Identification of the Basic Sites on Nitrogen-Substituted Microporous and Mesoporous Silicate Frameworks Using CO₂ as...

Area of Science:

• Materials Science
• Surface Chemistry
• Catalysis

Background:

• Understanding surface properties of porous materials is crucial for catalysis and adsorption.
• Nitrogen-substituted silicas offer tunable basicity for various applications.
• Characterizing active sites requires precise analytical techniques.

Purpose of the Study:

• To investigate the use of carbon dioxide (CO2) as a probe molecule for identifying surface basic sites.
• To differentiate between weak and strong basic interactions on nitrogen-substituted silicas.
• To evaluate CO2's potential for characterizing diverse surface functionalities.

Main Methods:

• Adsorption isotherm measurements.
• Heat of adsorption analysis.
• Infrared (IR) spectroscopy.
• Utilizing CO2 as a probe molecule.

Main Results:

• CO2 adsorption revealed weak hydrogen-bonded interactions with silanol (Si-OH) and silamine (Si-NH-Si) groups.
• Silamine sites showed preferential CO2 adsorption over silanol sites.
• SBA-15 mesoporous silica exhibited high linear CO2 adspecies due to abundant silanol groups.
• Carbamate adspecies were absent on silamino sites, while carbonate adspecies formed on alkali-exchanged zeolites and residual sodium ions.

Conclusions:

• CO2 is a suitable probe for identifying weak hydrogen-bonding sites on silica surfaces.
• CO2 can potentially probe strongly interactive nucleophilic sites, such as those from alkaline ions or methylated silamino groups.
• The study highlights CO2's versatility in surface characterization of modified silicas.