甲烷无氧脱氢芳构化催化剂Mo/HZSM-5的研究

催化剂Ｍｏ／ＨＺＳＭ－５在甲烷地氧脱氢芳构化反应中表现出很高的活性，用ＸＲＤ，ＢＥＴ经表面，ＮＨ３－ＴＰＤ及ＴＰＲ等手段，对不抽提前后催化剂上的活性ｏ的种进行了研究，ＸＲＤ结果表明，Ｍｏ物种高度分散于筛表面，随着Ｍｏ担载量的提高，ＢＥＴ比表面积有所下降；但氨水抽提后，比表面积有很大程度的恢复，ＮＨ３－ＴＰＤ结果表明，Ｍｏ物种优先占据分子筛中的强酸位。ＴＰＲ结果显示出有ＭｏＯ３晶相存在的催化剂较易被     

Synthesis,characterization, and catalytic activity in the <Emphasis Type="Italic">n</Emphasis>-heptane conversion over Pt/In–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> sol–gel prepared catalysts

Platinum catalysts supported on indium-doped alumina were prepared by the sol–gel method. The method allows the incorporation of In³⁺ in the alumina network. The indium-doped alumina supports showed narrow pore size distribution (5.4–4.0 nm) and high specific surface areas (258–280 m²/g). The ²⁷Al NMR-MAS spectroscopy identified aluminum in tetrahedral, pentahedral, and octahedral coordination; however, the intensity of the signal assigned to aluminum in pentahedral coordination diminishes with the increase of the content of indium. Total acidity determined by ammonia thermodesorption diminishes strongly in Pt/In–Al₂O₃ catalysts, suggesting a selective deposit of platinum over the acid sites of the support. The effect of the support in the platinum catalytic activity was evaluated in the n-heptane dehydrocyclization reaction. The selectivity patterns for such reaction were modified substantially in the doped Pt/In–Al₂O₃ catalysts, in comparison with the Pt-In/Al₂O₃–I coimpregnated reference catalyst. As an important result, the formation of benzene was suppressed totally over the indium-doped alumina sol–gel supports with a high content (3 wt%) of indium.     

Distribution of aluminum and boron in the periodical building units of boron-containing Beta zeolites

Various boron only ([B]-BEA) as well as aluminum- and boron-containing beta zeolites ([Al,B]-BEA) have been prepared and modified by ion exchange of ammonium, sodium, and nickel ions. The zeolite samples have been characterized by 11B, 27Al, and 29Si MAS as well as three of them by 11B and 27Al 3Q-MAS NMR spectroscopy. The quantitative contributions of defect-free Si(nX) (n = 2, 1, 0; X = Al, B) and Si(OH)x (x = 2, 1) sites to the NMR signal intensities were calculated from the various Si/(Al + B) ratios and relative 11B, 27Al, and 29Si NMR signal intensities using the special distribution of aluminum and boron in different periodical building units of the zeolite framework. The boron atoms are sitting exclusively in diagonal positions in the four-membered rings of [B]-BEA zeolites, while the aluminum atoms are situated both in diagonal and lone positions in the four-membered rings of [Al,B]-BEA zeolites. A higher part of boron atoms are positioned in framework-related deformed tetrahedral boron species than in lattice positions in the [B]-BEA than in the [Al,B]-BEA zeolites. All extraframework octahedral aluminum species are transformed back to lattice positions due to ion exchange from the protonated form to ammonium-, sodium-, or nickel-ions containing zeolites. Oppositely, trigonal boron leaves the zeolite structure completely during ion exchange.     

An investigation of the roles of surface aluminum and acid sites in the zeolite MCM-22

Ammonia adsorption studies reveal that the observed Lewis acidity in the zeolite MCM-22 is derived from at least two types of framework aluminum sites (AlF), that is, octahedral AlF and three-coordinate AlF. Comparative ammonia or trimethylphosphine (TMP) adsorption experiments with MCM-22 confirm that octahedral Al species gives rise to the signal at delta(iso) approximately 0 in the 27Al NMR spectrum; this is a superposition of two NMR signals from the different Al species on the water-reconstructed zeolite surface. A sharp resonance assigned to framework Al reversibly transforms on ammonia adsorption to delta(iso)27Al approximately 55 from tetrahedral AlF, while the broad peak is assigned to nonframework aluminum which results from hydrothermal treatment. This study also demonstrates the effectiveness of 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR spectroscopy as a technique for the study of zeolite reactions.     

