Kinetics and Mechanism of Paraffin Cracking with Zeolite Catalysts

Mechanisms of Hydrogen Exchange of Methane with H-Zeolite Y: An ab Initio Embedded Cluster Study

“…The Mulliken population (in Table 3) for the embedded cluster's transition state also agrees more closely to the free CH 5 + ion than the cluster predictions. To examine the net charge on the carbon atom, q net (C), the hydrogen atoms' charges were summed into their nearest heavy atom neighbors.…”

“…In our embedded cluster calculations, the transition state is significantly more ionic than the present and previous cluster predictions, with more than three times as much positive charge (0.67 au) effectively lying on the carbon atom. The charge is not 1.00 au as would be the case for an isolated CH 5 + molecule, because the adsorbate species is bound to the zeolite framework, yet it is positive enough to display definite ionic character. So the Madelung potential promotes the carbonium-character in the transition state of this hydrogen exchange reaction.…”

“…[1][2][3][4] The Brønsted acid site has been established as the primary active site for zeolite catalysis; however, little is still known of the mechanistic details for reactions occurring at these sites. For instance, in zeolite catalyzed cracking, isomerization, and alkylation of hydrocarbons, most proposed mechanisms involve proton transfers from the Brønsted site to the hydrocarbon adsorbate to form carbonium (pentacoordinated carbocations) or carbenium ions (tricoordinated carbocations) as reactive intermediates; [5][6][7][8][9] however, there is still little empirical confirmation of the nature of these intermediates.…”

The transformation of 1,2,4-trimethylbenzene A probe reaction to monitor external surface modifications of HZSM-5?

“…This represents a decrease in the first order rate constant by a factor of 2, which is less than the decrease observed for 1,3,5-TIPB cracking. The hexane cracking reaction is widely used to measure relative activities of cracking catalysts [28]. For HZSM-5 it has also been shown that diffusion limitations are negligible for crystal sizes smaller than 40 gm at 773 K [29][30][31].…”

Computationally Exploring Confinement Effects in the Methane-to-Methanol Conversion Over Iron-Oxo Centers in Zeolites

“…[18][19][20] While all these phenomena qualitatively explain an increase in selectivity, they are not able to quantitatively capture the influence of confinement on the reaction rate. Based on ideas about separating activation energies into contributions from the molecular adsorption process, and to the catalytic reaction itself, 21,22 research has more recently focused on quantifying the impact of confinement by separating entropy and enthalpy contributions to the two components. 14,23 As Gounder and Iglesia pointed out, 14 due to the temperature dependence of the Gibbs Free Energy, the effect of confinement on activation enthalpies will be dominant at the low temperatures encountered in the conversion of methane to methanol.…”