Intracrystalline Mesopores Research Articles

A Sustainable Template for Mesoporous Zeolite Synthesis

A generalized synthesis of high-quality, mesoporous zeolite (e.g., MFI-type) nanocrystals is presented, based on a biomass-derived, monolithic N-doped carbonaceous template. As an example, ZSM-5 single crystals with desirable large-diameter (12-16 nm) intracrystalline mesopores are synthesized. The platform provides scope to optimize template dimensions and chemistry for the synthesis of a range of micro-/mesoporous crystalline zeolites in a cost-effective and highly flexible manner.

Hierarchical high-silica zeolites as superior base catalysts

For more than four decades, the design of zeolite base catalysts has relied on the application of aluminium-rich frameworks exchanged with alkali metal cations (preferably Cs+). However, moderate activity associated with access and diffusion limitations, and high manufacturing costs associated with high caesium content (typically over 30%) have hampered their industrial implementation so far. Herein, we have discovered that high-silica USY zeolites outperform their Al-rich counterparts in a variety of base-catalysed reactions of relevance in the fine chemical industry, as well as in the upgrading of biofuels. The benefits of this class of materials are amplified upon the alleviation of diffusion constraints through the introduction of a network of intracrystalline mesopores by post-synthetic modification. For example, the resulting cation-free hierarchical USY provides an up to 30-fold Knoevenagel condensation activity compared to the benchmark Cs–X, and similar observations were made upon application in liquid-phase (nitro)aldol reactions. Moreover, in the gas-phase aldol condensation of propanal, high-silica zeolites provide superior activity, selectivity, and lifetime compared to caesium-containing zeolites and even a strong solid base such as MgO. We decouple the complex interplay between mesoporosity and intrinsic zeolitic properties such as crystallinity, and quantify the increase in catalyst effectiveness upon hierarchical structuring as a function of reactant size. The obtained results are a major step to resolve the drawbacks of zeolites catalysis and thereby revitalise their potential for industrial application.

Unstable-Fe-site-induced formation of mesopores in microporous zeolite Y without using organic templates

A novel organic template-free strategy for generating mesoporosity in Y zeolites is reported. It is revealed that Fe(3+) functioned as unstable sites in the Fe-NaY zeolite, which promotes deferrization-dealumination, leading to enhanced formation of intra-crystalline mesopores as well as desirable interconnectivity. The mesopore-enriched zeolite exhibits a remarkable ability in conversion of the bulky substrate.

Hierarchically tetramodal-porous zeolite ZSM-5 monoliths with template-free-derived intracrystalline mesopores

In this work, we report a unique synthesis of hierarchical zeolite ZSM-5 monoliths using polyurethane foam (PUF) as a rigid scaffold through a simple steam-assisted crystallization (SAC) method. The resultant monolithic ZSM-5 with a well crystalline structure possesses unique tetra-modal porosity (macropore/macropore/mesopore/micropore). The first modes of macroporosity (pore size of ∼33 μm) is formed after the removal of the PUF scaffolds. The second set of macropores with a size of 0.2–1.7 μm originate from the aggregation of ZSM-5 nanocrystals (crystal size of ∼500 nm) inside the macropores of the PUF scaffolds. The third level of porosity stems from the intracrystalline mesopores (∼53 nm) in each ZSM-5 crystal, along with the fourth intrinsic microporosity of zeolites. The mesopores are derived from the voids formed in the initial dry precursors, which are transformed and preserved as intracrystalline mesopores in each zeolite crystal. This type of ZSM-5 monolith possesses a high macroporosity (75.2%) and mechanical stability (1.2 MPa). The total surface area of the monolithic ZSM-5 is ∼226 m2 g−1 and the total pore volume is ∼0.21 cm3 g−1. The ZSM-5 monolith as a structured catalyst for methanol to propylene (MTP) conversion shows excellent catalytic performance with high methanol conversion (above 95%) and propylene selectivity (above 40%) at a high weight hourly space velocity (WHSV, 3.6 h−1). Importantly, the diffusion efficiency of catalyst is remarkably improved. After reaction for 5 h, the selectivity of propylene reaches a steady state.

