Development of Anharmonic Molecular Models and Simulation of Reaction Kinetics in Zeolite Catalysis

-- Entwicklung von anharmonischen Molekülmodellen und Kinetiksimulationen für Zeolithkatalyse -- Zeolithe katalysieren eine bemerkenswerte Kohlenwasserstoffchemie durch ihre einzigartige Topologie mit Poren von molekularer Größenordnung und verteilten aktiven Brønsted Säurezentren. Insbesondere industriell nutzbare Prozesse wie “Ethanol-to-Olefins” (ETO) und “Methanol-to-Olefins” (MTO) könnten ein fester Bestandteil der Wertschöpfungskette von vergaster Biomasse zu Petrochemikalien werden. Simulationen bieten direkte Einblicke in ansonsten kaum beobachtbare Vorgänge, wie zum Beispiel unter Reaktionsbedingungen arbeitende Katalysatoren, und erleichtern somit die zielgerichtete Gestaltung von neuen Materialien und Prozessen. Mehrskalenmodellierung von Zeolithkatalyse, wie in dieser Arbeit gezeigt, verbindet Theorien zur Beschreibung von Phänomenen auf verschiedensten Längen- und Zeitskalen. Grundlegende katalytische Eigenschaften auf molekularer Ebene werden durch Dichtefunktionaltheorie (DFT) und weitere Quantenmethoden bestimmt. Die Zustandssummen der statistischen Mechanik – einschließlich der Näherung des harmonischen Oszillators – verknüpfen diese fundamentalen Ergebnisse mit Thermodynamik. Dadurch abgeleitete Ratenkonstanten ermöglichen Kinetiksimulationen mit Reaktormodellen wie dem idealen Rührkessel und idealen Strömungsrohr. Die Untersuchung der eher unerforschten, aber vermutlich mangelhaften Qualität der Näherung des (molekularen) harmonischen Oszillators für Adsorptionsprozesse und Reaktionsbarrieren in Zeolithen zeigt die Bedeutung von Anharmonizität für Schwingungen sowie für unterdrückte Rotationen und Translationen (Dissoziationen). Um Anharmonizität zu quantifizieren wird eine neue allgemein anwendbare Methode zur Berechnung anharmonischer Korrekturen vorgestellt. Das neue Konzept bedient sich thermodynamischer $\lambda$-Pfadintegration ($\lambda$-TI) vom harmonisch zum DFT-basiert interagierenden System kombiniert mit krummlinigen internen Koordinaten in Systemen mit periodischen Randbedingungen. Seine Unabhängigkeit von jeglichen Reaktionskoordinaten macht dieses $\lambda$-TI Verfahren besonders nützlich für computergestützte (Zeolith-)Katalyse und vorteilhaft für die Betrachtung von Adsorption relativ zur Gasphase, da es zur Untersuchung beliebiger Zustände in der freien Energiehyperfläche angewendet werden kann. In zwei Schritten wird das $\lambda$-TI Verfahren unter Verwendung von DFT-basierter Molekulardynamik Simulation zur Untersuchung von Adsorption im Zeolith H-SSZ-13 und von Reaktionsbarrieren eingesetzt. Die Vorhersage experimenteller freier Adsorptionsenthalpien wird verbessert. Für Barrieren erweist sich das $\lambda$-TI Verfahren als vergleichbare Alternative zu einer etablierten, von der Reaktionskoordinate abhängigen, Methode. Maschinelles Lernen und damit verbundene Deskriptoren werden als vielversprechende Ergänzungen zum $\lambda$-TI Verfahren skizziert. Reaktionspfade der Ethanoldehydratisierung und eines wesentlichen Teils des autokatalytischen Olefinzyklus in ETO werden unter Verwendung der oben beschriebenen Mehrskalenmodellierung untersucht. Die Bildung von Diethylether wird bei niedrigeren Temperaturen mit abnehmender Selektivität für steigenden Umsatz beobachtet. Bei höheren Temperaturen verläuft die Ethanoldehydratisierung viel schneller als die Ethylierung des entstehenden Ethens. Hexenisomere bilden sich auf der gleichen Zeitskala wie Buten, wobei verzweigte Isomere bevorzugt werden, und 2-Methylpentenisomere am meisten zur Propenbildung durch Spaltung beitragen. Oberflächengebundene Alkoxygruppen ermöglichen den relevantesten (schrittweisen) Alkylierungsmechanismus unter den untersuchten Reaktionsbedingungen in H-SSZ-13 sowohl bei ETO als auch bei MTO. Außerdem wird die Relevanz einer Auswahl gängiger organischer Verunreinigungen für die Initiierung des MTO Prozesses in einem kinetischen Modell mit 107 Elementarschritten, darunter ein repräsentativer Teil des Olefinzyklus und die direkte Initiierung aus Methanol durch CO-vermittelte C−C Bindungsknüpfung, quantifiziert. Die Wirkung verschiedener Verunreinigungen auf die Olefinentwicklung variiert mit ihrer Art und Menge. Bereits extrem kleine Verunreinigungsmengen führen zu schnellerer Initiierung als die direkte C−C Bindungsknüpfung, die nur in vollständiger Abwesenheit von Verunreinigungen von Bedeutung ist

