Acid Nafion study

Moreover, the catalyst deactivation must also be considered in order to use these solid materials in industrial processes. Figure 13.8 shows the variation of catal54ic activity (2-butene conversion) with the time on stream obtained under the same reaction conditions on different solid-acid catalysts. It can be seen how all the solid-acids catalysts studied generally suffer a relatively rapid catalyst deactivation, although both beta zeolite and nafion-sihca presented the lower catalyst decays. Since the regeneration of beta zeolite is more easy than of nafion, beta zeolite was considered to be an interesting alternative. ... 

In addition to the discussed Br0nsted or Lewis superacidic activation in solution chemistry, there have been reports to suggest that superelec-trophilic species can be formed with solid acids, and even in biochemical systems. For example, Sommer and co-workers have found several examples in which HUSY zeolite has exhibited catalytic activity similar to liquid superacids (eqs 33-34).12 In the same study, the perfluorinated resinsulfonic acid Nafion-H (SAC-13) was found to give products consistent with the formation of the superelectrophile (36, eq 35). 

A CPC 600 calorimeter (15) has been used with heating or cooling speeds between 0.5 and 4°/mn. Acid Nafions 120 containing different water percentages have been studied. 

In view of these considerations and the limitations of polystyrenesulfonic acids, we studied superacidic Nafion-H perfluorosulfonic-acid-resin-caialyzed nitration of aromatics with nitric acid under conditions of azeotropic removal of water (azeotropic nitration) [12]. The azeotropic removal of water in Nafion- H -catalyzed nitration allows utilization of nitric acid to a significantly greater extent than do conventional conditions of nitration. Both fuming and concentrated nitric acid are effective (Table VII). 

Lewis acids supported on graphite as well as the perfluorinated resin sulfonic acids (Nafion-H) have been studied for their abilities to perform Friedel-Crafts alkylation of benzene and transalkylation of alkylbenzenes (Olah et aL, 1977). The graphite-supported materials were less stable than the perfluorinated resin sulfonic acid. Subsequent studies investigated the use of this latter resin in the methylation of phenols, benzene, and alkylbenzenes (Kaspi and Olah, 1978 Kaspi et aL, 1978), the nitration of aromatics (Olah et aL, 1978a), the rearrangement of allyl alcohols to aldehydes (Olah et al., 1978b), the pinacolone rearrangement (Olah and Meidar, 1978) and isomerizations of alkylbenzenes (Olah and Kaspi, 1978). 

Kim, M. H., Ghnka, C. J., Grot, S. A., and Grot, W. G. 2006. SANS study of the effects of water vapor sorption on the nanoscale structure of perfluorinated sulfonic acid (NAFION) membranes. 39(14), 4775-4787. 

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

Many superacid-catalyzed reactions were found to be carried out advantageously not only using liquid superacids but also over solid superacids, including Nafion-H or certain zeolites. We extensively studied the catalytic activity of Nafion-H and related solid acid catalysts (including supported perfluorooctanesulfonic acid and its higher ho-... 

Studies in Lewis acid and LiCi04 (or nafion-H) catalyzed ionic Diels-Alder reactions of chiral and achiral olefinic acetals respectively [96]... 

Photocatalytic oxidation of ethanol on Pt/ri02 and Nafion coated Ti02 catalysts were studied using in situ infrared IR techniques. Infrared studies show that the reaction produced acetaldehyde, acetic acid, acetate, formic acid, formate, and CO2/H2O. Modification of the Ti02 catalyst by Pt and Nafion slowed down the oxidation reaction through site blocking. Incorporation of Pt was found to favor formation of formate (HCOO ), indicating Pt decreases the rate of oxidation of formate more than that of its formation. 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

Ease of separation of tritiated products from a reaction medium is an important feature in the choice of labeling procedure. Sometime ago we used polymer-sup-ported acid and base catalysts [12, 13] to good effect and with the current interest in Green Chemistry one can expect to see more studies where the rate accelerations observed under microwave-enhanced conditions are combined with the use of solid catalysts such as Nafion, or zeolites. 