Mechanistic study of zeolites recrystallization into micro-mesoporous materials

The mechanism of mordenite recrystallization has been studied using multinuclear MAS NMR, X-ray diffraction, IR spectroscopy, transmission electron microscopy, thermogravimetric analysis and nitrogen adsorption–desorption. The recrystallization procedure involved mixing of zeolite with alkali solution containing cetyltrimethylammonium bromide (CTAB) surfactant, heating the mixture in the autoclave to 423K and hydrothermal treatment at 423K. The intermediate products were recovered during each reaction step and thoroughly investigated. The results pointed to dissolution/re-assembling mechanism involving (i) fast ion exchange of protons with sodium cations and zeolite desilication, (ii) the diffusion of CTAB inside the pores and the ion exchange of sodium with CTA-cations and (iii) the formation of micelles and the condensation of siliceous species around micelles both in the intracrystalline mesopores formed during mordenite desilication and on the external surface of zeolite crystallites.

Formation mechanism and catalytic application of hierarchical structured FeAlPO-5 molecular sieve by microwave-assisted ionothermal synthesis

At atmospheric pressure, FeAlPO-5 molecular sieve with hierarchical micro- and meso-porous structure has been ionothermally synthesized by microwave heating with eutectic mixture as the solvent. Among the synthesis parameters investigated, the ratio of P2O5/Al2O3, aluminum source, and heating methods significantly affected the mesoporosity of FeAlPO-5 molecular sieve. N2 Physisorption, SEM and TEM characterizations showed that the resultant material possessed inter- and intra-crystalline mesopores simultaneously. The larger intercrystalline mesopores in size resulted from the intercrystalline void space between bar crystals and nanoparticles. The calcination removed the organic species embedded in the crystals of FeAlPO-5 molecular sieve, leading to the formation of smaller intracrystalline mesopores. The catalytic properties of hierarchical FeAlPO-5 molecular sieve were investigated in the hydroxylation of phenol with microporous FeAlPO-5 as a reference. The results showed that the catalytic performance of the former catalyst was superior to the latter.

Micron-Sized Single-Crystal-like CoAPO-5/Carbon Composites Leading to Hierarchical CoAPO-5 with Both Inter- and Intracrystalline Mesoporosity

Commercial mesopores-containing carbon (carbon black BP2000) has been used as crystallization host as well as hard template of intracrystalline mesopores in CoAPO-5 materials prepared with N-methyldicyclohexylamine as specific structure-directing agent. As a result, micrometer-sized AFI-structured terminated-tip (pencil-like) hexagonal prims have been obtained. Despite the appearance of single crystals, the crystals are actually composites formed by a large number of elongated-shape CoAPO-5 and carbon nanodomains. Indeed, the removal of carbon by calcination leaves an inorganic Co-doped AlPO4-based material having a complex hierarchical pore system, formed by the expected microporosity for an AFI-structured material and both intra- and intercrystalline mesoporosity. In good agreement with the nanocomposite nature of these micrometer-sized crystals, the size of their fibers generated by burning the carbon matrix can be relatively controlled by the conditions of the calcination processes. Under optimized synthesis and calcination conditions, AFI-structured materials with high mesoporous surface area and pore volume can be afforded.

Highly stable hierarchical ZSM-5 zeolite with intra- and inter-crystalline porous structures

Highly stable hierarchical ZSM-5 zeolites with intra-crystalline mesopores and inter-crystalline macropores have been synthesized via a steam-assisted conversion in the presence of polyethylene glycol. XRD, SEM, TEM, N2 sorption and mercury intrusion measurement revealed that the irregular intra-crystal mesopores (2–10nm) in each zeolite nanocrystal were generated by the removal of encapsulated PEG, and the inter-crystalline macropores (50–90nm) were formed by the aggregation of the well crystallized ZSM-5 nanocrystals with a small size (50–200nm). The ZSM-5 zeolite nanocrystals were achieved by a dense-gel with high TPA+/SiO2, highly concentrated TPA+ can induce a burst in nucleation rate during the first step of steam-assisted conversion, which makes the dry gel easy to convert into zeolite nanocrystals, PEG arrested in the gel can be easily encapsulated by zeolite crystals during the steam-assisted conversion. Finally, these nanocrystals containing intra-crystalline mesopores aggregate into the bulky materials with an inter-crystalline macroporous network. In addition, the mesopore volume and external surface areas of Hier-ZSM-5 can be easily adjusted by changing the amount of PEG. Catalytic tests showed that Hier-ZSM-5 had a similar activity to Nano-ZSM-5, which must be collected by high-speed centrifugation; while had a higher activity than Con-ZSM-5, especially for the reactions involving large molecules. The high catalytic activity and easy separation from liquid mixture of Hier-ZSM-5 would facilitate its practical applications in chemical industry.

