Molecular Heterogeneous Catalysis, Wiley (2006), 352729662X

186 Chapter 4

acetonitrile appears to be product inhibited. The inhibition is stronger for the protonic zeolite system where the acetamide adsorption is stronger. The acetamide desorption, however, can be assisted by a concerted adsorption step with the reactant nitrile. The overall thermodynamics for product desorption over Zn2+ is exothermic. The analogous reaction, however, still remains endothermic for the protonic case. Zn2+ is preferred for this reaction because there is no product inhibition.

Two of the key reaction steps are the cleavage of H2O (see Fig. 4.23a) to produce an OH− intermediate that will attach to the C atom of the nitrile, and the subsequent proton transfer from C–OH to form the first NH bond (se Fig. 4.23b). Figures 4.23 compares the activation energies for the activation of water to produce OH− in both the absence and the presence of coadsorbed H2O.

Figure 4.23a. Comparison of the e ect of coadsorbed water on the generation of ZnOH+ and the zeolitic H+ by dissociation of water H2 O.[26].

Figure 4.23b. The most di cult step in initial hydrolysis is the proton transfer within the intermediate (iminol) to form the keto group. This reaction step is made more facile by synergetic e ect of a coadsorbed water molecule, which catalyzes the proton transfer from the OH to the NH group[26].

Figure 4.23b illustrates that the proton migration from the COH to form NH is assisted by coadsorbed water, which provides a low-energy path for proton transfer. The addition of a second water molecule here significantly lowers the activation barrier. Water directly participates in the transition state, providing a low-energy conduit for proton transfer by the rearrangement of the proton oxygen bonds around the water molecules. Proton transfer paths that proceed via consecutive H2O proton bond formation and breaking reactions are well known in bulk water, aqueous and hydroxylated metal surfaces (see Chapter 6) and in enzyme catalysis (see Chapter 7).

Shape-Selective Microporous Catalysts, the Zeolites 187

The presence of coadsorbed water enhances this proton transfer path by providing a more optimal transition-state structure that does not require the dramatic distortion of metal–adsorbate bond angles. In addition, water stabilizes charge transfer, thus lowering the activation energies.

The promotional e ect of protic polar molecules for proton transfer is very general. As an example, we mention the influence of methanol on the epoxidation of alkenes by Ti substituted in the zeolite lattice (see Scheme 4.1). The reaction is promoted by methanol, because it stabilizes the reactant structure and provides for a direct proton transfer path (see also Chapter 8 and ref. [27]). Alcohol or water formation restores the catalyst. Coadsorbed methanol assists proton transfer from the zeolite to the peroxide to produce the alcohol (ROH). The proton from (TiOHSi) is transferred to methanol and the proton from methanol assists the cleavage of the peroxide O–O bond.

Scheme 4.1 Enhancement of the epoxidation activity of framework Ti by coadsorbed methanol. The methanol proton is transferred to form ROH (schematic).

4.3 Redox Catalysis

Redox reactions can be catalyzed by reducible cations substituted into the framework of zeolitic systems as well as polymorphic AlPO4 systems or by cations not located in the framework but in the micropores. In Chapter 8 we will discuss more extensively catalysis by TixSi(1−x)O2 systems using peroxides. Here we will initiate the discussion on redox catalysis with CoxAl(1−x)PO4 oxidation catalysts where reducible ions such as Co3+ substitute for Al3+. Catalytic oxidation carried out with oxygen provides an opportunity to discuss radical-type chemistry. A second system that we will discuss is photochemical oxidation induced by the strong electrostatic field of ion-exchanged cations. We will subsequently discuss catalysis by Fe3+ and Fe2+ ion exchanged zeolites with comparisons to Zn2+ systems and the important role of the corresponding oxycation.

For the iron system we first discuss N2O decomposition and then describe the selective oxidation with N2O to produce benzene from phenol. The N2O decomposition reaction will be an example that illustrates additional complexity of catalytic systems, with selforganizing features.

