Zeolite Shape‐Selectivity in the <i>gem</i>‐Methylation of Aromatic Hydrocarbons

Lesthaeghe, David; Sterck, Bart De; Speybroeck, Véronique Van; Marin, Guy; Waroquier, Michel

doi:10.1002/anie.200604309

Cited by 157 publications

(150 citation statements)

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“…In this article, extended cluster models consisting of 46 and 52 T-atoms for H-ZSM-5 and H-beta, respectively, were used (represented in Figure 2) [14][15][16]. This approach has previously proven a reliable and computationally efficient method to predict rate coefficients and reaction barriers that are very close to experimental kinetic data [14,20]. In contrast to periodic calculations, in which the entire unit cell is taken into account, cluster calculations only consider a fragment of the crystallographic structure.…”

Section: Methodsmentioning

confidence: 99%

“…The exact mechanism underlying the conversion process has been debated for decades, but currently there is a consensus on an indirect mechanism in which methanol is converted to light olefins via repeated methylation and/or cracking reactions of a pool of hydrocarbons present inside the zeolite pores [6][7][8][9][10][11][12][13]. In every catalytic cycle proposed to date to explain olefin formation, methylations were found to be key reaction steps [14][15][16]. For these reasons methylations have received a lot of attention in several recent studies, in which they were examined from both an experimental and a theoretical angle [17][18][19][20][21].…”

Section: Introductionmentioning

confidence: 99%

“…The large-pore structure allows direct feeding of even larger species than in H-ZSM-5, making H-beta an ideal candidate for the study of mechanistic aspects, the activity of larger hydrocarbon pool species and the formation of coke precursors [14,[23][24][25]. However, because of its largepore structure, MTH-conversion in H-beta results in a product stream consisting predominantly of larger components such as hexamethylbenzene.…”

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confidence: 99%

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Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

Mynsbrugge

Visur

Olsbye

et al. 2012

Journal of Catalysis

129

126

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Benzene methylation by methanol is studied on acidic zeolites H-ZSM-5 (MFI) and H-beta (BEA) to investigate the influence of the catalyst topology on the reaction rate. Experimental kinetic measurements at 350 °C using extremely high feed rates to suppress side reactions show that methylation occurs considerably faster on H-ZSM-5 than on H-beta. Theoretical rate constants, obtained from first-principles simulations on extended zeolite clusters, reproduce a higher methylation rate on H-ZSM-5 and provide additional insight into the various molecular effects that contribute to the overall differences between the two catalysts. The calculations indicate this higher methylation rate is primarily due to an optimal confinement of the reacting species in the medium pore material. Co-adsorption of methanol and benzene is energetically favored in H-ZSM-5 compared with H-beta, to the extent that the stabilizing host-guest interactions outweigh the greater entropy loss upon benzene adsorption in H-ZSM-5 vs. in Hbeta.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

Mynsbrugge

Visur

Olsbye

et al. 2012

Journal of Catalysis

129

126

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“…[6] Consequently, numerous studies focused on the methylation by methanol, dimethyl ether or methoxides of aromatics and alkenes in the framework of the MTO process. [16,17,26,42,44,46,82,83] It should be mentioned that the static approach with finite cluster models as applied in most of these studies has been very successful in reproducing and predicting experimentally measured kinetic data. [16,17] However, a similar approach is not applicable for the study of methylation reactions in the large pore AFI structured H-SSZ-24 as will be demonstrated in this case study.…”

Section: Case Study Ii: Simulating Competing Pathways For Benzene Metmentioning

confidence: 99%

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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“…These zeolite-catalyzed alkylation reactions are known to be crucial steps in the HP mechanism as they are responsible for carbon incorporation and growth of HP species (see Scheme 2). [55][56][57][58][59][60][61][62] However methylation reactions are also of a more general interest for industrial application to produce for example xylenes. 63 For methylation reactions, two mechanisms are known, being the concerted and stepwise pathway.…”

Section: Introductionmentioning

confidence: 99%

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

Wispelaere

Bailleul

Speybroeck

2016

Catal. Sci. Technol.

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Title: Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions Advanced ab initio molecular dynamics techniques enable study of the infl uence of catalyst topology and acidity, reaction temperature and the presence of additional guest molecules on methylation reactions of benzene and propene, which are crucial reaction steps in the conversion of methanol into hydrocarbons. Our advanced simulations provide detailed insight into how operating conditions impact the competition of the concerted and stepwise methylation mechanism and thus the reaction kinetics. As featured in:See and mechanism of individual reaction steps at operating conditions. Herein we use state-of-the-art advanced ab initio molecular dynamics techniques to study the influence of catalyst topology and acidity, reaction temperature and the presence of additional guest molecules on elementary reactions. Such advanced modeling techniques provide complementary insight to experimental knowledge as the impact of individual factors on the reaction mechanism and kinetics of zeolite-catalyzed reactions may be unraveled. We study key reaction steps in the conversion of methanol to hydrocarbons, namely benzene and propene methylation. These reactions may occur either in a concerted or stepwise fashion, i.e. methanol directly transfers its methyl group to a hydrocarbon or the reaction goes through a framework-bound methoxide intermediate. The DFT-based dynamical approach enables mimicking reaction conditions as close as possible and studying the competition between two methylation mechanisms in an integrated fashion. The reactions are studied in the unidirectional AFI-structured H-SSZ-24, H-SAPO-5 and TON-structured H-ZSM-22 materials. We show that varying the temperature, topology, acidity and number of protic molecules surrounding the active site may tune the reaction mechanism at the molecular level. Obtaining molecular control is crucial in optimizing current zeolite processes and designing emerging new technologies bearing alternative feedstocks.

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Zeolite Shape‐Selectivity in the gem‐Methylation of Aromatic Hydrocarbons

Cited by 157 publications

References 38 publications

Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

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

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

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