Activities and stabilities of heterogeneous catalysts in selective liquid phase oxidations: recent developments

Applied Catalysis A: General 212 (2001) 175–187

Activities and stabilities of heterogeneous catalysts in selective liquid phase oxidations: recent developments I.W.C.E. Arends, R.A. Sheldon∗ Department of Biotechnology, Laboratory for Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Abstract Different strategies for the heterogenization of redox-active elements in solid matrices are reviewed. These include framework-substituted molecular sieves, amorphous mixed oxides by grafting or sol–gel methods, grafting or tethering to the inner walls of mesoporous molecular sieves, encapsulation by ship-in-a-bottle or other techniques and ion exchange in layered double hydroxides. The different approaches are illustrated by reference to recent developments involving a variety of metal catalysts — titanium, chromium, cobalt, manganese, iron, ruthenium, tungsten, molybdenum, vanadium and tantalum — in oxidations with O2 , H2 O2 and RO2 H as primary oxidants. Emphasis is placed on an evaluation of the stability of the various catalysts under reaction conditions, a conditio sine qua non for practical utility. Protocols for establishing heterogeneity are discussed. An analysis of experimental results leads to the conclusion that many of the systems described in the literature, particularly those involving oxometal species, are unstable towards leaching or the appropriate rigorous tests for heterogeneity have not been performed. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Selective oxidations; Heterogeneous catalysis; Zeolites; Redox molecular sieves

1. Introduction Catalytic oxidation in the liquid phase is widely used in bulk chemicals manufacture [1] and is becoming increasingly important in the synthesis of fine chemicals where traditional processes employing stoichiometric inorganic oxidants are under increasing environmental pressure [2–4]. Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes in combination with clean, inexpensive oxidants such as O2 , H2 O2, or RO2 H. However, heterogeneous catalysts have the advantage, compared to their homogeneous counterparts, of facile ∗ Corresponding author. Tel.: +31-15-2782675; fax: +31-15-2781415. E-mail address: [email protected] (R.A. Sheldon).

recovering and recycling. Moreover, site-isolation of active metal ions or complexes in inorganic matrices precludes their dimerization/oligomerization to less reactive ␮-oxo species and can, therefore, endow them with unique activities. An early example of the successful application of the site-isolation concept in liquid phase oxidation catalysis is the Ti(IV)/SiO2 catalyst (see later), commercialized by Shell in the 1970s for the production of propene oxide (reaction 1). (1) Another benchmark was the development of titanium silicalite (TS-1) by Enichem scientists in the mid-eighties (see later). The success of TS-1 as a catalyst for a variety of oxidations, including epoxidation,

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 5 5 - 3

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with 30% aqueous hydrogen peroxide led to frenetic activity world-wide on the synthesis of related heterogeneous catalysts for liquid-phase oxidations [5]. This review focuses on recent developments in this area. Emphasis is placed on a critical evaluation of the operational stability of these materials. In the final analysis interesting activities and selectivities are not enough; stability under operating conditions is a conditio sine qua non for industrial utility. A heterogeneous catalyst which simply releases its active species into solution, like Greek warriors from the Trojan horse, is likely to have limited practical utility. It is surprising, therefore, that many papers pay scant attention to this aspect.

2. Mechanisms of catalytic oxidations One-electron oxidants, e.g. Co(III), Mn(III), Ce(IV), Fe(III), Cu(II), etc. catalyze free radical autoxidation processes by promoting the decomposition of alkyl hydroperoxides into chain initiating alkoxy and alkyl peroxy radicals in one-electron transfer processes (reactions 2 and 3). Strictly speaking the metal ion acts as an initiator of free radical autoxidation, which proceeds further via reactions 4–6, rather than a catalyst. ROOH + MII → MIII (OH) + RO•

(2)

MIII OH + ROOH → MII + RO2 • + H2 O

(3)

RO• + R H → ROH + R•

(4)

R• + O2 → R O2 •

(5)







R O2 • + R H → R O2 H + R•

(6)

Metal ions which catalyze oxygen transfer reactions with H2 O2 or RO2 H can be divided into two types based on the active intermediate: a peroxometal or an oxometal complex [6] (Fig. 1). Peroxometal pathways usually involve early transition elements with d0 configuration, e.g. Mo(VI), W(VI), V(V), Ti(IV). Late or first row transition elements, e.g. Cr(VI), V(V), Mn(V), Ru(VI), Ru(VIII), Os(VIII), generally employ oxometal pathways. Some elements, e.g. vanadium, can employ oxometal or peroxometal pathways depending on the substrate. Reactions that typically involve peroxometal

