Zeolite Crystal Size Research Articles

Fine control of crystal morphologies of all-silica DDR in ethylenediamine-free gel with inorganic base as mineralizing agent

Abstract The size and morphology control of zeolite crystals is important which might have significant impact on their adsorption, separation and catalytic properties. Traditional hydrothermal synthesis of zeolite DDR usually yields big diamond-shaped crystals and there are few reports about its size and morphology control. In this study, microwave synthesis of all-silica DDR zeolite was systematically studied with 1-adamantane amine as template and inorganic base as mineralizing agent (KOH, NaOH and LiOH). The use of co-solvent (ethylenediamine) and fluorides were avoided which is environment-friendly. The use of inorganic bases also reduced synthesis cost. High quality micron sized DDR crystals with high yield were obtained in only a few hours which is the synergy of microwave heating and seeding, both are powerful methods on reducing synthesis time. A rich variety of crystal morphologies, like diamond, bipyramid, hexagonal prism and sphere, were obtained which can be controlled by the addition of metal cations, gel alkalinity, synthesis temperature and gel dilution. 29 Si NMR, FT-IR and nitrogen adsorption confirmed the formation of zeolite framework and its porosity. The fine control of crystal size and morphology of DDR zeolite may facilitate its application in adsorption, membrane separation and catalysis.

Influence of zeolite crystal size on selective conversion of n-alkane: Controlling intermediates’ diffusion distances inside the micropores

In this work, the alkali-treatment with NaOH solution was used to reduce the crystal sizes of original zeolite Y. Acidic supports of two bifunctional catalysts consisted of γ-alumina binder and zeolite Y (original or alkali-treated) with different crystal size distributions. Most of metal platinum was selectively deposited on the γ-alumina component via the electrostatic adsorption method. Given the above, it can be deduced that (de)hydrogenation reaction occurred inside the zeolite micropores at a low probability, while active protonic acid sites catalyzing the skeletal conversions of alkene intermediates were exclusively located on zeolite Y. Therefore, the number of active protonic sites that intermediates potentially encountered between two metal sites can be regarded as being proportional to diffusion distances of intermediates within the zeolite micropore structures. Moreover, the above-mentioned diffusion distances were greatly associated with the zeolite crystal sizes. Isomerization reactions of n-decane (n-C10) on two bifunctional catalysts were investigated, and results indicated that the catalyst, with alkali-treated zeolite as the acidic component, performed better than the catalyst, for which original zeolite provided the acid function, in restraining the secondary reactions. This can be attributed to the shortened diffusion distances of intermediates inside the zeolite micropores, which was caused by reducing the zeolite crystal size.

Influence of salt on nanozeolite-Y particles size synthesized under organic template-free condition

Abstract Zeolite-Y is one of the preferred micro-sized zeolite commonly used as a catalyst and for adsorption and separation. The size and shape of zeolite crystals have great influence on their properties therefore the synthesis of nano-size zeolite-Y has received much attention with straightforward methodology. In the present study, nano-sized zeolite-Y crystals were synthesized by the clear-solution (CS) and dense-gel (DG) methods in the presence of alkali metal chloride salt (i.e. NaCl) under organic template-free condition. The influence of the salt on the growth of nano zeolite-Y crystal size and morphology was studied systematically by varying the NaCl concentration. Depending on the method selected for synthesis (CS or DG), the growth of nano-size crystals was affected by the addition of NaCl content. In the CS method, the size of zeolite-Y crystals was found to decrease from 120 nm to 60 nm with an increase of salt content which signify the influence of salt in hindering diffusion of nutrients to the crystal nuclei in a homogeneous starting gel. Whereas, the spherical aggregates of well-shaped nanocrystals were formed with an increase of crystal size in the case of DG method. This is attributed to the suppression of the rate of crystal dissolution in dense-gel system upon introduction of salt ions. The nano-size zeolite-Y synthesized by both methods exhibited comparatively higher water adsorption capacity (15.4 ± 0.38–16.2 ± 0.41 mmol/g) than commercially available micro-size zeolite-Y (12.9 mmol/g) which can be attributed to the contribution of the meso/macropores arising from the inter-crystal voids. Hence, the synthesized nano-size zeolite-Y can be considered as promising adsorbents for water adsorption applications.

