High-silica zeolites: A relationship between energetics and internal surface areas

Eric C. Moloy, Lilian P. Davila, James F. Shackelford, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

37 Scopus citations

Abstract

High-silica zeolites are 5.6-15.5 kJ/mol less stable in enthalpy than α-quartz-the stable polymorph of silica under ambient conditions. Previous studies have correlated these energetic metastabilities to molar volumes and framework densities. In this study, we consider the question of whether these energetics might arise from a surface free energy term that originates from the large internal surfaces of these materials. Cerius2 molecular simulation software is used to calculate the internal surface areas. A linear relationship between formation enthalpy and internal surface area is found for α-quartz, α-cristobalite, and 17 zeolitic frameworks: AFI, AST, BEA, CFI, CHA, EMT, FAU, FER, IFR, ISV, ITE, MEI, MEL, MFI, MTW, MWW, and STT. The slope of the regression line has direct physical meaning: an average internal surface enthalpy of 0.093 ± 0.009 J/m2. This value is similar to a value of 0.100 ± 0.035 J/m2 for the average external surface free energy of amorphous silica obtained from various amorphous, but not microporous or mesoporous, phases reported in the literature. We conclude that it is physically reasonable to consider the metastability of anhydrous silica zeolites as resulting from their large internal surface area, that the average value of the surface enthalpy (or surface free energy) is similar for both internal and external surfaces, and that this quantity is not strongly dependent on the specific nature of the tetrahedral framework.

Original languageEnglish (US)
Pages (from-to)1-13
Number of pages13
JournalMicroporous and Mesoporous Materials
Volume54
Issue number1-2
DOIs
StatePublished - Jul 1 2002
Externally publishedYes

Keywords

  • Computer simulation
  • Internal surface area
  • Silica
  • Surface free energy
  • Zeolite

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials

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