LTL Zeolite Research Articles

A nanoscale investigation of the formation of mesostructured zeolites FAU and LTL

Nanostructured materials can be utilised as potential catalysts for the production of platform chemicals and renewable biofuels from biomass derived molecules. The formation of hierarchical meso-microporous zeolites LTL and FAU via the surfactant assisted tandem acid-base post-synthesis treatment has been investigated by time-resolved in situ synchrotron SAXS and WAXS, providing a new insight into the mechanism of the mesostructuring treatment. Based on the results of TEM and in situ synchrotron measurements, a model for the formation of the core-shell structure of LTL zeolite crystals is proposed. Complementary evaluation using FTIR, NMR and nitrogen adsorption, in conjunction with reaction studies on mesostructured zeolites, demonstrated a potential for enhanced catalytic performance of these materials owing to the increased accessibility of the active sites and reduced transport limitations.

A New Type of Acidic OH-Groups in the LTL Zeolite

Acidic properties of ion-exchanged LTL zeolites have been studied using FTIR spectroscopy, complemented by X-ray powder diffraction, SEM-EDX, XRF and N2 physisorption. Infrared spectra of the ion-exchanged zeolites show the presence of two intense bands of the bridging OH-groups: a narrow band at ~3640 cm−1 that is attributed to Si(OH)Al groups freely vibrating in 12 MR and a broad, intense band at ~3250 cm−1 that is assigned to bridging OH groups forming hydrogen bond with neighbouring oxygen atoms, e.g., in six-membered rings. The former can be selectively removed by caesium or rubidium cations with up to 3 Cs+ or Rb+ per unit cell readily ion-exchanged into the LTL zeolite, replacing an equivalent number of acidic OH-groups or K+ cations within the structure. The cation migration of the larger cation, evaluated by the Rietveld refinement method, occurs mostly via the main 12 MR channels. By contrast, less than 1 Li+ or Na+ cation per unit cell can be introduced under similar conditions. Accordingly, the concentration of Si(OH)Al groups in back-exchanged NH4-K-LTL with smaller cations (Li+, Na+) does not differ considerably from the concentration of Brønsted acid sites in the original NH4-K-LTL. Lower concentrations of acid sites have been detected in the samples back-exchanged with Cs+, Rb+ and K+. In addition, the acidic properties of NH4-LTL samples have been compared with a structurally related NH4-MAZ zeolite.

Zeofilters for potentially innovative sunscreen products: formulation, stability and spectroscopic studies

Exposure to both UVA and UVB radiation from the sun stands as the primary cause of skin cancers, contributing to the escalating yearly count of diagnosed cases. Sunscreen products, (i.e. organic and inorganic UV filters) have become ubiquitous in personal-care products, packaging, plastics, dyes, and many other sectors. Ideal UV filters should be photostable and dissipate the absorbed energy efficiently. However, most organic UV filters are photolabile upon protracted UV exposure, leading to the formation of photoproducts of unknown toxicity for human health and environment. The chemical environment surrounding UV filters may strongly affect their behaviour: the interaction with other components of sunscreen formulations may enhance their degradation. Particularly, the interaction between photochemically incompatible ingredients must be avoided. Therefore, much effort has been invested in developing more effective and safe sunscreens.Our team recently focused on the preparation and characterization of hybrid UVfs obtained by the encapsulation of organic UV filters into various zeolites differing for topology. The zeolite most efficient for the encapsulation of organic UV filters, in term of stability and efficiency in radiation filtration, resulted to be LTL zeolite. Here, hybrid materials realized after the encapsulation of octinoxate (OMC) and avobenzone (AVO) in LTL zeolite were tested in terms of skin permeation profiles, cytotoxicity and photostability once the materials are dispersed in cream formulations, so to mimicking the real condition of use.The results show that the hybrids were successfully incorporated into oil-in-water emulsions and the skin permeation tests displayed a very low UV filter permeation through pig skin and no UV filter accumulation in dermis layer.The photostability of the hybrid-based emulsions were compared with that of emulsions containing a mixture of bare OMC and AVO, highlighting a reduced decrease in UV screening efficiency upon irradiation for the samples containing the hybrid materials with respect to the free filters. This evidence confirms that the encapsulation in the zeolite frameworks prevents some of the photodegradation reactions. The result of this paper highlights that zeolite encapsulation is increasingly establishing as a viable way to stabilize UV filters and other photoactive molecules.

