Microporous Zeolite Research Articles

Computational Screening of Structure-Directing Agents for the Synthesis of Pure Silica ITE Zeolite.

"Shape" was the first criterion claimed to explain the specificity between organic structure-directing agents (OSDAs) and zeolite micropores. With the advent of computational chemistry methods applied to study the effectiveness of SDA-zeolite combinations, "energy" (mainly van der Waals) became the most commonly invoked concept to explain the zeolite phase selectivity. The lower the energy, the better the SDA. In this study, we rescue the concept of shape, and we combine it with the concept of energy within the frame of a SDA screening approach to identify new SDAs for the synthesis of cage-based ITE zeolite. Once we identify an appropriate shape fingerprint, filtering through the SDA database can be done quickly and accurately. With the shape selection, an automated Monte Carlo software allows us to assess the suitability using the force-field-calculated zeo-SDA energy. The computational approach can be promptly applied to other cage-based zeolites.

Enhancement in sintering driving force derived from in situ ordered structural collapse of mesoporous powders

AbstractIn this study, the sintering behaviors of a series of mesoporous silica (FDU‐12, SBA‐15, MCM‐41, and mesoporous silica nanoparticles) and microporous zeolite (ZSM‐5) using spark plasma sintering (SPS) technology were investigated. Highly transparent glass was prepared at a low temperature from all the powders. In particular, FDU‐12 type of mesoporous silica could be fully densified at an ultralow sintering temperature (910°C). It is suggested that the collapse of ordered mesostructures during SPS process dependents on the large pore size/pore wall thickness ratio, appropriate pore arrangement and amorphous frameworks of mesoporous silica, which implies that the enhancement in sinterability is possibly derived from the in situ collapse of ordered mesostructures. Such present findings pave a new way toward the construction of functional dense bulk materials from mesoporous powders.

Probing the pore structure of hierarchical EU-1 zeolites by adsorption of large molecules and through catalytic reaction

The adsorption of toluene and 1,3,5-trimethylbenzene and the catalytic transformation of 1,3,5-trimethylbenzene are applied as probing approaches to characterize the pore system of hierarchical EU-1 zeolites prepared using organofunctionalized fumed silica as the silicon source. The adsorption and diffusion of toluene and 1,3,5-trimethylbenzene are significantly improved in the hierarchical EU-1 zeolites compared with the conventional microporous EU-1 zeolite. The adsorption kinetics of toluene and 1,3,5-trimethylbenzene suggested that introducing mesopores significantly increases the rate of adsorption and improved the diffusion of large molecules. In the catalytic transformation of 1,3,5-trimethylbenzene, the conversion of 1,3,5-trimethylbenzene on the hierarchical EU-1 zeolites is doubled compared with the conventional microporous EU-1 zeolite, due to the improved diffusion of bulky molecules and enhanced accessibility of active sites in the hierarchical EU-1 structure. Although isomerization is the main reaction, differences are observed in the product ratios of isomerization to disproportionation between the hierarchical EU-1 zeolites and the microporous counterpart with different times on stream. The transformation of 1,3,5-trimethylbenzene over the hierarchical EU-1 zeolites has a higher isomerization to disproportionation ratio than that over the microporous EU-1 zeolite; this is due to the increased mesoporosity.

Importance of PCDD/F molecules’ polarizability and steric hindrance on their adsorption onto zeolites in a standard EN1948-1 sampling device for incinerator emission monitoring

Currently, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) monitoring at stationary sources uses short- and long-term sampling methods compliant with EN1948 standard. The XAD-2 resin is exposed to gas stream for 3–4 weeks (long term), and subsequently analysed in an accredited laboratory for 2 weeks. With the aim of developing an on-line mass spectrometer device using adsorption onto a set of different zeolites as specific adsorbents prior to mass analysis, the paper proposes to firstly test zeolites within the methods compliant with EN1948 standard.In addition to the non-specific external surface adsorption, zeolites possess micropores for adsorbing molecules specifically. Zeolites allowing selective adsorption of only tetra- and penta-chlorinated toxic congeners (TeCDD/F, PeCDD/F) were exposed to the gas flow of a household waste incinerator. As the extraction method of EN 1948 standard does not allow us to discern the two types of adsorption, the adsorbents have been tested at a sampling point located before the last filter. With high congener concentrations, it is expected that the dynamic adsorption properties of the zeolites can be deduced from the breakthrough of some congeners.The electronic interactions between zeolite surface area and PCDD/F involve the polarizability of the molecules. Thanks to the congener breakthrough comparing two zeolite structure types, it has been deduced that toxic TeCDD/F and PeCDD/F were adsorbed in the micropores of FAU-NaX zeolite due to steric compatibility of these congeners. Moreover, the partition of congeners between the condensate and gas phase was elucidated by considering the polarizability of PCDD/F molecules.

