Improved rhodium hydrogenation catalysts immobilized on silica

XPS and HR TEM Elucidation of the Diversity of Titania-Supported Single-Site Ir Catalyst Performance in Spin-Selective Propene Hydrogenation

Immobilized [Ir(COD)Cl]2-Linker/TiO2 catalysts with linkers containing Py, P(Ph)2 and N(CH3)2 functional groups were prepared. The catalysts were tested via propene hydrogenation with parahydrogen in a temperature range from 40 °C to 120 °C which was monitored via NMR. The catalytic behavior of [Ir(COD)Cl]2-Linker/TiO2 is explained on the basis of quantitative and qualitative XPS data analysis performed for the catalysts before and after the reaction at 120 °C. It is shown that the temperature dependence of propene conversion and the enhancement of the NMR signal are explained via a combination of the stabilities of both the linker and immobilized [Ir(COD)Cl]2 complex. It is demonstrated that the N(CH3)2-linker is the most stable at the surface of TiO2 under used reaction conditions. As a result, only this sample shows a rise in the enhancement of the NMR signal in the 100-120 °C temperature range.

TiO2 NPs-immobilized silica granules: New insight for nano catalyst fixation for hydrogen generation and sustained wastewater treatment

In heterogeneous catalytic processes, immobilization of the functional material over a proper support is a vital solution for reusing and/or avoiding a secondary pollution problem. The study introduces a novel approach for immobilizing R25 NPs on the surface of silica granules using hydrothermal treatment followed by calcination process. Due to the privileged characteristics of the subcritical water, during the hydrothermal treatment process, the utilized R25 NPs were partially dissolved and precipitated on the surface of the silica granules. Calcination at high temperature (700°C) resulted in improving the attachment forces. The structure of the newly proposed composite was approved by 2D and 3D optical microscope images, XRD and EDX analyses. The functionalized silica granules were used in the form of a packed bed for continuous removal of methylene blue dye. The results indicated that the TiO2:sand ratio has a considerable effect on the shape of the dye removal breakthrough curve as the exhaustion point, corresponding to ~ 95% removal, was 12.3, 17.4 and 21.3 min for 1:20, 1:10 and 1:5 metal oxides ratio, respectively. Furthermore, the modified silica granules could be exploited as a photocatalyst for hydrogen generation from sewage wastewaters under direct sunlight with a good rate; 75×10-3 mmol/s. Interestingly, after the ease separation of the used granules, the performance was not affected. Based on the obtained results, the 170°C is the optimum hydrothermal treatment temperature. Overall, the study opens a new avenue for immobilization of functional semiconductors on the surface of sand granules.

Small Molecules, Non-Covalent Interactions, and Confinement

This review gives an overview of current trends in the investigation of small guest molecules, confined in neat and functionalized mesoporous silica materials by a combination of solid-state NMR and relaxometry with other physico-chemical techniques. The reported guest molecules are water, small alcohols, and carbonic acids, small aromatic and heteroaromatic molecules, ionic liquids, and surfactants. They are taken as characteristic role-models, which are representatives for the typical classes of organic molecules. It is shown that this combination delivers unique insights into the structure, arrangement, dynamics, guest-host interactions, and the binding sites in these confined systems, and is probably the most powerful analytical technique to probe these systems.

Disentangling different modes of mobility for triphenylphosphine oxide adsorbed on alumina

Triphenylphosphine oxide (TPPO, 1) has been adsorbed on neutral alumina by dry grinding of the components in the absence of a solvent. The adsorption proves translational mobility of 1 on the surface of alumina. Different surface coverages from a densely packed monolayer (99% coverage) to a dilute sub-monolayer (25%) have been produced. The samples have been studied by diverse multinuclear ¹H, ¹³C, and ³¹P variable temperature solid-state nuclear magnetic resonance (NMR) techniques. The interactions of 1 with the surface are determined by hydrogen bonding of the P=O group to OH groups on the surface. The ³¹P solid-state NMR spectra prove that even at low temperatures, the molecules of 1 are highly mobile on the surface. Using T₁ and T₂ relaxation time analyses of the ³¹P resonance in the solid state at variable temperatures allowed the identification and quantification of two different modes of mobility. Besides the translational mobility that consists of jumps from one hydrogen-bonding OH site on the surface to an adjacent one, a rotational movement around the axis defined by the P=O group of 1 occurs.

Colloidal Palladium Nanoparticles for Selective Hydrogenation of Styrene Derivatives with Reactive Functional Groups

This article presents the catalysis investigation of octanethiolate-capped palladium nanoparticles (C8 PdNP) and phenylethanethiolate-capped palladium nanoparticles (PhC2 PdNP) for chemoselective catalytic hydrogenation reactions of styrene derivatives in the presence of other reducible functionalities. The results show that the C8 PdNP is highly active under mild reaction conditions (room temperature and atmospheric pressure) and selective for hydrogenating monosubstituted alkene groups without reducing other reactive functional groups such as nitro, halo, carbonyls, and so forth. In comparison, the noncovalent interactions between surface phenyl ligands and aromatic substrates are found to hinder the hydrogenation activity of PhC2 PdNP.

