Generation of active oxygen species on solid surfaces.Opportunity for novel oxidation technologies over zeolites
Gennady I.Panov a,*,Anthony K.Uriarte b ,Mikhail A.Rodkin b ,Vladimir I.Sobolev a
a
Boreskov Institute of Catalysis,Siberian Branch of RAS,630090Novosibirsk,Russian Federation
b
Solutia Inc.1P .O.Box 97,Gonzalez,FL 320560-0097,USA
beautifulpeople comAbstract
Generation of surface oxygen species and their role in partial oxidation reactions catalyzed by metal oxides are discusd.Main attention is paid to a new concept related to a recent discovery of remarkable ability of Fe complexes stabilized in a ZSM-5matrix to generate a new form of surface oxygen ( -oxygen)from N 2O.At room temperature, -oxygen exhibits a high reactivity typical for the active oxygen of monooxygenas,and mimics its unique ability to perform lective oxidation of hydro
carbons.This opens new opportunity for creating novel technologies bad on biomimetic strategy.A process of direct oxidation of benzene to phenol,recently demonstrated by Solutia on a pilot plant scale,is evidence of great potential of this approach.#1998Elvier Science B.V .All rights rerved.
Keywords:Oxidation of hydrocarbons;FeZSM-5;Nitrous oxide;Benzene to phenol;Biomimetic oxidation
1.Introduction
Oxygen activation on the catalyst surface is a necessary stage for the heterogeneous reactions of partial oxidation [1,2].Reactions of this kind are of great practical signi®cance.They are ud for manu-facturing such important chemicals as formaldehyde,acrolein,acrylic acid,phthalic and maleic anhydrides,etc.However,the number of industrial process practid today hardly exceeds a dozen.Compared to the large number of potentially uful reactions the achievements are very modest.
The main reason for the limited u of oxidation catalysis in organic synthesis is the dif®culty of ®nd-ing lective catalysts.A lective catalyst is suppod to perform a dual function:activate oxygen by gen-erating oxygen species of proper reactivity,and acti-vate the starting material to direct
the oxidation in the desired way.Optimization of the two functions is a dif®cult problem,since one cannot tune them inde-pendently by varying chemical composition of the catalysts.Often gaining in one aspect,we are losing in the other.This is the reason,why many reactions emingly simple are not realized with a reasonable lectivity.
First of all,this is true for the reactions of oxidative hydroxylation of hydrocarbons,many of which are carried out in industry via complex multi-stage pro-cess sometimes involving chemically illogical
steps.
Catalysis Today 41(1998)365±385
*Corresponding author.Fax:+73832355756/54/66;e-mail:panov@catalysis.nsk.su 1
Solutia Inc.±formerly chemical part of Monsanto 0920-5861/98/$32.00#1998Elvier Science B.V .All rights rerved.P I I S 0920-5861(98)00026-1
Methane oxidation to methanol and benzene oxidation to phenol are two notable examples.To obtain the hydroxylated products methane is®rst decompod to CO and H2,while benzene is alkylated to cumene within a three stage cumene technology.
At the same time natural enzymes monooxygenas (MO)can easily effect the transformations in a single step[3](to the continuous envy of chemists). The catalytic performance of MO is bad on a conceptually different approach.Nature rejected the idea of activation of the organic molecules, instead concentrating its skills on the activation of oxygen.Such activation is achieved with the help of the Fe-containing centers of MO.The reactivity of oxygen coordinated to the centers is so high that it can inrt into the non-activated C±H bonds of hydrocarbons under ambient conditions leading to the lective formation of hydroxylated products.
Attempts to understand the mechanism and to model unique MO functions are hindered by the inability of arti®cial systems to provide such a powerful oxygen activation coupled with the high lectivity.
军事训练目的
This paper considers active oxygen species on the surface of oxide catalysts as well as their role in the gas-pha partial oxidation.Reactions in the liquid pha are outside of our consideration.New concepts in this®eld are discusd in other reviews published in this issue.
This article has the following structure.First,we will brie¯y consider radical forms of oxygen species generated on pre-reduced metal oxides;then the cur-rent understanding of the surface oxygen participating in the partial oxidation over conventional catalysts will be discusd;and®nally,considerable attention will be given to the new concept formed within biomimetic approach to the oxidation catalysis.This concept is related to the remarkable species of oxygen (called -oxygen)generated on iron complexes in the ZSM-5zeolite matrix,which in many aspects rem-bles the active oxygen of MO.This opens a new opportunity for both better understanding of biological oxidation mechanisms and application of the biomi-metic strategy to develop new oxidation process.As one will e,such process are becoming clor to reality and one of them is at the commercial devel-opment stage.
