Monomolecular cracking of propane over acidic chabazite: An ab initio molecular dynamics and transition path sampling study

Bučko, Tomáš; Benco, Ľ.; Häfner, J.; Ángyán, János G.

doi:10.1016/j.jcat.2011.01.022

Cited by 102 publications

(161 citation statements)

References 51 publications

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“…[27][28][29][30][31][32][33][34][35][36][37] Only a limited number of studies have applied techniques that go beyond the static approach to describe chemical transformation in zeolites. [19,20,[38][39][40][41][42][43][44] The strength of a dynamical approach was earlier demonstrated by some of the present authors.We reported that the influence of the presence of multiple protic molecules on the methylation of benzene [44] and methoxide formation [42] in H-ZSM-5 has to be studied with molecular dynamics. These studies demonstrated that formation of protonated clusters of protic molecules has to be taken into account when modeling reactions under conditions of high reactant loadings, in particular in case of protic reactants that can play a solvent-like role.…”

supporting

confidence: 66%

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

Wispelaere

Ensing

Ghysels

et al. 2015

Chemistry A European J

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The methanol to olefins process is a show case example of complex zeolite-catalyzed chemistry. At real operating conditions, many factors such as framework flexibility, adsorption of various guest molecules and competitive reaction pathways, affect reactivity. In this paper we show the strength of first principle molecular dynamics techniques to capture this complexity by means of two case studies. Firstly, the adsorption behavior of methanol and water in H-SAPO-34 at 350 °C is investigated. Hereby we observed an important degree of framework flexibility and proton mobility. Secondly, we studied the methylation of benzene by methanol via a competitive direct and stepwise pathway in the AFI topology. Both case studies clearly show that a first principle molecular dynamics approach enables to obtain unprecedented insights into zeolite-catalyzed reactions at the nanometer scale.Keywords: ab initio calculations, molecular dynamics, proton mobility, methylation, zeolites 3 IntroductionChemical conversions in zeolites play an essential role in today's industrial catalysis. [1][2][3] Within the field of heterogeneous catalysis, the conversion of methanol to hydrocarbons (MTH) or olefins (MTO) over acidic zeolites received a lot of attention during the last decades, due to its relevance in the search for alternative processes to produce hydrocarbon products. [4][5][6][7] The MTO process has experimentally been developed in the past 3-4 decades and is currently being industrialized. [5] Methanol can be obtained from coal, natural gas or biomass and is in turn converted into ethene, propene or other hydrocarbons. The process proved extremely difficult to unravel at the nanoscale level due to various factors such as pressure, temperature and the occurrence of competing reaction mechanisms, which highly influence the catalytic performance of the system. Due to its complexity, it is a very challenging case study for theoretical modeling studies. [8] Currently, there is a consensus that a hydrocarbon pool (HP) mechanism operates during the methanol conversion process. Herein, an organic center is trapped in the zeolite pores and acts as co-catalyst. [9][10][11] The HP may be of aromatic or aliphatic nature and the two corresponding types of catalytic cycles are in close connection with each other, a concept that is called the dual cycle. [12] Depending on the catalyst topology and operating conditions either one or both cycles may be responsible for olefin formation during the methanol conversion process. [12][13][14] Theoretical contributions have proven to be indispensable within MTO research.Methodological developments and a steady increase in computer power, contributed to the fact that many properties, in particular rate coefficients of well-defined elementary reactions, are now routinely calculated with high accuracy. [8,[15][16][17] However, theoretical chemists are still confronted with paramount challenges to thoroughly explain experimental observations. The true challenge lies in linking the model system wit...

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supporting

confidence: 66%

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

Wispelaere

Ensing

Ghysels

et al. 2015

Chemistry A European J

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“…18,[53][54][55][56][57][58][59][60][61][62] The typical framework flexibility exhibited by zeolites, the influence of surrounding solvent molecules and entropic and temperature effects cannot be fully captured by well-established static methods and require an approach in which larger portions of the potential energy surface (PES) can be explored. In this study, we mainly aim to assess the influence of multiple protic molecules on methanol adsorption, activation and compared to pure methanol adsorption.…”

Section: Computational Detailsmentioning

confidence: 99%

Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy

et al. 2016

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The role of water in the methanol-to-olefins (MTO) process over H- has been elucidated by a combined theoretical and experimental approach, encompassing advanced molecular dynamics simulations and in-situ micro-spectroscopy. First principle calculations at the molecular level point out that water competes with methanol and propene for direct access to the Brønsted acid sites. This results in less efficient activation of these molecules, which are crucial for the formation of the hydrocarbon pool. Furthermore, lower intrinsic methanol reactivity towards methoxide formation has been observed. These observations are in line with a longer induction period observed from in-situ UV-Vis micro-spectroscopy experiments. These experiments revealed a slower and more homogeneous discoloration of H-SAPO-34, while insitu confocal fluorescence microscopy confirmed the more homogeneous distribution and larger amount of MTO intermediates when co-feeding water. As such it is show that water induces a more efficient use of the H-SAPO-34 catalyst crystals at the microscopic level. The combined experimental theoretical approach gives a profound insight into the role of water on the catalytic process at the molecular and single particle level.

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“…However, this falls beyond the scope of this article. Note that there are only a few studies applying MDbased techniques to study zeolite-catalysed reactions [56][57][58][59][60][61]. A comprehensive review on various theoretical procedures applied to study zeolite-catalysed reactions can be found in reference [62].…”

Section: Computational Detailsmentioning

confidence: 99%

How zeolitic acid strength and composition alter the reactivity of alkenes and aromatics towards methanol

Erichsen

Wispelaere

Hemelsoet

et al. 2015

Journal of Catalysis

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a b s t r a c tThis work encompasses a combined experimental and theoretical assessment of how zeolitic acid strength and composition affects acid-catalysed methylation reactions. Overall, higher methylation rates were observed over the material with higher acid strength. Co-reactions of methanol with benzene at 250°C over the two isostructural AFI materials H-SSZ-24 and H-SAPO-5 revealed large differences in selectivity. While the stronger acidic H-SSZ-24 mainly produced toluene and polymethylbenzenes, high yields of C 4+ aliphatics were observed over H-SAPO-5. These results strongly suggest that alkene methylation was preferred over H-SAPO-5 even at very low conversion during methanol/benzene co-reactions. Furthermore, a comparison of benzene and propene methylation at 350-400°C revealed a significantly faster rate of benzene than propene methylation in H-SSZ-24, whereas the rates of benzene and propene methylation were similar in H-SAPO-5. The observed difference in reactivity of the two hydrocarbons in both catalysts could be understood by careful analysis of various molecular dynamics simulations of the co-adsorbed complexes. The probability to form protonated methanol was, as expected, higher in the more acidic material. However, in H-SSZ-24, the probability for methanol protonation was higher when co-adsorbed with benzene than when co-adsorbed with propene, while the same was not observed in H-SAPO-5. Furthermore, it was found that benzene and methanol are more likely to form a reactive co-adsorbed complex in H-SSZ-24 compared to propene and methanol, while the opposite was observed for H-SAPO-5. This work shows that molecular dynamics simulations provide insights into the adsorption behaviour of guest molecules in large pore AFI materials. The obtained insights correlate with the experimentally observed reactivities.

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Monomolecular cracking of propane over acidic chabazite: An ab initio molecular dynamics and transition path sampling study

Cited by 102 publications

References 51 publications

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy

How zeolitic acid strength and composition alter the reactivity of alkenes and aromatics towards methanol

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