Intracrystalline Mesopores Research Articles

Synthesis and applications of ordered and disordered mesoporous zeolites: Present and future prospective

Zeolites are microporous aluminosilicates that are used extensively for a wide variety of industrial applications. Applications of microporous zeolites are limited to transformations of small molecules due to the pore diffusion limitation imposed by the microporous network structure. In order to overcome this limitation, zeolites with inter-/intra-crystalline mesopores are generally preferred. This review article presents the details of various strategies for the preparation of mesoporous zeolites with inter-/intra-crystalline mesopores. A wide range of soft templates and additives is described in details for their use in the synthesis of mesoporous ZSM-5 and Beta zeolites. A brief report on the applications of mesoporous zeolites in various acid catalyzed reactions, involving large organic molecules, is provided in this article. Another important feature of this article is that it summarizes the applications of mesoporous zeolites in the electrochemical detection of various physiologically important biomolecules and organic/inorganic water pollutants. Further, the role of mesoporous zeolites based electrode materials for direct methanol fuel cell application is presented. Finally, the potential role of mesoporous zeolites in the field of biomedical engineering, particularly as a bone implant material, is summarized.

Bifunctional Hierarchical Zeolite‐Supported Silver Catalysts for the Conversion of Glycerol to Allyl Alcohol

AbstractThe establishment of suitable processes for the conversion of glycerol into allyl alcohol is hindered by the fast deactivation of solid acids in the dehydration of the substrate to acrolein and by the requirement of hydrogen donors to enhance the selectivity of the subsequent reduction step. In this work, silver nanoparticles deposited onto a hierarchical ZSM‐5 zeolite are proved to be an effective bifunctional catalyst to conduct the two reactions in the gas phase and in the presence of hydrogen by using a continuous fixed‐bed reactor. The acidic function was accomplished by using a ZSM‐5 zeolite modified by facile alkaline and acid treatments, which decreased the amount of Lewis acid centers while preserving the amount of Brønsted acid centers, and introduced an auxiliary network of intracrystalline mesopores, thus boosting the selectivity to acrolein (62 %) and the resistance to coking. Upon screening of various metals supported on the aluminosilicate, silver was identified as a superior hydrogenation catalyst, enabling a relatively high activity with >50 % allyl alcohol selectivity. Tuning of the metal loading, temperature, pressure, and contact time led to 15 % yield of allyl alcohol, thus approaching the state‐of‐the‐art transfer hydrogenation systems, and stable behavior for 100 h on stream. Our results highlight the advantage of conducting the two transformations over a bifunctional material rather than over two separate single‐function solids.

Fabrication of intracrystalline mesopores within zeolite Y with greatly decreased templates

Zeolite Y with intracrystalline mesopores has been emerging as one of the most potential materials in the catalytic cracking of large molecules.

Synthesis of hierarchical TS-1 zeolites with abundant and uniform intracrystalline mesopores and their highly efficient catalytic performance for oxidation desulfurization

Hierarchical TS-1 zeolites with abundant intracrystalline mesopores are synthesized and show excellent catalytic performance in oxidizing bulky organic compounds.

Engineering growth defects: a new route towards hierarchical ZSM-5 zeolite with high-density intracrystalline mesopores

Engineering growth defects is applied as a novel strategy to prepare hierarchical ZSM-5 zeolites with high-density intracrystalline mesopores and excellent catalytic activities in conversions of bulky molecules.

Facile preparation of hierarchical titanium silicalite-1 (TS-1) with efficient oxidation of cyclic alkenes using PVA modified MWCNTs as templates

Hierarchical TS-1 (HTS-1) with highly connective mesoporous channels has been successfully synthesized through a simple one-pot method using polyvinyl alcohol (PVA) modified multi-wall carbon nanotubes as templates. The synthesized samples are characterized by X-ray diffraction, UV–vis spectra, scanning electron microscopy, transmission electron microscopy, FT-IR spectra and N2 adsorption/desorption. The HTS-1 contains rich intracrystalline mesopores (10–20 nm) and a large amount of open channels, which shows much better catalytic activity than conventional TS-1 in the epoxidation of cyclohexene and cyclooctene. Meanwhile, the introduction of PVA triggers the size of HTS-1 increase from ∼230 nm to ∼380 nm, which is also beneficial for the separation of nanosized zeolite crystals from the reaction products.

