Towards understanding sodium effect on USY zeolite

Samples of a commercial USY zeolite were exchanged with NaCl solution and then thermally treated under dry atmosphere. Resulting samples were evaluated by n-butane cracking, isopropylamine (i-pam) decomposition, TG–DSC, nitrogen adsorption, and powder XRD. Activation barriers for n-butane reactions were determined as a function of sodium load in the samples. The addition of sodium chloride has a marked negative effect upon material crystallinity, but dealumination rate does not seem to be affected, which suggests framework destruction mediated by sodium does not involve aluminum atoms. As sodium was loaded, samples were less able to retain water because of loss in crystallinity as shown by XRD and volume of micropore measurements. Activity to n-butane cracking decreases monotonically with sodium load, but selectivity to cracking, dehydrogenation, and hydride transfer products remained unaltered; that shows sodium chloride effect is not selective to a particular kind of acid site. The Brønsted acid site (BAS) density measured by i-pam decomposition exhibits a volcano-like profile with sodium load; at low sodium contents there is a gain in i-pam accessibility due to structural collapse but at higher loads BAS disappearance overcomes accessibility and density of acid sites decreases. The average pore size increases with sodium load. After thermal treatment in presence of sodium chloride, gates are opened in zeolite structure, allowing n-butane and isopropylamine to reach formerly inaccessible sites; hydrocarbon conversion can occur in spatially constrained environments where stabilization effects lead to low-energy/low-entropy activated complexes and apparent activation energy shows a minimum with sodium chloride load. Sodium chloride has an effect on Y zeolite even in dry conditions at temperatures similar to those predominant in FCC regenerator.

Figure optionsDownload high-quality image (58 K)Download as PowerPoint slideResearch highlights▶ Sodium destroys USY zeolite but does not increase dealumination rate. ▶ Sodium affects activity but not selectivity in n-butane cracking. ▶ Exchange, if exist, is not selective to a particular kind of Brønsted acid site. ▶ At low sodium loads accessibility of probe molecules increase but at higher loads destruction overcomes this initial effect. ▶ n-Butane activation may occur in spatially constrained environments exposed by sodium action on Y zeolite.