Hollow Zeolite Research Articles

Polypyrrole-coated hollow zeolite microcake as sulfur host for lithium‑sulfur batteries with improved electrochemical behaviors

Abstract The polysulfide shuttle phenomenon of lithium‑sulfur batteries restricts their practical applications. Herein, polypyrrole-coated hollow zeolite microcakes are first synthesized to reduce the polysulfide shuttle. The large surface area of hollow zeolite microcakes can increase the amount of sulfur storage. Moreover, the special hollow zeolite microcakes with polypyrrole coating enhance sulfur utilization. Consequently, the as-prepared composite electrode deliveres a capacity of 753 mAh g−1 after cycling 200 times at 0.1 A g−1, and a capacity of 460 mAh g−1 after cycling 500 times at 0.5 A g−1, which are much higher than those of the hollow zeolite microcakes without polypyrrole coating. These electrochemical characteristics make the composite a prospective sulfur host for lithium‑sulfur batteries.

Catalytic pyrolysis of poplar sawdust: Excellent hydrocarbon selectivity and activity of hollow zeolites

Hollow zeolites were investigated for catalytic fast pyrolysis (CFP) of biomass to produce hydrocarbon-rich bio-oil. A series of hollow ZSM-5 catalysts were synthesized via a dissolution-recrystallization strategy. The physicochemical properties of the catalysts were investigated by high-resolution transmission electron microscopy, N2 sorption, X-ray photoelectron spectroscopy, and ammonia temperature-programmed desorption experiments. The hollow zeolite was effective for increasing the hydrocarbon fraction in bio-oil. In particular, hollow HS-ZSM-5(50) afforded the highest hydrocarbon yield (6.8 wt%), which was ~3 times of that achieved with solid ZSM-5(50). The hollowness, acidity, and the presence of secondary wall mesopores in the hollow zeolite were found to affect bio-oil production. The hollow regions stabilized more active biomass intermediates and inhibited their repolymerization to coke, while the interior acid sites continually converted these intermediates to aromatic hydrocarbons. Secondary wall mesopores compromise the hollow space and hinder consecutive catalysis, resulting in phenols as the main product.

One-step synthesis of high-amount Fe-doped hollow MFI zeolite by Kirkendall effect in the presence of organic acid anions

Abstract Development of economical routes for synthesis of hollow zeolites with high amount of framework iron and high effectiveness factor is of great importance. Here, we provide a one-step and free-mesoporogen strategy to synthesize high-amount Fe-doped hollow silicate-1 with various cheap organic acids or derivative salts of organic acids. The contents of iron in zeolite and zeolite framework are ca. 3.3 wt% and 2.7 wt%, respectively. The hollow structures with the surface rich in iron have also been demonstrated. The presence of organic acid anions and the content of iron salts play the vital roles in the formation of hollow structure. In addition, the evolution process of Fe-doped hollow zeolite has been also investigated. The proposed mechanism based on the Kirkendall effect involves that high-amount iron-containing zeolite nanoparticles migrate outward relative to the nanoparticles interacted with carboxylate anions and carboxylate anions. The strategy can be successfully extended to TS-1 and ZSM-5. The high-amount Fe-doped hollow silicate-1 with high effectiveness factor exhibits an excellent catalytic performance for degradation reaction at mild conditions.

Facile synthesis of hierarchical hollow Mn-ZSM-5 zeolite for enhanced cyclohexane catalytic oxidation

ZSM-5 zeolite with hollow structure has gained considerable attention due to their unique properties and great potential applications in catalysis field. In this study, hierarchical hollow ZSM-5 zeolite was synthesized via a facile one-pot hydrothermal process during which a few amount of polyvinylpyrrolidone (PVP) was added. The synthesized ZSM-5 zeolite catalysts were characterized by the XRD, BET-BJH, TEM, FE-SEM, EDS, ICP-AES, XPS and intrusion/extrusion mercury analysis. The results showed that such hierarchical hollow ZSM-5 zeolite as micro-reactor after the modification by Mn species displayed excellent activity with a conversion of 98.7% and an overall selectivity of 97.6% for mild oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA oil) at 70 °C, making this type of catalyst a candidate for the production of industrial KA oil. Also, this approach is proved to be a desirable strategy for constructing hollow zeolite micro-ellipsoids by simple and cost-effective manner, thereby, providing a new avenue to be explored in catalysis field.

