SBA-15分子筛羟基结构性质及其催化活性的理论研究

采用密度泛函方法对SBA-15分子筛簇模型化合物进行了计算模拟,重点研究了簇模型化合物羟基的几何构型和电子结构性质。从键级、前线分子轨道、静电势和质子化能等方面探讨了表面羟基的酸碱及氧化还原性质。分子筛模型化合物的静电势图显示SBA-15分子筛表面孤立羟基的H原子为分子筛表面的L酸性位,而氢键羟基的O原子则为分子筛表面的B碱性位。分子筛的前线分子轨道研究发现,表面羟基是分子筛表面的氧化还原活性位。计算得到的羟基SiO-H键级及形成的氢键键级分别在0.677 5～0.710 5和0.055 7～0.092 6范围,计算羟基OH的质子化能在1 471～1 589 kJ.mol-1范围。考察分子筛表面的质子化能显示未参与形成氢键的羟基H质子具有较强的B酸性。     

Y型分子筛骨架外阳离子行为的FT-FIR研究

应用付里叶变换远红外光谱（ＦＴ－ＦＩＲ）方法研究了ＮＨ＾＋４交换、Ｎａ＾＋反交换和不同水蒸汽处理条件对ＮａＹ分子筛骨架外阳离子的影响。结果表明，在ＮＨ＾＋４、Ｎａ＾＋阳离子交换过程中，六方柱笼中Ⅰ位竞争激烈，积极参与交换。骨架外阳离子受影响较大，升温过程中分子筛空穴的吸附水被除去的同时阳离子发生了重新分布。较高温度下分子筛发生脱结构羟基，生成多种形式的铝阳离子。     

金属络合物制备分子筛负载金属催化剂的CO加氢反应性能

应用Ｙ分子筛吸附Ｆｅ（ＣＯ）５、Ｃｏ（ＣＯ）３ＮＯ和Ｍｏ（ＣＯ）６的温和热分解制备分子筛负载金属催化剂，从分子筛交换阳离子和吸附金属络合物之间的相互作用说明其ＣＯ加氢产物的分布特点。实验结果表明，分子筛内阳离子和吸附金属络合物间的相互作用显著影响制得催化剂的结构和反应的产物分布。由于金属络合物和分子筛内阳离子间的电子转移，使ＮｉＹ附载金属催化剂显示出高的ＣＯ转化率和甲烷选择性。在ＣｕＹ中，这种电子     

应用弦长度分布函数研究中孔分子筛的精细结构

弦长度分布(chord length distribution, 简称CLD)可以用来描述几何对象(球形颗粒、柱状孔、薄片等)的大小、形状和空间排布等特点. 原理上, 弦长度分布正比于由小角散射所得的相关函数的二次微分, 因此可以由相对散射强度进行计算. 在结构研究中, 将实验所得弦长度分布与理论计算某种确定几何体所得高度准确的弦长度分布进行比较, 可以获知对象的特征几何结构. 本文以中孔氧化硅分子筛为研究对象, 介绍了弦长度分布的计算方法, 以此精确计算了中孔分子筛的孔径分布, 得到了分子筛表面有机官能化对孔径的影响规律.     

甲硫氨酸-脑啡肽的活性位点

用ＡＭ１方法中计算静电势的ＰＭＥＰ子程序了甲硫氨酸－脑啡肽的活性位点。通过计算得到了整个分子的三维空间静电势分布和由静电势导出的各原子的电荷分布。进一步分析,确定模型分子活性位点为酪氨酸残在的叔氨原子和苯酚基、苯丙氨酸残基的苯基及部分氧原子。     

N2和Ar在改性的NaA分子筛上的吸附分离

采用水溶液离子交换法分别制备了不同Li^ 和Ca^2^ 交换度的A型分子筛,并在25℃下测定了各分子筛吸附剂的氮和氩的吸附等温线及穿透曲线．研究发现,锂交换和钙交换的两种分子筛的氮吸附量和氮氩分离选择性都随分子筛中阳离子交换度的增加而增大;在阳离子交换度较高时,锂离子和钙离子交换的A型分子筛的氮氩分离能力均优于NaA分子筛．穿透曲线的结果显示,所研究的各种吸附剂都存在一个最优的吸附分离压力,大约在0．6MPa．在0．6MPaT,接近100％阳离子交换度的CaNaA分子筛的动态氮氩分离性能优于相同交换度的LiNaA分子筛。     

胰岛素单体、双体分子静电势的计算

计算了三方二锌胰岛素晶胞不对称单位中两个胰岛素分子单体及其双体的分子静电势。结果表明:胰岛素两个单体分子的构象不同,其分子表面静电势也不相同,由于它们的构象是类似的,所以其电势分布也是接近的,单体胰岛素分子的表面静电势呈偶极型分布,双体的表面静电势基本保持单体的特征。文中还比较了两个单体分子及其集聚后的静电势变化。     

锂离子与芳香体系相互作用的量子化学研究

运用微扰MP2和密度泛函B3LYP方法在不同的基组水平上对锂离子与一系列以杂环为主的芳香化合物所形成复合物的可能构型进行了优化,获得了复合物的几何构型、电荷转移量、结合能等重要信息,对阳离子-π作用与阳离子-杂原子作用的竞争情况进行了讨论.特别是,我们发现此体系阳离子-π复合物的结合能和芳环单体中心上方一个点的静电势成很好的线性相关.揭示了锂离子与芳香杂环形成阳离子-π复合物的形成过程中静电势起着非常重要的作用;同时表明,可用芳环表面静电势来定量预测较复杂体系阳离子-π作用结合能的大小.     

