Nafion, silica supported

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).

To increase the surface area, the resin can be supported on porous carriers, or it can be directly incorporated into silica by a sol-gel preparation technique. Both methods have been used by Botella et al. (205), who compared several composite Nafion/silica samples with varying surface areas and Nafion loadings for isobutane/2-butene alkylation at 353 K. Furthermore, supported and unsupported Nafion samples were used. As expected, the unsupported resin with its low... 

This latter interpretation would mean that with the approach depicted in Fig. 10, the catalyst itself could be monitored. The authors reported that the silica-supported Nafion could not be observed in the beginning of their experiments and appeared in the spectra only after the catalyst interacted with octanol. This observation may indicate that the octyl groups promote the sticking of the catalyst particles onto the ATR probe, within the evanescent field. However, the example also shows that this approach may not be without problems, because it depends on the adsorption of the particles from the slurry reactor onto the ATR element. This process is accompanied by the adsorption of molecules on the catalyst surface and complicates the analysis. More important, as also indicated by the work of Mul et al. (74). this adsorption depends on the surface properties of the catalyst particles and the ATR element. These properties are prone to change as a function of conversion in a batch process and are therefore hardly predictable. 

Bronsted acids can also be fixed to the surfaces of many common support materials. Surface attached perfluorosulfonic acids have been reported123 as well as the perfiuorinated sulfonic acid resin (Nafion)-silica composites reported a year earlier.124 The former are prepared by reacting... 

Another solid-aeid catalyzed reaction in which the Nafion-silica composites are much more active than the non-supported pure resin and Amberlyst-15 is the addition-esterification of carboxylic acids to cyclic olefins. For example, the addition of saturated carboxylic acids to dicyclopentadiene, leading to starting esters for the flavor and fragrance industry [27]. 

A variety of other heterogeneously catalyzed Diels-Alder reactions has been reported. Nafion-H, a perfluorinated resinsulfonic acid, catalyzed several Diels-Alder reactions and the isolated yield of the adducts was 80-95% [46]. We have found that a recently described Nafion-silica composite catalyst containing 13 % (w/w) Nafion [47] was approximately 30 times more active than the pure resin in the Diels-Alder reaction of 2,3-dimethylbutadiene with 1,4-naphthoquinone [48]. We also showed that another strong heterogeneous Brpnsted acid, tung-stophosphoric acid supported on silica gel, is a very active catalyst of Diels-Alder reactions of quinones [49] and other enones [50]. 

Nafion is a perfluorinated polymer with sulfonic acid groups grafted to side chains, yielding acidity similar to that of sulfuric acid [5]. Nafion has not been extensively studied as a catalyst for isoparaffin alkylation, although it has shown good activity for a number of acid catalyzed reactions [6-9]. Nafion is available in both unsupported and supported forms. In the supported form, the polymer is impregnated on high surface area silica supports, which has been shown to improve accessibility to acid sites [10,11]. 

Rorvik et al. studied unsupported Nafion for isobutane/1-butene alkylation in a stirred liquid phase batch reactor [12]. The production of trimethylpentanes (the most desirable alkylate product) was shown to cease within 30 minutes of operation. More recently, silica-supported Nafion was used to catalyze the same reaction [13]. Once again, rapid deactivation with respect to trimethylpentane formation was observed. It was hypothesized that the strongest acid sites—the most active for alkylation—are also the first to be poisoned. 

Temporal conversion and selectivity profiles obtained over a silica-supported Nafion catalyst in a stirred reactor operated at supercritical conditions are shown in Figure 2. The experiment was carried out at 80 bar, 368 K, and 0.05 h OWHSV, using a 5 1 isoparaf-fm/olefm (FO) ratio and 2.4 fold molar excess of CO2 [(C02+I)/0=19j. At these conditions, a steady butene conversion (80%) and product selectivity are demonstrated for 48 hours on stream. The alkylate selectivity (TMP+DMFI) and overall Cg selectivity are constant at 27% and 75% respectively. Clearly operation in the near-supercritical region facilitates superior alkylate production over other published results using Nafion catalysts in the liquid phase... 