Fig. 1. Peroxometal vs. oxometal pathways.

pathways are olefin epoxidation and heteroatom oxidations. Oxometal species, on the other hand, display a broader range of activities, including benzylic and allylic oxidations. An important difference is that peroxometal pathways do not involve any change in oxidation state of the metal, i.e. the metal acts as a Lewis acid and activity is not restricted to variable valence elements. An oxometal pathway, in contrast, involves two-electron redox reactions of the metal ion. Furthermore, it should be emphasized that most metals, which catalyze oxygen transfer processes, via peroxometal or oxometal pathways, are also capable of catalyzing one-electron transfer processes with peroxides (see before). Consequently, competition from free radical processes is often observed, to a greater or lesser extent, in oxygen transfer processes. When alkyl hydroperoxides are used as oxidants homolytic versus heterolytic processes can be distinguished by the use of suitable probe molecules [7,8]. We also note that immobilization of a redox-active element in a solid matrix will probably not influence the oxidation mechanism, e.g. one-electron oxidants such as Co(III) will still catalyze free radical processes when incorporated in the framework of a molecular sieve.

3. Types of heterogeneous catalysts Various strategies can be employed for immobilizing redox-active elements in a solid (inorganic) matrix (see Fig. 2). Metal ions can be isomorphously substituted in framework positions of molecular sieves, e.g. zeolites, silicalites, aluminophosphates (APOs), silico-aluminophosphates (SAPOs), via hydrothermal synthesis or post synthesis modification [5,9,10].

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Fig. 2. Schematic representation of strategies for heterogenization of metal centre (M).

A wide variety of redox molecular sieves have been synthesized and characterized and their catalytic properties investigated [5]. The range of substrate molecules that are amenable to catalysis by molecular sieve materials was considerably expanded by the discovery, in 1992, of mesoporous (alumino) silicates, such as MCM-41 [11]. Amorphous mixed oxides can be prepared by impregnation (grafting) of metal compounds onto the surface of, e.g. silica [12–14] or by the sol–gel method [9]. The latter is equivalent to the hydrothermal synthesis of molecular sieves (but without the template) and can afford much higher levels of incorporation than the grafting technique. For example, grafting techniques result in the incorporation of ca. 2% titanium in silica while titania–silica aerogels or xerogels typically contain up to 20% titanium [15]. Alternatively, metal complexes can be tethered to the surface of a solid, e.g. silica, via a spacer ligand. Similarly, coordination complexes or organometallic species can be grafted or tethered to the internal surface of mesoporous molecular sieves. Examples of the former include the synthesis of surface-grafted Ti(IV) [16] and oxomanganese [17] by reaction of MCM-41 with titanocene dichloride (Cp2 TiCl2 ) and Mn2 (CO)10 , respectively, followed by calcination.

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Examples of the latter include the tethering of chiral molybdenum complexes to the internal surface of mesoporous ultra-stable zeolite Y (USY) [18] and manganese triazacyclononane complexes to silica or MCM-41 [19]. Another approach is to encapsulate metal complexes in a solid matrix. For example, the so-called ship-in-a-bottle concept, which involves the entrapment of a bulky complex in a zeolite cage, has been widely used to immobilize metal complexes of phthalocyanines, bipyridyls, and Schiff’s base type ligands [20]. Metal complexes can also be heterogenized by encapsulation in polydimethylsiloxane membranes [21] or by attachment to oxidatively resistant organic polymers such as polybenzimidazole [22,23]. Finally, metal ions can be immobilized by cation exchange into zeolites or acidic clays and oxoanions such as molybdate and tungstate can be exchanged into hydrotalcite-like anionic clays (so-called layered double hydroxides, LDHs) [24,25]. A major disadvantage of the cation exchange approach is the mobility of the metal ion which manifests itself in its facile leaching into solution.

4. The question of leaching As noted above, to have real synthetic utility a heterogeneous catalyst should be stable towards leaching of the active metal into the liquid phase under operating conditions. This is true for all types of catalytic reactions but leaching is particularly a problem in oxidation catalysis owing to the strong complexing and solvolytic properties of oxidants (H2 O2 , RO2 H) and/or products (H2 O, ROH, RCO2 H, etc.). Leaching is generally a result of solvolysis of metal–oxygen bonds, through which the catalyst is attached to the support (e.g. SiO2 ), by such polar molecules. Generally speaking, one can distinguish three different scenarios for heterogeneous catalysts in the liquid phase. 1. The metal does not leach and the observed catalysis is truly heterogeneous. 2. The metal does leach but is not an active catalyst; the observed catalysis is (predominantly) heterogeneous.

