Absence Of Organic Structure-directing Agent Research Articles

Synthesis of CHA zeolite in phenoxide media for CO2 capture

Synthesis of small-pore chabazite (CHA) zeolites in the absence of organic structure-directing agents (OSDA) has been proven difficult. A judicious mixed cation approach recently reported could expand the design space of CHA formation. In a seemingly opposite direction, we herein report an anion tuning approach for controlling the zeolite polymorphism in an OSDA-free system that effectively promotes hierarchical CHA formation in a competitive growth with MER. Phenoxide media, utilizing mild in-situ nucleophilic etching in cooperation with zeolite framework growth, allow beneficial kinetic control to facilitate CHA nucleation. Operando analyses reveal the crucial role of stabilized dimeric aluminate species as a key intermediate for CHA phase selection. The phenol-mediated CHA zeolites exhibit almost perfect 9 molecules per unit cell CO2 uptake capacity and no CH4 adsorption. This work offers an anion tuning strategy for controlling zeolite polymorphism as a supplement to the current phase-selection toolbox that utilizes mixed inorganic cations.

Highly efficient synthesis of zeolite chabazite using cooperative hydration-mismatched inorganic structure-directing agents.

Chabazite (CHA type) zeolite is notoriously difficult to synthesize in the absence of organic structure-directing agents owing to long synthesis times and/or impurity formation. The ability to tailor organic-free syntheses of zeolites is additionally challenging due to the lack of molecular level understanding of zeolite nucleation and growth pathways, particularly the role of inorganic cations. In this study, we reveal that zeolite CHA can be synthesized using six different combinations of inorganic cations, including the first reported seed- and organic-free synthesis without the presence of potassium. We show that lithium, when present in small quantities, is an effective accelerant of CHA crystallization; and that ion pairings can markedly reduce synthesis times and temperatures, while expanding the design space of zeolite CHA formation in comparison to conventional methods utilizing potassium as the sole structure-directing agent. Herein, we posit the effects of cation pairings on zeolite CHA crystallization are related to their hydrated ionic radii. We also emphasize the broader implications for considering the solvated structure and cooperative role of inorganic cations in zeolite synthesis within the context of the reported findings for chabazite.

Preparation of hydrophobic Ti-beta catalyst derived from Al-rich zeolite beta synthesized without using organic structure-directing agent and its enhanced catalytic performance for phenol oxidation

Crystalline microporous titanosilicates with multi-dimensional 12-ring channels are potentially useful as catalysts for the oxidation of phenol with H2O2. Among them, titanosilicate version of zeolite beta (Ti-beta), which has three-dimensional, 12-ring micropores, is one of the most promising catalytic materials. The Ti-beta catalysts are preferably with Al-free and Na-free chemical compositions, and they can be conventionally crystallized via direct hydrothermal synthesis with the aid of a dipiperidinium-type organic structure-directing agent (OSDA) or via the so-called fluoride method using tetraethylammonium in a fluoride-containing medium. Another route involves a post-synthesis treatment such as the isomorphous substitution of Ti for Al starting from Al-rich zeolite beta. One of the Al-rich beta was synthesized in the absence of OSDA. We succeeded in converting it to Ti-beta, denoted as (Ti)-beta_OSDAF, for the first time via deep dealumination along with framework stabilization followed by Ti-incorporation using TiCl4. The (Ti)-beta_OSDAF was much more effective catalyst for the phenol oxidation with H2O2 than the directly crystallized Ti-beta materials prepared in the conventional techniques. The high efficiency of phenol oxidation was ascribed to an appropriate balance between hydrophilicity and hydrophobicity realized by increasing hydrophobicity of titanosilicate catalysts. The bulk hydrophobicity reflects that of the vicinity of active Ti sites. The enhanced affinity of hydrophobic moiety of phenol molecules with the hydrophobic surface around the vicinity of active Ti sites should facilitate the diffusion of phenol molecules to the active sites inside the micropores. The hydrophobicity of Ti-beta samples prepared by the various techniques in this work differed substantially, depending on the amount of silanol groups within the zeolitic framework. Positive effect of EtOH as an additive in the reaction mixture on enhancing the shape-selectivity of the (Ti)-beta_OSDAF catalyst was obvious, probably due to the increase in steric restriction by the EtOH molecules inside the pores of the catalyst. Moreover, the phenylsilylation of (Ti)-beta_OSDAF catalyst drastically increased its hydrophobicity, which was found to be effective for enhancing para-selectivity and turnover number.