Mechanical properties of porous materials based on new triply periodic and minimal surfaces

In this study, the mechanical properties of eight new porous structures were investigated. These structures are based on triply periodic surfaces derived from zeolite frameworks AWW, CGF, HEU, NPT, FRA, IFY, LTL and TSC; the TPSs obtained from the last four frameworks were found to be new minimal. Samples of TSC-based porous structures were made of polyamide using 3D printing methods and their mechanical properties (loading curves, compression modulus, shear modulus, Poisson’s ratios, and yield strengths) were experimentally and numerically studied. Numerically calculated mechanical properties were found to be in good agreement with the experimental results. We revealed dependences of the mechanical characteristics on the relative density of the material and estimated mechanical properties of porous structures obtained from other seven zeolites. The results obtained enable one to choose porous materials that are the most suitable in terms of their mechanical properties for various engineering applications.

Hierarchical LTL Zeolite as an Efficient and Sustainable Solid Acid Catalyst for Replacing HCl in the Production of Polyurethane Intermediates.

In many industrially important reactions, caustic mineral acid catalysts have been successfully replaced with green solid acids such as zeolites. In this context, extensive efforts have been devoted to replacing HCl to produce methylenedianiline (MDA), a key intermediate in polyurethane production. Unfortunately, limited success has been achieved thus far due to low activity, selectivity towards the desired 4,4'-MDA, and rapid catalyst deactivation. Here we report that meso-/microporous hierarchical LTL zeolite exhibits unprecedentedly high activity, selectivity, and stability. The one-dimensional cage-like micropores of LTL promote the bimolecular reaction between two para-aminobenzylaniline intermediates to selectively produce 4,4'-MDA and inhibit the formation of undesired isomers and heavy oligomers. Meanwhile, the secondary mesopores alleviate mass transfer limitations, resulting in a 7.8-fold higher MDA formation rate compared to solely microporous LTL zeolite. Due to suppressed oligomer formation and fast mass transfer, the catalyst exhibits inappreciable deactivation in an industrially relevant continuous flow reactor.

Hierarchical LTL Zeolite as an Efficient and Sustainable Solid Acid Catalyst for Replacing HCl in the Production of Polyurethane Intermediates

AbstractIn many industrially important reactions, caustic mineral acid catalysts have been successfully replaced with green solid acids such as zeolites. In this context, extensive efforts have been devoted to replacing HCl to produce methylenedianiline (MDA), a key intermediate in polyurethane production. Unfortunately, limited success has been achieved thus far due to low activity, selectivity towards the desired 4,4′‐MDA, and rapid catalyst deactivation. Here we report that meso‐/microporous hierarchical LTL zeolite exhibits unprecedentedly high activity, selectivity, and stability. The one‐dimensional cage‐like micropores of LTL promote the bimolecular reaction between two para‐aminobenzylaniline intermediates to selectively produce 4,4′‐MDA and inhibit the formation of undesired isomers and heavy oligomers. Meanwhile, the secondary mesopores alleviate mass transfer limitations, resulting in a 7.8‐fold higher MDA formation rate compared to solely microporous LTL zeolite. Due to suppressed oligomer formation and fast mass transfer, the catalyst exhibits inappreciable deactivation in an industrially relevant continuous flow reactor.

Theoretical FRET Efficiency of an Antenna Material Containing Natural Dyes and Zeolite L.

We calculated the Förster resonance energy-transfer (FRET) efficiency of a theoretical host-guest composite formed by all-trans β-cryptoxanthin (BCRY), all-trans zeaxanthin (ZEA), and a zeolite-LTL (Linde Type L) nanochannel with the help of computational chemistry tools. Climate change demands urgently the development of novel renewable energies, and in such a context, artificial photosynthesis arises as a promising technology capable of contributing to satisfying humankind's energy needs. All artificial photosynthetic devices need antennas to harvest and transfer energy to a reaction center efficiently. Antenna materials integrated by highly fluorescent synthetic pigments embedded onto the nanochannels of a zeolite-LTL have already been shown experimentally to be very efficient supramolecular assemblies. However, research work computing the efficiency of an antenna made of nonfluorescent natural pigments and a zeolite-LTL nanochannel has not been undertaken yet, at least to our knowledge. Fortunately, natural dyes possess outstanding features to study them dynamically; they are environmentally friendly, inexpensive, ubiquitous, and abundant. Density functional theory (DFT) methods were chiefly employed along with the CAM-B3LYP functional and the 3-21G*/6-311+G(d,p) basis sets. The ONIOM method enabled geometry and energy calculations of dyes inside the zeolite-LTL (ZL) nanochannel. The Förster resonance energy-transfer (FRET) efficiency and the Förster radius of the composite were 40.9% and 24.9 Å, respectively. Theoretical findings suggested that this composite might contribute to diminishing costs and improving the environmental friendliness of an antenna system.

