Pactest

Pure Appl. Chem., Vol. 72, No. 7, pp. 1289–1304, 2000.
2000 IUPAC Cleaner industrial processes using hydrogen
peroxide*
Consultant to Solvay SA, c/o Solvay Interox Ltd., P.O. Box 7, Baronet Works,Warrington WA4 6HB, UK Abstract: Recent research progress in catalytic systems for potential use with hydrogenperoxide in industrial chemical synthesis is reviewed, with special focus on work publishedin the last five years. The main types of chemistry employed are critically appraised regardingtheir suitability for industrial exploitation. The most significant catalyst types are discussedin terms of the positive features identified to date, and the obstacles yet to be surmounted inorder to become more widely adopted. It is believed that fully inorganic systems have morescope for commercialization than those containing organic ligands or supports, however robust.
Critical targets are larger-pore analogs of titanium silicalite TS-1, more exploration of smectite-based materials, effective immobilization of activated metal peroxo systems, and improvementsin design and manipulation of polyoxometallate compounds. Cooperation between branchesof chemistry that have not traditionally worked closely together is advocated.
INTRODUCTION
This article is concerned with sustainable chemistry, and adheres to the objective green chemistrydefinitions already published [1], and discussed elsewhere in this issue.
There have, overall, been two related trends, from a factory-based approach to a plant-based approach, and from waste management (“cure”) to waste avoidance (“prevention”). One should notrush to denigrate the traditional “end-of-pipe” method of managing waste at the factory level, sincemany mature chemical sites do this very effectively, even in today’s stringent operating environment.
Rather, the message is that design of new processes and plants can benefit by following, as far aspossible, the principles of “atom utilization” or “atom economy” [2], to eliminate waste at source.
By any criteria, hydrogen peroxide has already made an appreciable contribution to a cleaner chemical industry. Its inherent chemistry as an oxidant [3] continues to give it a role beyond that ofoxygen, especially in selective oxidations. As the world’s leading producer, Solvay has played its part indiscovering the technology to maintain and accelerate this development (see box on next page). Thispaper seeks to review progress, particularly in catalytic oxidation methods for chemical synthesis basedon H O , analyzing the strengths, weaknesses, and current challenges in this area. Such a task is not easy for at least two reasons: firstly, there is a huge volume of work to be surveyed in a small space [4]; andsecondly, many of the academic studies do not reveal the full potential of the catalysts identified, sinceexperiments are not extended to high turnover (a critical parameter for industrial exploitation). Hence,there is inevitably an element of personal opinion in this review, for which the reader’s understanding isasked.
GENERAL ISSUES AND CHALLENGES
As indicated above, there are difficulties in comparing and appraising the commercial potential ofpublished work on peroxygen oxidations. In journals, the screening approach adopted at the research *Pure Appl. Chem. Vol. 72, No. 7, 2000. A special topic issue on green chemistry.
Solvay’s efforts towards clean catalytic processes
Over the last 15 years or so, Solvay (via the Interox joint venture before 1992) has, in addition tovery significant in-house R&D effort, supported a host of projects at research institutions throughoutWestern Europe and the United States. Below is a non-exhaustive list of collaborations, aimedeither at the discovery of new oxidation technology using hydrogen peroxide and its derivatives,or at mechanistic studies underpinning such advances. The author of this paper was personallyinvolved in over 90% of this work, and owes his own understanding of the subject in large measureto those named here.
Institution (UK)
Key Collaborators
Institution
Key Collaborators
Dr J. R. Lindsay SmithDr C. B. ThomasDr B. J. Keely Liverpool University Prof R. A. W. Johnstone stage gives results that typically fall far short of demonstrating industrial feasibility, since “friendly”conditions are used. For example, apart from the lack of information on turnover limits, it is quitecommon to find work done at high excess of substrate to peroxide (say, 5:1 molar or greater) which inliquid-phase oxidations is a serious drawback. Others have referred to the lack of suitable blankexperiments in many cases, and to insufficient product analysis and recycle data to prove that the activecatalyst remains within the material added [5]. In patents, the intention is often to maximize coverage 2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
Cleaner industrial processes using hydrogen peroxide without disclosing all details of the best embodiments, again making interpretation and evaluation difficult.
One sees many claims to cover a range of peroxygen oxidants when there is no evidence in the examplesthat they were even tried.
The choice of substrate is a special concern. Two common test reactions for electrophilic peroxygen systems are epoxidation and (organic) sulphide oxidation. In the former case, lots of results are reportedfor cyclooctene, which is not only about the easiest common olefin to epoxidize, but also forms ep-oxides readily with dioxygen and other oxidants which do not usually transfer single oxygen atoms.
