Mesoporosity of Zeolite Y: Quantitative Three-Dimensional Study by Image Analysis of Electron Tomograms

Research Progress on Modifications of Zeolite Y for Improved Catalytic Properties

Zeolite Y, as a solid acid catalyst with excellent performance, is a landmark in petroleum refining and chemical industry production–especially in catalytic cracking reactions. Improving the SAR of Y zeolite, enriching its pore structure, and modifying it with heteroatoms can realize the multifunctional catalysis of Y zeolite, improve the application value of it, and then meet the demands of petroleum refining. In this review, the synthesis of Y zeolites with high SAR, multistage pores, and heteroatom modification is summarized.

Dissolution Behavior and Varied Mesoporosity of Zeolites by NH4 F Etching

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH₄ F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano- and sub-nano scales. At the nano-scale, the hidden mosaics of zeolite structure predetermine the spatio-temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub-nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites.

Towards a comprehensive understanding of mesoporosity in zeolite Y at the single particle level

A full understanding of zeolite mesoporosity is not trivial yet is necessary to understand and optimize the catalytic performance of zeolites. The present work reports an integrated approach for the...

Chemical Imaging of Hierarchical Porosity Formation within a Zeolite Crystal Visualized by Small-Angle X-Ray Scattering and In-Situ Fluorescence Microscopy

Introducing hierarchical porosity to zeolites is vital for providing molecular access to microporous domains. Yet, the dynamics of meso- and macropore formation has remained elusive and pore space ill-characterized by a lack of (in situ) microscopic tools sensitive to nanoporosity. Here, we probe hierarchical porosity formation within a zeolite ZSM-5 crystal in real-time by in situ fluorescence microscopy during desilication. In addition, we introduce small-angle X-ray scattering microscopy as novel characterization tool to map intracrystal meso- and macropore properties. It is shown that hierarchical porosity formation initiates at the crystal surface and propagates to the crystal core via a pore front with decreasing rate. Also, hierarchical porosity only establishes in specific (segments of) subunits which constitute ZSM-5. Such space-dependent meso- and macroporosity implies local discrepancies in diffusion, performance and deactivation behaviors even within a zeolite crystal.

Effect of Pore Connectivity of Pore-Opened Hierarchical MOR Zeolites on Catalytic Behaviors and Coke Formation in Ethanol Dehydration

The hierarchical zeolite is one of the most promising materials for catalytic applications. However, the effect of its pore connectivity on catalytic behaviors and coke formation has not clearly been revealed. In this contribution, we demonstrate the visualization of the mesopore architecture in three-dimensional perspectives together with the pore connectivity network of pore-opened hierarchical mordenite (MOR), fabricated by the seed-assisted template-free synthesis followed by the fluoride treatment via the electron tomography (ET) technique. Interestingly, the pore-opened zeolites clearly display higher catalytic performance (approximately 80% of ethylene yield) in ethanol dehydration with respect to the parent one due to their additional pore-opened structures connected to the external surfaces of zeolites. In addition, the effect of pore connectivity network on the coke location and type obtained from ethanol conversion has been observed. It was found that the porous structure of the etched sample is directly connected to the external surface, and then, the large area of crystals can contribute to the reaction. Conversely, only a small amount of closed mesopores is observed inside the crystals in the case of the untreated sample, and therefore, the molecules cannot easily penetrate inside crystals for the catalytic reaction. These results open up promising perspectives for the development of hierarchical catalysts including fabrication by the template-free synthesis approach, pore-architecture characterization, and catalytic applications.