Novel connectivity tensor for a systematic assessment of topology and anisotropy of real membranes and microporous structures

In this work, an original and complete systematic topological assessment of anisotropy, connectivity and tortuosity of real 3D porous structures is presented. Several novelties are introduced here, like: the definition of the Average Effective Diffusivity Tensor (AEDT), allowing us to assess the desired topological parameters, the novel concept of connectivity tensor based on the inverse of the (diagonal) tortuosity tensor, the development of a new procedure of digital reconstruction of real 3D structures for CFD purpose, and a novel methodology for the connectivity tensor calculation, which avoids time-dependent diffusivities. After a detailed explanation of methodology and related validation using well-known 3D structures, we analyse two different real digitally-reconstructed 3D structures: LTL zeolite and clinoptilolite. As key-result, the internal voids distribution is characterised in terms of several existing and new topological parameters, all obtained from the mathematical properties of tortuosity and connectivity tensors. Among them, particular useful are the here-defined directionality factors, which indicate the preferential diffusional paths. The proposed approach is relatively easy and can be effectively applied not only to characterise existing materials, but also to design novel ones (in form of catalysts, selective membranes and/or adsorbents) with desired and predictable topological and mass transport properties.

Performance of BEA, FAU, LTL, MFI, and MOR zeolites in the removal of ethyl mercaptan traces from natural gas by Monte Carlo molecular simulation

Selective removal of ethyl mercaptan from natural gas is presently challenge in the gas industry. Nanoporous zeolite frameworks with different topologies have been known as the most appropriate compounds for the adsorption of ethyl mercaptan. Grand Canonical Monte Carlo (GCMC) simulation has been employed to predict the adsorption and separation properties of mixtures containing ethyl mercaptan under difficult experimental conditions. The adsorption isotherms of ethyl mercaptan in pure form and mixtures with methane on purely siliceous zeolites including BEA, FAU, LTL, MFI, and MOR have been obtained at the temperature of 298 K over the pressure range of 0–100 kPa. The results show that the adsorption capacity is a function of the pore diameter of zeolites. FAU has appeared as an adsorbent with high adsorption capacity. In contrast, it has shown considerably lower adsorption affinity for methane in comparison with ethyl mercaptan, confirming the competitive adsorption between them. Therefore, FAU could be the promising candidate for industrial desulfurization of natural gas. The effects of temperature, pressure, initial bulk concentration, and pore structure have been examined. Distribution snapshots displays the preferred positions of zeolites for the adsorption of sulfur species.

Chromic acid dealumination of zeolites

Commercial zeolites with CHA, MFI, and LTL framework topology are subjected to acid treatment with chromic acid (H 2 CrO 4 ) solutions with concentrations ranging between 0.1 and 10 w /%. As chromic acid forms polymeric species as a function of concentration, specific effects are obtained with these three zeolites, differing by their framework topology, pore size, Si/Al ratios and crystal size. The etched zeolites are characterized by X-ray diffraction, SEM, TEM, N 2 physisorption, ICP-AES, 27 Al and 29 Si MAS NMR, 29 Si{ 1 H} cross-polarization spectra, EDX. Their acidic properties are probed with pyridine and d 3 -acetonitrile by in situ FTIR. Results indicate that the 8 MR (CHA) and 10 MR (MFI) zeolites are more resistant to acid treatment with chromic acid than the 12 MR (LTL) zeolite. This paves the way for rational control of zeolite dealumination since adjusting chromic acid concentration will generate anions appropriate for treating a particular zeolite. • CHA, MFI, and LTL zeolites are subjected to treatment with chromic acid (H 2 CrO 4 ). • Chromic acid forms polymeric species as a function of concentration. • CHA and MFI zeolites resist more to the chromic acid treatment than the LTL zeolite. • Tuning the acid concentration yields anions apt for treating a particular zeolite.

