Journal of Hazardous Materials 162(2009)588–606
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Journal of Hazardous
Materials
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a
t
Review
Heterogeneous catalytic degradation of phenolic substrates:Catalysts activity
L.F.Liotta a ,∗,M.Gruttadauria b ,G.Di Carlo c ,G.Perrini d ,V.Librando d ,e
a
Istituto per Lo Studio dei Materiali Nanostrutturati (ISMN)-CNR via Ugo La Malfa,153,90146Palermo,Italy b
Dipartimento di Chimica Organica “E.Patern`o ”,Universit`a di Palermo,Viale delle Scienze,Pad.17-90128Palermo,Italy c Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro”,Universit`a di Palermo,Viale delle Scienze,Pad.17-90128Palermo,Italy d Dipartimento di Scienze Chimiche,Universit`a di Catania,Viale Doria 6,95127Catania,Italy
e Research Centre for Analysis,Monitoring and Minimization Methods o
f Environmental Risk and Department of Chemistry,University of Catania,Viale A.Doria 8,95125,Catania,Italy
a r t i c l e i n f o Article history:
Received 4February 2008
Received in revised form 28April 2008Accepted 20May 2008
Available online 28May 2008Keywords:AOPs
Catalytic wet peroxide oxidation Catalytic ozonation Catalytic wet oxidation Phenol Acetic acid
Metal-exchanged zeolites Hydrotalcite-like compounds
Metal-exchanged/clays and resins Activated carbon Mixed oxides Noble metals
CoO x /Al 2O 3-BaO catalysts
a b s t r a c t
This review article explored the catalytic degradation of phenol and some phenols derivates by means of advanced oxidation processes (AOPs).Among them,only the heterogeneous catalyzed processes based on catalytic wet peroxide oxidation,catalytic ozonation and catalytic wet oxidation were reviewed.Also selected recent examples about heterogeneous photocatalytic AOPs will be presented.
In details,the present review contains:(i)data concerning catalytic wet peroxide oxidation of phenolic compounds over metal-exchanged zeolites,hydrotalcites,metal-exchanged clays and resins.(ii)Use of cobalt-based catalysts,hydrotalcite-like compounds,active carbons in the catalytic ozonation process.(iii)Activity of transition metal oxides,active carbons and supported noble metals catalysts in the catalytic wet oxidation of phenol and acetic acid.
The most relevant results in terms of catalytic activity for each class of catalysts were reported.
©2008Elsevier B.V.All rights reserved.
Contents 5892.
Catalytic wet peroxide oxidation .................................................................................................................
..5892.1.5902.2.Hydrotalcite-like compounds ...............................................................................................................5912.3.5932.4.5953.5963.1.Some recent investigations on catalytic ozonation .........................................................................................5974.
Catalytic wet oxidation (CWO)......................................................................................................................5984.1.Catalytic wet oxidation of phenol ...........................................................................................................
5984.1.1.Transition metal oxides ............................................................................................................5984.1.2.Noble metal catalysts ..............................................................................................................
6004.2.Catalytic wet oxidation of 6025.
Recent reports on real effl
602
∗Corresponding author.Tel.:+390916809371;fax:+390916809399.E-mail address:liotta@pa.ismnr.it (L.F.Liotta).0304-3894/$–see front matter ©2008Elsevier B.V.All rights reserved.doi:10.1016/j.jhazmat.2008.05.115
L.F.Liotta et al./Journal of Hazardous Materials162(2009)588–606589 6.Summary and conclusions (603)
Acknowledgement (604)
References (604)
1.Introduction
Removing pollutants from industrial process waters and wastewaters is becoming an important area of research as the amount and quality of freshwater available in the world continues to decrease due to growing water demands and/or long periods of drought.Increasingly stricter wastewater discharge standards continue to be introduced worldwide in an effort to reduce the envi-ronmental impacts of industrial processes.Chemical and petroleum industries generate a wide variety of highly toxic organic wastes. Among organic pollutants phenol and phenol derivates,used as raw materials in petrochemical,chemical and pharmaceutical indus-tries,have received increased attention in the last years due to their toxicity.Some of the most toxic members of phenol compounds are the chlorinated and nitro-substituted phenols that are used as pesticides and anti-bacterials.
