Tetrahedral distortion and energetic packing penalty in "zeolite" frameworks: linked phenomena?

We report a computational study on the distortion of SiO4 tetrahedra in zeolite frameworks. For all previously observed frameworks, the tetrahedral mismatch was found to span a narrow range (1.0 x 10(-3) to 2.5 x 10(-2) angstroms2) of values, in contrast to the hypothetical frameworks, which were calculated to have a much wider range of mismatch values. The energy of the frameworks was not found to be a function of the tetrahedral distortion for the previously observed and moderately distorted (tetrahedral mismatch <2.5 x 10(-2) angstroms2) hypothetical frameworks. In contrast, the energy of the bulk of the hypothetical frameworks was shown to be a strong function of the tetrahedral distortion. The fact that the framework energies of some hypothetical frameworks lie much higher than both those of the observed frameworks and the values we would expect from our previously developed topological method (the so-called energetic packing penalty) is explained in terms of the tetrahedral distortion contribution to the framework energy which is negligible for the observed frameworks. Finally, it is hypothesized that the absence of tetrahedral distortion is pivotal for a framework to be experimentally realized, in which case a large fraction of hypothetical frameworks are unrealizable and will forever remain in the realms of the abstract.     

Assembly and Post-modification of Fe3O4@MIL-100(Fe) for Knoevenagel Condensation

Many efforts have been devoted to the integration of magnetic nanoparticles and metal organic frameworks, which makes it easy and simple to separate the nano-sized metal organic frameworks from liquid phase. Amino-functionalized magnetic metal organic frameworks[Fe₃O₄@MIL-100(Fe)-NH₂]were prepared by a stepwise assembly method followed by post-modification with electron-rich reagent. This magnetic catalyst was characterized by means of X-ray diffraction(XRD), transmission electron microscopy(TEM), scanning electron microscopy(SEM) and nitrogen adsorption, and tested in Knoevenagel condensation as a base catalyst. The magnetic catalyst exhibits a core-shell structure and can afford a high activity for the Knoevenagel condensation due to its bifunctional property and reduced diffusion limitation. Furthermore, it could be recovered magnetically and recycled three times. Although activity loss was observed in the recycle experiments, it could be reactivated by dispersing in a fresh modifier solution.     

Topology-Templated Synthesis of Crystalline Porous Covalent Organic Frameworks

A strategy is presented for the synthesis of crystalline porous covalent organic frameworks via topology-templated polymerization. The template is based on imine-linked frameworks and their (001) facets seed the C=C bond formation reaction to constitute 2D sp² carbon-conjugated frameworks. This strategy is applicable to templates with different topologies, enables designed synthesis of frameworks that cannot be prepared via direct polymerization, and creates a series of sp² carbon frameworks with tetragonal, hexagonal, and kagome topologies. The sp² carbon frameworks are highly luminescent even in the solid state and exhibit topology-dependent π transmission and exciton migration; these key fundamental π functions are unique to sp² carbon-conjugated frameworks and cannot be accessible by imine-linked frameworks, amorphous analogues, and 1D conjugated polymers. These results demonstrate an unprecedented strategy for structural and functional designs of covalent organic frameworks.     

Enhanced Long Persistent Luminescence by Multifold Interpenetration in Metal–Organic Frameworks

Metal–organic frameworks (MOFs) with long persistent luminescence (LPL) have attracted widespread attention due to potential applications in displays, anticounterfeiting, and so on. However, MOFs often have large pore size, which restricts the formation of efficient inter- and intramolecular interactions to realize LPL. Herein, a new approach to achieving LPL in MOFs by multifold interpenetration of discrete frameworks is reported. By comparison between threefold- and twofold-interpenetrating MOFs, it was found that the former, which have higher multiplicity and denser frameworks, can be endowed with enhanced inter- and intramolecular interactions, and thus enhanced LPL is obtained. Meanwhile, metal-cluster and heavy-halogen effects could also cause variations in LPL duration and color.     