Nanocrystalline mesoporous zeolite X with a considerable external surface area prepared via an ordered precursor: a potential adsorbent

A nanocrystal zeolite X with intracrystalline mesopores has been successfully prepared by using organofunctionalised mesoporous silica in hydrothermal system. The as-prepared zeolitic material persist the inherent framework structure of FAU zeolite, and present a crystallographic morphology of stacked three-dimensional intergrowths made up of rectangular nanocrystals. The intracrystalline mesopores are formed via a bond-blocking principle with hydrophobic carbon chains of organosilane inhibiting the growth of zeolite units and crystals in the channels of mesoporous silica during hydrothermal synthesis. The results showed that the zeolitic material has a considerable external surface area (247 m2g−1)and mesopore volume (0.44 cm3g−1), maintaining high Brunauer–Emmett–Teller surface area (991 m2g−1) and large total pore volume (0.71 cm3g−1), and clearly stating a larger adsorption capacity for p-xylene at relative higher pressure.

Preparation of Mesoporous Fe2O3-Supported ZSM-5 Zeolites by Carbon-Templating and their Evaluation as Photo-Fenton Catalysts to Degrade Organic Pollutant

Mesoporous Fe2O3-supported ZSM-5 zeolites were prepared by carbon-templating and subsequently evaluated as photo-Fenton catalysts to degrade a dye used as a model heavy organic pollutant. The synthesis procedure of the mesoporous ZSM-5 zeolites was performed employing a nucleating gel and carbon particles as mesopores template. Thereafter, the precursor salt of the iron oxide (Fe2O3) was impregnated and then calcined to obtain the final catalyst. For comparison purposes, a conventional Fe2O3-supported ZSM-5 zeolite was also prepared. The results showed that the amount of intracrystalline mesopores formed in the ZSM-5 crystals was influenced by the amount of carbon added into the synthesis mixture. In comparison to the conventional prepared catalyst, the mesoporous Fe2O3/ZSM-5 ones showed an improved performance in the degradation of the target organic pollutant by the photo-Fenton reaction, which was attributed to the improvement of their textural properties as consequence of the mesopores generation.

The comparison of mesoporous HZSM-5 zeolite catalysts prepared by different mesoporous templates and their catalytic performance in the methanol to aromatics reaction

Three kinds of mesoporous HZSM-5 were synthesized successfully by using a hydrothermal methodology with different mesoporous templates, namely, hexadecyl trimethyl ammonium bromide (CTAB), dimethyl distearylammonium chloride (DDAC) and carbon nanotube (CNT). The differences in crystallinity, textural property and acidity among these micro-mesoporous composite molecular sieves were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy techniques, transmission electron microscopy, nitrogen adsorption and NH3 temperature programmed desorption. The catalytic performances of these mesoporous HZSM-5 zeolites were evaluated by methanol to aromatics (MTA) reaction, and the conventional microporous HZSM-5 was also tested in this reaction for comparison. The results show that mesoporous HZSM-5 zeolite synthesized with CNT mesoporous templates exhibited the highest BTX yields and lowest rate of carbon deposition in MTA reaction among all catalysts. This improved catalytic performance is mainly attributed to not only the well-preserved strong acid sites, which would provide enough active sites for methanol aromatization, but also the generated intra-crystalline mesopores, which would facilitate the access of reactant to the active sites and improve the mass transfer of products. However, mesoporous HZSM-5 synthesized with CTAB and DDAC mesoporous templates did not present improved catalytic performances due to their disconnected micro-mesopores and inadequate strong acidic sites.

Full-crystalline hierarchical monolithic ZSM-5 zeolites as superiorly active and long-lived practical catalysts in methanol-to-hydrocarbons reaction

The efforts to obtain superior active and long-lived practical zeolite catalysts must be based on shaped forms rather than only powder, if considering necessary mechanical strength requirement in industrial application. The binders, incorporated as indispensable additives to keep intact of macroscopical body, are catalytically inert and their negative impacts in mass transfer are widely neglected. Here, we firstly revealed the remarkable mass transfer depression by incorporating silica sol as binders in shaped zeolitic catalyst. Then, by re-crystallizing the shaped binder/zeolite body with assistance of controlled organic template steam, the binders were converted into zeolitic phase, forming full-zeolitic monoliths. In re-crystallization, it was found obvious intra-crystalline mesopores were in situ created while maintaining high mechanical strength. A remarkable mass transfer advantage was revealed in such hierarchical monolith, which means more acid sites will be accessible after re-crystallization. Therefore, in performance evaluation, hierarchical full-zeolitic monolith demonstrated higher propylene yield and slower deactivation rate in C4 olefin cracking reaction. Moreover, highly prolonged cycle time, which was more than 2000h and three times as long as that of commercial catalyst, was realized in catalyzing methanol-to-hydrocarbons (MTH) under industrial condition. That is the first report on so long-lived zeolite catalyst without regeneration in methanol conversion reaction. Fundamental understanding in re-crystallization of monolithic material will contribute significantly to technical progress in developing more high-valued applicable catalysts and optimizing existing zeolite catalyst.

