Ind. Eng. Chem. Res. 2004, 43, 5089-5097
5089
Catalytic Wet Oxidation of Phenol with Mn-Ce-Based Oxide Catalysts: Impact of Reactive Adsorption on TOC Removal M. Abecassis-Wolfovich,† M. V. Landau,*,† A. Brenner,‡ and M. Herskowitz† Chemical Engineering Department, The Blechner Center for Industrial Catalysis and Process Development, and Biotechnology and Environmental Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
Catalytic wet oxidation of phenol solutions at low temperatures of 80-130 °C and space velocities of 1-100 h-1 using Mn-Ce catalysts was studied with an emphasis on the reactive adsorption mechanism and total organic carbon (TOC) removal. Eight catalysts (Mn/Ce ) 6:4) were activated under different conditions and promoted with alkali metals (K, Cs) or noble metals (Pt, Ru). The compositions and physical properties of all catalysts were measured. Preliminary runs were conducted in a batch reactor, but most experiments were carried out in a continuous-flow tricklebed reactor. Catalysts containing mixed Mn3O4-CeO2 phases pure and promoted with alkali metals displayed a higher activity and a higher adsorption of organic deposits on their surface. Noble metals had little effect on process performance. The adsorption capacity of the catalysts was found to be considerably higher than that reported for activated carbon. Furthermore, complete regeneration of a catalyst in three consecutive tests was demonstrated under relatively low temperature and with no loss of activity. The selectivity toward reactive adsorption was highest on Mn-Ce-Cs catalysts. Low space velocity yielded essentially complete adsorption of phenol, resulting in deposits on the catalyst surface. The conversion of phenol to water-soluble oxygenates was found to increase water toxicity. The catalytic reactive adsorption-regeneration process should become an attractive treatment method for phenol solutions and other complex waste streams. Introduction Catalytic wet oxidation (CWO) with heterogeneous catalysts is a method for the treatment of dilute aqueous waste streams containing a variety of organic pollutants such as phenols.1,2 It can be used to mineralize organic contaminants to CO2 and H2O or to convert them into nontoxic and biodegradable products. CWO has also become an alternative to the traditional treatment of wastewater with activated carbon, in a periodic adsorption-regeneration process.3,4 CWO needs to be conducted at relatively low temperature to exclude water evaporation and condensation, short space-time to save reactor volume, and with a high-recovery regeneration procedure to ensure economic feasibility. Many catalysts have been tested in CWO of phenols: Pt; Pd; Ru/C;5 CuO-ZnO/Al2O3;6,7 Ru/CeO2;8 Pt/CeO2;9 and bulk manganese-cerium oxide composites, pure10-12 or doped with Pt,13 Pt-Ag,14 or K.15,16 Only composites based on manganese-cerium oxides have displayed the ability to remove phenol efficiently at low temperatures of e100 °C. At low temperatures, Mn-Ce-based catalysts convert phenol to polymeric carbonaceous deposits produced on cerium-related sites that cause substantial catalyst deactivation due to surface blocking.10,12,17 The amount of carbon on a spent Mn-Ce catalyst could reach about 23 wt %.11 Introduction of Pt decreases the amount of carbonaceous deposits,13 whereas promoting the Mn-Ce catalyst with potassium achieves the opposite effect.15 * To whom correspondence should be addressed. E-mail:
[email protected]. † The Blechner Center for Industrial Catalysis and Process Development, Chemical Engineering Department. ‡ Biotechnology and Environmental Engineering.
