Controlling Crystal Morphology of Anisotropic Zeolites with Elemental Composition.

The morphology of zeolite crystals strongly affects their textural, catalytic, and mechanical attributes. However, controlling zeolite crystal morphology without using modifiers or structure-directing agents remains a challenging task because of our limited understanding of the relationships between zeolite crystal shape, crystallization mechanism, and composition of the starting synthesis mixture. In this study, we aimed at developing a general method for controlling the morphology of zeolites by assessing the impact of the Si/T molar ratio of the synthesis gel on the growth rate of zeolite crystals in various crystallographic directions and on the final crystal morphology of the UTL germanosilicate with a 2D system of intersecting 14- and 12-ring pores. Our results showed that flat UTL crystals progressively thicken with the Si/Ge molar ratio, demonstrating that Ge concentration controls the relative rate of crystal growth in the perpendicular direction to the pore system. The morphology of other zeolites and zeotypes with an anisotropic structure, including AFI (12R), IFR (12R), MWW (10-10R), and IWW (12-10-8R), can also be predicted based on their Si/T ratio, suggesting a systematic pattern across zeolite structures and in a wide range of zeolite framework elements. Combined, these findings introduce a facile and cost-efficient method for directly controlling crystal morphology of zeolites with anisotropic structures with a high potential for scale-up while providing further insights into the role of elemental composition in zeolite crystal growth.

Construction of Hierarchical Films via Layer-by-Layer Assembly of Exfoliated Unilamellar Zeolite Nanosheets.

Zeolites have been widely applied as versatile catalysts, sorbents, and ion exchangers with unique porous structures showing molecular sieving capability. In these years, it is reported that some layered zeolites can be delaminated into molecularly thin 2-dimensional (2D) nanosheets characterized by inherent porous structures and highly exposed active sites. In the present study, two types of zeolite nanosheets with distinct porous structures with MWW topology (denoted mww) and ferrierite-related structure (denoted bifer) are deposited on a substrate through the solution process via electrostatic self-assembly. Alternate deposition of zeolite nanosheets with polycation under optimized conditions allows the layer-by-layer growth of their multilayer films with a stacking distance of 2-3 nm. Furthermore, various hierarchical structures defined at the unit-cell dimensions can be constructed simply by conducting the deposition of mww and bifer nanosheets in a designed sequence. Adsorption of a dye, Rhodamine B, in these films, is examined to show that adsorption is dependent on constituent zeolite nanosheets and their assembled nanostructures. This work has provided fundamental advancements in the fabrication of artificial zeolite-related hierarchical structures, which may be extended to other zeolite nanosheets, broadening their functionalities, applications, and benefits.

Current State and Perspectives of Exfoliated Zeolites.

Zeolites are highly efficient industrial catalysts and sorbents with microporous framework structures. Approximately 10% of the frameworks, but eventually all in the long run, have produced both 3D crystals and 2D layers. The latter can be intercalated and expanded like all 2D materials but proved difficult to exfoliate directly into suspensions of monolayers in solution as precursors for unique synthetic opportunities. Successful exfoliations have been reported recently and are overviewed in this perspective article. The discussion highlights 3 primary challenges in this field, namely finding suitable 2D zeolite preparations that exfoliate directly in high yield, proving uniform layer thickness in solution and identifying applications to exploit the unique synthetic capabilities and properties of exfoliated zeolite monolayers. Four zeolites have been confirmed to exfoliate directly into monolayers: 3 with known structures-MWW, MFI, and RWR and one unknown, bifer with a unit cell close to ferrierite. The exfoliation into monolayers is confirmed by the combination of 5-6 characterization techniques including AFM, in situ and in-plane XRD, and microscopies. The promising areas of development are oriented films and membranes, intimately mixed zeolite phases, and hierarchical nanoscale composites with other active species like nanoparticles and clusters that are unfeasible by solid state processes.

Exfoliated Ferrierite-Related Unilamellar Nanosheets in Solution and Their Use for Preparation of Mixed Zeolite Hierarchical Structures.