4.3.1 Selective Oxidation of Alkanes Using the Reducible MxAl1−xPO4 Zeolitic Polymorphs

We will follow closely the analysis given by Labinger[28]. The selective oxidation of cyclic and linear alkanes with O2 over reducible MxAl1−xPO4 catalytic materials has been reported under mild conditions. Dugal et al.[29] designed Coand Mn-containing alu-

188 Chapter 4

minophosphates, where the Co or Mn substitute for Al framework positions, with microporous structures similar to those of zeolites. They discovered that the catalysts with the smallest pore diameters show the highest selectivities towards oxidation of the terminal carbon atoms (see Table 4.3).

In radical chain oxidation reactions, the relative rates of the termination steps differentiate oxidation steps and, hence, a ect the selectivity. The termination of tertiary peroxides is much faster than that of primary peroxides. The constraints of the zeolite micropore dimensions limit the geometry of the bimolecular encounter of alkane and oxyradical.

In zeolites with small dimensions, the initial interaction will involve the terminal methyl group of the alkane. The main products that form are the diacids. Radical-chain autoxidation proceeds by a sequence of initiation, chain propagation and chain termination steps. The initiation steps involve the formation of an OOH (OOH •) and a hydrocarbon free radical (R •). The formation of the reactive ROO • species occurs by reaction of the alkyl radical with O2. A reaction chain propagation step is

ROO • + RH −→ ROOH + R •

A chain termination step is

2R(H)OO • −→ ROH + (R–H)=O + O2

The metals participate in redox steps such as

Shape-Selective Microporous Catalysts, the Zeolites 189

In the zeolitic micropore, chain termination will also be suppressed because of the small likelihood that two peroxy radicals are present in the same small cavity. This favors the chain propagation reaction that depends on the CH bond strength, which in turn favors the primary oxidation reactions. While the primary carbenium ions are high in energy as compared with secondary or tertiary carbenium ions, the opposite order of stability is found for the alkyl radicals! Interestingly, oxidation reactions that take place in enzymes, which have been proposed to proceed through radical intermediates, are thought to be selective because the reactions are constrained by rebound within the enzyme cavity (see Chapter 7, page 328).

Radical reactions can be initiated also by radical centers generated in zeolites by heat treatment. Brønsted acidic sites, on the Si–OH–Al sites can be converted to highly reactive Lewis acid sites by high temperature with the elimination of water. The nature of these sites is still a matter of debate (see K¨uhl[31]). With hydrocarbons ESR-active radical cations are generated from such sites, which are part of catalytic reaction cycles. Such radicals may play a role in coke deposition and have been proposed also to play a role in catalytic cracking, Orchilles[32] and Corma[33] have studied the catalytic oxidation of hydrocarbons by such systems in detail, and have sustained catalysis over more than 4000 cycles. Dehydroxylation of the Si–OH–Al site creates three-fold coordinated Si and Al as well as an Si–O–Al center. In the dehydroxylation process also Lewis acidic AlO+ sites may have been generated that adsorb to the negatively charged Si–O–Al sites. Interaction with 2,5-dimethylhexa-2,4-diene (DMHD) produces ESR signals that can be readily followed. Leu and Rodriner[34] concluded the following reaction sequence, in which single-electron transfer sites (SETS) are regenerated:

DMHD + SETS −→ DMHD+ + SETS−

SETS− + O2 −→ SETS + O2 −

DMHD+ + O2 − −→ reaction products

Reaction products have not been analyzed, because product molecules remained adsorbed in the zeolite. The chemistry of this dark oxidation reaction is related to the photochemical reaction steps discussed above.

4.3.2 Photo Catalytic Oxidation

Cations such as Na+, Ba2+ or Ca2+ ion-exchanged into zeolites have been shown by Blatter et al.[30 to play an important role in the selective photo-oxidation of alkenes and alkanes. We learned earlier in Section 4.2.3 that the electrostatic field-induced polarization of an adsorbed molecule changes the adsorption intensity of vibrational transitions in the infrared spectroscopic regime. When an organic molecule and O2 adsorb on the cations, the energy of electron transfer between the organic molecule and the oxygen molecule is lowered, with important consequences for oxidation catalysis initiated by this electron excitation event. In the gas or liquid phase, charge transfer between O2 and the hydrocarbon to give O2 − and a positively charged hydrocarbon occurs via an electron excitation induced by UV or visible light. Subsequent oxidation steps occur through radical chain reaction pathways that result from OOH and R. They tend to have low selectivity.

