Zeolites as Solid Acid Catalysts

The effective area of the anionic aluminosilicate framework in the pores of a zeolite is at least 100 times the external surface area, and it can be as high as 1000 m g . Consequently zeolites are unusually effective as catalysts for reactions that are favored by aluminosilicate surfaces. Substitution of Si + by AP+ in a silica framework makes it acidic and, potentially, coordinatively unsaturated. Suppose, for example, that we heat the NH4 form of a zeolite. Ammonia is driven off, and one H+ remains to counterbalance each Al + that has substituted for a silicon. The protons are attached to oxygens of the aluminosilicate framework  

However, further heating drives off the elements of water as water vapor  

one A1 has lost H+ from the neighboring OH group, and one Si has lost 0H and therefore becomes coordinatively unsaturated, as well as positively charged. This Si can therefore act as catalytic site for the numerous organic reactions that are catalyzed by Lewis acids (i.e., electron-pair acceptors). 

Consider, for example, the zeolite-catalyzed dehydration of alcohols to give olefins  

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for ( ) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (m) cracking of hydrocarbons (i.e., loss of H2), and Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

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In contrast with the widespread application of zeolites as solid acid catalysts (see earlier), their use as solid base catalysts received scant attention until fairly recently [121]. This is probably because acid-catalyzed processes are much more common in the oil refining and petrochemical industries. Nonetheless, basic zeolites and related mesoporous molecular sieves can catalyze a variety of reactions, such as Knoevenagel condensations and Michael additions, which are key steps in the manufacture of flavors and fragrances, pharmaceuticals and other specialty chemicals [121]. Indeed, the Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate (Fig. 2.36) has become a standard test reaction for solid base catalysts [121]. 

The variable temperature measurements of MAS NMR offer very variable information on the dynamic character of the protons of zeolites as solid-acid catalysts. The temperature-dependent lineshape of MAS NMR spectra of acidic protons (acidic OH groups) in zeolites shows the first and unambiguous evidence for proton migration in zeolite structure. There is a possibility that the mobility of protons influences a catalytic activity of solid-acid catalysts. 

Zeolite molecular sieves are widely used as solid acid catalysts or catalyst components in areas ranging from petroleum refining to the synthesis of intermediates and fine chemicals (112,113). An important reason for their widespread use is the flexibility they oflFer regarding the tailoring of the concentration and nature of catalytically active sites and their immediate environments. We note that discrimination between chemical and structural aspects works well at a conceptual level, but one faces quite severe limitations as soon as one tries to separate the contributions of the two effects. The complexity arises because the chemical properties of a particular molecular sieve are connected with its framework density. 

Although there are some important differences between what we describe as 3-connected aluminium sites in our bb-matrices and what the active sites are thought to be in zeolites, we have begun a preliminary study of the activities of the Al, Ti and V-containing bb-catalysts as solid acid catalysts in the dehydration of alcohols. For this type of bench marking reaction, there are two parameters that can be used as preliminary indicators of catalytic activity lightoff temperatures and product selectivity. A plot of conversion versus temperature produces what is known as a lightoff curve. The temperature at which 50% of the maximum... 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

Zeolites have many uses, most importantly as cation exchangers (e.g., in water softening), as desiccants (i.e., drying agents), and as solid acid catalysts. 

Clays are naturally occurring minerals that are produced in enormous quantities and find a wide variety of applications including their use as catalysts [8-10, 20, 21]. They were widely used as solid acid catalysts in oil refining from the 1930s until the mid 1960s when they were replaced by the zeolites which exhibited better activity and selectivity. 

Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. 

Zeolites have found extensive applications in the field of petroleum cracking as solid acid catalysts [75]. With the introduction of redox metal atoms into the framework. 

Traffic fuels account for about one-third of oil use and thus development of improved and more sustainable traffic fuels is a priority worldwide. We will not discuss this problem here, but instead take only two examples to evidence that not only zeolite or mesoporous materials are used as solid acid catalysts but that other solid acid catalysts (ion-exchange resins in these specific cases) are also vhdely applied [218]. 