Calcination-Free Fabrication of Highly b-Oriented Silicalite-1 Zeolite Films by Secondary Growth in the Absence of Organic Structure-Directing Agents

Calcination-free fabrication of highly b-oriented silicalite-1 (S-1) zeolite films in the absence of organic structure-directing agents (OSDAs) is attractive but still challenging. Herein, for the first time, we show a novel route for calcination-free synthesis of a b-oriented S-1 zeolite film in the absence of OSDAs. The key to this success is a rational secondary growth of zeolite crystals on the surface of supports by avoidance of the formation of zeolite nuclei in the system due to the absence of OSDAs. Because of OSDA-free synthesis of S-1, it not only reduces the cost of zeolite films but also avoids the calcination of OSDAs in the synthesis, thus avoiding the formation of cracks in the zeolite films. In addition, the calcination-free fabrication is favorable for reduction of zeolite film defects such as a terminal OH group. As a result, the zeolite films obtained from this work exhibit higher hydrophobicity than the conventional one. Furthermore, the current polarization curve of this S-1 zeolite film shows excellent corrosion resistance. The aforementioned features would be potentially important for industrial application of zeolite films in the near future.

Alcohol-assisted synthesis of high-silica zeolites in the absence of organic structure-directing agents

In this work, we show for the first time that high-silica zeolites (MFI, TON, MTT, and *MRE) could be synthesized from a combined strategy of both zeolite seeding and alcohol filling in the absence of organic structure-directing agents (OSDAs). High-silica ZSM-5 zeolites with Si/Al ratios ranging from 38 to 240 (TF-Al-ZSM-5) could be synthesized via this route. The key to the success of this technique was the employment of an aluminosilicate precursor with a fully 4-coordinated aluminum species as the initial source, wherein the rearrangement and condensation of the silicate species, rather than the aluminate species, occurred during zeolite crystallization. In addition, heteroatoms, such as Fe and B, could be incorporated into the zeolite frameworks. Catalytic tests for the methanol-to-propylene (MTP) reaction exhibited good catalytic performance for TF-Al-ZSM-5, which was comparable to that of the aluminosilicate ZSM-5 zeolite synthesized with OSDAs. Hence, this method offers viable opportunities for the industrial production and catalytic application of high-silica zeolites in the future.

Sustainable Synthesis of Pure Silica Zeolites from a Combined Strategy of Zeolite Seeding and Alcohol Filling

AbstractCurrently, the synthesis of pure silica zeolites always requires the presence of organic structure‐directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.

Sustainable Synthesis of Pure Silica Zeolites from a Combined Strategy of Zeolite Seeding and Alcohol Filling.

Currently, the synthesis of pure silica zeolites always requires the presence of organic structure-directing agents (OSDAs), which direct the assembly pathway and ultimately fill the pore space. A sustainable route is now reported for synthesizing pure silica zeolites in the absence of OSDAs from a combined strategy of zeolite seeding and alcohol filling, where the zeolite seeds direct crystallization of zeolite crystals from amorphous silica, while the alcohol is served as pore filling in the zeolites. Very importantly, the alcohol could be fully washed out from zeolite pores by water at room temperature, which completely avoids calcination at high temperature for removal of OSDAs in the synthesis of pure silica zeolites.

Organotemplate-Free β Zeolites: From Zeolite Synthesis to Hierarchical Structure Creation.

