Polysiloxanes, sulfonated

The use of POSSs as a support for the extractant for selective extraction of heavy metals was studied.471 Sulfonated polysiloxanes were surveyed as solid acids and as supports for precious metal catalysts. 

Wieland S, Panster P (1997) Replacing liquid acids in fine chemical synthesis by sulfonated polysiloxanes as sohd acids and as supports for precious metal catalysts. Stud Surf Sci Catal 108 67... 

Replacing Liquid Acids in Fine Chemical Synthesis by Sulfonated Polysiloxanes as Solid Acids and as Supports for Precious Metal Catalysts... 

The use of solid acid catalysts, consisting of polysiloxanes bearing alkylsulfonic acid groups, is described, in organic synthesis and in technical applications. Although sulfonated polysiloxanes have been reported in the literature and have been found to show excellent activities in comparison to conventional polystyrene based cation exchange resins, no large scale technical use has become known thus far. 

Sulfonated polysiloxanes have been described and reported in the literature for their excellent activities in comparison with polystyrene based cationic exchange resins. ... 

A wide variety of solid-acid catalysts is available [17] acidic clays, zeolites, silica-occluded heteropoly acids, sulfonated polysiloxanes, Nafion (a sulfonated perfluoroalkyl resin) and Nafion-silica composites, and a variety of hybrid sulfonated mesoporous systems (see Chapter 3). 

The different approaches used to synthesize sulfonated polysiloxanes can be distinguished according to synthesis/modification of the solid siliceous network, to the organosilane precursor used, and to the generation of the active sulfonic acid group, as given in Figure 1. 

Aryl- and alkylsulfonated polysiloxanes are available. Aryl sulfonated polysiloxanes are generally synthesized by sulfonation of solid aryl polysiloxanes whereas alkyl sulfonated polysiloxanes are synthesized by starting either with an aqueous solution of the sulfonic acid-functionalized silane or with a highly crosslinked mercapto or polysulfan functionalized polysiloxane. Figure 3 gives an overview of the predominantly synthesized sulfonated siloxane units. 

Figure 1. Approaches to the synthesis of sulfonated polysiloxanes. Any combination of the first and second rows of the table is possible. The second row gives the organo-silane precursors required and whether post-reaction of the solid formed is necessary.

Figure 3. Functional subunits of sulfonated polysiloxanes as attached to the solid phase.

Sulfonated polysiloxanes are extremely stable in gas-phase reactions. No degradation of catalytic activity has been found from study of catalysts thermally pretreated under nitrogen up to 300 °C and also after hydrothermal pretreatment,... 

Sulfonated polysiloxanes have been tested in a variety of reactions. A typical reaction used to compare the acid strength of these catalysts with that of other solid acids is dehydration of isopropanol to yield propene [3]. The splitting of ethers, especially MTBE, at temperatures up to 200 °C has also been successfully demonstrated [14]. 

Sulfonated polysiloxanes might overcome the disadvantages of polystyrene-based cation-exchange resins but they have other intrinsic disadvantages arising from the siloxane matrix. Although their chemical and thermal stability is excellent and despite successful demonstration of their use in a variety of chemical reactions, no industrial use has yet been reported, mainly because production is more complicated and expensive and so cannot compete with much cheaper polystyrene-based materials produced on a large scale. 

Figure 1. Sulfonated amorphous solids (a) sulfonated polystyrene-silica hybrid prepared via the sol-gel technique. Si02-grafted groups (b) alkylphenylsulfonic acid (c) sulfonated polyphenylsilsesquioxane (d) perfluorosulfonic acid (e) benzylsulfonic acids attached to a chlorinated silica (f) sulfonated polysiloxane.

Because the grafting techniques described in Section 3.5.1.2 lead to rather low loading, it is of interest to incorporate functional groups directly and in large amounts into a material via a sol-gel process. Open and patent literature report colloidal silicas [23], aerogels [24] and xerogels [14] with 3-mercaptopropyl, 2-phenylethyl, or benzyl functional groups. These materials are valuable precursors to sulfonated polysiloxanes (Section 3.3). 

Acidic ion-Exchange Resins Cataiysts. Commercial solid acidic ion-exchange resins have been demonstrated to catalyze the carbonylation of alkenes and alcohols, when operated under proper conditions (65). The resins comprise various commercial sulfonated styrene-divinylbenzene resins, both in gel and in microreticular forms Dupont s Nafion NR50 and its Nafion-silica composite SAC 13 as well as Degussa s sulfonated polysiloxane resin Deloxan ASP. 

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