Nanocomposites Nafion/silica

Table 7. As can be seen, both Dowex and Deloxan led to poor enantioselec-tivities, which further decreased after catalyst recovery. Better results, which are comparable with those obtained in homogeneous phase, were obtained with Nation (Table 7) [53], although it was necessary to carry out the reaction at 60 °C due to the low copper content in the soHd. This low copper level is a consequence of the low surface area of this polymer (< 0.02 m g ) and, for this reason, a nafion-silica nanocomposite was used as the support [53]. With this catalyst, the reaction took place at room temperature and with similar enantioselectivity (Table 7).

Polymeric resin sulfonic acids including sulfonic acid resins complexed with Lewis acids and perfluorinated polymer resin acids (Nafion-H and Nafion-silica nanocomposites). 

In a similar study, Nafion-H beads, 10% Nafion-H on silica, and 13% Nafion-silica were compared in the adamantylation of toluene with 1-bromoadamantane.239 13% Nafion-silica nanocomposite exhibited the highest activity and showed achangeinthe isomeric ratio of the two alkylated products the para/me ta ratio shifted to lower values in prolonged reaction (Table 5.20). Acidity is known to exert a significant influence on... 

Nafion-silica nanocomposite catalysts have also been tested in the Friedel-Crafts acylation of aromatics with acyl chlorides.191,194,371 Anisole, toluene, and... 

Fries rearrangement—that is, the transformation of phenolic esters to isomeric hydroxyphenyl ketones—is related to Friedel-Crafts acylations.392,393 Olah et al.394 have found a convenient way to perform the Fries rearrangement of a variety of substituted phenolic esters in the presence of Nafion-H in nitrobenzene as solvent [Eq. (5.153)]. A catalytic amount of Nafion-H is satisfactory, and the catalyst can be recycled. In contrast, Nafion-silica nanocomposites, in general, exhibit low activities in the Fries rearrangement of phenyl acetate to yield isomeric hydroxyacetophe-nones.239,395 In a recent study, BF3-H20 was found to be highly efficient under mild conditions (80°C, 1 h) to transform phenolic esters of aliphatic and aromatic carboxylic acids to ketones (71-99% yields).396 In most cases the para-hydroxyphenyl isomers are formed with high (up to 94%) selectivity. 

Nafion-silica nanocomposites exhibit high selectivity in the synthesis of substituted 7-hydroxychromanones 158 in high yields691 [Eq. (5.247)]. Conversions increase with increasing Nafion loading and, consequently, Nafion SAC-80 (Nafion-silica with a Nafion content of 80 wt%) affords the highest yields. The catalysts could be recycled after treatment with nitric acid or hydrogen peroxide. 

Nafion-silica nanocomposites (5%, 13%, 20%, 40%, and 80% loading) have been applied in the synthesis of a-tocopherol (160) in various solvents695 696 [Eq. (5.251)]. Under optimized conditions, Nafion SAC-40 proved to be the best catalyst (91% yield of a-tocopherol using 0.6 wt% of catalyst). Catalyst recycling was possible after reactivation with oxidizing agents (nitric acid or hydrogen peroxide). 

Harmer and co-workers996,997 tested various Nafion-based samples in the dimerization of a-methylstyrene to form isomeric pentenes and a cyclic dimer, which are of industrial interest 13% and 40% Nafion-silica nanocomposites exhibited near-complete conversion and gave the cyclic dimer 300 with high selectivity [Eq. (5.360)], whereas compound 299a was the main product over Nafion NR50. In sharp contrast, isomeric pentenes 299 could be obtained in a continuous process (86% selectivity at conversion >95%). Similar findings were reported with Nafion immobilized in MCM-41 mesoporous silica. In kinetic studies, 13% Nafion-silica and the catalyst with... 

Due to the high conversion obtained with SAC 40 and especially with SAC 80, only a small amount of catalyst contamination was observed and the catalyst could be reused several times or regenerated by a washing procedure with acetone and diluted nitric acid solutions. Compared with the conversion of resorcinol using lower amounts of other solid acid catalysts such as Amberlyst 15 (74%), H-BEA (36%), H-US-Y (10%) and H-ZSM-5 (3%), the SAC 40 and SAC 80 Nafion/silica nanocomposites are the favoured catalysts. 

Tang, H. L., and M. Pan, 2008. Synthesis and characterization of a self-assembled nafion/silica nanocomposite membrane for polymer electrolyte membrane fuel cells. JPhys Chem C 112 11556-68. 

