Zeolite Crystal Size Research Articles

Deciphering the Link between Zeolite Crystal Size, Brønsted Acid Site Distribution, and Dual-Cycle Selectivity in Methanol-to-Olefins over Zeolite

Deciphering the Link between Zeolite Crystal Size, Brønsted Acid Site Distribution, and Dual-Cycle Selectivity in Methanol-to-Olefins over Zeolite

TEAOH-assisted cost-effective preparation of SSZ-13 using L zeolite as Al source

Developing an economical and high-efficiency synthesis pathway of SSZ-13 zeolite is of great significance for the ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) reactions. Herein, SSZ-13 zeolite were synthesized using L zeolite as the aluminum source in the presence of a mixed structure directing agent (SDA) composed of N,N,N, trimethtyl-1-adamantammonium hydroxide (TMAdaOH) and tetraethylammonium hydroxide (TEAOH). This synthesis strategy aims to accelerate the crystallization rate through interzeolite transformation, while less expensive TEAOH can play a cooperative structure-directing role, thus reducing the consumption of expensive TMAdaOH template. The results showed that pure SSZ-13 zeolites with high crystallinity can be obtained from L zeolite within 12 h at 160 ℃ under low TMAdaOH/SiO2 ratio of 0.05 (too low to carry out the synthesis on its own in the current synthesis system) in the presence of TEAOH. Moreover, the crystallization mechanism was also investigated by XRD, FT-IR, SEM and TG-DTG techniques. It was proposed that the crystallization of SSZ-13 occurs through the formation of SSZ-13 nuclei under the direction of TMAdaOH and promotion of TEAOH, subsequently TMAdaOH was incorporated in the framework to further induce the crystal growth of SSZ-13. Especially, the zeolite crystal size can be modulated by the amount of TEAOH added. In addition, After Cu2+ exchange, the obtained Cu-SSZ-13 sample exhibits superior catalysis performance in the NH3-SCR of NOx reaction, and the NOx conversion rate remained over 90 % across a broad range from 200 to 600 °C, demonstrating its significant application potentials in NOx removal from diesel vehicle exhausts.

Nanoscale Features of Zeolite Fillers Enhance the Depth of Cure, Optical Properties, and Mechanical Properties of 3D-Printed Polymer Composite Structures

3D printing provides the possibility for the preparation of structured zeolite nanomaterial monoliths, which can overcome the drawback of limited forms obtained from conventional shaping methods and make more efficient use of this material. Compared to other techniques, photopolymerization presents many advantages such as being environmentally friendly, having low energy consumption, and having excellent spatial and temporal controls. Nevertheless, a photocurable system with fillers still faces challenge for the light penetration issue. In this paper, the effect of different characteristics of LTA zeolite (crystal size and molar Si/Al ratio) are discussed. The results demonstrate that compared to microfillers, nanosized particles lead to better depth of cure (DOC), optical properties, and mechanical properties for the composites. Furthermore, the Si/Al ratio of LTA zeolites under study (1 ≤ Si/Al ≤ 2.8) also has an impact on the DOC, optical property of the composites, and the photopolymerization conversion. 3D printed zeolite-embedded monoliths were tested for water vapor sorption in nanopores (60 °C, relative humidity = 50%). This work enables a better understanding toward the fabrication of zeolite nanomaterials/polymer based composites by photopolymerization. It will expand the potential applications for 3D printing and zeolite shaping for applications in several domains such as decontamination and catalysis and in the biomedical field.