Direct exfoliation of layered zeolites into solutions of monolayers has remained unresolved since the 1990s. Recently, zeolite MCM-56 with the MWW topology (layers denoted mww) has been exfoliated directly in high yield by soft-chemical treatment with tetrabutylammonium hydroxide (TBAOH). This has enabled preparation of zeolite-based hierarchical materials and intimate composites with other active species that are unimaginable via the conventional solid-state routes. The extension to other frameworks, which provides broader benefits, diversified activity, and functionality, is not routine and requires finding suitable synthesis formulations, viz. compositions and conditions, of the layered zeolites themselves. This article reports exfoliation and characterization of layers with ferrierite-related structure, denoted bifer, having rectangular lattice constants like those of the FER and CDO zeolites, and thickness of approximately 2 nm, which is twice that of the so-called fer layer. Several techniques were combined to prove the exfoliation, supported by simulations: AFM; in-plane, in situ, and powder X-ray diffraction; TEM; and SAED. The results confirmed (i) the structure and crystallinity of the layers without unequivocal differentiation between the FER and CDO topologies and (ii) uniform thickness in solution (monodispersity), ruling out significant multilayered particles and other impurities. The bifer layers are zeolitic with Brønsted acid sites, demonstrated catalytic activity in the alkylation of mesitylene with benzyl alcohol, and intralayer pores visible in TEM. The practical benefits are demonstrated by the preparation of unprecedented intimately mixed zeolite composites with the mww, with activity greater than the sum of the components despite high content of inert silica as pillars.

From Colloidal Dispersions of Zeolite Monolayers to Effective Solid Catalysts in Transformations of Bulky Organic Molecules: Role of Freeze-Drying and Dialysis.

We investigated the properties and catalytic activity of zeolites with MWW topology obtained by unprecedented liquid exfoliation of the MCM-56 zeolite into solutions of monolayers and isolation/reassembly of the dispersed layers by various methods, with optional purification by dialysis or ammonium exchange. The layers were recovered by flocculation with alcohol or ammonium nitrate and freeze-drying. Flocculation alone, even with ammonium nitrate, did not ensure removal of residual sodium cations resulting in catalysts with low activity. Dialysis of the solutions with dispersed monolayers proved to be efficient in removing sodium cations and preserving microporosity. The monolayers were also isolated as solids by freeze-drying. The highest BET area and pore volume obtained with the freeze-dried sample confirmed lyophilization efficiency in preserving layer structure. The applied test reaction, Friedel-Crafts alkylation of mesitylene, showed high benzyl alcohol conversion due to increased concentration of accessible acid centers caused by the presence of secondary mesoporosity. The applied treatments did not change the acid strength of the external acid sites, which are the most important ones for converting bulky organic molecules. Zeolite acidity was not degraded in the course of exfoliation into monolayers, showing the potential of such colloid dispersions for the formation of active catalysts.

Detemplated and Pillared 2-Dimensional Zeolite ZSM-55 with Ferrierite Layer Topology as a Carrier for Drugs.

The present studies were conducted to show the potential of 2D zeolites as effective and non-toxic carriers of drugs. Layered zeolites exhibit adjustable interlayer porosity which can be exploited for controlled drug delivery allowing detailed investigation of the drug release because the structure of the carrier is known exactly. This study was conducted with model drugs ciprofloxacin and piracetam, and ZSM-55 with ca 1 nm thick layers, in detemplated and pillared forms. The release profiles differed from the commercial, crystalline forms of drugs-the release rate increased for ciprofloxacin and decreased for piracetam. To understand the dissolution mechanisms the release data were fitted to Korsmeyer-Peppas equation, showing Fickian (for pillared) and anomalous (for detemplated sample) transport. FT-IR studies showed that strong interaction carrier-drug may be responsible for the modified, slowed down release of piracetam while better solubility and faster release of ciprofloxacin was attributed to formation of the protonated form resulting in weaker interaction with the zeolite than in the pure crystalline form. Two independent tests on L929 mice fibroblasts (ToxiLight and PrestoBlue) showed that ZSM-55, in moderate concentrations may be safely used as a carrier of drug molecules, not having negative effect on the cells viability or proliferation rate.

Liquid dispersions of zeolite monolayers with high catalytic activity prepared by soft-chemical exfoliation.

The most effective approach to practical exploitation of the layered solids that often have unique valuable properties-such as graphene, clays, and other compounds-is by dispersion into colloidal suspensions of monolayers, called liquid exfoliation. This fundamentally expected behavior can be used to deposit monolayers on supports or to reassemble into hierarchical materials to produce, by design, catalysts, nanodevices, films, drug delivery systems, and other products. Zeolites have been known as extraordinary catalysts and sorbents with three-dimensional structures but emerged as an unexpected new class of layered solids contributing previously unknown valuable features: catalytically active layers with pores inside or across. The self-evident question of layered zeolite exfoliation has remained unresolved for three decades. Here, we report the first direct exfoliation of zeolites into suspension of monolayers as proof of the concept, which enables diverse applications including membranes and hierarchical catalysts with improved access.

