Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite

An Extrinsic-Pore-Containing Molecular Sieve Film: A Robust, High-Throughput Membrane Filter

MFI type zeolites with 10-membered-ring pores (ca. 0.55 nm) have the ability to separate p-xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non-zeolitic micropores (ca. 0.6-1.5 nm) and mesopores (ca. 2-7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non-zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p-xylene perm-selectivity and molar flux. Specifically, compared to a conventional, crack network-containing MFI membranes of similar thickness (ca. 1 μm), the mesoporous MFI membranes showed almost double p-xylene permeance (ca. 1.6±0.4×10^-7  mol m^-2  s^-1  Pa^-1 ) and a high p-/o-xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non-zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

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.

Behavior of a Metal Organic Framework Thin-Film at Elevated Temperature and Pressure as Studied with an Autoclave-Inserted Atomic Force Microscope

Bridging the gap in studying surface reactions, processes, and morphology and measuring at (catalytic) relevant conditions is crucial for our understanding of the working principles of porous crystalline materials. Scanning tunneling microscopy is limited because of the required conductivity of the sample, whereas atomic force microscopy (AFM) is often challenging in use owing to the physical mechanism underlying the technique. Herein, we report a tailor-made autoclave-inserted AFM, able to measure at ∼20 bar and ∼110 °C. First, we show the ability to obtain nanometer resolution on well-defined test samples at before-mentioned conditions. Second, to demonstrate the possibilities of analyzing morphological evolutions at elevated temperatures and pressures, we use this setup to measure the stability of a surface-anchored metal-organic framework (SURMOF) in-situ at pressures of 1-20 bar in the temperature range between 20 and 60 °C. It was found that the showcase HKUST-1 material has a good physical stability, as it is hardly damaged from exposure to pressures up to 20 bar. However, its thermal stability is weaker, as exposure to elevated T damaged the material by influencing the interaction between organic linker and metal cluster. In-situ measurements at elevated T also showed an increased mobility of the material when working at such conditions. Combining the strength of AFM at elevated T and p with ex-situ AFM and spectroscopic measurements on this MOF showcases an example of how porous materials can be studied at (industrially) relevant conditions using the autoclave-inserted AFM.

Uniformly Oriented Zeolite ZSM-5 Membranes with Tunable Wettability on a Porous Ceramic

Facile fabrication of well-intergrown, oriented zeolite membranes with tunable chemical properties on commercially proven substrates is crucial to broadening their applications for separation and catalysis. Rationally determined electrostatic adsorption can enable the direct attachment of a b-oriented silicalite-1 monolayer on a commercial porous ceramic substrate. Homoepitaxially oriented, well-intergrown zeolite ZSM-5 membranes with a tunable composition of Si/Al=25-∞ were obtained by secondary growth of the monolayer. Intercrystallite defects can be eliminated by using Na+ as the mineralizer to promote lateral crystal growth and suppress surface nucleation in the direction of the straight channels, as evidenced by atomic force microscopy measurements. Water permeation testing shows tunable wettability from hydrophobic to highly hydrophilic, giving the potential for a wide range of applications.

Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core-Shell Materials

The term "engineered zeolitic materials" refers to a class of materials with a rationally designed pore system and active-sites distribution. They are primarily made of crystalline microporous zeolites as the main building blocks, which can be accompanied by other secondary components to form composite materials. These materials are of potential importance in many industrial fields like catalysis or selective adsorption. Herein, critical aspects related to the synthesis and modification of such materials are discussed. The first section provides a short introduction on classical zeolite structures and properties, and their conventional synthesis methods. Then, the motivating rationale behind the growing demand for structural alteration of these zeolitic materials is discussed, with an emphasis on the ongoing struggles regarding mass-transfer issues. The state-of-the-art techniques that are currently available for overcoming these hurdles are reviewed. Following this, the focus is set on core-shell composites as one of the promising pathways toward the creation of a new generation of highly versatile and efficient engineered zeolitic substances. The synthesis approaches developed thus far to make zeolitic core-shell materials and their analogues, yolk-shell, and hollow materials, are also examined and summarized. Finally, the last section concisely reviews the performance of novel core-shell, yolk-shell, and hollow zeolitic materials for some important industrial applications.