Trimethylbenzenes, separation

The catalytic performances obtained during transalkylation of toluene and 1,2,4-trimethylbenzene at 50 50 wt/wt composition over a single catalyst Pt/Z12 and a dualbed catalyst Pt/Z 121 HB are shown in Table 1. As expected, the presence of Pt tends to catalyze hydrogenation of coke precursors and aromatic species to yield undesirable naphthenes (N6 and N7) side products, such as cyclohexane (CH), methylcyclopentane (MCP), methylcyclohexane (MCH), and dimethylcyclopentane (DMCP), which deteriorates the benzene product purity. The product purity of benzene separated in typical benzene distillation towers, commonly termed as simulated benzene purity , can be estimated from the compositions of reactor effluent, such that [3] ... 

Other MFl-type zeolite-sorbate systems are known to exhibit similar behavior. In a recent study, Yu et al. [34] reported that at saturated loadings of -hexane a single MFl-type zeolite unit cell has an overall volume expansion of 2.3%, which can correlate to shrinkage in non-zeoUtic pores up to 7 nm for a 1 tm crystal when isotropic expansion is assumed. It was demonstrated that, even in membranes with large number of defects, the crystallite swelling caused the membrane to achieve significant separation between n-hexane and trimethylbenzene, iso-octane and 2,2-dimethylbutane using pervaporation [34]. 

T he petroleum industry entered the field of aromatics production largely because the unprecedented demand for toluene for the manufacture of TNT at the outbreak of World War II in 1939 could not be met by other sources. As a result of its efforts, the industry supplied 75 to 85% of all the toluene which was nitrated for TNT production during the latter years of World War II. Since that time the petroleum refiners have remained in the field and at present they are major suppliers of toluene and xylenes. In Table I it is shown that in 1949 about 59% of the toluene and 84% of the xylenes produced in the United States were derived from petroleum sources. The petroleum industry has diversified its operations in the field of aromatics production until at present a variety of materials is offered. Table II presents a partial list of the commercially available aromatics, together with some of their uses. A number of other aromatics, such as methylethyl-benzene and trimethylbenzene, have been separated in small scale lots both as mixtures and as pure compounds. 

In the field of aromatic separation, the trend of research is toward isolation of pure compounds for chemical purposes. Benzene, toluene, and some of the C8 aromatics have been separated and used commercially. However, the physical properties of the C9 and Cio aromatic hydrocarbons found in reformed stocks show that other aromatics could be separated from these mixtures by distillation, crystallization, or extraction processes. It is reasonably certain that if sufficient demand develops for the pure compounds, processes for their separation will become available. Present information indicates that perhaps methylethylbenzenes and trimethylbenzenes could be isolated in relatively high purity by distillation from aromatic stocks obtained by hydroforming, but no information is available as to their industrial uses. Similarly, durene (1,2,4,5-tetramethylbenzene) possibly could be isolated from its homologs by crystallization. Furthermore, large... 

Difficulties met in separating chemical individuals from higher fractions of light oil and lower fractions of middle oil stimulated attempts at the direct nitration of solvent-naphtha, the name given to a mixture of isomers comprising xylenes, ethylbenzene, pseudo-cumene (1,2,4-trimethylbenzene), ethyltoluene and mesitylene. 

Chromatograms in Figure 6 show the separation of tri-methylbenzenes. As it was observed for dialkylbenzenes p -CD complexation not only improves selectivity towards trimethylbenzene isomers, but also works as an organic solvent by lowering their capacity factors. This makes the time of analysis shorter and detectability better (2j ). The improvement in the resolution of trimethyl benzenes due to the OC-CD complexation is not so obvious. 

Figure 6. Separation of trimethylbenzenes (a) without CD, (b) with 3.1 O M a-CD, and (c) with 2.7 x 10 1 M /J-CD. Conditions as in Figure 5. (Reprinted with permission from ref. 28. Copyright 1986 Elsevier Science Publishers.)...

After CLD modification, the zeolites exhibit excellent shape selectivity for separation and purification of various isomers. For instance, the pore-size-adjusted NaY zeolite after Si(OCH3)4 modification is very effective for the separation of methylnaphthalene and trimethylbenzene isomers. Because the pore size of NaY itself is large, before modification the zeolite shows no shape selectivity for the two methylnaphthalene isomers, and the adsorption capacities for 1-methylnaphthalene and 2-methylnaphthalene are similar. However, as the pore size decreases, the zeolite exhibits increasing adsorption capacity... 