Synthesis of Nanoscale Zeolites

The review is devoted to the features of the synthesis of zeolites with nanosized crystals, which are of great interest for the processes of petroleum chemistry, gas chemistry, and organic synthesis. On the basis of analysis of published data, a classification is proposed for methods of directional control of the size of zeolite crystals. Methods for qualitative and quantitative controlling the composition of the reaction mixtures crystallized into nanoscale zeolites are considered in detail, as well as the effect of crystallization conditions on the change in the dispersity of zeolite crystals.

Influence of MEL Zeolite Synthesis Conditions on the Physicochemical and Catalytic Properties in the Oligomerization Reaction of Butylenes

The influence of MEL zeolite synthesis conditions on the set of physicochemical properties and catalytic activity of the zeolites in the oligomerization reaction of butylenes has been studied. The number of synthesis stages and the concentration of tetrabutylammonium hydroxide template in the reaction mixture have been used as the variable parameters for the synthesis of MEL zeolites. It has been found that the use of two-stage crystallization comprising the low-temperature stage at 90°C and the high-temperature stage at 170°C promotes a decrease in the size of MEL zeolite crystals and provides a more complete involvement of the source materials in the crystallization process. The interval of template concentration in the reaction mixture in which the pure phase of zeolite MEL free of zeolite MFI can be obtained has been determined. It has been shown that during the two-stage crystallization of the reaction mixture with a template/SiO2 molar ratio of 0.2, MEL zeolite is formed with crystals of 200–300 nm in size, whose surface is depleted in aluminum. It has been found that the nanocrystalline zeolite MEL exhibits high resistance to deactivation in the oligomerization reaction of butylenes due to a decrease in the number of acid sites on the outer surface of nanocrystals. No significant differences in the product distribution have been revealed for the oligomerization of butylenes on zeolites of the MEL and MFI structural types possessing identical physicochemical properties.

Catalytic application of ferrierite nanocrystals in vapour-phase dehydration of methanol to dimethyl ether

The FER-type zeolite was found to be a reliable catalyst for DME synthesis via both direct CO2 hydrogenation or gas phase methanol dehydration. In this work, the effect of the crystal size of FER-type zeolites on the catalytic behavior during methanol-to-DME conversion is studied. A series of FER-type crystals ranging from 0.1 μm to 10 μm in size were synthesized and their physicochemical properties (e.g. crystal morphology, acidity, textural characteristics) analyzed. The catalytic performance of FER-type catalysts was studied in a wide range of reaction temperatures (120–280 °C). The set experimental data show that the catalytic effectiveness of FER-type catalysts in terms of methanol conversion, DME selectivity and coke resistance, can strongly be improved by reducing the zeolite crystal size.

Synthesis of LTA zeolites with controlled crystal sizes by variation of synthetic parameters: Effect of Na+ concentration, aging time, and hydrothermal conditions

Comprehensive synthetic studies of crystalline microporous LTA zeolite with controlled crystal sizes were carried out in this work. The sol–gel-mediated crystallization of an aluminosilicate solution can be affected by changes in the synthetic parameters including the Na+ concentration, which depends on the tetramethylammonium hydroxide (TMAOH)/NaOH ratio, the aging time of the synthesis solution, and the hydrothermal conditions. Modifying these synthetic parameters was found to significantly influence the crystal size and uniformity of the LTA zeolites. Variation of the TMAOH/NaOH ratio had the strongest effect on the crystal size. The higher the TMAOH/NaOH ratio, the smaller the average crystal size produced. The average sizes of the LTA zeolite crystals could be tuned from less than 100 nm to 1 μm with a uniform distribution by simply varying the TMAOH/NaOH ratio. For a given gel composition, variation of the aging time (0 h–72 h) of the synthesis solution had a significant influence on the average size and uniformity of the LTA zeolite crystals. As the aging time was increased, the resultant LTA zeolite crystals became much smaller and exhibited higher uniformity. When the hydrothermal temperature and time were varied, longer hydrothermal treatment at higher temperature produced larger crystals. However, with sufficiently long aging, the change of hydrothermal condition had a much less significant effect on the crystal size. Although the comprehensive synthetic studies produced LTA zeolites with controlled crystal sizes, all the zeolites were stable, showing no structural collapse upon ion-exchange to various metal cations and thermal treatment up to 700 °C. The present synthetic studies and characterizations may provide important insight into the sol–gel-mediated control of crystal sizes of LTA zeolite.

