Liquid phase calorimetric-adsorption analysis of Si-MCM-41: Evidence of strong hydrogen-bonding sites

The surface acidity of the molecular sieve Si-MCM-41 was investigated by liquid phase calorimetric-adsorption analysis (Cal-Ad), gas adsorption measurements (TG-TPD) and infrared spectroscopy using pyridine as the probe molecule. The results showed that Si-MCM-41 evacuated at 200 °C had one type of hydrogen bonding site (n1 = 2.27 mmol g−1 with ΔH1 = −95.3 kJ mol−1) that was much stronger than those in silica gel. These sites corresponded to the total OH groups of the material surface and were able to retain pyridine at higher temperatures than silica gel (400–600 °C). In addition, the material had a higher entropy value (ΔS1 = −250 J K−1 mol−1) than silica gel. This indicates that pyridine is more strongly adsorbed on the surface of Si-MCM-41 than silica-gel, which leads to better catalyst properties. Solid-state 29Si MAS-NMR measurements showed a high amount of silanol groups (52%) in the Si-MCM-41 structure (for the Q2 and Q3 species). These results suggest a highly polar environment caused by the hydroxyl groups that line the walls of the hexagonal long-range ordered pores.

Graphical abstractOne-site model was obtained by donor–acceptor adsorption and calorimetric analysis of pyridine for three adjacent silanol groups hydrogen-bonding to each other on the molecular sieve Si-MCM-41 surface.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Si-MCM-41 shows one type of hydrogen-bonding site much stronger than silica gel. ► Pyridine is more strongly adsorbed on the Si-MCM-41 surface than silica-gel. ► Si-MCM-41 structure leads to strong interactions between the adjacent silanol groups. ► A polar environment of hydroxyl groups lines the walls of the long-range pores. ► Hydrogen-bonding sites are primarily located inside the long-range ordering mesopores.