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Abstract  With P(CH3)3 as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Br�nsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH3)3] species can be stabilized by the first or second Br�nsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH3)3] species from the first to the second Br�nsted acidic site. [ABSTRACT FROM AUTHOR]

Highly regioselective removal of the 7-O-acyl groups of the peracylated flavones, isoflavones, and flavonols (PhSH, imidazole, NMP) followed by effective glycosylation with glycosyl trifiuoroacetimidates (BF[sub 3]·Et[sub 2]O) and cautious deprotection of the acyl groups under basic conditions afforded the desired 7-O-flavonoid glycosides in satisfactory yields. [ABSTRACT FROM AUTHOR]

The acid sites in dealuminated HZSM-5 zeolite with crystal sizes down to the nanoscale were firstly characterized by the probe molecule trimethylphosphine (TMP). As evidenced by the combination of 31P CP/MAS NMR, 27Al MAS and 1H→ 27Al CP/MAS NMR measurements, the Brönsted acid sites of both microsized and nanosized HZSM-5 could be decreased upon the dealumination of zeolitic framework after hydrothermal treatment. At the same time, the appearance of Lewis acid sites was observed. The dealuminated nanosized HZSM-5 is easier to form Lewis acid sites than microsized HZSM-5, and the type of Lewis acid sites in nanosized HSM-5 is more than one. In addition, the origin of Lewis acid sites is mainly associated with the aluminum at ca. 30 ppm in the 27Al MAS NMR spectra, and only a part of which in the dealuminated HZSM-5 zeolite acts as Lewis acid sites. [Copyright &y& Elsevier]

Variations in the microporous structure and non-framework aluminum (AlNF) distribution of HZSM-5 zeolite with a crystal size less than 100 nm dealuminated by thermal and hydrothermal treatment have been investigated by xenon adsorption and 129Xe NMR spectroscopy. The secondary pores in the nanosized HZSM-5 zeolite are observed directly via 129Xe NMR spectroscopy. As evidenced by 129Xe NMR and XPS, for the calcined microsized HZSM-5, most of the AlNF may deposit on its external surface, a small amount of AlNF may be located in its channels. However, for the calcined nanosized HZSM-5, most of the AlNF may be located in the channels, while a small amount of AlNF may be located on the external surface. After hydrothermal treatment at lower temperature, the distribution of AlNF in dealuminated nanosized HZSM-5 is heterogeneous. It is not only present in the channels and on the external surface as in the case of microsized HZSM-5, but also near the pore entrances at the outer edge of the crystallites. However, at higher steaming temperatures, more AlNF in both the microsized and the nanosized HZSM-5 zeolites will migrate to their external surface. [Copyright &y& Elsevier]

Methane structures endowed with different charges have been experimentally observed, but understanding of them currently remains rather limited. With the aid of ab initio calculations, a systematic work was carried out on the methane structures with different charges [image omitted] as well as on the structures of the neutral methane molecule bound to anions (O2 - and F- ) and cations (Na+ and Mg2 + ). The geometry of the neutral methane species was found to be well reserved in the negatively charged structures but severely distorted in the positively charged ones. The binding modes of the neutral methane species with cations (Na+ and Mg2 + ) and anions (O2 - and F- ) are threefold and onefold, respectively. The smallest and largest elongations of the C-H bonds in methane were observed in the binding with Na+ and O2 - , respectively. According to the C-H bond dissociation energies (BDE1s), the dissociation pathways were determined for all the structures, and are in agreement with the experimental data. It was further revealed that the first C-H BDE1s are heavily dependent on the charges of the methane species. Through the charge effects, the activations of the first C-H bonds in methane become facile; in addition, the selective activations and conversions can be fine-tuned as well. Accordingly, the charge effects have the potential to be employed in the effective utilisation of alkanes. [ABSTRACT FROM AUTHOR]

