Aluminosilicates acid catalysts

The general principle usually involves contact of an aldehyde or mixtures of aldehydes with ammonia at high temperature in the presence of an acidic catalyst. Aluminosilicate catalysts have been used (80USP4220783). A series of condensation reactions occurs with elimination of water and hydrogen and mixed products usually result. Acetaldehyde gives... 

The hydroamination of alkenes has been performed in the presence of heterogeneous acidic catalysts such as zeolites, amorphous aluminosilicates, phosphates, mesoporous oxides, pillared interlayered clays (PILCs), amorphous oxides, acid-treated sheet silicates or NafioN-H resins. They can be used either under batch conditions or in continuous operation at high temperature (above 200°C) under high pressure (above 100 bar). 

Catalytic cracking is a process that is currently performed exclusively over fluidized catalyst beds. The fluid catalytic cracking (FCC) process was introduced in 1942 and at that time replaced the conventional moving bed processes. These early processes were based on acid-treated clays as acidic catalysts. The replacement of the amorphous aluminosilicate catalysts by Faujasite-type zeolites in the early-1960s is regarded as a major improvement in FCC performance. The new acidic catalysts had a remarkable activity and produced substantially higher yields than the old ones. 

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]. 

Zeolites, which are aluminosilicates that can be regarded as being derived from AI2O3 and SiC>2, function as acidic catalysts in much the same way (Section 7.3). In addition, they catalyze isomerization, cracking, alkylation, and other organic reactions. A structurally related class of micro-porous materials based on aluminum phosphate (AIPO4) has also been developed (Section 7.7) like zeolites, they have cavities and channels at the molecular level and can function as shape-selective catalysts. 

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... 

Structural and Mechanistic Aspects of the Dehydration of Isomeric Butyl Alcohols over Porous Aluminosilicate Acid Catalysts... 

Over the past five years, we and our colleagues have undertaken an extensive study of the acid-catalyzed dehydration of the four isomeric butyl alcohols. In so doing, we compared the performance of crystalline, molecular-sieve acid catalysts (HZSM-5) in a range of crystal sizes (so as to vary diffusion path and active-site concentration) with that of amorphous aluminosilicate (AAS) gels in which the pore size is significantly larger. Our results, which permit the... 

Aluminosilicates (zeolites) are widely used as acidic and bifunctional catalysts. The formation of carbocationic intermediates is generally ascribed to the protons present in the open zeolite structure. A newer class molecular-sieve catalyst is the aluminophosphates. These may contain sihcon (SAPO) or metal (MeAPO) in their AiP04 frameworks. These framework substitutions in several cases generate protonic acidity that makes SAPO and MeAPO acid catalysts. There is already an extensive literature of this subject (20, 21). 

The fundamental carbonium ion-type reactions of olefins— including double bond and carbon skeleton isomerization, polymerization, isotopic exchange, and hydrogen transfer—have been reviewed earlier (62). The importance of a thorough understanding of the nature of olefin transformations over zeolite catalysts cannot be underestimated. Probably the most important and frequently recurring pattern is the transfer or redistribution of hydrogen that is observed with olefins over acidic crystalline aluminosilicate catalysts. 

The oximes of acetone, acetophenone, and cyclohexanone undergo rearrangement to the corresponding amides when they are passed over acidic crystalline aluminosilicates at 300-500° in continuous flow systems 151). As shown in Table XXIX, high conversions of oximes were generally observed, but selectivity to rearranged amides was variable. HY catalyst was most effective for this transformation. 

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... 

Microporous and mesoporous aluminosilicates are efficient acid catalysts for the alkylation of saccharides. Large-pore 12 MR tridirectional zeolites such as Y and Beta are reasonably active and selective in the synthesis of alkyl glycosides. The use of zeolites as acid catalysts for this reaction reduces the formation of oligomers as a result of shape-selectivity. As a consequence the mass balance of the reaction is relatively high compared with other catalysts, for example macropor-ous ion-exchange resins and /7-toluenesulfonic acid. 

Mesoporous aluminosilicates are of special interest as catalysts for the alkylation of ketoses, which are prone to degradation when treated with strong acid catalysts. Although disaccharides are generally subject to alcoholysis under strong acid catalysis, the use of MCM-41 as acid catalyst enables the selective preparation of alkylated disaccharides in good yield. 

Classify the following as semiconductor catalysts (S), acid catalysts (A), or insulators (I) Pd AI2O3 ZnO aluminosilicates MgO CoO zeoUtes... 

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. 

The acid forms of aluminosilicate zeolites have found wider use as acid catalysts than any other materials. Their outstanding utility derives from their relatively high acid strength, their high hydrothermal stability, their ability to impart shape selectivity to product distributions and the reproducibility with which they can be synthesised and modified. Each of these advantages stems directly from their crystalline structure. The two basic types of acid site types in microporous solids are Bronsted, which are protons located at bridging sites (Si-O-Al in zeolites, M-O-P in aluminophosphates) and Lewis, usually incompletely coordinated metal cations (especially aluminium in zeolites) in... 

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