Interest in the production of β zeolites in the absence of organic structure-directing agent (OSDA) has continued to grow consistently during the past decade. During this time, numerous strategies have been proposed to manipulate the hierarchy of zeolite pore structures in order to facilitate the transport of bulky reactants and to improve the accessibility of active sites in zeolite catalysts. In this work, we describe an organotemplate-free route to produce hierarchical β zeolites. Using OSDA-free β as the starting zeolites, we explored the applicability of various postsynthetic approaches to create hierarchical structures with mesoporosity, including framework stabilization, dealumination, conventional desilication, and hydrothermal desilication. While framework stabilization and dealumination were not effective in generating mesoporosity, they were necessary as modification steps to determine the efficacy of hierarchical structure creation. Compared to conventional desilication, hydrothermal desilication produces larger mesopores and much better preservation of microporosity and acidity because of the occurrence of recrystallization. The cost-effective, scalable production of organotemplate-free hierarchical β zeolites could greatly enhance the adoption of β zeolites in oil refining and petrochemical industries, where the advantages of hierarchically structured zeolites can dramatically improve catalytic performances in formulated catalysts.

OSDA-free synthesis of zeolite beta by magadiite hydrothermal conversion method and an insight into the changes of medium-range structure during crystallization

Abstract Zeolite beta has been successfully synthesized by magadiite conversion method in the absence of organic structure-directing agents. By using powder X-ray diffraction, fourier transform infrared spectroscopy and Raman spectroscopy, solid-state 27Al and 29Si MAS nuclear magnetic resonance spectroscopy and electron microscope, the crystallization behavior and changes of medium-range structure were investigated in detail. It was found that part of medium-range structure of magadiite remained during the crystallization process and part of seed crystals were dissolved, providing active surface for the formation of zeolite beta. Meanwhile, with the growth of zeolite beta, five-membered rings existing in magadiite were retained and their connection probably resulted in the formation of four-membered rings. An increasing number of Al atoms favourably entered the T1 and T2 positions which were mainly located in four-membered rings, while Si atoms resided in T3-T9 sites. The influence of reaction time, temperature, alkalinity in starting gel and the amount of seed crystals were also examined. With the reactive gel composition: 0.96 NaOH: SiO2: 0.0125 Al2O3: 240 H2O by adding 25 wt % seeds into the synthesis system, pure and highly crystalline zeolite beta could be obtained at 125 °C after 48 h.

Building Zeolites from Precrystallized Units: Nanoscale Architecture.

Since the early reports by Barrer in the 1940s on converting natural minerals into synthetic zeolites, the use of precrystallized zeolites as crucial inorganic directing agents to synthesize other crystalline zeolites with improved physicochemical properties has become a very important research field, allowing the design, particularly in recent years, of new industrial catalysts. This Review highlights how the presence of some crystalline fragments in the synthesis media, such as small secondary building units (SBUs) or layered substructures, not only favors the crystallization of other zeolites with similar SBUs or layers, but also permits control over important parameters affecting their catalytic activity (chemical composition, crystal size, or porosity, etc.). Recent advances in the preparation of 3D and 2D zeolites through seeding and zeolite-to-zeolite transformation processes will be discussed extensively in this Review, including their preparation in the presence or absence of organic structure-directing agents (OSDAs). The aim is to introduce general guidelines for more efficient approaches for target zeolites.

Solvent/OSDA-free transformation of unseeded aluminosilicate into various zeolites via mechanochemical and vapor treatments

Zeolites have been conventionally synthesized using organic structure directing agents (OSDA) in solvent. In this work, we synthesized various zeolites from unseeded aluminosilicate via mechanochemical and vapor treatments in the absence of OSDA and solvent. Various types of zeolites were successfully synthesized from starting materials with different Si/Al ratios. With an increase of Si/Al from 1 to 20, different phases appeared in the order of CAN, ANA, MOR, MFI and α-quartz. This work provides a new insight of zeolite synthesis.

Organic-Free, ZnO-Assisted Synthesis of Zeolite FAU with Tunable SiO2 /Al2 O3 Molar Ratio.

