Application of Multinuclear MAS NMR for the in†situ Monitoring of Hydrothermal Synthesis of Zeolites.

In situ MAS NMR studies on the monitoring of hydrothermal synthesis of zeolites are reviewed. The first part of the review contains information on the experimental techniques used for the in situ NMR studies in static and MAS conditions. In the second part, the main capabilities of the in situ 1 H, 11 B, 13 C, 14 N, 19 F, 23 Na, 27 Al, 29 Si and 31 P MAS NMR for the elucidation of the mechanism of hydrothermal synthesis of zeolites are examined and the data on NMR lines identification are summarized. In the last part the main application areas of the techniques are considered and illustrated with examples taken from the mechanistic studies of zeolites A, X, MFI and BEA synthesis. A cross-reference index between the materials studied, the experimental approaches used, the mechanistic information obtained, and the corresponding literature sources is established.

Features of the chemical structure of different organic matter pools in Haplic Chernozem of the Streletskaya steppe: 13C MAS NMR study.

The aim of the current research was to study structural features of four organic matter (OM) pools isolated by the modified method of granulo-densimetric fractionation from two Сhernozems. We purposed to relate these features to the OM allocation and the transformation processes. The pools included: 1) free light fraction located in an inter-aggregate space, 2) light fraction occluded inside the microaggregates, 3) OM bound with clay particles, and 4) OM bound with a residual heavy fraction left after light fractions and clay separation. Soils of contrasting land uses: steppe and long-term permanent bare fallow were selected to assess changes that occur in soil OM during the degradation. We used 13C CP/MAS NMR spectroscopy controlling the quantitativeness of spectra with the aid of 13C DP/MAS NMR. The obtained spectra of the studied fractions clearly differed in the proportion of main functional groups. The occluded OM is more aromatic compared to the free OM. The structural changes observed at transition from the free light fraction to the occluded one indicate an active decomposition of lignin and carbohydrates in the latter fraction. This provides in the occluded OM appropriate conditions for the formation of "young HA": high local concentration of substrates and spatial proximity of enzymes. At C-deficiency (bare fallow) the chemical structure of the occluded OM is very close to that of humic acids. The chemical structure of the clay bound OM reflects high content of products of microbial origin. OM of the residual heavy fraction differs from that of the clay: the proportions of main functional groups in it are more close to that of the free light fraction, but with higher carboxyl content. Heavy fractions also differ under acid treatment: the residue losses less carbon. The above-mentioned differences show that the division of heavy fractions into two components is reasonable. Various acid hydrolyzability indicates a predominance of strong chemical bonds in the occluded OM and the significant contribution of weak bonds in the clay OM, i.e., the occluded OM is highly condensed, in contrast, components of the clay OM are largely interconnected by hydrogen-, coordinate, hydrophobic and other relatively weak bonds. Soil degradation under extreme land use leads not only to OM scarcity, but also to its greater hydrolyzability, strong enrichment with aromatic fragments and depletion of carbohydrate and aliphatic fragments in all studied fractions. Degradation changes in the occluded OM are most pronounced. Our results demonstrate that the applied fractionation scheme coupled with quantitative 13C CP/MAS NMR spectroscopy is a very promising approach for evaluating processes of soil OM transformation and degradation.

Time-Resolved In†Situ MAS NMR Monitoring of the Nucleation and Growth of Zeolite BEA Catalysts under Hydrothermal Conditions.

Time-resolved 13 C, 23 Na, 27 Al, and 29 Si MAS NMR has been applied in situ for monitoring the hydrothermal synthesis of zeolite BEA. Isotopic labelling with 29 Si and 13 C isotopes has been used to follow the fate of siliceous species and structure directing agent ((13 CH3 -CH2 )4 NOH). Two mechanistic pathways, namely solution-mediated and solid-solid hydrogel rearrangement have been distinguished for two synthesis procedures studied. The mechanisms of structure-directing behavior of TEA+ cations in two reaction pathways have been elucidated. The results show that multinuclear MAS NMR can serve as a superior tool for monitoring hydrothermal synthesis of various solids including zeolites, zeotypes, mesoporous materials, metal-organic frameworks and so on and for the design of novel outstanding materials for different applications.

Liquid metals for boosting stability of zeolite catalysts in the conversion of methanol to hydrocarbons.

Methanol-to-hydrocarbons (MTH) process has been considered one of the most practical approaches for producing value-added products from methanol. However, the commonly used zeolite catalysts suffer from rapid deactivation due to coke deposition and require regular regeneration treatments. We demonstrate that low-melting-point metals, such as Ga, can effectively promote more stable methanol conversion in the MTH process by slowing coke deposition and facilitating the desorption of carbonaceous species from the zeolite. The ZSM-5 zeolite physically mixed with liquid gallium exhibited an enhanced lifetime in the MTH reaction, which increased by a factor of up to ~14 as compared to the parent ZSM-5. These results suggest an alternative route to the design and preparation of deactivation-resistant zeolite catalysts.

Catalysis by coke deposits: synthesis of isoprene over solid catalysts.

