THERMOCHEMICAL INVESTIGATION OF NATURAL FIBROUS ZEOLITES

Abstract

New experimental data on the heat capacities, enthalpies of dehydration and formation of some fibrous zeolites have been obtained. Natural samples of zeolites of the natrolite group including natrolite Na2Al2Si3O10 · 2H2O, tetranatrolite Na2Al2Si3O10 · 2H2O, scolecite CaAl2Si3O10 · 3H2O, mesolite Na2Ca2[Al2Si3O10] 3 · 8H2O and structural types of thomsonite NaCa2[Al5Si5O20] · 6H2O, edingtonite BaAl2Si3O10 · 4H2O and gonnardite Na0.16Ca1.95Al4Si6O20 · 6H2O were used for calorimetric study. The heat capacities of zeolites were measured by DSC from 120 K to dehydration temperatures and the resultant C0p(T) equations were calculated. The standard enthalpies of dehydration of mineral studied at 298.15 K were determined by two-step "transposed temperature drop" technique in a Calvet microcalorimeter: 115.3 ± 7.2 kJ/mol (natrolite), 114.6 ± 8.4 (tetranatrolite), 121.6 ± 8.8 (scolecite), 350.5 ± 18.6 (mesolite), 333.4 ± 23.2 (thomsonite), 88.6 ± 13.5 (edingtonite) and 315.8 ± 35.4 (gonnardite). The standard enthalpies of formation (in kJ/mol) from oxides and elements at T = 298.15 K were calculated using experimental data obtained by the "drop solution Calvet microcalorimetry": ΔHof(oxidcs) ΔHof(elements) Natrolite -372.6± 8.1 - 5769.9± 8.7 Tetranatrolite -349.4± 14.3 - 5743.7±14.6 Scolecite -200.3± 9.9 - 6100.7±10.5 Mesolite -748.9±25.0 -17943.9±27.2 ΔHof(oxides) ΔHof(elements) Thomsonit -529.4 ±22.2 -12464.8±23.0 Edingtonite -287.4±14.9 -12793.2±30.8 Gonnardite -396.4±19.7 -12198.6±20.9 The small anomalies in heat-capacity behaviour of thomsonite, edingtonite and gonnardite in the temperature interval of 120-210 K which are associated with phase transformations were observed by DSC method. The α-β metanatrolite transition at T = 836 ± 3 K was studied and the enthalpy was calculated (14.1 ± 10 kJ/mol). The enthalpy of the disordering reaction: natrolite - tetranatrolite was determined from experimental calorimetric data to be 23.2 kJ/mol.