Aromatization, aliphatic hydrocarbons, zeolites

One of the most significant stages in the development of zeolite catalysts was the synthesis by Mobil scientists (U.S. Patent 3,702, 866) of the zeolite now universally known as ZSM-5 (i.e. Zeolite Socony Mobil-5). This was the first - and most important - member of a new class of shape selective catalysts, which have made viable the production of synthetic gasoline . In this process, high-octane gasoline is produced by the catalytic conversion of methanol to a mixture of aromatic and aliphatic hydrocarbons (Derouane, 1980). Because of its unique combination of chemical nature and pore structure, ZSM-5 is a highly effective dehydration, isomerization and polymerization catalyst. 

Alcohol Dehydration using New Shape Selective Zeolites. - The shape selectivity of zeolites has been referred to and exploited in numerous ways. Nevertheless, within the last ten years a new class of zeolite (Type ZSM-5) has been reported, which possesses pore openings, intermediate in size between the large and small pore zeolites its sieving properties in respect of alkyl aromatics and various aliphatic hydrocarbons have been reported. 

Catalytic aromatisation of aliphatic hydrocarbons was first described by researchers at Mobil.2s In this process, termed M-2 forming , alkanes from ethane to high boiling point naphthalenes can be aromatised. The most effective catalyst for aromatisation was found to be the medium pore HZSM-5.26 Large pore zeolite and amorphous silica-alumina with broad pore size distributions gave only low yields of aromatics due to rapid coke formation. 

O Connor has written surveys of both homogeneous and heterogeneous catalysis while the reviews of Skupinska and Al-Jarallah et focus particularly on homogeneous catalysis. Recently, Minachev et al. have written about their work on the catalytic and physicochemical properties of the zeolites based on pentasils for oligomerizing lower olefins and paraffins into a mixture of aliphatic hydrocarbons of the composition Ce-Cio or into a concentrate of aromatic hydrocarbons, depending on the reaction conditions. 

The number of oxygen atoms bound to silicon and aluminum atoms in a zeolite pore opening determines its size. A ring of eight oxygen atoms characteristic of zeolite A, for example, is sufficiently large to admit and expel straight-chain aliphatic hydrocarbons, but is so small as to exclude aromatic and branched chain compounds ... 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeolite catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg+ aliphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equilibrium, ie, 102—104% of equilibrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions are temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydrocarbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

Although zeolites have been known for their adsorption properties for over a century, it was not until 1952, when the first synthetic zeolite was prepared, that their utility in chemical transformations was explored. Since that time, zeolites have been used for a multitude of purposes, and to this day, they are essential catalysts in the petroleum industry, converting large and small hydrocarbons into high-octane compounds. As an outgrowth of this work, zeolites have found utility in industrial fine chemical synthesis for the construction of aromatics, heterocycles, aliphatic amines, and ethers, and the photochemistry within zeolites has already grown out of its infancy. 

The external surfaces of H-FER [143-145], H-MFl [146, 147] and H-MOR [147, 148] have been studied by IR spectroscopy of adsorbed hindered aliphatic nitriles, pyridine [145, 146], lutidine and aromatic hydrocarbons [149]. Terminal silanols and Lewis acid sites exist at the external surface of H-FER, H-MFl and H-MOR. Interestingly, the acidity of the external silanol OH groups of zeolites can be enhanced with respect to those of silica, appearing similar to those of SA. The very... 

The alkylation of the naphthenic cation causes formation of complex aliphatic carbonium ions. Transformation of such intermediates according to Poustma [30] gives the molecules of light saturated hydrocarbons and aromatics. It is generally accepted that the formation of condensed aromatic rings being a coke precursors is difficult in the pores of ZSM-5 zeolite. The fact that the products of the toluene transformation reaction in all cases contained 1 - and 2-methylnaphthalene seems to prove their formation from the olefinic or naphthenic carbocations. Transformation of the naphthenic carbocations occuring in zeolite pores and on the external zeolite surface is the most probable source of methyinaphthalene isomers [23]. 

Harandi 1993). The hydrocarbons produced in this process are aliphatic and aromatic, and are predominantly in the gasoline range (C4 to Ci0). The gasoline thus produced is chemically conventional, with unleaded research octane numbers of 90 to 95. Activity, selectivity and catalyst decay strongly depend on the structure of the zeolite. 

The interaction of many hydrocarbons (both aliphatic and aromatic) with zeolites has been investigated. HY zeolite catalyses the conversion of cyclopropane at room temperature to isobutane. The proposed mechanism involves a non-classical protonated cyclopropane ion intermediate. At 200 cyclopropane isomerizes to propene and also forms aromatic species. Adsorption and transformation of but-ene has been widely studied. It is useful to draw a distinction between hydroxylated and dehydroxylated samples. On hydroxylated samples but-ene isomerizes and also oligo-The -OH groups vibrating at 3640 cm" were found to be... 

In two papers by Walsh and Rollman [14-C]labelled hydrocarbons were used to study the origin of carbonaceous deposits on zeolites. With feeds composed of an aliphatic + an aromatic hydrocarbon, the initial reaction involved in the formation of coke was the alkylation of aromatics by the olefmic fragments of alkane cracking. Since ZSM-5 and mordenite have the same framework A1 content, it was possible to compare directly the coke yields of these zeolites. Under the same experimental conditions it was found that C deposition on mordenite was almost two orders of magnitude greater than on ZSM-5. The differences were explained in terms of pore size. In the smaller ZSM-5 pore, the alkylaromatics, once formed were prevented from reacting further to produce coke, because of the spacial constraints. 

The acid-catalyzed aldol condensation of acetone is probably the most studied reaction of this type. As shown in Scheme 2, self-condensation of acetone yields a number of different products depending on the operating conditions and catalyst used. In particular, on acidic catalysts, the products are mainly aliphatic and aromatic hydrocarbons. A summary of the product distribution as a function of the reaction temperature on acidic zeolites can be found in Ref (65). 

The declining oil reserves have stimulated considerable efforts towards the exploration of alternative sources of energy and organic chemicals. One solution is to use the abundant supply of coal as a source of synthesis gas (CO + H2) which is readily converted to methanol (MeOH). MeOH can then be transformed into higher molecular weight hydrocarbons (olefins, aliphatics and aromatics) over shape-selective zeolite catalysts, the most successful of which in this respect is H-ZSM-5, capable of converting MeOH to hydrocarbons up to Cio- The selective synthesis of ethylene and propylene, the key intermediates for the production of detergents, plasticizers, lubricants and a variety of chemicals, proceeds over smaller pore zeolites such as chabazite and erionite. 

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