Hydrated Cs+-exchanged MFI zeolites: location and population of Cs+ cations and water molecules in hydrated Cs6.6MFI from in and ex situ powder X-ray diffraction data as a function of temperature and other experimental conditions

Extending our previous investigation of dehydrated, Cs-exchanged MFI zeolites (J. Phys. Chem. B 2006, 110, 97-106) to hydrated analogues, we have determined the crystal structures of members of the Cs(6.6)H(0.3)MFI.xH(2)O series, for 0 < x < 28, from synchrotron-radiation powder diffraction data. In the fully hydrated phase, three independent Cs(+) cations and six water molecules are identified in difference Fourier maps. The populations of the cations amount to 2.79/3.40/0.41 Cs/unit cell (uc) for the Cs1/Cs2/Cs3 sites, respectively, and those of the water molecules to 4/4/4/4/8/4 H(2)O/uc for the Ow1/Ow2/Ow3/Ow4/Ow5/Ow6 sites, respectively. Close to water saturation, the Cs3 and Ow6 sites are near each other (approximately 1.44 A) and are not occupied simultaneously. At saturation, Cs cations and water molecules form three interconnected Cs(H(2)O)(n) clusters and one (H(2)O)(4) cluster in the MFI channel system: Cs2(H(2)O)(5) centered at x/y/z approximately -0.018/0.146/0.546 (midway between the intersection and the straight channels), Cs1(H(2)O)(4) centered at approximately 0.056/0.240/0.889 (the zigzag channel openings), Cs3(H(2)O)(2) centered at approximately 0.228/0.25/0.899 (in the zigzag channel), and the (H(2)O)(4) cluster (in the zigzag channel) bonded to Cs1 and Ow1. (H(2)O)(4) and Cs3(H(2)O)(2) exclude each other. The Cs2(H(2)O)(5) clusters are connected through weak Ow5...Ow5' hydrogen bonds (2.88 A) and form polymeric chains in the straight channel direction (010). During progressive hydration this Cs2 cation enlarges its hydration shell, stepwise, from Cs2(H(2)O)(2) to Cs2(H(2)O)(3), to Cs2(H(2)O)(4), and finally to a Cs2(H(2)O)(5) cluster. During the dehydration process, these extraframework species migrate, and it is shown that for varying total H(2)O/uc loadings the individual populations of the Cs(+) cations and H(2)O molecules strongly depend on experimental and measurement (in situ vs ex situ) conditions. The shapes of the channels change also; except for T > 150 degrees C, in all the Cs(6.6)H(0.3)MFI.xH(2)O phases, the straight channel D10R (double 10-ring) pore openings (1.16 < epsilon < 1.23) become strongly elliptical. The framework structure of all the investigated phases conforms to orthorhombic Pnma space group symmetry. Hydration and dehydration in Cs(6.6)MFI are fully reversible processes. From a knowledge of the Cs(+) locations, we are able to estimate, by computer simulations, the positions of H(2)O molecules in Cs(6.6)H(0.3)MFI.28H(2)O. The maximum theoretically possible water loading in an hypothetical and idealized cationless [Cs(6.6)H(0.3)]MFI structure amounts to 48 H(2)O/uc (nine independent water species), which is in fair agreement with existing high-pressure data (47 H(2)O/uc). This value is to be compared with the water saturation capacity obtained in a structural refinement of sealed-tube diffraction data of a proton-exchanged H(6.9)MFI.38H(2)O (seven independent water molecules). In the crystal structure of this H-ZSM-5 phase, the straight channel openings are almost circular (epsilon = 1.08). From this we conclude that the main factor responsible for the flexibility of the MFI framework is the presence of the Cs(H(2)O)(n)() clusters residing in, or close to, the straight channel double 10-rings.     