Hollow zeolite microspheres as a nest for enzymes: a new route to hybrid heterogeneous catalysts.

In the field of heterogeneous catalysis, the successful integration of enzymes and inorganic catalysts could pave the way to multifunctional materials which are able to perform advanced cascade reactions. However, such combination is not straightforward, for example in the case of zeolite catalysts for which enzyme immobilization is restricted to the external surface. Herein, this challenge is overcome by developing a new kind of hybrid catalyst based on hollow zeolite microspheres obtained by the aerosol-assisted assembly of zeolite nanocrystals. The latter spheres possess open entry-ways for enzymes, which are then loaded and cross-linked to form cross-linked enzyme aggregates (CLEAs), securing their entrapment. This controlled design allows the combination of all the decisive features of the zeolite with a high enzyme loading. A chemo-enzymatic reaction is demonstrated, where the structured zeolite material is used both as a nest for the enzyme and as an efficient inorganic catalyst. Glucose oxidase (GOx) ensures the in situ production of H2O2 subsequently utilized by the TS-1 zeolite to catalyze the epoxidation of allylic alcohol toward glycidol. The strategy can also be used to entrap other enzymes or combination of enzymes, as demonstrated here with combi-CLEAs of horseradish peroxidase (HRP) and glucose oxidase. We anticipate that this strategy will open up new perspectives, leveraging on the spray-drying (aerosol) technique to shape microparticles from various nano-building blocks and on the entrapment of biological macromolecules to obtain new multifunctional hybrid microstructures.

Synthesis of all-silica hollow zeolites by selective demetallation

Exploiting the protective role of extra-framework aluminum species on the crystal rim, all-silica hollow zeolites are efficiently produced by demetallation.

Variation of Aluminium Distribution in Small-Sized ZSM-5 Crystals during Desilication.

Hollow ZSM-5 zeolites of size below one micrometer can be produced by desilication of crystals with aluminium zoning. The parent crystals have a core-shell structure: the core part has nearly no aluminium, whereas the aluminium content in the shell increases when extending to exterior surface. Transmission electron microscopy confirmed the preservation of the crystalline shell after base leaching, but could not identify its subtle change. An increase of the Si/Al ratio of the surface was detected upon leaching the parent material to form the hollow zeolite by using ambient pressure X-ray photoelectron spectroscopy and infrared spectroscopy of substituted alkylpyridines. 27 Al MAS NMR showed that base leaching results in a reduced percentage of distorted tetrahedrally coordinated aluminium. The reprecipitation of dissolved species occurs and tetrahedrally coordinated tin atoms can thus be introduced to the shell framework. Overall, the formation of hollow ZSM-5 zeolites by desilication involves not only the removal of silicon-rich core, but also a reduced percentage of exterior aluminium-related acid sites, which should be considered while using hollow zeolites in acid-catalyzed reactions.

Accelerating CO2 capture of highly permeable polymer through incorporating highly selective hollow zeolite imidazolate framework

AbstractThe implementation of high‐performance membranes in large‐scale CO2 capture has the potential to significantly decrease the capture cost and reduce the environmental footprints. However, highly permeable polymers rarely have sufficient selectivity for energy‐efficient carbon capture. In this study, zeolite imidazolate framework hollow nanoparticles (ZIF‐HNPs) were synthesized and embedded into highly permeable polymers as versatile fillers to prepare mixed matrix membranes (MMMs). The interior hollow architecture minimizes transport resistance of gas diffusion through the fillers while its molecular‐sieving shell provides high selectivity. With 28 vol% loading of ZIF‐HNPs, the membrane exhibits CO2 permeability of 7,128 Barrer and CO2/CH4 selectivity of 16.4 (57.7% and 31.4% higher than these of pristine membrane), which surpass the upper bound of the state‐of‐the‐art reported polymeric membranes. Meanwhile, we proposed a modified Maxwell model based on the hierarchical structure of the MMM to analyze the effects of cavity size and loading on gas transport behaviors within membranes.

