Relationships between the Hydrogenation and Dehydrogenation Properties of Rh‑, Ir‑, Pd‑, and Pt-Containing Zeolites Y Studied by In Situ MAS NMR Spectroscopy and Conventional Heterogeneous Catalysis

The intrinsic hydrogenation activities of homologous series of noble-metal-containing zeolites Y were studied by in situ solid-state NMR spectroscopy under semibatch conditions. For the hydrogenation of acrylonitrile, reaction rates in the sequence Pd/H,Na–Y > Rh/H,Na–Y > Pt/H,Na–Y > Ir/H,Na–Y were determined. The dehydrogenation of propane at these zeolites gave a sequence of the turnover frequencies of Ir/H,Na–Y > Rh/H,Na–Y > Pd/H,Na–Y, while Pt/H,Na–Y zeolites showed significantly higher activities. The temperature-programmed desorption of hydrogen (H₂-TPD) was utilized for studying the strength of H₂/metal interactions. The positions of the high-temperature peaks were arranged according to 2.8Pd/H,Na–Y (723 K) > 2.3Rh/H,Na–Y (713 K) > 4.7Ir/H,Na–Y (663 K). Comparison of these data indicates that strong H₂/metal interactions are accompanied by a preferred formation of surface hydrogen atoms, which are the reason for the high hydrogenation activity of Pd/H,Na–Y zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. In the case of the propane dehydrogenation, the strong H₂/Pd interactions in Pd/H,Na–Y zeolites hinder the desorption of the reaction product H₂, explaining the lower dehydrogenation activity of these zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. For the high catalytic activities of the Pt/H,Na–Y zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt clusters is discussed

Relationships between the Hydrogenation and Dehydrogenation Properties of Rh‑, Ir‑, Pd‑, and Pt-Containing Zeolites Y Studied by In Situ MAS NMR Spectroscopy and Conventional Heterogeneous Catalysis

The intrinsic hydrogenation activities of homologous series of noble-metal-containing zeolites Y were studied by in situ solid-state NMR spectroscopy under semibatch conditions. For the hydrogenation of acrylonitrile, reaction rates in the sequence Pd/H,Na–Y > Rh/H,Na–Y > Pt/H,Na–Y > Ir/H,Na–Y were determined. The dehydrogenation of propane at these zeolites gave a sequence of the turnover frequencies of Ir/H,Na–Y > Rh/H,Na–Y > Pd/H,Na–Y, while Pt/H,Na–Y zeolites showed significantly higher activities. The temperature-programmed desorption of hydrogen (H₂-TPD) was utilized for studying the strength of H₂/metal interactions. The positions of the high-temperature peaks were arranged according to 2.8Pd/H,Na–Y (723 K) > 2.3Rh/H,Na–Y (713 K) > 4.7Ir/H,Na–Y (663 K). Comparison of these data indicates that strong H₂/metal interactions are accompanied by a preferred formation of surface hydrogen atoms, which are the reason for the high hydrogenation activity of Pd/H,Na–Y zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. In the case of the propane dehydrogenation, the strong H₂/Pd interactions in Pd/H,Na–Y zeolites hinder the desorption of the reaction product H₂, explaining the lower dehydrogenation activity of these zeolites compared with Rh/H,Na–Y and Ir/H,Na–Y zeolites. For the high catalytic activities of the Pt/H,Na–Y zeolites, an effect of strongly chemisorbed hydrogen atoms inside the Pt clusters is discussed

Understanding the Early Stages of the Methanol-to-Olefin Conversion on H‑SAPO-34

Little is known on the early stages of the methanol-to-olefin (MTO) conversion over H-SAPO-34, before the steady-state with highly active polymethyl­benzenium cations as most important intermediates is reached. In this work, the formation and evolution of carbenium ions during the early stages of the MTO conversion on a H-SAPO-34 model catalyst were clarified via ¹H MAS NMR and ¹³C MAS NMR. Several initial species (i.e., three-ring compounds, dienes, polymethyl­cyclopentenyl, and polymethyl­cyclohexenyl cations) were, for the first time, directly verified during the MTO conversion. Their detailed evolution network was established from theoretical calculations. On the basis of these results, an olefin-based catalytic cycle is proposed to be the primary reaction pathway during the early stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based cycle may be involved in the MTO conversion for long times on stream