Topology‐Templated Synthesis of Crystalline Porous Covalent Organic Frameworks

A strategy is presented for the synthesis of crystalline porous covalent organic frameworks via topology‐templated polymerization. The template is based on imine‐linked frameworks and their (001) facets seed the C=C bond formation reaction to constitute 2D sp² carbon‐conjugated frameworks. This strategy is applicable to templates with different topologies, enables designed synthesis of frameworks that cannot be prepared via direct polymerization, and creates a series of sp² carbon frameworks with tetragonal, hexagonal, and kagome topologies. The sp² carbon frameworks are highly luminescent even in the solid state and exhibit topology‐dependent π transmission and exciton migration; these key fundamental π functions are unique to sp² carbon‐conjugated frameworks and cannot be accessible by imine‐linked frameworks, amorphous analogues, and 1D conjugated polymers. These results demonstrate an unprecedented strategy for structural and functional designs of covalent organic frameworks.     

Gas Hydrate Frameworks of Allen's Polyhedra. 1. Frameworks on 3–5 Nets

The structures of water molecule frameworks built up of 5¹², 5¹²6², 5¹²6³, and 5¹²6⁴ polyhedra are discussed. In the frameworks of such polyhedra, it is possible to distinguish layers; the centers of the polyhedra belonging to a layer are at the nodes of a planar network consisting of triangular, pentagonal, and hexagonal meshes. The structure of one layer entirely determines the structure of the whole framework. An analysis is given of the structure and topology of frameworks whose layers are constructed on pentagontrigonal nets. It is shown that these frameworks can be constructed from two types of polyhedral blocks closely packed in space.     

Multienzyme Biocatalytic Cascade Systems in Porous Organic Frameworks for Biosensing

Multienzyme biocatalytic cascade systems (MBCS) have attracted widespread research in the field of biosensing due to selective substrate transformations and signal amplification function. However, the poor stability of enzymes significantly restricts their effectiveness in practical applications. The spatial organization of MBCS within porous organic frameworks (POFs), such as metal–organic frameworks, covalent organic frameworks, and hydrogen-bonded organic frameworks, is regarded as a promising strategy to overcome these challenges. This advanced biotechnology sets up a POFs microenvironment for enzymes immobilization, and thus make it possible to shield the enzyme from the external stimulus by POFs-guided structural confinement. Simultaneously, the tailorable porous structure of POFs shell allows for the selective transport of substrates into interior enzymes, thereby accelerating the sensing process. Herein, we present the concept of this POFs-confined MBCS, wherein enzymes were completely encapsulated into, rather than adsorption onto, the POFs. We highlight the new strategies for MBCS spatial organization through rational POFs support, and describe how this new bio-nanosystem that integrates framework and enzymes functions can be designed as a versatile biosensing platform. In addition, the challenges and outlooks are also discussed.     

计算模拟研究金属有机骨架材料吸附分离C2、C3烃类气体

碳氢化合物在工业生产中发挥着重要的作用,其分离纯化过程是工业生产中重要的环节。低碳烃气体的物理化学性质十分相似,仅在分子尺寸和不饱和度等方面有微小差异,分离困难。传统的精馏等分离方式能耗高、有时效率较低。金属有机骨架材料由于其优异的性能(高比表面积、高孔隙率、结构尺寸可控)在吸附分离方面发挥了重要作用。计算模拟方法能够在微观层次上描述吸附分离过程,起到实验无法替代的作用。本文综述了计算模拟用于探索金属有机骨架吸附分离低碳烃的最新研究进展,探讨了其在金属有机骨架吸附分离低碳烃研究中存在的问题,并展望了发展前景。     

SAPO-5分子筛中硅的取代及其性能研究

本文采用三乙胺为模板剂,用水热法合成不同含硅量的SAPO-5分子筛,并用X射线衍射、扫描电镜、电子探针分析、真空重量吸附等方法对其进行了表征,结果表明硅已进入了骨架,硅同晶非等比取代磷和铝,并且分子筛的孔道是畅通的。 用微型反应器测定了AlPO_4-5及不同量硅取代的SAPO-5分子筛上的异丙苯裂解和邻二甲苯异构化催化反应,结果表明Si引入了催化活性很低的磷酸铝骨架后使其催化活性明显增加,进一步证实了硅是同晶非等比取代磷和铝,因而使分子筛骨架带负电性。 硅进入磷酸铝分子筛骨架的SAPO-5分子筛经红外光谱实验发现不仅具有表面羟基,而且都是酸性羟基。吸附吡啶后的红外光谱结果表明SAPO-5上同时存在B酸和L酸中心,B酸中心数目随着硅含量的增加而增加。     