Hydrogen peroxide adducts of triarylphosphine oxides

Five new hydrogen peroxide adducts of phosphine oxides (p-Tol₃PO·H₂O₂)₂ (1), (o-Tol₃PO·H₂O₂)₂ (2), (o-Tol₂PhPO·H₂O₂)₂ (3), (p-Tol₃PO)₂·H₂O₂ (4), and (o-TolPh₂PO)₂·H₂O₂ (5), and the water adduct (o-Tol₂PhPO·H₂O)₂ (6) have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The IR and ³¹P NMR data are in accordance with strong hydrogen bonding of the hydrogen peroxide. The mono- versus dimeric nature of the adduct assemblies has been investigated by DOSY NMR experiments. Raman spectroscopy of the symmetric adducts and the ν(O-O) stretching bands confirm the presence of hydrogen-bonded hydrogen peroxide in the solid materials. The solubilities in organic solvents have been quantified. Due to the high solubilities of 1-6 in organic solvents their ¹⁷O NMR spectra could be recorded in natural abundance, providing well-resolved signals for the P[double bond, length as m-dash]O and O-O groups. The adducts 1-5 have been probed regarding their stability in solution at 105 °C. The decomposition of the adduct 1 takes place by loss of the active oxygen atoms in two steps.

Structures and Dynamics of Secondary and Tertiary Alkylphosphine Oxides Adsorbed on Silica

The three secondary phosphine oxides [CH₂ =CH(CH₂ )₄ ]₂ HPO (1), [CH₂ =CH(CH₂ )₅ ]₂ HPO (2), and [CH₂ =CH(CH₂ )₆ ]₂ HPO (3), and two diphosphine dioxides, {[CH₂ =CH(CH₂ )₆ ]₂ PO(CH₂ )₇ }₂ (4) and {[CH₂ =CH(CH₂ )₆ ]₂ PO(CH₂ )₄ }₂ (5), incorporating long methylene chains, are described. The single crystal X-ray structures of 1, 2, and 5 have been determined. The phosphine oxides 3, 4, and 5 have been adsorbed on silica in submonolayer quantities to give 3 a-5 a. The ¹ H, ¹³ C, and ³¹ P solid-state NMR spectra of polycrystalline 3-5 have been analyzed and compared with those of 3 a-5 a. The changes of the solid-state NMR characteristics upon adsorption and the surface mobilities of the phosphine oxides are discussed.

Selective synthesis and stabilization of peroxides via phosphine oxides

MEKPO (methyl ethyl ketone peroxide) and other peroxides can be synthesized selectively and stabilized as hydrogen-bonded phosphine oxide adducts.

31P-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts

The synthesis of novel robust and stable iridium-based immobilized catalysts on silica-polymer hybrid materials (Si-PB-Ir) is described. These catalysts are characterized by a combination of 1D 31P CP-MAS and 2D 31P-1H HETCOR and J-resolved multinuclear solid state NMR experiments. Different binding situations such as singly and multiply coordinated phosphines are identified. Density functional theory (DFT) calculations are performed to corroborate the interpretation of the experimental NMR data, in order to propose a structural model of the heterogenized catalysts. Finally, the catalytic activity of the Si-PB-Ir catalysts is investigated for the hydrogenation of styrene employing para-enriched hydrogen gas.

Total Synthesis of K777: Successful Application of Transition-Metal-Catalyzed Alkyne Hydrothiolation toward the Modular Synthesis of a Potent Cysteine Protease Inhibitor

We report the total synthesis of K777 and a series of analogues via alkyne hydrothiolation catalyzed by Wilkinson's complex (ClRh(PPh3)3). The alkyne hydrothiolation reactions proceeded with excellent regio- and diastereoselectivity to generate the desired E-linear vinyl sulfides in high yield. The use of Ellman's auxiliary generates the requisite propargyl amines in excellent enantiomeric excess (ee) and obviates the use of L-homophenylalanine, an expensive unnatural amino acid. The vinyl sulfone derivatives exhibit a large difference in rate toward Michael addition. Kinetic data are consistent with rate-limiting nucleophilic attack to generate the carbanion intermediate.

Monometallic Ni(0) and Heterobimetallic Ni(0) /Au(I) Complexes of Tripodal Phosphine Ligands: Characterization in Solution and in the Solid State and Catalysis

The tridentate chelate nickel complexes [(CO)Ni{(PPh2 CH2 )3 CMe}] (2), [(CO)Ni{(PPh2 CH2 CH2 )3 SiMe}] (6), and [Ph3 PNi{(PPh2 CH2 CH2 )3 SiMe}] (7), as well as the bidentate complex [(CO)2 Ni{(PPh2 CH2 )2 CMeCH2 PPh2 }] (3) and the heterobimetallic complex [(CO)2 Ni{(PPh2 CH2 )2 CMeCH2 Ph2 PAuCl}] (4), have been synthesized and fully characterized in solution. All (1) H and (13) C NMR signal assignments are based on 2D-NMR methods. Single crystal X-ray structures have been obtained for all complexes. Their (31) P CP/MAS (cross polarization with magic angle spinning) NMR spectra have been recorded and the isotropic lines identified. The signals were assigned with the help of their chemical shift anisotropy (CSA) data. All complexes have been tested regarding their catalytic activity for the cyclotrimerization of phenylacetylene. Whereas complexes 2-4 display low catalytic activity, complex 7 leads to quantitative conversion of the substrate within four hours and is highly selective throughout the catalytic reaction.