2.Radical forms of oxygen
2.1.Formation of radical anions
Adsorbed oxygen on oxide surfaces can exist in a number of forms-molecular or atomic,neutral or charged.It is assumed that during the adsorption on the reduced surface,oxygen can accept electrons one by one going in succession all the way to the fully reduced O2À,which may differ only slightly from the lattice oxygen ions[1]:
Ions OÀ2and OÀare paramagnetic and can be obrved by the ESR.Ions O2À2and O2Àare not paramagnetic, and the neutral forms O2and O,though paramagnetic, do not give an ESR signal when on the solid surface. Radicals OÀ2and OÀare usually obtained by room temperature adsorption of O2or N2O on metal oxides that were initially reduced by CO or H2at high temperature.Bulk oxides with the oxygen of low lability(MgO,ZnO,TiO2,etc.),or silica-supported oxides of V,Mo,W are usually ud for this purpo. Generation of single-charged forms of oxygen is favored by low concentration of the transition metal when four electrons needed to convert dioxygen into O2Àare not easily available.
OÀ2radical is not thermally stable.When the tem-perature is raid above1508C,signals of OÀ2usua
lly disappear.OÀradical is more stable.Upon heating,it is consumed for reoxidation of the surface,but in some systems still can be obrved at temperatures as high as3008C.Becau of their potential role in the oxida-tion catalysis,chemistry of the radicals has been thoroughly studied,mainly in the1970s.The results of the studies have been reviewed by Lunsford[4]and in more detail later by Che and Tench
[5,6].(1)
366G.I.Panov et al./Catalysis Today41(1998)365±385
2.2.Stoichiometric reactions of OÀ
Surface radical OÀis highly reactive.It reacts with H2and CO even at the liquid nitrogen temperature. Becau in the prence of O2ion OÀeasily and reversibly forms OÀ3,it is assumed that the species are responsible for the low temperature isotope exchange of oxygen[7]:
16OÀ 18O2 16O18O18O À 18OÀ 16O18O
(2) Such an exchange on some of pre-reduced oxides occurs atÀ1908C.Heating in O2leads to the disap-pearance of both OÀand the low temperature isotope exchange activity.
Aika and Lunsford have studied stoichiometric reactions of alkanes and alkenes with the OÀradical on MgO[8,9].Very low concentration of OÀspecies (100±200nmol/g)required to develop a special tech-nique for analysis of the reaction products.After the reaction,the samples were gradually heated and the desorbed products were condend into a liquid nitro-gen trap.Then the trap was thawed out and the products were analyzed by GC.The reaction of C1±C4alkanes with OÀgave signi®cant amounts of dehydrogenation products(alkenes),but no oxyge-nated products such as alcohols,aldehydes or epox-ides were detected.The authors propod hydrogen abstraction as the initial step of the reaction yielding alkyl radicals,which quickly transformed into surface alkoxides(reaction4)or alkenes(reaction5):
C2H6 OÀ s3C2H 5 OHÀ s(3) C2H 5 O2À s3 OC2HÀ5 s eÀ(4) C2H 5 O2À 53C2H4 OHÀ s eÀ(5) Methane reacts with OÀon MgO surface at À130ÄÀ1508C with probable formation of methox-ide,which decompos upon heating to4508C with evolution of H2.The reaction also occurs on supported oxides of V,Mo and W[10].In[11],methanol was detected among thermodesorption products of the reaction over MoO3/SiO2.
In ca of alkenes C2±C4the initial stage of inter-action with OÀis also hydrogen abstraction[9].The resulted radicals can then follow different reaction pathways.1-Butene radical is oxidized to an alkoxide ion with subquent formation of butadiene as the main product.Radicals formed from ethylene and propylene apparently are being transformed into car-boxylate ions,which upon heating above4508C decompo with evolution of methane.
It should be noted that the u of thermodesorption procedure may give a distorted picture,since the desorbed compounds may be a result of condary transformations taking place at higher temperatures, rather than products of the primary reactions.
The role of OÀradical in the oxidation of methanol on SiO2supported MoO3has been recently evidenced [12,13].