A comparative study of glycerol dehydration catalyzed by micro/mesoporous MFI zeolites

The catalytic properties of monomodal microporous and bimodal micro-mesoporous zeolites were investigated in the gas-phase dehydration of glycerol. The desilication methodology used to produce the mesoporous zeolites minimized diffusion limitations and increased glycerol conversion in the catalytic reaction due to the hierarchical system of secondary pores created in the zeolite crystals. The chemical and structural properties of the catalyst were studied by X-ray diffraction, nitrogen adsorption–desorption isotherms, NH3-TPD and pyridine chemisorption followed by IR-spectroscopy. Although the aim was to desilicate to create mesoporosity in the zeolite crystals, the desilication promoted the formation of extra-framework aluminum species that affected the conversion of glycerol and the products distribution. The results clearly show that the mesoporous zeolites with designed mesopore structure allowed a rapid diffusion and consequently improved the reaction kinetics. However, especial attention must be given to the desilication procedure because the severity of the treatment negatively interfered on the Brønsted and Lewis acid sites relative concentration and, consequently, in the efficiency of the catalysis performed by these materials. On the other hand, during the catalytic reaction, the intracrystalline mesopores allowed carbonaceous compounds to be deposited herein, resulting in less blocked micropores and catalysts with higher long-term stability.

Cumene disproportionation over micro/mesoporous catalysts obtained by recrystallization of mordenite

Micro/mesoporous materials with different contribution of micro/mesoporosity have been studied in cumene disproportionation. The materials were prepared by recrystallization of mordenite and characterized by X-ray diffraction, nitrogen adsorption–desorption, transmission electron microscopy, infrared spectroscopy, and temperature-programmed desorption of ammonia. Micro/mesoporous materials with a low degree of recrystallization showed remarkably high activity and stability with respect to the parent and desilicated mordenites. The enhanced catalytic activity is due to a high zeolitic acidity combined with improved accessibility of active sites and transport of bulky molecules provided by uniform and homogeneously distributed intracrystalline mesopores. The high catalyst resistance to deactivation is due to two reasons: i) the improved transport of bulky products, which prevents the catalyst from the formation of “soft” coke inside the zeolite pores; ii) the coating of the zeolite with a mesoporous layer, which prevents the formation of “hard” coke on the external surface of the crystal.

Visualization of hierarchically structured zeolite bodies from macro to nano length scales

A major challenge in the implementation of laboratory-designed catalysts is the scale-up into technically relevant forms. Advanced characterization is essential to understand and optimize catalyst assembly and function in industrial reactors. This Article presents an integrated approach to visualizing millimetre-sized extrudates and granules of a hierarchical MFI-type zeolite, displaying trimodal networks of micropores (0.56nm), intracrystalline mesopores (∼10nm) and macropores (∼200-300nm). As exemplified for the conversion of methanol to olefins, the hierarchical zeolite yields a superior performance compared to its conventional analogue. The combination of dedicated specimen preparation with state-of-the-art optical, X-ray and electron-based microscopic and tomographic techniques proves a powerful methodology to reveal otherwise inaccessible information regarding structural organization over the whole range of length scales. It is expected that these tools will play a crucial role in the rationalization of scale-up principles in catalyst development.

