Large Zeolite Crystals Research Articles

Effect of preparation parameters on properties of metakaolin-based geopolymer activated by silica fume- sodium hydroxide alkaline blend

In this study, metakaolin was activated using an alkali activator containing silica fume and sodium hydroxide to obtain a metakaolin-based geopolymer. Silica fume was used to partially substitute metakaolin (0 to 25 wt% of metakaolin). Enough reactive silica was introduced using silica fume, which was added to sodium hydroxide solution before being mixed with metakaolin. The Na2O/Al2O3 molar ratio was adjusted in the range of 0.8 to 1.2 by varying the sodium hydroxide concentration, and the geopolymer paste specimens were cured at three different temperatures, i.e., 60, 70, and 80 °C. The specimens were analyzed by X-ray diffraction, Fourier transforms infrared spectroscopy, scanning electron microscopy, mercury intrusion porosimetry, energy dispersion spectroscopy, and compressive strength measurement techniques. The results revealed that geopolymer-zeolite composites were produced in the presence of less than 10 wt% silica fume and Na2O/Al2O3 molar ratios higher than 1. When the SF content was raised above 10%, a significant increase in the formation of the geopolymer phase developed, resulting in increased compressive strength. Mercury intrusion porosimetry measurements show the geopolymer composite with 10 wt% silica fume and a Na2O/Al2O3 molar ratio of 1. It contained large zeolite crystals embedded in its macroporous geopolymer matrix and had a pore size of 400 nm. Maximum compressive strength of 33 MPa was achieved at 15 wt% silica fume, Na2O/Al2O3 molar ratio of 1, and curing temperature of 70 °C.

Synthesis of Hierarchical MOR-Type Zeolites with Improved Catalytic Properties.

Hierarchical MOR-type zeolites were synthesized in the presence of hexadecyltrimethylammonium bromide (CTAB) as a porogen agent. XRD proved that the concentration of CTAB in the synthesis medium plays an essential role in forming pure hierarchical MOR-type material. Above a CTAB concentration of 0.04 mol·L−1, amorphous materials are observed. These hierarchical mordenite possess a higher porous volume compared to its counterpart conventional micrometer crystals. Nitrogen sorption showed the presence of mesoporosity for all mordenite samples synthesized in the presence of CTAB. The creation of mesopores due to the presence of CTAB in the synthesis medium does not occur at the expense of zeolite micropores. In addition, mesoporous volume and BET surface seem to increase upon the increase of CTAB concentration in the synthesis medium. The Si/Al ratio of the zeolite framework can be increased from 5.5 to 9.1 by halving the aluminum content present in the synthesis gel. These synthesized hierarchical MOR-type zeolites possess an improved catalytic activity for n-hexane cracking compared to large zeolite crystals obtained in the absence of CTAB.

Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

AbstractImproving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one‐dimensional zeolites (ZSM‐22 and mordenite) and a γ‐alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n‐heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low‐molecular‐weight reactants.

Impact of the Spatial Organization of Bifunctional Metal-Zeolite Catalysts on the Hydroisomerization of Light Alkanes.

Improving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one-dimensional zeolites (ZSM-22 and mordenite) and a γ-alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n-heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the "intimacy criterion" for the rational design of bifunctional catalysts for the conversion of low-molecular-weight reactants.

Dealumination of MOR Zeolites with Different Crystal Morphologies

The process of dealumination of mordenites differing in morphology and crystal size obtained under identical conditions from reaction mixtures of similar composition has been studied. Parameters that have been chosen to change the morphology and crystal size are the dry matter concentration in the reaction mixture, the pH of the reaction mixture, the crystallization time, the presence of crystalline seeds, and agitation during the synthesis. Dealumination has been performed by heat treatment at 700°C followed by acid treatment. It has been shown that the stability of the mordenite crystal framework to dealumination depends on the thickness of the primary needle crystal, which forms the basis of large zeolite crystals, not on the size of the zeolite crystal as a whole. With a decrease in the average needle crystal thickness from 250 to 90 nm, the Si/Al molar ratio in the crystal lattice, determined from 29Si NMR data, increases from 6.8 to 9.5. The dealumination is accompanied by a change in the texture of large mordenite crystals associated with a decrease in the packing density of the primary needles.

