The influence of surface acid density on the freezing behavior of water confined in mesoporous MCM-41 solids

✍ Scribed by Styliani Sklari; Hanna Rahiala; Vassilis Stathopoulos; Jarl Rosenholm; Philippos Pomonis

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 658 KB
• Volume: 49
• Category: Article
• ISSN: 1387-1811
• DOI: 10.1016/s1387-1811(01)00371-7

✦ Synopsis

The freezing behavior of water con®ned in the pores of ®ve mesoporous MCM-41 materials of controlled acidity has been studied using 1 H-NMR. The surface acidity of Si, Si/Al and Si/Ti-based MCM-41 solids was determined using NH 3 temperature programmed desorption experiments. The Full Width at Half Maximum of the 1 H-NMR signal in the temperature range 300 K > T > 200 K, corresponding to the spin±spin relaxation time 1=T 2 , was recorded at 5°i ntervals. The 1=T 2 values for all samples show a smooth increase as temperature drops down to T % 225±230 K. At this region the core water in the pores freezes leaving a layer of water between the pore wall and the core ice unfreezable. This shell of unfreezable water undergoes a continuous transition as temperature drops. Below T < 225±230 K the slope d1=T 2 =dT depends on the surface acid density. An energy relationship 1=T 2 A expQ=RT was used to calculate the Q and A values which show a compensation eect. The energy parameter Q assumes values between 14±19 kJ mol À1 and depends on the acid±acid site distance. A model of water molecules bridging the acid surface sites is proposed. The number N of water molecules bridging these sites is suggested to be related to Q via a charge ± hopping mechanism. The Q value becomes zero when the distance between the acid sites is $5.5

A which corresponds to the unfreezable shell water layer in the pores.