Pulsed field gradient NMR diffusion measurement in nanoporous materials

Labeling in diffusion measurements by pulsed field gradient (PFG) NMR is based on the observation of the phase of nuclear spins acquired in a constant magnetic field with purposefully superimposed field gradients. This labeling does in no way affect microdynamics and provides information about the probability distribution of molecular displacements as a function of time. An introduction of the measuring principle is followed by a detailed description of the ranges of measurements and their limitation. Particular emphasis is given to an explanation of possible pitfalls in the measurements and the ways to circumvent them. Showcases presented for illustrating the wealth of information provided by PFG NMR include a survey on the various patterns of concentration dependence of intra-particle diffusion and examples of transport inhibition by additional transport resistances within the nanoporous particles and on their external surface. The latter information is attained by combination with the outcome of tracer exchange experiments, which are shown to become possible via a special formalism of PFG NMR data analysis. Further evidence provided by PFG NMR concerns diffusion enhancement in pore hierarchies, diffusion anisotropy and the impact of diffusion on chemical conversion in porous catalysts. A compilation of the specifics of PFG NMR and of the parallels with other measurement techniques concludes the paper.

Complex Pore Structure of Mesoporous Zeolites: Unambiguous TEM Imaging Using Platinum Tracking

The article proposes a new way for visualization of mesopores and quantitative evaluation of the pore structure in zeolite crystals. The approach is based on platinum tracking inside the zeolite material after its incorporation from a gaseous precursor using an electron beam prior to preparing a TEM specimen by the focused ion beam technique. The pores in mesoporous silica and purely microporous zeolite Y were visualized in TEM images in a demonstration of the capabilities of the approach. Finally, platinum tracking was used for studying the pore structure of zeolite Y (CBV 720) containing mesopores both inside the crystal and those emerging at its surface, which were unambiguously distinguished from each other. The obtained sizes of the mesopores and the calculated material porosity are in good agreement with the results obtained by the low-temperature argon sorption isotherms method.

Hierarchical micro-/mesoporous zeolite microspheres prepared by colloidal assembly of zeolite nanoparticles

A novel template-free colloidal assembly method that combines colloidal zeolite (silicalite-1) suspensions in a water-in-oil emulsion with an evaporation-induced assembly process has been developed for preparing hierarchical micro-/mesoporous zeolite microspheres (MZMs). Such particles have an interconnected mesoporosity and large mesopore diameters (25–40 nm) combined with 5.5 Å diameter micropores of the zeolite nanoparticles. The method developed has the advantages of employing mild synthesis conditions, a short preparation time, and not requiring the use of a mesoporogen template or post-treatment methods. The method provides a new range of micro-/mesoporous zeolites with tunable mesoporosity dictated by the size of the zeolite nanoparticles. It also offers the possibility of combining several zeolite particle sizes or optionally adding amorphous silica nanoparticles to tune the mesopore size distribution further. It should be generally applicable to other types of colloidal zeolite suspensions (e.g. ZSM-5, zeolite A, beta) and represents a new route amenable for cost-effective scale-up.

Evaporation-driven colloidal assembly of silicalite-1 nanoparticles into well-defined micro-sized spheres at low temperature and preparation times.

Synthesis and Characterisation of Hierarchically Structured Titanium Silicalite-1 Zeolites with Large Intracrystalline Macropores

The successful synthesis of hierarchically structured titanium silicalite-1 (TS-1) with large intracrystalline macropores by steam-assisted crystallisation of mesoporous silica particles is reported. The macropore topology was imaged in 3D by using electron tomography and synchrotron radiation-based ptychographic X-ray computed tomography, revealing interconnected macropores within the crystals accounting for about 30 % of the particle volume. The study of the macropore formation mechanism revealed that the mesoporous silica particles act as a sacrificial macropore template during the synthesis. Silicon-to-titanium ratio of the macroporous TS-1 samples was successfully tuned from 100 to 44. The hierarchically structured TS-1 exhibited high activity in the liquid phase epoxidation of 2-octene with hydrogen peroxide. The hierarchically structured TS-1 surpassed a conventional nano-sized TS-1 sample in terms of alkene conversion and showed comparable selectivity to the epoxide. The flexible synthesis route described here can be used to prepare hierarchical zeolites with improved mass transport properties for other selective oxidation reactions.

