Emmanuel Klontzas

Reticular Synthesis of HKUST-like tbo-MOFs with Enhanced CH4 Storage

Successful implementation of reticular chemistry using a judiciously designed rigid octatopic carboxylate organic linker allowed the construction of expanded HKUST-1-like tbo-MOF series with intrinsic strong CH4 adsorption sites. The Cu-analogue displayed a concomitant enhancement of the gravimetric and volumetric surface area with the highest reported CH4 uptake among the tbo family, comparable to the best performing metal organic frameworks (MOFs) for CH4 storage. The corresponding gravimetric (BET) and volumetric surface areas of 3971 m(2) g(-1) and 2363 m(2) cm(-3) represent an increase of 115% and 47%, respectively, in comparison to the corresponding values for the prototypical HKUST-1 (tbo-MOF-1), and are 42% and 20% higher than that of tbo-MOF-2. High-pressure methane adsorption isotherms revealed a high total gravimetric and volumetric CH4 uptakes, reaching 372 cm(3) (STP) g(-1) and 221 cm(3) (STP) cm(-3), respectively, at 85 bar and 298 K. The corresponding working capacities between 5 and 80 bar were found to be 294 cm(3) (STP) g(-1) and 175 cm(3) (STP) cm(-3) and are placed among the best performing MOFs for CH4 storage particularly at relatively low temperature. To gain insight on the mechanism accounting for the resultant enhanced CH4 storage capacity, molecular simulation study was performed and revealed the presence of very strong CH4 adsorption sites near the organic linker with similar adsorption energetics as the open metal sites. The present findings support the potential of tbo-MOFs based on the supermolecular building layer (SBL) approach as an ideal platform to further enhance the CH4 storage capacity via expansion and functionalization of the quadrangular pillars.

A “turn-on”–turning-to-ratiometric sensor for zinc(II) ions in aqueous media

A potential fluorescent zinc(II) sensor was synthesized and its spectral profile was studied in aqueous solutions of increasing zinc(II) concentrations. Due to the inherent properties of its chromophore, the sensor exhibits a “turn on” response in solutions of nanomolar zinc(II) concentrations that turns to ratiometric, as zinc(II) ion concentrations reach micro/millimolar levels.

A cyclam-type “turn on” fluorescent sensor selective for mercury ions in aqueous media

The synthesis and spectral profile of a cyclam-type “turn on” fluorescent sensor selective for Hg2+ ions in aqueous media is described. Its properties are compared to those of a second probe with an N-deprotected cyclam system. The vast difference in ion selectivity between the two sensors reveals the influence of functional group modifications on the selectivity of fluorescent ion probes.

Ab Initio Study of the Adsorption of CO2 on Functionalized Benzenes

The interaction of carbon dioxide with a series of functionalized aromatic molecules was studied by using quantum mechanical methods (MP2), to examine the effect of the substituent on the adsorption of CO2 . Several different initial configurations of CO2 were taken into account for each functionalized benzene to locate the energetically most favorable configuration. To get a better estimation of the binding energies, we applied an extrapolation scheme to approach the complete basis set. CH2 N3 -, COOH-, and SO3 H-functionalized benzenes were found to have the strongest interaction with CO2 , and the corresponding binding energies were calculated to be -3.62, -3.65, and -4.3 kcal mol(-1) , respectively. Electrostatic potential maps of the functionalized benzenes and electron redistribution density plots of the complexes were also created to get a better insight into the nature of the interaction of CO2 with the functionalized benzenes. The functional groups that were examined can be potentially incorporated in organic bridging molecules that connect the inorganic corners in MOF.

Multiscale simulations reveal IRMOF-74-III as a potent drug carrier for gemcitabine delivery

A multiscale computational study is reported that investigates the microscopic behavior of the anti-cancer drug gemcitabine (GEM) stored in metal organic frameworks IRMOF-74-III and the functionalized OH-IRMOF-74-III. Accurate Quantum Mechanics calculations indicate that the GEM-MOF interaction energy in both host structures is suitable for drug adsorption and delivery with a slow release. Based on Grand-Canonical Monte Carlo simulations, the predicted maximum loading of GEM is three-fold greater than in lipid-coated mesoporous silica nanoparticles and similar to liposome nanocarriers. Finally, Molecular Dynamics simulations reveal slow diffusion of GEM inside the pores of both hosts, which is crucial for the controlled release of GEM. This work unravels the energetics and dynamics of GEM in MOFs and highlights the ability of the biocompatible (OH)-IRMOF-74-III to be used as a promising nano encapsulator for GEM delivery.

