New Approach to the Acidity Characterization of Pristine Zeolite Crystals by Ethylene Using Reversed-Flow Inverse Gas Chromatography (RF-IGC)

Abstract

The adsorption and diffusion are key parameters for the catalytic conversions of ethylene to hydrocarbons. Using microcrystals of beta zeolites and the reversed-flow inverse gas chromatography technique, a new approach is developed for the catalytic characterization of pristine crystals. On the basis of the monitoring of the dynamic concentration in a time-resolved way, this method allows one to follow the surface coverage and adsorption energies among other physicochemical criteria. Pristine crystals were synthesized to rule out the effect of surface defects. The impact of porosity and acidity was studied by ethylene adsorption over porous, nonporous, and acid materials. The acidity of the mother zeolite (acid and porous) was decreased by retro-exchanges and partial cokage. The modification of the ethylene sorption due to the Brønsted acidity was then highlighted. The local surface coverage was not affected by the presence of porosity, while a second adsorption phase of the ethylene was induced by the modification of the acidity. Moreover, a relation between the amount of Brønsted acidity and the ethylene lateral interactions measured by RF-IGC was evidenced.

Impact of Crystal Size on the Acidity and the Involved Interactions Studied by Conventional and Innovative Techniques

“…The observed minimum energies corresponds to the end of the monolayer and the beginning of the physisorption phase. It is only observed on acidic zeolites . In a previous study, we emphasized the importance of lateral interactions.…”

“…The value is higher for NS (0.30 cm 3 g –1 ). This corresponds to an ultramicroporous pore volume of diameter close to 0.15 nm . The crystal sizes reduction implies an increase of both the specific surfaces and mesoporous volumes.…”

“…With regard to perfectly crystallized materials (MC), the same correlation previously observed as a function of the Brønsted acidity is still valid. 26 Since the silanol contents are similar, the lateral interactions depend only on previously occupied Brønsted and Lewis sites. Moreover, these sites are in a fairly similar physicochemical environment (same textural properties).…”

“…In addition to these methods, our research team recently used reverse-flow inverse gas chromatography (RF-IGC) for the zeolites characterization. − A recent work was carried out in order to study the pristine β-microcrystals acidity . These materials have only micropores and very small external surfaces; the surface defects are therefore very few.…”

Simultaneous Characterization of Solid Acidity and Basicity of Metal Oxide Catalysts via the Solid-State NMR Technique

“…The acid–base reaction is the most common type of chemical reactions − and provides significant benefits for industrial applications, , so an accurate characterization of acidity and basicity is important for understanding the catalytic mechanism and designing efficient catalysts. The existing experimental methods for acidity–basicity characterization include Hammett indicators, , NH 3 and CO 2 temperature-programmed desorption (TPD), − microcalorimetry, , inverse gas chromatography, , infrared (IR) spectroscopy, ,, and solid-state NMR spectroscopy. − Among these methods, solid-state NMR spectroscopy offers unique quantitative information on the type and the strength of acids and bases. However, the acidity and basicity of a solid catalyst are always characterized separately.…”