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3. The metal leaches to form a (highly active) homogeneous catalyst; the observed catalysis is homogeneous in nature. For example, heterogeneous titanium-based epoxidation catalysts could fall into category 1 or 2 since titanium is known to be a mediocre homogeneous catalyst. It is generally accepted that Ti(IV)/SiO2 and TS-1 belong to category 1 but Ti-MCM-41, in oxidations with H2 O2 , clearly belongs to category 2 (see later). Chromium and vanadium-based catalysts, on the other hand, have generally been shown to belong to category 3, i.e. facile leaching occurs to generate active homogeneous catalysts. How does one distinguish between the various categories? As we have noted elsewhere [26] the definitive test for leaching is to filter the catalyst, at the reaction temperature, during the course of the reaction and allow the filtrate to react further. If no further reaction is observed this is strong evidence in support of heterogeneous catalysis. If the reaction mixture is allowed to cool to ambient temperature before filtration (which is common practice) readsorption of leached metal ions onto the solid support can occur or different oxidation states of the metal can be formed in solution. The common practice of claiming heterogeneity based on the observation that a catalyst can be recycled, in batch experiments, n times without loss of activity cannot be construed as rigorous proof as we discovered in the case of chromium-substituted molecular sieves [27]. If a very small amount of metal, e.g. 0.1%, is leached in each run, and is responsible for the observed catalysis, then the catalyst could be recycled ten or more times without any significant loss of activity. The leaching would become apparent in catalyst longevity experiments in continuous operation. Similarly, in cases where slow, continuous leaching occurs, categories 1 and 2 can only be distinguished in appropriate longevity tests.

5. Overview of recent developments 5.1. Titanium catalysts The silica-supported titania catalyst, consisting of ca. 2 wt.% TiO2 on silica, developed by Shell was the first truly heterogeneous epoxidation catalyst useful

for continuous operation in the liquid phase [28,29]. It forms the basis of the commercial process for the epoxidation of propene with ethylbenzene hydroperoxide. In contrast with soluble Ti(IV) compounds, which are rather mediocre catalysts, Ti(IV)/SiO2 exhibits relatively high activities and selectivities comparable to homogeneous molybdenum. It is now generally believed [30] that the titanium is attached to the silica surface by (at least) three silanoxy groups, consistent with the original proposal that TiCl4 with silica gel involves attachment to three Si–OH groups [31]. This can be considered as the main reason for its excellent stability. The superior catalytic activity of Ti(IV)/SiO2 was attributed [28] to both an increase in Lewis acidity of the Ti(IV), owing to electron withdrawal by silanoxy ligands and to site-isolation of discrete Ti(IV) centres on the silica surface preventing oligomerization to unreactive ␮-oxo species, which readily occurs with soluble Ti(IV) compounds. One property which Ti(IV)/SiO2 shares with the homogeneous catalysts is a marked sensitivity towards deactivation by strongly coordinating ligands, especially water [32]. For this reason Ti(IV)/SiO2 is an ineffective catalyst for epoxidations with aqueous hydrogen peroxide. Hence the discovery, by Enichem scientists [33–37] in the mid-eighties, of the remarkable activity of titanium silicalite-1 (TS-1) in, inter alia, selective epoxidations with 30% aqueous hydrogen peroxide under very mild conditions, constituted a milestone in oxidation catalysis. TS-1 contains Ti(IV) isomorphously substituted for silicon in the framework of silicalite-1 [38], a hydrophobic molecular sieve possessing a three-dimensional system of intersecting elliptical pores with diameters of 5.3×5.5 and 5.1×5.5 Å. TS-1 catalyzes the smooth epoxidation of relatively unreactive olefins, such as propene and even allyl chloride, with 60% aqueous H2 O2 at 40–50◦ C in methanol [39,40]. Its remarkable activity in (ep)oxidations with aqueous H2 O2 is attributed to site-isolation of Ti(IV) centers in the hydrophobic pores of silicalite which allows for the simultaneous adsorption of the oxidant and the hydrophobic substrate in the presence of water. The success of TS-1 stimulated the search for related materials [41]. Titanium silicalite-2, possessing the MEL structure was discovered by Ratnasamy and coworkers in 1990 [42] and exhibited similar properties to TS-1.

