Stability of Zeolites in Hot Liquid Water

Abstract

Zeolites Y and ZSM-5 with varying Si/Al ratios are treated in liquid water at 150 and 200 °C under autogenic pressure to assess their hydrothermal stability. The changes in the structure are characterized by atomic absorption spectroscopy, X-ray diffraction, scanning electron microscopy, argon physisorption, 27Al and 29Si MAS NMR spectroscopy, temperature-programmed desorption of ammonia, and pyridine adsorption followed by IR spectroscopy. During treatment in hot water, zeolite Y with a Si/Al ratio of 14 or higher is transformed into an amorphous material, and the rate of this degradation increases with increasing Si/Al ratio. In contrast, ZSM-5 is not modified under the same conditions. The main degradation mechanism is suggested to be hydrolysis of the siloxane bonds (Si−O−Si) as opposed to dealumination, which dominates under steaming conditions. In the resulting amorphous phase, Al remains tetrahedrally coordinated, but the micropore volume and concentration of accessible acid sites is reduced dramatically. The results demonstrate that potential structural changes of zeolites have to be considered when these materials are used as catalysts for aqueous phase conversion of biomass.

Improving Heterogeneous Catalyst Stability for Liquid-phase Biomass Conversion and Reforming

“…[72,73] As shown by detailed characterization using 27 Al and 29 Si solid-state NMR and NH 3 -TPD, siloxane bridge hydrolysis was the main deactivation route in the presence of hot liquid water while zeolites treated under steam suffered from dealumination. [74] After a treatment in hot water at 200°C, mimicking APR conditions, ZSM-5 was stable independent of its Si/Al ratio while degradation of zeolite Y increased with increasing Si/Al ratio, as opposed to the better stability of zeolites with higher Si/Al in the presence of steam. Finally, Al-rich zeolite Y could be stabilized by incorporation of lanthanum cations.…”

“…Finally, Al-rich zeolite Y could be stabilized by incorporation of lanthanum cations. [74] Recently, Gardner et al reported that testing the hydrothermal stability of zeolites in distilled water was not completely relevant to biomass conversion reaction conditions. [75] In fact, dry biomass contains about 1.5 wt.% of inorganic salts such as NaCl and the dissolution rate of metal oxides could be dramatically enhanced by cations in solution.…”

Continuous flow synthesis of ZSM-5 zeolite on the order of seconds

“…The high-energy X-ray total scattering experiments at SPring-8 were approved by the Japan Synchrotron Radiation Research Institute under Proposals 2015B0115, 2016A0115, and 2016B0115. 27 Al solid NMR spectra for the products synthesized at 240, 260, 280, and 300°C, respectively. ("tetra" and "octa" denote tetrahedral and octahedral coordination, respectively.…”

“…Fig. 5 A-D shows the 27 Al MAS NMR spectra of the products synthesized at 240, 260, 280, and 300°C, respectively. The 27 Al MAS NMR spectrum can serve as an accurate indicator of crystallinity because aluminum hydroxide with a gibbsite structure, whose peak appears at around 0 ppm, totally dissolves and is consumed only on complete crystallization (14).…”

“…Although it is considered that zeolites are susceptible to hot water owing to the accelerated hydrolysis of silanol groups at high temperatures (27,28), they can withstand the hot water environment if the residence time is short. We demonstrate herein the continuous flow synthesis of ZSM-5, where direct mixing of a welltuned precursor with pressurized hot water at high temperature was used to quicken the crystallization rate, resulting in the synthesis on the order of seconds.…”