The un-substituted phenol is usually taken as a model compound for advanced wastewater treatment studies.Several technologies are available to remove industrial organic wastes,such as biological,thermal and chemical treatments.Conventional bio-logical processes represent an environmentally friendly way of treatment with reasonable costs,however,they are not adequate to treat non-biodegradable wastewaters and,usually,require a long residence time for micro-organisms to degrade the pollutants. Among biological treatments,biodegradation of phenol influidized bed bioreactors has received considerable attention because of the superior performance and some inhere
nt advantages compared to suspended biomass reactors[1].Thermal treatments present many drawbacks,such as considerable emission of other hazardous compounds.Chemical treatments,which includeflocculation,pre-cipitation,adsorption on activated carbon,air stripping or reverse osmosis,are not resolving requiring a post-treatment[2].
Alternative pollutants destructive technologies are advanced oxidation processes(AOPs).
AOPs are characterized by a common chemical feature:the capa-bility of exploiting the high reactivity of HO•radicals in driving oxidation processes which are suitable for achieving the complete abatement and through mineralization of even less reactive pollu-tants.Contaminants are oxidized through four different reagents: ozone,hydrogen peroxide,oxygen and air or their combination. These procedures may also be combined with UV radiation.
To choose the most appropriate technology some aspects,such as the concentration and nature of the pollutants and the volume of wastewater,must be considered.
Moreover,an integrated process combining AOP,as a prelimi-nary treatment,with an inexpensive biological process,represents an interesting opportunity from an economical point of view[3].
AOPs constitute a promising technology for the treatment of wastewaters containing refractory organic compounds.Catalytic wet air oxidation(CWAO)is one of the most important advanced oxidation processes.AOPs include also many others techniques, such as methods based on ultrasound[4],plasma[5]and electro-hydraulic discharge[6]along with processes based on hydrogen peroxide(H2O2+UV,Fenton,photo-Fenton and Fenton-like pro-cesses),photolysis,photocatalysis and processes based on ozone (O3,O3+UV and O3+catalyst)[7].
Among the various AOPs processes proposed in the literature for the treatment of wastewaters containing refractory organic compounds,the present review will focus on the heterogeneous catalytic abatement of phenol and of some phenol derivates. Although several reviews on degradation of organic pollutants have been published in the last decades,the present work would sum-marize some AOPs technologies focusing only on heterogeneous catalytic degradation of phenol and of some phenol derivates high-lighting the catalysts activity and reaction conditions.
Oxidation of phenol furnishes more hydroxylated aromatic com-pounds that can be oxidized to quinones while further oxidation give a complex mixture of organic compounds as reported schemat-ically in Fig.1.
2.Catalytic wet peroxide oxidation
Wet peroxide oxidation processes using hydrogen peroxide as the oxidant have emerged as a viable alternative for the wastewater treatments of medium-high total organic carbon concentrations. Hydrogen peroxide does not form any harmful by-products,and it is a non-toxic and ecological reactant.Moreover,although hydro-gen peroxide is a relatively costly reactant,the peroxide oxidation compares very favourably to processes that use gaseous oxygen.The lack of a gas/liquid boundary removes mass-transfer limitations and the hydrogen peroxide acts as a free-radical initiator,
providing Fig.1.Simplified scheme for phenol oxidation.
590L.F.Liotta et al./Journal of Hazardous Materials162(2009)588–606
OH•radicals that promote the degradation of organics.This leads to reduce residence times and enables conversion under milder conditions.However,to enhance the decomposition of hydrogen peroxide to hydroxyl radicals the use of a catalytic system is highly desirable.Although the use of AOPs has been recently reviewed[7] with particular attention to the degradation of chlorophenols,in the present review the Fenton reaction based on the use of hydrogen peroxide combined with metal salts will be briefly introduced.
