Solid acids acidity enhancement

Treatment with hydrogen sulfide of metal-ion-exchanged solid acids enhances the acidity and catalytic activity. ... 

De la Hoz, A., Moreno, A. and Vazquez, E., Use of microwave irradiation and solid acid catalyst in an enhanced and environmentaly friendly synthesis of coumarin derivatives, Synlett, 1999,608-610. 

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. 

The enhanced diffusivity of polynuclear compounds in sc C02 has been utilized to enhance catalyst lifetimes in both 1-butene/isoparaffin alkylations (Clark and Subramaniam, 1998 Gao et al., 1996). The former may be catalyzed using a number of solid acid catalysts (zeolites, sulfated zeolites, etc.), and the use of sc C02 as a solvent/diluent permits the alkylations to be carried out at relatively mild temperatures, leading to the increased production of valuable trimethylpentanes (which are used as high-octane gasoline blending components). The enhancement of product selectivity in the latter process is believed to result from rapid diffusion of ethylbenzene product away from the Y-type zeolite catalysts, thus preventing product isomerization to xylenes. 

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 ... 

The results pointed in another direction. The effects of steric congestion in the substrates are the most important factor. One way to enhance the selectivity would therefore be to amplify the steric effects by forcing the reaction to occur close to or on the surface of a solid acid catalyst. This was found to be a valid conclusion, and by using zeolites as catalysts the mode of ring closure could be partially controlled by a proper choice of zeolite. For certain substrates, the regioselectivity was reversed by changing the catalyst from Mordenite to Zeolite-Y [82]. 

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. 

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. 

A remarkable synthesis of oxalate salts using a supercritical mixture of CO2 and CO under very drastic conditions (400 bar, 380°C) in the presence of solid 082(003) was reported [71]. Friedel-Crafts-type alkylations on zeolites [72] or other solid acid catalysts [73] have been studied using SCCO2 as the medium. Performing the reaction in the supercritical state was found to be superior to either liquid or gas phase processes. Using scCOj lead to enhanced catalyst life-... 

Limestone Utilization. At a scrubber inlet pH of about 5.2, the corresponding limestone utilization is normally 80 percent or higher for an adipic acid-enhanced system, as compared to 65 to 70 percent in unenhanced limestone systems at an equivalent S0 removal. Thus the quantity of waste solids generated is reduced in an adipic acid-enhanced system. Higher limestone utilization also contributes to more reliable scrubber operation by reducing the fouling tendency. This increased reliability is a very attractive feature of adipic acid-enhanced systems, since reliability problems have historically plagued limestone FGD. 

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