Catalytic Wet Oxidation of Phenol with Mn Ce-Based Oxide catalyst

Catalytic Wet Oxidation of Phenol with Mn-Ce-Based Oxide Catalysts:Impact of Reactive Adsorption on TOC Removal

M.Abecassis-Wolfovich,?https://www.360docs.net/doc/0018100154.html,ndau,*,?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-Sheva84105,Israel

Catalytic wet oxidation of phenol solutions at low temperatures of80-130°C and space velocities

of1-100h-1using 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 trickle-

bed reactor.Catalysts containing mixed Mn3O4-CeO2phases 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.1,2It can be used to mineralize organic contaminants to CO2and 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 adsorp-tion-regeneration process.3,4CWO needs to be con-ducted 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;5CuO-ZnO/Al2O3;6,7Ru/CeO2;8Pt/CeO2;9 and bulk manganese-cerium oxide composites,pure10-12 or doped with Pt,13Pt-Ag,14or K.15,16Only 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 cata-lysts convert phenol to polymeric carbonaceous deposits produced on cerium-related sites that cause substantial catalyst deactivation due to surface blocking.10,12,17The amount of carbon on a spent Mn-Ce catalyst could reach about23wt%.11Introduction of Pt decreases the amount of carbonaceous deposits,13whereas promoting the Mn-Ce catalyst with potassium achieves the op-posite effect.15

Pintar and Levec7measured the formation of poly-meric deposits on the catalyst surface during phenol CWO on CuO-ZnO/Al2O3.They assumed that poly-merization was promoted by the partial oxidation of phenol to aldehydes followed by condensation with phenol.Such polymerization usually occurs on basic catalysts.18It is therefore consistent with promotion of carbon deposition on Mn-Ce composites on cerium-related sites,10,12,17which are more basic than the surface of manganese oxide.19,20The polymeric deposi-tion 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 reached50-70%.7,13Mn-Ce-based catalysts were tested in phenol CWO at low temperature only in a batch reactor,10,11,13although it was already postulated that the process should be more efficient in a trickle-bed 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

*To whom correspondence should be addressed.E-mail: mlandau@bgumail.bgu.ac.il.

?The Blechner Center for Industrial Catalysis and Process Development,Chemical Engineering Department.

?Biotechnology and Environmental Engineering.5089

Ind.Eng.Chem.Res.2004,43,5089-5097

10.1021/ie049756n CCC:$27.50?2004American Chemical Society

Published on Web07/13/2004

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 200mL of3M aqueous sodium hydroxide(NaOH,97%, Aldrich Chemicals),and the resulting precipitate was separated by filtration,washed,dried in air at100°C for16h,and then calcined at350°C for3h under a vacuum of85mbar(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 at100°C.The samples were impregnated with aqueous solutions of potassium nitrate(KNO3,Aldrich Chemi-cals)or cesium nitrate(CsNO3,Aldrich Chemicals)to gain4wt%of the alkaline metal.The samples were evacuated at350°C for4h(Mn-Ce-K-1,Mn-Ce-Cs-1)or calcined in air under the same conditions(Mn-Ce-K-2,Mn-Ce-Cs-2).Addition of platinum or ruthe-nium was performed by incipient wetness impregnation

of Mn-Ce catalyst after treatment in a vacuum at350°C for3h with5mg/L aqueous solutions of H2PtCl6or H2RuCl6(Aldrich Chemicals)to gain3wt%of the noble metal.After the samples had been dried at100°C,they were activated in hydrogen for3h at350°C(Mn-Ce-Pt and Mn-Ce-Ru).