The system containing hydrogen peroxide and Fe(II)salts that in water solution form hydroxyl radicals following the reaction:
H2O2+Fe2+→Fe3++HO−+HO•
is known as the Fenton’s reagent,whichfinds wide application for wastewater treatment[8,9].The oxidizing efficiency of the Fen-ton reagent is the highest for pH ranging from2to5and for molar ratio about1:1.The mechanism of this reagent has not been fully explained because of the variety of Fe(II)a
nd Fe(III) complexes,numerous radical intermediate products and their con-secutive reactions.A significant role is played by the formation of Fe(III)ions,which decompose H2O2and produce HO2•radicals:
H2O2+Fe3+→Fe2++H++HO2•
In the solution of H2O2and Fe(II)salts,organics(RH)are oxi-dized during radical chain reactions.The main agents oxidizing and propagating the reactions are HO•radicals:
HO•+RH→H2O+R•
R•+H2O2→ROH+HO•
HO•radicals also decompose H2O2producing HO2•radicals: HO•+H2O2→H2O+HO2•
In the reaction of R•radicals with Fe(III)ions,carbocations R+ may be formed,while in conditions involving Fe(II)ions,carbanions may occur.The kinetic chain is terminated by reactions between radicals.
The major problem of these Fenton-type homogeneous catalytic systems is the tight pH control as well as the production of addi-tional toxic wastes,which need to be treated.
For these reasons,there has been a considerable interest in the development of heterogeneous catalysts for the oxidation of wastewater streams.
Here,we have focused the attention on different kind of supported transitions metals ions:metal-exchanged zeolites, hydrotalcite-like compounds,metal-exchanged/clays and resins.
2.1.Metal-exchanged zeolites
Zeolites are inorganic microporous and microcrystalline mate-rials capable of complexing small and medium-sized organic molecules.Relatively few works describe the use of zeolitic mate-rials containing metal active species tetrahedrally coordinated into the zeolitic framework for catalytic abatement of water pollutants (Table1).
The catalyst Fe/ZSM-5has been reported as a promising sys-tem for the treatment of phenolic aqueous wastes in presence of H2O2,allowing total elimination of phenol and significant total organic carbon(TOC)removal under mild working condi-tions[10].Moreover,this system remains active after successive runs.The Fenton-type decomposition of hydrogen peroxide over Fe/ZSM-5was later reported[11].Studies about hydro-gen peroxide decomposition and phenol oxidation were carried out using Cr(III),Fe(III),Bi(III),Ni(III)and Zn(III)complexes of N,N -bis(salicylidene)propane-1,3-diamine(Fig.
2)encapsulated
in
Fig.2.Structure of N,N -bis(salicylidene)propane-1,3-diamine.
Y-zeolite[12].Cr(III),Fe(III)complexes gave the best results,how-ever,degradation of phenol was not good.Authors concluded that the oxidation of phenol solely depended on the nature of the cen-tral metal ion present in the encapsulated complex and not on the H2O2decomposition ability of the catalyst.
Degradation of phenol was also studied with immobilized Fe(III)-HY as stable and efficient photo-Fenton catalyst(Fig.3)[13]. Different loadings of Fe(III)ions were immobilized on HY zeolite by impregn
ation and calcination.The effect of Fe loadings,hydrogen peroxide concentration and pH were studied.The results show that 0.25wt.%Fe(III)-HY was efficient in the degradation of phenol at pH 6.
In a different work a series of Fe-containing zeolitic materials, prepared by different methods,have been tested as hetero-geneous catalysts for the oxidation of phenolic solutions with hydrogen peroxide,under mild conditions[14].Fe-TS-1cata-lysts were synthesized through hydrothermal crystallization of wetness-impregnated Fe2O3-TiO2-SiO2xerogels.Moreover,other Fe-modified zeolitic materials were prepared and tested.Fe-TS-1 zeolite with a moderate Fe content(76Si/Fe molar ratio)showed the best results in terms of catalytic activity and loss of active species into the aqueous solutions.The stability of Fe species was shown to be strongly dependent on the Fe environment into the zeolitic framework,the synthetic route,and the temperature of the treatment.