Introduction of vanadium species in <Emphasis Type="Italic">β</Emphasis> zeolite by solid-state reaction: spectroscopic study of V speciation and molecular mechanism

V-containing β zeolites were prepared by solid-state reaction between V₂O₅ and β zeolite. The zeolite structure was analysed by XRD and N₂ physisorption. The V speciation was studied by chemical analysis and different spectroscopies (FT-IR, ²⁷Al-NMR, UV-Vis, EPR, photoluminescence). After calcination of V₂O₅-β zeolite mechanical mixtures at 500°C, three kinds of V species were identified: (i) oligomeric vanadates with octahedral V⁵⁺ easily removed by treatment with NH₄OAc, (ii) isolated vanadyl (V=O)²⁺ ions in axially distorted octahedral or square pyramidal environment, interacting with framework and/or extraframework Al nuclei and (iii) isolated V⁵⁺ in tetrahedral and octahedral environments, localized in framework defect sites. The amount of the latter species is higher when water vapor is present during calcination and when parent β zeolite contains a high concentration of defect sites generated by a strong acid pretreatment. Isolated V⁵⁺ are easily reduced to tetrahedral V⁴⁺ or to square pyramidal (V=O)²⁺. Possible models of the mechanism of formation of V species by solid-state reaction and further reduction are proposed.     

Sites are Separable in Garnets with ALCHEMI

 It is shown in this paper that – in contrast to the accepted belief in the literature – it is possible to determine if a minority component is located on the dodecahedral, octahedral or tetrahedral sites in a garnet single crystal. Previous literature regarded the dodecahedral sites indistinguishable form the tetrahedral sites. Our prediction about the separability is based upon dynamical Bloch-wave calculations and proved experimentally in a transmission electron microscope (TEM) on two different natural garnets (almandine and grossular). The crystallographic positions of the minority components are determined in an ALCHEMI experiment that makes use of the channeling of electrons in special directions of a single-crystal sample.     

Incorporation of Co(II) in dealuminated BEA zeolite at lattice tetrahedral sites evidenced by XRD, FTIR, diffuse reflectance UV-Vis, EPR, and TPR

A CoSiBEA zeolite is prepared by a two-step postsynthesis method that consists of first creating vacant T-sites with associated silanol groups by dealumination of TEABEA zeolite with nitric acid and then impregnating the resulting SiBEA zeolite with an aqueous solution of Co(NO3)2. The incorporation of Co into lattice sites of SiBEA is evidenced by XRD. The consumption of OH groups is monitored by FTIR. The presence of Co in its II oxidation state and in tetrahedral coordination is evidenced by diffuse reflectance UV-vis and EPR spectroscopy. The very high reduction temperature (1120 K) of cobalt in CoSiBEA zeolite determined by TPR confirms that Co interacts strongly with the zeolite support, consistent with lattice tetrahedral (T(d)) coordination.     

Orthorhombic aluminium oxyfluoride,AlOF

Crystals of the title compound were extracted from the bulk of grown SrAlF₅ crystals as unexpected inclusions that were identified as the long sought after aluminium oxyfluoride. The structure of AlOF is built up from tetrahedral and octahedral polyhedra. Each tetrahedron is bisected by a mirror plane, with the Al atom and two vertex anions in the plane. All tetrahedral vertices are positions of competing oxide and fluoride ions and are shared with octahedra. These shared vertices belong to two octahedral edges which join the octahedra to form infinite zigzag chains. The chains are strung along twofold screw axes that run parallel to the unit‐cell b axis. The remaining two octahedral vertices are occupied only by fluoride ions. A small deficiency in the occupation of the octahedral Al position was suggested by the refinement. However, the stoichiometry of the compound is AlOF within experimental uncertainty. The Al—F(O) distances are separated into three groups with average values of 1.652 (3) (tetrahedra), 1.800 (2) (octahedra) and 1.894 (2) Å (octahedra). This structure differs widely from the reported tetragonal phase Al_1−xO_1−3xF_1+3x (x = 0.0886) [Kutoglu (1992). Z. Kristallogr. 199 , 197–201], which consists solely of octahedral structural units.     

