Quantitative Zn, Al and H MAS NMR spectroscopy for the characterization of Zn species in ZSM-5 catalysts.

Zn MAS NMR spectroscopy was used to characterize the state of Zn in Zn-modified zeolites ZSM-5. Two Zn enriched zeolite samples were prepared: by solid-state exchange with metal Zn (Zn/ZSM-5 sample) and by ion exchange with zinc formate solution (ZnO/H-ZSM-5 sample), both containing 3.8 wt% Zn.

Mobility and separation of linear and branched C alkanes in UiO-66 (Zr) probed by H NMR and MD simulations.

The UiO-66 (Zr) metal-organic framework (MOF) is of notable interest due to its facile synthesis, robustness under a wide range of chemical and physical conditions and its capability to separate industrially relevant hydrocarbons mixtures. However, the knowledge of the molecular mechanisms behind these process remains limited. Here, we present a combined experimental (H NMR) and computational study of the molecular mobility, transport and adsorption of C alkanes isomers in a dehydroxylated UiO-66 (Zr) MOF.

Tuning the Mechanism of H/D Exchange for Isobutane on H-BEA by Loading Zn Species in Zeolite.

Kinetics of H/D hydrogen exchange between deuterated isobutane-d and Brønsted acid sites (BAS) of three zeolite samples (H-BEA, ZnO/H-BEA, Zn /H-BEA) were monitored with H MAS NMR in situ at 343-468 K. The regioselective H/D exchange in the methyl groups detected on H-BEA can be rationalized in terms of the mechanism of indirect exchange, which involves protonation of the intermediate olefin and further hydride abstraction from the other alkane molecule by the formed carbenium ion. Loading of Zn species in the zeolite results in a decrease of the rate and an increase of the activation energy of the exchange.

Butane isomers mobility and framework dynamics in UiO-66 (Zr) MOF: Impact of the hydroxyl groups in zirconia cluster.

UiO-66 (Zr) is a metal-organic framework (MOF) known for its thermal and chemical stability and wide range of adsorption-based applications. This MOF exhibits high separation selectivity for butane isomers. It has been earlier inferred that the separation performance of the material depends on the hydroxylation state of the zirconia cluster.

Dissecting the effects of water guest adsorption and framework breathing on the AlO(OH) centres of metal-organic framework MIL-53 (Al) by solid state NMR and structural analysis.

The relationship between the adsorption of water on MIL-53 (Al) MOF, the structural phase of MIL-53 (Al), and the quadrupole coupling constant of Al framework aluminium atom (QCC) of the MOF AlO(OH) centres (Al-sites) has been investigated by combining solid-state Al MAS NMR spectroscopy with XRD analysis and DFT calculations. It is established that Al QCC is primarily sensitive to water adsorption to the Al-sites and by a minor extent to the framework contraction/expansion interconversions. We thus conclude that the Al MAS NMR method is sensitive enough to differentiate the effects of pore contractions and water adsorption to Al-sites basing on the changes of the QCC value.

Isobutane Transformation to Aromatics on Zn-Modified Zeolites: Intermediates and the Effect of Zn and ZnO Species on the Reaction Occurrence Revealed by C MAS NMR.

To clarify the effects of different Zn species, zeolite topology and acidity (quantity of Brønsted acid sites, BAS) on alkane aromatization, isobutane transformation on Zn /H-ZSM-5, Zn /H-BEA, and ZnO/H-BEA zeolites has been monitored with C MAS NMR. The alkane transformation has been established to occur by aromatization and hydrogenolysis pathways. Zn species is more efficient for the aromatization reaction because aromatic products are formed at lower temperatures on Zn /H-BEA and Zn /H-ZSM-5 than on ZnO/H-BEA.

Molecular Insight into the Slow Dynamics of C Hydrocarbons in the Zeolitic-Imidazole Framework (ZIF-8).