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A serious shortcoming of TS-1 (and TS-2), however, is its restriction to substrates with kinetic diameters < 5.5 Å. Examples of the incorporation of titanium in large pore molecular sieves are Ti-beta and Ti-MCM-41. For example, Corma and coworkers [43] incorporated Ti in the framework of zeolite beta (pore dimensions 7.6 × 6.4 Å). The resulting Ti-, Al-beta catalyzes epoxidation with aqueous H2 O2 but the resulting epoxides undergo acid-catalyzed ring opening [43–46]. Aluminium-free Ti-beta is also active under these conditions but, depending on the conditions, can still lead to ring-opening [47–49]. The stability of epoxides towards (Lewis acidic) zeolites can be improved by modification of the zeolite or by performing the reaction in acetonitrile (a Lewis base) as a solvent [46]. Overall the activity of Ti-beta, in epoxidations with aqueous hydrogen peroxide, is lower than of TS-1. On the other hand, contrary to TS-1, epoxidation with tert-butyl hydroperoxide (TBHP) is possible. Titanium was also incorporated into mesoporous silicas such as MCM-41 [50–52] and MCM-48 [53]. The resulting materials catalyze epoxidations with both H2 O2 and TBHP. However, they are not stable towards leaching under reaction conditions, particularly with H2 O2 [54,55]. Various stategies [56–58] have been employed to enhance the hydrophobicity and hence, the stability and overall performance

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of these materials as catalysts for epoxidations with H2 O2 , albeit with only moderate success. In an alternative approach Maschmeyer et al. [16] grafted Ti(IV) species to the internal surface of MCM-41, by reaction with (Cp)2 TiCl2 and subsequent calcination. This material is more active in epoxidations with TBHP than Ti-MCM-41 prepared by hydrothermal synthesis, because all of the Ti(IV) sites are at the surface [59]. However, it is still not active with aqueous hydrogen peroxide as the oxidant. An overview of the activities of the various titanium catalysts is given in Table 1. In yet another novel approach Ti(IV) silsesquioxanes were prepared and shown to be excellent homogeneous catalysts for epoxidations with TBHP [30]. A heterogeneous variant was prepared by adsorbing the Ti(IV) silsesquioxane in the pores of MCM-41 that had been silylated with Ph2 SiCl2 to passivate the external surface. The resulting material was claimed to be a stable recyclable catalyst for epoxidations with TBHP [60]. The development of mesoporous silicas renewed the interest in the use of silica gels and supported oxides. Recently, Mayoral and coworkers prepared Ti(IV)/SiO2 catalysts by impregnating silica with Ti(O–i-Pr)4 followed by heating at 140◦ C in vacuo [61–63]. Under these conditions the catalyst retained isopropoxy groups. This material gave smooth

Table 1 Comparison of the different catalysts for cyclohexene epoxidation with hydroperoxides Catalyst

S/Ca

Alkyl hydroperoxides, R = tert-butyl 750 Ti → MCM-41e Ti ↑ MCM-41f 173 Ti-beta 202 Ti-aerogel 308 Ti–Si mixed oxide 596 Mesoporous, Ti–Si mixed oxide 1500 Ti ↑ SiO2 f 100 Hydrogen peroxide, R = H Ti-beta Hydrophobic Ti–Si mixed oxide Hydrophobic Ti–Si mixed oxide a

1470 640 1220

RO2 H/Cb

187 207 50 52 149 373 100 740 1280 60

S/RO2 Hc

4 0.83 4 5.9 4 4 1 2 0.5 20

Converted alkene (%)

TOF (h−1 )

3 50 8.5 17 24 19 66

5 87 3.4 35 29 570 3.7

26 16 11

130 20 –

Substrate to catalyst ratio. Hydroperoxide to catalyst ratio. c Alkene to hydroperoxide ratio. d Selectivity towards epoxide, based on converted alkene. e The right arrow (→) refers to titanium, substituted in the framework of MCM-41. f The up arrow (↑) refers to titanium grafted on the surface of MCM-41 or silica. b

Selectivity (%)d

Reference

[61–63] [16] [61–63] [15] [61–63] [56–58] [32] 86 66 Low

[48] [66] [67]