Catalyst Characterization.The chemical composi-tions of the catalysts(weight percentage,average of five measurements at different points of the solid)were measured by energy-dispersive X-ray spectroscopy(JEOL JEM5600scanning electron microscope).Surface areas and pore volumes were obtained from N2adsorption-desorption isotherms using the conventional BET and BJH methods.The calcined samples were outgassed under vacuum at250°C.Isotherms were measured at liquid nitrogen temperature with a NOVA-2000(Quan-tachrome,Version7.01)instrument.The phase compo-sitions of the catalysts were tested by X-ray diffraction (XRD).The XRD patterns were collected on a Philips diffractometer PW1050/70(Cu K R radiation)with a graphite monochromator at diffracted beam.Data were recorded at a0.02°step size for2s at each step.The peak positions and the instrumental peak broadening ( )were determined by fitting each diffraction peak by means of APD computer software.The crystal domain size was determined using the Scherrer equation where K)1.000, )0.1°,λ)0.154nm,and B is the peak broadening at2θ)30-45°for different manga-nese oxides and at2θ)48°for cerium oxide.Temper-ature-programmed reduction(TPR)and temperature-programmed oxidation(TPO)experiments were carried out in an AMI-100Catalyst Characterization System (Zeton-Altamira)equipped with a mass spectrometer for outlet component identification(Ametek1000).Catalyst (0.2g)was loaded and treated in10vol%H2-Ar,25 cm3/min(TPR),or5vol%O2-He,25cm3/min(TPO), as the temperature was increased from ambient to420°C at5°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),300mL,Bu¨chi]at100°C,under a10atm oxygen pressure,with a stirring speed of800rpm,a catalyst loading of2g/L,and a phenol concentration of0.2g/L. Catalyst(0.3g in a powder form to avoid any diffusion limitations)and110mL of distilled water were fed to the reactor,which was pressurized with oxygen to9 atm,and the mixture was then heated under agitation using an oil bath.After the desired reaction temperature had been attained,25mL of concentrated phenol solution(1.2g/L)was loaded into the reactor using an Eldex piston pump.One-milliliter samples were taken periodically for HPLC analysis(GBC,LC1205instru-ment,Zorbax ODS-5C18reverse-phase column,mobile phase acetonitrile/distilled water mixture(8:2vol ratio), 1cm3/min flow rate,UV spectrophotometer for detec-tion,λ)260nm)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 at80-130°C with10bar of oxygen pressure and a liquid hourly space velocity (LHSV)of1-100h-1in a fixed-bed reactor rig(Figure 1).The reactor was heated with an electric coil con-trolled by a Eurotherm PID controller.A K-type ther-mocouple was positioned in the center of the catalyst bed.Two independently controlled heating zones kept the axial temperature gradient in the reactor to less than5°C.The oxygen was fed by a Brooks mass controller to a preheater at a rate of25-100mL min-1 (STP conditions)before entering the reactor at the selected temperature.The tested solution containing1 g/L phenol(doubly distilled water)was fed to the reactor by a high-pressure metering pump(SSI HPLC series II isocratic)at20-52mL/h and reaction pressure. Catalyst pellets(0.3-11g,0.4-0.5mm 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 Apollo9000HS model TOC combustion analyzer (Tekmar Dohmann)equipped with a nondispersive infrared(NDIR)detector.Testing of phenol oxidation at100°C in a reactor loaded only with glass Raching rings showed that the phenol conversion was below1%. The phenol conversion(X PhOH,%)and TOC conversion

l)

Kλ

[(B2- 2)0.5cos(2θ/2)]

Figure1.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.

5090Ind.Eng.Chem.Res.,Vol.43,No.17,

2004

(X TOC,%)were calculated as

and

where C PhOH is the phenol concentration,with the subscripts0and t representing the initial concentration for the batch reactor or the concentration at the inlet for the fixed-bed reactor(C PhOH,0)and the final concen-tration for a batch reactor or the concentration at the outlet for the fixed-bed reactor(C PhOH,t).C TOC is the TOC concentration measured at the fixed-bed reactor inlet or outlet.The reaction rate constants of phenol trans-formation in the batch reactor were calculated assuming first-order kinetics with respect to phenol

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 Optima3000Perkin-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 ToxAlert10luminometer(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 pre-pared by reconstituting a vial of freeze-dried Vibrio fischeri cells,purchased from Merck,using1mL of2% (w/v)NaCl.Luminescence of a tested solution and a control having the same amounts of reconstituted bacteria at pH6-8were 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 carbon-aceous material adsorbed on the catalyst surface in continuous runs was calculated according to the time profile of CO2formed during oxidative regenerative treatment.In this treatment the spent catalyst was heated in the reactor under a50mL/min flow rate of oxygen and heating rate of3°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-Mac580 instrument equipped with a TCD detector and a Pora-pack Q packed column;L)6ft,o.d.)1/8in.,i.d.)3 mm).