Cationic surfactant-directed synthesis of hollow Beta zeolite with hierarchical structure

Hollow Beta zeolite with hierarchical meso/microporous has been synthesized via one-step hydrothermal method by using a designed cationic surfactant of N2-p-N2 as soft-template. The structure-directing ability of N2-p-N2 was studied by the DFT calculations. The hollow Beta zeolite with a hierarchical structure was characterized by a combination of X-ray diffraction, Fourier transform infrared spectroscopy, nitrogen adsorption-desorption isotherms, scanning electron microscopy and transmission electron microscopy. The structural characterization suggested that the positively charged quaternary ammonium group can interact with the anionic aluminosilicate species to induce the formation of Beta zeolites. The hollow zeolites possess the large inner cavity and hierarchical structure, which can significantly reduce diffusion limitations and facilitate accessibility of the active sites. FTIR spectra of 2,6-di-tertbutylpyridine results indicated that the hollow Beta zeolite with hierarchical structure provided increased external surface acid sites and exhibited superior catalytic property in the benzylation of benzyl alcohol with mesitylene. Such hollow zeolite with hierarchical structure has great potential in catalysis involving large molecules.

One-Pot Synthesis and Formation Mechanism of Hollow ZSM-5.

Coffin-shaped hollow ZSM-5 zeolite (HZZ) particles with shell thickness of about 200 nm and hollow diameter of approximately 1.5 μm were synthesized in one pot by using tetrapropylammonium bromide (TPABr), aluminum triisopropoxide Al[OCH(CH3 )2 ]3 and tetraethoxysilane (TEOS) as the structure-directing agent (SDA), aluminum and silica source, respectively. The appropriate molar ratios of TPABr/SiO2 and Si/Al as well as suitable crystallization temperature are the key factors for the formation of HZZ. The formation of the HZZ can be attributed to the existence of intrinsic density variation inside the initial amorphous aggregates and the Al zoning in the outer surface of the ZSM-5 particles. Amorphous silica with low crystallinity formed at early stages and low Al concentration, which has been subsequently dissolved and recrystallized on the ZSM-5 particle surface through Ostwald ripening, leading to the formation of HZZ. This approach, which uses a high concentration of SDA, will provide new possibilities and insight into the prospective fabrication of hollow zeolites.

Preparation of zeolite hollow fibers for high-efficiency cadmium removal from waste water

Geometry greatly affects performances in adsorption and catalytic reactions. Since the crystal structure of zeolite collapses at high temperature, it is difficult to synthesize hollow fibers from only zeolite by sintering. In this study, we synthesized zeolite hollow fibers using colloidal silica, which has excellent heat and chemical resistance, as an inorganic binder. The zeolite hollow fiber precursor was immersed in a colloidal silica aqueous solution and calcined at 600 °C. Additional silica coating significantly improves the mechanical robustness by increasing the fracture load by up to 200%, while the zeolite content, specific surface area/pore volume reduced slightly. These results show that colloidal silica functions as an effective inorganic binder without blocking the pores of the zeolite. The cadmium adsorption showed that for a 100-ppm Cd2+ solution, the breakthrough time at C/C0 = 0.05 was effectively extended by 20% (112 min) and 65% (153 min) for conventional hollow fiber and 3-channel designs, respectively, compared to the powder counterpart (93 min).

Transformation of Stöber Silica Spheres to Hollow Hierarchical Single-Crystal ZSM-5 Zeolites with Encapsulated Metal Nanocatalysts for Selective Catalysis.

