Hydrogen-bond-donating acidity and dipolarity/polarizability of surfaces within silica gels and mesoporous MCM-41 materials

Journal Article · Mon Nov 01 04:00:00 EST 1999 · Chemistry of Materials

DOI:https://doi.org/10.1021/cm990308t· OSTI ID:20004032

Spange, S; Zimmermann, Y; Graeser, A

Linear solvation energy (LSE) relationships are employed to characterize the internal polarities in the channels of a siliceous MCM-41 material and of a silica gel synthesized by a sol-gel process from tetramethoxysilane (TMOS). The polarity of the surfaces inside the silica gel and the mesopores of the MCM-41 can be quantitatively described by three independent terms: the dipolarity/polarizability ({pi}*), the HBD (hydrogen-bond-donating) acidity ({alpha}), and the HBA (hydrogen-bond-accepting) ability ({beta}). These terms are defined by means of the Kamlet-Taft solvent parameters {alpha}, {beta}, and {pi}* as a reference system. The following indicators have been encapsulated by the sol-gel process within the silica matrix or adsorbed to MCM-41: 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate, dicyanobis-(1,10-phenanthrolin)iron(II), and Michler's ketone. The solvatochromic UV/vis spectroscopic band shifts of these indicators correlate with the Kamlet-Taft solvent parameters {alpha} and {pi}*. The HBD acidity, {alpha}, within MCM-41 increases significantly with decreasing temperature, whereas the dipolarity/polarizability term, {pi}*, is temperature-independent. With 2 as indicator, an average hydrogen-bond acidity ({alpha}) is measured, whereas 3 is suitable to distinguish between differently acidic silanols within MCM-41 as shown by surface titration of the adsorbed indicator 3 with a sterically hindered base.

• Research Organization: Univ. of Technology Chemnitz (DE)
• OSTI ID: 20004032
• Journal Information: Chemistry of Materials, Journal Name: Chemistry of Materials Journal Issue: 11 Vol. 11; ISSN CMATEX; ISSN 0897-4756
• Country of Publication: United States
• Language: English

Related Subjects

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATION
36 MATERIALS SCIENCE
BINDING ENERGY
HYDROGEN
MOLECULAR SIEVES
PHENOLATES
SILANES
SILICA GEL
SOL-GEL PROCESS