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities. Graphic Abstract: [Figure not available: see fulltext.

Theoretical investigation of the olefin cycle in H-SSZ-13 for the ethanol-to-olefins process using ab initio calculations and kinetic modeling

The formation of the hydrocarbon pool (HCP) in the ethanol-to-olefins (ETO) process catalyzed by H-SSZ-13 is studied in a kinetic model with ab initio computed reaction barriers. Free energy barriers are computed using density functional theory (DFT) and post-Hartree–Fock methods with a complete basis set extrapolation applied to a hierarchy of periodic and cluster models. The kinetic model includes ethanol (EtOH) dehydration to ethene as well as olefin ethylations up to hexene isomers and the corresponding cracking reactions. Ethylation of ethene and of products thereof leads only to even-numbered olefins, while cracking can lead to propene and thus initiate the formation of olefins with an odd number of carbon atoms. During EtOH dehydration at 473.15 K we observe diethyl ether (DEE) formation for a short period of time where the DEE selectivity decreases monotonically with increasing EtOH conversion. At 673.15 K we find that EtOH dehydration occurs much faster than ethylation of the formed ethene, which takes considerably longer due to higher free energy barriers. Hexene isomers form on the same time scale as butene, where branched isomers are favored with 2-methyl-pentene isomers contributing most to the formation of propene through cracking. As in the methanol-to-olefins (MTO) process, the most relevant alkylation pathway is the stepwise mechanism via surface alkoxy species (SAS) on the zeolite catalyst. A comparison of ethylation with methylation barriers of up to heptene isomers forming nonene and octene isomers, respectively, shows that ethylation barriers are lower by around 11 kJ mol$^{-1}$ on average

Thermodynamics and reaction mechanism of urea decomposition

The selective catalytic reduction technique for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis and differential scanning calorimetry. A new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases. Thermodynamic data for triuret have been refined. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C is explained by formation of a eutectic mixture with urea