The activity of zeolite-supported nanocatalysts is dependent on both the dispersion, size, and location of metal nanoparticles around the zeolite and the size and pore structure of the zeolite. In this study, a synthetic approach was developed to encapsulate metal catalysts within hollow interiors of single-crystal ZSM-5. Briefly, Stöber silica spheres were synthesized and then transformed to single-crystal nano-ZSM-5 (Si/Al = 60), followed by growth of embedded metal nanoparticles and subsequently creation of a nanosized (30-50 nm shell thickness) hollow hierarchical zeolite structure. Metal nanoparticles such as Co, Cu, Cu-Zn, Fe, and Ni can be supported on the inner wall of the hollow zeolite and the surrounding satellite mesopores, without any particles present on the external zeolite surface. When evaluated as a catalyst for the Fischer-Trøpsch reaction, the Fe@h-ZSM5 catalyst shows high activity, sintering and coking resistance (50% longer stability than Fe@ZSM5), and secondary cracking reactions in the acid sites in the ZSM-5 shell, which reduce C5+ hydrocarbon selectivity and increase smaller-chain hydrocarbon selectivity. In addition, when Pt was further deposited inside the hollow structure, shape-selective alkene hydrogenation was demonstrated. These configured nanoscale zeolite catalysts have potential applications for reactions that involve supported metal nanoparticle catalysis, shape selectivity, or secondary cracking reactions.

Zeolite Nanoreactor for Investigating Sintering Effects of Cobalt-Catalyzed Fischer–Tropsch Synthesis

In this study, a hollow zeolite nanoreactor is used to promote uniform nanoparticle formation and inhibit sintering of active nanoparticles during Fischer–Tropsch synthesis. Cobalt oxide nanopartic...

Direct synthesis of hollow single-crystalline zeolite beta using a small organic lactam as a recyclable hollow-directing agent

Hollow zeolite beta can be directly synthesized by using some pentacyclic lactams as hollow-directing agents.

Hollow Zeolite Single Crystals: Synthesis Routes and Functionalization Methods

AbstractHollow zeolite single crystals represent a new class of materials that have received considerable attention over the last decade. Besides facilitating the transport of molecules in zeolite crystals, the cavity present in hollow crystals offers many possibilities for encapsulating active species and constitutes a closed space that can be used as a nanoreactor. However, despite the simplicity of the concept, the synthesis of hollow zeolites is difficult, as it necessitates to control the chemistry and the spatial distribution of elements throughout the crystals. Here, the different strategies that can be used to transform bulk zeolite crystals into hollow analogs are reported. In particular, selective dissolution as well as dissolution–recrystallization methods are discussed in detail in the case of zeolites with mobile five (MFI) framework type. More sophisticated fabrication strategies are also described for two other major zeolites, namely, zeolites Beta and Y. Additionally, methods to introduce metal nanoparticles with controlled size and composition in the cavity are also addressed. The review ends with concluding remarks, including major challenges and future outlooks in the field.

Highly Selective SSZ‐13 Zeolite Hollow Fiber Membranes by Ultraviolet Activation at Near‐Ambient Temperature

AbstractZeolitic membranes synthesized using organic structure‐directing agents (SDAs) require an activation step to remove the SDA and open their porosity. Activation is typically achieved by high‐temperature (>673 K) calcination. This process has multiple disadvantages, including coke formation due to incomplete removal of the SDA as well as the formation of cracks and other defects due to differential thermal expansion of the membrane layer and the underlying support material. Here we report that high‐performance hollow fiber membranes of the small‐pore (0.38 nm) zeolite SSZ‐13 can be produced via UV irradiation to decompose and remove the SDA. Remarkably, UV irradiation allowed complete removal of the bulky SDA (trimethyladamantylammonium hydroxide) from the pores at near‐ambient conditions, whereas membranes activated by calcination exhibited severe cracking. The UV‐activated SSZ‐13 membranes showed excellent H2/C3H8 and CO2/CH4 mixture selectivities (up to 810 and 110 whereas the conventionally activated membranes showed poor selectivity (<5). The combined demonstration of hollow fiber membrane synthesis and low‐temperature membrane activation of small‐pore zeolite membranes is a significant step in the effort to create reliable, scalable, and low‐cost fabrication processes for zeolite membranes for gas separations.