The combination of dicyclopentadienylzirconium dichloride and silver perchlorate activates armed glycosyl sulfoxides in dichloromethane between -20 °C and room temperature, but only very simple acceptors were studied [335]. Other Lewis and Bronsted acids studied include the environmentally benign europium, lanthanum and ytterbium triflates [336], certain polyoxometallates [337], sulfated zirconia [338] and Nafion H [338]. 

Studies on morphology and conclusions about observed levels of proton conductivity have also been carried out on PEMs other than Nafion and sulfonated poly(ether ketone). These include studies in which phenomenological examinations of relationships between conductivity and observed microstructure were carried out upon polymer systems where acid content was varied but the basic chemical structure was kept constant. In addition, other systems allowed... 

Also, discussions of a number of applications of Nafion are not included in this document and are, at most, mentioned within the context of a particular study of fundamental properties. A number of these systems are simply proposed rather than in actual commercial applications. Membranes in fuel cells, electrochemical energy storage systems, chlor-alkali cells, water electrolyzers, Donnan dialysis cells, elec-trochromic devices, and sensors, including ion selective electrodes, and the use of these membranes as a strong acid catalyst can be found in the above-mentioned reviews. 

Roche and co-workers used SAXS and small-angle neutron scattering (SANS) to study the morphology of 1200 EW Nafion membranes in the acid and neutralized forms with a range of water contents. ... 

A variation of the lamellar model was recently proposed by Haubold et al., in which synchrotron SAXS studies were performed on acid form Nafion 117 samples. The molecular weight of these samples was reported to be 250 000 g mol and the experiments were conducted on dry samples in air and samples equilibrated with water, methanol, and a range of water/methanol mixtures using an in situ flow cell. The most fundamental result of this study is that the data show the usual ionomer peak at = 1.4 nm which gives a Bragg spacing of d= Zjtlq ... 

Later, Hinatsu et al. studied the uptake of water, from the liquid and vapor states at various temperatures, in acid form Nafion 117 and 125, and Aciplex and Flemion membranes, although the latter two similar products will not be discussed here. These studies were motivated by a concern over the deleterious effects, involving either overly dry or overly wet membranes, on electrical conductivity within the context of polymer electrolyte fuel cells and polymer electrolyte water electrolyzers. 

Another noteworthy early IR spectroscopic study of Nafion (125 and 152) membranes, in the acid as well as various counterion forms, was that of Heitner-Wirguin, who assigned a number of characteristic bands and compared the spectra to those of sul-fonated polyethylene (Redcat) membranes. 

Risen et al. investigated cation—anion interactions using far IR spectroscopy (50—300 cm ) to study Nafion sulfonate membranes that were neutralized by cations in the series Na+, K+, Rb+, and Cs+ and the series Mg +, Ca +, Sr +, and Ba +, as well as the acid form." The spectra in this region for hydrated samples show a broad but well-defined band below 300 cm that is not present for the acid form. For both the monovalent alkali and divalent alkaline earth series, the band monotonically shifts to lower frequencies, f, such that foe where Mis the... 

Szentirmay et al. studied the microchemical environments of Nafion 117 in the acid and Na+ forms using Py and Ru(bpy)32+ probes in fully hydrated ( 40%) samples in various cation forms.Ru(bpy)32+ emission spectra cannot be interpreted in terms of environmental polarity in as straightforward a fashion as in the case of Py, but blue shifts can reflect this aspect. One of the results of this study was that the microenvironment polarities were such that Lj//i values for Py are between those for fluorocarbon and aqueous environments, and this conclusion was strengthened by the results of Ru(bpy)3 + probe studies, as well as the similar conclusion of Lee and Meisel. Another conclusion that was reached was that the SOa" clusters are chemically heterogeneous, an idea that was in line with the view of Yeager and Steck, who spoke of mixed interfacial regions. ... 

Cable and Moore performed DMA (dynamic mechanical analysis) studies of various Nafion membranes including the acid form. ° A tan <3 peak with maximum at 110 °C, referred to as Tg , was seen, and there is a suggestion of a shoulder on the low temperature side that might arise from another mechanism. As this membrane was dried at only 60 °C, the possibility of residual water incorporation exists. Moore and Cable concluded that the a relaxation was due to chain motions within and/or near the ion-rich domains and that the ji relaxation was... 

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