First principle kinetic studies of zeolite-catalyzed reactions relevant for the MTO process

The methanol-to-olefin (MTO) process, catalyzed by acidic zeolites provides an increasingly important alternative to the production of light olefins from crude oil. However, the various mechanistic proposals for methanol-to-olefin conversion have been strongly disputed for the past several decades. This work provides an overview of various mechanistic cycles to produce both propene and ethene. In all proposed reaction cycles for the MTO process, methylation reactions of various hydrocarbons have been shown to be one of the most important steps. The reaction rates are very much dependent on the particular hydrocarbon pool species that is methylated and on the topology of the zeolite. Within this contribution we will particularly highlight methylation reactions on aromatics and alkenes in ZSM-5, SAPO-34, ZSM-22 other related topologies, as these were proven to be successful catalysts for the MTO process

Unraveling the thermodynamic conditions for negative gas adsorption in soft porous crystals

Soft porous crystals (SPCs) are widely known for their intriguing properties and various counterintuitive phenomena such as negative linear compression, negative thermal expansion and negative gas adsorption (NGA). An intriguing case is the adsorption of methane in DUT-49 for which experimentally a drop in the amount of adsorbed particles was observed under increasing vapor pressure. It is yet unknown which specific systems can exhibit NGA under which thermodynamic conditions. Herein, a semi-analytical thermodynamic model is applied to determine the conditions required for NGA, including their sensitivity towards various system-specific parameters, and investigate the correlation with pressure-induced breathing. As such, it is found that certain non-breathing materials may exhibit breathing with NGA under application of a fixed mechanical pressure. Such meticulous control of multiple triggers for NGA can open the way to new applications such as tunable gas detection and pressure amplification

Molecular modeling within zeolite catalysis : the methanol-to-olefin process as a case study

Zeolites are amongst the most widely investigated and topical of inorganic materials. They are widely used in industry for a plethora of applications. Within this contribution we specifically highlight meticulous design of zeolites for catalysis. The Methanol to Olefins reaction is chosen as a case study to highlight the subtle interplay between various factors leading to catalysts with longer lifetime and/or better selectivity towards the desired olefins. Molecular modeling in close synergy with experimentalists is crucial in understanding the function and nature of the active site. However modeling efforts need to account for realistic working conditions such as the true nature of the feedstock, framework flexibility, temperature and pressure effects, competitive pathways,… Our approach consists in simulating complex chemical transformations in nanoporous materials using first principle molecular dynamics methods at real operating conditions, capturing the full complexity of the free energy surface. The subtle interplay between various factors enable to enhance the product selectivity or to suppress or enhance the aromatic cycle leading to more or less propene formation. Within this contribution, we will highlight the impact of zeolite acidity, feed composition, temperature, and post-synthetic modification to enhance the olefin selectivity. Recently it was shown that post-synthetic incorporation of alkaline earth metals in H-ZSM-5 may also enhance olefin selectivity. It is an example of how meticulously tuning the active site may impact the lifetime of the catalyst and product selectivity. Herein we give molecular level understanding in the nature of the active sites. Throughout the talk we show to importance of modeling techniques that account for realistic working conditions. The approach followed here may greatly impact catalysis science and reveal insights that were undiscovered so far

DMRG-CASPT2 study of the longitudinal static second hyperpolarizability of all-trans polyenes

We have implemented internally contracted complete active space second order perturbation theory (CASPT2) with the density matrix renormalization group (DMRG) as active space solver [Y. Kurashige and T. Yanai, J. Chem. Phys. 135, 094104 (2011)]. Internally contracted CASPT2 requires to contract the generalized Fock matrix with the 4-particle reduced density matrix (4-RDM) of the reference wavefunction. The required 4-RDM elements can be obtained from 3-particle reduced density matrices (3-RDM) of different wavefunctions, formed by symmetry-conserving single-particle excitations op top of the reference wavefunction. In our spin-adapted DMRG code chemps2 [https://github.com/sebwouters/chemps2], we decompose these excited wavefunctions as spin-adapted matrix product states, and calculate their 3-RDM in order to obtain the required contraction of the generalized Fock matrix with the 4-RDM of the reference wavefunction. In this work, we study the longitudinal static second hyperpolarizability of all-trans polyenes C$_{2n}$H$_{2n+2}$ [n = 4 - 12] in the cc-pVDZ basis set. DMRG-SCF and DMRG-CASPT2 yield substantially lower values and scaling with system size compared to RHF and MP2, respectively.Comment: 9 pages, 4 figure

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

Ranking the stars : a refined Pareto approach to computational materials design

We propose a procedure to rank the most interesting solutions from high-throughput materials design studies. Such a tool is becoming indispensable due to the growing size of computational screening studies and the large number of criteria involved in realistic materials design. As a proof of principle, the binary tungsten alloys are screened for both large-weight and high-impact materials, as well as for fusion reactor applications. Moreover, the concept is generally applicable to any design problem where multiple competing criteria have to be optimized

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

Zeolites are the workhorses of today's chemical industry. For decades they have been successfully applied, however many features of zeolite catalysis are only superficially understood and in particular the kinetics 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

Unravelling thermal stress due to thermal expansion mismatch in metal–organic frameworks for methane storage

Thermal stress is present in metal–organic frameworks undergoing temperature changes during adsorption and desorption. We computed the thermal pressure coefficient as a proxy for this phenomenon and discuss the impact of thermal expansion mismatch.Thermal stress is present in all systems undergoing temperature changes during their operation. Metal-organic frameworks (MOFs) are a class of porous, crystalline materials ideally suited for a wide range of adsorption-based technologies. The release and consumption of the heat of adsorption instigate temperature fluctuations and thermal stress in these materials that could induce disruptive volume changes. To bring these materials to engineering applications, it is of utmost importance to understand their thermal expansion behavior and the overall induced thermal stress due to thermal expansion mismatch with other components. In this work, we focus on a large group of MOFs known to have promising methane adsorption properties and predict their thermal expansion coefficients based on force field molecular dynamics simulations. Negative thermal expansion (NTE) behavior is predicted for all studied MOFs, and the magnitude of the NTE coefficients is found to be positively correlated with the degree of porosity of the frameworks. Finally, as a proxy for the thermal stress, the thermal pressure coefficient is calculated, which is found to be in the range between polymers and ceramics. Variations within the operating temperature range of MOFs are therefore expected to result in a relatively low thermal stress