Industrial demand, on the other hand, is most often for epoxidation of substrates at the very far end ofthe reactivity series [6]—terminal olefins and allylic compounds—and no conclusions can be drawnfrom cyclooctene results. Similarly, thioanisole (PhSMe) is a common test substrate for catalyzedperoxygen oxidation at sulfur, but the uncatalyzed reaction is quite significant, and again under someconditions dioxygen also oxidizes the sulfur.
After a long period when catalysis was essentially the province of heavy chemical production, it is now increasingly relevant to fine chemical manufacture, often for pharmaceutical/ veterinary or agro-chemical use. Such products need to be rigorously free from toxic metals, down to ppm or even ppblevels. This has implications for catalyst recovery and leach levels, which may be especially difficult toachieve for “immobilized” (heterogenized) homogeneous catalysts. This favors covalent over ionicbinding, or physical confinement of dissociable complexes.
A perennial challenge in H O oxidations is its strongly hydrophilic nature, and the mismatch between this and the hydrophobic character of most substrates. The use of “bridging” solvents such asacetonitrile, t-butanol and methanol is a partial answer, as is phase-transfer catalysis using two-phasepolar/non-polar mixtures. In academic studies, chlorinated solvents are still commonly used as theorganic component, whereas this is often not feasible in new industrial processes: alternatives such astoluene are employed, which may need different phase-transfer catalysts.
The difficulty of “mixed polarity” is magnified in heterogeneous systems, where surface affinity for all reactants must be provided such that effective local concentrations are maintained. This factorseems often to be overlooked, there being an implicit assumption that bulk concentrations of reactantswill be similar to those near the active site; good catalysts may be missed by neglecting this influence.
Finally, again related to hydrophilic conditions, the reducing properties of H O are often more evident in the presence of water, favoring decomposition and nonselective reactions in the presence ofcatalysts that perform well in organic media. Unfortunately, owing to safety constraints, and the simplefact that water is usually a co-product of reactions, the large-scale industrial use of organic H O solu- tions is not often attractive. This could change somewhat in the future, in the light of chemical engineer-ing developments such as reactive distillation.
CATALYST TYPES
In the remainder of this review the chief catalyst types will be discussed, in terms of the positive featuresidentified to date, and the obstacles yet to be surmounted in order to become more widely exploited.
Redox metal complexes
Most work in this area has been done with iron and manganese porphyrins [7], aimed largely at
oxygen transfer to form M(n+2)+ = O oxene intermediates. These were not thought at all relevant as
commercial catalysts until relatively recently, when the relative robustness of Mn(III) tetrakis(meso-
2,6-dichlorophenyl) porphyrin (MnTDCPP) was found, along with high-yielding preparative methods.
Whenever organic ligands are present in oxidation catalysts, some degradation over long time periods
almost invariably occurs. However, in the case of porphyrins, this has been mitigated by electron-
withdrawing meso-substituents such as are present in TDCPP, and in later generations by b-substitution
also [8]. Nature does not do this, instead providing steric shielding of the meso-positions, along with
2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
“site isolation”. On the other hand, metalloporphyrins in solution can be degraded by intermolecular “face to edge” contact between oxene and ligand. Furthermore, with H O as the oxidant a further problem can arise—that of homolytic rather than heterolytic cleavage of the M–O– O–H intermediate. This will generate free hydroxyl radical, an indiscriminate oxidant that will attack even the most unreactive ligands.
That is why H O is often found to be more aggressive to porphyrins than hypochlorite, even though the latter is a more powerful oxidant per se. Related to this, the electron-withdrawing substituents used to stabilizeporphyrins (and to make the oxene a more powerful electrophile) also make heterolytic cleavage lessfavorable, hence actually encouraging formation of oxidizing radicals. Such systems can no longerreally be called “biomimetic”. Some features of iron and manganese systems now need to be discussedseparately.
Manganese porphyrins mainly depend on a co-catalyst to assist heterolytic –O–O– cleavage, by donating electrons as an axial ligand below the porphyrin plane, and by assisting in proton transfer tothe –OH to create –O+H as leaving group. Imidazoles are the traditional choice, reflecting the role of histidine in natural peroxidases. However, in “free” systems, these are rapidly degraded. Alternativessuch as ammonium acetate and aliphatic amine N-oxides appear more practicable. MnTDCPP oxidizesterminal n-olefins such as non-1-ene with high turnover and little loss of catalyst—in fact, catalystdoses are so small as to make disposal after a single batch reaction potentially economic for fine chemi-cals. There is always, however, some competition between substrate oxidation and catalyst degradation,which is more serious for less reactive substrates, and masked in much literature work by the use oflarge excesses of substrate to H O . In very electron-deficient Mn porphyrins (such as poly-b-nitro [9], or poly-b-chloro [10]), the role of the axial ligand is diminished, and it may be that the M–O–O–Hspecies acts directly on the substrate, as has been shown for similar iron systems. The catalytic potentialof such systems remains to be established.
