Clays acid catalysts

Ans The reactions differ in boiling points because the number of carbon atoms of the hydrocarbons present in each fraction is different. In catalytic cracking reactions, solid acid catalysts (clay, rare earth exchanged zeolites, etc.) promote carbonium- and carbenium ion-based C-C bond cleavage, while in thermal aacking it is homolytic cleavage. 

Many other polymerization processes have been patented, but only some of them appear to be developed or under development ia 1996. One large-scale process uses an acid montmorrillonite clay and acetic anhydride (209) another process uses strong perfiuorosulfonic acid reski catalysts (170,210). The polymerization product ia these processes is a poly(tetramethylene ether) with acetate end groups, which have to be removed by alkaline hydrolysis (211) or hydrogenolysis (212). If necessary, the product is then neutralized, eg, with phosphoric acid (213), and the salts removed by filtration. Instead of montmorrillonite clay, other acidic catalysts can be used, such as EuUer s earth or zeoHtes (214—216). 

Since 1976, these forms of acids have become a significant environmental concern from both a physical handling and disposal perspective. This concern has fueled much development work toward soHd acid catalysts, including zeoHtes, siHca —aluminas, and clays (107,108). 

Reaction of 1 mole of aminals 352 with 4 mol of methyl 3-aminocrotonate in the presence of the solid acids montmorillonte clay (Kio) and ZF520 zeolite as strong Bronsted acidic catalysts, gave 1,4-dihydropyridines 353 and 2-methyl-4//-pyrido[l, 2-n]pyrimidin-4-one (99MI8). 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Cationic polymerization of cyclosiloxanes is well known but used much less frequently than anionic reactions. The most widely used catalysts include sulfuric acid and its derivatives, alkyl and aryl sulfonic acids and trifluoroacetic acid1 2,1221. Due to their ease of removal, in industrial applications acid catalysts are generally employed on supports such as bentonite clay or Fuller s earth. 

Acid-treated clay catalyst Engelhard F-24 was found to be very effective for the alkylation of diphenylamine (DPA) with an olefin such as a-methyl styrene (AMS) to obtain a mixture of mono and dialkylated diphenylamines (Chitnis and Sharma, 1995). For example, alkylation of DPA with AMS produced a mixture of 4-(a,a-dimethyl benzyl) diphenylamine, i.e. monocumyl-diphenylamine (MCDPA) and 4,4 -bis(a,a-dimethylbenzyl) diphenylamine, i.e. dicumyldiphenylamine (DCDPA) (Eqn.(l 1)). The dialkylated diphenylamine, i.e. DCDPA, is indu.strially important as an antioxidant and heat stabilizer. DCDPA is reported to be an ideal antioxidant for many materials like polyethylene, polypropylene, polyether polyol, polyacetals, nylon 6, synthetic lubricants, hot melt adhesives, etc. 

The various other grades of acid-treated clay catalysts like Engelhard F-25, F-34, F-44,F-54, F-124, F-224, G-62, Tonsil K 306, etc. were also found to be useful catalysts for the alkylation of DPA with AMS. This alkylation reaction was unsuccessful with macroporous... 

Shah et al. (1994) have studied the preparation of a class of compounds called Indans, by cross-dimerization of AMS with amylenes, using an ion-exchange resin and acid-treated clay catalysts (Eqns. (12) and (13)). Indans can be subsequently converted, e.g. by acetylation, into perfumric compounds having mu.sk odour. For example, 1,1,2,3,3-pentamethylindan, the product obtained by cross-dimerization of AMS and wo-amylene (Eqn. (12)), can be reacted with propylene oxide and /7 ra-formaldehyde to give an indan type isochroman musk compound, 6-oxa-l,l,2,3,3,8-hexamethyl-2,3,5,6,7,8-hexahydro-lH-benz(f)-indene, sold as Galaxolide commercially. 

The selectivity for cross-dimerization relative to the dimerization of AMS, was found to be better with the acid-treated clay catalyst Engelhard F-24 than with the ion-exchange resin catalyst Amberlyst-15. Also, the formation of undesired side products, i.e. diisoamylenes, was lower in the case of Engelhard F-24 than for Amberlyst-15. 

Dimerization of unsaturated fatty acids, to. so-called dimer acids, is widely practised in industry, where acid-treated clays are invariably used as a catalyst. In the case of oleic acid the major products are dimers, trimers, and isosteric acid. Koster et al. (1998) have investigated the relative importance of the various acid sites as well as structural and textural parameters of montmorrilonite. The interlamellar space dominates the oleic acid dimerization and the active site is the tetrahedrol substitution site. 