Integration of the CO2concentration versus time data using Polymath5.1(Simpson method)yielded the total amount of CO2produced during oxidative regeneration. The corresponding mass of phenol on the surface (m PhOH,s)was calculated from this value and compared with the mass of reacted phenol(m PhOH,r).The ratio between those two values represents here the selectivity

toward reactive adsorption(SRA)and was calculated

as

The catalyst adsorption capacity(CAC)was calculated

as

where m cat is the catalyst mass.When the regeneration

was performed after a small decline in phenol conver-

sion(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(X PhOH e30%),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 of5-10%. 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 of6:4established in previous investiga-

tions.12,22This mixed oxide was modified with potas-

sium,cesium,and noble metals(i.e.,platinum or

ruthenium)to examine its ability to accumulate car-

bonaceous 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 Table1.The

catalysts were treated(1)in air or(2)under vacuum

atmosphere,which caused a change in the structure of

manganese oxide component from Mn5O8to Mn3O4.

XRD diffractograms of samples Mn-Ce-1and Mn-Ce-2

(Figure2)shows sharper peaks for Mn-Ce-1,indicating

a lower dispersion of Mn3O4compared to Mn5O8(crystal

sizes of23-25and9-10nm,respectively)and similar

dispersion of the CeO2phase(4-5nm)(Table1).The

relatively high surface area of the mixed oxides(about

100m2.g-1)is determined mostly by the CeO2phase,

which displayed a much higher dispersion and com-

prised about60%of the catalyst by weight.Taking into

account the theoretical densities of Mn5O8and CeO2of

4.933and7.214g/cm3,respectively,the calculation of

corresponding surface areas according to formula

(where F is the density,d is the crystal diameter)gives 208m2/g for CeO2and122m2/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 is128m2/g.This figure compared with the weighted contributions of the calculated surface areas of the oxide components,174m2/g,means that～75%of the oxide crystal surface in the composite is accessible for reacting molecules.Evacuated Mn-Ce-1catalyst had a lower surface area relative to the air-treated Mn-Ce-2be-cause of the lower dispersion of the manganese oxide

X

PhOH (%))(1-C PhOH,t C PhOH,0)×100

X TOC (%))(1-C TOC,outlet

C

TOC,inlet)×100

-ln(1-X PhOH100))kt

SRA)

m

PhOH,s

m

PhOH,r

CAC)

m

PhOH,s

m

cat

SA)

6000

F d

Ind.Eng.Chem.Res.,Vol.43,No.17,20045091

phase (23-25nm,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 (<5wt %),this could indicate a change of the manganese and cerium oxide crystal packing mode during metal addi-tion.

Batch Reactor Experiments.The rate constants and phenol conversions of the different catalysts in batch reactor experiments are presented in Table 2.The manganese oxide component in the form of Mn 3O 4phase yielded a higher catalyst activity relative to that in the form of Mn 5O 8,which was obtained according to a known preparation procedure.22The increase of specific activity by 40%measured for Mn -Ce-1catalyst (vacuum treated)compared with Mn -Ce-2(air treated)corre-

sponds to a decrease of the oxidation state of the Mn ions from 3.2(Mn 5O 8)to 2.6(Mn 3O 4).The same trend was observed for Mn -Ce catalysts modified with alka-line metals.To examine the effect of the different phases on the redox cycle,TPO and TPR experiments were performed (Figure 3a,b).According to the TPO spectra (Figure 3a),treatment in a vacuum shifts the oxygen consumption peak to higher temperature as a result of

