Methylcyclopentane dehydrogenation

The alkylcyclopentane (AGP) to aromatics process (ACP ACH Ar) is less efficient than ACH dehydrogenation, owing to the slowness of the first step and to ACP ring opening. Under conditions where cyclohexane is converted to benzene with close to 100% efficiency, only 50—75% of methylcyclopentane may be converted to benzene. 

An example of a parallel-reaction network is the decomposition of cyclohexane, which may undergo dehydrogenation to form benzene and isomerization to form methylcyclopentane, as follows ... 

Fig. 1. Reaction composition profile. Reforming at 794 K, 2620 kPa. Zone A dehydrogenation zone zone B isomerization zone zone C hydrogenation and cracking zone. [Charge stock A, hexane (HEX) , benzene (BENZ) V, cyclohexane (CH) O, methylcyclopentane (MCP).]...

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

Sexton et al. (66) also examined the activity for the dehydrogenation of cyclohexane and conversion of methylcyclopentane of a series of PtSn alumina catalysts where the Sn/Pt ratio was varied. They found that the activity decreased as the Sn/Pt ratio increased. Selectivity for benzene formation from methylcyclopentane increased to a maximum at ca. 1.5 to 2.5 wt.% Sn (Sn/Pt = 4.9 to 8.2) and then declined. These conversions were conducted at normal pressures. 

Naphthene isomerization has been applied also to the conversion of methylcyclopentane to cyclohexane for subsequent dehydrogenation to benzene (24). 

Conversion of Methylcyclopentane over Single and over Coarse Particle Mixed Catalysts, at Dehydrogenative Conditions... 

The dehydrogenation of hexane to hexene or cyclohexane (reactions 5 and 6) only becomes appreciable at temperatures approaching 800 °K. The dehydrogenation to methylcyclopentane however appears to be thermodynamically feasible at temperatures as low as 350 °K. One cannot place too much reliance on this particular result since the affinity of formation of methylcyclopentane is known less accurately than the others. These three reactions, however, scarcely affect the synthesis of aromatic compounds in the reaction since the ethylenes and cycloparaffins are thermodynamically unstable relative to aromatic hydrocarbons above 550 °K, and they decompose spontaneously to form aromatics at this temperature. They can therefore only appear as intermediates in reaction (9) above 550 °K. 

The equilibria for isomerization reactions are much less temperature sensitive than those for dehydrogenation reactions, since the heats of reaction are relatively small. The equilibrium between methylcyclopentane and cyclohexane favors the former, indicating that the five-membered ring structure is more stable than the six-membered ring. In the equilibria between n-hexane and the methylpentanes, 2-methylpentane is the favored isomer over 3-methylpentane. This is reasonable from the simple statistical consideration that the substituent methyl group can occupy either of two equivalent positions in the former molecule, compared to one in the latter. 

In the reaction scheme in Figure 5.2, the dehydrocyclization of n-hexane proceeds via formation of n -hexene on dehydrogenation centers, followed by cyclization of the n-hexene to methylcyclopentane on acidic centers. The methylcyclopentane then is converted to benzene in the manner already described. Alternatively, it seems possible that a hexadiene may be an intermediate in the reaction sequence. Such a sequence would involve formation of a hexadiene on platinum sites, followed by cyclization on acidic centers to form a cyclic olefin, methylcyclopentene (1). 

A catalyst consisting of platinum dispersed on an acidic alumina is a very effective dual function catalyst, used in petroleum reforming of naphtha and also for paraffin isomerization. The conversion of naphtha constituents such as methylcyclopentane, MCP, to benzene, B, is desired in order to increase octane rating. The reaction pathway for conversion of MCP to B is illustrated in Fig. 3 . MCP is first dehydrogenated on a platinum site to the olefin of the same structure. The olefin then transfers to an acidic site where it is isomerized to cyclohexene. This olefin proceeds to a platinum site where it is dehydrogenated to B and H2. In the diagram, vertical movement represents hydrogen subtraction or addition and horizontal movement represents isomerization. 

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