Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.     

Combining the pair distribution function and computational methods to understand lithium insertion in Brookite (TiO2)

X-ray pair distribution function (PDF) methods and first-principles calculations have been combined to probe the structure of electrochemically lithiated TiO(2) Brookite. Traditional powder diffraction studies suggest that Brookite amorphizes upon lithium insertion, with the Bragg reflections disappearing. However, PDF analysis indicates that the TiO(2) framework connectivity is maintained throughout lithium intercalation, with expansions along the a and b axes. The Li(+) ions within the framework are poorly observed in the X-ray PDF, which is dominated by contributions from the more strongly scattering Ti and O atoms. First-principles calculations were used to identify energetically favorable Li(+) sites within the Brookite lattice and to develop a complete structural model of the lithiated material. This model replicates the local structure and decreased intermediate range order observed in the PDF data. The analysis suggests that local structural distortions of the TiO(2) lattice accommodate lithium in five-coordinate sites. This structural model is consistent with the observed electrochemical behavior.     

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.     

P-derived organic cations as structure-directing agents: synthesis of a high-silica zeolite (ITQ-27) with a two-dimensional 12-ring channel system

Recently, efforts have been made to synthesize large-pore, multidimensional zeolite frameworks as a basis for new catalysts to improve various hydrocarbon conversions. A new aluminosilicate zeolite, ITQ-27, has been prepared using the phosphorus-containing structure-directing agent, dimethyldiphenylphosphonium. Its crystal structure was determined in its calcined form by direct methods (FOCUS) on synchrotron powder diffraction data (lambda = 0.8702 A) after the unit cell and space group were determined from tilt electron diffraction experiments on individual microcrystals. The material crystallizes in space group Fmmm, where a = 27.7508(5) A, b = 25.2969(7) A, and c = 13.7923(4) A. The final model, refined by Rietveld methods, comprises seven unique T-sites forming a framework with straight 12-MR channels that are connected by 14-MR openings between them. (Corresponding 12-ring pore dimension is 6.94 A x 6.20 A.) Since access from one 14-MR opening to the next is through the 12-MR channel, the structure is best described as a two-dimensional, 12-MR framework.     

Spatial partitioning of the molecular wave function: Reexamination of the bond‐charge model of covalent binding

Owing to the complexity of many‐body quantum interactions, theoretical problems in chemistry almost always necessitate numerical methods of solution. In turn, and somewhat paradoxically, this reliance on computation often precipitates a need for approximate models that can provide a conceptual framework for the interpretation of the accurate numerical solutions. In this report, we reexamine and improve upon the simple bond‐charge (SBC) model of Parr and Borkman with the express purpose of examining its role as a suitable intuitive model for bonding energetics. We first discuss the role of spatial partitioning in the SBC model and the molecular‐structure approximations that are implicit within the model. We then develop an improved version of the model by including electrostatic repulsion between shared electron pairs; we refer to this enhanced version as a semiempirical charge‐cloud (SCC) model. Applying the SCC model to a broad range of diatomics, we find that it is able to predict dissociation energies on a par with simple variational calculations. As an illustrative application of the SCC model, we use it to estimate unknown dissociation energies and rotational constants for a number of alkali dimers. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

A generalized many-body expansion and a unified view of fragment-based methods in electronic structure theory

Fragment-based quantum chemistry methods are a promising route towards massively parallel electronic structure calculations in large systems. Unfortunately, the literature on this topic consists of a bewildering array of different methods, with no clear guiding principles to choose amongst them. Here, we introduce a conceptual framework that unifies many of these ostensibly disparate approaches. The common framework is based upon an approximate supersystem energy formula for a collection of intersecting (i.e., overlapping) fragments. This formula generalizes the traditional many-body expansion to cases where the "bodies" (fragments) share some nuclei in common, and reduces to the traditional many-body expansion for non-overlapping fragments. We illustrate how numerous fragment-based methods fit within this framework. Preliminary applications to molecular and ionic clusters suggest that two-body methods in which dimers are constructed from intersecting fragments may be a route to achieve very high accuracy in fragment-based calculations.     

