Monitoring and controlling zeolite synthesis via reactor-based solutions: a fed-batch strategy

Most conventional zeolite synthesis takes place in closed batch autoclaves that cannot be monitored or controlled during the process. Moreover, the study of time-dependent parameters of the synthesis with the conventional "cooling-opening" procedure not only reduces accuracy as a series of reactors (never 100% identical) needs to be started in parallel (and stopped at different times), it is also labor intense. Furthermore, the classic batch concept does not permit the intermediate addition of species without disrupting synthesis and the cooling-reheating effects. In this study, we developed a technique for zeolite synthesis monitoring in one-pot experiments using the sampling feature of fed-batch (FB) reactors. These one-pot syntheses can save time and ingredients instead of performing plenty of classic batch experiments. In addition, we could control and manipulate the zeolite synthesis by using the feeding function of the FB reactor and the intermediate addition of precursors at operational temperatures and pressures. Stannosilicate and zincosilicate syntheses were carried out via the FB reactor to investigate the intermediate timed-addition and the possibility of optimizing feeding rates of heteroatoms opposed to a classic synthesis, which faces challenges when a high amount of heteroatom precursor presents at the start. Finally, a modified FB platform was further developed to be able to monitor essential kinetic and synthetic parameters on-line (T, P, and also pH) on-line without intervention. For instance, pH profiles can allow one to estimate key events in zeolite synthesis, but in the art, these profiles are always measured ex situ (including cooling effects etc.).

Ultrafast synthesis of zirconium-porphyrin framework nanocrystals from alkoxide precursors

Porphyrinic metal-organic frameworks (MOFs) offer high surface areas and tunable catalytic and optoelectronic properties, making them versatile candidates for applications in phototherapy, drug delivery, photocatalysis, electronics, and energy storage. However, a key challenge for industrial integration is the rapid, cost-effective production of suitable sizes. This study introduces Zr(IV) alkoxides as metal precursors, achieving ultrafast (∼minutes) and high-yield (>90%) synthesis of three well-known Zr-based porphyrinic MOF nanocrystals: MOF-525, PCN-224, and PCN-222, each with distinct topologies. By adjusting linker-to-metal and modulator-to-metal ratios, we attain precise control over single-phase formation. Demonstrating alkoxides' potential, we synthesized nanosized PCN-224 at room temperature within seconds using a continuous multifluidic method. This advancement greatly simplifies porphyrinic MOF production, enabling broader industrial and scientific applications.

Continuous Synthesis and Processing of Covalent Organic Frameworks in a Flow Reactor

Covalent organic frameworks (COFs) are typically prepared in the form of insoluble microcrystalline powders using batch solvothermal reactions that are energy-intensive and require long annealing periods (>120 °C, >72 h). Thus, their wide-scale adoption in a variety of potential applications is impeded by complications related to synthesis, upscaling, and processing, which also compromise their commercialization. Here we report a strategy to address both the need for scalable synthesis and processing approaches through the continuous, accelerated synthesis, and processing of imine- and hydrazone-linked COFs using a flow microreactor. The flow microreactor is capable of unprecedented COF productivities, up to 61,111 kg m-3 day-1, and provides control over key stages of COF formation, including nanoparticle growth, self-assembly, and precipitation. Additionally, the technique successfully yields highly crystalline and porous COFs in versatile macroscopic structures such as monoliths, membranes, prints, and packed beds. We also show that a COF synthesized using the flow microreactor acts as an excellent photocatalyst for the photocatalytic degradation of perfluorooctanoic acid (PFOA) outperforming the degradation efficiency of its batch analogue and other classical photocatalysts such as titanium dioxide (TiO2).

