Assessment of the 3 D Pore Structure and Individual Components of Preshaped Catalyst Bodies by X-Ray Imaging
Résumé
Porosity in catalyst particles is essential because it enables reac-tants to reach the active sites and it enables products to leave the catalyst. The engineering of composite-particle catalysts through the tuning of pore-size distribution and connectivity is hampered by the inability to visualize structure and porosity at critical-length scales. Herein, it is shown that the combination of phase-contrast X-ray microtomography and high-resolution pty-chographic X-ray tomography allows the visualization and characterization of the interparticle pores at micro-and nanometer-length scales. Furthermore, individual components in preshaped catalyst bodies used in fluid catalytic cracking, one of the most used catalysts, could be visualized and identified. The distribution of pore sizes, as well as enclosed pores, which cannot be probed by traditional methods, such as nitrogen physisorption and isotherm analysis, were determined. Fluid catalytic cracking (FCC) is the most important conversion process in the refinery, upgrading the heavy fraction of oil into gasoline and volatile olefins. [1] The catalyst bodies are spheres made of various components with a diameter of approximately 100 microns. Reactants must diffuse into and through the pre-shaped catalyst body to reach an active site. Because large molecules must react on the surface of the catalytic particles that are encompassed in the preshaped catalyst bodies, these bodies contain a large fraction of interparticle space. Thus, the size and connectivity of the pores created by the interparticle space and the relative orientation of the various components within the preshaped particle play an important role in catalysis. The main FCC catalytic component is a steamed zeolite, H-USY, and other components include alumina, silica-alumina, and clays. [2] Zeolites are crystalline alumina silicates that contain Brønsted acid sites. [2] Although this microporous material [3] is already widely used in the refinery, mesoporous materials are also of interest for catalysis because mesopores in the FCC catalyst improve the diffusion of large molecules into the catalyst bodies, thus enhancing catalytic performance. Indeed, meso-structured zeolites type Y with excellent hydrothermal stability have been demonstrated, and the mesopores in individual zeo-lite crystals have been visualized by electron tomography. [4, 5] Nitrogen physisorption, often complemented by mercury porosimetry, and isotherm analysis are the most generally applied characterization methods to study porosity in heterogeneous catalysts. [6-8] Adsorption of molecules of a particular size provides direct insight into the accessibility of the interior of crystals and the pore-size openings. Although these methods are widely applied to probe large sample volumes, they yield only averaged parameters, and the information of individual particles is not accessible. On the other hand, electron micros-copy (EM) enables detection and analysis of individual catalyst particles. In the case of the FCC zeolite catalyst, electron to-mography has provided insight into the pore structure within the individual zeolite crystals. [9, 10] Such visualization of pore size and connectivity stimulated the design of new and optimized trimodal pore structures. [3] Recently, the morphology of commercial spent equilibrium fluid catalytic-cracking catalyst (ECAT) was characterized by X-ray tomography and the distribution of zeolites Y in the catalyst particles was inferred from X-ray fluorescence results. [11] Although the structural variation between individual particles was previously observed, [12, 13] as well as pore sizes of approximately 100 nm, [11] the distribution of the different components and the interparticle porosity within a single FCC catalyst body have not been quantified. Herein, we employ state-of-the-art synchrotron X-ray imaging techniques to analyze the size and connectivity of the 3 D pore structure in individual FCC catalyst bodies, as well as the padding of their components. We investigate an FCC composite catalyst body formed by 5 % La 2 O 3-exchanged zeolite type Y and metakaolin, which is a calcined kaolin clay consisting of aluminum silicate. In FCC catalysts, the clay is a diluent used to control the level of cracking activity and the rare-earth-exchanged zeolites are used to improve hydrothermal stability. [a] Dr.