Nanosized Zeolite P for Enhanced CO2 Adsorption Kinetics.

Downsizing zeolite crystals is a rational solution to address the challenge of slow adsorption rates for industrial applications. In this work, we report an environmentally friendly seed-assisted method for synthesizing nanoscale zeolite P, which has been shown to be promising for binary separations. The potassium-exchanged form of nanoagglomerates demonstrates dramatically enhanced CO2 adsorption capacity, improved diffusion rate, and separation performance. Single-component CO2 adsorption at equilibrium demonstrated higher CO2 uptake and faster adsorption kinetics (ca. 1400 s vs >130000 s) for nanosized zeolite (KP1) compared to its micron-sized (KP2) counterpart. The diffusion kinetics analysis revealed the relation between the crystal size and the transport mechanism. The micron-sized KP2 sample was primarily governed by a surface barrier resistance mechanism, while in KP1, the diffusion process involved both intracrystalline and surface barrier resistance, facilitating the surface diffusion process and enhancing the overall diffusion rate. Breakthrough curve analysis confirmed these findings as fast and efficient CO2/N2 and CO2/CH4 separations recorded for the nanosized sample. The results showed remarkably enhanced breakthrough time for KP2 vs KP1 in CO2/N2 (1.0 vs 10.9 min) and CO2/CH4 (1.1 vs 9.9 min) mixtures, along with much higher adsorption capacity for CO2/N2 (0.18 vs 1.33 mmol/g) and CO2/CH4 (0.18 vs 1.21 mmol/g) mixtures. The set of experimental data demonstrates the importance of zeolite crystal engineering for improving the gas separation performance of processes involving CO2, N2, and CH4.

Interplay between alkali-metal cations and silanol sites in nanosized CHA zeolite and implications for CO2 adsorption.

Silanols are key players in the application performance of zeolites, yet, their localization and hydrogen bonding strength need more studies. The effects of post-synthetic ion exchange on nanosized chabazite (CHA), focusing on the formation of silanols, were studied. The significant alteration of the silanols of the chabazite nanozeolite upon ion exchange and their effect on the CO2 adsorption capacity was revealed by solid-state nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR) spectroscopy, and periodic density functional theory (DFT) calculations. Both theoretical and experimental results revealed changing the ratio of extra-framework cations in CHA zeolites changes the population of silanols; decreasing the Cs+/K+ ratio creates more silanols. Upon adsorption of CO2, the distribution and strength of the silanols also changed with increased hydrogen bonding, thus revealing an interaction of silanols with CO2 molecules. To the best of our knowledge, this is the first evidence of the interplay between alkali-metal cations and silanols in nanosized CHA.

The role of mixed alkali metal cations on the formation of nanosized CHA zeolite from colloidal precursor suspension.

A clear understanding of the crystal formation pathways of zeolites remains one of the most challenging issues to date. Here we investigate the synthesis of nanosized chabazite (CHA) zeolites using organic template-free colloidal suspensions by varying the time of aging at room temperature and the time of hydrothermal treatment at 90 °C. The role of mixed alkali metal cations (Na+, K+, Cs+) on the formation of CHA in the colloidal suspensions was studied. Increasing the aging time of the precursor colloidal suspension from 4 to 17 days resulted in faster crystallization of CHA nanocrystals (3 h instead of 7 h at 90 °C) to afford significantly smaller particles (60 nm vs 600 nm). During the crystallization a considerable change in the content of inorganic cations in the recovered solid material was observed to coincide with the formation of the CHA nanocrystals. The Na+ cations were found to direct the formation of condensed and pre-shaped aluminosilicate particles in the colloidal precursor suspensions, while K+ cations facilitated the formation of secondary building units (SBUs) of the CHA type framework structure such as d6r and cha cages, and the Cs+ cations promoted the long-range crystalline order facilitating the crystallization of stable zeolite nanocrystals.

A Facile Route toward the Increase of Oxygen Content in Nanosized Zeolite by Insertion of Cerium and Fluorinated Compounds.

Enriching oxygen content within nanosized zeolite X (as synthesized Na-X) by insertion of cerium (ion exchanged Ce-X) and functionalization with bromoperfluoro-n-octane (fluorinated F-X) is reported. The materials were fully characterized by powder X-ray diffraction (XRD), dynamic light scattering (DLS), zeta potential, thermogravimetric analysis (TGA), nitrogen adsorption, and nuclear magnetic resonance (19F NMR). The O2 adsorption in the zeolite samples at various concentrations (0 to 165 Torr) at -196 °C was studied by in situ FTIR. The modification of nanosized zeolites did not alter their colloidal stability, crystallinity, porosity, and particle size distribution. The inclusion of cerium and bromoperfluoro-n-octane considerably increase the oxygen capacity by 33% for samples Ce-X and F-X in comparison to the as-synthesized Na-X zeolite. Further, toxicity tests revealed that these materials are safe, which opens the door for their implementation in medical applications, where controlled delivery of oxygen is highly desirable.

The preparation of hierarchical SAPO-34 crystals via post-synthesis fluoride etching.

SAPO-34 crystals are etched in fluoride medium. The interface between crystalline domains is dissolved and yields a hierarchical material with a system of straight intersecting mesopores that improve the access to micropore space.