Trimethylpentane, catalytic cracking

The trimethylpentanes are easily produced by alkylating isobutane with isobutylene, but unfortunately, the content of isobutylene produced by catalytic cracking is only about one-third of the total butylenes in the C4 stream, the remaining butylenes being butylene-1 and butylene-2. Although most of the butylene-2 tends to form trimethylpentanes, the butylene-1 must be isomerized to butylene-2, either in the alkylation reaction or in a separate previous reaction, before it will form trimethylpentane. If not isomerized, the butylene-1 when alkylated forms the much lower-octane material, dimethylhexane. 

P G Smimiotis, E. Ruckenstein. Comparison of the Performance of ZSM-5, b-Zeolite, Y, USY and Their Composites in the Catalytic Cracking of n-Octane, 2,2,4-Trimethylpentane and 1-Octene. Industrial Engineering Chemistry Research, Vol. 33, 800-813, 1994. 

The catalytic activity of the protonic forms of SAPO-37 and HY zeolite were compared in the cracking of n-heptane and 2,2,4-trimethylpentane. HY zeolite presents a higher initial activity which is in agreement with its higher acidity characterized by temperature programmed desorption of ammonia. This is confirmed by the fact that SAPO-37 exhibits a higher relative cracking activity (2,2,4-trimethylpentane/n-heptane) than HY zeolite. 

The catalytic behavior of an Al-ITQ-7 zeolite, with a three-dimensional system of large pore channels, has been evaluated for the liquid phase alkylation of isobutane with 2-butene, and compared to that of a Beta zeolite. In absence of deactivation (TOS=l min), zeolite ITQ-7 gives a higher proportion of C5-C7/C5+, obtained by cracking of Cs and specially of the bulky C9+. However, the main differences are observed in the distribution of the trimethylpentane (TMP) isomers. Although zeolite ITQ-7 is more selective to TMP in the C8 fraction than Beta, the most abundant isomers are 2,3,3- and 2,3,4-TMP instead of the primary 2,2,3-TMP or the thermodynamically favored 2,2,4-TMP. This is a clear shape selectivity effect, due to the smaller pore size of ITQ-7 as compared to Beta, and the fact that 2,3,3- and 2,3,4-TMP are the isomers with less restricted transition states and smaller diffusion problems. 

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