Figure 4 shows the butene conversion over the silica-supported Nafion catalyst in both liquid and supercritical phases at 368 K, an OWHSV of 0.05 h and an I/O ratio of 10. The liquid phase was maintained at a pressure of 26 bar, while the supercritical phase was maintained at 95 bar, with a 2.4 fold molar excess of carbon dioxide ( 70% total mole fraction CO2). In both cases, a high steady butene conversion is observed. However, the alkylate selectivity continuously declines to zero after 45 hours on stream, at which point the catalyst is only active for butene oligomerization. At the supercritical condition, the acid sites responsible for alkylation are kept active, extending the production of the desired trimethylpentanes. Similar results comparing liquid and supercritical phase runs were also seen on unsupported Nafion . 

Figure 5 compares the activity of the silica-supported SAC-13 with unsupported Nafion particles. Both catalysts show similar product selectivity at identical conditions. As expected, the butene turnover frequency, defined as the butene conversion rate per acid site, is enhanced fourfold when using the supported catalyst. Similar enhancements on supported Nafion have been reported for other reactions as well [10,11,24]. Since hydrated Nafion can conduct protons, acid sites that are hidden within the polymer may not be available for reaction, but may still be measured by the aqueous titration method. 

Okubo T., Nagamoto H. Low temperature preparation of nanostructured zirconia and YSZ by sol-gel processing. J. Mater. Sci. 1995 30 749-757 Okubo T., Takahashi T., Sadakata M., Nagamoto H. Crack-free porous YSZ membrane via controlled synthesis of zirconia sol. J. Membr. Sci. 1996 118 151-157 Palinko L, Torok B., Surya Prakash G.K., Olah G.A. Surface characterization of variously treated Nafion-H, Nafion-H supported on silica and Nafion-H silica nanocomposite catalysts by infrared microscopy. Appl. Catal. A Gen. 1998 174 147-153 Schmidt H., Wolter H. Organically modified ceramics and their applications. J. Non-Cryst. SoUds 1990 121 428 35... 

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 dimerization of isobutene carried out in a forced-flow polymeric catalytic membrane reactor was reported by D. Fritsch and co-workers. The authors prepared composite porous membranes consisting of a catalytic layer made of solid add catalysts, such as siUca supported Naflon , Nafion NR50, Amberlyst 15 and silica supported tungstophosphoric add dispersed in polymeric binders such as Teflon AF, Hyflon AD, polytrim-ethylsilylpropyne, or polydimethylsiloxane (PDMS), cast on microporous support membranes made of polyacrylonitrile (PAN) or Torlon . The membranes were assembled in the membrane reactor into which isobutene was fed in the retentate side with a build-up pressure of 4 bar. The liquid product was collected on the permeate side. 

To keep the consumption of the valuable platinum low, thin foils have been glued to a graphite support [34] or thin-layers of platinum have been sputtered on to a glass base [59]. Platinum can be used in a particle electrode by plating silica gel with platinum [60], or in a solid polymer electrolyte where platinum is incorporated into a nafion ion exchange membrane [61]. 

Reactivity of a number of solid acid catalysts that include zeolites, resin, nafion and HP As was determined for the direct reaction of ethylene with acetic acid to produce ethyl acetate (Table 1). It was established that the Keggin HSiW supported on silica is very active for the vapom phase reaction of acetic acid with ethylene at about 180°C, 145 psig with a high molar ratio of ethylene to... 

Using different DFT functionals and basis sets (Focsan et al. 2008, Lawrence et al. 2008) it was confirmed that the isotropic ()-methyl proton hyperfine couplings do not exceed 9MHz for the carotenoid radical cation, Car-. DFT calculations of neutral carotenoid radicals, Car formed by proton loss (indicated by ) from the radical cation, predicted isotropic P-methyl proton couplings up to 16 MHz, a fact that explained the large isotropic couplings observed by ENDOR measurements for methyl protons in UV irradiated carotenoids supported on silica gel, Nafion films, silica-alumina matrices, or incorporated in molecular sieves (Piekara-Sady et al. 1991, 1995, Wu et al. 

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