Patrick E. Fuller

Nanoparticle Core/Shell Architectures within MOF Crystals Synthesized by Reaction Diffusion

Metal–organic frameworks (MOFs) have attracted considerable attention on account of their applications in molecular separations, gas storage, catalysis, and chemical sensing. Recently, there has been a growing interest in using these highly ordered microporous architectures as host matrices or templates with incorporated metal or metal oxide nanoclusters or nanoparticles (NPs). Such hybrid NPMOF structures are promising materials for gas storage and catalysis. The strategies available for the incorporation of metal or metal oxide NPs into MOFs include solvent-free gasphase loading, solution impregnation, incipient wetness impregnation, solid grinding, and microwave irradiation. Most of these techniques, however, involve relatively cumbersome processes such as pretreating the MOF (e.g., by solvent exchange or MOF activation) or particle loading (by introduction of reducing agents, heating, irradiation, etc.), and none allows any spatial control over the NP loading within the MOF crystals. Here, we describe a procedure in which reaction-diffusion processes inside the MOF crystals mediate the deposition of NPs either in a uniform or in a location-specific fashion, with the latter leading to the formation of core/shell architectures (which are of interest in the context of multistep catalysis). In our method, cyclodextrin-based MOF (CD-MOF) crystals are immersed in a metal (here, Ag or Au) salt solution, and the OH counterions—which are homogenously distributed in the CD-MOF at concentrations of about 1.33m—reduce this salt to the respective metal NPs. By coupling the diffusion of salt precursors with their reduction inside the CD-MOFs it is possible to deposit the NPs only at the core of the MOF crystal and such that the thickness of the NP-free shell depends on the concentration of the HAuCl4 used. Subsequent deposition of another type of NPs gives rise to core/ shell architectures. NPs of all types can be readily liberated by dissolving the CD-MOFs in water—for the core/shell NP/CDMOFs, the release of the two different types of NPs is then sequential. Two types of millimeter-sized CD-MOFs were used. The first type was synthesized from g-cyclodextrin (g-CD) and RbOH following the reported procedures (see Experimental Section for details). As illustrated in Figure 1a–c, these Rb-CD-MOF single crystals (for powder X-ray diffraction (PXRD) spectra, see Section 1 in the Supporting Information) were rectangular prisms up to about 2 2 1 mm in size with nanosized cavities (ca. 1.7 nm across) and 1D channels connecting them (channel cross-section: ca. 8 8 ). The second type of MOFwas also made from g-CD, but CsOH was used as the alkali metal source. These Cs-CDMOF crystals also comprised cavities of approximately 1.7 nm in diameter connected by channels with cross-sections of about 8 8 2 (Figure 1a,b). Following the procedure described in the Experimental Section, Cs-CD-MOF single crystals were grown that had an overall truncated-octahedron shape and the diameters of these crystals were up to 5 mm (Figure 1d, see also the PXRD spectra in Section 2 of the Supporting Information). In the context of the present work, the key feature of the CD-MOFs is that they contain hydroxide counterions (one per metal center) which can work either alone or cooperatively with the cyclodextin units to reduce metal salt Figure 1. a) A unit cell of Rbor Cs-CD-MOF crystals synthesized from g-CD and RbOH or CsOH, respectively (red: oxygen; gray: carbon; purple: Rb or Cs). b) The 1D channels in Rbor Cs-CD-MOF crystals. c) and d) The optical images of the millimeter-sized Rb-CD-MOF and Cs-CD-MOF crystals, respectively.

Parallel Optimization of Synthetic Pathways within the Network of Organic Chemistry

