Polyaromatic species

Most frequently, the capillary electrochromatography system is equipped with a UV detector, so it is not surprising that — to the best of our knowledge — no paper dealing with the separation of aliphatic hydrocarbons has been published so far. On the other hand, quite a few reports deal with the separation of polyaromatic species. Nearly always the packing used is ODS and most of the separations are run isocrati-cally. 

The comparison between the 7, values of the two systems (Figure 10.5) shows that their behavior compared to HTT is different. At 280°C, the 7 , value of the system with the zeolite is twice as large as that for the APP/PER system. It was concluded that the materials formed at this temperature are structurally different. Indeed a slower 7) means that the molecular motions may be hindered and therefore that the structure may be more rigid. Between 280°C and 350°C, the Tx values decrease and become identical. At 350°C, both systems are organized in stacks of polyaromatic species and it may be proposed that the two materials are structurally, in the NMR sense, similar. At higher temperatures, the 7 , values of the systems with zeolite are always larger. The carbonization process of the two intumescent materials develops in different ways. The zeolite allows molecular motion and the carbonaceous shields keep mobile structures. 

The intracrystalline pore volume of the catalysts was evaluated by n-hexane sorption as shown in Fig. 6. Sorption capacities for samples SI to S3 are comparable to that of the zeolite before Ga impregnation and correspond to the value expected for an unaltered ZSM-5 type material (S10). Sorption capacity decreases for samples S3, S4, S5, and S6, because of intracrystalline volume blockage by coke deposits and possibly also (silica)-alumina debris [6] in the aged catalyt S6. In addition, the sorption rate for S6 is about twice the rate observed for the other samples, suggesting that adsorption occurs mostly at the external surface of the S6 catalyst crystallites. Thus, it appears that coke deposited on S6, probably as polyaromatic species, has almost blocked the channel pore mouths and/or practically occupied the whole intracrystalline pore volume. It explains the poor catalytic performance of S6. 

Figure 3 shows the results obtained with pyrene and substituted pyrenes. It is seen that the position and number of substituents do not affect the adsorption. Similar results were obtained in studies of chrysenes, benzopyrenes and also benzonaphthothiophene. This indicates that polyaromatic species without nitrogen heteroatoms are adsorbed via the tt-electrons in the large aromatic sheets, and thereby adsorption is unaffected by the degree of substitution. This shows that the sulfur does not play a part in the initial adsorption or coke formation. 

Bulky polyaromatic molecules tend to concentrate in recycle streams and are known to be prime coke precursors, as demonstrated in model studies by Latif (60). However, by means of a relatively small bleed stream, the concentration of these polyaromatic species can be contained at a low level, thereby minimizing their deleterious effect on catalyst stability. Alternatively, they can be substantially removed by adsorption on an active-carbon column. 

In helium quantitative yield of HCI. remainder residue and hydrocarbons, benzene is major volatile hydrocarbons product aliphatic hydrocarbons, benzene (major product), toluene, ethylbenzene, o-xyiene, monochlorobenzene styrene, vinyl tcriuerre. p-dichlorobenzene, o-dichlorobenzene, indene, 1,3.5-trichlorobenzene 1.2.4. richlorobenzene. naphthalene, u-methylnaphthalene. p-methylnaphthatene effect of ZnO. SnOj, and Ab03 on the yields of products Is also recorded HCI. CO2. ethene. ethane, propane, 1-butene. 2-butene. 1-pentene. cydopentene, n-pentane, 2-methylbulane, 1,3-pentadiene. 2-methyl-1,3-pentadiene, complex series (60 Identlfled) of aromatic and polyaromatic species including benzene, styrene, methylstyrenes, toluene, o-xytene, m-xylene, p-xylene, biphenyl, naphthalene, anthracene, phenanthrene. pyrene, etc. 

The intensity of the band at 1600 cm (the so-called coke band) which is considered as characteristic of polyaromatic species [3] increases slightly with ageing as well as those of the bands at 1450 cm and at 1355 cm whereas the intensity of the band at 1505 cm which corresponds to less condensed aromatics [8] decreases. Therefore, in addition to the decrease in coke content there is an increase in the degree of aromaticity of coke. Similar observations have been previously made by various authors on zeolites [9-13] as well as on other acid catalysts [14-16]. However, the sweeping under inert gas flow of the coked samples was always carried out at temperatures much higher than in this work. 

Particulate matter consists basically of three fractions, the relative amounts of which depend strongly on fuel quality and engine conditions. The first is dry soot, which is a low-hydrogen-content, polyaromatic species that, regardless of the catalyst, cannot be oxidized under normal diesel exhaust conditions. The second component is the soluble organic fraction (SOF), derived from unbumt fuel and lube oil, and consisting mostly of HCs in the C20-C28 range. The SOF... 

The formation of both p-SiC and carbon is evidenced by transmission electron microscopy (TEM) [1] [16]. The p-SiC phase appears as tiny crystals with a mean size of 2 nm. The carbon phase appears as randomly oriented 0.7 to 0.8 nm BSUs, which consist of polyaromatic species associated face to face and which could be related to coronene, C24H12 [16]. PCS pyrolytic residues contain 20 at.% residual hydrogen after pyrolysis at 850°C and 4... 

The intumescent structure consists of carbonaceous and polyaromatic species whose carbon organization in stacks is characteristic of a pre-graphitization stage. This carbon, formed from the additives, plays two different chemical parts in the fire retardancy process ... 

The Elbs reaction provides rapid access to polyaromatic systems, particularly due to the ease of preparing many substrates for the reaction. When the Elbs reaction proceeds smoothly, it may provide the best known means to synthesize a certain substrate. For example, 3-methylcholanthrene and derivatives have been shown to be potent carcinogenic agents. The reaction has been used to prepare several methylcholantlu ene analogues and other polyaromatic species, the products were isolated and tested in carcinogenic studies to probe characteristics of polyaromatics as potential carcinogenic... 

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