Staining of fluid-catalytic-cracking catalysts: localising Brønsted acidity within a single catalyst particle.

A time-resolved in situ micro-spectroscopic approach has been used to investigate the Brønsted acidic properties of fluid-catalytic-cracking (FCC) catalysts at the single particle level by applying the acid-catalysed styrene oligomerisation probe reaction. The reactivity of individual FCC components (zeolite, clay, alumina and silica) was monitored by UV/Vis micro-spectroscopy and showed that only clay and zeolites (Y and ZSM-5) contain Brønsted acid sites that are strong enough to catalyse the conversion of 4-fluorostyrene into carbocationic species. By applying the same approach to complete FCC catalyst particles, it has been found that the fingerprint of the zeolitic UV/Vis spectra is clearly recognisable. This almost exclusive zeolitic activity is confirmed by the fact that hardly any reactivity is observed for FCC particles that contain no zeolite. Confocal fluorescence microscopy images of FCC catalyst particles reveal inhomogeneously distributed micron-sized zeolite domains with a highly fluorescent signal upon reaction. By examining laboratory deactivated FCC catalyst particles in a statistical approach, a clear trend of decreasing fluorescence intensity, and thus Brønsted acidity, of the zeolite domains is observed with increasing severity of the deactivation method. By comparing the average fluorescence intensities obtained with two styrenes that differ in reactivity, it has been found that the Brønsted acid site strength within FCC catalyst particles containing ZSM-5 is more uniform than within those containing zeolite Y, as confirmed with temperature-programmed desorption of ammonia.

Catalytic activity in individual cracking catalyst particles imaged throughout different life stages by selective staining.

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.

Monitoring transport phenomena of paramagnetic metal-ion complexes inside catalyst bodies with magnetic resonance imaging.

An indirect magnetic resonance imaging (MRI) method has been developed to determine in a noninvasive manner the distribution of paramagnetic Co2+ complexes inside Co/Al2O3 catalyst extrudates after impregnation with Co2+/citrate solutions of different pH and citrate concentrations. UV/Vis/NIR microspectroscopic measurements were carried out simultaneously to obtain complementary information on the nature of the Co2+ complexes. In this way, it could be confirmed that the actual distribution of Co2+ inside the extrudates could be derived from the MRI images. By combining these space- and time-resolved techniques, information was obtained on both the strength and the mode of interaction between [Co(H2O)6]2+ and different Co2+ citrate complexes with the Al2O(3) support. Complexation of Co2+ by citrate was found to lead to a stronger interaction of Co with the support surface and formation of an eggshell distribution of Co2+ complexes after impregnation. By addition of free citrate and by changing the pH of the impregnation solution, it was possible to obtain the rather uncommon egg-yolk and egg-white distributions of Co2+ inside the extrudates after impregnation. In other words, by carefully altering the chemical composition and pH of the impregnation solution, the macrodistribution of Co2+ complexes inside catalyst extrudates could be fine-tuned from eggshell over egg white and egg yolk to uniform.

Noninvasive in situ visualization of supported catalyst preparations using multinuclear magnetic resonance imaging.

Multinuclear magnetic resonance imaging (MRI) is employed as a new noninvasive tool for monitoring supported catalyst preparation by visualizing precursor transport within the porous support. In particular, liquid phase 31P MRI experiments were used to visualize the dynamics of H3PO4 penetration into an alumina pellet and have revealed a strong interaction of H3PO4 with the support. Solid state 31P MRI was applied to map the distribution of the adsorbed phosphate inside the support after its drying. Comparison of the liquid phase and solid phase MRI results confirms the correlation of the phosphate distribution in the liquid phase during impregnation and the phosphate adsorbed on the support. The possibility to monitor the transport of metal atoms within the support by a direct detection of their NMR signal is demonstrated for 195Pt nucleus during impregnation of an alumina pellet with an aqueous solution of H2PtCl6. Other possible strategies for the utilization of MRI to characterize in situ the preparation of supported catalysts and other supported materials are briefly discussed.

Spatially resolved Raman and UV-visible-NIR spectroscopy on the preparation of supported catalyst bodies: controlling the formation of H2PMo11CoO40 5- inside Al2O3 pellets during impregnation.