The Role of Contact Interactions in the Rheological Behavior of a Fibrous Suspension

The model of structure formed by randomly arranged crossed fibres is proposed. The model makes it possible to calculate the number of contacts between the fibers and to estimate the mechanical characteristics of a structure, provided that contact adhesion forces are known. Within the framework of this model, experimental data obtained by the author and his coworkers on the forces of adhesion and friction between individual cellulose fibers as well on macroscopic rheological parameters of aqueous suspensions of these fibers, including those obtained in the presence of surface-active additives such as poly(ethylenimine) and tetraalkylammonium halides are analyzed.     

Isolation of template effects that control the structure and function of nonspherical, biotemplated Pd nanomaterials

Advances in nanotechnology have indicated that the passivant and the inorganic surface play a pivotal role in controlling the structure/function relationship of materials. Beyond standard materials-based methods, bioligands have recently demonstrated the production of unique nanomaterial morphologies for application under ambient conditions for multiple activities, such as catalysis and biosensing. We have recently demonstrated that a biotemplate technique could be employed to produce spherical and linear Pd nanostructures in water using a self-assembling peptide framework. The materials possessed high catalytic reactivity that was controlled by the three-dimensional structure of the composite materials. To investigate the effect of the peptide template on the reactivity of Pd nanostructures, an in depth analysis of the catalytic activity of Pd nanostructures fabricated via truncated templates is presented. The new templates were designed from portions of the original framework, which demonstrated unique synthetic and functionality control. Two different reactions, Stille C-C coupling and 4-nitrophenol reduction, were employed to ascertain the effect of template structure on the reactivity of synthesized Pd nanomaterials via changes in reagent diffusion through the bioscaffold. The results indicate that the peptide framework plays an important role and could be used to tune and optimize the functionality of the final composite materials for the target application.     

TEM investigation of the microporous compound VSB-1: Building units and crystal growth mechanisms

Surface fine structure and structural defects in the open framework material VSB-1 have been investigated by electron microscopy. Crystal growth phenomena are proposed by a building unit model: (i) a unit is formed by two building units; (ii) they are linked to form first channels; and (iii) the whole network is grown via a layer-by-layer growth mechanism. A planar defect was observed in high-resolution transmission electron microscope (HRTEM) image taken with the [0001] incidence, and diffuse streaks related to the presence of defects were observed in a series of electron diffraction (ED) patterns. The microstructure model derived from the defect structure gives information on crystal growth. These defects highlight an open site that could be the pillar of a new crystal growth process. The study of defects and crystal growth is important in understanding physical properties such as catalytic or magnetic properties, and in synthesising a new open framework structure.     

离子型共价有机框架材料的制备及对染料吸附性能的研究

茜素红(Alizarin Red,AR)作为蒽醌类染料中的重要组成,由于其具有优异的特性,在染料和酸碱指示剂等方面被广泛使用。但是AR具有毒性高、结构复杂以及化学需氧量(COD)值大等原因,使其成为了主要污染物之一,去除水体中的茜素红染料污染物已经成为了目前亟待解决的问题。共价有机框架材料作为一种新型的多孔有机材料,由于其具有比表面积大,孔径均一和可设计的独特优势,已经广泛应用吸附和分离等方面。因此,以三醛基间苯三酚和溴化乙锭为构筑单元,通过水热的方法合成一种二维离子型共价有机框架材料(TpEB-COF)。对制备的TpEB-COF进行相关表征,包括X射线衍射仪(XRD),扫描电镜(SEM)和能量色散谱仪(EDS)等。然后将制备的TpEB-COF作为固体吸附剂,将其应用对水中AR的吸附,研究了不同吸附时间和不同pH值对吸附过程的影响。实验结果证明制备的离子型共价有机框架材料具有良好的晶型结构。同时,对实验数据分析表明,离子型共价有机框架材料对于茜素红的吸附符合准二级动力学方程和Langmuir吸附模型,吸附效率为82.8%,最大吸附量为828 mg g-1。本研究不仅为共价有机框架材料的设计和合成奠定坚实的基础,而且拓展了离子型共价有机框架材料的应用范围,促进共价有机框架材料的发展。     

