Theoretical identification of the interactions between the zeolite framework and the hydrocarbon pool co-catalyst in methanol-to-olefin conversion

The rapidly increasing demand of oil-based chemicals calls for the development of new technologies based on other natural sources. Among these emerging alternatives, the methanol-to-olefin process (MTO) in acidic zeolites is one of the most promising. However, unraveling the reaction mechanism of such an extremely complex catalytic process like MTO conversion has been a challenging task from both experimental and theoretical viewpoint. For over 30 years the actual mechanism has been one of the most discussed topics in heterogeneous catalysis.[1] Instead of plainly following direct routes,[2-3] the MTO process has experimentally been found to proceed through a hydrocarbon pool mechanism, in which organic reaction centers act as homogeneous co-catalysts inside the heterogeneous acid catalyst, adding a whole new level of complexity to this issue.[4-5] Therefore, a more detailed understanding of the elementary reaction steps can be obtained with the complementary assistance of theoretical modeling. In this work, a complete supramolecular complex of both the zeolite framework and the co-catalytic hydrocarbon pool species is modeled through state-of-the-art quantum chemical techniques [6-7]. This approach provides a more detailed understanding of the crucial interactions between the zeolite framework and its contents, which form the driving forces for successful methanol-to-olefin conversion

Identification of the driving forces in methanol-to-olefin conversion by modeling the zeolite cage and contents

The rapidly increasing demand of oil-based chemicals calls for the development of new technologies based on other natural sources. Among these emerging alternatives, the methanol-to-olefin process (MTO) in acidic zeolites is one of the most promising. However, unraveling the reaction mechanism of such an extremely complex catalytic process like MTO conversion has been a challenging task from both experimental and theoretical viewpoint. For over 30 years the actual mechanism has been one of the most discussed topics in heterogeneous catalysis.[1] Instead of plainly following direct routes,[2-3] the MTO process has experimentally been found to proceed through a hydrocarbon pool mechanism, in which organic reaction centers act as cocatalysts inside the zeolite pores, adding a whole new level of complexity to this issue.[4-5] Therefore, a more detailed understanding of the elementary reaction steps can be obtained with the complementary assistance of theoretical modeling. In this work, a complete supramolecular complex of both the zeolite framework and the co-catalytic hydrocarbon pool species is modeled through state-of-the-art quantum chemical techniques [6-7]. This approach provides a more detailed understanding of the crucial interactions between the zeolite framework and its contents, which form the driving forces for successful methanol-to-olefin conversion. [1] Stocker, M., Microporous Mesoporous Mater. 29 (1999) 3. [2] Song, W.G., Marcus, D.M., Fu, H., Ehresmann, J.O., Haw, J.F., J. Am. Chem. Soc. 124 (2002) 3844. [3] Lesthaeghe, D., Van Speybroeck, V., Marin, G.B., Waroquier, M., Angew. Chem. Int. Ed. 45 (2006) 1714. [4] Dessau, R. M., J. Catal. 99 (1986) 111. [5] Dahl, I.M., Kolboe, S., Catal. Lett. 20 (1993) 329. [6] Lesthaeghe, D., De Sterck, B., Van Speybroeck, V., Marin, G.B., Waroquier, M., Angew. Chem. Int. Ed. 46 (2007) 1311. [7] McCann, D.M., Lesthaeghe, D., Kletnieks, P.W., Guenther, D.R., Hayman, M.J., Van Speybroeck, V., Waroquier, M., Haw, J.F. Angew. Chem. Int. Ed. 47 (2008) 5179

Deactivation of the catalyst during the MTO process from a molecular modeling perspective

Currently, the industrially important conversion process of methanol to olefins (MTO) forms a key process for the production of higher valued products that can easily be transported, such as ethylene and propylene. Methanol can be made from natural gas or coal via synthesis gas. Unraveling the underlying reaction mechanism of the complex MTO process has already shown to be very challenging. Recent ab initio calculations, in combination with experimental data, are in strong support of the “hydrocarbon pool model” as opposed to a direct (C-C coupling) route [1, 2]. The hydrocarbon pool has been described as a catalytic scaffold inside the zeolite building, consisting of polymethylbenzenes and their cationic derivatives. The continued growth of these initially active carbonaceous species within acidic zeolites, such as H-ZSM-5 and H-SAPO-34, is an undesired side effect resulting from secondary reactions for which at present no computational data exist whatsoever. The presence of these large species – coke precursors - inside or at the external cups of the periodic structure leads to blockage of the pores or channels and ultimately to the deactivation of the catalyst. An improved in-depth understanding of the underlying reaction mechanisms of coke formation is therefore desperately needed. A main problem is the generally poor characterization of coke, despite the great number of techniques (gas chromatography, mass spectroscopy) that can be used for locating and identifying the deposits [3, 4]. Because of this, it is not clear whether benzenoid species consisting of 3 rings can already be regarded as coke as opposed to large aromatic species present in the hydrocarbon pool that still allow an active route. Within this contribution possible reaction routes leading to the formation of naphthalene- and/or phenanthrene-like species are studied from theoretical viewpoint within various industrially relevant zeolite topologies. For each of these elementary steps reaction rates are evaluated based on energies and frequencies originating from reliable ab initio data. The latter were obtained by taking into account a large portion of the zeolites, as to be representative for the actual topology