A help rather than a hindrance: Carbonaceous deposits have been found to play a key role in the selective synthesis of isoprene from formaldehyde and isobutene over solid catalysts. They accumulate on the catalyst surface during the induction period and promote the interaction of the substrates at the steady state. The proposed mechanism shows the way forward for the design of efficient solid catalysts for the synthesis of isoprene.

Direct Identification of Zeolite Beta Polymorphs by Solid-State 29Si Nuclear Magnetic Resonance Spectroscopy.

Stacking disorder and polymorphism in zeolite and zeolite-like materials hinder their structural characterization. In this work, we propose an advanced approach based on applying "pure shift" solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy for the structural investigation of zeolitic materials containing intergrown polymorphs. The approach developed in the case study of zeolite beta allows for the resolution of 21 29Si signals, attributing them to non-equivalent T sites in polymorphs A, B, and C, reconstruction of individual 29Si magic angle spinning NMR spectra for each polymorph, and determination of the polymorph composition with higher accuracy than X-ray diffraction. The results reveal that two widely used synthetic routes for zeolite beta, alkaline and fluoride synthesis, lead to different polymorph compositions. These findings indicate that "pure shift" solid-state 29Si NMR can serve as a superior tool for the elucidation of polymorphism in zeolites.

Evaluation of Zeolite Acidity by 31P MAS NMR Spectroscopy of Adsorbed Phosphine Oxides: Quantitative or Not?

31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy of adsorbed alkyl-substituted phosphine oxides has witnessed tremendous progress during the last years and has become one of the most informative and sensitive methods of zeolite acidity investigation. However, quantitative evaluation of the number of sites is still a challenge. This study clarifies the main origin of errors occurring during NMR experiments, introduces the appropriate standards (both internal and external), and determines the relaxation parameters and the conditions for the acquisition and integration of spectra. As a result, a methodology for the quantitative measurement of the content of Brønsted and Lewis sites and the amount of internal and external silanol groups is established. The application of probe molecules of different sizes (namely, trimethylphosphine oxide (TMPO), tri-n-butylphosphine oxide (TBPO), and tri-n-octylphosphine oxide (TOPO)) is shown to be a good tool for distinguishing between the active sites inside the zeolite pores, mesopores, and on the outer crystal surface. The methodology proposed is verified on BEA zeolites different in composition, texture, and morphology.

Relation between Morphology and Porous Structure of SAPO-11 Molecular Sieves and Chemical and Phase Composition of Silicoaluminophosphate Gels.

The formation of silicoaluminophosphate gels using boehmite, Al isopropoxide, and di-n-propylamine as a template of silicoaluminophosphate gels as well as their subsequent crystallization into SAPO-11 molecular sieves was studied in detail using X-ray fluorescence spectroscopy (XRF), powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption-desorption methods. The effect of the chemical and phase composition of silicoaluminophosphate gels on the physicochemical properties of SAPO-11 molecular sieves was shown. The secondary structural units that the AEL lattice is composed of were found to be formed at the initial stage of preparation involving aluminum isopropoxide. Several approaches to control their morphology and secondary porous structure are also proposed.

Direct Observation of Tin in Different T-Sites of Sn-BEA by One- and Two-Dimensional 119Sn MAS NMR Spectroscopy.

The direct and quantitative identification of active sites is crucial for the development of zeolite catalysts and their implementation in industry. Herein we report on the application of one-dimensional 119Sn direct polarization (DP) and rotational echo double-resonance (REDOR) and two-dimensional 119Sn magic-angle tuning (MAT) NMR spectroscopy for the identification of different Sn sites in fully dehydrated Sn-BEA zeolite. It is demonstrated that 119Sn magic-angle spinning (MAS) NMR techniques, modified by Carr-Purcell-Meiboom-Gill (CPMG) echo-train acquisition allow to resolve three groups of NMR signals, which can be attributed to three groups of nonequivalent T-sites based on the existing theoretical predictions: (I) T9, T4, and T3; (II) T2, T1, and T8; and (III) T7, T5, and T6. Results suggest that the sites attributed to group III are the most populated in Sn-BEA samples obtained via the fluoride route. The attribution of NMR lines to different T-sites in the structure of BEA allows for the establishment of structure-reactivity relationship and therefore for further improvement of Sn-BEA catalysts.

Application of (119)Sn CPMG MAS NMR for Fast Characterization of Sn Sites in Zeolites with Natural (119)Sn Isotope Abundance.

(119)Sn CPMG MAS NMR is demonstrated to be a fast and efficient method for characterization of Sn-sites in Sn-containing zeolites. Tuning of the CPMG echo-train sequence decreases the experimental time by a factor of 5-40 in the case of as-synthesized and hydrated Sn-BEA samples and by 3 orders of magnitude in the case of dehydrated Sn-BEA samples as compared to conventional methods. In the latter case, the reconstruction of the quantitative spectrum without the loss of sensitivity is shown to be possible. The method proposed allows obtaining (119)Sn MAS NMR spectra with improved resolution for Sn-BEA zeolites with natural (119)Sn isotope abundance using conventional MAS NMR equipment.