Quantifying Morphology and Diffusion Properties of Mesoporous Carbon From High-Fidelity 3D Reconstructions

A reliable quantitative analysis in electron tomography, which depends on the segmentation of the three-dimensional reconstruction, is challenging because of constraints during tilt-series acquisition (missing wedge) and reconstruction artifacts introduced by reconstruction algorithms such as the Simultaneous Iterative Reconstruction Technique (SIRT) and Discrete Algebraic Reconstruction Technique (DART). We have carefully evaluated the fidelity of segmented reconstructions analyzing a disordered mesoporous carbon used as support in catalysis. Using experimental scanning transmission electron microscopy (STEM) tomography data as well as realistic phantoms, we have quantitatively analyzed the effect on the morphological description as well as on diffusion properties (based on a random-walk particle-tracking simulation) to understand the role of porosity in catalysis. The morphological description of the pore structure can be obtained reliably both using SIRT and DART reconstructions even in the presence of a limited missing wedge. However, the measured pore volume is sensitive to the threshold settings, which are difficult to define globally for SIRT reconstructions. This leads to noticeable variations of the diffusion coefficients in the case of SIRT reconstructions, whereas DART reconstructions resulted in more reliable data. In addition, the anisotropy of the determined diffusion properties was evaluated, which was significant in the presence of a limited missing wedge for SIRT and strongly reduced for DART.

Transmission electron microscopy as an important tool for characterization of zeolite structures

This review presents various TEM techniques including electron diffraction, high-resolution TEM and scanning TEM imaging, and electron tomography and their applications for structure characterization of zeolite materials.

Pd/Cu-Oxide Nanoconjugate at Zeolite-Y Crystallite Crafting the Mesoporous Channels for Selective Oxidation of Benzyl-Alcohols

Solid-state grinding of palladium and copper salts allowed the growth of palladium/copper oxide interface at the zeolite-Y surface. The hybrid nanostructured material was used as reusable heterogeneous catalyst for selective oxidation of various benzyl alcohols. The large surface area provided by the zeolite-Y matrix highly influenced the catalytic activity, as well as the recyclability of the synthesized catalyst. Impregnation of PdO-CuO nanoparticles on zeolite crystallite leads to the generation of mesoporous channel that probably prevented the leaching of the metal-oxide nanoparticles and endorsed high mass transfer. Formation of mesoporous channel at the external surface of zeolite-Y was evident from transmission electron microscopy and surface area analysis. PdO-CuO nanoparticles were found to be within the range of 2-5 nm. The surface area of PdO-CuO-Y catalyst was found to be much lower than parent zeolite-Y. The decrease in surface area as well as the presence of hysteresis loop in the N₂-adsoprtion isotherm further suggested successful encapsulation of PdO-CuO nanoparticles via the mesoporous channel formation. The high positive shifting in binding energy in both Pd and Cu was attributed to the influence of zeolite-Y framework on lattice contraction of metal oxides via confinement effect. PdO-CuO-Y catalyst was found to oxidize benzyl alcohol with 99% selectivity. On subjecting to microwave irradiation the same oxidation reaction was found to occur at ambient condition giving same conversion and selectivity.

Selectivity Control in the Tandem Aromatization of Bio-Based Furanics Catalyzed by Solid Acids and Palladium

Bio-based furanics can be aromatized efficiently by sequential Diels-Alder (DA) addition and hydrogenation steps followed by tandem catalytic aromatization. With a combination of zeolite H-Y and Pd/C, the hydrogenated DA adduct of 2-methylfuran and maleic anhydride can thus be aromatized in the liquid phase and, to a certain extent, decarboxylated to give high yields of the aromatic products 3-methylphthalic anhydride and o- and m-toluic acid. Here, it is shown that a variation in the acidity and textural properties of the solid acid as well as bifunctionality offers a handle on selectivity toward aromatic products. The zeolite component was found to dominate selectivity. Indeed, a linear correlation is found between 3-methylphthalic anhydride yield and the product of (strong acid/total acidity) and mesopore volume of H-Y, highlighting the need for balanced catalyst acidity and porosity. The efficient coupling of the dehydration and dehydrogenation steps by varying the zeolite-to-Pd/C ratio allowed the competitive decarboxylation reaction to be effectively suppressed, which led to an improved 3-methylphthalic anhydride/total aromatics selectivity ratio of 80 % (89 % total aromatics yield). The incorporation of Pd nanoparticles in close proximity to the acid sites in bifunctional Pd/H-Y catalysts also afforded a flexible means to control aromatic products selectivity, as further demonstrated in the aromatization of hydrogenated DA adducts from other diene/dienophile combinations.