Micropore diffusivities of NO and NH3 in Cu-ZSM-5 and their effect on NH3-SCR

Micropore diffusivities of NO and NH3 were experimentally measured for Cu-ZSM-5 using constant volumetric method at a temperature range from 373 K to 423 K. Intracrystalline diffusivities and effective diffusivities of NO were lower than those of NH3, and the activation energy for intracrystalline diffusivity of NO (21 kJ mol−1) was higher than that of NH3 (6.3 kJ mol−1). The intracrystalline diffusivity of NO in Cu-ZSM-5 catalysts at 373 K was independent of Cu content and Si/Al ratio of Cu-ZSM-5. We proposed an equation for calculation of effective diffusivity of NO in Cu-ZSM-5 which was validated by the experimental data. Kinetic studies for NH3-SCR and NH3 oxidation were carried out using Cu-ZSM-5 with different crystal thickness. The apparent reaction rate of NH3-SCR at 523 K depended on the zeolite crystal thickness; the rates by a small Cu-ZSM-5 (0.088 μm) was 2.75 times higher than that by a large crystal catalyst (2.7 μm), corresponding to the effectiveness factor of 0.36. The apparent reaction rate of NH3 oxidation did not depend on the zeolite crystal size. It is concluded that NH3-SCR is controlled by intracrystalline diffusion of NO, or in other words, the active sites in zeolite crystal are not fully utilized due to the NO diffusion limitation. The effective diffusivity of NO in Cu-ZSM-5 estimated by the catalytic study was close to that calculated using the NO diffusivity results, which supported our conclusion.

Properties Modification of Nanosized Hollow Zeolite Crystals by Desilication

AbstractA strategy to modify the properties of hollow zeolite was developed by leaching the “parent” zeolite in alkaline solution. A systematic study was performed to identify the effect of both the synthesis conditions of “parent” zeolite and the leaching conditions on the properties of hollow zeolite. Fine‐tuning the leaching conditions, i. e. the concentration of sodium hydroxide solution, allows control over the degree of hollowness of the zeolite, achieving the introduction of larger voids in different locations of individual crystals, which is a key factor if the crystals are to be used for catalytic applications. In addition, a hypothesis was proposed to explain the formation of hollow ZSM‐5 crystals. By replacing part of tetrapropylammonium hydroxide (TPAOH) with tetrapropylammonium bromide (TPABr) as the structure‐directing agent (SDA), the “parent” zeolites obtained were less stable in alkaline solution and formed hollow structures more easily. The size of hollow zeolite crystals could be easily controlled by adjusting the water content of the synthesis gel. Finally, crystals with double shells were prepared by a method involving two consecutive crystallizations followed by base leaching. Overall, fine‐tuning of zeolite synthesis and post‐synthesis conditions leads to control of the structure of hollow zeolite crystals.

Template-Free Synthesis of Hierarchical SSZ-13 Microspheres with High MTO Catalytic Activity.

Hierarchical SSZ-13 microspheres, with abundant mesoporous channels, have been successfully synthesized by addition of calcined SSZ-13 crystals in the starting gel in the absence of any organic templates. This strategy not only remarkably reduces the production cost and environmental pollution, but also significantly decreases the crystallization time as well as evidently reducing the zeolite crystal sizes in many cases. The synthesized sample was characterized by XRD, BET, ICP, SEM, TEM, TG, and NH3 -TPD, which indicates that the organic template free synthesized SSZ-13 exhibits high crystallinity, textural properties, and acidity. Moreover, the synthesized hierarchical SSZ-13 catalyst shows outstanding performance in the methanol-to-olefin (MTO) reaction, with slightly higher ethylene plus propylene selectivity and prolonged catalyst lifetime as compared to the templated SSZ-13 catalyst.

