On Comprehensive Understanding of Catalyst Shaping by Extrusion

On Comprehensive Understanding of Catalyst Shaping by Extrusion

CATALYSIS S. DEVYATKOV1, N.V. KUZICHKIN1, D.YU. MURZIN2 1. St. Petersburg State Technical University, Russia 2. Åbo Akademi University, Turku/Åbo, Finland S. Devyatkov N. V. Kuzichkin D. Yu. Murzin On comprehensive understanding of catalyst shaping by extrusion KEYWORDS: Sulfated zirconia, extrusion, zeta-potential, binder, alkylation. A brief introduction into catalyst shaping and its importance is given. Some challenges and key aspects Abstract are discussed. A special attention is paid to visualization of the structure of catalyst extrudates. Hydrous sulfated zirconia, as a promising catalyst for acid catalyzed transformations of hydrocarbons such as alkylation, is considered from the viewpoint of its shaping via extrusion. Rheology and zeta potential measurements are presented for this material, addressing also the influence of PVA addition as a surface-modifying agent. The quality of the green bodies of the shaped catalysts is shown to be strongly dependent on particles surface properties in molding masses. INTRODUCTION fi eld of heterogeneous catalysis. At the same time shaped catalysts particles (mm range) are applied industrially on a Immense importance of catalysis in chemical industry is macroscopic level. Thus, in addition to intrinsic kinetics, also manifested by the fact, that roughly 85-90% of all chemical transport phenomena constitute an essential part of catalytic products have seen a catalyst during a course of the engineering. Moreover, understanding of the events occurring production. The number of catalytic processes in industry at macroscopic or reactor level (i.e. 1 m scale) requires is very large with mainly heterogeneous catalysts applied. description at the mm (shaped catalyst pellet) and micron Such catalysts come in many different forms as porous solids. scale. Contribution of catalysts to the overall cost of a product The shaping of catalysts and supports is a key step in the made with an aid of catalysts on average is not very high catalyst preparation procedure. (ca. 3%). In theory catalysts are not consumed in a chemical Most academic research institutions, however, work only reaction, while in industrial practice many industrial catalysts with small quantities of powders, and do not consider the deactivate, requiring gradual replacement. Even taking such infl uence of the scale-up. At the same time information about replacement into account a part of catalyst sales in relation the distribution of components (for example zeolite and a to the gross domestic product (GDP) is marginal (ca. 0.1%). binder) inside an industrial catalyst is very important. At the same time the share of products made by catalysts in In general, in the process of catalyst design, more attention GDP could be as high as 25%. should be attributed to the stage of scale-up, including In the areas where heterogeneous catalysts are used in shaping. Very few academic groups in Europe and globally industry, almost one third is used in crude oil refi ning and have a proper infrastructure and skills to address the petrochemistry (1). Among the current trends driving catalysts engineering aspects of catalyst shaping on a quantitative demands in the future, the following should be mentioned: level. The required work needs expertize in catalyst improving process effi ciency and expansion of the feedstock preparation, elucidation of catalytic activity, characterization base to include coal, natural gas and biomass. In particular and imaging, rheology, kinetics and mass transfer. The lignocellulosic biomass, not competing with the food chain, complexity of the work apparently explains why catalyst has attracted a lot of attention. It is fair to say that utilization shaping is not in the focus of research done in academia. of such feedstock might require fundamental changes in One example of a sophisticated characterization work for processing technology. an industrial catalyst is related with fl uid catalytic cracking Microscopic understanding of catalysis (i.e. nm scale – catalysts. This examples involves integrated laser and electron active site) is the main goal of academics s working in the microscopy. In fl uorescence microscopy, a focused laser light Chimica Oggi - Chemistry Today - vol. 33(6) November/December 2015 57 scans a catalyst detecting fluorescence emission from the Small particle size increases activity by minimizing influence focal points. The technique contrary to electron microscopy of internal and external mass transfer. At the same time the has limited spatial resolution and solely reveals fluorescent bed pressure drop increases. Thus, there is an apparent structures. The integrated laser and electron microscope contradiction between the desire to have small