Conductive within zeolite channels

Conducting polymers have been grown within zeolites.174175 With channels as small as 0.3 nm, this allows assembly of single molecular wires. Conductivities of 10-9 S cm-1 for such wires contained within zeolites have been reported, and this increases to 10-2 S cm-1 when the polymer is extracted from the host structure. 

We have demonstrated that oxidative polymerization reactions leading to conducting polymers can be carried out within the channel systems of zeolites. Acidic zeolite forms are required to synthesize intrazeolite polyaniline by analogy to the oxidative coupling of aniline in acidic solutions. The presence of intrazeolite oxidants such as Cu(II) and Fe(III) ions is fundamental for the polymerization of pyrrole, thiophene and 3-methylthiophene. The degree of polymer chain oxidation and probably the chain lengths are influenced by the dimension ity of the zeolite channels. 

The formation of conducting polymers within the confines of zeolite channels has been shown to depend on several factors, including the nature of the cation, reaction pressure and the presence of acid sites. From a detailed study of pyrrole polymerisation it is possible to postulate the following reaction mechanism. Interaction of pyrrole with a Cu site initially gave a radical monomer species, (which presumably became associated with AIO4 sites in the framework) and a Cu site. Further creation of radical polymers leads to a polymerisation reaction in which aromatic polymer chains were formed. As the polymer chain lengthens the energy levels for the n system are lowered and thus oxidation... 

An elaboration of this technique, incorporating the concept of template-guided synthesis, is the use of nanoporous matrices such as zeolites and polycarbonates as a template within whole pores to perform the chemical polymerization of aniline monomers. For example, Wu and Bein254 have prepared nanofilaments of conducting PAn in the 3-nm-wide channels (pores) of the aluminosilicate host, MCM-41, through initial adsorption of aniline vapor into the dehydrated host followed by oxidation with (NH)2S208. 

The predominant importance of the cations in zeolites is that they form so-called active sites for selective interaction with guest molecules in sorption and catalytic processes. From the point of view of advanced material science [47] they play a significant role in the formation of quantum-sized clusters with novel optical or semiconducting properties. As they give rise to cationic conductivity, zeolites can be used as solid electrolytes, membranes in ion-selective electrodes and as host structures in solid-state batteries. Organometallic compounds and coordination complexes can be readily formed on these cations within the larger cages or channels and applied to gas separation, electron-transport relays and hybrid as well as shape-selective catalysis [48]. 

Samples of NaY with pyrolyzed (920 K) PAN show no measurable dc conductivity. This is not surprising because the polymer is encapsulated completely within the insulating zeolite, and b ause no external polymer coats the zeolite crystal surfaces. However, the conductivity of the pyrolyzed PAN extracted from the zeolite is at the order of 10 Scm l, almost identical with that of bulk PAN pyrolyzed at 800 K, and five orders of magnitude smaller than that of the bulk sample pyrolyzed at 920 K. In contrast to the bulk polymer, pyrolysis treatment conditions above about 870 K have little effect on the resulting conductivity of the extracted intrazeolite samples. We conclude that the spatial limitations within the zeoHte channels prevent the formation of more extended, graphitized structures with higher conductivity. 

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