Matthew D. Oleksiak

Controlling crystal polymorphism in organic-free synthesis of Na-zeolites.

Controlling polymorphism is critical in areas such as pharmaceuticals, biomineralization, and catalysis. Notably, the formation of unwanted polymorphs is a ubiquitous problem in zeolite synthesis. In this study, we propose a new platform for controlling polymorphism in organic-free Na-zeolite synthesis that enables crystal composition and properties to be tailored without sacrificing crystal phase purity. Through systematic adjustment of multiple synthesis parameters, we identified ternary (kinetic) phase diagrams at specific compositions (i.e., Si, Al, and NaOH mole fractions) using colloidal silica and sodium aluminate. Our studies identify multiple stages of zeolite phase transformations involving the framework types FAU, LTA, EMT, GIS, SOD, ANA, CAN, and JBW. We report an initial amorphous-to-crystalline transition of core-shell particles (silica core and alumina shell) to low-density framework types and their subsequent transformation to more dense structures with increasing temperature and/or time. We show that reduced water content facilitates the formation of structures such as EMT that are challenging to synthesize in organic-free media and reduces the synthesis temperature required to achieve higher-density framework types. A hypothesis is proposed for the sequence of phase transformations that is consistent with the Ostwald rule of stages, wherein metastable structures dissolve and recrystallize into more thermodynamically stable structures. The ternary diagrams developed here are a broadly applicable platform for rational design that offers an alternative to time- and cost-intensive methods of ad hoc parameter selection without a priori knowledge of crystal phase behavior.

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

Abstract 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.

Tailoring the physicochemical properties of zeolite catalysts

The physicochemical properties of zeolite catalysts, such as crystal topology, composition, size, and morphology, can have a marked effect on their performance for a broad range of reactions, notably in catalyst activity, hydrothermal stability, shape selectivity, and/or lifetime. There are relatively few zeolite framework types employed as commercial catalysts. Contributing factors include the high cost of synthesis and the difficulty of tailoring crystal nucleation and growth to achieve the desired properties. There is an increasing amount of structure performance data in the literature and patents that can be used to guide the identification of effective zeolite “formulations”; however, the challenges for realizing these materials are generally twofold: (i) growth mechanisms are not well understood, which often prohibits the control of zeolite crystallization, and (ii) the impracticality of most design schemes hinders their economic feasibility and their potential for facile implementation. These aspects are often overlooked in the design of zeolite catalysts, yet they are essential for any plans aimed at eventual commercialization. In this review, we summarize our recent findings in the area of zeolite synthesis and characterization, focusing specifically on practical routes to control zeolite crystallization in the absence of costly organics, tailoring crystal habit through the use of versatile and recyclable zeolite growth modifiers, and pioneering techniques in zeolite surface science as a platform to expand our knowledge of crystal growth mechanisms. These concerted efforts in rational design bridge fundamental and applied research towards the development of zeolites with improved catalytic performance.

Organic-Free Synthesis of a Highly Siliceous Faujasite Zeolite with Spatially Biased Q4(nAl) Si Speciation

We report the most siliceous FAU-type zeolite, HOU-3, prepared via a one-step organic-free synthesis route. Computational studies indicate that it is thermodynamically feasible to synthesize FAU with SAR=2-7, though kinetic factors seemingly impose a more restricted upper limit for HOU-3 (SAR≈3). Our findings suggest that a slow rate of crystallization and/or low concentration of Na+ ions in HOU-3 growth mixtures facilitate Si incorporation into the framework. Interestingly, Q4 (nAl) Si speciation measured by solid-state NMR can only be modeled with a few combinations of Al positioning at tetrahedral sites in the crystal unit cell, indicating the distribution of Si(-O-Si)4-n (-O-Al)n species is spatially biased as opposed to being random. Achieving higher SAR is desirable for improved zeolite (hydro)thermal stability and enhanced catalytic performance, which we demonstrate in benchmark tests that show HOU-3 is superior to commercial zeolite Y.

Synthesis Strategies for Ultrastable Zeolite GIS Polymorphs as Sorbents for Selective Separations.

Designing zeolites with tunable physicochemical properties can substantially impact their performance in commercial applications, such as adsorption, separations, catalysis, and drug delivery. Zeolite synthesis typically requires an organic structure-directing agent to produce crystals with specific pore topology. Attempts to remove organics from syntheses to achieve commercially viable methods of preparing zeolites often lead to the formation of impurities. Herein, we present organic-free syntheses of two polymorphs of the small-pore zeolite P (GIS), P1 and P2. Using a combination of adsorption measurements and density functional theory calculations, we show that GIS polymorphs are selective adsorbents for H2 O relative to other light gases (e.g., H2 , N2 , CO2 ). Our findings refute prior theoretical studies postulating that GIS-type zeolites are excellent materials for CO2 separation/sequestration. We also show that P2 is significantly more thermally stable than P1, which broadens the operating conditions for GIS-type zeolites in commercial applications and opens new avenues for exploring their potential use in processes such as catalysis.