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conversions with a variety of olefins, using TBHP as the oxidant. The activity of this material could be tuned by exchanging the surface isopropoxy groups with vic-diols such as ethane diol [64]. The best catalyst was obtained by treating the original Si–Ti(O–i-Pr) with tartaric acid. This catalyst was active in epoxidations with 30% H2 O2 at 80◦ C albeit with reactive olefins (cyclooctene and cyclohexene). Moreover, cyclohexene afforded mixtures of epoxide, cyclohexane-1,2-diol and allylic oxidation products. However, experiments involving filtration under the reaction conditions show that with Si–Ti (tartaric acid) the reaction takes place in both the homogeneous and the heterogeneous phase, while the original Si–Ti(O–i-Pr) catalyst was stable. A possible explanation for these observations is that in the latter material titanium is attached via three silanoxy groups, which, as was noted above, is a prerequisite for the stability of titania on silica, while after exchange with tartaric acid only two silanoxy anchors remain. Materials prepared by silylation of the surface hydroxyl groups in combination with a lower titanium loading, followed by exchange with tartaric acid, displayed a better stability in the reaction. However, the thus prepared catalysts were deactivated by the action of hydrogen peroxide. An alternative approach to new epoxidation catalysts involves the synthesis of amorphous mixed titania–silica oxides, containing up to 20 wt.% TiO2 , by the sol–gel technique [65]. These materials resemble the Shell catalyst, i.e. they are active with TBHP in anhydrous media but not with H2 O2 . An advantage with respect to the Shell catalyst is the much higher titanium loading which translates to a higher catalyst productivity. In an attempt to more closely mimic TS-1, hydrophobic mixed titania–silica oxides were prepared containing Si–C bonds by the sol–gel method [66–68] or subsequent trimethylsilylation of the surface [69]. However, as yet this has not yet resulted in catalysts that have sufficient activity and scope with aqueous hydrogen peroxide. In the field of asymmetric epoxidation, a heterogeneous version of the Sharpless [70] epoxidation of allylic alcohols has been described [71,72]. A complex formed between Ti(O–i-Pr)4 and a linear poly-ltartrate, in combination with zeolite 4A as a drying agent, catalyzed the enantioselective epoxidation of trans-2-hexen-1-ol with tert-butyl hydroperoxide

(92% yield and 79% ee). However, not all of the titanium remained complexed to the polymer on recycling and was uncomplexed or complexed with tartrate monomer/oligomer in solution. 5.2. Chromium catalysts In previous papers we already commented on the problems regarding the stability of chromium redox molecular sieves as catalysts in oxidation reactions (notably CrAPO-5, CrAPO-11 and CrS-1 but other framework-substituted chromium redox molecular sieves gave the same results) [26,27]. The alkyl hydroperoxides that are used as oxidants in the chromium catalyzed oxidation of alkylaromatics, led to breakdown of the Si–O–Cr or Al–O–Cr bonds in the molecular sieve and thus to small amounts of chromium in solution. Cr6+ in solution is an extremely active catalyst and concentrations as low as 1–2 ppm chromium may catalyze the reaction efficiently. Similarly, Cr(III) was incorporated in the framework of SAPO-37 [73]. This material was tested as catalyst in the liquid-phase oxidation of benzene using hydrogen peroxide and the oxidation of cyclohexane using tert-butylhydroperoxide. In spite of the framework stability under the reaction conditions, leaching of small quantities of chromium occurs and the observed catalytic reaction is mainly due to the chromium ions in solution. To our knowledge up till now no heterogeneous chromium catalysts have been reported, which have been unequivocally shown to be stable under liquid-phase oxidation conditions by performing the appropriate rigorous tests for leaching. Recently, a silica supported Schiff’s base complex with chromium was reported and applied to the aerobic oxidation of alkyl aromatics [74]. This reaction is a typical example of a free radical autoxidation, and is characterized by high background activity and induction periods of several hours. The authors performed a leaching test by filtering the catalyst hot after one hour, and then restarted the reaction. Because the conversion of ethylbenzene to acetophenone observed in this case was similar to that observed without catalyst, they concluded that this silica supported chromium acts as a truly heterogeneous catalyst. However, because of the uncertainty of induction periods (if the reaction mixture cools down chromium can be reduced to Cr3+ , which in low concentrations gives induction periods

