Article pubs.acs.org/IECR
Optimization of Reaction Parameters and Kinetic Modeling of Catalytic Wet Peroxidation of Picoline by Cu/SBA-15 V. Subbaramaiah, Vimal Chandra Srivastava,* and Indra Deo Mall Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667 Uttarakhand, India ABSTRACT: In this study, various copper loaded SBA-15 catalysts were synthesized by an impregnation method and characterized by UV−visible DRS, NH3-TPD and TGA. The synthesized catalysts were used for the catalytic wet peroxidation (CWPO) of 2-picoline in aqueous solution using hydrogen peroxide as an oxidant. Copper loading on SBA-15 enhanced the surface acidity by the generation of Lewis acid sites. Effects of various operating parameters such as initial pH, catalyst dosage, hydrogen peroxide dosage, initial 2-picoline concentration, and temperature have been investigated. At the optimum conditions of pH = 6, catalyst dosage = 0.5 g/L, stoichiometric ratio of H2O2/picoline = 1.3 (i.e., molar ratio of H2O2/picoline = 20.15) and temperature = 338 K, more than 99% 2-picoline removal efficiency was observed for a solution containing 100 mg/L of 2picoline by 5%Cu loaded SBA-15. Furthermore, the CWPO process was well-described by a two-step pseudo-first-order kinetic model. The power law model well described the dependence of rate constant on catalyst dosage. ruthenium, platinum, palladium, gold, etc.5 Also, catalyst supports play an important role in improving the properties of a catalyst in terms of activity, selectivity, and stability by manipulating its surface properties.14 SBA-15 has gained high attention in the field of catalysis owing to its good hydrothermal stability resulting from thicker pore walls.15 Many investigations have revealed excellent activity and stability of SBA-15 supported catalysts for the catalytic oxidation of organic compounds.3,10−12,16 In our previous study,12 copper impregnated SBA-15 (Cu/ SBA-15) catalyst was synthesized and characterized by temperature programmed reduction (TPR), H2-chemisorption, Fourier transform infrared (FTIR) spectroscopy, and field emission scanning electron (FE-SEM) microscopy. It was further used for CWPO of pyridine from aqueous solution. More than 97% pyridine removal and 92% total organic carbon (TOC) removal was achieved at optimum parametric conditions. The present study was planned to more deeply characterize Cu/SBA-15 catalysts in terms of thermal stability, presence of acid sites, metal coordination, etc., and also to check the efficacy of Cu/SBA-15 catalysts for mineralization of other nitrogenous compounds such as 2-picoline. In this study, Cu/SBA-15 was characterized by thermogravimetric analysis (TGA), NH3-temperature programmed desorption (NH3-TPD), UV−visible diffused reflectance spectrophotometer (UV-DRS) and pore size distribution. It was then used for the CWPO of 2-picoline bearing wastewater under atmospheric pressure using hydrogen peroxide as an oxidant for the optimization of operating parameters such as pH, catalyst dose, H2O2 dose, and temperature. Kinetics of the treatment process were studied and mechanism of the picoline mineralization by Cu/SBA-15 was explored.
