Soluble coke, components

With HZSM5, Va does not depend on the adsorbate. At low coke content Vr/Va is close to 1, which shows that deactivation is not due to pore blockage. However Vr/Va decreases when the coke content increases. Thus for 7 wt% coke Vr/Va = 0.3, which means that coke blocks the access of the adsorbates, and probably of the reactant, to a volume much greater than the volume it occupies. It must be noted that the significant decrease of Vr/Va is observed as soon as bulky non soluble coke components appear. Deactivation results probably from a pore blockage due to these non soluble coke molecules which surround the zeolite crystallites. 

Three examples have been chosen for describing coking of zeolites. The first concerns the formation of coke during the transformation of propene, of toluene and of a mixture propene toluene at 120°C and 450°C on a HZSM5 zeolite. All the coke components are soluble in methylene chloride. Most of them are located inside the pores indeed they are not dissolved by direct soxhlet extraction of the coked zeolite samples. 

The coke deposited in the catalyst in the MTG process in the 250 450 °C range is basically soluble in pyridine and has a structure constituted by methyl-substituted polycyclic aromatics. The molecular weight of the coke components and their H/C ratio increases with time on stream and temperature. The acidity deterioration occurs simultaneously with the increase in content of coke deposited (up to 1.6 % in weight with respect to the catalyst, at 350 °C) and is attributable to the irreversible occupation of acidic Bites. The coke components preferably occupy strongly acidic sites. 

The sequence of events in coke formation was studies in the model reaction [70] of H-Y zeolite with propene at 723 K. Under these drastic conditions the soluble white coke formed rapidly within 20 min and was converted into insoluble coke within 6h under inert gas without loosing carbon atoms in the deposit. Due to the larger pores in the Y-zcolites compared to the ZSM type zeolites used in the other studies mentioned so far, the structure of the aromatic molecules is somewhat different. The soluble coke in this system consisted of alkyl cyclopentapyrenes (C H2 -26. Type A) as the hydrogen-rich primary product and of alkyl benzoperylenes (C H2n- 2. Type B) and alkyl coro-nenes (CrtH2 -36. Type C) as matured components. The temporal evolution of the various products is presented in Fig. 14. It clearly emerges that the soluble coke fractions are precursors for the insoluble coke and that within the soluble coke fraction the final steps of dehydrogenation-polymerization are very slow compared to the initial formation of smaller aromatic molecules from propene. The sequential formation of precursors with decreasing C H ratio follows from the shift of the maximum in the abundance of each fraction on the time axis. 

As an example the degree of information obtained by the above methods on the coke formed from propene at 400-450°C on a HZSM-5 zeolite is presented. For a coke content of 3.5 wt% the atomic H/C ratio of coke is close to 1, i. e. comparable to that of benzene. IR spectroscopy shows the presence of aromatics (7). All the coke components are soluble in methylene chloride after dissolution of the zeolite in a hydrofluoric acid solution. Two families of compounds are found naphthalenes with 2 to 4 methyl groups (about 60 wt%) and fluorenes with 1 to 3 methyl groups (40 wt%) (8). 

The analysis of the soluble part of coke by GC/MS showed that the size and the degree of aromaticity of the coke components increased with time on stream (hence with the coke content). This is shown in figure 2 for HUSY. With this zeolite, the components of the soluble coke can be classified into three main families ... 

For similar coke contents the composition of coke strongly depends on the pore system of the zeolite, i.e., coking is a shape-selective process. At low coke contents, the major components are alkylbenzenes in HZSM5 and HERI, alkylnaphthalenes or fluorenes in HOFF, alkylbenzopyrenes in USHY and very polyaromatic compounds (insoluble in methylene chloride) in HMOR. At high contents, the coke components are polyaromatic in all zeolites. However, the molecules of components soluble in methylene chloride are very different as they take the size and shape of the cavities or of the channel intersections [38], as is shown in Figure 4. Whatever the coke content soluble coke molecules are trapped in cages... 

The chemical characterization of the soluble coke was obtained from mass spectral interpretation and from the analysis of key ion mass fragmentograms [12], as well by identification of homologous series and by injection of standards of respective components. Table 3 presents the identified compounds in the extracts. 

The composition of coke was determined at different coke contents through the method develop in our laboratory [4], i.e., dissolution of the zeolite in a hydrofluoric acid solution then recovery and GC/MS analysis of the part of coke soluble in methylene chloride. In this case, all the coke components are soluble in methylene chloride. It should be emphasized that practically no coke could be recovered in this solvent through a direct... 