Oxidation of phenol was also carried out with copper-modified zeolite(CuY-5)in wet hydrogen peroxide.The catalyst was pre-pared by ionic exchange from the protonic form of the commercial HY-5zeolite.The process was performed within the temper-ature range from50to80◦C and at atmospheric pressure. Other operating variables were:hydrogen peroxide
concentration Fig.3.A representation of photo-Fenton degradation of phenol over Fe(III)-HY[13].
L.F.Liotta et al./Journal of Hazardous Materials162(2009)588–606591
Table1
Catalytic wet peroxide oxidation of phenol substrates over metal-exchanged zeolites
Catalyst Phenol conversion and/or
TOC removal(%)reactive metal
Conditions References
Fe-aerosil20060(17)TOC Phenol0.069M,H2O2stoichiometric ratio1.5,catalyst
0.35g/L,180min,70◦C,pH2.5
[10]
Fe-ZSM-577(21)TOC As above[10]
Fe-aerosil20065(19)TOC As above,pH3.5[10]
Fe-ZSM-581(17)TOC As above,pH3.5[10]
Fe-ZSM-5100(46)TOC As above,catalyst1.5g/L pH3.5[10]
[Cr(salpn)]-Y15Phenol4.7g,H2O21.2g(30%),catalyst0.025g,80◦C,5h[12]
[Fe(salpn)]-Y24As above[12]
[Bi(salpn)]-Y5As above[12]
[Ni(salpn)]-Y<5As above[12]
[Zn(salpn)]-Y<5As above[12]
Fe(III)-HY>99Phenol10−4M,pH6,H2O210−3M,UV,60min[13]
Fe-TS-1(1)Fe0.64wt.%64TOC Initial TOC phenol765ppm,H2O2stoichiometric,
catalyst0.6g/L,air pressure1MPa,120min,100◦C
[14]
Fe-TS-1(2)Fe1.18wt.%66TOC As above[14]
Fe-TS-1(3)Fe4.43wt.%70TOC As above[14]
Fe-silicalite79TOC As above[14]
Fe-ZSM-568TOC As above[14]
Fe-NaY78TOC As above[14]
Fe-USY67TOC As above[14]
Fe-ZSM-5(by ion exchange)54TOC As above[14]
CuY-550Phenol0.01M,H2O20.03M,w cat0.1g dm−3,180min,
50◦C
[15]
CuY-570As above60◦C[15]
CuY-580As above70◦C[15]
CuY-580As above80◦C[15]
Cu/ZSM-5by hydrothermal synthesis92Phenol0.01M,H2O20.1M,w cat0.1g dm−3,180min,
80◦C
[16]
Cu/ZSM-5by hydrothermal synthesis85As above70◦C[16]
Cu/ZSM-5by hydrothermal synthesis75As above65◦C[16]
Cu/ZSM-5by hydrothermal synthesis68As above60◦C[16]
Cu/ZSM-5by hydrothermal synthesis46As above55◦C[16]
Cu/ZSM-5by hydrothermal synthesis36As above50◦C[16]
Cu/ZSM-5by ion-exchange synthesis96Phenol0.01M,H2O20.1M,w cat0.1g dm−3,180min,
80◦C
[16]
Cu/ZSM-5by ion-exchange synthesis81As above70◦C[16]
Cu/ZSM-5by ion-exchange synthesis70As above65◦C[16]
Cu/ZSM-5by ion-exchange synthesis56As above60◦C[16]
Cu/ZSM-5by ion-exchange synthesis33As above55◦C[16]
Cu/ZSM-5by ion-exchange synthesis20As above50◦C[16]
For each catalyst or catalysts group only some experimental conditions and the corresponding phenol degradation or TOC removal are reported for a qualitative comparison.