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of up to several hours), the action of heterogenized chromium ions needs to be addressed in more than one single experiment. A similar material (chromium attached to silica via complexation with a carboxylic acid derivatized spacer) published by the same authors, was not subjected to tests of heterogeneity, but is for the same reason subject to reasonable doubt [75]. 5.3. Cobalt catalysts The most studied, framework-substituted heterogeneous cobalt catalyst is undoubtedly Co in aluminophosphates [76,77]. While the isomorphous substitution of cobalt into several APO structures is commonly accepted, there is a controversy regarding the redox properties of cobalt-containing APOs involving Co(II) and Co(III) valence changes [76–78]. Apparently the ease of interconversion of Co(II) and Co(III) ions in the framework site is critically dependent upon the particular APO framework within which the cobalt is accommodated [79]. Cobalt catalysts are mainly used for the autoxidation of alkanes and alkylaromatics. Once subjected to liquid-phase oxidation conditions, leaching can be observed and in this respect the composition of the reaction mixture is critical. For CoAPO-5 it was shown that the molecular sieve dissolves during the oxidation of organic compounds in a strongly alkaline medium [80]. Similarly the use of acetic acid as a solvent results in leaching of cobalt from the CoAPO-5 framework [81,82]. In a careful study of the CoAPO-5 catalyzed aerobic oxidation of cyclohexane to adipic acid, it was revealed that the activity was entirely due to low concentrations of Co(II) leached by the acetic acid solvent [82]. On the other hand, in a strong apolar solvent (e.g. cyclohexane) only an extremely low concentration of Co could be detected in solution, and the autoxidation activity of CoAPO-5 could not be attributed to the dissolved cobalt [81]. However, this is only valid if the conversion of cyclohexane remains below 10%. At higher conversion acids, such as adipic acid, are formed in secondary reactions, and these cause Co leaching via complex formation. Furthermore, leaching tests are complicated by the “catalyst–inhibitor” phenomenon [1]. That means that a metal is a catalyst at low concentrations, but can be an inhibitor at higher concentrations. Separate studies of catalysis by (homogeneous)

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Co(II) showed that a sharp maximum in the catalytic activity was encountered at [Co(II)] = 0.17 mM [82]. Thomas et al. [83,84] obtained good results with CoAPO-18 (pore diameter 3.8 Å), as a catalyst for the oxidation of alkanes. In the case of linear alkanes such as n-hexane, regioselective terminal oxidation to the corresponding n-alkanoic acids was observed. When a mixture of cyclohexane (which is too large to enter the sieve) and n-hexane was oxidized with O2 in the presence of CoAPO-18 [85], no oxidation of cyclohexane occurred while at the same time the conversion of n-hexane was substantial. This was taken as proof of the heterogeneity of the reaction. In short, as long as no acids (acetic acid as solvent or formed as a product) or strong alkaline media are present, cobalt ions remain in framework positions. Therefore, in the case of cobalt, contrary to the chromium catalysts, under certain conditions heterogeneous oxidation can be observed. Recently, a heterogeneous cobalt catalyst was prepared by immobilizing a trimeric cobalt complex on the surface of MCM-41 [86]. This material gave good activity for the oxidation of cyclohexane to cyclohexanone and cyclohexanol (turnover frequency of 216 after 4 days) using the substrate as solvent. A filtration test revealed no further activity for the filtrate which led the authors to conclude that this material is stable under the reaction conditions, presumably with the caveat, at low conversions. 5.4. Manganese, iron and ruthenium catalysts Examples of framework-substituted manganese and iron molecular sieves for use in liquid phase oxidations have been reported [5]. In general the stability of these materials is very limited [26]. Recently Thomas et al. reported MnAPO-18 as a stable catalyst for the autoxidation of cyclohexane, when using the substrate as a solvent [85]. Similar to the stability of CoAPO-18 (see before), the use of acetic acid results in leaching of manganese from the framework, as was the case with manganese-substituted zeolites [87]. A successful example of the selective oxidation of benzene to phenol with iron-zeolites, the FeZSM-5, was reported [88]. However this reaction proceeds in the gas phase, at temperatures of 350–430◦ C using N2 O as the oxidant, where obviously leaching is non-existent.

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Fig. 3. Covalent attachment of Mn-triazacyclononanes to functionalized silica.