The separation effect for the mixture of 1,2,4-trimethylbenzene and 1,3,5-trimethyl-benzene is similar, and as the amount of Si(OCH3)4 used increases, the pore size of NaY zeolite is narrowed gradually, and the adsorption selectivity of the zeolite for 1,2,4-trimethylbenzene increases rapidly. When the Si(OCH3)4 amount used is 0.15 mL/g, the adsorption capacity of the zeolite for 1,2,4-trimethylbenzene remains almost unchanged, but the adsorption for 1,3,5-trimethylbenzene is very small, and the 1,2,4-trimethyl benzene adsorption selectivity is exceeds 90%, achieving an ideal separation effect as shown in Figure 6.24.[64] Comparison of the two systems reveals that to achieve an ideal separation effect, more modifier is required that is, the zeolite pore size should be narrower for separation of trimethylbenzene mixture because the size of the 1,3,5-trimethylbenzene molecule is smaller that that of 1-methylnaphthalene. 

In a typical experiment of the chloromethane/zeolite reaction on the ZSM-5 catalyst chloromethane converion varies from 98.5% to 100%. Over 240 compounds have been analytically separated from the reaction mixture and about 90% of the products have been identified. 1,2,4-trimethylbenzene is a major product, which constitutes about 45 wt% of the total liquid product. Tetra- and penta-methylbenzenes are also found but in substantially lower amounts. A few percent of the products contain chlorine, which are mostly 2-chloroalkanes presumably formed by Markovnikov addition of HCl to the terminal olefins, both being produced in... 

A distillation column with a partial reboiler and a total condenser is being used to separate a mixture of benzene, toluene, and 1,2,3-trimethylbenzene. The feed, 40 mol% benzene, 30 mol% toluene, and 30 mol% 1,2,3-trimethylbenzene, enters the column as a saturated vapor. We desire 95% recovery of the toluene in the distillate and 95% of the 1,2,3-trimethylbenzene in the bottoms. The reflux is returned as a saturated liquid, and constant molar overflow can be assumed. The column operates at a pressure of 1 atm. Find the number of equilibrium stages required at total reflux, and the recovery fraction of benzene in the distillate. Solutions of benzene, toluene, and 1,2,3-trimethylbenzene are ideal. 

Separation of Isomers. Sulfonationj-followed by desulfonation, has been used to separate mixtures of aromatic compounds not easily separable by distillation. Either or both steps can be selective. Hydrocarbons isolated pure by this method include meta-xylene (on a substantial commercial scale), l-methyl-3-ethylbenzene, 3,5-dimethyl-l-ethylbenzene, three meta-dibutylbenzene isomers, trimethylbenzenes, and dimethylnaph-thalene isomers in coal tar. This procedure has also been used to isolate 3-chlorotoluene, 2,5-dichIorotoluene, trichlorotoluene, " and trichloro-... 

Cyclodextrins (CDs) have recently found use as stationary phases in gas-solid chromatography (GSC) [1-8, 12-14] and in gas-liquid chromatography (GLC) [8-11], because of their selective separation capability. Their application to separations of stereoisomers (alkenes, pinenes) and positional isomers of aromatics (xylenes, trimethylbenzenes) has been found to be very advantageous. The inclusion process, which underlies selective separations, is, with cyclodextrins, also affected by the presence of water. It is well known that cyclodextrins form crystal hydrates and that the water of crystallization participates in the formation of inclusion complexes [15]. On the formation of an inclusion complex, the water molecules included in the CD cavity are liberated preferentially. This liberation is further enhanced under the dynamic conditions of gas chromatography. It can thus be assumed that water also plays an important role in the equilibrium processes between CD and a guest (sor-bate) in the gaseous state. 

With )8-CD, advantages of saturation of the carrier gas with water vapour can be demonstrated on a separation of trimethylbenzenes (Figure 6). The three isomers are well separated even in the dry carrier gas, but the presence of water vapour shortens the analysis almost three-fold, while the separation efficiency is preserved. 

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