Promoting Aromatic C-H Activation through Reactive Brønsted Acid-Base Pairs on Penta-Coordinated Al-Enriched Amorphous Silica-Alumina.

The surface acidity and local coordination environment of zeolites and amorphous silica-aluminas (ASAs) can promote acid-catalyzed C-H activation in many important hydrocarbon conversion reactions. Acid sites generated by penta-coordinated Al species (Al) can lead to enhanced acidity and changes in the surface coordination. We evaluated the potential of flame-derived ASAs with enriched Al for C-H activation using hydrogen/deuterium (H/D) exchange with benzene-.

Engineering the Distinct Structure Interface of Subnano-alumina Domains on Silica for Acidic Amorphous Silica-Alumina toward Biorefining.

Amorphous silica-aluminas (ASAs) are important solid catalysts and supports for many industrially essential and sustainable processes, such as hydrocarbon transformation and biorefining. However, the wide distribution of acid strength on ASAs often results in undesired side reactions, lowering the product selectivity. Here we developed a strategy for the synthesis of a unique class of ASAs with unvarying strength of Brønsted acid sites (BAS) and Lewis acid sites (LAS) using double-flame-spray pyrolysis.

Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small-Pore Zeolites.

The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite-catalyzed processes. Transport of small alkenes in the confined zeolite pores can become hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8-ring windows of zeolite SAPO-34 is characterized by a complementary set of first-principle molecular dynamics simulations, PFG-NMR experiments, and pulse-response temporal analysis of products measurements, yielding information at different length and time scales.

Carbon dioxide adsorption in open nanospaces formed by overlap of saponite clay nanosheets.

Nanoscale open spaces formed by partial overlap of two-dimensional nanosheets in clays, abundantly and ubiquitously available, possess reactive molecular sites such as nanosheet edges in their interior. Here, the capture and storage of CO molecules in open spaces within saponite clay are explored by solid-state nuclear magnetic resonance coupled with open space analysis using positronium. CO physisorption occurs on the nanosheet surfaces inside the open spaces under ambient conditions.

High population and dispersion of pentacoordinated Al species on the surface of flame-made amorphous silica-alumina.

Pentacoordinated Al (Al) species in silica-alumina are promising to promote the formation of acid sites or act as surface defects for tailoring single-atom catalysts. However, pentahedral coordination (Al) is rarely observed in conventionally prepared silica-alumina. Here, we show that high population and dispersion of Al species on the surface of amorphous silica-alumina (ASA) can be achieved by means of flame spray pyrolysis.

Molecular studies of Cs adsorption sites in inorganic layered materials: the influence of solution concentration.

Radioactive Cs released into a soil environment migrates along with groundwater in a manner dependent on Cs concentration. Data on the variation of Cs adsorption as a function of solution concentration are an essential prerequisite to successful decontamination work in Fukushima. To aid the ongoing decontamination work, the adsorption of Cs in aqueous solution across a wide Cs molarity range is studied for the case of saponite clay as adsorbent, an inorganic layered material that is an abundant mineral in the soil environment.

Separation of Anti-Phase Signals Due to Parahydrogen Induced Polarization via 2D Nutation NMR Spectroscopy.

The present work introduces a novel method for the selective detection of H NMR anti-phase signals caused by the pairwise incorporation of parahydrogen into olefins on noble-metal-containing catalysts. Via a two-dimensional (2D) nutation NMR experiment, the anti-phase signals of hyperpolarized H nuclei are separated due to their double nutation frequency compared to that of thermally polarized H nuclei. For demonstrating this approach, parahydrogen induced polarization (PHIP) was achieved via the hydrogenation of propene with parahydrogen on platinum-containing silica and investigated by in situ H MAS NMR spectroscopy under continuous-flow conditions, that is, the hydrogenation reaction was performed inside the magnet of the NMR spectrometer.

Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica.

The synthesis and single-crystal X-ray structures of the novel molybdenum imido alkylidene N-heterocyclic carbene complexes [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf)2] (3), [Mo(N-2,6-Me2C6H3)(IMes)(CHCMe2Ph)(OTf)2] (4), [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf){OCH(CF3)2}] (5), [Mo(N-2,6-Me2C6H3)(CH3CN)(IMesH2)(CHCMe2Ph)(OTf)](+)BArF(-) (6), [Mo(N-2,6-Cl2C6H3)(IMesH2)(CHCMe3)(OTf)2] (7) and [Mo(N-2,6-Cl2C6H3)(IMes)(CHCMe3)(OTf)2] (8) are reported (IMesH2=1,3-dimesitylimidazolidin-2-ylidene, IMes=1,3-dimesitylimidazolin-2-ylidene, BArF(-)=tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate, OTf=CF3SO3(-)). Also, silica-immobilized versions I1 and I2 were prepared. Catalysts 3-8, I1 and I2 were used in homo-, cross-, and ring-closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω-diynes.

Evidence of rutile-to-anatase photo-induced electron transfer in mixed-phase TiO2 by solid-state NMR spectroscopy.

With ethanol as a probe molecule, the surface sites of anatase and rutile can be distinguished using (13)C CP/MAS NMR spectroscopy, which offers an opportunity to investigate the transfer of photo-induced electrons from rutile to anatase in the mixed-phase TiO2.

Identification of tert-Butyl Cations in Zeolite H-ZSM-5: Evidence from NMR Spectroscopy and DFT Calculations.

Experimental evidence for the presence of tert-butyl cations, which are important intermediates in acid-catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with (1)H/(13)C magic-angle-spinning NMR spectroscopy, the tert-butyl cation was successfully identified on zeolite H-ZSM-5 upon conversion of isobutene by capturing this intermediate with ammonia.

