Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent

The role of the concentration and the nature of aluminium in the creation of hierarchical porosity in both commercial and synthesized MFI zeolites have been investigated through controlled mesoporosity development by desilication in alkaline medium. Framework aluminium controls the process of framework silicon extraction and makes desilication selective towards intracrystalline mesopore formation. An optimal molar Si/Al ratio in the range 25-50 has been identified; this leads to an optimal mesoporosity centred around 10 nm and mesopore surface areas of up to 235 m(2) g(-1) while preserving the intrinsic crystalline and acidic properties. At lower framework Si/Al ratios the relatively high Al content inhibits Si extraction and hardly any mesopores are created, while in highly siliceous ZSM-5 unselective extraction of framework Si induces formation of large pores. The existence of framework Al sites in different T positions that are more or less susceptible to the alkaline treatment, and the occurrence of re-alumination, are tentative explanations for the remarkable behaviour of Al in the desilication process. The presence of substantial extra framework Al, obtained by steam treatment, inhibits Si extraction and related mesopore formation; this is attributed to re-alumination of the extraframework Al species during the alkaline treatment. Removal of extraframework Al species by mild oxalic acid treatment restores susceptibility to desilication, which is accompanied by formation of larger mesopores due to the enhanced Si/Al ratio in the acid-treated zeolite.     

Aldol condensations over reconstructed Mg-Al hydrotalcites: structure-activity relationships related to the rehydration method

Two different rehydration procedures in the liquid or gas phase have been applied to reconstruct mixed oxides derived from calcined hydrotalcite-like materials to be used as catalysts for aldol condensation reactions. The as-synthesized hydrotalcite, its decomposition product, as well as the reconstructed solids upon rehydration were characterized by XRD, N(2) adsorption, He pycnometry, FTIR, SEM, TEM, (27)Al MAS-NMR and CO(2)-TPD (TPD=temperature-programmed desorption). Compared to the Mg-Al mixed oxide rehydrated in the gas phase (HT-rg), that rehydrated in the liquid phase (HT-rl) exhibits a superior catalytic performance with respect to the aldol condensation of citral with ketones to yield pseudoionones and in the self-aldolization of acetone. The textural properties of HT-rl and HT-rg differ strongly and determine the catalytic behavior. A memory effect led to a higher degree of reconstruction of the lamellar structure when the mixed oxide was rehydrated in the gas phase rather than in the liquid phase, although liquid-phase rehydration under fast stirring produced a surface area that was 26 times greater. This contrasts to typical statements in the literature claiming a higher degree of reconstruction in the presence of large amounts of water in the medium. CO(2)-TPD shows that the number of OH(-) groups and their nature are very similar in HT-rg and HT-rl, and cannot explain the markedly different catalytic behavior. Accordingly, only a small fraction of the available basic sites in the rehydrated samples is active in liquid-phase aldol condensations. Our results support the model in which only basic sites near the edges of the hydrotalcite platelets are partaking in aldol reactions. Based on this, reconstructed materials with small crystallites (produced by exfoliation during mechanical stirring), that is, possessing a high external surface area, are beneficial in the reactions compared to larger crystals with a high degree of intraplatelet porosity.     

Ion trap mass spectrometry of trimethylsilylamides following gas chromatography

Fatty acid amides are a class of compounds with newly discovered biological activity. The ion trap mass spectrometric characteristics of silylated fatty acid amides were examined. Silylation of primary fatty acid amides is required prior to gas chromatography owing to thermal instability of the underivatized compound. The trimethylsilylated amides do not yield a molecular ion under normal electron ionization conditions (70 eV). With methane as a chemical ionization gas, the [M+H]+ ion appears. The [M+H]+ ion also appears when the helium buffer gas pressure is increased in the ion trap. There are three fragments other than the [M+H]+ peak that are predominant in the ion trap mass spectra of these compounds. Two of the fragments have been reported previously, namely the m/z 59 and the [M-71]+ fragments. The fragment of m/z 72 was identified and is the result of a rearrangement. Isotopic labeling was used to confirm fragment identity and the composition of the rearrangement products. Fragmentation patterns were affected by the amide chain length and concentration.     

Ethers as ligands. Part VI. Transition metal(II) complexes with 18-crown-6

Summary Seven new coordination compounds are reported with the cyclicpolyether 18-crown-6 as the ligand,viz. [Mg(18-crown-6) (H₂O)₂](SbCl₆)₂, [M(18-crown-6)(MeNO₆)₂](SbCl₆)₂ with M is Ca²⁺ and Sr²⁺, [M(18-crown-6)(MeNO₂)](SbCl₆)₂ with M is Mn²⁺ and Co²⁺, and [M(18-crown-6)](SbCl₆)₂ with M is Ni²⁺ and Zn²⁺.     