Understanding the zeolites catalyzed isobutane alkylation based on their topology effects on the reactants adsorption

The easy deactivation of zeolites catalyzed isobutane alkylation, originated from the preferential adsorption of olefin against isobutane, limits their industrial applications. In this work, the adsorption properties of zeolites towards isobutane and 1-butene with different topologies and Si/Al ratios were investigated using CBMC simulations. Based on the calculated Henry coefficient and adsorption heat, the adsorption affinity decreases with the increase of Si/Al ratio in FAU-type zeolites, while for 1-butene the decrease is more obvious. The adsorption capacity of zeolites towards isobutane and 1-butene depends on the interaction between adsorbents and adsorbates at low pressures, while on the effective void volume of adsorbents at high pressures. A satisfactory relationship between the adsorption selectivity (SI/O) and catalytic stability of zeolites for isobutane alkylation was found. Compared to FAU and LTL zeolites with expansions in pores or channels, BEA-15 and MTW-39 zeolites exhibit a much higher SI/O and catalytic stability.

Theoretical Prediction from Classical Equations and Rational Synthesis of Ultrafine LTL Zeolite Nanocrystals

Much more attention has been paid to zeolite nanocrystals due to their use as catalysts and zeolite membranes as well as composites and hierarchical structures. Recently, zeolite nanocrystals of se...

A Density Functional Theory study of molecular hydrogen adsorption on Mg site in OFF type zeolite cluster

In this study, hydrogen molecule adsorption was investigated on additional framework of Mg atom in OFF type zeolite by way of DFT (Density Functional Theory). The adsorption enthalpy and adsorption energy, chemical potential, HOMO energy, electronegativity, LUMO energy, chemical hardness values have been computed on a 8T zeolite cluster model and compared with those of Mg exchanged ERI, LTL and DFT zeolite structures previously reported in literature. Hydrogen adsorption enthalpy value was computed as −17 kJ/mol. This value is meaningfully larger than the hydrogen molecule's liquefaction enthalpy value. This accordingly specifies that Mg-OFF zeolite structure appears to be an encouraging candidate cryoadsorbent for hydrogen storing.

Morphological effects on catalytic performance of LTL zeolites in acylation of 2-methylfuran enhanced by non-microwave instant heating

Morphological characteristics of zeolites can have significant effects on the catalytic reactions. However, the roles of zeolite morphology on catalytic reactions, particularly Friedel-Crafts acylation of 2-methylfuran, are still not clear. Hence, LTL-type zeolites with different morphologies are prepared by using bamboo leaf bio-silica source where the zeolites are used to elucidate the morphological properties in this acylation reaction. The results show that LTL zeolites with short-rod, cylindrical, stick-like and nanosized shapes can be prepared by simply tuning the water content of the precursor hydrogels and the crystallization time. The morphological properties of LTL zeolites are also found to have pronounced influences on their surface properties (surface areas, pore volumes, number of micropore channels, textural properties) and acidity (amount, strength and type) where these properties affect their catalytic activity in acylation of 2-methylfuran under non-microwave instant heating condition. Furthermore, the influence of temperature and heating time on the kinetics of catalytic acylation of 2-methylfuran, and the catalytic comparative study between nanosized LTL zeolite and several homogeneous and heterogeneous catalysts are also discussed.

Studies on the synthesis and characterization of polyaniline-zeolite nanostructures and their role in carbon monoxide sensing

Polyaniline was modified with zeolite LTL to increase its response towards carbon monoxide. The synthesis was carried out by chemical oxidative polymerization using ammonium peroxydisulfate (NH4)2S2O8 as oxidizing agent. Zeolite LTL and its ion exchanged forms (Na, Fe, Cu and H-form) were converted into composites. The dry mixed composites were subsequently pressed into pellets with a diameter of 12 mm and a nominal thickness of 2 mm, using hydraulic press machine at a pressure of ∼280 Mpa. The carbon monoxide response measurements were performed for various forms of PANI/LTL at various CO concentrations. The response was found to be maximum for PANI/Na-LTL.