Catalytic cracking of heavy atmospheric gas oil to light olefins over ZSM-5 zeolite: Effect of crystal size in the absence/presence of steam

The catalytic cracking of heavy atmospheric gas oil (AGO) has been investigated to study the effect of ZSM-5 zeolite crystal size on the selectivity to light olefin in the absence and presence of steam. Dodecane (used as model compound) and heavy AGO feed streams were utilized to study the influence of changing pore shape selectivity when comparing cracking performance by nano and micro ZSM-5 zeolites. The zeolites were characterized using BET, XRD NMR, NH3-TPD and Py-FTIR. In the presence of steam, the short pores length nano ZSM-5 crystals showed enhanced catalytic cracking activity and selectivity to light olefins. The nano ZSM-5 catalyst promoted higher selectivity to light olefins through β-scission while reducing the extent of hydrogenation reactions towards the formation of liquefied petroleum gas components (LPG). In contrast, the long pores length of micro ZSM-5 tended to favor the production of more naphtha, kerosene and LPG through hydrogen transfer routes, resulting in lower selectivity to light olefins. The resulting propylene/ethylene (P/E) ratios were altered by the type of feed used, cracking reaction temperature, influence of steam and catalyst crystal size. Specifically, higher P/E ratio was achieved over nano ZSM-5 zeolite, especially with the cracking of heavy AGO feed in the presence of steam, due to an increase of monomolecular reactions through β-scission cracking mechanism.

Synthesis of nanosized IM-5 zeolite and its CH4/N2 adsorption and separation

Reduction of zeolite crystal size from the micron scale to the nanoscale leads to a significant increase in specific surface area, favoring mass-transfer pathways. Here, we have prepared nanoscale (ca. 30–80 nm) IM-5@100 zeolite (IMF type) by a facile strategy of regulating the amount of Al source in the starting gel, and have investigated its CH4/N2 adsorption and separation properties. We found that on decreasing the amount of Al source, the crystal size of the product changed significantly, decreasing from strips of length about 1 μm to irregular nanocrystals. Compared with micron-sized IM-5@50, the mesopore volume of IM-5@100 was increased fourfold from 0.12 cm3/g to 0.49 cm3/g, its external surface area was increased from 38 m2/g to 118 m2/g, and its DFT pore size distribution based on N2 adsorption/desorption showed a mesopore distribution in the range 5–10 nm. Adsorption kinetics tests showed that nano-IM-5@100 can achieve adsorption equilibrium more swiftly and exhibits better mass-transfer performance. Moreover, IM-5@100 showed good adsorption capacity for CH4 (17.8 cm3/g) and high IAST selectivity (4.7) for CH4/N2 (50:50, v/v). Breakthrough experiments have indicated that IM-5@100 permits efficient practical separation of CH4/N2.

Solvent-free synthesis of zeolites: New insights into the mechanism and non-mechanochemical route

The solvent-free (SF) synthesis is a green alternative to the conventional method of synthesis for zeolites. In this study, we show for the first time the feasibility of carrying out the SF synthesis of silicalite-1 (F− route) without any grinding. We also monitor the changes during the synthesis thanks to multiple characterization techniques: XRD, TGA, SEM, EDX, and multinuclear NMR (13C, 19F, 29Si). We show how the duration of the thermal treatment impacts the zeolite crystal size and morphology, as well as the local degree of order probed by solid-state NMR. The obtained results provide new insights on the SF formation mechanism based on solid phase transformations. Our findings also show that once the organic structure directing agent (OSDA), here tetrapropylammonium, is incorporated, the crystals are formed and no other intermediate phases incorporating the OSDA exist. Further, we study the role of water in the synthesis using 17O NMR. For this purpose, we develop a simple and efficient 17O enrichment method of zeolite frameworks. The efficiency of this enrichment demonstrates the role of water in the hydrolysis and condensation reactions and its need for a successful synthesis.

Defect Sites in Zeolites: Origin and Healing.

This paper deals with the synthesis conditions–defect formation relationship in zeolites. Silicalite‐1 (MFI‐type) is used as a model material. Samples synthesized from a system with high basicity (at 100 °C), a system with moderate basicity (at 150 °C), and a fluoride‐containing system in neutral medium (at 170 °C) are compared. Well‐crystallized materials with sizes ≈0.1, 1–10, and 30–40 µm are obtained. The samples are analyzed by complementary methods providing information on the short‐ and long‐range order in the zeolite framework. A strong correlation between the number of point defects in the zeolite framework and preparation conditions is established. Silicalite‐1 synthesized under mild synthesis conditions from a highly basic system exhibits a larger number of framework defects and thus low hydrophobicity. Further, the calcined samples are subjected to aluminum and silicon incorporation by postsynthesis treatment. The Al/Si incorporation in the zeolite framework and its impact on the physicochemical properties is studied by XRD, TEM/SEM, solid‐state NMR, FTIR, and thermogravimetric analyses. The defects healing as a function of the number of point defects in the initial material and zeolite crystal size is evaluated. The results of this study will serve for fine‐tuning zeolite properties by in situ and postsynthesis methods.