ChemInform Abstract: Perspectives on Zeolite‐Encapsulated Metal Nanoparticles and Their Applications in Catalysis

AbstractReview: 155 refs.

Mesoporous MEL, BEA, and FAU zeolite crystals obtained by in situ formation of carbon template over metal nanoparticles

Highly crystalline zeolites with intracrystalline mesopores are synthesized through a straightforward and feasible templating method.

A green surfactant-assisted synthesis of hierarchical TS-1 zeolites with excellent catalytic properties for oxidative desulfurization.

Hierarchical TS-1 zeolites with uniform intracrystalline mesopores have been successfully synthesized through the hydrothermal method by using the green and cheap surfactant Triton X-100 as the mesoporous template. The resultant materials exhibit remarkably enhanced catalytic activity in oxidative desulfurization reactions compared to the conventional TS-1 zeolite.

New insights into hierarchical metal-containing zeolites; synthesis and kinetic modelling of mesoporous gallium-containing ZSM-5 for propane aromatization

Hierarchical (mesoporous) gallium-containing HZSM-5 zeolites were synthesized via incipient wetness impregnation, followed by top–down post-synthetic treatments; alkaline treatment, CTAB-mediated assembly of Ga-containing zeolite seed into MCM-41 mesostructure, or CTAB-mediated coating of Ga-containing zeolite with MCM-41 layer. It was observed that the resulting Ga-containing HZSM-5 with microporous/mesoporous hierarchical structure exhibited improved propane aromatization, as compared to the corresponding microporous sample. Mesoporous Ga-containing HZSM-5 displayed large external surface area (Smeso=265–350m2g−1), relatively preserved microporosity (Vmicro=0.08cm3g−1), and random intracrystalline mesopores of ∼7–8nm in size or ordered intercrystalline MCM-41 mesopores of ∼2–4nm in size, compared to Smeso of 194m2g−1 and Vmicro of 0.10cm3g−1 for the corresponding microporous sample. C3 conversion increased to 33.9%, 47.4% and 33.4% upon alkaline treatment, CTAB-mediated hydrolysis, CTAB-mediated coating, respectively, up from 30.3% (microporous Ga-containing ZSM-5). At comparable conversion level, mesoporous samples were more selective to BTX aromatics, with ∼56% compared to 48.9% for microporous sample. The apparent activation energy (Ei) for the cracking of propane over mesoporous Ga-containing HZSM-5 was 130kJmol−1, compared to 110kJmol−1 for microporous sample. Whereas, the Ei (dehydrogenation) over mesoporous and microporous Ga-containing HZSM-5 was 12.65kJmol−1 and 70kJmol−1, respectively. The Ei (propene to aromatics) over mesoporous and microporous Ga-containing HZSM-5 was 140kJmol−1 and 220kJmol−1, respectively. The improved C3 aromatization over mesoporous Ga-containing ZSM-5 was tentatively attributed to the hierarchical topology (mesoporosity), which resulted in substantial improvements in the mass-transport properties and (probably) facilitated the migration of extracrystalline Ga2O3 formed during impregnation into the micropores.

Development of hierarchical MCM-49 zeolite with intracrystalline mesopores and improved catalytic performance in liquid alkylation of benzene with ethylene

Hierarchical MCM-49 zeolite with intracrystalline mesopores was prepared through post-treating the conventional MCM-49 zeolite in hexadecyltrimethylammonium bromide (CTAB) and NaOH mixed solution. N2 adsorption and desorption, XRD, TEM, XRF, ICP and Py-IR techniques were applied to characterize the obtained samples. It was found that uniform intracrystalline mesopores were distributed in the MCM-49 crystals, and the intracrystalline meso-porosity could be modified by changing the post-treatment conditions such as CTAB concentration, NaOH concentration, temperature and time. Based on the characterization results upon the hierarchical MCM-49 zeolite samples obtained by post-treating different time, the formation of intracrystalline mesopores in MCM-49 zeolite was proposed, which followed a CTAB micelle directed desilication mechanism. By virtue of the intracrystalline mesopores, the hierarchical MCM-49 zeolite prepared under proper post-treatment condition exhibited superior stability than the conventional MCM-49 zeolite in liquid alkylation of benzene with ethylene.