(7) In the nineties many publications appeared on the immobilization of metal complexes in zeolite cages. Encapsulation was expected to greatly enhance the catalyst stability. A successful demonstration of the principle is a manganese-bis(bipyridyl) complex [89], which was encapsulated within the cages of zeolite Y. The resulting material catalyzed the epoxidation of cyclohexene with hydrogen peroxide (contrary to the corresponding homogeneous complex, which only displayed catalase activity with hydrogen peroxide). Encapsulated phthalocyanine complexes of iron [90] and ruthenium (see for the encapsulation of perfluorinated ruthenium phthalocyanines [91,92]) have also been synthesized and shown to catalyze oxidations with alkyl hydroperoxides. Although these materials seem to be stable under oxidative conditions, their main disadvantage is the relatively low loading of metal in the molecular sieve and the limited accessibility. The resulting low activity makes these so-called “ship-in-a-bottle” materials less suitable for industrially viable catalysis. In the case of manganese, various approaches have been used for heterogenizing homogeneous complexes. For example, an attractive catalyst was designed by tethering a Mn-1,4,7-trimethyl-1,4,7-triazocyclononane (tmtacn) complex (which is active in epoxidations with aqueous hydrogen peroxide) to the surface of MCM-41 or silica [19]. This material displayed reasonable activities in epoxidation reactions with hydrogen peroxide (Fig. 3). It is also possible to achieve heterogenization by membrane inclusion. In this way the chiral Jacobsen catalyst could be heterogenized [93]. However in order to prevent this

material from leaching, it was necessary to limit the swelling of the polymer by a careful choice of solvent. Alternatively a less soluble variant of the chiral salen ligand could be used for membrane inclusion. Mn(III) Schiff bases, in which the ligand contained a secondary amine function, were grafted onto the surface of mesoporous silica [94]. The activities and stabilities of the catalysts were investigated in the epoxidation of styrene [95]. The reaction carried out using sodium hypochlorite led to a collapse of the mineral mesoporous system due to the hydrolysis of silica under the basic conditions used (pH 11.3). As would be expected for these catalysts, hydrogen peroxide was not active as the oxidant. Recently, a hydrotalcite containing both Ru(III) and Co(II) ions was prepared [96]. This hydrotalcite consists of cationic brucite layers (in which the metal ratio of Ru:Co:Al was 0.3:3.0:1.0) separated by an anionic CO3 layer. This material was very active in the aerobic oxidation of benzylic, allylic, and secondary aliphatic alcohols to the corresponding aldehydes and ketones under mild conditions (60◦ C). The analogous ruthenium talcite without cobalt was considerably less active (aliphatic alcohols could not be oxidized). Although no leaching tests were reported, this reaction would appear to be truly heterogeneous, because the corresponding metal ions in solution are not expected to display this activity. Nonetheless, one would feel more comfortable with this conclusion if appropriate tests were performed. Another heterogeneous ruthenium catalyst, which displayed a remarkable activity for primary benzylic and allylic alcohols, is a triethylammonium perruthenate species tethered to the internal surface of MCM-41 [97]. This material could be recycled 12 times without loss of activity and filtration experiments were performed which seemed to indicate that the reaction is heterogeneous in nature (the corresponding homogeneous perruthenate is active in solution as well). The same perruthenate species when

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Fig. 4. Catalytic cycle in the bromide–tungstate–LDH catalyzed epoxidation of 1-methyl-1-cyclohexene.

bound to a polystyrene bead [98], led to a material which could not be recycled [97]. This was attributed to oxidative degradation of the polystyrene support. 5.5. Tungsten catalysts Recently, a hydrotalcite-like layered double hydroxide (MgAl or NiAl) exchanged with tungstate, WO4 2− LDH, was reported which displayed excellent activities for mild oxidative bromination reactions with bromide / H2 O2 at neutral or mildly basic pH [99]. This material mimics the action of the enzyme vanadium bromoperoxidase, which catalyzes the oxidative bromination of organic substrates in vivo. For the bromination of monochlorodimedone, a turnover frequency of 71 h−1 was observed. The activity of WO4 2− was more than 100 times higher when it was immobilized in the LDH compared to that observed in solution. This fact together with the observation that the recycled catalysts displayed the same activity, suggests that the oxidations are truly heterogeneous. Because the reaction is performed in water (pH 7–8) it can be used for the epoxidation of olefins via (base-mediated) ring closure which regenerates bromide ion (see Fig. 4). Examples are the epoxidation of 1-methyl-1-cyclohexene and the selective production of the 6,7-epoxide of linalool. The same materials are also active in the direct epoxidation of allylic alcohols with hydrogen peroxide [100]. In this case the best results were obtained when the active site was rendered more hydrophobic through exchange with p-toluenesulphonic acid. 5.6. Molybdenum catalysts Very few examples of active heterogeneous molybdenum catalysts are known. Molybdenum cannot