MCM-22分子筛酸性的DFT理论计算研究

本文利用量子力学中的密度泛函理论(DFT)计算，研究了MCM-22分子筛上骨架Al在8个不同的T位的分布和Br?nsted酸的落位及强度。所有计算基于分子筛的8T簇模型 (H₃SiO)₃Si-O(H)-T(OSiH₃)₃(T=Si，Al)，采用DFT的BLYP方法，所有原子均应用DNP基组。通过计算(Al，H)/Si替代能和质子亲和势，得出推论：MCM-22分子筛中骨架Al的最有利落位在T1，T4，T3和T8位。而形成Br?nsted-酸的最可能的位置为Al1-O3-Si4，Al4-O3-Si1，Al3-O11-Si2和Al8-O10-Si2桥基。Al1-O3H-Si4和Al4-O3H-Si1位的酸性强度接近，Al3-O11H-Si2和Al8-O10H-Si2位的酸性分别略低于和略高于前两个酸位。通过计算模板剂分子六次甲基亚胺(HMI)与B-酸中心的相互作用，进一步探讨了HMI对分子筛中Al落位的靶向作用。     

Beta分子筛中Al的分布和Brφnsted酸的酸性强度

基于量子化学中的密度泛函理论(DFT), 研究了Beta分子筛中骨架Al在9个不同T位的分布和Brφnsted酸的分布及其强度. 计算采用8T 簇模型和B3LYP/6-31G(d,p)方法, 得到了不同T位的Al/Si替代能、(Al, H)/Si替代能和质子亲和势, 以及碱性探针分子NH3在Beta分子筛酸性位的吸附能. 结果表明, 骨架Al和Brφnsted酸优生位于Beta分子筛的T5 和T6 位; 其中酸性最强的位置是Al5-O14-Si9, 最弱的位置是Al7-O3-Si1.     

High-temperature, high-pressure hydrothermal synthesis, crystal structure, and solid-state NMR spectroscopy of Cs2(UO2)(Si2O6) and variable-temperature powder X-ray diffraction study of the hydrate phase Cs2(UO2)(Si2O6) x 0.5H2O

A new uranium(VI) silicate, Cs2(UO2)(Si2O6), has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It crystallizes in the orthorhombic space group Ibca (No. 73) with a = 15.137(1) A, b = 15.295(1) A, c = 16.401(1) A, and Z = 16. Its structure consists of corrugated achter single chains of silicate tetrahedra extending along the c axis linked together via corner-sharing by UO6 tetragonal bipyramids to form a 3-D framework which delimits 8- and 6-ring channels. The Cs+ cations are located in the channels or at sites between channels. The 29Si and 133Cs MAS NMR spectra are consistent with the crystal structure as determined from X-ray diffraction, and the resonances in the spectra are assigned. Variable-temperature in situ powder X-ray diffraction study of the hydrate Cs2(UO2)(Si2O6) x 0.5H2O indicates that the framework structure is stable up to 800 degrees C and transforms to the structure of the title compound at 900 degrees C. A comparison of related uranyl silicate structures is made.     

β分子筛中Brønsted酸分布及酸性强度的量子化学ONIOM 理论计算研究

基于量子化学ONIOM [B3LYP/6-31G(d,p):UFF]计算方法, 研究了β分子筛中Brønsted酸的落位及其酸性强度. 计算采用22T簇模型, 得到了不同酸性位的(Al, H)/Si替代能和质子亲和势. 研究结果表明, Brønsted酸最有可能落位在Al(8)-O(11)-Si(4), Al(8)-O(4)-Si(1), Al(7)-O(7)-Si(2)和Al(9)-O(6)-Si(3)位置. 根据去质化能的计算, Al(7)-O(7)-Si(2)位置的酸性最强, Al(8)-O(11)-Si(4)的酸性最弱. 酸性的强弱顺序为Al(7)-O(7)-Si(2)＞Al(9)-O(6)-Si(3)＞Al(8)-O(4)-Si(1)＞Al(8)-O(11)-Si(4).     

β分子筛中BrΦnsted酸分布及酸性强度的量子化学ONIOM理论计算研究

基于量子化学ONIOM [B3LYP/6-31G(d,p):UFF]计算方法,研究了β分子筛中BrΦnsted酸的落位及其酸性强度.计算采用22T簇模型,得到了不同酸性位的(Al,H)/Si替代能和质子亲和势.研究结果表明,BrΦnsted酸最有可能落位在Al(8)-O(11).Si(4),Al(8)-O(4)-Si(1),Al(7)-O(7)-Si(2)和Al(9)-O(6)-Si(3)位置.根据去质化能的计算,Al(7)-O(7)-Si(2)位置的酸性最强,A1(8)-O(11)-Si(4)的酸性最弱.酸性的强弱顺序为Al(7)-O(7)-Si(2)＞Al(9)-O(6)-Si(3)＞Al(8)-0(4)-Si(1)＞Al(8)-O(11)-Si(4).     