Table 1.Textural and Structural Properties of Manganese -Ceria-Based Catalysts

crystal size (nm)catalyst surface area (m 2?g -1)

pore diameter

(nm)

pore volume (cm 3?g -1)

phase composition MnO x CeO 2Mn -Ce-19814.80.36Mn 3O 4,CeO 2254Mn -Ce-212811.50.37Mn 5O 8,CeO 2104Mn -Ce -K-17420.00.37Mn 3O 4,CeO 2244Mn -Ce -K-25422.80.25Mn 5O 8,CeO 210 4.5Mn -Ce -Cs-111114.60.40Mn 3O 4,CeO 223 4.5Mn -Ce -Cs-25319.30.25Mn 5O 8,CeO 210 4.5Mn -Ce -Pt 8314.00.32Mn 5O 8,CeO 225 5.0Mn -Ce -Ru

77

13.3

0.31

Mn 5O 8,CeO 2

24

4.5

Figure 2.X-ray diffractograms of Mn -Ce catalysts:(a)Mn -Ce-1,(b)Mn -Ce-2.

Table 2.Phenol Conversions and Rate Constants

Measured with Mn -Ce-Based Catalysts in Batch Reactor after 60min a

catalyst phenol conversion (%)rate

constant,k

(10-3L ?g -1?min -1)specific rate constant,k ′(10-6L ?m -2?min -1)

Mn -Ce-1808.990Mn -Ce-2707.765Mn -Ce -K-18713.2178Mn -Ce -K-2728.4155Mn -Ce -Cs-19317.8160Mn -Ce -Cs-2607.8147Mn -Ce -Pt 829.0108Mn -Ce -Ru

83

10.2

130

a

[C PhO ]0)0.2g ?L -1,T )100°C,P O 2)10bar,[C cat ])2g ?L -1.

Figure 3.(a)TPO and (b)TPR spectra of Mn -Ce-based catalysts.

5092Ind.Eng.Chem.Res.,Vol.43,No.17,

2004

the lower manganese oxidation state (2.6).In the TPR spectra (Figure 3b),the lack of the low-temperature peak that is assigned to the high-oxidation-state man-ganese species 15corresponded to the formation of the Mn 3O 4phase with a lower oxidation state of Mn ions and a lower dispersion.Most oxygen consumption during O 2treatment and removal during H 2treatment of the evacuated Mn -Ce-1catalyst proceeded at higher temperature (300-350°C)relative to that of air-calcined Mn -Ce-2(150-300°C).This could result in lower efficiency of evacuated Mn -Ce catalyst in phenol con-version to deep oxidation products,facilitating the reactive adsorption of phenol at the catalyst surface.The diminishing redox ability was further observed after addition of alkaline metals.This addition substantially decreased the amount of oxygen adsorbed in TPO experiments by evacuated Mn -Ce catalysts without changing the shape of the TPO (Figure 3a)and TPR (Figure 3b)spectra.

Introduction of K and especially of the more basic Cs strongly increased the rate of phenol conversion (Table 2).Treatment in a vacuum along with the addition of alkaline metals increased the rate of phenol oxidative conversion by a factor of 1.6(K)and 2.2(Cs).Introduc-tion of Pt to the more active evacuated Mn -Ce catalyst had no substantial effect on phenol conversion in agreement with the results of Hamoudi et al.13The same result was measured with Ru promoter.Both noble metals increased the specific rate of phenol conversion (normalized per unit of catalyst surface)by a factor of 1.7-2.

Contribution of Reactive Adsorption to TOC Removal.The evacuated Mn -Ce-based catalysts that displayed higher activities relative to the air-calcined samples were tested in a fixed-bed reactor.The phenol and TOC conversions and product distributions between solid (due to reactive adsorption)and liquid (due to partial oxidation to low-molecular-weight oxygenates)6,7are shown in Table 3.The parameters were measured in a steady-state https://www.360docs.net/doc/0018100154.html,parison of the phenol conversions in Tables 2and 3shows that,in both systems (batch and continuous mode),the same ranking of the samples according to their activities was obtained.In all of the continuous-mode experiments,70-94%of the converted phenol was removed through reactive adsorption and CO 2formation.The carbon mass balance based on the amount of carbon detected in the treated water and on that accumulated on the spent catalyst was 96-100%(Table 3).This indicates that negligible CO 2was formed at the selected testing conditions {T )100°C,WHSV (weight hourly space velocity))0.1g of phenol/[(g of catalyst)?h],LHSV )100h -1},so almost all of the organics removed from the water remained on the catalyst surface,as shown in Table 3.This obser-

