Surface processes during sorption of aromatic molecules on medium pore zeolites

Area of Science:

• Physical Chemistry
• Materials Science
• Catalysis

Background:

• Zeolites, particularly H/ZSM-5, are crucial in catalysis and separation due to their porous structure and acid sites.
• Understanding the adsorption and transport mechanisms of aromatic hydrocarbons within zeolites is vital for optimizing industrial processes.
• The interaction of volatile organic compounds (VOCs) like benzene, toluene, and xylenes with zeolite active sites influences their catalytic activity and selectivity.

Purpose of the Study:

• To investigate the elementary steps of sorption and transport for benzene, toluene, and o-, p-xylene on zeolite H/ZSM-5.
• To differentiate sorption mechanisms on external surface hydroxyl groups (SiOH) versus internal pore hydroxyl groups (SiOHAl).
• To elucidate the role of molecular structure and steric hindrance in the adsorption kinetics and pathways.

Main Methods:

• Utilized pressure modulation coupled with fast time-resolved infrared (IR) spectroscopy.
• Monitored the dynamic changes in molecular interactions with zeolite active sites.
• Analyzed sorption kinetics and equilibrium states of aromatic molecules on different hydroxyl groups.

Main Results:

• A common physisorbed state on the outer surface precedes sorption onto SiOH and SiOHAl acid sites.
• Aromatic molecules exhibit rapid equilibration in the physisorbed state, suggesting high surface mobility before chemisorption.
• Molecules with pore access (benzene, toluene, p-xylene) adsorb faster on internal SiOHAl sites than external SiOH sites; sterically hindered o-xylene shows enhanced adsorption on accessible external SiOH sites.

Conclusions:

• Sorption of benzene, toluene, and xylenes in H/ZSM-5 involves a mobile physisorbed intermediate on the external surface.
• Adsorption rates are dependent on the accessibility of internal (SiOHAl) versus external (SiOH) acid sites and molecular steric factors.
• The findings provide fundamental insights into zeolite-host guest interactions, crucial for designing advanced catalytic materials and separation processes.