Sulfonation sites

The most important types of these methods are the isomerizing rearrangements. According to whether the reaction occurs at the sulfone site or at the carbon site on the one hand, or at both sites on the other, one should distinguish between unifold and twofold transformations (Schemes 2 and 3). 

Recendy, Guiver et al. reported a number of derivatives of polysulfone and poly(aryl sulfone).172 188 Polysulfones were activated either on the ortho-sulfone sites or the ortho-ether sites by direct lithiation or bromination-lithiation. The lithiated intermediates were claimed to be quantitatively converted to azides by treatment with tosyl azides. Azides are thermally and photochemically labile groups capable of being transformed readily into a number of other useful derivatives. 

Based on this new model for the morphology of Nafion, the dimensional variations of the scattering entities with water content were used in simple space filling calculations to estimate the cluster diameter, the number of sulfonate sites per cluster, and the number of water molecules per cluster. The results of these model calculations showed that, for a given equivalent weight, the cluster diameter. 

Al-Omran and Rose controlled the location and extent of sulfonation on poly (ary lene ether) backbones by copolymerizing 4,4 -dichlorodiphenyl sul-fone, durohydroquinone, and hydroquinone to form random copolymers, where only the hydroquinone residue would be expected to be susceptible to sulfonation by sulfuric acid. Although these authors observed sulfonation at positions other than at the desired hydroquinone locations, designing sulfonation sites into a polymer backbone remains an attractive strategy for producing copolymers with known structures. This allows the chemical structure and composition of the material to dictate the extent of sulfonation rather than trying to externally control sensitive and sometimes unpredictable macromolecu-lar sulfonation reactions. 

Figure 38. Possible sulfonation sites on poly[(3-metb-ylpbenoxy) (pbenoxy)pbospbazenej.

Proceeding on the same line, Hagerdal et al. reported that perfluorinated resin supported sulfonic sites (NATION 501) can hydrolyze disaccharides [25]. In particular, these authors studied the effect of the addition of sodium chloride in the hydrolysis of cellobiose, a subunit of cellulose much more resistant to hydrolysis than sucrose. They observed that the presence of sodium chloride in water dramatically increased the conversion of cellobiose. Indeed, in the presence of 10 wt% of sodium chloride, 80% of cellobiose was converted at 95°C after 6 h. For comparison, when 1% of sodium chloride was added, only 50% of cellobiose was hydrolyzed. It should be noted that without addition of sodium chloride only 15% conversion was achieved, thus pushing forward the key role of sodium chloride on the reaction rate. Effect of salt on the reaction rate was attributed to a change of the pH caused by the release of proton in the reaction medium (due to an exchange of the supported proton by sodium). 

In a similar way, Mizota et al. grafted polymer chains functionalized with sulfonic sites over a polystyrene-type polymer. As observed above, the flexibility of the polymer chains allowed better accessibility of the catalytic sites and this solid acid catalyst was ten times more active than the conventionally used cross-linked resin in the hydrolysis of sucrose (Scheme 2) [27]. 

Although zeolites are stable in water, their narrow pore openings considerably limit their utilization in carbohydrate chemistry. In most cases, only the external surface is accessible to reactants. For these reasons, many investigations have focused on mesoporous silica-supported sulfonic sites (Fig. 1). The presence of uniform channel with large pore openings (2-9 nm) offer notable advantages over zeolites. 

As expected, mesoporous silica-supported sulfonic sites were able to catalyze the hydrolysis of cellobiose. Indeed, at 448 K, 90% of cellobiose was hydrolyzed within 30 min of reaction with an apparent activation energy ( = 130 kJ moF ) similar to that of reactions promoted by homogeneous organic acid catalysts [33]. The hydrolysis reaction rate is proportional to the concentration of hydrated... 

Remarkably, in 2002, Inagaki and co-workers reported that, starting from 1,2-bis (triethoxysilyl)benzene as a siliceous precursor, mesoporous benzene-silica with crystal-like pore walls (Ph-PMO) can be prepared (Fig. 2) [35]. Owing to their crystallinity, these new hybrid organic-inorganic materials were much more stable in water than the amorphous mesoporous silica-supported sulfonic sites described above [36-39]. 

Jacobs and coworkers reported the catalytic activity of a mesoporous silica-supported sulfonic sites in the esterification of sorbitol with lauric acid [130]. This reaction affords in a one step process the dilauryl isosorbide (dehydration of sorbitol/esterification). In contrast to zeolites, it is found that mesoporous silica-supported sulfonic sites afford the corresponding dilauryl isosorbide with 95% selectivity at 33% conversion (Scheme 13). 

Figure 40 Diffuse reflectance spectra of Ru(bpy) + adsorbed into ZrPS. Percent of sulfonate sites associated with Ru(bpy) + are as follows (A) 0.43% (B) 0.22% (C) 0.11% (D) 0.02%. (From Ref. 84. Copyright 1990 The American Chemical Society.)...