vation is different from the results reported by Hussain et al.15for phenol CWO in a batch reactor,where the mineralization selectivity toward CO 2formation reached 33%with Mn -Ce and 95%with K -Mn -Ce catalysts even at lower contact times.This difference in product distributions is a result of the different catalyst activa-tion procedure applied,as,in the present study,an evacuation procedure was used instead of air calcina-tion.This yielded a Mn phase with a lower oxidation state of the metal ions and a lower dispersion.Testing of the Mn -Ce-2catalyst that was calcined in air yielded a lower phenol conversion and a reduction of the deposits on the solids surface.In this case,only 68%of the removed organics remained in the catalyst (Table 3).The remaining 32%was converted to CO 2in agree-ment with the data reported previously.13,15Testing of the evacuated Mn -Ce catalysts showed that the alka-line metals increased the phenol conversion and the contribution of reactive adsorption to phenol conversion (Table 3).It was also found that Pt decreased this contribution (in agreement with the results of Hamoudi et al.13),whereas Ru had no effect.

Figure 4shows phenol conversions obtained with the evacuated Mn -Ce-1catalyst as a function of phenol load for various LHSVs.Two periods can be identified:(i)a steady-state period at a fixed phenol conversion and (ii)a deactivation period displaying a decline of phenol conversions to e 30%.The deactivation is a result of surface blocking with carbonaceous deposits formed in the catalyst pores by copolymerization of phenol with products of its partial oxidation.7Figure 5and Table 4compare the textural properties of fresh and spent Mn -Ce-1catalysts.After deactivation (phenol conversion e 30%),the wide pore size distribution of the fresh Mn -Ce-1catalyst with a maximum in range of 10-20nm was narrowed,and its maximum was shifted to 4nm because of the formation of a thick carbonaceous depos-its layer on the walls of catalyst pores.This is also confirmed by the decreasing values of the surface area,pore diameter,and pore volume (Table 4).

Effects of Additives,LHSV,and Temperature.The steady-state catalyst adsorption capacity (SS-CAC)and the total CAC (measured after X PhOH e 30%)depend on modifiers (K,Cs,Pt,Ru),contact time (LHSV),and temperature,as shown in Tables 5-7.

The effect of the addition of alkaline and noble metals on the catalyst operation time,SRA,SS-CAC,and total CAC is shown in Table 5.Addition of basic additives

Table 3.Phenol and TOC Conversions and Product Distributions Measured with Mn -Ce-Based Catalysts before Visible Deactivation a

conversion (%)

product distribution (%)catalyst phenol TOC liquid solid Mn -Ce-160462377Mn -Ce -K-173621284Mn -Ce -Cs-18275691Mn -Ce -Pt 70493070Mn -Ce -Ru 66522278Mn -Ce-2

52

37

29

50

a

Reaction conditions:[C PhOH ]0)1g ?L -1,T )100°C,P O 2)10bar,LHSV )100h -1.

Figure 4.Phenol conversion as a function of amount of phenol flow through catalyst at different LHSV (T )100°C,P O 2)10bar,[C PhOH ]0)1g ?L -1,Mn -Ce-1catalyst).

Ind.Eng.Chem.Res.,Vol.43,No.17,2004

5093

(K,Cs)reduced both steady-state and deactivation periods,hence reducing catalyst stability.18These find-ings and the steady-state conversions and product distribution presented in Table3indicate that decreased stability is caused by polymer accumulation.The de-crease of the steady-state period and SS-CAC seems to be a result of blocking of the basic sites related to K and especially Cs,being highly reactive in polymeriza-tion reactions.The results for unpromoted Mn-Ce catalyst and that promoted with noble metals,which are shown in Table5,were similar for the steady-state region(duration of operation period and SS-CAC). However,both the deactivation period and the total CAC were smaller with the promoters.This is in agreement with the results of Hamoudi et al.,13who found that Pt-promoted Mn-Ce catalyst is sensitive to deactivation.