Zeolite synthesis by the excess fluoride approach in the presence of piperidinium-based structure-directing agents

Here, we investigate the effects of fluoride concentration on the phase selectivity of crystallization in (alumino)silicate synthesis mixtures containing a series of N -alkyl- N -methylpiperidinium ions, where alkyl is methyl to hexyl, as organic structure-directing agents (OSDAs). Considerable diversity in the phase selectivity was observed when the HF/OSDA ratio in synthesis mixtures is 2.0 or higher. Using this excess fluoride approach, we synthesized five different aluminosilicate zeolites (i.e., EMM-10P, EU-1, MCM-22P, ZSM-12, and ZSM-58) that did not crystallize under the normal F − concentration conditions (HF/OSDA = 1.0). Our synthesis results also reveal that the same zeolite phase (i.e., levyne, nonasil, octadecasil, or ZSM-5) can crystallize over a wide range (0.5–2.0) of HF/OSDA ratios. More interestingly, F − concentration in the synthesis mixture was found to influence the crystal size and framework Al distribution of levyne. • Zeolite syntheses using piperidinium-based OSDAs in fluoride media are performed. • Notable changes in the zeolite product are observed at HF/OSDA ratios ≥ 2.0. • Zeolite crystal size varies with the F − concentration in synthesis mixtures. • F − concentration in the synthesis mixture is a zeolite framework Al distribution factor.

Modelling of an Adsorption Heat Storage System and Study of Operating and Design Conditions

An open system based on physical adsorption phenomena with humid air and zeolite 13X is herein discussed for residential heat storage purposes. A model has been developed to describe the conservation of mass and heat in the system. A simplified approach of a complete model describing both mass conservation in the macroporous and microporous domains is used based on the linear driving force (LDF) model. Local mass and heat transfer properties have been used. To describe the equilibrium, the Aranovich–Donohue isotherm model is selected. As an example, the developed model is compared and fitted to experimental data from a pilot scale system. A parametric study on operating and design parameters is given to understand their effect on the amount and/or duration of heat supply, concentration, and temperature profiles. The studied parameters are the inlet adsorbate concentration, fluid temperature, and velocity, as well as particle and zeolite crystal sizes. This analysis shows that an identification of values for the set of parameters tested can possibly suit the energy needs for a case study of domestic heat supply. Future work will focus on the optimization of these parameters.

Methane-selective oxidation to methanol and ammonia selective catalytic reduction of NOx over monolithic Cu/SSZ-13 catalysts: Are hydrothermal stability and active sites same?

This study investigated and compared the reactivities of NH3-SCR and methane-to-methanol reactions over the monolithic Cu/SSZ-13 catalysts with different zeolite crystal sizes before and after hydrothermal aging. It was found that the methanol yield decreases with the increase in crystal size in methane-to-methanol, and a severe deactivation was detected after aging. However, the NH3-SCR performance presents a higher thermal stability and even an enhanced low-temperature reactivity after aging. The fresh and aged catalysts were comprehensively characterized by XRD, SEM, N2 physisorption, NH3-TPD, H2-TPR, XPS, UV–vis-NIR and in-situ DRIFTS measurements. The results indicate that the isolated Cu2+ species, including those that interacted with two Al in 6MR and [CuII(OH)]+ motifs, and acid sites are main active centers for NH3-SCR. While CuxOy clusters, [CuII(OH)]+ motifs and Brønsted acid sites in the Cu-CHA frameworks could be important active sites for methane-to-methanol. Besides, the stable SCR performance is ascribed to the stable textural property, higher crystallinity of zeolite, increased NH3 storage capacity and isolated Cu2+ active sites. The loss of the catalytic activity in methane-to-methanol is mainly attributed to the decrease of Brønsted acid sites and CuxOy concentration and the break of zeolite crystal structure. This work concludes the differences in active sites and hydrothermal stability between NH3-SCR and methane-to-methanol reactions, it is very helpful to understand the two reactions.

