Solid acids aluminosilicates

We have also investigated the properties of several of our nanostructured catalysts as solid acids in reactions such as the dehydration of alcohols and transesterification reactions [99]. One of the best examples of atomically dispersed solid acid catalysts is aluminosilicates [100]. When aluminium is substituted into silicate frameworks and remains isolated from other A1 centers it can behave as a strong acid site [101]. 

Traditionally, solid acidic catalysts are applied in industry for the oligomerization of butenes and are still studied. MTS-type aluminosilicates,522 a NiCsNaY zeolite,523 and a silica-alumina containing 13% alumina524 proved to be active and selective catalysts. Moreover, deactivation rates of these catalysts are also favorable. Sulfated zirconia promoted with Fe and Mn was active and selective to yield primarily dimethylbutene isomers under supercritical conditions.525 A small amount of water improved productivity and decreased deactivation. A study showed that the blending octane number of Cg hydrocarbons is directly linked to the number of allylic hydrogens in the molecules.526... 

While they are not solvents and solutions in the usual sense of the word, it is convenient here to introduce the concept of solid acids and bases. For example, consider the class of compounds known as zeolites. These are aluminosilicate structures with variable amounts of A1( II). Si(IV), metal cations, and water (see Chapter 16). 

Aluminosilicates represent solid acids (42). In general, the nature of their acidity is quite clear and is associated with the chemical peculiarity of the substitution of a tetrahedral Si4 + ion by an Al3 + ion in the silicate structure. At the same time, the detailed structure of BASs and LASs in a particular system still remains the subject of experimental studies. Recently quantum-chemical methods have also come to find ever-increasing use in this field. To... 

Oxide catalysts fall into two general categories. They are either electrical insulators or they can act as semiconductors. Insulator oxides are those in which the cationic material has a single valence so they have stoichiometric M 0 ratios. The simple oxides, MgO, AI2O3 and Si02 and the more complex zeolites, which are aluminosilicates, fall into this category. These materials are not effective as oxidation catalysts and find most use as solid acids or bases.2-3... 

As described in Chapter 10, clays are laminar aluminosilicates. While these materials are most commonly used as solid acids their ion exchange capability provides a means of incorporating catalytically active metals into the laminar... 

Fio. 20. Solid acidity (Ho < 6.8) of aluminosilicate exchanged with cadmium ion... 

The reaction of mono- and poly-alcohols catalyzed by solid acids has been widely investigated. An important application is the synthesis of five membered cyclic ethers starting from di- or triols. Several authors described such cyclisation reactions, starting from 1,2,4-butanetriol (clay) [1], 1,2,5-pentatriol (pentasile, mordenite, erionite) [2]. Linear ethers like dimethyl ether are formed from methanol (modified aluminosilicate, zeolites) [3,4] or MTBE from methanol and i-butene (zeolite, resin) [5,6] The yields of the desired products are often quite high, e g over 90 % in the case of 1,2,4-butanetriol to 3-hydroxy-tetrahydrofiiran and about 60 % in the case of dimethyl ether. The reactions are either carried out in the presence of water as slurry process [1,2] at 150 - 200 °C or at temperatures > 300 °C in the gas phase with a fixed bed catalyst [2-4]... 

Surface area of the layer aluminosilicates available for adsorption of the reactants and the hydroxyl groups present on the surface contributing to the acidity, decide how successfully they can function as solid acids for catalysis. Consequently the study of surface acidity and its modification has attracted considerable attention. The effect of treatment of the clay with mineral and organic acids i.e., acid... 

The Diels-Alder reaction has been shown to be subject to catalysis by a wide range of solid catalysts (see Chapter 4 for some examples). Acidic mesoporous aluminosilicates can be used to catalyse selective Diels-Alder reactions such as that between cyclopentadiene with methyl acrylate. The zinc-exchanged version of the material is particularly effective and compares well to other more established solid acids such as the ion-exchanged clay Zn2+-K10 as well as homogeneous catalysts such as boron trifluoride (Table 2.7).50... 

