Zeolite polyacrylonitrile

Vapors of acrylonitrile were adsorbed into the dehydrated forms of different large- and medium-pore zeolites, to saturation levels of 46, 6, and 9 molecules of acrylonitrile per unit cell in dehydrated zeolite NaY, Na-mordenite, and silicalite, respectively. Subsequent reaction with radical initiator (aqueous solution of K2S2O8 and NaHSOs) produced intrazeolite polyacrylonitrile (no polymer was found in silicalite due to size constraints). The intrazeolite polyacrylonitrile could be recovered after dissolution of the host with dilute aqueous HF, and was very similar to bulk polyacrylonitrile. Gel permeation chromatography revealed a peak molecular weight of 19,000 for polyacrylonitrile recovered from the NaY host, and about 1,000 for the polymer from mordenite. 

Polyacrylonitrile is a very important polymer material. Using zeolites as a host template, acrylonitrile monomers can polymerize to form polyacrylonitrile in the zeolite... 

Industrial applications of zeolite membranes can be considered only for separations where they offer some unique advantage in terms of flux, selectivity, or thermal and chemical stability. The very high fluxes obtained with LTA membranes (typically 100 times higher than those obtained with a polyacrylonitrile membrane at the same FLO/alcohol selectivity) explain the rapid expansion of this type of application at the end of the 90s [8J. 

An interesting variant of the formation of nanometer-sized conductive strands is pyrolysis of intrazeohte polymers such as polyacrylonitrile in zeolite NaY,the color of which turns from white to grey-black [136]. The electronic spectriun of the extracted material is practically structureless with a feature at about 350 nm and resembles that of graphite. In this way graphite-like chains with conductivity in the order of 10 S cm can be prepared. 

Porous membranes can be made of polymers (polysulfones, polyacrylonitrile, polypropylene, silicones, perfluoropolymers, polyimides, polyamides, etc.), ceramics (alumina, silica, titania, zirconia, zeolites, etc.) or microporous carbons. Dense organic membranes are commonly used for molecular-scale separations involving gas and vapor mixtures, whereas the mean pore sizes of porous membranes is chosen considering the size of the species to be separated. Current membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), gas and vapor separation (GS), and pervaporation (PV). Figure 1 indicates the types and sizes of species typically separated by these different separation processes. 

Besides CNTs, another ID carbon nanostructure is carbon nanoflbers. For example, Shen et al. [156] prepared a series of hierarchical porous carbon libers with a BET surface area of 2,231 m g and a pore volume of 1.16 cm g. In this synthesis method, the polyacrylonitrile (PAN) nanofibers (prepared by dry-wet spinning) were selected as precnrsors, and pre-oxidation and chemical activation were involved to get the developed porosities. This type of material contained a large amount of nitrogen-containing groups (N content >8.1 wt%) and consequently basic sites, resulting in a faster adsorption rate and a higher adsorption capacity for CO2 than the commercial zeolite 13X that is conventionally used to capture CO2, in the presence of H2O (Fig. 2.27). 

Membranes with promising properties have been prepared from polyphosp-hazenes in combination with sulfonimide (polyphosphazene-based sulfonimide Chalkova et al., 2002) or with polyacrylonitrile (blended polyphosp-hazene/polyacrylonitrile Carter et al., 2002). Low methanol crossover was also seen with membranes prepared from poly(vinyl alcohol) that contained mordenite (a zeolite variety Libby et al., 2001). Various aspects of the work on composite membranes prepared from different polymers have been discussed in detail in a review by Savadogo (2004). 

As a last example of zeolite inclusion chemistry, we discuss the assembly of polyacrylonitrile (PAN) strands in different large-pore zeolites, zeolite Y and mordenite, and explore the pyrolysis reactions of the encapsulated polymer (Figure 6).88... 

Acrylonitrile vapor was adsorbed in the degassed (670 K, lO Torr) zeolite crystals at a vacuum line for 60 min at 298 K. To an aqueous suspension of the acrylonitrile-containing zeolite were added aqueous solutions of potassium peroxodisulfate and sodium bisulfite as radical polymerization initiatOTS. The zeolite frameworks could be dissolved with HF to recover the intrazeolite polyacrylonitrile (PAN). IR and NMR data show no damage to the polymers after this treatment For pyrolysis, the zeolite/PAN adducts were heated under nitrogen or vacuum to 920 and 970 K for extended periods. 

Infrared spectra of the zeolite/polymer inclusions and of PAN extracted from the zeolites show also peaks characteristic of the bulk polymer,including methylenic C-H stretching vibrations of the backbone (2940 cm l and at 2869 cm"l), and a band at 2240 cm l due to the pendant nitrile group. The spectra of the extracted intrazeolite polymers are indistinguishable from the spectrum of the bulk polymer. We conclude that the polymer formed in the zeolites is polyacrylonitrile. 

This study demonstrates the inclusion synthesis of polyacrylonitrile in the channel systems of NaY and Na-mordenite zeolites, and its pyrolysis to yield a conducting material consisting of nanometer size carbon filaments. These and related systems are promising candidates for low-field conductivity at nanometer scale (timensions. 

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