Rapid screening of the antimicrobial efficacy of Ag zeolites

A semi-quantitative screening method was used to compare the killing efficacy of Ag zeolites against bacteria and yeast as a function of the zeolite type, crystal size and concentration. The method, which substantially reduced labor, consumables and waste and provided an excellent preliminary screen, was further validated by quantitative plate count experiments. Two pairs of zeolite X and zeolite beta with different sizes (ca. 200nm and 2μm for zeolite X and ca. 250 and 500nm for zeolite beta) were tested against Escherichia coli (E. coli) and Candida albicans (C. albicans) at concentrations in the range 0.05–0.5mgml−1. Reduction of the zeolite crystal size resulted in a decrease in the killing efficacy against both microorganisms. The semi-quantitative tests allowed convenient optimization of the zeolite concentrations to achieve targeted killing times. Zeolite beta samples showed higher activity compared to zeolite X despite their lower Ag content, which was attributed to the higher concentration of silver released from zeolite beta samples. Cytotoxicity measurements using peripheral blood mononuclear cells (PBMCs) indicated that Ag zeolite X was more toxic than Ag zeolite beta. However, the trends for the dependence of cytotoxicity on zeolite crystal size at different zeolite concentrations were different for the two zeolites and no general conclusions about zeolite cytotoxicity could be drawn from these experiments. This result indicates a complex relationship, requiring the necessity for individual cytotoxicity measurements for all antimicrobial applications based on the use of zeolites.

Synthesis and characterization of tin, tin/aluminum, and tin/boron containing MFI zeolites

The synthesis and characterization of tin, tin/aluminum, and tin/boron containing MFI zeolites are reported. It is shown that tin uptake is high (near 100%) but decreases with the introduction of a second heteroatom. Additionally, the zeolite crystal size decreases and morphology changes with the introduction of a second heteroatom as evidenced by FE-SEM and XRD. UV-Vis confirms the tetrahedral framework coordination of tin within the zeolite and the absence of tin oxide clusters. Studies of methanol adsorption using in-situ IR spectroscopy indicate that tin-MFI sample with low tin contents possess lower acidity, and that tin/aluminum samples confirm aluminum within the zeolite through the presence of a Brönsted acid site. Comparison of spectra of adsorbed d-acetonitrile on the surface of these catalysts with the silicalite-1 is consistent with the tin centers in the framework possessing Lewis acidity.

Understanding the effect of zeolite crystal expansion/contraction on separation performance of NaA zeolite membrane: A combined experimental and molecular simulation study

Experimental measurements and molecular simulations were used to investigate the effect of adsorption-induced changes in zeolite crystal size on the separation performance of NaA zeolite membrane for dehydration of alcohols. The vapor permeation (VP) separations of water/IPA showed a dramatic decrease in selectivity due to increase of IPA flux as the feed water concentration decreased. However, in the case of water/methanol and water/ethanol mixtures, the alcohol fluxes were almost independent of the feed water concentration. Permporosimetry measurements as well as molecular simulations show that at low loading of water, NaA crystals contract slightly, while they expand at higher loadings, which significantly affects the intercrystalline defects. In addition, at low water loadings intercrystalline defects are not blocked by water adsorbed at the defect sites. Both methanol and ethanol can enter the zeolite to reduce the crystal contraction. However, isopropanol cannot enter the NaA crystal and is thus unable to mitigate the effects of low water loadings. Based on this knowledge, the presence of methanol or ethanol in the water/isopropanol mixtures with low water content was expected to improve the dehydration performance of NaA zeolite membrane. This result was also observed for the dehydration of water/other large molecular mixture. Our studies here provide an improved understanding of the permeation and separations for NaA zeolite membrane.

Synthesis of zeolite/carbon nanotube composite for gas separation

A hybrid composite of NaY zeolite and amine modified multi-walled carbon nanotube (MWCNT) has been synthesized by hydrothermal method. The obtained NaY/CNT composite (NC composite) was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and BET analysis. CO2, CH4 and N2 adsorption at two different temperatures and P < 5 bar on the composite was investigated by the volumetric method. The selectivity of the NC composite for CO2/CH4 and CO2/N2 has been studied and compared with pure NaY zeolite. Crystal structures of NC composite were similar to those of pure NaY zeolite, but the surface area and pore volume of the NC composite are enhanced. Incorporation of MWCNTs into NaY zeolite increases nucleation sites for the formation of NaY zeolite crystals, resulting in the smaller size of NaY zeolite crystals. Gas adsorption capacity and selectivity of NC composite increased because of enhancement of micropore volume. The results confirm that NC composite is a promising material for the separation and purification of gases.

