TY - JOUR
T1 - High-silica zeolites
T2 - A relationship between energetics and internal surface areas
AU - Moloy, Eric C.
AU - Davila, Lilian P.
AU - Shackelford, James F.
AU - Navrotsky, Alexandra
N1 - Funding Information:
We would like to thank N. Khosrovani and J. Liang of Molecular Simulations Inc. for their help with the Cerius 2 software. We also thank M.E. Davis and P. Piccione of the California Institute of Technology for valuable discussions and for their participation in earlier calorimetric studies. This work was supported by the National Science Foundation (grants DMR97-31782 and 01-01391). It benefited from the infrastructure provided by CHiPR, the Center for High Pressure Research, an NSF Science and Technology Center. Finally, we appreciate informal discussions with Robert Bell (The Royal Institution of Great Britain) and Stacey Zones (Chevron–Texaco).
PY - 2002/7/1
Y1 - 2002/7/1
N2 - 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.
AB - 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.
KW - Computer simulation
KW - Internal surface area
KW - Silica
KW - Surface free energy
KW - Zeolite
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U2 - 10.1016/S1387-1811(02)00328-1
DO - 10.1016/S1387-1811(02)00328-1
M3 - Article
AN - SCOPUS:0036639426
SN - 1387-1811
VL - 54
SP - 1
EP - 13
JO - Microporous and Mesoporous Materials
JF - Microporous and Mesoporous Materials
IS - 1-2
ER -