1. INTRODUCTION Picoline is a hazardous chemical which is used as a solvent and intermediates in the synthesis of pharmaceuticals, resins, insecticides, dyes and rubber accelerators, water proofing agents, niacin and niacinamide.1 Such industrial units and processes which manufacture or utilize picoline discharge a high concentration of organic pollutants such as pesticides, phenol, pyridine, picoline, and its derivatives.2 Conventional treatments based on thermal destruction and chemical or biological methods have limitations in applicability, effectiveness, and cost in the treatment of wastewater.2,3 Moreover, refractory organic compounds such as picoline need to be mineralized or destroyed. During the past decade, there were enormous research efforts in studying advanced oxidation processes (AOPs).4 Catalytic wet air oxidation (CWAO) is a well established alternative technique for treating toxic and hazardous organic wastewater with air or oxygen as an oxidant.5 However, it is an energy intensive process and works at elevated temperature (100−300 °C) and pressure (1−10 MPa).6 Fenton oxidation works at comparatively mild conditions at temperatures ≤ 353 K and atmospheric pressure.6,7 However, key shortcomings of restricted operation around pH 3, leaching of iron, and generation of huge quantities of sludge restrict its usage for many organic compounds.4,8 Catalytic wet peroxidation (CWPO) is a wellknown process that can be used for the degradation of organic pollutants with the help of a homogeneous or heterogeneous catalyst using hydrogen peroxide as oxidant under mild conditions with low energy consumption.9 Different alternative heterogeneous CWPO systems have been proposed using transition metals supported over various solid materials.4,9−12 Hydrogen peroxide is a nontoxic reactant that improves the oxidation efficiency of catalytic processes and reduces the requirement of critical reaction conditions.13 Copper is well-known to undergo a Fenton-type reaction and has been used successfully for treating various organic pollutants. Copper is cheaper and more easily available as compared to other traditional and costlier catalyst such as © XXXX American Chemical Society
Received: January 11, 2013 Revised: April 9, 2013 Accepted: June 4, 2013
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dx.doi.org/10.1021/ie400124d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. (a) UV−visible DRS of SBA-15 and Cu/SBA-15; (b) NH3-TPD profile of SBA-15 and Cu/SBA-15.
2.3. Experimentation and Analytical Methods. The experimental studies were carried out in a 250 mL three-neck round-bottom glass reactor. It was equipped with a total reflux system at the top (to prevent loss of 2-picoline vapor) and an oil bath at the bottom which itself was kept above a magnetic stirrer hot plate to keep the solution homogeneous during the experimental run. The hot plate was used initially for increasing the temperature of the solution to the desired temperature and then a proportional−integral−derivative (PID) controller was used to maintain the temperature constant during the experimental run. Each experimental run was started with 100 mL of 2-picoline solution of required concentration in the reactor; thereafter, this reaction mixture was heated and stabilized at the desired reaction temperature with the help of the oil bath and hot plate. This time was called as time zero (t = 0). At this time, the required amount of catalyst (Cu/SBA-15) and oxidation agent (hydrogen peroxide) was added to reaction mixture. After desired reaction time intervals, 1 mL samples were taken out and analyzed for residual 2-picoline and other intermediates. To study the effect of initial pH, the pH of the 2-picoline was adjusted using 0.1 M NaOH and H2SO4. The initial and residual 2-picoline concentration was determined using highperformance liquid chromatography (HPLC) supplied by Waters India, Ltd. HPLC was equipped with a C18 reverse phase column and a ultraviolet (UV) detector. Mixture of 80% methanol and 20% distilled water was used as a mobile phase in HPLC and its flow rate was 0.5 mL/min. Wavelength in the UV detector was kept at 262 nm for measurement of 2-picoline concentration. For identification of intermediate compounds, a mixture of 40% methanol and 60% milli-Q water (0.005 M sodium salt of hexane sulfonic acid) was used as a mobile phase. The analysis was carried out in isocratic mode at a flow rate of 1 mL/min, with column effluent being monitored at 254 nm. Short chain organic compounds were identified by an ion chromatograph equipped with a Metrosep A Supp 5-250 column with thermal conductivity detector. In this, the mobile phase used was a mixture containing Na2CO3 and NaHCO3 in water and having a molar ratio of 3.2/1. Cu leaching from Cu/ SBA-15 catalysts was also measured in each cycle using an ion chromatograph equipped with UV−vis detector. The degree of mineralization of 2-picoline was quantified by determining the total organic carbon (TOC) conversion. TOC was estimated