Probing Zeolite Crystal Architecture and Structural Imperfections using Differently Sized Fluorescent Organic Probe Molecules

AbstractInvited for the cover of this issue is the group of Bert M. Weckhuysen at Utrecht University. The image depicts the probe molecule‐based staining method applied to large zeolite crystals, and is drawn with inspiration from the well‐known surrealist artist Salvador Dalí. Read the full text of the article at 10.1002/chem.201700078.

Probing Zeolite Crystal Architecture and Structural Imperfections using Differently Sized Fluorescent Organic Probe Molecules.

A micro‐spectroscopic method has been developed to probe the accessibility of zeolite crystals using a series of fluorescent 4‐(4‐diethylaminostyryl)‐1‐methylpyridinium iodide (DAMPI) probes of increasing molecular size. Staining large zeolite crystals with MFI (ZSM‐5) topology and subsequent mapping of the resulting fluorescence using confocal fluorescence microscopy reveal differences in structural integrity: the 90° intergrowth sections of MFI crystals are prone to develop structural imperfections, which act as entrance routes for the probes into the zeolite crystal. Polarization‐dependent measurements provide evidence for the probe molecule's alignment within the MFI zeolite pore system. The developed method was extended to BEA (Beta) crystals, showing that the previously observed hourglass pattern is a general feature of BEA crystals with this morphology. Furthermore, the probes can accurately identify at which crystal faces of BEA straight or sinusoidal pores open to the surface. The results show this method can spatially resolve the architecture‐dependent internal pore structure of microporous materials, which is difficult to assess using other characterization techniques such as X‐ray diffraction.

Large Ferrierite Crystals as Models for Catalyst Deactivation during Skeletal Isomerisation of Oleic Acid: Evidence for Pore Mouth Catalysis.

Large zeolite crystals of ferrierite have been used to study the deactivation, at the single particle level, of the alkyl isomerisation catalysis of oleic acid and elaidic acid by a combination of visible micro-spectroscopy and fluorescence microscopy (both polarised wide-field and confocal modes). The large crystals did show the desired activity, albeit only traces of the isomerisation product were obtained and low conversions were achieved compared to commercial ferrierite powders. This limited activity is in line with their lower external non-basal surface area, supporting the hypothesis of pore mouth catalysis. Further evidence for the latter comes from visible micro-spectroscopy, which shows that the accumulation of aromatic species is limited to the crystal edges, while fluorescence microscopy strongly suggests the presence of polyenylic carbocations. Light polarisation associated with the spatial resolution of fluorescence microscopy reveals that these carbonaceous deposits are aligned only in the larger 10-MR channels of ferrierite at all crystal edges. The reaction is hence further limited to these specific pore mouths.

Limitations of the Zero‐Length Column Technique to Measure Diffusional Time Constants in Microporous Adsorbents

AbstractThe zero‐length column (ZLC) technique has been developed to measure the intracrystalline diffusivity of strongly adsorbed species in large zeolite crystals above 50 μm in the Henry's law range of sorption equilibrium. The ZLC is a macroscopic technique, and there is a need of large crystallites or pellets to measure the intracrystalline diffusivity Dc of fast diffusion species or the macropore diffusivity DP of weakly adsorbed species, respectively. Another limitation is that ZLC desorption curves produce similar concentration profiles (linear isotherms) in bidisperse adsorbents (pellets) under macropore or micropore diffusion control. Moreover, the forms of the response curves are very similar in both diffusion‐ and nonlinear equilibrium‐controlled processes, leading to some misinterpretations of ZLC experiments. In this work, two criteria are developed showing that, in order to macroscopically measure the micropore diffusion time constant $ r_c^2/{D_c} $ or the macropore diffusion time constant $ R_p^2(1 + K)/{D_P} $ , the time of the ZLC experiments t should be higher than 7.0 × 10−2$ r_c^2/{D_c} $ or 7.0 × 10−2$ R_p^2(1 + K)/{D_P} $ , respectively. The interpretation of the ZLC response curve data is also checked in two completely different regimes, showing that a single ZLC response curve is not enough to conclude if a system is under a kinetic or an equilibrium regime.

ChemInform Abstract: Recent Advances in Secondary Ion Mass Spectrometry of Solid Acid Catalysts: Large Zeolite Crystals under Bombardment.

