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Particle Characterization and Dynamics
Published in Wen-Ching Yang, Handbook of Fluidization and Fluid-Particle Systems, 2003
The reaction kinetics of catalytic cracking goes mainly through the intermediate of a carbenium ion. The first step of catalytic cracking begins with the reaction of a hydrocarbon molecule at the acidic sites of the FCC catalyst, forming a carbenium ion R-CH2+. This can occur from either adding a H+charge to an olefin molecule at the Bronsted acidic site, or removing a H-charge from a paraffin molecule at a Lewis acidic site. The three key catalytic reactions following the formation of the carbenium ion are isomerization, beta scission, and hydrogen transfer.
Advances in Refining Technologies
Published in Deniz Uner, Advances in Refining Catalysis, 2017
James E. Rekoske, Hayim Abrevaya, Jeffery C. Bricker, Xin Zhu, Maureen Bricker
In general, the hydrocracking reaction starts with the generation of an olefin or a cyclo-olefin on a metal site on the catalyst. Next, an acid site adds a proton to the olefin or cyclo-olefin to produce a carbenium ion. The carbenium ion cracks to a smaller carbenium ion and a smaller olefin. These products are the primary hydrocracking products. These primary products can react further to produce still smaller secondary hydrocracking products. The reaction sequence can be terminated at primary products by abstracting a proton from the carbenium ion to form an olefin at an acid site and by saturating the olefin at a metal site. Figure 1.6 illustrates the specific steps involved in the hydrocracking of paraffins. The reaction begins with the generation of an olefin and the conversion of the olefin to a carbenium ion. The carbenium ion typically isomerizes to form a more stable tertiary carbenium ion. Next, the cracking reaction occurs at a bond that is beta to the carbenium ion charge. The beta position is the second bond from the ionic charge. Carbenium ions can react with olefins to transfer charge from one fragment to the other. In this way, charge can be transferred from a smaller hydrocarbon fragment to a larger fragment that can better accommodate the charge. Finally, olefin hydrogenation completes the mechanism. This selectivity is due in part to a more favorable equilibrium for the formation of higher carbon number olefins. In addition, large paraffins adsorb more strongly. The carbenium ion intermediate causes extensive isomerization of the products, especially to a-methyl isomers, because tertiary carbenium ions are more stable. Finally, the production of C1 to C3 is low because the production of these light gases involves the unfavorable formation of primary and secondary carbenium ions. Other molecular species such as alkyl naphthenes, alkyl aromatics, and so on react via similar mechanisms, e.g., via the carbenium ion mechanism.
A review on reaction mechanisms and catalysts of methanol to olefins process
Published in Chemical Engineering Communications, 2022
Catalytic conversion of methanol to hydrocarbons (MTHc) over ZSM-5 was also investigated by Chang (1980). They indicated that the products including olefins, paraffins and aromatics made the reaction mechanism more complex. Most discussions were around the primary steps of MTHc process. Many investigators proposed that light olefins were as intermediates for creation of vast types of products (Keil 1999). According to this idea, Chen and Reagan (1979) demonstrated some evidence about auto-catalysis in MTO/MTP process. Afterwards, Chang (1980) approved auto-catalysis in MTO/MTP process by investigating behavior of C2-C5 olefins selectivity during initial steps of the reaction. Experimental data showed that at low conversions, a low amount of ethylene was produced while production of propylene and butenes was sharply increased with increasing residence time. Secondary reactions like oligomerization, isomerization and cycling were accelerated via carbenium ion chemistry. Derouane (1978) suggested that alkyl methyl ether molecules were converted to the olefins with the same carbon numbers by water elimination reaction. Kaeding and Butter (1975) proposed that ethylene was produced by methyl ethyl ether as an intermediate in MTO/MTP process.