The synthetic strategies of hierarchical TS-1 zeolites for the oxidative desulfurization reactions

With the increasingly stringent standards for limiting sulfide content in liquid fuels, oxidative desulfurization (ODS) has become a promising ultra-deep desulfurization process in fuel desulfurization. TS-1 zeolites show great potential as catalysts for ODS reactions, due to its remarkable oxidation activity at low temperatures and pressure. However, the inherent microporous structure of conventional TS-1 zeolites restricts the mass transportation and renders the active sites in the microporous space of TS-1 zeolites inaccessible for bulky aromatic organosulfur compounds. Fabrication of hierarchical TS-1 zeolites by incorporating meso-/macropores into microporous TS-1 zeolites is an effective strategy to improve mass transportability. In recent years, abundant efforts have been dedicated to developing synthetic strategies of hierarchical TS-1 zeolite, thereby improving its catalytic performance in the ODS process. This mini-review addresses the synthetic methods of hierarchical TS-1 catalysts and their catalytic performance in the ODS reactions. In addition, some current problems and prospects of synthesis routes for constructing hierarchical TS-1 catalysts have also been revised. We expect this mini-review to shed light on the more efficient preparation strategies of hierarchical TS-1 zeolites for the ODS process. Construction of hierarchical TS-1 zeolites for the oxidative desulfurization reactions.

Natural Aluminosilicate-based Y Zeolite for Catalytic Cracking of n-Hexadecane

Natural zeolite and kaolin are abundant in Indonesia especially in Java and Belitung islands. These materials generally contain high aluminosilicate minerals that can be used as silica and alumina sources for the formation of NaY zeolite. This research aimed to identify the modification of NaY zeolite structure using Bayat natural zeolite as alumina source and Belitung kaolin as silica source and its catalytic activity for n-hexadecane cracking. Pretreatment of materials were performed in several steps: activation, purification, fragmentation for Bayat–Klaten natural zeolite, and silica extraction for Belitung kaolin. The synthesis of NaY was performed hydrothermally using seed-assisted method with crystallization times of 24 and 48 h. The surface area analysis showed a typical surface area of microporous zeolite (i.e., a pore diameter of 1.897 nm) and surface area of 309.4 cm2/g. Meanwhile, powder X-ray diffraction characterization showed that the as-synthesized materials have the structure of NaY zeolite. The optimum crystallization time was 24 h with Si/Al ratio of 4.10. Furthermore, both synthetic and natural sourced NaY zeolites were converted into HY zeolites and employed as cracking catalyst of n-hexadecane in a fixed-bed reactor at 500 °C for 75 s. The natural sourced HY resulted in 59.80% conversion and 59.80% yield of gasoline product (C9–C12). These values were lower than the results given by synthetic HY (80%).

Fischer-Tropsch reaction within zeolite crystals for selective formation of gasoline-ranged hydrocarbons

Abstract Product selectivity control is attractive in Fischer-Tropsch (F-T) synthesis but it is still a challenge, because the F-T products follow the Anderson-Schulz-Flory (ASF) distribution with maximized gasoline-ranged (C5−C11) hydrocarbon selectivity at 45%. Herein, we report a strategy by optimizing the gasoline selectivity to outperform the ASF limitation. The key to this success is fixation of the metal nanoparticles within zeolite crystals (metal@zeolite), where the zeolite micropore adjusts the product selectivity. For example, the Ru@NaY exhibited the gasoline selectivity 64.3% in the F-T reaction, which is significantly higher than the ASF limitation and about 2 times of that (32.8%) over conventionally supported Ru catalyst (Ru/NaY). This investigation might offer an alternative route for the direct transformation of syngas to liquid fuels with controllable selectivities.