Zeolite FAU with tunable SiO2 /Al2 O3 molar ratio has been successfully synthesized in the absence of organic structure-directing agents (OSDA). Specifically, the addition of zinc species contributes to the feasible and effective adjustment of the framework SiO2 /Al2 O3 molar ratio between about 4 and 6 depending on the amount of zinc species added in the batch composition. In contrast, a typical OSDA such as tetramethylammonium hydroxide (TMAOH) has a limited effect on the SiO2 /Al2 O3 molar ratio of the zeolite. The role of zinc species is essential for the crystallization of zeolite FAU with a higher SiO2 /Al2 O3 molar ratio under the particular synthesis conditions. It is speculated that zinc species may suppress the incorporation of aluminum into the aluminosilicate framework, which is due to the Coulombic repulsive interaction. A higher SiO2 /Al2 O3 molar ratio is also found to be accompanied by a lower CO2 adsorption heat for CO2 /CH4 separation.

Seed-assisted, template-free synthesis of ZSM-5 zeolite from natural aluminosilicate minerals

Abstract The hydrothermal synthesis of zeolite from chemical Al/Si source in the presence of structure directing agent is considered to be a resource extensive, energy consuming, and environmental unfriendly process. Herein, we report a novel, cost-effective, and eco-friendly synthesis for the preparation of ZSM-5 zeolite from silicon-rich diatomite and aluminum-rich rectorite minerals in the absence of organic structure-directing agent and chemical Al/Si source. Synthesis factors (SiO2/Al2O3, Na2O/SiO2, and H2O/SiO2 molar ratios along with seed content) and crystallization behavior were systematically investigated, providing fundamental knowledges on the formation of ZSM-5 from natural minerals. Synthesis under the optimized conditions (SiO2/Al2O3 ratio of around 38, Na2O/SiO2 ratios of 0.12–0.15, H2O/SiO2 ratios of 15–25, and seed content above 0.5%) led to production of highly crystalline ZSM-5 zeolite with hexagonal prism morphology and large surface area. Various characterization techniques tracking the products at different crystallization times revealed that the crystallization process consisted of three stages: short induction stage (0–6 h), rapid growth stage (6–18 h), and stable growth stage (18–36 h). Moreover, the synthesized ZSM-5 crystals can be used as effective seed for producing the next generation of ZSM-5 zeolite.

Steam-dealuminated, OSDA-free RHO and KFI-type zeolites as catalysts for the methanol-to-olefins reaction

RHO and KFI-type zeolites are synthesized in the absence of organic structure-directing agents (OSDAs), post-synthetically dealuminated via high temperature (600–800 °C) steam treatments, and evaluated as catalysts for converting methanol-to-light olefins (MTO). The proton forms of the as-made zeolites deactivate rapidly when tested for the MTO reaction (conducted at 400 °C and atmospheric pressure) due to their high aluminum content. Steam treatments lead to improvements in olefin selectivities and catalyst lifetimes with samples steamed at 600 °C giving the best combination of lifetime and olefin selectivity. Zeolite characterizations by 27Al NMR, 29Si NMR and argon physisorption indicate that the steam treatments extract framework aluminum, leading to reductions in the total number of Brønsted acid sites and the creation of mesopores that can facilitate transport of reactants. Poisoning of the acid sites external to the 8MR pores of the zeolites by treatments with trimethylphosphite results in longer catalyst lifetimes but do not affect the observed olefin selectivities, suggesting that olefins do not undergo secondary dimerization or methylation reactions at these sites.

Synthesis of zeolites in the absence of organic structure-directing agents: factors governing crystal selection and polymorphism

Organic structure-directing agents (OSDAs) are commonly avoided in commercial zeolite synthesis because of the economic and environmental disadvantages associated with the synthesis and removal of organics occluded within zeolite micropores. Zeolite crystallization in OSDA-free media is the route by which microporous clays form in nature, and it is also the preferred method of producing zeolites in bulk for a wide range of applications. There are many synthesis parameters that influence zeolite crystallization, among which include the molar fractions of reagents (silica, alumina, and hydroxide ions), water content, temperature, synthesis aging and heating time, the selection of extraframework cations, the choice of silica and alumina sources, and the use of crystal seeds. In this review, we discuss zeolite framework types that form in OSDA-free solutions at these different synthesis conditions in an effort to highlight structure-property relationships while simultaneously emphasizing the areas where further studies are needed to optimize and/or discover new materials. Interestingly, fewer than 15% of the total reported zeolite structures have been prepared in the absence of OSDAs. For many of these structures, fundamental mechanisms governing their formation are not well understood. In addition, OSDA-free syntheses tend to be more susceptible to the formation of crystal polymorphs (or impurities) that can be generated through a series of structural transformations during the course of zeolite growth. Here we examine the driving forces for phase transitions and explore methods to control phase selection and polymorphism. In order to better facilitate comparisons among zeolite synthesis parameters, we have reinstituted the approach of using kinetic phase diagrams to identify conditions of phase stability.