The alcohol will come off in the void volume of the column since it has no attraction to the column. The amines will be retained, because at the pH of the acetate solution they are protonated and have a positive charge. As more mobile phase passes the through the column, its sodium ions begin to compete for the sulfonate sites with the bound amines. Through a mass effect, the amines are displaced down the column until, finally, they elute into the detector. The amine that has the strongest charge and binds the tightest is eluted last. 

We are interested in the application of polymers as adsorbents, ion exchangers, fuel cells, and permeable materials. In this regard, the first resins with some of these properties were obtained by D Aleleio in 1944 based on the copolymerization of styrene and divinylbenzene. Unfunctionalized polystyrene resins cross-linked with divinylbenzene (Amberlite) are widely applied as adsorbents [191,192], In addition, the polystyrene-divinylbenzene resins functionalized with sulfuric acid (sulfonation) to create negatively charged sulfonic sites are applied as cation exchangers, and treated by chloromethylation followed by animation produce anionic resins [193,194],... 

FIGURE 27.17 Water uptake from vapor phase (100% relative humidity [RH]) by Nafion 117 membrane and recast Nafion film at different equilibration temperatures (O) Nafion 117 membrane, (A) recast Nafion film, (o) heat-treated Nafion 117 membrane. Fitted curves second order polynomial X is the number of water molecules per sulfonate site. (Reproduced from Broka, K. and Ekdunge, P., J. Appl. Electrochem., 27, 117, 1997. With permission from Springer Science and Business Media.)... 

Figure 18. A proposed morphological representation of zinc stearate crystals interacting with Zn-S-EPDM molecules under relaxed and deformed states. Interactions occur between sulfonated sites on the polymeric backbone and polar sites on the surface of the crystals.

General Considerations. In order to study cation exchange equilibria it is necessary to determine the rate at which exchange equilibrium is attained. Exchange rates are relatively rapid for 1200 EW Nafion. A IT -form membrane, when immersed in aqueous NaCl solution, attains 90% conversion to the Na" "-form in less than two minutes (5). This time interval increases to 40 min for conversion to the Cs" "-form. This increase in equilibration time is attributable to the anomalously low diffusion coefficient of Cs" " in the polymer phase (6). Even in this case equilibration times of a few hours are sufficient to ensure complete reaction. Another factor to consider is whether all sulfonate sites are available for exchange with various cations. 

The first waters sorbed cause the sulfonate heads to dissociate, resulting in the formation of hydronium ions [26]. The water that hydrates the membrane forms counter-ion clusters localized on sulfonate sites with the sulfonate heads acting as nucleation sites [26]. Given the hydrophobic nature of the backbone, and the hydrophilic nature of the sulfonate heads, it is reasonable to consider that all water molecules sorbed by the membrane at this low water content are associated with the sulfonate heads. Moreover, the hydronium ions will be localized on the sulfonate heads, and, because the amount of water sorbed is insufficient for the formation of a continuous water phase, the conductivity will be extremely low. Figure 4.2a is a schematic... 

Usually a A parameter is defined as the number of water molecules per sulfonic site to characterize the degree of membrane hydration. Our MD simulations were carried out at various values of A to analyze the possible water structure in such environment [80]. As shown by Figure 17.8, at low water contents (A = 5), small water clusters are nucleated near the sulfonic hydrophilic sites, which are barely cormected with each other. Thus, at these conditions, proton transfer could be severely limited because of the poor contact between water clusters. At a much higher water concentration (A = 45, not shown), water forms a continuous phase [80] occupying almost all accessible sites on the surface thus oxygen diffusion will be reduced because of the low O2 solubility in water. When A is 24 (Figure 17.8), a value close to the amount... 

The preparation of sulfonated PEEK with a sulfonated phenyl group pendant from the main chain has been described in detail [78]. These compounds are useful in proton exchange membranes, particularly for fuel cells. The pendant phenyl group can provide an easily sulfonable site that may be sulfonated under mild conditions, providing the ability to precisely control the sulfonic acid content of the polymer. 

Metal ions that originate from other parts of the system (electrode, bipolar plates, coolant, BOP) or from the fuel or air can be absorbed into the membrane. NH3 present in the fuel or in air will migrate into the membrane to form NH4. Another source of is NO2, that can be present in air and can be electrochemically reduced at the cathode [141]. The sulfonate sites have a stronger affinity to cations except LF, than for protons, leading to exchange of the protons by the metal ion... 

Cationic contaminants tend to build up in the polymer electrolyte. This is because the sulfonate sites have a higher affinity for most other cations than protons and because most other cations do not partake in a suitable reaction to exit the polymer electrolyte phase [2,3]. In the case of ammonia, there is a suitable reaction at the cathode to remove ammonium ions from the system, but this reaction is likely slower than proton reduction. Some other metal ions, such as copper and cobalt, are electrochemically active in the fuel cell potential window and tend to "plate out" of the system. In general, once a cationic contaminant is in the polymer electrolyte phase it tends to stay there until the membrane has an acid treatment. 

The total charge concentration in the membrane is Q. If the membrane contains a blocking cation B with charge Zg, we will let i/h represent the ion fraction of H+ cations to sulfonate sites. This is also its charge fraction. The... 

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