Zeolite electrochemical reactions

Although limited by sensitivity, chemical reaction monitoring via less sensitive nuclei (such as 13C) has also been reported. In 1987 Albert et al. monitored the electrochemical reaction of 2,4,6-tri-t-butylphenol by continuous flow 13C NMR [4]. More recently, Hunger and Horvath studied the conversion of vapor propan-2-ol (13C labeled) on zeolites using 1H and 13C in situ magic angle spinning (MAS) NMR spectroscopy under continuous-flow conditions [15]. 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

Based on their regular pore structure, it should be possible to impart the molecular sieving capabilities of zeolites on the surface of electrodes in electrochemical reactions. Several research groups have addressed this issue by (i) developing ways to modify electrodes with thin zeolite films and (ii) by studying the resulting changes in electrode behaviour in electrochemical reactions. The subject has been reviewed.[91]... 

Diels-Alder reactions are thermal reactions requiring no catalysts (120). However, over the years both acid- and metal-based homogeneous or heterogeneous catalysts have been developed (121—127). Some catalysts used in Diels-Alder catalyzed reactions of butadiene are Fe(NO)2Cl—(CH3CH2)2A1C1, Pd[P(C H5)3]4, Cu(I) exchanged silica—alumina (128,129), large pore zeolites (130), and carbon molecular sieves. An electrochemical process has also been used to catalyze the self-condensation to vinylcyclohexene (131). When the asymmetric Ni catalyst (4) was used, specificity to the enantomeric (5)-4-vinylcyclohexene (132,133) was observed (26% enantiomeric excess). 

Aluminosilicate zeolites because of their structure, composition, and properties offer a superior ionic strength environment [172,173], Even though these materials are electronic insulators, when hydrated, they are solid solutions of high ionic mobility, and when dehydrated exhibit fair ionic conductivity (see Section 8.2.7) [38,112,119,172], The properties of aluminosilicate zeolites that are responsible for affecting the charge-transfer reactions in electrochemical systems are [172,174] ... 

When zeolites are hydrated shows a notable ionic conductivity [112], Consequently, since all electrode processes depend on the transport of charged species zeolites provide an excellent solid matrix for ionic conduction [172], In 1965 [175], Freeman established the possibility of using zeolites in the development of a functional solid-state electrochemical system, that is, a battery where a zeolite, X, was used as the ionic host for the catholyte, specifically, Cu2, Ag+, or Hg2+, and as the ionic separator in its sodium-exchanged form, that is, Na-X. Pressed pellets of Cu-X and Na-X were sandwiched between a gold current collector and a zinc anode. Then, the half-cell reactions are the oxidation of Zn —> Zn2+ + 2e and the reduction of Cu2+ + 2e —> Cu, with type X providing a solid-state ionic path for cationic transport [175], The electrochemical system obtained can be represented as follows (Au I Cu11 -XI Na-X I Zn). 

Debra R. Rolison is head of Advanced Electrochemical Materials at the Naval Research Laboratory (NRL). She received a B.S. in chemistry from Florida Atlantic University in 1975 and a Ph.D. in chemistry from the University of North Carolina at Chapel Hill in 1980 under the direction of Royce W. Murray. Dr. Rolison joined the Naval Research Laboratory as a research chemist in 1980. Her research at NRL focuses on the influence of nanoscale domains on electron- and charge-transfer reactions, with special emphasis on the surface and materials science of aerogels, electrocatalysts, and zeolites. Her program creates new nano structured materials and composites for catalytic chemistries, energy storage and conversion (fuel cells, supercapacitors, batteries, thermoelectric devices), and sensors. 

Palladium nanoparticles (nm-Pd) were synthesized by ship-in-a-bottle technique in supercages of NaA zeolite. The behaviors of electrodes of thin film of nm-Pd accommodated in NaA zeolite were characterized by cyclic voltammetry. The results illustrated that the nm-Pd possess particular properties for hydrogen reaction, i.e. in contrast to hydrogen absorption on massive palladium electrode, the surface processes of hydrogen adsorption-desorption become the dominant reaction on electrodes of thin film of nm-Pd. The processes of adsorption and desorption of carbon monoxide on the electrodes were studied using in situ electrochemical FTIR reflection spectroscopy. It has been revealed that in comparison with CO adsorbed on a massive Pd electrode, the IR absorption of CO adsorbed on nm-Pd particles accommodated in NaA zeolite has been enhanced to about 36 times. 

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

The electrochemical potential for redox reaction controls the situation where atoms of one element are available to be sorbed by a zeolite containing exchangeable cations of another element. Within the zeolite and even in the absence of water, aqueous reduction potentials are usually capable of deciding whether reaction will occur, with an error due to the difference between the zeolitic environment and aqueous solution of no more than 0.1 (or perhaps 0.2) V. Accordingly there is no question that alkali-metal vapors will reduce transition-metal ions within a zeolite, and that vapors of zinc, mercury, or sulfur will not reduce the cations of the alkali or alkaline-earth metals. 

The redox reactivity of divanadium salen615 and other complexes555 have been investigated. Disproportionation of [ V(salen) 2(//-0)] was observed under electrochemical conditions.6 The reduced, [Vin(salen)]+ complex was found to be the essential species in the catalysis of the electroreduction of 02 by four electrons in CH2C12.616 While [ V(salcn) 2(/x-0)] was proposed to be the active species in the redox reaction, more recently [Vin(salen)]+ was identified as a reservoir from which the active species forms.616 Schiff base complexes encapsulated in zeolite Y are catalytically active in the oxidation of thioanisole with H202.14 Acid-promoted disproportionation of a Vlv phenolate under anaerobic conditions was proposed as a model reaction for the vanadium uptake in tunicates.139... 

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