Solid Acidity of SAPO-n and MeAPO-n. - The largest difference between AlPO-n and AlPO-n substituted with metal cations (SAPO-n or MeAPO-n) is its solid acidity and, consequently, its ion-exchange properties. Solid acidity is caused by substitution of a part of A1 or P in framework with metal cations. However, the number of acid sites caused by metal substitution does not increase linearly as the amount of substituted metal cations increases. This is very much in contrast with aluminosilicate zeolites. It is well known that, in the case of zeolites, the number of acid sites increases linearly as the number of A1 increases in the framework. Consequently, the acidity is often expressed simply by the Si/Al ratio in the zeolite. However, the number of acid sites as well as their strength depends on the amount of substituted metal cations in a complicated way. This is because two sites for substitution, A1 and P, exist in the framework of AlPO-n, and the substituted cations are not always substituted at the same site. The acidity of SAPO-n or MeAPO-n has been studied in connection with the state of substituted metal. In this section, acidity of SAPO-n and MeAPO-n will be briefly reviewed. 

By the 1980s most of the aluminosilicate zeolites currently used industrially were known, and the emphasis shifted to the study of these materials using a range of powerful new techniques that came of age at this time. These included, in particular, solid state NMR, X-ray and neutron powder diffraction analysis, high resolution electron microscopy and computational methods. All were ideal for the study of structural details of solids that were rarely available, and never used in industrial applications, other than as microcrystalline powders. All these techniques are applicable to the bulk of the solid - this in turn makes up the (internal) surface, which is accessible to adsorbed molecules. Since the techniques are able to operate under any conditions of gas pressure, they may be used to extract structural details in situ under the operating conditions of ion exchange, adsorption and catalysis. In particular, zeolitic systems have proved ideal for the study, understanding and subsequent improvement of solid acid catalysts. 

Among the other mieroporous solid acids, SAPOs and MAPOs tend to possess acidities that are weaker than those found for aluminosilicates. Not all substituted aluminophosphates are weak acids, however. Magnesium-substituted AIPO4-36, for example, gives a strong performance in acid catalysed alkane cracking. " ... 

Whilst solid acids such as zeolites and other aluminosilicates have been the subject of considerable study in the context of alkane chemistry (e.g. catalytic cracking and alkene isomerisation), the design of more versatile materials which can be used in the fine chemical industry has been less thoroughly researched. However, the need for the production of solid acids to replace traditional protonic acids such as hydrogen fluoride, phosphoric and sulfuric acids in liquid phase processes is an increasingly important goal. Some progress has been made in this area and the commercial product Envirocat EPIC [19] provides an excellent example of a... 

As a catalyst support material, clays possess unique electronic and structural properties. They may be considered as solid acids, with electrostatic properties that can affect catalytic behavior and, possibly, the microstructure of polymers made with clay-supported catalysts. The layered structure of these aluminosilicates has also been reported to affect polymerization kinetics. Studies in this area have focused on three main effects polymerization activity and stability, polymer molecular weight, and polymer tacticity. 

The activated carbon, owing to its porosity and chemical surface composition, both of which may be appropriately controlled, is also recommended as a suitable catalyst support [179]. On the other hand, both silica and alumina oxides as well as natural amorphous aluminosilicates and zeolites are widely used as heterogeneous catalysts. These adsorbents having acid or (and) base sites are named as solid acid-base catalysts [183]. 

Solid acid catalysts such as clays and zeolites are also utilized for phenol acylation however, these processes suffer from catalyst deactivation problems and lack C-selectivity. In the acylation of phenol with acetic anhydride, HZSM-5 zeolite shows a very high ort/io-selectivity (48% o-HAP yield, <1% p-HAP yield), although phenyl acetate is isolated in only approximately 20% yield [115]. The SAR value has a remarkable influence on the selectivity of the process when the reaction is carried out in the presence of HZSM-5(30), HZSM-5(150), and HZSM-5(280) zeolites, the o-HAP yields are 42,40, and 15%, respectively, whereas the O-acylation is noticeably increased. These results mean that C-acylation requires higher Brpnsted acidity and that lower acidity leads to phenyl acetate formation. It must be noted that the reaction performed with an amorphous aluminosilicate acid catalyst gives mostly phenyl acetate without isomer selectivity. These results suggest that the C-acylation of phenol occurs in the channels of zeolites and not on the external surface. 

These remarkable aluminosilicates can be used as drying agents, ion-exchangers, and molecular sieves for gas separation. Their microporosity provides them with high surface area, and they can be converted into solid acids with superb catalytic activity. 

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

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