Symmetry-based 29Si dipolar recoupling magic angle spinning NMR spectroscopy: a new method for investigating three-dimensional structures of zeolite frameworks

A new 29Si solid-state MAS NMR experiment is described for investigating the framework structures of pure silica zeolites. The symmetry-based homonuclear dipolar recoupling sequence SR26411 has been incorporated into a two-dimensional NMR experiment to probe the Si-O-Si bonding connectivities and long-range Si-Si distances in zeolite frameworks. This dipolar recoupling sequence is shown to have a number of advantages over the J-coupling-based INADEQUATE experiment. For the clathrasil Sigma-2, it is demonstrated that there is excellent agreement between experimental double-quantum build-up curves obtained from a series of two-dimensional double-quantum correlation spectra and simulated curves which consider all Si-Si distances out to 8 A. This result suggests that this experiment could be used to solve zeolite frameworks with unknown structures.     

Structural principles of semiconducting Group 14 clathrate frameworks

We have performed a comprehensive theoretical investigation of the structural principles of semiconducting clathrate frameworks composed of the Group 14 elements carbon, silicon, germanium, and tin. We have investigated the basic clathrate frameworks, together with their polytypes, intergrowth clathrate frameworks, and extended frameworks based on larger icosahedral building blocks. Quantum chemical calculations with the PBE0 hybrid density functional method provided a clear overview of the structural trends and electronic properties among the various clathrate frameworks. In agreement with previous experimental and theoretical studies, the clathrate II framework proved to be the energetically most favorable, but novel hexagonal polytypes of clathrate II also proved to be energetically very favorable. In the case of silicon, several of the studied clathrate frameworks possess direct and wide band gaps. The band structure diagrams and simulated powder X-ray patterns of the studied frameworks are provided and systematic preliminary evaluation of guest-occupied frameworks is conducted to shed light on the characteristics of novel, experimentally feasible clathrate compositions.     

SAPO-5分子筛中硅的取代及其性能研究

本文采用三乙胺为模板剂,用水热法合成不同含硅量的SAPO-5分子筛,并用X射线衍射、扫描电镜、电子探针分析、真空重量吸附等方法对其进行了表征,结果表明硅已进入了骨架,硅同晶非等比取代磷和铝,并且分子筛的孔道是畅通的。 用微型反应器测定了AlPO₄-5及不同量硅取代的SAPO-5分子筛上的异丙苯裂解和邻二甲苯异构化催化反应,结果表明Si引入了催化活性很低的磷酸铝骨架后使其催化活性明显增加,进一步证实了硅是同晶非等比取代磷和铝,因而使分子筛骨架带负电性。 硅进入磷酸铝分子筛骨架的SAPO-5分子筛经红外光谱实验发现不仅具有表面羟基,而且都是酸性羟基。吸附吡啶后的红外光谱结果表明SAPO-5上同时存在B酸和L酸中心,B酸中心数目随着硅含量的增加而增加。     

CAVEAT: A program to facilitate the design of organic molecules

Summary A frequently encountered problem in the design of enzyme inhibitors and other biologically active molecules is the identification of molecular frameworks to serve as templates or linking units that can position functional groups in specific relative orientations. The program CAVEAT was designed to address this problem by searching 3D databases for such molecular fragments. Key innovations introduced in CAVEAT are a focus on relationships between bonds and the provision of automated methods to identify and classify structural frameworks. Performance has been a particular concern in formulating CAVEAT, since it is intended to be used in an interactive manner. The focus in this report is the design and implementation of the principal algorthms and the performance achieved.CAVEAT is available from the Office of Technology Licensing, University of California, Berkeley, CA, and from Molecular Simulations, Inc., Burlington, MA; further information is available from P.A. Bartlett.     

Flexible host frameworks with diverse cavities in inclusion crystals of bile acids and their derivatives

We have systematically investigated structures and properties of inclusion crystals of bile acids and their derivatives. These steroidal compounds form diverse host frameworks having zero‐, one‐ and two‐dimensional cavities, causing various inclusion behaviors towards many organic compounds. The diverse host frameworks exhibit the following guest‐dependent flexibility. First, the frameworks mainly depend on the included guests in size and shape. The size‐dependence is quantitatively estimated by the parameter PCcavity, which is the volume ratio of a guest molecule to a host cavity. The resulting values of PCcavity lie in the range of 42–76%. Second, each of the host frameworks has its own range of the values. Some guests can employ two different frameworks with the boundary values, explaining formation of polymorphic crystals. Third, the host frameworks are selected by host–guest interactions through weak hydrogen bonds, such as NH/π and CH/O. The weak hydrogen bonds play an important role for various selective inclusion processes. Fourth, the host frameworks are dynamically exchangeable, resulting in intercalation and polymerization in the cavities. These static and dynamic structures of the frameworks demonstrate great potential of crystalline organic inclusion compounds as functional materials. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 124–135; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20171     