It is interesting to note that the chemical properties of OÀradicals are quite the same in the gas pha[14], in solution[15]and on the surface.In all cas hydrogen abstraction is the dominating mechanism, despite the very different reactivity of OÀspecies among the different states.According to[10],the rate constant of methane interaction with OÀon the sur-face is7±8orders of magnitude lower than that in the gas pha.
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2.3.Stoichiometric reactions of OÀ2
Reactivity of OÀ2species is also very high,though much inferior to OÀ.As for the isotope exchange,OÀ2 is only capable of exchanging places with the gas pha molecules[7]:
16OÀ2 s 18O2 18OÀ2 s 16O2(6) There is no isotope scrambling,and identity of the exchanged particles is prerved.
On ZnO and V2O5/SiO2[16],OÀ2ion slowly reacts with propylene at room temperature,but gives no interaction with CO,H2and C2H4.On MgO OÀ2 radical apparently is more reactive[17].At1758C, the reaction with propylene proceeds to completion within2h.Acetaldehyde and methanol are detected in the products.Authors[17]believe that the®rst step of the reaction is hydrogen abstraction leading to allyl radical:
CH2 CHÀCH3 OÀ23CH2 CH CH2 HOÀ2
(7) Then the allyl radical probably reacts with surface oxygen ions to form acetate and formate ions.
G.I.Panov et al./Catalysis Today41(1998)365±385367
In ca of propane and 1-butene,oxygenated pro-ducts with the same number of carbon atoms as the reactant are found.It is assumed that the reactions also start with hydrogen abstraction.2.4.
Participation in catalytic reactions
The role of oxygen radicals in catalytic reactions is largely bad on assumptions and hypothes.Such radicals are formed on the pre-reduced oxides and are stable only at low temperatures.This is far from the conditions of catalytic reactions,which are carried out in the prence of O 2and usually above 3008C.However,one cannot rule out the existence of the radicals even under the conditions becau the catalyst surface is always partially reduced by the organic substrate [18,19].Also,it is assumed [5]that at elevated temperatures thermal activation of the charge transfer with formation of O Àspecies is pos-
sible:
(8)
The mechanism involving O Àis widely accepted for the catalytic dimerization of methane [20].Formation of ethane in the reaction of CH 4with O Àwas detected on MoO 3/SiO 2even at room temperature [10].Ethane is suggested to result from recombination of methyl radicals formed in the reaction with O À:CH 4 O À s 3 OH À s CH
3jell o
(9)
Frike et al.[21]studied the role of radical anions in the oxidation of butene to maleic anhydride over
V 2O 5±P 2O 5catalysts.Becau O À2and O À
are stabi-lized on vanadium cations their concentration decreas with increasing P 2O 5content.At the same time lectivity of the reaction grows and reaches a maximum at P:V 2:1.This allows to conclude that oxygen radicals are not lective.Though prently this is a widely accepted point of vi
ew [1,22],experi-mental results sometimes prompt authors to assume O Àspecies to be responsible for lective reactions,especially when nitrous oxide is ud as an oxidant [23,24].
Some authors consider neutral atomic oxygen O to be the active species in oxidation reactions.Such
mechanism was propod for ethane dehydrogenation in the prence of N 2O over Co 2 /MgO [25]and over ZSM-5[26],and also for the oxidative coupling of methane over modi®ed calcium oxide [27].
3.
Active oxygen on conventional partial oxidation catalysts
Contrary to the situation with radicals O Àand O À2,that can be easily identi®ed and studied at low tem-peratures,practical oxidation catalysis offers a situa-tion that is much more complicated to study.Classical catalysts for the lective oxidation leading to oxygenated products are usually complex systems bad on oxides of Mo or V [28].Oxidations are usually carried out at 350±5008C under conditions when different oxygen species can rapidly undergo transformations into one another.Oxygen isotope exchange provides a convincing evidence for such transformations.This met
hod was widely ud,espe-cially in the 1960±70s to obtain valuable information on the mechanism of oxygen activation [29,30].In many oxides important for the partial oxidation (V 2O 5,MoO 3,WO 3,Sb 2O 5,SnO 2,etc.),the exchange involves not only the surface oxygen,but also most of the lattice oxygen atoms.This fast exchange is a reason why despite signi®cant efforts,the real nature of the intermediate oxygen species in the oxidative catalysis remains unclear.Our knowledge in this area is limited by more or less substantiated hypothes that are generally qualitative.They will be brie¯y considered below.3.1.
