2.2.4- Trimethylpentane, cracking

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

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. 

Cracking of n- octane/2,2,4- trimethylpentane Al-M A reaction usefixl for discriminating between larger pore microporous materials based on reactants selectivity ratio. 46... 

Figure 6.22 Cracking reaction of C8 isomers on mordenite zeolite with various Si02 depositions. Reproduced with permission from [56]. Copyright (1985) Royal Society of Chemistry. O n-Octane A 3-methylheptane 0 2,2,4-trimethylpentane. (a) PtHM (b) SiPtHM (3.2%) (c) SiPtHM (3.4%) (d) SiPtHM (3.7%) (W/F) is defined as the amount of zeolite (W) divided by the flow rate (F — 0.091 h-1) is shown on the abscissa...

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 gas phase alkylation reaction of isobutane with 1-butene has been carried out on a series of Y zeolites dealuminated by treatment with HtEDTA solution. Dealuminated zeolites found to contain no extra-ftamework aluminium as evidenced by Al MAS NMR. However, only limited dealumination was achieved as attempts to further dealuminate results in structural collapse. Si MAS NMR spectra show the presence of amorphous siliceous species in highly dealuminated samples. Dealuminated Y zeolites are active for alkylation of isobutane with 1-butene but deactivated rapidly within first few minutes reaction. Mild dealumination improves the selectivity for trimethylpentanes but further dealumination results in the increase of cracked products and faster deactivation. The catalysts are active in the 60-80 C temperature range but beyond that catalyst deactivation was considerable. 

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. 

When butenes are used instead of ethylene, a lower temperature and a fine tuning of the acidity of the IL are required to avoid cracking reactions and heavy byproduct formation. The continuous butene alkylation has been performed for more than 500 h with no loss of activity and stable selectivity (80-90% isooctanes are obtained containing more than 90% trimethylpentanes MON = 90-95 RON = 95-98). A high level of mixing is essential for a high selectivity and then for a good quality alkylate. It has been shown that the addition of copper(I) chloride to the acidic chloro-aluminate improves the reaction performances [29]. 

Namba et al. studied the cracking of octane on H-ZSM-5 in the presence of other alkanes [27]. The reaction conditions were such that the conversion of octane obeyed first order kinetics and that the coverage of the active sites was low. The octane conversion was not affected by the presence of 3-methylheptane or 2,2,4-trimethylpentane in the feed. 3-Methylheptane is expected to diffuse rapidly through the ZSM-5 pores, while 2,2,4-trimethylpentane is excluded from the pores. Secondary shape-selectivity does not occur with these two molecules. However, the octane conversion dropped sharply with increasing partial pressures of 2,2-dimethylbutane in the feed. This strong inhibition caimot be the result of primaiy shape-selectivity, since the competing 2,2-dimethylbutane molecule should not be selectively adsorbed over octane. The explanation is that the slowly diffusing 2,2-dimethylbutane molecules retard the diffusion and, consequently, the conversion of the octane molecules. 

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