Longer wavelength visible light can be used instead of UV photons needed in the gas phase. This has the important advantage that the radical-generating reactions which compete with the desired oxidation radical chain reaction are now suppressed. For example,

190 Chapter 4

light with λ 600 nm will induce a selective oxidation reaction chain between O2 and toluene towards benzaldehyde, with no consecutive oxidation of benzaldehyde . The lower energy photons are not able to overcome the higher ionization potential of benzaldehyde (9.5 eV) compared with 8.8 eV for toluene, necessary for electron transfer towards oxygen.

Reactions are proposed to proceed through radical chain reactions and intermediate formation of the corresponding hydroperoxides that have been trapped at low temperature. These hydroperoxides can be produced with high selectivity because of the constraints on the recombination between molecules or radicals from the cavity. Thirdly, dehydration of peroxides occurs readily in the ionic zeolite environment via heterolytic mechanisms leading to carbonyl products without side reactions. The use of low-energy photons and a low-temperature environment also precludes homolysis of the peroxide bond, which minimizes non-selective gas-phase oxidation reactions. A representative reaction scheme for propane is shown in Fig. 4.24. The origin of the low-energy electron transfer reaction through cation interactions appears to be only partially the direct consequence of the high electric fields near the cations (order of magnitude is 0.6 ˚A−1), but can be considered to be the result of a confinement e ect[24b] as has been found theoretically for dimethylbutene oxidation. Reaction only occurs when cations with a specific size are properly located with respect to each other. One of the cations adsorbs O2 and the other one adsorbs the alkene. Their heats of adsoption have to be comparable.

Light-activated charge transfer takes place with low energy between O2 and the alkene oriented by interaction with the reaction such that there is overlap between the respective HOMO and LUMO orbitals. Adsorption on the cations overcomes the repulsive interaction between molecules when they approach so close that van der Waals radii overlap, with the result that a (photo) chemical reaction occurs. This can be considered again as an example of pre-transition state stabilization, that was discussed earlier in Section 4.2.1.

Figure 4.24. Proposed mechanism for photo-oxidation of alkanes.

4.3.3 The N2O Decomposition Reaction; Self-Organization in Zeolite Catalysis

We will initially examine N2O decomposition over single cationic metal atom centers and then subsequently continue the discussion on N2O decomposition over dimer oxycation species. The reaction energy diagrams for N2O decomposition over single Fe3+, Co3+

and Rh3+ centers established from ab initio density functional theory calculations are presented in Fig. 4.25 a and b[35]. The initial adsorption of N2O appears to be strongest

on Co3+, but the metal–oxygen bond generated as the result of N2O decomposition is strongest for [FeO]3+. Therefore, the reaction with the second N2O molecule to form O2

192 Chapter 4

is most di cult on the Fe3+ center but easier on Rh3+ or Co3+ centers to which atomic oxygen is more weakly bound. The small angle of N2O in the transition state (see Fig. 4.25b) implies that there is a small amount of electron donation towards N2O (remember that N2O− is isoelectronic with NO2). Figure 4.26 shows the [HOFe3+–O–Fe3+–OH]2+ dimer iron oxyhydroxy cation in contact with the negatively charged α-site of ZSM-5. Such dimers have been proposed as dominant species for N2O decomposition in ironexchanged zeolites. The hydroxylated dimer cation is thermodynamically stable for N2O decomposition in the presence of gas-phase water[36].

Figure 4.27. The intermediate structures and energies in the N2O decomposition reaction of binuclear iron(III+)oxydehydroxide[36]. Energies are reported in kJ/mol. The energies reported all refer to the reaction direction from the lower numbers to the higher number. For example, ∆E(I − > V ) =+23 kJ/mol.

Figure 4.28. N2 O decomposition over Fe/ZSM-5 shows oscillating behavior in the presence of water[37].

The energies for the N2O decomposition reaction on this iron-dimer cluster are shown in Fig. 4.27.

