Transition Metal versus Heavy Metal Synergy in Selective Catalytic Oxidations

Simultaneous framework incorporation of heavy metal ions such as Ru­(III) and Sn­(IV) into aluminophosphate architectures generates novel bimetallic active sites, which facilitate synergistic interactions, affording high degrees of selectivity and activity in the catalytic oxidations as compared with analogous bimetallic systems, in which transition metals, such as Co­(III) and Ti­(IV), have been similarly incorporated

Iridium–Bismuth Cluster Complexes Yield Bimetallic Nano-Catalysts for the Direct Oxidation of 3‑Picoline to Niacin

The reaction of Ir₃(CO)₉(μ₃-Bi), <b>1</b>, with BiPh₃ has yielded a iridium–bismuth cluster complex Ir₅(CO)₁₀(μ₃-Bi)₂(μ₄-Bi), <b>2</b>. The <i>first</i> examples of bimetallic iridium–bismuth nanoparticles have been subsequently synthesized from <b>1</b> and <b>2</b>, and these have been securely anchored onto the inner walls of mesoporous silica. These isolated, bimetallic iridium–bismuth nanoparticles display a superior catalytic performance, when compared to their analogous monometallic counterparts and equivalent physical mixtures, in the C–H activation of 3-picoline to yield niacin

Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Structure–property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and <i>in situ</i> FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO²⁺ ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems

Role of Isolated Acid Sites and Influence of Pore Diameter in the Low-Temperature Dehydration of Ethanol

Silicoaluminophosphates, SAPO-5 and SAPO-34, differ not only in their pore diameters and structural topology but also in their preferred mechanism of silicon substitution into the framework, which subsequently influences the nature of the acid sites for solid-acid-catalyzed transformations. This study combines ²⁹Si NMR, FTIR, and DFT calculations for probing the nature of the isolated acid sites, thereby affording structure–property correlations, in the low-temperature catalytic dehydration of ethanol to ethylene

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