Superacids solid, catalysts

Solid superacids may be made by treating ordinary solid add catalysts with strong Br0nsted or Lewis acids. For example, if freshly precipitated titanium hydroxide or zirconium hydroxide is treated with sulfuric acid and calcined in air at 500 °C. a very active solid acid catalyst results. The solids consist mainly of the metal dioxides with sulfate ions coordinated to the metal ions on the surface. Likewise, a superacid solid catalyst can be made by treating these metal oxides with antimony penlafluonde. Both catalysts contain both Br nsted and Lewis acid sites, and they arc sufficiently active to catalyze the isomerization of n-butane at room temperature.26... 

When using propanols for esterification, the alcohol and water form azeotropes making necessary secondary recovery and recycling of alcohol. An industrial process that is conducted in this way makes use of homogeneous catalyst (para-toluene sulfonic acid) lost after reaction [5]. A more efficient solution can found by employing an entrainer for breaking the azeotrope water/alcohol and superacid solid catalyst. Besides, the entrainer has a beneficial effect on the reaction rate, by increasing the amount of alcohol recycled to the reaction space [3]. 

For the esterification of FFA, superacid solid catalysts can be applied, such as ion-exchange resins (Amberlist, Nafion) or sulfated zirconia [22-24], The preferred environment is a reactive-distillation column. 

The isomerization of light paraffin using superacid solid catalysts is a clean way to increase the octane number of hydrocarbons. On this basis, sulfated metal oxides have attracted the attention of many research groups owing to their high activity in acid catalyzed reactions [1]. Sulfated zirconia was found to be a promising catalyst in this field and at the industrial level [2],... 

There is a real opportunity to reduce biodiesel production costs and environmental impact by applying modem catalyst technology, which will allow increased process flexibility to incorporate the use of low-cost high-FFA feedstock, and reduce water and energy requirement. Solid catalysts such as synthetic polymeric catalysts, zeolites and superacids like sulfated zirconia and niobic acid have the strong potential to replace liquid acids, eliminating separation, corrosion and environmental problems. Lotero et al. recently published a review that elaborates the importance of solid acids for biodiesel production. ... 

ACID AND SUPERACID SOLID MATERIALS AS NONCONTAMINANT ALTERNATIVE CATALYSTS IN REFINING... 

The tendency in the past decades has been to replace them with solid acids (Figure 13.1). These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous sihco-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nation (Figure 13.1). It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the sohd-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes. 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

In the case of C4-hydrocarbons, the use of acid or superacid solids will depend on both the acid strength required in each reaction and the reaction conditions required to optimize the thermodynamic equilibrium (Figure 13.3). For example, catalysts with very high acid strength could be substituted for a solid with a lower acidity by increasing reaction temperature. This has been proposed in both the isomerization of lineal alkanes and in the alkylation of isobutene with olefins, although the thermodynamic equilibrium should also be considered. 

A clear example of the possible use of acid and/or superacid solids as catalysts is the alkylation of isobutane with butenes. Isobutane alkylation with low-molecular-weight olefins is one of the most important refining process for the production of high-octane number (RON and MON), low red vapor pressure (RVP) gasoline. Currently, the reaction is carried out using H2SO4 or HF (Table 13.1), although several catalytic systems have been studied in the last few years. 

Cracking according to the second mechanism occurs by direct protonation of the paraffin with the solid catalyst acting as a superacid ... 

Solid acid catalysts such as mixed oxides (chalcides) have been used extensively for many years in the petroleum industry and organic synthesis. Their main advantage compared with liquid acid catalysts is the ease of separation from the reaction mixture, which allows continuous operation, as well as regeneration and reutilization of the catalyst. Furthermore, the heterogeneous solid catalysts can lead to high selectivity or specific activity. Due to the heterogeneity of solid superacids, accurate acidity measurements are difficult to carry out and to interpret. Up until now, the most useful way to estimate the acidity of a solid catalyst is to test its catalytic activity in well-known acid-catalyzed reactions. 

In discussing superacids as catalysts for chemical reactions, we will review both liquid (Magic Acid, fluoroantimonic acid, etc.) and solid (Nafion-H, etc.) acid-catalyzed reactions, but not those of conventional Friedel-Crafts-type catalysts. The latter reactions have been extensively reviewed elsewhere (see G. A. Olah, Friedel-Crafts Chemistry, Wiley, New York, 1972 G. A. Olah, ed., Friedel-Crafts and Related Reactions, Vols. I-IV, Wiley-Interscience, New York, 1963-1965). 

About 10 years have passed since this study began to be seriously undertaken, but the usage of solid superacids as catalysts is still limited. Table IX summarizes the acid-catalyzed reactions on sulfated metal oxides, i.e., cracking, isomerization, alkylation, acylation, esterification,... 