The N2O decomposition reaction is especially interesting because under particular conditions the reaction can be induced to oscillate (see Fig. 4.28[37]). If non-isothermal e ects

can be excluded, this implies the presence of an auto-catalytic elementary reaction step in the overall catalytic reaction cycle (see Chapter 8). In this case, the auto-catalysis results from N2O decomposition catalyzed by both mono-center and bi-center iron complexes.

There is ample experimental evidence for the presence of partial hydroxylated monomerand dimer-iron complexes in zeolites that decompose N2O. At high temperatures N2O

easily decomposes on a single-center Fe3+(OH)− cationic complex to form N2 and the Fe3+=O complex

N2O + Fe3+(OH)− −→ (OH)−Fe3+=O + N2

The formation of O2 by a consecutive reaction of this oxidized center with N2O requires a high activation energy (see Fig. 4.25a).

As we can deduce from Fig. 4.27, the recombination of two oxygen atoms on the bi-center iron complex is easy. The oscillatory time-dependent behavior of the overall

The overall result of reaction (d) and (e) is that one (OH)−Fe3+ species generates, in an inorganic reaction with (OH)−Fe3+=O, two (OH)−Fe3+ intermediates. Such overall stoichiometry defines an auto-catalytic reaction. An important conclusion from this analysis is that the catalytically reactive phase only establishes itself during the course of reaction. The catalytic system is therefore considered dynamic. Monomers and dimers are formed and disappear in reactions with N2O and desorption of O2. The dynamic patterns arise when these events are synchronized. This is mathematically similar to the Turing patterns discussed in Chapter 8. The decomposition reaction is the driving force for the self-organization of the inorganic system. As a corollary, the inorganic chemistry of the reactive phase cannot be established independently of the catalytic reaction.

Oscillating phenomena have also been observed for N2O decomposition by Cu-exchang- ed ZSM-5 zeolites[36]. A dynamic state consisting of monomeric Cu+ and dimeric

Cu2+OCu2+ is also proposed here.

Heyden et al.[39] suggested that hydrated and dehydrated monomolecular iron sites in Fe-ZSM-5 are responsible for N2O decomposition. They proposed that Z−[FeO]+ is a key intermediate. Furthermore, water strongly adsorbs to give Z−Fe(OH)2 +1. This deactivates the Z−[FeO]+ site. The activation energy for N2O decomposition in the presence of water increases steeply compared with the anhydrous situation, because water has to desorb from Z−Fe(OH)2 +1 in order for N2O reduction to occur. Hydration and subsequent dehydration of the oxy-iron complex may provide an alternative explanation for the oscillatory reaction found by El-Malki et al. shown in Fig. 4.28. If the reaction is not isothermal, the temperature fluctuations arising from the exothermic N2O decomposition reaction may lead to fluctuation in the water adsorption. This may provide an alternative explanation of the oscillatory kinetic behavior in the Fe3+-ZSM-5 system.

4.3.4 Oxidation of Benzene by N2O; the Panov Reaction

The catalytic oxidation of benzene to phenol in iron-containing zeolites is known as the Panov reaction[40] The ZSM-5 zeolitic system is the preferred matrix. There are several ways in which the catalyst can be activated for this reaction.

It is now well established that the active component of the catalytic reaction is mono– meric Fe2+. The Panov reaction consists of two reaction steps:

194 Chapter 4

The uniqueness of the ZSM-5 catalyst relates to its stabilization of Fe2+ cations in the selected (α) sites of the zeolite micropores. There is increasing evidence that non-lattice

alumina plays a promoting role, by potentially enhancing the relative stability of isolated Fe2+ centers[41] .

As a preliminary to our later comparison with biochemical systems (see Chapter 7), it is relevant to note here that the enzyme cytochrome P-450 also contains a single Fe center attached to a porphyrin system. Cytochrome P-450 catalyzes the reaction of methane to methanol. There is also an enzyme that contains a two-iron cationic center that catalyzes the same reaction. In the methane monooxygenase enzyme, two Fe cations are bridged by oxygen and charge compensated by glutamate and histidine groups[42].

