Engineering Lewis Acidity in Zeolite Catalysts by Electrochemical Release of Heteroatoms during Synthesis

Split Syntheses: Introducing Bottom-Up Control over Aluminum in SSZ-13 and ZSM-5 Zeolites

Zeolite synthesis is known as a difficult-to-control process, with many degrees of freedom that have a partially uncharted impact on the final product. Due to this, many zeolite scientists have regarded the initial mixing (aging) stage as the only time at which the chemical composition of a zeolite synthesis mixture can be impacted without heavily disrupting the delicate equilibria that are at play during crystallization. Recently, however, this view has started to change, with innovative techniques such as charge density mismatch or electro-assisted synthesis showing that the addition of new elements to the reactor midsynthesis might lead to new and surprising outcomes. In this manuscript, we show that by intermittent removal of certain fractions, notably Al-rich solids or Si-rich liquids, from the reaction medium during an interzeolite conversion from FAU-to-CHA (and FAU-to-MFI), one can control the Si/Al ratio of the final product, without heavily impacting the reaction time, particle size, or divalent cation capacity of the final product. This approach was named "split synthesis" and has led to several insights. By removing some Si-rich liquid phase after 40 min of synthesis, the Si/Al ratio of the daughter zeolite was lowered to a value of 20 (starting from 40), while the divalent cation capacity, a performance indicator for several acid and metal-catalyzed reactions, was kept maximized. On the other hand, when Al-rich solids were removed after 40 min (and in some cases colloidal silica was supplemented), we were able to rapidly synthesize small SSZ-13 zeolites with Si/Al ratios up to 180. These high-Si SSZ-13 zeolites had particle sizes in the range 100-150 nm and are traditionally difficult to crystallize in hydroxide medium. They showed a great olefin yield (6%) in the conversion of CO2 and H2 with ZnZrOx as cocatalyst.

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.).

Electrochemical Synthesis of Zeolite Coatings with Controlled Crystal Polymorphism and Self-Regulating Growth

Zeolite coatings are studied as molecular sieves for membrane separation, membrane reactors, and chemical sensor applications. They are also studied as anticorrosive films for metals and alloys, antimicrobial and hydrophobic films for heating, ventilation, and air conditioning, and dielectrics for semiconductor applications. Zeolite coatings are synthesized by hydrothermal, ionothermal, and dry-gel conversion approaches, which require high process temperatures and lengthy times (ranging from hours to days). Here, we report the first zeolite coatings synthesized via electrochemical deposition on a cathodic electrode, with controlled crystal polymorphism achieved within subhourly duration. We demonstrate this approach by developing sodium zeolite (e.g., sodalite (SOD), NaA (LTA), and Linde Type N (LTN)) coatings on a titanium electrode and extending the synthesis method to porous stainless steel. The coating morphology and crystallinity depend on the temperature, time, and applied current. The coating thickness is independent of the applied current, showing the presence of a self-regulating mechanism to ensure a uniform coating thickness across the metal surface. The electrochemical zeolite growth mechanism was elucidated with high-resolution transmission electron microscopy, and applications of the resultant zeolite coatings for oil/water separation and ethanol/water pervaporation were exploited. Electrochemical synthesis represents a novel, simple, fast, and environmentally friendly approach to preparing zeolite coatings. It can potentially be generalized for developing zeolite materials with diverse framework structures, morphologies, and orientations for substrates with complicated geometries.

Acceleration of Zeolite Crystallization: Current Status, Mechanisms, and Perspectives

Zeolites are important classes of crystalline materials and possess well-defined channels and cages with molecular dimensions. They have been extensively employed as heterogeneous catalysts and gas adsorbents due to their relatively large specific surface areas, high pore volumes, compositional flexibility, definite acidity, and hydrothermal stability. The zeolite synthesis normally undergoes high-temperature hydrothermal treatments with a relatively long crystallization time, which exhibits low synthesis efficiency and high energy consumption. Various strategies, e.g., modulation of the synthesis gel compositions, employment of special silica/aluminum sources, addition of seeds, fluoride, hydroxyl (·OH) free radical initiators, and organic additives, regulation of the crystallization conditions, development of new approaches, etc., have been developed to overcome these obstacles. And, these achievements make prominent contributions to the topic of acceleration of the zeolite crystallization and promote the fundamental understanding of the zeolite formation mechanism. However, there is a lack of the comprehensive summary and analysis on them. Herein, we provide an overview of the recent achievements, highlight the significant progress in the past decades on the developments of novel and remarkable strategies to accelerate the crystallization of zeolites, and basically divide them into three main types, i.e., chemical methods, physical methods, and the derived new approaches. The principles/acceleration mechanisms, effectiveness, versatility, and degree of reality for the corresponding approaches are thoroughly discussed and summarized. Finally, the rational design of the prospective strategies for the fast synthesis of zeolites is commented on and envisioned. The information gathered here is expected to provide solid guidance for developing a more effective route to improve the zeolite crystallization and obtain the functional zeolite-based materials with more shortened durations and lowered cost and further promote their applications.

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.