Pintar and Levec7 measured the formation of polymeric deposits on the catalyst surface during phenol CWO on CuO-ZnO/Al2O3. They assumed that polymerization was promoted by the partial oxidation of phenol to aldehydes followed by condensation with phenol. Such polymerization usually occurs on basic catalysts.18 It is therefore consistent with promotion of carbon deposition on Mn-Ce composites on ceriumrelated sites,10,12,17 which are more basic than the surface of manganese oxide.19,20 The polymeric deposition on the catalyst surface can be considered as the “reactive adsorption” of phenol caused by the catalytic material. Its contribution to TOC removal in phenol CWO reached 50-70%.7,13 Mn-Ce-based catalysts were tested in phenol CWO at low temperature only in a batch reactor,10,11,13 although it was already postulated that the process should be more efficient in a tricklebed reactor because of the higher solid-to-liquid ratio.7,21 No information has been published on the regeneration of Mn-Ce-based catalysts after deactivation during the CWO of phenol. The scope of the present study was to demonstrate a controlled reactive adsorption mechanism as a means to improving TOC removal in the CWO of phenol with Mn-Ce-based catalysts in a fixed-bed reactor. Oxidative regeneration procedure to recover the initial catalyst activity was also developed. The information obtained was compared to the conventional adsorptive removal of phenol from wastewater by activated carbon. Experimental Work Catalyst Preparation. Two samples of manganese and cerium oxide composite (atomic ratio Mn/Ce ) 6:4) were prepared by coprecipitation from mixed aqueous
10.1021/ie049756n CCC: $27.50 © 2004 American Chemical Society Published on Web 07/13/2004
5090 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
solutions of manganese(II) chloride (MnCl2‚4H2O, Sigma Chemical Co.) and cerium(III) chloride (CeCl3‚7H2O, Sigma Chemical Co.) as described by Imamura et al.22 One hundred milliliters of this solution was poured into 200 mL of 3 M aqueous sodium hydroxide (NaOH, 97%, Aldrich Chemicals), and the resulting precipitate was separated by filtration, washed, dried in air at 100 °C for 16 h, and then calcined at 350 °C for 3 h under a vacuum of 85 mbar (Mn-Ce-1) or in air (Mn-Ce-2). Addition of potassium or cesium to these Mn-Ce catalysts was done by incipient wetness impregnation that was performed after drying the Mn-Ce precipitate at 100 °C. The samples were impregnated with aqueous solutions of potassium nitrate (KNO3, Aldrich Chemicals) or cesium nitrate (CsNO3, Aldrich Chemicals) to gain 4 wt % of the alkaline metal. The samples were evacuated at 350 °C for 4 h (Mn-Ce-K-1, Mn-Ce-Cs1) or calcined in air under the same conditions (MnCe-K-2, Mn-Ce-Cs-2). Addition of platinum or ruthenium was performed by incipient wetness impregnation of Mn-Ce catalyst after treatment in a vacuum at 350 °C for 3 h with 5 mg/L aqueous solutions of H2PtCl6 or H2RuCl6 (Aldrich Chemicals) to gain 3 wt % of the noble metal. After the samples had been dried at 100 °C, they were activated in hydrogen for 3 h at 350 °C (Mn-CePt and Mn-Ce-Ru). Catalyst Characterization. The chemical compositions of the catalysts (weight percentage, average of five measurements at different points of the solid) were measured by energy-dispersive X-ray spectroscopy (JEOL JEM 5600 scanning electron microscope). Surface areas and pore volumes were obtained from N2 adsorptiondesorption isotherms using the conventional BET and BJH methods. The calcined samples were outgassed under vacuum at 250 °C. Isotherms were measured at liquid nitrogen temperature with a NOVA-2000 (Quantachrome, Version 7.01) instrument. The phase compositions of the catalysts were tested by X-ray diffraction (XRD). The XRD patterns were collected on a Philips diffractometer PW 1050/70 (Cu KR radiation) with a graphite monochromator at diffracted beam. Data were recorded at a 0.02° step size for 2 s at each step. The peak positions and the instrumental peak broadening (β) were determined by fitting each diffraction peak by means of APD computer software. The crystal domain size was determined using the Scherrer equation