Metal‐Helix Frameworks from Short Hybrid Peptide Foldamers

The potential of structured peptides has not been explored much in the design of metal‐organic frameworks (MOFs). This is partly due to the difficulties in obtaining stable secondary structures from the short α‐peptide sequences. Here we report the design, crystal conformations, coordination site dependent different silver coordinated frameworks of short α,γ‐hybrid peptide 12‐helices consisting of terminal pyridyl moieties and the utility of metal‐helix frameworks in the adsorption of CO₂. Upon silver ion coordination the 12‐helix terminated by the 3‐pyridyl derivatives adopted a 2:2 macrocyclic structure, while the 12‐helix terminated by the 4‐pyridyl derivatives displayed remarkable porous metal‐helix frameworks. Both head‐to‐tail intermolecular H‐bonds of the 12‐helix and metal ion coordination have played an important role in stabilizing the ordered metal‐helix frameworks. The studies described here open the door to design a new class of metal‐organic‐frameworks from peptide foldamers.     

Identification of a single light atom within a multinuclear metal cluster using valence-to-core X-ray emission spectroscopy

Iron valence-to-core Fe Kβ X-ray emission spectroscopy (V2C XES) is established as a means to identify light atoms (C, N, O) within complex multimetallic frameworks. The ability to distinguish light atoms, particularly in the presence of heavier atoms, is a well-known limitation of both crystallography and EXAFS. Using the sensitivity of V2C XES to the ionization potential of the bound ligand, energetic shifts of ~10 eV in the ligand 2s ionization energies of bound C, N, and O may be observed. As V2C XES is a high-energy X-ray method, it is readily applicable to samples in any physical form. This method thus has great potential for application to multimetallic inorganic frameworks involved in both small molecule storage and activation.     

Synthesis of open-framework zinc phosphates from organophosphorus amides

The use of hexamethylphosphoramide as a single source of amine and phosphorus has made it possible to synthesize new three-dimensional zinc phosphate frameworks under solvothermal conditions.     

Solvated Graphene Frameworks as High‐Performance Anodes for Lithium‐Ion Batteries

A solvent‐exchange approach for the preparation of solvated graphene frameworks as high‐performance anode materials for lithium‐ion batteries is reported. The mechanically strong graphene frameworks exhibit unique hierarchical solvated porous networks and can be directly used as electrodes with a significantly improved electrochemical performance compared to unsolvated graphene frameworks, including very high reversible capacities, excellent rate capabilities, and superior cycling stabilities.     

Recent applications of metal–organic frameworks in sample pretreatment

Metal–organic frameworks are promising materials in diverse analytical applications especially in sample pretreatment by virtue of their diverse structure topology, tunable pore size, permanent nanoscale porosity, high surface area, and good thermostability. According to hydrostability, metal–organic frameworks are divided into moisture‐sensitive and water‐stable types. In the actual applications, both kinds of metal–organic frameworks are usually engineered into hybrid composites containing magnetite, silicon dioxide, graphene, or directly carbonized to metal–organic frameworks derived carbon. These metal–organic frameworks based materials show good extraction performance to environmental pollutants. This review provides a critical overview of the applications of metal–organic frameworks and their composites in sample pretreatment modes, that is, solid‐phase extraction, magnetic solid‐phase extraction, micro‐solid‐phase extraction, solid‐phase microextraction, and stir bar solid extraction.     

Electronic energy inequalities for isoelectronic molecular systems

Within the Born-Oppenheimer approximation an inequality relation is derived between points of electronic energy hypersurfaces of pairs of isoelectronic molecules. The inequality is directly applicable to point pairs fulfilling a symmetry criterion for the nuclear frameworks and it may be extended to coordinate domains on both hypersurfaces. The result is applied to special examples of conformational problems, dissociation reactions and impurity —vacancy centres in solid clusters.