2D versus 3D MFI zeolite: The effect of Si/Al ratio on the accessibility of acid sites and catalytic performance

Through the synthesis of 2D MFI zeolite samples of Si/Al ratio ranged from 13 to 74 with inter-crystalline mesoporosity and their reference 3D counterparts, we have systematically studied and revealed the impact of Si/Al ratio on the inter-dependence of core intrinsic properties of structural porosity and acidity. It is apparent that mesopores in the 2D MFI zeolite play a critical role, dictating the accessibility and distribution of specific acid sites. It was found that, compared to their 3D counterparts, the 2D samples possess a three-times larger accessible surface area owing to the mesopores. Although having a slightly lower total number of acid sites, the 2D samples enjoy a higher percentage of accessible strong acid sites and weak Lewis acid sites. Consequently, in three selected liquid phase reactions, which had different acidity demands and molecular diffusion constraints, the 2D samples demonstrated much higher catalytic activities and resistance to deactivation. This study has, for the first time, established the relationship between Si/Al ratio and acidity for the 2D MFI zeolite, thus enabling rational selection of a Si/Al ratio for a targeted application.     

High resolution adsorption isotherms of N2 and Ar for nonporous silicas and MFI zeolites

The high resolution adsorption isotherms of N₂ (77.4 K) and Ar (87.3 K) have been measured for two nonporous silicas with different silanol contents (3.3 and 0.35 OH/nm²) and for two MFI zeolite with different Al contents (Si/Al=12.5 and 500). Silanol groups and Al sites (acid sites) gives the significant effect on the N₂ isotherms at submonolayer, but the Ar isotherms are independent of silanols and Al sites. The Ar isotherms, therefore, are preferable in calculation of microporosity of zeolites. The N₂ and Ar isotherms for MFI zeolite (Si/Al=500) have been measured at temperatures of 77–94 K, from which the differential adsorption energies of N₂ and Ar are calculated. The interaction of N₂ with channel surface of MFI zeolite is greater than that of Ar in the range of α _s =0.1–0.7. The hystereses are detected for the N₂ isotherm in p/p ^o=0.1–0.3 at 77.4 K and for the Ar isotherm in p/p ^o=3×10⁻⁴–2×10⁻³ at 87.3 K. However, it is difficult to explain the hysteresis phenomenon using differential adsorption energy.     

SAPO-5分子筛结构和酸性的研究

以三乙胺为模板剂, 在一定的实验条件下, 合成了一系列以硅原子非等比取代APO-5分子筛骨架中的磷和铝或选择取代磷的SAPO-5分子筛。它的酸性取决于硅取代磷, 铝原子的差值。对于差值为1.86的样品,在TPD实验中NH_3脱附峰面积是APO-5的7.4倍。应用多晶x射线衍射法在400 ℃测定了非等比取代和选择取代的NaSAPO-5的结构, 并经Rietveld法修正。其骨架与APO-5相似。空间群C_(6v)~2-P_6cc。结果说明, 非等比取代和选择取代SAPO-5分子筛的骨架中, 磷和铝的排列有较高的有序度。非等比取代和选择取代SAPO-5具有开口直径为0.8 nm左右的12元环直通道。由于阳离子位于六方柱中心, 则可认为HSAPO-5中的质子酸中心, 应机迂地分布在12元环直通道内壁上的O(2)附近。因此, 其晶内仅存在一类质子酸中心或活性中心。在NH_3-TPD曲线上只应出现一个NH_3脱附峰。这一推断与实验结果是一致的。     

An anomalous x-ray diffraction study of the hydration structures of Cs+ and I- in concentrated solutions

