Hydrogen Adsorption in Ultramicroporous Metal-Organic Frameworks Featuring Silent Open Metal Sites

In this study, we utilized an ultramicroporous metal-organic framework (MOF) named [Ni3(pzdc)2(ade)2(H2O)4]·2.18H2O (where H3pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H2) adsorption. Upon activation, [Ni3(pzdc)2(ade)2] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m2/g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H2 uptake saturation at 2.11 mmol/g at ∼0.15 bar. The affinity of this MOF for H2 was determined to be 9.7 ± 1.0 kJ/mol. In situ H2 loading experiments supported by molecular simulations confirmed that H2 does not bind to the open Ni(II) sites of [Ni3(pzdc)2(ade)2], and the high affinity of the MOF for H2 is attributed to the interplay of pore size, shape, and functionality.

Theoretical Assessment of the Mechanism and Active Sites in Alkene Dimerization on Ni Monomers Grafted onto Aluminosilicates: (Ni-OH)+ Centers and C-C Coupling Mediated by Lewis Acid-Base Pairs

Ni-based solids are effective catalysts for alkene dimerization, but the nature of active centers and identity and kinetic relevance of bound species and elementary reactions remain speculative and based on organometallic chemistry. Ni centers grafted onto ordered MCM-41 mesopores lead to well-defined monomers that are rendered stable by the presence of an intrapore nonpolar liquid, thus enabling accurate experimental inquiries and indirect evidence for grafted (Ni-OH)⁺ monomers. Density functional theory (DFT) treatments presented here confirm the plausible involvement of pathways and active centers not previously considered as mediators of high turnover rates for C₂-C₄ alkenes at cryogenic temperatures. (Ni-OH)⁺ species act as Lewis acid-base pairs that stabilize C-C coupling transition states by polarizing two alkenes in opposite directions via concerted interactions with the O and H atoms in these pairs. DFT-derived activation barriers for ethene dimerization (59 kJ mol^-1) are similar to measured values (46 ± 5 kJ mol^-1) and the weak binding of ethene on (Ni-OH)⁺ is consistent with kinetic trends that require sites to remain essentially bare at subambient temperatures and high alkene pressures (1-15 bar). DFT treatments of classical metallacycle and Cossee-Arlman dimerization routes (Ni⁺ and Ni²⁺-H grafted onto Al-MCM-41, respectively) show that such sites bind ethene strongly and lead to saturation coverages, in contradiction with observed kinetic trends. These C-C coupling routes at acid-base pairs in (Ni-OH)⁺ differ from molecular catalysts in (i) the type of elementary steps; (ii) the nature of active centers; and (iii) their catalytic competence at subambient temperatures without requiring co-catalysts or activators.

Porous Materials Confining Single Atoms for Catalysis

In recent years, single-atom catalysts (SACs) have received extensive attention due to their unique structure and excellent performance. Currently, a variety of porous materials are used as confined single-atom catalysts, such as zeolites, metal-organic frameworks (MOFs), or carbon nitride (CN). The support plays a key role in determining the coordination structure of the catalytic metal center and its catalytic performance. For example, the strong interaction between the metal and the carrier induces the charge transfer between the metal and the carrier, and ultimately affects the catalytic behavior of the single-atom catalyst. Porous materials have unique chemical and physical properties including high specific surface area, adjustable acidity and shape selectivity (such as zeolites), and are rational support materials for confined single atoms, which arouse research interest in this field. This review surveys the latest research progress of confined single-atom catalysts for porous materials, which mainly include zeolites, CN and MOFs. The preparation methods, characterizations, application fields, and the interaction between metal atoms and porous support materials of porous material confined single-atom catalysts are discussed. And we prospect for the application prospects and challenges of porous material confined single-atom catalysts.

Precise Identification and Characterization of Catalytically Active Sites on the Surface of γ-Alumina*

γ-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of γ-alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopy and solid-state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O)₅ Al(VI)-OH sites on the (100) segments of massively restructured (110) facets on typical rhombus-platelet γ-alumina as well as the (100) segments of irrational surfaces (invariably always present in all γ-alumina samples) responsible for its unique catalytic activity. Such (O)₅ Al(VI)-OH sites are also present on the macroscopically defined (100) facets of γ-alumina with elongated/rod-like geometry. The mechanism by which these sites lose -OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta-coordinate Al⁺³ O₅ sites is clarified. These coordinatively unsaturated penta-coordinate Al sites produce well-defined thermally stable Al-carbonyl complexes. Our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ-alumina and their role as catalytically active sites.