Approaching the self-consistency challenge of electrocatalysis with theory and computation

This opinion piece centers around challenges involved in developing first-principles electrochemical methods. In recent years, theory and computation have become indispensable tools to navigate the parameter space that controls the activity and stability of electrocatalytic materials and electrochemical devices. Viable methods process as input details on materials structure, composition and reaction conditions. Their output includes metrics for stability and activity, phase diagrams, as well as mechanistic insights on reaction mechanisms and pathways. The core challenge, connecting input to output, is a self-consistency problem that couples the electrode potential to variables for the electronic structure of the solid electrode, solvent properties and ion distributions in the electrolyte as well as specific properties of a boundary region in-between. We will discuss a theoretical framework and computational approaches that strive to accomplish this feat.     

SRSF shape analysis for sequencing data reveal new differentiating patterns

MotivationSequencing-based methods to examine fundamental features of the genome, such as gene expression and chromatin structure, rely on inferences from the abundance and distribution of reads derived from Illumina sequencing. Drawing sound inferences from such experiments relies on appropriate mathematical methods to model the distribution of reads along the genome, which has been challenging due to the scale and nature of these data.ResultsWe propose a new framework (SRSFseq) based on square root slope functions shape analysis to analyse Illumina sequencing data. In the new approach the basic unit of information is the density of mapped reads over region of interest located on the known reference genome. The densities are interpreted as shapes and a new shape analysis model is proposed. An equivalent of a Fisher test is used to quantify the significance of shape differences in read distribution patterns between groups of density functions in different experimental conditions. We evaluated the performance of this new framework to analyze RNA-seq data at the exon level, which enabled the detection of variation in read distributions and abundances between experimental conditions not detected by other methods. Thus, the method is a suitable supplement to the state-of-the-art count based techniques. The variety of density representations and flexibility of mathematical design allow the model to be easily adapted to other data types or problems in which the distribution of reads is to be tested. The functional interpretation and SRSF phase-amplitude separation technique give an efficient noise reduction procedure improving the sensitivity and specificity of the method.     

QM:QM embedding using electronic densities within an ONIOM framework: energies and analytic gradients

Accurate calculations of large systems remain a challenge in electronic structure theory. Hybrid energy techniques are a promising family of methods for treating such systems. Expanding on previous developments, we present a QM:QM electronic embedding model whereby the high-level region is polarized by the electron density of the low-level region within an ONIOM framework. A direct Coulomb embedding model as well a more computationally efficient model involving a density fitting expansion are considered. We also develop a generalized theory for the first derivatives of these classes of QM:QM electronic embedding schemes, which requires solution of a single set of self-consistent field response equations. Two initial test cases are presented and discussed.     

杂原子进入分子筛骨架结构的波谱学判据

概括了杂原子取代的分子筛内骨架杂原子的波谱学鉴定方法。根据杂原子进入骨架后所引起的晶体结构、表面酸性、电子跃迁规律、配位环境等微观性质的变化,重点讨论了XRD、FT-IR、DR UV-Vis、Raman、MAS-NMR、XAFS、ESR和XPS等波谱学方法在鉴别骨架杂原子时的应用原理和方法。     

自适应模糊偏最小二乘方法在药物构效关系建模中的应用

作为一种局部逼近方法,自适应神经模糊推理系统(ANFIS)适于为药物定量构效关系(QSAR)建模。描述药物分子结构的参数较多,常存在耦合关系,会增加建模难度,并影响模型的预报性能。为此,将ANFIS和偏最小二乘(PLS)相结合,先由PLS从样本数据中提取成分,再由ANFIS实现每对成分间的非线性映射,并基于输出误差进一步修正所提取的成分,使之对因变量具有最优的解释能力,由此构建为EB-AFPLS方法。该法已成功地应用于HIV-1蛋白酶抑制剂的QSAR建模,效果良好,显示出很强的学习能力,所建模型的预报性能也优于其它方法。     

NMR crystallography of zeolites: How far can we go without diffraction data?