Anomalous x-ray diffraction experiments were carried out on concentrated aqueous solutions of sodium iodide (6 molal) and cesium iodide (3 molal). Data were gathered at two energies below the absorption edges of the Cs+ and I- ions in order to avoid contributions from fluorescence. The statistics and quality of the raw data were improved by the use of a focusing analyzer crystal. Differences were taken between the data sets and used to calculate the hydration structures of Cs+ and I-. The structures found are more complex than anticipated for such large ions with relatively low charge densities and show evidence of ion-pair formation in both solutions. A two-Gaussian fit to the Cs+ data gives information about the Cs+-O and Cs+-I- correlations. The central position of the Gaussian representing the Cs+-O was fixed at 3.00 A, that is, the maximum of this contribution. The other parameters were allowed to vary freely, giving a Cs+-I- distance of 3.84+/-0.05 A and coordination numbers of 7.9 and 2.7, respectively, for the Cs+-O and Cs+-I- correlations. The results on the structure of I- in the 6 molal NaI aqueous solution were also fitted to a model based on Gaussians; this gives correlations for I- -O and I- -Na+ at 3.17+/-0.06 and 3.76+/-0.06 A with respective coordination numbers of 8.8 and 1.6. The structure of I- in the 3 molal CsI solution shows overlapping contributions due to I- -H, I- -O, and I- -Cs+. The best Gaussian fit gives two peaks centered at 3.00+/-0.08 and 3.82+/-0.04 A and shows that the latter two correlations are unresolved. The hydration structures are compared with those of other alkali and halide ions. The results are also found to be in good agreement with those obtained from standard x-ray diffraction and computer simulation.     

Photoluminescence response of terbium-exchanged MFI-type materials to Si/Al ratio, texture, and hydration state

Terbium-exchanged MFI zeolite type materials, i.e., microporous-mesoporous Zeotile-1 with the Si/Al ratio in the range 33-200, Zeogrid with the Si/Al ratio of 75, and nanocrystalline MFI with the Si/Al ratio of 75, were prepared via an ion exchange procedure. All of these zeolites were investigated by means of time-resolved photoluminescence techniques in various hydration states: as-synthesized (hydrated), calcined (heated at 450 degrees C in air), and rehydrated (after a six-month exposure to the atmospheric moisture). The photoluminescence decays and spectra were analyzed by discrete exponential fitting, distribution lifetimes analysis, and area-normalized time-resolved photoluminescence spectra. The results sustained a single average terbium species coordinated to both water molecules and framework oxygens in the hydrated zeolites. The framework contribution increased with the Si/Al ratio in Zeotile-1 and was greatest for the nanocrystalline MFI zeolite. For the calcined Zeotile-1 and Zeogrid, two main terbium species of different environments were found. For the nanocrystalline Tb3+-MFI, a distinct number of species could not be inferred, indicating a more heterogeneous distribution. Rehydration further differentiated among the Tb3+-exchanged zeolites. Photoluminescence line shape and decay of Tb3+-Zeotile-1 were between those of the hydrated and calcined states indicating a slow rehydration rate in contrast with the photoluminescence properties of Tb3+-MFI, which fully recovered the values of the hydrated state. Tb3+-Zeogrid presented an intermediate case: while the PL line shape was fully restored to that measured for the hydrated sample, the decay was still longer than that measured with the hydrated sample. Terbium photoluminescence response related to zeolite texture, Si/Al ratio, and hydration state suggest different sitting and location of terbium in Zeotile-1, Zeogrid, and nanocrystalline MFI materials. In mesoporous Zeotile-1 and Zeogrid, the results sustained two types of terbium sites: one on the internal surface of mesopores, the other inside the pores, while for the nanocrystalline MFI, terbium sites inside the pores predominate.     

Cs atoms on helium nanodroplets and the immersion of Cs+ into the nanodroplet

We report the non-desorption of cesium (Cs) atoms on the surface of helium nanodroplets (He(N)) in their 6(2)P(1/2) ((2)Π(1/2)) state upon photo-excitation as well as the immersion of Cs(+) into the He(N) upon photo-ionization via the 6(2)P(1/2) ((2)Π(1/2)) state. Cesium atoms on the surface of helium nanodroplets are excited with a laser to the 6(2)P states. We compare laser-induced fluorescence (LIF) spectra with a desorption-sensitive method (Langmuir-Taylor detection) for different excitation energies. Dispersed fluorescence spectra show a broadening of the emission spectrum only when Cs-He(N) is excited with photon energies close to the atomic D(1)-line, which implies an attractive character of the excited state system (Cs?-He(N)) potential energy curve. The experimental data are compared with a calculation of the potential energy curves of the Cs atom as a function of its distance R from the center of the He(N) in a pseudo-diatomic model. Calculated Franck-Condon factors for emission from the 6(2)P(1/2) ((2)Π(1/2)) to the 6(2)S(1/2) ((2)Σ(1/2)) state help to explain the experimental data. The stability of the Cs?-He(N) system allows to form Cs(+) snowballs in the He(N), where we use the non-desorbing 6(2)P(1/2) ((2)Π(1/2)) state as a springboard for ionization in a two-step ionization scheme. Subsequent immersion of positively charged Cs ions is observed in time-of-flight mass spectra, where masses up to several thousand amu were monitored. Only ionization via the 6(2)P(1/2) ((2)Π(1/2)) state gives rise to a very high yield of immersed Cs(+) in contrast to an ionization scheme via the 6(2)P(3/2) ((2)Π(3/2)) state. When resonant two-photon ionization is applied to cesium dimers on He droplets, Cs(2) (+)-He(N) aggregates are observed in time-of-flight mass spectra.     