Implementation of hierarchically porous zeolite-polymer membrane for Chromium ions removal

This work deals with the preparation of a zeolite/polymer flat sheet membrane with hierarchical porosity and ion-exchange properties. The performance of the prepared membrane was examined by the removal of chromium ions from simulated wastewater. A NaY zeolite (crystal size of 745.8 nm) was prepared by conventional hydrothermal treatment and fabricated with polyethersulfone (15% PES) in dimethylformamide (DMF) to obtain an ion-exchange ultrafiltration membrane. The permeate flux was enhanced by increasing the zeolite content within the membrane texture indicating increasing the hydrophilicity of the prepared membranes and constructing a hierarchically porous system. A membrane containing 0.8% zeolite gave the higher %Rejection of chromium ions (about 83.2%) for an initial chromium ion concentration of 150 ppm and trans-membrane pressure of 1 bar.

Investigation of Small‐Pore Zeolites via Vapor‐Phase Adsorption of n‐Hexane

AbstractCage‐based small‐pore zeolites are promising catalysts for catalytic applications such as methanol‐to‐olefins to produce light olefins and selective catalytic reduction to mitigate NOx emissions. The present paper reports our recent studies on vapor‐phase adsorption of n‐hexane in a series of small‐pore zeolites with cages such as 3A, 4A, 5A, ITQ‐29, SSZ‐13, SSZ‐16, SSZ‐17, SSZ‐36, SSZ‐50, ITQ‐12, ZK‐5, and Rho. The effects of zeolite compositions and crystal sizes on the adsorption kinetics and capacity are investigated, demonstrating that n‐hexane adsorption is a useful tool for characterizing the effective pore sizes and volumes of small pore zeolites.

Sustainable ammonium recovery from wastewater: Improved synthesis and performance of zeolite N made from kaolin

Abstract Although zeolite N ( K 12 A l 10 S i 10 O 40 C l 2 . 8 H 2 O ) is used commercially for the removal and recovery of ammoniacal nitrogen from wastewater, it has received limited attention in academic literature. The hypothesis was that if an improved understanding of zeolite N synthesis and resultant physical parameters and performance can be acquired, then the application of this material may be accelerated. This study successfully addressed existing research challenges including: maximizing the cation exchange capacity (CEC); selection of preferred synthesis conditions; control of the size, shape, and dispersion of zeolite N crystals; and improvement of ion exchange properties. Zeolite N was hydrothermally synthesised from kaolin clay added to solutions of potassium hydroxide and potassium chloride in a “one pot” process. CEC values increased to 563 meq/100 g (c.f. 528 meq/100 g in previous literature). Reaction between 120 and 140 °C for 2–4 h under static conditions was preferred as higher temperatures (180 °C) produced kalsilite. The KOH molarity (1.2–4.3 M) controlled the average zeolite crystal size, dispersion, and crystal shape; with highest molarity values optimal. Whereas, added KCl acted as a template which promoted zeolite N formation. Removal of KCl resulted in creation of zeolite F ( K 13 ( O H ) 3 A l 10 S i 10 O 40 . 13 H 2 O ) which also belonged to the EDI structural type. Ammonium ion exchange from aqueous solution was improved when diffusion restrictions were eased; as evidenced by using smaller zeolite crystals which were highly dispersed and not agglomerated. Not only was a greater cation capacity recorded but also the time to reach equilibrium was decreased.