Synthesis and Characterization of Nd-Doped ZSM-5 Zeolite from Diatom Mud

ZSM-5 zeolite was synthesized using diatom mud as the raw material, and then doped with different amounts of Nd2O3. Nd/ ZSM-5 zeolite was synthesized at °C for 24 h after stirring at high speed and aging in the synthesis mixture with the molar composition of 1.5 Nd2O3:25Na2O:150SiO2:1Al2O3:10TPABr:2500H2O.XRD confirmed Nd/ZSM-5 crystal phase. TEM showed that Nd/ZSM-5 zeolite crystal sizes are about 2-4μm. EDS shows Nd/ ZSM-5 zeolites chemical element compound. FT-IR analysis further showed that they are pentacyclic with typical absorption summit of ZSM-5 at about 547 cm-1.

Kinetic studies on high-pressure methylation of 2-methylnaphthalene over MTW-type zeolite with different crystal sizes

Methylation of 2-methylnaphthalene (2-MN) under high-pressure reaction conditions over MTW-type zeolites (Si/Al=100) was carried out using a batch-type reactor. Compared with macro-sized MTW-type zeolites, nano-sized MTW-type zeolites exhibited higher 2-MN conversion and dimethylnaphthalene (DMN) selectivity. In addition, coke formation on the catalyst could be suppressed due to improvement of the produced DMN solubility by increasing the bulk phase pressure, which enabled kinetic analysis of 2-MN methylation over MTW-type zeolites. Kinetics analysis of 2-MN methylation over MTW-type zeolites with different crystal sizes was successfully carried out, and reaction rate constants and activation energies for methylation and isomerization of 2-MN over MTW-type zeolites were obtained. The apparent reaction rate constants of 2-MN methylation were improved by decreasing MTW-type zeolite crystal size due to decrease in diffusion resistance. Moreover, the effectiveness factors for methylation of 2-MN were estimated based on the rate constants of 2-MN methylation and crystal sizes of the MTW-type zeolites. As a result, it was revealed that the crystal size of MTW-type zeolite (Si/Al=100) required for methylation of 2-MN to achieve the reaction-limiting condition was below 240nm.

Downsizing AFX Zeolite Crystals to Nanoscale by a Postmilling Recrystallization Method

Downsizing zeolites to obtain nanosized crystals for circumventing problems regarding molecular diffusion is a critical method for enhancing their functionalities, and therefore, such an approach has been the focus of significant attention. Here, we applied the postmilling recrystallization method for reducing the AFX aluminosilicate zeolite crystal sizes to the nanoscale. Highly crystalline AFX zeolite particles with crystal sizes smaller than 100 nm were obtained by the postmilling recrystallization method using the supernatant solution acquired during the preparation of the ingredient raw crystals used for the milling. The prepared AFX zeolite nanocrystals were found to possess better ion-exchange performance than micrometer-sized counterparts, highlighting the improved accessibility of the cations inside the micropores.

GIS-NaP1 zeolite microspheres as potential water adsorption material: Influence of initial silica concentration on adsorptive and physical/topological properties.