乙硫醇在MFI和MOR沸石中扩散行为的分子动力学模拟

采用分子动力学方法模拟计算了乙硫醇分子在MFI和MOR两种拓扑结构沸石中的动力学性质，比较了乙硫醇不同负载量和沸石晶格原子刚性和柔性两种状态下乙硫醇分子在沸石孔道中扩散系数的大小。模拟结果表明，对于这两种沸石，负载量增加，扩散系数减小。对于MFI沸石，在刚性和柔性沸石结构的情况下，扩散系数分别为3×10－10m2/s和4×10－9m2/s；对于MOR沸石，在刚性和柔性结构的情况下，扩散系数分别为2×10－7m2/s和3×10－8m2/s。对于MFI沸石，柔性结构下的扩散系数比刚性结构下的扩散系数要大，而对于MOR却呈现出相反的行为。对比这两种沸石，MOR中的扩散系数要比MFI沸石中大，这是因为乙硫醇分子在MOR12元环中扩散更易进行所致。相互作用能的计算结果表明，MFI中交叉型和直型孔道活性位相近，MOR孔道中12元环与4元环孔道相差较大。     

FER分子筛中铝原子的受限落位

分子筛是一类具有规则孔道或笼状结构的多孔材料,因其独特的结构和可调的酸性而广泛用于石油化工、精细化学品合成、现代煤化工等诸多行业.2006年Iglesia等在具有8元环孔道结构/侧口袋的FER和MOR分子筛上实现了无卤素添加、无贵金属存在条件下,由二甲醚羰基化合成乙酸甲酯的反应.乙酸甲酯通过进一步加氢可实现煤基乙醇的绿色生产.MOR分子筛通常具有较高的催化活性,但失活迅速;FER分子筛表现出良好的催化稳定性,但活性较低.如何在保证FER分子筛稳定性的前提下,进一步提升其羰基化活性是目前研究的热点.前期理论和实验研究发现,二甲醚羰基化反应活性与分子筛8元环孔道中的Br?nsted酸位密度存在正相关.因此,通过优化合成条件,选择性调控铝原子分布在"ferrierite"笼中,可以提高FER分子筛的羰基化反应活性.尽管研究者已在调节FER分子筛铝分布方面进行了大量研究,但对于不同T位上Al原子的精准识别以及对应Br?nsted酸位的可接触性还缺少系统和深入的认识.本文选取了几种代表性模板剂,分别在碱性和含氟体系下制备了系列FER分子筛样品,利用Rietveld精修和模拟退火算法,在原子水平揭示了模板剂种类以及合成介质变化对Al原子在不同T位分布的影响,并结合二甲醚羰基化反应进行了结构和性能的关联.首先选取不同尺寸大小的环状胺(环己胺、哌啶、吡啶、吡咯烷)和链状胺(乙二胺)合成了具有相似形貌、孔结构、酸密度的系列FER分子筛样品.以CHA-Na-FER为例,PXRD精修结果显示,Na+(平衡35％的骨架负电荷)分布在10元环孔道中与O1形成氢键,质子化的环己胺分布在"ferrierite"笼中,并且环己胺上的N与O3形成氢键.这说明与O1相连的T3位以及与O3相连的T1位都有可能是Al富集的位置.为了进一步验证该结论,本文还精修了吸附探针分子吡啶的样品CHA-Na-FER-Py-60h.原粉以及吸附吡啶样品的精修结果表明,T1位和T3位是样品中铝富集的位置.随后,运用相同方法研究了Py-Na-FER,PI-Na-FER,En-Na-FER和Pyrr-HF-FER样品中的Al落位,发现T1/T3位均是样品中Al富集的位置.此外,理论计算结果表明T1/T3位上Al原子的取代能较低,说明Al优先取代T1/T3位上的Si,这与精修结果相一致.前期理论模拟结果表明,FER分子筛中T2-O5和T4-O7位点的CO插入反应能垒较低,是二甲醚羰基化反应的活性位.本文吡啶吸附实验、热重分析以及PXRD精修结果表明,FER分子筛中大部分Al富集在T1/T3位,与T2/T4位相关的Br?nsted酸约占18％～30％.最后,对各样品进行了二甲醚羰基化反应评价,结果显示PI-Na-FER,Py-Na-FER,En-Na-FER和CHA-Na-FER催化剂的乙酸甲酯生成速率相近,约为0.10 mol/(mol H+?h).Pyrr-HF-FER催化剂的乙酸甲酯生成速率最高,可达到0.16 mol/(mol H+?h),这可能是由于Pyrr-HF-FER催化剂具有更多T2/T4位相关的Br?nsted酸.虽然Pyrr-HF-FER催化剂的乙酸甲酯生成速率较其他四个催化剂有一定提升,但其仍远低于MOR分子筛上乙酸甲酯生成速率(0.40 mol/(mol H+·h)).综上,有机模板剂的选择与合成介质的改变对FER分子筛中Al分布的调控作用是有限的,即Al原子总是优先分布于T1/T3位.而与T1和T3位相关的Br?nsted酸位不是二甲醚羰基化反应的活性位点.因此与MOR相比,FER分子筛在二甲醚羰基化反应中表现出较低的催化活性.     