Ce离子对苯在Y分子筛上吸附扩散行为的影响

采用液相离子交换法制备了不同稀土含量的Y型分子筛(HY、USY和NaY),研究了稀土铈(Ce)阳离子在Y型分子筛上吸附-脱附烃类分子(苯)过程中的作用机理与影响。通过X射线荧光光谱仪(XRF)、智能质量分析仪(IGA)、脱附指数的计算和巨正则蒙特卡罗模拟计算等多种表征计算方法,对引入稀土物种后,Y型分子筛对苯的饱和吸附量、吸附作用力、脱附热力学参数、苯在Y分子筛上的吸附势能分布及扩散行为等方面进行了研究。结果表明,Ce离子对苯在Y分子筛上脱附活化能的降低、吸附作用力的减弱以及吸附态由团聚态向分散态转变等方面具有显著影响,该作用构成了CeY分子筛催化剂在流化催化裂化(FCC)过程中能够优化轻质产品选择性的重要因素。     

ZSM-5沸石分子筛的吸附性能

对ZSM-5沸石分子筛的吸附性能进行了研究。测定了环己烷、正己烷、苯和水分子在ZSM-5沸石上的吸附等温线;发现室温下环己烷分子的吸附受到阳离子的空间障碍作用。计算了ZSM-5佛石单胞中的吸附分子数、等量吸附热、微分吸附功和微分吸附熵等值。吸附热曲线表明ZSM-5沸石分子筛的表面是不均匀的。     

On the Energetics of Ions in Carbon and Gold Nanotubes

We investigate the insertion of halide and alkali atoms into narrow single‐walled carbon nanotubes with diameters <9 Å by density functional theory; both chiral and non‐chiral tubes are considered. The atoms are stored in the form of ions; the concomitant charge transfer affects the band structure and makes originally semiconducting tubes conducting. The electrostatic interaction between a charge and the walls of the tube is explicitly calculated. The insertion energies and the positions of the ions are determined by a competition between electrostatic energy and Pauli repulsion. For comparison, we consider ions in gold nanotubes. Alkali ions follow the same principles in gold as in carbon tubes, but chloride is specifically adsorbed inside gold tubes.     

Theoretical study of the structure and properties of Na-MOR and H-MOR zeolite models

This work presents calculated structural parameters and energetic properties of Na- and H-mordenites (MORs) using cluster models with more than 400 atoms. These state of the art calculations were performed within the framework of density functional theory, using both the local density approximation and the generalized gradient approximation, employing all-electron basis sets. The most populated T3, T4, and T1 Al sites have been investigated using two different MOR models, each containing two isolated Al sites. A detailed analysis of the structures, three-dimensional contours of molecular electrostatic potential, binding energies of Na cations and protons, and Br?nsted O-H harmonic frequencies are presented. These properties are compared with available experimental results. The structural changes among Si/Al substitution as well as Na/H exchange are discussed.     

Aqueous NaCl and CsCl solutions confined in crystalline slit-shaped silica nanopores of varying degree of protonation

All-atom molecular dynamics simulations were conducted to study the dynamics of aqueous electrolyte solutions confined in slit-shaped silica nanopores of various degrees of protonation. Five degrees of protonation were prepared by randomly removing surface hydrogen atoms from fully protonated crystalline silica surfaces. Aqueous electrolyte solutions containing NaCl or CsCl salt were simulated at ambient conditions. In all cases, the ionic concentration was 1 M. The results were quantified in terms of atomic density distributions within the pores, and the self-diffusion coefficient along the direction parallel to the pore surface. We found evidence for ion-specific properties that depend on ion-surface, water-ion, and only in some cases ion-ion correlations. The degree of protonation strongly affects the structure, distribution, and the dynamic behavior of confined water and electrolytes. Cl(-) ions adsorb on the surface at large degrees of protonation, and their behavior does not depend significantly on the cation type (either Na(+) or Cs(+) ions are present in the systems considered). The cations show significant ion-specific behavior. Na(+) ions occupy different positions within the pore as the degree of protonation changes, while Cs(+) ions mainly remain near the pore center at all conditions considered. For a given degree of protonation, the planar self-diffusion coefficient of Cs(+) is always greater than that of Na(+) ions. The results are useful for better understanding transport under confinement, including brine behavior in the subsurface, with important applications such as environmental remediation.     