Recent advances in continuous flow synthesis of metal-organic frameworks and their composites

Metal-organic frameworks (MOFs) and their composites have garnered significant attention in recent years due to their exceptional properties and diverse applications across various fields. The conventional batch synthesis methods for MOFs and their composites often suffer from challenges such as long reaction times, poor reproducibility, and limited scalability. Continuous flow synthesis has emerged as a promising alternative for overcoming these limitations. In this short review, we discuss the recent advancements, challenges, and future perspectives of continuous flow synthesis in the context of MOFs and their composites. The review delves into a brief overview of the fundamental principles of flow synthesis, highlighting its advantages over batch methods. Key benefits, including precise control over reaction parameters, improved scalability and efficiency, rapid optimization capabilities, enhanced reaction kinetics and mass transfer, and increased safety and environmental sustainability, are addressed. Additionally, the versatility and flexibility of flow synthesis techniques are discussed. The article then explores various flow synthesis methods applicable to MOF and MOF composite production. The techniques covered include continuous flow solvothermal synthesis, mechanochemical synthesis, microwave and ultrasound-assisted flow synthesis, microfluidic droplet synthesis, and aerosol synthesis. Notably, the combination of flow chemistry and aerosol synthesis with real-time characterization is also addressed. Furthermore, the impact of flow synthesis on the properties and performance of MOFs is explored. Finally, the review discusses current challenges and future perspectives in the field of continuous flow MOF synthesis, paving the way for further development and broader application of this promising technique.

Engineering the physical properties and photocatalytic activities of a β-ketoenamine COF using continuous flow synthesis

Covalent Organic Frameworks (COF) having conjugated backbone are an interesting class of metal-free, visible light active, heterogeneous photocatalysts. Interestingly, synthesis of COF using continuous flow process has emerged as an efficient, alternative method when compared to the traditional batch process. Here, we demonstrate the possibility to engineer the physical properties and hence the adsorption and catalytic activities of a β-ketoenamine COF by varying monomer flow rate and microreactor design during the continuous flow synthesis. Crystallinity of the COF increases on varying the monomer flow rate from 100 (S-100) to 500 (S-500) and up to 1000 μLmin-1 (S-1000), in an S-shaped microreactor, resulting in an enhanced surface area: 525, 722 and 1119 m2g-1 respectively. The photophysical properties of the COF are also found to vary significantly with the change in flow synthesis conditions. S-1000 is characterized by the highest adsorption of MB, due to its high surface area and accessible pores. On the other hand, S-500 shows the highest photocurrent, a low recombination of photogenerated charges and the lowest charge transfer resistance. Thus, S-500 is found to be the best photocatalyst for the removal of a model pollutant (methylene blue, MB). Further, enhanced photocatalytic removal of MB using S-500 could be achieved by performing the photocatalysis in continuous flow.

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

Zeolite catalysts and adsorbents have been an integral part of many commercial processes and are projected to play a significant role in emerging technologies to address the changing energy and environmental landscapes. The ability to rationally design zeolites with tailored properties relies on a fundamental understanding of crystallization pathways to strategically manipulate processes of nucleation and growth. The complexity of zeolite growth media engenders a diversity of crystallization mechanisms that can manifest at different synthesis stages. In this review, we discuss the current understanding of classical and nonclassical pathways associated with the formation of (alumino)silicate zeolites. We begin with a brief overview of zeolite history and seminal advancements, followed by a comprehensive discussion of different classes of zeolite precursors with respect to their methods of assembly and physicochemical properties. The following two sections provide detailed discussions of nucleation and growth pathways wherein we emphasize general trends and highlight specific observations for select zeolite framework types. We then close with conclusions and future outlook to summarize key hypotheses, current knowledge gaps, and potential opportunities to guide zeolite synthesis toward a more exact science.

Pore engineering of micro/mesoporous nanomaterials for encapsulation, controlled release and variegated applications of essential oils

Essential oils have become increasingly popular in fields of medical, food and agriculture, owing to their strongly antimicrobial, anti-inflammation and antioxidant effects, greatly meeting demand from consumers for healthy and safe natural products. However, the easy volatility and/or chemical instability of active ingredients of essential oils (EAIs) can result in the loss of activity before realizing their functions, which have greatly hindered the widely applications of EAIs. As an emerging trend, micro/mesoporous nanomaterials (MNs) have drawn great attention for encapsulation and controlled release of EAIs, owing to their tunable pore structural characteristics. In this review, we briefly discuss the recent advances of MNs that widely used in the controlled release of EAIs, including zeolites, metal-organic frameworks (MOFs), mesoporous silica nanomaterials (MSNs), and provide a comprehensive summary focusing on the pore engineering strategies of MNs that affect their controlled-release or triggered-release for EAIs, including tailorable pore structure properties (e.g., pore size, pore surface area, pore volume, pore geometry, and framework compositions) and surface properties (surface modification and surface functionalization). Finally, the variegated applications and potential challenges are also given for MNs based delivery strategies for EAIs in the fields of healthcare, food and agriculture. These will provide considerable instructions for the rational design of MNs for controlled release of EAIs.

