A.H. Janssen

Generation, Characterization, and Impact of Mesopores in Zeolite Catalysts

Amongst the current developments in the field of hierarchical pore structures, the creation of mesopores in zeolite crystals is the most frequently employed way to combine micropores with mesopores in one material. In this review an overview is presented of the different approaches to generate and characterize mesopores in zeolite crystals and establish their impact on the catalytic action. Mesopores can be created via several routes from which steaming and acid leaching are the most frequently applied. Novel approaches using secondary carbon templates that are removed after synthesis have recently been launched. For the characterization of mesopores, nitrogen physisorption and electron microscopy are commonly used. More recently, it was shown that electron tomography, a form of three-dimensional transmission electron microscopy, is able to reveal the three-dimensional shape, size, and connectivity of the mesopores. The effect of the presence of mesopores for catalysis is demonstrated for several industrially applied processes that make use of zeolite catalysts: the cracking of heavy oil fractions over zeolite Y, the production of cumene and hydroisomerization of alkanes over mordenite, and synthesis of fine chemicals over Y, ZSM-5, and Beta. For these processes, the mesopores ensure an optimal accessibility and transport of reactants and products, while the zeolite micropores induce the preferred shape-selective properties.

Three‐Dimensional Transmission Electron Microscopic Observations of Mesopores in Dealuminated Zeolite Y

Supported by NWO under grant 98037. The research of AJK has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences (KNAW). The authors thank J.E.M.J. Raaymakers for the nitrogen physisorption measurements, A.J.M. Mens for the XPS measurements, J.A.R. van Veen and E.J. Creyghton for physical data and useful discussions and Shell International Chemicals and Zeolyst for the samples.

Plugged hexagonal templated silica: a unique micro- and mesoporous composite material with internal silica nanocapsulesElectronic supplementary information (ESI) available: Fig. S1: X-ray diffractogram of a PHTS material. Fig. S2: TEM images of SBA-15 and PHTS-2. Fig. S3: hydrothermal stabilities. See http://www.rsc.org/suppdata/cc/b2/b201424f/

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro- and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures.

Automated high-throughput electron tomography by pre-calibration of image shifts

Electron tomography is a versatile method for obtaining three‐dimensional (3D) images with transmission electron microscopy. The technique is suitable to investigate cell organelles and tissue sections (100–500 nm thick) with 4–20 nm resolution. 3D reconstructions are obtained by processing a series of images acquired with the samples tilted over different angles. While tilting the sample, image shifts and defocus changes of several µm can occur. The current generation of automated acquisition software detects and corrects for these changes with a procedure that incorporates switching the electron optical magnification. We developed a novel method for data collection based on the measurement of shifts prior to data acquisition, which results in a five‐fold increase in speed, enabling the acquisition of 151 images in less than 20 min. The method will enhance the quality of a tilt series by minimizing the amount of required focus‐change compensation by aligning the optical axis to the tilt axis of the specimen stage. The alignment is achieved by invoking an amount of image shift as deduced from the mathematical model describing the effect of specimen tilt. As examples for application in biological and materials sciences 3D reconstructions of a mitochondrion and a zeolite crystal are presented.

Development and Application of 3-Dimensional Transmission ElectronMicroscopy (3D-TEM) for the Characterization of Metal-Zeolite CatalystSystems

With electron tomography (3D-TEM) a 3D-reconstruction is calculated from a series of TEM images taken at a tilt angle range (tilting range) of +70° to −70°. The reconstruction can be visualized with contour surfaces that give information about the surface of the sample as well as with slices through the reconstruction that give detailed information on the interior of the sample. Electron tomography gives much more information than Scanning Electron Microscopy (SEM), since SEM gives only information about the surface of a sample. As a case study, the imaging of silver clusters on zeolite NaY is given. The reconstruction shows silver particles at the external surface as well as a silver particle in a mesopore of the zeolite crystallite. It is concluded that 3D-TEM comprises a breakthrough in the characterization of nano-structured solid catalysts.

A 3D-TEM study of the shape of mesopores in SBA-15 and modified SBA-15 materials

The mesopores in SBA-15 are curved on a mesoscopic length scale and even more so in the case of modified SBA-15, which implies that using the particle size of these types of materials in diffusion studies may strongly underestimate the path length relevant for intraparticle diffusion.

Electron tomography of molecular sieves

This chapter discusses electron tomography of molecular sieves. The chapter discusses prospects and limitations of different modes of electron microscopy. The use of three-dimensional transmission electron microscopy (TEM) is discussed—in particular, electron tomography (ET)—for the study of molecular sieves. The use of electron tomography in materials science is quite recent and exciting information on zeolites and mesoporous materials has already been obtained. ET can now be applied on a routine basis using an automated electron microscope in combination with image processing and visualization software. Three-dimensional information with nanometer resolution has been obtained for structural studies of molecular sieves. The application of electron tomography with zeolites has provided unique information on the nature of mesopores that have been obtained by secondary treatments or the use of carbon templates. Structural information obtained from ET should be complemented with other characterization techniques because of the intrinsically poor statistics of electron microscopy.

Three-dimensional transmission electron microscopy of disordered and ordered mesoporous materials

Abstract The use of 3D-TEM, in particular electron tomography, for the characterisation of mesoporous materials is introduced. In 3D-TEM a tilt series of the specimen collected in bright field mode comprises typically 150 images over tilt angles ranging from -70°C to +70°C. The tilt series are used to calculate a full 3D- image reconstruction of the specimen in question. The first example delt with comprises the study of zeolite Y crystals that contain mesopores. The pore shape, size and connectivity of the zeolite Y crystal is obtained with great clarity and detail. The second example involves SBA-15 materials. The curved nature of the pores in the particles of SBA-15 is clearly demonstrated from tilt series.

Quantitative morphological studies of mesoporous catalysts at nanometer scale resolution

We have combined electron tomography with posterior image processing for a quantitative morphological study of mesoporous catalysts at nanometer scale resolution. As a first example for the possibilities of the approach, mesopore size distributions of single crystallites of commercial USY and XVUSY zeolites were derived.