Use Of Organic Structure-directing Agents Research Articles

Conversion of NOx over Aluminosilicate Cu-CHA Zeolite Catalysts Synthesized Free of Organic Structure-Directing Agents

Cu-CHA zeolites have proven to be effective for NOx reduction, but a drawback in using CHA zeolites is the cost associated with using expensive organic structure-directing agents. To overcome this drawback, we are reporting here the synthesis of Cu-CHA zeolite catalysts in both their NH4-form as well as K-form that do not require the use of organic structure-directing agents. After comprehensive characterization by XRF, XRD, 27Al NMR spectroscopy, FE-SEM, SEM/EDS, N2-adsorption/desorption, NH3-TPD, H2-TPR, and XPS, the zeolite catalysts were tested for NOx conversion by NH3-selective catalytic reduction (NH3-SCR). Cu-NH4-CHA zeolite catalysts exhibited remarkable activity and thermal stability over a wide temperature window, outperforming their counterpart K-forms. Among the NH4-forms of CHA zeolite catalysts, the 0.1 M Cu-NH4-CHA showed the best catalytic performance, achieving 50% NOx conversion at a temperature as low as 192 °C, and reaching full conversion of NOx at 261 °C. These Cu-based CHA zeolite catalysts are promising thanks to their environmentally friendly synthesis and offer the opportunity of maximizing DeNOx strategies in applications for NOx pollution abatement.

Crystallization of potassium-zeolites in organic-free media

A ubiquitous phenomenon in zeolite synthesis is the generation of polymorphs leading to either single or multi-phase products. The use of organic structure-directing agents (OSDAs) often ensures a high degree of phase purity for a desired zeolite; however, OSDAs can be economically and environmentally prohibitive. The use of alkali metals as inorganic structure-directing agents (ISDAs) is a common alternative, although reductions in polymorph specificity and/or purity in organic-free media are significant limitations. Developing more controlled, facile synthesis strategies without the aid of OSDAs relies on understanding how zeolite growth conditions impact polymorph selection and alter the physicochemical properties of crystalline products. Relatively few studies of organic-free syntheses have systematically explored potassium growth media. In this study, we examine synthesis conditions that promote the formation of five K-zeolites: CHA, LTL, MER, EDI, and BPH frameworks. Time-resolved analyses reveal distinct trends in crystallization kinetics and interzeolite transformations for potassium compared to prior studies that have largely focused on Na-zeolites. We also highlight methods to achieve nanosized crystals and markedly reduce crystallization times for zeolites, such as chabazite, which are notoriously difficult to synthesize in organic-free media at reasonable timescales. Collectively, these findings identify routes to control K-zeolite polymorphism and provide general design strategies to tailor their crystallization.

Spontaneous Pillaring of Pentasil Zeolites.

Conventional methods to prepare hierarchical zeolites depend upon the use of organic structure-directing agents and often require multiple synthesis steps with limited product yield and Brønsted acid concentration. Here, it is shown that the use of MEL- or MFI-type zeolites as crystalline seeds induces the spontaneous formation of self-pillared pentasil zeolites, thus avoiding the use of any organic or branching template for the crystallization of these hierarchical structures. The mechanism of formation is evaluated by time-resolved electron microscopy to provide evidence for the heterogeneous nucleation and growth of sequentially branched nanosheets from amorphous precursors. The resulting hierarchical zeolites have large external surface area and high percentages of external acid sites, which markedly improves their catalytic performance in the Friedel-Crafts alkylation and methanol to hydrocarbons reactions. These findings highlight a facile, commercially viable synthesis method to reduce mass-transport limitations and improve the performance of zeolite catalysts.

Discovering Relationships between OSDAs and Zeolites through Data Mining and Generative Neural Networks.

Organic structure directing agents (OSDAs) play a crucial role in the synthesis of micro- and mesoporous materials especially in the case of zeolites. Despite the wide use of OSDAs, their interaction with zeolite frameworks is poorly understood, with researchers relying on synthesis heuristics or computationally expensive techniques to predict whether an organic molecule can act as an OSDA for a certain zeolite. In this paper, we undertake a data-driven approach to unearth generalized OSDA–zeolite relationships using a comprehensive database comprising of 5,663 synthesis routes for porous materials. To generate this comprehensive database, we use natural language processing and text mining techniques to extract OSDAs, zeolite phases, and gel chemistry from the scientific literature published between 1966 and 2020. Through structural featurization of the OSDAs using weighted holistic invariant molecular (WHIM) descriptors, we relate OSDAs described in the literature to different types of cage-based, small-pore zeolites. Lastly, we adapt a generative neural network capable of suggesting new molecules as potential OSDAs for a given zeolite structure and gel chemistry. We apply this model to CHA and SFW zeolites generating several alternative OSDA candidates to those currently used in practice. These molecules are further vetted with molecular mechanics simulations to show the model generates physically meaningful predictions. Our model can automatically explore the OSDA space, reducing the amount of simulation or experimentation needed to find new OSDA candidates.