In the case of iron, imidazole is not very useful in homogeneous systems, since the inactive trans- dicoordinated complex is readily formed. Accordingly, homolytic –O–O– cleavage is more of a factor.
Moreover, it appears that when oxygen transfer from the oxene does occur, it is in two discrete1-electron steps, as shown, for example, by isomerization of cis-stilbene to the trans-epoxide, and bythe greater tendency to hydroxylate aryl-substituted olefins along with the epoxidation. This is likely tobe due to the absence of FeV in the resonance equilibria: p+.FeIV=O ´ pFeIV-O. ´ p+.FeIII-O.
allowing significant radical character arising from the single-bonded species. Recent mechanistic work has also shown, however, that –O–O– cleavage is not necessary to obtain a catalytic system—theFe–O–O–H intermediate can transfer oxygen directly to substrates [11]. Where the oxene is formed, Feporphyrins have been shown to exhibit “oxo-hydroxo tautomerism” in the presence of water, which is afavored axial ligand. All in all, iron porphyrins may have the same catalytic potential as manganese,with less dependence on cocatalysts, but fewer studies on relevant substrates have been reported.
Water-soluble Mn [13] and Fe [14] porphyrins have been prepared, mainly by substitution of meso-phenyl substituents with charged groups such as quaternary ammonium or sulphonate.
Attempts to immobilize porphyrins on solid supports [15], whether by physical or chemical at- tachment, have met with limited success to date. While site isolation should improve catalyst life bypreventing face-to-edge contact, this is counterbalanced by the loss of activity usually seen. A likelymajor factor is lack of control over the relative surface affinity for reactants (and products). In Mnsystems, co-catalyst access is also often needed—tethering of the co-factor is not a promising approachowing to its own degradation, and to the dual role of axial ligand and proton donor required (thoughthese functions can be performed by different molecules).
2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
Cleaner industrial processes using hydrogen peroxide Ruthenium porphyrins cannot be used directly as catalysts with H O , but the latter can be used to generate N-oxides which are effective O-donors to Ru in these complexes. In such a way, RuIV-VI redoxchemistry can be accessed, again involving oxenes. This is very slow, however. There is a prospect of afaster RuIII-V couple [16] which has yet to be explored.
Recent work with phthalocyanines [17] has revealed new catalytic poten-
tial, a key advantage being their cheap and simple preparation, incorporating metals from all three rows of the periodic table. They can be readily prepared in zeolite cavities and on other supports [18]. They appear, however, to be moresuited to C–H oxidation reactions than to epoxidation.
Another relative newcomer as a ligand is sym-triazacyclononane (tacn)
and its N,N’,N”-trimethyl analog (tmtacn). These form stable complexes with all first-row transition metals, and the manganese complexes in particular offer elec-trophilic oxygen-transfer catalysis with H O [19], and radical hydroxylation of saturated hydrocarbons in acetic acid solution [82]. Once again, ligand degradation is not negligibleover long periods, and leads to a loss of selectivity as Mn is liberated in different forms. The conse-quences of this for catalysis of stain bleaching in domestic laundry are well known. Salen (Schiff-basetype) ligands are another class which have not achieved much success with H O owing to their easy By contrast, hypervalent metal complexes with tetragonal amide ligands are much more robust to oxidation, and recent reports suggest sig- nificant potential of iron amide complexes [20] as catalysts for H O oxi-
Hydrogen peroxide can be used as an oxidant in aromatic side-chain oxidations in acetic acid, catalyzed by cobalt and bromide salts [21] with
or without manganese, or with cerium and bromide [22]. The first of these
systems is analogous to dioxygen oxidations carried out at high pressure onvery large scale, to make terephthalic and isophthalic acids from the corre-sponding xylenes. The ability to use ambient pressure in general purpose plant, with H O as oxidant, is useful for fine chemicals. In this case, the second oxidation on a side chain is faster than the first,
tending to give toluic acid or aldehyde intermediates if a low excess of oxidant is used. Another side-
chain oxidation system uses hydrogen peroxide and hydrogen bromide irradiated with visible light
[23], the active species being atomic bromine. Here, the second oxidation is slower than the first, offer-
ing a selective route to dialcohols from xylenes, for example.