Catalytic conversion of ethene to butadiene or hydrogenation to ethane on HY zeolite-supported rhodium complexes: Cooperative support/Rh-center route

Rh(C₂H₄)₂ species grafted on the HY zeolite framework significantly enhance the activation of H₂ that reacts with C₂H₄ ligands to form C₂H₆. While in this case, the simultaneous activation of C₂H₄ and H₂ and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C₂H₆, the results obtained for Rh(CO)(C₂H₄)/HY materials exposed to H₂ convincingly show that the support-assisted C₂H₄ hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of C₂H₄ ligands on Rh sites and contributes significantly to the overall hydrogenation activity. This pathway does not require simultaneous activation of reactants on the same metal center and, therefore, is mechanistically different from hydrogenation chemistry exhibited by molecular organometallic complexes. We also demonstrate that the conversion of zeolite-supported Rh(CO)₂ complexes into Rh(CO)(C₂H₄) species under ambient conditions is not a simple CO/C₂H₄ ligand exchange reaction on Rh sites, as this process also involves the conversion of C₂H₄ into C₄ hydrocarbons, among which 1,3-butadiene is the main product formed with the initial selectivity exceeding 98% and the turnover frequency of 8.9 × 10^-3 s^-1. Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H₂, as these species significantly accelerate the formation of the C₄ hydrocarbons from C₂H₄ even without the presence of H₂ in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of C₂H₄ into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane.

Economizing on Precious Metals in Three-Way Catalysts: Thermally Stable and Highly Active Single-Atom Rhodium on Ceria for NO Abatement under Dry and Industrially Relevant Conditions*

We show for the first time that atomically dispersed Rh cations on ceria, prepared by a high-temperature atom-trapping synthesis, are the active species for the (CO+NO) reaction. This provides a direct link with the organometallic homogeneous Rh^I complexes capable of catalyzing the dry (CO+NO) reaction. The thermally stable Rh cations in 0.1 wt % Rh₁ /CeO₂ achieve full NO conversion with a turn-over-frequency (TOF) of around 330 h^-1 per Rh atom at 120 °C. Under dry conditions, the main product above 100 °C is N₂ with N₂ O being the minor product. The presence of water promotes low-temperature activity of 0.1 wt % Rh₁ /CeO₂ . In the wet stream, ammonia and nitrogen are the main products above 120 °C. The uniformity of Rh ions on the support, allows us to detect the intermediates of (CO+NO) reaction via IR measurements on Rh cations on zeolite and ceria. We also show that NH₃ formation correlates with the water gas shift (WGS) activity of the material and detect the formation of Rh hydride species spectroscopically.

Tuning supported Ni catalysts by varying zeolite Beta heteroatom composition: effects on ethylene adsorption and dimerization catalysis

The influence of zeolite heteroatom composition on the electron density and catalytic activity of a supported Ni cation is examined.

Variable Temperature and Pressure Operando MAS NMR for Catalysis Science and Related Materials

The characterization of catalytic materials under working conditions is of paramount importance for a realistic depiction and comprehensive understanding of the system. Under such relevant environments, catalysts often exhibit properties or reactivity not observed under standard spectroscopic conditions. Fulfilling such harsh environments as high temperature and pressure is a particular challenge for solid-state NMR where samples spin several thousand times a second within a strong magnetic field. To address concerns about the disparities between spectroscopic environments and operando conditions, novel MAS NMR technology has been developed that enables the probing of catalytic systems over a wide range of pressures, temperatures, and chemical environments. In this Account, new efforts to overcome the technical challenges in the development of operando and in situ MAS NMR will be briefly outlined. Emphasis will be placed on exploring the unique chemical regimes that take advantage of the new developments. With the progress achieved, it is possible to interrogate both structure and dynamics of the environments surrounding various nuclear constituents (¹H, ¹³C, ²³Na, ²⁷Al, etc.), as well as assess time-resolved interactions and transformations.Operando and in situ NMR enables the direct observation of chemical components and their interactions with active sites (such as Brønsted acid sites on zeolites) to reveal the nature of the active center under catalytic conditions. Further, mixtures of such constituents can also be assessed to reveal the transformation of the active site when side products, such as water, are generated. These interactions are observed across a range of temperatures (-10 to 230 °C) and pressures (vacuum to 100 bar) for both vapor and condensed phase analysis. When coupled with 2D NMR, computational modeling, or both, specific binding modes are identified where the adsorbed state provides distinct signatures. In addition to vapor phase chemical environments, gaseous environments can be introduced and controlled over a wide range of pressures to support catalytic studies that require H₂, CO, CO₂, etc. Mixtures of three phases may also be employed. Such reactions can be monitored in situ to reveal the transformation of the substrates, active sites, intermediates, and products over the course of the study. Further, coupling of operando NMR with isotopic labeling schemes reveals specific mechanistic insights otherwise unavailable. Examples of these strategies will be outlined to reveal important fundamental insights on working catalyst systems possible only under operando conditions. Extension of operando MAS NMR to study the solid-electrolyte interface and solvation structures associated with energy storage systems and biomedical systems will also be presented to highlight the versatility of this powerful technique.