This Perspective aims to inform the heterogeneous catalysis and materials science community about the recent advances in Time-of-Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) to characterize catalytic solids by taking large model H-ZSM-5 zeolite crystals as a showcase system. SIMS-based techniques have been explored in the 1980–1990's to study porous catalyst materials but, due to their limited spectral and spatiotemporal resolution, there was no real major breakthrough at that time. The technical advancements in SIMS instruments, namely improved ion gun design and new mass analyser concepts, nowadays allow for a much more detailed analysis of surface species relevant to catalytic action. Imaging with high mass and lateral resolution, determination of fragment ion patterns, novel sputter ion concepts as well as new mass analysers (e.g. ToF, FTICR) are just a few novelties, which will lead to new fundamental insight from SIMS analysis of heterogeneous catalysts. The Perspective article ends with an outlook on instrumental innovations and their potential use for catalytic systems other than zeolite crystals.

Recent advances in secondary ion mass spectrometry of solid acid catalysts: large zeolite crystals under bombardment

This Perspective aims to inform the heterogeneous catalysis and materials science community about the recent advances in Time-of-Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) to characterize catalytic solids by taking large model H-ZSM-5 zeolite crystals as a showcase system. SIMS-based techniques have been explored in the 1980-1990's to study porous catalyst materials but, due to their limited spectral and spatiotemporal resolution, there was no real major breakthrough at that time. The technical advancements in SIMS instruments, namely improved ion gun design and new mass analyser concepts, nowadays allow for a much more detailed analysis of surface species relevant to catalytic action. Imaging with high mass and lateral resolution, determination of fragment ion patterns, novel sputter ion concepts as well as new mass analysers (e.g. ToF, FTICR) are just a few novelties, which will lead to new fundamental insight from SIMS analysis of heterogeneous catalysts. The Perspective article ends with an outlook on instrumental innovations and their potential use for catalytic systems other than zeolite crystals.

The role of zeolite Fe-ZSM-5 porous structure for heterogeneous Fenton catalyst activity and stability

Four types of iron containing materials have been synthesized: conventional zeolite Fe-ZSM-5 (conv), hierarchical zeolite Fe-ZSM-5 (hier), small crystals (d=330nm) of zeolite Fe-ZSM-5 (nano) and ferric oxide species supported on the amorphous silica Fe/SiO2. Samples were prepared by hydrothermal treatment, polystyrene spheres were used as a template for Fe-ZSM-5 (hier) and Fe/SiO2. The materials were characterized by different techniques. Nature of iron-containing particles in the samples and stability of iron species in the reaction media were suggested by using thermodynamic considerations. All solid-phase Fe-containing samples as well as dissolved Fe(NO3)3 were tested in H2O2 decomposition reactions in absence or presence of iron-complexing agent Na2EDTA, which has been used to test the catalyst stability. Catalytic activity of ferric species in hydrogen peroxide decomposition for small 330-nm crystals of Fe-ZSM-5 was 1.4times higher than for large zeolite crystals, and significant decrease of the activity was observed for samples containing amorphous silica phase. Experimental results showed that ferric sites in zeolite were stable due to the limited diffusion of Na2EDTA in zeolite phase. Wet hydrogen peroxide oxidation of organic complexing agents by H2O2 using Fe-containing zeolites has a good potential for purification of nuclear waste water.

Selective staining of zeolite acidity: Recent progress and future perspectives on fluorescence microscopy

This perspective article highlights recent methodical approaches for probing acid sites in zeolites using selective chemical staining methods. Research and method development on model systems (large zeolite crystals) is presented in close relation to the investigation of industrially relevant catalyst particles for Fluid Catalytic Cracking (FCC) and zeolite-based extrudates. The article begins with an (1) introduction on characteristics of zeolites and industrial catalyst particles, followed by a methodical overview on (2) probing acidity in zeolites, including temperature programmed desorption (TPD), solid state nuclear magnetic resonance (NMR) spectroscopy, and infrared (IR) spectroscopy. Instrumental details on (3) fluorescence microscopy are provided to prepare for the focus on (4) selective staining by probe molecules and oligomerisation reactions to highlight or distinguish materials (i.e. zeolite vs. binder or matrix components) and visualise acid sites within zeolites. Confocal fluorescence yields, in contrast to the other discussed techniques, a high spatiotemporal resolution giving way to exciting prospects such as probing single (fluorescent) reaction products. (5) Concluding remarks and future perspectives envision how Brønsted and Lewis acid sites can be investigated selectively.Sections: (1) Introduction (2) Probing acidity – methodical overview (3) Confocal fluorescence microscopy – instrumentation (4) Selective staining (5) Concluding Remarks and Future Perspectives.

