation of phenol in aqueous medium.

MnO2/CeO2 catalyzed wet oxidation of phenol in a slurry reactor in the temperature range of 353-403 K and pressure of 2.04-4.76 MPa has been reported by A. J. Luna et al [11]. The total removal of phenol (2.08 g/L) was achieved within 40 minutes of reaction at 403 K and a catalyst loading of 6.0 g/L. Similarly a number of catalysts like Pt/Ce-Zr [12], MnO2/CeO2 [13], CuO+ZnO, CuO+NiO and CuO/Al2O3 [14], CuO [15] (La0.8Sr0.2) Mn0.98O3 [16], Fe2O3/SBA-15 [17] have been used for wet catalytic oxidation of phenol in aqueous medium. In this work cobalt oxide, which can oxidize higher concentration of phenol at lower temperature, was used as a catalyst for oxidative degradation of phenol in aqueous medium. Cobalt oxide has been used as heterogeneous catalyst in several reactions. The catalytic performance of cobalt oxide depend on various factors like preparation conditions, surface structures, oxidation state, surface area and size distribution and morphology of the particles.

Many synthetic routes can be applied for the preparation of cobalt oxide; however the mechanochemical process in solid state at room temperature is novel and suitable for preparation of cobalt oxide on large scale [18].

Results and Discussion

Characterization of Catalyst

Particle size of the catalyst determined by wet method of analysis was found in the range of 10-30 micron. BET surface area of the cobalt oxide was 63 m2g-1. Average pore radius was estimated by BJH method as described elsewhere [18]. About 88 % of the pores have the radius equal to or greater than 4.5 nm.

X-Ray diffraction pattern of the catalyst is given in Fig. 1. The observation of various peaks in XRD shows the crystalline nature of the cobalt oxide. Peaks at 2th = 18.8, 31.2, 45.5 and 59.1 are assigned to crystal lattice of Co3O4 [18]. As it is clear in the Fig. 1 that there is no change in the XRD of lab- prepared and used catalyst, so it can be concluded that the nature and composition of the cobalt oxide doesn't vary during the oxidation process.

FTIR spectra of the various samples of cobalt oxide are shown in Fig. 2. As there is no difference between the FTIR spectra of commercial Co3O4 (spectrum a) and lab prepared Co3O4 (spectrum b), therefore it is concluded that both samples have the same chemical composition. A very small absorption band in the spectrum of lab prepared catalyst (spectrum b) at ~2400 cm-1 could be due to the physically adsorbed carbon dioxide [18-20]. The absorption bands at 3500-3200 cm-1 and 1600 cm-1 in all these samples are assigned to stretching and bending vibrations of adsorbed water molecules. In the used catalyst absorption bands observed at 400-1200 cm-1 are the same as observed in commercial sample (a) and fresh lab prepared (b) and are assigned to vibration of cobalt-oxygen bonds [1]. Broad band at 3200 cm-1 and 1182 cm-1 in spectrum (c) are assigned to phenolic O-H and C-O stretching, respectively due to adsorbed phenol [21-24].

SEM is employed for the surface morphology of the catalyst and to know whether the particles of the catalyst are evenly distributed or packed up in aggregate. Scanning electron micrographs of the catalysts are shown in Fig 3. The particles of the catalyst are fine particles having uniform shapes. Uniform shapes of the catalyst particles play an important role in reproducibility of the catalytic results. On comparison of micrographs of fresh and used catalysts, it can be concluded that morphology of the catalysts don't change in oxidative degradation of phenol in aqueous medium. Furthermore, it can be seen that particle size of the catalyst also remains the same after reaction. It can also be noted from the scanning electron micrograph that particle size of the catalysts are in the range of micron, while on the basis of XRD it was concluded that these are in the range of nanometer scale.

Actually, sizes calculated by Scherer's equation on the basis of XRD show the crystallite size while SEM show the size of particles, which are aggregates of several crystallites.

Bulk oxygen and surface oxygen content of the prepared cobalt oxide catalyst were found as 3.158 x 10-4 and 8.249 x 10-5 g atom of oxygen per g of catalyst, respectively.

Time Profile

The time course study for the degradation/oxidation of phenol was monitored periodically. This investigation was carried out at 323 K for 120 minutes. Concentration of phenol was determined spectrophotometrically at wavelength 269 nm. With the increase in reaction time, concentration of phenol decreases as shown in Fig. 4, which shows that degradation/conversion of phenol increases with reaction time. Phenol was degraded to carbon dioxide and water. Formation of carbon dioxide was confirmed by lime water test. Percent degradation of phenol was calculated by equation 1.

equation

where Co is initial concentration and Ct is the concentration of phenol at particular time.

