Redox active zeolites

As a test reaction, we selected the bleaching of phenolphthalein at pH 10 with H2O2. Apart from the Mo-LDH, several redox-active zeolites were used, such as Mn2t-exchanged Y, or Y zeolites with entrapped Mn... 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

Synthetic zeolites have gained importance as industrial catalysts for cracking and isomerization processes, because of their unique pore structures, which allow the shape-selective conversion of hydrocarbons, combined with their surface acidity, which makes them active for acid-catalyzed reactions. Many attempts have been made to introduce redox-active TMI into zeolite structures to create catalytic activity for the selective oxidation and ammoxidation of hydrocarbons as well as for SCR of nitrogen oxides in effluent gases (69-71). In particular, ZSM-5 doped with Fe ions has attracted attention since the surprising discovery of Panov et al. (72) that these materials catalyze the one-step selective oxidation of benzene to phenol... 

Further variation of the stmctural and catalytic properties of four-coimected tetrahedral frameworks is obtained by the substitution of silicon or metal cations,giving materials known as SAPO s and MeAPO s, respectively. More than twenty metal aluminophosphate frameworks have been identified with Mg, Mn, Fe, Co, or Zn substituents. These give the possibility of framework redox activity (e.g. Fe +/Fe +) in catalysis as well as the usual Bronsted acidity. For further information about zeolitic and microporous phosphate frameworks see Porous Inorganic Materials and Zeolites) and recent reviews. ... 

In this context, we mention two zeolite-induced conversions of cyclopropane derivatives. Incorporation of tra -l,2-diphenylcyclopropane trans-Vi) and its 3,3-D2-isotopomer into the channels of a redox-active pentasil zeolite (Na-ZSM-5) generated exo,exo-l,3-diphenylallyl radical (24 ) and its 2-Di-isotopomer. This conversion is a zeolite-specific reaction it requires a series of reactions, including oxidation, ring opening, and deprotonation [70]. 

The common means of introducing redox catalytic activity in zeolites is by the substitution of framework atoms such as Si, A1 or P with redox-active metal cations. This has been accomplished by two different methods (1) hydrothermal synthesis and (2) post-synthesis modification. Irrespective of the method of preparation, with the notable exception of titanium silicalites, these redox metals in the framework are susceptible to leaching due to the solvolysis of M-O bonds [77]. Even the Ti silicalites suffer from leaching under basic conditions [76a]. 

Zeolites are microporous frameworks, and all of the ET chemistry that we have discussed is with molecules smaller than 13 A. The unique features of zeolites are their ion-exchanging ability, a stable structure upon dehydration and a pore/chan-nel structure that allows for a well-defined arrangement of molecules in space and the fact that redox-active atoms can be substituted on the framework. In most cases, the zeolite is an active host, influencing ET reactions via electrostatic fields or steric effects, a feature that is not found with the mesoporous and sol gel materials. Packing of molecules/ions in the intrazeolitic space with very high densities is also possible and was found to be important in charge propagation and electrochemistry. 

All these properties are important no less important is the possibility of introducing isolated, well defined active sites in accessible framework positions. More specifically, one can introduce acid, base, and/or redox active sites directly during the synthesis or by post-synthesis treatment [6]. Zeolites and zeotypes are prepared under hydrothermal conditions in basic or acid media, at a range of temperatures normally from 80 to 180 °C, and from a few hours to several days or even weeks. 

Redox-type zeolite catalysts can also be prepared in which the active sites occupy extra-framework positions and are obtained by different post-synthesis procedures, including anchoring the transition metal complexes (TMC) [92-94], CVD deposition, trapping the TMC within the pores during zeolite synthesis [95,96], or encapsulation of the TMC by the ship-in-a-bottle teehnique. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Obtaining a reliable measure of the number of unpaired electrons ( spin concentration ) in a sample is often extremely useful. Even reliable relative values measured across a series of samples can often provide useful information. There are various important applications, as may be illustrated by the following incomplete list ESR dating, the determination of oxidized polycyclic aromatic hydrocarbons and of environmental carbon in samples of ambient air, the influence of air pollution (e.g., SO2 and NO2) on plants and soils, quantification of NO2, RO2, and HOI radicals in air samples, radiation dosimetry, redox activities of zeolite catalysts, and the metabolism of spin probes in cells and tissues. 

The determination of several non-electroactive species is also possible at ZMEs, by means of an indirect approach briefly schematized in Fig. 7.10a. Zeolite films are doped with redox active species, mainly consisting of Cu ". According to mechanism 2 previously outlined, these species undergo an extra-zeolite charge transfer the presence in solution of catimis possessing dimensions suitable to enter the zeolite pores and presenting competitive affinity for the zeolite causes the expulsimi of initially included electroactive species and their reduction at the substrate. The extent of the current is consequently proportional to the concentration of the inert cationic analyte [181]. 

---