A finite element method with a large mesh-width for a stiff two-point boundary value problem

We describe a finite element method for computation of numerical approximations of the solution of the second order singularly perturbed two-point boundary value problem on [?1, 1]

$\text{? u″ + pu′ = f, u(?1) = u(1) = 0, 0 < ? ∠ 1, (′ =}\text{d}\text{dx}\text{)}$

On a quasi-uniform mesh we construct exponentially fitted trial spaces which consist of piece-wise polynomials and of exponentials which fit locally to the singular solution of the equation or its adjoint. We discretise the Galerkin form for the boundary problem using such exponentially fitted trial spaces. We derive rigorous bounds for the error of discretisation with respect to the energy norm and we obtain superconvergence at the mesh-points, the error depending on ?, the mesh-width and the degree of the piece-wise polynomials.     

Fast Field Echo imaging: an overview and contrast calculations

Current fast imaging techniques are based on gradient echo sequences with reduced flip angle excitation pulses and very short repetition times TR. Practical T2 values may be of the order of TR or longer. In this situation, a different image contrast can be obtained, depending on details of the sequence. Four essentially different versions of the basic Fast Field Echo (FFE) sequence can be distinguished and are described systematically in this article. For these sequences, image contrast formulas are presented. Practical imaging should tolerate small field inhomogeneities. This requirement can be satisfied by only three of the four versions. Numerical simulations are used to study the influence of a modified phase alternation scheme on image contrasts of two of the remaining sequences. The results of the calculations are verified by phantom studies on a 1.5-T whole-body imager. Implications for contrast in clinical images are discussed in relation to head images obtained on the same machine.     

Trends and challenges of refractometric nanoplasmonic biosensors: A review

Motivated by potential benefits such as sensor miniaturization, multiplexing opportunities and higher sensitivities, refractometric nanoplasmonic biosensing has profiled itself in a short time span as an interesting alternative to conventional Surface Plasmon Resonance (SPR) biosensors. This latter conventional sensing concept has been subjected during the last decades to strong commercialization, thereby strongly leaning on well-developed thin-film surface chemistry protocols. Not surprisingly, the examples found in literature based on this sensing concept are generally characterized by extensive analytical studies of relevant clinical and diagnostic problems. In contrast, the more novel Localized Surface Plasmon Resonance (LSPR) alternative finds itself in a much earlier, and especially, more fundamental stage of development. Driven by new fabrication methodologies to create nanostructured substrates, published work typically focuses on the novelty of the presented material, its optical properties and its use – generally limited to a proof-of-concept – as a label-free biosensing scheme. Given the different stages of development both SPR and LSPR sensors find themselves in, it becomes apparent that providing a comparative analysis of both concepts is not a trivial task. Nevertheless, in this review we make an effort to provide an overview that illustrates the progress booked in both fields during the last five years. First, we discuss the most relevant advances in SPR biosensing, including interesting analytical applications, together with different strategies that assure improvements in performance, throughput and/or integration. Subsequently, the remaining part of this work focuses on the use of nanoplasmonic sensors for real label-free biosensing applications. First, we discuss the motivation that serves as a driving force behind this research topic, together with a brief summary that comprises the main fabrication methodologies used in this field. Next, the sensing performance of LSPR sensors is examined by analyzing different parameters that can be invoked in order to quantitatively assess their overall sensing performance. Two aspects are highlighted that turn out to be especially important when trying to maximize their sensing performance, being (1) the targeted functionalization of the electromagnetic hotspots of the nanostructures, and (2) overcoming inherent negative influence that stem from the presence of a high refractive index substrate that supports the nanostructures. Next, although few in numbers, an overview is given of the most exhaustive and diagnostically relevant LSPR sensing assays that have been recently reported in literature, followed by examples that exploit inherent LSPR characteristics in order to create highly integrated and high-throughput optical biosensors. Finally, we discuss a series of considerations that, in our opinion, should be addressed in order to bring the realization of a stand-alone LSPR biosensor with competitive levels of sensitivity, robustness and integration (when compared to a conventional SPR sensor) much closer to reality.     

Diamond anvil cell for measurements in high magnetic fields

Abstract

We constructed a diamond anvil cell for pressures up to 100 GPa in magnetic fields up to 12 T. The cell can be operated at any temperature between 10 and 350 K. Loading by condensation of gases is possible.