Long-term self-assembly of inorganic layered materials influenced by the local states of the interlayer cations.

A wide variety of parameters as, e.g., temperature, humidity, particle size, and cation state are known to influence the agglomeration process of two-dimensional (2D) nanosheets, called self-assembly, in inorganic layered materials.

Anomalous diamagnetic susceptibility in 13-atom platinum nanocluster superatoms.

We are used to being able to predict diamagnetic susceptibilities χD to a good approximation in atomic increments since there is normally little dependence on the chemical environment. Surprisingly, we find from SQUID magnetization measurements that the χD per Pt atom of zeolite-supported Pt13 nanoclusters exceeds that of Pt(2+) ions by a factor of 37-50. The observation verifies an earlier theoretical prediction.

Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts.

Further progress in the field of heterogeneous catalysis depends on our knowledge of the nature and behavior of surface sites on solid catalysts and of the mechanisms of chemical reactions catalyzed by these materials. In the past decades, solid-state NMR spectroscopy has been developed to an important tool for routine characterization of solid catalysts. The present work gives a review on experimental approaches and applications of solid-state NMR spectroscopy for investigating Brønsted and Lewis sites on solid acids.

Vanadium phosphates on mesoporous supports: model catalysts for solid-state NMR studies of the selective oxidation of n-butane.

SBA-15 was utilized as mesoporous support for the dispersion of vanadium phosphate (VPO) compounds. Loading of SBA-15 with VPO compounds was found to be accompanied by decreasing (29)Si MAS NMR signals of Q(2) (Si(2Si,2OH)) and Q(3) (Si(3Si,1OH)) silicon species, which indicates coverage of the mesoporous support by the guest compounds. The (51)V MAS NNR spectra of the activated VPO/SBA-15 catalysts consist of patterns typical for the alpha(II)- and beta-phases of vanadyl orthophosphate.

Adsorption-desorption induced structural changes of Cu-MOF evidenced by solid state NMR and EPR spectroscopy.

Adsorption-desorption induced structural changes of Cu(bpy)(H(2)O)(2)(BF(4)),(bpy) (bpy = 4,4'-bipyridine) [Cu-MOF] have been evidenced by combined NMR and EPR spectroscopy. Upon adsorption of probe molecules even at a few mbar, EPR spectra show that they are activated to form complexes at Cu(II) sites, which results in a change of the Cu-MOF's structure as indicated by a high-field shift of the (11)B MAS NMR. After desorption, both EPR and (11)B MAS NMR spectra evidenced that the structure of the Cu-MOF reversibly shifted to the original state.

Insight into the mechanisms of the ethylbenzene disproportionation: transition state shape selectivity on zeolites.

The direct experimental evidence shows that ethylbenzene disproportionation is a transition state shape selective reaction on zeolites: a bimolecular reaction mechanism via diphenylethane-mediated pathway on large-pore zeolites X and Y (ca. 0.74 nm) and a monomolecular reaction mechanism on medium-pore zeolites ZSM-5 (ca.

Reactivity of surface alkoxy species on acidic zeolite catalysts.

[Reaction: see text]. A solid understanding of the mechanisms involved in heterogeneously catalyzed reactions is of fundamental interest for modern chemistry. This information can help to refine modern theories of catalysis and, in a very practical way, can help researchers to optimize existing industrial processes and develop new ones.

Beckmann rearrangement of 15N-cyclohexanone oxime on zeolites silicalite-1, H-ZSM-5, and H-[B]ZSM-5 studied by solid-state NMR spectroscopy.

By means of solid-state 15N NMR spectroscopy, evidence for the formation of nitrilium ions as intermediates of the Beckmann rearrangement of 15N-cyclohexanone oxime to epsilon-caprolactam on silicalite-1, H-ZSM-5, and H-[B]ZSM-5 is reported. The zeolites under study are characterized by different acid strengths (silicalite-1 < H-[B]ZSM-5 < H-ZSM-5). Depending on the nature of catalytically active surface OH groups, reactant and product molecules exist in the nonprotonated or protonated state.

On the reactivity of surface methoxy species in acidic zeolites.

The in situ preparation and isolation of surface methoxy species on acidic zeolites are followed by further investigations of their reactivity in heterogeneously catalyzed reactions. For the first time, the following solid-state NMR evidence for the high reactivity of surface methoxy species has been obtained: (i) Surface methoxy species react readily with ammonia on acidic zeolites at room temperature, by which methylamines and methylammonium cations are formed. (ii) The transformation of surface methoxy species to other alkoxy species can be achieved by the reaction of surface methoxy species and corresponding alkyl halides on acidic zeolites.

Effects of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms in dehydrated zeolite H,Na-Y.

The effect of adsorbate molecules on the quadrupolar interaction of framework aluminum atoms with the electric field gradient in dehydrated zeolite H,Na-Y has been studied by (27)Al MAS NMR and (27)Al MQMAS NMR spectroscopy at magnetic fields of 9.4 and 17.6 T.

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0 = 9.4 to 17.6 T.

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.

Mechanism studies of the conversion of 13C-labeled n-butane on zeolite H-ZSM-5 by using 13C magic angle spinning NMR spectroscopy and GC-MS analysis.

By using 13C MAS NMR spectroscopy (MAS = magic angle spinning), the conversion of selectively 13C-labeled n-butane on zeolite H-ZSM-5 at 430-470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13C-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl-substituted cyclopentenyl cations and condensed aromatic compounds. In situ 13C MAS NMR and complementary ex situ GC-MS data provided evidence for a monomolecular mechanism of the 13C-label scrambling, whereas both isobutane and propane are formed through intermolecular pathways.