Igor L. Moudrakovski

Tuning clathrate hydrates for hydrogen storage

The storage of large quantities of hydrogen at safe pressures is a key factor in establishing a hydrogen-based economy. Previous strategies—where hydrogen has been bound chemically, adsorbed in materials with permanent void space or stored in hybrid materials that combine these elements—have problems arising from either technical considerations or materials cost. A recently reported clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (∼ 2 kbar) required to produce the material make it impractical. The synthesis pressure can be decreased by filling the larger cavity with tetrahydrofuran (THF) to stabilize the material, but the potential storage capacity of the material is compromised with this approach. Here we report that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to ∼4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability. The tuning mechanism is quite general and convenient, using water-soluble hydrate promoters and various small gaseous guests.

Anhydrous proton conduction at 150 °C in a crystalline metal–organic framework

Metal organic frameworks (MOFs) are particularly exciting materials that couple porosity, diversity and crystallinity. But although they have been investigated for a wide range of applications, MOF chemistry focuses almost exclusively on properties intrinsic to the empty frameworks; the use of guest molecules to control functions has been essentially unexamined. Here we report Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. Proton conduction in β-PCMOF2 was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores and reached 5 × 10(-4) S cm(-1) at 150 °C in anhydrous H(2), as confirmed by electrical measurements in H(2) and D(2), and by solid-state NMR spectroscopy. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)(0.45)) was also incorporated into a H(2)/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.

Facile Proton Conduction via Ordered Water Molecules in a Phosphonate Metal−Organic Framework

A new phosphonate metal-organic framework (MOF) with a layered motif but not that of the classical hybrid inorganic-organic solid is presented. Zn(3)(L)(H(2)O)(2)·2H(2)O (L = [1,3,5-benzenetriphosphonate](6-)), henceforth denoted as PCMOF-3, contains a polar interlayer lined with Zn-ligated water molecules and phosphonate oxygen atoms. These groups serve to anchor free water molecules into ordered chains, as observed by X-ray crystallography. The potential for proton conduction via the well-defined interlayer was studied by (2)H solid-state NMR spectroscopy and AC impedance spectroscopy. The proton conductivity in H(2) was measured as 3.5 × 10(-5) S cm(-1) at 25 °C and 98% relative humidity. More interestingly, an Arrhenius plot gave a low activation energy of 0.17 eV for proton transfer, corroborating the solid-state NMR data that showed exchange between all deuterium sites in the D(2)O analogue of PCMOF-3, even at -20 °C.

Complex gas hydrate from the Cascadia margin

Natural gas hydrates are a potential source of energy and may play a role in climate change and geological hazards. Most natural gas hydrate appears to be in the form of ‘structure I’, with methane as the trapped guest molecule, although ‘structure II’ hydrate has also been identified, with guest molecules such as isobutane and propane, as well as lighter hydrocarbons. A third hydrate structure, ‘structure H’, which is capable of trapping larger guest molecules, has been produced in the laboratory, but it has not been confirmed that it occurs in the natural environment. Here we characterize the structure, gas content and composition, and distribution of guest molecules in a complex natural hydrate sample recovered from Barkley canyon, on the northern Cascadia margin. We show that the sample contains structure H hydrate, and thus provides direct evidence for the natural occurrence of this hydrate structure. The structure H hydrate is intimately associated with structure II hydrate, and the two structures contain more than 13 different hydrocarbon guest molecules. We also demonstrate that the stability field of the complex gas hydrate lies between those of structure II and structure H hydrates, indicating that this form of hydrate is more stable than structure I and may thus potentially be found in a wider pressure–temperature regime than can methane hydrate deposits.