There have been several attempts at encapsulation [24] of some of the above types of complex [and others such as Mn(bpy) ]: in zeolite cavities (e.g., zeolite Y) [25]; smectite interlayers (e.g., mont- morillonite, layered double hydroxide) [26]; mesopore channels (e.g., MCM-41) [27]; amorphous silica[28]; or membranes (e.g., polydimethylsiloxane) [29]—using many synthetic approaches. So far, mod-erately good catalysts have resulted in a few cases. Transport of reactants and products within thesupport, and space around the active site, are common limitations for “ship in a bottle” catalysts [86],which are more difficult to solve than for framework-substituted catalysts. Only quite low loadings canbe tolerated, to ensure adequate mobility. Smectites have the option of pillaring to increase interlayervolume, and are underexplored, maybe because many more catalysis groups specialize in zeolite/mesopore synthesis and characterization.
Metal peroxo systems
This section refers to electrophilic peroxo complexes of d0 metals, which are formed by several elementsunder Ti, V, Cr, and Mn in the periodic table, the relevant oxidation state being favored as one moves tothe left and down each row. Nucleophilic d8 peroxo complexes, while also catalytic, are not believed tocompare with these in commercial potential.
2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
Catalysis using tungsten (VI) and molybdenum (VI) complexes, through
formation of peroxo-metal intermediates, has been known for over 50 years, butimportant advances continue to be made in understanding and improving this mode of H O activation. It is, in fact, one of the most versatile systems, and comes closest to classical organic peracids in its range of applications. The key catalytic interme- diate is an h -peroxometal species (one or two peroxo groups are usually attached in this way) which acts on the substrate either directly or via protonation to give the M–O–O–H species. Substrates may be coordinated to the metal center (e.g., in alcohol oxidation, byhydride abstraction) or uncoordinated (e.g., in epoxidation, according to the consensus of opinion).
Both molybdenum and tungsten work effectively in aqueous systems, unlike many catalysts, owing to their high affinity for H O . If the substrate is hydrophobic, two-phase systems are commonly used, the peroxo-complex being taken into organic by suitable ligands and/ or cationic phase transferagents. The simple complexes are moderately active in epoxidation [88], N-oxidation, alcohol oxida-tion etc, with tungsten being better except for the last of these. Activitycan sometimes be raised by increasing temperature (to reflux in wa- ter), but this is not a good option for epoxidation owing to hydrolytic ring-opening. A key discovery, by Venturello [30], was the use of phos- phate/ tungstate mixtures, which epoxidized terminal olefins at mod- erate temperatures. This boosts activity by making the peroxo-inter- mediate asymmetric, through nonbonded tungsten-oxygen interaction, facilitating oxygen transfer [31]. Industrial use of this system is cer- Since the discovery of this structural feature, it has been repro- duced in many other complexes, including XM and XM types (X = P, As, S) [33]. There are draw- backs, however. The non-bonded interaction is the basis for the stability of the complex, which there-fore dissociates after the peroxo-group is lost, suggesting that a significant excess of H O be main- tained. This can lead to further oxidation by Hock reaction with the epoxide, giving –C–-C– cleavage(though this itself can be a desirable transformation [34]. Hydrolytic ring-opening can be a problem forsensitive epoxides, owing to acidity of the medium and/ or Lewis acid character of the d0 metal center.
Excess H O with tungsten, and particularly molybdenum, complexes can lead under some condi- tions to liberation of singlet oxygen—a particularly convenient and controllable source of the latter[35].
Simple immobilization of molybdenum or tungsten complexes on solids gives materials which
oxidize “easy” substrates such as sulfides or electron-rich olefins [36], but as noted earlier, these are notof much interest for industrial chemical synthesis. In principle, it should be possible to attach XW2complexes covalently to solid supports. This remains a serious and worthwhile research target, butsuccessful catalysts based on this feature have not yet been reported.
A much more recently-discovered (by Herrmann et al.) catalyst based on peroxo-metal chemistry is methylrhenium (VII) oxide (MeReO , MTO) [37]. The alkyl substitution is critical to catalytic
activity, which is mainly lost on degradation—owing to formation of unreactive rhenate, ReO -. MTO is a stronger Lewis acid than molybdenum or tungsten complexes, and catalyzes many reactions as such,including olefin metathesis. Its strong electrophilic nature makes the peroxo complexes good oxygen-transfer species to olefins, etc., but the acidity also increases ring-opening, giving diol rather than ep-oxide as the product. However, unusual effects of azine ligands, including pyridines, bipyridines, etc.,have been found, which appear to accelerate epoxidation but inhibit acid-catalyzed ring-opening [38].
The system is further complicated by gradual oxidation of these ligands to N-oxides [39], a commonreaction of peroxo-metal oxidants. A full explanation is probably still lacking, but a lot of good infor-mation has been generated.