More information was obtained by direct analysis of the coke components recovered after dissolution of the zeolite in hydrofluoric acid solutions. Thus during cyclohexene conversion on USHY at 350 C the coke was totally soluble in methylene chloride and composed of alkyl pyrenes and alkylnaphtenopyrenes [8]. At 450 0 only 40 % of coke was soluble in methylene chloride, 60 % appearing as black particles constituted of highly polyaromatic compounds. Soluble coke contained alkylcyclopentapyrenes and alkylindenopyrenes. The composition of coke formed from propene [7] in the same conditions was practically identical. In contrast the coke formed from 1-hexene on HZSM5 at 320°C contained mainly alkylindanes and alkylnaphtalenes [42]. 

Coke formed from toluene. By CP/MAS- C-NMR it was shown that on LaY at 350°C coke formed from toluene had a polynuclear aromatic structure with few methyl substituents [6]. Coke formed on USHY at 350°C and 450°C was recovered from the coked zeolite by treatment with a solution of hydrofluoric acid [9]. For T = 350°C (wt % coke = 8.2) all the coke components were soluble in methylene chloride (R = 100 %) while for T = 450°C R was very low 55 % and 10 % for 3 and 14 wt % coke. At 350 C pyrene was the main coke component while at 450 C the main components of the soluble coke were (like from n-heptane, propene or cyclohexene) cyclopentapyrenes and indenopyrenes. A mechanism involving the alkylation of pyrenes by small olefins (C2-C olefins were observed in the reaction products) was proposed to explain the formation of these latter compounds. The number of coke molecules which caused the complete deactivation of the zeolite at 350°G was close to the number of strong acid sites while at 450 C it was about twice smaller which could show a blockage of the access to sites of supercages unoccupied by coke molecules. However, another explanation could be that the bulky insoluble coke molecules formed at 450 C occupy two supercages. 

The rate of coke formation on HZSM5 was much lower than on USHY (10 times at 450 C) and the composition of coke was different. At 450 C, (1.6 wt % coke) all the coke components were soluble with methyl-pyrenes as major components [49]. 

Coking kinetics has been the subject of various researches reported in the literature, with special emphasis on the mechanism of coke formation, interconversion of the solubility class components during conversion, role of these components in coke formation, influence of structural properties on coking rate and yields, development of correlations, among others. 

In the case of low temperature tar, the aqueous Hquor that accompanies the cmde tar contains between 1 and 1.5% by weight of soluble tar acids, eg, phenol, cresols, and dihydroxybenzenes. Both for the sake of economics and effluent purification, it is necessary to recover these, usually by the Lurgi Phenosolvan process based on the selective extraction of the tar acids with butyl or isobutyl acetate. The recovered phenols are separated by fractional distillation into monohydroxybenzenes, mainly phenol and cresols, and dihydroxybenzenes, mainly (9-dihydroxybenzene (catechol), methyl (9-dihydtoxybenzene, (methyl catechol), and y -dihydroxybenzene (resorcinol). The monohydric phenol fraction is added to the cmde tar acids extracted from the tar for further refining, whereas the dihydric phenol fraction is incorporated in wood-preservation creosote or sold to adhesive manufacturers. Naphthalene Oils. Naphthalene is the principal component of coke-oven tats and the only component that can be concentrated to a reasonably high content on primary distillation. Naphthalene oils from coke-oven tars distilled in a modem pipe stiU generally contain 60—65% of naphthalene. They are further upgraded by a number of methods. 

MINOR NONHYDROCARBON COMPONENTS. Sulfur compounds and gum (insoluble or soluble) are present in jet fuels and can affect coke formation. The effects of sulfur are not always consistent, but, in general, sulfur is not considered to be a problem until 1% or more is present. However, no more than 0.4% of sulfur is permitted in current fuels specifications. Gum causes a small increase in coke deposits, but this effect is insignificant for fuels meeting gum specifications. 

In studies running parallel to carbonization with catalysts, Mochida et al. (78, 81) studied the separation of parent feedstocks into benzene-soluble (BS) and benzene-insoluble fractions(BI) and attempted co-carbonization of blends of these fractions. In addition, certain fractions were hydrogenated or alkylated and these alkylated fractions themselves hydrogenated. Mochida et al. (78-81) were looking for Compatibility between fractions, that is the ability of a blend of fractions to carbonize to a needle-coke. The commercial application of good compatibility could involve the use of low percentage additions of an active fraction (component) to produce a good needle-coke. 

Table 1. Main Famihes of Components in the Coke Soluble in Methylene Chloride, obtained at various temperatures from propene and isobutene...

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