Triggered Gate Opening and Breathing Effects during Selective CO2 Adsorption by Merlinoite Zeolite.

Zeolites with flexible structures that adapt to coordinate extraframework cations when dehydrated show a rich variety of gas adsorption behavior and can be tuned to optimize kinetics and selectivity. Merlinoite zeolite (topology type MER) with Si/Al = 3.8 has been prepared in Na, K, and Cs forms and its structural response to dehydration measured: the unit cell volumes decrease by 9.8%, 7.7%, and 7.1% for Na-, K-, and Cs-MER, respectively. Na-MER adopts Immm symmetry, while K- and Cs-MER display P42/nmc symmetry, the difference attributed to the preferred locations of the smaller and larger cations. Their performance in CO2 adsorption has been measured by single-component isotherms and by mixed gas (CO2/CH4/He) breakthrough experiments. The differing behavior of the cation forms can be related to structural changes during CO2 uptake measured by variable-pressure PXRD. All show a "breathing" transition from narrow to wide pore forms. Na- and Cs-MER show non-Type I isotherms and kinetically-limited CO2 adsorption and delivery of pure CH4 in CO2/CH4 separation. However, K-MER shows good uptake of CO2 (3.5 mmol g-1 at 1 bar and 298 K), rapid adsorption and desorption kinetics, and promising CO2/CH4 separation. Furthermore, the narrow-to-wide pore transition occurs rapidly and at very low pCO2 via a "triggered" opening. This has the consequence that whereas no CH4 is adsorbed from a pure stream, addition of low levels of CO2 can result in pore opening and uptake of both CO2 and CH4, although in a continuous stream the CH4 is replaced selectively by CO2. This observed cation size-dependent adsorption behavior derives from a fine energetic balance between different framework configurations in these cation-controlled molecular sieves.

Tuning Zeolite Properties for a Highly Efficient Synthesis of Propylene from Methanol.

A series of nanosized ZSM-5 samples was synthesized at 170, 150, 120, and 100 °C. Experimental data show that the decrease of crystallization temperature leads to significant changes in zeolite properties. Crystals synthesized at 100 °C exhibit many framework defects with lower acid-site density, strength, and a larger external surface area. The selectivity to light olefins and the propylene-to-ethylene ratio increases as the crystallization temperature decreases. A propylene-to-ethylene ratio of above 6 with the highest selectivity to propylene of 53 % was obtained over ZSM-5 catalyst prepared at 100 °C. The stability of the nanosized zeolite in methanol to olefins (MTO) was also improved compared to the industrial sample with a similar Si/Al ratio. This catalytic performance is a result of the decrease in the acid-site density, strength, and the crystals' size, providing a shorter diffusion path and larger external surface area. The presence of structural defects and a different external surface in the crystals has been shown to play an important role in the MTO catalyst performance.

3D Study of the Morphology and Dynamics of Zeolite Nucleation.

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na-rich aluminosilicate system. A detailed time-series EMT-type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core-shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels.