Reactivity of Alkanes on Zeolites:  A Computational Study of Propane Conversion Reactions

Abstract

In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activation barriers is within 1 kcal/mol. Reaction activation barriers do change as zeolite acidity changes, however. Linear relationships were found between activation barriers and zeolite deprotonation energies. Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.

Proton exchange of small hydrocarbons over acidic chabazite: Ab initio study of entropic effects

“…In agreement with our calculations and with the results of other theoretical studies [33,34,36], the activation energies for H-exchange involving either CH 3 or CH 2 groups of propane were found to be similar within the limits of experimental error (see Table 1). Thus the observed regioselectivity of propane cannot be explained in terms of activation energy differences.…”

“…Again, the calculated activation energies for all alkanes were very similar, with the largest difference being 6 kJ/mol. In a series of papers, Zheng et al [35][36][37][38] published studies on proton exchange for the same series of short alkanes as considered in this study. Structures were optimized using the B3LYP method, and energies were calculated using the CBS-QB3 composite energy method, providing complete basis set (CBS) total energy estimates.…”

“…Esteves et al [33] Ryder et al [34] Zheng and Blowers [35][36][37][38] Level of theory B3LYP/6-31G* MP2/6-31G* BH&HLYP/6-31++G** B3LYP/6-31G*, CBS-QB3 significant trends in the activation energies in the series of short alkanes. Thus, at first sight, the theoretical results seem to be in disagreement with the experimental results.…”

n‐Butane Monomolecular Cracking on Acidic Zeolites: A Density Functional Study

“…Zheng and Blowers 19,36,39 studied the effects of the proton acidity on the hydrocarbon cracking reaction barriers with the 3T cluster. Blaszkowski et al 40 inves- Figure 5 The pathways of the n-butane monomolecular dehydrogenation reaction on the acidic zeolites: (a) primary dehydrogenation reaction and (b) secondary dehydrogenation eaction.…”

Adsorption of amino acids on MFI-type zeolite: DFT calculations and experimental results

“…in the case of protolytic cracking of alkanes [2,3]. In order to model different pH-values Alanine and Phenylalanine are applied in their protonated, zwitterionic and deprotonated state.…”