Thermodynamics and reaction mechanism of urea decomposition

Selective catalytic reduction (SCR) for automotive applications depends on ammonia production from a urea-water solution by thermolysis and hydrolysis. In this process, undesired liquid and solid by-products are formed in the exhaust pipe. The formation and decomposition of these by-products have been studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Based on a previously published reaction mechanism by Brack et al. [1], a new reaction scheme is proposed that emphasizes the role of thermodynamic equilibrium of the reactants in liquid and solid phases [2]. The observed phenomenon of liquefaction and re-solidification of biuret in the temperature range 193–230 °C can be explained by formation of a eutectic mixture with urea. According to DSC data, the direct decomposition of urea to ammonia and isocyanic acid can be ruled out. The dominant route is a self-polymerisation of urea to biuret and triuret. Biuret and triuret decomposition are dominated by thermodynamic equilibria with gaseous isocyanic acid. For this, thermodynamic data of triuret have been refined. The apparent melting point of biuret at 193 °C is explained by the formation of a eutectic mixture within the urea-biuret-triuret-cyanuric acid ensemble. Furthermore, DSC data shows that cyanuric acid sublimates without decomposition at temperatures above 300 °C. Numerical simulations of the TGA and DSC experiments are performed by a multi-phase tank reactor model (DETCHEMMPTR [3]). The new reaction mechanism describes well the main features (decomposition steps and calorimetry) and dependencies (on heating rate and surface area) of the decompositions of urea, biuret, triuret and cyanuric acid. [1] W. Brack, B. Heine, F. Birkhold, M. Kruse, G. Schoch, S. Tischer and O. Deutschmann, “Kinetic modeling of urea decomposition based on systematic thermogravimetric analyses of urea and its most important by-products”, CES 106, 1–8 (2014). [2] S. Tischer, M. Börnhorst, J. Amsler, G. Schoch and O. Deutschmann, “Thermodynamics and reaction mechanism of urea decomposition”, PCCP, in press, DOI: 10.1039/C9CP01529A (2019). [3] www.detchem.co

Prospects of Heterogeneous Hydroformylation with Supported Single Atom Catalysts

[EN] The potential of oxide-supported rhodium single atom catalysts (SACs) for heterogeneous hydroformylation was investigated both theoretically and experimentally. Using high-level domain-based local-pair natural orbital coupled cluster singles doubles with perturbative triples contribution (DLPNO-CCSD(T)) calculations, both stability and catalytic activity were investigated for Rh single atoms on different oxide surfaces. Atomically dispersed, supported Rh catalysts were synthesized on MgO and CeO2. While the CeO2-supported rhodium catalyst is found to be highly active, this is not the case for MgO, most likely due to increased confinement, as determined by extended X-ray absorption fine structure spectroscopy (EXAFS), that diminishes the reactivity of Rh complexes on MgO. This agrees well with our computational investigation, where we find that rhodium carbonyl hydride complexes on flat oxide surfaces such as CeO2(111) have catalytic activities comparable to those of molecular complexes. For a step edge on a MgO(301) surface, however, calculations show a significantly reduced catalytic activity. At the same time, calculations predict that stronger adsorption at the higher coordinated adsorption site leads to a more stable catalyst. Keeping the balance between stability and activity appears to be the main challenge for oxide supported Rh hydroformylation catalysts. In addition to the chemical bonding between rhodium complex and support, the confinement experienced by the active site plays an important role for the catalytic activity.X-ray absorption experiments were performed at the ALBA Synchrotron Light Source (Spain), experiment 2019023278. Beamline scientists L. Simonelli and C. Marini are gratefully acknowledged for their contribution to beam setup. E. Andrés, E. Martínez-Monje, I. López, and M. García-Farpón (ITQ) are acknowledged for their assistance with XAS data acquisition. J. Ternedien (MPI-KOFO) is acknowledged for the performance of XRD experiments. N. Pfänder (MPI-CEC) is acknowledged for his contribution to STEM characterization. The authors acknowledge support by the state of Baden-Württemberg through bwHPC (bwUnicluster and JUSTUS, RV bw17D01). The authors gratefully acknowledge support by the GRK 2450. Financial support from the Helmholtz Association is also gratefully acknowledged. The experimental work received funding from the Max Planck Society and the Spanish Ministry of Science, Innovation and Universities (projects SEV-2016-0683 and RTI2018-096399-A-I00). B.B.S. acknowledges the Alexander von Humboldt Foundation for a postdoctoral scholarship.Amsler, J.; Sarma, BB.; Agostini, G.; Prieto González, G.; Plessow, P.; Studt, F. (2020). 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Combining Theoretical and Experimental Methods to Probe Confinement within Microporous Solid Acid Catalysts for Alcohol Dehydration