The phosphorus-modified ZSM-5/ZSM-11 intergrowth zeolites has been used for the catalytic conversion of methanol to hydrocarbons. As evidenced by MAS NMR and NH3-TPD, the modification by phosphorus leads a distinct decrease of the BrØnsted acid sites and the acid strength of the catalysts in comparison with the parent ZSM-5/ZSM-11, which causes a dramatic improvement of the selectivity towards propylene. At 400 °C, the ratio of propylene to ethylene can reach about 8.0. A comparison with the conventional ZSM-5 zeolite has been also carried out in the present study. Phosphorus-modified ZSM-5/ZSM-11 intergrowth zeolites show good catalytic performances in the conversion of methanol to hydrocarbons. The selectivity of propylene on p-modified ZSM-5/ZSM-11 is higher than on p-modified ZSM-5. This may be related to the proper acid strength of intergrowth zeolites.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Density functional calculations have been employed to investigate the locating and binding of lanthanum cation, i.e., La(OH)[sub 2][sup +], on HZSM-5 zeolite. Through geometry optimization, it was determined that lanthanum ions are favorably accommodated in the two 6-T rings of the straight channels (Clusters 1 and 2, see Sec. III A for details). Cluster 1 was found to exist in prior to Cluster 2 due to the preference of Al substitution in the T11 site (Cluster 1) rather than in the T8 site (Cluster 2). Geometry-optimization of Cluster 1 containing another two lanthanide ions Nd[sup 3+] and Yb[sup 3+] was also carried out and it was found that a monotonic decrease in Ln–O bond length will take place as the atomic number increases, conforming well to the rule of lanthanide contraction. Some of the optimized parameters are comparable to the corresponding experimental values in Y zeolite, which confirms that the optimized configurations are acceptable. The average frequencies of hydroxyls attached to La[sup 3+] or Yb[sup 3+] in Cluster 1 fall at 3609.16 and 3579.76 cm-1, respectively, with the gap of these two frequencies close to that in the sodalite cage of Y zeolite. Compared to H-form zeolite, the charges on both Al and O atoms in Ln-ZSM-5 zeolite show an obvious increase, which will undoubtedly lead to a stronger mutual interaction and hence enhance the stability of the [AlO[sub 4]][sup -] anion. Moreover, the Ln(OH)[sub 2][sup +] seem to have thickened the zeolite framework, which can effectively retard the process of dealumination. Through the evaluation of the possibility for dimer formation, it turned out that when the exchange degree arrived to approximately 0.28, lanthanum monomers began to aggregate into dimers, and were completely converted into dimers when the exchange degree approached 0.60. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations were performed to study the detailed reaction mechanism of the cross-metathesis of ethylene and 2-butylene over heterogeneous Mo/HBeta catalysts. The whole process is divided into two stages: the initiation of Mo–carbene species from Mo-oxo precursors and the propagating process by these active sites to yield propylene. The formation of initial Mo–carbene takes place via the endothermic addition and the subsequent decomposition of the oxametallacyclobutane intermediate. In the propagating stage to yield the final products, Mo＝CHCH3 firstly reacts with ethylene to form Mo ＝CH2, which would further react with 2-butylene to give another propylene molecule. The oxidation states of the Mo species have great influences on the reactivities associated with these two stages. It is unfavorable for MoIV-oxo precursors to produce Mo–alkylidene species compared with MoVI and MoV sites. The energy barriers indicate that the MoVI and MoV–alkylidene species could catalyze the olefin metathesis reaction, but MoV ones are more preferred to be the active sites. The calculation results are consistent with our previous XPS results. [ABSTRACT FROM AUTHOR]

The effect of support acidity on the cross-metathesis of ethylene and 2-butylene to propylene over heterogeneous Mo/HBeta catalyst was studied by density functional theory (DFT) calculations. The zeolite acidity was mimicked by changing the terminating Si–H bond lengths. Both of the initiating formation and propagating processes of Mo-carbene were investigated. It is found that there is a correlation between the zeolite acidity and the activation barrier. The influence of acidity on the initial Mo-carbene formation process is greater than that on the next process catalyzed by this active site. Theoretical calculations reveal that the heterogeneous catalyst with more acidic support could have better olefin metathesis activities, which is consistent with the experimental findings in the literature. DFT calculations indicate that the olefin metathesis could occur much easier on heterogeneous catalyst with more acidic support.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Fe/ZSM-5 zeolite has shown great potential in the selective oxidations of hydrocarbons such as methane and benzene. The various competing active sites of Fe/ZSM-5 zeolite are reviewed, including the mono-iron, oxygen-bridged [Fe, X] (X = Fe, Al) and peroxide species; in addition, the influences of H2 pretreatment are considered. For the mono-iron species, the H2 molecules are chemisorbed on the Fe(III) sites via the η2-binding mode. Both high- and low-spin Fe(III) ions play an important role during the H2 reduction process whereas the former predominates in the N2O decomposition process. As the calculated energy barriers indicate, the Fe(III) ions are facile to be reduced by H2 pretreatment and therefore the active site of the mono-iron species should be in the FeO(OH) form. Instead, the oxygen-bridged [Fe, X] and peroxide species remain stable by H2 pretreatment. The suitable oxygen-bridged [Fe, X] structures are screened out by comparisons with the experimental data and energy considerations from computational aspects. The geometries are in good agreement with the experimental data; meanwhile, it provides sound explanations to the distribution of the iron valence states, the thermodynamic facilitation of the “alfa-oxygen” generation by the introduction of extra-lattice Al ions as well as the shift of the Fe–Fe distances from ca. 3.06 to 2.53 Å. The superoxide species exists in Fe/ZSM-5 zeolite but not with the presence of extra-lattice Al ions. As the temperature increases, it gradually converts into the peroxide species and probably is the precursor of the peroxide species, one of the competing active sites in Fe/ZSM-5 zeolite. The clarification of active sites lays a solid foundation on the understanding of the catalytic processes and improvement of the Fe/ZSM-5 catalyst, one of the promising candidates to meet the industry challenges. [ABSTRACT FROM AUTHOR]