Large Zeolite H-ZSM-5 Crystals as Models for the Methanol-to-Hydrocarbons Process: Bridging the Gap between Single-Particle Examination and Bulk Catalyst Analysis

The catalytic, deactivation, and regeneration characteristics of large coffin-shaped H-ZSM-5 crystals were investigated during the methanol-to-hydrocarbons (MTH) reaction at 350 and 500 °C. Online gas-phase effluent analysis and examination of retained material thereof were used to explore the bulk properties of large coffin-shaped zeolite H-ZSM-5 crystals in a fixed-bed reactor to introduce them as model catalysts for the MTH reaction. These findings were related to observations made at the individual particle level by using polarization-dependent UV-visible microspectroscopy and mass spectrometric techniques after reaction in an in situ microspectroscopy reaction cell. Excellent agreement between the spectroscopic measurements and the analysis of hydrocarbon deposits by means of retained hydrocarbon analysis and time-of-flight secondary-ion mass spectrometry of spent catalyst materials was observed. The obtained data reveal a shift towards more condensed coke deposits on the outer zeolite surface at higher reaction temperatures. Zeolites in the fixed-bed reactor setup underwent more coke deposition than those reacted in the in situ microspectroscopy reaction cell. Regeneration studies of the large zeolite crystals were performed by oxidation in O2 /inert gas mixtures at 550 °C. UV-visible microspectroscopic measurements using the oligomerization of styrene derivatives as probe reaction indicated that the fraction of strong acid sites decreased during regeneration. This change was accompanied by a slight decrease in the initial conversion obtained after regeneration. H-ZSM-5 deactivated more rapidly at higher reaction temperature.

Influence of crystal size and probe molecule on diffusion in hierarchical ZSM-5 zeolites prepared by desilication

The improvement of molecular transport properties of hierarchical H-ZSM-5 obtained by desilication was evidenced by studying the desorption of o-xylene and isooctane by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique enabled monitoring simultaneously bands associated with the molecular probes and the zeolite, using powdered sample masses as low as 1 mg. Two H-ZSM-5 samples with markedly different crystal sizes were investigated. The first sample was commercial and consisted of small crystallites ( ca. 250 nm). The second sample were laboratory-made large crystals with coffin-like shape ( ca. 17 × 4 × 4 μm 3). The hierarchical derivatives of the small and large zeolite crystals displayed 250 and 120 m 2 g −1 of mesopore surface area, respectively, in contrast to the 62 and 5 m 2 g −1 of the parent counterparts. The data based on o-xylene desorption were partly disguised by site-desorption limitations. Desorption experiments using isooctane evidenced a 4-fold reduction in the characteristic diffusion path length on both mesoporous small and large zeolites with respect to their purely microporous analogues. These results confirm the substantial potential for improvement of commercial nanocrystalline zeolites in diffusion-limited reactions upon the introduction of intra-crystalline mesoporosity by post-synthesis modification.

ChemInform Abstract: Detection of Carbocationic Species in Zeolites: Large Crystals Pave the Way

AbstractReview: 43 refs.

Detection of Carbocationic Species in Zeolites: Large Crystals Pave the Way

Large zeolite crystals have been used as model systems for the investigation of diffusion and catalytic reactivity phenomena in microporous host materials for at least two decades. However, their potential in assisting the detection of elusive reactive intermediates appears to have been underestimated. Herein, we show that a complementary use of vibrational and optical spectroscopy in combination with theoretical calculations allows for the unambiguous identification of transient carbocationic species generated upon the acid-catalyzed oligomerization of styrene derivatives within zeolite H-ZSM-5. Thanks to the mediated diffusion of the reactant in the large H-ZSM-5 crystals and minimal external surface the reaction intermediates can be accumulated within zeolite micropores in sufficient concentrations to allow their detection by synchrotron-based IR microspectroscopy. The UV/Vis and IR spectra display strong polarization dependence of on the molecular alignment of the dimeric styrene carbocations imposed by the zeolite channels and cages that can be rationalized in terms of the electronic and vibrational transitions of the intrazeolite carbocations. Based on these findings, a molecular-level picture of the macroscopic arrangement of the reaction intermediates confined within microporous zeolite matrices can be devised.

