Hydroisomerization Model Compound Studies

A number of studies on model compounds such as n-octane, n-hexadecane, and n-CM were reported from Chevron Research and Technology44 47 using their successful SAPO-11 catalyst. SAPO-11 has an intermediate pore size with a one-dimen- 

FIGURE 10.24 Dependence of the viscosity index of substituted n-paraffins on the position of the substituent. 

Source R. T. Sanderson, Viscosity-Temperature Characteristics of Hydrocarbons, Industrial and Engineering Chemistry 41 368 (1949). 

Miller (Chevron) published results from early work that highlighted the selectivity of the platinum form of SAPO-11 catalyst compared to a number of others. These others were amorphous silica-alumina, from which one would expect little or no selectivity, ZSM-5, HY, and Na-Beta zeolites. All the catalysts carried 1 wt. % platinum and the feed employed was n-octane. He found that at 30% conversion, only SAPO-11, the amorphous silica-alumina, and the HY catalysts exhibited better than 94% selectivity for feed isomerization to isooctanes. ZSM-5 and Na-Beta catalysts behaved poorly in this regard. Selectivity for dimethylhexanes was low. SAPO-11 also produced equal quantities of 2- and 3-methyl heptanes, whereas the other catalysts favored 3-methyl heptane, with a ratio close to that favored by thermodynamics. SAPO-11 also produced one of the lowest levels of doubly-branched hexanes (Table 10.1646) and the predominant ones formed were those separated by more than one carbon—only minor amounts of the less thermally stable (bond breaking here can produce tertiary carbonium ions) geminal-dimethyl (2,2 and 3,3-) ones were formed. Noble metal presence was a key to success since replacement of the hydrogenation metal platinum by pallodium did not alter the isomeri-zation selectivity much, but replacement by nickel led to very poor isomerization. 

With the larger n-hexadecane (n-C16) molecule, just below the start of the lube carbon number range of about C20, as feed, SAPO-11 was found to be clearly superior to the other catalysts selected since at 94% conversion, selectivity for isomerization was about 85% versus about 70% for the Pt-Si02-Al203 catalyst 

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A systematic study of the mechanism of the hydroisomerization reaction has been undertaken in our laboratories in order to elucidate the various steps involved in such reactions. Ethylcyclohexane has been chosen as a model compound for the study of cycloalkanes, inasmuch as it may form a variety of isomers, of which the alkylcyclohexanes can be readily investigated. In order to determine the path through which such reactions may proceed, ethylcyclohexane having a labeled carbon atom in the a position of the side chain has been hydroisomerized. It was reasoned that if the hydroisomerization to dimethylcyclohexanes proceeds by rearrangement... 

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