HIGH-TEMPERATURE CALORIMETRY OF MCM-41

Thermochemical stability of pure-silica MCM-41 molecular sieves (pore size 2.5-5 nm) was studied by high temperature solution and transposed temperature drop calorimetry at 977 K. Though the pore size varies by approximately 2 nm (almost a factor of 2), there are minimal differences in enthalpy among these structures both at 977 and 298 K. At 298 K they are destabilized by less than 15 kJ/mol relative to quartz, which implies an energy only about 1 kJ/mol higher than siliceous zeolite Y, whose pore size is 0.74 nm. Results are compared with those for other microporous silicas and implications for structure/stability relations are discussed.