Solid acids with enhanced acidity

The following subchapters cover various solid superacids, including perfluorinated sulfonic acid resins (Nafion resins). Furthermore, in the past, various attempts have been made to obtain solid superacids by either (a) enhancing the intrinsic acidity of a solid acid by treatment with a suitable co-acid or (b) physically or chemically binding a liquid superacid to an otherwise inert surface. We will briefly review some of these attempts because most of these catalysts rapidly lose activity and need to be regenerated. 

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

Friedel-Crafts alkylation of mesitylene with 2-propanol Polysiloxane-supported solid acid Solvent tunability Selectivity enhancement Hitzler et al. (1998a, 1998b)... 

Sulfated zirconia is a good example of a structural Lewis acid which has been chemically treated to enhance acidity. It has been extensively studied as a solid acid catalyst for vapour phase reactions and we1112 and others14 have found that a mesoporous version of this material is a particularly effective catalyst for liquid phase Friedel-Crafts alkylation reactions and to a lesser extent Friedel-Crafts benzoylations. The commercial (MEL Chemicals Ltd) material SZ999/1 shows a nitrogen isotherm characteristic of a mesoporous solid (surface area 162 m2g, pore volume 0.22 cm3g )- Whereas microporous and mesoporous materials are capable of rapidly catalysing the alkylation of benzene with various alkenes (Table 1), on reuse only the mesoporous... 

Enhancing the Activity of Solid Acid Catalysts with Supercritical Reaction Media Experiments and Theory ... 

Bentonite, whose main ingredient is montmorillonite, is one kind of layer structure clay mineral. It is an ideal material for preparation of pillared clays. If metal cations with high catalytic activity were intercalated in the interlayers of montmorillonite, a new type of solid acid catalyst can be obtained. The catalytic activity of the catalyst is closely associated with its porosity, specific surface area and surface total acidity. One of the effective ways to enhance catalytic activity is to prepare catalyst with appropriate metal cations and structure. 

Since azides are smoothly reduced on solid supports with SnCybenzenethiol/TEA (5 mmol/ 25 mmol/25 mmol for 1 mmol resin-bound azide rt, 2 this alternative procedure has been exploited for the synthesis of oligoureas on resin using a-azido acids.Similarly, azides are efficiently reduced to amines with dithiothreitol (DTT),P 1 a procedure that was successfully transferred to the reduction of a-azidoacyl peptides on resin for the SPPS with a-azido acids.In the case of sterically hindered azides, addition of small amounts of 2-sulfanylethanol to the DTT/DIPEA mixture or the use of DBU as a base enhances the reaction rates (Scheme Other reductions of azides include the use of H2S/pyridine/... 

The results for other conditions for polystyrene pyrolysis were reported. For example, pyrolysis on different catalysts was shown to lead to modifications of the yield of specific components in the pyrolysate. During the pyrolysis of PS on solid acid catalysts, the increase of contact time and surface acidity enhanced the production of ethylbenzene. Pyrolysis in the presence of water increases the yield of volatile products and that of monomer [30]. Studies on the generation of polycyclic aromatic hydrocarbons (PAHs) in polystyrene pyrolysates also were reported [36]. It was demonstrated that the content in PAHs in polystyrene pyrolysates increases as the pyrolysis temperature increases. The analysis of the end groups in polystyrenes with polymerizable end groups (macromonomers) was reported using stepwise pyrolysis and on-line methylation [46]. 

By integrating optimized acid sites with superior mass transport characteristics and a pore architecture that reduces pore-mouth plugging, a catalyst with enhanced performance can be created. Figure 5 demonstrates that both the catalyst selectivity and lifetime are significantly improved. As shown in figure 6, which compares the performance of Exelus solid-acid catalyst with other commercially available systems, the new catalyst system is easily able to achieve a step-change in performance over other solid-acid catalysts. 

The addition of further o-methoxy groups to the p-alkoxybenzyl alcohol framework naturally results in linkers with enhanced acid sensitivities. One of the first linkers of this type was 2-(4-hydroxy-3-methoxyphenoxy)acetic acid (11) developed by Sheppard and colleagues for the Fmoc solid phase synthesis of protected peptide acids for fragment condensation [41]. Treatment of peptidyl resins with... 

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