Silica-occluded

HPAs, however, is their solubility in polar solvents or reactants, such as water or ethanol, which severely limits their application as recyclable solid acid catalysts in the liquid phase. Nonetheless, they exhibit high thermal stability and have been applied in a variety of vapor phase processes for the production of petrochemicals, e.g. olefin hydration and reaction of acetic acid with ethylene [100, 101]. In order to overcome the problem of solubility in polar media, HPAs have been immobilized by occlusion in a silica matrix using the sol-gel technique [101]. For example, silica-occluded H3PW1204o was used as an insoluble solid acid catalyst in several liquid phase reactions such as ester hydrolysis, esterification, hydration and Friedel-Crafts alkylations [101]. HPAs have also been widely applied as catalysts in organic synthesis [102]. 

Several zeolites in the II-form, two activated clays, a silica-alumina, a sulfonic acid resin and a silica-occluded heteropoly acid were tested in the reaction of cyclohcxcne and toluene (excess) at 110 °C [64]. The ortho / meta / para ratio of the mixtures strongly depends on the structure of the catalysts involved. With zeolite H-USY and Filtrol-24 as active catalysts the meta / para ratio is found to be about 2 1, in agreement with the thermodynamic equilibrium, and the ortho-isomer is essentially absent.By contrast 11-Bcta and H-mordenite gave a meta /para ratio of 1 4.5. As H-USY appeared to be a good isomerization catalyst for the cyclohexyltoluenes, the mechanism may involve ortho / para-alkylation followed by isomerization. Researchers of UOP (Dcs Plaines, USA) found a separation method for meta / para cyclohexyltoluenc (undisclosed technique). Altogether the results open a new low-waste route to 3-methylbiphenyl. 

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). 

Figure 2. Nitrogen adsorption isotherm and pore-size distribution of silica-occluded...

Liquid-phase Organic Reactions over Silica-occluded H3PWj2O40... 

Silica-occluded H3PW12O40 can be used as an insoluble solid-acid catalyst in several liquid-phase reactions such as ester hydrolysis [15c], esterification [15dj, hydration [15d], alkylation of phenol [15d] in polar media, and Friedel-Crafts type alkylation and transalkylation [16] in non polar media. 

Turnover frequency/ 0 mmol meq min (based on the total amount of acid), silica-occluded H3PW,204q (1.0 g, H3PW12O40 content 11 % wlw). 

Silica-occluded H3PW12O40 is a microporous material with a relatively sharp pore-size distribution with a peak at 0.55 nm (Figure 2). This microporous property seems to be favorable for molecular shape-selective reactions. Indeed, silica-occluded H3PW12O40 acts as a shape-selective solid-acid catalyst in the solvent-free alkylation of phenol with formaldehyde (Scheme 1). 

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