Modified Dendritic Mesoporous Silica Nanospheres Composites: Superior Pore Structure and Acidity for Enhanced Hydrodesulfurization Performance of Dibenzothiophene

Dendritic mesoporous silica nanospheres (DMSNs) with a center-radical pore structure were successfully fabricated with ZSM-5 seeds to synthesize hierarchically meso–microporous ZSM-5/DMSN (ZD) materials which were used as the support for preparing HDS catalysts. The catalytic activities were evaluated by adopting DBT as the model oil. ZD composites and the corresponding catalysts were well characterized by XRD, XPS, HRTEM, and other techniques. The characterization results manifested that the specific center-radical pore structure was retained after incorporation of microporous zeolite ZSM-5; thus, ZD exhibited a wide pore diameter of about 17 nm, which was definitely preferable for mass transfer in the S removal process. Meanwhile, the open pore channels enhanced the accessibility of the active sites on the internal surface of the catalysts. Introducing ZSM-5 seeds into the framework of DMSNs also improved the acidity and modulated the metal–support interaction as well. As a result, NiMo/ZD series catalysts demonstrated high HDS activities, of which NiMo/ZD-3 achieved the highest HDS efficiency of 98.3%. The superior catalytic performance not only originated from the large center-radical pore structure and good acidity of support but also related to the suitable metal–support interaction and perfect dispersion of the metallic active sites.

Определение характеристик поверхности Zn-модифицированного цеолита типа H-ZSM-5

The surface properties of microporous aluminosilicates are of primary importance in chemical technology and catalysis. The determination of the surface area, pore volume, and pore size and size distribution for microporous aluminosilicates is a complex scientific and technical problem related to both accurate measurements and the correct choice of a mathematical model that adequately describes the physicochemical processes occurring on the test surface. Currently, t-plot, as-plot, Dubinin-Radushkevich model, Dillimore model and Horvath-Kawazoe model are frequently used, each of which has certain advantages and certain disadvantages, to determine micropores in the material. It should be noted that t-plot model found the most frequent use. This is a consequence of the good correlation of volumes and pore areas obtained with its help with theoretically calculated values for various materials. In this point, the article provides a calculation of Zn-modified zeolite H-ZSM-5 surface properties using the t-plot model, for which nitrogen adsorption isotherms were originally obtained. The nitrogen adsorption isotherms of the initial zeolite H-ZSM-5 can be attributed to the first type of isotherms, and the modified samples to the fourth type of isotherms according to Brunauer’s classification. It was found that the initial zeolite is a typical microporous zeolite of H-ZSM-5 type, while the modified samples are micro- and mesoporous materials. Calculation of nitrogen adsorption isotherms using the t-plot model allows to determine the volume of micro and mesopores, which varied in the range of 0.11–0.14 cm3/g for micropores and 0.04–0.07 cm3/g for mesopores. Based on the obtained data, it can be concluded that when zeolite is modified with zinc at a zinc concentration less than 15 wt.%, micropores are not filled with zinc, while mesopores are predominantly filled and an additional surface of zinc oxide is formed on the external surface of the zeolite.

Conversion of the Propane-Butane Fraction into Arenes on MFI Zeolites Modified by Zinc Oxide and Activated by Low-Temperature Plasma.

The effect of modification of MFI zeolite 1–5 wt.% ZnO activated by plasma on acid and catalytic properties in the conversion of the propane–butane fraction into arenes was investigated. The high-silica zeolites with silicate module 45 were synthesized from alkaline alumina–silica gels in the presence of an ‘X-oil’ organic structure-forming additive. The modification of the zeolite with zinc was carried out by impregnating the zeolite granules in the H-form with an aqueous solution of Zn(NO3)2. The obtained zeolites were characterized by X-ray phase analysis and IR spectroscopy. It is shown that the synthesized zeolites belong to the high-silica MFI zeolites. The study of microporous zeolite-containing catalysts during the conversion of C3-C4 alkanes to aromatic hydrocarbons made it possible to establish that the highest yield of aromatic hydrocarbons is observed on zeolite catalysts modified with 1 and 3% ZnO and amount to 63.7 and 64.4% at 600 °C, respectively, which is 7.7–8.4% more than on the original zeolite. The preliminary activation of microporous zeolites modified with 1–5% ZnO and plasma leads to an increase in the yield of aromatic hydrocarbons from the propane–butane fraction; the maximum yield of arenes is observed in zeolite catalysts modified with 1 and 3% ZnO and activated by plasma, amounting to 64.9 and 65.5% at 600 °C, respectively, which is 8.9–9.5% more than on the initial zeolite. The activity of the zeolite catalysts modified by ZnO and activated by plasma show good agreement with their acid properties. Activation of the zeolites modified by 1 and 3% ZnO and plasma leads to an increase in the concentration of the weak acid sites of the catalyst to 707 and 764 mmol/g in comparison with plasma-inactivated 1 and 3% ZnO/ZKE-XM catalysts at 626 and 572 mmol/g, respectively.