Effect of Temperature

The effect of temperature on degradation/oxidation on phenol was investigated in the range of 313-333K, while keeping the initial concentration of phenol (0.71M), amount of catalyst (0.2g) and partial pressure of oxygen (101 kPa) constant. As it is clear in Fig 5, degradation of phenol increases with temperatures. 323 K temperature and 0.71 M concentration of phenol were selected as optimum parameters.

Effect of Partial Pressure of Oxygen The system was tested in relation to the effect of partial pressure of oxygen on degradation of phenol. An oxygen partial pressure range of 51-101 kPa was investigated to degrade the pollutant, phenol, with initial concentration of 0.71M at 323 K. and 0.2 g of catalyst. Fig. 6 indicates that oxidation/degradation of phenol increases with partial pressure of oxygen. For determination of various partial pressure of oxygen, nitrogen was admixed with oxygen having the total flow rate of 60 mLmin-1. The partial pressure of oxygen was calculated by

equation

where Fo2 and 2 FN2 represent the flow rate of oxygen and nitrogen respectively.

Kinetic Studies

Langmuir-Hinshelwood kinetic model can be used to describe the degradation of phenol catalyzed by cobalt oxide, in which it is assumed that the degradation reaction is taking place at the surface of catalyst particles. Recently this model has been described for the reaction involving liquid and gaseous reactants at the surface of catalyst, although it was initially developed to describe the reaction involving the gaseous reactants [5, 18, 25-26].

According to this kinetic model, the rate of reaction, r, is proportional to the fraction of the surface covered by substrate, th,

equation

where PhOH, t and kr represent concentration of phenol, time and rate constant respectively.

Considering the Langmuir equation for th, equation (3) can be expressed as (partial pressure of oxygen is kept constant)

equation

K is adsorption equilibrium constant. Similarly considering Temkin equation for th, equation (3) can be expressed as:

equation

K and n are empirical constants, which depend on several factors. K can be defined as the adsorption or distribution coefficient and represent the phenol adsorbed on the surface of catalyst. The slop, 1/n which, range between 0 and 1 for favorable adsorption on surface, is a measure of surface heterogeneity. The surface is considered more heterogeneous as its value gets closer to zero. A value of 1/n above one is indicative of cooperative adsorption, while 1/n below one indicates normal Langmuir adsorption isotherm [27-30].

Combining the two constants together, equation (7) can be simplified as

equation

Time profile data at various temperatures from figure was subjected to kinetic analysis according to Langmuir, Temkin and Freundlich models by linear and non-linear least square method.

For non-linear least square method Curve Expert 1.4 software was used. Rate of the reaction was calculated by applying third order polynomial to the time profile data, using Curve Expert software. Freundlich model was found to be applicable to the data. Fig. 7 shows the applicability of Fruendlich Model, equation (8) by non-linear least square method using Curve expert software, from which apparent rate constants, krApp and 1/n at different temperatures were determined. The values of rate constants and 1/n are listed in Table-1. Fig. 8 shows the application of Freundlich Model, equation (9) by linear method, from which rate constants and 1/n were determined as listed in Table-1. It is evident from Table-1 that apparent rate constant of degradation of phenol increases with temperature.

The value of slope (1/n) is less than one, showing the heterogeneous nature of the catalyst surface. Similarly time profile data at various partial pressure of oxygen from Fig 6 was subjected to kinetic analysis according to equation (9), using Curve Expert software. Values of apparent rate constants and 1/n at various partial pressure of oxygen were determined as listed in Table-2. To investigate the adsorption nature of oxygen at the surface of catalyst, apparent rate constants at various partial pressure of oxygen from Table-2 were subjected to kinetic analysis according to various kinetic models. Only Freundlich model was applicable to the data from which true rate constant for degradation of phenol was determined and was found 5.57 x 10-4 min -1.

Value of 1/n was calculated as 0.205, which shows that adsorption of oxygen is also taking place at heterogeneous sites.

Table-1: Rate constants determined by application of Freundlich Model to time profile data at various temperatures at 101 kPa partial pressure of oxygen.

###krApp (min-1)###1/n

T (K)###Non-linear###Linear###Non-linear###Linear

###method###method###method###method

313###0.011###0.010###0.704###0.623

323###0.012###0.012###0.433###0.425

333###0.012###0.013###0.313###0.333

Table-2: Rate constants determined by application of Freundlich Model to time profile data at various partial pressure of oxygen at 323 K temperature.

###krApp (min-1)###1/n

pO2###Non-linear###Linear###Non-linear###Linear

(kPa)###method###method###method###method

51###0.00646###0.00644###0.745###0.741

64###0.00713###0.00712###0.573###0.574

84###0.00863###0.000863###0.521###0.524

101 0.01220###0.01210###0.448###0.446

Experimental

Materials

Phenol (Merck, 822296), cobalt nitrate (Acros, A0225837), and ammonium bicarbonate (Merck, 1336) were used as received. Nitrogen and oxygen were supplied by BOC Pakistan Ltd. and were further purified by passing through traps (C. R. S. Inc. 202268) to remove traces of water. Traces of oxygen in nitrogen gas were removed by using specific oxygen traps (C. R. S. Inc. 202223).

