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

Un modèle à criticalité auto-régulée de la magnétosphère terrestre

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Lithium-Beryllium-Boron : Origin and Evolution

The origin and evolution of Lithium-Beryllium-Boron is a crossing point between different astrophysical fields : optical and gamma spectroscopy, non thermal nucleosynthesis, Big Bang and stellar nucleosynthesis and finally galactic evolution. We describe the production and the evolution of Lithium-Beryllium-Boron from Big Bang up to now through the interaction of the Standard Galactic Cosmic Rays with the interstellar medium, supernova neutrino spallation and a low energy component related to supernova explosions in galactic superbubbles.Comment: 28 pages, 7 figures, to be published in a special memorial volume of Physics Reports in honor of David Schram

Relationship between the Clinical Frailty Scale and short-term mortality in patients ≥ 80 years old acutely admitted to the ICU: a prospective cohort study.

BACKGROUND: The Clinical Frailty Scale (CFS) is frequently used to measure frailty in critically ill adults. There is wide variation in the approach to analysing the relationship between the CFS score and mortality after admission to the ICU. This study aimed to evaluate the influence of modelling approach on the association between the CFS score and short-term mortality and quantify the prognostic value of frailty in this context. METHODS: We analysed data from two multicentre prospective cohort studies which enrolled intensive care unit patients ≥ 80 years old in 26 countries. The primary outcome was mortality within 30-days from admission to the ICU. Logistic regression models for both ICU and 30-day mortality included the CFS score as either a categorical, continuous or dichotomous variable and were adjusted for patient's age, sex, reason for admission to the ICU, and admission Sequential Organ Failure Assessment score. RESULTS: The median age in the sample of 7487 consecutive patients was 84 years (IQR 81-87). The highest fraction of new prognostic information from frailty in the context of 30-day mortality was observed when the CFS score was treated as either a categorical variable using all original levels of frailty or a nonlinear continuous variable and was equal to 9% using these modelling approaches (p < 0.001). The relationship between the CFS score and mortality was nonlinear (p < 0.01). CONCLUSION: Knowledge about a patient's frailty status adds a substantial amount of new prognostic information at the moment of admission to the ICU. Arbitrary simplification of the CFS score into fewer groups than originally intended leads to a loss of information and should be avoided. Trial registration NCT03134807 (VIP1), NCT03370692 (VIP2)

Ab initio study of the growth of fused bicyclic species within a zeolite-type catalyst: the influence of confinement and material composition

In the search for greener and cheaper manufacturing routines for the key chemical compounds ethylene and propylene, the conversion process of methanol to olefins (MTO) is becoming more and more vital. Methanol can be made from natural gas or coal via synthesis gas. The transformation to the smaller olefins occurs over microporous solid acids, such as zeolites and zeotype materials. ZSM-5 and SAPO-34 are currently the most applied representatives in industry. Other materials however also show promising product distributions [1]. Unraveling the underlying reaction mechanism of the complex MTO process has already shown to be very challenging. 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 [2,3]. The hydrocarbon pool has been described as a catalytic scaffold inside the zeolite building, consisting of polymethylbenzenes [4] or alkenes [5] and their cationic derivatives. The continued growth of these initially active carbonaceous species within the channels and pores of the periodic structure is an undesired side effect resulting from secondary reactions, leading to blockage and ultimately to the deactivation of the catalyst. It is however not clear whether larger aromatics should be regarded as passive coke or rather as hydrocarbon pool species that still allow an active route. Theoretical calculations can provide important insights regarding this activity behavior. In order to model the growth process of aromatic species, methylation reactions at fused bicyclic species are examined by means of advanced computational methods. A protonated high-silicon zeolite with chabazite topology [6] is compared with its silicoaluminophosphate analogue (SAPO-34), allowing to study the influence of material composition and the comparison with experimental data [7]

Theoretical simulations elucidate the role of naphthalenic species during methanol conversion within H-SAPO-34

The role of naphthalenic species during the methanol-to-olefins (MTO) process in a silicoaluminophosphate zeolitic material exhibiting the chabazite topology (H-SAPO-34) has been studied from first principles. These species could either act as active olefin-eliminating compounds or as precursors for deactivating species. Results incorporating van der Waals contributions for finite large clusters point out that successive methylation steps of naphthalenic compounds are feasible. The calculated intrinsic activation barrier is relatively independent of the number of methyl groups already attached on the aromatic compound and is approximately 140 kJ mol(-1). The influence of the composition of the catalyst and hence the acidic strength on the intrinsic chemical kinetics was investigated in detail through comparison with the isostructural high-silicon material. Apparent chemical kinetics, starting from adsorbed methanol on the acid site, were also computed. The initiation steps of the side-chain route starting from a trimethylated naphthalenium ion were also examined. The actual side-chain methylation exhibits a high barrier and hence this mechanism involving methylated naphthalenes is not expected to be an active ethene-eliminating route in H-SAPO-34

The effect of confined space on the growth of naphthalenic species in a chabazite-type catalyst: a molecular modeling study

Methylation reactions of naphthalenic species over the acidic microporous zeolite with chabazite topology have been investigated by means of two-layered ab initio computations. Large cluster results combined with van der Waals contributions provide thermodynamic and kinetic results of successive methylation steps. The growth of fused bicyclic species is important as these can act as hydrocarbon pool species within the methanol-to-olefin (MTO) process, but ultimately leads to the deactivation of the catalyst. The influence of the confined space of the zeolite pore on the resulting transition state or product shape selectivity is investigated in detail