MTO is less tolerant of water than Mo and W systems, and requires strong to anhydrous H O for best results. The ratio of H O to H O and catalyst influences equilibria between mono and diperoxo 2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
Cleaner industrial processes using hydrogen peroxide complexes, which can exhibit different activities—more so than molybdenum, for example, where themono-peroxo tends to disproportionate. Degradation probably occurs via loss of a proton from themethyl group, then electrophilic attack on the H C=Re bond.
In addition to forming a fibrous solid polymer itself, monomeric MTO has been successfully supported on a range of solid surfaces, retaining its catalytic properties [40]. None of these catalysts hasbeen developed for industrial use to date, and gradual degradation of the MTO is obviously a concern iflong lifetimes are to be reached, but there remains scope for further research to this end.
Somewhat different properties to the above are offered by vanadium (V) complexes [41], which
also readily form peroxo-complexes with H O , but which are generally more selective catalysts with t- butyl hydroperoxide (TBHP) than with hydrogen peroxide (though, for example, enantioselective S-oxidation can be achieved with chiral V-complexes and H O ). Indeed, the h -peroxo complexes of vanadium and molybdenum are themselves good catalysts for TBHP oxidations.
With H O , vanadium exhibits some 1-electron (VV-IV) redox chemistry, introducing free-radical character into its reactions. This makes epoxidation non-stereoselective, and can also changechemoselectivity. For example, when substituted in silicalites (see later), titanium (TS-1 and –2), oxi-dizes toluene mainly at the nucleus, giving cresols, whereas vanadium (VS-1 and –2) oxidizes more atthe side-chain to give benzylic products—taken as evidence of parallel electrophilic and radical mecha-nisms [42]. This radical character can be useful, for example, in alcohol oxidations, where vanadiumsystems are more active than molybdenum, especially toward primary alcohols. Another chemoselectivityeffect in silicalites concerns allylic alcohols, which are mainly epoxidized by TS-1 but undergo mainlyalcohol oxidation by VS-1 [43].
Oxygen can be used as co-oxidant, since the radical intermediates can capture oxygen from the atmosphere, even to the extent where H O is acting more as a radical initiator than a stoichiometric oxidant. The Fenton-like activity of vanadium complexes with azine carboxylic acids (2-picolinic acid,4-heptyl-2-picolinic acid, and pyrazinecarboxylic acid) has been quite thoroughly explored [44], evenextending to attack on methane [45].
A great deal of work has recently been reported on mimics of vanadium bromoperoxidase
enzymes. Bromide with V/H O systems can provide an effective system for halogenation and for hy- dride abstractions such as alcohol oxidation, at moderate pH (uncatalyzed bromide and H O only works in strong acid) [46]—molybdenum behaves similarly [47], as does MTO [83]. There is evidencefor a bound active halogen species in both enzyme and mimics.
A final practical note is that, owing to the 1-electron chemistry, much more decomposition of excess H O is caused in V systems than in W, Mo and Re. This requires either better control of addition rates, or the effective capture and use of the oxygen generated, having regard for safety issues.
Polyoxometallates and heteropolyanions
This is a group of polynuclear oxoanion complexes usually based ontungsten or molybdenum. They often include structural heteroatoms, whichmay be di- to pentavalent, and one or more main atoms can be substitutedby transition metals, giving additional 1- or 2-electron redox chemistry.
Common structure types are Keggin (XM ), Wells-Dawson (X M ), and “sandwich” (M X.Y .XM ). They have the attraction of being fully inorganic and therefore not prone to oxidative degradation, though theequilibria involved in their formation are subtle and intricate. A bewilderingrange of structure options exist, where unit size, main and heteroatom,substituent, degree of substitution, and topomerism can all be varied.
Adding the Keggin complex, [PW O ]3–, catalyzes epoxidations with H O . However, it has been shown that the active species are the same as in the Venturello system (see earlier), arising from breakdown of the Keggin structure [48]. This illustrates two factors involved with 2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
polyoxometallate dissociation. Firstly, addition of H O itself promotes dissociation, since it is a strong “ligand” for W and Mo. Secondly, dissociation is a nucleophilic process, and occurs readily when thecomplex has a relatively low charge. In fact, the silicon analog, [SiW O ]4–, is much more stable, and does not catalyze epoxidation. Hydrolytic stability is therefore enhanced by high negative charges, andhence by lower-valent heteroatoms or substituents. Trivalent heteroatoms include B and Al, and diva-lent Zn and Co—these are very much more robust. As usual, there is also a cost—higher negative chargeinhibits catalysis of electrophilic oxidations.