be incorporated in the tetrahedral positions of the framework of molecular sieves. For example, a recently reported example of MoS-1 [101], was shown by us to be completely erroneous [102]. Similar to the tungsten LDHs (see before), polyperoxomolybdates can be heterogenized on a layered double hydroxide [103]. This material generates singlet oxygen from H2 O2 and therefore performs oxygenations of (di)olefins with particular selectivities. Mo(VI) complexes of thermally and oxidatively stable polybenzamidazoles were shown to be recyclable catalysts for epoxidations with tert-butyl hydroperoxide although stability towards slow leaching of molybdenum was not rigorously demonstrated and the observed activity could be due to low concentrations of soluble molybdenum [104] (see Fig. 5). 5.7. Vanadium catalysts Many examples of isomorphous substitution of vanadium in the framework of e.g. APOs and silicalites have been reported [5]. However, under liquid phase oxidation conditions, a (small) fraction of the vanadium is leached from the lattice, leading to a homogeneous reaction which dominates the heterogeneous reaction [26,105]. Vanadium incorporated in mesoporous silicas and vanadium xerogels, especially in combination with hydrogen peroxide, gives an extremely unstable material [106–108]. Vanadium “ship-in-a-bottle” catalysts, whose stability does not depend on the V–O–Si bonds in the framework and, therefore, could be expected to exhibit greater stability, have also been shown to be susceptible to leaching [102,109–111]. Leaching was similarly observed with vanadium supported on silica catalyst [112] and vanadomolybdophosphate polyoxometalate, H5 PV2 Mo10 O40 , supported on mesoporous MCM-41

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Fig. 5. Proposed structures for Mo complexes supported on polybenzimidazole (PBI) [104].

[113]. In the latter example hydrocarbon oxidation was performed using aldehyde as a stoichiometric oxidant in combination with oxygen. Leaching was shown to be dependent on the aldehyde concentration in solution, which could possibly be the result of formed acid (stoichiometric product of aldehyde) in solution. 5.8. Other examples Recently, Basset and coworkers developed silica grafted tantalum ethoxides, which, in the presence of tartrate esters, were effective as heterogeneous catalysts for enantioselective epoxidation of allylic alcohols with tert-butyl hydroperoxide [114]. For example, trans-2-hexen-1-ol gave the corresponding epoxide in 85–94% ee. Compared to the analogous asymmetric Sharpless catalyst, pentavalent tantalum, is better able to coordinate to two tartrate oxygens, allyl alkoxide

Fig. 6. Tantalum–tartaric acid catalyzed asymmetric epoxidation of allylic alcohols.

and tert-butylperoxo group in addition to being bonded to the surface (see Fig. 6).

6. Conclusions and prospects In the expectation that TS-1 was the progenitor of a broad range of redox molecular sieves, with novel activities, much attention has been devoted to the development of such materials. The initial euphoria was subsequently tempered by the realization that many of these materials are not stable towards leaching under oxidizing conditions. Highly polar, protic molecules that are present as the oxidant (H2 O2 , RO2 H) or are inevitably formed in the reaction (H2 O) can leach the metal by solvolysis of the metal–oxygen bonds that anchor it to the surface. It is generally believed that surface attachment through three (metal–oxygen) bonds is a prerequisite for good stability. Another salient feature, emerging from an analysis of experimental data presented in the literature, appears to be that when the incorporated element is present as an oxometal species e.g. chromyl, vanadyl, molybdenyl, it is more susceptible to leaching. A possible explanation is that the attack of protic molecules at oxometal centres produces dramatic changes in coordination geometries and bond angles, leading to less stable coordination environments around the metal. Variable valence metals, e.g. cobalt, manganese, iron, that involve one-electron redox processes and

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free radical autoxidation pathways, appear to be more stable to solvolysis. This only applies to oxidations carried to low conversions (< 10%), however. At higher conversions the metal ion is susceptible to leaching by the action of strongly coordinating (corrosive) secondary oxidation products, notably carboxylic acids. By the same token, the use of acetic acid as a solvent leads to rapid leaching of the metal ion. In short, the quest for heterogeneous catalysts, for liquid phase oxidations, that have unique activities and/or selectivities and are operationally stable, continues.

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