Constructing two dimensional amide porous polymer to promote selective oxidation reactions

Rare example of carboxamide linked 2D COF has been synthesized and applied as robust organic catalyst for selective oxidation reactions with no metal add-ons. The reaction undergoes free radical mechanism aided by the conjugated π-cloud and amide functionality present in the COF.

Recent advances in the textural characterization of hierarchically structured nanoporous materials

This review focuses on important aspects of applying physisorption for the pore structural characterization of hierarchical materials such as mesoporous zeolites.

Effect of vanadium contamination on the framework and micropore structure of ultra stable Y-zeolite

Y-zeolites are the main component of fluid catalytic cracking (FCC) catalyst for conversion of crude petroleum to products of high demand including transportation fuel. We investigated effects of vanadium which is present as one of the impurities in FCC feedstock on the framework and micropore structure of ultra-stable (US) Y-zeolite. The zeolite samples were prepared and characterized using standard techniques including: (1) X-ray diffraction, (2) N2 adsorption employing non local density functional theory method, NLDFT, (3) Transmittance and Pyridine FTIR, (4) Transmittance electron microscopy (TEM), and (5) (27)Al and (29)Si MAS-NMR. Results revealed that in the presence of steam, vanadium caused excessive evolution of non inter-crystalline mesopores and structural damage. The evolved mesopore size averaged about 25.0nm at 0.5wt.% vanadium loading, far larger than mesopore size in zeolitic materials with improved hydrothermal stability and performance for FCC catalyst. A mechanism of mesopore formation based on accelerated dealumination has been proposed and discussed. Vanadium immobilization experiments conducted to mitigate vanadium migration into the framework clearly showed vanadium is mobile at reaction conditions. From the results, interaction of vanadium with the passivator limits and decreases mobility and activity of vanadium into inner cavities of the zeolite capable of causing huge structure breakdown and acid sites destruction. This study therefore deepens insight into the causes of alteration in activity and selectivity of vanadium contaminated catalyst and hints on a possible mechanism of passivation in vanadium passivated FCC catalyst.

Larger voids in mechanically stable, loose packings of 1.3μm frictional, cohesive particles: Their reconstruction, statistical analysis, and impact on separation efficiency

Lateral transcolumn heterogeneities and the presence of larger voids in a packing (comparable to the particle size) can limit the preparation of efficient chromatographic columns. Optimizing and understanding the packing process provides keys to better packing structures and column performance. Here, we investigate the slurry-packing process for a set of capillary columns packed with C18-modified, 1.3μm bridged-ethyl hybrid porous silica particles. The slurry concentration used for packing 75μm i.d. fused-silica capillaries was increased gradually from 5 to 50mg/mL. An intermediate concentration (20mg/mL) resulted in the best separation efficiency. Three capillaries from the set representing low, intermediate, and high slurry concentrations were further used for three-dimensional bed reconstruction by confocal laser scanning microscopy and morphological analysis of the bed structure. Previous studies suggest increased slurry concentrations will result in higher column efficiency due to the suppression of transcolumn bed heterogeneities, but only up to a critical concentration. Too concentrated slurries favour the formation of larger packing voids (reaching the size of the average particle diameter). Especially large voids, which can accommodate particles from>90% of the particle size distribution, are responsible for a decrease in column efficiency at high slurry concentrations. Our work illuminates the increasing difficulty of achieving high bed densities with small, frictional, cohesive particles. As particle size decreases interparticle forces become increasingly important and hinder the ease of particle sliding during column packing. While an optimal slurry concentration is identified with respect to bed morphology and separation efficiency under conditions in this work, our results suggest adjustments of this concentration are required with regard to particle size, surface roughness, column dimensions, slurry liquid, and external effects utilized during the packing process (pressure protocol, ultrasound, electric fields).

