Para-xylenes

The fully side-chaia chloriaated products, 1,3-his(trichioromethy1)henzene [881-99-1] and 1,4-his(trichioromethy1)henzene [68-36-0] are manufactured by exhaustive chlorination of meta and para xylenes. For the meta compounds, ring chlorination cannot be completely eliminated ia the early stages of the reactioa. The xyleae hexachlorides are iatermediates ia the manufacture of the xylene hexafluorides and of iso- and terephthaloyl chloride [100-20-9] (see Phthalic acids). 

Distill a small quantity each day to obtain relatively pure o-xylene from a mixture of ortho and para xylene, having boiling points of 142.7°C and 138.4°C, respectively. The feed is 15 Ib-mols (about 225 gallons) per batch, at 0.20 mol fraction para. The desired residue product is 0.020 in the kettle, while the distillate is to be 0.400 mol fraction para. A distillation column equivalent to 50 theoretical plate is to be used. 

The data in Example 1.2 are in moles of the given component per mole of mixed feed. These are obviously calculated values. Check their consistency by using them to calculate the feed composition given that the feed contained only para-xylene and chlorine. Is your result consistent with the stated molar composition of 40% xylene and 60% chlorine ... 

Properties There are three xylene isomers, commonly known as orr/zo-xylene, meta-xylene, and para-xylene. They are all colorless liquids. The orr/zo-isomer boils at 144°C, the meta- at 139.1°C, and the para-at 138.5°C. 

Because orr/zo-xylene is more readily isolated and purified (by distillation), it costs less than para-xylene. Like all petrochemicals, prices depend on the price of crude oil but in early 2001, mixed-xylene was about 17 cents/lb while para-xylene was only about 15 cents due to high manufacturing capacity and low demand for use for making terephthalic acid. In the extremely high volumes in which such chemicals are sold, fractions of a penny difference in price can be very important. 

The generation of PPV and corresponding derivatives via the dihalide approach is possible not only in solution reaction, but also - via the gas phase -in a so-called chemical vapor deposition (CVD) process. In this process, the vapor of a dichlorinated para-xylene (a,a or a,a) is pyrolyzed at moderately low pressures (0,1-0,2 torr) to form a chlorinated para-xylylene intermediate, which then condenses and polymerizes on a suitable, cooled substrate. The coating of the chlorinated precursor polymer can be heated to eliminate HCl, to form PPV 60 (or a PPV derivative) [88]... 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

In an attempt to increase Tg of the poly[bis(o-carboxyphenoxy)alkanes], Anastasiou and Uhrich (2000a) replaced the alkane moiety by ortho-, meta-, and para-xylenes producing poly[o-/m-bis(p-carboxyphenoxy)xylene]s (Po-p-CPX, and Pm-p-CPX) and poly[o-/m-/p-bis(o-carboxyphenoxy)xylene]s (Po-o-CPX, Pm-o-CPX, and Pp-o-CPX). They found Po-p-CPX to be relatively insoluble and were unable to synthesize poly[p-bis(p-carboxy-phenoxy)xylene] because of the insolubility of the dicarboxylic acid (Anastasiou and Uhrich, 2000a). Po-o-CPX and Pm-o-CPX demonstrated the most favorable solubility and neither exhibited a melting temperature. All of the polymers synthesized had Tgs between 71 and 101°C (Anastasiou and Uhrich, 2000a). 

Paralene [para-xylene] Also called Gorham and also spelled parylene. A process for coating articles with poly-p-xylene. The vapor of di-p-xylylene is pyrolyzed at 550°C, yielding p-xylyl free radicals, -CHj-CgH CH, which deposit and polymerize on cooled surfaces. Developed by W. F. Gorham at Union Carbide Corporation. 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

The effect of crystal size, 2r, in STOP is demonstrated in Figure 10. These data for three zeolites having similar activity, but with crystal sizes differing by nearly two orders of magnitude, show a significant increase in para-xylene selectivity with increasing crystal size. The primary product selectivity is enhanced and secondary isomerization is retarded. 

In view of the difficulty of measuring the diffusivity of o-xylene at the reaction temperature, 482°c, we have used the diffusivity determined at 120°C. For a series of ZSM-5 catalysts, the two D-values should be proportional to each other. Para-xylene selectivities at constant toluene conversion for catalysts prepared from the same zeolite preparation (constant r) with two different modifiers are shown in Figure 11. The large effect of the modifier on diffusivity, and on para-selectivity, is apparent. 

Para-selectivity for a wide variety of ZSM-5 preparations of comparable activity are shown in Figure 12. These data include results for unmodified HZSM-5 s of varying crystal size as well as chemically modified HZSM-5 s. Since the activity of these catalysts is nearly identical, these data clearly establish the major role of diffusion in the para-xylene content of the xylenes produced in TDP. We have examined in more detail the effect of the concentration of one of these chemical modifiers, MgO. 

Both para-xylene selectivity and r2/D (tQ 3) increase smoothly with MgO level for a series of large crystal, Mg modified HZSM-5 catalysts, and again para-xylene selectivity increases with tQ 3 (Figure 13, Table IV). However, these catalysts appear to be significantly different from the catalysts just discussed, defining a separate functional dependence on r2/D (tQ 3). These differences will be shown to be attributable to differences in acid activity of this series of catalysts. 

The affinity of [2.2]paracyclophane for nitrosonium cation is much greater than that of para-xylene, presumably owing to stacking interaction between the aromatic rings in the 7i-complex. Low isotope effect on the aromatic carbon... 

The diester/diacid component of PBT is made by oxidizing para-xylene. Oxidation followed by esterification leads to dimethyl terephthalate. 

Para-Xylene Oxidation (wt.%) Phenol Oxidation (wt.%) TBHP Conv. (TOF,h ) H2O2 Conv. (TOF,h )... 

The isomers, called ortho-xylene, meta-xylene, and para-xylene, each have unique properties. Two such properties are the freeze points, at which xylenes turn from liquid to crystals, and the boiling points, at which xylenes turn from liquid to vapor. These two properties figure importantly in the apparatus used to separate xylene isomers from each other. Mixed xylenes, a commonly traded commodity, is a combination of the three isomers. 

To the dismay of toluene lovers, if there are any, the volume growth of benzene has overshadowed that of toluene, and toluenes major use is to make benzene in hydrodealkylation and toluene disproportionation units. About 50% of the toluene recovered in the United States is used this way. Conversion to para-xylene is also of growing importance. 

Ortho-xylene can be separated by distillation ethylbenzene is only 3.9°F from para-xylene, but by using very tall, multitrayed distillation columns (200 feet high with 300 trays), it too can be separated fairly... 

Even though the boiling temperatures of meta- and para-xylene are close together, their freezing points, i.e., the temperatures at which the liquid starts freezing, i.e., turning to crystals, are not. Meta-xylene crystallizes at -54.2 F and para-xylene at +55.9°F, a spread of more than 100 degrees. 

F, in a holding tank. At that temperature, para-xylene crystals form and grow in a liquid-solid mixture like slush. The key to good solid-liquid separation is large crystal growth. The larger the crystals, the better the separation because of the next step. 

When the crystals have grown sufficiently, the slush is put in a centrifuge. The spinning action permits the para-xylene to separate from the mother liquor, so-called because the crystals come out of the liquid. At this stage, the para-xylene crystals, C2[ tA filter cake 2.x. this point, have a purity of 80-90%, due to the mother liquor that coats the crystal surface. (Thafs the reason for big crystals—less surface area for the mother liquor to coat.)... 

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