Effect of Mesoporosity, Acidity and Crystal Size of Zeolite ZSM‐5 on Catalytic Performance during the Ex‐situ Catalytic Fast Pyrolysis of Biomass

AbstractThe catalytic performance of a set of technical, zeolite ZSM‐5 extruded materials have been studied in a bench scale unit for the ex‐situ Catalytic Fast Pyrolysis (CFP) of lignocellulosic biomass. The set of catalysts include micro‐ and mesoporous materials, with and without ZrO2‐promotion, with different Si/Al ratios and with micro‐ and nanosized zeolite crystals. Mesoporosity, acidity and crystal size play a key role on the overall catalytic performances in terms of activity and selectivity (i. e., in their ability to obtain the highest bio‐oil fraction with the lowest oxygen content), and also importantly, stability. Detailed post‐mortem bulk and micro‐spectroscopic studies of the solid catalysts complement the catalytic testing. The obtained results point towards coke deposits as the main cause for catalyst deactivation. Details of the nature, formation and evolution of these coke deposits revealed essential insights, which serve to evaluate the design of catalysts for the ex‐situ CFP of biomass. In particular the mesoporous catalysts are overall better preserved with increasing time‐on‐stream and deactivate later than their microporous counterparts, which suffer from pore blockage. Likewise, it was seen that ZrO2‐promotion contributed positively in the prevention against deactivation by hard coke spreading, thanks to its enhanced Lewis acidity. The zeolite's crystal size is another important characteristic to combine the different components within catalyst bodies, ensuring the proper interaction between zeolite, promoter and binder. This is illustrated by the nanocrystalline ZrO2/n‐ZSM‐5‐ATP material, which shows the best catalytic performance.

Designer Synthesis of Ultra-Fine Fe-LTL Zeolite Nanocrystals

Nanosized zeolites with larger external surface area and decreased diffusion pathway provide many potential opportunities in adsorption, diffusion, and catalytic applications. Herein, we report a designer synthesis of ultra-fine Fe-LTL zeolite nanocrystals under very mild synthesis conditions. We prepared Fe-LTL zeolite nanocrystals synthesized using L precursor. The precursor is aging at room temperature to obtain zeolite L nuclei. In order to investigate more details of Fe-LTL zeolite nanocrystals, various characterizations including X-ray diffraction (XRD), inductively coupled plasma (ICP), diffuse reflectance ultraviolet-visible (UV-Vis) spectroscopy, confirm the tetrahedral Fe3+ species in the zeolite framework. Besides, scanning electron microscope (SEM), Fourier transform infrared spectrometer (FT-IR), dynamic light scattering (DLS) indicate that the average particle size of Fe-LTL zeolite crystals is approximately 30 nm. Thus, ultra-fine Fe-LTL zeolite with large external surface area and shorter diffusion pathway to the active sites might have great potential in the near future.

Understanding HZSM-5's shape-selectivity improvement through tiny autonomous dealumination

Abstract The decisive factor that determines HZSM-5's shape-selective capability is obscure due to the interference from zeolite acidity, pore size and crystal size. p-xylene selectivity was found to increase from 45% to 82% in toluene alkylation reaction with the same HZSM-5 catalyst but storage period difference. However, no instrument detectable zeolite acid sites strength, type and quantity changes were observed. NMR revealed slight progressive increase of extra-framework aluminum by dealumination, and nitrogen adsorption-desorption isotherm verified the tiny decrease of micropore volume. Pulse chromatography tests confirmed p-xylene diffusion resistance increase in HZSM-5's channels. The finding made it possible to clarify the interactions between shape-selectivity improvement, p-xylene diffusion resistance increase and extra-framework aluminum while excluding all the other interfering factors, indicating the diffusion resistance control of molecules in HZSM-5's channels is the key to improve its shape-selectivity.

