Proton conduction in zeolites

ERIK KROGH ANDERSEN, INGE G. KROGH ANDERSEN AND ERIK SKOU 

Zeolites are crystalline aluminosilicates. They are composed of cations (generally metal ions, but also ammonium ions or hydrogen ions) and an aluminosilicate anion framework. The formula for zeolites - usually written as the content of one unit cell - is 

In this formula M is a cation with valence m. The M cations are usually referred to as extra framework cations (in the following they are called EFC-ions). The square brackets contain the formula for the anion framework. This is a three-dimensional network of silicon and aluminium atoms, both tetrahedrally coordinated to oxygen atoms. The anion framework forms cages and channels in which the EFC-ions and absorbed molecules (water) are located. 

Excellent compilations of structural properties of zeolites are found in publications by Mortier and Meier Olson 

It has been known for many years that zeolites conduct electricity, and it has been assumed without experimental verification that the conduction was by migration of ions and not by electrons. The investigations of the conductivity of zeolites fall into three periods. 

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Protonic conduction in ammonium zeolites and in hydrogen zeolites... 

Unfortunately HU As is the only proton-conducting hydrate for which available oxygen diffusion coefficient data allow direct verification of the vehicle mechanism. For other compounds there is just an indication that this may be the mechanism. These are e.g. diffusion bottleneck considerations for ionic conduction in zeolites or the exceptionally high temperature factors for the water oxygen in and some heteropoly-... 

Fig. 6 Illustration of Nafion-acid functionalized zeolite Beta nanocomposite membranes helping to increase the proton conductivity and decrease the methanol crossover (A) H2O and CH3OH diffusion reduced by zeolite flow resistance (B) sulphonic acid functionalized zeolite nanoparticles increases proton conductivity of composite membranes. (View this art in color at www. dekker. com.)...

In this study protonated large pore zeolites of different structures (HY, HBeta and HMordenite) and framework Si-to-Al ratios were used in liquid phase in a batch reactor. The zeolites were calcined at 500°C and the hydrolysis was conducted at 75°C. The procedure was optimised in terms of solvent, activation, type and amount of catalyst for the hydrolysis of nitroacetanilides, currently carried out with 10 % sulphuric acid [14], and then extended to other substituted amides. The reaction, followed by GC with nitrobenzene as internal standard, was clean and no by-products or degradation were detected. 

The nature and mobility of ions and solvent in zeolite cages will affect the ac-impedance of the material. This effect can be utilized for zeolite-based sensor concepts where a zeolite film is coated on interdigitated electrodes. For example, it was shown that the impedance of a film of proton-conducting H-ZSM-5 is influenced by the presence of ammonia (Figs. 15, 16).[126,127] The ammonia is protonated in the zeolite, thus producing much larger ammonium ions with different mobilities in the zeolite that can be detected by impedance spectroscopy. The detection of ammonia is of interest for automotive applications where the selective catalytic reduction of NOx by ammonia is envisioned. 

However, for liquid phase reactions, it is very difficult to correlate the results wiHi BrOnsted or Lewis acidity as the reaction conditions used are different from those used for characterization. When the Diels-Alder reaction is conducted in a solvent, it appears that the maximum for the activity of HY zeolites is obtained for a Si/Al ratio of 15. This maximum was also observed for esterification of carboiQ lic acids (14), methylthiolation of phenol with dimethyldisulfide (15), acylation of toluene with benzoic acids (15) or dehydration of fhictose (15), and in solvents such as alcohols, water or hydrocarbons. If we assume that Lewis species are transformed to Brdnsted ones in the presence of water as solvent, this would thus mean that the Diels-Alder reaction is preferentially catalyzed by BrSnsted species, the maximum observed at Si/Al=15 for HY zeolites being a good balance between the niunber 6ind the strength of the protonic species. 

Eguizabal A, Lemus J, Urbiztondo M, Moschovi A.M, Ntais S, Nikolakis V, Soler J, Pina MP. Ammonium based ionic liquids immobilized in large pore zeolites Encapsulation procedures and proton conduction performance. J Power Sources 2011 196(9) 4314-4323. 

Han W, Kwan SM, Yeung KL. Zeolite applications in fuel eeUs Water management and proton conductivity. Chem Eng JW12 187 367-371. 

Conductivities in the range from 10" to 10 Q" cm have been observed most zeolites tested for proton conduction have conductivities in the range 10 to cm" . 

HUP and many other natural or synthetic compounds belong to the torbernite mineral group of general formula M, y(U02X04)(H30)y. 3H2O where X is P or As and M" " is a cation such as K, NH4", Na, Li, Ca, Pb, Ag or AP. The role of interlayer cations and water molecules in torbernite is similar to that in zeolites, clay minerals, some feldspars and micas, jarosites, autunites and clathrate compounds. In all these materials solid solutions between protonic and ionic derivatives are readily observed ion exchange and protonic conductivity can thus be expected. 

One of the figures from the chapter on zeolites in this book is included here also to show one of the limitations of the method. In the experiment shown in Fig. 27.3, proton conducting lithium chabazite is purged with argon in order to detect a possible electronic conductivity. The treatment is seen to lower the conductivity by almost three orders of magnitude. It... 

S.D. Mikhailenko, S.M.J. Zaidi, S. Kaliaguine, Development of zeolite based proton conductive membranes for use in direct methanol fuel cells. Report in Natural Resoiu ces Canada, Ottawa, 1997... 

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