Optical Investigation of the Intergrowth Structure and Accessibility of Brønsted Acid Sites in Etched SSZ-13 Zeolite Crystals by Confocal Fluorescence Microscopy

Template decomposition followed by confocal fluorescence microscopy reveals a tetragonal-pyramidal intergrowth of subunits in micrometer-sized nearly cubic SSZ-13 zeolite crystals. In order to accentuate intergrowth boundaries and defect-rich areas within the individual large zeolite crystals, a treatment with an etching NaOH solution is applied. The defective areas are visualized by monitoring the spatial distribution of fluorescent tracer molecules within the individual SSZ-13 crystals by confocal fluorescence microscopy. These fluorescent tracer molecules are formed at the inner and outer crystal surfaces by utilizing the catalytic activity of the zeolite in the oligomerization reaction of styrene derivatives. This approach reveals various types of etching patterns that are an indication for the defectiveness of the studied crystals. We can show that specially one type of crystals, denoted as core-shell type, is highly accessible to the styrene molecules after etching. Despite the large crystal dimensions, the whole core-shell type SSZ-13 crystal is utilized for catalytic reaction. Furthermore, the confocal fluorescence microscopy measurements indicate a nonuniform distribution of the catalytically important Brønsted acid sites underlining the importance of space-resolved measurements.

Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H‐ZSM‐5 Crystals by Time‐ and Space‐Resolved Optical and Fluorescence Microspectroscopy

A combination of in-situ optical and fluorescence microspectroscopy has been employed to investigate the oligomerization of styrene derivatives occurring in the micropores of coffin-shaped H-ZSM-5 zeolite crystals in a space- and time-resolved manner. The carbocationic intermediates in this reaction act as reporter molecules for catalytic activity, since they exhibit strong optical absorption and fluorescence. In this way, reactant selectivity and restricted transition-state selectivity for 14 substituted styrene molecules can be visualized and quantified. Based on a thorough analysis of the time- and space-resolved UV/Vis spectra, it has been revealed that two main parameters affect the reaction rates, namely, the carbocation stabilization effect and the diffusion hindrance. The stabilization effect was tested by comparison of the reaction rates for 4-methoxystyrene versus 4-methylstyrene and in the series 4-bromo-, 4-chloro and 4-fluorostyrene; in both cases less electronegative substituents were found to accelerate the reaction. As to the steric effect, bulkier chemical groups bring down the reaction rate, as evident from the observation that 4-methoxystyrene is more reactive than 4-ethoxystyrene due to differences in their diffusivity, while heavily substituted styrenes, such as 3,4-dichlorostyrene and 2,3,4,5,6-pentafluorostyrene, cannot enter the zeolite pore system and therefore do not display any reactivity. Furthermore, beta-methoxystyrene and trans-beta-methylstyrene show limited reactivity as well as restricted reaction-product formation due to steric constraints imposed by the H-ZSM-5 channel system. Finally, polarized-light optical microspectroscopy and fluorescence microscopy demonstrate that dimeric styrene compounds are predominantly formed and aligned within the straight channels at the edges of the crystals, whereas a large fraction of trimeric carbocations along with dimeric compounds are present in the straight channels of the main body of the H-ZSM-5 crystals. Our results reinforce the observation of a non-uniform catalytic behavior within zeolite crystals, with specific parts of the zeolite grains being less accessible and reactive towards reactant molecules. The prospects and potential of this combined in-situ approach for studying large zeolite crystals in the act will be discussed.

Synthesis and Characterization of Large NaX Zeolite Crystals by Continuous Crystallization

NaX zeolite crystals of 50 µm were grown by a continuous crystallization with seed crystal (10 ㎛) in a mother solution having a 3.5Na2O : Al2O3 : 2.1SiO2 : 1000H2O composition. The seed crystal morphology of NaX zeolite shows that an octahedron is formed composed of 8 equilateral triangles which have an average lattice constant of a = 24.9911 Å with a SiO2/Al2O3 molar ratio of 2.1~2.4. In order to grow crystals of zeolite X to an appropriate size by the continuous method, the mother solution was supplied with various seed contents (3~20 wt%) in an autoclave at 90°C after 7, 12, 16, 19 and 24 days.