A new approach of in situ continuous-flow laser-hyperpolarized 129Xe MAS NMR together with 13C MAS NMR is designed and applied successfully to study the adsorption and reaction kinetics in the nanospace. Methanol conversion in CHA nanocages has been investigated in detail for proof of principle demonstrating the prospect of in situ NMR of reaction kinetics. Our findings well elucidates that the reaction intermediate can be identified by 13C MAS NMR spectroscopy, meanwhile the kinetic and dynamic processes of methanol adsorption and reaction in CHA nanocages can be monitored by one- and two-dimensional hyperpolarized 129Xe MAS NMR spectroscopy under the continuous-flow condition close to the real heterogeneous catalysis. The kinetic curves and apparent activation energy of the nanocages involving the active site are obtained quantitatively. The advantages of hyperpolarized 129Xe with much higher sensitivity and shorter acquisition time allow the kinetics to be probed in a confined geometry under real working conditions. [ABSTRACT FROM AUTHOR]

One- and two-dimensional 129Xe NMR spectroscopy has been employed to study the porosity of cocrystallized MCM-49/ZSM-35 zeolites under the continuous flow of hyperpolarized xenon gas. It is found by variable-temperature experiments that Xe atoms can be adsorbed in different domains of MCM-49/ZSM-35 cocrystallized zeolites and the mechanically mixed counterparts. The exchange of Xe atoms in different types of pores is very fast at ambient temperatures. Even at very low temperature two-dimensional exchange spectra (EXSY) show that Xe atoms still undergo much faster exchange between MCM-49 and ZSM-35 analogues in the cocrystallized zeolites than in the mechanical mixture. This demonstrates that the MCM-49 and ZSM-35 analogues in cocrystallized zeolites may be stacked much closer than in the physical mixture, and some parts of intergrowth may be formed due to the partially similar basic structure of MCM-49 and ZSM-35. [ABSTRACT FROM AUTHOR]

Highly dispersed Pt nanoparticles have been confined in the mesochannels of SBA-15 silica by a simple “one-pot” co-assembly method. Metallic Pt can be encapsulated in the mesochannels by changing the amount of Pt ions in the starting materials. The catalytic performances of Pt–SBA-15 were evaluated for methylcyclohexane dehydrogenation. Compared with conventional supported Pt–SiO2 catalysts, the Pt–SBA-15 catalysts show higher stability. TEM studies show that the confinement effect of ordered mesochannels of SBA-15 may restrict the further growth of Pt nanoparticles during the reaction. [ABSTRACT FROM AUTHOR]

Novel Ti-containing mesoporous materials with wormhole structure (Ti-WMS) can be assembled from preformed titanosilicate-1 precursors with long-chain alkylamines as structure directors. The obtained products were characterized by a series of techniques including powder X-ray diffraction, transmission electron microscopy, FT-infrared spectroscopy, UV−visible spectroscopy, N2sorption, and hyperpolarized 129Xe NMR at variable temperature. The catalytic properties of the materials were investigated by oxidative desulfurization reactions. The results show that Ti-WMS, whose synthesis utilizes zeolite precursors as total or part of the silica and titanium sources, is a pure mesoporous phase. However, the prepared materials contain additional micropores except the uniform mesopores, which is confirmed by N2sorption and 129Xe NMR. Infrared spectra indicate that there are zeolitic primary and secondary building units in the pore walls of Ti-WMS. UV−visible spectroscopy results confirm the existence of active Ti4species. Ti-WMS is very active in the oxidation of bulky sulfides such as benzothiophene, dibenzothiophene, and 4,6-dimethyl dibenzothiophene, with activities similar to Ti-HMS. However, in thiophene oxidation, which is not diffusion-limited, Ti-WMS exhibits much higher catalytic activity than Ti-HMS because of the presence of micropores and zeolite-like active sites. [ABSTRACT FROM AUTHOR]