Synthesis of Catalyst

Cobalt oxide was prepared by mechanical mixing and milling of Co(NO3)2.6H2O and NH4HCO3 (molar ratio 2:5) in agate mortar until color of the powder was unchanged. Then the milled powder was thoroughly washed with distilled water and dried at 383 K for 24 hours. The resulted powder was calcined with a programmable furnace at the rate of 5 K/min to reach 573 K and was kept at that temperature for 3 hours.

Characterization Techniques

The prepared catalyst was characterized by physical and chemical methods. Physical characterization includes particle size, surface area, XRD, SEM and FTIR analysis, while chemical characterization is the determination of oxygen content. Particle size analyzer (Analysette 22 Compact, Fritsch, Germany) was used for particle size analysis using wet method of analysis.

BET surface area of the catalyst was determined using a Quanta Chrome (Nova 2200e) surface area and pore size analyzer. Samples were degassed at 383 K for 2 hours prior to analysis.

X-Ray diffraction (XRD) patterns were recorded using X-Ray Diffractometer, JEOL (JDX-3532) Japan, using Cu-K(alpha) radiation with a tube voltage of 40 KV and 20 mA with 2th ranges from 0 to 70deg.

The FTIR spectra were recorded in KBr medium using IR Prestige 21, Shimadzu, Japan instrument in the range of 400-4500 cm-1.

SEM analysis was carried out by using

Scanning Electron Microscope (JOEL-JSM 5910 Japan). For this purpose the catalysts were mounted on the sample stubs and coated with gold foil using gold coating machine (JEOL-JSM-420, Japan). The samples were then automatically analyzed using computer software.

The oxygen content was determined by dissolving 2.5 g of potassium iodide in 20 mL of 36% acetic acid solution followed by addition of about 0.2 g of the catalyst. The solution was allowed to stand for 15 minutes under the atmosphere of molecular nitrogen to liberate iodine. After filtration, the liberated iodine was titrated against 0.1 N sodium thiosulfate solution using starch as indicator.

For determination of surface oxygen content about 0.2 g of the catalyst and about 2 g of potassium iodide were added to 15 mL buffer solution of pH 7.1 and was vigorously shaken in atmosphere of molecular nitrogen for 15 minutes. The reaction mixture was then filtered, acidified with 1 N HCl and the liberated iodine was titrated against 0.1 N sodium thiosulfate solution, using starch as indicator. Following expression was used for determination of oxygen content [5, 18, 26].

equation

Oxidation Protocol

Catalytic oxidation reaction was performed in magnetically stirred Pyrex three necks round bottom batch reactor equipped with reflux condenser and mercury thermometer. The reaction temperature was maintained at desired value by using hot plate. Typically 10 mmol phenol was taken in 15 mL (0.71 M) of water and the reaction mixture was saturated with molecular oxygen by passing oxygen gas at the rate of 60 mL per minute for 30 minutes. After getting the required temperature, 200 mg of the catalyst was loaded to the reactor, while keeping the temperature and flow of oxygen constant at desired value. After the desired interval of time catalyst was separated from reaction mixture with Whatman Glass micro fiber filter No. 1825 055 using glass syringes. Analysis of the reaction mixture was carried out by UV-Visible spectrophotometer (Shimadzu UV-160A, Japan).

Conclusions

Cobalt oxide prepared by mechanochemical process in solid phase at room temperature can be used as effective catalyst for oxidative degradation of phenol in aqueous medium. In most of the catalytic reactions, the same results can not be reproduced under similar reaction condition due to variation in size and shape of the catalyst particles. In present case due to uniform size and shape almost same results can be reproduced. In catalytic oxidative degradation, the nature of the cobalt oxide catalyst doesn't alter; hence it can be used as catalyst again and again. In oxidative degradation reaction Langmuir-Hinshelwood type of mechanism is operative. According to Langmuir-Hinshelwood mechanism reaction proceed in two steps. In first step both reactants adsorb at the surface of catalyst followed by reaction between the adsorbed reactants in second step. Adsorption of phenol and oxygen at the surface of catalyst follow the Freundlich adsorption model.

Cobalt oxide acts as heterogeneous catalyst in this investigation, which can be easily separated from the reaction mixture by simple filtration.

Acknowledgment

Higher Education Commission (HEC) Pakistan is acknowledged for providing financial assistance under Indigenous PhD 5000 and IPFP Program (Muhammad Saeed).

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1Department of Chemistry, Abdul Wali Khan University, Mardan 23200, Pakistan., 2National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan., pksaeed2003@yahoo.com
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Author:Saeed, Muhammad; Ilyas, Mohammad; Siddique, Mohsin
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Geographic Code:9PAKI
Date:Jun 30, 2012
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