The effect of increasing LHSV on phenol and TOC conversions,CAC,and SRA are presented in Table6 for the Mn-Ce-1catalyst.The increase in LHSV had little effect on SS-CAC and total CAC.It substantially affected the period of steady-state operation,phenol conversion,and SRA,as shown in Table6.In addition, as the space time increased,the phenol transformation gradually shifted to reactive adsorption.The increase of space time(lower LHSV)also decreased the specific load(grams of phenol on stream per gram of catalyst) as shown in Figure4,allowing longer operation time and higher phenol conversion.On the basis of the TOC balance,it can be concluded that the higher phenol conversion caused by higher amounts of carbonaceous solids deposited on the catalyst surface(Table6)also results in a higher SRA.It is important to mention that the Mn-Ce-1catalyst at100°C removed95-100% phenol from contaminated water at LHSVs of10-1h-1 with SS-CACs of120-130mg of phenol/g of catalyst (Table6).

The effect of temperature on the phenol and TOC conversions and on the CAC and SRA are presented in Table7for the Mn-Ce-1catalyst.Decreasing the reaction temperature from130to80°C for a steady LHSV gradually enhanced the SRA,SS-CAC,and total CAC.The same trend was obtained when the LHSV was decreased for a steady temperature.Although the phenol conversion decreased with temperature,the selectivity toward reactive adsorption(SRA)increased, due to the fact that there is a higher amount of unreacted phenol that is free to be adsorbed on the catalyst surface(which has a high affinity to adsorb carbonaceous deposits).In this process,more polymeric deposits are adsorbed with lower deactivation ability yielding an increase of SS-CAC and total CAC to170 and420mg of phenol/g of catalyst,respectively(Table 7).Decreasing SS-CAC,total CAC,and SRA with increasing temperature,in the absence of CO2emission, corresponded to production of more liquid products and not to oxidation of carbonaceous deposits.

For a deeper understanding of the observed effects of additives,LHSV,and temperature on the performance of the Mn-Ce catalysts and the reactive-adsorption phenomena,it will be helpful to collect direct informa-tion about the nature of carbonaceous deposits as a

Figure5.Pore size distributions of fresh,spent,and regenerated Mn-Ce-1catalyst.

Table4.Textural Properties of Mn-Ce-1Catalyst

sample surface area

(m2?g-1)

pore diameter

(nm)

pore volume

(mL?g-1)

fresh9814.80.36 spent56 3.80.05 regenerated11511.20.30 Table5.Steady-State and Deactivation Periods,Phenol Adsorption Capacities,and Selectivities toward Reactive Adsorption as a Function of Catalyst Composition a

catalyst steady-

state

period

(h)

deacti-

vation

period

(h)

SS-CAC

(mg of

phenol/g of

catalyst)

total CAC

(mg of

phenol/g of

catalyst)

SRA

(mg of

phenol/

g of reacted

phenol)

Mn-Ce-1 2.514.4120300500

Mn-Ce-K-10.610.540310670

Mn-Ce-Cs-10.6550320810

Mn-Ce-Pt 2.56130270550

Mn-Ce-Ru 2.5 4.2150240660

a Reaction conditions:LHSV)100h-1,[C PhO]0)1g L-1,T) 100°C,P O

2

)10bar.

Table6.Phenol Adsorption Capacity and Reactive Adsorption Selectivity of Mn-Ce-1Catalyst as a

Function of Catalyst Composition a

LHSV(h-1):11050100 phenol conversion b(%)100957560 TOC conversion b(%)100886246 SS-CAC(mg of phenol/g of catalyst)120130130120 total CAC(mg of phenol/g of catalyst)300300290300 SRA(mg of phenol/g of reacted phenol)1000690500500 steady-state period(h)7876 2.5 deactivation period(h)340402514.4 product distribution b(%)

liquid071723 solid100938377

a Reaction conditions:[C PhO]0)1g?L-1,T)100°C,P O

2)10

bar.b Before visible deactivation.Table7.Phenol Adsorption Capacity and Reactive Adsorption Selectivity of Mn-Ce-1Catalyst as a

Function of Reaction Temperature a

temperature(°C):

LHSV(h-1):

80

10

100

10

80

100

100

100

130

100 phenol conversion b(%)8995526075 TOC conversion b(%)8488474647 SS-CAC(mg of phenol/g of catalyst)160130170120110 total CAC(mg of phenol/g of catalyst)420300320300280 SRA(mg of phenol/g of reacted phenol)790690530500410 product distribution b(%)

liquid57102337 solid9593907763

a Reaction conditions:[C PhO]0)1g?L-1,P O

2

)10bar.b Before visible deactivation.