Since the early work, catalytic activity has now been shown for a variety of polyoxometallates, at least some of which appear to act in the undissociated form. Several substituted “lacunary” Keggincomplexes have been studied [49], though the activity in epoxidations has usually been low for terminalolefins, where reported. The “sandwich” type has probably shown the greatest promise for epoxidations,in the form of the two complexes [WZnMnII (ZnW O ) ]12– [50] and [(WZnRhIII )(ZnW O ) ]10– [51], the latter offering lower H O decomposition and greater stability.
Keggin complexes with a single transition metal substituent can give both radical and electro- philic reactions depending on the substituent, but reported activities are not of great interest industrially.
A W-peroxo derivative of an intact singly substituted Keggin structure has now been discovered, b - [CoIIO )W O (O ) ]10– [52]. This appears, from cyclohexenol oxidation results, to be a relatively nu- cleophilic oxidant, as expected. Hydroxylations of alkanes are catalyzed by the substituted Kegginstructure, [g-SiW {Fe(OH) } O ]6– [53], and it is established by NMR that the 1,2-Fe topomer (with vicinal Fe atoms) is the main active component. Similar results are reported for oxygen oxidations withH PV Mo O , with the 1,2-V topomer being the best catalyst of phenol and alcohol oxidation, among others [54]. Both of these catalysts seem to involve cooperation between two 1-electron oxidizing spe-cies, as may others with this feature [55]. Mixed Mo/V complexes up to PV Mo catalyze phenol hy- droxylation by H O [56], and the dependence of o-:p- ratio on V:Mo ratio may well be related to An important development in the practical use of polyoxometallates, aimed thus far at paper pulp bleaching, is successful self-assembly—including self-repair and self-re-assembly after reaction, evenif dissociation occurs at an intermediate stage [57]. This makes use of the thermodynamic stability ofthe complexes under given conditions, once an effective catalytic structure can be matched to thoseconditions. A lot of laborious research is needed to achieve this match, but it is ultimately one of themost valuable properties of polyoxometallates, and should ensure their adoption for many catalyticprocesses in future. This same feature suggests enormous potential in immobilized systems, as yetlargely untapped. A limitation of polyoxometallates is their high equivalent weight as oxidizing inter-mediates. For this reason, true catalytic cycles, rather than stoichiometric generation/ use/ regenerationloops, remain a key target.
Zeolitic and smectitic materials
This section addresses heterogeneous catalysts with no homogeneous analog, as distinct from immobilizedhomogeneous catalysts. Some excellent critical reviews covering one or both areas have recently beenpublished [58].
TS-1 is a titanium-substituted aluminium-free silicalite with 5.5Å channels (MFI structure, analogous
to ZSM-5), found to catalyze many H O oxidations [59]. After the first reports of titanium silicalite in
the early 1980s, there was a huge research effort worldwide to find the many analogous materialsbelieved to be waiting to be discovered. However, as time has progressed, TS-1 itself seems more andmore unusual. Hence, this effort has not abated, but a large amount of it has in fact been applied tofinding out why TS-1 works so well, before being in a position to make new breakthroughs [60]. This initself has fuelled progress on characterization techniques for such materials [61].
2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
Cleaner industrial processes using hydrogen peroxide From the beginning, at least three features were potentially important: hydrophobic environment, tetrahedral geometry, and constrained reaction site.
All of these have turned out to be relevant. Work on solvent dependence, and on Ti/ Si xerogels with varying hydrophobicity [62], confirms the importance of the active site environment, while not achieving comparable catalysis by the latter route. In fact, TS-1 has been commercialized for oxidation of phenol to catechol/quinol, and for in situ oxidation of ammonia to hydroxylamine in production of caprolactam from cyclohexanone (via the oxime). It is likely also to be used for epoxidation of olefinsin the future, an application made more attractive by the reduction in price of H O during the 1990s.
Methanol or water are the usual solvents [63]. For many applications, residual acidity must be sup-pressed, as it leads to fouling of the catalyst by overoxidized on hydrolyzed products. Post-addition ofalkali-metal or other cations helps, without much impact on activity, implying that acid sites are mainlyat the surface [64]. Site-isolation of the titanium atoms appears important, and the lower limit of 40:1Si:Ti excess (m/m) for good catalysts agrees well with this criterion—increased H O decomposition and less substrate oxidation are otherwise seen. Regeneration of TS-1 can be done effectively in theliquid phase, without recourse to re-calcination, by use of H O in the absence of substrate [65]— presumably this helps to degrade “heavies” fouling the channels into smaller molecules which can bedesorbed. The physical form of the catalyst is critical, as in all industrial process, and the very small TS-1 crystallites required for high activity must themselves be securely supported on a suitable substrate[87].
