Cyclopentanes dehydrogenation

Wanae and Walters [69] studied the thermal degradation of cyclopentane in a static system at 438-548 °C and under pressure of 38-244mmHg. It is shown that under these conditions the cyclopentane conversion degree was below 25%. There are some other works [70, 71] devoted to cyclopentane dehydrogenation here, however, the cyclopentadiene yield does not exceed 3-11%. 

Catalytic Reforming. Worldwide, approximately 30% of commercial benzene is produced by catalytic reforming, a process ia which aromatic molecules are produced from the dehydrogenation of cycloparaffins, dehydroisomerization of alkyl cyclopentanes, and the cycHzation and subsequent dehydrogenation of paraffins (36). The feed to the catalytic reformer may be a straight-mn, hydrocracked, or thermally cracked naphtha fraction ia the... 

My initial plan involved dehydrogenation of cyclic alkanes to give stable arenes, particularly of cyclopentane to give cyclopentadienyl derivatives. My own experimental efforts with Pt were unavailing, but Jeimifer Quirk, a graduate student of mine working on Ir, took an interest in the project and found a small yield of Cp... 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

In the case of iridium, complex [IrH2(PPh3)2(acetone)2] BF4 (11) was the first to carry out catalytically the dehydrogenation of cycloalkanes [13, 14]. However, it was later realized that the halocarbons used as solvents reacted with 11 to produce the stable species [HL2lr(p-Cl)2(. i-X)IrL2H]BF4 (X = Cl (14) or H (15)) [16] (Scheme 13.8), and that elimination of the solvent by running the reactions in neat alkane not only improved yields but also permitted the activation of other previously unreactive cycloalkanes, such as methyl- and ethyl-cyclopentane. However, it was also noted that the system in some cases was not catalytic, due mainly to decomposition of the catalyst at the temperatures employed [16]. 

Starting from C5 molecules, dehydrocyclization (into cyclopentane and derivatives of cyclopentane) is also possible. From C6 on up, aromatization also occurs. These two reactions comprising a dehydrogenation step are only observable at temperatures which on most metals are higher than the region where hydrogenolysis (hydrocracking) is first observed. 

The ability of a catalyst to promote isomerization plays two roles in reforming it increases the amount of branched chain paraffins in the product and it converts naphthene hydrocarbons with cyclopentane rings into cyclohexane ring naphthenes which are necessary for the formation of aromatics by dehydrogenation. 

In each case, as the temperature was raised to 200 K, a markedly changed VEEL spectrum was observed, which was attributed to the formation of cyclopentene by dehydrogenation (see Section VI.B). Avery s study of the adsorption of cyclopentane was continued to 260 K, whereby a much simpler spectrum was obtained, convincingly attributed to the formation of the 175-C5H5 (i75-cyclopentadienyl) structure adsorbed flat on the surface. The 200 K spectrum of the species on Ru(0001) may even contain some features characteristic of this species (strong bands at 758 and 3057 cm 1). 

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. 

Figure 6. Initial rates vs Pt contents of the Pt/TiC>2 specimens for liquid methanol (A) or 1-propanol. (B) dehydrogenation at 298 K cyclopentane-deuterium exchange in gas phase at 263 K (C) oxygen isotope heteroexchange at 298 K over non-preoxidized (D) or preoxidized (E) samples.

Interpretation of the optimum metal content for these reactions. As already mentioned an optimum Pt content was found for dehydrogenation of liquid alcohols and cyclopentane-deuterium exchange in gas phase. Also, with Pt/Ti02 samples which had not been preoxidized and which were accordingly non-stoichiometric according to conductivity measurements, the same optimum content was found for the initial rate of OIE, whereas this rate decreased as a function of Pt content for preoxidized samples (44). 

Figure 4.4 Kinetic curves for conjugated dehydrogenation of cyclopentane at (a) 450 °C and (b) 600 °C (1 unreacted cyclopentane 2 cyclopentene yield (per involved cyclopentane) and 3 cyclopen-tadiene yield (per involved cyclopentane)).

These studies determined the range of parameters of cyclopentane conjugated dehydrogenation with hydrogen peroxide [75],... 

Conjugated dehydrogenation of cyclopentane to cyclopentadiene passes through an intermediate formation of cyclopentene, and kinetic regularities of this reaction are identical to those for cyclohexane ... 

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