Four types of N-heterocyclic ring systems were successfully constructed from the reactions of 1-alkynyl Fischer carbene complexes (OC)5MC(OEt)CCPh (1) (M = Cr, W) and substituted pyrazolinones (2). Reactions of 1 with 3-methyl-2-pyrazolin-5-one (2a), 3-n-propyl-2-pyrazolin-5-one (2b), 3,4-dimethyl-2-pyrazolin-5-one (2c), 3,4-trimethylene-2-pyrazolin-5-one (2d), or 3,4-tetramethylene-2-pyrazolin-5-one (2e) generated three kinds of Fischer aminocarbene complexes (3–5), and reactions of 1 with phenyl-substituted pyrazolinones, i.e., 3-phenyl-2-pyrazolin-5-one (2f) and its tautomer 3-phenyl-3-pyrazolin-5-one (2g), gave Fischer alkoxycarbene complexes (6) as the major products and aminocarbene complexes of types 3–5 as the minor products. Multiple tautomerism of pyrazolinones is attributed to the versatile formation of N-heterocyclic Fischer carbene complexes. Oxidative demetalation of complexes 3–6 with pyridine N-oxide or m-chloroperoxybenzoic acid efficiently afforded organic carbonyl products, and thus, strongly fluorescent syn-mixed-bimanes were prepared. The present findings constitute an alternative new method to synthesize mixed bimanes and other novel N-heterocyclic compounds. [ABSTRACT FROM AUTHOR]

A novel in situ autoreduction route has been developed, by which monodispersed silver nanoparticles with tunable sizes could be easily fabricated on silica-based materials, especially inside the channels of mesoporous silica (MPS). 13C CP/MAS NMR spectroscopy was employed to monitor the whole assembly process. It was demonstrated that the amino groups of APTS (aminopropyltriethoxyl silane)-modified MPS can be used to anchor formaldehyde to form novel reducing species (NHCH2OH), on which Ag(NH3)2NO3 could be in situ reduced. Monodispersed silver nanoparticles were thus obtained. In situ XRD and in situ TEM experiments were used to investigate and compare the thermal stabilities of silver nanoparticles on the external surface of silica gels (unconfined) and those located inside the channels of SBA-15 (confined). It was observed that unconfined silver nanoparticles tended to agglomerate at low temperatures (i.e., lower than 773 K). The aggregation of silver nanoparticles became more serious at 773 K. However, for those confined silver nanoparticles, no coarsening process was observed at 773 K, much higher than its Tammann temperature (i.e., 617 K). Only when the treating temperature was higher than 873 K could the agglomeration of those confined silver nanoparticles happen with time-varying via the Ostwald ripening process. The confinement of mesopores played a key role in improving the thermal stabilities of silver nanoparticles (stable up to 773 K without any observable coarsening), which is essential to the further investigations on their chemical (e.g., catalytic) properties. [ABSTRACT FROM AUTHOR]

Abstract: Reactions of O-ethyl lactim ∼(CH2)3–N223c; with 1-alkynyl Fischer carbene complexes (OC)5Me:glyph name="tbnd" />CPh (M=Cr, W) afforded biscarbene complexes with an azabicyclo[3.2.0]heptene core. Under a nitrogen atmosphere, the resultant chromium biscarbene complex gave a SiO2-promoted rearrangement complex in 86% yield. Thermal annelation of the rearrangement product followed by hydrolysis over SiO2 formed a cyclopentenone derivative. A cyclopentadiene derivative was obtained as a rare example of intermediate Fischer carbene species, suggesting a possible reaction pathway for the thermal annelation of the rearrangement complex. Stepwise oxidation of the chromium biscarbene complexes with an azabicyclo[3.2.0]heptene core with pyridine-N-oxide (PNO) under mild controlled conditions generated partially and completely demetalated carbonyl products, respectively. The key carbene complexes and demetalated compounds were structurally characterized by X-ray crystallographic studies. [Copyright &y& Elsevier]

The deactivation process of methane dehydroaromatization (MDA) reaction has been followed by various physical chemistry methods. [ABSTRACT FROM AUTHOR]