Periodic DFT calculations of the stability of Al/Si substitutions and extraframework Zn2+ cations in mordenite and reaction pathway for the dissociation of H2 and CH4

The local stability of Al atoms replacing Si in the zeolite framework is compared for all inequivalent tetrahedral (T) sites in mordenite. For Al/Si substitutions in two T sites the stable location of the compensating extraframework Zn(2+) cation forming a Lewis acid site is determined. In the most stable Zn-MOR structures Zn(2+) is located in a small ring (5MR, 6MR) containing two Al/Si substitutions. In less stable structures the Al atoms are placed at larger distances from each other and Zn(2+) interacts with only one Al site. The simulated adsorption of H(2) and CH(4) shows that adsorption strength decreases with increasing stability of the Zn(2+) Lewis site. A higher adsorption strength is observed for Zn(2+) deposited in the 5MR than for the 6MR. The reactivity of a series of stable Zn(2+) Lewis sites is tested via the dissociative adsorption of H(2) and CH(4). The heterolytic dissociation of the adsorbed molecule on the extraframework Zn(2+) cation produces a proton and an anion. The anion binds to Zn(2+) and proton goes to the zeolite framework, restoring a Br?nsted acid site. Because bonding of the anion to Zn(2+) is almost energetically equivalent for Zn(2+) in any of the extraframework positions the dissociation is governed by stabilizing bonding of the proton to the framework. Those structures which can exothermically accommodate the proton represent reaction pathways. Due to the repulsion between the proton and Zn(2+) the most favorable proton-accepting O sites are not those of the ring where Zn(2+) is deposited, but O sites close to the ring. Large differences are observed for neighboring positions in a- and b-directions and those oriented along the c-vector. Finally, among the stable Zn(2+) Lewis sites not all represent reaction pathways for dehydrogenation. For all of them the dissociation of H(2) is an exothermic process. In structures exhibiting the highest reactivity the Al/Si substitutions are placed at a large distance and the Zn(2+) cation interacts with O-atoms next to Al in the T4 site of the 5MR. This Lewis site is strong enough to break the C-H bond in the CH(4) molecule.     

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

²⁷Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co^II ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM‐5. Furthermore, the effect of Na⁺ ions together with tetrapropylammonium cation (TPA⁺) in the ZSM‐5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA⁺ as a structure‐directing agent (i.e., in the absence of Na⁺ ions) led to 55–90 % of Al atoms located at the channel intersection, regardless the presence or absence of Al pairs [Al?O?(Si?O)₂?Al sequences in one ring] in the zeolite framework. The presence of Na⁺ ions in the synthesis gel did not modify the Al location at the channel intersection (55–95 % of Al atoms) and led only to changes in i) the distribution of framework Al atoms between Al pairs (decrease) and single isolated Al atoms (increase), and ii) the siting of Al in distinguishable framework tetrahedral sites.     