Hierarchical structure and catalytic properties of a microspherical zeolite with intracrystalline mesopores

An effective microspherical zeolite MFI for large molecules was prepared for the first time. The material is found in a microspherical form 5.0μm in size, and the microspheres are composed of 20–30-nm nanoparticles. The nanoparticles further contain uniform intracrystalline mesopores of ∼3.0nm. The mesopores in the material are generated in the course of crystallization, owing to bond-blocking from silanized silicon dioxide, and the mesoporous porosity can be effectively adjusted though different degrees of silanization. During the synthesis, an excess of grafted organic species on the silica surface inhibits the formation of zeolitic phases. The formation of the microsphere is attributed to the glomeration of halogen anions and/or the salts present during the synthesis. Fourier transform infrared spectrophotometry and 29Si and 27Al nuclear magnetic resonance results confirm that the existence of silanol groups is directly related to the rich crystalline defect sites. The adsorption of alkali molecules with different kinetic diameters (0.5–0.8nm) indicates that the mesopores generated in the zeolite catalyst markedly increase the accessibility of micropores and the availability of acid sites, which is confirmed by high performances observed in catalytic conversions, such as cumene cracking, benzylation of benzene, and methanol conversion to gasoline. The introduction of auxiliary mesoporosity to the zeolitic catalyst is believed to improve molecule transport and enhance the accessibility of active sites in the microporous structure.

Skeletal isomerization of 1-butene over micro/mesoporous materials based on FER zeolite

Skeletal isomerization of 1-butene has been studied over composite micro/mesoporous zeolitic catalysts prepared by the recrystallization of ferrierite (FER) in alkaline solutions in the presence of cethyltrimethylammonium bromide and by the desilication of FER in alkaline solutions. Desilication procedure led to the creation of large mesopores in zeolite crystal with broad distribution of size. Recrystallization procedure resulted in two types of mesoporosity: uniform intracrystalline mesopores homogeneously distributed along the crystal and intercrystalline ordered mesoporous layers covering zeolite crystal. Recrystallized materials showed improved catalytic performance in skeletal isomerization of 1-butene with respect to both parent and desilicated ferrierites. The effect is due to the improved accessibility of the acid sites and the creation of new pore mouths, in which the active carbonaceous species responsible for selective pseudo-monomolecular pathway can be formed. The improved stability of catalytic activity with time on stream is due to coating of ferrierite crystals with mesoporous layes, which prevent the deactivation of the catalysts. The best catalytic performance was achieved over composite micro/mesoporous materials with intermediate degree of recrystallyzation due to proper balance between zeolitic and mesoporous phases and the amount and density of the acid sites.

Polyelectrolyte–Surfactant Complex as a Template for the Synthesis of Zeolites with Intracrystalline Mesopores

Mesoporous zeolite silicalite-1 and Al-ZSM-5 with intracrystalline mesopores were synthesized with polyelectrolyte-surfactant complex as the template. Complex colloids were first formed by self-assembly of the anionic polymer poly(acrylic acid) (PAA) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) in basic solution. During the synthesis procedure, upon the addition of the silica source, microporous template (tetrapropylammonium hydroxide), and NaCl, these PAA/CTA complex colloids underwent dissociation and gave rise to the formation of hollow silica spheres with mesoporous shells templated by CTAB micelles and PAA domains as the core. Under hydrothermal treatment, the hollow silica spheres gradually merged together to form larger particles with the PAA domains embedded as the space occupant, which acted as a template for intracrystalline mesopores during the crystallization of the zeolite framework. Amphiphilic organosilane was used to enhance the connection between the PAA domain and the silica phase during the synthesis. After calcination, single crystal-like zeolite particles with intracrystalline mesopores of about 5-20 nm were obtained, as characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and N(2) adsorption measurements. With the addition of an aluminum source in the synthesis, mesoporous zeolite Al-ZSM-5 with intracrystalline mesopores was also synthesized, and enhanced catalytic property was observed with mesoporous Al-ZSM-5 in acetalization of cyclohexanone with methanol.

Zeolite with trimodal porosity by desilication of zeolite nanocrystals aggregate

Zeolite with trimodal porosity can be synthesized by desilication of zeolite nanocrystal aggregate. In the desilication process, the originally existed intercrystalline mesopores of zeolite nanocrystal aggregate were enlarged into large mesopore, and the new small intracrystalline mesopore channel was created, thus the Zeolite with trimodal porosity was formed. The structure of resulted zeolite, both on aggregate and mesopore level can be fine tuned by the desilication degree.

