Aromatic molecules, coke formation

Nor did catalytic cracking escape the probing attention of Paul Emmett. At Johns Hopkins his students used labeled molecules extensively to examine the nature of secondary reactions in the cracking of cetane over amorphous silica-alumina and crystalline zeolites. They demonstrated that small olefins (e.g., propylene) are incorporated extensively into higher-molecular-weight molecules, especially aromatics, and are the primary source of coke formation on these catalysts. 

A higher concentration of basic and polar molecules, i.e. nitrogen compounds that are readily adsorbed on to the catalyst acidic sites, leading to an instant, but temporary deactivation. Polycyclic aromatics also strongly contribute to coke formation. 

At 450°C the formation of coke compared to that of the other products is very slow. Whatever the reactant the main coke components are alkylpyrenes. However these alkylpyrenes result probably through different reaction paths one involving aromatics and alkene would be responsible for coke formation from the propene-toluene mixture and from propene, the other involving only reactions of aromatics would be responsible for coke formation from toluene [8]. The same coke molecules are formed through both paths because their size and their shape are imposed by the size and the shape of channel intersections. 

Also polycyclic aromatics and other organic and non-strippable molecules which lead to coke formation are considered reversible (regenerate) catalyst poisons [11]. 

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. 

Deactivation of light naphtha aromatization catalyst based on zeolite was studied, by kinetic analysis, micropore volume analysis and model reactions. Coke accumulates at the entrance of zeolite channel, blocks it and hinders reactant molecule to access active sites in zeolite channel. Our own stabilization technique passivates coke-forming sites at the external surface of the zeolite. This minimizes the coke formation at the entrance of zeolite channel and increases on-stream stability. The stabilized catalyst enabled us to develop a new light naphtha aromatization process using an idle heavy naphtha reformer that is replaced by CCR process. 

High selectivities to para-nitrotoluene can be obtained using acid zeolites at mild reaction conditions. However, the catalysts rapidly deactivate by two main mechanisms. At the low temperature range, the dominant form of deactivation is the plugging of the pores by the aromatic molecules. As the temperature increases, the adsorption of hydrocarbon species and pore plugging decreases, but the catalysts deactivate by coke formation. 

The nature of crude oils depends on their source. Initial separation into components is carried out by atmospheric and vacuum distillation. Heavy ends are particular boiling point cuts, which can include atmospheric gas oil (250-350°C), atmospheric residues (350°C+) vacuum gas oil (350-5S0°C) and vacuum residues (5S0°C+). The descriptions are based on boiling points and, within a particular distillation cut, various chemical species can be identified. These include saturated and unsaturated hydrocarbons, aromatic and polyaromatic hydrocarbons and inorganic atoms such as V, Ni, and S, which are associated with large organic molecules [5]. As a result of this complexity, the composition of the boiling cuts is often described in terms of their content of oils, resins and asphaltenes [6,7,8], the relative amounts of which vary depending on the cut and the source of the crude [6] Of these species, asphaltenes are particularly important in the present context since they are known to be associated with heavy coke formation [7,8]. 

Coke formation therefore leads to selective deactivation in two ways it affects the metal sites more than it affects the acid sites and it constrains access to the zeolite by increasing the diffusion resistance for larger molecules. This results in a selectivity shift to lighter products and a decrease in the aromatics selectivity, paraffin to olefin ratio and conversion. 

---