Nano-Assembled Mordenite Zeolite with Tunable Morphology for Carbonylation of Dimethyl Ether

Morphology dependent catalytic performance has been reported pervasively in heterogeneous catalysis, especially on nano-materials. Microporous zeolites with different shapes and sizes tend to exhib...

Development of mesoporous ZSM-5 zeolite with microporosity preservation through induced desilication

Alkali treatment is a widely used strategy to create mesopores for microporous zeolites to decrease the diffusion limitation in micropores. However, many of the micropore losses existed due to the uncontrollable destruction of framework. In this paper, a series of mesoporous ZSM-5 with well-maintained micropores was synthesized through induced desilication strategy. To induce the diffusion of alkali solution for the homogenizing desilication and mesopore formation, abundant and uniform small mesopores were firstly built for ZSM-5 by protective desilication in tetrapropylammonium hydroxide solution. The pore structure and acidity of the obtained ZSM-5 were characterized by XRD, TEM, N2 physisorption, NH3-TPD and Py-IR. Through induced desilication, the external surface area of obtained ZSM-5 increased from 42 m2 g−1 of parent ZSM-5 to 148 m2 g−1 and only 17% of micropore surface area was sacrificed, which was far lower than 37% of the loss in traditional NaOH treatment. Meanwhile, the strong acid sites were well preserved and the external acid sites were reduced. Compared with the sample treated by NaOH solution, the obtained catalyst exhibited higher catalytic stability and aromatics, especially PX selectivity. Good regenerated performance with long catalytic lifetime and high liquid hydrocarbon yield could also be found for the obtained catalyst. The strategy proposed in this paper is simple and ingenious in tailoring the porosity since it only relies on the desilication method, which is different from the usual coupling of dealumination and desilication.

Melting-assisted Synthesis of Hierarchical SSZ-13 Zeolite

In order to reduce the diffusion resistance of the reactants and products in the traditional microporous SSZ-13 zeolite, a hierarchical SSZ-13 zeolite was synthesized with melting-assisted method. The hierarchical SSZ-13 zeolite was easily and quickly synthesized by ammonium fluoride solid-phase grinding under solvent-free condition. The obtained SSZ- 13 samples were characterized by XRD, SEM, BET and EDS. The results clearly indicate that the etched zeolite contains micropores and mesopores, and in addition, no significant decrease of crystallinity is observed for the sample etched with ammonium fluoride. Using this method, other zeolites with CHA, MFI, and TON topologies were etched successfully.

Identification of New Templates for the Synthesis of BEA, BEC, and ISV Zeolites Using Molecular Topology and Monte Carlo Techniques.