Assemblies of Mesoporous FAU‐Type Zeolite Nanosheets

Holey sheets: Nanosheet assemblies of FAU-type zeolite X with intracrystalline mesopores are prepared (see picture). This material is synthesized by soft-templating and combines micro, meso, and macropores in a hierarchically interconnected manner.

Assembly of a hierarchical zeolite-silica composite by spray drying

We report a one-step spray drying synthesis of a multimodal composite based on a polymer-templated silica matrix enclosing a mesoporous ZSM-5 zeolite. The first level of porosity, comprising the silica matrix with 13–25 nm mesopores, is generated by evaporation-induced assembly of the silica nanoparticles together with the non-ionic surfactant using spray drying. The mesoporous zeolite contributes with micropores (0.56 mm) and intracrystalline mesopores (∼7 nm) introduced by controlled alkaline leaching. The nanostructured composite, featuring agglomerated zeolite microgranules uniformly covered by the silica, displayed tuneable properties of the matrix mesoporosity and particle size by changing both the silica source and the spray drying conditions. The structural and acidic properties of the zeolite phase were not altered by the assembly method. The technique was successfully applied to other zeolite structures (USY, mordenite and ferrierite) with no impact in the characteristics of the matrix. This fast, versatile, reproducible, economical and scalable technique represents a powerful strategy for creating hierarchically structured materials with adjustable micro-, meso- and macroporous properties and enhanced agglomeration.

Synthesis of nanosized ZSM-5 zeolite with intracrystalline mesopores

A series of nanosized ZSM-5 zeolite samples with intracrystalline mesopores has been synthesized using organosilane functioned fumed silica as silica source and tetrapropylammonium hydroxide as microporous structure directing agent. The organic moiety linked on the surface of fumed silica blocks the growth of zeolite crystal, forming the nanosized zeolite crystal and intracrystalline defects inside the nano-crystal. The formation of the mesopores is by bond-blocking function in crystallization. The XRD results indicate that the samples are high crystalline ZSM-5 and the TEM images show that the samples are nanosized with the size ca. 20nm and the intracrystalline mesopores about 3nm exist inside the nanosized crystal.

Multimodal Zeolite‐Beta‐Based Catalysts with a Hierarchical, Three‐level Pore Structure

Hole diggers: The hierarchically structured porous solid-acid catalyst described in this report possess a remarkable pore system, encompassing well-defined macrochannels, interconnected mesopores, intracrystalline mesopores, and tunable zeolite micropores. Importantly, the catalyst exhibits very strong acidity and superior catalytic activity for esterification reactions.

Synthesis of mesoporous MOR materials by varying temperature crystallizations and combining ternary organic templates

An approach, which involved perturbations of synthesis conditions in crystallization temperature and organic template composition, has been proposed for derivation of mesopores from bulk structural defects formed in mordenite (MOR) phases, based on the character of intrinsic crystal defects and tensional distortions of two compressed 8-membered rings in MOR framework. Consequently, hierarchically porous materials with MOR structures were prepared by varying temperature synthesis and using ternary organic micelles templates. The synthesized materials were characterized by powder X-ray diffraction, nitrogen adsorption at 77 K and transmission electron microscopy. These crystalline materials combined the advantages of both mordenites with the micropore system and mesoporous materials by featuring an additional intracrystalline mesopore system in MOR single crystals, resulting in a hierarchical pore distribution in range of 1.7–80.0 nm together with the significant specific surface areas. In particular, individual crystal morphologies of typical mesoporous MOR materials showed the defect-rich structures responding essentially to the mesoporosity; the mesoporous materials were substantiated matchable well to normal mordenites in acid capacity and acid strength distribution. Furthermore, several factors acting on intracrystalline mesopore formation were elucidated for explicating the perturbation synthesis approach, and whole reaction routes for synthesizing different solid products under related crystallization conditions were summarized coupling with a demonstration of part TEM micrographs. The overall results suggested that the investigative approach of perturbation imposed on crystal growth provides an alternative route for direct hydrothermal synthesis of mesoporous zeolite materials and implied that bulk structural defects within single crystals lead to the formation of intracrystalline mesopores in various sizes.