Direct Space Methods for Powder X-ray Diffraction for Guest-Host Materials: Applications to Cage Occupancies and Guest Distributions in Clathrate Hydrates

Structural determination of crystalline powders, especially those of complex materials, is not a trivial task. For non-stoichiometric guest-host materials, the difficulty lies in how to determine dynamical disorder and partial cage occupancies of the guest molecules without other supporting information or constraints. Here, we show how direct space methods combined with Rietveld analysis can be applied to a class of host-guest materials, in this case the clathrate hydrates. We report crystal structures in the three important hydrate crystal classes, sI, sII, and sH, for the guests CO(2), C(2)H(6), C(3)H(8), and methylcyclohexane + CH(4). The results obtained for powder samples are found to be in good agreement with the experimental data from single crystal X-ray diffraction and (13)C solid-state NMR spectroscopy. This method is also used to determine the guest disorder and cage occupancies of neohexane and tert-butyl methyl ether binary hydrates with CH(4) in the structure H clathrate hydrates. The results are found to be in good agreement with the results from the (13)C solid-state NMR and molecular dynamics simulations. It is demonstrated that the ab initio crystal structure determination methodology reported here is able to determine absolute cage occupancies and the dynamical disorder of guest molecules in clathrate hydrates from powdered crystalline samples.

CdS magic-sized nanocrystals exhibiting bright band gap photoemission via thermodynamically driven formation.

CdS magic-sized nanocrystals (MSNs) exhibiting both band gap absorption and emission at 378 nm with a narrow bandwidth of approximately 9 nm and quantum yield (QY) of approximately 10% (total QY approximately 28%, in hexane) were synthesized via a one-pot noninjection approach. This CdS MSN ensemble is termed as Family 378. It has been acknowledged that magic-sized quantum dots (MSQDs) are single-sized, and only homogeneous broadening contributes to their bandwidth. The synthetic approach developed is ready and highly reproducible. The formation of the CdS MSQDs was carried out at elevated temperatures (such as 90-140 degrees C) for a few hours in a reaction flask containing bis(trimethylsilyl)sulfide ((TMS)(2)S) and Cd(OAc)(OA) in situ made from cadmium acetate dihydrate (Cd(OAc)(2).2H(2)O) and oleic acid (OA) in 1-octadecene (ODE). Low OA/Cd and high Cd/S feed molar ratios favor this formation, whose mechanism is proposed to be thermodynamically driven. (13)C solid-state cross-polarization magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) demonstrates that the capping ligands are firmly attached to the nanocrystal surface via carboxylate groups. With the cross-polarization from (1)H of the alkyl chains to surface (113)Cd, (113)Cd NMR is able to distinguish the surface Cd (471 ppm) bonding to both -COO(-) and S and the bulk Cd (792 ppm) bonding to S only. DOSY-NMR was used to determine the size of Family 378 ( approximately 1.9 nm). The present study provides strategies for the rational design of various MSNs.

A Channel‐Free Soft‐Walled Capsular Calixarene Solid for Gas Adsorption

The use of molecular recognition for gas storage and separation is of great interest in applied chemistry, since this can lead to new applications in energy conservation and controlled consumption. Metal–organic frameworks, covalent organic frameworks, nanoporous organic polymers, as well as dipeptide aggregates with nanochannels, have received considerable attention in regard to gas adsorption. Unlike nanochannel structures, void spaces in van der Waals assemblies based on calixarenes are formed naturally by the flexible substituents attached to core phenol units with restricted mobility. The simple para-tert-butylcalix[4]arene showed promising results both for molecular recognition and gas adsorption. However, the small size of the cavity as well as relatively easy transitions between lowand highdensity polymorphs prompted many research groups to search for other calixarene-based structures. Keeping a reasonable balance between the rigid part of the calixarene molecule and the length of the flexible chains in paraacylcalix[4]arenes has made it possible to isolate and characterize crystalline van der Waals nanocapsular forms of parahexanoyl (C6OH) and para-octanoyl-calix[4]arenes (C8OH, Figure 1). In both cases there is an absence of porosity and hence no nanochannels in which potential guest molecules may flow in and out of the material. Unlike C6OH, the C8OH nanocapsular form is stable up to the melting point of the material (ca. 183 8C) and does not need the presence of stabilizing guest molecules. The role of guest is played here by the acyl arms of the host calixarene molecule. The difference in the crystallographic density of the nanocapsular form of C8OH with the corresponding guest-free, dense structure of the same compound (1.140 versus 1.200, respectively) clearly indicates the availability of the calixarene pocket for the inclusion of small molecules. We therefore decided to study the general availability of void space in C8OH capsules for the adsorption of gas molecules, the influence of the temperature on the ability to entrap gases by these nanocontainers, and the possible selectivity of the adsorption. Figure 2 shows adsorption isotherms measured volumetrically at room temperature for linear alkanes C1–C4, ethylene, as well as nitrogen, oxygen (measured up to 4 bar only for safety reasons), and carbon dioxide in C8OH. The same isotherms for subcritical gases at normalized pressure (p/ps) are shown in the Supporting Information. The affinity of the Figure 1. para-Octanoylcalix[4]arene, its crystalline lattice, and capsular structure.