Dicks, A., Duke, M and Diniz da Costa, J.C. (2004) Modified polyaniline-Nafion -silica nanocomposites for DMFC, in Proceeding of the 1st Nanomaterials Conference (eds S. Coombs and A.L. Dicks), ARC Centre for Functional Nanomaterials, Brisbane, Australia, pp. 10-13. 

Many efforts have been made to introduce new developments relating to established technologies and properly new technologies (Hommeltoft, 2001) and other catalysts have been evaluated as alternative for this process, such as solid materials like zeolites and Lewis and Bronsted acid in different solid supports (Feller et al., 2003, Feller et al., 2004, Feller Lercher, 2004, Platon Thomson, 2005, Thompson Ginosar, 2005, Guzmto et al., 2006), heteropolyacids (Zhao et al, 2000) and Nafion silica nanocomposite (Kumar et al., 2006, a-b. Shen et al, 2010). 

The solid acid catalyzed adamantylation of substituted benzene derivatives was studied with the aim to achieve high para regioselectivities. Of various acidic resins, zeolite HY, sulfated zirconia, perfluorolkanesulfonic acids, and phosphotungstic acid, Amberlyst XN-1010 was found to be the catalyst of choice to afford para-substituted alkylbenzenes with selectivities exceeding 70%.398 In further systematic studies with a Nafion-H-silica nanocomposite catalyst,399 and various... 

Solid Catalysts. Nafion-H is an active catalyst for acylation with aroyl halides and anhydrides.60,61 The reaction is carried out at the boiling point of the aromatic hydrocarbons. Yields with benzoyl chloride using 10-30% Nafion-H for benzene, toluene, and p-xylene are 14%, 85% and 82%, respectively. Attempted acylation with acetyl chloride, however, led to HC1 evolution and ketene formation. Nation resin-silica nanocomposite materials containing a dispersed form of the resin within silica exhibits significantly enhanced activity in Friedel-Crafts acylations.62,63... 

A comparative study of nanocomposites (16% Nafion-silica and commercial SAC-13) has been performed by Hoelderich and co-workers169 in the alkylation of isobutane and Raffinate II. Raffinate II, the remaining C4 cut of a stream cracker effluent after removal of dienes, isobutane, propane, and propene, contains butane, isobutylene, and butenes as main components. High conversion with a selectivity of 62% to isooctane was found for Nafion SAC-13 (batch reactor, 80°C). Both the quality of the product and the activity of the catalysts, however, decrease rapidly due to isomerization and oligomerization. Treating under reflux, the deactivated catalysts in acetone followed by a further treatment with aqueous hydrogen peroxide (80°C, 2 h), however, restores the activity. 

Recently, we found that a new class of solid acid catalyst, a Nafion resin/silica nanocomposite effectively catalyzes the Pechmann reaction (65). Nafion resin is a tetrafluoroethylene and perfluoro-2-(fluorosulfonylethoxy)propyl vinyl ether copolymer and can be made more suitable for organic catalysis. In the novel nanocomposites of Nafion resin, the nanometer-sized Nafion particles are entrapped within a porous silica matrix. These materials combine the excellent solid-acid... 

The Claisen rearrangement of allyl phenyl ethers proceeds in the presence of the Nafion-H and silica/Nafion-H nanocomposites. The 2-methyldihydrobenzofuran is formed as a major product (75%) in the presence of the Nafion-H beads the minor product is the orf/zo-allylphenol (25%) (equation 10). However, the ortto-allylphenol is formed as the major product in the presence of the Nafion/H-silica nanocomposites. 

Nafion-H-silica nanocomposite (13% Nafion-H) catalyzed Fries rearrangement of phenyl acetate at high temperatures gives phenol, ort/zo-acetylphenol, para-acetylphenol and para-acetylphenyl acetate in a ratio of 45 5 24.5 25.5 (equation 69). The rearrangement in the presence of added phenol gives exclusively the para-acetylphenol, showing that the Fries rearrangement under these conditions is intermolecular in nature. ... 

Harmer, MA Farneth, WE Sun, Q High surface area Nafion resin/silica nanocomposites a new class of solid-acid catalysts. J. Am. Chem. Soc. 1996, 118, 7708-7715. 

Laufer, M. C., Hausmann, H., Holderich, W. F. 2003. Synthesis of 7-hydroxycoumarins by Pechmann reaction using Nafion resin/silica nanocomposites as catalysts. J. Catal. 218(2) 315-320. 

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