Bond energy
In the early studies,signi®cant attention was paid to discover a correlation between the catalytic properties and bonding energy of the surface oxygen of oxides [31±34].The studies allowed Boreskov and co-workers [1,35]to formulate some uful rules for prediction of the catalytic activity,especially for the deep oxidation.A correlation was established between the heat of dissociative O 2adsorption,q (O 2),and the rate of oxidation of various molecules (H 2,CH 4,C 2H 6,etc.)to H 2O and CO 2.The reason for this correlation is that the heat of oxygen adsorption contributes to the
368G.I.Panov et al./Catalysis Today 41(1998)365±385
二建考试科目都有哪些
activation energy of reaction:E E 0 q O 2
(10)
The highest oxidation rate is obrved for oxides of Co,Cu,Mn,Ni with the lowest value of q (O 2) 16±20kcal/mole,whereas the lowest rate is obrved for titanium oxide for which q (O 2)is the highest at 60kcal/mole.It is interesting to note that for all the reactions coef®cients in Eq.(10)are clo to 0.5[36].This indicates that the deep oxidation involves active oxygen species of the same nature,though their bonding energies to the surface are different.
In partial oxidation,the role of oxygen bond energy is also very important,but not so straightforward.For ef®cient catalysts,q (O 2)should be in a certain optimal range.Low bonding energy would favor deep oxida-tion,high bonding energy would reduce the reaction rate.Experiments have shown that q (O 2)for lective oxidation catalysts lies in the range of 50±60kcal/mole O 2[19].3.2.
The role of adsorbed and lattice oxygen
jokingTheoretically,both adsorbed and lattice oxygen can take part in the oxidation.It is widely agreed that t
he lattice oxygen plays an important role in lective oxidation.Indeed,it was shown in a number of works [18,19,37±39]that partial oxidation catalysts can rve by themlves as a source of active oxygen and for some time perform the oxidation in the abnce of O 2in the gas pha.The amount of consumed oxygen could correspond to veral mono-layers.Subquent reoxidation restores the depleted oxygen.Experiments using 18O [38,39]are especially illustrative of the lattice oxygen participation.
A general scheme of catalytic oxidation including two stages of oxygen transformation was suggested in [18]:1)a primary activation with the formation of highly reactive electrophilic species and 2)subquent incorporation of the species into the
lattice:
sherry怎么读
(11)
Here,O s is partially reduced species of oxygen which could be reprented by the surface radicals discusd
visitorsabove.Such electrophilic species are mostly respon-sible for the deep oxidation,the attack on multiple bonds being especially easy [40].O latt.is a lattice oxygen which,according to the majority of opinions,is responsible for the partial oxidation.
According to this scheme,the catalyst lectivity should be controlled by the ratio of the rates of primary oxygen activation and its subquent incor-poration into the lattice.Indeed,a wealth of experi-mental data shows that the surface of deep oxidation catalysts (Co 3O 4,CuO,MnO 2,NiO)contains large quantities of looly bound oxygen,which is different from the lattice oxygen.At the same time,partial oxidation catalysts (MoO 3,V 2O 5,WO 3,Sb 2O 5and compositions bad on them)provide no evidence of looly bound oxygen [1,19].3.3.
Role of oxygen diffusion
A number of theories explaining lectivity of oxide catalysts are bad on the important role of the surface and bulk oxygen diffusion [37,40±43].It is assumed that oxygen is supplied to the surface sites not directly from the gas pha,but via volume diffusion [41]or diffusion from the other surface sites.A consistent remote control concept was propod by Delmon and coworkers to explain a synergetic effect in the oxida-tion of ole®ns over multicomponent catalysts [43].According to this concept a donor Sb 2O 4,adsorbs the oxygen from the gas pha,which is then transferred by a spill-over mechanism to an acceptor MoO 3,which hosts active and lective sites.
The study of paraf®n partial oxidation catalysts gave different obrvations [44].In this ca,to avoid overoxidation,the active sites (usually vanadium ions)must be isolated in the inert matrix of the support to prevent oxygen diffusion from the neighboring sites.A detailed study of vanadium oxide supported on a variety of carriers [45]concluded that the prence of V±O±V fragments was esntial for the lective oxidation.