Natural abundance 15N NMR by dynamic nuclear polarization: fast analysis of binding sites of a novel amine-carboxyl-linked immobilized dirhodium catalyst

A novel heterogeneous dirhodium catalyst has been synthesized. This stable catalyst is constructed from dirhodium acetate dimer (Rh2(OAc)4) units, which are covalently linked to amine- and carboxyl-bifunctionalized mesoporous silica (SBA-15-NH2-COOH). It shows good efficiency in catalyzing the cyclopropanation reaction of styrene and ethyl diazoacetate (EDA) forming cis- and trans-1-ethoxycarbonyl-2-phenylcyclopropane. To characterize the structure of this catalyst and to confirm the successful immobilization, heteronuclear solid-state NMR experiments have been performed. The high application potential of dynamic nuclear polarization (DNP) NMR for the analysis of binding sites in this novel catalyst is demonstrated. Signal-enhanced (13)C CP MAS and (15)N CP MAS techniques have been employed to detect different carboxyl and amine binding sites in natural abundance on a fast time scale. The interpretation of the experimental chemical shift values for different binding sites has been corroborated by quantum chemical calculations on dirhodium model complexes.

Solid-state NMR concepts for the investigation of supported transition metal catalysts and nanoparticles

In recent years, solid-state NMR spectroscopy has evolved into an important characterization tool for the study of solid catalysts and chemical processes on their surface. This interest is mainly triggered by the need of environmentally benign organic transformations ("green chemistry"), which has resulted in a large number of new catalytically active hybrid materials, which are organized on the meso- and nanoscale. Typical examples of these catalysts are supported homogeneous transition metal catalysts or transition metal nanoparticles (MNPs). Solid-state NMR spectroscopy is able to characterize both the structures of these materials and the chemical processes on the catalytic surface. This article presents recent trends both on the characterization of immobilized homogeneous transition metal catalysts and on the characterization of surface species on transition metal surfaces.

Parahydrogen-induced polarization in heterogeneous catalytic processes

Parahydrogen-induced polarization of nuclear spins provides enhancements of NMR signals for various nuclei of up to four to five orders of magnitude in magnetic fields of modern NMR spectrometers and even higher enhancements in low and ultra-low magnetic fields. It is based on the use of parahydrogen in catalytic hydrogenation reactions which, upon pairwise addition of the two H atoms of parahydrogen, can strongly enhance the NMR signals of reaction intermediates and products in solution. A recent advance in this field is the demonstration that PHIP can be observed not only in homogeneous hydrogenations but also in heterogeneous catalytic reactions. The use of heterogeneous catalysts for generating PHIP provides a number of significant advantages over the homogeneous processes, including the possibility to produce hyperpolarized gases, better control over the hydrogenation process, and the ease of separation of hyperpolarized fluids from the catalyst. The latter advantage is of paramount importance in light of the recent tendency toward utilization of hyperpolarized substances in in vivo spectroscopic and imaging applications of NMR. In addition, PHIP demonstrates the potential to become a useful tool for studying mechanisms of heterogeneous catalytic processes and for in situ studies of operating catalytic reactors. Here, the known examples of PHIP observations in heterogeneous reactions over immobilized transition metal complexes, supported metals, and some other types of heterogeneous catalysts are discussed and the applications of the technique for hypersensitive NMR imaging studies are presented.

2H-solid-state-NMR study of hydrogen adsorbed on catalytically active ruthenium coated mesoporous silica materials

(2)H solid-state NMR measurements were performed on three samples of ruthenium nanoparticles synthesized inside two different kinds of mesoporous silica, namely SBA-3 silica materials and SBA-15 functionalized with -COOH groups and loaded with deuterium gas. The line-shape analyses of the spectra reveal the different deuteron species. In all samples a strong -OD signal is found, which shows the catalytic activity of the metal, which activates the D-D bond and deuterates the -SiOH groups through the gas phase, corroborating their usability as catalysts for hydrogenation reactions. At room temperature the mobility of the -Si-OD groups depends on the sample preparation. In addition to the -Si-OD deuterons, the presence of different types of deuterons bound to the metal is revealed. The singly coordinated -Ru-D species exhibit several different quadrupolar couplings, which indicate the presence of several non-equivalent binding sites with differing binding strength. In addition to the dissociated hydrogen species there is also a dihydrogen species -Ru-D(2), which is attributed to defect sites on the surface. It exhibits a fast rotational dynamics at all temperatures. Finally there are also indications of three-fold coordinated surface deuterons and octahedrally coordinated deuterons inside the metal.