Zeolites are widely used as solid acid catalysts for a number of organic transformations, such as the cracking of n-paraffins which are catalysed by Bronsted acid sites. " In the case of zeolites, the so-called bridging hydroxyl groups in the i-OH-Als configuration as shown in Eq. (1) are known to act as Brdnsted acid sites and they are responsible for the ability of zeolites to catalyse the reactions. Therefore, the characterization of acidic properties of solid acids is of great importance in discussing the catalytic properties of solid acids. 

The metallosilicates molecular sieves, [Ga]-ZSM-5, [B]-ZSM-5, and [B,A1]-ZSM-5 are also used as solid-acid catalysts as well as zeolites. There is a possibility that the mobility of protons is influenced by incorporating Ga and B atoms into the ZSM-5 zeolite skeleton instead of A1 atom, because the extent of incorporation of the isomorphous replacement affects the catalytic properties, e.g. the activities of the zeohtes. Quantification can be attained by appropriate test reactions. ... 

Industrial methods for the synthesis of esters involve the use of mineral acids such as H2SO4, H3PO4 and ion exchange resins. In the recent years zeolites are finding more application as solid acid catalysts in several organic reactions. The application of solid acids like oxides and zeolites in the place of mineral acids is highly desirable as such processes are enviromnentally safe. 

Sheemol, V. N., Tyagi, B., and Jasra, R. V. 2004. Acylation of toluene using rare earth cation exchanged zeolite (3 as solid acid catalyst. J. Mol. Catal. A Chem. 215 201-208. 

Zeolite catalysts are used widely in the area of chemical industry because they have high activity as solid acid catalysts and shape-selectivity based on their small pore size at molecular level. Many researchers have reported their interesting catalytic properties for the production of many chemical compounds [1-3]. Especially, the formation of para-dialkylbenzenes such as p-xylene has been studied a lot [4-7] because para-dialkylbenzenes are valuable components for polymers. The selective formation of p-xylene by alkylation of toluene and... 

Many of the reactions catalysed involve the transfer of protons to and from the zeolite (Figure 2.8). As this is the characteristic function of an acid, the H-forms of the zeolite (those in which the adsorbed ions are H+) are sometimes referred to as solid acid catalysts. Synthetic zeolites are often prepared in concentrated solutions of sodium hydroxide, which leaves Na" as the counter ion in the zeolite pores, the Na-zeolite. The usual way to prepare H-zeolites is to exchange these metal ions for ammonium ions (NH4" ), and then heat the ammonium form of the zeolite to about 500 °C. Ammonia (NH3) is then driven off, leaving H+ ions to balance the negative charges on the silicate structure. This is the reverse of the reaction shown in Figure 2.8, when the base B is NH3. 

P-14 - A study on the use of zeolite Beta as solid acid catalyst in liquid and gas phase esterification reactions. The influence of the hydrophobicity of the catalyst... 

Zeolites. Zeolites are usually used as solid-acid catalysts. However, alkali ion-exchanged zeolites, especially faujasites, are weak bases and show unique... 

Metal oxide catalytic materials currently find wide application in the petroleum, chemical, and environmental industries, and their uses have significantly expanded since the mid-20th century (especially in environmental applications) [1,2], Bulk mixed metal oxides are extensively employed by the chemical industries as selective oxidation catalysts in the synthesis of chemical intermediates. Supported metal oxides are also used as selective oxidation catalysts by the chemical industry, as environmental catalysts, to selectively transform undesirable pollutants to nonnox-ious forms, and as components of catalysts employed by the petroleum industry. Zeolite and molecular sieve catalytic materials are employed as solid acid catalysts in the petroleum industry and as aqueous selective oxidation catalysts in the chemical industry, respectively. Zeolites and molecular sieves are also employed as sorbents for separation of gases and to trap toxic impurities that may be present in water supplies. Significant molecular spectroscopic advances in recent years have finally allowed the nature of the active surface sites present in these different metal oxide catalytic materials to be determined in different environments. This chapter examines our current state of knowledge of the molecular structures of the active surface metal oxide species present in metal oxide catalysts and the influence of different environments upon the structures of these catalytic active sites. 

Thomas, B. Sugunan, S. Rare-earth (Ce, La, Sm, and RE ) exchanged Na-Y zeolites and K-10 clay as solid acid catalysts for the synthesis of benzoxazole via Beckmann rearrangement of salicylaldoxime. Micropor. Mesopor. Mater. 2006, 96, 55-64. 

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

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