(0.008–0.254mol dm−3)and catalyst loadings(0.05–0.4g).The ini-tial phenol concentration was0.01mol dm−3.Good results were obtained at70–80◦C,complete phenol conversion was achieved in 150min wh
en the oxidant supplied for the reaction was close to the stoichiometric amount for phenol oxidation.The amount of copper leached during the test(after3h)was4.8%,minimal but not neg-ligible.The results show that the used catalyst entirely eliminated phenol and could be reused in successive runs,without significant loss of activity[15].The influence of different methods of Cu/ZSM-5preparation on the catalytic performances,in terms of phenol conversion and metal leaching,was addressed in a recent paper [16].The activity and stability of Cu/ZSM-5catalyst prepared by direct hydrothermal synthesis resulted higher than the activity of the catalyst by ion exchange.
Since the degradation of phenol furnishes a mixture of car-boxylic acids,the catalytic wet oxidation with hydrogen peroxide of diluted formic,acetic and propionic acid solutions were also mentioned in the present review.A study of catalytic wet per-oxide oxidation of carboxylic acids was undertaken using a Fe(III)-containing zeolite,Fe/ZSM-5and the catalytic results were compared with the behaviour of homogeneous Fe(III)catalysts in the same experimental conditions[17].A comparison of the reac-tivity of heterogeneous and homogeneous Fenton-type catalysts showed that the solid catalyst has a higher rate of conversion of the propionic acid as well as a lower sensitivity with respect to pH.Indeed,a maximum activity in propionic acid conversion was observed around a pH of4,precipitation of iron hydrox-ide occurring at higher pH values.However,the solid Fe(III) catalyst showed two main drawback
s:a higher rate of hydro-gen peroxide decomposition to water and oxygen was observed as well as some leaching of iron,especially at high reaction temperature.
As an advantage aspect of this approach,metal-exchanged zeo-lites are easily prepared,although no complete phenol removal was achieved.
2.2.Hydrotalcite-like compounds
Hydrotalcite-like compounds,which are known also as lay-ered double hydroxides or anionic clays,represent a group of important inorganic materials usable in many applications.The structure of the hydrotalcite-like compounds is very similar to that of brucite Mg(OH)2,in which each magnesium cation is octahe-drally surrounded by hydroxyls.Their chemical composition can
592L.F.Liotta et al./Journal of Hazardous Materials 162(2009)
588–606
Fig.4.Layered crystal structure of hydrotalcite-like compounds.
be expressed by the general formula M 1−x II M x III (OH)2A x /n n −·y H 2O,where M II and M III are divalent and trivalent metal cations and A n −is an n -valent anion,respectively.These compounds have a layered crystal structure composed of positively charged hydrox-ide layers [M 1−x II M x III (OH)2]x +and interlayers containing anions and water molecules (Fig.4).The value of x represents a por-tion of trivalent metal cations substituted in hydroxide layers and usually corresponds to 0.20<x <0.35.Hydrotalcite-like compounds exhibit anion-exchange anions in the interlayers may be exchanged for the other ones.At temperatures of approxi-mately 300–500◦C,hydrotalcite-like compounds are decomposed to form mixed oxides of M II and M III metals.In aqueous solu-tions,a rehydration of these mixed oxides takes place,which is accompanied by recovering of the layered hydrotalcite structure and incorporating of the anions from solution into interlayers.This unique property of hydrotalcite-like compounds can be employed for preparation of compounds intercalated with various anions or in removal of anions from solutions.The often used group name “hydrotalcite-like compounds”is related to the mineral hydro-talcite (Mg 6Al 2(OH)16CO 3·4H 2O).There are some other natural minerals and a great number of synthetic compounds with an analogous layered crystal structure combining various M II and M III metal cations in hydroxide layers and various anions intercalated in the interlayers.
