Octane catalysts silica/alumina ratio

The effect of ZSM-5 as an octane additive to a cracking catalyst was studied in both a small fixed-bed reactor and a fluidized-bed pilot plant. Analyses of the products of these tests were used to determine the reaction chemistry. It was found that by maximizing the ratio of isomerization activity to hydrogen transfer activity, gasoline octane was increased with a minimum of yield loss. This could be accomplished by increasing the silica-to-alumina ratio of the additive zeolite. 

S. J. 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. 

Isomer ratios (Table 10.20) for the isomerization on n-CM by Isodewaxing to -15°C and by the Pt-silica-alumina catalyst to +22°C, then solvent dewaxed to -15°C, show that from the Pt-SAPO-11 catalyst, the 4-methyl isomer amounts to about 50% more than the 2-methyl case and from the Pt-silicon-alumina, the 4-methyl constitutes are three times as much as the 2-methyl. In the case of n-octane, Pt-SAPO-11 gave about equal quantities of the 2- and 3-isomers, and a similar situation occurred with n-hexadecane, where the 2-, 3-, 4-, and 5- isomer yields were just about equal. 

---