Alkali treatment upon MCM-49 zeolite with various contents of HMI in the presence of CTAB and application in anisole acylation with acetic anhydride

A series of MCM-49 zeolites with various contents of hexamethyleneimine (HMI), obtained by calcination at different temperatures, were alkali-treated in NaOH solution containing cetyltrimethylammonium bromide (CTAB). Changes in structural and acidic properties of the resultant samples were systematically investigated by transmission electron microscopy and scanning electron microscopy (TEM and SEM), X-ray diffraction (XRD), N2 and Ar adsorption/desorption isotherms measurements, ammonia temperature-programmed desorption (NH3-TPD) and pyridine adsorption/desorption followed by infrared measurements (Py-IR), as well as 29Si MAS NMR techniques. It was revealed that only crevices were developed on the crystals of MCM-49 zeolite samples with more HMI, resulting in the increase of acid concentrations; whereas intracrystalline mesopores (ca. 3nm) could be formed for the samples with less HMI at the expense of framework destruction. A bit of ordered mesoporous materials (OMMs) could also be detected from the latter samples, because the presence of CTAB could induce the re-assembly of dissolved species through the formation of micelle, originating from the desilication by NaOH. When the modified MCM-49 samples were used as catalysts for anisole acylation with acetic anhydride, due to the positive changes in acidities and pore structure properties, they exhibited better catalytic performances for the reaction than the unmodified HMCM-49 catalyst.

Mesoporous Zr-Beta zeolites prepared by a post-synthetic strategy as a robust Lewis acid catalyst for the ring-opening aminolysis of epoxides

Mesoporous Zr-Beta exhibits a remarkable catalytic activity and regio-selectivity to β-amino alcohols in epoxide aminolysis. Intra-crystalline mesopores greatly promote the reaction through enhanced mass transfer and suppress the catalytic deactivation.

Design of Hierarchical Zeolite Catalysts for the Manufacture of Polyurethane Intermediates

This study undertakes the design of hierarchically structured zeolites for the synthesis of methylenedianiline (MDA) mixtures via the liquid-phase condensation of aniline with formaldehyde. Affordable acid and base treatments enabled the controlled generation of an auxiliary network of intracrystalline mesopores within commercial zeolites of different frameworks, among which the FAU topology is identified as superior. As the micropore structure of the zeolite can be preserved, the unique shape selectivity is retained while improved mass transport in the hierarchical crystals leads to a 7-fold increased activity and a 3-fold prolonged lifetime before thermal regeneration is necessary. The linear correlation between the MDA yield and the product of the Brønsted acidity and the external surface area emphasizes the need to balance acid site and pore quality in the design of an optimal catalyst. Hierarchical USY zeolites outperform the state-of-the-art delaminated zeolites and mesoporous aluminosilicates. These results offer a viable solution for the replacement of the industrially applied homogeneous catalyst (HCl), resulting in an efficient and environmental friendly technology for the production of polyurethanes.

Effective removal of Mg2 + and Ca2 + ions by mesoporous LTA zeolite

The Mg2+ and Ca2+ exchange properties in the meso-zeolite LTA were investigated. These meso-zeolite materials with intracrystalline mesopores ca. 3nm were prepared using organic functioned fumed silica as silica source. The Dual-site Langmuir (D-L) expression appropriately describes the equilibrium data for the Ca2+ and Mg2+ exchange. The fitting results with D-L model suggest that two kinds of ion-exchange sites in the meso-zeolites exist, one in the part of intrinsic micropore structure (internal surface area) and the other one in the part of established mesopore structure (external surface area). The latter site has lower enthalpy values of Ca2+ or Mg2+ exchanges than the former one. Ca2+ or Mg2+ exchange kinetics is conducted at 273K, 288K and 308K. The results show the emergence of mesopores lowering the activation energy of ion exchange and accelerating Mg2+ exchange extremely in the materials. The diffusion rate of hydrated Mg2+ in the meso-zeolite LTA increases by 17.5 times, higher than that of conventional LTA at 273K. And the meso-zeolite LTA possesses large Mg2+ competitive ability in a solution containing Ca2+ and Mg2+. These properties make the meso-zeolite LTA a suitable material which could be used effectively in hard water softening and detergent building for Mg2+ removing.

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.