Theoretical study of P2O5 polymorphs at high pressure: hexacoordinated phosphorus

Binary oxides of elements belonging to groups 13-15 are of special relevance from a fundamental point of view as well as because of their technological applications as a basis in zeotypes, glasses, or semiconductors. Aluminum oxide, Al2O3, crystallizes in the corundum structure, which is stable at low and high pressures, with the Al showing octahedral coordination. Silicon oxide, SiO2, crystallizes in phases with tetrahedral Si coordination at low pressures as alpha-quartz, but at high pressures, octahedral coordination is stable in the stishovite polymorph. The only known binary phosphorus(V) oxides have tetrahedral P coordination. We have studied the stability of different phases of P2O5 at high pressure, applying density functional theory methodology within the local density and generalized gradient approximations and a plane-wave basis set. Our results indicate that the most stable form of P2O5 at high pressure could be one with hexacoordinated phosphorus. All of the high-pressure phases of Al, Si, and P can be described as a different linking of the same rutile-type blocks.     

Correlation between structure and catalytic activity of manganese oxides in liquid-phase oxidation of 1-octene

We have studied the correlation between the crystal structure and the catalytic activity of manganese oxides MnO, MnO₂, Mn₃O₄, and Mn₂O₃ in liquid-phase oxidation of 1-octene by molecular oxygen. The catalytic activity decreases in the series of oxides with octahedral coordination environment for the manganese atoms MnO−Mn₂O₃−MnO₂. The oxide Mn₃O₄ (with mixed tetrahedral and octahedral environment for the Mn atoms) catalyzes the process according to a different mechanism. L'vov Polytechnic State University, 12 S. Bandery ul., L'vov-13 290646, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 34, No. 5, pp. 324–327, September–October, 1998.     

氟置换高岭石层间羟基的能量最小化模拟

为了进一步澄清高岭石中结构无序的成因以及氢键对它们的影响程度,利用补充了氟参数的CLAYFF力场,对层间羟基不同分数的氟置换进行了能量最小化模拟.结果发现:四面体底氧起皱的原因是四、八片层不匹配引起的Al—O(连接氧)键拉伸以及维持四面体自身外形的需要;四面体旋转的原因与Newnham的解释类似.八面体上下三原子对旋转是由于:(1)四、八面体层的不匹配,具体地说是连接氧/内部羟基氧与八面体铝之间的O—Al—O键角(θ1)和Al—O—Al键角(θ2),层间羟基氧与八面体铝之间的O—Al—O键角(θ4)和Al—O—Al键角(θ5)的增大,以及八面体共棱O—Al—O键角(θ3)的减小;(2)铝硅斥力引起的θ1、θ2变小和θ3变大;(3)(1)和(2)中所有键角变化引起的结构调整;(4)高岭石特殊的网状结构共同引起的.八面体O-O共享棱的缩短和铝更靠近层间羟基氧同样也是(1)-(4)作用的结果;θ1、θ2、θ4和θ5增大和θ3减小还引起了八面体展平.层间氢键对四面体底氧起皱、八面体展平和八面体上下三原子对旋转起阻碍作用,而对四面体旋转起促进作用.此外,当氟对层间羟基的置换摩尔分数较低时(xF=0-0.7),高岭石层间距并不明显随氟的增加而增加,这说明了高岭石的水合过程可能并不需要氟化铵的加入.     

Zur Koordination des Aluminiums im β-LiAlO2

On the Coordination of Al in β-LiAlO₂ By using ²⁷Al-NMR and the determination of the contribution of Al₂O₃ to the molar refractivities, the octahedral coordination of Al in α-LiAlO₂ respectively the tetrahedral coordination in γ-LiAlO₂ has been confirmed. For β-LiAlO₂ the results show that there only tetrahedrally coordinated Al occurs, contrary to earlier statements given in literature.     

Fe- and Cr-substituted mullites: Mössbauer spectroscopy and rietveld structure refinement

Fe-substituted mullite samples Al_4.5-yFe_ySi_1.5O_9.75 (y = 0.25 and 0.50) have been prepared using a sol-gel route involving Si and Al alkoxides and iron nitrate as precursors. Mössbauer spectroscopy reveals the presence of three different Fe(III) sites but does not allow to conclude concerning the ion environments, tetrahedral or octahedral. Rietveld refinements reveal that Fe ions are distributed between octahedral and tetrahedral positions, the proportion of octahedral iron increasing from y = 0.25 to y = 0.50. On the other hand, for Cr-substituted mullite, Cr ions arc mainly located on octahedral positions. This difference between Fe and Cr has been attributed to the fact that Cr shows a clear trend to occupy octahedral sites compared to Fc and that it is incorporated inside the mullite structure only at high temperature.     

