Zeolite redox centers

Since the discovery in the early 80 s of the remarkable catalytic activity of Ti-modified silicalite-1 (TS-1) in the selective oxidation of organic substrates by dilute H2O2, the field of transition metal modified zeolites grew tremendously as shown in a number of recent reviews [156,235,236]. In addition to its hydrophobicity, the major role of the zeolite matrix is the stabilization of isolated redox centers. However, the limited accessibility of these sites precluded the use of large substrate molecules. The discovery of crystalline mesoporous silicate was immediately perceived as an ideal solution to these limitations. 

Zeolites are crystalline but versatile materials. They may be modified in many ways they can be tuned over a wide range of acidity and basicity, and of hydrophylicity and hydrophobicity, many cations can be introduced by ion exchange and isomorphous substitution is possible also allowing build-in of isolated redox centers (e.g. Ti) in the lattice. Moreover metal crystallites and metal complexes can be entrapped within the microporous environment. There is for instance much progress in enantioselective synthesis on chiral catalysts immobilized in microporous or mesoporous materials [16]. 

Both the Brpnsted acid sites as the copper sites inside the zeolite are important for the SCR reaction, the acid sides adsorb NH3 to form and the Cu is an essential redox center for the NHj-SCR reaction. To get an optimal NH3-SCR activity, the Cu exchange level should be well balanced, as both these functions are essential in the catalyst [20]. How the Cu exactly behaves during the SCR reaction is still under debate, but it is generally accepted that Cu should be in its ionic state. CuO clusters, which may appear in the zeolite, because of imperfect synthesis or hydrothermal deactivation, are more active in catalyzing NH3 oxidation at high temperatures, yielding a negative effect on the selectivity for NO in NH3-SCR [18,21],... 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

The Cr A and several other zeolites containing transition metal ions, which may exist in two or more valence states, were also found to be oxidation catalysts. One such system of note is the copper containing Type Y zeolite, the redox chemistry of which was studied in several recent investigations (2, 3.4, 5). These studies established the range of conditions at which copper exists in divalent, monovalent, or zerovalent state and in particular determined the reduction conditions in hydrogen and carbon monoxide atmospheres for a complete conversion of Cu Y to Cu Y but no further to Cu°. The Cu ions in type Y zeolite were reported to be specific adsorption centers for carbon monoxide ( 6), ethylene ( 7), and to catalyze the oxidation of CO (8). In the present work the Cu ions were also found to be specific adsorption centers for oxygen. 

Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties a uniform intra-crystalline microporosity that provides aceess to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. 

The singular porosity of MOFs allows for a significant redox conductivity that, in contrast with zeolites and other microporous aluminosilicates, can involve all units of the material. This is the case of Cu-- and ZrF -based MOFs with terephtalic acid in these materials, both metal centers and organic units are potentially electroactive in contact with suitable electrolytes. 

Although the mobility of MePc complexes should be strongly reduced in the intracrystalline faujasite supercages, truly site-isolated metal centers can be obtained this way. Moreover, Fc and Mn (salts or exchanged zeolites) which arc active for one-elcctron redox processes as in Fenton free radical H2O2 decomposition (see reaction),... 

The examination of zeolites modified with transition metals as catalysts of methanol conversion have revealed the close coimection of redox and acid-base functions of these catalytic systems. In faet, methyl alcohol can react by two pathways that are supposed to be rather independent. The acid centers of zeolites are commonly believed to be responsible for dehydration of methanol to dimethyl ether while the oxidative sites account for the formation... 

Catalytic N2O decomposition was investigated over FeZSM-5 prepared by a novel exframework method. This method comprises the introduction of Fe in the MFI framework, followed by calcination and steam treatment to extract the iron to extra-framework positions. The ex-framework method induces superior activity for direct N2O decomposition, compared to FeZSM-5 catalysts prepared by solid and aqueous ion-exchange methods. The extraordinary catalytic performance of ex-FeZSM-5 is attributed to the highly dispersed state of the Fe in the zeolite matrix and the Fe(III)/Fe(II) redox behaviour of a significant fraction of the iron centers in the catalyst. 

Here, we will first present briefly the general chemistry at work in deNOx catalysis. Then we wiU focus on the various specific nanometric issues in the domain. NOx decomposition mainly takes place on a metal center, as do most redox reactions in heterogeneous catalysis. We will therefore focus on how nanometric size influences the role of metal particles in this domain. It is very much linked to interaction with the oxide support, which wiU be the subject of the following part Zeolites are very specific oxide supports, controlling the distance between reactants and imposing chemical reactivity within the nanometer scale. Metal particles formed in zeoHtes have their size controlled between 0.5 and several dozens of nm, sometimes with Angstrom accuracy. We wiU next deal with three-way catalysts, mainly used for automotive deNOx. They involve very complex solids, where particle size is a key issue for activity. Finally, we wiU mention quickly new nanometric sohds like carbon nanotubes, which have appeared recently in deNOx catalysis. 

A conceptual model has been proposed (Fig. 53), in which the redox potential of the Fe + center was modified to suppress the irreversible conversion to the Fe +—O—Fe + /jl-oko complex in favor of the /u.-peroxo species Fe " "—O—O—Fe " ". It has been suggested that via this route the formation of the ferryl species, 0=Fe, would be facilitated. Four catalysts have been prepared on these principles iron perhaloporphyrins complexes Keggin structures with iron in the framework and with proximate iron centers and crystalline iron zeolite materials. 

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