Enthalpies of formation of microporous titanosilicates ETS-4 and ETS-10

The energetics of microporous titanosilicates ETS-4 (K1.13Na3.92Ti3.07Si8.1 7O25 . 8.64H(2)O) and ETS-10 (K0.61Na1.09Ti1.10Si4.98O13 . 2.89H(2)O) has b een investigated by high-temperature drop solution calorimetry using lead b orate as the solvent at 974 K. Combining the measured heats of drop solutio n with the published enthalpies of drop solution and formation for the cons tituent oxides, the standard enthalpies of formation from the oxides and fr om the elements (DeltaH(f.el)(0)) for both phases were derived for the firs t time. The obtained values (in kJ/mol) are as follows: DeltaH(f.ox)(0)(ETS -4) -818.5 +/- 13.1, DeltaH(f.el)(0)(ETS-4) = -14, 642.8 +/- 15.9, DeltaH(f .ox)(0) (ETS-10) = -262.2 +/- 3.1, and DeltaH(f.el)(0)(ETS-10) = -6995.2 +/ - 6.1. Comparison between the DeltaH(f.ox)(0) (or DeltaH(f.el)(0)) values o f the two phases suggests that ETS-4 is thermodynamically more stable than ETS-10 with respect to the oxides (or the elements) at 298 K and I atm. Thi s behavior can largely be attributed to the higher degree of hydration of E TS-4 than that of ETS-10. (C) 2001 Elsevier Science B.V. All rights reserve d.