Chemically intuited, large-scale screening of MOFs by machine learning techniques

A novel computational methodology for large-scale screening of MOFs is applied to gas storage with the use of machine learning technologies. This approach is a promising trade-off between the accuracy of ab initio methods and the speed of classical approaches, strategically combined with chemical intuition. The results demonstrate that the chemical properties of MOFs are indeed predictable (stochastically, not deterministically) using machine learning methods and automated analysis protocols, with the accuracy of predictions increasing with sample size. Our initial results indicate that this methodology is promising to apply not only to gas storage in MOFs but in many other material science projects.Machine learning: Quickly screening materials for effective gas storageThe gas storage properties of metal-organic frameworks can now be quickly and accurately predicted by artificial intelligence. George Froudakis at the University of Crete has developed a machine learning approach to predict the H2/CO2 adsorption properties of metal-organic frameworks (MOFs), highly porous materials promising for catalysis and gas storage, based on their chemical structure. Previous methods were either too slow, or not accurate enough. Here, Froudakis and his team encoded ‘chemical intuition’ into their algorithm by training it to recognize certain structural features in MOFs with known properties. Then, when they applied the method to large-scale screening tests of new MOFs they found their predictions matched with experimental data. With this technique, it is hoped that new materials for CO2 sequestration or hydrogen storage will be discovered more quickly.

Directed assembly of a high surface area 2D metal–organic framework displaying the augmented “kagomé dual” (kgd-a) layered topology with high H2 and CO2 uptake

The rigid and highly aromatic hexatopic, carboxylate-based organic linker H6L, under solvothermal reaction conditions with CoCl2·6H2O, directs the assembly of two new 2D MOFs (denoted as 1 and 2) with the rare kgd-a layered topology, as revealed by single crystal X-ray diffraction measurements. A unique 3-connected dinuclear cluster, Co2(–COO)3Cl, was observed in 1 and this MOF was found to be stable upon solvent removal, in contrast to 2, where single Co2+ cations serve as 3-c nodes and the structure collapses upon activation. A detailed Ar sorption measurement at 87 K revealed that 1 has an apparent BET surface area of 1299 m2 g−1 with narrow pore size distribution, centered at 6.9 A. A very high H2 and CO2 uptake is observed reaching 209.9 cm3 (STP) g−1 and 106.8 cm3 (STP) g−1 at 77 K/1 bar and 273 K/1 bar, respectively, which is attributed to the combination of small pore size and the high density of aromatic rings in 1.

Antimony speciation in spirits stored in PET bottles: identification of a novel antimony complex

Total antimony and its +V and +III oxidation state species were determined in twelve spirit samples (Greek raki and tsipouro) stored in polyethylene terephthalate bottles. Reliable and reproducible results were obtained following direct analysis by using ICP-MS providing total Sb concentrations between 0.4–4 μg L−1. Antimony speciation analysis by LC-ICP-MS was also assessed, showing the presence of both inorganic Sb species along with an unknown Sb complex, which was the predominant species in all samples analysed. The structure of this complex was investigated by using liquid chromatography with high-resolution tandem mass spectrometry. The analysis gave evidence for an acetaldehyde–bisulphite pyruvate Sb complex with the formula: C7H14O12S2Sb. The proposed ligands are organic substances expected to be present in the raki matrix. In addition, the influence of high temperature storage conditions and extended exposure times up to two weeks, on Sb migration from PET bottles into raki samples was investigated. Total Sb and Sb species content was determined by ICP-MS and LC-ICP-MS, respectively. The concentrations determined were in the range of 5.6 to 28 μg Sb per L spirit after a week of storage at 60 °C. In which case, inorganic Sb(V) and Sb(III) became the predominant species in comparison to the “novel” organic Sb complex.

Author Correction: Chemically intuited, large-scale screening of MOFs by machine learning techniques

A correction to this article has been published and is linked from the HTML version of this article.

Reticular Chemistry and the Discovery of a New Family of Rare Earth (4, 8)-Connected Metal-Organic Frameworks with csq Topology Based on RE4(μ3-O)2(COO)8 Clusters

In recent years, the design and discovery of new metal-organic framework (MOF) platforms with distinct structural features and tunable chemical composition has remarkably enhanced by applying reticular chemistry rules and the molecular building block (MBB) approach. We targeted the synthesis of new rare earth (RE)-MOF platforms based on a rectangular-shaped 4-c linker, acting as a rigid organic MBB. Accordingly, we designed and synthesized the organic ligand 1,2,4,5-tetrakis(4-carboxyphenyl)-3,6-dimethyl-benzene (H4L), in which the two methyl groups attached to the central phenyl ring lock the four peripheral carboxyphenyl groups to an orthogonal/vertical position. We report here a new family of RE-MOFs featuring the novel inorganic building unit, RE4(μ3-O)2 (RE: Y3+, Tb3+, Dy3+, Ho3+, Er3+, and Yb3+), with planar D2h symmetry. The rigid 4-c linker, H4L, directs the in situ assembly of the unique 8-c RE4(μ3-O)2(COO)8 cluster, resulting in the formation of the first (4, 8)-c RE-MOFs with csq topology, RE-csq-MOF-1. The structures of the yttrium (Y-csq-MOF-1) and holmium (Ho-csq-MOF-1) analogues were determined by single-crystal X-ray diffraction analysis. Y-csq-MOF-1 was successfully activated and tested for Xe/Kr separation. The results show that Y-csq-MOF-1 has high isosteric heat of adsorption for Xe (33.8 kJ mol-1), with high Xe/Kr selectivity (IAST 12.1, Henry 12.9) and good Xe uptake (1.94 mmol g-1 at 298 K and 1 bar), placing this MOF among the top-performing adsorbents for Xe/Kr separation.