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.     

A new surface structural approach to ion adsorption: tracing the location of electrolyte ions

Electrolyte ions differ in size leading to the possibility that the distance of closest approach to a charged surface differs for different ions. So far, ions bound as outersphere complexes have been treated as point charges present at one or two electrostatic plane(s). However, in a multicomponent system, each electrolyte ion may have its own distance of approach and corresponding electrostatic plane with an ion-specific capacitance. It is preferable to make the capacitance of the compact part of the double layer a general characteristic of the solid-solution interface. A new surface structural approach is presented that may account for variation in size of electrolyte ions. In this approach, the location of the charge of the outersphere surface complexes is described using the concept of charge distribution in which the ion charge is allowed to be distributed over two electrostatic planes. It was shown that the concept can successfully describe the pH dependent proton binding and the shift in the isoelectric point (IEP) in the presence of variety of monovalent electrolyte ions, including Li(+), Na(+), K(+), Cs(+), Cl(-), NO(-)(3), and ClO(-)(4) with a common set of parameters. The new concept also sheds more light on the degree of hydration of the ions when present as outersphere complexes. Interpretation of the charge distribution values obtained shows that Cl(-) ions are located relatively close to the surface. The large alkali ions K(+), Cs(+), and Rb(+) are at the largest distance. Li(+), Na(+), NO(-)(3), and ClO(-)(4) are present at intermediate positions.     

A型沸石中Lowenstein规则的能学研究

通过模拟退火方法,使用协合分子力学力场对Si、Al分布分别为4:0序、两种3:1序和随机分布的NaA型沸石结构进行了能量最小化计算,获得了不同结构的位能及其生成热大小。计算结果表明,4:0序结构的位能和生成热在所讨论的几种序结构中最低,从而在理论上证实了Lowenstein规则是分子筛结构中能量最小化的自然结果。     

水在NanZSM-5型分子筛中吸附的研究: 分子模拟

利用分子动力学(MD)模拟退火的方法和巨正则系综Monte Carlo模拟方法(GCMC)研究了水在NanZSM-5型分子筛中的吸附行为, 计算结果与文献中报道的实验结果吻合较好. 在此基础上, 进一步预测了水在不同硅铝比的NanZSM-5型分子筛中的吸附性质, 计算结果显示: 分子筛骨架上的硅铝比会显著影响水分子的吸附量和吸附等温线, 随着硅铝比的降低, 水的吸附量增加; 水分子的吸附位置是在钠离子和铝原子的周围, 平均每个钠离子周围吸附4个水分子, 而当水的吸附量增大时, 水分子与分子筛骨架上的氧原子之间发生了氢键作用; 在吸附量相同的条件下, 水的吸附热随着硅铝比的降低而升高.     

Incidence of partial charges on ion selectivity in potassium channels

Potassium channels are membrane proteins known to select potassium over sodium ions at a high diffusion rate. We conducted ab initio calculations on a filter model of KcsA of about 300 atoms at the Hartree-Fock level of theory. Partial charges were derived from the quantum mechanically determined electrostatic potential either with Merz-Kollman or Hinsen-Roux schemes. Large polarization and/or charge transfer occur on potassium ions located in the filter, while the charges on sodium ions remain closer to unity. As a result, a weaker binding is obtained for K(+) ions. Using a simplified version of a permeation model based on the concerted-motion mechanism for ion translocation within the single-file ion channel [P. H. Nelson, J. Chem. Phys. 117, 11396 (2002)], we discuss how differences in polarization effects in the adducts with K(+) and Na(+) can play a role as for ionic selectivity and conductance.     

Insights into metal borohydride and aluminohydride bonding: X-ray and neutron diffraction structures and a DFT and charge density study of [Na(15-crown-5)][EH(4)] (E = B, Al)

The neutron and X-ray structures of [Na(15-crown-5)][BH(4)] and [Na(15-crown-5)][AlH(4)], respectively, are reported, along with a topological analysis of their DFT-computed charge densities that explores the bonding between the anionic complex hydride [EH(4)](-) (E = B, Al) and the counterion [Na(15-crown-5)](+). In each case, the interaction is weak and mainly electrostatic in nature; however, notable differences are observed in the manner in which [BH(4)](-) and [AlH(4)](-) bind to the metal, which explains their different coordination modes. A range of unconventional E-H···H-C contacts is revealed to play an important role in the overall bonding and crystal packing of both complexes. These interactions can be classified as weak dihydrogen bonds based on the atoms in molecules approach.     

镧系元素的静电排斥参数和价轨道电离势的半经验公式

根据由光谱数据求得的镧系元素静电排斥参数和价轨道电离势,用最小二乘法拟合出它们的半经验公式,可用以求得各种组态下的镧系元素原子和离子的未知VOIP值与未知Slater和Racah参数值。