Exploiting Noncovalent Interactions in an Imine-Based Covalent Organic Framework for Quercetin Delivery

Covalent organic frameworks (COFs) are a new class of nanoporous polymeric vector showing promise as drug-delivery vehicles with high loading capacity and biocompatibility. The interaction between the carrier and the cargo is specifically tailored on a molecular level by H-bonding. Cell-proliferation studies indicate higher efficacy of the drug in cancer cells by nanocarrier delivery mediated by the COF.

Molecularly imprinted mesoporous organosilica.

We have prepared molecularly imprinted mesoporous organosilica (MIMO) using a semicovalent imprinting technique. A thermally reversible covalent bond was used to link a bisphenol A (BPA) imprint molecule to a functional alkoxysilane monomer at two points to generate a covalently bound imprint precursor. This precursor was incorporated into a cross-linked periodic mesoporous silica matrix via a typical acid-catalyzed, triblock copolymer-templated, sol-gel synthesis. Evidence of imprint sites buried in the pore walls was found through careful characterization of the imprinted material and its comparison to similarly prepared non-imprinted mesoporous organosilica (NIMO) and pure periodic mesoporous silica (PMS). After thermal treatment, the imprinted material (MIMO-ir) removed more than 90% of appropriately sized bisphenol species from water, yet showed significantly lower binding for both smaller and larger molecules containing phenol moieties. Identically treated NIMO-ir showed much poorer retention behavior than MIMO-ir for the same bisphenol species and behaved only slightly better than PMS-ir.

Methanol incorporation in clathrate hydrates and the implications for oil and gas pipeline flow assurance and icy planetary bodies

One of the best-known uses of methanol is as antifreeze. Methanol is used in large quantities in industrial applications to prevent methane clathrate hydrate blockages from forming in oil and gas pipelines. Methanol is also assigned a major role as antifreeze in giving icy planetary bodies (e.g., Titan) a liquid subsurface ocean and/or an atmosphere containing significant quantities of methane. In this work, we reveal a previously unverified role for methanol as a guest in clathrate hydrate cages. X-ray diffraction (XRD) and NMR experiments showed that at temperatures near 273 K, methanol is incorporated in the hydrate lattice along with other guest molecules. The amount of included methanol depends on the preparative method used. For instance, single-crystal XRD shows that at low temperatures, the methanol molecules are hydrogen-bonded in 4.4% of the small cages of tetrahydrofuran cubic structure II hydrate. At higher temperatures, NMR spectroscopy reveals a number of methanol species incorporated in hydrocarbon hydrate lattices. At temperatures characteristic of icy planetary bodies, vapor deposits of methanol, water, and methane or xenon show that the presence of methanol accelerates hydrate formation on annealing and that there is unusually complex phase behavior as revealed by powder XRD and NMR spectroscopy. The presence of cubic structure I hydrate was confirmed and a unique hydrate phase was postulated to account for the data. Molecular dynamics calculations confirmed the possibility of methanol incorporation into the hydrate lattice and show that methanol can favorably replace a number of methane guests.