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Silver-modified zeolites have demonstrated promising properties for hydrocarbon feedstock valorization and the oxidation of organic compounds. Within the frame of the scientific interest for selective oxidation of light alkenes on metal-modified zeolites, the transformation of propene on Ag/H-ZSM-5 zeolite in the presence of molecular oxygen has been monitored by 13C (CP) MAS NMR spectroscopy at 296–773 K. Silver-allyl intermediate species have been detected by 13C CP/MAS NMR, indicating that Ag+ sites are involved in propene transformation. It is assumed that oxygen and propene compete for the Ag+ adsorption sites, thus prompting propene adsorption on zeolite Brønsted acid sites (BAS) and its further transformation by oligomerization and conjunct polymerization. Among the products of propene oxidation by molecular oxygen, only carbon dioxide has been detected by 13C MAS NMR, implying high activity of Ag/H-ZSM-5 in alkene total oxidation. It is thus inferred that silver-modified zeolite is not a promising catalyst for propene selective oxidation to propene oxide and carbonyl group containing organics. [ABSTRACT FROM AUTHOR]

67Zn MAS NMR spectroscopy was used to characterize the state of Zn in Zn-modified zeolites ZSM-5. Two 67Zn enriched zeolite samples were prepared: by solid-state exchange with metal 67Zn (Zn2+/ZSM-5 sample) and by ion exchange with zinc formate solution (ZnO/H-ZSM-5 sample), both containing ca. 3.8 wt% Zn. The elemental analysis, TEM, and quantitative BAS and aluminum analyses with 1H and 27Al MAS NMR have shown that Zn2+/ZSM-5 contains zinc in the form of Zn2+ cations, while both ZnO species and Zn2+ cations are present in ZnO/H-ZSM-5 besides BAS. 67Zn MAS NMR has detected the signal of Zn in a tetrahedral environment from ZnO species for both the activated and hydrated ZnO/H-ZSM-5 zeolite. The signal of Zn in an octahedral environment was detected for the hydrated Zn2+/ZSM-5 and ZnO/H-ZSM-5 zeolites. This signal may belong to zinc cation [HOZn]+ or Zn(OH)2 species surrounded by water molecules. Quantitative 67Zn MAS NMR analysis has shown that only 27 and 38% of zinc loaded in the zeolite is visible for the activated and hydrated ZnO/H-ZSM-5 zeolite, and 24% of Zn is visible for the hydrated Zn2+/ZSM-5. Zinc in the form of ZnO species is entirely visible in both the activated and hydrated ZnO/H-ZSM-5 zeolite, while Zn2+ cations are not detected at all for the activated sample and only 29% of Zn2+ cations is visible for the hydrated zeolite. Detection of only a part of Zn2+ cations in the form of [HOZn]+ or Zn(OH)2 species in octahedral environment presumes only partial hydrolysis of the bond of Zn2+ cation with framework oxygen and further solvation of the Zn species formed at hydrolysis by the adsorbed water. [ABSTRACT FROM AUTHOR]

Kinetics of H/D hydrogen exchange between deuterated isobutane‐d10 and Brønsted acid sites (BAS) of three zeolite samples (H‐BEA, ZnO/H‐BEA, Zn2+/H‐BEA) were monitored with 1H 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. The loaded Zn species provide the tuning effect on the reaction occurrence, changing the mechanism from the indirect one to the mechanism of the direct exchange. [ABSTRACT FROM AUTHOR]

13C MAS NMR spectroscopy is a powerful technique to study the mechanisms of hydrocarbon transformations on heterogeneous catalysts. It can reliably identify the surface intermediates and the adsorbed products based on the analysis of their 13C chemical shifts, δ(13C). However, the unambiguous assignment of the detected signals is always a challenge due to the uncertainty of the nature of the surface intermediates formed and the mechanism of adsorbed species interaction with active sites. The way to solve this problem is the application of DFT calculations to predict chemical shifts for the expected intermediate hydrocarbon species. Herein, the methodology for δ(13C) chemical shift calculations for adsorbed species has been proposed. It includes: (i) zeolite framework optimization with periodic DFT (pPBE); (ii) medium-sized cluster geometry optimization with hybrid GGA (PBE0), and (iii) σ(13C) values calculation followed by δ(13C) estimation using the linear regression method. It is inferred that the TPSS/cc-pVTZ method provides the best computational cost/accuracy ratio for the set of adsorbed hydrocarbon species that was previously detected experimentally on the surface of Zn-containing zeolites. The drawbacks of the computation method have also been revealed and discussed. [ABSTRACT FROM AUTHOR]