The physicochemical processes that occur during the preparation of CoMo-Al2O3 hydrodesulfurization catalyst bodies have been investigated. To this end, the distribution of Mo and Co complexes, after impregnation of gamma-Al2O3 pellets with different CoMoP solutions (i.e., solutions containing Co, Mo, and phosphate), was monitored by Raman and UV-visible-NIR microspectroscopy. From the speciation of the different complexes over the catalyst bodies, insight was obtained into the interaction of the different components in the impregnation solution with the Al2O3 surface. It is shown that, after impregnation with a solution containing H2PMo11CoO40(5-), the reaction of phosphate with the Al2O3 leads to the disintegration of this complex. The consecutive independent transport of Co2+ complexes (fast) and Mo6+ complexes (slow) through the pores of the Al2O3 is envisaged. By the addition of extra phosphate and citrate to the impregnation solution, the formation of the desired heteropolyanion can be achieved inside the pellets. Ultimately, the H2PMo11CoO40(5-) distribution could be controlled by varying the aging time applied after impregnation. The power of a combination of spatially resolved spectroscopic techniques to monitor the preparation of supported catalyst bodies is illustrated.

UV-Vis microspectroscopy: probing the initial stages of supported metal oxide catalyst preparation.

A UV-vis microspectroscopy methodology for monitoring the speciation and macrodistribution of catalyst-precursor species inside catalyst-support bodies at the initial stages of catalyst preparation has been developed. The setup is based upon optical-fiber technology and allows spatially resolved analysis of bisected catalyst bodies. The potential of this tool is demonstrated by two pore-volume impregnation studies involving Ni2+ d-d transition bands and Cr6+ charge-transfer bands.

Insights into the preparation of supported catalysts: a spatially resolved Raman and UV-Vis spectroscopic study into the drying process of CoMo/gamma-Al2O3 catalyst bodies.

Spatially resolved Raman and UV-vis-NIR microspectroscopy have been used as tools to study the preparation process of supported catalyst bodies. Detailed spectroscopic information on the local coordination geometry of two different metallic species along with their macro-distribution over the catalyst body has been obtained, enabling a good understanding of the physicochemical processes occurring during the drying process of impregnated gamma-Al(2)O(3) bodies. The formation and decomposition of the Keggin-type complex H(x)PMo(11)CoO(40)((7-)(x)-), which is considered to be a potential precursor for CoMoS(2)/gamma-Al(2)O(3) HDS catalysts, inside gamma-Al(2)O(3) bodies is shown to be a function of the composition of the impregnation solutions, the aging time, and the drying conditions applied. This knowledge has been successfully applied to prepare samples with a well-defined distribution of the bimetallic complex, that is, either egg-shell, egg-yolk, or homogeneous distributions. The Raman results are presented in a semiquantitative way by subtraction of a reference spectrum of a sample containing a known amount of H(x)PMo(11)CoO(40)((7-)(x)-) from the spectra recorded along the cross-section of the catalyst bodies.

Envisaging the physicochemical processes during the preparation of supported catalysts: Raman microscopy on the impregnation of Mo onto Al2O3 extrudates.

Raman microscopy has been applied to study the preparation of shaped Mo/Al(2)O(3) catalysts. The speciation of different Mo complexes over gamma-Al(2)O(3) support bodies was followed in time after pore volume impregnation with aqueous solutions containing different Mo complexes. The addition of NO(3-) to the impregnation solutions allows for a quantitative Raman analysis of the distribution of different complexes over the catalyst bodies as this ion can be used as an internal standard. After impregnation with an acidic ammonium heptamolybdate (AHM) solution, the strong interaction between Mo(7)O(24)(6-) and Al(2)O(3) results in slow transport of this complex through the support and extensive formation of Al(OH)(6)Mo(6)O(18)(3-) near the outer surface of the support bodies. This may be prevented by decreasing the interaction between Mo and Al(2)O(3). In this way, transport is facilitated and a homogeneous distribution of Mo is obtained on a reasonable time scale. A decrease in interaction between Mo and Al(2)O(3) can be achieved by using alkaline impregnation solutions or by the addition of complexing agents, such as citrate and phosphate, to the impregnation solution. In general, time-resolved in situ Raman microscopy can be a valuable tool to study the physicochemical processes during the preparation of supported catalysts.