Elucidating the Role of Tetraethylammonium in the Silicate Condensation Reaction from Ab Initio Molecular Dynamics Simulations.

The understanding of the formation of silicate oligomers in the initial stage of zeolite synthesis is important. The use of organic structure-directing agents (OSDAs) is known to be a key factor in the formation of different silicate species and the final zeolite structure. For example, tetraethylammonium ion (TEA+) is a commonly used organic template for zeolite synthesis. In this study, ab initio molecular dynamics (AIMD) simulation is used to provide an understanding of the role of TEA+ in the formation of various silicate oligomers, ranging from dimer to 4-ring. Calculated free-energy profiles of the reaction pathways show that the formation of a 4-ring structure has the highest energy barrier (97 kJ/mol). The formation of smaller oligomers such as dimer, trimer, and 3-ring has lower activation barriers. The TEA+ ion plays an important role in regulating the predominant species in solution via its coordination with silicate structures during the condensation process. The kinetics and thermodynamics of the oligomerization reaction indicate a more favorable formation of the 3-ring over the 4-ring structure. The results from AIMD simulations are in line with the experimental observation that TEA+ favors the 3-ring and double 3-ring in solution. The results of this study imply that the role of OSDAs is not only important for the host–guest interaction but also crucial for controlling the reactivity of different silicate oligomers during the initial stage of zeolite formation.

Synthesis of Discrete CHA Zeolite Nanocrystals without Organic Templates for Selective CO2 Capture.

Small-pore zeolites such as chabazite (CHA) are excellent candidates for the selective separation of CO2 ; however, the current synthesis involves several steps and the use of organic structure-directing agent (OSDA), increasing their cost and energy requirements. We report the synthesis of small-pore zeolite crystals (aluminosilicate) with CHA-type framework structure by direct synthesis in a colloidal suspension containing a mixture of inorganic cations only (Na+ , K+ , and Cs+ ). The location of CO2 molecules in the host structure was revealed by 3D electron diffraction (3D ED). The high sorption capacity for CO2 (3.8 mmol g-1 at 121 kPa), structural stability and regenerability of the discreate CHA zeolite nanocrystals is maintained for 10 consecutive cycles without any visible degradation. The CHA zeolite (Si:Al=2) reaches an almost perfect CO2 storage capacity (8 CO2 per unit cell) and high selectivity (no CH4 was adsorbed).

Organotemplate-free synthesis of hierarchical beta zeolites

This paper reports a straightforward approach to synthesizing hierarchical beta zeolites by exposing microporous zeolites to hydrothermal conditions amenable for beta crystallization, which simultaneously creates hierarchical pore structure while maintaining crystallinity and framework integrity without the use of organic structure-directing agents, pore-directing agents, or mesoporous templates. The obtained hierarchical beta zeolites possess enlarged mesopores and exhibit improved catalytic performance for the conversion of bulky molecules in the liquid-phase conversion of benzyl alcohol with mesitylene. A sequential hierarchical structure formation and recrystallization mechanism is proposed to elucidate the creation of mesopores in beta zeolites with excellent preservation of acidity and microporosity.

Zeolites with continuously tuneable porosity.

Zeolites are important materials whose utility in industry depends on the nature of their porous structure. Control over microporosity is therefore a vitally important target. Unfortunately, traditional methods for controlling porosity, in particular the use of organic structure-directing agents, are relatively coarse and provide almost no opportunity to tune the porosity as required. Here we show how zeolites with a continuously tuneable surface area and micropore volume over a wide range can be prepared. This means that a particular surface area or micropore volume can be precisely tuned. The range of porosity we can target covers the whole range of useful zeolite porosity: from small pores consisting of 8-rings all the way to extra-large pores consisting of 14-rings.