Molecular orbital calculation of Al and Mg Kα chemical shifts

The octahedral and tetrahedral coordination shifts of Mg and Al Kα were systematically measured using a high resolution spectrometer. To explain these shifts, we applied SCC-DV-Xα molecular orbital calculation to several cluster models. The results of the calculations agreed well with the observed shifts. The possibility of determining coordination by Mg or Al Kα shifts is confirmed after parameter survey of atomic distance.     

Computer simulation of the structure of the hydration shells of calcium and calcium-water-zeolite a

A potential function is suggested to describe the interaction of the calcium ion with the water molecule using the tetrahedral model of the water molecule. Monte Carlo simulations of small clusters Ca(H₂O)_n (n≤20) and analyses of the resulting F-structures showed that the coordination number of Ca is 8. The structure of water adsorbed in the α-cavity of zeolite CaA depends predominantly on interactions with Ca²⁺ ions. The water molecule forms one hydrogen bond with an oxygen atom of the framework; the molecules are not hydrogen-bonded with each other. In this respect the structure of water in the Ca form of zeolite A resembles that in the Na form but differs from that in the K form. Institute of Physical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 88–97, January–February, 1996 Translated by L. Smolina     

Quantum chemical models and electronic structure of active centers of heterogeneous polymerization of olefins

Conclusions The peculiarities of the catalytic activity in olefin polymerizations which can find explanation in terms of the concepts suggested in this work are as follows. First, the low catalytic activity of the individual organometal compounds of group IV-VI transition metals is indicative [53] of the important role of the coordination state of the transition metal in AC, which, according to Cosse's model, must be octahedral (tetrahedral for individual metal-alkyl compounds MR_n). Second, the activity of a catalytic system depends essentially on the nature of the ligand environment of the metal in AC. The catalysts based on titanium halides display the highest activity. Third, the results of [19, 20] show that the highly active catalytic centers of homogeneous Ziegler-Natta's systems are “cation-like” Zr(IV) complexes Cp2Zr⁺-R. All these features find explanation in terms of the concept of the competitive contributions from the AC metal s and d orbitals to the active M-R bond. Thus a transition of AC environment from tetrahedral to octahedral may be compared with a change in transition metal AO hybridization:d ³s¹ (tetrahedron) ⇒d ³s¹ (octahedron). Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 2, pp. 391–404, March–April, 2000.     

Particle Size Limit for Concomitant Tuning of Size and Shape of Platinum Nanoparticles

Based on a polyol process, one-step synthetic procedures were investigated for concomitant control of the shape and size of platinum nanoparticles (Pt NPs). Cubic and octahedral/tetrahedral particles of different sizes (5–10 nm) were synthesized using these procedures. Further attempts to control the shape of the NPs below 3–4 nm failed. It was found that 3–4 nm is the particle size limit for Pt NPs above which the particle size and shape can be concomitantly controlled.     

An in situ Al K-edge XAS investigation of the local environment of H+- and Cu+-exchanged USY and ZSM-5 zeolites

Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Br?nsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.     

Cation distribution in co-doped ZnAl2O4 nanoparticles studied by X-ray photoelectron spectroscopy and 27Al solid-state NMR spectroscopy

Co(x)Zn(1-x)Al(2)O(4) (x = 0.01-0.6) nanoparticles were synthesized by the citrate sol-gel method and were characterized by X-ray powder diffraction and transmission electron microscopy to identify the crystalline phase and determine the particle size. X-ray photoelectron spectroscopy and (27)Al solid-state NMR spectroscopy were used to study the distribution of the cations in the tetrahedral and octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles as a function of particle size and composition. The results show that all of the as-synthesized samples exhibit spinel-type single phase; the crystallite size of the samples is about 20-50 nm and increases with increasing annealing temperature and decreases with Co-enrichment. Zn(2+) ions are located in large proportions in the tetrahedral sites and in small proportions in the octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles. The fraction of octahedral Zn(2+) increases with increasing Co concentration and decreases with increasing particle size. Besides the tetrahedral and octahedral coordinations, the presence of the second octahedrally coordinated Al(3+) ions is observed in the nanoparticles. The change of the inversion parameter (2 times the fraction of Al(3+) ions in tetrahedral sites) with Co concentration and particle size is consistent with that of the Zn fraction in octahedral sites. Analysis of the absorption properties indicates that Co(2+) ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The inversion degree of Co(2+) decreases with increasing particle size.