Hydrotalcite-like compounds of general formula CuM II AlCO 3,where M II =Co 2+,Ni 2+,Cu 2+,Zn 2+and Fe 2+,were synthesized by coprecipitation and characterized with XRD and IR.These materi-als were studied in the phenol hydroxylation by hydrogen peroxide in liquid phase [18](Table 2).It was found that the un-calcined compounds have higher activities than those calcined.CuAlCO 3-HTLcs with Cu/Al ratio of 2,3and 4were prepared and used in order to study the effect of the M(II)/M(III)ratio.Results showed that phenol conversion increased with the Cu/Al ratio.However,HTLcs with higher ratio cannot be synthesized.The catalyst having Cu/Al =3efficiently oxidized phenol,giving high yields of the cor-responding diphenols.A reaction mechanism was also proposed.Authors hypothesized that H 2O 2might first adsorb on the surface of the layered structure of CuM II AlCO 3and then get one electron from Cu 2+ions to produce Cu 3+,HO •and HO −.HO •can further react with phenol to produce diphenols.
Catalytic hydroxylation of phenol over ternary hydrotal-cites containing Cu,Ni and Al with different (Cu +Ni)/Al ratios was carried out,leading to the formation of cate-chol and hydroquinone [19].The influence of various reaction parameters,such as reaction temperature,pH,solvent and hydrotalcites calcination temperatures (150,400,600,800◦C,respectively)was investigated.The best results were obtained over the samples [Ni 0.14Cu 0.61Al 0.25(OH)2](CO 3)0.13·0.96H 2O and [Ni 0.30Cu 0.38Al
0.32(OH)2](CO 3)0.16·1.23H 2O labelled as CuNiAl3-5and CuNiAl2-1,respectively.Authors attributed this high activ-ity to the large concentration of copper in the former catalyst,while to a larger specific surface area in the latter catalyst.More-over,a decrease of the copper concentration of the catalyst having a similar (Cu +Ni)/Al atomic composition decreased the activity.Both catalysts showed higher activity at 65◦C.Higher temperatures decreased the conversion of phenol because of the competitive thermal decomposition of H 2O 2.The higher activity observed at pH 5was ascribed to a better stabilization of the hydroxyl radical.Among the calcined samples,hydrotalcites treated at 800◦C exhib-ited the maximum activity,although their activities were lower
Table 2
Catalytic wet peroxide oxidation of phenol substrates over hydrotalcite-like compounds Catalyst Phenol conversion (%)Conditions
References CuMgAlCO 3
41Cu/M II /Al =1.5/1.5/1,60◦C,1h,pH 7,phenol/H 2O 2(molar ratio)1:60,phenol 0.5g,catalyst 0.05g [18]CuZnAlCO 338As above [18]CuCoAlCO 345As above [18]CuNiAlCO 347As above [18]CuCuAlCO 3
53As above [18]CuAlCO 3(Cu/Al 2/1)47As above [18]CuAlCO 3(Cu/Al 3/1)53As above [18]CuAlCO 3(Cu/Al 4/1)56As above [18]CuAlCO 3(Cu/Al 3/1)4850◦C [18]CuAlCO 3(Cu/Al 3/1)3840◦C [18]CuAlCO 3(Cu/Al 3/1)2830◦C
[18]CuNiAl2-122Phenol/H 2O 2(mol)2.0,catalyst 10mg,65◦C,2h pH 5[19]CuNiAl3-524As above
[19]CuNiAl2-140As above,phenol/H 2O 2(mol)1.0[19]CuNiAl2-156As above,phenol/H 2O 2(mol)0.5[19]CuNiAl3-541As above,phenol/H 2O 2(mol)1.0[19]CuNiAl3-561
As above,phenol/H 2O 2
(mol)0.5
[19]Mg 2Al-FePcTs 88(catechol)Borate buffer pH 10.0,30◦C,0.1%catalyst/catechol ratio,H 2O 23%wt.,2h [20]Mg 3Al-FePcTs 73(catechol)As above [20]Mg 4Al-FePcTs
63(catechol)
As above
[20]
For each catalyst or catalysts group only some experimental conditions and the corresponding phenol degradation or TOC removal are reported for a qualitative comparison.
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