In [46,47],a hypothesis was put forward in which the partial oxidation is carried out with the oxygen species bound to the transition metal cations by a double bond M=O and giving IR adsorption in the range of 900±1100cm À1.Indeed,such oxygen is
G.I.Panov et al./Catalysis Today 41(1998)365±385369
prent on partial oxidation catalysts,but not found on the catalysts of deep oxidation such as oxides of Co, Ni,Fe.
In conclusion,we may note that identi®cation of the active oxygen species participating the oxidation(concentration,charge,bond energy,etc.) is still an open question.Rapid interconversion of the species prevents a reliable answer to the question. Even in ca when the reaction occurs by using lattice oxygen(in the abnce of O2)one cannot completely rule out a hypothesis that actual intermedi-ate species is still a looly bound oxygen like OÀ.As was mentioned earlier,such forms can be generated as a result of charge transfer from O2Àion to metal cation.
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4.Active oxygen on FeZSM-5zeolite Zeolites are crystalline alumosilicates and as such are of no interest for redox reactions.Unlike other oxides,they have an intracrystalline system of micro-pores of molecular dimensions.Chemical composi-tion of zeolites can be modi®ed by transition metals due to their incorporation either into the crystal lattice or into the micropore space.Such modi®cations have been extensively ud in the last decade.Several new catalysts of redox type with incorporated metal in ZSM-5zeolites and their analogues act more ef®-ciently than conventional oxide systems.S
ome of the examples are titanium,and vanadium silicalites for the liquid pha oxidation with H2O2[48,49], vanadium silicalites for oxidative dehydrogenation of hydrocarbons[50],CuZSM-5for decomposition of NO[51]and CoZSM-5for decomposition of N2O [52].
Modi®cation with Fe is a special ca.Unlike other metals,who oxides initially exhibit some activity of the same sort as that after the introduction into the zeolite,iron atoms in ZSM-5matrix acquire funda-mentally new features compared to the free Fe2O3. The iron los its ability to activate O2,but acquires special af®nity towards nitrous oxide,causing its decomposition in a particular way[53].This reaction gives ri to the formation of a new form of surface oxygen(called -oxygen[54])which,as one can e further,is behind a unique performance of FeZSM-5 zeolites in the oxidation with N2O.4.1.Formation of -oxygen.The role of iron Authors of the®rst papers discovering -oxygen formation at N2O decomposition related this phenom-enon either to the prence of a single electron donor±acceptor sites in ZSM-5zeolite[55]or to the admix-ture of transition metals[56].Later,a hypothesis was also suggested[57]to correlate -oxygen with coor-dinatively unsaturated aluminum atoms of the zeolite (Lewis acid sites).Detailed studies on the meachanism of N2O decomposition performed over ZSM-5zeolites [58]and their ferrosilicate analogs[53]have shown that formation of -oxygen relates to the prence of iron.
The role of iron is illustrated by Fig.1.It shows the results of a temperature programmed N2O decompo-sition over ZSM-5zeolites with different iron content. Experiments were carried out in a static unit equipped with a microreactor,which provides high accuracy of adsorption and kinetic measurements.The following procedure was ud.After a standard pretreatment in vacuum and oxygen at5508C,the samples were cooled to508C and0.4Torr of N2O was introduced into the reaction space.At time A the reactor was opened and,after the adsorption equilibrium was established,the heating was turned on(time B). The composition of the gas pha was followed by mass-spectrometry.
The spectrum for the sample containing almost no iron(Fig.1(a))is typical for conventional catalytic systems.In this ca raising the temperature to around 2008C leads to complete desorption of N2O and restoration of the initial pressure,which remains unchanged up to4508C.Above this temperature, decomposition of N2O takes place with the evolution of O2and N2into the gas pha.
There is a drastically changed picture for the sample containing0.056wt%Fe(Fig.1(b)).Here,after the initial adsorption,P(N2O)does not reach its original value becau at1508C nitrous oxide starts to decom-po,which is evidenced by the evolution of dinitro-gen.This picture is even more pronounced for the sample with0.35wt%Fe.In this ca,the decom-position becomes noticeable eve
n at lower tempera-tures.An unusual feature of this low temperature decomposition is that only N2is evolved into the gas pha while oxygen remains completely bound to the active sites to form -oxygen:
370G.I.Panov et al./Catalysis Today41(1998)365±385
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