Fluid catalytic cracking (FCC) is the major conversion process used in oil refineries to produce valuable hydrocarbons from crude oil fractions. Because the demand for oil-based products is ever increasing, research has been ongoing to improve the performance of FCC catalyst particles, which are complex mixtures of zeolite and binder materials. Unfortunately, there is limited insight into the distribution and activity of individual zeolitic domains at different life stages. Here we introduce a staining method to visualize the structure of zeolite particulates and other FCC components. Brønsted acidity maps have been constructed at the single particle level from fluorescence microscopy images. By applying a statistical methodology to a series of catalysts deactivated via industrial protocols, a correlation is established between Brønsted acidity and cracking activity. The generally applicable method has clear potential for catalyst diagnostics, as it determines intra- and interparticle Brønsted acidity distributions for industrial FCC materials.
The liquid crystal phase behavior of a suspension of charged gibbsite [Al(OH)3] platelets is investigated. By variation of the ionic strength, we are able to tune the effective thickness-to-diameter ratio of the platelets in suspension. This enables us to experimentally test the liquid crystal phase transition scenario that was first predicted a decade ago by computer simulations for hard platelets (Veerman, J. A. C.; Frenkel, D. Phys. Rev. A 1992, 45, 5632), that is, the isotropic (I) to nematic (N) and isotropic to columnar (C) phase transitions in one colloidal suspension. In addition to the shape-dependent thermodynamic driving force, the effect of gravity is important. For example, a biphasic (I-N) suspension becomes triphasic (I-N-C) on prolonged standing. This effect is described by a simple osmotic compression model.
We study interfacial phenomena in a colloidal dispersion of sterically stabilized gibbsite platelets, exhibiting coexisting isotropic and nematic phases separated by a sharp horizontal interface. The nematic phase wets a vertical glass wall and polarized light micrographs reveal homeotropic surface anchoring both at the free isotropic-nematic interface and at the wall. On the basis of complete wetting of the wall by the nematic phase, as found in our density functional calculations and computer simulations, we analyze the balance between Frank elasticity and surface anchoring near the contact line. Because of weak surface anchoring, the director field in the capillary rise region is uniform. From the measured rise (6 m) of the meniscus at the wall we determine the isotropic-nematic surface tension to be 3 nN=m, in quantitative agreement with our theoretical and simulation results. (IN) transition of anisometric colloidal particles. The spontaneous orientational ordering of the particles in the nematic phase is attributed to the strongly anisotropic excluded volume pair interactions that favor parallel alignment of the particles and overcome the orientational entropy of the isotropic state. However, the precise particle shape matters, as ordering of thin rods can be quantitatively described at the level of second virial theory, yielding a density jump of 20% at the IN transition, whereas-to quote Onsager-''we can hardly hope for more than that our result will describe concentrated solutions of (platelike) particles qualitatively rather than quantitatively '' [2]. Indeed, computer simulations [3] have shown that for (infinitely thin) hard platelets the density jump is only 8%.In recent years the entropy-driven IN transition in suspensions of sterically stabilized [4] as well as charge stabilized colloidal platelets [5] has been studied both experimentally and theoretically [6]. To explain that the IN interface is easily deformed under gravity, it was estimated from a scaling relation that the interfacial tension in these systems could be as low as 0:01 N=m [7]. In this Letter we present a measurement of the IN interfacial tension in suspensions of colloidal platelets and carry out density functional calculations [8] and Monte Carlo (MC) simulations of the free IN interface and wetting at a wall using a microscopic model of platelets with continuous degrees of freedom. For detailed studies of interfacial properties of rectangular platelets with discrete orientations (Zwanzig model) see Refs. [9,10]. We find indeed a very low IN interfacial tension from the capillary rise measured in experiment, in quantitative agreement with our theoretical and simulation results. Our theory and simulations show complete wetting of the wall by the nematic phase, with its onset occurring only remarkably close to bulk IN coexistence.We use a model system of hard disks consisting of sterically stabilized colloidal gibbsite [Al OH 3 ] platelets dispersed in toluene [4], with average diameter D 240 nm and thickness L 18 nm, and a polyd...
We are studying the phase behavior of a suspension of platelike colloids which has a very broad size distribution, particularly in thickness. This suspension exhibits an isotropic-nematic phase separation over a notably wide range of particle concentrations, displaying a remarkable phenomenon. In part of the coexistence region, phase separation yields a nematic upper phase in coexistence with an isotropic bottom phase. If the nematic phase is isolated and diluted, the reverse situation is observed such that the isotropic phase now becomes the upper phase. We show that these phenomena can be explained by a pronounced fractionation with respect to platelet thickness.
The magnetic-field-induced orientational order in the isotropic phase of colloidal gibbsite ͓Al͑OH͒ 3 ͔ platelets is studied by means of optical birefringence and small-angle x-ray scattering ͑SAXS͒ techniques. The suspensions display field-induced ordering at moderate field strengths ͑a few Tesla͒, which increases with increasing particle concentration. The gibbsite particles align their normals perpendicular to the magnetic field and hence possess a negative anisotropy of their diamagnetic susceptibility ⌬ . The results can be described following a simple, Onsager-like approach. A simplified model is derived that allows one to obtain the orientational distribution function directly from the scattering data. However, it leads to an underestimate of the diamagnetic susceptibility anisotropy ⌬ . This accounts for the difference between the ⌬ values provided by the two experimental techniques ͑SAXS and magneto-optics͒. The order of magnitude ⌬ ϳ 10 −22 J/T 2 lies in between that of goethite suspensions and that of suspensions of organic particles.
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