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Title: Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n -Butane in Brønsted Acidic Zeolites


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Van der Mynsbrugge, Jeroen, Janda, Amber, Mallikarjun Sharada, Shaama, Lin, Li-Chiang, Van Speybroeck, Veronique, Head-Gordon, Martin, and Bell, Alexis T. Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n -Butane in Brønsted Acidic Zeolites. United States: N. p., 2017. Web. doi:10.1021/acscatal.6b03646.
Van der Mynsbrugge, Jeroen, Janda, Amber, Mallikarjun Sharada, Shaama, Lin, Li-Chiang, Van Speybroeck, Veronique, Head-Gordon, Martin, & Bell, Alexis T. Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n -Butane in Brønsted Acidic Zeolites. United States. doi:10.1021/acscatal.6b03646.
Van der Mynsbrugge, Jeroen, Janda, Amber, Mallikarjun Sharada, Shaama, Lin, Li-Chiang, Van Speybroeck, Veronique, Head-Gordon, Martin, and Bell, Alexis T. Fri . "Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n -Butane in Brønsted Acidic Zeolites". United States. doi:10.1021/acscatal.6b03646.
@article{osti_1361159,
title = {Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n -Butane in Brønsted Acidic Zeolites},
author = {Van der Mynsbrugge, Jeroen and Janda, Amber and Mallikarjun Sharada, Shaama and Lin, Li-Chiang and Van Speybroeck, Veronique and Head-Gordon, Martin and Bell, Alexis T.},
abstractNote = {},
doi = {10.1021/acscatal.6b03646},
journal = {ACS Catalysis},
number = 4,
volume = 7,
place = {United States},
year = {Fri Mar 17 00:00:00 EDT 2017},
month = {Fri Mar 17 00:00:00 EDT 2017}
}
  • Experimental measurements of the rate coefficient (kapp) and apparent enthalpies and entropies of activation (ΔHapp and ΔSapp) for alkane cracking catalyzed by acidic zeolites can be used to characterize the effects of zeolite structure and alkane size on the intrinsic enthalpy and entropy of activation, ΔHint‡ and ΔSint‡. To determine ΔHint‡ and ΔSint‡, enthalpies and entropies of adsorption, ΔHads-H+ and ΔSads-H+, must be determined for alkane molecules moving from the gas phase to Brønsted acid sites at reaction temperatures (>673 K). Experimental values of ΔHapp and ΔSapp must also be properly defined in terms of ΔHads-H+ and ΔSads-H+. We reportmore » here a method for determining ΔHads-H+ and ΔSads-H+ in which the adsorption site is represented by a fixed volume that includes the proton. Values of ΔHads-H+ and ΔSads-H+ obtained from Monte Carlo simulations are in good agreement with values obtained from experimental data measured at 300–400 K. An important feature of the simulations, however, is their ability to account for the redistribution of alkane adsorbed at protons in different locations with increasing temperature. Values of ΔHint‡ and ΔSint‡ for the cracking of propane through n-hexane, determined from measured values of kapp and ΔHapp and simulated values of ΔHads-H+ and ΔSads-H+, agree well with values obtained independently from quantum mechanics/molecular mechanics calculations. Application of our method of analysis reveals that the observed increase in kapp with increasing n-alkane size is due primarily to a decrease in ΔHint‡ with increasing chain length and that ΔSint‡ is independent of chain length.« less
  • The pathway to monomolecular C-C bond cracking over Broensted acid sites has been characterised theoretically using the AM1 molecular orbital method. The model describes not only the transition of the proton from the acid site to the paraffin, but also the collapse of the resulting carbonium ion directly to a smaller alkene and alkane. This model predicts that the protolysis is not driven by an acid-base pair type reaction, as witnessed with ammonia and alkene activation. The data amassed allowed a quantitative description of the primary product distributions for the alkanes butane and hexane which shows good agreement with experimentalmore » data. 25 refs., 4 figs., 3 tabs.« less
  • The cracking of n-butane catalyzed by the zeolite HZSM-5 has been characterized by measurements of the conversion determined with a flow reactor at temperatures of 426-523 C and n-butane partial pressures of 0.01-1.00 atm. The primary products, each formed in a first-order reaction, are H[sub 2] + butenes; methane + propylene; and ethane + ethylene. In the limit approaching zero conversion, each compound in each stated pair was formed at approximately the same rates as the other. Propane and a small amount of isobutane were formed as secondary products in second-order reactions. The results are consistent with the occurrence ofmore » two simultaneous mechanisms: (1) a monomolecular mechanism proceeding through the pentacoordinated carbonium ion formed by protonation of the n-butane at the two position and (2) a bimolecular hydride transfer proceeding through carbenium ion intermediates. The former proceeds almost to the exclusion of the latter in the limit approaching zero n-butane conversion. The limiting product distribution characterizes the intrinsic selectivity of the collapse of the carbonium ion; at 496 C, the relative rates of decomposition of the carbonium ion to give H[sub 2] + butenes, methane + propylene, and ethane + ethylene are 30 [plus minus] 6, 36 [plus minus] 4, and 34 [plus minus] 5, respectively, with the corresponding activation energies all being approximately 140 kJ/mol. These results provide the first demonstration of stoichiometric dehydrogenation accompanying paraffin cracking.« less