Facile Fabrication of ZSM-5 Zeolite Catalyst with High Durability to Coke Formation during Catalytic Cracking of Paraffins

Post-synthetic HNO3 treatment of ZSM-5 zeolite synthesized in the absence of organic structure-directing agent removes framework Al selectively from the external surface, producing a unique ZSM-5 zeolite catalyst that has very few acid sites on its external surfaces. The resulting external acid sites can be readily deactivated at the early stage of the reaction, giving a potentially long-life catalyst as a result of a high durability to coke formation during the cracking of hexane or other paraffin molecules.

A Working Hypothesis for Broadening Framework Types of Zeolites in Seed-Assisted Synthesis without Organic Structure-Directing Agent

Recent research has demonstrated a new synthesis route to useful zeolites such as beta, RUB-13, and ZSM-12 via seed-assisted, organic structure-directing agent (OSDA)-free synthesis, although it had been believed that these zeolites could be essentially synthesized with OSDAs. These zeolites are obtained by adding seeds to the gels that otherwise yield other zeolites; however, the underlying crystallization mechanism has not been fully understood yet. Without any strategy, it is unavoidable to employ a trial-and-error procedure for broadening zeolite types by using this synthesis method. In this study, the effect of zeolite seeds with different framework structures is investigated to understand the crystallization mechanism of zeolites obtained by the seed-assisted, OSDA-free synthesis method. It has been found that the key factor in the successful synthesis of zeolites in the absence of OSDA is the common composite building unit contained both in the seeds and in the zeolite obtained from the gel after heating without seeds. A new working hypothesis for broadening zeolite types by the seed-assisted synthesis without OSDA is proposed on the basis of the findings of the common composite building units in zeolites. This hypothesis enables us to design the synthesis condition of target zeolites. The validity of the hypothesis is experimentally tested and verified by synthesizing several zeolites including ECR-18 in K-aluminosilicate system.

Critical Factors in the Seed‐Assisted Synthesis of Zeolite Beta and “Green Beta” from OSDA‐Free Na+–Aluminosilicate Gels

Organic structure-directing agent (OSDA)-free synthesis of zeolite beta is a subject of both scientific and industrial interest. Herein, we report a comprehensive investigation into the effects of various parameters on the seed-assisted crystallization of zeolite beta in the absence of OSDA. The crystallization behavior of "OSDA-free beta" is strongly governed by the chemical composition of the starting Na(+)-aluminosilicate gel as well as by the Si/Al ratios of the calcined beta seed crystals, which are prepared using tetraethylammonium hydroxide (TEAOH). Furthermore, OSDA-free beta seed crystals can be used to form zeolite beta, termed "green beta". XRD, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, and ²⁷Al magic angle spinning NMR analyses showed that the OSDA-free beta and green beta were of high purity and crystallinity. The nitrogen adsorption-desorption of OSDA-free beta and green beta revealed higher surface areas and larger volumes in the micropore region than those of the beta seeds synthesized with OSDA after calcination. These results provide a robust and reliable process for the environmentally friendly production of high-quality zeolite beta in a completely OSDA-free Na(+)-aluminosilicate system.

Synthesis of MTW-type Zeolites in the Absence of Organic Structure-directing Agent

Abstract The synthesis of MTW-type aluminosilicate zeolites in the absence of organic structure-directing agents (OSDAs) was successfully attained by the addition of calcined ZSM-12 seeds to (Li, Na)-aluminosilicate gels. In this system, Li cation plays a crucial role in the growth of the MTW crystals.