Mechanism of Benzene Hydroxylation on Tri-Iron Oxo-Centered Cluster-Based Metal-Organic Frameworks.

High-valent Fe(IV)-oxo species derived upon reactions of N2O with Fe(II) centers-embedded in the framework of tri-iron oxo-centered-based metal-organic frameworks (MOFs)- selectively affect the conversion of benzene-to-phenol via electrophilic addition to arene C-H bonds akin to oxygen transfer mechanisms in the P450 enzyme. The Fe(II) species identified by Mössbauer spectroscopy can be titrated in situ by the addition of NO to completely suppress benzene oxidation, verifying the relevance of Fe(II) centers. Observed inverse kinetic isotope effects in benzene hydroxylation preclude the involvement of H atom transfer steps from benzene to the Fe(IV)-oxo species and instead suggest that the electrophilic iron-oxo group adds to an sp2 carbon of benzene, resulting in a change in the hybridization from sp2-to-sp3. These mechanistic postulates are affirmed in Kohn-Sham density functional calculations, which predict lower barriers for additive mechanisms for arene oxidation than H atom abstraction steps. The calculations show that the reaction proceeds on the pentadectet spin surface and that a non-innocent ligand participates in the transfer of the H atom. Following precedent literature which demonstrates that these Fe(IV)-oxo species react with C-H bonds in alkanes via hydrogen atom abstraction to form alcohols, it appears that iron(IV)-oxo species in MOFs exhibit duality in their reactions with inert hydrocarbon substrates akin to enzymes-if the C-H bonds are in saturated aliphatic hydrocarbons, then activation occurs via hydrogen abstraction, while if the C-H bonds are aromatic, then activation occurs by addition rearrangement.

In Situ Imaging of Faujasite Surface Growth Reveals Unique Pathways of Zeolite Crystallization.

Zeolite crystallization occurs by complex processes involving a variety of possible mechanisms. The sol gel media used to prepare zeolites leads to heterogeneous mixtures of solution and solid states with diverse solute species. At later stages of zeolite synthesis when growth occurs predominantly from solution, classical two-dimensional nucleation and spreading of layers on crystal surfaces via the addition of soluble species is the dominant pathway. At earlier stages, these processes occur in parallel with nonclassical pathways involving crystallization by particle attachment (CPA). The relative roles of solution- and solid-state species in zeolite crystallization have been a subject of debate. Here, we investigate the growth mechanism of a commercially relevant zeolite, faujasite (FAU). In situ atomic force microscopy (AFM) measurements reveal that supernatant solutions extracted from a conventional FAU synthesis at various times do not result in growth, indicating that FAU growth predominantly occurs from the solid state through a disorder-to-order transition of amorphous precursors. Elemental analysis shows that supernatant solutions are significantly more siliceous than both the original growth mixture and the FAU zeolite product; however, in situ AFM studies using a dilute clear solution with a lower Si/Al ratio revealed three-dimensional growth of surfaces that is distinct from layer-by-layer and CPA pathways. This unique mechanism of growth differs from those observed in studies of other zeolites. Given that relatively few zeolite frameworks have been the subject of mechanistic investigation by in situ techniques, these observations of FAU crystallization raise the question whether its growth pathway is characteristic of other zeolite structures.

In situ imaging of two-dimensional surface growth reveals the prevalence and role of defects in zeolite crystallization.

Zeolite crystallization predominantly occurs by nonclassical pathways involving the attachment of complex (alumino)silicate precursors to crystal surfaces, yet recurrent images of fully crystalline materials with layered surfaces are evidence of classical growth by molecule attachment. Here we use in situ atomic force microscopy to monitor three distinct mechanisms of two-dimensional (2D) growth of zeolite A where we show that layer nucleation from surface defects is the most common pathway. Direct observation of defects was made possible by the identification of conditions promoting layered growth, which correlates to the use of sodium as an inorganic structure-directing agent, whereas its replacement with an organic results in a nonclassical mode of growth that obscures 2D layers and markedly slows the rate of crystallization. In situ measurements of layered growth reveal that undissolved silica nanoparticles in the synthesis medium can incorporate into advancing steps on crystal surfaces to generate defects (i.e., amorphous silica occlusions) that largely go undetected in literature. Nanoparticle occlusion in natural and synthetic crystals is a topic of wide-ranging interest owing to its relevance in fields spanning from biomineralization to the rational design of functional nanocomposites. In this study, we provide unprecedented insight into zeolite surface growth by molecule addition through time-resolved microscopy that directly captures the occlusion of silica nanoparticles and highlights the prevalent role of defects in zeolite crystallization.

Regulating Nonclassical Pathways of Silicalite-1 Crystallization through Controlled Evolution of Amorphous Precursors.

Differentiating mechanisms of zeolite crystallization is challenging owing to the vast number of species in growth solutions. The presence of amorphous colloidal particles is ubiquitous in many zeolite syntheses, and has led to extensive efforts to understand the driving force(s) for their self-assembly and putative roles in processes of nucleation and growth. In this study, we use a combination of in situ scanning probe microscopy, particle dissolution measurements, and colloidal stability assays to elucidate the degree to which silica nanoparticles evolve in their structure during the early stages of silicalite-1 synthesis. We show how changes in precursor structure are mediated by the presence of organics, and demonstrate how these changes lead to significant differences in precursor-crystal interactions that alter preferred modes of crystal growth. Our findings provide guidelines for selectively controlling silicalite-1 growth by particle attachment or monomer addition, thus allowing for the manipulation of anisotropic rates of crystallization. In doing so, we also address a longstanding question regarding what factors are at our disposal to switch from a nonclassical to classical mechanism.

Bridging the Gap between Structurally Distinct 2D Lamellar Zeolitic Precursors through a 3D Germanosilicate Intermediate.

There is broad scientific interest in lamellar zeolitic materials for a large variety of technological applications. The traditional synthetic methods towards two-dimensional (2D) zeolitic precursors have made a great impact in the construction of families of related zeolites; however, the connection between structurally distinct 2D zeolitic precursors is much less investigated in comparison, thereby resulting in a synthetic obstacle that theoretically limits the types of zeolites that can be constructed from each layer. Herein, we report a Ge-recycling strategy for the topotactic conversion between different 2D zeolitic precursors through a three-dimensional (3D) germanosilicate. Specifically, the intermediate germanosilicate can be constructed within 150 min by taking advantage of its structural similarity with the parent lamellar precursor. This process enables the conversion of one 2D zeolite structure into another distinct structure, thus overcoming the synthetic obstacle between two families of zeolitic materials.

Transient modes of zeolite surface growth from 3D gel-like islands to 2D single layers.

Zeolite crystallization occurs by multifaceted processes involving molecule attachment and nonclassical pathways governed by the addition of amorphous precursors. Here, we use scanning probe microscopy to monitor zeolite LTA crystallization in situ with a spatiotemporal resolution that captures dynamic processes in real time. We report a distinctive pathway involving the formation of gel-like islands from supersaturated solutions comprised of (alumino)silicate molecules. Three-dimensional assembly and evolution of these islands constitutes a unique mode of growth that differs from classical theories. Time-resolved imaging also reveals that growth can occur by (nearly) oriented attachment. At later stages of crystallization, a progressive transition to lower supersaturation shifts growth to a layered mechanism involving two-dimensional nucleation and spreading of layers. Here, we show that LTA crystallization occurs by multiple pathways, thereby reconciling putative hypotheses of growth mechanisms while also highlighting new modes of nonclassical crystallization that may prove relevant to other zeolites and related materials.