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 degrees C had one type of hydrogen bonding site (n(1) = 2.27 mmol g(-1) with Delta H-1 = -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 degrees C). In addition, the material had a higher entropy value (Delta S-1= -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 Si-29 MAS-NMR measurements showed a high amount of silanol groups (52%) in the Si-MCM-41 structure (for the Q(2) and Q(3) species). These results suggest a highly polar environment caused by the hydroxyl groups that line the walls of the hexagonal long-range ordered pores. (c) 2010 Elsevier Inc. All rights reserved.