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

The freezing behavior of water confined in the pores of five mesoporous MCM -41 materials of controlled acidity has been studied using H-1-NMR. The sur face acidity of Si, Si/Al and Si/Ti-based MCM-41 solids was determined usin g NH3 temperature programmed desorption experiments. The Full Width at Half Maximum of the H-1-NMR signal in the temperature range 300 K > T > 200 K, corresponding to the spin-spin re:laxation time 1/T-2 was recorded at 5 deg rees intervals. The 1/T-2 values for all samples show a smooth increase as temperature drops down to T approximate to 225-230 K. At this region the co re water in the pores freezes leaving a layer of water between the pore wal l and the core ice unfreezable. This shell of unfreezable water undergoes a continuous transition as temperature drops, Below T < 225-230 K the slope d(1/T-2)/dT depends on the surface acid density. An energy relationship (1/ T-2) = A exp(Q/RT) was used to calculate the Q and A values which show a co mpensation effect. The energy parameter Q assumes values between 14-19 kJ m ol(-1) and depends on the acid-acid site distance. A model of water molecul es bridging the acid surface sites is proposed. The number N of water molec ules bridging these sites is suggested to be related to Q via a charge - ho pping mechanism. The Q value becomes zero when the distance between the aci d sites is similar to5.5 Angstrom which corresponds to the unfreezable shel l water layer in the pores. (C) 2001 Elsevier Science B.V. All rights reser ved.