Solid catalysts can be used at elevated temperatures, though their acidities are much weaker than those of liquid ones. From this point of view, solid superacids based on Lewis acids and liquid superacids discussed in Sections II—1V are not sufficiently stable Nafion-H is also unsatisfactory, its maximum operating temperature being below 200°C. A new type of the sulfate-supported metal oxides is more stable because of preparatory heat treatment at high temperatures, but elimination of the sulfate is sometimes observed during reaction, thus it is hoped to synthesize superacids with the system of metal oxides. Another type of superacid, tungsten or molybdenum oxide supported on zirconia, has been prepared by a new preparation method, and its stability is satisfactory so far. It is hoped that the preparation method will be extensively applied to other metal oxides for new solid superacids. 

Another type of solid superacid is based on perfluorinated resin sulfonic acids, such as the acid form of DuPont s Nafion resin, a copolymer of a perfluorinated epoxide and vinylsulfonic acid, or higher perflu-oroalkanesulfonic acids such as perfluorodecanesulfonic acid, CF3(CF2) 03H. Such solid catalysts were found to be very efficient in alkylation of aromatic hydrocarbons and other Friedel-Crafts reactions. A comprehensive review is available on the application of Nafion-H in organic catalysis. ... 

Chloromethane is an important industrial chemical. Olah et al. [56] have reported the selective catalytic monochlorination of methane to chloromethane over superacidic sulfated zirconia solid catalysts, for example 804 /Zr02, Pt/ S04 7Zr02, and Fe/Mn/S04 7Zr02- The reactions were conducted in a continuous-flow reactor under atmospheric pressure. At 200 °C with 30 % chlorine the selectivity to chloromethane was > 90 %.The selectivity could be enhanced by adding platinum. The only by-product was CH2CI2. The latter is formed by the subsequent chlorination of chloromethane. No chloroform or carbon tetrachloride was formed. The authors postulated that chlorination occurs by an electrophilic insertion of an electron-deficient, metal coordinated, chlorine molecule into the C-H bond of methane. One drawback of the process was that above 225 °C, part of the metal was removed as the metal chloride [56]. Formation and subsequent loss of volatile metal chlorides is a major pitfall that should be avoided during vapor-phase chlorination over solid catalysts. 

Arata, K. and Hino, M. 1990. Solid catalyst treated with anion—XVIII. Benzoylation of toluene with benzoyl chloride and benzoic anhydride catalysed by solid superacid of sulphate-supported alumina. Appl. Catal. 59 197-204. 

Alkylation of methane by olefins under superacid catalysis was demonstrated both in solution chemistry under stable ion conditions and in heterogeneous gas-phase alkylations over solid catalysts using a flow system. Not only propylene and butylenes but also ethylene could be used as alkylating agents. 

On these bases it was accepted that, indeed, a new type of solid superacids was obtained and this opened new perspectives in the use of friendly solid catalysts for carrying out, reactions involving very strong acid sites under mild conditions. However, when research on this subject had progressed it became necessary to question the superacidity of sulfated metal oxides on the basis of the following observations. The use of Ho values for... 

Gu, X. Chen, T. Wan, Y. et al. Study on Synthesis of n-Butylacetate with S04 " /Zr02-Ti02-La Solid Superacid as Catalyst. Petrochem.Thchnol. 2002, 31(5), 353-... 

Acids are not limited to liquid (or gaseous) systems. Solid acids also play a significant role. Acidic oxides such as silica, silica-alumina, etc. are used extensively as solid acid catalysts. New solid acid systems that are stronger than those used conventionally are frequently called solid superacids. 

Many superacid-catalyzed reactions were found to be carried out advantageously not only using liquid superacids but also over solid superacids, including Nafion-H or certain zeolites. We extensively studied the catalytic activity of Nafion-H and related solid acid catalysts (including supported perfluorooctanesulfonic acid and its higher ho-... 

To solve some of the environmental problems of mixed-acid nitration, we were able to replaee sulfuric acid with solid superacid catalysts. This allowed us to develop a novel, clean, azeotropic nitration of aromatics with nitric acid over solid perfluorinated sulfonic acid catalysts (Nafion-H). The water formed is continuously azeotroped off by an excess of aromatics, thus preventing dilution of acid. Because the disposal of spent acids of nitration represents a serious environmental problem, the use of solid aeid eatalysts is a significant improvement. 

In these (and other) solid superacid catalyst systems, bi- or multi-dentate interactions are thns possible, forming highly reactive intermediates. This amounts to the solid-state equivalent of protosolvation resulting in superelectrophilic activation. 

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. 

Solid superacid catalysts, proposed as replacements for catalysts such as hydrogen fluoride and aluminum chloride for processes such as alkylation and acylation (Misono and Okuhara, 1993). 

Misono, M., and T. Okuhara (1993). Solid Superacid Catalysts. Chemtech, November, 23-29. 

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