catalyst (iLEM) developed by Weckhuysen and co-workers (2) is particles (less diffusional length, higher activity) and to utilize an imaging tool, combining strengths of both methods. large particles displaying lower pressure drop. In order to generate a fluorescence signal, fluorogenic For reactors with fixed beds, often applied in industry, relatively thiophene oligomerization is used, catalysed by acidic sites large particles are in use (several mm) to avoid pressure drop. of zeolites. The product, oligomerized thiophene, emits green fluorescence. In this way location of acid sites can be made. Extrusion (Figure 2) is the most economic and commonly Moreover, fluorescence intensity can be related to the carried out shaping technique for catalysts and supports. strength of acid sites. The method allows a spatial resolution During extrusion, a wet paste from a hopper at the top is of ca. 20 nm. The iLEM setup shown in Figure 1a consists of a forced through a die. The emerging ribbon, passing through custom-designed laser scanning fluorescence microscopy holes in the die plate, is cut to the desired length using a (FM), mounted on a side port of a TEM. The laser beam is suitable device. The pressure, which is developed in the screw perpendicular to the path of the electron beam. For FM extruder as the paste moves towards the die, is affected by imaging, the grid with the catalyst sample is rotated to face the screw geometry and the paste rheology. the laser beam, while FM is partially retracted and the grid is Too viscous pastes can block the extruder, while the opposite tilted to enable TEM imaging. leads to unstable extrudates. The technique was applied to study a fluid catalytic cracking Usually the catalyst powders obtained after the thermal catalyst. The active phase, zeolite Y, is embedded in a matrix treatments behave like sand, i.e. do not have by themselves consisting of clay, silica, and alumina. Using iLEM technique the required moldability and plasticity, even when water is location of active acidic centres could be determined by added. Various additives are used in formulation of pastes, relation to fluorescence and could be correlated to the such as: a) compounds for improving the rheological catalyst particle structure. Two different areas were found behaviour (clays or starch); b) binders (aluminas or clays); in the catalyst, one associated with the zeolite component c) peptizing agents to de-agglomerate the particles (dilute giving the fluorescent products, while the other areas are acetic or nitric acid); and d) combustible materials to increase mainly composed of the matrix material (Figure 1 b, c). the porosity (carbon black, starch, etc.). The operating variables include mixing time, additive content, water content, ageing and extrusion temperature. The quality of the extrudates also depends on the drying and calcination procedure. Special shapes (trilobates, rings, hollow cylinders, monoliths or honeycombs) can be obtained using proper dies. Figure 2. Principle of extrusion. Catalyst forming is seldom considered in academic research. It is, however, extremely important for preparation of catalysts, influencing the final performance. Information about catalyst forming is typically proprietary and primarily empirical, leading Figure 1. a) The iLEM setup, and iLEM analysis of a sectioned to uncertain/sub-optimal processes, decreased quality FCC catalyst particle: b) FM image, c) TEM image, taken from and increased costs. It is, thus, apparently clear, that solid the same region (2) Copyright WILEY. When the zeolitic structure is exposed to steam, Al atoms from engineering approaches are needed also in this field. the framework can be extracted creating extra-framework Al Although extrusion is thoroughly studied in ceramic science species. This results in lower amounts of Brønsted acid sites and, the use of porous materials (3–5), such as for example zeolites thus, eventual deactivation. The iLEM images of hydrothermally deactivated catalysts show much lower fluorescence intensity relevant for catalysis is much more rare. compared to the fresh FCC catalyst particles. In addition, a Compared to other preparation methods extrusion process large fraction of the crystals is damaged and the large regions affords high throughput at relatively low costs and gives a of clay are lost. variety of possible extrudate shapes (Figure 3). SHAPING OF CATALYSTS Main principles of extrusion The shape and size of the catalyst particles should promote catalytic activity, strengthen the particle resistance to crushing and abrasion, minimize the bed pressure drop, Figure 3. Shapes of extrudated catalysts. lessen fabrication costs and distribute dust build-up uniformly. 58 Chimica Oggi - Chemistry Today - vol. 33(6) November/December 2015 The downside of the

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