9 research outputs found
Accelerated synthesis of Sn-BEA in fluoride media:effect of H<sub>2</sub>O content in the gel
Sn-BEA synthesis in concentrated gels results in 2.5–4 fold reduction of crystallization time and formation of smaller zeolite crystals.</p
119Sn MAS NMR Study of the Interaction of Probe Molecules with Sn-BEA: The Origin of Penta- and Hexacoordinated Tin Formation
119Sn CPMG MAS NMR was applied to study the adsorption of acetonitrile, methanol, isopropanol, isobutanol and water over Sn-BEA enriched with 119Sn isotope. Two signals observed at ca. -422 and -443 ppm over dehydrated samples were attributed to tetracoordinated framework tin sites with strong and weak Lewis acidity, respectively. The adsorption of acetonitrile and methanol resulted in observation of pentacoordinated tin species, due to the formation of 1:1 adsorption complexes over both Sn-sites. Water adsorption led first to formation of pentacoordinated tin species, which were further converted into hexacoordinated species at longer reaction times. The latter transformation was found to be kinetically limited and was attributed to chemical interaction of tin sites with water, such as hydrolysis of Si-O-Sn bonds. The adsorption of isopropanol and isobutanol was accompanied by the formation of pentacoordinated Sn species in the case of weak sites and hexacoordinated Sn over sites with strong Lewis acidity, pointing to the possibility of dissociative adsorption of secondary alcohols over strong Sn-sites
Production Process and Cost Accounting (Mizoguchi Commemorative Issue)
<sup>119</sup>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 <sup>119</sup>Sn MAS NMR spectra with improved resolution
for Sn-BEA zeolites with natural <sup>119</sup>Sn isotope abundance
using conventional MAS NMR equipment
<sup>119</sup>Sn MAS NMR Study of the Interaction of Probe Molecules with Sn-BEA: The Origin of Penta- and Hexacoordinated Tin Formation
<sup>119</sup>Sn CPMG MAS NMR was applied to study the adsorption
of acetonitrile, methanol, isopropyl alcohol, isobutyl alcohol, and
water over Sn-BEA enriched with <sup>119</sup>Sn isotope. Two signals
observed at ca. −422 and −443 ppm over dehydrated samples
were attributed to tetracoordinated framework tin sites with strong
and weak Lewis acidity, respectively. The adsorption of acetonitrile
and methanol resulted in the observation of pentacoordinated tin species,
due to the formation of 1:1 adsorption complexes over both Sn sites.
Water adsorption led first to formation of pentacoordinated tin species,
which were further converted into hexacoordinated species at longer
reaction times. The latter transformation was found to be kinetically
limited and was attributed to chemical interaction of tin sites with
water, such as hydrolysis of Si–O–Sn bonds. The adsorption
of isopropyl alcohol and isobutyl alcohol was accompanied by the formation
of pentacoordinated Sn species in the case of weak sites and hexacoordinated
Sn over sites with strong Lewis acidity, pointing to the possibility
of dissociative adsorption of secondary alcohols over strong Sn sites
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 <sup>119</sup>Sn 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 <sup>119</sup>Sn direct polarization (DP) and rotational echo double-resonance
(REDOR) and two-dimensional <sup>119</sup>Sn magic-angle tuning (MAT)
NMR spectroscopy for the identification of different Sn sites in fully
dehydrated Sn-BEA zeolite. It is demonstrated that <sup>119</sup>Sn
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
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