Nuclear magnetic resonance (NMR) crystallography—an approach to structure determination that seeks to integrate solid-state NMR spectroscopy, diffraction, and computation methods—has emerged as an effective strategy to determine structures of difficult-to-characterize materials, including zeolites and related network materials. This paper explores how far it is possible to go in determining the structure of a zeolite framework from a minimal amount of input information derived only from solid-state NMR spectroscopy. It is shown that the framework structure of the fluoride-containing and tetramethylammonium-templated octadecasil clathrasil material can be solved from the 1D ²⁹Si NMR spectrum and a single 2D ²⁹Si NMR correlation spectrum alone, without the space group and unit cell parameters normally obtained from diffraction data. The resulting NMR-solved structure is in excellent agreement with the structures determined previously by diffraction methods. It is anticipated that NMR crystallography strategies like this will be useful for structure determination of other materials, which cannot be solved from diffraction methods alone.     

Application of QM simulations and multivariate analysis in the study of alkene reactivity in the zeolite H‐ZSM5

Reported herein are the results of an investigation into the effect of the extended framework of the zeolite ZSM‐5 on the reaction energetics and structures of (a) the physisorbed complex formed between the zeolite and six alkenes, (b) the corresponding chemisorbed alkoxide intermediate and (c) the transition states (TS) connecting the two. For this, quantum mechanical (QM) simulations of ZSM‐5 in the presence and absence of the zeolite framework have been employed. A 46T density functional theory (DFT) cluster model and a 3T:46T DFT:UFF ONIOM model are used to represent the former scenario and a simple 3T DFT cluster model for the latter. The structural implications of neglecting the zeolite framework have been rigorously compared using the multivariate statistical method principal components analysis (PCA). This method allows one to assess the correlated nature of the changes in structure along the reaction coordinate, for multiple different alkenes, in a facile, reliable way. Copyright © 2008 John Wiley & Sons, Ltd.     

Electronic Structure of (8,0) Gold Nanotubes

The electronic structure of (8,0) gold nanotubes have been studied using quantum field theory methods in the framework of the Hubbard model. An expression for the Fourier transform of the anticommutator Green function, the poles of which determine the energy spectrum of the system under consideration, has been derived. The energy spectrum demonstrates that the (8,0) gold nanotube has metal-like electronic structure. The peaks of the calculated density of states correspond to Van Hove singularities. The optical absorption spectrum is presented, and the energy of the first direct optical transition is 0.55 eV.     

Current Production Processes on Metallic Electrodes in Contact with Oxygen-Conducting Solid Electrolytes with Face-Centered Cubic Lattice of the Fluorite Type

The data concerning the processes of current production that occur in the electrode system O₂, Pt/O²⁻, which are available from relevant literature sources, are considered. Major progress in the understanding of the kinetics of an oxygen reaction has been attained with the application of model electrodes and cells, as well as at the expense of the application of such physical methods as a radiotracer method and atomic force microscopy. A discussion of model notions on the specific features of the occurrence of an oxygen reaction, which are formed on the basis of consideration of experimental results is the major subject matter of this communication. It is claimed that modernization of an existing model with allowance made for the fact of the existence, on the surface of an electrolyte with a face-centered cubic lattice, of a layer with another structure gives one a chance to also describe experimental results that cannot be described in the framework of an existing model.__________Translated from Elektrokhimiya, Vol. 41, No. 7, 2005, pp. 787–803.Original Russian Text Copyright © 2005 by Shkerin.