Porous crystalline materials for memories and neuromorphic computing systems

Porous crystalline materials usually include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs) and zeolites, which exhibit exceptional porosity and structural/composition designability, promoting the increasing attention in memory and neuromorphic computing systems in the last decade. From both the perspective of materials and devices, it is crucial to provide a comprehensive and timely summary of the applications of porous crystalline materials in memory and neuromorphic computing systems to guide future research endeavors. Moreover, the utilization of porous crystalline materials in electronics necessitates a shift from powder synthesis to high-quality film preparation to ensure high device performance. This review highlights the strategies for preparing porous crystalline materials films and discusses their advancements in memory and neuromorphic electronics. It also provides a detailed comparative analysis and presents the existing challenges and future research directions, which can attract the experts from various fields (e.g., materials scientists, chemists, and engineers) with the aim of promoting the applications of porous crystalline materials in memory and neuromorphic computing systems.

An Efficient Continuous Flow Synthesis for the Preparation of N-Arylhydroxylamines: Via a DMAP-Mediated Hydrogenation Process

The selective hydrogenation of nitroarenes to N-arylhydroxylamines is an important synthetic process in the chemical industry. It is commonly accomplished by using heterogeneous catalytic systems that contain inhibitors, such as DMSO. Herein, DMAP has been identified as a unique additive for increasing hydrogenation activity and product selectivity (up to >99%) under mild conditions in the Pt/C-catalyzed process. Continuous-flow technology has been explored as an efficient approach toward achieving the selective hydrogenation of nitroarenes to N-arylhydroxylamines. The present flow protocol was applied for a vast substrate scope and was found to be compatible with a wide range of functional groups, such as electron-donating groups, carbonyl, and various halogens. Further studies were attempted to show that the improvement in the catalytic activity and selectivity benefited from the dual functions of DMAP; namely, the heterolytic H2 cleavage and competitive adsorption.

Non-fouling flow reactors for nanomaterial synthesis

This review provides a holistic description of flow reactor fouling for wet-chemical nanomaterial syntheses. Fouling origins and consequences are discussed together with the variety of flow reactors for its prevention.

Semiflow Microwave Heating Reactor with Resonator Moving Mechanism Applied to Zeolite Synthesis

A semiflow microwave (MW) heating reactor similar to a flow reactor system was developed. Slurry raw materials in the reaction tube were heated continuously and cooled rapidly by moving a thin MW resonator instead of flowing slurry raw materials. From highly viscous mother slurries, Linde-type A (LTA) and faujasite (FAU)-type zeolite nanoparticles of small crystal grains were synthesized quickly. Results show that this heating system can synthesize hydroxy-sodalite (SOD)-type zeolite from coal fly ash particles including those larger than 50 μm. Numerical calculations using the COMSOL Multiphysics program revealed the thermal distribution of liquids of various viscosities using the semiflow MW heating reactor.

Application of a chemical clock in material design: chemically programmed synthesis of zeolitic imidazole framework-8

Here we show a time-programmed and autonomous synthesis of zeolitic imidazole framework-8 (ZIF-8) using a methylene glycol-sulfite clock reaction. The induction period of the driving clock reaction, thus, the appearance of the ZIF-8 can be adjusted by the initial concentration of one reagent of the chemical clock. The autonomously synthesized ZIF-8 showed excellent morphology and crystallinity.