Detection of CO2 and O2 by iron loaded LTL zeolite films

Detection of oxygen and carbon dioxide is important in the field of chemical and biosensors for atmosphere and biosystem monitoring and fermentation processes. The present study reports on the preparation of zeolite films doped with iron nanoparticles for detection of CO2 and O2 in gas phase. Pure nanosized LTL type zeolite with monomodal particle size distribution loaded with iron (Fe-LTL) was prepared under hydrothermal condition from colloidal precursor suspensions. The zeolite was loaded with iron to different levels by ion exchange. The Fe-LTL suspensions were used for preparation of thin films on silicon wafers via spin coating method. The reduction of the iron in the zeolite films was carried out under H2 flow (50% H2 in Ar) at 300 °C. The presence of iron nanoparticles is proved by in situ ultra-violet-visible spectroscopy. The properties of the films including surface roughness, thickness, porosity, and mechanical stability were studied. In addition, the loading and distribution of iron in the zeolite films were investigated. The Fe-LTL zeolite films were used to detect O2 and CO2 in a concentration dependent mode, followed by IR spectroscopy. The changes in the IR bands at 855 and 642 cm–1 (Fe‒O‒H and Fe‒O bending vibrations) and at 2363 and 2333 cm–1 (CO2 asymmetric stretching) corresponding to the presence of O2 and CO2, respectively, were evaluated. The response to O2 and CO2 was instant, which was attributed to great accessibility of the iron in the nanosized zeolite crystals. The saturation of the Fe-LTL films with CO2 and O2 at each concentration was reached within less than a minute. The Fe-LTL films detected both oxygen and carbon dioxide in contrast, to the pure LTL zeolite film.

Formation of copper nanoparticles in LTL nanosized zeolite: spectroscopic characterization.

The state of copper species stabilized in nanosized LTL zeolite subjected to various post-synthesis treatments was unveiled by a range of spectroscopic techniques. FTIR and UV-Vis studies demonstrated that the reduction process of copper in the LTL nanosized zeolite leads to the formation of different species including Cu2+, Cu+ and Cu nanoparticles (Cu NPs). The adsorption of probe molecules (NO and CO) was used to selectively monitor the copper species in the LTL nanosized zeolite upon oxidation and reduction post-synthesis treatments. Both the Cu2+ and Cu+ species were probed by NO and CO, respectively. The amount of Cu+ in the LTL zeolite nanocrystals was about 43% as determined by FTIR, while the amount of Cu NPs was about 55% determined by the UV-Vis spectroscopic characterization. These results were complemented by EPR, 29Si and 63Cu MAS NMR spectroscopic data. The EPR spectroscopy was further applied to monitor the effective reduction of the Cu2+ species and their re-oxidation, while the 63Cu MAS NMR verified the presence of Cu NPs in the LTL nanosized zeolite crystals.

Synthesis of nano-sized LTL zeolite by addition of a Ba precursor with superior n-octane aromatization performance

Nano-sized LTL zeolites obtained by the Ba-assisted method show improved catalytic performance in n-octane aromatization reaction.

Multi step energy transfer between three Si_LTL and SiGe_LTL zeolite-loaded dyes

We report a novel potential light-harvesting antenna material constructed with three dyes loaded on two types of zeolite LTL (SiGe_LTL and Si_LTL). Fluorescence resonance energy transfer (FRET) occurs from acridine hydrochloride (Ac) to Thionine acetate (Th) via Acriflavine hydrochloride (AF), which acts as an antenna to receive and transfer energy from Ac to Th. We compared multistep FRET systems based on dye-loaded SiGe_LTL paired with an Si_LTL-based system. Our results show that in both cases, FRET efficiency increases with Th loading and decreases with increasing AF loading. Moreover, the zeolite LTL microenvironment causes a red shift in the fluorescence spectra of the three SiGe_LTL-loaded-dyes compared to those of the Si_LTL-based dyes.

A density functional theory study of hydrogen adsorption on Be-, Mg-, and Ca-exchanged LTL zeolite clusters.

Hydrogen molecule adsorption on frameworks consisting of alkaline earth metal atoms (Be, Mg, or Ca) in LTL zeolite was investigated via density functional theory. A 24T zeolite cluster model was used in this study. HOMO and LUMO energy, chemical potential, chemical hardness, electronegativity, adsorption energy, and adsorption enthalpy values were calculated. The Mg-LTL and Ca-LTL clusters were found to have much lower chemical potentials and adsorption energies than those of the Be-LTL cluster. Additionally, the calculations indicated that the Mg-LTL and Ca-LTL clusters are softer (considering their lower chemical hardness values) and more chemically reactive than the Be-LTL cluster. The calculated hydrogen adsorption enthalpies were -14.7 and -9.4kJ/mol for the Mg-LTL and Ca-LTL clusters, respectively, which are significantly larger than the enthalpy of liquefaction for the hydrogen molecule. These results imply that the Mg-LTL and Ca-LTL zeolite structures are promising cryoadsorbents for hydrogen storage. Graphical abstract Hydrogen adsorption was theoretically investigated on Be-, Ca- and Mg-LTL clusters. Ca- and Mg-LTL zeolites are potential cryoadsorbent materials for hydrogen storage.