The presence of organic structure directing agents (templates) in the synthesis of zeolites allows the synthesis to be directed, in many cases, toward structures in which there is a large stabilization between the template and the zeolite micropore due to dispersion interactions. Although other factors are also important (temperature, pH, Si/Al ratio, etc.), systems with strong zeolite-template interactions are good candidates for an application of new computational algorithms, for instance those based in molecular topology (MT), that can be used in combination with large databases of organic molecules. Computational design of new templates allows the synthesis of existing and new zeolites to be expanded and refined. Three zeolites with similar 3-D large pore systems, BEA, BEC, and ISV, were selected with the aim of finding new templates for their selective syntheses. Using a training set of active and inactive templates (obtained from the literature) for the synthesis of target zeolites, it was possible to select chemical descriptors related to activity, meaning a good candidate template. With a discriminant function defined upon MT, the screening through a database of organic molecules led to a small subset (preselection) of candidate templates for the synthesis of BEA, BEC, and ISV. As far as we know, this is the first time that topological/topochemical descriptors, which do not consider 3-D information on the molecules, have been used to predict the activity of zeolite structure directing agents (SDAs). Following the prediction of SDAs using MT, an automated approach of sequential template filling of micropores based on a combination of Monte Carlo and lattice energy minimization was applied for all the candidate templates in the three zeolites. Two results can be obtained from this: an evaluation of the quality of the molecular topology QSAR models leading to the preselection of templates, and a final selection of candidate templates for the selective synthesis of BEA, BEC, and ISV. Regarding the latter, a good template will be that which maximizes the zeolite-template dispersion interactions with one, and only one, of the three zeolites. The presented methodology can be used to find alternative (maybe cheaper or perhaps more selective) templates than those already known.

Experimental and molecular dynamics simulation studies of an anti-hyperlipidemic drug release from microporous zeolites differing in Si/Al content

Abstract The effect of different framework topologies and aluminum contents on the release behavior of probucol was investigated using BEA, MFI and NaX as the microporous zeolite drug carriers. Post-drug loading changes in the textural properties of the zeolites and the crystalline state of probucol were evaluated using nitrogen adsorption–desorption measurements, differential scanning calorimetry and X-ray diffraction analysis. Results indicated significant drug amorphization upon loading in all zeolite types, with drug deposition occurring within the micropore channels and the external surface area of the microporous carriers. Molecular dynamics simulations were employed to delineate drug diffusion within the framework structure, showing that PB diffused more easily through the BEA pore network, compared to MFI, while drug's movement within NaX was almost negligible. The same trend was observed in the in vitro drug release profiles in simulated gastric fluid, indicating that aluminum content is a determinant factor in the extend of probucol release. NaX containing the highest aluminum content demonstrated less than 5% of probucol release, in contrast to BEA containing the lowest aluminum content for which almost total drug release was observed.

In-situ synthesis of hierarchical lamellar ZSM-5 zeolite with enhanced MTP catalytic performance by a facile seed-assisted method

A seed-assisted route has been applied to prepare the hierarchical lamellar ZSM-5 zeolite with good porosity and catalytic performance in methanol to propylene (MTP) reaction. In this method, ZSM-5 crystal seeds instead of organic quaternary ammonium were used to direct the formation of ZSM-5 nuclei and conventional surfactant cetyltrimethylammonium bromide (CTAB) employed as the mesoporogen. The process for the formation of hierarchical lamellar ZSM-5, including the effect of the ratio of CTAB/SiO2, initial gel aging temperature, synthesis temperature and synthesis time were discussed in detail. The results suggested that mesoporous structure was first obtained and then transformed into MFI structure in-situ during the hydrothermal synthesis process. The obtained hierarchical lamellar ZSM-5 zeolite displays an organized flack-like nanosheet stacks morphology with regular intercrystal mesopores of 3–7 nm, possessing large surface area and mesopore volume. Moreover, the synthesized hierarchical ZSM-5 zeolite has perfect catalyst lifetime in methanol to propylene (MTP) reaction than conventional microporous ZSM-5 zeolite, which could be ascribed to the hierarchical mesoporous structure accommodating more bulky molecules and accessible acid sites in the catalytic reaction.

Fault zone structures and strain localization in clinoptilolite-tuff (Nižný Hrabovec, Slovak Republic)

Clinoptilolite is a natural micro-porous zeolite composed of silica and alumina tetrahedra that commonly forms as a devitrification product of glass in volcanic tuffs. Besides applications in the agriculture, building and manufacturing industries, clinoptilolite has been widely studied in veterinary and human medicine because of its positive effects on health. Based on field observations and microstructural investigations, we show, for the first time, strain localization features along faults in natural zeolite tuffs in a clinoptilolite-tuff open-pit mine at Nižný Hrabovec (Slovak Republic). The faults are localized along pre-existing joints with plumose structures that acted as a pathway for the infiltration of mainly Mn- and K-rich fluids. Displacement along the faults created structures that are indicative of both velocity hardening (accommodated by dissolution precipitation creep on SC/SCC′ foliation) and velocity weakening (accommodated by multi-generations of ultracataclasites along principal slip surfaces) behaviors. Rock-fluid interaction was characterized by a high-mobility of K-rich fluids, with K-feldspar decorating the SC/SCC′ foliation, infiltrating fractures in the damage zone of the faults and precipitating as idiomorphic crystals in open cavities and along the fault surface. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast-cortex grains in cataclasites and truncated grains along principal slip surfaces probably suggest that at least some of the faults formed during seismic slip.