Progress in seed-assisted synthesis of zeolites without using organic structure-directing agents

Recent progress in the development of methods that do not require the use of organic structure-directing agents (OSDAs) has enabled the synthesis of useful zeolites under the use of zeolite seed crystals. This novel approach has attracted considerable attention as a low-cost, environmentally friendly technique for the production of zeolites. In this review, the conventional seed-assisted synthesis of zeolites in addition to the seed-assisted preparation of zeolite membranes are introduced, and an overview of recent developments in the seed-assisted, OSDA-free synthesis of various zeolites, such as beta, ZSM-12, TTZ-1, levyne, and so on, is presented. Although some mechanistic studies have been reported, the details of seed-assisted synthesis remain under investigation; recent OSDA-free syntheses have been estimated to be kinetically controlled systems. A new working hypothesis, designated as the “composite building unit (CBU) hypothesis” was proposed by us in order to broaden the range of zeolite types that can be prepared via seed-assisted synthesis. This hypothesis focuses on the structural relationship between the seed (target) zeolite and the zeolite obtained from a reactant gel without seeds. It was confirmed that several zeolites, such as ZSM-11, ECR-18, and so on, can be synthesized without OSDAs for the first time, by applying this hypothesis. The requirements for the successful synthesis of zeolites are introduced in addition to the unique features of the zeolites that are obtained by the seed-assisted synthesis method.

Preparation of RTH-type zeolites with the amount and/or kind of organic structure-directing agents (OSDA): Are OSDAs indispensable for the crystallization?

Although the RTH-type borosilicate, [B]-RTH, has been originally synthesized by using 1,2,2,6,6-pentamethylpiperidine (PMP) as organic structure-directing agent (OSDA) in combination with ethylenediamine (EDA), we have established the synthesis method for preparing the RTH-type zeolites without using any OSDAs. To confirm whether the OSDAs is indispensable for this zeolite, the conditions for synthesizing the RTH-type borosilicate have been investigated in detail; the RTH-type borosilicate was synthesized with the amount and/or kind of organic amines varied. First, the effect of the amount of PMP on the crystallization of the [B]-RTH was investigated with the amount of EDA constant. Second, [B]-RTH was synthesized by using diverse amines in place of PMP in the presence of EDA. The effect of the use of OSDAs on the direct incorporation of heteroatoms such as Al, Ga and Fe into the RTH framework was also investigated. Finally, the effectiveness of the use of OSDAs for the RTH-type zeolites in terms of the diversification of the RTH-type zeolite was described.

Further investigations on the synthesis of pure-silica molecular sieves via the use of organic structure-directing agents

Several organic structure-directing agents (SDAs) that are variations of a 4,4′-trimethylenebis( N-methyl, N-R 1-piperidinium) moiety were synthesized and used for the synthesis of pure-silica molecular sieves. Pure-silica MTW, MFI, BEA and the MCM-41-type mesoporous materials could be prepared by changing only the R 1 group. When R 1 was pentyl or hexyl, MFI was crystallized, suggesting that each SDA molecule spans two channel intersections. For R 1 groups smaller than pentyl, MTW was formed. Pure-silica BEA could easily be prepared when R 1 was benzyl or cyclohexylmethyl. Numerous variations of the organics used to crystallize BEA were explored and the features necessary to structure direct the formation of BEA elucidated. When R 1 was either octyl or nonyl, the MCM-41-type mesoporous materials were formed, indicating that these organics can act as surfactants. The results presented provide further evidence that organic molecules can serve as structure-directing agents for the synthesis of pure-silica molecular sieves.

Synthesis of Ordered Silicates by the Use of Organic Structure-Directing Agents

Microporous, crystalline silicates can be synthesized using organic structure-directing molecules whose function is to organize silicate species into particular arrangements that then spontaneously self-assembles into the final crystalline materials. The porosity is obtained by removal of the organic component. The authors discuss the molecular properties (size, rigidity, hydrophobicity) necessary for the organic component to interact with aqueous silicate species and prepare microporous silicates. It is shown that a strategy to construct large organic molecules in order to prepare large pore crystalline silicates could involve the use of two charge centers if these functionalities are distributed within the molecules such to prevent aggregation in aqueous media. A charge distribution that allows aggregation of the organic molecules at synthesis conditions directs the formation of a locally amorphous, mesoporous silicate rather than a crystalline, microporous material.

Synthesis of Porous Silicates

The issues of importance and future concern in the synthesis of porous silicates and porous materials that contain a large fraction of silica, e.g. zeolites and other crystalline molecular sieves, are reviewed. The thermodynamics of zeolite synthesis are discussed, including a detailed thermodynamic analysis of the synthesis of pure-silica ZSM-5. The kinetics of porous silicate synthesis are reviewed, with particular emphasis on the control of porous structure formation through the use of organic structure-directing agents. Ordered mesoporous materials are discussed in the context of distinguishing their features from zeolites in order to describe further the unique properties of each class of material. Finally, several unresolved issues in the understanding of the synthesis process are outlined, the resolutions of which would aid in the synthesis of porous silicates by design.