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). 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

Traditionally, the production of LABs has been practiced commercially using either Lewis acid catalysts, or liquid hydrofluoric acid (HF).2 The HF catalysis typically gives 2-phenylalkane selectivities of only 17-18%. More recently, UOP/CEPSA have announced the DetalR process for LAB production that is reported to employ a solid acid catalyst.3 Within the same time frame, a number of papers and patents have been published describing LAB synthesis using a range of solid acid (sterically constrained) catalysts, including acidic clays,4 sulfated oxides,5 plus a variety of acidic zeolite structures.6"9 Many of these solid acids provide improved 2-phenylalkane selectivities. 

Alumina, silica, clays, and zeolites are increasingly used as acidic or basic supports [26], Cycloaddition reactions often require Lewis-acid catalysts if good yields are to be obtained. Clay and doped silica gel catalysts have emerged as useful alternatives to the use of Lewis acids. Cycloaddition offuran (5) under solvent-free conditions, catalyzed by K10 montmorillonite, results in a decrease in the reaction time the endo-exo relationship is no different that obtained by use of classical heating (Scheme 9.2) [27]. 

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. 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

During the period 1940-1962, the cracking catalysts used most widely commercially were the aforementioned acid-leached clays and silica-alumina. The latter was made in two versions low alumina (about 13% AI2O3) and high alumina (about 25% AI2O3) contents. High-alumina-content catalysts showed a higher equilibrium activity level and surface area. 

Simple ways to catalyze nitrations and sulfonylations of aromatic compounds are of great interest since these reactions are carried out industrially on large scale. Clays and zeolites with defined pore structures and channels as acidic catalysts have been utilized in nitrations [109] and Friedel-Crafts sulfonylations [110]. 

Prior to 1938, gasoline was obtained from thermal-cracking plants then the Houdry fixed-bed catalytic cracking process led to the development of a fluidized-bed process by Standard Oil for the catalytic production of motor fuels (4-8). Acid-treated clays of the montmorilIonite type were the first fluid-cracking catalysts widely employed by the industry. However, the ever greater demand for aviation fuels during the 1939-1945 period prompted the search for more active and selective catalysts. Research on novel catalyst... 

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. 

However, when it comes to the more important 2,2-diaryl derivatives (1.25), the routes illustrated in Figure 1.7 are not very useful. For these derivatives the almost universally adopted synthetic method involves the reaction of a l,l-diarylprop-2-yn-l-ol (1.24) with a substituted phenol or naphthol in the presence of an acid catalyst. The acid catalyst can be alumina, an acidic clay or Nafion for heterogeneous reactions, or trifluoroacetic acid, p-toluenesulfonic acid and dodecylbenzenesulfonic acid for reactions carried out in solution. The alkynols are prepared by reaction of a benzophenone (1.22) with a Na or Li derivative of an alkynide, such as the trimethylsilyl acetylide (1.23), (Figure 1.8). ... 

With this purpose, several different types of solid acid catalysts have been investigated for the acylation of aromatics, but the best performances have been obtained with medium-pore and large-pore zeolites (3-9). In general, however, the use of acylating agents other then halides, e.g., anhydrides or acids, is limited to the transformation of aromatic substrates highly activated towards electrophilic substitution. In a previous work (10), we investigated the benzoylation of resorcinol (1,3-dihydroxybenzene), catalyzed by acid clays. It was found that the reaction mechanism consists of the direct 0-benzoylation with formation of resorcinol monobenzoate, while no primary formation of the product of C-benzoylation (2,4-dihydroxybenzophenone) occurred. The latter product formed exclusively by... 

The Friedel-Crafts acylation of aromatic compounds is an important synthesis route to aromatic ketones in the production of fine and specialty chemicals. Industrially this is performed by reaction of an aromatic compound with a carboxylic acid or derivative e.g. acid anhydride in the presence of an acid catalyst. Commonly, either Lewis acids e.g. AICI3, strong mineral acids or solid acids e.g. zeolites, clays are used as catalysts however, in many cases this gives rise to substantial waste and corrosion difficulties. High reaction temperatures are often required which may lead to diminished product yields as a result of byproduct formation. Several studies detail the use of zeolites for this reaction (1). 

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