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C–H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework ZnEF2+ cations, framework oxygens (OF2−), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C–H bond cleavage performs with a determination coefficient R2 = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R2 = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C–H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property–activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials. [ABSTRACT FROM AUTHOR]

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 27Al framework aluminium atom (QCC) of the MOF AlO4(OH)2 centres (Al-sites) has been investigated by combining solid-state 27Al MAS NMR spectroscopy with XRD analysis and DFT calculations. It is established that 27Al 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 27Al 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. [ABSTRACT FROM AUTHOR]

The influence of the model and method choice on the DFT predicted 13C NMR chemical shifts of zeolite surface methoxide species has been systematically analyzed. Twelve 13C NMR chemical shift calculation protocols on full periodic and hybrid periodic–cluster DFT calculations with varied structural relaxation procedures are examined. The primary assessment of the accuracy of the computational protocols has been carried out for the Si–O(CH3)–Al surface methoxide species in ZSM-5 zeolite with well-defined experimental NMR parameters (chemical shift, δ(13C) value) as a reference. Different configurations of these surface intermediates and their location inside the ZSM-5 pores are considered explicitly. The predicted δ value deviates by up to ±0.8 ppm from the experimental value of 59 ppm due to the varied confinement of the methoxide species at different zeolite sites (model accuracy). The choice of the exchange–correlation functional (method accuracy) introduces ±1.5 ppm uncertainty in the computed chemical shifts. The accuracy of the predicted 13C NMR chemical shifts for the computational assignment of spectral characteristics of zeolite intermediates has been further analyzed by considering the potential intermediate species formed upon methane activation by Cu/ZSM-5 zeolite. The presence of Cu species in the vicinity of surface methoxide increases the prediction uncertainty to ±2.5 ppm. The full geometry relaxation of the local environment of an active site at an appropriate level of theory is critical to ensure a good agreement between the experimental and computed NMR data. Chemical shifts (δ) calculated via full geometry relaxation of a cluster model of a relevant portion of the zeolite lattice site are in the best agreement with the experimental values. Our analysis indicates that the full geometry optimization of a cluster model at the PBE0-D3/6-311G(d,p) level of theory followed by GIAO/PBE0-D3/aug-cc-pVDZ calculations is the most suitable approach for the calculation of 13C chemical shifts of zeolite surface intermediates. [ABSTRACT FROM AUTHOR]

MIL-53 (Al) is an example of a MOF with a flexible framework featuring a broad range of applications such as hydrocarbon adsorption, separation and catalysis. Such processes are strongly influenced by the flexibility of the framework and thus require monitoring of the interrelation between the guest dynamics and framework breathing events. Here, we demonstrate that breathing of the framework can be monitored by probing the isobutane guest dynamics with 2H solid-state nuclear magnetic resonance (2H NMR). We show that a stepwise change in the 2H NMR anisotropic line shape to an isotropic one occurs at 253 K, simultaneous with a notable change of the quadrupole coupling constant Q Al0 of the 27Al framework aluminium atom. The stepwise variation of Q Al0 is indicative of a breathing event in the MIL-53 (Al) framework structure. Such synchronous change is rationalized in terms of the sharp switch of the guest mobility from anisotropic to isotropic rotation due to the pore expansion upon structural phase transition. It is thus inferred that the dynamics of isobutane confined in the pores represents a very sensitive probe to follow breathing of the MIL-53 (Al) framework with loaded guest molecules. [ABSTRACT FROM AUTHOR]

