Hydrogen bonding of water confined in mesoporous silica MCM-41 and SBA-15 studied by 1H solid-state NMR.
The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling.
Freie Universität Berlin, Institut für Chemie, Takustrasse 3, 14195 Berlin, Germany.
Bob
B
Grünberg
Thomas
T
Emmler
Egbert
E
Gedat
Ilja
I
Shenderovich
Gerhard H
GH
Findenegg
Hans-Heinrich
HH
Limbach
Gerd
G
Buntkowsky
D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry; D016062:Porosity; D011522:Protons; D012822:Silicon Dioxide / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
C509968:MCM-41; D011522:Protons; C509969:SBA-15; D014867:Water; D012822:Silicon Dioxide; D009584:Nitrogen