5094Ind.Eng.Chem.Res.,Vol.43,No.17,

2004

function of those parameters,as well as of oxygen pressure.This investigation is now in progress.

A comparison with data on activated carbon3,4,24 shows the advantage of using Mn-Ce catalysts for phenol treatment by CWO.CWO could remove95-100%of the phenol at100°C and LHSV1-10h-1 (Tables6and7),which corresponds to a treatment

capacity up to12L?(kg of catalyst)-1?h-1.This is higher than the typical treatment capacity with activated carbon reported to be4.6-5.5L?(kg of carbon)-1?h-1at the same degree of organics removal.4In addition,the SS-CAC obtained was120-170mg of phenol/g of catalyst,compared with the50-100mg/g capacity of activated carbons.24,25There is,therefore,a great po-tential for further improvement of SS-CAC and treat-ment capacity of Mn-Ce-based mixed oxide catalysts by increasing their surface area,which was relatively low in the samples used because of the low dispersion of the manganese oxide phase.This should yield an economical alternative to the conventional carbon ad-sorption process,compensating for the need to work at elevated temperature and pressure that should be optimized.

Oxidative Regeneration of Mn-Ce-Based Cata-lysts.The concentration profiles of CO2evolved during the oxidative treatment of spent Mn-Ce-1and Mn-Ce-Pt catalysts in oxygen flow are shown in Figure6. The burning off of coke deposits from Mn-Ce-1catalyst starts at～100°C,but the burning rate was relatively low.Increasing the temperature to270°C yielded the highest rate of carbon removal,so that the regeneration process was completed in5h.In the presence of platinum,which is known as an efficient combustion catalyst,the regeneration was completed in about the same period of time but at a much lower temperature (180°C).As shown in Figure6,this regeneration included a short-term temperature jump to～210°C, caused by fast heat evolution that could not be compen-sated by heat exchange.The regeneration treatment fully restored the catalyst textural parameters by complete removal of the coke deposits,as shown for Mn-Ce-1material in Figure5and Table4.The burning of the carbonaceous deposits at less than300°C had no effect on the catalyst structure or the dispersion of the metal oxide phases(XRD).It yielded a complete recovery of the catalyst performance in consecutive CWO-regeneration cycles,as demonstrated for the Mn-Ce-1catalyst in Figure7.The SS-CAC and steady-state operation period for CWO with fresh and regener-ated catalyst in three consecutive runs conducted at100°C and an LHSV of100h-1were in the range of118-122mg of phenol/g of catalyst and2.4-2.6h,respec-tively.This indicates strongly a full recovery of the catalysts.In activated carbon regeneration treatments, the spent activated carbon is regenerated at elevated temperatures(700-1000°C)in steam flow,3,4which leads to degradation of the carbon structure and loss of adsorption capacity.The regeneration temperature of activated carbon could be decreased to240-300°C24by its promotion with transition metal oxides as oxidation catalyst.However,the insertion of such metal oxides decreases the breakthrough adsorption capacity.The breakthrough capacities for the promoted carbon mea-sured in ref24were88-56and62-45mg of phenol/g of carbon,respectively,for the fresh and regenerated carbon.

Leaching of Catalysts and Toxicity Impact.Sev-eral samples of the outlet stream were analyzed for the presence of metal ions that could leach during the process.The extent of leaching was found to be insig-nificant(Mn,0.1mg/L;Ce,0.26mg/L),summing up to less than0.2%of the catalyst amount.Because several experiments resulted in partial conversion of phenol and TOC,the toxicity of the intermediates was also ana-lyzed.This was done for water collected after oxidative treatment with Mn-Ce-1catalyst at100%and60% phenol conversions and for the inlet stream(1000mg/L phenol).The results are presented in Table8,where the toxicity is expressed as percent inhibition of biolumi-nescence.The toxicity of the inlet stream is in agree-ment with reported values for phenol solutions.26The outlet samples for the different conversions show that, at a phenol conversion of60%,the TOC conversion was only46%,which is evident for formation of water-soluble oxygenates as a result of partial phenol oxidation(as no CO2was detected).This might result in intermedi-ates that are more toxic than the parent compounds.27 The outlet stream with the full phenol conversion showed a decrease in the inhibition percentage,which is in agreement with the full TOC conversion.However, the outlet stream with60%phenol conversion showed