The family of zeolitic-imidazole framework (ZIF) materials is currently considered for the challenging separation of C4 hydrocarbons. However, yet, the microscopic diffusion mechanism for these hydrocarbons in these narrow gate porous materials remains elusive by conventional methods due to its very slow nature. Experimental (solid-state H nuclear magnetic resonance-NMR) and computational (molecular dynamics-MD) approaches were applied together to derive slow diffusional dynamics of -butane and 1-butene in ZIF-8.

Molecular Mobility of Tert-butyl Alcohol Confined in a Breathing MIL-53 (Al) Metal-Organic Framework.

We present a detailed solid-state NMR characterization of the molecular dynamics of tert-butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53(Al). Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two axes and librations of the molecule as a whole about the axis passing through the TBA C-O bond.

Dynamics of propene and propane in ZIF-8 probed by solid-state H NMR.

We present a detailed H NMR characterization of molecular mobility of propene and propane propagating though the microporous ZIF-8, a zeolitic imidazolate framework renowned for its outstandingly high separation selectivity for industrially relevant propene/propane mixtures. Experimental characterization of both propene and propane diffusivity in ZIF-8 has been provided. Using H NMR spin relaxation analysis, the motional mechanisms for propene and propane guests trapped within the ZIF-8 framework have been elucidated.

Methane Activation on H-ZSM-5 Zeolite with Low Copper Loading. The Nature of Active Sites and Intermediates Identified with the Combination of Spectroscopic Methods.

Cu-modified zeolites have enormous potential as the catalysts facilitating the conversion of methane to methanol. It becomes important to investigate the active sites and the reaction mechanisms involved. In this paper, several spectroscopic methods such as UV-vis diffuse reflectance spectroscopy (UV-vis DRS), pulse electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared spectroscopy, and solid-state (C MAS) NMR have been employed to characterize the state of the Cu sites and the intermediates formed during the catalyst activation and methane-to-methanol transformation on Cu/H-ZSM-5 zeolite with low (0.

H Solid-State NMR Spectroscopy Reveals the Dynamics of a Pyridine Probe Interacting with Coordinatively Unsaturated Metal Sites of MIL-100(Al) Metal-Organic Frameworks.

Coordinatively unsaturated metal sites (CUS) play an important role in catalysis by metal-organic frameworks (MOF). Being an intrinsic part of the framework the CUS take the role of acidic sites active in industrially relevant processes such as condensation or oxidation reactions. The key step of such reactions represents the coordination of the reagents to CUS.

H/D exchange of molecular hydrogen with Brønsted acid sites of Zn- and Ga-modified zeolite BEA.

Kinetics of hydrogen H/D exchange between Brønsted acid sites of pure acid-form and Zn- or Ga-modified zeolites beta (BEA) and deuterated hydrogen (D(2)) has been studied by (1)H MAS NMR spectroscopy in situ within the temperature range of 383-548 K. A remarkable increase of the rate of the H/D exchange has been found for Zn- and Ga-modified zeolites compared to the pure acid-form zeolite. The rate of exchange for Zn-modified zeolite is one order of magnitude higher compared to the rate for Ga-modified zeolite and two orders of magnitude larger compared to the pure acid-form zeolite.

In situ high temperature MAS NMR study of the mechanisms of catalysis. Ethane aromatization on Zn-modified zeolite BEA.

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism.

Significant influence of Zn on activation of the C-H bonds of small alkanes by Brønsted acid sites of zeolite.

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn.

In situ (1)H and (13)C MAS NMR kinetic study of the mechanism of H/D exchange for propane on zeolite H-ZSM-5.

The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and variously deuterated propanes (propane-d(8), propane-1,1,1,3,3,3-d(6), propane-2,2-d(2)) have been monitored in situ by (1)H MAS NMR spectroscopy within the temperature range of 503-556 K. The contribution of intramolecular hydrogen transfer to the H/D exchange in the adsorbed propane was estimated by monitoring the kinetics of (13)C-labeled carbon scrambling in propane-2-(13)C in situ with (13)C MAS NMR at 543-573 K. Possible mechanisms of the exchange have been verified on the basis of the analysis of the variation of protium concentration in both the methyl and the methylene groups of propane in dependence of the reaction time.

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.