Catalytic transformations play a vital role in the implementation of chemical technologies, particularly as society shifts from fossil-fuel-based feedstocks to more renewable bio-based systems. The dehydration of short-chain alcohols using solid acid catalysts is of great interest for the fuel, polymer, and pharmaceutical industries. Microporous frameworks, such as aluminophosphates, are well-suited to such processes, as their framework channels and pores are a similar size to the small alcohols considered, with many different topologies to consider. However, the framework and acid site strength are typically linked, making it challenging to study just one of these factors. In this work, we compare two different silicon-doped aluminophosphates, SAPO-34 and SAPO-5, for alcohol dehydration with the aim of decoupling the influence of acid site strength and the influence of confinement, both of which are key factors in nanoporous catalysis. By varying the alcohol size from ethanol, 1-propanol, and 2-propanol, the acid sites are constant, while the confinement is altered. The experimental catalytic dehydration results reveal that the small-pore SAPO-34 behaves differently to the larger-pore SAPO-5. The former primarily forms alkenes, while the latter favors ether formation. Combining our catalytic findings with density functional theory investigations suggests that the formation of surface alkoxy species plays a pivotal role in the reaction pathway, but the exact energy barriers are strongly influenced by pore structure. To provide a holistic view of the reaction, our work is complemented with molecular dynamics simulations to explore how the diffusion of different species plays a key role in product selectivity, specifically focusing on the role of ether mobility in influencing the reaction mechanism. We conclude that confinement plays a significant role in molecular diffusion and the reaction mechanism translating to notable catalytic differences between the molecules, providing valuable information for future catalyst design

One Pot Cooperation of Single Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization - Hydrosilylation Process

[EN] Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru-1/CeO2 and Rh-1/CeO2 SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov alpha-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (>95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.X-ray absorption experiments were performed at the ALBA Synchrotron Light Source (Spain), experiments 2018082961 and 2019023278. L. Simonelli and C. Marini (CLAESSALBA beamline) are thanked for beamline setup. E. Andres, M. E. Martinez, M. Garcia, and I. Lopez (ITQ), are acknowledged for their assistance with XAS experiments. J. Buscher, J. Ternedien, B. Spliethoff, and C. Wirtz (MPI-KOFO) are acknowledged for the performance of XPS, XRD, BF-TEM and 2H NMR experiments, respectively. I. C. de Freitas (MPIKOFO) is thanked for assistance with Raman spectroscopy. J. M. Salas (ITQ) is gratefully acknowledged for his contribution to CO-FTIR experiments. J. J. Barnes and Shell (Amsterdam) are acknowledged for kindly providing an industrial olefin mixture as feed. Authors are thankful to F. Schuth for the provision of lab space and continued support. Part of the HRSTEM and EDX-STEM studies were conducted at the Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon, Universidad de Zaragoza, Spain. R.A. gratefully acknowledges the support from the Spanish Ministry of Economy and Competitiveness (MINECO) through project grant MAT2016-79776-P (AEI/FEDER, UE) and from the European Union H2020 programs "ESTEEM3" (823717). The authors acknowledge support by the state of Baden-Wurttemberg through bwHPC (bwUnicluster and JUSTUS, RV bw17D01), by the GRK 2450 and by the Helmholtz Association. This research received funding from the Max Planck Society, and the Fonds der Chemische Industrie of Germany. Funding from the Spanish Ministry of Science, Innovation and Universities (Severo Ochoa program SEV-2016-0683 and grant RTI2018096399-A-I00) is also acknowledged. B.B.S. acknowledges the Alexander von Humboldt Foundation for a postdoctoral scholarship. Open Access funding is provided by the Max Planck Society.Sarma, BB.; Kim, J.; Amsler, J.; Agostini, G.; Weidenthaler, C.; Pfaender, N.; Arenal, R.... (2020). One Pot Cooperation of Single Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization - Hydrosilylation Process. Angewandte Chemie International Edition. 59(14):5806-5815. https://doi.org/10.1002/anie.201915255S580658155914Liang, S., Hao, C., & Shi, Y. (2015). The Power of Single-Atom Catalysis. ChemCatChem, 7(17), 2559-2567. doi:10.1002/cctc.201500363Liu, J. (2016). Catalysis by Supported Single Metal Atoms. 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Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe