Theoretical investigation of n-butane isomerization in metal-substituted aluminosilicates

Der stetige Anstieg an Treibhausgasen in unserer Atmosphäre, welche der Grund für den konstanten Anstieg der Temperatur der Erde sind, beruht zu einem großen Teil auf der Emission dieser Gase, z.B. von Kohlenstoffdioxid (CO$_2$), durch industrielle Prozesse. Speziell in der petrochemischen Industrie wird eine große Menge an CO$_2$ durch Flaring von ungewollten leichten Kohlenwasserstoffen, wie z.B. Butan, produziert. Um das Problem des Klimawandels, welches auf dem erhöhten Ausstoß von Treibhausgasen beruht, zu bekämpfen, is es von hoher Wichtigkeit, diese Nebenprodukte effizient zu nutzen. Im Fall von Butan ist eine katalytische Isomerisierung zu Isobutan möglich. Isobutan ist ein sehr viel wertvolleres Molekül aufgrund der diversen Verwendungsmöglichkeiten in der Industrie, z.B. zur Verbesserung der Oktanzahl von Benzin oder in der Synthese von Methyl-tert-butyl-ether (MTBE) via Isobutylen. Jedoch enthalten gegenwärtig für diese Reaktion verwendete Katalysatoren toxische Komponenten, wodurch deren Verwendung gefährlich und umweltschädlich ist. Daher ist eine gründliche und breite Forschung notwendig, um effizientere, billigere und umweltfreundlichere Katalysatoren für die Isomerisierung von n-Butan zu Isobutan zu entdecken. In dieser Arbeit wird dieses Problem rechnerisch mithilfe von sehr genauen Dichtefunktionaltheorie (DFT) Rechnungen angegangen. Die zugrundeliegenden Mechanismen der n-Butan Isomerisierung werden mithilfe eines Modell-Katalysators, H-SSZ-13 (CHA), untersucht, welcher kostengünstig und einfach zu berechnen ist. Die zwei in der Literatur ausgiebig diskutierten Reaktionspfade für die Isomerisierung von 2-Buten, nämlich der monomolekulare und der bimolekulare Mechanismus, werden in dem Zeoliten H-SSZ-13 optimiert und miteinander verglichen. Zusätzlich wurden zwei weitere Reaktionsmechanismen vorgestellt, der (intermolekulare) Wasserstoff-Transfer-Mechanismus und der Methyl-Transfer-Mechanismus. Ersterer zeigt eine Möglichkeit auf, wie Olefine, welche während der Reaktion gebildet werden, die Reaktion selbst katalysieren können, während letzterer einen zweiten Reaktionspfad beschreibt, durch welchen die Bildung von ungewünschten Nebenprodukten der Reaktion erklärt werden können. Die Reaktionsbarrieren wurden für alle Mechanismen mithilfe sehr genauer Methoden berechnet. Diese Barrieren zeigen, dass für den Zeolit H-SSZ-13 mit einer Barriere von 152 kJ/mol bei einer Temperatur von 400 °C der monomolekulare Mechanismus gegenüber dem bimolekularen Mechanismus bevorzugt wird. Der Wasserstoff-Transfer-Mechanismus hat eine eher hohe freie Barriere der freien Energie von 203 kJ/mol, während die Barriere für den Methyl-Transfer-Mechanismus mit 227 kJ/mol sehr hoch und daher bei den betracheten Reaktionsbedingungen nicht plausibel ist. Diese Reaktionsmechanismen werden anschließend in einer Reihe von verschiedenen Zeoliten und Zeotypen von CHA, AFI und MOR neu optimiert. Für den monomolekularen Mechanismus werden abhängig von dem konkreten Zeoliten unterschiedliche Reaktionspfade als optimal berechnet. Für Zeolite, welche eine höhere Azidität aufweisen, wie es für AFI der Fall ist, wird der bimolekulare Mechanismus als konkurrierend oder sogar als dominant gegenüber den monomolekularen Mechanismus berechnet. Die Barrieren des Wasserstoff-Transfer-Mechanismus reichen von 181 kJ/mol bis 236 kJ/mol, was bedeutet, dass diese im Fall von stark aziden Zeoliten berücksichtigt werden müssen, während der Methyl-Transfer-Mechanismus nur Barrieren von $\geq$217 kJ/mol aufzeigt. Alle Resultate werden schlussendlich mittels linearer Skalierungsbeziehungen zusammengefasst, welche sich auf die Adsorptionsenergie von Ammoniak als Deskriptor beziehen. Es ist bekannt, dass diese Skalierungsbeziehungen Tendenzen in der Reaktivität von Zeolit-Katalysatoren voraussagen können, und dies gilt auch hier für die in dieser Arbeit untersuchten Reaktionsmechanismen

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

Combining theoretical and experimental methods to probe confinement within microporous solid-acid catalysts for alcohol dehydrations

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.</p

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

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