l)
Kλ [(B - β ) cos(2θ/2)] 2
2 0.5
where K ) 1.000, β ) 0.1°, λ ) 0.154 nm, and B is the peak broadening at 2θ ) 30-45° for different manganese oxides and at 2θ ) 48° for cerium oxide. Temperature-programmed reduction (TPR) and temperatureprogrammed oxidation (TPO) experiments were carried out in an AMI-100 Catalyst Characterization System (Zeton-Altamira) equipped with a mass spectrometer for outlet component identification (Ametek 1000). Catalyst (0.2 g) was loaded and treated in 10 vol % H2-Ar, 25 cm3/min (TPR), or 5 vol % O2-He, 25 cm3/min (TPO), as the temperature was increased from ambient to 420 °C at 5 °C/min. Catalyst Testing Procedures. The wet oxidation of phenol (Sigma Chemical Co.) was carried out in batch and fixed-bed reactors. The preliminarily tests were performed in a stirred autoclave reactor [steady state (SS), 300 mL, Bu¨chi] at 100 °C, under a 10 atm oxygen
Figure 1. Fixed-bed reactor rig for phenol oxidation: 1, oxygen cylinder; 2, flowmeter; 3, preheater; 4, fixed-bed reactor; 5, temperature controller; 6, separator and liquid sampling cell; 7, gas sampling; 8, nitrogen cylinder; 9, liquid pump; 10, feed tank.
pressure, with a stirring speed of 800 rpm, a catalyst loading of 2 g/L, and a phenol concentration of 0.2 g/L. Catalyst (0.3 g in a powder form to avoid any diffusion limitations) and 110 mL of distilled water were fed to the reactor, which was pressurized with oxygen to 9 atm, and the mixture was then heated under agitation using an oil bath. After the desired reaction temperature had been attained, 25 mL of concentrated phenol solution (1.2 g/L) was loaded into the reactor using an Eldex piston pump. One-milliliter samples were taken periodically for HPLC analysis (GBC, LC 1205 instrument, Zorbax ODS-5 C18 reverse-phase column, mobile phase acetonitrile/distilled water mixture (8:2 vol ratio), 1 cm3/min flow rate, UV spectrophotometer for detection, λ ) 260 nm) to determine the phenol concentration. Selected catalysts were tested in a continuous mode using a fixed-bed reactor (SS, 20-mm i.d., 24-cm length). CWO was performed at 80-130 °C with 10 bar of oxygen pressure and a liquid hourly space velocity (LHSV) of 1-100 h-1 in a fixed-bed reactor rig (Figure 1). The reactor was heated with an electric coil controlled by a Eurotherm PID controller. A K-type thermocouple was positioned in the center of the catalyst bed. Two independently controlled heating zones kept the axial temperature gradient in the reactor to less than 5 °C. The oxygen was fed by a Brooks mass controller to a preheater at a rate of 25-100 mL min-1 (STP conditions) before entering the reactor at the selected temperature. The tested solution containing 1 g/L phenol (doubly distilled water) was fed to the reactor by a high-pressure metering pump (SSI HPLC series II isocratic) at 20-52 mL/h and reaction pressure. Catalyst pellets (0.3-11 g, 0.4-0.5 mm fraction) were diluted with quartz particles of the same size in order to keep a fixed volume of catalyst layer inside the isothermal zone. The liquid effluent was collected in periods of time and analyzed by HPLC to detect the residual phenol concentration. The liquid outlet stream was also analyzed for TOC (total organic carbon) using an Apollo 9000 HS model TOC combustion analyzer (Tekmar Dohmann) equipped with a nondispersive infrared (NDIR) detector. Testing of phenol oxidation at 100 °C in a reactor loaded only with glass Raching rings showed that the phenol conversion was below 1%. The phenol conversion (XPhOH, %) and TOC conversion
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5091
(XTOC, %) were calculated as
(
XPhOH (%) ) 1 -
)
CPhOH,t × 100 CPhOH,0
and
(
XTOC (%) ) 1 -
)
CTOC,outlet × 100 CTOC,inlet
where CPhOH is the phenol concentration, with the subscripts 0 and t representing the initial concentration for the batch reactor or the concentration at the inlet for the fixed-bed reactor (CPhOH,0) and the final concentration for a batch reactor or the concentration at the outlet for the fixed-bed reactor (CPhOH,t). CTOC is the TOC concentration measured at the fixed-bed reactor inlet or outlet. The reaction rate constants of phenol transformation in the batch reactor were calculated assuming first-order kinetics with respect to phenol
(
-ln 1 -
)
XPhOH ) kt 100
To examine the leaching of the metal ingredients, the effluent stream was also analyzed for the presence of Mn and Ce by an inductively coupled plasma (ICP) technique, using an Optima 3000 Perkin-Elmer ICPOES system. In addition, toxicity bioassays based on the relative inhibition of bioluminescence of marine bacteria were conducted. Toxicity was determined by the Vibrio fischeri bioluminescence assay method, using a model ToxAlert 10 luminometer (Merck K G a A, Darmstadt, Germany). The Vibrio fischeri bioluminescence assay has been successfully employed for the evaluation of toxicity in complex waste samples and has shown a high sensitivity to the presence of toxicants as well as a good correlation to other toxicity bioassays (Bulich et al.23). A working solution of luminescent bacteria was prepared by reconstituting a vial of freeze-dried Vibrio fischeri cells, purchased from Merck, using 1 mL of 2% (w/v) NaCl. Luminescence of a tested solution and a control having the same amounts of reconstituted bacteria at pH 6-8 were conducted simultaneously. Toxicity was calculated as the percent decrease in luminescence of the tested solution, compared to the control, and is termed here the percent inhibition. Catalyst Regeneration. The amount of the carbonaceous material adsorbed on the catalyst surface in continuous runs was calculated according to the time profile of CO2 formed during oxidative regenerative treatment. In this treatment the spent catalyst was heated in the reactor under a 50 mL/min flow rate of oxygen and heating rate of 3 °C/min up to the required temperature and kept at this temperature until no carbon dioxide was detected at the reactor outlet. A gas sample was taken every few minutes from the exit pipe of the system and analyzed by GC (Gow-Mac 580 instrument equipped with a TCD detector and a Porapack Q packed column; L ) 6 ft, o.d. ) 1/8 in., i.d. ) 3 mm). Integration of the CO2 concentration versus time data using Polymath 5.1 (Simpson method) yielded the total amount of CO2 produced during oxidative regeneration. The corresponding mass of phenol on the surface (mPhOH,s) was calculated from this value and compared with the mass of reacted phenol (mPhOH,r). The ratio
between those two values represents here the selectivity toward reactive adsorption (SRA) and was calculated as
SRA )
mPhOH,s mPhOH,r
The catalyst adsorption capacity (CAC) was calculated as
CAC )
mPhOH,s mcat
