3D Nanoscale Imaging and Quantitative Analysis of Zeolite Catalysts

Abstract

Zeolites are crystalline microporous aluminosilicates, one of the most versatile and widely used class of materials.The unique physico-chemical properties of zeolites are found to be irreplaceable in many industrial processes such as separation, adsorption and catalysis. To exploit their full potential and optimize their properties for specific applications, zeolites are often subjected to several post-synthesis modifications. The work presented in this thesis aims to provide a deeper understanding of the nanostructure of zeolite Y crystals and their modifications and shaping into an industrially relevant metal supported catalyst body. Advanced electron microscopy techniques, with focus on electron tomography (ET) and image analysis, enabled identification of hidden structural features and quantitative analysis of important zeolite Y properties at the nanoscale. The recent advances in electron tomography and image analysis for structural characterization of various catalytic materials (metal loaded - zeolites, silicas, carbon) were reviewed. The progress towards obtaining reliable and unique quantitative information of nanoscale catalysts’ features through image analysis, the development of faster reconstruction algorithms and atomic scale ET, is highlighted through a number of examples. The leading role of zeolite Y in oil refining processes and in petrochemistry stems from its acidic nature, its crystalline structure composed of micropores (ca. 1nm diameter), and its pronounced stability under harsh reaction conditions. By introducing mesopores into microporous zeolite Y mass transfer of molecules throughout the crystals is facilitated which is beneficial for catalyst performance. A new approach for obtaining a hierarchical mesopore system in zeolite Y was developed. By submitting the commercially available steamed and acid leached zeolite Y to base leaching, an additional network of small mesopores was created next to zeolite Y micropores (from zeolite synthesis) and larger mesopores (from steaming and acid leaching). The hierarchical nature of the trimodal porosity proved beneficial for hydrocracking performance of this material. 3D imaging of zeolite crystals using electron tomography was crucial to unfold the true nature of this complex mesopore network. A deeper understanding of the structure of large mesopores (introduced after steaming and acid leaching) through quantitative assessment of properties unavailable to techniques other than ET was obtained. Image analysis of electron tomograms enabled differentiation and quantification of open, closed and constricted mesopores, calculation of mesopore tortuosity as well as the size of intact microporous domains, which are considered to limit mass transport through zeolite crystals. The nanostructure of the bifunctional Pt/zeolite Y catalyst was studied in depth. Semi-automated image analysis enabled measurement of size distributions and Pt particle-particle distances in 3D, and revealed large variations in Pt loading between individual zeolite crystals, up to a factor of 35. This discovery calls for re-evaluation of catalyst preparation methods and suggests potential for lowering the nominal loading with noble metals. Structural characterization of the bifunctional Pt/zeolite Y catalyst in its ‘final’ industrially applied form, where Pt particles are supported on extrudates composed of zeolite Y and alumina binder was carried out. Preliminary results of the method designed to controllably deposit Pt particles on either zeolite or alumina phase are presented.