Figure6.CO2concentration profiles and temperature variations

during oxidative regeneration of spent Mn-Ce-1and Mn-Ce-Pt catalysts.Figure7.Performance of Mn-Ce-1catalyst after oxidative regeneration cycles.

Table8.Toxicities of the Inlet and Outlet Streams

conversion(%)

sample phenol TOC

inhibition

(%) inlet0041

outlet11001006

outlet2604689 Ind.Eng.Chem.Res.,Vol.43,No.17,2004

5095

a high inhibition,indicating an increase of water toxicity caused by the partial phenol conversion to water-soluble oxygenates.This phenomenon was also observed by Santos et al.,28who found that the products of phenol transformation (hydroquinone and p -benzoquinine)caused an increase in toxicity and also had a synergistic toxicity effect.This indicates that catalytic reactive adsorption -regeneration or complete combustion might be the preferred conversion routes in Mn -Ce-based CWO treatment of phenol solutions.Conclusions

Catalytic wet oxidation of phenol solutions using Mn -Ce-based catalysts can result in three different routes:partial oxidation to soluble oxygenates,deposition (reac-tive adsorption)on the catalyst surface,and full com-bustion to CO 2.Partial conversion of phenol to water-soluble oxygenates is undesirable because it might increase water toxicity and thus limit further applica-tion of a biological process.Evacuation of freshly preci-pitated Mn -Ce composite at 350°C reduces its redox ability.This suppresses the partial oxidation of phenol,makes the contribution of the full combustion route insignificant,and substantially increases the efficiency of reactive adsorption.At space velocities lower than 10h -1at a temperature of 100°C,complete adsorption of phenol was achieved as a result of deposition on the catalyst surface.The reactive adsorption capacity was found to be substantially higher than that reported for conventional activated carbon treatments.Low-temper-ature oxidative regeneration of Mn -Ce-based catalysts showed full recovery of its activity and adsorption capacity in consecutive runs,superior to activated carbon regeneration,which is usually conducted at higher temperatures and result in partial loss of ad-sorption ability.Because of its advantageous adsorption characteristics,low temperature,and efficient regenera-tion,a controlled reactive adsorption -regeneration process using improved Mn -Ce-based catalysts could become an economical and efficient alternative to con-ventional carbon adsorption for the treatment of phenol solutions and other waste streams.Acknowledgment

The authors gratefully acknowledge Dr.A.I.Eren-burg for conducting the XRD measurements.Notation

C )concentration,g ?(L of solution)-1d )crystal diameter,nm

k )reaction rate constant,L ?g -1?min -1

k ′)specific reaction rate constant,L ?m -2?min -1LHSV )liquid hourly space velocity,h -1m )mass,g or mg

P O 2)oxygen pressure,bar

SRA )selectivity to reactive adsorption of phenol,mg of adsorbed phenol/g of reacted phenol

SS-CAC )catalyst capacity to reactive adsorption during steady-state period,mg of adsorbed phenol/g of catalyst T )reaction temperature,°C

total CAC )total catalyst capacity to reactive adsorption,mg of adsorbed phenol/g of catalyst t )reaction time,min

WHSV )weight hourly space velocity,g of phenol ?(g of catalyst)-1?h -1X )conversion (%)

F )crystal density,g/cm 3Sub-and Superscripts

0)initial time at t )0or inlet cat )catalyst PhOH )phenol r )reacted

s )on the surface t )reaction time

TOC )total organic carbon

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Received for review March28,2004

Revised manuscript received May17,2004

Accepted June3,2004

IE049756N Ind.Eng.Chem.Res.,Vol.43,No.17,20045097