Zeolite crystal size effects of Au/uncalcined TS-1 bifunctional catalysts on direct propylene epoxidation with H2 and O2

Understanding the nature of Au-Ti bifunctional catalysis is important yet challenging for improving direct propylene epoxidation with H2 and O2 to propylene oxide (PO). Herein, we report a facile strategy to fabricate four different crystal sized TS-1 with the presence of highly active isolated Ti4+ species by adjusting the amount of TPAOH template. The resultant uncalcined zeolite samples, i.e., TS-1-B, are employed to load Au via deposition–precipitation with urea for obtaining similar sized Au immobilized on TS-1-B catalysts and thus excluding the size effects of Au active sites. The zeolite crystal size effects are further probed, and then decreasing the crystal size gives rise to the simultaneous promotion in PO formation rate and selectivity, as well as H2 efficiency. These are attributed to the Au-Ti electronic synergy based on UV–vis and XPS measurements. The insights reported here could benefit the development of high-efficiency Au-Ti bifunctional catalysts for this reaction.

Hydroisomerization of n-heptane on a new kind of bifunctional catalysts with palladium nanoparticles encapsulating inside zeolites

The bifunctional catalysts capable of regulating the metal dispersion and location through encapsulating palladium (Pd) nanoparticles inside Beta and mordenite (MOR) zeolites were used for n-heptane hydroisomerization. Several characterizations such as X-ray powder diffraction (XRD), scanning electron microscope (SEM), N2-adsorption and so on suggested that the difference in the phase, textural property, morphology, and acidity of zeolites between the catalysts with encapsulation structure and prepared by impregnation method was negligible whether Beta- or MOR-series catalysts. Nevertheless, the higher Pd dispersion, smaller Pd particle size, and higher aggregation resistance can be achieved on the encapsulation catalysts as determined by H2-pulse and transmission electron microscopy (TEM). Compared to the palladium-loading catalyst prepared by conventional impregnation method, the encapsulation structure guaranteed the higher selectivity and yield in n-heptane isomerization due to the significantly improved balance and intimacy. Furthermore, the catalyst with the complete encapsulation structure for the Beta-series catalysts and the catalyst with partial encapsulation structure for the MOR-series catalysts respectively exhibited the highest isomer selectivity and yield. All results may be resulted from that the zeolite topology and crystal size acted a significant role in determining the intimacy of the bifunctional catalysts with encapsulation structure, due to the different diffusion hurdles of the channels structure.

CO2 hydrogenation using bifunctional catalysts based on K-promoted iron oxide and zeolite: influence of the zeolite structure and crystal size

The influence of the zeolite structure and crystal size on bifunctional tandem catalysts combining K-promoted iron oxide (K/Fe3O4) with different zeolites has been studied for the CO2 hydrogenation reaction at 320 °C and 25 bar.

Enhancing the removal performance of Cd(Ⅱ) from aqueous solutions by NaA zeolite through doped thiourea reduced GO which is trapped within zeolite crystals

In this study, thiourea reduced graphene oxide/NaA zeolite composite (tGO-Zeo) was successfully prepared by a facile two-step synthetic route and used as an efficient adsorbent for removal of Cd(II) from aqueous solution. Characterization of the composite material showed that tGO was entrapped within zeolite crystalline particles and intimately interacted with the zeolite framework. Furthermore, the presence of tGO considerably reduced zeolite crystal size and altered crystal morphology, turning cubic zeolite crystals of micrometer size into nanometer irregular particles. Batch adsorption experiment for Cd(II) removal was conducted and the adsorption performance of tGO-Zeo and Zeo (zeolite) were determined. It was shown that doped tGO significantly enhanced the adsorption capacity of zeolite by 31.54%. The adsorption mechanism differs for tGO-Zeo and Zeo, and tGO-Zeo has higher affinity for Cd(II). The adsorption capacity of tGO-Zeo with a tGO doping amount of 1.26% (wt%) was estimated to be 196 mg/g (1.74 mM/g) at 30 °C and superior to most previously reported zeolite based composites, which often contain large amounts of dopants. The effects of pH and ionic strength on adsorption suggest that ion exchange and coordination are responsible for Cd(II) adsorption by tGO-Zeo. Humic acid can slightly increase Cd(II) adsorption performance of tGO-Zeo, which may be due to tGO-Zeo—humic acid—Cd(II) complexes formation. Therefore, tGO-Zeo composite is a cost-effective and eco-friendly adsorbent for efficient removal of Cd(II) ions from aqueous solutions.