2. METHODS AND MATERIALS 2.1. Materials. All the chemicals used in the study were of analytical reagent (AR) grade. Tetraethylorthosilicate (TEOS) (Aldrich, Germany), pluronic P123 (PEO20-PPO70-PEO20, Aldrich, Germany), copper sulfate (SD fine chemicals, India), 2-picoline (SD fine chemicals, India), HCl (Ranken, India), 30 wt % hydrogen peroxide (Ranken, India), etc. were purchased from various companies. 2.2. Synthesis of Catalyst and Its Characterization. Copper containing SBA-15 samples with various copper loading namely 5%, 10%, and 20%Cu/SBA-15 were synthesized in two-step procedures as per the method reported earlier.12 The textural possessions of the samples were analyzed at 77 K by N2 adsorption−desorption isotherms. Before the analysis, the sample was degassed at 300 °C for ∼6 h. The surface area was obtained in a relative pressure range of 0.05 to 0.30, using Brunauer−Emmett−Teller (BET) model. Total pore volumes were calculated at a relative pressure of 0.99. The average pore diameter was determined by assuming all pores in the sample are parallel and cylindrical. UV−visible spectra were measured by diffused reflectance spectra (DRS) recorded using a UV−visible spectrophotometer equipped with an integrating sphere attachment (UV-2450 Shimadzu). Spectra were recorded using BaSO4 as a reference standard material in the range from 200 to 800 nm. NH3-TPD experiments were performed to determine the acidity of catalyst using Micrometric Chemisorb 2720 instrument. An amount of 100 mg of each catalyst was loaded in the quartz U-tube reactor and preheated at 200 °C for 2 h under a flowing helium stream (20 mL/min), then the catalyst was cooled down to room temperature. The pretreated samples were then saturated with 10% NH3/He (v/v) mixture at flow rate of 40 mL/min for 60 min. The physisorbed NH3 was removed by flushing the catalyst with helium at a flow rate of 20 mL/min for 1 h before starting the TPD analysis. Experimental runs were recorded with a thermal conductivity detector by heating the sample from 30 to 800 °C at a heating rate of 10 °C/min flowing He at flow rate of 20 mL/min. TGA experiment were carried out under air atmosphere with air flow rate of 200 mL/min, in the temperature range of room temperature to 1000 °C with a heating rate of 10 °C/min. Aluminum was used as a reference material. B
dx.doi.org/10.1021/ie400124d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. (a) Pore size distribution of SBA-15 and Cu/SBA-15; (b) thermo-gravimetry analysis (TG) of SBA-15 and Cu/SBA-15.
200−400 °C is attributed to the desorption of ammonia from medium acid sites. Ammonia desorption at higher temperature (>400 °C) is due to the existence of superacidic sites on the catalyst.24 Hence, it may be concluded that Cu/SBA-15 samples are highly acidic in nature. It may be seen in Figure 1b that the peak intensity increased with an increase in weight percentage of copper loading. Copper loading on SBA-15 enhanced the surface acidity by the generation of Lewis acid sites, to which ammonia molecules get bonded by donor−acceptor bond.22 Since Cu/SBA-15 samples have superacidic Lewis acid sites, there is high likelihood that it can interact with basic 2-picoline molecules and enhance its oxidation.25 Textural properties of catalyst samples (SBA-15 and Cu/ SBA-15) were studied by BET surface area and Barrett− Joyner−Halenda (BJH) pore size distribution by N 2 adsorption−desorption isotherms. All the samples exhibited international union of pure and applied chemistry (IUPAC) type IV-isotherms nature that was reported in our previous publication.12 A sharp inflection in the relative pressure (P/P0) between 0.5 and 0.8 corresponds to capillary condensation within uniform mesopores, and is a function of the pore diameter.26 With increasing copper loading into SBA-15 the height of hysteresis loop decreased due to deceased pore volume indicating the introduction of metal species within the mesopores of the support.27 The surface area of the SBA-15 decreased significantly after metal loading. Surface areas were found to be 650 m2/g, 569 m2/g, 486 m2/g, and 313 m2/g for bare SBA-15, 5%Cu/SBA-15, 10%Cu/SBA-15, and 20%Cu/ SBA-15, respectively. High copper content (20%Cu/SBA-15) SBA-15 was found to contain inkbottle type of pores.28 The pore size distribution for SBA-15 and Cu/SBA-15 is shown in Figure 2a. The average pore diameter, which was calculated from the BJH model, decreased with an increase in Cu loading. It was found to be 6.0 nm, 5.8 nm, 5.2 nm, and 5.0 nm for bare SBA-15, 5%Cu/SBA-15, 10%Cu/SBA-15, and 20%Cu/SBA-15, respectively. Pore volume also decreased after the loading of Cu into SBA-1513 indicating loading of copper species into the mesoporous channels of SBA-15.28
using a TOCVCPH-analyzer (Shimadzu 5500A). TOC estimation process involved catalytic oxidation followed by quantification of the CO2 formed through nondispersive infrared (NDIR) detector.
3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The UV-DRS is used for the characterization of metal ion coordination and its existence in intra- and/or extra-framework position of metal containing mesoporous material.17 The UV-DRS spectra of SBA-15 and copper containing SBA-15 samples are depicted in Figure 1a. Three band regions can be identified in the spectra of the Cu/ SBA-15 samples. Small peaks in the first UV-DRS spectrum band region (200−230 nm) of Cu/SBA-15 sample can be attributed to surface Cu2+ species suggesting incorporation of some Cu2+ ions into the framework of SBA-15.18 The broad band centered at about 260−280 nm can be ascribed to the charge-transfer between mononuclear Cu2+ and oxygen. Small peaks in the band region around 340−380 nm may be due to the presence of [Cu−O−Cu]n-type surface clusters.19 The samples had lower intensity reflectance in the range of 600− 800 nm suggesting that copper oxide was better dispersed on mesoporus silica (not shown in the figure).20 The pure silica SBA-15 exhibited a weak peak at 216 nm which is typically due to siliceous materials.21 The TPD study has been used most commonly to investigate the binding of adsorbates on the catalytic surface. The higher is the temperature of desorption peak, the more strongly the adsorbate is bound to the surface. NH3-TPD profile of bare SBA-15 exhibited only a peak at 105 °C (desorption temperature below 200 °C) indicating the presence of weak acid sites. No additional desorption occurred at higher temperature indicating that SBA-15 has no acid sites except a weak acid site.22 A desorption peak at 105 °C may be due to the free hydroxyl group on the surface of the catalyst.23 Cu/ SBA-15 samples exhibited all three type of acidic sites on their surfaces. The peak below 200 °C corresponds to the desorption of ammonia from weak acid sites of Cu/SBA-15; and a peak at C
dx.doi.org/10.1021/ie400124d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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al.12 and Tu et al.33 reported that Cu particles having maximum dispersion capacity of 5 wt % and at higher loading particles become enlarged due to sintering. Higher loading causes Cu particles to get confined deep into the channels that may not participate in the oxidation reaction. Also, sometimes Cu particles positioned at the start of the pores block the active Cu particles which are positioned deep into the pores. Cu particles which are directly accessible to reactant particles such as those which are positioned at or near the pore opening act as the reaction sites. Because of the above reasons, 5%Cu/SBA-15 was taken as optimum loading for CWPO of 2-picoline. This 5% Cu/SBA-15 was used for further study to optimize various operating parameters. 3.3. Effect of pH on the Catalytic Performance. For checking the effect of pH on the degradation of 2-picoline by 5%Cu/SBA-15, the pH of the solution was varied in the range of 3−10 and other parameters such as catalyst dose (1 g/L), molar ratio of H2O2/picoline = 1.3 times the stoichiometric level, 2-picoline concentration (100 mg/L), and reaction time (5 h) were kept constant throughout this study. The results are depicted in Figure 3a. It may be seen that the maximum 2-