Iron-Based Metal-Organic Frameworks as a Theranostic Carrier for Local Tuberculosis Therapy.

PURPOSE: The purpose of the study was initial evaluation of applicability of metal organic framework (MOF) Fe-MIL-101-NH2 as a theranostic carrier of antituberculous drug in terms of its functionality, i.e. drug loading, drug dissolution, magnetic resonance imaging (MRI) contrast and cytotoxic safety. METHODS: Fe-MIL-101-NH2 was characterized using X-ray powder diffraction, FTIR spectrometry and scanning electron microscopy. The particle size analysis was determined using laser diffraction. Magnetic resonance relaxometry and MRI were carried out on phantoms of the MOF system suspended in polymer solution. Drug dissolution studies were conducted using Franz cells. For MOF cytotoxicity, commercially available fibroblasts L929 were cultured in Eagle's Minimum Essential Medium supplemented with 10% fetal bovine serum. RESULTS: MOF particles were loaded with 12% of isoniazid. The particle size (3.37-6.45 µm) depended on the micronization method used. The proposed drug delivery system can also serve as the MRI contrast agent. The drug dissolution showed extended release of isoniazid. MOF particles accumulated in the L929 fibroblast cytoplasmic area, suggesting MOF release the drug inside the cells. The cytotoxicity confirmed safety of MOF system. CONCLUSIONS: The application of MOF for extended release inhalable system proposes the novel strategy for delivery of standard antimycobacterial agents combined with monitoring of their distribution within the lung tissue.

Pillaring of layered zeolite precursors with ferrierite topology leading to unusual molecular sieves on the micro/mesoporous border.

Layered zeolite materials with FER layer topology can produce various condensed and expanded structures including zeolite frameworks, FER and CDO, their interlayer expanded forms (IEZ), and organic-intercalated and pillared derivatives. This work concerns pillaring of the surfactant-swollen derivative with a gallery height of ca. 2.5 nm between layers by treatment with tetraethylorthosilicate (TEOS) at room and elevated temperatures. The materials obtained at 100 °C and higher showed unusual properties including 2 nm pores on the micro/mesoporous border and disordered layer packing indicated by the absence of distinct low angle interlayer peaks at d-spacing >3 nm (∼3° 2θ Cu Kα radiation) in the X-ray diffraction pattern (XRD). TEOS treatment at room temperature produced a pillared molecular sieve with the expected mesoporous characteristics, namely a pore size of around 3 nm and a high intensity low angle (001) peak at 2.3° 2θ, and a d-spacing of 3.8 nm, in the XRD. The characterization aiming to elucidate the nature of the obtained unusual products included gas adsorption isotherms, aberration corrected (Cs-corrected) Scanning Transmission Electron Microscopy (STEM) studies and 29Si solid state NMR. BET surface area values decreased with the temperature of TEOS treatment from approximately 1200 m2 g-1 to ∼900 and 600 m2 g-1, at room temperature, 100 °C, and 120 °C, respectively. The 29Si solid state NMR revealed the presence of both Q3 ((SiO)3SiOX, X = H or minus charge) and Q4 ((SiO)4Si) centers giving separated signals up to the pillaring step. After pillaring at 100 °C and calcination, the nominal intensity ratios Q4 : Q3 were 2.17 and 2.61 but the signals were merged into one broad peak indicating the structural heterogeneity of Si-O coordination. The microscopy showed the presence of FER layers in the samples but the overall structure and composition were not well-defined. The observed unusual disorganization and possible partial degradation of layers during pillaring may result from the combination of high temperature, alkalinity (surfactant hydroxide) and siliceous composition of the layers. The obtained pillared products are of interest for the preparation of larger pore catalysts and sorbents or controlled drug delivery.

Characterization of Co and Fe-MCM-56 catalysts for NH3-SCR and N2O decomposition: An in situ FTIR study.