where mcat is the catalyst mass. When the regeneration was performed after a small decline in phenol conversion (the start of visible deactivation), the ratio of phenol amount on the surface to the amount of catalyst was defined as the steady-state catalyst adsorption capacity (SS-CAC). When the regeneration was performed after full deactivation (XPhOH e 30%), the ratio was defined as the total catalyst adsorption capacity (total CAC). SRA and CAC measurements and calculations were performed three times with an accuracy of 5-10%. Results and Discussion Catalyst Properties. Mn-Ce catalysts containing different promoters (that affect polymeric deposition) were tested in a batch reactor. The catalysts tested included a mixed oxide with optimal CWO Mn/Ce atomic ratio of 6:4 established in previous investigations.12,22 This mixed oxide was modified with potassium, cesium, and noble metals (i.e., platinum or ruthenium) to examine its ability to accumulate carbonaceous deposits during phenol CWO due to different surface basicity (K, Cs) and oxidation activity (Pt, Ru). The texture parameters of the different catalysts as well as their phase compositions are listed in Table 1. The catalysts were treated (1) in air or (2) under vacuum atmosphere, which caused a change in the structure of manganese oxide component from Mn5O8 to Mn3O4. XRD diffractograms of samples Mn-Ce-1 and Mn-Ce-2 (Figure 2) shows sharper peaks for Mn-Ce-1, indicating a lower dispersion of Mn3O4 compared to Mn5O8 (crystal sizes of 23-25 and 9-10 nm, respectively) and similar dispersion of the CeO2 phase (4-5 nm) (Table 1).The relatively high surface area of the mixed oxides (about 100 m2.g-1) is determined mostly by the CeO2 phase, which displayed a much higher dispersion and comprised about 60% of the catalyst by weight. Taking into account the theoretical densities of Mn5O8 and CeO2 of 4.933 and 7.214 g/cm3, respectively, the calculation of corresponding surface areas according to formula
SA )
6000 Fd
(where F is the density, d is the crystal diameter) gives 208 m2/g for CeO2 and 122 m2/g for Mn5O8. The weight ratio of these phases in the Mn-Ce catalyst treated in air is CeO2/Mn5O8 ≈ 1.5, and the measured surface area is 128 m2/g. This figure compared with the weighted contributions of the calculated surface areas of the oxide components, 174 m2/g, means that ∼75% of the oxide crystal surface in the composite is accessible for reacting molecules. Evacuated Mn-Ce-1 catalyst had a lower surface area relative to the air-treated Mn-Ce-2 because of the lower dispersion of the manganese oxide
5092 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 1. Textural and Structural Properties of Manganese-Ceria-Based Catalysts catalyst
surface area (m2‚g-1)
pore diameter (nm)
pore volume (cm3‚g-1)
phase composition
Mn-Ce-1 Mn-Ce-2 Mn-Ce-K-1 Mn-Ce-K-2 Mn-Ce-Cs-1 Mn-Ce-Cs-2 Mn-Ce-Pt Mn-Ce-Ru
98 128 74 54 111 53 83 77
14.8 11.5 20.0 22.8 14.6 19.3 14.0 13.3
0.36 0.37 0.37 0.25 0.40 0.25 0.32 0.31
Mn3O4,CeO2 Mn5O8,CeO2 Mn3O4,CeO2 Mn5O8,CeO2 Mn3O4,CeO2 Mn5O8,CeO2 Mn5O8,CeO2 Mn5O8,CeO2
crystal size (nm) MnOx CeO2 25 10 24 10 23 10 25 24
4 4 4 4.5 4.5 4.5 5.0 4.5
Figure 2. X-ray diffractograms of Mn-Ce catalysts: (a) MnCe-1, (b) Mn-Ce-2. Table 2. Phenol Conversions and Rate Constants Measured with Mn-Ce-Based Catalysts in Batch Reactor after 60 mina
catalyst Mn-Ce-1 Mn-Ce-2 Mn-Ce-K-1 Mn-Ce-K-2 Mn-Ce-Cs-1 Mn-Ce-Cs-2 Mn-Ce-Pt Mn-Ce-Ru a
specific rate phenol rate constant, k′ (10-6 conversion constant, k (%) (10-3 L‚g-1‚min-1) L‚m-2‚min-1) 80 70 87 72 93 60 82 83
8.9 7.7 13.2 8.4 17.8 7.8 9.0 10.2
90 65 178 155 160 147 108 130
[CPhO]0 ) 0.2 g‚L-1, T ) 100 °C, PO2 ) 10 bar, [Ccat] ) 2 g‚L-1.
phase (23-25 nm, Table 1). It is important to note that the final treatment affected only the manganese oxide phase and not the cerium oxide phase. Modification of Mn-Ce mixed oxide with alkaline and noble metals created substantial changes of its surface area and pore diameter, with no effect on the crystal dimensions of the main phases or appearance of novel phases in the case of alkaline metals (Table 1). Because the amount of inserted modifiers was relatively small (