An important di erence between the benzene oxidation reaction and methane activation, is the absence of an isotope e ect in the benzene oxidation reaction. In the enzyme, CH4 activation is initiated by hydrogen abstraction. This initiates a radical-type reaction. Benzene oxidation in the Panov system, however, follows a very di erent reaction path.

Figure 4.29. (a) The speculated reaction path for the oxidative transformation of benzene to phenol in

the Panov reaction[43]. (b) Rates of phenol formation and N2 O decomposition as a function of Fe content in ZSM-5[44].

The unique feature of the Fe2+=O bond is its rather high bond energy of 250–290 kJ/mol. This is in contrast to the low bond energy in Fe3+=O (90 kJ/mol). This is found when Fe3+ is part of the oxycationic complex as well as the monomer.

Interestingly the [FeO]2+ cation has been proposed[65] as the active oxidation species in the Fenton reagent[66] that, amongst others, hydroxylates aromatic substrates in the

Shape-Selective Microporous Catalysts, the Zeolites 195

waterphase. Molecular orbital analysis of hydrated [FeO]2+ shows that the HOMO of [FeO]2+ has 2π character and the σ-type lone pair orbital is unoccupied. The reactant has, therefore, a strong electron donative capacity into the antibonding orbitals of the substrate CH orbitals.

Comparing the Fe3+ with the Fe2+ system, there is a di erence of the order of 400 kJ/mol for the recombination energy of two oxygen atoms to form O2. This would result in significantly slower O2 evolution at the Fe2+ centers than from the Fe3+-containing dimeric centers. This may also explain the uniqueness of Fe2+ for the benzene oxidation reaction. Oxygen generated by N2O decomposition has to react with benzene and cannot recombine to O2. The recombination reaction to give O2 has to be suppressed without, however, suppressing the steps involved in the oxidation of benzene. The subsequent steps involved in the oxidation of benzene are shown in Fig. 4.29a.

The only step in the overall oxidation reaction cycle which is endothermic is step 2, which involves the direct insertion of oxygen into the C–H bond of benzene. This is costly since it requires the loss of aromaticity in the benzene ring. All other steps in the cycle are exothermic. Furthermore, matrix e ects are absent in this reaction. The main role of the lattice appears to be to stabilize Fe2+ and prevent over-oxidation of N2O decomposing Fe3+ oxyhydroxy dicationic clusters. The overall result is that the rate-limiting step for phenol formation is the rate of desorption of phenol. The relative concentration of the di erent sites varies with Fe loading, as illustrated in Fig. 4.29b. Whereas the rate of phenol formation increases steeply with the Fe content, when the Fe concentration is low, at higher Fe content N2O decomposition increases, but phenol production is constant.

Whereas in the zeolite system the [Fe2+O] species is produced from N2O, in the biochemical system the biochemical oxidation step of the enzyme with O2 is coupled to an electrochemical reduction: the overall reaction for cytochrome P-450 that converts the alkane to an alcohol is

RCH + O2 + 2H+ + 2e −→ RCOH + H2O

A similar stoichiometry holds for the reaction with the two-Fe center methane–monooxy- genase enzyme.

The di erence in the chemical environment and the valency of Fe may explain why the zeolite system does not produce methanol from CH4 whereas the enzymes do.

4.4 The Zeolite Catalytic Cycle. Adsorption and Catalysis in Zeolites; the Principle of Least Optimum Fit

The dependence of the overall rate of the catalytic reaction on adsorption is extremely important in analyzing the kinetics for the overall rate of a zeolite-catalyzed reaction. We have already met this subject in Chapter 2 when analyzing the basis of the Sabatier principle. A proper understanding of adsorption e ects is essential for establishing a theory of zeolite catalysis that predicts the dependence of kinetics on zeolite-micropore shape and connectivity.

Catalysis consists of a reaction cycle made up of several elementary reaction steps. For a zeolite, the catalytic cycle contains at least the following four elementary steps: adsorption, di usion, substrate activation and desorption.

We will discuss in Section 4.6 explicitly the role of di usion. In principle, di usion e ects can always be experimentally excluded by selecting crystallites so small that chemical