The entire chemical-synthetic knowledge created since the days of Lavoisier to the present can be represented as a complex network (Figure 1a) comprising millions of compounds and reactions. While it is simply beyond cognition of any individual human to understand and analyze all this collective chemical knowledge, modern computers have become powerful enough to perform suitable network analyses within reasonable timescales. In this context, a problem that is both fundamentally interesting and practically important is the identification of optimal synthetic pathways leading to desired, known molecules from commercially available substrates. In either manual searches or semiautomated search tools, such as Reaxys, this procedure is done by back-tracking the possible syntheses step-by-step. Such “manual” methods, however, give virtually no chance of finding an optimal pathway, as the number of possible syntheses to consider is very large (for example, ca. 10 within five steps). Moreover, the problem becomes dramatically more complex when one aims to optimize the syntheses of multiple substances simultaneously when, for example, a company producing N products would strive to design synthetic pathways sharing many common substrates/intermediates and minimizing the overall synthetic cost (Figure 1a). As we show herein, however, judicious combination of combinatorial optimization with network search algorithms allows the parallel optimization of tens to thousands of syntheses. The algorithms we describe traverse the network of organic chemistry (henceforth, NOC or simply the network) probing different synthetic paths according to the cost criterion as defined by a combination of labor cost and the cost of staring materials. In a specific case study, we show that our optimization can reduce the cost of an existing synthetic company (here, ProChimia Surfaces) by almost 50%. Overall, this communication is the first instance in which synthetic optimizations are based on the entire body of synthetic knowledge as stored in the NOC and combined with economical descriptors (that is, prices). While each of the individual reactions in the NOC is known, the network search algorithms create new chemical knowledge in the form of near optimal reaction sequences; notably, the syntheses that are optimal for making any molecule individually can be different from those optimizing the synthesis of this and other molecules simultaneously. Our analyses are based on a network of about 7 million reactions and about 7 million substances derived as described in the first communication in this series (also see Refs. [1, 2]). While in our earlier analyses of NOC, the simple dot–arrow representation was typically sufficient, the analysis of specific syntheses involving multiple substrates and/or products requires the so-called bipartite-graph representation with two types of nodes: those corresponding to specific substances (blue dots in Figure 1b), and those representing the reactions (black dots in Figure 1b). This representation of the NOC captures the causal synthetic dependencies and accounts for the fact that a viable synthesis (see the Supporting Information, Section 2) cannot proceed without all of the necessary reactants, which must either be synthesized by another suitable reaction or purchased. Also, as our network searches are intended to compare the actual costs of syntheses, we have linked the NOC to a test Figure 1. The network of organic chemistry and its bipartite wiring plan. a) Small fraction of the network (ca. 0.025%) centered on six target compounds (red). Computational methods described herein allow for the identification of near optimal synthesis plans (inset) despite the size and complexity of the network. b) Illustration of the mapping from a list of chemical reactions to a directed, bipartite network.

Metal–Organic Frameworks for Oxygen Storage

We present a systematic study of metal-organic frameworks (MOFs) for the storage of oxygen. The study starts with grand canonical Monte Carlo simulations on a suite of 10,000 MOFs for the adsorption of oxygen. From these data, the MOFs were down selected to the prime candidates of HKUST-1 (Cu-BTC) and NU-125, both with coordinatively unsaturated Cu sites. Oxygen isotherms up to 30 bar were measured at multiple temperatures to determine the isosteric heat of adsorption for oxygen on each MOF by fitting to a Toth isotherm model. High pressure (up to 140 bar) oxygen isotherms were measured for HKUST-1 and NU-125 to determine the working capacity of each MOF. Compared to the zeolite NaX and Norit activated carbon, NU-125 has an increased excess capacity for oxygen of 237% and 98%, respectively. These materials could ultimately prove useful for oxygen storage in medical, military, and aerospace applications.

Transport into Metal-Organic Frameworks from Solution Is Not Purely Diffusive**

Chemistry in motion: a combination of confocal microscopy and reaction-diffusion modeling provided a powerful toolkit with which solution transport into metal-organic framework crystals was studied. Commonly used pure diffusion models are insufficient to describe this process and, instead, it is necessary to account for the interactions of the guest molecules and the MOF scaffold.

Chemical Network Algorithms for the Risk Assessment and Management of Chemical Threats

A network of chemical threats: Current regulatory protocols are insufficient to monitor and block many short-route syntheses of chemical weapons, including those that start from household products. Network searches combined with game-theory algorithms provide an effective means of identifying and eliminating chemical threats. Language: en

Utilization of Metal-Organic Frameworks for the Management of Gases Used in Ion Implantation

Metal-Organic Frameworks (MOFs) are porous extended crystalline structures comprised of organic ligands and metal units. By changing the identity of the organic ligand and metal unit utilized in the MOF synthesis, the structure, surface area, pore size, and reactivity of the MOF can be modulated. This structural flexibility means that MOFs can potentially be used in a wide range of storage, separation, and catalytic applications. There is a high level of interest in the storage, delivery, capture, and purification of ultra high-purity hazardous gases used in electronics manufacturing (electronic gases). This paper will discuss the use of MOFs as an ideal platform for product and process innovation in the electronic gas sector.