Two-step preparation of iron and cobalt-containing MCM-56 zeolites has been undertaken to evaluate the influence of their physicochemical properties in the selective catalytic reduction (NH3-SCR or DeNOx) of NO using NH3 as a reductant. Zeolites were prepared by the selective leaching of the framework cations by concentrated HNO3 solution and NH4F/HF mixture and consecutively, introduction of Co and Fe heteroatoms, in quantities below 1wt%. Further calcination allowed to obtain highly dispersed active species. Their evaluation and speciation was realized by adsorption of pyridine and NO, followed by FTIR spectroscopy. Both Fe-MCM-56 zeolites showed excellent activities (maximum NO conversion 92%) with high selectivity to dinitrogen (above 99%) in the high temperature NH3-SCR process. High catalytic activity of Fe-MCM-56 zeolites was assigned to the formation of stable nitrates, delivering NO to react with NH3 at higher temperatures and suppressing the direct NO oxidation. It was found that more nitrates was formed in Fe-MCM-56 (HNO3) than in Fe-MCM-56 (HF/NH4F) and that could compensate for the lower Fe loading, resulting in very similar catalytic activity of both catalysts. At the same time both Co-and Fe-MCM-56 zeolites were moderately active in direct N2O decomposition, with maximum N2O conversion not higher than 80% and activity window starting at 500°C. This phenomenon was expected since both types of catalysts contained well dispersed active centers, not beneficial for this reaction.

Layer like porous materials with hierarchical structure.

Many chemical compositions produce layered solids consisting of extended sheets with thickness not greater than a few nanometers. The layers are weakly bonded together in a crystal and can be modified into various nanoarchitectures including porous hierarchical structures. Several classes of 2-dimensional (2D) materials have been extensively studied and developed because of their potential usefulness as catalysts and sorbents. They are discussed in this review with focus on clays, layered transition metal oxides, silicates, layered double hydroxides, metal(iv) phosphates and phosphonates, especially zirconium, and zeolites. Pillaring and delamination are the primary methods for structural modification and pore tailoring. The reported approaches are described and compared for the different classes of materials. The methods of characterization include identification by X-ray diffraction and microscopy, pore size analysis and activity assessment by IR spectroscopy and catalytic testing. The discovery of layered zeolites was a fundamental breakthrough that created unprecedented opportunities because of (i) inherent strong acid sites that make them very active catalytically, (ii) porosity through the layers and (iii) bridging of 2D and 3D structures. Approximately 16 different types of layered zeolite structures and modifications have been identified as distinct forms. It is also expected that many among the over 200 recognized zeolite frameworks can produce layered precursors. Additional advances enabled by 2D zeolites include synthesis of layered materials by design, hierarchical structures obtained by direct synthesis and top-down preparation of layered materials from 3D frameworks.

Synthesis of 'unfeasible' zeolites.

Zeolites are porous aluminosilicate materials that have found applications in many different technologies. However, although simulations suggest that there are millions of possible zeolite topologies, only a little over 200 zeolite frameworks of all compositions are currently known, of which about 50 are pure silica materials. This is known as the zeolite conundrum--why have so few of all the possible structures been made? Several criteria have been formulated to explain why most zeolites are unfeasible synthesis targets. Here we demonstrate the synthesis of two such 'unfeasible' zeolites, IPC-9 and IPC-10, through the assembly-disassembly-organization-reassembly mechanism. These new high-silica zeolites have rare characteristics, such as windows that comprise odd-membered rings. Their synthesis opens up the possibility of preparing other zeolites that have not been accessible by traditional solvothermal synthetic methods. We envisage that these findings may lead to a step change in the number and types of zeolites available for future applications.

The ADOR mechanism for the synthesis of new zeolites.

A novel methodology, called ADOR (assembly-disassembly-organisation-reassembly), for the synthesis of zeolites is reviewed here in detail. The ADOR mechanism stems from the fact that certain chemical weakness against a stimulus may be present in a zeolite framework, which can then be utilized for the preparation of new solids through successive manipulation of the material. In this review, we discuss the critical factors of germanosilicate zeolites required for application of the ADOR protocol and describe the mechanism of hydrolysis, organisation and condensation to form new zeolites starting from zeolite UTL. Last but not least, we discuss the potential of this methodology to form other zeolites and the prospects for future investigations.

The aqueous colloidal suspension of ultrathin 2D MCM-22P crystallites.

The action of a tetrapropylammonium hydroxide solution on lamellar zeolite precursor MCM-22P produced a stable aqueous colloidal suspension which was shown by X-ray diffraction, small angle X-ray scattering and atomic force microscopy to contain ultrathin two-dimensional (2D) crystallites, including one-unit cell thick (i.e., 2.5 nm) monolayers.

Application of quasi-equilibrated thermodesorption of linear and di-branched paraffin molecules for detailed porosity characterization of the mono-layered zeolite MCM-56, in comparison with MCM-22 and ZSM-5.