A series of fluorinated nanosized HZSM-5 catalysts were prepared by impregnating the zeolite with NH4F solution. The samples were characterized by XRD, XRF, FTIR-Py, TPD, 27Al and 29Si MAS-NMR. The effect of calcination temperature on the structure and acidity was studied in the range of 400–500 °C. When the calcination temperature increases, more framework aluminum atoms are removed due to the fluorination. As a result, the number of acid sites and the acid strength decrease, but the ratio of the total Brönsted acid sites to the total Lewis acid sites (BT/LT) increases. The fluorinated nanosized HZSM-5 catalysts exhibit better stability in the methylation of biphenyl (BP) with methanol. Moreover, the stability increases with the increase of the calcination temperature. When the fluorinated nanosized HZSM-5 is calcined at 500 °C, the conversion of BP can keep at about 10% for 50 h. The enhancement of stability may be due to the decrease of the acid strength and the increase of the ratio of BT/LT. [ABSTRACT FROM AUTHOR]

Density functional calculations were carried out on a series of metal-exchanged and H-form ZSM-5 zeolite clusters with or without H2O molecules adsorbed. By comparing the H2O adsorption on H-form and metal exchanged ZSM-5 zeolite clusters, the model for H2O adsorption on M/ZSM-5 zeolites was put forward, confirmed by the consistency of our frequency calculations with previous IR experiments. The binding energies of the metal cations, which decreased as La/ZSM-5>Ca/ZSM-5>Mg/ZSM-5>K/ZSM-5>Rb/ZSM-5>Na/ZSM-5>Zn/ZSM-5 (>H/ZSM-5), determined the relative hydrothermal stabilities of different metal exchanged ZSM-5 zeolites. Detailed analysis on the three parts to the binding energies was performed, concluding that the part between the metal cations and the zeolite framework has played the most important role, which can be reflected by the consistency of the sequences of the elongated of MㅡOf distances and the binding energies. Here the mean square deviation ... was adopted to characterize the [AlO4]− tetrahedron, which showed that the process of H or M falling off was facilitated at higher water pressure. When the Brϕnsted acidic proton was exchanged with the metal cations or adsorbed with some water, the [AlO4]− tetrahedron can restore somewhat towards ideal regularity, and this is probably the reason why 27Al MAS NMR signals are illegible in H/ZSM-5 zeolite and will be much improved by the introduction of some water or the exchange with metal cations. [Copyright &y& Elsevier]

[π-C5H5NC16H33]3[PW4O16] was reported to be an excellent epoxidation catalyst which exhibited a unique reaction-controlled phase transfer behavior. In the paper, the composition and structural changes of the reaction-controlled phase transfer catalyst during and after reaction have been investigated by 31P NMR spectroscopy. The 31P MAS NMR confirmed that the original catalyst was a mixture of heteropoly tungstophosphates. When the catalyst reacted with hydrogen peroxide, the species {PO4[WO(O2)2]4}3-, [(PO4){WO(O2)2}2{WO(O2)2(H2O)}]3- and [(PO3(OH)){WO(O2)2}2]2- were detected by in situ31P NMR. It was also found that the P/W ratios and quaternary ammonium cations had great influence on the composition of heteropoly tungstophosphates. Although the catalyst with [(C18H37)2N(CH3)2]+ was not a reaction-controlled phase transfer catalyst, it could be precipitated from the reaction solution when acetone was subsequently added to the solution. The 31P MAS NMR spectra of the recovered catalysts revealed that they had more low P/W ratio heteropoly tungstophosphates than fresh catalysts. [ABSTRACT FROM AUTHOR]

The hydrogen reduction of the red-ox Fe/ZSM-5 zeolite is an essential step for catalyst preparation. In this letter, various adsorption modes of the H2molecules on the Fe(III)/ZSM-5 zeolite were first explored by density functional theory, with their exact configurations provided. The adsorption energies revealed that the two configurations with Fe(III) at the sextet states are the main ones, consistent with the results of inelastic neutron scattering (INS) experiments. On such basis, the vibrational and orbital analyses were made, which will be valuable for the future studies on the Fe/ZSM-5 systems. [ABSTRACT FROM AUTHOR]

The article discusses the study on the development of a catalytic system for the one-pot aerobic oxidation of methane at 80°C, with combined redox couples. The activation of methane with Pd2 initiates an electron-transfer chain, which carries the electrons from methane to oxygen similar to the biological oxidation processes. Relevant information related to the study are further discussed.