An elementary chromatic reduction for gain graphs and special hyperplane arrangements

A gain graph is a graph whose edges are labelled invertibly by "gains" from a group. "Switching" is a transformation of gain graphs that generalizes conjugation in a group. A "weak chromatic function" of gain graphs with gains in a fixed group satisfies three laws: deletion-contraction for links with neutral gain, invariance under switching, and nullity on graphs with a neutral loop. The laws lead to the "weak chromatic group" of gain graphs, which is the universal domain for weak chromatic functions. We find expressions, valid in that group, for a gain graph in terms of minors without neutral-gain edges, or with added complete neutral-gain subgraphs, that generalize the expression of an ordinary chromatic polynomial in terms of monomials or falling factorials. These expressions imply relations for chromatic functions of gain graphs. We apply our relations to some special integral gain graphs including those that correspond to the Shi, Linial, and Catalan arrangements, thereby obtaining new evaluations of and new ways to calculate the zero-free chromatic polynomial and the integral and modular chromatic functions of these gain graphs, hence the characteristic polynomials and hypercubical lattice-point counting functions of the arrangements. We also calculate the total chromatic polynomial of any gain graph and especially of the Catalan, Shi, and Linial gain graphs.Comment: 31 page

Growth of naphthalenic HP species: influence of the CHA topology from a molecular modeling perspective

The conversion of methanol to olefins (MTO) forms a key process for the production of higher valued products that can easily be transported, such as ethylene and propylene. Unraveling the underlying reaction mechanism of this complex process has already shown to be very challenging. Recent ab initio calculations, in combination with experimental data, are in strong support of the “hydrocarbon pool (HP) model” as opposed to a direct route.1 The HP has been described as a catalytic scaffold inside the zeolite building, consisting of polymethylbenzenes and their cationic derivatives. The exact nature and reactivity of the HP species is still unclear, however, and is probably highly dependent on zeolite topology. Within this contribution the growth of naphthalenic species through successive methylations are studied in the SSZ-13 catalyst from a theoretical viewpoint. The influence of space limitations imposed by the zeolite framework is investigated in detail. Reaction rates and kinetic parameters are evaluated based on energies and frequencies originating from reliable ab initio data. The latter were obtained by taking into account a large portion of the zeolite, as to be representative for the actual topology

Naphthalene derivatives in the MTO process from a molecular modeling perspective: reactive species or coke?

Currently, basic chemicals in polymer industry are mainly produced by thermal cracking of petroleum, but a promising alternative has been found: methanol-to-olefins (MTO). Methanol can be made from natural gas via syngas, but also from biomass. Molecular modeling of the MTO process has been a challenging topic, yet the reaction mechanism of the active route is starting to get unraveled based on the ‘hydrocarbon pool’ hypothesis [1], where aromatic species play a fundamental role as co-catalytic species within the zeolite pores and cages. All catalysts face the problem of deactivation due to coke formation [2]. This is a major threat for the application of the process and the need for a reliable kinetic model of the coke deposition to optimize the reaction conditions is, therefore, high. Experimentally, it is found that the deactivation is a result of the presence of voluminous polyaromatic compounds in the cages of the catalyst. For SAPO-34, which has a chabasite topology, this are phenantrene- and pyrene-like species, which show no activity towards olefin production. The topology of the catalyst is a crucial aspect regarding the coking issue: ZSM-5 only shows a blocking of the channels in the external cups, while a chabasite topology is subject to internal coking [3]. As of yet, the boundary region between active hydrocarbon pool species and deactivating coke remains uncharacterized. In this contribution, this question will be answered for naphtalenic compounds by remodeling the active route for ethylene and propylene production and comparing the activities with the original side-chain mechanism [1]. An other topic of examination is the influence of the formation of such compounds on the propene/ethene selectivity ratio [4]. And finally, the chemical composition of the catalyst, which clearly has an influence on the activity and coking rate of the catalyst [5] will be investigated by comparing the behavior of naphtalenic molecules in SSZ-13 chabasite and SAPO-34