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Abstract: The control of the inter-particle porosity in polycrystalline zeolite bodies remains a challenge. The present study reports on the preparation of centimeter-sized zeolite bodies (32×2mm pellets) with reduced inter-particle porosity. The bodies were prepared by assembling zeolite colloidal nanocrystals with monomodal particle size distribution and sub-colloidal aluminum hydroxide binder. A secondary growth treatment under hydrothermal conditions was performed in order to fill up the inter-particle voids and obtain polycrystalline zeolite materials with textural characteristics close to those of single zeolite crystals. Initial material, intermediates and ultimate product were studied with a combination of complementary methods revealing a substantial reduction even an elimination of the inter-particle porosity. Particular attention was paid to the chemical evolution of the binder and the migration of aluminum species, employing X-ray powder diffraction, 27Al-MAS, 27Al-3QMAS NMR and X-ray fluorescence spectroscopy analyses. Evidences for insertion of aluminum from the binder into the zeolite framework during secondary growth step were found. Consequently the ultimate pellets comprised aluminum-rich zeolite beta crystals and very low binder content. [ABSTRACT FROM AUTHOR]

All industrially available zeolites are obtained from hydrogel systems. Unfortunately the level of understanding of the events preceding zeolite crystallization is far from satisfactory. In this respect, revealing the nature of the processes taking place in the precursor gel is of paramount importance to understanding zeolite nucleation. The investigation of the gel structure, however, is a difficult task due to the complexity of the object in terms of both composition and topology. Therefore, a combination of hyperpolarized (HP) 129Xe NMR-N2 adsorption-high-resolution transmission electron microscopy-energy-dispersive spectrometry methods complemented by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and chemical analyses has been employed to study the changes in composition and structure of sodium hydroxide rich aluminosilicate gel yielding zeolite A. The role of each component in the system and the entire sequence of events during the induction, nucleation, and crystallization stages have been revealed. The high concentration of sodium hydroxide in the studied system has been found to control the size and structure of the gel particles in the beginning stage. During the initial polymerization of aluminosilicate species a significant part of the sodium hydroxide is expelled from the gel into the solution, which restricts extensive polymerization and leads to formation of small aluminosilicate particles with open pore structure. The induction period that follows is marked by incorporation of Na back in the bulk gel. The combined action of the Na ion as a structure-directing agent and the hydroxyl group as a mobilizer results in partial depolymerization of the gel and formation of voids with mesopore sizes. The nucleation maximum coincides temporally with development of pores with sizes in the range of 2-5 nm. The amorphous gel undergoes into crystalline zeolite only after these pores have disappeared and the chemistry of the gel has evolved to reach the stoichiometric zeolite composition. It was established unambiguously by high-resolution transmission electron microscopy and HP 129Xe NMR that the nucleation of zeolite occurs in the solid part of the system and the succeeding crystallization commences only after the nuclei are released into the liquid, which is consistent with the autocatalytic mechanism. Also this investigation has demonstrated the unrivaled sensitivity of HP 129Xe NMR that is capable of identifying presence of small amounts of crystalline zeolite material in amorphous medium with detection limit extending below 1 wt %. [ABSTRACT FROM AUTHOR]

The aim of this work was to develop a sustainable single-step process involving supercritical CO 2 as an anti-solvent to recrystallize sodium bicarbonate from an aqueous solution. First, a feasibility study validated the capacity to recrystallize sodium bicarbonate from a hydroalcoholic solution, under a pressure of at least 150 bar, an atomization jet solution velocity of at least 2.6 m·s−1 and a mass flowrate ratio X (CO 2 over solution) of at least 21.6. The crystals obtained were long, thick and zigzag-shaped. In a second step, an optimization of operating conditions was proposed to obtain crystals exhibiting an isometric habit. The most favourable conditions leading to the thickest and shortest crystals were 150 bar, 50 °C, an atomization velocity of 2.6 m·s−1, and a solute concentration of 0.45 g·(kg solvent)−1. No significant influence of higher pressure or higher atomization velocity was observed. [Display omitted] • Implementation of a single-step process of sodium bicarbonate recrystallization using supercritical CO 2 • Formation of a novel crystal morphology: long and thick zigzag shapes • A mechanism of Crystallization by Particle Attachment (CPA) suggested [ABSTRACT FROM AUTHOR]

Gasoline direct injection (GDI) sprays are complex multiphase flows. When compared to multi-hole diesel sprays, the plumes are closely spaced, and the sprays are more likely to interact. The effects of multi-jet interaction on entrainment and spray targeting can be influenced by small variations in the mass fluxes from the holes, which in turn depend on transients in the needle movement and small-scale details of the internal geometry. In this paper, we present a comprehensive overview of a multi-institutional effort to experimentally characterize the internal geometry and near-nozzle flow of the Engine Combustion Network (ECN) Spray G gasoline injector. In order to develop a complete picture of the near-nozzle flow, a standardized setup was shared between facilities. A wide range of techniques were employed, including both X-ray and visible-light diagnostics. The novel aspects of this work include both new experimental measurements, and a comparison of the results across different techniques and facilities. The breadth and depth of the data reveal phenomena which were not apparent from analysis of the individual data sets. We show that plume-to-plume variations in the mass fluxes from the holes can cause large-scale asymmetries in the entrainment field and spray structure. Both internal flow transients and small-scale geometric features can have an effect on the external flow. The sharp turning angle of the flow into the holes also causes an inward vectoring of the plumes relative to the hole drill angle, which increases with time due to entrainment of gas into a low-pressure region between the plumes. These factors increase the likelihood of spray collapse with longer injection durations. [ABSTRACT FROM AUTHOR]

Abstract: The performance of zeolite crystals in different industrial processes often depends on their size and shape. Several physico-chemical parameters can have significant impact on their particulate properties (especially morphology). Amongst the chemical parameters the Si/Al ratio is one of most important variable together with the Na+ ions (alkalinity) content. The present study is devoted to the effect of Na on the morphology of zeolite A (LTA-type) crystals. Initial hydrogels were heated at 80°C in reactors made of HDPE, under static conditions. After separation by centrifugation, samples of solid and liquid phase were characterized using several analytical methods such as: atomic absorption spectroscopy, X-ray diffraction, laser light scattering, optical, scanning and transmission electron microscopy. The results show that the increase of alkalinity in starting system causes two effects: (a) increase of the number of nuclei (crystals) in system through additional nucleation at surface and subsurface area of gel particles, and (b) the growth of crystals which have more irregular shape (rounded edges). [Copyright &y& Elsevier]

Abstract: Smooth and homogeneous monolayer Silicalite-1 seeds were deposited on silicon substrate surfaces via the Langmuir–Blodgett (LB) technique. The LB films were prepared by vertical dipping leaving part of the substrate uncoated. Neither substrate modification nor additives were required to produce densely packed monolayer seeds. The degree of coverage and packing of the seed layers were dependent on the target pressure used for film deposition. Dense Silicalite-1 films with controlled thickness in the range 100–650nm were synthesized after secondary synthesis. The (0 k 0) reflections dominated in all the XRD patterns, suggesting that the prepared polycrystalline films were preferentially b-oriented perpendicular to the substrate surface. The degree of in-plane orientation was preserved with increasing film thickness. All secondary growth films were very smooth with a clear boundary between the seeded and non seeded substrate sections. The Silicalite-1 films were highly stable upon both ultrasonic treatment and calcination. [Copyright &y& Elsevier]