Trimethylpentanes distribution

Butylenes. Butylenes are the primary olefin feedstock to alkylation and produce a product high in trimethylpentanes. The research octane number, which is typically in the range of 94—98, depends on isomer distribution, catalyst, and operating conditions. 

The effect of butene isomer distribution on alkylate composition produced with HF catalyst (21) is shown in Table 1. The alkylate product octane is highest for 2-butene feedstock and lowest for 1-butene isobutylene is intermediate. The fact that the major product from 1-butene is trimethylpentane and not the expected primary product dimethylhexane indicates that significant isomerization of 1-butene has occurred before alkylation. 

Observed product distributions, however, make it clear that there must exist reaction pathways in addition to those of the sort in (24) and (25). Thus m-xylene is also a product from 2,3,4-trimethylpentane. This is illustrative of a reaction for which an adsorbed cyclo-C4 intermediate has been suggested (89, 95, 98, 8,185,188, 189). 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

The distributions of products within a certain carbon number fraction are far from equilibrium. In the Cg-fraction, for example, the dimethylhexanes would be thermodynamically favored over the trimethylpentanes, but the latter are predominant. The distribution within the trimethylpentanes is also not equilibrated. 2,2,4-TMP would prevail at equilibrium over the other TMPs, constituting 60-70% of the product, depending on the temperature. Furthermore, 2,2,3-TMP as the primary product is found in less than equilibrium amounts. Qualitatively, the same statement is valid for the other carbon number distributions. Products with a tertiary carbon atom in the 2-position dominate over other isomers in all fractions. 

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 1-butene conversion and product distribution obtained at 25°C after 1 h of alkylation reaction of isobutane on JML-I50 and Beta catalysts are summarized in Table 6.1. The conversion (97%) with JML-I50 catalyst is higher than that (86%) with zeolite Beta. The primary products with the above catalysts are Cs compounds (59.9% with JML-I50 and 62% with Beta). The Cg products mainly consist of trimethylpentanes (TMPs), 58.7% for JML-I50 and 73% for zeolite Beta. The TMP/DMH (dimethylhexane) ratios are 13.5 for JLM-I50 and 4.1 for Beta, demonstrating that the selectivity of JML-I50 is higher than that of zeolite Beta. The yields of alkylate are 6.6 mL and 5.2 mL for JML-I50 and Beta zeolite, respectively. The weights of alkylate produced per weight of butene fed to the reactor are 1.13 and 0.95 for JML-I50 and zeolite Beta, respectively. 

The main products formed by the catalytic alkylation of isobutane with ethylene (HC1—AICI3, 25-35°C) are 2,3-dimethylbutane and 2-methylpentane with smaller amounts of ethane and trimethylpentanes.13 Alkylation of isobutane with propylene (HC1—AICI3, — 30°C) yields 2,3- and 2,4-dimethylpentane as the main products and propane and trimethylpentanes as byproducts.14 This is in sharp contrast with product distributions of thermal alkylation that gives mainly 2,2-dimethylbutane (alkylation with ethylene)15 and 2,2-dimethylpentane (alkylation with propylene).16... 

In addition to saving acid the additive appeared to improve the octane number by more than 0.1 MDN as was Indicated by a few spot checks of alkylate during the run. The improvements generally arose from a slight Increase In the Cg fraction, a rise In the trimethylpentane concentration and changes of the trl-methylpentane distribution. The octane analyses are not nearly as extensive as the tltratable acidity determinations and the Improvements are noted as being consistent with what would be estimated from plant correlations and the observed reduction In acid composition. 

Figure 2. Distribution of trimethylpentanes in trimethylpentane family as function of temperature (Amberlyst XN-IOIO/BF, catalyst)...

On the other hand, the 1-5 ring closure-ring enlargement process is supported by the initial formation of aromatics from a number of alkanes with only five carbon atoms in a linear chain (22, 25, 26, 33, 70, J32), by the easy aromatization of substituted cyclopentanes (63, 69, 132-134), and by the identical aromatic product distributions from 2,2,4-trimethylpentane and... 

The catalytic behaviour in the alkylation reaction of Ce-Y zeolite has been compared with those obtained on superacid catalysts such as Nafion-H (Rorbik et al. 1995). They observed that both catalysts show a high initial activity for the alkylation reaction with a rapid decrease in the alkylation selectivity. Nevertheless, the Ce-Y catalyst showed the highest molar ratio of trimethylpentanes (TMP) to dimethylhexanes and can be considered a better catalyst for the alkylation reaction. However, the different distribution of TMP and especially the highest formation of the isomer 2,2,4-trimethylpentane (2,2,4-TMP) on Nafion-H has been explained by the acidity distribution. It is likely that the strongest acid sites in a zeolite deactivate first. Since 2,2,4-TMP is formed mainly in the first minutes on zeolitic materials, its formation is probably more dependent on the presence of strong acidic sites than any of the other isomers. 

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. 

Trimethylbenzene transalkylation Trimethylbenzene-phenol transalkylation Trimethylpentane synthesis Trimethylsilylcyanation, asymmetric Tris (acetylacetonato)Cr Trivalent distribution in MCM-22 Trivalent distribution in MFI Trivalent-template interaction TS-1 Ol-P-14 Il-P-15 27-P-ll... 

Figure 2 shows the results of activity experiments, with LCH-Y and the supported metal catalysts Pt(0.5) and Pd(0.5). The product distribution is very similar with the three catalysts. The selectivity towards trimethylpentanes (TMP) is 50% approximately, being the main type of component of the C5+ fraction. At about 25 minutes on oil, the selectivity to this type of compound decreases, and increases towards the dimethylhexenes (DMH=). This is due to coke deposition that deactivates the acid sites, decreasing the hydrogen transfer... 

Browarzik et al calculated asphaltenes flocculation at high pressures for methane + crude oil - - 2,2,4-trimethylpentane [i-octane] using continuous thermodynamics where 2,2,4-trimethylpentane acts as a precipitant. The asphaltene flocculation was considered to be a liquid -b liquid equilibrium. Browarzik et al applied the van der Waals equation of state. The polydispersity of the crude oil was considered to be described by the solubility parameter of the Scatchard-Hildebrand theory. Within this distribution the asphaltenes represent the species with the highest solubility parameters. The calculated results were compared to experimental data. For oils with a very low content of asphaltenes the model describes the experimental flocculation data reasonably well. However, on contrary to the experimental results, the model predicts the asphaltenes to show a higher flocculation tendency with increasing asphaltenes content of the crude oil. Based on these comparisons further work was undertaken by Browarzik et al and the associates formed... 

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