To clarify the issue on Brønsted acidity enhancement by the presence of zinc species in H‐ZSM‐5 zeolite (G.D. Qi, et al. Angew. Chem.‐Int. Edit. 2016, 55, 15826), the influence of Zn2+ cationic species, introduced into the zeolite, on the strength of Brønsted acid sites (BAS) has been analyzed by solid‐state NMR and FTIR techniques using trimethylphosphine (TMP) and CO as probe molecules. Comparative analysis of the acidity for H‐ZSM‐5 and Zn2+/H‐ZSM‐5 zeolites has been performed. TMP adsorbed on both zeolite samples exhibits same signals at 6.3 and −4.5 ppm in 1H and 31P MAS NMR spectra, respectively, assigned to protonated TMP. FTIR spectroscopy has shown that both zeolite samples, H‐ZSM‐5 and Zn2+/H‐ZSM‐5, display the same red frequency shift (ΔνOH⋅⋅⋅CO=−318 cm−1) of the O−H stretching band and the same blue frequency shift (ΔνCO=+32 cm−1) of the CO stretching band for CO adsorption complex with BAS. The results obtained do not provide any evidence for the existence of some specific BAS with enhanced acid strength for Zn2+‐exchanged zeolite compared to the strength inherent to pure acid‐form zeolite. It is thus inferred the strength of BAS in H‐ZSM‐5 zeolite is not affected by the modification of the zeolite with Zn2+ cationic species. [ABSTRACT FROM AUTHOR]

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. In MOFs the mechanism of the reagent interaction with CUS is poorly understood. Herein, we characterize the interaction of a widely used acidity probe pyridine with CUS of MIL‐100(Al) MOF by means of the 2H solid‐state NMR spectroscopy. 2H NMR reveals that pyridine species, which are interacting with CUS and the ones which are coordinated to the Al−OH site, exhibit different motional behavior. 2H NMR line shape as well as T1, T2 relaxation analyses for [D5]pyridine adsorbed in MIL‐100(Al) allowed us to perform a detailed characterization of pyridine dynamics in both states including the kinetics of the exchange process between these adsorption states. [ABSTRACT FROM AUTHOR]

This work reports new physical insights of the thermodynamic parameters and mechanisms of possible interactions occurring in polymers subjected to high-pressure CO2. ATR-FTIR spectroscopy has been used in situ to determine the thermodynamic parameters of the intermolecular interactions between CO2 and different functional groups of the polymers capable of specific interactions with sorbed CO2 molecules. Based on the measured ATR-FTIR spectra of the polymer samples subjected to high-pressure CO2 (30 bar) at different temperatures (300–340 K), it was possible to characterize polymer–polymer and CO2–polymer interactions. Particularly, the enthalpy and entropy of the formation of the specific non-covalent complexes between CO2 and the hydroxy (–OH), carbonyl (C=O) and hydroxyimino (=N–OH) functional groups of the polymer samples have been measured. Furthermore, the obtained spectroscopic results have provided an opportunity for the structure of these complexes to be proposed. An interesting phenomenon regarding the behavior of CO2/polymer systems has also been observed. It has been found that only for the polyketone, the value of enthalpy was negative indicating an exothermic process during the formation of the CO2–polymer non-covalent complexes. Conversely, for the polyoxime and polyalcohol samples there is a positive enthalpy determined. This is a result of the initial polymer–polymer interactions requiring more energy to break than is released during the formation of the CO2–polymer complex. The effect of increasing temperature to facilitate the breaking of the polymer–polymer interactions has also been observed. Hence, a mechanism for the formation of CO2–polymer complexes was suggested based on these results, which occurs via a two-step process: (1) the breaking of the existing polymer–polymer interactions followed by (2) the formation of new CO2–polymer non-covalent interactions. [ABSTRACT FROM AUTHOR]

Five crude oil samples with different physical properties have been studied with respect to asphaltene stability. The attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopic imaging approach of n-heptane-induced precipitation has been used to monitor crude oil behaviour under dilution with a flocculation agent. For each sample, the dynamics of asphaltene precipitation has been observed by applying this chemical imaging method. Based on these data, the stability of crude oil samples has been compared and the correlation between asphaltene stability and crude oil properties has been proposed. [ABSTRACT FROM AUTHOR]

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 (D2) has been studied by 1H MAS NMR spectroscopy in situwithin 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. This promoting effect of metal on the rate of H/D exchange was rationalized by a preliminary dissociative adsorption of molecular hydrogen on metal oxide species or metal cations. The adsorbed hydrogen is further involved in the exchange with the acid OH groups located in vicinity of metal species. The role of different metal species in the possible mechanisms of the exchange with involvement of zeolite Brønsted acid sites and metal species is discussed. [ABSTRACT FROM AUTHOR]