Comparison of the Catalytic Activity of Surface-Immobilized Copper Complexes with Phosphonate Anchoring Groups for Atom Transfer Radical Cyclizations and Additions

Covalent immobilization of molecular catalysts onto metal oxide surfaces through linker groups is a common strategy for heterogenizing homogeneous catalysts with the expectation that the immobilized catalyst will have properties similar to those of its molecular counterpart. However, the catalytic properties of the immobilized species are often quite different compared to their soluble counterparts in ways that are difficult to predict. This phenomenon is poorly understood and could be due to a variety of factors, including steric shielding of the complex by the surface, changes to the coordination sphere upon immobilization, or a lack of conformational flexibility of the immobilized complexes. Here, we tested the effect of surface immobilization on the catalytic activity and selectivity of atom transfer radical additions and cyclizations. In this study, we varied the proximity of the phosphonate anchoring group to the Cu center by attachment at varying positions of chelating nitrogen ligands such as 1,10-phenanthroline (phen), tris(pyridylmethyl)amine, and 2,9-dimethyl-1,10-phenanthroline as ligand scaffolds. Catalytic testing revealed that in cases where the anchoring group is remote from the catalytic center, as is the case for Cu(phen), the immobilized catalyst functions overall slightly better than its homogeneous counterpart (resulting in higher yields). However, for complexes in which the linker group is close to the active center, the catalytic performance of the immobilized complex was generally worse when immobilized than when in solution (decreased yield upon immobilization). Potential explanations of these observations are discussed. This study very clearly demonstrates the highly complex nature of immobilized catalysts and highlights the need for more in-depth comparisons between immobilized and soluble organometallic catalysts

Accessibility of Reactants and Neighborhood of Mo Species during Methane Aromatization Uncovered by Operando NAP-XPS and MAS NMR

One-step nonoxidative methane dehydroaromatization is a facile process to generate aromatics and CO-free hydrogen. Despite their high activity and aromatics selectivity, Mo/HZSM-5 catalysts suffer from a continuous deactivation, hampering their application, yet the cause is intensively debated. Employing a combination of characterizations including, but not restricted to, high-resolution electron microscopy, operando NAP-XPS, and MAS NMR spectroscopy, we endeavored in this contribution to get deeper insight into the nature of active sites and origin of catalyst deactivation. Our results indicated (i) an irreversible reaction-induced MoOx particle sintering, (ii) reversible buildup/removal of coke species, (iii) no quantitative correlation between the deactivation rate and the presence/loss of Brønsted acid sites, and (iv) that coke accumulation occurs almost exclusively on Mo instead of Brønsted acid sites. Deactivation is explained by partial blocking of Mo species by coke, which diminishes the accessibility of methane to active sites and successive narrowing and/or blocking of pores hindering the diffusion of larger reaction products (e.g., naphthalene) to the outer surface. Active sites for aromatics formation are referred to as highly dispersed Mo species (mononuclear and tiny subnanometer oxy- and/or oxycarbidic Mo clusters) located inside the micropores on/or close to Brønsted sites

A Straightforward Descriptor for the Deactivation of Zeolite Catalyst H‑ZSM‑5

ZSM-5 is a widely used zeolite catalyst and is employed industrially for the methanol to gasoline (MTG) process. Even so, deactivation of ZSM-5 by coke formation constitutes a major technical and also fundamental challenge. We investigate the deactivation of a range of ZSM-5 catalysts through catalytic testing, physicochemical characterization, and powder X-ray diffraction (XRD). It is demonstrated that the unit cell changes upon deactivation. Periodic density functional theory is used to show that the change is induced by certain methyl substituted benzenes in the channel intersection in ZSM-5. This finding is corroborated by Rietveld refinement of XRD data obtained for deactivated catalysts. We are able to establish a direct correlation between the difference in the length of the <i>a</i>- and <i>b</i>-unit cell vectors and the total amount of coke, the remaining acidity, and the remaining surface area of the catalysts. This <i>a</i>- minus <i>b</i>-parameter is a straightforward descriptor that carries the essential information regarding the degree of deactivation of a ZSM-5 catalyst, and a routine measurement of a diffractogram of the catalyst can be used to quantitatively assess the degree of deactivation