The pore characteristics of zeolite samples including two kinds of ZSM-5 crystals as a base case and the unique mono-layered MCM-56 in different structural forms have been studied by the new method QE-TPDA (quasi-equilibrated temperature-programmed desorption and adsorption) in comparison with the standard nitrogen adsorption. Both approaches produce consistent results in terms of micro- and meso-porous features as well as quantitative pore volume values. The benefits of QE-TPDA include fast data acquisition (hours) and small sample size (milligrams). It is very flexible in using various hydrocarbons as probe molecules, which may reveal additional details associated with pores, their internal environment and dimensions/shape of the sorbate molecules. Hence, QE-TPDA is a valuable complementary tool for porosity characterization of the ever increasing diversity of porous materials and their pore structures. This was demonstrated by the results for the desorption of nonane and 2,2-dimethyloctane (DMO). The latter showed an additional maximum in the intermediate temperature range (between 'micro-' and 'mesopore' regions) which could be attributed to adsorption in the subsurface micropores (i.e. the pore mouths) where DMO could be partially adsorbed with t-butyl groups remaining on the outside. This was also reflected in the discrepancy between apparent volumes of micro- and mesopores calculated from the nonane and DMO experiments. Pillared MCM-56 revealed visibly enhanced subsurface micropore adsorption compared to the parent (mono-layer MWW) and MCM-22 (multi-layered MWW) consistent with the expected increase in the content of external 12 ring surface cups.

Swelling and pillaring of the layered precursor IPC-1P: tiny details determine everything.

The influence of swelling (i.e. the size of tetraalkylammonium surfactant molecule, the presence of tetrapropylammonium hydroxide (TPAOH), pH) and pillaring (i.e. the ratio between the swollen precursor IPC-1P and tetraethyl orthosilicate) conditions on the structure and textural properties of the resulting materials was studied in detail for IPC-1P, which is the layered precursor of zeolite PCR. The swelling of IPC-1P proceeds efficiently under basic conditions both in mixed C(n)H(2n+1)N(CH3)3Cl/TPAOH systems and in C(n)H(2n+1)N(CH3)3OH (n = 8, 10, 12, 14, 16, 18) solutions at pH = 13-14. The intercalation of C(n)H(2n+1)N(+)(CH3)3 in IPC-1P resulted in the formation of expanded materials with interlayer distances growing with increasing length of the alkyl chain in C(n)H(2n+1)N(CH3)3(+): 1.59-1.86 (n = 8) < 1.89-2.11 (10) < 2.05-2.26 (12) = 2.08-2.26 (14) < 2.37-2.43 (16) < 2.57-2.71 (18) Å. IPC-2 zeolite was formed during calcination of IPC-1P samples swollen in C(n)H(2n+1)N(CH3)3OH solution, while PCR zeolite can be obtained by calcination of IPC-1P treated with either C(n)H(2n+1)N(CH3)3Cl/TPAOH or C(n)H(2n+1)N(CH3)3Cl. The pillaring of IPC-1P samples swollen with C(n)H(2n+1)N(CH3)3OH provided mesoporous materials with narrow pore size distribution in the range 2.5-3.5 nm. Pillared materials derived from the samples swollen in the presence of TPAOH were characterized by a broader pore size distribution. The optimal TEOS/IPC-1PSW ratio being sufficient for the formation of well-ordered pillared derivatives characterized by improved textural properties (S(BET) = 878 m(2) g(-1), V(total) = 0.599 cm(3) g(-1)) was found to be 1 : 1.5.

High acidity unilamellar zeolite MCM-56 and its pillared and delaminated derivatives.

The unilamellar form of zeolite MWW, MCM-56, which is obtained by direct hydrothermal synthesis has been studied with regard to acidity and porosity in its original and post-synthesis modified pillared and delaminated forms. The acidity measured by FTIR was found to be only slightly lower than the highly active 3-D MWW forms, MCM-22 and MCM-49. Pivalonitrile adsorption, which is a measure of spatial openness, showed 50% accessibility vs. <30% for MCM-22/49. It highlights the potential of MCM-56 as a layered material with increased access to acid sites because it does not entail laborious post-synthesis modification. Swelling, pillaring and delamination of MCM-56 are facile but result in a reduction in the number of Brønsted acid sites (BAS) while increasing accessibility to pivalonitrile. The delamination procedure involving sonication and acidification of the highly basic mother liquor produces the most visible increase in surface area and access to all BAS. The accompanying doubling of the solid yield and the decrease in absolute number of BAS suggest significant precipitation of dissolved silica generated during swelling and sonication in high pH medium. The viability of separating surfactant covered layers upon sonication with the consequence of exposing hydrophobic hydrocarbon tails to aqueous environment is addressed.