TGA of bare and copper loaded SBA-15 was carried out to study the thermal stability of the catalyst. Cu/SBA-15 degradation pattern shows three different weight loss zones can be shown in Figure 2b. First weight loss zone (∼14% for SBA-15; ∼15% for 5%Cu/SBA-15; ∼20% for 10%Cu/SBA-15; ∼22% for 20%Cu/SBA-15) identified at ∼100 °C is due to loss of physically absorbed water (condensed, physically adsorbed, in the silica channels and cavities as well as the water coordinated to copper complexes). Second weight loss zone (∼4% for SBA-15; ∼7% for 5%Cu/SBA-15; ∼5% for 10%Cu/ SBA-15; ∼8% for 20%Cu/SBA-15) was recognized in the range of 200−400 °C. It was found that the second degradation zone (200−400 °C) accounted for maximum weight loss (≤10%). This was mainly attributed to the fact that a small portion of templates still remain in the SBA-15 framework during calcination. Zhang et al. (2012)18 studied TGA for 0.03Cu/ SBA-15 after calcination at 500 °C for 6 h under air atmosphere. They found four weight loss zones: below 120 °C (1.6%), 120−224 °C (18.6%), 224−337 °C (8.2%), and above 337 °C (4.1%). Many other authors have observed similar degradation zones around 150−300 °C. This has been attributed to the decomposition of template in SBA-15.29,30 This weight loss may also be due to the decomposition of surfactant,31 as well as due to combustion of carbon species. Subsequent third weight loss (∼2% for SBA-15; ∼7% for 5% Cu/SBA-15; ∼11% for 10%Cu/SBA-15; ∼13% for 20%Cu/ SBA-15) around 570−770 °C may be due to the dehydroxylation of Si−OH.31 This study showed that Cu/SBA-15 catalysts may be treated thermally (for example, in catalytic reactions) up to a maximum temperature of around 500 °C without any significant decomposition.32 3.2. Picoline OxidationPreliminary Experiment. Initially few preliminary experiments were performed for the oxidation of 2-picoline with oxidant (H2O2) and without catalyst. In these experiments, only 7% oxidation of 2-picoline was achieved. In addition, runs were performed without oxidant and with catalyst to distinguish effect of adsorption. Only a marginal effect was observed for the removal of 2-picoline bearing wastewater. Stepwise addition of hydrogen peroxide (30%) was also tested. It was found that if the total amount of added H2O2 was kept constant, single-step, two-step, or three step H2O2 additions did not significantly alter the overall oxidation of 2-picoline bearing wastewater. Therefore, most of the experiments were carried out with a one-step addition of H2O2 at the desired stoichiometric H2O2/picoline molar ratio. As per the following reaction, complete oxidation of 1 mol 2picoline requires 15.5 mol of H2O2: 2C6H 7N + 31H 2O2 → 2CO2 + 38H 2O + N2
Figure 3. Effect of various variables on 2-picoline oxidation by 5%Cu/ SBA-15; (a) Effect of pH (T = 328 K, stoichiometric ratio of H2O2/ picoline = 1.3, Co = 100 mg/L, 5%Cu/SBA-15 dose = 1 g/L); (b) effect of catalyst dose (T = 328 K, stoichiometric ratio of H2O2/ picoline = 1.3, Co = 100 mg/L); (c) effect of H2O2 dose (T = 328 K, 5%Cu/SBA-15 dose = 0.5 g/L, Co = 100 mg/L); (d) effect of initial 2picoline concentration (T = 328 K 5%Cu/SBA-15 dose = 0.5 g/L, stoichiometric ratio of H2O2/picoline = 1.3).
(1)
Different copper loaded SBA-15 (5%Cu/SBA-15, 10%Cu/ SBA-15, and 20%Cu/SBA-15) were tested for the oxidation of 2-picoline bearing wastewater using hydrogen peroxide as oxidant. Other reaction parameters were kept constant: pH ≈ 6, catalyst dose = 1 g/L, 2-picoline initial concentration = 100 mg/L and temperature = 328 K. A molar ratio of H2O2/ picoline was kept constant at 1.3 times the stoichiometric level. 2-Picoline degradation was found to be 82%, 85%, and 75% for 5%Cu/SBA-15, 10%Cu/SBA-15, and 20%Cu/SBA-15, respectively. The difference in catalytic activity of 5%Cu/SBA-15 and 10%Cu/SBA-15 was very marginal, and beyond 10%-Cu/SBA15, 2-picoline degradation was decreased. TOC removal was observed to be 65%, 69%, and 58% for 5%Cu/SBA-15, 10%Cu/ SBA-15, and 20%Cu/SBA-15, respectively. Subbaramaiah et
picoline degradation was achieved at pH 6, and the degradation efficiency decreased with both an increase and a decrease in pH from pH = 6.0. 2-Picoline (pKa = 5.16) gets protonated in the acidic medium and deprotonated at higher pH. At lower pH (