GIS-NaP1 zeolite samples were synthesized using seven different Si/Al ratios (5–11) of the hydrothermal reaction mixtures having chemical composition Al2O3:xSiO2:14Na2O:840H2O to study the impact of Si/Al molar ratio on the water vapour adsorption potential, phase purity, morphology and crystal size of as-synthesized GIS-NaP1 zeolite crystals. The X-ray diffraction (XRD) observations reveal that Si/Al ratio does not affect the phase purity of GIS-NaP1 zeolite samples as high purity GIS-NaP1 zeolite crystals were obtained from all Si/Al ratios. Contrary, Si/Al ratios have remarkable effect on the morphology, crystal size and porosity of GIS-NaP1 zeolite microspheres. Transmission electron microscopy (TEM) evaluations of individual GIS-NaP1 zeolite microsphere demonstrate the characteristic changes in the packaging/arrangement, shape and size of primary nano crystallites. Textural characterisation using water vapour adsorption/desorption, and nitrogen adsorption/desorption data of as-synthesized GIS-NaP1 zeolite predicts the existence of mix-pores i.e., microporous as well as mesoporous character. High water storage capacity 1727.5 cm3 g−1 (138.9 wt.%) has been found for as-synthesized GIS-NaP1 zeolite microsphere samples during water vapour adsorption studies. Further, the total water adsorption capacity values for P6 (1299.4 mg g−1) and P7 (1388.8 mg g−1) samples reveal that these two particular samples can absorb even more water than their own weights.

Synthesis of hierarchical zeolite Beta with low organic template content via the steam-assisted conversion method

We report the synthesis of hierarchical zeolite Beta with low organic template (tetraethylammonium hydroxide, TEAOH) content by the steam-assisted conversion (SAC) method. Neither zeolite seeds nor the secondary mesoporogen were required. Only a bit amount of organic template (denoted as TEAOH/SiO2 ratio=0.06) was used to obtain highly crystalline zeolite Beta, which was remarkably lower than the reported lowest value (0.10). Physicochemical properties of the resulting samples were investigated by comprehensive characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, solid state 27Al MAS NMR spectra, nitrogen adsorption–desorption, X-ray fluorescence, temperature-programmed desorption of ammonia and infrared spectra of pyridine adsorption. These characterization results showed that the SAC method led to hierarchical crystalline zeolite Beta aggregates assembled by 20–40nm nanosized crystals with controllable SiO2/Al2O3 ratios among 29–47 by varying the compositions of synthetic gels with the Na2O/SiO2 ratio of 0.12 and the H2O/dry gelratios at 0.20–0.30. These zeolite Beta aggregates possessed large external surface area (158–180m2/g) and mesoporous volume (0.27–0.30cm3/g) resulted from the formation of intercrystalline mesopores and of the small zeolite crystal sizes. The obtained hierarchical zeolite Beta showed 41.2% conversion in the esterification reaction of levulinic acid with ethanol and four catalytic runs without obvious decrease in conversion, which were better than those on the zeolite Beta samples from the conventional hydrothermal synthesis. This was due to the promotion of the accessibility of reactants to the acid sites distributed on the external surface of zeolite nanocrystals and of the prominent diffusion in intercrystalline mesopores.

The assessment of pore connectivity in hierarchical zeolites using positron annihilation lifetime spectroscopy: instrumental and morphological aspects

Recent studies demonstrated the power of positron annihilation lifetime spectroscopy (PALS) to characterise the connectivity and corresponding effectiveness of hierarchical pore networks in zeolites. This was based on the fractional escape of ortho-positronium (Ps), formed within the micropore framework, to vacuum. To further develop this technique, here we assess the impact of the positron implantation energy and of the zeolite crystal size and the particle morphology. Conventional measurements using fast positrons and beam measurements applying moderated positrons both readily distinguish purely microporous ZSM-5 zeolites comprised of single crystals or crystal aggregates. Unlike beam measurements, however, conventional measurements fail to discriminate model hierarchical zeolites with open or constricted mesopore architectures. Several steps are taken to rationalise these observations. The dominant contribution of Ps diffusion to the PALS response is confirmed by capping the external surface of the zeolite crystals with tetraethylorthosilicate, which greatly enhances the sensitivity to the micropore network. A one-dimensional model is constructed to predict the out-diffusion of Ps from a zeolite crystal, which is validated experimentally by comparing coffin-shaped single crystals of varying size. Calculation of the trends expected on the application of fast or moderated positrons indicates that the distinctions in the initial distribution of Ps at the crystal level cannot explain the limited sensitivity of the former to the mesopore architecture. Instead, we propose that the greater penetration of fast positrons within the sample increases the probability of Ps re-entry from intercrystalline voids into mesopores connected with the external surface of zeolite crystals, thereby reducing their fractional escape.