负载铯催化剂上苯酚与甲醇醚化制苯甲醚

研究了不同酸碱中心、载体、前躯体和负载量对负载铯催化剂上苯酚与甲醇醚化制苯甲醚反应行为的影响。结果表明,碱性中心比酸性中心具有更高的苯甲醚选择性,碱性中心的阳离子影响催化剂的苯甲醚选择性。载体影响铯离子的电子结合能,从而影响催化剂的醚化活性;铯离子的电子结合能越低,催化剂醚化活性越低;载体影响催化剂强碱性位数量,从而影响苯甲醚选择性;强碱性位数量越多,副反应越容易发生,苯甲醚选择性越低。不同前躯体制备的Cs/Si O2由于表面相对铯原子数量不同而活性不同;Cs/Si O2的单层负载量为1.0 mmol/g,超过单层负载量后催化剂的平均活性显著下降。     

Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent

The role of the concentration and the nature of aluminium in the creation of hierarchical porosity in both commercial and synthesized MFI zeolites have been investigated through controlled mesoporosity development by desilication in alkaline medium. Framework aluminium controls the process of framework silicon extraction and makes desilication selective towards intracrystalline mesopore formation. An optimal molar Si/Al ratio in the range 25-50 has been identified; this leads to an optimal mesoporosity centred around 10 nm and mesopore surface areas of up to 235 m(2) g(-1) while preserving the intrinsic crystalline and acidic properties. At lower framework Si/Al ratios the relatively high Al content inhibits Si extraction and hardly any mesopores are created, while in highly siliceous ZSM-5 unselective extraction of framework Si induces formation of large pores. The existence of framework Al sites in different T positions that are more or less susceptible to the alkaline treatment, and the occurrence of re-alumination, are tentative explanations for the remarkable behaviour of Al in the desilication process. The presence of substantial extra framework Al, obtained by steam treatment, inhibits Si extraction and related mesopore formation; this is attributed to re-alumination of the extraframework Al species during the alkaline treatment. Removal of extraframework Al species by mild oxalic acid treatment restores susceptibility to desilication, which is accompanied by formation of larger mesopores due to the enhanced Si/Al ratio in the acid-treated zeolite.     

Theoretical study of stable structures and photoelectron spectra of mass-selected Al12Cs-, Al11Cs2-, and Al10Cs3- clusters

The geometric and electronic structures of the ground and low-lying states for the Al(12)Cs(-), Al(11)Cs(2) (-), and Al(10)Cs(3) (-) clusters were examined using the density functional theory. Semi-icosahedral structures of the Al(12)Cs(-) and Al(11)Cs(2) (-) clusters were found as the ground state. The most stable structure of the Al(10)Cs(3) (-) cluster is a distorted icosahedron structure. The vertical detachment energy of these clusters and the anion photoelectron spectra (PES) were compared. The peaks of the anion PES were assigned on the basis of the shell model. The single peak of 3.1-3.2 or 2.5-2.7 eV for the Al(12)Cs(-) or Al(11)Cs(2) (-) cluster, respectively, is observed due to the electron detachment from the 2p or 1f or 1f+2p shells. Two large peaks of 2.1 eV and 3.1-3.3 eV correspond to the electron detachments from the 1f+2p and 2p, and 1d+1f shells, respectively. It was found that a second peak appears with the hybridization of the 1d and 1f shells due to the distortion from the icosahedral structure in the Al(10)Cs(3) (-) cluster.     

Crystal structure of zeolite MCM-68: a new three-dimensional framework with large pores

The crystal structure of the aluminosilicate MCM-68 was solved from synchrotron powder diffraction data by the program FOCUS. The unit cell framework contains Si100.6Al11.4O224. This material crystallizes in space group P42/mnm, where, after Rietveld refinement, a=18.286(1) A and c=20.208(2) A. A three-dimensional framework is found that contains continuous 12-ring channels and two orthogonal, intersecting, undulating 10-ring channels. Rietveld refinement of the model coordinates optimizes the framework geometry, to match the observed intensity profile by Rwp=0.1371, R(F2)=0.1411. It is not possible to determine the location of approximately 0.84 K+ cations remaining in the unit cell after the material is steamed and then dehydrated. The framework model also successfully predicts observed electron diffraction data in two projections, and the tetragonal projection can be determined independently from these data by direct methods. The calculated density of the framework structure is 1.66 g/cm3, and the T-site framework density is 16.6 T/1000 A3.