Patterned microfluidic devices for rapid screening of metal-organic frameworks yield insights into polymorphism and non-monotonic growth

Metal-organic frameworks (MOFs) are porous crystalline structures that are composed of coordinated metal ligands and organic linkers. Due to their high porosity, ultra-high surface-to-volume ratio, and chemical and structural flexibility, MOFs have numerous applications. MOFs are primarily synthesized in batch reactors under harsh conditions and long synthesis times. The continuous depletion of metal ligands and linkers in batch processes affects the kinetics of the oligomerization reaction and, hence, their nucleation and growth rates. Therefore, the existing screening systems that rely on batch processes, such as microtiter plates and droplet-based microfluidics, do not provide reliable nucleation and growth rate data. Significant challenges still exist for developing a relatively inexpensive, safe, and readily scalable screening device and ensuring consistency of results before scaling up. Here, we have designed patterned-surface microfluidic devices for continuous-flow synthesis of MOFs that allow effective and rapid screening of synthesis conditions. The patterned surface reduces the induction time of MOF synthesis for rapid screening while providing support to capture MOF crystals for growth measurements. The efficacy of the continuous-flow patterned microfluidic device to screen polymorphs, morphology, and growth rates is demonstrated for the HKUST-1 MOF. The effects of solvent composition and pH modulators on the morphology, polymorphs, and size distribution of HKUST-1 are evaluated using the patterned microfluidic device. Additionally, a time-resolved FT-IR analysis coupled with the patterned microfluidic device provides quantitative insights into the non-monotonic growth of MOF crystals with respect to the progression of the bulk oligomerization reaction. The patterned microfluidic device can be used to screen crystals with a longer induction time, such as proteins, covalent-organic frameworks, and MOFs.

Luminescent organic porous crystals from non-cyclic molecules and their applications

Organic porous crystals from small and non-cyclic organic molecules can be constructed by various intermolecular weak interactions. Owing to their precise stacking types, intermolecular interaction and pore microstructure, the relationship...

Controlling Nucleation Pathways in Zeolite Crystallization: Seeding Conceptual Methodologies for Advanced Materials Design

A core objective of synthesizing zeolites for widespread applications is to produce materials with properties and corresponding performances that exceed conventional counterparts. This places an impetus on elucidating and controlling processes of crystallization where one of the most critical design criteria is the ability to prepare zeolite crystals with ultrasmall dimensions to mitigate the deleterious effects of mass transport limitations. At the most fundamental level, this requires a comprehensive understanding of nucleation to address this ubiquitous materials gap. This Perspective highlights recent methodologies to alter zeolite nucleation by using seed-assisted protocols and the exploitation of interzeolite transformations to design advanced materials. Introduction of crystalline seeds in complex growth media used to synthesize zeolites can have wide-ranging effects on the physicochemical properties of the final product. Here we discuss the diverse pathways of zeolite nucleation, recent breakthroughs in seed-assisted syntheses of nanosized and hierarchical materials, and shortcomings for developing generalized guidelines to predict synthesis outcomes. We offer a critical analysis of state-of-the-art approaches to tailor zeolite crystallization wherein we conceptualize whether parallels between network theory and zeolite synthesis can be instrumental for translating key findings of individual discoveries across a broader set of zeolite crystal structures and/or synthesis conditions.

Revealing scenarios of interzeolite conversion from FAU to AEI through the variation of starting materials

Interzeolite conversion, which refers to the synthesis of zeolites using a pre-made zeolite as the starting material, has enabled promising outcomes that could not be easily achieved by the conventional synthesis from a mixture of amorphous aluminum and silicon sources. Understanding the mechanism of interzeolite conversion is of particular interest to exploit this synthesis route for the preparation of tailor-made zeolites as well as the discovery of new structures. It has been assumed that the structural similarity between the starting zeolite and the target one is crucial to a successful interzeolite conversion. Nevertheless, an image as to how one type of zeolite evolves into another one remains unclear. In this work, a series of dealuminated FAU zeolites were created through acid leaching and employed as the starting zeolites in the synthesis of AEI zeolite under various conditions. This experimental design allowed us to create a comprehensive diagram of the interzeolite conversion from FAU to AEI as well as to figure out the key factors that enable this kinetically favourable crystallization pathway. Our results revealed different scenarios of the interzeolite conversion from FAU to AEI and pinpointed the importance of the structure of the starting FAU in determining the synthesis outcomes. A prior dealumination was proven effective to modify the structure of the initial FAU zeolite and consequently facilitate its conversion to the AEI zeolite. In addition, this strategy allowed us to directly transfer the knowledge obtained from the interzeolite conversion to a successful synthesis of the AEI zeolite from dealuminated amorphous aluminosilicate precursors. These results offer new insights to the design and fabrication of zeolites via the interzeolite conversion as well as to the understandings of the crystallization mechanisms.

