Comprehensive study of the macropore and mesopore size distributions in polymer monoliths using complementary physical characterization techniques and liquid chromatography.

Poly(styrene-co-divinylbenzene) monolithic stationary phases with two different domain sizes were synthesized by a thermally initiated free-radical copolymerization in capillary columns. The morphology was investigated at the meso- and macroscopic level using complementary physical characterization techniques aiming at better understanding the effect of column structure on separation performance. Varying the porogenic solvent ratio yielded materials with a mode pore size of 200 nm and 1.5 µm, respectively. Subsequently, nano-liquid chromatography experiments were performed on 200 µm id × 200 mm columns using unretained markers, linking structure inhomogeneity to eddy dispersion. Although small-domain-size monoliths feature a relatively narrow macropore-size distribution, their homogeneity is compromised by the presence of a small number of large macropores, which induces a significant eddy-dispersion contribution to band broadening. The small-domain size monolith also has a relatively steep mass-transfer term, compared to a monolith containing larger globules and macropores. Structural inhomogeneity was also studied at the mesoscopic level using gas-adsorption techniques combined with the non-local-density-function-theory. This model allows to accurately determine the mesopore properties in the dry state. The styrene-based monolith with small domain size has a distinctive trimodal mesopore distribution with pores of 5, 15, and 25 nm, whereas the monolith with larger feature sizes only contains mesopores around 5 nm in size.

Highly selective water adsorption in a lanthanum metal-organic framework.

We present a new metal-organic framework (MOF) built from lanthanum and pyrazine-2,5-dicarboxylate (pyzdc) ions. This MOF, [La(pyzdc)1.5(H2O)2]⋅2 H2O, is microporous, with 1D channels that easily accommodate water molecules. Its framework is highly robust to dehydration/hydration cycles. Unusually for a MOF, it also features a high hydrothermal stability. This makes it an ideal candidate for air drying as well as for separating water/alcohol mixtures. The ability of the activated MOF to adsorb water selectively was evaluated by means of thermogravimetric analysis, powder and single-crystal X-ray diffraction and adsorption studies, indicating a maximum uptake of 1.2 mmol g(-1) MOF. These results are in agreement with the microporous structure, which permits only water molecules to enter the channels (alcohols, including methanol, are simply too large). Transient breakthrough simulations using water/methanol mixtures confirm that such mixtures can be separated cleanly using this new MOF.

Predicting adsorption on metals: simple yet effective descriptors for surface catalysis.

We present a simple and efficient model for predicting the adsorption of molecules on metal surfaces. This heuristic model uses six descriptors for each metal (number of d-electrons, surface energy, first ionization potential and atomic radius, volume and mass) and three for each adsorptive (HOMO-LUMO energy gap, molecular volume and mass). Strikingly, despite its simplicity and low computational cost, this model predicts well the chemisorption of a range of adsorptives (H2, HO˙, N2, CO, NO, O2, H2O, CO2, NH3 and CH4) on a range of metals (Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, W, Ir, Pt and Au) as calculated with DFT and taken from the literature. Using only a third of the data for fitting, the rest of the data were predicted with Q(2) = 0.91-0.95 and RMSEP = 0.94-1.16 eV. Furthermore, we measured experimental adsorption data for CO, CO2, CH4, H2, N2 and O2 on Ni, Pt and Rh supported on TiO2. Using the same descriptors, we then constructed a model for this experimental data set. Once again, the model explained the data well, with R(2) = 0.95 and Q(2) = 0.86, respectively.

Sieving di-branched from mono-branched and linear alkanes using ZIF-8: experimental proof and theoretical explanation.

We study the adsorption equilibrium isotherms and differential heats of adsorption of hexane isomers on the zeolitic imidazolate framework ZIF-8. The studies are carried out at 373 K using a manometric set-up combined with a micro-calorimeter. We see that the Langmuir model describes well the isotherms for all four isomers (n-hexane, 2-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane). The linear and mono-branched isomers adsorb well, but 2,2-dimethylbutane is totally excluded. Plotting the differential heat of adsorption against the loading shows an initial plateau for n-hexane and 2-methylpentane. This is followed by a slow rise, indicating adsorbate-adsorbate interactions. For the di-branched isomers the differential heat of adsorption decreases with loading. To gain further insight, we ran molecular simulations using the grand-canonical Monte Carlo approach. Comparing the simulation and the experimental results shows that the ZIF framework model requires blocking of the cages, since 2,2-dimethylbutane cannot fit through the sodalite-type windows. Practically speaking, this means that ZIF-8 is a highly promising candidate for enhancing gasoline octane numbers at 373 K, as it can separate 2,2-dimethylbutane and 2,3-dimethylbutane from 2-methylpentane. Our results prove the potential of ZIF-8 as a new adsorbent that can be employed in the upgrade of the Total Isomerization Process for the production of high octane number gasoline, by blending di-branched alkanes in the gasoline.

Poly(vinylidene fluoride)/nickel nanocomposites from semicrystalline block copolymer precursors.

The fabrication of nanoporous poly(vinylidene fluoride) (PVDF) and PVDF/nickel nanocomposites from semicrystalline block copolymer precursors is reported. Polystyrene-block-poly(vinylidene fluoride)-block-polystyrene (PS-b-PVDF-b-PS) is prepared through functional benzoyl peroxide initiated polymerization of VDF, followed by atom transfer radical polymerization (ATRP) of styrene. The crystallization of PVDF plays a dominant role in the formation of the block copolymer structure, resulting in a spherulitic superstructure with an internal crystalline-amorphous lamellar nanostructure. The block copolymer promotes the formation of the ferroelectric ß-polymorph of PVDF. Selective etching of the amorphous regions with nitric acid leads to nanoporous PVDF, which functions as a template for the generation of PVDF/Ni nanocomposites. The lamellar nanostructure and the ß-crystalline phase are conserved during the etching procedure and electroless nickel deposition.