Aromatization from dehydrogenation

Chemicals directly based on propane are few, although as mentioned, propane and LPG are important feedstocks for the production of olefins. Chapter 6 discusses a new process recently developed for the dehydrogenation of propane to propylene for petrochemical use. Propylene has always been obtained as a coproduct with ethylene from steam cracking processes. Chapter 6 also discusses the production of aromatics from LPG through the Cyclar process. ... 

Research is also being conducted in Japan to aromatize propane in presence of carhon dioxide using a Zn-loaded HZSM-5 catalyst/ The effect of CO2 is thought to improve the equilibrium formation of aromatics by the consumption of product hydrogen (from dehydrogenation of propane) through the reverse water gas shift reaction. 

Beck et al. (1997) used GC-MS to monitor the increase in the proportion of retene in pine tars with increasing temperature, and 13C-NMR to monitor the increased aromatic signal resulting from dehydrogenation and decarboxylation reactions. These data allow approximate determinations of the production temperature of a tar from the molecular composition. Diterpenoid molecules of probable pine origin have been detected in many archaeological contexts and some detailed compositional studies have appeared (Robinson et al, 1987 Heron and Pollard, 1988 Beck et al., 1989 Reunanen et al., 1989 Beck et al, 1994). 

The inverse electron demand Diels-Alder [4- -2]-cycloaddition of imidazoles to electron-poor dienes to yield imidazo[4,5-i pyridazines, reported in CHEC-II(1996), has been further developed. It was reported that the reaction of267 with tetrazines 268 was fruitless. However, 267 reacted with excess of 268 to yield aromatic 271 along with 1,4-dihydrotetrazine 270. Most likely, 271 arose from dehydrogenation of first-formed 269 by an extra equivalent of 268 <2001T5497> (Scheme 18). 

In the middle thirties the reactions of naphtha and certain compounds known to be present in naphtha were being studied in university and industrial laboratories. One of the problems was to find a catalyst that was capable of synthesizing an aromatic from a paraffin. It was reasoned that the hydrogenation-dehydrogenation oxide-type catalysts such as molybdenum oxide and chromium might possess suitable activity at temperatures well below those employed in thermal reforming. 

The principal source of toluene is catalytic reforming of refinery streams. This source accounts for ca 79% of the total toluene produced. An additional 16% is separated from pyrolysis gasoline produced in steam crackers during the manufacture of ethylene and propylene. The reactions taking place in catalytic reforming to yield aromatics are dehydrogenation or aromatization of cyclohexanes, dehydroisomerization of substituted cyclopentanes, and the cyclodehydrogenation of paraffins. The formation of toluene by these reactions is shown. 

Although obtained only in low yields upon troublesome chromatography of product mixtures that contained much polymeric material, the tricyclic benzofuran and benzothiophene lactones (61) were shown to be isolable products from attempted Diels-Alder reactions on the allene ester precursors shown in Equation (32) <85JCS(Pl)747>. Although it was noted in the case of the two thiophenes that the tricyclics appeared to be forming from a precursor (presumably a dihydro form) on the chromatographic column, it was not possible to convert the crude suspected cycloaddition adducts directly into the aromatics by dehydrogenation with DDQ. Complex mixtures were obtained instead. It is possible that the actual dienophiles in these Diels-Alder reactions are alkynes. In a related study, the bis-lactone (62) was also obtained (Equation (33)) <86H(24)88l>. 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

However, the major source of these hydrocarbons is now petroleum. Although aromatic compounds do occur naturally in petroleum, they are mainly obtained by the process of catalytic reforming, in which aliphatic hydrocarbons are aromatized through dehydrogenation, cyclization and isomerization. The process, which is also known as hydroforming, is carried out under pressure at 480-550 °C in the presence of a catalyst, typically chromium(III) oxide or alumina. Benzene is thus produced from... 

At the same time as the lower paraffins were being pressed into service, the second world war led to the manufacture of aromatics from petroleum. New methods of isolating, isomerizing, and dehydrogenating petroleum naphthenes were devised on the basis of petroleum techniques. During the war, manufacture of toluene and xylene was established since then, benzene has been added, because the growing demands of the chemical industry could not be met from the conventional source, coke-oven tar. 

A major factor in the rapid commercial utilization of catalytic reforming processing for upgrading low octane naphthas, and the production of aromatics from petroleum sources, has been the development of more active and selective dual-functional catalysts. These catalysts contain a very active hydrogenation-dehydrogenation agent such as platinum, in combination with an acidic oxide support such as alumina or silica-alumina. 

Since the concentration of aromatics in crude oils is low, production of aromatics from crude oil generally necessitates additional aromatization of naphthenic and aliphatic hydrocarbons by dehydrogenation and cyclization. Before the development of modem crude oil refining technologies, crude oils rich in aromatics, mainly from Southeast Asia, were commonly resorted to, especially for the production of toluene. 

The formation of aromatics from alkanes with less than six carbon atoms in their main chain (eg, methylpentanes) is also possible but proceeds much less readily than direct Ce-dehydrocyclization (13,85). This mechanism should involve a sort of dehydrogenative bond shift isomerization. The optimum hydrogen pressure (Fig. 5a) for aromatization of heptane isomers with less than six carbon atoms... 

For a better comprehension of this model, we started with a well-known process the production of aromatics from propene, involving different reaction mechanisms, like cyclization, ohgomerization, and dehydrogenation reactions resulting in aromatics. In this process, there are several other secondary reactions, such as carbonization and cracking [1-9]. 

---