Molecular Bottom-Up Approaches for the Synthesis of Inorganic and Hybrid Nanostructures

Chemical routes for the synthesis of nanostructures are fundamental in nanoscience. Among the different strategies for the production of nanostructures, this article reviews the fundamentals of the bottom-up approaches, focusing on wet chemistry synthesis. It offers a general view on the synthesis of different inorganic and hybrid organic–inorganic nanostructures such as ceramics, metal, and semiconductor nanoparticles, mesoporous structures, and metal–organic frameworks. This review article is especially written for a wide audience demanding a text focused on the basic concepts and ideas of the synthesis of inorganic and hybrid nanostructures. It is styled for both early researchers who are starting to work on this topic and also non-specialist readers with a basic background on chemistry. Updated references and texts that provide a deeper discussion and describing the different synthesis strategies in detail are given, as well as a section on the current perspectives and possible future evolution.

Continuous Fluidic Techniques for the Precise Synthesis of Metal-Organic Frameworks

The continuous fluidics-based synthesis of metal-organic frameworks (MOFs) has attracted considerable attention, resulting in advancements in the reaction efficiency, a continuous production of complex structures, and access to products that are difficult or impossible to attain by bulk synthetic routes. This Minireview discusses the continuous fluidics-based synthesis of MOFs in terms of reaction process control, and is divided into three chapters dealing with the efficient synthesis of high-quality MOFs, the confined-space synthesis of MOF composites with diverse morphologies, and the selective synthesis of metastable products. The products of continuous fluidic synthetic process are introduced (e. g., uniform products, composites, fibers, membranes, and metastable products with advantageous properties that cannot be obtained by bulk synthesis), and their usefulness is demonstrated by referencing representative examples.

Synthetic and natural MOR zeolites as high-capacity adsorbents for the removal of nitrous oxide

N₂O is typically present as a trace gas in chemical processes, but its emission causes serious environmental issues. We herein demonstrate that ion-exchanged mordenite zeolites (framework code: MOR) can exhibit high capacities for N₂O adsorption under ambient conditions. In particular, a natural MOR zeolite gives an adsorption capacity as high as 0.34 mmol-N₂O per g-zeolite (1 atm, 25 °C), representing the best performing material among all zeolite-based adsorbents reported so far. The results contribute toward a comprehensive understanding of the structure-activity relationship and offer insights to establishing a zeolite-based adsorption system for enriching or removing N₂O.

Ultrafast and continuous-flow synthesis of AFX zeolite via interzeolite conversion of FAU zeolite

Continuous-flow synthesis of AFX zeolite is achieved with the residence time of 10 min, using FAU zeolite as the silica and alumina source, and acid-leached seeds which promote secondary nucleation of AFX.

Accelerated Development of Colloidal Nanomaterials Enabled by Modular Microfluidic Reactors: Toward Autonomous Robotic Experimentation

In recent years, microfluidic technologies have emerged as a powerful approach for the advanced synthesis and rapid optimization of various solution-processed nanomaterials, including semiconductor quantum dots and nanoplatelets, and metal plasmonic and reticular framework nanoparticles. These fluidic systems offer access to previously unattainable measurements and synthesis conditions at unparalleled efficiencies and sampling rates. Despite these advantages, microfluidic systems have yet to be extensively adopted by the colloidal nanomaterial community. To help bridge the gap, this progress report details the basic principles of microfluidic reactor design and performance, as well as the current state of online diagnostics and autonomous robotic experimentation strategies, toward the size, shape, and composition-controlled synthesis of various colloidal nanomaterials. By discussing the application of fluidic platforms in recent high-priority colloidal nanomaterial studies and their potential for integration with rapidly emerging artificial intelligence-based decision-making strategies, this report seeks to encourage interdisciplinary collaborations between microfluidic reactor engineers and colloidal nanomaterial chemists. Full convergence of these two research efforts offers significantly expedited and enhanced nanomaterial discovery, optimization, and manufacturing.