Other elements have been used in place of titanium in making silicalite catalysts; many of these are probably not true framework-substituted structures. ZrS-1 [66], SnS-1 [67] and MoS-1 [68] allshow some catalytic activity, but fail to improve upon TS-1 for any particular oxidation. VS-1 doessome useful additional chemistry, as noted earlier [42,43]. CrS-1 shows activity in alcohol oxidations,but this has now been shown to arise from homogeneous catalysis from Cr leaching [83,111]. TS-2 isbased on the MEL (ZSM-11) structure, and exhibits the same range of chemistry as TS-1, with somevariations; these are not large enough to have caused its industrial exploitation to date.
TS-1 is a valuable and versatile catalyst, but with an important drawback of substrate size limita- tion. The zeolite channels will not accept o- or m- disubstituted aromatics, alicyclic terpenes or tertiaryaliphatic compounds, and simple alicyclics or branched aliphatics pass with difficulty, restricting des-orption of product and therefore reaction rate. For this reason alone, the search for alternatives is boundto continue.
The most useful such material found to date is Ti-b, a large-pore zeolite (6.4–7.6Å) [70] containing
framework Ti, though the cavity size also accommodates other forms of Ti. By comparison with TS-1,
it certainly accepts large substrate molecules, but it is not as hydrophobic, and does not exclude solvent
from the reaction site—hence solvent effects are very significant [71]. The lower hydrophobicity favors
acid-catalyzed ring-opening of epoxides, giving diols or their monoethers as the main products from
olefins [72]. The acidity can be suppressed by working in the mildly-basic acetonitrile as solvent
(selectivity is more solvent-dependent for Ti-b, and trifluoroethanol is also a good choice), and by
addition of cations [73]. In the latter case, activity is reduced, which confirms that the acidity is in the
cavities, adjacent to active sites. Presence of solvent in the cavities also reduces activity, and Ti-b is
slower than TS-1 at oxidizing small substrates: in fact, it is a better catalyst for TBHP reactions, the
latter being more hydrophobic than H O . However, Ti-b is now quite well enough understood to find
application in fine chemistry as a convenient catalyst for larger molecule oxidations, etc. [74].
Furthermore, the cavities are large enough to allow some surface modification, to increase hydrophobicityor attach other functional groups, while maintaining a viable volume.
The incorporation of titanium into even larger cavities [75], such as the mesomorphous MCM and HMS series made using liquid crystal templates, yields materials which, according to reports to date, 2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
are still less good with H O and better with TBHP than Ti-b, for reasons similar to those discussed. The same options for modifying surface [76] and spatial properties exist. The subject of design of large-porematerials has been reviewed [84], and several catalysts based on them investigated [85], results beinglargely consistent with the picture presented here.
Zeotypes APO and SAPO have been explored to some extent as supports for liquid phase oxidationcatalysts. APOs such as VPI-5 can incorporate a variety of transition metals such as vanadium (Psubstitution) cobalt (Al substitution or cation exchange), or Cr, and are large-pore materials. However,the structures are not particularly robust, and rely for their integrity on a large amount of coordinatedwater. Overall, published results do not demonstrate any advantage over zeolitic supports. CrAPO suffersCr leaching in use with H O , just as CrS-1 [58c,69].
A rather more encouraging theme is that of metal (IV,V) phosphates such as zirconium, tin, cerium, and va-
nadium. Taking zirconium phosphate as an example, this
readily forms a layered (smectitic) structure in which the
interlayer spaces (7.6 Å spacing) contain strong Brønsted
acid sites, corresponding to the protons in the empirical
formula ZrPO (OH). Such materials, whether overtly crys-
talline or amorphous according to XRD spectroscopy, ac-tivate H O towards electrophilic oxidations. Examples include phenol hydroxylation [77], where performancecompares well with TS-1, and Bäyer-Villiger reactions. In some cases, acetic acid is found to be the bestsolvent, and there is evidence that peracetic acid is formed in the interlayers and reacts directly with thesubstrate there. In other cases, acetonitrile is a good solvent, and the chemistry appears to arise fromprotonated hydrogen peroxide itself. The interlayer spaces are relatively hydrophilic and acidic, soolefins are more likely to give diols than epoxides in this system. However, these spaces can be modi-fied by pillaring (organic or inorganic) to give a range of heights and polarities. Some variation is alsopossible by templating with nonionic or cationic surfactants. Like polyoxometallates, there is enormousscope for structure manipulation. Combinations of major and minor elements, crystallinities, etc. can bemade, but the potential of this type of catalyst for use with H O has been much less explored to date.
One of their chief attractions compared to TS-1 is the greater mobility and size of substrates achievable.