Synthesis, characterisation, and catalytic evaluation of hierarchical faujasite zeolites: milestones, challenges, and future directions

The preparation of hierarchical faujasite catalysts is challenging yet rewarding.

Creation of mesostructured hollow Y zeolite by selective demetallation of an artificial heterogeneous Al distributed zeolite crystal

A hollow mesoporous Y zeolite has been fabricated via a selective demetallation method based on an artificial heterogeneous Al distribution strategy.

Structural analysis of hierarchically organized zeolites

Advances in materials synthesis bring about many opportunities for technological applications, but are often accompanied by unprecedented complexity. This is clearly illustrated by the case of hierarchically organized zeolite catalysts, a class of crystalline microporous solids that has been revolutionized by the engineering of multilevel pore architectures, which combine unique chemical functionality with efficient molecular transport. Three key attributes, the crystal, the pore and the active site structure, can be expected to dominate the design process. This review examines the adequacy of the palette of techniques applied to characterize these distinguishing features and their catalytic impact.

Genesis of Delaminated-Zeolite Morphology: 3-D Characterization of Changes by STEM Tomography

Zeolite delamination increases the external surface area available for catalyzing the conversion of bulky molecules, but a fundamental understanding of the delamination process remains unknown. Here we report morphological changes accompanying delamination on the length scale of individual zeolite clusters determined by 3-D imaging in scanning transmission electron microscopy. The results are tomograms that demonstrate delamination as it proceeds on the nanoscale through two distinct key steps: a chemical treatment that leads to a swelled material and a subsequent calcination that leads to curling and peeling off of delaminated zeolite sheets over hundreds of nanometers. These results characterize the direct, local, 3-D morphological changes accompanying delaminated materials synthesis and, with corroboration by mercury porosimetry, provide unique insight into the morphology of these materials, which is difficult to obtain with any other technique.

Life and death of a single catalytic cracking particle

Fluid catalytic cracking (FCC) particles account for 40 to 45% of worldwide gasoline production. The hierarchical complex particle pore structure allows access of long-chain feedstock molecules into active catalyst domains where they are cracked into smaller, more valuable hydrocarbon products (for example, gasoline). In this process, metal deposition and intrusion is a major cause for irreversible catalyst deactivation and shifts in product distribution. We used x-ray nanotomography of industrial FCC particles at differing degrees of deactivation to quantify changes in single-particle macroporosity and pore connectivity, correlated to iron and nickel deposition. Our study reveals that these metals are incorporated almost exclusively in near-surface regions, severely limiting macropore accessibility as metal concentrations increase. Because macropore channels are "highways" of the pore network, blocking them prevents feedstock molecules from reaching the catalytically active domains. Consequently, metal deposition reduces conversion with time on stream because the internal pore volume, although itself unobstructed, becomes largely inaccessible.

Tailoring and visualizing the pore architecture of hierarchical zeolites

This review provides an overview of the different synthesis methods and microscopy techniques for tailoring and visualizing the pore architecture of hierarchical zeolites.

Morphological analysis of disordered macroporous-mesoporous solids based on physical reconstruction by nanoscale tomography

Solids with a hierarchically structured, disordered pore space, such as macroporous-mesoporous silica monoliths, are used as fixed beds in separation and catalysis. Targeted optimization of their functional properties requires a knowledge of the relation among their synthesis, morphology, and mass transport properties. However, an accurate and comprehensive morphological description has not been available for macroporous-mesoporous silica monoliths. Here we offer a solution to this problem based on the physical reconstruction of the hierarchically structured pore space by nanoscale tomography. Relying exclusively on image analysis, we deliver a concise, accurate, and model-free description of the void volume distribution and pore coordination inside the silica monolith. Structural features are connected to key transport properties (effective diffusion, hydrodynamic dispersion) of macropore and mesopore space. The presented approach is applicable to other fixed-bed formats of disordered macroporous-mesoporous solids, such as packings of mesoporous particles and organic-polymer monoliths.