Ultrafast Encapsulation of Metal Nanoclusters into MFI Zeolite in the Course of Its Crystallization: Catalytic Application for Propane Dehydrogenation

Encapsulating metal nanoclusters into zeolites combines the superior catalytic activity of the nanoclusters with high stability and unique shape selectivity of the crystalline microporous materials. The preparation of such bifunctional catalysts, however, is often restricted by the mismatching in time scale between the fast formation of nanoclusters and the slow crystallization of zeolites. We herein demonstrate a novel strategy to overcome the mismatching issue, in which the crystallization of zeolites is expedited so as to synchronize it with the rapid formation of nanoclusters. The concept was demonstrated by confining Pt and Sn nanoclusters into a ZSM-5 (MFI) zeolite in the course of its crystallization, leading to an ultrafast, in situ encapsulation within just 5 min. The Pt/Sn-ZSM-5 exhibited exceptional activity and selectivity with stability in the dehydrogenation of propane to propene. This method of ultrafast encapsulation opens up a new avenue for designing and synthesizing composite zeolitic materials with structural and compositional complexity.

Chiral Macroporous MOF Surfaces for Electroassisted Enantioselective Adsorption and Separation

The development of surfaces with chiral features is a fascinating challenge for modern materials science, especially when they are used for chiral separation technologies. In this contribution, the design of hierarchically structured chiral macroporous zeolitic imidazolate framework-8 (ZIF-8) electrodes is presented. They are elaborated by an electrochemical deposition-dissolution technique based on the electrodeposition of metal through a colloidal crystal template, followed by controlled electrooxidation. This generates locally metal cations, which can interact with a chiral ligand present in the solution to form metal-organic frameworks (MOFs). The macroporous structure facilitates the access of the chiral recognition sites, located in the mesoporous MOF, and thus helps to overcome mass transport limitations. The efficiency of the designed functional materials for chiral adsorption and separation can be fine-tuned by applying an adjustable electric potential to the electrode surfaces. This hierarchical chiral ZIF-8 structure was deposited at the walls of a microfluidic device and used as a stationary phase for enantioselective separation. The potential-controlled interaction between the stationary phase and the chiral analytes allows baseline separation of two enantiomers. This opens up interesting perspectives for using hierarchically structured chiral MOFs as an efficient material for the selective adsorption and separation of chiral compounds.

Continuous-Flow Synthesis of ZIF-8 Biocomposites with Tunable Particle Size

Zeolitic imidazolate framework (ZIF) biocomposites show the capacity to protect and deliver biotherapeutics. To date, the progress in this research area is based on laboratory batch methods. Now, the first continuous flow synthetic method is presented for the encapsulation of a model protein (bovine serum albumin, BSA) and a clinical therapeutic (α1-antitrypsin, AAT) in ZIF-8. The in situ kinetics of nucleation, growth, and crystallization of BSA@ZIF-8 were studied by small-angle X-ray scattering. By controlling the injection time of ethanol, the particle growth could be quenched by ethanol-induced crystallization from amorphous particles to ZIF-8 crystals. The particle size of the biocomposite was tuned in the 40-100 nm range by varying residence time prior to introduction of ethanol. As a proof-of-concept, this procedure was used for the encapsulation of AAT in ZIF-8. Upon release of the biotherapeutic from the composite, the trypsin inhibitor function of AAT was preserved.

Continuous liquid-phase synthesis of nickel phosphide nanoparticles in a helically coiled tube reactor

Liquid-phase synthesis of Ni₂P in different flow types.

Metal organic frameworks as solid catalysts for liquid-phase continuous flow reactions

This Feature Article describes the recent developments in the use of MOFs as catalysts under continuous flow conditions illustrating that these materials can meet the required stability.