Other smectitic materials include clays, for example, montmorillonite (acid sites), and layered double hydroxides, for example, gibsonite (base sites). These have been employed as supports for othercatalysts, the former for cationic species such as bpy and tacn complexes, the latter (which are onlystable over a narrow neutral to alkaline pH range) for anionic species such as polyoxometallates andmetal peroxo complexes.
Given the enormous progress in biotechnology, and the receptiveness of much of the chemical industrytowards it, such a review as this should address this area. In fact, oxidoreductase enzymes are plentiful—many are well characterized and readily isolated—but they are hard to use here.
Firstly, most enzymes are not designed to withstand significant concentrations (≥ 1%) of H O . In nature, for example, H O is often generated from dioxygen reduction by oxygenases, but invariably there is co-production of catalase, which destroys the H O very efficiently without release of other active oxidants. Peroxidase enzymes do exist, of course, using H O itself to carry out organic oxida-
tions, but again these work naturally with small peroxide concentrations, and are not very robust tooxidation—supported or immobilized catalysts have a strictly limited life. In some cases, such asligninases, self-destruction is even a normal part of the mode of action. This drawback could conceiv- 2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
Cleaner industrial processes using hydrogen peroxide ably be overcome by using whole cell systems rather than isolated enzymes: such expertise is notwidespread in the chemical industry at present.
Secondly, the low concentration limit and high catalyst molecular weight mean that space yield is poor and recovery/ recycle of enzyme is awkward. Hence, peroxidases as such are not particularlyattractive as catalysts in industrial oxidation. This applies particularly to haem-based systems. Vana-dium and molybdenum enzymes are somewhat more robust, but also less active towards substrates ofindustrial interest (see Issues section). Peroxidases do have other applications in synthesis [78].
Of distinct interest, however, are hydrolase (lipase, esterase) enzymes—not for catalyzing H O
reactions directly but for forming more electrophilic intermediates, through acylation of H O (or “es- terification” of acids with H O to give peracids). These enzymes are much more robust, and one in particular, Candida Antarctica lipase, is outstandingly so, such that it has a long lifetime in immobilized
form (Novozym™ 435). A range of peracids has been generated this way, either from their acids or
from lower alkyl esters. Peracetic acid works reasonably well, but longer-chain analogs suit the enzyme
better, especially C and greater. Methyl oleate reacts in two stages, the intermediate peracid epoxidizing
itself to form 9,10-epoxystearic acid [80]. Furthermore, acid-sensitive substrates can be oxidized by apercarbonic acid intermediate generated from dialkyl (e.g., dimethyl) carbonates and H O ; after reac- tion, only alcohol and CO are left [81].
CONCLUSIONS
The author being, on the whole, an optimist, this section is likely to be a mixture of what is desirable andwhat is likely. The first point should serve to convince on this point: greater cooperation between thepractitioners of homogeneous and heterogeneous catalysis must be achieved. There is now a lot ofrigorous study of the behavior of molecules at active sites by the “molecular catalysis” camp. There alsoremain many naïve attempts to immobilize catalysts with little regard for the influence of the surface orsteric environment, including the apparent assumption that surface concentrations of reactants will besimilar to those in the bulk liquid medium. Heterogeneous catalysis expertise (mainly, of course, fromthe gas phase) has a bigger part to play than so far realized. Maybe the mutually beneficial progress incharacterization techniques can form the bridge between these two estranged disciplines.
To be more specific on the systems discussed, the basic belief is that industry will increasingly focus research, and particularly development, on fully inorganic systems. Hence, there will be littleimpact at the large scale from catalytic complexes with organic ligands, or from enzymes, even thoughthese will improve further in robustness. Some uses of supported organic complexes in fine chemicaloxidations, on the other hand, are quite possible.
The four main “battlegrounds”, upon which new territory may be claimed for catalytic technol- ogy using hydrogen peroxide, are believed to be as follows: Further progress is due on surface modification of large-pore materials to get Ti catalysts closer tothe performance of TS-1 for large molecules.
There is a good possibility of successful immobilization of interactive XM peroxo complexes (especially for M = W) to obtain Venturello activity in heterogeneous form. MTO will remainessentially a small-scale catalyst, owing to the organic content as noted above.
The potential for smectitic or layered catalytic materials will be more fully explored, as the fieldof zeolites and mesopores is increasingly crowded.
Polyoxometallate-supported catalytic species—containing redox metals such as Mn, Co, and V—will become much more important, as ways to optimize reactivity and to manage association/dissociation equilibria are mastered.
The story continues to unfold, and will test the accuracy of these predictions.
2000 IUPAC, Pure and Applied Chemistry 72, 1289–1304
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