Properties Modification of Nanosized Hollow Zeolite Crystals by Desilication

AbstractA strategy to modify the properties of hollow zeolite was developed by leaching the “parent” zeolite in alkaline solution. A systematic study was performed to identify the effect of both the synthesis conditions of “parent” zeolite and the leaching conditions on the properties of hollow zeolite. Fine‐tuning the leaching conditions, i. e. the concentration of sodium hydroxide solution, allows control over the degree of hollowness of the zeolite, achieving the introduction of larger voids in different locations of individual crystals, which is a key factor if the crystals are to be used for catalytic applications. In addition, a hypothesis was proposed to explain the formation of hollow ZSM‐5 crystals. By replacing part of tetrapropylammonium hydroxide (TPAOH) with tetrapropylammonium bromide (TPABr) as the structure‐directing agent (SDA), the “parent” zeolites obtained were less stable in alkaline solution and formed hollow structures more easily. The size of hollow zeolite crystals could be easily controlled by adjusting the water content of the synthesis gel. Finally, crystals with double shells were prepared by a method involving two consecutive crystallizations followed by base leaching. Overall, fine‐tuning of zeolite synthesis and post‐synthesis conditions leads to control of the structure of hollow zeolite crystals.

Pt nanoparticles encapsulated in a hollow zeolite microreactor as a highly active and stable catalyst for low-temperature ethanol steam reforming

High dispersion of Pt nanoparticles encapsulated in hollow Pt@HBZ microreactor has been successfully fabricated through the function of resorcinol-formaldehyde layer during the synthesis process. The low-temperature ethanol steam reforming (LT-ESR) on the Pt@HBZ was studied using various techniques with a focus on the structural characterization of the catalysts, reaction mechanism, and understanding of the structure-activity relationship. The results of steady-state-activity testing, comparisons with those from Pt-beta and Pt-SiO2 catalysts showed that the Pt@HBZ exhibited superior activity, stability and low by-products with negligible deactivation phenomena for 15 h during LT-ESR. The hindering effect of zeolite shell could effectively improve the concentration of relatively large molecules (i.e., ethanol and acetaldehyde) in the hollow micro-environment and promote the molecules reacted with Pt active sites to achieve a complete catalytic conversion. Moreover, due to the “purification effect” for the undesired products (CO, CH4, acetaldehyde and unreacted ethanol), the existence of a large number of microreactors in the reaction system can obtain high-purity hydrogen production.

Preparation of hollow zeolite NaA/chitosan composite microspheres via in situ hydrolysis-gelation-hydrothermal synthesis of TEOS

In situ hydrolysis-gelation-hydrothermal (HGH) synthesis of tetraethylorthosilicate (TEOS) technique was developed to prepare hollow zeolite NaA/chitosan composite microspheres. The chitosan solution coated calcium alginate microspheres served as template to generate hollow structure, which were pre-modified by oleic acid and coated by TEOS. Furthermore, the calcium alginate microspheres were prepared by a simple homemade double T-junction mixer. During the hydrothermal process, the TEOS hydrolyzed and provided silica source for the zeolite NaA shell, meanwhile the inner calcium alginate microsphere core dissolved by the alkaline synthesis mixture and left the hollow structure. The obtained products were characterized by XRD, FT-IR, SEM, TG et al. techniques. The preparation method for calcium alginate microspheres template was simple and the preparation process had no NaA crystal seeds been involved. The hollow size could be adjusted by controlling the synthesis parameters of calcium alginate/chitosan microspheres. In addition, the functional magnetic γ-Fe2O3 nanoparticles could be introduced into the cavity during synthesis of calcium alginate/chitosan microspheres and guest magnetic γ-Fe2O3 nanoparticles had no effect on the properties of host zeolite NaA. The obtained functional magnetic hollow NaA/chitosan microspheres had decent adsorption performance for Cu2+ ions and were easy to recycle.

Big World in the Small Space: Constructing Hollow Zeolite Microspheres Using a Sustainable Template

Hollow zeolite microspheres have attracted considerable interest due to their unique properties and great potential applications. In this work, we report a simple and cost-effective approach for constructing hollow zeolite microspheres based on a biomass-derived template, i.e., carboxymethylcellulose sodium (CMC). As an example, a hollow ZSM-5 microsphere with a hollow core smaller than 1 μm in diameter and a complete crystal shell is synthesized. This approach overcomes most of the limitations associated with the existing methods, such as complex operations and costly spherical templates. By studying the growth process in detail, a possible formation mechanism is proposed. The crystallization of the hollow zeolite microsphere is through a “surface to core” process, in which the interaction between the CMC and the zeolite gel and the decomposition of the polymer network play the critical roles.