Pd-modified beta zeolite for modulated hydro-cracking of low-density polyethylene into a paraffinic-rich hydrocarbon fuel

The catalytic properties of a microporous beta zeolite and Pd-loaded analogue were assessed in the cracking of low-density polyethylene (LDPE). The idea behind this study was to investigate the potential of zeolite beta from a different perspective, aiming at producing a hydrocarbon fuel, i.e. formed by saturated hydrocarbons, without the addition of external H2 but using the self-generated hydrogen, or hydrogen-transfer reactions. For this, a catalytic system was carefully prepared and compared to the bare beta zeolite; a protonic Pd-containing beta zeolite has a similar texture, structure and acidity features as in the base zeolite. The characterisation showed that this catalyst was achieved. The operando FTIR catalytic results indicated that the Pd-beta zeolite reaches a high selectivity towards the C2-C5 and aromatics fractions, with better control of the composition of the C2-C5 fraction leading to high isoparaffinic content, and a relatively low share of olefinic compounds. Additionally, the retardation in the coke forming reactions was evidenced, due to the high Pd-hydrogenation efficiency. Further insights were rationalized about the coke content and the catalyst auto-regeneration. From the process point of view, it was found that the use of a beta zeolite having well-dispersed Pd is an effective way to upgrade the LDPE into a paraffinic-rich hydrocarbon fuel.

Oxidative Dehydrogenation of Propane over Vanadium-Containing Faujasite Zeolite.

Oxidative dehydrogenation (ODH) of light alkanes to olefins—in particular, using vanadium-based catalysts—is a promising alternative to the dehydrogenation process. Here, we investigate how the activity of the vanadium phase in ODH is related to its dispersion in porous matrices. An attempt was made to synthesize catalysts in which vanadium was deposited on a microporous faujasite zeolite (FAU) with the hierarchical (desilicated) FAU as supports. These yielded different catalysts with varying amounts and types of vanadium phase and the porosity of the support. The phase composition of the catalysts was confirmed by X-ray diffraction (XRD); low temperature nitrogen sorption experiments resulted in their surface area and pore volumes, and reducibility was measured with a temperature-programmed reduction with a hydrogen (H2-TPR) method. The character of vanadium was studied by UV-VIS spectroscopy. The obtained samples were subjected to catalytic tests in the oxidative dehydrogenation of propane in a fixed-bed gas flow reactor with a gas chromatograph to detect subtract and reaction products at a temperature range from 400–500 °C, with varying contact times. The sample containing 6 wt% of vanadium deposited on the desilicated FAU appeared the most active. The activity was ascribed to the presence of the dispersed vanadium ions in the tetragonal coordination environment and support mesoporosity.

Salt inclusion and deliquescence in salt/zeolite X composites for thermochemical heat storage

Abstract Composites of commercially available zeolite Na-X beads and the salts NaCl, CaCl2, MgCl2, and MgSO4 (loadings up to 24 mol mol−1 or 18 wt.-%) were prepared by ion-exchange followed by incipient wetness impregnation and characterized by elemental analysis (XRF), XRD, N2 sorption, mercury porosimetry, and SEM. During loading of the zeolite with salt, the Donnan effect leads to an inclusion of salt ions into the zeolitic micropores. Salt is included up to a threshold loading of ~20 mol mol−1 depending on the nature of salt. Thermochemical properties are determined at 30 °C and a humidity of 3–21 g kg−1. Both water loading lift and heat storage density decrease with increasing salt loading below the deliquescence humidity of the salt. Above the deliquescence humidity, absorption into a salt solution within the secondary pores of the zeolite beads contributes to the water uptake. Heat storage densities up to 275 kWh m−3 (153% of the salt-free zeolite) are reached at high salt loadings and sorption humidity.